Abstract:
A liquid reductant dosing module for a combustion exhaust system is disclosed comprising an enclosed reservoir comprising a top, a bottom, one or more sides, an inlet, and an outlet. A resistive wire rod heater is disposed in that reservoir, comprising a hollow member extending along a first axis between the bottom of the reservoir and the top of the reservoir. Along that member there is disposed resistive metal wire such that when electric current is applied to the resistive metal wire, a greater portion of electric power is distributed as heat proximate to the bottom of the reservoir than is distributed as heat proximate to the top of the reservoir.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to a reservoir for a fluid dosing system. More specifically, the invention relates to a reservoir for holding a reducing agent for introduction into a combustion exhaust gas. 
         [0002]    The emission of nitrogen oxide (NO x ) compounds in engine exhausts has long been the focus for health professionals and regulatory agencies worldwide. In many locations, regulations require stringent reductions of NO x  levels in new equipments. NO x  emissions may be found in a variety of systems such as internal combustion engines, gas turbine exhaust, lean burn engines, industrial boilers, process heaters or other process streams. 
         [0003]    In order to reduce NO x  emissions, it is known to use a selective catalytic reduction (SCR) device to treat an exhaust flow and to significantly reduce NO x  emissions. In an SCR system a reducing agent, for example urea solution, is dosed into the exhaust gas flow upstream of an SCR catalyst. This reducing agent is then usually reacted in the presence of a catalyst downstream of the injection point in an SCR device. Within the SCR device NO x  compounds are then reduced to nitrogen. WO2004111401 discloses such a device. 
         [0004]    The general operation of an SCR device is shown in  FIG. 1 , in which a diesel engine  1  produces an exhaust flow comprising various exhaust gases  3 . The exhaust gases are conveyed through an exhaust system, indicated generally at  5 , comprising an oxidation catalyst device  7 , a selective reduction catalyst device  9  and a slip catalyst  11 . 
         [0005]    The oxidation catalyst device  7  is a flow through device that consists of a canister containing a honeycomb-like structure or substrate. The substrate has a large surface area that is coated with an active catalyst layer. This layer contains a small, well dispersed amount of precious metals such as platinum or palladium. As the exhaust gases traverse the catalyst, carbon monoxide, gaseous hydrocarbons and liquid hydrocarbon particles (unburned fuel and oil) are oxidized, thereby reducing harmful emissions. 
         [0006]    The SCR device  9  performs SCR treatment of NO x  using ammonia derived from a source of urea as a chemical reductant. A slip catalyst  11  may be located downstream of the SCR device  9  to clean up any unreacted ammonia. 
         [0007]    Urea for the SCR device  9  is stored in a tank  13  which is in fluid communication with the exhaust system  5 . A pump  15  is provided to pump urea from the tank  13  to the exhaust system  5 . The supply of urea is controlled by a control unit  17 , for example the engine control unit, which receives engine speed and other engine parameters from the engine  1 . An injection device  19  (also referred to herein as a fluid dosing device) is used to inject the urea into the exhaust flow. 
         [0008]    As a 32.5% urea solution freezes at −11.5° C., urea delivery systems must be adapted for delivery of liquid urea to the vehicle exhaust system under conditions that would normally cause the liquid urea to freeze. One solution would be to simply heat the storage tank  13 . However, this can require substantial quantities of energy to maintain the entire storage tank  13  in a liquid state and can take significant time to thaw if the tank has become completely frozen. An alternative arrangement is to place a smaller reservoir downstream of the storage tank that can be unfrozen quickly and/or maintained as liquid more efficiently since it contains a smaller amount of liquid urea. Such an arrangement is shown in  FIG. 2 , with urea flowing from storage tank  23  into dosing reservoir  33  through inlet  25 , and then pumped out of dosing reservoir  33  by pump  15  through outlet  27  from where it flows to the exhaust stream as shown in  FIG. 1 . Alternatively, the dosing reservoir can be positioned adjacent to and in physical contact with the storage tank or even inside the storage tank so that heat from the heated dosing reservoir during prolonged periods of operation will help thaw the storage tank or maintain it in a liquid state. 
