循环流化床锅炉（CFB）炉内石灰石脱硫技术是其最具竞争力的优点之一，但随着环保要求日益严格，传统炉内脱硫方式难以满足SO2超低排放的要求。为了达标排放，燃用低硫煤的CFB锅炉大幅增加钙硫比，却会引起NOx催化生成和燃烧效率降低等问题。燃用高硫煤的机组更是被迫增设二级湿法脱硫系统，将导致机组经济性下降，含盐废水排放等诸多问题。因此，有必要进一步挖掘CFB锅炉炉内脱硫潜力，以实现仅依靠炉内脱硫接近或达到SO2超低排放，从而提高机组的环保经济性。石灰石颗粒粒径对于炉内脱硫效率有重要影响，但是此前对于最优粒径的选择存在明显分歧。本文以脱硫剂的粒径效应为核心，基于石灰石动力学实验和实炉测试结果，构建一维脱硫模型，揭示了分离器效率与最优粒径间的匹配关系，并据此提出循环流化床炉内高效脱硫技术路线。论文主要工作和结论如下： （1） 搭建大容量热重反应试验台，测量了250 ~ 2000 ppmSO2浓度下，0~ 600 μm范围内7个窄筛分粒径档石灰石的硫化反应速率，并据此建立了石灰石脱硫经验模型，拟合得到CaO与SO2表观反应级数约为0.6，能够较好的预测特定温度、低SO2浓度条件下，不同粒径石灰石脱硫反应转化率随时间的变化规律。（2） 根据文献中Geldart B类颗粒A型噎塞数据，提出B类颗粒快速床经验判据，进而判定循环流化床炉膛稀相区流态仍然是快速流态化。据此建立快速床条件下颗粒团返混子模型，修正颗粒停留时间，进而更为准确地预测超细石灰石炉内脱硫效率。根据外置换热床（EHE）内气固流动规律建立分区模型，修正了边壁区和中心区磨耗速率的差异。（3） 实炉试验发现当CFB锅炉采用高效旋风分离器后，细颗粒存量增加，停留时间相应增大。因此，超细石灰石可以在更低钙硫比条件下实现高效脱硫甚至SO2超低排放。（4） 模型预测结果表明，不同分离效率下最优脱硫剂粒径选择存在明显差异。高分离效率条件下超细石灰石可以在更低钙硫比条件下实现超低排放。石灰石反应活性、床压降、一次风率和给煤含硫量增加都有利于提高脱硫效率。此外，当石灰石从炉膛中加入时，EHE内颗粒磨耗对于脱硫效率几乎没有影响。但当石灰石从EHE内加入时，EHE内颗粒磨耗则有助于提高脱硫效率。
Desulfurization by limestone injection into the furnace is one of the most competitive merits of circulating fluidized bed (CFB) boilers. However, the stringent emission regulation for power station boilers taking effect in China, conventional desulfurization methods are difficult to meet the requirement of ultra-low emission of SO2. For those CFB boilers burning low-sulfur fuels, a considerable increase in Ca/S molar ratios is deemed necessary to meet the discharge standard, which will inevitably lead to the increased in NOx and the decline in combustion efficiency. Worsely, for those CFBs burning high-sulfur fuels, it is forced to install the second-staged wet flue gas desulfurization (WFGD) systems. Saline wastewater and fine particulate matters have a potential to cause other problems stemming from the operation of WFGD systems, which will greatly increase the investment and operation costs. Therefore, it is absolutely necessary to further enhance the desulfurization performance of CFB furnaces, and identify a cost-effective technical route to the ultralow SO2 emission only through the high-efficiency desulfurization in CFB furnaces, which could enhance the competitiveness of CFB boilers.Limestone particle size has a significant impact on the desulfurization efficiency in CFB furnaces, but there still exist conflicting opinions about the optimum sorbent particle size in previous studies. Therefore, the effects of limestone particle sizes on desulfurization process were detailed analyzed in this dissertation. Based on the sulfation reactivity of limestones obtained in bench-scale experiments and the desulfurization performance of ultrafine limestone in CFB boilers, a one dimensional CFB combustion model was established to reveal the optimum matching relation between the cyclone efficiency and the optimum particle size. Furthermore, a technical roadmap was drawn for the cost-effective control of SO2 emission. The main conclusions obtained include:(1) A large-capacity thermal gravimetric analyzer was developed to obtain the sulfation reaction rate under a wide range of particle sizes (0~600 μm) and SO2 concentrations (250 ~2000 ppm), and then an empirical sulfation model was established based on the experimental results. The fitting reaction order of SO2 was found to be roughly 0.6, which can well predict the sulfation process of different limestone particle sizes under low SO2 concentrations and a specific temperature.(2) Based on the measurement data of saturation carrying capacity in the literature, an empirical equation was fitted to distinguish the fluidization state in the upper CFB furnace. Then, a solid back-mixing submodel was established to modify the residence time of fine particles in the fast fluidization, which can better predict the desulfurization efficiency of ultrafine limestone. According to the non-uniform gas-solid flow in an external heat exchanger, a zoning submodel was also developed to correct the differences of the attration rate in the wall zone and the central zone.(3) Field test results showed that when the high-efficiency cyclones applied to CFB boilers, the mass inventory of fine particles can be significantly increased and the residence time will be extended accordingly. Thus the ultrafine limestone can be used to achieve the high desulfurization efficiency and even the ultralow SO2 emission. (4) Modeling results showed that the optimum particle size of limestone varies significantly with the cyclone collection efficiency. With the high-efficiency cyclones, the ultrafine limestone can achieve the ultralow SO2 emission with a lower Ca/S molar ratio compared to the coarse limestone. Bed pressure drop, sulfation reactivity of limestone, primary air ratio and sulfur content of coal all have a positive effect on the desulfurization efficiency. Besides, the particle attrition in EHE barely affects the SO2 removal when the limestone is injected in furnace. However, the desulfurization efficiency will be increased if the limestone is added in EHE.
