The exhaust gases discharged from diesel engines contain a large amount of particulates composed mainly of carbon. The particulates, when released into the air, cause environmental pollution. Therefore, there are already developed techniques for fitting, into the exhaust gas system of diesel engine, a particulate-trapping unit containing a filter for particulate capturing, to remove the particulates in exhaust gas before their release into the air.
In using the particulate-trapping unit, the captured particulates accumulate increasingly in the filter, which causes an increase in back pressure and a subsequent reduction in engine performance. In order to prevent this phenomenon, the filter is subjected to a treatment such as backwash, combustion or the like at a timing when the particulates have accumulated to a certain level or while the filter is being used, to remove the particulates and regenerate the filter activity, continuously.
The method for removing the particulates by backwash, however, has a problem that the whole system associated becomes complicated and large. Meanwhile, the method for removing the particulates by combustion has also had the following problem. That is, part of the particulates remains as an ash in the filter after the filter regeneration by combustion; the ash is accumulated in the filter by repeated or long-term regeneration treatment by combustion and reacts with the filter material at high temperatures appearing in the subsequent use or regeneration treatment by combustion, of the filter; as a result, the filter undergoes melting at temperatures lower than the melting point of the filter material.
The above problem has specifically been as follows: the particulates accumulated in the filter contain the P, S, Ca, Na, Zn, Cr, Fe, Ni, Cu, etc. originating from fuels, engine oils, pipes, etc.; compounds and/or composites containing the above elements remain as an ash in the filter even after the combustion of the filter; and this ash causes the filter melting mentioned above. In particular, in the case of a filter made of an oxide ceramic material, it reacts with the above ash easily, which is a serious problem.
Under such a situation, for example, JP-A-10-33923 proposes a technique of using a filter having a ceramic particles layer formed thereon in such a state that the ceramic particles layer is easily peeled, wherein the ash accumulated on the ceramic particles layer is peeled and removed together with the ceramic particles layer by, for example, the pressure of exhaust gas.
Also, National Publication of International Patent Application No. 6-506138 discloses a technique of using a filter having a film on the surface of each filter passage, to prevent the penetration of a material to be filtered, into the pores of the filter and make easy the backwash of the filter.
Further, Japanese Patent No. 2926187 discloses a gas filter for ceramic comprising a filter substrate and a filter layer strongly bonded to at least the filtration side surface, wherein the pore diameters of the filter substrate and the filter layer are controlled to increase the bonding strength of the filter layer.
Furthermore, JP-A-6-198148 discloses a technique of, in the method for producing an inorganic porous film comprising a porous substrate and a thin layer composed of inorganic fine particles, formed on the porous substrate, controlling the average pore diameter and maximum pore diameter of the porous substrate and the thickness of the thin film to prevent the generation of pinholes and cracks.
Also, JP-B-3-42936 discloses a catalyst for purification of exhaust gas, used for selective reduction of particulate matter in diesel engine exhaust gas containing So2, which catalyst comprises a refractive three-dimensional structure having a gas-filtrability and a porous inorganic layer formed on the structure, having a vanadium oxide and a noble metal loaded thereon in a dispersed state.
However, in the technique described in JP-A-10-33923, the ceramic particles layer is formed on the filter in such a state that the ceramic particles layer is peeled from the filter surface by a relatively weak force such as exhaust gas pressure or the like, and the ceramic particles layer is peeled by an exhaust gas current even when no ash is accumulated on the ceramic particles. Therefore, it is necessary to keep on feeding the ceramic particles. As a result, the ceramic particles accumulate in the filter quickly, incurring a decrease in filtration area and an increase in pressure loss; therefore, removal of ceramic particles by backwash or the like must be conducted frequently, which significantly reduces the practical utility of the technique and moreover makes indispensable a large backwash system or a ceramic particles-feeding system. Further in the technique, no investigation is made on the affinity between the ceramic particles and the ash.
In the technique disclosed in National Publication of International Patent Application No. 6-506138, it is a prerequisite to subject the accumulated particulates to backwash to regenerate the filter, and the film formed on each filter passage merely prevents the physical penetration of particulates into the pores of the porous material constituting the filter. No investigation is made on the problem caused by the accumulation of the ash generated upon combustion of particulates. No investigation is made, either, on the affinity between the particulates and the film.
Also in the gas filter for ceramic, described in Japanese Patent No. 2926187, it is a prerequisite to subject the accumulated particulates to backwash to remove them. No investigation is made on the problem caused by the accumulation of the ash generated upon combustion of accumulated particulates. In the gas filter, it is preferred to form the filter layer, for example, in such a manner that a material for filter layer is rubbed into the surface of the filter substrate in such a thickness as part of the filter substrate surface is exposed. With such a filter layer, however, it is impossible to avoid the direct contact between the accumulated material and the filter substrate; therefore, the filter is unsuitable as a filter regeneratable by combustion of accumulated particles.
As to the technique disclosed in JP-A-6-198148, the Examples of the literature show one level for the binder amount used in production of a porous thin film. However, since use in diesel exhaust gas is not the prerequisite for the porous thin film, it is not considered in the technique to control the bonding strength of the thin layer so that the thin layer can withstand exhaust gas heat, vibration and, depending upon the case, backwash. Further, no investigation is made on the problem caused by ash accumulation inherent in use in diesel exhaust gas; therefore, no disclosure is made on the affinity between the ash and the thin layer.
In the catalyst for exhaust gas purification, disclosed in JP-B-3-42936, a catalyst substance is loaded on the porous inorganic layer in order to burn and remove the particulates accumulated on in the filter. Therefore, the porous inorganic layer is appropriately designed as a base for loading a catalyst thereon. However, no consideration is made on the affinity between the layer and the ash remaining after combustion of particulates.
The present invention has been made in view of the above situation and aims at providing a ceramic-made filter regeneratable by combustion and removal of captured particulates, wherein the melting of the filter caused by the ash remaining and accumulating on the filter after the particles captured by the filter have been burnt, can be prevented easily without using a large system or the like.