Abstract:
A system for storage and dosing of ammonia includes solid ammonia storage material capable of binding and releasing ammonia reversibly by adsorption/absorption. The system includes a start-up storage unit and a main storage unit, both of which hold ammonia storage material. A start-up heating device is arranged to heat the start-up storage unit to generate gaseous ammonia by thermal desorption from the solid storage material. A main heating device arranged to heat the main storage unit to generate gaseous ammonia by thermal desorption from the solid storage material. A controller modulates operation of the heating devices such that the main and start-up heating devices are not simultaneously activated.

Description:
BACKGROUND 
     Selective catalytic reduction (SCR) is commonly used to remove NO x  (i.e., oxides of nitrogen) from the exhaust gas produced by internal engines, such as diesel or other lean burn (gasoline) engines. In such systems, NO x  is continuously removed from the exhaust gas by injection of a reductant into the exhaust gas prior to entering an SCR catalyst capable of achieving a high conversion of NO x . 
     Ammonia is often used as the reductant in SCR systems. The ammonia is introduced into the exhaust gas by controlled injection either of gaseous ammonia, aqueous ammonia or indirectly as urea dissolved in water. The SCR catalyst positioned in the exhaust gas stream causes a reaction between NO x  present in the exhaust gas and a NO x  reducing agent (e.g., ammonia) to reduce/convert the NO x  into nitrogen and water. 
     In many applications, such as SCR systems for vehicles, for example, the storage of ammonia in the form of a pressurized liquid in a vessel may be too hazardous and a storage method involving absorption in a solid may circumvent the safety hazard of anhydrous liquid ammonia. For example, metal ammine salts are ammonia absorbing materials, which can be used as solid storage media for ammonia, which in turn, for example, may be used as the reductant in SCR to reduce NO x  emissions from internal combustion engines in vehicles, see e.g., U.S. Pat. No. 8,088,201 and WO 1999/01205. The ammonia can be released from the ammine salts through thermal desorption, e.g., by external heating of a storage container, see e.g., id. and U.S. Patent App. Pub. NO. 2010/0086467. The ammonia is released from an either adsorptive or absorptive solid storage medium, among others Sr(NH 3 ) 8 Cl 2  or Ca(NH 3 )Cl 2  in granular form, in a storage container and temporarily stored as a gas in a buffer volume. The amount of ammonia to be supplied to a reaction volume in the vehicle&#39;s exhaust system is dosed under the control of an electronic controller according to the current operating state of the engine. 
     In vehicular applications, the SCR system typically includes one or more main storage units and a smaller start-up storage unit. Heating devices are arranged to heat the main storage units and start-up storage unit separately to generate gaseous ammonia by thermal desorption. The amount of ammonia to be desorbed from the storage medium can, for example, be controlled by a feed-back control in which the pressure in the storage container is measured by a pressure sensor. The heater can be cycled on and off to maintain the pressure in the storage container at or near a target pressure. The start-up storage unit is generally much smaller than the main storage units so that it reaches the pressure threshold more rapidly than the main storage units and, accordingly, can begin supplying gaseous ammonia to the SCR system in a shorter period of time. However, due to its limited storage capacity, the start-up storage unit is not suitable as a long-term source of ammonia for the SCR system. 
     Simultaneously operating the heating units for the start-up storage unit and one or more main storage units may place excessive loads on the vehicle&#39;s electrical system. Accordingly, it is desirable to provide a method for modulating operation of the main and start-up storage units to reduce the load placed on the vehicle&#39;s electrical system. 
     SUMMARY 
     Aspects and embodiments of the present technology described herein relate to one or more systems and methods for release of stored ammonia. 
     According to at least some embodiments of the present technology, a system is provided for storage and dosing of ammonia. The system is of the type that includes solid ammonia storage material capable of binding and releasing ammonia reversibly by adsorption/absorption. The system includes a start-up storage unit and a main storage unit, both of which hold ammonia storage material. A start-up heating device is arranged to heat the start-up storage unit to generate gaseous ammonia by thermal desorption from the solid storage material. A main heating device arranged to heat the main storage unit to generate gaseous ammonia by thermal desorption from the solid storage material. A controller modulates operation of the heating devices such that the main and start-up heating devices are not simultaneously activated. 
     According to at least some embodiments, the system includes a dosing valve arranged to control ammonia flow from the storage units to a consuming system. The controller is configured to control operation of the dosing valve according to a demand. 
