i Valorization of Biomass Waste to Produce Bioethanol by Rose Amira Binti Karim Dissertation submitted in partial fulfilment of the requirements for the Bachelor of Engineering (Hons) (Chemical Engineering) MAY 2013 Universiti Teknologi PETRONAS Bandar Seri Iskandar 31750 Tronoh Perak Darul Ridzuan
100
Embed
Valorization of Biomass Waste to Produce Bioethanolutpedia.utp.edu.my/8414/1/Dissertation FYP -Rose Amira...supervisor, Dr Asna Md Zain. The supervision and support that she gave truly
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
i
Valorization of Biomass Waste to Produce Bioethanol
by
Rose Amira Binti Karim
Dissertation submitted in partial fulfilment of
the requirements for the
Bachelor of Engineering (Hons)
(Chemical Engineering)
MAY 2013
Universiti Teknologi PETRONAS
Bandar Seri Iskandar
31750 Tronoh
Perak Darul Ridzuan
ii
CERTIFICATION OF APPROVAL
Valorization Of Biomass Waste To Produce Bioethanol
by
Rose Amira Binti Karim
A project dissertation submitted to the
Chemical Engineering Programme
Universiti Teknologi PETRONAS
in partial fulfilment of the requirement for the
BACHELOR OF ENGINEERING (Hons)
(CHEMICAL ENGINEERING)
Approved by,
_____________________
(Dr Asna Binti Md Zain)
UNIVERSITI TEKNOLOGI PETRONAS
TRONOH, PERAK
May 2013
iii
CERTIFICATION OF ORIGINALITY
This is to certify that I am responsible for the work submitted in this project, that the
original work is my own except as specified in the references and acknowledgements,
and that the original work contained herein have not been undertaken or done by
unspecified sources or persons.
___________________________________________
ROSE AMIRA BINTI KARIM
iv
ACKNOWLEDGEMENT
In the name of Allah, the Most Gracious and the Most Merciful. Praise to Him the
Almighty that in His will and given strength, the final year project is successfully
completed within the allocated eight months period. Upon completing the project
undertake, I owe a great thanks to a great many people for their help and support, as well
as their contribution in time, effort, advice, supervise, discuss and help during the
period.
First and foremost, deepest appreciation to PETRONAS Research Sdn. Bhd (PRSB).
and Process Technology R&D Department for giving the opportunity for me to perform
experimental working for my project at PRSB. Special thanks to my beloved and helpful
supervisor, Dr Asna Md Zain. The supervision and support that she gave truly helped
the progression and smoothness of the internship program. The co-operation is much
indeed appreciated. Thank you madam!
My grateful appreciation also goes to Head of Process Technology Department, Dr
Mahpuzah Binti Abai who always gives me advice and support throughout the program.
Not to forget, a deep sense of gratitude to my former internship supervisor, Dr Azlan
Shah Bin Haji Hussain for his help and willingness in facilitating upon completing the
project.
Last but not least, endless thanks to those who helped me to finish up my project directly
or indirectly for their guidance, sharing and support throughout the entire two semesters.
Their help are much appreciated.
v
ABSTRACT
There are various methods that can be implemented to produce biofuel specifically
bioethanol. The bioethanol can be produced from cellulose and hemicelluloses that may
be originate from various sources of biomass such as Empty Fruit Bunches (EFB),
mesocarp fiber, shell and palm kernel cakes. The studies and research works were
focused on the production of bioethanol from oil palm waste, EFB using bacteria,
Saccharomyces Cerevisiae ATCC 96581 as fermentation aid to expand the usage of oil
palm waste and to enhance the production of bioethanol. The purposes of this study are
to investigate the effect of FPU loading, pH value and temperatures by using celluloses.
