Application of Solid-oxide Water Electrolysis Tech in Large-scale Hydrogen Production Systems
From：
Zhonglin International Group Date：07-31 942 Belong to：Industry Related
Preface
In the process of accelerating carbon neutrality, countries around the world are actively developing and demonstrating the use of renewable energy and nuclear power to produce CO2 free hydrogen gas, and further producing fossil fuel alternatives such as hydrocarbons and ammonia. However, considering global economic activities and regional situations, it is believed that this new trend towards a sustainable development society will take a considerable amount of time. Therefore, it is necessary to gradually reduce the proportion of fossil fuel power generation in the energy structure while fully tapping into the potential of fossil fuels. One of them is the utilization of waste heat. In thermal power generation and nuclear power, the Energy conversion efficiency of combined cycle thermal power generation is more than 55%, while that of nuclear power is about 33%. The energy that cannot be used for power generation is discharged in the form of heat energy. If these thermal energy can be effectively utilized, it is possible to reduce the CO2 emissions per unit of fuel consumption.
&Nbsp& Nbsp& Nbsp; In addition, many areas in the industrial system also use fossil fuels and emit CO2. For example, the carbon dioxide emissions in the steel industry are very high, and there is a great demand for carbon reduction. During the ironmaking process, using coke to reduce iron ore and extract iron will generate CO2. Japan is developing a hydrogen reduction ironmaking technology that replaces coke with hydrogen, which is a new technology for the future. Not only the steel industry, but various other industries are currently striving to improve energy efficiency and reduce CO2 emissions by effectively utilizing waste heat.
Effective utilization of waste heat is crucial for reducing CO2 emissions, and one of the hydrogen production methods using thermal energy is high-temperature steam electrolysis. Electrolysis of water can produce CO2 free hydrogen through renewable energy. In particular, the difference between high-temperature steam electrolysis and other Electrolysis of water methods is that water is electrolyzed in the form of steam, and heat energy can be used as part of the energy needed for electrolysis. Therefore, both the waste heat from existing power generation and industrial systems can be utilized, and the hydrogen produced can also be utilized in the process. It is generally believed that Electrolysis of water is highly compatible with these large systems. In addition, because the Electrolysis of water tank using high-temperature steam electrolysis technology operates at high temperature, the larger the scale, the higher the efficiency, and is very suitable for large-scale hydrogen production.
Therefore, Toshiba Energy Systems Co., Ltd. is promoting a series of developments from materials to system design and production to achieve hydrogen production systems utilizing high-temperature steam electrolysis technology. This paper introduces the characteristics of high temperature steam electrolysis technology using solid oxide Electrolytic cell (SOEC) (hereinafter referred to as solid oxide Electrolysis of water technology) and the development of Toshiba energy system. In addition, this article also describes an uation application that
Solid-oxide water electrolysis technology
2.1 Comparison of various Electrolysis of water methods
&Nbsp& Nbsp& Nbsp; Electrolysis of water methods can be divided into several types according to the type of electrolyte and operating temperature, mainly including alkaline Electrolysis of water operated at low temperature, PEM (polymer electrolyte membrane) Electrolysis of water and solid oxide Electrolysis of water operated at high temperature. Alkaline Electrolysis of water and PEM Electrolysis of water are carried out at a low temperature below 100 ℃. Among them, alkaline Electrolysis of water can increase the battery area and has been widely commercialized; The characteristic of PEM Electrolysis of water is that it can pass the current with high current density, so it can build a compact system and obtain high-purity hydrogen. On the other hand, solid oxide Electrolysis of water can electrolyze Electrolysed water vapor at a high temperature above 500 ℃, which is characterized by high electrolysis efficiency because it can use heat energy to compensate part of the energy needed for electrolysis. This will be discussed in detail in the next section.
