Title:
Exhaust Gas Treatment
Kind Code:
A1


Abstract:
A device 1, capable of being placed in-line in the exhaust conduit of an IC engine upstream of an SCR catalyst, which produces a gaseous hydrolysis product, containing ammonia, which is added to the exhaust gas in a controlled manner to pass therewith through the SCR catalyst to reduce the NOx content of the exhaust gas. The device 1 has an inlet 2 and an outlet 3 for the exhaust gas flowing therethrough and comprises an outer body 4, which forms a pressure barrier, and passing through the outer body 4 is an inner body 5 which comprises a flowpath longitudinally therethrough from the inlet 2 to the outlet 3. The device 1 is split into two sections, a reactor section with an inlet for pressurised aqueous urea and comprising an upper and lower area connected by a plurality of tubes 10 which pass through the exhaust gas for heat exchange and wherein the hydrolysis of urea occurs at elevated temperature and pressure and a reservoir section connected to the reactor section via a pressure release valve to allow the gaseous hydrolysis product to pass from the reactor to the reservoir. The device further comprises a valve 17 for dosing the gaseous hydrolysis product into the exhaust gas.



Inventors:
Barber, Graham Richard (West Midlands, GB)
Buckberry, Clive (Warwick, GB)
Coates, James (Cambridgeshire, GB)
Davey, Stuart Charles (Warwickshire, GB)
Heyes, Keith James (Worcestershire, GB)
Lupkes, Berno (Worcestershire, GB)
Sealy, Mark (Warwickshire, GB)
Watton, James (Worcestershire, GB)
Johnson, Martin Stanley (Warwickshire, GB)
Application Number:
11/815414
Publication Date:
12/25/2008
Filing Date:
02/16/2006
Assignee:
IMI Vision Limited (Alcester, Warwickshire, GB)
Primary Class:
Other Classes:
60/303, 60/299
International Classes:
F01N3/18
View Patent Images:
Related US Applications:



Primary Examiner:
WALTER, AUDREY BRADLEY
Attorney, Agent or Firm:
The Ollila Law Group LLC (Lafayette, CO, US)
Claims:
1. A unitary device for generating and feeding gaseous hydrolysis product comprising ammonia, formed by the hydrolysis of an aqueous solution of urea (as hereinbefore defined) at elevated temperature and pressure, into the exhaust gas of an IC engine as it flows through the exhaust system of the engine to an SCR catalyst, the device being adapted to be placed in the exhaust system so that the exhaust gas will flow through it during use, and comprising a) a housing having an inlet for the exhaust gas and an outlet for the exhaust gas; b) a reaction vessel located at least partially within the housing between the inlet and the outlet for containing an aqueous solution of urea and arranged such that, in use, the vessel and therefore the urea solution become heated by means of heat exchange with the exhaust gas as it flows from the inlet to the outlet; c) a urea solution inlet to the reaction vessel and a gaseous hydrolysis product outlet from the reaction vessel; d) a reservoir for receiving and storing gaseous hydrolysis product; e) a valve in the outlet from the reaction vessel and adapted to permit the contents of the reaction vessel, in use, to attain an elevated pressure as it becomes heated, and periodically to discharge gaseous hydrolysis product into the reservoir; and f) a conduit for interconnecting the reservoir and the exhaust system, the conduit including a valve to selectively control the feed of hydrolysis product stored in the reservoir into the exhaust gas via the conduit.

2. The device according to claim 1 wherein the valve in the outlet is placed at least partially outside the housing such that it is at least partially protected from direct exposure to the hot exhaust gasses

3. The device according to claim 1 wherein the conduit for interconnecting the reservoir and the exhaust system is placed at least partially outside the housing such that it is at least partially protected from direct exposure to the hot exhaust gasses

4. The device according to claim 1, wherein the valve in the outlet actuates in response to the pressure within the reaction vessel.

5. The device according to claim 4 wherein the valve in the outlet is a mechanical back pressure valve and allows excess gas to pass through once the pressure within the reaction vessel exceeds a specific predetermined pressure.

6. The device according to claim 4 wherein the valve in the outlet is actuated via a control system in response to a signal received from a pressure transducer situated in the reaction vessel indicating the pressure therein is above a specific value.

7. The device according to claim 1, wherein the valve in the outlet is actuated via a control system in response to a signal received from a temperature sensor situated in the reaction vessel indicating the temperature of the aqueous solution of urea therein is above a specific value.

8. The device according to claim 1, further comprising an SCR catalyst within the unitary device.

9. The device according to claim 8 wherein the downstream end of the SCR catalyst is coated with a catalyst that converts any un-reacted ammonia in the exhaust gas into harmless gasses such that ammonia is not released into the environment.

10. The device according to claim 1, wherein the gaseous hydrolysis product is introduced substantially on the axis of the exhaust gas flow path substantially perpendicularly to the direction of the exhaust gas flow.

11. The device according to claim 10 wherein a number of radially spaced inlets are situated adjacent to one another substantially perpendicularly to the flow.

12. The device according to claim 1, wherein the point of introduction of the gaseous hydrolysis product is substantially at the mouth of a truncated conical section of the flowpath and the flow of exhaust gas and gaseous hydrolysis product into the cone induces mixing.

13. The device according to claim 1, wherein the flowpath between the point of introduction of the gaseous hydrolysis product and the SCR catalyst has at least one substantially 90 degree bend therein causing turbulence in the flowpath.

14. The device according to claim 1, further comprising a substantially cylindrical vortex chamber, upstream of the SCR catalyst, wherein the exhaust gas and gaseous hydrolysis product enter the vortex chamber substantially perpendicularly to its radius and exit the chamber along its central axis, the vortex within the chamber further inducing mixing of exhaust gas and gaseous hydrolysis product.

15. The device according to claim 1, further comprising an oxidation catalyst within the single unit and through which the exhaust gas flows prior to introduction of the gaseous hydrolysis product.

16. The device according to claim 1, further comprising a diesel particulate filter and through which the exhaust gas flows prior to introduction of the gaseous hydrolysis product.

17. The device according to claim 1, further comprising: an oxidation catalyst within the single unit and through which the exhaust gas flows prior to introduction of the gaseous hydrolysis product; and a diesel particulate filter and through which the exhaust gas flows prior to introduction of the gaseous hydrolysis product, wherein the diesel particulate filter is upstream of the oxidation catalyst

18. A device according to claim 1, further comprising a NOx sensor placed within the exhaust gas flow to measure the quantity of NOx therein.

19. The device according to claim 18 wherein the NOx sensor is placed in the exhaust flow upstream of the point of introduction of the gaseous hydrolysis product and the signal is used to predict the required volume of the gaseous hydrolysis product required to be dosed into the gas to effect its removal.

20. The device according to claim 18 wherein the NOx sensor is placed in the exhaust flow downstream of the SCR catalyst and a greater or lesser amount of gaseous hydrolysis product will be dosed into the exhaust gas depending whether the sensed NOx level is above or below a target level.

21. The device according to claim 1, further comprising an ammonia sensor placed downstream of the SCR catalyst to measure ammonia slip.

22. The device according to claim 1, wherein the device comprises an outer body, which forms a pressure barrier, and passing through the outer body is an inner body which comprises a flowpath longitudinally therethrough with an inlet and an outlet for the exhaust gas, the outer and inner bodies forming two chambers therebetween, one substantially above the inner body and one substantially below the inner body, the two chambers connected by at least one fluid passageway.

23. The device according to claim 22 wherein the two chambers and the at least one fluid passageway comprise the reaction vessel.

24. The device according to claim 23 wherein the at least one fluid passageway is in thermal contact with the exhaust gas.

25. The device according to claim 24 wherein the at least one fluid passageway between the two chambers passes through the exhaust gas flowpath formed by the inner body.

26. The device according to claim 25 wherein the at least one fluid passageway passes around the sides of the inner body.

27. The device according to claim 22, wherein the inner and outer bodies extend beyond the reaction vessel, the volume defined between said inner and outer bodies in their extended sections being separated from the reaction vessel at one end by a bulkhead and enclosed at the other end to form a reservoir area abutting the reaction vessel and through which the inner body passes.

28. The device according to claim 27 wherein the valve in the outlet is located in the bulkhead separating the reaction vessel and the reservoir.

29. The device according to claim 22, where a by-pass valve is provided to selectively bypass a proportion of the exhaust gas so that it does not directly heat the fluid passageways of the reaction vessel.

30. The device according to claim 29 wherein the inner body comprises two exhaust gas flowpaths, only one of which is in thermal contact with the fluid passageways of the reaction vessel.

31. The device according to claim 1, wherein the device comprises a rear section comprising two substantially cylindrical upright tubes and an enclosed cavity therebetween, said tubes adapted to house the reaction vessel and reservoir respectively.

32. The device according to claim 31 wherein the tube housing the reaction vessel has an inlet for the exhaust gas and is in fluid communication with the enclosed cavity such that the hot exhaust gasses flow in the inlet, over the reaction vessel and exit the tube into the enclosed cavity.

33. The device as claimed in claim 31 wherein at least a part of the tube housing the reservoir is in direct fluid contact with the hot exhaust gasses.

34. The device as claimed in claim 33 wherein the reservoir is isolated from direct heat contact with the part of the tube in direct fluid contact with the hot exhaust gasses by an air gap.

35. The device as claimed in claim 34 wherein heat transfer through the wall of the tube containing the reservoir and across the air gap is sufficient to maintain the reservoir at a high enough temperature to prevent solidification of salts out of the hydrolysis gas.

36. The device as claimed in claim 31, wherein the enclosed cavity has an opening therein for the exhaust gas to pass through prior to entering a diesel particulate filter and/or an oxidation catalyst.

37. The device according to claim 31, wherein the catalysts and at least one mixing means is attached to the exterior of the rear section via a framework.

38. The device according to claim 37 further comprising an outer casing that fits over the catalysts and mixing elements forming a treatment enclosure.

39. The device according to claim 38 wherein the outlet for the exhaust gas passes from the treatment enclosure through the enclosed cavity in the rear section to allow the exhaust gas to exit the unit for eventual discharge.

40. The device according to claim 31, wherein the reservoir and reaction vessel abut a manifold plate, said manifold plate providing a barrier between a hot area below it and a cooler area above it.

41. The device according to claim 40 wherein the valves and any sensors are placed at least partially in the cooler area such that their electronics and some other function critical parts can be protected from direct exposure to the hot environment.

42. The device according to claim 40 wherein the manifold plate includes a heat shield between the hot area and the cooler area.

43. The device according to claim 41 wherein the valves and any sensors have covers sealed thereover to prevent water ingress into the electronics.

44. The device according to claim 43 wherein said covers comprise a thermally conductive material and include a number of cooling fins to assist in removing any heat from this area.

45. The device according to claim 31, mounted on a commercial vehicle such that the rear section is closest the centre of the vehicle and treatment enclosure extends outwards therefrom such that, in event of a collision, the treatment area forms a sacrificial ‘crumple zone’ to absorb the energy of impact and protect the pressurised reaction vessel and reservoir from direct impact.

46. The device according to claim 1, further provided with a heating element for the reservoir.

47. A hydrolysis gas reservoir for receiving ammonia containing gas from a hydrolysis reaction vessel, the reservoir comprising a body and a manifold, said manifold having passageways therein to accommodate various sensors and at least one valve and having heating means associated therewith to maintain said manifold at an elevated temperature.

48. The reservoir according to claim 47 wherein the heating means comprises one or more electric heating elements.

49. The reservoir according to claim 48 wherein the heating means comprises a plurality of finger heaters inserted substantially radially into the manifold.

50. The reservoir according to claim 47, wherein the heating means is adapted to maintain the manifold at a temperature in the range 130 to 300 degrees centigrade.

51. The reservoir according to claim 50 the heating means is adapted maintain the manifold at a temperature in the range 210 to 230 degrees centigrade.

52. The reservoir according to claim 47, wherein attached to a passageway of the manifold is a pressure relief valve that releases the hydrolysis product from the reservoir should the pressure therein exceed a certain value.

53. The reservoir according to claim 52 wherein any gas being released via the pressure relief valve is released into a small reservoir of water to condense the gaseous hydrolysis product and prevent it being released directly into the atmosphere.

54. The reservoir according to claim 52 wherein any gas being released via the pressure relief valve is released directly into the exhaust gas flow.

55. The reservoir according to claim 47, wherein attached to a port of the manifold is a dosing valve for dosing the ammonia-containing gas into an exhaust gas stream.

56. The reservoir according to claim 47, wherein the manifold has a valve seat therein between two of said passageways, one passageway leading from the interior of the reservoir and forming a valve inlet and the other passageway exiting the side of the manifold forming a valve outlet.

