加压气化过程煤中矿物转化对典型有害元素迁移行为的影响

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

揭示特定工况下有害元素的迁移规律、转化机理及影响因素,对建立气化过程中有害元素污染过程控制方法,实现煤炭的清洁转化具有重要的理论指导意义。煤中矿物质是有害元素赋存的主要载体,在加压气化过程中矿物的转化会直接影响有害元素的迁移。研究针对典型褐煤,在自制固定床加压气化实验考察了特定气化条件下煤中矿物质转变对有害元素F,Hg,As,Se,Cd,Pb,U和Be演化行为的影响。结合FactSage热力学模拟,解析了气化过程灰渣中有害元素的演变规律。结果表明,有害元素在原煤中主要与矿物质结合,并在气化过程呈3种不同形式的迁移行为。有害元素As易于富集在直径略小于10μm的细颗粒矿物质中;F倾向于与碱性元素结合,并可能被吸附于层状矿物质中;Pb与黄铁矿伴存。经加压气化后,F,Hg,Se和Cd,尤其是F和Hg,相对富集系数RE<1,呈耗散趋势。As的RE随气化温度升高显著增大,由耗散转变为富集;而Pb则相反。U和Be在灰渣中富集,RE始终大于1。在矿物转化过程中,黄铁矿分解促进亲硫性有害元素Hg,As,Se,Cd和Pb的析出;高岭石等黏土矿物的转化有利于F的挥发。温度高于800℃后,白云母、钙基矿物质的分解进一步影响As,Pb,U和Be的迁移。热力学模拟表明,As,U,Be倾向于以砷酸盐、砷化镁、氧化铀和铝酸铍形式富集在气化灰渣中,在灰渣的堆放及无害化处理过程中需给予关注。

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(<10μm)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.