EXHAUST GAS ENERGY RECOVERY CONVERTER

Abstract

An energy recovery converter for exhaust gases or waste heat is provided. The converter includes a membrane electrode assembly (MEA), an exhaust gas having a first molecular oxygen content, and an external electrical load. The MEA includes a first electrode, a second electrode and an oxygen ion conductive membrane sandwiched between the first and second electrodes. Each of the first and second electrodes includes at least one oxidation catalyst configured to promote an electrochemical reaction. The second electrode of the MEA is exposed to the exhaust gas and the first electrode of the MEA is exposed to a gas having a second molecular oxygen content. The second molecular oxygen content is higher than the first molecular oxygen content. The external electrical load is connected between the first and second electrodes of the MEA.

Claims

1. An energy recovery converter for exhaust gases or waste heat, the converter comprising: a membrane electrode assembly including a first electrode, a second electrode and an oxygen ion conductive membrane sandwiched between the first and second electrodes, each of the first and second electrodes including at least one oxidation catalyst configured to promote an electrochemical reaction; an exhaust gas having a first molecular oxygen content, the second electrode of the membrane electrode assembly being exposed to the exhaust gas and the first electrode of the membrane electrode assembly being exposed to a gas having a second molecular oxygen content, the second molecular oxygen content being higher than the first molecular oxygen content; and an external electrical load connected between the first and second electrodes of the membrane electrode assembly.

2. The energy recovery converter according to claim 1, wherein a fuel is mixed with oxygen rich gas and burned in order to produce the exhaust gas.

3. The energy recovery converter according to claim 1, wherein the at least one oxidation catalyst is a platinum/palladium catalyst.

4. The energy recovery converter according to claim 1, further comprising a first chamber containing at least one reduction catalyst.

5. The energy recovery converter according to claim 4, wherein the at least one reduction catalyst is selected from the group consisting of rhodium and platinum.

6. The energy recovery converter according to claim 5, wherein the at least one reduction catalyst is rhodium.

7. The energy recovery converter according to claim 6, wherein the exhaust gas includes hydrocarbons, nitrogen oxides and water, and wherein, in the first chamber, the hydrocarbons and water in the exhaust gas are converted to water, carbon dioxide, nitrogen and carbon monoxide by the rhodium catalyst.

8. The energy recovery converter according to claim 7 wherein oxygen from the gas having the second molecular oxygen content enters the first electrode and is oxidized by the at least one oxidation catalyst contained therein to produce oxygen ions, wherein the oxygen ions are conducted through the oxygen ion conductive membrane and electrons are released to the external electrical load, and wherein the electrons are transferred from the external electrical load to the second electrode and combine with the oxygen ions to oxidize the carbon monoxide and any unreacted hydrocarbons at their respective reaction potentials.

9. A method for purifying exhaust gas from a mechanical engine using a converter, the converter comprising: a catalytic reduction chamber; a membrane electrode assembly including a first electrode, a second electrode and an oxygen ion conductive membrane sandwiched between the first and second electrodes, each of the first and second electrodes including at least one oxidation catalyst configured to promote an electrochemical reaction; an exhaust gas containing hydrocarbons, nitrogen oxides and water, the exhaust gas having a first molecular oxygen content, the second electrode of the membrane electrode assembly being exposed to the exhaust gas, the first electrode of the membrane electrode assembly being exposed to a gas having a second molecular oxygen content such that oxygen contained within the gas enters the first electrode, the second molecular oxygen content being higher than the first molecular oxygen content; and an external electrical load connected between the first and second electrodes of the membrane electrode assembly, the method comprising: transporting the exhaust gas to the catalytic reduction chamber, such that the hydrocarbons, nitrogen oxides and water of the exhaust gas are converted to reaction gases, the reaction gases being elemental hydrogen, carbon monoxide, carbon dioxide and elemental nitrogen; transporting a stream containing the reaction gases, water and any unburned hydrocarbons from the catalytic reduction chamber to the second electrode; oxidizing the oxygen from the gas having the second molecular oxygen content to produce oxygen ions, such that the oxygen ions are conducted through the oxygen ion conductive membrane and electrons are simultaneously released to the external electrical circuit; and transporting the electrons from the external electrical circuit to the first electrode to combine with the oxygen ions and for an oxidation reaction with the carbon monoxide and unburned hydrocarbons at their respective reaction potentials.

