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What are the Contaminants in Biogas and Biomethane?

January 24, 2024

Biogas is typically a mixture of gases including methane (CH4), carbon dioxide (CO2), hydrogen sulphide (H2S), volatile organic compounds (VOCs), and atmospheric gases such as nitrogen (N2), oxygen (O2), and water (H2O). The actual composition of the biogas may depend on the origin and nature of the waste or biomass feedstock being processed and on the actual performance of the digestion process.

However, to use biogas as a fuel source either through electricity generation or injection into the natural gas grid, it needs to be purified to very low threshold levels, to ensure that the biogas is of the appropriate purity. Therefore, to remove some of these impurities, raw biogas has to undergo several processing steps to upgrade it to a direct combustion fuel or renewable natural gas. 

What are these Impurities?

There are many different types of contaminants or impurities that may be found in Biogas or Biomethane streams.

Hydrogen sulphide (H2S), and any sulphur compounds, such as mercaptans, that may come from the degradation of sulphur containing biomass feed, can cause corrosion issues within engines and equipment and so impact downstream operations. These compounds are typically removed at early stages in biogas upgrading.

Volatile organic compounds or VOCs typically come from household waste and are particularly common in landfill gas and some types of agricultural biogas. They need to be removed due to their impact on the downstream processes.

Siloxanes in biogas are volatile organosilicon chemicals that typically come from waste from household products, such as detergents, shampoos, deodorants, toothpastes and cosmetics. These siloxanes contain a Si-O-Si linkage which when combusted, can generate silicon dioxide (SiO2). This deposits into engines and other components causing scaling and reduces their power efficiency, so requiring more frequent maintenance. 

Ammonia can also be produced through biological degradation of nitrogenous containing biomass. However, Ammonia at high concentrations is considered as an inhibitor of methane production.

Formaldehyde may also be formed during any incomplete combustion process of longer chain organics.

What Impurities Can be Removed with Activated Carbon?

These contaminants can be effectively removed from Biogas or Biomethane streams using activated carbons. However, for the most effective impurity removal, different carbon types are typically required, depending on whether the prime contaminant is an organic or inorganic impurity or compound. 

Organic Impurities

Some of the typical organic impurities that can be effectively removed used activated carbon include the following:

Alpha-pinene Aromatic Compounds
Benzene Carene
Cumene Cymene
D3: Hexamethylcyclotrisiloxane D4: Octamethylcyclotrisiloxane
D5: Decamethylcyclotrisiloxane  D6: Dodecamethylcyclotrisiloxane
Dodecane Ketones?
L2 or MM or HDMS: Hexamethyldisiloxane L3 or MDM: Octamethyltrisiloxane
L4 or MD2M: Decamethylterasiloxane L5 or MD3M: Dodecamethylpentasiloxane
Limonene Methyl ethyl ketone
Naphthalene Oils
Pinene Silanes
Siloxanes Solvents
Terpenes TMS Tetramethyl silane
Toluene Trimethyl silanol

Inorganic Impurities

For inorganic impurities, an impregnated activated carbon is typically used to provide the most effective removal. Such inorganic impurities are as follows:

Ammonia Hydrogen Sulphide (H2S) Formaldehyde
Mercaptans Sulphur Dioxide (SO2) Sulphur compounds

For other pollutants of a more specialised or inorganic nature, see our ‘’What are Air Pollutants’’ or our “What are Volatile Organic Compounds (VOC’s)” articles.

How can we help?

The most effective carbon to be used for treating a Biogas or Biomethane stream may depend on the nature of the gas to be treated, the plant operating process conditions, the particular impurities to be removed and the required treatment objective to be met.

If this is required for a new treatment process, why not consider the use of a mobile carbon filter. These are activated carbon filters that can be both used on-site as a Biogas treatment vessel and then transported to and from the site, without the need for any on-site carbon exchange.

If you need help with the choice of activated carbon, the support of our mobile carbon filter service or just some further advice, please contact us.

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