Precise biogas flow measurement

Paul Boughton

Scott Rouse reveals how to overcome the challenges of changing gas composition.

Harnessing biogas from landfills is a quickly growing source of this renewable energy. The USA’s Environmental Protection Agency (EPA) estimates that there are approximately 6,000 landfills in the USA, contributing an estimated 650 billion cubic feet of methane per year. Landfill gas, containing mostly CH4 and CO2, is produced by wet organic waste, decomposing under anaerobic conditions in a landfill that is covered and mechanically compressed by the weight of the material that is deposited from above. This material prevents oxygen exposure thus allowing anaerobic microbes to thrive. As seen in Fig. 1, this gas builds up and is slowly released into the atmosphere.

The flow measurement challenge in such applications is the fact that the composition of biogas varies depending upon the source. Most flow meters are calibrated for one specific gas mix composition; they cannot provide accurate mass flow meter readings if the composition changes without sending the meter back to the factory for recalibration.

How to measure biogas and manage variable composition

Since the biogas composition is critical to its energy-producing value, facilities need to assess the best flow meter measurement technology to manage the compositional changes. Many companies with varying technologies are interested in measuring the biogas as it leaves the landfill or digester tank, but this is a challenging application for many reasons. Firstly varying gas compositions make accurate measurements difficult because most meters are calibrated for one gas or mixture; when the composition changes, the flow measurements are no longer accurate and the meter must be recalibrated.

Secondly, low pressure makes differential pressure devices such as orifice plates unsuitable since they require a fairly large differential pressure to operate.

Finally, biogas is often very dirty with a high moisture and particulate content, which can clog up devices such as annubars and orifice plates, and gum up turbine meters and similar instruments that have moving parts.
Traditionally, thermal mass flow meters have been the instrument of choice. They offer reasonable accuracy for the price (2% of reading) and use a convenient insertion design that eliminates pressure drop. They also have no moving parts and can measure both high and low flows with a 100:1 turndown.

Although such meters do many things well, one thing they cannot do is account for changes in biogas composition. These flow meters must be calibrated for a specific biogas mix and rapidly lose accuracy if gas composition changes, which means the instrument must be sent back to the factory to recalibrate for the changing gas composition – wasting time, resources and money. One way to account for variable composition is the use of a continuous real-time sampling system integrated with a flow meter.

New metering technology 

Recently, thermal technology has undergone significant advancements, moving from two-sensor to four-sensor technology (see Fig. 3), which yields unprecedented accuracy for thermal insertion flow meters of +/-0.75% of reading (far better than the 2.0% reading possible previously with other thermal technologies). New four-sensor quadratherm technology, pioneered by Sierra, is emerging as an optimal solution for accurately measuring and managing biogas even with its changing gas composition.

Along with this new four-sensor technology, traditional analogue measurement circuits, such as the Wheatstone bridge, have been superseded by more powerful hyper-fast microprocessors that run comprehensive flow-measurement algorithms to compute mass flow. This proprietary algorithm set serves as the ‘brain’ of the mass flow meter, using inputs from the four sensors to solve the first law of thermodynamics for the sensor in the biogas flow stream. This allows for precise flow measurement and also enables the management of gas composition because recalibration every time the gas changes is no longer required – a breakthrough in mass flow measurement.

By combining four-sensor technology with this algorithm set, the meter has the ability to change gas and compositions without losing accuracy (see Fig. 2.). This new technology creates many benefits. The meter can hold up to four user-customisable gas mixtures onboard and store biogas composition in a proprietary gas library, easily accessed through user software. Engineers and operators have access to this gas library, which contains all the gas properties needed to make algorithmic gas mass flow rate calculations.

Once sampling has determined the biogas composition, operators can use a simple software tool to create and name a proprietary biogas mixture. This allows operators and engineers to use just one meter with one calibration for varying gas compositions. 

Finding the best flow meter for this biogas measurement technology is critical for optimising the energy yields of biogas production. With these new advancements in four-sensor quadratherm technology, operators now have higher accuracy with changing gas composition and more field flexibility. This allows them to monetise biogas and use it to get the top efficiency out of cogeneration gas engines and enables highly accurate custody transfer of gas to the collection system.

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Scott Rouse is VP of Product Management at Sierra Instruments.

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