Besides, this study also aimed to produce bioethanol from EFB by using Simultaneous
Saccharification and Fermentation (SSF) method. Prior fermentation process, enzymatic
saccharifications of EFB need to be done to investigate the highest amount of
monomeric sugars; glucose and fructose produced from EFB aided by enzyme,
Trichoderma Reesei. Three sets of experiment were performed; in first set, the sample
was hydrolyzed with pretreatment with sodium hydroxide solution then being subjected
to sulfuric acid solution. Pretreatment process is necessary to remove lignin from the
EFB that could hinder the saccharification of EFB to produce sugars and bioethanol. At
the second set of the experiment, the fermentation process which is SSF method was
performed aided with Saccharomyces Cerevisiae under anaerobic conditions. The result
has shown that bioethanol has been produced from the process and the highest amount
of bioethanol produced was 0.42 mg/ml at 58 hours. Final sets of experiment were
performing to examine the effect of mass loading, pH value and also FPU loading of the
celluloses in producing bioethanol. From optimization works of these various
fermentation parameters it was found that the highest ethanol production from cellulose
(Avicel Ph 101) which ranges from 3.1 mg/mL to 4.6 mg/mL can be achieved at pH 4,
217 FPU, 5.0g of cellulose loading with an agitation rate of 100 rpm for 60 hours
incubation.
vi
TABLE OF CONTENTS
ACKNOWLEDGEMENT ........................................................................................................ iv
ABSTRACT ............................................................................................................................. v
TABLE OF CONTENTS ......................................................................................................... vi
LIST OF FIGURES.................................................................................................................viii
LIST OF TABLES ................................................................................................................... xi
Bioethanol is one form of renewable energy source that is fast gaining position as
potential fuel to power automotive engine. Unlike gasoline which is refined through
distilling crude oil, ethanol can be synthesized from a wide variety of biological
materials such as wheat, corn, barley, wood and sugar cane. In fermentation process,
baker’s yeast is used to breakdown starch (carbohydrate) into bioethanol and carbon
dioxide as a by-product. Fuel ethanol contains 10% of ethanol mixed with 90% gasoline
which is commonly known as E10 in United States of America. Because the ethanol is a
high-octane fuel with high oxygen content (35% oxygen by weight), it allows the engine
to complete the combustion of fuel, resulting in fewer emissions and has replaced lead
as an octane enhancer in petrol2.
Bioethanol is an ethanol synthesized from biomass and it is renewable. Therefore
bioethanol has some advantages over petrol as a fuel such as it can help to reduce the
amount of carbon monoxide produced by the vehicle thus improving air quality and
reduce the emission of greenhouse gases to the atmosphere. Other than that, it gives
benefits to Malaysian agriculture such as increase the plantation of the crops such as
palm oil, sugar cane and also provides job vacancies to the farmers. These benefits, in
turn, could serve to stabilize and improve financial stability for farmers, which would
increase the economic well-being of rural and other agriculture-dependent sectors of
Malaysia. In addition, it also can give job opportunities whether directly or indirectly in
all aspects of ethanol production; from farming to transportation and manufacturing.
2Mohammad J. Taherzadeh., Ethanol from Lignocellulose: Physical Effect of Inhibitors. Chalmers University of Technology; 1999.
3
Bioethanol can be produced from the fermentation process of biomass aided by bacteria
to decompose the biomass. There are two key parameters take place on how biomass is
transformed to bioethanol:
1. Enzymatic hydrolysis is a chemical process in which acid is used to convert
starch (complex sugars) into monomeric sugars such as glucose and fructose.
The feedstock must first be hydrolyzed into glucose before proceed with the
fermentation process for bioethanol production3. In the biomass-to-bioethanol
process, acids and enzymes are used to catalyze this reaction.
2. Fermentation is a biological process in which sugars such as glucose, fructose
and sucrose are converted into cellular energy and thereby produce ethanol and
carbon dioxide as waste products. Fermentation reaction occurs in the presence
of yeast or bacteria, which feed on sugars as nutrient. Ethanol and carbon dioxide
are being produced as the sugar is consumed. The simplified fermentation
reaction of 6-carbon sugar is:
C6H12O6 bacteria 2CH3CH2OH + 2CO2
Eq. (1)
(Glucose) (Ethanol) (Carbon dioxide)
The sugar formed in the enzymatic hydrolysis reaction is fermented into bioethanol. The
common microorganisms use in the fermentation process is Saccharomyces cerevisiae,
which is known as ordinary baking yeast. It is the critical element in the fermentation
process that converts sugar into alcohol4. Beside glucose, it also has the ability to
ferment mannose as well since soft wood also contains substantial amounts of mannose.