57. The reservoir according to claim 56 wherein a valve actuator and associated valve armature are connected to said manifold, the valve actuator operable to move the valve armature on and off the valve seat thereby allowing or preventing flow therethrough.

58. The device according to claim 47, wherein the reservoir manifold has means for attaching it to a manifold plate such that the valves and sensors protrude through the manifold plate into the cool area above it.

59. The reservoir according to claim 58 wherein said means for attaching the manifold to the manifold plate comprise a plurality of flanges adapted to take a screw or bolt.

60. A system for the reduction of NOx in the exhaust gas of an IC engine comprising a reactor for producing ammonia, a reservoir to temporarily store ammonia a means of introducing ammonia to the exhaust gas and an SCR catalyst, the reservoir including a body and a manifold, said manifold having passageways therein to accommodate various sensors and at least one valve and having heating means associated therewith to maintain said manifold at an elevated temperature.

61. A device for generating gaseous hydrolysis product comprising ammonia, formed by the hydrolysis of an aqueous solution of urea at elevated temperature and pressure, the device being adapted to be placed in the exhaust system so that the exhaust gas will flow through it during use, and comprising a) a first substantially upright and cylindrical tube enclosed at its upper end and open at its lower end and having an inlet and an outlet on its sides for the exhaust gas; b) an elongate reaction vessel located in the tube for containing an aqueous solution of urea and arranged such that, in use, the vessel and therefore the urea solution become heated by means of heat exchange with the exhaust gas as it flows from the inlet to the outlet; and c) a urea solution inlet to the reaction vessel and a gaseous hydrolysis product outlet from the reaction vessel; wherein said reaction vessel is attached to the upper enclosed end of the first tube and sealingly engages with the first tube at its lower end preventing the exhaust gas from escaping out of the open lower end of the first tube.

62. The device according to claim 61 wherein the reaction vessel is provided with a structurally weak point in its upper end that will rupture at a lower pressure that the rest of the reaction vessel ensuring that, in the case of excessive pressure build up in the reaction vessel, the structurally weak point will rupture and the gas in the reaction vessel will expand therethrough forcing the reaction vessel downwards.

63. The device according to claim 61 wherein the reaction vessel has a circumferential seal attached to the outer surface of its lower end and the said seal slides in the tube as the reaction vessel expands and contracts.

64. The device according to claim 61 wherein the tube has a circumferential seal attached to the inner surface of its lower end and the reaction vessel slides past the seal as it expands and contracts.

65. The device according to claim 61, wherein the device further comprises a second substantially upright and cylindrical tube having an enclosed upper end and an open lower end, said second tube housing a substantially elongate reservoir to collect the gaseous hydrolysis product produced in the reaction vessel, said reservoir being attached to the upper enclosed end of the tube and sealingly engaging with the tube at its lower end.

66. The device according to claim 65 wherein the exterior of the second tube is at least partially heated by the hot exhaust gasses.

67. The device according to claim 65 wherein the reservoir is provided with a structurally weak point in its upper end that will rupture at a lower pressure that the rest of the reservoir ensuring that in the case of excessive pressure build up in the reservoir the structurally weak point will rupture and the gas in the reservoir will expand therethrough forcing the reservoir downwards.

68. The device according to claim 65, wherein reservoir has a circumferential seal attached to the outer surface of its lower end and the said seal slides in the tube as the reservoir expands and contracts.

69. The device according to claim 65, wherein the tube has a circumferential seal attached to the inner surface of its lower end and the reservoir slides past the seal as it expands and contracts.

70. The device according to claim 65, wherein the first and second substantially upright tubes form the two substantially upright tubes.

Description:

The present invention relates to an apparatus for reducing emissions of Nitrogen oxides (NOx) in exhaust gasses of an internal combustion (IC) engine.

The introduction of reagents into the flow of an exhaust gas of an IC engine prior to the gas passing through a catalyst in order to effect selective catalytic reduction (SCR) of NOx is well known.

The known systems principally fall into one of two categories, those which introduce gaseous ammonia into the exhaust conduit and those which introduce into the exhaust conduit a liquid reagent which decomposes into ammonia gas in the conduit.

The introduction of gaseous ammonia into exhaust gasses for SCR purposes has been known for a long time in association with static systems, for example the after-treatment of flue gas in power plants. Over time, the benefit of SCR has been realised in mobile solutions, initially in the shipping industry and more recently in the motor vehicle industry. Where the application is mobile, for example a motor vehicle, there are, however, safety implications in carrying a sufficiently large supply of ammonia on board to cope with requirements over an acceptable period of time. For example a rupture of the ammonia vessel, for example in a crash, could cause the release of large volumes of ammonia into the atmosphere. In addition there are additional risks of ammonia release when handling and refilling the ammonia vessel, for example at roadside service stations.

One solution to this problem has been to inject a liquid reagent into the hot exhaust gas where it decomposes into ammonia. The liquid reagent is, at ambient temperatures, a stable medium, but it decomposes at elevated temperatures to form at least ammonia gas. It is preferably an aqueous solution of urea or related substance such as biuret or ammonium carbamate, collectively referred to, and defined, herein as “urea”. While this solution to the problem provides a satisfactory result, there are a number of problems associated with it. Firstly, the liquid is injected through a nozzle as a fine spray of droplets into the fast flowing exhaust gas in which it preferably fully decomposes into at least ammonia gas prior to contacting the SCR catalyst. As this is not an instantaneous process, there needs to be a minimum separation distance between the injector and the SCR catalyst to allow sufficient time to allow the full decomposition of the liquid into gas prior to it contacting the SCR catalyst. Secondly is the problem of precipitation of solids from the urea solution throughout the system and especially in the injector nozzle and catalyst. Solid formation in the nozzles tends to occur particularly where dormant urea solution has resided at a high temperature under minimal pressure for a period of time in the injector nozzle. The solids may frequently block the nozzles, calling for complex control systems either to purge the nozzle, e.g. with pressurised air, or to re-circulate the urea so that it does not have the requisite time at elevated temperature for the precipitation to occur. Solidification of solids on the catalyst which occurs particularly when the liquid is dosed at low temperatures below about 180 degrees C. reduces the efficiency of the catalyst and increases the back pressure the catalyst creates within the exhaust system and therefore in time the catalyst will need replacing.

An alternative solution to the problem has been proposed in U.S. Pat. No. 6,361,754 and comprises hydrolysing aqueous urea under pressure at a high temperature so that it decomposes into at least gaseous ammonia and then introducing the gaseous ammonia into the exhaust conduit. While this is an efficient method of preparing ammonia gas in situ, as the heating is dependant on the reactor being placed in the exhaust conduit and the pressure under which the urea is being maintained will vary depending on the dosing of the gas into the exhaust, it is very hard to maintain a stable reaction and ammonia concentration within the hydrolysis gas will vary. Also, all components of the system, of which there are many, need to be maintained at a minimum temperature and pressure to prevent the precipitation of solids. The operational pressure of the system is directly linked to the dosing and if compensated by continual supply of aqueous urea, to maintain constant ammonia concentration in the hydrolysis gas, then in times of peak demand the aqueous urea may pass fully through the reactor and be dosed directly into the exhaust.

U.S. Pat. No. 6,399,034 discloses an alternative solution which utilises decomposition of an alternative reagent, for example an aqueous solution of ammonium carbamate, which decomposes at a lower temperature, and stores the ammonia gas produced in an intermediate storage vessel and doses the gas from that vessel into the exhaust. The aqueous solution is heated by the engine cooling system which is capable of providing the lower heat requirements to decompose ammonium carbamate, but which would not be sufficient to hydrolyse urea at the rate required for NOx reduction in the exhaust of an IC engine.

Both of the above solutions and similar solutions used in static applications in the power industry are complex assemblies containing a large number of parts and interconnecting tubing which firstly need to be maintained at an appropriate temperature to prevent precipitation of solids from the liquid phase and deposition of solids from the gaseous phase, and which secondly are hard to retrofit to existing vehicles. This is commonly done by thermally lagging all the interconnecting tubing and by heating the interconnecting tubing, which while suitable for static systems running constantly under steady state conditions, is not suitable for commercial vehicles which operate under a stop/start regime. In particular, at start-up, until enough exhaust gas has passed through the exhaust conduit in order to heat it to the required temperature, there is a danger of depositions of solids creating blockages in the conduit and causing the gas pressure in it to rise to dangerous levels.

A further problem with existing systems is that, because of the number of parts, retrofitting them to existing vehicles is a complex procedure.

It is the purpose of the present invention to mitigate some of the above problems by providing a simplified apparatus for the production of ammonia gas for use in SCR systems of IC engines, especially but not exclusively diesel engines.

According to a first aspect of the present invention there is provided a unitary device for generating and feeding gaseous hydrolysis product comprising ammonia, formed by the hydrolysis of an aqueous solution of urea (as hereinbefore defined) at elevated temperature and pressure, into the exhaust gas of an IC engine as it flows through the exhaust system of the engine, the device being adapted to be placed in the exhaust system so that the exhaust gas will flow through it during use, and comprising

a) a housing having an inlet for the exhaust gas and an outlet for the exhaust gas;
b) a reaction vessel located at least partially within the housing between the inlet and the outlet for containing an aqueous solution of urea and arranged such that, in use, the vessel and therefore the urea solution become heated by means of heat exchange with the exhaust gas as it flows from the inlet to the outlet;
c) a urea solution inlet to the reaction vessel and a gaseous hydrolysis product outlet from the reaction vessel;
d) a reservoir for receiving and storing gaseous hydrolysis product;
e) a valve in the outlet from the reaction vessel and adapted to permit the contents of the reaction vessel, in use, to attain an elevated pressure as it becomes heated, and to discharge gaseous hydrolysis product into the reservoir; and
f) a conduit for interconnecting the reservoir and the exhaust system, the conduit including a valve to selectively control the feed of hydrolysis product stored in the reservoir into the exhaust gas via the conduit.

Preferably the valves are placed at least partially outside the housing such that they are at least partially protected from direct exposure to the hot exhaust gasses.

By having substantially all the components within the same unit, heat transfer from the exhaust gas by conduction quickly heats the components upon start up and maintains them at an elevated temperature preventing ammonium carbamate and other solids from solidifying from the gaseous hydrolysis product thereby preventing potential blockages of the system and maintaining a system which can operate safely whilst producing gas under pressure. Where the valves are partially placed outside of the unit the parts of the valve through which the hydrolysis product flows is maintained at an elevated temperature sufficient that the hydrolysis product stays in gaseous form as it passes therethrough.

As the device, in its preferred form, can be supplied as essentially one unit it is simple to fit both for new builds and as a retrofit to existing vehicles as it simply replaces a section of the current exhaust conduit.

The valve (e) in the outlet from the reaction vessel is adapted to permit the contents of the reaction vessel, in use, to attain an elevated pressure as it becomes heated. The valve may take a number of forms. In one preferred arrangement the valve (e) actuates in response to the pressure within the reaction vessel and preferably periodically discharges gaseous hydrolysis product into the reservoir. This can be an active actuation where the pressure is measured in the reaction vessel and the valve is actuated via a control system depending on the signal received from a pressure transducer situated in the reaction vessel. Alternatively this can be a passive actuation where the valve is self actuating when a preset pressure occurs on its inlet side, i.e. it is a simple mechanical back pressure valve. By maintaining a substantially constant pressure within the reactor vessel the concentration of the ammonia within the hydrolysis gas remains substantially constant.

In an alternative preferred arrangement the valve actuates in response to the temperature of the aqueous solution of urea. This is preferably done by measuring the temperature within the aqueous urea solution and actuating the valve in response to the measured temperature. As the reaction occurs within the reaction vessel and the pressure rises the temperature within the solution can also be raised until both are elevated, and as there is a direct relationship between the two, control of the release of the gaseous hydrolysis product can be based on either. The valve for controlling the release of the gaseous hydrolysis product is preferably placed in a bulkhead between the reaction vessel and the reservoir.

In a preferred arrangement of the present invention the system further comprises a sensor placed within the exhaust gas flow to measure the quantity of NOx therein. The NOx sensor may be upstream of the point of introduction of the ammonia containing hydrolysis gas or downstream of the SCR catalyst and would either measure the NOx output of the engine or the NOx output of the vehicle respectively. If the NOx output of the engine is measured then the signal is used to predict the required volume of the gaseous hydrolysis product required to be dosed into the gas to effect its removal (i.e. open loop control), whereas if the NOx output of the vehicle is sensed then more or less gaseous hydrolysis product will be dosed into the exhaust gas depending whether the sensed NOx level is above or below a target level (i.e. closed loop control). In an alternative arrangement an ammonia sensor is placed downstream of the SCR catalyst to measure ammonia slip (the amount of ammonia passing un-reacted through the SCR catalyst). The control system can then sense if too much ammonia containing hydrolysis gas is being added to the exhaust flow and reduce the amount accordingly.

