What is a Biogas Plant?

The question of "What is a Biogas Plant?" is becoming a popular question as biogas plants, also known as anaerobic digestion plants, become more common, and people hear about them talked about through various routes.

We set about answering the question by making a video in which we go further to describe what goes in inside a biogas facility, stage by stage.

The video which follows is the result of that exercise in providing information, and the video also explains upgrading biogas to become biomethane, because this is becoming such a common feature of many UK biogas plants:

In case for any reason you are unable to watch the video, we have copied the text from the video below:

What is a Biogas Plant? (Transcript)

Biogas is mostly methane gas, and when methane burns it produces a clean heat source, without harmful by-products. It is also a "renewable" energy, and because it is renewable it is also said to be "green" and eco-friendly.

The best way to understand what a biogas plant is, is to know what happens inside a biogas plant, which we will now explain.

1 - Organic material arrives at the biogas plant.

These are also known as anaerobic digestion facilities. The organic material delivered may include animal manure, food waste, agricultural residues, or wastewater solids (sludge).

The organic materials are the "input", or "feed" for the biogas plant. Each individual biogas plant is tailor-made for the feed materials which it will accept. The accepted materials may be from just one source or a combination of several types of organic food source.

2 - Organic material is broken down in a "digester".

The digester is a big tank or multiple tanks for wet digestion, (where the biomass is mixed in water).

Or, the digester may be a long tube, for plug-flow digestion.

And, sometimes (for a dry digester which operates on biomass solids), the digester reactor vessel is configured like a tunnel.

A tunnel type dry digester may be rounded, with large paddles to move the feedstock through the digester.

Or, the digester may have a flat floor for batch-by-batch production, and be large enough for feedstock loading and unloading vehicles to work in.

The digester is always airtight, and is usually equipped with mixing and heating equipment.

Naturally occurring microorganisms grow in the zero-oxygen environment.

They use the organic matter as their food, and break down (digest) the organic matter.

At most AD plants the digester is continuously fed with organic materials (the feedstock) and biogas, and the remaining liquid and solids, are also simultaneously discharged.

3 - Raw biogas is produced.

The biogas flows out of the digester continually and is made of mostly methane. It also contains carbon dioxide, water vapour, and small amounts of what are known as, "trace" compounds.

"Natural gas" which is made from fossil fuels is methane like biogas, the difference is that natural gas contains none of the impurities we just mentioned, two slides ago.

The most damaging impurities, if not removed from the biogas, are usually hydrogen sulphide, and siloxanes.

Biogas can replace "natural gas", but first it must be processed to as far as possible, remove everything but the methane.

The degree of processing which takes place in the anaerobic digestion plant varies according to the use intended for the upgraded biogas.

Upgraded biogas of the highest quality is called "biomethane" or "renewable natural gas".

4 - After digestion in the Anaerobic Digestion Plant (Biogas Facility), which can take anything from 5 days to 60 days, the biogas is used in one of many ways.

Biogas may be used to: Produce heat, electricity, vehicle fuel. Or, it may be used for injection in the gas grid (natural gas pipelines), after it has been upgraded to become "biomethane".

5 - However, the biogas accounts for only a small proportion of what is left, after digestion. 

The digester residue is known as "digestate" comprising solid fibres and a liquid.

The solids and liquids can be used to provide marketable products, such as fertiliser, compost, soil amendments or animal bedding.

These are called "co-products", and can be sold for a profit.

We hope that you now have a clear understanding of what a biogas plant does.

If you would like to read more, we suggest the following sources.

For the original American Biogas Council article upon which this was based, go to https://anaerobic-digestion.com/abiogasc

For our detailed article, visit our A.D. website at: https://anaerobic-digestion.com/anaerobic-digestion-basics

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