Patent Publication Number: US-2013239554-A1

Title: Exhaust gas treatment system having a solid ammonia gas producing material

Description:
FIELD OF THE INVENTION 
     Exemplary embodiments of the invention relate to exhaust gas treatment systems for internal combustion engines and, more particularly, to an exhaust gas treatment system having a pressurized vessel that is selectively activated to heat a solid ammonia gas producing material into an ammonia gas. 
     BACKGROUND 
     The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NO x ”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components. 
     One type of exhaust treatment technology for reducing CO and HC emissions is an oxidation catalyst device (“OC”). The OC device includes a flow-through substrate and a catalyst compound applied to the substrate. One type of exhaust treatment technology for reducing NO x  emissions is a selective catalytic reduction (“SCR”) device that may be positioned downstream of the OC device. The SCR device includes a substrate, having a SCR catalyst compound applied to the substrate. 
     In one approach, a reductant is typically sprayed into hot exhaust gases upstream of the SCR device. The reductant may be an aqueous urea solution that decomposes to ammonia (“NH 3 ”) in the hot exhaust gases and is adsorbed by the SCR device. The ammonia then reduces the NO x  to nitrogen in the presence of the SCR catalyst. However, the SCR device also needs to reach a threshold or light-off temperature to effectively reduce NO x . During a cold start of the engine, the SCR device has not attained the respective light-off temperature, and therefore generally may not effectively remove NO x  from the exhaust gases. 
     Several drawbacks may exist when spraying an aqueous urea solution into the exhaust gas. For example, the tanks that store the aqueous urea may be heavy and bulky, and therefore add weight and cost to a vehicle. Also, during certain operating conditions, such as low ambient temperatures, the aqueous urea solution may become frozen (i.e. below the freezing temperature of the urea solution which is usually at about negative 12° C.). This causes the urea solution to lose the ability to be injected into the exhaust gas stream by an injector. Thus, in order to maintain the effectiveness of the injector, an electrical heater may need to be provided for thawing the urea solution, which also adds weight and cost to a vehicle. Accordingly, it is desirable to provide an efficient, cost-effective approach for effectively removing NO x  from the exhaust gas. 
     SUMMARY OF THE INVENTION 
     In one exemplary embodiment of the invention, an exhaust gas treatment system for an internal combustion engine is provided, including an exhaust gas conduit, a pressurized vessel, a selective catalytic reduction (“SCR”) device, and a control module. The internal combustion engine has a plurality of pistons and an engine off condition that indicates that the pistons are generally stationary. The exhaust gas conduit is in fluid communication with, and configured to receive an exhaust gas from the internal combustion engine during operation. The pressurized vessel stores a solid ammonia gas producing material. The pressurized vessel is selectively activated to heat the solid ammonia gas producing material into an ammonia gas. The ammonia gas is released into the exhaust gas conduit. The SCR device is in fluid communication with the exhaust gas conduit and is configured to receive the ammonia gas. The SCR device has a SCR temperature profile and a SCR light-off temperature. The control module is in communication with the internal combustion engine and the pressurized vessel. The control module receives a signal indicating the engine off condition. The control module includes a memory for storing a value indicating a target amount of the ammonia gas released into the exhaust gas conduit by the pressurized vessel and loaded on the SCR device. The control module includes control logic for determining if the internal combustion engine is in the engine off condition based on the signal. The control module includes control logic for determining the SCR temperature profile. The control module includes control logic for determining if the SCR temperature profile is below a threshold value if the internal combustion engine is in the engine off condition. The threshold value indicates that the SCR device is a specified amount below the SCR light-off temperature. The control module includes control logic for determining if the pressurized vessel has released the target amount of the ammonia gas into the exhaust gas conduit if the SCR temperature profile is below the threshold value. The control module includes control logic for deactivating the pressurized vessel if the pressurized vessel has released the target amount of the ammonia gas. 
     The above features and advantages and other features and advantages of the invention are readily apparent from the following detailed description of the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features, advantages and details appear, by way of example only, in the following detailed description of embodiments, the detailed description referring to the drawings in which: 
         FIG. 1  is a schematic diagram of an exemplary exhaust gas treatment system; and 
         FIG. 2  is a process flow diagram illustrating a method of activating a pressurized vessel to heat a solid ammonia gas producing material into an ammonia gas. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     Referring now to  FIG. 1 , an exemplary embodiment is directed to an exhaust gas treatment system  10 , for the reduction of regulated exhaust gas constituents of an internal combustion (“IC”) engine  12 . The exhaust gas treatment system described herein can be implemented in various engine systems that may include, but are not limited to, diesel engine systems, gasoline engine systems, and homogeneous charge compression ignition engine systems. In the example as illustrated, the engine  12  includes a plurality of pistons  16 . For example, the engine  12  may be an eight cylinder or a twelve cylinder engine, however it is to be understood that any number of pistons  16  may be used as well. 
