Gaseous Fuels - Biogas and Hydrogen

What is Biogas?
In 1776, for the first time, the Italian Physicist, Volta, demonstrated methane in the marsh gas, generated from organic matter in bottom sediments of ponds and streams. Under anaerobic conditions, the organic materials are converted through microbiological reactions into gases (fuel) and organic fertilizer (sludge). The mixture of gases is composed of 63 per cent methane, 30 per cent CO₂, 4 per cent nitrogen and 1% hydrogen sulphide and traces of hydrogen, oxygen and carbon monoxide. Methane is the main constituent of biogas. It is also referred to as biofuels, sewerage gas, Klar gas, sludge gas, will-o-the wisp of marsh lands, fool's fire, gobar gas (cow dung gas), bioenergy and fuel of the future (Dasilva, 1981). About 90 per cent of energy of substrate is retained in methane. Biogas is used for cooking and lighting purposes in rural sector. It is devoid of smell and burns with a blue flame without smoke.

Biogas Technology in India
Many developing countries are encouraging for installation of biogas plants to meet the demand of fuel. India is one of the pioneer country in biogas technology where biogas research and plant construction has been carried out over the past 30 years. The experiments were initiated at the IARL, New Delhi, in 1939. However, there are many other institutions where research and development programmes are carried out such as Khadi and Village Industries Commission (KVIC) Bombay, the Gobar gas Research Station (Ajitmal, Etawah), the Planning Research and Action Institutes (Lucknow), and National Environmental Engineering Research Institute (Nagpur).

In 1951, for the first time, biogas plants were constructed with the target of 8,000 units before 1973. KVIC played a major role in construction of biogas plants during 1960s. Government of India launched an All India Coordinated Project with the target of 1,00,000 units by 1978, but the number could reach to 50,000 units only.

Up to 1993, Non-Conventional Energy Development Agency (NEDA) of Uttar Pradesh has installed about 100 night soil based biogas plant throughout the state. One of the unique example is the Rajapurwa biogas plant of Kanpur where about 1400 kg human waste from 50-seat toilet complex is pooled through underground pipelines at one place. The waste is processed through a digester into about 55 m³ methane gas. This quantity is enough to run an 8 HP engine and pumpset and to provide fuel to some biogas lamps and cooking burners to about 3500 dwellers. For maintenance of the plant, each family pay some nominal charges.

The sewage plant at Okhla (New Delhi) has 15 digesters of 5665 m³ capacity each, and produces 17,000 m³ gas per day. The gas generated is equivalent to about 10,000 liters of kerosene per day, where 1 m³ biogas has energy potential equal to about 2/3 liters of kerosene.

The Dadar Sewage Treatment Plant (Bombay) produces about 2,800 m² biogas per day from sewage. From this plant pipe lines are connected to about 500 houses, located about 5 km away from the station.

In Sonepat District of Haryana, over 300 biogas units of 2 to 10 m³ capacity have been set up for cooking and lighting purposes. The Haryana Government provided 25 per cent subsidy per plant for setting up the units.

National Sugar Institute (Kanpur) has developed methods for production of biogas from bagasse and other agricultural residues. This plant was built up in 1960s. In this plant, 12 steel digesters, each having capacity of 50 m³, are set up to which about 14 tonnes of mixture of agricultural wastes and cattle manure and 28 tonnes of water are fed to begin the biogas production.

Himachal Pradesh Government set up the National Project on biogas, under which about 12, 292 units in 1987-89, and 3,505 units in 1989-90 were setup.

Government policy is to provide direct subsidy of 25 per cent of the cost of plants and encourage banks to pay loans for remaining costs to aspirants. High construction costs, and cost of cement and steel digesters and rates of interest on loans are some of the factors that discourage villagers to setting biogas plants. It has been pointed out that in villages, support of minimum number of cattle (at least 5) is necessary to run a small home digester plant. Five cattle generate dung to produce about 2m³ biogas plant to meet the demand of cooking and lighting for a family of 4-5 people. In addition, attempts are also being made to develop a small engine powered with methane gas.

