Purification of Biogas
Most effluents and solid wastes contain sulphates, which give rise to presence of H2S in the biogas. The engines to be fuelled by biogas, can tolerate H2S content of up to 1000 ppm, beyond which the H2S can cause rapid corrosion.
Although biogas generated from MSW is generally not expected to contain high percentage of H2S, adequate arrangements for cleaning of the gas have to be made in case it is beyond 1000 ppm. Systems being used to remove H2S from biogas are based on Chemical, Bio-chemical processes or physical processes, which are described below:
15.4.4.1 Chemical Processes
Chemical processes are based on absorption of H2S by Alkali, Iron or Amines. The most widely used process for desulphurisation is the Amine process because it selectively absorbs H2S from biogas and can be carried out at near atmospheric pressure. This can reduce the H2S content to 800 ppm. The raw biogas is treated through an absorber column against tri-ethanol amine solution. The absorber has one or more packing beds of polypropylene rings to provide better contact between gas and the liquid media. Amine solution while reacting with biogas, gets saturated with H2S and CO2 and is sent to the stripper column wherein it gets regenerated by stripping off the H2S and CO2 by heating with steam. The sour gases are let off to a chimney. The regenerated amine is ready for reuse.
15.4.4.2 Bio-chemical Processes
These processes uses secondary treated effluent to clean the biogas. This effluent is sprayed from the top of the absorber columns while the raw biogas is blown in from the bottom. The effluent cleans the biogas and is then sent to a aeration tank where the H2S is converted into sulphates. The effluent from this aeration tank is partly supplemented by fresh treated effluent and partly disposed off. The formation of elemental sulphur is outside the scrubber and therefore ensures availability of the scrubber without choking effect.
15.4.4.3 Summary of Gas Cleaning Methods
A summary of the different methods being used for purification of biogas is given in Table 15.3.
Table 15.3 Summary of Gas Cleaning Methods
Compound - Process type - Process alternatives available
H2O
Adsorption 1. Silica gel 2. Molecular sieves, and Alumina
Absorption 1. Ethylene glycol (at low temperature -20oF)
2. Selexol Refrigeration Chilling to -4oF
Hydrocarbons
Adsorption Activated carbon
Absorption 1. Lean oil Absorption 2. Ethylene glycol, and 3. Selexol
(All at low temperatures of -20oF)
Combination Refrigeration with ethylene Glycol plus activated carbon
Absorption
CO2 and H2S
Absorption 1. Organic solvents 2. Alkaline salt solutions 3. Alkanolamines
Adsorption 1. Molecular sieves 2. Activated carbon
Membrane separation Hollow fiber membrane
H2S removal with Sulphur recovery
Bio-chemical process
