27 Energy from waste – Biogasification

Dr. Yogalakshmi K. N

Objectives:

1. To understand the process of biogas generation

2. To study about the types of biogas plants used in biogas generation

3. To gain knowledge about the factors influencing the biogas generation

4. To realize the merits and demerits of the biogasification process

5. To gain knowledge about the limitations of biogas technology

1.0 Biogas

Gas produced during the anaerobic degradation of organic matter, manure or waste material is termed as biogas. Biogas comprises of methane, carbon dioxide and other trace gases produced by the process of biogasification. Biogasification is also termed as methane fermentation. In earlier times, biogas was produced from cow dung and decayed plant matter. Additionally, a variety of other feedstocks such as cow dung, cattle dung, dead plant and animal material, food waste, kitchen waste, biomass, sewage, manure, green waste is also digested in digester for biogas generation. Large scale application of biogasification process was used for the treatment of wastewater sludge. The application of biogasification shifted to municipal solid waste since 1930s and 40s with the purpose of stabilizing the waste and to recover energy from the organic fraction of waste material. Municipal solid waste contains abundant chemical energy in the form of carbon which can be efficiently used as a fresh source of energy. Moreover, biogasification will also reduce the amount of waste destined to the landfills thereby increasing the life of landfills.

2.0 History of biogas

Flickering lights under the surface of swamps due to the gas generated during the decay of dead organic matter led to the discovery of biogas in the 17th century by Van Helmot. In 1776, Volta reported that the biogas can be produced by decaying plant material and is flammable at specific conditions. The biogas generation from cow dung by anaerobic digestion at 35 ⁰C was first started by Guoan in 1884. He was successful in producing around 100 L of biogas per cubic meter of cow dung. The biogas plants started becoming popular in 1970s and 80s when biogasification schemes were proposed by government for the treatment of night soil and animal manures.

3.0 Process description

The anaerobic digestion occurs in four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. The organic components of the waste which is generally represented by carbohydrates, proteins and lipids are degraded by the action of anaerobic microorganisms. In addition to the biological process certain interrelated physical and chemical processes also occurs in the digester. The first step of degradation (i.e.) hydrolysis uses facultative anaerobes and converts the carbohydrates into sugars; proteins to amino acids and ammonia via de-aminisation; and lipids to fatty acids and glycerol. The gas production at the stage can rise upto 80% carbon dioxide and 20% hydrogen.

Figure 1 Anaerobic digestion process

The second stage converts the organic acids also termed as volatile fatty acids into acetic acids and its derivatives, carbon dioxide and hydrogen. This stage is generally terms as the acid stage that involves acetogenic bacteria. Some microorganism converts carbohydrates into acetic acid in the presence of carbon dioxide and hydrogen. The end of the stage is marked by the decrease in hydrogen and carbon dioxide levels. The last stage of anaerobic digestion, methanogenesis plays an important role in generation of methane gas. It is the phase during which methanogenic bacteria convert the acetic acids and its derivative into carbon dioxide and methane. The methane generating microorganisms are promoted during low hydrogen levels. Two groups of microorganisms are active during this stage: mesophilic bacteria which are active in the temperature range of 30-35⁰C and thermophilic bacteria in the range of 45-65 ⁰C. The gas produce during the methanogenesis stage comprises of 60% methane and 40% carbon dioxide.

3.1Microorganisms involved in biogasification

Biogasification involves a mixed population of microorganisms. The successful biogas generation occurs due to the synergistic action of the microorganisms. Several microorganisms are involved in the process of gas generation. Hydrolytic bacteria are mostly used for breakdown of complex substances especially cellulose and hemi cellulosic fibers into simple substances. Among the microbial population, 90% population is acidogenic bacteria. They degrade the polymers to acids and acetates to facilitate easy utilization of substrate by methanogens. Acid-formers grow vigorously and tolerate a wide variety of environmental conditions. Due to their wide tolerance, they rarely act as a rate-limiting factor in biogasification. Unlike the acid-formers, the methanogens are slow growers and require less nutrition. Simple organic compounds are sufficient to provide nutrients for methanogenic bacteria. They rely on polymer and acid stage for carbon and ammonia for nitrogen needs. Methanogens are more sensitive to environmental factors. Atmospheric oxygen is one main factor that affects the methanogens even at low concentrations. Likewise, nitrites and nitrates also inhibit the growth of the methanogens. Optimum pH level required for the survival of methanogens is 7. However, the tolerance level extends from pH 4.5 or 8.0.

ethane producing bacteria (methanogens) transforms the acids by two types of reactions: fermentation of short-chain fatty acids and some alcohols; and a respiration in which H2, CO2, and certain simple organic compounds are oxidized anaerobically, coupled with the reduction of CO2 to CH4.

