Flow meter selection for biogas flow measurements

Green energy sources are a necessity. Fossil fuels will run out one day and research is being conducted into alternative energy sources in many areas. Waste wood is used in power plants via wood gasification installations. And biofuels are now being produced from corn, rapeseed, but also from used cooking fats and more.

Gas composition of biogas and landfill gas

Biogas is the gas that is released when biowaste or another substance is fermented. It can serve as an alternative for natural gas. A fairly large part of this biogas consists of methane (CH4), the rest is carbon dioxide (CO2). The fermentation process is exothermic – it releases heat. Partly because of this, the biogas is always saturated with water vapour, while low concentrations of corrosive gases, such as hydrogen sulphide (H2S), can also be formed.

Another form of biogas is landfill gas. This is the gas produced in a landfill. The composition and challenges for flow measuring landfill gas and biogas are similar.

We are dealing with dirty, wet gases that contain corrosive gases and liquids. Until now, there were not many suitable flow meters for many of these applications.

Which flow meter for biogas

For the wet biogas (wet = raw, from the source, it still contains all kinds of components that will be removed later to obtain a usable gas composition) a few measuring techniques can be considered. Assuming that droplets are present turbine flowmeters or rotor meters are usually not chosen. Because those droplets give pollution and extra corrosive conditions.

So which flow meter do you choose for biogas? We have been using Transus’ ultrasonic flow meters for the last few years. The new UIM-3/3F ultrasonic flow meters use a patented Broadband Continuous Wave system for signal processing which ensures that each packet of data sent is checked on reception. This makes the Transus flowmeter insensitive to droplets in the gas, as the scatter on those particles is filtered out and only the good measurements are used for the reading. Furthermore, the mechanical design is very robust and proven to withstand aggressive media. Transus achieves this by using a stainless steel meter body and highly efficient titanium transducers, which make the measurement reliable and accurate. With these ultrasonic flowmeters, even calculation of mass flow and molecular weight is possible. Therefore, a methane content calculation with this flow meter is also possible and is already provided in the supplied flow computer.

Photo: an ultrasonic flowmeter, Transus UIM 3

Other options for flow meters for measuring biogas are vortex flowmeters and pitot tube flow meters, with integrated pressure and temperature compensation if required. These have the disadvantage that they have parts that hinder the flow. And with aggressive and abrasive gases, these types can suffer from wear and tear. But it can be an option if the gas composition lends itself to it.

Dew point and moisture content

The droplets form because the dew point is too high. One can decide to treat the gas and lower the dew point. Although the relative humidity will still balance around 100 %, there is no fluid fraction anymore. In such cases turbine flowmeters and thermal insertion flowmeters can be used. However, with thermal insertion flowmeters one should always be aware of the fact that this type of flowmeter reacts to variations in the gas composition.

A possible alternative is a differential pressure flowmeter. But this is only possible if the process pressure is so high that pressure drop is acceptable. Usually, however, the process pressure is very limited, so an instrument with a minimal pressure drop is preferred.

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