The Study Of Gasification Mechanism Of Lignite In CO₂ Atmosphere

Gasification with CO2 as the gasifying agent is an important technology to improve the utilization of low-rank lignite and reduce CO2 emissions.The CO2 gasification process of lignite is difficult to explain the mechanism of CO2 gasification at the molecular level by using experiment alone.Hence,performing simulations the CO2 gasification process of lignite by using the molecular dynamics based on the reactive force field(Reax FF).In order to obtain the formation mechanism of CO,CH4 and H2,both generated radicals and elementary reactions of free radicals are analyzed.Using the lignite model proposed by Wolfrum as the initial structural unit,and then construct two models for CO2-gasification and direct pyrolysis processes.By Reax FF,the two models are simulated at 3000 K for 1 ns.C++ programs are developed to process Reax FF trajectories.The results show that CO2 can increase the amount of gas product CO,CH4,and H2 by reacting with lignite,which is consistent with previous studies.Chain carbon radicals react with CO2 to form the intermediate Rn-CO2,Rn-O-C-O and Rn-O in the CO2 gasification process of lignite.These intermediates further release CO.Both Rn-O and Rn-O-C-O structures are important precursors for the formation of CO.In the CO2 gasification process,CH4 molecules are formed by the cross-linking reactions between methyl radicals and “active hydrogen atoms”.Methyl radicals come from methoxy group of lignin moieties methyl-containing chain radical formed by the cleavage of lignite.H2 is mainly formed by the cross-linking reactions of two active hydrogen atoms.In the pyrolysis process,the H atoms on the methyl groups and methylene are fall off to form free H atoms.However,for the CO2 gasification process,a large amount of COH radicals are formed.These COH radicals indirectly increase the concentration of active hydrogen atoms in the CO2 gasification of lignite.However,it promots the formation of CH4 and H2.This paper explains the formation mechanism of small molecular gas in the gasification process from the molecular level,which provides the theoretical support for studying the gasification of CO2.