ADIABATIC REVERSE COMBUSTION IN A PACKED-BED

Downward propagation of a combustion front in a packed bed of wood particles, with air supply from below, is examined theoretically and experimentally. Using a single-step reaction, with a kinetic model of char oxidation as the dominant mechanism, and assuming local thermal equilibrium, but allowing a local chemical nonequilibrium between the solid and gas phases, the front speed u(F), the adiabatic temperature T-r, and the extent of solid consumption [p](s)(r)/[p](s)(n) are determined as functions of the entering air pole-velocity (u)(g). Both oxygen-limited and fuel-limited regimes, with the boundary marked by the stoichiometric burning, are examined. In the oxygen-limited regime, T-r, U-F, [P](s)(r)/[P](s)(n), and the thickness of the front delta(F), all increase with [u](g)(n). In the fuel-limited regime, the reverse occurs and the extinction at high [u](g)(n) is predicted. In the oxygen-limited regime, where the bed is not yet fluidized, the experimental results are in good agreement with the predictions.