FILTER CANDLE WITH MINERAL ADDITIVE

Abstract

The present invention relates to a filter candle element for the dedusting of industrial gases having improved properties concerning stability and environmental sustainability. This filter candle element comprises a filter body which is composed of inorganic fibers, and a mineral additive which is accumulated to the inorganic fibers. The mineral additive preferably comprises zeolite. Another aspect of the invention relates to a method of manufacturing a filter candle element for dedusting industrial gases. The method comprises the following steps: production of a slurry comprising inorganic fibers and a mineral additive; sucking in the slurry onto a suction core to form the filter candle element; drying of the formed filter candle element.

Claims

1. A filter candle element for the dedusting of industrial gases, comprising: a filter body which is composed of inorganic fibers, and a mineral additive which is accumulated to the inorganic fibers.

2. The filter candle element according to claim 1, wherein the inorganic fibers are arranged in a fiber matrix.

3. The filter candle element according to claim 2, wherein the mineral additive is evenly distributed in the fiber matrix.

4. The filter candle element according to claim 1, wherein the mineral additive has a weight accounting for 1% to 8% of the weight of the filter body.

5. The filter candle element according to claim 1, wherein the mineral additive comprises zeolite.

6. The filter candle element according to claim 5, wherein the zeolite is doped with a catalytically active component.

7. The filter candle element according to claim 6, wherein the catalytically active component comprises a catalytically active metal or a catalytically active metal compound.

8. A method of manufacturing a filter candle element for dedusting industrial gases, comprising the following steps: producing a slurry comprising inorganic fibers and a mineral additive; sucking in the slurry onto a suction core to form the filter candle element; and drying the formed filter candle element.

9. The method of manufacturing a filter candle element according to claim 8, wherein the production of the slurry comprises the following steps: dispersion of the inorganic fibers and the mineral additive in powder form in a water bath; and circulation of the slurry.

10. The method of manufacturing a filter candle element according to claim 8, wherein the mineral additive comprises zeolite, which was doped with a catalytically active component prior to the production of the slurry.

Description

[0024] Below, preferred exemplary embodiments of the present invention are described on the basis of the following figures, wherein:

[0025] FIG. 1 shows a diagonal view of a filter candle element according to the invention;

[0026] FIG. 2 shows a cross-section through a filter candle element according to the invention;

[0027] FIG. 3 shows a scanning electron microscope image of the inorganic fibers with accumulated mineral additive;

[0028] FIG. 4 shows a framework structure of zeolite (ZSM-5);

[0029] FIG. 5 shows a schematic view of an industrial filter system.

[0030] FIG. 1 shows a filter candle element 1 according to the invention, comprising inorganic fibers 5 and the mineral additive 6. The candle element 1 is designed in the shape of a hollow body. The shown filter candle element 1 comprises a central area 2. A collar 3 borders on the upper end of the central area 2. A bottom 4 borders on the lower end of the central area 2. The shown central area 2 is designed in the shape of a long hollow cylinder. The collar 3 likewise has the shape of a hollow cylinder. The height of this hollow cylinder is clearly smaller than the length of the central area 2. Furthermore, it can be recognized that the collar 3 projects outwards beyond the central area 2. The internal side of the collar 3 is flush with the internal side of the central area 2. Thus, the collar 3 has a greater wall thickness as compared to the central area 2. The upper end of the central area 2 and the collar 3 form a T-shaped collar area. The T-shaped design of the collar area has several advantages. For one thing, the filter candle element 1 can be inserted in the end plate of a filter system. Due to the T-shaped design of the collar area it is further achieved that the filter candle element 1 is more resistant to bending moments and bending stresses in the clamping area. The bottom 4 is designed as a spherical cap. The latter is flush with the central area 2 both on the inside and the outside. The filter candle element 1 is closed on its lower end by means of the bottom 4. In a preferred exemplary embodiment, the central area 2, collar 3 and bottom 4 of the filter candle element 1 are designed as one piece. The advantage of this is that crude gas cannot penetrate through possible transitions between the three areas 2, 3, 4 into the interior of the filter candle element 1.

[0031] The shown filter candle element 1 comprises inorganic fibers 5 forming a filter body. Expediently, the filter body has a porous structure. Dust particles contained in the crude gas can be retained or accumulated to the filter body upon passage through the filter candle element 1. Preferably, the inorganic fibers 5 are mineral fibers, high-temperature wool, vitreous fibers or carbon fibers.

