【作者】 熊斌；

【导师】 卢啸风；

【作者基本信息】 重庆大学 ， 热能工程， 2009， 博士

【摘要】 循环流化床(CFB)燃烧技术以其广泛的燃料适应性、较低的氮氧化物排放、较高的脱硫效率、良好的负荷调节性能等优点成为新一代洁净煤燃烧技术。随着CFB燃烧技术的不断发展,CFB锅炉容量也在快速增大。国外已建成CFB锅炉最大容量已达460MW,并已完成了600MW及800MW的超临界大型CFB锅炉设计工作。我国也在积极开展600MW循环流化床锅炉的研制工作。但CFB锅炉的大型化也带来了许多问题:分离器数量不断增多,使得分离器布置困难,进入各个分离器的烟气量相差较大;炉膛容积增大,导致二次风穿透性不佳,布风均匀性差;CFB锅炉尺寸继续放大的风险也越来越大。此外,现有的几种大型CFB锅炉结构布置方案,国外均已申请了相关专利。为解决我国CFB锅炉大型化所面临的技术问题及知识产权问题,迫切需要研究CFB锅炉大型化时采用新的、具有自主知识产权的结构布置方案。在总结和分析大型CFB锅炉结构形式的基础上,重庆大学首次提出了一种“炉膛包围分离器”的新型CFB锅炉布置方案。即在两个炉膛中间布置一个双侧进风的方形旋风分离器,同时分离器下面还布置有与返料机构相联合的一体化外置式换热器(EHE)。与传统CFB锅炉布置相比,此新型布置方案具有对二次风射程要求低,布置紧凑等优点。锅炉放大时,采用多模块组合方式,每个模块可独立运行,其给煤和排渣可单独控制,大大降低了锅炉放大的风险。为了验证此新型CFB锅炉布置方案的可行性,对其循环系统的气固流动进行了研究,其主要包括:①新的CFB锅炉系统运行的气固流动特性及其运行稳定性的研究;②新型外置式换热器内气固流动特性的研究;③双进口方形分离器性能的研究。新的CFB锅炉系统运行的气固流动特性及其运行稳定性的试验结果表明,双炉膛运行时,两个炉膛流化特性相近,相互干扰小,整个系统运行稳定。双进口方形分离器能高效地将物料分离下来。一体化外置式换热器返料均匀,排料调节灵活可靠,基本能顺利地将立管中的循环物料排入任一炉膛。整个锅炉具有良好的压力平衡系统,自平衡特性良好。新型CFB锅炉布置方案中采用了一种全新的非机械阀式外置式换热器。通过控制流入各个换热仓室的固体物料流量,从而达到对各换热仓室中布置的受热面的换热情况进行单独调节。同时把外置床和返料机构(loop seal)结合在一起,保证向两个炉膛的返料。对这种一体化外置式换热器及其返料机构中的物料流动特性进行了冷态试验研究。试验结果表明这种外置式换热器有很好的物料分流和流量控制特性。可以通过调节运行参数和结构参数来控制两个换热室、EHE和loopseal以及两个返料口之间的物料流量和比例。这主要是由于流化风速的改变而引起颗粒夹带速率及各仓室压力分布的变化所致。同时还建立了孔口两侧压差与通过孔口处固体流率之间的经验关系式,定义了孔口流量系数新的关系式。双进口方形分离器的性能是新型CFB锅炉成功的关键因素。在截面尺寸为400mm×400mm的冷态试验台上对分离器性能及分离器内的气固流动进行了研究。当入口风速为22.4m/s、入口颗粒浓度为4.9g/m3时,双进口方形分离器的切割粒径为15μm,临界粒径为75μm,阻力系数为1.7。同时,分离器的性能还受到入口风速及入口颗粒浓度的影响。分离器边角处的即时分离现象对于提高CFB锅炉中高入口颗粒浓度下分离器效率是有利的。在试验研究的基础上,对分离器内的气固流动进行了数值模拟,数值计算的分离器效率和压降与试验结果基本吻合。同时采用数值模拟的方法对分离器结构进行了优化。双进口方形分离器内的流场与圆形分离器相似,具有Rankine涡的特点,在边角处出现局部小漩涡。研究了中心筒插入深度、入口尺寸、直筒段长度、锥体长度对分离效率的影响,通过结构优化,分离器的切割粒径可在6μm左右。方形分离器内旋流较弱,压降较小,可以认为不是分离器性能的主要考虑因素。冷态试验及数值模拟结果表明一体化外置式换热器的流量控制特性及双进口方形分离器的性能能够满足新型CFB锅炉布置方案的运行要求,但仍需锅炉放大后的检验。应用上述研究成果,对一台35t/h工业循环流化床锅炉的结构布置方案进行了研究,并完成了相关设计。以期将此新型CFB锅炉放大到工业锅炉规模,在工业锅炉上进行一系列的测试,积累经验和关键数据,为此新型CFB锅炉的进一步放大打下坚实的基础。更多还原

