Rapid gasification of nascent char in steam atmosphere during the pyrolysis of Na- and Ca-Ion-exchanged brown coals in a drop-tube reactor

Several recent studies on in situ steam gasification of coal suggest a possibility of extremely fast steam gasification of char from rapid pyrolysis of pulverized brown coal. The unprecedented rate of char steam gasification can be achieved by exposing nascent char, that is, after tar evolution (temperature range>600°C), but before devolatilization (<900°C), to steam in the presence ofNa and/orCa dispersed in/on the char. In this study, we conducted rapid pyrolysis experiments using ion-exchanged Loy Yang brown coal samples, that is, H-form coal with Na/Ca contents<0.001 wt%,Na-form coal with Na content=2.8 wt%and Ca-form coal with Ca content=3.2 wt %. These samples were pyrolyzed in an atmospheric drop-tube reactor at a temperature of 900°C, inlet steam concentration of 50 vol. %, and a particle residence times of 2.8 s. The char yields from the pyrolysis ofNa-formand Ca-formcoals were as low as 12 and 33% on the respective coal carbon bases, and accounted for only 18 and 53% of the char yields from the full devolatilization of the respective coals at 900°C. In addition, the pyrolysis also consumed asmuch as 0.7-1.1mol ofH₂Opermol of coal C. On the other hand, the nascent char fromthe H-form coal allowed carbon deposition from the nascent tar, resulting in a char yield as high as 115% of that fromthe full devolatilization. The chars from theNa-form andCa-form coals also acted as catalysts for steam reforming of tar,whichwas evidenced by significant negative synergistic effects of blending of H-form coal with Na-form coal or Ca-form coal on the tar and soot yields.