Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-10-02
2004-05-18
Tran, Hien (Department: 1764)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S235000, C422S172000, C422S177000, C422S180000, C060S274000, C060S301000
Reexamination Certificate
active
06737033
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method for the catalytic removal of a pollutant contained in an exhaust gas of a combustion system. A predetermined amount of a reagent is thereby introduced into the exhaust gas as a function of the concentration of the pollutant per unit time and is reacted with the pollutant at a catalytic converter. The invention also relates to a device for carrying out the method.
The combustion of a fossil fuel or of refuse in a combustion system leads to the formation of not inconsiderable quantities of pollutants, such as nitrogen oxides, hydrocarbons, carbon monoxide, sulfur oxide and, in particular, dioxins and furans. These may pass into the environment via the exhaust gas from the combustion system. Examples of a pollutant-emitting combustion system of this nature include a boiler system, a coal-fired, oil-fired or gas-fired fossil power plant, a gas turbine, or an internal-combustion engine, in particular a diesel engine. Refuse incineration plants also emit the above-mentioned pollutants.
On account of strict statutory regulations which limit the amount of the above-mentioned pollutants which may be released, all the above-mentioned combustion systems require additional treatment of the exhaust gases in order to reduce, the level of pollutants contained therein. To this end, a wide variety of catalytic converters which convert the pollutants into compounds which are not hazardous have been developed in the past.
For example, to reduce the level of pollutants in the exhaust gas from a spark-ignition engine (Otto cycle), catalytic converters which contain precious metals are known, at which hydrocarbons and carbon monoxide together with nitrogen oxides are converted to form carbon dioxide, nitrogen and/or water. Catalytic converters for breaking down dioxins and/or furans are also known, comprising a catalytically active material based on titanium dioxide. At those catalytic converters the dioxins and/or furans are oxidized with residual oxygen which is present in the exhaust gas to form compounds which are not harmful.
If the exhaust gas composition is unsuitable, it may be necessary for an additional reagent to be added to the exhaust gas, which reagent reacts with the pollutant to be eliminated at a suitable catalytic converter, forming harmless compounds. By way of example, to break down nitrogen oxides in oxygen-containing exhaust gases, a suitable reducing agent has to be added to the exhaust gas as a reagent. The reducing agent reduces the nitrogen oxides which are present in the exhaust gas to form harmless nitrogen even in the presence of oxygen. This reaction can be catalyzed by a so-called DeNOx catalytic converter based on titanium dioxide with added vanadium pentoxide, molybdenum trioxide, and/or tungsten trioxide, a which uses the selective catalytic reduction process to react nitrogen oxides with a suitable reducing agent, such as ammonia, to form nitrogen and water.
To fully break down the pollutant in the exhaust gas, the reagent which is added separately must be added in a stoichiometric amount with respect to the concentration of the pollutant. However, since in combustion systems the concentration of the pollutant in the exhaust gas often fluctuates over time and since, in addition, other factors of the exhaust gas, such as temperature or pressure, influence: the reaction of the pollutant with the reagent, excessive quantities of the reagent are metered in relatively frequently. This leads to the reagent being emitted into the environment together with the exhaust gas. This emission of reagent, which is also known as reagent slippage or simply slippage, has to be avoided if the reagent itself is harmful. In addition, excessive metering of the reagent also always means that operation is uneconomic, since expensive reagent remains unused in the exhaust gas.
Therefore, considerable technical outlay is devoted to attempting to meter the reagent in stoichiometric proportions which are as exact as possible and adapted to the current state of the exhaust gas. In this context, to determine the amount of reagent which is to be introduced into the exhaust gas per unit time, it is not only necessary for the concentration of the pollutant in the exhaust gas to be either directly measured or calculated on the basis of characteristic curves derived from relevant operating parameters of the combustion system, but rather, in addition, it is also necessary to take into account relevant operating parameters of the catalytic converter, e.g. catalytic converter temperature, catalytic activity or storage capacity, and parameters of the exhaust gas, e.g. pressure, gas composition or temperature, using suitable analysis methods. A technically complex method of this nature for determining the amount of a reagent metered into an exhaust gas containing nitrogen oxides is known, for example, from U.S. Pat. No. 5,628,186 (see European Patent EP 0 697 062 B1).
However, a complex method of this nature with an expensive measurement analysis system is inappropriate for retrofitting in relatively old combustion systems in particular for cost reasons. Also, in particular in the exhaust system of internal-combustion engines used to drive vehicles, there is no further additional space for sensor elements which are required to record parameters that are specific to the exhaust gas or catalytic converter. In addition, the required introduction device has to be of highly dynamic design in order to achieve exact metering, so that it can respond quickly and accurately under every operating state of the combustion system with corresponding exhaust gas parameters. This also entails excessive costs.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method and device, which overcomes the above-mentioned disadvantages of the heretofore known devices and methods of this general type and which provides for a particularly inexpensive and simple method for the catalytic removal of a pollutant contained in an exhaust gas of a combustion system by reacting a reagent, without allowing, in the process, any significant slippage or excessive consumption of reagent to occur. It is a further object to also provide a device for carrying out the method which is particularly simple, takes up little space and, in addition, is inexpensive.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of catalytically removing a pollutant contained in an exhaust gas of a combustion system, which comprises:
determining a time average for a concentration of a pollutant in the exhaust gas;
providing a catalytic converter designed for substantially complete conversion of the pollutant given stoichiometric metering of a reagent with respect to the pollutant;
introducing a substoichiometric amount of reagent, in dependence on the time average for the concentration of the pollutant, into the exhaust gas; and
conducting the exhaust gas with the reagent to the catalytic converter and reacting the reagent with the pollutant at the catalytic converter.
In other words, the time average, or temporal mean, is formed for the concentration of the pollutant, and, in the case of a catalytic converter which, given stoichiometric metering of the reagent, is designed for substantially complete conversion of the pollutant, the reagent is introduced into the exhaust gas in substoichiometric proportions with respect to the average.
In a first step, the invention is based on the fact that it is possible to avoid reagent slippage if the pollutant is not converted completely, but rather only to a significant extent. In fact, in such a case, the reagent can be metered in substoichiometric proportions according to the desired conversion level throughout the entire operating period of the combustion system. This offers sufficient security in the event of minor fluctuations in the concentration of the pollutant in the event of unpredictable operating states of the combustion system. However, with a p
Hofmann Lothar
Mathes Wieland
Siemens Aktiengesellschaft
Tran Hien
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