WO2008150370A1

WO2008150370A1 - Stoichiometric engine system utilizing reformed exhaust gas

Info

Publication number: WO2008150370A1
Application number: PCT/US2008/006455
Authority: WO
Inventors: Weidong Gong; Martin L. Willi; Balamurugesh Thirunavukarasu
Original assignee: Caterpillar Inc.
Priority date: 2007-05-31
Filing date: 2008-05-21
Publication date: 2008-12-11
Also published as: US20080295501A1

Abstract

A power system (10) is provided having an engine (12) configured to combust a substantially stoichiometric air/fuel mixture and produce a flow of exhaust. The power system also has an exhaust passageway (24) fluidly connected to the engine. In addition, the power system has a catalytic device (34) situated within the exhaust passageway. Furthermore, the system has an exhaust gas recirculation loop (36) fluidly connected to the exhaust passageway. The system further has a steam fuel reformer (42) situated within the exhaust gas recirculation loop.

Description

STOICHIOMETRIC ENGINE SYSTEM UTILIZING REFORMED

EXHAUST GAS

Technical Field The present disclosure is directed to a stoichiometric engine system, and more particularly, to a stoichiometric engine system utilizing reformed exhaust gas.

Background

Internal combustion engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art exhaust a complex mixture of air pollutants. The air pollutants are composed of solid particulate matter and gaseous compounds including nitrous oxides (NOx) and carbon monoxide (CO). In addition, some types of fuels such as, for example, diesel fuels often contain sulfur that, at times, convert to potentially corrosive and environmentally unfriendly byproducts. Due to increased attention on the environment, exhaust emission standards have become more stringent, and the amount of solid particulate matter and gaseous compounds emitted to the atmosphere from an engine is regulated depending on the type of engine, size of engine, and/or class of engine. U.S. Patent No. 6,508,209 to Collier, Jr. ("the '209 patent") discloses a natural gas powered engine system employing an oxidation catalyst and exhaust gas recirculation (EGR) system to reduce emissions in the engine exhaust. In the system, air and natural gas fuel is combined and combusted in a plurality of combustion chambers. Exhaust from some of the combustion chambers is directed to the oxidation catalyst where carbon monoxide is converted to carbon dioxide and released into the atmosphere. Exhaust from the rest of the combustion chambers is mixed with natural gas fuel and recirculated back to the intake of the engine. By recirculating exhaust gas back to the engine, the peak combustion temperature in the combustion chambers is lowered, which reduces NOx production. In order to prevent engine misfires, which can result from combusting an air/fuel mixture containing a significant amount of recirculated exhaust, the exhaust/fuel mixture is conditioned in a steam fuel reformer. In addition, any water vapor remaining in the exhaust/fuel mixture may be removed by condensation in a heat exchanger before the exhaust/fuel mixture is combined with ambient air and combusted in the combustion chambers.

Although the system in the '209 patent may reduce emissions in the engine exhaust, it does not address the removal of particulate matter and other hydrocarbon emissions from the exhaust before the exhaust is released into the atmosphere. Many petroleum based fuels generate particulate matter and other hydrocarbons when combusted. Such particulate matter and other hydrocarbon emissions are considered pollutants and are strictly regulated. By not providing a means to reduce such emissions, the engine system might not meet exhaust emissions standards.

The disclosed system is directed to overcoming one or more of the problems set forth above.

Summary of the Invention

In one aspect, the present disclosure is directed toward a power system that includes an engine configured to combust a substantially stoichiometric air/fuel mixture and produce a flow of exhaust. The power system also includes an exhaust passageway fluidly connected to the engine. In addition, the power system includes a catalytic device situated within the exhaust passageway. Furthermore, the system includes an exhaust gas recirculation loop fluidly connected to the exhaust passageway. The system further includes a steam fuel reformer situated within the exhaust gas recirculation loop. Consistent with another aspect of the disclosure, a method is provided for treating exhaust gas. The method includes combusting a substantially stoichiometric fuel/air mixture. The method also includes catalyzing an exhaust gas generated by the combustion of the air/fuel mixture. In addition, the method includes redirecting at least a portion of the exhaust gas so that it is combusted with the fuel/air mixture. The method further includes enriching the redirected exhaust gas.

Brief Description of the Drawings

Fig. l is a diagrammatic illustration of a power system according to an exemplary disclosed embodiment of the present disclosure; and

Fig. 2 is a diagrammatic illustration of a power system according to another exemplary disclosed embodiment of the present disclosure.

