FLASHCHAIN THEORY FOR RAPID COAL DEVOLATILIZATION KINETICS .5. INTERPRETING RATES OF DEVOLATILIZATION FOR VARIOUS COAL TYPES AND OPERATING-CONDITIONS

In this paper, FLASHCHAIN predictions based on the extended submodel for coal constitution developed in part 4 are evaluated for transient devolatilization. The complete dynamic behavior of 15 coals is evaluated at temperatures from 500 to 1200 K and reaction times to 10 s. The dynamics are depicted for extended heating periods at temperatures too low to achieve ultimate yields, as well as for nonisothermal decomposition during rapid thermal transients. In almost all cases the predictions are within experimental uncertainty throughout. They also depict several important aspects of the dependence on coal rank, including a shift in the onset of devolatilization to higher temperatures with coals of higher rank, and an initial abundance of gases for low rank coals and of tars for bituminous coals. Based on its quantitative performance, the theory is also used to rigorously define nominal devolatilization rates for diverse coal types and broad ranges of operating conditions. At all conditions, the apparent activation energies are far too low to associate with any pyrolytic scission of model bridge compounds, such as ethylene linkages in polynuclear aromatics. These results also prove that heat and mass transport resistances are not responsible for the low values, because this theory is completely free of these considerations. Rather, the combined impact of chemical kinetics, the statistics for depolymerization and cross-linking, and flash distillation obscures any conceivable relations among rates of weight loss or tar evolution and the rates of the chemical reactions that underlie devolatilization. Whereas nominal rates are rather insensitive to coal type variations, they vary significantly with reaction time during isothermal devolatilization and, especially, with changes in heating rate. In fact, the FLASHCHAIN predictions show that a single pair of rate parameters cannot possibly represent the devolatilization behavior of any coal for different heating rates, at odds with other recent assignments.