A two-stage SOFC stack system with carbon capture

Abstract

The present invention discloses a two-stage SOFC stack system with carbon capture, belonging to the field of solid oxide fuel cell technology. The system includes a two-stage SOFC stack, a reformer / burner, and an ammonia reactor. The reformer / burner is used to pre-reform the gas obtained by mixing natural gas and water vapor after heat exchange, providing fuel for the anode of the first-stage SOFC stack. The air after heat exchange is passed into the cathode of the first-stage SOFC stack. After the first-stage SOFC stack generates electricity through electrochemical reaction, the anode fuel tail gas is condensed through a heat exchanger. The liquid water produced can be recycled to produce water vapor. The remaining gas is decarbonized through an ammonia reactor and then, together with part of the ammonia, is passed into the anode of the second-stage SOFC stack as fuel after heat exchange. The cathode tail gas of the first-stage SOFC stack is passed into the cathode of the second-stage SOFC stack after heat exchange. The present invention can effectively improve fuel utilization and power generation efficiency, and achieve carbon capture and self-heating maintenance.

Description

Technical Field

[0001] The present invention relates to the technical field of solid oxide fuel cells, and in particular to a two-stage SOFC stack system with carbon capture. Background Art

[0002] A solid oxide fuel cell (SOFC) is an all-solid-state electrochemical power generation device that can directly convert the chemical energy stored in a fuel and oxidant into electricity at high temperatures in an efficient and environmentally friendly manner. Compared to several common fuel cells (such as proton exchange membrane fuel cells), SOFCs utilize an all-solid-state battery structure, offering the highest power generation efficiency and outstanding energy conversion efficiency advantages. Natural gas is a common fuel for SOFC systems. China has a large and well-established natural gas pipeline network, making it a suitable fuel source for distributed power generation in SOFC systems. Typically, natural gas is heated and reformed with water vapor in an external reformer before being fed into the SOFC stack. However, limitations in stack manufacturing processes and electrochemical properties result in relatively low system fuel utilization, which in turn reduces SOFC system power generation efficiency. By adopting a series-connected multiple stack configuration, where the fuel from the previous stack is processed before being fed into the next stack, the system fuel utilization and power generation efficiency can theoretically be improved. However, due to the structural constraints of various stack systems, improvements in fuel utilization and power generation efficiency are limited. For example, Chinese patent publication number CN216773297U discloses a burner-free solid oxide fuel cell system, comprising a multi-stage stack structure connected in series. A fuel separator extracts hydrogen and carbon monoxide from the anode tail gas of the previous SOFC stack and passes them into the anode of the next SOFC stack for electrochemical reaction. Its disadvantages are: (1) the air heater and fuel heater consume additional electricity, reducing the overall power generation efficiency of the system; (2) the fuel separator process is complex and requires additional pressurization, further increasing the overall energy consumption and complexity of the system. In addition, the exhaust gas of the natural gas-fueled SOFC system contains a large amount of carbon dioxide. In order to achieve clean utilization of the system, effective and low-energy carbon capture of carbon dioxide is required. Summary of the Invention

[0003] The technical problem solved by the present invention is how to effectively improve the fuel utilization rate and power generation efficiency of the SOFC system and to effectively and low-energy-consumption capture the carbon dioxide generated during the operation of the SOFC system.