10. The method according to claim 9, wherein the at least one oxidation catalyst is a platinum/palladium catalyst.

11. The method according to claim 9, wherein the catalytic reduction chamber contains a catalyst selected from the group consisting of rhodium and platinum.

12. The method according to claim 11, wherein the catalyst is rhodium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The following detailed description of preferred embodiments of the present invention will be better understood when read in conjunction with the appended drawing. For the purposes of illustrating the invention, there is shown in the drawing an embodiment which is presently preferred. It is understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:

[0014] FIG. 1 shows a representative graph of exhaust gas temperatures of a piston-type internal combustion engine as a function of engine speed;

[0015] FIG. 2 provides a schematic view of a three-way catalytic converter according to a preferred embodiment of the present invention;

[0016] FIG. 3 provides a schematic view of a catalytic converter system including a piston-type internal combustion engine according to a preferred embodiment of the present invention;

[0017] FIG. 4 provides a schematic view of a catalytic converter system including a piston-type internal combustion engine according to another preferred embodiment of the present invention;

[0018] FIG. 4A provides an enlarged view of a section of an MEA stack assembly of the catalytic converter system shown in FIG. 4;

[0019] FIG. 5 provides a schematic view of a catalytic converter system including a turbine engine according to a preferred embodiment of the present invention;

[0020] FIG. 5A provides an enlarged view of enlarged view of an MEA stack of the catalytic converter system shown in FIG. 5; and

[0021] FIG. 6 provides a schematic view of a catalytic converter system including a scram jet engine according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0022] Referring to FIG. 2, there is shown an energy recovery converter 20 for exhaust gases or waste heat, namely a catalytic converter 20, and more particularly a three-way type catalytic converter 20, according to the present invention. The catalytic converter 20 reduces toxic gases and pollutant in exhaust gas, such as from an internal combustion engine, into less-toxic or harmful exhaust gas. The catalytic converter 20 includes a first chamber 12 and a second chamber 22. The first chamber 12 includes one or more reduction catalysts (and is therefore a catalytic reduction chamber) and the second chamber 22 includes one or more oxidation catalysts (and is therefore a catalytic oxidation chamber). Examples of the reduction catalysts include, but are not limited to, rhodium, platinum, transition metal macrocycles and chalgogenides. In a preferred embodiment, the reduction catalyst of the first chamber 12 is rhodium metal, and more particularly finely divided rhodium metal.

[0023] Engine exhaust gas or waste heat flow 11, herein generally referred to as exhaust gas, is initially provided to the first chamber 12 of the catalytic converter 20. In the first chamber 12, the reduction catalyst of the first chamber 12 reduces unburned hydrocarbons and water in the exhaust gas to form elemental hydrogen H.sub.2, by the following reaction:

Hydrocarbons+H.sub.2O.fwdarw.H.sub.2+CO

[0024] The elemental hydrogen produced in this reaction then reacts with nitrogen oxides in the exhaust gas, thereby reducing them to elemental nitrogen, by the following reaction:

2NO+2H.sub.2.fwdarw.N.sub.2+2H.sub.2O

[0025] The carbon monoxide formed from the reduction of the hydrocarbons and water is also available to reduce nitrogen oxides in the exhaust gas to elemental nitrogen and carbon dioxide, by the following reactions:

2NO+2CO.fwdarw.N.sub.2+2CO.sub.2

2CO+NO.sub.2.fwdarw.N.sub.2+2CO.sub.2

[0026] The stream 15 of reaction gases, water and partially combusted hydrocarbons then exits the first chamber 12 and travels to the second chamber 22. The second chamber 22 is equipped with, includes or is formed by an oxygen conductive membrane electrode assembly (MEA) stack 9. The MEA stack 9 includes first and second electrodes 14, 16 which sandwich an ion conductive membrane 18. The first electrode 14 is a cathode and the second electrode 16 is an anode. Each of the first and second electrodes 14, 16 contains a catalyst, and more preferably an oxidation catalyst. Examples of the oxidation catalysts that may be used include, but are not limited to, platinum, palladium, metal oxides, transition metal macrocycles and chalgogenides. Preferably, the oxidation catalyst contained in each of the first and second electrodes 14, 16 is a platinum/palladium catalyst.

