Non-Nulling Measurements of Flue Gas Flows in a Coal-Fired Power Plant Stack

摘要

Exhaust flows from coal-fired stacks are determined by measuring the flue gas velocity at prescribed points in the stack cross section. During the last 30+ years these velocity measurements have been made predominantly using S-type pitot probes. These probes are robust and inexpensive; however, S-probes measure only two components of the velocity vector and can give biased results if the stack flow has significant yaw and pitch angles. Furthermore, S-probe measurements are time intensive, requiring probe rotation (or nulling) at each traverse point to find the yaw angle. The only EPA-sanctioned alternatives to the S-probe are 5-hole probes (i.e., the prism probe and sphericalprobe) that also require yaw-nulling. We developed a non-nulling technique applicable to the spherical probe and two custom designed 5-hole probes that reduce testing time and may improve measurement accuracy. The non-nulling technique measures all 3 components of velocity without rotating the probe. We assessed the performance of these 5-hole probes in a coal-fired stack at the high-load (16 m/s) and the low-load (7 m/s). For the spherical probes, the non-nulling results and the nulling results were in excellent mutual agreement (< 0.1%). For the custom probes, the non-nulling and nulling results were inconsistent: the differences were 5% at the high load and 10% at the low load. We speculate that the nulling data for the custom probes were flawed because the non-nulling data for all the probes accurately determined the yaw and pitch angles at high and low loads. Our results demonstrate that the non-nulling technique can accurately measure flue gas flows in a coal-fired stack.