2.2 Principle and characteristics of solid oxide Electrolysis of water
&Nbsp& Nbsp& Nbsp; In principle, the operation mode of solid oxide Electrolysis of water can vary between endothermic/thermal neutral/exothermic according to the electrolysis conditions, as shown in Figure 1. The horizontal axis represents current density, while the vertical axis represents heat, with a current density of 0A/cm2 as the boundary. The left side is the electrolysis reaction zone, and the right side is the fuel cell reaction zone. Electrolytic reaction is essentially a Endothermic reaction, but joule heat will be generated when current flows through the Electrolytic cell. At the same time, when the balance between reaction heat and Joule heat changes, the operating mode also changes. Due to insufficient heat energy in the endothermic operation mode, if high-temperature waste heat can be utilized, higher electrolysis efficiency can be achieved. On the other hand, the exothermic operation mode can provide sufficient heat energy without the need for external heat supply. Conversely, if there is excess heat, it will lead to a decrease in efficiency.
Since the balance between Joule heat and reaction heat generated when the current flows through the Electrolytic cell will change, the endothermic/thermal neutral/exothermic operation mode can be changed according to the operation method and environmental conditions.
The electrochemical cell used for solid oxide Electrolysis of water (hereinafter referred to as“ solid oxide Electrolysis of water cell”) mainly uses solid oxide as electrolyte and forms electrodes on both sides: hydrogen generation electrode (cathode) and oxygen generation electrode (anode).
Solid oxide electrolytes usually have the function of conducting oxygen ions and are an airtight and dense structure, commonly used materials such as stable zirconia. The cathode has the function of dissociating water molecules to generate hydrogen molecules and conducting electrons and ions, and has a porous structure for diffusing water to the reaction Active site and for moving the generated hydrogen. The cathode material is a mixture of metal components responsible for catalytic activity and electron conduction, such as nickel (Ni), and electrolyte components responsible for ion conduction. The anode extracts electrons from the oxide ions moved from the cathode through the electrolyte to generate oxygen molecules and conduct electrons and ions, and has a porous structure for the movement of oxygen generated at the reaction Active site. Anode materials mainly use metal oxides with catalytic activity and electron and ion conductivity.
As for the shape, the Electrolytic cell used for alkaline Electrolysis of water and PEM Electrolysis of water is of flat plate type, while the solid oxide Electrolysis of water cell can be made into any shape such as flat plate type, cylindrical type, cylindrical flat plate type, honeycomb type, etc. On the other hand, since it is difficult to increase the effective reaction area of each Electrolytic cell to 1000 cm2 or more, it is necessary to increase the reaction area by combining the stacking and modularization of multiple Electrolytic cell< Br/>
2.4 Hydrogen production system using solid-oxide water electrolysis
Since the hydrogen production using solid oxide Electrolysis of water is Electrolysed water steam at a high temperature of 600~1000 ℃, the water steam needs to be heated. On the other hand, the hydrogen and oxygen generated by solid oxide Electrolysis of water have extremely high heat energy, so it is necessary to cool the generated hydrogen and oxygen below room temperature for storage and utilization. By exchanging heat with low-temperature water or steam, this thermal energy can be recovered and reused for steam generation and overheating, achieving energy recycling and improving system efficiency.
&Nbsp& Nbsp& Nbsp; Figure 3 shows a conceptual diagram of the system process. The system mainly consists of pure water manufacturing equipment, boilers, heat exchangers, preheaters, electrolysis modules, coolers, gas-liquid separators, rectifiers, etc. Due to the large temperature difference, the optimization design of the number of heat exchangers and heat recovery efficiency is crucial for improving system efficiency.
Hydrogen and oxygen generated by solid oxide Electrolysis of water maintain extremely high heat energy, which can be recovered through heat exchange and used for the generation and overheating of water vapor, so that the discharged energy can be reused to improve system efficiency.
3.1 Low cost and large-scale construction of Electrolytic cell materials
So far, Toshiba Energy System has been developing electrode materials and Electrolytic cell structures suitable for high-temperature steam electrolysis, which can conduct steam electrolysis efficiently and maintain its performance for a long time. In the uation of small Electrolytic cell used for performance uation, the initial electrolytic performance of current density over 0.8A/cm2 was achieved under the operating conditions of 700 ℃ and 1.3V (hot neutral point) of electrolytic voltage; In addition, in the continuous operation (fixed temperature and current density) using the Electrolytic cell, the voltage rise rate was successfully suppressed below 0.3% per 1000 hours. If a 10% voltage rise rate is used as the design operating life, the expected life is about 4-5 years.
