Operational Characteristics And Optimization Of Cold End System In Large-scale Indirect Dry Cooling Unit

In northern China,coal is abundant but water is scarce.In order to reduce the consumption of water,coal-fired power plants in northern China usually adopt dry cooling technology to their cold end systems.Dry cooling technology mainly comprises two kinds,direct dry cooling technology and indirect dry cooling technology Compared to direct dry cooling technology,indirect dry cooling technology can make the cold end system maintain lower back pressure and operating cost.Hence,indirect dry cooling technology gradually becomes preferred for the coal-fired power plants in northern water-lacking area.The power generating unit adopted indirect dry cooling technology is called indirect dry cooling unit,whose performances are susceptible to ambient conditions.When there is ambient wind,the air flow field around and in the natural draft dry cooling tower of cold end system is deteriorated,which reduces heat dissipation capacity of the tower.The reduction of heat dissipation capacity leads to the increase of turbine back pressure and then causes the rise of operating coal consumption.Moreover,the variation of ambient temperature could also affect unit's operating performances significantly.On the other hand,with the accelerated adjustment of China's energy structure,wind and solar power are developing rapidly,and the installed capacity of wind and solar power is increasing year by year.In order to absorb the new energy power generation,the traditional coal-fired power generating units need to be further involved in the peak shaving operation.For the many indirect dry cooling units located in northern water-lacking area,off-design operation has become new normal.As aforementioned,for the indirect dry cooling unit,its performances are easily affected by ambient conditions and its off-design operation becomes normal.Therefore,this paper takes the important part of the indirect dry cooling unit-cold end system as research subject,investigating its operational characteristics under variable working conditions,and optimizing the way it operates.A super-critical indirect dry cooling unit in northern China is taken as example.A three-dimensional numerical calculating model for natural draft dry cooling tower of cold end system is established,and a theoretical calculating model for the thermal circulating system is built.Through an iterative algorithm these two calculating models are combined to comprise the steady mathematical model for the cold end system.Furthermore,on the base of mechanisms of the condenser and the natural draft dry cooling tower,the dynamic mathematical model for the cold end system is established by means of lumped parameter method as well as mass and energy conservation.Since the performances of indirect dry cooling units are susceptible to ambient conditions,this paper utilize the steady mathematical model to obtain cold end system's operating parameters under different ambient wind speeds and ambient temperatures.According to the air flow and temperature fields around and in the natural draft dry cooling tower as well as the turbine's back pressures,the operational characteristics of the cold end system are studied.By calculating the unit's standard coal consumption rates,the operational economy is analyzed.The results of sensitivity analysis show that even though the ambient wind could deteriorate air flow and temperature fields,the ambient temperature has a greater impact on the unit's operational economy.In order to reduce the adverse effects of ambient wind,an optimization operating strategy is proposed.Under ambient wind condition,mass flow rates of cooling air are different among air-cooled heat exchanger sectors.So,the optimization operating strategy redistributes circulating cooling water among sectors,so that the turbine's back pressure and the unit's standard coal consumption rate are decreased.The optimal distribution of circulating cooling water is calculated on the base of evolution strategies.According to the results,the optimization operating strategy has a good economic benefit and could reduce the unit's standard coal consumption rate by 2.50 g/kW·h at most.Since the off-design operation of indirect dry cooling units becomes normal,this paper utilizes the steady mathematical model to investigate the operational characteristics of cold end system under different loads.In order to decrease unit's standard coal consumption and improve operating economy,an adjusting approach is proposed for the mass flow rate of circulating cooling water.By means of try and error method,the optimal mass flow rates of circulating cooling water under different loads which corresponds to the minimum unit's standard coal consumption rate are obtained.Since the off-design operation of indirect dry cooling units becomes normal,this paper utilizes the dynamic mathematical model so that the transient behavior of cold end system under changing ambient wind and load can be obtained.Moreover,for the load decreasing process,four adjusting strategies for circulating cooling water are proposed.For these four adjusting strategies,the cold end system's response times and the unit's standard coal consumptions during the load decreasing process are calculated respectively.According to the response times and standard coal consumptions,the merits and demerits of the four strategies are discussed.