Reaction Characteristics Of The Simultaneous Calcination And Sulfation Of Limestone In Circulating Fluidized Bed Boilers

Limestone is widely used to capture SO2 in circulating fluidized bed(CFB)boilers.But the desulfurization efficiency and calcium utilization in CFB boilers are not high enough to meet the increasingly strict emission standards.In fact there are still many uncleared questions in this filed,a typical one is that the calcination-then-sulfation reaction process has been followed by most investigators,while the simultaneous calcination/sulfation reaction is the realistic reaction process to limestone.Thus more investigations on the in-situ sulfur capture are still necessary.Based on the customized constant-temperature thermalgravimetric analyzer,the differences between the two reaction processes were investigated.The characteristics of the simultaneous calcination/sulfation reaction were studied in detail.A simultaneous calcination/sulfation reaction model was established.The main conclusions are as follows:(1)The differences between the simultaneous calcination/sulfation reaction(designated as the simultaneous reaction)and the calcination-then-sulfation reaction process(designated as the sequential reaction)were investigated.In both the two reaction processes,the mass of the limestone particle decreased first then increased,thus there was a minimum mass point in each process,but the minimum mass point in the simultaneous reaction was higher.In the sequential reaction,the minimum mass point is the end of the calcination reaction and the beginning of the sulfation reaction.While in the simultaneous reaction,the minimum mass point is at the moment when the mass-loss caused by the calcination reaction equals the mass-growth caused by the sulfation reaction.The mass of particles in the simultaneous reaction was always higher than that in the sequential reation,and the sulfation ratio over 90min of the simultaneous reaction was higher too.Since the simultaneous reaction is the realistic process and it is different from the sequential rection,the investigation on sulfation of limestone should always follow the simultaneous process.(2)The SO2 in flue gas can slow down the calcination reaction,and the mechanism of this phenomenon was revealed.Tests showed that the porosity of CaO formed in atmosphere containing SO2 was lower than that without SO2.The diffusion resistance of CO2 was higher and the effectiveness factor of calcination was lower in the calcination process with SO2.A possible mechanism was proposed to explain the effect of SO2 on calcination:the calcination usually occurred from particle surface to inside,and a CaO layer formed in the outer layer of the particle,the pore in the CaO layer served as the pathway for CO2 diffusing from particle inside to outside;when limestone particles were calcined in atmosphere containing SO2,CaSO4 formed in the pore of the CaO layer,which can fill or block the pores and increase the CO2 diffusion resistance,raise the CO2 concentration on the calcination site and slow down the calcination reaction.(3)The effect of temperature,H2O and particle size on the simultaneous reaction were investigated as well as the mechanisms.The impeding effect of SO2 on calcination reaction became weaker with the rise of temperature.The reason should be that the calcination stage was shorter at higher temperatures,the CaSO4 formed in this stage was less and thus the filling of CaSO4 to the pores became weaker.The early stage sulfation rate increased with the rise of temperature,but the sulfation rate decreased faster,which lead to a lower final sulfation ratio at higher temperatures.H2O can decrease the activation energy and accelerate the calcination reaction.It was found that the acceleration of calcination by 15%H2O was stronger than the deceleration caused by 0.3%SO2.H2O can increase the sulfation rate in the slow sulfation stage significantly.while has little effect on the fast sulfation stage.With the increase of particle size,both of the calcination and the sulfation reaction slowed down.(4)Considering the high CO2 and SO2 concentrations in oxy-fuel combustion atmosphere,the simultaneous reaction under oxy-fuel conditions was investigated too.The differences between the two reaction processes still existed in oxy-fuel combustion.In the range of 890?950?,the final sulfation ratio over 90 min of reaction reached the maximum at 890? in the simultaneous reaction,while in the sequential reaction,it reached the maximum at 925?.H2O can lengthen the fast sulfation stage in oxy-fuel atmosphere,which was different from that in the air-fuel atmosphere.(5)Based on the random pore concept,a simultaneous reaction model was established,which included the interaction of the calcination reaction and the sulfation reaction.The results of the model fit well with the testes.Based on the model,the reason why the sulfation rate decreased in the reaction process was analyzed.For a typical 400?m particles,the decrease of the sulfation rate in the inner 0?150?m layer was mainly due to the decrease of SO2 concentration,which was caused by the accumulation of CaSO4 or sintering of CaO.While in the 150?200?m layer,the decrease of the sulfation rate should be due to the decrease of all the pore surface area,the SO2 concentration and the Ca2+ concentration.