ORNL/CP-102173

DESIGN AND INITIAL DEVELOPMENT OF MONOLITHIC CROSS-FLOWCERAMIC HOT-GAS FILTERS

Vhginie Vaubert and David P. StintonOak Ridge National Laboratory

Chris Barra and Santosh LimayeLoTEC, Inc

ABSTRACTAdvanced, coal-fueled, power generation systems utilizing

pressurized fluidized bed combustion (PFBC) and integratedgasification combined cycle (IGCC) technologies are currentlybeing developed for high-efficiency, low emissions, and low-cost power generation. In spite of the advantages of thesepromising technologies, the severe operating environmentoften leads to material degradatiotr and loss of performance inthe barrier filters used for particle entrapment. To address thisproblem, LoTEC Inc., and Oak Ridge National Laboratory arejointly designing and developing a monolithic cross-flowceramic hot-gas filter. The filter concept involves a trulymonolithic cross-flow design that is resistant to delamination,can be easily fabricated, and offers flexibility of geometry andmaterial make-up.

During Phase I of the program, a thermo-mechanicalanafysis was performed to determine how a cross-flow filterwould respond both therrnafly and mechanically to a series ofthermal and mechanical loads. The cross-flow filter mold wasdesigned accordingly, and the materials selection wasnarrowed down to Cao.$%(J.SZr4p@2.4(C.$50) and 2A1@s-3Si02 (mullite). A fabrication process was developed usinggelcasting technology and monolithic cross-flow filters werefabricated. The program focuses on obtaining optimum filterpermeability and testing the corrosion resistance of thecand~date materials.

INTRODUCTIONCleaning of the combustion gas stream is a key issue for

the successful development of sever-al advanced, coal-fueled,gas turbine technologies. Effective removaf of the flyash isnecessary in order to protect the turbines, minimizedeterioration of chemical clean-up process units, and meetenvironmental standards.

The thermal efficiency of power generation is maximizedwhen the removai of all (gas, liquid, and soiid) impurities isconducted at or above the operating temperature of thecombustor or gasitier. In PFBC systems, hot-gas filters mustoperate in a temperature range of 840 to 870”C and at apressure of 1 MPa. For IGCC systems, the operatingtemperature will be 650° to 870”C at a pressure of about 3MPa. The gas stream may contain chIonde, sulfide, phosphateand afkafi species. The ash particles may contain mixed andpure oxides of Si, Al, Ca, Fe, and alkali elements; sulfates ofsodium, potassium, and cafcium; and compounds of traceelements. Thus, prospective filter elements must be resistantto a wide variety of chemistries.

RECEIVEDfvw25w0s7-1

As the ash layer builds up on the filter surface, thepressure drop across the filter increases and the ash layer mustbe periodically dislodged and collected. This operation isperformed by pulsing a clean, typically cooler, gas streamfrom the downstream side of the filter element. This pulsecleaning and other process transients expose the filter elementto cyclic mechanical and thermaf stresses. The filter elementsmust have, therefore, excellent thermal shock resistance andsignificant toughness.

Many commercially availabie, porous ceramic materialsare cumently being evafuated for use in PFBC and IGCCpower generation systems]. While several of these materialsdemonstrate sufficient filtration capability, virtually all of theceramics tested show signs of thermal fatigue andfor excessivecorrosion in accelerated testing environments that potentiallylimit their survivability. Research conducted during the pastdecade has shown that different failure mechanisms exist foroxide and non-oxide materials. In oxide-based candle filters,failure occurs by thermaf fatigue. Corrosion occurs innonoxide-based candle filters primarily due to the reaction ofthe silica in the grain boundary or binder phases with theeffluent gases that contain volatile alkafi vapors that formlower melting amorphous phases2’3’4’5.

