Presently used methods to measure CO2 and other emissions from smoke stacks have errors of 20 % or more depending on the level of swirl in the flow. The objective of this project is to measure stack emissions with an uncertainty on the order of 1% at a reasonable cost. [1, 2, 3]
NIST and power industry partners represented by the Electric Power Research Institute (EPRI) are using the NIST smoke stack simulator and field tests at power plant stacks to show that NIST designed sensors and methods reliably attain 2 % uncertainty, regardless of the swirl level in the flow. NIST smoke stack profiling methods and sensors are resistant to plugging by solids and water in smoke stacks, are 4x faster, and are 10x more accurate than presently used methods.
The Fluid Metrology Group is using its smoke stack simulator (See Fig. 1.) and field tests in operating smoke stacks to critically test conventional and improved ways of measuring the flow of stack gases.
In the NIST stack simulator, 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 0.8% (at a 95% confidence level) while it was installed in the same inlet and outlet pipes.
Figure 1 shows a tee connecting the reference section to the downstream test section. (One leg of the tee is a small "dead" volume.) The tee generates counter-rotating vortices in the test section, thereby simulating the flow generated by a typical connection between a power plant's pollution control system and its stack. Obstacles (such as perforated plates) can be inserted in the test section to further complicate the flow. The exhaust fans generate flow velocities typical of power-plant stacks (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 smoke stack simulator generates known (±0.8 %) flows for evaluating:
Improved profiling methods and sensors:
NIST and industrial collaborators developed new methods (e.g. the NIST non-nulling or NNN method) that are faster, more accurate, and easier to apply than the presently used methods. The NNN method and new probe designs have been tested in the NIST wind tunnel, the Smoke Stack Simulator, and in multiple field tests at power plants. The stack simulator is preferable for assessing the accuracy of the NNN method because it has a low uncertainty reference (0.65 %). Field tests are essential for demonstrating the practicality, robustness, and time efficiency of the NNN method.
Stack simulator results:
Field test results:
NIST stack profiling methods and sensors do not suffer from plugging, are 4x faster, and are 10x more accurate than presently used methods. Presently used methods have errors of 20 % or more depending on the level of swirl while the new method will reliably attain 2 % uncertainty, regardless of the swirl level. Ongoing work on sensor calibration will reduce the NNN uncertainty to 1 %, a 20x improvement over the presently used method.
We are also exploring alternative methods of measuring flue gas flows such as: