Effects of in-situ interactions between steam and coal on pyrolysis and gasification characteristics of pulverized coals and coal water slurry

The effects of water on the pyrolysis and gasification characteristics of coal water slurry (CWS) and pulverized coals with different ranks have been studied in the present work. Rapid pyrolysis characteristics (i.e. char yield, char structure evolution) of raw carbonaceous materials with varied water contents were investigated by using a high frequency furnace at 800-1200 °C. Moreover, gasification characteristics of the pyrolysis char were studied by using a thermogravimetric analyzer (TGA). The results indicate that at the pyrolysis temperature of 800 °C, the char yield of Wu-ran-cha-bu (WRCB) lignite slightly decreased with increasing the water content from 2.1 wt.% to 13.98 wt.%, while that of Yun-nan (YN) lignite showed a more significant decrease with increasing water content from 2.2 wt.% to 11.54 wt.%. This could be attributed to a higher coal reactivity of YN lignite than that of WRCB lignite. When the pyrolysis temperature was at or above 1000 °C, due to more significant in-situ coal-steam interactions, a lower char yield of both the pulverized lignites were observed with increasing water content from 2 wt.% to above 10 wt.%. CWS char presented a higher graphitization degree than pulverized coal char. The carbon microcrystalline structure factors (i.e. L002/d002 value) of pyrolysis char increased with coal rank, which might reflect the variation trend of the graphitization degree from low rank coal to high rank coal. During the rapid heat treatment processes, the water evaporation and the in-situ steam-char gasification were favorable for void formation for both pulverized coals with high water contents and CWS. Notably, during the gasification processes of three different rank coals at 1200 °C, a more significant inhibition effect of residual ash on CWS char was observed compared to the pulverized parent coal char with high carbon conversion (x > 0.9).