Abstract:
A process for reducing nitrogen oxides on a SCR catalyst disposed in an exhaust gas system of an internal combustion engine is performed by a device adapted to introduce ammonia (NH 3 ) into the exhaust gas flow of the exhaust gas system before it reaches the SCR catalyst. The device includes a heatable pressure-tight converter, an NH 3  store and a control unit with control signals processing engine operating characteristics and determining therefrom the NO x  output for controlling a timed valve for injection of an NH 3  dose.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to the field of reducing primary injurious substances generated in internal combustion engines by means of a catalyst. The invention relates, in particular, to a process for reducing nitrogen oxides (NO x ) on a SCR catalyst, disposed in the exhaust gas system of an internal combustion engine, with ammonia (NH 3 ) which is preferably introduced into the exhaust gas flow before the catalyst. The invention further relates to a device for introducing ammonia (NH 3 ) into the exhaust gas flow of an internal combustion engine for reducing nitrogen oxides (NO x ) contained in the exhaust gas flow on a SCR catalyst comprising a NH 3  source, a supply line for introducing the NH 3  into the exhaust gas flow and a dosing device. 
     Apart from carbon monoxide (CO) and hydrocarbons (HC), in particular the nitrogen oxides (NO x ) are among the environmentally harmful, directly emitted, primary injurious substances which are generated during the operation of internal combustion engines, in particular Diesel engines. The use of three-way catalysts, such as are used in Otto engines and gas engines, cannot be used in the exhaust of Diesel engines due to an oxygen excess. For this reason, for the reduction of the nitrogen oxide emission in Diesel engines a selectively operating SCR (Selective Catalytic Reduction) catalyst has been developed in which, in the presence of an added reducing agent, namely ammonia (NH 3 ), the expelled nitrogen oxides are reduced to N 2  and H 2 O. 
     This type of reducing nitrogen oxide emissions in stationary Diesel engines has been found to be useful. In these stationary installations in the exhaust gas system of the internal combustion engine is disposed a SCR catalyst and the NH 3  to be added to the exhaust gas flow before the SCR catalyst is added by injection. In such installations the NH 3  is supplied in the form of a gas or aqueous solution. In the case in which NH 3  is supplied as an aqueous solution, a thermolytic splitting takes place in the exhaust gas flow, or in the SCR catalyst, in order to release the NH 3  necessary for the reduction of nitrogen oxides. The introduction of the reducing agent takes place via a dosing device which is adjusted as a function of the anticipated NO x  quantity in the exhaust gas flow of the Diesel unit. When using NH 3  either as the gas or as aqueous solution, the use of skilled personnel is required since associating with these substances is not without danger with respect to their handling as well as also with respect to their toxicity. 
     Due to the not undangerous association with NH 3  in gaseous form or as an aqueous solution, it has become customary to provide the NH 3  required for the NO x  reduction by introducing aqueous urea solution into the exhaust gas flow. The thermohydrolytic splitting with the liberation of NH 3  as reducing agent takes place through the heat of the exhaust gas flow or of the catalyst. In addition to the difficulties relating to the process of the unchecked feed of this reducing agent in the form of an aerosol, a further disadvantage entailed in obtaining NH 3  in this way is the formation of undesirable byproducts such as, for example, isocyanic acid. The fact that the freezing point of such a reducing agent is approximately −13° C., moreover, stands in the way of using aqueous urea solutions for the NO x  reduction of Diesel engine exhaust gases in mobile units, for example in utility vehicles or passenger motor vehicles. The winter-worthy use of this reducing agent is therefore only possible by mixing in additives which decrease the freezing point. However, the conversion of these additives in the SCR catalyst can lead to the emission of undesirable secondary injurious substances. 
     It is furthermore necessary to carry along a relatively large quantity of aqueous urea solution since the urea necessary for the reduction is only present in the aqueous solution at a ratio of at best 1:3 with respect to the H 2 O. 
     The use of NH 3  carried along in pressure tanks or as aqueous solution for removing nitrogen in mobile Diesel units is not possible in the event of an accident due to the danger inherent in these substances. 
