Anaerobic digester plants

There has been quite a rush to build these plants, which take food waste, sewage sludge or other organic waste products and with the help of the right bacteria, produce biogas containing typically around 40% methane. This is normally burnt in gas fired internal combustion engines, sometimes with a small amount of liquid fuel as well. They have been widely used on farms in Europe for years, with relatively small scale plants. They are not without their problems, and there are safety codes from German, Swiss, and Austrian sources picking out the hazards, and making recommendations about design and operation. Unhelpfully, the recommendations are not identical. For those who wish to wade through 157 pages of German, the Austrian government guidelines are available as a pdf here with the title ‘Beurteilung von Biogasanlagen’>

There have clearly been incidents, summarised by a German colleague with the terse comment ‘ what do you expect, with chemical plants run by farmers?’

Some 5 years ago, the Renewable Energy Association came to BSI with a request for a British standard on these plants, following some incidents here. A project was started, but came soon to a full stop, as we could not get full cooperation from all the necessary sources, and the REA and ADBA did not work together to agree a scope.

These plants are chemical processes, and have the unique feature in my experience of having no ‘off ‘ button. As built, you cannot shut them down quickly, because the operators don’t want to kill the bacteria by injecting some suitable chemical, and they are installed with no means of cooling the large liquid mass. They have limited storage, so if the engine goes down, you have to flare the gas for many hours, or release it to atmosphere.

They also sometimes have the novel feature of flexible membrane roofs to the low pressure storage. Doubtless these are cheap, but you can’t pressure test them properly, as the textile stretches under pressure. Their vulnerability was demonstrated by a fire in June 2016 at a facility in Wallingford, Oxfordshire. This plant has a nameplate capacity of 2.4MW, so in round numbers generates 6m3 of gas/minute. Its not clear if the engines continued to run during and after the thunderstorm set fire to the gas storage unit.

See this report

To me this technology has all the hallmarks we have seen before, of designs bought in from European sources as ‘well established and safe’ when in fact UK versions were much larger, and not as safely designed as they ought to be.