WO2004079171A1

WO2004079171A1 - Improvements in engine emissions

Info

Publication number: WO2004079171A1
Application number: PCT/GB2004/000823
Authority: WO
Inventors: Martin Vierkotter; Russell Stuart Avens; Berno Lupkes; Clive Henry Buckberry; Martin S. Johnson; Keith James Heyes
Original assignee: Imi Vision Limited
Priority date: 2003-03-01
Filing date: 2004-03-01
Publication date: 2004-09-16
Also published as: GB2414692A; GB2414692B; GB0517654D0

Abstract

Apparatus for introducing ammonia into the exhaust pipe of a diesel IC engine whereby harmful NOx in the exhaust gasses may be selectively reduced using an SCR catalyst comprises a number of spaced hollow fins (1, 4) that extend laterally into the exhaust pipe and that are filled with a sintered material (5, 6) that defines a network of passageways extending within each fin (1, 4) from a lower inlet end (2) to an upper outlet end (3). The ammonia is produced in situ by feeding an aqueous solution of urea to the inlet end (2) of the fins, the urea thermally decomposing into, inter alia, ammonia during passage of the solution towards the outlet end (3), via the passageways, where the ammonia issues from the fins and is entrained by the exhaust gasses.

Description

IMPROVEMENTS IN ENGINE EMISSIONS

This invention relates to a method of, and apparatus for, reducing emissions of Nitrogen Oxides (NOx) in exhaust gasses of an internal combustion ("IC") engine.

Currently there are a number of commercially available selective catalytic reduction (SCR) systems which aim to reduce NOx emissions. These are principally for use with diesel engines. In one such system ammonia is introduced into the exhaust gas, for example into the exhaust conduit of the engine, where it mixes with the NOx in the exhaust which then flow together through the SCR device resulting in reduced NOx in the exhaust gas. Commonly the ammonia is derived from a liquid reagent, for example aqueous urea, which decomposes in situ in the exhaust gas to produce ammonia. It is common for liquid reagent to be introduced into the exhaust gas using a combination of an atomizing nozzle and a dosing unit. The dosing unit feeds the reagent at the required rate to the atomising nozzle dependant on the quantity of NOx and the atomizing nozzle introduces small droplets of the liquid reagent into the exhaust gas.

There are a number of problems associated with such systems. Firstly the atomised droplets of urea solution are introduced into a fast flowing gas stream in which hydrolysis and pyrolysis of the urea into ammonia preferably fully occur before reaching the SCR device. This does not happen instantaneously and a minimum critical separation distance between the point of introduction of urea and the SCR device is needed to allow this to happen. This affects the compactness of the system and ultimately the cost. Secondly, between the atomising nozzle and the SCR a mixing plate is often provided which creates turbulence in the exhaust flow, slowing it and allowing the atomised urea solution more time to fully evaporate before entering the SCR, this adds to the complexity and size of the system. Thirdly, there can be a problem with residual urea in the nozzle breaking down and solids precipitating, resulting in the need to regularly clean or flush the system. Finally, the spray from an atomising nozzle can cause droplets of urea to be deposited on the inside of the exhaust (due to its cooler temperature) which can cause problems with corrosion and necessitate regular cleaning.

Alternative methods use, as the source of gaseous ammonia, liquefied or compressed ammonia gas. However ammonia is a hazardous material and while this may be a viable solution for fixed IC engines, it is not deemed a safe approach for mobile IC engines, for example in commercial vehicles.

It is an object of the present invention to provide a urea delivery method and apparatus for use in an SCR system of an IC engine which eliminates or mitigates the above problems.

According to one aspect of the present invention, there is provided a method of introducing ammonia into NOx-containing exhaust gases flowing through the exhaust conduit of an IC engine wherein an aqueous solution of urea or other substance, e.g. ammonium carbamate, that decomposes upon heating into, inter alia, ammonia (herein collectively referred to as "urea" for convenience) is supplied to a reactor located at least partially in the exhaust conduit and heated by the exhaust gases, the reactor comprising an inlet end to which said solution is fed, an outlet end located in the conduit, and a multiplicity of passageways interconnecting said inlet and outlet ends, whereby the solution passes along the passageways from the inlet end towards the outlet end during which the urea is thermally decomposed into, inter alia, ammonia which issues from the outlet end into the exhaust gasses.

