Method and apparatus for emission control

A converter for purifying exhaust gases from lean-burn engines, in particular for controlling the amount of NO.sub.x and soot from a diesel engine in transient operation such as a vehicle. The converter contains a catalyst bed with a catalyst effective for NO.sub.x reduction with a chemical reductant when the catalyst bed is with a certain temperature window and when the ratio between the molar amount of chemical reductant and NO.sub.x is above a certain minimum ratio. The catalyst bed is heated or cooled to a temperature within the temperature window and a switching valve is provided for reverse flowing the exhaust gases through the converter to maintain the catalyst bed at a temperature within the temperature window for a longer time than is possible with a conventional non-flow-reversing converter. A reductant delivery system adds chemical reductant to the exhaust gases in an appropriate amount so that the ratio between the molar amount of chemical reductant and NO.sub.x is above the certain minimum ratio when the exhaust gases pass over the catalyst bed. A soot trap may be provided in series with the catalyst bed in the converter, said reverse flowing of the exhaust gases through converter heating and continuously maintaining the soot trap at or above the ignition temperature of the soot.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a method and apparatus for purifying
 exhaust gases from lean-burn engines, in particular for controlling the
 amount of NO.sub.x and soot after a diesel engine.
 2. Brief Description of the Prior Art
 Diesel engines are more efficient than gasoline engines and emit lesser
 amounts of greenhouse gas. However, their emissions contain large amounts
 of nitrogen oxides (hereunder sometimes abbreviated NO.sub.x) and
 particulates (hereunder sometimes called soot). A diesel engine can be
 operated to emit less NO.sub.x or soot, but there is trade-off between the
 amount of soot and NO.sub.x. For example, if the engine is operated to
 reduce the amount of soot in the exhaust gases, the amount of NO.sub.x
 increases.
 Diesel engines operate under lean-burn conditions. As a result, the exhaust
 gas has a high concentration of oxygen so that conventional three-way
 catalyst and oxygen sensor technology used with stoichiometrically fueled
 gasoline engines cannot be used for treating diesel exhaust gases. A
 number of so called lean-NO.sub.x catalysts have been developed which are
 selective for NO.sub.x reduction with organic chemical reductants. These
 catalysts have a relatively narrow temperature window in which they are
 selective for NO.sub.x reduction; above which, the reductant is oxidized
 without effective NO.sub.x control. For high efficiency NO.sub.x
 reduction, in addition to having an effective temperature window, the
 catalysts also require that the reductant be present in a certain molar
 ratio to the NO.sub.x.
 The temperature of the exhaust gases from a diesel engine in transient
 operation, such as in a vehicle, varies from about 100 to 700.degree. C.
 Until now, there has been no practical converter capable of NO.sub.x
 reduction over that range of normal operating conditions. Initially there
 is a problem in bringing a selected lean-NO.sub.x catalyst to a
 temperature within the temperature window that it is selective for
 NO.sub.x reduction and then there is problem in preventing it from being
 overheated. There is also a problem in providing the chemical reductant in
 the right proportion with respect to the NO.sub.x.
 Suitable reductants with lean-NO.sub.x catalysts are hydrocarbons,
 oxygenated organic compounds or carbon monoxide. Additional hydrocarbons
 or other reductant must be provided with lean-NO.sub.x catalysts as the
 amount of hydrocarbons in diesel exhaust is low.
 Another NO.sub.x removal technology, which has been used for diesel exhaust
 aftertreatment, includes the selective catalytic reduction of NO.sub.x
 with a nitrogen containing compound such as urea or ammonia. Like the
 lean-NO.sub.x catalysts, the known catalysts for selective catalytic
 reduction of NO.sub.x (hereunder sometimes abbreviated SCR catalysts)
 provide for effective removal of NO.sub.x within some temperature window
 and with a sufficient amount of ammonia or other nitrogen containing
 reductant added to the exhaust gas. The temperature window for efficient
 operation of a SCR catalyst is typically wider than for a lean-NO.sub.x
 catalyst, yet there are no SCR catalytic systems that can provide for high
 NO.sub.x removal efficiency over the entire range of diesel exhaust
 temperatures.
 A conventional catalytic converter for automotive exhaust aftertreatment
 includes a catalyst supported on a ceramic or metallic block or monolith
 with a plurality of straight, open channels for gas passage. In a
 conventional converter, the temperature inside the catalyst bed follows
 the temperature of the exhaust gases with some time delay. The temperature
 of the catalyst bed may then rise above the temperature of the exhaust
 gases since the reduction of NO.sub.x is an exothermic reaction. The
 exhaust gas parameters change quickly, with engine load and speed or
 during vehicle acceleration and deceleration, so that the temperature of
 the lean-NO.sub.x or SCR catalyst in the catalyst bed may fit the
 temperature window for NO.sub.x reduction for a while, but this favorable
 condition is not maintained long. Because the exhaust gases change in
 temperature with engine operating parameters, the catalyst quickly becomes
 overheated above the temperature window at which it is effective for
 NO.sub.x reduction, or overcooled below that window.
 At low temperatures typical for low load mode of engine operation, the
 lean-NO.sub.x and SCR catalysts do not provide for appreciable NO.sub.x
 conversion. Hence at low temperatures, some of the hydrocarbons used as
 reductants over lean-NO.sub.x catalysts or the nitrogen containing
 compounds (e.g., ammonia or urea) used as reductants over SCR catalysts
 may be emitted with the exhaust gases, increasing the environmental
 hazard. At high temperatures, the hydrocarbon reductants over
 lean-NO.sub.x catalysts or nitrogen containing compounds over SCR
 catalysts quickly react with oxygen thus reducing the process selectivity
 for NO.sub.x reduction.
 To some limited extent, the temperature of the exhaust gases can be
 controlled before or inside the converter. For example, a cooler can be
 installed in the exhaust pipe before the converter to control the
 temperature of the exhaust gases during engine acceleration. However, the
 cooler system is expensive, requires a suitable coolant and consumes
 energy. A heater system similarly adds to the cost and decreases engine
 efficiency.
