【作者】 王正阳；

【导师】 孙绍增；

【作者基本信息】 哈尔滨工业大学 ， 热能工程， 2009， 博士

【摘要】 循环流化床(CFB)燃烧技术是一种适合我国以煤为主的能源结构的低成本清洁燃烧技术,在近年得到较快的发展。由于循环流化床内气固流动的复杂性,目前对于在回料流和二次风射流等因素作用下提升管内的气固流动特性还缺乏系统研究,而且现有的研究大多局限于圆形截面的流化床,对于矩形或方形截面流化床的流动特性还需要进行更深入的研究。作者设计和建立了一套方形循环流化床冷态试验系统,研究了回料气固两相流和二次风射流的流动特性及其对提升管整体和局部流动特性的影响。通过数值模拟的方法,更好地认识了不同形式二次风射流作用下提升管内气固流动特性。在此基础上,通过对一台440t/h燃用低挥发份贫煤的循环流化床锅炉进行的系统分析和相关燃烧优化实验,考察了回料流与二次风射流对循环流化床锅炉炉内气固混合及燃烧情况的影响。采用压力梯度测量,光纤探针技术对颗粒浓度及速度的测量,以及射流气体示踪实验等手段研究了方形循环流化床提升管气固流动特性的影响因素。结合回料流和主流流动形式来分析床层底部流动的不对称分布以及因此造成的对床层中上部区域气固流动的影响。研究了在实验物料中加入部分粗大颗粒改善底部一次风偏流的现象。研究了不同颗粒循环流率下,回料口高度的不同对系统压力梯度分布和局部颗粒浓度分布的影响,同时还分析了空气分级条件下回料口高置所产生的影响。系统地研究了不同形式二次风射流,包括墙式、角式、中心水平杆式和中心竖直柱式布置二次风对床内气固流动的影响,以及射流在床内的扩散与分布。在进行空气分级之后,床层底部由于一次风速的降低以及二次风射流形成的布幕作用而形成较高浓度的密相区。不同形式的二次风射流附近存在不同的颗粒浓度分布,当射流距离布风板较近时还将影响到底部颗粒浓度的分布。实验发现,气体饱和夹带流率随二次风率的增加而降低,随二次风高度的增加而减少。当系统颗粒循环流率超过空气分级下系统饱和夹带流率时,系统压力梯度趋于呈S形分布,底部压力梯度将不再变化,密相区高度随颗粒循环流率的增加而提高,其底部压力梯度大小主要同底部流化风速有关。在添加粗砂颗粒的空气分级工况下,粗大颗粒在床层底部颗粒群中占有更高的比例,而在上部区域及出口颗粒群中则降低了。本文还利用分形分析的手段研究了二次风射流对提升管局部和整体流动的波动特性的影响。利用气体示踪技术,对回料风和二次风射流在床内的扩散与分布进行了测量,提出改进的提升管内水平墙式二次风射流穿透深度的计算模型,模型预测值同实验结果吻合较好。采用双流体模型对所研究的冷态方形循环流化床实验台部分工况的气固流动进行了模拟。计算结果同实验数据在趋势上吻合较好。这些工作有助于我们更深入地了解射流对床内整体和局部气固流动的影响。在一台440t/h超高压循环流化床锅炉上进行了回料流及二次风射流对床内气固混合和燃烧情况的影响的研究,研究了一二次风比例、二次风上中下三层之间分配、二次风前后墙之间分配,减小二次风管径提高二次风速度以及调整床温、床压和氧气浓度等方法对飞灰含碳量的影响规律,为提高低挥发分煤循环流化床锅炉燃烧效率提供了借鉴。更多还原

【Abstract】 As a low-cost and effective clean combustion technology, circulating fluidized bed (CFB) combustion technology has been developed greatly. Due to the complexity of gas-solid flow field in the bed, both the effects of return gas-solid flow and secondary air (SA) injection on the hydrodynamics of riser are not fully understood, especially when the riser cross-section is square or rectangular. By the experiments on a square cold circulating fluidized bed model and the numerical simulation, detailed study has been performed on the return gas-solid flow and the SA injection as well as the hydrodynamics in the riser. Furthermore, some optimization combustion tests were done on a 440t/h CFB boiler to evaluate the effects of return gas-solid flow and SA distribution on the coal combustion in the bed.By pressure drop analysis, local solids concentration and particle velocity distribution measurement and jet’s dispersion tracing, the hydrodynamics of the square riser has been researched on a cold CFB model. When the CFB system was operated without air-staging, the thesis focused on the heterogeneous gas-solid distribution in the bed. The back feeding particles penetrate the gas-solid stream in the bed and get to front wall, which leads to a relatively denser region near the front wall in the bottom bed. More work has been done on the effects of gas velocity (U0) and solids circulation rate (Gs) on the asymmetry of solids concentration and particle velocity distribution in the riser. Adding some coarse particles into the bed material makes the lateral distribution of solids more symmetrical. With constant gas velocity, the solids circulation rate was taken as a variable to research the flow properties in the bed with different solids inlet height. Similar work was performed when the air-staging was adopted.Four SA injection modes have been investigated in the paper, i.e. traditional wall SA, corner SA, internal SA with a horizontal SA duct and internal SA with a vertical SA duct. A denser bottom zone is formed when the primary air was reduced and some up-rising solids deflected to the lower bed by the cut-off effects of SA jets. SA jets has limited influenced region especially when the SA nozzles are far away from the distributor. When the Gs is beyond the saturation carrying capacity of gas with air-staging, the pressure drop at the bottom bed doesn’t change any more and S shaped pressure drop profile along the riser is formed, no matter what SA mode is used. When the coarse particle is added into the CFB system, higher coarse particle fraction is found in the bottom bed for the case of air-staging, while the coarse particle fraction in the external circulating particles decreases.Using CO2 as tracer, the dispersion of loop-seal fluidizing air and SA jets have investigated combined with the local solids concentration distribution. A modified model for calculating the SA jet’s penetration is developed when the core-annulus gas-solid flow field is considered. The predicted value by the model has good agreement with the experimental results.The dynamic behavior of gas-solid flow in the experimental CFB setup is predicted based on the theory of Euler-Euler gas-solid two-phase flow and the kinetic theory of granular flows. The simulation includes the operation case with air-staging and without air-staging. The SA modes include Wall SA, Corner SA and Internal SA. The simulation results are in agreement with the experimental results qualitatively.Some optimizing tests were performed on a 440t/h CFB boiler to get more information about the effects of return gas-solid flow and SA injection on the flow properties and coal combustion in a CFB boiler. Some operating parameters have been chosen for combustion optimization, e.g. the loading rate, coal characteristics, oxygen component in the flue gas, SA rate, SA distribution along the riser and distribution between the front wall and the rear wall. Smaller SA nozzles were adopted to enhance the SA jet’s penetration for increasing the oxygen concentration in the central region of furnace, which is beneficial for the char burn-out.更多还原