CN113711401A - Fuel cell grading for molten carbonate fuel cells - Google Patents

Abstract

Provide for when in CO₂Use of fuel in operating molten carbonate fuel cells with improved utilizationSystems and methods for fuel cell grading to reduce or minimize current density variations. The fuel cell staging can be reduced in CO₂The amount of transport of surrogate ions that occurs when operating molten carbonate fuel cells at conditions of increased utilization.

Description

Fuel cell grading for molten carbonate fuel cells

Technical Field

Provide for when in CO₂A method of fuel cell classification is used when operating molten carbonate fuel cells at conditions of increased utilization.

Background

The subject matter disclosed and claimed herein is the achievement of activity within the scope of a joint Research agreement between ExxonMobil Research and Engineering Company and fuel cell Energy Company, Inc.

Molten carbonate fuel cells utilize hydrogen and/or other fuels to generate electricity. Hydrogen may be provided by reforming methane or other reformable fuel in a steam reformer, such as a steam reformer located upstream of or integrated within a fuel cell. Fuel may also be reformed in an anode cell in a molten carbonate fuel cell that may be operated to produce conditions suitable for reforming fuel in the anode. Yet another option is to perform some reforming both externally and internally in the fuel cell. The reformable fuel can encompass a hydrocarbonaceous material that can be reacted with steam and/or oxygen at elevated temperature and/or pressure to produce a gaseous product that includes hydrogen.

The basic structure of a molten carbonate fuel cell comprises a cathode, an anode, and a matrix located between the cathode and the anode, the matrix comprising one or more molten carbonates for use as an electrolyte. During normal operation of a molten carbonate fuel cell, molten carbonate partially diffuses into the pores of the cathode. This diffusion of molten carbonate into the cathode pores provides an interface region where CO is present₂Can be converted into CO₃ ^2-For transport across the electrolyte to the anode.

Conventionally, cathodes for molten carbonate fuel cells are typically constructed with a single layer. The properties of a single layer cathode are selected based on several types of properties. First, the cathode needs to have sufficient structural stability to maintain the integrity of the cathode layer, while also providing sufficient electrical conductivity to allow fuel cell operation. This typically results in the use of materials such as nickel for the electrodes, which are oxidized and (optionally) lithiated during initial operation of the fuel cell. With respect to other properties, it is desirable to have a pore size small enough to provide sufficient wetting of the cathode by the electrolyte to again provide suitable conductivity.

One difficulty with conventional electrode materials, such as (optionally lithiated) nickel oxide, is that nickel oxide is susceptible to polarization under fuel cell operating conditions. During fuel cell operation, polarization of the cathode may result in additional voltage loss across the cathode. The effect of polarization can be mitigated by increasing the operating temperature of the fuel cell, but this results in a reduction in operating life. Accordingly, it is desirable to have a molten carbonate cathode structure that can reduce polarization, as this can provide reduced voltage loss and/or can allow for operating temperature reduction.

Us patent 9,077,007 describes a method for operating a molten carbonate fuel cell for integrated power generation and chemical production. The method includes operating the fuel cell at a reduced level of fuel utilization. Configurations are also described that include fuel cells operating in parallel or passing an anode flow and a cathode flow through a series of fuel cells.

Disclosure of Invention

In one aspect, a method for generating power is provided. The method comprises adding H₂An anode input stream of reformable fuel, or a combination thereof, is introduced into a first anode stage of the plurality of molten carbonate fuel cell stages. The method further comprises adding O₂And CO₂Is introduced into a first cathode stage of the plurality of molten carbonate fuel cell stages. The method further comprises passing a second anode input stream into a second anode stage of the plurality of molten carbonate fuel cell stages. The method further comprises outputting an intermediate cathode into a second cathode stage of the plurality of molten carbonate fuel cell stages. The method further comprises operating the plurality of molten carbonate fuel cell stages to produce: i) the average current density was 60mA/cm²Or greater power; ii) anode exhaust from the plurality of molten carbonate fuel cell stages, the anode exhaust comprising H₂CO and CO₂(ii) a And iii) cathode off-gas from the plurality of molten carbonate fuel cell stages, the cathode off-gas comprising CO in an amount of 2.0 vol% or less₂H in an amount of 1.0 vol% or more₂O and O in an amount of 1.0 vol% or more₂At least one of the first cathode stage and the second cathode stage operates at a transfer rate of 0.97 or less.

Drawings

Fig. 1 shows an example of fuel cell staging in a co-current cross-flow configuration.

Fig. 2 shows an example of fuel cell staging in a counter-current cross-flow configuration.

Figure 3 shows an example of a portion of a molten carbonate fuel cell stack.

Fig. 4 shows an example flow pattern for a molten carbonate fuel cell in which the anode flow direction is aligned approximately perpendicular to the cathode flow direction.

FIG. 5 shows a reaction at CO₂Under the condition of improved utilization rateCO in the cathode of a fuel cell operating as a single stage₂Concentration mode.

FIG. 6 shows a reaction at CO₂CO in cathodes of two fuel cell stages operating with increased utilization₂Concentration mode.

Detailed Description

SUMMARY

In various aspects, methods for treating a disease associated with CO are provided₂Systems and methods for using fuel cell staging to reduce or minimize current density variations when operating molten carbonate fuel cells with increased utilization. In some aspects, in CO₂Operating the fuel cell or series of fuel cell stages with increased utilization corresponds to operating at least one fuel cell stage at a transfer rate of 0.97 or less, or 0.95 or less, or 0.90 or less. Additionally or alternatively, in CO₂CO from the output stream of the last cathode stage during operation with increased utilization₂The amount may comprise 2.0 vol% or less, or 1.0 vol% or less, or 0.8 vol% or less CO₂。

When operating molten carbonate fuel cell stacks as a single stage to increase CO₂In terms of utilization, it has been found that a large number of surrogate ion transmissions can occur. The substitute ion transport refers to the removal of carbonate ions (CO)₃ ^2-) Transport of the foreign ions across the molten carbonate electrolyte.

Conventional operating conditions for molten carbonate fuel cells generally correspond to conditions in which the amount of transport of surrogate ions is reduced, minimized, or absent. The amount of surrogate ion transport can be quantified based on the transfer rate of the fuel cell. The transfer rate is defined as the fraction of ions that are transported across the molten carbonate electrolyte, which ions correspond to carbonate ions, but not to hydroxide ions and/or other ions. A simple method for determining the transfer rate can be based on a) measuring the CO at the cathode inlet₂The change in concentration compared to the cathode outlet is compared to b) the amount of carbonate ion transport required to achieve the current density produced by the fuel cell. Notably, this definition of the transfer rateSuppose CO₂Return transport from anode to cathode is minimal. It is believed that such return transmissions are minimal for the operating conditions described herein. For CO₂Concentration, the cathode input stream and/or the cathode output stream can be sampled, wherein the sample is transferred to a gas chromatograph for CO determination₂And (4) content. The average current density of the fuel cell may be measured in any convenient manner.

Under normal operating conditions, the transfer rate may be relatively close to 1.0, such as 0.98 or higher, and/or such as substantially free of surrogate ion transport. A transfer rate of 0.98 or higher means that 98% or more of the ionic charge transported across the electrolyte corresponds to carbonate ions. Notably, the charge of a hydroxide ion is-1, while the charge of a carbonate ion is-2, thus requiring the transport of two hydroxide ions across the electrolyte to produce the same charge transfer as the transport of one carbonate ion.

Operating a molten carbonate fuel cell at a transfer rate of 0.95 or less (or 0.97 in at least one stage of a multi-stage configuration) can increase the effective amount of carbonate ion transport achieved compared to conventional operating conditions, even though a portion of the current density produced by the fuel cell is due to transport of ions other than carbonate ions. To operate the fuel cell at a transfer rate of 0.97 or less, or 0.95 or less, the CO must be consumed in the fuel cell cathode₂. It has been found that such CO in the cathode₂Consumption is often local. Many regions within the fuel cell cathode may still have sufficient CO₂For normal operation. These regions contain additional CO that is desired to be transported across the electrolyte (e.g., for carbon capture)₂. However, when operating under conventional conditions, CO in such regions₂And are generally not transported across the electrolyte. By selecting operating conditions with a transfer rate of 0.97 or less, or 0.95 or less, sufficient CO is present₂Can be used to transport additional CO₂And the depleted region may be based on a surrogate ion transport operation. This can increase the capture of CO from the cathode input stream₂Practical limits on the amount.

Using MCFC to provideHigh CO content₂One difficulty with capture rates is that fuel cell operation may be kinetically limited if one or more of the reactants required for fuel cell operation are present in small amounts. For example, when CO is used₂75% or more CO is achieved at a cathode input stream content of 4.0 vol% or less₂The utilization rate 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 CO is present₂Evenly distributed throughout the cathode. In contrast, the concentration within the cathode typically varies due to a variety of factors, such as the flow patterns in the anode and cathode. CO 2₂The concentration variation may result in some part of the CO of the cathode₂The concentration is significantly below 1.0 vol%.

Conventionally, CO is predicted in the cathode₂The consumption of (b) may result in a reduction in voltage and a reduction in current density. However, it has been found that removal of CO is due to₃ ^2-The current density may be in CO, with transport of ions other than CO across the electrolyte₂Is maintained when consumed. For example, a portion of the ions transported across the electrolyte may correspond to hydroxide ions (OH)^-). Transport of surrogate ions across the electrolyte may allow the fuel cell to maintain a target current density even at CO transported across the electrolyte₂This is also the case in the case of insufficient amounts.

One of the advantages of transporting the surrogate ions across the electrolyte is that even when there is not a sufficient amount of CO₂The fuel cell can also continue to operate where the molecules are kinetically available. This may allow for additional CO₂From cathode to anode, even if CO is present in the cathode₂The amount is generally considered insufficient for normal fuel cell operation as well. This may allow the fuel cell to measure CO₂Operating with near 100% utilization, and calculated CO₂Utilization (based on current density) of CO that can be measured₂The utilization is at least 3% higher, or at least 5% higher, or at least 10% higher, or at least 20% higher. Notably, alternative ion transport may allow the fuel cell to operate with over 100% calculated CO₂The utilization rate is correspondingCurrent density operation.