On the other hand, considering the practicality, it is necessary to reduce the cost of Electrolytic cell. The factors that affect cost include the constituent materials, the amount of material used, manufacturing conditions, dimensions, and output. This time, Toshiba Energy System changed some of the components of its high durability Electrolytic cell into cheaper materials, trial produced an improved Electrolytic cell, and compared its performance with the traditional Electrolytic cell.
Figure 4 shows the comparison of the initial electrolytic performance (current voltage characteristics at 700 ℃ operating temperature) of the trial produced Electrolytic cell. The electrolytic characteristics of the traditional specification and the improved specification are compared in a small Electrolytic cell. The results show that when the electrolytic voltage is 1.3V, the current density of the traditional specification is 1.0A/cm2, and the improved specification is slightly lower, 0.9A/cm2. The performance is almost the same. The change of materials has little effect on the initial electrolytic performance of the Electrolytic cell. In addition, for batteries with improved specifications, a size of 110mm× was trial produced as shown in Figure 4; 110mm large Electrolytic cell.
Comparing the initial electrolytic performance of large Electrolytic cell with that of small Electrolytic cell, the results show that when the electrolytic voltage is 1.3V, the current density of small Electrolytic cell is 0.9A/cm2, while that of large Electrolytic cell is 0. 7A/cm2, performance has decreased. It is speculated that the factors leading to performance degradation are the contact resistance between the battery and the power supply components, as well as the current distribution and flow distribution within the electrode surface. Through this trial production of large Electrolytic cell, Toshiba Energy System is promoting the optimization of manufacturing process and Electrolytic cell specifications, so as to achieve a yield rate of more than 90%. In the future, it will strive to reduce costs while improving performance and yield.
3.2 uation of thermal and mass balance of hydrogen production system
According to 2The equipment configuration of the hydrogen production system used in the solid oxide Electrolysis of water described in Section. 4 has produced a process flow diagram of the electrolysis system with a power capacity of 500kW. At the same time, the heat and mass balance calculation was carried out, and the unit consumption of hydrogen production of the whole system and the power consumption of system components (BOP: Balance of Plant) other than the Electrolytic cell module, such as water vapor generation, heat exchange and gas-liquid separation, were uated.
Table 1 shows the power consumption details of the 500kW level electrolysis system. In the 500kW electrolysis system, the rated electrolytic power of the Electrolytic cell is assumed to be 400kW, the fluid composition and flow at that time are set, and the heat and mass balance is calculated. For 400kW of electrolytic power, the power required for BOP is approximately 89kW, with the electric boiler responsible for generating water vapor having the highest power consumption, which requires approximately 80kW. If high-temperature steam can be supplied from outside, electric boilers, water supply pumps, pure water manufacturing equipment, etc. can be removed, which is expected to reduce the unit energy consumption of hydrogen production.
In addition, Figure 5 shows the results of the heat and mass balance study assuming that the system is partially operational. The horizontal axis is the electrolytic power (rated value is 100%) supplied to the Electrolytic cell, and the vertical axis is the required power and hydrogen production unit consumption (converted value). As a system operating condition, the fluid composition and flow at the inlet side remain constant under rated operating conditions, only changing the electrolytic power supplied to the Electrolytic cell.
In order to understand the impact of partial load operation on BOP power consumption, this study does not consider the heat absorption, heat release of the Electrolytic cell and thermal compensation to the module caused by the change of electrolytic power. It is assumed that the electrolytic power supplied to the electrolytic module is constant. It can be seen from Figure 5 that even if the electrolytic power supplied to the Electrolytic cell changes, the power consumption of the BOP does not change significantly, and always remains at a constant value of about 90kW. In addition, when converted to hydrogen production unit consumption, as the load rate decreases, the unit consumption increases, and the unit consumption significantly increases when the load rate is below 50%. This is because the power consumption of BOP does not significantly change during partial load operation.
In the future, Toshiba Energy Systems will continue to research system operating conditions and methods that can address various issues, such as reduced responsiveness due to significant differences in time constants when changing fluid conditions based on power fluctuations; Alternatively, the pressure loss and flow distribution of heat exchangers and stacks may fluctuate significantly, requiring the system to adapt to broader operating conditions.
