Abstract:
An improved SCR system for controlling NOx levels in internal combustion engine exhaust, comprising a least one ammonia sensor disposed at an intermediate longitudinal location in an SCR catalyst and in communication with a System Control Module (SCM). The ammonia measurement permits calculation of ammonia storage on catalyst sites via a stored SCM algorithm. Locating the ammonia sensor midway in the catalyst allows for optimum control of NOx reduction and permits the portion of the catalyst downstream of the sensor to be treated as a slip catalyst, thus minimizing or eliminating the need for a second slip catalyst and housing, and reducing the size, volume, complexity, and cost of an SCR system. In-brick ammonia sensor permits the system to manage engine exhaust to a desired NOx conversion level and ammonia slip target value, thus minimizing the rate of consumption of ammonia while meeting required limits for NOx emissions.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to systems for controlling the level of nitrogen oxides (NOx) in internal combustion engine exhaust; more particularly, to such systems for catalytically reducing NOx to N 2  by reaction with ammonia; and most particularly, to an improved system having an ammonia sensor disposed within the catalyst for feedback control to manage exhaust levels of NOx and ammonia. 
       BACKGROUND OF THE INVENTION 
       [0002]    Reducing and controlling engine emissions of oxides of nitrogen are important considerations in modern internal combustion engines, both spark-ignited and compression-ignited. NOx emissions are an element of smog production, and emissions limits mandated by state governments and/or the federal government are likely to become even more stringent in the future. 
         [0003]    One known approach to reducing NOx emissions is to reduce NOx formation by reducing combustion temperatures, such as by recirculation of exhaust gas into the engine firing chambers to dilute the combustion mixture. Even under the best of control, however, untreated engine exhaust typically contains an unacceptable level of NOx. Thus, another approach is to strip NOx from the exhaust via one or more aftertreatment devices. 
         [0004]    Aftertreatment systems are known in the art which can convert NOx to elemental N 2  by selective catalytic reduction (SCR) in the presence of a suitable reductant, for example, ammonia (NH 3 ) in accordance with the following equations: 
         [0000]      NO+NO 2 +2NH 3 →2N 2 +3H 2 O   (Eq. 1) 
         [0000]      4NO+O 2 +4NH 3 →4N 2 +6H 2 O   (Eq. 2) 
         [0000]      2NO 2 +O 2 +4NH 3 →3N 2 +6H 2 O   (Eq. 3) 
         [0000]    Typically, ammonia is provided (“dosed”) to the catalyst via decomposition of urea (typically aqueous urea) in accordance with the following equations: 
         [0000]      CO(NH 2 ) 2  (aqueous)→CO(NH 2 ) 2 +H 2 O   (Eq. 4) 
         [0000]      CO(NH 2 ) 2 →NH 3 +HNCO   (Eq. 5) 
         [0000]      HNCO+H 2 O→NH 3 +CO 2    (Eq. 6) 
         [0000]    or a net reaction of: 
         [0000]      CO(NH 2 ) 2 +H 2 O→2NH 3 +CO 2    (Eq. 7) 
         [0005]    It will be recognized that specific molar match of ammonia to NOx is desired to convert all NOx while slipping no excess ammonia to atmosphere. In practice, this has proved to be very difficult to achieve. For example, in a prior art SCR system first and second catalyst bricks are required, typically disposed in two separate sequential housings. The first brick is designated as the SCR catalyst and the second brick is designated as the “slip” catalyst for oxidizing residual exhaust ammonia, which is known to happen due to one or more of three causes: 
         [0006]    First, excess ammonia in the tailpipe exhaust can be due to incomplete reaction of the SCR as shown in Eqs. 1-3. 
         [0007]    Second, in the SCR catalytic reaction mechanism, NOx reacts with ammonia stored on the catalyst. Ammonia storage capacity is highly dependent on temperature of the catalyst, with capacity at low temperatures being significantly greater than at higher temperatures. Because of this effect, even when dosing is greatly reduced or even stopped completely during hot exhaust transients, unreacted ammonia can be desorbed from the SCR catalyst and pass into the tailpipe exhaust. 
         [0008]    Lastly, dosing at low temperatures can lead to solid or liquid urea deposits in the exhaust system which, upon subsequent heating, can lead to additional ammonia release unaccounted for in the dosing control. 
