Power generation method

A fuel cell module includes at least one fuel cell stack comprising a number of fuel cell units. An inlet is configured to receive an oxidant flow for supplying the fuel cell stack. An outlet is configured to exhaust an exhaust oxidant flow generated by the fuel cell stack. A recirculation path is configured to convey at least about thirty percent (30%) of the exhaust oxidant flow from the outlet to the inlet as a recirculated exhaust flow, for combination with the new oxidant flow to form the oxidant flow to the fuel cell stack.

BACKGROUND OF THE INVENTION

The invention relates generally to fuel cell modules and, more particularly, to fuel cell modules for use in combined cycle power generation.

Fuel cells, for example solid oxide fuel cells (SOFCs), are energy conversion devices that produce electricity by electrochemically combining a fuel and an oxidant across an ion conducting layer. Many types of fuel cells, such as SOFCs, have high operating temperatures. For power generation applications, large numbers of fuel cells arranged in stacks are used to generate electric power. Stacks of high temperature fuel cells require large quantities of inlet oxidant, for example air, heated to the operating temperature of the fuel cells, for example in excess of 600 degrees Celsius. In addition, heating occurs within the fuel cell stack, creating a thermal gradient across the stack and thereby subjecting the stack to thermal stresses. The high temperature oxidant exhaust is conveyed downstream.

Presently, heat exchangers are used to transfer some of the excess heat from the exhaust oxidant to the inlet oxidant flow. However, heat exchangers are costly, bulky and possess a limited lifetime, due to the extreme thermal stresses that these devices experience. Accordingly, it would be desirable to design a fuel cell module for use in combined cycle power generation that heats the inlet oxidant flow without using a heat exchanger. It would further be desirable to design a fuel cell module for use in combined cycle power generation having a reduced thermal gradient across the fuel cell stack, to increase the lifetime of the stack.

BRIEF DESCRIPTION

Briefly, in accordance with one embodiment of the present invention, a fuel cell module is disclosed. The fuel cell module includes at least one fuel cell stack comprising a number of fuel cell units. An inlet is configured to receive an oxidant flow for supplying the fuel cell stack. An outlet is configured to exhaust an exhaust oxidant flow generated by the fuel cell stack. A recirculation path is configured to convey at least about thirty percent (30%) of the exhaust oxidant flow from the outlet to the inlet as a recirculated exhaust flow, for combination with the new oxidant flow to form the oxidant flow to the fuel cell stack.

A combined cycle power system embodiment includes at least one fuel cell stack comprising a number of fuel cell units. The combined cycle power system further includes the inlet, outlet and recirculation path. A tail gas burner is adapted to receive a remaining portion of the exhaust oxidant flow from the outlet.

A power generation method includes supplying an oxidant flow to the inlet of the fuel cell module, supplying a fuel flow to the fuel cell module, and recirculating at least about thirty percent (30%) of an exhaust oxidant flow from the outlet of the fuel cell module to the inlet of the fuel cell module as a recirculated exhaust flow. The method further includes combining the recirculated exhaust flow with a new oxidant flow to form the oxidant flow supplied to the inlet. A remaining portion of the exhaust oxidant flow is exhausted from the outlet. The supplying of the oxidant and fuel flows and recirculation of at least about thirty percent (30%) of the exhaust flow are controlled to achieve an equivalence ratio of at least about 0.2.

DETAILED DESCRIPTION

A fuel cell module50embodiment of the invention is described with respect toFIG. 1. As indicated, fuel cell module50includes at least one fuel cell stack10, which includes a number of fuel cell units20. The exemplary arrangement of two fuel cell stacks10shown inFIG. 1is purely illustrative, and the number and arrangement of fuel cell stacks10, as well as the number and arrangement of fuel cell units10within each stack10vary based on the requirements of the specific application, such as desired power output and spatial constraints. Fuel cell module50further includes an inlet12, which is configured to receive an oxidant flow for supplying the fuel cell stacks10, and an outlet14, which is configured to exhaust an exhaust oxidant flow generated by the fuel cell stacks10. Fuel cell module50further includes a recirculation path30, which is configured to convey at least about thirty percent (30%) of the exhaust oxidant flow from outlet14to inlet12, as a recirculated exhaust flow. The recirulated exhaust flow is combined with the new oxidant flow to form the oxidant flow to fuel cell stack10.

