METHOD AND APPARATUS FOR CONTROLLING GASEOUS HYDROLYSIS

PRODUCTION

TECPiNICAL FIELD The present invention relates to a method for reducing emissions of Nitrogen

Oxides (NO _x ) in exhaust gases of an internal combustion (IC) engine, and in particular, to controlling the conversion of urea to a gaseous hydrolysis product containing ammonia for the reduction of the emissions.

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 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 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. 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". It has been recognized in the truck industry that the use of aqueous

urea is the most sensible solution for mobile SCR. Consequently, the use of aqueous urea is currently the industry standard.

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 for sufficient 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, cold spots on the exhaust, 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 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 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.

A problem exists with this configuration in that the demand of ammonia varies with driving conditions. For example during high engine demands, which may occur during an incline or acceleration demands a higher concentration of ammonia to lower the emissions OfNO _x in the exhaust gases to an acceptable level. Unlike stationary hydrolysis reactors where the energy available for the reactor is directly controlled, the present invention uses the exhaust conditions of the vehicle as the source of energy to raise the temperature of the urea in order to perform the hydrolysis. The present invention overcomes the difficulty of controlling the rate of the reaction without being able to control the energy available for the reactor along with other problems and an advance in the art is achieved.

SUMMARY OF THE INVENTION A method for controlling the production of a gaseous hydrolysis product from a urea solution in a reaction vessel is provided according to an embodiment of the invention. The reaction vessel comprises a liquid level of urea solution and a gaseous hydrolysis product having a gas ratio, wherein the gas ratio contains at least some amount of ammonia gas. The method is characterized by raising the liquid level of the urea solution within the reaction vessel to increase the rate of gaseous hydrolysis product production and lowering the liquid level of the urea solution within the reaction vessel to decrease the rate of gaseous hydrolysis product production.

Another method for controlling the production of a gaseous hydrolysis product from a urea solution in a reaction vessel is provided according to an embodiment of the invention. The reaction vessel comprises a liquid level of urea solution and a gaseous hydrolysis product having a gas ratio, wherein the gas ratio contains at least some amount of ammonia gas. The method is characterized by momentarily increasing the ratio of ammonia in the gaseous hydrolysis product by increasing the pressure within the reaction vessel and momentarily decreasing the ratio of ammonia in the gaseous hydrolysis product by decreasing the pressure within the reaction vessel.

Another method for controlling the production of a gaseous hydrolysis product from a urea solution in a reaction vessel is provided according to an embodiment of the

invention. The reaction vessel comprises a liquid level of urea solution and a gaseous hydrolysis product having a gas ratio, wherein the gas ratio contains at least some amount of ammonia gas. The method is characterized by releasing the gaseous hydrolysis product from the reaction vessel at a controlled rate to a reservoir. The gaseous hydrolysis product is then released independently from the reservoir into an engine exhaust at a dosing rate. The method is further characterized by momentarily increasing the dosing rate when the gas ratio of the gaseous hydrolysis product is ammonia lean and momentarily decreasing the dosing rate when the gas ratio of the gaseous hydrolysis product is ammonia rich.

4

ASPECTS

In an embodiment of the invention, raising the liquid level of the urea solution comprises increasing the rate at which a pump delivers the urea solution to the reaction vessel. In another embodiment of the invention, lowering the liquid level of the urea solution comprises decreasing the rate at which the pump delivers the urea solution to the reaction vessel.

In another embodiment of the invention, lowering the liquid level of urea solution within the reaction vessel comprises momentarily discharging a portion of the urea solution from the reaction vessel.

In another embodiment of the invention, the method further comprises momentarily increasing the ratio of ammonia in the gaseous hydrolysis product by increasing the pressure within the reaction vessel.

In another embodiment of the invention, the method further comprises momentarily decreasing the ratio of ammonia in the gaseous hydrolysis product by decreasing the pressure within the reaction vessel.

In another embodiment of the invention, the method further comprises releasing the gaseous hydrolysis product from the reaction vessel at a controlled rate to a reservoir; subsequently releasing the gaseous hydrolysis product from the reservoir into an engine exhaust at a dosing rate; momentarily increasing the dosing rate when the gas ratio of the gaseous hydrolysis product is ammonia lean; and momentarily decreasing the dosing rate when the gas ratio of the gaseous hydrolysis product is ammonia rich.

In another embodiment of the invention, the method further comprises determining the gas ratio using a gas ratio sensor.

In another embodiment of the invention, the method further comprises tracking an engine load profile to predict swings in the gas ratio. In another embodiment of the invention, the method further comprises determining the gas ratio using a gas ratio algorithm.

In another embodiment of the invention, the method further comprises releasing the gaseous hydrolysis product from the reaction vessel at a controlled rate into the reservoir, wherein the reservoir has a nominal working reservoir pressure; and maintaining a pre-determined reservoir pressure, wherein the pre-determined reservoir pressure is based at least on information from an engine management system.