Benefits from Biogas Plants
In Asia, biogas is used mainly for cooking and lighting purposes. In addition, there are many other advantages in installing the biogas plants. It is used in internal combustion engines to power water pumps and electric generators. It is also used as fuel in fuel type refrigerators. Sludge is used as fertilizer. The most economical benefits are minimizing enviornmental pollution and meeting the demand of energy for various purposes.

Feedstock Materials
There are two sources of biomass i.e. plant and animal for biogas production. Biomass from plant origin is aquatic and terrestrial ones, derived from various sources (see Sources of wastes). Biomass from animals are cattle dung, manure from poultry, goats and sheep, and slaughter house and fishery wastes. In most of biogas plants cattle dung is used for gas production. There are many cases where gobar gas plants are in operation, and sullage gas production is started. Beside cattle dung (gobar), agricultural wastes containing cellulosic and lignocellulosic materials are also being used.

Biogas Production : Anaerobic Digestion
Anaerobic digestion is described earlier under Anaerobic digestion. Anaerobic digestion is carried out in an air tight cylindrical tank which is known as digester. A digester is made up of concrete bricks and cement or steel. It has a side opening (charge pit) into which organic materials for digestion are incorporated. There lies a cylindrical container above the digester to collect the gas.

In India, single stage digester is set up in gobar gas plant. However, in other countries single-stage, two stage and multistage digester(s) are set up to accomplish digestion at high rate.

A diagram of a single stage digester for biogas/gobar gas production is shown in Fig. 20.2. In biogas plant, a concrete tank is built up which has the concrete inlet and outlet basins. Fresh cattle dung is deposited into a charge pit, which leads into the digestion tank. Dung remains in tank. After 50 days, sufficient amount of gas is accumulated in gas tank, which is used for house-hold purposes. Digested sludge is removed from the basin and is used as fertilizer. Usually digesters are buried in soil in order to benefit from insulation provided by soil. In cold climate, digester can be heated by the installations provided from composting for the agricultural wastes.

Anaerobic digestion is accomplished in 3 stages, solubilization, acidogenesis and methanogenesis. These stages are characterized by 3 groups of bacteria.

Solubilization
It is the initial stage, when feedstock is solubilized by water and enzymes. The feedstock (cattle dung and other organic polymers) is dissolved in water to make slurry. The complex polymers are hydrolyzed into organic acids and alcohols by hydrolytic fermentative bacteria which are mostly anaerobes (Fig. 20.3A).

Acidogenesis
During this stage, the second group of bacteria i.e. facultative anaerobic and H₂ producing acidogenic bacteria convert the simple organic material via oxidation/reduction reactions into acetate, H₂ and CO₂. These substances serve as food for the final stage. Fatty acid is converted into acetate, H₂ and CO₂ via acetogenic dehydrogenation by obligate H₂ producing acetogenic bacteria. There is other group of acetogenic bacteria which produce acetate and other acids from H₂ and CO₂ via acetogenic hydrogenation (Fig. 20.3B I and II).

Methanogenesis
This is the final stage of anaerobic digestion where acetate and H₂ plus CO₂ are converted by methane producing bacteria (methanogens) into methane, carbon dioxide, water and other products (Fig. 20.3C III and IV).

Different species of methanogens are involved in break-down of complex organic matter into acetate or other organic acids. Acetate is one of the substrates of methanogenic bacteria. Hydrogen with CO₂ is a general substrate for methanogenesis. Numbers of these bacteria differ with type of substrates. For example, counts of 10 - 10⁶ per ml and 10⁵ - 10⁸ per ml of hydrogen utilizing bacteria were determined from the pig waste and sewage sludge digesters, respectively (Hobson and Shaw, 1974; Smith, 1966).

