Characterization of coal char and biomass char reactivities to oxygen

Studies were undertaken to quantify the extent of carbon deactivation during the combustion of coal and biomass chars at high temperatures. To this end, chars extracted at successive residence times from a flow reactor were subjected to oxidation tests in order to determine char reactivity as a function of conversion under zone II burning conditions. Measured char conversion rates during oxidation tests in a thermogravimetric analyzer were combined with surface areas determined from gas adsorption measurements to yield intrinsic reactivities as functions of char conversion under zone I burning conditions. These reactivities were characterized using a heterogeneous reaction mechanism that not only accurately describes variations in char reactivity with conversion at low temperatures but also predicts high enough reaction rates at high temperatures to yield the mass loss rates observed in the laminar flow reactor. Results indicate that char intrinsic reactivity decreases progressively during oxidation at high temperatures. Coal-char reactivity just subsequent to devolatilization in an environment containing 12 mol % O-2 at nominally 1650 K was about 33% higher than the reactivities of chars at later extents of conversion. In 8 mol % oxygen at nominally 1200 K, the reactivity of a biomass-char at 92% conversion was about 45% higher than the reactivity of the biomass char at 98% conversion. Results indicate that mass transport limitations become evident at oxidation temperatures as low as 900 K. Calculations indicate that a quasi-steady burning rate is not established until relatively late in burnoff, suggesting that char oxidation models that use rates based on steady state adsorbed oxygen concentrations may not accurately predict the times for char burnout under all reaction conditions.