In this study, empty fruit bunches (EFB) of oil palm has been chosen to be the substrate
for the fermentation process due to its abundance and low cost of processing. Besides, it
also cleans, non-toxic and renewable. EFB is one of the lignocellulosic materials
3 Kamaruddin,H., H.Mohamad, D.Ariffin and S.Johari. An estimated availability of oil palm biomass in Malaysia. PORIM Occ. Paper Palm Oil Res. Inst. Malaysia 37:1997 4 Laundry, C.R.,Townsend, J.P., Hartl, D.L. and Cavalieri, D. Ecological and evolutionary genomics of Saccharomyces cerevisiae. Molecular Ecology. 2006. Volume 15.p.575-591.
4
consists primary of cellulose and hemicellulose component. It was obtained from Sze
Tech Engineering Sdn Bhd located in Padang Jawa, Selangor with fiber length of 0.5-
1.0 inch.
1.2 Problem Statement
Carbon dioxide is one of the major atmospheric contributors to the greenhouse effect.
Greenhouse effect refers to the Earth’s trapping of the sun’s incoming solar radiation,
causing warming of the Earth’s atmosphere. Current analysis suggests that the
combustion of fossil fuels is a major contributor to the increase in the carbon dioxide
concentration, such contributions being 2 to 5 times the effect of deforestation
(Kraushaar & Ristinen). Carbon dioxide and other so-called greenhouse gases allow
solar energy to enter the earth’s atmosphere, but reduce the amount of energy that can
radiate back into space, trapping energy and heat causing to global warming5. Figure 2
shows the concentration of greenhouse gases emitted such as carbon dioxide, methane
and chlorofluorocarbons. It shows that carbon dioxide has the largest concentration
followed by methane gas and chlorofluorocarbons. Hence, the opportunity to reduce
dependence on fossil fuels, while reducing carbon dioxide is of strategic important
today.
5 Kraushaar & Ristinen, Energy and problems of a technical society, 2nd ed (1993)
5
Figure 2: Analysis data for concentration of greenhouse gases emitted to
atmosphere (Valentas et al., 2009)
One of the environmental benefits of replacing fossil fuels with biomass-based fuels is
that the energy obtained from biomass does not contribute to global warming. All
combustion process would produce carbon dioxide as a byproduct, including fuels
produced by biomass. Nonetheless, as because plants use carbon dioxide from the
atmosphere to grow; for photosynthesis process, carbon dioxide released during
combustion is balanced by that absorbed during the annual growth of the plants.
Increase the usage of renewable fuels like ethanol will help to counter the pollution and
global warming effects of burning gasoline. Use of 10% ethanol-blended fuels results in
a 6-10% carbon dioxide reduction and higher level of ethanol can further reduce the net
quantity of carbon dioxide emitted into the atmosphere. Ethanol reduces greenhouse
gases emissions relative to gasoline by between 40% to 62% depending on agricultural
practices and production technologies.6 Thus, more carbon dioxide will be absorbed by
crop growth.
Gasoline and diesel is a liquid mixture distilled from crude oil. They consist of blends of
different hydrocarbon chains. During the process of refining, groups of hydrocarbon
6 Coad.L., Bristow,M.,(2011).Ethanol’s Potential Contribution to Canada’s Transportation Sector. The conference board of Canada.pp: 68
6
chains with similar molecular size are separated based on the difference in their boiling
points. Many of these are toxic and volatile compounds such as benzene, toluene, and
xylenes which are responsible for the health hazards and pollution associated with
combustion of petroleum – based fuels. The largest single contributor to the rise of man-
made greenhouse gases is, of course, the burning of oil and gas to power vehicles,
machinery, and produce energy and warmth. Carbon monoxide, nitrogen oxides, sulfur
oxides and particulates are the main concerns nowadays. A key environmental benefit of
using biofuels as an additive to petroleum-based transportation fuels can give a
reduction in these harmful emissions.
