Test set #3: EU Standard EN-13211
This European standard specifies a manual reference method for determining HgT concentration in the flue gas of a power plant stack. Similar to the U.S. RM, a specific-volume sample of the flue gas stream is extracted isokinetically from a stack over a certain period of time at a controlled flow rate. Particulate matter in the sampled gas stream is collected on a filter. The particle-free flue gas stream passes through a series of impingers (absorbers), which contain solution(s) for collecting gas-phase Hg. At the end of the sampling period, the filter and impinger solutions are analyzed in a laboratory.
The sampling equipment consists of a heated probe and a sampling nozzle made of titanium, a filter housing, a series of impingers, a suction unit with a gas metering device, and a flow controller. Depending on the type of impingers used, two different sampling arrangements may be employed: main stream and side stream. In the main stream arrangement, shown in Figure 9, all sampled flue gas flows through the impingers, while in the side stream arrangement only some of the flue gas is passed through the impingers. Depending on the arrangement, one or two suction units may be used.
9. Gently down the stream. A schematic of the EU sampling train with the main stream arrangement. Source: Lehigh University Energy Research Center
Figure 10 compares the HgT values measured by the two EN-13211 sampling trains and the two paired OHM trains. The correlation coefficient R2 of 0.9521 indicates a very good agreement between the data sets. The scatter in the HgT concentrations measured when low-Hg coal was fired is considerably lower than when high-Hg coal was burned.
10. Less mercury, less scatter. EN-13211 compared to OHM on a wet basis. Source: Lehigh University Energy Research Center
Again, the test data were divided into high-Hg and low-Hg data sets when calculating test statistics. The results show that the average HgT values measured by the EN-13211 and OHM methods were very close (within 0.5 µg/sm3) for both the high-Hg and low-Hg coals. The higher precision of the OHM could be attributed to the larger number of samples it used (two paired trains vs. two single trains for EN-13211).
The average bias between the EN-13211 and OHM readings was 2.9%. The bias for the high-Hg coal was 2.5%. For the low-Hg coal, the error was larger (3.8%) but still well within the ±RSD and ±RCI intervals of the OHM. As a result, one could conclude that the bias between EN-13211 and OHM is not statistically significant.
Comparison shopping
All mercury monitors and sorbent traps tested at Armstrong performed very well, although some performed better than others. All of the methods were close in terms of precision. Yet somewhat better precision was obtained on low-Hg coal, compared with high-Hg coal. This can be attributed to the low-Hg coal's less-variable mercury content. The maximum bias was in the 10% range. Also, the precision of the OHM results obtained at Armstrong is comparable to that obtained at other sites.
Also, don't forget that sootblowing interferes with Hg measurement. When sootblowing was initiated at Armstrong, the gas-phase mercury concentration measured by all Hg CEMs suddenly dropped. The cause of the drop is believed to be Hg adsorption/absorption on solid particles that were dislodged from tube surfaces by sootblowing and then entered the flue gas stream. To obtain good-quality, repeatable Hg measurements, you should not activate sootblowers during Hg testing.
Each plant is different, so it is difficult to extrapolate results from one power plant to another even if they both burn the same coal or blend of coals. This is why the various manufacturers were not identified by name. Because one size does not fit all, you have to test Hg monitors at your particular plant (using approved procedures) to determine their actual measurement performance characteristics (accuracy, precision, and drift).
—Dr. Nenad Sarunac (ns01@lehigh.edu) is a principal research engineer and associate director of Lehigh University's Energy Research Center. Dr. Domenico Cipriano (cipriano@cesiricerca.it) is the head and product leader of the emissions monitoring group at CESI Ricerca, Milan, Italy. Jeffrey Ryan (ryan.jeff@epa.gov) is a senior scientist in the Air Pollution Prevention and Control Division of The Office of Research and Development of the U.S. Environmental Protection Agency. John Schakenbach (schakenbach.john@epa.gov) is an environmental scientist at the Clean Air Markets Division of the U.S. Environmental Protection Agency.
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