Utilization of Agricultural Residue with a newly designed Biochar reactor

UTILIZATION OF AGRICULTURAL RESIDUE WITH A NEWLY DESIGNED

BIOCHAR REACTOR

A Process Engineering Project Report

Presented to the

Department of Chemical Engineering

Faculty of Chemical and Materials Engineering

College of Engineering

Kwame Nkrumah University of Science and Technology, Kumasi

by

Benjamin, Opare Donald

Godwyll, Richmond Kojo Annan

Obeng, Sika Kwame

Paintsil, Henry Odoom

Yeboah-Asuamah, Gerald

in Partial Fulfilment of the Requirements

for the Course

Process Engineering Project

April, 2011.

ABSTRACT

Agricultural residue has become a menace to the environment for a long time. This project,

however, seeks to utilise these residues by converting them into useful resources. The

thermo-chemical conversion of biomass (agricultural residue) under oxygen-controlled

conditions to produce biochar has been seen to be one major way of managing these

residues. Biochar has proved to be a good way of managing these agricultural residues since

it is able to improve soil properties, increase crop yield and also play a major role in

climatic and environment sustainability. Using a locally designed Pyrolysis system, five

different biochar samples were produced from sawdust and shavings of wood species like

Wawa, Odum, Teak, Asanfena etc., as well as from rice husk. An average yield of 1.42%,

16.2%, 28.26%, 26.09% and 44.8% biochar was obtained for Wawa stain sawdust, Odum

sawdust, wood shavings, Asanfena wood shavings and rice husk respectively. From the

analysis conducted on the five different samples produced, we found them to contain

appreciable amounts of soil nutrients like phosphorus, potassium, magnesium, calcium and

sodium, which are very essential for plant growth. The results obtained indicate that

agricultural residue can be pyrolysed into biochar to serve as a nutrient source and as well

as to sequester carbon.

ACKNOWLEDGEMENT

We first want to give thanks to the Almighty God who in His loving kindness has made this

project a success. We also want to appreciate Dr. Moses Y. Mensah who has been not only

a great supervisor but also a wonderful father despite his busy schedules. We also want to

thank the following people and group of persons who supported us in several ways to make

our project a wonder:

Dr. E. Yeboah, CSIR-Soil Research Institute, Kwadaso

Mr. E .Agbeko, Mechanical Engineering Department, KNUST

Mr. Edward Calys Tagoe, CSIR-Soil Research Institute, Kwadaso

Ato Fanyin-Martin, Postgraduate Student of Department Of Chemical Engineering, KNUST

Technicians in the PD Laboratory, Chemical Engineering Department, KNUST

We finally want to express our profound gratitude to all persons who in one way or the

other supported us and made this project a success.

TABLE OF CONTENT

CHAPTER ONE PAGE

1.0 INTRODUCTION....................................................................................... 1

1.1 General Objectives....................................................................................... 6

1.2 Specific Objectives........................................................................................ 6

CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 BIOCHAR.................................................................................................. 7

2.1.1 History of Biochar................................................................................ 8

2.1.2 Chemical Composition........................................................................ 9

2.1.3 Types of Biochar................................................................................. 10

2.1.4 Importance of Biochar......................................................................... 10

2.1.5 Limitations of Biochar Uses................................................................. 17

2.2 RAW MATERIALS

2.2.1 Biomass.................................................................................................... 18

2.2.1.1 Chemical Composition of Biomass........................................................ 19

2.2.1.1.1 Organic components ………………………………………………... 20

2.2.1.2 Saw Dust and Wood Shavings........................................................... 23

2.2.1.2.1 Science of sawdust………………………………………… 24

2.2.1.2.1 Properties of Saw Dust and Wood Shavings.................................. 25

2.3 Process Technology

2.3.1 Pyrolysis................................................................................................ 30

2.3.1.1 History of Pyrolysis.......................................................................... 31

2.3.1.2 Pyrolysis Process Types....................................................................... 32

2.3.1.3 Processes of Pyrolysis..................................................................... 37

2.3.2 Chemical Reactor............................................................................. 39

2.3.2.1 Types of Pyrolyzers...................................................................... 40

2.3.3 Temperature Control and Measurement

2.3.3.1 Thermocouples.............................................................................. 45

2.3.3.1.1 Principles of thermocouples......................................................... 46

2.3.3.1.2 Types of thermocouples................................................................ 48

2.3.3.1.3 Operation of thermocouples......................................................... 51

2.3.3.1.4 Aging of Thermocouples.................................................................. 56

2.3.3.1.5 Advantages and Disadvantages of Thermocouples...................... 57

2.3.3.1.6 Problems to be aware of when using a thermocouple................. 57

2.3.3.1.7 Calibrating Thermocouples…………………………………….. 60

2.3.3.2 Refractory Brick.................................................................................. 61

2.3.3.3 Fibre Glass........................................................................................... 63

2.3.4 Block diagram of process........................................................................ 68

2.4 Compost........................................................................................................ 69

2.4.1 Types of compost....................................................................................... 70

2.4.2 Materials used for making compost........................................................ 71

2.4.3 Uses of compost............................................................................................ 72

CHAPTER THREE

METHODOLOGY

3.1 BIOMASS COLLECTION AND PREPARTION

3.1.1 Biomass Collection............................................................................................ 75

3.1.2 Drying.............................................................................................................. 75

3.1.3 Determination of pH......................................................................................... 77

3.1.4 Moisture Content Determination.......................................................................78

3.2 REACTOR OPERATIONS

3.2.1 HEATING OF REACTOR................................................................................82

3.2.2 BIOMASS WEIGHING..................................................................................... 83

3.2.3 CHARGING OF THE REACTOR....................................................................84

3.2.4 DISCHARGING OF BIOCHAR........................................................................ 86

3.2.5 WEIGHING OF DRIED BIOCHAR...................................................................88

CHAPTER FOUR

4.0 RESULTS

4.1 THE pH AND MOISTURE CONTENT OF THE RAW MATERIAL S USED...89

4.2 4.1 THE pH AND YIELD OF THE CHARRED MATERIAL... ...........................90

4.3 TEMPERATURE MEASUREMENT......................................................................93

CHAPTER FIVE

5.0 DISCUSSION..............................................................................................................101

5.1 RAW MATERIALS AND CHAR OBTAINED................ .......................................101

5.2 TEMPERATURE DISTRIBUTION……………………………………………….108

5.3 BIOCHAR ANALYSIS (CHEMICAL PROPERTIES)……….…………… …… 109

CHAPTER SIX

6.1 CONCLUSION...........................................................................................................111

6.2 RECOMMENDATION..............................................................................................112

6.3 REFERENCE..............................................................................................................113

6.4 APPENDIX................................................................................................................117

LIST OF TABLES

TABLE TITLE PAGE

2.1 Mechanical and physical properties of some selected wood species.. 27

2.2 Summary of the various types of pyrolysis.......................................... 35

2.3 Summary of thermocouple types.......................................................... 48

2.4 Seebeck’s coefficients for some materials........................................... 55

2.5 Properties of E and G –types fibre glass............................................. 66

4.1 Moisture content and pH of various raw materials............................ 89

4.2 Yield and pH of Biochar........................................................................ 90

4.3 Summary of feed and yield of various raw materials.......................... 92

4.4 Temperature readings during pyrolysis of wawa saw dust............... 93

4.5 Temperature readings during pyrolysis of wood shavings............ 94

4.6 A 48 Hour Temperature monitoring of 24kg Wood shavings........... 95

4.7 Chemical Analysis on Some Biochar Types........................................ 100

LIST OF FIGURES

FIGURE TITLE PAGE

1-1 A section of the 2nd October, 2010’s of Daily Graphic..................... 3

1-2 A typical dump site near a household at Ayiom................................. 4

1-3 Waste management in Ayiom............................................................... 5

2-1 A comparison of soils in the Tropics and that in the Amazons…… 9

2-2 Simple carbon cycle.............................................................................. 15

2-3 Manipulated carbon cycle.................................................................... 16

2-4 Hardwood fibre structure..................................................................... 19

2-5 Softwood structure............................................................................... 19

2-6 The molecular structure of cellulose................................................... 21

2-7 The chemical structure of softwood hemicellulose............................ 21

2-8 The structure of the three main components of lignin....................... 23

2-9 Pyrolysis flow chart.............................................................................. 31

2-10 Bubbling fluidized bed pyrolyzer......................................................... 41

2-11 Circulating fluidized bed pyrolyzer.................................................... 42

2-12 Rotating plate pyrolyzer....................................................................... 43

2-13 Rotating cone pyrolyzer....................................................................... 44

2-14 Typical thermocouple circuit............................................................... 52

2-15 Fibre glass.............................................................................................. 63

2-16 Flow diagram of biochar production................................................... 68

3-1 Drying of odum sawdust...................................................................... 76

3-2 Covering of sawdust............................................................................. 76

3-3 Covered sawdust................................................................................... 77

3-4 Biochar reactor...................................................................................... 81

3-5 Heating of reactor................................................................................ 83

3-6 Weighing of sawdust............................................................................. 84

3-7 Feeding Of Reactor.............................................................................. 85

3-8 Taking of Temperature after Feeding Of Reactor............................ 86

3-9 Discharged Biochar in a Wheel Barrow............................................. 87

4-1 A diagram showing various levels of temperature measurement..... 99

4-2 A cross section of the reactor.............................................................. 100

5-1 Odum sawdust (A) Biochar............................................................... 102

5-2 Odum sawdust Biochar....................................................................... 103

5-3 Wawa stain biochar............................................................................. 104

5-4 Biochar from wood shavings……………………………………….. 106

5-5 Biochar from rice husk……………………………………………. 108

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CHAPTER ONE

INTRODUCTION

Agriculture is a very important sector in the Ghanaian economy. Sixty percent of the

population depends on agriculture as their only source of money hence they invest a lot in

fertilizer to be able to have a good harvest. The government spends a lot of money to import

fertilizer in to the country. It spent over GH¢ 20 million in 2008, covering 72,795 metric

tons of fertilizers, under a fertilizer subsidy programme and has an annual import bill on

fertilizer estimated at more than $50 million now. [Kofi Yeboah (October 2nd 2010), Daily

Graphic].Considering the amount spent on fertilizer importation, it has become incumbent

on researchers and some concerned literates in the country to provide strategies to combat

this problem. In an attempt to find a solution to this problem, some concerned citizens in the

country have come out with a technology known as biochar. This technology is one of the

few that is relatively inexpensive, widely applicable and quickly scalable.

Biochar is a 2,000 year-old practice that converts agricultural waste into a soil enhancer that

can hold carbon, boost food security and discourage deforestation. The process creates a

fine-grained, highly porous charcoal that helps soils retain nutrients and water. Biochar is

found in soils around the world as a result of vegetation fires and historic soil management

practices. Intensive study of biochar-rich dark earths in the Amazon (terra preta), has led to

a wider appreciation of biochar’s unique properties as a soil enhancer. A once worthless and

costly by-product (in most countries) is now a valuable resource. Through biochar, biomass

becomes a sustainable and value-added product for urban and rural agriculture and forest

communities while creating jobs, improving soil and reducing forest fire hazards.

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Biochar provides a unique opportunity to improve soil fertility and nutrient-use efficiency

using locally available and renewable materials in a sustainable way. Biochar has unique

properties that make it not only a valuable soil amendment to sustainably increase soil

health and productivity, but also an appropriate tool for sequestering atmospheric carbon

dioxide in soils for the long term in an attempt to mitigate global warming. . Biochar also

improves water quality and quantity by increasing soil retention of nutrients and

agrochemicals for plant and crop utilization. More nutrients stay in the soil instead of

leaching into groundwater and causing pollution. A once worthless and costly by-product

(in most countries) is now a valuable resource. Through biochar, biomass becomes a

sustainable and value-added product for urban and rural agriculture and forest communities

while creating jobs, improving soil and reducing forest fire hazards

The novelty, importance and the excitement of having the first biochar reactor in the

country was captured in the back page of the 2nd October 2010 edition of the Daily Graphic.

3

Fig 1-1. A section of the 2nd October, 2010’s edition of the Daily Graphic announcing

the building of the first biochar reactor in the country.

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The second part of this project is the use of house hold waste to produce compost. The

compost would then be combined with the biochar to further enrich its used as fertilizer. For

the purpose of this project house hold waste from the people of Ayiom is going to be sorted

and collected with the help of Zoomlion. The waste will then be used as compost. The

biochar will then be added to the compost and then supplied to the community for use on

their farms. This is going to help improve sanitation in the community hence improving the

standard of living of the people.

The pictures below show the improper ways of disposing of waste in the Ayiom

community.

Fig 1-2 A typical dump site near a household at Ayiom

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Fig 1-3 Waste Management in Ayiom

Ayiom, being a small farming community with potential rich biomass dumps, serves as a

sampled area to realize the significance of the biochar – compost blend. This is because the

household wastes of the people will be composted and blended with the biochar and then

used on their farms. Thus, the soil fertility, plant growth and yield will be effectively

demonstrated.

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General Objectives

� To serve as supplement to enrich compost through sorting of household waste.

� To reduce the need for chemical fertilizer.

� To help fight global warming and provide economic value in a future of carbon-

restrained economy.

� To play a major role in environmental waste management.

� To help check the emission of greenhouse gases.

Specific Objectives

� To provide an alternative way of managing agricultural waste.

� To improve the value of compost used in soil enrichment.

� To produce fertilizer at a reduced price for farmers.

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CHAPTER TWO

2.0 LITERATURE REVIEW

Biochar- The new frontier inspired by the fascinating properties of Terra Preta de Indio,

biochar was identified as a soil amendment that has the potential to revolutionize concepts

of soil management. While "discovered" may not be the right word, as biochar or bio-char

(also called charcoal or biomass-derived black carbon, in the context of agricultural

application sometimes called agrichar or agric-char, which we do not adopt due to the wider

applicability of biochar for environmental management beyond agriculture) has been used

in traditional agricultural practices as well as in modern horticulture, never before has

evidence been accumulating that demonstrates so convincingly that biochar has very

specific and unique properties that make it stand out among organic soil amendments.