         [0009]    The slip catalyst acts to oxidize excess ammonia, converting it into elemental nitrogen, per the following mechanism: 
         [0000]      4NH 3 +3O 2 →2N 2 +6H 2 O   (Eq. 8) 
         [0000]    Less desirably, excess ammonia may also be further oxidized back into NOx, per the following mechanisms: 
         [0000]      4NH 3 +5O 2 →4NO+6H 2 O   (Eq. 9) 
         [0000]      2NH 3 +2O 2 →N 2 O+3H 2 O   (Eq. 10) 
         [0010]    A prior art SCR system may further include, after the SCR or slip catalyst, an NOx sensor which is additionally cross-sensitive to ammonia, and the system is close-loop controlled to maintain slipped ammonia below a predetermined level by regulating the dosing rate of aqueous urea. 
         [0011]    Such a prior art SCR system has several shortcomings. First, significant inadequacies in the prior art require a slip catalyst element and volume of slip catalyst to offset lack of optimal dosing control. Second, due to hysteresis the system cannot be responsive to abrupt changes in NOx load or catalyst temperature, which occur frequently in actual engine usage. Third, when the sensor is placed after the slip catalyst, ammonia sensed by the ammonia sensor is by definition lost to atmosphere, as the system has no means for absorbing or oxidizing slipped ammonia after the sensor. 
         [0012]    What is needed in the art is an improved method and apparatus for determining and controlling the ammonia level resident in the exhaust leaving the SCR catalyst. 
         [0013]    What is further needed in the art is an improved method and apparatus for eliminating the need for a slip catalyst or minimizing slip catalyst volume as well as minimizing the amount of slip ammonia lost to atmosphere. 
         [0014]    It is a principal object of the present invention to optimize the consumption of urea in an SCR system to a targeted ammonia slip average and/or slip maximum while meeting required targets for NOx emissions. 
         [0015]    It is a still further object of the present invention to reduce the size, volume, complexity, and cost of an SCR system. 
       SUMMARY OF THE INVENTION 
       [0016]    Briefly described, an improved SCR system in accordance with the invention comprises a least one ammonia sensor disposed at an intermediate longitudinal location in an NOx-reducing SCR catalyst and in communication with an Engine Control Module (ECM). Locating the ammonia sensor within the catalyst allows for optimal NOx reduction and permits the downstream NOx catalyst to be treated as an effective slip catalyst, thus minimizing or eliminating the need for a second slip catalyst and housing, and reducing the size, volume, complexity, and cost of the SCR system. Continuous measurement of ammonia concentration in exhaust at an intermediate location in the SCR catalyst permits calculation of ammonia storage amounts on active catalyst sites throughout the catalyst brick, and thus permits the engine exhaust to be managed to a desired NOx conversion level and ammonia slip target value. Further, placing the ammonia sensor closer to the point of urea introduction reduces hysteresis in the SCR system, allowing faster response to NOx load changes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0018]      FIG. 1  is an elevational view, partially in cutaway, of the exhaust treatment catalysts in a prior art SCR; 
           [0019]      FIG. 2  is a schematic drawing of an SCR system improved in accordance with the present invention; 
           [0020]      FIG. 3  is a graphical representation of key factors in SCR control; and 
           [0021]      FIGS. 4   a,    4   b,  and  4   c  are elevational views, partially in cutaway, of three alternative embodiments of the improved SCR catalytic converter shown in  FIG. 2 . 
       
    
    
       [0022]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein are not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    The benefits and advantages of the present invention may be better appreciated by first considering a prior art SCR system. 
         [0024]    Referring to  FIG. 1 , a catalyst assembly  10  in a prior art SCR system comprises a Selective Catalytic Reduction (SCR) unit  14  and an optional ammonia slip catalyst  16  connected in sequence to treat raw engine exhaust  18  from an internal combustion engine  20 , especially a diesel or gas engine operated with excess oxygen in the exhausted gases, and to discharge treated engine exhaust  22  to atmosphere  24 . 
         [0025]    SCR  14  includes a first selective catalyst  30  disposed in a first housing  32  for selectively reducing NOx to N 2  in the presence of NH 3  and O 2 , as described above, in known fashion. An atomizing nozzle  34  prior to SCR  14  receives a reductant solution  38  containing urea and/or ammonia from a source  40  and sprays atomized reductant solution  42  into exhaust  18 . 
         [0026]    Slip catalyst  16 , a second selective catalyst, is disposed in a second housing  46  for receiving intermediate exhaust  48  containing excess ammonia which is oxidized by catalyst  16  to minimize the amount of ammonia slipped to atmosphere  24  in treated exhaust  22 . An exhaust gas sensor  50  sensitive to both NOx and NH 3  is disposed between first and second SCR housings  32 , 46  and monitors levels of NOx and NH 3  in intermediate exhaust  48  and communicates with a System Control Module  54  for feedback control of dosing rate of reductant solution  38  from source  40  based upon the sensed NOx levels in the exhaust. (Description and illustration of the storage and dosing apparatus is omitted for clarity.) An optional ammonia sensor  56  may be provided after slip catalyst  16  to monitor actual slip ammonia levels in treated exhaust  22 . 
