Gas separation: processes – Filtering – Coated or chemically treated filter
Reexamination Certificate
2001-12-18
2004-05-11
Lawrence, Frank M. (Department: 1724)
Gas separation: processes
Filtering
Coated or chemically treated filter
C055S484000, C055S524000, C055S527000, C110S216000
Reexamination Certificate
active
06733575
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a hot gas filtration system comprising high temperature and corrosion; resistant filters.
BACKGROUND OF THE INVENTION
Hot gas filtration is for example used in the furnace exhaust of gases resulting from the combustion of fossil fuels. These fuels contain many impurities. Many filters have been used to reduce or eliminate the impurities in such furnace exhaust gases. However, there is still a need for improved filters which are capable of withstanding higher temperatures and pressures for removing particulates from hot combustion gases.
Especially in coal-fired power generation systems and more particularly in combined cycle power technologies, there is a big need for high temperature resistant and corrosion resistant filters. These technologies are based on the combustion and gasification of coal whereby the gas turbine is driven by the coal-derived gas.
Typical combined cycle power systems are pressurized fluidized bed combustor (PFBC) based systems, integrated gasification combined cycle (IGCC) based systems and hybrid cycle based systems. Some of these systems are already in operation, others are in development or at the demonstration stage.
A critical step in all these systems is the filtration of hot gases for the removal of particulates and other contaminants.
In these technologies, an effective and reliable hot gas filtration is of the utmost importance not only to meet the environmental emission requirements but also to protect the gas turbine components against fouling and erosion.
For such systems high performance, reliable hot gas filtration systems operating at high temperatures (200-900° C.) with a high corrosion resistance are required.
Particulates and contaminants such as sulfur, alkali metals and heavy metals have to be removed by the filter. Also the concentration of hydrogen halides such as HCl and HF is preferably low to protect the components of the turbine. This can be a serious problem when high chlorine coals are gasified.
Up to now, the filtration of gases is a limiting factor for coal-fired combustion systems, because there are no filters which meet the above mentioned requirements.
Presently available filters for the filtration of hot gases are for example ceramic filters. A considerable drawback of this type of filters is their limited temperature range,(250-450° C.) in which they can be operated. Ceramic filters also have the disadvantage that they suffer from thermal fatigue and high temperature corrosion, particularly in high temperature oxidising environments. The service life of ceramic filters is limited because the gas phase may react with the amorphous binderphase and because of oxidation of non-oxide based ceramics. Phase transitions may further put restrictions on the service life of such filters.
Still another drawback of ceramic filters is their limited shock resistance, either mechanical or thermal shock resistance. This can cause problems, for example during pulse cleaning.
An alternative for ceramic filters is the use of metallic filters. However, the known metallic filters feature the disadvantage that they are only suitable in reducing environments at low or intermediate temperatures (350-600° C.). When these filters are exposed to an oxidising environment, they suffer considerably from corrosion. Also impurities, such as sulfur or sulfur containing compounds or hydrogen halides may only be present in low concentrations in order to avoid corrosion.
WO 9532048 describes a filter comprising FeAl or FeAl
3
powder. The resistance is improved when a certain amount of Cr is added.
SUMMARY OF THE INVENTION
It is an object of the present invention to avoid the drawbacks of the prior art.
It is another object to provide a coal-fired power generation system comprising a number of high temperature and corrosion resistant filters. It is also an object to provide filters for such a system which can be used both in an oxidising or in a reducing atmosphere, which can withstand repeated temperature cycles and which has a high resistance against thermal and mechanical shocks.
Moreover, the invention aims to provide a system having a high filter performance, a high reliability and a long-term durability.
According to a first aspect of the present invention a coal-fired power generation system comprising means for the production of coal-derived gas and a filter system for the filtration of said coal-derived gas is provided.
The filter system comprises at least one high temperature and corrosion resistant filter. This filter comprises a filter medium and filter caps. The filter medium comprises at least one layer of metal fibers which has been sintered. Both the filter caps and the metal fibers are made from an iron-chromium-aluminium (Fe—Cr—Al) based alloy.
The diameter of the metal fibers is preferably between 4 and 30 &mgr;m, more preferably the diameter is between 8 &mgr;m and 22 &mgr;m.
The weight of the filter medium is preferably between 600 and 1500 g/m
2
and more preferably between 750 and 1200 g/m
2
. The filter medium has a porosity between 60 and 85% and preferably between 65 and 80%. In order to improve the resistance of the filter a protective Al
2
O
3
layer may be formed on the surface of the filter medium and on the other elements being part of the filter. This protective layer may be obtained by preoxidising the filter.
In one embodiment the filter medium comprises a non-woven layer of metal fibers. The web is sintered and preferably compacted. In a further step, the sintered medium is welded to form the filter.
In another embodiment a filter medium comprising at least a first and a second layer is provided. Each layer comprises a web of metal fibers. The first layer, at the flow in side of the filter medium comprises metal fibers with a diameter between 4 and 12 &mgr;m. The diameter of the fibers of the second layer, this is the layer at the flow out side, is between 12 and 30 &mgr;m.
It is preferred that the weight of the first layer is between 20 and 60% of the total weight of the filter medium. More preferably, the weight of the first layer is between 40 and 60% of the total weight.
The first and second layer are brought into contact with each other to form a layered structure. This layered structure is sintered and compacted. In a subsequent step, the sintered and compacted filter medium is welded to form the filter.
In a preferred embodiment a mesh is fixed to the filter medium as a support layer.
The mesh may be fixed,to the layer situated at the flow out side.
In an alternative way the mesh may be sandwiched between a first and a second layer of metal fibers.
The layered structure comprising the layer or layers of metal fibers and comprising the mesh is then sintered in a subsequent step.
As mentioned before, all components being part of the filter such as the metal fibers, the filter caps, and when a mesh is present also the mesh, are made from a Fe—Cr—Al based alloy.
A first group of Fe—Cr—Al based alloys comprises 15 to 25 wt % Cr and 4 to 6 wt % Al. Preferably the Al content is between 4.8 and 5.7 wt %.
A preferred alloy composition is a Fe—Cr—Al based alloy further comprising Y. This alloy is known as Fecralloy®.
The Y content ranges from 0.03 to 0.5 wt % and is preferably between 0.08 and 0.35 wt %. Most preferably, the Y content is between 0.25 and 0.35 wt %. Another possible alloy composition of this group is a Fe—Cr—Al based alloy further comprising at least one additional element selected form the group consisting of Sc, Y, Ti, Zr, Hr, Ta and the lanthanides, for example La or Ce.
The content of the additional element or the sum of the additional elements is between 0.01 and 1 wt %.
A second group of Fe—Cr—Al based alloys comprises up to 15 wt % Cr and 20 to 60 wt % Al. These alloys further comprise at least one additional element selected from the group consisting of Sc, Y, Ti, Zr, Hf, V, Nb, Ta and the lanthanides.
The Fe—Cr—Al based alloys show good corrosion resistance and high temperature resistance characteristics.
This resistance may further be improved by th
Lefever Ignace
Losfeld Ronny
Vandamme Johan
Foley & Lardner
Lawrence Frank M.
N.V. Bekaert S.A.
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