Monitoring Gas from Bio-digesters - Dr Roger James Riley

Bio-digesters are used extensively to process sewage sludge and other waste streams in order to reduce the disposal problem. As a by-product of this process large quantities of methane gas and carbon dioxide are produced. It is common practice to simply flare off this excess methane gas. This is rather like burning dollar bills, since with some investment the methane can be used to generate energy that can pay for part or all of the running costs of the plant.

The methane can be used as fuel for a gas engine in a CHP system which can generate heat for the digesters and electricity, either to power the plant or for sale to the grid.

One of the problems of using the gas to generate electricity is that the gas quality can vary. The efficiency of the engine will depend on the methane concentration, and this can vary depending on the digester cycle. More worryingly, the digester gas can contain high concentrations of hydrogen sulphide. This is a very corrosive and dangerous gas. Too high a concentration of hydrogen sulphide can seriously damage an engine and any repair will be an expensive business.

It thus makes sense to continuously monitor the gas composition. This can give information about the efficiency of the digestion process, can help in correctly running the generator engine, and can give a warning of high hydrogen sulphide concentrations before the engine is damaged. The monitor can even switch the gas stream to a hydrogen sulphide scrubber if the concentration goes over a pre-set limit.