WO2000017499A1

WO2000017499A1 - Method and device for catalytically reducing emission concentrations in the exhaust gas of a combustion system

Info

Publication number: WO2000017499A1
Application number: PCT/DE1999/003082
Authority: WO
Inventors: Ronald Neufert
Original assignee: Siemens Aktiengesellschaft
Priority date: 1998-09-24
Filing date: 1999-09-23
Publication date: 2000-03-30
Also published as: DE19843960A1

Abstract

A problem encountered in computer-controlled diesel catalytic converters is the slowing down of reactant activity. The invention thus provides for the dosage (S10) of the reactant (10) to be coupled (S2, S8) by means of a functional relationship to operation-relevant parameters of the combustion system, for said functional relationship to be corrected (S4, S5, S7) by autoadjustment via emission measurements and for the reactant (10) to be dosed rich of stoichiometry in relation to the calculated emission concentration of the exhaust gas (6). This prevents a slowing down of reactant activity.

Description

description

Method and device for the catalytic reduction of the pollutant content in the exhaust gas of an incineration plant

The invention relates to a method for catalytically reducing the pollutant content in the exhaust gas of an incineration plant, in which a reactant is added to the exhaust gas and is reacted with the pollutant on a catalyst. By means of a functional relationship, the amount of pollutants emitted by the incineration plant per time and the addition amount of the reactant are calculated from operational parameters of the incineration plant. In addition, the functional relationship is checked and corrected accordingly.

Furthermore, the invention relates to a device for the catalytic removal of a pollutant in the exhaust gas of an incineration plant with a catalytic converter through which the exhaust gas can flow for reacting a reactant with the pollutant, a metering device for introducing the reactant into the exhaust gas and with a control unit for controlling the reactant throughput in the dosing device. The control unit is designed to calculate the amount of pollutants emitted by the incineration plant per unit of time from operational parameters of the incineration plant by means of a functional relationship. The control unit is also designed for checking and correcting the functional relationship.

A functional relationship is understood to be a mathematical function that assigns a value to a state that is defined by one or more variables. The functional relationship can in particular take the form of a characteristic curve or a multi-dimensional characteristic diagram. The use of fossil fuels in combustion plants, especially in internal combustion engines for traction in a motor vehicle, poses great problems in areas with high vehicle densities, particularly in the industrialized countries, due to the pollutant content in the exhaust gas. To reduce the pollutants in the exhaust gas from gasoline engines, catalysts containing noble metals are known, on which hydrocarbons and carbon monoxide are converted with nitrogen oxides and residual oxygen to carbon dioxide, nitrogen and water. In order to reduce the pollutant emissions of diesel engines, work is currently underway in many places on the development of a regulated diesel catalytic converter. It should make it possible to significantly reduce the nitrogen oxide content in the exhaust gas of diesel engines. A so-called DeNOx catalyst is preferably used, which converts the nitrogen oxides contained in the exhaust gas with a suitable reactant, usually ammonia, to environmentally friendly nitrogen and water using the selective catalytic reduction (SCR) process. Here, the reactant or a precursor of the reactant is introduced into the exhaust gas upstream of the catalyst in the flow direction of the exhaust gas and then enters the catalyst in a preferably homogeneous mixture with the nitrogen oxides contained in the exhaust gas.

Combustion systems for traction of vehicles are operated with variable load and speed. This means that the amounts of nitrogen oxide generated per unit of time and the exhaust gas mass flows and temperatures are subject to large fluctuations. There is currently no known solution to adjust the amount of reactant to be introduced into the exhaust gas per unit of time in such a way that high separation rates for the nitrogen oxides are achieved regardless of the operating state of the internal combustion engine, while the emission of the reactant into the environment disappears at the same time. To make matters worse, a reagent such as ammonia is poisonous and, even at concentrations of only a few ppm, represents a considerable odor nuisance for humans. For this reason to avoid the emission of ammonia into the environment, the so-called reagent slip.