[0032] In addition to the filter body the shown filter candle element 1 comprises a mineral additive 6. Preferably, silica sand, rock flour, lime or zeolite is used. According to the invention, the mineral additive 6 is accumulated to the inorganic fibers 5. This deposition leads to a stiffening of the fiber matrix, in which the inorganic fibers 5 are arranged. The advantage thereof is that the strength and compression stability of the filter body is increased. In particular, the radial compression stability of the filter body and the bending moment of the filter body are increased in the longitudinal direction. In another exemplary embodiment according to the invention it is also possible that the filter candle element 1, in addition to the filter body composed of inorganic fibers 5 and the mineral additive 6, also includes a reinforcing element which is embedded in the filter body. The reinforcing element may preferably be made of metallic foam, a perforated plate, a wire grating or an expanded metal. An additional reinforcing element may contribute to further increase the stability of the filter candle element 1.

[0033] FIG. 2 shows a cross-section of the filter candle element 1 according to the invention of FIG. 1. Here, it may be a cross-section of the central area 2, but also of the collar 3 or the bottom 4. The cross-section has the shape of a circular ring. Moreover, the inorganic fibers 5 are shown. In part they lie on top of each other; in part they are interwoven with each other. The three-dimensional interwoven structure, comprising the inorganic fibers 5 as struts and the shown cavities 7, constitutes the so-called fiber matrix. The fiber matrix forms the porous structure of the filter body. Furthermore, the fine-particle mineral additive 6 is shown. It is distributed uniformly over the fiber matrix. For example, the mineral additive 6 may be present in powder form. FIG. 2 shows the individual powder particles. Furthermore, it is shown that the mineral additive 6 fills up part of the cavities 7 of the fiber matrix. Thereby, the filter body is additionally stiffened.

[0034] The amount of added mineral additive 6 defines the increase in the stability of the filter body. It is expedient if the weight of the mineral additive 6 in the filter candle element 1 amounts to about 1% to 8% of the weight of the filter body. This is to say that the percentage by weight of the mineral additive 6, on the one hand, and that of the inorganic fibers 5, on the other hand, is preferably at a ratio from 0.01:1 to 0.08:1 in the filter candle element 1. It is particularly preferred that the weight of the mineral additive 6 in the filter candle element 1 amounts to about 2% to 6% of the weight of the filter body. The selected ranges of quantities added are selected such that, on the one hand, a significant increase in the stability of the filter body, preferably of its radial compression stability and its bending moment in the longitudinal direction is noticeable. On the other hand, limiting the quantity of added mineral additive 6 serves the purpose of minimizing the disadvantages in the pressure loss behavior of the filter candle element 1 as much as possible. The addition of the mineral additive 6 basically reduces the porosity and air permeability of the filter candle element 1. A porosity of at least 80% is advantageous for the filter candle element 1 according to the invention. Such a porosity is achieved for the cited quantities added. Lower porosities would result in a considerably increased pressure loss. In addition, limiting the quantity of the added mineral additive 6 to the above cited ranges has the advantage that the increase in weight of the filter candle element 1 is limited due to the higher density. Thus, less additional weight must be borne by the end plate of the filter system.

[0035] FIG. 3 shows a scanning electron microscope image of inorganic fibers 5 in a filter candle element 1 according to the invention. The image is an example of a section of the fiber matrix. The mineral additive 6 is accumulated to the inorganic fibers 5. Four inorganic fibers 5 can be seen in the section. The shown fibers have different thicknesses. The fibers 5 lie partially on top of each other. Furthermore, several cavities 7, 70 are shown. The fibers 5 and the cavities 7, 70 build up the porous interwoven structure (fiber matrix) of the filter body, as already described in connection with FIG. 2. Different-sized fragments 8 made of the mineral additive 6 are accumulated to the fibers 5. In a cavity 70 a deposition 9 of several fragments 8 made of a mineral additive 6 is shown. The cavity 70 is formed by three fibers 5 lying on top of each other. The fragments 8 have different shapes. For example, fragments 8 are shown in the shape of platelets and little rods of different dimensions. Finally, FIG. 3 is provided with a scale. The thickness of the fibers 5 is in the range of a few micrometers. The dimensions of the fragments 8 are in the range from a few tenths of a micrometer up to few micrometers. The length of the fibers 5 is in the range from 50 μm to 1 mm.