         [0027]    Referring now to  FIG. 2 , an exemplary improved SCR system  100  in accordance with one aspect of the present invention comprises a dosing apparatus assembly  102 , a catalyst assembly  110 , and a system controller  154 . 
         [0028]    Dosing apparatus assembly  102  includes a tank  158  for storing reductant solution  38 , a high level sensor  160 , a temperature sensor  164 , and a solution heater  166  in communication with system controller  154  which may be an Engine Control Module. A supply line  168  leading from a solution filter  170  and containing a pressure sensor  172  is connected to catalyst assembly  110  at a solenoid pump/injector  174  for supplying reductant solution  38  to atomizer  134 . Preferably, supply line  168  is heated by an electric heater  176 . 
         [0029]    Some elements of catalyst assembly  110  are similar to their counterparts in prior art catalyst assembly  10 . Assembly  110  comprises a Selective Catalytic Reduction (SCR) unit  114  comprising both a selective NOx reduction first catalyst  130  and a second catalyst  116 . Catalysts  130 , 116  may be disposed in separate sequential housings as in the prior art but preferably are disposed in a common housing  132 , all elements being connected in sequence to treat raw engine exhaust  18  from an internal combustion engine  20 , especially a diesel engine, and to discharge treated engine exhaust  122  to atmosphere  24 . 
         [0030]    Preferably, catalysts  116 , 130  are provided as porous or channeled monoliths known in the art, and used herein, as “bricks”. 
         [0031]    Selective catalyst  130  is disposed in housing  132  for selectively reducing NOx to N 2  in the presence of NH 3  and O 2 , as described above. Atomizing nozzle  134  sprays atomized reductant solution  42  into exhaust  18 . 
         [0032]    (Note that an SCR system in accordance with the invention contemplates provision of ammonia from any source. For exemplary purposes, the present discussion refers only to urea as the source, and to liquid urea dosing as the means of introduction, but it should be understood that all other ammoniacal chemical reductants and apparatus for supplying them having the net effect of providing ammonia to the SCR catalysts are fully comprehended by the invention.) 
         [0033]    Second catalyst  116 , preferably also disposed in housing  132  and downstream of selective NOx catalyst  130 , receives treated exhaust which may contain slip ammonia and unreacted NOx. 
         [0034]    In a first embodiment (see  FIG. 4   a ), catalyst  116   a  is an extension of selective NOx catalyst  130   a,  and slip ammonia is controlled as described below. 
         [0035]    In a second and third embodiments (see  FIGS. 4   b  and  4   c ), second catalysts  116   b,   116   c  are selective NOx catalysts similar to catalysts  130   b,   130   c.  However, optionally, catalysts  116   b,   116   c  may also have ammonia-oxidizing capability like a prior art ammonia slip catalyst. In this configuration, excess ammonia is oxidized by catalysts  116   b,   116   c,  and NOx passing beyond catalysts  130   b,   130   c  is also reduced by catalysts  116   b,   116   c,  thereby minimizing the amount of ammonia and NOx slipped to atmosphere  24  in treated exhaust  122 . 
         [0036]    An important feature of the present invention is an ammonia sensor  180  disposed between first and second catalysts  130 , 116  and in communication with a System Control Module (SCM)  154  for feedback control of dosing rate of reductant solution from tank  158 . As used herein, the term “ammonia sensor” should be taken to mean any device capable of determining the ammonia content in exhaust gases. Note that prior art catalyst assembly  10  becomes a functional embodiment of the present invention when provided with an ammonia sensor  180  in place of NH 3 /NOx sensor  50  in accordance with the present invention, even though catalysts  30  and  16  are disposed in separate housings. 
         [0037]    Exemplary ammonia sensors are disclosed in the prior art; see, for example, U.S. Pat. No. 7,069,770 B2, and Published US Patent Application Nos. 20060200969 A1 and 20070080074 A1, the relevant disclosures of which are incorporated herein by reference. Such sensors are presented as only exemplary, and any other ammonia sensing device is fully anticipated by the invention. 
         [0038]    The present invention affords at least two distinct advantages. 
