Method of operating a solid oxide fuel cell system and solid oxide fuel cell system

Abstract

The present invention relates to a method for operating a solid oxide fuel cell system (2). In this method, a fuel supply flow (FF) is supplied to a fuel cell module (4), the fuel cell module (4) discharges an exhaust fuel flow (EF), an air supply flow (AF) is supplied to the fuel cell module, and the fuel cell module (4) discharges an exhaust flow (EA). To improve the sequestration of CO2 from the exhaust gas, in this method, at least a portion of the exhaust fuel flow (EF) is further supplied to an afterburner (26), oxygen from an oxygen source (28) is supplied into the afterburner (26), the exhaust fuel flow (EF) and oxygen are burned in a stoichiometric ratio, and the combustion gas (FG) from the afterburner (26) is used in a superheater (30) to superheat the exhaust flow (EA).

Description

Technical Field

[0001] The present invention relates to a method of operating a solid oxide fuel cell (SOFC) system. The method includes providing a fuel supply stream to a fuel cell module such that the fuel cell module discharges an exhaust fuel stream, and providing an air supply stream to the fuel cell module such that the fuel cell module discharges an exhaust (or exhaust air) stream.

[0002] The present invention also relates to a solid oxide fuel cell system. The fuel cell module included in the system is configured to receive a fuel supply stream from a fuel supply line and discharge an exhaust fuel stream through an exhaust fuel line, and is configured to receive an air supply stream from an air supply line and discharge an exhaust stream through an exhaust air line.

[0003] Isolating (or separating) CO2 from the exhaust gas of a fuel cell system based on the oxidation of hydrocarbons using prior methods (e.g., amine absorption method) was complex and energy-intensive. The reason was that fuel was oxidized using clearly excessive air. Downstream of the fuel cell, the exhaust gas is mixed with air and unreacted fuel components are oxidized. As a result, in the flue gas, CO2 occupies a relatively small volume fraction together with other components (N2, H2O, etc.). Since CO2 is strongly diluted, its isolation becomes very complex, and thus, a high degree of system complexity, additional operating materials (e.g., amines), and / or relatively high energy consumption have been required. The isolation of CO2 can be performed with a loss of system efficiency (e.g., driving a pump, pressure loss, etc.), but the purity of the isolated CO2 was limited in a single-stage process.

[0004] Currently, the most common method of CO2 sequestration involves removing CO2 after an oxidation process using air (usually combustion). This often involves a fuel / air mixture with excess air (carbon capture after combustion). As a result, CO2 is present in relatively low concentrations in the combustion gases, requiring significant effort to remove. The loss of effort, cost, and efficiency was substantial. Furthermore, complete CO2 sequestration in a single stage was impossible. In addition, depending on the materials used for operation (e.g., amine absorption of combustion gases), there were concerns about the environmental impact.

[0005] Other methods include stoichiometric (or stoichiometric) oxidation using oxygen and water sequestration by downstream condensation. Because these methods are applied at high temperatures, the components are subjected to high-temperature stress. In some cases, the high temperatures are lowered by using the reaction products as a working fluid in the heat exchange process.

[0006] Patent Document 1 describes a solid oxide fuel cell (SOFC) system with carbon recovery. This system includes an afterburner, which is fluidly connected to the fuel cell module and located downstream of the outlet. The SOFC module discharges oxygen-depleted air and depleted fuel. The depleted fuel flow is led to an output manifold, where a first portion of the depleted fuel is sent to a fluidly connected afterburner, and a second portion of the depleted fuel forms a recirculation flow and is returned to the fuel inlet manifold. The afterburner also receives depleted air but is configured to promote combustion of residual oxides from the depleted air and the first portion from the depleted fuel to generate an exhaust flow, which contains CO2, nitrogen oxides (NOx), nitrogen (N2), and H2O. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] U.S. Patent Application Publication No. 2022 / 0246966 (US20220246966A1) [Overview of the project] [Problems that the invention aims to solve]

[0008] The object of the present invention is to provide an improved system for sequestering CO2 from the exhaust gas of a solid oxide fuel cell system.

[0009] The object of the present invention can be achieved by the independent claims. Advantageous developments and modifications of the present invention are described in the dependent claims. [Means for solving the problem]

[0010] According to the present invention, a method for operating a solid oxide fuel cell system is provided, and this method includes, A fuel supply flow is supplied to the fuel cell module, causing the fuel cell module to discharge an exhaust fuel flow. By supplying an air supply flow to the fuel cell module, the fuel cell module is made to discharge an exhaust (or exhaust air) flow. At least a portion of the exhaust fuel flow is supplied to the afterburner (or re-combustion device), oxygen from an oxygen source is supplied to the afterburner, and the exhaust fuel flow and oxygen are combusted in a stoichiometric ratio (or stoichiometrically). The combustion gases from the afterburner are used in a superheater (or superheater) to superheat the exhaust flow. Each step is included.

