Advanced Ceramic Materials for Use in High-Temperature Particulate

Westinghouse Science & Technology Center, 1310 Beulah Road, Pittsburgh, Pennsylvania 15235 ... development of the second-generation porous ceramic fil...
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Ind. Eng. Chem. Res. 1996, 35, 3384-3398

Advanced Ceramic Materials for Use in High-Temperature Particulate Removal Systems M. A. Alvin† Westinghouse Science & Technology Center, 1310 Beulah Road, Pittsburgh, Pennsylvania 15235

First-generation monolithic porous ceramic filter materials have experienced thermal fatigue, high-temperature creep, and a loss of material strength when operated for extended periods of time in advanced coal-fired combustion and gasification systems. Fiber-reinforced and advanced ceramic composites potentially provide a means to mitigate the degradation mechanisms encountered by monolithic filter matrices and ultimately extend operating life. In this paper we will review advancements which have recently been made during the development of the second-generation porous ceramic filter materials and provide insight into the performance of the filter elements during bench-scale qualification testing and operation in demonstration plant test facilities. Introduction High-temperature, high-pressure particle control is required to meet environmental and turbine equipment specifications in advanced coal-fired power generation systems. Direct application of high-performance, hightemperature particulate control ceramic filters is expected not only to be beneficial to the advanced fossil fuel processing technology but also to select hightemperature industrial and waste incineration processes. Porous ceramic filters are currently being designed to meet performance, life, and cost constraints of the process application in which they are used. For each application, the porous ceramic filters must withstand variation in the effluent gas stream chemistry, changes in the composition and loadings of the entrained fines, and oscillations in the effluent gas stream temperature and possibly pressure, while maintaining high particulate removal efficiencies, high flow capacity, and relatively low pressure drop characteristics. During process operation, the ceramic filters must also withstand stresses resulting from mechanical clamping and support structures, as well as vibration and thermal stresses imparted from pulse gas cleaning. The criteria for successful use and operation of porous filters as a viable advanced particulate removal concept therefore requires not only thermal, chemical, and mechanical stability of the ceramic materials but also long-term structural durability of each filter element and high reliability of integrated process design features. Development and production of highly durable and reliable filters are dependent not only on properties of the material used in the construction of each filter element but also on the relative stability of that material under corrosive high-temperature, high-pressure environments which contain gas-phase sulfur, alkali, and chloride phases [Alvin et al. (1991)]. In addition, during production of the filter body, manufacturing and process control must be exercised to produce a quality-assured component that meets both design dimensional tolerances and compositional criteria. With respect to filter materials, strength, thermal shock resistance, thermal expansion, modulus of elasticity, fracture toughness, thermal conductivity, hardness, density, and chemical reactivity are important criteria in the selection of a viable, durable, filter material. †

Telephone: 412-256-2066. Fax: 412-256-2121.

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Many of these material properties are available for dense ceramics but have not been established for the porous matrices which are currently being considered for filter fabrication. As shown in Table 1, oxides, mixed oxides, and nonoxide matrices are utilized to construct porous rigid barrier, cross-flow, candle, or tube filter elements. Materials that were initially considered for use in the construction of the cross-flow filters included alumina/ mullite, cordierite, aluminosilicate foam, cordieritesilicon nitride, reaction-bonded silicon nitride, and sintered silicon nitride. As technology moved from the stacked-plate cross-flow filter design to the production of monolithic units, alternate materials and processes were utilized to fabricate the cross-flow filter concept. Similarly, as technology progressed, utilization of the cross-flow filter alumina/mullite matrix and processing techniques were transitioned into the manufacture of alumina/mullite candle filters. Currently, the alumina/ mullite matrix, clay-bonded silicon carbide materials are used to commercially manufacture ceramic candle filters. Alternate materials which have been used to fabricate candle filters include fireclay, aluminosilicate fibers, alumina, chemically vapor infiltrated silicon carbide (CVI-SiC), and filament wound matrices. Numerous advanced materials and processes are currently being considered in the production of prototypic candle filter elements. First-Generation Filter Materials The alumina/mullite (Al2O3/3Al2O3‚2SiO2) matrix consists of mullite rods that are surrounded by a grain boundary amorphous or glassy phase which contains corundum (Al2O3) and anorthite (CaAl2Si2O8). Cordierite (Mg2Al4Si5O8) when properly fired contains very little glass. The aluminosilicate foam is comprised principally of glass fibers which are coated and held together by a mullite wash. Cordierite-silicon nitride (CSN) is initially formed from a mixture of cordierite and silicon nitride (Si3N4). Upon firing, a Mg-SiAlON and a glass phase are expected to result. In this material, cordierite and the resulting glassy grain boundary phase can be considered as the “binder” which holds the silicon nitride grains together. Reaction-bonded silicon nitride (RBSN) is manufactured through a process in which silicon is nitrided in the presence of iron, forming a microstructure in which the Si3N4 grains are directly bonded to © 1996 American Chemical Society

Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3385 Table 1. Porous Ceramic Filter Materials cross flow filters Coors alumina/mullite GTE cordierite GTE cordierite-silicon nitride Foseco aluminosilicate foam AiResearch sintered silicon nitride AiResearch reaction-bonded silicon nitride DuPont SiC-SiC composite Blasch injection-molded alumina/SiC Blasch injection-molded mullite-bonded alumina CeraMem cordierite bag filters 3M Nextel 312, 440, 550, 610, 720 tube filters Asahi cordierite

each other. In contrast, sintered silicon nitride (SSN) is manufactured via direct sintering of silicon nitride grains with oxide additives. Unlike the porous alumina/ mullite, cordierite, CSN, and SSN filter materials, RBSN does not contain grain boundary phases. Commercially available clay-bonded silicon carbide candle filters generally contain silicon carbide support grains (500-1000 µm) that are held together with an aluminosilicate or clay binder or a crystallized glass phase. A fibrous aluminosilicate mat, or a finer silicon carbide grain layer (80 µm), is applied as an outer surface skin or membrane to provide a barrier to micron and submicron particles in the process gas stream. In the clay-bonded silicon carbide matrices, an oxide-based binder phase bonds adjacent grains to each other. During the processing of these materials, the potential exists for additional grain boundary phases to form. The oxide-based binder in the clay-bonded silicon carbide filter materials contains relatively high concentrations (0.5-10%) of sodium, iron, potassium, calcium, and/or titanium. Efforts have been focused on producing a second-generation clay-bonded silicon carbide matrix which has increased strength, as well as enhanced creep and corrosion resistance during use at either high-temperature and/or in-process gas streams which contain gas-phase alkali species. Alternately, binderless (sintered) silicon carbide materials have been used to fabricate candle filters. These materials contain relatively low concentrations of iron, nickel, and/or chromium within the grain boundary phase(s). Mixed oxides as clay-bonded aluminosilicates or fireclay materials have also been used in the production of first-generation candle filters. These matrices contain relatively high concentrations of sodium, potassium, titanium, iron, and chlorine. Alternately, relatively pure candle filter matrices have been formed from R-alumina bonded with aluminosilicate phases. Similar to the alumina/mullite matrix, aluminosilicate fibers have been utilized to manufacture both cross-flow and candle filters. In order to contain the fibers and increase the strength of the fibrous aluminosilicate candles, a toughened outer skin is applied over the support structure. The toughened skin typically is a blend of both fibers and a silica and alumina binder phase. Advanced Second-Generation Filter Materials Recent advancements have led to the production of filament wound and fiber-reinforced CVI-SiC or SiC-

