Research On Operation Characteristics Of Double Reheat Boiler And Optimization Of Reheat Steam Temperature Adjustment

High parameters,large capacity and flexibility will be the mainstream direction for the development of coal-fired units in the future.Ultra-supercritical double reheat units will be one of the main types.However,the system structure of ultra-supercritical double reheat units is complicated.The main,the primary reheat and the secondary reheat steam temperature of the double reheat boiler have poor controllability under variable load.Therefore,it is of great significance to improve the operational controllability and flexibility of high-parameter and large-capacity coal-fired power generating units,and to ensure that the steam temperature of the units under off-design variable operating conditions meets the requirements of safe and economic operation.According to the actual fuel situation of a 660MW ultra-supercritical double reheat unit,the paper carries out the investigation on several different blending ratios and design coal currently used in power plants,and compares the effects of different coal influence on the thermal performance and combustion conditions in the double reheat unit.The paper adopts the thermodynamic simulation software THERMOFLOW and 1998 version ofthermal performance calculation method of the former Soviet Union,and investigates the the boiler swing burner,flue gas recirculation and flue gas effects on the stability of the boiler main steam,primary and secondary reheated steam temperatures.The influence of baffle opennings of flue recirculation on the thermal performance of the double reheat unit,according to the characteristics of the double reheat unit in different operating conditions,a compound temperature regulation scheme under the range of full load conditions is proposed.In addition,in view of the slow response of the primary and secondary reheated steam temperature adjustment and the easy overshoot of the adjusted steam temperature under the flue gas recirculation of the double reheat unit,it is proposed to obtain the change of the enthalpy increase during the operation as the feedforward signal.A new method for adjusting the quality of the primary and secondary reheat steam temperature of the double reheat unit is proposed.The composite temperature control scheme proposedcan ensure that the main steam and primary and secondary reheat steam temperatures of the power generation unit meet the operating requirements for the steam temperature control.In addition,the paper proposes to add a feedforward control signal for the enthalpy increase of the planten supreheater through the flue gas recirculation temperature adjustment method.The control system can adjust the steam temperature according to the relationship between the flue gas recirculation rate,the reheated steam temperature and the enthalpy value of the platen superheater.The change in enthalpy at the inlet and outlet of the boiler planten panel superheater is used as the feedforward signal to indirectly reflect the overshoot and undershoot of the flue gas recirculation adjustment means of the double reheat unit,and effectively solve the problem of the flue gas recirculation temperature adjustment means.The problem of steam temperature hysteresis improves the regulation quality of the primary and secondary reheated steam temperature,and ensures the safety and economy operation in this power plant.