EP4732360A1

EP4732360A1 - Method for operating a solid oxide fuel cell system and a solid oxide fuel cell system

Info

Publication number: EP4732360A1
Application number: EP24749206.9A
Authority: EP
Inventors: Tobias RETHER
Original assignee: Siemens Energy Global GmbH and Co KG
Current assignee: Siemens Energy Global GmbH and Co KG
Priority date: 2023-08-17
Filing date: 2024-07-26
Publication date: 2026-04-29
Also published as: GB2632689A; CN121713293A; KR20260040356A; GB202312583D0; AU2024324249A1; WO2025036682A1

Abstract

The invention relates to a method for operating a solid oxide fuel cell system (2), comprising the steps of providing a fuel feed stream (FF) to a fuel cell module (4) and the fuel cell module (4) expelling an exhaust fuel stream (EF), and providing an air feed stream (AF) to the fuel cell module (4) and the fuel cell module (4) expelling an exhaust air stream (EA). In order to improve the separation of CO2 from the exhaust gas, the method comprises further the steps of feeding at least a portion of the exhaust fuel stream (EF) to an afterburner (26), feeding in the afterburner (26) oxygen from an oxygen source (28) and burning the exhaust fuel stream (EF) and the oxygen in a stochiometric ratio, and using flue gas (FG) from the afterburner (26) in a superheater (30) to superheat the exhaust air stream (EA).

Description

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

The invention relates to a method for operating a solid oxide fuel cell (SOFC) system, comprising the steps of providing a fuel feed stream to a fuel cell module and the fuel cell module expelling an exhaust fuel stream, and providing an air feed stream to the fuel cell module and the fuel cell module expelling an exhaust air stream.

The invention also relates to a solid oxide fuel cell system comprising a fuel cell module configured to receive a fuel feed stream from a fuel feed line and to expel an exhaust fuel stream through an exhaust fuel line, also configured to receive an air feed stream from an air feed line and to expel an exhaust air stream through an exhaust air line.

The separation of CO2 from the exhaust gas of fuel cell systems based on the oxidation of hydrocarbons using conventional methods (e.g. , amine scrubbing) is complex and energy intensive. The reason for this lies in the oxidation of the fuel with a clear excess of air. Downstream of the fuel cell, the exhaust gas is mixed with the air and the unreacted fuel components are oxidized. This results in a flue gas in which CO2 takes up a smaller volume fraction alongside the other components (N₂, H₂0, etc. ) . Due to the strong dilution of CO2, its separation is quite complex and requires a high system complexity, additional operating materials (e.g. , amine) and/or higher energy consumption. The separation of CO2 may also take place under efficiency losses of the system (e.g. , drive of pumps, pressure losses, etc. ) and the purity of the separated CO2 is limited in a single-stage process.

At present, the most common methods for CO2 separation involve the removal of CO2 after an oxidation process (usually combustion) using air. This is often based on a fuel/air- mixture with excess air (post-combustion carbon capture) . As a result, the CO2 is present in the flue gas in relatively low concentrations, and it takes great efforts to remove it. The efforts, costs and loss of efficiency are significant. Moreover, it is not possible to separate the CO2 completely in one step. In addition, there are possible environmental impacts due to the used operating materials (e.g. , amine scrubbing of the flue gas) .

Other methods involve stoichiometric oxidation with oxygen and downstream separation of the water content by condensation. These methods are applied at high temperatures and hence, the components are exposed to high temperature stress. In some cases, the high temperatures are lowered by using the reaction products as working media in a heat exchange process.

US20220246966A1 describes a solid oxide fuel cell (SOFC) system with carbon capture. The system includes an afterburner in fluid communication with the fuel cell module and disposed downstream of the outlet. The SOFC module outputs oxygen depleted air and depleted fuel streams . The depleted fuel stream is routed to an output manifold, wherein a first portion of the depleted fuel is sent to the fluidly coupled afterburner and a second portion of the depleted fuel forms a recycle stream back to the fuel inlet manifold. The afterburner, which also receives depleted air, is configured to facilitate the combustion of the remaining oxidant from the depleted air and the first portion of the depleted fuel in order to produce an exhaust stream, which includes CO2, nitrous oxides (NOx) , nitrogen (N₂) , and H₂0.

The object of the invention is to provide an improved system for separation of CO2 from the exhaust gas of solid oxide fuel cell systems. The obj ect of the invention is achieved by the independent claims . The dependent claims de scribe advantageous development s and modifications of the invention .