[0027] Ambient air from the surrounding environment is introduced into the second chamber 22 through the MEA stack 9. More particularly, under the oxygen partial pressure differential between ambient air and the exhaust gas, the oxygen contained within the ambient air enters the anode 16 where it is oxidized by the catalyst contained therein. The resulting oxygen ions are conducted through the ion conductive membrane 18, as the released electrons are simultaneously released to an external circuit, such as an external load 19 or energy storage device 17. The circuit is completed at the cathode 14, where electrons from the external circuit combine with the oxygen ions conducted through the ion conductive membrane 18, for an oxidation reaction with carbon monoxide and any unreacted hydrocarbons at their respective reaction potentials.

[0028] Specifically, the partially combusted (i.e., any remaining unburned) hydrocarbons react with the oxygen conducted through the MEA stack 9 and are converted to carbon dioxide and water, and the carbon monoxide generated in the first chamber 12 and introduced into the second chamber 22 reacts with the oxygen conducted through the MEA stack 9 to form carbon dioxide, by the following reactions:

2CO+O.sub.2.fwdarw.2CO.sub.2

Hydrocarbons+O.sub.2.fwdarw.H.sub.2O+CO.sub.2

[0029] In addition to the oxidation reactions in the second chamber 22, power is also produced by the converter 20 under the oxygen partial pressure differential between ambient air and the exhaust gas. Power drawn under the Nernst potential results in an increase in the oxygen content in the exhaust stream, due to the additional oxygen introduced into the converter 20 via the MEA stack 9. More particularly, in addition to the electrochemical reaction potentials of the unreacted combustion products, the high temperature of the exhaust and its low oxygen partial pressure relative to the oxygen partial pressure of oxygen in ambient air produces a voltage in accordance with the Nernst equation:

[00001]$\begin{array}{cc}{V}_{\mathrm{open}\mathrm{circuit}}=\frac{\mathrm{RT}}{2F}\ln \left(P_{\mathrm{ratio}}\right)& \mathrm{Equation}1\end{array}$

[0030] where, R is the gas constant (8.31 J/mol-K), T is the cell operating temperature, F is Faraday's constant (96,487 C/mol). The hot exhaust gas is a source of heat of expansion for the oxygen of the ambient air which is conducted through the MEA stack 9 into the second chamber 22 under the partial pressure differential.

[0031] For example, oxygen is 20% of the ambient atmosphere and the partial pressure of oxygen in air is 2.94 psi. On the other, an exhaust gas stream, such as from an internal combustion engine, typically contains approximately 0.5% oxygen and the partial pressure of oxygen is 0.0735 psi. The resulting ratio of oxygen partial pressures between ambient air and a typical engine exhaust stream is therefore 40. As reflected in FIG. 1, the temperature of the exhaust stream of a representative engine running at 2,000 rpm is 500° C. The resulting open circuit voltage for the MEA stack 9, calculated by the Nernst equation, is therefore 123 mV. The resulting electrical energy may be supplied directly to the external load 19 or it may be stored within an energy storage device 17 until needed.

[0032] Referring to FIG. 3, there is shown a further embodiment of an energy recovery converter 30 for exhaust gases or waste heat, namely a catalytic converter system 30, according to the present invention. The converter system 30 includes a piston-type internal combustion engine 42, a first chamber 25 and a MEA stack 24. The internal combustion engine may be any conventional engine, and includes an exhaust gas manifold 44 through which exhaust gas 28 generated by the engine 42 travels. The exhaust gas 28 generated by the internal combustion engine 42 includes partially combusted fuel (i.e., hydrocarbons), carbon monoxide, nitrous oxide (NO.sub.x), carbon dioxide and residual oxygen. The converter system 30 functions as a three-way type catalytic converter capable of recovering waste energy from the exhaust gas stream 28.

