Rapid conversion of tar and char from pyrolysis of a brown coal by reactions with steam in a drop-tube reactor

Yallourn brown coal particles were heated at rates in the order of 10(3) K s(-1) and pyrolyzed in two different reactors, a drop-tube reactor (DTR) an d a Curie-point reactor (CPR). In DTR the vapor-phase secondary (extra-part icle) reactions of volatiles occurred concurrently with the primary reactio ns within the particle, while in CPR the volatiles were swept out of the he ating zone immediately after formed so that their secondary reactions were efficiently suppressed. The char yields for the pyrolyses in these two reac tors were described by the same functions of temperature. This enabled to e valuate the decrease or increase in the yield of volatile products due to t he secondary reactions that proceeded within the volatiles' residence for t ime shorter than 2 s. The evaluation was done by a parameter, Delta Y(i), d efined as Delta Y(i) = Y(i)(DT)Y(i)(CP) where Y(i)(DT) and Y(i)(CP) are the yields of volatile product i for pyrolyses in DTR and CPR, respectively. D elta Y(tar) was found to decrease with increasing pyrolysis temperature and reached -19 mol-C per 100 mol-C in the coal, while it was negligible at te mperatures lower than 873 K. Delta Y(tar) at 1173 K was much lower than tha t expected when Y(tar)(DT) was assumed to decrease only due to dealkylation to form gaseous hydrocarbons and deoxygenation to form carbon monoxide, -1 0 mol-C. The difference between the above two Delta Y(tar)s was explained w ell by the reaction of tar with steam (water formed by the primary pyrolysi s) occurring above 1073 K and resulting in a considerable decrease in Delta Y(H2O) and the corresponding increase in Delta Y(CO) and Delta Y(H-2). The addition of steam to the carrier nitrogen gas further promoted the reducti on of Y(tar)(DT) and Delta Y(tar) to 1 and - 23 mol, respectively. The redu ction of Delta Y(tar) for the pyrolysis of acid washed coal was much less s ignificant than that for the original coal. Thus the observed steam reformi ng of tar was found to be catalyzed by Fe, Ca and/or Mg species that were i nitially dispersed in the coal matrix as ion-exchanged cations. The additio n of steam also increased the total conversion of carbon into volatiles at 1173 K from 44 to 56 mel, indicating the gasification of char by steam with in an estimated residence time of coal/char particles. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.