2.1. BIOCHAR

Biochar is just charcoal made from biomass (which is plant material and agricultural

waste).Hence the name ‘biochar’. It is a fine-grained charcoal produced from pyrolysis: the

slow burning of organic matter in a low- or no-oxygen environment. As a soil amendment,

biochar creates a recalcitrant soil carbon pool that is carbon-negative, serving as a net

withdrawal of atmospheric carbon dioxide stored in highly recalcitrant soil carbon stocks.

The enhanced nutrient retention capacity of biochar-amended soil not only reduces the total

fertilizer requirements but also the climate and environmental impact of croplands

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2.1.1History of Biochar

The term ‘biochar’ was coined in recent times, but the origins of the concept are ancient.

Throughout the Amazon Basin there are regions—up to two metres in depth—of terra preta.

This is a highly fertile dark-coloured soil that has for centuries supported the agricultural

needs of the Amazonians.

Analyses of the dark soils have revealed high concentrations of charcoal and organic matter,

such as plant and animal remains (manure, bones and fish). Terra preta’s productivity is due

to good nutrient retention and a neutral pH, in areas where soils are generally acidic.

Interestingly, terra preta exists only in inhabited areas, suggesting that humans are

responsible for its creation. What has not been confirmed is how terra preta was created so

many years ago.

Many theories exist. A frontrunner is the suggestion that ancient techniques of slash-and-

char are responsible for the dark earth. Similar to slash-and-burn techniques, slash-and-char

involves clearing vegetation within a small plot and igniting it, but only allowing the refuse

to smoulder (rather than burn). Combined with other biomass and buried under a layer of

dirt, the smouldering char eventually forms terra preta. It is from these hypotheses of early

slash-and-char practices that modern scientists have developed methods for producing

biochar.

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Fig 2-1 A comparison of soils in the tropics and that in the amazons

2.1.2 Chemical Composition

Biochar mostly is made up of carbon and hydrogen. The carbon left in the biochar may be

about 40% of the total carbon in the material used. The chemical composition of the biochar

may vary slightly depending on the type of material used and the temperature to which the

feed material is heated changes the chemical composition of biochar.

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2.1.3 Types of Biochar

Not all biochar is the same. The type of biochar varies with biomass type, in many cases

rice, wood or bark has been used and production parameters, such as the rate of pyrolysis

and kiln size. As an example, biochar made from manure will have a greater nutrient

content than that formed from wood chips. A wood based biochar, on the other hand, will

remain more stable for a longer time. Higher firing temperatures will result in a greater

amount of micro porosity and adsorptive capacity, therefore a better potential for adsorption

of toxic substances and soil rehabilitation. Hence the raw material used determines the type

of biochar.

2.1.4 Importance of Biochar

Biochar has been popularized by its potential role in climate change mitigation. Biochar is

rich in carbon and, depending on its ultimate use, the biochar may retain the carbon, thereby

delaying or completely preventing the release of the carbon back into the atmosphere in the

form of carbon dioxide gas. The benefits of biochar go beyond this, however, extending to

the agricultural sector and to various types of waste management.

2.1.4.1 Benefits to the agricultural sector and waste management

The agricultural sector can benefit from biochar in two ways: soil improvement and

animal and crop waste disposal.

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The following benefits occur with additions of biochar to the soil:

• Enhanced plant growth

• Suppressed methane emission

• Reduced nitrous oxide emission

• Reduced fertilizer requirement

• Reduced leaching of nutrients

• Stored carbon in a long term stable sink

• Reduced soil acidity: biochar raises soil pH

• Reduced aluminium toxicity

• Increased soil aggregation due to increased fungal hyphae

• Improved soil water handling characteristics

• Increased soil levels of available Ca, Mg, P, and K

• Increased soil microbial respiration

• Increased soil microbial biomass

• Stimulated symbiotic nitrogen fixation in legumes

• Increased arbuscular mycorrhyzal fungi

• Increased cation exchange capacity

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A second benefit of biochar production to the agricultural sector (and some industries, such

as the paper industry) is the fact that it uses organic waste. Left to accumulate, animal and

crop waste can contaminate ground and surface waters. Waste management practices are

aimed at preventing such contamination, but they can become costly. Biochar presents an

attractive alternative if the economic costs can be kept below those of waste management.

By accepting organic material as its input, the biochar production process transforms waste

into a resource. The pyrolysis process reduces the weight and volume of the feedstock, and

by operating at a temperature above 350˚C, it also removes potential pathogens that can be a

problem if directly applied to soils. Green urban waste and waste from some industrial

processes, such as paper milling, can also be used.

2.1.4.2 Climate Change Mitigation

Biochar has been given a lot of attention recently as one means of addressing climate

change. It has the capacity to do so in three ways: the storage of carbon over long periods;

the reduction of greenhouse gases such as methane (CH4) and carbon dioxide (CO2) that can

be generated from waste disposal, waste processing or recycling; and the production of

renewable energy.

Through the production process, around 50 per cent of the feedstock’s carbon content is

retained in the biochar. This compares to the 10 to 20 per cent that remains in biomass after

5 to 10 years of natural decay, and the less than 3 per cent that remains in ash after complete

burning. Some analysts have suggested that ‘up to 12% of the total anthropogenic [carbon]

13

emissions by land use change can be off-set annually in soil, if slash-and-burn is replaced

by slash-and-char’. If it proves practicable to replace traditional slash-and-burn practices

with slash-and-char methods, biochar may present a real quantifiable and verifiable option

for storing carbon in the long term.

At the same time, it has the potential to reduce emissions from other activities that might

need to take place in the absence of the biochar option. These other activities are the waste

disposal process described above and any recycling process. Both can be sources of

greenhouse gas emissions, either as carbon dioxide from transport and processing, or

methane from landfill sites.

Finally, the pyrolysis process also produces viable forms of renewable energy. The syngas

and bio-oils that result from the biochar production process, and the generated heat, can be

used either to produce electricity, or as fuel. Not only does this represent a renewable

energy alternative but it also improves the energy efficiency of the pyrolysis process.

Moreover, it has been calculated that ‘the emission reductions associated with biochar

additions to soil appear to be greater than the fossil fuel offset in its use as fuel’.

2.1.4.3 MANIPULATING THE CARBON CYCLE

Carbon dioxide (CO2) is removed from the atmosphere through photosynthesis and stored in

organic matter. When plants grow they utilize sunlight, CO2, and water (H2O) to synthesize

organic matter and release oxygen (O2). This accumulated organic matter is returned to the

atmosphere by decomposition of dead plant tissue or disturbances, such as fire, in which

14

large amounts of organic matter are oxidized and rapidly transferred into CO2 (Figure 2-2).

Terrestrial carbon is primarily stored in forests. In undisturbed full-grown forest

ecosystems, the turnover time of carbon takes decades, and uptake by photosynthesis and

release by decay is balanced.

Reduced decomposition is an advantage of biochar. Thus, biochar formation has important

implications for the global carbon cycle. In natural and agro ecosystems, incomplete

burning produces residual charcoal. As the soil carbon pool declines due to cultivation, the

more resistant biochar fraction increases as a portion of the total carbon pool and may

constitute up to 35 percent of the total. Carbon dating of charcoal has shown some to be

over 1500 years old, fairly stable, and a permanent form of carbon sequestration.

Biochar can be produced by thermo-chemical conversion of biomass. Burning biomass in

the absence of oxygen produces biochar and products of incomplete combustion (PIC). The

PIC includes burnable gases such as H2 and CH4. These gases can be used to fuel the

conversion of biomass into biochar and/or renewable energy generation (Figure 2-3).

Larger molecules can be condensed into bio-oil and also used as a renewable fuel. The

resulting biochar consists of mainly carbon and is characterized by a very high recalcitrance

against decomposition. Thus biochar decelerates the second part of the carbon cycle (decay,

mineralization) and its non-fuel use would

establish a carbon sink.

15

Figure 2-2 Simple Carbon Cycle.

The figure above shows a simplified version of the carbon cycle in vegetation and soil.

Plants take CO2 from the atmosphere to synthesize tissue (plant biomass). As long as

biomass is growing it accumulates carbon. During decomposition of dead biomass and

humus the carbon is released as CO2. In undisturbed ecosystems the accumulation and

release of CO2 is in equilibrium.

16

\

Figure 2-3 Manipulated Carbon Cycle

Figure2-3 illustrates the manipulated carbon cycle due to biochar carbon sequestration.

Biochar is recalcitrant against decomposition and remains in the soil for centuries or

millennia. Thus pyrolysis can transfer 50% of the carbon stored in plant tissue from the

active to an inactive carbon pool. The remaining 50% of carbon can be used to produces

energy and fuels. This enables carbon negative energy generation if re-growing resources

are used. (i.e. with each unit if energy produced CO2 is removed from the atmosphere)

(Christoph Steiner, University of Georgia, Biorefining and Carbon Cycling Program,

Athens, GA 30602, USA)

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2.1.5 LIMITATION TO BIOCHAR USES

Despite the potential benefits that biochar presents, there are limits to its potential

production and usage. A major limitation to the production of biochar is that the biomass

used cannot be drawn from just any agricultural (or industrial or urban) waste. Some studies

have estimated that no more than three per cent of available biomass is suitable for

producing biochar. On a global scale, using all aboveground biomass would sequester only

0.56 gigatonnes of carbon per year, just one third of what is emitted each year from land use

change, or less than a tenth of annual fossil fuel emissions.

If plants are grown specifically for the production of biochar (instead of using waste), then

the plants must have a growth rate matched to the rate of planned biochar production. Fast

growing plants deliver the best productivity, but these also mature earlier and may begin to

decay sooner. The most efficient way to capture the carbon used by the plant in

photosynthesis would be to harvest it before the growth rate begins to taper.

Also, the purpose for the produced biochar will change the potential benefits, so it must be

clear from the beginning whether the goal is to improve soil nutrient retention, sequester

carbon or manage waste. Whatever the objective, the process will be optimised for that

purpose in order to maximise financial return. This is often to the detriment of other

benefits. By targeting soil improvement, the resulting biochar may not produce any usable

renewable energy; or if bio-energy production is the main objective, the resultant biochar

may be too unstable to store any carbon long-term. Such trade-offs are not to be neglected

as the ultimate profitability of the process will determine its potential net benefit.

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2.2 RAW MATERIALS

There are wide ranges of biomasses that can serve as potential biochar feedstocks: e.g.

wood waste, timber, agricultural wastes, manure, rice husks and straw, leaves, food wastes,

paper sludge, green waste, distillers grain, bagasse (sugar cane residue,)peanut hull, grasses,

cocoa husk, coconut husk, nut shell(groundnut, hazel nut, palm kernel) and many others.

The type of material used will also depend on the availability of the biomass material.

2.2.1 Biomass

In ecological studies, biomass is the mass of living organisms present in a particular area or

ecosystem.

Annually, photosynthesis is said to store 5-8 times more energy in biomass than humanity

currently consumes from all sources. Biomass is currently the fourth largest energy source

in the world – primarily used in less developed countries and could in principle become one

of the main energy sources in the developed world.

In the world of energy production, the term biomass refers to renewable sources of energy

that come from living organisms as well those that have recently died. This eliminates the

fossil fuels such as coal and petroleum because, while produced from ancient biomass, these

fuels are bound in the earth's crust and are not part of the carbon cycle. Burning biomass

does not release carbon into the atmosphere that adds to the amount of carbon already

present in the normal carbon cycle. However, combustion of biomass to produce energy

does put the carbon into the atmosphere faster than natural processes can accommodate and

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reabsorb.

Different substances meet the definition of biomass. These include wood, both directly

harvested and waste products from other processes, grasses, grains, oils produced from trees

or vegetables, household garbage, and more. Industries are typically more selective when

they define biomass to mean only that which is useful to their particular type of business.

2.2.1.1 Chemical Composition of Biomass

From a chemical perspective wood can be regarded as a mixture or polymers combined with

a small fraction of minerals. The chemical composition depends on many characteristics

like the wood species, the amount of bark and the woods geographical origin. Usually there

are two groups of wood distinguished; hard- and softwood. Both groups have different

chemical, physical and biological properties, but within these groups also large variations

are possible

Figure 2-4 Hardwood fibre structure Figure2-5 Soft wood structure

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2.2.1.1.1 Organic components

The numerous different polymers present in the organic fraction of the fuel are generally

divided in three main groups, cellulose, hemicellulose and lignin. The proportions of these

groups depend on the type of wood, but normally mass fractions range from 40 to 50% for

cellulose, from 15 to 25% for hemicellulose and between 20 and 30% for lignin. Together

these three groups from 90 to 95% of wood. The other 5 to 10% consists of mineral matter

and a few other organic compounds. The next sections describe the three groups in more

detail.

� Cellulose

Cellulose is the main component of woody biomass. Cellulose consists of long polymers

built with a C6-monomer as base structure. This structure is the same for both hard and

softwood. The cellulose chains can reach polymerization grades up to 10.000 resulting in a

high anisotropic material with a strong crystalline structure. Thermal degradation of

cellulose normally starts around 350 °C. This temperature depends on the presence of side

groups and branches along the chains. During pyrolysis the polymer chains can be broken

down into smaller polymers, but also in gas species that are much smaller than one

monomer.

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Fig 2-6 The molecular structure of cellulose

� Hemicellulose

Hemicellulose is in most aspects similar to cellulose. The hemicellulose polymers form a

crystalline structure and the same types of chemical bonds are present. The base structure of

hemicellulose consists of C5-monomers and the polymerization grade is in the order of 100.

Compared to cellulose, hemicellulose is a more branched polymer. This results in a lower

thermal stability, the thermal degradation of hemicellulose starts around 270 °C.

Figure 2-7 The chemical structure of softwood hemicellulose.

The chemical structure of hemicellulose differs for hard- and softwood. Where branches in

softwood consist of merely arabinose (C5H10O5), hardwood hemicellulose contains large

amounts of acetyl groups (-COCH3). Figure 5 and figure 6 show the chemical structure of

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cellulose and a hemicellulose chain respectively. The C6- and C5-structure of the monomers

give the chains either a hexagonal or a pentagonal shape.