 Modern diesel vehicles are often supplied with catalytic or non-catalytic,
 filters capable of removing diesel exhaust particulates or soot. A popular
 commercial filter includes a ceramic monolith with a plurality of straight
 channels, opposite ends of which are opened or closed in checkerboard
 fashion. The particulates gradually accumulate on the filter walls. The
 filter can be regenerated by raising the temperature of the exhaust gases
 and burning the particulates off. The regeneration can be catalytically
 activated through the addition of metal oxides to the diesel fuel or by
 depositing an appropriate metal oxide catalyst on the filter ceramic
 substrate. For a non-catalytically activated filter, the typical
 temperature required for initial ignition of diesel particulates is in
 excess of 600.degree. C. This temperature can be reduced to about
 350-400.degree. C. when the filter is catalytically activated. The
 temperature developed during the filter regeneration cannot be easily
 controlled as it depends on the amount of particulates accumulated. At
 high particulate capacity, the temperature can increase up to 1,200 to
 1,400.degree. C. during regeneration, which may cause the ceramic support
 to break down or the catalyst washcoat to be destroyed. Filter
 regeneration could be substantially improved if the operating temperature
 of the soot filter was above the soot ignition temperature most of the
 time so that filter regeneration occurred continuously. However, this is
 not easily achievable in a conventional converter, where the filter
 temperature follows that of the exhaust gases, and, therefore, can be very
 low for extended periods of time, allowing the soot to accumulate on the
 walls of the filter.
 BRIEF SUMMARY OF THE INVENTION
 In view of the above, it is an object of the present invention to provide a
 practical method and apparatus for purifying exhaust gases from a
 lean-burn engine, with a catalyst selective for NO.sub.x reduction. It is
 another object to provide such a method and apparatus that can accommodate
 wide variations in exhaust temperatures depending on the operating
 conditions of the engine. It is a still further object of the invention to
 provide for an improved method and apparatus for diesel soot removal that
 allows for continuous combustion of accumulated soot even when the
 temperature of the exhaust gases is low. Other objects and features of the
 invention will be in part apparent and in part pointed out hereinafter.
 The present invention provides a converter containing a catalyst bed with a
 catalyst which is effective for NO.sub.x reduction. The invention also
 concerns a method for periodically reversing the flow of exhaust gases
 from a lean-burn engine through the converter to maintain the catalyst in
 a temperature window that is selective for the reduction of NO.sub.x with
 a chemical reductant. Said method also includes the addition of chemical
 reductant to the exhaust gases as they are reverse flowed through the
 converter such that the chemical reductant is present in the amount
 required for the catalyst to be efficient in NO.sub.x reduction.
 The invention further provides a soot trap which may be in series with the
 catalyst bed in the converter discussed above, or provided as a portion of
 the converter. The method includes the addition of a chemical reductant to
 the exhaust gases as they are reverse flowed through the converter for use
 in heating and then continuously maintaining the soot trap at or above the
 ignition temperature of the soot.
 The invention summarized above comprises the method and constructions
 hereinafter described, the scope of the invention being indicated by the
 subjoined claims.

DETAILED DESCRIPTION OF THE INVENTION
 In accordance with the present invention, a converter 10 is provided for
 controlling the amount of NO.sub.x in exhaust gases from a lean-burn
 engine. Converter is particularly designed for use with a lean-burn engine
 under transient operation such as a diesel engine in a vehicle but can
 include stationary engines.
 The major pollutants in exhaust gases from a diesel engine are NO.sub.x and
 soot. The exhaust gases, however, also include hydrocarbons (hereunder
 sometimes abbreviated HC), carbon monoxide, water vapor, possibly some
 SO.sub.2 and a high concentration of oxygen.
 Converter 10 is a substantially closed container 12 with an inlet 14 and an
 outlet 16 between which is packed a catalyst bed 18 comprising catalyst
 capable of reducing NO.sub.x to N.sub.2 in a gas permeable solid material.
 The gas permeable solid material can be any material used as a catalyst
 substrate in an automobile exhaust purifier. For example, the gas
 permeable solid material can be formed from alumina, mullite, cordierite,
 zirconia or mixture thereof or from some other ceramic material that
 possesses high thermal stability and is resistant to thermal shock.
 Converter 10 can be thermally insulated using state-of-the-art means and
 gas permeable solid material may be provided as a randomly packed material
 of any suitable shape such as spheres, cylinders, Rashig rings, etc. or as
 a monolith having a random structure such as a ceramic, metallic, etc.
 porous foam, or an ordered structure with corrugated or wave channels,
 etc. It is preferred that the gas permeable solid material be formed as a
 monolith with straight, parallel channels to facilitate passage of the
 exhaust gases. The channels can have different sizes along the length of
 the monolith, but it is preferred that the channels be essentially equal
 in size.
 A suitable catalyst for use in converter 10 must have a temperature window
 within which it is effective for NO.sub.x reduction with a chemical
 reductant. For use in the present invention, the catalyst must be at least
 partly effective at destroying or oxidizing harmful compounds in the
 exhaust gases with the added chemical reductant and/or with CO and
 hydrocarbons present in the exhaust gases to non-harmful compounds such as
 carbon dioxide, water and nitrogen. The catalyst should be effective for
 NO.sub.x reduction in the presence of water, SO.sub.2, and such other
 materials as may be found in the exhaust gases. Suitable lean-NO.sub.x
 catalysts presently available and prospective lean-NO.sub.x catalysts
 include low temperature noble metal catalysts such as Pt/Al.sub.2 O.sub.3,
 Pt/SiO.sub.2 or Pt on ion-exchanged zeolite. These catalysts provide for
 NO.sub.x reduction within a temperature window between 200 and
 250-300.degree. C. Many zeolite based catalysts, such as those obtained
 through incorporation of transition metals (for example Fe, Cu and Co) to
 ZSM-5 or zeolite-Y structure can be effective within a temperature window
 from 300 to 500.degree. C. Some alumina supported catalysts such as, for
 example, AgAlO.sub.2 /Al.sub.2 O.sub.3, Sn/.gamma.-Al.sub.2 O.sub.3 can be
 applied in a higher temperature window, i.e., between 400 and 600.degree.
 C. Highly acidic alumina is, itself, active for NO.sub.x reduction by
 hydrocarbons in a temperature window between 550 and 650.degree. C.
