Study On Oxidative Cracking Performance Of Thermal Extracted Residue Of Low-rank Coal

It is of great significance to effectively using fossil resources and protecting environment by development high-efficiency conversion and high-value utilization technology of low-rank coal,based on its resource characteristic.This paper focuses on the preparation of coal-based humic acid（HA）by a combination of thermal extraction of coal and the oxy-cracking of extract residual coal.Xilinguole lignite（XLGL）,Shenfu sub-bituminous coal（SF）and Xiaolongtan lignite（XLT）were used as raw materials in this paper.Firstly,the coal was extracted by solvent at high temperature to remove the small molecular phase,which is easily oxidized into CO₂,thereby obtaining the thermal extract residual coal（TER）.Then,the obtained TER was oxy-cracked in aqueous to prepare HA.The effects of reaction temperature,oxygen initial pressure,coal/alkali ratio and reaction time on the oxy-cracking performances of TER were investigated.Further,the correlation between TER structure and its oxy-cracking reactivity were discussed by combined with the characterizations of products.The results show that,（1）the content of original humic acid（RHA）in XLGL,SF and XLT is low,4.2,6.3 and 4.0%,respectively,though these coals have low metamorphism and contain a large amount of oxygen.RHAs mainly consist of aromatic structure with a large amount of oxygen-containing functional groups,such as phenolic hydroxyl group,carboxyl group,ether etc.（2）The thermal extraction temperature significantly affects the structure of TER and its oxidative cracking performance.Lower than the initial pyrolysis temperature of coal,the extract yield is low.Because the obtained TER retains the macromolecular network structure of coal cross-linked by bridge bond,its oxy-cracking reactivity is high.Above the initial pyrolysis temperature of coal,the thermal extraction yield significantly increases,and the oxy-cracking reactivity of TER greatly reduces.The suitable thermal extraction temperature are 300and 250°C for XLGL and XLT,and 350°C for SF.（3）For the oxy-racking of TER,the reaction temperature and the initial oxygen pressure are the main influencing factors.Increasing the temperature and the initial oxygen pressure can promote the oxy-cracking of TER and its intermediate products,and affect the yield of target product HA.Meanwhile the CO₂ yield is also greatly increasing.KOH shows a good catalysis on the oxy-cracking of TER,and can promote the dissolution of depolymerized products.Appropriately increasing the amount of KOH is beneficial to increase the yield of HA.However,reaction time has little effect on oxy-cracking,and too long reaction time is disadvantageous to increasing HA yield.Suitable oxy-cracking temperature of TER are 120°C（XLGL-TER-300°C）and 160°C（SF-TER-350°C）.Other optimized conditions are TER/KOH=3/1.5,O₂ 3.0 MPa,1.5 h.Under optimized conditions,the conversion of XLGL-TER-300°C,XLT-TER-250°C and SF-TER-350°C are 68.08,80.23 and 63.32%,and the HA yields reach 51.28,64.34 and 41.87%,and the CO₂ yields are 2.29,6.33 and 4.69%,respectively.（4）The oxy-cracked HA from TER has a high content of aromatic structures,in which mainly consist of 2～3ring condensed aromatic nucleus,and contains a large number of functional groups such as hydroxyl,carboxyl and carbonyl groups.The acidic groups are mainly carboxyl groups.Compared with RHA,the aliphatic structure and carboxyl group of oxy-cracked HA are lower,but there are more carbonyl groups in the oxy-cracked HA.（5）The carbon balance analyses of oxy-cracking show that the carbon recovery of XLGL,SF and XLT are 85.46,84.12 and 96.11%,respectively.（6）The oxy-cracking activity of TER should be related to the bridge structure of methylene and ether bonds.The higher the content of methylene and ether bonds,the easier it is to be oxidized and depolymerized under relatively mild conditions to obtain the target product HA,and reduce the usage of alkali and CO₂ yield.In addition,it was also found that the effect of thermal extraction temperature on the oxy-cracking conversion of TER and the yield of CO₂ is inconsistent.There is a specific thermal extraction temperature range for each coal,in which the oxy-cracking of the obtained TER has high conversion and low CO₂yield.Therefore,the preparation technology of coal-based HA combining the thermal extraction of coal and the oxy-cracking of extract residue is a high-value utilization approach of low-rank coal with good application prospects.