Sour Gas is any gas that contains impurities which form acidic combustion products. Biogas may include carbon dioxide, hydrogen Sulphide and methyl mercaptan which form Carbonic and Sulphuric acids on combustion. These acidic byproducts severely corrode engine bearings, exposed copper and mild steel engineering components. Most Anaerobic Digesters will produce a Gas with 40% carbon dioxide and 0.3 - 2% hydrogen sulphide [H2S](3 - 20,000 ppm) and significant mercaptan.
The operational limit for most engines is 200 ppm hydrogen sulphide. Given a suitable high TBN oil, four stroke conventional multi cylinder engines can be operated at 400 ppm hydrogen sulphide with occasional 1000 ppm events but only as long as the engine is running. Every time the engine stops and goes cold, hydrogen sulphide condensation takes place and the crankcase oil should be changed. Gas cleaning can be used to reduce hydrogen sulphide from 10,000 ppm to 500-1000 ppm and enable four stroke engines to be operated with special (expensive) oils. However gas cleaning is expensive (a mercaptan filter can cost £400 for a throw away can) and practical experience over several decades has shown that as soon as the gas cleaning unit breaks down the engine bearings collapse. Ferric Chloride can be added to the waste stream in digesters but this is very expensive in the long run and the chlorine corrodes pipework and can kill off valuable micro-organisms in the downstream processes however it does reduce 'struvite' formation.
Control of H2S is compounded by the fact that there is no reliable hydrogen sulphide sensor on the market for continuous duty in a sewage treatment plant environment - reason, the sensors are degraded by the Hydrogen Sulphide in about 4-6 months. This means that an engine protected by an H2S sensor is dependent upon highly disciplined sensor maintenance for decades - this is unlikely to occur on most sites.
Landfill gas may contain less sulphur if ferric oxide from rusting metal food cans is present, however, chloride production may be high in these localities leading to hydrochloric acid production on combustion.
The occupational Exposure Safety Limit for H2S is only 10ppm.
Practically Green sells the most advanced safety warning alarms on the market.
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Powered CHP units purpose built for Sour Gas Applications
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