     At least some embodiments can include at least one one-way valve interconnecting the main storage unit with the start-up storage unit. The one-way valve prevents back-flow of ammonia from the start-up storage unit to the main storage unit. 
     According to at least some embodiments of the present technology, the controller is configured to monitor the pressure in the start-up storage unit and activate the start-up heating unit until the pressure in the start-up storage unit reaches a first pressure threshold. The controller can further be configured to turn off the start-up heating unit until the pressure in the start-up storage unit drops below a second pressure threshold. 
     Some embodiments of the present technology are directed a method for storing and dosing of ammonia from a solid ammonia storage material capable of binding and releasing ammonia reversibly by adsorption/absorption. The method includes providing a main storage unit and a start-up storage unit that hold ammonia storage material. The method also includes heating the start-up storage unit until the pressure in the start-up storage unit reaches a first pressure threshold and thereafter heating the main storage unit while simultaneously not heating the start-up storage unit until either the pressure in the start-up storage unit falls below a second pressure threshold or the pressure in the main storage unit reaches a third pressure threshold. According to at least some embodiments, if the pressure in the start-up storage unit falls below the second pressure threshold before the pressure in the main unit reaches the third pressure threshold, the method modulates operation of the start-up and main heating units until the pressure in the main storage unit reaches a third pressure threshold. Further, according to at least some embodiments, once the pressure in the main storage unit reaches the third pressure threshold, the method controls heating of the main storage unit to maintain the pressure in the main storage unit between the third pressure threshold and a fourth pressure threshold which is lower than the third threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an exemplary ammonia storage and dosing system according to at least one embodiment of the present technology. 
         FIG. 2  is a schematic illustration of an exemplary ammonia storage and dosing system according to at least one embodiment of the present technology, with the main storage unit being composed of a plurality of sub-units connected in parallel. 
         FIG. 3  is a schematic illustration of an exemplary ammonia storage and dosing system according to at least one embodiment of the present technology, with the main storage unit being composed of a plurality of serially connected sub-units. 
         FIG. 4  is a flow diagram of an exemplary method for determining modulating power according to at least one embodiment of the present technology. 
         FIGS. 5A and 5B  are graphs illustrating an exemplary power modulation strategy according to at least one embodiment of the present technology. 
     
    
    
     DETAILED DESCRIPTION 
     Various examples of embodiments of the present technology will be described more fully hereinafter with reference to the accompanying drawings, in which such examples of embodiments are shown. Like reference numbers refer to like elements throughout. Other embodiments of the presently described technology may, however, be in many different forms and are not limited solely to the embodiments set forth herein. Rather, these embodiments are examples representative of the present technology. Rights based on this disclosure have the full scope indicated by the claims. 
       FIG. 1  is a schematic diagram illustrating an embodiment of an ammonia storage and dosing system  10  according to at least one embodiment of the present technology. The ammonia storage and dosing system  10  includes a main storage unit  12  and a start-up storage unit  14 . In the example of  FIG. 1 , the main storage unit  12  is made up of a single storage container which holds ammonia storage material. In other embodiments (see, e.g.,  FIGS. 2 and 3 ), the storage material of the main storage unit  12  is held in more than one container. The start-up storage unit  14  can be relatively small compared to the main storage unit  12 , to facilitate rapid start up. The number and the size of the other containers that make up the main storage unit  12  can vary in accordance with design and performance parameters, including for example, the desired total ammonia amount reserves and the start-up time of the main storage unit. The coupling of several storage containers can be performed in a variety of different ways in combination with passive one-way valves, active valves, pressure sensors, pressure switches etc. Additional details and considerations regarding the size and number of storage units are provided in U.S. Patent Application Pub. No. 2010/0086467 (the “467 Publication”), the disclosure of which is hereby incorporated in its entirety. 
     The storage units  12 ,  14  are fluidly connected to a dosing valve  18  by which ammonia from the storage units  12 ,  14  is dosed according to a demand to an ammonia consuming process or system  22 , such as a selective catalytic reduction (SCR) system, is in the desired proportion. An electronic control unit  20  controls operation of the dosing valve  18  to control delivery of ammonia from the storage and dosing system  10  to consuming system  22   
     Heating devices  26 ,  28 , such as electrical heaters, are provided for heating the main storage unit  12  and the start-up storage unit  14 , respectively. The heating units  26 ,  28  can be placed inside the main storage unit  12  and the start-up storage unit  14  containers. The ECU  20  is operable to control (e.g., switch on or off, and/or regulate) the heating devices  26 ,  28  independently from each other, e.g., by controlling the power supplied to them. 