In summary, suitable filter materials must have sufficientstrength and toughness to tolerate thermal stresses caused bypulse cleaning and process transients. They must be relativelyinert toward the ash components so that ash filter cake can beeasily dislodged and the pressure drop across the filter can bekept within acceptable limits. The materials must afso havesufficient chemicaf resistance to survive in high-pressure gasstreams containing steam and alkali vapor.

Filter designs include candle, fabric, and cross-flow.Filter systems Iarge enough to clean the typical volume of gasproduced by an IGCC and PFBC systems will be veryexpensive. Therefore cross-flow filters, which have a veryhigh filter areahohrme ratio, are of particular interest.However, they are complex in design and extensivedevelopment will be required to bring the technology tocommercialization. Typically, such a filter is fabricated bylaminating several layers of material containing channels inorthogonal directions. As a result, such filters tend todelaminate and are susceptible to breakage during operation.’LoTEC, Inc. and ORNL are jointly developing a new cross-flow, hot-gas filter that addresses most of the challengesassociated with hot-gas filtration. This filter design is basedon fabricating a monolithic, cross-flow ceramic body usinggelcasting technology to avoid delamination problems Thematerials considered for this design have low thermal

Portions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.

expansion resulting in excellent thermal shock resistance. Thetwo primary candidate materials being considered for thisapplication are mullite (as a benchmark) and alkaline oralkaline earth zirconium phosphates (NZP) ceramics that havea very low thermal expansion. If successful, this research isexpected to result in a robust hot-gas filter that has excellentchemical stability and a very high resistance to thermal shockand delamination.

PROJECT DESCRIPTIONOver the last few years, LoTEC, Inc. has developed a

unique method for fabricating cross-flow filters in a singlestep without using laminates. The flexibility of the processallows control of various geometric parameters includlngchannel size, shape, wall thickness, and taper. The fabricationof LoTEC’S monolithic, cross-flow filter utilizes gelcasting, aprocess licensed from ORNL. The constraints of the PFBCtest facility at Westinghouse require ceramic filters of specificdimensions (3O.5X3O.5X1O.2 cm). However, the rest of thevariables -channel size, shape, material properties (strength,toughness, and thermal expansion)- can be optimized. Theoptimization of the filter was realized by improving its designand fabrication process.

The design development was based on mechanical andthermal modeling of the filter, which helped determine theoptimum filter properties as well as materiak selection. Theprocess development effort focused on identifying parametersthat improve the overall mechanical strength, increase thestrain to failure, and increase the number of connected poressince the low pressure drop required during gas cleaningnecessitates significant porosity levels. Several porosity-forming techniques have been pursued: porosity formation byliquid-phase separation, air entrapment, and pore formation byfugitive additives. The current effort also includes mechanicaltesting of corroded mullite and NZP samples.

DESIGN DEVELOPMENT

Materials Selection. The primary criteria for selectingmaterials for hot-gas filters include corrosion resistance,thermal shock resistance, high strength, and high toughness.Candidate materials that provide a good compromise amongall these criteria are mtdlite, NZP, and high-purity alumina.

The NZP family of ceramics [NaZrzP301z and itsisostructural analogs (e.g., CaZr,P@zJ] providescompositions with ultra-low thermal expansion (1 ppm/°C)and high melting temperatures ( 1850”C). The very 10wcoefficient of thermal expansion (CTE) results in very highthermal shock resistance. The materials also have highmelting temperatures and superior thermal insulationcapabilities compared to most low thermal expansionceramics.9 In addhion to low and tailorable thermafexpansion, NZP materials can be chemically ahered to reducethe thermal expansion anisotropy. It has been shown that thethermaJ expansion, as well as the thermal expansionanisotropy of these materials, can be reduced significantly bytailoring the chemical composition within the NZP family ofceramics. The composition, Cal.$r.&p& (x= 0.5, C$5t))exhibits minimaf microcracking, whereas compositions ofvalues x = O and x = 1.0, exhibit a high degree of anisotropy

and microcracking. Thus, one can control the microcrackingassociated with the thermal expansion ani sotrop y to improvethe mechanical properties by controlling the chemistry of NZPmaterials. Typically, compositions with a high degree ofanisotropy show low flexural strength (30 MPa), whilecompositions that have lower anisotropy have higher tlexuralstrength (75 MPa).10 In addhion, the thermal shock resistanceof these materials is very high. These materials can bequenched from 1400”C without any significant loss ofstrength. The major properties of alumina, mullite and CS-50are summarized in Table 1.

