WO2007099372A1

WO2007099372A1 - Ammonia precursor conversion reactor

Info

Publication number: WO2007099372A1
Application number: PCT/GB2007/050093
Authority: WO
Inventors: Jeremy Temple Gidney; Martyn Vincent Twigg
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2006-03-01
Filing date: 2007-03-01
Publication date: 2007-09-07
Also published as: EP1989411A1

Abstract

A reactor for converting an aqueous ammonia precursor to a product comprising ammonia for supplying ammonia to a selective catalytic reduction catalyst disposed in an exhaust system of a lean burn internal combustion (IC) engine or gas turbine, which reactor comprising an electrically heated element and at least one conduit for conveying the aqueous ammonia precursor from an inlet end to an outlet end thereof, the arrangement being such that aqueous ammonia precursor carried in the conduit is brought into relatively close thermal contact with the electrically heated element, whereby the aqueous ammonia precursor is rapidly heated and converted to a product containing ammonia which is ejected from the outlet end in gaseous form.

Description

AMMONIA PRECURSOR CONVERSION REACTOR

The present invention relates to a reactor for converting an aqueous ammonia precursor to a product comprising ammonia (NH₃) for supplying ammonia to a selective catalytic reduction catalyst disposed in an exhaust system of a lean burn internal combustion engine or gas turbine.

Lean-burn internal combustion (IC) engines, such as diesel engines, lean-burn gasoline engines, and gas turbines, produce combustion exhaust gases containing various levels OfNO_x. Legislated emission standards provide limits on the amount of pollutants such as NO_x it is permissible to emit to atmosphere.

Advances in engine technology and management have led to a reduction in NO_x emissions. Two particular measures adopted are to reduce peak combustion temperatures through fuelling strategies; and to restrict the fuel-burning rate by lowering the oxygen content by, for example, exhaust gas recirculation (EGR). The latter approach is being used on some production diesel engines, but there can be problems such as fouling of the EGR valve with soot causing them to malfunction.

With diesel engines it has also been possible to employ multiple fuel injections coupled with very high pressures that permit very small nozzle sizes. This arrangement gives ultra fine fuel aerosols in the cylinder and a series of smoother spontaneous of combustion events. These technologies tend to homogenise the fuel burning process so lower peak combustions temperatures, and hence minimise the amount of NO_x formed.

Perhaps the ultimate form of homogeneous combustion is that in which fuel is effectively pre-vaporised, as in the process known as "homogeneous charge compression combustion" (HCCI) and one of its several variants. Diesel HCCI engines are being developed, and they produce only very small amounts of NO_x. However, there are technical difficulties in operating engines in this mode, and so it is highly desirable to have an effective means of reducing NO_x in lean exhaust gas to nitrogen (N₂). In practice, it is difficult catalytically to reduce NO_x in lean exhaust gases. One method that is being adopted is selectively to reduce NO_x using an SCR catalyst and a reductant, such as NH₃.

Whilst it is possible to store NH₃ in liquid form on board a vehicle, this is deemed undesirable for safety and other reasons. The approach that is being adopted is to carry on vehicles an NH₃ precursor, and particularly an aqueous solution of urea ((NH₂)₂CO), and to inject this solution directly into hot exhaust gas, so the (NH₂)₂CO is decomposed in the presence of water vapour, releasing NH₃ and carbon dioxide (CO₂), a "hydrolysis catalyst" placed upstream of the SCR catalyst and downstream of the injection point may help this process; or to generate NH₃ "off-line" by pyrolysing solid (NH₂)₂CO or some other precursor such as solid "ammonium carabamate". The NH₃ generated "off-line" is then injected into the exhaust gas as desired. These approaches all have practical difficulties associated with formation of linear and cyclic polymeric materials derived from (NH₂)₂CO that foul injection systems causing them to malfunction, and in extreme cases prevent reduction of NO_x present in the exhaust gas.

