Biogas production and utilization

AEN 3200 Farm Practice Course

Biogas Production and Utilization

CONTENT

1. Introduction1.1 What is biogas?1.2 Composition of biogas1.3 Calorific value of biogas1.4 What can biogas do?1.5 Why do we need biogas?

2. Is there any Potential to Produce Biogas in Sri Lanka?2.1 Livestock byproducts2.2 Municipal solid wastes2.3 Human excreta2.4 Kitchen wastes2.5 Abattoirs2.6 Agricultural byproducts2.7 Industrial wastes

3. Important Definitions…3.1 Anaerobic digestion (degradation)3.2 Methanogenesis 3.3 Biomethanation3.4 Organic Matter

4. History of Biogas

5. How is Biogas Produced?5.1 Microbial process of biogas production5.2 Microbes involved in anaerobic digestion

5.2.1 Hydrolytic fermentative microorganisms5.2.2 Acidogenic fermentative microorganisms5.2.3 Methanogenic fermentative microorganisms

6. Factors Affecting Biomethanation

6.1 Important factors6.1.1 Anaerobiosis6.1.2 Temperature6.1.3 pH6.1.4 Substrate composition6.1.5 C/N ratio6.1.6 Micronutrients6.1.7 Toxins and inhibitors6.1.8 Hydraulic retention time6.1.9 Total solids

6.2 Essential requirements for anaerobic digestion

7. Biogas Production7.1 Compounds that can produce biogas7.2 Raw materials suitable for biomethanation7.3 Degradability of organic compounds

S Wijetunga Page 1 4/8/2023


8. Digester for Biogas Production8.1 Parts of a digester

8.1.1 Digester8.1.2 Gasholder8.1.3 Piping system8.1.4 Inlet and outlet

8.2 Types of digesters8.2.1 Based on origin8.2.2 Based on dome8.2.3 Based on the way of putting feeding materials

9. Utilization of Biogas9.1 Major uses of biogas

9.1.1 Biogas for cooking9.1.2 Biogas for lighting9.1.3 Biogas for engines

9.2 Treatment of biogas9.3 Storage of biogas

10. Design of a Biogas Plant

11. Advantages and Environmental Aspects of Anaerobic Digestion11.1 Advantages of anaerobic digestion11.2 Environmental aspects of biogas technology11.3 Uses of digested slurry

References



1. Introduction

Biogas production has several advantages in terms of economically as well as environmentally. Therefore, it has to be very important to know about production and utilization of this environmentally friendly technology. This manual explains the most of important aspects of biogas production and utilization.

1.1 What is biogas?Biogas is a mixture of gases produced by microorganisms under anaerobic degradation (digestion) of organic water. Major component of biogas is methane. Methane is a combustible gas and it can be used for energy generation.

1.2 Composition of biogas

Methane (CH4) 50% - 70%Carbon dioxide (CO2) 25% - 40%Various other gases (H2S, NH3, H2O vapour) 0% - 5%

In addition to above gases N2, CO, O2, and H2 slight concentrations occasionally present in biogas.

Biogas is often named according to its origin. Biogas is named as landfill gas when they originate from landfill sites and named as sewage gas when they originate from sewage sludge.

1.3 Calorific value of biogasThe calorific value of biogas varies according to its percentage of methane since methane is the major component of biogas that can produce energy. Other constituents do not produce energy and they absorb energy, which are produced by the combustion of biogas.

The calorific value of pure methane is 36000kJ/m3. Each 10% of methane can change the calorific value by 3600kJ/m3. For example, calorific value of biogas containing of 70% of methane is 25200kJ/m3. The actual calorific value of biogas is a function of its methane percentage, pressure and temperature. The actual values are very important parameter for the performance of the engines, which are running on biogas.

1.4 What can biogas do?It can easily be used as an energy source especially for cooking, lighting, generating electricity and motive power. In addition, digested material, which comes out from the digesters, can be used as organic manure and there are various beneficial effects to environment in biogas production.

1.5 Why do we need biogas?Production of biogas is a process of generation of renewable energy and a process of waste management as well.



Energy is very important for the development of a country. With the development, the demand for energy is also increasing. And also increase of population makes high demand on energy. Therefore, we have to spend lot of money for importing the petroleum since Sri Lanka do not have fossil fuel. Although we use biomass for major energy source, lots of petroleum fuels are needed for industries and to generate electricity. The energy sources and their contribution and the sectors of energy usage and their percentages are given below.

Sources of Energy Contribution %BiomassPetroleumHydropower

702505

Consumption Category Consumption %DomesticIndustryOther

651322

Table 01: Sources of energy and consumption

Almost all the biomass energy is used for household purposes, especially cooking. The limited amount of biomass is used for industries such as bricks & tile, tea, rubber, coconut, etc. Use of biomass as an energy source is compelled to increase deforestation. Therefore, use of biogas as an energy source for household purposes reduce the deforestation of the country.

In year 2000, cost of petroleum imports was Rs 67187 million and it is considerably high when compared with national income. Considerable amount of petroleum (basically LP gas) is also used for cooking and it can be replaced by biogas. As such the use of biogas save the national income, which is needed to import petroleum fuels.

Uses of firewoods in unventilated kitchens create health problems in housewives due to inhalation of unidentified substances in smokes that are produced by burning of biomass. But biogas burners do not produce other gases than carbon dioxide. Therefore, use of biogas improves the health of the housewives.

