Patent Application
20110016848
The present invention relates, generally, to systems and methods for treating diesel exhaust gases and, more particularly, to systems and methods for treating non-stationary diesel exhaust gases.
Diesel engines tend to have low emissions of gas phase hydrocarbons and carbon monoxide, however, they tend to have relatively high emissions of nitrogen oxides (NOx) and particulates. As standards for NOx and particulate emissions become more stringent, it is increasingly important to devise strategies for reducing these emissions.
In diesel engines, a DPF can be used downstream of the engine to filter particulates from the engine exhaust. If too much soot collects in the DPF, the soot can burn in an uncontrolled manner and may crack or melt the DPF. This phenomenon is sometimes referred to as a “runaway” or uncontrolled regeneration. Other problems can result from excessive soot accumulation in the DPF as well, such as an increase in engine backpressure, which can have an adverse effect on engine operation and affect fuel consumption.
To avoid aforementioned problems such as runaway regenerations, the DPF is periodically cleaned by burning off the soot, through a so-called active regeneration operation in a controlled manner that does not ordinarily do serious damage the DPF. Carbon in the filter reacts with NO2 in the exhaust gas stream by the reactions NO2+C->NO+CO and 2NO2+2C->N2+CO2. To achieve the desirable reaction, 2NO2+2C->N2+CO2, the carbon soot particles typically require temperatures in excess of 500-550° C. Those temperatures are much higher than typical diesel exhaust temperatures.
A solution to this problem is to install a heater, such as a burner or electrical coils, upstream of the DPF to raise the exhaust stream temperature to a temperature suitable for regeneration, a technique typically referred to as “active” regeneration. Active regeneration results in some efficiency losses and thermal stress on the filter, and it is desirable to limit its use. Also, care must be taken to ensure that “runaway” regeneration, essentially uncontrolled combustion, does not occur and cause damage to the filter.
An alternative to active regeneration is so-called passive regeneration. The reaction between the accumulated particles and oxygen in the exhaust stream (usually in the form of O2 and NO2) ordinarily naturally results in some of the particles oxidizing. However, at ordinary exhaust temperatures, passive regeneration generally occurs too slowly to remove accumulated particles. To provide sufficient passive regeneration to keep the DPF operating efficiently, a catalyst can be used. The catalyst can be a so-called diesel oxidation catalyst (DOC) that can be provided upstream of or on the DPF and causes NO in the exhaust stream to convert to NO2, thereby facilitating passive regeneration when the NO2 reacts with the particles in the DPF.
To reduce NOx emissions, NOx catalysts such as selective catalytic reduction catalysts (SCR) or lean NOx catalysts (LNC) can be provided in the exhaust stream. SCR catalysts are presently most common and can be very efficient at reducing NOx emissions to N2 using NH3 over a catalyst such as zeolite or V/Ti. However, these catalysts typically operate most efficiently at relatively high temperatures, such as >300° C., which is often above the exhaust temperature for a diesel engine used in a vehicle application.
It is desirable to provide a system and method for treating diesel exhaust gases that can efficiently remove particulate matter and reduce NOx emissions.
According to an aspect of the present invention, a system for treating diesel exhaust gases comprises an engine for producing a diesel exhaust stream, a diesel particulate filter downstream of the engine and adapted to remove particulates from the exhaust stream, a heater upstream of the filter adapted to deliver energy in sufficient amounts to the exhaust stream to cause active regeneration of the filter, and a diesel oxidation catalyst upstream of the heater adapted to cause passive regeneration of the filter when the exhaust stream is above a light-off temperature of the diesel oxidation catalyst.
According to another aspect of the invention, a system for treating diesel exhaust gases comprises an engine for producing a diesel exhaust stream, a diesel particulate filter downstream of the engine and adapted to remove particulates from the exhaust stream, a heater upstream of the filter adapted to deliver energy to the exhaust stream, and a NOx reduction catalyst downstream of the heater.
According to yet another aspect of the invention, a method for treating diesel exhaust gases comprises producing a diesel exhaust stream with a diesel engine, removing particulates from the exhaust stream with a diesel particulate filter downstream of the engine, periodically delivering energy from a heater to the exhaust stream upstream of the filter in sufficient amounts to cause active regeneration of the filter, and periodically heating a diesel oxidation catalyst upstream of the heater to a light-off temperature of the diesel oxidation catalyst to cause passive regeneration of the filter.