While transport of the surrogate ions may allow the fuel cell to maintain a target current density, it has further been found that transport of the surrogate ions across the electrolyte may also reduce or minimize the life of the molten carbonate fuel cell. It is therefore desirable to mitigate this loss of fuel cell life. It has been unexpectedly found that the use of multiple fuel cell stages can allow for increased CO₂Trapping rate while reducing or minimizing the amount of surrogate ion transport.

In some aspects, increased CO₂The capture rate may be defined based on the amount of transfer rate, such as a transfer rate of 0.97 or less, or 0.95 or less, or 0.93 or less, or 0.91 or less. Maintaining operating conditions with a transfer rate of 0.97 or less, or 0.95 or less, may also typically result in CO in the cathode output stream₂The concentration is 2.0 vol% or less, or 1.5 vol% or less, or 1.0 vol% or less. Higher CO in cathode output stream₂At concentrations, there is generally not sufficient CO to result in lower values of transfer rate₂Local consumption.

The presence of CO may also be indicated by other factors₂The increase in capture rate, but such other factors are not generally indicative of CO per se₂Sufficient conditions for the increase of the capture rate. For example, when using lower CO₂Concentration of CO when cathode is fed to the stream₂CO the capture rate of which can correspond in some aspects₂The utilization is 70% or higher, or 75% or higher, or 80% or higher, such as up to 95% or possibly still higher. CO 2₂May correspond to CO resulting in the cathode input stream containing₂CO at 5.0 vol% or less, or 4.0 vol% or less (e.g., as low as 1.5 vol% or possibly less)₂And (4) source. The exhaust gas from natural gas turbines being CO-containing₂Examples of streams, CO thereof₂CO in an amount of usually 5.0 vol% or less₂Or 4.0 vol% or less. Additionally or alternatively, enhanced CO₂The capture rate may correspond to operating conditions that produce a substantial current density, such as 60mA/cm, using a molten carbonate fuel cell²Or larger,Or 80mA/cm²Or greater, or 100mA/cm²Or greater, or 120mA/cm²Or greater, or 150mA/cm²Or greater, or 200mA/cm²Or greater, e.g. up to 300mA/cm²Or may be higher. It is noteworthy that the surrogate ion transport may also be indicated by a decrease in the operating voltage of the fuel cell, since the surrogate ion transport reaction pathway has a lower theoretical voltage than the reaction pathway using carbonate ions.

Conventionally, CO in cathode exhaust of molten carbonate fuel cells₂The concentration being maintained at a relatively high value, e.g. 5 vol% CO₂Or greater, or 10 vol% CO₂Or larger, or possibly higher. In addition, molten carbonate fuel cells are typically operated at 70% or less CO₂The utilization value operates. When any of these conditions are present, the primary mechanism of charge transport across the molten carbonate electrolyte is the transport of carbonate ions. While transport of surrogate ions (such as hydroxide ions) across the electrolyte may occur under such conventional conditions, the amount of surrogate ion transport is marginal, corresponding to a current density of 2% or less (or equivalently, a transfer rate of 0.98 or more).

As an alternative to describing the operating conditions in terms of transfer rate, the operating conditions may be based on measured CO₂Utilization and "calculated" CO based on average current density₂Utilization is described. In this discussion, measured CO₂Utilization corresponding to CO removed from cathode input stream₂The amount of (c). This can be done, for example, by using gas chromatography to determine CO in the cathode input stream and the cathode output stream₂The concentration is determined. This may also be referred to as actual CO₂Utilization rate, or simply CO₂Utilization ratio. In this discussion, the calculated CO₂The utilization ratio is defined as all current densities produced at the fuel cell are based on CO₃ ^2-Transport of ions across the electrolyte (i.e. based on CO)₂Ion transport of) to CO occurring in the case of generation of₂Utilization ratio. Measured CO₂Utilization and calculated CO₂The difference between the utilization rates can be used alone to characterize the amount of surrogate ion transport, and/or these values can be used to calculate the transfer rate, as described above.

Has been found to increase CO₂Utilization of the amount of surrogate ion transport generated when operating a molten carbonate fuel cell can be performed by using more than one fuel cell stage to perform the increased CO₂Utilization is reduced or minimized. Operating multiple fuel cell stages to achieve increased CO even with the same (or preferably) smaller fuel cell area₂Utilization may reduce or minimize CO in cathode flow mode₂The amount of change in concentration. This may result in a corresponding reduction in the amount of surrogate ion transport. For example, when operating a fuel cell stage in a cross-flow configuration of an anode gas flow and a cathode gas flow, the amount of surrogate ion transport in the corners of the fuel cell corresponding to the anode inlet and the cathode outlet can be reduced or minimized. Such reduction in surrogate ion transport may be beneficial, for example, to facilitate increasing the operational life of one or more fuel cells.

When multiple fuel cell stages are used in series, the anode and cathode flows through the cells can be arranged in a variety of ways (e.g., with co-current or counter-current flow). In co-current flow, the fuel cell corresponding to the first stage of anode flow is also the first stage of the cathode. In counterflow, a fuel cell corresponding to a first stage of the anode (or cathode) corresponds to a last stage and/or a stage different from the first stage of the cathode (or anode). In addition, flow within a single fuel cell may be characterized. For example, in a cross-flow configuration, the flow direction in the anode of a given fuel cell may be oriented substantially perpendicular to the flow direction in the anode. The cross-flow configuration is opposite to the aligned-flow configuration, in which the flow direction in the anode may be oriented along a flow axis that is substantially the same as the flow direction in the cathode. According to the aspects, various combinations of fuel cells (e.g., combinations of fuel cell stacks) can be arranged in series and/or parallel. In such aspects, for fuel cells arranged in series, any convenient combination of co-current, counter-current, cross-current, and/or aligned flow may be used.

Fig. 1 shows an example of a series of fuel cells (e.g., a series of fuel cell stacks) arranged in a co-current, cross-flow configuration. In the example shown in fig. 1, fuel cell 120, fuel cell 130 and fuel cell 140 are arranged to operate in series. Both the cathode input stream 119 and the anode input stream 115 enter the fuel cell 120, which is the first fuel cell stage. The first stage cathode intermediate output 129 and the first stage anode intermediate output 125 then enter a second fuel cell 130. The second stage cathode intermediate output 139 and the second stage anode intermediate output 135 then enter a third fuel cell stage 140. In the example shown in fig. 1, the third fuel cell 140 corresponds to the last stage, and thus the output from the third fuel cell 140 corresponds to the cathode output 149 and the anode output 145. As shown in fig. 1, in each of the fuel cells 120, 130, and 140, the flow direction of the anode flow is substantially orthogonal to the flow direction of the cathode flow. Thus, the flow in the fuel cells 120, 130 and 140 corresponds to a cross-flow orientation.

Figure 2 shows an example of a series of fuel cells (e.g., a series of fuel cell stacks) arranged in a counter-current, cross-flow configuration. In the example shown in fig. 2, fuel cell 220, fuel cell 230 and fuel cell 240 are arranged to operate in series. The cathode input stream 219 enters the fuel cell 220 as a first cathode fuel cell stage. The first stage cathode intermediate output 229 enters the fuel cell 230 as a second cathode fuel cell stage. The second stage cathode intermediate output 239 enters the fuel cell 240, which is a third cathode fuel cell stage. In the example shown in fig. 2, fuel cell 240 corresponds to the last cathode fuel cell stage, and thus the output from the cathode of fuel cell 240 corresponds to cathode output 249. In contrast to fig. 1, anode input 215 enters fuel cell 240, which is the first anode fuel cell stage. This produces a first stage anode intermediate output 255 that goes to fuel cell 230 as a second anode fuel cell stage. The second stage anode intermediate output 265 enters the fuel cell 220, which corresponds to the third (last) fuel cell stage of the anode flow. The output from the anode of fuel cell 220 corresponds to anode output 245. As shown in fig. 2, in each of the fuel cells 220, 230, and 240, the flow direction of the anode flow is substantially orthogonal to the flow direction of the cathode flow. Thus, the flow in fuel cell 220, fuel cell 230, and fuel cell 240 corresponds to a cross-flow orientation.

In some aspects, any convenient type of electrolyte suitable for operating a molten carbonate fuel cell may be used. Many conventional MCFCs use a eutectic carbonate mixture as the carbonate electrolyte, such as 62 mol% lithium carbonate and 38 mol% potassium carbonate (62% Li)₂CO₃/38％K₂CO₃) Or 52 mol% lithium carbonate and 48 mol% sodium carbonate (52% Li)₂CO₃/48％Na₂CO₃) The eutectic mixture of (1). Other eutectic mixtures may also be used, such as 40 mol% lithium carbonate and 60 mol% potassium carbonate (40% Li)₂CO₃/60％K₂CO₃) The eutectic mixture of (1). Although eutectic mixtures of carbonates may be conveniently used as electrolytes for various reasons, off-eutectic mixtures of carbonates may also be suitable. Typically, such off-eutectic mixtures may comprise various combinations of lithium carbonate, sodium carbonate, and/or potassium carbonate. Optionally, other metal carbonates, such as other alkali metal carbonates (rubidium, cesium carbonate) or other types of metal carbonates, such as barium carbonate, bismuth carbonate, lanthanum carbonate or tantalum carbonate, may be included in the electrolyte in minor amounts as additives.

Notably, the structure of molten carbonate fuel cells may also have an effect on the rate at which degradation occurs. For example, the open area of the cathode surface available to receive cathode gas may affect the rate at which degradation occurs. To make electrical contact, at least a portion of the cathode current collector is typically in contact with the cathode surface in the molten carbonate fuel cell. The open area of the cathode surface (adjacent to the cathode current collector) is defined as the percentage of the cathode surface that is not in contact with the cathode current collector. A typical value for open area is about 33% for conventional molten carbonate fuel cell designs. This is due to the nature of conventional cathode current collector arrangements, which correspond to a plate-like structure resting on the surface of the cathode, wherein a portion of the plate-like structure has openings that allow the cathode gas to diffuse into the cathode. In various aspects, additional benefits may be obtained by using a cathode current collector that provides a greater open area (e.g., 45% or greater, or 50% or greater, or 60% or greater, such as up to 90% or possibly still higher open area) at the cathode surface.

