OCR Text |
Show char , the maximum values of T being also identical. This has found to 5 be the case for all temperatures , as illustrated on figure 11. At that point of the discussion one may thus conclude , that the presence of a pyrolysable fraction of the coal molecule , from the point of view of the onset of coal combustion, acts as a double cutting sword : a) Clearly the higher oxidabiliry of that pyrolysable fraction and the heat released by its oxidation already at low temperatures promote coal ignition. Therefore the coal ignition temperatures are always lower than those of the corresponding chars, as shown on figure 9 and Table TJ. Therefore also, as a general rule, higher rank coals ignite at higher temperatures than lower rank ones (Table II). b) On the other hand, the endothermic character of pyrolysis as well as the shielding effect of the escaping volatiles and their subsequent gas phase oxidation, resulting in a decrease of oxygen disposable for the combustion of the solid carbon, exert a relatively strong inhibiting effect on the reaction rate of the latter during "whole coal" ignition. 2.3. Maximum coal or char combustion rate. For the study of the combustion rate with respect to temperature, oxygen concentration and particle size, the maximum value of that rate (V ) has been retained, since it corresponds to the maximum of c, max r T which is more accurately measured, and also since it is found to be the same whether the initial particle is coal or char. For internal combustion (constant volume combustion) the experimental value of V (expressed in grams reacted per unit time and per gram of carbon present) should be related to intrinsic combustion rate V . (expressed in grams reacted per unit time und per unit specific sur- SX face S.) by the expression : V = r,. V .. S. = 1. S.. k .. P* (2) c si I l si O v ' 7-20 |