US20090173061A1

US20090173061A1 - OPTIMIZED REDUCTION OF NOx EMISSIONS FROM DIESEL ENGINES

Info

Publication number: US20090173061A1
Application number: US11/971,267
Authority: US
Inventors: Frederic Vitse; Dan Hancu; Benjamin Hale Winkler
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2008-01-09
Filing date: 2008-01-09
Publication date: 2009-07-09
Also published as: WO2009088636A1

Abstract

A system is provided for operating a diesel engine with reduced emissions of NOx. The system comprises a fuel tank adapted to directly or indirectly supply a first premixed fuel stream and a second premixed fuel stream, wherein each fuel stream comprises a primary fuel component and a reductant component. An engine is in fluid communication with the fuel tank, whereby the engine is configured to receive the first premixed fuel stream and create an exhaust stream. The system includes an emission treatment unit to treat the exhaust stream, and a separation unit configured to receive the second premixed fuel stream. The separation unit is also configured to separate the second premixed fuel stream into a first fraction stream and a second fraction stream, and supply the first fraction stream to the exhaust stream. The first fraction stream comprises a higher concentration of the reductant component than the second fraction stream. A temperature sensor measures the temperature of the exhaust stream, whereby the concentration of the reductant component in the exhaust stream is controlled based on the measured temperature of the exhaust stream. The invention further provides a method for operating a diesel engine with reduced emissions of NOx. The method includes supplying a first premixed fuel stream to an engine, wherein the engine is configured to create an exhaust stream. A second premixed fuel stream is supplied to a separation unit, wherein the first and second premixed fuel streams each comprise a reductant component and a primary fuel component. At least a portion of the second premixed fuel stream is separated into a first fraction stream and a second fraction stream via the separation unit, wherein the first fraction stream comprises a higher concentration of the reductant component than the second fraction stream. At least a portion of the first fraction stream is supplied to the exhaust stream. The temperature of the exhaust stream is measured, and the concentration of the reductant component in the exhaust stream is controlled based on the measured temperature. The selective catalytic reduction of NOx present in the exhaust stream is performed.

Description

FIELD OF THE INVENTION

This invention relates to the reduction of diesel engine NOx emissions, and more specifically to on-board optimization of the composition of reducing agents injected into the engine exhaust for an improved selective catalytic reduction of NOx.

BACKGROUND OF THE INVENTION

Current emission control regulations necessitate the reduction of pollutant species in diesel engine exhaust. NOx, principally NO and NO₂, contributes to smog, ground level ozone formation and acid rain. NO is produced in large quantities at the high combustion temperatures associated with diesel engines. NO₂is formed principally by the post oxidation of NO in the diesel exhaust stream. Exhaust aftertreatment devices achieve NOx reduction by using a reductant agent. The reductant agent is added to the exhaust gas entering the aftertreatment device and reacts with NOx over a catalyst in a process of selective catalytic reduction. In the known selective catalytic reduction process, NOx is reduced to N₂through a reaction with NH₃(or urea as a source of NH₃) over a selective catalyst.
Various technologies have been investigated for NOx removal from lean-burn engine exhaust. As noted above, one technology involves the selective catalytic reduction (SCR) of NOx with ammonia as a reducing agent. One disadvantage of this technology is that it requires a source of ammonia which may be either liquid ammonia stored under high pressure or an aqueous solution of urea which decomposes prior to or on the SCR catalyst to produce ammonia. This limits ammonia-SCR to large stationary applications. Another disadvantage is the need to develop the infrastructure to supply ammonia or urea to vehicles using this technology.
Different methods for treating diesel engine exhaust with reducing agents for the selective catalytic reduction of NOx have been attempted. However the achievement of lower emissions of NOx especially under transient conditions continues to present a technical challenge.

SUMMARY OF THE INVENTION

Briefly stated, embodiments of the present invention provide methods and systems for improved emission control in internal combustion engines fueled by a diesel fuel. Embodiments of the invention provide for lower emissions of NOx through an on-board optimization of the supply of reducing agents to the engine exhaust. The embodiments of the present invention are particularly useful under engine exhaust transient conditions, for example, when the engine exhaust temperature is changing due to engine warm-up or due to a change in engine operating parameters.
Briefly stated, in accordance with one embodiment of the invention, there is provided a system for operating a diesel engine with reduced emissions of NOx comprising a fuel tank adapted to directly or indirectly supply a first premixed fuel stream and a second premixed fuel stream, each fuel stream comprising a primary fuel component and a reductant component; an engine in fluid communication with the fuel tank, wherein the engine is configured to receive the first premixed fuel stream and create an exhaust stream; an emission treatment unit to treat the exhaust stream; a separation unit configured to receive the second premixed fuel stream, the separation unit also configured to separate the second premixed fuel stream into a first fraction stream and a second fraction stream, and supply the first fraction stream to the exhaust stream, wherein the first fraction stream comprises a higher concentration of the reductant component than the second fraction stream; and a temperature sensor to measure the temperature of the exhaust stream, whereby the concentration of the reductant component in the exhaust stream is controlled based on the measured temperature of the exhaust stream.
In accordance with another embodiment of the invention, there is provided a method for operating a diesel engine with reduced emissions of NOx including supplying a first premixed fuel stream to an engine, wherein the engine is configured to create an exhaust stream; supplying a second premixed fuel stream to a separation unit, wherein the first and second premixed fuel streams each comprise a reductant component and a primary fuel component; separating at least a portion of the second premixed fuel stream into a first fraction stream and a second fraction stream via the separation unit, wherein the first fraction stream comprises a higher concentration of the reductant component than the second fraction stream; supplying at least a portion of the first fraction stream to the exhaust stream; measuring the temperature of the exhaust stream; controlling the concentration of the reductant component in the exhaust stream based on the measured temperature of the exhaust stream; and performing a selective catalytic reduction of NOx present in the exhaust stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary system for operating a diesel engine with reduced emissions in accordance with an embodiment of the invention.

