How to treat biogas?
A typical biogas cleaning process involves the removal of coarse and fine particles, H2S, water, siloxane (landfill gas and some digester gas), and CO2 (biomethane production). At the same time, a gas holder is often needed as a buffer to smooth the flow and regulate the pressure. An emergency flare is needed for excess biogas or during maintenance. For the landfill, a backup flare may be needed.
Particles can be easily removed in coarse and fine filters. For digester gas, it is important to have a coarse filter immediately after the digester to remove the potential foam within the biogas. For biogas engine, a fine filter is compulsory to reduce the wear and tear.
A variety of methods are used for H2S removal, named desulphurisation. A distinction can be drawn between biological, chemical and physical desulphurisation methods as well as between rough and fine desulphurisation, depending on the application. The method, or combination of methods, used will depend on how the biogas is to be subsequently utilised.
In order to protect the gas utilisation equipment from severe wear and destruction and to meet the requirements of the downstream purification stages, water vapor must be removed from the biogas. The amount of water or water vapor that biogas can take up depends on the temperature of the gas. The relative humidity of biogas in the digester is 100%, which means that the biogas is saturated with water vapor. The methods that enter into consideration for the drying of biogas are condensation drying, adsorption drying (silica gel, activated carbon) and absorption drying (glycol dehydration). Condensate drying is most commonly used.
Siloxanes are a subgroup of silicones containing Si-O bonds with organic radicals. They are widely used for a variety of industrial processes and added to consumer products, including detergents, medical products and devices, shampoos, cosmetics, paper coatings, and textiles. Siloxanes are often treated by adsorption with activated carbon or activated alumina. They are often treated after desulphurisation and drying process, as H2S and water vapor are likely to consume the adsorption capacity before siloxanes removal.
Biogas holder is required for digester gas and industrial biogas system to stablise the volume and pressure. For efficient use of electricity generation, a common rule of thumb is 4 hours storage capacity. For landfill gas, gas holder is not compulsory as the gas well and pipe provides some storage capacity, however, it is still recommended to have a small gas holder to regulate the flow and pressure. Due to the corrosive natural of biogas, gas bag is preferred to steel gas tank. the gas bag can be housed within a steel or concrete structure, or even within a protective layer of membrane structure, which is commonly named double membrane gas holder. An external load can be exerted on the gas bag, so that the pressure is stablised.
The emergency flare is compulsory to avoid direct discharge of biogas into the atmosphere. The emergency flare is often controlled by the gas level within the gas holder and/or the gas pressure. It should start automatically when the biogas level or pressure is too high. A flame detector is required. If the flame extinguishes due to various reasons (composition, flow change, weather etc.), the flare should automatically restart. Temperature monitoring or control is equipped for advanced model to ensure complete destruction of methane while little production of NxO.
So far CO2 removal is not very relevant in New Zealand.