� Lignin

Lignin is the least homogeneous wood compound and consists of a lot of different chemical

structures. It has the highest energy content of the three fractions. Lignin has also a

relatively large carbon content compared to cellulose and hemicellulose, and is as a result

the most charring biomass component. Lignin does not have a well defined crystalline

structure like cellulose and hemicellulose, but is amorphous and highly cross-linked,

meaning that the polymer chains are highly interconnected.

According to Rao the thermal degradation of lignin, which starts around 390 °C, takes place

over a wide temperature range, due to the many different chemical structures present in

lignin.

Lignin can be subdivided into three different groups of lignin. These structures are indicated

with Lig-C, Lig-O and Lig-H and are shown below. The Lig-C structure is representative

for softwood lignin. Hardwood lignin contains more metoxyl (-OCH3) groups and can be

represented by a mixture of Lig-O and Lig-H.

23

Figure 2-8 The structure of the three main components of lignin.

2.2.1.2 Saw Dust and Wood Shavings

Sawdust is composed of fine particles of wood. This material is produced from cutting with

a saw, hence its name. It has a variety of practical uses, including serving as mulch, or as an

alternative to clay cat litter, or as a fuel, or for the manufacture of particleboard.

Historically, it has been treated as a by-product of manufacturing industries and can easily

be understood to be more of a hazard, especially in terms of its flammability. It has also

been used in artistic displays, and as scatter. It is also sometimes used in bars in order to

soak up spills, allowing the spill to be easily swept out the door. The common types of

wood sawdust in Ghana include those obtained from Mahogany, Onyina, Wawa, Odum,

Sapele, Teak, Cedar and some red woods.

24

2.2.1.2.1 Science of sawdust

The main by-product of sawmills, unless reprocessed into particleboard, burned in a

sawdust burner or used to make heat for other milling operations, sawdust may collect in

piles and add harmful leachates into local water systems, creating an environmental hazard.

This has placed small sawyers and environmental agencies in a deadlock.

Questions about the science behind the determination of sawdust being an environmental

hazard remain for sawmill operators (though this is mainly with finer particles), who

compare wood residuals to dead trees in a forest. Technical advisors have reviewed some of

the environmental studies, but say most lack standardized methodology or evidence of a

direct impact on wildlife. They don’t take into account large drainage areas, so the amount

of material that is getting into the water from the site in relation to the total drainage area is

minuscule.

Other scientists have a different view, saying the "dilution is the solution to pollution"

argument is no longer accepted in environmental science. The decomposition of a tree in a

forest is similar to the impact of sawdust, but the difference is of scale. Sawmills may be

storing thousands of cubic metres of wood residues in one place, so the issue becomes one

of concentration.

Water-borne bacteria digest organic material in leachates, but use up much of the available

oxygen. This high "biological oxygen demand" can suffocate fish and other organisms.

There is an equally detrimental effect on beneficial bacteria, so it is not at all advisable to

25

use sawdust within home aquariums, as was once done by hobbyists seeking to save some

expense on activated charcoal.

But of larger concern are substances such as lignin and fatty acids that protect trees from

predators while they are alive, but can leach into water and poison wildlife. Those types of

things remain in the tree and, as the tree decays, they slowly get broken down. But when

sawyers are processing a large volume of wood and large concentrations of these materials

get out into the runoff, they cause toxicity and are toxic to a broad range of organisms.

These impacts on the ecology have called for the increased use of sawdust in Biochar

technology.

2.2.1.2.1 Properties of Saw Dust and Wood Shavings

The properties of sawdust which makes it susceptible for charring are dependent on the type

of wood from which it was obtained. When attempting to complete a wood project you must

be careful of your choice of wood. Numerous species take on different characteristics. All

are composed of 60% cellulose and 28% lignin. These substances make up the fibrous and

woody cell walls of plants and trees and are held together by cementing properties. The

individual consistencies and colours are the elements remaining of about 12%. Other

characteristics are due to the way that the wood is sawed and cured.

The physical properties (other than appearance) are moisture content, shrinkage, density,

permeability, and thermal and electrical properties.

26

Moisture content is a major factor in the processing of wood because it influences all

physical and mechanical properties, and durability and performance during use. Normal in-

use moisture content of processed wood that has been dried ranges 8–13%. Moisture

content for wood is expressed on either a fractional or percentage basis. Moisture content is

defined as the ratio of the mass of water contained in the wood to the mass of the same

sample of dry wood.

Shrinkage occurs when wood loses moisture below the fibre saturation point. Above that

point, wood is dimensionally stable. The amount of the shrinkage depends on its direction

relative to grain orientation and the amount of moisture lost below the fibre saturation point.

Wood shrinks significantly more in the radial and tangential directions than in the

longitudinal direction.

The density of wood is determined by the amount of cell wall substance and the volume of

voids caused by the cell cavities (lumens) of the fibres. Density can vary widely across a

growth or annual ring. The percentage of early wood and latewood in each growth ring

determines the overall density of a wood sample.

Permeability is a measure of the flow characteristics of a liquid or gas through wood as a

result of the total pressure gradient. Permeability is influenced by the anatomy of the wood

cells, the direction of flow (radial, tangential, and longitudinal), and the properties of the

fluid being measured. Permeability is also affected by the species, by whether the wood is

sapwood or heartwood, and by the chemical and physical properties of the fluid.

27

The primary thermal properties of wood are conductivity, specific heat, and coefficient of

thermal expansion. The conductivity of wood is determined by density, moisture content,

and direction of conduction. Thermal conductivity in the transverse directions (radial and

tangential) is approximately equal. Conductivity in the longitudinal direction is greater than

in the transverse directions. For most processing operations, the dominant heating direction

is transverse. Thermal conductivity is important to wood processing because heating—

whether for drying, curing, pressing, or conditioning- is an integral step. Specific heat of

wood is dependent on moisture content and, to less extent, on temperature.

WOOD TYPES AND THEIR PROPERTIES

Table 2.1 Mechanical and Physical Properties of Some Selected Wood Species

A. African Mahogany

Mechanical Properties

Quality Value Unit

Young's modulus 0 – 10000 MPa

Tensile strength 32.5 – 101 MPa

Compressive strength 36 - 58.5 MPa

Bending strength 36 – 126 MPa

Physical Properties

Quantity Value Unit

28

Thermal expansion 3.6 – 40 e-6/K

Thermal conductivity 0.14 - 0.31 W/m.K

Density 0 – 490 kg/m3

Shrinkage 1 – 1.3 %

B. Teak


Quality Value Unit


Tensile strength 95 – 155 MPa

Compressive strength 48 – 91 MPa


Physical Properties

Quantity Value Unit

Thermal conductivity 0.19 - 0.38 W/m.K


Shrinkage 0.6 - 0.6 %

29

C. Wawa


Quality Value Units


Tensile strength 11 – 80 MPa

Compressive strength 24 - 50.5 MPa


Physical Properties

Quality Value Unit


30

2.3 PROCESS TECHNOLGY

2.3.1 Pyrolysis

Pyrolysis is a thermo chemical decomposition of organic material at elevated temperatures

in the absence of oxygen (Demirbas and Arin, 2002). The solid, termed variously as char,

biochar, charcoal or coke, is generally of high carbon content and may contain around half

the total carbon of the original organic matter. The volatiles can be partly condensed to give

a liquid fraction leaving a mixture of so-called ‘non-condensable’ gases.

The term char is used generally to describe the solid product of pyrolysis, charcoal will be

used for more traditional processes with wood as feedstock, and biochar will be used where

the intention is for the char to be used as a soil amendment. Char contains varying carbon

content, typically ranging 60-90% (Gaur and Reed, 1995). Some is ‘fixed-carbon’ in terms

of its proximate analysis, some present in a remaining volatile portion; inorganic material in

char is termed ash. Liquid products from biomass pyrolysis are frequently termed bio-oil.

Organic liquid product is generally hydrophilic containing many oxygenated compounds

and is present, sometimes as a single aqueous phase, sometimes phase-separated, together

with water produced in the pyrolysis reaction or remaining from the feedstock (Demirbas

and Arin, 2002). The gas product is termed synthesis gas, shortened to syngas. It is

generally composed of carbon dioxide, carbon monoxide, methane, hydrogen and two-

carbon hydrocarbons in varying proportions.

Fig 2-9 Pyrolysis Flow chart

2.3.1.1 History of pyrolysis.

Mankind has used pyrolysis and related processes for thousands of years. The earliest

known example is the

Magnon man) some 38,000 years ago. In the Bronze Age intentionally produced charcoal

was used for smelting metals and charcoal is still heavily used in metallurgy today. For

thousands of years charcoal has been a preferred cooking fuel. Prior to the development of

petrochemicals, pyrolysis, or ‘wood distillation’, was a source of many valuable organic

compounds for industrial and medicinal uses; some high value liquid products, such as

flavorings, are still produced by wood pyrolysis. Pyrolysis and gasification proces

been used to extract liquid and gas products from coal since Victorian times and the

31

Pyrolysis Flow chart

History of pyrolysis.


known example is the production of charcoal (a fuel for cooking and

some 38,000 years ago. In the Bronze Age intentionally produced charcoal



trochemicals, pyrolysis, or ‘wood distillation’, was a source of many valuable organic


, are still produced by wood pyrolysis. Pyrolysis and gasification proces



a fuel for cooking and cave drawings by Cro-

some 38,000 years ago. In the Bronze Age intentionally produced charcoal



trochemicals, pyrolysis, or ‘wood distillation’, was a source of many valuable organic


, are still produced by wood pyrolysis. Pyrolysis and gasification processes have


32

technology for producing a synthetic crude oil from coal is well established. It is only more

recently that biomass and organic wastes have become a focus as feeds for pyrolysis and

related thermal treatment processes for energy recovery or bio-fuel production; the

technologies are still relatively undeveloped. Char has also been used in agriculture for

thousands of years. The fertile terra preta (dark earth) soils of the Amazonian region result

from incorporation of char into otherwise poor soils. The resulting soils have long-lasting

fertility that has been related to the stability of carbon in the soil (Lehmann et al, 2009). It is

this observation coupled with the search for carbon sequestration techniques for climate

change mitigation that has led to recent interest in pyrolysis-derived char, or biochar.

2.3.1.2 Pyrolysis Process Types

There are two main classes of process for biomass pyrolysis. They are fast pyrolysis and

slow pyrolysis. In addition to the two, there may be other technologies. Introduced briefly

below are the two main types and a number of other related technologies.

Fast Pyrolysis

Fast pyrolysis is characterized by high heating rates and short vapor residence times. This

generally requires a feedstock prepared as small particle sizes and a design that removes the

vapors quickly from the presence of the hot solids. There are a number of different reactors

configurations that can achieve this including ablative systems, fluidized beds, stirred or

moving beds and vacuum pyrolysis systems. A moderate (in pyrolysis terms) temperature of

around 500°C is usually used. Development of fast pyrolysis progressed rapidly following

33

the oil crises of the 1970’s as a way of producing liquid fuel from an indigenous renewable

resource, primarily wood, and the process is designed to give a high yield of bio-oil.

Slow Pyrolysis

Slow pyrolysis can be divided into traditional charcoal making and more modern processes.

It is characterized by slower heating rates, relatively long solid and vapour residence times

and usually a lower temperature than fast pyrolysis, typically 400°C. The target product is

often the char, but this will always be accompanied by liquid and gas products although

these are not always recovered.

Traditional processes, using pits, mounds or kilns, generally involve some direct

combustion of the biomass, usually wood, as heat source in the kiln. Liquid and gas

products are often not collected but escape as smoke with consequent environmental issues.

Developments through the late 19th and early 20th centuries led to industrial scale

processes using large retorts operated in batch or continuous modes. These allow recovery

of organic liquid products and recirculation of gases to provide process heat, either

internally or externally. Prior to the widespread availability of petrochemicals, such

processes were used to generate important organic liquid products, in particular acetic acid

and methanol. Other developments in the later 20th century led to slow pyrolysis

technologies of most interest for biochar production. These are generally based on a

horizontal tubular kiln where the biomass is moved at a controlled rate through the kiln;

these include agitated drum kilns, rotary kilns and screw pyrolyzer. In several cases these

have been adapted for biomass pyrolysis from original uses such as the coking of coal with

production of towns gases or the extraction of hydrocarbons from oil. Although some of

34

these technologies have well-established commercial applications, there is as yet little

commercial use with biomass in biochar production.

Other Technologies

This section covers a brief review of technologies other than slow and fast pyrolysis that

maybe used for thermal treatment of biomass and char production.

The term ‘intermediate pyrolysis’ has been used to describe biomass pyrolysis in a certain

type of commercial screw-pyrolyzer – the Haloclean reactor. This reactor was designed for

waste disposal of electrical and electronic component residues by pyrolysis. When used for

biomass it has performance similar to slow pyrolysis techniques, although somewhat

quicker. Very fast pyrolysis is sometimes referred to as ‘flash pyrolysis’, usually in the

context of laboratory studies involving rapid movement of substrate through a heated tube

under gravity or in a gas flow. Higher temperatures and shorter residence times than fast

pyrolysis are used, the main product distributions are similar to fast pyrolysis. Flash

carbonization is a different process involving partial combustion of a packed bed of biomass

in a pressurized reactor with a controlled air supply. A high yield of char and gas are

reported with no liquid product formed under the reaction conditions. The technology is

currently being commercialized by Carbon Diversion Incorporated (CDI, 2009).

Gasification is an alternative thermo-chemical conversion technology suitable for treatment

of biomass or other organic matter including municipal solid wastes or hydrocarbons such

as coal. It involves partial combustion of biomass in a gas flow containing a controlled level

of oxygen at relatively high temperatures (500-800°C) yielding a main product of

35

combustible syngas with some char. Although designed to produce gas, under some

conditions gasifiers can produce reasonable yields of char and have been proposed as an

alternative production route to pyrolysis for biochar.

Hydrothermal carbonization is a completely different process involving the conversion of

carbohydrate components of biomass (from cellulose) into carbon-rich solids in water at

elevated temperature and pressure. Under acidic conditions with catalysis by iron salts the

reaction temperature may be as low as 200°. The process may be suitable for concentration

of carbon from wet waste streams that would otherwise require drying before pyrolysis,

making it complementary to pyrolysis and a potential alternative to anaerobic digestion for

treatment of some wastes.

