Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
2000-10-24
2002-09-03
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S286000, C060S295000, C060S303000
Reexamination Certificate
active
06442932
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 contained in the off-gass from a combustion system, in particular in an exhaust gas from an internal combustion engine operated with excess air, for example a diesel engine.
During the combustion of a fossil fuel, such as petroleum or coal, in a combustion system, in particular of diesel fuel in a diesel engine, nitrogen oxides that are hazardous to the environment are formed, inter alia. To reduce the emissions of nitrogen oxides to the environment, the use of a catalytic converter disposed in the off-gas line of a combustion system is known, inter alia, in the power plant sector. The catalytic converter is used to catalytically convert the nitrogen oxides contained in the off-gas into substances that are not hazardous.
In an internal combustion engine that is operated with excess air, nitrogen oxides are removed from the exhaust gas using, for example, a 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 DeNO
x
catalytic converter, which reducing agent converts the nitrogen oxides which are contained in the exhaust gas, in the presence of oxygen, at the catalytic converter to form harmless nitrogen and water. The reducing agent used is generally ammonia. The reducing agent is introduced into the exhaust gas in the form, for example, of a reducing agent solution from which the actual reducing agent is released. In the case of ammonia, a reducing agent solution of this type is, for example, an aqueous urea solution. In this respect, see the Siemens brochure, titled “SINO
x
, Stickoxidminderung für stationäre Dieselmotoren” [SINO
x
, Nitrogen Oxide Abatement For Steady-State Diesel Engines], 1997, order No. A96001-U91-A232.
When reducing the levels of nitrogen oxides using the SCR process, it is always necessary for an amount of reducing agent that is adapted to the current nitrogen oxide emissions to be introduced into the exhaust gas. This on the one hand leads to a high conversion rate for the nitrogen oxides at the catalytic converter and on the other hand prevents too much reducing agent from being introduced, which then leaves the catalytic converter together with the exhaust gas and enters the environment. The emission of reducing agent into the environment is also known as slippage. This phenomenon is to be avoided in particular when using ammonia, in order to avoid additional pollution of the environment.
Particularly in the case of combustion systems that are not operated in a steady state, it is difficult to determine the amount of reducing agent that needs to be introduced per unit time. Examples of combustion systems which are operated in a non steady state include diesel engines which are used in the automotive sector and are operated with frequent load changes. Therefore, the emission of nitrogen oxides may vary considerably within short periods of time. Consequently, it is also necessary for the amount of reducing agent metered in to vary quickly and to be accurately adjusted. The amount of reducing agent to be introduced therefore needs to be controlled according to demand. The current demand is determined on the basis of parameters that characterize the operating state of the combustion system. In a diesel engine, these parameters are, for example, the engine speed, the torque, the operating temperature or the fuel consumption. It is known from Published, Non-Prosecuted German Patent Application DE 19 536 571 A1 to additionally use parameters which characterize the operating state of the catalytic converter. Examples of these parameters are the capacity of the catalytic converter to store the reducing agent, the operating temperature and/or the catalytic activity of the catalytic converter.
The amount of reducing agent required for conversion of the nitrogen oxides is determined from the various parameters, for example on the basis of a characteristic diagram. To determine the volume of reducing agent solution which is to be metered per unit time, when using a reducing agent solution it is additionally necessary to take into account the properties, for example the concentration, of this solution. Generally, the amount of reducing agent is determined in such a way that slightly less reducing agent than is required for conversion of the nitrogen oxides is fed to the catalytic converter, so that slippage is avoided under all circumstances. The catalytic converter is therefore operated at below the conversion rate that is theoretically possible. The conversion rate indicates the proportion of nitrogen oxides that are reduced at the catalytic converter.
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 off-gas from a combustion system that overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, in which slippage of the reducing agent is reliably avoided and, at the same time, a high conversion rate for the nitrogen oxides at the catalytic converter is achieved.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for a catalytic reduction of nitrogen oxides contained in a medium flow output from a combustion system. The method includes the steps of:
a) channeling an off-gas towards and through a catalytic converter;
b) metering a reducing agent solution into the off gas before it flows through the catalytic converter;
c) determining an amount of dissolved reducing agent to be metered per unit time according to demand; and
d) using a density of the reducing agent solution to determine a volume of the reducing agent solution to be metered per unit time.
According to the invention, to achieve the object in the method for the catalytic reduction of nitrogen oxides, in particular of nitrogen oxides contained in the exhaust gas from an internal combustion engine operated with excess air, the off gas flows through a catalytic converter. A reducing agent solution is metered into the off gas before it flows through the catalytic converter, and the amount of dissolved reducing agent which is to be metered per unit time is determined according to demand. The density of the reducing agent solution is used to determine the volume of reducing agent solution that is to be metered per unit time.
The invention is based on the consideration that, if the reducing agent solution is metered volumetrically, the amount of reducing agent metered in is influenced by the density of the reducing agent solution. Density fluctuations occur primarily in the event of temperature changes. As a result of the density being included, the inaccuracy of metering caused by density fluctuations is largely prevented and a high metering accuracy is achieved. As a result, the desired conversion rate is achieved with the maximum possible accuracy without slippage occurring.
The reducing agent solution used is preferably an aqueous urea solution. The urea solution is metered into the hot off gas. In the process, the actual reducing agent, namely ammonia, is released from the dissolved urea. Together with the nitrogen oxides, the ammonia enters the catalytic converter, where it reduces the nitrogen oxides to form nitrogen on the catalytically active surface.
To determine the density, it is preferable to measure the temperature of the reducing agent solution and to control the volume of reducing agent solution to be metered as a function of the density derived from the temperature. The temperature is the significant determining parameter for the density and is easy to determine using standard temperature sensors. Since the reducing agent solution is in liquid form and is virtually incompressible, the density is substantially unaffected by the influences of pressure.
The temperature measurement may be carried out as early as in a reservoir f
Hofmann Lothar
Pajonk Günther
Weigl Manfred
Wissler Gerhard
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