# Patent

## Title
Reforming catalyst pattern for fuel cell operated with enhanced CO2 utilization

## Abstract

A reforming element for a molten carbonate fuel cell stack and corresponding methods are provided that can reduce or minimize temperature differences within the fuel cell stack when operating the fuel cell stack with enhanced CO2 utilization. The reforming element can include at least one surface with a reforming catalyst deposited on the surface. A difference between the minimum and maximum reforming catalyst density and/or activity on a first portion of the at least one surface can be 20% to 75%, with the highest catalyst densities and/or activities being in proximity to the side of the fuel cell stack corresponding to at least one of the anode inlet and the cathode inlet.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to U.S. Patent Application No. 62/773,485, filed Nov. 30, 2018 and entitled “Reforming Catalyst Pattern for Fuel Cell Operated with Enhanced CO2 Utilization.” The entirety of the aforementioned application is incorporated by reference herein.


FIELD
Systems and methods are provided for operating molten carbonate fuel cell stacks for enhanced CO2 utilization when operating with low CO2 content cathode input streams. The fuel cell assemblies can include internal reforming assemblies with a reforming catalyst distribution that reduces or minimizes temperature gradients within the fuel cells.
BACKGROUND
This application discloses and claims subject matter made as a result of activities within the scope of a joint research agreement between ExxonMobil Research and Engineering Company and FuelCell Energy, Inc. that was in effect on or before the effective filing date of the present application.
Molten carbonate fuel cells utilize hydrogen and/or other fuels to generate electricity. The hydrogen may be provided by reforming methane or other reformable fuels in a steam reformer, such as a steam reformer located upstream of the fuel cell or integrated within the fuel cell. Fuel can also be reformed in the anode cell in a molten carbonate fuel cell, which can be operated to create conditions that are suitable for reforming fuels in the anode. Still another option can be to perform some reforming both externally and internally to the fuel cell. Reformable fuels can encompass hydrocarbonaceous materials that can be reacted with steam and/or oxygen at elevated temperature and/or pressure to produce a gaseous product that comprises hydrogen.
One of the attractive features of molten carbonate fuel cells is the ability to transport CO2 from a low concentration stream (such as a cathode input stream) to a higher concentration stream (such as an anode output flow). During operation, CO2 and O2 in an MCFC cathode are converted to a carbonate ion (CO32−), which is then transported across the molten carbonate electrolyte as a charge carrier. The carbonate ion reacts with H2 in the fuel cell anode to form H2O and CO2. Thus, one of the net outcomes of operating the MCFC is transport of CO2 across the electrolyte. This transport of CO2 across the electrolyte can allow an MCFC to generate electrical power while reducing or minimizing the cost and/or challenge of sequestering carbon oxides from various COx-containing streams. When an MCFC is paired with a combustion source, such as a natural gas fired power plant, this can allow for additional power generation while reducing or minimizing the overall CO2 emissions that result from power generation.
For fuel cell assemblies with internal reforming sections located within the fuel cell stack, various types of catalyst distributions are known. For example, U.S. Pat. No. 8,822,090 describes a reforming catalyst pattern and corresponding flow scheme. The catalyst pattern and flow scheme are described as providing an improved temperature distribution, in part by causing additional reforming to occur near the center of the fuel cell stack.
U.S. Patent Application Publication 2015/0093665 describes methods for operating a molten carbonate fuel cell with some combustion in the cathode to provide supplemental heat for performing additional reforming (and/or other endothermic reactions) within the fuel cell anode. The publication notes that the voltage and/or power generated by a carbonate fuel cell can start to drop rapidly as the CO2 concentration falls below about 1.0 mole %. The publication further state that as the CO2 concentration drops further, e.g., to below about 0.3 vol %, at some point the voltage across the fuel cell can become low enough that little or no further transport of carbonate may occur and the fuel cell ceases to function.
SUMMARY
In an aspect, a method for producing electricity is provided. The method can include passing a fuel stream comprising a reformable fuel into a fuel stack comprising a first surface. The first surface can include a first portion that includes a reforming catalyst. In some aspects, the reforming catalyst density on the first portion of the first surface can have a difference between a maximum catalyst density and a minimum catalyst density of 20% to 75%. In some aspects, the reforming catalyst density on the first portion of the first surface can have a difference between a maximum catalyst activity and a minimum catalyst activity of 20% to 75%. At least a portion of the reformable fuel can be reformed in the presence of the first surface to produce reformed hydrogen. At least a portion of the reformable fuel, at least a portion of the reformed hydrogen, or a combination thereof can be introduced into an anode of a molten carbonate fuel cell. The method can further include introducing a cathode input stream comprising O2, H2O, and CO2 into a cathode of the molten carbonate fuel cell. A direction of flow in the cathode of the molten carbonate fuel cell can be substantially orthogonal to a direction of flow in the anode of the molten carbonate fuel cell. The molten carbonate fuel cell can be operated at a transference of 0.97 or less and an average current density of 60 mA/cm2 or more to generate electricity, an anode exhaust comprising H2, CO, and CO2, and a cathode exhaust comprising 2.0 vol % or less CO2, 1.0 vol % or more O2, and 1.0 vol % or more H2O.
In another aspect, a fuel cell stack is provided. The fuel cell stack can include a molten carbonate fuel cell comprising an anode and a cathode. The fuel cell stack can further include a reforming element associated with the anode. The reforming element can include a first surface, with the first surface including a first portion comprising a reforming catalyst. A reforming catalyst density on the first portion of the first surface can correspond to a monotonically decreasing catalyst density. Additionally or alternately, the reforming catalyst density on the first portion of the first surface can have a difference between a maximum catalyst density and a minimum catalyst density of 20% to 75%. The fuel cell stack can further include a separator plate between the anode and the reforming element.



BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 shows an example of a configuration for molten carbonate fuel cells and associated reforming and separation stages.
FIG. 2 shows another example of a configuration for molten carbonate fuel cells and associated reforming and separation stages.
FIG. 3 shows an example of a portion of a molten carbonate fuel cell stack.
FIG. 4 shows a flow pattern example for a molten carbonate fuel cell with an anode flow direction that is aligned roughly perpendicular to a cathode flow direction.
FIG. 5 shows an example of a flow pattern within a reforming element.
FIG. 6 shows an example of a reforming catalyst density profile in a reforming element.
FIG. 7 shows a comparison of reforming catalyst density profiles for modeling of fuel cell behavior.
FIG. 8 shows a temperature profile for operation of a molten carbonate fuel cell stack with a conventional reforming catalyst pattern under conditions for elevated CO2 utilization.
FIG. 9 shows a temperature profile for operation of a molten carbonate fuel cell stack with an improved reforming catalyst pattern under conditions for elevated CO2 utilization.
FIG. 10 shows a comparison of reforming catalyst density profiles for modeling of fuel cell behavior.
FIG. 11 shows a temperature profile for operation of a molten carbonate fuel cell stack with a conventional reforming catalyst pattern under conditions for elevated CO2 utilization.
FIG. 12 shows a temperature profile for operation of a molten carbonate fuel cell stack with another conventional reforming catalyst pattern under conditions for elevated CO2 utilization.
FIG. 13 shows a temperature profile for operation of a molten carbonate fuel cell stack with an improved reforming catalyst pattern under conditions for elevated CO2 utilization.