WO 2004/079171 discloses an apparatus for introducing ammonia into the exhaust pipe of a diesel internal combustion engine for the purposes of selectively reducing NO_x on a selective catalytic reduction (SCR) catalyst. The apparatus comprises a number of spaced hollow fins that extend laterally into the exhaust pipe and that are filled with sintered material that defines a network of passageways extending within each fin from a lower end to an upper outlet end.

JP 2004-270609 discloses a device for raising the efficiency of hydrolytic decomposition of urea in aqueous solution in an exhaust system of an internal combustion engine comprising a SCR catalyst comprising an inner tube on an upstream side of the catalyst disposed within an exhaust pipe and an injector for injecting the urea solution into the tube interior.

JP 2004-108185 discloses an exhaust emission control device for a diesel engine comprising an ammonia generating device including a urea hydrolysis catalyst to generate ammonia gas from urea solution using the exhaust heat of the diesel engine, a pump to supply of urea solution to the ammonia generating device, and an ammonia gas tank to store ammonia gas generated in the ammonia generating device.

US patent no. 6,361,754 discloses an exhaust system for a lean burn engine comprising a urea solution holding tank, a pump for pumping the urea solution into a pressurised coiled line disposed within an exhaust pipe that carries the exhaust gas to atmosphere, an additional heater, a surge vessel and a valve for controlling the passage of ammonia to a nozzle, which is also disposed within the exhaust pipe. The urea is hydro lysed as the coiled line is heated by the exhaust gas. If necessary, an additional heater can be used. The nozzle can be a simple low-pressure nozzle or a high-pressure injector.

US patent no. 6,399,034 discloses a device adapted to introduce ammonia into the exhaust gas flow of an exhaust gas system before it reaches a SCR catalyst. The device includes a heatable pressure-tight converter, an ammonia store and a control unit with control signals processing engine operating characteristics and determining therefrom the NO_x output for controlling a timed valve for injection of an ammonia dose.

Our WO 99/55446 discloses a method of reducing the content of nitrogen oxides in the exhaust gas of a lean burn engine, comprising passing the exhaust gas over a SCR catalyst which catalyses the reduction of the NO_x by ammonia to nitrogen and which adsorbs and desorbs ammonia during the engine cycle and adsorbing ammonia intermittently on the catalyst during the engine cycle. The adsorbed ammonia is available to reduce cold start NO_x.

We have now developed a reactor for converting an aqueous ammonia precursor to a product comprising ammonia, carbon dioxide and steam for supplying ammonia to a selective catalytic reduction catalyst disposed in an exhaust system of a lean burn internal combustion engine which enables the supply of ammonia to be controlled more accurantely and is independent of the exhaust gas temperature, thus reducing or avoiding such problems associated with the prior art. According to one aspect, the invention provides a reactor for converting an aqueous ammonia precursor to a product comprising ammonia for supplying ammonia to a selective catalytic reduction catalyst disposed in an exhaust system of a lean burn internal combustion (IC) engine or gas turbine, which reactor comprising an electrically heated element and at least one conduit for conveying the aqueous ammonia precursor from an inlet end to an outlet end thereof, the arrangement being such that aqueous ammonia precursor carried in the conduit is brought into relatively close thermal contact with the electrically heated element, whereby the aqueous ammonia precursor is rapidly heated and converted to a product containing ammonia which is ejected from the outlet end in gaseous form.

By "converting" herein we include the processes of pyro lysis, thermolysis and hydrolysis.

In one embodiment, the conduit twists around the electrically heated element, thereby promoting the relatively close thermal contact between the aqueous ammonia precursor and the electrically heated element.

The aqueous ammonia precursor can contain such additives as are commonly used in the field, e.g. for automotive applications. Such a product is known as "AdBlue".

The electrically heated element is such that it is capable of very rapidly, i.e. "flash", heating the aqueous urea solution to a high temperature e.g. 400-500⁰C prior to ejecting the products from the outlet end of the conduit.