[0011] Furthermore, the present invention provides a fuel cell system which includes a fuel cell module, an afterburner, and a superheater. The above fuel cell module is configured to receive a fuel supply flow from a fuel supply line and discharge an exhaust fuel flow through an exhaust fuel line, and to receive an air supply flow from an air supply line and discharge an exhaust flow through an exhaust line. The afterburner described above is fluidly connected to the fuel cell module, and is configured to receive at least a portion of the exhaust fuel flow and to be connected to an oxygen source, and is configured to perform stoichiometric combustion of the exhaust fuel flow and oxygen. The superheater described above is positioned in the exhaust line and is fluidly connected to the combustion gas line from the afterburner, and the superheater is configured to superheat the exhaust flow using the combustion gas from the afterburner.

[0012] This invention draws its basic idea from the combination of stoichiometric oxygen-fuel afterburning and a solid oxide fuel cell module. Stoichiometric combustion means that maximum combustion efficiency is achieved by the optimal oxygen-fuel mixing level. In the afterburner, at least a portion of the exhaust fuel flow is mixed with external pure or near-pure oxygen (i.e., the oxygen flow used in the afterburner is different from the air supply flow or exhaust flow). Oxygen for oxygen-fuel combustion can be obtained from an air isolation process or an electrolysis process. Oxygen extraction may be performed separately and / or with a time delay from the operation of the fuel cell. This means that an oxygen tank can be used for longer-term operation due to the lower oxygen supply. One of the major advantages of the proposed method of operating the solid oxide fuel cell system is that a moderate combustion temperature is obtained and oxygen demand is reduced due to the low fuel ratio and high reactive component ratio in the exhaust fuel flow.

[0013] By definition, the stoichiometric ratio refers to the precise ratio of air to combustible gas or vapor when complete combustion occurs. The stoichiometric ratio can be adjusted in advance based on the type of fuel, the amount of fuel, and the amount of oxygen. This adjustment can also be made during operation. There are two possibilities: if there is too much oxygen, it can be detected at the burner exhaust (e.g., by a lambda sensor). If there is too little oxygen, combustible components (mainly hydrogen in the exhaust gas) can be detected (e.g., by using Raman scattering and / or thermal conductivity). In addition, there are many model-based methods for determining the precise amount of oxygen based on the amount, composition, and current of the fuel and providing this as a set value to the control system.

[0014] By incorporating a stoichiometrically and electrochemically controlled oxidation process and catalytically stoichiometric oxygen-fuel combustion downstream of the SOFC module, the demand for oxygen is significantly reduced. Furthermore, compressed oxygen or oxygen extraction systems require a much smaller system size. Using oxygen from the electrolysis process allows for the recovery of electrolytic energy used for CO2 sequestration (either immediately or with a time delay). If energy is also required for oxygen extraction, the amount of energy needed is also reduced.

[0015] By reusing heat from oxygen fuel combustion at various points within the SOFC system, the efficiency loss caused by the CO2 sequestration (or carbon sequestering) process can be significantly reduced. The CO2 and water sequestrated at the end of this process exist in a nearly pure form, thus opening up the possibility of direct recirculation. The fuel flexibility of the SOFC system allows for the use of renewable hydrocarbon sources (e.g., biogas). In this case, the proposed method can be used to remove CO2 from the atmosphere (biogas) with minimal effort. The effort and efficiency loss incurred can be significantly reduced compared to existing methods known in the prior art. The two-stage oxidation process at an appropriate temperature reduces the load on components compared to single-stage high-temperature oxygen fuel combustion without an upstream SOFC module.

[0016] Preferably, the combustion gases are cooled below the boiling point of water in a water condenser downstream of the superheater. For this reason, a water condenser is located downstream of the superheater. The reaction product leaving the catalytic afterburner is a mixture of superheated water vapor and CO2. To separate the CO2 from the water vapor, the water needs to condense.

[0017] In a preferred embodiment, heat from the combustion gases is used to preheat the fuel supply flow. Thus, condensation heat is transferred to the fuel, preheating it in the first stage. This means that, in terms of system configuration, the fuel supply line is connected to a water condenser, which is configured to preheat the fuel supply flow using heat from the combustion gases.

[0018] In a preferred embodiment, heat from the exhaust flow is used to preheat the fuel supply flow for optimal utilization of the heat source in the fuel cell system. For this purpose, a fuel preheater is provided in the fuel supply line, connected to the exhaust line, and configured to preheat the fuel supply flow using heat from the exhaust flow. The preheater raises the fuel temperature to a higher temperature level.

[0019] Preferably, the high temperature of the exhaust flow is further utilized to preheat the air supply flow, using heat from the exhaust flow. For this purpose, an air preheater is provided in the air supply line, and the air preheater is connected to the exhaust line, configured to preheat the air supply flow using heat from the exhaust flow.

