Content – Other fuels
Removal of H2S from the feed-gas by dry or wet oxidation (Liquid-phase oxidation)
Dry oxidation of H2S
By adding oxygen
A small amount of oxygen (around 6 %) is released into the system. The result is that the sulphide is oxidized to sulfur.
2H2S + O2 → 2S + 2H2O
If too much oxygen is added, the mixture with methane becomes explosive.
The concentration of H2S can be reduced by 95% to below 50 ppm by this method.
H2S is removed from the gas by using an absorbent. For biogas upgrading iron oxide is used, which reacting with the H2S. When the iron oxide is covered by sulfur, the adsorbent is replaced. The process is simple, but it some energy is lost when recovering the iron oxide.
For further use of the gas in steam reforming the content of H2S needs to be very low to avoid damaging the catalytic converter inside the reformer. One solution is to heat the gas to around 400 ○C and using zinc oxide as the absorbent reducing the H2S content to below 1ppm.
Wet oxidation of H2
Wet oxidation of H2S from biogas is carried out by physical or chemical absorption.
Physical absorption by adding solvents.
This may be water, but it requires large quantities to remove small amounts of H2S, and therefore often NaOH is added to increase the secretion. The by product from this process is sodium sulfide. This represents a waste issue.
Chemical absorption takes place by means of iron salt solutions, such as ferric chloride (FeCl3). This is an effective way to reduce high concentrations of H2S. FeCl3 may be added directly and removes H2S down to 10 ppm.
In larger systems for H2S removal it is common to use amine as absorbent. Amine liquid captures H2S from the gas in an absorber, and the amine is regenerated in a desorber by application of heat. The freed H2S usually is processed further into pure sulfur.
Carbondioxide (CO2) removal
To remove CO2 from biogas, chemical or physical absorption is often used.
Chemical absortion of CO2
Various absorbents (amine or alkaline solutions) that captures CO2 may be added to the biogas. The absorbent is recycled by adding heat during a desorption prosess. Close to 100% absorption of the CO2 may be obtained by this method.
Physical absorbtion of CO2
The simplest form of CO2 absorption is by means of high pressure water. Compressed biogas is added into pressurized water within a reactor. Both CO2 and H2S dissolves in the water and are removed from the bottom of the reactor.The amount of CO2 which is separated depends on the reactor design and process conditions.
Removal of water/drying of biogas
Drying of biogas mainly is done by cooling, absorption (as glycol) or adsorption.