The invention provides a number of very useful advantages. Firstly, it injects the reductant as ammonia per se. Problems have been reported with injector nozzles becoming blocked with ammonium cyanate crystals or biuret when aqueous urea solution per se is injected into an exhaust gas. The invention also avoids the problem of such crystals forming in the SCR catalyst substrate monolith, thus avoiding any back pressure problems and reduction in SCR catalyst activity. Secondly, because a relatively small quantity of aqueous ammonia precursor is heated at any one time, the power demand from the electrically heated element required to produce the ammonia-containing product is relatively low.

Thirdly, because the aqueous ammonia precursor is heated independently of the engine it is possible to inject ammonia into an exhaust gas independent of the temperature of an exhaust gas flowing in the exhaust system. This is an advantage over prior art cited above that rely on the temperature of the exhaust gas to convert urea disposed in a reactor in the exhaust gas flow. Accordingly, it is possible to contact the SCR catalyst, e.g. an Fe/beta zeolite catalyst, with ammonia prior to turning over the engine (provided that it may contact the catalyst directly, in the absence of flowing exhaust gas to convey it to the catalyst once the engine is turned over). In this way, NO_x emitted at "cold start" can combine with the adsorbed ammonia to form e.g. ammonium nitrite or ammonium nitrate. The NO_x stored is later converted on the SCR catalyst as the temperature of the SCR catalyst increases sufficiently to catalyse its reduction. This process enables "cold start" NO_x to be controlled much earlier than in systems where urea injection is delayed until such time as the exhaust gas is hot enough to enable the urea to be converted to ammonia.

In one embodiment, the conduit is a tube of relatively small bore. The tube can be made from a relatively thermally conductive material such as a metal, e.g. stainless steel.

Alternatively, the conduit can be a defined by a metal block, e.g. of iron or preferably stainless steel construction to maximise thermal conductivity. In the latter embodiment, the conduit can be formed e.g. by drilling into the metal block or the metal block can be formed of at least two pieces which, when assembled, define the conduit. In this embodiment, the element can be inserted into a suitable aperture in the block and a thermally conductive fluid such as a mineral oil can be used to improve thermal contact between the electrically heated element and the internal surface of the aperture. Alternatively, where the electrically heated element is a glow plug or similar, the aqueous ammonia precursor may be introduced into a coaxial chamber in which the glow plug sits, so that the aqueous ammonia precursor contacts an external surface of the glow plug.

The conduit may have an internal cross-sectional dimension, e.g. diameter, of from 0.1 -5mm, such as 0.5 to 3.0mm. Where a plurality of conduits are used, the plurality may consist of a range of different internal cross-sectional dimensions.

In one embodiment, the twisting conduit is spirally wound around the electrically heated element. Of course the twisting cross section can be any desirable shape as well as circular, such as oval, square, rectangular or triangular.

In a particular embodiment, the electrically heated element is a glow plug from a diesel engine.

US 5,240,688 discloses the use of enzymes such as urease as a catalyst for the hydrolysis of (NH₂)₂CO for use in a NO_x reduction system. The enzyme can be added per se or added to an inert carrier particle. However the process disclosed in US 5,240,688 requires the (NH₂)₂CO solution to be heated to 310⁰F (approximately

154°C) at which temperature urease denatures and is rendered inactive.

WO 99/65858 discloses the use of a substance to lower the hydrolysis reaction temperature of (NH₂)₂CO such that the NH₃ produced may be used in a NO_x reduction system. In one embodiment, (NH₂)₂CO is fed to a reactor heatable to from 20 to

90⁰C, which reactor containing urease water-insolubly located on a granular support.

The hydrolysate is then fed into the exhaust gas stream.

DE 44 25 420 Al also discloses the use of urease as a catalyst for hydrolysing (NH₂)₂CO for use in a NO_x reduction system, and suggests maintaining the reactor at a low temperature (from 40 to 65°C) and feeding the ammonia produced directly into the exhaust gas. However, the systems described in WO 99/65858 and DE 44 25 420 Al would both suffer from disadvantages when the ammonia they produce is fed into low temperature exhaust gas, as typically found on passenger cars. Often when the exhaust gas temperature is low sufficient heat is not available to completely vaporise the aqueous ammonia solution in the short time necessary before reaching the SCR catalyst that leads to maldistribution OfNH₃ across the SCR catalyst and significantly lowers NO_x reduction efficiency.

