WO2008078059A1

WO2008078059A1 - Method and apparatus for selective catalytic nox reduction

Info

Publication number: WO2008078059A1
Application number: PCT/GB2006/004924
Authority: WO
Inventors: David Mark Heaton
Original assignee: Perkins Engines Company Ltd
Priority date: 2006-12-22
Filing date: 2006-12-22
Publication date: 2008-07-03

Abstract

The present disclosure provides an apparatus and method for removing NOx compounds from a gas stream. The apparatus includes a precursor body (5) capable of generating a reductant when subjected to an atmospheric glow discharge, the reductant being capable of reducing NOx; at least one electrode (7) and at least one conductor (9). The electrode (7) and the conductor (9) are arranged to pass an atmospheric glow discharge through the precursor body (5), whereby a reductant is generated and provided to the gas stream so as to reduce NOx therein. The present disclosure further provides a system for reducing NOx compounds in a gas stream in an internal combustion engine (11) and/or in a static hydrocarbon burning equipment, the system comprising the apparatus of the disclosure.

Description

Method and apparatus for selective catalytic NOx Reduction

Technical Field

The present disclosure relates to an apparatus and method for removing nitrogen oxide species (NOx) from a gas stream. In particular the disclosure relates to the selective catalytic reduction of NOx species in an exhaust gas after-treatment system, such as in the exhaust duct of an exhaust system of an internal combustion engine for example.

Background

Internal combustion engines and static hydrocarbon burning equipment tend to emit, via their exhaust systems, nitrogen oxides commonly referred to as NOx compounds or species. Whilst efforts are being expended towards reducing exhaust system emissions at source, catalytic converters and filters (or traps) in the exhaust systems of such equipment are in common usage in helping to meet increasingly strict environmental legislation and public expectations.

A catalytic converter is a device used to reduce the emission of certain compounds from an internal combustion engine. A catalytic converter provides an environment for a chemical reaction wherein toxic combustion products are converted to less toxic gases.

To reduce NOx in the exhaust gases of a compression ignition engine, such as a diesel engine for example, or indeed in the flue gas of boilers used in power generation and industry, in static hydrocarbon burning equipment for example, it is necessary to change the composition of the exhaust gas. Selective Catalytic Reduction, or SCR, is a process where a gaseous or liquid reducing agent, or reductant, is added to the gas stream and is passed over a catalyst. The reductant reacts with NOx in the exhaust gas stream to form water and nitrogen gas.

The most commonly used reductants in SCR systems are ammonia or urea in gaseous, liquid or molten form. Ammonia vapour when used as a reducing agent is passed into the gas stream of an exhaust system and passes over a catalyst. NOx compounds in the gas stream are chemically reduced by the ammonia to form nitrogen and water.

Various pre-cursors capable of providing ammonia to a gas stream have been previously proposed. AdBlue, for example, is a solution of urea in demineralised water used as an operating fluid in diesel-powered vehicles to improve emissions. AdBlue is stored in a separate tank to the fuel for the vehicle and is dosed into the hot exhaust gas in a specific catalytic converter. The oxides of nitrogen formed at combustion are converted into elementary nitrogen and water. This process is the underpinning of a SCR system.

The use of aqueous urea can be problematic. In winter, in the operation of motor vehicles, the exhaust gas temperature drops because of water evaporating in the exhaust gas and the 30-35% aqueous urea solution used in the art has a freezing point of about -1 1⁰C, thus freezing of the urea solution can become a problem. At lower temperatures, particularly at the freezing point of diesel fuel, the operation of the motor vehicle is no longer ensured. Whilst the freezing point of the urea solution can be lowered using additives, additives such as ammonium formiate, for example, are usually particularly corrosive, so that their use poses new problems.