In another embodiment of the invention, the pre-determined reservoir pressure is less than the nominal working reservoir pressure when the engine management system indicates a high engine load. In another embodiment of the invention, the pre-determined reservoir pressure is more than the nominal working pressure when the engine management system indicates a low engine load.

In another embodiment of the invention, the method further comprises raising the liquid level of the urea solution within the reaction vessel to increase the rate of gaseous hydrolysis product production and lowering the level of the urea solution within the reaction vessel to decrease the rate of gaseous hydrolysis product production.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an apparatus for producing a gaseous hydrolysis product according to an embodiment of the invention.

Figure 2 is a schematic representation of a control system including the apparatus of Figure IV

Figure 3 is a perspective view of a second design of the apparatus according to an embodiment of the invention. Figure 4 is a perspective view of the rear of the apparatus of Figure 2.

Figure 5 is a perspective view of the apparatus shown in Figure 2 with the outer cover removed.

Figure 6 is a cross section through the reservoir and reaction vessel of Figure 2.

Figure 7 is a cross section of a reaction vessel of the invention with heat exchange fins according to an embodiment of the invention.

Figure 8 shows another reaction vessel of the invention according to an embodiment of the invention.

Figure 9 shows a reservoir according to an embodiment of the invention.

Figure 10 is a graph relating the rate of the chemical reaction to that of boiling with respect to the reactor liquid temperature.

Figure 11 shows how the gas ratio changes with engine load. Figure 12 displays a method for controlling the reservoir control pressure in response to the engine load.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 - 12 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 and 2 an apparatus 100 is shown capable of being placed in-line in an exhaust conduit 104 of an IC engine, for example that found on a diesel vehicle, typically upstream of an SCR catalyst. The apparatus comprises a reaction vessel 101, a reservoir 106, and at least one dosing nozzle 108. The apparatus 100 produces a gaseous hydrolysis 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. The exhaust conduit 104 has an inlet 111 and an outlet 112 for the exhaust gas flowing there through. The reaction vessel 101 of the apparatus 100 can be used for hydrolyzing an aqueous solution of urea at elevated temperature and pressure so that it decomposes to form a gaseous hydrolysis product typically containing a ratio of ammonia gas, carbon

dioxide, and steam. The ratio of ammonia gas, carbon dioxide, and steam, herein referred to as the "gas ratio" can be measured or determined by known techniques.

The reaction vessel 101 has a lower section 1 19 and an upper section 120. The lower section 119 has an inlet 102 for receiving a supply of aqueous urea solution delivered by a pump 103. The aqueous urea solution can be stored in a storage vessel 113 (only shown in Figure 2). While previous applications have provided an inlet such as 102, the present invention provides for the inlet/outlet 102, which also allows a supply of aqueous urea solution to be discharged from the reaction vessel 101 so that rapid lowering of the aqueous level is now possible. This will be discussed more below. The upper section 120 has an outlet 121 for releasing the gaseous hydrolysis product.

In use the aqueous solution of urea is fed into the reaction vessel 101 via the inlet 102 in the lower section 119 by the pump 103. The level of aqueous urea in the reaction vessel can be measured by a level sensor 125 (only shown in Figure 2).

The exhaust gas from the engine, which has a temperature up to around 550 degrees centigrade (dependent on engine load), passes through the exhaust conduit 104. The exhaust gas flows around the reaction vessel 101, raising the temperature of the liquid contained therein by heat exchange. As the temperature rises, the hydrolysis reaction accelerates and the gaseous hydrolysis product starts to collect in the headspace above the liquid level in the upper section 120. As the temperature rises further the reaction accelerates and the gaseous hydrolysis product starts to collect a head of pressure in the head space. Pressurizing the reaction vessel 101 allows the temperature of the aqueous urea solution to rise above the temperature at which it would otherwise boil. The reaction vessel outlet 121 in the upper section 120 includes a valve 105 which opens passively at a predetermined set pressure, preferably in the region of about 15 to 20 bar, ideally about 17 bar. Thus the pressure in the reaction vessel 101 is elevated above atmospheric pressure but is maintained below a certain value (in this case about 17 bar)7 which gives a good reaction rate without the need to contain excessive pressures.

Alternatively the valve 105 may be active, i.e. it may operate in response to a pressure sensor 122. The pressure sensor 122, is shown to be located in the inlet/outlet 102. Positioning the pressure sensor 122 where the liquid urea is relatively cooler prevents the pressure sensor 122 from being subjected to high temperatures. It should

be understood however, that the pressure sensor 122 could be located anywhere within the reaction vessel 101.

