Mathematical modelling of Collie coalpyrolysis considering the effect of steam producedin situ from coal inherent moisture and pyrolytic water

摘要

Low-rank coals have high inherent moisture content, and high yield of pyrolytic water formed from thefunctional groups in the coal during pyrolysis. Steam produced from coal moisture can lead to significantin-situ char gasification during pyrolysis under conditions where there are strong and prolonged interactionsbetween the char and the in-situ steam. This phenomenon can be encountered in many practical processesyet is often overlooked in pyrolysis modelling. Using Collie coal as an example, this study presents amathematical model on low-rank coal pyrolysis, considering primary and secondary coal pyrolysis reactions,heat transfer, mass transports, as well as char gasification and volatiles cracking/reforming by steamproduced in situ during pyrolysis due to both coal inherent moisture and pyrolytic water. Under conditionswhere there are strong and prolonged interactions between the pyrolysis products (particularly char) andsteam produced in situ from the coal inherent moisture and pyrolytic water, lower char yields from bothraw and demineralised coal are predicted, in agreement with the experimental data, under the currentexperimental conditions. This effect increases with increasing coal inherent moisture content, but decreaseswith increasing particle size due to the slower heating rate incurred during pyrolysis and the pore diffusioneffect during char gasification for the large particles. Volatile steam reforming competes with char gasificationfor available steam, especially at high temperatures and for large particles. Experiments as wellas model prediction also clearly demonstrate that char steam gasification reactivity data should be obtainedfrom experiments carried out on the in-situ basis as the in-situ reactivity is significantly higher than that onthe ex-situ basis.