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

The pre-oxidation combustion technology and catalytic combustion technology of pulverized coal can not only reduce the emissions of greenhouse gases and NO_x, 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.