CFD Simulation Of Coal Pyrolysis In A Coupled Downer-hopper Reactor

China is rich in coal resources,which are the main source of primary energy.In recent years,with the increasingly serious problem of environmental pollution and the rapid growth of energy demand for economic development,the clean and efficient utilization of coal resources has become an important issue currently affecting the sustainable development of China's energy and environment.Advocated by the lowcarbon economy and sustainable development,Chinese Academy of Sciences has launched a strategic priority research program entitled “Demonstration of Key Technologies for Clean and Efficient Utilization of Low-rank Coal”.Institute of Process Engineering has developed the “Coal Topping” process,where the optimized design and operation of the key reactors can improve the utilization efficiency of coals.Focusing on the process of coal pyrolysis in the downer and the solid hopper,this thesis employs an Eulerian multifluid model,coupling mass,momentum and energy transfer and chemical reaction model together,to study flow,heat transfer and reaction characteristics within the reactors.Present thesis derives a model of particle size density variation suitable for the multifluid model according to the mass conservation at the single particle level.The model is then implemented into FLUENT via UDF and coupled with the multifluid model.Meanwhile,the parameters associated with the particle size,such as interphase drag coefficient,heat transfer coefficient and multi-component kinetic parameters directly call the instantaneous particle size,density value during CFD calculation.Then,thesis studies the impact of different pyrolysis reaction kinetic models on the simulation results,by employing the first order global reaction model and the twocompeting reaction model.The results are as follows:(1)It was shown that with the decreasing of the particle size,the interphase heat transfer coefficient increases,the heating rate increases,the reaction rate increases,and the conversion rate of coal becomes higher during the reaction.The influence of changing particle size should be consideration during simulating coal pyrolysis,(2)The pyrolysis process of coal has a relatively lower activation energy and reaction rate under a low temperature,and tends to produce more solid char products;In contrast,the pyrolysis process of coal has a relatively higher activation energy and reaction rate under a high temperature,and tends to produce more light gas products.Therefore,the two-competing reaction model is more consistent with the real pyrolysis process of coal.