Biogas Plant Design

Introduction:
Biogas is based upon the use of dung to produce gas which is used as domestic fuel especially in rural areas. This technique is based on the decomposition of organic matter in the absence of air to yield gas consisting of methane (55%) and carbon dioxide (45%) which can be used as a source of energy.
Biogas is a gaseous fuel which may be directly burnt in a gas burner for the use as a cooking fuel or source of direct heat for thermal applications. It has a calorific value of 5871 kCal/m³.

Biogas Plant

The plant where biogas is produced is called biogas plant. The design of the plant is tailored to use cattle dung as feed stock. Cattle dung and water is mixed in the ratio of 1:1 and the resultant slurry containing 9% total solids is fed to the biogas plant by gravity. This slurry is retained in the digester for a period of 35-50 days. During this retention period, 35-40 liters of biogas is recovered per kg of dung fed.

Classification of Biogas Plants

Biogas generates from the disintegration of organic substrate and being lighter than air, rises upwards. This gas is collected in various types of drums. Therefore, according to the method of gas storage, biogas plants are of two types:

1. Floating dome biogas plants
2. Fixed dome biogas plants

Floating dome design: This design consists of a tank or a well with a partition wall to prevent shorting of influent fresh dung slurry with the outgoing spent slurry. The gas produced is trapped under a plastic or a metallic drum. With the continuous production more gas is trapped under this bell and the drum rises. This acts as a gas storage unit and when the tap above is released, the gas is discharged at more or less constant pressure.

Fixed dome design: This is designed to reduce the overall cost of the biogas plant. In this, the slurry is fed to a spherical masonry plant. When the gas is produced owing to the rigid nature of the masonry dome, the trapped gas exerts a pressure on the slurry surface and a corresponding amount of slurry is displaced into a wide outlet and inlet. As a result, the pressure of gas stored varies significantly.

 

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