Abstract:

Limestone decomposition and desulfurization in a circulating fluidized bed boiler occurs simultaneously with calcination/sulfidation. A random pore model, which considered the calcination of CaCO₃, the sintering of CaO and the sulfation of CaO simultaneously, was established, and the sulfation model of CaO was based on the solid state ion diffusion in the CaSO₄ product layer. The results of the model are well matched with the results from the TGA test, thus the model can be used to investigate the kinetics of the simultaneous calcination sulfation (SCS) reaction. The SCS reaction contains a mass loss stage and a mass gain stage, which were divided by the minimum mass point. The minimum mass point got higher with the increase of the SO₂ concentration. The calcination reaction occurred in a layer of the particle, which is different from the homogeneous reaction model or the unreacted-core shrinking model. The SO₂ in the calcination atmosphere can react with the CaO layer and form CaSO₄, which can fill the pore of the CaO layer and narrow the pore width, increase the CO₂ diffusion resistance and slow the calcination reaction. The sulfation of the calcined CaO got slower with the reaction time. For a particle with a radius of 200 μm sulfated in 0.4% SO₂ atmosphere, the decrease of the sulfation rate in the part of the inner 150 μm was mainly caused by the lower SO₂ concentration here. There was a zone where the SO₂ exhausted. With the sulfation reaction proceeding, more CaSO₄ accumulated in the outer layer of the particle, which increased the diffusion resistance of SO₂ in the outer layer of the particle, caused the exhausting zone of SO₂ being larger and the sulfation rate of the total particle being slower.

Key words: limestone, reaction kinetics, circulating fluidized bed, calcination, sulfation, model

摘要：

循环流化床锅炉内石灰石分解、脱硫发生的是同时煅烧/硫化反应。建立了石灰石同时煅烧/硫化反应的随机孔模型，综合考虑石灰石的分解、烧结和硫化，且CaO硫化反应基于CaSO₄产物层固态离子扩散模式。模型计算结果与实验测试结果吻合良好，并采用该模型研究了同时煅烧/硫化反应的特性。石灰石的同时煅烧/硫化反应包含连续的质量下降阶段和质量上升阶段，且质量最低点随着SO₂浓度的增加而升高。石灰石颗粒的煅烧反应发生在颗粒的壳层内，煅烧反应更符合区域反应模型而不是均相反应或缩核反应模型。煅烧环境中的SO₂与CaO层反应生成CaSO₄，导致CaO层内孔径和孔隙率减小，CO₂外扩散阻力增大，从而导致煅烧反应减慢。颗粒硫化反应速度随时间减慢，主要是颗粒内SO₂耗尽导致的，颗粒外层不断积累的CaSO₄减小了SO₂扩散通道，增加了SO₂孔内扩散阻力，使颗粒内SO₂耗尽区不断增大，颗粒的硫化反应速度不断下降。

关键词: 石灰石, 反应动力学, 循环流化床, 煅烧, 硫化, 模型