More recently solid ammonia pre-cursors have been described for use in reducing NOx compounds in gas streams. In addition to alleviating the problem of freezing of aqueous pre-cursors, a further advantage of using solid precursors as opposed to gaseous or liquid forms is the higher relative storage density of a solid precursor as opposed to a liquid one. Compared to conventional urea solution, AdBlue for example, the relative storage density of solid urea granules is circa 2.2 times and for solid urea is circa 3.8 times. In turn this allows for smaller reagent volumes, or longer refilling periods, possibly even allowing refilling to be extended to engine service intervals depending on engine out NOx levels and the storage space available for the pre-cursor.

Previously described systems have been provided in which a NOx reducing agent has been formed by thermal degradation of the solid ammonia pre-cursor. In such systems a thermal reactor is required between the dosing system and the catalyst, or trap, to dissociate the urea from solid to gaseous ammonia and isocyanic acid (the latter then hydrolysing with steam to form ammonia).

The current disclosure is aimed at overcoming some or all of the disadvantages associated with the prior art.

Summary of the Disclosure

According to a first aspect of the present disclosure, there is provided an apparatus for removing NOx compounds from a gas stream. The apparatus includes a precursor body capable of generating a reductant when subjected to an atmospheric glow discharge, the reductant being capable of reducing NOx and at least one electrode located proximate to the precursor body, the electrode being operable to produce an atmospheric glow discharge. The apparatus further includes at least one conductor spaced away from the at least one electrode and adapted to lead current away from the electrode, wherein the at least one electrode and the at least one conductor are arranged to conduct the atmospheric glow discharge past the precursor body.

According to a second aspect of the present disclosure, there is provided a system for selective catalytic reduction, the system comprising an apparatus according to the first aspect of the present disclosure arranged in an exhaust duct of an exhaust system of an internal combustion engine.

According to a third aspect of the present disclosure, there is provided a system for selective catalytic reduction, the system comprising an apparatus according to the first aspect of the present disclosure arranged in an exhaust duct of an exhaust system of a static hydrocarbon burning equipment.

According to a fourth aspect of the present disclosure, there is provided a method of removal of NOx compounds from a gas stream comprising: providing a precursor body capable of generating a reductant when subjected to an atmospheric glow discharge, the reductant being capable of reducing NOx compounds; positioning at least one electrode proximate to the precursor body; positioning at least one conductor at a location spaced away from the at least one electrode, such that the precursor body is in the conductive path between the at least one electrode and the at least one conductor; generating an atmospheric glow discharge from the electrode and leading current away from the electrode through the conductor such that the atmospheric glow discharge is conducted past the precursor body and reductant is formed from the precursor body; and providing the reductant to the gas stream.

Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

Brief Description of the Drawings

Various embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic diagram of a first embodiment of an apparatus for removing NOx compounds from a gas stream;

Fig. 2 shows a schematic diagram of a second embodiment of an apparatus for removing NOx compounds from a gas stream; and

Fig. 3 shows a schematic diagram of a first embodiment of a system for removing NOx compounds from a gas stream.

Detailed Description of the Disclosure

Referring to the drawings, each embodiment of the apparatus is shown schematically. In addition, each embodiment of the apparatus is described when in use in the exhaust system of an internal combustion engine. However, it will be understood that the disclosure is not limited to internal combustion engine applications. Each embodiment of the apparatus of the present disclosure includes a precursor body capable of generating a reductant when subjected to an atmospheric glow discharge, the reductant being capable of reducing NOx; at least one electrode located proximate to the precursor body, the electrode being operable to produce an atmospheric glow discharge; and at least one conductor spaced away from the at least one electrode and adapted to lead current away from the electrode, wherein the at least one electrode and the at least one conductor are arranged to conduct the atmospheric glow discharge past the precursor body.

Each embodiment of the apparatus depicted in the accompanying figures further includes a container 1 adapted to store the precursor body 5. More specifically, the apparatus includes a gas tight container 1 including a housing, or canister, 3 in which a precursor body 5 is located. The precursor body 5 is located between an electrode 7 and a conductor 9 such that an atmospheric glow discharge generated between the electrode 7 and the conductor 9 is conducted past the precursor body 5. Unless otherwise stated, the housing 3 is formed of an electrically insulating material.