In order to draw the portion of the exhaust oxidant flow from outlet14and move this recirculated exhaust flow through recirculation path30, a fuel cell module50according to a particular embodiment further includes a blower32positioned in recirculation path30and configured to blow the recirculated exhaust flow. Exemplary blowers include radial and/or axial flow turbomachines and fluid entrainment devices, such as ejectors or eductors. The blower32may be mechanically or electrically driven.

As indicated inFIG. 1, the exemplary fuel cell module50is further configured to receive a fuel flow and to exhaust spent fuel. Both fuel and oxidant are supplied to each of the fuel cell stacks10. As known to those skilled in the art, this can be accomplished in a variety of ways, and fuel cell module50is not limited to any particular arrangement of fuel cell stacks10.

Fuel cell units20are well known and are not described in detail herein. Briefly, fuel cell units20(or fuel cells) are energy conversion devices that produce electricity by electrochemically combining a fuel and an oxidant, such as air, across an ion conducting layer. More particularly, each fuel cell unit20includes an anode, an electrolyte, and a cathode (not shown), arranged for example in a tubular or planar configuration. Exemplary types of fuel cell units20include solid oxide fuel cells20(SOFCs), molten carbonate fuel cells20, and proton exchange membrane fuel cells20(PEMs).

Fuel cell module50provides a number of benefits. A number of types of fuel cell units20, such as SOFCs, have high operating temperatures, for example above at least about 600 degrees Celsius for SOFCs. Thus, efficient conversion of the fuel and oxidant within fuel cell stack10typically requires inlet12temperatures in excess of about 800 degrees Celsius for such high temperature fuel cell units20. Moreover, the electrochemical processes occurring within fuel cell stack10further heat the exhaust oxidant flows, generating a thermal gradient across fuel cell stack10and thereby subjecting fuel cell stack10to thermal stress, shortening its lifetime. For power systems applications, tremendous volumes of oxidant flow are typically required, for example inlet12oxidant flows on the order of hundreds of kilograms/second are typical for power utility sized units. Heating such massive oxidant flows typically requires large heat transfer rates. Several conventional fuel cell stack arrangements use the excess heat of the exhaust oxidant flow to at least partially heat the oxidant flow to the inlet via a heat exchanger (not shown). Briefly, the hot exhaust oxidant flow passes through one chamber of the heat exchanger, while the oxidant flow passes through another chamber of the heat exchanger. The two chambers are separated by a thermally conductive barrier, and heat flows from the hot exhaust oxidant flow to the oxidant flow through the barrier, providing at least a portion of the heat necessary to heat the oxidant flow. Problems associated with this use of heat exchangers include cost, size, losses and the limited lifetime of the heat exchangers due to the large thermal stresses experienced. Beneficially, the fuel cell module50of the present invention obviates the use of a heat exchanger by heating the oxidant flow to the inlet12by direct mass exchange.

