Gas separation – Specific media material – Ceramic or sintered
Reexamination Certificate
1999-03-22
2001-04-10
Simmons, David A. (Department: 1724)
Gas separation
Specific media material
Ceramic or sintered
C055S524000, C428S446000, C428S698000
Reexamination Certificate
active
06214078
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to permeable refractory products suitable for high temperature applications and especially to refractory products having a sufficient permeability and durability for use in high temperature gas filtration applications. More particularly, the invention relates to a permeable refractory structure in the form of a matrix of recrystallized silicon carbide. The invention has particular utility in connection with hot gas filtration in solid fuel fired gas turbines used for electrical power generation.
BACKGROUND OF THE INVENTION
Fossil fuels have had limited use as the energy source for turbines due to particulate matter that is produced in their combustion. This matter can cause the erosion and destruction of the turbine blades. At the high temperature required for efficient operation, often in excess of 900° C., deterioration of blades may be rapid resulting in down time and expensive repairs.
One method of overcoming this problem is to utilize heat exchangers where the combustion gases are passed through a system that utilizes a secondary flow path where “clean” air is separated by plates or tubes through which the heat is transferred. The clean air is then utilized to drive the turbine blades. The use of heat exchangers to drive turbines has been limited due to the lack of suitable materials and the inherent loss of efficiency in transferring heat.
A second approach to this problem has been to filter the combustion gas, removing the particulate matter. While some success has been achieved, only limited use has developed due to the lack of suitable materials to construct filter tubes that have sufficient permeability and high temperature strength.
Extensive work has been done in testing materials and designing hot gas filtration systems for this purpose. One material which has shown promise is clay-bonded silicon carbide which has good chemical resistance and good physical properties at elevated temperatures. However, this material has limits in that it loses strength at very high temperatures and oxidation produces a glass formation that decreases permeability and can seriously restrict filtration. High temperature creep also deforms and damages these filter tubes.
Recrystallized silicon carbide has been extensively used for kiln furniture and structural shapes in high temperature applications. This type of silicon carbide has much higher strength, both at room temperature and elevated temperature, than clay bonded silicon carbide. However, commercial recrystallized silicon carbide is produced with the highest density possible to provide the maximum achievable strength for its normal use as a structural refractory. This high density restricts both the pore size and the permeability of the material making it unsuitable for a gas filter.
The unique refractory product of the present invention, however, along with the method for making the product, resolves the problems indicated above and provides other features and advantages heretofore not obtainable.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a porous refractory product suitable for use in filtering high temperature combustion products wherein the high temperature stream contains significant unwanted particulate matter. The refractory product comprises a matrix of recrystallized silicon carbide having a controlled porosity by volume of from between about 15% and about 40% and a pore size below about 250 &mgr;m, and preferably less than 200 &mgr;m.
In order to obtain the above objective, the inventors have combined the technology used in producing ceramic filters for a broad range of low temperature applications with the technology used for producing commercial, structural, recrystallized silicon carbide refractories. It must be appreciated that applicants' invention is formed of recrystallized silicon carbide, and is not a sintered silicon carbide product.
Ceramic filters can be produced by combining controlled sizes of ceramic grains with a glass former which, upon firing, forms a monolith with a controlled porosity. Both permeability and pore size can be closely controlled by controlling the size and range of the ceramic grains and the amount of glass former utilized.
In the manufacture of recrystallized silicon carbide, the particle size of the silicon carbide and the rheology of the casting slip are controlled to obtain the maximum density. This provides maximum strength but limits the permeability and the pore size obtained.
By utilizing closely controlled trimodal distribution of silicon carbide grain sizes, a casting can be produced in which the coarse grains form a matrix that produces a relatively open structure. During the firing process, the fine portion of the silicon carbide sublimes resulting in recrystallization with virtually no change in total net size. By altering furnace conditions, grain size packing and the fine portion, both the pore size and the permeability can be closely controlled. The method of the present invention also allows one to produce a one-piece monolithic filter structure. Thus, end pieces (caps) and collars are integrally formed—there is no need to separately produce these portions of the filter.
Filters with more restricted pore size can be produced by using the above cited technique to produce a substrate structure which is then coated with a thin layer of silicon carbide or other ceramic material which when re-fired forms a bonded membrane that can restrict pore size to 2 &mgr;m.
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Bulley Michael J.
Vlach Thomas J.
Way Philip S.
Ferro Corporation
Hopkins Robert A.
Rankin, Hill Porter & Clark LLP
Simmons David A.
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