Abstract: Under the“double carbon”goal of“carbon dioxide emissions striving to reach its peak by 2030 and striving to achieve carbon neutrality by 2060”, China will further promote the adjustment of energy structure based on coal, and build a clean, low-carbon, safe and efficient energy system with new energy sources such as wind power and solar power generation as the main body. Coal-fired generating units will gradually change from the main power supply which is in the basic position and plays a supporting role to the basic load power supply and the regulating power supply, and play the role of basic support and safety, while achieving clean, efficient,advanced and energy-saving. The deep utilization of flue gas waste heat of coal-fired boiler is an important measure to save energy, reduce carbon emissions, reduce pollution, improve quality and efficiency. The three main stages of flue gas waste heat utilization of coal-fired boilers in China,and their main technical characteristics have been introduced, and the difference between the back pressure unit without low pressure（less than back pressure exhaust pressure） and low vacuum regenerative system,and the extraction condensing unit and pure condensing unit with multi-stage perfect regenerative system in the deep recovery and utilization of flue gas waste heat have been analyzed. Taking a coal-fired thermal power plant with two back pressure units as an example, this paper introduces the design selection, operation practice and optimization improvement of adding ultra low temperature economizer to two circulating fluidized bed boilers. According to the principle of energy balance, mass balance and equivalent enthalpy drop, the simplified calculation formula of economic and technical indicators that meet the production needs is derived from the empirical formula of boiler flue gas waste heat recovery. More than five years of production and operation practice has proved that these simplified calculation formulas are basically consistent with the economic and technical indicators obtained by common empirical formulas, which can be used to guide operation.After adding ultra low temperature economizers to two circulating fluidized bed boilers in the thermal power plant, the boiler flue gas heat can be recovered by 6 100.5 kW and4 381.4 kW respectively under 100% BMCR and 80% BMCR load conditions. These heats can be used to heat low-temperature desalted make-up water, which can be squeezed out to replace the 9 519.6 kg/h, 6 837.0 kg/h higher quality back pressure exhaust.If all the operating boilers have been operated under BMCR conditions or the back pressure units have been operated under full load conditions and still cannot meet the needs of heat users, these exhaust steams are used to increase heating. Under 100% BMCR and 80% BMCR load conditions,the mass flow of heating steam can be increased by 9 888.6 kg/h and 7 102.0 kg/h respectively, and the water consumption of desulfurization system can be reduced by 9 204.4 kg/h and 6 610.6 kg/h, and the comprehensive thermal efficiency of fuel can be increased by 1.85% and 1.66% respectively. Calculated on an annual basis of 5 000 hours of operation, after deducting the auxiliary power to increase the cost of electricity and maintenance, inspection and cleaning, the annual non-tax revenue can be increased by 8.909 million yuan, with a static investment recovery period of less than 1 year;At the same time, each year can reduce SO2emissions 2 158.9 t,NOx emissions 2 251.3 t, dust emissions 438.7 t, CO2emissions 57 613.4 t, reduce carbon emissions 15 712.7t, energy saving and emission reduction effect is obvious. If the operation of the thermal power plant back pressure unit can meet the needs of all heat users in the industrial park at any time, the recovery of flue gas waste heat can reduce the boiler main steam output, in the boiler 100% BMCR and 80% BMCR load conditions can save coal（ design coal）1 139.4 kg/h, 829.4 kg/h, fuel comprehensive thermal efficiency increased by 2.63%,2.35%; annual expenditure can be saved 2.037 million yuan, SO2, NOx, dust, CO2, carbon emissions reductions were 697.0 t, 726.8 t,141.6 t, 4 474.4 t, 1 220.3 t.If the back pressure machine is used to drive the motor or screw power generation technology and other thermal power cogeneration methods, the heat of the back pressure exhaust steam required to heat the desalting water from the ambient temperature to the ultra low temperature economizer inlet temperature of about 75 °C is stepped, deeply and rationally utilized. Under the 100% BMCR and 80% BMCR load conditions of the boiler, the power generation power can be increased by 1 631.3 kW and 1 186.1 kW through the thermal power cogeneration device, respectively. Increase heating steam mass flow6 243.0 kg/h, 4 462.2 kg/h or save the amount of coal（ design coal） 719.4 kg/h, 521.1 kg/h, fuel integrated thermal efficiency increased by more than 1.39%, the annual income of 7.914 million yuan or save spending3.584 million yuan;At the same time, the annual reduction of SO2emissions 2 553.6 t, 1 632.4 t, NOx emissions 2 662.9 t, 1 702.2 t, dust emissions 518.9 t, 2 374.4 t, CO2emissions 41 653.3 t, 8 166.6 t, carbon emissions 11 360.0 t, 2 227.3 t, energy saving and carbon reduction effect is more obvious, more significant economic, ecological and social benefits.The operation practice of the thermal power plant for more than 5 years has proved that increasing heat supply and saving coal are integrated, and the actual income and expenditure are between the two. At the same time, according to the actual situation of the site, the problems of corrosion, wear and ash accumulation in the operation of the ultra low temperature economizer are continuously improved. The thermal power plant selects reasonable flue gas velocity considering different boiler load conditions, optimizes the heat exchange tube structure of the ultra low temperature economizer, proposes and implements the control of the inlet water temperature and outlet flue gas temperature of the ultra low temperature economizer, and increases the automatic control and protection logic circuit. A row of anti-wear false tubes is set on the upper part of the ultra low temperature economizer and an anti-wear tile is set on the first three rows of heat exchange tubes, which has achieved good results. It provides practical reference and practical evidence for the optimization design and improvement of flue gas waste heat deep recovery and utilization of coal-fired back pressure units.