Patent Application
20170218821
The invention relates to a device for generating ammonia from an ammonia precursor solution. A device of said type may for example be used for generating ammonia that is utilized for exhaust-gas purification in an exhaust-gas treatment device. Exhaust-gas treatment devices in which ammonia is utilized for exhaust-gas purification are for example commonly used for the purification of exhaust gases of diesel internal combustion engines in motor vehicles. Nitrogen oxide compounds in the exhaust gas of the internal combustion engine are reduced with the aid of the ammonia. As ammonia precursor solution, use is typically made of urea-water solution. An ammonia precursor solution of said type has the advantage over ammonia that it can be stored in a tank without problems. What is particularly commonly used is a urea-water solution with a urea content of 32.5%, which is available under the trade name AdBlue®.
The conversion of ammonia precursor solution into ammonia generally necessitates special technical measures Ammonia is obtained from the ammonia precursor solution for example at certain temperatures. The conversion takes place in a particularly effective manner in a temperature range between 200° C. and 550° C.
The temperature range for the conversion of ammonia precursor solution into ammonia may be expanded, and in particular lowered, by the presence of catalytic converters, such that the conversion can even take place in a temperature range between 140° C. and 250° C. The thermal conversion of ammonia precursor solution into ammonia is normally referred to as thermolysis or as thermolytic conversion. If the conversion is additionally assisted by way of a catalytic converter, this is referred to as hydrolysis or hydrolytic conversion.
In any case, the ammonia precursor solution must be heated in order to be converted into ammonia. Heat energy is required for this purpose. Normally, the ammonia precursor solution is thus metered directly into the exhaust gases of an internal combustion engine. The heating is then realized by way of the exhaust gases. Here, there is however the problem that, in particular in the case of diesel internal combustion engines, the exhaust gases are commonly at relatively low temperatures and therefore (at least in many load states of the diesel engine, in particular also at idle) cannot provide enough thermal energy to convert ammonia precursor solution into ammonia in an effective manner Where possible, it has then been the approach to avoid dosing of ammonia in said time periods. If this was not possible, this problem has generally, in the past, been counteracted with the aid of suitable measures for increasing the temperature in an exhaust-gas treatment device. The heating of the exhaust gases is particularly commonly performed with the aid of an electric heater. A further known method for increasing the temperature of the exhaust gases is the intentional shifting of the operating point of the internal combustion engine such that the internal combustion engine generates exhaust gases at relatively high temperature. Here, there is however the problem that, in particular if ammonia precursor solution is not fully converted into ammonia, deposits of the ammonia precursor solution can form in the exhaust-gas treatment device. Such deposits can (permanently) adversely affect the exhaust gas purification, impair the efficiency and consumption, and/or even damage the exhaust-gas treatment device. Furthermore, a situation may arise in which the exhaust gas temperature is too low to be able, by way of the described known measures for increasing the temperature, to adequately increase the exhaust-gas temperature for the conversion of ammonia precursor solution into ammonia. This applies in particular to cold-start phases of an internal combustion engine because, in cold-start phases, the exhaust gases of the internal combustion engine are intensely cooled by the thermal mass of the internal combustion engine and of the exhaust-gas treatment device.
It is an object of the present invention to specify a device for generating ammonia from an ammonia precursor solution, which device is improved in relation to the prior art and is in particular capable of providing ammonia substantially independently of the temperatures prevailing in an exhaust-gas treatment device.
Said objects are achieved by means of a device according to the features of claim 1. Further advantageous embodiments of the invention are specified in the dependent claims. The features specified individually in the claims may be combined with one another in any desired technologically meaningful way and may be supplemented by explanatory facts from the description, with further embodiments of the invention being highlighted.
The invention relates to a device for generating ammonia from an ammonia precursor solution, having a reaction space with an inflow connector through which an exhaust-gas flow can flow into the reaction space, having an outflow connector through which an ammonia-containing gas flow can exit the reaction space, and having a supply device by way of which selectively an ammonia precursor solution or a fuel can be supplied into the reaction space.