Methane is a green house gas by which increase the global temperature 25 times more than carbon dioxides does. As such production of methane is not environmentally friendly and it can cause to increase global temperature. The estimated anthropogenic green house effect is 15% from methane and 60% from carbon dioxide. The atmospheric methane concentration is increasing at a rate of 0.8-1.0 % per year. The almost all methane emissions are from low land paddy fields, ruminant excreta and landfill sites. Ruminant excreta naturally produce methane and it increases the methane concentration in the atmosphere. But if we can produce methane from animal excreta in control condition and it can be used for energy purposes. Then methane emission to atmosphere is reduced and decreases the global worming. Lowland paddy cultivation is also one reason to increase methane emission to atmosphere. It has been found that emission of methane from paddy cultivation varies from 20-150Tg per year. And also researches showed that methane emission from paddy fields is higher after harvesting due to degrading of



straw at the field. Therefore, if we can use straw for biogas production methane emission can be reduced and useful energy could be generated. In the same way, the organic wastes, which are a large part of land filling, can be used for biogas production and it will also reduce the emission of methane to the atmosphere.

Conventional energies such as petroleum, coal and LP gases are not unlimited. The estimated recovable years for petroleum, coal and gases are 43, 232 & 65 years respectively. Therefore, it is beneficial to find out good sources of renewable energies for our requirements. In considering Sri Lanka almost all hydropower capabilities has already been used and wind and biomass energy sources, as renewable energy is a good option.



2. Is there any Potential to Produce Biogas in Sri Lanka?

The biogas production basically depends on the availability of substrates that can go through anaerobic digestion. Since Sri Lanka is an agricultural country there are so many good sources of substrates.

2.1 Livestock byproductsCattle, buffalo, pig and poultry are the major groups of livestock that can produce good substrates for biogas production. In 1995 statistics shows that cattle, buffalo, pigs and poultry population in the country is 1704000, 764000, 87000 and 9573000 in numbers, respectively. Therefore, production potential of biogas in Sri Lanka is very high. Cattle and buffalo rearing in open space, especially dry zone farmers, is the major problem for collecting of cow dung that badly affect to the biogas production. Use of total available animal excreta can generate 27782400MJ of energy per day (@ 252MJ per cubic meter of biogas). This value is equivalent to 86.82MW of electricity.

2.2 Municipal solid wastesIn most town areas the waste disposal is a big problem. The urban wastes contain more than 80% of organic wastes, which can easily be used for biogas production. This helps to make pleasant and healthier environment in urban areas.

The total availability of solid wastes in Sri Lanka is about 2425MT/day (Ministry of environment, 1996). The production rate may vary with population growth rate and economic changes. As a guide value, waste production in low-income countries is 0.4kg/person/day; middle-income countries 0.5-0.9kg/person/day and industrialized countries 0.7-1.8kg/person/day.

The amount of organic wastes that can be obtained from municipal solid wastes in Sri Lanka is about 1940T/day (if organic fraction of MSW is 0.8). The energy potential that could be generated from MSW through biogas production is 101.5MW of electricity equivalent.

2.3 Human excretaHuman excreta are good source but production of biogas using human wastes is not a culturally accepted in Sri Lanka. And handling is also a problem. By diverting toilet outlet directly to digester can prevent handling practice. Digested wastes coming from digester do not have any harmful organisms and they all were destroyed due to high temperature in the digester.

Total potential of biogas production is about 518000m3/day, which is equivalent to 40.8MW of electricity.

2.4 Kitchen wastesKitchen wastes could also easily be used for biogas production but available amount of wastes in a single kitchen is very small and they only do not enough for biogas production. However, kitchen wastes generated from hostels, hospitals, factories, etc., can easily be used for biogas generation. The biogas production rate is 93L/kg



with a 30 days retention time from a conventional Indian type digester. The methane content is observed as 58%.

2.5 AbattoirsAbattoirs produce a variety of wastes such as blood, soft offal, meat, tallow, bone meal, etc., and liquid effluents at cleaning operations. These fractions could also be used but potential is not exactly known.

2.6 Agricultural by productsLots of agricultural by products can be used for biogas production. Since Sri Lanka cultivates rice, the rice straw can be used as substrates for biogas production. It is estimated that the rice straw production is about 2000000MT per annum. Most of this straw is burnt removing vast amount of nutrients from the field. However, use of rice straw as a raw material for biogas has several benefits, energy generation and production of enriched manure as well as avoiding of releasing methane from paddy fields by uncontrolled anaerobic digestion.

Potential energy generation from rice straw is about 64MW electricity equivalent (use of 100% of straw).

2.7 Industrial wastesWastes and wastewater coming from industries such as beverages, food, milk, sugar, rubber, coconut, etc., can be used. The potential of energy generation from these sectors are high.



3. Important Definitions…

3.1 Anaerobic digestion (degradation)Anaerobic digestion is the process by which organic matter is transformed into methane or reduced organic components such as ethanol, lactic acid etc, by the microorganism in the absence of oxygen (air). This is a microbial process.

3.2 MethanogenesisMethanogenesis is a biological process by which organic matter is transformed into methane by microorganisms in the absence of air.

3.3 BiomethanationWhen Methanogenesis process is housed in a reactor to create a technological process, it is known as biomethanation.

3.4 Organic matterOrganic matter is the compound containing carbon atoms usually in chain. In other words, matter made essentially from carbon linked together. Organic matter forms the better part of living organisms.



4. History of Biogas

Anecdotal (Unpublished) evidence indicates that biogas was used for heating bath water in Persia during the 16th Century BC.

Benjamin Franklin described as early as 1764 that he was able to light a large surface of shallow muddy late in New Jersy.