According to still another aspect of the invention, a method for treating diesel exhaust gases comprises producing a diesel exhaust stream with a diesel engine, removing particulates from the exhaust stream with a diesel particulate filter downstream of the engine, causing NOx reduction in the exhaust stream with a NOx reduction catalyst, and delivering energy from a heater upstream of the NOx reduction catalyst sufficient to raise a temperature of the NOx reduction catalyst to a temperature in an optimal range for NOx reduction by the NOx reduction catalyst.
The features and advantages of the present invention are well understood by reading the following detailed description in conjunction with the drawing in which like numerals indicate similar elements and in which:
A system for treating diesel exhaust gases according to an aspect of the present invention is seen in
A diesel oxidation catalyst (DOC) 31 is disposed upstream of the heater 29 and is adapted to cause passive regeneration of the DPF when the exhaust stream is above a light-off temperature of the DOC. If the heater 29 includes a diesel oxidation catalyst, the DOC 31 is a different diesel oxidation catalyst. A diesel oxidation catalyst forming part or all of the heater 29 will be of a type adapted to raise temperatures upstream of the DPF 25 to temperatures sufficient to cause active regeneration. The DOC 31 will typically be of a type that is adapted to cause NO in the exhaust stream to form NO2, usually at ordinary diesel exhaust temperatures.
In the system 21 according to this aspect, regeneration of the DPF 25 can occur through passive regeneration when exhaust stream temperatures are above the light-off temperature of the DOC 31 and, when exhaust stream temperatures are too low to permit for adequate passive regeneration, the heater 29 can be operated to cause active regeneration. It is ordinarily desirable to increase exhaust stream temperatures to about 650° C. upstream of the DPF 25 for active regeneration, however, an oxidation catalyst can be provided on the DPF, in which case temperature of the exhaust stream can be lower, such as around 600-625° C.
A NOx reduction catalyst 33 (shown in phantom) can be provided downstream of the heater 29. The NOx reduction catalyst 33 will ordinarily be provided downstream of the DPF 25. The heater 29 can be adapted to elevate the exhaust stream temperature to a temperature in an optimal range for NOx reduction by the catalyst, such as a temperature greater than 300° C. When an oxidation catalyst is provided on the DPF 25, it can be of a type designed to produce a desired ratio of the NO/NO2 (usually about 1:1) for a fast, low temperature NOx reduction across the NOx reduction catalyst. Providing an oxidation catalyst on the DPF 25 can lower the temperature required for active regeneration of the DPF, and can condition the exhaust gas going into the NOx reduction catalyst 33. In addition, the oxidation catalyst on the DPF 25 can be formulated to increase passive regeneration activity in the DPF when the catalyst light-off temperature is met, regardless whether there is DOC upstream of the heater.
A controller 35, such as a suitable computer, can be provided for controlling operation of the heater 29. A temperature sensor 37 can be provided for sensing a temperature in the exhaust stream. The temperature sensor 37 can send a signal corresponding to the temperature in the exhaust stream to the controller 35, and the controller can control the heater in response to the temperature signal. For example, if the temperature sensor 37 sends a signal to the controller 35 corresponding to a temperature below the light-off temperature of the DOC, and the controller determines that the temperature has been below the light-off temperature for a sufficient period of time (not necessarily a continuous length of time) such that, according to models for the particular system which may consider other factors such as backpressure, it is expected that regeneration of the DPF 25 is required, the controller can initiate control the heater 29 to deliver energy sufficient to cause active regeneration of the DPF.
A signal to the controller 35 from the temperature sensor 37 can also be used to cause the controller to control the heater 29 to deliver only enough energy to elevate the exhaust stream temperature to an optimal temperature range for NOx reduction. The controller 35 may also receive a signal from a NOx sensor 39 that can provide a signal to the controller to cause the controller to control the heater 29 to deliver sufficient heat to the exhaust stream to reduce the NOx level. The controller 35 can control the heater 29 in response to a variety of signals and inputs, including one or both of temperature and NOx levels from the temperature sensor 38 and the NOx sensor 39. There can, of course, be multiple temperature sensors, multiple NOx sensors, and multiple other sensors throughout the system 21 and the heater 29 can be controlled to optimize catalyst performance or regeneration as required.
