|Purification of Biogas (GTZ, 1985)|
Dissolved H2S is contained in the fermentation slurry.
An equilibrium is set up between the dissolved H2S and the
H2S in the gas phase. The dissolved H2S in high
concentrations can be toxic to the bacteria in the slurry. It can inhibit the
production of biogas and cause its composition to alter.
Remedies - put less sulphur-rich material in the plant, dilute with water. In less serious cases stir vigorously (to drive H2S out of the slurry).
Corrosion by H2S
The presence of H2S gas in biogas makes it corrosive to
metal parts. Iron is subject to surface attack, although not major corrosion.
Galvanized parts are similarly subject to surface corrosion.
The effect on non-ferrous metals in components, such as pressure regulators, gas meters, valves and mountings, is much more serious. They are very quickly corroded. These materials also corrode in gas engines (seals and valves).
Corrosion by SO2 from H2S
The combustion product SO2 combines with water vapour and badly corrodes the exhaust side of burners, gas lamps and engines. Burning biogas in stoves and boilers can also result in damage to the chimney.
The acid which is formed corrodes engine parts in the combustion chamber, exhaust system and in various bearings. This is enhanced by frequent starts, short running times and the relatively low temperatures when starting up and after cutting off the engine. The water cooling system also provides the means (water needed to form sulphuric acid) for corrosion.
Running engines with H2S -containing gas can reduce the service time to the first general overhaul by about 10-15%.
Engine oil changes
The suphur content of biogas used in gas engines shortens the time between oil changes. SO2 from combustion and water vapour both dissolve in the lubricating oil. The oil becomes acidic and its properties change. It loses its ability to lubricate and sometimes corrodes metal components. Under continuous operating conditions the interval between oil changes is reduced to 200 - 250 hours.
If biogas is burned for cooking and lighting in poorly ventilated rooms' the occupants