Classification of biogas plants depends upon the plants design and mode of working.
Classification of biogas plants depends upon the plants design and mode of working. One common way to classify them is
- Movable type drum plant
- Continuous type plant
- Batch type plant
BATCH TYPE BIOGAS PLANT
Batch type biogas plants are appropriate where daily supplies of raw waste materials are difficult to be obtained. A batch loaded digester is filled to capacity sealed and given sufficient retention time in the digester. After completion of the digestion, the residue is emptied and filled again. Gas production is uneven because bacterial digestion starts slowly, peaks and then tapers off with growing consumption of volatile solids. This difficulty can overcome by having minimum to digester so that at least one is always in operation. This problem can also minimize by connecting batch loaded digester in series and fed at different times so that adequate biogas is available for daily use. The salient features of batch-fed type biogas plants are:
- Gas production in batch type is uneven.
- Batch type plants may have several digesters for continuous supply of gas.
- Several digesters occupy more space.
- This type of plants require large volume of digester, therefore, initial cost becomes high.
- This plant needs addition of fermented slurry to start the digestion process.
CONTINUOUS TYPE BIOGASS PLANT
In continuous type biogas plant, the supply of the gas is continuous and the digester is fed with biomass regularly. Continuous biogas plants may be single stage, double stage or multiple stages. Digestion of waste materials in a single chamber or digester is called single stage process, in two chambers or digester is called multi stage process. In double stage process, acidogenic and methanogenic stage are physically separated into two chambers. Thus, the first stage of acid production is carried out in a separate chamber and only diluted acids are fed into the second chamber where biomethanation takes place. In single stage, acidogenic and methanogenic stage are carried out in the same chamber without barrier. These plants are economic, simple and easy to operate. These plants are generally for small and medium size biogas plants. However, the two stage biogas plants are costlier, difficult in operation and maintenance but they produce more gas.
These plants are preferred for larger biogas plant system. The important features of continuous type biogas plants are:
- Gas production is continuous.
- Retention period is less
- Fewer problems as compared to batch type
- Small digestion chambers are required
MOVABLE DRUM TYPE PLANTS
This also known as floating dome type biogas plant. The conventional movable drum type comprises a masonry digester with an inlet on one side for feeding slurry and an outlet on the other side for removing digested slurry. The gas collects in a steel gasholder which is inverted over the slurry and moves up and down depending upon accumulation and discharge of gas guided by a central guide pipe. This movable gas holder is made of steel. The gas holder is painted by anticorrosive painting at least once in year. This plant helps in consistent pressure which can be adjusted by regulating weight. The main drawback of this is that metal cost is large and maintenance cost is also high. To tackle this problem the scientists have created high density polyethylene.
- Constant gas pressure
- No problem of gas leakage
- Higher gas production
- Scum problem is less