Effect Of CO₂ And H₂O On Oxy-fuel Combustion Of Lignite In Fluidized Bed And Kinetic Analysis

Oxy-fuel combustion has the advantages of relatively low cost,easy scale,and the ability for retrofit.Combined with the advantages of fluidized bed（FB）combustion,such as fuel flexibility,low cost denitrification and desulfurization in furnace,fluidized bed oxy-fuel combustion holds potentials to realize CO₂ reduction in large scale and with low cost.Investigation on O₂/CO₂ FB combustion is significant for the development and extension of this technology.Unlike conventional combustion,oxy-fuel combustion uses O₂ mixed with recycled flue gas（mainly CO₂ and H₂O）as oxidizer instead of air,and the CO₂ concentration in the exhaust flue gas is more than 90%after water condensation.Meanwhile,under the oxy-wet combustion atmosphere,the volume concentration of water vapor can reach more than 40%,which is much higher than that of conventional air combustion,where steam concentrations are in the range of 6¹0%when firing high quality coal.That is,the flue gas in conventional air combustion is mainly N₂,while it consists primarily of CO₂ and H₂O in oxy-fuel combustion.Unlike N₂,both CO₂ and H₂O will react with coal char,especially at high temperature,thus the effect of gasification reaction on the combustion process cannot be ignored.However,Most of the existing researches focus on the combustion characteristics of oxy-fuel conditions,such as burnout rate,ignition characteristics,and so on.Few studies have been conducted on the effects of CO₂ and H₂O under oxy-fuel conditions,especially on the competitive mechanism of oxidation and gasification during oxy-fuel combustion.Therefore,it is of great significance to analyze the effects of CO₂ and H₂O on oxy-fuel combustion in fluidized beds.In this paper,the following three contents are studied by means of experimental study and theoretical analysis:1)the effect of different pyrolysis atmospheres（100%N₂,50%N₂/50%CO₂and 50%N₂/50%H₂O）on the structure characteristics of lignite char in fluidized bed;2)the effects of CO₂ and H₂O on oxy-fuel combustion of lignite char in fluidized bed and kinetic analysis;3)the effects of CO₂ and H₂O on the gasification of lignite char in fluidized bed and kinetic analysis.Results indicate that different pyrolysis atmospheres have a great influence on the structure of coal char.The char after pyrolysis has a high degree of fragmentation.The pore structure of CO₂ char and 50%N₂/50%H₂O char is looser than that of N₂ char.It shows that there are bulging bubbles on the surface of char particles,a large number of micropore and mesopore,and larger specific surface area and pore volume.After pyrolysis,the functional groups of chars are much lower than that of raw coal,especially the oxygen-containing functional groups,but the peak number and strength of the functional groups of the three kinds of coal chars are not significantly different.The amorphous carbon content of three kinds of coal chars is slightly higher than that of graphite carbon.The defect peak（D peak）and graphite peak（G peak）area ratios A_D/A_G of three kinds of coal chars is similar,with the largest CO₂ char and the smallest N₂ char.The half-width of G peak W_G and peak position difference P_G-P_D of three kinds of chars have similar conclusions.With the increase of bed temperature and O₂ concentration,lignite char displays similar combustion characteristics under O₂/H₂O,O₂/CO₂ and O₂/N₂ atmospheres（the burnout time decreases,while the peak and average reaction rate increase）.At low oxygen concentration,the combustion performances of lignite char in O₂/H₂O atmosphere are worser than those in O₂/CO₂and O₂/N₂ atmospheres.However,as the oxygen concentration increases,the combustion performances in O₂/H₂O atmosphere are significantly improved,and become better than those observed in O₂/CO₂ atmosphere.The calculation result for the activation energy based on the shrinking core model shows that the order of activation energy under different atmospheres is:O₂/CO₂>O₂/H₂O>O₂/N₂.Furthermore,char gasification in oxy-fuel combustion cannot be neglected,especially in the O₂/H₂O atmosphere.The active sites occupied by gasification agents significantly increase as the bed temperature increases,and the competitive effect of gasification on oxidation is enhanced.As the reaction temperature increases,the gasification reactivity increases,which is manifested by an increase in the concentration of gas products,the acceleration of carbon conversion rate and the decrease of total gasification time.The average reaction rate in50%CO₂/50%H₂O mixture is slower than the reaction rate in 50%N₂/50%H₂O atmosphere,which indicates that CO₂ and H₂O compete for the same active reaction sites on the char surface.Furthermore,with an increase of temperature,the competition capacity of CO₂ gasification over H₂O gradually increases,as a result,CO₂ gasification occupies more active sites than H₂O when the temperature is higher than 887^oC.Calculations of the activation energy in the kinetically controlled region based on the shrinking core model reveal that the activation energy of the gasification is the lowest in the CO₂/H₂O atmosphere,followed by the N₂/H₂O atmosphere,and the N₂/CO₂ atmosphere is the highest.Results obtained in the paper provide a new thinking direction for studying the effects of CO₂ and H₂O in oxy-fuel combustion of fluidized beds.It also can provide a deeper insight into the coal combustion behavior under oxy-CFB condition and will be meaningful for the further development of this technology.