Method and device for catalytic nitrogen oxide reduction of motor vehicle exhaust

Abstract

A method and a device is provided for NO reduction in motor vehicle exhaust by reduction on a catalyst. The hydrogen required for the NO reduction is produced directly on-board the motor vehicle by water vapor reformation and/or by partial oxidation of hydrocarbons, for example, methanol, diesel fuel, or gasoline on a catalyst. The device includes an adjustable heater for the catalyst for water vapor reformation or for partial oxidation.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of patent application Ser. No. 09/916,287, filed Jul. 30, 2001, which is a continuation of patent application Ser. No. 08/585,685, filed Jan. 16, 1996, the entire disclosures of which are incorporated herein by reference. Priority is claimed based on German Patent Application No. 195 00 788.3, filed Jan. 13, 1995.

FIELD OF THE INVENTION

The invention relates to a method and a device for catalytic reduction of nitrogen oxides for mobile applications and, more particularly, for reducing nitrogen oxides in the exhaust of motor vehicles by reduction on a catalyst with the addition of hydrogen. The hydrogen required for nitrogen oxide reduction is generated on-board the vehicle by water vapor reformation and/or by partial oxidation of hydrocarbons, for example, methanol, diesel fuel, or gasoline on a catalyst.

For the operation of motor vehicles with gasoline and, especially for diesel engines, the observation of the applicable legal emission guidelines is indispensable. In this connection, catalytic NOx reduction using hydrogen is being used advantageously.

This catalytic removal of nitrogen oxides from the combustion exhaust from motor vehicles is performed using hydrogen on suitable catalysts with the reaction
2NO+2H₂→N₂+2H₂O.

In the known method for removing nitrogen oxides by NO_x reduction, the hydrogen required for the reaction is carried in the vehicle, for example in compressed gas tanks, liquid hydrogen tanks, or metal hydride storage devices. The disadvantage of this method is that large heavy containers are required to transport the hydrogen. The cumbersome containers also have a narrowly limited capacity, hence requiring short intervals between refills.

European Patent document EP 0 537 968 A1 describes a device for catalytic reduction of nitrogen oxides in motor vehicle exhaust by addition of hydrogen. Hydrogen generation takes place aboard the motor vehicle by partial oxidation or reformation of methanol on a suitable catalyst. The heating of these catalysts is accomplished by placing them in the hot exhaust stream from the engine.

This device has the following disadvantages: 1) with the engine cold, for example shortly after starting, the catalyst is not yet active for reformation or partial oxidation; 2) the engine must be adjusted in such fashion that an excessive exhaust temperature is avoided throughout the entire lifetime of the engine, even briefly, in order to prevent irreversible deactivation of the catalyst; and 3) it has a relatively cumbersome size.

Hence, the object of the invention is to provide a method and a device to generate hydrogen in a motor vehicle which overcomes the disadvantages of the prior art described above.

These objects are achieved by the method for reducing nitrogen oxides in the exhaust of motor vehicles by reduction on a catalyst with the addition of hydrogen. The hydrogen required for nitrogen oxide reduction is generated on-board the vehicle by water vapor reformation and/or by partial oxidation of hydrocarbons, for example, methanol, diesel fuel, or gasoline on a catalyst. The catalyst is adjustably heated for water vapor reformation or for partial oxidation. These objects are further achieved by the device for reduction of nitrogen oxides in the exhaust of motor vehicles by catalytic reduction, including a reactor containing a catalyst on which the nitrogen oxide reduction is performed by the addition of hydrogen; a device for generating hydrogen located on-board the motor vehicle and including a reactor for water vapor reformation of hydrocarbons on a catalyst and/or a reactor for partial oxidation of hydrocarbons on a catalyst. An adjustable heating device is provided for the- reactor for water vapor reformation and for the reactor for partial oxidation.

According to the present invention, the catalyst used to generate the hydrogen is adjustably heated for reformation of hydrocarbons.

The device according to the present invention includes an adjustable, especially an electrical, heating device by which, independently of the hot engine exhaust, the catalyst can be brought to a predetermined temperature and kept there.

The adjustable heating of the catalyst according to the invention for reformation of hydrocarbons has the following advantages: 1) by preheating the catalyst, its activation can occur as soon as the engine starts; 2) it is possible to keep the catalyst operationally ready (standby operation) even when the vehicle is parked for some time; 3) the catalyst can be operated at its optimum operating temperature through temperature regulation; 4) in contrast to known devices which heat the catalyst using hot engine exhaust, the temperature can be regulated flexibly and independently of the operating state of the engine, and damage to the catalyst (deactivation) is always avoided; and 5) in contrast to known devices, the result is a compact unit with small dimensions which can easily be integrated as a separate module into an existing device, for example through the use of a flange.

