DEVOLATILIZATION OF LARGE COAL PARTICLES UNDER FLUIDIZED-BED CONDITIONS

The rate of devolatilization of various sizes of coal particles is an important parameter in the design and interpretation of the performance of fluidized bed combustors. Combined with data on char combustion kinetics, bed mixing and residence times of the gaseous and particulate phases, devolatilization data are a primary input to combustion models. It is proposed here that the phenomenon of volatiles release from a coal can be represented by a single reaction rate equation of the nth order Arrhenius type: dV/dt = k (V(infinity) - V)n, where n is the order of reaction, not zero, k is the devolatilization rate constant, V is the fractional amount of volatiles evolved to time t, and V(infinity) is the fractional amount of volatiles evolved as t --> infinity. A group of nine coals ranging in type from semianthracite to subbituminous, having particle sizes from 1.4 to 29 mm were devolatilized under simulated fluidized bed combustion conditions. Order of reaction and devolatilization rate constants were calculated for each coal type and particle size combination and a relationship between rate and size determined. The order of the devolatilization reaction was constant for each coal over the size range examined, ranging between 0.4 and 0.6 for the various coals. A relationship to estimate total devolatilization time of various sizes of coal particle was derived from the nth order rate expression and shown to be of similar form to a previously published semi-empirical relationship, tau = Kd(N). The rate relationships are used to estimate the coal particle size below which substantial freeboard volatiles will occur.