The devices designed in particular as a component which is positioned externally or adjacent to an exhaust line with the main exhaust-gas flow of an internal combustion engine. The device is typically connected to a line branch which branches off from an exhaust line of an exhaust-gas treatment device of an internal combustion engine. Said line branch then opens into the inflow connector of the device. The line branch preferably branches from the exhaust-gas treatment device upstream of an exhaust gas turbocharger which is integrated in the exhaust line. A supply line is typically connected to the outflow connector of the device, via which supply line the ammonia-containing gas flow generated by the device is supplied to the exhaust-gas treatment device. The supply line opens, preferably downstream of the turbocharger in the exhaust-gas flow direction, into the exhaust-gas treatment device of the internal combustion engine. The device is thus particularly preferably integrated in an exhaust-gas bypass which bypasses the turbocharger. This has the advantage that a pressure difference prevails between the inflow connector and the outflow connector, which pressure difference ensures that the exhaust gases that are conducted into the inflow connector are forced through the device with a pressure gradient.
The reaction space refers to a (at least one) cavity within the device, in which cavity the conversion reaction of ammonia precursor solution into ammonia takes place. The reaction space may comprise a component or various functional components which promote the conversion reaction. These are for example (a) special catalytic converters, (b) structures which influence the exhaust-gas flow and the movement of the ammonia and of the ammonia precursor solution in the reaction space, and/or (c) sensors etc.
The supply device preferably comprises at least one nozzle by way of which the ammonia precursor solution and/or a fuel can be metered into the reaction space. The supply device preferably furthermore comprises at least one valve by way of which the supply of ammonia precursor solution and/or of fuel can be (actively) controlled. The valve has the task in particular of controlling the amount of ammonia precursor solution supplied and the amount of fuel supplied. This may be realized for example by way of an adaptation of the opening time of the at least one valve. Here, the duration of the opening time is proportional to the supplied amount of ammonia precursor solution and/or proportional to the supplied amount of fuel.
The supply device of the device described here has the property that both ammonia precursor solution and fuel can be supplied by way of said supply device. This means that, by way of the supply device, ammonia precursor solution and fuel can be supplied to the reaction space in parallel and/or with a time offset with respect to one another.
The ammonia precursor solution has already been described in detail further above. A “fuel” refers in particular to a hydrocarbon-containing fluid (liquid and/or gas) which can be burned together with oxygen and which thus contributes to an intense temperature increase. Fuels are for example the fuels that are commonly used in motor vehicles, such as for example gasoline or diesel. Diesel fuel is particularly preferably used as fuel for the device, because diesel fuel is available in any case in motor vehicles with a diesel internal combustion engine (for which the device described here is particularly suitable).
By way of the supply device of the device being discussed here, which supply device performs both the supply of ammonia precursor solution and the supply of fuel, it is possible for fuel to be supplied to the device. Fuel can be burned in the device such that the temperature in the device is increased and lies in the temperature range required for the conversion of ammonia precursor solution into ammonia. The increase of the temperature with the aid of fuel is particularly efficient because suitable fuel (for example diesel fuel) is generally available in any case in motor vehicles. The temperature increase is realized in particular by virtue of the fuel being dispensed onto a catalytically active surface, where the fuel ignites.
Furthermore, the temperature increase may also be achieved by virtue of the fuel being ignited by way of a flame or a spark. The temperature increase with the aid of fuel can in particular be considerably faster than the temperature increase with the aid of an electric heater. Specifically, in a motor vehicle, it is generally the case that there is only a limited availability of electrical energy, because electrical energy must be generated in a cumbersome manner by way of a generator.
Furthermore, the described device has the great advantage that only a small (branched-off) part of the exhaust gases of an internal combustion engine flows through the device, and must be heated in the device, if the exhaust-gas temperature lies below the temperature required for the reaction of ammonia precursor solution into ammonia. The heating of the exhaust gases in the device can thus be performed with considerably reduced energy consumption than in devices which (only) permit heating of the entire exhaust-gas flow of an internal combustion engine.
The device is particularly advantageous if the supply device comprises a dosing valve which has a first feed line for ammonia precursor solution and a second feed line for a fuel.
