ABSTRACT
Abstract
A high efficiency fuel cell system includes a topping fuel cell assembly that includes a topping cathode portion and a topping anode portion, as well as a bottoming fuel cell assembly that includes a bottoming cathode portion and a bottoming anode portion. The assembly also includes a flue gas generating device configured to provide flue gas to the topping cathode portion and/or the bottoming cathode portion, and an oxidizer assembly configured to (i) oxidize anode exhaust output from the bottoming anode portion with air and/or oxygen to generate carbon dioxide-containing exhaust and (ii) generate waste heat for heating the flue gas before the flue gas is provided to the topping cathode portion and/or the bottoming cathode portion. A separation assembly is configured to receive the carbon dioxide-containing exhaust from the oxidizer assembly and to separate carbon dioxide from the carbon dioxide-containing exhaust.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation of U.S. patent application Ser. No. 14/732,032, filed Jun. 5, 2015, the entirety of which is incorporated herein by reference.
BACKGROUND
This invention relates to fuel cell power production systems and, in particular, to a multi-stack high-efficiency fuel cell system with carbon dioxide capture capability and method of operating same. The systems of the present invention may be used with any types of fuel cells, and particularly with molten carbonate fuel cells and solid oxide fuel cells.
World energy consumption is increasing with average energy use growing at about 1.1% per year until 2040 according to the IEA (International Energy Agency). Currently, over 85% of the energy is supplied from fossil fuels. Fossil fuels used for electricity, transportation and heating require combustion, resulting in carbon dioxide emissions into earth's atmosphere. The carbon dioxide concentration in the atmosphere has almost doubled since humans started using fossil fuels and increasing carbon dioxide concentrations in the atmosphere is considered to be a major cause of global warming trends. In fact, the world is on a track to increase total carbon dioxide in the atmosphere by 20% by 2040, resulting in a projected global temperature rise of 3.6 C. Sustainable and efficient use of fossil fuels, as well as capture of emitted carbon dioxide will help to slow the increase of carbon dioxide concentrations in earth's atmosphere. Development of fuel cells, which output lower levels of emissions, as an alternative method of heat and electricity production to conventional fossil fuel-based combustion power plants is ongoing.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte matrix, which conducts electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
In building fuel cell systems, individual fuel cells are stacked together to form a fuel cell stack. The number of fuel cells determines the power rating of the fuel cell stack. To provide systems with higher power ratings, a number of fuel cell stacks are utilized and the outputs of the fuel cell stacks are combined to provide the desired power output. In certain fuel cell systems, the fuel cell stack(s) may be organized in one or more fuel cell stack modules, each of which includes one or more fuel cell stacks housed in an enclosure or a containment structure.
A multi-stack fuel cell system may include a fuel cell stack module with multiple fuel cell stacks housed within a common enclosure. In a system of this design developed for high temperature fuel cell stacks and, in particular, for molten carbonate fuel cell stacks, a box-like containment structure is employed as the enclosure and the fuel cell stacks may be arranged along the length of the containment structure. Each fuel cell stack within the fuel cell module may have inlet manifolds for receiving fuel and oxidant gases needed to operate the fuel cell stack and outlet manifolds for conveying spent fuel and oxidant gases as anode and cathode exhausts from the fuel cell stack. The containment structure of the fuel cell module includes fuel and oxidant gas inlet ports that communicate through ducts with the respective fuel and oxidant gas inlet manifolds of the fuel cell stacks, and fuel and oxidant gas outlet ports that communicate through ducts with the oxidant and fuel gas outlet manifolds. Alternative arrangement of fuel cell stacks within a containment structure that does not require inlet and outlet manifolds is described in U.S. Pat. No. 8,962,210, assigned to the same assignee herein.
In internally reforming fuel cells, a reforming catalyst is placed within the fuel cell stack to allow direct use of hydrocarbon fuels such as pipe line natural gas, liquefied natural gas (LNG), liquefied petroleum gas (LPG), bio-gas, methane containing coal gas, etc. without the need for expensive and complex external reforming equipment. In an internal reformer, water and heat produced by the fuel cell are used by the reforming reaction, and hydrogen produced by the reforming reaction is used in the fuel cell. The heat produced by the fuel cell reaction supplies heat for the endothermic reforming reaction. Thus, internal reforming is used to cool the fuel cell stack.
Two different types of internally reforming fuel cell designs have been developed and used. The first type of an internally reforming fuel cell is a Direct Internally Reforming (DIR) fuel cell module, in which direct internal reforming is accomplished by placing the reforming catalyst within an active anode compartment of the fuel cell. A second type of internally reforming fuel cell utilizes Indirect Internal Reforming (IIR), which is accomplished by placing the reforming catalyst in an isolated chamber within the fuel cell stack and routing the reformed gas from this chamber into the anode compartment of the fuel cell. An internally reforming molten carbonate fuel cell system, also called Direct Fuel Cell (DFC), incorporating both the DIR and IIR, has evolved as the choice for environmentally friendly power generation and is the leading commercial option for green power. Carbonate power plants have lower emissions of greenhouse gases and particulate matter than conventional combustion-based power plants. Carbonate power plants emit little NOx gas, SOx gas, or particulate matter. Carbonate power plants have been designated âultra-cleanâ by the California Air Resources Board (CARB).
SUMMARY
It is an objective of the present invention to provide a high efficiency system including fuel cells for utilizing flue gas output from a flue gas producing system and for outputting exhaust gas from which carbon-dioxide can be easily separated and captured. More particularly, the present invention provides a fuel cell system that utilizes flue gas output from a flue gas producing system and which provides for separation and transfer of CO2 from the flue gas to produce a CO2-rich gas stream with CO2 concentrations approaching 90% on dry basis.
It is also an objective of the present invention to provide a cost-effective, modular fuel cell system, in which overall fuel utilization of 80% or greater and electrical and conversion system efficiency of 55% or greater are achieved while maintaining thermal balance within the fuel cells.
It is also a further objective of the present invention to provide a fuel cell system that utilizes between about 80% and 100% of the fuel provided to the system.
It is yet a further objective of the present invention to provide a fuel cell system in which partially spent fuel from the exhaust of a topping fuel cell module(s) is supplied to a bottoming fuel cell module(s).
It is also a further objective of the present invention to provide a controller for a fuel cell system that balances the pressure between a cathode portion and an anode portion of a fuel cell module(s) to eliminate the need for an anode pressure booster and/or a cathode pressure booster.
It is still yet a further objective of the present invention to provide a controller for a fuel cell system that adjusts carbon dioxide utilization to greater than 90%, while maintaining high electrical output in a topping fuel cell module(s) and a bottoming fuel cell module(s).
