Thermal mass flow meters provide an accurate and dependable measurement of biogas and offer distinct advantages over alternative flow meters. In any of the following biogas applications the moisture content of the gas should be considered.
Biogas is a natural occurring gas produced from the decomposition of organic material. The gas consists of a mixture of methane and carbon dioxide with small amounts of other gases including hydrogen sulfide, nitrogen, and trace amounts of other gases. Composition can range from 35% methane, 65% carbon dioxide to 70% methane, 30% carbon dioxide. Biogas can also be wet and dirty. Sources of biogas include digesters in wastewater treatment plants, landfill operations, and waste-to-energy plants.
Wastewater Treatment Plant
In a wastewater treatment plant, the incoming sewage containing organic solids is separated from the liquid. This sludge goes through various forms of treatment which may include anaerobic digestion. Large wastewater treatment facilities commonly use anaerobic digesters. In the anaerobic digester, the microorganisms decompose in the absence of oxygen which reduces the amount of sludge and produces biogas. The composition of biogas from a digester will be approximately 60-65% methane with 35-40% carbon dioxide. The biogas is burnt in an engine and a digester can produce enough biogas almost to meet the energy demands of the plant.
Read more information on wastewater digester gas in the Sage application brief, Wastewater Digester Gas at Wastewater Treatment Plant.
Landfill gas (LFG) is primarily produced by the decomposition of the organic material in municipal solid waste. The organic material is typically food scraps, waste paper, and cardboard. A collection system includes extraction wells and a series of collection manifolds which connect to the main manifold and a blower. There are various options for disposal of the LFG. A small landfill may flare the LFG on the property. Larger landfill operations may feed the LFG to a boiler or engine to produce energy. In some cases, the LFG can be treated and fed into a natural gas pipeline. The maximum flow of LFG occurs after five to seven years and will then decline. The ratio of methane and carbon dioxide will also change over the life of the landfill.
More information about the measurement of landfill gas is available in the Sage application brief, Landfill Gas Monitoring, Recovery, and Flaring.
The term “waste-to-energy plants” refers to facilities taking an organic material and converting it to biogas. The organic material may be agricultural waste such as livestock manure and byproducts from food and vegetable processing. A large covered lagoon is frequently used rather than a separate digester vessel.
Biogas Flow Meter
The accurate and reliable measurement of biogas flow rates can be challenging. Much depends on the source of the biogas and application specific issues must be considered when using thermal mass flow meters. Advantages of thermal mass flow meters when measuring biogas include:
- Mass flow measurement without the need for pressure and temperature correction
- Digester gas flow may be at low velocities. The thermal mass flow meter has excellent low-velocity sensitivity while other flow meter types do not accurately measure low flow rates.
- Digester gas is often dirty leading to the need to maintain flow meters. Thermal mass flow meters can include a retraction device permitting removal of the probe for periodic cleaning.
Digester gas can be wet and contain condensed moisture. Condensed moisture can cause spiking in a thermal mass flow meter. Often the condensed moisture can successfully be removed with the use of a moisture drop out chamber. This vessel provides a reduction in velocity and change in direction, causing moisture droplets to separate from the gas stream.
The thermal mass flow meter makes an accurate and reliable biogas flow meter, providing many advantages over alternative measurement technologies.