Detailed Description

Fig. 1 illustrates an exemplary power system 10 having an engine 12 that combusts a mixture of air and fuel to generate a mechanical output and a flow of exhaust. For the purposes of this disclosure, engine 12 is depicted and described as a natural gas powered engine. One skilled in the art will recognize, however, that engine 12 may be any other type of internal combustion engine such as, for example, another gaseous fuel-powered engine, a gasoline engine, a four-stroke diesel engine, or any engine capable of being powered by a petroleum based or non-petroleum based fuel. Engine 12 may include an engine block 14 defining a plurality of cylinders 16, an air/fuel intake manifold 18 fluidly connecting cylinders 16 to an air/fuel intake passageway 20, and an exhaust manifold 22 fluidly connecting cylinders 16 to an exhaust passageway 24. A piston (not shown) may be slidably disposed within each cylinder 16 to reciprocate between a top-dead-center position and a bottom-dead-center position, and a cylinder head (not shown) may be associated with each cylinder 16. Cylinder 16, the piston, and the cylinder head may form a combustion chamber 26 fluidly connected to air/fuel intake manifold 18 and exhaust manifold 22 via fluid passageways 28 and 30, respectively. In the illustrated embodiment, engine 12 includes six such combustion chambers 26. However, it is contemplated that engine 12 may include a greater or lesser number of combustion chambers 26 and that combustion chambers 26 may be disposed in an "in-line" configuration, a "V" configuration, or in any other suitable configuration.

Power system 10 may also include an exhaust treatment system 32 for removing and/or reducing the amount of pollutants in the exhaust produced by engine 12 and released into the atmosphere. Exhaust treatment system 32 may include a catalyzed member 34 situated within exhaust passageway 24 and an exhaust gas recirculating (EGR) loop 36 for directing a predetermined portion of exhaust back to air/fuel intake 20 of engine 12. Catalyzed member 34 may include a structure coated with or otherwise containing a catalyst to reduce the by-products of combustion. In one example, the structure may be coated with a 3 -way catalyst that supports the reduction of particulate matter, hydrocarbons, carbon monoxide (CO), and NOx. The catalyst may include, for example, a base metal oxide, a molten salt, or a precious metal that catalytically reacts with particulate matter, CO, and NOx.

EGR loop 36 may include components that cooperate to redirect a portion of the exhaust generated by engine 12 from exhaust passageway 24 to air/fuel intake passageway 20. Specifically, EGR loop 36 may include an inlet port 38, a recirculation valve 40, a reformer 42, an EGR cooler 44, and a discharge port 46. In addition, inlet port 38 may be fluidly connected to reformer 42 via a fluid passageway 48, steam fuel reformer 42 may be fluidly connected to EGR cooler 44 via a fluid passageway 50, and discharge port 46 may be fluidly connected to EGR cooler 44 via fluid passageway 52. Furthermore, recirculation valve 40 may be disposed within fluid passageway 48, between inlet port 38 and reformer 42. It is contemplated that inlet port 38 may be located upstream or downstream of any turbochargers present (if any) and/or additional emission control devices disposed within exhaust passageway 24 (not shown) such as, for example, particulate filters and catalytic devices. Recirculation valve 40 may be located to regulate the flow of exhaust through EGR loop 36. Recirculation valve 40 may be any type of valve such as, for example, a butterfly valve, a diaphragm valve, a gate valve, a ball valve, a globe valve, or any other valve known in the art. In addition, recirculation valve 40 may be solenoid-actuated, hydraulically-actuated, pneumatically-actuated or actuated in any other manner to selectively restrict the flow of exhaust through fluid passageway 48.

Reformer 42 may remove or reduce the amount of water vapor in the exhaust gas flowing through EGR loop 36. In particular, reformer 42 may facilitate a reaction between exhaust gas flowing through reformer 42, and a fuel over a catalyst. The catalyst may be any material that may promote a reforming reaction such as, for example, nickel. In addition, the fuel may be any petroleum based or non-petroleum based fuel or natural gas. The fuel may be supplied from a source (not shown) such as, for example, a tank, a pipeline, or any other source known in the art capable of supplying fuel to EGR loop 36. Furthermore, the fuel may be directed to EGR loop 36 via a fuel passageway 54 upstream of reformer 42. It is contemplated that the source supplying fuel to EGR loop 36 may be either the same source supplying fuel to air/fuel intake passageway 20 or a separate source. It is further contemplated that any percentage including 100 percent of the fuel combusted by engine 12 may be supplied via fuel passageway 54.

The reaction mechanism between the exhaust gas and fuel may be summarized in the following equations:

CH₄ + H₂ o^v » co + 3H₂ (i) CO + H₂O^V O CO₂ + H₂ (2)

As shown in equation 1 , methane (CH₄) in the fuel may combine with water vapor (H₂O) in the exhaust gas to produce carbon monoxide (CO) and hydrogen (H₂). The shift reaction summarized by equation 2 may kinetically follow the reaction summarized by equation 1. In particular, the water vapor (H₂O) remaining in the exhaust gas and fuel mixture after the first reaction may combine with carbon monoxide (CO) also produced by the first reaction to generate carbon dioxide (CO₂) and hydrogen (H₂), thereby removing the remaining water vapor from the exhaust gas and enriching the exhaust gas with hydrogen and carbon monoxide.

