Study On Desulfurization Mechanism Of Ca-based Sorbent In CFBB

Circulating Fluidized Bed Boiler(CFBB)has become the preferred furnace for inferior coal fuel combustion due to its wide fuel adaptability,high combustion efficiency and large load regulation range.The sulfur fixation in the furnace is an important part of the circulating fluidized bed to fix sulfur.At present,there are still problems such as low utilization rate of sulfur-fixing agent and improvement of economic and environmental benefits.The type of sulfur-fixing agent,particle size,furnace temperature and atmosphere are important factors affecting the utilization efficiency of sulfur-fixing agent.Most of the previous studies focused on the effects of various factors on the calcination and sulfur-suppressing single reaction,but the true reaction situation should also be taken seriously,that is,the calcination and sulfurization reaction occur simultaneously.Therefore,this study comprehensively considers the difference in particle size of traditional calcium-based sulfur-fixing agent limestone and solid waste-based calcium-based sulfur-fixing agent calcium carbide slag under CFBB simulated flue gas,temperature distribution unevenness in furnace and high concentration under oxy-combustion conditions.The effects of particle size,temperature and CO2 concentration on the calcination and sulfation of calcium-based sulfur-fixing agents were systematically investigated.Synchronous reaction modes were studied by comparing the similarities and differences between simultaneous and sequential calcination sulfation.The interaction of the two reactions and the mechanism of sulfur fixation in the calcium-based sulfur-fixing were further explored.The main content and conclusions are as follows.(1)Jincheng limestone and Xiangyuan carbide slag were taken as research objects.The effects of particle size and CO2 concentration on the thermal calcination behavior of two sulfur-fixing agents were studied by non-isothermal experiments.The kinetic calculations were carried out.The study shows that the activation energy of limestone decreases as particle size decreases.However,when the particle size is reduced to 0.45-0.90 mm,the activation energy increases with the decrease of the particle size,because the particle size is too small,which is not conducive to the heat transfer of the bed.For limestone,the increase of CO2 concentration makes the chemical equilibrium shifts to the reverse reaction direction,which is not conducive to the calcination reaction;for carbide slag,the increase of CO2 concentration is beneficial to the carbonation reaction of Ca(OH)2 and hinder the further calcination of Ca CO3 formed.(2)The interaction and mechanism of the simultaneous calcination and sulfation were discussed by comparing the different reaction behaviors of limestone in the simultaneous and sequential reaction modes.It was found that Ca SO4 generated by the sulfation blocked the pore passage and hindered the calcination of Ca CO3.The sulfation slowed down the calcination.With the decrease of particle size,the retarding effect decreases,and disappears when the particle size is 0.105-0.200 mm.The final desulfurization performance of simultaneous calcination sulfation(CS)is worse than that of sequential CS,and the difference is about 25%.The reason is the content of Ca O is insufficient in the control stage of sulfation reaction,and the specific surface area and effective pore size produced by calcination in simultaneous CS are smaller than that produced by calcination in sequential CS,so the resistance is greater in the later stage of diffusion control.The effects of particle size,temperature and CO2 concentration on the simultaneous CS were further studied by isothermal experiments.The particle size is reduced,and the limestone has better sulfur-fixing performance as the particle size decreases.The calcium utilization rate of 0.105-0.200 mm is 43.6%,while the calcium utilization rate of 2.50-2.75 mm is only 7.4%.The diffusion resistance of the product layer after the calcination of Ca CO3 is the main reason for restricting the sulfur-fixing performance of large-size limestone;The calcination in the low temperature zone is the main reason for restricting the sulfur fixation performance in the simultaneous CS,because the equilibrium of the calcination reaction moves to the reverse reaction at this time,and there is not enough Ca O for sulfation.In the high temperature zone,the sulfur fixation performance decreases with increasing temperature.The calcium utilization rate at 850 °C is 7.2% higher than that at 950 °C,which may be related to the sintering and high sulfur fixation rate of Ca O at high temperature;The high CO2 concentration has obvious inhibition to calcination in the actual operation of CFBB,the competition process of sulfation reaction and carbonation reaction will directly affect the sulfur fixation performance of limestone.(3)The effects of particle size,temperature and CO2 concentration on the reaction behavior of carbide slag were studied by isothermal experiments.The products of the reaction process were characterized by XRD.The results show that the particle size has little effect on the simultaneous CS of carbide slag.In the high temperature zone(850 °C,950 °C),the sulfur fixation performance of carbide slag increases with increasing temperature,and the calcium utilization rate can reach 54.8% and 63.5%.In the low temperature zone(650 °C,750 °C),the Ca CO3 formed after the carbide slag enters,and the Ca CO3 is difficult to be completely decomposed.The carbonation reaction competes with the sulfation reaction,which is not conducive to the occurrence of the sulfation reaction;When the CO2 concentration is greater than 70% The generated Ca CO3 is difficult to be completely decomposed,and the Ca O used for the sulfurization reaction is insufficient to hinder the sulfation.When the CO2 concentration is within 35%,the sulfur fixation performance of the calcium carbide slag in the early stage decreases with the increase of the CO2 concentration,and after the Ca CO3 is completely decomposed,Ca O continues to solidify sulfur,resulting in similar calcium utilization rate about 55%.In this paper,by studying the sulfur-fixing performance and mechanism of calcium-based sulfur-fixing agent in furnace,it is obtained that the sulfation of limestone can slow down the calcination in simultaneous CS,and the effect of the calcination on the sulfurization reaction makes the final sulfur-fixing performance of limestone lower than that of the sequential CS.From the point of view of simultaneous CS reaction mechanism,the influence mechanism of various factors on the sulfur fixation performance of sulfur fixing agent was studied,which provided theoretical basis for the actual sulfur fixation of calcium-based sulfur fixing agent in furnace.