Synthetic diesel – 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.

or

By absorption

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 (FeC_l3). This is an effective way to reduce high concentrations of H₂S. FeC_l3 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.