Process Description Biogas upgrading by means of Vacuum Pressure Swing Adsorption Process (VPSA)
Biogases from landfills, digesters etc mainly contains of methane (CH4), Carbon dioxide (CO2), Nitrogen (N2), Oxygen (O2), Hydrogen Sulfide (H2S), water (H2O) and traces of hydrocarbons and hallucinated carbons (CFKW’s). In order to upgrade the biogas to a natural gas (NG) quality the H2S, CFKW’s and most of the CO2 need to be removed. The HADETEC process used activated carbons and molecular sieves for removal of the impurities.
Here below the different process steps are described in more detail:
The raw biogas is collected from the landfill or the digester plant in a main header and most of the free water is removed in a conventional water separator. The biogas is fed with a blower at approx 20-40 mbarg to the upgrading plant. Depending on the environmental conditions and the temperature of the biogas the gas needs to be cooled to approx 20 deg C. During cooling the available water will condensate and is separated in a water separator.
Compression
The next step is compression of the gas. The separation process for CO2 removal has it optimum at approx 6 barg. A feed gas compressor provides the biogas at approx 7 barg During compression water in the gas condensates and is separated in a water separator. The heat of compression is left in the gas in order to operate the H2S removal process on an elevated temperature.
H2S-removal
The removal of H2S will be achieved in a chemisorption process with activated carbon. The activated carbon is impregnated with a catalyst which helps the process. The H2S in the gas reacts to elemental Sulphur according following reaction: 8H2S + 4O2 --> S8 + 8 H2O The elemental Sulphur is adsorbed on the activated Carbon.
CFKW removal
For removal of CFKW a activated carbon process based on Temperature Swing Adsorption will be used. The CFKW’s are adsorbed on activated carbon and the regeneration is done by increasing the temperature of the activated Carbon bed to approx 200 deg C. The CFKW’s will vaporise and can be flared. The heat for regeneration can be produced by means of steam or by burning part of the product (NG) gas.
CO2 removal
The VPSA process for CO2 removal a 3 or 4 bed PSA process is used. The CO2 is adsorbed on a molecular sieve and the Methane goes through up to the NG quality. After a certain Cycle time the adsorber vessel is saturated and regeneration is required. The regeneration is achieved by reducing the pressure to below atmosphere with a vacuum pump At this low pressure the CO2 will desorb from the molecular sieve and will be blown to the atmosphere. In order to improve the methane yield one or two pressure equalisation steps are introduced. With VPSA technology Methane recovery figures can be reached up to 98%. Methane content in the product gas is strongly depending on the feed gas concentration. In case the feed gas contains high percentage of nitrogen the methane content in the product gas will be low as well since Nitrogen is very difficult to separate from the feed gas.
Odorisation
Because NG is an odourless gas an odorant need to be added for safety reasons. In Europe the commonly used Odorant is TetraHydroThiophane (THT). Based on the produced capacity.
Mixing
The product gas comes available from the VPSA process with fluctuating purities. In order to have a homogenous concentration of the product gas the gas is mixed in a mixing vessel. Downstream the mixing vessel the gas can be injected in the natural gas grid or boosted up to a higher pressure for storage or fuel gas application.