These and other objectives are achieved by a high efficiency fuel cell system that includes topping fuel cell stack(s) and bottoming fuel cell stack(s) and that has high fuel utilization during operation and provides for efficient concentration and separation of CO2 from anode exhaust of the bottoming fuel cell stack(s). The present invention has two possible configurations for efficient CO2 concentration and separation from the exhaust of the high efficiency fuel cell system. In one configuration of the system, flue gas from a flue gas generating device, such as a fossil fueled power plant or a chemical process plant, is partially or entirely fed to the cathodes of the system as inlet oxidant gas, with or without additional supplemental air. The fuel cells of this system are molten carbonate fuel cells and a significant part of the CO2 present in the flue gas is transported to the anode side by the electrochemical fuel cell reaction and combined with the CO2 already present in the fuel. The highly concentrated CO2 present in anode exhaust output from the bottoming fuel cell stacks is separated in a separation assembly and output for storage or for use in another process. The remaining CO2-depleted anode exhaust can be recycled as fuel to the fuel cell or used for heat generation or in another chemical process.
In another configuration of the system, air from outside or dried air is fed to the cathodes of the system as inlet oxidant gas. The fuel cells of this system configuration can be solid oxide fuel cells. Alternatively, phosphoric acid fuel cells or any other type of fuel cells may be used in this system configuration. In this configuration, CO2 concentration is very high in the anode exhaust of the bottoming fuel cell stack(s) due to the high fuel utilization (about 80% or greater), and the CO2 present in the anode exhaust is separated by a separation assembly and output for storage or for use in another process. The remaining CO2-depleted exhaust can be recycled as fuel to the fuel cell or used for heat generation or in another chemical process.
Some embodiments of the present invention are characterized by a high efficiency fuel cell system adapted to receive flue gas from a flue gas generating device and to capture carbon dioxide from the flue gas, the high efficiency fuel cell system comprising: a topping fuel cell assembly comprising a topping cathode portion and a topping anode portion; a bottoming fuel cell assembly comprising a bottoming cathode portion and a bottoming anode portion, wherein the bottoming anode portion receives anode exhaust output from the topping anode portion; and a separation assembly configured to receive carbon dioxide-containing exhaust and to separate carbon dioxide from the carbon dioxide-containing exhaust. In the system, the carbon dioxide-containing exhaust is one of anode exhaust output from the bottoming anode portion and a gas derived from the anode exhaust output from the bottoming anode portion, and at least one of the topping cathode portion and the bottoming cathode portion receives at least a portion of the flue gas output from the flue gas generating device.
In the system of the present invention, the topping cathode portion and the bottoming cathode portion each receive a portion of the flue gas output from the flue gas generating device in parallel. Alternatively, one of the topping cathode portion and the bottoming cathode portion receives at least a portion of the flue gas output from the flue gas generating device and generates cathode exhaust, and the other one of the topping cathode portion and the bottoming cathode portion receives the cathode exhaust generated by the one of the topping cathode portion and the bottoming cathode portion. For example, the topping cathode portion receives at least a portion of the flue gas output from the flue gas generating device and generates cathode exhaust, and the bottoming cathode portion receives the cathode exhaust generated by the topping cathode portion. In some embodiments, the topping cathode portion and the bottoming cathode portion are adapted to receive the flue gas in parallel or in series, and the system further comprises a controller for controlling the flow of the flue gas to the topping cathode portion and the bottoming cathode portion to be in parallel or in series.
In some embodiments, the carbon dioxide-containing exhaust is anode exhaust output from the bottoming anode portion, the separation assembly outputs separated carbon dioxide and separately outputs separated anode exhaust with a reduced amount of carbon dioxide, and the separated anode exhaust is recycled to the topping anode portion for use as fuel. The system may also include a conduit and a flow control member for bleeding off a small amount of the separated anode exhaust to remove inert gases prior to recycling the remaining separated anode exhaust to the topping anode portion.
In some embodiments, the system further comprises an oxidizer assembly for oxidizing anode exhaust output from the bottoming anode portion with one or more of air and oxygen to generate the carbon dioxide-containing exhaust and to generate waste heat for heating the flue gas before the flue gas is provided to the at least one of the topping and bottoming cathode portions, and the separation assembly outputs separated carbon dioxide and separately outputs a separated gas with a reduced amount of carbon dioxide.
In certain embodiments, the separation assembly comprises: a condenser for cooling the carbon dioxide-containing exhaust to separate water from the carbon dioxide-containing exhaust and to output a water separated carbon dioxide containing exhaust, and a carbon dioxide separator for separating carbon dioxide from the water separated carbon dioxide-containing exhaust to output a separated gas with a reduced carbon dioxide content and to separately output carbon dioxide suitable for one or more of sequestration and external use. The carbon dioxide separator separates the carbon dioxide using one or more of compression and cryogenic cooling to generate liquid carbon dioxide, solvent washing, and a membrane process.
The system further comprises a controller. In some embodiments, the controller controls the flow rate of the flue gas to the one or more of the topping cathode portion and bottoming cathode portion to support a fuel cell cathode side electrochemical reaction in each of the topping and bottoming fuel cell assemblies and to achieve a predetermined overall CO2 utilization. The predetermined overall CO2 utilization is 50% or greater, and in some embodiments 85% or greater. In some embodiments, the controller controls the flow rates of flue gas and fuel through the topping fuel cell assembly and bottoming fuel cell assembly so that pressures between the topping anode portion and the topping cathode portion are balanced and the pressures between the bottoming anode portion and the bottoming cathode portion are balanced. In some embodiments, the bottoming anode portion receives supplemental fuel, and the controller controls the amount of supplemental fuel conveyed to the bottoming anode portion, wherein the controller controls the amount of current generated in the bottoming fuel cell assembly by controlling the amount of supplemental fuel conveyed to the bottoming anode portion. In certain embodiments, the controller controls one or more of:
(a) the amount of current generated in the topping and bottoming fuel cell assemblies so that as operating time of the high efficiency fuel cell system increases, the amount of current generated by the topping fuel cell assemblies decreases and the amount of current generated by the bottoming fuel cell assemblies increases;
(b) the amount of current generated in the bottoming fuel cell assembly based on electricity needs so that the amount of current generated in the bottoming fuel cell assembly is increased when the electricity need increases;
(c) the flow of the flue gas to be conveyed to the topping fuel cell assembly and to the bottoming fuel cell assembly in parallel or in series;
(d) the flow rate of flue gas to the topping fuel cell assembly and to the bottoming fuel cell assembly to achieve a predetermined overall CO2 utilization;
(e) the pressure between the topping fuel cell module and the bottoming fuel cell module such that the pressure is balanced;
(f) the amount of fuel feed provided to each of the first and second topping fuel cell modules;
(g) the amount of supplemental fuel provided to the bottoming fuel cell assembly;
(h) recycling of separated exhaust output from the separation assembly to the topping anode portion; and
(i) the amount of supplemental air supplied to one or more of the topping cathod
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation of U.S. patent application Ser. No. 14/732,032, filed Jun. 5, 2015, the entirety of which is incorporated herein by reference.
