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O2/Ar预热处理和K原子对预热焦炭燃烧机理的影响机制

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

    摘要
  • 摘要: 煤粉预热燃烧技术和催化燃烧技术可在保证预热焦炭高燃烧效率的同时降低温室气体及氮氧化物的排放。采用原子偶极矩校正的Hirshfeld(ADCH)和静电势(ESP)方法深入揭示O2/Ar预热处理和K原子对预热焦炭(SC-Ar和SC-K-O2Ar)结构特性的影响，并通过密度泛函理论(DFT)探究SC-Ar和SC-K-O2Ar的燃烧机理。结果表明，焦炭内K和酚官能团的存在有效改变了SC-Ar和SC-K-O2Ar的ADCH和ESP特性，其中，C6、C7、C8和C9的ADCH电荷分别由0.18e、0.03e、−0.25e和−0.01e变为−0.41e、0.01e、0.11e和−0.16e。在ADCH有色图中，由于O原子和K原子对电子的吸附能力显著强于C原子，所以，羟基中的H原子和K原子的正电荷特性更显著。K原子的ADCH电荷为0.68e，—OH中的H原子的ADCH电荷为0.34e。由所构建的焦炭燃烧路径可知，SC-K-O2Ar和SC-Ar的燃烧本质为焦炭内芳香环的断裂及重组。在SC-K-O2Ar的燃烧过程中，主要存在2条反应路径(Path 1和Path 2)。其中Path 1的决速步为 \textIM5\xrightarrow\textTS6\textIM6 ，能垒为325.01 kJ/mol，略低于Path 2中决速步( \mathrmIM2'\xrightarrow\mathrmTS3'\mathrmIM3' )的能垒(360.98 kJ/mol)，说明SC-K-O2Ar的燃烧主要遵循反应路径Path 1。SC-Ar的燃烧路径亦有2条(Path 3和Path 4)，Path 3的决速步为 \textR\xrightarrow\textTS1\textIM1 ，能垒为430.34 kJ/mol，Path 4的决速步为 \textR\xrightarrow\mathrmTS1'\mathrmIM1' ，能垒为425.09 kJ/mol，略低于Path 3的决速步能垒，说明SC-Ar的燃烧主要遵循路径Path 4。Path 4的决速步能垒低于Path 1，因此，SC-K-O2Ar的燃烧比SC-Ar更易发生，说明O2/Ar预热处理和K原子可有效促进预热焦炭的燃烧。

     

    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 \textIM5\xrightarrow\textTS6\textIM6 , and the corresponding value was 325.01 kJ/mol. It slightly lowered than that of in path 2 ( \mathrmIM2'\xrightarrow\mathrmTS3'\mathrmIM3' , 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 \textR\xrightarrow\textTS1\textIM1 , the energy barrier is 430.34 kJ/mol. The rate-determining step in path 4 is \textR\xrightarrow\mathrmTS1'\mathrmIM1' , 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.

     

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