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating the process of water vapor reformation from methanol.

FIG. 2 illustrates a reactor for performing water vapor reformation from methanol.

FIG. 3 is a schematic block diagram illustrating the process of partial oxidation of methanol.

FIG. 4a illustrates a reactor for performing partial oxidation of methanol.

FIG. 4b illustrates a hydrogen generator 50 and a reactor for nitrogen oxide reduction.

FIGS. 5a and 5b illustrate another reactor for performing partial oxidation or for water vapor reformation.

DETAILED DESCRIPTION OF THE DRAWINGS

H₂ generation for NO reduction takes place aboard the motor vehicle by reformation of hydrocarbons. Methanol is especially suitable for this purpose. Diesel fuel or gasoline may also be used for reformation.

One advantageous reaction in this regard is the water vapor reformation from methanol by the equation
CH₃OH+H₂O→3H₂+CO₂
and partial oxidation of methanol by the equation
CH₃OH+½O₂→2H₂+CO₂.

The first reaction is endothermal and can be performed with catalysts that are known of themselves, for example catalysts with the active components copper and zinc, at only 200 to 400° C.

The second reaction is exothermal and is likewise performed with the support of catalysts that are known of themselves, for example Pt, Pd, Ru, or CuZnO.

In both reactions, depending on the operation of the reactors, CO is produced in a ‘range from several hundred ppm to several percent.

The processes of water vapor reformation and partial oxidation of other hydrocarbons are known and proceed in accordance with comparable equations but under different reaction conditions (mainly, higher temperatures) and possibly with other catalysts. However, for the process performance and reactor variations the same remarks apply in theory as described below for the special case of methanol.

In one advantageous embodiment, hydrogen generation can also be performed by a mixed reaction composed of water vapor reformation and partial oxidation.

Water Vapor Reformation from Methanol (FIGS. 1 and 2)

FIG. 1 shows the process of water vapor reformation from methanol. Initially, a water-methanol mixture is delivered from a tank, preferably with a molar ratio of methanol to water of 1:1 to 1:2. The tank can be under ambient pressure. Delivery is by means of a pump that assumes the metering function directly as a function of the load through a suitable control (rpm regulation, for example). Alternatively, the liquids can be delivered through a constantly operating pump, with subsequent metering being performed for example by solenoid valves, operating comparably to the injection valves in a motor vehicle. Then, the water-methanol mixture is evaporated in an evaporator and fed to the reactor. In the reactor, with the addition of heat to the catalyst, the conversion into hydrogen and CO₂ and H₂ takes place. According to the invention, direct heating of the catalyst is provided. Preferably, an electrical resistance heater can be used here as a heat source. The addition of heat to the reactor can be adjusted, for example, by a temperature regulator in the catalyst. Possibly the CO component of the gas product can then be lowered in another reaction stage and/or the hydrogen yield can be increased, for- example, by means of the known shift reaction:
CO+H₂O→H₂+CO₂.

Instead of water and methanol being stored as a mixture in the same tank, the water and methanol can be stored separately from one another in different tanks and delivered from them.

FIG. 2 shows one advantageous embodiment of a reactor for performing the water vapor reformation of methanol or other hydrocarbons. It is in the form of a tube 10, preferably with an inside diameter of 5 to 50 mm. The reactor is divided into three stages 12, 14 and 16. In the center 14 is the main reaction stage in which a catalyst 18 is located. This is where the water vapor reformation takes place. The catalyst 18 is present in the form of loose material but monolithic materials can be used as well. An evaporator section 12 is located upstream from the main reaction stage 14, in which the water-methanol mixture is evaporated as it enters. An aftertreatment stage 16 is located downstream from the main reaction stage 14. This is where the CO that is produced is reduced by means of a shift reaction.

The figure also shows an electrical heater 20 according to the invention by which the catalyst in the main reaction zone is heated. In this case, the electrical heater is located in the reaction chamber itself but can also be located in the outer area of the tube. In addition, the heating of the reactor can also be performed by the hot exhaust from the internal combustion engine of the motor vehicle. In this case, the hot exhaust stream is guided over ribs (22, shown in dotted lines) provided on the outside wall of the reactor.

The figure also shows that the evaporator stage 12 is also heated by the above-mentioned heating arrangement or electrical heater 20. However, embodiments are also possible in which the individual stages including the aftertreatment stage 16 can be heated independently of one another. Thus the evaporator stage 12 in particular can be heated more strongly than the reformer stage (main reaction stage) 14. The aftertreatment stage 16 is heated only slightly, if at all, so that the corresponding heating is not shown here.

