关键词: 石灰石, 反应动力学, 循环流化床, 煅烧, 硫化, 模型

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