Particulate trap for diesel engine

Details Particulate trap for diesel engine Particulate trap for diesel engine

2000-06-20

2004-10-26

Tran, Hien (Department: 1764)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

Waste gas purifier

C422S171000, C422S180000, C422S181000, C055SDIG003

Reexamination Certificate

active

06808688

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a particulate trap for trapping and removing particulate matters, such as carbon particles, contained in exhaust gases of diesel engines.
2. Description of the Background Art
Engines that use petroleum-based fuel as their energy source burn fuel to convert heat energy into mechanical energy. Generally, the combustion is incomplete and the exhaust gas contains particulate matter (hereinafter called PM) consisting mainly of carbon particles. If the exhaust gas is emitted without proper treatment, it causes air pollution.
Researchers and engineers have invented many devices that trap and remove PM emitted from diesel engines of cars, for example, filters to prevent air pollution. However, engines emit exhaust gases containing corrosive gases such as sulfur oxides (SO
x
) with a wide range of temperature. This makes it difficult to select the material of filters. Because PM in an exhaust gas comprises extremely fine particles, there is particular focus on the size of pores in filters.
A cordierite-ceramic foamed body has been developed as a typical filter material. The material has extremely fine pores and hence is advantageous in trapping PM securely. This structure of fine pores requires a large effective trapping surface. To meet this requirement, a honeycomb structure has been used as one of the solutions.
Because cordierite has low thermal conductivity, when a filter is regenerated by burning trapped PM to remove them and allowing the filter to be restored to its original condition, it may cause local heating, posing problems such as cracking or melting.
Another material that has been developed in recent years is a metallic porous body that has a three-dimensional network structure formed of an Fe—Cr—Al, Ni—Cr—Al, or Fe—Ni—Cr—Al-based metal. Having high thermal conductivity, these metallic porous bodies are free from local heating, a drawback of the foregoing cordierite. Consequently, they can be regenerated with equalized temperatures. However, when they are produced with fine pores equivalent to those of a cordierite-ceramic foamed body, they produce extremely heavy filters because of the high specific gravity.
To overcome this disadvantage, various concepts have been disclosed on the structure of a filter using a metallic porous body having a three-dimensional network structure (hereinafter called “a metallic porous body”). A published Japanese patent application Tokukaihei 6-257422 discloses a structure in which two or four concentric cylindrical filter elements formed of a metallic porous body are used together with a heater provided between the filter elements. This structure allows effective combustion and removal of trapped PM and enables equalized regeneration of filters, prolonging the life of filters.
However, when the filters are used continuously in cars, industrial machines, and so on, a plurality of filters must be provided in parallel because while a filter is regenerating, another filter must work to trap PM. In addition, the filter regeneration requires provision of burners or electric heaters for burning PM together with a complex electrical control system for controlling them.
Another published Japanese patent application Tokukaihei 10-159552 discloses a technique that burns, or oxidizes, trapped PM without using a burner or electric heater. This technique uses a catalytic converter in which an oxidizing catalyst is carried by a honeycomb flow-through monolith. The catalytic converter is placed upstream of a filter that traps PM. Hereinafter, in a path for emitting an exhaust gas, the side nearer to the engine is called “the upstream side,” and the side farther from the engine “the downstream side.” The catalytic converter placed at the upstream side oxidizes NO in the exhaust gas into NO
2
, which in turn burns PM trapped by the filter.
The device disclosed in Tokukaihei 10-159552 can burn and remove PM trapped by the filter at an exhaust gas temperature of about 250° C. or higher by oxidizing NO in the exhaust gas into NO
2
. However, driving in ordinary urban districts often result in prolonged idling, during which time the temperature of the exhaust gas is low. If a catalyst carrier has large thermal capacity, prolonged time is needed to heat the catalyst carrier to a temperature at which PM can burn. As a result, the device is unable to burn the trapped PM completely.
Even if the catalytic converter oxidizes NO into NO
2
, the NO
2
sometimes reverts to NO because of the drop in the exhaust gas temperature before the NO
2
reacts on the PM accumulated on the filter. This phenomenon reduces the efficiency of the device.
SUMMARY OF THE INVENTION
An object of the present invention is to offer a particulate trap in which one or more metallic porous bodies having less fine pores and smaller thermal capacity per unit volume than a cordierite-ceramic foamed body forms a catalytic converter and a filter. This structure solves the foregoing problems and produces a particulate trap excellent in durability and advantageous in cost.
The present invention uses a catalytic converter in which an oxidizing catalyst is carried on the surface of the framework of one or more metallic porous bodies having an average pore diameter not less than 500 &mgr;m and not more than 2,000 &mgr;m. This catalytic converter is placed upstream of the filter that traps PM. The metallic porous body has larger porosity than a ceramic foamed body, so it can carry a larger amount of catalysts per unit volume. In addition, the metallic porous body has the advantage of large thermal conductivity and small thermal capacity per unit volume.
Consequently, even when the temperature of an exhaust gas rises in a short time while the car is operating, a catalytic converter with a small volume can oxidize NO effectively and burn trapped PM, because the temperature of the catalytic converter can be raised easily. A catalytic converter with an average pore diameter not less than 500 &mgr;m can suppress the reduction in catalytic action resulting from the accumulation of PM. A catalytic converter with an average pore diameter not more than 2,000 &mgr;m can properly maintain the distance between the catalyst carried on the surface of the framework of the catalytic converter and the passing NO molecules, thereby providing sufficient oxidizing reaction for burning PM.
It is desirable that a catalytic converter have a porosity not less than 90% and not more than 98%. The porosity not less than 90% can reduce the thermal capacity per unit volume of the catalytic converter, causing the temperature rise of the catalytic converter in a short time. It is desirable, however, that the porosity be not more than 98% because a porosity exceeding 98% cannot produce sufficient strength.
It is desirable that the catalytic converter carry an oxidizing catalyst having an amount not less than 0.15 grams and not more than 1.5 grams per unit volume of one liter of the catalytic converter (hereinafter merely expressed as g/l). The amount not less than 0.15 g/l can gives sufficient reaction for burning PM. The amount not more than 1.5 g/l is desirable in terms of economic consideration, because the oxidizing reaction shows a saturation tendency when an excessive amount of oxidizing catalysts is given.
It is desirable that the catalytic converter and the filter be formed of a material made of an Fe—Cr—Al, Ni—Cr—Al, or Fe—Ni—Cr—Al-based metallic material, which has small thermal capacity per unit volume. It is also desirable to use the same type (the same composition) of metallic material for forming the catalytic converter and the filter. This is because the catalytic converter and the filter are used under the same conditions, and the use of the same type of material eliminates the need of independent measures for thermal expansion, corrosion, and oxidation.
It is desirable that the filter have an average pore diameter not larger than that of the catalytic converter, because this condition enables effective trapping of PM.
When the catalyti

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