candle filters Schumacher Dia Schumalith F40 Schumacher Dia Schumalith FT20 Pall Vitropore 442T Pall Vitropore 326 Coors alumina/mullite 3M CVI-SiC composite DuPont SiC-SiC composite DuPont filament-wound PRD-66 IF&P fibrosic Textron nitride-bonded SiC B&W alumina-based CFCC Westinghouse/Techniweave sol-gel Pall Aerotech iron aluminide Kyocera zirconia/mullite Kyocera mullite Third Millennium silicon carbide Kaiser Aerotech non-oxide CFCC

SiC composite candle filter elements. The CVI-SiC composite filter produced by 3M consists of three layerssan outer open-mesh confinement layer, a middle filtration mat, and an inner triaxial braided fabric layer which forms the structural support matrix of the filter element. Within the confinement and filtration mat layers, an ∼1-2 µm layer of silicon carbide is deposited which encapsulates Nextel 312 or alumina-based fibers. Alternately, an ∼100 µm layer of silicon carbide is deposited along the Nextel 312 triaxial braid in the support matrix. The SiC-SiC composite filter produced by the DuPont Lanxide Corp. consists of an ∼10-20 µm silicon carbide layer that is chemically vapor infiltrated (CVI) along an ∼2-5 µm interface coating which encapsulates ∼15 µm Nicalon fibers. Fine-grain silicon carbide grit is utilized to form an outer membrane along the surface of the SiC-SiC composite filter matrix. Originally, the DuPont SiC-SiC composite candle was constructed using two plys of Nicalon felt. In order to more efficiently utilize the CVI-SiC process, production of the candle was modified to include the use of a singleply felt layer and a mesh screen support layer. As a hybrid construction, an enhanced phase was added to the interface coating in the mesh screen support, and the fine-grain silicon carbide grit was utilized to form the outer membrane. Several issues have been raised as to the oxidative stability of the CVI-deposited silicon carbide, as well as the stability of the interface coating, and the Nextel or Nicalon fibers in either the 3M or DuPont matrices during use in high-temperature, oxidizing process applications. The oxide-based, filament-wound matrix produced by the DuPont Lanxide Corp. consists of a layered cordierite, mullite, cristobalite, and corundum microstructure. An amorphous phase is also present in the asmanufactured filament-wound filter matrix. Due to the differences in the thermal coefficient of expansion for mullite, cordierite, and corundum which are present in the matrix, a microcracked structure is formed after high firing. Critical issues in demonstrating successful long-term viability of the porous ceramic filters are the chemical, thermal, and mechanical stability of the various materials at process operating conditions. In the following section, the response of the monolithic first-generation filter materials during operation in the pressurized fluidized-bed combustion (PFBC) environment at the American Electric Power (AEP) Demonstration Plant in Brilliant, OH, is presented. Several of the secondgeneration fiber-reinforced and filament-wound filter

3386 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 Table 2. W-PFBC Hot Gas Filter Testing at AEP PFBC Test Segment

operating time, h operating temp, °C no. of filter elements Schumacher F40 Schumacher FT20 Pall Vitropore 442T Coors alumina/mullite 3M CVI-SiC composite DuPont PRD-66

1 10/92-12/92

2 7/93-9/93

3 1/94-4/94

4 7/94-10/94

5 1/95-3/95

464 730-790

1295 620-790

1279 650-780

1705 660-760

1110 760-845

384

384

384

258 8 8 8 3 3

5 153 98 10 22

elements were also installed and tested in the Westinghouse Advanced Particulate Filtration (W-APF) system at AEP. The response of the advanced secondgeneration filter materials to PFBC operating conditions will also be described. Subsequently, a discussion is presented which reviews the qualification testing which was conducted in the Westinghouse high-temperature, high-pressure (HTHP) PFBC and bench-scale flowthrough facilities which explored the response of the various second-generation filter materials to extended simulated process and thermal transient conditions and high-temperature oxidizing environments containing gas-phase alkali. Many of the material response results that were obtained during qualification testing in the Westinghouse HTHP PFBC test facility have been directly observed during field operation. Field Testing Testing in the W-APF system was initiated at the AEP Demonstration Plant in October 1992. Five hightemperature test campaigns were conducted primarily utilizing first-generation, monolithic, Schumacher Dia Schumalith F40 clay-bonded silicon carbide candle filters (Table 2). During conduct of the last two test segments, advanced second-generation candle filters were installed and operated in the W-APF system. With the exception of ash bridging, which generally failed the filter elements, the stability and long-term viability of the “aged” or “conditioned” candle filter materials were demonstrated during operation in the PFBC gas environment at AEP. A summary of the residual strength and microstructural and phase changes which occurred during operation of the first- and secondgeneration candle filters follows [Alvin et al. (1995a,b), Mudd et al. (1995)]. Schumacher Dia Schumalith F40 The Schumacher Dia Schumalith F40 filter material consists of an aluminosilicate binder phase that encapsulates and bonds together silicon carbide grains, forming 40-50 µm pores within a 15 mm thick support wall. An ∼100 µm thick, aluminosilicate fibrous membrane is applied to the outer surface of the filter body, in order to prevent fines penetration into and/or through the porous ceramic filter wall. During test operations at AEP, the integrity of the membrane was retained, imparting complete barrier filtration characteristics to the Schumacher Dia Schumalith 40 filter matrix. Typically 288-384, 1.5 m clay-bonded silicon carbide Schumacher Dia Schumalith F40 candle filters were installed and operated in the W-APF in Test Segments 1-5. Schumacher Dia Schumalith F40 surveillance candles successfully achieved 5855 h of operation in the W-APF system at Tidd. The residual bulk strength of

Figure 1. Schumacher Dia Schumalith F40 strength profile.