[0004] The technical solution adopted by the present invention is as follows: a two-stage SOFC stack system with carbon capture, comprising a heat exchanger I, a mixer, a heat exchanger II, a reformer / burner, a first-stage SOFC stack, a condenser, an ammonia reactor I, a heat exchanger III, a second-stage SOFC stack, a heat exchanger IV and an ammonia reactor II; natural gas is introduced from the low-temperature side inlet of the heat exchanger I, and the low-temperature side outlet of the heat exchanger I is connected to the inlet of the mixer; deionized water is introduced into the heat exchanger II, the low-temperature outlet of the heat exchanger II is connected to the inlet of the mixer, and the outlet of the mixer is connected to the reforming inlet a of the reformer / burner; the fuel inlet a of the first-stage SOFC stack is connected to the reforming outlet b of the reformer / burner, the fuel outlet b of the first-stage SOFC stack is connected to the high-temperature side inlet of the heat exchanger II, and the high-temperature side outlet of the heat exchanger II is connected to the inlet of the condenser; the gas outlet of the condenser is connected to the inlet of the ammonia reactor I, and the outlet of the ammonia reactor I is connected to The low-temperature gas side inlet of the heat exchanger III is connected, and the low-temperature side outlet of the heat exchanger III is connected to the fuel inlet a of the second-stage SOFC stack; air passes into the heat exchanger IV, and the low-temperature side outlet of the heat exchanger IV is connected to the cathode inlet c of the first-stage SOFC stack; the cathode side outlet d of the first-stage SOFC stack is connected to the high-temperature inlet of the heat exchanger III, and the high-temperature outlet of the heat exchanger III is connected to the cathode side inlet c of the second-stage SOFC stack; the anode side outlet b of the second-stage SOFC stack is connected to the inlet e of the reformer / burner, and the cathode side outlet d of the second-stage SOFC stack is connected to the inlet d of the reformer / burner; the combustion side outlet f of the reformer / burner is connected to the high-temperature inlet of the heat exchanger IV; the high-temperature side outlet of the heat exchanger IV is connected to the high-temperature side inlet of the heat exchanger I, and the high-temperature side outlet of the heat exchanger I is connected to the inlet of the ammonia reactor II, and the outlet of the ammonia reactor II discharges exhaust gas.

[0005] As a further improvement of the present invention, the system further includes a compressor I and a compressor II, wherein one end of the compressor I serves as a natural gas inlet, and the other end is connected to the low-temperature side inlet of the heat exchanger I; one end of the compressor II serves as an air inlet, and the other end is connected to the low-temperature side inlet of the heat exchanger II.

[0006] As a further improvement of the present invention, the system also includes a proportional valve, the outlet of the compressor I is connected to the inlet of the proportional valve, and the outlet of the proportional valve is connected to the combustion side inlet c of the reformer / burner.

[0007] As a further improvement of the present invention, the system further comprises a constant flow pump, one end of which serves as a deionized water inlet and the other end of which is connected to the inlet of the heat exchanger II.

[0008] As a further improvement of the present invention, the system further includes ultrapure water equipment, the outlet of the condenser is connected to the inlet of the ultrapure water equipment, and the outlet of the ultrapure water equipment is connected to the inlet of the constant flow pump.

[0009] As a further improvement of the present invention, the combustion chamber in the reformer / burner uses catalytic combustion to ensure the stability of the combustion reaction.

[0010] As a further improvement of the present invention, the second-stage SOFC stack has an ammonia cracking function, and utilizes the ammonia from the ammonia reactor I as fuel.

[0011] As a further improvement of the present invention, the heat exchanger I, the heat exchanger II, the heat exchanger III and the heat exchanger IV are all printed circuit board type heat exchangers.

[0012] Compared with the prior art, the present invention has the following advantages:

[0013] (1) This system adopts a two-stage SOFC stack series mode. Through reasonable configuration, the system energy efficiency can be maximized and the system complexity can be reduced. The anode tail gas of the first-stage SOFC stack is dehydrated by condensation and the carbon dioxide is removed by ammonia solution before entering the anode side of the second-stage stack. Due to the increase in the available fuel concentration at the inlet of the second-stage SOFC stack, the voltage drop of the second-stage SOFC stack can be effectively reduced, the power of the second-stage SOFC stack can be increased, and the overall power generation efficiency of the system can be improved.