The valve 122 releases the excess pressure from the reaction vessel 101 into a reservoir 106. According to one embodiment of the invention, the exhaust gas flows by a small heated pipe 109 and keeps the ammonia containing hydrolysis product in its gaseous state. The heated pipe 109 may also contain a heating element 110. According to another embodiment of the invention, the reservoir 106 is substantially insulated and provided with a separate means for keeping the gaseous hydrolysis product hot (see Figure 9). Referring now to Figure 2, the pump 103 delivers aqueous urea solution from a holding tank 113 into the lower section 119 of the reaction vessel 101 via the inlet 102. The pump 103 is controlled by a controller 133 which is also connected electrically to the level sensor 125; reaction vessel outlet valve 105; dosing valve 107; reaction vessel pressure sensor 122; and an engine management system 134 as indicated by the dashed lines in Figure 2. The engine management system 134 logs and controls the performance characteristics of the IC engine in known manner. It should be understood that while the controller 133 and engine management system 134 are described in relation to the apparatus 100, these components likewise may be coupled to the apparatus' that are described below. The controller 133 and engine management system 134 are omitted from the Figures that follow for the purpose of clarity.

Referring to Figures 3 to 6, an alternative embodiment of gas treatment apparatus 300 is shown which operates in a substantially similar manner to the embodiment described previously. While the controller 133 and engine management system 134 are not shown in Figures 3-6, it should be understood that these are omitted solely for clarity. It should be understood that these components can by coupled to the apparatus 300 in a similar manner as coupled to the apparatus 100. The exhaust gas of an IC engine flows through the apparatus 300 from an inlet 365 to an outlet 366. The exhaust enters the inlet 365 containing NO _x and leaves the outlet 366 substantially free on NO _x . The apparatus 300 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 365 it passes a NO _x sensor 312 before entering a first cylindrical tube 367 containing a hydrolysis reaction vessel 368

(see Figure 6). The hot exhaust gasses exit the tube 367 through an opening therein and enter an enclosed cavity 369. As the hot exhaust gasses pass over the reaction vessel 368 it absorbs heat from the gasses and becomes elevated in temperature. The reaction vessel 368 has an inlet 370 at its lower end through which an aqueous solution of urea is supplied. The aqueous solution is delivered from a holding tank 310 by a pump 311, both shown schematically in Figure 6 only.

As the reaction vessel 368 becomes heated the aqueous solution of urea starts to hydrolyze and a hydrolysis product forms in the head space above the level of the urea. The reaction vessel 368 is provided with a pressure relief valve 371 in its upper end which allows the gaseous hydrolysis product to pass from the reaction vessel 368 to a reservoir 372 if the pressure in the reaction vessel 368 exceeds a pre-determined pressure.

The tube 367 has a closed upper end (with an opening therein through which the pressure relief valve 371 projects). The reaction vessel 368 is attached to the apparatus 300 by its upper end.

The enclosed cavity 369 has a passageway in one of its walls (not shown) allowing the exhaust gas to exit the cavity 369 and pass through an oxidation catalyst 374 where a percentage of the NO in the exhaust gas is oxidized into NO ₂ . The exhaust gas then exits the oxidation catalyst and enters a truncated conical section 375 which reduces in diameter.

A feed tube 376 leads from the reservoir 372 into the conical section 375 and the hydrolysis product is dosed through the feed tube 376 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 product. After the conical section 375 the exhaust gasses pass around a 90° bend 382 and flows into a cylindrical vortex mixer 383. The exhaust gases enter the vortex mixer 383 tangentially and exit along its central axis into an SCR catalyst 384 wherein the hydrolysis product mixes with the NO _x converting it substantially to nitrogen and water. The exhaust gas exits the SCR catalyst 384 and expands into the interior of the apparatus 300 enclosed by cover 385. The treated exhaust gasses then exit the apparatus 300 via the outlet 366 which passes through the enclosed cavity 369. Arranged in proximity to the exit 366 are a NO _x sensor 313 and an ammonia sensor 314.

The flow of hydrolysis product from the reservoir 372 into the conical section 375 via the tube 376 is controlled by a dosing valve 377 (as will be described in further detail shortly) attached to an upper manifold 378 of the reservoir 372. The reservoir 372 is located in a tube 379 and positioned such that there is an air gap between the reservoir 372 and the tube 379. Part of the outer surface of the tube 379 forms a wall of the enclosed cavity 369 and as such is in direct contact with the hot exhaust gasses. In use the reservoir 372 becomes heated by heat transfer from the exhaust gas through the tube 379 and across the air gap. The reservoir 372 is elongate in shape and similar to the reaction vessel 368 will expand in length. The reservoir 372 is attached at its upper end and free to expand at its lower end. A sliding seal 380 is provided to retain the lower end of the reservoir 372. A heater 381 is situated at the lower end of the reservoir 372 to allow for additional heating to supplement the heat from the exhaust gasses. The pressure release valve 371 and the dosing valve 377 are maintained in a cooler area and are separated from the warmer area by a manifold plate 386, which may either be of a thermally shielding material or may include a thermal shield. The pressure relief valve 371 and the dosing valve 377 have covers 387, 388 sealed there over maintaining them in a clean and dry environment.