According to a more particular embodiment, fuel cell module50further includes an inlet path16, which is configured to supply a new oxidant flow to inlet12, as exemplarily shown inFIG. 1. Fuel cell module50further includes an exhaust path18, which is configured to exhaust a remaining portion of the exhaust oxidant flow from outlet14. For this embodiment, recirculation path30is configured to convey at least about fifty percent (50%) of the exhaust oxidant flow from outlet14to inlet12for combination with the new oxidant flow to form the oxidant flow through inlet12. Because of the recirculated exhaust flow, the oxidant flow at inlet12has a reduced oxygen concentration relative to the oxygen concentration of the new oxidant flow through inlet path16. More particularly, the oxygen concentration cinletof the oxidant flow is governed by the following formula:
cinlet/c∞=[1+(1−φ)BR]/[1+BR],
where φ is the equivalence ratio, which is the stoichiometric fraction of fuel with respect to the oxidant, BR is the blowing ratio of the mass flow, dm30/dt, through recirculation path30to the mass flow, dm16/dt, through inlet path16, and c∞ is the oxygen concentration of the new oxidant flow (i.e., at the inlet12absent any recirculation of the oxidant). The above expression for the oxygen concentration cinletneglects sight variations in the molecular weight of the exhaust stream with respect to the inlet stream. For example, for an exemplary equivalence ratio φ=0.2 and a blowing ratio BR=1, the ratio of the oxygen concentration at the inlet to that of the new oxidant flow through inlet path16is cinlet/c∞=90%.

According to more particular embodiments, recirculation path30is configured to convey at least about seventy-five percent (75%), and still more particularly at least about eighty percent (80%), of the exhaust oxidant flow from outlet14to inlet12. The ratio of recirculated to exhausted oxidant flow varies based on the specific system parameters, such as the flow rates, fuel cell type, and blower power consumption.

Exemplary recirculation paths30, inlet paths16, and exhaust paths18comprise piping of suitable cross-section to accommodate the oxidant flows, which vary in magnitude depending on the application. Further, the piping is selected to withstand the high temperatures involved, for example to withstand temperatures at least about 800 degrees Celsius. Exemplary high temperature piping materials include ferritic stainless steel, Iron/Chromium (Fe/Cr), Molybdenum/Manganese/Aluminum (Mo/Mn/Al), and Titanium/Yttrium/Lanthanum (Ti/Y/La) alloys, barrier coatings and coated alloys. Such high temperature piping materials are resistant to contamination of the fuel cell.

As noted above, the electrochemical processes occurring within the fuel cell stacks in conventional stack arrangements generate a thermal gradient across the fuel cell stacks, subjecting them to thermal stress and thereby shortening their lifetimes. Beneficially, recirculation of a portion of the exhaust oxidant flow through recirculating path30increases the mass flow rate through fuel cell stack10, reducing the thermal gradient across fuel cell stack10, thereby reducing the thermal stress on fuel cell stack10, which increases its lifetime. Moreover, because the oxidant flow is oxygen depleted, the rate of reaction is more uniform across fuel cell stack, further reducing the thermal gradient across the stack10, and hence reducing the thermal stress on the stack10. In addition, because the oxidant flow is oxygen depleted, corrosion within fuel cell stack10is reduced, enhancing both its lifetime and its resistance to performance degradation. However, oxygen depletion can increase losses due to concentration polarization, so types of fuel cell units20for which concentration losses dominate, such as PEM fuel cells may be less desirable than other types of high temperature fuel cells, such as SOFCs and molten carbonate fuel cells, at high recirculation rates (i.e., for increased oxygen depletion).

The benefits of fuel cell module50are enhanced in a combined cycle power generation configuration. An exemplary combined cycle power system60embodiment is illustrated inFIG. 2. As shown, combined cycle power system60includes at least one fuel cell stack10and further includes inlet12and outlet14, all of which are described above with respect to fuel cell module50. As explained above the number and arrangement of fuel cell stacks10vary based on the specific requirements of the power system, such as power output and spatial requirements. Exemplary fuel cell units20for the stacks10include solid oxide fuel cells (SOFCs)20and molten carbonate fuel cells20. Combined cycle power system60further includes a recirculation path30, which is configured to convey at least about thirty percent (30%) of the exhaust oxidant flow from outlet14to inlet12as a recirculated exhaust flow. In addition, combined cycle power system60includes a tail gas burner (also known as a combustor or anode tail gas oxidizer)40, which is adapted to receive a remaining portion of the exhaust oxidant flow from outlet14, as indicated inFIG. 2.