Conditions for molten carbonate fuel cell operation with alternative ion transport

In various aspects, operating conditions of a molten carbonate fuel cell (e.g., a cell that is part of a fuel cell stack) can be selected to correspond to a transfer rate of 0.97 or less, or 0.95 or less, such that the cell simultaneously transports carbonate ions and at least one type of surrogate ion across the electrolyte. In addition to the transfer rate, operating conditions that may indicate that the molten carbonate fuel cell is operating with the transport of alternative ions include, but are not limited to, CO of the cathode input stream₂Concentration, CO in cathode₂Utilization, current density of the fuel cell, voltage drop across the cathode, voltage drop across the anode, and O in the cathode input stream₂And (4) concentration. In addition, the anode input stream and the fuel utilization in the anode can generally be selected to provide a desired current density.

Typically, to induce surrogate ion transport, it is desirable to have CO in at least a portion of the cathode while operating the fuel cell to provide a sufficiently high current density₂The concentration is sufficiently low. CO in the cathode₂A sufficiently low concentration generally corresponds to low CO in the cathode input stream₂High concentration, high CO₂Utilization and/or high average current density. However, such conditions alone are not sufficient to indicate a transfer rate of 0.97 or less, or 0.95 or less.

For example, a molten carbonate fuel cell having a cathode open area of about 33% with 19 vol% CO₂Cathode inlet concentration, 75% CO₂Utilization and average current density of 160mA/cm 2. These conditions correspond to the calculated CO₂Utilization and measured CO₂The difference between the utilization rates is less than 1 percent. Therefore, it cannot be easily derived from high CO₂The presence of the utilization and high average current density infers that there is a large amount of surrogate ion transport/transfer rate of 0.97 or less, or 0.95 or less.

As another example, a molten carbonate fuel cell with a cathode open area between 50% and 60% has 4.0 vol% CO₂Cathode inlet concentration, 89% CO₂Utilization rate and 100mA/cm²Current density operation of. These conditions correspond to a transfer rate of at least 0.97. Therefore, it cannot be easily derived from high CO₂Utilization and low CO in cathode input stream₂The presence of a combination of concentrations infers that there is a transfer rate of 0.95 or less/bulk surrogate ion transport.

As yet another example, a molten carbonate fuel cell having a cathode open area between 50% and 60% has 13 vol% CO₂Cathode inlet concentration, 68% CO₂Utilization and a current density of 100mA/cm 2. These conditions correspond to a transfer rate of at least 0.98.

In this discussion, operating the MCFC to transport surrogate ions across the electrolyte is defined as operating the MCFC such that more than a minimum amount of surrogate ions are transported. Under various conventional conditions, small amounts of substitute ions may be transported across the MCFC electrolyte. Such surrogate ion transport under conventional conditions may correspond to a transfer rate of 0.98 or higher, which corresponds to surrogate ion transport corresponding to a fuel cell current density of less than 2.0%. In contrast, in this discussion, operating an MCFC to cause surrogate ion transport is defined as operating the MCFC at a transfer rate of 0.95 or less, such that a current density of 5.0% or more (or a calculated CO of 5.0% or more)₂Utilization) corresponds to a current density based on transport of the surrogate ions, or 10% or more, or 20% or more, such as up to 35% or possibly more. Notably, in some aspects, operating with multiple fuel cell stages may reduce the CO that would otherwise be achieved in any single fuel cell stage₂The severity of the conditions required for capture rate. Thus, by operating in multiple stages, some have increased CO₂The operating conditions for the capture rate may correspond to 0.97 orLower transfer rates.

In this discussion, operating the MCFC to cause a large number of surrogate ion transmissions (i.e., operating at a transfer rate of 0.95 or less, or 0.97 or less, within one or more stages of a multi-stage configuration) is further defined to correspond to operating the MCFC with a voltage drop across the anode and cathode suitable for generating electrical power. The total electrochemical potential difference for the reaction in the molten carbonate fuel cell was-1.04V. For practical reasons, MCFCs are typically operated at voltages close to 0.7V or about 0.8V to generate a current. This corresponds to a combined voltage drop across the cathode, electrolyte and anode of about 0.34V. To maintain stable operation, the combined voltage drop across the cathode, electrolyte and anode can be less than 0.5V, such that the current produced by the fuel cell is at a voltage of 0.55V or higher, or 0.6V or higher.

With respect to the anode, one condition for operation with a large amount of surrogate ion transport may be having 8.0 vol% or more, or 10 vol% or more of H in the region where the large amount of surrogate ion transport occurs₂And (4) concentration. According to the aspect, this may correspond to a region near the anode inlet, a region near the cathode outlet, or a combination thereof. In general, if H in the anode region₂Too low a concentration will not have sufficient driving force to produce a large amount of surrogate ion transport.

Conditions suitable for the anode may also include providing H to the anode₂A reformable fuel, or a combination thereof; and operating at any convenient fuel utilization that produces the desired current density, including fuel utilizations in the range of 20% to 80%. In some aspects, this may correspond to traditional fuel utilization, such as 60% or more, or 70% or more, such as up to 85% or possibly even higher fuel utilization. In other aspects, this may correspond to H being selected to provide H with an increase₂Content and/or increased H₂And CO (i.e., syngas), such as 55% or less, 50% or less, or 40% or less, such as low as 20% or possibly even less. H in the anode output stream₂Content and/or H in the anode output stream₂And COThe combined amount may be sufficient to allow the desired current density to be produced. In some aspects, H in the anode output stream₂The amount may 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 even higher. Additionally or alternatively, H in the anode output stream₂And CO may be present in a combined amount of 4.0 vol% or more, or 6.0 vol% or more, or 10 vol% or more, such as up to 20 vol% or possibly even more. Optionally, H in the anode output stream when the fuel cell is operating at low fuel utilization₂The amount may be in a higher range, such as 10 vol% to 25 vol% H₂And (4) content. In such aspects, the syngas content of the anode output stream can be correspondingly higher, such as 15 vol% to 35 vol% H₂And the combined content of CO. According to the aspects, the anode can be operated to increase the amount of electrical energy generated, increase the chemical energy generated (i.e., H generated by reforming available in the anode output stream)₂) Or the anode may be operated using any other convenient strategy compatible with operating a fuel cell to cause surrogate ion transport.

Except that there is sufficient H in the anode₂Outside the concentration, one or more locations within the cathode need to have sufficiently low CO₂Concentration such that a more favorable carbonate ion transport pathway is not readily available. In some aspects, this may correspond to CO in the cathode outlet stream (i.e., cathode exhaust gas)₂The concentration is 2.0 vol% or less, or 1.0 vol% or less, or 0.8 vol% or less. Notably, 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 still lower CO in a localized area of the cathode₂And (4) concentration. For example, in a cross-flow configuration, CO is present at the corners of the fuel cell adjacent to the anode inlet and cathode outlet₂The concentration may be lower than at corners of the same fuel cell adjacent the anode outlet and the cathode outlet. Similar CO₂Local variations in concentration may also occur in fuel cells having co-current or counter-current configurations.

With low concentration of CO₂In addition, the local region of the cathode may also have 1.0 vol% or more, or 2.0 vol% or more of O₂. In the fuel cell, O₂For forming hydroxide ions that allow for the transport of substitute ions. If there is not enough O₂The fuel cell will not operate because the carbonate ion transport and the surrogate ion transport mechanisms both rely on O₂Availability of (c). With respect to O in the cathode input stream₂In some aspects, this may 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 (e.g., 1.0 vol% or more, or 2.0 vol% or more) should also be present for the surrogate ion transfer to take place. Without being bound by any particular theory, if no water is available in the cathode when attempting to operate with a large amount of surrogate ion transport, the rate of degradation of the fuel cell appears to be much faster than the rate of deactivation observed with sufficient available water due to surrogate ion transport. Notably, since air is generally used as O₂From a source of, and due to, H₂O is one of the products produced during combustion, so there is usually a sufficient amount of water available in the cathode.

Due to operation in molten carbonate fuel cells to increase CO₂During capture rates, uneven distribution of cathode gas and/or anode gas, it is believed that one or more of the corners and/or edges of a molten carbonate fuel cell will typically have a significantly higher density of surrogate ion transport. One or more corners may correspond to CO in the cathode₂At a concentration below the average, or H in the anode₂A location where the concentration is above the average, or a combination thereof.

In this discussion, a fuel cell may correspond to a single cell in which an anode and a cathode are separated by an electrolyte. The anode and cathode may receive an input gas stream to facilitate respective anode and cathode reactions to transfer charge across the electrolyte and generate electrical power. A fuel cell stack may represent a plurality of cells in an integrated unit. Although a fuel cell stack may contain a plurality of fuel cells, the fuel cells may typically be connected in parallel and may (substantially) function as if they collectively represent a single fuel cell of larger size. When an input stream is delivered to the anode or cathode of a fuel cell stack, the fuel cell stack may contain flow channels for dividing the input stream between each of the cells in the stack and flow channels for combining the output streams from the individual cells. In this discussion, a fuel cell array may be used to refer to a plurality of fuel cells (e.g., a plurality of fuel cell stacks) arranged in series, parallel, or in any other convenient manner (e.g., in a combination of series and parallel). The fuel cell array may comprise one or more stages of fuel cells and/or fuel cell stacks, wherein the anode/cathode output from a first stage may be used as the anode/cathode input to a second stage. It is noted that the anodes in a fuel cell array need not be connected in the same manner as the cathodes in the array. For convenience, the input to the first anode stage of the fuel cell array may be referred to as the anode input of the array, and the input to the first cathode stage of the fuel cell array may be referred to as the cathode input of the array. Similarly, the output from the last anode/cathode stage may be referred to as the anode/cathode output from the array. In aspects in which the fuel cell stack contains a separate reforming element, it is noted that the anode input stream may first pass through the reforming element before entering one or more anodes associated with the reforming element.

It should be understood that reference to the use of fuel cells herein generally refers to a "fuel cell stack" made up 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 often be "stacked" together in a rectangular array called a "fuel cell stack". Other types of elements, such as reforming elements, may also be included in the fuel cell stack. Such a fuel cell stack may generally employ a feed stream and distribute the reactants to all of the individual fuel cell elements, and the product may then be collected from each of these elements. When considered as a unit, the fuel cell stack in operation can be considered as a whole even if it is made up of many (typically tens or hundreds) of individual fuel cell elements. These individual fuel cell elements can typically have similar voltages (because reactant and product concentrations are similar), and when the elements are electrically connected in series, the total power output can be from the sum of all currents in all cell elements. The stacks may also be arranged in a series arrangement to generate high voltages. The parallel arrangement may increase the current. The systems and methods described herein may be used with a single molten carbonate fuel cell stack if a sufficiently large volume of fuel cell stack is available to treat a given exhaust stream. In other aspects of the invention, multiple fuel cell stacks may be desired or required for a variety of reasons.