     From DE 34 22 175 A1 as well as DE 42 00 514 A1 processes are known which relate to the “just-in-time” production of NH 3  for the reduction of NO x . The superior concept evident in these patents comprises using specific substances which thermolytically split off NH 3 , the handling and toxicity of which are quite safe, in order to split off the required quantity of NH 3  in accordance with the particular requirements and to inject it into the exhaust gas flow. The generation of NH 3  takes place by heating such an NH 3 -splitting compound, for example ammonium carbamate. The adaptation to the particular operating state of the motor, as described in DE 34 22 175 A1, takes place thereby that by controlling the calorific power, which for example acts on the carbamate, the NH 3  generation and thus the NH 3  quantity added to the exhaust gas flow can be regulated. 
     Such a regulatable NH 3  generator comprises essentially a storage tank for the compound splitting off NH 3 , a decomposition chamber in which the action of the heat takes place, as well as a takeout line or supply line for the NH 3 -containing gas. For heating the decomposition chamber, in particular electric heating [systems], for example resistance heating bodies or infrared radiators are provided. 
     The use of this prior known process or this prior known device is suitable for stationary installations which in general are only subject to few load changes and even then only to predetermined load changes, however, for deployment in mobile use, such as for example in utility vehicles or passenger vehicles, the use of this prior known technology is not appropriate since the reaction time of the system is too slow and thus the system too inert in order to be able to do justice to the unexpected motor load changes occurring in rapid succession in street traffic with correspondingly varying NO x  emissions. If the operating state of the motor is acquired at a specific time and, based on this acquisition, a specific NH 3  dosing is calculated, the substance splitting off the NH 3  must first be heated in order to produce the NH 3 -gas mixture. However, in street traffic, in particular in city traffic, an engine is subject to continuous and unexpected load changes such that the quantity of NH 3  which is lastly introduced does not match the operating state of the engine which has changed in the meantime. If an insufficient quantity of NH 3  is introduced, the NO x  reduction can only be carried out to a limited extent. If the NH 3  is introduced in excess, unconsumed NH 3  exits the catalyst. 
     SUMMARY OF THE INVENTION 
     Building on the discussed prior art, the invention is therefore based on the task of proposing a process as well as a device for the NO x  reduction of exhaust gases of an internal combustion engine using a SCR catalyst, which is not only suitable for mobile application but with which the above listed disadvantages are avoided. 
     This task is solved according to the invention through a process according to the species thereby that the process comprises the following steps: 
     initial heating of a substance or substance mixture at ambient temperature and capable of thermolytically splitting off NH 3  disposed in a pressure-tight receptacle (converter) to that temperature range at which the splitting-off of NH 3  takes place (splitting-off temperature range), 
     maintaining the splitting-off temperature range until an internal pressure builds up in the converter, 
     intermediately storing the split-off NH 3  or NH 3 -gas mixture and, after reaching a predetermined reduction gas pressure in the NH 3  storage, 
     acquiring engine operating characteristics according to which the NO x  content in the exhaust gas flow can be calculated, based on which characteristics subsequently the determination of the NH 3  dosing, necessary for the reduction of the calculated NO x  content, takes place, 
     injection of a predetermined dose of the stored NH 3  into the exhaust gas flow of the internal combustion engine with the last two process steps being repeated with optional frequency during the operation of the engine. 
     The task is further solved according to the invention thereby that as the NH 3  source a heatable pressure-tight converter is provided in which is disposed a substance or a substance mixture splitting off NH 3  thermolytically and that the dosing device is preceded by an NH 3  store for the intermediate storage of the NH 3 split off from the substance through the addition of heat, which dosing device is acted upon with control signals by a control unit processing the engine characteristics and determining on that basis the NO x  output. 
     By providing a process which makes possible the use as NH 3  precursor of a substance or a substance mixture which, in terms of its handlability and its toxicity is unobjectionable, and which, by building up an internal pressure in the converter, utilizes the establishment of an equilibrium state for the termination (in time) of a further NH 3  splitting, a process is created with which only a specific NH 3  quantity is made available. At any given time a specific NH 3  quantity is therefore available which can be injected into the exhaust gas flow in order to reduce NO x . The intermediately stored quantity of NH 3  is so small that even in the case of a hypothetical destruction of the converter in the event of an accident it can be classified as unobjectionable due to its rapid mixing with ambient air. 