         [0009]    In any case, a liquid reductant reservoir (whether it is storage tank  13  or smaller dosing reservoir  33 ) will need to be heated in order to provide liquid reductant to the exhaust system. One proposed approach has been to use a submerged ceramic PTC heater in the reservoir. However, this approach provides heat at the bottom of the reservoir, but not at the top. As liquid reductant is dosed into the exhaust system, the level in the reservoir drops, resulting in a cavity of dead air space forming between the frozen reductant toward the top of the reservoir and a level of liquid underneath, which can result in poor heat transfer to the remaining frozen urea. Further, as the liquid urea level continues to drop, the ceramic PTC heater element may itself become exposed to air, at which point its self-regulating heating function will result in restricted power to the heater for further melting of frozen urea. 
         [0010]    An alternative heater approach is to use a vertical ceramic PTC rod heater that runs from the top to the bottom of the urea reservoir. This approach can provide good heating near the top of the reservoir, but may not heat the bottom quickly enough to provide liquid urea for exhaust treatment right after vehicle startup. Also, as the urea level drops during dosing, exposure of the top of the ceramic PTC rod heater to the air can result in power reduction due to the heater&#39;s self-regulating function. 
         [0011]    Therefore, there is a need in the art for providing heat to a reservoir for use in a dosing system that addresses the above mentioned problems. 
       SUMMARY OF THE INVENTION 
       [0012]    Therefore, according to the present invention, there is provided a dosing module for a combustion exhaust system comprising an enclosed reservoir comprising a top, a bottom, one or more sides, an inlet, and an outlet. A resistive wire rod heater is disposed in that reservoir, comprising a hollow member extending along a first axis between the bottom of the reservoir and the top of the reservoir. Along that member there is disposed resistive metal wire such that when electric current is applied to the resistive metal wire, a greater portion of electric power is distributed as heat proximate to the bottom of the reservoir than is distributed as heat proximate to the top of the reservoir. 
         [0013]    The above-described dosing module provides greater heat at the bottom of the reservoir for rapid melting of liquid reductant to be dosed to an exhaust system while still providing some power (or at least conductance of heat) to the top of the reservoir to melt any residual frozen reductant. These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0015]      FIG. 1  depicts a known SCR system. 
           [0016]      FIG. 2  depicts a known reservoir configuration for dosing liquid reductant as part of an SCR system. 
           [0017]      FIG. 3  shows an exemplary liquid reductant dosing module according to the invention with a heat conductor attached at one end of the heater. 
           [0018]      FIGS. 4A and 4B  show an exemplary heater and pickup tube assembly for a liquid reductant dosing module according to the invention with a heat deflector assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Referring now to the Figures, the invention will be described with reference to specific embodiments, without limiting same. Turning now to  FIG. 3 , there is shown dosing reservoir  33  having a heater therein according to an exemplary embodiment of the invention. The heater is shown in cut-away fashion in  FIG. 3  with hollow tube  32  having windings of resistive metal wire  39  thereon. The resistive metal wire may be on either the inner surface or the outer surface of tube  32 . In one exemplary embodiment, it is on the inner surface, which may provide protection against damage to the wire. The tube  32  may be of any material that is compatible with the reductant composition in the reservoir and the temperature of the wire windings  39  when heated, although in one exemplary embodiment it is a heat-conductive metal such as stainless steel. The resistive metal wire  39  may be of any metal having a resistance in the range that renders it useful for a heating element in this application, which in an exemplary embodiment is in the range of 1 ohm to 1.5 ohm. The total length of the resistive metal wire  39  depends on the configuration of the reservoir and environmental conditions. In one exemplary embodiment, the total wire length ranges from 1 to 1.5 meters. 
