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

The high oxygen content in lignite increases the yield of H2O during the thermal conversion process,hence,the hydrogen consumption per unit target product is increased.To circumvent this problem,the oxygen content should be kept in the oil as phenolic oxygen,which might leads 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 H2O and CO2.In the second stage,the as-phaltenes,pre-asphaltenes (PAA) and THFI transfer their oxygen content to n-hexane soluble substances and also to H2O (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 study provides some useful insights for understanding the existing chemical problems in direct coal liquefaction.