Preferably the device is close coupled to an SCR catalyst, or optionally is contained within one and the same unitary housing with an SCR catalyst connectable in line in the exhaust system. Preferably the downstream end of the SCR catalyst is coated with a catalyst that converts any un-reacted ammonia in the exhaust gas so that ammonia is not released into the environment.

Preferably the exhaust gas flowpath between the point of introduction of the hydrolysis gas and the SCR catalyst is shaped to induce mixing of the hydrolysis gas with the exhaust gas. Preferably the ammonia gas is introduced substantially on the axis of the exhaust gas flow path and is introduced in a direction substantially perpendicular to the direction of the flow. Preferably a number of radially spaced inlets are situated adjacent one another substantially perpendicular the flow. Preferably the point of introduction is substantially at the mouth of a truncated conical section of the flowpath and the flow of exhaust gas and hydrolysis gas into the cone induces mixing. Preferably the flowpath between the point of introduction of the hydrolysis gas and the SCR catalyst has at least one substantially 90 degree bend causing turbulence in the flowpath further inducing mixing. Preferably the exhaust gas and hydrolysis gas enter a substantially cylindrical vortex chamber, upstream of the SCR catalyst, substantially perpendicularly to the radius of the chamber and exits the chamber along its central axis, the vortex within the chamber further inducing mixing of exhaust gas and hydrolysis gas.

Preferably, contained within the same unit as the device, is an oxidation catalyst through which the exhaust gas flows prior to the addition of the hydrolysis gas, Preferably the oxidation catalyst is sized to oxidise a proportion of the Nitric Oxide in the exhaust gas that a favourable mixture, preferably approximately 50/50, of NO and NO2 is achieved in the exhaust gas. Preferably the oxidation catalyst, device and SCR are all contained within one unit having an exhaust inlet and an exhaust outlet and connectible in line in the exhaust system of a vehicle.

Preferably, downstream of the oxidation catalyst is a diesel particulate filter. Preferably the diesel particulate filter is contained in one and the same unit as the oxidation catalyst, device and SCR catalyst.

By containing the device within the same housing as the catalysts and optionally a filter the entire exhaust treatment system, comprising the functions of removing diesel particulates, preparing the exhaust for NOx treatment, adding an appropriate reagent to the exhaust gas and then passing the admixture through or over a catalyst to reduce the NOx content of the exhaust gas can be performed by one unit which may be supplied to the vehicle manufacturer as a unit ready for incorporation into the vehicle. Preferably the aforementioned NOx sensor is also contained within the same unitary housing.

Preferably the reservoir is provided with a heater to increase its temperature.

In one arrangement of the invention the device comprises an outer body, which forms a pressure barrier, and passing through the outer body is an inner body which comprises a flowpath longitudinally therethrough with an inlet and an outlet for the exhaust gas, the outer and inner bodies forming two chambers therebetween. In a preferred arrangement, one of the chambers is situated substantially above the inner body and the other is situated substantially below the inner body. The two chambers are connected by at least one fluid passageway, the two chambers and the at least one fluid passageway comprising the reaction vessel.

The fluid passageway(s) and optionally at least a section of the walls of the two chambers are, in use, in thermal contact with the exhaust gas.

Preferably the fluid passageway(s) between the two chambers passes through the exhaust gas flowpath formed by the inner body and preferably comprises a number of tubes. Alternatively the fluid passageway(s) may pass around the sides of the inner body.

Preferably, the inner and outer bodies extend beyond the reaction vessel and the volume defined between said inner and outer bodies in their extended sections is separated from the reaction vessel by a bulkhead. The extended section of the inner and outer sections is enclosed on the other end to form an enclosed reservoir area abutting the reaction vessel and through which the inner body passes.

In this arrangement the device is optionally further provided with a by-pass valve to selectively bypass a proportion of the exhaust gas so that it does not directly heat the fluid passageways of the reaction vessel whereby the heat input to the reaction vessel can be varied. This enables the output of the reaction vessel to be controlled as the demand fluctuates. Preferably the inner body comprises two exhaust gas flowpaths, only one of which is in thermal contact with the fluid passageways of the reaction vessel and the by-pass valve controls the proportion of the exhaust gas flow which passes through each exhaust gas flowpath.

In a second arrangement the device comprises, in part, a rear section comprising two substantially cylindrical upright tubes and an enclosed cavity therebetween. The two upright tubes contain the reaction vessel and reservoir respectively. The tube containing the reaction vessel has an inlet for the exhaust gas and is in fluid communication with the enclosed cavity. The hot exhaust gasses enter flow through the tube passing over the reaction vessel and heat it. The outer surface of the tube containing the reservoir is partially in direct fluid contact with the hot exhaust gasses but the reservoir itself is insulated from the direct heat, preferably by an air gap. Heat transfer through the wall of the tube containing the reservoir and across the air gap is sufficient to maintain the reservoir at a sufficiently high temperature to prevent condensation of the hydrolysis gas or solidification of salts out of the hydrolysis gas during operation. The enclosed cavity has an opening therein for the exhaust gas to pass through prior to entering a diesel particulate filter and/or an oxidation catalyst.

Preferably attached to the exterior of the rear section via a framework, are the catalysts and mixing elements and optionally the diesel particulate filter. An outer casing fits over these components forming a treatment enclosure. Preferably an outlet from the unit passes from the treatment enclosure through the enclosed cavity in the rear section to allow the exhaust to exit the unit for eventual discharge.

Preferably the device is mounted on a commercial vehicle such that the rear section is closest to the centre of the vehicle and the treatment enclosure extends outwards therefrom such that, in event of a collision, the treatment enclosure and components therein form a sacrificial ‘crumple zone’ to absorb the energy of impact and protect the pressurised reaction vessel and reservoir from direct impact

Preferably the top of the reservoir and reaction vessel abut a manifold plate, said manifold plate providing a barrier between the hot area below it (the two tubes) and the cooler area above it. Preferably all the valves and sensors are placed in, or pass through, the cool area such that their electronics and some other function critical parts can be protected from direct exposure to the hot environment. Preferably the manifold plate includes a heat shield between the hot area and the cool area. Preferably the valves and sensors have a cover sealed thereover such that the exterior of the device can be washed down without affecting the electronics. Preferably the covers are of a thermally conductive material and include a number of cooling fins to assist in removing any heat from this area. Preferably the covers are made of aluminium.

In use the system is attached via a urea inlet line to a urea pump and a urea solution tank. Preferably the urea tank is provided with a quality sensor to detect the quality of urea and create an alert or render the device inoperable if the urea is not of the correct quality, for example if it does not have the correct percentage of urea. Equally the sensor will be able to detect if a different substance, e.g. water or diesel has been deliberately or inadvertently put into the urea tank. In one arrangement the pump is integral with the urea tank. Preferably the urea tank and urea line between the urea tank and the reaction vessel inlet is heated to prevent urea freezing in the urea line.

According to a second aspect of the invention there is provided a hydrolysis gas reservoir for receiving ammonia containing gas from a hydrolysis reaction vessel, the reservoir comprising a body and an upper manifold, said upper manifold having passageways therein to accommodate various sensors and at least one valve and having heating means associated therewith to maintain said upper manifold at a substantially constant temperature, thereby preventing blockages of the passageways therein by deposition of solid ammonia salts formed at lower temperatures.

Preferably the heating means comprises electric heating elements, more preferably the heating means comprises a plurality of elongate cartridge heaters inserted substantially radially into the upper manifold.

Preferably the heating means maintain the upper manifold at a temperature in the range 100 to 300 degrees centigrade. More preferably the heating means maintain the upper manifold at a temperature in the range 180 to 220 degrees centigrade, ideally 200 degrees.

Preferably attached to a passageway of the upper manifold is a pressure relief valve that releases the hydrolysis product from the reservoir should the pressure therein exceed a certain value. Preferably any gas being released therefrom is released into a small reservoir of water to condense the ammonia and prevent it being released directly into the atmosphere. Alternatively the gas may be vented directly into the exhaust gas stream, ideally prior to the oxidation catalyst.

In one arrangement, attached to a passageway of the upper manifold, there is preferably a dosing valve for dosing the hydrolysis gas into the exhaust gas stream.

In an alternative preferred arrangement the manifold has a valve sat therein between two of the passageways, one leading from the interior of the reservoir and forming a valve inlet and the other exiting the side of the manifold forming an outlet. Preferably a valve actuator and associated valve armature are connectable to the manifold, the valve actuator operable to move the valve armature on and off the valve seat thereby preventing or allowing flow.

Preferably located in passageways into the upper manifold are a temperature sensor and/or a pressure sensor.

Preferably the upper manifold is welded to the body. Preferably the upper manifold has means for attaching it to a manifold plate according to the first aspect of the invention such that the valves and sensors protrude through the manifold plate into the cool area above it. Preferably said means for attaching the upper manifold to the manifold plate comprise a plurality of flanges adapted to take a screw or bolt.

According to a third aspect of the present invention there is provided a device for generating gaseous hydrolysis product comprising ammonia, formed by the hydrolysis of an aqueous solution of urea (as hereinbefore defined) at elevated temperature and pressure, the device being adapted to be placed in the exhaust system so that the exhaust gas will flow through it during use, and comprising

  • a) a first substantially upright and cylindrical tube enclosed at its upper end and open at its lower end and having an inlet and an outlet on its sides;
  • b) an elongate reaction vessel located in the tube for containing an aqueous solution of urea and arranged such that, in use, the vessel and therefore the urea solution become heated by means of heat exchange with the exhaust gas as it flows from the inlet to the outlet; and
  • c) a urea solution inlet to the reaction vessel and a gaseous hydrolysis product outlet from the reaction vessel;
    wherein said reaction vessel is attached to the upper enclosed end of the first tube and sealingly engages with the first tube at its lower end preventing the exhaust gas from escaping out of the open lower end of the first tube.

As the reaction vessel is elongate its primary direction of thermal expansion and contraction will be along its axis. As the reaction vessel is only attached by one end it is free to expand within the first tube as it heats up and contract as it cools down. In addition, in the case of a rupture in the vessel anywhere apart from the bottom, as tube is enclosed at its upper end the expansion of the gas as it exits the ruptured reaction vessel will tend to force the reservoir out of the open bottom end of tube.

Preferably the reaction vessel or the relief valve is provided with a structurally weak point in its upper end of the reaction vessel/relief valve assembly that will rupture at a lower pressure than the rest of the reaction vessel ensuring that in the case of excessive pressure build up in the reaction vessel the structurally weak point will rupture and the gas in the reaction vessel will expand therethrough forcing the reaction vessel downwards and enhancing the effect of the downwards projection of the reaction vessel due to the restraint of the tube.

In one preferred arrangement the reaction vessel has a circumferential seal attached to the outer surface of its lower end and the said seal slides in the tube as the reaction vessel expands and contracts. In an alternative preferred arrangement the tube has a circumferential seal attached to the inner surface of its lower end and the reaction vessel slides past the seal as it expands and contracts.

Preferably the device further comprises a second substantially upright and cylindrical tube having an enclosed upper end and an open lower end, said second tube housing a substantially elongate reservoir to collect the gaseous hydrolysis product produced in the reaction vessel and said reservoir attached to the upper enclosed end of the tube and sealingly engaging with the tube at its lower end.

The reservoir is able to expand and contract in a similar way as the reaction vessel.

Preferably the exterior of the second tube is at least partially heated by the hot exhaust gasses.

Preferably the reservoir is provided with a structurally weak point in its upper end that will rupture at a lower pressure than the rest of the reservoir ensuring that in the case of excessive pressure build up in the reservoir the structurally weak point will rupture and the gas in the reservoir will expand therethrough forcing the reservoir downwards and enhancing the effect of its downwards projection due to the restraint of the tube.

In one preferred arrangement the reservoir has a circumferential seal attached to the outer surface of its lower end and the said seal slides in the tube as the reservoir expands and contracts. In an alternative preferred arrangement the tube has a circumferential seal attached to the inner surface of its lower end and reservoir slides past the seal as it expands and contracts.

Preferably the first and second substantially upright tubes form the two substantially upright tubes of the rear section of the second arrangement of the first embodiment of the invention.

