OCR Text |
Show Actually this is of no importance for the purpose here, since any way coal cannot persist as coal at high temperature, due to pyrolysis; therefore the largest part of the coal curve (shown by the heavy dotted line) has only a virtual meaning : it represents the heat released under condition of frozen pyrolysis. and will never constitute, like in the case of char, a steady state condition. Between the heat release curves of coal and char, one can imagine the virtual existence of an infinite number of curves, each one characterized by a given advancement of pyrolysis, i.e. by a well defined value of \ comprised between zero and unity, one of 'which is represented on the schema of figure 15. Let us assume the grid temperature to be initially equal to T . , higher than the grid temperature for which the critical ignition point of coal is reached, but lower than the grid temperature for which the critical char extinction temperature is reached. A virtual steady state point s. situated on the (virtual part of the) coal curve should be reached; actually this virtual steady state point will shift to a lower value s® located on a lower coal curve (indicated on the graph as £ = 0 and O ) and corresponding to a lower reaction rate of combustion due to the shielding-off of oxygen by the onset of pyrolysis. The heat released by the combustion of the coal may well increase the grid temperature, bringing it from its initial value T to a higher value T _ , corresponding to a shift of g, 1 g,*- point s° towards point s°. Actually, due to progressing pyrolysis, the figurative point (either s. or the more realistic points s° or s°) cannot remain on a ( ? = 0) curve but will progressively shift towards virtual curves of increasing pyrolysis advancement (or to the corresponding ,f \\ and O " curves as far as shielding-off of O remains effective) until it reaches a given (0<|<1) curve (the one represented on the schema) for "which the upper intersection with the Q.. curve corresponds to the critical extinction point (s ) of that partly devolatilized coal. At e that moment the fast combustion rate drops to a slow oxidation rate, Le. the , partly devolatilized, partly burnt coal extinguishes. 7-30 |