 For urea or ammonia based NO.sub.x reduction systems, common SCR metal
 oxide catalysts include TiO.sub.2 --WO.sub.3 --V.sub.2 O.sub.5. Such
 catalysts typically provide for NO.sub.x reduction in a temperature window
 between about 250 and about 450-500
 C. At lower temperatures, the rate of NO.sub.x reduction is low, thus the
 NO.sub.x passes through the converter unaffected, while the nitrogen
 containing compounds, such as ammonia, add to the pollutants in the
 exhaust gases.
 As described below, converter 10 may have more than one catalyst bed 18. It
 will be understood that when more than one catalyst is used, the catalysts
 may be mixed and distributed in the gas permeable solid material.
 Alternatively, the catalysts may be provided in separate catalyst beds for
 sequential passing of the exhaust gases.
 When the catalyst is a lean-NO.sub.x catalyst, the chemical reductant may
 be selected from linear, cyclic and aromatic hydrocarbons, oxygenated
 organic compounds such as aldehydes, alcohols and ketones, or carbon
 monoxide, provided the selection is operative to reduce the NO.sub.x in
 the temperature window of the lean-NO.sub.x catalyst selected for catalyst
 bed 18. Mixtures of hydrocarbons, such as found in diesel fuel, may be
 used and, for reasons of cost, may be preferred. When the catalyst is a
 SCR metal oxide catalyst, the chemical reductant is a nitrogen containing
 compound such as ammonia or urea.
 When the catalyst is a lean-NO.sub.x catalyst and the chemical reductant is
 a hydrocarbon fuel, the hydrocarbon reductant can be preliminarily
 pre-treated in a partial oxidation reactor to obtain mixtures of
 oxygenated organic compounds such as aldehydes, alcohols and ketones,
 which may be more effective at NO.sub.x reduction than the hydrocarbons.
 Alternatively, oxygenated organic compounds such as acetaldehyde,
 formaldehyde, methanol, ethanol, acetone or the like can be specially
 supplied and carried onboard the vehicle for admixture with the diesel
 exhaust gases.
 Within the temperature window of the lean-NO.sub.x catalyst, the chemical
 reductant (taken as HC) either reduces the NO.sub.x or is oxidized by the
 oxygen. The chemistry of NO.sub.x reduction is represented by the
 following generalized process:
EQU NO.sub.x +HC.fwdarw.CO.sub.2 +H.sub.2 O+N.sub.2 (Process 1).
 The competing process is the reaction of the organic compound with oxygen:
EQU O.sub.2 +HC.fwdarw.H.sub.2 O+CO.sub.2 (Process 2).
 The ratio between the rate of Process 1 and the rate at which the HC is
 consumed through combined Processes 1 and 2 determines the catalyst's
 selectivity for NO.sub.x reduction.
 With the lean-NO.sub.x catalysts mentioned above, selectivity of the
 catalyst for NO.sub.x reduction within the temperature window requires
 that the reductant be in molar excess to the NO.sub.x. Preferably, the
 molar ratio of the reductant taken as C.sub.1 is equal to or greater than
 2:1, more preferably between about 2:1 and 8:1; however, the chemical
 reductant may be in even greater excess so long as the HC emissions after
 the converter are within acceptable levels.
 Within the temperature window for the SCR catalysts, the reductant is
 typically ammonia. If urea is used as the reductant, the urea is initially
 decomposed to ammonia before the NO.sub.x reduction begins. At optimal
 operating temperature, the reduction of the NO and NO.sub.2 components of
 NO.sub.x can be described by the following generalized process:
EQU NO+NO.sub.2 +NH.sub.3 +O.sub.2.fwdarw.N.sub.2 +H.sub.2 O (Process 3)
 At excessively high temperature, the ammonia is oxidized to N.sub.2, NO or
 N.sub.2 O, thus decreasing the selectivity:
 NH.sub.3 +O.sub.2.fwdarw.N.sub.2, NO, N.sub.2 O+H.sub.2 O (Process 4)
 For SCR catalysts, substantial reduction of NO.sub.x can be obtained when
 the molar ratio of the reductant taken as N to NO.sub.x is higher than
 about 0.5:1, preferably between about 0.5 and 0.8 to 1. At ratios higher
 than 1:1, a substantial quantity of unreacted ammonia, urea or other
 nitrogen containing compound may pass through the converter even when the
 catalyst is within the temperature window for selective reduction of the
 NO.sub.x. When the molar ratio is below about 0.5:1, less of the NO.sub.x
 is reduced.
 Converter 10 is inserted into an exhaust line 20 with a switching valve 22
 for reversing the flow of the exhaust gases between inlet 14 and outlet
 16. One or more ports 24 may be provided for adding reductant to the
 exhaust gases. Ports 24 may be upstream of converter 10, in converter 10,
 or both.
 The initial stage of the method according with the present invention
 includes heating up catalyst bed 18 so that at least a fraction of the bed
 reaches a temperature in the temperature window at which the catalyst is
 effective with regards to NO.sub.x reduction. As hot exhaust gases from
 the engine pass through converter 10, the temperature of the bed will
 follow the temperature of the exhaust gases with a time delay. This
 process can be accelerated with an electric heater or burner provided
 upstream of converter 10 for boosting the temperature of the exhaust gases
 or with an electric heater or burner provided in converter 10 or catalyst
 bed 18 for heating the catalyst bed directly.
 If the temperature in catalyst bed 18 exceeds the temperature window of the
 lean-NO.sub.x or SCR catalyst, the initial stage of the method according
 with the present invention involves cooling down the bed by decreasing the
 temperature of the exhaust gases either through deceleration (or load
 decrease) of the engine, or through the use of a cooler installed before
 or inside converter 10.
 One or more temperature sensors 26 should be provided inside converter 10
 for monitoring the temperature in catalyst bed 18. The temperature
 information from sensors 26 is provided to a controller 28. When the
 temperature in at least a portion of catalyst bed 18 reaches a temperature
 in the temperature window at which the catalyst is effective with respect
 to NO.sub.x reduction, controller 28 activates both switching valve 22 and
 a reductant delivery system 30.