Table 2.2 Summary of The Various Types Of Pyrolysis.

Process Liquid

(bio-oil) Solid (biochar) Gas (syngas)

Slow pyrolysis

• Long residence times

• Low-moderate

reactor temperature

30%

(70% water) 35% 35%

36

Intermediate pyrolysis

• Low-moderate

reactor temperature

• Moderate hot vapour

residence time

50%

(50% water) 25% 25%

Fast pyrolysis

• Moderate reactor

temperature

(~500°C)

• Short hot vapour

residence time (<2 s)

75%

(25% water) 12% 13%

Gasification

High reactor temperature

(>800°C)

• Long vapour

residence time

5% tar

(contains 5%

water)

5%

(not used as

biochar)

90%

37

2.3.1.3 Processes of Pyrolysis

The pyrolysis of biomass.

All biomass resources are composed primarily of cellulose (typically 30 to 40 percent of

dry weight), hemicelluloses (25 to 30 percent of dry weight), and lignin (12 to 30 percent

of dry weight), but the percent of each compound differs significantly among biomass

resources. This heterogeneity creates variability in the yields of pyrolysis products.

Cellulose is converted to char and gases (CO, CO2, H2O) at low temperatures (< 300oC),

and to volatile compounds (tar and organic liquids, predominantly levoglucosan) at high

temperatures (> 300oC). The yield of light hydrocarbons (i.e., C1 - C4) is negligible below

500°C but increases substantially at high temperatures. At temperatures above 600°C, tar

yields drop, gas yields increase, and the pyrolysis of cellulose is complete.

Hemicellulose is the most reactive component of biomass and decomposes between 200

and 260oC. The decomposition of hemicellulose is postulated to occur in two steps—the

breakdown of the polymer into water soluble fragments followed by conversion to

monomeric units and decomposition into volatile compounds. Hemicelluloses produce

more gases and less tar than cellulose, and no levoglucosan. They also produce more

methanol and acetic acid than cellulose.

Lignin is a highly linked, amorphous, high molecular weight phenolic compound which

serves as cement between plant cells and is the least reactive component of biomass.

The time required to pyrolyze biomass resources is controlled by the rate of pyrolysis of

lignin under operating conditions. Decomposition of lignin occurs between 280°C and

38

500°C, although some physical and/or chemical changes (e.g., depolymerization, loss of

some methanol) may occur at lower temperatures. At slow heating rates, lignin loses only

about half of its weight at temperatures below 800°C. Pyrolysis of lignin yields more char

and tar than cellulose.

For wood, the decomposition of the major components occurs separately and

sequentially with the hemicellulose decomposing first and the lignin last. Up to 200°C,

moisture is removed, volatile products such as acetic acid and formic acid are released, and

non-condensable gases such as CO and CO2 are produced. Between 200 and 280°C, further

decomposition of the char and wood occur resulting in the release of pyroligneous acids,

water and non-condensable gases. Separation of tar occurs. Between 280 and 500°C, release

of combustible volatile products (CO, CH4, H2, formaldehyde, formic acid, methanol, and

acetic acid) occurs. Char formation decreases and the carbon content of the char increases.

Condensable tar is released. Above 500°C, carbonization is complete. Secondary reactions

begin if the materials are not removed from the reaction zone as quickly as they form.

As temperatures increase, char production decreases (to a steady level above 650°C) and

the carbon content of the char increases. Hydrocarbon gas yields (e.g., C2H6, C3H6)

increase up to about 660°C and then decline, probably due to thermal cracking. The time

required to obtain a given conversion level decreases with increasing temperature.

39

2.3.2 CHEMICAL REACTOR

A chemical reactor is a device which is used to contain controlled chemical reactions.

Reactions take place inside the reactor, in conditions which can be monitored and controlled

for safety and efficiency. These types of reactors are used in the production of chemicals

such as components of pharmaceutical compounds, and they can operate in several different

ways. A number of scientific specialty companies produce chemical reactors and

accessories such as replacement components for damaged devices.

Chemical reactors can be designed as either tanks or pipes, depending on the needs, and

they can vary in size considerably. Small bench top chemical reactor designs are intended

for use in labs, for example, while large tanks can be used to make chemicals on an

industrial scale. The design also includes a variety of features which can be used to control

conditions inside the reactor.

With a batch chemical reactor, the components of the reaction are added to the reactor and a

controlled reaction is allowed to take place. When the reaction is finished, the batch can be

removed and the reactor can be prepared for another round. This type of reactor works best

when people need chemicals on a small scale, as for example when research chemists are

preparing compounds for pharmaceutical research.

Continuous chemical reactors operate continuously, as long as the materials needed for the

reaction are supplied. These are used to create a steady supply of a needed chemical.

Continuous reactors are commonly used in the manufacture of industrial chemicals, when

the need for a chemical is high and very consistent. These reactors are periodically shut

40

down for maintenance or when they are not needed, in which case special steps may need to

be taken when they are restarted so that their functionality will not be impaired.

These devices are designed by chemical engineers who are familiar with the needs of

chemical reactors and the various ways in which they can be used. For special applications,

an engineer may design a custom reactor which is specifically built for the purpose, in

which case the engineer is also involved in the design of the space where the reactor will be

used, to ensure that it conforms with safety guidelines and to confirm that the space has

been properly designed to accommodate the chemical reactor

2.3.2.1 Types of pyrolyzers

There are a number of different pyrolysis reactors (physical containers where the reaction is

performed).

1. Bubbling Fluidized Bed Pyrolyzers

Bubbling fluidized bed pyrolyzers have been popular because they are simpler to design and

construct compared with other reactor designs. They also have good gas-to-solids contact,

good heat transfer, good temperature control and a large heat storage capacity.

In a fluidized bed pyrolyzer, a heated sand medium in a zero-oxygen environment quickly

heats the feedstock (biomass) to 850º F, where it is decomposed into solid char, gas,

vapours and aerosols which exit the reactor by the conveying fluidizing gas stream. After

exiting the reactor zone, the charcoal can be removed by a cyclone separator and stored. The

41

scrubbed gases, vapours and aerosols enter a direct quenching system where they are

rapidly cooled (< 125º F) directly with a liquid immiscible (two liquids that don’t mix) in

bio-oil or indirectly using chillers (heat exchanger). The condensed bio-oil is collected and

stored, and the non-condensable gas (syngas) may be recycled or used as a fuel to heat the

reactor. High liquid yields (about 60 percent weight of biomass on a dry basis) can be

typically recovered. Small feedstock particle sizes are needed (< 2-3 mm) to ensure that the

high heat rate requirement is fulfilled. The particle heating rate is the major factor limiting

the rate of the pyrolysis reaction. Prior to recycling the syngas and residual bio-oil, aerosol

droplets may be further scrubbed in an electrostatic precipitator to remove finer particulates

and aerosols. The syngas (a medium Btu gas) may be burned to provide necessary heat to

the reactor.

Fig 2-10 Bubbling Fluidized Bed Pyrolyzer

42

2. Circulating Fluidized Bed Pyrolyzers

Circulating fluidized bed pyrolyzers are similar to bubbling fluidized bed reactors but have

shorter residence times for chars and vapours. The short residence times encountered in the

reactor result in higher gas velocities faster vapour and char escape and higher char content

in the bio-oil than bubbling fluidized beds. However, they have higher processing capacity,

better gas-solid contact and improved ability to handle solids that are more difficult to

fluidize but are less commonly used. The heat supply typically comes from a secondary char

combustor.

Fig 2-11 Circulating Fluidized Bed Pyrolyzers

43

3. The Rotating Plate Pyrolyzers

The rotating plate pyrolysis reactors function on the premise that, while under pressure, heat

transferred from a hot surface can soften and vaporize the feedstock in contact with it –

allowing the pyrolysis reaction to move through the biomass in one direction. With this

arrangement, larger particles, including logs, can be pyrolyzed without pulverizing them.

The most important feature is that there is no requirement for an inert gas medium, thereby

resulting in smaller processing equipment and more intense reactions. However, the process

is dependent on surface area, so scaling can be an issue for the larger facilities.

Fig 2-12 The Rotating Plate Pyrolyzer

4. Rotating Cone Pyrolyzer

In a rotating cone pyrolysis reactor, biomass particles at room temperature and hot sand are

introduced near the bottom of a cone at the same time. They are mixed and transported

upwards by the rotation of the cone. The pressures of outgoing materials are slightly above

atmospheric levels. Rapid heating and short gas phase residence times can be easily

achieved in this reactor.

44

Fig 2-13 Rotating Cone Pyrolyzer

(Samy Sadaka and A.A Boateng (2009): Pyrolysis and Bio-Oil - FSA1052. [online].

University of Arkansas, United States Department of Agriculture and County Governments

Cooperating.)

45

2.3.3 TEMPERATURE CONTROL AND MEASUREMENT

In the production of biochar temperature control and measurement is paramount. The

temperature at which the pyrolyzer is operated will determine the quality and quantity of

products to be obtained .Some materials usually used in the temperature control may be

factored during the construction of the reactor and some may be used during the pyrolysis

process. The temperature regulation materials may include thermocouple, fibre glass,

refractory brick and refractory mortar.

2.3.3.1Thermocouple

A thermocouple is a sensor for measuring temperature. It consists of two dissimilar metals,

joined together at one end. When the junction of the two metals is heated or cooled a

voltage is produced that can be correlated back to the temperature.

In 1822, an Estonian physicist named Thomas Johann Seebeck discovered that when any

conductor (such as a metal) is subjected to a thermal gradient, it will generate a small

voltage. Thermocouples make use of this so-called Peltier-Seebeck effect.

Thermocouples produce an output voltage which depends on the temperature difference

between the junctions of two dissimilar metal wires. It is important to appreciate that

thermocouples measure the temperature difference between two points, not absolute

temperature.

46

In most applications, one of the junctions — the "cold junction" — is maintained at a

known (reference) temperature, whilst the other end is attached to a probe.

The relationship between the temperature difference and the output voltage of a

thermocouple is nonlinear and is given by a complex polynomial equation (which is fifth to

ninth order depending on thermocouple type). To achieve accurate measurements some type

of linearization must be carried out, either by a microprocessor or by analogue means.

2.3.3.1.1 Principles of Thermocouples

� Voltage–temperature relationship

For typical metals used in thermocouples, the output voltage increases almost linearly with

the temperature difference (∆T) over a bounded range of temperatures. For precise

measurements or measurements outside of the linear temperature range, non-linearity must

be corrected. The nonlinear relationship between the temperature difference (∆T) and the

output voltage (mV) of a thermocouple can be approximated by a polynomial:

………………1

The coefficients an are given for n from 0 to between 5 and 13 depending upon the metals.

In some cases better accuracy is obtained with additional non-polynomial terms.

47

� Laws of thermocouples

1. Law of homogeneous material

A thermoelectric current cannot be sustained in a circuit of a single homogeneous material

by the application of heat alone, regardless of how it might vary in cross section. In other

words, temperature changes in the wiring between the input and output do not affect the

output voltage, provided all wires are made of the same materials as the thermocouple.

2. Law of intermediate materials

The algebraic sum of the thermoelectric emfs in a circuit composed of any number of

dissimilar materials is zero if all of the junctions are at a uniform temperature. So if a third

metal is inserted in either wire and if the two new junctions are at the same temperature,

there will be no net voltage generated by the new metal.

3. Law of successive or intermediate temperatures

If two dissimilar homogeneous materials produce thermal emf1 when the junctions are at

T1 and T2 and produce thermal emf2 when the junctions are at T2 and T3 , the emf

generated when the junctions are at T1 and T3 will be emf1 + emf2

48

2.3.3.1.2 Types of Thermocouples

Thermocouples are available in different combinations of metals, usually referred to by a

letter, e.g. J, K etc. Each combination has a different temperature range and is therefore

more suited to certain applications than others. Although the thermocouple calibration

dictates the temperature range, the maximum range is also limited by the diameter of the

thermocouple wire.

Table 2.3 Summary of Thermocouple Types

Type Positive

Material

Negative

Material

Accuracy***

Class 2

Range, °C

(extension) Comments

B Pt, 30%Rh Pt, 6%Rh 0.5%

>800°C

50 to 1820

(1 to 100)

Good at high

temperatures, no

reference junction

compensation

required.

C** W, 5%Re W, 26%Re 1%

>425°C

0 to 2315

(0 to 870)

Very high

temperature use,

brittle

D** W, 3%Re W, 25%Re 1%

>425°C

0 to 2315

(0 to 260)

Very high

temperature use,

brittle

E Ni, 10%Cr Cu, 45%Ni 0.5% or -270 to General purpose,

49

1.7°C 1000

(0 to 200)

low and medium

temperatures

G** W W, 26%Re 1%

>425°C

0 to 2315

(0 to 260)

Very high

temperature use,

brittle

J Fe Cu, 45%Ni 0.75% or

2.2°C

-210 to

1200

(0 to 200)

High temperature,

reducing

environment

K* Ni, 10%Cr

Ni, 2%Al

2%Mn

1%Si

0.75% or

2.2°C

-270 to

1372

(0 to 80)

General purpose

high temperature,

oxidizing

environment

L** Fe Cu, 45%Ni 0.4% or

1.5°C 0 to 900

Similar to J type.

Obsolete - not for

new designs

M** Ni Ni, 18%Mo 0.75% or

2.2°C -50 to 1410

N* Ni, 14%Cr

1.5%Si

Ni,

4.5%Si

0.1%Mg

0.75% or

2.2°C

-270 to

1300

(0 to 200)

Relatively new

type as a superior

replacement for K

Type.

P** Platinel II Platinel II 1.0% 0 to 1395 A more stable but

expensive

50

substitute for K &

N types

R Pt, 13%Rh Pt 0.25% or

1.5°C

-50 to 1768

(0 to 50)

Precision, high

temperature

S Pt, 10%Rh Pt 0.25% or

1.5°C

-50 to 1768

(0 to 50)

Precision, high

temperature

T* Cu Cu, 45%Ni 0.75% or

1.0°C

-270 to 400

(-60 to 100)

Good general

purpose, low

temperature,

tolerant to

moisture.

U**

Cu Cu, 45%Ni

0.4% or

1.5°C 0 to 600

Similar to T type.