For the purposes of the present invention, unless otherwise specified, the term "fuel cell" shall be understood to also refer to and/or be defined to include reference to a fuel cell stack consisting of a set of one or more individual fuel cell elements having a single input and output, which is also the way fuel cells are typically employed in practice. Similarly, unless otherwise specified, the term one or more fuel cells should be understood to also refer to and/or be defined to encompass a plurality of individual fuel cell stacks. In other words, all references in this document may interchangeably refer to "operation of a fuel cell stack" as a "fuel cell" unless specifically noted. For example, the volume of exhaust gas produced by a commercial scale combustion generator may be too large to be processed by a conventionally sized fuel cell (i.e., a single stack). To treat the entire exhaust, a plurality of fuel cells (i.e., two or more individual fuel cells or fuel cell stacks) may be arranged in parallel such that each fuel cell can treat (approximately) an equal portion of the combustion exhaust. Although a plurality of fuel cells may be used, each fuel cell may generally operate in a substantially similar manner, taking into account that the fraction of the combustion exhaust gas of each fuel cell is (substantially) equal.

Example of molten carbonate fuel cell operation: cross-current orientation of cathode and anode

Figure 3 shows a generalized 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. To isolate a fuel cell from adjacent fuel cells in the stack and/or other elements in the stack, the fuel cell includes separators 310 and 311. In fig. 3, a fuel cell 301 comprises an anode 330 and a cathode 350 separated by an electrolyte matrix 340 containing an electrolyte 342. In various aspects, the cathode 350 can correspond to a bi-layer (or multi-layer) cathode. The anode current collector 320 provides electrical contact between the anode 330 and the other anodes in the fuel cell stack, while the cathode current collector 360 provides similar electrical contact between the cathode 350 and the other cathodes in the fuel cell stack. In addition, the anode current collector 320 allows gas to be introduced and exhausted from the anode 330, while the cathode current collector 360 allows gas to be introduced and exhausted from the cathode 350.

During operation, CO₂And O₂Together into the cathode current collector 360. CO 2₂And O₂Diffuses into the porous cathode 350 and travels to the cathode interface region near the boundary of the cathode 350 and the electrolyte matrix 340. In the cathode interface region, a portion of the electrolyte 342 may be present in the pores of the cathode 350. CO 2₂And O₂Can be converted into carbonate ions (CO) near/in the cathode interface region₃ ^2-) The carbonate ions may then be transported across the electrolyte 342 (and thus across the electrolyte matrix 340) to facilitate the generation of electrical current. In the aspect where the transport of the substitute ion takes place, a part of O₂May be converted to a surrogate ion (e.g., a hydroxide ion or a peroxide ion) for transport in the electrolyte 342. After transport across the electrolyte 342, carbonate ions (or surrogate ions) may reach the anode interface region near the boundary of the electrolyte matrix 340 and the anode 330. In the presence of H₂In the case of (2), the carbonate ion can be converted back to CO₂And H₂O, thereby releasing electrons for forming the current generated by the fuel cell. H₂And/or suitable for forming H₂Is introduced into the anode 330 through the anode current collector 320.

The direction of flow within the anode of the molten carbonate fuel cell may have any convenient orientation relative to the direction of flow within the cathode. One option is to use a cross-flow configuration such that the flow direction within the anode is at an angle of approximately 90 ° relative to the flow direction within the cathode. This type of flow configuration may have practical benefits because the use of a cross-flow configuration may allow the manifolds and/or conduits of the anode inlet/outlet to be located on a different side than the manifolds and/or conduits of the cathode inlet/outlet of the fuel cell stack.

Fig. 4 schematically shows an example of a top view of a fuel cell cathode, while arrows indicate the flow direction 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).

Since the anode and cathode flows are oriented at about 90 ° with respect to each other, the anode and cathode flow patterns can contribute to having different reaction conditions at various portions of the cathode. The different conditions can be illustrated by considering the reaction conditions in the four corners of the cathode. In the illustration of FIG. 4, the reaction conditions described herein are qualitatively similar to CO at 70% or higher (or 80% or higher)₂Reaction conditions for fuel cells operating at utility.

The corner 482 corresponds to a portion of the fuel cell near the entry point of the cathode input stream and the anode input stream. Thus, CO in corner 482₂(in the cathode) and H₂The concentration (in the anode) is relatively high. Based on the high concentration, it is expected that the portion of the fuel cell near the corner 482 will operate under expected conditions, wherein substantially no ions other than carbonate ions are transported across the electrolyte.

The corner 484 corresponds to a portion of the fuel cell near the entry point of the cathode input stream and near the exit point of the anode output stream. In locations near the corner 484, the amount of current density may be due to H in the anode₂The decrease in concentration is limited depending on the fuel utilization. However, sufficient CO should be present₂Such that any ions transported across the electrolyte correspond substantially to carbonate ions.

Corner 486 corresponds toAt a portion of the fuel cell near the exit point of the anode output stream and near the exit point of the cathode output stream. In a position near corner 486, due to H₂(in the anode) and CO₂The concentration (in the cathode) is low and the current is expected to be small or non-existent due to the low driving force of the fuel cell reaction.

Corner 488 corresponds to a portion of the fuel cell near the entry point of the anode input stream and near the exit point of the cathode output stream. It is expected that the relatively high availability of hydrogen gas at locations near corner 488 will produce a substantial current density. However, due to CO₂At relatively low concentrations, the transport of significant amounts of hydroxide ions and/or other surrogate ions may occur. According to the aspect, a large number of alternative ion transports may be used to calculate the CO₂The utilization rate is increased by 5% or more, or 10% or more, or 15% or more, or 20% or more. Additionally or alternatively, the transfer rate may be 0.95 or less, or 0.90 or less, or 0.85 or less, or 0.80 or less. The transport of a large number of surrogate ions across the electrolyte may temporarily allow a higher current density to be maintained at a location near corner 488. However, transport of the surrogate ions may also degrade the cathode and/or anode structure, resulting in a decrease (and possibly no) in current density over time at a location near corner 488. Notably, the amount of lifetime degradation is not as severe at lower amounts of surrogate ion transport (e.g., 0.96 or higher, or 0.98 or higher transfer rates).

It has been found that when surrogate ion transport becomes significant at one or more locations within the fuel cell, the fuel cell will begin to degrade rapidly. This is believed to be due to degradation of one or more sites and not providing any further current density. Since one or more regions cease to contribute to the required current density, the rest of the locations in the fuel cell must be operated at a higher current density in order to maintain a constant total (average) current density for the fuel cell. This may cause the region for the transport of the substitute ions to grow, resulting in deterioration of the swelling portion of the fuel cell and eventually stopping the operation. Alternatively, the fuel cellMay result in a reduction in the overall current density from the cell, which is also undesirable. Operating multiple fuel stages may reduce at CO₂The capture rate increases the amount of surrogate ion transport that occurs within the fuel cell stage during the period, thereby extending the useful life of the fuel cell.

Anode input and output

In various aspects, the anode input stream of the MCFC can comprise hydrogen, a hydrocarbon (e.g., methane), a hydrocarbon-containing or hydrocarbon-like compound that can contain heteroatoms other than C and H, or a combination thereof. The source of hydrogen/hydrocarbon-like compounds may be referred to as a fuel source. In some aspects, a majority of the methane (or other hydrocarbon, hydrocarbon-containing, or hydrocarbon-like compound) fed to the anode can typically be fresh methane. In this description, fresh fuel (e.g., fresh methane) refers to fuel that is not recycled from another fuel cell process. For example, methane that is recycled from the anode outlet stream back to the anode inlet may not be considered "fresh" methane, but may be described as recovered methane.

The fuel source used may be shared with other components, such as using a portion of the fuel source to provide CO-containing for cathode input₂The stream of (a). The fuel source input may comprise water in proportion to the fuel, the water being suitable for reforming hydrocarbon (or hydrocarbon-like) compounds in a reforming section that produces hydrogen. For example, if methane is used for reforming to produce H₂The molar ratio of water to fuel may be from about one-to-one to about ten-to-one, such as at least about two-to-one. Typical ratios for external reforming are four to one or more, but typical values for internal reforming may be lower. Is about H₂Being part of the fuel source, in certain optional aspects, additional water may not be required in the fuel, as H₂Oxidation at the anode tends to produce H that can be used to reform the fuel₂And O. The fuel source may also optionally contain components incidental to the fuel source (e.g., the natural gas feed may contain some amount of CO as an additional component₂). For example, the natural gas feed may contain CO₂、N₂And/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 alternative potential source of CO in the fuel entering the fuel cell assembly may be CO generated by steam reforming of a hydrocarbon fuel to the fuel prior to entering the fuel cell assembly.

More generally, various types of fuel streams may be suitable for use as an anode input stream for an anode of a molten carbonate fuel cell. Some fuel streams may correspond to streams containing hydrocarbons and/or hydrocarbon-like compounds that may also include heteroatoms other than C and H. In this discussion, unless otherwise specified, reference to a fuel stream containing hydrocarbons for an MCFC anode is defined to include fuel streams containing such hydrocarbon 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), and combinations thereof. Still other additional or alternative examples of potential fuel streams for anode input may include biogas-type streams, such as methane produced by natural (bio) decomposition of organic materials.

In some aspects, molten carbonate fuel cells may be used to treat an input fuel stream, such as a natural gas and/or hydrocarbon stream, that has a low energy content due to the presence of diluent compounds. For example, some sources of methane and/or natural gas may contain significant amounts of CO₂Or other sources of inert molecules such as nitrogen, argon or helium. Due to the presence of increased amounts of CO₂And/or inerts, the energy content of the fuel stream based on the source can be reduced. Difficulties may arise in conducting combustion reactions using low energy content fuels, such as to power combustion-powered turbines. However, molten carbonate fuel cells can generate electricity based on low energy content fuel sources, with reduced or minimal impact on fuel cell efficiency. The presence of the additional gas volume may require additional heat to raise the temperature of the fuel to the temperature required for reforming and/or anode reactions. In addition, due toEquilibrium nature of water gas shift reaction, additional CO, in the anode of a fuel cell₂Will be on the presence of H in the anode output₂And the relative amount of CO. However, inert compounds may otherwise have only minimal direct impact on the reforming and anode reactions. CO in fuel streams for molten carbonate fuel cells₂And/or the amount of inert compound (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 alternatively, CO in fuel streams for molten carbonate fuel cells₂And/or the amount of inert compound 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.