DETAILED DESCRIPTION OF THE EMBODIMENTS
Overview
In various aspects, a reforming element for a molten carbonate fuel cell stack is provided that can reduce or minimize temperature differences within the fuel cell stack when operating the fuel cell stack with a low CO2 content cathode feed and with enhanced CO2 utilization. The reforming element can include at least one surface with reforming catalyst deposited on the surface. In order to reduce or minimize temperature differences within the fuel cell stack, the difference between the minimum and maximum reforming catalyst density on a first portion of the at least one surface can be 20% to 75%, or 20% to 70%, or 25% to 65%, with the highest catalyst densities being in proximity to the side of the fuel cell stack corresponding to at least one of the anode inlet and the cathode inlet. Additionally or alternately, the difference between the minimum and maximum reforming catalyst activity on a first portion of the at least one surface can be 20% to 75%, or 20% to 70%, or 25% to 65%, with the highest catalyst activities being in proximity to the side of the fuel cell stack corresponding to at least one of the anode inlet and the cathode inlet. For example, for a catalyst density pattern that is aligned with the anode flow, a maximum catalyst density and/or activity can be present at the edge of the catalyst pattern that is closest to the side of the fuel cell stack corresponding to the anode inlet (i.e., in proximity to the anode inlet). Optionally but preferably, the reforming catalyst density and/or activity can vary monotonically across the first portion of the at least one surface. Optionally, a second portion of the at least one surface in the reforming element can correspond to a portion of the surface in proximity to the cathode inlet or anode inlet side of the reforming element. In particular, if the catalyst pattern in the first portion is aligned based on the anode flow pattern, the second portion can be in proximity to the cathode inlet. Alternatively, if the catalyst pattern in the first portion is aligned based on the cathode flow pattern, the second portion can be in proximity to the anode inlet. The second portion of the at least one surface can have a separate catalyst profile, such as a relatively constant catalyst density profile and/or activity profile, to account for the increased fuel cell activity at the cathode inlet. This reforming catalyst pattern within the reforming element, corresponding to a first portion and an optional second portion, can allow the fuel cell stack to operate with elevated CO2 capture while having a temperature difference of 70° C. or less, or 50° C. or less, or 45° C. or less, or 40° C. or less across the fuel cell stack. The temperature difference may be measured at the separator plate between the reforming element and the anode. By reducing or minimizing temperature differences within a fuel cell stack, higher average operating temperatures can be used while reducing or minimizing the presence of localized “hot spots” within the fuel cell stack that could damage the structures in the fuel cell stack.
Additionally or alternately, in various aspects the anode of the fuel cell can include at least one surface containing reforming catalyst. In such aspects, a first portion of the at least one surface can have difference between the minimum and maximum reforming catalyst density and/or catalyst activity on a first portion of the at least one surface of 20% to 75%, or 20% to 70%, or 25% to 65%, or 40% to 75%, or 40% to 70%, or 40% to 65%. In such aspects, the highest catalyst densities and/or activities can be in proximity to the side of the fuel cell stack corresponding to the anode inlet. Optionally but preferably, the reforming catalyst density and/or activity can vary monotonically across the first portion of the at least one surface. Optionally, a second portion of the at least one surface in the reforming element can correspond to the cathode inlet side of the reforming element. The second portion of the at least one surface can have a separate catalyst profile, such as a relatively constant catalyst density profile and/or activity profile, to account for the increased fuel cell activity at the cathode inlet.
With regard to catalyst activity, it is noted that the nature of the catalyst pattern and the orientation of gas flows relative to the catalyst pattern can alter the resulting catalyst activity. For example, one option for providing a desired catalyst density on a surface can be to provide catalyst as a series of (substantially) parallel lines of catalyst particles. The resulting activity from such a parallel line catalyst pattern can vary depending on the orientation of the gas flow relative to the direction of the lines of catalyst particles. In particular, the catalyst activity can be greater for a gas flow that is roughly aligned with (such as substantially parallel to) the lines of catalyst particles, while a skew or substantially perpendicular gas flow relative to the lines of catalyst particles could lead to varying amounts of reduction in catalyst activity.
In contrast to conventional fuel cell operation, in various aspects, molten carbonate fuel cells can be operated to have elevated CO2 capture, such as a transference of 0.97 or less, or 0.95 or less. Operating with a transference of 0.97 or less, or 0.95 or less, can lead to a temperature distribution across the fuel cell that differs from conventional operation. In particular, due to depletion of CO2 in the cathode, it has been discovered that a portion of the current density in the fuel cell can be due to alternative ion transport. Such alternative ion transport is typically accompanied by greater waste heat generation. If a conventional reforming catalyst pattern is used, the waste heat due to the alternative ion transport can result in significant unexpected temperature variations.
It has been discovered that the additional waste heat due to alternative ion transport can accumulate near the cathode outlet. It is believed this is due in part to the depletion of CO2 near the cathode outlet. This dependence of the temperature profile on the nature of the reaction in the cathode is in contrast to conventional fuel cell operation, where the temperature pattern is roughly dependent on the flow patterns in the cathode and the anode. As a result, using a conventional reforming catalyst pattern during elevated CO2 consumption in the cathode can result in unexpected hot spots within the fuel cell. For example, when the flows within the cathode and anode are oriented to be roughly orthogonal, it has been further discovered that using a reforming catalyst pattern based on the expected anode reaction profile can result in a substantial hot spot in the fuel cell near the corner of the fuel cell corresponding to both the anode outlet and the cathode outlet. This is due to a combination of the excess waste heat from alternative ion transport and the reduced amount of cooling from conventional catalyst patterns near the corner corresponding to the anode outlet and cathode outlet.
In order to overcome the difficulties associated with excess waste heat due to alternative ion transport, an alternative reforming catalyst pattern can be used so that a greater amount of reforming occurs near the anode outlet. This can be achieved, for example, by using a reforming catalyst pattern with a reduced amount of variation between the maximum and minimum catalyst density. For example, the difference between the maximum and minimum in reforming catalyst density can correspond to 20% to 75% of the maximum catalyst density, or 20% to 70%, or 25% to 65%. Additionally or alternately, a reforming catalyst pattern can be used with a difference between the maximum and minimum reforming catalyst activity of 20% to 75% of the maximum catalyst activity, or 20% to 70%, or 25% to 65%. This can allow for increased reforming near the anode outlet, resulting in additional cooling that can mitigate the additional waste heat generated due to alternative ion transport.
An example of a suitable catalyst density profile and/or catalyst activity profile can be a monotonically decreasing profile. In such a catalyst density/catalyst activity profile, the catalyst density and/or catalyst activity is at a maximum value at the anode inlet. The catalyst density and/or activity then either stays roughly the same or decreases along the direction from the anode inlet to the anode outlet. Examples of a monotonically decreasing profile include a catalyst density and/or activity profile with a constant slope (i.e., a constantly decreasing amount of catalyst density and/or activity); or a profile where the reforming catalyst density and/or activity corresponds to a series of steps, with the catalyst density/activity being roughly constant within each step. Still other catalyst patterns can also be used, such as combinations of steps and regions of decreasing catalyst density and/or activity.
Using an improved reforming catalyst pattern can be beneficial when operating an MCFC to have enhanced or elevated CO2 utilization. One difficulty in operating an MCFC with elevated CO2 utilization is that the operation of the fuel cell can potentially be kinetically limited if one or more of the reactants required for fuel cell operation is present in low quantities. For example, when using a cathode input stream with a CO2 content of 4.0 vol % or less, achieving a CO2 utilization of 75% or more corresponds to a cathode outlet concentration of 1.0 vol % or less. However, a cathode outlet concentration of 1.0 vol % or less does not necessarily mean that the CO2 is evenly distributed throughout the cathode. Instead, the concentration will typically vary within the cathode due to a variety of factors, such as the flow patterns in the anode and the cathode. The variations in CO2 concentration can result in portions of the cathode where CO2 concentrations substantially below 1.0 vol % are present.
Conventional operating conditions for molten carbonate fuel cells typically correspond to conditions where the amount of alternative ion transport is reduced, minimized, or non-existent. The amount of alternative ion transport can be quantified based on the transference for a fuel cell. The transference is defined as the fraction of ions transported across the molten carbonate electrolyte that correspond to carbonate ions, as opposed to hydroxide ions and/or other ions. A convenient way to determine the transference can be based on comparing a) the measured change in CO2 concentration at the cathode inlet versus the cathode outlet with b) the amount of carbonate ion transport required to achieve the current density being produced by the fuel cell. It is noted that this definition for the transference assumes that back-transport of CO2 from the anode to the cathode is minimal. It is believed that such back-transport is minimal for the operating conditions described herein. For the CO2 concentrations, the cathode input stream and/or cathode output stream can be sampled, with the sample diverted to a gas chromatograph for determination of the CO2 content. The average current density for the fuel cell can be measured in any convenient manner.
Under conventional operating conditions, the transference can be relatively close to 1.0, such as 0.98 or more and/or such as having substantially no alternative ion transport. A transference of 0.98 or more means that 98% or more of the ionic charge transported across the electrolyte corresponds to carbonate ions. It is noted that hydroxide ions have a charge of −1 while carbonate ions have a charge of −2, so two hydroxide ions need to be transported across the electrolyte to result in the same charge transfer as transport of one carbonate ion.
In contrast to conventional operating conditions, operating a molten carbonate fuel cell with transference of 0.95 or less (or 0.97 or less when operating with increased open area and/or reduced unblocked flow cross-section) can increase the effective amount of carbonate ion transport that is achieved, even though a portion of the current density generated by the fuel cell is due to transport of ions other than carbonate ions. In order to operate a fuel cell with a transference of 0.97 or less, or 0.95 or less, depletion of CO2 has to occur within the fuel cell cathode. It has been discovered that such depletion of CO2 within the cathode tends to be localized. As a result, many regions within a fuel cell cathode can still have sufficient CO2 for normal operation. These regions contain additional CO2 that would be desirable to transport across an electrolyte, such as for carbon capture. However, the CO2 in such regions is typically not transported across the electrolyte when operating under conventional conditions. By selecting operating conditions with a transference of 0.97 or less, or 0.95 or less, the regions with sufficient CO2 can be used to transport additional CO2 while the depleted regions can operate based on alternative ion transport. This can increase the practical limit for the amount of CO2 captured from a cathode input stream.
One of the advantages of transport of alternative ions across the electrolyte is that the fuel cell can continue to operate, even though a sufficient number of CO2 molecules are not kinetically available. This can allow additional CO2 to be transferred from cathode to anode even though the amount of CO2 present in the cathode would conventionally be considered insufficient for normal fuel cell operation. This can allow the fuel cell to operate with a measured CO2 utilization closer to 100%, while the calculated CO2 utilization (based on current density) can be at least 3% greater than the measured CO2 utilization, or at least 5% greater, or at least 10% greater, or at least 20% greater. It is noted that alternative ion transport can allow a fuel cell to operate with a current density that would correspond to more than 100% calculated CO2 utilization.
Although transport of alternative ions can allow a fuel cell to maintain a target current density, it has further been discovered that transport of alternative ions across the electrolyte can also reduce or minimize the lifetime of a molten carbonate fuel cell. Thus, mitigation of this loss in fuel cell lifetime is desirable. It has been unexpectedly discovered that managing the temperature within a fuel cell by modifying the reforming catalyst density and/or activity can reduce or minimize such loss in fuel cell lifetime.
In some aspects, elevated CO2 capture can be defined based on the amount of transference, such as a transference of 0.97 or less, or 0.95 or less, or 0.93 or less, or 0.90 or less. Maintaining an operating condition with transference of 0.97 or less can typically also result in a CO2 concentration in the cathode output stream of 2.0 vol % or less, or 1.5 vol % or less, or 1.0 vol % or less. At higher CO2 concentrations in the cathode output stream, there is typically not sufficient local depletion of CO2 to result in lower transference values.
The presence of elevated CO2 capture can also be indicated by other factors, although such other factors are by themselves typically not a sufficient condition to indicate elevated CO2 capture. For example, when using a lower CO2 concentration cathode input stream, elevated CO2 capture can in some aspects correspond to a CO2 utilization of 70% or more, or 75% or more, or 80% or more, such as up to 95% or possibly still higher. Examples of lower concentration sources of CO2 can correspond to CO2 sources that result in cathode input streams containing 5.0 vol % or less of CO2, or 4.0 vol % or less, such as down to 1.5 vol % or possibly lower. The exhaust from a natural gas turbine is an example of a CO2-containing stream that often has a CO2 content of 5.0 vol % or less of CO2, or 4.0 vol % or less. Additionally or alternately, elevated CO2 capture can correspond to operating conditions where the molten carbonate fuel cell is used to generate a substantial amount of current density, such as 60 mA/cm2 or more, or 80 mA/cm2 or more, or 100 mA/cm2 or more, or 120 mA/cm2 or more, or 150 mA/cm2 or more, or 200 mA/cm2 or more, such as up to 300 mA/cm2 or possibly still higher. It is noted that alternative ion transport can also be indicated by a reduced operating voltage for a fuel cell, as the reaction pathway for alternative ion transport has a lower theoretical voltage than the reaction pathway that uses carbonate ions.
Conventionally, the CO2 concentration in the cathode exhaust of a molten carbonate fuel cell is maintained at a relatively high value, such as 5 vol % CO2 or more, or 10 vol % CO2 or more, or possibly still higher. Additionally, molten carbonate fuel cells are typically operated at CO2 utilization values of 70% or less. When either of these conditions are present, the dominant mechanism for transport of charge across the molten carbonate electrolyte is transport of carbonate ions. While it is possible that transport of alternative ions (such as hydroxide ions) across the electrolyte occurs under such conventional conditions, the amount of alternative ion transport is de minimis, corresponding to 2% or less of the current density (or equivalently, a transference of 0.98 or more).
As an alternative to describing operating conditions in terms of transference, the operating conditions can be described based on measured CO2 utilization and “calculated” CO2 utilization based on average current density. In this discussion, the measured CO2 utilization corresponds to the amount of CO2 that is removed from the cathode input stream. This can be determined, for example, by using gas chromatography to determine the CO2 concentration in the cathode input stream and the cathode output stream. This can also be referred to as the actual CO2 utilization, or simply as the CO2 utilization. In this discussion, the calculated CO2 utilization is defined as the CO2 utilization that would occur if all of the current density generated by the fuel cell was generated based on transport of CO32− ions across the electrolyte (i.e., transport of ions based on CO2). The difference in measured CO2 utilization and the calculated CO2 utilization can be used individually to characterize the amount of alternative ion transport, and/or these values can be used to calculate the transference, as described above.
