Theoretical oxygen requirement for coal combustion: relationship with its calorific value

The general relationship between heat of combustion (Q) of fuel and the theoretical oxygen requirement (Or) for complete combustion is well known. A revaluation of such a relation in the case of coal has yielded a novel and more accurate correlation. Graphical studies revealed a three-phase relationship between Q and Or, from brown coal/lignite to anthracite in the normal path of coalification, each of which has been found to be remarkably linear in their respective regimes, marked by two turning points, one at about 84 and the other at 92 wt% carbon. A similar curve observed for atomic H/C′ ratio vs rank was claimed to truly represent the genetic path of coal. Interpolation of the two relationships, aided by previous interpretation of structural changes of coal across its genetic path in terms of changes in H/C ratio, has led to the following correlation: Q (MJ/kg)=13.03×Or+0.93+1.98(H/C′−0.80), which is uniformly applicable to all coals, irrespective of rank (C: 65–97 wt%) or petrographic composition. A derivative of the correlation giving the calorific value directly on the dry basis has also been developed: Qd=13.03×Ord+(100−MM)×(0.238Hd/Cd−0.0065)−0.007 ash, using the weight ratio of hydrogen to carbon instead of the atomic ratio. Its accuracy has been found to be one of the best amongst all ‘ultimate’ formulae, (SD 0.16 MJ/kg on dry basis). The success of the new-found correlation is ascribed to the H/C term, the role and function of which are brought out in full.