Wastewater Biogas systems can benefit from cost-effective and accurate wet gas thermal flow meters By Tom Morjig A typical anaerobic digester in a municipal wastewater facility processes human waste, FOG (fat, oil, and grease), and other organic material, and heats this sludge to about 38°C. The heated sludge creates biogas, a methane gas mixture (CH4-CO2) containing a signi¿cant amount of water Yapor. 'aily and seasonal temperature Yariations can affect the density of the water Yapor. :arm temperatures cause Yery small water particles, while cool temperatures create large ones with increased water entrainment in the pipe. 5eliable Àow measurement is the leading indicator of digester operation and health. Historically, wastewater facilities haYe ignored digester data, because the information was unreliable. 1ow that Àow meter manufacturers are focusing on the unique requirements of the wet gas enYironment, plant operators can learn to trust Àow meter data. They can then use digester production information to aYoid upsets and taNe pre-emptiYe correctiYe action. Measuring biogas in digesters and other wet gas applications Typically, wet gas measuring applications remoYe water and then use any Àow meter that can measure a dry Àow or Yelocity. Two common methods haYe been employed to alleYiate the issue of condensation affecting Àow meter performance: 1. Mechanical refrigeration systems/ desiccant dehumidi¿er systems, worN by remoYing moisture through absorption or adsorption. They typically haYe a high initial cost, a moderate-to-high maintenance cost and operational energy costs. 2. The heat tracing method raises the process Àow temperature by warming biogas pipes. This Neeps the water Yapor in smaller particles, while lowering the CH4-CO2 concentration. :hen installed in a Class , 'iYision or 2 area, these systems haYe an oYerall high cost, eYen though the initial cost and maintenance/opera- www.esemag.com 29-Advantages for Biogas Enviro.indd 45 Figure 1. Annual wet gas measured at three digesters (blue) using conventional thermal flow meters compared to the local ambient temperature (orange). tion costs can appear moderate. For example, a modest three digester system would consume oYer 2, annually in energy costs. Based on ./N:h for a cm pipe with cm of insulation coYering 4 m, raising the pipe temperature ° would require : of heat. :hen considering any Àow metering technology, additional requirements beyond the initial deYice should be included as part of the oYerall cost of the deYice. Biogas applications typically inYolYe low Yelocities, which is one of the maMor criteria in selecting a Àow monitoring technology. Ultrasonic meters are an option, but their typically high purchase price has essentially blocNed them from the marNet. ,n addition, they require bypass plumbing and periodic tear-downs for cleaning. :hile ultrasonic deYices intrinsically measure actual Àow or Yelocity, they must be pressure and temperature corrected for mass Àow, which is the required measurement to meet regulations. Thermal Àow meters are frequently used because of their ease-of-installation and low Àow capabilities. HoweYer, they are sensitiYe to moisture in the Àow stream, and additional processes must be used to control condensation. ,n considering product life cycle and functionality issues, insertion thermal meters are generally the technology of choice. The Nature of Biogas ,n consistently warm temperatures, water particle si]e is less than 2 microns. :ater-saturated biogas has a mbar partial pressure that represents . per cent water in the methane mix at sea leYel air pressure ( mbar). :ater Yapor fogging density, or liquid water content (LWC) in g/m3, is proportional to the cooling rate in a pipe. The heat used in a thermal sensor is typically enough to Yapori]e the particle. When the biogas pipe coming from the digester cools to less than 21°C, saturated water partial pressure drops to 2 mbar, or 2. per cent water Yapor. As the pipe cools, particles conYerge and get larger (with many of these contacting the thermal sensors). (Yen more particles contribute to the liquid Àow along the bottom of the pipe. 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This is four-times higher than a standard 75°C Kurz meter and much higher than other thermal meters at approximately 10°C to 50°C. It is important to note that the heated sensor is well below the 537°C auto-ignition point for methane gas. Liquid impacting the hot sensor is immediately vaporized to steam. The power required to vaporize the water at a given Àow rate is de¿ned by the LWC, gas velocity, and sensor effective area. Figure 2 compares three different thermal meters installed on a digester, where red is the biogas temperature, blacN is the dry Àow, blue is the Kurz WGF, purple is the Kurz standard meter, and orange is a popular thermal competitor. During the warmest part of the day, all the wet gas meters converge and are reading about six per cent high compared to the treated dry Àow meter. During cooler periods, the readings separate out and start over-reporting at various levels. Separation is related to the LWC condensation density in the pipe, which is highest at lower temperatures. FOG dosing to achieve more consistent gas production, alter biogas production to maximize electrical savings based on daily utility rates change, and minimize gas Àaring. With cost-effective, reliable wet gas Àow measurements, they can be ready to accurately report biogas sourc- Conclusion 5eliable and accurate Àow meters offer many bene¿ts. %y comparing the output from each digester against total biogas production on the fuel/destructor side of the facility, digester and facility issues, such as imbalances and leaks can be detected. Plant operators can adjust sludge and NETZSCH Pumps & Systems - Solutions you can trust www.esemag.com 29-Advantages for Biogas Enviro.indd 47 es as laws limiting greenhouse gases are introduced and enforced. Tom Morjig is with Kurtz Instruments. The company is represented in Canada by SPD Sales. 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