Five wastewater treatment plants (WWTP) across the city of Charlotte, N.C., support more than 805,000 customers. The WWTPs operate 24/7 and produce methane-rich biogas, which is potentially a renewable energy source. The city used biogas generated during wastewater treatment on a small scale for process heating, but any remaining biogas was sent to flare. The city began considering plans to further exploit this essentially free renewable energy.
Gathering Data
In 2013, the city embarked on a combined heat and power project to generate up to 15% of its largest facility’s electrical power needs from biogas. After initial testing of the facility, possible expansion of the raw material input stream was considered for inclusion of fats, oils and grease (FOG) from schools, hospitals, restaurants and other large public buildings. The business plan for this project required specific biogas data, but existing measurement devices lacked reliability.
To determine operational control of the planned gas engine, the city needed to know the quantity, and more importantly, the quality of biogas that could be reliably produced from the facility’s anaerobic digesters. Using external feedstock (such as FOG) would introduce additional variables, which would require constant monitoring of digester gas as an integral part of the control strategy. Anaerobic bacteria will only thrive under certain conditions, and can be destroyed when feedstock and temperature are unfavorable. Measuring biogas flow and analyzing the methane content, however, had proven problematic and expensive for the city.
Biogas, also referred to as digester gas, leaves the facility’s anaerobic digesters at a comparatively low flow rate, at an elevated temperature when compared with ambient air temperature, and near atmospheric pressure. The biogas is saturated with water vapor, contains dirt particles and varies in composition. This creates a potentially corrosive condensate that will coat the inside of the pipe and pipework components. Variation in gas composition will increase with the addition of FOG. Measuring elements for instrumentation exposed to the gas can become fouled, and sampling lines for analysis can become restricted or blocked.
When evaluating flow metering technologies for the project, the city considered thermal mass technology, but this type of meter is not suited to wet biogas applications. The technology is highly flow profile dependent, inferring mass flow rate from a single-point mass velocity in the pipe cross section. The long, straight inlet and outlet pipe runs required for accurate measurement are often unavailable. Changes in gas composition and the presence of condensed water droplets also can cause errors in readings. Flow conditioner plates are not a realistic solution because of the wet, dirty nature of the gas.
Measurement Solution
In an effort to find an alternative solution capable of reliable and continuous monitoring of anaerobic digester biogas, the city began looking at transit time ultrasonic technology. Since the early 1980s, transit time has been used to measure the flow of wet, dirty, low-pressure and variable composition refinery flare gas. In conjunction with temperature and pressure data, the technology also can compute the average molecular weight of flare gas. Because it was originally developed specifically for the oil and gas and chemical industries, the instrumentation available initially did not offer the right technical or commercial solution for biogas applications, including landfill gas and coal-bed methane. The Proline Prosonic Flow B200 ultrasonic flowmeter from Endress + Hauser was developed specifically for biogas applications.
In March 2013, the city of Charlotte purchased, installed and began operation of its first B200. Used in applications worldwide since 2010 as a solution for the continuous monitoring of anaerobic digester gas, it provides reliable data for the safe and efficient operation of individual digesters and the downstream combustion process, where additional revenue can be earned from the combustion of biogas to generate electrical energy from this renewable resource.
Functional pairs of ultrasonic transducers, defined by the meter size, are mounted within the meter spool. A series of ultrasonic pulses are transmitted almost simultaneously in alternate directions, and their individual transit times are measured. When the biogas is static, the pulse transit times are identical both in the upstream and downstream directions. When flow is present, pulse transit times in the upstream direction increase, and by the same amount, times in the downstream direction decrease. The transit time difference, determined with nanosecond precision, is proportional to flow velocity and volume flow rate.
If upstream and downstream transit times are stored, summed and halved, one gets the average transit time over a distance which, if precisely known, yields an accurate sound velocity calculation. Ultrasonic transducers are in a constant state of oscillation at ultrasonic frequency, and as such, their emitting surfaces effectively repel liquid droplets and solid materials.
The B200 uses an integral PT1000 temperature sensor to determine biogas temperature. The assumption is that biogas leaving the digester is saturated with respect to water vapor, and therefore the dew point equates to the measured temperature. The B200 corrects the measured sound velocity for the presence of water vapor, ignoring all other trace components of the gas, to calculate the methane fraction of the gas, the assumed balance being carbon dioxide. When corrected volume flow rate and methane fraction are known, it only requires another stage of calculation, using an internationally recognized algorithm, to produce net and gross heating values, energy flow and Wobbe Index. For the safe and efficient management of a combustion process, such as a gas engine, these additional data are valuable to operations staff.
Immediately after installation, the city collected expected readings for both flow rate and methane content. It later purchased the additional meters needed for complete coverage of the project to ensure collection of adequate information on the cause of any declines in output or quality of gas available to the facility. Only by monitoring individual digester output can the overall scheme be effectively managed. Since 2013, neither the original B200 nor those installed subsequently have required any maintenance.
Acknowledgements: Travis Hunnicutt, city of Charlotte; Andy Campbell, city of Charlotte; and Scott Oliver, Carotek
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