     The exhaust gas treatment system  10  generally includes one or more exhaust gas conduits  14 , and one or more exhaust treatment devices. In the embodiment as illustrated, the exhaust gas treatment system devices include a hydrocarbon adsorber  20 , an electrically heated catalyst (“EHC”) device  22 , an oxidation catalyst device (“OC”)  24 , a selective catalytic reduction device (“SCR”)  26 , and a particulate filter device (“PF”)  30 . As can be appreciated, the exhaust gas treatment system of the present disclosure may include various combinations of one or more of the exhaust treatment devices shown in  FIG. 1 , and/or other exhaust treatment devices (not shown), and is not limited to the present example. 
     In  FIG. 1 , the exhaust gas conduit  14 , which may comprise several segments, transports exhaust gas  15  from the IC engine  12  to the various exhaust treatment devices of the exhaust gas treatment system  10 . The hydrocarbon adsorber  20  includes for example, a flow-through metal or ceramic monolith substrate. The substrate can include a hydrocarbon adsorber compound disposed thereon. The hydrocarbon adsorber compound may be applied as a wash coat and may contain materials such as, for example, zeolite. The hydrocarbon adsorber  20  is located upstream of the EHC device  22 , the OC device  24 , and the SCR device  26 . The hydrocarbon adsorber  20  is configured for reducing the emissions of HC during an engine cold start condition when the EHC device  22 , the OC device  24  and the SCR device  26  have not heated to the respective light-off temperatures and are not active, by acting as a mechanism for storing exhaust emission components. Specifically, the zeolite-based material is used to store fuel or hydrocarbons during a cold start. 
     The OC device  24  is located downstream of the hydrocarbon adsorber  20  and may include, for example, a flow-through metal or ceramic monolith substrate that may be packaged in a stainless steel shell or canister having an inlet and an outlet in fluid communication with exhaust gas conduit  14 . The substrate can include an oxidation catalyst compound disposed thereon. The oxidation catalyst compound may be applied as a wash coat and may contain metals such as platinum (“Pt”), palladium (“Pd”), perovskite or other suitable oxidizing catalysts, or combination thereof. The OC device  24  treats unburned gaseous and non-volatile HC and CO, which are oxidized to create carbon dioxide and water. 
     In the embodiment as illustrated, the EHC device  22  is disposed within the OC device  24 . The EHC device  22  includes a monolith  28  and an electrical heater  32 , where the electrical heater  32  is selectively activated and heats the monolith  28 . The electrical heater  32  is connected to an electrical source (not shown) that provides power thereto. In one embodiment, the electrical heater  32  operates at a voltage of about 12-24 volts and at a power range of about 1-3 kilowatts, however it is understood that other operating conditions may be used as well. The EHC device  22  may be constructed of any suitable material that is electrically conductive such as a wound or stacked metal monolith  28 . An oxidation catalyst compound (not shown) may be applied to the EHC device  22  as a wash coat and may contain metals such as Pt, Pd, perovskite or other suitable oxidizing catalysts, or combination thereof. 
     The SCR device  26  may be disposed downstream of the OC device  24 . In a manner similar to the OC device  24 , the SCR device  26  may include, for example, a flow-through ceramic or metal monolith substrate that may be packaged in a stainless steel shell or canister having an inlet and an outlet in fluid communication with the exhaust gas conduit  14 . The substrate may include an SCR catalyst composition applied thereto. The SCR catalyst composition may contain a zeolite and one or more base metal components such as iron (“Fe”), cobalt (“Co”), copper (“Cu”) or vanadium (“V”) which can operate efficiently to convert NO x  constituents in the exhaust gas  15  in the presence of a reductant such as ammonia (“NH 3 ”). 
