Intelligent biogas analysers control AD desulphurisation

Basildon , UK - With increasing volumes of waste now being processed to generate ‘renewable energy’ through anaerobic digestion, it’s important for plant operators to ensure that capacities are maintained and optimised.

The primary purpose of an AD plant is to generate electricity from waste product such as animal slurry, food waste and crops. This electricity can then be sold to the National grid in return for cash, based on what are called “Feed-in-Tariffs” (FITS) which are determined by the government. For example, an AD plant that produces between 250 and 500Kw of electricity will receive 13.6p per Kwh.

When the waste material is stored in anaerobic conditions (no oxygen) the organic matter breaks down in the presence of bacteria and releases biogas which will typically contain from 50 to 70% methane.

The balance is a mixture of carbon dioxide, water vapour and hydrogen sulphide (H2s), with the H2s in concentrations up to around 5,000ppm (parts per million). H2s is an unwanted bi-product of the process and smells of rotten eggs but more importantly, if concentrations occur at higher levels of around 1,000ppm, it can be fatal if inhaled.

Another issue with H2s is that although the biogas produced contains enough methane for it to be used as a fuel in combined heat and power units (CHP), when it’s burned the presence of H2s in the gas combines with the water vapour to form a weak sulphuric acid and this would corrode the generator.

The usual solution to the problem is for AD sites to install scrubbers which clean and scrub the gas to reduce the H2s content to acceptable levels. However, installing gas scrubbers for the desulphurisation process can be very expensive and it also involves continuous chemical dosing and constant maintenance.

Also, the CHP plant represents the largest capital outlay of an AD project and the engine manufacturer’s warranty is usually invalidated if the H2s concentration exceeds certain levels, typically 300 to 500 ppm.

An alternative to H2s scrubbers is microbial-desulphurisation which relies on injecting very small quantities of air into the biogas (providing it does not exceed 1 - 2%, the anaerobic process within the sludge will be unaffected). The oxygen breaks down the H2s into water vapour and elemental sulphur.

This process is widely used as a low cost alternative to more expensive large scale scrubbers but requires constant attention from the plant operators. Too much air will kill the bacteria and stop the AD process, too little air and the H2s concentration will quickly rise.

Another option is to use biogas analysers and these are routinely used on AD sites to monitor and record the amount of methane and H2s within the biogas as it leaves the digester.

Plant operators rely on the analyser to warn them of low levels of methane or particularly high levels of H2S caused by unstable anaerobic conditions which could result in a plant shutdown and ultimately, huge financial loses.

A recent development in this area has been the UK introduction of a device to automatically control micro-biological desulphurisation, by control and instrumentation company Allison Engineering.

The Awi-Flex analyser, it said, is already established as the market leader for biogas analysis on AD applications with thousands of units installed in Germany.

The multi-function biogas analyser is supplied with a small air compressor which regulates air injection to the digester - above the sludge level - to between 0.5 to 1% volume and breaks down the H2s to less harmful elemental sulphur and water.

By accurately measuring the O2 and H2s in the biogas and using a combination of PI and fuzzy logic control, H2s levels can be reduced by up to 80%. The device is designed to provide measurement for CH4, CO2, H2S, O2 and H2, with up to five measuring points.