In addition, each embodiment of the apparatus shown and described is provided with at least one high voltage (HV) electrode 7 for producing an atmospheric glow, or glow-like, discharge in the container 1. In the embodiments described, each HV electrode is connected to a power supply (not shown) suitable for the particular application of the apparatus. For example, the power supply may be a 12V power supply when used for vehicle applications, or a 230V (or 240V) power supply when used for generator applications. In one embodiment, the power supply may be adapted to generate square-wave pulses at a frequency within the range of 1 kHz to 200 kHz. In one embodiment, the voltage is generated at a frequency within the range of 18 kHz to 30 kHz. In another embodiment, the frequency range is 20 kHz to 25 kHz. In another embodiment the range for the open circuit output voltage provided by the power supply is between 5 kV and 25 kV, optimally the open circuit output voltage is 10 kV. A frequency of 20 kHz is outside of the normal human audio range upper limit of 16-18 kHz and is a relatively safe frequency in conjunction with the current flow of the apparatus.

The apparatus will also work effectively at higher frequencies, for example 38 MHz, but the aforementioned frequency ranges allow for human comfort and practical advantages. The disclosed frequency ranges have a strong advantage over higher frequencies which would be expensive to achieve at the necessary power levels and which would require circuits generally less robust and which cannot readily be miniaturised.

Fig. 1 shows a first embodiment of an apparatus according to the present disclosure, including the features already described above. In this first embodiment, the HV electrode is a point electrode 7 which is held by a support means (not shown), spaced apart from the precursor body 5. During use, the gap between the electrode 7 and the precursor body 5 may increase due to a reduction in the size of the precursor body 5 upon electrical dissociation of at least a portion of the precursor body 5. The electrode 7 is, at least initially and prior to electrical dissociation of at least a portion of the precursor body 5, spaced from the precursor body 5 by a distance of between 2mm and 6mm.

In all of the embodiments of the apparatus shown, a conductor 9 is provided in the container 1 , spaced away from the HV electrode 7. The conductor 9 is adapted to lead current from the HV electrode 7 out of the container 1 and return it to the power supply. For the preferred embodiments shown, the conductor 9 takes the form of a counter electrode, unless otherwise stated. However, it should be understood that other suitable conductors may be used as well as the counter electrodes described. In the embodiment illustrated in Fig. 1 , the counter electrode is formed by a plate of conducting material.

Fig. 1 further depicts an engine 11 the exhaust duct 19 of which is connected to a catalyst 13. Further, the housing 3 of container 1 includes an outlet 15 which is connected by supply means 17 to exhaust duct 19.

Fig. 2 shows a second embodiment of the apparatus of the present disclosure. The second embodiment shares the majority of its features with the first embodiment, and the same reference numerals are used for shared features. However, in the second embodiment the apparatus further includes an electrically or pneumatically actuated valve 21 , controlled by a controller 23.

The present disclosure also provides a system for selective catalytic reduction, the system including an apparatus according to the first aspect of the present disclosure arranged in an exhaust duct of an exhaust system of an internal combustion engine.

Fig. 3 shows a first embodiment of a system for removing NOx compounds from a gas stream according to the present disclosure. Again, features shared with the first and second embodiments of the apparatus of the disclosure are designated by the same reference numbers. The depicted exhaust-gas aftertreatment system is of an internal combustion engine 11. An exhaust gas aftertreatment system, such as a catalyst 13, is provided in exhaust duct 19. Control element 23 is operable to supply gaseous ammonia from container 1 and the exhaust gas stream from engine 11 may be caused to flow past catalyst 13.

A control unit 25, including at least one engine control unit 27 and an exhaust gas aftertreatment control unit 29 is also provided. Engine control unit 27 is operable to provide triggering signals to a fuel metering system 31. Exhaust gas aftertreament control unit 29 is operable to exchange signals with engine control unit 27. Furthermore, exhaust gas aftertreatment control unit 29 is operable to provide triggering signals to an actuator element 23 situated in the exhaust duct 19 upstream from or in the exhaust gas aftertreatment system 13.

