OCR Text |
Show not be completely decoupled from reactivity in this last study, it cannot be stated unambiguously that volatile matter does not play a role in the ignition of low-volatile chars from different coals. Figure 5 presents the results on neat bituminous coals and their coal-water slurries burning in one combustor with a common burner. The straight line in Figure 5 represents the best linear fit of ignition delay time versus parent coal volatile matter content for the seven neat bituminous coals tested (see Table IB). What is striking about these data, determined with the multiphase fuel combustor, is that their trend seems to scale very well with the neat bituminous coal data shown in Figure 4 determined with the plane flame furnace. The reader should be reminded that the method of determination and definition of ignition time is different for each combustor. In the multiphase fuel combustor, ignition time is visually determined (via HeNe laser line-of-sighting) and is defined as the standoff distance between the burner nozzle and the flame front at the furnace centerline. Ignition time is evaluated by the ratio of the standoff distance to the coal/air flow velocity. In the plane flame furnace, ignition time is chemically and physically determined by direct sampling that determines either the instant of 1% carbon burnoff or the maximum rate of temperature rise, respectively. The data in Figures 4 and 5 indicate that the three sets of data for the two types of combustors compare quite favorably. Hence, ignition data from the multiphase fuel furnace combustion tests have been calibrated with noncombustion dTGA burning profile data via plane flame furnace combustion data. Figure 5 superimposes coal-water slurry ignition data onto those for the neat parent coals as a function of volatile matter content. This allows the estimation of the effect on ignition of the 30% dilution of the parent coal in water. The ignition delay caused by the presence of water amounts to 6-8 milliseconds. With this information, an estimation can be made of what this moisture-induced delay translates into in terms of an apparent loss in volatile matter content: 30% dilution in water corresponds to an apparent loss in coal volatile matter content of about 3 percentage points. Experiments reported elsewhere [11] have established that every percentage point loss in parent coal volatility must be compensated for by about 15°C air preheat in order to match the ignition stability of a slurry 22-19 |