Thermal Research And Optimization Of Flue Gas Waste Heat Utilization System In Secondary Reheating Power Unit

In order to improve the thermal economy of thermal power units,many energy-saving and emission reduction measures are applied in engineering practice.Among them,secondary reheat and flue gas waste heat utilization are effective in practice.However,the existing research only proposed the technical scheme for the flue gas waste heat utilization,and did not fully study the thermal efficiency of these technical schemes.What’s more,the optimization of the related technologies has not been fully optimized.Therefore,this paper will carry out comprehensive thermal efficiency analysis and further optimization for the related flue gas waste heat utilization scheme.This paper studied the thermal economy of the system without optimization reference system,series optimization system and parallel optimization system through Visual Basic programming.The computation mainly includes the heat consumption of system and these related parameters affecting the heat consumption.The calculation process divides the system into two parts,the boiler side and the steam turbine side.The calculation of air preheater is the core of boiler side thermal calculation.The equivalent enthalpy drop method is used to calculate the steam turbine side.On one hand,this paper studied the variable condition features of reference system,the optimization of series-parallel system.These variable condition features included flue gas recirculation rate,primary air rate,primary air temperature and flue gas cooler power allocation.On the other hand,this paper studied SLMAP(superposed liquid medium air preheater)contribution to waste heat utilization.And then,the parallel optimization system of steam turbine is optimized,providing a reference for practical engineering applications.Under the condition of changing recirculation rate of the flue gas(GRR),the thermal economy of the system without optimization reference system,series optimization system and parallel optimization system are calculated.The results show that the series and parallel optimization system will greatly improved thermal efficiency of the unit.Under 100%load,GRR 12%,the coal consumption of series and parallel optimization system of will be 14.374 g/kWh and 17.425 g/kWh lower than the reference system.With the decrease of GRR and load,the coal consumption of the series and parallel optimization system will be reduced compared with that of the reference system.This shows that the series and parallel optimization system have better thermal efficiency under high load and high GRR.The coal consumption of series and parallel optimization system increases with the increase of primary air rate,indicating that the thermal economy of the unit is decreasing.When the increment of primary air is 20%,the coal consumption of the parallel optimization system increases by no more than 0.5g/kWh,and the coal consumption of the series optimization system increases by no more than 2g/kWh.This indicates that the thermal economy of the series optimization system is greatly influenced by the change of primary air rate,while the thermal economy of the parallel optimization system is less affected by the change of primary air rate.The coal consumption of the series and parallel optimization system increases with the increase of primary air temperature,and the increase of coal consumption is no more than 1 g/kWh.When the power distribution of the flue gas cooler is changed,the total coal consumption of the unit is not more than 0.2 g/kWh.It shows that changing the power distribution of flue gas cooler has little effect on the thermal economy of the unit.In the parallel optimization system,the preset liquid phase dielectric cooler is used to ensure the safe operation of the air preheater.Its advantages in thermal economic is improving the power of waste heat utilization,boiler efficiency and thermal efficiency.Under 100%load and 0%GRR,the equipment is t4.18 g/kWh lower than the non-set coal consumption.With the increase of GRR.the decrease of coal consumption is decreasing,and with the increase of load,the decrement of coal consumption is also decreasing.This indicates that the thermal efficiency of conditions with preset liquid phase medium air preheater is increases in the low load and low GRR.Finally,the optimization of the turbine side of the parallel optimization system is applied,which mainly changes the water intake point of the high-energy side and the power distribution of two low temperature economizer.The calculation results show that the closer the intake point is to the heater with high pressure and the higher the power distribution of the economizer is to the high-pressure stage,the better the thermal economy of the unit.Finally,the recommended optimization method is not to change the high pressure level or to replace water intake point to the 3rd high pressure heater and appropriately increase the power ratio of the high pressure level side.In this paper,the thermal economy of series and parallel optimization system has been comprehensively studied,and the parallel optimization system has been optimized.These research have theoretical research and practical application value.