Referring to Figure 7 a reaction vessel 713 for use in an exhaust gas treatment apparatus, for example for use in the SCR system described with reference to Figures 1 - 6, is shown comprising an elongate body 714 with a bulbous head section 715 and a conical lower section 716. During use the reaction vessel 713 is heated by heat transfer from the hot exhaust gasses of an engine (not shown) to hydrolyze the aqueous urea therein. The reaction vessel 713 has a level sensor 717 entering at its top and extending downwards there from into the aqueous urea within the reaction vessel 713. It should be understood however, that the level sensor 717 could enter from the bottom of the reaction vessel 713 and extend upwards. The liquid level sensor 717 is situated on the central axis of the reaction vessel 7T3. By placing the liquid level sensor 717 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 717 measures the liquid level 718 on a continuous scale. The reaction vessel 713 has an inlet 719 for the supply of pressurized aqueous urea and an outlet 720 which leads to a pressure control valve (not shown). The reaction vessel 713 has a baffle 721 situated in its head section

715 above the liquid level and below the outlet 720. In the event of any splashing of the reagent within the reaction vessel 713, for example due to motion of the vehicle the baffle 721 prevents splashes of liquid from exiting from the outlet 720. The liquid level 718 may be controlled by controlling the volume of aqueous urea pumped into the reactor via inlet 719 dependant on the sensed liquid level. The heat transfer from the hot exhaust gas is dependent on the wetted surface area of the reaction vessel 713. The geometry of the conical section 716 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 reaction vessel 713 a number of heat exchange fins 722 are shown on the external surface of the reaction vessel 713. The surface area of the fins 722 changes in relation to the height of the reaction vessel 713 and thus the heat input to the aqueous urea can be controlled by varying the liquid level 718. For additional heat transfer to the liquid heat exchange fins 723 fins are shown inside the reaction vessel 713 to increase the contact surface area between the reaction vessel body 714 and the aqueous urea within the reaction vessel 713. The reaction vessel 713 is also provided with temperature 724 and pressure 725 sensors to monitor the temperature and pressure of the gas within the reaction vessel 713.

Referring to Figure 8 another reaction vessel 801 is shown according to an embodiment of the invention. The reaction vessel 801 comprises a lower section 819, a central section 818, and an upper section 820. According to an embodiment of the invention, the reaction vessel 801 is fed an aqueous urea solution by a pump 803. The pump 803 draws aqueous urea from a supply (not shown) and pumps the urea through a supply conduit 850. The supply conduit is coupled to an inlet/outlet 802, which is located in the lower section 819. An exhaust conduit 804 is provided which provides energy to the reaction vessel

801. As the exhaust conduit 804 raises the temperature within the reaction vessel 801, the hydrolysis reaction accelerates producing a gaseous hydrolysis product. The gaseous hydrolysis product flows through a plurality of conduits 831 to the upper section 820 where a head of pressure develops. Because the reaction vessel 801 is divided into sections, substantially un-reacted aqueous urea may be discharged momentarily from the reaction vessel 801 through the supply conduit 850. The sections 818, 819, and 820 substantially prevent any reacted or partially reacted hydrolysis

product from being discharged. The reaction vessel 801 is also provided with temperature and pressure sensors (not shown) similar to the sensors discussed above.

Referring to Figure 9, a reservoir 906 is provided according to an embodiment of the invention. As mentioned briefly above, in some embodiments, the reservoir 906 is substantially insulated. In the embodiment shown in Figure 9, the reservoir 906 comprises an inner vessel 960 and an outer vessel 961, and a cavity 962 located between the inner and outer vessels 960, 961. In the presently preferred embodiment depicted, the cavity 962 substantially surrounds the inner vessel 960 and the outer vessel 961 substantially surrounds the cavity 962. Also shown in FIG. 9, the reservoir 906 is provided with at least one connecting portion 963 that connects the inner vessel 960 to the outer vessel 961. In the embodiment depicted, the connecting portion 963 is a separate structure that connects the inner and outer vessels 960, 961 , however, in an alternative embodiments the connecting portion can be totally or partially provided as an integral portion of the inner vessel 960, the outer vessel 961, or on each of the inner and outer vessels 960, 961.

