A heating furnace flue gas waste heat cascade utilization and deep recovery system

Abstract

The application discloses a heating furnace flue gas waste heat step utilization and deep recovery system, which sequentially passes high-temperature flue gas through multiple heat exchange devices of different energy levels: firstly, the high-temperature flue gas enters a high-efficiency steam superheater to produce superheated steam for power or power generation of a plant area; medium-temperature flue gas enters an ORC power generation unit to convert low-grade heat energy into electric energy; low-temperature flue gas is used for preheating combustion-supporting air and coal gas; finally, flue gas with a temperature reduced to the vicinity of a dew point can also be used for preheating cold billets or drying materials entering a furnace, so that all-round recovery of heat energy from high-grade to low-grade heat energy is realized; the system can dynamically adjust flue gas flow through the heat exchange devices of all levels according to real-time collected flue gas temperature data at the outlets of the devices and link the flue gas flow with a hot air supplementary combustion device, so that when load fluctuates, process requirements such as air / coal gas preheating temperature can be preferentially ensured, and the flue gas temperature can be actively and accurately controlled above the acid dew point, thereby ensuring efficient and stable operation of the system and safety of the equipment under all working conditions.

Description

Technical Field

[0001] This application relates to the field of energy saving and waste heat recovery technology for heating furnaces, and in particular to a system for the cascade utilization and deep recovery of waste heat from heating furnace flue gas. Background Technology

[0002] Industrial heating furnaces are among the core energy-consuming equipment in the industrial sector, accounting for more than 30% of total industrial energy consumption. During operation, the high-temperature flue gas (typically reaching 1000℃~1200℃) generated by fuel combustion carries a large amount of waste heat. Direct emission of this gas not only causes serious energy waste but also exacerbates thermal pollution. Therefore, flue gas waste heat recovery has become a key technological means to improve the energy efficiency of heating furnaces and reduce carbon emissions.

[0003] Currently, common industrial methods for utilizing flue gas waste heat mainly fall into two categories: one is to preheat combustion air or gas through heat exchange equipment and directly return the heat to the furnace to achieve energy conservation; the other is to generate steam through waste heat boilers for power generation or production processes. In recent years, with the development of Organic Rankine Cycle (ORC) technology, the use of medium-temperature flue gas for power generation has also gradually been applied.

[0004] However, existing technologies still have the following shortcomings:

[0005] On the one hand, some waste heat recovery systems focus on utilizing heat energy of a single grade, failing to achieve cascade matching of high-temperature, medium-temperature, and low-temperature flue gas. For example, Chinese patent application CN117628734A discloses a high-low temperature heat pump system for producing steam based on cascade waste heat from flue gas of coal-fired units. This system uses a combination of high-temperature and low-temperature heat pumps to recover waste heat from flue gas before and after the desulfurization tower. Although this scheme achieves an improvement in the grade of heat energy, it relies on the power consumption of the compressor, essentially exchanging high-grade electrical energy for low-grade heat energy, and its overall energy efficiency is not optimal. At the same time, this system is mainly designed for the desulfurization environment at the tail end of coal-fired power plants, and its adaptability to the operating conditions of heating furnace flue gas in the steel and metallurgical industries is poor. Moreover, its temperature range coverage is discontinuous, failing to effectively utilize the medium- and high-temperature flue gas above 350℃ for power generation.

[0006] On the other hand, existing systems have shortcomings in operation control, particularly lacking effective corrosion prevention and control measures when flue gas temperature fluctuates. Chinese patent application CN119508833A discloses a multi-element solid and hazardous waste energy resource synergistic cascade utilization system in a waste-free urban green energy valley model. This system generates high-temperature flue gas through the combustion of solid and hazardous waste, which is then used for subsequent desorption and roasting processes. This scheme focuses on solid and hazardous waste treatment, with waste heat recovery only as a byproduct. The flue gas stability is poor; when the flue gas temperature drops below the acid dew point, it easily leads to sulfuric acid corrosion in the tail-end equipment, affecting the system's lifespan and safe operation.

[0007] In addition, there are waste heat recovery technologies targeting different temperature ranges of flue gas: in the high-temperature range, waste heat is converted into process energy or electricity through thermal energy conversion equipment; in the medium-temperature range, it is used to preheat the combustion medium to reduce fuel consumption; and in the low-temperature range, waste heat is attempted to be used in auxiliary processes. However, the application in the low-temperature range is limited by technical and economic constraints and has not yet been widely promoted. Based on this, the inventors recognized that although waste heat recovery technology is widely used, existing systems mostly adopt single-stage or two-stage heat exchange modes, resulting in unreasonable energy grade matching and insufficient cascade utilization. This leads to the downgrading of high-grade heat energy for use and the failure to effectively recover medium- and low-grade waste heat. Therefore, it is necessary to propose a new technical solution to address the problems existing in the current technology. Summary of the Invention

[0008] This application provides a system for the cascade utilization and deep recovery of waste heat from heating furnace flue gas, which solves the problems of unreasonable energy utilization grade matching and low recovery efficiency in existing flue gas waste heat recovery systems.