Alexander Volta was the first researcher describing the formation of inflammable gases in (low temperature) marshes and lake sediments scientifically. His paper was published in Italy in 1776. The importance of these findings was fully recognized by the scientific community and his letters were translated into German after two years (1778). Therefore, Volta is considered as the inventor of biogas.

In 1804, Dalton gave the correct chemical formula for methane.

In 1875, Popoff found that river sediments could produce biogas at temperature as low as 6C and with increasing of temperature up to 50C the gas production was stimulated. He also observed that the composition of biogas did not change with temperature.

The first digestion plant was built at leper colony in Bombay, India in 1859.

Gayon, a pupil of Pasteur, recoded a success in his experiments with animal manure in 1883-84. In same period, Louis Pasteur concluded that anaerobic manure fermentation might supply gas for heating and illumination.

Based on the findings that higher temperatures stimulate the biogas formation, heating systems were developed to increase the digester temperature. In between 1914 and 1921, Imhoff and Blunk took patents for heating devices (heat exchangers) to increase the temperature in the digester.

In 1936, Bushwell made his basic experiments on manure digestion in combination with most possible types of organic waste and he became the father of co-digestion.

The first full scale agricultural biogas installation developed in 1938 by Isman and Descellion in Algeria.

Towards the end of the Second World War when the fuel was limited, anaerobic digestion of liquid manure and sewage sludge became quite popular

France, Germany are operating biogas plants, specially large size plants, with higher technical standard mainly on sewage works. Half of gas was utilized to run engines.

Today biogas production has become a standard technology in wastewater treatment and upgrading of biowaste from household and agriculture. The development of the last 20 years allows not only low cost gas production but also it’s



upgrading and efficient – utilization in gas engines to produced electricity and fuel vehicles.

In the field of biogas production (anaerobic digestion) from wastes the India and China are recognized as world leaders. The period from 1973 – 1985 showed the rapid and worldwide development of simple AD systems for methane production as an energy source. In 1973, India, China and South East Asia rapidly and massively expended their AD units to answer their increased energy cost. The both countries have large number of small size biogas digesters for supplying the energy needs in especially rural people.



5. How is Biogas Produced?

Biogas is produced by microorganisms when they are degrading (digesting) of organic substances under anaerobic condition.

5.1 Microbial process of biogas productionIn fact, no known microbe can produce methane (major component of biogas) other than

Acetate Carbon dioxide & Hydrogen Methanol Formate Carbon monoxide and Some methylated amines.

Therefore, any organic matter should be converted to one or more of above substances before produce the methane. There are three distinct groups of bacteria that can involve in the process of anaerobic digestion. Together with these three groups of bacteria finally organic matter converted to biogas and digested material, which is almost different from initial material. And some instances, indigestible material can also be seen in the digested material.

5.2 Microbes involved in anaerobic digestionThree groups of bacteria involved in anaerobic digestion are,

i. Hydrolytic fermentative microorganisms. (Hydrolysis or liquefaction)ii. Acetogenic fermentative bacteria. (Acedogenesis)iii. Methanogenic bacteria (Methanogenesis)

5.2.1 Hydrolytic fermentative microorganisms These groups of microorganisms can hydrolyze high molecular substances in to low molecular substances, for example, proteins to amino acid, polysaccharides to oligo and monosaccharides, and lipids to free fatty acids. Process performed by these microorganisms is called as Hydrolysis or liquefaction.

5.2.2 Acetogenic fermentative microorganismsAcetogenic bacteria obtain their energy from the oxidation of organic acids, alcohols and volatile acids with more than two carbon atoms, for example, caproate, butyrate and propionate. End products of hydrolysis process are converted to volatile fatty acids, H2, CO2, NH3, ethanol, and methanol. The process performed by microorganism is called as Acedogenesis.

5.2.3 Methanoganic fermentative microorganismsMethanogenic bacteria are the ultimate group in the process of anaerobic digestion. They produce the most reduced form of carbon, namely, methane. A detailed scheme for break down of organic compounds is given in the following diagram. Approximately 70% of the methane is formed from VFA, 30% form H2, CO2 by



methanogenic bacteria. The process performed by these groups of microorganism is referred as Methane formation.

According to above three groups of bacteria process of biogas production could be divided into three steps

o Hydrolysiso Fermentationo Methane formation.

Figure 01: Flow sheet of methanogenesis



6. Factors Affecting Biomethanation

6.1 Important factorsSeveral factors are governing the methanogenesis process in anaerobic digestion process. Important factors are briefly explained in the following sections.

6.1.1 Anaerobiosis (anaerobic condition) Methanogenesis is a strict anaerobic process. The major groups of bacteria actively engaged in methanogenesis will die in the present of O2. But some researchers have found some aerobic facultative bacteria in the digesters. These bacteria do not have any role in the main degradative reactions of the digestion but they may have some role in sugar fermentation. Most probably, these bacteria may use the oxygen in the system, reducing the system, to be suitable for the growth of the methanogenic bacteria.

6.1.2 Temperature Methane is formed in nature over a wide range of temperatures. Three different temperature varies are distinguished

Psychrophilic temperature - 10 - 25oC Measophilic temperature - 30 - 37oC Thermophilic temperature - 50 - 65oC

In low temperatures gas production rate as well as the amount of gas production is low and in high temperatures biogas production is high. In most cases, measophilic temperature range is used. The measophilic digesters could be converted to thermophilic or vise versa.

However, the change should be in smoothly (slow change). The sudden temperature changes badly affects to the digester activities and slow or stop the gas production. To convert the measophilic process to thermophilic process, it will take 10-20 days.

