| Coal gasification technology, especially in supercritical water (SCW), has received much attention over decades as an effective method for a clean conversion of coal into fuel gas, such as H2, CH4and other products. However, the effect of SCW on coal pyrolysis and gasification has not yet well understood. In this dissertation, we have investigated the reaction mechanism of coal pyrolysis and catalytic gasification in supercritical water using a combined ReaxFF reactive force field and the density functional theory (DFT) method. Our work would not only be helpful for the application of SCW coal gasification technology, but also help facilitate the conceptual design and analysis of chemical processes based on SCW reaction media. Firstly, the effects of supercritical water on the linear chain cleavage in coal, aromatic ring openings as well as hydrogen production have been studied, and the results showed that the water clusters inSCW weaken the C-O linear bonds and Cï¼C bonds in aromatic rings. After the aromatic rings break into small cyclic structures, such as quaternary rings and ternary rings, the water clusters in SCW further weaken their Cï¼C ring bonds to induce the small cyclic rings to open. During this process, the water clusters in SCW turn into H radical-rich water clusters after providing OH radicals to the cyclic rings. This is the main source for the production of hydrogen molecules in SCW-coal system. The combination of H radicals produced by coal with water clusters in SCW is another pathway which forms H radical-rich water clusters. Under the catalysis of water molecules or clusters, H radical-rich water clusters decompose into H2and OH radicals. These OH radicals further bind with coal intermediates and result in the breaking of coal intermediates into smaller products. Therefore, the cooperative effects between SCW and coal form a virtuous circle, which greatly enhances the reaction rate of coal gasification, promotes the production of small molecules, and increases the yield of hydrogen.Secondly, the stabilities of Na2CO3and K2CO3catalysts on coal with and without SCW, and the catalytic mechanism of Na2CO3in the SCW coal gasification process has been investigated. The results showed that Na2CO3are more stable than K2CO3on coal, but the stability of Na2CO3is strongly reduced as the cluster gets larger. In supercritical water system, the dispersion and stability of Na2CO3catalyst on coal support is strongly improved. During coal gasification, Na2CO3transforms with supercritical water into NaOH and NaHCO3, which is beneficial for hydrogen production. The transformation process has been studied via thermodynamics and kinetics ways. The selectively catalytic mechanism of NaOH and the intermediate form of sodium-based catalyst in the water-gas shift reaction for higher hydrogen production has also been investigated. Furthermore, NaOH can transform back to Na2CO3after catalyzing the water-gas shift reaction. Thus, the cooperative effects between supercritical water and Na2CO3catalyst form a benignant circle which greatly enhances the reaction rate of coal gasification and promotes the production of hydrogen. |