Acetic acid:

CH₃COOH →CH₄+CO₂

Methyl alcohol:

4CH₃OH →3CH₄ +CO₂ + 2H₂O

The production of CH4 through respiration involving the incomplete oxidation of alcohol to acetic acid, coupled with the reduction of CO2 to CH4, can be exemplified by the reaction by Methanobacterium omelianski.

2CH₃CH₃OH +CO₂ → 2CH₃COOH +CH₄

The reduction of CO2 with molecular hydrogen is:

4H₂+CO₂ → CH₄ + 2H₂O

4.0 Products of biogasification

4.1 Composition and Properties of biogas

The biogas is composed of 55% to 65% methane and 34% to 44% is carbon dioxide. H2S, N2, and H2O are other gases present in the biogas. Methane, hydrogen and carbon monoxide can be used as a fuel. The heating value of raw biogas ranges from 18,630 to 26,080 kJ/m3. The biogas should be free of hydrogen sulphide to avoid corrosion in the engine. The biogas can be cleaned, compressed and used as natural gas. Biogas is termed a methane when they are cleaned and brought to natural gas standards.

Table 1 Composition of biogas

Component	Concentration (by volume)
Methane	55-60%
Carbon dioxide	35- 40%
Water	2-7%
Hydrogen sulfide	20-20,000 ppm (2%)
Ammonia	0-0.05%
Nitrogen	0-2%
Oxygen	0-2%
Hydrogen	0-1%

Table 2 General features of biogas

Energy content	6- 6.5 kWh/m3
Fuel equivalent	0.6- 0.651 oil/m3 biogas
Explosion limit	6- 12 % biogas in air
Ignition temperature	650- 750℃
Critical pressure	75-89 bar
Critical temperature	-82.5℃
Normal density	1.2 kg/m3
Smell	Bad eggs

4.2 Residues

The digested sludge left after the release of combustible biogas make up the residue of biogasification process. The digested sludge consists of two major constituents: supernatant and settled solids (sludge). The supernatant contains a mixture of suspended colloidal solids and bacteria in suspension in aqueous phase. Since these solids are biodegradable they are unstable and hence need proper treatment before discharge into the environment. Generally, a portion of the supernatant is either used in slurry preparation or recirculated to digester. Recirculation is done to maintain the microbial population inside the reactor, complete degradation of waste and utilization of nutrients. The remaining supernatant can be discharged into the land. Thick solid sludge is either dewatered or spread on to a sand bed to concentrate the solid. The dried solids are used in agriculture.

5.0 Types of biogas plants

Three types of biogas plants are popularly used for the generation of biogas. They are

Balloon type bio-gas plants
Fixed- dome type of biogas plant
Floating gas holder type of biogas plant

5.1 Balloon type plants:

The balloon digester is made up of heat-sealed plastic or rubber bag with an inlet and outlet. The material used in balloon construction should be weather and UV resistant. Reinforced plastic, RMP (red mud plastic), Trevira and butyl are the materials by which balloon type biogas reactor is made. The balloon functions as a digester and gas holder. It acts like a fixed dome plant when the gas space inside the balloon becomes full. The movement of balloon skin helps in agitation of digester contents. Agitation favors good gas production. The balloons must be provided with safety valves to avoid damage of the balloon skin. At increased gas pressure, the balloon skin gets damaged. The total life of the balloons is around 2-5 years.

5.1.1 Advantages of balloon type biogas plant:

Cheap and economical Low fabrication cost ease of transportation

These installations are shallow and hence suitable for use in areas with a high groundwater table Cleaning, emptying and maintenance of these digesters is easy

Feedstock that are difficult to digest are also used in this digester

5.1.2 Disadvantages:

Gas pumps are required during low gas pressure conditions
Scum formed inside the digester cannot be removed during operation
Life span of plastic balloon is very short
Balloons are more susceptible to mechanical damage Balloons are not locally available
Balloon plants are recommended in places where the temperature is even and high

5.2 Fixed dome type biogas plant:

Fixed dome biogas plants are cemented structure with central digester tank fitted with an inlet and an outlet chamber. The feed is mixed with required amount of water in a mixing tank which is generally present above the ground level. After mixing the contents (slurry) are fed into the digester through an inlet chamber. The inlet chamber is sloppy and is generally constructed underground. The digester is a huge tank with a dome like ceiling fitted with an outlet valve. The biogas generated is collected through this outlet. The digester contents termed as spent slurry is pumped out through an outlet chamber into an overflow tank. The reactor is operated under batch condition, where the digester is partially filled with slurry and left idle for two months. During this period, the anaerobic bacteria hydrolysis the waste and ferments them. Subsequent to fermentation, acidogenic and methanogenic bacteria decompose the waste and generate biogas. The biogas generated during the process gets accumulated in the dome. As more and more gas accumulate in the dome, pressure build inside the system. Due to this pressure, the biogas forces the spent slurry into the overflow tank. From the overflow tank, the slurry is removed manually. The reactor should be continuously fed to obtain continuous gas supply.