FIG. 2 is a schematic diagram of an exemplary system for operating a diesel engine with reduced emissions in accordance with an alternative embodiment of the invention.

FIG. 3 is a schematic diagram of an exemplary for operating a diesel engine with reduced emissions in accordance with an alternative embodiment of the invention.

FIG. 4 is a schematic diagram of an exemplary for operating a diesel engine with reduced emissions in accordance with an alternative embodiment of the invention.

FIG. 5 is a schematic diagram of a separation unit in accordance with an alternative embodiment of the invention.

FIG. 6 illustrates a method for operating a diesel engine with reduced emissions in accordance with an embodiment of the invention.

FIG. 7 is a graph illustrating the reduction of NOx in diesel and diesel/ethanol blends at different temperatures.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of an exemplary system 10 for reducing NOx emissions from an engine in accordance with one embodiment of the invention. The system 10 includes an engine 12 that is directly supplied with a first premixed fuel stream 14 from a fuel tank 16. An on-board separation unit 18 is directly supplied with a second premixed fuel stream 20 from the fuel tank 16. If desired, the fuel tank 16 may be adapted to indirectly supply the first premixed fuel stream 14 to the engine 12 and the second premixed fuel stream 20 to the separation unit 18 via a single premixed fuel stream (not shown) exiting the fuel tank that is split, or comprises a slip stream, to form the first and second premixed fuel streams. The premixed fuel streams 14 and 20 each include a primary fuel component, generally comprising a fuel suitable for operation of engine 12, and a reductant component suitable for use in NOx reduction as discussed below. The separation unit 18 separates the reductant component from the primary fuel component in the second premixed fuel stream 20.
The fuel tank 16 may be adapted to supply the first premixed fuel stream 14 to a first fuel pump 22, wherein the first fuel pump 22 is adapted to pump the first premixed fuel stream to the engine 12. The fuel tank 16 is also adapted to supply the second premixed fuel stream 20 to a second fuel pump 24. The second fuel pump 24 pumps the second premixed fuel stream 20 to the separation unit 18. A portion of the first premixed fuel stream 14 is burnt in the engine 12 during operation of the engine and an emission of exhaust gases containing NOx is produced thereby. The exhaust gases, thus produced, are discharged through an exhaust stream 30. The exhaust stream 30 carries the exhaust gases to an emission treatment unit 32 where the exhaust stream is treated by selective catalytic reduction. The resulting treated exhaust stream 38 containing reduced NOx emissions, is exhausted into the atmosphere.
The systems and methods of the invention allow for the use of one fuel tank 16 for carrying the fuel and reductant together instead of requiring an extra storage tank for the SCR reductant in addition to the fuel tank. This is advantageous from an implementation and distribution point of view. For example, the system can be installed on existing locomotive engines.
Various separation techniques may be used to achieve the separation of the reductant component from the primary fuel component, including but not limited to selective filtration, membrane-assisted separation, ultra-filtration, vibration and ultrasonic-assisted separation, centrifugation, and evaporation. For example, the separation unit 18 may be a flash, distillation and/or membrane separation unit. Additional examples of separation units include cross-flow reactors, parallel plate reactors with substantially flat membranes, cylindrical reactors with wound-up membranes, and fiber-based membrane reactors. The separation unit 18 separates the second premixed fuel stream 20 into a first fraction stream 42 and a second fraction stream 44. Because the separation unit 18 serves to separate the reductant component from the primary fuel component, the first fraction stream 42 includes a higher concentration of the reductant component than the second fraction stream 44. The first fraction stream 42 is supplied to the emission treatment unit 32 via exhaust stream 30. The separation unit 18 may supply the first fraction stream 42 to a first fraction pump 45, wherein the first fraction pump is adapted to pump at least a portion of the first fraction stream to the emission treatment unit 32 via exhaust stream 30, as shown in FIG. 1. In another embodiment of the invention (not shown), a portion of the first fraction stream 42 may be sent back to the fuel tank 16 or to the engine 12 directly.
A temperature sensor 46 is used to measure the temperature of the exhaust stream 30. The temperature sensor 46 may be a thermocouple, a thermistor, a resistance temperature detector (RTD), an infrared temperature probe, or another temperature sensing element operating either in direct or indirect contact with the measured media, as known in the art. A range of installation options of the temperature sensor 46 is possible, including, but not limited to measurements taken directly in exhaust stream 30, measurements taken through a contact with exhaust stream 30 conduit wall, or contact-free measurements via infrared radiation.
The temperature sensor 46 sends a signal representing the temperature of the exhaust stream 30 to a second premixed fuel controller 47. The second premixed fuel controller 47 integrates the processed information and determines if the system parameters are indicative of proper control of the exhaust stream 30, and further determines whether there is a need for supply of reductants and/or fuel to the exhaust stream 30 and emission treatment unit 32. For example, the temperature sensor 46 may send a signal to the second premixed fuel controller 47 indicating that the temperature of the exhaust stream 30 is decreasing. As a result, the second premixed fuel controller 47 will cause the second fuel pump 24 to increase the flow of the second premixed fuel stream 20 entering the separation unit 18. Consequently, the amount of the first fraction stream 42 supplied to the exhaust stream 30 and subsequently supplied to the emission treatment unit 32, is increased, and the concentration of the reductant component in the exhaust stream 30 and emission treatment unit 32 is increased. Conversely, if the temperature sensor 46 sends a signal to the second premixed fuel controller 47 indicating that the temperature of the exhaust stream 30 is increasing, the concentration of the reductant component in the exhaust stream and emission treatment unit 32 may be decreased by reducing the flow of the second premixed fuel stream entering the separation unit 18.
The temperature sensor 46 may alternatively be used to directly control the amount of reductant component flow, i.e. the first fraction stream 42 flow, into the exhaust stream 30 and emission treatment unit 32. The temperature sensor 46 sends a signal representing the temperature of the exhaust stream 30 to a first fraction controller 64. The first fraction controller 64 regulates the flow of the first fraction stream 42 entering the exhaust stream 30, based on this signal. The first fraction pump 45 is in communication with the first fraction controller 64, and the controller 64 directly controls and monitors the operation of the first fraction pump to adjust the flow of the first fraction stream 42. Accordingly, the concentration of the reductant component in the exhaust stream 30 and emission treatment unit 32 is adjusted based on the temperature of the exhaust stream.
