Investigation of the effects of O2/Ar pretreatments and K addition on the combustion characteristics of pretreated char

Abstract

The pre-oxidation combustion technology and catalytic combustion technology of pulverized coal can not only reduce the emissions of greenhouse gases and NOx, but also can ensure the high combustion efficiency of pulverized coal. Therefore, the effects of O2/Ar pretreatment and K addition on the structural characteristics of pretreated char (SC-Ar and SC-K-O2Ar) were investigated by the employment of ADCH and ESP. And the combustion mechanisms of SC-Ar and SC-K-O2Ar were determined through DFT (density functional theory) method. The study results demonstrated that the O2/Ar pretreatment with K addition altered the ADCH and ESP of SC-Ar and SC-K-O2Ar. Among them, the ADCH charges of atoms C6, C7, C8 and C9 altered from 0.18e, 0.03e, −0.25e. and −0.01e. to −0.41e., 0.01e, 0.11e and −0.16e. In the ADCH diagram, the positive charge characteristics of H in —OH and K were more pronounced. The obvious charge characteristics could be attributed to the stronger electron adsorption of atoms O and K than C. According to the combustion pathway, there were two pathways (path 1 and path 2) for the combustion of SC-K-O2Ar. The rate-determining step of path 1 was \begin{document}$ {\text{IM5}}\xrightarrow{{{\text{TS6}}}}{\text{IM6}} $\end{document}, and the corresponding value was 325.01 kJ/mol. It slightly lowered than that of in path 2 (\begin{document}$ {{{\mathrm{IM}}2'}}\xrightarrow{{{{{\mathrm{TS}}3'}}}}{{{\mathrm{IM}}3'}} $\end{document}, 360.98 kJ/mol). Therefore, the appropriate combustion route of SC-K-O2Ar was path 1. The combustion of SC-Ar also had two reaction pathways, which were called path 3 and path 4. The rate-determining step in path 3 was\begin{document}$ {\text{R}}\xrightarrow{{{\text{TS1}}}}{\text{IM1}} $\end{document}, the energy barrier is 430.34 kJ/mol. The rate-determining step in path 4 is \begin{document}$ {\text{R}}\xrightarrow{{{{{\mathrm{TS}}1'}}}}{{{\mathrm{IM}}1'}} $\end{document}, the energy barrier is 425.09 kJ/mol, slightly lower than that in path 3. Path 4 is the appropriate route for the combustion of SC-Ar. The energy barrier of rate-determining step in path 4 is lower than that in path 1. Therefore, the combustion of SC-K-O2Ar occurs more easier than SC-Ar, illustrating that the O2/Ar pretreatment with K addition could effectively promote the combustion of pretreated char.

Keywords

• char
• combustion
• o2/ar pretreatment
• density functional theory