A method for coupling waste heat recovery and iron powder reduction preheating in green hydrogen production process

Abstract

This invention discloses a method for coupling waste heat recovery and iron powder reduction preheating in the green hydrogen production process. It utilizes renewable energy-driven water electrolysis to produce hydrogen, recovering low-grade waste heat (60-90°C) from the electrolyzer cooling system. This waste heat is then pumped up to 120-180°C and used to preheat the iron oxide feedstock to 200-400°C. The preheated iron oxide reacts with electrolyzed hydrogen in a reduction reactor at 800-1200°C to generate iron powder. Compared to traditional systems where waste heat from hydrogen production is directly discharged and reduction preheating is independently powered, this invention improves overall energy efficiency by 15%-25% and reduces reduction preheating energy consumption by 20%-30%.

Description

Technical Field

[0001] This invention belongs to the field of green metallurgy and new energy technology, specifically involving a coupled method for preheating raw materials in the process of hydrogen reduction of iron oxide to prepare iron powder using waste heat generated during the electrolysis of renewable energy. Background Technology

[0002] With the global energy structure transformation, the scale of renewable energy power generation such as wind and solar power has grown rapidly. However, due to the intermittency and volatility of renewable energy, the grid's peak-shaving capacity is insufficient, resulting in a large amount of wind and solar power being unable to be connected to the grid for consumption during certain periods, forming the phenomenon of "wind and solar curtailment".

[0003] In the metallurgical field, traditional blast furnace ironmaking uses coke as a reducing agent, resulting in large carbon emissions and making it a key area for carbon reduction in the steel industry. Hydrogen metallurgy technology uses hydrogen instead of carbon as a reducing agent, with water as the only reaction product, fundamentally eliminating carbon emissions from the ironmaking process and is considered a crucial pathway for the steel industry to achieve carbon neutrality.

[0004] Currently, hydrogen metallurgy technology mainly uses water electrolysis to produce hydrogen as a reducing agent. The water electrolysis process (especially alkaline electrolysis and proton exchange membrane electrolysis) generates a large amount of waste heat. The operating temperature of the electrolyzer is typically between 60 and 90°C, and the cooling system needs to continuously remove this heat to maintain a stable tank temperature. This waste heat is of low grade and has traditionally been discharged directly into the environment via cooling water, resulting in energy waste.

[0005] Meanwhile, the process of reducing iron oxide with hydrogen to produce iron powder needs to be carried out at a relatively high temperature (usually 800~1200℃), and the raw materials need to be preheated before entering the reduction reactor. In the existing technology, the preheating of raw materials usually relies on an external heat source (such as electric heating or gas heating), which consumes a lot of energy.

[0006] In the existing technology, there are reports of integrating water electrolysis for hydrogen production with hydrogen metallurgy. For example, CN201910812334A (Northeastern University) discloses an electric all-hydrogen flash reduction direct steelmaking system and process, which realizes the process connection between water electrolysis for hydrogen production and hydrogen reduction for steelmaking. There are also studies on waste heat recovery from hydrogen-based vertical shaft furnaces, such as recovering waste heat from furnace top gas and cooling section gas through an improved hydrogen-based direct reduction vertical shaft furnace (HDRSF) process for preheating raw materials.

[0007] However, existing technologies have the following shortcomings:

[0008] 1. Single level of waste heat utilization: There is a lack of effective energy coupling between the waste heat from electrolytic hydrogen production and the hydrogen reduction process, and the waste heat from hydrogen production is not fully utilized for the preheating of raw materials in the reduction process;

[0009] 2. Inappropriate energy matching: There is a large temperature difference between the waste heat temperature of hydrogen production by electrolysis (60~90℃) and the preheating temperature required for hydrogen reduction (usually 200~800℃), resulting in low direct heat exchange efficiency;

[0010] 3. High system independence: The hydrogen production system and the reduction system operate independently, lacking a unified energy management and scheduling strategy, and cannot achieve optimal allocation of abandoned power resources.

[0011] Therefore, there is an urgent need to develop a method that can efficiently couple the waste heat from the green hydrogen preparation process with the iron powder reduction preheating process, so as to achieve the cascade utilization of energy and the overall improvement of system energy efficiency. Summary of the Invention

[0012] The purpose of this invention is to provide a method for coupling waste heat recovery and iron powder reduction preheating in the green hydrogen production process. By constructing a coupled process of "waste heat recovery from electrolytic hydrogen production - thermal energy grade improvement - iron oxide preheating - hydrogen reduction", the technical problems of waste heat in hydrogen production, high energy consumption in reduction preheating, and low system energy efficiency in the prior art are solved.

