Patent Application
20070278199
1. Field of the Invention
The present invention related generally to emission controls and more particularly to systems for reducing particles in exhaust streams.
2. Description of the Prior Art
When a fuel burns incompletely, pollutants such as particles and hydrocarbons are released into the atmosphere. The United States Environmental Protection Agency has passed regulations that limit the amount of pollutants that, for example, diesel trucks, power plants, engines, automobiles, and off-road vehicles can release into the atmosphere.
Currently, industries attempt to follow these regulations by adding scrubbers, catalytic converters and particle traps to their exhaust systems. However, these solutions increase the amount of back pressure exerted on the engine or combustion system, decreasing performance. In addition, the scrubbers and particle traps themselves become clogged and require periodic cleaning to minimize back pressure.
Radiation sources and heaters have been used in exhaust systems, for example, to periodically clean the particle traps or filter beds. Others solutions have included injecting fuel into the filter beds or exhaust streams as the exhaust enters the filter beds to combust the particles therein. However, the filter beds can be sensitive to high temperatures and the radiation sources and heaters must be turned off periodically.
An exhaust system comprises a combustion chamber and a radiation source. The radiation source is arranged with respect to the combustion chamber, either inside or outside of the chamber, so as to be able to produce radiation within the combustion chamber. The radiation source can comprise a resistive heating element, a coherent or incoherent infrared emitter, or a microwave emitter, for example. The microwave emitter can be tuned to a particular molecular bond. Where the radiation source is disposed outside of the combustion chamber, the radiation source can either heat the chamber walls to reradiate into the chamber, else the combustion chamber can include a radiation transparent window.
Particles in an exhaust stream passing through the combustion chamber are heated by the radiation to an ignition point and are consequently removed from the exhaust by burning. Microwave radiation tuned to excite a molecular bond found in the particles can be particularly effective for heating the particles rapidly. Additional air or fuel can be added to the combustion chamber, as needed, to promote better combustion. Once a flame front is established in the combustion chamber, the combustion reaction can become self-sustaining so that further radiation from the radiation source is no longer required.
In some embodiments, the combustion chamber has a non-circular cross-section perpendicular to a longitudinal axis of the chamber. In some of these embodiments, the cross-section is at least partially parabolic to focus heat from the burning particles back into a hot zone within the combustion chamber where the particle burning preferentially occurs. The combustion chamber can be thermally insulated to better retain heat in order to maintain the combustion reaction. The exhaust system can also comprise a thermally insulated exhaust pipe leading to the combustion chamber to further reduce the loss of heat from the exhaust stream before particle burning can occur. In some embodiments, a reverse flow heat exchanger is placed in fluid communication with the combustion chamber so that heat is transferred to the incoming exhaust stream from the combusted exhaust stream exiting the combustion chamber. In certain embodiments, the reverse flow heat exchanger is also thermally insulated.
One advantage of certain embodiments of the present invention is the absence of a particle filter or trap within the combustion chamber. While prior art systems have attempted to trap particles and then periodically clean the trap or filter, these systems create significant back-pressure as such traps and filters obstruct the exhaust flow, especially as they become plugged with particles. Continuously burning the particles in the combustion chamber without the use of such traps or filters provides a more simple design that additionally reduces back-pressure.
A vehicle comprising an internal combustion engine and the exhaust system described above is also provided. The exhaust system can serve as either or both of a muffler and a catalytic converter. Thus, the combustion chamber can also include a catalyst. In some embodiments, the combustion chamber and/or the reverse flow heat exchanger can be sized to act as a resonating chamber to serve as a muffler. For example, the combustion chamber can have a diameter greater than a diameter of the exhaust pipe leading into the combustion chamber. The vehicle can also comprise a controller configured to control the radiation source.
The system described herein can be implemented in a variety of settings where particles are present in a gas stream. Some embodiments include automobile exhaust systems, diesel exhaust systems, power plant emission systems, fireplace chimneys, off-road vehicle exhaust systems, and the like.