the Schumacher Dia Schumalith F40 material was determined via room temperature and process temperature testing of C-rings in compression and tension. As shown in Figure 1, the process temperature bulk strength of the Schumacher Dia Schumalith F40 material decreased during the initial 1000-2000 h of PFBC operation. With continued operation, however, the residual bulk strength of the Schumacher Dia Schumalith F40 material remained constant. The residual or “conditioned” strength was considered to result from the complete or nearly complete crystallization of the binder phase that encapsulated the silicon carbide grains, as well as the bond posts or ligament surfaces (Figure 2). Previous use of alternate clay-bonded silicon carbide candle filters indicated that creep crack growth occurred primarily at the base of the flange, and failure of the elements resulted after 500-1000 h of operation in the ∼830 °C circulating pressurized fluidized-bed combustion environment [Alvin et al. (1994)]. In order to ascertain whether the Schumacher Dia Schumalith F40 filter material experienced creep during operation at AEP, the overall length of the surveillance filter elements was measured after the various test campaigns and compared to the initial, as-manufactured lengths. After 5855 h of operation in the W-APF at AEP, two 1.5 m surveillance filters were observed to have elongated by 5-7 mm. Cracks as a result of high-temperature creep were not evident within the densified section of the flange nor along the remainder of the filter body.

Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3387

lag required to initiate high-temperature creep in the Vitropore 442T clay-bonded silicon carbide candle filter matrix. Schumacher Dia Schumalith FT20 Eight improved, high-temperature, creep-resistant Schumacher Dia Schumalith FT20 clay-bonded silicon carbide candles were installed and operated in the W-APF system in Test Segment 4 at AEP. The initial bulk strength of the 10 mm wall Schumacher Dia Schumalith FT20 material was comparable to the higher strength, 15 mm wall, Schumacher Dia Schumalith F40 filters which had been manufactured in 1992. As shown in Table 3, the residual bulk strength of the Schumacher Dia Schumalith FT20 matrix after 1705 h of successful operation in the W-APF at AEP was comparable to that of the initial bulk strength of the filter matrix. Coors Alumina/Mullite

Figure 2. Crystallization of the Schumacher Dia Schumalith F40 filter matrix after (a) 3038 and (b) 5855 h of operation in the W-APF system at AEP.

Pall Vitropore 442T The Pall Vitropore 442T material also consists of silicon carbide grains that are bonded together via an oxide-based binder to form the 10 mm filter support wall. A finer grit silicon carbide layer is applied to the outer surface of the filter element, forming the barrier filter’s membrane. During operation at AEP in Test Segment 4, eight Pall Vitropore 442T clay-bonded silicon carbide candle filters were installed and operated in the W-APF system. Similar to the “conditioning” experienced by the Schumacher Dia Schumalith F40 filters, the Pall Vitropore 442T clay-bonded silicon carbide filters experienced a loss of bulk material strength after 1705 h of operation in the PFBC gas environment (Table 3). Three of the Pall Vitropore 442T filter elements which had been operated in the W-APF in Test Segment 4 were reinstalled and acquired an additional 1110 h of operating life in Test Segment 5. Post-test inspection indicated that the 2815 h PFBC-exposed Pall Vitropore 442T filters elongated by 17-20 mm. Cracks as a result of high-temperature creep were not evident directly below the flange nor along the external surface of the filter elements. During operation in Test Segment 5, 150 Pall Vitropore 442T candle filter elements were installed in the W-APF system. All candles remained intact after 1110 h of operation. Post-test characterization of 17 elements indicated that the filters elongated by 0-4 mm. Since all of the Pall Vitropore 442T filter elements utilized at AEP had been identically manufactured, the difference in elongation which resulted between the two test segments was considered to reflect “aging” or the time

Eight Coors alumina/mullite P-100A-1 filters were installed in the W-APF system during conduct of Test Segment 4 at AEP. All Coors alumina/mullite filter elements remained intact after 1705 h of operation in the 660-760 °C PFBC gas environment. The residual bulk strength of the 1705 h, PFBC-exposed, Coors alumina/mullite filter matrix was comparable to that of the as-manufactured filter matrix (Table 3). Three of the Test Segment 4 Coors alumina/mullite filters were installed in the W-APF system prior to initiating Test Segment 5. Post-test inspection indicated that all three of the Coors alumina/mullite filters remained intact, achieving 2815 h of successful operation. In addition, 95 newly manufactured Coors alumina/ mullite filters were installed in the W-APF system during conduct of Test Segment 5 at AEP. After 1110 h of PFBC operation at temperatures of 760-845 °C, two filters failed, while the remaining Coors alumina/ mullite filters remained intact. One of the Coors alumina/mullite filters failed directly below the flange, while the second filter fractured at approximately 1000 mm below the flange. Characterization of the residual bulk strength of the 1110 h, PFBC-exposed, Coors alumina/mullite filter elements indicated that the Coors alumina/mullite matrix lost strength during operation in the 760-845 °C oxidizing environment. This implied a sensitivity of the Coors alumina/mullite matrix to higher process operating temperatures which may either induce or accelerate phase changes or enhance crack propagation properties along the i.d. surface and/or through the 10 mm filter wall. A further reduction in the residual bulk strength along the i.d. surface and/or through the filter wall was exhibited within the Coors alumina/ mullite filter matrix that experienced 2815 h of operation in the PFBC environment (Test Segments 4 and 5). As shown in Figure 3, extensive mullitization resulted along the pore cavity walls of the Coors alumina/mullite filter matrix as both operating time and process temperature increased. As determined by qualitative X-ray diffraction analysis, minor dissociation of mullite that was present in the as-manufactured Coors filter material was initiated early during process operation. Simultaneously, a reduction and/or depletion of the as-

3388 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 Table 3. Candle Filter Residual Bulk Matrix Strength strength, psi filter matrix

candle filter location

S153/317B S504/322B S436/321B S193/318B S065/314B S106/317B S324/319B S109/317B S215/378B S447/322B S455/322B S523/322B S418/321B S422/322B S228/318B

Test Segment no.

