Combustion behavior and kinetics of synthetic and coal-derived chars: Comparison of theory and experiment

Abstract

A theoretical model describing the combustion of carbonaceous particles at intermediate and high temperatures (1250–2600 K) has been developed and compared with experimental results. The analysis includes the effects of pore diffusion and growth, inert mineral matter, gas-phase heat and mass transfer, and treats solid spherical and cenospheric char particles. The combustion dynamics are described by time-dependent equations for particle temperature, radius, ash layer thickness, and a number of intraparticle conversion variables. These are coupled to pseudo-steady equations for gas-phase transport and internal reaction and diffusion. Model predictions were contrasted with combustion measurements obtained for several synthetic chars and two coal-derived chars. The synthetic chars were glassy carbons produced in the form of solid monodisperse spheres. Cenospheric synthetic chars were also produced. All synthetic chars were mineral free. The coal-derived chars contained mineral matter and were considered spherical. In each case, apparent and intrinsic rate parameters were estimated by direct application of the combustion model to experimental data gathered under conditions of moderate to large Thiele modulus. These rate parameters were compared with previous results using a simpler model that does not consider pore enlargement and the temperature dependence of gas-phase properties. The comparison indicated that the latter of the above phenomena affects the predicted apparent rates by a factor of about 2 and the intrinsic rates by a factor of 4 at high temperature. At low temperatures, accounting for the temperature dependence of the gas phase did not appear to influence the calculated apparent rates, and the difference in the intrinsic rates was attributed to structural changes in the char. The estimated kinetic parameters were subsequently used to generate temperature-time profiles of single burning particles. The results obtained are in good agreement with the experimentally observed behavior of the synthetic and coal-derived chars. The validity of the intrinsic rate calculation was tested by simulating the combustion behavior of particles burning in the kinetically limited (low Thiele modulus) regime; however, more definitive tests are needed.