Preferably, the inlet end is located externally of the conduit, e.g. of the exhaust pipe, through which the exhaust gases flow and is preferably located below the level of the outlet end so that the urea solution passes through the passageways in a generally upwards direction.

According to another aspect of the present invention, there is provided apparatus for use in the method of the invention, the apparatus comprising a reservoir for containing an aqueous solution of urea, a reactor adapted to be mounted on the exhaust conduit of an IC engine and comprising an inlet end for said solution, an outlet end remote from the inlet end and adapted to be located in the exhaust conduit, a multiplicity of passageways interconnecting said inlet and outlet ends, and means for feeding said solution to said inlet end, whereby in use the solution passes from the inlet end towards the outlet end via the passageways.

Preferably, the means for feeding the solution to the inlet comprises a volumetric displacement pump, although other means for feeding the solution at a controlled feed rate may be used, for example a pressurised supply of the solution in conjunction with a control valve.

Preferably, each of the passageways, at the outlet end, opens directly into the exhaust gasses, that is to say that each passageway has an outlet over which the exhaust gasses pass and at which they entrain the ammonia gas issuing from each outlet.

According to yet another aspect of the invention, there is provided an IC engine incorporating, in its exhaust conduit, apparatus of the invention as defined above. The IC engine may be the engine of a diesel powered vehicle, for example a commercial road vehicle.

In a preferred arrangement, the reactor comprises a porous material, sealed on its faces apart from at the inlet and outlet ends, defining the passageways and having pores of a suitable cell size to ensure an even flow of the urea solution through the reactor, for example Grade 2733 metal coated polyurethane foam from Recemat. Alternatively the reactor may comprise a rigid outer shell of a heat conducting material, for instance a metal, in which small pieces of metal are compressed or sintered, or a stack of perforated sheets through which the solution will slowly permeate, thus effecting an appropriate dwell time of the urea in the passageways to allow production of ammonia. In general, the reactor is preferably made of a material with a high thermal conductivity and it preferably has a large surface area to volume ratio so as to increase the transfer efficiency of the thermal energy from the hot exhaust gas which causes the evaporation of the solvent part of the urea solution and the pyrolysis of the urea resulting in, principally, water, ammonia and isocyanic acid, all in gaseous form.

In a preferred arrangement, within the reactor and between the area in which the pyrolysis occurs and the point of introduction of the ammonia into the exhaust, there is provided a hydrolysis catalyst through which the gaseous water, ammonia and isocyanic acid must pass. The catalyst functions to promote the reaction of the isocyanic acid and water to form ammonia and carbon dioxide, thus maximising the amount of ammonia available, from the same quantity of urea, at the point of entry of the exhaust gas into the SCR catalyst. The hydrolysis catalyst may be any one or more of a number of substances known in the art, for example silica, alumina and/or titania. In another preferred arrangement, the quantity of ammonia produced is controlled in response to a measured quantity of NOx in the exhaust, the NOx being measured either directly or indirectly determined based on measured engine parameters, supplied from the engine management system. The quantity of ammonia produced is preferably controlled by the flow of urea into the passageways and the temperature of the exhaust gas.

In an alternative arrangement, the amount of ammonia produced can be controlled by the speed of the exhaust gas over the surface and the temperature of the reactor, thereby effecting a passive system whereby external inputs from the engine are not required enabling the apparatus to be easily retrofitted to existing vehicles.

In another preferred arrangement, a separate means of heating the reactor is provided, using internal or external heating elements. This decreases the lime required for the reactor to reach the requisite temperature and secondly also functions to aid converting any residual urea to gasses after the flow of exhaust gas ceases thereby preventing the passageways of the reactor becoming blocked. This may be facilitated by additionally or alternatively pre-heating the urea solution, eg by means of heal exchange with the engine coolant water, to, say, a temperature of around 80 - 90 degrees C.

