4.5 Desulphurization and Filtering of Biogas

Biogases from different materials contain different percentages of hydrogen sulphide H₂S, i.e. 0.10 . . . 0.50% Vol (1000 . . . 5000 ppm). As H₂S is corrosive to metals especially in connection with water or humidity, its content should be as low as possible when used as a fuel in engines. Some engine manufacturers specify a maximum allowable value of 0.15% Vol; others allow more or give no data.

H₂S can be removed by filtering with earth or with iron oxide (e.g. filings) whereby the filters need to be regenerated or the material exchanged periodically [24]. Recent experiments in a large biogas plant in Ferkessedougou, Ivory Coast [25], have revealed that by purging a small amount of air into the gas holder or store and allowing a reaction time of about 25 . . . 30 hours, a substantial
percentage, i.e. about 80%, of the H₂S is reduced to elementary sulphur which is deposited on surfaces within the plant or on the floating scum. The amount of air allowed into the gas holder/store needs however to be well dosed, preferably with a small dosage pump. A mean value for the constant air supply is approx. 0.4 % Vol of the constant gas production for a reduction of approx. 80% of the H₂S, e.g. from 0.5% Vol H₂S to 0.1% Vol, which is adequate for engine operation.

Depending on the type of biogas plant and piping, some indispensable solids can be drawn with the gas to the mixer. A simple filter in the form of a larger container filled with washed rubble or a tissue filter with no measurable pressure loss is recommendable in any system.

initial H2S content		stoichiometric amount of oxygen as vol. % of biogas production	stoichiometric amount of air as vol % of biogas production
in ppm	in vol. %
500	0.05	0.025	0.125
1000	0.10	0.050	0.250
1500	0.15	0.075	0.375
2000	0.20	0.100	0.500
2500	0.25	0.125	0.625
3000	0.30	0.150	0.750

a) Stoichiometric amounts of oxygen or air to be added for an 85% reduction of the H₂S content for different initial H₂S content values.

b) H₂S reduction from initially 1500 ppm as a function of added air.