Therefore, according to a second aspect, the invention provides an exhaust system for treating exhaust gas containing nitrogen oxides from a lean burn IC engine or a gas turbine, which system comprising an exhaust pipe for conveying an exhaust gas from the engine to atmosphere, a SCR catalyst disposed in the flow path of the exhaust pipe, a first reactor according to the invention and a source of aqueous ammonia precursor.

In one embodiment, the exhaust pipe comprises a spur in which is disposed the first reactor. In another embodiment, the reactor includes a shield to prevent it from being impinged upon directly by exhaust gas components such as particulate matter.

The aqueous ammonia precursor for use in the present invention can be selected from the group consisting of urea ((NH₂)₂CO), ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate, ammonium carbamate and aqueous ammonia per se.

In one embodiment, wherein the aqueous ammonia precursor is urea, the exhaust system according to the invention comprises a first container for the aqueous urea solution and means for supplying the aqueous urea solution to a second reactor comprising a natural or synthetic urease-based catalyst for hydrolysing the aqueous urea to NH₃ and CO₂ at temperatures below 95°C and a conduit for conveying the urea hydro lysate containing NH₃ to the first reactor for injecting the hydro lysate into an exhaust line for carrying the exhaust gas upstream of a selective catalytic reduction (SCR) catalyst. In another embodiment, a second container is disposed in the conduit between the second reactor and the first reactor for receiving the aqueous urea hydrolysate.

The exhaust system may include means, when in use, for controlling the supply of aqueous ammonia precursor to the conduit, e.g. using an electrically operable valve such as a solenoid valve, and operation of the electrically heated element to provide ammonia in response to a demand signal from a suitable NO_x sensor disposed in the exhaust gas flow. Such means can include a suitably preprogrammed computer chip and processor or it can be integrated with a vehicle's electronic control unit (ECU). A suitable closed loop feedback system using sensors with cross sensitivity for both NO_x and ammonia can also be used to control the supply of ammonia to reduce or prevent emission of ammonia to atmosphere.

In a further aspect, the invention provides an automotive vehicle comprising a lean burn IC engine, such as a diesel engine or a lean burn gasoline engine, and an exhaust system according to the invention. The vehicle can be a passenger vehicle or heavy-duty diesel vehicle.

In order that the invention may be more fully understood, embodiments thereof will now be described with reference to the accompanying drawings, in which:

Figure 1 is a Computer Aided Design (CAD) drawing of a reactor according to the present invention;

Figure 2 is a photograph of the reactor of Figure 1;

Figure 3 is a photograph showing a second working embodiment of a reactor according to the present invention;

Figure 4 is a close-up image of the reactor of Figure 3;

Figure 5 is a schematic drawing of a first exhaust system according to the invention; and Figure 6 shows a two -stage urea hydrolysis vaporisation unit for use on a passenger car.

Figure 1 shows a CAD drawing of a reactor according to the present invention, wherein aqueous ammonia precursor is conveyed to the reactor via a 3 mm OD stainless steel pipe. The aqueous ammonia precursor is passed through a metal block to a coaxial chamber surrounding a glow plug. A photograph of the reactor is shown in Figure 2. In use, the electric heating element is maintained at a temperature of about 500°C by supplying it with a current of 20 amps at 12.5 volts. Liquid entering the coaxial chamber reactor is instantly gasified to produce a strongly alkaline mist.

Referring to Figure 3, there is shown a second embodiment of a reactor according to the present invention comprising a diesel glow plug of the type used in a diesel engine cylinder head around which has been spirally wound an stainless steel capillary tube with an outlet end at the top of the picture and an inlet end in the foreground at the foot of the picture including a threaded connector for connection to an aqueous urea solution supply. Figure 4 shows the arrangement in greater detail.