Obsolete - not for

new designs

Materials codes: Al = Aluminium, Cr = Chromium, Cu = Copper, Mg = Magnesium, Mo =

Molybdenum, Ni = Nickel, Pt = Platinum, Re = Rhenium, Rh = Rhodium, Si = Silicon, W =

Tungsten

51

FACTOR THAT AFFECT THE CHOICE OF A THERMOCOUPLE

Thermocouples measure wide temperature ranges and can be relatively rugged; therefore

they are very often used in industry. The following criteria are used in selecting a

thermocouple:

• Temperature range

• Chemical resistance of the thermocouple or sheath material

• Abrasion and vibration resistance

• Installation requirements (may need to be compatible with existing equipment;

existing holes may determine probe diameter)

2.3.3.1.3 Operation of Thermocouples

The basis of thermocouples was established by Thomas Johann Seebeck in 1821 when he

discovered that a conductor generates a voltage when subjected to a temperature gradient.

To measure this voltage, one must use a second conductor material which generates a

different voltage under the same temperature gradient. Otherwise, if the same material was

used for the measurement, the voltage generated by the measuring conductor would simply

cancel that of the first conductor. The voltage difference generated by the two materials can

then be measured and related to the corresponding temperature gradient. It is thus clear that,

based on Seebeck's principle; thermocouples can only measure temperature differences and

need a known reference temperature to yield the absolute readings.

52

There are three major effects involved in a thermocouple circuit: the Seebeck, Peltier, and

Thomson effects.

The Seebeck effect describes the voltage or electromotive force (EMF) induced by the

temperature difference (gradient) along the wire. The change in material EMF with respect

to a change in temperature is called the Seebeck coefficient or thermoelectric sensitivity.

This coefficient is usually a nonlinear function of temperature.

Peltier effect describes the temperature difference generated by EMF and is the reverse of

Seebeck effect. Finally, the Thomson effect relates the reversible thermal gradient and EMF

in a homogeneous conductor.

Thermocouple Circuit

A typical thermocouple circuit can be illustrated as follows:

Fig 2-14 Typical Thermocouple Circuit

53

Suppose that the Seebeck coefficients of two dissimilar metallic materials, metal A and

metal B, and the lead wires are SA, SB, and SLead respectively. All three Seebeck coefficients

are functions of temperature. The voltage output Vout measured at the gage (see schematic

above) is,

...............2

where TRef is the temperature at the reference point, TTip is the temperature at the probe tip.

Note that mathematically the voltage induced by the temperature and/or material mismatch

of the lead wires cancels, whereas in reality the lead wires will introduce noise into the

circuit.

If the Seebeck coefficient functions of the two thermocouple wire materials are pre-

calibrated and the reference temperature TRef is known (usually set by a 0°C ice bath), the

temperature at the probe tip becomes the only unknown and can be directly related to the

voltage readout.

If the Seebeck coefficients are nearly constant across the targeted temperature range, the

integral in the above equation can be simplified, allowing one to solve directly for the

temperature at the probe tip,

54

........................3

In practice, vendors will provide calibration functions for their products. These functions

are usually high order polynomials and are calibrated with respect to a certain reference

temperature, e.g., 0 °C (32 °F). Suppose that the coefficients of the calibration polynomials

are a0, a1, a2, ..., an. The temperature at the probe tip can then be related to the voltage

output as,

…………..4

Note that the above formula is effective only if the reference temperature TRef in the

experiment is kept the same as the reference temperature specified on the data sheet.

Furthermore, these coefficients are unit sensitive. Make sure to use the vendor-specified

temperature unit (i.e. Celsius/centigrade, Fahrenheit, or Kelvin) when plugging in numbers.

Again, a thermocouple is a relative not absolute temperature sensor. In other words, a

thermocouple requires a reference of known temperature which is provided by ice water in

the above illustration. While ice water is an easy to obtain and well known reference, it is

not practical outside of a laboratory. Thus, common commercialized thermocouples often

include another temperature sensor, such as thermistor, to provide the reading of the

reference (room/surrounding) temperature.

55

Table 2.4 Seebeck’s Coefficients of Some Materials

Material Seebeck

Coefficient Material

Seebeck

Coefficient Material

Seebeck

Coefficient

Aluminium 3.5 Gold 6.5 Rhodium 6.0

Antimony 47 Iron 19 Selenium 900

Bismuth -72 Lead 4.0 Silicon 440

Cadmium 7.5 Mercury 0.60 Silver 6.5

Carbon 3.0 Nichrome 25 Sodium -2.0

Constantan -35 Nickel -15 Tantalum 4.5

Copper 6.5 Platinum 0 Tellurium 500

Germanium 300 Potassium -9.0 Tungsten 7.5

56

2.3.3.1.4 Aging of Thermocouples

Thermo elements are often used at high temperatures and in reactive furnace atmospheres.

In this case the practical lifetime is determined by aging. The thermoelectric coefficients of

the wires in the area of high temperature change with time and the measurement voltage

drops. The simple relationship between the temperature difference of the joints and the

measurement voltage is only correct if each wire is homogeneous. With an aged

thermocouple this is not the case. Relevant for the generation of the measurement voltage

are the properties of the metals at a temperature gradient. If an aged thermocouple is pulled

partly out of the furnace, the aged parts from the region previously at high temperature enter

the area of temperature gradient and the measurement error is significantly increased.

However an aged thermocouple that is pushed deeper into the furnace gives a more accurate

reading.

2.3.3.1.5 Advantages and Disadvantages of Thermocouples

Advantages with thermocouples

• Capable of being used to directly measure temperatures up to 2600 oC.

• The thermocouple junction may be grounded and brought into direct contact with

the material being measured.

57

Disadvantages with thermocouples

• Temperature measurement with a thermocouple requires two temperatures be

measured, the junction at the work end (the hot junction) and the junction where

wires meet the instrumentation copper wires (cold junction). To avoid error the cold

junction temperature is in general compensated in the electronic instruments by

measuring the temperature at the terminal block using with a semiconductor,

thermistor, or RTD.

• Thermocouples operation is relatively complex with potential sources of error. The

materials of which thermocouple wires are made are not inert and the thermoelectric

voltage developed along the length of the thermocouple wire may be influenced by

corrosion etc.

• The relationship between the process temperature and the thermocouple signal

(millivolt) is not linear.

• The calibration of the thermocouple should be carried out while it is in use by

comparing it to a nearby comparison thermocouple. If the thermocouple is removed

and placed in a calibration bath, the output integrated over the length is not

reproduced exactly.

2.3.3.1.6 Problems Associated with the use of Thermocouple

� Connection problems. Many measurement errors are caused by unintentional

thermocouple junctions. Remember that any junction of two different metals will

cause a junction. If you need to increase the length of the leads from your

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thermocouple, you must use the correct type of thermocouple extension wire (egg

type K for type K thermocouples). Using any other type of wire will introduce a

thermocouple junction. Any connectors used must be made of the correct

thermocouple material and correct polarity must be observed.

� Lead Resistance. To minimize thermal shunting and improve response times,

thermocouples are made of thin wire (in the case of platinum types cost is also a

consideration). This can cause the thermocouple to have a high resistance which can

make it sensitive to noise and can also cause errors due to the input impedance of the

measuring instrument. A typical exposed junction thermocouple with 32 AWG wire

(0.25 mm diameter) will have a resistance of about 15 Ohms / meter. If

thermocouples with thin leads or long cables are needed, it is worth keeping the

thermocouple leads short and then using thermocouple extension wire (which is

much thicker, so has a lower resistance) to run between the thermocouple and

measuring instrument. It is always a good precaution to measure the resistance of

your thermocouple before use.

� Decalibration is the process of unintentionally altering the makeup of thermocouple

wire. The usual cause is the diffusion of atmospheric particles into the metal at the

extremes of operating temperature. Another cause is impurities and chemicals from

the insulation diffusing into the thermocouple wire. If operating at high

temperatures, check the specifications of the probe insulation.

� Noise: The output from a thermocouple is a small signal, so it is prone to electrical

noise pick up. Most measuring instruments (such as the TC-08) reject any common

mode noise (signals that are the same on both wires) so noise can be minimised by

59

twisting the cable together to help ensure both wires pick up the same noise signal.

Additionally, the TC-08 uses an integrating analogue to digital converter which

helps average out any remaining noise. If operating in an extremely noisy

environment, (such as near a large motor) it is worthwhile considering using a

screened extension cable. If noise pickup is suspected first switch off all suspect

equipment and see if the reading changes.

� Common Mode Voltage: Although thermocouple signal are very small, much

larger voltages often exist at the input to the measuring instrument. These voltages

can be caused either by inductive pick up (a problem when testing the temperature

of motor windings and transformers) or by 'earthed' junctions. A typical example of

an 'earthed' junction would be measuring the temperature of a hot water pipe with a

non insulated thermocouple. If there are any poor earth connections a few volts may

exist between the pipe and the earth of the measuring instrument. These signals are

again common mode (the same in both thermocouple wires) so will not cause a

problem with most instruments provided they are not too large. For example, the

TC-08 has a common mode input range of -4 V to +4 V. If the common mode

voltage is greater than this then measurement errors will result. Common mode

voltages can be minimised using the same cabling precautions outlined for noise,

and also by using insulated thermocouples.

� Thermal Shunting: All thermocouples have some mass. Heating this mass takes

energy so will affect the temperature you are trying to measure. Consider for

example measuring the temperature of liquid in a test tube: there are two potential

problems. The first is that heat energy will travel up the thermocouple wire and

60

dissipate to the atmosphere so reducing the temperature of the liquid around the

wires. A similar problem can occur if the thermocouple is not sufficiently immersed

in the liquid, due to the cooler ambient air temperature on the wires; thermal

conduction may cause the thermocouple junction to be a different temperature to the

liquid itself. In the above example a thermocouple with thinner wires may help, as it

will cause a steeper gradient of temperature along the thermocouple wire at the

junction between the liquid and ambient air. If thermocouples with thin wires are

used, consideration must be paid to lead resistance. The use of a thermocouple with

thin wires connected to much thicker thermocouple extension wire often offers the

best compromise.

2.3.3.1.7 Calibrating Thermocouples

In order to achieve accurate readings from a thermocouple, it’s essential to calibrate the

device accordingly. Typically, thermocouples are standardized by using 0 degrees C as a

reference point, and many devices can adjust to compensate for the varying temperatures at

thermocouple junctions.

To calibrate a thermocouple, various types of measuring equipment, standards, and

procedures must be in place. First, a control temperature must be established that is stable

and provides a constant temperature; it must be uniform and cover a large enough area that

the thermocouple can adequately be inserted into it (such as an ice bath). Sources of

controlled temperatures are called fixed points. A fixed point cell is composed of a metal

61

sample within a graphite crucible, with a graphite thermometer submerged in the metal

sample. When this metal sample reaches the freezing point, it maintains a very stable

temperature. The freezing point occurs when a material reaches the point between the solid

and liquid phase. A reference junction temperature must also be established; typically, 0 C

is used. A measuring instrument, such as Fluke 702 calibrator, can be used to measure

thermocouple output.

2.3.3.2 Refractory Brick

Refractory brick, also known as fire brick, is a type of specialized brick which is designed

for use in high heat environments such as kilns and furnaces. High quality refractory brick

has a number of traits which make it distinct from other types of brick.

The primarily important property of refractory brick is that it can withstand very high

temperatures without failing. It also tends to have low thermal conductivity, which is

designed to make operating environments safer and more efficient. Furthermore, refractory

brick can withstand impact from objects inside a high heat environment, and it can contain

minor explosions which may occur during the heating process. It may be dense or porous,

depending on the design and the intended utility.

This brick product is made with specialty clays which can be blended with materials such as

magnesia, silicon carbide, alumina, silica, and chromium oxide. The exact composition of

refractory brick varies, depending on the applications it is designed for, with manufacturers

disclosing the concentrations of ingredients and recommended applications in their catalogs.

62

Using refractory brick which is not designed for the application can be dangerous, as the

bricks may fail cracking, exploding, or developing other problems during use which could

pose a threat to safety in addition to fouling a project.

Even though it is specifically designed for high heat environments, refractory brick will

eventually start to fail. It can crack, flake, or break down over time, necessitating regular

inspection of environments where this product is used. If damaged bricks are identified,

they need to be removed and replaced with new bricks to ensure that the device operates as

intended, and to reduce the risk of injuries, equipment failure, and other problems. The

bricks can also accumulate soot and other materials through routine use, and they may need

to be scrubbed down periodically.

APPLICATION

Some places where refractory brick can appear include: fireplaces, wood stoves, cremation

furnaces, ceramic kilns, furnaces, forges, and some types of ovens. The earliest refractory

bricks were developed around the 1800s, with several inventors contributing radical

reworkings to make such products safer and more reliable. Companies continue to

experiment with recipes and manufacturing process to develop even better products which

will increase efficiency and safety while cutting down on maintenance costs.

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2.3.3.3 FIBRE GLASS

Fibreglass is commonly used as an insulating material. It is also used as a reinforcing agent

for many polymer products; the resulting composite material, properly known as fibre-

reinforced polymer (FRP) or glass-reinforced plastic (GRP), is also called "fibreglass" in

popular usage. A somewhat similar, but more expensive technology used for applications

requiring very high strength and low weight is the use of carbon fibre.

Fig 2-15 Fibre Glass

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TYPES OF FIBRE GLASS

The types of fibreglass most commonly used are mainly E-glass (alumino-borosilicate glass

with less than 1 wt% alkali oxides, mainly used for glass-reinforced plastics), but also A-

glass (alkali-lime glass with little or no boron oxide), E-CR-glass (alumino-lime silicate

with less than 1 wt% alkali oxides, has high acid resistance), C-glass (alkali-lime glass with

high boron oxide content, used for example for glass staple fibres), D-glass (borosilicate

glass with high dielectric constant), R-glass (alumino silicate glass without MgO and CaO

with high mechanical requirements), and S-glass (alumino silicate glass without CaO but

with high MgO content with high tensile strength).

E-glass and S-glass. The differences between them are their physical properties and

compositions. E-glass comes from calcium whereas S-glass is created with magnesium. S-

glass also has a higher tensile and compressive strength (the maximum stress it can take

before breaking).