[0036] FIG. 3 shows that the fragments 8 are uniformly accumulated to the fibers 5. The term “uniformly” in this connection also includes the random distribution of a plurality of fragments 8 of different sizes and shapes on the surface of the fibers 5. This results in a stiffening of the fiber matrix. This increases the stability of each individual fiber 4 and thus also the stability of the filter body. For example, the fragments 8 may correspond to the powder particles of a mineral additive 6 available in powdered form. Further fragments 8 may be accumulated to the fragments 8 accumulated already to the fibers 5, preferably at locations where fibers 5 are lying directly on top of each other or contact each other. This results in depositions 9 of fragments 8. These depositions 9 project into the cavities 7, 70 of the three-dimensional fiber matrix. Thus, they fill up part of the cavities 7, 70. In addition, this results in a stiffening of the filter body.

[0037] In a particularly preferred exemplary embodiment of the filter candle element 1 according to the invention, the mineral additive 6 comprises zeolite. Zeolites are crystalline tectosilicates.

[0038] FIG. 4 shows the framework structure 10 of a zeolite. It has characteristic uniform pores 11, cages 12 and channels 13. In the shown example, the pores 11 have a decagonal structure. Because of this structure the pores 11 have a considerably larger diameter than the shown pentagonal and hexagonal structures. Cages 12 are partially formed from the pentagonal and hexagonal structures. As indicated by the dashed line in FIG. 4, for example, a cage 12 may extend over several pentagonal and/or hexagonal structures. In addition, the course of the channels 13 is indicated. Finally, cations 15 are shown. They ensure the electric charge neutrality of the zeolite.

[0039] The realization of the framework structure 10 will be explained in more detail below. The chemical composition of the zeolites will be described by the following chemical formula: M.sup.n+.sub.x/n [(AlO.sub.2).sub.x—(SiO.sub.2).sub.y].wH.sub.2O. It follows therefrom that zeolites are composed of SiO.sub.4— or AlO.sub.4 tetrahedrons. Therefore, zeolites are aluminosilicates. These tetrahedrons are connected to one another via all four oxygen atoms. One of these tetrahedrons is situated at each corner 14 of the polygons shown in FIG. 4. Thereby, a microporous framework structure 10 is created. For this reason, zeolites belong to the substance class of tectosilicates. Depending on the type of zeolite a characteristic structure made of pores 11, cages 12 and/or channels 13 is created. Due to the content of aluminum, zeolites have a negative framework charge. This negative charge is leveled out by cations M.sup.n+) 15. These may be enclosed in the defined pores 11, cages 12 and/or channels 13 of the framework structure 10. In naturally occurring zeolites these are cations 15 of the metals of the first and second main groups.

[0040] Depending on how the channels 13 are interconnected, zeolites are subdivided into three groups: zeolites having a one-dimensional channel system, which are characterized by channels 13 that are not interconnected; zeolites having a two-dimensional channel system, which are characterized in that the channels 13 are interconnected to form a layered system; zeolites having a three-dimensional channel system. In the present invention, from the first group, zeolites of the mordenite-(MOR-) type and the Linde-type-L-(LTL-) type are preferably used; from the second group zeolites of the brewsterite-(BRE-) type are used; from the third group, zeolites of the beta-(BEA-) type, pentasile-(MFI-) type, LiLSX-(FAU-) type, chabasite-(CHA-) type, erionite-(ERI-) type are used.

[0041] The framework structure 10 shown in FIG. 4 corresponds to a zeolite of the MFI-type (ZSM-5-type). The shown zeolite also belongs to the group of zeolites with a three-dimensional channel system. As indicated in FIG. 4, the system of pores 11 is connected by linear channels 13 and crossing, angled channels 13.

[0042] The cations 15 may be replaced by metals, metal compounds or precious metal compounds via the channels 13 of the zeolites. This takes place e. g. by means of ion exchange or chemical treatment. Preferably, doping takes place with copper, cobalt or iron, with copper, cobalt or iron compounds or with precious metal compounds. The cited substances and/or compounds are retained or even embedded in the pores 11 and/or cages 12. As a result of such doping the zeolites may include a catalytically active component. This has the advantage that the filter candle element 1 according to the invention may be used for the catalytic denitrification of the crude gas. Thus, not only the effect of the increased strength may be reached, but also a catalytic function may be integrated in the filter candle element 1. The catalytically active centers are then immobilized in the filter candle element 1. An advantage is that the addition of the catalyst may take place in one working step with the manufacturing process of the filter candle element 1. The characteristic of zeolite of being able to include other compounds in its framework structure 10 remains unaffected even at temperatures of 600° C. This makes possible an unlimited use of zeolites as a carrier of catalytically active components, even at an increased permanent operating temperature.