[0014] (2) This system absorbs carbon dioxide from the anode tail gas of the first-stage SOFC stack through an ammonia solution. Ammonia absorption has the characteristics of reducing decarbonization energy consumption, being non-corrosive, having low absorbent cost, and not producing organic products. In addition, part of the ammonia gas escaping from the solution can be used as fuel to supplement the second-stage SOFC stack. Its low-temperature treatment feature facilitates the condensation and dehydration process, and can also significantly reduce the thermal energy consumption of the system, thereby maximizing the energy efficiency and long-term stability of the system.

[0015] (3) This system introduces additional fuel into the combustion chamber of the reformer / burner through a proportional valve to maintain the system's self-heating balance. At the same time, through reasonable energy flow distribution, it maximizes the utilization of system heat energy and reduces system complexity;

[0016] (4) This system condenses the anode tail gas of the first-stage SOFC stack and then purifies it to meet the water-to-carbon ratio required by the system, solving the problem of the source of deionized water in the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] Figure 1 This is a system schematic diagram of the SOFC two-stage stack system with carbon capture according to the present invention;

[0019] In the figure, 1-compressor I, 2-heat exchanger I, 3-mixer, 4-constant flow pump, 5-heat exchanger II, 6-proportional valve, 7-reformer / burner, 8-first stage SOFC stack, 9-condenser, 10-ammonia reactor I, 11-heat exchanger III, 12-second stage SOFC stack, 13-heat exchanger IV, 14-compressor II, 15-ammonia reactor II, 16-ultrapure water equipment. DETAILED DESCRIPTION

[0020] like Figure 1As shown, the SOFC two-stage stack system with carbon capture of the present invention includes a compressor I1, a heat exchanger I2, a mixer 3, a constant flow pump 4, a heat exchanger II5, a proportional valve 6, a reformer / burner 7, a first-stage SOFC stack 8, a condenser 9, an ammonia reactor I10, a heat exchanger III11, a second-stage SOFC stack 12, a heat exchanger IV13, a compressor II14, an ammonia reactor II15 and an ultrapure water device 16. Among them, the outlet of compressor I1 is connected to the low-temperature side inlet of heat exchanger I2, and the low-temperature side outlet of heat exchanger I2 is connected to the inlet of mixer 3; the outlet of constant flow pump 4 is connected to the low-temperature side inlet of heat exchanger II5, and the low-temperature side outlet of heat exchanger II5 is connected to the inlet of mixer 3, and the outlet of mixer 3 is connected to the reforming side inlet a of reformer / heat exchanger 7; the fuel side inlet a of the first-stage SOFC stack 8 is connected to the reforming side outlet b of reformer / burner 7, and the fuel side outlet b of the first-stage SOFC stack 8 is connected to the high-temperature side inlet of heat exchanger II5, and the high-temperature side outlet of heat exchanger II5 is connected to the inlet of condenser 9. The gas outlet of the condenser 9 is connected to the inlet of the ammonia reactor I10, the outlet of the ammonia reactor I10 is connected to the low-temperature gas side inlet of the heat exchanger III11, and the low-temperature side outlet of the heat exchanger III11 is connected to the fuel side inlet a of the second-stage SOFC stack 12; the outlet air of the compressor II14 is connected to the low-temperature side inlet of the heat exchanger IV13, and the low-temperature side outlet of the heat exchanger IV13 is connected to the cathode side inlet c of the first-stage SOFC stack 8; the cathode side outlet d of the first-stage SOFC stack 8 is connected to the high-temperature side inlet of the heat exchanger III11, and the high-temperature side outlet of the heat exchanger III11 is connected to the high-temperature side inlet of the heat exchanger III11. The high-temperature side outlet is connected to the cathode side inlet c of the second-stage SOFC stack 12; the anode side outlet b of the second-stage SOFC stack 12 is connected to the inlet e of the reformer / combustor 7, and the cathode side outlet d of the second-stage SOFC stack 12 is connected to the inlet d of the reformer / combustor 7; the combustion side outlet f of the reformer / combustor 7 is connected to the high-temperature side inlet of heat exchanger IV13; the high-temperature side outlet of heat exchanger IV13 is connected to the high-temperature side inlet of heat exchanger I2, and the high-temperature outlet side of heat exchanger I2 is connected to the inlet side of ammonia reactor II15, and the outlet side of ammonia reactor II15 discharges exhaust gas. The outlet of compressor I1 is connected to the inlet of proportional valve 6, and the outlet of proportional valve 6 is connected to the combustion side inlet c of reformer / combustor 7. The outlet of condenser 9 is connected to the inlet and of ultrapure water equipment 16, and the outlet of ultrapure water equipment 16 is connected to the inlet of constant flow pump 4.

