Fuels - Part II

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Bio gas : Bio gas is made from organic waste matter after it is decomposed. The decomposition breaks down the organic matter, releasing various gases. The main gases released are methane, carbon dioxide, hydrogen and hydrogen sulphide. Bacteria carry out the decomposition or fermentation. The conditions for creating bio gas has to be anaerobic that is without any air and in the presence of water. The organic waste matter is generally animal or cattle dung, plant wastes, etc. These waste products contain carbohydrates, proteins and fat material that are broken down by bacteria.  The waste matter is soaked in water to give the bacteria a proper medium to grow. Absence of air or oxygen is important for decomposition because bacteria then take oxygen from the waste material itself and in the process break them down.

There are two types of bio gas plants that are used in India. These plants mainly use cattle dung called “gobar” and are hence called gobar gas plant. Generally a slurry is made from cattle dung and water, which forms the starting material for these plants. The two types of bio gas plants are :
1. Floating gas-holder type
2. Fixed dome type

Floating gas holder type of plant : The diagram below shows the details of a floating gas holder type of bio gas plant.  

A well is made out of concrete. This is called the digester tank T. It is divided into two parts. One side has the inlet, from where slurry is fed to the tank. The tank has a cylindrical dome H made of stainless steel that floats on the slurry and collects the gas generated. Hence the name given to this type of plant is floating gas holder type of bio gas plant. The slurry is made to ferment for about 50 days. As more gas is made by the bacterial fermentation, the pressure inside H increases. The gas can be taken out through outlet pipe V.  The decomposed matter expands and overflows into the next chamber in tank T.  This is then removed by the outlet pipe to the overflow tank and is used as manure for cultivation purposes.

Fixed dome type of plant : The diagram below shows the details of a fixed dome type of bio gas plant.  

A well and a dome are made out of concrete. This is called the digester tank T. The dome is fixed and hence the name given to this type of plant is fixed dome type of bio gas plant. The function of the plant is similar to the floating holder type bio gas plant. The used slurry expands and overflows into the overflow tank F.  

Advantages and disadvantages of both the bio gas plants :

  • In the floating gas holder type of plant, the floating chamber is made of stainless steel. This is expensive and needs continuous maintenance and supervision for non-rust. This does not arise in the fixed – holder type of bio gas plant as everything here is made of concrete.

  • Fixed dome type of bio gas suffers from a disadvantage that its volume is fixed. So if the gas pressure increases inside, it may cause damage to the concrete dome. This does not happen in the floating holder type of bio gas plant.

Uses of bio gas :

  • Bio gas is used as cooking fuel.  This is because bio gas burns without smoke, has high calorific value, can be piped into kitchens directly from a plant and is cheaper in cost.

  • Bio gas can be used to run electric engines such as pumps, as they cause less air pollution.

  • Bio gas can be used for street lighting as they do not cause any smoke and the illumination obtained can be made to be quite adequate.

 

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