Bioethanol is used as fuel oxygenates to improve combustion characteristic. Ethanol
reduces pollution through the volumetric displacement of gasoline and by adding
oxygen to the combustion process which reduces exhaust emissions to the atmosphere.
Hence, the production of bioethanol from empty fruit bunches can spur economic
growth because it expands the usage of oil palm and also can reduce the cost in
producing fossil fuels.
1.3 Objective of Study
As bioethanol has a huge potential as a substituting agent to gasoline in car fuel and give
further advantages to the environment, this research is carried out in order to produce
bioethanol from empty fruit bunches (EFB) of oil palm.
1.4 Scope of Study
To achieve the objective, there are three scopes that have been identified:
i. To find the Filter Paper Unit (FPU) value of cellulase derived from Trichoderma
reesei
ii. To investigate the effect on the production of bioethanol at different
temperatures, pH value and FPU loading by using cellulose.
7
CHAPTER 2: LITERATURE REVIEW
2.1 Overview
Worldwide, biomass is the fourth largest energy resource after coal, petroleum and
natural gas. Biomass is biological material from living organism, most often referring to
plants or plant-derived materials. Plants use light energy from the sun to convert carbon
dioxide and water to sugars through a photosynthesis process. It remains the largest
biomass energy source today for example dead trees, branches, tree stumps, wood chips
and even municipal solid waste.7
Another type of plant matter, called cellulosic biomass, is made up of very complex
sugar, and it is not generally used for food. Cellulosic biomass consists of three main
components which are lignin, hemicellulose and cellulose. Among these components,
the largest portion is cellulose which covers from 38% to 50% followed by
hemicellulose (23%-32%) and lignin (15%-25%).8
Biomass energy currently contributes 9-13% of the global energy supply accounting for
45±10 EJ per year (Thomas, 2000). Biomass energy includes both traditional uses such
as a ring for cooking and heating and modern uses such as producing electricity and
steam, and liquid bio-fuels. Use of biomass energy in modern ways is estimated at 7 EJ
(exajoule) a year, while the remainder is in traditional uses. Biomass energy is derived
from renewable resources. Ethanol derived from biomass, one of the modern forms of
biomass energy, has the potential to be a sustainable transportation fuel, as well as a fuel
oxygenate that can replace gasoline (Wang, 2000). Shapouri et al. (1995, 2002) reported
that the energy content of ethanol was higher than the energy required producing
ethanol. Kim and Dale (2002) also estimated the total energy requirement for producing
ethanol from corn grain at 560 kJ MJ−1 of ethanol, indicating that ethanol used as a
7Biomass Energy Centre. http://www.biomassenergycentre.org.uk. Retrieved on December 6, 2012 8 Valentas.K.,(2009). Biofuel from Cellulosic Biomass: An Overview of Current Technologies & Economic Feasibility. Biotechnology Institute,University of Minnesota.pp 1-5.
liquid transportation fuel could reduce domestic consumption of fossil fuels, particularly
petroleum. The world ethanol production in 2001 was 31 giga litres (GL) (Berg, 2001).
The major producers of ethanol are Brazil and the United States, which account for
about 62% of world production.
As a renewable energy source, biomass can either be used directly or indirectly to
convert into another type of energy product such as biofuel. The estimated biomass
production in the world is 146 billion tons a year.9 Furthermore, Malaysia is one of the
largest producers of palm oil in the region and among the biggest income earners to the
country for many years. With the rapid growth of palm oil production in Malaysia, the
amount of biomass residues generated also has shown a corresponding increase.
In 2010, the oil palm planted area in the country is 4.8 million hectares. In 2011,
Malaysian oil palm accounted for just 1.97% which is about five million hectares of the
total 253.9 million hectares. It makes up to71% of agriculture land or 14.3% of total
land area.10
The overall average of 18.03 tones Fresh Fruit Bunches (FFB) per hectare
of palm oil plantation has been produced from the oil palm industry (Choo, 2011).