Figure 5 shows an exhaust system according to the invention referred to generally by the numeral 10 comprising a diesel engine 12, an exhaust pipe 14 for conveying a flowing exhaust gas to atmosphere 15. A ceramic monolith substrate 16 coated with a SCR catalyst, such as Fe/beta zeolite is disposed in the flow path of the exhaust gas. Upstream of substrate monolith 16 is spur 18 arranged at an acute angle, e.g. 45°, to the exhaust pipe and exhaust gas flow from the engine. In enlargement, is seen a reactor 20 as shown in Figures 3 and 4 disposed in a tube 21 set back from the join with the exhaust pipe 14 by about 10-20 mm. Supply of aqueous urea solution 22 from container 24 to reactor 20 via line 25 is controlled by pump 26 and valve 28 which are both under control of an ECU. The ECU also controls the actuation of the electrically heated element of reactor 20 via electrical connection 30. The ECU is the engine control unit and this feature is represented by dashed line 32. The embodiment shown in Figure 6 includes an electronic controller (50) powered by for example a 12 volt DC electricity supply (51) having a number of input signals (52) and output voltages (53) which are actuated so the unit functions correctly under all possible operating conditions, or not function when conditions are not appropriate for it to operate. Urea solution, for example 32% by weight, is stored in a reservoir tank (01) that is prevented from freezing in cold weather, as sensed by a thermocouple (TCl), by applying a voltage to an immersion heater (03). Should the amount of solution in the tank (01) be less than a predetermined level as determined by a level sensor (Ll), the operator, e.g. driver, is alerted via the electronic controller unit. If the urea solution level in the tank falls below a certain minimum level, the unit does not operate.

Urea solution is pumped via a pump (02) at a pressure between 4.5 and 20 bar to a hydrolysis reactor (09) that is maintained between 20 and 50⁰C, as determined by a thermocouple (TC2), by applying a voltage to the immersion heater (11). The hydrolysis reactor is 2.5 inches in diameter and 6.0 inches long and contains, between wire mesh retaining packs, urease incorporated into porous hydraulic cement pellets of about 5 inches diameter. The residence time in the hydrolysis reactor is sufficient for complete hydrolysis of the urea to take place so effectively no urea is passed via the solenoid control valve (33) to the reactor 31 according to Figure 1 into the exhaust gas system upstream of an iron exchanged zeolite SCR catalyst coated on to a cordierite monolithic substrate having 400 cell/square inch (40).

A laboratory demonstration unit of hydrolysis reactor was assembled with reference to Figure 6 using components suitable for a light duty diesel vehicle application. A Grundfos DME2 self-priming diaphragm reciprocating pump (02) capable of providing 0.5 cc/sec of a urea solution (10%). The stainless steel hydrolysis unit (09) had dimensions of 38 x 120 mm packed with immobilised Jack

Bean urease (Fluka) prepared by vacuum impregnation of porous ceramic pieces (4 mm x 5 mm). The unit had a stainless steel tube (6 mm diameter x 1 m) welded to its outside surface through which water was passed at 40°C recycled from a thermostatted Grant FH16-D heated recirculating pump. The fresh urea solution entering the hydrolysis unit had a neutral pH (6.9) and that exiting the unit had a pH of 9.5.

The exhaust system was tested on a passenger car powered by a two -litre four cylinder diesel engine, fuelled with standard diesel fuel containing less than 50 ppm sulphur. There was 80% NO_x conversion over the European Stage 3 test cycle. It was found conversion was improved by having a platinum based oxidation catalyst in the upstream position (60), and it was also found beneficial in the early part of the test cycle if the SCR catalyst had been pre-dosed with NH₃ since this improved its low temperature performance in the slow speed urban part of the test cycle. With NH₃ pre-dosing and an upstream oxidation catalyst, conversions of more than 90% were achieved.

