Numerical study of pore structure effect on SO2-CaO reactions☆
Liang Ma 1,Liyong Cao 2,Rong He 1,*
【摘 要】ABSTRACT The effect of varying pore structures on the kinetics of SO2-CaO reactions is not fully understoodin the previous studies.Com bining fractal pore model,gas molecular movement model and two-stage reaction model,a new desu lfurization model is estab lishedin this paper.Fractal pore model is used to simulate CaO particle and gas molecular movement model is used to sim ulate gas diffusion in pores.Fractal dim ension is used to characterize com plexity of pore structure instead of tortuosity factor.It is found that the reaction is significan tly affected by pore structures.A modu lus φ is in troduced to characterize the relationship between varying pore structures and apparen t reaction parameters.And this relationshipis verified by therm o-gravimetric analysis(TGA)data.Com paring to the previous models,the effectof varying pore structure on the kinetics of the reaction isdescribed more accurately by the desulfurization model. 【期刊名称】中国化学工程学报（英文版） 【年(卷),期】2015(000)004 【总页数】7
【关键词】Keyw ords:Desu lfurization model Gas diffusion Fractal Pore structure
ARTICLE INFO Article history:Received 18 Sep tem ber 2013 Receivedin revisedform 24 January 2014 Accepted 11 February 2014 Availab le on line 27 Decem ber 2014
?2014 The Chemical Industry and Engineering Society ofChina,and Chemical Industry Press.Allrigh ts reserved.
☆Supported by the National Natural Science Foundation of China(51176096).
1.Introduction
SO2 is a pollu tan t in flue gas and desu lfuration has been experim entally and numerically investigated by som e researchers[1-3].In SO2 em ission con trol,lim estone is w ell-know n and comm only used to reactwithfluegas.The lim estone(CaCO3)iscalcined(heated to decompose in to CO2 and porousCaO)and then su lfated[4].The su lfation reaction ofporousCaOwith SO2 isa typicalnon-catalyticand heterogeneous reaction,and a com plex physicaland chemicalp rocess.In general,there are two stages du ring the su lfation reaction[5,6].In the early stage,the reactan t gases diffuse into pores and react with CaO on the CaO surface.The first-stage reaction can be represented by:
And the reaction rate-lim iting step rep resen ts the com plex coupling of chemical reaction and gas diffusion in pores[7].How ever,w hen a con tinuous CaSO4 product layer form ed on the CaO surface,the Ca2+and O2-diffuse to the gas-so lidin terface through the product layer and
react with SO2 as:
In the second-stage reaction,the rate-lim iting step changes to ion diffusion through product layer[8].The resistance of diffusion through product layer is far greater than that of gas diffusion in pores[9,10].The first stage reaction plays a dom inan t role in the fluegasdesu lfurization.Therefore,the study of the first stage su lfation reaction of CaO is more im portant for engineering application.
Gas diffusion in pores is strongly affected by pore structure[11]and pore diffusion has an im portant effect on gas-solid reaction kinetics[12].The mo lar vo lum e of the product CaSO4 is about 3 times of the mo lar vo lum e of the reactan t CaO.The form ation of CaSO4 product layer on CaO surface has two effects.First,the volumetric expansion results in pore structure variation,even pore closure.Pore structure variation during the reaction has an im portant effect on gas diffusion and the kinetics of such reaction[13].Second,CaSO4 covers the fresh CaO surface and b locks the first-stage reaction.Therefore,the key of studying the reaction mechanism is the effect of varying pore structures on the kinetics of the reaction.
To include the effect of varying pore structures on the kinetics of the su lfation reaction,porous CaO particle model shou ld be able to reflect pore characteristics of natural CaO and the gasdiffusion in pores shou ld be simulated accurately at the molecu lar level.The previous desu lfu
Numerical study of pore structure effect on SO2-CaO reactions☆