         [0020]    The dosing reservoir also includes a liquid reductant pickup tube  31  for drawing liquid reductant from near the bottom of reservoir  33  and discharging out the top to the vehicle exhaust. Pickup tube  31  may be of a suitable material, and may be of the same material as hollow tube  32 , e.g., stainless steel. The pickup tube is shown in  FIG. 3  as being inside the hollow tube  32 , which can help to ensure that it is quickly thawed by the heat from the resistive metal wire  39 , but the pickup tube  31  can also be positioned adjacent to the tube  32  or anywhere else in the reservoir  33 . 
         [0021]    The resistive metal wire  39  is configured so that when electric current is applied to it, a greater portion of electric power is distributed as heat proximate to the bottom of the reservoir than is distributed as heat proximate to the top of the reservoir. One way this can be accomplished is by placing a greater density of metal wire toward the bottom of the tube  32  than toward the top of tube  32  and applying a voltage differential between an electrical leads  40  at opposite ends of the wire, shown in  FIG. 3  to both be at the top end of the tube  32 . In an exemplary embodiment of the invention, the distribution of electric power is broken down into multiple zones, and  FIG. 3  depicts three such zones: a low power density zone  34  proximate to the top of tube  32  where the spacing of the courses as the wire winds along the tube  32  are relatively far apart, a high power density zone  36  proximate to the bottom of tube  32  where the spacing of the courses as the wire winds along the tube  32  are relatively close together, and a medium power density zone  35  in between zones  34  and  36  where the spacing of the courses as the wire winds along the tube  32  is between the spacing for zone  34  and the spacing for zone  36 . The proportion of the length of tube  32  taken up by each of the zones  34 ,  35 , and  36  may vary depending on design parameters. In one exemplary embodiment, each of zones  34 ,  35 , and  36  covers approximately one third of the total length of tube  32 . The distribution of power along the tube  32  can vary depending on the configuration of the reservoir, position of the heater, etc., but the power in zone  36  should be sufficiently high to rapidly melt frozen reductant upon a cold vehicle start while the power in zone  34  should be sufficiently low to avoid overheating when the level of liquid reductant in the reservoir drops before the storage tank melts sufficiently to refill it. In one exemplary embodiment it is 0-1 watts/in 2  of the tube surface area along the upper zone of tube  32 , 2.5-3.5 watts/in 2  along the middle zone of tube  32 , and 8-10 watts/in 2  along the bottom zone of tube  32 . In an embodiment where there is zero power distribution along the upper zone of tube  32 , the upper electric lead attaches at the interface of zones  34  and  34  instead of at the top of zone  34 , and the wire in zone  34  acts only as a conductor of heat generated in the powered portions of the wire. 
         [0022]    In an alternate exemplary embodiment of the invention, the power distribution profile along the tube  32  is achieved through the use of materials of different resistivities that have been electrically connected to one another. For example, in the three-zone embodiment shown in  FIG. 3 , the wire in zone  36  would have a higher resistivity than the wire in zone  35 , which would have a higher resistivity than the wire in zone  34 . In this embodiment, zone  34  could still be given zero power by positioning the upper lead at the interface of zones  34  and  34  and having the wire in zone  34  functioning only as a heat conductor, in which case its electrical resistivity is not relevant since it is not part of the electric circuit. In an exemplary implementation of this embodiment, the wire in zone  35  has a resistivity of 0.25 ohm-m to 0.5 ohm-m and the wire in zone  36  has a resistivity of 1.5 ohm-m to 2.5 ohm-m, and the wire in zone  34  has a resistivity of 0 ohm-m to 0.25 ohm-m. Resistivity may be varied by choosing different materials having different resistivities and/or varying the wire diameter. 
         [0023]    In yet another alternate exemplary embodiment, the power distribution profile along the tube  32  can be achieved through the use of separate control circuits for different zones along the tube  32 . In this embodiment, each zone may have its own set of electric leads connected to an independent controller and/or control circuit so that the power to each zone is directly and independently regulated by a controller and/or a control circuit to provide a greater distribution of power proximate the bottom of tube  32  than proximate the top of tube  32 . 