Figure 2 Fixed dome type biogas plant

The volume of fixed dome biogas plant is around 5 – 200 m3 and the biogas volume generated is not higher than 20 m3. The pressure inside the digester ranges between 60-150 mbar.

5.2.1 Advantages:

Cheap and economical
Can be constructed easily
The plant is cheaper than the floating drum plants by 20-30%
The durability of the plant is more than 20 years.
Cost of construction and maintenance is low

5.2.2 Disadvantage:

Requires more excavation work low reliability

5.3 Floating Gas Holder Type Biogas Plant:

The floating gas holder type biogas plant consists of two chambered (inner and outer chamber) underground digester fitted with two long cement pipes: an inlet and outlet pipe. Similar to fixed dome type, floating gas holder biogas plant also consists of a mixing tank where the feed stock is made into a slurry by mixing it with required quantity of water. Mixing tank is always located above the ground level. From the mixing tank the slurry is fed into the digester through the inlet pipe. The slurry is left in the digester for two months for decomposition of waste and biogas generation. The gas generated is collected in a drum like structure inverted above the digester. The drum is usually made of steel. Unlike the fixed dome biogas plant, the gas holder in floating type is not fixed and it keeps moving up and down depending on the amount of biogas generated inside the digester. The drum floats over the digester up to a certain level. The gas holder has a gas outlet through which gas is collected and used. The gas builds pressure and pushes the digested slurry to the outlet chamber tank through inlet chamber. When the outlet chamber is overfull with spent slurry the excess is sent to overflow tank through outlet pipe.

5.3.1 Advantages:

Requires less excavation work
Constant gas pressure simplifies the digester design The system is highly reliable

Source: http://www.nathmotors.com/biogas-types.html

Figure 3 Floating gas holder type biogas plant

5.3.2 Disadvantages:

Expensive. High capital and maintenance cost
The drum tends to rust
This type of reactor requires higher maintenance
The durability of the digester is low

6.0 Factors influencing biogas generation

The factors that influence biogas generation are discussed below:

pH: The pH plays a significant role in generation of biogas. During the process of biogas generation, pH is continuously altered by the different processes involved inside the reactor. During fermentation, when huge amount of organic acids is produced, the pH in the digester drops below 5. At pH below 5, the methane formation is inhibited. The volatile fatty acid measured as acetic acid should be below 2000 mg/L. The pH further rises at the end of acetogenesis process to 7.2- 7.5, making the condition favourable for the growth of methanogenesis bacteria. At certain times, increasing ammonia concentration acts as a buffer to increase the pH up to 8.2. Thus, to achieve high methane production, the pH of the feedstock is always should be between pH 6 and 7.
Temperature: Like pH, temperature also greatly influences methanogenesis. The rate of methane production increases with increase in temperature. Increased temperature also causes the accumulation of free ammonia inside the reactor. The digester should be operated in the mesophilic and thermophilic range. The process of fermentation and the growth of methanogens are enhanced at the temperature range of 29°C to 41°C or 49°C to 60°C. The temperature fluctuation inside the reactor should not exceed than 1℃ for mesophils and 0.5℃ for thermophils.
Pressure: The pressure required for enhanced biogasification is 1.1 to 1.2 bar absolute.
Loading rate: Loading rate is defined as the amount of raw material fed per unit volume of digester per day. Loading rate of around 0.2 kg/m3 of digester capacity is recommended. Overloading or over feeding might inhibit the methane production. Hence it is essential to optimize the solid waste loading rate in the digester. Limited or low feeding results in low gas production and make the process economically ineffective.
Solid concentration: Good biogas generation is achieved at solid concentration of 10%. The ratio of solid to water should always be maintained as 1:9. The reactor should be fed at the same time every day to maintain the solid concentration. Moreover, during solid waste digestion, cow dung is also added to the digester in the ratio of 1:1.
Retention time: The time period during which the waste is retained inside the digestion unit for methane production is termed as retention time. The retention time for the biogas digester ranges between from 35 days to 50 days. In batch reactors, the retention time is distinct unlike digester operated in continuous mode. Complete digestion is achieved at longer retention time. Longer the retention time, larger would be the size of digester. Retention time is determined based on two factors: process temperature and substrate type.
C/N ratio: carbon to nitrogen ratio should be around 30:1. Minimum of 2% phosphorous is also required for better enhanced efficiency. It is essential to maintain C/N ratio to achieve good microorganism growth and high gas generation. Any alteration in C/N ratio lead to loss of fertilizer quality and low methane generation. For example, during the conditions of high nitrogen and low carbon, the carbon is consumed faster and the bacterial cells are starved to death due to non availability of carbon. Nitrogen in the dead cells returns back to the solution adding to the nitrogen content.
Nutrients: To achieve better biogasification, nutrients like nitrogen, sulfur, phosphorus, potassium, calcium, magnesium are requiring in addition to copper and oxygen. Trace elements such as manganese, molybdenum, cobalt, zinc, nickel and selenium are other essential nutrients required to facilitate bacterial growth. The municipal solid waste, agricultural residue and other biodegradable waste originally possess these nutrients and hence does not require external source of nutrients.
Water Content: A water content of 90% should be maintained within the digester. Water content depends on the nature of substrate digested in the digester. Too much water content will lead to the reduction of gas yield per unit volume of the reactor. Likewise, at low water concentration, the volatile fatty acids accumulate, reduce the pH and hinder the gas production process. Also, thick scum formation occurs during low water content.
Nature of feedstock: Waste materials rich in carbohydrates such as cellulose and semi-cellulose and sufficient proteins results in high gas production. Polysaccharides preferably generate more gas than proteins.
Reaction Period: The biogas production is determined by diameter to depth ratio of digesters. Diameter to depth ratio of 0.66 – 1 results in better gas production. Around 80-90% of total gas production occurs within 3-4 weeks in digeters operated at optimum conditions.
Stirring of Digester contents: Stirring of digesters will increase the contact of the substrate with the bacteria, thereby resulting in increased gas production and improved fermentation. Stirring also avoids settling of solids and scum formation. Scum formation avoids gas release. This phenomenon is more popular in vegetable waste. Continuous feeding minimizes the problem of scum formation as they break up the charge and provide stirring. Occasional mixing can also be provided. This will mix the masses floating on top to settle down with the deposits.

