Studies On Depolymerization Of Low-rank Coal And Its Structure

Owing to the crisis of energy resources and the environmental issues arising from the process of energy utilization,it is imperative to transform the energy structure.As a carbon source with abundant reserves and wide distribution,the utilization rate of low-rank coal is low due to its structural limitation,and the clean,efficient,and high-value conversion of low-rank coal is imminent.The structure of a substance determines its reactivity and application.Based on the above,this paper explores the structure and evolution laws of low-rank coal in different transformation processes through depolymerization methods while preparing chemicals,and provides new insights into the structural model of low-rank coal,which can further provide theoretical support for the conversion and industrial application of low-rank coal.As a primary source of low-rank coal,biomass has significant potential to replace traditional fossil energy resources.The molecular structure of lignin,the most stable component of biomass,is importantly related to that of low-rank coal.By exploring the structural similarities and differences between lignin and low-rank coal,we attempt to provide new insights into the evolution of low-rank coal and the"coal-like"utilization of lignin.The main studies and conclusions are as follows.（1）Low-rank coals evolved from plant bodies,and there must be some common relationship between biomass as a future emerging energy resource and low-rank coals in fossil energy resources.The physical characterizations and stepwise depolymerizations of structurally stable lignin fraction and lignite revealed some structural similarities between the two,but with their own individual characteristics.However,lignin is rich in oxygen,mostly phenols and aromatic ethers,with abundant C_al-O bridge bonds and aromatic structures substituted with methoxy and phenolic hydroxyl groups,and long-chain aliphatic acids in the form of free state,weak bonded or aromatic ring side chains.In contrast,lignite is rich in carbon,with mostly C_al-C_al bridge bonds and aromatic structures dominated by alkyl-substituted aromatic carbons,and a large number of long-chain aliphatic acids existed in the form of aromatic ring side chains.The bridge chain length and the abundance of long-chain alkanes in lignite were all better than those in lignin.The above experimental results indicated that the deoxygenation reactions occurred in the process of coal formation from the plant,and a large amount of methoxy and phenolic hydroxyl groups on the aromatic rings,as well as a large number of ether bond structures were removed from lignin,and the structurally stable long-chain alkane structures and fused-ring aromatic structures were retained and enriched,and accompanied by occurrence of cross-linking between structural units.（2）Compared with pyrolysis and liquefaction,alkali-oxygen oxidation has unique advantages in the conversion of low-rank coals to prepare chemicals because of its low reaction temperature and high conversion rate,and it is suitable for the structural properties of low-rank coals with high oxygen content.It is particularly important to understand the correlation between structural properties and oxidation reactivity of low-rank coals.By stepwise alkali-oxygen oxidation of low-rank coal,Xiaolongtan coal,it was found that as the oxidation reaction proceeded,the fused-ringed aromatic structures were enriched in humic acids,while the long-chain alkane structures were enriched in the unreacted residues.Based on special structures that were stripped out,a structural model of Xiaolongtan lignite with a molecular formula of C₃₆₆₁H₃₄₃₅O₁₀₃₆N₈₁S₇ and molecular weight of 65301 amu was constructed.There are 43.7%single-,39.3%double-,14.7%triple-,and 2.4%quadruple-ring aromatic clusters,amount which there 15.9%pyrrole-and 2.4%pyridines-based aromatic clusters.The controllable oxidation avoids the peroxidation reaction of aromatic clusters and the long-chain alkanes,thereby gaining a deeper understanding of the structure and oxidation process of lignite.（3）Direct coal liquefaction is a conversion method to capture free radicals and get a high yield of oils by hydrogenation reaction.However,the selection of types of coal during liquefaction is also a key factor in influencing the liquefaction result.And what structure of coal is suitable for direct coal liquefaction is a critical issue.The structure of Naomaohu coal,typical low-rank coal used for liquefaction,has been studied,and a structural model of Naomaohu coal with a molecular size of 128755 amu has been constructed by combining physical characterization analyses and oxidation depolymerizations,in which the ratio of single-,double-,triple-and quadruple-aromatic rings is 149:164:102:63.The triple-and quadruple-aromatic clusters are mainly connected by C-C bonds consisting of 2-4 carbon atoms,while the single-and double-aromatic clusters are mainly connected to the triple-and quadruple-aromatic clusters by ether and ester bonds.The proportions of aromatic and aliphatic carbon in Naomaohu coal were 62.60%and 31.03%,respectively,and the average size of aromatic ring structures was about 2 rings.Finally,it was determined that the high H/C ratio,a large amount of oxygen-containing structures（ether and ester bonds,and other weak bonds）,the small-size aromatic clusters structures composed of abundant 1～4 rings,and the C2-C4 alkyl bridge bonds are favorable to the liquefaction behavior of Naomaohu coal.（4）Pyrolysis is the most widely utilized method,involving almost all thermal conversion processes.The study of the evolution characteristics and pathways of low-rank coal during pyrolysis can provide theoretical guidance for low-rank coal grading and fractionation utilization and the coalification process.In this work,the oxidative depolymerization of low-rank coal and pyrolyzed coal at different temperatures,combined with physical characterization,was used to deduce the structural evolution characteristics and pathways during pyrolysis,revealing the correlation between aromatic cluster size and pyrolysis temperature.As the pyrolysis temperature proceeded to 500℃,the aliphatic side chains and bridges scission as well as the aromatic structures were enriched and grew up in the low-rank coal matrix.The layer spacing(d₀₀₂)varied very little,while the aromatic lamellae size（L_a）and stacking height（L_c）increased significantly.The average size of aromatic clusters gradually evolved from single-and double-ring aromatic structures of low-rank coals to triple-and quadruple-ring aromatic clusters by pyrolysis at 500℃.By oxidative depolymerization,it was found that the reaction time of pyrolyzed char samples was prolonged and the yield of the product of benzene carboxylic acids was increased by increasing the pyrolysis temperature.It indicated that the coal structure became more complex and less reactive after pyrolysis,and a large number of aromatic ring structures that were not directly connected with oxygen atoms were generated,especially the precursor structures of benzene carboxylic acids containing 2～4-COOH.