 When sensors 26 determine that the temperature inside catalyst bed 18 has
 reached the temperature window of the lean-NO.sub.x or SCR catalyst,
 reductant delivery system 30 adds reductant to the exhaust gases through
 ports 24. Ports 24 may be in exhaust line 20, in converter 10 or in
 catalyst bed 18. More particularly, the addition of reductant can be made
 in exhaust line 20 before converter 10 or directly into the converter or
 both before and inside the converter. Alternatively, when the catalyst is
 a lean-NO.sub.x catalyst and the reductant is a hydrocarbon, the reductant
 can be added directly in the cylinders of the diesel engine. In-cylinder
 addition is preferably performed by means of recurring injection pulses of
 reductant during each cycle of piston movement. The timing of reductant
 injection can be a function of the crank angle after top dead center of
 the piston. For example, this crank angle can be chosen between 45 and
 120.degree..
 Reductant delivery system 30 may include an injection pump or compressed
 air injector. Delivery system 30 can also include a partial oxidation
 reactor for converting hydrocarbon reductant to oxygenated organic
 compounds (i.e. aldehydes, alcohols, ketones and so on), which may be more
 effective with regard to NO.sub.x reduction over a particular
 lean-NO.sub.x catalyst. The partial oxidation reactor may be installed in
 reductant delivery system 30 and can include a specific catalyst for
 partial oxidation of hydrocarbons, selected for example from the group
 consisting of oxides of base metals supported on alumina.
 When the temperature inside catalyst bed 18 reaches a temperature within
 the catalyst temperature window for effective NO.sub.x reduction, as
 delivery system 30 adds reductant to the exhaust gases, controller 28
 causes switching valve 22 to begin to periodically reverse the flow of the
 exhaust gases between inlet 14 and outlet 16. By continuing to
 periodically reverse the flow of exhaust gases through catalyst bed 18,
 the required temperature of the catalyst operation is sustained for a
 longer time than with a conventional non-flow-reversing operation. This
 allows for maintaining catalyst bed 18 at a temperature within the
 temperature window for the lean-NO.sub.x or SCR catalyst as the exhaust
 gases vary in temperature with changing engine operating parameters of
 speed and load.
 The amount of reductant added to the exhaust gases should be commensurate
 with the amount of NO.sub.x treated. For lean-NO.sub.x catalysts, the
 reductant must be taken in molar excess to the NO.sub.x, such that the
 number of carbon atoms supplied by the reductant combined with unburned
 hydrocarbons in the exhaust gases is greater than the number of NO.sub.x
 molecules. The preferred ratio of moles of reductant taken as C.sub.1 to
 the number of moles of NO.sub.x should be greater than one and preferably
 between 2 and 8. For the SCR process, the molar ratio of reductant and
 NO.sub.x taken as a ratio of number of nitrogen atoms should be less than
 one, preferably between 0.8 and 1.0. The molar ratio between the reductant
 and NO.sub.x can be controlled using a flow meter 32. Information from
 flow meter 32 is supplied to controller 28 which signals the amount of
 reductant to be added by reductant delivery system 30. The exhaust gas
 flow rate can be measured directly or computed based on the operating
 parameters of the engine such as its rotating speed, acceleration and
 deceleration, pressure in the intake pipe and so on. The means for
 controlling reductant delivery system 30 can also include a NO.sub.x
 sensor 34. This sensor can directly measure the NO.sub.x concentration in
 the exhaust gas. It is preferred, however, that the NO.sub.x concentration
 be predictively computed based on the operating parameters of the engine
 (e.g. its rotating speed, acceleration and deceleration, torque, pressure
 in the intake pipe, the opening of the injection pump and so on). The
 amount of NO.sub.x can be further calculated as a product of measured or
 predicted NO.sub.x concentration and total exhaust gas flow rate
 determined with help of flow meter 32.
 Controller 28 is programmed to calculate the amount of reductant necessary
 for efficient NO.sub.x reduction based on the flow rate and NO.sub.x
 concentration of the exhaust gases, and temperature inside catalyst bed
 18, obtained with flow meter 32, NO.sub.x sensor 34 and temperature sensor
 26. One of the simplest methods of control, which can be used for the
 system including lean-NO.sub.x catalyst, includes a proportional
 adjustment of the amount of added reductant to the total flow rate of
 exhaust gas. In this case, the concentration of the reductant will be
 approximately constant at all conditions of engine operation and the
 method does not require NO.sub.x sensor 34. An alternative method, which
 can be used with converters having a lean-NO.sub.x catalyst as well as
 those with a SCR catalyst, includes control of the amount of reductant
 proportionally to NO.sub.x flow rate, which may be determined with the
 help of NO.sub.x sensor 34 and flow meter 32.
 If the temperature in catalyst bed 18 exceeds the predetermined catalyst
 temperature window, controller 28 may be programmed to signal reductant
 delivery system 30 to reduce amount of reductant added to the exhaust
 gases. Lowering the amount of reductant, decreases the energy released due
 to the exothermic oxidation of the reductant and, thus, causing the
 temperature in converter 10 to fall until it is within the temperature
 window at which the catalyst is effective with regards to NO.sub.x
 reduction. In another situation, when the temperature measured by
 temperature sensor 26 is below the predetermined catalyst temperature
 window, controller 28 may be programmed to increase the amount of
 reductant thus causing the temperature of catalyst bed 18 to rise because
 of the exothermic oxidation of the reductant, tending to restore the
 temperature of the bed to a temperature within the catalyst temperature
 window. The appropriate amount of the reductant can be selectively
 injected through different ports 24 using control valves 36. For example,
 delivery of the reductant can be completely cut off from converter inlet
 14 and outlet 16 if the temperature in the boundaries of converter 10 near
 inlet 14 and outlet 16 exceeds the range of the catalyst temperature
 window, while continuing the addition of reductant through ports 24 inside
 converter 10 if the temperature there remains within the temperature
 window.
 A soot trap 38 may be included in converter 10, placed in series with the
 converter or used alone as more particularly described below. Soot trap 38
 filters soot from the exhaust gases by physically trapping the particles
 in their structure. Soot trap 38 may comprise a packing such as a
 temperature resistant metal or mineral wool. Of particular interest,
 however, are the ceramic monoliths traversed by parallel flow passages in
 which at any one time a passage, which is open on an end face, is closed
 on the other end face so that macroporously designed passage walls act as
 filter surfaces.
 From time to time the accumulated soot in soot trap 38 must be oxidized to
 regenerate the soot trap and prevent unacceptable levels of exhaust back
 pressure on the engine. The most common regeneration methods employ
 thermal means (engine operation modifications mainly by throttling intake
 or exhaust flow and/or addition of auxiliary heat via burners, electric
 heaters, etc.), aerodynamic means, or catalytic means (catalytic fuel
 additives, catalytic coatings, exhaust catalyst/oxidant injection, etc.).
