Exothermicity of coal during low temperature oxidation process based on quantum chemical calculation

Abstract

The molecular structure is the fundamental factor affecting the oxidation exothermic properties of coal. Shuiyu (SY) bituminous coal was characterized by the combination of quantum chemical calculation, proximate and ultimate analysis, fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and temperature programmed and differential scanning calorimetry experiment. The three-dimensional molecular model of SY coal was constructed and optimized, and the low-temperature oxidation exothermicity of coal was studied. The results were as follows: the metamorphism degree of SY coal was high, in which the carbon elements mainly existed in the form of tetra-substituted aromatics and cycloparaffin, and the nitrogen and sulfur elements mainly existed in the form of aromatic pyrrole and thiophene. The ratios of ether-oxygen bond (C—O), carbonyl group (C=O) and carboxyl group (COO—) in the carbon-oxygen functional groups were about 1.5∶1∶1.5, and the hydrogen bonds in coal were mainly self-associated by hydroxyl. The molecular formula of SY coal three-dimensional layered structure model was defined as C203H140O18N2 (molecular weight is 2 893.01), during the low-temperature oxidation process, the six active groups in coal were determined as:、、、、、, respectively. In the low-temperature oxidation stage of coal, the first three active groups did not occur ring-opening reaction, and mainly generated cyclohexanone and H2O, while the last three active groups underwent multi-step reactions and the final products were CO and CO2. During the above oxidation reaction, the reaction heats of the complete oxidation reaction to generate CO and CO2 were \begin{document}$ \Delta {H_{2}}$\end{document}=−591.88 kJ/mol and \begin{document}$ \Delta {H_{3}}$\end{document}=−718.10 kJ/mol, while the minimum reaction heat of the incomplete oxidation reaction was \begin{document}$\Delta {H_{\min }}$\end{document}=−366.99 kJ/mol, and the maximum reaction heat was \begin{document}$\Delta {H_{\max }} $\end{document}=−535.07 kJ/mol. Combined with the O2 consumption rate, CO generation rate and CO2 generation rate, the exothermicity of SY coal in different low-temperature oxidation stages could be calculated. Compared with the exothermicity measured by differential scanning calorimetry experiment, the calculated value was basically consistent with the experimental value. It indicates the feasibility of using quantum chemistry method to calculate the exothermicity of coal.

Keywords

• exothermicity
• low-temperature oxidation
• coal structure
• quantum chemistry calculation
• active group