BACKGROUND
This invention relates to fuel cell power production systems and, in particular, to a multi-stack high-efficiency fuel cell system with carbon dioxide capture capability and method of operating same. The systems of the present invention may be used with any types of fuel cells, and particularly with molten carbonate fuel cells and solid oxide fuel cells.
World energy consumption is increasing with average energy use growing at about 1.1% per year until 2040 according to the IEA (International Energy Agency). Currently, over 85% of the energy is supplied from fossil fuels. Fossil fuels used for electricity, transportation and heating require combustion, resulting in carbon dioxide emissions into earth's atmosphere. The carbon dioxide concentration in the atmosphere has almost doubled since humans started using fossil fuels and increasing carbon dioxide concentrations in the atmosphere is considered to be a major cause of global warming trends. In fact, the world is on a track to increase total carbon dioxide in the atmosphere by 20% by 2040, resulting in a projected global temperature rise of 3.6 C. Sustainable and efficient use of fossil fuels, as well as capture of emitted carbon dioxide will help to slow the increase of carbon dioxide concentrations in earth's atmosphere. Development of fuel cells, which output lower levels of emissions, as an alternative method of heat and electricity production to conventional fossil fuel-based combustion power plants is ongoing.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte matrix, which conducts electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
In building fuel cell systems, individual fuel cells are stacked together to form a fuel cell stack. The number of fuel cells determines the power rating of the fuel cell stack. To provide systems with higher power ratings, a number of fuel cell stacks are utilized and the outputs of the fuel cell stacks are combined to provide the desired power output. In certain fuel cell systems, the fuel cell stack(s) may be organized in one or more fuel cell stack modules, each of which includes one or more fuel cell stacks housed in an enclosure or a containment structure.
A multi-stack fuel cell system may include a fuel cell stack module with multiple fuel cell stacks housed within a common enclosure. In a system of this design developed for high temperature fuel cell stacks and, in particular, for molten carbonate fuel cell stacks, a box-like containment structure is employed as the enclosure and the fuel cell stacks may be arranged along the length of the containment structure. Each fuel cell stack within the fuel cell module may have inlet manifolds for receiving fuel and oxidant gases needed to operate the fuel cell stack and outlet manifolds for conveying spent fuel and oxidant gases as anode and cathode exhausts from the fuel cell stack. The containment structure of the fuel cell module includes fuel and oxidant gas inlet ports that communicate through ducts with the respective fuel and oxidant gas inlet manifolds of the fuel cell stacks, and fuel and oxidant gas outlet ports that communicate through ducts with the oxidant and fuel gas outlet manifolds. Alternative arrangement of fuel cell stacks within a containment structure that does not require inlet and outlet manifolds is described in U.S. Pat. No. 8,962,210, assigned to the same assignee herein.
In internally reforming fuel cells, a reforming catalyst is placed within the fuel cell stack to allow direct use of hydrocarbon fuels such as pipe line natural gas, liquefied natural gas (LNG), liquefied petroleum gas (LPG), bio-gas, methane containing coal gas, etc. without the need for expensive and complex external reforming equipment. In an internal reformer, water and heat produced by the fuel cell are used by the reforming reaction, and hydrogen produced by the reforming reaction is used in the fuel cell. The heat produced by the fuel cell reaction supplies heat for the endothermic reforming reaction. Thus, internal reforming is used to cool the fuel cell stack.
Two different types of internally reforming fuel cell designs have been developed and used. The first type of an internally reforming fuel cell is a Direct Internally Reforming (DIR) fuel cell module, in which direct internal reforming is accomplished by placing the reforming catalyst within an active anode compartment of the fuel cell. A second type of internally reforming fuel cell utilizes Indirect Internal Reforming (IIR), which is accomplished by placing the reforming catalyst in an isolated chamber within the fuel cell stack and routing the reformed gas from this chamber into the anode compartment of the fuel cell. An internally reforming molten carbonate fuel cell system, also called Direct Fuel Cell (DFC), incorporating both the DIR and IIR, has evolved as the choice for environmentally friendly power generation and is the leading commercial option for green power. Carbonate power plants have lower emissions of greenhouse gases and particulate matter than conventional combustion-based power plants. Carbonate power plants emit little NOx gas, SOx gas, or particulate matter. Carbonate power plants have been designated âultra-cleanâ by the California Air Resources Board (CARB).
SUMMARY
It is an objective of the present invention to provide a high efficiency system including fuel cells for utilizing flue gas output from a flue gas producing system and for outputting exhaust gas from which carbon-dioxide can be easily separated and captured. More particularly, the present invention provides a fuel cell system that utilizes flue gas output from a flue gas producing system and which provides for separation and transfer of CO2 from the flue gas to produce a CO2-rich gas stream with CO2 concentrations approaching 90% on dry basis.
It is also an objective of the present invention to provide a cost-effective, modular fuel cell system, in which overall fuel utilization of 80% or greater and electrical and conversion system efficiency of 55% or greater are achieved while maintaining thermal balance within the fuel cells.
It is also a further objective of the present invention to provide a fuel cell system that utilizes between about 80% and 100% of the fuel provided to the system.
It is yet a further objective of the present invention to provide a fuel cell system in which partially spent fuel from the exhaust of a topping fuel cell module(s) is supplied to a bottoming fuel cell module(s).
It is also a further objective of the present invention to provide a controller for a fuel cell system that balances the pressure between a cathode portion and an anode portion of a fuel cell module(s) to eliminate the need for an anode pressure booster and/or a cathode pressure booster.
It is still yet a further objective of the present invention to provide a controller for a fuel cell system that adjusts carbon dioxide utilization to greater than 90%, while maintaining high electrical output in a topping fuel cell module(s) and a bottoming fuel cell module(s).
These and other objectives are achieved by a high efficiency fuel cell system that includes topping fuel cell stack(s) and bottoming fuel cell stack(s) and that has high fuel utilization during operation and provides for efficient concentration and separation of CO2 from anode exhaust of the bottoming fuel cell stack(s). The present invention has two possible configurations for efficient CO2 concentration and separation from the exhaust of the high efficiency fuel cell system. In one configuration of the system, flue gas from a flue gas generating device, such as a fossil fueled power plant or a chemical process plant, is partially or entirely fed to the cathodes of the system as inlet oxidant gas, with or without additional supplemental air. The fuel cells of this system are molten carbonate fuel cells and a significant part of the CO2 present in the flue gas is transported to the anode side by the electrochemical fuel cell reaction and combined with the CO2 already present in the fuel. The highly concentrated CO2 present in anode exhaust output from the bottoming fuel cell stacks is separated in a separation assembly and output for storage or for use in another process. The remaining CO2-depleted anode exhaust can be recycled as fuel to the fuel cell or used for heat generation or in another chemical process.
