Corrosion resistant engine for odour control with electricity generation

An Ajax sour gas engine was used to control the biogas odour from an anaerobic digester and to generate electrical power. The 65% methane biogas was analysed for hydrogen sulphide by Dräger tubes and organosulphide compounds by GC-MS. Only low molecular weight sulphur compounds contributed to the odour of the biogas. The hydrogen sulphide concentrations averaged 13,700 mg m−3 and methyl mercaptan 0.277 mg m−3 in the biogas and the Ajax engine reduced these to 6.1 and 0.015 mg m−3, respectively. Dilution of the biogas by air in the engine, together with oxidation during combustion, accounted for the lowering of the hydrogen sulphide concentration. There was no oxidation of the mercaptan. Tuning the engine to provide a slightly lean burn reduced hydrogen sulphide concentration to 2.7 mg m−3. A simple stack dispersion model indicates that a further tenfold reduction in exhaust concentration at ground level can be achieved by a 5 m chimney and a 100-fold reduction from a 15 m chimney. The higher chimney gives concentrations around the odour threshold of hydrogen sulphide.