Thermo-mechanical analvsisThe finite-element model contained three layers of the

cross-flow filter and is shown in Figure 2. The middle layerwas exposed to the cold backpulse used for dislodging ashparticulate buildup, while the two surrounding layers wereexposed to the particulate containing hot gas. The initial

temperature of the hot gas was assumed to be 870”C for theentire filter. The model examined three thermal-loading

conditions with cold backpulses of 25°C for one second, 25°Cfor five seconds, and 100”C for one second.

The same-finite element model used for determiningtemperature differentials throughout the filter was also used todetermine the thermal and mechanical stresses induced duringhot-gas filtration. The analysis was performed using NZP,mrdlite, and alumina, to provide a comparison amongcandidate hot-gas filter materials.

ResultsThe finite-element anafysis results indicate that the

minimum temperature of the filter due to the pulse is 854”C,which corresponds to a temperature gradient over themembrane of only 16°C. hr actual environment, cold-gasbackpulsing is on the order of a tenth of a second, whichwould lead to even smaller temperature gradients.

Thermal stresses generated by the cold-gas pulses werefound to be as high as 355 MPa for alumina. This eliminatedahtmina as a candidate material to be used for the design ofcross-flow filters. The values of the thermal stresses found formtdlite and CS-50 were respectively ’75 and 15 MPa.

Table 1: Properties of ahtmina, mullite and CS-50

Alumina Mullite CS-50

Strength, of (MPa) 350 200 75

Young’s modulus, (MPa) 375 180 75

Coeff. Th. Exp. (pprn/°C) 8 5 3

Th. shock, AT (“C) 300 400 750

Thermal IS(MPa) 355 75 15

Channel and filter desiznlnititdly, a mold was designed which allowed fabrication

by gelcasting of a 5x5x5 cm, cross-flow body with rectangularcross-flow channels of 0.4 mm. The thermo-mechanicalanalysis was based on this initial mold design. AS shown inFigure 1, the thermal stresses found with the model wereextremely large at the corners of the channels; these should bediminished by rounding oft’ the corners in the next designiteration.

-221 MPa71792283785286788279771127

Figure 1: Thermaf stresses distribution in alumina

As shown in Figure 2 the maximum mechanical stressesoccur in the center of the wall. The mechanical stressesapplied on the membrane are proportional to the inverse of thesquare of the thickness. so by increasing the thickness of themembrane, the mechanical stresses will diminish. The initialfinite element analysis has demonstrated that the thermalgradients for a thin-walled (0.4 mm), hot-gas filter are

relatively small (less than 50°C). However, as the wallthickness is increased to accommodate higher mechmicalstresses, the thermal gradients are expected to be more severe.

A new mold has been designed with larger dimensionsIOX10x15 cm, and thicker walls of 3 mm. The channel shapehas been changed from circular to more rectangular in order toincrease the overall surface area. A photograph of prototypesmade with both the old (small) and new (large) mold designsis shown in Figure 3. Along with dimensional clmngcs in themold, there have been minor changes in the mold materials.The mold walls have changed from plain aluminum to ananodized aluminum in order to increase surface scratchresistance and improve mold release,

Figure 2: Mechanical stresses in alumina

. .i

Figure 3: Comparison of first generation (smalIer) to currenthot gas filter design.