Thermophilic digestion process has several advantages. These are

Rapid metabolic activity, help to - Reduce the retention time- Increasing the loading rate- Small digester volume- Increase rate of methane production (1.5 times faster than

measophilic). High temperature, helps to

- Kill the pathogenic organisms- Improve dewaterability.

Low temperature, psychrophilic, waste digestion is slower than measophilic and thermophilic digestion.



6.1.3 pHA pH value between 6.5 and 7.7 has been found to be optimum for the process. Methanogenesis beyond the range of 6.5 – 8.0 pH has been found to be less yield of biogas

6.1.4 Substrate composition Depending on the composition of the substrate (feedstock), the rate and amount of gas production and digesting process rate will vary. If we have the composition of the substrate, it is possible to calculate the amounts of gas, which may be produced on the basis of a simple carbon balance.

Theoretical yield of biogas (m3/kg is destroyed) from various compounds of organic matter are 0.886 (carbohydrates), 1.535 (fat), and 0.587 (proteins) with methane content of 50, 70 and 84% respectively.

6.1.5 C/N ratio Nitrogen is essential for cell growth and it can controls the pH by releasing the NH3

from Nitrogenous compounds.

The optimum range of C/N ration for methane production is 25:1 to 30:1. This shows that bacteria consume carbon 25 to 30 times faster than Nitrogen. Therefore, optimum ratio of C/N is essential for smooth operation of digester. The C/N ratios of important feeds stocks are given in the following table.

Organic Substrate Carbon (dry wt %) Nitrogen (dry wt %) C/N RatioStraw (rice)Fallen leavesStalks (corn)WeedsSheep excretaCattle dungHorse excretaSwine excretaHuman faeces

42414014167.3107.82.5

0.631.00.750.540.550.290.420.650.85

67:141:153:127:129:125:124:113:13:1

Table 02: Composition of different raw materials

The different substrates can be mixed together to achieve the proper C/N ratio for anaerobic digestion. If we have various raw materials, they can be mixed up to get desired C/N ratio. Following example clearly show the method of calculating required amount of nitrogen sources to get desired C/N ratio of law nitrogen raw materials.

Example: Calculate the quantity of urea (46% of N) required to make the 1000kg of rice straw for biogas digester with C/N, 30:1.

Rice straw contains carbon, 42% and nitrogen, 0.63%. The urea (CO(NH2)2) contains nitrogen, 46% and carbon, 20%. Required amount of urea can be calculated as follows.



6.1.6 Micronutrients In addition to nitrogen, the phosphorous is also essential for the better growth of microorganisms. The ratio of C:P of 100-200 is said to be optimum. Micronutrients such as Nickel (100 nM) Cobalt (50nM) and Molybdenum (50 nM) are important. Molybdenum may enhance the joint effect of Nickel and Cobolt. Iron (2nm) and Copper (4nm) are necessary for enhancing the performance of the digestion process.

6.1.7 Toxins and inhibitors It has been found that metal irons exert a toxic effect they exceed the required concentration. If they are in the digester more than tolerable limit, it should be diluted by adding water and regularly flushed out.

--SO 4 500 ppmCyanide <25 mg/LDetergent compounds 20-40 ppmSodium chloride 40 000 ppmAmmonia (Not NH+

4 ) 1500 - 3000 ppmCopper 100 mg/LChromium 200 mg/LNickel 200-500 mg/LSodium 3500-5500 mg/LPotassium 2500-4500 mg/LCalcium 2500-4500 mg/LMagnesium 1000-1500 mg/L

6.1.8 Hydraulic retention time (HRT)Hydraulic Retention Time expresses the volume of fluids in the reactor per volume of fluid passing into and out of the reactor on a daily basis. It indicates the contact time allowed between the substrate and microorganisms in the system. Maintaining option retention time is an important for efficient conversion of organic matter to methane.

Use of long retention time would result inefficient use of digestion capacity. The substrates for microorganisms are not sufficient and their growth is slow down.

Short retention time is not sufficient for microorganisms to degrade the whole substrate. The retention time may very depending on the composition of the substrate. For example, cellulose material required 10 days for digestion and for lignocellulose material is non degradable even at 30 days. In most cases 30-40 days of retention time is suitable.

6.1.9 Total solids



Total solid content of the substrate is very important in biogas production. The optimal total solid level for biogas production was observed as 10%. But considerable gas production can be obtained up to 30% of solid level. The increase of solid level leads to reduce the gas production. To obtained desirable amount of biogas production from higher total solid levels, long duration of retention times should be practiced.

When we are using animal excreta (specially cow dung) and green refuse, 1:1 ratio of water to substrate could be practiced as a general value. Approximate total solid contents of several substrates are given below.

Substrates Total Solid %Water hyacinthPlant wastesGrassesPoultry manure (fresh)Poultry manure (dry)Pig manureHuman faecesNews papersCow dung

1175

30-803590142793

20-25Table 03: Total solids in different raw materials

6.2 Essential requirements for anaerobic digestion pH near neutral C/N 25 – 30 / 1 Organic substrates of digestion Total solids in the substrates (10% is best but up to 30% is possible) Free from toxic compounds Initial material to initiate the digestion process (inoculums), in most cases

cow dung (ruminant fluid) or sewage sludge or digested material from an anaerobic digester could be used.

7. Biogas Production



7.1 Compounds that can produce biogasThere are several compounds, which can be easily anaerobically degraded. The following table shows the compound for biogas production and its anaerobic biodegradability, biogas yields and percentage of methane in the biogas.