[0021] The system operates as follows: Compressor I1 increases the pressure of desulfurized natural gas and feeds it into the system pipeline. The desulfurized natural gas is then heated by heat exchanger I2 and enters mixer 3. Deionized water is pressurized by constant flow pump 4 and then enters heat exchanger II5 for heating and vaporization. It then enters mixer 3 and mixes evenly with the desulfurized natural gas. The mixed gas enters the reforming side a inlet of the reformer / combustor 7, where it absorbs combustion heat from the reformer / combustor 7 for partial pre-reforming. The reformed gas enters the anode side a of the first-stage SOFC stack 8. Compressor II14 increases the pressure of air and feeds it into the system pipeline. The air is then heated by heat exchanger IV13 and enters the cathode side c of the first-stage SOFC stack 8. The heated fuel and air enter the first-stage SOFC stack 8, reacting and outputting electrical energy. In the first-stage SOFC stack 8, the high-temperature fuel exhaust gas from the anode side (b) that has not fully reacted enters heat exchanger II5 to reduce its temperature before entering condenser 9. In condenser 9, water is removed from the anode exhaust gas. The removed water then enters ultrapure water equipment 16 to reduce its conductivity and is then recycled as the system's deionized water source. The remaining gas in condenser 9 enters ammonia reactor I10, where ammonia reacts with and removes carbon dioxide from the remaining gas. The remaining gas, primarily consisting of hydrogen, carbon monoxide, and some ammonia, enters heat exchanger III11 to be heated to the anode inlet temperature of the second-stage SOFC stack 12. Once the carbon dioxide enters the ammonia reactor, the ammonia and carbon dioxide undergo a chemical reaction, primarily as follows: NH3 + H2O + CO2 → NH4HCO3. The primary product, ammonium bicarbonate, can be used as fertilizer, a fermentation agent, and in related fields. The ammonia absorption method offers advantages such as reduced decarbonization energy consumption, no corrosion issues, low absorbent cost, and no organic product generation. Furthermore, some of the ammonia that escapes from the solution can be used as fuel for the second-stage SOFC stack 12. The second-stage SOFC stack 12 features ammonia cracking capabilities, utilizing some of the ammonia that escapes from the ammonia reactor I10, thereby improving the second-stage stack's power generation efficiency. The catalyst at the stack's anode catalyzes the cracking of ammonia, with the reaction mechanism being: 2NH₃ → N₂ + 3H₂. The cathode exhaust from the first-stage SOFC stack 8 serves as a high-temperature heat source and enters the heat exchanger III11, where it is cooled to the cathode inlet temperature of the second-stage SOFC stack 12. The fuel and air enter the second-stage SOFC stack, reacting to generate electricity. Unreacted high-temperature exhaust from the anode side b and cathode side d of the second-stage SOFC stack 12 enter the combustion chamber of the reformer / combustor 7. Since the concentration of the fuel decreases after passing through the second-stage SOFC stack 12, in order to maintain the heat balance of the system, part of the desulfurized natural gas is pressurized by the compressor I1 and the required flow is adjusted by the proportional valve 6 before entering the combustion chamber in the reformer / burner 7 for combustion.

