Design And Research Of Flue Gas Recirculation Retrofit Plans For A 1000MW Double Reheat Unit

Aiming at the problem that the reheat steam temperature is lower than the design value of the Guodian Taizhou 1000MW ultra-supercritical double reheat unit,the flue gas recirculation technology is proposed to regulate the reheat steam temperature.According to the different extraction and inlet points of the recycled flue gas,four schemes of flue gas recirculation are worked out,which are to extract flue gas from the economizer or the induced draft fan of the boiler and then introduce it into the bottom or upper part of the furnace.The research method combining thermal calculation with numerical simulation is adopted.The influence of flue gas recirculation on boiler operation is analyzed by studying the variation of boiler operation parameters under different schemes.The results show that the reheat steam temperature can be effectively increased by introducing the recirculation flue gas into the bottom of the furnace.When the recirculation rate reaches 10%,the primary and secondary reheat steam temperatures have met the design requirements,which shows that this scheme can solve the problem of low reheat steam temperature of the boiler.At the same time,through the numerical simulation,it is found that the furnace outlet flue gas temperature slightly decreases,the concentration of O₂ and NO decreases,and the concentration of CO increases,which shows that flue gas recirculation can reduce NO emission.Under the condition of maximum output（BMCR）,with the increase of 5%flue gas recirculation rate,the primary and secondary reheat steam temperature rises about 5.9℃and 6.4℃,the flue gas temperature at furnace outlet decreases about 10℃,the O₂,CO and NO concentrations decrease about 4.1%,22.47%and 6.1%.The scheme of introducing recirculation flue gas into the upper part of the furnace cannot obviously increase the reheat steam temperature,but can only reduce the furnace outlet flue gas temperature,the slagging at the furnace outlet and the NO emission.Under the same recirculation rate,the furnace outlet flue gas temperature decreases about 40℃,the O₂,CO and NO concentrations decrease about2.35%,13.68%and 4.8%.The lower the boiler load,the more obvious the increase of reheat steam temperature.Under the same recirculation rate,the increase of reheat steam temperature at low load is about 1.7 times of that at high load.The recirculation process for the scheme of extracting flue gas from the back of economizer does not include air preheater and induced draft fan.The recirculating system is simple and the boiler efficiency changes little.With the increase of 5%flue gas recirculation rate,the power of induced draft fan motor increases about 5.6%and the maximum wear thickness of economizer tube wall increases about 18.33%.The recirculation process of the scheme of extracting flue gas from the back of induced draft fan includes air preheater and induced draft fan.The recirculating system is more complicated and the boiler efficiency drops obviously.Under the same recirculation rate,the boiler efficiency decreases about 0.59 percentage points,the power of induced draft fan motor increases about 18.9%and the maximum wear thickness of economizer tube wall increases by about 21.03%.The results show that the scheme of extracting flue gas from the back of economizer and introducing it into the bottom of furnace is suitable for this unit.According to the calculation,the motor power of recirculating fan under this scheme is 520 kW,with the increase of 5%flue gas recirculation rate,the auxiliary power consumption rate increases about 0.04 percentage points and the standard coal consumption for unit power supply decreases by about 1.13 g/（kW·h）.This show that the flue gas recirculation can improve the economy of the double reheat unit.