Partial Oxidation of Methanol (FIGS. 3 and 4)

FIG. 3 shows the process of partial oxidation of methanol. Initially, a water-methanol mixture is supplied from a tank, preferably with a molar ratio of methanol to water of 1:0 to 1:2. The tank in this case can be at ambient pressure. Delivery is by means of a pump which may assume the metering function directly as a function of load through the use of a suitable control, for example, rpm regulation. Alternatively, the liquids can be delivered through a constantly operating pump, with subsequent metering being performed for example by solenoid valves comparable to the injection valves in a motor vehicle. Then, the water-methanol mixture is evaporated, air is fed to the vapor mixture through a compressor, for example a diaphragm pump, and the mixture is admitted to the reactor. In the reactor the exothermal conversion to hydrogen and CO₂ and H₂ takes place on a catalyst. Advantageously, the CO component of the gas product can then be reduced in a subsequent reactor stage and/or the hydrogen yield can be increased, for example by means of the known shift reaction:
CO+H₂O→H2+CO₂.

As in the process described above for water vapor reformation, water and methanol can be used instead, stored as a mixture in the same tank, or they can be stored separately from one another in different tanks and be delivered from them. The water can also be supplied advantageously only after partial oxidation. This has the advantage that the CO content is additionally reduced.

FIGS. 4a and b show a concrete embodiment of a reactor for performing partial oxidation of methanol and an arrangement with a reactor for nitrogen oxide reduction 48. It is in the form of a tube 30 whose inside diameter is preferably 5 to 50 mm.

Once again the main reaction stage 32 is located in the center, together with the catalyst 34 on which partial oxidation takes place. According to the invention, adjustable heating is provided for the catalyst in the form of an electrical resistance heater 36.

In the tubular reactor, connected upstream from the main reaction stage 32, an evaporator stage 38 is located that is heated by an electrical heater 40. Instead of or in addition to this heating, a feed device 46 can be provided with which the hot product gases that are produced during partial oxidation on the catalyst can be supplied for example in a countercurrent on the outside wall of the reactor.

Between the evaporator stage 38 and the main reaction stage 32 is a mixing stage 42 in which the methanol-water mixture in vapor form is mixed with air admitted from the outside and conducted into the reforming stage 32. The reforming stage is followed by a gas aftertreatment stage 44 in which further reaction of the remaining methanol with water vapor takes place and/or the resultant CO is reduced with water vapor in a shift reaction. This stage is heated only slightly, if at all, so that the corresponding heatinq is not shown here.

FIG. 4b shows an H₂ generator which is configured to deliver gas to a reactor for nitrogen oxide reduction 48.

FIGS. 5a and 5b show another reactor whose theoretical design can be used both for water vapor reformation and for partial oxidation of hydrocarbons. FIG. 5a shows a section through the reactor in the radial direction. FIG. 5b shows a section in the axial direction along line AB in FIG. 5a. Sample dimensions are given (in mm). The reactor consists of a compact cylindrical body including a plurality of axial holes. Four of these axial holes contain the catalyst, as loose material for example. The four additional axial holes, which in this embodiment have a smaller diameter by comparison with the holes for the catalyst, constitute the evaporator stage. Another hole is provided in the center of the reactor. This center hole contains a heating cartridge. Since this heating cartridge is surrounded by both the evaporator holes and the catalyst holes, it can be used both for evaporation and for heating the catalyst according to the present invention optionally, the outer jacket of the reactor can be provided with thermal insulation in order to keep heat loss as low as possible. Reformation of methanol then takes place in such fashion that the methanol/oxygen mixture is conducted into the holes in the evaporator and then is conducted in the opposite direction through the holes that are filled with catalyst.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example, and is not to be taken by way of limitation. The spirit and scope of the present invention are to be limited only by the terms of the appended claims.

Claims

1. A device for reducing nitrogen oxides in an exhaust stream of a motor vehicle via a catalytic reduction, comprising:

a nitrogen oxide reduction reactor containing a catalyst on which nitrogen oxide reduction of the nitrogen oxides in the exhaust stream is performed via the addition of hydrogen;

a hydrogen generating device arranged on-board said motor vehicle for generating hydrogen, said hydrogen generating device including at least one of a water vapor reformation reactor for water vapor reformation of hydrocarbons and a partial oxidation reactor for partial oxidation of hydrocarbons, said hydrogen generating device being arranged so as not to be in thermal contact with the exhaust stream of the motor vehicle;

an adjustable heating device coupled with at least one of the water vapor reformation reactor for water vapor reformation and the partial oxidation reactor for partial oxidation.

2. The device according to claim 1, wherein said adjustable heating device is an electrical heater.

3. The device according to claim 2, wherein the electrical heater is at least one of a resistance heater and a heating cartridge.

