WO2009098484A1

WO2009098484A1 - Urea-hydrolysis and injection system for exhaust gas treatment

Info

Publication number: WO2009098484A1
Application number: PCT/GB2009/000353
Authority: WO
Inventors: Mark Edward Byers Sealy
Original assignee: Norgren Limited
Priority date: 2008-02-06
Filing date: 2009-02-06
Publication date: 2009-08-13
Also published as: GB0802181D0

Abstract

A system (1) for reducing a NOx content of an exhaust stream is provided. The system (1) includes an exhaust conduit (5) and a reaction vessel (120) located proximate the exhaust conduit (5). The reaction vessel (120) includes a liquid reagent inlet (8) and a gaseous hydrolysis outlet (9). A reservoir (15) is provided in fluid communication with the reaction vessel (120) and adapted to receive the gaseous hydrolysis product. The reservoir (15) is also in fluid communication with the exhaust conduit (5). A liquid reagent conduit (50) is also provided in fluid communication with the exhaust conduit (5) and in fluid communication with a liquid reagent supply (35). The liquid reagent decomposes into at least ammonia.

Description

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SYSTEM FOR EXHAUST GAS TREATMENT

TECHNICAL FIELD

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

BACKGROUND OF THE INVENTION

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) OfNO_x 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 gases 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 realized 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, known as "wet spray systems". 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 pressurized 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 Celsius (356° Fahrenheit) 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. Additionally insufficient ammonia is produced at times of low engine load and cold start up due to insufficient temperature in the exhaust gas to cause the urea to fully decompose into ammonia.

An alternative solution to the problem has been proposed in United States Patent 6,361,754 and comprises hydrolyzing 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. United States Patent 6,399,034 discloses an alternative solution which utilizes 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 hydrolyze urea at the rate required for NO_x 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.

WO 2006/087555 discloses a system for generating and feeding gaseous hydrolysis product comprising ammonia into the exhaust gas of an IC engine as it flows through the exhaust system of the engine. In this system 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 there through. However, the system is more complex than known wet spray systems and only offers a substantial NO_x reducing performance improvement over wet spray systems in cold start up conditions.

It is an object of the present invention to at least mitigate the above problems.

SUMMARY OF THE INVENTION

According to the present invention, a system for reducing a NO_x content of an exhaust stream is provided. The system includes an exhaust conduit and a reaction vessel located proximate the exhaust conduit. The reaction vessel includes a liquid reagent inlet and a gaseous hydrolysis outlet. A reservoir is provided in fluid communication with the reaction vessel and adapted to receive the gaseous hydrolysis product. The reservoir is also in fluid communication with the exhaust conduit. A liquid reagent conduit is also provided in fluid communication with the exhaust conduit and in fluid communication with a liquid reagent supply. The liquid reagent decomposes into at least ammonia.

A method for reducing a NO_x content of an exhaust stream is provided. The method comprises the step of generating a gaseous hydrolysis product including at least ammonia from a liquid reagent. The method also comprises the step of introducing the gaseous hydrolysis product into the exhaust stream to reduce the NO_x content of the exhaust stream. The method also comprises the step of introducing the liquid reagent into the exhaust stream to reduce the NO_x content of the exhaust stream.

ASPECTS

Preferably, the system further comprises a liquid reagent valve coupled to the liquid reagent conduit and in fluid communication with the exhaust conduit.

Preferably, the system further comprises a liquid reagent pump coupled to the liquid reagent conduit.

Preferably, the system further comprises a pressure release valve communicating a gaseous hydrolysis product between the reaction vessel and the reservoir. Preferably, the system further comprises an SCR catalyst.

Preferably, the system further comprises a NO_x sensor.

Preferably, the system further comprises an ammonia sensor. Preferably, the step of generating a gaseous hydrolysis product comprises the steps of: supplying a reaction vessel with a liquid reagent; and heating the reaction vessel under pressure to hydrolyze the liquid reagent into at least ammonia.

Preferably, the step of heating the reaction vessel comprises using the exhaust stream to heat the reaction vessel.

