Patent Description:
The present disclosure relates to carbon dioxide (CO<NUM>) separation in direct molten carbonate fuel cells ("DFC").

In a CO<NUM> separation system for a DFC, the CO<NUM>-rich anode exhaust stream also contains water vapor and unused fuel, including mostly hydrogen and carbon monoxide (CO). To make the stream ready for CO<NUM> capture (i.e., separation) for sequestration or use, some processing or post-treatment is required. <CIT> discloses a system and method for molten carbonate type fuel cell power generation and exhaust gas recovery. <CIT> discloses an apparatus and thermal management method for molten carbonate fuel cells. <CIT> discloses a high temperature fuel cell stack system, such as a solid oxide fuel cell system, with an improved balance of plant efficiency which includes a thermally integrated reformed, combustor and a fuel cell stack. <CIT> discloses a high temperature fuel cell system having sectional circuit of the anode waste gas and outward transfer of gas components. <CIT> discloses integrated power generation and chemical production using fuel cells.

In a first aspect of the present invention, there is provided a fuel cell system including a fuel cell having an anode and a cathode, wherein the anode is configured to output an anode exhaust gas. The system further includes a condenser configured to receive and condense the anode exhaust gas, to separate water from the anode exhaust gas to form a dried anode exhaust gas, and to separately output the water and the dried anode exhaust gas. The system further includes a pressure swing adsorption unit configured to receive the dried anode exhaust gas, and to output a hydrogen stream and a separate CO<NUM> stream. The system further comprises an oxidizer configured to receive a first portion of the hydrogen stream from the pressure swing adsorption unit and air from an air supply, and to output an oxidized hydrogen stream. The system further comprises a heat exchanger configured to receive and transfer heat from the oxidized hydrogen stream to a cathode inlet stream received by the cathode.

Ina second aspect of the present invention, there is provided a method of processing fuel cell exhaust includes, at a condenser, receiving anode exhaust gas from an anode of a fuel cell, outputting a dried anode exhaust gas stream, and separately outputting a water stream. The method further includes, at a first compressor, compressing the dried anode exhaust gas stream and outputting a compressed anode exhaust gas stream. The method further includes, at a pressure swing adsorption ("PSA") unit, receiving the compressed anode exhaust gas stream, outputting a hydrogen stream, and separately outputting a CO<NUM> stream. The method further includes, at an oxidizer, receiving a first portion of the hydrogen stream and air from an air supply and outputting an oxidized hydrogen stream. The method further includes, at a heat exchanger, transferring heat from the oxidized hydrogen stream to a cathode inlet stream received in the cathode.

These and other advantageous features will become apparent to those reviewing the disclosure and drawings.

Referring generally to the figures, disclosed herein is a fuel cell subsystem for post-processing fuel cell anode exhaust gas to provide CO<NUM> sequestration.

Conventionally, combustibles in an anode exhaust gas may be reacted in an oxidizer. Oxygen rather than air is supplied to the oxidizer because nitrogen present in air may dilute the CO<NUM> in the anode exhaust gas. An air separation subsystem must be incorporated to provide the necessary oxygen to the oxidizer. However, when using oxygen, water is injected as a coolant in the oxidizer to maintain the oxidizer at a desired temperature level (e.g., to avoid overheating a catalyst). The oxidizer generates an oxidizer exhaust including at least water and CO<NUM>. Heat generated in the oxidizer is then used to preheat a cathode inlet stream. After recuperative heat exchange, the anode exhaust/oxidizer exhaust stream is cooled down in a condenser to remove water. A condenser downstream from the oxidizer separates and removes the injected water and any other water present in the exhaust stream, generating oxidizer exhaust with a higher concentration of CO<NUM> ready for sequestration. In one example, when feeding oxygen to an oxidizer in a fuel cell system using greenhouse gas ("GHG") from a pulverized coal ("PC") boiler steam cycle power plant, the CO<NUM> stream for sequestration contains approximately <NUM>% CO<NUM> and <NUM>% water, with <NUM>% fuel utilization. When air is fed to the oxidizer rather than oxygen, the CO<NUM> content is reduced to approximately <NUM>%.