     In the example as shown in  FIG. 1 , a pressurized vessel  40  is provided for storing a solid ammonia gas producing material  42 . In one embodiment, the solid ammonia gas producing material  42  is ammonium carbamate or ammonium carbonate. The pressurized vessel  40  is selectively activated to heat the solid ammonia gas producing material  42  into an ammonia gas that is injected or released into the exhaust gas conduit  14 . In the exemplary embodiment as shown in  FIG. 1 , the pressurized vessel  40  includes a plurality of heaters  44  that are located along the side walls  46  of the pressurized vessel  40 . In one example, the heaters  44  are 200 Watt resistive elements that act as heaters. The pressured vessel  40  also includes a flash heater  48  that the solid ammonia gas producing material  42  rests upon. A space  50  exists in the pressure vessel  40  between the pressure vessel  40  and the solid gas producing material  42 . In one embodiment, the heaters  44  are activated to heat the solid gas producing material  42  to a temperature ranging from about 60° C. to about 100° C. Then, the flash heater  48  may be activated to heat the solid gas producing material  42  to a relatively high temperature (i.e., in one embodiment to about 110° C.). The temperature created by activation of the flash heater  48  creates a decomposition of the solid gas producing material  42  at the interface between the solid gas producing material  42  and the flash heater  48 . Specifically, the activation of the flash heater  48  converts the solid gas producing material  42  into an ammonia gas and carbon dioxide (“CO 2 ”). The mixture of ammonia gas and carbon dioxide are fed through a tube  52  that is connected to the exhaust gas conduit  14 . The mixture of ammonia gas and the carbon dioxide are then dosed or released into the exhaust gas conduit  14 . Specifically, the ammonia gas and carbon dioxide are released into the exhaust gas conduit  14  and directed towards the SCR device  26 . 
     The pressure vessel  40  also includes a pressure transducer  54  that is used to monitor the pressure of the space  50  located internally of the pressure vessel  40 . Specifically, the space  50  eventually reaches a threshold pressure as the solid gas producing material  42  decomposes into the ammonia gas. The threshold pressure indicates the solid gas producing material  42  is being converted into the ammonia gas and carbon dioxide at a rate that results in a steady supply of ammonia gas that is required by the SCR device  26 . That is, the pressure vessel  40  includes a normally closed solenoid valve  56  that is opened in the event the pressure transducer  52  detects that the pressure within the space  50  has exceeded the threshold pressure. The opening of the solenoid valve  56  allows for the ammonia gas and carbon dioxide to enter the exhaust gas conduit  14 . Thus, the threshold pressure creates the dispersion or gas propagation needed to create a target amount of ammonia gas released into the exhaust gas conduit  14  that is loaded on the SCR device  26 . Specifically, in one example, the target amount of ammonia gas may represent a saturation amount of ammonia gas that is stored by the SCR device  26 . The saturation amount represents a maximum amount of ammonia gas the SCR device  26  is capable of storing, however it is to be understood that the target amount of ammonia gas may be other quantities as well. 
     The PF device  30  may be disposed downstream of the SCR device  26 . The PF device  30  operates to filter the exhaust gas  15  of carbon and other particulates. In various embodiments, the PF device  30  may be constructed using a ceramic wall flow monolith filter  23  that may be packaged in a shell or canister constructed of, for example, stainless steel, and that has an inlet and an outlet in fluid communication with exhaust gas conduit  14 . The ceramic wall flow monolith filter  23  may have a plurality of longitudinally extending passages that are defined by longitudinally extending walls. The passages include a subset of inlet passages that have and open inlet end and a closed outlet end, and a subset of outlet passages that have a closed inlet end and an open outlet end. Exhaust gas  15  entering the filter  23  through the inlet ends of the inlet passages is forced to migrate through adjacent longitudinally extending walls to the outlet passages. It is through this wall flow mechanism that the exhaust gas  15  is filtered of carbon and other particulates. The filtered particulates are deposited on the longitudinally extending walls of the inlet passages and, over time, will have the effect of increasing the exhaust gas backpressure experienced by the IC engine  12 . It is appreciated that the ceramic wall flow monolith filter is merely exemplary in nature and that the PF device  30  may include other filter devices such as wound or packed fiber filters, open cell foams, sintered metal fibers, etc. 
