The amounts of CO2 and other pollutants emitted by a coal-fired power plant are measured using a continuous emissions monitoring system (CEMS) permanently installed in the exhaust smokestack. The pollutant flux is the product of the pollutants concentration and the flow in the stack. The concentration measurements are traceable to certified reference standards; however, the complex velocity fields (i.e., swirling flow with a skewed velocity profile) in stacks make accurate flow measurements difficult. Therefore, the CEMS flow monitor, which commonly consist of a single-path ultrasonic flow meter, must be calibrated at least once a year by a procedure called a relative accuracy test audit (RATA). This calibration is generally performed using a differential pressure probe called an S-type pitot probe. However, S-probes are not accurate when used in complex velocity fields. NIST developed a 1/10th scale-model smokestack simulator (SMSS) to quantify the uncertainty of diverse stack flow measurement techniques. The SMSS generates complex, stack-like flows, but with an expanded uncertainty (95 % confidence level) in flow velocity of 0.7 %. Using the SMSS, we assessed S-probe-based RATAs and both single and two-crossing-path (X-pattern) CEMS ultrasonic flow monitors. Remarkably, the X-pattern ultrasonic CEMS deviated by only 0.5 % from the NISTs flow standard. In contrast, a single-path ultrasonic CEMS deviated from NIST standards by 14 % to 17 % in a highly distorted flow. Deviations for the S-probe RATAs ranged from 5 % to 6 %.
2018 International Symposium on Fluid Flow Measurement (ISFFM)
March 21-23, 2018
RATA, CEMS, stack flows, S-probe, Ultrasonic Flow Monitors, SMSS, Scale-Model Smokestack Simulator