Preferably, the step of supplying the reaction vessel with a liquid reagent comprises controlling the supply based on an engine demand. Preferably, the method further comprises the step of storing the gaseous hydrolysis product in a reservoir.

Preferably, the step of introducing the liquid reagent into the exhaust stream comprises determining a NO_x level in the exhaust stream and selectively introducing at least one of the liquid reagent and the gaseous hydrolysis product based on the NO_x level.

Preferably, the method further comprises the step of measuring an ammonia content in the exhaust stream.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of the system in accordance with the invention.

Figure 2 is a longitudinal cross section through the system of Figure 1. Figure 3 is a schematic representation of a control system including the system of Figure 1.

Figure 4 is a perspective view of a third design of system according to the invention.

Figure 5 is a perspective view of the rear of the system of Figure 4. Figure 6 is a perspective view of the system shown in Figure 4 with the outer cover removed.

Figure 7 is a cross section through the reservoir and reaction vessel of Figure 4. DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 - 7 and the following description depict specific examples to teach those skilled in the art how to make and use the best mode of the invention. For the purpose of teaching inventive principles, some conventional aspects have been simplified or omitted. Those skilled in the art will appreciate variations from these examples that fall within the scope of the invention. Those skilled in the art will appreciate that the features described below can be combined in various ways to form multiple variations of the invention. As a result, the invention is not limited to the specific examples described below, but only by the claims and their equivalents. Referring to Figures 1 to 3, a system 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, according to an embodiment of the invention. The system 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 NO_x content of the exhaust gas. According to an embodiment of the invention, the system 1 has an inlet 2 and an outlet 3 for the exhaust gas flowing there through and comprises an outer body 4, which forms a pressure barrier, and passing through the outer body 4 is an inner body, in the form of an exhaust conduit, 5 which comprises a flow path longitudinally there through from the inlet 2 to the outlet 3. According to an embodiment of the invention, the system 1 is split into two sections; the first section, comprising a reaction vessel 120, is for hydrolyzing 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 there between, substantially above the inner body 5 and substantially below the inner body 5 respectively. The lower chamber 7 has an inlet 8 for receiving a supply of aqueous urea solution delivered by a pump 32 (shown in Figure 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 flow paths between the lower 7 and upper 6 chambers. The upper 6 and lower 7 chambers and the tubular elements 10 together form an enclosed reaction vessel 120 in which the hydrolysis reaction occurs. In use, the aqueous solution of urea, or other liquid reagent that decomposes into at least ammonia, is fed into the reaction vessel 120 via the inlet 8 in the lower chamber 7 by the pump 32. Although urea is described, as mentioned above, the liquid reagent could comprise other substances, such as for example, biuret or ammonium carbamate. Therefore, the present invention should not be limited to liquid urea. According to an embodiment of the invention, the level of aqueous urea in the reaction vessel 120 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 120 the level sensor 11 may extend into the lower chamber 7 through one of the passageways. Additionally, or alternatively, a second level sensor 12 may be placed in the lower chamber (Figures 3 and 4).

The exhaust gas from the engine, which, in some embodiments, has a temperature up to around 550 degrees centigrade (1,022 degrees Fahrenheit), 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 (140 degrees Fahrenheit)) 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, pressurizing the reaction vessel 120 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 can include a valve 13 which opens passively at a predetermined set pressure, preferably in the region of approximately 15 to 20 bar, ideally approximately 17 bar. Thus, the pressure in the reaction vessel 120 is elevated above atmospheric pressure but is maintained below a certain value (in this case approximately 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 18 within the header section of the reservoir 15 (as will be described in further detail shortly).

According to an embodiment of the invention, the valve 13 releases the excess pressure from the reaction vessel 120 into the second section of the system 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 15 and keeps the ammonia containing hydrolysis product in its gaseous state. The reservoir 15 has an outlet 16 and a dosing valve 17 associated therewith. The system is further provided with a pressure sensor 18 to sense the pressure in the reservoir 15.