Various sensors which are operable to supply signals to the exhaust gas aftertreatment control unit 29 and the engine control unit 27 may also be provided. For example, at least one sensor 33 may be provided, which sensor 33 is operable to supply signals characterising the state of the internal combustion engine 11. A further sensor 35 is operable to provide signals characterising the state of the fuel metering system 31.

A temperature sensor 37 is operable to detect the temperature of the exhaust gas aftertreatment system. A NOx sensor 39 is operable to detect NOx concentration downstream of catalyst 13.

Embodiments of the system of the present disclosure include features of the apparatus of the disclosure as depicted and described in accordance with Fig. 1 or Fig. 2 above.

Although various embodiments of the present disclosure have been described above, further modifications are also possible. For example, although the HV electrode 7 has been described as being a pin, or point, electrode, it may also take other forms. For example, a mesh electrode could be used as the HV electrode 7. The mesh electrode would act as a pin electrode having a large surface area. Thus, the mesh electrode could cover a larger surface area of the precursor body 5.

As explained above, each embodiment includes at least one HV electrode 7 and at least one conductor 9. It is also possible for a plurality of HV electrodes to be used in the disclosure.

In one embodiment, where multiple HV electrodes are utilised, each electrode in the array is provided with a dedicated electrical stabilising element (not shown). The stabilising elements are operable to stabilise the electrodes by providing a resistance between the power supply and each HV electrode. The stabilising elements can be simple resistors, or else they may be elements which provide an inductive or capacitive resistance. The provision of the stabilising elements ensures effective operation of the HV electrodes. Without the stabilising elements, the atmospheric glow discharge could favour particular electrodes and areas of the container 1.

Alternatively, or in addition, a plurality of conductors could be provided with each conductor providing a path to ground for the atmospheric discharge(s) emanating from the HV electrode(s).

The container 1 may include a first end and a second end, with the at least one electrode 7 being located at an end of the container 1. In this arrangement, the at least one conductor 9 is located at the other, opposing end, of the container 1 or, alternatively, is located at the same end of the container 1 as the at least one electrode 7. In another embodiment, the container 1 is a substantially sealed unit. More specifically the container 1 includes a closed, gas tight container 1 and an outlet means 15 therein through which gaseous material in the container 1 may be caused to flow.

In other embodiments of the present disclosure, the apparatus includes a catalyst 13 past which the gas stream may be caused to flow.

When referred to herein the gas stream flowing "past" the catalyst may be taken to mean that the gas stream flows around or through the catalyst and/or that the gas stream, or at least constituents thereof, are absorbed onto and/or into the catalyst.

In an embodiment of the present disclosure, the catalyst 13 is a reduction catalyst, or de-nitration catalyst, capable of selective catalytic reduction of NOx compounds.

The catalyst 13 composition may consist of many active metals and support materials to meet the requirements of the present disclosure. The catalyst 13 may be a vanadium-based catalyst and/or a zeolite catalyst with zeolites in the washcoat for example. Other suitable catalysts will be known to the skilled artisan.

In an embodiment of the present disclosure the apparatus even further includes a supply means 17 for connecting the container 1 in fluid communication with the catalyst 13, and being adapted to supply reductant to the catalyst 13.

The supply means 17 may be configured to engage with the outlet means 15 of the container 1. More specifically the supply means 17 may be a conduit or a duct. The supply means may be a hose or pipe for example.

The apparatus is operable such that an increase in the gas pressure within the container 1 provided by the electrical dissociation of the pre-cursor body 5 is sufficient to cause reductant to flow through the outlet means 15 from the container 1 to the supply means 17.

Alternatively, or in addition, the supply means 17 includes a transfer means (not shown) operable to actively transfer reductant from the container 1 to the catalyst 13. The transfer means may be a pump for example.

