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Why thermal mass meters offer the solution for accurate and safe biogas measurement

Author : David Bowers, ABB

16 June 2021

After years of heavy reliance on fossil fuels, societal and environmental pressures are leading governments worldwide to look for new ways of meeting their energy demands. One of these alternative fuel sources is biogas, a side product of anaerobic digestion in wastewater treatment processes.

A biogas plant in Germany – Image: Shutterstock
A biogas plant in Germany – Image: Shutterstock

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In this article, David Bowers, Process Flow Products Manager for ABB Measurement & Analytics, explains why thermal mass flowmeters offer water and sewage treatment utilities the ideal solution for safely and accurately measuring biogas flows.

Recent declarations about the urgent need to reduce global greenhouse gas emissions have made it increasingly apparent that fossil fuels are no longer an acceptable or sustainable solution to meeting the world’s energy requirements. As a result, there has been a growing movement towards finding alternative methods of generating energy at scale whilst minimising environmental impact.

One solution that is finding favour is biogas. Classified as a renewable energy source, biogas is produced when organic matter is digested down by micro-organisms through a process of anaerobic digestion. As the organic matter is broken down, it produces biogas – a mixture of methane, carbon dioxide and traces of other contaminant gases.

This mixture can be combusted to produce energy, either through burning in boilers or recycling through a combined heat and power system. Often the electricity and waste heat generated from these processes are used to warm the digesters or to heat buildings, with excess being sold to suppliers or fed back into the local grid, providing an additional revenue stream.

How is biogas produced?

Utility companies and sewage treatment works use anaerobic digestion as part of their wastewater treatment processes to treat biodegradable waste and sewage sludge, also known as biomass.

Anaerobic digestion is a proven technology for sewage sludge treatment. During the anaerobic digestion process, microorganisms break down the organic matter contained in the sludge and convert it into biogas. At the same time, the sludge is stabilised and its dry matter content is enhanced.

An aerial view of a biogas plant in Czechia – Image: Shutterstock
An aerial view of a biogas plant in Czechia – Image: Shutterstock

Accurate measurement of biogas production enables the efficiency of the anaerobic digestion process itself to be calculated. For optimum efficiency to be achieved, it is particularly important to strike the right balance. Adding excessive biomass may inactivate the biomass, requiring a cost-intensive restart, while administering too little can impair electricity and heat generation, potentially reducing revenue.

The ability to correctly measure biogas flows is also important where Combined Heat and Power (CHP) plants are used to help generate on-site electricity and heating. In order to maximise the electricity and heat generated from biogas, there needs to be a consistent supply of biogas to the CHP plant from the anaerobic digestion process. For this to happen, the flow of biogas needs to be accurately measured in order to achieve a balance and calculate the amounts of natural gas that may be needed to increase the biogas heating value.

In both cases, difficulties can arise that can affect many of the technologies commonly used to measure biogas flows.

In many cases, variations in the quantity and pressure of the biogas produced can be caused by factors such as the time of year, temperature changes during the digestion process and the amount of sewage sludge entering the treatment works. This can make it difficult for flowmeter technologies that measure volume flow and rely on pressure and temperature compensation to obtain an accurate mass flow measurement.

The quality of the biogas itself can also pose problems, with high levels of particulates and moisture that can impair the performance of orifice plates or meters with moving parts such as turbine flowmeters.

With biogas containing a potentially explosive mix of gases including methane (CH4), there is also the need to ensure that any devices used to measure flow provide suitable protection to help guard against damage to plant and equipment and injury to personnel.

These problems can be overcome by using well installed thermal mass flowmeters which are suitable for use in potentially explosive applications. Capable of measuring mass flow directly, they can provide a mass flow rate in kg/h without any additional measurements or calculations.

Image: Shutterstock
Image: Shutterstock

How do thermal mass flowmeters work?

Thermal mass flowmeters for biogas applications work on the hot film anemometer, or thermal dispersion principle.

A heated platinum resistor is maintained at a constant over-temperature in relation to an unheated platinum sensor that is the same temperature as the gas flow. One of these resistors assumes the temperature of the flowing gas, whereas the other resistor is electrically heated and, at the same time, cooled by the gas mass flow. A control circuit applies heat to the resistor so that a constant temperature difference exists between the resistors. With a known and constant gas composition, the mass flow can be determined based on the relationship between the heater current and flow rate with no need for additional pressure and temperature compensation.

With no moving parts, thermal mass flowmeters are ideal for handling challenging conditions characteristic of biogas, eliminating the need for maintenance to deal with problems such as clogging.

As well as being suitable for measuring low flow rate, thermal mass flow meters are better suited to monitoring the anaerobic digestion process due to their low pressure loss. Because thermal mass flowmeters take measurements using two small probes on the end of an insert, they form only a minor obstruction in the surrounding flow. This means that correctly sized thermal mass flowmeters offer an extremely small pressure drop, which is important in this application where the gas pressure can drop to very low levels.

With a turndown of 100:1 they can also offer highly accurate measurement at particularly low and variable loads, with the resulting data able to be used to help optimise the efficiency of the anaerobic digestion process. It is incredibly important to have this low turndown particularly when working with flammable gases such as methane. These need to be operated at a low pressure to reduce the likelihood of a fire. Using an ATEX rated thermal mass flowmeter provides an added safeguard against the risk of explosion in biogas measurement applications.

Image: Shutterstock
Image: Shutterstock

Installation tips for top thermal mass metering performance

To optimise measurement performance in anaerobic digestion processes, it is important to ensure thermal mass flow meters are installed correctly. Installation should be behind a foam trap and gravel filter to maximise measurement accuracy and reduce contact with condensate. Installing the meter in a 90° position can also afford protection against measurement interference caused by condensate. Further interference can also be avoided by fitting a water trap to capture water and moisture build up.

Summary

With the cost of energy produced by conventional sources continuing to escalate, the use of biogas to help address on-site energy requirements and as an additional potential revenue stream offers an increasingly attractive proposition for water utilities looking to save costs and boost profits.

With their inherent operational benefits, plus a low cost of ownership through the elimination of peripheral temperature and pressure equipment and the reduced need for maintenance, thermal mass flowmeters offer the ideal solution for biogas measurement applications.

About the author:

David Bowers has been Product Manager UK and Ireland for Pressure and Process Flow at ABB Measurement & Analytic for the past seven years. In total, David has 31 years’ experience in the instrumentation industry.


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