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 10³ K s^-1 and pyrolyzed in two different reactors, a drop-tube reactor (DTR) and a Curie-point reactor (CPR). In DTR the vapor-phase secondary (extra-particle) reactions of volatiles occurred concurrently with the primary reactions within the particle, while in CPR the volatiles were swept out of the heating zone immediately after formed so that their secondary reactions were efficiently suppressed. The char yields for the pyrolyses in these two reactors were described by the same functions of temperature. This enabled to evaluate the decrease or increase in the yield of volatile products due to the secondary reactions that proceeded within the volatiles' residence for time shorter than 2 s. The evaluation was done by a parameter, ΔY(i), defined as ΔY(i) = Y(i)_DTY(i)_CP where Y(i)_DT and Y(i)_CP are the yields of volatile product i for pyrolyses in DTR and CPR, respectively. ΔY(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 temperatures lower than 873 K. ΔY(tar) at 1173 K was much lower than that expected when Y(tar)_DT was assumed to decrease only due to dealkylation to form gaseous hydrocarbons and deoxygenation to form carbon monoxide, -10 mol-C. The difference between the above two ΔY(tar)s was explained well by the reaction of tar with steam (water formed by the primary pyrolysis) occurring above 1073 K and resulting in a considerable decrease in ΔY(H₂O) and the corresponding increase in ΔY(CO) and ΔY(H₂). The addition of steam to the carrier nitrogen gas further promoted the reduction of Y(tar)_DT and ΔY(tar) to 1 and -23 mol, respectively. The reduction of ΔY(tar) for the pyrolysis of acid-washed coal was much less significant than that for the original coal. Thus the observed steam reforming of tar was found to be catalyzed by Fe, Ca and/or Mg species that were initially dispersed in the coal matrix as ion-exchanged cations. The addition of steam also increased the total conversion of carbon into volatiles at 1173 K from 44 to 56 mol, indicating the gasification of char by steam within an estimated residence time of coal/char particles.