Fuel cell reforming system with carbon dioxide removal

Abstract

A power generation system includes a fuel cell including an anode that generates a tail gas. The system also includes a hydrocarbon fuel reforming system that mixes a hydrocarbon fuel with the fuel cell tail gas and to convert the hydrocarbon fuel and fuel tail gas into a reformed fuel stream including CO.sub.2. The reforming system further splits the reformed fuel stream into a first portion and a second portion. The system further includes a CO.sub.2 removal system coupled in flow communication with the reforming system. The system also includes a first reformed fuel path coupled to the reforming system. The first path channels the first portion of the reformed fuel stream to an anode inlet. The system further includes a second reformed fuel path coupled to the reforming system. The second path channels the second portion of the reformed fuel stream to the CO.sub.2 removal system.

Claims

1. A method of generating electric power with a power generation system utilizing a fuel cell, said method comprising: channeling a fuel to an inlet of a solid oxide fuel cell anode; generating a tail gas stream including carbon dioxide (CO.sub.2) in the solid oxide fuel cell anode and discharging the tail gas stream from an outlet of the fuel cell anode; channeling the tail gas stream to a hydrocarbon fuel reforming system; mixing a hydrocarbon fuel with the tail gas stream to form a fuel/tail gas mixture stream; at least partially converting the fuel/tail gas mixture stream into a reformed fuel stream including CO.sub.2; splitting the reformed fuel stream including CO.sub.2 into a first portion and a second portion, wherein the first portion includes a hot partially reformed fuel stream; channeling the first portion of the reformed fuel stream including CO.sub.2 to the inlet of the solid oxide fuel cell anode; channeling the second portion of the reformed fuel stream to a CO.sub.2 removal system; wherein channeling the second portion of the reformed fuel stream including CO.sub.2 to a CO.sub.2 removal system comprises decreasing a CO.sub.2 concentration in the first portion of the reformed fuel stream including CO.sub.2 comprising: channeling the second portion of the reformed fuel stream including CO.sub.2 to a solvent-based CO.sub.2 absorber; transferring at least a portion of the CO.sub.2 in the second portion of the reformed fuel stream including CO.sub.2 to a CO.sub.2-lean solvent stream, and forming a CO.sub.2-rich solvent stream and a reformed fuel stream without CO.sub.2; and channeling the reformed fuel stream without CO.sub.2 into the hot partially reformed fuel stream.

2. The method in accordance with claim 1, wherein decreasing a CO.sub.2 concentration in the first portion of the reformed fuel stream including CO.sub.2 further comprises: channeling the CO.sub.2-rich solvent stream to a rich solvent reboiler; adding heat into the rich solvent reboiler from at least one of a combustion engine exhaust stream and a heat pump.

3. The method in accordance with claim 2, wherein adding heat into the rich solvent reboiler from at least one of a combustion engine exhaust stream and a heat pump comprises: splitting the reformed fuel stream including CO.sub.2 into a third portion; and channeling the third portion of the reformed fuel stream including CO.sub.2 to a combustion engine and generating the engine exhaust stream.

4. The method in accordance with claim 1, wherein decreasing a CO.sub.2 concentration in the first portion of the reformed fuel stream including CO.sub.2 further comprises transferring heat from the CO.sub.2-lean solvent stream to the CO.sub.2-rich solvent stream through a solvent recuperator, thereby preheating the CO.sub.2-rich solvent stream prior to entry into the rich solvent reboiler.

Description

DRAWINGS

(1) These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

(2) FIG. 1 is a schematic view of an exemplary combined cycle power plant that includes a fuel cell; and

(3) FIG. 2 is a continuation of FIG. 1.

(4) Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of the disclosure. These features are believed to be applicable in a wide variety of systems comprising one or more embodiments of the disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein.

DETAILED DESCRIPTION

(5) In the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings.

(6) The singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise.

(7) “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.

(8) Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.

(9) The carbon dioxide (CO.sub.2) removal systems embedded within the fuel cell reforming-and-recirculating systems described herein provide a cost-effective method for generating electric power. The embodiments described herein facilitate increased thermal efficiency through heat recovery. The embodiments described herein also facilitate decreased CO.sub.2 discharge through methods and equipment that include, e.g., and without limitation, solvent-based CO.sub.2 absorption. Specifically, the devices, systems, and methods described herein include a system of fuel recirculation and heat recovery devices that facilitate increased utilization of the fuel in the anode of the fuel cell. Also, the devices, systems, and methods described herein facilitate heat recovery from the fuel cell reformation, a combustion engine exhaust, and the CO.sub.2 absorption process. Moreover, the devices, systems, and methods described herein further facilitate increased thermal efficiency through reducing reliance on additional parasitic loads, i.e., without introducing increased auxiliary power loads. Therefore, overall plant efficiencies may exceed 65% for at least some of the embodiments described herein. Furthermore, the devices, systems, and methods described herein are scalable to any electric power output rating, including large-scale centralized power generation and local or distributed generation. In addition, the CO.sub.2 removal systems and the fuel cell reforming-and-recirculating systems described herein may be used with solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs).