It is desirable to maximise the efficiency of heat transfer from the exhaust gas to the reactor, and therefore to the urea solution, as a minimum temperature is required for pyrolysis to occur. This temperature can easily be reached under normal running conditions but under start up conditions where the exhaust gas may only have a temperature of around 200°C it is harder to effect. This problem may be solved by careful selection of the materials of construction of the reactor and maximising its surface area.

In one preferred embodiment of the present invention, the reactor comprises a plurality of hollow metal fins, mounted on a base containing a small liquid reservoir, into each of which small pieces of metal material (e.g. shavings of 316L stainless steel) are compressed and then sintered to form a plurality of passageways within the fins. Optionally the sintered pieces of metal material may extend into the reservoir in the base, the material in the reservoir creating some resistance to the flow of aqueous urea solution and thus aiding its equal distribution to all fins. The reactor is preferably positioned in the exhaust such that the base is flush with the wall of the exhaust pipe and the fins protrude into the hot exhaust gas flow. The fins are open at the end protruding into the exhaust gas flow to enable the ammonia to issue therefrom into the exhaust gas and mix therewith prior to passing through the SCR catalyst. Preferably a baffle through which the fins pass is positioned between the base and the inlet and outlet ends of the fins such thai it creates an amount of turbulence downstream of the reactor to enhance mixing of the ammonia and exhaust gas and also to give some structural strength to the fins.

In another preferred embodiment, the reactor is in the shape of a hollow annular cylinder made of a porous material, sealed on the outer and inner cylindrical surfaces, but the pores being open at both ends of the cylinder. The open porous face of one end (the inlet end) is supplied through a manifold with aqueous urea solution and ammonia issues from the other open porous end (the outlet end). Preferably the manifold is supplied with a flowpath through it such that the exhaust gas, in addition to passing over the outside surface of the reactor, can flow through the centre of the reactor and therefore pass over the inside surface thus maximising the heated surface area and therefore heat transfer to the porous element. Preferably the device is placed axially in the exhaust with the manifold (inlet) end being the downstream end.

In another preferred embodiment, the reactor consists of a plurality of stacked units comprising flat dish shaped structures, each containing a shallow (eg around 0.25mm to 2mm) reservoir area supplied with urea from a supply pipe and supporting, above the urea reservoir, a disc shaped porous element defining passageways between its lower and upper surfaces, the upper surface of the porous element and the lower surface of the dish structure being thus maximised to facilitate heat transfer to the urea. Preferably, the angle of the reactor can be varied to alter the flow of hot gas over it. Preferably, the dish and porous element combined form a lenticular shape creating a venturi effect between the stacked units, which further increases the heat transfer rate. Preferably, the porosity of the elements in the stacked units varies to ensure equal distribution of the urea solution amongst them, thus overcoming gravitational effects encouraging the liquid reagent to drain towards the lowermost unit.

In yet another preferred embodiment, the reactor comprises a plurality of open-ended, small diameter, eg capillary, tubes arranged in parallel, preferably spaced, relationship, each of which defines one of the aforesaid passageways. The tubes are preferably substantially straight.

According to another aspect of the present invention there is provided a

NOx reduction device for incorporation into the exhaust system of an IC engine, the device comprising a substantially unitary housing containing a reactor as described above and, located adjacent thereto in the housing, an SCR catalyst. The dimensions of the housing preferably correspond to those of respective pre-existing SCR housings whereby it is relatively easy to retrofit to existing vehicles and removes the need either to replace existing exhaust piping or to drill into existing exhaust piping. Where the reactor includes a catalyst for promoting the hydrolysis of hydrogen isocyanate, such a catalyst may be omitted from the SCR catalyst and the additional space afforded in so doing may be utilised to accommodate the reactor in, for example, the SCR housing itself.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 and 2 are, respectively, perspective and cross sectional diagrams of a reactor constructed according to the invention which have, respectively, three and five fins for exposure in the exhaust gas;