In accordance with the invention there is provided a method for operating a solid oxide fuel cell system, comprising the steps of :

- providing a fuel feed stream to a fuel cell module and the fuel cell module expelling an exhaust fuel stream,

- providing an air feed stream to the fuel cell module and the fuel cell module expelling an exhaust air stream, wherein feeding at least a portion of the exhaust fuel stream to an afterburner , feeding in the afterburner oxygen from an oxygen source and burning the exhaust fuel stream and the oxygen in a stochiometric ratio ,

- using flue ga s from the afterburner in a superheater to superheat the exhaust air stream .

In accordance with the invention there is also provided a fuel cell system comprising :

- a fuel cell module configured to receive a fuel feed stream from a fuel feed line and to expel an exhaust fuel stream through an exhaust fuel line , also conf igured to receive an air feed stream from an air feed line and to expel an exhaust air stream through an exhaust air line , wherein

- an afterburner in fluid communication with the fuel cell module , wherein the afterburner is configured to receive at least a portion of the exhaust fuel stream and is also connected to an oxygen source , wherein the afterburner i s configured for stochiometric combustion of the exhaust fuel stream and the oxygen , and

- a superheater arranged on the exhaust air line and in fluid communication with a flue gas line from the afterburner , wherein the superheater is configured to superheat the exhaust air stream us ing flue gas from the af terburner . The essential idea of the present invention is the combination of a stoichiometric oxy-fuel post-combustion with a solid oxide fuel cell module. Stoichiometric combustion means that an optimum oxygen and fuel mix levels lead to maximum combustion efficiency. In the afterburner, at least part of the exhaust fuel stream is mixed with an external pure or almost pure oxygen (i.e. , the oxygen stream used in the afterburner is different from the air feed stream or the exhaust air stream) . The oxygen for the oxy-fuel combustion can originate from an air separation process or an electrolysis process. The oxygen extraction can take place separately and/or with a time delay to the fuel cell operation. This means, that as the oxygen feed is low, oxygen tanks could be used for longer operation periods. One great advantage of the suggested method for operating a solid oxide fuel cell system is that the low proportion of fuel and the high proportion of reaction components in the exhaust fuel stream result in moderate combustion temperatures as well as in reduced oxygen demand .

Per definition, the stoichiometric ratio is the exact ratio between air and flammable gas or vapor at which complete combustion takes place. The stoichiometric ratio can be preadjusted based on the type of fuel, amount of fuel as well as amount of oxygen. The adjustment can be done during operation as well. There are two possibilities: if there is too much oxygen, the oxygen will be detectable in the burner exhaust - e.g. a lambda sensor; and if there is too little oxygen, combustible components (mostly hydrogen in the exhaust gas) would be detectable, e.g. via Raman scattering and/or thermal conductivity. In addition, there are also many model-based approaches to determine the exact amount of oxygen based on the fuel quantity, composition, and current, and to provide it as a setpoint to a control system.

The oxygen demand is significantly reduced by the installation of the stoichiometric, electrochemically controlled oxi- dation process downstream of the SOFC module and the catalytic stoichiometric oxy-fuel combustion. Furthermore, compressed oxygen or oxygen extraction systems require considerably smaller system sizes. Using oxygen from an electrolysis process, results in the recovery of the electrolysis energy used for CO2 sequestration (instantly or with a time delay) . As the extraction of oxygen also requires energy, its quantity is also reduced.

The reuse of the heat from the oxyfuel combustion at various points in the SOFC system significantly reduces the loss of efficiency due to the C0₂ sequestration process. The separated products CO2 and water, at the end of the process, are almost in pure form. This opens possibilities for direct recycling. Due to the fuel flexibility of an SOFC system, renewable hydrocarbon sources can be used (e.g. , biogas) . In such a case, CO2 can be removed from the atmosphere (biogas) with little effort using the suggested method. The efforts involved and the loss of efficiency are considerably lower than with the present methods known from the prior art. Due to the two-stage oxidation process at moderate temperatures, the component load is lower than with single-stage high- temperature oxyfuel combustion without an upstream SOFC module .

Preferably, the flue gas is cooled below the boing point of water in a water condenser downstream of the superheater. For this purpose, a water condenser is arranged downstream of the superheater. The reaction products leaving the catalytic afterburner are a mixture of superheated water vapour and C0₂. To separate the C0₂ from the water vapour, the water must be condensed out.

In a preferred embodiment, the heat from the flue gas is utilized to preheat the fuel feed stream. The condensation heat is thereby transferred to the fuel and preheats in a first step. With regard to the system design this means that the fuel feed line is connected to the water condenser and the water condenser is configured to utilize the heat from the flue gas to preheat the fuel feed stream .