An exhaust system comprises a combustion chamber and a radiation source to facilitate the combustion of particles within the chamber. Once ignited, the combustion can continue so long as the concentration of particles in the exhaust entering the chamber remains sufficiently high. The disclosed device can replace both the muffler and the catalytic converter in a vehicle exhaust system and offers reduced back pressure for better fuel economy and lower maintenance costs. The device requires little to no maintenance and is self-cleaning.
One advantage of certain embodiments of the present invention is the absence of an obstructing particle filter or trap within the combustion chamber 110. A particle trap or filter is obstructing if it would at least partially restrict the flow of an exhaust gas through the combustion chamber 110. By not restricting the flow of exhaust gas through the combustion chamber 110, embodiments of the invention serve to reduce back-pressure compared with prior art systems.
Radiation source 120, in the illustrated embodiment, comprises a resistive heating element wrapped around the outside of the combustion chamber 110. In another embodiment, the radiation source 120 is placed externally along the longitudinal length of the combustion chamber 110. In some embodiments, a controller (not shown) for the radiation source 120 is provided to control the power to the radiation source 120 and to turn off the radiation source 120 when not needed, such as when no exhaust is flowing. Alternative radiation sources are discussed below with reference to
In operation, an exhaust gas containing particles, such as carbonaceous particles like soot, flows through the combustion chamber 110. The radiation source 120 heats the wall of the combustion chamber 110 which re-radiates infrared (IR) radiation into the interior of the combustion chamber 110. Some of the IR radiation is absorbed by the particles in the exhaust gas as they traverse the combustion chamber 110. When the particles reach a temperature at which they ignite, about 800° C. for carbonaceous particles, the particles burn completely, leaving no residue. Accordingly, essentially particle-free exhaust leaves the combustion chamber 110.
The heat produced by the combustion of the particles can make the continuing reaction self-sustaining so that the radiation source 120 is not necessary. A thermocouple (not shown) can be placed on or in the combustion chamber 110 in order to monitor the temperature of the combustion reaction to provide feedback to a controller (not shown) for controlling the power to the radiation source 120. As noted above, the combustion chamber 110 can be shaped to focus IR radiation from the combustion reaction onto a focal point or line within the combustion chamber 110 to create a hot zone that helps to sustain the continuing reaction in the absence of external heating.
In some embodiments, the inlet 320 and/or the combustion chamber 310 are thermally insulated by the thermal insulation 340 to retain as much heat as possible in the exhaust gas as the gas enters the combustion chamber 310. It will be appreciated that insulation 340 can be similarly applied to the other embodiments disclosed herein. For example, a blanket of insulation 340 can be wrapped around the radiation source 120 and combustion chamber 110 of
Radiation source 350 can be, for example, a coherent or incoherent IR emitter or microwave emitter, such as a Klystron tube. Unlike a resistive heating element, radiation source 350 can be configured to emit radiation directionally and/or within a desired range of wavelengths. Accordingly, radiation transparent window 360 is provided to allow radiation to pass directly into the combustion chamber 310. In some embodiments, the radiation transparent window 360 extends completely around the circumference of the combustion chamber 310.
As noted, radiation source 350 can be tuned to produce radiation within a desired range of wavelengths. Thus, the radiation can be tuned to excite specific molecular bonds that are known to be present in the particles of the exhaust stream. For example, microwave radiation can be tuned to excite carbon-hydrogen bonds or carbon-carbon bonds where the particles in the exhaust are known to include such bonds. Tuning the radiation in this manner can heat particles to their ignition temperature more quickly and with less energy.
The radiation transparent window 360 is constructed using a material that can withstand the heated exhaust gases within the combustion chamber 310. In some embodiments, radiation transparent window 360 is a microwave transparent window constructed using fiberglass, plastic, polycarbonate, quartz, porcelain, or the like. In other embodiments, the radiation transparent window 360 is an IR transparent window constructed using, for instance, sapphire.