[0043] FIG. 5 shows a schematic view of an industrial filter system 20 for the denitrification of industrial gases. A filter house 21, several pipelines 22a, 22b, 22c, a urea tank 23 with nozzle 24, a mixing tower 25, fan 26 and a vent 27 are shown. The crude gas side 211 is situated in the lower part of the shown filter house 21. A plurality of filter candle elements 1 is arranged there. The clean gas side 212 which is isolated from the crude gas side 211 is located in the upper part of the filter house 21.

[0044] First of all, the crude gas passes into the mixing tower 25 via the pipeline 22a. There, urea solution from a urea tank 23 is injected via a nozzle 24. The solution expediently contains 30% to 35%, preferably 32.5% urea (CH.sub.4N.sub.2O). As is shown, the cross-section of the mixing tower 25 is clearly larger than the cross-section of the pipelines 22a or 22b. This cross-sectional enlargement is necessary so that a sufficiently strong mixing of the crude gas with the urea solution can take place. The mixture is then introduced into the crude gas sides 211 of the filter house 21 via pipeline 22b. The length of pipeline 22b is chosen such that further mixing of the crude gas and the urea solution may take place. Additional mixers may also be expediently built into the filter system. The use of a urea solution has the advantage that the urea, as opposed to ammonia, is non-corrosive and non-toxic. The ammonia required for denitrification is generated by hydrolysis of the urea solution.

[0045] A plurality of filter candle elements 1 according to the invention is located on the crude gas side 211 of the filter house 21. Preferably, filter candle elements 1 with zeolite are used. Particularly preferably, zeolite which is doped with a catalytically active component is used. The filter candle elements 1 are clamped with their collar 3 in the end plate 213 of the filter house 21.

[0046] A negative pressure between 0.010 bar and 0.050 bar, expediently between 0.012 bar and 0.020 bar is produced on the clean gas side 212 of the filter house 21 by the fan 26. The mixture of crude gas and injected urea solution is sucked through the filter candle element 1 due to the pressure gradient between the crude gas side 211 and the clean gas side 212. As a result, the crude gas is dedusted on the filter body. In addition, the crude gas is denitrified by selective catalytic reduction (SCR). According to the invention, preferably zeolite is used as a catalyst for the SCR. It is particularly preferred that the zeolite itself is doped with a catalytically active component. The catalytically active component may cause an advantageous oxidation of nitrogen oxide (NO) into nitrogen dioxide (NO.sub.2). After a significant portion, expediently about half of the NO has oxidized into NO.sub.2, denitrification may take place by means of the so-called fast catalytic reduction (“fast SCR”). In this connection, using ammonia (NH.sub.3) obtained from the urea solution, NO and NO.sub.2 are reduced to molecular nitrogen (N.sub.2) and water (H.sub.2O) in the presence of the zeolite as the catalyst. The latter reaction preferably takes place via the framework structure 10 of the zeolite. The fast catalytic reduction favored by the catalytically doped zeolite takes place faster by an order of magnitude than the NO.sub.2-poor SCR. On the crude gas side 211, the crude gas expediently has a temperature between 300° C. and 600° C., preferably between 350° C. and 450° C.

[0047] Finally, the dedusted and denitrified gas is conveyed through pipeline 22c and to vent 27 by means of the negative pressure produced by the fan 26. Finally, the dedusted and denitrified gas is discharged via the vent 27.

LIST OF REFERENCE NUMERALS

[0048] 1 filter candle element [0049] 2 central area [0050] 3 collar [0051] 4 bottom [0052] 5 inorganic fibers [0053] 6 mineral additive [0054] 7 cavity [0055] 70 cavity with deposition 9 [0056] 8 fragments [0057] 9 deposition of fragments 8 [0058] 10 framework structure of a zeolite [0059] 11 pores of the zeolite [0060] 12 cages of the zeolite [0061] 13 channels of the zeolite [0062] 14 corners of polygons [0063] 15 cations [0064] 20 industrial filter system [0065] 21 filter house [0066] 22a, b, c pipelines [0067] 23 urea tank [0068] 24 nozzle [0069] 25 mixing tower [0070] 26 fan [0071] 27 vent [0072] 211 crude gas side of filter house 21 [0073] 212 clean gas side of filter house 21 [0074] 213 end plate