Cordierite ceramic filter

Details Cordierite ceramic filter Cordierite ceramic filter

1999-11-10

2001-07-17

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Refractory

C501S009000, C501S119000, C055S523000

Reexamination Certificate

active

06261982

ABSTRACT:

The present invention relates to a ceramic filter. More particularly, it relates to a ceramic filter useful for removing dust in a high temperature gas discharged from e.g. a pressure fluidized bed boiler, a coal gasification furnace or a garbage or industrial waste incinerator.
A dust-removing apparatus employing a ceramic tube as a filter is considered to be a key technology for realizing pressure fluidized bed combined power generation or coal gasification power generation as a clean and highly efficient power generation system of next generation wherein coal is used as a fuel, and there has been active competition in its research and development in various countries of the world. Further, its application to an incinerator for garbage or industrial waste is also being studied from the viewpoint of the environmental pollution represented by dioxin, which has been regarded as a serious problem in recent years.
On the other hand, a ceramic tube is susceptible to breakage due to various loads during its use or during its handling before use because of its brittleness attributable to low fracture toughness of ceramics, and accordingly it has been regarded to be very important to secure the reliability in its practical use for an extended period of time. It has been attempted to improve the strength by improving the material for the ceramics tube itself or to develop a technology for inspecting the material before use. However, there are still many problems to be solved.
Most expected for a ceramic tube to be used as a filter, is a material using cordierite as an aggregate, since cordierite not only is excellent in heat resistance but also has a very small thermal expansion coefficient and thus has high durability against thermal stress. For example, U.S. Pat. No. 5,073,178 discloses a ceramic filter for removing a dust from a high temperature dust-containing gas, which comprises a cordierite aggregate as the main component and describes a preferred construction and characteristics such as strength.
As one of the most important characteristics among functions as a filter, the pressure loss being low may be mentioned. In order to increase the efficiency for removing particles in a high temperature gas, it is advisable to reduce the pressure loss by increasing the porosity. On the other hand, from the viewpoint of the reliability for a long period of time, the strength of the filter tube is desired to be as high as possible, and from this viewpoint, the porosity should preferably be small. Namely, in the control of the material by the porosity, the pressure loss and the strength are in a mutually opposing relationship, and it is attempted to improve the strength within an allowable range in consideration with the porosity obtained from the required pressure loss.
However, as the practical application to a high pressure fluidized bed coal power generation is approaching in recent years, the necessity is increasing for a ceramic filter tube which is excellent in reliability and has higher strength. However, it has been difficult to present a ceramic tube having a long term reliability by the structural control of the material solely based on the porosity. Accordingly, it has been required to exactly know the factors which essentially control the pressure loss of a ceramic filter and to present an improved ceramic filter i.e. a ceramic filter which is substantially superior in strength to the conventional ceramic filter and has a low pressure loss.
Accordingly, it is an object of the present invention to provide a cordierite ceramic filter having a low pressure loss and high strength, by clarifying the structural controlling factors.
The present invention provides a ceramic filter comprising at least 50 mass % of a cordierite aggregate having a particle size of at least 74 &mgr;m and a binder, wherein said binder comprises at least 10 mass % of a fine powder of cordierite having a particle size of less than 74 &mgr;m and &bgr;-spodumene, and the mass ratio of said &bgr;-spodumene to said fine powder of cordierite is from 0.6 to 2.5, and wherein said ceramic filter has a pressure loss coefficient m′ of at least 30×10
−8
cm
2
as represented by the following formula and as calculated with respect to pores having diameters corresponding to at least {fraction (1/10)} of the volume-based median pore diameter:
m
′=(
V
p
/S
p
)
2
×(&rgr;)
wherein V
p
(cm
3
/g) is a cumulative pore specific volume, S
p
(cm
2
/g) is a cumulative pore specific surface area, and &rgr; is a cumulative porosity.
Now, the present invention will be described in detail with reference to the preferred embodiments.
The ceramic filter of the present invention essentially comprises a binder which comprises at least 10 mass % of a fine powder of cordierite having a particle size of less than 74 &mgr;m and &bgr;-spodumene, and at least 50 mass % of a cordierite aggregate having a particle size of at least 74 &mgr;m. If the cordierite aggregate having a particle size of at least 74 &mgr;m is less than 50 mass %, or if the cordierite aggregate having a particle size of less than 74 &mgr;m is at least 50 mass %, the thermal expansion coefficient of the ceramic filter tends to be high, whereby the properties as a ceramic filter tend to deteriorate such that a property such as thermal shock resistance tends to deteriorate, or the diameters of pores to be formed tend to be small. Further, it is essential that the binder comprises at least 10 mass % of a fine powder of cordierite having a particle size of less than 74 &mgr;m and -spodumene, and the mass ratio of the &bgr;-spodumene to the fine powder of cordierite (hereinafter referred to as the &bgr;-spodumene ratio) is from 0.6 to 2.5. If the &bgr;-spodumene ratio is outside the above range, sintering tends to hardly proceed, and it tends to be difficult to obtain adequate strength as a filter.
However, the object of the present invention to satisfy both the strength and the reduction of pressure loss of a ceramic filter can not be accomplished solely by these requirements, because merely by improving the sintering property of the ceramic filter to secure the strength, only a ceramic filter having a low porosity is obtainable, and the pressure loss will exceed the practically acceptable level, whereby a practically useful ceramic filter can not be obtained. Namely, for the ceramic filter of the present invention, it is essential that the pressure loss coefficient m′ (hereinafter referred to as the coefficient) calculated by the following formula is at least 30×10
−8
cm
2
;
m
′=(V
p
/S
p
)
2
×(&rgr;)
wherein V
p
(cm
3
/g) is a cumulative pore specific volume, S
p
(cm
2
/g) is a cumulative pore specific surface area, and &rgr; is a cumulative porosity. Namely, by adjusting the coefficient to be at least 30×10
−8
cm
2
, it is possible to present a ceramic filter which has a low pressure loss within a practically useful level and which satisfies both the practical level of pressure loss and the practical level of strength, even if it may be a ceramic filter having a low porosity as a result of improving the strength.
Here, the cumulative pore specific volume V
p
(cm
3
/g) and the cumulative pore specific surface area S
p
(cm
2
/g) are pore specific volume and a pore specific surface area recalculated by using values of only pores having diameters corresponding to at least {fraction (1/10)} of the volume-based median pore diameter, based on the results of the measurement of the pore distribution by a mercury porosimeter. A porosity is usually one measured by e.g. an Archimedes method and is not usually a cumulative physical amount. However, the porosity obtained by recalculation by using values of only pores having pore diameters corresponding to at least {fraction (1/10)} of the volume-based median pore diameter, is taken as a cumulative porosity &rgr;, which is thus distinguished from a usual term “porosity”. Specifically, the cumulative porosity &rgr;= (porosity)× (cumulat

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