Still other examples of potential sources of the anode input stream may correspond to refinery and/or other industrial process output streams. For example, coking is a common process used in many refineries to convert heavier compounds to a lower boiling range. Coking typically produces an exhaust gas containing a variety of compounds that are gases at room temperature, including CO and various Cs₁–C₄A hydrocarbon. Such an exhaust gas may be used as at least a portion of the anode input stream. Other refinery off gas streams may additionally or alternatively be suitable for inclusion in anode input streams, such as light ends (C) produced during cracking or other refinery processes₁–C₄). Still other suitable refinery streams may additionally or alternatively comprise CO-containing or CO₂The refinery stream of (1), which also contains H₂And/or reformable fuel compounds.

Still other potential sources for anode input may additionally or alternatively comprise streams having increased water content. For example, an ethanol output stream from an ethanol plant (or another type of fermentation process) may comprise ethanol prior to final distillationA substantial portion of H₂And O. This H₂O generally has only minimal impact on the operation of the fuel cell. Thus, a fermentation mixture of alcohol (or other fermentation product) and water may be used as at least a portion of the anode input stream.

Biogas or biogas is another additional or alternative potential source of anode input. The biogas may comprise primarily methane and CO₂And is typically produced by decomposing or digesting organic matter. Anaerobic bacteria can be used to digest organic matter and produce biogas. Impurities, such as sulfur-containing compounds, may be removed from the biogas prior to use as an anode input.

The output stream from the MCFC anode can comprise H₂O、CO₂CO and H₂. Optionally, the anode output stream may also contain unreacted fuel (e.g., H) in the feed₂Or CH₄) Or inert compounds as further export components. The anode output stream may be subjected to one or more separations to separate CO₂With potentially valuable components (e.g. H) as input to another process₂Or CO) rather than using such output streams as a fuel source for providing heat for the reforming reaction or as a combustion fuel for heating the cell. H₂And/or the CO may be used as a synthesis gas for chemical synthesis, a hydrogen source for chemical reactions, and/or a fuel to reduce greenhouse gas emissions.

Cathode input and output

Conventionally, molten carbonate fuel cells can operate based on drawing a desired load while consuming a portion of the fuel in the fuel stream delivered to the anode. The voltage of the fuel cell can then be controlled by the load, the fuel input to the anode, the air and CO supplied to the cathode₂And the internal resistance of the fuel cell. CO may be conventionally provided to the cathode in part by using the anode exhaust as at least a portion of the cathode input stream₂. In contrast, the present invention may use separate/different sources for the anode input and the cathode input. By removing any direct connection between the composition of the anode input stream and the cathode input stream, andthe other options become available for operating the fuel cell, such as generating excess syngas, increasing the carbon dioxide capture rate, and/or increasing the overall efficiency (electrical plus chemical) of the fuel cell, etc.

In various aspects, the MCFC can be operated to cause surrogate ion transport across the electrolyte of the fuel cell. CO of cathode input stream to cause transport of substitute ions₂The amount may 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%.

CO-containing suitable for use as cathode input stream₂An example of a stream of (b) may be an output stream or exhaust stream from a combustion source. Examples of combustion sources include, but are not limited to, sources based on natural gas combustion, coal combustion, and/or combustion of other hydrocarbon-based fuels, including biologically-derived fuels. Additional or alternative sources may include other types of boilers, fired heaters, furnaces, and/or other types of devices that burn carbonaceous fuel to heat another substance (e.g., water or air).

Other potential sources of the cathode input stream may additionally or alternatively comprise biologically produced CO₂The source of (a). This may comprise, for example, CO generated during processing of the biologically-derived compound₂E.g. CO produced during ethanol production₂. Additional or alternative examples may include CO produced by combustion of biologically produced fuels (e.g., combustion of lignocellulose)₂. Still other additional or alternative potential CO₂The source may correspond to an output stream or exhaust stream from various industrial processes, such as a CO-containing stream produced by a plant for manufacturing steel, cement, and/or paper₂The stream of (a).

Yet another additional or alternative potential CO₂The source may be CO-containing from a fuel cell₂The stream of (a). CO-containing from fuel cells₂Can correspond to a cathode output stream from a different fuel cell, an anode output stream from a different fuel cell, a recycle stream from a cathode output to a cathode input of a fuel cell, and/or from fuel electricityThe anode of the cell outputs a recycle stream to the cathode input. For example, an MCFC operating in a standalone mode under normal conditions may produce CO₂Cathode exhaust gas at a concentration of at least about 5 vol%. This CO-containing₂May be used as the cathode input to an MCFC operating in accordance with an aspect of the present invention. More generally, the generation of CO from cathode exhaust may additionally or alternatively be used₂Other types of fuel cells outputting, and other types of CO-containing power generators not produced by "combustion" reactions and/or combustion power generators₂The stream of (a). Optionally but preferably, CO from another fuel cell₂May be from another molten carbonate fuel cell. For example, for the cathodes of the molten carbonate fuel cells connected in series, the output from the cathode of a first molten carbonate fuel cell may be used as the input to the cathode of a second molten carbonate fuel cell.

CO removal₂In addition, the cathode input stream may also comprise O₂To provide the components required for the cathode reaction. Some cathode input streams may be based on air as a component. For example, the combustion exhaust stream may be formed by combusting a hydrocarbon fuel in the presence of air. This combustion exhaust stream or another type of cathode input stream having an oxygen content based on the contained 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 alternatively, 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, the cathode input stream may have a suitable oxygen content for conducting the cathode reaction. In some aspects, this may correspond to an oxygen content of about 5 vol% to about 15 vol%, such as about 7 vol% to about 9 vol%. For many types of cathode input streams, CO₂And O₂Can 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. The oxygen-containing air stream may be mixed with CO having a low oxygen content₂And (4) combining sources. For example, the exhaust stream produced by burning coal may contain low levels of oxygen, which mayTo mix with air to form a cathode inlet stream.

In addition to CO₂And O₂In addition, the cathode input stream may be composed of inert/non-reactive species (e.g., N)₂、H₂O and other typical oxidant (air) components). For example, for cathode input derived from combustion reaction exhaust, if air is used as part of the oxidant source for the combustion reaction, the exhaust may contain typical components of air, such as N₂、H₂O and other small amounts of compounds present in air. Depending on the nature of the fuel source used for the combustion reaction, additional species present after combustion based on the fuel source may comprise H₂O, oxides of nitrogen (NOx) and/or oxides of sulfur (SOx), and other compounds present in the fuel and/or which are partial or complete combustion products of compounds present in the fuel, such as CO. These materials may be present in amounts that do not poison the cathode catalyst surface but they may reduce the overall cathode activity. This reduction in performance is acceptable or the material interacting with the cathode catalyst can be reduced to acceptable levels by known contaminant removal techniques.

O present in a cathode input stream (e.g., an input cathode stream based on combustion exhaust gas)₂The amount may be sufficient to advantageously provide the oxygen required for the cathode reaction in the fuel cell. Thus, O₂May advantageously be CO in the exhaust gas₂At least 0.5 times the amount of (c). Optionally, additional air may be added to the cathode input as needed to provide sufficient oxidant for the cathode reaction. N in cathode exhaust when using some form of air as oxidant₂The amount of (c) may be at least about 78 vol%, for example at least about 88 vol% and/or about 95 vol% or less. In some aspects, the cathode input stream may additionally or alternatively contain compounds commonly regarded as contaminants, such as H₂S or NH₃. In other aspects, the cathode input stream can be purified to reduce or minimize the level of such contaminants.

Suitable temperatures for MCFC operation may be between about 450 ℃ and about 750 ℃, such as at least about 500 ℃, e.g., an inlet temperature of about 550 ℃ and an outlet temperature of about 625 ℃. Heat can be added to or removed from the cathode input stream, if desired, prior to entering the cathode, for example to provide heat for other processes, such as reforming the fuel input to the anode. For example, if the source of the cathode input stream is a combustion exhaust stream, the temperature of the combustion exhaust stream may be higher than the desired temperature of the cathode inlet. In such aspects, heat may be removed from the combustion exhaust before the combustion exhaust is used as a cathode input stream. Alternatively, the combustion exhaust gas may be at a very low temperature, for example, after a wet gas scrubber on a coal-fired boiler, in which case the combustion exhaust gas may be below about 100 ℃. Alternatively, the combustion exhaust may be from the exhaust of a gas turbine operating in a combined cycle mode, wherein the gas may be cooled by generating steam to run a steam turbine for additional power generation. In this case, the gas may be below about 50 ℃. Heat may be added to the combustion exhaust gas at a temperature lower than the desired temperature.

Alternative molten carbonate fuel cell operating strategies

In some aspects, when operating an MCFC to cause surrogate ion transport, the anode of the fuel cell may be operated at conventional fuel utilization values of about 60% to 80%. Operating the anode of a fuel cell at a relatively high fuel utilization rate may be beneficial in increasing the electrical efficiency (i.e., the electrical energy produced per unit of chemical energy consumed by the fuel cell) when attempting to produce electricity.

In some aspects, the electrical efficiency of the fuel cell is reduced to provide other benefits (such as increasing the H provided in the anode output stream)₂Amount) may be beneficial. This may be beneficial, for example, if it is desired 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, molten carbonate fuel cells may be operated to increase syngas and/or hydrogen production. Can be generated by an exothermic electrochemical reaction in the anode for the generation of electricityThe heat required to perform the endothermic reforming reaction should be provided. This excess heat may be used in situ as a heat source for reforming and/or another endothermic reaction, rather than attempting to transfer heat generated by one or more exothermic fuel cell reactions away from the fuel cell. As a result, more efficient use of thermal energy and/or a reduced need for additional external or internal heat exchange may be achieved. Efficient generation and use of such thermal energy (substantially in situ) can reduce system complexity and reduce components while maintaining favorable operating conditions. In some aspects, the amount of reforming or other endothermic reaction may be selected to have an endothermic heat demand comparable to or even greater than the excess heat generated by the exothermic reaction or reactions, rather than significantly lower than that typically described in the prior art.