Claims

CLAIMS:

1. A reactor for converting an aqueous ammonia precursor to a product comprising ammonia for supplying ammonia to a selective catalytic reduction catalyst disposed in an exhaust system of a lean burn internal combustion (IC) engine or gas turbine, which reactor comprising an electrically heated element and at least one conduit for conveying the aqueous ammonia precursor from an inlet end to an outlet end thereof, the arrangement being such that aqueous ammonia precursor carried in the conduit is brought into relatively close thermal contact with the electrically heated element, whereby the aqueous ammonia precursor is rapidly heated and converted to a product containing ammonia which is ejected from the outlet end in gaseous form.

2. A reactor according to claim 1, wherein the conduit twists around the electrically heated element, thereby promoting the relatively close thermal contact between the aqueous ammonia precursor and the electrically heated element.

3. A reactor according to claim 2, wherein the twisting conduit is spirally wound around the electrically heated element.

4. A reactor according to claim 1, 2 or 3, wherein the arrangement is such that the electrically heated element heats the aqueous ammonia precursor solution to 400- 500⁰C prior to ejection from the outlet end of the conduit.

5. A reactor according to claim 1, 2, 3 or 4, wherein the conduit comprises a tube of relatively small bore.

6. A reactor according to claim 5, wherein the tube is made from a relatively thermally conductive metal, such as stainless steel.

7. A reactor according to claim 1, 2, 3 or 4, wherein the conduit is defined by a metal block.

8. A reactor according to claim 7, wherein the metal block is constructed from stainless steel.

9. A reactor according to any preceding claim, wherein an internal cross- sectional dimension of the conduit is from 0.1 to 5mm.

10. A reactor according to any preceding claim, wherein the electrically heated element is a glow plug from a diesel engine.

11. An exhaust system for treating exhaust gas containing nitrogen oxides from a lean burn IC engine or a gas turbine, which system comprising an exhaust pipe for conveying an exhaust gas from the engine to atmosphere, a SCR catalyst disposed in the flow path of the exhaust pipe, a first reactor according to any preceding claim and a source of aqueous ammonia precursor.

12. An exhaust system according to claim 11, wherein the exhaust pipe comprises a spur in which is disposed the first reactor.

13. An exhaust system according to claim 11 or 12, wherein the aqueous ammonia precursor is selected from the group consisting of urea ((NH₂)₂CO), ammonium carbonate, ammonium carbamate or ammonium hydrogen carbonate, ammonium formate and aqueous ammonia per se.

14. An exhaust system according to claim 13, wherein the aqueous ammonia precursor is urea, which system comprising a first container for the aqueous urea solution and means for supplying the aqueous urea solution to a second reactor comprising a natural or synthetic urease-based catalyst for hydrolysing the aqueous urea to NH₃ and CO₂ at temperatures below 95°C and a conduit for conveying the urea hydro lysate containing NH₃ to the first reactor for injecting the hydro lysate into an exhaust line for carrying the exhaust gas upstream of a selective catalytic reduction (SCR) catalyst.

15. An exhaust system according to claim 14, wherein a second container is disposed in the conduit between the second reactor and the first reactor for receiving the aqueous urea hydrolysate.

16. An exhaust system according to any of claims 11 to 15, comprising means, when in use, for controlling the supply of aqueous ammonia precursor to the conduit, and operation of the electrically heated element to inject ammonia into an exhaust gas carried in the exhaust pipe in response to a demand signal from a suitable NO_x sensor disposed in the exhaust gas flow.

17. An exhaust system according to claim 16, wherein the control means comprises a suitably pre-programmed computer chip and processor.

18. An automotive vehicle comprising a lean burn IC engine and an exhaust system according to any of claims 11 to 17.

19. An automotive vehicle according to claim 18, wherein the engine is a diesel engine.

20. A method of introducing ammonia into an exhaust gas of a lean burn internal combustion engine or gas turbine for reducing NO_x on a selective catalytic reduction catalyst, which method comprising bringing an aqueous ammonia precursor into relatively close thermal contact with an electrically heated element, rapidly heating the aqueous ammonia precursor with the electrically heated element to convert the aqueous ammonia precursor to a product containing ammonia and ejecting the product into the exhaust gas in gaseous form.