Catalytic Reaction And Transfer Characteristics Of Chemical Energy Storage Process By Methane Reforming

Solar energy is the most abundant and clean renewable energy. Highly effective use ofsolar energy can be able to alleviate the unreasonable energy consumption and energyshortage in China, reducing the energy dependence degree on the foreign countries. The solarenergy has many primary characteristics, such as low energy density, intermittent anddynamic supply, which lead to the solar energy gathered by the solar concentrators exhibitingnon-continuous and non-steady-state features. Therefore, it is necessary to develop the heatstorage technology. Nowadays, sensible heat and phase-change heat storage techniques arethe main approaches for high temperature solar energy storage. These two methods have lowenergy density and hard to storage at room temperature, which limit their applications in thenear future. Thermochemical energy storage is the new technology for high temperatureenergy storage, so it has the important research significance and application prospect.CO₂/CH₄reforming thermochemical energy storage has been increasingly attractedinternational community attentions, due to its superior characteristics of converting solarenergy with low energy density and instability into stable thermochemical energy. In addition,the high temperature reaction condition can effectively supplement the shortage of solarenergy thermal utilization at high temperature higher than600oC. Moreover, it can improvethe enthalpy value up to23.6%of stable fuel chemical energy. According to the principle ofenergy grade and temperature match, CO₂/CH₄reforming thermochemical energy storage canimprove the energy grade and quality, realizing the active control of energy storage.Meanwhile, it can also employ the green house gases of CO₂and CH₄to generate cleanhydrogen energy during the energy gathering-storage-utilization cycle, which can achieve thegoals of renewable energy use in large-scale and stable fuel chemical energy storage in activecontrol. In other words, it has a wide application prospect with the energy conservation andemission reduction.Thermochemical energy storage is a complicated thermal fluid reaction dynamics processwith multiphase and multi-physics field. It is different from other common CO₂/CH₄reforming conditions due to the aim of high energy storage efficiency and severe reactioncondition. Hence, the catalyst employed in this process is faced with the demand of moreresistant to high temperature and long term stability. In this paper, the viewpoints andmethods of chemical reaction engineering were performed to study the reaction principle,catalytic mechanism in the process of CO₂/CH₄reforming thermochemical energy storage. Anumerical model of shell and tube reformer based on the background of high temperature heat transfer and storage platform in the solar power tower plant by using molten carbonate asthermal medium was constructed. The effects of reformer geometric parameters and reactionconditions on the energy storage efficiency were conducted. In addition, the numerical modelbased on the ordered packed bed reactor was also designed to analyze the influence ofdifferent stacks on the law of heat and mass transfer.In this paper, the theoretical analysis for CO₂/CH₄reforming was firstly proposed. It isfound that the optimum reaction condition for energy storage efficiency is CO₂/CH₄=1, T=800°C and P=1atm, in which the main reaction degree is higher than90%and the amountof surface carbon deposit occurred in the side reaction holds at a low level. In order to solvethe key problems such as process stability and mechanisms in the CO₂/CH₄reforming energystorage, Pt-Ru/γ-Al₂O₃catalysts with superior catalytic and stability performance wereprepared, and the variation tendency of the stability and carbon deposition were alsodiscussed. The orthogonal experimental results show that the catalysts possess higher energystorage efficiency and the catalysts of Pt-Ru/γ-Al₂O₃have a superior catalytic performance at800°C during the500h stability test. It is proved that the prepared catalysts have the abilitybeing operated in the long life. The results of tests of BET （N2-adsorption-desorption）, XPS（X-ray photoelectron spectroscopy） and SEM （scanning electron microscope） havedemonstrated that the stability of the catalyst is superior. It is inferred that the bimetallicmechanism between Pt and Ru leads to the high catalytic performance, that is to say, thebimetallic catalysts can not only improve the poison tolerance of active components Pt in theredundant CO reaction system but also enhance the antioxidant ability of components Ru inthe high temperature oxygen-enriched environment. The obtained results also demonstratethat some poisoned carbon species on the catalyst surface would change into the active carbonwith increasing the reaction time, and the main surface carbon deposits are some carbon typewith a certain activity. In all, the experimental results revealed that the selected bimetalliccatalysts can work continuously in a stable state at the high temperature, which is possible tobe utilized for closed-loop cycle of the solar thermochemical energy storage in future industryapplications.Based on the practical application field of CO₂/CH₄reforming to store thermochemicalenergy, a numerical model of chemical catalytic reactor for thermochemical energy storagewas built. The Fluent software was used to conduct the numerical analysis of the storageprocess of catalytic reaction to investigate the impact of some factors, such as the structureparameters of the reactor and the catalytic reaction conditions. The results show that theconsiderable enhanced energy storage efficiency is achievable by increasing the tube length, by decreasing the tube diameter, by increasing the length of catalyst packed bed, bydecreasing the Reynolds number, by decreasing the activation energy, by decreasing the molefraction of the CH₄in inlet flow, by decreasing the operating pressure and by increasing inlettemperature and wall temperature. Furthermore, detailed heat and mass transfer mechanismsand contours for the CO₂/CH₄reforming were discussed.A preliminary study for the ordered stacked structures was performed to investigate theheat transfer, fluid flow and chemical reaction characteristics in the catalytic packed bed,aiming in increasing the space utilization of the catalytic packed bed. First, the heat transferand flow characteristics of a single-ball model is conducted and compared with the classicempirical equations. Then, three different ordered stacked structures are investigated to studythe overall heat transfer and flow characteristics, local heat transfer and flow characteristicsand CH₄conversion per surface area. The numerical results present that the local flowresistance characteristic is dominated by the form drag of catalysts. It was found that thestacked structure with N=2.16has the best heat transfer rate and CH₄conversion per surfacearea, which is the most efficient way to storage energy under investigated conditions.