Based on this figure, palm oil plantation areas has produced more than 66.63 million
tonnes of biomass residues such as Empty Fruit Bunches (EFB), mesocarp fiber, shell,
palm kernel cakes, trunks and Palm Oil Mill Effluent (POME) in 2010 (Goh et al.,
2009). The EFB represent about 9% of this total. They are the residue left after the fruit
bunches has been processed to extract oil at oil mills.
In a country that has significant amount of agricultural activities, biomass can be a very
promising alternative source of renewable energy. With increased awareness on
reducing greenhouse gas emissions, conversion of biomass residues into renewable such
as ethanol, biogas, syngas and bio hydrogen has attracted global responsiveness. The
conversion of biomass to this functional compound involve two reaction processes
9 Schenk,Justin;et al.(2012).Wood Fired Plants Generate Violations. Wall Street Journal. Retrieved on December 6,2012. 10 Palm Oil Facts and Figures 2011. http://www.simedarbyplantation.com/Palm-Oil.pdf
9
which are biochemical which involve chemicals or enzymes and fermentation and also
thermochemical processes; gasification to syngas and pyrolysis.
2.2 Bioethanol as a Fuel
Bioethanol is one form of renewable energy source that is fast gaining foothold as
potential fuel to power automotive engine. In comparing to gasoline which is refined
through distilling crude fossil fuel, bioethanol can be synthesized from the starchy parts
of natural plants. Nowadays, ethanol is one of the most widely used biofuel today. Fuel
ethanol has been called ‘gasohol’; the most common blends contain 10% ethanol mixed
with 90% gasoline. It also can be used in a mixture with gasoline (3-22% ethanol) with
no modification of the engine (Taherzadeh, 1999). Because the ethanol is a high-octane
fuel with high oxygen content (35% oxygen by weight), it allows the engine to combust
the fuel completely, resulting in fewer emissions. Since ethanol is produced from plants
that harness the energy from the sun, ethanol is also considered as a renewable fuel.
Therefore, ethanol has many advantages as an automotive fuel.11
Although ethanol production from corn and sugar bagasse can still expand greatly, its
primary used mainly for animal feed, food domestics and beverage industries. Besides,
the feedstock may not always be in surplus. Making ethanol from cellulose and
hemicellulose dramatically expands different types and amount of available feedstock.
This includes many materials now regarded as wastes requiring disposing, as well as
corn stalks and wood chips.
Brazil is the world frontrunner in the use of ethanol as an automobile fuel. More than 11
billion litres of ethanol for fuel are produced from sugar cane bagasse each year. About
15% of the vehicles with spark ignition engines (the type normally fueled by gasoline)
run on ethanol and the rest use a blend of 20% ethanol in gasoline. Ethanol was
introduced to reduce Brazil’s dependence on expensive foreign oil, and provides an
11http://www.comalc.com/fuel_ethanol.htm
10
additional market for domestic sugar producers. Beneficial effects on air quality have
been an added bonus to the country.
The Clean Air Act Amendments of 1990 authorized the sale of oxygenated fuels in areas
of the country with unhealthy levels of carbon monoxide. Since that time, there has been
strong demand for ethanol as an oxygenate that blended with gasoline. In United States,
ethanol blends make up about 12% of the total gasoline market. In some parts of
America, there are projects handled to test the viability of replacing diesel fuel with
ethanol. Support for fuel ethanol is a key factor in the current U.S. because of its
beneficial effect on air quality. Oxygenated fuel such as ethanol blends, mandated in
certain regions to reduce carbon monoxide emissions or ozone.12
Today, there are more
than 55 domestic fuel ethanol production facilities located in 22 states across the country
with annual capacity of approximately 1.8 billion gallons.
Ethanol has an octane number of 113 compared with 107 for methanol and 86 to 94 for
gasoline, allowing a higher compression ratio in the gasoline engine. Furthermore, it
also can be used in reformulated gasoline13
. The blending octane value of ethanol can
actually be much higher than that neat of ethanol, and the blending octane value
increases with lower octane-base gasoline. Therefore, ethanol is an excellent additive to
prevent engine knock and improve the performance of the engine.
