Investigation Of In-situ Coal Char Combustion Kinetics And A General Model For Char Combustion

Char combustion takes up 90% of the whole coal combustion time and its burn-out degree determines the efficiency of coal utilization.Thus,it is quite necessary to study the rate of char combustion.Various models including the global power-law kinetics,the two-step semi-global kinetics and three-step semi-global kinetics,have been proposed in the past decades to predict char combustion rate.The kinetic parameters of char combustion are required in all models,but the values of kinetic parameters in these models were not consistent.Besides,during coal combustion,char structure is tending to be ordered and graphitized during heat treatment at high temperatures,which would decrease the intrinsic reactivity and char combustion rate.Therefore,it is vital for predicting char combustion rate to obtain the exact kinetic parameters and the variation of intrinsic reactivity during char combustion.This thesis has investigated in-situ char combustion kinetics using the indigenously developed micro drop-fluidezed bed.Moreover,the mechanisms of char reactivity variation and structure evolution during the heat treatment were investigated to clarify the internal relationship between char reactivity and its structure.A general model was proposed and built for char combustion simulation and prediction.This thesis firstly investigated the char combustion kinetics using the micro-fluidized bed(MFB)in the absence of mass and heat transfer limitations.TGA was adopted to check the reliability of MFB.The kinetic parameters measured by MFB were similar to those measured by TGA,demonstrating the reliability of MFB on measuring char combustion kinetics.The experimental and modeling results suggested that the combustion in TGA and MFB were both in the kinetic regime when the reaction temperature was low.As the temperature increased,the char reaction rate in TGA was limited by oxygen diffusion rate due to its pile-up effects.While for MFB,the effective mass and heat transfer ensured char combustion still in the kinetic regime at higher temperatures and the test time was greatly shortened.Therefore,MFB could cover the limitations of TGA on measuring char combustion kinetics at higher temperatures.Then we indigenously developed a micro drop-fluidezed bed,which coupled a micro drop tube furnace and a micro fluidized bed.Coal was firstly pyrolyzed in the micro drop tube furnace and then the produced char with high temperature was combusted in the micro fluidized bed.Based on the developed micro drop-fluidezed bed,the combustion kinetics of in-situ char and cooling char(commonly used in previous literatures)were investigated.In-situ char showed better reactivity than cooling char.With the decrease of the cooling rate,the char reactivity decreased.The results of cooling char characterization indicated that char deactivation during the cooling process was independent of specific surface area,but was correlated with the loss of active sites.There was a good linearity between the cooling char reactivities and their active surface areas.The kinetic parameters between in-situ char and rapid cooling char were compared.When the cooling rate was fast(>100 K/s),the cooling process had little impact on the measured kinetic parameters.However,when the cooling rate was slow(1 K/s),the measured activation energy was significantly increased.The mechanisms of char reactivity decativation during the heating process were investigated using the micro drop-fluidezed bed.In order to study the effects of different heating rates on char reactivity and structure,the holding process after rapid heating was avoided.The results showed that char deactivation during the heating process was related to the break of aliphatic chain with poor thermal stability,the coalescence of carbon matrix edge and the loss of active sites.Meanwhile,the holding process caused further decrease of char reactivity,reducing the gap between rapid heating char and slow heating char.Based on the experimental data,a model which illustrated the relationship between char reactivity and the heating rate,was proposed.The instantaneous reactivity of char with different structures during heating was predicted according to this proposed model.The results indicated that char deactivation rate was correlated with both the heating rate and char structure.The coal with a lower degree of graphitization was more sensitive to heating rate due to its more edge sites in carbon structure and provision of more active sites for oxidation.The reactivity index Fz was used to represent the char structural features.A model for char deactivation rate during heating process was established.Finally,the numerical simulation of char combustion was studied.On one hand,the impacts of activation energy's value on CBK(Carbon burnout kinetics)simulation results were analyzed.On the other hand,the CBK model was modified by adding the effects of different structures on char deactivation rate.The results suggested that there were big differences in the burnout time and particle temperature predicted by the CBK model when the activation energy varied much(±30 kJ/mol).Besides,as the char combustion rate was controlled by the chemical reaction rate during the late stage of char combustion,the impacts of activation energy's value on char combustion rate were increased.The modified CBK model(M-CBK)was used to predict different kinds of coal burnout processes at high temperatures in the previous literatures.The original CBK model was also used for comparison.The results indicated that M-CBK could better predict different coals burnout processes in most cases.The simulated char reactivity during char combustion process through M-CBK was closer to actual combustion process when the effects of carbon structure during thermal annealing on char reactivity were taken into account.Therefore,the M-CBK model was suitable for more types of coal.