Optimization Design Of Large-Scale Coal-fired Units Under Variable Load Condition

With the increase of unit capacity and the change of social power consumption in our country, an increasing number of capacity power generating units were put into the peak regulation of power network, to meet the load changing needs of different enterprises and users. Under off-design condition, the main steam temperature at high load condition will remain constant or changed a little; but at lower load, the main steam temperature and reheated steam temperature will decrease largely, especially when the load is lower than 70%. Therefore, it is necessary to increase the reheat steam temperature to ensure economic efficiency and security of units under off-design condition.The recirculation flue gas, from the tail of the boiler back into combustion chamber, can change the velocity and the temperature of the whole flue gas system, and further will change the heat transfer rate of the radiant heating surface and convection heating surface and it can also reducing emissions of pollutants. In this paper, the thermodynamic calculation on a 600 MW coal-fired boiler was analyzed, and the optimization of coal input and recirculation were carried out to reduce the net coal consumption.Firstly, based on the research of thermodynamic calculation methods, the calculation model and method with flue gas recirculation was established, some formulas was revised and the influence of recirculation temperature on calculation results was considered.Then, the heat absorption and flue gas outlet temperature of each heating surface under different recirculation and load condition were concretely analyzed and some suggestions to reheater and superheater arrangement were made. Besides, this paper proposed another gas recirculation system in which the flue gas is extracted between low temperature reheater and economizer, with the higher flue gas temperature, which can improve the combustion stability and reduce the formation of NOx.Finally, the influence of different recirculation model on boiler economy efficiency was analyzed through the calculation results under different load condition. Besides, an optimization is carried out and the optimum recirculation rate and coal input were reported.