In another configuration of the system, air from outside or dried air is fed to the cathodes of the system as inlet oxidant gas. The fuel cells of this system configuration can be solid oxide fuel cells. Alternatively, phosphoric acid fuel cells or any other type of fuel cells may be used in this system configuration. In this configuration, CO2 concentration is very high in the anode exhaust of the bottoming fuel cell stack(s) due to the high fuel utilization (about 80% or greater), and the CO2 present in the anode exhaust is separated by a separation assembly and output for storage or for use in another process. The remaining CO2-depleted exhaust can be recycled as fuel to the fuel cell or used for heat generation or in another chemical process.
Some embodiments of the present invention are characterized by a high efficiency fuel cell system adapted to receive flue gas from a flue gas generating device and to capture carbon dioxide from the flue gas, the high efficiency fuel cell system comprising: a topping fuel cell assembly comprising a topping cathode portion and a topping anode portion; a bottoming fuel cell assembly comprising a bottoming cathode portion and a bottoming anode portion, wherein the bottoming anode portion receives anode exhaust output from the topping anode portion; and a separation assembly configured to receive carbon dioxide-containing exhaust and to separate carbon dioxide from the carbon dioxide-containing exhaust. In the system, the carbon dioxide-containing exhaust is one of anode exhaust output from the bottoming anode portion and a gas derived from the anode exhaust output from the bottoming anode portion, and at least one of the topping cathode portion and the bottoming cathode portion receives at least a portion of the flue gas output from the flue gas generating device.
In the system of the present invention, the topping cathode portion and the bottoming cathode portion each receive a portion of the flue gas output from the flue gas generating device in parallel. Alternatively, one of the topping cathode portion and the bottoming cathode portion receives at least a portion of the flue gas output from the flue gas generating device and generates cathode exhaust, and the other one of the topping cathode portion and the bottoming cathode portion receives the cathode exhaust generated by the one of the topping cathode portion and the bottoming cathode portion. For example, the topping cathode portion receives at least a portion of the flue gas output from the flue gas generating device and generates cathode exhaust, and the bottoming cathode portion receives the cathode exhaust generated by the topping cathode portion. In some embodiments, the topping cathode portion and the bottoming cathode portion are adapted to receive the flue gas in parallel or in series, and the system further comprises a controller for controlling the flow of the flue gas to the topping cathode portion and the bottoming cathode portion to be in parallel or in series.
In some embodiments, the carbon dioxide-containing exhaust is anode exhaust output from the bottoming anode portion, the separation assembly outputs separated carbon dioxide and separately outputs separated anode exhaust with a reduced amount of carbon dioxide, and the separated anode exhaust is recycled to the topping anode portion for use as fuel. The system may also include a conduit and a flow control member for bleeding off a small amount of the separated anode exhaust to remove inert gases prior to recycling the remaining separated anode exhaust to the topping anode portion.
In some embodiments, the system further comprises an oxidizer assembly for oxidizing anode exhaust output from the bottoming anode portion with one or more of air and oxygen to generate the carbon dioxide-containing exhaust and to generate waste heat for heating the flue gas before the flue gas is provided to the at least one of the topping and bottoming cathode portions, and the separation assembly outputs separated carbon dioxide and separately outputs a separated gas with a reduced amount of carbon dioxide.
In certain embodiments, the separation assembly comprises: a condenser for cooling the carbon dioxide-containing exhaust to separate water from the carbon dioxide-containing exhaust and to output a water separated carbon dioxide containing exhaust, and a carbon dioxide separator for separating carbon dioxide from the water separated carbon dioxide-containing exhaust to output a separated gas with a reduced carbon dioxide content and to separately output carbon dioxide suitable for one or more of sequestration and external use. The carbon dioxide separator separates the carbon dioxide using one or more of compression and cryogenic cooling to generate liquid carbon dioxide, solvent washing, and a membrane process.
The system further comprises a controller. In some embodiments, the controller controls the flow rate of the flue gas to the one or more of the topping cathode portion and bottoming cathode portion to support a fuel cell cathode side electrochemical reaction in each of the topping and bottoming fuel cell assemblies and to achieve a predetermined overall CO2 utilization. The predetermined overall CO2 utilization is 50% or greater, and in some embodiments 85% or greater. In some embodiments, the controller controls the flow rates of flue gas and fuel through the topping fuel cell assembly and bottoming fuel cell assembly so that pressures between the topping anode portion and the topping cathode portion are balanced and the pressures between the bottoming anode portion and the bottoming cathode portion are balanced. In some embodiments, the bottoming anode portion receives supplemental fuel, and the controller controls the amount of supplemental fuel conveyed to the bottoming anode portion, wherein the controller controls the amount of current generated in the bottoming fuel cell assembly by controlling the amount of supplemental fuel conveyed to the bottoming anode portion. In certain embodiments, the controller controls one or more of:
(a) the amount of current generated in the topping and bottoming fuel cell assemblies so that as operating time of the high efficiency fuel cell system increases, the amount of current generated by the topping fuel cell assemblies decreases and the amount of current generated by the bottoming fuel cell assemblies increases;
(b) the amount of current generated in the bottoming fuel cell assembly based on electricity needs so that the amount of current generated in the bottoming fuel cell assembly is increased when the electricity need increases;
(c) the flow of the flue gas to be conveyed to the topping fuel cell assembly and to the bottoming fuel cell assembly in parallel or in series;
(d) the flow rate of flue gas to the topping fuel cell assembly and to the bottoming fuel cell assembly to achieve a predetermined overall CO2 utilization;
(e) the pressure between the topping fuel cell module and the bottoming fuel cell module such that the pressure is balanced;
(f) the amount of fuel feed provided to each of the first and second topping fuel cell modules;
(g) the amount of supplemental fuel provided to the bottoming fuel cell assembly;
(h) recycling of separated exhaust output from the separation assembly to the topping anode portion; and
(i) the amount of supplemental air supplied to one or more of the topping cathode portion and the bottoming cathode portion so as to control the temperature and concentration of carbon dioxide in a cathode inlet gas conveyed to the one or more of the topping cathode portion and the bottoming cathode portion.
