【作者】 赵争辉；

【导师】 陈鸿伟；

【作者基本信息】 华北电力大学 ， 热能工程， 2014， 博士

【摘要】 化石燃料燃烧产生的S02和C02被认为是分别导致酸雨和全球气候变暖的主要原因,而以化石燃料为主要能源的电力生产是最大的S02和C02排放点源。石灰石等钙基吸收剂因来源广、价格低、吸收容量大被广泛用来脱除电厂烟气中的S02,进一步研究发现钙基吸收剂还具备循环脱除烟气中C02的能力。基于前人研究成果,本文提出利用钙基吸收剂先循环脱碳再脱硫的思想,并设计了循环流化床运行流程,其中钙基吸收剂先循环脱碳再脱硫,烟气先脱硫再脱碳,既避免了烟气中S02对脱碳的不利影响,又实现了两种气体的相继脱除,是一项环保经济可行的方案。本文采用热重分析仪(TGA)和固定床实验等实验设备,从基础动力学入手,探讨了粒径和煅烧气氛对钙基吸收剂脱碳和脱硫的影响以及吸收剂经历的碳化循环次数对其脱硫的影响。由于吸收剂脱碳能力随循环次数的增加而下降,采用蒸汽水合的方法提高钙基吸收剂脱碳和脱硫能力。在模拟真实烟气气氛下,采用鼓泡流化床设备,研究了钙基吸收剂循环脱碳再脱硫特性以及吸收剂流化效果和颗粒磨损特性。基于“以废治废”的思想,利用煤灰与CaO水热化合反应,合成具有更大比表面积和比孔容积的吸收剂,并利用TGA进行脱碳和脱硫能力测试。实验过程借助了扫描电镜、比表面积分析仪、粒径分析仪和X射线衍射分析仪等辅助设备研究样品在不同反应阶段的特性。研究发现,在180目-400目的粒径范围内,粒径对碳化反应影响较小,而对硫化反应影响较大,粒径越小硫化率越高。煅烧过程中CO2浓度对碳化反应影响较大,而对硫化反应影响不大。钙基吸收剂碳化率随碳化循环次数的增加而下降,但硫化率受碳化循环次数的影响不大,40次循环后的吸收剂硫化率与新鲜剂硫化率接近,说明脱碳失效后的吸收剂剂仍是良好的脱硫剂。蒸汽水合法对钙基吸收剂激活作用明显,且吸收剂具有可重复水合激活特性。碳化失效的吸收剂每经历一次水合作用,其碳化活性可恢复至接近新鲜剂水平,且随后的活性下降规律与新鲜剂相同。水合后的吸收剂硫化率远高于新鲜剂硫化率。水合作用在吸收剂颗粒表面产生了大量裂缝和破碎,一方面有利于气体扩散和增大产物可自由生长的外表面积,另一方面由于增大了吸收剂磨损速度,不利于流化床运行。本文证实了钙基吸收剂流化床运行时团聚固结问题的存在。天然石灰石在鼓泡流化床中运行一段时间后,颗粒团聚固结导致流化效果恶化,甚至出现“死床”的现象。这是由于钙基吸收剂高温下自身的特性所造成的,粉末状新鲜石灰石在固定床上煅烧后即发生结块现象。石灰与铝酸钙水泥混合并制粒成球型颗粒的吸收剂,具有更好的循环稳定性和脱硫能力,更好的抗磨损和抗团聚性能,但是该吸收剂初始几次脱碳能力低于天然吸收剂。基于“以废治废”的思想以及受火山灰反应可生成高比表面积产物的启发,将煤灰与CaO在热水中混合搅拌加速火山灰反应,得到比表面积和比孔容积明显提高的钙基吸收剂。火山灰反应产物主要是CaSiO3,呈网状结构,其发达程度与CaO/煤灰配比、水热化合时间以及CaSO4或NaOH的添加量有关。合成剂比天然吸收剂具有更好是循环稳定性,但合成剂中CaO配比不宜过低,否则合成剂中有效CaO含量过低,将降低整体脱碳脱硫能力。经过多次脱碳循环失效的合成剂仍然具有良好的脱硫能力。添加少量NaOH会明显降低合成剂循环脱碳能力,但却能大幅提高脱硫能力,这是由于脱硫反应除了受气体扩散控制外,还受固态离子扩散控制,Na+离子的加入可造成更多的晶格缺陷,加速了离子扩散速率,从而提高脱硫能力。更多还原