Figure 3 is a cross sectional view of a reactor having seven fins, shown in situ in the exhaust pipe and having baffles, constructed in accordance with the invention;

Figure 4 is a cross section on A-A of figure 3 showing a detail of the baffles;

Figure 5 and 6 are, respectively, longitudinal and cross sectional diagrams of another reactor constructed in accordance with the invention;

Figure 7 is a diagram of a modification to the reactor shown in figures 5 and 6; Figure 8 is a cross sectional view of yet another reactor shown in situ in the exhaust pipe and constructed in accordance with the invention;

Figure 9 is a diagram of a NOx reduction apparatus incorporating a reactor constructed in accordance with the invention;

Figure 10 is a diagram of a unitary NOx reduction unit incorporating a reactor constructed in accordance with the invention; and

Figure 11 is a diagram of yet another reactor constructed in accordance with the invention.

Referring to Figures 1 and 2, a reactor is shown comprising a plurality of hollow stainless steel fins, two of which are designated 1 and 4, which define internally a multiplicity of capillary passageways extending from a lower inlet end 2 to an upper outlet end 3. An aqueous solution of urea enters the reactor fins 1, 4 at the inlet end 2 and flows towards the outlet end 3. The fins 1, 4 are filled with a material that defines the passageways which allow the passage of the urea solution therethrough but which increases the dwell time of the urea solution within each passageway. As shown, two materials may be used 5, 6, one of which 5 is a hydrolysis catalyst and the other of which 6 consists of small shavings of 316L grade stainless steel. The materials 5, 6 are placed in the fins 1, 4 and then compressed and sintered to retain them in place, the compressing and sintering steps effecting a porous material with an average pore size of approximately 100 μm. During transition of urea solution from inlet to outlet, as a result of heat absorbed through the walls of the fins 1 and 4, the urea solution decomposes to produce ammonia. It is believed that the decomposition of aqueous urea happens in three phases, namely the evaporation of the water in the solution, the pyrolysis of the urea into ammonia and isocyanic acid, and then the hydrolysis of the isocyanic acid with the water released in the evaporation to form ammonia and carbon dioxide, thus enabling the aqueous urea solution to be substantially totally converted to ammonia and carbon dioxide, both of which are gases. The evaporation and pyrolysis phases can occur simultaneously.

The fins 1 and 4 are shaped to facilitate heat transfer. The leading edge 7 of each fin 1, 4, which faces the direction of the exhaust gas flow, has a curved radius, as does the corner where the outlet end 3 meets the leading edge 7. Further (not shown) , the trailing edge of each fin 1, 4 may taper. This geometry minimises turbulence, which can move the exhaust gas flow away from the fins, and aids the exhaust gas flow to pass in thermal contact along the side face 8 of each fin 1, 4. As shown in Figure 2, the height of the fins may differ in order to optimise heat transfer from the exhaust gasses whilst improving the distribution of the ammonia in the gasses. The base of the reactor 9 contains a cavity 10 which forms a reservoir and feed zone for the urea solution to the fins 1 and 4. The cavity 10 is filled with the material 6 which aids even distribution to all the fins 1 and 4.

Referring to Figures 3 and 4, a reactor with seven fins constructed substantially as described with reference to Figures 1 and 2 is shown mounted in place in an exhaust pipe 11. The reactor is clamped in place by clamp plate 12 which has a port 13 therein to receive the urea solution. In addition the reactor is provided with three baffles 14, 15, 16 which increase the effective surface area to volume ratio and cause a change in the momentum of the exhaust gasses thereby improving the thermal energy transfer from the exhaust gasses to the fins. The trailing ends 17, 18, 19 of the baffles are shaped to facilitate the mixing of the ammonia with the exhaust gas by creating some downstream turbulence. This is achieved here by the curved configuration directing the exhaust gas flow upwards within the pipe 11 but equally may be achieved by creating rotational or other turbulence. On the other hand, the leading ends of the baffles, whilst shown as being curved (see Figure 4), might be flat and extend horizontally so as to minimise turbulence of the exhaust gasses as they enter the reactor.