FIG. 2 displays an alternative embodiment wherein the second premixed fuel stream 20 is split, or comprises a fuel slip stream 21, that is supplied to the exhaust stream 30. The system includes a three way valve 48 which directs the flow of the second premixed fuel stream 20 into the separation unit 18, and also directs the flow of the fuel slip stream 21 into exhaust stream 30.
The second premixed fuel controller 47 controls the operation of the three way valve 48 to select whether the second premixed fuel stream 20 is supplied to the separation unit 18, or whether the fuel slip stream is supplied to the exhaust stream 30. Through control of the three way valve 48, the concentration of the reductant component in the exhaust stream 30 and emission treatment unit 32 may be increased or decreased based on the temperature of the exhaust stream, thereby providing an optimum concentration of the reductant component in the exhaust stream and emission treatment unit for the selective catalytic reduction of NOx.
In operation, the temperature sensor 46 sends a signal representing the temperature of the exhaust stream 30 to the second premixed fuel controller 47. The second premixed fuel controller 47 integrates the processed information and determines if the system parameters are indicative of proper control of the exhaust stream 30, and further determines whether there is a need for supply of reductants and/or fuel to the exhaust stream 30 and emission treatment unit 32. For example, the temperature sensor 46 may send a signal to the second premixed fuel controller 47 indicating that the temperature of the exhaust stream 30 is decreasing. As a result, the second premixed fuel controller 47 will cause the three way valve 48 to allow flow of the second premixed fuel stream 20 into the separation unit 18, and cease flow of the fuel slip stream 21 to the exhaust stream 30. Consequently, the amount of the first fraction stream 42 supplied to the exhaust stream 30, and subsequently supplied to the emission treatment unit 32 is increased, and the concentration of the reductant component in the exhaust stream 30 and emission treatment unit 32 is increased.
Conversely, the temperature sensor 46 may send a signal to the second premixed fuel controller 47 indicating that the temperature of the exhaust stream 30 is increasing. As a result, the second premixed fuel controller 47 will cause the three way valve 48 to cease flow of the second premixed fuel stream 20 into the separation unit 18, and if necessary, allow flow of the fuel slip stream 21 to the exhaust stream 30. Consequently, the amount of the first fraction stream 42 supplied to the exhaust stream 30, and subsequently supplied to the emission treatment unit 32 is decreased, and the concentration of the reductant component in the exhaust stream 30 and emission treatment unit 32 is decreased.
In the embodiments of the invention, the second fuel pump 24 and three way valve 48 may be used independently or simultaneously to adjust the flow of the second premixed fuel stream.
In an alternative embodiment shown in FIG. 3, the first premixed fuel stream 14 may be split, or comprise a fuel slip stream 15 that is supplied to exhaust stream 30. A third fuel pump 49 is adapted to pump the fuel slip stream 15 to the exhaust stream 30. A third premixed fuel controller 50 controls the flow of the fuel slip stream 15 into the exhaust stream 30 based on the temperature of the exhaust stream. For example, the temperature sensor 46 may send a signal to the third premixed fuel controller 50 indicating that the temperature of the exhaust stream 30 is decreasing. As a result, the third premixed fuel controller 50 will cause the third fuel pump 49 to decrease the flow of the fuel slip stream 15 supplied to the exhaust stream 30. Consequently, the amount of premixed fuel supplied to the exhaust stream 30 and subsequently supplied to the emission treatment unit 32, is decreased. Assuming that the flow of the second premixed fuel stream 20 into the separation unit 18 remains constant or is being increased, the ratio of the reductant component to the fuel component in exhaust stream 30 will increase. Accordingly the concentration of the reductant component in the exhaust stream 30 and emission treatment unit 32 is increased.
Conversely, the temperature sensor 46 may send a signal to the third premixed fuel controller 50 indicating that the temperature of the exhaust stream 30 is increasing. As a result, if necessary, the third premixed fuel controller 50 will cause the third fuel pump 49 to increase the flow of the fuel slip stream 15 supplied to the exhaust stream 30. Consequently, the amount of premixed fuel supplied to the exhaust stream 30 and subsequently supplied to the emission treatment unit 32, is increased. Assuming that the flow of the second premixed fuel stream 20 into the separation unit 18 remains constant or is being decreased, the ratio of the reductant component to the fuel component in exhaust stream 30 will decrease. Accordingly the concentration of the reductant component in the exhaust stream 30 and emission treatment unit 32 is reduced.
The first fuel pump 22, second fuel pump 24, first fraction pump 45, and third fuel pump 49, may each be an electrically actuated fuel pump. In another embodiment of the invention, the pumps 22, 24, 45 and 49 may be a fuel injector.
Referring to FIG. 4, a condenser 54 may be disposed in fluid communication with the separation unit 18 for condensing at least a portion of the first fraction stream 42 exiting the separation unit. Accordingly, the reductant component in the first fraction stream 42 can be condensed in the condenser 54 and injected into the exhaust stream 30 as a liquid, or it can be condensed and then stored in a holding tank 58, as shown in FIG. 4. The condensed first fraction stream 42′ is supplied to the exhaust stream 30. The second fraction fuel stream 44 is recycled to the fuel tank 16. In the embodiments of the invention, complete separation of the reductant component from the primary fuel component, while desired, is not necessary because most fuels can also be used as reductants in hydrocarbon based SCR. In another embodiment of the invention (not shown), a portion of the second fraction stream 44 may be sent to the engine 12 directly and burnt as the main fuel of the engine. In yet another embodiment of the system (not shown), if the system 10 is equipped with a diesel particulate filter that uses a fuel burner for regeneration, the second fraction 44 may be sent to the fuel burner.
Referring to FIGS. 1-4, the amount of reductant component that is separated can also be controlled using a NOx sensor 60 that is placed down stream of the emission treatment unit 32. The NOx sensor 60 may be used to increase or decrease the concentration of the reductant component in the exhaust stream 30 and emission treatment unit 32, to provide an optimum concentration of the reductant component for the selective catalytic reduction of NOx.