(A) Methanogens. Methanogens are a unique group of bacteria. They are obligate anaerobes and have slow growth rate. They play a major role in breakdown of substrate into gas form. They are the only organisms which can anaerobically catabolize acetate and H₂ to gaseous products in the absence of exogenous electron acceptors other than CO₂ or light energy. In their absence, effective degradation would cease because of accumulation of non-gaseous, reduced fatty acid and alcohol products of the fermentative and other H₂ using bacteria that have almost the same energy content as the original organic matter (Me Inerney and Bryant, 1981). In morphology they are of different types such as cocci, bacillii, spirilli and sarcinea. Batch et al (1979) have classified the methanogens as below :

Order 1. Methanobacteriales

Family. Methanobacteriaceae

Methanobacterium formicicum

M. bryantii

M. thermoautotrophicum

Methanobrevibacter ruminantium

M. arboriphilus

M. smithii

Order 2. Methanococcales

Family. Methanococcaceae

Methanococcus vanielii

M. voltae

Order 3. Methanomicrobiales

Family 1. Mathanomicrobiaceae

Methanomicrobium mobile

Methanogenium cariaci

M. marisnigri

Methanospirillum hungatei

Family 2. Methanosarcinaceae

Methanosarcina barken

All the bacteria require H₂ and formate (except M. bryantii, M. thermoautotrophicum and M. arboriphilus) for growth and methane production, whereas M. barkeri requires (besides H₂) methanol (CH₃OH), methyl amine (CH₃NH₂) and acetate for their growth. Thus, the methanogens are either autotrophs or utilize simple organic compounds as formate, acetate, and methyl amine and occupy the terminal position in anaerobic food chain.

(B) Mechanism of Methane Formation. Metabolically the methanogens are very peculiar. Carbon dioxide fixation, Calvin cycle, serine or hexulose pathway are absent in them. The mechanism of methane formation is not well understood. Ralph Wolfe (1979) found that several new coenzymes are involved which are not present in any other group of bacteria. These coenzymes are methyl coenzymes M, hydroxy methyl coenzyme M, coenzyme F420, coenzyme F430, component B, corririoids, methanofuran or carbon dioxide reducing factor and methanopterin and formaldehyde activating factor.

A detailed description of biochemistry and pathways of methane formation is out of scope of this text. Therefore, only reactions are given in this connection. Primary reaction in which carbon monoxide takes part is as below :

CO + H₂O CO₂ + H₂

The secondary reaction takes place in the presence of sufficient hydrogen:

CO₂ + 4H₂ CH₄ + 2H₂O

Other reactions showing methane formation from various substrates are given below:

4CH₃ OH 3CH₄ + CO₂ + 2H₂O
(Methanol)

4HCOOH CH₄+3CO₂ + 2H₂O
(Formate)

CH₃ COOH 12 CH₄ + 12 CO₂-
(Acetate)

Biogas Production from Different Feedstocks
Although there are various types of biomaterials differing in chemical composition, yet gobar (cow dung) is most popular in India. Besides gobar, other materials which would be successful in the coming years are agricultural wastes, municipal wastes, industrial wastes and some of the aquatic biomass.

Salvinia
Significance of Salvinia is described earlier (see Salvinia). This fern can be a feedstock material for biogas production. Fermentation of Salvinia starts within 7-9 days on putting under water in suitable container. Biogas yield is about 0.1 liters /kg fresh weight for 4 weeks. Air dried weed produces about 1 liter/kg for 90 days and fermentation is continued for 3 months. Thereafter, gas yield gradually declines.

Daily requirement of gas is 0.4 liter per capita. Two tonnes of air dried Salvinia, therefore, can meet the fuel requirement of a small family for 3-5 months. Special advantage of using Salvinia is that unpleasant odors do not come out. This will certainly provide cheap and clean burning fuel and reduce the increasing pressure on forests.

Water Hyacinth
This weed can be used for the production of laboratory and domestic fuel. Its decomposition rate is higher than that of cow dung for gas production. Water hyacinth is totally decomposed within 3 days in summer while cow dung takes 8 days.

The ratios between total gas evolved by water hyacinth and cow dung under the identical conditions in summer and winter are 5:3 and 5:1, respectively. Methane production under varied conditions is given in Table 20.3.

Table 20.3. Composition of methane in biogas produced by water hyacinth under the different conditions. *, Chopped water hyacinth; **, Water hycianth blended into slurry; (amendment of Ni (3.95 mg/ kg wet mass) and Cd (12.66 mg/Kg wet mass); (Source : Haque and Sharma, 1980).

