Thermodynamic And Kinetic Analyses On Low Temperature Oxidation Process Of Inner Mongolia Lignite

The high surface activity of lignite makes it easy to undergo low temperature oxidation,which will eventually lead to its spontaneous combustion.The high water content in lignite will reduce its combustion heat value and increase transportation cost,which give large scale applications a big problem.During drying process,different drying techniques and drying conditions will have different effects on physicochemical structures,which affect its low temperature oxidation behavior.This paper selected Inner Mongolia lignite as feedstock,and took the release content and cumulative amount of CO₂ as an index to evaluate the low temperature oxidation reactivity.The effects of drying techniques,temperature and medium on the low temperature oxidation activity of lignite were investigated.In addition,oxidation methods such as constant temperature,cyclic oxidation in air and solution of nitric acid and hydrogen peroxide oxidation were employed to prepare coal samples with different physicochemical structures to investigate the factors influencing release content of CO₂.And the thermodynamic and kinetics analyses on low temperature oxidation process.The following conclusions were obtained:?1?The bond energies of the different chemical bonds in lignite were summarized.The results showed that the bond energy of C-H bond in the-CH₂-are about 85 kJ/mol that is lower than that in–CH₃?95 kJ/mol?,but the energy required to break–CH₃ attached to the ether bond and the ester group is about 80kJ/mol.The energy required to break the carboxyl group and the carbonyl group attached to the aliphatic hydrocarbon is 75 kJ/mol.In addition,the?G and?H results of chemical reaction during low temperature oxidation showed that carbonyl group mainly takes oxidation reaction,and the carboxyl group mainly takes decomposition reaction.?2?The effects of drying techniques,temperature and medium on low temperature oxidation behavior of lignite were investigated.The index evaluating the low temperature oxidation behavior of lignite was the CO₂ release content and cumulative amount.The results showed that CO₂ release amount of the coal samples dewatered by microwave and fixed bed techniques under N₂ atmosphere tend to decrease with the increase of drying temperature.CO₂ release content of lignite dewatered by hydrothermal dewatering increases significantly.When lignite was dewatered by hydrothermal dewatering under 230 ?,its CO₂ release content increased significantly.?3?The constant temperature oxidation,cyclic oxidation in air,and solution of nitric acid and hydrogen peroxide oxidation methods were employed to compare coal sample with different structures for investigating their low temperature oxidation behavior.The results shows that after the Inner Mongolia lignite is oxidized by constant temperature and cyclic oxidation in air,the relative contents of-CH-,-CH₂-,and-CH₃ decrease significantly,and the CO₂ cumulative amounts also decrease significantly.After the Inner Mongolia lignite is oxidized by solution of nitric acid and hydrogen peroxide,the relative contents of-CH-,-CH₂-,and-CH₃ and oxygen-containing groups increase significantly,which will lead to the significant increase of CO₂ cumulative release amount.?4?The kinetics analyses on the process of CO₂ release have been conducted.The results show that the first-order kinetic model can be employed to describe CO₂ release process.The low temperature oxidation process is carried out in the different stages,and the activation energy of the first stage is significantly lower than that of the activation energy of second stage.1The activation energy of the first stage of coal samples oxidized by constant temperature,cyclic oxidation in air,and solution of nitric acid and hydrogen peroxide oxidation is about 12.4 kJ/mol,which indicates that the chemical reaction at first stage were prone to occur.2b.The second stage of low temperature oxidation was mainly controlled by physicochemical structures.For hydrothermally dewatered coals,the second stage of low temperature oxidation process is mainly affected by diffusion rate dependent on pore volume and specific surface area.The activation energy of the chemical reaction second stage of coal samples oxidized by constant temperature,cyclic oxidation in air,and solution of nitric acid and hydrogen peroxide oxidation is mainly controlled by chemical structure.The inflection point of the temperature entering the second stage is consistent with the change trend in the activation energy of the second stage.In a word,the chemical reaction with a larger activation energy needs to occur at higher temperature.This shows that the second stage of low temperature oxidation is mainly controlled by chemical structure.