[0013] To achieve the above objectives, the present invention adopts the following technical solution:

[0014] A method for coupling waste heat recovery and iron powder reduction preheating in a green hydrogen preparation process includes the following steps:

[0015] S1: The hydrogen and oxygen are produced by using renewable energy waste to drive the water electrolysis hydrogen production device, and the waste heat generated in the electrolysis process is recovered through the electrolyzer cooling system to obtain primary heat transfer water with a temperature of 60~90℃.

[0016] S2: The primary heat transfer medium water obtained in step S1 is fed into the heat pump system, and the heat pump is used to improve the heat energy grade, outputting a secondary heat transfer medium with a temperature of 120~180℃.

[0017] S3: The secondary heat medium obtained in step S2 is introduced into the preheating device to preheat the iron oxide raw material, so that the temperature of the iron oxide is raised from room temperature to 200~400℃.

[0018] S4: The hydrogen gas obtained in step S1 is heated to 800~1200℃ and then introduced into the reduction reactor to react with the preheated iron oxide to generate iron powder and water vapor.

[0019] S5: The waste residue generated by the reduction reaction is separated by magnetic separation to obtain magnetic iron powder and non-magnetic ferric oxide tailings. The non-magnetic tailings are collected as ferric oxide by-products and used for the production of pigments or catalyst carriers.

[0020] S6: The system features intelligent coupling control, establishing a coupled model of hydrogen production power, waste heat output, and preheating load. It adjusts the electrolyzer operating load in real time based on fluctuations in abandoned power, while maintaining the stability of the iron oxide preheating temperature through heat pump frequency conversion control and preheating device heat medium flow regulation. Attached Figure Description

[0021] Figure 1 This is a process flow diagram of the coupling method described in this invention;

[0022] The reference numerals in the accompanying drawings include: 1. Wind farm; 2. Alkaline water electrolysis hydrogen production unit; 3. Electrolyzer; 4. Plate heat exchanger; 5. High-temperature heat pump system; 6. Rotary preheater; 7. Electric heater; 8. Hydrogen reduction reactor (fluidized bed); 9. Magnetic separation device; 10. Cyclone dust collector; 11. Condensation and dehydration device; 12. PLC-based distributed control system; 13. Thermal storage device; 14. PEM electrolyzer. Detailed Implementation

[0023] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. Example 1

[0024] like Figure 1 As shown, the waste heat recovery and iron powder reduction preheating coupling method in the green hydrogen preparation process of the present invention specifically includes:

[0025] Step 1: Hydrogen production by electrolysis and waste heat recovery

[0026] A green hydrogen production and iron powder reduction coupled system was constructed as part of a wind farm in Northwest China. When the wind power output exceeds the grid's absorption capacity, approximately 5-20 MW of power is wasted, driving the alkaline water electrolysis hydrogen production unit. The electrolyzer operates at a temperature of 75-85℃, with a single cell producing approximately 1000 Nm³ / h of hydrogen. The electrolyzer cooling system uses a closed-loop circulating water cooling system, with the cooling water outlet temperature at approximately 85℃. Heat is recovered through a plate heat exchanger to obtain primary heat transfer water at 80℃, with a flow rate of approximately 50 m³ / h, recovering approximately 4.65 MW of waste heat.

[0027] Step Two: Enhancing Thermal Energy Quality

[0028] The primary heat transfer medium, water, enters the high-temperature heat pump system. The heat pump uses a transcritical CO2 cycle, with an evaporation temperature set at 70°C and a condensation temperature set at 150°C. The heat pump compressor consumes approximately 1.2MW of electrical energy to raise the low-grade heat energy in the primary heat transfer medium to 150°C, outputting secondary heat transfer medium (heat transfer oil) at a flow rate of approximately 30m³ / h, providing approximately 3.5MW of heating capacity, with a coefficient of performance (COP) of approximately 2.9.

[0029] Step 3: Preheating iron oxide

[0030] Secondary heat transfer medium (150℃ heat transfer oil) enters the rotary preheater, where it indirectly exchanges heat with the iron oxide feedstock (mainly Fe2O3, particle size ≤10mm). The iron oxide feed rate is 10t / h, and it is preheated from room temperature (25℃) to 350℃ in approximately 30 minutes. The preheated iron oxide then enters the hydrogen reduction reactor through a closed conveying system.