From DE 43 15 278 AI a method is known in which the amount of reactant introduced into the exhaust gas per unit of time is set depending on operationally relevant parameters of the engine, the catalyst and the exhaust gas. A functional relationship is stored in the form of a characteristic map in a control unit associated with the catalytic converter, with the aid of which the nitrogen oxide emissions of the engine and the amount of the reactant to be injected per unit of time are calculated from the operationally relevant parameters. However, this method does not take into account that the amount of nitrogen oxide actually generated by the engine e.g. due to signs of aging or other long-term effects may differ from the quantity calculated using the map. It is therefore not guaranteed in this process that a slippage of reactant is excluded.

DE 195 36 571 AI describes a method in which a map is continuously corrected. Under suitable conditions, a sensor measures the pollutant concentration in the exhaust gas of the internal combustion engine and compares it with a value that was calculated from the operationally relevant parameters using the characteristic diagram. If the calculated value deviates from a tolerance limit, the map is corrected. However, the amount of nitrogen oxide generated by the internal combustion engine and the amount of reactant required to break it down are subject to such rapid and strong fluctuations and are dependent on so many parameters that even with this method, a reactant slip is not reliably avoided.

The object of the invention is to provide a method and a device for the catalytic reduction of the pollutant content in the exhaust gas of an incineration plant, with which the reaction agent slip can be reliably avoided. With regard to the method, this object is achieved by a method in which a) a reactant is added to the exhaust gas and reacted with the pollutant on a catalytic converter, b) the functional quantity of pollutant emissions emitted by the incinerator is calculated from operational parameters of the incinerator by means of a functional relationship c) the addition of the reactant is calculated from the calculated amount of pollutant, d) the functional relationship is checked and corrected accordingly, according to the invention, a sub-stoichiometric amount of the reactant, based on the amount of pollutant calculated for the time unit, is added becomes.

The invention is based on the consideration that a reactant slip can occur when a larger amount of reactant is added to the exhaust gas than can be converted or stored on the catalyst. In the case of a catalyst designed for complete conversion of the pollutant, this is possible if there is more than the stoichiometric amount of the reactant based on the pollutant in the catalyst. This can be done by adding the reagent to the exhaust gas in a stoichiometric amount. Or it happens because the reactant stored in the catalyst is released due to changes in the parameters of the catalyst. Because of the enormous complexity of the underlying causes, both circumstances are extremely difficult to identify from measurements and calculations. Therefore, it makes sense not to look for the solution to the problem in measurement and calculation, as has been done so far, but to start directly with the metering of the reactant. In a further consideration, the invention is based on the fact that the complete removal of the pollutant present in the exhaust gas is not necessarily to be striven for, but a significant reduction in the pollutant is already a significant improvement. This applies particularly to the reduction of nitrogen oxides in the exhaust gas of diesel engines in vehicles. So far, the exhaust gases from diesel engines have generally escaped into the environment without removing nitrogen oxide. Therefore, a significant reduction in nitrogen oxides in the exhaust gas from diesel engines in vehicles means a significant relief for the environment. The prerequisite for this is that the long-known problem of reactant slip has to be solved. The invention achieves both in an effective manner: the nitrogen oxides are reduced by metering in the reactant onto the catalyst and a slip of the reactant is prevented by the sub-stoichiometric metering.

The sub-stoichiometry relates to the amount of pollutants in the exhaust gas calculated from a functional relationship. It acts on a faulty calculation

Overdosing of the reactant counter. To avoid a slippage of reactant, it is therefore no longer absolutely necessary by the invention that the amount of pollutant emitted by the incinerator per unit of time corresponds exactly to the calculated amount of pollutant. This is particularly advantageous when installing a dosing device, for example in a vehicle: before the dosing device is started up for the first time, the functional relationship to be stored in the control unit of the dosing device does not have to be individually adapted to the combustion system in question. It is sufficient if the functional relationship is adapted to the relevant series of the incinerator. A functional relationship belonging to a series can easily be determined by the manufacturer of the combustion system. This fact makes it much easier to equip vehicles with the metering device. In particular, the retrofitting of old vehicles possible without extensive analysis of the operational parameters of the engine.