In some aspects, any convenient type of electrolyte suitable for operation of a molten carbonate fuel cell can be used. Many conventional MCFCs use a eutectic carbonate mixture as the carbonate electrolyte, such as a eutectic mixture of 62 mol % lithium carbonate and 38 mol % potassium carbonate (62% Li2CO3/38% K2CO3) or a eutectic mixture of 52 mol % lithium carbonate and 48 mol % sodium carbonate (52% Li2CO3/48% Na2CO3). Other eutectic mixtures are also available, such as a eutectic mixture of 40 mol % lithium carbonate and 60 mol % potassium carbonate (40% Li2CO3/60% K2CO3). While eutectic mixtures of carbonate can be convenient as an electrolyte for various reasons, non-eutectic mixtures of carbonates can also be suitable. Generally, such non-eutectic mixtures can include various combinations of lithium carbonate, sodium carbonate, and/or potassium carbonate. Optionally, lesser amounts of other metal carbonates can be included in the electrolyte as additives, such as other alkali carbonates (rubidium carbonate, cesium carbonate), or other types of metal carbonates such as barium carbonate, bismuth carbonate, lanthanum carbonate, or tantalum carbonate.
In this discussion, a reforming element refers to a reforming stage located within a fuel cell stack. A reforming element can receive a fuel feed of reformable fuel and convert at least a portion of the reformable fuel into hydrogen. After conversion of the reformable fuel to hydrogen, the hydrogen (plus optionally remaining reformable fuel) can be passed into one or more anodes associated with the reforming element. In this discussion, the temperature uniformity within a fuel cell stack can be determined based on the temperature of a separator plate located between the anode of a fuel cell and an adjacent associated reforming element.
In this discussion, reforming catalyst density is defined as the average weight of reforming catalyst per unit area on a surface. In aspects where reforming catalyst is loaded onto a surface in a non-continuous manner, such as be loading grains or particles of catalyst on a surface, the unit area for determining the reforming catalyst density at a location can be a square with a side length of 25 times a characteristic length of the particle. The characteristic length for the particle can be an average diameter or an average length along the longest axis of the particle. Such a square can then be used to form a tesselation, and the catalyst density can be calculated in a discrete manner per unit square. For a catalyst pattern without discrete particles, or for catalyst patterns where a characteristic length is not otherwise available, the square for defining a tesselation can have a side length of 1 mm. In this discussion, when determining a difference between the minimum reforming catalyst density and the maximum reforming catalyst density across the fuel cell (such as across a first portion of a surface in a reforming element), the difference can be determined based on normalizing the reforming catalyst density so that the maximum density corresponds to a value of 100. The difference between the maximum density and the density at a given location can then correspond to the percentage difference in reforming catalyst density between the locations.
Conditions for Molten Carbonate Fuel Cell Operation with Alternative Ion Transport
In various aspects, the operating conditions for a molten carbonate fuel cell (such as a cell as part of a fuel cell stack) can be selected to correspond to a transference of 0.97 or less, thereby causing the cell to transport both carbonate ion and at least one type of alternative ion across the electrolyte. In addition to transference, operating conditions that can indicate that a molten carbonate fuel cell is operating with transport of alternative ions include, but are not limited to, CO2 concentration for the cathode input stream, the CO2 utilization in the cathode, the current density for the fuel cell, the voltage drop across the cathode, the voltage drop across the anode, and the O2 concentration in the cathode input stream. Additionally, the anode input stream and fuel utilization in the anode can be generally selected to provide the desired current density.
Generally, to cause alternative ion transport, the CO2 concentration in at least a portion of the cathode needs to be sufficiently low while operating the fuel cell to provide a sufficiently high current density. Having a sufficiently low CO2 concentration in the cathode typically corresponds to some combination of a low CO2 concentration in the cathode input flow, a high CO2 utilization, and/or a high average current density. However, such conditions alone are not sufficient to indicate a transference of 0.97 or less, or 0.95 or less.
For example, a molten carbonate fuel cell with a cathode open area of roughly 33% was operated with a CO2 cathode inlet concentration of 19 vol %, 75% CO2 utilization, and 160 mA/cm2 of average current density. These conditions corresponded to a difference between calculated CO2 utilization and measured CO2 utilization of less than 1%. Thus, the presence of substantial alternative ion transport/a transference of 0.97 or less, or 0.95 or less, cannot be inferred simply from the presence of a high CO2 utilization and a high average current density.
As another example, a molten carbonate fuel cell with a cathode open area of between 50% and 60% was operated with a CO2 cathode inlet concentration of 4.0 vol %, 89% CO2 utilization, and 100 mA/cm2 of current density. These conditions corresponded to a transference of at least 0.97. Thus, the presence of a transference of 0.95 or less/substantial alternative ion transport cannot be inferred simply from the presence of high CO2 utilization in combination with low CO2 concentration in the cathode input stream.
As still another example, a molten carbonate fuel cell with a cathode open area of between 50% and 60% was operated with a CO2 cathode inlet concentration of 13 vol %, 68% CO2 utilization, and 100 mA/cm2 of current density. These conditions corresponded to a transference of at least 0.98.
In this discussion, operating an MCFC to transport alternative ions across the electrolyte is defined as operating the MCFC so that more than a de minimis amount of alternative ions are transported. It is possible that minor amounts of alternative ions are transported across an MCFC electrolyte under a variety of conventional conditions. Such alternative ion transport under conventional conditions can correspond to a transference of 0.98 or more, which corresponds to transport of alternative ions corresponding to less than 2.0% of the current density for the fuel cell.
In this discussion, operating an MCFC to cause alternative ion transport is defined as operating an MCFC with a transference of 0.95 or less, so that 5.0% or more of the current density (or, 5.0% or more of the calculated CO2 utilization) corresponds to current density based on transport of alternative ions, or 10% or more, or 20% or more, such as up to 35% or possibly still higher. It is noted that in some aspects, operating with increased open area can reduce or minimize the amount of alternative ion transport under conditions that would otherwise result in a transference of 0.95 or less. Thus, by operating with increased open area and/or reduced unblocked flow cross-section, some operating conditions with elevated CO2 capture/substantial alternative ion transport may correspond to a transference of 0.97 or less.
In this discussion, operating an MCFC to cause substantial alternative ion transport (i.e., to operate with a transference of 0.95 or less, or 0.97 or less with increased open area) is further defined to correspond to operating an MCFC with voltage drops across the anode and cathode that are suitable for power generation. The total electrochemical potential difference for the reactions in a molten carbonate fuel cell is  ̃1.04 V. Due to practical considerations, an MCFC is typically operated to generate current at a voltage near 0.7 V or about 0.8 V. This corresponds to a combined voltage drop across the cathode, electrolyte, and anode of roughly 0.34 V. In order to maintain stable operation, the combined voltage drop across the cathode, electrolyte, and anode can be less than  ̃0.5 V, so that the resulting current generated by the fuel cell is at a voltage of 0.55 V or more, or 0.6 V or more.
With regard to the anode, one condition for operating with substantial alternative ion transport can be to have an H2 concentration of 8.0 vol % or more, or 10 vol % or more in the region where the substantial alternative ion transport occurs. Depending on the aspect, this could correspond to a region near the anode inlet, a region near the cathode outlet, or a combination thereof. Generally, if the H2 concentration in a region of the anode is too low, there will be insufficient driving force to generate substantial alternative ion transport.
Suitable conditions for the anode can also include providing the anode with H2, a reformable fuel, or a combination thereof; and operating with any convenient fuel utilization that generates a desired current density, including fuel utilizations ranging from 20% to 80%. In some aspects this can correspond to a traditional fuel utilization amount, such as a fuel utilization of 60% or more, or 70% or more, such as up to 85% or possibly still higher. In other aspects, this can correspond to a fuel utilization selected to provide an anode output stream with an elevated content of H2 and/or an elevated combined content of H2 and CO (i.e., syngas), such as a fuel utilization of 55% or less, or 50% or less, or 40% or less, such as down to 20% or possibly still lower. The H2 content in the anode output stream and/or the combined content of H2 and CO in the anode output stream can be sufficient to allow generation of a desired current density. In some aspects, the H2 content in the anode output stream can be 3.0 vol % or more, or 5.0 vol % or more, or 8.0 vol % or more, such as up to 15 vol % or possibly still higher. Additionally or alternately, the combined amount of H2 and CO in the anode output stream can be 4.0 vol % or more, or 6.0 vol % or more, or 10 vol % or more, such as up to 20 vol % or possibly still higher. Optionally, when the fuel cell is operated with low fuel utilization, the H2 content in the anode output stream can be in a higher range, such as an H2 content of 10 vol % to 25 vol %. In such aspects, the syngas content of the anode output stream can be correspondingly higher, such as a combined H2 and CO content of 15 vol % to 35 vol %. Depending on the aspect, the anode can be operated to increase the amount of electrical energy generated, to increase the amount of chemical energy generated (i.e., H2 generated by reforming that is available in the anode output stream), or operated using any other convenient strategy that is compatible with operating the fuel cell to cause alternative ion transport.
In addition to having sufficient H2 concentration in the anode, one or more locations within the cathode need to have a low enough CO2 concentration so that the more favorable pathway of carbonate ion transport is not readily available. In some aspects, this can correspond to having a CO2 concentration in the cathode outlet stream (i.e., cathode exhaust) of 2.0 vol % or less, or 1.0 vol % or less, or 0.8 vol % or less. It is noted that due to variations within the cathode, an average concentration of 2.0 vol % or less (or 1.0 vol % or less, or 0.8 vol % or less) in the cathode exhaust can correspond to a still lower CO2 concentration in localized regions of the cathode. For example, in a cross-flow configuration, at a corner of the fuel cell that is adjacent to the anode inlet and the cathode outlet, the CO2 concentration can be lower than a corner of the same fuel cell that is adjacent to the anode outlet and the cathode outlet. Similar localized variations in CO2 concentration can also occur in fuel cells having a co-current or counter-current configuration.
In addition to having a low concentration of CO2, the localized region of the cathode can also have 1.0 vol % or more of O2, or 2.0 vol % or more. In the fuel cell, O2 is used to form the hydroxide ion that allows for alternative ion transport. If sufficient O2 is not present, the fuel cell will not operate as both the carbonate ion transport and alternative ion transport mechanisms are dependent on O2 availability. With regard to O2 in the cathode input stream, in some aspects this can correspond to an oxygen content of 4.0 vol % to 15 vol %, or 6.0 vol % to 10 vol %.
It has been observed that a sufficient amount of water should also be present for alternative ion transport to occur, such as 1.0 vol % or more, or 2.0 vol % or more. Without being bound by any particular theory, if water is not available in the cathode when attempting to operate with substantial alternative ion transport, the fuel cell appears to degrade at a much more rapid rate than the deactivation rate that is observed due to alternative ion transport with sufficient water available. It is noted that because air is commonly used as an O2 source, and since H2O is one of the products generated during combustion, a sufficient amount of water is typically available within the cathode.
Due to the non-uniform distribution of cathode gas and/or anode gas during operation of a molten carbonate fuel cell for elevated CO2 capture, it is believed that one or more of the corners and/or edges of the molten carbonate fuel cell will typically have a substantially higher density of alternative ion transport. The one or more corners can correspond to locations where the CO2 concentration in the cathode is lower than average, or a location where the H2 concentration in the anode is greater than average, or a combination thereof.
In this discussion, a fuel cell can correspond to a single cell, with an anode and a cathode separated by an electrolyte. The anode and cathode can receive input gas flows to facilitate the respective anode and cathode reactions for transporting charge across the electrolyte and generating electricity. A fuel cell stack can represent a plurality of cells in an integrated unit. Although a fuel cell stack can include multiple fuel cells, the fuel cells can typically be connected in parallel and can function (approximately) as if they collectively represented a single fuel cell of a larger size. When an input flow is delivered to the anode or cathode of a fuel cell stack, the fuel cell stack can include flow channels for dividing the input flow between each of the cells in the stack and flow channels for combining the output flows from the individual cells. In this discussion, a fuel cell array can be used to refer to a plurality of fuel cells (such as a plurality of fuel cell stacks) that are arranged in series, in parallel, or in any other convenient manner (e.g., in a combination of series and parallel). A fuel cell array can include one or more stages of fuel cells and/or fuel cell stacks, where the anode/cathode output from a first stage may serve as the anode/cathode input for a second stage. It is noted that the anodes in a fuel cell array do not have to be connected in the same way as the cathodes in the array. For convenience, the input to the first anode stage of a fuel cell array may be referred to as the anode input for the array, and the input to the first cathode stage of the fuel cell array may be referred to as the cathode input to the array. Similarly, the output from the final anode/cathode stage may be referred to as the anode/cathode output from the array. In aspects where a fuel cell stack includes separate reforming elements, it is noted that the anode input flow may first pass through a reforming element prior to entering one or more anodes associated with the reforming element.
It should be understood that reference to use of a fuel cell herein typically denotes a “fuel cell stack” composed of individual fuel cells, and more generally refers to the use of one or more fuel cell stacks in fluid communication. Individual fuel cell elements (plates) can typically be “stacked” together in a rectangular array called a “fuel cell stack.” Additional types of elements can also be included in the fuel cell stack, such as reforming elements. This fuel cell stack can typically take a feed stream and distribute reactants among all of the individual fuel cell elements and can then collect the products from each of these elements. When viewed as a unit, the fuel cell stack in operation can be taken as a whole even though composed of many (often tens or hundreds) of individual fuel cell elements. These individual fuel cell elements can typically have similar voltages (as the reactant and product concentrations are similar), and the total power output can result from the summation of all of the electrical currents in all of the cell elements, when the elements are electrically connected in series. Stacks can also be arranged in a series arrangement to produce high voltages. A parallel arrangement can boost the current. If a sufficiently large volume fuel cell stack is available to process a given exhaust flow, the systems and methods described herein can be used with a single molten carbonate fuel cell stack. In other aspects of the invention, a plurality of fuel cell stacks may be desirable or needed for a variety of reasons.
For the purposes of this invention, unless otherwise specified, the term “fuel cell” should be understood to also refer to and/or is defined as including a reference to a fuel cell stack composed of a set of one or more individual fuel cell elements for which there is a single input and output, as that is the manner in which fuel cells are typically employed in practice. Similarly, the term fuel cells (plural), unless otherwise specified, should be understood to also refer to and/or is defined as including a plurality of separate fuel cell stacks. In other words, all references within this document, unless specifically noted, can refer interchangeably to the operation of a fuel cell stack as a “fuel cell.” For example, the volume of exhaust generated by a commercial scale combustion generator may be too large for processing by a fuel cell (i.e., a single stack) of conventional size. In order to process the full exhaust, a plurality of fuel cells (i.e., two or more separate fuel cells or fuel cell stacks) can be arranged in parallel, so that each fuel cell can process (roughly) an equal portion of the combustion exhaust. Although multiple fuel cells can be used, each fuel cell can typically be operated in a generally similar manner, given its (roughly) equal portion of the combustion exhaust.
Example of Molten Carbonate Fuel Cell Operation: Cross Flow Orientation for Cathode and Anode