According to a fourth aspect of the present invention there is provided a device for generating gaseous hydrolysis product comprising ammonia, formed by the hydrolysis of an aqueous solution of urea (as hereinbefore defined) at elevated temperature and pressure, for feeding into the exhaust gas of an IC engine as it flows through the exhaust system of the engine, the device being adapted to be placed in the exhaust system so that the exhaust gas will flow through it during use, and comprising

a) a housing having an inlet for the exhaust gas and an outlet forth exhaust gas;
b) a reaction vessel located at least partially within the housing between the inlet and the outlet for containing an aqueous solution of urea and arranged such that, in use, the vessel and therefore the urea solution become heated by means of heat exchange with the exhaust gas as it flows from the inlet to the outlet;
c) a urea solution inlet to the reaction vessel and a gaseous hydrolysis product outlet from the reaction vessel;
d) a pump for pumping urea solution into the reaction vessel via the urea solution inlet; and
e) control means for controlling the pump in response to changing NOx output from the IC engine;
wherein, in response to an increase in said NOx output, the control means controls the pump to increase the level of urea solution in the reactor vessel, thereby increasing the surface area of urea solution available for heat exchange with the exhaust gas so as to increase the rate of production of gaseous hydrolysis product in the reactor vessel.

Preferably, in response to a decrease in said NOx output, the control means controls the pump to decrease the level of urea solution in the reactor vessel, thereby decreasing the surface area of urea solution available for heat exchange with the exhaust gas so as to decrease the rate of production of gaseous hydrolysis product in the reactor vessel.

Preferably, the device further comprises a sensor placed within the exhaust gas flow to measure the quantity of NOx therein.

Advantageously, the NOx sensor may be upstream or downstream of the SCR catalyst and would either measure the NOx output of the engine or the NOx output of the vehicle respectively. If the NOx output of the engine is measured then the signal is used to predict the required volume of the gaseous hydrolysis product required to be dosed into the gas to effect its removal (i.e. open loop control), whereas if the NOx output of the vehicle is sensed then more or less gaseous hydrolysis product will be dosed into the exhaust gas depending whether the sensed NOx level is above or below a target level (i.e. closed loop control)

Alternatively, engine management data, for example torque, engine speed, and/or throttle setting, are interrogated in order to deduce the NOx output of the vehicle.

Preferably, the device includes a reservoir for receiving and storing gaseous hydrolysis product. More preferably, the device includes a conduit for interconnecting the reservoir and the exhaust system. Most preferably, the conduit includes valve means to selectively control the feed of hydrolysis product stored in the reservoir into the exhaust gas via the conduit.

Preferably, level and/or temperature and/or pressure sensors are provided in the reactor.

Preferably, all the sensors required in the reactor are provided in a single cluster, removable in its entirety to minimise the number of access points required in the pressurised reactor. Preferably there is additionally a quality sensor provided in the reservoir and optionally in the urea storage tank to monitor the quality (for example the concentration) of the urea. Preferably the level sensor also acts as the quality sensor.

Preferably the device is provided with ammonia sensors downstream of the SCR catalyst to measure the ammonia slip. Preferably temperature sensors are provided inside the SCR catalyst to measure the temperature of the catalyst. Preferably there are also sensors provided upstream and/or downstream of the SCR catalyst to fully measure the temperature changes of the exhaust gas as it passes through the catalyst.

Preferably, the device includes a valve in the outlet from the reaction vessel, the valve being adapted to cause the contents of the reaction vessel, in use, to attain an elevated pressure as it becomes heated, and to discharge gaseous hydrolysis product into the reservoir

The valve may take a number of forms. In one preferred arrangement the valve actuates in response to the pressure within the reactor. This can be an active actuation where the pressure is measured in the reactor and the valve is actuated via a control system depending on the signal received from a pressure transducer situated in the reactor. Alternatively this can be a passive actuation where the valve is self actuating when a preset pressure occurs on its inlet side, i.e. it is a simple mechanical back pressure valve. In an alternative preferred arrangement the valve actuates in response to the temperature of the aqueous solution of urea. This is preferably done by measuring the temperature within the aqueous urea solution and actuating the valve in response to the measured temperature. As the reaction occurs within the reaction vessel and the pressure rises the temperature within the solution also rises until both are elevated, and as there is a direct relationship between the two, control of the release of the gaseous hydrolysis product can be based on either. The valve for controlling the release of the gaseous hydrolysis product is preferably placed in the bulkhead between the reactor and the reservoir.

In a preferred arrangement the device further includes an auxiliary heating means for heating the reservoir, thereby enabling the reservoir to become heated prior to the engine being started, or alternatively enabling the reservoir to be maintained at an elevated temperature when the engine is switched off. The auxiliary heating means is preferably an electrically powered heater or a diesel burning heater.

Preferably the device further comprises a bypass valve which can selectively control the proportion of the exhaust gas which is in thermal contact with the reactor to control the heat input into it.

Preferably the device is adapted for use with mobile, for example vehicle, engines. As the hydrolysis reaction favours fairly stable conditions then in such applications, and due to the transient operating conditions, it necessary to have a reservoir to store some of the hydrolysis product so the system can respond quickly to changes in the requirement for said hydrolysis product. This results in a residual volume of hot, pressurised, ammonia containing, hydrolysis gas in the reservoir when the engine is shut down. The content of the hydrolysis gas will depend on the reagent which is initially used which may for example be aqueous urea or ammonium carbamate. Both these reagents and a number of others will result in a hydrolysis gas containing steam and carbon dioxide as well as the ammonia. As the reservoir cools below 60 degrees, ammonia and carbon dioxide will react to form ammonium carbamate which will then at least partially dissolve in the water which forms as the steam condenses. Preferably the reservoir acts as a secondary reactor to, when the engine is re-started, heat the contents therein to evaporate the water and decompose the ammonium carbamate into the carbon dioxide and ammonia from which it formed.

Preferably there is a holding area into which, in response to a desire to reduce the liquid volume within the reactor, an amount of the aqueous urea is moved for temporary holding. Preferably the holding area is separate from the aqueous urea storage tank.

Preferably when it is required to increase the liquid-volume within the reactor, if there is any liquid in the holding area, the reactor is filled from the holding area until it is empty upon which, if further filling is required, the reactor will be filled from the aqueous urea storage tank.

Preferably the holding area is maintained at a temperature above which solids form within the liquid.

Preferably, both the reactor and the reservoir are heated by heat exchange with the exhaust gas.

Preferably, the device includes a catalyst arranged within the reactor to advance the rate of hydrolysis of the aqueous solution. More preferably the catalyst is arranged on a substrate. Most preferably the substrate is conical or frustoconical.

According to a fifth aspect of the present invention there is provided a method of controlling the generation of a gaseous hydrolysis product comprising ammonia, and the feeding of that product into the exhaust gas of an IC engine, the method comprising the steps of:

a) providing a housing having an inlet for the exhaust gas and an outlet for the exhaust gas;
b) providing a reaction vessel located at least partially within the housing between the inlet and the outlet for containing an aqueous solution of urea and arranged such that, in use, the vessel and therefore the urea solution become heated by means of heat exchange with the exhaust gas as it flows from the inlet to the outlet;
c) providing a urea solution inlet to the reaction vessel and a gaseous hydrolysis product outlet from the reaction vessel;
d) providing a pump for pumping urea solution from into the reaction vessel via the urea solution inlet;
the method further comprising the steps of:
e) hydrolysing an aqueous solution of urea (as hereinbefore defined) at elevated temperature and pressure within the reactor vessel;
j) determining the level of NOx in the exhaust gas;
g) controlling the pump to increase the level of urea solution in the reactor vessel in response to an increase in NOx levels in the exhaust system, thereby increasing the surface area of urea solution available for heat exchange with the exhaust gas so as to increase the rate of hydrolysis in the reactor vessel.

According to a sixth aspect of the present invention there is provided a thermo-hydrolysis reactor for producing ammonia-containing gas by heating an aqueous solution of urea (as hereinbefore defined), the reactor comprising an elongate vessel having a middle tubular section, an enlarged lower section having an inlet therein for the solution, and an enlarged upper section having an having an outlet therein for the ammonia-containing gas, said reactor being adapted such that, in use, heat transmitted through the walls of the reactor from an external heat source heats the solution therein causing it to hydrolyse producing said ammonia-containing gas.

The reactor is designed for use with liquid reagents which hydrolyse to form ammonia-containing gas; in particular the reactor is designed for use with aqueous solutions containing urea.

Preferably, in use, the thermo-hydrolysis reactor is heated by heat exchange with the hot exhaust gasses of an internal combustion engine.

Preferably the level of the aqueous solution of urea in the reactor is variable and the reactor is configured such that, as the level of the aqueous solution of urea in the reactor increases, the wetted surface area to volume ratio of the reactor also increases.

In a preferred arrangement the enlarged lower section has conical sides and the ratio of the maximum diameter of the lower conical section to the diameter of the tubular section, and the angle of the sides of the lower conical section, define the relationship between fill level and wetted surface area of the reactor.

Preferably the reactor is provided with a level sensor to detect the level of the reagent within the reactor. In one arrangement the level sensor passes through the lower end of the reactor and extends substantially vertically upwards into it, thereby maintaining the majority of the sensor substantially at the temperature of the liquid within the reactor. Alternatively the level sensor passes through the upper end of the reactor and extends substantially vertically downwards into it.

Preferably, situated within the reactor below the level of the outlet and above the level of the solution is a baffle to prevent splashes of aqueous urea from entering the ammonia-containing gas outlet.

Preferably a catalyst is placed in the reactor vessel to promote the hydrolysis of the aqueous solution of urea. More preferably the catalyst extends from below the level of the aqueous solution of urea within the reactor to above the level of the aqueous solution of urea thereby enabling the contact area of the catalyst to be varied by changing the volume of aqueous solution of urea within said reactor

Additionally the reactor may have a plurality of heat exchange fins on its exterior and/or interior. In one preferred arrangement the heat exchange fins placed on the interior of the reactor are made of a hydrolysis catalyst.

Preferably the reactor is provided with a supplementary heater such that, if necessary, the reactor may be heated by both heat exchange with the exhaust gas and the supplementary heater. Preferably the reactor is provided with temperature and pressure sensors to sense the temperature and pressure within the reactor.

According to the a seventh aspect of the present invention there is also provided a NOx-reduction system including a reactor as defined above and a road vehicle containing such a system.

According to an eighth aspect of the present invention there is provided an apparatus for generating an ammonia-containing gas for use in the selective catalytic reduction of NOx contained in the exhaust gases of an IC engine, the apparatus comprising:

a) a hydrolysis reactor for containing an aqueous solution of urea (as hereinbefore defined)
b) means for heating the solution to an elevated temperature by way of heat exchange with said exhaust gases, whereby the urea is hydrolysed and the ammonia containing gases are liberated;
c) valve means operable between a substantially closed position for enabling the pressure of the ammonia-containing gas to attain a predetermined elevated pressure within the reactor, and an open position when the gas is above said predetermined pressure;
d) a reservoir having an inlet for receiving all of the ammonia-containing gas discharged from the reactor when said valve is in its open position, and an outlet for feeding ammonia-containing gas to the exhaust gases, the reservoir serving to store: ammonia-containing gas during operation of the IC engine and, following the IC engine being switched off, ammonia-containing gas condensate; and
e) means for heating the reservoir,
the arrangement being such that on cold start-up of the IC engine, the means for heating the reservoir is operable to decompose the condensate into ammonia-containing gas.

By decomposing the condensate into ammonia-containing gas a source thereof is thereby provided at cold start-up of the IC engine for use in NOx reduction before the hydrolysis reactor reaches its normal working elevated temperature and pressure.

Preferably, during normal operation the reservoir is maintained at a pressure above the pressure within the exhaust conduit.

The reservoir, by providing a store of ammonia containing gas during normal operation of the system enables a fast response to transient changes in the demand for ammonia to be dosed as the load on the engine changes. While it is relatively easy to use a system without a reservoir and where the ammonia is effectively produced “on demand” in a situation where there is little or only gradual changes in the demand on the system, in a highly dynamic operating situation such as that found onboard a commercial or a passenger vehicle there will normally be a time lag between a change in engine operating conditions and the ammonia-containing gas supply being matched to those conditions due to the finite time taken to hydrolyse the reagent “on demand”. By placing the reservoir between the point of generation of ammonia-containing gas product and point of introduction to the exhaust, the requirement for ammonia containing-gas to be dosed into the exhaust can be substantially met in real time. In addition the separation of the reservoir from the reactor ensures that the operating conditions within the reactor are kept constant, i.e. the pressure within the reactor does not fluctuate as a result of dosing the ammonia-containing gas into the exhaust, therefore resulting in a substantially consistent gas product mixture exiting the reactor.

