Process for reducing nitrogen oxides on SCR catalyst

Details Process for reducing nitrogen oxides on SCR catalyst Process for reducing nitrogen oxides on SCR catalyst

2000-01-27

2002-06-04

Griffin, Steven P. (Department: 1754)

Chemistry of inorganic compounds

Modifying or removing component of normally gaseous mixture

Mixture is exhaust from internal-combustion engine

C060S274000, C060S286000, C060S295000, C423S239100

Reexamination Certificate

active

06399034

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to the field of reducing primary injurious substances generated in internal combustion engines by means of a catalyst. The invention relates, in particular, to a process for reducing nitrogen oxides (NO
x
) on a SCR catalyst, disposed in the exhaust gas system of an internal combustion engine, with ammonia (NH
3
) which is preferably introduced into the exhaust gas flow before the catalyst. The invention further relates to a device for introducing ammonia (NH
3
) into the exhaust gas flow of an internal combustion engine for reducing nitrogen oxides (NO
x
) contained in the exhaust gas flow on a SCR catalyst comprising a NH
3
source, a supply line for introducing the NH
3
into the exhaust gas flow and a dosing device.
Apart from carbon monoxide (CO) and hydrocarbons (HC), in particular the nitrogen oxides (NO
x
) are among the environmentally harmful, directly emitted, primary injurious substances which are generated during the operation of internal combustion engines, in particular Diesel engines. The use of three-way catalysts, such as are used in Otto engines and gas engines, cannot be used in the exhaust of Diesel engines due to an oxygen excess. For this reason, for the reduction of the nitrogen oxide emission in Diesel engines a selectively operating SCR (Selective Catalytic Reduction) catalyst has been developed in which, in the presence of an added reducing agent, namely ammonia (NH
3
), the expelled nitrogen oxides are reduced to N
2
and H
2
O.
This type of reducing nitrogen oxide emissions in stationary Diesel engines has been found to be useful. In these stationary installations in the exhaust gas system of the internal combustion engine is disposed a SCR catalyst and the NH
3
to be added to the exhaust gas flow before the SCR catalyst is added by injection. In such installations the NH
3
is supplied in the form of a gas or aqueous solution. In the case in which NH
3
is supplied as an aqueous solution, a thermolytic splitting takes place in the exhaust gas flow, or in the SCR catalyst, in order to release the NH
3
necessary for the reduction of nitrogen oxides. The introduction of the reducing agent takes place via a dosing device which is adjusted as a function of the anticipated NO
x
quantity in the exhaust gas flow of the Diesel unit. When using NH
3
either as the gas or as aqueous solution, the use of skilled personnel is required since associating with these substances is not without danger with respect to their handling as well as also with respect to their toxicity.
Due to the not undangerous association with NH
3
in gaseous form or as an aqueous solution, it has become customary to provide the NH
3
required for the NO
x
reduction by introducing aqueous urea solution into the exhaust gas flow. The thermohydrolytic splitting with the liberation of NH
3
as reducing agent takes place through the heat of the exhaust gas flow or of the catalyst. In addition to the difficulties relating to the process of the unchecked feed of this reducing agent in the form of an aerosol, a further disadvantage entailed in obtaining NH
3
in this way is the formation of undesirable byproducts such as, for example, isocyanic acid. The fact that the freezing point of such a reducing agent is approximately −13° C., moreover, stands in the way of using aqueous urea solutions for the NO
x
reduction of Diesel engine exhaust gases in mobile units, for example in utility vehicles or passenger motor vehicles. The winter-worthy use of this reducing agent is therefore only possible by mixing in additives which decrease the freezing point. However, the conversion of these additives in the SCR catalyst can lead to the emission of undesirable secondary injurious substances.
It is furthermore necessary to carry along a relatively large quantity of aqueous urea solution since the urea necessary for the reduction is only present in the aqueous solution at a ratio of at best 1:3 with respect to the H
2
O.
The use of NH
3
carried along in pressure tanks or as aqueous solution for removing nitrogen in mobile Diesel units is not possible in the event of an accident due to the danger inherent in these substances.
From DE 34 22 175 A1 as well as DE 42 00 514 A1 processes are known which relate to the “just-in-time” production of NH
3
for the reduction of NO
x
. The superior concept evident in these patents comprises using specific substances which thermolytically split off NH
3
, the handling and toxicity of which are quite safe, in order to split off the required quantity of NH
3
in accordance with the particular requirements and to inject it into the exhaust gas flow. The generation of NH
3
takes place by heating such an NH
3
-splitting compound, for example ammonium carbamate. The adaptation to the particular operating state of the motor, as described in DE 34 22 175 A1, takes place thereby that by controlling the calorific power, which for example acts on the carbamate, the NH
3
generation and thus the NH
3
quantity added to the exhaust gas flow can be regulated.
Such a regulatable NH
3
generator comprises essentially a storage tank for the compound splitting off NH
3
, a decomposition chamber in which the action of the heat takes place, as well as a takeout line or supply line for the NH
3
-containing gas. For heating the decomposition chamber, in particular electric heating [systems], for example resistance heating bodies or infrared radiators are provided.
The use of this prior known process or this prior known device is suitable for stationary installations which in general are only subject to few load changes and even then only to predetermined load changes, however, for deployment in mobile use, such as for example in utility vehicles or passenger vehicles, the use of this prior known technology is not appropriate since the reaction time of the system is too slow and thus the system too inert in order to be able to do justice to the unexpected motor load changes occurring in rapid succession in street traffic with correspondingly varying NO
x
emissions. If the operating state of the motor is acquired at a specific time and, based on this acquisition, a specific NH
3
dosing is calculated, the substance splitting off the NH
3
must first be heated in order to produce the NH
3
-gas mixture. However, in street traffic, in particular in city traffic, an engine is subject to continuous and unexpected load changes such that the quantity of NH
3
which is lastly introduced does not match the operating state of the engine which has changed in the meantime. If an insufficient quantity of NH
3
is introduced, the NO
x
reduction can only be carried out to a limited extent. If the NH
3
is introduced in excess, unconsumed NH
3
exits the catalyst.
SUMMARY OF THE INVENTION
Building on the discussed prior art, the invention is therefore based on the task of proposing a process as well as a device for the NO
x
reduction of exhaust gases of an internal combustion engine using a SCR catalyst, which is not only suitable for mobile application but with which the above listed disadvantages are avoided.
This task is solved according to the invention through a process according to the species thereby that the process comprises the following steps:
initial heating of a substance or substance mixture at ambient temperature and capable of thermolytically splitting off NH
3
disposed in a pressure-tight receptacle (converter) to that temperature range at which the splitting-off of NH
3
takes place (splitting-off temperature range),
maintaining the splitting-off temperature range until an internal pressure builds up in the converter,
intermediately storing the split-off NH
3
or NH
3
-gas mixture and, after reaching a predetermined reduction gas pressure in the NH
3
storage,
acquiring engine operating characteristics according to which the NO
x
content in the exhaust gas flow can be calculated, based on which characteristics subsequently the determination of the NH
3
dosing, necessary for the reduction of the calculated NO
x
c

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