The NOx sensor 60 measures the concentration of NOx in the treated exhaust stream 38 exiting the emission treatment unit 32. The NOx sensor 60 sends a signal representing the NOx concentration in the treated exhaust stream 38 to a second premixed fuel controller 47. The second premixed fuel controller 47 integrates the processed information and determines if the system parameters are indicative of proper control of the treated exhaust stream 38, and may further determine whether there is a need for supply of reductants to the emission treatment unit 32. Accordingly, the second premixed fuel controller 47 regulates the flow of the second premixed fuel stream 20, entering the separation unit 18, based on the signal received from the NOx sensor 60. The second fuel pump 24 is in communication with the second premixed fuel controller 47, and the controller directly controls and monitors the operation of the second fuel pump to inject a portion of the second premixed fuel stream 20 into the separation unit 18. Similarly, the three way valve 48 may be in communication with the second premixed fuel controller 47 as described above, whereby the controller controls and monitors the operation of the valve to adjust the flow of the second premixed fuel stream 20 into the separation unit 18, and also adjust the flow of the fuel slip stream 21 into exhaust stream 30.
Referring to FIGS. 1, 3 and 4, the NOx sensor 60 may alternatively be used to directly control the amount of reductant flow, i.e. the first fraction stream 42 flow, into the emission treatment unit 32. The NOx sensor 60 sends a signal representing the NOx concentration in the treated exhaust stream 38 to the first fraction controller 64. The first fraction controller 64 regulates the flow of the first fraction stream 42 entering the emission treatment unit 32, based on this signal. The first fraction pump 45 is in communication with the first fraction controller 64, and the controller 64 directly controls and monitors the operation of the first fraction pump to inject at least a portion of the first fraction stream 42 into the emission treatment unit 32 via exhaust stream 30. The NOx sensor 60 may be used instead of, or in addition to the temperature sensor 46 to control the concentration of the reductant component in the exhaust stream 30 and emission treatment unit 32.
The fuel component is effective as a reductant in the SCR reduction of NOx at high temperatures, e.g. temperatures between approximately 200° C. and approximately 600° C., and is ineffective at low temperatures. In contrast, the reductant component is effective throughout a wider range of temperatures, e.g. temperatures between about 100° C. and about 600° C. Accordingly, in the systems and methods of the invention, the amount of the reductant component supplied to the exhaust stream 30 will be minimal at higher temperatures at which the fuel component alone is effective to act as a reductant. Thus, the reductant component can be efficiently utilized and primarily supplied to the exhaust stream 30 and emission treatment unit 32 at lower temperatures when it is needed.
The systems and methods of the invention may be configured to increase the concentration of the reductant component in the exhaust stream 30 and emission treatment unit 28 at temperatures between approximately 100° C. and approximately 200° C., and/or configured to decrease the concentration of the reductant component in the exhaust stream and emission treatment unit at temperatures between approximately 200° C. and approximately 600° C. In preferred embodiments, the systems and methods of the invention are configured increase the concentration of the reductant component in the exhaust stream 30 and emission treatment unit 28 at temperatures between approximately 100° C. and approximately 300° C., and/or configured to decrease the concentration of the reductant component in the exhaust stream and emission treatment unit at temperatures between approximately 300° C. and approximately 600° C. These temperature ranges can be optimized depending on specific system and method parameters, such as the composition of the premixed fuel that is used.
In other embodiments, the systems and methods of the invention may be configured to supply the second premixed fuel stream 20 to the separation unit 18 only at temperatures between approximately 100° C. and approximately 200° C., and/or configured to cease the flow of the second premixed fuel stream to the separation unit at temperatures between approximately 200° C. and approximately 600° C. In preferred embodiments, the systems and methods of the invention may be configured to supply the second premixed fuel stream 20 to the separation unit 18 only at temperatures between approximately 100° C. and approximately 300° C., and/or configured to cease the flow of the second premixed fuel stream to the separation unit at temperatures between approximately 300° C. and approximately 600° C.
Furthermore, the systems and methods of the present invention may be designed to supply the slip fuel streams 15 or 21 to the exhaust stream 30 only at temperatures between about 200° C. and about 600° C., and/or configured to cease the flow of the slip fuel streams 15 and 21 at temperatures between about 100° C. and about 200° C. In preferred embodiments, the systems and methods of the present invention may be designed to supply the slip fuel streams 15 or 21 to the exhaust stream 30 only at temperatures between about 300° C. and about 600° C., and/or configured to cease the flow of the slip fuel streams 15 and 21 at temperatures between about 100° C. and about 300° C. These temperature ranges can be optimized depending on specific system and method parameters, such as the composition of the premixed fuel that is used.
The systems and methods of the invention preferably maintain the concentration of the reductant component in the exhaust stream 30 between about 1 ppm and about 20,000 ppm, at an exhaust stream temperature between about 200° C. and about 600° C. More preferably, the systems and methods of the invention maintain the concentration of the reductant component in the exhaust stream 30 between about 300 ppm and about 5000 ppm, at an exhaust stream temperature between about 275° C. and about 450° C.
Structurally, the controllers 47, 50 and 64 as shown in FIGS. 1-4, may each be a conventional microcomputer, including conventional components such as a microprocessor unit, input/output ports, read-only memory, random access memory, read-out displays, and conventional data bus. Furthermore, the controllers 47, 50 and 64 may each include a micro-controller or a solid-state switch to communicate with the sensors 46 and 60. In one embodiment, the controllers 47, 50 and 64 may each be an electronic logic controller that is programmable by a user. In another embodiment, each controller 47, 50 and 64 may include an analog-to-digital converter accessible through one or more analog input ports.
As will be recognized by those of ordinary skill in the art, the controllers 47, 50 and 64 may be embodied in several other ways. In one embodiment, the controllers 47, 50 and 64 may include a logical processor (not shown), a threshold detection circuitry (not shown) and an alerting system (not shown). Typically, the logical processor is a processing unit that performs computing tasks. It may be a software construct made up using software application programs or operating system resources.
If the separation unit 18 operates via membrane separation, the unit may be selected from a non-exclusive list including reverse osmosis membrane separation systems, electro-kinetic separation systems, pervaporation systems, perstraction systems and the like. Perstraction involves the selective dissolution of particular components (i.e. the reductant component in the second premixed fuel stream 20) into the membrane, the diffusion of those components through the membrane and the removal of the diffused components from the downstream side of the membrane by use of a liquid sweep stream.
