直流锅炉中间点焓值多平衡点现象及特性分析

Characteristic Analysis on Multiple Balance Points Phenomenon of Intermediate Point Enthalpy in Once-through Boilers

分析直流锅炉工质-能量平衡特性,特别是其中间点焓值特性,是建立其有效数学模型和优化控制系统的基础。过热汽温控制系统是否投入自动,会导致直流锅炉的工质-能量平衡特性出现重大差异。过热汽温控制系统投入自动后,中间点焓值在小范围内变化时,不是一个确定的值,控制系统能够通过调节减温水流量,维持锅炉水煤比整体恒定,将此现象描述为中间点焓值的多平衡点现象。对2台超超临界机组的设计数据进行分析,发现水煤比一定时中间点焓值会在设计值的-4%^+3%之间变化。针对此现象,在DCS逻辑中对中间点焓值设定值进行补偿,是提高直流锅炉控制品质的有效措施,也可以为直流锅炉建模和优化控制系统提供支持。

It is the basis of modeling and optimizing control system to analyze the characteris- tics of working medium-energy balance in once-through boiler, especially to analyze the char- acteristics of intermediate point enthalpy. Whether superheated steam temperature control system （SSTCS） put into automation or not will lead to a significant difference on the charac- teristics of working medium-energy balance in once-through boiler. When SSTCS is put into automation, the intermediate point enthalpy changes in a small scale, the control system is able to maintain the fundamental stability of water to coal ratio by modulating the flow of wa- ter, which can be described as the multiple balance points phenomenon of intermediate point enthalpy. Analysis on the design data of two ultra supercritical units shows that intermediate point enthalpy varies from --4~ to -t-3~ of design value if the ratio of water to coal can remains a fixed value. So amending the set point value of intermediate point enthalpy in DCS logic is the effective approaches to improve the control performance of control system for once-through boiler. And the analysis conclusion can provide support for modeling and optimization of control system in once-through boiler.