[0020] In a preferred embodiment, the exhaust fuel flow is branched into a first portion that goes to the afterburner and a second portion that mixes with the fuel supply flow. Thus, the system is equipped with a manifold downstream of the fuel cell module that branches the exhaust fuel flow into a first portion that goes to the afterburner and a second portion that remains in the exhaust fuel line, the latter of which is fluidly connected to the fuel supply line. In this way, unreacted fuel components are recycled to provide steam for the reforming process and some of the heat is reused.

[0021] In other preferred embodiments, in the last stage before entering the fuel module, the fuel supply stream passes through a reformer (or reforming unit). In the reformer, integrated with the fuel supply line before the fuel cell module, excess thermal energy is partially combined with chemical fuel energy by means of a steam reforming process. The so-called reformate (or reformed oil) exits the previous reformer at the temperature level of the SOFC module.

[0022] Hereinafter, an example of an embodiment of the present invention will be illustrated with reference to the attached drawings.

Brief Description of the Drawings

[0023] [Figure 1] FIG. 1 is a diagram illustrating a SOFC system with CO2 sequestration.

Mode for Carrying Out the Invention

[0024] The illustrated system configuration exemplifies the components of a SOFC (or solid oxide fuel cell) system 2 with subsequent carbon recovery (or CO2 sequestration). Through a blower 6 provided in an air supply line 8, an air supply stream AF (air feed) is sent to the cathode side of a solid oxide fuel cell module (or SOFC module) 4. The air supply stream AF is heated to the inlet temperature when passing through an air preheater 10. Heat is transferred from the high-temperature side to the air supply stream AF through the heat transfer surface of the air preheater 10. The heated air supply stream AF passes through the cathode of the SOFC module 4 and releases oxygen stoichiometrically to the anode side of the SOFC module 4 through the electrolyte.

[0025] On the fuel side, as illustrated by arrow F1, the fuel feed (FF) in the fuel supply line 16 first goes to the water condenser 12. There, the fuel feed (FF) is preheated to absorb condensation heat. Then, as illustrated by F2, the preheated fuel feed (FF) leaves the condenser 12 and flows into the fuel preheater 14 located in the fuel supply line 16. Inside the fuel preheater 14, the fuel temperature rises to an even higher temperature level. After leaving the fuel preheater 14, the fuel feed (FF) mixes with the recirculated exhaust fuel (EF), which contains steam, CO2, and unreacted fuel, in the manifold 18, and finally passes through the reformer 20. There, the excess thermal energy is partially combined with the chemical fuel energy through the steam reforming process. A mixture of CO2, CO, H2O, H2, and CH4 (also referred to as reformate) leaves the reformer 20 at the temperature level required for SOFC module 4.

[0026] In the SOFC module 4, a portion of the reformate is electrochemically and stoichiometrically converted at an operating temperature in the range of approximately 500 to 1000°C. The proportions of water vapor and CO2 increase while the proportions of fuel components such as H2, CO, and CH4 decrease. This conversion occurs in a range of up to 60 to 80%. The exhaust fuel flow EF, containing low-heat anode exhaust gas that is very rich in water vapor and CO2, leaves the anode side of the SOFC fuel cell module 4 via the exhaust fuel line 21. The exhaust fuel flow EF then branches into two parts at the manifold 22. At this point, a portion of the exhaust fuel flow EF circulates through the upstream exhaust fuel line 21 and is returned to the reformer 20 using the recirculation blower 24. In this way, approximately 60% of the exhaust fuel flow EF is recirculated.

[0027] The rest of the exhaust fuel flow EF is sent to the afterburner 26, where it is stoichiometrically oxidized with oxygen through a catalytically assisted process. Due to the low proportion of fuel and the high proportion of reactive components such as CO2 and H2O, this process takes place at a moderate temperature and the demand for oxygen is low. Oxygen for the combustion process is supplied from the oxygen source 28. This oxygen can be supplied from an air separation process or electrolysis process that takes place outside the SOFC system 2.

[0028] The combustion gas FG (flue gas) leaves the catalytic afterburner 26 via the combustion gas line 29 and contains a mixture of superheated water vapor and CO2. The thermal energy in the combustion gas FG leaving the oxygen fuel (or oxy fuel) combustion is exchanged with several media in a longitudinal order. In order to isolate CO2 from water vapor, the water needs to be condensed and discharged as a liquid phase. First, in the superheater (or superheater) 30, the excess thermal energy stored in the combustion gas FG is transferred to the exhaust air EA (exhaust air) flowing from the SOFC module 4 via the exhaust line 31, and as a result, the exhaust air EA is superheated. This exhaust air EA then transfers its thermal energy to the fuel supply flow FF as a high-temperature gas flow in the fuel preheater 14, and then to the air supply flow AF in the air preheater 10. Both the fuel preheater 14 and the air preheater 10 are located in the exhaust line 31.

