OCR Text |
Show 7 furnace, the temperature of which could be kept constant. This could be explained by the difference in specific heat between CO2 or H20 and N2 in the combustion gas, resulting the difference of an actual flame temperature. Estimating the adiabatic flame temperature against 02 conc. in the secondary stream, as shown in Fig. 7, we could easily note that 02/C02 combustion essentially had a lower reaction temperature due to higher specific heat compared wi th air combustion. Figure 7 also indicates that more than 40% of 02 conc. in the secondary stream will be needed t9 get similar flame temperature as air combustion, on 02/C02 combustion. This will be the reason why poor char burnout occurs on 02/C02 combustion wi th lower 02 conc. in the secondary stream. When 02 conc. in the secondary stream became high, therefore, the char burnout was improved so significantly, as was shown in Fig.5. Flame stability was also affected by the 02 conc. in the secondary stream, as well. It was found that the boundary by flame ignition between flame and coal dust paricles could not be clarified by human eyes due to lower luminousity, otherwise a flame could be detected by a flame scanner, when 02 conc. in the secondary stream was below 30%. The difference of flame behavior was recognized by the comparison between air combustion and 02/C02 combustion, as shown in (a) and (b) of Fig.B, respectively. On air combustion, the ignition delay could be aparently found in the flame. Changing oxidant from air to the mixture of 02/recycled flue gas, pulverized coal flame became dark and it's luminousity or darkness strongly depended on 02 conc. in the secondary stream. These results suggested that 02 conc. in the secondary stream should be determined from the viewpoint of the flame stability and char burnout. On the contrary, 02 conc. in piping and/or ducting should be limited from the safety point of view. The design criterion did not be issued for the mixture of 02/C02 at present time, however, an oxygen supply manufacture had a design criterion for the mixture of 02/N2' depended on company experience and so on. An allowable limit on 02 conc. was decided to be 30% for 02/N2 mixture, in a pipe |