An anaerobic digestor refers to an airtight tank where anaerobic bacteria, or those that thrive in the absence of oxygen, are used to breakdown organic wastes into smaller molecular compounds for use elsewhere. These anaerobic bacteria generate both methane (CH4) and carbon dioxide (C02) gases in near equal volumes as they digest the waste material. In modern AD this biogas is captured and converted into energy to provide heat, power and sanitation.
Anaerobic digestion and biogas technology can supply 30% of the UK’s domestic gas demand. That is the degree of importance that the Worlds industry trade organisation ADBA placed on the ability of a fully supported UK biogas industry back in 2015. The process is now a continually important area of focus as the initiative can reduce CO2 emissions, provide a wide range of post COVID employment opportunities for city and rural occupants, produce low carbon energy 24/7 and provide a resource to fertilise land and reduce polluting farm run off.
According to David Newman, President of the World Biogas Association, “If we do not address methane emissions from organic waste, all our efforts to tackle the climate crisis will fail. AD is one of those ready to go, ready to scale technologies, the path we take is clear”.
Biogas is the main desirable product from anaerobic digestion of organic carbon. The key to small scale digestion is generating and utilising the energy on site to help to meet our 2023 targets as set by the Government.
The risk of explosion or corrosion from toxic gases and moist air respectively, means that the systems components must be corrosion resistant and manufactured from suitable materials that can handle this type of environment, for example non-corrosive polypropylene fans should be used to exhaust biogas from the digestor to the combined heat and power engine to enable safe and effective processing.
Aggressive chemicals produced during the anaerobic digestion process attack steel and corrosion can set in very quickly. If a fan for example starts to develop signs of rusting, processes such as welding are out of the question due to the high concentrations of combustible methane and carbon dioxide gases that can be readily ignited by a simple welding spark. The fermentation and oxidation processes used by these digestors whether concrete or steel, create the perfect environment for corrosion both at the liquid and gas stage, while sludge tanks face similar challenges. Waste management industries such as AD require corrosion management strategies that extend their lifetime, as well as maintenance free periods. Non metallic solutions and components, as mentioned previously and high-performance coatings, are well positioned to deliver benefits to the asset owners and AD operators.
All agricultural AD/biogas systems employ two distinct processes that must work together:
As the process of AD produces hazardous gases and operates under the absence of oxygen there are risks for those maintaining these systems. Handling biogas requires caution. CH4 is combustible with air at concentrations between 5 percent and 15 percent, known as the lower explosive limit and the upper explosive limit, respectively. Biogas systems typically produce CH4 concentrations in the range of 45 percent to 70 percent and introducing air into the biogas handling system could bring the CH4 concentrations into the explosive range, presenting risk of an explosion in the presence of an ignition source. H2S is a toxic gas that can cause severe health effects or death. An example of the risk of death occurs in maintenance. Empty digestors will be low in oxygen and some residual gases may still be present even though the digestate and slurry has been removed. These gases must be mechanically exhausted out to ensure it is safe for maintenance staff to occupy.
Aside from the process of delivering high quality biogas and digestate, operators have a wide range of things to consider and maintain to ensure the AD process remains safe and effective. Corrosion, explosion hazards and ventilation are key aspects to consider. Preventing the presence of explosive mixtures from escaping seams and joints is one aspect but controlling ignition sources is another issue which could cause fatal within a few seconds. The Occupational Safety and Health administration (OSHA) defines something immediately dangerous to life and health (IDHL) to be the concentration of toxic or corrosive substances posing an immediate threat to life. AD is an interesting and expansive topic to research. It will be interesting to see if the British Government continue to throw weight behind the sector especially in line with their commitment to being net zero by 2050 and closer to home, the initiative to reduce food waste from land fill by 2030. Naturally, prevention is the best cure, but where this is not achieved, AD is the better option to incineration or disposal. Learn more about anaerobic digestion legislation here.
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The Axair ‘S’ range fans are designed to provide high air flow rate against medium system pressures.
The Axair STORM ‘ST’ range fans are designed to provide a relatively low air flow rate against elevated system pressures.
The ATEX ranges are available for Category 3G Zone 2 applications and can be supplied with EEx nA or EEx d motors to order.