[0029] The steam / CO2 mixture exiting the superheater 30 is cooled below the boiling point of water in the condenser 12, while transferring heat to the fuel supply flow FF as described above. Depending on the thermal equilibrium conditions, additional heat exchangers can be provided for further waste heat utilization. The water and CO2 exist in a nearly pure form. This water is released into the reservoir 32, and the CO2 is stored in the CO2 reservoir 34. In general, this solid oxide fuel cell system 2 is configured to optimally reuse the heat contained within the system.

Claims

1. A method for operating a solid oxide fuel cell system (2), A fuel supply flow (FF) is supplied to the fuel cell module (4) so ​​that the fuel cell module (4) discharges an exhaust fuel flow (EF). An air supply flow (AF) is supplied to the fuel cell module (4) so ​​that the fuel cell module (4) discharges an exhaust flow (EA). At least a portion of the exhaust fuel flow (EF) is supplied to the afterburner (26), oxygen from the oxygen source (28) is supplied into the afterburner (26), and the exhaust fuel flow (EF) and oxygen are burned in a stoichiometric ratio. The combustion gas (FG) from the afterburner (26) is used in the superheater (30) to superheat the exhaust flow (EA). method.

2. The method according to claim 1, wherein the combustion gas (FG) is cooled to below the boiling point of water in a water condenser (12) downstream of the superheater (30).

3. The method according to claim 2, wherein heat from the combustion gas (FG) is used to preheat the fuel supply flow (FF).

4. The method according to any one of claims 1 to 3, wherein the heat from the exhaust flow (EA) is used to preheat the fuel supply flow (FF).

5. The method according to any one of claims 1 to 4, wherein the heat from the exhaust flow (EA) is used to preheat the air supply flow (AF).

6. The method according to any one of claims 1 to 5, wherein the exhaust fuel flow (EF) is branched into a first portion that goes toward the afterburner (26) and a second portion that is mixed with the fuel supply flow (FF).

7. The method according to any one of claims 1 to 6, wherein the fuel supply flow (FF) passes through a reformer (20) before flowing into the fuel cell module (4).

8. A solid oxide fuel cell system (2), comprising a fuel cell module (4), an afterburner (26), and a superheater (30), The fuel cell module (4) is configured to receive a fuel supply flow (FF) from a fuel supply line (16) and discharge an exhaust fuel flow (AF) via an exhaust fuel line (21), and to receive an air supply flow (AF) from an air supply line (8) and discharge an exhaust flow (EA) via an exhaust line (31). The afterburner (26) is fluidly connected to the fuel cell module (4), and is configured to receive at least a portion of the exhaust fuel flow (EF) and to be connected to an oxygen source (28), and is configured to perform stoichiometric combustion of the exhaust fuel flow (EF) and oxygen. The superheater (30) is positioned in the exhaust line (31) and is fluidly connected to the combustion gas line (29) from the afterburner (26), and the superheater (30) is configured to superheat the exhaust flow (EA) using the combustion gas (FG) from the afterburner (26). Solid oxide fuel cell system (2).

9. The solid oxide fuel cell system (2) according to claim 8, wherein a water condenser (12) is located downstream of the superheater (30) in the combustion gas line (29), and the water condenser (12) is configured to cool the combustion gas (FG) to a temperature below the boiling point of water.

10. The solid oxide fuel cell system (2) according to claim 9, wherein the fuel supply line (16) is connected to the water condenser (12), and the water condenser (12) is configured to use heat from the combustion gas (FG) to preheat the fuel supply flow (FF).

11. A solid oxide fuel cell system (2) according to any one of claims 8 to 10, wherein a fuel preheater (14) is located in the fuel supply line (16), and the fuel preheater (14) is connected to the exhaust line (31), and is configured to use heat from the exhaust flow (EA) to preheat the fuel supply flow (FF).

12. A solid oxide fuel cell system (2) according to any one of claims 8 to 11, wherein an air preheater (10) is located in the air supply line (8), and the air preheater (10) is connected to the exhaust line (31), and is configured to use heat from the exhaust flow (EA) to preheat the air supply flow (AF).

13. A solid oxide fuel cell system (2) according to any one of claims 8 to 12, wherein a manifold (22) is provided downstream of the fuel cell module (4), and the manifold (22) is configured to branch the exhaust fuel flow (EF) into a first portion directed toward the afterburner (26) and a second portion remaining in the exhaust fuel line (21) which is fluidly coupled to the fuel supply line (16).

14. The solid oxide fuel cell system (2) according to any one of claims 8 to 13, wherein the reformer (20) is integrated with the fuel supply line (16) before the fuel cell module (4).

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