In a preferred embodiment , which aims at on optimal use of the heat sources within the fuel cell system, the heat from the exhaust air stream is utili zed to preheat the fuel feed stream . For this purpose , a fuel preheater is arranged on the fuel feed line and the fuel preheater is connected to the exhaust air line and configured to utilize the heat from the exhaust air stream to preheat the fuel feed stream . The preheater raises the fuel temperature to a higher temperature level .

Preferably, the high temperature of the exhaust air stream is further used, whereby the heat from the exhaust air stream is utilized to preheat the air feed stream . For this purpose , an air preheater is arranged on the air feed line and the air preheater is connected to the exhaust air line and configured to utilize the heat from the exhaust air stream to preheat the air feed stream .

In a preferred embodiment , the exhaust fuel stream is split into a first part , which i s directed to the afterburner , and a second part , which i s mixed with the fuel feed stream . Therefore , the system comprises manifolds downstream f rom the fuel cell module , conf igured to split the exhaust fuel stream into a first part directed to the afterburner and a second part remaining in the exhaust fuel line , which is fluidly coupled to the fuel feed line . This way the unreacted fuel fraction is recycled, steam for the reforming proces s is provided and also part of the heat is reutilized .

In another preferred embodiment , in a last step before entering the fuel module , the fuel feed stream pas ses through a reformer . In the reformer , which is integrated in the fuel feed line before the fuel cell module , the exces s thermal energy is partly bound in chemical fuel energy by a steam re forming process. The so-called reformate leaves the prereformer at the temperature level for the SOFC module.

An embodiment of the invention is now described, by way of example only, with reference to the accompanying drawing, of which the only figure shows a SOFC system with CO2 sequestration .

The system design in the figure shows the components of an SOFC system 2 with subsequent carbon capture (C02 sequestration) . An air feed stream AF is delivered to the cathode side of a solid oxide fuel cell module 4 via the blower 6 arranged on an air feed line 8. As the air feed stream AF passes through the air preheater 10 and is heated to the inlet temperature. The heat transfer takes place via the heat transfer surface of the air preheater 10 from the hot side to the air feed stream AF. The heated air feed stream AF passes the cathode of the SOFC module 4 and releases its oxygen stoichi- ometrically to the anode side of the SOFC module 4 through an electrolyte .

On the fuel side, a fuel feed stream FF in fuel feed line 16 is first directed to a water condenser 12 shown by the arrow Fl. There the fuel feed stream FF absorbs the condensation heat and is thereby preheated for the first time. The preheated fuel feed stream FF leaves the condenser 12, as shown by F2 , and enters a fuel preheater 14, which is arranged on the fuel feed line 16. In the fuel preheater 14 the fuel temperature is raised to a yet higher temperature level. After leaving the fuel preheater 14, in manifolds 18 the fuel feed stream FF is mixed with a recirculate of an exhaust fuel stream EF, which contains mixture of steam, CO2 and unreacted fuels, and finally passes through the reformer 20. There, the excess thermal energy is partly bound in chemical fuel energy by a steam reforming process. A mixture of CO2, CO, H₂O, H₂, CH₄, called reformate, leaves the reformer 20 at the temperature level required for SOFC module 4. In the SOFC module 4, part of the reformate is converted electrochemically and stoichiometrically at operating temperature of approx. 500-1000°C. The proportion of water vapour and C02 is increased and at the same time the proportion of the fuel components such as H₂, CO and CH₄ is reduced. The conversion takes place at a maximum of 60-80%. An exhaust fuel stream EF containing low-calorific anode exhaust gas, which is highly enriched with water vapour and CO2 , leaves the anode side of the SOFC fuel cell module 4 via an exhaust fuel line 21. The exhaust fuel stream EF is then split into two parts in manifolds 22. Part of this exhaust fuel stream EF is recirculated through the exhaust fuel line 21 upstream and fed back to the reformer 20 using a recirculation blower 24. About 60% of the exhaust fuel stream EF is recycled this way .

The other part of the exhaust fuel stream EF is fed into a afterburner 26, where it is stoichiometrically oxidized with oxygen in a catalytically supported process. Due to the low proportion of fuel and the high proportion of reaction components such as CO₂ and H₂O, the process takes place at moderate temperatures and the oxygen demand is low. The oxygen for the combustion process is provided from an oxygen source 28. The oxygen can be provided from an air separation process or from an electrolysis process carried out outside the SOFC system 2.