Although bioethanol fuel gives many advantages, but there are also disadvantages
contribute from it. One of the most evident disadvantages of ethanol is that the majority
of cars in Malaysia are designed to run on petrol. Petrol consists of over one thousand
chemical compounds which are mostly petroleum based. Petrol fuels require an
extensive range of operating conditions. This includes climate, altitude and driving
patterns. This means that the properties of petrol must be balanced to give a satisfactory
performance over a range of different driving conditions. This is detrimental in
12 http://www.comalc.com/fuel_ethanol.htm 13 Eva-Lena Jakobsson (2002). Optimization of the pretreatment of wheat straw for production of bioethanol.Ph.D. Thesis. Department of Chemical, Lund University.
11
ascertaining the right amount of ethanol that goes in to petrol as it can adversely affect
the balance and performance of a vehicle.
Besides, ethanol has lower energy content and suitable for cleansing usage. It cannot be
used in two strokes engines as it will clean up the lubricating oil off the cylinder walls
and this may leads to overheating. Recent study in United States from Cornell
University has shown that 71% more energy is required to produce a litre of ethanol than
the energy contained in a litre of ethanol.
2.3 Ethanol Production
2.3.1 Empty fruit bunch (EFB)
Empty Fruit Bunch is composed of 45-50% cellulose and about equal amounts (25-35%)
of hemicellulose and lignin (Deraman, 1993). Due to oil palm empty fruit bunch is
available in large quantities and contain high amount of cellulose, so empty fruit bunch
fiber is appears to be a potential substrate for enzyme and other chemical production
(Deraman, 1993). Table 1 below shows the composition of EFB under dry matter basis
and fresh matter basis.
12
Table 1 Composition of EFB under dry matter basis and fresh matter basis
Parameter Dry matter basis
(mean)
Fresh wt. basis (mean)
Ash (%) 6.30 2.52
Oil (%) 8.90 3.56
Carbon (%) 42.80 17.12
Nitrogen (%) 0.80 0.32
Diphosphorous pentoxide (%) 0.22 0.09
Potassium oxide (%) 2.90 1.16
Magnesium oxide (%) 0.30 0.12
Calcium oxide (%) 0.25 0.10
Boron (mg/L) 10 4
Copper (mg/L) 23 9
Zinc (mg/L) 51 20
Ferum (mg/L) 473 189
Manganese (mg/L) 48 19
C/N ratio 54 54
Source: Cheng et al. (2007
13
Figure 3: An empty fruit bunch (top) and its fibrous form (bottom)
In order to obtain the best advantages from the application of EFB in the field, inorganic
supplements are also required. They are given for immature and mature plants (Gurmit
et al. 1999).From the analysis done before, it is stated that one tonne of EFB (fresh
weight) would have a fertilizer content equivalent of 3.8 kg urea, 3.9 kg rock phosphate,
18 kg muriate of potash and 9.2 kg kieserite. At current fertilizer prices, this would have
a monetary value of RM12.0014
2.3.2 Chemical treatment
In order to produce sugars from the biomass, the biomass needs to be pre-treated with
acids or enzymes to open up the plant structure and reduce the size of the feedstock.
Pretreatment (steam, alkali or acid treatment) may reduce the indigenous microflora
particularly required in simultaneous saccharification and fermentation (SSF), where
key enzymes must be pre-induced for a quick start of lignocellulose breakdown and
fungal growth (Tangerdy and Szakacs, 2003). Rita Rani et al. (2006) mentioned that
pretreatment of substrate increased the cellulase yields by 33%. The one of the most
14 Chan, K.W., Chow,M.C., MA,A.N., and Yusof Basiron (2002). The global challenge of GHG emission on carbon reduction:palm oil industry.Paper presented at the 2002 National Seminar on Palm Oil Milling, Refining Technology, Quality & Environment. 19-20 August 2002.12 pp,.