Preferably the reactor is solely heated by thermal heat transfer with the exhaust gas effecting a simple heating system utilising the “free” energy available in the exhaust. To that end, the reactor is preferably placed within the exhaust conduit such that there is direct contact between the hot exhaust gas and at least a part of the exterior surface of the reactor.

Alternatively the reactor may be heated at least in part by electric means. Preferably the reactor is initially heated by both heat exchange with the exhaust conduit and electric means and, once the exhaust reactor is at operating temperature and pressure, the electric heating means is turned off and the reactor is maintained at operating temperature and pressure by heat exchange with the exhaust gas only.

In another alternative preferred arrangement the reactor is preheated by electric heating means prior to the IC engine being started such that the reactor can produce ammonia-containing gas substantially immediately from the time the IC engine is started.

When the IC engine is turned off there is a residual volume of ammonia-containing gas within the reservoir and the ammonia-containing gas will continue to be produced for short time. As the reservoir cools the pressure of the ammonia-containing gas in the reservoir drops and the H2O condenses on the surface of the reservoir. As the temperature and pressure further drop some of the ammonia and carbon dioxide will combine to produce ammonium carbamate which then dissolves in the condensed water forming a solution of ammonium carbamate. The pressure and temperature within the reactor will also drop and the gas product contained within the reactor will undergo a similar process, the aqueous ammonium carbamate mixing with the aqueous urea solution within the reactor. As the ammonia-containing gas product within the reservoir cools and condenses, the pressure within the reactor will drop to substantially atmospheric pressure, preferably slightly below atmospheric pressure, as will the pressure within the reactor, thereby substantially eliminating the danger of ammonia escaping form the system while the engine is not running. This is particularly important for mobile IC engines, for example commercial or passenger vehicles where the vehicle may be stored within an enclosed space, for example a garage where any ammonia escaping from a pressurised system would be in a contained environment creating a build up of contained ammonia.

During cold start, i.e. when the IC engine is started from ambient temperature there is a time period before the reactor will produce ammonia-containing gas product for use in the NOx reduction process resulting in a time lag before ammonia is available for use in the NOx reduction process. This time lag is the result of a combination of several factors including: the time taken for the exhaust gas to reach its normal operating temperature (compounded by the fact that IC engines are normally started under no load or very light load conditions therefore taking longer to reach normal operating temperatures), the coefficient of thermal transfer between the exhaust gas and the liquid reagent within the reactor and the ratio of volume of liquid within the reactor to head space above the liquid. The result is that meeting requirements of emission standards is difficult during initial start up. As emissions standards are becoming ever increasingly stringent, this period during which the NOx is untreated will become unacceptable.

On cold start, heat is applied to the reservoir which then acts as a secondary reactor, evaporating the condensed water and thermally decomposing the ammonium carbamate dissolved therein to create ammonia and carbon dioxide gas thus reverting the contents of the reservoir back to their original state prior to the IC engine being shut down. When operational the reservoir is maintained at an elevated temperature to prevent the gasses therein condensing. Preferably the reservoir is maintained at a substantially constant temperature.

Preferably, heat is supplied to the reservoir by heat transfer from the hot exhaust gas both during normal operation and cold start-up. For that purpose the reservoir is preferably located such that a part of it protrudes through, or forms a part of, the exhaust conduit. As the heat and time required for the cold-start reaction in the reservoir is much less than that needed to drive the hydrolysis reaction in the reactor, the gas from the reservoir can be made available much sooner for introduction to the exhaust.

However, preferably, an electric heating element is provided for initially heating the contents of the reservoir which may be used in isolation or in combination with the heat supplied by the hot exhaust gasses.

In another preferred arrangement the electric heater is used on start up to supplement the heat transfer from the hot exhaust gas thus enabling a faster reaction of the aqueous ammonium carbamate. Preferably once the system is up to operational temperature the electric heating element is not used and the temperature of the reservoir is substantially maintained by the exhaust gas. In periods of low engine load when the exhaust gas is relatively cool the electric heater may be used to supplement the heating effect of the hot exhaust gas. When an electric heater is used during old start the heater is preferably turned on before the IC engine is started such that the aqueous ammonium carbamate within the reservoir is substantially thermally decomposed into ammonia-containing gas such that it is immediately available on start up of the engine.

In one preferred arrangement the reservoir is isolated from direct contact with the hot exhaust gas by an air gap, optionally containing an insulating material, and is provided with an electric heating element. Heat transfer across the air gap is sufficient to produce the majority of the heat needed to maintain the reservoir at an elevated temperature under operating conditions and the electric heater is used in start up and, if needed, to supplement the heating effect of the heat transfer with the exhaust gas.

Preferably the reservoir has a means of losing heat to the environment such that a balance of heat input to heat output can be achieved approximately at its operating temperature such that continued input of heat does not cause the reservoir to continue to rise.

In an alternative arrangement where it is preferable to control the temperature of the reservoir independently of the exhaust gas temperature, the reservoir is placed completely externally of the exhaust conduit and preferably is heated by means of its proximity to the exhaust conduit. Preferably an electric heater is provided for use on start up to thermally decompose the aqueous ammonium carbamate as described above. The electric heater, or an additional heater, may optionally also heat the entire outer surface of the reservoir to ensure no re-condensation of the gas occurs during start up. Preferably, once the system is up to operational temperature the electric heating element(s) is not used and heat input to the reservoir is provided by radiated and conducted heat from the exhaust gas. Where more accurate control of the temperature of the reservoir is required a variable cooling circuit is provided operable to remove excess heat and maintain the reservoir at a substantially constant temperature less than the temperature of the exhaust gas. Preferably this cooling circuit is either a part of the engine cooling circuit or has a heat exchanger to transfer heat to the engine cooling or lubrication circuit. Preferably the reservoir is maintained at a temperature between 125 and 250 degrees centigrade, more preferably between 180 and 225 degrees centigrade.

In one preferred arrangement the reservoir is substantially positioned externally from the exhaust conduit but has a section that extends into the exhaust conduit for heat transfer arranged such that any liquid within the reservoir drains toward the section extending into the exhaust conduit. On start up any liquid within this section is directly acted on by the hot exhaust gas converting it to ammonia-containing gas. Preferably the part of the reservoir extending into the exhaust conduit comprises a heat pipe.

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

FIG. 1 is a perspective view of the device in accordance with the invention;

FIG. 2 is a longitudinal cross section through the device of FIG. 1;

FIG. 3 is a transverse cross section through a first reaction vessel design of the device of FIG. 1;

FIG. 4 is a schematic representation of a control system including the device of FIG. 1;

FIG. 5 is a transverse cross section through a second reaction vessel design of the device of FIG. 1;

FIG. 6 is a longitudinal cross section through the device of FIG. 1 incorporated with an SCR catalyst;

FIG. 7 is a longitudinal cross section through the device of FIG. 1 incorporated within a housing, the housing containing the device, a particulate removal device and an SCR catalyst;

FIG. 8 is a perspective view of a second design of the device with variable heating;

FIG. 9 is a transverse cross section of the device of FIG. 8;

FIG. 10 is a perspective view of the device of FIG. 1 including a diesel particulate filter;

FIG. 11 is a perspective view of a third design of device according to the invention;

FIG. 12 is a perspective view of the rear of the device of FIG. 11;

FIG. 13 is a perspective view of the device shown in FIG. 11 with the outer cover removed;

FIG. 14 is a cross section through the reservoir and reaction vessel of FIG. 11;

FIG. 15 is a perspective view of a reservoir upper manifold according to the present invention;

FIG. 16 is an exploded assembly drawing of the reservoir upper manifold of FIG. 15 and its associated components;

FIG. 17 is a perspective view of the assembled reservoir upper manifold of FIG. 15 and its associated components;

FIG. 18 is a schematic representation of a control system according to the present invention.

FIG. 19 is a cross section of a reactor according to the invention;

FIG. 20 is a cross section of a reactor of the invention with heat exchange fins;

FIG. 21 is an cross section of an alternative reactor of the invention with a supplementary heater; and

FIG. 22 is a cross section of another reactor of the invention.

FIG. 23 is an embodiment of a system according to the invention;

FIG. 24 is an alternative embodiment of the invention incorporating a heat pipe;

FIG. 25 is an alternative embodiment of the invention incorporating electric heating;

FIG. 26 is an alternative embodiment of the invention incorporating electric heating and a heat pipe;

FIG. 27 is an alternative embodiment of the invention incorporating a cooling system;

FIG. 28 is a vertical cross section showing a reservoir adjacent the exhaust conduit; and

FIG. 29 is a horizontal cross section through the arrangement shown in FIG. 28.

Referring to FIGS. 1 to 4 a device 1 is shown capable of being placed in-line in the exhaust conduit of an IC engine, for example that found on a diesel vehicle, upstream of an SCR catalyst. The device 1 produces a gaseous product which is added to the exhaust gas in a controlled manner to pass therewith through the SCR catalyst to reduce the NOx content of the exhaust gas. The device 1 has an inlet 2 and an outlet 3 for the exhaust gas flowing therethrough and comprises an outer body 4, which forms a pressure barrier, and passing through the outer body 4 is an inner body 5 which comprises a flowpath longitudinally therethrough from the inlet 2 to the outlet 3. The device 1 is split into two sections, the first section is for hydrolysing an aqueous solution of urea at elevated temperature and pressure so that it decomposes to form a gaseous hydrolysis product containing ammonia gas. In the first section the outer 4 and inner 5 bodies form two chambers 6, 7 therebetween, substantially above the inner body and substantially below the inner body respectively. The lower chamber 7 has an inlet 8 for receiving a supply of aqueous urea solution delivered by a pump 32 (shown in FIG. 4 only) The upper chamber 6 has an outlet 9 for gaseous hydrolysis product. The lower 7 and upper 6 chambers are connected by a plurality of tubular elements 10 which pass through the inner body 5 and which form fluid flowpaths between the lower 7 and upper 6 chambers. The upper 6 and lower 7 chambers and the tubular elements 10 together form and enclosed reaction vessel in which the hydrolysis reaction occurs.

In use the aqueous solution of urea is fed into the reaction vessel via the inlet 8 in the lower chamber 7 by the pump 32. The level of aqueous urea in the reaction vessel is measured by a level sensor 11. Although the level sensor 11 is shown to be only in the upper chamber 6, for greater control over the liquid level within the reaction vessel it may extend into the lower chamber 7 through one of the passageways or alternatively a second level sensor 12 may be placed in the lower chamber (FIGS. 3 and 4).

The exhaust gas from the engine, which has a temperature up to around 550 degrees centigrade (dependent on engine load), passes over the tubes 10 and the upper and lower surfaces of the lower 7 and upper 6 chambers respectively, raising the temperature of the liquid contained therein by heat exchange. As the temperature rises the hydrolysis reaction starts to occur (at approximately 60 degrees centigrade) and the gaseous hydrolysis product starts to collect in the headspace above the liquid level in the upper chamber 6. As the temperature rises further the reaction accelerates and a head of pressure builds up in the head space, pressurising the reaction vessel and allowing the temperature of the aqueous urea solution to rise above the temperature at which it would otherwise boil. The reaction vessel outlet 9 in the upper chamber 6 includes a valve 13 which opens passively at a predetermined set pressure, preferably in the region of 15 to 20 bar, ideally 17 bar. Thus the pressure in the reaction vessel is elevated above atmospheric pressure but is maintained below a certain value (in this case 17 bar), which gives a good reaction rate without the need to contain excessive pressures.

Alternatively the valve 13 may be active, i.e. it may operate in response to a pressure sensor 14 within the header section of the reservoir 15 (as will be described in further detail shortly).

The valve 13 releases the excess pressure from the reaction vessel into the second section of the device which comprises a reservoir 15 which surrounds the inner body 5. The passage of hot exhaust gas through the inner body 5 heats the reservoir and keeps the ammonia containing hydrolysis product in its gaseous state. The reaction vessel has an outlet 16 and a dosing valve 17 associated therewith. The device is further provided with a pressure sensor 18 to sense the pressure in the reservoir 15.

Referring now to FIG. 4, the pump 32 delivers aqueous urea solution from a holding tank 35 into the lower chamber 7 via the inlet 8. The pump 32 is controlled by a controller 33 which is also connected electrically to the level sensors 11, 12; reaction vessel outlet valve 13; dosing valve 17; reaction vessel pressure sensors 14, 18; and an engine management system 34 as indicated by the dashed lines in FIG. 4. The engine management system 34 logs and controls the performance characteristics of the IC engine in known manner.

The device 1 is operable as follows. The controller 33 receives a supply of data from the engine management system 34, the data including, for example, engine speed, torque, ignition timing and throttle position. This data is used to calculate the NOx level in the engine exhaust according to known techniques, such as executing algorithms on the engine management data or referencing look up tables. Given the NOx level in the exhaust, the controller 33 then calculates the volume of ammonia gas required to react with the prevailing level of NOx established in the exhaust.

