Study On Catalytic Combustion Characteristics And Quantum Chemistry Mechanism About Methane/moist Air In Micro-channel

With the rapid development of micro-electromechanical systems?MEMS? and miniaturization of the devices, micro-combustors have attracted the attention of many researchers in the field of miniature energy system. Due to the decrease of the channel size, the flow and combustion are different from the conventional scale, serious free radical loss and heat loss are caused because the flame mutual coupling with the inside wall in the microscale combustion. Catalytic combustion can effectively solve the free radical flameout, because it is flameless combustion. Moreover, catalytic combustion has the characteristics of high efficiency and energy saving, emissions near zero pollution, which have attracted attention in the field of micro combustion.Currently, there are many experiment and numerical researches about methane catalytic combustion in microscale channels, while the study of methane/moist air is relatively less, and the research on microscopic mechanism of methane/moist air catalytic combustion based on density functional theory?DFT? is even less. Thus, the study of methane/moist air catalytic combustion characteristics and the reaction mechanism in micro channels, can lay the foundation of the optimization of the micro-combustors and the development of the new catalysts.Aiming at the problems of low combustion efficiency and stable combustion in micro combustors, this paper carries out a comprehensive theory research on the flow characteristics of methane/moist air, the inside wall structures and the micro reaction mechanism of methane/moist air in micro-channels.Firstly, the flow characteristics of methane/moist air, such as velocity and boundary layer properties are studied in the parallel plate and tube micro-channels with Matlab software. The the definition method of stagnation flow thickness is derived based on the boundary layer thickness. The influence of air/carbon ratio and water /carbon ratio on methane/moist air flow characteristics are investigated under different temperature and inlet velocity in the micro-channels. The changes of velocity, wall friction coefficient and boundary layer thickness are obtained. It is firstly derived that:?1?Under the flow condition,the stagnation flow thickness, ?s, can be expressed with the maximum boundary layer thickness ?, i.e., ?s=K??0<K<1?, thus, the stagnation flow thickness is ?s=K1?0.1 and ?s=K2?1 for the microplate channel and the microtube channel, respectively;?2?It is required to compare diffusion velocity ud with flow velocity uf under the combustion condition, when ud>uf, stagnation flow exists.Secondly, the catalytic combustion characteristics of methane/moist air are numerical studied in the smooth and nonsmooth microtubes, the best microtube structure is determined under the conditions in this paper. The influence of the groove size, number and shape on methane/moist air catalytic combustion in microtube reactors are analyzed, and the characteristics of temperature, velocity, mass fraction of methane and methane conversion rate are obtained. It is discovered that the slight protuberances on the inside wall can promote the heat transfer and the free radical diffusion between the wall and space, moreover, low velocity recirculation zones which can prolong fuel residence time are formed, thus, CH₄ reaction rate and conversion rate are higher in microtubes with protuberances than that of smooth tube, the micro-tube with five triangular protuberances has the peak efficiency.Then, the interactions between the various free radical components are sutdied based on DFT, the activation mechanism of CH₄ is investigated on the catalysts Pt₂, Pt Ni and Pt?111? surface. Through the contrast of reaction potential energy curves and kinetic parameters, it is found that, compared with the bimetallic Pt Ni, the dissociation of CH₄ is not easy to form carbon deposition on the dimers Pt₂, and more favorable with the thermodynamics. In the process of methane activation on Pt?111? surface, the most stable adsorption configurations and the order of adsorption energies is CH₄*?T?<<CH₃*?T?<H*?fcc?<CH₂*?B?<CH*?fcc?<C*?fcc?, the most abundant component is CH?s? in the process of methane activation, which provides the basic condition for the next mechanism research of methane catalytic combustion.Finally, the mechanisms of methane/moist air catalytic combustion on Pt?111? surface to form CO₂ and H₂O are studied systematically. The micro mechanisms and reaction pathways of CH₄ combustion to form CO₂ are abtained for the first time under the conditions with and without H₂O.?1? Without H₂O, there are four reaction pathways for methane/air catalytic combustion to form CO₂, and the reaction pathway?4?CH₄*?CH₃*?CH₂*?CH*?HCO*?HCOO*?CO₂* is the most likely one to ocuur.?2?With H₂O, there are two pathways to form CO₂, namely, pathway?5?CH₄*?CH₃*?CH₂*?CH*+OH*?CHOH*?CO*+OH*?COOH*?CO₂* and pathway?6? CH₄*?CH₃*?CH₂*?CH*?C*+OH*?COH*?CO*+OH*?COOH*?CO₂*. It is found that pathway?5?is more thermodynamics favorable,which is also the most likely to ocuur in the six reaction pathways(?1?⁶).