Additionally or alternatively, the fuel cell may be operated such that the temperature difference between the anode inlet and the anode outlet may be negative rather than positive. Thus, rather than increasing the temperature between the anode inlet and the anode outlet, a sufficient amount of reforming and/or other endothermic reactions can be performed to lower the output stream temperature from the anode outlet than the anode inlet temperature. Further additionally or alternatively, additional fuel may be supplied to the heater and/or internal reforming stage (or other internal endothermic reaction stage) of the fuel cell such that the temperature difference between the anode input and the anode output may be less than a desired temperature difference based on the relative demand for one or more endothermic reactions and the combined exothermic heat generation of the cathode combustion reaction and the anode reaction for generating electricity. In aspects where reforming is used as an endothermic reaction, operating the fuel cell to reform excess fuel may allow for increased syngas and/or increased hydrogen production relative to conventional fuel cell operation while minimizing system complexity for heat exchange and reforming. The additional syngas and/or additional hydrogen can then be used in various applications, including chemical synthesis processes and/or collection/reuse 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 may be significantly greater than the amount of heat generated per mole of hydrogen generated by the reforming reactionThe heat consumed. Net reaction of hydrogen (H) in molten carbonate fuel cells₂+1/2O₂＝>H₂O) can have a reaction enthalpy of about-285 kJ/mol hydrogen molecule. At least a portion of this energy may be converted to electrical energy within the fuel cell. However, the difference between the reaction enthalpy and the electrical energy produced by the fuel cell (approximately) can become heat within the fuel cell. This energy may alternatively be expressed as the current density of the cell (current per unit area) multiplied by the difference between the theoretical maximum voltage and the actual voltage of the fuel cell, or<Current density>(Vmax-Vact). This energy is defined as the "waste heat" of the fuel cell. As an example of reforming, the enthalpy of reforming (CH) of methane₄+2H₂O＝>4H₂+CO₂) It may be about 250kJ/mol methane or about 62kJ/mol hydrogen molecules. From a thermal balance perspective, each electrochemically oxidized hydrogen molecule can generate enough heat to produce more than one hydrogen molecule by reforming. In conventional configurations, this excess heat may result in a significant temperature difference from the anode inlet to the anode outlet. The excess heat can be consumed by performing a matching amount of reforming reaction rather than using it to increase the temperature in the fuel cell. The excess heat generated in the anode may be supplemented with the excess heat generated by the combustion reaction in the fuel cell. More generally, excess heat may be consumed by performing an endothermic reaction at the fuel cell anode and/or in an endothermic reaction stage that is thermally integrated with the fuel cell.

According to the aspects, the amount of reforming and/or other endothermic reactions can be selected relative to the amount of hydrogen reacted in the anode to achieve a desired thermal ratio for the fuel cell. As used herein, "thermal ratio" is defined as the amount of heat generated by the exothermic reaction in the fuel cell assembly (including the exothermic reaction in both the anode and cathode) divided by the endothermic demand of the reforming reaction occurring within the fuel cell assembly. Expressed mathematically, the heat ratio (TH) is Q_EX/Q_ENWherein Q is_EXIs the sum of the heat generated by the exothermic reaction, and Q_ENIs the sum of the heat consumed by the endothermic reaction occurring within the fuel cell. It should be noted thatThe heat generated by the thermal reaction may correspond to any heat generated as a result of a reforming reaction in the cathode, a water gas shift reaction, a combustion reaction (i.e., oxidation of fuel compounds), and/or an electrochemical reaction in the cell. The amount of heat generated by the electrochemical reaction can be calculated based on the ideal electrochemical potential of the fuel cell reaction on the electrolyte minus the actual output voltage of the fuel cell. For example, the ideal electrochemical potential for the reaction in an MCFC is considered to be about 1.04V based on the net reaction occurring in the cell. During MCFC operation, the output voltage of the cell is typically below 1.04V due to various losses. For example, a common output/operating voltage may be about 0.7V. The heat generated may be equal to the electrochemical potential of the cell (i.e., -1.04V) minus the operating voltage. For example, when the output voltage in a fuel cell reaches-0.7V, the heat generated by the electrochemical reaction in the cell may be-0.34V. Thus, in this case, the electrochemical reaction will generate-0.7V of electrical energy and-0.34V of thermal energy. In such an example, a power of 0.7V is not considered Q_EXIs included. In other words, thermal energy is not electrical energy.

In various aspects, the thermal ratio may 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 ratios may also be calculated for different units within the fuel cell stack, such as fuel cells or assemblies of fuel cell stacks. For example, the thermal ratio may be calculated for a fuel cell (or fuel cells) within a fuel cell stack along with an integrated reforming stage and/or integrated endothermic reaction stage element that is close enough to the fuel cell(s) to be integrated from a thermal integration perspective.

From a thermal integration perspective, the width of a feature in a fuel cell stack may be the height of a single fuel cell stack element. It is noted that the individual reforming stages and/or the individual endothermic reaction stages may have different heights in the stack compared to the fuel cells. In such a case, the height of the fuel cell element may be used as the characteristic height. In this discussion, an integrated endothermic reaction stage may be defined as a stage that is thermally integrated with one or more fuel cells such that the integrated endothermic reaction stage may use heat from the fuel cells as a source of heat for reforming. Such an integrated endothermic reaction stage can be defined as being positioned at a height 10 times less than the height of the stack elements of the fuel cell that provide heat to the integrated stage. For example, the integrated endothermic reaction stage (e.g., reforming stage) may be positioned at a height that is 10 times less than the height of the stack elements from any thermally integrated fuel cell, or may be 8 times less than the height of the stack elements, or may be 5 times less than the height of the stack elements, or may be 3 times less than the height of the stack elements. In this discussion, an integrated reforming stage and/or an integrated endothermic reaction stage that represents a stack element adjacent to a fuel cell element may be defined as a height of about one stack element height or less from an 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 or less may be lower than that typically sought when using MCFC fuel cells. In aspects of the invention, the heat ratio may be reduced to increase and/or optimize syngas production, hydrogen production, production of another product by an endothermic reaction, or a combination thereof.

In various aspects of the present invention, the operation of the fuel cell may be characterized based on the thermal ratio. When the fuel cell is operated to have a desired thermal ratio, the molten carbonate fuel cell may 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 alternatively, the thermal ratio may 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 alternatively, in some aspects, the fuel cell can be operated to have an exotherm between anode input and anode output of about 40 ℃ or less, such as about 20 ℃ or less, or about 10 ℃ or less. Still further additionally or alternatively, the fuel cell may be operated to have an anode outlet temperature that is about 10 ℃ lower to about 10 ℃ higher than the temperature of the anode inlet. Still further additionally or alternatively, the fuel cell can be operated such that its anode inlet temperature is higher than the anode outlet temperature, such as at least about 5 ℃ higher, or at least about 10 ℃ higher, or at least about 20 ℃ higher, or at least about 25 ℃ higher. Still further additionally or alternatively, the fuel cell can be operated such that its anode inlet temperature is about 100 ℃ or less, or about 80 ℃ or less, or about 60 ℃ or less, or about 50 ℃ or less, or about 40 ℃ or less, or about 30 ℃ or less, or about 20 ℃ or less higher than the anode outlet temperature.

Operating the fuel cell at a thermal ratio less than 1 may cause the temperature of the entire fuel cell to drop. In some aspects, the amount of reforming and/or other endothermic reactions can be limited such that the temperature drop from the anode inlet to the anode outlet can be about 100 ℃ or less, such as about 80 ℃ or less, or about 60 ℃ or less, or about 50 ℃ or less, or about 40 ℃ or less, or about 30 ℃ or less, or about 20 ℃ or less. A limitation on the temperature drop from the anode inlet to the anode outlet may be beneficial, for example, to maintain a sufficient temperature to allow complete or substantially complete conversion of the fuel in the anode (by reforming). In other aspects, additional heat may be supplied to the fuel cell (e.g., by heat exchange or combustion of additional fuel) such that the anode inlet temperature is about 100 ℃ or less higher than the anode outlet temperature, such as about 80 ℃ or less higher, or about 60 ℃ or less higher, or about 50 ℃ or less higher, or about 40 ℃ or less higher, or about 30 ℃ or less higher, or about 20 ℃ or less higher, due to a balance between the heat consumed by the endothermic reaction and the additional external heat supplied to the fuel cell.

The amount of reforming may additionally or alternatively depend on the availability of reformable fuel. For example, if the fuel comprises only H₂Then reforming does not occur because of H₂Has been reformed and cannot be further reformed. The amount of syngas "produced by the fuel cell may be defined as the difference between the Lower Heating Value (LHV) value of the syngas in the anode input and the LHV value of the syngas in the anode output. Produced synthesis gas LHV (sg n)et) — (LHV (sg out) -LHV (sg in)), where LHV (sg in) and LHV (sg out) refer to the LHV of syngas in the anode inlet stream or stream and the LHV of syngas in the anode outlet stream or stream, respectively. Is provided with a large amount of H₂May be limited in potential syngas production because the fuel contains a significant amount of H that has been reformed₂Rather than containing additional reformable fuels. Low heating value is defined as the conversion of fuel components to gas phase, complete oxidation products (i.e., gas phase CO)₂And H₂O products) enthalpy of combustion. For example, any CO present in the anode input stream₂Do not contribute to the fuel content of the anode input because of CO₂Has been completely oxidized. For this definition, the amount of oxidation that occurs in the anode due to the anode fuel cell reaction is defined as the H in the anode as part of the electrochemical reaction in the anode₂Oxidation of (2).

An example of a method for operating a fuel cell at a reduced thermal ratio may be a method of performing excess reforming of fuel in order to balance the generation and consumption of heat in the fuel cell and/or consume more heat than is generated. Reforming reformable fuel to form H₂And/or the CO may be an endothermic process, while the anodic electrochemical oxidation reaction and one or more cathodic combustion reactions may be exothermic. During conventional fuel cell operation, the amount of heat consumed to supply the feed components required for fuel cell operation may generally be less than the amount of heat generated by the anodic oxidation reaction. For example, conventional operation at a fuel utilization of about 70% or about 75% will result in a thermal ratio substantially greater than 1, such as at least about 1.4 or greater, or 1.5 or greater. Thus, the output stream of the fuel cell may be hotter than the input stream. Unlike conventional operation of this type, the amount of fuel reformed in the reforming stage associated with the anode may be increased. For example, the additional fuel may be reformed such that the heat generated by the exothermic fuel cell reaction may be (approximately) balanced by and/or consume more heat than is generated by the heat consumed in reforming. This can result in a significant excess of hydrogen relative to the amount of hydrogen oxidized in the anode for power generationAnd may 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.

