| OCR Text |
Show Only when the grid temperature is higher than the one corresponding to the char extinction temperature (upper tangency of curve Q-with the char curve \\ = 1) may the coal combustion be complete, since under these conditions the coal combustion will be achieved as char combustion. This case is illustrated on figure 15 for the grid temperature T _ , the shift of T _ towards T being again due to heat transfer g»3 g>3 g»4 from the burning coal to the grid. The incomplete combustion of char followed by extinction at grid temperatures close to that for which the critical char ignition point is reached , has also been observed experimentally and may be seen on figure 8. This may be attributed either to the fact that this char is actually not a completely devolatilized coal , or to a decrease in the char combustion rate due to a decrease in reactivity of the char matrix with progressing burn-out. The higher burn-out of coal compared to that of char at the same grid temperature follows from the fact that the fraction of carbon burnt (F ) in the case of coal, also accounts for the combust - CD ion of the volatiles of the coal : upon subtracting this contribution of volatiles combustion from the F , values obtained for coal, one finds cb with reasonable agreement the F values measured on char. CONCLUSIONS 1. Under typical conditions of particle size, temperature and oxygen concentrations prevailing in industrial pulverized coal flames, "whole coal" ignition seems to be most frequent. 2. The presence, in the solid fuel molecule, of a pyrolysable fraction, has a promoting effect on ignition, decreasing substantially the ignition temperatures of coal compared to those of the corresponding char. 3. In the case of "whole coal" ignition, the pyrolysis reactions and the subsequent gas phase oxidation of the pyrolysis products , especially in the case where the latter takes the form of a deflagration, may 7-31 |