(10) FIG. 1 is a schematic view of an exemplary electric power generation system, e.g., a combined cycle power plant 100 that includes a fuel cell 102. FIG. 2 is a continuation of FIG. 1. Fuel cell 102 is one of a solid oxide fuel cell (SOFC) and a molten carbonate fuel cell (MCFC). Fuel cell 102 generates direct current (DC) electric power 104 by electrochemically combining a fuel and an oxidant (both discussed further below) across an ionic conducting layer (not shown). The ionic conducting layer, i.e., the electrolyte of fuel cell 102, may be a liquid or solid. Fuel cell 102 may be positioned in electrical series in an assembly of fuel cells 102 (only one shown in FIG. 1) to produce power at useful voltages or currents. Fuel cell includes the electrolyte and two electrodes, i.e., an anode 106 and a cathode 108. The electrochemical reactions that produce DC electricity 104 generally take place at electrodes 106 and 108 where a catalyst (not shown) is disposed to speed the reactions. Electrodes 106 and 108 may be constructed as, e.g., and without limitation, channels and porous layers (neither shown) to increase the surface area for the chemical reactions to occur. The electrolyte carries electrically charged particles from one electrode to the other and is otherwise substantially impermeable to both fuel and oxidant. Anode 106 includes an inlet 110 and an outlet 112. Cathode 108 includes an inlet 114 and an outlet 116. Anode 106 discharges a tail gas stream 118 from anode outlet 112 that includes water, hydrogen, carbon monoxide (CO), carbon dioxide (CO.sub.2), and unreacted fuel, if any.

(11) Combined cycle power plant 100 also includes a fuel preheater 120 coupled in flow communication with anode outlet 112. Fuel preheater 120 is coupled in flow communication with a carbonaceous, i.e., hydrocarbon fuel source (not shown) that, in the exemplary embodiment, channels natural gas stream 122 to fuel preheater 120. Alternatively, any fuel that enables operation of combined cycle power plant 100 and fuel cell 102 may be used.

(12) Combined cycle power plant 100 further includes a hydrocarbon fuel reforming system 124 configured to mix natural gas stream 122 with tail gas stream 118 downstream of fuel cell anode outlet 112. Hydrocarbon fuel reforming system 124 also converts natural gas stream 122 with entrained tail gas stream 118 into a partially reformed fuel stream 126 including CO.sub.2, and a substantially fully reformed fuel stream 128.

(13) Hydrocarbon fuel reforming system 124 includes a first reformer 130 that is coupled to fuel cell anode outlet 112 and fuel preheater 120. First reformer 130 receives natural gas stream 122 with entrained tail gas stream 118 and coverts them to a hot partially reformed fuel stream 126 including CO.sub.2. Hydrocarbon fuel reforming system 124 also includes an air/fuel heat exchanger 132 coupled in flow communication with first reformer 130. Air/fuel heat exchanger 132 receives a portion of hot partially reformed fuel stream 127 with at least a portion of CO.sub.2 removed and transfers heat from another fluid stream (discussed further below) to stream 127 to continue to increase the heat content of stream 127. Hydrocarbon fuel reforming system 124 further includes at least one downstream reformer 134 coupled in flow communication with air/fuel heat exchanger 132 and fuel cell anode inlet 110. Downstream reformer 134 is configured to substantially complete reformation of hot reformed fuel stream 127 into fully reformed fuel stream 128 prior to channeling into fuel cell anode 106. Therefore, combined cycle power plant 100 defines a first reformed fuel path 135 that includes air/fuel heat exchanger 132 and downstream reformer 134.

(14) In at least some alternative embodiments of hydrocarbon fuel reforming system 124, first reformer 130 is the only reformer and it substantially completes reformation of hot reformed fuel stream 127 or stream 126 into fully reformed fuel stream 128 prior to channeling into fuel cell anode 106. Also, in at least some alternative embodiments of hydrocarbon fuel reforming system 124, a plurality of strings of first reformer 130-heat exchanger 132-reformer 134 are coupled in a serialized and/or parallel configuration. Each heat exchanger 132 may also be referred to as a “booster” heat exchanger that is used to counter the otherwise tendency for the decrease in temperature of streams 126, 127, and 128 due to the endothermic nature of the reforming reaction. Moreover, alternatively, air/fuel heat exchanger 132 may be integrated into downstream reformer 134 to define a unity device.