[0009] To achieve the above objectives, this application provides the following technical solution:

[0010] This application provides a system for the cascade utilization and deep recovery of waste heat from heating furnace flue gas, including a control module, a multi-stage heat exchanger module connected in series and a hot air combustion supplement device that interacts with the control module, wherein:

[0011] The multi-stage heat exchanger module includes a steam superheater, an evaporator, an ORC power generation unit, an air preheater, a gas preheater, and a cold billet preheater connected sequentially at the flue gas outlet of the heating furnace. High-temperature flue gas discharged from the flue gas outlet of the heating furnace flows through each stage of equipment and undergoes heat exchange and cooling at each stage, achieving cascaded recovery of flue gas waste heat. The steam superheater uses the high-temperature flue gas to heat saturated steam into superheated steam suitable for the power system, outputting flue gas at the first-stage temperature. The evaporator uses the first-stage temperature flue gas to heat water into saturated steam suitable for the process system, outputting the second-stage temperature flue gas. The ORC power generation unit includes an ORC evaporator, which uses the second-stage temperature flue gas to heat the working fluid of the ORC power generation unit and outputs a third-stage temperature flue gas. The air preheater uses the third-stage temperature flue gas to preheat the air used for combustion in the heating furnace and outputs a fourth-stage temperature flue gas. The gas preheater uses the fourth-stage temperature flue gas to preheat the gas used for combustion in the heating furnace and outputs a fifth-stage temperature flue gas. The cold billet preheater uses the fifth-stage temperature flue gas to preheat the metal billet to be processed and discharges end flue gas.

[0012] The control module is configured to: collect the outlet flue gas temperature data of each stage of the multi-stage heat exchanger series module to dynamically adjust the flue gas flow rate output to each stage of the equipment so that the terminal flue gas temperature is higher than the acid dew point temperature; and, when the flue gas temperature of the terminal flue gas reaches the critical value of the acid dew point temperature, control the hot air combustion device to start to raise the flue gas temperature to the set safe temperature.

[0013] Furthermore, in the above technical solution, the control module includes a central controller and multiple temperature sensors connected to the central controller for collecting flue gas temperature data; the multiple temperature sensors include: a first temperature sensor located at the flue gas outlet of the steam superheater, a second temperature sensor located at the flue gas outlet of the evaporator, a third temperature sensor located at the flue gas inlet of the ORC evaporator, a fourth temperature sensor located at the flue gas outlet of the air preheater, a fifth temperature sensor located at the flue gas outlet of the gas preheater, and a sixth temperature sensor located at the exhaust port of the cold billet preheater.

[0014] Furthermore, the multi-stage heat exchanger series module also includes:

[0015] An ORC bypass valve is connected in parallel between the flue gas inlet and the flue gas outlet of the ORC evaporator.

[0016] An air preheating bypass valve is connected in parallel between the flue gas inlet and the flue gas outlet of the air preheater;

[0017] A cold billet preheating bypass valve is connected in parallel between the flue gas inlet and the flue gas outlet of the cold billet preheater;

[0018] The ORC bypass valve, air preheating bypass valve, and cold billet preheating bypass valve are respectively connected to the control output terminal of the central controller. The central controller dynamically adjusts the opening degree of each bypass valve according to the collected flue gas temperature data to control the flow rate of flue gas flowing through the corresponding heat exchange equipment.

[0019] Furthermore, the central controller is configured to: when the terminal flue gas temperature at the outlet of the cold billet preheater is lower than the acid dew point critical value, preferentially adjust the opening of the cold billet preheating bypass valve to reduce the flue gas flow through the cold billet preheater, so as to ensure that the air preheating temperature and the gas preheating temperature meet the process requirements.

[0020] Furthermore, the hot air combustion supplement device is a natural gas burner, and the hot air outlet of the hot air combustion supplement device is connected to the flue gas duct of the exhaust port; the hot air combustion supplement device is signal-connected to the control output terminal of the central controller; the central controller is configured to: when the end flue gas temperature continues to drop to the preset start threshold after adjusting the cold billet preheating bypass valve, start the hot air combustion supplement device and dynamically adjust the combustion supplement amount to stabilize the end flue gas temperature above the set safe temperature.