         [0039]    First, sensing ammonia rather than nitrogen oxides in the treated exhaust permits direct feedback flow control of ammonia-generating reductant via an algorithm programmed into SCM  154 . The algorithm factors in at least the exhaust temperature at the first catalyst inlet (temperature sensor  182 ), the exhaust temperature at the second catalyst outlet (temperature sensor  184 ), the flow rate of reductant, and the instantaneous concentration of exhaust ammonia, as well as various engine operating parameters, to calculate the new flow rate. 
         [0040]    Second, placing the ammonia sensor ahead of second catalyst  116  allows the algorithm also to calculate the instantaneous ammonia loading already in both catalyst bricks  116 , 130 , as a part of calculating a new reductant flow rate, and further allows ammonia-oxidizing catalysts  116   b,   116   c  to oxidize ammonia sensed by sensor  180 . Thus, the algorithm is able to minimize consumption of reductant and formation of atmospheric slip ammonia while maximizing conversion of NOx to N 2  in treated exhaust  122 . Note that an optional second ammonia sensor  186  after second catalyst  116  may also be included to monitor tailgas ammonia levels in treated exhaust  122  directly, to confirm proper operation of the control system  100 . 
         [0041]    Referring to  FIG. 3 , the sensing and control advantages of the present invention are shown. The actual concentrations of ammonia and NOx in tailgas exhaust  122  are shown as a function of the rate of urea dosing. As ammonia concentration (curve  188 ) increases, NOx concentration (curve  190 ) decreases. By severely overdosing the system with urea, the concentration of NOx can be reduced to near zero (SCR efficiency=100%) but with a large amount of slip ammonia being released to atmosphere. Use of an ammonia sensor permits correlation of curves  184 , 186  at all points such that actual levels of ammonia and NOx in exhaust  122  are known, and the system may be controlled to any desired value of either one. On the other hand, in prior art systems ( FIG. 1 ) herein a tailgas sensor  50  ( FIG. 3  curve  192 ) is sensitive to both NOx and NH 3 , the sensor output is the same for two values of the curve, e.g., points  194 , 196 , and therefore cannot distinguish whether to further increase or to decrease urea flow. 
         [0042]    A third advantage of the present invention is that both first catalyst  130  and second catalyst  116  may be disposed together in a single housing  132 , thus reducing the cost and volume of an SCR system. 
         [0043]    Referring to  FIGS. 4   a - 4   c,  various configurations of catalysts and ammonia sensor are possible within the scope of the present invention. In each case, the first and second catalysts  130 , 116  have NOx-reducing capability, and optionally second catalysts  116  may also have ammonia-oxidizing capability. 
         [0044]      FIG. 4   a  shows a single monolithic brick catalyst disposed in housing  132  and having a well  198  formed at an intermediate longitudinal location for receiving ammonia sensor  180 , which location defines portions of the brick upstream of sensor  180  as SCR catalyst  130   a  and portions downstream of sensor  180  as slip catalyst  116   a.    
         [0045]      FIG. 4   b  shows SCR catalyst  130   b  and slip catalyst  116   b  abuttingly disposed in housing  132  and having a well  198  formed at the abutting location for receiving ammonia sensor  180 . 
         [0046]      FIG. 4   c  shows SCR catalyst  130   c  and slip catalyst  116   c  sequentially disposed in housing  132  and having a gap  199  formed therebetween for receiving ammonia sensor  180 . 
         [0047]    In operation, ammonia, preferably formed from a urea solution, is supplied (dosed) via nozzle  134  and exhaust  18  into SCR catalyst assembly  114  at a controlled dispensing rate. Ammonia reacts with NOx at reaction sites on first and second catalysts  130 , 116 , reducing NOx in exhaust  122  to a predetermined level. Ammonia sensor  180  senses the level of ammonia entrained in exhaust gas leaving first catalyst  130  and sends a signal to SCM  154 , which is being provided with various relevant engine operating parameters. SCM  154  is programmed with an algorithm describing exhaust ammonia levels as a function of engine operating data over the range of engine speeds and temperatures in which the engine can operate, as well as test-bed generated performance data showing ammonia storage three-dimensionally within first and second catalysts  130 , 116  as a function of at least exhaust temperature, exhaust flow rate, and NOx level. The SCM determines whether the level of ammonia being experienced in the exhaust by sensor  180  is within predetermined limits and also predicts, from the present urea dosing rate and the expected near-term engine conditions, whether the present ammonia level is a) insufficient, b) correct, or c) excessive to meet present and anticipated needs in order to meet desired levels of NOx and ammonia in tailpipe exhaust  122 . The SCM then adjusts the liquid urea dosing rate accordingly and begins another calculation cycle. 
         [0048]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.