CFB锅炉内石灰石同时煅烧/硫化反应动力学及孔结构演变模拟

Kinetic Model of the Simultaneous Calcination/Sulfation Reaction and the Pore Change of Limestone in CFB Boilers

综合考虑石灰石的分解、CaO烧结和硫化,建立石灰石同时煅烧/硫化反应的随机孔模型。模型计算结果与实验测试结果吻合良好,并采用该模型研究温度、粒径和SO2浓度等因素对同时煅烧/硫化反应动力学特性的影响。循环流化床内石灰石煅烧反应同时发生硫化反应,所生成的CaSO4阻碍了煅烧反应,主要通过两种机制:孔壁面CaSO4产物层阻碍CO2从煅烧部位向颗粒孔道扩散,以及CaSO4阻碍颗粒孔道内CO2向颗粒外扩散。硫化反应中,由于CaSO4的生成而导致的颗粒表层孔隙的快速堵塞是颗粒整体硫化速度下降、硫化程度低于理论值的主要原因。850~910℃范围内反应温度升高明显加速石灰石分解的速度;温度升高增加快速硫化反应阶段的反应速度,但由于颗粒表层以更快的速度发生堵塞而更早地进入慢速硫化反应阶段,导致反应进行到90min时的硫化转化率变化不大。颗粒粒径下降加速硫化反应速度;对于0.4%SO2气氛中850℃下的反应,粒径对煅烧速度在100s前后的影响不同。在100s之前,硫化反应进行程度较低,煅烧反应速度受CaSO4的阻碍作用弱,煅烧速度随粒径减小而增加;100s之后CaSO4对煅烧速度的阻碍作用显现,粒径越小,CaSO4积累量越大,对煅烧速度的阻碍越明显,致使100s后煅烧速度随粒径减小而下降。

A random pore model which considered the calcination of CaCO3,the sintering of CaO and the sulfation of CaO simultaneously was established.The results of the model are well matched with the results from the test,thus the model was used to investigate the effect of temperature,particle size and SO2 concentration on the kinetics of the simultaneous calcination/sulfation reaction.The CaSO4 formed simultaneously while the limestone is calcined,and the CaSO4 retards the calcination by two ways,one is that the CaSO4 on the pore increases the diffusion resistance of CO2 from the calcine site to the pore of the particle,another is that the CaSO4 increases the resistance of CO2 diffusion from the particle inside to the particle outside in pore.The block of pores at the surface of the particle is the main reason for the decrease of the sulfation rate.In the range of 850~910℃,higher temperature increases the calcination rate;the sulfation rate in the fast sulfation stage is faster at higher temperatures,while the pores at the particle surface get blocked earlier,the time to reach the slow sulfation stage is shorter,which make the sulfation ratio at 90 min change little.The sulfation rate is faster when decreasing the particle size;under typical conditions of 850℃and 0.4%SO2,the effect of particle size on the calcination rate is different before and after 100 s.Before 100 s,the sulfation ratio is low,the effect of CaSO4 on the calcination rate is insignificant,thus the calcination rate increases when decreasing the particle size;while after 100 s,the sulfation ratio is high and the effect of CaSO4 on the calcination rate is significant,since sulfation ratio increases faster with smaller particles,the retarding effect of CaSO4 on the calcination reaction is more obvious for smaller particles,thus the calcination ratio decreases when decreasing the particle size.