Investigation On Interactions Between High-temperature Gas Solid Reaction And Particle Behavior

Interactions between high-temperature gas solid reaction and particle behavior are key factors that affect the efficiency and performance of gasifier.In this work,particle behaviors including the molting slag on particle surface,microstructural evolution,particle concentration and particle fluctuation and aggregation were studied to analyze their effects on heat and mass transfer in gas-solid phase reactions.Typical high-temperature gas solid reaction was investigated to supply theoretical basis and guidance for high-efficiency operation of gasifier.The highlights in this work include:1.Ash-to-slag transformation and char-slag interactions of coal char were investigated during high-temperature gasification process.In respect to the liquid product on particle surface,new reaction model was developed to describe the carbon conversion in later reaction stage.The reaction of a single char particle at lower temperature corresponded to shrinking core model.The particle surface was blocked by molten slag when the reaction temperature was above ash fusion temperatures,which resulted in the decrease of reaction rate and residual carbon in slag.In this condition,two reaction modes were developed to describe the reaction process of coal char particle at high temperatures.Moreover,a new reaction model(in respect to the watermelon model)was proposed to predict the carbon conversion in later reaction stage when the particle surface was covered with slag.The results showed that the performance of new model is better than the shrinking core model to describe the carbon conversioin in later reaction stage.2.Effect of ash fusion on gasification kinetics of co-gasification of high ash coal or biomass with petcoke was revealed.The change of particle temperature during the reaction process is not evident because of the poor reactivity of petcoke.Therefore,the heating effect of molten slag on petcoke particle is neglectable.Meanwhile,the petcoke particles adhered on slag surface would decrease the reaction surface area.Therefore,the reaction rate decreased when the particle was gasified on molten slag surface.The reaction rate of petcoke particles can be improved when adding catalyst into slag.During the co-gasification of chicken manure ash and petcoke,it was found that the reaction rate could be improved for 5 times compared without chicken manure ash.But the improve of reaction rate was not evident when adding excess ash into the mixture.It was found the molten salt on particle surface blocked the contact between carbon and gasifying agent.Therefore,the molten product on particle decreased reaction area and not in favor of the improvement of reactivity.3.The gasification characteristics of petcoke particle were analyzed and the microstructural evolution of single particle was studied to explain the reaction character at different reaction temperatures.The results showed that the pores on particle surface showed none to less to developed tendency.At the same carbon conversion,the pores were more developed when the reaction is at higher temperatures.This evolution also demonstrated the reaction rate evolution at different reaction temperatures.Appling Transmission Electron Microscope(TEM),electron spectroscopy(EDS)and Raman analysis to analyze petcoke particle,it was found that the non-uniform distribution of ash and graphitization resulted to the reaction rate differences in various mirco-areas.This result also explained the pore development during the gasification process.4.Effects of particle concentration on gasification kinetics of coal char and petcoke particle swarms were revealed.The reaction rate of anthracite and petcoke particle swarm decreased with particle concentration.The increase of inter-particle diffusion resistance and decrease of gasifying agent concentration in particle swarm resulted to the decrease of reaction rate.The reaction rate of lignite and bituminous char particle swarm slightly changed with particle concentration.It was found that the particle temperature evidently changed for these samples when the reactioin time is limited.The increase of particle temperature and the decreas of gasifying agent concentration played contrast effect on reaction rate when increase the particle concertration.Moreover,it was found that the diffusion resistance in thermogravimetric analysis is significantly increased compared with in high temperature stage experiments.5.Chemical reaction induced particle fluctuation,collision and aggregation were found.The fluctuation velocity of petcoke particle was up to 2000?m/s in combustion process at the temperature of 1400?.The irregular geometry of particles and the non-uniform reactive sites on particle surface caused these phenomenons.The physical chemical properties of particles resulted in the different reaction rate in microareas and the non-uniform release velocity of reaction product.The petcoke particle would bear reactive force because of the non-uniform release velocity.The increase of reaction temperature promoted this reactive force and the fluctuation velocity would increased with reaction temperature.The evolution of fluctuation velocity with reaction temperature obay Arrhenius equation.The diffusion resistance in the inner side is higher than that in the outside of two close particles.The concentration of gasifying agent in the inner side was lower than that in the outside of two close particles.Thesefore,the reaction rate and product release velocity is different in different positions.The two close particles would bear reactive force and finally aggregated together.Based on two particles aggregation,it was found that three and more partilces would aggregated together to form liner structure.The characteritics of product release velocity in different positions resulted in different drag force for the third particle.Therfore,the third(or n th)particle would aggregated with the aggregate to form liner structure.