Mathematical modelling of Collie coal pyrolysis considering the effect of steam produced in situ from coal inherent moisture and pyrolytic water

Abstract

Low-rank coals have high inherent moisture content, and high yield of pyrolytic water formed from the functional groups in the coal during pyrolysis. Steam produced from coal moisture can lead to significant in-situ char gasification during pyrolysis under conditions where there are strong and prolonged interactions between the char and the in-situ steam. This phenomenon can be encountered in many practical processes yet is often overlooked in pyrolysis modelling. Using Collie coal as an example, this study presents a mathematical 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 steam produced in situ during pyrolysis due to both coal inherent moisture and pyrolytic water. Under conditions where there are strong and prolonged interactions between the pyrolysis products (particularly char) and steam produced in situ from the coal inherent moisture and pyrolytic water, lower char yields from both raw and demineralised coal are predicted, in agreement with the experimental data, under the current experimental conditions. This effect increases with increasing coal inherent moisture content, but decreases with increasing particle size due to the slower heating rate incurred during pyrolysis and the pore diffusion effect during char gasification for the large particles. Volatile steam reforming competes with char gasification for available steam, especially at high temperatures and for large particles. Experiments as well as model prediction also clearly demonstrate that char steam gasification reactivity data should be obtained from experiments carried out on the in-situ basis as the in-situ reactivity is significantly higher than that on the ex-situ basis.