Study On The Transformation Of Oxygen-Containing Functional Group And Its Mechanism During Lignite Direct Liquefaction

The utilization of lignite is limited due to high moisture,high ash yield,high oxygen content,and low calorific value character.The technology of direct coal liquefaction turns lignite into liquid fuel and oxygen-containing chemicals.It takes advantage of the high volatile and high oxygen properties of lignite and increases the added value of lignite utilization.Hulunbuir lignite was selected as raw material in this study.The rule of migration,distribution and transformation of oxygen from lignite to liquefaction products,such as liquefied oil?Oil-O?,asphaltene,preasphaltene?PAA-O?,gas phase products?Gas-O?,liquefaction residues?THFI-O?and water?H₂O-O?was researched under different liquefaction temperatures,liquefaction time and with/without catalyst Fe₇S₈.In addition,the formation mechanism of phenols in liquefied oil was revealed.The contents of carboxyl and phenolic hydroxyl were quantified by chemical titration method.Combing with FTIR,carbonyl and ether bonds were quantified.Elemental oxygen content was analyzed with elemental analyzer.The main research conclusions are as follows:1.The coal conversion and the yield of oil were 75.80 wt%and 31.28 wt%during liquefaction reaction at 450? for 60 min without catalyst.The coal conversion and the yield of oil were 88.71 wt%and 49.29 wt%with Fe₇S₈ under the same conditions.2.Oil-O.Phenolic compounds is the main form of oxygen in liquefied oil.The order of main phenolic content is o-cresol>m/p-cresol>phenol.Without catalyst,the m-cresol and 1-naphthol can be detected at 300?.With the addition of 3 wt%catalyst Fe₇S₈,phenolic substances were produced at 200?,mainly o-cresol.In addition,the maximum yield of phenolic compounds with Fe₇S₈ was increased by 40.58%than that without catalyst addition.The main reason is that the catalyst Fe₇S₈ increases the production rate of hydrogen radical,reduces the condensation reaction of phenolic hydroxyl groups,and increases the yield of phenolic products.3.H₂O-O.The content of H₂O-O increased firstly then decreased with the increase of liquefaction temperature or the reaction time.H₂O-O can reach up to36.05 wt%at 450? for 60 min without catalyst and reach up to 20.32 wt%when3wt%catalyst Fe₇S₈ was added at 450?.4.PAA-O.The content of carboxyl and phenolic hydroxyl groups decreased and the contents of carbonyl and ether bonds increased with the increase of reaction time at 450? without catalyst.Finally,the oxygen-containing functional groups in PAA are mainly carbonyl and ether bonds.Catalyst has no effect on the content and type of PAA-O.5.THFI-O.With the increase of liquefaction temperature and reaction time,the oxygen-containing functional groups in THFI partially were decomposed into gas,some of them transformed to PAA and Oil.The ether bond is the main form of THFI-O at the end of liquefaction reaction,regardless of the presence or absence of catalyst.The aromatic ether bonds formed by the conversion of functional groups are stable in structure and unfavorable in the reaction.6.Gas-O.The occurrence forms of Gas-O were CO and CO₂.The catalyst Fe₇S₈ has little effect on the content of Gas-O.In summary,at the normal operation condition of coal liquefaction?450? and 6 Mpa initial hydrogen pressure?,the more oxygen element trended to migrate into the Oil-O rather than H₂O-O when 3 wt%Fe₇S₈ was added.The proportion of oxygen in phenolic products was 5.5 wt%.It is indicated that it is possible to extract phenolic oxygenates from oil.The research conclusions may provide a theoretical basis for further application of liquefaction industrialization.