PROCESS DEVELOPMENT

Fabrication Technology

Gelcasting. As mentioned earlier, fabrication ofLoTEC’S, monolithic, cross-flow tilter waspossible due to aunique process called gelcasting. Gelcasting involves theaddition of two monomers to a water-based ceramic slurryfollowed by polymerization of these monomers usingappropriate activators and a catalyst. The polymerizationprocess ngidizes the slurry, transforming it from a liquid to asemi-rigid gel. Once the gelation process is complete, thecomponent is dried and sintered. The advantages of thegelcasting process are 1) production of a strong, machinable,green-body ceramic, 3) fabrication of both simple andcomplex shapes, 4) production of near-net shape partsresulting in a decrease of the machining time and cost. As aresult, gelcasting has become an attractive and cost-effectivesolution for fabricating complex-shaped components.

Powder urocessimz. To achieve a strong, intercomecteclmatrix for porous material, it was necessmy to reduce theparticle size of CS-50 in order to increase particle packing andsinterability. An average particle size of 1.5 ~ was obtainedand solids loading of 78 wt~o solids were obtained withoutsignificantly increasing the viscosity of the shmry. Submicronmullite powder was obtained commercially (MULSM,Baikowski , Charlotte, NC) and used as received. Othercommercial mullite powders (Aremco) used essentially for thecorrosion study were wet milled down to 1 Vm in size.

Drying, bkauing and sintering. Because water istrapped in the gelled polymer network, @lng is a slow andcritical step in obtaining crack-free gelcast components. Dueto the thin-walled nature of the first generation cross-flow

filter, d~ing has not been a m~jor constraint in obtainingcrack-free components, However, as indicated by the thermo-mechanical analysis, the thickness of the walls in the secondgeneration filter was increased, as well as the overalldimensions of the filter. These modifications introducedlonger drying schedules using rate-controlled, weig-ht-lossmeasurements. Binder bum-out schedules were adapted to thenew tilter dimensions, and sintering times and temperatureswere adapted to the particle size utilized.

Porosity FormationMuch work is being devoted to obtaining acceptable

porosity in order to achieve optimum permeability for hot-gasfiltration. Several pore-forming techniques are still beingevaluated.

Porous Ceramics by L]quid Phase Separation. In thisprocess, porous, monolithic, ceramic bodies containingreticulated macropores are produced by first forming amixture of ceramic powder and two organic polymers,specifically dibutyl sebacate and low-density polyethylene. i1The temperature of this mixture is raised above the miscibilitylimit to form a liquid solution. While cooling, the polymersphase-separate and the ceramic particles migrate with thepolyethylene. Further cooling provides a three-dimensional,interpenetrating structure involving a ceramic-enrichedpolyethylene phase and dibutyl sebacate. The dibutyl sebacateis then extracted using organic solvents such as hexane. andthe remaining porous structure is fired to form a reticulatedmacroporous body. It has been shown by Breznyl 1 that porevoIumes as high as 70’%0with pore sizes in the 10 to100-pmrange can be achieved using this technique. The pore volumeand size is accurately controlled by adjusting the ratio ofpolyethylene to dibutyl sebacate. The efficacy of using thistechnique to form a cross-flow, monolithic filter is still beingevaluated, including whether the existing mold can be adaptedfor this purpose.

Air Entrapment. An air entrapment technique applied togelcasting to retain air bubbles in the gelled slurry. 12 Severalsamples of CS-50 and mullite have been successfully foamed,bisqued, and sintered. A scanning electron micrograph of agelcast CS-50 foam is shown in Figure 4. The entrapped airtends to produce closed porosity unless the bubbles are inclose enough proximity so that they eventually join. Toincrease the connectivity between pores, several processingparameters are being studied to obtain more uniform andsmaller bubbles. Variations in the amount of foaming agent,mixing temperatures, speed of the mixer, amounts of catalystand initiator for polymerizing the slurry are being evaluated.Different sintering temperature and atmospheres are also beingevaluated to increase the amount of open porosity duringsintenng. This process is very promising in terms of volumeof pores obtained, shape of the pores, and strength of the cells.