In certain embodiments, the topping fuel cell assembly has a greater number of fuel cells than the bottoming fuel cell assembly so that the topping fuel cell assembly utilizes more fuel than the bottoming fuel cell assembly, and wherein overall fuel utilization by the high fuel cell system is 80% or greater.
In certain embodiments, high efficiency fuel cell system will use fresh or dried fresh air in the topping and bottom cathodes instead of receiving and using flue gas from a flue gas generating device. In this mode of operation, the exhaust from the anodes of the bottoming fuel cells is conveyed to a gas separation assembly where carbon dioxide and water are separated from the unutilized fuel. Separated carbon dioxide can be sequestered or used for other applications. The separated unutilized fuel, with reduced carbon dioxide and water can be pressurized and combined with the fuel feed to the topping module. The fuel cells in this system would be solid oxide fuel cells. However, this system may also be used with other types of fuel cells, including phosphoric acid fuel cells.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which:
FIG. 1 shows a high efficiency fuel cell system including a first topping fuel cell module, a second topping fuel cell module, a bottoming fuel cell module, and a carbon capture assembly; and
FIG. 2 shows an alternative embodiment of the high efficiency fuel cell system of FIG. 1 .
DETAILED DESCRIPTION
As shown and described below, the present invention provides a cost-effective, modular fuel cell system, in which capture of excess carbon dioxide is achieved while maintaining higher overall fuel utilization, higher electrical power output and improved conversion system efficiency. In accordance with the present invention, the fuel cell system includes a plurality of fuel cell stacks or a plurality of fuel cell stack modules including one or more topping stack(s) or topping stack module(s) and one or more bottoming stack(s) or bottoming stack module(s), and wherein the one or more topping stack(s)/module(s) receive fresh fuel and partially spent fuel from the exhaust of the one or more topping fuel cell stack(s)/module(s) is supplied to the one or more bottoming fuel cell stack(s)/module(s). The fuel cell system also receives and utilizes carbon dioxide-containing flue gas output from one or more flue gas generating devices as inlet oxidant gas, which can be provided to the cathodes of the one or more topping stack(s)/module(s) and/or one or more bottoming stack(s)/module(s) in parallel or in series. During the electrochemical reactions in the fuel cell stacks of the system, most of the carbon dioxide contained in the flue gas is extracted and output with anode exhaust from the bottoming fuel cell stack(s)/module(s). In the present invention, anode exhaust from the bottoming fuel cell stack(s)/module(s) is conveyed to a carbon dioxide capture assembly, where carbon dioxide is separated and captured, and a remaining gas stream comprising unused fuel may be mixed with fresh fuel and supplied to the one or more topping fuel cell stack(s)/module(s). In certain alternative embodiments, anode exhaust from the bottoming fuel cell stack(s)/module(s) is oxidized in an anode exhaust oxidizer (AGO) and thereafter conveyed to a carbon dioxide capture assembly, where carbon dioxide is separated from the AGO exhaust and stored away or used externally. In both embodiments of the present invention, up to 90% of carbon dioxide fed to the system could be separated and concentrated in the anode exhaust, and the carbon dioxide is separated from the anode exhaust for capture and sequestration. Moreover, the carbon dioxide utilization in the topping and bottoming fuel cell stack(s)/module(s) is controlled and adjusted to maintain the overall carbon dioxide utilization of 50% or greater and in some embodiments 85% or greater, or approaching 90%, while still maintaining high voltages in the topping and bottoming fuel cell stack(s)/module(s).
In the configuration of the system of the present invention, fuel utilization in the topping and bottoming fuel cell stacks/modules are controlled within desired limits while still increasing the overall fuel utilization and electrical efficiency of the system. Specifically, the size of the topping fuel cell stack(s)/module(s) and the fuel utilized are made greater than the size of the bottoming fuel cell stack(s)/module(s). In certain embodiments of the invention, the system includes a topping fuel cell stack or module with two or more stacks and a bottoming fuel cell stack or module with two or more stacks, wherein the topping fuel cell stack has a greater number of fuel cells stacked in series than the bottoming fuel cell stack so that the topping stack utilizes more fuel in the fuel feed than the bottoming stack. In other embodiments of the invention, the system includes multiple topping fuel cell modules and one or more bottoming fuel cell modules, wherein each fuel cell module comprises one or more fuel cell stacks, and the number of topping fuel cell modules is greater than the number of bottoming fuel cell modules. In some embodiments, the topping fuel cell stack/modules consume about two thirds of the fuel and the bottoming fuel cell stack(s)/module(s) consume the remaining about one third of the fuel in the fuel feed. The bottoming fuel cell stack(s)/module(s) may receive additional fresh fuel, which may be dry fuel or may be premixed with water vapor. In addition, the current density in the bottoming fuel cell stack(s)/module(s) is lower than the current density in the topping fuel cell stack/modules.
In the present invention, current generated in the topping stack(s)/module(s) and in the bottoming stack(s)/module(s) may be controlled over operating time of the system. Specifically, over time, current generated in the topping stack(s)/module(s) may be reduced while current generated in the bottoming stack(s)/module(s) is increased. In the bottoming stack(s)/module(s), current generated is changed based on the amount of methane or other fresh fuel supplied to the bottoming stack(s)/module(s). Moreover, current generated in the bottoming stack(s)/module(s) may be adjustable to adapt to customer needs for heat and electricity. For example, when the customer need for electricity is greater, the output of the bottoming stack(s)/module(s) may be increased. In addition, the oxidizer that oxidizes anode exhaust conveyed from the bottoming stack(s)/module(s) may include design features that allow the processing of the anode exhaust from the bottoming stack(s)/module(s) at various output levels of the bottoming module. For example, when the bottoming stack(s)/module(s) generate less power, there will be more excess fuel supplied in the anode exhaust to the anode exhaust oxidizer, and thus, the anode exhaust oxidizer is designed to be capable of handling large amounts of unutilized fuel. In certain embodiments, the system may be adapted to use the heat in the plant exhaust of the high efficiency fuel cell system for heating the high pressure natural gas from a gas distribution system in an Energy Recovery Generation System before pressure is let down through a turbine. Use of heat from the fuel cell power plant for heating the high pressure natural gas from a gas distribution system is discussed in U.S. Pat. No. 8,080,344, assigned to the same assignee herein.
In some embodiments, the pressure of the anode exhaust from the topping fuel cell stack(s)/module(s) supplied to the bottoming fuel cell stack(s)/module(s) may be controlled using an anode pressure booster, and the pressure of the cathode exhaust from the bottoming fuel cell stack(s)/module(s) supplied to the topping fuel cell stack(s)/module(s) may be controlled using a cathode pressure booster. In this way, the pressure differential between the anode and cathode streams in the topping module may be controlled using the anode pressure booster and/or the cathode pressure booster. However, in other embodiments, the pressures between the anodes and the cathodes in the topping and bottoming fuel cell stack(s)/module(s) is balanced such that one or more of the anode pressure booster and the cathode pressure booster is eliminated from the system.