     Pressure sensors  30 ,  32  monitor the pressure in the main and start-up storage units  12 ,  14 , respectively and produce signals indicative of the sensed pressures. The ECU  20  is coupled to the pressure sensors  30 ,  32  for receipt of the pressure signals. The ECU  20  is also configured to control the dosing valve  18 , e.g., by providing it with a variable dosing target value (for example, the dosing target value prescribes a certain degree of opening of the dosing valve). 
     According to at least some embodiments, the main storage unit  12  and the start-up storage unit  14  can be fluidly coupled by a valve. In some embodiments, the valve can be a passive, one-way valve  36 . The one-way valve  36  closes when the pressure downstream of it (i.e., the pressure in the start-up storage unit  14 ) is higher than that upstream of it (i.e. lower than the pressure in the main storage unit  12 ), and opens when the upstream pressure becomes higher than the down-stream pressure. Accordingly, the one-way valve  36  allows the main storage unit  12  to resaturate the smaller (rapid) start-up storage unit  14  with ammonia, e.g., in situations where the system  10  is powered off (because the pressure in the main storage unit  12  will be higher than that in the start-up storage unit  14 , when the main storage unit  12  is more saturated), or where heating of the start-up storage unit  14  has ceased while the main storage unit  12  continues to be heated. This increases the likelihood that the smaller, start-up storage unit, is available to make a rapid start-up. At the same time the one-way valve  36  prevents ammonia from being introduced into the main storage unit  12  from the smaller start-up storage unit  14  when the pressure in the latter is higher, particularly during start-up. 
     In the embodiment illustrated in  FIG. 1 , the ammonia storage and dosing system  10  is used to supply reductant (i.e., ammonia) for selective catalytic reduction (SCR) of NO x  in the exhaust emitted by an internal combustion engine  40 . The ECU  20  (or another controller) controls delivery of ammonia from the storage and dosing system  10  and into the exhaust system  42  through the dosing valve  18 . The dosing valve  18  is positioned in the exhaust system  42  upstream from a catalyst  44 . As the ammonia is injected into the exhaust system  42 , it mixes with the exhaust gas and this mixture flows through the catalyst  44 . The catalyst  44  causes a reaction between NO x  present in the exhaust gas and a NO x  reducting agent (e.g., ammonia) to convert the NO x  into nitrogen and water which then passes out of the tailpipe  48  and into the environment. While the system  10  has been described in the context of SCR for engine exhaust, it will be appreciated that the system could be used to supply ammonia in other applications, such as ammonia used as an energy carrier for a fuel cell or ammonia used as a reactant or additive in a chemical reaction, as described in greater detail in the aforementioned 467 Publication. 
       FIGS. 2 and 3  illustrate exemplary ammonia storage and dosing systems  10 B,  10 C where the main storage unit  12  is composed of a plurality of sub-units  12   a ,  12   b ,  12   c . In  FIG. 2 , the sub-units  12   a ,  12   b ,  12   c  are connected in parallel, while in  FIG. 3 , the sub-units are serially connected. The embodiments of  FIGS. 2 and 3  use many components that are the same or similar to the components described above in connection with  FIG. 1 . Accordingly, like reference numbers have been used to identify similar components and the systems of  FIGS. 2 and 3  will only be briefly described. Regarding the other features, reference is made to the detailed description of  FIG. 1  above, which also applies to the embodiments of  FIGS. 2 and 3 . 
     In both the embodiments of  FIGS. 2 and 3 , the main storage unit  12  is composed of a plurality (three in the illustrated examples) of sub-units  12   a ,  12   b ,  12   c , each of which has respective heating devices  26   a ,  26   b ,  26   c . The ECU  10  can individually and selectively operate the heating units  26   a ,  26   b ,  26   c  to control heating of the individual sub-units  12   a ,  12   b ,  12   c.    
     In the embodiment of  FIG. 2 , the sub-units  12   a ,  12   b ,  12   c  are fluidly connected in parallel. Each sub-unit  12   a ,  12   b ,  12   c  is equipped with its own passive one-way valve  36   a ,  36   b ,  36   c . The up-stream side of each one-way valve  36   a ,  36   b ,  36   c  is connected with the ammonia outlet  50   a ,  50   b ,  50   c  of a respective sub-unit  12   a ,  12   b ,  12   c . The downstream sides of all the one-way valves  36   a ,  36   c ,  36   d  are joined together in a point of parallel coupling  52 . Pressure sensors  30   a ,  30   b ,  30   c  monitoring the pressure in each of the respective sub-units  12   a ,  12   b ,  12   c  and deliver responsive pressure signals to the ECU  20 . 