It is then possible for a feed-in line for ammonia precursor solution to be connected to the first feed line and for a feed-in line for fuel to be connected to the second feed line. In the supply device there is preferably provided at least one valve by way of which the amount of fuel and the amount of ammonia precursor solution supplied to the device can each be precisely set (preferably separately from one another). The supply device particularly preferably comprises a multi-way valve by way of which selectively ammonia precursor solution or fuel can be supplied. The supply device preferably has a common nozzle by way of which both fuel and ammonia precursor solution can be metered. It is then the case that selectively ammonia precursor solution or fuel passes through the nozzle into the reaction space of the device. The supply of fuel results in a temperature increase in the device. When the device has heated up to a sufficient extent, ammonia precursor solution is supplied. The ammonia precursor solution is converted into ammonia in a particularly effective manner in the heated device. If the temperature of the device falls again, the supply of ammonia precursor solution can be ended again and fuel can be supplied to the device or to the reaction space of the device again, such that the temperature is increased again to the temperature required for the generation of ammonia, and metering of ammonia precursor solution can be performed again.
The device is particularly advantageous if, in the reaction space, there is arranged an impingement structure toward which the supply device is oriented and which is impinged on by the supplied ammonia precursor solution.
An impingement structure of said type may for example be in the form of a honeycomb body through which the exhaust-gas flow can flow, the face surface of which honeycomb body is impinged on by the ammonia precursor solution and/or by the fuel. An impingement structure of said type is in particular arranged within the device such that it is heated in an effective manner by the fuel and/or by the exhaust gases entering the device. The impingement structure is preferably not in direct contact with an outer wall of the device. The outer wall of the device is possibly cool because it is in contact with the surroundings. The impingement structure is preferably arranged in cantilevered or at least partially cantilevered form in the reaction space. The impingement structure is preferably arranged so as to at least partially span the cross section of the reaction space.
The device is furthermore advantageous if the impingement structure is provided with a coating which catalyzes both a hydrolysis of ammonia precursor solution into ammonia and an exothermic reaction of fuel with oxygen.
A coating of said type comprises, for example, one of the following components of catalytically active material: a) titanium dioxide, aluminum dioxide and gold; b) titanium dioxide, platinum and palladium.
A catalytically active coating which promotes an exothermic reaction of the fuel with oxygen is particularly advantageous because a coating of said type reduces the ignition temperature of the fuel, and it is thus possible in particular for local temperature peaks in the device to be prevented. If appropriate, it is also possible by way of a catalytically active coating to avoid the need for an open flame for the conversion of the fuel in the device. Also, in this way, the thermal loads within the device can be kept low. Furthermore, it is possible for no separate and/or electrical ignition to be required for the initiation of the combustion, with the conversion of the fuel rather starting spontaneously upon the supply of the fuel into the reaction space of the device. The oxygen required for the combustion of the fuel normally enters the reaction space with the exhaust gas through the inflow connector. In particular in the case of lean-burn diesel internal combustion engines, a high oxygen fraction is normally encountered in the exhaust gas. As an alternative or in addition to the oxygen, it is also possible for incompletely oxidized exhaust-gas constituents, such as carbon monoxide or nitrogen monoxide, to be converted with the fuel.
The device is furthermore advantageous if the supply device comprises a common nozzle by way of which ammonia precursor solution and fuel are sprayed into the reaction space, wherein the nozzle generates different spray patterns with ammonia precursor solution and with fuel.
Here, the expression “common” nozzle means that both the ammonia precursor solution and the fuel pass into the reaction space through the same nozzle. A spray pattern refers here in particular to the shape of a spray cone that is formed when ammonia precursor solution and/or fuel emerge(s) through an outlet opening of the nozzle. The fuel that is used and the ammonia precursor solution may have different flow characteristics. In particular with regard to the viscosity, the density, the temperature capacity and/or the surface tension, the fuel that is used and the ammonia precursor solution generally differ greatly. It is therefore possible for a nozzle for the supply of ammonia precursor solution and fuel to be designed such that the spray pattern generated by the nozzle with fuel and the spray pattern generated by the nozzle with ammonia precursor solution differ from one another (e.g. with regard to the spray direction, the spray cone angle, the spray droplet size, etc.).