The at least one electrode 7 may be spaced from the precursor body 5 at a distance of 2mm to 10mm.

More specifically, the at least one electrode 7 may be spaced from the precursor body 5 at a distance of 2mm to 8mm, and even more specifically the distance is 5mm to 6mm.

Alternatively, the at least one electrode 7 may be in contact with at least a portion of the pre-cursor body 5.

In one arrangement, the at least one electrode 7 may be a point electrode located at least partially within the container 1.

Alternatively, the at least one electrode 7 may be a plate electrode located at least partially in the container 1. In an embodiment of the present disclosure, the at least one conductor 9 is a point electrode located at least partially within the container 1.

Alternatively, the at least one conductor 9 is a plate electrode located at least partially in the container 1.

The at least one conductor 9 provides a path to ground such that the atmospheric glow discharge is conducted away from the at least one electrode 7 to the at least one conductor 9. In this way, the electric discharge is directed away from the apparatus, the container 1 for example.

In one arrangement of the apparatus, the at least one conductor 9 is a mesh of conductive material.

In other embodiments of the disclosure, the reductant is gaseous ammonia (NH₃).

The precursor body 5 may be capable of generating gaseous ammonia, or a chemical intermediate which is itself capable of further reaction to generate gaseous ammonia, upon being subjected to an atmospheric glow discharge.

More specifically, the precursor body 5 is capable of electrical dissociation, which electrical dissociation of the precursor body 5 generates gaseous ammonia, or a chemical intermediate which is itself capable of further reaction to generate gaseous ammonia.

The precursor body 5 may include a member of the selected consisting of urea, ammonium carbamate, ammelide, ammeline, ammonium cyanate, biuret, cyanuric acid, isocyanic acid, melamine, tricyanourea and combinations of any number of these. Optimally the pre-cursor body 5 is urea.

The chemical intermediate which is itself capable of further reaction to generate gaseous ammonia may be isocyanic acid (CHNO) for example. Isocyanic acid hydrolyses with steam to provide ammonia and carbon dioxide.

In an embodiment of the present disclosure the precursor body 5 is a solid.

The solid is in the form of a monolith, of a rod shape for example. Alternatively, the solid is in particulate or granular form.

The precursor body 5 may be mounted on a conductive material.

Alternatively, or in addition, the precursor body 5 may be impregnated onto the conductive material.

In one embodiment, the conductive material is the at least one conductor 9.

Advantageously the precursor body 5 may be regeneratable i.e. once the precursor body 5 has been substantially completely electrically dissociated, a new precursor body 5 may be provided in the apparatus.

The at least one electrode 7 may be connected to an AC voltage supply generating an AC voltage in a frequency within the range of 1 kHz to 200 kHz. More specifically, the voltage supply may be pulsed, or modulated. The power supply may be switched on and off for very short periods. For example, the on and off periods could be in the range of 10ms to 1s, with a maximum cycle time (the beginning of the on period to the end of the off period) of 3s.

In other embodiments of the system of the disclosure, the internal combustion engine 11 is a compression ignition engine, a diesel engine for example.

More specifically the internal combustion engine 11 is a motor vehicle engine.

In an embodiment of the present disclosure, the system further includes at least one further catalytic converter (not shown).

More specifically, the system further includes a so-called two-way catalytic converter capable of oxidation of carbon monoxide to carbon dioxide and oxidation of unburnt hydrocarbons (unburnt and partially burnt fuel) to carbon dioxide and water.

More specifically, the two-way catalytic converter is a diesel oxidation catalyst.

In addition, it is possible for a plurality of catalytic converters to be provided for different pollutants, or a combination of at least one catalytic converter and at least one particle filter to be used. In another embodiment, the system further includes at least one sensor (33, 35, 37, 39). More specifically the system further includes at least one NOx sensor 39 for determining the levels (i.e. concentration) of NOx compounds in the exhaust system.