FIG. 3 shows a general example of a portion of a molten carbonate fuel cell stack. The portion of the stack shown in FIG. 3 corresponds to a fuel cell 301 and an associated reforming element 380. In order to isolate the fuel cell from adjacent fuel cells in the stack and/or other elements in the stack, the fuel cell includes separator plates 310 and 311. For example, separator plate 310 separates the fuel cell 301 from reforming element 380. An additional separator plate 390 is located above reforming element 380. In FIG. 3, the fuel cell 301 includes an anode 330 and a cathode 350 that are separated by an electrolyte matrix 340 that contains an electrolyte 342. Anode collector 320 provides electrical contact between anode 330 and the other anodes in the fuel cell stack, while cathode collector 360 provides similar electrical contact between cathode 350 and the other cathodes in the fuel cell stack. Additionally anode collector 320 allows for introduction and exhaust of gases from anode 330, while cathode collector 360 allows for introduction and exhaust of gases from cathode 350.
During operation, CO2 is passed into the cathode collector 360 along with O2. The CO2 and O2 diffuse into the porous cathode 350 and travel to a cathode interface region near the boundary of cathode 350 and electrolyte matrix 340. In the cathode interface region, a portion of electrolyte 342 can be present in the pores of cathode 350. The CO2 and O2 can be converted near/in the cathode interface region to a carbonate ion (CO32−), which can then be transported across electrolyte 342 (and therefore across electrolyte matrix 340) to facilitate generation of electrical current. In aspects where alternative ion transport is occurring, a portion of the O2 can be converted to an alternative ion, such as a hydroxide ion or a peroxide ion, for transport in electrolyte 342. After transport across the electrolyte 342, the carbonate ion (or alternative ion) can reach an anode interface region near the boundary of electrolyte matrix 340 and anode 330. The carbonate ion can be converted back to CO2 and H2O in the presence of H2, releasing electrons that are used to form the current generated by the fuel cell. The H2 and/or a hydrocarbon suitable for forming H2 are introduced into anode 330 via anode collector 320.
In the example portion of a fuel cell stack shown in FIG. 3, at least a portion of the H2 and/or hydrocarbon can be passed into anode collector 320 from reforming element 380. Reforming element 380 can include one or more surfaces that include a reforming catalyst. The reforming catalyst on a first portion of at least one surface in the reforming element can correspond to reforming catalyst with a catalyst density pattern as described herein. Optionally, reforming catalyst can be present on a first portion of a surface within anode 330 (not shown), with the reforming catalyst in anode 330 having a catalyst density pattern as described herein.
The flow direction within the anode of a molten carbonate fuel cell can have any convenient orientation relative to the flow direction within a cathode. One option can be to use a cross-flow configuration, so that the flow direction within the anode is roughly at a 90° angle relative to the flow direction within the cathode. This type of flow configuration can have practical benefits, as using a cross-flow configuration can allow the manifolds and/or piping for the anode inlets/outlets to be located on different sides of a fuel cell stack from the manifolds and/or piping for the cathode inlets/outlets.
FIG. 4 schematically shows an example of a top view for a fuel cell cathode, along with arrows indicating the direction of flow within the fuel cell cathode and the corresponding fuel cell anode. In FIG. 4, arrow 405 indicates the direction of flow within cathode 450, while arrow 425 indicates the direction of flow within the anode (not shown).
Because the anode and cathode flows are oriented at roughly 90° relative to each other, the anode and cathode flow patterns can contribute to having different reaction conditions in various parts of the cathode. The different conditions can be illustrated by considering the reaction conditions in the four corners of the cathode. In the illustration in FIG. 4, the reaction conditions described herein are qualitatively similar to the reaction conditions for a fuel cell operating with a CO2 utilization of 75% or more (or 80% or more).
Corner 482 corresponds to a portion of the fuel cell that is close to the entry point for both the cathode input flow and the anode input flow. As a result, the concentration of both CO2 (in the cathode) and H2 (in the anode) is relatively high in corner 482. Based on the high concentrations, it is expected that portions of the fuel cell near corner 482 can operate under expected conditions, with substantially no transport of ions other than carbonate ions across the electrolyte.
Corner 484 corresponds to a portion of the fuel cell that is close to the entry point for the cathode input flow and close to the exit point for the anode output flow. In locations near corner 484, the amount of current density may be limited due to the reduced concentration of H2 in the anode, depending on the fuel utilization. However, sufficient CO2 should be present so that any ions transported across the electrolyte substantially correspond to carbonate ions.
Corner 486 corresponds to a portion of the fuel cell that is close to the exit point for the anode output flow and close to the exit point for the cathode output flow. In locations near corner 486, due to the lower concentrations of both H2 (in the anode) and CO2 (in the cathode), little or no current would be expected due to the low driving force for the fuel cell reaction.
Corner 488 corresponds to a portion of the fuel cell that is close to the entry point for the anode input flow and close to the exit point for the cathode output flow. The relatively high availability of hydrogen at locations near corner 488 would be expected to result in substantial current density. However, due to the relatively low concentration of CO2, a substantial amount of transport of hydroxide ions and/or other alternative ions can occur. Depending on the aspect, the substantial amount of alternative ion transport can increase the calculated CO2 utilization by 5% or more, or 10% or more, or 15% or more, or 20% or more. Additionally or alternately, the transference can be 0.97 or less, or 0.95 or less, or 0.90 or less, or 0.85 or less, or 0.80 or less. The transport of substantial amounts of alternative ions across the electrolyte can temporarily allow higher current densities to be maintained at locations near corner 488. However, the transport of alternative ions can also degrade the cathode and/or anode structures, resulting in lower (and possibly no) current density over time at locations near corner 488. It is noted that at lower amounts of alternative ion transport (such as a transference of 0.96 or more, or 0.98 or more), the amount of lifetime degradation is not as severe.
It has been discovered that when alternative ion transport becomes significant at one or more locations within the fuel cell, the fuel cell will quickly begin to degrade. This is believed to be due to the one or more locations degrading and not providing any further current density. As region(s) stop contributing to the desired current density, the remaining locations in the fuel cell have to operate at higher current densities in order to maintain a constant overall (average) current density for the fuel cell. This can cause the region for transport of alternative ions to grow, resulting in an expanding portion of the fuel cell that degrades and eventually stops working. Alternatively, degradation of a portion of the fuel cell can result in reduced total current density from the cell, which is also undesirable. Operating a fuel cell with an improved reforming catalyst pattern can reduce temperature variations due to uneven distribution of alternative ion transport. This can reduce or minimize the amount of degradation due to alternative ion transport that occurs during elevated CO2 capture, allowing for longer fuel cell lifetimes.
Anode Inputs and Outputs
In various aspects, the anode input stream for an MCFC can include hydrogen, a hydrocarbon such as methane, a hydrocarbonaceous or hydrocarbon-like compound that may contain heteroatoms different from C and H, or a combination thereof. The source of the hydrogen/hydrocarbon/hydrocarbon-like compounds can be referred to as a fuel source. In some aspects, most of the methane (or other hydrocarbon, hydrocarbonaceous, or hydrocarbon-like compound) fed to the anode can typically be fresh methane. In this description, a fresh fuel such as fresh methane refers to a fuel that is not recycled from another fuel cell process. For example, methane recycled from the anode outlet stream back to the anode inlet may not be considered “fresh” methane, and can instead be described as reclaimed methane.
The fuel source used can be shared with other components, such as a turbine that uses a portion of the fuel source to provide a CO2-containing stream for the cathode input. The fuel source input can include water in a proportion to the fuel appropriate for reforming the hydrocarbon (or hydrocarbon-like) compound in the reforming section that generates hydrogen. For example, if methane is the fuel input for reforming to generate H2, the molar ratio of water to fuel can be from about one to one to about ten to one, such as at least about two to one. A ratio of four to one or greater is typical for external reforming, but lower values can be typical for internal reforming. To the degree that H2 is a portion of the fuel source, in some optional aspects no additional water may be needed in the fuel, as the oxidation of H2 at the anode can tend to produce H2O that can be used for reforming the fuel. The fuel source can also optionally contain components incidental to the fuel source (e.g., a natural gas feed can contain some content of CO2 as an additional component). For example, a natural gas feed can contain CO2, N2, and/or other inert (noble) gases as additional components. Optionally, in some aspects the fuel source may also contain CO, such as CO from a recycled portion of the anode exhaust. An additional or alternate potential source for CO in the fuel into a fuel cell assembly can be CO generated by steam reforming of a hydrocarbon fuel performed on the fuel prior to entering the fuel cell assembly.
More generally, a variety of types of fuel streams may be suitable for use as an anode input stream for the anode of a molten carbonate fuel cell. Some fuel streams can correspond to streams containing hydrocarbons and/or hydrocarbon-like compounds that may also include heteroatoms different from C and H. In this discussion, unless otherwise specified, a reference to a fuel stream containing hydrocarbons for an MCFC anode is defined to include fuel streams containing such hydrocarbon-like compounds. Examples of hydrocarbon (including hydrocarbon-like) fuel streams include natural gas, streams containing C1-C4 carbon compounds (such as methane or ethane), and streams containing heavier C5+ hydrocarbons (including hydrocarbon-like compounds), as well as combinations thereof. Still other additional or alternate examples of potential fuel streams for use in an anode input can include biogas-type streams, such as methane produced from natural (biological) decomposition of organic material.
In some aspects, a molten carbonate fuel cell can be used to process an input fuel stream, such as a natural gas and/or hydrocarbon stream, with a low energy content due to the presence of diluent compounds. For example, some sources of methane and/or natural gas are sources that can include substantial amounts of either CO2 or other inert molecules, such as nitrogen, argon, or helium. Due to the presence of elevated amounts of CO2 and/or inerts, the energy content of a fuel stream based on the source can be reduced. Using a low energy content fuel for a combustion reaction (such as for powering a combustion-powered turbine) can pose difficulties. However, a molten carbonate fuel cell can generate power based on a low energy content fuel source with a reduced or minimal impact on the efficiency of the fuel cell. The presence of additional gas volume can require additional heat for raising the temperature of the fuel to the temperature for reforming and/or the anode reaction. Additionally, due to the equilibrium nature of the water gas shift reaction within a fuel cell anode, the presence of additional CO2 can have an impact on the relative amounts of H2 and CO present in the anode output. However, the inert compounds otherwise can have only a minimal direct impact on the reforming and anode reactions. The amount of CO2 and/or inert compounds in a fuel stream for a molten carbonate fuel cell, when present, can be at least about 1 vol %, such as at least about 2 vol %, or at least about 5 vol %, or at least about 10 vol %, or at least about 15 vol %, or at least about 20 vol %, or at least about 25 vol %, or at least about 30 vol %, or at least about 35 vol %, or at least about 40 vol %, or at least about 45 vol %, or at least about 50 vol %, or at least about 75 vol %. Additionally or alternately, the amount of CO2 and/or inert compounds in a fuel stream for a molten carbonate fuel cell can be about 90 vol % or less, such as about 75 vol % or less, or about 60 vol % or less, or about 50 vol % or less, or about 40 vol % or less, or about 35 vol % or less.
Yet other examples of potential sources for an anode input stream can correspond to refinery and/or other industrial process output streams. For example, coking is a common process in many refineries for converting heavier compounds to lower boiling ranges. Coking typically produces an off-gas containing a variety of compounds that are gases at room temperature, including CO and various C1-C4 hydrocarbons. This off-gas can be used as at least a portion of an anode input stream. Other refinery off-gas streams can additionally or alternately be suitable for inclusion in an anode input stream, such as light ends (C1-C4) generated during cracking or other refinery processes. Still other suitable refinery streams can additionally or alternately include refinery streams containing CO or CO2 that also contain H2 and/or reformable fuel compounds.
Still other potential sources for an anode input can additionally or alternately include streams with increased water content. For example, an ethanol output stream from an ethanol plant (or another type of fermentation process) can include a substantial portion of H2O prior to final distillation. Such H2O can typically cause only minimal impact on the operation of a fuel cell. Thus, a fermentation mixture of alcohol (or other fermentation product) and water can be used as at least a portion of an anode input stream.
Biogas, or digester gas, is another additional or alternate potential source for an anode input. Biogas may primarily comprise methane and CO2 and is typically produced by the breakdown or digestion of organic matter. Anaerobic bacteria may be used to digest the organic matter and produce the biogas. Impurities, such as sulfur-containing compounds, may be removed from the biogas prior to use as an anode input.
The output stream from an MCFC anode can include H2O, CO2, CO, and H2. Optionally, the anode output stream could also have unreacted fuel (such as H2 or CH4) or inert compounds in the feed as additional output components. Instead of using this output stream as a fuel source to provide heat for a reforming reaction or as a combustion fuel for heating the cell, one or more separations can be performed on the anode output stream to separate the CO2 from the components with potential value as inputs to another process, such as H2 or CO. The H2 and/or CO can be used as a syngas for chemical synthesis, as a source of hydrogen for chemical reaction, and/or as a fuel with reduced greenhouse gas emissions.
The anode exhaust can be subjected to a variety of gas processing options, including water-gas shift and separation of the components from each other. Two general anode processing schemes are shown in FIGS. 1 and 2.
FIG. 1 schematically shows an example of a reaction system for operating a fuel cell array of molten carbonate fuel cells in conjunction with a chemical synthesis process. In FIG. 1, a fuel stream 105 is provided to a reforming stage (or stages) 110 associated with the anode 127 of a fuel cell 120, such as a fuel cell that is part of a fuel cell stack in a fuel cell array. The reforming stage 110 associated with fuel cell 120 can be internal to a fuel cell assembly. In some optional aspects, an external reforming stage (not shown) can also be used to reform a portion of the reformable fuel in an input stream prior to passing the input stream into a fuel cell assembly. Fuel stream 105 can preferably include a reformable fuel, such as methane, other hydrocarbons, and/or other hydrocarbon-like compounds such as organic compounds containing carbon-hydrogen bonds. Fuel stream 105 can also optionally contain H2 and/or CO, such as H2 and/or CO provided by optional anode recycle stream 185. It is noted that anode recycle stream 185 is optional, and that in many aspects no recycle stream is provided from the anode exhaust 125 back to anode 127, either directly or indirectly via combination with fuel stream 105 or reformed fuel stream 115. After reforming, the reformed fuel stream 115 can be passed into anode 127 of fuel cell 120. A CO2 and O2-containing stream 119 can also be passed into cathode 129. A flow of carbonate ions 122, CO32−, from the cathode portion 129 of the fuel cell can provide the remaining reactant needed for the anode fuel cell reactions. Based on the reactions in the anode 127, the resulting anode exhaust 125 can include H2O, CO2, one or more components corresponding to incompletely reacted fuel (H2, CO, CH4, or other components corresponding to a reformable fuel), and optionally one or more additional nonreactive components, such as N2 and/or other contaminants that are part of fuel stream 105. The anode exhaust 125 can then be passed into one or more separation stages. For example, a CO2 removal stage 140 can correspond to a cryogenic CO2 removal system, an amine wash stage for removal of acid gases such as CO2, or another suitable type of CO2 separation stage for separating a CO2 output stream 143 from the anode exhaust. Optionally, the anode exhaust can first be passed through a water gas shift reactor 130 to convert any CO present in the anode exhaust (along with some H2O) into CO2 and H2 in an optionally water gas shifted anode exhaust 135. Depending on the nature of the CO2 removal stage, a water condensation or removal stage 150 may be desirable to remove a water output stream 153 from the anode exhaust. Though shown in FIG. 1 after the CO2 separation stage 140, it may optionally be located before the CO2 separation stage 140 instead. Additionally, an optional membrane separation stage 160 for separation of H2 can be used to generate a high purity permeate stream 163 of H2. The resulting retentate stream 166 can then be used as an input to a chemical synthesis process. Stream 166 could additionally or alternately be shifted in a second water-gas shift reactor 131 to adjust the H2, CO, and CO2 content to a different ratio, producing an output stream 168 for further use in a chemical synthesis process. In FIG. 1, anode recycle stream 185 is shown as being withdrawn from the retentate stream 166, but the anode recycle stream 185 could additionally or alternately be withdrawn from other convenient locations in or between the various separation stages. The separation stages and shift reactor(s) could additionally or alternately be configured in different orders, and/or in a parallel configuration. Finally, a stream with a reduced content of CO2 139 can be generated as an output from cathode 129. For the sake of simplicity, various stages of compression and heat addition/removal that might be useful in the process, as well as steam addition or removal, are not shown.