     The parallel connection arrangement of  FIG. 2  enables selective depletion of individual ones of the sub-units  12   a ,  12   b ,  12   c , by only heating the sub-unit(s) to be depleted. The parallel connection and the one-way valves  36   a ,  36   b ,  36   c  ensure that a sub-unit  12   a ,  12   b ,  12   c  that is depleted to a greater extent than another sub-unit is not resaturated on the other sub-unit&#39;s expense. However, resaturation of the start-up storage unit  14  is governed by that sub-unit from among all the sub-units  12   a ,  12   b ,  12   c  that is least depleted (because it will produce the highest pressure). This, in turn, enables the start-up functionality to be particularly safe and long-running. By the use of passive one-way valves  36   a ,  36   b ,  36   c , this function can be achieved automatically without the need to actively switch between the sub-units  12   a ,  12   b ,  12   c.    
       FIG. 3  is a diagram similar to  FIG. 2 , except that the sub-units  12   a ,  12   b ,  12   c  are connected serially. Each sub-unit  12   a ,  12   b ,  12   c  is equipped to a passive one-way valve  36   a ,  36   b ,  36   c . The upstream side of each one-way valve  36   a ,  36   b ,  36   c  is connected with the ammonia outlet  50   a ,  50   b ,  50   c  of its respective sub-unit  12   a ,  12   b ,  12   c , and with the downstream side of the one-way valve  36   a ,  36   b ,  36   c  of the sub-unit preceding in the direction of flow (however, the one way-valve  36   a  of the most upstream sub-unit  12   a  is not connected to a one-way valve of a preceding sub-unit, because there is no preceding sub unit; similarly, the downstream side of the one-way valve  36   a  of the most downstream sub-unit  12   a  is not connected to the upstream side of a subsequent sub-unit&#39;s one-way valve, because there is no subsequent sub-unit). The down-stream side of the one-way valve  36   a  of the most downstream sub-unit  12   a  forms the outlet of the main storage unit  12  and is connected to the dosing valve  18 . As in  FIG. 1 , there is no pressure sensor upstream the (most downstream) one-way valve  36   a.    
     As with  FIG. 2 , such an arrangement also enables selective depletion of individual ones of the sub-units  12   a ,  12   b ,  12   c , by only heating the sub-unit(s) to be depleted. However, while in  FIG. 2  the sub-units are equitable, and the order of depleting the sub-units can be freely chosen, the serial connection of  FIG. 3  establishes a sequential order between the sub-units  12   a ,  12   b ,  12   c . In order to achieve the functionality mentioned above (selective depletion without resaturation of sub-units of the main storage unit, but with resaturation of the start-up storage unit), the sub-units can be heated sequentially, from the upstream to the downstream sub-units (i.e. from  12   c  to  12   b  to  12   a ). The one-way valves  36   a ,  36   b ,  36   c  between the sub-units  12   a ,  12   b ,  12   c  ensure that a sub-unit (e.g.  12   c ) that is already depleted to a greater extent than the downstream sub-unit(s) (e.g.  12   a  and  12   b ) is (are) not resaturated on the other sub-units&#39; expense. However, resaturation of the start-up storage unit  14  is governed by the downstream sub-unit(s) (e.g.  12   b  and  12   a ). This, in turn, enables the start-up functionality to be particularly safe and long-running. Again, by the use of passive one-way valves  36   a ,  36   b ,  36   c , this functionality can be achieved automatically, without any actively performed switching between the sub-units  12   a ,  12   b ,  12   c . Using an arrangement like the one showed in  FIG. 3  gives the possibility of exploiting ammonia that would otherwise be unusable. The last fraction of ammonia in a storage unit (e.g.  12   c ) will not be able to maintain dosing pressure under normal operating conditions, and hence the system will change to the next storage unit in the series (e.g.  12   b ). While the system is depleting ammonia from a downstream sub-unit (e.g.  12   b ), an upstream sub-unit (e.g.  12   c ) can be heated up again. Although it usually cannot be used on its own for normal operation, it will still be possible to withdraw more of the ammonia in the upstream sub-unit (e.g.  12   c ), hereby increasing the storage capacity of the system; the ammonia released will then act as a supplement to the ammonia from the downstream sub-unit (e.g.  12   b ). In some cases the ammonia released from the upstream sub-unit (e.g.  12   c ) will be used to resaturate the down-stream sub-unit (e.g.  12   b ). 