When one NOx sensor 39 is provided, it is advantageously located upstream of the catalyst, that is to say, pre-catalyst, in the exhaust system. When a plurality of NOx sensors 39 are provided, at least one NOx sensor may be located pre-catalyst, whilst at least one further NOx sensor 39 may be provided downstream, that is to say, post-catalyst in the exhaust system.

As an alternative to providing the system with NOx sensors 39, the engine map may be programmed into the Engine Control Unit 27 or computer.

A further embodiment of the present disclosure provides a system for selective catalytic reduction, the system including an apparatus according to the first aspect of the present disclosure arranged in an exhaust duct 19 of an exhaust system of a static hydrocarbon burning equipment.

More specifically, the static hydrocarbon equipment may be a fossil fuel burning furnace or power station for example.

In this arrangement, the SCR system, or unit, is generally located between the furnace economiser and the air heater. Industrial Applicability

The various embodiments of the present disclosure described herein are intended for use in the exhaust systems of internal combustion engines and the like. The various embodiments of the present disclosure are further intended for use in the exhaust systems of static hydrocarbon burning equipment, for example fossil fuel burning power stations, and the like.

In use, the system according to the present disclosure is operable to perform selective catalytic reduction of a gas stream produced, for example, by an internal combustion engine 11. The exhaust gas stream from the engine 11 , is caused to flow through an exhaust duct 19 to a catalytic converter 13 capable of reducing NOx compounds to elemental nitrogen and water in the presence of a suitable reductant. The system may include an apparatus according to the first aspect of the present disclosure, whereby a precursor body 5, of solid urea for example, is electrically dissociated by the passing of an atmospheric glow discharge to the precursor body 5. The electrical dissociation of the precursor body 5 generates gaseous ammonia which is then caused to flow from the apparatus, preferably by way of a supply conduit 17, into the exhaust duct 19 upstream of the catalyst 13. By way of the catalyst 13, NOx compounds are reduced by the ammonia and subsequently emitted as nitrogen and water from the system via the exhaust duct 19.

Referring to Fig. 1 , the precursor body 5 is a solid rod of urea. The precursor body 5 is located between electrode 7 and conductor 9, such that when an atmospheric glow discharge is produced by electrode 7 in the gap between the electrode 7 and the conductor 9, at least a portion of the pre-cursor body 5 is caused to electrically dissociate from a solid form to gaseous ammonia (NH₃). The electrical dissociation of the precursor body 5 may be caused by the atmospheric (electric) glow discharge passing at least a portion of the precursor body, thereby causing a change in the physical state of the body 5 from a solid to a gas.

When referred to herein, the atmospheric glow discharge "passing", or being conducted "past", the pre-cursor body may be taken to mean that the atmospheric glow discharge travels around or through the pre-cursor body so as to electrically dissociate the pre-cursor body.

It will be understood that the precursor body may alternatively be a particulate solid capable of generating ammonia when electrically dissociated. The particulate material may form a layer of precursor material on the surface of the conductor 9, or may alternatively be impregnated into conductor 9.

The gaseous NH₃ formed when the precursor body 5 is electrically dissociated, is caused to flow through an outlet 15 in housing 3 by an increase in the gas pressure within the housing 3. The increase in the pressure within the housing 3 is, in turn, caused by at least a portion of the precursor body changing from a first physical state to a gaseous physical state, thereby increasing the pressure within the housing 3 of container 1. The gaseous ammonia flows through a supply means 17, a hose for example, into exhaust duct 19, upstream of a reduction catalyst 13. The reductant (gaseous ammonia) is mixed with the gas stream from engine 11 in the exhaust duct 19, and the gas stream/red uctant mixture is caused to flow past catalyst 13. The NOx compounds in the gas stream are thus converted to water and elemental nitrogen for emission from the exhaust duct 19. W

The volume of container 1 is designed so that there is sufficient ammonia available to remove NOx from the exhaust gas of an internal combustion engine with the help of a catalyst system 13 within the interval between servicings of the vehicle.