(15) Combined cycle power plant 100 also includes fuel cell anode outlet 116 coupled to air/fuel heat exchanger 132 to transfer heat from a hot oxidizing stream 136 to hot partially reformed fuel stream 127. In the exemplary embodiment, stream 136 includes vitiated air, i.e., air depleted of oxygen. Air/fuel heat exchanger 132 is coupled in flow communication with an air preheater 138 such that a cooled vitiated air stream 140 discharged from heat exchanger 132 is channeled to air preheater 138. A fresh air stream 142 is channeled through air preheater 138 and residual heat is transferred from cooled vitiated air stream 140 to fresh air stream 142 to produce a hot fresh air stream 144 that is channeled to fuel cell cathode inlet 114. Exhausted vitiated air stream 146 is vented.

(16) Combined cycle power plant 100 further includes a bottoming, i.e., low temperature cycle, i.e., a combustion engine 148 coupled in flow communication with first reformer 130. Hot reformed fuel 126 with entrained CO.sub.2 is channeled to combustion engine 148 that may include, for example, without limitation, a reciprocating 4-stroke engine, reciprocating 2-stroke engine, an opposed piston 2-stroke engine, or a gas turbine engine. Combustion engine 148 may be mechanically coupled to an electric power generator or other load, e.g., without limitation, a pump and a compressor. Combustion engine 148 generates an engine exhaust stream 150 that includes residual heat that may be used as described further below.

(17) Combined cycle power plant 100 also includes a solvent-based CO.sub.2 removal system 152 coupled in flow communication with hydrocarbon fuel reforming system 124. Hydrocarbon fuel reforming system 124 is configured to split hot reformed fuel stream 126 with entrained CO.sub.2 into a first portion and a second portion. The first portion of stream 126 including partially reformed fuel stream 127 is channeled toward air/fuel heat exchanger 132 through first reformed fuel path 135. The second portion of hot reformed fuel stream 126 with entrained CO.sub.2 is channeled toward fuel preheater 120. Fuel preheater 120 transfers heat into natural gas stream 122 from hot reformed fuel stream 126 with entrained CO.sub.2. Therefore, combined cycle power plant 100 defines a second reformed fuel path 153 that includes fuel preheater 120 and solvent-based CO.sub.2 removal system 152. Combined cycle power plant 100 also defines a third reformed fuel path 155 that includes combustion engine 148.

(18) Solvent-based CO.sub.2 removal system 152 includes a gas recuperator 154 coupled in flow communication with fuel preheater 120. Gas recuperator 154 is configured to extract heat from reformed fuel stream 126 with entrained CO.sub.2 after it is channeled from fuel preheater 120 and transfer the heat to another fuel stream (discussed further below). Gas recuperator 154 is also coupled in flow communication with a solvent-based CO.sub.2 absorber 157 that substantially removes CO.sub.2 from stream 126 to generate a reformed fuel stream 156 without CO.sub.2. Stream 156 is channeled back to gas recuperator 154 for received heat from incoming stream 126 and then being channeled back to hydrocarbon fuel reforming system 124 between first reformer 130 and air/fuel heat exchanger 132 such that CO.sub.2 channeled into fuel cell anode inlet 110 is substantially reduced. Water removal preferably occurs prior to admission of stream 126 into absorber 157 through a cooling device (not shown).

(19) Solvent-based CO.sub.2 removal system 152 also includes a solvent regeneration system 158 that includes a solvent recuperator 160 coupled in flow communication with a solvent-based CO.sub.2 absorber 157. Solvent regeneration system 158 also includes a rich solvent reboiler 162 coupled in flow communication with solvent recuperator 160. Solvent regeneration system 158 further includes a heat pump 164 thermally coupled to rich solvent reboiler 162 and solvent-based CO.sub.2 absorber 157.

(20) In the exemplary embodiment, partially reformed fuel stream 126 with entrained CO.sub.2 is channeled from fuel preheater 120 to gas recuperator 154. Heat is transferred from stream 126 to reformed fuel stream 156 without CO.sub.2. Hot reformed fuel 156 without CO.sub.2 is channeled from gas recuperator 154 to mix with the portion of hot partially reformed fuel stream 126 with CO.sub.2 not diverted to solvent-based CO.sub.2 removal system 152 to form hot partially reformed fuel stream 127 with significantly less CO.sub.2 than stream 126. Cooled stream 126 with CO.sub.2 is channeled from gas recuperator 154 to solvent-based CO.sub.2 absorber 156. Solvent absorbs CO.sub.2 from stream 126 and generates reformed fuel stream 156 without CO.sub.2 that is channeled back to gas recuperator 154.

