By-product reacts badly

There is one major problem in using digester gas for generation, and that is the make-up of the gas itself. The gas produced by the digesters is not only variable, but contains one compound which crucially affects engine operation - hydrogen sulphide.

Most engines can accept gas with a hydrogen sulphide content of about 400 ppm, but in digester gas the levels can be more than four times this amount. `Sewage produces normally about 1800 ppm of hydrogen sulphide', points out Brian Muggleton, assistant manager at Stoke Bardolph. `Hydrogen sulphide affects the alkalinity reserve of the lubricating oil, and if this starts to break down you get corrosion in the engine.'

The normal method of dealing with the problem is to install some form of prior treatment such as gas scrubbers to remove the offending element, but at Stoke Bardolph they are trying a different approach. Since the hydrogen sulphide affects the lubricant, not the metal, they are experimenting with frequent oil changes.

Working with engineers from Deutz UK and scientists at Mobil Oil, the staff began a programme of oil changes every ten days. The results were promising, and they are now considering intervals between changes of 500 hours. Downtime for a full oil change is less than one hour.

So far the new system seems to be working well. Each oil change costs just over £200 per engine, but is still seen as a viable alternative to a gas scrubber.