In an alternative arrangement, the precursor body 5 may be located in the exhaust duct 19 such that the gas stream from engine 11 is caused to flow past the precursor body 5 and then past the catalyst 13. In this arrangement, the electrode 7 and the conductor 9 are arranged in the exhaust duct 19 and the electrical dissociation of the precursor body 5, or at least a part thereof, occurs upstream of catalyst 13 in the exhaust duct 19. In this arrangement, the container 1 is dispensed with and the precursor body 5 is located in the main gas stream from the engine 11.

Referring to Fig.2, gaseous ammonia formed by the electrical dissociation of precursor body 5 is metered into the exhaust gas line 19 between engine 11 and catalyst 13 by activation and deactivation of valve 21 by controller 23. In this way the quantity of reductant supplied to the catalyst 13 is adjustable. The quantity of reductant (reducing agent) supplied may be determined on the basis of the operating characteristics of the internal combustion engine, such as the rotational speed and the quantity of fuel injected, as well as variables which characterise the state of the exhaust- gas aftertreatment system, for example, the exhaust gas temperature upstream from, within and/or downstream from the exhaust-gas aftertreatment system.

In either or both of the depicted embodiments of the apparatus of the present disclosure, the reductant i.e. gaseous ammonia, may be actively transported through supply line 17. The active transport means may be a pump for example. The apparatus of the present disclosure facilitates a fast and controllable supply of reductant gas to the exhaust duct 19 of the exhaust system such that an on-demand supply of reductant is readily available in response to the operating conditions of the engine 11 and to changes therein and to changes in the concentration of NOx compounds in the exhaust gas stream in exhaust duct 19.

In other embodiments of the present disclosure, the gas stream may be caused to flow past a further catalyst.

Referring to the system depicted in Fig. 3, exhaust gases from engine 11 enter the environment through exhaust duct 19. An exhaust gas aftertreatment system, a catalyst 13, is provided in exhaust duct 19. Control element 23 supplies gaseous ammonia from container 1 to exhaust duct 19 upstream of catalyst 13, such that nitrogen oxides from the exhaust gas stream from engine 11 are caused to flow past catalyst 13.

A control unit 25, including at least one engine control unit 27 and an exhaust gas aftertreatment control unit 29 is also provided. Engine control unit 27 provides triggering signals to a fuel metering system 31. Exhaust gas aftertreament control unit 29 exchanges signals with engine control unit 27. Furthermore, exhaust gas aftertreatment control unit 29 provides triggering signals to an actuator element 23 situated in the exhaust duct 19 upstream from or in the exhaust gas aftertreatment system 13.

Various sensors which supply signals to the exhaust gas aftertreatment control unit 29 and the engine control unit 27 may also be provided. For example, at least one sensor 33 may be provided to supply signals characterising the state of the internal combustion engine 11. A further sensor 35 supplies signals characterising the state of the fuel metering system 31.

A temperature sensor 37 detects the temperature of the exhaust gas aftertreatment system. Temperature sensor 37 is located downstream of the catalyst 13. A NOx sensor 39 detects NOx concentration downstream of catalyst 13 such that the quantity of reductant supplied to the catalyst 13 may be increased or decreased in response to the sensor output.

In response to the signals received from the sensors, engine control unit 27 computes trigger signals to be applied to fuel metering system 31 , which then meters the correct quantity of fuel for internal combustion engine 11. During combustion, nitrogen oxides, NOx, may be formed in the exhaust gas. The NOx compounds are converted to nitrogen and water within reduction catalyst 13. In order to facilitate this conversion, a reductant (reducing agent) must be supplied to the exhaust gas stream upstream from the catalyst 13. In the depicted embodiment, the reductant is supplied to the exhaust duct 19 from container 1 via actuator 23. In one embodiment, the reductant is gaseous ammonia generated by electrical dissociation of a solid urea precursor body in container 1.

The optimum temperature for efficient operation of the SCR system is in the range of 25O⁰C to 500⁰C. Advantageously the temperature range is 300⁰C to 400⁰C.