Due to the aging of the incineration plant, it is necessary to check and correct the functional relationship from time to time. This is done by measuring the amount of pollutants in the exhaust gas and comparing the measured value with the value calculated from the operationally relevant parameters. Suitable conditions such as stationary operation. The functional relationship is then corrected in such a way that the measured value and the calculated value lie within a tolerance range. This process is also made considerably easier by the invention: While it is necessary without the invention to continuously check the functional relationship, at least at short intervals, in order to avoid a possible divergence of the calculated and actual amount of pollutants due to aging or malfunction of the engine, this is no longer necessary with general substoichiometric dosing. Due to the substoichiometric dosing, there is sufficient scope to prevent a slippage of reactant if the calculation is incorrect. The correction is therefore only necessary in larger time intervals. It can be carried out, for example, in legally prescribed emissions tests, during routine checks, or in the case of vehicles during visits to the workshop. Thus, the invention eliminates the need to permanently install a pollutant sensor in the exhaust duct.

In an advantageous embodiment of the invention, an amount of the reagent reduced by a factor of 0.1 to 0.8 compared to the calculated stoichiometric amount is metered into the exhaust gas. This, and in particular if the metered amount is only 60% or less of the calculated stoichiometric amount, ensures that there is no reactant slip in the exhaust gas even due to greater inaccuracies in the metering of the reactant. The exhaust gas behind the catalytic converter is advantageously checked for the presence of the reactant. The amount of the reagent added is expediently set such that there is essentially no reagent in the exhaust gas behind the catalyst. This control of the reactant slip prevents a reactant slip even in the event of a defect in the engine, the catalyst or the metering device. In addition, a defect that may have arisen can be detected by a check and further emissions of reactant can be prevented.

The measurement of the reactant slip is advantageously carried out with an external measuring device if necessary. This can be done without great effort during the legally prescribed exhaust gas examinations or for vehicles during a stay in the workshop. This is particularly expedient in the case of vehicles retrofitted with the metering device, since there is no need to retrofit a sensor which is sensitive to the reactant.

Alternatively, the reactant slip can of course be monitored by a sensor permanently installed in the exhaust duct behind the catalytic converter. In this method, a reaction agent slip which may be caused by a defect is recognized immediately and can be corrected by correcting the

Dosage of the reactant can be corrected quickly and automatically.

The specified method is particularly suitable for the removal of nitrogen oxides according to the selective catalytic reduction (SCR) method. Ammonia or an ammonia-releasing substance, in particular urea, is advantageous here as a suitable reactant.

Furthermore, the specified method is particularly suitable for use together with a supply operated with excess air. Internal combustion engine for traction of a motor vehicle, in particular together with a diesel engine.

With regard to the device, the object is achieved by a device which comprises a) a metering device for introducing the reactant into the exhaust gas, b) a catalytic converter (8) through which the exhaust gas can flow for converting the reactant with the pollutant, and c) a control unit for control of the reactant throughput in the dosing device, the control unit being designed for calculating the amount of pollutant emitted per unit of time from the incineration plant by means of a functional relationship from operationally relevant parameters of the incineration plant and for checking and correcting the functional relationship, and wherein the control unit according to the invention is designed for the substoichiometric dosing of the reactant in relation to the calculated pollutant content.

With a control unit designed in this way, the stoichiometric amount of the reactant resulting from the calculated amount of pollutant can be reduced and this reduced amount can be entered into the exhaust gas of the combustion system. Such sub-stoichiometric metering leads to the safe avoidance of a reagent slip.

In a further advantageous embodiment of the invention, the control unit is equipped with an input device with which a reduction factor can be entered which is used to calculate the desired sub-stoichiometric input quantity. The sub-stoichiometry relates to the calculated amount of pollutants. This input device can be designed in the form of an interface to which an external control unit, for example a control unit of an exhaust gas test system or a computer, is connected. can be closed. This external control unit transmits the desired reduction factor to the control unit of the dosing device. However, the input device can also be designed in such a way that the reduction factor is set directly, for example manually on the control unit of the metering device.

