Using Thermal Mass Flow Meters For Biogas Measurement

At many industrial and municipal facilities, the production of biogas is a key part of operation, and biogas measurement is necessary to ensure facility efficiency and safety. Biogas is primarily produced by the biological breakdown of organic matter in the absence of oxygen – a process known as anaerobic digestion. Feed materials include biomass, manure, sewage, plants and plant waste, grease trap contents and the organic fraction of municipal solid waste. “Biogas” in this post can refer to biogas plants, landfill gas and digester gas. These gases are used to fuel boilers that produce heat or steam and to fuel generators or gas turbines to produce electricity. In all forms of biogas production, safe and reliable gas flow measurement is essential in the collection, disposal or re-use of biogas.

Issues With Gas Flow Measurement

The difficulty in obtaining good flow measurements of any gas is the simple fact that gases are compressible, and thus the volume of the gas is dependent upon the pressure and temperature at the point of measurement. As the principle of the Ideal Gas Law states, gas volume is proportional to the temperature and inversely proportional to the pressure. This complicates gas flow measurement because many standard gas flow measurement technologies measure the flow at the actual operating pressure and temperature. When comparing natural gas usage at various combustion sources for energy-management systems, on the other hand, the desired goal is to measure the flow relative to a defined pressure and temperature — at standard conditions (standard temperature and pressure; STP).

Using Thermal Mass Flow Meters for Biogas Measurement

Biogas composition is typically a mixture of methane and carbon dioxide, with the potential trace concentration of other gases depending upon the application. Typically, this ratio is 65% methane and 35% carbon dioxide. Biogas can come from a number of sources including anaerobic digesters, landfill operations, and organic-industrial-waste processing. Other distinguishing issues with biogas are that the gas is often wet and may also be dirty. Biogas measurement systems frequently operate at relatively low pressures and low flow rates.

A Thermatel® TA2 thermal mass flow meter measuring biogas in the field.

A Thermatel® TA2 thermal mass flow meter measuring biogas in the field.

The combination of low flow, low pressure, and a wet and dirty gas rules out flow measurement technologies, due to lack of sensitivity at low flow rates and difficulties with the potential buildup of particulate matter on the flow element. By comparison, thermal mass flow meters are particularly well-suited for biogas/digester gas-flow measurement, due to the low flow sensitivity and low pressure drops. The use of an insertion probe with a retractable probe assembly eases the periodic removal of the probe for cleaning.

There are many flow measurement technologies that can be used for biogas measurement. However, thermal mass flow meters provide certain advantages in terms of mass flow measurement, turndown, flow sensitivity, low pressure drop and ease in installation. In fact, thermal mass flow meters tend to allow for very economical installations, thereby providing the lowest installed cost compared to other technologies that require pressure and temperature compensation.

Magnetrol® has created a flow solutions site, flow.magnetrol.com, where you can explore technology like Thermatel® thermal mass flow meters as well as solutions for energy management and efficiency.

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