Chemical Reactions in Bio-gas Generator

In the bio-gas generator cattle dung like cow dung, poultry or piggery dropping, and night soil are used for the generation of the gas. The cattle feed is mixed with water to form the slurry, which is sent to the biogas plant. Here the process of fermentation occurs on the slurry due to which the combustible gas is produced that can be used for various cooking and heating applications.

The byproduct of these chemical reactions is the manure, which is far better in quality compared to the green manure produced from the dung by other processes. Thus the cattle dung used in the bio-gas plants is used completely without leaving any wastes, thus helping in a great way keeping the environment clean. To produce the biogas on continuous basis the mixture slurry has to be fed continuously.

The slurry is the mixture of the cattle dung and the water, which are mixed in the mixture in the ratio of 1:1. When there is no air is present in the biogas plants, two type of bacteria act on the slurry: acid forming bacteria and gasifying bacteria. Let us see how both these bacteria act:

1) Acid forming bacteria: The acid forming bacteria are formed due to the liquefaction of the cattle feed. When cattle feed is mixed with water, its liquefaction occurs. The acid forming bacteria are a set of saprophytic bacteria that are produced by the process of extracellular bacteria enzyme. These bacteria can exist, develop and multiply in wide range of conditions. The acid forming bacteria convert carbohydrates, proteins, and fats into too the volatile acids and release carbon dioxide.

2) Gasifying bacteria: After the liquefaction process, the next process is that of gasification, which is carried out by the gasifying or methane bacteria. These bacteria act on the acids produced in the previous phase with the help of intracellular bacteria enzyme and convert them into methane and carbon dioxide. This gas can be removed from the dome of the biogas plant and it can be used for various purposes. The flow of the gas can be controlled by the valve.

The amount of the gas and the quality of the biogas produced from the biogas plant depends on a number of factors. These are: temperature of the substrate, rate of loading of the slurry, concentration of the cattle dung, detention period, pH value of the slurry, nutrients concentration, amount of toxic substance present etc.

The amount of the methane gas produced in the biogas plant depends on the fermentation material used in it. If cattle feed is used in the slurry, the methane gas produced is about 55 to 60% along with 40-45% of carbon dioxide and some quantity of hydrogen sulfide. If night soil is used, the percentage of methane gas can be about 65%.

1) Book: Thermal Engineering by R. K. Rajput