Properties of FIBRE GLASS

The basis of textile-grade glass fibres is silica, SiO2. In its pure form it exists as a polymer,

(SiO2) n. It has no true melting point but softens at 2,000 °C (3,630 °F), where it starts to

degrade. At 1,713 °C (3,115 °F), most of the molecules can move about freely. If the glass

is then cooled quickly, they will be unable to form an ordered structure. In the polymer, it

forms SiO4 groups that are configured as a tetrahedron with the silicon atom at the centre

65

and four oxygen atoms at the corners. These atoms then form a network bonded at the

corners by sharing the oxygen atoms.

The vitreous and crystalline states of silica (glass and quartz) have similar energy levels on

a molecular basis, also implying that the glassy form is extremely stable. In order to induce

crystallization, it must be heated to temperatures above 1,200 °C (2,190 °F) for long periods

of time.

Although pure silica is a perfectly viable glass and glass fibre, it must be worked with at

very high temperatures, which is a drawback unless its specific chemical properties are

needed. It is usual to introduce impurities into the glass in the form of other materials to

lower its working temperature. These materials also impart various other properties to the

glass that may be beneficial in different applications. The first type of glass used for fibre

was soda lime glass or A glass. It was not very resistant to alkali. A new type, E-glass, was

formed; this is an alumino-borosilicate glass that is alkali free (<2%). This was the first

glass formulation used for continuous filament formation. E-glass still makes up most of the

fibreglass production in the world. Its particular components may differ slightly in

percentage, but must fall within a specific range. The letter E is used because it was

originally for electrical applications. S-glass is a high-strength formulation for use when

tensile strength is the most important property. C-glass was developed to resist attack from

chemicals, mostly acids that destroy E-glass.

66

Table 2.5 Properties of E and G Types Fibre Glass

Property E-glass S-glass

Density, kg/m 2580 2490

Tensile Strength, GPa 3.45 4.6

Tensile Modulus, GPa 72.5 85.0

Elongation,% 4.9 5.7

The high surface area to weight ratio make glass fibres useful in many situations. However

a large surface area is not always a good idea as its performance can be severely impaired

by contact with moisture and chemicals.

They are often used in high stress situations because they can undergo a higher elongation

than carbon fibres before breaking. Whereas Kevlar’s weakness is that its strength is only

along the fibre axis, fibreglass products, because of the underlying amorphous structure,

have similar properties in all directions. The picture shows the microstructure of a glass

fibre.

Uses of Fibreglass:

Uses for regular fibreglass include mats, thermal insulation, electrical insulation, sound

insulation, reinforcement of various materials, tent poles, sound absorption, heat- and

corrosion-resistant fabrics, high-strength fabrics, pole vault poles, arrows, bows and

crossbows, translucent roofing panels, automobile bodies, hockey sticks, surfboards, boat

67

hulls, and paper honeycomb. It has been used for medical purposes in casts. Fiberglas is

extensively used for making FRP tanks and vessels. Fiberglas is also used in the design of

Irish stepdance shoes.

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2.3.4 BLOCK DIAGRAM OF PROCESS

Fig 2-15 Flow Diagram of Biochar Production

BIOMASS

COLLECTION

FARMLAND

WASTE WOOD

WASTE

CHIPPING

DRYING

SOIL

APPLICATION

COMPOST

GAS BIOCHAR

SLOW

PYROLYSIS

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2.4 Compost

Compost fertilizer a product that provides helpful nutrients to soil, plants, flowers and

vegetables. This type of fertilizer is generally formed through the controlled decomposition

of organic material such as plants, fruits, and vegetables. While all organic material

eventually decomposes, composting speeds up the process and helps forms a nutrient-rich

soil.

Compost fertilizer helps soil retain water, promotes the healthy development of root

systems in plants, and provides an ecologically and environmentally friendly means of

disposing of food waste and lawn clippings. It has been estimated that food and lawn waste

account for approximately 30% of all waste in landfills. Converting waste to compost

fertilizer helps to free up valuable and limited space in landfills.

Compost is generally made by collecting organic waste and material in a container, often

called a compost bin. The compost fertilizer is most effective when there is a proper balance

between green ingredients, such as green clippings, manure, vegetable and fruit waste and

brown waste such as wood, dried leaves, sawdust, straw and paper. In the compost mix

green components provide nitrogen, while the brown ingredients provide carbon.

Once the ingredients are combined in the compost bin, water is added and the ingredients

are mixed together. At this point in the process, compost activator is often added, which is a

liquid high in nitrogen, to speed up the decomposition process. When the compost pile is

formed, microbes in the mix start to multiply and break down the organic components of the

material, causing the compost pile to heat up.

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When properly maintained, compost fertilizer should be ready for use in approximately two

to four weeks. The completed compost should have a dark brown colour and smell earthy

and musty like fresh soil. If the pile has a bad smell like garbage, it is more than likely not

ready and may need additional material added to it to correct the balance of the overall

mixture.

Compost fertilizer has several different applications. It is often combined with existing soil,

which allows the nutrients in the compost to slowly release into the soil. It can be used very

similarly to mulch and applied around flowers and plants to reduce the growth of weeds and

retain water around the plants. Compost fertilizer may also be used as part of a potting soil

mix.

2.4.1 Types of Compost

Compost can be broken into three basic types based on its quality and usage.

These are Biological, Commercial, and Industrial.

Biological – This is the highest quality compost and therefore the most

beneficial in improving soils, preventing disease, making compost tea, etc.

This is the product that experienced gardeners often call black gold because it

is so valuable to plants. The bag will have holes in it so that air can enter and

the beneficial microbes can breathe and be kept alive.

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Commercial – This is a middle grade of compost made from sewage sludge,

construction debris, etc. It will be in a sealed bag and may have a sour or stale

odour. The better manure based composts may be found here.

Industrial – This is the lowest grade of products called compost. It is made

from industrial wastes like boiler ash. It is often very black and sometimes will

rub off in your hand. It often contains fillers like sawdust and rice hulls which

are chemically burned black from the industrial waste. It may be extremely

alkaline and high in toxic salts.

2.4.2 Materials used for making compost

Compost can be made from a number of materials. Listed below are some of these

materials:

� Kitchen waste

� Lawn clippings (use thin layers so they don't mat down)

� Chopped leaves (large leaves take a long time to break down)

� Shredded branches

� garden plants (use disease free plants)

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� shredded paper

� weeds (before they go to seed)

� straw

� newspapers

� wood ash (sprinkle lightly between layers)

� hay

2.4.3 Uses of compost

Compost is a valuable material that does many jobs and does them well. There always seem

to be the need for more compost than is available. The uses include:

• NUTRIENT SOURCE FOR THE SOIL ORGANISMS: Compost is the best, all around

source of organic material as a nutrient for soil organisms. The composting process has

"pre- digested" a lot of the material making it easier for the soil organisms to assimilate

as nutrients. Composted material is also cleaner relative to pathogens, salts and other

toxic materials.

Most garden soils are low in organic content. Making the soil richer in organic content,

increasing the quantity, diversity and activity of the soil organisms. One of the activities

of the soil organisms is the consumption of this organic matter. The feeding of the soil

organisms needs to be on-going. Generally speaking, the soil organisms in regions with

a relatively short growing season can consume a 2" (5 cm) layer of compost or

composted manure per year. In regions in which the growing season is extended or

73

continuous, they can consume twice that much. It takes a lot of organic material to feed

the soil organisms.

• NUTRIENT SOURCE FOR THE PLANTS: It is often said that compost is a soil

amendment and not a fertilizer. This is mostly an issue of semantics. The manufacture

and sale of synthetic fertilizers is big business. Trade associations prevailed on a number

of State governments to pass legislation that prohibits the sale of manures and composts

as "fertilizers". The justification was that manures and composts are generally low in

their content of mineral nutrients and particularly in terms of the trace elements. The

NPK numbers for a typical compost would be somewhere around 0.5-0.5-0.5.

In practice, NPK numbers are only half the story. Plant nutrition involves the processing

of minerals and other nutrients by the soil organisms. Feeding the garden means feeding

the soil organisms and compost is an excellent source of these nutrients. At the use

level, compost contains adequate levels of the various mineral nutrients but can be shy

of trace elements. Fish emulsion or fish meal usually contain enough of the trace

elements that supplemental feedings with either material would correct possible

deficiencies.

• SOIL AMENDMENT TO IMPROVE STRUCTURE: Compost is the organic material

of choice to improve soil structure. It helps the soil particles to bond and form soil

aggregates; it helps in the way these aggregates retain and release plant nutrients; and it

helps to create better porosity. Compost, better than any other material, helps clay soil to

become workable.

• SOIL AMENDMENT TO IMPROVE DRAINAGE: Organic material, in general, will

amend slow draining clay soils to drain faster and sharp draining sand soils to drain

74

slower. Compost is of particular value as a soil amendment to improve drainage in that

it works so well and is so easy to use.

• MULCH TO MINIMIZE PROBLEMS WITH WEEDS: A good 2" (5cm) layer of

compost works very well as mulch but with the understanding that this mulch layer is

temporary. The compost layer is eventually consumed by the soil organisms,

particularly by earthworms, adding to the life of the soil.

The mulch layer serves as a physical barrier keeping weed seeds from getting into the

soil and as a light barrier keeping weed seeds that are already in the soil from

germinating.

• MULCH TO MINIMIZE LOSS OF SOIL MOISTURE: Compost acts like a sponge and

retains moisture. A layer of compost on the soil surface will catch and hold moisture

that would otherwise evaporate from the soil. This helps to minimize the loss of soil

moisture. The flip side is that it also tends to catch and hold water that would otherwise

enter to irrigate the soil. Too thick a layer of compost as a mulch can create watering

problems.

• MULCH TO INSULATE SOIL: Compost has good insulating properties and a layer of

compost slows down changes in soil temperatures as a result of changes in the weather.

Soil organisms do better when temperature changes are gradual.

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CHAPTER THREE

3.0 METHODOLOGY

3.1 BIOMASS COLLECTION AND PREPARTION

3.1.1 Biomass Collection

Raw Material used

Saw dust, wood shavings and rice husk.

Source:

Saw dust- AG Timbers, Kaasi, Kumasi in the Ashanti region Ghana.

Wood shavings - project site at FABI

Rice Husk-Asuanse

3.1.2 Drying

Procedure

i.A sample of the saw dust was first collected and tested for its moisture content.

ii.The saw dust was spread on a tarpaulin to dry.

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Fig 3-1 Drying of Odum sawdust

Fig 3-2 Covering of sawdust

77

Fig 3-3 covered saw dust

3.1.3 Determination of pH

pH is a measure of the acidity or alkalinity of an aqueous solution. Pure water is said to be

neutral, with a pH close to 7.0 at 25 °C (77 °F). Solutions with a pH less than 7 are said to

be acidic and solutions with a pH greater than 7 are basic or alkaline.

The pH of the samples used and the char obtained were determined.

Apparatus and Materials

pH meter, glass beaker, Standard buffer solutions of known pH values - standards used were

pH of 4.0, Distilled water, teaspoon or small scoop, glass stiffing rod,

78

Procedure

I. Known masses of 0.2g of the samples were weighed and put in the glass beakers.

II. 12 ml of distilled water was added to the samples and stirred to obtain a mixture

and the beakers covered with watch glasses.

III. The samples were allowed to stand for a minimum of one hour, while stirring every

10 to 15 minutes to allow the pH of the samples to stabilize.

IV. The pH meter was then standardized by means of the standard solutions provided.

V. Prior to immersing the electrode into the samples, the samples were stirred well with

a glass rod.

VI. The electrode was then placed into the mixture and the solutions gently turned to

make good contact between the solutions and the electrode. The electrode was not

placed in the samples itself but only in the slurry.

VII. The electrode was immersed till the meter reading stabilized.

VIII. The pH values were then read and recorded to 2 decimal places.

IX. The electrode was then rinsed well with distilled water

3.1.4 Moisture Content Determination

Apparatus/Equipment

Crucible, Analytical balance, Oven, desiccator

Procedure

i. An empty crucible was first measured and its mass noted as m1.

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ii. A sample of the raw materials was put into the crucible and weighed .Its mass was

then recorded as m2.

iii. The sample in the crucible was put in an electric oven and heated to a temperature of

about 115oC. After every hour, the sample in the oven was removed and cooled over

a period of 45 minutes in a desiccator (to prevent the sample from absorbing

moisture from the atmosphere). The mass of the sample was then taken. This was

repeated until a constant mass was obtained and recorded as m3.

iv. The moisture content was obtained using the relation:

weightinitial

weightinlosscontentMoisture

=

10012

32 ×−−

=mm

mmM

……………..6

N/B: The moisture content was determined for samples before and after drying them.

80

3.2 REACTOR OPERATIONS

Description of Reactor

The reactor is made up of two cylindrical iron drums, one mounted on top of the other. It

has two openings: one for feeding the reactor and the other for collecting the char after it

has been produced.

The reactor has insulating materials that prevent it from loosing heat to the environment.

They are:

� Fibre glass: Attached to the inner lining of the iron drum is coarse fibre glass. It has

the ability to withstand heat to about 1200oC. The outer part of the iron drum is

insulated with fine fibre glass.

� Refractory mortar: This layer comes after the fibre glass. Its properties allow it to

withstand temperatures of about 1400oC

The iron drum is coated outwardly with aluminium sheets. The lower portion of the reactor

has grates or mesh that prevents the feed from entering the hearth. The hearth is the section

at the bottom part of reactor, where the char is collected after pyrolysis and also where the

fire is lit. Attached to the reactor, is a reflux pipe that returns the product gases to the reactor

during pyrolysis. These gases help to sustain the pyrolysis process.

81

Fig3-4 Biochar Reactor

82

Calibration of Thermocouple

• A basic calibration process involves heating water to 30 degrees C in a thermo bath.

• The thermocouple was turned on and each of the two multimeter leads are attached

to one end of the thermocouple—at this point, the multimeter registed one

microvolt.

• One junction of the thermocouple was then placed into the thermo bath. The voltage

was then recorded once the multimeter reading becomes stable.

