Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2001-10-30
2004-07-06
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S239100, C423S358000, C422S109000, C422S168000, C422S172000
Reexamination Certificate
active
06759021
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for introducing at least one gaseous reducing agent for a nitrogen oxide into a gas mixture, thermal energy being supplied to at least one solid starting material of the reducing agent, so as to form the reducing agent, and the reducing agent and the gas mixture being brought into contact with one another. A method of this type is known, for example, from U.S. Pat. No. 4,731,231. As well as the method, the invention provides a device for carrying out the method.
The combustion of a fossil energy carrier in the presence of air involves the formation of the undesirable byproduct nitrogen oxide. With a view to making the combustion environmentally friendly, it is desirable to suppress the formation of nitrogen oxide or to subsequently remove nitrogen oxides from an exhaust gas from the combustion. Subsequent removal of the nitrogen oxides is achieved, for example, by admixing a reducing agent for the nitrogen oxides with the exhaust gas. In the method which is known from U.S. Pat. No. 4,731,231, cyanuric acid is introduced into the exhaust gas as reducing agent. On a surface of a catalytic converter, the cyanuric acid reacts with nitrogen oxide to form molecular nitrogen and carbon dioxide. According to this method, the reducing agent (cyanuric acid) is obtained through sublimation of a solid starting material of the reducing agent, for example cyanuric acid. The cyanuric acid is prepared in a heatable vessel.
For effective reduction of nitrogen oxide, it is necessary to introduce into the exhaust gas a quantity of reducing agent which corresponds to a level of nitrogen oxide in the exhaust gas.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method for introducing a gaseous reducing agent for nitrogen oxides into a gas mixture, in which the amount of reducing agent which is to be introduced into the gas mixture can be adjusted in a simple way.
To achieve the object, the invention provides a method for introducing at least one gaseous reducing agent for a nitrogen oxide into a gas mixture, thermal energy being supplied to at least one solid starting material of the reducing agent, so as to form the reducing agent, and the reducing agent and the gas mixture being brought into contact with one another, characterized in that the thermal energy is supplied to at least one body made from the starting material.
The gas mixture is, for example, the exhaust gas from an internal-combustion engine, in particular a diesel or spark-ignition engine. The gas mixture may also be the off-gas from a fossil-fired powerplant.
The gaseous reducing agent, is, for example, ammonia. Ammonia has the advantage that it reacts (in the presence of a catalyst) with nitrogen oxides to form molecular nitrogen. The only byproduct formed is carbon dioxide.
One example of a starting material of the reducing agent is urea. Urea liberates ammonia at a temperature which lies above the melting point of the urea. The only byproduct of hydrolysis of the urea (decomposition in the presence of water, which is automatically present in the exhaust gas from an internal-combustion engine) is carbon dioxide. To this extent, urea can be regarded as an ideal starting material for ammonia. However, urea is hygroscopic and there are therefore restrictions on its storage.
A hygroscopic property of a material can be diminished by reducing a reactive surface area of the material for the absorption of water. Therefore, the starting material is provided in the form of a body. In particular, a body with a compacted starting material is used. The body consists, for example, of a compressed powder of the starting material. Melted-down starting material is also conceivable. The starting material may also be embedded in a solid matrix. The material of the matrix is preferably hydrophobic. Moreover, it should be easy to convert the material into environmentally friendly reaction products.
In a particular configuration of the invention, the thermal energy is provided by a means for controlling the temperature of the body. The thermal energy is supplied to a surface of the body. In particular, the thermal energy is supplied with the aid of a temperature-controllable holding device for the body by thermal conduction.
By way of example, it is also possible to supply the thermal energy by convection or thermal radiation. In a further configuration, the thermal energy is provided by the gas mixture.
In a particular configuration of the method, a surface of the body at which the formation of the reducing agent takes place and the gas mixture are brought into contact with one another. By way of example, the exhaust gas from the internal-combustion engine flows within an exhaust pipe. The flowing gas mixture, which has been heated by the combustion, is guided past the body. The temperature of the gas mixture fluctuates, for example, between 350° and 600° C. These temperatures are sufficient to liberate ammonia from urea. In this configuration, the thermal energy is provided through convection by the gas mixture.
In a further configuration, the temperature-control means functions as a catalyst for the formation of the reducing agent. By way of example, the temperature-control means is a device for holding the body comprising the starting material. The holding device functions, for example, as a hydrolysis catalyst for urea.
In a particular configuration, ammonia is formed as reducing agent in the method. Aliphatic ammonia derivatives (amines) are also conceivable. In particular, the starting material of the reducing agent used is urea. Other starting materials, such as for example ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate, ammonium formate, ammonium oxalate, ammonium hydroxide, cyanuric acid, urea-formaldehyde, melamine and mixtures of these compounds.
In particular, a body which consists of a compacted starting material is used in the method. This is, for example, a compressed powder of the starting material or the starting material in melted-down form, in which case the body is formed through solidification of a melt in a mold or on a device for dissipation of heat. A body of this type has a less hygroscopic nature than pulverulent, loose starting material. The body can be stored successfully.
To carry out this method in practice, it is also advantageous to monitor the amount of starting material which is present. Monitoring takes place, for example, with the aid of a light barrier which indicates a filling level of the starting material or a size of the body made from the starting material.
In a particular configuration of the method, the body is pressed against the holding device. This ensures that the body is in constant contact with the means for controlling the temperature of the body. In particular, in the method a level of nitrogen oxide in the gas mixture is measured, and a quantity of thermal energy is supplied as a function of this level. This ensures that as much nitrogen oxide as possible is broken down. At the same time, this prevents excessive quantities of reducing agent, which could pollute the environment, from being released. The level of nitrogen oxide is determined, for example, using a gas sensor.
The quantity of thermal energy is determined, for example, by means of a heating output of the temperature-control means.
In a further configuration of the invention, it is also possible to cool the body comprising the starting material. By way of example, the body is heated by the exhaust gas from the internal-combustion engine to such an extent that too much reducing agent is liberated. By cooling the body, therefore, it is possible to reduce or regulate the amount of starting material which is converted. The cooling is effected, for example, by a Peltier element connected to the temperature-control means or by an increased supply of air. It is therefore possible to achieve optimum reduction of the level of nitrogen oxide in any phase of operation of an internal-combustion engine.
The introduction of gaseous reducing agent
Berthold Thomas
Preu Gabriele
Medina Maribel
Siemens Aktiengesellschaft
Silverman Stanley S.
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