The present invention further provides improved thermal uniformity in the fuel cells of the system, particularly in the fuel cells of the bottoming stack(s)/module(s). In some embodiments, the cells in the bottoming stack(s)/module(s) use a different amount and a different distribution of direct internal reforming (DIR) catalyst than the cells in the topping stack(s)/module(s) in order to improve thermal uniformity of the cells in the bottoming stack(s)/module(s). Specifically, the cells in the bottoming stack(s)/module(s) have a gradually increasing direct internal reforming catalyst loading from the anode inlet region to the anode outlet region. In contrast, the cells in the topping stack(s)/module(s) have DIR catalysts distributed uniformly from the anode inlet to the outlet region.
The systems of the present invention and described in more detail below are particularly suited for use with molten carbonate fuel cell stacks. However, these systems may be adapted for use with solid oxide fuel cell stacks and with other types of fuel cells. For example, the system of the present invention may be adapted for use with solid oxide fuel cells and/or with phosphoric acid fuel cells so that the system receives fresh air or dried fresh air as inlet oxidant gas, which is provided to the cathodes of the one or more topping stack(s)/module(s) and/or one or more bottoming stack(s)/module(s) in parallel or in series. During the electrochemical reactions in the fuel cell stacks of the system, most of the carbon dioxide contained in the fuel and in the inlet oxidant gas when the fuel cells are carbonate fuel cells, is output from the anodes of the bottoming stack(s)/module(s) as anode exhaust, which is conveyed to the carbon dioxide capture assembly (gas separation assembly), where carbon dioxide is separated and captured, and the remaining gas stream comprising unused fuel may be mixed with fresh fuel and supplied to the one or more topping fuel cell stack(s)/module(s). In some alternative embodiments, the anode exhaust from the bottoming fuel cell stack(s)/module(s) is oxidized in the anode exhaust oxidizer (AGO) with oxygen and then conveyed to the carbon dioxide capture assembly, where carbon dioxide is separated from the AGO exhaust and stored away or used externally. These systems are suitable for use with solid oxide fuel cells in the topping and bottoming fuel cell stack(s)/module(s) to result in high efficient fuel utilization while also separating and capturing carbon dioxide from the system exhaust. Moreover, these systems may instead utilize phosphoric acid fuel cells, or other types of fuel cells.
FIG. 1 shows an illustrative embodiment of a modular fuel cell system 100 of the present invention. The fuel cell system 100 shown in FIG. 1 has improved performance efficiencies, and in particular, higher fuel utilization and power output compared to conventional fuel cell systems, and also provides manufacturing and operational cost efficiencies compared to conventional fuel cell systems described in the prior art. The system 100 captures carbon dioxide from exhaust generated by the system, reducing the overall carbon-footprint of the system 100 . The improved performance, cost efficiencies and carbon capture capability of the fuel cell system 100 are described in more detail herein below.
The fuel cell system 100 of FIG. 1 includes a topping fuel cell assembly and a bottoming fuel cell assembly. In FIG. 1 , the topping fuel cell assembly includes first and second topping fuel cell modules
102 , 112 while the bottoming fuel cell assembly includes a bottoming fuel cell module 122 . The first topping fuel cell module 102 comprises one or more first fuel cell stacks 102 A, and each fuel cell stack having two or more cells and having a first topping cathode portion 104 and a first topping anode portion 106 . Although in FIG. 1 , the first topping fuel cell module 102 is shown as having one fuel cell stack, it is understood that in some embodiments, the first fuel cell topping module 102 includes two or more fuel cell stacks having similar configurations and being disposed within a common containment structure. The fuel cell stacks in the first topping fuel cell module 102 may include high temperature molten carbonate fuel cell stacks. In each cell of each stack 102 A, the first topping cathode portion 104 and the first topping anode portion 106 are separated by molten carbonate electrolyte (not shown) stored in an electrolyte matrix.
In FIG. 1 , the first topping fuel cell stack 102 A is an internally reforming fuel cell stack and includes either direct internal reforming, indirect internal reforming or a combination of both direct and indirect internal reforming. In the present illustrative embodiment, the first topping fuel cell stack 102 A includes one or more internal reforming units 148 receiving a portion of the fuel feed and conveying reformed or partially reformed fuel, including hydrogen and carbon monoxide, to a fuel turn manifold 106 A, which directs the fuel to the first topping anode portion 106 of the cells of the stack where the fuel undergoes an electrochemical reaction with oxidant gas passing through the first topping cathode portion 104 . In the illustrative embodiment of FIG. 1 , the first topping cathode portion 104 receives oxidant gas, e.g., flue gas generated by a flue gas generating device, provided to the common containment structure through an open stack face. However, in other embodiments, the oxidant gas may be supplied to the first topping cathode portion 104 through a cathode inlet manifold (not shown).
As shown in FIG. 1 , the first topping cathode portion 104 conveys cathode exhaust to a cathode outlet manifold 108 . Cathode exhaust is then conveyed from the cathode outlet manifold 108 via suitable ducting to a heat exchanger 136 which can be provided either inside or outside the first topping fuel cell module 102 . The first topping anode portion 106 conveys anode exhaust to an anode outlet manifold 110 . Anode exhaust is then conveyed from the anode outlet manifold 110 to outside of the first topping fuel cell module 102 for use in the bottoming fuel cell module 122 .
As mentioned above, the first topping stack 102 A includes one or more internal reforming units 148 . Hydrogen, carbon dioxide and carbon monoxide are produced from fuel passing through the one or more internal reforming units 148 . Reformed or partially reformed fuel is then supplied to the first topping anode portion 106 of the stack via fuel turn manifold 106 A. In the first topping anode portion 106 of the stack, hydrogen electrochemically reacts to produce water, carbon monoxide either reacts electrochemically to form carbon dioxide or chemically with water to produce hydrogen and carbon dioxide. Direct internal reforming (DIR) may also be provided in each cell of the first topping stack 102 A by placing reforming catalyst in an anode compartment(s) of the first topping anode portion 106 , and in particular, by placing the reforming catalyst in corrugations of an anode current collector of the anode compartment(s) of each cell in the stack.
As discussed above, the first topping fuel cell module 102 may include multiple first topping fuel cell stacks. The number of fuel cell stacks in each fuel cell module is determined based on the number needed to provide the desired power output. The number of fuel cells in each fuel cell stack may be determined by the required output, size and weight of the stacks and ease of transportation.