Pore Formation by Fugitive Organic Addition. Themost effective fugitive pore former was potato starch13obtained from Lyckeby Sttirkelsen, Sweden. LoTEC, Inc.,was supplied with two separate starches, Mikrolys and

‘.

t

Trecomex, with average particle sizes of 20 and 55~m,respectively. Both Mikrolys and Trecomex starch were addedto CS-50 slurry in amounts of 30, 40, and 50 vol%. With thesmaller-sized Mikrolys starch, all but the 50 VO1’%0sampleswere crack- free through the sintering process. Samples withthe larger Trecomex starch exhibited some cracking in the 40and 50 voiYo loading after sintering. Firing temperaturesranged from 1000”C while bisquing to 1500”C duringsintering. Scanning electron microscope images of varioussamples showing high levels of uniform porosity are shown inFigure 5. It can be seen that in the highly porous samples, theporosity is both spherically shaped and uniform throughout thesample, which is desirable for hot-gas filtration.

Figure 4: SEM picture of CS-50 gelcast foam

Figure 5: CS-50, + 50 vol% starch, sintered at 1300”C.

CORROSION RESISTANCECorrosion/oxidation resistance of the [Ilter components is

needed for durability and reliability. SiC and Si3N4 areexpected to form a surface layer of SiO~ at high temperatures.T~s silica layer is responsib~e for the ~xid&n res&mce ofthe material. However, any species that reacts with the silicalayer will change the transport properties and alter thecorrosion resistance of the material 3’ ‘. Many Oxide-basedmaterials are inherently stable and are more likely to retaintheir physical integrity during an alkali attack.2

Recent work at ORNL has shown that CS-50 has a highchemical corrosion resistance to molten sahl~’i5. Lee et al.have studied the stability of several NZP compositions,including CS-50, in a corrosive environment containingNazSOt at 1000°C. Their study showed that CS-50 remainedstructurally intact and did not exhibit significant weightchanges after 100 hours of exposure. Current work involvesmechanical testing of dense corroded ftexure bars of mulliteand CS-50 to obtain a more quantitative comparison.Additionally, several muilite compositions have been studied,since it is presumed that SiOz-rich mullite is more susceptibleto corrosion degradation than is alumina-rich. Flexurestrength will be measured to evaluate the damage of alkaliexposure on the material’s strength. The samples arethoroughly cleaned in a solvent such as acetone and dried,Once dry, the specimens are then weighed and dimensionallymeasured. The sodium sulfate loading is performed by

heating the specimens on a hot plate at 90°C and bydispensing drops of saturated Na2SO~ solution onto thesurface. The samples are left to dry for an hour, weighed, andmore sodium sulfate is added until the loading reaches 8mg/cm2. The samples are then heated in a furnace at 1000”Cfor 100 hr with 02 at 200 cm3/min flowing over them. Thesamples are weighed, washed in hot distilled water for twohours to dissolve any residual salt and sodium sulfate, andthen weighed again. The identification of the phases presenton the sample’s surface is performed before and after corrosionby standard x-ray diffraction (XRD) and electron microscopy.The corrosion task will also be extended to include evacuationof filter components tested in the simulated PFBC Facility atWestinghouse for any degradation as well as corrosion.

CONCLUSIONSDue to excellent thermal shock resistance of the NZP

family of ceramics and their promising corrosion resistance,the development of cross-flow hot-gas filters was initiated.During the initial program, the design of a truly monolithiccross-flow filter was improved by the results of a thermo-mechanical analysis, The thickness of the walls wasincreased, the channels were rounded and the materialsselection was reduced to two candidates, CS-50 and mullite.Gelcasting technology was used to fabricate a cross-flowshape without laminations and in a single step.

There have been significant improvements in the porositylevels and uniformity, and more effort will be devoted tofurther optimize the porosity for hot gas-filters requirements.Future efforts will also involve tailoring the surface porosity toprevent ash penetration and blinding of the material.Evaluation and comparison of the mechanicrd properties ofCS-50 and mullite after corrosion is under way.

a’

ACKNOWLEDGMENTSThis work was sponsored by the U.S. Department of

Energy SBIR/S’ITR program.