【Abstract】 SO2and CO2, genetated by the combustion of fossil fuels, are considered as the main contributors towards acid rain and global warming, separately. Electric power plants which use fossil fuels are the largest global source of SO2and CO2emissions. CaO-based sorbents such as limestone are widely used to retain SO2in coal-fired flue gas, due to its ease of availability, low price and large absorption capacity. Further studies have revealed that CaO-based sorbents can also capture CO2cyclically in the flue gas at high temperature. Based on previous work of other researchers, this thesis proposes the idea of using CaO-based sorbent to capture CO2cyclically and then to retain SO2. The flow chart of running with circulating fluidized beds is also ploted. To avoid the negative effect of SO2on carbonation and realize sequential removal of both gases, fresh sorbents are used to capture CO2cyclically first and then the spent ones are used to retain SO2, while flue gas is desulphated first and then decarbonated. It is obvious that using CaO-based sorbent to capture CO2and SO2sequentially is a feasible scheme and will have great economic advantages.With the equipments of thermogravimetric analyzer (TGA) and fixed bed, this paper begins with basic kinetic research, investigating the effect of particle size and calcination atmosphere on CO2and SO2capture, and the effect of CO2looping cycles on SO2capture. As CO2carrying capacity decreases with increasing cycles, steam hydration is used to reactivate CaO-based sorbent. Under simulated flue gas atmosphere, the sequential CO2and SO2carrying capacity of CaO-based sorbent is tested in a bubbling fluidized bed and the fluidized effect and particle attrition is observed. With the idea of "treating waste with waste", coal ash and CaO are hydrated in hot water to synthesize new sorbents with specific surface area and pore volume enhanced dramatically and their CO2and SO2carrying capacity is tested using TGA. During all experimental process, a lot of auxiliary equipment such as electron microscopy scanner, surface area analyzer, particle size analyzer and X-ray diffraction analyzer, are used to identify properties of the sorbent at different stage.The results show that the particle size in the range of180-400mesh has little effect on CO2capture while having a great effect on SO2capture, with SO2carrying capacity increasing with decreasing particle size. Calcination condition such as CO2concentration has a large effect on CO2capture while little on SO2capture. CO2carrying capacity decreases as the number of cycles increases, while the cycle number has little influence on SO2capture. For example, the SO2carrying capacity of sorbent that has experienced40cycles is close to that of fresh sorbent. That means the CaO-based sorbent which is spent in CO2capture is still active in SO2capture and can be reused. It is proved that the CO2carrying capacity of spent CaO-based sorbent can be enhanced to the equivalent of fresh sorbent by steam hydration, and the new carrying capacity decreases in next few cycles at the same rate as fresh sorbent. This process can be repeated. The SO2carrying capacity of steam reactived sorbent is much higher than that of fresh sorbent. The hydration process generates a large amount of cracks on the particle surface, which benefits gas diffusion and increases the outer surface area where products can grow freely, but also intensifies sorbent attrition which is bad for fluidization.This thesis identifies the problem of sorbent agglomeration in fluidized beds. When the natural limestone runs in bench scale bubbling fluidized bed for few cycles, the sorbent particles agglomerate and fluidization deteriorates, even resulting in a "dead area". Apart from the reason of a small reactor size, agglomeration is a built-in problem of CaO-based sorbent at high temperature, as it can be seen that fresh limestone powder cakes after the first calcination in a fixed bed. Although the CaO-based pellets have a lower CO2carrying capacity than natural limestone for the first few cycles, they show much better performace in cycling stability, SO2carrying capacity, anti-attrition, and anti-agglomerationBased on the idea of "treating waste with waste" and inspired by that a pozzolanic reaction can enhance surface area, coal ash and CaO were stirred together in hot water for few hours to synthesize surface area and activity improved CaO-sorbent. The main product of the pozzolanic reaction is CaSiO3, showing network structure, and its development is related to the ratio of CaO/coal ash, hydration time, amount of CaSO4and NaOH. The synthesized sorbent has a better cycling stability than natural sorbent, however, the CaO/coal ash ratio should not be too low, otherwise the free CaO content of the synthesized sorbent decreases and consequently the overall CO2and SO2carrying capacity. The synthesized sorbent which experienced multi-cycles still keep a high SO2carrying capacity. Adding a small amount of NaOH decreases cyclic CO2carrying capacity of synthesized sorbent but enhances SO2carrying capacity dramatically. The reason is that sulphation reaction is controlled not only by gas diffusion but also solid-state iron diffusion. Na+ions generate more crystal lattice defects which can accelerate iron diffusion rate in product layer, and consequentially enhance overall SO2carrying capacity.更多还原