The topping fuel cell assembly of the fuel cell system 100 also includes the second topping fuel cell module 112 , which includes one or more second fuel cell stacks. As with the first topping fuel cell module 102 , FIG. 1 shows a single fuel cell stack 112 A being included in the second topping fuel cell module 112 , but it is contemplated that two or more second topping fuel cell stacks 112 A may be included in the second fuel cell module 112 and housed in the same containment structure. The second topping fuel cell stacks in the second topping fuel cell module 112 may comprise high temperature molten carbonate fuel cells.
As shown in FIG. 1 , the second topping fuel cell stack 112 A includes a second topping cathode portion 114 and a second topping anode portion 116 which in each cell of a stack is separated by an electrolyte matrix storing therein molten carbonate electrolyte (not shown). Fuel entering the second topping anode portion 116 is reformed internally to produce partially or fully reformed fuel including hydrogen and carbon monoxide, which then undergoes an electrochemical reaction with oxidant gas passing through the second topping cathode portion 114 . In the second topping anode portion 116 of the stack, hydrogen electrochemically reacts to produce water, and carbon monoxide either reacts electrochemically to form carbon dioxide or chemically with water to produce hydrogen and carbon dioxide. The second topping fuel cell stack 112 A includes indirect internal reforming, direct internal reforming or a combination of both direct and indirect internal reforming. In the present illustrative embodiment, the second topping fuel cell stack 112 A includes one or more reforming units 150 receiving a portion of the fuel feed and conveying reformed or partially reformed fuel, including hydrogen and carbon monoxide, to a fuel turn manifold 116 A, which directs the fuel to the second topping anode portion 116 of the stack where the fuel undergoes the electrochemical reaction with the oxidant gas. In FIG. 1 , the second topping cathode portion 114 receives oxidant gas, e.g., flue gas generated by the flue gas generating device, provided to the common containment structure through an open cathode inlet stack face. However, in other embodiments, the oxidant gas may be supplied to the second topping cathode portion 114 through a cathode inlet manifold (not shown).
As shown in FIG. 1 , the second topping cathode portion 114 conveys cathode exhaust to a cathode outlet manifold 118 . Cathode exhaust is then conveyed from the cathode outlet manifold 118 via suitable ducting to a heat exchanger 138 which can be provided either inside or outside the second topping fuel cell module 112 . The second topping anode portion 116 conveys anode exhaust to an anode outlet manifold 120 . Anode exhaust is then conveyed from the anode outlet manifold 120 to outside of the second topping fuel cell module 112 for use in the bottoming fuel cell module 122 .
As mentioned above, the second topping fuel cell stack 112 A includes one or more internal reforming units 150 . Hydrogen, carbon dioxide and carbon monoxide are produced from fuel passing through the one or more internal reforming units 150 . Reformed or partially reformed fuel is then supplied to the second topping anode portion 116 of the stack via the turn manifold 116 A. Direct internal reforming (DIR) may also be provided in the second topping stack 112 A by placing reforming catalyst in the anode compartment(s) of the second topping anode portion 116 , and in particular, by placing the reforming catalyst in corrugations of an anode current collector of the anode compartment(s) in each cell of the second topping stack 112 A.
Similarly as described above with respect to the first topping fuel cell module 102 , the second topping fuel cell module 112 may include multiple second topping fuel cell stacks having similar construction. The number of fuel cell stacks per module is determined based on the number needed to provide the desired power output. The number of fuel cells in each fuel cell stack may be determined by the required output, size and weight of the stacks and ease of transportation.
As shown in FIG. 1 , the fuel cell system 100 includes the bottoming fuel cell assembly, which, in FIG. 1 , comprises the bottoming fuel cell module 122 , including one or more fuel cell stacks 122 A housed by a common containment structure. Each fuel cell in the bottoming fuel cell stack 122 A includes a bottoming cathode portion 124 and a bottoming anode portion 126 , which in each cell of a stack are separated by an electrolyte matrix storing therein molten carbonate electrolyte (not shown).
The bottoming fuel cell stack 122 A is an internally reforming fuel cell stack and includes direct internal reforming, indirect internal reforming or a combination of both direct and indirect internal reforming. In the embodiment of FIG. 1 , the bottoming fuel cell stack 122 A includes direct internal reforming catalyst placed in the anode compartment(s) of the bottoming anode portion 126 , and in particular, in corrugations of an anode current collector of the anode compartment(s) in each cell of the bottoming fuel cell stack 122 A. Although in the embodiment shown in FIG. 1 , the bottoming fuel cell stack 122 A does not include indirect internal reforming, in other embodiments, the bottoming fuel cell stack may include one or more reforming units for further reforming the anode exhaust received from the first and second topping modules before the reformed anode exhaust is conveyed to the bottoming anode portion 126 .
In FIG. 1 , the bottoming cathode portion 124 receives oxidant gas, e.g., flue gas generated by a flue gas generating device, provided to the common containment structure through an open cathode inlet stack face. However, in other embodiments, oxidant gas may be supplied to the bottoming cathode portion 124 through a cathode inlet manifold. The bottoming cathode portion 124 conveys cathode exhaust into a <figure-callout id="128" label="cathode outlet manifold" file
CLAIMS
Claims ( 20 )
What is claimed is:
1. A high efficiency fuel cell system comprising:
a topping fuel cell assembly comprising a topping cathode portion and a topping anode portion;
a bottoming fuel cell assembly comprising a bottoming cathode portion and a bottoming anode portion, wherein the bottoming anode portion receives anode exhaust output from the topping anode portion;
a flue gas generating device configured to provide a portion of the flue gas to each of the topping cathode portion and the bottoming cathode portion in parallel;
an oxidizer assembly configured to (i) oxidize anode exhaust output from the bottoming anode portion with air and/or oxygen to generate carbon dioxide-containing exhaust and (ii) generate waste heat for heating the flue gas before the flue gas is provided to the topping cathode portion and/or the bottoming cathode portion; and
a separation assembly configured to receive the carbon dioxide-containing exhaust from the oxidizer assembly and to separate carbon dioxide from the carbon dioxide-containing exhaust.
2. The high efficiency fuel cell system of claim 1 , further comprising a controller configured to control a temperature in the oxidizer assembly by controlling an amount of the air and/or oxygen provided to the oxidizer assembly.
3. The high efficiency fuel cell system of claim 1 , wherein:
the separation assembly comprises a condenser, and a carbon dioxide separator;
the condenser is configured to condense out water and output water separated carbon dioxide-containing exhaust to the carbon dioxide separator, and
the carbon dioxide separator is configured to separate carbon dioxide from the water separated carbon dioxide-containing exhaust.
4. The high efficiency fuel cell system in accordance with claim 3 , wherein the carbon dioxide separator is configured to separate the carbon dioxide using compression and cryogenic cooling to generate liquid carbon dioxide.