C/M-15

R2-325 R5-325 R2-360

high-temp (732 °C) C-ring testing compression tension

Schumacher Dia Schumalith F40s1991 Production Lot 1300 ( 213 1907 ( 111 1 464 1120 ( 123 1 464 1096 ( 116 1 464 1147 ( 119 1 464 940 ( 60 1 464 1180 ( 98 1 464 1083 ( 148 1438 ( 108 1 464 1140 ( 120 1424 ( 162 1, 2 1760 908 ( 72 1117 ( 91 1, 2 1760 794 ( 50 709 ( 71 1, 2 1760 793 ( 58 711 ( 89 1, 2 1760 793 ( 39 1016 ( 134 1, 2, 3 3038 720 ( 57 944 ( 172 1, 2, 3, 4 4734 870 ( 145 816 ( 112 1, 2, 3, 4, 5 5855 592 ( 56 763 ( 87

B/T-1 B/M-1 B/B-1 A/B-6 B/B-45 B/B-9 B/B-41 B/M-22 A/T-22 B/T-22 C/T-22 B/T-16 B/B-16 A/T-16

S199/315E S039/312E

operating time, h

room-temp C-ring testing compression tension

Schumacher Dia Schumalith FT20 2296 ( 261 2268 ( 167 1705 2283 ( 184 2370 ( 238

4

B/M-20 C/T-1

Pall Vitropore 442T 2857 ( 186 1705 2311 ( 231 1110 2569 ( 132

4 5

2574 ( 177 2034 ( 139 2277 ( 156

Coors Alumina/Mullite P-100A-1 2575 ( 182 2721 ( 415 1705 2475 ( 189 2903 ( 289 1110 2079 ( 140 2392 ( 130

DC-013 DC-003 DC-056

B/M-16 A/T-17

4 5

DC-068

A/T-1

5

1110

1958 ( 68

2544 ( 214

DC-002

B/M-17

4, 5

2815

2097 ( 119

2063 ( 178

1416 ( 127 1226 ( 116 1172 ( 134 1245 ( 108 1056 ( 131 1230 ( 127 1137 ( 101 1132 ( 112 1064 ( 72 1028 ( 94 989 ( 77 968 ( 96 890 ( 65 1031 ( 105 777 ( 57 891 ( 52a

2328 ( 228

1778 ( 246 1873 ( 174 1418 ( 122 973 ( 121 885 ( 54 1252 ( 241 1284 ( 199 1139 ( 160 1208 ( 99 966 ( 47a

3034 ( 148 3041 ( 238

2708 ( 360 3102 ( 272

3430 ( 221 2453 ( 187 2721 ( 138 2454 ( 214a

3029 ( 149 2138 ( 180 2201 ( 212 2115 ( 238a

3107 ( 276 2738 ( 161 2368 ( 93 2512 ( 107a 1800 ( 135 2079 ( 85a 2200 ( 141 2360 ( 116a

3353 ( 231 3291 ( 246 2636 ( 238 2717 ( 167a 2542 ( 196 2659 ( 109a 2146 ( 362 2287 ( 251a

strength, psi filter ID no. D-99 D-132 D-237

candle filter location

B/M-7 B/M-8

Test Segment no.

4 5

operating time, h

1705 1110

room-temp C-ring testing compression tension

high-temp (732 °C) C-ring testing compression tension

high-temp (843 °C) C-ring testing compression tension

DuPont PRD-66 1219 ( 162 1265 ( 188 1830 ( 238 1725 ( 320 1533 ( 202 1380 ( 188

1277 ( 178 1884 ( 142 1897 ( 256

NAb NAb 1872 ( 230

1304 ( 327 1642 ( 401 1356 ( 104

NAb NAb 1460 ( 197

diametral O-ring testing filter ID no. 43-1-2 43-1-6 45-18-02 a

candle filter Test operating location Segment no. time, h

B/M-15 C/T-18

4 5

1705 1110

room-temp strength, psi composite triaxial braid

high-temp (732 °C) strength, psi composite triaxial braid

high-temp (843 °C) strength, psi composite triaxial braid

3M CVI-SiC Composite 1341 ( 254 14026 ( 2012 1060 ( 219 11012 ( 1795 NA NA 1696 ( 195 18220 ( 1356 1429 ( 159 15599 ( 2246 NA NA 2333 ( 415 18975 ( 3117 2225 ( 361 18001 ( 3745 1850 ( 299 16173 ( 2245

High-temperature strength testing conducted at 834 °C. b NA: not available.

manufactured anorthite and glass phases resulted, leading to the production of trace and minor concentrations of alumina, cordierite, and cristobalite within the bulk matrix. Retention of the bulk matrix strength with time is expected to depend on the rate of cristobalite and cordierite grain growth, which could ultimately lead to the formation of microcracks and loss of material strength. Since phase conversion appears to have stabilized within the first 1100 h of PFBC operation, additional microcrack formation and/or loss of bulk material strength would be attributed to thermal fatigue (i.e., increased pulse cleaning intensity or duration) and/or thermal shock of the Coors alumina/mullite filter matrix during high-temperature PFBC process operation.

3M CVI-SiC Composite The 3M CVI-SiC composite filter consists of three layerssan outer open-mesh confinement layer, a middle filtration mat, and an inner triaxial braided fabric layer which forms the structural support matrix of the filter element. Within the confinement and filtration mat layers, an ∼1-2 µm layer of silicon carbide is deposited which encapsulates Nextel 312 or alumina-based fibers. An ∼100 µm layer of silicon carbide is deposited along the Nextel 312 triaxial braid in the support matrix. Three 3M CVI-SiC composite filters were initially installed in the W-APF in Test Segment 4, and after 1015 h of operation, two of the elements failed at the

Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3389

Figure 3. Crystallization of the Coors alumina/mullite filter matrix after (a) 1110 h of PFBC operation in Test Segment 4 and (b) 2815 h of PFBC operation in Test Segments 4 and 5.

base of their flange due to ash bridging. The third 3M filter element, which remained intact, achieved 1705 h of PFBC test operation prior to termination of Test Segment 4 at AEP. Post-test, room-temperature, gas flow resistance measurements of the intact PFBC-exposed 3M filter indicated that the pressure drop across the element was ∼220 in-wg at a gas face velocity of 10 ft/min. The roomtemperature pressure drop across the ash-coated 3M filter was relatively high in comparison to alternate filter elements which were exposed to similar PFBC test conditions. The high-pressure drop across the 3M PFBC-exposed candle filter was attributed to the adherence of the dust cake layer along and within the outer confinement layer, as well as through the filtration mat, and triaxial support braid. Characterization of the 3M CVI-SiC matrix via scanning electron microscopy/energy-dispersive X-ray analysis (SEM/EDAX) indicated that minor changes in the morphology of the filter matrix had occurred after 1705 h of operation in the 660-760 °C W-APF system at AEP. Due to deposition of the thin 99.99% particle collection efficiency during steady-state, simulated PFBC operation. Similarly, the initial pressure drop across the elements at process temperature, dust cake removal efficiency, and the as-manufactured strength of the flange and mechanical sealing/mounting of the element within the filter holder are assessed. In addition, Westinghouse qualifies the performance of the various filters under extreme conditions as accelerated pulse cycling which monitors the performance of the matrix with respect to thermal fatigue, as well as exposure to thermal transient events which simulate rapid startup or shutdown ramps experienced during process upset conditions. In order to evaluate the performance of the advanced second-generation filter elements, an array which included a DuPont PRD-66, a 3M CVI-SiC composite, and a DuPont SiC-SiC composite filter was installed in the Westinghouse HTHP PFBC test facility in Pittsburgh, PA. Initially, the filter array was heated to temperatures of 843 °C prior to initiating thermal transient testing. A series of seven increasing-severity thermal transients were delivered to the array which reduced the outer surface temperature of the filter elements by

