Effect of mineral transformation in coal on migration behavior of typicallyharmful elements during pressurized gasification

In this study,the diffusion,migration,and transformation mechanism of harmful elements and their controlling factors under specific gasification conditions were revealed,which is considered to have important theoretical significance for establishing methods to reduce the pollution of harmful elements in the coal gasification process,thereby realizing the clean conversion of coal. Minerals in coal are the carrier of most harmful elements,their transformation will directly affect the migration of harmful elements. A typical lignite was used to carry out the pressurized gasification in a fixedbed reactor,and the effects of mineral transformation on the migration behavior of harmful elements,such as F,Hg,As,Se,Cd,Pb,U and Be,were systematically investigated. Furthermore,the species of harmful elements enriched in slags were analyzed by Factsage simulation. The results showed that the harmful elements were mainly combined with minerals in raw coal,and their transformation demonstrated three different forms during gasification. The fine mineral particles(<10 μm)were easy to enrich some harmful elements,especially As. The F with a relatively high abundance in coal was in various forms. It tended to combine with alkaline elements and/or be adsorbed in layered minerals. The Pb was found to coexist with pyrite. After pressurized gasification,the RE of F,Hg,Se and Cd,especially F and Hg,was less than 1. The RE of As and Pb changed significantly with the temperature increase. The RE of As remarkably increased from dissipation to enrichment,while the trend of Pb was opposite. The harmful elements U and Be were relatively enriched in slags with RE>1. In the process of mineral transformation,pyrite decomposition promotes the release of sulfur affinity elements such as Hg,As,Se,Cd and Pb,while the transformation of kaolinite and other clay minerals is conducive to the volatilization of F. The transformation of muscovite and calciumbased minerals in coal ash would further affect the migration of As,Pb,U and Be over 800 ℃. The As,U and Be tended to enrich in the slag in the forms of arsenates,magnesium arsenide,uranium oxide and beryllium aluminates,respectively,after pressurized gasification,which should be a matter of concern and should be controlled in the process of ash stacking and harmless treatment.