Referring to Figures 5, 6 and 7, the reactor comprises an inlet 20 to which aqueous urea solution is supplied, a manifold 21 containing flowpaths 22, 23 which distribute the solution urea to one end of an annular metallic, eg stainless steel, casing 24 having sealed external 25 and internal 26 sidewalls to prevent the urea solution escaping laterally. The casing 24 is filled with a material of high thermal conductivity and which has a porosity designed to enable the required dwell time of the urea solution in the reactor to ensure its decomposition to ammonia as described above (for example a 45 micron pore size material manufactured from sintered bronze balls, available from Carbis Filtration Limited) . The annular casing 24 is open at each end allowing for supply of the urea solution via flowpaths 22, 23 at one end and the passage of ammonia gas into the exhaust from the other. The reactor is placed axially in the exhaust gas flow such that the exhaust gas flows around the outside of the element and through its centre, escaping through holes 27, 28. This ensures good heat contact on both surfaces 25, 26 of the annular casing 24. In certain IC engine conditions, specifically idle condition, the temperature of the exhaust gas is considerably reduced and there may not be sufficient heat transfer to fully complete the conversion of aqueous urea to ammonia and other gases within the reactor and small droplets of liquid may emerge from the outlet end of the reactor. To prevent this, a catchment disc 29 is provided which is made of a rigid but loose structured material (for example Grade 0610 metal foam available from Recemat International BV) such that any liquid droplets will flow into it, be broken up and evaporate. As described with reference to Figures 1 and 2, the annular casing 24 may contain a hydrolysis catalyst to promote the production of further ammonia from isocyanic acid.

Referring to Figure 8, a reactor is shown in which a central supply passageway 30 has an inlet 31 supplied with urea solution. Sealingly connected to the central passageway 30 are three metallic dishes 31, 32, 33 into ^' which the urea solution passes through holes 34, 35, 36. The lower end of the supply tube 30 is capped with a cap 37 to prevent the liquid flowing straight through it. Seated in each dish 31, 32, 33 is a disc of porous material 38, 39, 40, eg of a type referred to earlier, through which the urea solution passes generally upwardly and thermally decomposes into, inter alia, ammonia gas as it passes therethrough. The device is shown secured in an exhaust pipe 41 by a nut 42.

Referring to Figure 9, a system is shown in which an exhaust gas bypass 43 is connected to the main exhaust manifold 44, the main exhaust manifold 44 containing the reactor 45 for the production of ammonia. A variable fluid flow control valve 46 is placed at the point of separation of the bypass 43 from the main manifold 44 which can direct a variable amount of the flow through the bypass 43 thus reducing the flow of exhaust gas over the reactor 45. The bypass 43 then rejoins the main manifold 44 where the gasses mix before passing through the SCR catalyst 47.

Referring to Figure 10, an NOx SCR unit is shown in which a housing 48 is formed between two flanges 49, 50 defining the upstream and downstream ends, each flange provided with holes whereby they may be bolted to matching flanges of the exhaust pipe (not shown) . Encased in the central section of the body 48 is the SCR catalyst material 51 and situated at one end (which will be the upstream end) and located directly in the path of the exhaust gas flow is a reactor 52 connected via a feed tube 53 for the supply of urea solution. The reactor 52 may take the form of any reactor in accordance with the present invention.

Referring to Figure 11, a reactor is shown in which a number of straight metal capillary tubes 54 each defining an elongate passageway are connected to a manifold 55 which is supplied with urea solution via inlet 56. The reactor is placed in the hot exhaust gas such that the capillary tubes 54 are healed by the flow of the hot gas whereby the urea solution is decomposed into, inter alia, ammonia.

