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Show 2 8 % and 3 5 % oxygen mixtures were generally identical in appearance, stable and luminous with an orange/white colour. In the case of the 21/79% 0 2/C02 mixture on the other hand, the coal flame established in the combustor, although stable, was more transparent with a dull orange/red colour. Because of the lower luminosity of the 21/79% 02/C02 pulverised coal flame, the automated burner management system was operated with the flame scanners by passed; since the scanners were designed to sense a more luminous and higher frequency radiation signal. These observations suggest that the 21/79% 02/C02 combustion mixture produces flames with significantly lower radiant heat transfer characteristics (see also below). 4.2 Flame temperatures, total and radiant heat flux Figure 2 shows the suction pyrometer measurements of the centre line flame temperatures in the vertical combustor. The axial temperature profiles indicate progressively hotter flames with increasing oxygen concentration in the 0 2/C02 combustion medium. Compared to the 2 1 % 0 2 by volume 0 2/C02 mixture, the equivalent pulverised coal flame in air shows considerably higher temperatures that fall between the values measured for the 28 and 3 5 % oxygen flames. A s noted in some work reported by Japanese researchers (2), the higher specific heat of the C 0 2 gas compared to nitrogen will lead to lower flame temperatures compared to an equivalent oxygen concentration in an air burning medium. In addition, the data in Table 2 also show that the total mass flux of combustion gas used for fuel combustion in the 21/79% 0 2/C02 medium at 478.7 kg/h is also ~ 4 5 % higher than the 330 kg/h gas flow used for coal combustion in air. Thus, a combination of the higher mass flow and specific heat for the 0 2/C02 medium would therefore lead to much lower flame temperatures than anticipated for fuel combustion in an equivalent air, or higher oxygen concentration and nitrogen rich combustion medium. The data in Table 2 and Figure 2 also show that from considerations of a similar heat capacity (mass flow x specific heat) of the combustion medium, the flame temperatures for the 28/72% 0 2/C02 mixture comes closer to simulating the flame temperatures measured for coal combustion in air. Figure 3 shows values of the axial profiles of total incidental heat flux measured at the combustor wall. The measured value of the total heat flux is the sum of both the convective and radiant heat transfer fluxes emitted from the flame. Figure 4 shows the equivalent data for the axial profiles of the radiant heat flux. Comparison of the data in Figures 3 and 4 show that in the flame zone, the radiant heat flux forms a significant component of the total heat transfer flux to the combustor wall. In a similar manner to the trends noted for the flame temperatures in Figure 2, the total incident heat and radiant fluxes can be expected to increase with the oxygen concentration in the 02/C02 combustion medium and with intermediate values between those measured for the 28 and 35%o oxygen tests anticipated for the air run. While this anticipated trend appears to hold consistently for most of the measured heat flux data in Figures 3 and 4, much lower heat flux values towards the combustor outlet have been reported for the 35/65% 0 2/C02 run. Because of the sensitivity of the total and radiant heat flux measurements to flame impingement on the probes (which is determined by the furnace draft setting; see Section 3), it is likely that some of these data are enoneous measurements. Hence, it is likely that the measured values of the total 9 |