[0033] The first chamber 25 preferably contains a reduction catalyst, and more preferably finely divided rhodium metal. As such, the first chamber 25 is a catalytic reduction chamber 25 and functions primarily to reduce hydrocarbons and water in the exhaust gas to elemental hydrogen and carbon monoxide, and to reduce nitrogen oxides to nitrogen, carbon dioxide and water.

[0034] Thus, the exhaust gas stream 36 leaving the catalytic reduction chamber 25 contains water, carbon monoxide, any remaining fuel and partially combusted hydrocarbons, nitrogen and carbon dioxide. Ideally, only trace amounts of oxygen remain, since any available oxygen would have reacted with the carbon monoxide or hydrocarbons. The exhaust gas stream 36 exiting the catalytic reduction chamber 25 then passes through the MEA stack 24.

[0035] The MEA stack 24 essentially serves as an oxidation reaction chamber for oxidizing the exhaust gas stream 36. The MEA stack 24 includes an oxygen ion conductive membrane 31 sandwiched between first and second electrodes 30, 32. Each of the first and second electrodes 30, 32 contains a catalyst, preferably an oxidation catalyst, and more preferably a platinum/palladium catalyst. The membrane 31 essentially serves as an oxygen ion conductive barrier between ambient air and the exhaust gas flow 36, with the first electrode 30 being positioned within and in contact with the flow of the exhaust gas stream 36 and second electrode 32 being exposed to ambient air. Under the oxygen partial pressure differential between ambient air and the exhaust gas stream 36, oxygen enters the second electrode 32 and is oxidized therein by the oxidation catalyst. The resulting oxygen ions are conducted through the membrane 31 as the released electrons are conducted externally to and through the external load 19. The circuit is completed at the first electrode 30, where electrons from the external circuit combine with oxygen ions conducted through the membrane 31 for oxidation reactions (by the oxidation catalyst contained therein) with carbon monoxide and unreacted hydrocarbons at their respective reaction potentials to form a mixture 38 containing water and an increased content of carbon dioxide, as discussed above with respect to the embodiment of FIG. 2.

[0036] The catalytic reduction chamber 25 and MEA stack 24 therefore serve the requirements of a catalytic converter by promoting oxidation and reduction of the reaction products in the engine exhaust gas stream. There is also an increase in the oxygen content, due to the excess oxygen which is conducted through the MEA stack 24 under the ambient air/exhaust gas oxygen pressure differential. Electrical energy produced by the conduction of oxygen through the MEA stack 24 may be supplied directly to the external load or it may be stored within energy storage device 17 until needed.

[0037] Referring to FIG. 4, there is shown another embodiment of an energy recovery converter for exhaust gases or waste heat according to the present invention. The converter 70 comprises an assembly comprising series and parallel electrical connections of MEA stacks in order to generate power at useful voltage and current levels in the form of a MEA stack assembly 73, in addition to a piston-type internal combustion engine 72 and a first chamber 75. Similar to that of the embodiment of FIG. 3, the internal combustion engine includes an exhaust gas manifold 74 through which exhaust gas 78 generated by the engine 72 travels. The exhaust gas 78 generated by the internal combustion engine 72 includes partially combusted fuel (i.e., hydrocarbons), carbon monoxide, nitrous oxide (NO.sub.x), carbon dioxide and residual oxygen. The first chamber 75 contains a reduction catalyst, preferably finely divided rhodium, and functions primarily to reduce hydrocarbons and water in the exhaust gas 78 to elemental hydrogen and carbon monoxide, and to reduce nitrogen oxides to nitrogen, carbon dioxide and water.

[0038] Thus, the exhaust gas stream 76 leaving the catalytic reduction chamber 75 contains water, carbon monoxide, any remaining fuel and partially combusted hydrocarbons, nitrogen, and carbon dioxide. Ideally, only trace amounts of oxygen remain, since any available oxygen would have reacted with the carbon monoxide or hydrocarbons. The exhaust gas stream 76 exiting the catalytic reduction chamber 75 then passes through an MEA stack assembly 73.