Referring to <FIG>, a post-treatment system is shown according to an embodiment not forming part of the invention. The process includes recovering hydrogen such that, after providing the required heat to a the cathode inlet stream, excess hydrogen is isolated as a co-product. According to another embodiment not forming part of the invention, the excess hydrogen is recycled to a DFC anode as supplementary fuel.

A fuel cell system <NUM> includes a first fuel cell <NUM> having a cathode <NUM> (i.e., a first cathode) and an anode <NUM> (i.e., a first anode). According to an embodiment, the first fuel cell <NUM> may be a DFC. The anode <NUM> outputs an anode exhaust gas, including at least CO<NUM>, hydrogen, water, and CO. A first heat exchanger <NUM> receives the anode exhaust gas from the DFC and partially cools the anode exhaust gas. The first heat exchanger <NUM> then outputs a first partially-cooled gas. The first partially-cooled gas is transformed through a high-temperature ("HT") CO shift reaction (e.g., water-gas shift reaction) in a first shift reactor <NUM>, forming a first shifted gas, which is received by a second heat exchanger <NUM>. The first shift reactor <NUM> is configured to operate at a first temperature in a range of approximately <NUM> to <NUM>. The first shift reactor <NUM> may be configured to shift CO and water into CO<NUM> and hydrogen, such that the first shifted gas has a higher concentration of CO<NUM> and hydrogen than the first partially-cooled gas. The second heat exchanger <NUM> partially cools the first shifted gas and outputs a second partially-cooled gas. The second partially-cooled gas is transformed through a low-temperature ("LT") CO shift reaction in a second shift reactor <NUM>, forming a second shifted gas, which is received by a third heat exchanger <NUM>. The second shift reactor <NUM> is configured to operate at a second temperature in a range of approximately <NUM> to <NUM>, such that the first temperature is higher than the second temperature. The second shift reactor <NUM> may be configured to shift CO and water into CO<NUM> and hydrogen, such that the second shifted gas has a higher concentration of CO<NUM> and hydrogen than the second partially-cooled gas. The third heat exchanger <NUM> cools the second shifted gas to a desired temperature and outputs a cooled gas. According to an exemplary embodiment, the temperature of the cooled gas is based on a range of temperatures acceptable by an oxidizer <NUM> downstream from the third heat exchanger <NUM>.

The cooled gas is mixed with air, rather than oxygen, which is provided (i.e., injected) by an air supply <NUM> (i.e., first air supply, controlled air supply, etc.), forming a mixed gas. According to an embodiment, the air supply <NUM> may be controlled to establish a preferred ratio of air to any one of CO<NUM>, hydrogen, water, and/or CO making up the cooled gas. This preferred ratio may be based on the requirements of the oxidizer. The mixed gas is then fed to the oxidizer <NUM>, which is configured to perform a preferential oxidation reaction for conversion of CO to CO<NUM>. Preferential oxidation is a chemical process for removing CO. This process uses a low-temperature shift reactor (e.g., similar to the second shift reactor <NUM>) followed by a staged preferential oxidizer for oxidizing CO using oxygen in the presence of a noble metal catalyst (e.g., platinum, palladium-cobalt, palladium-copper, gold, etc.). The oxidizer <NUM> outputs an oxidized gas containing CO<NUM> for sequestration and generates heat due to the reaction. A fourth heat exchanger <NUM> receives the oxidized gas from the oxidizer <NUM> and cools the oxidized gas, forming, at least in part, an anode inlet stream <NUM>. According to an embodiment, the oxidizer <NUM> generates exhaust, separate from the oxidized gas containing CO<NUM>. Because exhaust from the oxidizer <NUM> does not form part of the oxidized gas output, air may be used for the oxidizer, eliminating the need for an air separation unit and/or water injection (e.g., for oxidizer temperature control).

