| OCR Text |
Show is the initial amount of carbon present in the sample, which, as already indicated , is obtained from complete combustion of the sample by the "total combustion test" (in practice, t = c« corresponds to the end of that test). The evolution of V as a function of time is illustrated on figure c 5_ in the case of Beulah lignite coal at two different grid temperatures higher than the ignition temperature. In figure 5 A, the volatiles oxidation (V.O.) still precedes the moment of ignition; the small temperature peak accompanying V.O. characterizes it as a true exothermic phenomenon and not as a mere, pyrolysis reaction. At the higher temperature shown on the right hand side of that figure, pyrolysis and volatiles oxidation occurs after ignition of the lignite. During pyrolysis and volatiles oxidation the solid carbon combustion rate V is "inhibited" by the fact that the c diffusion of oxygen into the pores is greatly counteracted by the escaping pyrolysis products as "well as by the oxygen consuming oxidation of the . ,8. latter (-). Only towards the end of the pyrolysis and volatiles oxidation may the oxygen become entirely disposable for combustion of the solid: the combustion rate V then steeply rises to its maximum value (V ) c c, max "which is accompanied by a maximum value of the temperature T . Similar behaviour in the case of other coals, different particle sizes and oxygen concentration, are shown on figures 6 and 7 . On these figures the evolution of the ratio X /X in the outlet gas stream is also shown ; both figures correspond to "whole coal" ignition and the oxidation of the volatiles is of the explosion type, a premixed gas phase flame being visible close to the coal layer (see figure 7) . The CO/CO ratio depends on different parameters : a) The oxidation of the solid carbon matrix in itself yields as well CO as o CO (-), the decomposition of the oxygen-carbon surface complexe into CO rather than into CO normally increasing with temperature. b) However, with increasing temperature, the oxidation of CO into CO in the gas phase also increases. 7-12 |