     By acquiring the engine operating characteristics and, if appropriate, the concentrations of exhaust gas components, calculation of the NO x  mass flow in the exhaust gas becomes possible and, based on it, the calculation of the NH 3  quantity. Since a sufficient quantity of NH 3  is intermediately stored, a process is created which permits the NH 3  output quasi-simultaneously with the NH 3  determination. The process according to the invention as well as the device according to the invention are therefor primarily suited for the NO x  reduction of mobile Diesel units subject to unpredictable load changes. 
     The invention combines the advantages of introducing the NH 3  as gas into the exhaust gas flow, the carrying along of substances or substance mixtures which, with respect to their handling and toxicity are unobjectionable at ambient temperature, as well as making immediately available the required NH 3 . The process according to the invention as well as the device according to the invention are suitable for use of substances, splitting off NH 3  substantially free of residues and decomposable thermolytically, such as for example ammonium carbamate (NH 2 CO 2 NH 4 ) as well as for the use of substances reversibly NH 3 -absorbing/desorbing and thus splitting off NH 3  such as for example of an iron (II) ammine sulfate. 
     For the initial heating as well as for maintaining the splitting-off temperature range of the converter, heat generated during operation of the internal combustion engine, is used and it is useful to assign to the converter a heating coil which is connected in particular when using ammonium carbamate to the cooling water circulation of the internal combustion engine. During operation of the internal combustion engine, the cooling water is at a temperature between 80 and 110° C., which corresponds to an internal converter pressure of approximately 6.5-8 bars when attaining a state of equilibrium. The converter is therefore implemented so as to be suitably pressure-tight for operation at such internal pressures. It is advisable to take a safety margin into account. 
     An embodiment example provides that the intermediate storage of the split-off NH 3  takes place in the converter itself. In a further embodiment example as the intermediate store a separate NH 3  store is provided whose storage pressure is below the operating pressure of the converter. 
     For better dosing of the NH 3  quantity to be removed, the converter usefully comprises a pressure-reducing valve. As the dosing device usefully a timed valve is provided. 
     A further development provides that the maximum NH 3  dosing is slightly smaller than that NH 3  dosing determined to be optimal According to such an implementation it is avoided that, in spite of the quasi-simultaneous NH 3  introduction, upon an abrupt load change potentially non-converted NH 3  exits the rear side of the SCR catalyst. To a limited extent, an emission of excess NH 3  could also be reduced through an oxidation catalyst succeeding the SCR catalyst. 
     The converter comprising the substance splitting off NH 3  can be connectable by means of rapid fasteners to the heating means provided for heating and maintaining the temperature as well as to the NH 3  supply line. In this way the rapid exchange of the converter can be carried out, if the substance splitting off NH 3  contained therein or the substance mixture splitting off NH 3  contained therein is consumed. It is further possible to provide that the converter is comprised of a heating unit and a reaction receptacle with the reaction receptacle being detachable from the heating unit. The heating unit, connected for example to the cooling water circulation, remains in the motor vehicle such that only the reaction receptacle proper is exchangeable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following detailed description, reference will be made to the attached drawings in which: 
     FIG. 1 is a schematic representation of a device of the present invention for introducing ammonia into the exhaust gas flow of an internal combustion engine for reducing nitrogen oxides contained in the exhaust gas flow, 
     FIG. 2 is a diagram representing the decomposition of ammonium carbamate and ammonium hydrogen carbonate in the temperature range of 35 to 130° C., 
     FIG. 3 is a diagram representing the decomposition of iron (II) triammine sulfate monohydrate in the temperature range of 30 to 450°C., and 
     FIG. 4 is a diagram representing the pressure and temperature curve over time when using ammonium carbamate with repeated gas removal. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The nitrogen-removal device  1  depicted schematically in FIG. 1 comprises a converter  2 , a supply line  3  for introducing NH 3  into the exhaust gas flow  7  of a Diesel engine  4  and a timed valve  5  provided as a dosing device, as well as a SCR catalyst  6 . 
     To control the timed valve  5  a memory-programmed control unit  8  is provided which is acted upon by signals of sensors receiving the engine operating characteristics as well as by signals of a sensor receiving the catalyst temperature. The control unit is connected at the output side via a control line  9  with the timed valve  5  such that the timed valve  5  is driven by the control unit  8 . 