Compounds Anaerobic bio degradability

Bio gas yield(m3/kg COD destroyed)

Methane (%)

SugarsStarchCellulose

ExcellentExcellentPoor – good

0.79 50

Proteins Excellent 0.96 53Fats/Lipids Excellent 1.43 71Volatile Fatty Acids (Short chain with two to five “C” atoms)

Excellent 0.75 50(for Acetic Acid)

Table 04:Rate of biogas production in different compounds

7.2 Raw materials suitable for Biomethanation

1. Agricultural wastes: Plant residues (straws, husks, cobs, hulls, etc) and Animal byproducts (horse, cattle, goat, sheep, pig, poultry, etc)

2. Agro industrial wastes (Bagasses, coir mill wastes, etc) 3. Forestry and forest product industries (leaves, twigs, sawdust, wood wastes) 4. Energy crops: Agricultural crops and aquatic weeds (Salvinia, Water

Hyacinth)5. Municipal Bio waste: Municipal solid waste, sewage sludge sewage including

industrial effluent6. Other wastes such as petrochemical, organic chemical, leather, soap, etc.7. Food industrial wastes (milk, fruit, vegetable processing)

7.3 Degradability of organic compounds

Compound DurationLignin Hardly NoticeableCellulose Several weeksHemi celluloseFatProteins

A few days

Low molecular sugarsVolatile Fatty AcidsAlcohols

A few hours

Table 05: Degradability of different compounds

8. Digester for Biogas Production



8.1 Parts of a digesterBiogas Unit and Plant are other names for biogas digester. The biogas plant consists of basically four parts

The digester where organic matter is digested The gas holder where the biogas is collected and stored Piping system which helps to transport of biogas to the place where

its use. Inlet and outlet for feeding the feedstock and removing the digested

material

8.1.1 DigesterThe digester is a tank, normally circular in shape and it is normally constructed underground. The digester should be entirely air light to prevent the entering of oxygen to the digester and leaving out of biogas produced from the digester. It also should be waterproof. The digester should meet followings also;

Absolutely leak proof for liquid and gas Strong enough to withstand slurry pressure Resistance to corrosion Heat insulation Provisions for fittings such as gas outlet, inlet, outlet, etc,.

8.1.2 GasholderThere are two types of gasholders namely fixed dome type and floating dome type. In fixed dome type, the biogas accumulates in the dome over the digester slurry. The slurry in the digester serves as a reversible displacement medium. The biogas accumulating in the dome pushes out a portion of the slurry into a higher auxiliary compartment. The digester slurry floor back into the dome by gravity as the biogas is consumed. In floating done type, the inverted tank is placed over the digester tank and the gas released from the digester tank accumulates in the inverted tank over the slurry.

8.1.3 Piping systemThe piping system helps to transport the biogas where it is used. PVC based pipes are the most suitable pipes for gas transportation. Metal-based pipes may be corroded since H2S and water vapour are present in the biogas.

8.1.4 Inlet and outletInlet is used to feed the feeding material to the digester. In most cases, before diverting the feeding material, they are mixed with water. Digested materials come out from the digester through outlet.

8.2 Types of digestersThere are several types of digesters, fabricated by the scientist in various countries specially China, India, Vietnam, Taiwan, German, etc. These digesters could be categorized into groups depending on the country where it is constructed, types of the dome and the way of putting the feeding material in to digester.



8.2.1 Based on origin China type (fixed dome type) Indian (Floating dome) Vietnam/Taiwan (The bag digesters is essentially a long cylinder

made of PVC), Inlet, outlet and gas outlet pipes are attached to the bag. The construction cost is very much less compare to other digesters.

8.2.2 Based on the dome o Fixed dome: This digester has been developed in china. The

biogas accumulates in the upper portion of the digester. The pressure of the gas is varied depending on the consumption. The digester is constructed to maintain the pressure of the gas equal or below 120 cm of water. The retention time for both cow and pig manure normally is 35-40 days. The total solid concentration of the feeding material is about 7 – 10 %. The construction of digester is somewhat difficult and skilled masonry labour is required.

o Floating dome: Floating dome types has been developed in India in 1950. The produced gas accumulates in a tank, which was placed on the slurry in inverted position. The digesters are designed for 30,40,55 days retention times. The gasholder (drum) was originally made of milled steel, until fiberglass reinforced plastic was introduced successful, to overcome the problem of corrosion. The pressure of the gas available depends on the weight of the gasholder per unit area, and usually varies between 4-10cm of water pressure. This pressure is sufficient to carry the gas up to a length of 20 to 100 m depend on the size.

8.2.3 Based on the way of putting feeding materialso Batch System: A digester is loaded periodically with designed

quantity of waste material, water and initial material (inoculums or activated sludge) to initiate the microbial activity in the digester. The daily gas production increases initially thereafter, it starts falling down, after use over a period. When quantity of gas decreases below the rated capacity, the digester is unloaded and recharged with fresh material.

If the raw material (waste material) contains total solids more than 10%, the system is referred as dry batch system. The researches have shown that total solid up to 32% could be used for dry batch fermentation. In batch systems, the biogas production is initially increasing and after some period, (depending on the feeding material) the production will be reduced. Therefore, uniform gas production cannot be achieved using this system.

o Continuous system: The continuous digesters are more suitable for regularly and sufficiently available wastes, which can be mixed with water homogeneously to form liquid slurry, such materials are cattle dung human and animal excreta or the materials, which can flow after digestion.

A fixed amount of waste is charge into the biogas plant daily and gas production from its digester is constant and equal to gas produced from the



daily charge. The daily effluent from the digester is equal to daily feed material.

