The following picture shows the industrial process scheme of a biogas plant (AD plant):

How can Biogas be produced?

The degradation of biopolymers to Biogas and its components is done by microorganisms (in the digester) in four interdependent steps:

1. Hydrolysis:

In the first step water-insoluble biopolymers are fractionalized into basic monomer modules or in other dissoluble fragments. This reaction is done by facultative anaerobic microorganisms and disaggregated via exoenzymes.

2. Acidification:

The acidification takes place at the same time as the hydrolysis step (1st step). The low molecular compounds, which were built in the first step, are implemented in short-chain organic acids, alcohols, H₂ and CO₂.

3. Acetogenesis phase:

In the third step organic acids and alcohols are transformed by obligate H2-building acetogenic microorganisms to acetate/acetic acid, H₂ and CO₂, respectively.

4. Methanogenis phase:

Strictly anaerobic methane bacteria build methane and CO₂ from acetic acid (~ 70 %) as well as from CO₂ and H₂ (~ 30%). For methane bacteria these reactions are an important energy source.

Biogas is made from…

Biogenic substrates like liquid manure, sewage, maize, grass, sunflower, biodegradable waste, slaughterhouse waste, vegetable and fruit waste,…..

How can Biogas be used?

There are various possibilities for the utilisation of “Biogas”.

• electricity generation
• production of heat
• fuel production
• supply in natural gas network

The pros and cons of Biogas:

Pros:

• green energy production (electricity, heat)
• substitute of fossil fuels
• reduction of greenhouse gas emissions
• reduction of disposal costs (biodegradable waste as substrate)
• subsidies by the green energy law
• energy savings (covering of own energy demand)
• saving of chemical fertilizer (closing of nutrient cycle)
• increase of plants tolerance
• CO₂ neutral energy production

Cons:

• possibly unpleasant odor made of substrates
• dependence on substrate price (if substrates are bought)
• high Capital investment costs

References:

Biogas Research and Consulting groupe, http://www.codigestion.com/www.codigestion.com (accessed 16 September 2013)