In the process of accelerating carbon neutrality, countries around the world are actively developing and demonstrating the use of renewable energy and nuclear power to produce CO2 free hydrogen gas, and further producing fossil fuel alternatives such as hydrocarbons and ammonia. However, considering global economic activities and regional situations, it is believed that this new trend towards a sustainable development society will take a considerable amount of time. Therefore, it is necessary to gradually reduce the proportion of fossil fuel power generation in the energy structure while fully tapping into the potential of fossil fuels. One of them is the utilization of waste heat. In thermal power generation and nuclear power, the Energy conversion efficiency of combined cycle thermal power generation is more than 55%, while that of nuclear power is about 33%. The energy that cannot be used for power generation is discharged in the form of heat energy. If these thermal energy can be effectively utilized, it is possible to reduce the CO2 emissions per unit of fuel consumption.
&Nbsp& Nbsp& Nbsp; In addition, many areas in the industrial system also use fossil fuels and emit CO2. For example, the carbon dioxide emissions in the steel industry are very high, and there is a great demand for carbon reduction. During the ironmaking process, using coke to reduce iron ore and extract iron will generate CO2. Japan is developing a hydrogen reduction ironmaking technology that replaces coke with hydrogen, which is a new technology for the future. Not only the steel industry, but various other industries are currently striving to improve energy efficiency and reduce CO2 emissions by effectively utilizing waste heat.
Effective utilization of waste heat is crucial for reducing CO2 emissions, and one of the hydrogen production methods using thermal energy is high-temperature steam electrolysis. Electrolysis of water can produce CO2 free hydrogen through renewable energy. In particular, the difference between high-temperature steam electrolysis and other Electrolysis of water methods is that water is electrolyzed in the form of steam, and heat energy can be used as part of the energy needed for electrolysis. Therefore, both the waste heat from existing power generation and industrial systems can be utilized, and the hydrogen produced can also be utilized in the process. It is generally believed that Electrolysis of water is highly compatible with these large systems. In addition, because the Electrolysis of water tank using high-temperature steam electrolysis technology operates at high temperature, the larger the scale, the higher the efficiency, and is very suitable for large-scale hydrogen production.
Therefore, Toshiba Energy Systems Co., Ltd. is promoting a series of developments from materials to system design and production to achieve hydrogen production systems utilizing high-temperature steam electrolysis technology. This paper introduces the characteristics of high temperature steam electrolysis technology using solid oxide Electrolytic cell (SOEC) (hereinafter referred to as solid oxide Electrolysis of water technology) and the development of Toshiba energy system. In addition, this article also describes an uation application that
Solid-oxide water electrolysis technology
2.1 Comparison of various Electrolysis of water methods
&Nbsp& Nbsp& Nbsp; Electrolysis of water methods can be divided into several types according to the type of electrolyte and operating temperature, mainly including alkaline Electrolysis of water operated at low temperature, PEM (polymer electrolyte membrane) Electrolysis of water and solid oxide Electrolysis of water operated at high temperature. Alkaline Electrolysis of water and PEM Electrolysis of water are carried out at a low temperature below 100 ℃. Among them, alkaline Electrolysis of water can increase the battery area and has been widely commercialized; The characteristic of PEM Electrolysis of water is that it can pass the current with high current density, so it can build a compact system and obtain high-purity hydrogen. On the other hand, solid oxide Electrolysis of water can electrolyze Electrolysed water vapor at a high temperature above 500 ℃, which is characterized by high electrolysis efficiency because it can use heat energy to compensate part of the energy needed for electrolysis. This will be discussed in detail in the next section.
