OCR Text |
Show non-gray nature of soot radiation. The temperature obtained from the absolute intensity is given from the lower curve and will be anomalously low because of attenuation of the particle radiation by the surrounding volatile cloud. For Montana lignite particles the soot formation is small, the two curves are close to each other and show that for the duration of the volatile flame the particle temperature is rising toward its peak value. A similar - result is found for the dashed curves of Illinois #6 coal, but in this case the two-color pyrometry reading is sometimes considerably higher, suggesting that significant amounts of soot are formed. The important finding, from a viewpoint of ash vaporization, is the observation of particle temperature suppression during the period of devolatilization. The particle temperature remains fairly uniform during char combustion showing a minor peak whose position shifts from the initial to the final stages of char burnout as the oxygen partial pressure from 100 to 15 percent. Variations in temperature between particles was significant, with an average deviation of about 8 percent about the mean temperature. The effect of oxygen partial pressure on the temperature measured at 50 percent burnout is shown in Figure 7. The temperature of the particles exceed the 1700 K furnace temperature by amounts that increase with increasing oxygen partial to a maximum in pure oxygen of 2820 K for Montana lignite and 3120 K for the Illinois #6 bituminous coal. Figure 8 shows examples of the output of the time resolved PMT signals at the two-color wavelengths for Illinois #6 bituminous particles burning at 1700 K. The traces show a sharp increase in signal coincident with particle ignition. The signals then peak and decay during burnout. As the oxygen partial pressure is lowered a second peak emerges in the trace. The double peak was observed only for the bituminous coal at 1700 K and only for lower oxygen concentrations. This secondary peak is observed at very highly sooting conditions and its duration corresponds approximately to the duration of the sooting volatile flame formed during the initial stages of combustion. The above findings are consistent with observations derived from shadow photography (14) and holography (15) of burning coal particles showing that during the initial phases of combustion, volatiles are released and burn either in jets, or more commonly, in envelope flames surrounding the individual coal particles. The volatiles also form soot, in concentrations which 3-12 |