9. Utilization of Biogas



9.1 Major uses of biogasThe biogas could be used to obtain thermal energy, mechanical energy and electrical energy. The thermal energy is required for cooking, lighting, boiling of water, bakery industry, Ironing, etc. The mechanical energy is required for water pumping, running of engines, etc. Electric energy (electricity) could be generated by coupling a generator to an engine, which is running in biogas.

9.1.1 Biogas for cookingIn Sri Lanka, very large amount of energy is used for cooking. Almost all biomass energy is used for cooking. Biomass utilization for cooking easily could be replaced by adopting the use of biogas for cooking.

Pure methane burns in a mixture of 91% air and 9% methane. The biogas, however, needs approximately 93% air to burn completely. For this reason, the normal LPG burners used commonly in the houses are not fit for biogas. Special burners with bigger holes have been designed for use biogas as fuel. The bigger holes enable to draw higher percentages of air from the atmosphere, needed for efficient burning of the biogas with the correct air an biogas mixture, the flame temperature can reach as high as 8000C. Since the pressure of the biogas is normally low (less than 35cm) the bigger holes is essential to provide sufficient amount of gas to the burning point. The biogas burner should be designed to work at 7-10cm of water column. Generally about 0.25-0.35m3 of biogas is required for cooking for a person per day.

To ensure that the flame does not “lift off”, the ratio of the total area of burner parts to the area of the injector orifice should be between 80 and 200:1.

9.1.2 Biogas for lightingThe lamp with a mantle could be used for lighting purposes. The biogas consumption for lighting is about 0.15 m3 per hour for 100-candle power mantle lamp (equivalent to 60W).

9.1.3 Biogas for enginesBiogas can be used as a fuel in stationary and mobile engines to supply motive power, pump water, drive machinery (ex: threshers, grinders) or generate electricity. Both, spark and compression engines could be operated in biogas. The spark engines are easily modified to run on biogas by using a gas carburetor. Ignition system need not be altered, other than minor timing adjustment. Spark ignition engines can run entirely on biogas after starting on petrol and initial heating. The biogas supply pipe is attached to air manifold between air cleaner and carburetor. After the engine is run on petrol for 5 to 10 minutes the biogas supply valve is opened slowly and petrol supply valve closed simultaneously. In two-stroke cycle engine the lubrication is done by adding lubricant with fuel, which become difficult, therefore, these engines cannot be converted to biogas.

In the diesel engines the engine is usually started with pure diesel fuel and the biogas increased gradually until it comprises around 20% of fuel intake with 80% biogas, engine performance is good and 20% more hose power is delivered the with diesel alone. Approximately 0.43m3 of biogas is required for running a one-horse power engine for hour. Electricity also could be produced using biogas. Normally 0.75 m3 of Biogas is required to produce 1kwh



Thermal efficiency is very low in engines running on biogas, about 25-30%. Therefore, extra amount of thermal energy could be used to heat the digester or for space heating of animal sheds, green houses and buildings.

9.2 Treatment of biogasThe raw biogas produced in a digester is normally treated in order to remove water H2S, dust & CO2. The choice of cleaning method employed and the compounds to be removed depends on the type of end use of the gas. If biogas is used for any purpose, the water removal is essential. Simple arrangements could be employed in the gas distribution system to trap water in the biogas.

By removing the H2O in the biogas, quality of flames both burners and lamps could be improved. Hydrogen sulfide and water removed is essential when biogas is used in engines. Removing H2S and water could prevent damaging to the engines by corrosion. CO2 removal is employed when a high quality gas is required. This can be the case when the biogas is to be used as a substitute for natural gas or when sensitive equipment is used.

H2S removal using iron oxide pellets is one method. In this technique, in which the gas is led through a box of pellets, the iron oxide pellets reacts with the H2S in the biogas according to the following reaction

Fe2O3 + 3H2S Fe2S3 + 3H2O

Regeneration of the iron pellets is done with oxygen

2Fe2O3 + 3O2 2Fe2O3 + 6S

Normally two boxes of pellets are installed in the gas line; one box is loaded whereas the other is regenerated. When the iron pellets are completely covered with sulfide, the pellets are replaced. H2S could be removed by absorption of H2S in water or organic solvents. High consumption of water limits the use of this method. Chemical absorption with a diluted sodium hydroxide and iron chloride solutions are also employed to remove the H2S. By removing the CO2, biogas energy volume of biogas could be increased. And petrol and diesel engines works properly if the CH4 in the biogas more than 90-95%. Presence of CO2 in the biogas when biogas is used as fuel for vehicles, lower the output power, take up space in the storage cylinders and freezing at valves are major problems. Therefore, all or most of CO2 should be removed when biogas is used for vehicles or engines.

9.3 Storage of biogasBiogas could be compressed for the use of vehicles. However, it has to be utilized energy to compress the biogas. Therefore, for small biogas plants, it is not economically viable. Biogas cannot be liquefied easily because the boiling point of methane is very much less. It is about -1620C. Methane is the major components in the biogas.

10. Design of a Biogas Plant



The biogas plants are designed based on type of waste material available for digestion and the quantity of gas required. The gas requirements for various purposes are given below.

