| OCR Text |
Show It should be emphasized here that the so-called "exploding popcorn" effect reported by others was only rarely seen in these experiments. Only about one in twenty particles actually blew apart during devolatilization. Many of these "explosions" appeared to result from pieces of ash or pyrite imbedded in the coal. As devolatilization neared completion, the particles became rigid again and retained whatever shape they had assumed when soft. They also shrank somewhat, even if there was no oxygen. In spite of this slight shrinking, the char was always larger than the original coal size. The devolatilized char was characteristically very porous and friable. Under oxidizing conditions, it appeared to us that devolatilization was essentially complete and char combustion was just beginning when the particles reached 60-65% of their original weight. Only in the case of combustion in pure air did we see a clearly defined volatiles flame. This seems to indicate that the volatiles may burn rather slowly in combustors where the freeboard oxygen concentration is significantly less than 21 mole % (typically it is <5 mole % ). Our experiments clearly indicated that swelling is a strong function of the devolatilization conditions. The degree of swelling was greater for lower reactor temperatures and lower oxygen concentrations. It was not clear whether the temperature effect was due to the absolute magnitude of the temperature or its effect on the particle heating rate. Recent discussions in the literature favor the latter explanation. Qualitatively, it appeared that at lower reactor temperatures (and thus lower heating rates) the devolatili-zing coal had more time to froth up and expand with the volatile bubbles. The effect of initial particle size on swelling was not so clear cut. The Phase I data seem to indicate that smaller particles swelled more. Data from Phase II, however, which were collected for closely sized small particles, seem to indicate less swelling than would be expected from the Phase I results. As is discussed later, this could be due to a difference in ash contents. 24 |