Accordingly, in times of increased engine demand, for example high engine speed and/or torque, the controller 33 controls the pump 32 to increase the rate of delivery of aqueous solution into the reaction vessel. This results in an increase in the level of aqueous solution within the reaction vessel. Thus, a greater surface area of the inside of the reaction vessel becomes wetted by the aqueous solution. The resulting increase in the heated wetted area in the reactor vessel, ie the total surface area of aqueous solution directly exposed to heat from the exhaust, causes increased heat transfer from the exhaust gas to the aqueous solution. This in turn generates an increased rate of production of gaseous hydrolysis product.

In this manner, the controller 33 delivers an increased volume of aqueous solution into the reaction vessel in response to an increase in the level of NOx in the exhaust gas.

Conversely, in times of decreased engine demand, the controller 33 controls the pump 32 to decrease the rate of delivery of aqueous solution into the reaction vessel. This results in a reduced rate of production of gaseous hydrolysis product.

In times of increased load, the NOx levels in the exhaust increase. This increases the demand for ammonia gas, in response to which the controller 33 controls the pump 32 to increase the rate of delivery of aqueous solution to the reaction vessel. However, increased engine load also delivers an increase in exhaust gas temperature and flow rate. In particular, a high engine speed will lead to a high exhaust gas flow rate and high torque operation will increase the exhaust gas temperature. Consequently, in times of high load, an increase in the heated, wetted area of aqueous solution in the reaction vessel is observed concurrently with an increased exhaust gas temperature and/or flow rate. Accordingly not only is the rate of production of gaseous hydrolysis product increased by virtue of the increased heated wetted area, but also by the increased rate of heat transfer per unit area delivered by the increase in exhaust gas temperature and/or flow rate. In this way the increased volume of aqueous solution in the reactor vessel is balanced by the increased rate of gaseous hydrolysis product. This leads to a reduction in the volume of aqueous solution and thereby a stabilisation in the level of aqueous product within the reaction vessel.

However, there exist engine operating conditions where the increased rate of delivery of aqueous solution is not matched by an increased exhaust gas temperature and/or pressure. For example, where an engine is under high load driving the vehicle up a steep incline, NOx levels in the exhaust will increase. However, upon reaching the end of the incline the engine may operate at tick over or very low load, for example in negotiating a decline. The exhaust temperature will consequently decrease leading to a mismatch in exhaust conditions and rate of aqueous solution delivery into the reaction vessel. Under such circumstances, the controller 33 controls the pump 32 to pump aqueous solution from the reaction vessel into a holding reservoir (not shown for clarity). The solution is held in the holding vessel until such time as the demand for ammonia increases at which point the controller 33 controls the pump 32 to pump the solution from the holding vessel into the reaction vessel. The holding vessel may be heated by an auxiliary heating means in order to prevent condensation of the gaseous hydrolysis product. The holding vessel is evacuated before the pump pumps aqueous solution from the tank in order to retrieve the heat retained in the solution in the holding vessel by virtue of its earlier heating in the reaction vessel.

In the event that engine load rapidly increases following a period of low load, for example beginning a steep incline having previously completed a low load descent, the second level sensor 11 is provided to ensure that the level of aqueous solution in the reaction vessel does not become dangerously low.

As discussed above the gaseous hydrolysis product is released into the reservoir 15 when the headspace pressure in the reaction vessel rises above 17 bar. The dosing of the gas from the reservoir is controlled as follows. The valve 17 is operable in response to a signal from the controller 33 to open and allow some of the gas within the reservoir 15 to enter the exhaust gas flowing through the inner body 5 to flow therewith through an SCR catalyst (not shown) positioned downstream of the device. The controller 33 monitors the reservoir pressure via pressure sensor 18 and calculates the required opening of the valve (for the given pressure) to introduce the required volume of hydrolysis product (or component thereof) dictated by the engine exhaust conditions. Optionally the reservoir temperature is also monitored as will be discussed in further detail shortly.

Accordingly, the reservoir 15 acts as a buffer between the reaction vessel and the IC engine exhaust. The reservoir depletes and replenishes so as to allow for the lag in the control of the rate of production of gaseous hydrolysis product in response to the prevailing exhaust conditions.

Referring to FIG. 5 an alternative arrangement of the reaction vessel section is shown in which the inner body 5 sits within the outer body 4 and is arranged such that there is an upper chamber 6 and a lower chamber 7 substantially above and below the inner body 5, and a passageway 19 formed between the walls of the inner 5 and outer 4 chambers making them in fluid communication. The reaction vessel acts in substantially the same manner as described in reference to FIGS. 1 to 4 and the fluid is heated as it passes from the lower to the upper chamber by conduction through the walls of the inner body 5. Optionally a combination of the two designs may be utilised whereby the fluid passes from the lower chamber 7 to the upper chamber 8 through the tubes 10 (see FIG. 3) and through the passageways 19.

Referring to FIG. 6 the device as described in relation to FIGS. 1 to 4 is shown incorporated with an SCR Catalyst 20. The SCR catalyst 20 is contained within an open ended housing 21 which fits around the outer body 4 of the device. It may be attached by any means known in the art, for example a simple screw thread or bayonet type fitting. The outlet 3 of the device feeds directly into the SCR catalyst, the exhaust gasses passing therethrough and exiting the extended form of the device via SCR outlet 22. In an alternative arrangement (not shown) the outer wall 4 of the device extends beyond the reservoir 15 and in its extended region forms a housing for the SCR Catalyst.

Referring to FIG. 7 the device as described in relation to FIGS. 1 to 4 is shown incorporated within a common housing with the SCR catalyst and a particulate removal device. The common housing 23 has an inlet 24 and an outlet 25 and located therebetween a particulate removal device 26, the device of the invention 27 and the SCR catalyst 28. The particulate removal device may be any such device known in the art, for example a diesel particulate filter.

Referring to FIGS. 8 and 9 the device as described in FIGS. 1 to 4 is shown which additionally comprises a heat exchange bypass. The inner body 5 is split in two longitudinally by means of a dividing plate 29 to create a heat exchange section containing the tubes 10 and a bypass section 30, and a diverter flap 31 is placed upstream of dividing plate 29. The diverter flap 30 is movable by controller 100 (see FIG. 4) to selectively allow a varying amount of the exhaust gas to flow over the tubes 10 thereby controlling the heat input into the reaction vessel. This enables the speed at which the hydrolysis occurs, and therefore the speed at which the ammonia-containing gas is produced, to be controlled by way of the position of the diverter plate in addition to, or instead of, the rate of delivery of aqueous solution to the reaction vessel.

Referring to FIG. 10 a gas treatment apparatus is shown particularly useful for treating the exhaust gas of a commercial vehicle engine. An outer housing comprising endplates 41, 42 is split into three sections by plates 43 and 44 forming two end sections 45, 46 and a central section 47. An exhaust inlet 48 passes through plate 41 and section 45 and opens into an oxidation catalyst 49 situated in central section 47 and extending between plates 44 and 45. The outlet of the oxidation catalyst passes through plate 44 opening into end section 46, the exhaust gas expanding as it does so. Also located in the central section 47 between plates 43 and 44 are two SCR catalysts 50, 51 their inlets being in end section 46 and their outlets discharging into end section 45 such that the exhaust gas entering end section 46 then passes through the SCR catalysts into end section 45. Situated in plate 44 and leading from end section 45 is an inlet into a closed end baffle drum 52 which has a number of outlets 53 in the side of the drum opening into central section 47. Also located in central section 47 is a second closed end baffle drum 54 which has a plurality of inlets 55 in the side of the drum 54 and an outlet 56 leading from the drum 54 and passing through end section 46 and out of the apparatus for discharge to atmosphere. Located within end section 46 is a hydrolysis reaction vessel 57 as described in relation to FIG. 10 which has an inlet 58 for a pressurised supply of urea and an outlet 59 for ammonia containing gas. The reaction vessel 57 is heated by heat exchange with the exhaust gas circulating within the end section 46 as it passes from the outlets of the oxidation catalyst 49 to the inlets of the SCR catalysts 50, 51. Also located within end section 46, but separated from the gas flow therein by a baffle plate 60, is a valve unit 61 containing a pressure control valve for controlling the pressure within the reaction vessel and a dosing valve for controlling the flow of ammonia containing gas to two injection points 62. Each injection point 62 injects ammonia containing gas into the exhaust gas stream prior to it passing through the two SCR catalysts 50, 51 wherein the ammonia reacts with the NOx in the exhaust as on the surface of the SCR catalysts 50, 51 reducing the NOx content of the exhaust to a level acceptable for discharge to atmosphere via outlet 56. The pressure control valve of the valve unit 61 has an outlet leading a gas reservoir 63 which provides a buffer of ammonia containing gas ready to be dosed into the exhaust gas via the dosing control valve of the valve unit 61 and the injection points 62. The gas reservoir 63 is situated in the central section 47 of the apparatus in which the exhaust gas passing from baffle drum 52 to baffle drum 54 is circulating and is thereby heated by heat exchange with the hot exhaust gas.

Referring to FIGS. 11 to 14, an alternative embodiment of gas treatment device 64 is shown which operates in a substantially similar manner to the embodiment described previously. The exhaust gas of an IC engine flows through the device 64 from an inlet 65 to an outlet 66. The exhaust enters the inlet 65 containing NOx and leaves the outlet 66 substantially free on NOx. The device 64 may be attached to a commercial or passenger vehicle and connected in line in the existing vehicle exhaust system.

When the exhaust gas passes through the inlet 65 it passes a NOx sensor 112 before entering a first cylindrical tube 67 containing a hydrolysis reaction vessel 68 (see FIG. 14). The hot exhaust gasses exit the tube 67 through an opening therein and enter an enclosed cavity 69. As the hot exhaust gasses pass over the reaction vessel 68 it absorbs heat from the gasses and becomes elevated in temperature. The reaction vessel 68 has an inlet 70 at its lower end through which an aqueous solution of urea is supplied. The aqueous solution is delivered from a holding tank 110 by a pump 11, both shown schematically in FIG. 14 only.

As the reaction vessel 68 becomes heated the aqueous solution of urea starts to hydrolyse and hydrolysis gasses form in the head space above the level of the urea. The reaction vessel 68 is provided with a pressure relief valve 71 in its upper end which allows the hydrolysis gas to pass from the reaction vessel 68 to a reservoir 72 if the pressure in the reaction vessel 68 exceeds 17 bar.

The tube 67 has a closed upper end (with an opening therein through which the pressure relief valve 71 projects). The reaction vessel 68 is attached to the device by its upper end.

The enclosed cavity 69 has a passageway in one of its walls (not shown) allowing the exhaust gas to exit the cavity 69 and pass through an oxidation catalyst 74 where a percentage of the NO in the exhaust gas is oxidised into NO2. The exhaust gas then exits the oxidation catalyst and enters a truncated conical section 75 which reduces in diameter.

A feed tube 76 leads from the reservoir into the conical section 75 and the hydrolysis gas is dosed through the feed tube 76 into the exhaust gas at the open end of the cone. As the flow reduces mixing is induced between the exhaust gas and the hydrolysis gas. After the conical section 75 the exhaust gasses pass around a 90° bend 82 and flows into a cylindrical vortex mixer 83. The exhaust gasses enter the vortex mixer 83 tangentially and exit along its central axis into an SCR catalyst 84 wherein the hydrolysis gas mixes with the NOx converting it substantially to nitrogen and water. The exhaust gas exits the SCR catalyst 84 and expands into the interior of the device enclosed by cover 85. The treated exhaust gasses then exit the device via the outlet 66 which passes through the enclosed cavity 69. Arranged in proximity to the exit 66 are a NOx sensor 113 and an ammonia sensor 114.

The flow of hydrolysis gas from the reservoir 72 into the conical section 75 via the tube 76 is controlled by a dosing valve 77 (as will be described in further detail shortly) attached to an upper manifold 78 of the reservoir 72. The reservoir 72 is located in a tube 79 and positioned such that there is an air gap between the reservoir 72 and the tube 79. Part of the outer surface of the tube 79 forms a wall of the enclosed cavity 69 and as such is in direct contact with the hot exhaust gasses. In use the reservoir becomes heated by heat transfer from the exhaust gas through the tube 79 and across the air gap. The reservoir 72 is elongate in shape and similar to the reaction vessel 68 will expand in length. The reservoir 72 is attached at its upper end and free to expand at its lower end. A sliding seal 80 is provided to retain the lower end of the reservoir 72. A heater 81 is situated at the lower end of the reservoir to allow for additional heating to supplement the heat from the exhaust gasses. The pressure release valve 71 and the dosing valve 77 are maintained in a cooler area and are separated from the warmer area by a manifold plate 86, which may either be of a thermally shielding material or may include a thermal shield. The pressure relief valve 71 and the dosing valve 77 have covers 87, 88 sealed thereover maintaining them in a clean and dry environment.

