A model for the devolatilization of a coal particle sufficiently large to be controlled by heat transfer

Abstract

This study follows previous experimental work showing that the shrinking-core model applies to the pyrolysis (i.e., heating in the absence of oxygen) of particles (diam. ≈ 14 mm) of a bituminous coal or a lignite in a fluidized bed at 700–950 °C. These experimental facts are in accord with the production of volatile matter being endothermic and not thermoneutral, as often assumed. Also, the rate at which volatile matter is produced in the presence of oxygen (i.e., devolatilization) or in its absence (pyrolysis) is demonstrated here to be controlled not at all by mass transfer, but by heat conduction to a moving reaction front inside a coal particle, provided its diameter exceeds ∼3 mm. The resulting steady-state model of devolatilization indicates that six dimensionless groups are required to describe the rate of (I) coal → char + volatile matter for a nonfragmenting and nonswelling coal. This reaction occurs for a total time of c 1 r 0 + c 2 r 0 2 for a coal of radius r 0 . Experimental measurements of the times for pyrolysis in a bed of hot sand fluidized by N 2 indicate this dependence on r 0 holds much better than the usually accepted one of a r 0 n . The measurements also indicate that the enthalpy change of reaction (I) is 9.4 ± 4.0 MJ/(kg of volatile matter) and the thermal conductivity of char is 1.2 ± 0.5 W m −1 K −1 . In addition, it was deduced that reaction (I) occurs at ∼500 K and that η (of the heat released by burning the volatile material, a fraction η is fed back to a parent coal particle devolatilizing in a bed fluidized by, e.g., air) is less than unity. These numerical considerations enable the model of devolatilization to be simplified; it turns out that a heated particle's core of virgin coal shrinks mostly with a constant velocity, characterized by only four dimensionless groups—i.e., two of them usually turn out to be unimportant. The important groups include a Biot number for the char, Bi char, and Ω = 1 + η Δ H comb / Δ H ˜ v . Given that η = 0 and Ω = 1 for pyrolysis, the kinetics in that case are characterized entirely by three dimensionless groups, including Bi char. The conditions for the shrinking core model to apply to devolatilization and pyrolysis are discussed and are found not to apply, e.g., toward the end of devolatilization and for coals with low volatile contents. The model was used to calculate when reaction (I) starts after an initial heating period for a coal particle. The results are compared with experimental measurements (using gas chromatography of the volatiles released) of when various species (e.g., CO, CO 2, NO x , C 2H 4, C 2H 6, HCN) first appear from a pyrolyzing coal particle. These measurements confirm that reaction (I) can be assumed to occur at a mean temperature of ∼500 K.