3392 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 Table 4. Candle Filter Failure Mechanisms Experienced during Operation in the W-APF at AEP filter element

max hours of operation (test segment)

failure mechanism

Schumacher Dia Schumalith F40

5855 (1, 2, 3, 4, 5)

ash bridging

Schumacher Dia Schumalith FT20 Pall Vitropore 442T Coors alumina/mullite P-100A-1

1705 (4)

none experienced

2815 (4, 5) 2815 (4, 5)

none experienced possible thermal fatigue

3M CVI-SiC composite

1705 (4)

ash bridging

DuPont PRD-66

1705 (4)

divot formations; midbody and flange failure

Figure 6. Strengths of the as-manufactured, thermal transient, and accelerated pulse cycled second-generation candle filters as determined via C-rings tested in compression or tension.

6-110 °C and 20-240 °C within the first 5 and 60 s, respectively, after transient initiation. Nine maximumseverity thermal transients followed. Subsequently, the array was subjected to 10 accelerated pulse cleaning cycles and a final maximum-severity thermal transient. After 42 h of thermal transient testing, the filter array was slow cooled and the elements were removed for destructive characterization. As shown in Figure 6, both the high-temperature compressive and tensile strengths of the DuPont PRD-66 and 3M CVI-SiC composite filter materials increased, while the hightemperature compressive and tensile strengths of the DuPont SiC-SiC composite decreased. What is expected to have occurred is that crystallization and/or oxidation was initiated in the DuPont PRD-66 and 3M CVI-SiC composite filter materials, causing a strengthening of the matrices, similar to what had been identified during operation of the filter elements in the W-APF system at AEP. Characterization of the resulting morphology and phase composition via SEM/EDAX and X-ray diffraction analyses has not been completed for the thermal transient-tested DuPont PRD-66 and 3M CVI-SiC filter materials. However, when the DuPont SiC-SiC composite was subjected to SEM/EDAX analyses, several interesting changes were observed as a result of HTHP

comment maximum elongation of 5-7 mm; reduction of strength; conditioned strength of 800-1200 psi; binder crystallization; bowing under load; failure at the base of the flange during bridging; failure along end cap when i.d. filled with fines improved high-temperature creep-resistant binder; no loss of strength indicated maximum elongation of 17-20 mm; reduction in bulk strength midbody or failure at the base of the flange; end cap failure when i.d. filled with fines; loss of strength experienced at elevated temperatures failure at the base of flange during ash bridging; failure along end cap when i.d. filled with fines; removal of CVI-SiC layer along end cap; apparent strength increase due to fines filling matrix wall and/or phase crystallization/oxidation failure occurred at the base of the flange when ash encapsulated filter holder; body thinned where divots formed; divots may be responsible for midbody fracture; apparent strength increase due to fines filling matrix wall and/or phase crystallization; minimal adherence of ash along the outer filtration surface; minimal impact of bridging and ash i.d. bore filling

PFBC thermal transient testing. These included removal of the interface layer that was originally deposited around the Nicalon fibers in the single-ply felt layer of the DuPont SiC-SiC composite (Figure 7); evidence of oxidation of the CVI-SiC outer surface; and removal of the interface layer particularly along the outer fiber bundle or tow, directly beneath the CVI-SiC encapsulating layer in the mesh support screen. Melting or “sintering” of adjacent Nicalon fibers with the enhanced phase that was added during the manufacture of the mesh screen support layer was also evident. Retention of the interface layer along the interior of the fiber bundle or tow in the mesh screen support layer remains to be determined. Based on the resulting load versus deflection curves that were generated during C-ring strength testing, the fracture toughness of the DuPont SiC-SiC composite appeared to be reduced after 42 h of thermal transient testing. Loss of fracture toughness was primarily attributed to removal of the interface layer in the singleply felt and mesh screen support layers and to the sintering of the Nicalon fibers in the mesh screen support layer. In order to identify the viability and thermal fatigue resistance of the advanced second-generation filters, a second set of DuPont PRD-66, 3M CVI-SiC composite, and DuPont SiC-SiC composite candles was installed in the Westinghouse HTHP PFBC simulator and subjected to 3514 accelerated pulse cleaning cycles during a period of 197 h. The temperature of the filter array was maintained at 843 °C while accelerated pulse cycling was conducted. As shown in Figure 8, the interface layer that initially surrounded the Nicalon fibers in the single-ply felt layer of the DuPont SiC-SiC filter matrix was removed after 197 h of accelerated pulse cycle testing. Similarly, crack formations resulted along the outer periphery of the Nicalon fibers. Typically, the cracks had rounded tips, as well as segmented “step-like” characteristics. “Halo-like” areas were readily evident along the periphery of the Nicalon fibers in the single-ply felt. These areas were generally enriched with oxygen and effectively demarcated the location to which the cracks penetrated. Bonding of the Nicalon fiber to the inner surface of the CVI-SiC encapsulating shell often resulted near the crack formations.

Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3393

Figure 7. Morphology of the DuPont SiC-SiC composite hybrid filter matrix: (a) as-manufactured single-ply felt layer; (b) as-manufactured mesh screen support layer; (c) presence of the interface layer in the as-manufactured single-ply felt layer; (d) removal of the interface layer in the single-ply felt layer after thermal transient testing.