[0021] Furthermore, the ORC power generation unit also includes an expander, a generator, a condenser, and a working fluid pump; the working fluid outlet of the ORC evaporator is connected to the inlet of the expander, the outlet of the expander is connected to the inlet of the condenser, the outlet of the condenser is connected to the inlet of the working fluid pump, and the outlet of the working fluid pump is connected to the working fluid inlet of the ORC evaporator; the working fluid pump is equipped with a frequency converter, and the frequency converter is signal-connected to the control output terminal of the central controller; the working fluid of the ORC power generation unit is pentane; the central controller is configured to: adjust the flow rate of the working fluid pump through the frequency converter based on the flue gas temperature data of the second-stage temperature flue gas collected by the temperature sensor installed at the inlet of the ORC evaporator, so as to achieve adaptive matching between the power generation and the flue gas temperature at the inlet of the ORC evaporator.

[0022] Furthermore, the heating furnace has a flue gas outlet for discharging high-temperature flue gas; the steam superheater has a flue gas inlet, a flue gas outlet, a saturated steam inlet, and a superheated steam outlet, the superheated steam outlet being connected to a power system, and the flue gas inlet of the steam superheater being connected to the flue gas outlet of the heating furnace; the evaporator has a flue gas inlet, a flue gas outlet, and a saturated steam outlet, the saturated steam outlet of the evaporator being connected to a process system, and the flue gas inlet of the evaporator being connected to the flue gas outlet of the steam superheater; the ORC evaporator has a flue gas inlet, a flue gas outlet, a working fluid inlet, and a working fluid outlet, the ORC... The flue gas inlet of the evaporator is connected to the flue gas outlet of the evaporator; the air preheater has a flue gas inlet, a flue gas outlet, and an air outlet, the flue gas inlet of the air preheater is connected to the flue gas outlet of the ORC evaporator, and the air outlet is used to connect to the air inlet of the heating furnace; the gas preheater has a flue gas inlet, a flue gas outlet, and a gas outlet, the flue gas inlet of the gas preheater is connected to the flue gas outlet of the air preheater, and the gas outlet is used to connect to the gas inlet of the heating furnace; the cold billet preheater has a flue gas inlet and a flue gas outlet, the flue gas inlet of the cold billet preheater is connected to the flue gas outlet of the gas preheater.

[0023] Furthermore, the steam superheater and evaporator adopt silicon carbide ceramic tube heat exchangers; the ORC evaporator adopts 316L stainless steel shell and tube heat exchangers; the air preheater and gas preheater adopt 310S stainless steel plate heat exchangers; and the cold billet preheater adopts a composite structure of 316L stainless steel and corrosion-resistant coating.

[0024] Compared with the prior art, this application has at least the following beneficial effects:

[0025] 1. This application establishes a complete physical degradation and utilization chain from high-temperature flue gas (above 1000℃) to low-temperature flue gas by sequentially connecting a steam superheater, evaporator, ORC power generation unit, air preheater, gas preheater, and cold billet preheater at the flue gas outlet of the heating furnace. High-temperature flue gas is used to produce high-grade superheated steam; medium-temperature flue gas is converted into high-efficiency electricity through ORC power generation; and low-temperature flue gas is used to preheat air / gas and cold billets. This achieves "temperature-matched, tiered utilization" of flue gas energy. The refined matching of the principle avoids efficiency losses caused by high energy underutilization or idle energy in the intermediate temperature range, and improves the waste heat recovery efficiency. In addition, the control module configured in this application collects the flue gas temperature data of the outlet of each level of equipment in real time, dynamically adjusts the flue gas flow through each level of heat exchange equipment and links the hot air combustion device. Thus, when the load fluctuates, it can prioritize the process requirements such as air / gas preheating temperature, and actively and accurately control the exhaust gas temperature above the acid dew point, ensuring the efficient and stable operation of the system and the safety of the equipment under all working conditions.

[0026] 2. The steam superheater and evaporator in this application adopt silicon carbide ceramic tube heat exchangers, which are suitable for high-temperature flue gas environments. The ORC evaporator adopts a 316L stainless steel shell and tube heat exchanger, the air preheater and gas preheater adopt 310S stainless steel plate heat exchangers, and the cold billet preheater adopts a composite structure of 316L stainless steel and corrosion-resistant coating (such as polytetrafluoroethylene) to ensure that the flue gas temperature is stable in the range above the acid dew point and avoid sulfuric acid condensation. This application can achieve efficient cascade utilization and deep recovery of waste heat from heating furnace flue gas by selecting materials to adapt to different flue gas temperature ranges, intelligently controlling and regulating heat distribution and corrosion prevention, and recovering medium-temperature waste heat in ORC units, thus achieving both energy-saving benefits and operational reliability. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the embodiments will be briefly described below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. It should be understood that the specific shapes and structures shown in the drawings should not generally be regarded as limiting conditions for implementing this application. For example, based on the technical concepts disclosed in this application and the exemplary drawings, those skilled in the art are able to easily make conventional adjustments or further optimizations to the addition / reduction / classification, specific shapes, positional relationships, connection methods, and size ratios of certain units (components).

