Quantum Chemistry Study On Pyrolysis Of Coal-related Heteroatom-containing Model Compounds And Removal Of Sulfur-containing Compounds

The oxygen, nitrogen and sulfur in coal have great influence on coal ultilization. The content and form of oxygen in coal affect the properties of coal directly, and the NOx and SO2 released during coal pyrolysis has been an important environmental problem. More importantly, the pyrolysis gas with sulfur-containing compounds affects the subsequent processes, such as leading to the poisoning of catalysts, the corrosion of equipment, and so on. So how to transfer oxygen in coal to chemical products, and control the emission of these harmful gases effectively has been being a difficult problem to coal chemical industry. Recently, it is possible to study the migration of oxygen, nitrogen and sulfur during coal pyrolysis, and the conversion and removal of sulfur-containing compounds in pyrolysis gas from molecular structure and reaction mechanism with the rapid development of quantum chemistry.The conception of coal-related model compounds was proposed and a series of reasonable oxygen-containing, nitrogen-containing and sulfur-containing model compounds were selected based on the information of pyrolysis pouducts of typical power coal. The pyrolysis mechanisms of these model compounds were studied in detail by using quantum chemistry method, and the migration and release rules of oxygen, nitrogen and sulfur in coal during pyrolysis were summarized. The conversion mechanism of organic sulfur-containing compounds in pyrolysis gas, and the desulfurization reaction mechanism of H2S under the effect of desulfurizer were studied.The main results were listed following:1. Anisole was selected as coal-related oxygen-containing model compounds with ether-oxygen functional group. There were two parallel consecutive paths to the formation of o-, p-cresoles via anisole pyrolysis isomerizaiton reaction. Three resonance configurations of phenoxy radical existed in the pyrolysis of anisole provided theoretical basis for the important intermediates o-, p-methyl-cyclohexadienone, which resulted in the formation of o-, p-cresoles via different intramolecular hydrogen migration paths.Benzoic acid and benzaldehyde were selected as coal-related oxygen-containing model compounds with carboxyl and carbonyl functional groups, respectively. The release of CO₂, CO was corresponding to decarboxylation and decarbonylation in coal, activation energy of decarboxylation was lower than that of decarbonylation, which indicated that CO2 was easier to escape than CO. The intramolecular hydrogen migration mechanism was more feasible than radical mechanism via bond homolysis for decarboxylation of benzoic acid by kinetic analysis, which showed that decarboxylation process was not directly related to cross-linking reaction in coal.2. Quinoline and isoquinoline were selected as coal-related nitrogen -containing model compounds with six-membered ring, and a rational reaction mechanism involving eight reaction paths and a common tautomeric intermediate was proposed to study the pyrolysis of them. The conformational tautomerism of 1-indene imine intermediate played an important role in the mechanism, which decided that both of the reactants had the same pyrolysis products and toal disappearance rate. The existence of tautomeric intermediate and its significant effect could only be found by quantum chemistry calculation.3. Thiophene and benzenethiol were selected as coal-related sulfur -containing model compounds, and the reaction paths of releasing H₂S via thiophene and benzenethiol pyrolysis were determined. The migration of sulfur in benzenethiol resulting in the formation of H₂S,CS and thiophene was studied, respectively, and the lowest energy path was H of the thiol group migrated to the ipso C and S radical was eliminated by beta scission reaction, then the S radical combined with H radical formed during coal pyrolysis resulting in the formation of H₂S. Comparing with the pyrolysis of the two sulfur-containing model compounds, it could be got that a lower activation enery was needed during benzenethiol pyrolysis than thiophene, which showed that thiophenes were more stable than the thiol compounds.4. The intramolecular hydrogen migration widely appeared in the pyrolysis paths of oxygen-containing, nitrogen-containing and sulfur-containing model compounds, and played an important role, which could be summarized as following: (I) offering more reasonable reaction paths for the formation of products; (II) interpreting experimental results more reasonable; (III) needing less activation energy than radical mechanism; (IV) adjusting the distribution of products.5. The two-step and one-step reaction mechanisms with and without COS intermediate were proposed to study the hydrolysis mechanism of CS₂, respectively, it showed that the two mechanisms are competitive in the process of CS₂ converting into H₂S.H₂S released by coal pyrolysis and converted by organic sulfur-containing compound hydrolysis must be removed, and Fe2O3, ZnO, CaO and CeO2 were selected to study the desulfurization reaction. A detailed desulfurization reaction mechanism was proposed, in which two types of self regeneration processes occurred on the desulfurizer surface accelerated the desulfurization reaction. The catalytic effect and the action of desulfurizer as reactant were analyzed respectively, and the desulfurization reaction process of every desulfurizer was understood in essence. It was proposed that increasing the ratio of amphoteric surface with both metal and oxygen of desulfurizer by modifying desulfurizer could improve desulfurization performance.