What is the difference between biogas and biomethane?

Biogas is produced via anaerobic digestion by micro-organisms and is a mixture of CO2 and CH4. Biomethane is biogas that has been upgraded to a high-qualitative gas with minimum 96% CH4. While biogas is most common used in combined heat and power systems (CHP), biomethane can be injected in the gas grid to replace natural gas.
 

What are typical pollutants in biogas?

Aside from CO2 and CH4, biogas also contains impurities. We can divide them in two main groups: components containing sulphur, mainly H2S, and VOCs such as terpenes and siloxanes. H2S is the result from degradation of sulphurous organic components. Terpenes are mainly caused by digestion of green wastes. Siloxanes however often originate from Si-containing components in cosmectics and silicones.
 

Why do impurities need to be removed?

In general, these impurities decrease the quality of the gas. More problematic is their potential to cause damage to other equipment. H2S for example causes corrosion of the engine and acidification of the engine oil. VOCs need to be removed because they can block and degrade membranes for biomethane upgrading. Among the VOCs, siloxanes are especially damaging since Si-oxides are formed in the gas engine which leads to severe abrasion damage.
 

How can activated carbon help in purification of biogas and biomethane?

Both H2S and VOCs can be removed with activated carbon to values below the required levels. Each group however requires a different removal solution. VOCs can be removed with standard activated carbon, but specifically impregnated types are required for H2S removal to enable the correct chemical reaction. The process conditions will determine how well the carbon works.