 Catalytic fuel additives such as ceria and copper oxide reduce the
 ignition temperature to about 350 to 550.degree. C. in a so-called passive
 soot trap. The engine can then be adjusted to operate at higher soot
 emissions and lower NO.sub.x ; however, this technology requires the
 cooperation of the petroleum industry for the addition of ceria and copper
 oxide to the fuel and increases fuel costs. The metal oxides in the fuel
 may also corrode the engine and exhaust system and be harmful to the
 environment. In addition, the emission of NO.sub.x is still high.
 Soot trap 38, like catalyst bed 18, can be brought to the ignition
 temperature of the soot by the addition of reductant or by an external
 thermal source. Reverse flowing of the exhaust gases through soot trap 38
 assists in heating the trap and maintaining it at the ignition temperature
 of the soot.
 Various embodiments of the present invention are described below with
 specific reference to the accompanying drawings.
 First Embodiment
 As shown in FIG. 1, a gas permeable solid material, such as a monolith, is
 provided in converter 10 between inlet 14 and outlet 16. Catalyst bed 18
 comprises a catalyst with a temperature window in which it is effective at
 reducing NO.sub.x with a reductant. The catalyst is deposited on the
 monolith. Switching valve 22 reverses the flow of the exhaust gases from a
 lean-burn engine through converter 10 between inlet 14 and outlet 16. A
 reductant, such as diesel fuel when the catalyst is a lean-NO.sub.x
 catalyst, may be added to the exhaust gases through port 24 upstream of
 converter 10 or through port 24 into catalyst bed 18.
 In operation, catalyst bed 18 may be heated with an external source until a
 portion of the catalyst reaches a temperature at which the catalyst is
 effective for NO.sub.x reduction with the reductant. Thereafter, the
 exhaust gases are reverse flowed through converter 10 in a continuous
 series of cycles while additional reductant is added through port(s) 24 in
 an amount sufficient to reduce the NO.sub.x and to maintain a portion of
 the catalyst in catalyst bed 18 at a temperature within the temperature
 window.
 Second Embodiment
 A preferred embodiment of the present invention is shown in FIG. 2 wherein
 a soot trap 38 is placed between two beds of a high-temperature
 lean-NO.sub.x catalyst 18H. The system of operation includes injection of
 a hydrocarbon reductant or the like at port 24 upstream of converter 10.
 Ideally, the reductant is diesel fuel. After initially warming up beds 18H
 with an external thermal source, the system can maintain optimal catalyst
 temperature for a long time. The exhaust gases containing NO.sub.x and
 hydrocarbon react with the catalyst surface to produce primarily N.sub.2.
 Soot trap 38 will be regenerated continuously or periodically depending on
 the temperature requirements for oxidizing the soot. A periodic
 temperature increase for filter regeneration will be created naturally
 from the temperature excursion of the exhaust gases at high engine torque,
 or it will be forced by an increase in the amount of hydrocarbon reductant
 added to the exhaust gases. Catalyst bed 18H downstream of soot trap 38
 will promote oxidation of excess hydrocarbons by oxygen. To avoid sulfate
 build-up and formation of insoluble particulates, the diesel fuel should
 preferably have reduced sulfur content. With sulfur-free fuel (synthetic
 diesel, for instance), converter 10 may reduce NO.sub.x and soot emissions
 to nearly zero. A conventional, non-flow reversal converter would be
 impractical because of its inability to retain heat at low exhaust
 temperatures.
 Third Embodiment
 In the embodiment shown in FIG. 3, converter 10 includes a soot trap 38
 flanked by low temperature noble metal lean-NO.sub.x catalyst beds 18L.
 Boundary beds of gas permeable solid material 40 flank catalyst beds 18L.
 Gas permeable solid material 40 is not catalytically active and serves as
 a heat exchanger. This arrangement may be preferred for treatment of
 exhaust gases from light duty diesel engines characterized by low soot
 emission. The boundary inert beds 40 prevent catalyst beds 18L from being
 overheated and thereby maintain the catalyst temperature within the
 temperature window where the catalyst is selective for NO.sub.x reduction
 during temperature excursions of the exhaust gases from vehicle
 acceleration.
 Fourth Embodiment
 FIG. 4 shows a fourth possible configuration for converter 10. Converter 10
 includes two packed beds 40 and 18L of an gas permeable material arranged
 as cylindrical monoliths with straight-through channels. Upper bed 40 is
 not catalytically active and serves as a heat exchanger. Lower monolith
 18L has a noble metal lean-NO.sub.x catalyst deposited over its internal
 surface and thereby provides for NO.sub.x reduction with a hydrocarbon
 reductant. According to this arrangement, converter container 12 is
 divided into two plenums 42 by separation plates 44. The plenums are
 connected with a U-bend passage 46 at the bottom of container 12. The
 exhaust gases alternatively pass in one direction through one-half 48 of
 monoliths 40 and 18L and in an opposite direction through the other half
 50 of monoliths 40 and 18L. The structure of monolith channels together
 with separating plates 44 prevent gas leakage between halves 48, 50. The
 hydrocarbon reductant is introduced through port 24 in inlet 14 of
 converter 10 and through port 24 in U-bend passage 46 and port 24 in
 separation plates 44. Switching valve 22 is installed above container 12
 and provides for periodic flow reversal of the exhaust gases between
 plenums 42 and halves 48, 50 of monoliths. Switching valve 22 may have an
 intermittent drive such as described in SAE paper 99FL-288 "Development of
 a compact reverse-flow catalytic converter for diesel dual fuel LEV" by M.
 Zheng, E. Mirosh, W. Klopp, D. Ulan, M. Pardell, P. Newman, Yu. Matros and
 G. Bunimovich. Alternative designs for switching valve 22 include means
 for continuously rotating the valve and separation plates 44. Other
 possible configurations may include continuous rotating of catalyst bed
 18L and inert monolith 40 with regards to fixed inlet 14 and outlet 16 as
 described in U.S. Pat. No. 5,768,888 to Matros et al.