[0028] Figure 1 This is a schematic diagram illustrating the operating principle of the waste heat recovery and deep utilization system for heating furnace flue gas provided in this application, as one embodiment. Detailed Implementation

[0029] The present application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] In the description of this application: unless otherwise stated, "a plurality of" means two or more. The terms "first," "second," etc., in this application are intended to distinguish the objects referred to and do not have any special meaning in terms of technical connotation (e.g., they should not be construed as an emphasis on importance or order). Expressions such as "including," "comprising," and "having" also mean "not limited to" (certain units, components, materials, steps, etc.).

[0031] To address the issues of low efficiency and mismatched energy utilization grades in current flue gas waste heat recovery systems for heating furnaces, this application proposes a comprehensive, tiered deep recovery system. This system utilizes the flue gas temperature gradient to achieve a comprehensive, tiered utilization process: high-grade thermal energy is preferentially converted into high-value energy, medium-grade thermal energy is used for medium preheating, and low-grade thermal energy serves process requirements. The system sequentially passes high-temperature flue gas (above 1000℃) through multiple heat exchangers of different energy levels: first, it enters a high-efficiency steam superheater to produce superheated steam for plant power or electricity generation; medium-temperature flue gas enters an organic Rankine cycle (ORC) generator set, converting medium- and low-grade thermal energy into electricity; low-temperature flue gas is used to preheat combustion air and coal gas; finally, flue gas with a temperature near the dew point can be used to preheat cold billets or dry materials entering the furnace. Through this principle of "temperature matching and tiered utilization," the system achieves comprehensive recovery of thermal energy from high to low grades, maximizing the overall energy efficiency of the entire heating furnace system and significantly reducing energy consumption per ton of steel.

[0032] like Figure 1As shown, the waste heat recovery system for heating furnace flue gas proposed in this application mainly includes a control module, a series module of multi-stage heat exchange equipment that interacts with the control module, and a hot air combustion supplement device.

[0033] The multi-stage heat exchanger module consists of a steam superheater, an evaporator, an ORC power generation unit, an air preheater, a gas preheater, and a cold billet preheater, connected sequentially at the flue gas outlet of the heating furnace. High-temperature flue gas exiting the furnace flows through each stage of equipment, undergoing heat exchange and cooling at each stage to achieve cascaded recovery of waste heat. The steam superheater uses high-temperature flue gas to heat saturated steam into superheated steam usable in the power system, outputting flue gas at the first-stage temperature. The evaporator uses the first-stage temperature flue gas to heat water into saturated steam usable in the process system, outputting flue gas at the second-stage temperature. The ORC power generation unit includes an ORC evaporator, which uses the second-stage temperature flue gas to heat the working fluid in the ORC power generation unit, outputting flue gas at the third-stage temperature. The air preheater uses the third-stage temperature flue gas to preheat the air used for combustion in the heating furnace, outputting flue gas at the fourth-stage temperature. The gas preheater uses the fourth-stage temperature flue gas to preheat the gas used for combustion in the heating furnace, outputting flue gas at the fifth-stage temperature. The cold billet preheater uses the fifth-stage temperature flue gas to preheat the metal billet to be processed, and then discharges the final flue gas. The control module is used to collect the outlet flue gas temperature data of each stage of the multi-stage heat exchanger series module to dynamically adjust the flue gas flow rate output to each stage of the equipment, so that the final flue gas temperature is higher than the acid dew point temperature; and, when the flue gas temperature of the final flue gas reaches the critical value of the acid dew point temperature, it controls the hot air combustion supplement device to start to raise the flue gas temperature to the set safe temperature.

[0034] Therefore, this application establishes a complete physical degradation and utilization chain from high temperature (above 1000℃) to low temperature (end flue gas) by setting up a steam superheater, evaporator, ORC power generation unit, air preheater, gas preheater, and cold billet preheater sequentially connected at the flue gas outlet of the heating furnace. After the high-temperature flue gas is discharged from the heating furnace outlet, it flows sequentially through the steam superheater, evaporator, ORC evaporator, air preheater, gas preheater, and cold billet preheater, realizing the cascade recovery of flue gas waste heat. The ORC power generation unit is connected in series between the evaporator and the air preheater. The intelligent control module, with a central PLC controller as its core, dynamically adjusts the valve opening and the start and stop of auxiliary devices by collecting flue gas temperature data at each stage to ensure stable system operation. This system utilizes high-temperature flue gas to produce high-grade superheated steam, medium-temperature flue gas to generate high-efficiency electricity via ORC power generation, and low-temperature flue gas to preheat air / gas and cold billets. This achieves precise matching of flue gas energy according to the principle of "temperature matching and tiered utilization," avoiding efficiency losses caused by underutilization of high-energy or idle energy in intermediate temperature ranges. Simultaneously, the control module configured in this application dynamically adjusts the flue gas flow through each stage of heat exchange equipment by collecting real-time flue gas temperature data at the outlet of each stage and links with the hot air combustion supplementary device. This ensures that during load fluctuations, it prioritizes meeting process requirements such as air / gas preheating temperature while proactively and precisely controlling the exhaust gas temperature above the acid dew point, guaranteeing efficient and stable operation and equipment safety under all operating conditions.

