Narrow stack emissions: Errors in flow rate measurement due to disturbances and swirl

Abstract

European legislation continues to drive down emission limit values, making the emission measurement of narrow stacks of increasing importance. However, the applicable standards (EN ISO 16911-1 and EN 15259) are poorly validated for narrow stacks, and the effect of flow disturbances on the described methods are largely unknown. In this article, measurement errors are investigated in narrow stacks with flow disturbances and swirl, both experimentally and through computational fluid dynamics (CFD) simulations. The results indicate that measurement errors due to misalignment of the flow with typical measuring probes (pitot tubes) are small compared to errors resulting from the positioning of these probes in the measurement plane. Errors up to 15% are reported using the standardized methods, while the measurement error is both smaller and more predictable when using additional measurement points. Implications: Current international standards provide methods to measure emissions from industrial stacks. With increasingly small emission limit values, the accuracy of these measurements is becoming considerably more important. The data from this study can be used to inform revisions of these standards, in particular with respect to flow disturbances in narrow stacks, and can help law-and policy-makers to obtain insight into the uncertainties of emission measurements in these specific situations.

Modelling of uncertainties of an emission concentration measurement in stacks

“…The velocity fields computed by MPPICFoam was observed to be identical as given by simpleFoam in [5], what is however in correspondence with the assumption that the particles are not influencing the fluid flow and that both solvers are consistent in resolution of basic fluid flow.…”

“…In Figure 1, the straight supply pipe, a), single 90° elbow, b), and double 90° elbows c) are depicted. Note, that these topologies are similar as used in paper [4] and [5], where simulated velocity fields were used for discussion of uncertainty of flow rate measurements with Pitot tube or ultrasonic flow measurement. The diameter D equals 0.75 m, since the computational cost for original diameter 1.5 m as used in [4] and [5] has been shown as too high for finishing all defined combinations of parameters in reasonable time frame.…”