As noted above, the various types of separations performed on the anode exhaust can be performed in any convenient order. FIG. 2 shows an example of an alternative order for performing separations on an anode exhaust. In FIG. 2, anode exhaust 125 can be initially passed into separation stage 260 for removing a portion 263 of the hydrogen content from the anode exhaust 125. This can allow, for example, reduction of the H2 content of the anode exhaust to provide a retentate 266 with a ratio of H2 to CO closer to 2:1. The ratio of H2 to CO can then be further adjusted to achieve a desired value in a water gas shift stage 230. The water gas shifted output 235 can then pass through CO2 separation stage 240 and water removal stage 250 to produce an output stream 275 suitable for use as an input to a desired chemical synthesis process. Optionally, output stream 275 could be exposed to an additional water gas shift stage (not shown). A portion of output stream 275 can optionally be recycled (not shown) to the anode input. Of course, still other combinations and sequencing of separation stages can be used to generate a stream based on the anode output that has a desired composition. For the sake of simplicity, various stages of compression and heat addition/removal that might be useful in the process, as well as steam addition or removal, are not shown.
Cathode Inputs and Outputs
Conventionally, a molten carbonate fuel cell can be operated based on drawing a desired load while consuming some portion of the fuel in the fuel stream delivered to the anode. The voltage of the fuel cell can then be determined by the load, fuel input to the anode, air and CO2 provided to the cathode, and the internal resistances of the fuel cell. The CO2 to the cathode can be conventionally provided in part by using the anode exhaust as at least a part of the cathode input stream. By contrast, the present invention can use separate/different sources for the anode input and cathode input. By removing any direct link between the composition of the anode input flow and the cathode input flow, additional options become available for operating the fuel cell, such as to generate excess synthesis gas, to improve capture of carbon dioxide, and/or to improve the total efficiency (electrical plus chemical power) of the fuel cell, among others.
In various aspects, an MCFC can be operated to cause alternative ion transport across the electrolyte for the fuel cell. In order to cause alternative ion transport, the CO2 content of the cathode input stream can be 5.0 vol % or less, or 4.0 vol % or less, such as 1.5 vol % to 5.0 vol %, or 1.5 vol % to 4.0 vol %, or 2.0 vol % to 5.0 vol %, or 2.0 vol % to 4.0 vol %.
One example of a suitable CO2-containing stream for use as a cathode input flow can be an output or exhaust flow from a combustion source. Examples of combustion sources include, but are not limited to, sources based on combustion of natural gas, combustion of coal, and/or combustion of other hydrocarbon-type fuels (including biologically derived fuels). Additional or alternate sources can include other types of boilers, fired heaters, furnaces, and/or other types of devices that burn carbon-containing fuels in order to heat another substance (such as water or air).
Other potential sources for a cathode input stream can additionally or alternately include sources of bio-produced CO2. This can include, for example, CO2 generated during processing of bio-derived compounds, such as CO2 generated during ethanol production. An additional or alternate example can include CO2 generated by combustion of a bio-produced fuel, such as combustion of lignocellulose. Still other additional or alternate potential CO2 sources can correspond to output or exhaust streams from various industrial processes, such as CO2-containing streams generated by plants for manufacture of steel, cement, and/or paper.
Yet another additional or alternate potential source of CO2 can be CO2-containing streams from a fuel cell. The CO2-containing stream from a fuel cell can correspond to a cathode output stream from a different fuel cell, an anode output stream from a different fuel cell, a recycle stream from the cathode output to the cathode input of a fuel cell, and/or a recycle stream from an anode output to a cathode input of a fuel cell. For example, an MCFC operated in standalone mode under conventional conditions can generate a cathode exhaust with a CO2 concentration of at least about 5 vol %. Such a CO2-containing cathode exhaust could be used as a cathode input for an MCFC operated according to an aspect of the invention. More generally, other types of fuel cells that generate a CO2 output from the cathode exhaust can additionally or alternately be used, as well as other types of CO2-containing streams not generated by a “combustion” reaction and/or by a combustion-powered generator. Optionally but preferably, a CO2-containing stream from another fuel cell can be from another molten carbonate fuel cell. For example, for molten carbonate fuel cells connected in series with respect to the cathodes, the output from the cathode for a first molten carbonate fuel cell can be used as the input to the cathode for a second molten carbonate fuel cell.
In addition to CO2, a cathode input stream can include O2 to provide the components necessary for the cathode reaction. Some cathode input streams can be based on having air as a component. For example, a combustion exhaust stream can be formed by combusting a hydrocarbon fuel in the presence of air. Such a combustion exhaust stream, or another type of cathode input stream having an oxygen content based on inclusion of air, can have an oxygen content of about 20 vol % or less, such as about 15 vol % or less, or about 10 vol % or less. Additionally or alternately, the oxygen content of the cathode input stream can be at least about 4 vol %, such as at least about 6 vol %, or at least about 8 vol %. More generally, a cathode input stream can have a suitable content of oxygen for performing the cathode reaction. In some aspects, this can correspond to an oxygen content of about 5 vol % to about 15 vol %, such as from about 7 vol % to about 9 vol %. For many types of cathode input streams, the combined amount of CO2 and O2 can correspond to less than about 21 vol % of the input stream, such as less than about 15 vol % of the stream or less than about 10 vol % of the stream. An air stream containing oxygen can be combined with a CO2 source that has low oxygen content. For example, the exhaust stream generated by burning coal may include a low oxygen content that can be mixed with air to form a cathode inlet stream.
In addition to CO2 and O2, a cathode input stream can also be composed of inert/non-reactive species such as N2, H2O, and other typical oxidant (air) components. For example, for a cathode input derived from an exhaust from a combustion reaction, if air is used as part of the oxidant source for the combustion reaction, the exhaust gas can include typical components of air such as N2, H2O, and other compounds in minor amounts that are present in air. Depending on the nature of the fuel source for the combustion reaction, additional species present after combustion based on the fuel source may include one or more of H2O, oxides of nitrogen (NOx) and/or sulfur (SOx), and other compounds either present in the fuel and/or that are partial or complete combustion products of compounds present in the fuel, such as CO. These species may be present in amounts that do not poison the cathode catalyst surfaces though they may reduce the overall cathode activity. Such reductions in performance may be acceptable, or species that interact with the cathode catalyst may be reduced to acceptable levels by known pollutant removal technologies.
The amount of O2 present in a cathode input stream (such as an input cathode stream based on a combustion exhaust) can advantageously be sufficient to provide the oxygen needed for the cathode reaction in the fuel cell. Thus, the volume percentage of O2 can advantageously be at least 0.5 times the amount of CO2 in the exhaust. Optionally, as necessary, additional air can be added to the cathode input to provide a sufficient oxidant for the cathode reaction. When some form of air is used as the oxidant, the amount of N2 in the cathode exhaust can be at least about 78 vol %, e.g., at least about 88 vol %, and/or about 95 vol % or less. In some aspects, the cathode input stream can additionally or alternately contain compounds that are generally viewed as contaminants, such as H2S or NH3. In other aspects, the cathode input stream can be cleaned to reduce or minimize the content of such contaminants.
A suitable temperature for operation of an MCFC can be between about 450° C. and about 750° C., such as at least about 500° C., e.g., with an inlet temperature of about 550° C. and an outlet temperature of about 625° C. Prior to entering the cathode, heat can be added to or removed from the cathode input stream, if desired, e.g., to provide heat for other processes, such as reforming the fuel input for the anode. For example, if the source for the cathode input stream is a combustion exhaust stream, the combustion exhaust stream may have a temperature greater than a desired temperature for the cathode inlet. In such an aspect, heat can be removed from the combustion exhaust prior to use as the cathode input stream. Alternatively, the combustion exhaust could be at a very low temperature, for example after a wet gas scrubber on a coal-fired boiler, in which case the combustion exhaust can be below about 100° C. Alternatively, the combustion exhaust could be from the exhaust of a gas turbine operated in combined cycle mode, in which the gas can be cooled by raising steam to run a steam turbine for additional power generation. In this case, the gas can be below about 50° C. Heat can be added to a combustion exhaust that is cooler than desired.
Additional Molten Carbonate Fuel Cell Operating Strategies
In some aspects, when operating an MCFC to cause alternative ion transport, the anode of the fuel cell can be operated at a traditional fuel utilization value of roughly 60% to 80%. When attempting to generate electrical power, operating the anode of the fuel cell at a relatively high fuel utilization can be beneficial for improving electrical efficiency (i.e., electrical energy generated per unit of chemical energy consumed by the fuel cell).
In some aspects, it may be beneficial to reduce the electrical efficiency of the fuel cell in order to provide other benefits, such as an increase in the amount of H2 provided in the anode output flow. This can be beneficial, for example, if it is desirable to consume excess heat generated in the fuel cell (or fuel cell stack) by performing additional reforming and/or performing another endothermic reaction. For example, a molten carbonate fuel cell can be operated to provide increased production of syngas and/or hydrogen. The heat required for performing the endothermic reforming reaction can be provided by the exothermic electrochemical reaction in the anode for electricity generation. Rather than attempting to transport the heat generated by the exothermic fuel cell reaction(s) away from the fuel cell, this excess heat can be used in situ as a heat source for reforming and/or another endothermic reaction. This can result in more efficient use of the heat energy and/or a reduced need for additional external or internal heat exchange. This efficient production and use of heat energy, essentially in-situ, can reduce system complexity and components while maintaining advantageous operating conditions. In some aspects, the amount of reforming or other endothermic reaction can be selected to have an endothermic heat requirement comparable to, or even greater than, the amount of excess heat generated by the exothermic reaction(s) rather than significantly less than the heat requirement typically described in the prior art.
Additionally or alternately, the fuel cell can be operated so that the temperature differential between the anode inlet and the anode outlet can be negative rather than positive. Thus, instead of having a temperature increase between the anode inlet and the anode outlet, a sufficient amount of reforming and/or other endothermic reaction can be performed to cause the output stream from the anode outlet to be cooler than the anode inlet temperature. Further additionally or alternately, additional fuel can be supplied to a heater for the fuel cell and/or an internal reforming stage (or other internal endothermic reaction stage) so that the temperature differential between the anode input and the anode output can be smaller than the expected difference based on the relative demand of the endothermic reaction(s) and the combined exothermic heat generation of the cathode combustion reaction and the anode reaction for generating electrical power. In aspects where reforming is used as the endothermic reaction, operating a fuel cell to reform excess fuel can allow for production of increased synthesis gas and/or increased hydrogen relative to conventional fuel cell operation while minimizing the system complexity for heat exchange and reforming. The additional synthesis gas and/or additional hydrogen can then be used in a variety of applications, including chemical synthesis processes and/or collection/repurposing of hydrogen for use as a “clean” fuel.
The amount of heat generated per mole of hydrogen oxidized by the exothermic reaction at the anode can be substantially larger than the amount of heat consumed per mole of hydrogen generated by the reforming reaction. The net reaction for hydrogen in a molten carbonate fuel cell (H2+1⁄2O2⇒H2O) can have an enthalpy of reaction of about −285 kJ/mol of hydrogen molecules. At least a portion of this energy can be converted to electrical energy within the fuel cell. However, the difference (approximately) between the enthalpy of reaction and the electrical energy produced by the fuel cell can become heat within the fuel cell. This quantity of energy can alternatively be expressed as the current density (current per unit area) for the cell multiplied by the difference between the theoretical maximum voltage of the fuel cell and the actual voltage, or <current density>*(Vmax−Vact). This quantity of energy is defined as the “waste heat” for a fuel cell. As an example of reforming, the enthalpy of reforming for methane (CH4+2H2O⇒4H2+CO2) can be about 250 kJ/mol of methane, or about 62 kJ/mol of hydrogen molecules. From a heat balance standpoint, each hydrogen molecule electrochemically oxidized can generate sufficient heat to generate more than one hydrogen molecule by reforming. In a conventional configuration, this excess heat can result in a substantial temperature difference from anode inlet to anode outlet. Instead of allowing this excess heat to be used for increasing the temperature in the fuel cell, the excess heat can be consumed by performing a matching amount of the reforming reaction. The excess heat generated in the anode can be supplemented with the excess heat generated by the combustion reaction in the fuel cell. More generally, the excess heat can be consumed by performing an endothermic reaction in the fuel cell anode and/or in an endothermic reaction stage heat integrated with the fuel cell.
Depending on the aspect, the amount of reforming and/or other endothermic reaction can be selected relative to the amount of hydrogen reacted in the anode in order to achieve a desired thermal ratio for the fuel cell. As used herein, the “thermal ratio” is defined as the heat produced by exothermic reactions in a fuel cell assembly (including exothermic reactions in both the anode and cathode) divided by the endothermic heat demand of reforming reactions occurring within the fuel cell assembly. Expressed mathematically, the thermal ratio (TH)=QEX/QEN, where QEX is the sum of heat produced by exothermic reactions and QEN is the sum of heat consumed by the endothermic reactions occurring within the fuel cell. Note that the heat produced by the exothermic reactions can correspond to any heat due to reforming reactions, water gas shift reactions, combustion reactions (i.e., oxidation of fuel compounds) in the cathode, and/or the electrochemical reactions in the cell. The heat generated by the electrochemical reactions can be calculated based on the ideal electrochemical potential of the fuel cell reaction across the electrolyte minus the actual output voltage of the fuel cell. For example, the ideal electrochemical potential of the reaction in an MCFC is believed to be about 1.04 V based on the net reaction that occurs in the cell. During operation of the MCFC, the cell can typically have an output voltage less than 1.04 V due to various losses. For example, a common output/operating voltage can be about 0.7 V. The heat generated can be equal to the electrochemical potential of the cell (i.e.  ̃1.04 V) minus the operating voltage. For example, the heat produced by the electrochemical reactions in the cell can be  ̃0.34 V when the output voltage of  ̃0.7 V is attained in the fuel cell. Thus, in this scenario, the electrochemical reactions would produce  ̃0.7 V of electricity and  ̃0.34 V of heat energy. In such an example, the  ̃0.7 V of electrical energy is not included as part of QEX. In other words, heat energy is not electrical energy.