The combustion chamber 410 may comprise air intake 450 and/or fuel intake 460. In some embodiments, air intake 450 is configured to introduce oxygen to the combustion chamber to aid the combustion reaction in the event that there is not enough oxygen present in the exhaust as it enters the combustion chamber 410. In other embodiments, fuel intake 460 introduces fuel into the combustion chamber to burn and, thus, heat the exhaust as it enters through inlet 420. It will be appreciated that adding fuel with or without air can, in some instances, replace the need for a radiation source. In such embodiments, a spark generator or other ignition source can be employed to ignite the combustion reaction with the added fuel.
In certain embodiments, the combustion chamber 410 additionally comprises at least one catalyst 470 to catalyze oxidation and/or reduction reactions in the exhaust stream. The catalyst 470 can include platinum, rhodium, and/or palladium deposited on a honeycomb substrate or ceramic beads. In these embodiments, the combustion chamber 410 is configured to additionally function as a catalytic converter in the exhaust system 400. It will be understood that heating the exhaust gas in the presence of the catalyst 470 can advantageously improve the completeness of the reaction being catalyzed.
In some embodiments, the combustion chamber 515 has a parabolic or partially parabolic cross-section 535 perpendicular to a longitudinal axis to create a hot zone. The combustion chamber 515 also comprises a radiation source 540. In some embodiments, the radiation source 540 is a microwave emitter, such as a Klystron tube. Alternatively, radiation source 540 is an IR emitter. In some embodiments, a radiation transparent window separates the radiation source 540 from the combustion chamber 515.
In some embodiments, the combustion chamber 515 further comprises at least one catalyst 545 configured to catalyze oxidation and/or reduction reactions of undesirable gases in the exhaust stream such as NOx compounds. In those embodiments where the heat exchanger 510 is configured to act as a muffler, and the combustion chamber 515 comprises catalyst 545, it will be appreciated that the exhaust system 500 can replace both the muffler and the catalytic converter in a conventional vehicle exhaust system. Advantageously, because the combustion chamber 515 burns the particles present in the exhaust stream, it will be further appreciated that the exhaust system 500 can additionally replace a particle trap in a conventional vehicle exhaust system. One of skill in the art will also recognize that the exhaust systems disclosed herein can also be applied to clean exhaust streams from non-vehicular sources such as power plants, fireplace chimneys, industrial and commercial processing, and the like.
It should be noted that in some embodiments the catalyst 545 comprises a substrate, such as a grating, with a surface coating of a catalytic material that is placed over an opening 550 of the heat exchanger 510. While such a catalyst 545 may at least partially restrict the flow of exhaust gas through the combustion chamber 515, the catalyst is not a particle trap or filter. Specifically, openings in the grating are too large to trap or filter the particles in the exhaust entering the chamber 515. Additionally, such a catalyst 545 cannot collect particles for two reasons. First, particles are eliminated from the exhaust before the exhaust reaches the opening 550. Second, even if a particle survives the combustion reaction and adheres to the catalyst 545, the restriction around the particle would cause a local increase in temperature which would cause the particle to burn and not be retained thereon.
Likewise, some embodiments that employ a microwave emitter as the radiation source 540 include a microwave-blocking grating (not shown) either across the opening 550 or further downstream along the exhaust path to prevent microwaves from propagating out of the exhaust system 500. For essentially the reasons discussed above, although such a microwave-blocking grating may at least partially restrict the flow of exhaust gas through the combustion chamber 515, the microwave-blocking grating is not a particle trap or filter. The openings of the grating are too large to trap or filter particles in the exhaust, particles are eliminated from the exhaust before the exhaust reaches the microwave-blocking grating, and any particles that survive and adhere to the microwave-blocking grating simply burn off.
In the foregoing specification, the present invention is described with reference to specific embodiments thereof, but those skilled in the art will recognize that the present invention is not limited thereto. Various features and aspects of the above-described present invention may be used individually or jointly. Further, the present invention can be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the specification. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive. It will be recognized that the terms “comprising,” “including,” and “having,” as used herein, are specifically intended to be read as open-ended terms of art.
