褐煤加氢反应中氧的分布、迁移与转化规律

Distribution,migration and transformation of oxygen during the hydrogenation reaction of lignite

褐煤中较高的氧元素质量分数在加氢反应过程中极易生成水,从而增加了单位目标产物生成过程中的氢耗。反之,若将氧元素以酚羟基的形式保留在油品中,辅之以提酚工艺可将褐煤中的氧元素得到高值化利用。为了提高褐煤中O元素在液体产物中有机化合物的质量分数,在实验分析的基础上,将褐煤加氢反应过程分为3个阶段,以氧元素物料衡算为约束,通过定量分析各阶段加氢反应产物中含氧官能团物质的量的变化,揭示了褐煤中氧的热转化规律。第1阶段,氧元素的迁移方向为从四氢呋喃不溶物(THFI)转移至H_(2)O和CO_(2),相应温度范围为室温~400℃;第2阶段,沥青质与前沥青质(PAA)和四氢呋喃不溶物中的氧元素主要迁移至正己烷可溶物和H_(2)O中,同时,该阶段正己烷可溶物中酚类氧出现峰值,对应的温度区间为400~450℃,并在450℃保持30 min;第3阶段,随着加氢液化反应过程持续,氧元素迁移方向转变为由正己烷可溶物转移至沥青质与前沥青质以及四氢呋喃不溶物。同时,由于发生了明显的缩聚反应,正己烷可溶物中的酚类化合物产率逐渐减少,对应的反应阶段为450℃下保持30~60 min。通过上述3个阶段产物中含氧质量化分析,表明褐煤加氢反应产物中含氧官能团赋存形式不仅受化学环境影响,而且还与反应条件直接相关。基于对含氧官能团物质的定性定量分析结果,褐煤中含氧官能团在加氢反应热转化过程中变化规律,将有助于认识和理解煤直接液化工艺过程中的化学问题。

The higher oxygen content in lignite increases the yield of H_(2)O during the thermal conversion process and hence,the hydrogen consumption per unit target product is increased.To circumvent this,the oxygen content should be kept in the oil as phenolic oxygen,which might lead to the valuable utilization of oxygen content via phenol extraction.To enhance the utilization of oxygen content,a three-stage hydrogenation of lignite and the quantitative analysis of thermal transformation rules of oxygen-containing functional groups in each stage are presented by keeping the material balance restriction.In the first stage(room temperature-400℃),the oxygen is transferred from tetrahydrofuran insoluble substances(THFI)to yield H_(2)O and CO_(2).The asphaltenes and pre-asphaltenes(PAA),and THFI transfer their oxygen content to n-hexane soluble substances and also to yield H_(2)O(400-450℃and maintain 30 min).Intriguingly,the value of phenolic oxygen in oil products has reached the peak.In the third stage,the direction of oxygen migration is reversed,i.e.,the oxygen is transferred from n-hexane soluble substances to PAA and THFI.Concurrently,the phenolic compounds in the oil are gradually reduced due to the condensation reaction at 450℃in a certain retention time period(30-60 min).Based on the qualitative and quantitative analysis of oxygen-containing functional groups,the conversion of oxygen-containing functional groups in lignite during the thermal hydrogenation is understood.The analysis of oxygen content in the products at every stage shows that both the chemical environment and the conditions of the process are important to govern the nature of oxygen-containing functional groups in lignite and products.This present study might yield the useful insights for understanding the existing chemical problems in the direct coal liquefaction.