 Fifth Embodiment
 The fifth embodiment of the present invention may be useful for purifying
 exhaust gases from a diesel engine operated on a low-sulfur diesel fuel
 having a sulfur concentration less than 50 ppm. In this embodiment as
 shown in FIG. 5, soot trap 38 is inserted between noble metal, low
 temperature lean-NO.sub.x catalyst bed 18L and base metal, high
 temperature lean-NO.sub.x catalyst bed 18H. The noble metal, low
 temperature lean NO.sub.x catalyst monolith bed 18L allows for quick
 ignition of the converter, while the high temperature lean-NO.sub.x
 catalyst bed 18H takes over NO.sub.x reduction when the temperature in the
 low temperature lean-NO.sub.x bed 18L exceeds the temperature window for
 the noble metal catalyst. After the high temperature lean-NO.sub.x
 catalyst has reached the temperature window at which it is effective for
 NO.sub.x reduction, the reductant is mainly injected into the high
 temperature catalyst bed 18H. At this time, the temperature in soot trap
 38 rises sufficiently for continuous combustion of accumulated soot
 particles. The process of continuous soot regeneration is enhanced by
 NO.sub.2 produced through NO oxidation over the noble metal catalyst bed
 18L. The noble metal catalyst oxidizes NO in exhaust gas to NO.sub.2 which
 catalyses the process of soot removal in the soot trap. The hydrocarbons
 that were not oxidized over the base metal, high temperature lean-NO.sub.x
 catalyst bed 18H are effectively removed as they pass through noble metal
 catalyst 18L located in outlet 16 of converter 10.
 Sixth Embodiment
 FIG. 6 shows a possible configuration of converter 10 operated in
 accordance with the fifth embodiment. This configuration is similar to
 that shown in FIG. 4 except that it includes soot trap 38 and high
 temperature catalyst monolith 18H. Reductant (e.g., hydrocarbon) injection
 is provided through port 24 into converter inlet 14 and through port 24
 into the middle of u-bend passage 46.
 EXAMPLE 1
 A converter as shown in FIG. 1 may be used for purification of exhaust
 gases released after a medium size heavy-duty truck with a naturally
 aspirated ISUZU engine having a displacement volume 8.2 L. The engine may
 be installed on a dynamometer and tested using a standard 13 mode Japanese
 procedure for heavy-duty diesel engines. Table 1 describes the engine
 parameters and characteristics of the exhaust gases during each mode of
 the test.
 TABLE 1
 Engine and exhaust gas parameters for 13 mode Japanese test:
 Mode Engine Engine Engine Exhaust Exhaust NOx
 HC
 Test duration speed torque power flow rate temperature concentration
 concentration
 mode (sec) (rpm) (Nm) (KW) (g/s) (.degree. C.) (ppm)
 (ppm)
 1 306 560 5 0.3 40 150 160
 210
 2 124 1120 103 12.1 76 200 200
 110
 3 102 1120 206 24.2 76 250 300
 95
 4 316 560 4 0.2 39 110 150
 180
 5 115 1680 105 18.5 110 212 195
 120
 6 147 1680 206 36.2 108 330 250
 115
 7 119 2240 208 48.8 138 420 220
 135
 8 109 2240 312 73.2 132 550 230
 180
 9 162 1680 316 55.6 97 530 340
 200
 10 136 1680 422 74.2 108 550 340
 110
 11 129 1680 501 88.1 134 600 660
 100
 12 115 2240 417 97.8 180 650 520
 115
 13 240 1680 26 4.6 104 200 160
 135
 As shown in Table 1, the total duration of the test is 2120 sec. In modes
 1-6 and 13, which take up of about 60% of total test time, the engine is
 operated at relatively low speed and torque so that the temperature of the
 exhaust gases is below 400.degree. C. In highly loaded modes of operation
 7-12, the exhaust gases rise to 650.degree. C. The standard procedure
 includes initial warming up the engine in highly loaded mode such that the
 converter is preheated to 450.degree. C. or higher before the test begins.
 The concentration of NO.sub.x in diesel exhaust gases varies from 160 to
 660 ppm depending on engine operating conditions while the concentration
 of hydrocarbons does not exceed 200 ppm during all modes of operation
 (Table 1).
 Converter 10 installed after the engine operates according to the general
 embodiment shown in FIG. 1 and has the particular configuration shown in
 FIG. 3 except that only one catalytically active cylindrical monolith is
 included. The monolith is 14 inches (355.6 mm) in diameter and 10 inches
 (254 mm) in length. It is made from a standard metallic substrate formed
 from two sheets of metal foil, 0.5 mm thick, and rolled together into a
 spiral shape. One of the sheets is flat and the other is corrugated. The
 straight-through channels formed between the corrugated and flat foil
 sheets are approximately 1.5 mm in size. The cell density of the channels,
 in a cross-section of the block, is about 200 cells per square inch (200
 cpsi) and the total volume of block is about 25 L.
 A washcoat of a base metal, high temperature lean-NO.sub.x catalyst is
 applied to the block. The catalyst used in this example is a
 tungsten/silver aluminate catalyst, W/AgAlO.sub.2 /Al.sub.2 O.sub.3,
 prepared according to the method described in a publication by Nakatsuji
 et al. (T. Nakatsuji, R. Yasukawa, K. Tabata, K. Ueda, M. Niwa, "Catalytic
 reduction of NO.sub.x in exhaust gas from diesel engines with secondary
 fuel injection." Applied catalysis, B: Environmental 17 (1998), 333-335).
 The method includes immersing alumina powder in a silver and tungsten
 nitrate water solution, followed by thermal treatment of the wetted powder
 in humidified air at 800.degree. C. The powdery catalyst is then mixed
 with alumina sol and milled to into a slurry, forming the washcoat. The
 metallic monolith is the coated with 200 g/L of the washcoat and then
 calcined in air at 500-600.degree. C. The catalyst is stable with regard
 to any water and SO.sub.2 in the exhaust gases and provides for high
 activity toward NO.sub.x reduction with diesel fuel within a temperature
 window between 350 and 600.degree. C. The catalyst performance is further
 improved when the diesel fuel reductant is pretreated over a partial
 oxidation catalyst to produce several hundred ppm of acetaldehyde in the
 exhaust gas. The publication mentioned above by Nakatsuji et al. also
 describes a method for partial oxidation of diesel fuel. FIG. 7 presents
 an example of NO.sub.x reduction curves plotted based on the data obtained
 in this publication.
