Synergetic effect of H2O and SO2 on calcination kinetics of limestone during simultaneous calcination/sulfation reaction in CFB boilers

doi:10.11949/j.issn.0438-1157.20180450

Abstract

Abstract:

The simultaneous calcination/sulfation reaction is the real reaction process of limestone in circulating fluidized bed (CFB) boiler. To obtain the true calcination process of limestone in CFB, the combined effect of H₂O and SO₂ on the calcination kinetics and pore structure of limestone during simultaneous calcination/sulfation reaction under CFB conditions was studied in a constant-temperature reactor. H₂O (0-15%) can accelerate the calcination of CaCO₃. The SO₂ in flue gas decreased the calcination rate of limestone particles. This phenomenon was explained by a mechanism based on the measurement of pore structure, namely that SO₂ reacted with CaO layer, and the formed CaSO₄ would fill or block the pore in CaO layer, decrease the pore volume, increase the diffusion resistance of CO₂, and consequently impede the calcination reaction. H₂O and SO₂ can work synergistically on changing the calcination rate,and an increase of the calcination rate was found under 15% H₂O and 0.3% SO₂ compared to that without either. This may be because intrinsic reaction played a major role in the rate controlling step of calcination, and H₂O accelerated the intrinsic calcination directly. The effect of other factors like temperature and particle size on the calcination rate in the presence of H₂O and SO₂ were also tested. H₂O also accelerated the sintering of CaO significantly, and along with SO₂, the pore volume and surface area of CaO decreased further.

Key words: sorbents, kinetics, circulating fluidized bed, limestone, calcination, sulfation, pore structure, H₂O

摘要：

采用自制恒温热重分析仪，研究了CFB工况下石灰石同时煅烧/硫化反应中H₂O和SO₂对石灰石煅烧动力学和孔结构的协同作用。煅烧环境中的H₂O能够促进石灰石的分解，但SO₂会减慢石灰石分解速度，且测试发现SO₂使煅烧后颗粒的孔容积下降，分解反应的效率因子减小。基于此提出SO₂减缓煅烧反应的机理：高温下，石灰石颗粒外层首先分解并生成多孔CaO层，其中的孔隙作为内部CaCO₃分解产生CO₂的外扩散通道，当煅烧气氛中含有SO₂时，颗粒的CaO层与SO₂反应生成CaSO₄，堵塞了CaO中的孔隙，增加了CO₂扩散的阻力，从而减缓了其分解速度。当石灰石在含有15% H₂O和0.3% SO₂的环境中分解时，其分解速度比不含二者的环境下快，而比含15% H₂O但不含SO₂的环境下慢，说明H₂O和SO₂对改变石灰石分解的速度有协同效应，但15% H₂O的作用比0.3% SO₂的作用更大。对效率因子的计算表明，该现象可能由于石灰石煅烧反应的速度控制步骤中本征反应速度的影响比扩散阻力的作用更大，而H₂O能够直接加速煅烧反应的本征速度。温度、粒径等均能够影响石灰石同时煅烧/硫化反应的中的煅烧速度。H₂O还能够促进CaO的烧结，并且H₂O和SO₂在降低石灰石煅烧产物的孔面积和孔容积上具有叠加效应。

关键词: 吸附剂, 动力学, 循环流化床, 石灰石, 煅烧, 硫化, 孔结构, H₂O

CLC Number:

TK221

CHEN Liang, ZHAO Fan, YAN Guangjing, WANG Chunbo. Synergetic effect of H₂O and SO₂ on calcination kinetics of limestone during simultaneous calcination/sulfation reaction in CFB boilers[J]. CIESC Journal, 2018, 69(9): 3859-3868.

陈亮, 赵帆, 闫广精, 王春波. H₂O和SO₂对CFB内石灰石同时煅烧/硫化反应中煅烧动力学的协同作用[J]. 化工学报, 2018, 69(9): 3859-3868.

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Synergetic effect of H₂O and SO₂ on calcination kinetics of limestone during simultaneous calcination/sulfation reaction in CFB boilers

H₂O和SO₂对CFB内石灰石同时煅烧/硫化反应中煅烧动力学的协同作用

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