In a pervaporation separation unit 18, the second premixed fuel stream 20 can be fed into the separation unit whereby the reductant component passes through the membrane and evaporates on the permeate side of the membrane, while the primary fuel component remains on the retentate side and is recycled to the fuel tank 16 as the second fraction stream 44. The evaporation of the reductant component would be driven by a vacuum applied to the retentate side of the membrane. The reductant component in the first fraction stream 42 is sent directly to the exhaust stream 30. Alternatively, the reductant in the first fraction stream 42 is condensed in a condenser 54, and optionally sent to a holding tank 58, prior to being fed to the exhaust stream 30, as shown in FIG. 4.
During a pervaporation process, a partial pressure is generated at the permeate side of the membrane by means of a vacuum pump, or by means of an inert gas flow. The components of the liquid that move through the membrane, e.g. the reductant component in the premixed fuel stream 20, are vaporized by the low pressure, removed and condensed. The pervaporation process relies on vacuum or sweep gas on the permeate side to evaporate or otherwise remove the permeate from the surface to the membrane. The feed to the pervaporation unit is in the liquid and/or gas state. When the feed is in the gas state the process can be described as vapor permeation.
The membranes disposed in the separation unit 18 may be selected from a non-exclusive list including polymeric, ceramic, carbon, and hybrids of these and may be homogenous or heterogeneous, symmetric or asymmetric in structure, solid or liquid, may carry a positive or negative charge or be neutral or bipolar. Furthermore, the membranes can be used in any convenient form such as sheets, tubes or hollow fibers. Flat sheet membranes can be packaged as spiral wound module elements or pleated cartridges, or be used in single sheets in plate-and-frame systems. Tubular or hollow fiber configurations are formed into bundles and potted at one or both ends. Multiple separation elements in spiral wound, plate and frame, or hollow fibers configurations, can be employed either in series or in parallel.
In one embodiment, the separation unit 18 includes a membrane with differential permeability. Differential permeability in this case means that the permeability of the reductant component through the membrane is substantially different than that of the primary fuel component and the difference is to such an extent that a separation of the two components occurs. In many cases, the reductant component comprises chemical species having molecules of substantially smaller size than the molecules of fuel that are present in the primary component. Differential permeability in some embodiments is thus achieved by selecting, from those available membranes in the art, for example, a membrane having a pore size sufficient to allow the smaller component to move through the membrane while excluding the larger component. Transport of the reductant component through one such membrane may be effected by convection or by diffusion of individual molecules, induced by an electric field or concentration, pressure or temperature gradient. In one embodiment of the invention, the membrane separation unit 18 may be an ultrafiltration membrane separation unit and the driving force for transport of the reductant components of the fuel across the membrane may be a pressure differential. The membrane materials in such ultrafiltration membrane separation unit 18 may include polymeric materials such as polysulfone, polypropylene, nylon 6, polytetrafluoroethylene (PTFE), PVC, acrylic copolymer and the like. In another embodiment of the invention, inorganic materials such as ceramics, carbon based membranes, zirconia and the like may be used in the ultrafiltration based membrane separation systems. In yet another embodiment of the invention, the separation of the reductant component from the primary fuel component may be accomplished using selective facilitated transport membranes. Selective facilitated transport membranes with typically high reductant/fuel selectivity of about 200 may typically include cross-linked polyvinylalcohol-containing AgNO₃membranes and Ag+ exchanged perfluorosulfonic acid membranes.
In another embodiment of the invention, the separation unit 18 is a flash separation unit as illustrated in FIG. 5. In this embodiment, the separation unit 18 flashes the lower boiling point reductant component from the fuel mixture. This is accomplished by feeding the second premixed fuel stream 20 into a heat exchanger 80 via second fuel pump 24 (not shown). A pressure control valve 82 located downstream of the heat exchanger 80, which actuates is set to actuate at some pressure above the vapor pressure of the reductant component. The energy required for the flash could be provided by a split stream of the exhaust, heated engine coolant, or electrically. The reductant component and primary fuel component then enter a flash vessel 84 where the liquid primary fuel component and gaseous reductant component are separated. A flash vessel sensor 86 measures the pressure within the flash vessel 84 and sends a signal representing the measured pressure, to the pressure control valve 82. The pressure control valve 82 controls the pressure within the flash vessel 84 based on this signal. The second fraction fuel stream 44 is recycled to the fuel tank 16 and the first fraction stream 42 is sent directly to the exhaust stream 30, or condensed in a condenser 54, optionally stored, and then sent to the exhaust stream as shown in FIG. 4.
The fuel used in the embodiments of the invention include any fuel suitable for operation of the engine 12, such as gasoline. In one embodiment of the invention, the fuel may be normal diesel fuel. In another embodiment of the invention, the fuel may be a renewable fuel. In one embodiment of the invention, the renewable fuel is green diesel fuel. In another embodiment of the invention, the renewable fuel may be biodiesel, which consists of fatty acid methyl esters and may be made from vegetable oil, animal fat, or waste grease. Biodiesel is typically used as a blend with conventional diesel. In another embodiment of the invention, ethanol may also be blended into diesel fuel. Ethanol/diesel, ethanol/biodiesel and ethanol/gasoline fuel blends are readily available on the market, so no additional infrastructure would be required to mix the reductant with the fuel if desired.
