Crystallization Behavior And Deposited Particle Reaction Mechanism On The High Temperature Molten Slag Surface

Slag flow is the key factor of stable operation and high-efficient conversion for the entrained flow gasifier.In this research,high temperature molten slag and coal char were chosen as samples.Based on the technical method of high temperature stage microscope,the law of crystallization behavior,characteristics of crystal growth and the enrichment characteristics of element were investigated.The gasification and combustion characeristics of deposited chars on the molten slag surface were investigated.In addition,the bubble phenomenon and mechanism on the interface of char and slag was observed and studied during char gasification on the molten slag surface.The main research contents are given as bellow:1.The laws of crystallization during continuous cooling process were investigated by using the high temperature stage microscope.Results showed that Shenfu coal slag formed crystal of cube shape at low cooling rate and rectangle crystal at high cooling rate,and Tianye coal slag formed snowflake crystals on the slag surface.The crystallization behavior of Shenfu coal slag during molten isothermal process was observed in the in situ experiment.The internal reaction of slag promoted the crystallization behavior,and the growth time of crystal invreased with the increasing molten isothermal temperature.The crystal size decreased with decreasing molten temperature.The law of element enrichment was analyzed by using Time-of-flight Secondary Ion Mass Spectrometry?ToF-SIMS?.The main element?Al,Ca and Si?which formed the crystal uniformly distributed on the surface and inside the cystal.Na mainly enriched on the crystal surface,K was rich in the amorphous phase and Mg uniformly enriched on the crystal surface and inside the crystal.2.The laws of the crystal growth during continuous cooling process and isothermal porcess were investigated.Results showed the number of nucleus increased and the growth rate of crystal showed an increasing linear relationship with increasing cooling rate.During isothermal process,the number of nuclei of acicular crystal decreased with the increasing cooling rate before isothermal process while the number of nuclei of rectangle crystal increased.The growth rates of two crystals during isothermal process first increased and then decreased with increasing the cooling rate.The ratio of length and width under both continuous cooling and isothermal process increased with the decrease of supercooling degree.3.The gasification of coal char on the molten slag surface was investigated.Results showed that reaction rate of coal char on the molten slag surface increased clearly.The reactivity index of char particle on the molten slag surface was nearly twice for the raw char,which indicated that molten slag could promote the char gasification.With the analyses of heat transfer,the mechanism was revealed that char particle on the molten slag surface had the higher reaction temperature.Based on the above mechanism,the shrinking particle model?SPM?was modified with the area correction factor and and reaction temperature,and the model prediction showed good agreement with the experimental data,which proved modified model was suitable for the char gasification on the molten slag surface.4.The char combustion on the molten slag surface was investigated by the in situ experiment.Results showed that the complete combustion time of char on molten slag surface more than the time of raw char,which was prolonged about 20%to 25%,indicating that molten slag hindered the char combustion.With the analysis of heat balance,temperature of char particle on the molten slag surface was lower than raw char particle.The temperature difference between raw char and char particle on the molten slag surface increased during combustion.The shrinking particle model?SPM?was modified with the area correction factor and reaction temperature,and the mode prediction showed good agreements with the experimental data,which proved modified model was suitable for the char combustion on the molten slag surface.5.Bubble phonomenon during char gasification with CO₂ on the molten slag surface was observed in the in situ experiment.The ratio of cumulative volume and total gaseous product?Vb/Vg?increased with the increase of initial char particle size.The decrease of char particle size on the molten slag surface promoted probability of bubble formation.With the diffusion and reaction theory of porous medium,the mechanism of bubble formation was proposed that the carbon dioxide diffused into the char particle and reacted with carbon to produce CO through the pore channel.Then the gaseous product diffused onto the interface of char and slag to form bubbles floating on the molten slag surface.The results of theoretical analysis showed that the initial bubble formation time and the particle size were proportional to the initial char particle size,which showed good agreement with the experiment data.