2.2 Principle and characteristics of solid oxide Electrolysis of water
&Nbsp& Nbsp& Nbsp; In principle, the operation mode of solid oxide Electrolysis of water can vary between endothermic/thermal neutral/exothermic according to the electrolysis conditions, as shown in Figure 1. The horizontal axis represents current density, while the vertical axis represents heat, with a current density of 0A/cm2 as the boundary. The left side is the electrolysis reaction zone, and the right side is the fuel cell reaction zone. Electrolytic reaction is essentially a Endothermic reaction, but joule heat will be generated when current flows through the Electrolytic cell. At the same time, when the balance between reaction heat and Joule heat changes, the operating mode also changes. Due to insufficient heat energy in the endothermic operation mode, if high-temperature waste heat can be utilized, higher electrolysis efficiency can be achieved. On the other hand, the exothermic operation mode can provide sufficient heat energy without the need for external heat supply. Conversely, if there is excess heat, it will lead to a decrease in efficiency.
Since the balance between Joule heat and reaction heat generated when the current flows through the Electrolytic cell will change, the endothermic/thermal neutral/exothermic operation mode can be changed according to the operation method and environmental conditions.
The electrochemical cell used for solid oxide Electrolysis of water (hereinafter referred to as“ solid oxide Electrolysis of water cell”) mainly uses solid oxide as electrolyte and forms electrodes on both sides: hydrogen generation electrode (cathode) and oxygen generation electrode (anode).
Solid oxide electrolytes usually have the function of conducting oxygen ions and are an airtight and dense structure, commonly used materials such as stable zirconia. The cathode has the function of dissociating water molecules to generate hydrogen molecules and conducting electrons and ions, and has a porous structure for diffusing water to the reaction Active site and for moving the generated hydrogen. The cathode material is a mixture of metal components responsible for catalytic activity and electron conduction, such as nickel (Ni), and electrolyte components responsible for ion conduction. The anode extracts electrons from the oxide ions moved from the cathode through the electrolyte to generate oxygen molecules and conduct electrons and ions, and has a porous structure for the movement of oxygen generated at the reaction Active site. Anode materials mainly use metal oxides with catalytic activity and electron and ion conductivity.
As for the shape, the Electrolytic cell used for alkaline Electrolysis of water and PEM Electrolysis of water is of flat plate type, while the solid oxide Electrolysis of water cell can be made into any shape such as flat plate type, cylindrical type, cylindrical flat plate type, honeycomb type, etc. On the other hand, since it is difficult to increase the effective reaction area of each Electrolytic cell to 1000 cm2 or more, it is necessary to increase the reaction area by combining the stacking and modularization of multiple Electrolytic cell< Br/>
2.4 Hydrogen production system using solid-oxide water electrolysis
Since the hydrogen production using solid oxide Electrolysis of water is Electrolysed water steam at a high temperature of 600~1000 ℃, the water steam needs to be heated. On the other hand, the hydrogen and oxygen generated by solid oxide Electrolysis of water have extremely high heat energy, so it is necessary to cool the generated hydrogen and oxygen below room temperature for storage and utilization. By exchanging heat with low-temperature water or steam, this thermal energy can be recovered and reused for steam generation and overheating, achieving energy recycling and improving system efficiency.
&Nbsp& Nbsp& Nbsp; Figure 3 shows a conceptual diagram of the system process. The system mainly consists of pure water manufacturing equipment, boilers, heat exchangers, preheaters, electrolysis modules, coolers, gas-liquid separators, rectifiers, etc. Due to the large temperature difference, the optimization design of the number of heat exchangers and heat recovery efficiency is crucial for improving system efficiency.
Hydrogen and oxygen generated by solid oxide Electrolysis of water maintain extremely high heat energy, which can be recovered through heat exchange and used for the generation and overheating of water vapor, so that the discharged energy can be reused to improve system efficiency.
3.1 Low cost and large-scale construction of Electrolytic cell materials
So far, Toshiba Energy System has been developing electrode materials and Electrolytic cell structures suitable for high-temperature steam electrolysis, which can conduct steam electrolysis efficiently and maintain its performance for a long time. In the uation of small Electrolytic cell used for performance uation, the initial electrolytic performance of current density over 0.8A/cm2 was achieved under the operating conditions of 700 ℃ and 1.3V (hot neutral point) of electrolytic voltage; In addition, in the continuous operation (fixed temperature and current density) using the Electrolytic cell, the voltage rise rate was successfully suppressed below 0.3% per 1000 hours. If a 10% voltage rise rate is used as the design operating life, the expected life is about 4-5 years.
