September 15, 2006

How accurate are your reported emissions measurements?

Joseph J. Macak III, Mostardi Platt Environmental

Coal's unique issues

Because the accuracy of measurements taken on coal-fired plants tends to have a wider range, even higher uncertainty margins are the result. Once again, the uncertainty present in all parameters used in calculations should be addressed on a case-by-case basis. Continuous flow monitoring devices have the most uncertainty; quite often, comparing the relative accuracy of data from flow monitoring with that of emissions testing flow traverses indicates that significant adjustments may be needed to get the monitors back into a valid range.

To simplify monitoring, many coal plants sample and analyze their fuel daily and measure their consumption of it annually instead of tracking sulfur dioxide (SO₂) emissions directly. These facilities calculate SO₂ emission rates from fuel flow, fuel heating value, and fuel sulfur content. Because the molecular weight of sulfur is 32, and the molecular weight of sulfur dioxide is 64, every pound of sulfur contained in the fuel is theoretically converted into two pounds of SO₂.

Direct measurements of SO₂ emissions from coal-fired units with SO₂ analyzers, in conjunction with exhaust flow rate monitors, often produce numbers (in lb/hr) that are significantly higher than the theoretical values calculated from fuel sampling and analysis. This discrepancy is typically related to inaccuracies in continuous exhaust flow measurement techniques rather than inaccuracies in the SO₂ analyzers.

For example, if the coal being burned has a sulfur content of 0.65% by weight and a heating value of 9,650 Btu/lb (HHV), a heat input of 3,300 mmBtu/hr would result in a maximum emission rate of 4,446 lb/hr (assuming 100% conversion of the sulfur in the coal to SO₂). However, if the actual sulfur conversion rate is 97%, the emission rate would fall to 4,313 lb/hr. If the CEM system and exhaust flow rate monitor are operating accurately, the measured SO₂ emission rate should agree with the 4,313 lb/hr rate determined from the fuel analysis. But if the exhaust flow monitor was reading 10% high, the CEM system would erroneously calculate an SO₂ emission rate of 4,744 lb/hr, reflecting the uncertainty present in the system.

The proximity of the stack monitoring location to duct breaching and flow disturbances can cause significant stratification problems in some coal-fired units with physical constraints. Accordingly, these plants often create stratification test profiles to determine the best location for the stack probe. The stratification problems arise when load testing is conducted at only one or two load points rather than across the load range of the unit. As volumetric flow through the boiler varies, the flow profiles in the stack could change as well, leading to increased uncertainty.

—Joseph J. Macak III is a principal consultant for Mostardi Platt Environmental (Oak Brook, Ill.). He can be reached at 630-993-2127 or jmacak@mostardiplattenv.com.