摘要
随着循环流化床锅炉的快速发展,高温灰渣的冷却问题越来越受到人们的重视。作为循环流化床锅炉的重要辅机,灰渣冷却装置的正常运行至关重要。为了满足高参数大容量CFB锅炉(特别是600MW超临界CFB锅炉)对冷渣器的要求,作者在综合考虑目前大型循环流化床锅炉冷渣器的实际现场运行情况与存在的主要问题的基础上,研制出一种混流式流化床冷渣器,并申请国家发明专利一项。研制过程中,作者自行设计搭建了混流式流化床冷渣器的可视化冷态试验台,采用电厂筛分灰渣作为工作床料,对混流式流化床冷渣器内的灰渣流动特性与冷渣器结构特性等进行了系统的冷态试验研究,解决了以下关键问题:
①选择仓与冷却仓内的灰渣流动特性及其影响因素;
在选择仓内,灰渣床层高度、流化风量和隔墙高度是决定灰渣分选特性的关键因素;在冷却仓内,非均等配风有利于灰渣颗粒的定向流动与混合,存在着一个最有利的平均流化速度及流化速度梯度的控制范围;通过监测冷渣器悬浮空间压力变化可以有效地预测冷渣器内扬析量的变化,悬浮空间压力可作为一个检测扬析量大小的参考标准。
②混流式流化床冷渣器不同结构下的运行特性。
采用模块化分隔墙等方式,改变试验台中冷渣器的结构,通过分析不同结构下的运行特性,得到了冷渣器结构布置的优化方案:在保证足够的床层高度(满足受热面布置要求)的前提下,排渣口尽量布置低端,保证灰渣的顺利排放;第一分隔墙高度存在一个合理的布置范围,使粒度分布和分选率达到有效的优化组合;其余分隔墙高度在布置上沿灰渣流动方向呈阶梯逐步下降的趋势。
基于冷态试验结果,采用CFD软件完成了混流式流化床冷渣器的冷态数值模拟计算。计算结果显示:
①运用FLUENT软件中两相流动欧拉模型,能够正确反映冷渣器内部气固两相流动的特征;
②不同粒度颗粒在相同用风工况下颗粒运动效果截然不同;在非均等配风状态下,颗粒定向流动存在一个有利的流化速度梯度控制范围,与冷态试验结果相一致。
此外,借助研究室的研究成果,确定了冷却仓的传热特性;在上述研究成果基础之上,对混流式流化床冷渣器结构进行了全面优化,同时根据实际CFB锅炉的底渣粒径分布特性,设计了一台15t/h混流式流化床冷渣器,并投入工业制造与安装。
With the rapid development of the circulating fluidized bed boiler, an increasing attention is paid to the high-temperature ash cooling. As an important auxiliary equipment of the circulating fluidized bed (CFB) boiler, the normal running of ash cooling equipment is critical. In order to meet the requirements of the high parameter and large capacity CFB boiler, especially the 600MW supercritical CFB Boiler, the author developed an omnibus-flow fluidized bed bottom ash cooler and has already declared a nation invention patent. The research is based on the current actual scene operations and the major problems of the large-scale circulating fluidized bed boiler ash coolers.
In the development of the new ash cooler, a visual cold experimental apparatus was designed and set up. A series of researches and analysis on the running characteristics and structural characteristics of the new ash cooler was carried out with the bed material of screening bottom ash. Through the research, the key problems were resolved as follows:
①Flowing characteristics of the ash in the selective chamber and cooling chambers and the influencing factors;
The key factors of effect on particles selecting are the bed height、fluidized airflow and the height of separation partitions in the selective chamber. Non-uniform air distribution makes for the particles directional flow and mixture in bubbling beds. There is a most favorable control range of the average fluidized velocity and velocity gradient. The elutriation can be predicted effectually by monitoring the variation of the pressure in suspension space. The pressure in suspension space can be regarded as a reference for the elutriation measure.
②Running characteristics with different structures of the omnibus-flow fluidized bed bottom ash cooler.
In the research, the structure of the ash cooler is changed by using modularized separation partitions. an optimization scheme of the structure arrangement in the ash cooler is got by analyzing running characteristics with different structures: On the premise of ensuring adequate bed height(to meet the demand of heating surface layout),ash discharge port should be located lower to help ash discharge smoothly; a rational range of the height of the first separation partition can be selected to get the optimized combination of the particle size attribution and ash flux,and he height layout of other separation partitions took a decreasing trend along ash flowing. Based on the cold experimental tests, the cold numerical simulation of the omnibus-flow fluidized bed bottom ash cooler was completed by using CFD software.
The simulation results indicate that:
①By using Two-phase Flow Eulerian Model in software Fluent., the simulation to the gas-solids flow characteristics in the bottom ash cooler with numerical simulation could reflect the behavior of the gas-solids flow in the bottom ash cooler;
②Different sizes particles have distinct motion effects with the identical fluidized airflow; there is a favorable control range of the fluidized velocity gradient with the non-uniform air distribution in the particles directional flow. The result corresponds with the cold test results.
Moreover, heat transfer characteristics of cooling chambers were determined based on the research result of the laboratory. On the basis of the above research result, the structure of the ash cooler was optimized completely; according to particle size attribution characteristics of the actual bottom ash in CFB boilers, the industrial design of the omnibus-flow fluidized bed bottom ash cooler with 15t/h was carried out, and it will be put into industrial manufacture and installation.
引文
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