     A control module  60  is operably connected to and monitors the engine  12  and the exhaust gas treatment system  10  through a number of sensors. The control module  60  is also operably connected to the electrical heater  32  of the EHC device  22 , the engine  12 , and the pressurized vessel  40 . An engine off condition occurs if the pistons  16  are generally stationary within the respective cylinders of the engine  12 . In the embodiment as shown, the control module  60  is in communication with an ignition switch  70 . The ignition switch  70  sends a signal to the control module  60  that is indicative of the engine off condition. Specifically, the ignition switch  70  includes a key-on state and a key-off state, where the key-off state coincides with the engine off condition. In the key-on state, electrical power is supplied to a propulsion system of a vehicle (not shown in  FIG. 1 ). In the key-off state, electrical power is not supplied to the propulsion system. It should be noted that while the terms key-on and key-off are used, a key may not be employed with the ignition switch  70  in some embodiments. For example, in one embodiment the ignition switch  70  may be activated by proximity to a fob (not shown) that is carried by a user instead of a key. Thus, the key-off state exists when power is supplied to the propulsion system and the key-off state exists when power is not supplied to the propulsion system, regardless of whether an actual key is employed. It should also be noted that while an ignition switch  70  is illustrated, other approaches may be used as well to determine the engine off condition. 
       FIG. 1  illustrates the control module  60  in communication with two temperature sensors  62  and  64  located in the exhaust gas conduit  14 . The first temperature sensor  62  is situated upstream of the SCR device  26 , and the second temperature sensor  64  is located downstream of the SCR device  26 . The temperature sensors  62  and  64  send electrical signals to the control module  50  that each indicate the temperature in the exhaust gas conduit  14  in specific locations. 
     The control module  60  includes control logic for monitoring the first temperature sensor  62  and the second temperature sensor  64  and for calculating a temperature profile of the SCR device  26 . Specifically, the first temperature sensor  62  and the second temperature sensor  64  are averaged together to create the temperature profile of the SCR device  26 . The control module  60  includes control logic for determining if the SCR device  26  is below a threshold temperature. The threshold temperature is below a light-off or minimum operating temperature of the SCR device  26  (i.e. in one embodiment the light-off temperature is about 200° C.). Specifically, the threshold temperature is a specified amount below the light-off temperature of the SCR device  26 . That is, the SCR device  26  has been cooled to the threshold temperature such that ammonia gas may be stored on the SCR device  26 . In one example, the threshold temperature ranges from between 100° C. to about 150° C., however it is understood that the threshold temperature may include other ranges as well. 
     The control module  60  also includes control logic for determining if the SCR device  26  has the target amount of ammonia gas loaded therein. Specifically, in one embodiment, the control module  60  includes control logic for determining if the engine  12  is in the engine off condition by receiving the signal from the ignition switch  70 . In the event that the engine  12  is in the engine off condition, then the control module  60  includes control logic for determining if the temperature profile of the SCR device  26  is below the threshold temperature. That is, the control module  60  includes control logic for determining if the SCR device  26  is cooled to the threshold temperature such that ammonia gas may be stored on the SCR device  26  when the engine  12  is in the engine off condition. In the event that the SCR device  26  is below the threshold temperature, then the control module  60  also includes control logic for determining the amount of ammonia gas that has been released into the exhaust gas conduit  14  by the pressurized vessel  40 . 
     In the event that the control module  60  determines that the SCR device  26  has the target amount of ammonia gas loaded therein, then the control module  60  includes control logic for deactivating the pressurized vessel  40 . Specifically, the control module  60  includes control logic for deactivating the flash heater  48 , which in turn ceases the decomposition of the solid gas producing material  42  into the ammonia gas and carbon dioxide. This in turn deactivates the dosing or injection of the ammonia gas into the exhaust gas conduit  14 . In the event that the control module  60  determines that the SCR device  26  does not have the target amount of ammonia gas loaded therein, the control module  60  includes control logic for continuing to keep the flash heater  48  of the pressurized vessel  40  activated to produce the ammonia gas. 
     The control module  60  includes control logic for monitoring the pressure transducer  54 . The pressure transducer  54  monitors the pressure of the space  50  located internally of the pressure vessel  40 . The space  50  eventually reaches the threshold pressure as the solid gas producing material  42  decomposes into the ammonia gas. Once the control module  60  determines that the threshold pressure has been attained, the normally closed solenoid valve  56  is opened. The ammonia gas and carbon dioxide is then released into the exhaust gas conduit  14 . 
     The control module  60  also includes control logic for selectively activating or deactivating the EHC device  22  based on the temperature profile of the SCR device  26 . Specifically, if the temperature profile of the SCR device  26  is above the light-off temperature, then the electrical heater  32  is deactivated, and no longer heats the EHC device  22 . However, as long as the temperature profile of the SCR device  22  is below the light-off temperature the electrical heater  32  is activated or remains activated, and heat is provided to the SCR device  26 . 