In various aspects, a thermal ratio can be determined for any convenient fuel cell structure, such as a fuel cell stack, an individual fuel cell within a fuel cell stack, a fuel cell stack with an integrated reforming stage, a fuel cell stack with an integrated endothermic reaction stage, or a combination thereof. The thermal ratio may also be calculated for different units within a fuel cell stack, such as an assembly of fuel cells or fuel cell stacks. For example, the thermal ratio may be calculated for a fuel cell (or a plurality of fuel cells) within a fuel cell stack along with integrated reforming stages and/or integrated endothermic reaction stage elements in sufficiently close proximity to the fuel cell(s) to be integrated from a heat integration standpoint.
From a heat integration standpoint, a characteristic width in a fuel cell stack can be the height of an individual fuel cell stack element. It is noted that the separate reforming stage and/or a separate endothermic reaction stage could have a different height in the stack than a fuel cell. In such a scenario, the height of a fuel cell element can be used as the characteristic height. In this discussion, an integrated endothermic reaction stage can be defined as a stage heat integrated with one or more fuel cells, so that the integrated endothermic reaction stage can use the heat from the fuel cells as a heat source for reforming. Such an integrated endothermic reaction stage can be defined as being positioned less than 10 times the height of a stack element from fuel cells providing heat to the integrated stage. For example, an integrated endothermic reaction stage (such as a reforming stage) can be positioned less than 10 times the height of a stack element from any fuel cells that are heat integrated, or less than 8 times the height of a stack element, or less than 5 times the height of a stack element, or less than 3 times the height of a stack element. In this discussion, an integrated reforming stage and/or integrated endothermic reaction stage that represents an adjacent stack element to a fuel cell element is defined as being about one stack element height or less away from the adjacent fuel cell element.
A thermal ratio of about 1.3 or less, or about 1.15 or less, or about 1.0 or less, or about 0.95 or less, or about 0.90 or less, or about 0.85 or less, or about 0.80 or less, or about 0.75 of less, can be lower than the thermal ratio typically sought in use of MCFC fuel cells. In aspects of the invention, the thermal ratio can be reduced to increase and/or optimize syngas generation, hydrogen generation, generation of another product via an endothermic reaction, or a combination thereof.
In various aspects of the invention, the operation of the fuel cells can be characterized based on a thermal ratio. Where fuel cells are operated to have a desired thermal ratio, a molten carbonate fuel cell can be operated to have a thermal ratio of about 1.5 or less, for example about 1.3 or less, or about 1.15 or less, or about 1.0 or less, or about 0.95 or less, or about 0.90 or less, or about 0.85 or less, or about 0.80 or less, or about 0.75 or less. Additionally or alternately, the thermal ratio can be at least about 0.25, or at least about 0.35, or at least about 0.45, or at least about 0.50. Further additionally or alternately, in some aspects the fuel cell can be operated to have a temperature rise between anode input and anode output of about 40° C. or less, such as about 20° C. or less, or about 10° C. or less. Still further additionally or alternately, the fuel cell can be operated to have an anode outlet temperature that is from about 10° C. lower to about 10° C. higher than the temperature of the anode inlet. Yet further additionally or alternately, the fuel cell can be operated to have an anode inlet temperature greater than the anode outlet temperature, such as at least about 5° C. greater, or at least about 10° C. greater, or at least about 20° C. greater, or at least about 25° C. greater. Still further additionally or alternately, the fuel cell can be operated to have an anode inlet temperature greater than the anode outlet temperature by about 100° C. or less, or about 80° C. or less, or about 60° C. or less, or about 50° C. or less, or about 40° C. or less, or about 30° C. or less, or about 20° C. or less.
Operating a fuel cell with a thermal ratio of less than 1 can cause a temperature drop across the fuel cell. In some aspects, the amount of reforming and/or other endothermic reaction may be limited so that a temperature drop from the anode inlet to the anode outlet can be about 100° C. or less, such as about 80° C. or less, or about 60° C. or less, or about 50° C. or less, or about 40° C. or less, or about 30° C. or less, or about 20° C. or less. Limiting the temperature drop from the anode inlet to the anode outlet can be beneficial, for example, for maintaining a sufficient temperature to allow complete or substantially complete conversion of fuels (by reforming) in the anode. In other aspects, additional heat can be supplied to the fuel cell (such as by heat exchange or combustion of additional fuel) so that the anode inlet temperature is greater than the anode outlet temperature by less than about 100° C. or less, such as about 80° C. or less, or about 60° C. or less, or about 50° C. or less, or about 40° C. or less, or about 30° C. or less, or about 20° C. or less, due to a balancing of the heat consumed by the endothermic reaction and the additional external heat supplied to the fuel cell.
The amount of reforming can additionally or alternately be dependent on the availability of a reformable fuel. For example, if the fuel only comprised H2, no reformation would occur because H2 is already reformed and is not further reformable. The amount of “syngas produced” by a fuel cell can be defined as a difference in the lower heating value (LHV) value of syngas in the anode input versus an LHV value of syngas in the anode output. Syngas produced LHV (sg net)=(LHV(sg out)−LHV(sg in)), where LHV(sg in) and LHV(sg out) refer to the LHV of the syngas in the anode inlet and syngas in the anode outlet streams or flows, respectively. A fuel cell provided with a fuel containing substantial amounts of H2 can be limited in the amount of potential syngas production, since the fuel contains substantial amounts of already reformed H2, as opposed to containing additional reformable fuel. The lower heating value is defined as the enthalpy of combustion of a fuel component to vapor phase, fully oxidized products (i.e., vapor phase CO2 and H2O product). For example, any CO2 present in an anode input stream does not contribute to the fuel content of the anode input, since CO2 is already fully oxidized. For this definition, the amount of oxidation occurring in the anode due to the anode fuel cell reaction is defined as oxidation of H2 in the anode as part of the electrochemical reaction in the anode.
An example of a method for operating a fuel cell with a reduced thermal ratio can be a method where excess reforming of fuel is performed in order to balance the generation and consumption of heat in the fuel cell and/or consume more heat than is generated. Reforming a reformable fuel to form H2 and/or CO can be an endothermic process, while the anode electrochemical oxidation reaction and the cathode combustion reaction(s) can be exothermic. During conventional fuel cell operation, the amount of reforming needed to supply the feed components for fuel cell operation can typically consume less heat than the amount of heat generated by the anode oxidation reaction. For example, conventional operation at a fuel utilization of about 70% or about 75% produces a thermal ratio substantially greater than 1, such as a thermal ratio of at least about 1.4 or greater, or 1.5 or greater. As a result, the output streams for the fuel cell can be hotter than the input streams. Instead of this type of conventional operation, the amount of fuel reformed in the reforming stages associated with the anode can be increased. For example, additional fuel can be reformed so that the heat generated by the exothermic fuel cell reactions can either be (roughly) balanced by the heat consumed in reforming and/or consume more heat than is generated. This can result in a substantial excess of hydrogen relative to the amount oxidized in the anode for electrical power generation and result in a thermal ratio of about 1.0 or less, such as about 0.95 or less, or about 0.90 or less, or about 0.85 or less, or about 0.80 or less, or about 0.75 or less.
Either hydrogen or syngas can be withdrawn from the anode exhaust as a chemical energy output. Hydrogen can be used as a clean fuel without generating greenhouse gases when it is burned or combusted. Instead, for hydrogen generated by reforming of hydrocarbons (or hydrocarbonaceous compounds), the CO2 will have already been “captured” in the anode loop. Additionally, hydrogen can be a valuable input for a variety of refinery processes and/or other synthesis processes. Syngas can also be a valuable input for a variety of processes. In addition to having fuel value, syngas can be used as a feedstock for producing other higher value products, such as by using syngas as an input for Fischer-Tropsch synthesis and/or methanol synthesis processes.
In some aspects, the reformable hydrogen content of reformable fuel in the input stream delivered to the anode and/or to a reforming stage associated with the anode can be at least about 50% greater than the net amount of hydrogen reacted at the anode, such as at least about 75% greater or at least about 100% greater. Additionally or alternately, the reformable hydrogen content of fuel in the input stream delivered to the anode and/or to a reforming stage associated with the anode can be at least about 50% greater than the net amount of hydrogen reacted at the anode, such as at least about 75% greater or at least about 100% greater. In various aspects, a ratio of the reformable hydrogen content of the reformable fuel in the fuel stream relative to an amount of hydrogen reacted in the anode can be at least about 1.5:1, or at least about 2.0:1, or at least about 2.5:1, or at least about 3.0:1. Additionally or alternately, the ratio of reformable hydrogen content of the reformable fuel in the fuel stream relative to the amount of hydrogen reacted in the anode can be about 20:1 or less, such as about 15:1 or less or about 10:1 or less. In one aspect, it is contemplated that less than 100% of the reformable hydrogen content in the anode inlet stream can be converted to hydrogen. For example, at least about 80% of the reformable hydrogen content in an anode inlet stream can be converted to hydrogen in the anode and/or in an associated reforming stage(s), such as at least about 85%, or at least about 90%. Additionally or alternately, the amount of reformable fuel delivered to the anode can be characterized based on the Lower Heating Value (LHV) of the reformable fuel relative to the LHV of the hydrogen oxidized in the anode. This can be referred to as a reformable fuel surplus ratio. In various aspects, the reformable fuel surplus ratio can be at least about 2.0, such as at least about 2.5, or at least about 3.0, or at least about 4.0. Additionally or alternately, the reformable fuel surplus ratio can be about 25.0 or less, such as about 20.0 or less, or about 15.0 or less, or about 10.0 or less.
Comparative Example—Conventional Catalyst Patterns
During conventional operation of a molten carbonate fuel cell, the cathode is typically operated with a CO2 concentration in the cathode input feed of 8 vol % or more and/or a CO2 utilization of 70% or less. Such operation typically corresponds to a cathode exhaust that includes 5.0 vol % or more CO2. During such conventional operation, the anode is operated with an excess of fuel at a fuel utilization of 60% or more. For operation under such conventional conditions, a variety of reforming catalyst patterns have been developed in order to improve fuel cell operation. These catalyst patterns are based on an expected temperature profile during conventional fuel cell operation. The expected temperature profile is based on a corresponding expectation of where the reaction will occur within a fuel cell. In particular, at a typical or conventional fuel utilization of 60% to 70%, some depletion of fuel will occur as the fuel moves within the anode from the anode inlet to the anode outlet. Similarly, for a CO2 utilization of 40% to 60%, some reduction in the amount of CO2 available for reaction will occur as CO2 is depleted in moving from the cathode inlet to the cathode outlet. Because the concentration of reaction components in both the anode and the cathode can contribute to the driving force of the reaction in a fuel cell, it is expected that the amount of reaction will be highest near the cathode inlet and/or the anode inlet, with the amount of reaction being reduced in the direction of the anode outlet and in the direction of the cathode outlet. Based on this, conventional catalyst patterns are designed to provide additional reforming near the cathode inlet and anode inlet, while reducing or minimizing reforming near the anode outlet and the cathode outlet. This results in balancing the location of the exothermic reaction in the fuel cell with the endothermic reaction in the reforming element.
In order to illustrate a conventional reforming catalyst pattern and the relationship to the anode flow and cathode flow in the fuel cell, FIG. 5 shows an example of a flow pattern within a reforming element. In the flow pattern shown in FIG. 5, the reformable fuel 521 is initially passed through a channel 530 on the side of the reforming element 510. The channel 530 is in proximity to the cathode inlet of the fuel cell stack. The channel 530 is separated from a remaining or first portion of reforming element 510 by a barrier 535. The catalyst density in channel 530 can be relatively uniform, such as due to having no reforming catalyst in channel 530. More typically, channel 530 can have a non-zero reforming catalyst density, such as reforming catalyst density corresponding to the average reforming catalyst density in the first portion 540, the minimum catalyst density in first portion 540, or any other convenient catalyst density. In the example shown in FIG. 5, after traveling down the channel 530 (in the direction of arrow 507), the reformable fuel 521 is passed 545 from channel 530 to first portion 540 of reforming element 510, where it flows through first portion 540 in the direction of arrow 508. The hydrogen produced by reforming and/or remaining portions of unreformed fuel can then exit the reforming element (not shown) and be passed into one or more anodes (via anode collectors) by any convenient method, such as via one or more manifolds or conduits. Arrow 550 shows the direction of flow within an anode associated with the reforming element 510, while arrow 560 shows the direction of flow within a cathode associated with the reforming element 510.
FIG. 6 shows an example of a reforming catalyst pattern suitable for use on a surface that includes reforming catalyst. The catalyst pattern is formed based on parallel lines of catalyst particles, with the direction of flow being substantially parallel the lines of catalyst particles during most portions of the flow path. The pattern shown in FIG. 6 corresponds to a pattern suitable either for the reforming catalyst in the entire reforming element, or alternatively for a portion such as first portion 510 in a reforming element. In FIG. 6, the reforming catalyst density is highest at the left end, and decreases to no catalyst density at the right end. As indicated by arrow 550, the reforming catalyst pattern shown in FIG. 6 corresponds to having an elevated concentration of reforming catalyst near the anode inlet and little or no reforming catalyst near the anode exit. For a flow pattern similar to FIG. 6, this means that the reformable fuel initially encounters little or no catalyst density, with the highest catalyst density being present just before the reformable fuel exits from the reforming element. This can allow the amount of reforming that occurs to be at a maximum near the anode inlet and a minimum near the anode outlet. For operation where excess fuel is present in the anode and excess CO2 is present in the cathode, the oxidation of fuel in the anode can be at the highest near the anode inlet. Thus, the maximum amount of endothermic reforming (in the reforming element) occurs near the maximum amount of exothermic oxidation in the anode. It is noted that if an average catalyst density is used in an initial channel in proximity to the cathode inlet, such as the initial channel 510 in FIG. 5, the catalyst pattern in FIG. 6 can provide substantial reforming in proximity to both the anode inlet and the cathode inlet.
U.S. Pat. No. 8,822,090 describes another type of reforming catalyst pattern. For the catalyst pattern in U.S. Pat. No. 8,822,090, two separate catalyst density variations are present, with one being aligned with the cathode flow and the other being aligned with the anode flow. In U.S. Pat. No. 8,822,090, the fuel enters the reforming element on a side that is in proximity to the cathode outlet side of the fuel cell stack. The catalyst density increases as the fuel moves from the cathode outlet side of the reforming element toward the cathode outlet side. An example of a catalyst pattern is shown with variations in reforming catalyst density based on placement of reforming catalyst particles. The maximum catalyst density corresponds to placement of particles with a frequency of one particle per two potential placement sites and a minimum catalyst density of one particle per sixty-four potential placement sites. This corresponds to a variation from maximum to minimum of over 90%, under the definitions provided herein. It is noted that in addition to the variation along the direction of cathode flow, there is a second gradient along the direction of anode flow in the anode associated with the reforming element.
Example 2—Reforming Catalyst Pattern for Use with Elevated CO2 Utilization
A molten carbonate fuel cell model that included aspects to represent fluid flows and heat transfer within the fuel cell was used to determine the impact of catalyst pattern on temperature differences within the fuel cell. The model was constructed using a commercially available process modeling platform. For the model used in this example, the model represented a system with a reforming element, an associated fuel cell having an anode and a cathode, and a separator plate between the reforming element and the associated fuel cell. The flow pattern in the reforming element corresponds to the flow pattern shown in FIG. 5. Similar to FIG. 5, the reforming element included an initial channel in proximity to the cathode inlet with a constant catalyst density (corresponding to the average in the first portion), and a first portion with a catalyst pattern that was aligned with the flow pattern in the anode.
