Vigilance is the path to safety
Like most things in life, biogas is not 100% safe. But, when handled properly, it is a clean and easily controlled source of renewable energy. Veolia UK's technical director, Greg Turner, gives an overview of safety precautionsBiogas is an important renewable energy source. It results from the anaerobic digestion (AD) of soluble carbon - chemical oxygen demand (COD) - a process in which microbiological activity breaks down organic matter and converts it to gas.
If anaerobic conditions occur, and a digestible carbon source is also present, biogas will be produced. There are three main sources of biogas:
- Landfill - organic waste disposed of in landfill sites is covered when more organic waste is deposited on top. This has the dual effect of compressing the moist waste and starving it of oxygen. This is the perfect breeding ground for anaerobic microbes. The biogas builds up over time, until it reaches the point where it starts to be released into the atmosphere. Landfill-generated biogas is usually around 50% methane, so is weaker than other forms of biogas
- High-strength waste from industrial processes - this includes the AD of the highly soluble carbon waste produced by industries, including food, drinks, chemical and paper
- The digestion of semi-solids - typically, this occurs within sludges at municipal water and waste treatment plants. It also relates to the digestion of food waste and co-digestion, which involves mixed waste streams. As a general rule, digestion reduces sludge solids quantity by a factor of around five, resulting in a material suitable for composting or further heat treatment. It also reduces the quantity of sludge and therefore the overall cost, as it avoids expensive destruction/disposal costs of large quantities of sludge
Despite having been around for quite some time, the waste-to-fuel biogas conversion process must still be treated with caution at all times. The associated risks result primarily from methane and CO2, which tend to be the main constituents of biogas.
There are three major safety hazards involved:
- Damage to downstream equipment
Methane in biogas forms explosive mixtures in air. The explosive limits range from 6% to 14% methane. Where there is the potential to achieve these air-to-methane levels, it is vital for the gas to be completely contained, well away from any potential ignition source and explosive conditions.
For this reason, the electrical switch gear and the instrumentation surrounding a biogas installation must be approved for an explosion-proof environment. The installation has to be gas tight, with the gas generation point securely ducted to a containment vessel. This is typically a gasometer or bag that prevents the gas being in contact with the air.
In addition, the system must have a safety valve such as a flare, which will safely dispose of gas when it exceeds the needs of the generation equipment, or when the generators are offline for maintenance. As is the case in any industrial environment where flammable vapours are present, plant operators also need to be aware of the dangers presented by many seemingly innocuous devices, such as mobile phones.
This is because they can generate static electricity, which has the potential to cause an explosive reaction by providing an ignition source. All potential sources of static electricity should be well earthed. Also, care should be taken when operating under abnormal circumstances, such as during maintenance, not to create explosive conditions for eaxmple air mixture with chemicals such as peroxide.
The main constituents of biogas are methane and CO2, and therein lies the problem. Methane is lighter than air, so will collect in void spaces at higher levels - in other words, from the ceiling downwards. By contrast, CO2 is heavier than air, so it collects from the floor upwards. Both gases displace oxygen, so present a potential hazard to anyone entering this environment.
Another danger arises if any form of sulphate or sulphur-bearing material in the soluble carbons is used as feedstock for the reactor, because hydrogen sulphide will be generated as a result. Hydrogen sulphide has a pungent, rotten-egg smell up to around 10ppm.
Above that level it overpowers the olfactory organs, with potentially lethal consequences. Basically, if you can smell the gas, it is unpleasant, but not likely to be lethal. But, if you know gas is there but cannot smell it, you are in trouble. The solution is simple - anyone working in or around a biogas installation must wear a gas monitor at all times. The monitor will provide a life-saving early warning if dangerous gas levels are detected, and so helps to confirm that there is uncontaminated air available for the wearer to breathe.
There are two prime culprits in potential damage to plant - hydrogen sulphide and siloxanes. In addition to hydrogen sulphide's asphyxiation potential, if left untreated this chemical can cause the generating equipment to corrode to a dangerous extent.
Hydrogen sulphide can be removed by using caustic soda to wet scrub it out, but this is an expensive process. It also produces an effluent that requires safe disposal.
Fortunately, thanks to modern best practice techniques and technological advances, a far more efficient option is now available. This is a bioscrubber that uses aerobic thiobacillus bacteria to remove hydrogen sulphide and produces natural sulphur as a result. Bioscrubbing is economically and environmentally preferable to wet scrubbing.
Siloxanes are formed from the anaerobic decomposition of materials like detergents, soaps and industrial silicon products. There are seven principal types of siloxane found in biogas. All have a detrimental effect on the generator.
They coat it with mineral deposits - mostly silica or silicates - that reduce the effectiveness of engine oil's lubricating properties. Again, left untreated, a build-up of siloxanes could create a considerable safetyhazard.
Dealing with siloxanes is a relatively straightforward process. Activated carbon is used to absorb the siloxanes from the gas stream, thereby preventing them from getting into the engine and damaging its lubrication system.
Reducing the siloxanes with carbon has another positive spin-off, because maintenance of the generating engine is required less frequently.
And that is not all. Infection is a further potential hazard to guard against. Biogas conversion is a biological reaction process that maximises the digestion capability of bacteria.
Any operator coming in contact with these bacteria risks contamination by pathogens, with potential dangers to their health. High levels of hygiene are therefore essential for the prevention of cross-infection.
It is important to stress that all of the potential hazards detailed above can be avoided by implementing precautionary safety measures. But achieving a safe working environment requires the commitment and expertise of disciplined operators, together with a health and safety ethos that pervades the entire organisation.
When working with biogas, the message is simple: do it properly and you will be okay; drop your guard for a moment and you are in for trouble. There is no way of getting round it - safety must always be the number one priority.