[0031] Step 4: Hydrogen reduction to produce iron powder

[0032] Hydrogen produced by electrolysis is heated to 950°C by an electric heater and then undergoes a reduction reaction with preheated iron oxide in a fluidized bed reduction reactor.

[0033] The main reaction is: Fe₂O₃ + 3H₂ → 2Fe + 3H₂O

[0034] The reactor operates at a temperature of 850–950°C and a pressure of atmospheric pressure, with a hydrogen excess coefficient of 1.5. The iron powder generated by the reduction reaction has a metallization rate of ≥95% and is discharged after being cooled under nitrogen protection. The reaction tail gas (containing H2, H2O, and a small amount of N2) is subjected to cyclone dust removal, condensation, and dehydration. The hydrogen is then returned to the reactor under pressure, and the condensate is treated and returned to the electrolytic cell for makeup water.

[0035] Step 5: Waste treatment and resource utilization

[0036] The small amount of incompletely reduced material and entrained fine powder produced by the reduction reaction are separated by magnetic separation. The magnetic part is returned to the reduction reactor for further reduction, while the non-magnetic part (mainly Fe2O3) is sorted and sold as a by-product of ferric oxide for use in pigments, catalyst carriers and other fields.

[0037] Step Six: Intelligent Coupling Control of the System

[0038] A PLC-based distributed control system is established to monitor key parameters in real time, such as waste power, electrolyzer current, cooling water temperature, heat pump operating parameters, and preheater outlet temperature. Specifically, in the coupled model, the waste heat output Q_recovery and the electrolyzer operating current I are related as Q_recovery = k × I² × R_cell × η_cool (where k is the heat recovery efficiency coefficient of the cooling system, and η_cool is taken as 0.6~0.85); the preheating load Q_preheat = m_ore × c_p × (T_target - T_amb). The control system uses the target deviation ΔT of T_preheat_out as the controlled variable, and the electrolyzer current command I_cmd and the heat pump compressor frequency f as manipulated variables. It outputs the adjustment signal using PID or lookup table methods to maintain ΔT within ±10℃.

[0039] When the power curtailment fluctuates, the system automatically adjusts the number of electrolytic cells in operation and the load per cell. At the same time, through the frequency converter of the heat pump compressor and the heat medium flow regulating valve of the preheater, the preheating temperature of iron oxide is controlled within the range of 350±10℃ to ensure the stable operation of the reduction reaction. Example 2

[0040] Unlike Example 1, this example employs proton exchange membrane (PEM) water electrolysis for hydrogen production. The PEM electrolyzer operates at a relatively low temperature (60-70°C) but boasts high current density and fast response, making it more suitable for coupling with fluctuating renewable energy sources. The PEM electrolyzer's cooling water outlet temperature is approximately 65°C, and the primary heat transfer medium temperature is 60°C, which is then boosted to 140°C via a heat pump system to preheat iron oxide to 300°C. Due to the rapid response characteristics of the PEM electrolyzer, the system can complete load adjustment within 30 seconds of a change in abandoned power, making it more suitable for highly volatile wind power scenarios. Example 3

[0041] Unlike Example 1, this example adds a heat storage device between the heat pump system and the preheating device. Phase change heat storage materials (such as molten salt or paraffin) are used to store the secondary thermal energy output by the heat pump. When a sudden drop in abandoned power leads to a decrease in hydrogen production load, the heat storage device releases heat to maintain the stable operation of the preheating device. The heat storage capacity is designed for 2 hours of full-load operation, effectively resolving the contradiction between the volatility of renewable energy and the continuity of the reduction process. Beneficial effects

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1. High energy utilization efficiency: The low-grade waste heat of 60~90℃ in the electrolytic hydrogen production process is upgraded by heat pump and then used for iron oxide preheating, realizing the cascade utilization of energy and improving the overall energy efficiency of the system by 15~25%.

[0044] 2. Reduce energy consumption in the reduction process: Preheating the iron oxide before it enters the reduction reactor can reduce the hydrogen heating load in the reduction reactor and reduce the energy consumption of electric heating or gas heating by about 20-30%.

[0045] 3. Enhance the capacity for waste power utilization: By establishing a coupled system of hydrogen production, waste heat, and preheating, waste power is converted into dual products of hydrogen energy and thermal energy, thereby improving the comprehensive utilization efficiency and economy of waste power resources.

[0046] 4. Achieving near-zero carbon iron powder production: The entire process uses renewable energy waste electricity as energy and hydrogen as a reducing agent. The reaction products are only iron powder and water, with no CO2 emissions, achieving near-zero carbonization in iron powder production.