In this example, modeling results are provided for molten carbonate fuel cells with two different types of catalyst patterns for a first portion of the reforming element. One type of catalyst pattern corresponds to the catalyst pattern shown in FIG. 5. A second type of catalyst pattern corresponds to a catalyst pattern with a difference between maximum and minimum catalyst density of roughly 63%.
FIG. 7 shows three types of catalyst density profiles for a first portion of a reforming element. In FIG. 7, the x-axis corresponds to the flow axis in the anode, with 0 corresponding to the anode inlet and 1 corresponding to the anode outlet. Line 910 corresponds to the conventional catalyst profile shown in FIG. 5. Line 920 corresponds to the catalyst profile used for the second fuel cell that was modeled. As shown in FIG. 7, line 920 has a ratio of minimum to maximum catalyst density of 60/160, or 3/8. This corresponds to a difference between maximum and minimum of 62.5%. By contrast, the difference between maximum and minimum for line 910 is effectively 100%, as the catalyst density drops to zero at the anode outlet in line 910. FIG. 7 also shows line 930, which is similar to the profile in line 920, but with a continuous monotonic decrease in catalyst density, as opposed to the step-wise monotonic decrease in catalyst density shown in line 920.
In the models, fuel cells using the catalyst profiles described by line 910 for the first portion of the surface in the reforming element were modeled under conditions that resulted in elevated CO2 utilization. The model conditions were a cathode inlet CO2 concentration of 3.8 vol %, a fuel utilization of 50%, an actual CO2 utilization of  ̃81%, an average cathode temperature of roughly 903.15° K (630° C.), and a current density of 90 mA/cm2. The input stream into the reformer included 29 vol % H2, 9.0 vol % CO2, 41 vol % H2O, and 20 vol % CH4, with the balance corresponding to nitrogen. This resulted in an input stream into the anode input for each fuel cell that included 51 vol % H2, 9 vol % CO2, 24 vol % H2O, 9 vol % CH4, and 6 vol % CO. The cathode input stream for each fuel cell included 3.8 vol % CO2, 11 vol % O2, and 10 vol % H2O.
In the models, fuel cells using the catalyst profiles described by line 920 for the first portion of the surface in the reforming element were modeled under conditions that resulted in elevated CO2 utilization. The model conditions were a cathode inlet CO2 concentration of 4.5 vol %, a fuel utilization of 50%, an actual CO2 utilization of  ̃86%, an average cathode temperature of roughly 903.15° K (630° C.), and a current density of 90 mA/cm2. The input stream into the reformer included 29 vol % H2, 9.0 vol % CO2, 41% H2O, and 20 vol % CH4, with the balance corresponding to nitrogen. This resulted in an input stream into the anode input for each fuel cell included 53 vol % H2, 9 vol % CO2, 23 vol % H2O, 8 vol % CH4, and 8 vol % CO. The cathode input stream for each fuel cell included 4.5 vol % CO2, 11 vol % O2, and 10 vol % H2O.
FIG. 8 shows the temperature variation at the separator plate for the fuel cell that was modeled with the comparative reforming catalyst density corresponding to line 910 of FIG. 7. In FIG. 8, the x-axis corresponds to the anode flow direction, while the y-axis corresponds to the cathode flow direction. As shown in FIG. 8, the temperature gradient across the fuel cell is greater than 80° K, or greater than 80° C. This is due to the excess waste heat generated by alternative ion transport in the corner of the fuel cell corresponding to the cathode outlet and the anode outlet. Based on the catalyst pattern corresponding to line 910, the catalyst density is close to zero near the corner corresponding to the cathode outlet and anode outlet. Thus, there is only a minimal amount of the endothermic reforming reaction available to balance the additional waste heat generated due to alternative ion transport.
FIG. 9 shows the temperature variation at the separator plate for the fuel cell that was modeled with the reforming catalyst density corresponding to line 920 of FIG. 7. By reducing the difference between minimum and maximum reforming catalyst density, the temperature variation across the fuel cell is reduced to roughly 37° K, or roughly 37° C.
Example 3—Reforming Catalyst Pattern for Use with Elevated CO2 Utilization
Similar to Example 2, a molten carbonate fuel cell model that included aspects to represent fluid flows and heat transfer within the fuel cell was used to determine the impact of catalyst pattern on temperature differences within the fuel cell. The model was constructed using a commercially available process modeling platform. For the model used in this example, the model represented a system with a reforming element, an associated fuel cell having an anode and a cathode, and a separator plate between the reforming element and the associated fuel cell. The flow pattern in the reforming element corresponds to the flow pattern shown in FIG. 5. Similar to FIG. 5, the reforming element included an initial channel in proximity to the cathode inlet with a constant catalyst density (corresponding to the average in the first portion), and a first portion with a catalyst pattern that was aligned with the flow pattern in the anode.
In this example, modeling results are provided for molten carbonate fuel cells with three different types of catalyst patterns for a first portion of the reforming element. One type of catalyst pattern corresponds to the catalyst pattern shown in FIG. 5. A second type of catalyst pattern corresponds to a catalysts pattern with a difference between maximum and minimum catalyst density of roughly 100%, like the first type of catalyst pattern in this example. A third type of catalyst pattern corresponds to a catalyst pattern with a difference between maximum and minimum catalyst density of roughly 43%.
FIG. 10 shows four types of catalyst density profiles for a first portion of a reforming element. In FIG. 10, the x-axis corresponds to the flow axis in the anode, with 0 corresponding to the anode inlet and 1 corresponding to the anode outlet. Line 910 corresponds to the conventional catalyst profile shown in FIG. 5 and in FIG. 7 from Example 2 above. As noted above, the difference between maximum and minimum catalyst density for this catalyst profile is effectively 100%, as the catalyst density drops to zero at the anode outlet in line 910. Similarly, line 1010 corresponds to another catalyst profile having a difference between maximum and minimum catalyst density as effectively 100%, since here too, the catalyst density drops to zero at the anode outlet in line 1010. As shown in FIG. 10, line 1020 has a ratio of minimum to maximum catalyst density of approximately 80/160, or 1/2. This corresponds to a difference between maximum and minimum of approximately 50%. FIG. 10 also shows line 1030, which is similar to the profile in line 1020, but with a continuous monotonic decrease in catalyst density, as opposed to the step-wise monotonic decrease in catalyst density shown in line 1020.
In the models, fuel cells using the catalyst profiles described by line 1010 for the first portion of the surface in the reforming element were modeled under conditions that resulted in elevated CO2 utilization. The model conditions were a cathode inlet CO2 concentration of 4.3 vol %, a fuel utilization of 50%, an actual CO2 utilization of  ̃85%, an average cathode temperature of roughly 903.15° K (630° C.), and a current density of 110 mA/cm2. The input stream into the reformer included 7 vol % H2, 2 vol % CO2, 60 vol % H2O, and 30 vol % CH4, with the balance corresponding to nitrogen. This resulted in an input stream into the anode input for each fuel cell that included 50 vol % H2, 9 vol % CO2, 10 vol % H2O, 10 vol % CH4, and 5 vol % CO. The cathode input stream for each fuel cell included 4.3 vol % CO2, 11 vol % O2, and 10 vol % H2O.
In the models, fuel cells using the catalyst profiles described by line 1020 for the first portion of the surface in the reforming element were modeled under conditions that resulted in elevated CO2 utilization. The model conditions were a cathode inlet CO2 concentration of 4.4 vol %, a fuel utilization of 50%, an actual CO2 utilization of  ̃85%, an average cathode temperature of roughly 903.15° K (630° C.), and a current density of 150 mA/cm2. The input stream into the reformer included 10 vol % H2, 3 vol % CO2, 58% H2O, and 29 vol % CH4 with the balance corresponding to nitrogen. This resulted in an input stream into the anode input for each fuel cell included 51 vol % H2, 9 vol % CO2, 25 vol % H2O, 10 vol % CH4, and 6 vol % CO. The cathode input stream for each fuel cell included 4.3 vol % CO2, 11 vol % O2, and 10 vol % H2O.
In the models, fuel cells using the catalyst profiles described by line 1030 for the first portion of the surface in the reforming element were modeled under conditions that resulted in elevated CO2 utilization. The model conditions were a cathode inlet CO2 concentration of 4.4 vol %, a fuel utilization of 50%, an actual CO2 utilization of  ̃85%, an average cathode temperature of roughly 903.15° K (630° C.), and a current density of 150 mA/cm2. The input stream into the reformer included 10 vol % H2, 3 vol % CO2, 58% H2O, and 29 vol % CH4 with the balance corresponding to nitrogen. This resulted in an input stream into the anode input for each fuel cell included 51 vol % H2, 9 vol % CO2, 25 vol % H2O, 10 vol % CH4, and 6 vol % CO. The cathode input stream for each fuel cell included 4.3 vol % CO2, 11 vol % O2, and 10 vol % H2O.
FIG. 11 shows the temperature variation at the separator plate for the fuel cell that was modeled with the comparative reforming catalyst density corresponding to line 910 of FIG. 10. In FIG. 11, the x-axis corresponds to the anode flow direction, while the y-axis corresponds to the cathode flow direction. As shown in FIG. 11, the temperature gradient across the fuel cell is greater than 100° K, or greater than 100° C. This is due to the excess waste heat generated by alternative ion transport in the corner of the fuel cell corresponding to the cathode outlet and the anode outlet. Based on the catalyst pattern corresponding to line 910, the catalyst density is close to zero near the corner corresponding to the cathode outlet and anode outlet. Thus, there is only a minimal amount of the endothermic reforming reaction available to balance the additional waste heat generated due to alternative ion transport.
Similar to FIG. 11, FIG. 12 shows the temperature variation of the separator plate for the fuel cell that was modeled with the reforming catalyst density corresponding to line 1010 of FIG. 10. As shown in FIG. 12, the temperature gradient across the fuel cell is approximately 70° K or 70° C., which is due to the excess waste heat generated by alternative ion transport in the corner of the fuel cell corresponding to the cathode outlet and the anode outlet. Based on the catalyst pattern corresponding to line 1010, the catalyst density is close to zero near the corner corresponding to the cathode outlet and anode outlet. Thus, there is only a minimal amount of the endothermic reforming reaction available to balance the additional waste heat generated due to alternative ion transport.
FIG. 13 shows the temperature variation at the separator plate for the fuel cell that was modeled with the reforming catalyst density corresponding to line 1030 of FIG. 10. By reducing the difference between minimum and maximum reforming catalyst density, the temperature variation across the fuel cell is reduced to roughly 38° K, or roughly 38° C.
ADDITIONAL EMBODIMENTS
Embodiment 1
A method for producing electricity, the method comprising: passing a fuel stream comprising a reformable fuel into a fuel stack comprising a first surface, the first surface comprising a first portion comprising a reforming catalyst, a reforming catalyst density on the first portion of the first surface having a difference between a maximum catalyst density and a minimum catalyst density of 20% to 75%; reforming at least a portion of the reformable fuel in the presence of the first surface to produce reformed hydrogen; introducing at least a portion of the reformable fuel, at least a portion of the reformed hydrogen, or a combination thereof into an anode of a molten carbonate fuel cell; introducing a cathode input stream comprising O2 and CO2 into a cathode of the molten carbonate fuel cell, a direction of flow in the cathode of the molten carbonate fuel cell being substantially orthogonal to a direction of flow in the anode of the molten carbonate fuel cell; and operating the molten carbonate fuel cell at a transference of 0.97 or less and an average current density of 60 mA/cm2 or more to generate electricity, an anode exhaust comprising H2, CO, and CO2, and a cathode exhaust comprising 2.0 vol % or less CO2, 1.0 vol % or more H2O, and 1.0 vol % or more O2.
Embodiment 2
The method of Embodiment 1, wherein the cathode input stream comprises 5.0 vol % or less of CO2, or wherein the cathode exhaust comprises 1.0 vol % or less of CO2, or a combination thereof.
Embodiment 3
A method for producing electricity, the method comprising: passing a fuel stream comprising a reformable fuel into a fuel stack comprising a first surface, the first surface comprising a first portion comprising a reforming catalyst, a reforming catalyst density on the first portion of the first surface having a difference between a maximum catalyst activity and a minimum catalyst activity of 20% to 75%; reforming at least a portion of the reformable fuel in the presence of the first surface to produce reformed hydrogen; introducing at least a portion of the reformable fuel, at least a portion of the reformed hydrogen, or a combination thereof into an anode of a molten carbonate fuel cell; introducing a cathode input stream comprising O2 and CO2 into a cathode of the molten carbonate fuel cell, a direction of flow in the cathode of the molten carbonate fuel cell being substantially orthogonal to a direction of flow in the anode of the molten carbonate fuel cell; and operating the molten carbonate fuel cell at a transference of 0.97 or less and an average current density of 60 mA/cm2 or more to generate electricity, an anode exhaust comprising H2, CO, and CO2, and a cathode exhaust comprising 2.0 vol % or less CO2, 1.0 vol % or more H2O, and 1.0 vol % or more O2.
Embodiment 4
The method of any of the above embodiments, wherein the transference is 0.95 or less, or 0.90 or less.
Embodiment 5
The method of any of the above embodiments, wherein the reforming catalyst on the first portion of the first surface comprises a monotonic catalyst density variation.
Embodiment 6
The method of any of the above embodiments, wherein the maximum catalyst density is in proximity to the anode inlet, or wherein the maximum catalyst density is in proximity to the cathode inlet.
Embodiment 7
The method of any of the above embodiments, wherein the minimum catalyst density is in proximity to the anode outlet, or wherein the minimum catalyst density is in proximity to the cathode outlet.
Embodiment 8
The method of any of the above embodiments, wherein the fuel cell stack comprises a reforming element associated with the anode, wherein the first surface comprises an interior surface of the reforming element, and wherein a temperature variation within the fuel cell stack at a separator plate between the reforming element and the anode is optionally 70° C. or less (or 40° C. or less).
Embodiment 9
The method of any of Embodiments 1-7, wherein the first surface comprises an interior surface of the anode, and wherein a temperature variation within the fuel cell stack at a separator plate between the anode and another element is optionally 70° C. or less (or 40° C. or less).
Embodiment 10
The method of any of the above embodiments, wherein the first surface further comprises a second portion, the second portion optionally comprising at least one of a constant catalyst density and a constant catalyst activity, and wherein the second portion is in proximity to the cathode inlet, or wherein the second portion is in proximity to the anode inlet.
Embodiment 11
The method of any of the above embodiments, wherein the reforming catalyst comprises a plurality of lines of catalyst particles, and wherein reforming the at least a portion of the reformable fuel comprises flowing the at least a portion of the reformable fuel over the catalyst particles in a direction that is substantially parallel to the lines of catalyst particles.
Embodiment 12
A fuel cell stack comprising: a molten carbonate fuel cell comprising an anode and a cathode; a reforming element associated with the anode, the reforming element comprising a first surface, the first surface comprising a first portion comprising a reforming catalyst, a reforming catalyst density on the first portion of the first surface comprising a monotonically decreasing catalyst density, the reforming catalyst density on the first portion of the first surface having a difference between a maximum catalyst density and a minimum catalyst density of 20% to 75% (or 25% to 70%, or 25% to 65%); and a separator plate between the anode and the reforming element, wherein the first surface optionally further comprises a second portion, the optional second portion being in proximity to the cathode inlet or in proximity to the anode inlet, the optional second portion comprising a constant catalyst density.
Embodiment 13
The fuel cell stack of Embodiment 12, wherein the maximum catalyst density is in proximity to the anode inlet, or wherein the maximum catalyst density is in proximity to the cathode inlet.
Embodiment 14
The fuel cell stack of Embodiment 12 or 13, wherein the minimum catalyst density is in proximity to the anode outlet, or wherein the minimum catalyst density is in proximity to the cathode outlet.
Embodiment 15
The fuel cell stack of any of Embodiments 12 to 14, wherein the reforming catalyst comprises substantially parallel lines of catalyst particles
All numerical values within the detailed description and the claims herein are modified by “about” or “approximately” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.
Although the present invention has been described in terms of specific embodiments, it is not necessarily so limited. Suitable alterations/modifications for operation under specific conditions should be apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations/modifications that fall within the true spirit/scope of the invention.