Hydrogen or syngas may be extracted from the anode exhaust as a chemical energy output. Hydrogen can be used as a clean fuel that does not produce greenhouse gases when burned or burned. In contrast, for hydrogen, CO, produced by reforming of hydrocarbons (or hydrocarbon-containing compounds)₂Will have been "trapped" in the anode loop. Additionally, hydrogen can be a valuable input to various refinery processes and/or other synthesis processes. Syngas can also be a valuable input to various processes. In addition to having fuel value, syngas can also be used as a feedstock for the production of other higher value products, such as by using syngas as an input to a Fischer-Tropsch (Fischer-Tropsch) synthesis and/or methanol synthesis process.

In some aspects, the reformable hydrogen content of the reformable fuel delivered to the anode and/or the input stream delivered to the reforming stage associated with the anode can be at least about 50% greater than the net content of hydrogen reacted at the anode, such as at least about 75% greater or at least about 100% greater. Additionally or alternatively, the reformable hydrogen content of the fuel delivered to the anode and/or delivered to the input stream of the reforming stage associated with the anode can be at least about 50% greater than the net content of hydrogen reacted at the anode, such as at least about 75% greater or at least about 100% greater. In various aspects, the ratio of the reformable hydrogen content of the reformable fuel in the fuel stream relative to the 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 alternatively, the ratio of the 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 the anode inlet stream can be converted to hydrogen in the anode and/or in one or more associated reforming stages, such as at least about 85% or at least about 90%. Additionally or alternatively, the amount of reformable fuel delivered to the anode can be characterized based on a Lower Heating Value (LHV) of the reformable fuel relative to the LHV of hydrogen oxidized in the anode. This may 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 alternatively, 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.

Example 1

A steady state fuel cell model created using a commercially available process modeling platform was used to simulate the cathode flow patterns for both fuel cell configurations. In a first configuration, the pair size is 250cm²Is modeled. In a second configuration, the combined size of the pairs connected in series in a co-current cross-flow fashion was 166cm²Two fuel cells (about 2/3 of a single fuel cell area) were modeled. For each configuration, the anode input gas into the first stage was modeled as 72% H₂18% of CO₂And 10% of H₂O (by moles). The cathode input gas was modeled as 4% CO₂10% of O₂10% of H₂O and 76% of N₂(on a molar basis). In each configuration, the first (or unique) level is modeled as 120mA/cm²Current density of 85%, fuel utilization of 85% and CO of 72%₂And (5) utilization rate operation. For the second configuration, the first stage has 120mA/cm²Current density and fuel utilization of 85%, while the second stage has 120mA/cm²Current density of 70% and fuel utilization. Net CO on both stages of the second configuration₂The capture or utilization is 72%, so that the same net content of CO is used in both configurations₂. However, due to the CO in the first configuration₂Consumption, a significant amount of additional surrogate ion transport also occurs. This results in a decrease in the transfer rate. This explains that the larger fuel cell area in the first configuration has the same area as the smaller fuel cell area in the second configurationThe origin of the current density.

Based on the flow patterns modeled according to the first configuration, fig. 5 shows CO in the cathode at steady state₂And (4) concentration. The anode and cathode flow directions are also indicated in the figure. As shown in FIG. 5, although the concentration in the cathode output stream was about 1.0 mol% (based on 72% CO)₂Utilization rate) but CO in the cathode₂Distributed in a highly non-uniform manner. Specifically, at the corners corresponding to the anode inlet and cathode outlet, CO₂The concentration drops almost to zero, while at the corners corresponding to the cathode inlet and the anode outlet, CO₂The concentration was almost 4.0 mol%. Due to CO on the whole cathode₂Significant change in concentration, CO in the cathode₂The amount of surrogate ion transport may increase in localized areas of low concentration.

FIG. 6 shows the cathode CO in steady state for the two stages of the second configuration₂And (4) concentration. As shown in FIG. 6, CO is present in both cathode stages₂The variation in concentration is reduced or minimized. Further, in FIG. 5, the lowest concentration corresponds to CO of 0.08 mol% or less₂And (4) concentration. In contrast, the lowest concentration presented in fig. 6 is 0.3 mol%.

Example 2

In this example, two configurations using two molten carbonate fuel cells are used. In a first configuration, two fuel cells are arranged in parallel such that the two fuel cells receive half of the anode input stream and half of the cathode input stream. In the parallel configuration, both fuel cells operate under the same operating conditions. The operating conditions were selected so that the current density of each of the parallel fuel cells reached 90mA/cm². In a second configuration, two fuel cells are arranged in series with respect to the cathode flow. Thus, in a second configuration, each cell receives half of the anode input stream, while the first fuel cell receives all of the cathode input stream. The cathode exhaust from the first fuel cell is then used as the cathode input stream to the second fuel cell. For the second configuration, the first fuel cell was operated at 120mA/cm²Operating and the second fuel cellAt 60mA/cm²And (5) operating. The fuel utilization is the same for both configurations.

As explained above, the net current densities for operating the parallel and series configurations are the same, since the average of the two cells in both configurations is 90mA/cm². However, by using the series configuration, the transfer rate increases from 0.683 for the parallel configuration to 0.693 for the series configuration. This increase in transfer rate indicates a decrease in the amount of surrogate ion transport in the tandem configuration. Thus, even for the same current density generation, when operating with a large amount of surrogate ion transport, a staged fuel cell can provide unexpected benefits by reducing or minimizing the amount of surrogate ion transport. It is worth noting, however, that even with a reduced amount of net surrogate ion transport, the operating voltage of the first cell in the series configuration is lower than the operating voltage of the parallel configuration. This shows that even though the net surrogate ion transport is reduced, this reduction is based on an increase in the first stage of the series configuration followed by a larger decrease in the second stage.

Further embodiments

Embodiment 1. a method for generating electrical power, the method comprising: will include H₂An anode input stream of reformable fuel, or a combination thereof, is introduced into a first anode stage of a plurality of molten carbonate fuel cell stages; will comprise O₂And CO₂Is introduced into a first cathode stage of the plurality of molten carbonate fuel cell stages; passing a second anode input stream into a second anode stage of the plurality of molten carbonate fuel cell stages; passing an intermediate cathode output into a second cathode stage of the plurality of molten carbonate fuel cell stages; and operating the plurality of molten carbonate fuel cell stages to produce: i) the average current density was 60mA/cm²Or more, ii) anode exhaust from the plurality of molten carbonate fuel cell stages, the anode exhaust comprising H₂CO and CO₂And iii) cathode exhaust from the plurality of molten carbonate fuel cell stages, the cathode exhaust comprising CO in an amount of 2.0 vol% or less₂In an amount of 1.0 vol% or moreH₂O and O in an amount of 1.0 vol% or more₂At least one of the first cathode stage and the second cathode stage operates at a transfer rate of 0.97 or less.

Embodiment 2. the method of embodiment 1, wherein the first cathode stage operates at a transfer rate of 0.97 or less and the second cathode stage operates at a transfer rate of 0.97 or less.

Embodiment 3. the method of any of the preceding embodiments, wherein the at least one of the first cathode stage and the second cathode stage operates at a transfer rate of 0.95 or less, or 0.90 or less.

Embodiment 4. the method of any of the preceding embodiments, wherein the second anode input stream comprises at least a portion of an intermediate anode output stream, optionally comprising an output from the first anode stage.

Embodiment 5. the method of embodiment 4, further comprising: passing a second anode intermediate output into a third anode stage, the anode exhaust comprising an output from the third anode stage; and passing a second cathode intermediate output into a third cathode stage, the cathode exhaust comprising an output from the third cathode stage, wherein the first, second, and third anode stages are arranged in a co-current manner or in a counter-current manner with respect to the first, second, and third cathode stages.

Embodiment 6. the method of any of the preceding embodiments, wherein the transfer rate of the first cathode stage is different from the transfer rate of the second cathode stage.

Embodiment 7. the method of any of the preceding embodiments, wherein at least the first anode stage, the second anode stage, the first cathode stage, and the second cathode stage are operated in a co-current cross-flow manner; or wherein at least the first anode stage, the second anode stage, the first cathode stage and the second cathode stage are operated in a counter-current cross-flow manner.

Embodiment 8 the method of any preceding embodiment, wherein the anode exhaust comprises an anode output from the second anode stage, or wherein the cathode exhaust comprises a cathode output from the second cathode stage, or a combination thereof.

Embodiment 9. the method of any of the preceding embodiments, wherein the intermediate cathode output comprises an output from the first cathode stage.

Embodiment 10. the method of any of the preceding embodiments, wherein the combined measured CO in the first cathode stage and the second cathode stage during operation of the plurality of molten carbonate fuel cells₂The utilization rate is 75% or higher, or 80% or higher.

Embodiment 11 the method of any of the preceding embodiments, wherein the electricity is generated at an average current density of 100mA/cm2 or higher, or 120mA/cm2 or higher, or 150mA/cm2 or higher.

Embodiment 12 the method of any preceding embodiment, wherein the voltage drop across the cathode is 0.4V or less, or wherein power is generated at a voltage of 0.55V or more, or a combination thereof.

Embodiment 13. the method of any of the preceding embodiments, wherein the fuel utilization in the anode is 60% or more, or wherein the fuel utilization in the anode is 55% or less, or wherein H in the anode exhaust is₂A concentration of 5.0 vol% or more, or wherein H in the anode off-gas₂And CO at a combined concentration of 6.0 vol% or more, or a combination thereof.

Embodiment 14. the method of any of the preceding embodiments, wherein the fuel cell is operated at a thermal ratio of 0.25: 1.0; or wherein the amount of the reformable fuel introduced into the first anode stage, the internal reforming element associated with the first anode stage, or the combination thereof, is at least about 75% more than the amount of hydrogen reacted in the first anode stage to generate electricity; or a combination thereof.