 For the conditions of the experiments shown in FIG. 7 the 200 cpsi catalyst
 monolith was tested with a diesel engine exhaust gas containing 900 ppm of
 NO.sub.x, 11% O.sub.2, 7% H.sub.2 O and 40 ppm SO.sub.2. The organic
 reductant obtained after partial oxidation of diesel fuel had a
 concentration of about 3,600 ppm taken as C1. The points in FIG. 7
 correspond to the experimental data obtained at different space velocities
 of catalyst monolith. Space velocity is equal to the catalyst volume (L)
 divided by exhaust gas flow rate taken as L/h at normal conditions. The
 solid curves in FIG. 7 are obtained from a mathematical model, which
 estimates rate of reaction of NO.sub.x reduction and hydrocarbon
 oxidation. The model is described by the following system of equations:
 ##EQU1##
 where x.sub.HC and x.sub.NO conversion of hydrocarbon reductant and
 NO.sub.x, respectively, k.sub.o1 : pre-exponent, E.sub.1 : activation
 energy of reaction of HC oxidation, T: absolute temperature, K; R:
 universal gas constant, a, and n empirical coefficients, C.sub.o,HC :
 initial concentration of organic reductant, k.sub.o,2 and E.sub.2 :
 parameters, characterizing rate of inhibition of NO.sub.x reduction with
 increase in temperature.
 The parameters of the model and dimensions are given in the following
 table.
 TABLE 2
 Coefficient Dimension Value
 k01 s.sup.-1 3.4 .times. 10.sup.8
 E.sub.1 Cal/mole 25,500
 A -- 50.8
 N -- 0.46
 k.sub.o,2 8.2 .times. 10.sup.7
 E.sub.2 Cal/mole 15,600
 R Cal/mole/.degree. K. 1.987
 Referring to FIG. 4, the process control in the discussed example is
 performed using a single thermocouple sensor 26 located under catalyst bed
 18L in u-bend passage 46. The reductant is equally distributed between two
 injection ports 24, one in u-bend passage 46 and another in converter
 inlet 14. The total amount of reductant injected in each of the test modes
 is preset initially based on flow rate of the exhaust gases. The
 concentration of the reductant is thereby preset to be a constant 3000
 ppm, during each operated mode. The preset amount of injected reductant is
 continuously corrected during the test based on the data from the control
 thermocouple sensor 26. Controller 28 uses the proportional change of the
 reductant stream according with the formula:
EQU .DELTA.C=k(T.sub.catalyst -T.sub.actual),
 where .DELTA.C is correction in the concentration of the reductant
 injected, k: controller gain, T.sub.catalyst : preset required temperature
 in catalyst bed, T.sub.actual : actual temperature measured by temperature
 sensor 26. The coefficient k was preset to be equal 20 ppm/.degree. C. The
 amount of reductant was then calculated from the corrected concentration
 C.sub.corrected =C.sub.preset +.DELTA.C, and the exhaust flow rate in each
 mode.
 FIG. 8 characterizes the behavior of a flow reversing and a non-flow
 reversing converter during the standard test. The reversing converter is
 operated with a continuous series of cycles with a period between valve
 switching equal to 10 sec and total cycle period 20 sec. The transient
 curves in FIG. 8 were determined based on a two-phase model of a fixed bed
 reactor. The model is described in equations (8)-(10) in Reference 1
 below. The heat and mass transfer parameters of metallic monolith were
 obtained from Reference 2, while the catalyst kinetics were approximately
 described by the two equations given above regarding the derivatives of HC
 and NO.sub.x conversion. The kinetic parameters are listed in Table 2.
 REFERENCES
 Matros, Yu. Sh. and G. A. Bunimovich, "Reverse-Flow Operation in Fixed Bed
 Catalytic Reactors.", Catalysis Review--Science Engineering, 1996, 38 (1),
 1-68.
 Day, J. P., "Substrate Effects on Light-Off. Part II, Cell Shape
 Contribution, SAE paper 971024, 1997.
 Curves 1 and 2 in FIG. 8 represent the predicted temperature in the
 connecting space of the converter where temperature sensor 26 is located.
 Curve 1 corresponds to reversing flow operation while curve 2 to
 conventional operation without flow reversing. Curve 3 represents the
 temperature of exhaust gases in the converter inlet. Curves 4 and 5 show
 the NO.sub.x concentration in the outlets for flow reversing and non-flow
 reversing converters, respectively, while line 6 is inlet concentration of
 NO.sub.x after the engine. FIG. 8 also shows increase in the emission of
 NO.sub.x cumulated during the test time. This emission was calculated
 based on NO.sub.x concentration and flow rate for inlet exhaust gas (curve
 7) and outlet gas after flow reversing and non-flow reversing converters
 (curves 8 and 9, respectively). According with the test procedure, the
 engine is initially warmed up, preheating the catalyst bed to 450.degree.
 C. This initial temperature is within the temperature window of the
 lean-NO.sub.x catalyst applied, so the controller 28 begins to
 periodically switch flow direction and the reductant is supplied to the
 inlet and middle part of the converter in the amount corresponding to
 total concentration in the exhaust gas 3000 ppm. After beginning the test,
 the exhaust temperature quickly falls to 150.degree. C. according with a
 preset engine load and speed in mode # 1 (Table 1). Thereafter the
 temperature of the exhaust gases and inlet concentration of NO.sub.x
 (curves 3 and 6, FIG. 8) change in a step-wise fashion according with the
 engine parameters prescribed by the other 12 modes of the test (Table 1).
 Total NO.sub.x amount cumulatively produced during the test (curve 7, FIG.
 8) gradually increases up to about 90 g.
 The temperature in the conventional non-flow-reversing converter (curve 2,
 FIG. 8) closely follows the temperature of the exhaust gases. Therefore,
 this converter is cooled down quickly during mode 1 and than remains at
 low temperature for a half of the test time during modes 2-6. The
 conventional converter also loses heat quickly during the last test mode
 13. In highly loaded modes 11 and 12, the converter temperature exceeds
 that of the exhaust gases. This kind of temperature behavior leads to very
 low NO.sub.x conversion during the low load modes, because the temperature
 in catalyst bed lies below the temperature window at which the catalyst is
 effective for NO.sub.x reduction. At highly loaded modes of engine
 operation, the actual temperature of the catalyst in the catalyst bed is
 higher than the catalyst operating window and that also contributes to low
 NO.sub.x reduction efficiency. With the conventional converter, as it can
 be seen by comparing curves 7 and 9 in FIG. 8, total NO.sub.x emission is
 only 13% below the inlet exhaust emission.