According to one aspect of the presently preferred embodiment, the outer vessel 961 in combination with the cavity between the inner and outer vessels is configured to thermally insulate the inner gas 964. According to another aspect of the presently preferred embodiment, the outer vessel 961 is configured to insulate the inner gas and inner vessel 960. According to yet another aspect of the presently preferred embodiment, the outer vessel 961 is configured to insulate the outer gas 965. According to still another aspect of the presently preferred embodiment, the outer vessel 961 is configured to contain an outer gas 965 within the cavity 962. According to yet a further aspect of the present invention, the outer vessel 961 is configured to cooperate with the inner vessel to contain the inner gas 964 in the event of an over pressurization of the inner gas 964.

Without a consistent source of heat, even with the reduced thermal inertia properties of the inner vessel 960 and the insulating effects of the outer gas 965 in the presently preferred embodiment, the temperature and pressure of the inner gas 964 may deviate from the optimal normal. While a heating element 910 or reactor may be used to return the inner gas 964 to its normal pressure range and temperature, this may require excessive energy and/or excessive time.

According to one aspect of the presently preferred embodiment, in the event the inner gas 964 deviates from its optimal normal temperature and normal pressure range, the reservoir 906 of the presently preferred embodiment is configured to quickly return the inner gas 964 to its normal pressure range and temperature, while consuming minimal energy in the process. According to another aspect of the presently preferred embodiment, once a deviation from the normal temperature and normal pressure range has occurred, the reservoir 906 is configured to utilize supplemental heating to quickly return the inner gas 964 to its normal temperature and normal pressure range.

In the presently preferred embodiment, the reservoir 906 includes a vaporizer 970 that quickly vaporizes condensate that forms in the inner vessel 960 when the inner gas 964 cools. The vaporizer 970 may be used alone or in conjunction with at least one heating element 910 to more quickly return the inner gas 964 to its optimal temperature and pressure, while, at the same time, saving energy. When condensate is not present in the inner vessel 960, the vaporizer 970 transfers very little heat to the inner gas 964, thus, preventing the inner gas 964 from being overheated.

Methods for controlling the production of hydrolysis for the above devices and reactors are as follows. While the description below makes specific reference to Figures 3-6 for the purpose of clarity, it should be understood that the various methods for controlling productivity are equally applicable to any of the devices or reactors previously described. Accordingly, the discussion below should not be limited to operations on the apparatus 300, but should apply equally to any of the devices or reactors already described.

According to an embodiment of the invention, the controller 133 receives a supply of data from the engine management system 134, the data including, for example, engine speed, torque, and throttle position. This data is 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. Given the NO _x level in the exhaust and information on the conditions of the catalyst, the controller 133 then calculates the volume of ammonia gas 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, a higher volume of ammonia gas is required. Conversely, in times

of decreased engine demand, for example low engine speed and/or torque, a lower volume of ammonia gas is required. A difficulty in the past has been the ability to control the production of ammonia to meet the required demand. This is because the engine exhaust provides the energy to the apparatus 300. Accordingly, the energy supplied to the apparatus 300 varies with varying engine conditions. Therefore, the conventional method of controlling the rate of production of ammonia by controlling the energy supply is not available with the current invention. The present invention provides a number of methods for controlling the rate of production of the ammonia without being able to control the energy source. For example, in times of increased engine demand the controller 133 can make a calculation about how much the NO _x output has increased based on engine parameters provided by the engine management system 134. The controller 133 can then measure the conditions of the catalyst downstream to calculate the most advantageous rate for dosing the exhaust stream with gaseous hydrolysis product, hereinafter, this will be referred to as the "dosing rate" and provides for a controlled rate in which to mix the gaseous hydrolysis product with the exhaust gas. If the dosing rate is increased, the gaseous hydrolysis product in the reservoir 372 decreases because the gaseous hydrolysis product is leaving the reservoir at a substantially faster rate than is being provided. This decrease in gaseous hydrolysis product in the reservoir 372 causes a fall in the pressure within the reservoir 372. The controller 133 can then determine the ideal aqueous urea level to be provided to the reaction vessel 368 to replenish the supply of gaseous hydrolysis product in the reservoir 372.

Accordingly, when the controller 133 determines that the aqueous urea level provided to the reaction vessel 368 needs to be increased based on the determination described above, the controller 133 controls the pump 311 to increase the rate of delivery of aqueous solution into the reaction vessel 368. This results in an increase in the level of aqueous solution within the reaction vessel 368. Thus, a greater surface area of the inside of the reaction vessel 368 becomes wetted by the aqueous solution. The resulting increase in the heated wetted area in the reactor vessel 368, 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 times of increased load, the NO _x levels in the exhaust increase. This increases the demand for ammonia gas, in response to which the controller 133 controls the pump 311 to increase the rate of delivery of aqueous solution to the reaction vessel 368. 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 engine load, an increase in the heated, wetted area of aqueous solution in the reaction vessel 368 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.

