Hydrogen sulfide is a colorless gas that creates a blue flame when burned and a toxic gas, sulfur dioxide (SO2). H2S in a concentration of 10 parts per million (ppm) can be distinguished by smell. The density of H2S gas is 1,192 kg / m3 higher than that of air and due to its weight it settles on the ground. The hydrogen sulphide contained in biogas causes odours, corrosiveness, and sulphur emissions when the gas is burned. If the biogas is to be used in internal combustion engines, turbines or fuel cells, the removal of hydrogen sulphide from the biogas may be required to protect the equipment and avoid damages. Hydrogen sulfide is flammable, smells like rotten eggs, has a sweet taste and is noticeable at concentrations of 0.002 mg / l. However, in concentrations greater than 500 ppm it affects the ability of the olfactory nerve to perceive and thus prevents its detection through this sense, making it more dangerous.

The biogas contains hydrogen sulfide (H2S) in ranges from 500 to 5,000 ppm, which can reach 2% by volume or more. It is important to reduce H2S to avoid corrosion damage to biogas recovery equipment. Electricity generators generally require a biogas with an H2S content of maximum 100 ppm. The hydrogen sulphide must be removed to avoid any corrosion in parts of the plant. The removal also protects catalysts which are used from an impact by sulphur containing compounds and a subsequent deactivation.

Hydrogen sulphide (H2S) is always present in biogas. The concentrations vary with the feedstock. The concentration of hydrogen sulphide in the biogas is a function of the digester feed substrate and inorganic sulphate content. Wastes which are high in proteins containing sulphur-based amino acids (methionine and cysteine) can significantly influence biogas hydrogen sulphide levels.

HYDROGEN SULPHIDE REMOVAL TECHNIQUES

Hydrogen sulphide can be removed from biogas by activated carbon. The selection of the filter size and the related activated carbon type depends on the chemical composition of the biogas as well as on the physical parameters such as humidity and temperature.