Selective catalytic reduction catalysts are typically in the form of pellets or beads in a container, or coated on the walls of a monolithic structure, for example a monolithic structure in a honeycomb configuration. Monolithic structures are well known in the art and are typically composed of ceramic or metal material forming open channels from the inlet to outlet, with channels in some cases having turns and bends. The catalyst material is typically formed into a sol or colloidal dispersion in a liquid carrier and then applied to internal surfaces of the monolithic metal or ceramic substrate to form a layer of catalyst coating on these internal surfaces. The cell size and shape of the monolithic structure is selected to obtain the desired surface area, pressure drop, and heat and mass transfer coefficients required for a particular application. Such parameters are readily ascertainable to one of skill in the art. In accordance with the invention, the channels can be of any shape suitable for ease of production and coating, and appropriate flow of the exhaust stream 30. For example, for metal substrates, channels may be corrugated into straight, sinusoidal, or triangular shapes, and/or may include a herringbone or zig-zag pattern. For a ceramic substrate, the channels may be, for example, square, triangular, or hexagonal, or any shape that can be formed by extrusion or other methods of manufacture known in the art. Channel diameters are typically in the range of about 0.001 inches to about 0.2 inches.
Typical active catalytic components of the NOx SCR catalyst include metals of platinum and silver. High surface area refractory oxide supports or zeolites may be included. Typical refractory oxide supports are alumina, alumina with additives such as Si, Ca, Ba, Ti, La or other components to provide increased thermal stability. In addition, modifying components such as, for example, Na, Co, Mo, K, Cs, Ba, Ce, and La, may be used to improve the selectivity of the reaction, by reducing the oxidation activity of the catalyst. Additional NOx selective reduction catalyst compositions may contain Cu, Co, Ni, Fe, Ga, La, Ce, Zn, Ti, Ca, Ba, Ag or mixtures thereof, or Pt, Ir, Rh or mixtures thereof.
The monolithic metal substrate can be formed of parallel plates, multiple tubular elements, corrugated metal foil, honeycomb, or multi-cellular monolith and is made of a corrosion resistant metallic alloy suitable for high temperature service in aggressive environments characteristic of automotive exhaust. Such alloys include, but are not limited to, oxidation-resistant high temperature ferritic Cr—Al alloys.
In yet another embodiment of the invention, Fischer-Tropsch diesel may be used as a renewable fuel that at times may be produced from biomass. Fischer-Tropsch or gas-to-liquid (GTL) fuels are typically created by a Fischer Tropsch process that makes liquid diesel fuel from a synthetic mix of gases including CO and H₂. Typical Fischer-Tropsch fuels may contain very low sulfur and aromatic content and very high cetane numbers. Fischer-Tropsch diesel fuels typically reduce regulated exhaust emissions from the engines and the vehicles where this fuel is used. Additionally, the low sulfur content of these fuels may enable use of advanced emission control devices.
In embodiments of the invention, oxygenate reductants may be mixed with fuel in the fuel tank 16, whereby the fuel tank delivers a premixed fuel stream comprising a primary fuel component and an oxygenate reductant component. Oxygenate reductants are hydrocarbons that include one or more oxygen atoms in their molecules. Suitable oxygenate reductants may include alcohols, aldehydes, ketones, ethers, esters, or combinations thereof. The alcohols may include methanol, ethanol, iso-propanol and the like. Preferably, the oxygenate reductant is ethanol. Alcohols form neither particulate matter nor deposits when exposed to temperatures characteristic of diesel exhaust. Moreover, alcohols, such as methanol or ethanol or iso-propanol are sufficiently soluble in diesel fuel to enable the requisite quantity of the reductant to be conveyed to an emission treatment system via the engine fuel itself. The concentration of the reductants in the premixed fuel may typically be in the range of about 0.5 percent to 20 percent by weight of the total fuel.
In one embodiment of the invention, hydrocarbon reductants may be used in order to aid in the production of oxygenated hydrocarbons, as represented by equation (1) below.
Hydrocarbons(HC)+O₂=>oxygenated HC (1)
NOx+oxygenated HC+O₂=>N₂+CO₂+H₂O (2)
The hydrocarbon reductants may include diesel fuel, partially cracked diesel fuel, gasoline, propene, ethanol, diesel fuel, or any other suitable hydrocarbons and the oxygenated hydrocarbons may include methanol, ethanol, propanol, butanol, pentanol, hexanol, methanal, ethanal, propanal, butanal, propenal, acetone, 2-butanone, and 3-penten-2-one and any combination thereof. Although the lean-NOx reducing reaction is a complex process comprising many steps, one of the reaction mechanisms for lean NOx catalysts may be summarized as follows. A hydrocarbon-enriched reductant may be converted to an activated, oxygenated hydrocarbon that may interact with the NOx compounds to form organo-nitrogen containing compounds.
The principles of the invention are not limited to any particular type of engine. One of ordinary skill will recognize that other embodiments of the invention may be suited for many of the combustion-powered vehicles. For example, internal combustion engines that are used in railroad locomotives, in vehicles that run on roads such as trucks, municipal transport vehicles, city buses, cars and other passenger vehicles or in ships may be installed with this type of reductant supply system. The engine may also be a liquid fueled engine, a compression ignition engine, a gasoline engine, and any combination thereof. The gasoline engine may include a lean burn gasoline engine. A lean burn engine is one that produces an oxygen rich exhaust, which is defined as an exhaust having a higher molar ratio of oxygen than the total molar ratio of reductive compounds such as carbon-monoxide, hydrogen, hydrocarbons, and oxygenated hydrocarbons. Examples of such lean burn engine systems may include diesel engines, some natural gas or alternative fuel engines, liquid or gaseous-fueled turbine engines and various lean burn gasoline engine systems.
FIG. 6 illustrates an exemplary method for operating a diesel engine with reduced emissions of NOx in accordance with one embodiment of the invention. As will be appreciated by one of ordinary skill in the art, the method may represent one or more of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various steps or functions illustrated herein may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the objects, features and advantages of the invention, but is provided for ease of illustration and description. Although not explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending on the particular strategy being used.
To this end, beginning at block 102, a first premixed fuel stream 14 comprising a reductant component and a primary fuel component is supplied to an engine 12, wherein the engine is configured to create an exhaust stream. A second premixed fuel stream 20 comprising a reductant component and a fuel component is supplied to a separation unit 18 as in block 104. Referring to block 106, at least a portion of the second premixed fuel stream 20 is separated into a first fraction stream 42 and a second fraction stream 44 by passing the fuel stream 20 through the separation unit 18, wherein the first fraction stream comprises a higher concentration of the reductant component than the second fraction stream. At least a portion of the first fraction stream 42 is supplied to the exhaust stream 30, as shown in block 108. Referring to block 110, the temperature of the exhaust stream 30 is measured. As indicated in block 112, the concentration of the reductant component in the exhaust stream is controlled based on the measured temperature of the exhaust stream. Referring to block 114, the selective catalytic reduction of NOx present in the exhaust stream is performed.

