碳酸钙直接硫化反应产物层固态离子扩散机理研究

Investigation on Solid-state Ions Diffusion in the Product Layer of Direct Sulfation Reaction of Limestone

循环流化床富氧燃烧因其可对再循环烟气温度有效控制,被称为最有工业前景的控制温室气体技术之一。在该燃烧方式下,煤燃烧产生的SO2仍采用石灰石除掉,但该过程属于石灰石的直接硫化反应。此反应目前争议较多的为硫化反应产物层中的扩散物质种类问题。文中采用向石灰石中添加Na2CO3的方法,对其循环流化床富氧燃烧工况下的硫化特性进行研究,目的在于探讨硫化反应产物层中的扩散机理及添加剂对石灰石硫化反应的作用。结果表明Na2CO3对石灰石在富氧气氛下直接硫化反应具有明显的促进作用,而且随反应进行,这种促进作用相对更加显著。SEM分析表明经Na2CO3处理后的石灰石硫化后所形成的CaSO4产物层中CaSO4晶粒较原始石灰石硫化后所形成CaSO4晶粒小,且晶粒之间排列更不规则。Na2CO3的添加导致硫化产物层中缺陷浓度增加,从而加速了产物层中的离子扩散率,是使石灰石直接硫化的最终转化率和硫化速率升高的原因。这也给石灰石直接硫化反应产物层中固态离子扩散理论提供了进一步的依据。

The oxy-fuel combustion in CFB is regarded as one of the most promising technologies for controlling the greenhouse gases because of its effective control on recycled gas temperature. The sulfation of limestone is a direct sulfation reaction because of high CO2 concentration in oxy-fuel combustion. There are many arguments about this reaction, and, diffusion in sulfation product layer is one of the most difficult to understand. The sulfation of limestone modified by Na2CO3 in CFB oxy-fuel combustion was studied to find the diffusion mechanism in sulfation product layer and the influence of Na2CO3 on sulfation of limestone. It showed that Na2CO3 could improve the sulfation of limestone and the effect is getting better with reaction in oxy-fuel combustion. SEM photos showed that the CaSO4 crystal formed by limestone modified by Na2CO3 were smaller and more disordered than that of formed by original limestone. The improvement of void defect concentration in CaSO4 product layer for limestone modified by Na2CO3 would increase the diffusivity in product layer, and this maybe the cause that modified limestone could get higher reaction rate and final conversion. Also, it maybe provides the further evidence for the solid-state ions diffusion in sulfation product for the direct sulfation of limestone.