An energy-saving system using electric heat pump to recover the waste heat from coal-fired exhaust flue gas for district heating, comprising coal-fired boilers, steam turbine, condenser, heat-supply network heat exchanger, condensate tank, condensate water pump, dust remover, flue gas waste heat recovery tower, stack, waste heat recovery solution pump, anti-corrosion and high-efficiency water-water plate heat exchanger, electric heat pump, NaOH storage tank, NaOH preparation device, NaOH solution pump, check valve, flue gas inlet valve, condensate pool, flue gas bypass valve, and multiple valves and connecting pipes;
- inner wall of the flue gas waste heat recovery tower is attached with insulation material;
- the flue gas waste heat recovery tower mainly consists of inlet section of the flue gas, filler layer, spray layer, demister and exhaust outlet section;
- the inlet section of the flue gas is located in lower section of the flue gas waste heat recovery tower, and the exhaust outlet section is located at top of the flue gas waste heat recovery tower;
- the demister is located in upper and interior section of the flue gas waste heat recovery tower, and the filler layer is located in middle section and interior of the flue gas waste heat recovery tower;
- the spray layer is located in upper section and interior of the flue gas waste heat recovery tower, and the spray layer is located below the demister and above the filler layer;
- high-temperature flue gas produced by the coal-fired boiler is filtered by the dust remover, then controlled by the flue gas inlet valve, and enters the flue gas waste heat recovery tower through the inlet section of the flue gas waste heat recovery tower, the high-temperature flue gas flows upwards in the tower;
- circulating water is sprayed at top by the spray nozzle in the spray layer;
- high-temperature flue gas after the filler layer is contacted with spray water, where the intensive direct contact heat and mass transfer occurs between gas and liquid phases;
- water vapor in high temperature flue gas will condensate and release heat;
- circulating water will absorb heat from high-temperature flue gas and drop into the bottom of the flue gas waste heat recovery tower, one part of the condensate that returned to the bottom of the tower continues to participate in the circulation as circulating water, and another part is controlled by valve located at the bottom of the flue gas waste heat recovery tower and flows into the condensate pool located under the flue gas waste heat recovery tower, to recover the flue gas condensate and condensing water;
- after heat exchange, the moisture in the low-humidity flue gas will be caught by the demister, and the relatively dry flue gas is discharged from the outlet section of the flue gas and enters the stack, to achieve the purpose of recovering waste heat of flue gas condensation;
- the NaOH storage tank is connected with the NaOH preparation device, lye in the NaOH storage tank enters the NaOH preparation device, and desulfurized NaOH solution from the NaOH preparation device will be circulated by the NaOH solution pump, and is controlled jointly by valve and the check valve, and then, enters the flue gas waste heat recovery tower;
- at the same time, a large amount of SO2 gas in the flue gas has dissolved in condensate of the flue gas waste heat recovery tower;
- the condensate will react with NaOH to achieve the purpose of desulfurization;
- the electric heat pump is indirectly connected by anti-corrosion and high-efficiency water-water plate heat exchanger to the circulating water at the bottom of the flue gas waste heat recovery tower;
- circulating water is sent from the bottom of the flue gas waste heat recovery tower by the waste heat recovery solution pump or without the waste heat recovery solution pump to the anti-corrosion and high-efficiency water-water plate heat exchanger, which will exchange heat with the circulating water from the evaporator side of the electric heat pump;
- the circulating water heats up return water from heat-supply network;
- circulating water pipelines have several valves based on demand;
- after heat exchange process, low-temperature waste heat circulating water will be pumped back to the flue gas waste heat recovery tower and continues to exchange heat with flue gas by spraying;
- low-temperature waste heat circulating water pipelines also have several valves based on demand;
- return water from district heating network is controlled by valve and flows into the electric heat pump, which will be connected indirectly by anti-corrosion and high-efficiency water-water plate heat exchanger and exchange heat with high-temperature waste heat circulating water;
- electric heat pump uses electric energy from power plant as driving energy;
- the return water of the heat-supply network from the electric heat pump enters the heat-supply network heat exchanger for further heating until the water temperature is heated up to the required supply temperature and then the hot water will be sent to the heat-supply network;
- the coal-fired boiler is connected with the steam turbine;
- the steam generated by a coal-fired boiler enters the steam turbine for power generation and it is divided into two parts:
- exhaust steam and extraction steam;
- the exhaust steam enters the condenser;
- the extraction steam flows into the heat-supply network heat exchanger, to heat up the return water of the heat-supply network, and after heat exchange and condensation, it enters the condensation tank;
- the condensed water in the condenser and the condensed water separated from the condensation tank will be sent to a condensate system for treatment and then returned to the coal-fired boiler for reuse;
- condensate water pump is installed at the output pipe of the condensation tank.