In addition to the hardware provided for the delivery of gaseous ammonia as described above, the system 1 is also provided with a liquid reagent conduit 50 for dosing a liquid reagent directly into the exhaust gas flow. In some embodiments, the liquid reagent may comprise aqueous urea, for example. Although the description below refers to the aqueous urea, it should be appreciated that the present invention is not limited to aqueous urea and those skilled in the art will readily recognize suitable alternatives. According to the embodiment shown, urea is pumped along the conduit 50 by urea pump 52. However, it should be appreciated that other methods for delivering the urea along the urea conduit 50 could be used. Injection of the urea into the exhaust gas stream is controlled by urea valve 54 in a known manner. This method of delivering urea is well known in the art and is commercially available, for example from Bosch GmbH. The urea can decompose in the exhaust stream to form at least ammonia, which can decrease the amount OfNO_x in the exhaust stream.

Referring now to Figure 3, the system 1 is shown according to another embodiment of the invention. In the embodiment shown, a tank 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 Figure 3. The engine management system 34 logs and controls the performance characteristics of the IC engine in known manner.

The system 1 is operable as follows. The controller 33 receives a supply of data from the engine management system 34, the data may include, for example, engine speed, torque, ignition timing, throttle position, and exhaust temperature. This data can be used to calculate the NO_x level in the engine exhaust according to known techniques, such as executing algorithms on the engine management data or referencing look up tables, for example. Given the NO_x level in the exhaust, engine data and exhaust gas temperature, the controller 33 then calculates the volume of either ammonia gas or urea required to react with the prevailing level OfNO_x 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 120. This results in an increase in the level of aqueous solution within the reaction vessel 120. Thus, a greater surface area of the inside of the reaction vessel 120 becomes wetted by the aqueous solution. The resulting increase in the heated wetted area in the reactor vessel 120, i.e., 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 120 in response to an increase in the level OfNO_x in the exhaust gas. Alternatively, or additionally, depending on engine load, the controller 33 can control the urea pump 52 and urea valve 54 to deliver aqueous urea from the holding tank 35 directly into the exhaust flow. The urea can react with the exhaust stream to decompose into at least ammonia.

Conversely, in times of decreased engine demand, the controller 33 can control the pump 52 and urea valve 54 to stop the delivery of aqueous urea directly into the exhaust flow, and can control the pump 32 to decrease the rate of delivery of aqueous solution into the reaction vessel 120. This results in a reduced rate of production of gaseous hydrolysis product.

In this way the controller is able to best match the production of ammonia, be it by injecting urea directly into the exhaust or by generating ammonia for injection into the exhaust gas flow, to the prevailing engine load and exhaust gas temperature conditions.

Referring to Figures 4 to 7, an alternative embodiment of gas treatment system 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 system 64 from an inlet 65 to an outlet 66. The exhaust enters the inlet 65 containing NO_x and leaves the outlet 66 substantially free OfNO_x. In some embodiments, the system 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 NO_x sensor 112 before entering a first cylindrical tube 67 containing a hydrolysis reaction vessel 68 (see Figure 7). The hot exhaust gases exit the tube 67 through an opening therein and enter an enclosed cavity 69. As the hot exhaust gases pass over the reaction vessel 68, the reaction vessel 68 absorbs heat from the gases and becomes elevated in temperature.

According to an embodiment of the invention, the reaction vessel 68 has an inlet 70 at its lower end through which an aqueous solution of urea is supplied. The aqueous solution can be delivered from a holding tank 110, or other suitable storage device, by a pump 111, both shown schematically in Figure 7 only.

As the reaction vessel 68 becomes heated the aqueous solution of urea starts to hydrolyze and hydrolysis gases form in the head space above the level of the liquid urea.

According to an embodiment of the invention, 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 a threshold pressure. The threshold pressure may comprise any desired pressure; however, in some embodiments, the threshold pressure is approximately 17 bar, for example. The tube 67 has a closed upper end with an opening therein through which the pressure relief valve 71 projects. According to the embodiment shown, the reaction vessel 68 is attached to the system by its upper end.

