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Show 10 Special Diagnostic Instrumentation Advanced instrumentation techniques are very beneficial in conducting combustion diagnostic tests, particularly those involving the generation of O2 profiles at the economizer exit to evaluate combustion uniformity. The use of this special instrumentation will be illustrated using test data acquired during a recent boiler emissions and coal quality impact field test program conducted at a 250 MW tangentially-fired unit. A grid of 24 sample probes (12 per duct) was installed in the divided economizer exit duct as shown on the right side of Figure 7. Gas samples are drawn individually through each probe in the grid to define the gas composition at each location in the duct. A computer graphics program is used to generate the emissions contour plots in Figure 8 to evaluate combustion uniformity. Until recently, generating just one of the profiles shown in Figure 8 for a 24-point sample grid, would require up to a half-day of pOint-by-point sampling, data reduction and plotting. However, Fossil Energy Research Corp. has developed a real time multi-point O2 sampling, data acquisition and graphical display system that will continuously monitor and display the O2 profile shown in the upper portion of Figure 8. The system, shown on the left side of Figure 7, allows up to 12 points in a duct to be sampled and analyzed simultaneously. The PC-based data acquisition and custom sample system updates the screen display at 10-second intervals, computes averages over user selectable time intervals and provides storage of the O2 profiles to a hard disk for later recall. With the use of this custom instrumentation, the test engineer can make air register, mill bias or fan bias adjustments and immediately see the impact on the O2 profile and boiler combustion uniformity. This instrumentation has been very effectively used to optimize NOx emissions by means of combustion tuning. Fossil Energy Research has found that it can significantly reduce the time needed to tune a boiler by using two way radio communications between the equipment operator making air register adjustments on the burner deck and the test engineer in the truck who can observe the change in the O2 profile. The test engineer can direct the burner adjustments until uniform O2 and NOx emissions have been achieved. The diagnostic approach outlined above was very effectively applied during an emissions and coal quality impact test program conducted at a 250 MW tangentially-fired unit. The "as-found" diagnostic tests revealed non-uniform combustion conditions and a large "02 split" (as shown previously in Figure 8). The O2 ranged from a low of 1.8% on the right side of the boiler to a high in excess of 4.60/0 on the left side. This O2 imbalance was not due to air infiltration since it was also evident in the NO emissions contour which ranged from 380 ppm to 550 ppm (dry basis corrected to 3% 02). Discussions with the plant engineering personnel indicated that a coal flow imbalance had been previously confirmed but coal pipe orifices had not been replaced pending an upcoming outage. Although the primary objective of the field test program was to evaluate the emissions and boiler performance of a candidate test coal, one day of testing at the end of the test program involved a preliminary effort to offset the coal flow imbalance by adjusting secondary air dampers to compensate. An interactive testing approach using the multipoint O2 analyzer and two way radio communications was very successful as indicated in Figure 9. The O2 gradient was reduced to a 0.6% spread (2.2 to 2.8% variation) which |
