Thermochemical history in the genetic path of coal: derivation of a novel correlation for heat of combustion

Abstract

A plot of the heat of combustion of coal ( Q) vs sum-total of the C and H contents (on Parr's basis), for bright coals, has been found to show three clear-cut linear phases in the transition from lignite to anthracite, marked by two turning points, one at about 84–85 and the other at about 92 wt% C. Significantly, analogous curves showing exactly the same pattern of variation of Q vs rank, have also been found to emerge from the plots of Q vs C or Q vs O. The first long linear step (AB) encompasses the high-volatile (hv) low rank coals (C: 65–84 wt%), the second (BC), the middle rank coals (C: 84–92 wt%) and the third (CD), the highest rank coals (C: 92–97 wt%), tending to end up linearly at the point for graphite. Attention is drawn to the one-to-one correspondence of the above thermochemical paths with what had been observed earlier in the “genetic path” of coal, traced out from a plot of atomic H/C′ ratio vs rank for the same range and types of coals, showing the same three linear steps, demarcated by the same two turning points. The Q vs (C+H) relationship is considered more significant because of its direct relevance to the C–H matrix of coal and its progressive structural change as shown by the sequential changes of its atomic H/C′ ratio. Such interpolation of the thermochemical and genetic paths has led to the development of the following novel correlation, holding good from lignite to anthracite: Q( MJ/kg)=0.512( C+H)−11.19+6.75( H/C′−0.78) which, in turn, simplifies to Eq. (7): Q=0.512 ( C+H)+81.07 H/C−16.46, with atomic H/C′ ratio replaced by its weight ratio (H/C). Parallel correlations have also been developed based on Q vs C or Q vs O relationships but the Q vs (C+H) has been found to be the most accurate, and is recommended for general use. Allowing for contributions of heat due to S and mineral-matter (MM) transformation to ash, it has also been possible to logically deduce a derivative (Eq. (8)) giving calorific value on dry basis, directly from dry coal data: Q d =0.512 ( C d + H d )+(81.07 H d +C d −16.46)×(100− MM)/100+12 S d −0.007 Ash d The “genetic” correlations have been found to apply excellently well to all coals irrespective of their rank, region and variable petrographic composition, predicted values seldom deviating from experimental values by more than 0.5%. Its (Eq. (8)) potential accuracy is likely to be far better as assessed from a special test conducted on US coals. The success of the newly found correlations, is attributed mainly to the role of its H/C term, significance of which is discussed in full vis a vis rank and petrographic factors of coal.