[0022] By adjusting proportional valve 6, the flow rate of desulfurized natural gas entering the combustion chamber is varied, addressing low fuel concentration and insufficient system heat in the combustion chamber, thereby providing sufficient heat for self-heating. The combustion chamber in the reformer / burner 7 utilizes catalytic combustion, i.e., by adding a three-way catalyst to the combustion chamber, to address the low combustible gas content in the anode tail gas, which makes direct combustion difficult, thereby improving the stability of the combustion reaction.

[0023] The ultrapure water equipment 16 in the system purifies the water removed from the condenser 9, reducing its ionic conductivity before recycling it, thus resolving the problem of sourcing the deionized water required by the system. The exhaust gas from the reformer / burner 7 passes through heat exchangers IV13 and I2, providing heat to the air and desulfurized natural gas and lowering its temperature. It then enters the ammonia reactor II15, where carbon dioxide is further removed from the gas. Finally, the exhaust gas is discharged from the system. The heat exchangers required in the system—including heat exchangers I2, II5, III11, and IV15—are all printed circuit board-type heat exchangers, which improve heat exchange efficiency and reduce pressure drop.

[0024] The feasibility and benefits of the present invention are described below through specific examples.

[0025] Table 1 shows the initial operating conditions of a single-stack SOFC system fueled by natural gas, Table 2 shows the simulation results of this system, Table 3 shows the initial operating conditions of a two-stage SOFC system fueled by natural gas and carbon capture, Table 4 shows the simulation results of this system, Table 5 shows the initial operating conditions of the system described in patent CN216773297U, and Table 6 shows the simulation results of this system.

[0026] Table 1. Initial operating conditions of a single-stack SOFC system fueled by natural gas.

[0027]

[0028] Table 2. Simulation results of a single-stack SOFC system fueled by natural gas

[0029]

[0030] Table 3. Initial operating conditions of a natural gas-fueled SOFC system with a two-stage stack and carbon capture.

[0031]

[0032]

[0033] Table 4. Simulation results of a natural gas-fueled SOFC system with a two-stage stack and carbon capture

[0034]

[0035] Table 5. Initial operating conditions of the system described in patent CN216773297U

[0036] To facilitate comparison with this patent, two stacks are used for simulation based on the system framework, and it is assumed that there is a fuel separator that can completely separate carbon dioxide and water from carbon monoxide and hydrogen at high temperature.

[0037]

[0038] Table 6. Simulation results of the system described in patent CN216773297U

[0039]

[0040]

[0041] Simulation results demonstrate the superiority of this system in terms of carbon emissions and power generation efficiency. Compared to a single-stack SOFC system fueled by natural gas, this system reduces carbon emissions by 16.8 g / min and increases AC power generation efficiency by 11.9%. Compared to the system described in patent CN216773297U, this system reduces carbon emissions by 0.2 g / min and increases AC power generation efficiency by 85.9%. Furthermore, the two-stage SOFC system with carbon capture can produce 20.9 g / min of deionized water, meeting the system's required water-to-carbon ratio.

[0042] The embodiments of the present invention are described in detail above with reference to the accompanying drawings, but the present invention is not limited thereto. Various changes that can be made within the scope of knowledge possessed by technicians in the relevant technical field without departing from the spirit of the present invention are all within the scope of protection of the claims of the present invention.

