Energy-saving method and system for deep utilization of low-temperature waste heat of whole system of cogeneration device

The invention provides an energy-saving method and system for deep utilization of low-temperature waste heat of a total system of a combined heat and power generation device, and the system comprises the combined heat and power generation device, a C4 aromatic hydrocarbon combination device and an ethylene cracking device, desalted water heated by a slag cooler of the CFB boiler sequentially exchanges heat with an extraction tower top gas phase in an aromatic hydrocarbon extraction device boundary area, desalted water subjected to heat exchange from a low-temperature economizer of the CFB boiler and flue gas exhausted by the CFB boiler, and then is fed into a high-pressure deaerator of the combined heat and power generation device for deoxidization; chilled water supplied by an ethylene cracking device exchanges heat with a low-temperature air pre-heater of a CFB boiler in a boundary area of a combined heat and power generation device, so that air at a suction inlet is heated and then enters a large-scale pub