The enclosed cavity 69 can be provided with 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_x in the exhaust gas is oxidized 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 area reduces, mixing is induced between the exhaust gas and the hydrolysis gas. After the conical section 75, the exhaust gases pass around a 90° bend

82 and flows into a cylindrical vortex mixer 83. The exhaust gases enter the vortex mixer 83 tangentially and exit along its central axis into an SCR catalyst 84 wherein the hydrolysis gas mixes with the NO_x converting it substantially to nitrogen and water. The exhaust gas exits the SCR catalyst 84 and expands into the interior of the system enclosed by cover 85. The treated exhaust gases then exit the system via the outlet 66 which passes through the enclosed cavity 69. Arranged in proximity to the exit 66 are a NO_x sensor 112 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 gases. 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. According to an embodiment of the invention, 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 gases. 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 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 there over maintaining them in a clean and dry environment.

The system 64 is also provided with a urea delivery tube 113 for delivering aqueous urea from the holding tank 110 to the exhaust gas stream. The tube 113 has a urea pump 115 and an injection nozzle 117 of known design. In this way both gaseous ammonia and aqueous urea can be introduced into the exhaust gas stream so as to best match the provision of ammonia to the prevailing engine operating conditions.

The detailed descriptions of the above embodiments are not exhaustive descriptions of all embodiments contemplated by the inventors to be within the scope of the invention. Indeed, persons skilled in the art will recognize that certain elements of the above-described embodiments may variously be combined or eliminated to create further embodiments, and such further embodiments fall within the scope and teachings of the invention. It will also be apparent to those of ordinary skill in the art that the above-described embodiments may be combined in whole or in part to create additional embodiments within the scope and teachings of the invention. Thus, although specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. The teachings provided herein can be applied to other gas treatment systems, and not just to the embodiments described above and shown in the accompanying figures. Accordingly, the scope of the invention should be determined from the following claims.

Claims

CLAIMS We claim:

1. A system ( 1 ) for reducing a NO_x content of an exhaust stream, comprising: an exhaust conduit (5); a reaction vessel (120) including a liquid reagent inlet (8) and a gaseous hydrolysis outlet (9); a reservoir (15) in fluid communication with the reaction vessel (120) and in fluid communication with the exhaust conduit (5); and a liquid reagent conduit (50) in fluid communication with the exhaust conduit (5) and in fluid communication with a liquid reagent supply (35) that decomposes into at least ammonia.

2. The system (1) of claim 1, further comprising a liquid reagent valve (54) coupled to the liquid reagent conduit (50) and in fluid communication with the exhaust conduit (5).

3. The system (1) of claim 1, further comprising a liquid reagent pump (52) coupled to the liquid reagent conduit (50).

4. The system ( 1 ) of claim 1 , further comprising a pressure release valve (13) communicating a gaseous hydrolysis product between the reaction vessel (120) and the reservoir (15).

5. The system (1) of claim 1, further comprising an SCR catalyst (84).

6. The system (1) of claim 1, further comprising a NO_x sensor (112).

7. The system (1) of claim 1, further comprising an ammonia sensor (114).

8. A method for reducing a NO_x content of an exhaust stream, comprising the steps of: generating a gaseous hydrolysis product including at least ammonia from a liquid reagent; introducing the gaseous hydrolysis product into the exhaust stream to reduce the

NO_x content of the exhaust stream; and introducing the liquid reagent into the exhaust stream to reduce the NO_x content of the exhaust stream.

9. The method of claim 8, wherein the step of generating a gaseous hydrolysis product comprises the steps of: supplying a reaction vessel with a liquid reagent; and heating the reaction vessel under pressure to hydrolyze the liquid reagent into at least ammonia.

10. The method of claim 9, wherein the step of heating the reaction vessel comprises using the exhaust stream to heat the reaction vessel.

11. The method of claim 9, wherein the step of supplying the reaction vessel with a liquid reagent comprises controlling the supply based on an engine demand.

12. The method of claim 8, further comprising the step of storing the gaseous hydrolysis product in a reservoir.

13. The method of claim 8, wherein the step of introducing the liquid reagent into the exhaust stream comprises determining a NO_x level in the exhaust stream and selectively introducing at least one of the liquid reagent and the gaseous hydrolysis product based on the NO_x level.

14. The method of claim 8, further comprising the step of measuring an ammonia content in the exhaust stream.