       FIG. 4  is a flow chart illustrating at least one embodiment of a method  400  for operating an ammonia storage and distribution system of the types described above. Certain aspects of the present technology relate to a method for modulating activation of multiple heating elements in an ammonia storage and distribution system, particularly during start up in order to quickly raise the pressure the start-up storage unit and maintain operating pressure in the start-up storage unit while charging at least one of the main storage units during period. 
     The method begins in step  405 . Control is then passed to the step  410  where the method monitors for the occurrence of a start-up condition. A start-up condition can, for example, be indicated by a request to provide ammonia dosing when the system  10 ,  10 B,  10 C has been inactive for a predetermined period of time or when the pressure in all of the storage containers is below a required delivery threshold, for example. 
     When a start-up condition occurs, control is passed to step  415  where the method deactivates the main heating unit  26  and activates start-up heating unit  28 . Control is then passed to step  420  where the method checks to see if the pressure P S  in the start-up storage unit has reached a first preselected pressure threshold P 1 . The method  400  continues to heat the start-up storage unit  14  until its pressure P S  reaches the first pressure threshold P 1 . 
     Once pressure in the start-up storage unit rises to the first pressure threshold P 1 , control is passed to step  425 . In step  425 , the method  400  deactivates the start-up heating unit  28  and activates the main heating unit  26 . 
     Control is then passed step  430 , where the method checks to determine if the pressure P S  in the start-up storage unit  14  has dropped below a second pressure threshold P 2 . If the pressure P S  in the start-up storage unit  14  has dropped below a second pressure threshold P 2 , control is returned to step  415 , which causes deactivation of the main heating unit  26  and activation of the start-up heating unit  28 . The method  400  then continues from step  415  in the manner described above. 
     If, in the step  430 , the method  400  determines that the pressure P S  in the start-up storage unit  14  has not dropped below the second pressure threshold P 2 , control is passed to step  440 . In step  440 , the method  400  checks to determine if the pressure P M  in the main storage unit  12  has reached a third predetermined pressure threshold P 3 . If it has not, control is returned to step  430 . 
     The method continues to loop through steps  430  and  440  until either the pressure P S  in start-up storage unit  14  drops below the second pressure threshold P 2 , or the pressure P M  in the main storage unit  12  rises above the third pressure threshold P 3 . As will be appreciated, during this time, the start-up heating unit  28  is inactive and the main heating unit  26  is active. If the pressure P S  in the start-up storage unit  14  drops below the second pressure threshold P 2  before the pressure P M  in the main storage unit  12  rises above the third threshold P 3 , control is returned to step  415 , which causes deactivation of the main heating unit  26  and activation of the start-up heating unit  28 . The method then continues from step  415  in the manner described above. Conversely, if the pressure P M  in the main storage unit  12  rises above the third pressure threshold P 3  before pressure in the start-up storage unit falls below the second pressure threshold P 2 , control is passed to the step  445 , where operation is switched to the main mode. For example, the method can set a software flag to indicate that the system has transitioned from the start-up mode to the main mode. 
     Control is then passed to step  450 , where the method checks for a shut-down signal, which is used to indicate that the system  10  is to be deactivated. For example, the system  10  can be deactivated when the consuming system  22  is not operating. In this respect, when the consuming system  22  is an SCR system for an engine, the system  10  can be deactivated when the engine is not running. The shut-down signal can also be used to deactivate the system  10  when one of the storage units  12 ,  14  needs to be replaced or refilled, for example. If a shut-down signal is detected, control is passed to step  455  where the main and start-up heating devices  26 ,  28  are both deactivated. 
     Otherwise, control is passed to step  460 , where the method determines if the pressure P M  in the main storage unit exceeds a fourth pressure threshold P 4 . If the pressure P M  in the main storage unit exceeds the fourth pressure threshold P 4 , control is passed to step  465  where the method deactivates the main heating unit  26 . Control is then returned to step  450 . 
     Conversely, if the pressure P M  in the main storage unit does not exceed the fourth pressure threshold P 4 , control is passed to step  470 . In step  470  the method determines if the pressure P M  in the main storage unit  12  has dropped below the third fourth pressure threshold P 3 . If the pressure P M  in the main storage unit  12  is below the third fourth pressure threshold P 3 , control is passed to step  475 , where the method activates the main heating unit  26 . Control is then returned to step  450 . 