Embodiment 15 the method of any one of the preceding embodiments, wherein the cathode input stream comprises 5.0 vol% or lessCO of₂Or wherein the cathode exhaust comprises 1.0 vol% or less CO₂Or a combination thereof.

Alternative embodiments

Alternative embodiment 1. a method for generating electrical power, the method comprising: will include H₂An anode input stream of reformable fuel, or a combination thereof, is introduced into a first anode stage of a plurality of molten carbonate fuel cell stages; will comprise O₂And CO₂Is introduced into a first cathode stage of the plurality of molten carbonate fuel cell stages; passing a second anode input stream into a second anode stage of the plurality of molten carbonate fuel cell stages; passing an intermediate cathode output into a second cathode stage of the plurality of molten carbonate fuel cell stages; and operating the molten carbonate fuel cell stage to produce: i) the average current density was 80mA/cm²Or more, ii) anode exhaust from the plurality of molten carbonate fuel cell stages, the anode exhaust comprising H₂CO and CO₂And iii) cathode exhaust from the plurality of molten carbonate fuel cell stages, the cathode exhaust comprising CO₂1.0 vol% or more of O₂And 1.0 vol% or more of H₂O, wherein CO measured across a combination of the plurality of molten carbonate fuel cell stages₂A utilization rate of 70% or more (or 75% or more, or 80% or more), and wherein the calculated CO in the plurality of molten carbonate fuel cell stages is calculated based on the current density₂Utilization ratio of CO measured as compared to said combination₂The utilization is 5% or more (or 10% or more, or 20% or more) greater.

Alternative embodiment 2. the method of alternative embodiment 1, wherein the cathode input stream comprises 5.0 vol% or less CO₂(or 4.0 vol% or less), or wherein the cathode exhaust comprises 1.0 vol% or less CO₂Or a combination thereof.

Alternative embodiment 3. a method for generating electrical power, the method comprising: will include H₂Reformable fuel or a mixture thereofIntroducing the combined anode input stream into a first anode stage of the plurality of molten carbonate fuel cell stages; will comprise O₂And 5.0 vol% or less of CO₂(or 4.0 vol% or less) of a cathode input stream is introduced into a first cathode stage of the plurality of molten carbonate fuel cell stages; passing a second anode input stream into a second anode stage of the plurality of molten carbonate fuel cell stages; passing an intermediate cathode output into a second cathode stage of the plurality of molten carbonate fuel cell stages; and operating the molten carbonate fuel cell stage to produce: i) the average current density was 80mA/cm²Or more, ii) anode exhaust from the plurality of molten carbonate fuel cell stages, the anode exhaust comprising H₂CO and CO₂And iii) cathode exhaust from the plurality of molten carbonate fuel cell stages, the cathode exhaust comprising CO in an amount of 1.0 vol% or less₂Wherein the measured CO across the combination of the plurality of molten carbonate fuel cell stages₂A utilization rate of 70% or more (or 75% or more, or 80% or more), and wherein the calculated CO in the plurality of molten carbonate fuel cell stages is calculated based on the current density₂Utilization ratio of CO measured as compared to said combination₂High utilization rate of the calculated CO₂Utilization ratio of CO optionally measured in comparison with the combination₂The utilization ratio is 5.0% or more (or 10% or more, or 20% or more).

Alternative embodiment 4. the method of any of the preceding alternative embodiments, wherein the second anode input stream comprises at least a portion of an intermediate anode output stream, the intermediate anode output optionally comprising an output from the first anode stage.

Alternative embodiment 5. the method of alternative embodiment 4, further comprising: passing a second anode intermediate output into a third anode stage, the anode exhaust comprising an output from the third anode stage; and passing a second cathode intermediate output into a third cathode stage, the cathode exhaust comprising an output from the third cathode stage, wherein the first, second, and third anode stages are arranged in a co-current manner or in a counter-current manner with respect to the first, second, and third cathode stages.

Alternative embodiment 6. the method of any of the preceding alternative embodiments, wherein the measured CO in the first cathode stage₂Utilization rate different from the CO measured by the combination₂Utilization ratio.

Alternative embodiment 7. the method of any of the preceding alternative embodiments, wherein at least the first anode stage, the second anode stage, the first cathode stage, and the second cathode stage are operated in a co-current cross-flow manner; or wherein at least the first anode stage, the second anode stage, the first cathode stage and the second cathode stage are operated in a counter-current cross-flow manner.

Alternative embodiment 8. the method of any of the preceding alternative embodiments, wherein the anode exhaust comprises anode output from the second anode stage, or wherein the cathode exhaust comprises cathode output from the second cathode stage, or a combination thereof.

Alternative embodiment 9. the method of any of the preceding alternative embodiments, wherein the intermediate cathode output comprises an output from the first cathode stage.

Alternative embodiment 10 the method of any of the preceding alternative embodiments, wherein the electricity is generated at a current density of 120mA/cm2 or higher (or 150mA/cm2 or higher).

Alternative embodiment 11 the method of any of the preceding alternative embodiments, wherein the voltage drop across the cathode is 0.4V or less, or wherein power is generated at a voltage of 0.55V or more, or a combination thereof.

Alternative embodiment 12 the method of any of the preceding alternative embodiments, wherein the fuel utilization in the anode is 60% or greater, or wherein the fuel utilization in the anode is 55% or less.

Alternative embodiment 13. the method of any of the preceding alternative embodimentsWherein H in the anode exhaust₂A concentration of 5.0 vol% or more, or wherein H in the anode off-gas₂And CO at a combined concentration of 6.0 vol% or more, or a combination thereof.

Alternative embodiment 14. the method of any of the preceding alternative embodiments, wherein the fuel cell is operated at a thermal ratio of about 0.25: about 1.0; or wherein the amount of the reformable fuel introduced into the first anode stage, the internal reforming element associated with the first anode stage, or the combination thereof, is at least about 75% more than the amount of hydrogen reacted in the first anode stage to generate electricity; or a combination thereof.

All numbers in the description and claims herein are to be modified by the value indicated as "about" or "approximately" in view of experimental error and variations that may be expected by one of ordinary skill in the art.

Although the present invention has been described with specific embodiments, it is not necessarily limited thereto. Suitable changes/modifications to the 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 as fall within the true spirit/scope of the invention.

Claims (15)

1. A method for generating power, the method comprising: will include H₂An anode input stream of reformable fuel, or a combination thereof, is introduced into a first anode stage of a plurality of molten carbonate fuel cell stages; will comprise O₂And CO₂Is introduced into a first cathode stage of the plurality of molten carbonate fuel cell stages; passing a second anode input stream into a second anode stage of the plurality of molten carbonate fuel cell stages; passing an intermediate cathode output into a second cathode stage of the plurality of molten carbonate fuel cell stages; and operating the plurality of molten carbonate fuel cell stages to produce: i) the average current density was 60mA/cm²Or more, ii) anode exhaust from the plurality of molten carbonate fuel cell stages, the anode exhaust comprising H₂CO and CO₂And iii) cathode exhaust from the plurality of molten carbonate fuel cell stages, the cathode exhaust comprising CO in an amount of 2.0 vol% or less₂H in an amount of 1.0 vol% or more₂O and O in an amount of 1.0 vol% or more₂At least one of the first cathode stage and the second cathode stage operates at a transfer rate of 0.97 or less.

2. The method of claim 1, wherein the first cathode stage operates at a transfer rate of 0.97 or less and the second cathode stage operates at a transfer rate of 0.97 or less.

3. The method of any one of the preceding claims, wherein the at least one of the first cathode stage and the second cathode stage operates at a transfer rate of 0.95 or less, or 0.90 or less.

4. The method of any of the preceding claims, wherein the second anode input stream comprises at least a portion of an intermediate anode output stream, the intermediate anode output optionally comprising an output from the first anode stage.

5. The method of claim 4, further comprising: passing a second anode intermediate output into a third anode stage, the anode exhaust comprising an output from the third anode stage; and passing a second cathode intermediate output into a third cathode stage, the cathode exhaust comprising an output from the third cathode stage, wherein the first, second, and third anode stages are arranged in a co-current manner or in a counter-current manner with respect to the first, second, and third cathode stages.

6. The method of any one of the preceding claims, wherein the transfer rate of the first cathode stage is different from the transfer rate of the second cathode stage.

7. The method of any one of the preceding claims, wherein at least the first anode stage, the second anode stage, the first cathode stage, and the second cathode stage are operated in a co-current cross-flow manner; or wherein at least the first anode stage, the second anode stage, the first cathode stage and the second cathode stage are operated in a counter-current cross-flow manner.

8. The method of any of the preceding claims, wherein the anode exhaust comprises an anode output from the second anode stage, or wherein the cathode exhaust comprises a cathode output from the second cathode stage, or a combination thereof.

9. The method of any preceding claim, wherein the intermediate cathode output comprises an output from the first cathode stage.

10. The method of any of the preceding claims, wherein the combined measured CO in the first cathode stage and the second cathode stage during operation of the plurality of molten carbonate fuel cells₂The utilization rate is 75% or higher, or 80% or higher.

11. The method of any one of the preceding claims, wherein electricity is generated at an average current density of 100mA/cm2 or higher, or 120mA/cm2 or higher, or 150mA/cm2 or higher.

12. The method of any one of the preceding claims, wherein the voltage drop across the cathode is 0.4V or less, or wherein power is generated at a voltage of 0.55V or more, or a combination thereof.

13. The method of any of the preceding claims, wherein the fuel utilization in the anode is 60% or more, or wherein the fuel utilization in the anode is 55% or lessOr wherein H in the anode exhaust gas₂A concentration of 5.0 vol% or more, or wherein H in the anode off-gas₂And CO at a combined concentration of 6.0 vol% or more, or a combination thereof.

14. The method of any preceding claim, wherein the fuel cell is operated at a thermal ratio of 0.25: 1.0; or wherein the amount of the reformable fuel introduced into the first anode stage, the internal reforming element associated with the first anode stage, or the combination thereof, is at least about 75% more than the amount of hydrogen reacted in the first anode stage to generate electricity; or a combination thereof.

15. The method of any one of the preceding claims, wherein the cathode input stream comprises 5.0 vol% or less of CO₂Or wherein the cathode exhaust comprises 1.0 vol% or less CO₂Or a combination thereof.

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