EXAMPLE 1

Exhaust treatment of a diesel engine was performed at different temperatures using 100% diesel, a diesel/ethanol blend containing 90% diesel and 10% ethanol (90/10 blend) and a diesel/ethanol blend containing 50% diesel and 50% ethanol (50/50). The diesel and diesel/ethanol blends were used to reduce NOx in a temperature-controlled emission treatment unit. The results are displayed in FIG. 7. The selective reduction catalyst was a silver based catalyst on washcoated alumina, supported on a monolith. A simulated exhaust stream was used that contained NO, CO, H₂O, O₂and N₂. When injected into the exhaust stream for NOx SCR, the 90/10 blend showed at least a 30% NOx reduction to N₂at an exhaust stream temperature of 430° C., but below a 15% NOx reduction to N₂at an exhaust stream temperature of 375° C. When the alcohol-rich 50/50 blend was injected into an exhaust stream having a low temperature of 375° C., NOx reduction to N₂increased to above 40%. Thus the alcohol-rich fuel blend when used for exhaust NOx SCR treatment resulted in significantly reduced emissions of NOx.
All cited patents, patent applications, and other references are incorporated herein by reference in their entirety.
All ranges disclosed herein are inclusive of the endpoints, and the endpoints are combinable with each other.
It is to be noted that the terms “first,” “second,” and the like as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The modifiers “about” and “approximately” used in connection with a quantity are inclusive of the stated value and have the meaning dictated by the context (e.g., include the degree of error associated with measurement of the particular quantity). The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

1. A system for operating a diesel engine with reduced emissions of NOx comprising:

a fuel tank adapted to directly or indirectly supply a first premixed fuel stream and a second premixed fuel stream, each fuel stream comprising a primary fuel component and a reductant component;

an engine in fluid communication with the fuel tank, wherein the engine is configured to receive the first premixed fuel stream and create an exhaust stream;

an emission treatment unit to treat the exhaust stream;

a separation unit configured to receive the second premixed fuel stream, the separation unit also configured to separate the second premixed fuel stream into a first fraction stream and a second fraction stream, and supply the first fraction stream to the exhaust stream, wherein the first fraction stream comprises a higher concentration of the reductant component than the second fraction stream; and

a temperature sensor to measure the temperature of the exhaust stream, whereby a concentration of the reductant component in the exhaust stream is controlled based on the measured temperature of the exhaust stream.

2. The system according to claim 1, further comprising:

a second premixed fuel controller that receives a signal from a sensor, the controller regulates the flow of the second premixed fuel stream into the separation unit in accordance with the received signal.

3. The system according to claim 2, further comprising:

a second fuel pump in communication with the second fuel controller to pump at least a portion of the second premixed fuel stream into the separation unit.

4. The system according to claim 2, further comprising:

a fuel slip stream off of the second premixed fuel stream, wherein the fuel slip stream is supplied to the exhaust stream; and

a three way valve in communication with the second fuel controller to adjust the flow of the second premixed fuel stream into the separation unit, and adjust the flow of the fuel slip stream into the exhaust stream.

5. The system according to claim 2, wherein the signal received by the second premixed fuel controller represents the temperature of the exhaust stream measured by the temperature sensor.

6. The system according to claim 2, further comprising:

an NOx sensor to measure the concentration of NOx in the treated exhaust stream; and

wherein the signal received by the second premixed fuel controller represents the NOx concentration measured by the NOx sensor.