[0047] 5. High system stability: Through intelligent coupling control and thermal storage buffering, it effectively copes with the volatility of renewable energy and ensures the continuous and stable operation of the reduction process.

Claims

1. A method for coupling waste heat recovery and iron powder reduction preheating in a green hydrogen preparation process, characterized in that, Includes the following steps: S1: The hydrogen and oxygen are produced by using renewable energy waste to drive the water electrolysis hydrogen production device, and the waste heat generated in the electrolysis process is recovered through the electrolyzer cooling system to obtain primary heat transfer water with a temperature of 60~90℃. S2: The primary heat transfer medium water obtained in step S1 is fed into the heat pump system, and the heat pump is used to improve the heat energy grade, outputting a secondary heat transfer medium with a temperature of 120~180℃. S3: The secondary heat medium obtained in step S2 is introduced into the preheating device to preheat the iron oxide raw material, so that the temperature of the iron oxide is raised from room temperature to 200~400℃. S4: The hydrogen gas obtained in step S1 is heated to 800~1200℃ and then introduced into the reduction reactor to react with the preheated iron oxide to generate iron powder and water vapor. S5: The waste residue generated by the reduction reaction is separated by magnetic separation to obtain magnetic iron powder and non-magnetic ferric oxide tailings. The non-magnetic tailings are collected as ferric oxide by-products and used for the production of pigments or catalyst carriers. S6: The system features intelligent coupling control, establishing a coupling model of hydrogen production power, waste heat output, and preheating load. It adjusts the electrolyzer operating load in real time based on fluctuations in abandoned power, while maintaining the stability of the iron oxide preheating temperature through heat pump frequency conversion control and preheating device heat medium flow regulation.

2. The coupling method according to claim 1, characterized in that, The water electrolysis hydrogen production device mentioned in step S1 is an alkaline electrolyzer or a proton exchange membrane electrolyzer, and the operating temperature of the electrolyzer is 60~90℃.

3. The coupling method according to claim 1, characterized in that, The heat pump system mentioned in step S2 is a CO2 transcritical heat pump or a high-temperature heat pump with a coefficient of performance (COP) ≥ 2.

5.

4. The coupling method according to claim 1, characterized in that, The preheating device mentioned in step S3 is a rotary preheater, a fluidized bed preheater, or a vertical preheater, and the preheating method is indirect heat exchange.

5. The coupling method according to claim 1, characterized in that, The reduction reactor mentioned in step S4 is a fluidized bed reduction reactor, a vertical shaft furnace reduction reactor, or a rotary kiln reduction reactor, with a reduction reaction temperature of 800~1200℃ and a hydrogen excess coefficient of 1.2~2.

0.

6. The coupling method according to claim 1, characterized in that, Between steps S2 and S3, there is also a heat storage buffer step: the secondary heat medium output by the heat pump is introduced into the phase change heat storage device for heat storage. When the power abandonment decreases and the heat supply of the secondary heat medium is insufficient, the stored heat is released from the heat storage device to maintain the heat source supply of the preheating device. The phase change heat storage material is molten salt or paraffin-based phase change material, and the heat storage capacity is designed for 1 to 3 hours of full-load operation.

7. A waste heat recovery and iron powder reduction preheating coupling system in a green hydrogen preparation process, characterized in that, include: Renewable energy generation equipment used to supply surplus electricity; An electrolysis water hydrogen production device, electrically connected to the aforementioned renewable energy power generation device, is used for electrolysis water hydrogen production; An electrolyzer cooling system, connected to the water electrolysis hydrogen production device, is used to recover waste heat from electrolysis; a heat pump system, connected to the electrolyzer cooling system, is used to improve the heat energy grade. A preheating device, connected to the heat pump system, is used to preheat the iron oxide raw material; a hydrogen reduction reactor, connected to both the water electrolysis hydrogen production device and the preheating device, is used to reduce iron oxide with hydrogen to produce iron powder; a waste residue sorting device, connected to the hydrogen reduction reactor, is used to separate ferric oxide byproducts; and an intelligent control system, connected to the sensors and actuators of the renewable energy power generation device, the water electrolysis hydrogen production device, the heat pump system, the preheating device, and the reduction reactor, is used to adjust the operating load of the electrolyzer based on the abandoned power signal and to adjust the frequency of the heat pump compressor and the flow rate of the heat medium based on the feedback signal of the preheating device outlet temperature.

8. The coupling system according to claim 7, characterized in that, It also includes a heat storage device, which is installed between the heat pump system and the preheating device.

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