The disclosure offers improved reductant supply to a reduction catalyst over existing proposals due to the rapid electrical dissociation of a solid ammonia-precursor body when an atmospheric glow discharge is applied to same. Thus, a solid precursor may be utilised in a selective catalytic reduction system, providing increased ammonia storage density over aqueous precursors and avoiding problems associated with the use of a liquid precursor such as freezing for example. Furthermore, electrical dissociation occurs rapidly thereby allowing for a rapid, on-demand production of reductant in response to the changing operating characteristics of the engine.

Although preferred embodiments of this disclosure have been described herein, improvements and modifications may be incorporated without departing from the scope of the following claims.

Claims

1. An apparatus for removing NOx compounds from a gas stream, the apparatus comprising: a precursor body capable of generating a reductant when subjected to an atmospheric glow discharge, the reductant being capable of reducing NOx; at least one electrode located proximate to the precursor body, the electrode being operable to produce an atmospheric glow discharge;

. and at least one conductor spaced away from the at least one electrode and adapted to lead current away from the electrode, wherein the at least one electrode and the at least one conductor are arranged to conduct the atmospheric glow discharge past the precursor body.

2. An apparatus according to claim 1 , further including a container adapted to store the precursor body.

3. An apparatus according to claim 1 or claim 2, further including a catalyst past which the gas stream may be caused to flow.

4. An apparatus according to claim 3, further including a supply means for connecting the container in fluid communication with the catalyst and being adapted to supply reductant to the catalyst.

5. An apparatus according to any preceding claim, wherein the at least one electrode is spaced from the precursor body at a distance of 2 to 10mm.

6. An apparatus according to any one of claims 2-5, wherein the at least one electrode is a point electrode located at least partially within the container.

7. An apparatus according to any one of claims 2-6, wherein the at least one conductor is a point electrode located at least partially within the container.

8. An apparatus according to any one of claims 1 -7, wherein the at least one conductor is a plate electrode.

9. An apparatus according to any one of claims 1-7, wherein the at least one conductor is a mesh of conductive material.

10. An apparatus according to any preceding claim, wherein the reductant is gaseous ammonia (NH₃).

11. An apparatus according to any preceding claim, wherein the precursor body is urea.

12. An apparatus according to any preceding claim, wherein the precursor body is a solid.

13. An apparatus according to any preceding claim, wherein the precursor body is mounted on a conductive material.

14. An apparatus according to claim 13, wherein the precursor body is impregnated onto the conductive material.

15. An apparatus according to claim 13 or claim 14, wherein the conductive material is the at least one conductor.

16. An apparatus according to any preceding claim, wherein the at least one electrode is connected to an AC voltage supply generating an AC voltage in a frequency within the range of 1 kHz to 200 kHz.

17. An apparatus according to claim 16, wherein the voltage supply is pulsed.

18. A system for selective catalytic reduction, the system comprising an apparatus according to any one of claims 1 -17 arranged in an exhaust duct of an exhaust system of an internal combustion engine.

19. A system according to claim 18, wherein the internal combustion engine is a compression ignition engine.

20. A system for selective catalytic reduction, the system comprising an apparatus according to any one of claims 1 -17 arranged in an exhaust duct of an exhaust system of a static hydrocarbon burning equipment.

21. A method of removal of NOx compounds from a gas stream comprising: providing a precursor body capable of generating a reductant when subjected to an atmospheric glow discharge, the reductant being capable of reducing NOx compounds; positioning at least one electrode proximate to the precursor body; positioning at least one conductor at a location spaced away from the at least one electrode, such that the precursor body is in the conductive path between the at least one electrode and the at least one conductor; generating an atmospheric glow discharge from the electrode and leading current away from the electrode through the conductor such that the atmospheric glow discharge is conducted past the precursor body and reductant is formed from the precursor body; and providing the reductant to the gas stream.

22. A method according to claim 21 , wherein the gas stream is caused to flow past a catalyst.