The Study Of Pore Structure In The Simultaneous Calcination/Sulfation Of Limestone

Sulfur dioxide was one of the major pollutants in air and the main cause of acid rain.The technology of limestone injection into furnaces was used as an important desulfurization method with the advantage of the lower cost and the higher efficiency.The desulfurization reaction of limestone in the furnace was composed of two reactions:calcining and sulfation.For the desulfurization reaction of limestone in CFB,most studies divided the calcination and sulfation of limestone as two separate processes which was different from the actual reaction in CFB.In this paper,the kinetics and pore structures and sulfation reaction mechanism in the simultaneous calcination/sulfation of limestone under circulating fluidized bed boiler conditions was studied.A constant-temperature reactor was employed.The mechanism to describe the microstructure evolution of limestone particles was proposed.The random pore model used to calculate the sulfation reaction mechanism was built.The results showed that:in the initial reaction stage,a small amount of CaSO₄ formed in the surface of CaO layer,and few pores was plugged;in the mid calcination stage,the volume of closed pore increased with the increase of the thickness and area of CaSO₄ layer,and the diffusion resistance of CO₂ in pore also increased,resulting in a decrease of calcination rate;in the final stage,because of the accumulation of CaSO₄,the pore plugging was accelerated;in the initial stage of sulfation reaction,the sulfation reaction near the surface of the calcined product of simultaneous calcination sulfation reaction was controlled by the chemical reaction,the sulfation reaction near the center of the calcined product was controlled by diffusion;in the final stage,the sulfation reaction was controlled by diffusion;the smaller initial surface area reduced the sulfation reaction rate at the initial stage and extended the reaction time which was under the chemical reaction control stage,resulting in the improvement of the final conversion;the larger initial porosity increase the chemical reaction rate and the diffusion coefficient,therefore the concentration of SO₂ in the intra of the particle was increased resulting in the improvement of the final conversion.