Within the mesh screen support layer, thin CVI-SiC bands which followed the contour of the Nicalon fibers, the enhanced phase, and perhaps the interface layer were evident. Near the periphery of the fiber bundle or tow (i.e., adjacent to the CVI-SiC encapsulating layer), as well as within the bundle, irregularly shaped Nicalon fibers were evident. Melting of the fibers was frequently observed, as well as mottling of the fiber surface. These were considered to result as a response or reaction of the Nicalon fibers with the enhanced phase that was included in the mesh screen support layer. Adjacent to the CVI-SiC encapsulating layer, the melted fibers formed an interconnected network which readily formed cracks during fast fracture. Void formations that were observed in the fractured mesh screen support layer may have resulted from fiber pullout during sample preparation or alternately reflected removal of the interface phase during exposure to simulated HTHP PFBC process operating conditions. Notably the Nicalon fibers in the mesh screen support layer of the DuPont SiC-SiC composite filter matrix did not exhibit crack formations along their periphery. Oxidation of the outer surface of the CVI-SiC encapsulating layer was again evident after 197 h of acceler-

ated pulse cycling in the Westinghouse HTHP PFBC test facility. As previously discussed, the reduced fracture toughness of the DuPont SiC-SiC filter matrix after 197 h of accelerated pulse cycling was primarily attributed to removal of the interface layer in the single-ply felt and mesh screen support layers and to the sintering of the Nicalon fibers in the mesh screen support layer. After ∼800 h of steady-state, thermal transient and accelerated pulsing of DuPont SiC-SiC candles in the simulated HTHP PFBC process environment which contained ash and an equivalent of 20 ppm gas-phase sodium chloride at 1 atm pressure, further changes were evident within the filter material [Alvin (1996)]. These included an apparent swelling of the Nicalon fibers within the single-ply felt layer. As a result, the fibers fully, or nearly completely, filled the void in the CVISiC encapsulating shell. Bonding of the Nicalon fibers to the inner wall of the CVI-SiC shell was also apparent in the single-ply felt layer. Corrosion Testing Westinghouse has previously demonstrated the relative thermal/chemical stability of the oxide-based alu-

3394 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996

Figure 8. Morphology of the DuPont SiC-SiC composite hybrid filter material after 197 h of exposure to HTHP PFBC accelerated pulse cycle conditions: (a) single-ply felt layer; (b) mesh screen support layer. Table 5. Strength Characterization of the Steam/Air and Alkali/Steam/Air-Exposed 3M CVI-SiC Composite Minicandles strength at 870 °C, psi C-rings tested in compression filter ID no.

exposure

3M-453707 3M-453715 3M-453710 3M-453708 3M-453708

as-manufactured (15 mm) as-manufactured (25.4 mm) steam/air (15 mm) alkali/steam/air (15 mm) alkali/steam/air (25.4 mm)

time, h

400 400 400

temp, °C

compressive load, lb

composite matrix

triaxial support braid

870 870 870

3.3 ( 0.36 4.0 ( 0.14 2.10 ( 0.62 1.7 2.85 ( 0.64

1739 ( 259 1183 ( 78 987 ( 250 717 762 ( 151

11784 ( 1476 8561 ( 463 8533 ( 1546 6298 5808 ( 695

mina/mullite filter matrix during exposure to hightemperature, oxidizing conditions which contain gasphase alkali and steam. In order to demonstrate the stability of the oxide-based DuPont PRD-66 filter material, minicandles, which were manufactured with an ∼75 mm section of open filtration area, were exposed for 400 h at 870 °C to 5-7% steam/air and 20 ppm NaCl/ 5-7% steam/air flow-through testing. During the 400 h exposure, the minicandles were subjected to pulse cycling at 20 min intervals. High-temperature, flow-through testing was similarly conducted for the non-oxide-based, advanced, secondgeneration filters using minicandles of the 3M CVISiC composite matrix and 70 mm diameter × ∼2-3 mm thick disks of the DuPont SiC-SiC composite matrix. Post-test characterization of each material included C-ring compression testing at 870 °C of the as-manufactured and flow-through tested minicandles and 4-point bend, 1/4-point flexural strength testing at 870 °C of the as-manufactured and flow-through tested filter disks. SEM/EDAX analyses have been completed for both the 3M CVI-SiC and DuPont SiC-SiC tested materials. A discussion of these results follows. 3M CVI-SiC Composite Matrix After exposure to either the high-temperature steam/ air or alkali/steam/air flow-through environment, the 3M minicandles appeared to be intact, retaining their initial configuration. What was readily apparent, how-

ever, was the very brittle nature of the outer confinement layer along the alkali/steam/air-exposed 3M CVISiC composite material. In several locations, the fiber bundles of the confinement layer weave were broken or missing. During preparation of the 3M CVI-SiC matrix for strength testing, two as-manufactured minicandles were cut into 15 or 25.4 mm sections for high-temperature strength testing of C-rings in compression. These sections were removed from the open filtration area of the minicandle. Similarly, a 25.4 mm O-ring section was removed from the reinforced filter matrix directly below the flange. All three layers of the as-manufactured 3M CVI-SiC composite matrix remained bonded together during preparation of either the 15 and 25.4 mm C-ring or 25.4 mm O-ring samples. Similar C-ring and O-ring samples were removed from the steam/air and alkali/steam/air-exposed 3M CVI-SiC minicandles. During sample preparation, the three composite layers of the steam/air-exposed 3M CVI-SiC matrix remained bonded together, while debonding occurred along both the 15 and 25.4 mm C-ring sections that were removed from the alkali/steam/airexposed 3M filter material. What was also apparent for both the steam/air and alkali/steam/air-exposed 3M minicandles was the extreme difficulty that was encountered during cutting of the reinforced material directly below the flange. As shown in Table 5, the applied compressive load required

Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3395

Figure 9. Accelerated oxidation of the 3M CVI-SiC composite filter material after 400 h of exposure at 870 °C to the 20 ppm NaCl/5-7% steam/air flow-through test environment.

to fail C-rings that were removed from the 3M CVISiC composite filtration area, and the resulting strength

decreased after 400 h of exposure in the 870 °C steam/ air of alkali/steam/air flow-through environment. The resulting load versus deflection curves that were generated during C-ring compressive strength testing tended to indicate that a reduction in the fracture toughness of the embrittled, 3M CVI-SiC composite matrix resulted after exposure at high temperature to the steam/ air and alkali/steam/air test conditions. SEM/EDAX analyses of the steam/air and alkali/ steam/air-exposed 3M CVI-SiC filter matrices indicated that the ∼1-2 µm SiC layer which had been deposited along the outer confinement layer and the aluminabased filtration mat fibers had generally been removed. The ∼100 µm CVI-SiC deposited layer which coated the triaxial support braid generally was intact but was enriched with oxygen (Figure 9). Due to the very thin nature of the interface layer that initially coated the Nextel and alumina-based fibers, resolution and identification of its presence in either the as-manufactured or flow-through tested minicandles could not be made using SEM/EDAX analytical techniques. As shown in

Figure 10. Morphology of the 3M CVI-SiC composite filter material: (a) as-manufactured material; (b) embrittled material after 400 h of exposure at 870 °C to 20 ppm NaCl/5-7% steam/air; (c) melt formation of the Nextel and CVI-SiC matrix; (d) melt formation of the alumina fibers and the CVI-SiC matrix.