Cooler 44 may be configured to cool the exhaust flowing through EGR loop 36. Cooler 44 may include a liquid-to-air heat exchanger, an air-to-air heat exchanger, or any other type of heat exchanger known in the art for cooling an exhaust flow. It should be understood that the size of cooler 44 may be reduced because the reaction facilitated by reformer 42 may be an endothermic reaction. The heat required for the reaction may be provided by thermal energy contained within the recirculated exhaust gas, thereby cooling the exhaust gas. Because of the cooling effect of the endothermic chemical reaction facilitated by reformer 42, it is contemplated that cooler 44 may be omitted, if desired.

Fig. 2 illustrates another exemplary embodiment of power system 10 that positions catalyzed member 34 downstream of EGR loop 36. Positioning catalyzing member 34 downstream of EGR loop 36 may decrease the size of reformer 42 and the amount of fuel needed to remove or reduce water vapor from the recirculated exhaust gas. This is because the reaction facilitated by catalyzing member 34 may generate water vapor and increase the amount of water vapor in the exhaust. In configurations where catalyzing member 34 is positioned upstream of EGR loop 36, exhaust gas being recirculated back to engine 12 through EGR loop 36 may include the water vapor generated by catalyzing member 34 in addition to water vapor that may already be present in the exhaust gas. Therefore, more fuel and a larger reformer 42 may be required to react with and remove the additional amount of water vapor. However, in configurations where catalyzing member 36 is positioned downstream of EGR loop 36, water vapor may be generated in the portion of the exhaust gas that may not be recirculated back to engine 12. In such a configuration, reformer 42 may not need to handle the additional water vapor generated by catalyzing member 34.

It should be understood that positioning catalyzing device 34 downstream of EGR loop 36 may require additional emission control devices such as, for example, a particulate filter 56 located within exhaust passageway 24 upstream of EGR loop 36. Fuels that are combusted by engine 12 may include particulate matter or other elements that may corrode or foul the elements of EGR loop 36. It is contemplated that particulate filter 56 and/or other emission control devices may alternatively be situated within fluid passageway 48 of EGR loop 36, if desired.

Industrial Applicability

The disclosed engine system may reliably and efficiently remove or reduce emissions from exhaust that are released into the atmosphere. In particular, the combination of a 3 -way catalyst and an EGR loop may effectively reduce or remove NOx, hydrocarbons, and carbon monoxide from the engine exhaust. By utilizing a reformer, water vapor may be removed from the recirculated exhaust gas, which may increase the engine combustion flame speed. In addition, the reformer may enrich the recirculated exhaust gas, which may also increase the engine combustion flame speed. The increased flame speed combined with the increased heat capacity of the recirculated gas may ensure that the air/fuel mixture entering the engine may be stably combusted at temperatures adverse to NOx formation.

By utilizing a 3 -way catalytic device, particulate matter and other hydrocarbon emissions may be removed or reduced from the exhaust before it is released into the atmosphere. Therefore, the system may be powered by various petroleum based or non-petroleum based fuels and still meet stringent emissions standards.

It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed system without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.

Claims

1. A power system (10), comprising : an engine (12) configured to combust a substantially stoichiometric air/fuel mixture and produce a flow of exhaust; an exhaust passageway (24) fluidly connected to the engine; a catalytic device (34) situated within the exhaust passageway; an exhaust gas recirculation loop (36) fluidly connected to the exhaust passageway; and a steam fuel reformer (42) situated within the exhaust gas recirculation loop.

2. The power system of claim 1, further including a fluid passageway fluidly connected to the exhaust gas recirculation loop upstream of the steam fuel reformer, the fluid passageway being configured to direct at least a portion of the fuel to be combusted by the engine through the steam fuel reformer.

3. The power system of claim 1 , wherein the catalytic device includes a 3 -way catalyst.

4. The power system of claim 1, further including a heat exchanger (44) situated within the exhaust recirculation loop downstream of the steam fuel reformer.

5. The power system of claim 1, wherein the catalytic device is located downstream of the exhaust gas recirculation loop.

6. The power system of claim 1, wherein the catalytic device is located upstream of the exhaust gas recirculation loop.

7. A method for treating exhaust gas, comprising: combusting a substantially stoichiometric air/fuel mixture; catalyzing an exhaust gas generated by the combustion of the air/fuel mixture; redirecting at least a portion of the exhaust gas so that the redirected exhaust gas is combusted with the air/fuel mixture; and enriching the redirected exhaust gas/fuel mixture.

8. The method of claim 7, further including catalyzing the exhaust gas before redirecting at least a portion of the exhaust gas.

9. The method of claim 7, further including catalyzing the exhaust gas after a portion of the exhaust gas has been redirected.

10. The method of claim 7, further including mixing at least a portion of the fuel to be combusted with the redirected exhaust gas before the redirected exhaust gas mixes with air.