On the other hand, considering the practicality, it is necessary to reduce the cost of Electrolytic cell. The factors that affect cost include the constituent materials, the amount of material used, manufacturing conditions, dimensions, and output. This time, Toshiba Energy System changed some of the components of its high durability Electrolytic cell into cheaper materials, trial produced an improved Electrolytic cell, and compared its performance with the traditional Electrolytic cell.
Figure 4 shows the comparison of the initial electrolytic performance (current voltage characteristics at 700 ℃ operating temperature) of the trial produced Electrolytic cell. The electrolytic characteristics of the traditional specification and the improved specification are compared in a small Electrolytic cell. The results show that when the electrolytic voltage is 1.3V, the current density of the traditional specification is 1.0A/cm2, and the improved specification is slightly lower, 0.9A/cm2. The performance is almost the same. The change of materials has little effect on the initial electrolytic performance of the Electrolytic cell. In addition, for batteries with improved specifications, a size of 110mm× was trial produced as shown in Figure 4; 110mm large Electrolytic cell.
Comparing the initial electrolytic performance of large Electrolytic cell with that of small Electrolytic cell, the results show that when the electrolytic voltage is 1.3V, the current density of small Electrolytic cell is 0.9A/cm2, while that of large Electrolytic cell is 0. 7A/cm2, performance has decreased. It is speculated that the factors leading to performance degradation are the contact resistance between the battery and the power supply components, as well as the current distribution and flow distribution within the electrode surface. Through this trial production of large Electrolytic cell, Toshiba Energy System is promoting the optimization of manufacturing process and Electrolytic cell specifications, so as to achieve a yield rate of more than 90%. In the future, it will strive to reduce costs while improving performance and yield.
3.2 uation of thermal and mass balance of hydrogen production system
According to 2The equipment configuration of the hydrogen production system used in the solid oxide Electrolysis of water described in Section. 4 has produced a process flow diagram of the electrolysis system with a power capacity of 500kW. At the same time, the heat and mass balance calculation was carried out, and the unit consumption of hydrogen production of the whole system and the power consumption of system components (BOP: Balance of Plant) other than the Electrolytic cell module, such as water vapor generation, heat exchange and gas-liquid separation, were uated.
Table 1 shows the power consumption details of the 500kW level electrolysis system. In the 500kW electrolysis system, the rated electrolytic power of the Electrolytic cell is assumed to be 400kW, the fluid composition and flow at that time are set, and the heat and mass balance is calculated. For 400kW of electrolytic power, the power required for BOP is approximately 89kW, with the electric boiler responsible for generating water vapor having the highest power consumption, which requires approximately 80kW. If high-temperature steam can be supplied from outside, electric boilers, water supply pumps, pure water manufacturing equipment, etc. can be removed, which is expected to reduce the unit energy consumption of hydrogen production.
In addition, Figure 5 shows the results of the heat and mass balance study assuming that the system is partially operational. The horizontal axis is the electrolytic power (rated value is 100%) supplied to the Electrolytic cell, and the vertical axis is the required power and hydrogen production unit consumption (converted value). As a system operating condition, the fluid composition and flow at the inlet side remain constant under rated operating conditions, only changing the electrolytic power supplied to the Electrolytic cell.
In order to understand the impact of partial load operation on BOP power consumption, this study does not consider the heat absorption, heat release of the Electrolytic cell and thermal compensation to the module caused by the change of electrolytic power. It is assumed that the electrolytic power supplied to the electrolytic module is constant. It can be seen from Figure 5 that even if the electrolytic power supplied to the Electrolytic cell changes, the power consumption of the BOP does not change significantly, and always remains at a constant value of about 90kW. In addition, when converted to hydrogen production unit consumption, as the load rate decreases, the unit consumption increases, and the unit consumption significantly increases when the load rate is below 50%. This is because the power consumption of BOP does not significantly change during partial load operation.
In the future, Toshiba Energy Systems will continue to research system operating conditions and methods that can address various issues, such as reduced responsiveness due to significant differences in time constants when changing fluid conditions based on power fluctuations; Alternatively, the pressure loss and flow distribution of heat exchangers and stacks may fluctuate significantly, requiring the system to adapt to broader operating conditions.