More particularly, the combined cycle power system60is further configured to supply a fuel flow to the fuel cell stacks10and to exhaust spent fuel from fuel cell stacks10. As shown, tail gas burner40is adapted to receive an exhaust fuel flow from fuel cell stack10, as indicated inFIG. 2, and to combust the exhaust fuel flow and the remaining portion of the exhaust oxidant flow to produce heat. For this embodiment, combined cycle power system60further includes an energy cycle unit42, which is adapted to receive heat from tail gas burner40. Exemplary energy cycle units42include a steam turbine, a thermoelectric generator, a heat recovery unit, and a Stirling engine. According to a particular embodiment, energy cycle unit42is a gas turbine42. For the embodiment shown inFIG. 2, energy cycle unit42is a gas turbine42, and combined cycle power system60further includes a compressor44for compressing an oxidant, such as air, to supply a new oxidant flow to inlet12. For the particular embodiment illustrated, gas turbine42is configured to power compressor44. For another particular embodiment, also illustrated byFIG. 2, combined cycle power system60further includes a bottoming cycle unit46, for example a steam turbine46, adapted to receive heat from energy cycle unit42, for example a gas turbine42.

In order to draw the portion of the exhaust oxidant flow from outlet14and move this recirculated exhaust flow through recirculation path30, a combined cycle power system60according to a particular embodiment further includes a blower32positioned in recirculation path30and configured to blow the recirculated exhaust flow. According to a more particular embodiment, energy cycle unit42is configured to power blower32, either directly or indirectly (for example, via a motor).

According to a particular embodiment, inlet12, outlet14, recirculation path30, and blower32are adapted to achieve an equivalence ratio φ of at least about 0.2 and more particularly, of at least 0.3 or 0.4. As noted above, the equivalence ratio φ is the stoichiometric fraction of fuel with respect to the oxidant. For conventional fuel cell arrangements, equivalence ratios are typically limited to φ=0.2 or less, to provide adequate air-cooling for the fuel cell stack. However, the incorporation of recirculation path30into combined cycle power system60desirably permits selection of higher equivalence ratios, while providing adequate air-cooling for the stack10. For combined cycle power system60, it is desirable to adapt inlet12, outlet14, recirculation path30, and blower32to achieve about the maximum equivalence ratio for the energy cycle unit42. In this manner, the temperature of the combusted fuel cell module tail gases is raised to about the maximum allowable turbine inlet temperature, thereby increasing the efficiency of the conversion of fuel cell tail gases into additional power system60output. By “adapted,” it is meant that the recirculation path30and inlet12and outlet14(and the associated piping) are sized and fitted with controls, for example valves (not shown), to convey and control the respective fuel and oxidant flow rates to achieve the desired equivalence ratio φ. Similarly, blower32is controlled to provide a blowing ratio BR, of the mass flow, dm30/dt, through recirculation path30to the mass flow, dm16/dt, through inlet path16, to achieve the desired equivalence ratio φ.

In addition to the benefits discussed above with respect to fuel cell module50, an additional benefit of combined cycle power system60is that tail gas burner40incinerates the exhaust fuel flow from fuel cell stacks10in the exhaust oxidant flow from fuel cell stacks10at a higher flame temperature due to the reduced net oxidant flow required to thermally manage the stack(s)10, increasing the efficiency of energy cycle unit42. In other words, by recirculating at least a portion of the exhaust gas flow to inlet12, and more particularly by recirculating large portions of the exhaust gas flow to inlet12, the overall cycle equivalence ratio φ is increased. The overall cycle equivalence ratio φ is a key driver for efficiency gain in energy cycle unit42and hence for combined cycle power system60.