     The control module  60  also includes control logic for monitoring the temperature of the EHC device  22 . Specifically, the control module  60  may monitor the temperature of the EHC device  22  by several different approaches. In one approach, a temperature sensor (not shown) is placed downstream of the EHC device  22  and is in communication with the control module  60  for detecting the temperature of the EHC device  22 . In an alternative approach, the temperature sensor is omitted, and instead the control module  60  includes control logic for determining the temperature of the EHC device  22  based on operating parameters of the exhaust gas system  10 . Specifically, the temperature of the EHC device  22  may be calculated based on the exhaust flow of the engine  12 , an input gas temperature of the engine  12 , and the electrical power provided to the electrical heater  32 . The exhaust flow of the engine  12  is calculated by adding the intake air mass of the engine  12  and the fuel mass of the engine  12 , where the intake air mass is measured using an intake air mass flow sensor (not shown) of the engine  12 , which measures air mass flow entering the engine  12 . The fuel mass flow is measured by summing the total amount of fuel released into the engine  12  over a given period of time. The fuel mass flow is added to the air mass flow rate to calculate the exhaust flow of the engine  12 . 
     The control module  60  includes control logic for determining if the temperature of the EHC device  22  is above a threshold or EHC light-off temperature. In one exemplary embodiment, the EHC light-off temperature is about 250° C. If the temperature of the EHC device  22  is above the EHC light-off temperature, then the control module  60  includes control logic for de-energizing an electrical source (not shown) of the electrical heater  32 . 
     The SCR device  26  stores the ammonia gas during the engine off condition. This is because the SCR device  26  has been cooled to the threshold temperature, which is a specified amount below the respective light-off temperature of the SCR device  16 . Thus, the ammonia gas will not react with the SCR catalyst composition that is disposed on the substrate of the SCR device  26  before a cold start of the engine  12 . The SCR device  26  continues to store the ammonia gas before a cold start of the engine  12 . During the engine on condition, but prior to attaining the light-off temperature, the SCR device  26  generally acts as a NO x  adsorber. That is, the SCR device  26  is generally able to adsorb NO x  released into the exhaust gas  15  as the engine  12  operates. 
     The SCR device  26  is eventually heated to the light-off temperature during operation of the engine  12 , which generally effectively reduces the amount of NO x  in the exhaust gas  15 . Specifically, the NO x  in the exhaust gas  15  is reduced to nitrogen after light-off of the SCR device  26 . As discussed above, in one embodiment the oxidation catalyst compound applied to the EHC device  22  and the OC device  24  may contain metals such as Pt, Pd, or perovskite. These types of oxidation catalysts may convert NO to NO 2  at a relatively high rate during cold start of an engine when compared to some other types of oxidation catalyst compounds that are currently available. The majority of NO x  emitted from the engine  12  is in the form of NO, however it should be noted that NO 2  is more easily adsorbed than NO by the SCR device  26 . Thus, the conversion of NO to NO 2  at a relatively high rate may facilitate or improve the reduction of NO x  in the exhaust gas  15  by the SCR device  26  once the SCR device  26  is heated to the light-off temperature. 
     The EHC device  22  is also positioned downstream of a front face  74  of the OC device  24  such that hydrocarbons in the exhaust gas  15  do not substantially interfere with the generation of NO to NO 2  by the EHC device  22 . In the embodiment as shown, the EHC device  22  is located within the OC device  24 . Specifically, the OC device  24  is employed in an effort to treat unburned gaseous and non-volatile HC and CO upstream of the EHC device  22 . Hydrocarbons in the exhaust gas  15  may interfere with the conversion of NO to NO 2  by the EHC device  22 . Thus, the placement of the OC device  24 , or a portion thereof, upstream of the EHC device  22  facilitates reducing the amount of NO x  in the exhaust gas  15  by reducing or substantially eliminating hydrocarbons that interfere with NO 2  generation. 
     Moreover, the hydrocarbon adsorber  20  is configured for reducing the amount of HC that reaches the EHC device  22  and the OC device  24  during a cold start, which also facilitates or improves the reduction of NO x  in the exhaust gas  15 . The hydrocarbon adsorber  20  acts as a mechanism for storing fuel or hydrocarbons during a cold start. That is, the hydrocarbons are adsorbed by the hydrocarbon adsorber  20  prior to reaching the EHC device  22  and the OC device  24 . Thus, the hydrocarbon adsorber  20  may also facilitate reducing the amount of NO x  in the exhaust gas  15  by reducing or substantially eliminating hydrocarbons that interfere with NO 2  generation. 
