ReaxFF molecular dynamic simulation of primary and secondary reactions involving in sub-bituminous coal pyrolysis for tar production

ReaxFF molecular dynamic simulation combined with experimental verification was performed to understand the overall reaction mechanism,especially the primary and secondary reactions involving in tar formation of sub-bituminous coal pyrolysis.Quantitative relationship at atomic level is clarified between bond breakage of functional groups and products generation,revealing that the amount and order in forming each product are subject to the number of corresponding functional groups and their bond energies respectively.The primary breakage of-C-O-and-C-C-bridge-bonds present in initial coal macromolecular generates molecular of heavy tar,whereas heavy tar can be converted into light tar through cracking side chain of aromatic rings and cyclic hydrocarbons at increased pyrolysis temperatures.At very high temperatures the cracking of short-chain hydrocarbons and residual atoms connecting to aromatic rings further occurs to generate light tar and gas.The remaining aromatic-ring fragments of heavy tar are likely cross-linked to form char.Furthermore,the simultaneous evolution tendency of tar yield and tar quality under different pyrolysis temperatures and heating rates is obtained at molecular level.For obtaining high yield and quality of tar,appropriately high temperature as well as suitable heating rate are needed to compromise the high yield of primary tar and high quality of secondarily upgraded products.

Secondary cracking of volatile and its avoidance in infrared-heating pyrolysis reactor

This study aims to compare the pyrolysis behavior of Huadian oil shale in two infrared heating fixed bed reactors with different directions of infrared beam.Our previous work has shown that fast pyrolysis of oil shale conducted in the shallow fixed bed infrared heating reactor(co-current)presented the massive secondary reactions,which lowered the shale oil production(Siramard et al.,2017).Conversely,the cross-current infrared achieved shale oil yields higher than the Fischer Assay oil yield(13.07 wt%of dry basis),such as 117.7%of the Fischer Assay yield at our realized highest heating rate of 7℃/s under a specified pyrolysis temperature of 550℃.The shale oil from the cross-current infrared heating reactor was obviously heavier than the oil obtained from the cocurrent heating reactor.Thus,the infrared cross heating evidently suppressed the secondary reactions toward volatile.Our realized shale oil yield could reach 13.67 wt%or 122.5%of the Fischer Assay yield under reducing pyrolysis pressure of 0.6 atm,indicating that lower pressure is also beneficial to the release of volatile and reduction of the secondary cracking reactions.This work shows essentially that the infrared cross heating provides an effective merge of the advantages from quick heating and minimization of secondary cracking reactions to enable the shale oil yields being higher than the Fischer Assay oil yield.