5. The high efficiency fuel cell system in accordance with claim 3 , wherein the carbon dioxide separator is configured to separate the carbon dioxide using solvent washing.
6. The high efficiency fuel cell system in accordance with claim 3 , wherein the carbon dioxide separator is configured to separate the carbon dioxide using a membrane process.
7. The high efficiency fuel cell system in accordance with claim 1 , wherein one of the topping cathode portion and the bottoming cathode portion receives at least a portion of the flue gas output from the flue gas generating device and generates cathode exhaust, and the other one of the topping cathode portion and the bottoming cathode portion receives the cathode exhaust generated by the one of the topping cathode portion and the bottoming cathode portion.
8. The high efficiency fuel cell system in accordance with claim 7 , wherein the topping cathode portion receives at least a portion of the flue gas output from the flue gas generating device and generates cathode exhaust, and the bottoming cathode portion receives the cathode exhaust generated by the topping cathode portion.
9. The high efficiency fuel cell system in accordance with claim 1 , wherein the topping cathode portion and the bottoming cathode portion are configured to selectively receive the flue gas in parallel or in series, and wherein the system further comprises a controller configured to selectively control a flow of the flue gas to the topping cathode portion and the bottoming cathode portion to be in parallel or in series.
10. The high efficiency fuel cell system in accordance with claim 1 , further comprising a controller configured to a the flow rate of the flue gas to the topping cathode portion and/or the bottoming cathode portion to support a fuel cell cathode side electrochemical reaction in each of the topping and bottoming fuel cell assemblies and to achieve an overall carbon dioxide utilization of 50% or greater.
11. The high efficiency fuel cell system in accordance with claim 1 , further comprising a controller configured to control flow rates of flue gas and fuel through the topping fuel cell assembly and bottoming fuel cell assembly such that pressures between the topping anode portion and the topping cathode portion are balanced and pressures between the bottoming anode portion and the bottoming cathode portion are balanced.
12. The high efficiency fuel cell system in accordance with claim 1 , wherein overall fuel utilization by the high efficiency fuel cell system is 80% or greater.
13. The high efficiency fuel cell system in accordance with claim 1 , further comprising:
a supplemental fuel source configured to supply supplemental fuel to the bottoming anode portion; and
a controller configured to control an amount of current generated in the bottoming fuel cell assembly by controlling an amount of supplemental fuel conveyed to the bottoming anode portion.
14. The high efficiency fuel cell system in accordance with claim 1 , further comprising a controller configured to control an amount of current generated in the topping and bottoming fuel cell assemblies such that, as operating time of the high efficiency fuel cell system increases, the amount of current generated by the topping fuel cell assemblies decreases and the amount of current generated by the bottoming fuel cell assemblies increases.
15. The high efficiency fuel cell system in accordance with claim 1 , further comprising a controller configured to control a flow rate of flue gas to the topping fuel cell assembly and to the bottoming fuel cell assembly to achieve a predetermined overall carbon dioxide utilization.
16. The high efficiency fuel cell system in accordance with claim 1 , further comprising a controller configured to control an amount of supplemental air supplied to the topping cathode portion and/or the bottoming cathode portion so as to control the temperature and concentration of carbon dioxide and oxygen in a cathode inlet gas conveyed to the topping cathode portion and/or the bottoming cathode portion.
17. A method of operating a high efficiency fuel cell system, the method comprising:
providing a topping fuel cell assembly comprising a topping cathode portion and a topping anode portion;
providing a bottoming fuel cell assembly comprising a bottoming cathode portion and a bottoming anode portion,
providing a portion of the flue gas to each of the topping cathode portion and the bottoming cathode portion in parallel;
receiving, at the bottoming anode portion, anode exhaust output from the topping anode portion;
oxidizing, with an oxidizer assembly, anode exhaust output from the bottoming anode portion with air and/or oxygen to generate carbon dioxide-containing exhaust;
heating the flue gas before the flue gas is provided to the topping cathode portion and/or the bottoming cathode portion using waste heat generated during said oxidizing; and
receiving, at a separation assembly, the carbon dioxide-containing exhaust from the oxidizer assembly and separating carbon dioxide from the carbon dioxide-containing exhaust.
18. A high efficiency fuel cell system comprising:
a topping fuel cell assembly comprising a topping cathode portion and a topping anode portion;
a bottoming fuel cell assembly comprising a bottoming cathode portion and a bottoming anode portion, wherein the bottoming anode portion receives anode exhaust output from the topping anode portion;
a flue gas generating device configured to provide flue gas to the topping cathode portion and/or the bottoming cathode portion;
an oxidizer assembly configured to (i) oxidize anode exhaust output from the bottoming anode portion with air and/or oxygen to generate carbon dioxide-containing exhaust and (ii) generate waste heat for heating the flue gas before the flue gas is provided to the topping cathode portion and/or the bottoming cathode portion; and
a separation assembly configured to receive the carbon dioxide-containing exhaust from the oxidizer assembly and to separate carbon dioxide from the carbon dioxide-containing exhaust,
wherein one of the topping cathode portion and the bottoming cathode portion receives at least a portion of the flue gas output from the flue gas generating device and generates cathode exhaust, and the other one of the topping cathode portion and the bottoming cathode portion receives the cathode exhaust generated by the one of the topping cathode portion and the bottoming cathode portion.
19. The high efficiency fuel cell system in accordance with claim 18 , wherein the topping cathode portion receives at least a portion of the flue gas output from the flue gas generating device and generates cathode exhaust, and the bottoming cathode portion receives the cathode exhaust generated by the topping cathode portion.
20. A high efficiency fuel cell system comprising:
a topping fuel cell assembly comprising a topping cathode portion and a topping anode portion;
a bottoming fuel cell assembly comprising a bottoming cathode portion and a bottoming anode portion, wherein the bottoming anode portion receives anode exhaust output from the topping anode portion;
a flue gas generating device configured to provide flue gas to the topping cathode portion and/or the bottoming cathode portion;
an oxidizer assembly configured to (i) oxidize anode exhaust output from the bottoming anode portion with air and/or oxygen to generate carbon dioxide-containing exhaust and (ii) generate waste heat for heating the flue gas before the flue gas is provided to the topping cathode portion and/or the bottoming cathode portion; and
a separation assembly configured to receive the carbon dioxide-containing exhaust from the oxidizer assembly and to separate carbon dioxide from the carbon dioxide-containing exhaust,
wherein the topping cathode portion and the bottoming cathode portion are configured to selectively receive the flue gas in parallel or in series, and wherein the system further comprises a controller configured to selectively control a flow of the flue gas to the topping cathode portion and the bottoming cathode portion to be in parallel or in series.
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