14
common chemical treatment is by adding sulfuric acid. There are lignin-hemicellulose
networks in cellulose fibers. This network interrupts the enzymatic biodegradation of
cellulose and hinders the saccharification of EFB to monomeric sugars. To accomplish
more effective enzymatic hydrolysis, this network should be removed. In this case,
sulfuric acid can resolve hemicelluloses and activate the enzymatic activity to cellulose
(Esteghlalian et al, 1996).
Alkali is also being used to treat lignocellulosic biomass. To overcome the lignin barrier,
lignocelluloses are initially pretreated with alkali to dissolve the lignin caused by the
breakdown of ether linkage (Lee 1997). In the case of pretreatment of corn stover by
aqueous ammonia, 70 -85% lignin was removed, and 40-60% hemicelluloses were
solubilized (Kim et al., 2003). Efficient delignifying agent should remove a maximum
amount of lignin and minimum of sugars (not more than 5%) (Taherzadeh and Karimi,
2007). Chemical alkali pretreatment at ambient temperatures is simple and time-saving
and seems to have strong commercial possibilities (Kim and Holtzapple, 2005).
2.3.3 Cellulose, hemicellulose and lignin
Biomass wastes contain a complex mixture of carbohydrate polymers from the plant cell
walls known as cellulose, hemi cellulose and lignin. Typically, this contains 30-50%
cellulose, 15-35% hemicellulose and 10-30% of lignin (Lynd et al., 2002). Cellulose,
C6H10O5 is the structural component of the primary cell wall that organized into long,
unbranched microfibrils that give support to the cell wall15
.Cellulose from wood pulp
has typical chain lengths between 300 and 1700 units; cotton and other plant fibres have
chain lengths ranging from 800 to 10,000 units (Klemm et al. 2005 and Bailey et al.,
1986).
15 Crawford, R.L. (1981). Lignin Biodegradation and Transformation.New York.,John Wiley and Sons.
15
Figure 4: Arrangement of fibrils, microfibrils and cellulose in plant cell walls
Source: (Klemm et al. 2005 and Bailey et al., 1986)
Figure 4 shows the arrangement of fibrils, microfibrils and cellulose in plant cell walls.
In micro fibrils, the multiple hydroxide groups bonded with each other, holding the
chains firmly together and contributing to their high tensile strength. In cell walls, this
strength is important as they are meshed into a carbohydrate matrix that helps in keeping
the plants rigid and tough.
A hemicellulose is any of several heteropolymers (matrix polysaccharides) such as
arabinoxylans that present in almost all plant cell walls. While cellulose is in a form of
crystalline, strong and resistant to hydrolysis, hemicellulose has a random, amorphous
structure with little strength. Hemicellulose has a molecular weight that is lower than
that of cellulose and they have a weak undifferentiated structure compared to crystalline
cellulose16
. It is primarily composed of the 5-carbon sugars and xylose.
16 Scurlock., Jonathan.,(2004). Bioenergy Feedstock Characteristics, Oak Ridge National Laboratory. Retrieved from http://bioenergy.ornl.gov/papers/misc/biochar_factsheet.html
16
Figure 5: Structure of hemicellulose
Source: (Huber et al., 2006)
Lignin as an irregular polymer forms a network in which cellulose and hemicellulose
fibre are embedded and also provides structural integrity in plants (Huber et al., 2006).
Due to complex structure of lignocellulose, it is resistant to most chemicals and
hydrolysis, which definitely form a barrier for its utilization (Lynd et al., 2002; Zhu et
al., 2006). It remains as residual compound after the sugars in the biomass have been
converted to ethanol. Figure 6 shows the arrangement of cellulose, lignin and
hemicellulose in a plant cell wall that involve in ethanol synthesize.
Figure 6: Typical plant cell wall arrangement
Source: (Huber et al., 2006)
17
2.3.4 Enzymatic hydrolysis
Enzymes are more efficient agents of hydrolysis than are acids (Elwyn et al., 1955).
Cellulase enzyme is used to break up cellulose into glucose or other aligosaccharide
compounds (Acharya et al. 2008). The cellulase system in fungi comprises of three