Research On Catalytic Depolymerization Of Low-Rank Coal

The extensive use of fossil fuels is the main cause of global warming, but it is still inevitable to use coal containing high carbon content as primary energy source of China for a long period of time due to the occurrence state of energy resources in China. In the traditional conversion process of coal, basically all the chemical bonds in coal are broken and then recombined, which leads to considerable input and loss of energy and produces substantial emissions of carbon dioxide. Accordingly, innovation of coal conversion and development of conversion process of coal with low carbon content is strategically important for honoring international commitment of carbon emission reduction in the future and ensuring energy supply and security in critical production areas during rapid economic development of China.The reserve of low-rank coal（lignite and sub-bituminous coal） is large in China. At present, the proved reserves and output of low-rank coal account for more than 50% of coal in China. In comparison with medium and high-rank coals, the main structure unit in low-rank coal is bicyclic or tricyclic aromatic, which are connected by a large amount of bridging bonds. If the high-value constituents in coal can be separated from coal as many as possible under mild conditions, and the chemical production process of coal-based polymer is developed on this basis, it is expected to achieve high-value utilization of coal with low carbon emissions.Coal is not out of date, but the utilization method of coal. Through the action of the catalyst, the binding energy of the chemical bond in coal can be changed, so that more aromatic small molecules can be dissociated from macromolecular structure of coal and then be released as products under relatively mild conditions. The idea of catalytic pyrolysis is not new, but it is worthy of a study to judge whether the catalysts added in coal influence the rupture of chemical bonds. Therefore, the highly-dispersed catalysts must contact with coal molecules in depth in order to investigate the effect of catalysts on coal pyrolysis.In this work, a new method was applied to quickly add catalysts in large coal particles, and we focused on three aspects on this basis:（1） whether the catalysts and the addition method promotes depolymerization of coal at low temperatures and increases the yield of tar;（2） the change in characteristics of pyrolysates under the influence of catalysts;（3） the action mechanism of catalysts for depolymerization of coal.The coal was pyrolyzed after addition of Fe and Mo-based catalysts. Through the study of yield and composition of the resulting tar, gas, char, the main conclusions were drawn as follows:1. By temperature programming, 14 coals in two series were depolymerized and catalyzed by Fe-based catalyst at low temperatures in kettle-type furnace. In the presence of Fe-based catalyst, the conversion and tar yield of the most lignite and sub-bituminous coal were changed in varying degrees, indicating that the coal pyrolysis was influenced by the addition of the catalyst in this work. The increase in conversion and tar yield for the most coals demonstrates that the coal pyrolysis can be promoted by catalysts, namely, the achievement of the catalytic depolymerization of coal.2. The tar was analyzed by elemental analysis, TG and GC-MS. The Fe-based catalyst increased yield of phenols in tar from NeiMonggol A, and the Mo-based catalyst increased yields of alkane and dicyclic aromatic hydrocarbon in the tar. The yield of alkane in tar from Xinjiang A significantly increased, and lightening of the tar occurred under the influence of Fe- and Mo-based catalysts.3. The polymerization degree of char was analyzed by infrared, Raman and XRD techniques. The polymerization and graphitization in char was inhibited by Fe and Mo-based catalysts during catalytic depolymerization of NeiMonggol A and Xinjiang A.4. By chromatography, it is found that the content of CO₂ in coal gas increased and that calorific value of coal gas decreased during catalytic pyrolysis of NeiMonggol A and Xinjiang A.5. Fe-based catalyst has catalytic cracking and hydrogenation effects on coal pyrolysis, and the dominance degree of the two effects is related to the structure characteristics of coal. The catalytic cracking and hydrogenation effects of the catalyst were dominant during pyrolysis of NeiMonggol A and Xinjiang A, respectively. The catalytic mechanism of Fe-based catalyst is given as follows:（1） iron atom promotes the rupture of chemical bonds by electron-rich functional groups;（2） iron atom controls the rate of hydrogen release through affinity interaction with hydrogen, which increases the chance that hydrogen captures molecular fragments. Hence, the iron atom restrains the polycondensation in char and increases yield of tar.6. The main performance of catalytic depolymerization of Mo-based catalyst is hydrogenation with a certain degree of selectivity, namely, promoting hydrogenation of tar. Fe-based catalyst has hydrogenation effect on molecular fragments in tar and char, which significantly decreases condensation degree of char.7. The N₂ adsorption and CO₂ gasification reactivity characterization of char were carried out. The results indicate that Fe and Mo-based catalysts change the pore structure of char by altering the process of coal pyrolysis and thus affect the gasification behavior of char. Fe-based catalyst showed a more significant hydrogenation effect on Xinjiang A, which remarkably increased specific surface area of char, and thus the gasification yield of char increased by 1.2 times.8. The catalysts not only influence the coal pyrolysis but also have catalytic effects on conversion of sulfur in coal. Compared with the corresponding raw coal, the removal ratio of inorganic and organic sulfur in NeiMonggol A significantly decreased after addition of the catalysts, which lead to decline in total removal ratio of sulfur. The catalysts promoted the removal of inorganic sulfur in NeiMonggol B but had promotion and inhibition effects on removal of organic sulfur. Generally, chlorides and iron nitrate had propitious and inhibition effects on the removal of organic sulfur, respectively.