 Curve 1 (FIG. 8) demonstrates that reversing the flow through the converter
 makes it possible to retain the temperature within the catalyst
 temperature window both at low and high load of engine operation.
 Comparing curves 6, 4 and 5 in FIG. 8 shows that the NO.sub.x reduction
 efficiency in a reversed flow converter exceeds that of conventional
 converter during all modes of test. The NO.sub.x reduction efficiency for
 flow reversing operation is as much as 50% during low load modes 2-6 while
 the conventional converter does not indicate any appreciable NO.sub.x
 reduction at all. Some conversion gain is also indicated for flow
 reversing operation at highly loaded modes 11 and 12. Over the entire
 test, the reversing flow converter provides for NO.sub.x emission
 reduction by 26% thus increasing the NO.sub.x removal efficiency by two
 times compared with the conventional converter. As other calculations
 show, this gain can be higher at higher amounts of catalyst or higher
 catalyst activity with regards to NO.sub.x reduction.
 FIG. 8 also illustrates potential benefits of using the present invention
 in its second, third and fifth embodiments of the invention (FIGS. 2, 3
 and 5), which combine a soot trap with lean-NO.sub.x catalysts. The
 ability of a reversing flow converter to retain high temperature in the
 center of a packed bed allows for continuously maintaining a high
 temperature in the soot trap installed between the beds of NO.sub.x
 reduction catalyst. Maintaining sufficiently high temperatures by
 controlling the amount of injected reductant creates favorable conditions
 for continuous or periodic combustion of soot accumulated in the trap.
 EXAMPLE 2
 In this example, the converter with the configuration shown in FIG. 3 is
 installed in a passenger vehicle with diesel engine having a 2-L
 displacement volume. The engine and converter are tested in a dynamometer
 for a European Motor Vehicle Emission Group standard test procedure for
 light-duty vehicles. This procedure includes a continuous transient change
 of engine speed for a 1200 sec test time and models low load urban driving
 with frequent idling, accelerating and decelerating. When the test begins,
 the engine and converter are cold and the exhaust gases have a temperature
 below 100.degree. C. during first 60 sec. The following 1000 sec of the
 test correspond to low torque operation and the temperature of the exhaust
 gases does not exceed 300.degree. C. during this extended time. During the
 last 150 sec of the test, the engine load increases sharply, causing the
 temperature of the exhaust gases to rise to 500.degree. C. The gas flow
 rate after the particular engine considered in this example is about 150
 L/sec for the low load fraction and then increases up to 80 L/sec during
 the last fraction of the test. The NO.sub.x concentration averages about
 100 ppm at the initial fraction of the test and increases up to several
 hundred ppm during about 50 sec of the loaded test fraction. The
 concentration of the hydrocarbons in the exhaust gases does not exceed 100
 ppm at any time.
 According to FIG. 4, the converter includes a cylindrical ceramic block 18L
 of a supported noble metal, lean NO.sub.x catalyst and a similar block of
 inert non-catalytic material placed above the catalyst. The diameter of
 each monolith is 8 inches (203 mm). The length of the catalyst bed is 6
 inches (152 mm). Total volume of catalyst is 5 L. The length of inert
 block was varied between 0 to 6 inches. The catalyst and inert blocks are
 formed from cordierite and have a standard cell density 200 CPSI. The
 channels for gas passage through the inert monolith have square
 cross-section with each side being about 1.5 mm. The size of the channels
 in the catalytic monolith is reduced by 0.1 mm due to the thickness of
 catalyst washcoat. The washcoat includes Pt/-Al.sub.2 O.sub.3 prepared
 through incipient wetness impregnation and has a 2% platinum loading. This
 catalyst provides for NO.sub.x reduction within a temperature window
 between 150 and 350.degree. C. With diesel fuel used as a reductant the
 maximum NO.sub.x conversion is observed at temperature around 220.degree.
 C. and at a ratio between numbers of moles of diesel fuel reductant taken
 as HC and NO.sub.x equal to 6 with NO.sub.x concentration about 500 ppm.
 The absolute number for the NO.sub.x conversion at space velocity 40,000
 h.sup.-1 is in the range of 30%. The catalyst is stable with regard to
 water and SO.sub.2 in diesel exhaust gas.
 The converter as shown in FIG. 4 was studied with and without periodic flow
 reversal using similar mathematical model described in the Example 1. The
 concentration of hydrocarbons in the inlet of the converter was maintained
 fixed at 2400 ppm. In some modeling runs the reductant was injected only
 in the inlet of the converter, while in other runs it was injected also
 through port 24 in U-bend passage 46. Equal feed of hydrocarbon reductant
 was assumed in both injection points. The flow reversals and reductant
 feeding was set to begin after the temperature of exhaust gases (FIG. 4)
 reached 200.degree. C. The period of flow reversal was equal to 20 sec.
 Table 3 illustrates the NO.sub.x conversion results with and without flow
 reversal at different lengths of inert monolith and different number of
 injection points.
 TABLE 3
 Run # 1 2 3 4
 Number of injections 1 2 2 2
 Height of inert bed, 0 0 3 6
 inches
 NO.sub.x conversion
 non-reversing 11 12 13 13
 reversing 16 19 24 27
 Similar to Example 1, Table 3 shows an increase in NO.sub.x conversion for
 flow reversing operation compared with the traditional non-reversing
 operation. The reversing flow converter may provide for as much as 1.5-2
 times more NO.sub.x reduction depending on the volume of inert material
 and number of injection points. The benefits of flow reversing are
 increased with increased ceramic material length and by using a
 distributed reductant injection.
 In view of the above, it will be seen that the several objects of the
 invention are achieved and other advantageous results attained. As various
 changes could be made in the above method and constructions without
 departing from the scope of the invention, it is intended that all matter
 contained in the above description or shown in the accompanying drawings
 shall be interpreted as illustrative and not in a limiting sense.