3396 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996

Figure 11. Morphology of the cross-sectioned DuPont SiC-SiC composite single-ply felt layer after 400 h of exposure at 870 °C to 5-7% steam/air flow-through test conditions. Table 6. Strength Characterization of the Steam/Air and Alkali/Steam/Air-Exposed DuPont PRD-66 Minicandles C-rings testes in compression at 870 °C filter ID no.

exposure

time, temp, h °C

D-282 as-manufactured (15 mm) D-289 steam/air 400 (15 mm) D-288 alkali/steam/air 400 (15 mm)

load, lb

strength, psi

9.65 ( 0.72 1352 ( 135 870

11.88 ( 0.71 1541 ( 146

870

8.58 ( 1.70 1093 ( 265

Figure 10, exposure of the 3M CVI-SiC composite filter matrix to the 20 ppm NaCl/5-7% steam/air environment led to the formation of a “melt-like” bond between the Nextel or alumina fibers and the remaining encapsulating CVI-SiC shell. Aluminum- and silicon-rich micron, nodular formations were evident along the Nextel and alumina-based fibers in all three composite layers of the steam/air-exposed 3M CVI-SiC composite matrix. DuPont PRD-66 Filament-Wound Matrix After 400 h of exposure in the 870 °C steam/air and alkali/steam/air environment, the DuPont PRD-66 minicandles generally remained intact. However, along the outer membrane surface of the filtration area of the alkali/steam/air-exposed minicandle, numerous longitudinal cracks were evident. In addition, a 1 × 1.5 cm section of the membrane and several of the underlying structural support fibers had been removed along the alkali/steam/air-exposed minicandle. As shown in Table 6, the high-temperature, compressive strength of the filtration area in the brittle PRD66 filter matrix appeared to slightly increase after 400 h of exposure to the steam/air flow-through environ-

Figure 12. Morphology of the cross-sectioned DuPont SiC-SiC single-ply felt layer after 400 h of exposure at 870 °C to the 20 ppm NaCl/5-7% steam/air flow-through test environment: (a) oxidation of the CVI-SiC surface; (b) depletion of the interface layer and formation of the oxygen-enriched phase surrounding the Nicalon fibers; (c) glazed membrane.

ment, while a slight decrease in strength was observed after 400 h of exposure in the alkali/steam/air environment. To date, SEM/EDAX analyses have not been performed on either the steam/air or alkali/steam/airexposed DuPont PRD-66 minicandles. DuPont SiC-SiC Composite Matrix As previously discussed, the oxidation of the CVISiC layer that is deposited along Nicalon fibers to form

Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3397 Table 7. 4-Point Bend, 1/4-Point Flexural Strength of the Steam/Air and Alkali/Steam/Air-Exposed DuPont SiC-SiC Filter Matrixa membrane tested in compression,b psi filter matrix as-manufactured steam/air alkali/steam/air alkali/steam/air

25 °C 13118 ( NAf NAf NAf

870 °C

1697d

3128d

12068 ( 5427 ( 749e 3953 ( 90e 7269 ( 557d

membrane tested in tension,c psi 25 °C 7269 ( NAf NAf NAf

272d

870 °C 4981 ( 1801d 4450 ( 541e 3092 ( 304e 2106 ( 454d

a Span: 1.57 in. Cross-head speed: 0.02 in./min. b Pulse-cycled surface strength (open-mesh screen). c Membrane-coated surface strength (single-ply felt mat). d Bend bars were cut parallel to the open-mesh screen ribs. e Bend bars were cut on a diagonal to the open-mesh screen ribs. f NA: not available.

Table 8. High-Temperature Filter Material Stability filter matrix

thermal fatigue

thermal shock

accelerated pulse cycling

thermal transient testing

Coors alumina/mullite P-100A-1

intact; reduced strength

susceptible

Schumacher Dia Schumalith F40 Pall Vitropore 442T

intact; reduced strength

intact

intact; possible strength loss

intact

3M CVI-SiC composite intact; strength increaseb DuPont PRD-66 DuPont SiC-SiC composite

a

intact; strength increaseb

flange failure; matrix intact; strength strengtheningb increaseb intact; strength decrease; butt seam rupture; loss of interface coating; loss of interface; crack formations along melting/bonding of periphery of fibers; bonding fibers with enhanced of fibers to CVI-SiC; phase melting of enhanced phase along fibers; surface oxidation

flow-through oxidation/corrosion steam/air

alkali/steam/air

conversion of conversion of amorphous amorphous phase phase to albite/anorthite; to anorthite; strength increase strength increase eutectic formation; plastic NTa deformation NTa eutectic formation; plastic deformation expected strength loss; strength loss; accelerated surface oxidation oxidation; eutectic formation; embrittlement strength increase strength loss; membrane cracking and spalling strength loss; strength loss; accelerated surface oxidation; oxidation; eutectic bonding of fibers formation; embrittlement; to CVI-SiC removal of interface layer

NT: not tested. b Morphology changes to be determined.

the dual single-ply felt or hybrid (i.e., single-ply felt/ mesh screen support layer) DuPont SiC-SiC composite matrix has been shown to occur during exposure to simulated HTHP PFBC process conditions. Due to the thickness of the CVI-SiC deposit (i.e., ∼10-20 mm), oxidation may represent a long-term degradation mechanism for the DuPont SiC-SiC composite filter matrix. Alternately, removal of the interface layer, crack formations along the periphery of the Nicalon fibers, melting of the fibers in the presence of the enhanced phase, reduced fracture toughness, and a resulting, relatively embrittled material are likely to directly impact the short-term stability and viability of the DuPont SiCSiC composite filter matrix. After 400 h of exposure of the DuPont SiC-SiC hybrid matrix to the 870 °C flow-through, steam/air environment, melting or bonding of the Nicalon fibers to the inner surface of the CVI-SiC encapsulating shell resulted in the single-ply felt layer (Figure 11). Generally, the interface layer that initially surrounded the Nicalon fibers was considered to have been removed. Oxidation of the CVI-SiC outer surface was also observed (Figure 12). Oxidation of the CVI-SiC outer surface was accelerated when the DuPont SiC-SiC material was exposed for 400 h at 870 °C to the 20 ppm NaCl/5-7% steam/air flow-through test environment. Depletion of the interface layer along the Nicalon fibers in the single-ply felt was also evident after 400 h of exposure at 870 °C to the 20 ppm NaCl/5-7% steam/ air flow-through environment. In contrast to the steam/ air exposure, oxidation of the outer surface of the Nicalon fibers in the single-ply felt occurred during the alkali/steam/air exposure. Oxidation was expected to

