AEN 3200 Farm Practice Course
Biogas Production and Utilization
CONTENT
1. Introduction
1.1 What is biogas?
1.2 Composition of biogas
1.3 Calorific value of biogas
1.4 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 byproducts
2.2 Municipal solid wastes
2.3 Human excreta
2.4 Kitchen wastes
2.5 Abattoirs
2.6 Agricultural byproducts
2.7 Industrial wastes
3. Important Definitions…
3.1 Anaerobic digestion (degradation)
3.2 Methanogenesis
3.3 Biomethanation
3.4 Organic Matter
4. History of Biogas
5. How is Biogas Produced?
5.1 Microbial process of biogas production
5.2 Microbes involved in anaerobic digestion
5.2.1 Hydrolytic fermentative microorganisms
5.2.2 Acidogenic fermentative microorganisms
5.2.3 Methanogenic fermentative microorganisms
6. Factors Affecting Biomethanation
6.1 Important factors
6.1.1 Anaerobiosis
6.1.2 Temperature
6.1.3 pH
6.1.4 Substrate composition
6.1.5 C/N ratio
6.1.6 Micronutrients
6.1.7 Toxins and inhibitors
6.1.8 Hydraulic retention time
6.1.9 Total solids
6.2 Essential requirements for anaerobic digestion
7. Biogas Production
7.1 Compounds that can produce biogas
7.2 Raw materials suitable for biomethanation
7.3 Degradability of organic compounds
8. Digester for Biogas Production
8.1 Parts of a digester
8.1.1 Digester
8.1.2 Gasholder
8.1.3 Piping system
8.1.4 Inlet and outlet
8.2 Types of digesters
8.2.1 Based on origin
8.2.2 Based on dome
8.2.3 Based on the way of putting feeding materials
9. Utilization of Biogas
9.1 Major uses of biogas
9.1.1 Biogas for cooking
9.1.2 Biogas for lighting
9.1.3 Biogas for engines
9.2 Treatment of biogas
9.3 Storage of biogas
10. Design of a Biogas Plant
11. Advantages and Environmental Aspects of Anaerobic Digestion
11.1 Advantages of anaerobic digestion
11.2 Environmental aspects of biogas technology
11.3 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 biogas
The 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 %Biomass
Petroleum
Hydropower / 70
25
05
Consumption Category / Consumption %
Domestic
Industry
Other / 65
13
22
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 byproducts
Cattle, 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 wastes
In 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 excreta
Human 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 wastes
Kitchen 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 Abattoirs
Abattoirs 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 products
Lots 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 wastes
Wastes 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 Methanogenesis
Methanogenesis is a biological process by which organic matter is transformed into methane by microorganisms in the absence of air.
3.3 Biomethanation
When Methanogenesis process is housed in a reactor to create a technological process, it is known as biomethanation.
3.4 Organic matter
Organic 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 6°C and with increasing of temperature up to 50°C 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.