Electric resistance heating devices – Heating devices – Continuous flow type fluid heater
Reexamination Certificate
1998-07-02
2001-08-28
Walberg, Teresa (Department: 3747)
Electric resistance heating devices
Heating devices
Continuous flow type fluid heater
C165S170000, C422S168000
Reexamination Certificate
active
06282371
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the reduction of nitrogen oxides and the thermal oxidation of organics, and more particularly, to a device and method for the reduction of nitrogen oxides and the thermal oxidation of organics in a net oxidizing environment of a gas and particulate matter stream from industrial and vehicle exhaust.
BACKGROUND OF THE INVENTION
Destruction or conversion of atmospheric pollutants in industrial gas streams and internal combustion engine exhaust streams has been a long-standing research and development goal. Such atmospheric pollutants include products of incomplete combustion, such as carbon monoxide and unburned hydrocarbons, oxides of nitrogen [“NOx”], and carbonaceous particulate matter [“PM”].
Lean-burning engines, such as diesel engines and lean-burning gasoline or natural gas engines, often emit levels of pollutants above regulatory limits. In response to air quality regulations, vehicle manufacturers employ pollution control devices in internal combustion engine exhaust systems to reduce these emissions. Traditional gasoline engine pollution control devices employ a ceramic honeycomb monolith or a packed bed of pellets having a coating of a noble metal catalyst. Such devices catalyze the reactions of carbon monoxide and unburned hydrocarbons with oxygen, typically at approximately 260° C. to 427° C. (500° F. to 800° F.). Other devices employ catalysts that also catalyze the reaction of oxides of nitrogen. Unfortunately, two factors render such catalytic devices alone insufficient for treating vehicle engine exhaust (especially diesel engine) and similar industrial emissions. First, the catalytic devices are ineffective at destroying PM, which is present in engine gas streams, especially those from diesel engines. Second, the PM and other particulates deposit on the monolith, thereby preventing gaseous constituents from reaching the catalytic material, or possibly deactivating or poisoning the catalyst. In general, conventional three-way-catalysts fail to reduce NOx under lean-burn (that is, oxygen-rich) conditions common to many internal combustion engines.
Internal combustion engines are the subject of regulations limiting NOx emissions. The simultaneous emission limits for both particulate matter and NOx presents a unique problem because the two pollutants typically have an inverse relationship in engine exhaust. Internal combustion engines generally can be configured and tuned to produce an exhaust stream having low PM and high NOx concentrations or, alternatively, high PM and low NOx concentrations. Traditionally, engines that employ oxidation catalyst devices may be adjusted to minimize NOx formation because of the catalysts' inability to reduce NOx. Such adjustments may compromise engine efficiency and performance.
Although not generally employed in reducing NOx emissions from internal combustion engines, various techniques exist for reducing NOx emissions from gas streams in other applications. One technique for reducing NOx emissions is selective catalytic reduction (SCR), which reduces NOx in the presence of a reducing agent, such as of ammonia (NH
3
), over a catalyst. Typically, selective catalytic NOx reduction is employed with exhaust stream temperatures in the range of 288° C.-427° C. (550° F.-800° F.). SCR catalysts have the limitations discussed herein above.
Another approach for removing NOx is selective non-catalytic reduction (SNCR), which employs a chemical that selectively reacts, in the gas phase, with NOx in the presence of oxygen at a temperature greater than 621° C. (1150° F.). Chemical NOx reduction agents used in such processes include ammonia (NH
3
), urea (NH
2
CONH
2
), cyanuric acid (HNCO)
3
, iso-cyanate, hydrazine, ammonium sulfate, atomic nitrogen, melamine, methyl amines, and bi-urates.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system and method or reducing pollutant emissions from internal combustion engines and industrial exhaust gas streams. Specifically, an object of the present invention is to provide a system and method for reducing NOx, and oxidizing PM and oxidizable constituents in an engine or industrial exhaust stream. It is an object to provide such a system and method in a single, compact device, especially one suitable for use in a mobile vehicle engine.
It is another object of the present invention to provide a device and method for the integrated, substantially-simultaneous oxidation and reduction, especially thermal oxidation of organics and catalytic reduction of NOx, of an exhaust gas stream.
It is a further object of the present invention to provide a system and method for reducing NOx emissions from an engine exhaust stream under lean-burn (that is, oxygen-rich) conditions.
It is yet a further object of the present invention to provide a thermal oxidation and catalytic reduction system having a catalytic surface to reduce NOx under lean-burn conditions.
It is yet a further object of the present invention to provide film injection techniques to enhance contact between a reactant and a catalyst.
It is yet a further object of the present invention to provide techniques for the injection of supplementary fuel into a system for the thermal oxidation of organics and catalytic NOx reduction in such a manner that the performance of the NOx reduction catalyst is enhanced.
According to the present invention, a thermal oxidation and catalytic reduction system arranged in a compact, multi-spiral, recuperative configuration is provided that includes two interspaced, coiled sidewalls that form a spiral inlet passage and a spiral outlet passage, and a central chamber. A thermal oxidation zone, which is preferably disposed in the central chamber, may be located between the inlet and outlet (that is, entrance and exit) passages, which form a spiral, counter-current heat exchanger. A matrix of porous inert media may be disposed within each one of the spiral passages and in the central chamber. The oxidation reaction zone, which is in flow communication with the spiral inlet passage and spiral outlet passage, receives heat primarily by convection from the oxidized gases and loses heat primarily by radiation to the matrices, which are in intimate contact with the gas stream.
The thermal oxidation and catalytic reduction system utilizes a catalytic surface to reduce NOx. Preferably a lean-NOx catalyst is employed in the appropriate regions of the thermal oxidation and catalytic reduction system—that is, proximate the inlet (for conditions in which the inlet gas stream is within or belowthe range at which the particular catalyst may be effective) and the outlet. The catalytic surface may be disposed either on the sidewalls forming the spiral passages, on the media, or in a combination of the media and sidewalls. In the embodiment in which the catalytic surface is disposed on one or more sidewalls, the matrix may be omitted from the passage adjacent to the catalytic surface.
The matrices foster stable oxidation of the reacting gas at low temperatures (for example 788° C.-1093° C. (1450° F.-2000° F.)) within the reaction zone of a thermal oxidizer portion of the present system) compared with premixed flames. Thus, the system according to the present invention diminishes the formation of oxides of nitrogen. Further, the matrices provide a highly radiative environment and long residence times, which promote the destruction of gas phase organics, CO, and PM. Further, the geometry of the present invention provides regions that have temperature ranges that are well-suited for a wide variety of NOx reduction techniques. Specifically, the relatively smooth temperature profile of the gas stream within the spiral passages, compared with combustion processes using (for example) premixed flames, provides relatively long residence times within a wide range of temperatures to enable the present invention to employ a broad range of emission control techniques, especially those relating to NOx reduction. Further, the present invention, because of
Edgar Bradley L.
Martin Richard J.
Page Dorriah L.
Paul Partha P.
Campbell Thor
Luedeka,Neely & Graham PC
Walberg Teresa
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