As shown in <FIG>, the system <NUM> further includes an EHS <NUM> (also referred to as a second fuel cell). The EHS <NUM> includes a cathode <NUM> (i.e., a second cathode), an anode <NUM> (i.e., a second anode), and a proton exchange membrane ("PEM") <NUM> disposed between the cathode <NUM> and the anode <NUM>. The anode <NUM> receives the cooled anode inlet stream <NUM> from the fourth heat exchanger <NUM>. At the anode <NUM>, at least a portion of the hydrogen present in the anode inlet stream <NUM> is selectively oxidized to positively charge hydrogen ions (H+), which are then transferred to the cathode <NUM> through the PEM <NUM>. According to an embodiment the oxidizer <NUM> , the air supply <NUM>, and the heat exchanger <NUM> may be removed from the system <NUM> shown in <FIG> by incorporating a High Temperature Membrane ("HTM") operating in excess of <NUM> (e.g., as PBI or solid acid membrane) as a PEM. Referring still to <FIG>, in the cathode <NUM>, H+ is reduced to gaseous hydrogen due to the absence of an oxidant. Therefore, the EHS <NUM> selectively generates and outputs a hydrogen stream <NUM> from the anode inlet stream <NUM>. The hydrogen stream <NUM> is generated as co-product and may be used in the system <NUM> or exported. According to an embodiment, each of the shift reactors <NUM>, <NUM> are configured to maximize hydrogen recovery in the corresponding high-temperature and low-temperature CO shift reactions and prevent carbon monoxide poisoning of an EHS catalyst. According to another embodiment, the hydrogen stream <NUM> may be compressed (e.g., electrochemically), with relatively low energy input. Advantageously, the transfer across the PEM <NUM> utilizes a minimum energy input and does not require any moving parts. According to an embodiment, the EHS <NUM> may recover approximately <NUM>% of the hydrogen from the anode exhaust gas from the first fuel cell <NUM>.

The anode <NUM> of the EHS <NUM> generates a second anode exhaust gas. The second anode exhaust gas may be fed to a condenser <NUM>, which separates the second anode exhaust gas into a CO<NUM> stream <NUM> and a water stream (i.e., condensate) <NUM>. The CO<NUM> stream <NUM> from the condenser <NUM> is then fed through a CO<NUM> compressor <NUM> to liquefy at least a portion of the CO<NUM> stream <NUM>, generating a highly concentrated CO<NUM> supply <NUM> suitable for sequestration and/or export (i.e., transportation) to a point of use (e.g., for food processing). According to an embodiment, after removal of water in the condenser <NUM> to the water stream <NUM>, the CO<NUM> stream <NUM> includes approximately <NUM>% CO<NUM> and approximately <NUM>% water.

As shown in <FIG>, at least a portion of the hydrogen stream <NUM> may be oxidized using air to generate heat, according to another embodiment. A first portion <NUM> of the hydrogen stream <NUM> generated by the cathode <NUM> of the EHS <NUM> is fed to an oxidizer <NUM> (i.e., a second oxidizer) and is oxidized with air from an air supply <NUM> (i.e., a second air supply). The oxidization generates an oxidized hydrogen stream <NUM>, including at least heat and water and is fed through a fifth heat exchanger <NUM>. The fifth heat exchanger <NUM> transfers heat from the oxidized hydrogen stream <NUM> to preheat a cathode inlet stream <NUM> (e.g., desulfurized GHG from coal-fueled power plants), received by the first cathode <NUM> of the first fuel cell <NUM>. According to another embodiment, the oxidized hydrogen stream <NUM> may be used to preheat a cathode inlet stream received by the cathode <NUM> of the EHS <NUM> or any other cathode. The oxidized hydrogen stream <NUM> may then be outputted from the system <NUM>.

In the embodiment shown in <FIG>, the first portion <NUM> of the hydrogen stream <NUM> used to heat the cathode inlet stream <NUM> includes approximately <NUM>% of the hydrogen generated by the cathode <NUM>. The remaining second portion <NUM> (e.g., approximately <NUM>% of the hydrogen stream <NUM>) is generated as co-product and may be used in the system <NUM> or exported. The percentage of the hydrogen stream <NUM> forming each portion <NUM>, <NUM> may vary according to other exemplary embodiments. According to an exemplary embodiment, the first portion <NUM> of the hydrogen stream <NUM> may be limited to an amount necessary to provide a desired level of preheat to the cathode inlet stream <NUM>. According to another embodiment, the second portion <NUM> of the hydrogen stream <NUM> (e.g., hydrogen not fed to the second oxidizer <NUM> to preheat the cathode inlet stream <NUM>) may be recycled (e.g., fed) to the first anode <NUM> of the first fuel cell <NUM>, thereby reducing the natural gas fuel input required to operate the first fuel cell <NUM>.

