Various Types Of Biogas Plants

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

1. Movable type drum plant
2. Continuous type plant
3. 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.

Advantages:

• Constant gas pressure
• No problem of gas leakage
• Higher gas production
• Scum problem is less

More Efficient Renewable Resource.

New biofuel process generates energy 20 times higher than existing methods, and uses agricultural waste:

A new biofuel production process created by Michigan State University researchers produces energy more than 20 times higher than existing methods. The results, published in the current issue of Environmental Science and Technology, showcase a novel way to use microbes to produce bio fuel and hydrogen, all while consuming agricultural wastes.

Gemma Reguera, MSU microbiologist, has developed bioelectrochemical systems known as microbial electrolysis cells, or MECs, using bacteria to breakdown and ferment agricultural waste into ethanol. Reguera's platform is unique because it employs a second bacterium, which, when added to the mix, removes all the waste fermentation byproducts or non-ethanol materials while generating electricity.

Similar microbial fuel cells have been investigated before. However, maximum energy recoveries from corn stover, a common feedstock for biofuels, hover around 3.5 percent. Reguera's platform, despite the energy invested in chemical pretreatment of the corn stover, averaged 35 to 40 percent energy recovery just from the fermentation process, said Reguera, who co-authored the paper with Allison Spears.

"This is because the fermentative bacterium was carefully selected to degrade and ferment agricultural wastes into ethanol efficiently and to produce byproducts that could be metabolized by the electricity-producing bacterium," Reguera said. "By removing the waste products of fermentation, the growth and metabolism of the fermentative bacterium also was stimulated. Basically, each step we take is custom-designed to be optimal."

The second bacterium, Geobacter sulfurreducens, generates electricity. The electricity, however, isn't harvested as an output. It is used to generate hydrogen in the MEC to increase the energy recovery process even more, Reguera said.

"When the MEC generates hydrogen, it actually doubles the energy recoveries," she said. "We increased energy recovery to 73 percent. So the potential is definitely there to make this platform attractive for processing agricultural wastes."

Reguera's fuel cells use corn stover treated by the ammonia fiber expansion process, an advanced pretreatment technology pioneered at MSU. AFEX is an already proven method that was developed by Bruce Dale, MSU professor of chemical engineering and materials science. Dale is currently working to make AFEX viable on a commercial scale.

In a similar vein, Reguera is continuing to optimise her MECs so they, too, can be scaled up on a commercial basis. Her goal is to develop decentralised systems that can help process agricultural wastes. Decentralised systems could be customised at small to medium scales (such as compost bins and small silages, for example) to provide an attractive method to recycle the wastes while generating fuel for farms.

Worldwide, the market for biofuel production is expected to reach $100 billion by 2018, compared to $35 billion just a decade earlier.