Conversely, in times of decreased engine demand, the controller 133 performs a similar determination as described above. If the controller 133 determines that the level of aqueous urea in the reaction vessel needs to be decreased, the controller 133 controls the pump 311 to decrease the rate of delivery of aqueous solution into the reaction vessel 368. This results in a reduced rate of production of gaseous hydrolysis product.

In times when the engine demand decreases at such a rate that simply decreasing the rate of delivery of aqueous solution into the reaction vessel 368 is not sufficient, the rate of production may be decreased by rapidly lowering the reactor liquid level. There are a number of situations where it may be advantageous to rapidly lower the liquid level. These include, but are not limited to: (1) where the engines load is rapidly reduced (vehicle stops accelerating and starts braking); (2) where the vehicle starts a long hill descent (in this situation, there is little/no requirement for ammonia and yet there can be considerable heat in the exhaust system if the exhaust brake is used); and (3) on rapid shut down of the engine. During these times, the rate of hydrolysis production may not be reduced quickly enough to a sufficient level simply by either decreasing or ceasing the rate of delivery of aqueous solution into the reaction vessel 368. According to an embodiment of the invention, when the rate of production of the gaseous hydrolysis needs to be rapidly decreased, the aqueous solution may be discharged from the reaction vessel 368. This allows for a faster response to decreased

demands in hydrolysis production. For example, if the reaction vessel 801 is used, the aqueous urea may be temporarily discharged into the lower section 819. This substantially reduces the rate of the hydrolysis reaction.

While the above discussion addresses controlling the rate of production of the hydrolysis, the gas ratio may not be adequately controlled simply by controlling the rate of production of the hydrolysis. This is because within the reaction vessel 368 there are at least three activities occurring simultaneously. They are: the chemical decomposition of urea to ammonia and other gases; the boiling of the aqueous solution; and the superheating of the gaseous hydrolysis product. A problem exists however, because these three activities can occur at different rates. Additionally, the different rates are, in large part, driven by the temperature of the liquid in the reaction vessel 368, which in the current invention, varies with the exhaust conditions and the pressure within the reaction vessel 368.

Therefore, simply varying the amount of liquid urea solution provided to the reaction vessel 368 by the pump 311 may not provide adequate control over the production of ammonia. For example, if the rate of the chemical decomposition exceeds the rate of boiling for a period of time, the gas ratio produced will temporarily contain more ammonia gas than the equilibrium condition. This is known as an "ammonia rich" gas ratio and if not compensated for, will cause a temporary over dosing of ammonia into the system. Similarly, if the rate of boiling exceeds the rate of chemical decomposition for a period of time, the gas ratio will contain less ammonia gas than is required for a proper reduction in NO _x in the exhaust gas. This is known as an "ammonia lean" gas ratio. In this situation, because there is less than the equilibrium condition, under dosing of ammonia into the system may result, unless it is compensated for. According to an embodiment, the present invention provides increased control over the ammonia gas ratio by varying the pressure within the reaction vessel 368.

Figure 10 shows how the rate of the chemical decomposition can differ from the rate of boiling with temperature. As Figure 10 shows, in general, during transient changes to the reactor's condition, if the temperature of the boiling liquid is too high, the reaction vessel 368 momentarily produces an ammonia rich release, and if the temperature is too low, the reaction vessel 368 produces an ammonia lean release. Therefore, it is desired to control the temperature of the boiling liquid in the reaction

vessel 368. However, as stated above, the reaction vessel 368 depends on the engine exhaust for its energy source. Therefore, the temperature at which these two activities occur must be controlled in an alternative way.

According to an embodiment of the invention, the production of ammonia, and in particular, the ratio of the gases produced in the gaseous hydrolysis product is controlled by varying the pressure of the reaction vessel 368. This can be accomplished by using the pressure sensor (not shown). The pressure sensor is coupled to the controller 133. The controller 133 can control the pressure at which valve 371 activates to allow gases to pass from the reaction vessel 368 into the reservoir 372. The valve 371 provides for releasing the gaseous hydrolysis from the reaction vessel 68 to the reservoir 372 at a controlled rate. Controlling the pressure within the reaction vessel 368 provides the advantage of controlling the gas release ratio by controlling the temperature at which boiling occurs. For example, a high pressure, for example around 20 bar, will increase the temperature at which boiling occurs to about 210 degrees centigrade. Similarly, a low pressure, for example around 10 bar will lower the boiling point to about 170 degrees centigrade. By adjusting the pressure at which valve 71 activates, the gas ratio can be controlled to improve gas ratio during transient changes to the reaction vessel's operating conditions. When a higher ammonia ratio is desired, the pressure within the reaction vessel 368 can be increased. Similarly, when a lower ammonia ratio is desired, the pressure within the reaction vessel 368 can be decreased.