• The water temperature was increased to 35 degrees C, and the voltage was recorded

again. This process was repeated by increasing the temperature by five degree

increments and the voltage recorded, until 60 degrees C was reached.

• After all the measurements were taken, the voltage for the thermocouple type at the

room’s temperature was determined. The given figure was then added to each of the

recorded voltage values gathered previously. A curve-fitting method was used to fit

a line to the recorded data—the slope of the line became the voltage increase per

each degree of temperature increase.

3.2.1 HEATING OF REACTOR

A mixture of dry woods and wood shavings were first fed to the reactor and then lit. Two

openings (hearth and chimney) of the reactor were also opened to facilitate the complete

burning of the wood into ashes. While the burning took place, the temperature was

periodically checked along the reactor using a thermocouple, to know whether the desired

83

temperature had been obtained. The heating was allowed to take place until no smoke was

observed flowing out of the chimney. At this point the heating material is completely burnt.

Fig 3-5 Heating of Reactor

3.2.2 BIOMASS WEIGHING

Equipment/Apparatus:

Shovel, Sack, Mass balance,

84

Procedure:

i. The saw dust / wood shavings were collected with a shovel and bagged.

ii. The sack containing the saw dust and wood shavings was then hanged on a mass

balance and its mass recorded.

iii. The bag of saw dust / wood shavings was then sent to the reactor .

Fig 3-6 Weighing of saw dust

3.2.3 CHARGING OF THE REACTOR

After the wood had been burnt completely and the desired temperature reached, the lower

opening of the reactor was closed. The feed was then fed through the upper opening, and the

opening tightly closed for the charring process to begin. The temperature within the reactor

was periodically checked to monitor the process.

85

Fig 3-7 feeding of reactor

86

Fig 3-8 Taking of Temperature after Feeding Reactor.

3.2.4 DISCHARGING OF BIOCHAR

Apparatus/Equipments

Metallic pan, trowel.

87

Procedure

i. With the feeder and the chimney closed, the hearth was opened.

ii. The grate was pushed down for the biochar to fall into the hearth.

iii. The trowel was used to collect the biochar into a pan.

iv. Water was then sprinkled on the biochar to prevent combustion.

v. The wet biochar was then spread on a tarpaulin and dried.

Fig 3-9 Discharged Biochar in a Wheel Barrow

88

3.2.5 WEIGHING OF DRIED BIOCHAR

The dried biochar was then collected and weighed. The yield was determined by the

following relation:

Biochar Yield=��

�� ∗ 100% .............................7

89

CHAPTER FOUR

4.0 RESULTS

4.1 THE pH AND MOISTURE CONTENT OF THE RAW MATERIAL S USED.

pH and moisture content analysis of raw material

Mass of raw material= 0.2g

Volume of distilled water used= 12ml

Table 4.1 pH and Moisture Content Of Various Raw Materials

Raw Material pH Temperature ,o C Moisture

Content,%

Wood shavings B1 6.6 28.8



Average Wood shavings B 6.6

Wawa Stain A1 7.59 26.7 5.34

Wawa Stain A2 7.43 26.6 5.34

Average Wawa Stain A 7.51

Wawa Stain B 7.8 26.8 5.84

Wawa Stain C1 10.83 28.7 10.34

Wawa Stain C2 10.83 27.9 10.34

Wawa Stain C3 10.81 29.8 10.34

90

Average Wawa Stain C 10.82

Odum A1 7.68 26.6 7.74

Odum A2 7.74 26.8 7.74

Average Odum A 7.71

Odum C1 7.59 26.7 6.81

Odum C2 7.56 26.5 6.81

Average Odum B 7.58

4.2 THE PH AND YIELD OF THE CHARRED MATERIAL

Table 4.2 pH and Yield of Biochar

Mass of Raw

Material Used Char pH Temperature, o C Yield, %

-

Wood shavings A1 8.17 26.9

- Wood shavings A2 8.15 27

Average Wood shavings

A 8.16

22kg


29.9



Average Wood shavings 7.92

91

B

Wawa Stain A1 8.23 27.1 insignificant

Wawa Stain A2 8.19 27.3

Average Wawa Stain A 8.21

14kg

Wawa Stain C1 10.83 28.7

1.42



Average Wawa Stain C 10.83

14kg

Wawa Stain C1* 10.76 28.1

1.42



Average Wawa Stain C* 10.74

36kg

Odum A1* 8.34 26.4

Not properly

charred

Odum A2* 8.48 26.9

Average Odum A 8.41

15kg

Odum B1 8.73 27.2

15.78 Odum B2 8.75 27.5

Average Odum B 8.74

15kg

Odum C1 8.57 27.1 16.67

Odum C2 8.53 27.1

Average Odum B 8.55

92

Where C* has more ash

pH of bio oil from wood shavings =5.00 at 28.4 o C

Odum A*, 39 kg of the material was used

wood shavings B was 94% charred

Table 4.3 Summary of feed and yield of various raw materials

Raw Material/Feed Quantity Fed, kg Mass of Biochar

Obtained, kg Yield,%

Wawa stain 14 Ash(56.741g)+

Char(142.226) 1.42

Odum A 36 Not properly

charred -

Odum B 15 2.367 15.78

Odum C 15 2.5 16.67

Wood Shavings A - - -

Wood Shaving B 22 7 29.9

Wood Shavings C 24 6.5 27.08

Wood Shavings D 23 6.0 26.09

Rice husk A 10 4.2 42

Rice husk B 10 4.0 40

93

Rice husk C 130 59 45.38

NB: A, B, C and D are different samples of the raw material in question

4.3 Temperature Measurement

4th February, 2011, Time of firing -12:15 PM 15 kg of wawa

Table 4.4 temperature reading during pyrolysis of wawa sawdust

Time 13:00 13:03 13:05 13:30 13:35 13:40

Level Temperature,

o C

Temperature,

o C

Temperature,

o C Ts, o C

T1 721.5 769.2 779.8 807.2 813.6 816.3 84.9

T2 385.8 562.7 607.2 741.2 802.8 814.5 72.8

T3 341.3 359.3 361.8 513.6 562.5 571.5 49.4

94

9th February, 2011 22 kg of wood shavings

Table 4.5 Temperature Reading During Pyrolysis of Wood Shavings

Time

11:50 12:00 12:10 12:48 12:58 11/02/

12:15

Level

Ti, oC Ts, oC Ts, oC Ti, oC (after feeding) Ts ,oC (after

feeding) Ti, oC

T1 1161.7 77.2 126.3 556.5 101.6 93.6

T2 968.3 64.5 87.3 80.3 79.9 827.4

T3 881.3 50.3 83.6 201.3 89.9 233.7

T4 790.9 53.2 72.3 82.7 70.2 98.1

T5 880.6 55.1 97.1 144.8 85.5 92.4

T6 670.2 52.6 57.2 145.7 68.1 86.3

Ts - temperature at the surface of the reactor

Ti- temperature inside the reactor

T1, T2, T3, T4, T5, T6 are temperatures at levels 1, 2, 3, 4, 5, 6 respectively as shown in the

diagram

95

4.6 A 48 Hour Temperature monitoring of 24kg Wood shavings(C)

Time T1 T2 T3 T4 T5 T6

14:35 517.2 620.4 708.9 516.9 591.8 420.4

Feeding at 15:00

15:00 549.5 82.3 128.4 85.3 105.7 83.4

16:00 265.4 213.4 128.3100.7 100.7 97.6 124.5

17:00 310.0 92.1 169.7 148.8 115.2 139.2

18:00 255.7 115.7 317.4 165.1 192.6 115.1

19:00 261.4 118.6 317.7 168.5 185.3 140.5

20:00 262.7 140.4 283.6 166.3 206.9 139.2

21:00 243.5 140.1 318.3 171.3 208.6 137.4

22:00 224.9 147.9 350.6 172.6 213.1 144.6

23:00 188.3 227.8 355.2 212.2 205.4 138.8

00:00 190.9 310.7 402.2 224.1 185.3 134.5

01:00 198.6 319.2 209.0 205.0 174.0 124.3

02:00 138.9 443.5 412.5 180.9 166.4 119.44

96

03:00 153.8 431.4 405.1 265.9 153.6 110.7

04:00 154.7 422.4 387.4 219.8 152.9 108.6

05:00 157.6 408.2 369.4 244.4 149.2 111.2

06:00 154.3 407.4 349.6 167.5 135.4 98.6

07:00 163.8 417.7 351.1 158.5 125.8 93.2

08:00 152.8 357.7 249.7 212.2 92.4 123.9

09:00 150.6 358.4 251.5 183.3 92.5 120.1

10:00 146.8 368.5 256.8 174.1 88.4 113.8

11:00 150.7 318.9 249.5 166.7 87.9 110.2

12:00 144.4 371.3 254.7 153.8 80.0 105.3

13:00 161.4 316.7 321.5 151.3 84.4 104.9

14:00 140.8 324.0 254.2 141.3 88.1 110.8

15:00 148.8 322.9 253.8 140.5 87.6 119.4

16:00 185.8 265.4 250.3 144.0 90.6 112.3

17:00 127.6 344.8 230.1 186.5 86.4 108.9

18:00 133.3 402.8 336.6 99.0 107.3 79.4

97

19:00 132.4 406.8 336.1 95.6 106.4 78.2

20:00 136.4 408.6 331.2 90.4 103.3 77.2

21:00 139.3 410.4 332.0 92.9 103.3 76.4

22:00 134.4 411.4 332.3 91.2 102.7 75.6

23:00 139.9 410.3 330.8 87.3 100.3 74.9

00:00 139.1 374.1 225.3 114.9 73.1 94.7

01:00 122.9 398.5 243.5 115.7 71.4 93.1

02:00 128.3 373.0 238.5 107.8 65.4 87.1

03:00 123.2 377.5 241.7 112.5 65.6 89.5

04:00 125.4 381.2 258.7 112.3 64.3 86.2

05:00 132.1 362.1 221.1 104.5 55.1 74.2

06:00 127.4 389.9 239.4 111.2 68.1 88.2

07:00 128.0 413.4 251.6 112.9 65.2 82.6

08:00 107.4 427.0 364.7 92.9 109.2 80.4

09:00 120.5 423.9 377.5 96.1 114.6 85.2

10:00 117.4 382.5 338.8 96.8 117.4 89.4

98

11:00 110.1 414.5 355.5 98.1 120.2 90.9

12:00 95.6 433.5 374.5 96.7 211.7 92.2

13:00 101.2 433.2 380.2 98.7 123.3 94.3

14:00 101.7 407.3 386.8 99.9 125.1 96.9

15:00 98.1 379.1 365.3 100.7 125.9 96.1

99

Figure 4-1 A diagram showing the various levels at which temperature is measured.

100

Fig 4-2 A cross section of the reactor shown above.

4.4 BIOCHAR ANALYSIS

Table 4.7 Chemical Analysis on Some Biochar Types

BIOCHAR pH(1:5) ppm P ppm K Ca Mg K Na

Rice Husk

218.45 1945.09 19.49 6.68 6.67 2.68

Wood shavings 5.45 77.33 1570.17 19.49 4.54 4.31 2.68

Wood shavings D 7.35 164.24 1851.36 17.09 5.61 7.02 2.76

Odum sawdust 10.15 1.91 2038.82 40.58 13.62 11.16 6.28

Wawa sawdust 11.35 2.55 2202.7 24.3 52.07 61.41 6.7

NB: K, P, Mg, Ca and Na are some of the plant nutrients that can be found in biochar.

101

CHAPTER FIVE

5.0 DISCUSSION

5.1 RAW MATERIALS AND CHAR OBTAINED

ODUM A (36kg)

There was very little ash produced little char and lots of uncharred saw dust. When the

uncharred saw dust was removed from the reactor and exposed to the atmosphere, it began

to burn. This could be attributed to the fact that the was the presence of oxygen in the

surrounding atmosphere.

Problems

1. Initial temperature was too low, therefore charring was incomplete.

2. Mass of feed was too much

3. Inability to measure the temperature due to the absence of drilled holes on the sides

of the reactor.

4. Inability of heat to go through the feed due to small pore spaces of saw dust.

5. Feeding the reactor was difficult because the reactor was very hot and part of the

feed was lost during the process.

102

Fig 5-1 Odum sawdust (A) Biochar

ODUM B (15kg Odum)

There was a lot of ash and a considerable amount of charred saw dust.

Problems

1. The large amount of ash produced compared to the char gotten could be attributed to

the large amount of oxygen that was present in the reactor. This oxygen may have

entered through the drilled holes on the side of the reactor which were not blocked

during the pyrolysis process.

2. High temperature of the reactor burnt the first sample of saw dust that reached it

before complete feeding was done.

103

3. The volume occupied by the saw dust compared to the reactor volume was small.

Hence there was accumulation of oxygen at the unoccupied volume of the reactor.

Fig 5-2 Odum sawdust Biochar

WAWA STAIN SAWDUST

For 14kg of wawa stain sawdust used, 56.741g of ash and 142.226g of biochar. This gave a

yield of 1.42%. The ash produced may have resulted from the oxygen that may have entered

the reactor. It may also have resulted from the combustion of the sawdust at the time of

feeding the reactor. The feeding of the reactor with the sawdust took some considerable

amount of time. This was because of the large amount of smoke and heat that came out of

the reactor through the feeder.

104

The delay in the feeding process resulted in the accumulation of large amount of oxygen in

the reactor. This caused the combustion of the sawdust into ashes.

Also, from the yield obtained from the wawa sawdust compared to the odum sawdust, it can

be said that the wawa sawdust gave a less amount of char. Hence, the wawa sawdust should

not be recommended for mass production of biochar. The wawa sawdust can thus be said to

be a non–economical biochar feed stock.

Fig 5-3 Wawa stain biochar.

