Global demand for biogas and biomethane will grow significantly in the coming years, accounting for between 12 % and 20 % of the total energy market by 2040, according to recent analysis by the International Energy Agency (IEA). The IEA World Energy Outlook states that these gases can help ‘to decarbonise parts of the energy system’, ‘displace the traditional use of biomass’, ‘facilitate the rise of wind and solar’ and help communities ‘meet clean energy commitments’.
Although there is currently a significant production cost differential between biomethane and natural gas and, indeed, between biomethane and wind and solar energy, this situation is likely to change in the near future. In recent years, there has been a significant increase in the number of biomethane production plants, with capital as well as operating costs for biomethane plants becoming more competitive. Perhaps as importantly, national policy decisions and international climate commitments are creating a more favorable background against which commercial decisions for the development of new facilities can be made.
Biogas is produced from organic matter using a process of anaerobic digestion. Biogas is generally produced in biodigesters, or at landfill waste sites and wastewater and sewage plants using gas recovery systems. Feedstocks include animal and crop waste, municipal waste and sewage sludge. In each case, biogas contains a high concentration of water vapor, at around 2 %, plus a mix of methane, carbon dioxide and trace quantities of other gases; the concentration of methane varies from 45% to 75%, depending on the feedstock and method of digestion.
To create biomethane, the biogas has to be upgraded by passing it under pressure through a series of high-efficiency, semi-permeable polymeric membrane units. These separate the methane as a retentate that cannot pass through the membrane pores, from the carbon dioxide as the permeate, which passes through the pores along with other trace gases. Biomethane must have a methane concentration greater that 96 % to be suitable for injection into national gas distribution grids. Other, but less common, separation techniques use selective solvents with specialized additives, water scrubbing or pressure swing adsorption.
Although each separation process is different, they all need the biogas and final biomethane product to have as low a moisture content as possible. For example, in the membrane process, the biogas is sequentially passed through a dehydration dryer – normally either a refrigeration or desiccant-based system – followed by a scrubber, activated carbon filter and compressor, before it enters the membrane separator. Careful monitoring and control of moisture content allows the energy consumption of dryers and dehumidifiers to be minimized, reduces maintenance costs and potentially enables the operating life of desiccant materials to be extended.
Similarly, controlling the trace moisture content of biomethane is critical before it is injected into downstream gas distribution networks, to eliminate the danger of corrosion or ice formation under high gas transmission pressures. Accelerated corrosion can be a particular problem if condensate combines with other contaminants, such as carbon dioxide or hydrogen sulphide; this can lead to the formation of acidic compounds that will exacerbate the effects of corrosion in downstream equipment.
To protect distribution networks there are, therefore, commercially binding specifications that control acceptable levels of trace moisture, often with penalty clauses if these limits are exceeded. Although there is currently no common pan-European standard governing the injection of biomethane into national grids, most countries require moisture dew point to be monitored by online analyzers. Beyond this, individual nations apply different criteria. For example, in France and Italy, dew point at process pressure must be less than -5 °C, while in the UK it has to be less than -10 °C.
The next-generation Advanced Quartz Crystal Microbalance QMA601 Analyzer from Michell Instruments is designed to provide reliable, fast and accurate measurement of trace moisture content in biomethane and natural gas. The analyzer utilizes a new generation of precision crystal oscillators, guaranteeing a highly accurate measurement that is completely insensitive to changes in background gas composition. The QMA601 can offer reliability, simplicity and greatly reduced cost of ownership from trusted and proven Quartz Crystal technology.
An alternative approach is to use Michell Instruments’ Easidew ceramic metal-oxide dew-point sensors. There are already a large number of these simple and cost-effective instruments installed in biomethane plants around the world. Each Easidew sensor uses water-vapor measurement as the basis for calculating precise dew-point temperature and offers high levels of accuracy and repeatability in a compact unit that is easy to install and maintain.
All Michell Instruments’ dew-point sensors are supported by outstanding technical support and can be supplied as part of a sensor-exchange and calibration service, to ensure that your biomethane production process operates without disruption.
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With almost 50 years' experience in the development of innovative precision instruments, we are the application experts in humidity measurements for all biogas and biomethane applications. If you would like to discuss your requirements, please contact our team today.
Found this interesting? Take a look at Moisture Measurement for Hydrogen and Natural Gas Blending
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