A further advantage can be achieved in that the control unit has an interface for the transmission of data obtained by means of a pollutant sensor and / or a reaction agent sensor. Such an interface can be used to input measured values or the results of measurements into the control unit of the dosing device. This is advantageous, for example, for carrying out the method of correcting the characteristic map on the basis of pollutant measurements which are carried out with an external measuring device. This interface is also advantageous when testing the exhaust gas for reactant slip by means of an external sensor, since the reduction factor can then be set immediately by the control unit in such a way that no more reactant slip occurs.

Embodiments of the invention are explained in more detail with reference to three figures. Show it:

1 shows a flowchart of the method, which comprises the method steps of calculating the pollutant content of the exhaust gas on the basis of a map, correcting the map and calculating the substoichiometric dosage;

2 shows a flowchart which, in addition to the method steps shown in FIG. 1, includes the control of the reactant slip;

3 shows a diesel engine with a downstream device for the catalytic reduction of the nitrogen oxide content in the exhaust gas. In the method according to FIG. 1, operation-relevant parameters of the incineration system are queried in a step S1 by a control unit, which is assigned to a reagent dosing device. The incinerator is a diesel engine of a motor vehicle. The operational parameters of the motor are available in a possibly existing electronic control of the motor or on appropriate sensors. These parameters relevant to the operation of the engine can be, for example, fuel consumption, exhaust gas mass flow, exhaust gas temperature and air throughput and additionally include speed, torque and accelerator pedal position. From these parameters, the amount of pollutant emitted by the engine per time is calculated in a method step S2 using a map.

As a simple example, the accelerator pedal position P, the fuel consumption B and the exhaust gas temperature T can be mentioned as operationally relevant parameters. The amount of pollutant S emitted by the engine is then calculated as an approximation

S = (B + cP) f (T),

where c is a vehicle-specific constant and f is a correction function that takes the exhaust gas temperature into account.

For more precise calculations, characteristic curves are helpful which link the amount of pollutants and individual operationally relevant parameters and which are made available by the engine manufacturers. From these characteristics, the person skilled in the art can easily create a functional relationship, for example in the form of a map, from which the amount of pollutant emitted by the engine can be calculated. Such a map is stored in the control unit of the metering device. In a third step S3 of the method, a decision is made as to whether the functional relationship should be checked. This happens, for example, when an exhaust gas control is carried out by an external exhaust gas test system and the control unit of the exhaust gas test system is connected to the control unit of the metering device. If a pollutant sensor is permanently installed in the exhaust gas duct of the engine, the measured value of the pollutant sensor, which is located in the exhaust gas duct in front of the catalytic converter, is interrogated from time to time by the control unit of the metering device. The decision depends on the type of pollutant sensor. If the sensor is a slow sensor, it is queried at longer intervals whenever a usable measured value can be expected from the sensor. Whether the query is carried out can also depend, for example, on the operational parameters of the motor. For example, the establishment of a usable measurement value may require constant operating conditions to exist for a prolonged period.

If the measured value of the pollutant sensor is not queried, the control unit of the metering device calculates the stoichiometric dose of the reactant for a unit of time in relation to the calculated amount of exhaust gas in step S8 of the method. In step S9, this calculated dose is reduced by a factor of 0.4, so that the reduced dose only makes up 60% of the stoichiometric dose. In the subsequent process step S10, the reactant is introduced into the exhaust duct of the engine in substoichiometric metering. The input can be continuously sub-stoichiometric or in short batches super-stoichiometric.

If, on the other hand, the measured value of the pollutant sensor is queried in method step S4, the measured value is compared with the calculated value in the subsequent method step S5. In method step S6, a decision is made as to whether the calculated value is within a tolerance range around the measured value lies. If this is the case, method step S8 is the next step. Otherwise, in process step

57 the functional relationship is corrected in such a way that the calculated pollutant value comes within the tolerance range around the measured value. In the further course of the process, the dosage in the process steps is as described above

58 and S9 are calculated and then dosed substoichiometrically in method step S10.