[0035] In this application, the ORC evaporator of the ORC power generation unit is connected in series between the evaporator and the air preheater. The inlet flue gas temperature is controlled in the medium temperature range. The flow rate is adjusted by the working fluid pump frequency converter to achieve adaptive matching between the power generation and the flue gas parameters such as the inlet flue gas temperature of the ORC evaporator.

[0036] In a preferred embodiment of this application, the control module includes a central controller (using a programmable logic controller, PLC) and multiple temperature sensors connected to the central controller for collecting flue gas temperature data. Specifically, the multiple temperature sensors include: a first temperature sensor located at the flue gas outlet of the steam superheater, a second temperature sensor located at the flue gas outlet of the evaporator, a third temperature sensor located at the flue gas inlet of the ORC evaporator, a fourth temperature sensor located at the flue gas outlet of the air preheater, a fifth temperature sensor located at the flue gas outlet of the gas preheater, and a sixth temperature sensor located at the exhaust port of the cold billet preheater, i.e., an exhaust port flue gas temperature sensor. The central controller achieves coordinated control of heat distribution and corrosion prevention by collecting data on the flue gas temperatures at the steam superheater outlet, evaporator outlet, ORC evaporator inlet, air preheater outlet, gas preheater outlet, and exhaust port.

[0037] In this application, the multi-stage heat exchanger series module further includes: an ORC bypass valve, an air preheating bypass valve, and a cold billet preheating bypass valve. Specifically: the ORC bypass valve is connected in parallel between the flue gas inlet and outlet of the ORC evaporator; the air preheating bypass valve is connected in parallel between the flue gas inlet and outlet of the air preheater; and the cold billet preheating bypass valve is connected in parallel between the flue gas inlet and outlet of the cold billet preheater.

[0038] The aforementioned ORC bypass valve, air preheating bypass valve, and cold billet preheating bypass valve are connected to the control output of the central controller. The central controller dynamically adjusts the opening of each bypass valve based on the collected flue gas temperature data to control the flue gas flow through the corresponding heat exchange equipment. Furthermore, when the terminal flue gas temperature at the outlet of the cold billet preheater is lower than the acid dew point critical value, the central controller prioritizes adjusting the opening of the cold billet preheating bypass valve to reduce the flue gas flow through the cold billet preheater, ensuring that the air preheating temperature and gas preheating temperature meet process requirements. It also prevents sulfuric acid condensation corrosion by controlling the exhaust gas temperature to be higher than the acid dew point or activating the hot air combustion supplementary device. Specifically, the control module regulates the flue gas flow through the cold billet preheating bypass valve to ensure that the exhaust gas temperature is higher than the acid dew point. When the flue gas temperature falls below a set threshold, the hot air combustion supplementary device is automatically activated until the flue gas temperature rises back to a safe range. Simultaneously, the cold billet preheater employs a corrosion-resistant coating to extend its service life.

[0039] In this application, the steam superheater and evaporator employ silicon carbide ceramic tube heat exchangers, suitable for high-temperature flue gas environments. The ORC evaporator uses a 316L stainless steel shell-and-tube heat exchanger. The air preheater and gas preheater use 310S stainless steel plate heat exchangers, while the cold billet preheater uses a composite structure of 316L stainless steel and a corrosion-resistant coating (such as polytetrafluoroethylene) to ensure that the exhaust gas temperature remains stable above the acid dew point, preventing sulfuric acid condensation. Simultaneously, the heat exchange area at each stage is optimized according to the heat exchange requirements of the corresponding flue gas temperature range, ensuring a balance between waste heat recovery efficiency and equipment corrosion resistance.

[0040] In this application, the ORC power generation unit also includes an expander, a generator, a condenser, and a working fluid pump. The working fluid outlet of the ORC evaporator is connected to the inlet of the expander, the outlet of the expander is connected to the inlet of the condenser, the outlet of the condenser is connected to the inlet of the working fluid pump, and the outlet of the working fluid pump is connected to the working fluid inlet of the ORC evaporator. The working fluid pump is equipped with a frequency converter, which is connected to the control output of the central controller. Preferably, the working fluid of the ORC power generation unit is pentane. The central controller can adjust the flow rate of the working fluid pump through the frequency converter based on the flue gas temperature data of the second-stage temperature flue gas collected by the temperature sensor installed at the inlet of the ORC evaporator, so as to achieve adaptive matching between the power generation and the flue gas temperature at the inlet of the ORC evaporator, effectively converting the waste heat of the mid-temperature flue gas into electrical energy for output to the plant power grid. At the same time, the central controller is also connected to the pressure sensor signal used to monitor the working fluid pressure of the ORC, and together with the temperature sensor installed at the inlet of the ORC evaporator, achieves adaptive adjustment of the working fluid pump flow rate.