Referring to FIGS. 15 to 17, a reservoir upper manifold 89 for use in a gas reservoir containing ammonia and carbon dioxide for use in an SCR process is shown. It can be used, for example, as the reservoir in the system described with reference to FIGS. 12 to 14. The upper manifold forms the upper end of the reservoir 72 (see FIG. 14) and is welded to the reservoir body 90. The upper manifold 89 has a plurality of passageways in it adapted to accommodate associated components. Passageway 91 is the gas inlet to the reservoir and is fed with a supplied of ammonia containing gas via tube 92. Passageway 93 is the inlet for a dosing valve 94. The gas from the reservoir enters the valve 94 through port 93 and exits through pipe 95 which passes back into the reservoir and passes through the reservoir body 90 via a bulkhead fitting (not shown). The dosing valve 94 is operable to control the flow of ammonia containing gas from the reservoir into the exhaust gas flow of an IC engine. Passageway 96 accommodates a safety valve 97 which opens above a preset pressure and vents excess gas pressure out of the passageway 96, past the safety valve 97 and sideways out through passageway 98 from where it flows through a tube 99 into the exhaust gas flow.

Passageway 100 accommodates a reservoir pressure sensor 101. Passageway 102 accommodates a fitting 104 to accept a reservoir temperature sensor 103 which detects the temperature of the gas within the reservoir. The same sensor (or a second sensor) can also monitor the temperature of the upper manifold itself.

The upper manifold 89 has a plurality of ports 105 in its sides to accommodate heating elements 106. If the ammonia and carbon dioxide gasses cool down in the presence of each other then they can form solid salts, e.g. ammonium carbamate, which can block the valves resulting in not only the inability to dose the gas into the exhaust gas but also a possibly dangerous increase in pressure within the reservoir. Alternatively a build up of solids may occur on the sensors 101, 103 causing them to malfunction, again possibly leading to a dangerous increase in pressure within the reservoir. The heaters 106 are operated to maintain the upper manifold 89 at a raised temperature to prevent solidification of any salts in any of the passageways therethrough. The heaters maintain the upper manifold 89 above 130° C., ideally at a substantially constant temperature of 220° C. Between the upper manifold and the components is a thermal barrier 107 to protect the components from heat radiated directly from the upper manifold. The upper manifold has a number of threaded holes 108 therein for attaching a cover 109 to it. The thermal barrier also acts as a gasket and seals the cover 109 over to the manifold, thus the reservoir can be washed, for example with a powerful spray of water, without water ingress into the associated components 92, 94, 101, 104 and any associated electronics. The cover 109 is made of aluminium has a plurality of cooling fins to assist in rapid heat loss from this section maintaining the components within their working temperature range.

In use the device 64 is operable as follows. A controller (not shown in FIGS. 11 to 17 for clarity) is provided which is electrically connected to the dosing valve 77, reservoir pressure sensor 101, reservoir temperature sensor 103, heaters 81, 106, NOx sensors 112, 113 and ammonia sensor 114.

The device 64 may optionally be provided with an analogue level sensor for measuring the exact level of aqueous solution in the reaction vessel 68, the level sensor also being connected electrically to the controller. The reaction vessel may also have optional temperature and pressure sensors for communicating to the controller the reaction vessel conditions in order to control an active pressure release valve in place of the passive unit described above.

In distinction to the first embodiment, the controller of the second embodiment receives a signal from the NOx sensor 112 rather than calculating the exhaust NOx levels by derivation from engine load data.

The volume of ammonia gas required to react with the NOx level detected in the exhaust gas is calculated and the pump 111 controlled accordingly to increase or decrease the rate of flow of aqueous solution into the reactor. The level of aqueous solution in the reaction vessel rises or lowers accordingly, thereby altering the rate of heat transfer between the aqueous solution and the exhaust gas as described previously.

The controller also monitors downstream NOx levels in the exhaust by way of NOx sensor 113 in order to ensure that NOx consumption is maximised. Like wise the controller monitors ammonia levels in the exhaust gas exiting the device 64 by way of an ammonia sensor 114 in order to minimise the risk of ammonia slip.

The controller also monitors the reservoir temperature and pressure by way of temperature sensor 103 and pressure sensor 101. When the reservoir temperature and/or pressure fall below predetermined values, the reservoir heater 81 is operated to raise the reservoir temperature in order to prevent the gaseous hydrolysis product condensing.

This is particularly advantageous at cold start-up of the IC engine as the residual condensate in the reservoir is heated to provide ammonia for delivery into the exhaust before the exhaust gas has raised the temperature of the reactor sufficiently to cause hydrolysis of the aqueous solution.

In a further embodiment the reaction vessel includes a conical catalyst substrate. The varying cross-sectional area of the substrate with height further emphasises the effect of altering the rate of hydrolysis by changing the level of aqueous solution in the reaction vessel. Alternatively, the substrate may have a form other than conical, for example cylindrical in order to deliver a particular change in reaction rate per unit increase in the liquid height.

Referring now to FIG. 18, a control methodology 200 is shown for controlling the devices 1, 64 described above. The methodology is described hereafter with reference to the gas treatment device 64 but is equally applicable to the gas treatment device 1. A demand generator 202 receives a catalyst condition signal 204 from the exhaust catalyst, a NOx sensor signal 206 from the NOx sensor 112, an engine condition signal 208, from the engine management system (not shown for clarity) and a demand signal 210.

The demand generator calculates a required ammonia output rate and delivers an ammonia output signal 212. The ammonia output signal is delivered to a dosing valve control 214 and a pump control 216. The dosing valve control 214 outputs a dosing valve signal 218 to command the opening and closing of the dosing valve 77. In order to calculate the required dosing valve signal 218, the dosing valve control 214 receives a reservoir pressure signal 220 and a reservoir temperature signal 222 from the reservoir pressure sensor 101 and reservoir temperature sensor 103. The reservoir pressure signal 220 is also delivered to the pump control 216 in addition to an integral and differential of the pressure signal. The pump control 216 outputs a pump signal 224 to control the pump 111. In order to generate the pump signal 224, the pump control 216 may optionally also receive a reactor level signal 226 from a reactor level sensor (not shown for clarity). The reservoir temperature signal 222 is also delivered to a reservoir heater control 228 which generates a reservoir heater signal to control the reservoir heater 81.

Optionally, a reactor pressure sensor delivers a reactor pressure signal 232 to a reactor pressure control 234 which outputs a pressure relief valve signal 236 to an active pressure relief valve for venting gaseous hydrolysis product from the reactor into the reservoir. This optional control methodology is only required when an active pressure relief valve is used in place of a passive valve.

It will be appreciated that various components and control methods are described in respect of one or other of the embodiments. Nonetheless any of the measurement and control features described above are interchangeable between embodiments.

Referring to FIG. 19 a thermo-hydrolysis reactor 1901 is shown, capable of being placed in-line in the exhaust conduit of an IC engine, for example that found on a diesel vehicle, upstream of a selective catalytic reduction (SCR) catalyst. The thermo-hydrolysis reactor may for example be used in the SCR system described with reference to FIGS. 11 to 18. The thermo-hydrolysis reactor 1901 produces an ammonia-containing gaseous product which is added to the exhaust gas in a controlled manner to pass therewith through an SCR catalyst to reduce the NOx content of the exhaust gas. The reactor 1901 comprises an elongate body 1902 with enlarged upper 1903 and lower 1904 sections. The reactor 1901 is provided with an inlet 1905 for the supply of aqueous urea solution and an outlet 1906 for the removal of the ammonia-containing gas. The release of the ammonia-containing gas via the outlet 1906 is controlled by a pressure control valve in the outlet line (not shown). Entering the reactor 1901 from the top is a level sensor 1907, the output of which is used to control a pump (not shown) supplying inlet 1905 to maintain the urea liquid level 1908 between lower 1909 and upper 1910 liquid level measurement points. Also entering the top of the reactor are a pressure 1911 and temperature 1912 sensor. In use, the reactor 1901 is heated by heat transfer with hot exhaust gas. The aqueous solution of urea becomes heated and starts to decompose forming hydrolysis gasses comprising ammonia, carbon dioxide and steam. As the hydrolysis gases collect in the upper section 1903 of the reactor they are prevented from leaving by the pressure control valve in the outlet line and thus the pressure in the reactor increases to the set pressure of the control valve. The increase in pressure allows for a further increase in temperature, the increased temperature and pressure resulting in a shortened-hydrolysis time. Eventually the pressure in the reactor 1901 exceeds the set pressure of the pressure control valve whereby “excess” ammonia-containing gas issues from the outlet 1906 via the control valve for use in the SCR process. A reactor of this design is particularly appropriate for use in a mobile application, for example on board commercial vehicle as, due to its tall, thin geometry, the liquid level in the reactor will remain substantially unaffected by such factors as the vehicle being on an incline, centrifugal force of the vehicle following a radial path or the reagent sloshing due to uneven motion of the vehicle. All the sensors 1907, 1911, 1912 comprise a single sub assembly which is attached to the reactor at one end, thereby giving a single access point enabling simple replacement should any of the sensors fail.

Referring to FIG. 20 a reactor 2013 for use in an exhaust gas treatment apparatus, for example for use in the SCR system described with reference to FIGS. 11 to 18, is shown comprising an elongate body 2014 with a bulbous head section 2015 and a conical lower section 2016. During use the reactor 2013 is heated by heat transfer from the hot exhaust gasses of an engine (not shown) to hydrolyse the aqueous urea therein. The reactor 2013 has a level sensor 2017 entering at its top and extending downwards therefrom into the aqueous urea within the reactor 2013. The liquid level sensor 2017 is situated on the central axis of the reactor 2013. By placing the liquid level sensor 2017 on the central axis as the liquid moves slightly from side to side the level at the central axis should not change significantly. Preferably the liquid level sensor 2017 measures the liquid level 2018 on a continuous scale. The reactor 2013 has an inlet 2019 for the supply of pressurised aqueous urea and an outlet 2020 which leads to a pressure control valve (not shown). The reactor 2013 has a baffle 2021 situated in its head section 2015 above the liquid level and below the outlet 2020. In the event of any splashing of the reagent within the reactor 2013, for example due to motion of the vehicle the baffle 2021 prevents splashes of liquid from exiting from the outlet 2020. The liquid level 2018 may be controlled by controlling the volume of aqueous urea pumped into the reactor via inlet 2019 dependant on the sensed liquid level. The heat transfer from the hot exhaust gas is dependent on the wetted surface area of the reactor 2013. The geometry of the conical section 2016 allows for a specific non linear relationship of heat transfer to liquid level to be achieved. To assist heat transfer from the exhaust gas to the reactor 2013 a number of heat exchange fins 2022 are shown on the external surface of the reactor 2013. The surface area of the fins 2022 changes in relation to the height of the reactor 2013 and thus the heat input to the aqueous urea can be controlled by varying the liquid level 2018. For additional heat transfer to the liquid heat exchange fins 2023 fins are shown inside the reactor 2013 to increase the contact surface area between the reactor body 2014 and the aqueous urea within the reactor 2013. The reactor 2013 is also provided with temperature 2024 and pressure 2025 sensors to monitor the temperature and pressure of the gas within the reactor 2013.

Referring to FIG. 21 a reactor 2126 for use in an exhaust gas treatment apparatus, for example for use in the SCR system described with reference to FIGS. 11 to 18, is shown comprising an elongate body 2127 with a bulbous head section 2128 and a conical lower section 2129. During use the reactor 2126 is heated by heat transfer from the hot exhaust gasses of an engine (not shown) to hydrolyse the aqueous solution of urea therein. The reactor has a level sensor 2130 entering at its top and extending downwards therefrom into the aqueous solution of urea within the reactor. The reactor 2126 has an inlet 2131 in the lower section 2129 and an outlet 2132 in the upper section 2128, said inlet 2131 and outlet 2132 comprising bulkhead fittings 2133, 2134 for attaching the reactor to a bulkhead 2135 which may for example be the exhaust conduit. The lower section 2129 of the reactor 2126 contains a supplementary heating element 2136 which is situated below the liquid level 2137, said liquid level 2137 being maintained within a range detected by the liquid level sensor 2130. The supplementary heater 2136 is used during start up to enhance the heating capacity of the hot exhaust gas to decrease the time taken for the reactor 2126 to reach its operating conditions of temperature and pressure measured by temperature and pressure sensors 2138, 2139. Outlet 2132 leads to a pressure controller which, in use, maintains an elevated pressure within the reservoir 2126. A hydrolysis catalyst 2140, for example tungsten vanadium, is provided within the reactor below the level 2137 of the urea solution. Alternatively (not shown) the catalyst may extend from below the liquid level to above the liquid level whereby variation of the liquid level exposes the aqueous urea to a greater or a lesser surface area of the catalyst.