The calculation of the pollutant quantity in the exhaust gas can be incorrect in the event of strong engine load changes that lead to large fluctuations in the pollutant mass flow. In such a case, it may also be impossible to correct the calculation due to the lack of usable measured values. In this case, the substoichiometric metering (method step S9) ensures that the addition of the reactant is kept low, so that despite the incorrect calculation and the impossibility of correction, no reactant slip occurs. Furthermore, in the method described in FIG. 1, it is generally not necessary for a calculated pollutant quantity value to be within a narrow tolerance range around the value actually present. The general sub-stoichiometric dosing prevents the reagent from being added too much and the associated reagent slip. This is of particular importance when the method is carried out in older vehicles, for example where there is no electronic engine control. With these vehicles, engine parameters that are not relevant to operation can be called up, which makes the calculation of the pollutant quantity value difficult and inaccurate. The substoichiometric dosing makes it possible to tolerate a further error range of the calculated value around the actually present pollutant quantity value.

The correction process of the functional relationship shown in the process steps S4 to S7 ensures that the calculated and actually present pollutant amount of exhaust gas does not move significantly apart over time. This danger exists, for example, with an old engine, in which some parameters relevant to operation, such as accelerator pedal position and speed, do not lead to the same pollutant levels in the exhaust gas as with a new engine. The correction can either be started automatically by the control unit of the metering device if a pollutant sensor is permanently installed in the exhaust gas duct, or it is carried out during an exhaust gas inspection during a visit to the workshop. This option is particularly advantageous when retrofitting the dosing device in old vehicles, since in this case no pollutant sensor has to be installed subsequently.

The process shown in FIG. 1 ensures that no reactant passes through the catalyst when the components of the catalyst unit are intact. However, if the catalyst or part of the control unit of the metering device is defective, a slippage of reactant cannot be ruled out. FIG. 2 shows a test and correction method which prevents the slippage of the reactant in this case too. Process steps S1 to S10 inclusive of FIG. 2 are identical to those in FIG. 1. The pollutants to be reduced are nitrogen oxides, the reactant is ammonia and the catalyst is a DeNOx catalyst. In step S15 of the method it is checked whether the condition for a reagent measurement behind the catalytic converter is fulfilled. This condition is that a sensor of a stationary, ie external exhaust gas test system has been introduced into the exhaust gas duct and the control unit of the exhaust gas test system has been connected to the control unit of the metering device. If the condition is met, the measurement for reactants in the exhaust gas behind the catalytic converter is carried out in the subsequent step S16. If the test S17, whether a reactant slip is present, is positive, the calculation of the dosage of the reactant (method step S9) is carried out in a method step S13 corrected so that the dosage (step S10) is reduced. In this way it is ensured that - if necessary after a few cycles S1 to S18 - the dosage has been reduced so that there is no longer any reagent slip.

In Figure 3, a diesel engine is explained with a connected device for catalytic nitrogen oxide reduction, which works according to the specified method. The diesel engine 1 operated with excess air for traction of a motor vehicle has an interface 3 at which the current values of operationally relevant parameters can be tapped. A fuel / air mixture for combustion is made available to the diesel engine via a fuel supply 4 and an air supply 5. The exhaust gas 6 of the diesel engine is passed through an exhaust duct 7 and further through a catalytic converter 8. The catalytic converter 8 is designed as a so-called DeNOx catalytic converter, which uses the known SCR process to decompose nitrogen oxides into molecular nitrogen and water using the ammonia reactant. The required amount of ammonia is obtained by hydrolysis from metered urea. The quantity of pollutants and reactants in the exhaust gas is monitored with a device 30 and control unit 31 of a stationary exhaust gas test system connected to the exhaust gas duct 7.

For the metering of the urea, a metering device 9 is provided, which for the reactant 10 (= urea) comprises a storage container 11, a feed line 12, a metering valve 13 and an injection nozzle 14. to

A control unit 18 is provided for controlling the dosing device 9. The current values for speed, position of the accelerator pedal and engine temperature are available to the control unit via outputs 19, 20 and 21 of the interface of the diesel engine. The control unit 18 controls the metering valve 13 of the metering device 9 according to the method described in FIG. 1. A correspondingly reduced amount of reactant 10 is added to the exhaust gas 6 per unit time in accordance with an adjustable reduction factor compared to the stoichiometric amount of reactant 10. The reduction factor can be input via an input device 22 of the control unit 18. The metered urea breaks down in the exhaust gas 6 by hydrolysis into ammonia and residues, ammonia as a reactant reacting with the nitrogen oxides on the catalyst 8.

