By Antti Heikkila
Waste management presents a significant challenge to communities, with wastes increasingly being viewed as resources. Organic wastes are a particular concern because of their potential to produce methane, a powerful greenhouse gas. However, the gases from the anaerobic digestion of organic wastes can be contained and utilized as a source of renewable energy.
Combustion of the gas produces renewable heat, or, in a combined heat and power (CHP) engine, it produces both electricity and heat. Biogas can also be upgraded to biomethane (>99% methane) for compression and injection into the natural gas grid or for use as a transport fuel. Anaerobic digestion plant operators can also generate income by exporting electricity to the grid, by selling heat or biogas to local businesses or communities, and by charging a gate fee for incoming waste materials. The digestate produced by such plants is rich in nutrients and can be used as a soil fertilizer and conditioner.
Regardless of the process, all plants need to optimize biogas production, while minimizing costs, waste and down time. However, biogas is corrosive and potentially explosive, so in the past it has not been possible to conduct in-line monitoring. Until recently, the only solution has been to extract samples for analysis outside of the process.
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Seeking to enable effective process optimization, Vaisala developed the MGP261, an in situ biogas monitoring instrument, for the simultaneous measurement of methane, carbon dioxide and humidity. It is Ex certified up to Zone 0/1, which enables in-line installation in pipes and ducts where explosive atmospheres exist, with the area surrounding the pipes classified as Zone 1.
Anaerobic digestion
Four main processes take place inside the digester to produce biogas. All of these processes are mediated by different groups of bacteria, and a key feature of effective biogas process optimization is the maintenance of a healthy balance of these microorganisms. The four main processes are:
Hydrolysis – Complex organic matter such as proteins, carbohydrates and fats are broken down by bacterial enzymes into sugars, fatty acids and amino acids.
Acidogenesis – Various fermentation reactions convert larger molecules into organic acids, alcohols, ammonia, carbon dioxide, hydrogen and hydrogen sulfide.
Acetogenesis – Fermented products are oxidized into simpler forms such as acetate and carbon dioxide.
Methanogenesis – Archaea (single cell organisms) convert hydrogen and acetic acid into methane and carbon dioxide.
Any disruption to the last two processes will result in a lowering of biogas yield and can be detected by changes in the methane to carbon dioxide ratio. Hence, the requirement for continuous monitoring.
Process monitoring for improved efficiency
Biogas is typically 50% – 75% methane, with the majority of the remainder being carbon dioxide and water vapour, with small amounts of other gases. By monitoring methane it is possible to measure the successful operation of the plant. By monitoring the methane to carbon dioxide ratio, the plant operator is provided with a continuous real-time indicator of digester behaviour and the status of the digester’s microorganisms.
Data on methane and carbon dioxide can be used in a number of ways. It can be used by the operator to adjust loading rate and feedstock type if possible, in order to improve the status of the bacteria. Where the biogas is being used by an engine, the measurements can be used to optimize engine performance. Where it is being refined for injection into the grid, the data can be used to inform the biomethane upgrading process.
Of course, it is also possible to extract samples from the reactor for subsequent laboratory analysis. This can provide an accurate indication of process conditions, but the delay (and cost) of doing so means that timely or automatic action to optimize the process is not possible. In-line monitoring of biogas methane and carbon dioxide helps to reduce the requirement for laboratory analysis.
Why monitor biogas humidity?
Humidity in biogas represents a potential problem for a number of reasons. Humidity in gas may condense with changing pressure or temperature, in the pressure regulator or in transfer pipes, for example. Such condensation can cause serious damage and must be avoided. Similarly, excessive humidity in biogas fed to the CHP engine increases moisture in engine oil and results in a need to replace engine oil more frequently.
Obviously, engine downtime for service maintenance or repairs should be minimized because this may result in flaring which can reduce revenue.
Humidity is also a serious consideration in the operation of activated carbon filters, because they are designed to work within specific humidity ranges. Carbon filters are commonplace because biogas impurities such as hydrogen sulfide, siloxanes and a range of other organic gases need to be removed to prevent damage to the engine, or to generate biomethane of sufficient purity to be suitable for gas-to-grid applications.
Excessive humidity causes carbon filters to wear out prematurely, resulting in a costly requirement for refilling. Some plants need to change them several times a year. However, too little humidity can also be a problem for some filters, resulting in the inefficient operation of the carbon filter.
Why measure in situ?
In the past, the only option was to employ biogas analyzers which extract a sample for subsequent measurement by electrochemical or fixed wavelength infrared instruments. These technologies require frequent recalibration, which can be costly, labour-intensive, and potentially harm the plant’s capability to monitor continuously. Pumps and gas tubing are required and it is necessary to dry the sample to prevent the errors and potential damage incurred by condensation. These instruments are therefore unable to measure sample humidity.
This also means that the measurements from extractive instruments are given on a dry basis. Such readings, by definition, will be higher than those from an in situ probe measuring on a wet basis. Vaisala’s MGP261 can provide measurements on either basis.
Sample extraction in cold climates also risks freezing of the sample line, which inhibits flow and leads to erroneous data. This problem could be rectified by trace heated lines, but they are expensive.
Summary
Effective biogas process optimization requires in situ monitoring of the key parameters, methane, carbon dioxide and humidity. Monitoring technology can help derive more value from waste, improve the profitability of biogas plants, help reduce waste, lower greenhouse gas emissions and recycle agricultural nutrients.
Antti Heikkila is with Vaisala. This article appears in ES&E Magazine’s February 2020 issue.
