FLASHCHAIN THEORY FOR RAPID COAL DEVOLATILIZATION KINETICS .2. IMPACT OF OPERATING-CONDITIONS

In this paper, the theory formulated in a companion paper is evaluated against the extensive data base for high-volatile bituminous coals. Thermal history effects are illustrated for heating rates from 1 to 10(3) K/s and temperatures to 1200 K by using transient and ultimate weight loss, tar yields, and tar molecular weight distributions (MWDs). The examination of pressure effects incorporates data from vacuum to 6.9 MPa, including gas yields and the yields and average molecular weights of tar; transient weight loss showing nominally equal devolatilization rates at all pressures; and the suppression of yield enhancements by faster heating at elevated pressures. Ultimate weight loss for sizes to one millimeter is also examined. In every respect, model predictions are within the discrepancies among measurements for similar samples from different laboratories. Hence, the influences of all of the important operating conditions on the devolatilization of high-volatile bituminous coals can be understood in terms of only four mechanisms: (1) the coal macromolecule disintegrates into primary fragments which are widely distributed in size; (2) a phase equilibrium establishes the mole fraction of tar fragments in a gas stream which is convected out of the particle with no transport resistance (the flash distillation analogy); (3) reactions in the condensed phase convert labile bridges into char links and simultaneously expel noncondensible gases, establishing the convective flow and inhibiting the formation of tar precursors; and (4) recombination reactions in the condensed phase reincorporate tar precursors into char. This evaluation also spawns two distinct interpretations of processes in the condensed phase. One is reminiscent of the two-component hypothesis, in which only a fraction of the original aromatic nuclei become susceptible to tar formation at some stage of devolatilization and the rest are inert. The other envisions a thorough disintegration of the coal macromolecule into relatively small fragments. The latter scenario yields an abundance of tar precursors, so a competition between flash distillation and repolymerization into larger involatile gragments determines the tar yields.