Purpose Gas requirementCookingLightingDriving engines

0.25 m3 per person per day0.12-0.15 m3 per hour per lamp0.75 m3 per kWh

Table 06: Gas requirement for various purposes

Source Manure /day (kg)

Gas/kg/day(m3)

Retention Time(days)

Total Solids(%)

Cattle 10 0.036 30-45 14-25Buffalo 15 0.036 30-45 14-25Pig 2.25 0.078 45-55 18-27Chicken 0.18 0.062 30-45 20-30Adult Human 0.4 0.070 45-55 17-20

Table 07: Waste materials and their gas generations

Based on the above information such as waste material available, their gas productions rate, retention time, gas requirement for cooking, lighting etc, the biogas digesters could be designed. In the case of straw, one kilogram of straw approximately can produce 1L of biogas per day but this may vary with the retention time.

According to the number of animals that farmer having, the gas production from the wastes, which is produced by excreta of the animals, could be calculated. And also number of family members and their gas requirement decide the total gas requirement for the family.

Now it is easy to find out which is less, total amount of gas produced form available quantity of waste or the quantity of gas required per day. Then, it can be found the limiting parameter for determining the volume the digester. That means, if gas consumption is less than gas production, the volume of digester will be decided by the amount of gas consumption. If the gas production is less than gas consumption, the volume of gas digester will be decided by the gas production. Which is less, it will decide the digester size (volume).

Example: Farmer having six members is going to construct a biogas unit. He has five cows and two buffaloes. Design a biogas unit for the farmer. The biogas unit should be able to provide gas for cooking for the family and for lighting of two lamps for 2 hours.

First, it has to be calculated the gas production and gas consumption.



Gas production:No. Of animals are seven (five cows, two buffaloes)Amount of excreta (cow dung) per day is 70 Kg. Total gas production is 2.52 m3/day (70 x 0.036)

Gas consumption:For cooking, the gas requirement is (0.25 x 6) 1.5 m3

The gas requirement for lighting two lamps for three hrs is (0.15 x 2 x 2) 0.6m3

Total gas consumption is 2.1 m3 /day

The digester could be designed based on the gas production as well as gas consumption. But the size of the digester would be large when digester is designed in considering the gas production and hence the cost of construction is high. When the digester is designed based on gas consumption, the size of the digester would be small compared to the digester built based on gas production. Since the gas consumption is less than gas production, the digester is designed based on gas consumption. Hence, the cost of construction would be less and it is the suitable size. Therefore, the design of the digester should be done based on, gas consumption in this case.

Total gas consumption is 2.1 m3 /day

Required amount of raw material (Cow dung) is 58.3kg(2.1/0.036) and it could be considered as 60Kg.

Normally, before feed the raw material to the digester the raw materials should be mixed with water to make the desired level of total solids in the raw materials. The amount of total solids (TS) in the raw materials for better anaerobic digester and biogas production is about 10%. That mean, the 90% of water is required for digestion. Researches now have shown considerable biogas production even at 30% of total solid concentrations. However, in the design of a biogas unit, TS concentration should be considered as 10%.

Normally, cow dung has about 14 – 25 % of TS. By mixing cow dung with water at 1:1 ratio, the desired TS concentration can be achieved. Therefore, the amount of water that has to be applied is 60kg.

The weight of total feeding material is about 120 Kg. (60kg cow dung + 60kg of water).

Now it has to be calculated the volume of the digester. If the density of cow dung water mixture digester is about 1080kg/m3 the volume of the mixture is about 0.111 m3 (120/1080). This amount of mixture of raw materials can produce gas for one-day requirement for the family. The retention time for the cow dung in tropical condition is about 40 days.

The total volume of the digester (feeding compartment) is 4.44 m3 ((120x40)/1080). Retention time is the average time a unit volume of feeding material (liquid) remains



in a digester (vessel) operated a continuous way. It means, the cow dung water mixture is in the digester and produce gas for about 40 days.

Normally, volume of the dome (basically this space is for storage of gas) is about ¼ of the volume of the feeding material. Therefore total volume is 5.55m3

(4.44/4+4.444=5.55m3).

Actual volume should be more than 10% of the theoretical volume. Therefore actual volume of the digester would be about (5.55 x 10/100 + 5.55) = 6.105) 6.0m3.

Therefore, the total volume of the digester is about 6m3. Using following simple equations, the dimensions of the digester can be found.

Figure 02: Sketch of the digester

(A) Diameter of the digester =

(B) Height of the dome =

(C) Height of the cylindrical part =

(D) Depth of bottom =

Total height = 0.50+0.95+0.30=1.75m


D

C

A

B


11. Advantages and Environmental Aspects of Anaerobic Digestion

11.1 Advantage of Anaerobic digestion Produces large amount of methane gas which could be used as energy

source Digested materials (out come of the digester) are almost odorless Digested material has good fertilizer value and can be used as soil

conditioners Reduces organic content of waste materials by 30-80% and produce an

established sludge (slurry) for ultimate disposal Weed seed are destroyed and pathogens are either destroyed or greatly

reduced in number Rodents and flies are not attracted to the end product of the process Provide a sanitary method for disposal of human and animal wastes Helps conserve scare local energy resources such as forests Produce valuable source of energy and it could be used for cooking

purposes and make smoke free environment in the kitchen

11.2 Environmental aspects of biogas technologyThe use of biogas technology in the agricultural sector is influenced by environment, social (income, establishing and employment) and hygienic issues. In the following the only environmental aspects of biogas technology in the agriculture are discussed.

1. Anaerobic treatment of animal manure with co-substrates increases the quality of the digested manure: It will improve the handling of manure and increased yield. In addition, the range of application possibilities and the acceptance by farmers in increased. Many farmers have observed yield, after using digested manure, increase of 2 to 3% compared to untreated manure. By co-fermentation, nutritive value of crops could be easily increased.