An analysis of the exhaust gas from the incineration plant is carried out by an exhaust gas test system which comprises a device 30 for connection to the exhaust gas duct 7, a control unit 31 and a pollutant sensor 32 for detecting the nitrogen oxide concentration by changing the conductivity. The pollutant sensor 32 measures the amount of nitrogen oxide in the exhaust gas 6 of the engine 1. It is connected to the control unit 31 of the exhaust gas test system via an interface 33. The control unit 31 of the exhaust gas test system is connected via an interface 35 to the control unit 18 of the metering device 9 and transmits the pollutant quantity value to the control unit 18. The control unit 18 carries out the method described in FIG. 1 for correcting the functional relationship.

In addition, the reactant slip is measured in the device 30 of the exhaust gas test system by means of a reactant sensor 36. The measured value is passed on to the control unit 31 of the exhaust gas test system via an interface 37.

The amount of reactant in the exhaust gas 6 downstream of the catalytic converter 8 is passed to the control unit 18 of the metering device via the interface 35. According to the method described in FIG. 2, the control unit 18 adjusts the reactant metering in such a way that no reactant slip occurs.

Claims

claims

1. A process for the catalytic reduction of the pollutant content in the exhaust gas (6) of an incineration plant (1), in which a) a reaction agent (10) is added to the exhaust gas (6) (SlO) and is reacted with the pollutant on a catalyst (8), b) by means of a functional relationship from operationally relevant parameters of the incineration plant, the amount of pollutant emitted per unit of time is calculated c) from the calculated amount of pollutant, an addition amount of the reactant (10) is calculated (S2, S8), d) the functional relationship is checked (S4 -S6) and is corrected accordingly (S7), characterized in that a sub-stoichiometric amount of the reactant (10), based on the amount of pollutant calculated for the time unit, is added to the exhaust gas (6) (S9, S10).

2. The method according to claim 1, d a d u r c h g e k e n n z e i c h n e t that an amount of the reagent (10) reduced by a factor of 0.1 to 0.8 compared to the calculated stoichiometric amount is metered.

3. The method according to claim 1 or 2, characterized in that the exhaust gas (6) behind the catalyst (8) for the presence of the reaction agent (10) is checked (S16), and that the amount added is set in such a way (S18, S2, S8, S9) that there is essentially no reactant (10) in the exhaust gas behind the catalyst (8).

4. The method according to claim 3, characterized in that the check (S16) is carried out if necessary with an external test device (17).

5. The method according to any one of claims 1 to 4, characterized in that the reactant (10) is ammonia or an ammonia-releasing substance, in particular urea, and as a pollutant nitrogen oxides according to the method of selective catalytic reduction on a DeNOx catalyst (8) be eliminated.

6. The method according to any one of claims 1 to 5, so that the combustion system is an internal combustion engine (1) operated with excess air for traction of a motor vehicle.

7. Device for the catalytic removal of a pollutant in the exhaust gas (6) of an incineration plant (1), a) with a catalytic converter (8) through which the exhaust gas (6) can flow for converting a reactant (10) with the pollutant, b) with a metering device ( 9) for introducing the reactant (10) m the exhaust gas (6) and c) with a control unit (18) for controlling the reactant throughput m of the metering device, the control unit (18) for calculating the unit of time emitted by the incinerator The amount of pollutant is designed by means of a functional relationship from operationally relevant parameters of the incineration plant (1) and for checking and correcting the functional relationship, characterized in that the control unit (18) for the substoichiometric metering of the reactant (10) m in relation to the calculated Pollutant content is designed.

8. The device according to claim 7, characterized in that the control unit (18) has an input device (22) with which em reduction factor can be entered, which is used to calculate the substochiometric input quantity.

9. The device according to claim 7 or 8, that the control unit (18) has an interface (35) for the transmission of data obtained by means of a pollutant sensor (32) and / or a reactant sensor (36).