     The converter  2  comprises a heating unit  10  as well as a reaction receptacle  11 . The heating unit  10  comprises a heating coil  12  which is integrated into the cooling water circulation of the Diesel engine  4  via a supply line  13  and an outlet line  14 . During the operation of the Diesel engine  4  the cooling water flowing through the heating coil  12 , as a rule, has a temperature between 80 and 100° C. which can potentially also increase up to 110° C. For the rapid heating of the heating unit  10  or of the reaction receptacle  11  contained therein, the supply and outlet lines  13 ,  14  are integrated into the so-called small cooling water circulation of the Diesel engine  4 . 
     The reaction receptacle  11  is a pressure-tight receptacle into which a predetermined quantity of ammonium carbamate  15 . has been introduced. The reaction receptacle  11  is closed pressure-tight. The reaction receptacle  11  depicted in FIG. 1 is closable such that after the consumption of the ammonium carbamate  15  it can be opened and newly filled. 
     By heating the ammonium carbamate  15  as a consequence of the flow through the heating unit  10  of, as a rule, cooling water of 80 to 100° C., the ammonium carbamate  15  is decomposed into NH 3 .and carbon dioxide (CO 2 ). As is evident in the diagram represented in FIG. 2 the splitting-off temperature of the ammonium carbamate  15  starts at approximately 40° C. 
     Consequently, ammonium carbamate can be readily handled at ambient temperatures without special requirements being necessary with respect to handlability or toxicity of this substance. In the temperature range of 80 to 100° C., typical for cooling water temperatures of Diesel engines, thus the nearly complete decomposition of the ammonium carbamate  15  for the production of the reduction gas mixture comprising the NH 3  takes place. 
     Likewise, in FIG. 2 is also shown the decomposition of ammonium hydrogen carbonate—a further substance suitable for carrying out the invention, and it is evident that a decomposition of these substances splitting off NH 3  only starts at approximately 50° C. and that the complete decomposition occurs at approximately 130° C. Based thereon, it is evident that ammonium hydrogen carbonate can also be used for splitting off NH 3  under the cited conditions. However, for the effective utilization of the ammonium hydrogen carbonate it would be recommended to act upon the heating unit  10  with its heating coil  12  with a warmer medium, for example to integrate it into the oil circulation of the Diesel engine  4 . 
     Corresponding to FIG. 2, FIG. 3 represents in a diagram the deammination of iron (II) triammine sulfate monohydrate. When using iron (II) triammine sulfate monohydrate as an example of an iron ammine sulfate a substance is being used which is reversibly NH 3  absorbing/desorbing. The use of such a substance can be useful since, compared to the two previously cited substances, upon heating exclusively NH 3  is split off; gaseous byproducts, such as for example CO 2 , are not formed in this splitting off. During a reduction of the heat supply, such as for example when switching off the Diesel engine, the split-off NH 3  is bound again. 
     The reducing gases generated in the reaction receptacle  11 —NH 3  and CO 2  in the embodiment example shown in FIG. 1 using ammonium carbamate  15 —initially remain in the reaction receptacle  11 . 
     With increasing decomposition of the ammonium carbamate  15  by maintaining the splitting-off temperature in the reaction receptacle  11 , the internal pressure in it rises up to approximately 8 bars at a temperature of approximately 100° C. As a function of the internal temperature of the reaction receptacle  11  a state of equilibrium is established upon the cited internal pressure being reached such that no further ammonium carbamate  15  is decomposed. Thus, the reaction receptacle  11  also forms simultaneously an NH 3  store from which NH 3  can be drawn off as a portion of the gas mixture comprising NH 3  and CO 2 . Drawing off a specific reduction quantity leads simultaneously to a reduction of the internal pressure in the reaction receptacle  11  such that the decomposition of further ammonium carbamate  15  leads to the splitting off of NH 3 . The decomposition of additional ammonium carbamate  15  continues until said equilibrium state is established again. By utilizing the developing equilibrium state in the reaction receptacle  11  a means is created to stop the NH 3  production after a specific NH 3  quantity has been attained without requiring additional control mechanisms for this purpose. 