4. The device according to claim 1, wherein said hydrogen generating device includes said water vapor reformation reactor, and further wherein a catalyst in the water vapor reformation reactor contains one of copper and zinc as active components for water vapor reformation.

5. The device according to claim 1, wherein said hydrogen generating device includes said water vapor reformation reactor, and further wherein the water vapor reformation reactor further comprises an evaporator stage for water vapor reformation, said evaporator stage being located upstream from a main reaction stage in which the water vapor reformation on a catalyst occurs.

6. The device according to claim 1, wherein said hydrogen generating device includes said water vapor reformation reactor, and further comprising an aftertreatment stage located in said water vapor reformation reactor for water vapor reformation downstream from a main reaction stage in which the water vapor reformation on a catalyst occurs,

wherein in said aftertreatment stage one of a CO produced by the main reaction stage is reduced by a shift reaction and a hydrogen yield is increased.

7. The device according to claim 1, wherein said hydrogen generating device includes said water vapor reformation reactor, and further wherein said water vapor reformation reactor for water vapor reformation is a tube having an inside diameter between 5 to 30 mm.

8. The device according to claim 6, further comprising an evaporation stage for the water vapor reformation reactor, and means for independently heating said evaporation stage, said main reaction stage, and said aftertreatment stage from one another for water vapor reformation.

9. The device according to claim 1, wherein said hydrogen generating device includes said partial oxidation reactor, and further wherein said partial oxidation reactor comprises an evaporator stage for partial oxidation, said evaporator stage being located upstream of a main reaction stage in which partial oxidation on a catalyst occurs.

10. The device according to claim 9, wherein said adjustable heating device includes an electrical heater for heating said evaporator stage of the partial oxidation reactor.

11. The device according to claim 1, wherein said hydrogen generating device includes said partial oxidation reactor, and further comprising a feed device with which product gases produced during said partial oxidation on a catalyst are guided against an outer wall of said partial oxidation reactor.

12. The device according to claim 9, further comprising a feed device with which product gases produced during said partial oxidation on the catalyst are guided against an outer wall of said partial oxidation reactor.

13. The device according to claim 1, wherein said hydrogen generating device includes said partial oxidation reactor, and further comprising an aftertreatment stage provided in said partial oxidation reactor for partial oxidation downstream from a main reaction stage in which said partial oxidation occurs on a catalyst; and

wherein in said aftertreatment stage CO concentration is reduced by a shift reaction and a further reaction with residual hydrocarbons with water vapor occurs.

14. The device according to claim 9, further comprising an aftertreatment stage provided in said partial oxidation reactor for partial oxidation downstream from said main reaction stage in which said partial oxidation occurs on the catalyst; and

wherein in said aftertreatment stage CO concentration is reduced by a shift reaction and a further reaction with residual hydrocarbons with water vapor occurs.

15. The device according to claim 1, wherein said hydrogen generating device includes said partial oxidation reactor, and further wherein said partial oxidation reactor for partial oxidation is a tube having an inside diameter of between 5 to 50 millimeters.

16. The device according to claim 9, wherein said partial oxidation reactor for partial oxidation is a tube having an inside diameter of between 5 to 50 millimeters.

17. The device according to claim 1, wherein said at least one of said partial oxidation reactor for partial oxidation and said water vapor reformation reactor for water vapor reformation is a cylindrical block having a plurality of axial holes, a catalyst thereof being located in at least one of said axial holes and an evaporator stage thereof constituting at least one further of said axial holes.

18. The device according to claim 17, wherein a heating cartridge is located in a central one of said axial holes in said at least one of said partial oxidation reactor for partial oxidation and said water vapor reformation reactor for water vapor reformation.

19. The device according to claim 1, wherein said hydrogen generating device is a separate module arranged on-board the motor vehicle.

20. A device for reducing nitrogen oxides in an exhaust stream of a motor vehicle via a catalytic reduction, comprising:

a nitrogen oxide reduction reactor containing a catalyst on which nitrogen oxide reduction of the nitrogen oxides in the exhaust stream is performed via the addition of hydrogen;

a hydrogen generating device arranged on-board said motor vehicle for generating hydrogen, said hydrogen generating device including at least one of a water vapor reformation reactor for water vapor reformation of hydrocarbons and a partial oxidation reactor for partial oxidation of hydrocarbons, said hydrogen generating device being a separate module arranged so as not to be in thermal contact with the exhaust stream of the motor vehicle;

an adjustable heating device coupled with at least one of the water vapor reformation reactor for water vapor reformation and the partial oxidation reactor for partial oxidation and operating to regulate temperature independently of an operating state of the motor vehicle.