Claims

1. A two-stage SOFC stack system with carbon capture, characterized in that: The invention comprises a heat exchanger I (2), a mixer (3), a heat exchanger II (5), a reformer / combustor (7), a first-stage SOFC stack (8), a condenser (9), an ammonia reactor I (10), a heat exchanger III (11), a second-stage SOFC stack (12), a heat exchanger IV (13) and an ammonia reactor II (15); natural gas is introduced from the low-temperature side inlet of the heat exchanger I (2), and the low-temperature side outlet of the heat exchanger I (2) is connected to the inlet of the mixer (3); deionized water is introduced into the heat exchanger II (5), and the low-temperature outlet of the heat exchanger II (5) is connected to the inlet of the mixer (3); ) is connected to the inlet of the first-stage SOFC stack (8), the outlet of the mixer (3) is connected to the reforming inlet a of the reformer / combustor (7); the fuel inlet a of the first-stage SOFC stack (8) is connected to the reforming outlet b of the reformer / combustor (7), the fuel outlet b of the first-stage SOFC stack (8) is connected to the high-temperature side inlet of the heat exchanger II (5), the high-temperature side outlet of the heat exchanger II (5) is connected to the inlet of the condenser (9); the gas outlet of the condenser (9) is connected to the inlet of the ammonia reactor I (10), the outlet of the ammonia reactor I (10) is connected to the heat exchanger II I (11), the low-temperature gas side inlet of the heat exchanger III (11) is connected, the low-temperature side outlet of the heat exchanger III (11) is connected to the fuel inlet a of the second-stage SOFC stack (12); air is passed into the heat exchanger IV (13), the low-temperature side outlet of the heat exchanger IV (13) is connected to the cathode inlet c of the first-stage SOFC stack (8); the cathode side outlet d of the first-stage SOFC stack (8) is connected to the high-temperature inlet of the heat exchanger III (11), and the high-temperature outlet of the heat exchanger III (11) is connected to the cathode side inlet c of the second-stage SOFC stack (12); the second The anode side outlet b of the SOFC stack (12) is connected to the inlet e of the reformer / combustor (7), and the cathode side outlet d of the second-stage SOFC stack (12) is connected to the inlet d of the reformer / combustor (7); the combustion side outlet f of the reformer / combustor (7) is connected to the high-temperature inlet of the heat exchanger IV (13); the high-temperature side outlet of the heat exchanger IV (13) is connected to the high-temperature side inlet of the heat exchanger I (2), and the high-temperature side outlet of the heat exchanger I (2) is connected to the inlet of the ammonia reactor II (15), and the outlet of the ammonia reactor II (15) discharges exhaust gas.

2. The SOFC two-stage stack system with carbon capture according to claim 1, characterized in that: The invention also includes a compressor I (1) and a compressor II (14), wherein one end of the compressor I (1) serves as a natural gas inlet and the other end is connected to the low-temperature side inlet of the heat exchanger I (2); and one end of the compressor II (14) serves as an air inlet and the other end is connected to the low-temperature side inlet of the heat exchanger II (5).

3. The SOFC two-stage stack system with carbon capture according to claim 2, characterized in that: It also includes a proportional valve (6), the outlet of the compressor I (1) is connected to the inlet of the proportional valve (6), and the outlet of the proportional valve (6) is connected to the combustion side inlet c of the reformer / burner (7).

4. The SOFC two-stage stack system with carbon capture according to claim 1, characterized in that: It also includes a constant flow pump (4), one end of which serves as a deionized water inlet, and the other end of which is connected to the inlet of the heat exchanger II (5).

5. The SOFC two-stage stack system with carbon capture according to claim 4, characterized in that: It also includes an ultrapure water device (16), the outlet of the condenser (9) is connected to the inlet of the ultrapure water device (16), and the outlet of the ultrapure water device (16) is connected to the inlet of the constant flow pump (4).

6. The SOFC two-stage stack system with carbon capture according to claim 1, characterized in that: The combustion chamber in the reformer / burner (7) uses a catalytic combustion method to ensure the stability of the combustion reaction.

7. The SOFC two-stage stack system with carbon capture according to claim 1, characterized in that: The second-stage SOFC stack (12) has an ammonia cracking function and can utilize the ammonia gas from the ammonia reactor I (10) as fuel.

8. The SOFC two-stage stack system with carbon capture according to claim 1, characterized in that: The heat exchanger I (2), the heat exchanger II (5), the heat exchanger III (11) and the heat exchanger IV (13) are all printed circuit board type heat exchangers.

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