Referring to FIG. 22 a reactor 2241 for use in an exhaust gas treatment apparatus, for example for use in the SCR system described with reference to FIGS. 11 to 18, is shown having an enlarged upper section 2242 and lower section 2243. The reactor 2241 contains an aqueous solution of urea up to a level 2244 detected by level sensor 2245 which extends upwards from the bottom of the reactor 2241. The reactor has an aqueous urea inlet 2246 in its lower section for supplying the reactor with a supply of aqueous urea which in use, becomes heated by means of heat exchange with hot exhaust gas through the walls of the reactor 2241. The reactor 2241 is attached at its upper end to the exhaust conduit 2247 and a pressure regulating valve 2248, situated outside the conduit 2247 is in communication with the interior of the reactor 2241 through the conduit 2247. The valve 2248 has an outlet 2249 through which the ammonia containing hydrolysis gas passes for use in SCR of NOx in exhaust gasses. The reactor 2241 has a slosh baffle 2250 to help prevent splashes of the aqueous solution from entering the valve via the reactor outlet 2251.

Referring to FIG. 23 a system of the invention is shown which comprises a reactor 2301 fed through an inlet 2302 with a supply of pressurised aqueous urea solution. The urea is approximately 32% urea by volume, ideally AdBlue available from GreenChem Holdings B.V. The rate of supply is regulated by a pump 2303 which is controlled in response to a liquid level indicator (not shown) situated within the reactor 1301 to maintain the reactor 2301 in a partially full condition. The reactor 2301 is situated within the exhaust conduit 2304 of an IC engine such that the flow of hot exhaust gas passes over the reactor 2301 heating the urea therein. As the temperature rises the urea starts to break down by hydrolysis producing ammonia-containing gas, thus raising the pressure in the head space in the reactor 2301 above the liquid level. A pressure control valve 2305 is situated towards the top of the reactor 2301 and once the pressure within the reactor 2301 reaches a set pressure, preferably about twenty bar, any excess gas produced passes through the pressure control valve 2305 thereby maintaining the pressure within the reactor 2301 substantially constant. The temperature is also maintained substantially constant giving substantially constant operating conditions for the hydrolysis process. After passing through the pressure control valve the ammonia-containing gas enters a reservoir 2306 which is situated partially within, and partially outside of, the exhaust conduit 2304. The reservoir 2306 has one section within the exhaust conduit 2304 which is heated by the exhaust gas passing over it which prevents the ammonia-containing gas from condensing or crystallising during normal operation of the engine, and has a second section external to the flow of the exhaust gasses which allows for heat loss from the reservoir 2306 such its temperature is lower than that of the reactor 2301. The ammonia-containing gas is however still maintained at an elevated temperature and at a pressure above those of the exhaust gasses. A valve 2307 is controlled to release gas from the reservoir 2306 into the exhaust gas flowing through the conduit 2304 via nozzle 2308. The ammonia-containing gas then passes with the exhaust gasses through an SCR catalyst (not shown) where it reacts with the NOx in the exhaust gas on the surface of the SCR catalyst resulting in reduced NOx emissions. When the IC engine is shut down and therefore the exhaust gasses stop flowing, it loses heat to its environment and the system will gradually cool down. As it does so the hydrolysis process will stop and the ammonia-containing gas within the reservoir 2306 will start to condense, eventually forming a pool of aqueous solution of ammonium carbamate (which may also contain a small amount of ammonia and carbon dioxide) in the base of the reservoir. As the condensation occurs the pressure within the reservoir 2306 will drop eventually reaching a pressure which is approximately atmospheric pressure or slightly below. When the IC engine is restarted the hot exhaust gas will start to flow over the reactor 2301 and the reservoir 106 thereby heating them. However the reactor 2301 will take a finite amount of time to reach its operating pressure and temperature before it can produce more ammonia-containing gas. In the interim, the pool of aqueous ammonium carbamate in reservoir 2306 will be thermally decomposed and revert back into its previous gaseous form and be available for use in a shorter time than the new ammonia-containing gas produced by the reactor 2301. This allows for ammonia containing gas to be applied to the hot exhaust gas for SCR sooner after start-up of the engine, reducing the NOx emissions in the initial period prior to the reactor producing ammonia-containing gas.

Referring to FIG. 24 another embodiment of the invention is shown in which a reactor 2401 is fed in the same way as in FIG. 23 by conduit 2402 and pump 2403. In this embodiment the reactor 2401 ends at back pressure valve 2405 located adjacent to, but externally of, the exhaust conduit 2404. The majority of the reservoir 2406 is situated externally of the exhaust conduit 2404 and has a valve 2407 for controlling the flow of the ammonia-containing gas from the reservoir 2406 into the exhaust conduit 2404 via a nozzle 2408 to mix with the hot exhaust gas passing therein. The reservoir includes a small heat pipe 2409 which extends through the exhaust conduit 2404 and is in direct contact with the exhaust gas. The general operation of the system is as described in reference to FIG. 1. The reservoir 2401 is shaped such that when the IC engine is shut down and the cooling of the system condenses the ammonia-containing gas, the condensate will collect in the heat pipe 2409. On start up, therefore, the condensate is all in contact with the hot exhaust gas. In addition, a supplementary electric heater 2410 is provided such that additional heat can be put into the condensate to accelerate its re-conversion back to gaseous form, thereby reducing the NOx emissions by further reducing the time lag between start up of the IC engine and having ammonia-containing gas ready for addition to the system for use in SCR. During normal running of the system, once it is up to temperature the electric heater 2410 is turned off, the reservoir being maintained at an elevated temperature by heat transfer conduct between the hot exhaust gasses and the part of the reservoir 2406 within the exhaust conduit 2404.

Referring to FIG. 25 another embodiment of the system is shown in which the reservoir 2501, conduit 2502, pump 2503, and pressure control valve 2505 operate in the same manner as their corresponding parts in FIG. 24. The reservoir 2506 is situated completely externally form the exhaust conduit 2504 and as such is not directly heated by the hot exhaust gasses. The reservoir is joined to the exhaust conduit via valve 2507 and nozzle 2508 to allow the ammonia-containing gas as within the reservoir to be applied to the exhaust gas prior to them flowing together through an SCR catalyst (not shown). The reservoir 2506 is heated by an electric heater 2511 which raises the temperature of the reservoir 2506 to, and maintains it at, at a temperature above which the gasses therein will start to condense. The heater is controlled to maintain the reservoir at a substantially constant temperature in the region of 200 degrees centigrade.

Referring to FIG. 26 a system is shown which is a combination of FIGS. 24 and 25, and the components work in the same manner. The reservoir 2606 is provided with a small heat pipe 2609 situated at the bottom of the reservoir 2606 but external to the exhaust conduit 2604. When the IC engine is turned of and the ammonia-containing gas within the reservoir 2606 condenses, the resulting solution will collect in the heat pipe 2609. The heat pipe is provided with an electric heater 2610 which is of a high power to quickly reconvert the solution to ammonia-containing gas ready for dosing. The reservoir 2606 is also provided with a general heater 2611, which may be of lower power for the general heating of the reservoir 2606.

Referring to FIG. 27 a system of the invention is shown which comprises a reactor 2701 fed by an inlet 2702 with a supply of pressurised aqueous urea. The flow of supply is regulated by a pump 2703 which is controlled in response to a liquid level indicator 2712 situated within the reactor 2701 to maintain the reactor 2701 in a partially full condition. The reactor 2701 is situated within the exhaust conduit 2704 of an IC engine such that the flow of hot exhaust gas passes over the reactor 2701 heating the urea therein hydrolysing it to produce reaction gasses which are a mixture of ammonia, H2O and CO2. A pressure control valve 2705 is situated towards the top of the reactor 2701 and once the pressure within the reactor 2701 reaches a set pressure, preferably about twenty bar, any excess gas produced passes through the pressure control valve 2705 thereby maintaining the pressure within the reactor 2701 substantially constant. After passing through the pressure control valve the ammonia-containing gas enters a reservoir 2706 which is situated partially within, and partially outside of, the exhaust conduit 2704. The reservoir 2706 has one section within the exhaust conduit 2704 which is heated by the exhaust gas passing over it which prevents the ammonia-containing gas from condensing or crystallising, and has a second section external to the flow of the exhaust gasses which allows for heat loss from the reservoir 2706 such that its operating temperature will be lower than that of the reactor 2701. As the reservoir 2706 may be in an environment which has an elevated temperature, the natural temperature loss through the reservoir 2706 to its environment may not be sufficient, and there is no possibility to control the final temperature as it will be dependent on ambient temperature. Therefore the reservoir 2706 is surrounded by a cooling coil 2713 which is pumped by a variable speed pump 2714 through a heat exchanger 2715. The heat exchanger 2715 is in turn cooled by heat exchange with the cooling system of the IC engine which typically maintains a fairly constant temperature. The speed of pump 2714 can be controlled to maintain a substantially constant temperature within the reservoir 2706. A valve 2707 is controlled to release ammonia-containing gasses from the reservoir 2706 into the exhaust gas flowing through the conduit 2704 via nozzle 2708. The ammonia-containing gas then passes with the exhaust gasses through an SCR catalyst (not shown) where they convert the NOx in the exhaust. When the IC engine is shut down and therefore the exhaust gasses stop flowing, as it loses heat to its environment, the system will gradually cool down. As it does so the hydrolysis process will stop and the ammonia-containing gas within the reservoir 2706 will start to condense, eventually forming a pool of aqueous solution of ammonium carbamate (which may also contain a small amount of ammonia and carbon dioxide) in the base of the reservoir. When the IC engine is restarted the hot exhaust gas will start to flow over the reactor 2701 and the reservoir 2706 heating them up. The pump 2714 will not start to circulate the cooling fluid within the coil 2713 until the reservoir reaches its operating parameters. The reservoir 2701 will function as described above and will take a finite amount of time to reach its operating pressure and temperature before it can produce more ammonia-containing gas. In the interim, the pool of aqueous ammonium carbamate within the reactor 2706 will be thermally decomposed and revert back into its previous gaseous form and be available for use in a shorter time than the new ammonia-containing gas being produced by the reactor 2701. This allows for the ammonia containing gas to be applied to the hot exhaust gas sooner to start up of the engine, reducing the NOx emissions in the initial period prior to the reactor producing ammonia-containing gas. An auxiliary heater 2716 in the reactor 2701 can be used during start up to supplement the heat from the exhaust gasses to decrease the time taken for the reactor to reach operating parameters.

Referring to FIGS. 26 and 27, the introduction of the heater 2716 of FIG. 27 into the system of FIG. 26 would enable a system wherein prior to the starting of the IC engine, heaters 2716, 2609 and 2611 could be powered to bring the system up to operating temperature such that it is ready to apply ammonia-containing gas to the exhaust gasses as soon as the IC engine is started, thereby eliminating any delay between the starting of the engine, and therefore the production of NOx, and its reduction by the system of the invention.

Referring to FIG. 28 a section view of the reservoir 2806 of a system is shown located in a chamber 2817 adjacent the exhaust conduit 2804. An air gap 2818 separates the reservoir 2806 from the conduit and heat transfer across the air gap 618 heats the reservoir 2806. The rate of heat transfer across this gap may be controlled by adding an insulation material in the air gap 2818. The reservoir 2806 has an inlet and outlet with associated dosing valve (not shown). The reservoir 2806 has a heating element 2811 associated therewith. The heater 2811 can be used prior to, or during, start up to heat the condensate in the reservoir 2806 and revert it to its gaseous state ready for dosing into the exhaust gas flowing through the conduit 2804.

Referring to FIG. 29 a top view of FIG. 28 is shown. The reservoir 2906 is located in a chamber 2917 adjacent the exhaust conduit 2904 and has an air gap 2918 surrounding it. A first part 2919 of the surface of the chamber 2917 forms is in contact with the hot exhaust gasses and the remainder of the surface chamber 2917 is exposed to the atmosphere and heat is lost through that part. Preferably the ratio of the surface area of the first part 2919 to the remainder of the surface is such that at some operating conditions an equilibrium of heat input to heat lost is achieved so that the reservoir 2906 is maintained at a substantially constant temperature.