Biogas technology is a renewable form of energy that utilizes various organic wastes in the absence of oxygen to produce combustible mixture of methane and carbon dioxide gases, mineralized water and organic fertilizer (bio-slurry) (Gautam et al., 2009. In other words, the technology recovers biogas by harnessing anaerobic degradation pathways controlled by micro-organisms. The gas is principally a mixture of methane (CH4) and carbon dioxide (CO2), and other trace gases such as hydrogen sulphide (H2S) (Singh and Sooch, 2004; Shin et al., 2005). Methane makes up the combustible part of biogas. It is a colorless, odourless and highly inflammable gas, second only to hydrogen in the energy released per gramme of fuel burnt; hence it’s potential as a household energy source. It has a boiling point of -162°C and burns with a smokeless blue flame and is non-toxic. Methane is also the main constituent (77-90%) of natural gas.
Chemically, methane belongs to the alkanes (saturated hydrocarbons) and is the simplest possible form of these. At normal temperature and pressure, methane has a density of approximately 0.75 kg/m3. Pure methane has an upper calorific value of 39.8 MJ/m3, which corresponds to 11.06 kWh/m3 (Batzias, 2004).
Small-scale domestic biogas is one of decentralized renewable energy technologies (RETs) advocated for rural people. A biogas digester consists of one or more airtight reservoirs into which a suitable feedstock (cow dung, human waste, or any other organic waste) is placed, either in batches or by continuous feed. Small-scale digesters for household use are commonly made of concrete, bricks, metal, or plastic. Larger commercial biogas digesters are made mainly of bricks, mortar, and steel. The end product is the gas, which can be connected to a household stove for cooking, to a light fixture with a gauze mantle for lighting, or to other appliances with simple natural gas plumbing. The gas burns like liquefied petroleum gas.
Various designs of biogas plants and sizes exist throughout the world (Tom and Michael, 2011). The Fixed dome design originated with the Chinese. The main part of the plant is a brick and cement digester in which the organic inputs are collected and anaerobically converted into a combustible gas. Connected to the digester, there is an inlet tank in which the inputs get mixed with water prior to feeding into the digester; and a compensation tank where the by-product (bio-slurry), gradually accumulates until it overflows to a composting pit. The floating drum type plants, (Indian design) have an underground well-shaped digester with inlet and outlet connections through pipes at its bottom on either side of a partition wall (Rijal, 1985). An inverted drum (gas holder), is
placed in the digester, and rests on the wedge shaped support and guide frame at the level of a partition wall. This drum can move up and down along a guide pipe with the accumulation and disposal of gas, respectively. The weight of the drum applies pressure on the gas to make it flow through the pipeline to the point of use (Singh and Sooch, 2004). The Tubular or the low-cost polythene digester is adapted from the Taiwanese model. In such, the substrate flows through a tubular polyethylene or PVC bag (the reactor) from the inlet to the outlet. The gas is collected by means of a gas pipe connected to a reservoir. Most household digesters range between 4 to 16 m3 in size, with a few institutional and community ones, with a capacity of 36 and 54 m3 (Singh and Sooch, 2004).
2.7.1 The Process – Biogas Formation (Methanogenesis)
Biogas is produced by methanogenic bacteria while acting upon biodegradable material under anaerobic conditions. Complete decomposition of organic matter to CH4 and CO2 under oxygen-depleted conditions is complicated and is an interaction between different bacteria. The process takes place in three stages: hydrolysis, acidogenesis and methanogenesis. During hydrolysis (first step) long-chain molecules, such as protein, carbohydrate and fat polymers are broken down to monomers (small molecules). Different specialized bacteria produce a number of specific enzymes that catalyze the decomposition. During the second stage (fermentation/acidogenesis), acid-producing bacteria convert the simplified compounds into acetic acid (CH3CO2H), hydrogen (H2), and carbon dioxide (CO2). Approximately 50% of the monomers (glucose, xylose, amino acids) and long-chain fatty acids (LCFA) are broken down to CH3CO2H. 20% is
converted to CO2 and H2, while the remaining 30% is broken down into short-chain volatile fatty acids (VFA). In this process, the facultative anaerobic bacteria utilize oxygen and carbon, thereby creating anaerobic conditions necessary for methanogenesis.
The third and last step (methanogenesis) is undertaken by the methanogenic bacteria or methanogens. In this stage, the obligatory anaerobes decompose compounds with low molecular weight, (CH3COOH, H2, CO2), to form CH4 and CO2 (Gate, 1999). Two different groups of bacteria are responsible. One group degrades acetic acid to methane and the other produces methane from carbon dioxide and hydrogen. Under stable conditions, around 70% of methane production comes from the degradation of acetic acid, while the remaining 30% comes from carbon dioxide and hydrogen (Gate, 1999).
The bacteria involved in the fermentation process are sensitive to a range of variables that ultimately determine gas production. Temperature is the most critical consideration. Most digesters operate in the mesophylic range (35-40°C), but others are designated to operate in the thermophylic range of 50-60°C, and a few at psychrophylic range (15- 25°C) (Paustian et al., 2006). Loading rate and retention period of material are also important. Other factors likely to affect methanogenesis are pH and nutrient content of the slurry (Forster-Carneiro et al., 2008). The resulting biogas consists of CH4 and CO2, and traces of H2S. Its exact composition vary according to the substrate used in the methanogenesis process, but as an approximate guide, when cattle dung is the main substrate for fermentation, the resulting gas will be between 55-66% CH4, 40-45% CO2, plus a negligible amount of H2S and H2 (KVIC, 1993).