     The split-off NH 3  is removed from the reaction receptacle  11  via a pressure reducing valve  16  and supplied via the supply line  3  to the timed valve  5 . 
     In FIG. 1 a further variant is shown in dashed lines, in which between the pressure reducing valve  16  and the timed valve  5  an additional separate NH 3  store  17  is provided. This NH 3  store  17  serves for the intermediate stock-pile of NH 3  and is laid out for an internal pressure of approximately 3.5 bars; this internal pressure is below the operating pressure of the converter  2 . The output of the NH 3  store is connected to the input of the timed valve  5 . 
     In a further development, not shown, with the reaction receptacle  11  or the NH 3  store  17  additionally a pressure sensor is associated whose special steel membrane, flush at the front, is directly in contact with the medium in the particular receptacle. With the aid of such a pressure sensor the internal pressure of the particular receptacle  11  or  17  can be acquired; the measurement signals of such a pressure sensor can be utilized for determining the remaining available ammonium carbamate quantity. If in the corresponding receptacle  11  or  17  a predetermined internal pressure does not build up in a given time interval, this permits a conclusion regarding the fact that only an insufficient ammonium carbamate as NH 3  precursor is available in the reaction receptacle  11 . Such a state is subsequently indicated to the driver so that he can exchange the reaction receptacle  11  or the converter  2  for a newly filled one. 
     The device  1  for the nitrogen reduction functions in the following way: 
     After starting the Diesel engine  4  its small cooling water circulation is initially heated in a relatively short time to 80 to 100° C., potentially up to 110° C. By connecting the heating unit  10  with its heating coil  12  to the small cooling water circulation, thus after a brief time, the decomposition of the ammonium carbamate  15  in the reaction receptacle  11  into a gas mixture comprising NH 3  and CO 2  also starts. This decomposition or splitting process continues until the above addressed equilibrium state between the ammonium carbamate  15  and the decomposition gases—NH 3  and CO 2 —has been established. 
     During operation of the Diesel engine  4  representative engine operating characteristics, based on which the NO x  output can be determined, are acquired via sensors. These signals act upon the control unit  8 . In the control unit  8  a determination of the NO x  output of the Diesel engine  4  takes place and the determination of the correspondingly required NH 3  quantity for reduction of the output NO x . The quantity of NH 3  intended for the reduction of the NO x  is used in order to drive the timed valve  5  accordingly so that either directly from the reaction receptacle  11  or the NH 3  store  17  the required NH 3  dosing can be removed and injected into the exhaust gas system  7  before the SCR catalyst  6 . Before the SCR catalyst  6  the injected gas mixture comprising NH 3  is already mixed with the NO x —containing exhaust gas so that an effective reduction of the NO x  can already take place upon the entry of this mixture of exhaust gas and reduction gas into the SCR catalyst  6 . 
     Due to the direct conversion of the particular calculated NO x  output, a corresponding supply of already available NH 3  is ensured such that even with rapid and unexpected load changes the supplied NH 3  dosing is appropriately dimensioned. 
     The pressure and temperature curve over time is depicted in FIG. 4 using ammonium carbamate  15  under repeated gas removal. It is evident in this diagram that the pressure build-up in the reaction receptacle  11  (in the example only 250 cm 3  content) in each instance is slightly lengthened over time due to the continuously decreasing ammonium carbamate quantity as NH 3  precursor. For the time utilization of a converter  2  filled with ammonium carbamate  15  the initial loading ratio V F /V K  is critical where V F  is the solid volume of the ammonium carbamate and V K  is the converter volume. In the diagram shown in FIG. 4 the initial loading ratio V F /V K  is only 0.2. 
     With the process according to the invention the NO x  emissions of Diesel engines during mobile use can be reduced to below 0.4 g/km. If such an NO x  output is attained, which is below the so-called Euro-III limit value, a filling of approximately 1 kg ammonium carbamate in a converter with three liter capacity is sufficient in order to reduce on a driving distance of 2000 km the NO x  emission to below said limit value. In this example reference is made to a Diesel engine of 2 liter engine displacement. In correspondingly greater dimensioned converters an NO x  reduction for approximately 10,000 driven kilometers could also be achieved such that the renewal intervals of a reaction receptacle  11  need only be carried out relatively rarely, for example at the interval of the oil changes.