Recently, works done at the National Space Technology Laboratory, Florida (U.S.A.) revealed that 1 Kg dried water hyacinth produced 374 liters of biogas in which the proportion of methane was 60-80 per cent. Moreover, one hectare area can produce 600 kg of dry biomass per day which can generate 2,29,400 liters of biogas/ha/day.

In India, works on biogas production by water hyacinth has gained much momentum. Research works are being carried out at the Hissar Agricultural University (Hissar), IIT., (Madras), R.R. Laboratory (Jorhat) and Central Mechanical Engineering Research Institute (CMERI) (Durgapur). CMERI has designed a continuous feed type plant of capacity of 3,00 0 liters gas/day, in which initially 1,800 kg coarsely chopped semi-dried water hyacinth is used. Thereafter, plant is required to feed 40 kg raw materials. Peak of biogas generation period arrives within 13-18 flays. Total available energy from water hyacinth is estimated as 1.334 x 10⁹ KCal per year plus Rs. 2.25 million worth complex (Pawar, 1983).

Gas produced from water hyacinth by methanogens in an air tight tank contains 70 per cent methane and 30 per cent CO₂. Carbon dioxide can be removed before its use for cooking, heating, lithting and source of power for engines, tractors and cars. Increase in recreational value of water bodies from which hyacinth is to be removed can be an additional advantage of exploiting it for gas production. The hyacinth biomass contains substantial amounts of nutrients, the removal of which may keep a water body clean and less eutrophic.

Municipal Sewage

The potential of municipal sewage in India and its future prospects are described elsewhere (see Sources of wastes). New techniques have been developed to make available sullage gas for cooking purpose or industrial activities. The Okhla Sewage Disposal Works (New Delhi) receives 25 m³ sewage per second and generates 18,000 m³ of gas. In energy terms, it is equivalent to about 4.161 million liters of kerosene/year. From the plant, gas pipe lines are given to about 700 families of the surrounding villages. It is hoped, if whole sewage generated from the city is digested anaerobically to yield biogas, it could meet about 20 per cent fuel demand of the city (Sharma, 1984).

Factors Affecting Methane Formation

Following are the factors affecting methane production :

1. Slurry : Proper solubilization of organic materials (the ratio between solid and water) should be 1:1 when it is house hold things.
2. Seeding : In the beginning seeding of slurry with small amount of sludge of another digester activates methane evolution. Sludge contains acetogenic and methanogenic bacteria.
3. pH : For the production of sufficient amount of methane, optimum pH of digester should be maintained between 6-8 as the acidic medium lowers methane formation.
4. Temperature : Fluctuation in temperature reduces methane formation, because of inhibition in growth of methanogens. In case of mesophilic digestion, temperature should be between 30°C and 40°C but in case of thermophilic ones, it should be between 50°C and 60°C.
5. Nitrogen Concentration : Excess amount of nitrogen inhibits growth of bacteria, and thereby lowers methane production. Therefore, use of such materials should be discouraged.
6. Carbon-nitrogen (C : N) Ratio : Improper C:N ratio lowers methane production. Maximum digestion occurs when C:N ratio is 30:1. Amendment of nitrogen or carbon substrates should be done exogenously according to chemical nature of substrates used in fermentation.
7. Creation of Anaerobic Conditions : It is obvious that methane production takes place in strictly anaerobic condition, therefore, the digesters should be totally airtight. In Indian conditions, digesters are buried in soil.
8. Addition of Algae : Ramamoorthy and Sulochana (1989) have found an enhancement in biogas production from cow dung on addition of the green alga, Zygogonium sp. The amount of biogas produced from the alga was twice (344 ml/g dry algae) of that obtained from cow dung (179/g dry cow dung) alone. Also, the duration of gas evolution increased with increasing the proportion of slurry. The caloric value of the gas was 4,800 KCal/ m³ and the percentage of methane was 55.4 per cent. Generally in winter season gas production is lowered to some extent. Therefore, addition of algae holds promise to get biogas in sufficient amount even in winter season also.