have resulted either from outward diffusion of oxygen from the as-manufactured Nicalon fibers or from reaction of the fiber surface with the flow-through gases. Frequently, halo-like rings which were enriched with oxygen resulted around the periphery of the Nicalon fibers. Minor crack formations were evident in these areas. Retention of the enhanced phase that encapsulated the Nicalon fibers in the bundle or tow in the mesh screen support layer in the DuPont SiC-SiC hybrid filter matrix resulted during exposure to either the 5-7% steam/air or 20 ppm NaCl/5-7% steam/air flowthrough test environment. In localized areas, retention of the interface layer may be evident within the interior of the bundle. In contrast, the fine-grain SiC grit which formed the membrane of the SiC-SiC composite filter matrix tended to form a glassy phase during the high-temperature, alkali/steam/air flow-through exposure. The formation of the glassy phase reduced gas flow permeability through the matrix. The glazed surface which was enriched with sodium could conceivably serve as a potential site for collection and adherence of ash fines at process operating temperatures, ultimately causing blinding of the filter element surface. The residual strength of the hybrid DuPont SiC-SiC matrix was determined at 870 °C via 4-point bend, 1/4point flexural strength testing of bend bars that were removed from the flow-through tested filter disks. Approximately 60% of the as-manufactured strength remained along the surface of the material that had been subjected to pulse cleaning, while only 42% of the as-manufactured strength remained along the mem-

3398 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996

brane-coated surface that sorbed alkali during the 400 h alkali/steam/air, flow-through test exposure (Table 7). Based on the load versus deflection curves that were generated during high-temperature, flexural strength testing, the fracture toughness of the low fiber volume, DuPont SiC-SiC filter matrix appeared to decrease after 400 h of exposure in either the 870 °C steam/air or alkali/steam/air environment. Summary and Conclusions Table 8 summarizes the results which were obtained during HTHP PFBC and corrosion testing at Westinghouse for both the monolithic and advanced secondgeneration porous ceramic filter materials. The failure mechanisms of the oxide-based materials include thermal fatigue and/or thermal shock for the monolithic matrices, while component construction and low load bearing capabilities primarily impact the integrity of the advanced second-generation filter materials. The nonoxide-based materials tend to undergo oxidation, particularly in the presence of gas-phase alkali, leading to enhanced creep of the monoliths and embrittlement in the advanced second-generation filter materials. Generally, the monolithic materials tend to lose bulk strength during initial exposure to accelerated pulse cycling and thermal transient qualification testing, similar to what has been experienced in the field. The DuPont PRD-66 and 3M CVI-SiC composite materials, however, tend to experience an increase in their residual bulk strength during accelerated pulse cycling and thermal transient qualification testing, again similar to what has been observed during field testing. Removal of silicon carbide along the outer confinement layer of the 3M CVI-SiC filter was observed not only after 197 h of HTHP PFBC accelerated pulse cycling but also after 1100 h of operation in the W-APF system at AEP. Exposure time at temperature in the oxidizing environment is expected to impact the rate of removal and/or oxidation of the CVI-deposited SiC matrix. Although the DuPont SiC-SiC composite material tends to embrittle under oxidizing conditions, as well as lose bulk strength, the residual strength of the DuPont matrix remains relatively high in comparison to the conditioned strengths of the monolithic materials. The residual strength of the 3M CVI-SiC composite material also significantly exceeds the conditioned monolithic filter material strength. Although residual strengths of the non-oxide, fiberreinforced porous ceramic filter materials exceed those of the monoliths, the load bearing capability of the advanced second-generation filter materials is substantially lower than that of the monoliths. This may impact the ability of the advanced filters to remain intact if ash becomes lodged within the filter i.d. bore or bridges between adjacent filter elements. Similarly, the ability of the advanced filters to be easily and successfully removed from the holder mounts, which

generally become encased with deposited ash, may also make disassembly labor intensive. Although several critical issues need to be addressed prior to long-term use of the advanced second-generation filter materials, 5855 h of successful operation have been demonstrated for the monolithic clay-bonded silicon carbide filter elements under 620-845 °C PFBC conditions. The viability of the monoliths to successfully operate in an oxidizing environment where temperatures are g900 °C may require the use of high-temperature creep-resistant materials. Similarly, active oxidation of the non-oxide matrices may result in the 500700 °C gasification environment, and therefore the use of oxide-based filters may be required. Many of these issues will be addressed in the near future during operation of the W-APF systems in the Foster Wheeler pressurized circulating fluidized-bed combustion test facility in Karhula, Finland, as well as at the Sierra Pacific integrated gasification and combined cycle (IGCC) test facility in Reno, NV, and at the Southern Company Service combustion and gasification test facilities in Wilsonville, AL. Acknowledgment The continued support and guidance of DOE/METC are gratefully acknowledged. Literature Cited Alvin, M. A.; Lippert, T. E.; Lane, J. E. Assessment of Porous Ceramic Materials for Hot Gas Filtration Applications. Am. Ceram. Soc. Bull. 1991, 70 (9), 1491-1498. Alvin, M. A.; Tressler, R. E.; Lippert, T. E.; Diaz, E. S.; Smeltzer, E. E. Durability of Ceramic Filters. Paper presented at the CoalFired Power Systems 94sAdvances in IGCC and PFBC Review Meeting, Morgantown, WV, June 21-23, 1994. Alvin, M. A.; Lippert, T. E.; Diaz, E. S.; Smeltzer, E. E. Filter Component Assessment. Paper presented at the Advanced CoalFired Power Systems ‘95 Review Meeting, Morgantown, WV, June 27-29, 1995a. Alvin, M. A.; Lippert, T. E.; Diaz, E. S.; Smeltzer, E. E. Thermal and Chemical Stability of Ceramic Candle Filter. Paper presented at the Advanced Coal-Fired Power Systems ‘95 Review Meeting, Morgantown, WV, June 27-29, 1995b. Alvin, M. A.; Smeltzer, E. E.; Sanjana, Z. N.; Oberst, J. P.; Lippert, T. E. Stability of the DuPont SiC-SiC Composite Filter Matrix during Exposure to Simulated PFBC Conditions. Work performed at Westinghouse under DOE Cooperative Agreement No. DE-FC02-92CE40994, Jan 10, 1996. Mudd, M. J.; Hoffman, J. D.; Reinhart, W. P. Tidd Hot Gas Clean Up Program. Final Report, American Electric Power Service Corp., work performed under Contract No. DE-FC21-89MC26042, July 1995.

Received for review March 5, 1996 Revised manuscript received May 29, 1996 Accepted June 6, 1996X IE960128I

X Abstract published in Advance ACS Abstracts, August 15, 1996.