Furthermore, it is also possible for ammonia precursor solution and fuel to be provided at the nozzle with different pressures. The pressure prevailing at the nozzle also influences the spray pattern at the nozzle. For example, the ammonia precursor solution may be provided at the nozzle with a pressure of up to 8 bar, whereas the fuel is provided at the nozzle with a significantly different pressure, for example (depending on the specific pump-nozzle system and a desired spray form) with a relatively low pressure (e.g. lower than 5 bar) and/or with a relatively high pressure (e.g. at least 20 bar, at least 50 bar or even more than 200 bar). Then, the fuel is generally atomized very much more finely than the ammonia precursor solution. Normally, it is in the case that a more widely angled spray cone forms for the fuel than for the ammonia precursor solution. Furthermore, it is then also possible for the formation of different spray cones to be promoted by way of the construction of the nozzle. This may be realized for example by way of suitable diversions of the ammonia precursor solution and/or of the fuel in the nozzle. Such diversions have a different effect on the different liquids (fuel and ammonia precursor solution), such that this promotes different spray patterns.
It is possible for a carrier gas, in particular compressed air, to be used for the metering of ammonia precursor solution and/or fuel. It is also possible in this way for the predefined pressure during the metering of the ammonia precursor solution and/or of the fuel to be influenced or adapted.
The device is furthermore advantageous if different impingement regions for ammonia precursor solution and for fuel are provided within the reaction space.
Such different impingement regions may be realized for example by way of a correspondingly targeted configuration of the different spray patterns for ammonia precursor solution and for fuel. It is particularly preferable if, for ammonia precursor solution, as a spray pattern, a central conical spray cone is generated which impinges centrally on an impingement structure in the device and, for fuel, a widened spray cone is generated in the case in which no fuel is sprayed in a central region, resulting in a ring-shaped impingement region for fuel on the impingement structure, wherein the ring-shaped impingement region for fuel is arranged around a central impingement region for ammonia precursor solution.
With such a configuration of the different impingement regions for ammonia precursor solution and for fuel, it is possible for in each case targetedly suitable different coatings to be provided on the impingement structure in the different impingement regions, wherein a first catalytically active coating which promotes the catalytic conversion of ammonia precursor solution into ammonia is provided in the first impingement region in which ammonia precursor solution impinges on the impingement structure, whereas a second catalytically active coating which promotes the conversion reaction of fuel for heat generation purposes is provided in the second impingement region.
The device is furthermore advantageous if an electric heater is arranged in the reaction space.
By way of an electric heater, the device can be heated in a particularly effective manner In particular, it is possible for the device to be heated even in the presence of particularly low temperatures at which the generation of heat with the aid of fuel would not be possible at all, for example because a minimum temperature for the catalytic conversion of the fuel with oxygen has not yet been reached.
It is particularly advantageous if the electric heater simultaneously forms the impingement structure within the device. The increased temperature within the device is required in particular in the region of the impingement structure because it is there that cooling by way of the impinging ammonia precursor solution occurs. Furthermore, an increased temperature is required specifically on the impingement structure in order to effect the conversion reaction of the ammonia precursor solution. The electric heater contributes to the capability of maintaining a minimum temperature and/or a heat range for the dosing. Furthermore, owing to the contact with the ammonia precursor solution, this promotes faster heat transport into the droplets. At the same time, a high level of turbulence can be generated, such that, for example, so-called “Leidenfrost effects” can be substantially or even entirely eliminated. Furthermore, it is thus also possible for an adequately high heat capacity to be provided in order that the impinging droplets themselves do not effect significant cooling of the electric heater/impingement structure, and thus the conversion reaction can take place in an effective manner uniformly and/or quickly.
The device is particularly advantageous if the electric heater is an electrically heatable honeycomb body.
An electrically heatable honeycomb body is suitable in particular for forming, with the aid of an electric heater, an impingement structure which can at the same time be flowed through by the exhaust-gas flow. An electric honeycomb body may for example be a heating coil formed from metallic foils, which heating coil spans the cross section of the device or a cross section of the reaction space of the device.
Furthermore, the device is advantageous if the inflow connector is arranged tangentially at the reaction space.
By way of a tangential arrangement of the inflow connector, it can be ensured that the exhaust gas flowing into the inflow connector generates a vortex flow within the device. Said vortex flow firstly has the advantage that particularly good mixing of ammonia precursor solution, fuel and exhaust gas takes place. Furthermore, the vortex flow centers the fuel and/or the ammonia precursor solution within the device and ensures that the fuel and/or the ammonia precursor solution does not come into contact, or comes into contact only to a small extent, with an outer wall of the device. It is thus possible for the formation of deposits of the fuel and/or of the ammonia precursor solution on the outer wall of the device to be prevented.