7.0 Advantages of biogas as a fuel

It can readily mix with the air.
It is light in weight and has high calorific value.
Its thermal efficiency is higher due to uniform distribution. Biogas is a fuel with high octane number.
It causes less pollution and the capital cost of the plant is low. It is used as household fuels.
It is non toxic to skin.
As it produces residue is produced, it is considered clean
It is cost effective fuel and can be easily supplied through the pipe lines. It also generates electricity.

8.0 Advantages of biogas technology

The biogas as a fuel can effectively replace fire wood and cow dung used as fuel, thus aiding in curbing deforestation and soil erosion.
Biogas can also replace fossil fuels and thereby decreases the greenhouse gas and other harmful gas emissions.
Biogas can be utilized in a biogas plant as a source of energy, thereby reducing the harmful impact of methane on the biosphere.
In biogas technology, the waste and cattle dung are placed in a confined space which prevents contamination of surface and groundwater.
The waste material and cattle dung are ultimately transformed into natural fertilizer, which is rich in organic matter and is easily utilized by the crop and plants. This helps in reducing soil depletion.
Biogas technology produces energy in the form of heat, light and electricity. It reduces the contamination due to pathogens, worm eggs and Flies.

9.0 Limitations of biogas technology

High capital cost
Unrealistic
High expectation of users
Solar, micro, hydro and other renewable technologies show positive side than biogas technology

10.0 Biogas generation from solid wastes: Human activity leads to the generation of huge amount of organic waste. These wastes generally include household food waste, agricultural waste, human and animal waste etc. when the organic waste is degraded in the absence of oxygen using biologically active microbes, it produces methane and the process is termed as anaerobic digestion. The main advantage of this process is that the methane produced can be used as fuel and the residue can serve as a manure, as it contains high amount of nutrients.

Table 3 Biogas production from digestion of common wastes

Around 0.037 m3 of biogas per kg of cow dung is produced. The calorific value of biogas is 21000 to 23000 KJ/m3 or about 25200 KJ/kg of gas. Post gas generation, the waste material retains its value as fertilizer. It can also be used as animal feed after certain processing.

Table 4 Quantity of dung required for various plant sizes

11.0 Summary

To summarize, in this module we have discussed about

biogas, its composition and properties process of biogas generation
types of biogas digesters
factors influencing biogas generation biogas generated from waste material advantages and disadvantages of biogas limitations of biogas technology

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Reference

Brian Herringshaw, Ohio State Univercity, M. Tech. Thesis On “a study of Biogas Utilization Efficiency Highlighting Internal Combustion Electrical Generator Units”
Diaz, L.F., G.M. Savage, G.J. Trezek, and C.G. Golueke, “Biogasification of Municipal Solid Wastes”, Journal of Energy Resources Technology, 103:180-185, June 198
Harilal S. Sorathia, Dr. Pravin P. Rathod, Arvind S. Sorathiya (2012) “Bio-gas generation and factors affecting the bio-gas generation – a review study” International Journal of Advanced Engineering Technology
http://www.unep.or.jp/Ietc/Publications/spc/Solid_Waste_Management/Vol_I/17-Chapter11.pdf
http://www.nathmotors.com/biogas-types.html