REFERENCES

1 Alvin, M. A., “Performance and Stability of Porous CeramicCandle Filters During PFBC Operation”, Materials at HighTemperature, Proceedings of the 2’@International Workshopon Corrosion in Advanced Power Plants Mar 3-5, 14, (3), 355-364, 1997.

2 Alvin, M. A., Lippert, T. E., and Lane, J. E., “Assessment ofPorous Ceramic Materials for Hot Gas FiltrationApplications,” Ceram. Bull., 70, (9), 1491-1498, 1991.

3 Jacobson, N. S., “Corrosion of Silicon-Based Ceramics in

Combustion Environments,” J. Am. Ceram. Sot., 76, (l), 3-28, 1993.

4 Pickrell, G. R., Sun, T., Brown, J. J., “High TemperatureCorrosion of SiC and SisNi,” Fuel Processing Technology, 44,213-236,1995.

5 Fox, D.S., Smialek, J. L., “Burner Rig Hot Corrosion ofSilicon Carbide rmd Silicon Nitride,” J. Am. Ceram. Sot., 73(2), 303-311,1990.

b Epstein, M., “ Overview of Dust Filtration of Dust fromCord-Derived Reducing Gases at High Temperature, ” in: Proc.Second EPRI Worshop m Filtration of Dust From Coal-Derived Reducing and Combustion Gases at HighTemperature, R. C. Bedick, M. Epstein, and R. A. Brown,eds., Electric Power Research Institute, Palo Alto, CA, Mar.11-13, 1992.

7 Alvin, M. A., Tressler, R. A., Lippert, T. E., and Diaz, E. S.,“Evaluation of Ceramic Filter Material, Selection forApplication.” Proceedings of the Coal-Fired Power Systems -

Advances in IGCC and PFBC Review Meeting,DOEJMETC-93/6131, 1993. NTIS DE93000289, NationalTechnical Information Service, Springfield, VA.

8 Janney, M. A., Omatete, O. 0., Walls, C. A., Nunn, S. D.,Ogle, R. J., Westmorekmd, G, “Development of Low-toxicityGelcasting Systems”, J. Am. Ceram Sot., 81, (3), 1998.

9 Limaye, S. Y., “New Ultra-Low Thermal Expansion, HighlyThermal Shock-Resistant Ceramic,” U.S. Pat. No. 5488018,1989.

10 Jackson, T. B., Limaye S. Y., and Porter W. D. “The effectsof thermal cycling on the physical and mechanical propertiesof [NZP] ceramics”, Ceram. Trans., 52, 63-79, 1995.

11 Brezney, R., Spomitz, R. M., “Method of Making

Microcellular Ceramic Bodies,” U.S. Patent No. 5,427,721,

1995.

12 Sepulveda. P., “Gelcasting Foams for Porous Ceramics,”Am. Ceram Sot. Bull., 76, (10), 61-65, 1997.

13 Lyckfeldt, O., Ferreira, J. M. F., “Processing of PorousCeramics by Starch Consolidation,” J. Eur. Ceram. Sot., 18,131-140, 1998.

14Lee, W. Y., Stinton, D. P,, and JosIin, il. L., “Interaction ofLow-Expansion NZP Ceramics with Na2S04 at 1000 C,” J.Am. Ceram. Sot., 79, (2), 484-86, 1996.

15 Lee, W. Y., Cooley, K. M., Brendt, C. C., Joslin, D.L., andStinton, D.P., “High Temperature Chemical Stability of

plasma Sprayed C%.&r&r&@zi Coatings on Nicalon/SiCCeramic Matrix Composites and Ni-Based SuperalloySubstrates, J. Am. Ceram. Sot., 79, (10), 2759-62, 1996.