[0039] In this embodiment, the MEA stack assembly 73 is configured in a plurality of sections, preferably three sections 57a, 57b and 57c. The sections 57a, 57b, 57c are preferably electrically connected in series by connectors 59 to yield a higher net output voltage. An enlarged view of section 57a is shown in FIG. 4A, with sections 57b and 57c being identical to section 57a. Referring to FIG. 4A, each section 57a, 57b, 57c of the MEA stack assembly 73 includes a first plurality of electrodes 58 which are electrically connected in parallel with each other and to a negative terminal 53, and a second plurality of electrodes 54 which are electrically connected in parallel with each other and to a positive terminal 51. A membrane 52 is positioned between each pair of first and second electrodes 58, 54. Each of the first and second electrodes 58, 54 contains a catalyst, preferably a platinum/palladium catalyst. The first plurality of electrodes 58 are exposed to ambient air and the second plurality of electrodes 54 are in contact with the exhaust gas.

[0040] Due to the ambient air/exhaust gas oxygen pressure differential, oxygen from the ambient air enters the first plurality of electrodes 58. The entering oxygen is oxidized by the catalyst contained within each of the first electrodes 58, the electrons are released to the negative terminal 53 and the oxygen ions 56 are conducted through the respective membrane 52 to the respective second electrode 54. The electrons are then routed through an external circuit via the negative and positive terminals 53 and 51 and ultimately to the respective second electrodes 54, where they combine with the oxygen ions for an oxidation reaction with carbon monoxide and any unreacted hydrocarbons at their respective reaction potentials by the catalyst contained within the second electrodes 54, in the same manner as described above with respect to FIGS. 2 and 3. The second electrodes 54 therefore serve the requirements of a catalytic converter by promoting oxidation reactions of products in the engine exhaust gas stream. As such, the partially combusted fuel (e.g., hydrocarbons) and carbon monoxide undergo reactions with the oxygen conducted through the MEA stack assembly 73 to form a mixture 78 containing water and an increased content of carbon dioxide.

[0041] In addition to the oxidation reactions, power is also produced from the oxygen partial pressure differential between the abundant oxygen available from ambient air and the depleted oxygen content of the engine exhaust gas stream in accordance with the Nernst equation, as described above with respect to FIG. 2. The expansion of oxygen through the converter under the partial pressure differential generates electricity directly and results in an increase in oxygen content in the exhaust stream. The electrical energy produced by the MEA stack assembly 73 may be supplied directly to external load 19 or it may be stored within an energy storage device (not shown in FIG. 4) until needed.

[0042] Referring to FIG. 5, there is shown another embodiment of an energy recovery converter for exhaust gases or waste heat according to the present invention. The converter is part of and configured to extract energy from a turbine engine 90. The turbine engine 90 includes air intake section 60, a turbine air compressor section 62, a combustion chamber 66, a catalytic reduction section (not shown, but similar to the catalytic reduction section 25 shown in FIG. 3) and a power turbine section 64. Fuel and a portion of the compressed air from the compressor section 62 are supplied to the combustion chamber 66 where the fuel is burned. The rest of the compressed air flows around the outside of the combustion chamber 66.

[0043] The exhaust in the combustion chamber 66 includes water (H.sub.2O) and carbon dioxide (CO.sub.2) from the hydrocarbon fuel/oxygen burning reaction. Further, nitrogen and oxygen in the ambient intake air react at high temperatures in the combustion chamber 66 to produce nitrous oxide (NO.sub.x). Incomplete fuel combustion products, such as carbon monoxide (CO) and hydrocarbons (HC), are also present in the engine exhaust in the combustion chamber 66. The combustion process renders the exhaust substantially depleted of oxygen.

[0044] The converter includes a MEA stack assembly, and more particularly first and second MEA stacks 68. The first and second MEA stacks 68 are positioned in the wall of the combustion chamber 66, particularly in the post-ignition region. An enlarged view of the one of the MEA stacks 68 is shown in FIG. 5A. Each of the MEA stacks 68 includes an ion conductive membrane 65 sandwiched between a pair of electrodes 61, 63. Each MEA stack 68 is positioned such that the first electrode 61 is exposed to the turbine engine combustion exhaust gases and the second electrode 63 is exposed to the compressed air flow.