Furthermore, the device is advantageous if the reaction space is divided by a cylindrical diverting element into a cylindrical gap and a central chamber, wherein the cylindrical gap and the central chamber are connected to one another by way of a diverting region, wherein the inflow connector is arranged at the cylindrical gap, the exhaust-gas flow from the cylindrical gap is conducted into the central chamber through the diverting region, and the supply device supplies the precursor solution and the fuel in an axial direction into the central chamber through the diverting region.
The entire device preferably has a cylindrical housing. Here, the supply device is arranged on a face surface of the cylindrical housing, wherein the supply device is oriented in an axial direction toward said cylindrical housing. The outflow connector is preferably arranged on the opposite face side of the cylindrical housing. The inflow connector opens into the cylindrical housing through a circumferential surface in a tangential direction. The reaction space is arranged within the cylindrical housing of the device. The diverting element is then arranged in the direction toward the outflow connector proceeding from the supply device, which diverting element separates the cylindrical (ring-shaped) gap from the central chamber of the reaction space. The cylindrical gap is preferably open only in the direction of the supply device, and closed off on the opposite side. The exhaust gas that enters through the inflow connector therefore passes from the cylindrical gap into the central chamber in a diverting region in the vicinity of the supply device. In the diverting region there is preferably also a perforated screen, wherein the exhaust-gas flow passes through said perforated screen on the path from the cylindrical gap into the central chamber. Said perforated screen preferably has a large central opening through which the metering device meters the fuel and/or the ammonia precursor solution into the central chamber. The spray cone of the supply device, as described further above, preferably extends through said central opening. It is preferable for a multiplicity of small openings to be arranged around the central opening, which small openings promote the entry of the exhaust gas into the central chamber. The impingement structure already described is arranged in an axial direction and in the direction of the outflow connector proceeding from the supply device (downstream of the diverting element), which impingement structure spans the cross section of the device and of the reaction space and may also comprise an electric heater. Additional catalytic converter substrate bodies may be situated so as to follow the impingement structure in an axial direction, which additional catalytic converter substrate bodies contribute to the conversion of the ammonia precursor solution into ammonia. Sensors and further components by way of which the generation of ammonia in the device can be assisted and/or monitored may also be arranged there. A sensor of the device may comprise in particular a temperature sensor and/or a lambda probe by way of which an oxygen fraction in the gas can be determined.
The externally arranged cylindrical diverting element has the advantage that thermal insulation of the central chamber with respect to the outer wall of the device is realized, such that the ammonia precursor solution that is supplied into the central chamber cannot come into contact with the outer wall of the device. Furthermore, by way of the cylindrical diverting element, heating of the central chamber is realized, because the exhaust gas flowing in through the inflow connector flows around and heats the central chamber (through the cylindrical gap) before said exhaust gas enters the central chamber through the diverting region.
Also proposed is an exhaust-gas treatment device for the purification of the exhaust gases of an internal combustion engine, having a device for generating ammonia as described here, wherein the inflow opening is connected by way of a line branch to an exhaust line of the exhaust-gas treatment device, and the outflow opening is connected by way of a supply line to the exhaust line, wherein, through the inflow opening, between 0.1% and 5.0% of the exhaust gas from the internal combustion engine flows into the reaction space.
As already described, the line branch branches off from the exhaust line preferably upstream of the turbocharger, whereas the supply line opens into the exhaust line downstream of the turbocharger. For this reason, it is advantageous for only a relatively small exhaust-gas flow to be branched off from the main exhaust line and to flow into the device through the inflow connector. The main exhaust-gas flow can then be used in the turbocharger to generate mechanical energy for the supercharging of the internal combustion engine. Furthermore, such a small exhaust-gas partial flow of between 0.1% and 5% of the exhaust gas can be heated in a particularly effective manner with the aid of the fuel and/or with the aid of an electric heater, because relatively little thermal energy is required for this purpose.
An SCR catalytic converter on which nitrogen oxide compounds in the exhaust gas can be reduced with the aid of the generated ammonia to form non-harmful substances is preferably arranged in the exhaust gas treatment device.
