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Show Figures 12 and 13 show the exit concentration of S 0 2 and C O in the flue gas. For the 0 2/C02 gas mixtures, the exit concentration of S 0 2 increases with the oxygen concentration in the inlet combustion gas mixture. While this trend is similar to that noted for N O x formation in the 02/C02 gas flames, the lowest S 0 2 concentration was measured for coal combustion in air. The data for the C O composition in the exit flue gas show that the 0 2/C02 combustion gas mixtures generate twice the values measured for coal combustion in air. However, the higher measured C O concentrations in the 30-40 ppmv range for the 0 2/C02 gas mixtures are low enough to be of no concern from an operational stand point in a coal burning plant. 4.4 Combustion efficiency and ash residues During the trials, isokinetic particulate samples were extracted from the flue gas at the locations of the cyclone and boiler outlet. The ash samples extracted from the cyclone outlet were analysed to determine the unburned carbon content. These values of the unburned carbon in fly ash reported in Table 2 ranged from 0.2-3.3 %, with the highest level of a 3.3% unburned carbon monitored for the 21/79% 02/C02 tests. Based on these carbon in ash values, the overall combustion efficiency of the coal was calculated using the following equation: Combustion Efficiency (%) = 100 - [A x C/(VM + FC)] where A is the per cent ash in the coal, C is the per cent unburned carbon in the fly ash, VM is the per cent volatile matter in the coal and F C is the per cent fixed carbon. All of the above parameters, expressed on a dry basis, are obtained from the proximate analysis of the feed coal reported in Table 1. From the above, the calculated values of the combustion efficiencies for the trials ranged from 99.31-99.95 % showing good fuel burnout. A s noted for the carbon in fly ash values, the lowest combustion efficiency was measured for the 21/79% 0 2/C02 tests. Visual observations of the ash samples collected during and after the trials also indicated that the samples from the 21/79 % 02/C02 tests assumed a more greyish/brown appearance, compared to a more beige/brownish colour for the ash samples from the other tests. More detailed analysis of the ash samples recovered from the combustor bottom, cyclone, the heat let down boiler and bag house is ongoing. The largest amount of fly ash was recovered from the cyclone catch and the finest ash samples, with a texture similar to that of a fine talcum powder, was recovered from the bag filter. The coarsest ash deposits with larger partially fused ash agglomerates were recovered from the furnace bottom. Inspection of the combustor and burner region at the end of the tests almost always indicated a build up of coarse and partially fused ash deposits on the refractory surfaces. The build-up was usually heavier in the near burner region, becoming more sparse towards the combustor exit. The partially fused ash deposits in all cases could be readily removed from the refractory walls of the combustor by light scraping. In the case of the 35/65%) 0 2/C02 combustion test, there was some evidence of the formation of a 11 |