[0045] The membrane 65 of each MEA stack 68 is an oxygen ion conductive barrier between ambient air and the exhaust gas flow. Each of the first and second electrodes 61 and 63 contains a catalyst, preferably an oxidation catalyst, and more preferably a platinum/palladium catalyst. Under the oxygen partial pressure differential between compressed air and the exhaust gas, oxygen enters the second electrode 63 and is oxidized therein, and the resulting oxygen ions are conducted through the membrane 65 while the released electrons are conducted externally through a load (not shown in FIG. 5). The circuit is completed at the first electrode 61, where electrons from the external circuit combine with the oxygen ions for oxidation reactions with carbon monoxide and any unreacted hydrocarbons in the exhaust stream at their respective reaction potentials to form a mixture containing water and an increased carbon dioxide content. The first electrode 61 of each MEA stack 68 serves the requirements of a catalytic converter by promoting oxidation of the reaction products in the engine exhaust stream.

[0046] In addition to the oxidation reactions, power is also produced from the oxygen partial pressure differential between the abundant oxygen available from ambient air and the depleted oxygen content of the engine exhaust stream in accordance with the Nernst equation. The expansion of oxygen through the converter under the partial pressure differential generates electricity directly and results in an increase in the oxygen content in the gases exiting the combustion chamber 66.

[0047] Referring to FIG. 6, there is shown another embodiment of an energy recovery converter for exhaust gases or waste heat according to the present invention. The converter is part of and configured to extract energy from a scram jet engine. The scram jet engine includes an air inlet section 70, an isolator section 72, a combustor section 76 and a nozzle section 74. The converter also includes a MEA assembly, namely first and second MEA stacks 78. The MEA stacks 78 partially separate the flow through the combustion section 76 into a fuel burning region and an air flow-through region. Fuel and a portion of the air from isolator section 72 are supplied to the fuel burning region. The remaining portion of the air passes on the opposite side of each MEA stack 78, such that a first electrode 71 of each stack 78 is exposed to the combustion gases and a second electrode 73 of each stack 78 is exposed to oxygen-rich air flow provided directly from the isolator section 72.

[0048] A membrane 75 which is positioned between the first and second electrodes 71, 73 of each MEA stack 78 is an oxygen ion conductive barrier which separates oxygen-rich air flow exposed to the second electrode 73 from the oxygen-depleted post-combustion gas flow exposed to the first electrode 71. Each of the first and second electrodes 71, 73 contains a catalyst, preferably an oxidation catalyst, and more preferably a platinum/palladium catalyst. Under the oxygen partial pressure differential between the air from the isolator section 72 and the exhaust gas, oxygen enters the second electrode 73 and is oxidized therein by the catalyst contained therein, and the resulting oxygen ions are conducted through the ion conductive membrane 75 to the first electrode 71, while the released electrons are conducted externally through a load (not shown in FIG. 6). The circuit is completed at the first electrode 71, where electrons from the external circuit combine with the oxygen ions conducted through the ion conductive membrane 75 for oxidation reactions with carbon monoxide and any unreacted hydrocarbons in the combustion gas stream at their respective reaction potentials. The converter includes a catalytic reduction section and exhaust gas composition as described above with respect to FIGS. 3-5.

[0049] The first electrode 71 serves the requirements of a catalytic converter by promoting oxidation of the reaction products in the engine exhaust stream. As such, the partially combusted fuel (e.g., hydrocarbons) and carbon monoxide undergo reactions with the oxygen conducted through the MEA to form a mixture containing water and an increased carbon dioxide content.

[0050] In addition to the oxidation reactions, power is also produced from the oxygen partial pressure differential between the abundant oxygen available from ambient air and the depleted oxygen content of the fuel combustion products, in accordance with the Nernst equation. The expansion of oxygen through the converter under the partial pressure differential generates electricity directly.

[0051] Any of the above-described converters/systems may be suitable for supplying processing heat to an industrial facility. An exemplary facility would one in which heat is needed for processing and separating metal from raw ore, wood or paper drying processes. Essentially, an industrial processing facility that produces oxygen-depleted gas from the oxygen-rich gas in the process of burning fuel as a source of process heat. The residual heat and incomplete combustion products are often wasted.

[0052] It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.