In a method for treating diesel exhaust gases according to an aspect of the invention, a diesel exhaust stream is produced and particulates are removed from the exhaust stream with a diesel particulate DPF 25 downstream of the engine 23. A heater 29 periodically delivers energy to the exhaust stream upstream of the DPF 25 in sufficient amounts to cause active regeneration of the DPF. A DOC 31 upstream of the heater 29 is periodically heated to a light-off temperature of the DOC to cause passive regeneration of the DPF. The DOC 31 can be heated to the light-off temperature by any suitable means, such as by increasing the load on the engine 23 so that the engine exhaust stream is hotter, or heating the exhaust stream with another heater (not shown). A NOx reduction catalyst 33 can be provided to remove NOx from the exhaust stream downstream of the heater 29.
A temperature in the exhaust stream can be sensed by the temperature sensor 37 which can send a signal corresponding to the temperature in the exhaust stream to the controller 35 for controlling operation of the heater 29. The heater 29 can be controlled by the controller 35 in response to the temperature signal for tuning the temperature of the exhaust stream to achieve optimal DPF 25 regeneration temperatures (whether for a catalyzed or uncatalyzed DPF) and/or optimal temperatures for NOx reduction by the particular NOx reduction catalyst 33.
The system 121 can be arranged to tune the amount of energy delivered to the exhaust stream by the heater 129 to achieve optimal performance of the NOx reduction catalyst 133. In addition to being able to deliver sufficient energy to the exhaust stream to optimize performance of the NOx reduction catalyst, the heater 129 can be arranged to tune the amount of energy delivered to the exhaust stream by the heater to cause active regeneration of the DPF 125, usually about 650° C. upstream of the DPF for an uncatalyzed DPF and about 600-626° C. for a catalyzed DPF having a diesel oxidation catalyst coating.
A DOC 131 (shown in phantom) can be provided upstream of the heater 129. The DOC 131 can be adapted to cause passive regeneration of the DPF 125 when the exhaust stream is above a light-off temperature of the DOC.
A controller 135 can be provided for controlling operation of the heater 129. A temperature sensor 137 can be provided for sensing a temperature in the exhaust stream and sending a signal corresponding to the temperature in the exhaust stream to the controller 135. The controller 135 can control the heater 129 in response to the temperature signal to achieve optimal performance of the NOx reduction catalyst and/or to cause active regeneration of the DPF. Other sensors, such as a NOx sensor 139, can be provided to provide input to the controller 135 for determining when operation of the heater 129 should be initiated, usually based upon modeling for the particular system involved.
In a method for treating diesel exhaust gases according to an aspect of the present invention, a diesel exhaust stream is produced by an engine 123. Particulates are removed from the exhaust stream with the diesel particulate DPF 125 downstream of the engine 123. The NOx reduction catalyst 133 reduces NOx emissions in the exhaust stream. Energy can be delivered to the exhaust stream from a heater 129 upstream of the NOx reduction catalyst 133 sufficient to raise a temperature of the NOx reduction catalyst to a temperature in an optimal range for NOx reduction by the NOx reduction catalyst. Energy can also/instead be delivered to the exhaust stream from the heater 129 in sufficient amounts to elevate the temperature of the exhaust stream to a temperature sufficient to cause active regeneration of the DPF 125. A controller 135 can control operation of the heater 129 to tune delivery of energy to the exhaust gas stream to obtain optimal temperatures for NOx reduction and/or to reach temperatures for active regeneration of the DPF 125.
In the present application, the use of terms such as “including” is open-ended and is intended to have the same meaning as terms such as “comprising” and not preclude the presence of other structure, material, or acts. Similarly, though the use of terms such as “can” or “may” is intended to be open-ended and to reflect that structure, material, or acts are not necessary, the failure to use such terms is not intended to reflect that structure, material, or acts are essential. To the extent that structure, material, or acts are presently considered to be essential, they are identified as such.
While this invention has been illustrated and described in accordance with a preferred embodiment, it is recognized that variations and changes may be made therein without departing from the invention as set forth in the claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US08/59127 | 4/2/2008 | WO | 00 | 10/1/2010 |