Adjusting the pressure can be used as the sole means of control over the production of ammonia or can be used in combination with adjusting the level of the liquid urea solution. By combining the above methods, the control over the production of ammonia is increased. By varying the level of the aqueous solution within the reaction vessel 368, the rate of production is controlled. By controlling the pressure within the reaction vessel 368, the consistency of the gas release ratio is improved.

In order to determine how the pressure should be controlled as described above, the gas ratio must be determined. According to one embodiment, a gas ratio sensor (not shown) may be used. However, in certain applications, a gas ratio sensor may not be feasible and therefore, another method for determining the gas ratio must be used.

According to an embodiment of the invention, the gas ratio can be determined using the following gas ratio algorithm. As described above, within the reaction vessel 368, at least three activities are occurring. The chemical reaction of the liquid urea reacts with water according to the following equation:

NH ₂ CONH ₂ + H ₂ O «→ 2NH ₃ + CO ₂ ( 1 )

Along with the chemical reaction that occurs, the water within the reaction vessel boils, along with the superheating of the gaseous hydrolysis product. During the process of boiling the water, the water changes phases from a liquid to a gas. During this phase change, the temperature and pressure of the fluid are dependent upon one another, i.e., Psat _H2 o ⁼ f(T|jquid)- I ⁿ other words, the saturation pressure of the water is a function of the liquid temperature. Psat^o can be determined using steam tables, for example. The total pressure within the reaction vessel 368 may be determined using the pressure sensor described above. Once the total pressure within the reaction vessel 368 is determined, the partial pressures of the components within the system can be determined using Dalton's law of partial pressures:

Preaccor ⁼ (Psat ⁺ δPsupeiheaOfEO ⁺ PNH3(T _gas ) + Pcθ2(T _gas ) (2) Where:

P _reactor ⁼ total pressure within the reaction vessel 368 P _sat ⁼ saturation pressure of water

δP _superheat ⁼ superheating pressure correction due to superheated water P _NH3 ⁼ Partial pressure of ammonia Pco ₂ ⁼ Partial pressure of carbon dioxide

T _gas - temperature of the gas in the reaction vessel 368 It should be understood that the AP^ _p ^^^o term can be ignored in equation 2 when the reaction vessel 368 is mostly filled with liquid urea. When this term is used in equation 2, it can be estimated using known methods. For example, it has been found that δP _superbea , _H2 o is a function of the temperature of the gas (T _gas ) and the temperature of the boiling liquid (T| _iquid ). These two can be measured using temperature sensors.

P ₅₈ , can be determined using steam tables, for example. Using steam tables and a least squares fit, the saturation pressure of water can be calculated as:

P _sal = 0.47609 x (T _liquld J - 75.042 x {τ _iiφιid } + 3900.8 x T _liquid - 29230 (3) where temperature is measured in degrees Celsius.

From the measured reaction vessel pressure (P _reacto r) _> and the liquid and gas temperatures, equation (2) may be solved for (P _N H ₃ + Pco ₂ ) as the only unknowns left. However, at this point, the partial pressures may not be determined individually. Using equation (1), the proportions of the molecular weight of the two gas products can be calculated as about 43.6% ammonia and about 56.4% carbon dioxide. It should be understood that these values may vary due to incomplete reactions. Based on this calculated ratio, the ratio of the densities of the two gas components can be written as

According to the ideal gas equation:

P _i = m _i (R _< /M _i )T _ga /V (4)

Where:

Pi = The partial pressure of component i πij = mass of gas component i (kg) R ₀ = Universal gas constant (8314.5 J/kg/K

Mj = molecular mass of component I (kg/kmol)

T _gas ⁼ Temperature of the gases (K)

V = total volume occupied by the gas (m ³ )

Using equation 4 and applying the known values the following derivation can be used. Because the gases occupy the same volume equation 4 can be rearranged and put in terms of density and the partial pressures OfNH ₃ and CO ₂ added to become:

PNH ₃ + Pco ₂ = known from equation 2 = p _NH3 (R _o /M _NH3 )T _gas + p _C o ₂ (R _o /Mco ₂ )T _gas (5)

Because we have a known ratio of the densities equation (5) can be solved for in terms of the density of one of the components, for example ammonia.

PNH3 = (PNH3 + Pco2)/T _ga /732.6 (6)

Based on the known ratio of the densities, the mass fractions of the gas components (gas ratio) can be calculated. Based on the gas ratio calculated using the gas ratio algorithm, the pressure within the reaction vessel 368 can be adjusted to maintain a desired gas ratio. While the above mentioned methods for controlling the ammonia production provide control, the general rate of change of engine load often exceeds the rate of change of the reaction vessel's operating condition. This results in moments of over or under production and ammonia rich or lean output. These effects are largely compensated for by the reservoir 372, which acts as both a buffer storage and a gas mixing device. Further compensation can also be made by measuring, calculating, or predicting the changes in stored gas ratio so that the dosing rate can be adjusted accordingly.

