Stacks

Stacks Improving Accuracy of Plant Emission Monitors Major industrial facilities are required to continuously monitor plant emissions to the atmosphere. The continuous emissions monitoring systems (CEMs) in use today are highly sensitive to flow conditions in the vicinity of the monitor probes. If turbulent, swirling flow exists, measurement accuracy is degraded. For optimal accuracy, the flow passing the monitor should be unidirectional and uniform in profile.
At Gainesville Regional Utilities' Deerhaven Station, cyclonic flow in the chimney was an occasional issue under certain operating conditions. During these times, the plant had to modify their operating conditions to minimize the flow-related problem. GRU contracted Airflow Sciences Corporation to determine the root cause of the flow problem and develop a cost-effective design solution.
Airflow engineers utilized a computational fluid dynamics (CFD) flow model of the chimney and duct system in the analysis. This model provided a clear understanding of the flow profiles that set up the cyclonic non-uniform flow. Model results are shown in Figure 1. At the monitor location, a highly non-uniform velocity distribution was present.
The geometry of the chimney inlet ductwork did not allow the two incoming flow streams to join uniformly. The result was a biased velocity profile which sets up the corkscrewing, cyclonic flow in the chimney. Model results were confirmed via comparison to plant measurements (relative accuracy test audit, or RATA, data), shown in Figure 2.
Figure 2. Baseline Velocity Profile at Monitor Location
Figure 1. Baseline Model Results

The model was then used to evaluate a number of design changes to improve the flow. The final design, installed in the plant in March, 1999, reduced the cyclonic flow and significantly improved the velocity profile at the monitor position. Stack test data after the installation is shown in Figure 3.	Figure 3. Post Installation Performance - Velocity Profile at Monitor Location
Since the installation, the plant has not experienced any issues with the CEMs' accuracy or repeatability due to cyclonic flow.

Improving Accuracy of Plant Emission Monitors