Much of an AD plant is covered by the Dangerous Substances and Explosive Atmospheres Regulations 2002 (the DSEAR regulations): according to the HSE, these are concerned with ‘protection against risks from fire, explosion and similar events arising from dangerous substances used or present in the workplace’. The DSEAR regs also cover ‘gases under pressure’ and ‘substances that are corrosive to metals’ – both clearly relevant to AD facilities.
“In general everything comes down to the DSEAR regs,” says Euan Munro, a chartered engineer working for environmental consultancy SLR Consulting Ltd. He has written a useful paper on the environmental and safety hazards of AD plants for the Institution of Chemical Engineers (see links).
Munro points out that a couple of major accidents have resulted from PRV [pressure relief valve] failures, and “there were further failures upstream which led to high pressures.” These caused roof failure. And “if you have a flammable gas, it’s probably going to find an ignition source.”
They are some of the key hazards, he adds. “You’ve got a certain ATEX zone or hazardous area around the pressure relief valves, around the roof, and maybe some other pieces of the gas handling equipment.” For ATEX zoning, the risk of explosion is defined according to the composition of the gas and its pressure.
While the digester itself is typically sited in open air, presenting a relatively low risk of gas concentration, much of the associated equipment is enclosed: “There’s instrumentation in the head space,” says Munro; “high-level probes, and pressure transmitters – so they would need to be ATEX-rated. In terms of protecting people, you wouldn’t typically have a gas monitor around the gantries, but you would expect gas monitors around containerised equipment” – this might include a gas engine or a biogas boiler.
If the plant is contributing to the gas grid, it would include an upgrader (to remove CO2 and other contaminants), propane tanks (to increase the calorific value of the biogas) and an NEF [network entry facility] or GEU [grid entry unit]; all of these are explosion risks which would require ATEX-rated gas sensors and other equipment.
Munro makes the point that most health and safety legislation in the UK is goal-setting rather than prescriptive. “There are IGEM [Institute of Gas Engineers and Managers] standards,” and “in Germany there are some standards about zoning – around areas like a PRV or a roof – which are essentially like an industry code of practice (an ICOP)”.
Fire is not the only hazard linked to anaerobic digestion facilities: almost every other category of hazard is conceivable in an AD facility, from drowning to falling, trip hazards, burns from hot pipework, and entanglement or pinch hazards associated with rotating machinery.
More specific risks come from contamination due to spills of feedstock (the organic material going into the digester) or digestate (any material leaving the digester). And a release of gas can result in other problems: biogas typically contains not just methane but also carbon dioxide, which may pool in an area with a risk of asphyxiation; and it is also likely to contain toxic levels of gases such as hydrogen sulphide (H2S) — this has in fact resulted in one UK fatality.
Activated carbon filters are often used for odour reduction and to remove impurities such as hydrogen sulphide and VOCs [volatile organic compounds] from the biogas. Filters may also be impregnated with chemicals to improve their performance. Some of these reactions are exothermic (that is, they generate heat) but under normal conditions the gas flow through the filter keeps the temperature at a safe level.
The Environment Agency has produced a technical guide to highlight the fire risks from such filters, and it identifies three key risk factors:
●High VOC concentrations: some VOCs, such as ketones and acetates, may cause secondary heating
●Type and grade of activated carbon used: certain impregnation materials can cause localised hot spots
●Low flow rate, allowing the temperature to rise.
The EA guidance says: “there is a particular risk when a carbon filter is disconnected, because it may only take a gentle breath of air to enter the carbon filter to start a fire when other risk factors are present”.
It also says: “Temperature monitoring may not be sensitive enough to detect localised changes in temperature, whereas carbon monoxide monitoring is a much better early indicator”.
The EA also gives the example of an incident where oxygen was injected into a carbon filter to increase the life of the activated carbon, leading to a fire in the facility, and adds, “Carbon filters can be fitted with fire prevention measures such as one-way valves, vacuum relief valves and pressure relief valves. However, there generally appears to be little understanding of fire risk, and it is therefore not clear how widespread fire prevention measures are understood or have been implemented”.
The EA suggests that at a minimum it would expect the operator to demonstrate:
●Adequate design of the carbon filter;
●How they monitor the effectiveness of the filter to determine when the carbon needs replacing;
●What other monitoring is in place for inlet/outlet gases, visual checks, airflow and residence time;
●Understanding of the risk of fire and outline what fire prevention measures are in place.
Hydrogen sulphide has another nasty side-effect, in that it combines with oxygen in the presence of water to produce sulphuric acid (H2SO4): this is hazardous to health, and attacks concrete and metal structures; it will even corrode most grades of stainless steel – although some (such as 904L) are affected only mildly at normal temperatures – and specialist coatings may be needed to protect the head space of the digester.
Lightning strikes might seem an improbable risk, but a lightning protection system should be installed: a commercial facility in Oxfordshire was struck by lightning in 2016, leading to a significant fire.
Finally, “maintenance activities have presented a risk in the past,” says Munro, and ‘hot work’ can be a particular issue: an explosion at a commercial AD plant in Nottingham in 2017 was caused by workers welding in the pasteuriser.
Environmental safety – www.is.gd/xelalu
Operational lessons – www.is.gd/kakuya
IGEM regulations – www.is.gd/gmbYui
Risk management – www.is.gd/sikive
Fire prevention – www.is.gd/ofekid
BOX: MALT TO TRUCK
Glenfiddich, the single malt Scotch whisky, is set to decarbonise its transport operations, pioneering a circular economy approach with natural gas-powered IVECO Stralis trucks fuelled by distillery waste products. William Grant & Sons’ engineers have developed a way to use the by-products of its distilling process, while minimising the carbon output of its operations. By loading waste matter from the distillery into an anaerobic digester, the company can capture the biogas given off as it breaks down. This is subsequently cleaned and used as low-carbon, low-particulate fuel for its fleet of three methane-powered delivery trucks (pictured, with Dufftown site leader Kirsty Dagnan and distilleries director Stuart Watts). Left-over solids can then be used to fertilise the barley fields of its farmer growing partners and enrich the soils through carbon sequestration, pulling CO2 away from the earth’s atmosphere and thus creating a circular economy.