         [0024]    The various alternate embodiments for achieving a power distribution profile may be used independently or in combination. For example, sections of wire may have different winding densities and be made of different material, or zones of wire connected to independent control circuits may be made of materials having different resistivities, or all three embodiments may be used in combination. 
         [0025]    In yet another exemplary embodiment of the invention, the resistive metal wire  39  has a positive temperature coefficient of resistivity (TCR) that enables self-regulation of the temperature of the wire for a given voltage applied to it. Wire with a positive TCR will exhibit greater levels of resistivity at greater temperatures and lower levels of resistivity at lower temperatures. In this case, as temperature of the wire starts to rise, either because liquid reductant level in the reservoir has dropped causing a portion of the wire to be exposed to air or because the liquid reductant itself has been heated, the resistivity starts to rise so that for a given voltage, current flow through the wire is regulated and less heat will be generated. In an exemplary embodiment, the resistive metal wire  39  has a TCR ranging from 0.1 ohm-m/° C. to 0.15 ohm-m/° C. In an alternate embodiment, or in conjunction with the use of positive TCR wire, this type of self-regulation can be achieved and/or enhanced by controlling the current supplied to the resistive metal wire with a Wheatstone bridge control circuit to control current to the heater based on the resistance that it sees applied across the bridge. For example, when used with resistive metal wire that may have a positive TCR, but where the TCR is not large enough to provide self-regulation on its own, a Wheatstone bridge, which will see increasing resistivity of the resistive metal wire  39  as temperature of the wire increases, will reduce the current provided to the resistive metal wire  39 , thereby enhancing the temperature regulation of the wire. 
         [0026]    With continuing reference to  FIG. 3 , there is shown an exemplary embodiment where there is an annular-shaped heat conductor  38  surrounding tube  32 , having heat-conducting fins  37  extending radially therefrom. The heat conductor  38  and heat-conducting fins  37  can be made from any heat-conducting material such as aluminum or stainless steel. In an exemplary non-limiting embodiment, the heat conductor  38  and heat-conducting fins  37  are effective to reduce the effective power density in zone  36  to 0.5 to 1.5 watts/in 2 . Materials such as aluminum, which are susceptible to reaction with reductants like urea, may be have a thin layer to provide protection from the reductant, such as a polytetrafluoroethylene coating or a layer of anodized aluminum. 
         [0027]    The heated liquid reductant dosing reservoir according to the invention provides more heat toward the bottom of the reservoir where it is needed, and that heat can also be advantageously carried to other portions of the reservoir by convection. In some situations, however, because the pickup tube  31  extends below the bottom of tube  32 , it may be desirable to provide more heat below the bottom of the tube  32  to thaw or maintain liquid at the inlet of the pickup tube  31 . One way to provide such heat distribution is with a deflector. The deflector serves to deflect toward the reservoir bottom some of the convective heat transfer that would normally go from the bottom of the reservoir to the top of the reservoir, thus forcing more heat at the bottom near the inlet of pickup tube  31 . In one exemplary embodiment, the deflector may be a simple cup-shaped member with a hole in the bottom of the cup in which the tube  32  would be disposed with the cup-shaped member inversely mounted near the bottom of tube  32 . An alternative exemplary deflector design is shown in  FIGS. 4A and 4B . The deflector assembly is shown in an exploded view in  FIG. 4A , with bottom piece  44  having side openings  45 , bottom openings  46 , and central opening  47 , and a top piece having collar portion  43  adapted to fit around tube  32  and flange portion  42  having openings  41 . Openings  41 ,  45 , and  46  may optionally be fitted with a filter media such as a stainless steel mesh filter (not shown). The deflector is assembled with the tube  32  engaging bottom piece central opening  47  and top piece collar  43 . The upper edge of bottom piece  44  is joined to the outer periphery of the upper piece flange portion  42  by means known in the art, such as crimping, welding, gluing, and the like. 
         [0028]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.