Study Of Reaction Mechanism Of Catalytic And Non-catalytic Gasification Based On Density Functional Theory

Gasification is a clean and efficient solid fuel conversion technology.It uses gasification agent to convert solid fuel into gaseous fuel while removing most of the pollutants in solid fuel.Understanding the interaction among gasification agents,metal catalysts,and char edges is the key to revealing the mechanism of gasification.Density functional theory can describe the gasification mechanism from the chemical microscopic level,thereby explaining the special phenomena in the gasification experiment.In this thesis,density functional theory is used to study the mechanism of char gasification,and the reaction path of H₂O and CO₂ with char in the process of noncatalytic gasification is searched.The catalytic mechanisms of alkali metals and alkaline earth metals are similar.Select Na,Fe,and CuO catalysts that can represent alkali metals/alkaline earth metals,transition metals,and metal oxides respectively,and search for their catalytic reaction paths with char.Through energy and wave function analysis,describe the mechanism of gasification reaction and metal catalytic reaction,and explain the special phenomena in the experiment.Previous theories cannot effectively explain the difference in activation energy between H₂O gasification and CO₂ gasification reactions.In view of this,this thesis compares the overall reaction path of H₂O and CO₂ gasification,analyzes the difference between the aromaticity of different carbon-oxygen complexes,and describes the mechanism of H on the char edge during the H₂O gasification reaction.It is found that the energy span of the optimal reaction path of H₂O and char is287.6k J/mol,which is smaller than the energy span of the optimal path of CO₂reaction of 352.7k J/mol.The rate-determining step of CO₂ gasification is in the desorption process,while the rate-determining step of H₂O gasification is in the hydrogen transfer process.These can explain the difference in the kinetics of the water vapor(H₂O and D₂O)of different H isotopes and char reactions in the previous experiments.The aromaticity of the desorption precursor formed in the process of adsorption and hydrogen transfer has a linear relationship with its desorption energy barrier.There is no additional H introduced in the CO₂ gasification reaction path,while in the H₂O gasification reaction path,the H generated by H₂O occupies the carbon sites at the edge of the char,weakening the aromaticity of the desorption precursor and making the desorption process of H₂O gasification reaction more effective.This is the essential reason why steam gasification is easier than CO₂gasification.The previous explanations that alkali metals can increase the rate of gasification reaction but cannot effectively reduce the activation energy are not perfect.In order to better understand the catalytic properties of alkali metals,this article takes Na as an example and searches for the catalytic reaction paths in Armchair char edges model and Zigzag char edges model.And two types of Na-containing catalytic active centers,CNa and C-O-Na,are considered in each model.Through energy analysis,the influence of Na on the adsorption and desorption stages of gasification and the activation energy of the overall gasification reaction is discussed;through the reduced density gradient function and bond orders analysis,the principle of Na catalytic gasification is revealed.It is found that CNa and C-O-Na,the two catalytic active centers,have mutual conversion.In the early stage of the reaction,free Na can react with CO₂ to generate C-O-Na groups.This confirms the conjectures put forward by previous people through experimental phenomena.Na can significantly promote the dissociation and adsorption process of CO₂ on the Armchair and Zigzag edges.Na can reduce the activation energy of the dissociation and adsorption process by231.9k J/mol.During the desorption process,Na showed a strong catalytic effect on the Armchair edge,which reduced the activation energy by 204.2k J/mol,but had no effect on the Zigzag edge.Under the catalysis of Na,both the Zigzag and Armchair edges participate in the reaction.Compared with the case where only the Zigzag edges participate in the reaction in noncatalytic gasification,Na increases the carbon sites of the reaction,resulting in a significant increase in the reaction rate.However,the Armchair edge reaction activation energy containing Na is still not significantly lower than that of noncatalytic gasification,so the overall reaction activation energy of Na is still not significantly reduced.This result can well explain the phenomenon that alkali metals can significantly increase the gasification reaction rate in the experiment,but the decrease in activation energy is not obvious.In the process of CO₂ dissociation and adsorption,the catalytic mechanism of Na is:under the combined action of the attraction of Na to O and the repulsive force of C atoms,the C-O bond in CO₂ is easier to break,thereby promoting the dissociation and adsorption of CO₂.The catalytic principle of Na in the desorption process is:in the presence of Na,the gasification adsorption process can form a stable H-C-O group.This structure may weaken the aromaticity of the char edge,thereby promoting the desorption of CO.Aiming at the ignorance of the effect of Fe and char by the existing catalytic gasification mechanism,this thesis searches for the catalytic reaction path of Fe at the edge of char.Through the analysis of the molecular orbital change of the key reaction process,it reveals the effect of Fe and char in the process of Fe catalytic desorption.Different from the previous ones mentioned in the catalytic gasification mechanism of alkali metals and alkaline earth metals:metals have little effect on the desorption process.Fe has a significant catalytic effect on the adsorption process,desorption process,and overall reaction of gasification.Fe has a unique gasification reaction path.It is easy to connect with C on the edge of the char to form a C-Fe bond during the gasification process.The catalytic active center of Fe is the C-Fe-n CO structure directly connected to the char.It is also closer to the structure detected in the experiment.Rather than the C-O-Fe structure proposed by the predecessors based on the catalytic mechanism of alkali metals and alkaline earth metals.In the process of CO₂ adsorption,Fe adsorbs CO₂ molecules through electrostatic interaction,and the d orbital of Fe can easily bond with C in CO₂.For the desorption process,Fe can destroy the aromaticity of the char edge,making the overall carbon ring looser.During the reaction process,Fe has an ability to actively destroy the C-C bond,and the active d orbital of Fe can transfer to the anti-bonding orbital of the C-C bond connected to the carbon-oxygen complexes,making it easier to break,thereby promoting the carbon-oxygen complexes to the form of CO is desorbed from the char.In order to explain the high-rate solid-solid direct reaction between CuO and char from a theoretical level,this thesis established a cluster model of copper oxide,searched for the reaction paths of Cu₄O₄-char,O₂-char,and Cu₄O₄ oxygen decoupling process,and compared.Comparing and analyzing the oxygen decoupling mechanism and the solid-solid direct reaction mechanism in chemical looping reaction,it is found that the solid-solid direct reaction mechanism is much easier than the oxygen decoupling mechanism.Cu₄O₄ requires an energy of 78k J/mol to release oxygen,and at least 390.1k J/mol of energy is required to react between oxygen and char.The direct reaction of Cu₄O₄ with char requires only a minimum energy of 145k J/mol.The dissociation and dispersion effect of Cu on O₂ and the transmission of oxygen may be the catalytic nature of copper in chemical looping reactions.Compared with the direct adsorption of oxygen to char,the initial adsorption process of Cu₄O₄ clusters is easier.Oxygen and the hydrogen peroxide clustered together in the cluster form some very unstable dangling bonds after being adsorbed on the char,so that the overall energy is increased,and the energy of O-O bond breakage needs to be overcome to form a CO₂ precursor,while the oxygen dispersed in the CuO clusters can be easily transferred to the char and converted into CO₂ precursors.