摘要
循环流化床是近20年来发展起来的一种高效清洁燃烧技术,它有效地解决了节能与环保两方面的问题,代表了当今燃烧技术的一大进步。旋风分离器作为保证循环流化床锅炉具有一定颗粒循环倍率、燃烧效率和脱硫效率的关键设备,其性能的优化一直是CFB锅炉发展的一项关键技术。
为了适应循环流化床锅炉的大型化,目前世界上出现了多种旋风分离器的改进形式。在分离器入口烟道的改进方面,我国四川白马电厂从法国Alstom公司引进的300MW CFB锅炉中,其分离器入口烟道突破了以前的设计,采用了独特的分离器长进口管,这种措施可确保足够大的颗粒加速度和颗粒预分离,大大地提高了分离效率,降低了烟气中的飞灰含炭量。
为了消化吸收国外先进技术,作者以Alstom公司设计的300MW CFB锅炉改进型长烟道和传统型短烟道两种分离器入口烟道为原型,按相应几何比例缩放,制作了两个入口烟道模型,将其安装在一套循环流化床锅炉冷模试验装置上,进行了冷态试验研究,同时以数值模拟对试验研究进行补充,研究的主要内容有:
①对长、短烟道气固两相流动特性,包括烟道压降,颗粒的运动轨迹和加速性能进行试验研究。
②分离器分别配长烟道和短烟道时,对其分离效率和压降进行对比试验研究。
③对长、短烟道内颗粒运动轨迹,两种烟道分别配分离器时,分离器内部流场和分离器分离效率进行数值模拟,并与试验结果进行比较。
试验研究得到的主要结论有:
①长、短烟道都能对固体颗粒进行加速,且长烟道的加速性能明显优于短烟道的加速性能。
②在相同入口风速下,配长烟道的分离器分离效率比配短烟道的分离器分离效率高。
③在相同工况下,长烟道的压降比短烟道的压降要大,配长烟道的分离器压降比配短烟道的分离器压降大。
在冷态试验的基础上,数值模拟得到的主要结论有:
①在流场方面,短烟道内的空气流场比长烟道的空气流场稳定,长烟道在转角处的烟道内壁处会形成回流;配短烟道的分离器内部流场比配长烟道的分离器的内部流场的稳定性和对称性好。
②模拟得到的颗粒在长、短烟道内的运动轨迹和试验结果基本一致,认为试验中所选取的示踪粒子是正确的。
③配长烟道的分离器分离效率要高于配短烟道的分离器分离效率,这与试验结果一致。同时,从烟道入口不同位置进入的相同粒径的颗粒的分离效率有所不同,且配长烟道的分离器由于射入位置的不同而导致分离效率的差异比配短烟道的分离器要大。
As a highly-efficient clean combustion technology, the Circulating Fluidized Bed (CFB) boiler, which effectively solves the problems of energy saving and environmental protection, has developed over 20 years, and it represents the great progress in combustion technology. The cyclone separator is the key equipment for warranting the certain particle circulation rate, good combustion and desulfurization efficiency of the circulating fluidized bed boiler, therefore the optimization of its performance is the key technology in the development of CFB boiler.
To meet the demand of scale-up of the CFB boiler, a lot of improvements of cyclone separators have been made in the world these days. As for the improvements of cyclone separator’s inlet duct, Alstom 300MW CFB boiler, which was imported by Sichuan BAIMA power station, used a unique duct that made a breakthrough compared with the previous designs of its inlet duct of cyclone. The new design adopts long inlet duct to help particles obtain enough high acceleration and pre-separation, which improves separation efficiency greatly and reduces carbon content of fly ash in flue gas.
In order to digest and absorb the advanced foreign technology, the model inlet ducts used the modified long inlet duct and the traditional short inlet duct devised by Alstom Company as their prototypes. The model inlet ducts were reduced according to a certain ratio in geometry size and were installed on a circulating fluidized bed boiler’s cold test rig. Cold tests were carried out on the cold test rig, and based on the cold tests, numerical simulation were carried out as well. The main tasks of the study are as follows:
①Cold tests on gas solid flow characteristics, including the pressure drop and the acceleration performance of particle phases in the two inlet ducts were carried out.
②Measuring and computing the separation efficiency and the pressure drop of the cyclone connected with the two different inlet ducts.
③Simulating the motion trajectories of particles in the two different inlet ducts, the flow fields and the separation efficiency of the cyclone connected with the two different inlet ducts, and the simulation results were compared with the results of cold tests.
The results of the cold tests mainly include:
①Particles in the two kinds of inlet ducts were accelerated and the acceleration performance of particles in the long inlet duct was better than the one in the short inlet duct.
②The separation efficiency of the cyclone connected with the long inlet duct was higher than that with the short inlet duct.
③The pressure drop of the long inlet duct was higher under the same operating conditions; the pressure drop of the cyclone connected with the long inlet duct was higher under the same operating conditions.
Based on the cold tests, the simulation results indicate:
①The stability of flow fields in the short inlet duct was better than that in the long inlet duct, and there were reflux in the long inlet duct; the stability and the symmetry of flow field in the cyclone connected with the long inlet duct was better than that with the short inlet duct.
②Motion trajectories of particles obtained by numerical simulation in the two kinds of the cyclone’s inlet ducts was almost the same as the ones photographed by a high-speed camera, which proved that the tracer particles selected were appropriate.③The separation efficiency of the cyclone connected with the long inlet duct was higher than that with the short inlet duct, which was the same as the results of cold tests; the separation efficiency of particles that injected from different positions of inlet duct’s cross section was different; the difference of separation efficiency was caused by different injection positions of cyclone, and the difference was greater in the case of the cyclone connected with long inlet duct.
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