     A method of operating the exhaust gas treatment system  10  will now be explained. Referring to  FIG. 2 , an exemplary process flow diagram illustrating an exemplary process of operating the exhaust gas treatment system  10  is generally indicated by reference number  200 . Process  200  begins at step  202 , where the control module  60  includes control logic for monitoring the engine  12  for an engine off condition. Specifically, referring to  FIG. 1 , in one embodiment, an engine off condition occurs if the pistons  16  are generally stationary within the respective cylinders. In one exemplary embodiment, an ignition switch  70  is in communication with the control module  60 , and is used to indicate if the engine on or engine off condition has occurred, however it is to be understood that other approaches may be used as well to determine the engine off condition. If the engine  12  is not in the engine off condition, process  200  may then terminate. Process  200  may proceed to step  204  in the event the engine  12  is in the engine off condition. 
     In step  204 , the control module  60  includes control logic for monitoring a temperature profile of the SCR device  26 . Specifically, referring to  FIG. 1 , the control module  60  is in communication with two temperature sensors  62  and  64  located in the exhaust gas conduit  14 , where the first temperature sensor  62  is situated upstream of the SCR device  26 , and the second temperature sensor  64  is located downstream of the SCR device  26 . The control module  60  includes control logic for monitoring the first temperature sensor  62  and the second temperature sensor  64  and for calculating a temperature profile of the SCR device  26 . Specifically, the first temperature sensor  62  and the second temperature sensor  64  are averaged together to create the temperature profile of the SCR device  26 . The threshold temperature is below a light-off or minimum operating temperature of the SCR device  26 . Specifically, the threshold temperature is a specified amount below the light-off temperature of the SCR device  26 , such that ammonia gas may be stored on the SCR device  26 . If the SCR device  26  is above the threshold temperature, process  200  may continue to monitor the temperature profile of the SCR device  26 . In the event that the SCR device  26  is below a threshold temperature, process  200  may then proceed to step  206 . 
     In step  206 , the control module  60  includes control logic for determining if the SCR device  26  has a target amount of ammonia gas loaded therein. Specifically, the control module  60  includes control logic for monitoring the amount of ammonia gas that has been released into the exhaust gas conduit  14  by the pressurized vessel  40  decomposing the solid gas producing material  42  into an ammonia gas and carbon dioxide. In the event that the control module  60  determines that the SCR device  26  has the target amount of ammonia gas loaded therein, then process  200  may proceed to step  208 . In step  208 , the control module  60  includes control logic for deactivating the pressurized vessel  40 . Specifically, the control module  60  includes control logic for deactivating the flash heater  48  if the flash heater  48  has been activated. Deactivation of the flash heater  48  will cease the decomposition of the solid gas producing material  42  into the ammonia gas and carbon dioxide. This in turn deactivates the dosing or injection of the ammonia gas into the exhaust gas conduit  14 . Process  200  may then terminate. In the event that the control module  60  determines that the SCR device  26  does not have the target amount of ammonia gas loaded therein, process  200  may then proceed to step  210 . 
     In step  210 , the control module  60  includes control logic for monitoring the pressure transducer  54 . The pressure transducer  54  is used to monitor the pressure of a space  50  located internally of the pressure vessel  40  as the space  50  eventually reaches a threshold pressure. The threshold pressure indicates the solid gas producing material  42  is being converted into the ammonia gas and carbon dioxide at a rate that results in a steady supply of ammonia gas that is required by the SCR device  26 . That is, the pressure vessel  40  includes the normally closed solenoid valve  56  that is opened in the event the pressure transducer  52  detects that the pressure within the space  50  has exceeded the threshold pressure. Process  200  may then proceed to step  212 . 
     In step  212 , the control module  60  includes control logic for determining if the threshold pressure has been attained. In the event that the threshold pressure has not been attained, process  200  may return to step  210 , where the control module  60  continues to monitor the pressure transducer  54 . In the event the threshold pressure has been attained, process  200  may then proceed to step  214 . In step  214 , a normally closed solenoid valve  56  is opened. The ammonia gas and carbon dioxide may then enter the exhaust gas conduit  14 . Process  200  may then terminate. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the application.