7. The system according to claim 1, further comprising:

a fuel slip stream off of the first premixed fuel stream, wherein the fuel slip stream is supplied to the exhaust stream; and

a third premixed fuel controller that receives a signal from a sensor, the controller regulates the flow of the fuel slip stream into the exhaust stream in accordance with the received signal.

8. The system according to claim 7, further comprising:

a third fuel pump to pump at least a portion of the fuel slip stream into the exhaust stream.

9. The system according to claim 7, wherein the signal received by the third premixed fuel controller represents the temperature of the exhaust stream measured by the temperature sensor.

10. The system according to claim 7, further comprising:

wherein the signal received by the third premixed fuel controller represents the NOx concentration measured by the NOx sensor.

11. The system according to claim 1, further comprising:

a first fraction controller that receives a signal from a sensor, the controller regulates the flow of the first fraction stream into the exhaust stream in accordance with the signal.

12. The system according to claim 11, further comprising:

a first fraction fuel pump in communication with the first fraction controller to pump at least a portion of the first fraction stream into the exhaust stream.

13. The system according to claim 11, wherein the signal received by the first fraction controller represents the temperature of the exhaust stream measured by the temperature sensor.

14. The system according to claim 11, further comprising:

wherein the signal received by the first fraction controller represents the NOx concentration measured by the NOx sensor.

15. The system according to claim 1, further comprising:

a condenser disposed in fluid communication with the separation unit for condensing at least a portion of the first fraction stream exiting the separation unit.

16. The system according to claim 15, further comprising:

a holding tank disposed in fluid communication with the condenser unit to store the first fraction stream exiting the condenser unit.

17. The system according to claim 1, wherein the emission treatment unit is a selective catalytic reduction treatment unit.

18. The system according to claim 1, wherein the separation unit is a membrane, distillation or flash separation unit.

19. The system according to claim 1, wherein the reductant component comprises an alcohol, aldehyde, ketone, ether or ester or a combination thereof.

20. The system according to claim 19, wherein the reductant component is ethanol.

21. The system according to claim 1, wherein the premixed fuel comprises diesel fuel, biodiesel fuel, green diesel fuel, or Fischer-Tropsch fuel, or any combination thereof.

22. The system according to claim 21, wherein the premixed fuel comprises biodiesel fuel.

23. The system according to claim 1, wherein the engine is a liquid fueled engine, a compression ignition engine, or a gasoline engine, or any combination thereof.

24. A method for operating a diesel engine with reduced emissions of NOx comprising:

supplying a first premixed fuel stream to an engine, wherein the engine is configured to create an exhaust stream;

supplying a second premixed fuel stream to a separation unit, wherein the first and second premixed fuel streams each comprise a reductant component and a primary fuel component;

separating at least a portion of the second premixed fuel stream into a first fraction stream and a second fraction stream via the separation unit, wherein the first fraction stream comprises a higher concentration of the reductant component than the second fraction stream;

supplying at least a portion of the first fraction stream to the exhaust stream;

measuring a temperature of the exhaust stream;

controlling the concentration of the reductant component in the exhaust stream based on the measured temperature of the exhaust stream; and

performing a catalytic reduction of NOx present in the exhaust stream.

25. The method according to claim 24, wherein the first and second premixed fuel streams are directly or indirectly supplied from a fuel tank, and the method further comprising:

supplying the second fraction stream to the fuel tank.

26. The method according to claim 24, wherein controlling the concentration of the reductant component comprises:

regulating the flow of the second premixed fuel stream into the separation unit.

27. The method according to claim 26, wherein controlling the concentration of the reductant component further comprises:

regulating the flow of a fuel slip stream off of the second premixed fuel stream, wherein the fuel slip stream flows into the exhaust stream.

28. The method according to claim 27, wherein the flow of the second premixed fuel stream and the fuel slip stream is regulated with a three way valve.

29. The method according to claim 24, wherein controlling the concentration of the reductant component comprises:

regulating the flow of a slip fuel stream off of the first premixed fuel stream, wherein the fuel slip stream flows into the exhaust stream.

30. The method according to claim 24, wherein controlling the concentration of the reductant component comprises:

regulating the flow of the first fraction stream into the exhaust stream.

31. The method according to claim 24, further comprising:

measuring a concentration of NOx in a treated exhaust stream exiting the emission treatment unit; and

regulating the flow of the second premixed fuel stream into the separation unit in accordance with the measured concentration.

32. The method according to claim 24, further comprising:

regulating the flow of the first fraction stream into the exhaust stream in accordance with the measured concentration.

33. The method according to claim 24, further comprising:

condensing at least a portion of the first fraction stream exiting the separation unit.

34. The method according to claim 33, further comprising:

storing at least a portion of the condensed first fraction stream in a holding tank.

35. The method according to claim 24, wherein at least a portion of the second premixed fuel stream is separated by membrane, distillation or flash separation or a combination thereof.

36. The method according to claim 24, wherein the reductant component comprises an alcohol, aldehyde, ketone, ether or ester, or a combination thereof.

37. The method according to claim 36, wherein the reductant component is ethanol.

38. The method according to claim 24, wherein the premixed fuel comprises diesel fuel, biodiesel fuel, green diesel fuel, Fischer-Tropsch fuel, and any combination thereof.

39. The method according to claim 38, wherein the premixed fuel comprises biodiesel.

40. The method according to claim 24, wherein the concentration of the reductant component in the exhaust stream is maintained between approximately 1 ppm and approximately 20,000 ppm, when the measured temperature of the exhaust stream is between approximately 200° C. and approximately 600° C.

41. The method according to claim 24, wherein the concentration of the reductant component in the exhaust stream is increased when the measured temperature of the exhaust stream is decreasing.

42. The method according to claim 24, wherein the concentration of the reductant component in the exhaust stream is decreased when the measured temperature of the exhaust stream is increasing.