## Claims

1. A method for producing electricity, the method comprising:
passing a fuel stream comprising a reformable fuel into a fuel cell stack comprising a first surface, the first surface comprising a first portion comprising a reforming catalyst, a reforming catalyst density on the first portion of the first surface having a difference between a maximum catalyst density and a minimum catalyst density of 20% to 75%;
reforming at least a portion of the reformable fuel in the presence of the first surface to produce reformed hydrogen;
introducing at least a portion of the reformable fuel, at least a portion of the reformed hydrogen, or a combination thereof into an anode of a molten carbonate fuel cell;
introducing a cathode input stream comprising O2 and CO2 into a cathode of the molten carbonate fuel cell; and
operating the molten carbonate fuel cell at a transference of 0.97 or less and an average current density of 60 mA/cm2 or more to generate electricity, an anode exhaust comprising H2, CO, and CO2, and a cathode exhaust comprising 2.0 vol % or less CO2, 1.0 vol % or more H2O, and 1.0 vol % or more O2.

2. The method of claim 1, wherein the reforming catalyst on the first portion of the first surface comprises a monotonic catalyst density variation.

3. The method of claim 1, wherein the maximum catalyst density is in proximity to an anode inlet, or wherein the maximum catalyst density is in proximity to a cathode inlet.

4. The method of claim 1, wherein the minimum catalyst density is in proximity to an anode outlet, or wherein the minimum catalyst density is in proximity to a cathode outlet.

5. The method of claim 1, wherein the fuel cell stack comprises a reforming element associated with the anode, wherein the first surface comprises an interior surface of the reforming element.

6. The method of claim 5, wherein a temperature variation within the fuel cell stack at a separator plate between the reforming element and the anode is 70° C. or less.

7. The method of claim 1, wherein the first surface comprises an interior surface of the anode.

8. The method of claim 7, wherein a temperature variation within the fuel cell stack at a separator plate between the anode and another element is 70° C. or less.

9. The method of claim 1, wherein the first surface further comprises a second portion.

10. The method of claim 9, wherein the second portion is in proximity to a cathode inlet, or wherein the second portion is in proximity to an anode inlet.

11. The method of claim 9, wherein the second portion comprises a constant catalyst density.

12. The method of claim 1, wherein the cathode input stream comprises 5.0 vol % or less CO2, or wherein the cathode exhaust comprises 1.0 vol % or less CO2, or a combination thereof.

13. The method of claim 1, wherein the transference is 0.95 or less.

14. The method of claim 1, wherein a direction of flow in the cathode of the molten carbonate fuel cell is orthogonal to a direction of flow in the anode of the molten carbonate fuel cell.