[0041] In a preferred embodiment of this application, the hot air combustion supplement device is a natural gas burner, which ensures that the exhaust gas temperature rises back to a safe range by rapidly adjusting the combustion supplementation amount. Specifically, the hot air outlet of the hot air combustion supplement device is connected to the flue gas duct of the exhaust port; the hot air combustion supplement device is signal-connected to the control output terminal of the central controller; the central controller is configured to: when the terminal flue gas temperature continues to drop to a preset start-up threshold after adjusting the cold billet preheating bypass valve, start the hot air combustion supplement device and dynamically adjust the combustion supplementation amount to stabilize the terminal flue gas temperature above the set safe temperature.

[0042] The operation process of this system is briefly described as follows:

[0043] 1. System initial state: The heating furnace starts up, emitting high-temperature flue gas of approximately 1000℃. The PLC powers on and performs a self-test, all temperature sensors begin collecting data, all electric regulating valves are at their preset openings, the ORC working fluid pump is in standby mode, and the natural gas hot air combustion supplement device is shut down.

[0044] 2. High-temperature section energy recovery: High-temperature flue gas (approximately 1000℃) first enters the steam superheater, where saturated steam is heated into superheated steam, which is then sent to the power system to drive the steam turbine to generate electricity (at this point, the flue gas temperature drops to approximately 650℃). Subsequently, the flue gas enters the evaporator to heat the saturated steam for use in the process system. After passing through the evaporator, the flue gas temperature further decreases (to approximately 350℃).

[0045] 3. Mid-temperature ORC power generation: Flue gas cooled to approximately 350°C enters the ORC evaporator, heating the organic working fluid pentane. The PLC collects the inlet flue gas temperature signal of the ORC evaporator in real time and adjusts the working fluid pump flow rate through the frequency converter, causing the pentane to vaporize into high-pressure steam at approximately 130°C. The high-pressure pentane steam enters the expander, driving it to rotate at high speed, which in turn drives the generator to produce electricity, which is then output to the plant's power grid. The low-pressure pentane steam after performing work enters the condenser, condenses into liquid, and is then pressurized again by the working fluid pump and sent back to the ORC evaporator, completing the cycle. During this process, the flue gas temperature drops to approximately 220°C.

[0046] 4. Low-Temperature Preheating Utilization: The flue gas, cooled to approximately 220°C, flows sequentially through the air preheater and gas preheater, preheating the combustion air and gas respectively. The preheated medium is then returned to the furnace, improving combustion efficiency. At this point, the flue gas temperature drops to approximately 150°C, and it subsequently enters the cold billet preheater, preheating the metal billet to be processed from room temperature to approximately 100°C, reducing the heating load on the furnace. The final exhaust gas temperature is approximately 130°C, higher than the acid dew point (approximately 110°C), and the system operates normally.

[0047] 5. Abnormal Operating Condition Handling: When the production load decreases, causing the flue gas temperature to approach the acid dew point temperature, for example, dropping to 115℃, the PLC prioritizes adjusting the cold billet preheating bypass valve to reduce the amount of flue gas flowing through the cold billet preheater, allowing the flue gas temperature to rise back to 120℃. If the flue gas temperature continues to drop to 110℃ due to a sudden drop in ambient temperature, the PLC immediately activates the natural gas hot air combustion supplementary device to rapidly raise the flue gas temperature by burning natural gas, stabilizing the flue gas temperature above 120℃ and preventing sulfuric acid condensation from corroding the equipment.

[0048] Therefore, the waste heat recovery system for heating furnace flue gas provided in this application uses a PLC as the control core. The control input terminals of the PLC are connected to the first to sixth temperature sensors respectively, and the control output terminals of the PLC are connected to the opening control terminals of the ORC bypass valve, the air preheating bypass valve, and the cold billet preheating bypass valve respectively, the start / stop and power control terminals of the hot air combustion device, and the frequency converter of the working fluid pump, so as to adjust the opening of each valve, control the operation of the combustion device, and adjust the flow rate of the working fluid pump according to each temperature signal. This application uses a PLC to collect real-time flue gas temperature data from the steam superheater outlet, evaporator outlet, ORC evaporator inlet, air preheater outlet, gas preheater outlet, and exhaust port. It dynamically adjusts the opening of the ORC bypass valve, air preheating bypass valve, and cold billet preheating bypass valve, prioritizing that the air preheating temperature and gas preheating temperature meet process requirements. When the exhaust gas temperature is lower than the acid dew point, the flue gas flow is first controlled by adjusting the cold billet preheating bypass valve. If the temperature continues to decrease, the natural gas hot air combustion supplementary device is activated to quickly adjust the combustion supplementary amount to raise the flue gas temperature back to a safe range, preventing sulfuric acid condensation and corrosion of the equipment. Simultaneously, the PLC adjusts the working fluid pump flow based on the ORC evaporator inlet flue gas temperature signal to ensure power generation efficiency and system stability.

