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Today, inconsistencies between measured CO2 emissions and emissions calculated from coal consumption are on the order of 10% to 20%. The objective of this project is to develop the technical basis for measuring the CO2 flux with an uncertainty on the order of 1% at a reasonable cost. [1,2,3]


The Flow Metrology Group is using the smoke stack simulator shown in Fig. 1 to critically test conventional and alternative ways of measuring the flow of stack gases. The inlet cone and reference (upstream) section of the simulator draw in ambient (outside) air and generate a swirl-free, fully-developed turbulent flow. The reference section features an 8-path ultrasonic flow meter that NIST calibrated with an uncertainty of 0.5% (at a 95% confidence level) while it was installed in the inlet section.

diagram of nist's smoke stack simulator test bed

Figure 1. NIST's smoke stack simulator. The test section has a length of 28 m and a diameter of 1.2 m (4 feet) which is approximately 1/10th the diameter of a power plant stack.

Figure 1 shows a right-angle bend with a "dead" volume between the reference section and the downstream test section. The bend simulates a typical connection between a power plant's pollution control system and its stack. The bend generates counter-rotating vortices in the test section. Obstacles (such as perforated plates) can be inserted in the test section to further complicate the flow. The exhaust fans generate flows of 6 m/s to 25 m/s in the 28 m-long test section which has a diameter of 1.2 m (4 ft).

The test section is instrumented to quantify the errors and uncertainties encountered when EPA-approved Continuous Emission Monitoring (CEM) protocols are used to measure the CO2 flux. Thus, it accommodates 1-path and 2-path ultrasonic flow meters at various angles as well as automated surveys using S-probes and 3D-probes following EPA Relative Accuracy Test Audit protocols.

We seek to reduce the errors in flow measurements by using

  • a laser doppler anemometer to measure interference between pitot tubes and the walls
  • computational fluid dynamics to model/predict flow-dependence of swirl generated at stack entrance
  • computational fluid dynamics to model/predict meter responses to swirling flows
  • 3-path and 4-path ultrasonic flowmeters

We are exploring alternative methods of measuring flue gas flows such as

Created March 9, 2014, Updated March 2, 2020