24 Biogas Production from Biomass and Its Applications
Prof. A.K Jain and Dr. Dhanya M.S
Learning Objectives
This module helps to understand
- Microbiology and Chemistry of biogas production
- Factors affecting the performance of a biogas plant
- type of biogas plants
- Biogas plant design
- Application of biogas
- Application of digested slurry
1.INTRODUCTION
Biomass is defined as organic matter including crop and forest residues, animal excreta, poultry droppings, human excreta and microbial cell mass that are renewable in nature. Biomass serves as food, fodder, fibre, fuel, bedding, building material and also adds to soil organic matter. The organic wastes such as animal wastes and crop residues that are abundantly available are potential feed stocks for biogas production.
Anaerobic digestion of biomass materials is a bio-chemical conversion process. The product of this process is a gaseous fuel, called biogas, having 55-65% Methane and 25-45% Carbon dioxide with some trace gases such as Hydrogen, Hydrogen Sulphide, etc. It is medium calorific value (CV) gaseous fuel with CV of 18-22 MJ/Nm3 and gives self-sustaining flame on ignition. It can be used for thermal application especially cooking, lighting and for running IC engines. The waste material in the process gets stabilized and all the inorganic nutrients such as Nitrogen, Potassium, Phosphorous etc. are retained in the process. Thus the digested effluent slurry becomes a value added organic fertilizers and also soil conditioner.
2.MICROBIOLOGY AND CHEMISTRY OF BIOGAS PRODUCTION
Chemical composition of organic wastes consists of carbohydrates (cellulose, starch, and hemicellulose), proteins, lipids and lignin. Anaerobic digestion primarily occurs in three stages.
- Hydrolysis: In the first stage carbohydrates which are primarily occur as polysaccharides are hydrolyzed by extra cellular enzymes to monosaccharides. The monomer sugars are mainly glucose with some fructose and mannose. These simple sugars are converted to volatile fatty acids (VFA) namely acetic, propionic, butyric, valeric, lactic and succinic acids. Ethanol, propanol and butanol are also produced in addition to carbon dioxide and hydrogen. The lignin remains practically unutilized.
- Acidogenesis: The second stage involves the conversion of VFA, ketones and lower alcohols to higher organic acids such as propionic acid and butyric acid with the help of acid forming bacteria. If the acetic acid is produced along with carbon dioxide and hydrogen during the process, it is known as acetogenesis.
- Methanogenesis: In the third stage methanogenic bacteria results in the production of a mixture of methane and carbon dioxide from VFA. The methane forming bacteria are obligate anaerobes and need highly reduced environmental conditions for growth. Dissolved oxygen, even at a concentration of 0.01 mg/litre, completely inhibits their growth.
Following reactions illustrate the conversion of VFA, alcohol and ketone to methane and Carbon dioxide.
CH2COOH (acetic acid) | → CH4 | + CO2 | |
4CH3CH2COOH (propionic acid) + 2H2O | → 7CH4 | + 5CO2 | |
2CH3CH2CH2COOH (butyric acid) + 2H2O | → 5CH4 | + 3CO2 | |
2CH3CH2CHO (propionaldehyde) | → 3CH4 | + CO2 | |
CH3COCH3 (acetone)+ H2O | → CH4 | + CO2 | |
CO2 + 4H2 | → CH4 | + 2H2O | |
The above equations show that the fermentation of acetic, propionic, butyric acids, propionaldehyde and acetone will result in the same end products i.e. methane and carbon dioxide.
The methane production from acetic acid is known as aceticlastic methanogenesis while methane production from hydrogen is known as hydrogenotrophic methanogenesis.
In the formation of methane from this process, 72% of methane comes from acetic acid and remaining methane from other routes.
3.Factors affecting the performance of a biogas plant
Because anaerobic digestion is a complicated biological process, it is very sensitive to several environmental factors such as temperature, pH, loading rate, solid concentration, nutrient in the feed slurry and toxicity.
