| At present,the most widely used flue gas desulfurization process in China is limestonegypsum method.This method needs to consume a large amount of limestone resources,and the over-mining of limestone causes serious damage to the surface vegetation and ecological environment,and the by-product desulfurization gypsum is difficult to handle,and long-term piling or landfill will cause secondary pollution to the environment.The resource utilization of desulfurization gypsum to produce quicklime and sulfur-containing by-products using CaSO4 gas-phase reductive decomposition technology not only recovers the calcium and sulfur resources contained in desulfurization gypsum,but also provides a quicklime preparation process with low CO2 emissions.Therefore,the technology shows good prospects for industrial applications in terms of both economic value and environmental impact.However,the technology still has the problems of high reaction temperature and unclear reaction conditions,and the characteristics of CaSO4 gas-phase reduction decomposition and reaction mechanism are still unclear,which cannot guide industrial production.In this paper,the macroscopic reaction characteristics,thermodynamic and kinetic properties,microscopic reaction paths of CaO preparation by gas-phase reductive decomposition of CaSO4 and the influence of typical impurity components in desulfurization gypsum on the characteristics of CaSO4 gas-phase reductive decomposition are investigated with typical gas reductants CO and H2,respectively.Finally,the calcium cycle desulfurization process system was constructed,and the environmental impact evaluation and economic analysis of the process system were carried out,which initially proved the feasibility of the calcium cycle-based resourceful desulfurization process.This work provides a theoretical reference for the industrial application of technologies related to the resource utilization of desulfurization gypsum to produce quicklime and sulfurcontaining by-products.The properties of the CO/H2 reductive decomposition of CaSO4 reaction were investigated.The results show that the reducing atmosphere contributes to the reduction of the theoretical decomposition temperature of CaSO4.In addition,the effects of reaction temperature,CO/H2 concentration,PCO2/P（CO/H2）partial pressure ratio and the addition of O2 on the reductive decomposition characteristics of CaSO4 were investigated.The macroscopic reaction mechanism of the reductive decomposition of CaSO4 by CO/H2 to prepare CaO was clarified.Based on the above reaction mechanism,the high-temperature reaction characteristics of the intermediate product CaS with CO2 in the reaction atmosphere were investigated.It was found that CaSO4 can be completely decomposed to CaO when the reaction temperature t≥1000℃,CO/H2 concentration≥2%and Pco2/P（CO/H2）≥8.Increasing the partial pressure of CO2 in the reaction atmosphere is beneficial to improve the selectivity of CaO in the product,but it will reduce the rate of CaSO4 reduction decomposition.The addition of O2 can not only promote the decomposition of CaSO4,but also improve the selectivity of CaO in the product.The reaction mechanism of CaO preparation by gas phase reductive decomposition of CaSO4:CaSO4 first reacts with CO/H2 to form CaO and CaS.Subsequently,CaS reacts with unreacted CaSO4 and CO2 in the reaction atmosphere to form CaO.When O2 is added to the reaction atmosphere,CaS can also react with O2 to form CaSO4,and CaSO4 then decomposes to CaO according to the above reaction mechanism.The study of the high-temperature reaction characteristics of CaS and CO2 revealed that the reaction rate of CaS and CO2 accelerated with the increase of the reaction temperature as well as the concentration of CO2.When the reaction temperature t<1000℃,a small amount of CaSO4 is produced due to the disproportionation reaction of CaSO3.The kinetic properties of CaO preparation by gas-phase reductive decomposition of CaSO4 were investigated.The apparent activation energies for the reductive decomposition of CaSO4 under two atmospheres of 2%CO+20%CO2+N2 and 2%H2+20%CO2+N2 were 38.77 kJ/mol and 41.56 kJ/mol,respectively.The model functions were G（α）=[-ln（1-α）]0.574 and G（α）=[-ln（1-α）]0.572,respectively.The total reaction rate of CaO preparation by reductive decomposition of CaSO4 under the above two reaction atmospheres was controlled by the nucleation and growth rate of CaO.The isothermal kinetic analysis of the reaction of CaS with CO2 shows that the reaction activation energy is about 95.54 kJ/mol,the prefactor A=0.76557s1,the reaction kinetic model is a reduced nucleus model,and the modified model function G（α）=1-（1-α）1/1.368.The microscopic reaction pathway of CaO preparation by gas-phase reductive decomposition of CaSO4 was investigated.During the reduction of CaSO4 by CO,CO gradually takes away the O atoms in the[SO4]2-group until the CaS structure is formed.S2-and Ca2+ in the CaS structure are tightly bound by electrostatic force and have strong chemical inertness.The energy barrier of CaSO4→CaS reaction is high,and the rate-controlling step is CaSO3→CaSO2.During CaS disproportionation reaction,S2-continuously takes away O atoms from adjacent groups to form CaSO3.CaSO3 is unstable at high temperatures and undergoes decomposition to form CaO and SO2.CaS→CaSO is the rate-controlling step of the reaction,and the decomposition of the third CaSO3 on the surface in CaSO3 decomposition reaction is the rate-controlling step of the process.The effects of four typical impurity components in desulfurization gypsum,SiO2,MgO,Al2O3 and Fe2O3,on the preparation of CaO from CaSO4 by CO/H2 reduction decomposition and the calcination-adsorption characteristics of desulfurization gypsum in five cycles were investigated.It was found that MgO,Al2O3 and SiO2 had almost no effect on the preparation of CaO by the reductive decomposition of CaSO4,while Fe2O3 could greatly increase the reaction rate.Fe2O3 could reduce the initial reaction temperature of CaSO4 decomposition to CaS and CaO and Fe2O3 was converted to Ca2Fe2O5 during the reductive decomposition of CaSO4.The catalytic mechanism of Fe2O3 was analyzed and obtained as follows:firstly,Fe2O3 generated Ca2Fe2O5 in the reaction process,and Ca2Fe2O5 itself has a certain catalytic effect.In addition,the addition of Fe2O3 promoted the formation of liquid-phase eutectic in the process of CaO preparation by the reductive decomposition of CaSO4,and the diffusion of Fe3+ ions in the liquid eutectic state promoted the ionization of CaSO4 and CaS,which promoted the solid-solid reaction of CaS and CaSO4 in the rate-controlling step,and the overall reaction rate was increased by increasing the reaction rate in the rate-controlling step.The CaO content in quicklime,the product of 5 cycles of desulfurization gypsum decomposition,was much higher than the standard of first-class quicklime,and the quicklime prepared by reduction decomposition of desulfurization gypsum had strong desulfurization performance,and the desulfurization performance did not decay significantly during the 5 cycles of adsorptiondecomposition.Finally,a calcium cycle desulfurization process system based on the reductive decomposition of desulfurization gypsum was constructed,and the environmental impact evaluation and economic analysis of the process system were carried out.It was found that the desulfurization process and the desulfurization gypsum calcination process are the key processes in the above process system.The key substance in the desulfurization process is the electricity consumption,while the key substance in the desulfurization gypsum calcination process is the gas consumption.Compared with limestone-gypsum FGD technology,the calcium cycle FGD process system has a lower operating cost,although the initial investment is higher.According to the "3R" principle in circular economy,the purpose of electricity consumption reduction is achieved by improving the reactivity of desulfurizer and reducing the system pressure drop and desulfurization liquid to gas ratio. |
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