OCR Text |
Show generate electric power, and is used to good advantage as a test bed for ideas which might improve the performance of PENELEC's larger, more efficient, plants. The average pressure in the boiler furnace is below atmospheric and, because of its age, there is inleakage of air at various locations in the furnace and convection section. The tests were conducted with a variety of coal samples obtained especially for the test program. The majority of runs were made with blends of various proportions of surfacemined and underground-mined medium volatile bituminous coals. The test chosen as the principal example for discussion here was conducted using unblended underground-mined coal (test no. 2, October 8, 1991). The average proximate analysis of two composites of samples collected from the coal feed pipes during the run was (wt%, as received): moisture, 1.1; ash, 12.7; volatile matter, 25.6; and fixed carbon (difference), 60.6. The ultimate analysis was (wt%, as received): carbon, 74.7; hydrogen, 4.2;" nitrogen, 1.3; sulfur, 2.0; oxygen (difference) 4.0; moisture, 1.1; and ash, 12.7. The coal had a higher heating value of 30.4 MJ/kg (as received), and a Hardgrove grindability of 90. 81 ± 3 wt% of the particles were smaller than 75 J.lm. Results of two determinations of the free swelling index were 7.5 and 8. Additional information on the fuels and test conditions may be found in the final report of the results of the weathered coal tests (PENELEC, 1992). Among the data collected were measurements of unburned carbon in the fly ash. Estimates of the corresponding combustible heat losses, as fractions of the coal heat input, are shown in Figure 2. The fraction of heat input lost as unburned char varied from 2.3 to 3.5%. The size distribution of unburned carbon was found by sieve analysis of the fly ash, followed by determination of the carbon content of each size fraction. The size distribution of unburned carbon in the fly ash collected during the test using the unblended, undergroundmined, medium volatile bituminous coal is shown in Figure 3. Carbon was present at significant concentration over a wide range of particle sizes, from 38 to 500 J.lm. In an effort to obtain additional information relevant to carbon burnout, the boiler was revisited two years after the weathered coal tests, to measure the time-dependent oxygen content of postflame gas using a PtI02(reference)IZr02102(sample)IPt electrochemical cell (AACC/Zircoa, Solon, OH; now Marathon Monitors, Cincinnati, OR). The electrolyte is a yttria-stabilized zirconia tube 9 mm in diameter, 900 mm long, with electrodes formed by 4 |