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Flow, Level & Pressure Measurement

Thermal Dispersion Gas Mass Flowmeters Find Energy Efficient Uses

May 16, 2012
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While thermal dispersion gas mass flowmeters have long had a proven track record in municipal wastewater treatment process applications, they are now finding additional municipal and industrial process applications driven by multiple U.S. government initiatives. New clean water, air emission and greenhouse gas regulations from the U.S. Environmental Protection Agency (EPA) and alternative energy mandates by the U.S. Department of Energy (DOE) are requiring process engineers to upgrade their processes and plants.

A Southern California manufacturer of thermal mass flowmeters has been following and responding to this trend for some time and working closely with customers who are tasked with meeting these new pollution control, monitoring and reporting requirements. In addition, the high cost of energy is driving and providing economic justification for more municipal water and industrial process plants to harvest by-product waste gases to fuel on-site CHP and co-generation electric power systems.

“Newer applications in areas such as bio-mass, landfill gas recovery, coal mine and coal bed methane recovery and other green energy processes, as well as clean coal gasification and ethanol production are ideal for thermal flowmeters. They can measure extremely low flows, have wide turndowns, and are accurate in these often mixed-gas composition applications. Furthermore, there is no necessity to generate a differential pressure and they do not create a large pressure drop,” senior member of the technical staff, Fluid Components International (FCI) LLC, San Marcos, Calif., Jim DeLee says.

It’s well known that the differential pressure (dP) method calculates fluid flowrate by measuring the pressure drop across a pipe restriction and this technique has a long history in industrial applications. However, when measuring the flow of compressible gas materials, volumetric flow is not very meaningful. And, to infer mass flow requires adding temperature and pressure sensors and a mass flow computer. Thermal dispersion mass flowmeters, on the other hand, measure mass flowrate directly.

Why It Works Well

Thermal dispersion technology places two thermowell-protected platinum RTD temperature sensors in the process stream. In the constant power thermal dispersion technique, one RTD is heated while the other senses the actual process temperature. The temperature differential between the two sensors is measured and is directly proportional to the mass flowrate of the fluid. It’s a highly accurate technology, and because there are no moving parts to wear or foul, there is virtually no maintenance.

Thermal dispersion’s direct mass flow measurement technology is well suited to provide flow rate and totalized flow data for process control, emissions measurements and regulatory compliance as well as for carbon trading and greenhouse gas reduction incentives and credits. These instruments measure flowrate over a wide flow range, feature up to a 1000:1 turndown and can be applied to pure gases or mixed-composition gases as well as clean or dirty, dry or wet gas installations. Thermal dispersion mass flowmeters do not have line size limitations. They can be installed in line sizes as small as 0.25 inch to the largest of stacks.

“Often new applications occur because the EPA or other regulatory agencies issue regulations that mandate new or improved gas flow measurement, control and reporting. Our customers look to us for solutions to meet these mandates,” DeLee says. “Additionally, when oil and energy costs increase, plant engineers look to improve process efficiencies and usee alternative energy sources,” DeLee adds.

Municipal wastewater treatment plants still remain one of the largest users of thermal flowmeters. “Their primary applications are in aeration basin air flow control and digester processes,” DeLee says. “In aeration, air is pumped into basins to sustain the microorganisms that treat the sewage. Accurate measuring and control of the air flow is essential to an effective process and minimizing the plant’s energy costs,” DeLee said. If volumetric flowmeters, such as dP, are installed in this process, they can be thrown off by seasonal ambient temperature changes and generate a pressure drop that requires additional energy expenses to overcome.”

The anaerobic digesters, used by many municipal wastewater treatment facilities, generate digester gas, which is a mixed-composition of methane, CO2 and trace gases. While historically flared, rising energy costs and GHG regulations have provided the incentive to waste water treatment plant facilities to capture and utilize the gas. Today, the methane is fed to engines and turbines to produce electricity to supply on-site power or be exported to the public power grid.

“Thermal flowmeters are widely deployed and the industry’s preferred solution for digester gas measurements throughout this process,” Delee says.

Thermal mass flowmeters are also applied in other environmentally-sensitive industries and processes. Landfills, for example, use thermal flowmeters to measure the amount of methane that is flared or is produced in recovery systems that are producing electric power. Similar uses of thermal flowmeters are found in coalmine methane recovery and at refineries and power plants.

Other Notable Uses

“We believe there is a synergy arising from the need for greenhouse gas monitoring, electric power co-generation and alternative energy resource development that is expanding the applications for thermal mass flowmeters. This trend is expanding from the waste water treatment plant sector into other impacted and opportunistic industries,” DeLee concludes. Some of these applications are described briefly as follows:

In ethanol production and refining, thermal mass flowmeters accurately measure fuel gas, air flows and waste gases in lines operating with variable temperatures and flow rates to optimize ethanol process productivity. Production is a distillation process relying on boilers whose efficiency is optimized by controlling air-to-fuel ratio using flowmeters. In addition to producing ethanol, this process generates waste gases with volatile organic compounds that are monitored with flowmeters.

Thermal mass flowmeters in coal mine methane recovery systems measure the extracted gas, support efficient operation of co-gen engines or methane oxidizer systems and provide data for GHG reporting and incentive credits. Recovery and use of methane gas from coal mining is creating a new energy resource and reducing a major source of green-house gases. Three major sources of coal mine methane (CMM) are degasification systems (drainage type), both pre-mine and gob, ventilation air (VAM) and abandoned or closed mines.

Thermal mass flowmeters ensure the efficiency of on-farm digesters and biogas production processes. Fecal waste from dairy farms, swine ranches, cattle feed lots and other livestock operations can be processed in on-farm digesters to produce biogas for use as fuel gas. Rather than emit methane into the environment, farmers capture the gas for on-site power-plant operations or to export to the public power grid.

Thermal mass flowmeters measure biogas from biomass fermentation and recovery operations. A byproduct of organic waste from fruit and vegetable peelings or meat preparation in the food and beverage industry, biogas is a mix of methane and carbon dioxide, as well as water and trace amounts of hydrogen sulfide. Crop, food or agricultural waste is digested under anaerobic conditions in a reactor tank or fermentation tower with the biogas used as fuel for heating or to power an electricity generating engine.

This article originally appeared in the May 2012 issue of Water/Waste Processing.

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