Corrosion and explosive management in anaerobic digestion processes
14 September 2021
An anaerobic digester (AD) 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 and CO2 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.
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Anaerobic digestion and biogas technology can supply 30% of the UK’s domestic gas demand. That is the degree of importance that the UK Anaerobic Digestion and Bioresources Association (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 using the energy on site to help to meet our 2023 targets as set by the Government.
So how does the process work?
All agricultural AD/biogas systems employ two distinct processes that must work together:
- The biological process such as the hydrolysis and acidogenesis processes involves the microbial population that breaks down biodegradable organic material and converts a portion of it into CH4-rich biogas and digestate. Just as livestock farmers work to maintain proper animal nutrition, consistent feeding, and comfortable living conditions to maximise livestock growth or food production, Anaerobic Digestion or biogas system operators must also work to maintain proper AD nutrition, including consistent and high-quality feed and sustainable living conditions for their microorganisms. If these conditions are consistently achieved, the AD will efficiently produce high-quality biogas and digestate.
- The mechanical process involves the accompanying conveyors, pumps, fans and blowers, piping, tanks and other equipment that move and process the incoming organic material and the recovered biogas and digestate. The mechanical process also includes equipment for biogas utilisation. Just as farm operators must maintain their tractors, implements and manure-handling equipment to properly maintain farm production, AD/biogas system operators must also maintain the mechanical systems to ensure consistent production of high-quality biogas and digestate.
Corrosion & explosion management
The risk of explosion or corrosion from toxic gases and moist air respectively, means that the system’s 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 digester 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 digesters 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.
Fundamentals of biogas safety
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.
Faye Brophy, Axair Fans UK
H2S is a toxic gas that can cause severe health effects or death. An example of the risk of death occurs in maintenance. Empty digesters 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 prove 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 UK Government continues 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.
About the author:
With over 10 years’ experience in corrosive and hazardous environments, Faye Brophy heads up the Industrial division at Axair Fans UK Limited.
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