Referring now to <FIG>, a post-treatment system is shown according to another embodiment. In this system, as with earlier exemplary embodiments, hydrogen present in anode exhaust gas is separated and recovered.

A fuel cell system <NUM> includes a fuel cell <NUM> having a cathode <NUM> and an anode <NUM>. According to an exemplary embodiment, the fuel cell <NUM> may be a DFC substantially same as the first fuel cell <NUM>. The anode <NUM> outputs an anode exhaust gas, including at least CO<NUM>, hydrogen, water, and CO. A first heat exchanger <NUM> receives the anode exhaust gas from the DFC and partially cools the anode exhaust gas. The first heat exchanger <NUM> outputs a first partially-cooled gas. The first partially-cooled gas is transformed through a high-temperature CO shift reaction in a first shift reactor <NUM>, forming a first shifted gas, which is received by a second heat exchanger <NUM>. The first shift reactor <NUM> is configured to operate at a first temperature in a range of approximately <NUM> to <NUM>. The first shift reactor <NUM> may be configured to shift CO and water into CO<NUM> and hydrogen, such that the first shifted gas has a higher concentration of CO<NUM> and hydrogen than the first partially-cooled gas. The second heat exchanger <NUM> partially cools the first shifted gas and outputs a second partially-cooled gas. The second partially-cooled gas is transformed through a low-temperature CO shift reaction in a second shift reactor <NUM>, forming a second shifted gas, which is received by a condenser <NUM>. The second shift reactor <NUM> is configured to operate at a second temperature in a range of approximately <NUM> to <NUM>, such that the first temperature is higher than the second temperature. The second shift reactor <NUM> may be configured to shift CO and water into CO<NUM> and hydrogen, such that the second shifted gas has a higher concentration of CO<NUM> and hydrogen than the second partially-cooled gas. The condenser <NUM> separates the second shifted gas into a dried (e.g., dehydrated) anode exhaust gas stream <NUM>, containing at least CO<NUM> and hydrogen, and a separate water stream (i.e., condensate) <NUM>. For example, substantially all of the water is removed from the anode exhaust gas stream when forming the dried anode exhaust gas stream <NUM>. The dried anode exhaust gas stream <NUM> from the condenser <NUM> is then fed through a compressor <NUM>, forming a compressed anode exhaust gas stream, which is then fed through a third heat exchanger <NUM>, to further cool the compressed anode exhaust gas stream. According to another exemplary embodiment, the third heat exchanger <NUM> may be disposed upstream from the compressor <NUM> (e.g., between the condenser <NUM> and the compressor <NUM>) and is configured to cool the dried anode exhaust gas stream <NUM>.

The system <NUM> includes a pressure swing adsorption ("PSA") unit <NUM>. The PSA unit <NUM> is configured to receive the compressed anode exhaust gas stream from the third heat exchanger <NUM> and separate the stream into a hydrogen stream <NUM> and a CO<NUM> stream <NUM>. In the PSA unit <NUM>, the gases other than hydrogen (e.g. mostly CO<NUM> and some water) are adsorbed by an adsorbent bed media at high pressures and a pure hydrogen stream <NUM> is outputted from the PSA unit <NUM> at a pressure close to (e.g., substantially the same as) an inlet pressure of the compressed anode exhaust gas stream received at the PSA unit <NUM>. The hydrogen stream <NUM> is generated as co-product and may be used in the system <NUM> or exported. After the adsorbent bed media in the PSA unit <NUM> reaches its maximum adsorbent capacity, it is purged to remove the adsorbed gases, which generate the CO<NUM> stream <NUM>. This purging occurs by de-sorption, accomplished by lowering the pressure to near atmospheric pressure of approximately <NUM> psia. The CO<NUM> stream <NUM> is then fed to a CO<NUM> compressor <NUM> to liquefy at least a portion of the CO<NUM> stream <NUM>, generating a sequestered CO<NUM> supply <NUM>.