Wood Shavings (B and C)

The wood shavings used were generally composed of a mixture of Red woods (African

mahogany and Sapelewood), Wawa and Teak. It took about three days to char because

moderates temperatures of about 350 Deg. Celsius were recorded during the pyrolysis

process. Very little oil was seen seeping from the sides of the reactor (drilled holes provided

105

for temperature readings). Due to this, collection of oil was difficult. After the char was

removed, it had about 98% charred material and about 2% partially charred material. The

char also had some charcoal in it due to the wood used to heat up the reactor. Using 22kg of

wood shavings (B) and 24kg of wood shavings (C) a yield of 29.9% and 27.08% was

obtained respectively. Since 22kg wood shaving (B) gave a yield of 29.9%, it was expected

that the 24kg wood shavings would have given about the same percentage yield (7.2 kg) of

biochar, but that was not the case. The follow reasons may have accounted for the reduction

in yield:

• Variation in mass of the various wood types.

• High temperature: The high temperature in the reactor made feeding difficult.

During feeding, the first batch of feed that was allowed into the reactor began to

char even before feeding was complete. The wood shavings which were charred

before the feeding was complete were burnt into ashes due to the large amount of

oxygen that was allowed into the reactor during feeding. The large amount of

oxygen was as a result of the time used during feeding.

Thus, it took a long time feeding the reactor with the 24kg wood shavings compared to the

22kg wood shavings.

106

Fig 5-4 Biochar from wood shavings

Wood Shavings D

It is generally composed of Asanfra (Asanfena), Wawa and Teak and with this it took about

two days to char. A lot of oil was observed seeping from the holes. Some oil was also

observed dripping from the vent cap and the flange (some oil was actually collected from

the flange about 10ml). The char obtained was oily. This is because Asanfra is noted to be

an oil bearing wood (Dr. E. Yeboah, pers. comm. 25th March 2011). Using 23kg of the

wood shavings a yield of 26.09% was obtained.

107

Rice Husk

A yield of 42.0% char was obtained from 10kg of rice husk. The charring process took two

days and more oil was obtained.

In comparison with that obtained for saw dust and wood shavings the rice husk produced a

higher yield of char. This is partly due to the fact that it takes less time to feed 10kg of rice

husk than 10kg of wood shavings. The longer the time taken to feed the reactor, the more air

enters and subsequently, more ash is produced, which reduces the yield. In addition, the rice

husks have smaller air space between them than the wood shavings, therefore do not allow

more oxygen to accumulate within its pore spaces, hence this enhances pyrolysis and reduce

combustion.

Rice husk is noted as a good insulating material for building . This explains why less ash is

obtained even at higher temperatures compared to wood shavings.

108

Fig 5-6 Biochar from rice husk

5.2 Temperature Distribution

Temperature distribution within the reactor is uneven. Tables 4-4 and 4-5 depict this

observation.

A lot of fuel (e.g. firewood, palm kennel shell and husks, etc) is required to obtain higher

temperatures. Therefore the mass of the fuel used is proportional to the heat produced.

It was observed that the temperature within the reactor drops drastically during charging

(feeding of reactor) and rises gradually with time after charging.

109

During charring, it is observed that the uppermost portion of the reactor (Fig.4-1), T6 is

hotter than the immediate lower portion, T5. This is due to the accumulation of hot dense

gases at that point within the reactor.

During a 48 hour observation (appendix E) on the changes of temperature within the

reactor during the process, the following observations were made:

1. Temperature drops significantly during the night. This is a result of the high rate of

heat loss to the surrounding cold air as a result of the recycle tube (pipe) not lagged.

2. Rain also has a heat loss effect on the process.

3. Temperatures within and outside the reactor rises as the surrounding atmosphere

increases in temperature due to reduced or no heat loss from the reactor.

5.3 BIOCHAR ANALYSIS (CHEMICAL PROPERTIES)

The pH of the biochar has an influence on the availability of plant nutrients (major and

minor). Major nutrients tend to be less available in soils with low pH (acidic) and less

available in soils with high pH. From the analysis (appendix C), wawa sawdust was found

to have a higher pH (basic) and thus can be inferred to provide more macronutrients when

applied to the soil. Also, the high pH value obtained suggests that there was greater ash

content compared to the other biochar samples. Major nutrients such as K, P, Ca and Na are

needed in larger quantities by plants for proper growth and development. From table 4.7, it

can be deduced that wawa stain biochar contains more macronutrients compared to the rice

husk, odum saw dust and wood shavings biochar. The major nutrients that are displayed in

110

table 4.7 are found to be water soluble. They dissolved in soil water and made available to

plants for easy absorption and use. The pHs of most of the biochar was found to be basic.

This suggests that they can be applied to soils which are more acidic to neutralize them.

Biochars from wawa saw dust and odum saw dust can thus be best applied to acidic soils.

111

CHAPTER SIX

6.1 CONCLUSION

Agricultural waste such as rice husk, wood shavings and sawdust, which are usually, burnt

in huge heaps on farmlands and dumpsites cause the destruction of soil nutrients and micro

organisms. On the other hand, if these materials are left to decay, they decompose into

CO2and CH4, which promote global warming. However, through biochar technology, these

materials are made useful.

As a product from pyrolysis of such materials, it retains most of it carbon contents, as well

as contains good amounts of most major and minor soil nutrients (appendix A) to support

plant growth. Thus, may be used as a complete fertilizer or to enrich soil fertilizers. This

will go a long way to reduce the cost incurred by the government in importing fertilizers.

Also, the pH of biochar is influenced largely by the feedstock type. The use and application

of biochar should be guided by the purposeful selection of feedstock. Temperature gradients

exist within the biochar reactor being highest at the source of heat and declining further

away from the heat source. The existence of temperature gradient influences uniform

charring of the feedstock. Thus the efficiency of the charring process is influenced by the

quantity of feedstock in the reactor, moisture content and the residence time of the feedstock

in the reactor.

112

6.2 RECOMMENDATIONS

� An alternative method of feeding the reactor should be provided. A sliding spout

feeder may be employed.

� Protective clothing such as goggles, nose masks and heat gloves should be

provided.

� The reflux tube should be properly lagged to reduce heat loss.

113

6.3 REFERENCES

1. Wikipedia (2010): Saw Dust. [Online].aadet.com.Available from:

http://www.aadet.com/article/sawdust .[Accessed 2010 November 5]

2. Parliamentary library (2009): the basics of biochar [online]. Parliament of

Australia. Available from: <http://www.aph.gov.au/privacy.htm>[Accessed

2010 October 4]

3. Conversion and Resource Evaluation Ltd: Biochar - The Processes and

Benefits [online]. Environmental Improvement Solutions. Available from:

http://www.enviro-news.com/article/ > [Accessed 2010 October 9]

4. Samy Sadaka, S.S. (2007). Pyrolysis [online]. Sun Grant Initiative and the

University of Tennesee. Available from :<

http://bioweb.sungrant.org/General/Biopower/Technologies/Pyrolysis>[Acce

sses2010 September 29.Last modified 2008 September 9].

5. Samy Sadaka (edited by Marie Walsh), S.S (2007) Pyrolysis [online]. Sun

Grant Initiative and the University of Tennesee. Available from:

<http://bioweb.sungrant.org/Technical/Biopower/Technologies/Pyrolysis/>

[Accessed 2010 October 9.] Last modified 2008 November 15]

6. Wikipedia: Thermocouples [online]. Answers Corporation. Available from :<

http://www.answers.com/library/Wikipedia-cid-83243> [Accessed 2010

October 9]

7. Control and Instrumentation.com: Thermocouple types [online].Available

from :<http://www.controlandinstrumentation.com/resources/thermocouple-

types.html>. [Accessed 2010 October 8]

114

8. Efunda (2010) Thermocouples Theory [online]. Available from :<

http://www.efunda.com/designstandards/sensors/thermocouples/thmcple_intr

o.cfm.htm> [Accessed 2010 October 9]

9. MATBASE(2009):Wood[online].Available from:

<http://www.matbase.com/material/wood/> [Accessed 2010 October 13]

10. Sci-Tech Encyclopedia (2010): Wood properties. [online]. Answers

Corporation. Available from :< http://www.answers.com/topic/wood-

properties> [Accessed 2010 October 13]

11. Resources, Tools and Basic Information for Engineering and Design of

Technical Applications: Thermocouples [online].The Engineering Toolbox.

Available from: < http://www.engineeringtoolbox.com/thermocouples-

d_496.html> [Accessed 2010 October 15]

12. PBWORKS: Welcome to a Gardening with Biochar FAQ! [online].Available

from :< http://biochar.pbworks.com/w/page/FrontPage> [Accessed 2010

October 18]

13. New Energy and Fuel (2010): Torrefaction – A New Process In Biomass and

Biofuels[online].WordPress.Available from:<http://newenergy and

fuel.com/http:/newenergyandfuel/com/2008/11/19/torrefaction---a-new-

process-in-biomass-and-biofuels/>[Accessed 2010 November 4]

14. Smith, S.E (2010): What Is Refractory Brick? [online].wiseGEEK. Available

from :< http://www.wisegeek.com/what-is-refractory-brick.htm>. [Accessed

2010 November 10].

115

15. Wikipedia (2010).Fibreglass usage and properties.[online].Madabout

kitcars.com. Available from:

<http://www.madaboutkitcars.com/article/fibreglass/fiberglass/plastic/grp/sil

ica>. [Accessed 2010 November 10]

16. Samy Sadaka and A.A Boateng (2009): Pyrolysis and Bio-Oil - FSA1052.

[online]. University of Arkansas, United States Department of Agriculture

and County Governments Cooperating. Available from :<

http://www.uaex.edu> [Accessed 2010 November 8]

17. Gefran (2006) Thermal technologies [online] Available from

<http://www.gefran.com/en/tecnologies/tecnology.html> [Accessed 2010

October 20]

18. The engineering toolbox (2009) Resources, Tools and Basic Information for

Engineering and Design of Technical Applications [online] Available from

<http://www.engineeringtoolbox.com/thermocouples-d_496.html>[Accessed

2010 October17]

19. Omega engineering technical reference (2010) Thermocouple [online]

Available from<http:// www.omega.com/thermocouple.html> [Accessed

2010 October 20]

20. Spiritus-temporis (2005) Thermocouple [online] Available from

<http://www.spiritus-temporis.com/principle-of-operation.html>[Accessed

2010 November 4]

116

21. Connecting industry ThomasNet(2009) Instruments and control-calibrating

thermocouples. [online] Available from <http:// www.thomasnet

.com/articles/calibrating thermocouples.> [Accessed 2010 November 2]

22. B.A. Lerch, M.V. Nathal and D.J. Keller (2002).Thermocouple Calibration

and Accuracy in a Materials Testing Laboratory. National Aeronautics and

Space Administration Glenn Research Center

23. Brownsort, P.A (2009). Biomass Pyrolysis Process: Review of scope, control

and variability. UK Biochar Research Centre. (UKBRC).

24. Van de Weerdhof, M.W (2004) Modeling the pyrolysis process of biomass

particles PhD Thesis Eindhoven University of Technology

25. Johannes,L and Stephen, J (2009), Biochar For Environmental Management,

Earth scan publications, UK

26. Steiner, Christoph, Wenceslau G. Teixeira, Lehmann. J, Nehls T, Jeferson L,

Vasconcelos de Macêdo, Winfried E. H. Blum, Wolfgang Z, Long term

effects of manure, charcoal and mineral fertilization on crop production and

fertility on a highly weathered Central Amazonian upland soil, pp. 275 -291.

117

6.3 APPENDICES

Appendix A: Some Plants Nutrients

Major Nutrients Symbols

Nitrogen N

Phosphorus P

Potassium K

Calcium C

Magnesium Mg

Sulfur S

Sodium Na

Minor Nutrients Symbols

Boron B

Copper Cu

Iron Fe

Chlorine Cl

Manganese

Molybdenum

Zinc

Appendix B: Biochar type versus Yield

0

5

10

15

20

25

30

35

40

45

50

29.9

118

Mn

Mo

Zn

Appendix B: Biochar type versus Yield

29.927.08 26.09

4240

45.38

15.78

16.67

1.420

Appendix C: pH of some Biochar samples

Appendix D: Composition

a. Amount of Sodium (g) in some Biochar samples

0

2

4

6

8

10

12

Rice Husks

6.25

0

1

2

3

4

5

6

7

Rice Husks

shavings

2.68

119

Appendix C: pH of some Biochar samples

Appendix D: Composition of some major nutrients in Biochar samples

Amount of Sodium (g) in some Biochar samples

Wood

shavings

Wood

shavings D

Odum

sawdust

Wawa

sawdust

5.45

7.35

10.15

11.35

Wood

shavings

Wood

shavings D

Odum

sawdust

Wawa

sawdust

2.682.76

6.28

6.7

of some major nutrients in Biochar samples

b. Amount of Potassium (ppm) in some Biochar samples

c. Amount of Calcium (g) in some Biochar samples

0

500

1000

1500

2000

2500

Rice Husks

1945.09

0

5

10

15

20

25

30

35

40

45

Rice Husks

19.49

120

Amount of Potassium (ppm) in some Biochar samples

Amount of Calcium (g) in some Biochar samples

Wood

shavings

Wood

shavings D

Odum

sawdust

Wawa

sawdust

1570.17

1851.36

2038.822202.7

Wood

shavings

Wood

shavings D

Odum

sawdust

Wawa

sawdust

19.4917.09

40.58

24.3

d. Amount of Magnesium (g) in some Biochar samples

e. Amount of Phosphorus (ppm) in some Biochar samples

0

10

20

30

40

50

60

Rice Husks

6.68

0

50

100

150

200

250

Rice Husks

218.45

121

Amount of Magnesium (g) in some Biochar samples

Phosphorus (ppm) in some Biochar samples

Wood

shavings

Wood

shavings D

Odum

sawdust

Wawa

sawdust

4.54 5.61

13.62

52.07

Wood

shavings

Wood

shavings D

Odum

sawdust

Wawa

sawdust

77.33

164.24

1.91 2.55

f. Amount of Potassium (g) in some Biochar samples

0

10

20

30

40

50

60

70

Rice Husks

6.67

122

Amount of Potassium (g) in some Biochar samples

Wood

shavings

Wood

shavings D

Odum

sawdust

Wawa

sawdust

4.317.02

11.16

61.41

Appendix E: Display of temperature distribution in the biochar reactor over a 48 hour

period.

123

: Display of temperature distribution in the biochar reactor over a 48 hour : Display of temperature distribution in the biochar reactor over a 48 hour

124

Utilization of Agricultural Residue with a newly designed Biochar reactor

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