An exhaust-gas treatment device of said type is particularly preferably used for the purification of the exhaust gases of a diesel internal combustion engine. The exhaust-gas treatment device and the device are in particular also suitable for exhaust-gas purification in watercraft, rail vehicles, agricultural machines and construction machines etc. The device and the exhaust-gas treatment devices are particularly suitable in applications in which the exhaust gases of an internal combustion engine which is operated predominantly in the part-load range and in the low-load range are purified, because low exhaust-gas temperatures are particularly commonly encountered then.
The invention and the technical field will be explained in more detail below on the basis of the figures. The figures show particularly preferred exemplary embodiments, to which the invention is however not restricted. It is pointed out in particular that the figures, and in particular the dimensional relationships illustrated in the figures, are merely schematic. In the figures:
The outflow connector 4 is arranged on the device 1 so as to be situated opposite the supply device 5, through which outflow connector an ammonia-containing gas flow can emerge from the device 1. The inflow connector 3 is arranged on the circumferential surface of the device 1, via which inflow connector and exhaust-gas flow can enter the device. The reaction space 2 of the device 1 is divided by a diverting element 18 into a cylindrical gap 19 and a central chamber 20. The cylindrical gap 19 and the central chamber 20 are connected to one another by way of a diverting region 21 in the region of the supply device 5. In the diverting region 21 there is also provided a perforated screen 36 which has a central opening through which the supply device 5 can supply ammonia precursor solution and/or fuel into the central chamber 20 of the reaction space 2. The perforated screen 36 furthermore has additional, relatively small openings (arranged around the central opening) through which the exhaust-gas flow can pass into the central chamber 20.
The inflow connector 3 is preferably arranged tangentially at the device 1. In this way, a vortex flow 28 is generated within the reaction space 2 or within the cylindrical gap 19 and the central chamber 20.
Within the device 1, an impingement structure 9 is arranged downstream of the diverting element 18 and the central chamber 20 as viewed in the exhaust-gas flow direction, which impingement structure forms a first impingement region 14 and a second impingement region 15, wherein the first impingement region 14 is provided for receiving ammonia precursor solution and the second impingement region 15 is provided for receiving fuel. In the present embodiment of a device 1, the impingement structure 9 is in the form of an electric heater 16, and particularly preferably in the form of an electrically heatable honeycomb body 17. Arranged downstream of the impingement structure 9 in the flow direction is at least one catalytic converter substrate body 32 which may comprise coatings for the chemical conversion of the ammonia precursor solution and/or of the fuel. Furthermore, a sensor 33 is also provided in the device 1, by way of which sensor the conversion of ammonia precursor solution into ammonia in the device 1 can be monitored. The sensor 33 may for example comprise a temperature sensor and/or a lambda probe by way of which an oxygen content in the gas can be determined.
By way of the described device, the particularly reliable provision of ammonia for an exhaust-gas aftertreatment device is ensured even in the presence of particularly low exhaust-gas temperatures. At the same time, a particularly small amount of energy is required for this purpose, and ammonia can be provided in the form of an ammonia precursor solution and converted into ammonia by the device. The device is suitable in particular for the purification of diesel exhaust gases of internal combustion engines which are often operated in the part-load range.
1 Device
2 Reaction space
3 Inflow connector
4 Outflow connector
5 Metering device
6 Dosing valve
7 First feed line
8 Second feed line
9 Impingement structure
10 Nozzle
11 Spray pattern
12 First coating
13 Second coating
14 First impingement region
15 Second impingement region
16 Electric heater
17 Heatable honeycomb body
18 Diverting element
19 Cylindrical gap
20 Central chamber
21 Diverting region
22 Axial direction
23 Exhaust-gas treatment device
24 Motor vehicle
25 Line branch
26 Exhaust line
27 Internal combustion engine
28 Vortex flow
29 Turbocharger
30 Precursor solution tank
31 Fuel tank
32 Catalytic converter substrate body
33 Sensor
34 Intake line
35 Supply line
36 Perforated screen
37 SCR catalytic converter
38 Housing
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2014 108 875.1 | Jun 2014 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/062400 | 6/3/2015 | WO | 00 |