(21) Also, in the exemplary embodiment, CO.sub.2-rich solvent stream 166 is channeled from solvent-based CO.sub.2 absorber 157 to solvent recuperator 160 such that heat is transferred to stream 166 from another stream (discussed further below). Heated CO.sub.2-rich solvent stream 166 is channeled to rich solvent reboiler 162 that further heats and boils stream 166 to release the absorbed CO.sub.2 and channel the released CO.sub.2 to a collection system (not shown). A CO.sub.2-lean solvent stream 168 is channeled to solvent recuperator 160 to transfer heat to CO.sub.2-rich solvent stream 166 prior to stream 168 being channeled back to solvent-based CO.sub.2 absorber 157 to repeat the cycle. The thermal source for rich solvent reboiler 162 is engine exhaust stream 150 that is vented to atmosphere upon exiting reboiler 162. Alternatively, the heat required for the reboiling operation may be supplied heat pump 164 alone or in conjunction with stream 150. In the embodiments with heat pump 164, it is disposed between solvent-based CO.sub.2 absorber 157 and rich solvent reboiler 162 so that heat of CO.sub.2 absorption is removed from the working solvent and pumped to the CO.sub.2-loaded solvent for regeneration, thereby facilitating maintaining the working solvent cool and regenerating the CO.sub.2-loaded solvent. Additional water removal from the solvent streams may also be needed prior to returning CO.sub.2-lean solvent stream 168 to absorber 157 through a cooling device (not shown).

(22) In alternative embodiments, additional apparatus and systems may be added to combined cycle power plant 100 to enhance electric power generation. For example, in some alternative embodiments a trim cooler (not shown) or other heat exchange device may be used to recover heat in solvent-based CO.sub.2 removal system 152 and/or hydrocarbon fuel reforming system 124. Moreover, in some alternative embodiments, an Organic Rankine cycle (ORC) (not shown) may be used to receive engine exhaust stream 150, diverted hot partially reformed fuel stream 126 with CO.sub.2, and/or hot partially reformed fuel stream 127 to boil a Rankine fluid, e.g., a water-steam cycle. Engine exhaust stream 150 is then vented to atmosphere.

(23) Also, in alternative embodiments, and regenerative CO.sub.2 removal systems other than solvent-based CO.sub.2 removal system 152 that enable operation of combined cycle power plant 100 as described may be used. For example, and without limitation, CO.sub.2 removal systems such as solid-adsorbent CO.sub.2 removal systems and pressure-swing adsorption (PSA) CO.sub.2 removal systems may be used.

(24) The above-described carbon dioxide (CO.sub.2) removal systems embedded within the fuel cell reforming-and-recirculating systems described herein provide a cost-effective method for generating electric power. The embodiments described herein facilitate increased thermal efficiency through heat recovery. The embodiments described herein also facilitate decreased CO.sub.2 discharge through CO.sub.2 absorption. Specifically, the devices, systems, and methods described herein include a system of fuel recirculation and heat recovery devices that facilitate increased utilization of the fuel in the anode of the fuel cell. Also, the devices, systems, and methods described herein facilitate heat recovery from the fuel cell reformation, a combustion engine exhaust, and the solvent-based CO.sub.2 absorption process. Moreover, the devices, systems, and methods described herein further facilitate increased thermal efficiency through reducing reliance on additional parasitic loads, i.e., without introducing increased auxiliary power loads. Therefore, overall plant efficiencies may exceed 65% for at least some of the embodiments described herein. Furthermore, the devices, systems, and methods described herein are scalable to any electric power output rating, including large-scale centralized power generation and local or distributed generation. In addition, the solvent-based carbon dioxide (CO.sub.2) removal systems and the fuel cell reforming-and-recirculating systems described herein may be used with solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs).

(25) An exemplary technical effect of the methods, systems, and apparatus described herein includes at least one of: (a) increasing a thermal efficiency of a combined cycle power plant that includes a fuel cell through heat recovery, including heat recovery from fuel cell reformation, combustion engine exhaust, and solvent-based CO.sub.2 absorption; (b) decreasing CO.sub.2 discharge from a combined cycle power plant that includes a fuel cell through solvent-based CO.sub.2 absorption; (c) reducing an increase in auxiliary power consumption by the combined cycle power plant; and (d) increasing utilization of fuel in the anode of a fuel cell.

(26) Exemplary embodiments of combined cycle power plants that include a fuel cell and a solvent-based carbon dioxide (CO.sub.2) removal systems embedded within associated fuel cell reforming-and-recirculating systems are described above in detail. The solvent-based carbon dioxide (CO.sub.2) removal systems embedded within associated fuel cell reforming-and-recirculating systems, and methods of operating such systems and devices are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the methods may also be used in combination with other systems requiring heat recovery and CO.sub.2 removal methods, and are not limited to practice with only the solvent-based CO.sub.2 removal systems, fuel cell reforming-and-recirculating systems, and methods as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many other combined cycle power plant heat recovery and CO.sub.2 removal applications that are currently configured to receive and accept fuel cells, e.g., and without limitation, distributed generation facilities in remote areas and industrial facilities.

(27) Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.

(28) This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.