3.1 Temperature: Temperature isan extremely important factor which controls the growth of micro-organisms involved in the production of biogas. Three thermal zones based on microbial activity are identified as thermophilic zone (above 45°C), mesophilic zone (20°-45°C) and psychrophilic zone (below 20°C). Effective digestion is carried out in both thermophilic and mesophilic temperatures zones. In India, the biogas plants operate generally under mesophilic range, though in winter months, in northern parts of the country (viz. in the states of Punjab, Himachal Pradesh, Haryana and J&K), the temperature falls to the lower mesophilic or psychrophilic range and biogas(methane) generation is adversely affected.
3.2 pH: It indicates whether the material inside the digester is acidic or alkaline. Optimum value of pH for the proper production of biogas is 6.8 to 7.5. It affects the population and functioning of the microorganisms. During normal operation, pH is naturally maintained by the bicarbonate buffer system. If the equilibrium is altered and too much organic acid is formed, the conditions become unfavourable for methane forming bacterial to survive and methane formation stops.
3.3 Loading Rate: It is the amount of waste added per day per unit volume of digester. Available data indicates that the process is smooth with organic loading rate of 1.6 to 3.2 kg Volatile Solids/day-m3(digester volume).
3.4 Total Solid Concentration of Feed Slurry: It is the percentage of solid content in the influent slurry after mixing with water. The optimum solid concentration is 8-10%. It affects the digestion.
process and the flow of slurry inside the plant. When the substrate is in fluid state, it becomes easier for bacteria to come in contact with it. Thus the digestion process is accelerated.
3.5 C/N ratio: Most important is Carbon Nitrogen weight ratio. It is the ratio of Carbon to Nitrogen by weight, in the digestion mixture. It depends on the feed material used. Different feed materials have different C/N ratios. The recommended C/N in the digestion mixture is 20:1. Higher C/N increases the ammonia in the digester which is very toxic for the microorganisms. It has been observed that concentration of ammoniacal nitrogen above 5000 mg/litre is toxic and ceases methane formation. Although gas formation decreases as the level goes above 3000 mg/litre. Ammonia is also food for microorganism and its optimum value is 500-3000 mg/litre. Lower C/N decrease the ammonia content and also adversely affects the microbial activity.
3.6 Hydraulic Retention Time:. The hydraulic retention time (HRT) of any biogas plant is the time period for which the digestion mixture stays in the digester to produce biogas. HRT of any biogas plant is fixed on the basis that about 80% of the total potential of biogas is recovered from digestion mixture. The design of a biogas plant depends upon its HRT that depends on the atmospheric temperature.
The recommended HRT(s)for different zones of the country are as under:
- Plants based on 30 days HRT: This is recommended for states of Andhra Pradesh, Goa, Kerala, Karnataka, Maharashtra, Pondicherry and Tamil Nadu.
- Plants based on 40 days HRT: This is recommended for states of Bihar, Gujarat, Haryana, Punjab, Jammu, Madhya Pradesh, Orissa, Rajasthan, Uttar Pradesh and West Bengal.
- Plants based on 55 days HRT: This is recommended for states of Himachal Pradesh, North-eastern states, Sikkim, Kashmir area of J & K state, hilly districts of Uttar Pradesh and other hilly areas having long, severe winters.
3.7. Toxic substances: These are the materials which may harm the bacteria through their toxic effects. Common toxic substances are heavy metals like zinc, copper, iron, nickel, etc., pesticides, detergents, phenyl, sulphides and ammonia.
3.8 Types of Feed stock: It is the raw material fed into the plant. The most popular feed material is cow dung. Other materials such as buffalo, pig and sheep wastes, poultry droppings, human excreta, vegetable wastes from household, organic materials from farms can also be used. A proper mixture of different materials should be used to get the ideal C/N ratio of 20-30:1.
3.9 Initial inoculum: It is the bio-digested slurry collected from an already well stabilized digester which has rich bacterial population. It is added to the influent slurry to start digestion process in a new plant to be commissioned. It accelerates the digestion process. Around 10% base on the weight of the digestion mixture is recommended as initial inoculums.
4.Types of biogas plants
The commonly used biogas plants in the country are:
Floating drum type Fixed dome type
4.1 Floating-Drum Type Biogas Plants
In this biogas plant, the digester is made below the ground with inlet and outlet pipes on the sides of a partition wall. The gas holder is a steel drum which is inverted and placed in the digester that moves up and down depending on gas generation. The pressure on the gas produced by the gas holder makes it to flow to the utilization point along the gas pipeline. The common example of floating drum type biogas plant is KVIC (Khadi Village and Industrial Commission) plants. The different components of KVIC biogas plants are shown in Fig. 1.
