How does a biogas plant work?

Biogas is created whenever organic waste is digested without the presence of oxygen. This happens in so called digesters. These digesters are often part of an entire biogas plant. The terms biogas plant, biogas digester, biogas fermenter, or biogas tank are sometimes used interchangeably. In this article we use the term biogas plant to indicate all the installations used to facilitate the production, capture, and processing of biogas.

As stated, a biogas digester is essential in this process, but there are often times more installations that are used in a biogas plant. Some biogas plants have multiple digesters, some just have one. Other components you might find at a biogas plant could be a reception area, a storage tank for manure, a post-digester, a gas holder, the combined heat and power unit, or the gas upgrading unit. In this article we will explore some of the installations one might find on a biogas plant.

The reception area

A reception area is where the raw organic material is first received and stored, for instance plant material, green waste, or food waste. Sometimes the organic waste needs some pre-treatment to establish the ideal mixture for gas production. This happens in the reception area.

The storage tank

The storage tank for manure can also be found on a biogas plant. This functions somewhat the same as the reception area, but this is only used for manure. When the digester needs input, manure is pumped from the storage tanks into the digester. From here on the manure will be called slurry and it will be digested, creating the biogas.

The digester

In the digester the organic waste (or sometimes called slurry) is mixed, heated up, and sealed from oxygen, to create the ideal circumstances for optimal gas production. The digester is constantly fed with new material. This means there is also a constant outflow of material. To extract as much gas as possible there can be different set ups regarding the digester(s). There can be multiple digester which are connected to collect the overflow of the digestate. These so called post-digesters can vary in size and structure. This is done to extract the optimal amount of gas which could not be extracted with using only the original biogas digester. There can be multiple digesters or multiple post-digesters at a biogas plant.

The gas holder / gas upgrading unit

Like the digester, the gas holder is also an essential element of a biogas plant, since there needs to be a way to store the captured gases. When the gas is stored it can also be upgraded in an upgrading unit These can be integrated within a gas holder. Certain elements such as carbon hydroxide and hydrogen sulfide are removed to purify the gas. Once the gas is needed, it can be retracted to either be added to the natural gas pipelines, produce electrical power, heat, or liquefied natural gas / compressed natural gas.

Combined heat and power unit (CHP)

In a CHP biogas is converted into electricity via a generator. This can be used to provide the power for the biogas plant itself or other facilities. The heat that is generated during this process is also captured and used for different purposes such as heating households, or even absorption cooling.

These are some of the elements you can find on a biogas plant. Not all of the units are always present, and the composition of these units can vary. As said, the digester is essential in a biogas plant. The following article will dive deeper into how a biogas digester works.