Claims

1. A method of introducing ammonia into NOx-containing exhaust gasses flowing through the exhaust conduit of an IC engine wherein an aqueous solution of urea (as hereinbefore defined) is supplied to a reactor located at least partially in the exhaust conduit and heated by the exhaust gases, the reactor comprising an inlet end to which said solution is fed, an outlet end located in the conduit, and a multiplicity of passageways interconnecting said inlet and outlet ends, whereby the solution passes along the passageways from the inlet end towards the outlet end during which the urea is thermally decomposed into, inter alia, ammonia which issues from the outlet end into the exhaust gasses.

2. A method according to claim 1 wherein the inlet end is located below the level of the outlet end so that the urea solution passes through the passageways in a generally upwards direction.

3. A method according to claim 1 or claim 2 wherein the inlet end is located externally of the conduit.

4. A method according to any of claims 1 to 3 wherein the passageways are in the form of interconnected pores or other interstices defined by a mass of thermally stable and conductive material.

5. A method according to claim 4 wherein the thermally stable and conductive material comprises compressed and/or sintered paniculate material.

6. A method according to any one of claims 1 to 3 wherein the passageways are discrete from one another and are defined by a multiplicity of open-ended, elongate, thermally stable and conductive capillary tubes arranged in side-by-side relationship.

7. A method according to any one of claims 1 to 6 wherein each of the passageways, at the outlet end, opens directly into the exhaust gasses whereby the gasses pass over, and contact, a multiplicity of respective outlets defined by the passageways at which the ammonia is entrained by the gasses.

8. Apparatus for use in a method as claimed in claim 1, the. apparatus comprising a reservoir for containing an aqueous solution of urea, a reactor adapted to be mounted on the exhaust conduit of an IC engine and comprising an inlet end for said solution, an outlet end adapted to be located in the exhaust conduit, a multiplicity of passageways interconnecting said inlet and outlet ends, and means for feeding said solution to said inlet end, whereby in use the solution passes from the inlet end towards the outlet end via the passageways.

9. Apparatus according to claim 8 wherein the passageways are in the form of interconnected pores or other interstices defined by a mass of thermally stable and conductive material.

10. Apparatus as claimed in claim 9 wherein the thermally stable and conductive material comprises compressed and/or sintered particulate material.

11. Apparatus as claimed in claim 9 or claim 10 wherein the said material is comprised in a plurality of fins arranged in spaced, substantially parallel relationship whereby, in use, the exhaust gasses can flow over both major faces of each fin, the inlet and outlet ends of each fin being defined by, respectively, opposed edges of each fin.

12. Apparatus as claimed in claim 9 or claim 10 wherein the said material is comprised in an annular cylindrical member, the opposed ends of which define, respectively, the inlet and outlet ends.

13. Apparatus as claimed in claim 9 or claim 10 wherein the said material is comprised in a plurality of discs arranged in spaced, substantially parallel relationship, the opposed major faces of each disc defining said inlet and outlet ends and the discs being supported by a common supply conduit for delivering urea solution to the inlet end of each disc.

14. Apparatus as claimed in claim 8 wherein the passageways are discrete from one another and are defined by a multiplicity of open- ended, elongate, metal capillary tubes arranged in side-by-side relationship.

15. Apparatus as claimed in any one of claims 8 to 14 further comprising one or more heating elements for heating the reactor independently of the heating caused by the exhaust gasses.

16. Apparatus as claimed in any one of claims 8 to 15 wherein the reactor further comprises within the passageways a catalyst for promoting the hydrolysis of hydrogen isocyanate.

17. An NOx reduction device for incorporation into the exhaust system of an IC engine, the device comprising a unitary housing containing apparatus as claimed in any one of claims 8 to 16 and, located adjacent thereto in the housing, an SCR catalyst.

18. An IC engine incorporating, in its exhaust conduit, apparatus as claimed in any one of claims 8 to 16 or a device as claimed in claiml7.

19. An IC engine as claimed in claim 18 wherein the urea is preheated prior to introduction to the reactor.

20. An IC engine as claimed in claim 19 wherein the method of preheating is by heat exchange directly, or indirectly, with the engine coolant fluid.

21. A vehicle, for example a diesel-powered road vehicle, having an IC engine as claimed in any one of claims 18 to 20.