Biogas production is a vital part of the operation at many industrial and municipal facilities, 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, plant and plant waste, grease trap contents, and the organic fraction of municipal solid waste.
“Biogas” in this article may refer to biogas plants, landfill gas, and digester gas. These gases fuel boilers that produce heat or steam and fuel generators or gas turbines to produce electricity. In all forms of biogas production, safe and reliable gas flow measurement is essential for the collection, disposal, or reuse of biogas.
Utility companies and sewage treatment works use anaerobic digestion to treat biodegradable waste and sewage sludge to produce biogas. Biogas is a mixture of methane, carbon dioxide, and other contaminant gases, a by-product of the microorganisms breaking down the organic matter in the sewage sludge. In addition, the sewage sludge is stabilized, and its dry matter content is reduced. Today much of the biogas produced in anaerobic digestion plants is from those on municipal wastewater treatment sites. However, they are also used to break down and recycle food waste.
The biogas produced must be measured to run an efficient anaerobic digestion process. The rate of digestion can alter depending on the weather, the time of year, and the amount of biomass added. Thermal mass flow meter can accurately measure the mass and composition of biogas. However, they also help control the process by adding oxygen or air to the digester to ensure the bacteria thrive. Biogas production is one of many uses for thermal mass flow meters. The mass flow is a critical measurement for determining the flow of gases and controlling combustion.
Mass flow can also be crucial for maintaining many industrial processes, including in the water industry, the amount of ozone delivered to water to clean the bacteria from it or carbonizing the water to adjust its pH. In industry, they are used to measure the use of compressed air, or in the food and beverage industry, the amount of nitrogen used as a preservative in packaged foods. Thermal mass flow meters are, therefore, commonly used for measuring the flow of gases.
Concerns with Gas Flow Measurement
The difficulty in obtaining good flow measurements of any gas is a simple fact that gases are compressible. Thus, the gas volume depends upon the pressure and temperature at the point of size. As the Ideal Gas Law principle states, the 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 flow at actual operating pressure and temperature. By comparing the use of natural gas to various combustion sources for energy management systems, on the other hand, the objective sought is to measure flow against a set pressure and temperature — under standard conditions (standard temperature and pressure; STP).
How Thermal Mass Flow Meters Work for Biogas Measurement
The composition of biogas is usually a mixture of methane and carbon dioxide, with the potential concentration of trace amounts of other gases depending on the application. Typically, this ratio is 65% methane and 35% carbon dioxide. Biogas can come from several sources, including anaerobic digesters, landfill operations, and organic industrial-waste processing. Other distinctive issues with biogas are that the gas is often wet and can also be dirty. In addition, biogas measurement systems frequently operate at relatively low pressures and flow rates.
Many flow measurement technologies lack sensitivity at low flows/pressures and can be challenging to clean if buildup occurs. By comparison, thermal mass flow meters are particularly well-suited for biogas/digester gas-flow measurements due to the low flow sensitivity and pressure drops. In addition, using an insertion probe with a retractable probe assembly eases the periodic removal of the probe for cleaning.
Many flow measurement technologies can be used for biogas measurement. However, thermal mass flow meters provide certain advantages regarding mass flow measurement, turndown, flow sensitivity, low-pressure drop, and ease of installation. In addition, thermal mass flow meters allow for very economical installations, thereby providing the lowest installed cost compared to other technologies requiring pressure and temperature compensation.
Accurately measuring biogas flow rates can take time and effort. It all depends on the source of the biogas, and the user must consider application-specific issues when using thermal mass flow meters.
Benefits of thermal mass flow meters in biogas measurement include:
- Gas flow from the digester may be at low velocity. The thermal mass flow meter has excellent sensitivity at low speeds, while other biogas flow meters do not accurately measure low flow rates.
- The gas in the digester is often dirty, so the flow meters need to be maintained. Thermal mass flow meters may include a retraction device that allows the probe to be removed for periodic cleaning.
The thermal mass flow market is one of the fastest-growing flow meter segments. It will continue to rise due to environmental regulations and cost savings associated with energy management. Although the oil and gas industry is the primary driver, these flow meters can be found in many industrial, municipal, and commercial facilities.
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