The example of combined cycle power system60shown inFIG. 3is provided for purely illustrative purposes. The specific temperatures, ratios, and volumes will vary based on system requirements. For this specific example, air is supplied to compressor44, which operates at a compression ratio of about 15:1 to supply a new oxidant flow of about 100 kg/s of compressed air to inlet12. About 411 kg/s of the exhaust oxidant through outlet14, which exhaust is at a temperature of about 849 degrees Celsius, is recirculated via recirculation path30and mixed with the new oxidant flow. This corresponds to an oxidant recirculation ratio of about eighty percent (80%) (and to a blowing ratio of about 411%) and raises the inlet oxidant temperature to about 749 degrees Celsius. The remaining portion of the exhaust oxidant flow, about 100 kg/s, is exhausted to combustor40with the spent fuel, with both the spent fuel and the exhaust oxidant having a temperature of about 849 degrees Celsius. Combustor40burns the spent fuel with the exhaust oxidant to drive a gas turbine42at an inlet temperature of about 1360 degrees Celsius.

For the example ofFIG. 3, the cell voltage for each of the fuel cell units20is about 0.7 V, the stack10efficiency is about fifty percent (50%), and the fuel utilization of the stack10is about seventy five percent (75%). The fuel cell units20are SOFCs. The average temperature within the stack10is about 800 degrees Celsius, and the thermal gradient across the stack10is about 100 degrees Celsius. The cycle efficiency, assuming ideal thermodynamics and ninety percent (90%) polytropic efficiency for the turbomachinery, is about seventy percent (70%), with a fuel cell work split of about 71% and a turbine work split of about 29%. Higher efficiencies are enabled by capturing the heat rejected by the energy cycle unit42(at 620 degrees Celsius) in bottoming cycle unit46(for example, in a steam turbine46), as indicated inFIG. 2.

A power generation method embodiment of the invention is described with reference toFIGS. 1 and 2. As indicated inFIG. 1, the power generation method includes supplying an oxidant flow to inlet12of fuel cell module50. The power generation method further includes supplying a fuel flow to the fuel cell module50and recirculating at least about thirty percent (30%) of an exhaust oxidant flow from outlet14to inlet12, as a recirculated exhaust flow. The supply of the oxidant and fuel flows and recirculation of at least about thirty percent (30%) of the exhaust flow are controlled to achieve an equivalence ratio of at least about 0.2. The power generation method also includes exhausting a remaining portion of the exhaust oxidant flow from outlet14.

For a particular embodiment, the power generation method further includes blowing the recirculated exhaust flow at a blowing ratio BR. For this embodiment, the supply of the oxidant and fuel flows are controlled, and the blowing ratio BR is selected to achieve an equivalence ratio of at least about 0.2 and, more particularly, of at least about 0.3, 0.4 or 0.5. As noted above, the recirculation of a portion of the exhaust flow to inlet12permits running at a higher equivalence ratio φ, while providing adequate air cooling of the fuel cell stacks10. Consequently, a higher efficiency for the overall cycle is achieved.

For the embodiment depicted inFIG. 2, the power generation method also includes supplying the remaining portion of the exhaust oxidant flow to combustor40, supplying an exhaust fuel flow from fuel cell module50to combustor40, combusting the exhaust fuel flow with the remaining portion of the exhaust oxidant flow to generate heat, and supplying the heat to energy cycle unit42. Beneficially, the reduced overall oxidant flow required to thermally manage the stacks10increases the flame temperature at which combustor40incinerates the exhaust fuel flow from the fuel cell module50, increasing the efficiency of energy cycle unit42.

For a more particular embodiment, the supply of the oxidant and fuel flows are controlled, and the blowing ratio BR is selected to achieve about the maximum equivalence ratio for the energy cycle unit42. The equivalence ratio is limited by the allowable inlet temperature of energy cycle unit42. Thus, “maximum equivalence ratio” refers to the maximum equivalence ratio allowable under the constraint of the maximum allowable inlet temperature of energy cycle unit42.