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Tuesday April 19 2022

Biogas Monitoring – Systems, Sensors, and Processes Explained

With a finite supply of fossil fuels that exact a heavy toll on our environment and the contentious use of nuclear power, it’s no surprise that everyone is focusing their attention on renewable sources of fuel and power.

Arguably one of the best sources of renewable energy is biogas, which is produced in anaerobic digestion plants by fermenting organic waste products. Biogas production is carbon neutral – or can even be carbon negative with certain organic feedstocks – and is a much greener source of gas for heat, power or fuel for vehicles which are the most challenging energy sectors to decarbonise.

Biogas production is a major industry across Europe thanks to early Government support to grow and develop the technology. And, with the UK government’s Green Gas Support Scheme (GGSS), biogas production here is rapidly growing, ensuring we can improve the nation’s energy security and all access more renewable sources of power as we work towards the Government’s net-zero ambitions.

Anaerobic digestion plants are solid investments. Not only can they produce electricity and heat to power your other enterprises, significantly cutting your energy bills or even selling power back into the grid, but upgraded biogas into biomethane can be injected into the national gas transmission network for heating and/or for bio-CNG/LNG vehicle refuelling application, further producing a renewable energy source whilst disposing of waste. Better still, the digestate can be used as a rich source of fertiliser and CO2 from the upgrade process can be captured delivering increased value streams.

Biogas is a valuable renewable resource, but the fermentation process must be closely monitored and managed to realise its full potential.

Biogas Plant Monitoring

Biogas is produced in a complex and sensitive process, with carefully balanced biological reactions producing a rich mix of gases. These reactions must be closely monitored to ensure the anaerobic digester is operating at full potential and isn’t at risk of the process slowing down or even collapsing.

Microorganisms anaerobically break down the waste organic matter to produce biogas through four consecutive biological processes – hydrolysis, acidogenesis, acetogenesis and methanogenesis. If any one of these processes is negatively affected in any way, there is an immediate effect on all of the other processes and the biogas plant will become unstable.

By using sensors and monitors – which measure gases, temperatures, pH, ammonia, levels of feed going in, agitation, FOS/TAC ratio, etc – you can fully understand what is happening in a biogas plant and help maintain a stable process.

Process monitoring can help provide an overall picture of the biogas process, identify any potential instabilities in anaerobic digesters before they occur and ensure that the microorganisms aren’t inhibited in any way.

The costs of monitoring are far less than the lost revenue you’d face from having to re-establish a biologically destabilised plant. In a worst-case scenario, if a biogas plant totally crashed, it could take several months to drain and then restart the process. Without effective monitoring, the financial consequences can be devastating.

Biogas digester monitoring equipment

Sensors and monitors are required at each stage of the biogas process to help maintain the delicate equilibrium and maximise production.

From measuring the potential of the material you are feeding into a biogas plant by understanding the chemical oxygen demand (COD) of the substrate, through to the make-up and value of the gas and fertilisers at the end of the process, careful management and analysis is key.

Production in biogas plants is broken down into four key stages. In the first phase, the waste material is made available, stored and treated as required and then fed into the bioreactor. In the second phase, anaerobic fermentation takes place in the digestor producing the biogas. In the third phase, the gas is treated, stored and then utilised. In the final, fourth phase, the remnants of the fermentation process are then also used – usually as a valuable fertiliser for the agricultural sector.

Using a variety of process measurement technologies that offer online monitoring, coupled with photometric cuvette tests and easy-to-use titrators, ensures you can maintain stable reactions throughout each stage of the biogas plant cost effectively.

Without careful monitoring, biogas plants are often underloaded, i.e. the materials being fed into the system to feed the microorganisms are too low and the gas generation no longer becomes cost-effective.

On the opposite scale, the effects of overloading – or overfeeding – can be catastrophic. Ironically, that deluge of material can slow down or stop the biological fermentation process and may even cause a complete collapse of the system, resulting in a costly restart of the plant.

Monitoring the biogas process

One of the most important elements of monitoring biogas production is actually measuring the gas levels at each stage of the process as these can act as early indicators of any potential problems.

Biogas is primarily made up of methane and carbon dioxide but can include small amounts of potentially dangerous by-products or other gases depending on what feedstocks are used in the anaerobic digester.

Biogas operators need to closely monitor the gases produced at each stage to fully control the fermentation process and determine what further treatments the gas needs. By understanding if the methane and carbon dioxide levels are too high or too low at the early stages of the anaerobic digestion, you can maximise the efficiency of the plant and deliver the best returns.

Critically, careful gas monitoring inside and outside the plant is also paramount to ensure the safety of your people. The flammable nature of methane, the risk of asphyxiation from carbon dioxide and the poisonous nature of some of the other gases produced as a by-product of the process means that plants must also have ambient air monitoring to check for leaked gases accumulating and posing a potential threat to safety.

Maximising biogas yields with FOS/TAC ratio

Another key measurement in biogas production is understanding the FOS/TAC ratio which provides a valuable insight into the stability of the digester and, ultimately, how much gas will be produced.

The measurement of volatile organic acids (FOS) or total inorganic carbon (TAC) can be used individually to understand the state of a digester but you can get a more accurate picture of the state of the plant by using both results as a ratio of each other.

The FOS/TAC ratio is now commonly used as it provides a simple measurement for understanding the stability of the biogas process and indicates what corrective action needs to be taken. It is also a less time-consuming and less costly way of measuring the quality of biogas plants compared to other tests.

By accurately measuring the FOS and TAC and then calculating the ratio, biogas plant operators can get a value that should ideally be between 0.3-0.4, but every digester will have a unique optimal ratio. Above that range and there is too much material being fed into the system and below the range, there is too little input.

How do you maintain biogas digester?

While the biogas process is a carefully balanced and complex biological reaction, a range of online monitoring tools and testing systems make it simple to ensure biogas plants remain stable, productive and profitable.

Anaerobic digestion plants are already proven on the continent and are rapidly growing in popularity across the UK because they provide a renewable source of energy, tackle waste and can deliver a solid return on investment – especially with the latest Government incentives.

Every biogas plant is unique and measurements, monitoring and ratios will vary based on a wide range of factors. To maximise the potential of your biogas plant, you will need expert advice to understand the range of monitoring tools and how best to apply them to your process.

Birch Solutions has extensive experience in operating AD biogas plants and maintaining optimum levels of efficiency from the support of our in-house bioresource experts, and this resource and experience is available to help you maximise the potential from your own AD biogas plant assets – irrespective of feedstocks. Birch Solutions – trusted to deliver excellence!

For advice on how to monitor your biogas plant, get in touch.