The flue gas FG leaving the catalytic afterburner 26 through a flue gas line 29 contains a mixture of superheated water vapour and CO₂. The heat energy in the flue gas FG leaving the oxyfuel combustor is exchanged with several media in cascading sequence. To separate the CO₂ from the water vapour, the water must be condensed out and discharged as a liquid phase. First, in s superheater 30, the excess thermal energy stored in the flue gas FG is transferred to the exhaust air stream EA coming from the SOFC module 4 through an exhaust air line 31, thus superheating the exhaust air stream EA. The exhaust air stream EA, in turn, transfers the thermal energy as a hot gas stream in the fuel preheater 14 to the fuel feed stream FF and in the air preheater 10 to the air feed stream AF, wherein the fuel preheater 14 and the air preheater 10 are both arranged on the exhaust air line 31 .

The water vapour / CO2 mixture exiting the superheater 30 is cooled in the condenser 12 below the boiling point of the water while transferring heat to the fuel feed stream FF, as explained above . Depending on the thermal equilibrium situa- tion , additional heat exchangers could be provided for further waste heat utilisation . Water and CO2 are pre sent in almost pure form . The water is di scharged into a water reservoir 32 and the CO2 is stored in a CO2 reservoir 34 . In general , the solid oxide fuel cell system 2 is designed for an optimal reuse of the heat contained in the system .

Claims

Patent claims

1. A method for operating a solid oxide fuel cell system (2) , comprising the steps of:

- providing a fuel feed stream (FF) to a fuel cell module (4) and the fuel cell module (4) expelling an exhaust fuel stream (EF) ,

- providing an air feed stream (AF) to the fuel cell module (4) and the fuel cell module (4) expelling an exhaust air stream (EA) , wherein feeding at least a portion of the exhaust fuel stream (EF) to an afterburner (26) , feeding in the afterburner (26) oxygen from an oxygen source (28) and burning the exhaust fuel stream (EF) and the oxygen in a stochio- metric ratio,

- using flue gas (FG) from the afterburner (26) in a superheater (30) to superheat the exhaust air stream (EA) .

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

3. The method according to claim 2, wherein the heat from the flue gas (FG) is utilized to preheat the fuel feed stream (FF) .

4. The method according to any of the preceding claims, wherein the heat from the exhaust air stream (EA) is utilized to preheat the fuel feed stream (FF) .

5. The method according to any of the preceding claims, wherein the heat from the exhaust air stream (EA) is utilized to preheat the air feed stream (AF) .

6. The method according to any of the preceding claims, wherein the exhaust fuel stream (EF) is split into a first part, which is directed to the afterburner (26) , and a second part, which is mixed with the fuel feed stream (FF) .

7. The method according to any of the preceding claims, wherein before entering the fuel module (4) , the fuel feed stream (FF) passes through a reformer (20) .

8. A solid oxide fuel cell system (2) comprising:

- a fuel cell module (4) configured to receive a fuel feed stream (FF) from a fuel feed line (16) and to expel an exhaust fuel stream (AF) through an exhaust fuel line (21) , also configured to receive an air feed stream (AF) from an air feed line (8) and to expel an exhaust air stream (EA) through an exhaust air line (31) , wherein

- an afterburner (26) in fluid communication with the fuel cell module (4) , wherein the afterburner (26) is configured to receive at least a portion of the exhaust fuel stream (EF) and is also connected to an oxygen source (28) , wherein the afterburner (26) is configured for stochiometric combustion of the exhaust fuel stream (EF) and the oxygen, and

- a superheater (30) arranged on the exhaust air line (31) and in fluid communication with a flue gas line (29) from the afterburner (26) , wherein the superheater (30) is configured to superheat the exhaust air stream (EA) using flue gas (FG) from the afterburner (26) .

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

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

11. The solid oxide fuel cell system (2) according to any of the claims 8 or 10, wherein a fuel preheater (14) is arranged on the fuel feed line (16) and the fuel preheater (14) is connected to the exhaust air line (31) and configured to utilize the heat from the exhaust air stream (EA) to preheat the fuel feed stream (FF) .

12. The solid oxide fuel cell system (2) according to any of the preceding claims, wherein an air preheater (10) is arranged on the air feed line (8) and the air preheater (10) is connected to the exhaust air line (31) and configured to utilize the heat from the exhaust air stream (EA) to preheat the air feed stream (AF) .

13. The solid oxide fuel cell system (2) according to any of the preceding claims, comprising manifolds (22) downstream from the fuel cell module (4) , configured to split the exhaust fuel stream (EF) into a first part directed to the afterburner (26) and a second part remaining in the exhaust fuel line (21) , which is fluidly coupled to the fuel feed line (16) .

14. The solid oxide fuel cell system (2) according to any of the preceding claims, comprising a reformer (20) integrated in the fuel feed line (16) before the fuel cell module (4) .