[0049] In this application, the preheating media of the air preheater and the gas preheater are returned to the heating furnace to improve combustion efficiency; the superheated steam generated by the steam superheater is transported to the power system, and the saturated steam generated by the evaporator is transported to the process system; the electrical energy generated by the ORC power generation unit is output to the plant power grid, forming a closed loop of waste heat utilization of flue gas from high temperature to low temperature, maximizing the recovery of flue gas energy.

[0050] In summary, this application provides a system for the cascade utilization and deep recovery of waste heat from boiler flue gas, featuring full-process cascade utilization, optimized integration of multiple devices, and intelligent dynamic control. Through measures such as material selection to adapt to different flue gas temperature ranges, intelligent control to regulate heat distribution and prevent corrosion, and ORC unit recovery of medium-temperature waste heat, it achieves efficient cascade utilization and deep recovery of waste heat from boiler flue gas, combining energy-saving benefits with operational reliability. It overcomes the bottlenecks of existing recovery systems, such as unreasonable energy grade matching, low system integration, and outdated operation control, significantly improving boiler energy efficiency and reducing energy consumption.

[0051] The technical features of the above embodiments can be combined in any way (as long as there is no contradiction in the combination of these technical features). For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described; these embodiments not explicitly written should also be considered to be within the scope of this specification.

[0052] The present application has been described in a relatively specific and detailed manner above through general descriptions and specific embodiments. It should be understood that, based on the technical concept of the present application, several conventional adjustments or further innovations can be made to these specific embodiments; however, as long as they do not depart from the technical concept of the present application, the technical solutions obtained by these conventional adjustments or further innovations also fall within the protection scope of the claims of the present application.

Claims

1. A system for the cascade utilization and deep recovery of waste heat from heating furnace flue gas, characterized in that, It includes a control module, a multi-stage heat exchanger module connected in series and a hot air combustion supplement device that interacts with the control module, wherein: The multi-stage heat exchanger module includes a steam superheater, an evaporator, an ORC power generation unit, an air preheater, a gas preheater, and a cold billet preheater connected sequentially at the flue gas outlet of the heating furnace. High-temperature flue gas discharged from the flue gas outlet of the heating furnace flows through each stage of equipment and undergoes heat exchange and cooling at each stage, achieving cascaded recovery of flue gas waste heat. The steam superheater uses the high-temperature flue gas to heat saturated steam into superheated steam suitable for the power system, outputting flue gas at the first-stage temperature. The evaporator uses the first-stage temperature flue gas to heat water into saturated steam suitable for the process system, outputting the second-stage temperature flue gas. The ORC power generation unit includes an ORC evaporator, which uses the second-stage temperature flue gas to heat the working fluid of the ORC power generation unit and outputs a third-stage temperature flue gas. The air preheater uses the third-stage temperature flue gas to preheat the air used for combustion in the heating furnace and outputs a fourth-stage temperature flue gas. The gas preheater uses the fourth-stage temperature flue gas to preheat the gas used for combustion in the heating furnace and outputs a fifth-stage temperature flue gas. The cold billet preheater uses the fifth-stage temperature flue gas to preheat the metal billet to be processed and discharges end flue gas. The control module is configured to: collect the outlet flue gas temperature data of each stage of the multi-stage heat exchanger series module to dynamically adjust the flue gas flow rate output to each stage of the equipment so that the terminal flue gas temperature is higher than the acid dew point temperature; and, when the flue gas temperature of the terminal flue gas reaches the critical value of the acid dew point temperature, control the hot air combustion device to start to raise the flue gas temperature to the set safe temperature.

2. The waste heat recovery system for heating furnace flue gas according to claim 1, characterized in that, The control module includes a central controller and multiple temperature sensors connected to the central controller for collecting flue gas temperature data. The multiple temperature sensors include: a first temperature sensor located at the flue gas outlet of the steam superheater, a second temperature sensor located at the flue gas outlet of the evaporator, a third temperature sensor located at the flue gas inlet of the ORC evaporator, a fourth temperature sensor located at the flue gas outlet of the air preheater, a fifth temperature sensor located at the flue gas outlet of the gas preheater, and a sixth temperature sensor located at the exhaust port of the cold billet preheater.