Source: http://www.sahanipower.com/architectural-framework/
4.1.1 Characteristics of KVIC Biogas Plants
- The generation of gas is at constant pressure and it has higher efficiency.
- The life of digester is on an average of 30 years.
- No skilled labor is required for the construction of digester
- Requires less excavation work during installation
- The defects and leakages in gas holder can be easily trace out and can be solved But the limitations of floating drum biogas plant are
- High initial cost of installation
- High maintenance cost.
- Workshop facility is needed for fabrication of gasholder.
- The life of gas holder is short , i.e. around 5-7 years.
- The moving drum is exposed over the ground and so the above ground space cannot be used for other activities.
- The winter temperature effect gas generation inside the digester.
4.2 Fixed-Dome Type Biogas Plants
In this biogas plant, the digester and the gas holder is fixed. The common models are:
- Janta Model Biogas Plants
- Deenbandhu Model Biogas Plants
4.2.1 Janta Model Biogas Plant
- This biogas plant is completely a brick masonry structure. The digester has a dome shape top which act as gas holder. The inlet and outlet pipes are connected to digester. The outlet discharges spent digested slurry to the displacement chamber. The limit of size of Janata Model Biogas Plant is 15 m3 per day. The Schematic diagram of a Janta biogas plant is shown in Figure 2.
4.2.2 Deenbandhu Model Biogas Plant
The Deenbandhu model biogas plant was made with the principle that by reducing the surface area installation cost can be reduced without affecting the biogas plant efficiency. This model has two spheres joined, one segment act as digester and other is gas holder. The spent digested slurry is discharged to outlet displacement chamber. The size of these plants is recommended up to 6 m3 per day. The different components of Deenbandhu Model Biogas Plant are shown in Figure 3.
Earlier fixed dome type is commonly used as family size gas plants and floating drum type as the community and Institutional type biogas plants especially KVIC floating drum type. KVIC type family size plants were also available.
4.2.3 Characteristics of Fixed Dome Type Biogas Plant
- Relatively cheaper
- The maintenance cost is low due to absence of moving parts. No metallic parts are required.
- The plant has high life expectancy
- The plant is completely below the ground and the above space can be utilized. There is less impact of low temperature due to lack of any metallic structure.
- But the disadvantages of fixed dome plant are
- The biogas is produced at variable pressure and so reduced efficiency of gas appliances.
- The skilled labour is required (in construction of dome)
- The land need to be excavated more for installing the plant
- Difficulty in doing the repair work.
5. Biogas plants are also classified on the basis of the plant capacity as follows:
- Family size biogas plants: This type of biogas plants are installed at individual family level. The size of these plants is recommended up to 6 m3 per day.
- Community biogas plants: These types of plants are installed by a Village Panchayat/Municipal Committee for any Village/Town/City. The biogas produced from this type of plant is distributed to the people living in that locality/community. The size of these plants is recommended to be more than 15 m3 per day.
- Institutional biogas plants: These types of biogas plants are installed by an institution such as religious institution like Gurudwara/Mandir/Gowshala or educational institution likes school/college or industry. The biogas produced from these plants is used for the respective institution. The size of these plants is recommended to be more than 15 m3 per day.
6.Characteristics of Biogas:
6.1 Composition: Biogas is a mixture of a number of gases, whose composition is as follows:
i. | Methane (CH4) | 55-65% |
ii. | Carbon dioxide (CO2) | 30-40% |
iii. | Hydrogen (H2) | 1-5% |
iv. | Nitrogen (N2) | 1% |
v. | Hydrogen sulphide (H2S) | 0.1% |
vi. | Oxygen (O2) | 0.1% |
6.2 Calorific Value: Methane is the major combustible constituent in biogas. Caloric value (CV) of methane at normal condition of temperature and pressure is 39.6 MJ/Nm3. The CV of biogas depends on gas composition. Normally the CV of biogas falls in the range of 18-22 MJ/Nm3. It is medium CV value gas compared to other gaseous fuels such as LPG or natural gas.
6.3 Density of Biogas: The density of biogas also depends on the gas composition. The density of biogas is slightly less than air and falls in the range of 1.18-1.23 kg/Nm3.