【Abstract】 The circulating fluidized bed (CFB) combustion technology with the advantages of extensive fuel flexibility, low NOx emission, high desulfuring efficiency and good load adjustable capability and so on, has become the new generation clean coal combustion technology. With the development of CFB combustion technology, the capacity of a CFB boiler has become larger and larger. Until now, the largest 460MW CFB boiler was built and the design of 600-800MW super-critical CFB boilers had been complete in overseas. In China, the development work of 600MW CFB boiler is carried out. Current large CFB boiler usually uses the structural arrangement that several separators are laid around one or two furnaces. So the main problems we faced are as the number of separators of a large CFB boiler increases more and more, the arrangement of separators becomes more difficult. And the flow rate of flue gas to each separator is very different. Also, larger furnace can result in bad penetration of the Second Air. Therefore, the risk of scale-up of CFB boiler becomes higher. The existing arrangement schemes of foreign large-scale CFB boiler are granted patents. So a new domestic CFB boiler’s scaled up scheme which has independent intellectual property is needed to solve those problems.By summarizing and analyzing the structure forms of CFB boiler, a new type of CFB boiler arrangement scheme called“furnaces around separator”was put forward by Chongqing University. The largest difference with other schemes is identified that a square cyclone separator with double inlets is laid between two furnaces, and a new integrated external heat exchanger (EHE) is used under the separator. Thus, this scheme has many advantages comparing with traditional arrangement schemes such as low request for the penetration of the Second Air. The risk of scale-up can be effectually reduced by adopting module combination. And every module can be independently controlled such as the coal supply. In order to verify the feasibility of this new CFB boiler arrangement scheme, gas-solid flow in the solid circulation system of this boiler is investigated. The main research works include the gas-solid flow characteristics of system operation and operation stability of this new boiler, gas-solid flow characteristics of the EHE and the performance of the square separator with double inlets.Experimental study on the gas-solid flow characteristics of system operation and operation stability of this new boiler was conducted. Preliminary results indicated that the fluidization characteristics of the two furnaces were quite adjacent. The disturbance between the two furnaces was small and the whole solid circulation system could run steadily. The square separator separated solids efficiently and the integrated external heat exchanger could successfully discharge the solids from the standpipe into both of the furnaces. The solid circulation system had excellent characteristics of pressure balance.A new type of pneumatically controlled EHE was developed and used in this new CFB boiler scheme. The major feature of this EHE was that the solids flowed to two heat transfer chambers could be controlled. Thereby the distribution of the quantity of heat transfer could be easily adjusted. Simultaneously, the EHE and recycle device were connected in order to recycle the solids back to two furnaces steadily. Experiments completed in the visible cold EHE test rig showed that the solid mass flow rate to the EHE or the loop seal, to two heat transfer chambers, to two exports of solids could be controlled by the aeration airflow into each chamber and geometrical dimensions like the height of partitions. That was because the change of fluidizing velocity of each chamber led to the change of particle entrainment rate and pressure distribution in the EHE. In addition, an empirical correlation between the solid mass flow rate and pressure drop across an orifice was proposed and a new orifice discharging coefficient was defined according to the experimental data and theoretical analysis.The performance of the square separator with double inlets is a key factor of the success of this new CFB boiler arrangement scheme. Experiments on the performance and gas-solid flow of the square separator have been conducted in a cold test rig with a separator cross section 400mm×400mm. Experimental results indicated that with the inlet velocity of 22.4m/s and the inlet solid concentration of 4.9g/m3, the cut size was 15μm, the critical size was 75μm, and the pressure drop coefficient was 1.7. The performance was also affected by the inlet velocity and solid concentration. Especially, the instantaneous separation occurred at the corner was very significant for the improvement of the collection efficiency with high inlet solid concentration for CFB boiler.On the basis of experimental study, the gas-solid flow in the square separator was numerically simulated. The calculated grade collection efficiency and pressure drop have reasonable agreement with the experimental data. And numerical simulation was used to optimize the configuration and dimension of the square separator. The flow field in square separator shows a feature of Rankine eddy and local vortex exits at the corners. By optimization of the configuration and dimension, the cut size is about 6μm. The pressure drop of the square separator is small and is not considered to be the main factor of the performance.Experimental and numerical results indicated that the control characteristics of solid flow rate in EHE and the performance of square separator with double inlets met the operation requirements of this new CFB boiler arrangement scheme well, but it will be verified when this new CFB boiler is scaled up. A new 35t/h industrial boiler was designed according to the above results. First, the laboratory bench scale CFB boiler arrangement scheme will be scaled up to industrial boiler. Some test will be conducted on the industrial boiler. Those tested data and experience will be used to scale up the new CFB boiler to a larger commercial boiler.更多还原