Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2000-06-19
2002-06-25
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C060S295000, C060S303000
Reexamination Certificate
active
06408619
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method and a device for the catalytic conversion of nitrogen oxides in the exhaust gas of a combustion system, in particular of a diesel engine.
The combustion of fossil fuel such as petroleum or coal in a combustion system, in particular of diesel fuel in a diesel engine, may involve the production of not inconsiderable amounts of pollutants which may be discharged via the exhaust into the environment and cause damage there. Of these pollutants, the nitrogen oxides in particular present a not inconsiderable problem, since they are believed to be one of the factors contributing to acid rain and the dying of forests.
To reduce the amount of nitrogen oxides emitted into the environment it is known to use catalysts arranged in the exhaust line of a combustion system for the catalytic conversion of the nitrogen oxides contained in the exhaust gas into harmless substances.
For an internal combustion engine operated with excess air, such as a diesel engine, it is known to remove the nitrogen oxides from the exhaust gas by the process of selective catalytic reduction (SCR process). In this case, a reducing agent is introduced into the exhaust gas before it flows through a so-called SCR or DeNOx catalyst, said reducing agent converting the nitrogen oxides contained in the exhaust gas in the presence of oxygen at the catalyst into harmless nitrogen and water. Ammonia is generally used here as the reducing agent. It is also known to supply the reducing agent by introducing into the exhaust gas a reducing agent solution, from which the actual reducing agent is released. In the case of ammonia, such a reducing agent solution is, for example, an aqueous urea solution or ammonia water. See in this respect the Siemens company publication: SINOx, Stickoxidminderung für stationäre Dieselmotoren [Nitrogen Oxide Reduction For Stationary Diesel Engines], 1997, order No.: A96001-U91-A232.
In the reduction of nitrogen oxides by the SCR process, an amount of reducing agent that is adapted to the current nitrogen oxide emission must constantly be introduced into the exhaust gas in order to achieve a high conversion rate of the nitrogen oxides at the catalyst, without being accompanied by an appreciable leakage of reducing agent, which in the case of ammonia would be harmful for the environment.
In particular, in the case of non-stationary combustion systems, such as diesel engines for the traction of trucks or passenger cars, which are operated with frequent load changes, it proves to be extremely difficult to determine the amount of reducing agent to be introduced. Since high-speed sensors for directly determining the nitrogen oxide concentration in the exhaust gas are not so far known, the nitrogen oxide concentration must be determined from the parameters characterizing the operating state of the combustion system, such as rotational speed, torque or control rod displacement. For this purpose, the concentration of the nitrogen oxides is respectively determined for defined operating states of the combustion system on a test bench and are stored in a characteristic map in a monitoring unit. This characteristic map is then used during operation to read from the parameters characterizing the operating state of the combustion system the concentration of the nitrogen oxides and to meter in a corresponding amount of reducing agent or reducing agent solution. However, especially when there are highly fluctuating exhaust gas temperatures or when the combustion system or the catalyst is ageing, it is always necessary to maintain a certain safety margin with respect to the amount of reducing agent actually required in order to be certain of avoiding any leakage of the reducing agent. Therefore, slightly less reducing agent than would be necessary for conversion of the nitrogen oxides is always fed in. To avoid leakage, the system operates below the conversion rate that is theoretically possible.
For more accurate metering and to reduce the safety margin, it is also known from German patent application DE 195 36 571 A1 to take into consideration parameters of the exhaust gas such as the temperature, pressure or chemical composition, as well as parameters characterizing the catalyst, such as the storage capacity, for example for reducing agent, temperature, catalytic activity or structure, in the determination of the amount of reducing agent to be metered in. Considering in particular the storage capacity of the catalyst allows a further reduction of the safety margin, and consequently an increase in the conversion rate, since then the reducing agent stored by the catalyst at low temperatures by adsorption and given off to the exhaust gas at high temperatures by desorption is also taken into account.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method and a device for the catalytic reduction of nitrogen oxides contained in the exhaust gas of a combustion system, that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, in which it is certain that reducing agent leakage is avoided and at the same time the conversion rate of the nitrogen oxides is increased in comparison with the prior art.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for catalytically reducing nitrogen oxides in the exhaust gas of a combustion system, in particular of a diesel engine, in which method an amount of a reducing agent solution to be metered is determined in dependence on parameters characterizing the operating state of the combustion system, the exhaust gas and/or a catalyst and is introduced into the exhaust gas upstream of the catalyst in the direction of flow of the exhaust gas, the reducing agent is released from the reducing agent solution and converted with the nitrogen oxides at the catalyst, and according to the invention the concentration of the reducing agent solution is determined discontinuously during and/or before the reducing agent solution is introduced into the exhaust gas and the amount to be metered is adapted to the concentration determined. The method of reducing nitrogen oxides in the exhaust gas of a combustion system accordingly comprises the steps of providing a source of reducing agent and an exhaust gas line, introducing into the exhaust gas line upstream of the catalyst in the direction of flow of the exhaust gas an amount of reducing agent metered in dependence on the concentration of reducing agent in said source and at least one parameter characterizing the operating state of the combustion system, the exhaust gas and/or the catalyst, and flowing the exhaust gas and the reducing agent over the catalyst, wherein the concentration of the reducing agent in the source is determined discontinuously during and/or before the reducing agent is introduced into the exhaust gas and the amount to be metered is adapted to the concentration determined.
The invention is based on the observation that, even with an amount of reducing agent to be metered in that has been calculated with the storage capacity of the catalyst and the composition of the exhaust gas taken into consideration, under certain operating conditions a leakage of reducing agent can occur. The leakage of reducing agent occurs to an increased extent when the actual reducing agent is released from a reducing agent solution, which may be a consequence of different concentrations of the reducing agent solutions used. Since, in conventional metering methods, a fixed concentration is taken as a basis, different concentrations resulting from production reasons or due to effects of evaporation lead to not inconsiderable deviations of the amount of reducing agent actually introduced in comparison with the amount intended. These metering errors when introducing the reducing agent solution can be avoided, however, by using the concentration of the reducing agent solution in the determination of the amount of reducing agent actu
Hofmann Lothar
Pajonk Günther
Weigl Manfred
Wissler Gerhard
Lerner Herbert L.
Siemens Aktiengesellschaft
Tran Binh
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