Programmable heating and quenching for enhancing coal pyrolysis tar yield: A ReaxFF molecular dynamics study

In this paper, the impact of the programmable heating and quenching (PHQ) method on coal pyrolysis tar yield was investigated via ReaxFF molecular dynamics method. Firstly, thermostatic pyrolysis simulations of coal were performed to determine the range of maximum (Tmax) and minimum (Tmin) temperatures for the PHQ method. Then, the effects of the ratio of heating time (th) and cooling time (tc), Tmax, and Tmin on pyrolysis tar yield were investigated. The results showed that reducing th/tc, Tmax, and Tmin helped to improve tar yield. Finally, the effects upon pyrolysis products were compared between the conventional continuous heating (CCH) method and the PHQ method. It was revealed that 37.67 % increase in tar yield was achieved with the PHQ method compared to the CCH method. The main basic reactions during coal pyrolysis were analyzed to reveal the mechanism of the PHQ method on tar yield. There were three ways to increase tar yield with the PHQ method: (1) the elevated production of OH radicals during each heating stage facilitated the decomposition of char into tar; (2) the reduction of temperature during each quenching stage hindered the condensation reactions among tar molecules; (3) the reactivity of the tar was also diminished.