7.Biogas plant design
Main applications of biogas are thermal application i.e. for cooking. It is also used for engine application i.e. for running an unaltered compression ignition engine in dual fuel mode i.e. diesel and biogas, for running a Spark ignition engine on biogas alone and for lighting purpose. The application norms for these uses are as under and can be used for computing the biogas requirement.
Cooking | 0.4 Nm3/Adult Person-day |
Lighting | 0.12 Nm3/h for a 100 Cd Intensity lamp |
Engine (SI) | 0.5 Nm3/HP-h or 0.75 Nm3/kWh |
Other parameters for designing the Biogas Plant are assumed on the basis of the reported data in the literature
Collectable cow dung | 15 kg/Cow-day |
Total Solids content in fresh dung | 16% |
Bio-gas production from cow dung | 0.04 Nm3/kg fresh dung |
HRT (for Punjab) | 40 days |
Dung-Water ratio in Digestion mixture | 1:0.8 |
Density of Digestion mixture | 1100 kg/m3 |
Volume of gas holder | 50% of plant capacity |
Vol. of digester | 20% excess of Design Value |
Example: A biogas plant is to be designed for meeting the cooking requirement of a family of 6 adult members and running an engine-generator of 5 kW capacity for 4 hours a day using cow dung as the feed stock. Find out;
- Quantity of Bio-Gas required per day
- Quantity of cattle dung, no of cows, required
- Volume of digester
- Volume of gas holder
Solution:
8.Application of biogas
Biogas is a gaseous fuel with a heating value of around 20 MJ/Nm3. It produces self-sustaining smokeless flame on ignition. It can therefore, be used for following applications:
- Kitchen fuel for cooking through a specially designed biogas burner. A KVIC Designed Biogas Double deluxe burner having a capacity of 450 Litres/HR for each burner for Commercial is shown in Figure 5.
- It can totally replace liquid petroleum fuel (Petrol) in spark ignition engines.
- It can be used to run any CI engine under dual fuel mode (biogas & diesel). In dual fuel operation diesel replacement of up to 80% can be achieved with making any major alteration of the existing diesel engine. Engine derating of around 10% is observed in the engine operation.
- It can be used for lighting purpose.
- It can also be used as industrial intermediate feed stock.
- After removing Carbon dioxide, the gas can be filled in cylinders and can be used as replacement of CNG.
9.Bottling of Biogas
Since biogas contains 55-65% methane and 35-40% carbon dioxide, it cannot be liquefied like liquefied petroleum gas (LPG) under normal pressure and temperature as the critical temperature for liquefaction of methane is -82.1 ºC at 4.71 MPa pressure. In such situations, removing carbon dioxide and compressing and filling it into cylinders makes it easily usable for Automobiles applications, three wheelers, cars, pick up vans etc. and also for stationary engines for various applications at remote locations. Since CNG technology has already been in use, therefore, bio-methane (upgraded biogas) which is nearly same as CNG, can be used for all applications for which CNG is being used.
Carbon dioxide and other water soluble gases can be removed through high pressure water scrubbing. If biogas is bubbled through water at pressure of approximately 3400 kPa, the Carbon dioxide is dissolved. It may be followed by chemical absorption to get CO2 free gas if required.
The upgraded biogas (free from CO2) has its calorific value as 35-36 MJ/Nm3 against 20 MJ/Nm3 of raw biogas. Upgraded biogas can be successfully used as a substitute of CNG in internal combustion engines.
10.Application of digested slurry
The spent biogas slurry is the digested slurry coming out from a biogas plant. It is a valuable material as a manure and as a soil conditioner. It is useful in agricultural production. The following are the various applications of digested slurry
Organic Manure
Composting material Humus
Used in seed coating Vermiculture
Aquaculture
Value added products
CONCLUSION
- Animal wastes, human excreta, crop residues etc. can be converted to Biogas through anaerobic digestion process.
- Biogas can be used as fuel in domestic sector for cooking, fuel for internal combustion engines for mechanical work or for electricity generation and also for lighting purpose in remote areas.
- Effluent slurry from the plant is rich in nutrients than fresh dung and is thus a very good fertilizer and soil conditioner.
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