Investigation on the structure evolution and thermochemical conversion characteristics of lignite after different pretreatment

Abstract

Despite the importance of hydrothermal treatment dewatering (HTD) as a means of utilizing lignite efficiently and cleanly, there have been relatively few studies on its downstream combustion and gasification characteristics. Particularly, what kind of thermochemical process is more favourable, which has not been investigated. At the same time, conventional flue gas treatment (FTD) is a relatively mature lignite pretreatment technology and is selected for comparative analysis. The present study examined the thermochemical characteristics and physicochemical structure of upgraded lignite by HTD and FTD processes. A combined process with great practical application foreground is proposed. Results showed that both HTD and FTD processes can profit dehydration and deoxidation of lignite. HTD process has a better effect in terms of dehydration because of the high amount of O-containing functional group removal. In terms of aromaticity and coal-rank, upgraded lignite treated with HTD is inferior to that treated with FTD. The comprehensive combustion characteristics indexes of upgraded lignite are increased by HTD and decreased by FTD, indicating that the HTD process promotes the combustion reactivity of upgraded lignite. HTD and FTD processes both decrease the gasification reactivity of upgraded lignite. Compared with the HTD, the FTD process is more likely to reduce the gasification rate. Compared with effective coal rank and the concentration of oxygen functional groups, the revived microstructure of upgraded lignite has a greater impact on combustion and gasification reactivity. The HTD process was more prone to the breakdown of chemical groups rather than crosslinking. These findings are consistent with the results of kinetic analysis. In general, compared with FTD, upgraded lignite by HTD process is in favor of downstream thermochemical application. The comparison of different thermochemical characteristics, including combustion, gasification, pyrolysis, are analyzed. The effective application with combined process of HTD and pyrolysis and waste water coke slurry have industrial application prosperity.