2. Anaerobically treated manure increases the range of application possibilities in terms of time, crops and housing: Digested manure has high content of Ammonium Nitrogen. The increased ammonia content of digested manure combined with a slightly increased pH causes a higher risk of ammonia loss in treated manure compared to untreated manure. Therefore, digested manure must be handled more carefully. The loss of N (NH3 commissions) could be prevented by injecting the digested manure to the soil or by cover the manure. NH3 and NH4 are changed with pH. At lower pH, due to high concentration of H+, the NH3 is converted to NH4, which cannot evaporate. However, in high pH levels, NH3 is high and it has high evaporation capacity.

Undigested manure most probably has phytotoxic compounds, which creates necroses and scleroses on the plant leaves. Through anaerobic digestion, phytotoxic acids are degraded and dry matter content is decreased. Therefore, digested manure can be applied to a growing field, which usually has a high demand for nutrients. Odor causing substances are also degraded in the same way and therefore anaerobic digestion produces odorless digested material.



3. Reduction of weed seeds: The AD process lowers the ability of seeds to germinate.

4. Improvement and stabilization of soil fertility: Organic fertilizer and mineral fertilizer differentiate in not only nutrient content, composition and variance but also in qualitative aspects, while, organic fertilizer contributes directly to the humus for making process of the soil, mineral fertilized does not. Lack of humus alternately results in desertification. A productive soil system needs a balance between incoming humus and degrading of humus. Manure, compost or any organic fraction will increase or stabilize the humus level in the soil. During AD process, most of the low molecular substances (less than 1000g/Mol) are degraded, while lignin substances still contributes to the humus pool of the soil.

5. Reduction of pollutants: Organic compounds can be reduced through the anaerobic treatment process. Tests have shown that organic carbon compound, mainly resulting from the use of pesticides, can be degraded. (trichlormethane, tetrachlormethane, 2-4D,etc).

6. Contribution to the water resource protection: Since the increased ammonia content in the digested manure an accelerated plant up take occurs. Roots prefer, NH3 than nitrate, if they are available at same time. When NH3 is more and faster absorbed by the plants it cannot be transformed into nitrate and leached downward in direction of the ground water level. Thus, ground water pollution through nitrate is prevented.

7. Contribution to the climate change protection: Methane is the second most important greenhouse gas in the world, with a global worming potential of 25 times higher than CO2. It has been estimated that CH4 emissions from agriculture contributes about 33% to the global green house effect. About 7% alone result from animal excrement, which is similar to 20-30 million tones of methane per year. Through anaerobic treatment of animal excretion, a renewable source of energy is generated. It has an important duel climatic effect.

The use of renewable energy reduces the CO2 emission through a reduction of the demand for fossil fuel.

Coal Heavy mineral oil Mineral oil Gas0.33 0.28 0.26 0.20

Table 08: CO2 emissions different fuels kg/kwh

At the same time, the process can diminish uncontrolled methane generation by capturing methane as biogas.

Smaller agricultural biogas plants reduce the use of forest resources for household energy purposes and thus slow down deforestation, soil degradation, and resulting natural catastrophes (disasters) like flooding or desertification.

Nitrous Oxide has high global worming potential. It has found the anaerobic digestion reduce the Nitrous oxide emissions also.



8. Biogas composition and emissions after burning in comparison with other fuels are given in the following table and it shows that biogas has clear emissions after burning.

Pollutants SO2

kg/TJNOX

kg/TJDustkg/TJ

CO2

g/TJBaP*

g/TJMineral OilGasCoalWood industry(Efficient burning)Wood private(Inefficient)StrawBiogas

1403

300100

30

1703

9090

18064

60

34050

20220

100

100

2003

9070

100130

300

30050

1-

300013

-

--

(*Cancer causing substance, Benzo (a) Pyren)Table 09: Emissions after burning of different fuels

9. Resource protection, an appropriate technology: Fossil fuels are limited and contribute to the greenhouse effect. Biogas is renewable and can help to reduce the climate change a well as support the protection and conservation of limited resources. Gases as fuel have one big advantage in computing to other fuels. There is no need for refining and processing of the fuel, and the exhaust usually does not need an expensive and sophisticated clearing facility.

10.Reduction of waste disposal: AD contributes strongly to a closed nutrient cycle system, where nutrients are not lost but reused in the agriculture. At the same time, energy is generated. Therefore, participation of the agricultural sector is and should be a major and important step in a sound waste management.

11.3 Use of digested slurry Digested slurry could be used as a feed for fish. Digested slurry could be used for substrates for mushroom production. Valuable organic fertilizer with valuable micronutrients Digested manure could be easily handled since there is no odour. Slurry could be used as insecticide for insect pests

ReferencesAjith de alwis (2001).Study on the potential of biogas in Sri Lanka. ITDG South Asia.Anonymous, (2000). Annual report, central Bank of Sri Lanka.

Anonymous, (1998). National Environmental action plan 1998 – 2001. Ministry of forestry & Environment, Sampathpaya, Baththaramulla, Sri Lanka.

Bardiya, N., Somayaji, P. & Knanna,S. (196). Bio methanation of Banana peel and pineapple waste. Bioresource Technology,58, 73-76.

Mathur,A.N. and Rathore, N.S. (1992). Biogas management and utilization, Himanshu publications,5k51,Ram singh K1 Badi, Sector 11, Udaipur(Rajasthan).


Biogas production and utilization

Documents

production of biogas

utilization of biogas

treatment of biogas

storage of biogas

biogas plant

history of biogas

combustion of biogas

major component of biogas