3. The waste heat recovery and cascade utilization system for heating furnace flue gas according to claim 2, characterized in that, The multi-stage heat exchanger series module also includes: An ORC bypass valve is connected in parallel between the flue gas inlet and the flue gas outlet of the ORC evaporator. An air preheating bypass valve is connected in parallel between the flue gas inlet and the flue gas outlet of the air preheater; A cold billet preheating bypass valve is connected in parallel between the flue gas inlet and the flue gas outlet of the cold billet preheater; The ORC bypass valve, air preheating bypass valve, and cold billet preheating bypass valve are respectively connected to the control output terminal of the central controller. The central controller dynamically adjusts the opening degree of each bypass valve according to the collected flue gas temperature data to control the flow rate of flue gas flowing through the corresponding heat exchange equipment.

4. The waste heat recovery system for heating furnace flue gas according to claim 3, characterized in that, The central controller is configured to: when the terminal flue gas temperature at the outlet of the cold billet preheater is lower than the acid dew point critical value, prioritize adjusting the opening of the cold billet preheating bypass valve to reduce the flue gas flow through the cold billet preheater, so as to ensure that the air preheating temperature and the gas preheating temperature meet the process requirements.

5. The waste heat recovery system for heating furnace flue gas according to claim 4, characterized in that, The hot air combustion supplement device is a natural gas burner, and the hot air outlet of the hot air combustion supplement device is connected to the flue gas duct of the exhaust port; the hot air combustion supplement device is signal-connected to the control output terminal of the central controller; the central controller is configured to: when the end flue gas temperature continues to drop to the preset start threshold after adjusting the cold billet preheating bypass valve, start the hot air combustion supplement device and dynamically adjust the combustion supplement amount to stabilize the end flue gas temperature above the set safe temperature.

6. The waste heat recovery system for heating furnace flue gas according to claim 2, characterized in that, The ORC power generation unit also includes an expander, a generator, a condenser, and a working fluid pump; the working fluid outlet of the ORC evaporator is connected to the inlet of the expander, the outlet of the expander is connected to the inlet of the condenser, the outlet of the condenser is connected to the inlet of the working fluid pump, and the outlet of the working fluid pump is connected to the working fluid inlet of the ORC evaporator; the working fluid pump is equipped with a frequency converter, and the frequency converter is signal-connected to the control output terminal of the central controller; The working fluid of the ORC power generation unit is pentane; The central controller is configured to adjust the flow rate of the working fluid pump through the frequency converter based on the flue gas temperature data of the second-stage temperature flue gas collected by the temperature sensor installed at the inlet of the ORC evaporator, so as to achieve adaptive matching between the power generation and the flue gas temperature at the inlet of the ORC evaporator.

7. The system for cascade utilization and deep recovery of waste heat from heating furnace flue gas according to claim 1, characterized in that, The heating furnace has a flue gas outlet for discharging high-temperature flue gas; The steam superheater has a flue gas inlet, a flue gas outlet, a saturated steam inlet, and a superheated steam outlet. The superheated steam outlet is used to connect to the power system. The flue gas inlet of the steam superheater is connected to the flue gas outlet of the heating furnace. The evaporator has a flue gas inlet, a flue gas outlet, and a saturated steam outlet. The saturated steam outlet of the evaporator is used to connect to the process system, and the flue gas inlet of the evaporator is connected to the flue gas outlet of the steam superheater. The ORC evaporator has a flue gas inlet, a flue gas outlet, a working fluid inlet, and a working fluid outlet, and the flue gas inlet of the ORC evaporator is connected to the flue gas outlet of the evaporator. The air preheater has a flue gas inlet, a flue gas outlet, and an air outlet. The flue gas inlet of the air preheater is connected to the flue gas outlet of the ORC evaporator, and the air outlet is used to connect to the air inlet of the heating furnace. The gas preheater has a flue gas inlet, a flue gas outlet and a gas outlet. The flue gas inlet of the gas preheater is connected to the flue gas outlet of the air preheater, and the gas outlet is used to connect to the gas inlet of the heating furnace. The cold billet preheater has a flue gas inlet and a flue gas outlet, and the flue gas inlet of the cold billet preheater is connected to the flue gas outlet of the gas preheater.

8. The system for cascade utilization and deep recovery of waste heat from heating furnace flue gas according to claim 1, characterized in that, The steam superheater and evaporator are equipped with silicon carbide ceramic tube heat exchangers. The ORC evaporator uses a 316L stainless steel shell and tube heat exchanger. The air preheater and gas preheater are made of 310S stainless steel plate heat exchangers. The cold billet preheater adopts a composite structure of 316L stainless steel and corrosion-resistant coating.

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