Multi-Scale Modeling For Reaction Process Of Low- Concentration Methane Catalytic Combustion

Catalytic combustion technology is the most effective method to utilize low concentration methane, compared with the traditional flame combustion, the catalytic combustion has many advantages, such as the high efficient heat utilization, the reduction of harmful gas emission and lower combustion temperature with the aid of the flameless combustion of catalyst. Hence it is benefit to improve the environmental quality and the energy efficiency through the study on the reaction process of methane catalytic combustion. If using the monolithic reactor for methane catalytic combustion process, it involves the interaction of micro-scope, meso-scope and macro-scope in the reactor, which affect the catalytic performance of monolithic reactor. In order to investigate more about the relationship among different scales in the reactor, which can be guide to optimization of reactor design and operating conditions effectively, the monolithic reactor multi-scale model is built in this work. At the same time, the structure of monolithic reactor and operating conditions are also optimized.Based on the gas-solid phase transfer functions, the reaction-diffusion model in single particle of meso-scope is built, the simulated result and the experimental data are compared to prove the validity of this model. And then internal diffusion phenomenon is investigated through the single particle reaction-diffusion model. Results show that the internal diffusion resistance is obvious in the catalyst particle, hence the reaction process of methane in the catalyst particle is an internal diffusion control process obviously. In addition, the time reached the steady state of the reaction in the particle is only 0.1s, it can ignore the unsteady influence on the reaction process, at the same time, external diffusion effect is not exist in this reaction. Furthermore, the model investigate the influence of catalyst particle diameter, average pore diameter and catalyst porosity on the distribution of parameters and internal diffusion. Results indicate that with the increase of catalyst particle diameter, the internal diffusion resistance enhanced, so that the center catalytic reaction rate decreased, when the catalyst diameter reduce to 0.4mm, the internal diffusion is still exist which should not be neglected. Large average pore diameter and catalyst porosity lead to the decrease of internal diffusion resistance. Based on the simulation results, the single particle reaction-diffusion model was simplified by ignoring the effect the external diffusion.According to the simplified single particle model, the two dimension single channel model of methane catalytic combustion process was established which described the mass and heat transfer in the channel also catalytic reaction and transfer process in the catalyst. The model was proved by the experiment data. Then the influence of cell per square inch and wall thinness were discussed for catalytic reaction process. Results show that when the wall thinness is constant, with the increases of cell per square inch the methane conversion and temperature in the reactor increases. When the wall thinness increases a certain value, the internal diffusion phenomenon is serious in the catalyst leads to the small enhancement of methane at the same cell per square inch.Finally, the whole reactor multi-scale model of methane catalytic combustion process was built which not only describes the process of mass and heat transfer in the channel but also considers the heat transfer process among the channels. Through the grid independence test and the comparison of simulation result with experiment data, the whole reactor multi-scale model of methane catalytic combustion was proved which could describe the reaction process accurately. Based on this whole model, the effect of internal diffusion process on the distribution of the flow field were investigated in the reactor, and the influence of methane concentration, inlet flow and inlet temperature on the methane conversion and flow field distribution were forecasted.