Pore structure evolution during lean-oxygen combustion of pyrolyzed residual from low-rank coal and its effect on internal oxygen diffusion mechanism

Abstract

The lean-oxygen combustion of incompletely pyrolyzed char dominates at the combustion front of coalfield fires. Using non-local-density functional theory (NLDFT) and Barrette-Joynere-Halenda (BJH) analysis based on N2 and CO2 composited adsorption isotherms, the pore (0.4–300 nm) structure characteristics of pyrolyzed char during lean-oxygen combustion were obtained. The results indicate that the main pore morphology changes from irregular ink-bottle shape of raw coal to cylindrical shape of pyrolyzed char, then to slit-shaped and wedge-shaped pores under the effect of oxidative expansion, and finally coalesce into amorphous macropores during combustion. For the weak-caking coal, softened colloids diffuse and block the mesopores, leading to the reverse development of mesopores into smaller-sized pores. The pore development changes from the generation of secondary micropores in pyrolysis stage to the progressive expansion mode of mesopores in initial combustion stage, then to the free coalescence mode of macropores in late combustion stage. The dominant internal oxygen diffusion mechanism transforms from Knudsen diffusion to transition diffusion at 650 °C, and the carbon-oxygen reaction interfaces shift from the inner surface of deep micropores to the shallow mesopores and macropores. Finally, 650 °C is determined as the early-warning temperature for the accelerating spread of coalfield fires.