Quantum Chemical Study On The Mechanism Of Char-steam Gasification

Coal gasification is a hot spot in the field of coal chemical industry,in which the reaction activity of steam gasification is high and can produce more clean combustible hydrogen,so it is widely used in industry.Limited to the experimental methods,the mechanism of steam gasification has not been fully studied,especially in the process of hydrogen production,compared with CO2 gasification.As a reliable theoretical calculation method,quantum chemical calculation can be used to simulate the reaction process of coal-steam gasification directly from the molecular level,which provides a theoretical basis for the experimental study.In this paper,the reaction mechanism of char gasification with steam is studied by quantum chemical method with level M06-2X/6-31G(d,p).The four coal char models of Zigact,Zig,Armact and Arm are simultaneously optimized by geometric optimization,transition state retrieval and IRC verification.The reaction process of the four models is analyzed carefully,and a variety of reaction paths are identified.Each reaction path map and energy barrier map were plotted,and the similarities and differences between different paths and models were analyzed and compared.The four char models need to undergo three reaction processes: H2 O molecule adsorption,H atom transfer and CO molecular desorption.For the Zigact model,there are two reaction paths,in which the reaction path of Zigact1-Zigact7 is the best reaction path and the limited step is the desorption of CO molecules.For the Zig model,there are three reaction paths,and two steps of H transfer process are needed.The reaction path of Zig1-Zig12 is the best reaction path,and the limited step is the desorption of CO molecules.The final structure energy to Zigact model are-48.8(Zigact4)and-12.8(Zigact7)k J/mol,which can not react alone.For the Armact model,there is only one reaction path and the limited step is the transfer of H atom.For the Arm model,there are three reaction paths,and two steps of H transfer process are needed.The reaction path of Arm1-Arm10 is the best reaction path,and the CO step is the desorption process.The applicability of the Armact model and the Arm model are both good.In general,the zigzag models have better reactivity than the armchair models,which the minimum energy required are lower.Increasing the number of the zigzag active sites in the actual gasification will help to reduce the reaction cost.The limited step of most reaction paths is CO desorption.So we can add H atoms after adsorption and increasing the number of CO desorption steps to strength the process of CO desorption,which willhelp to improve the gasification efficiency.