It has been observed that when the engine changes from a period of low load activity to a period of high load activity, the rate of productivity generally needs to be momentarily increased. This increase in production is usually accompanied by an ammonia rich period followed by an ammonia lean period due to the changes in operating conditions in the reaction vessel 368. By tracking the engine load profile over time, swings in the gas ratio can be predicted. The controller 133 can adjust the rate at which the dosing valve 377 releases gaseous hydrolysis from the reservoir 372 into the exhaust stream. The dosing rate can be adjusted to compensate for this swing in the gas ratio (See Figure 11). The controller 133 can either calculate the current gas ratio, based on either the recent engine load profile from the engine management system 34, from the gas ratio algorithm described above, which uses temperature and pressure measurements from the reaction vessel 368, or from a gas ratio sensor (not shown), the dosing rate can be adjusted to compensate for changes in the gas ratio. For example, when the gas ratio has an ammonia rich composition, the dosing rate can be momentarily decreased. This is because the gaseous hydrolysis product contains a

greater amount of ammonia than the equilibrium ratio. Similarly, when the gas ratio has an ammonia lean composition, the dosing rate can be momentarily increased. By adjusting the dosing rate, an adequate amount of ammonia gas can be released into the exhaust stream during times of both ammonia rich and ammonia lean gas ratios. As mentioned above, the dosing rate can also be adjusted based on a gas ratio received from a gas ratio sensor (not shown).

As discussed above the gaseous hydrolysis product is released into the reservoir 372 when the headspace pressure in the reaction vessel 368 rises to the desired threshold. The dosing of the gaseous hydrolysis product from the reservoir 372 is controlled as follows. The valve 377 is operable in response to a signal from the controller 133 to open and allow some of the gaseous hydrolysis product within the reservoir 372 to enter the exhaust gas flowing through the conical section 375 to flow therewith through an SCR catalyst (not shown) positioned downstream of the dosing point. The controller 133 monitors the reservoir pressure via pressure sensor (not shown) and calculates the required opening of the valve (for the given pressure) to introduce the required mass of hydrolysis product (or component thereof) dictated by the engine exhaust and the catalyst conditions.

Accordingly, the reservoir 372 acts as a buffer between the reaction vessel 368 and the IC engine exhaust. The reservoir 372 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.

According to another embodiment of the invention, the production of hydrolysis product is controlled by predicting what productivity is actually required by referencing the pressure in the reservoir 372 and the engine's condition. For example, the pressure in the reservoir 372 has a working range between about 5 and 15 bar, with a nominal working pressure of about 10 bar. The controller 133 can be set to control the production of hydrolysis (as described above) in order to maintain the pressure within the reservoir 372 at a predetermined level. The pressure maintained in the reservoir 372 may be set based on information from the engine management system 134. For example, if the engine load is high, and therefore a greater amount of ammonia is needed, the reservoir control pressure may be allowed to move to about 8 bar. This is lower than the mid-range operating pressure, which results in a lower than normal

amount of hydrolysis within the reservoir 372 (See Figure 12). The reservoir control pressure is set low anticipating that the engine will come off load in the future and the reactor will then momentarily over produce. If the reservoir control pressure were set to a high value based on the high engine load there could be an excess supply of gas to the reservoir 372. Additionally, the reaction vessel would contain an excess amount of both liquid urea solution and gaseous hydrolysis. By allowing the pressure in the reservoir 372 to run lower than mid-range, the reaction vessel 368 has more spare capacity to deal with this period of over-production.

Similarly, if the engine load is low, and therefore a reduced amount of ammonia is currently needed, the controller 133 may allow the pressure in the reservoir 372 to move to about 12 bar. This is a higher pressure than the mid-range operating pressure. The pressure in the reservoir 372 is allowed to run high anticipating that the engine load may increase in the future needing a rapid increase in ammonia. If the pressure in the reservoir 372 were held at a low pressure based on the low engine load and therefore, low ammonia demand, when the engine load increased there would not be a sufficient supply of ammonia to handle the new demand. When the engine load is in the middle range, the controller 133 may set the pressure in the reservoir 372 at about 10 bar. Under mid-load conditions, the engine load could go either up or down. One advantage to controlling pressure in this way is that the size of the reservoir 372 may be reduced. Once, the desired pressure in the reservoir 372 is determined, the controller 133 can adjust the liquid urea solution level, the pressure in the reaction vessel 368, the dosing rate, or a combination thereof in order to control the productivity rate to maintain the determined pressure in the reservoir 372. By predicting what productivity will be required in the future, the controller 133 can minimize the swings in the production of the gaseous hydrolysis product.

While the above description for controlling the production of hydrolysis has been described in relation to the apparatus 300, it should be understood that the above methods could be used in relation to any of the devices or reaction vessels.

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 methods of controlling the production of a desired component, 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.