Oxygen Migration And Regulation During Lignite Pyrolysis

Pyrolysis is an important process of conversion and utilization of lignite,but the high oxygen content of lignite(15%-30%)make useless water to generate in pyrolysis,resulting in the decline of tar yield and quality,affecting the subsequent processing and utilization of pyrolysis products.It is of great theoretical and practical significance to study the law of oxygen migration during lignite pyrolysis and further regulate oxygen to tar or char.At present,there are few studies on oxygen migration and regulation during lignite pyrolysis,especially qualitative and quantitative studies on oxygen-containing functional groups in lignite.In this paper,a set of feasible and relatively accurate qualitative and quantitative methods of oxygen-containing functional groups in lignite is proposed,which is constrained by material balance.The oxygen migration of lignite under isothermal pyrolysis at 650 °C is studied.Further,the effects of preheating and acid pretreatment on oxygen migration and mechanism are discussed.The main results obtained in this paper are as follows:1.Combining chemical analysis with infrared analysis,the oxygen-containing functional groups in lignite can be qualitatively and quantitatively analyzed with relatively high accuracy.The content of oxygen-containing functional groups measured by this method occupies 85.70% of the total organic oxygen.2.The migration route of oxygen in lignite at 650 °C was studied with material balance as constraint.It was found that most of the organic oxygen in lignite migrated to char and pyrolysis water,accounting for 43.22 wt.% and 21.70 wt.% of the total organic oxygen,respectively,and only 2.47 wt.% of the total oxygen was in tar.Further quantitative analysis showed that phenolic oxygen in tar occupied 0.39 wt.% of the total organic oxygen.3.Compared with pyrolysis without preheating,the oxygen in lignite preheating at 200 °C transferred to water and char decreased by 7.55 wt.% and 1.43 wt.%,respectively.The oxygen transferred to tar and gas increased by 6.66 wt.% and 1.61 wt.%,respectively,and phenolic oxygen in tar doubled.The decrease of hydrogen bond in lignite preheating process leads to the increase of phenolic compounds in n-hexane-soluble matter in pyrolysis tar.Further in-situ infrared diffuse reflectance spectroscopy analysis shows that OH···?,OH···N and hydroxyl self-associating hydrogen bond in coal break during preheating to form free OH?,which results in the increase the content of phenol and cresol in phenolic compounds.4.The distribution of oxygen in lignite products is directly affected by acid pretreatment.With the progressing of acid treatment,the oxygen migrated to char and pyrolysis gas decreased,while the oxygen migrated to tar increased.Compared with raw coal,the oxygen increment of coal treated with HCl-HF is the largest(5.07 wt.%).The oxygen increment of light components of tar(n-hexane-soluble matter)is 0.40%,some of which is from the increase of polyphenols.Further study on the mechanism of acid pretreatment on oxygen migration pathway in lignite pyrolysis shows that: on one hand,remove of mineral elements is beneficial to the migration of carboxylic compounds to tar instead of gas.On the other hand,acid pretreatment make the pore structure of raw coal destroy and the H/C ratio increase,which regulated the oxygen transfer to tar instead of char.