BIOGAS REVIEW >>

Biogas is the CH4/CO2 gaseous mix evolved from digesters. To utilise this gas, the digesters are constructed and controlled to favour methane production and extraction

Simple oil drum batch digester Indian ‘gobar gas’ digester

Chinese ‘dome’ for small-scale use

Some organic material, e. g Some organic material, e.g. lignin, and all inorganic inclusions do not digested in the process. Properties Molecular formula CO2 Molar mass 44.01 g·mol−1 Appearance Colorless gas Odor Odorless Density 770 kg/m3 (liquid at 56 atm and 20 °C) 1.977 kg/m3 (gas at 1 atm and 0 °C) Properties Molecular formula CH4 Molar mass 16.04 g·mol−1 Appearance Colorless gas Odor Odorless Density 0.656 g/L at 25 °C, 1 atm 0.716 g/L at 0 °C, 1 atm

How much gases are produced for every kg of raw material ?

44 16 162 3(1/162) 3(1/162) 1/162 3(1/162)x44 3(1/162)x16 3(1/162)x44 1.98 3(1/162)x16 0.7 0.4 0.4

The temperature ranges (1) psicrophilic, about 20 0C, (2) mesophilic, about 35 0C (3) thermophilic, about 55 0C. In tropical countries, the digestion is psicrophilic, with average temperature btw 20 - 30 0C. (retention times at least 14 days). In colder climates, the process is slower, the digesters is heated by using part of the biogas output. Few digesters operate at 55 0C (unless the purpose is to digest material rather than produce excess biogas).

BIOGAS biochemical processes hydrogenesis. Insoluble biodegradable materials, e.g. cellulose, polysaccharides and fats, are broken down to soluble carbohydrates and fatty acids ( about a day at 25 0C in an active digester). acidogenesis Acid forming bacteria produce mainly acetic and propionic acid ( takes about one day at 25 0C). methanogenesis Methane forming bacteria complete the digestion to a maximum ∼70%CH4 and minimum ∼30%CO2 with trace amounts of H2 and perhaps H2S ( about 14 -20 days at 25 0C)

The methane forming bacteria are sensitive to pH, so conditions should be mildly acidic (pH 6.6–7.0) but not more acidic than pH 6.2. Nitrogen should be present at 10% by mass of dry input and phosphorus at 2%.

Question example: During anaerobic digestion, glucose is transformed into methane through a series of steps. The overall reaction is (CH2O)6 → 3CH4 + 3CO2. Calculate the percentage of methane produced.

ANSWER: Percentage >>> original is 100% MW :. 180. 16 ANSWER:  Percentage >>> original is 100% MW : 180 16 44 Moles: 100/180 (100/180)x3x16 (100/180)x3x44 26.67 73.33 CH4 = 27 % CO2 = 73% For every 100 kg of glucose, it will produce 27 kg of CH4 and 73 kg CO2

BIOGAS : Digester volume The volume of fluid in the digester is m0 the mass of dry solid the density of dry matter in the fluid , =(∼50 kg/m3). The volume of the digester is tr is the retention time in the digester (∼8–20 days).

Gas volume & energy output c is the biogas yield per unit dry mass (0.24 m3/ kg) The volume of biogas is Energy output Hb the heat of combustion per unit volume biogas (20MJ/m3

Typical parameters of animal waste

Example -1 By using the table given, calculate the volume of a biogas digester suitable for the output of 6000 pigs, assuming a retention time of 20 days.

ANSWER

(2) Calculte the power available from the digester of example 1. (assume a burner efficiency is 0.6. Biogas volume : Energy output 1 J = 2.8. 10 -7 kwh