99/01354 Pyrolysis/gasification of agricultural residues by carbon dioxide in the presence of different additives: influence of variables : Encinar, J. M. et al. Fuel Process. Technol., 1998, 55, (3), 219–233

Abstract

A century-long held notion was that the ignition of coal particles always occurs in the gas phase, as in a totally pyrolysing material. Recent experimental results disprove such a sequence, at least for small bituminous coal particles. Coals undergo only partial pyrolysis and hence ignition could occur in one of two possible modes: (i) exothermic gasification due to heterogeneous oxidation (regime I), and (ii) endothermic pyrolysis and subsequent oxidation of the pyrolysate in the gas phase (regime II).A steady-state ignition theory is developed for partially pyrolysing solid particles. The theory is formulated on an asymptotic basis: regime I is assumed to proceed independent of regime II. For regime I, Semenov's thermal theory is used to obtain the heterogeneous ignition temperature (HIT) and the solutions reveal that an increase in particle size and oxygen concentration results in decrease of HIT. For regime II, an adiabatic criterion is invoked to define the gas ignition temperature (GIT). The solutions reveal that a decrease in particle size and an increase in oxygen concentration increases the GIT. Superimposing the results for HIT and GIT, the conditions for transition of ignition phase (TIP) are found. The Pittsburgh seam bituminous particles of size below 350 μm may not undergo gas phase ignition, confirming recent experimental results.