     Accordingly, once the method transitions from the start-up mode to the main mode, the method continues to loop through steps  450 - 470  until a deactivation signal is detected in step  450 . As the method loops through steps  450 - 470 , it modulates the main heater on and off to maintain the pressure in the main storage unit  12  between the third and fourth pressure thresholds P 3 , P 4 . 
     With reference to  FIGS. 5A and 5B , the above method and system operate to initially raise the pressure P S  in the start-up storage unit  14  to the first pressure threshold P 1  by activating the start-up heating unit  28 . In the illustrated embodiment, this occurs between time T=0 and T=200. It will be appreciated that the times reflected in  FIGS. 5A and 5B  are merely provided for illustration purposes. Once the pressure P S  in the start-up storage unit  14  reaches the first pressure threshold P 1 , operation of the start-up heating unit  28  is modulated on and off to maintain the pressure P S  in the start-up storage unit between the first and second pressure thresholds P 1 , P 2 . The first and second pressure thresholds are selected to maintain the pressure P S  in the start-up storage unit  14  around its activation pressure. 
     Once the pressure P S  in the start-up storage unit  14  reaches the first pressure threshold P 1  (e.g., at T=200 in the illustrated example), the start-up heating unit  28  is turned off and the main heating unit  26  is turned on. The main heating unit  26  remains active until either the pressure P S  in the start-up storage unit  14  drops below the second pressure threshold P 2  or the pressure P M  in the main storage unit  12  rises above the third pressure threshold P 3 . 
     If the pressure P S  in the start-up storage unit  14  falls below the second pressure threshold P 2  before the pressure P M  in the main storage unit  12  reaches the third pressure threshold P 3  (e.g., at about T=400 in the illustrated example), then the main heating unit  26  is deactivated and the start-up heating unit  28  is reactivated. 
     During the start-up mode, the main and start-up heating units  26 ,  28  are modulated on and off in the above manner unit the pressure P M  in the main storage unit  12  reaches the third pressure threshold P 3 . Once the pressure P M  in the main storage unit  12  reaches the third pressure threshold P 3  (e.g., at about T=750 in the illustrated example), the system/method transitions from the start-up mode to the main mode. In the main mode, the start-up heating unit  28  remains off, while the main heating unit  26  is modulated on and off to maintain the pressure P M  in the main storage unit  12  between the third and fourth pressure thresholds P 3 , P 4 . The third and fourth pressure thresholds P 3 , P 4  are selected to maintain the pressure P M  in the main storage unit  12  around its activation pressure. 
     Accordingly, during the start-up mode, the start-up heating unit  28  is controlled to initially raise pressure  PS  in the start-up storage unit  14  to its activation pressure. Once the activation pressure is reached, the start-up heating unit  28  is modulated on and off to maintain pressure  PS  in the start-up storage unit  14  at or about its activation. When the start-up heating unit  28  is cycled off, the main heating unit  26  is cycled on to gradually raise the pressure  PM  in the main storage unit  12  towards its activation pressure. During the start-up mode, the main and start-up heating units  26 ,  28  are modulated to maintain the start-up storage unit  14  around its activation pressure, while raising the pressure  PM  in the main storage unit  12  during the times that the start-up heating unit  28  is off. Accordingly, the pressure in the start-up heating unit  28  can be quickly raised to the activation pressure so that the system can begin supplying reductant from the start-up storage unit  14 . Once the pressure  PM  in the main storage unit  12  reaches its activation pressure, the system/method transitions to the main mode where reductant is supplied from the main storage unit  12 . During the main mode, the start-up heating unit  28  remains off, while the main heating unit  26  is modulated on and off to maintain the pressure in the main storage unit  12  at or about its activation pressure. Because the heating units  26 ,  28  are never active at the same time, the power requirements, e.g., from a vehicle power system, can be reduced. 
     While this disclosure has been described as having exemplary embodiments, this application is intended to cover any variations, uses, or adaptations using the general principles set forth herein. It is envisioned that those skilled in the art may devise various modifications and equivalents without departing from the spirit and scope of the disclosure as recited in the following claims. Further, this application is intended to cover such departures from the present disclosure as come within the known or customary practice within the art to which it pertains. While this disclosure has been described as having exemplary embodiments, this application is intended to cover any variations, uses, or adaptations using the general principles set forth herein. It is envisioned that those skilled in the art may devise various modifications and equivalents without departing from the spirit and scope of the disclosure as recited in the following claims. Further, this application is intended to cover such departures from the present disclosure as come within the known or customary practice within the art to which it pertains.