Pressure effect on char combustion in different rate-control zones: initial rate expression

Effect of pressure on the initial rate of coal char combustion was studied in a fixed bed and analyzed in terms of theoretical rate equations. Char particles of 0.25-0.5 mm diameter were combusted over temperature and pressure ranges of 559-1273 K and 0.1-1.6 MPa, respectively. Char combustion proceeded in first order with respect to oxygen. The combustion rate increased proportionally with pressure in the chemical-kinetics control zone (Zone I), exhibited a nonlinear increase with pressure in the internal-diffusion control zone (Zone II), and was invariant with pressure in the external-diffusion control zone (Zone III). Theoretical rate equations involving explicit pressure terms were developed for predicting the combustion rate under elevated pressure. For given properties of char (d(p), rho(c), epsilon and k(v)) and operating conditions (C-o, u(g,o), T and P), the combustion rate estimated from the developed rate equations agreed well with the experimental results. (C) 1999 Elsevier Science Ltd. All rights reserved.