According to an embodiment, the system <NUM> may transform a portion of the hydrogen stream <NUM> in the same way as the hydrogen stream <NUM> as shown in <FIG>. For example, as shown in <FIG>, in accordance with the invention, a first portion <NUM> of the hydrogen stream <NUM> generated by the PSA unit <NUM> is fed to an oxidizer <NUM> and oxidized with air from an air supply <NUM>. The oxidization generates an oxidized hydrogen stream <NUM>, including at least heat and water and is fed through a fourth heat exchanger <NUM>. The fourth heat exchanger <NUM> transfers heat from the oxidized hydrogen stream <NUM> to preheat a cathode inlet stream <NUM> (e.g., desulfurized GHG from coal-fueled power plants), received by the cathode <NUM> of the first fuel cell <NUM>. The oxidized hydrogen stream <NUM> may be outputted from the system <NUM>. Similarly to <FIG>, the first portion <NUM> of the hydrogen stream <NUM> may be limited to an amount necessary to provide a desired level of preheat to the cathode inlet stream <NUM>. According to another exemplary embodiment, a remaining second portion <NUM> of the hydrogen stream <NUM> (e.g., hydrogen not fed to the oxidizer <NUM> to preheat the cathode inlet stream <NUM>) may be recycled (e.g., fed) to the anode <NUM> of the fuel cell <NUM>, thereby reducing the natural gas fuel input required to operate the fuel cell <NUM>.

With regard to either system <NUM>, <NUM>, according to another exemplary embodiment, a process for sequestering CO<NUM> may include consuming all hydrogen and other combustibles in an oxidizer and utilizing the energy content for preheating a cathode inlet stream.

In certain embodiments not forming part of the invention, a fuel cell system includes a fuel cell having an anode and a cathode, an oxidizer, and an electrochemical hydrogen separator. The oxidizer is configured to receive anode exhaust gas from the anode and air from a controlled air supply and react the anode exhaust gas and the air in a preferential oxidation reaction. The separator is configured to receive oxidized gas from the oxidizer and to form separate streams of hydrogen and CO<NUM> from the remaining gas. A condenser is configured to receive the CO<NUM> stream from the oxidizer and condense the stream to separate water and liquefy CO<NUM>.

In other embodiments, a fuel cell system includes a fuel cell having an anode and a cathode, a condenser, and a pressure swing adsorption unit. The condenser is configured to receive and condense anode exhaust gas from the anode and separate a water stream from the remaining condensed gas. A compressor receives and compresses the remaining condensed gas and feeds compressed gas to the pressure swing adsorption unit. The pressure swing adsorption unit separates a hydrogen stream and a CO<NUM> stream. The CO<NUM> stream is received by a second compressor configured to liquefy CO<NUM>.

As utilized herein, the terms "approximately," "about," "substantially", and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of the invention as recited in the appended claims.

The terms "coupled," "connected," and the like as used herein mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another.

References herein to the positions of elements (e.g., "top," "bottom," "above," "below," etc.) are merely used to describe the orientation of various elements in the Figures.

Claim 1:
A fuel cell system (<NUM>) comprising:
a fuel cell (<NUM>) comprising an anode (<NUM>) and a cathode (<NUM>), wherein the anode is configured to output an anode exhaust gas;
a condenser (<NUM>) configured to receive and condense the anode exhaust gas, to separate water (<NUM>) from the anode exhaust gas to form a dried anode exhaust gas (<NUM>), and to separately output the water and the dried anode exhaust gas;
a pressure swing adsorption unit (<NUM>) configured to receive the dried anode exhaust gas and to output a hydrogen stream (<NUM>) and a separate CO<NUM> (<NUM>) stream;
an oxidizer (<NUM>) configured to receive a first portion of the hydrogen stream (<NUM>) from the pressure swing adsorption unit and air from an air supply (<NUM>), and to output an oxidized hydrogen stream (<NUM>); and
a heat exchanger (<NUM>) configured to receive and transfer heat from the oxidized hydrogen stream (<NUM>) to a cathode inlet stream (<NUM>) received by the cathode (<NUM>).