循环流化床煤灰综合建筑应用---外文文献及翻译(可编辑)

1、循环流化床煤灰综合建筑应用- 外文文献及翻译外文文献及翻译Utilization of CFB Fly Ash for Construction ApplicationsR. E. Conn and K. SellakumarFoster Wheeler Development CorporationLivingston, NJA. E. BlandWestern Research InstituteLaramie, WYABSTRACTDisposal in landfills has been the most common means of handlingash in circulat

2、ingfluidizedbed CFBboilerpower plants.Recently,largerCFBboilers with generatingcapacities up to 300 MWearecurrentlybeingplanned,resultinginincreasedvolumes and disposalcostof ash byproduct.Studieshave shown that CFBashes do not pose environmentalconcernsthatshould significantlylimittheirpotentialuti

3、lization.Many uses ofCFBash are being investigated by Foster Wheeler, which can provide morecost-effective ash management.Constructionapplicationshave been identifiedas one ofthemajoruses forCFB ashes.Typically,CFB ashcannotbe usedasacementreplacement in concrete due to its unacceptably high sulfur

4、content.However, CFB ashes can be used for other construction applications thatrequireless stringentspecificationsincludingsoilstabilization,roadbase, structuralfill,and syntheticaggregate.Inthisstudy,potentialconstructionapplicationswere identifiedforflyashes from severalCFBboilersfiringdiversefuel

5、ssuch as petroleumcoke,refusederivedfuelRDFand coal.The compressive strength ofhydratedflyashes was measuredinordertoscreentheirpotentialforuseinvariousconstructionapplications.Based on the results ofthis work,the effects ofboth ashchemistry and carbon content on utilization potential were ascertain

6、ed.Actualbeneficialusesofashesevaluatedinthisstudyarealsodiscussed.INTRODUCTIONCFB combustion has developed into a mature technology for burningawiderangeoffuels,whilestillachievingstrictairemissionsrequirements.Typically,fuelsareburned in aCFB boilerwiththeaddition of limestone to capture SO2 in a

7、solid form. With larger CFBboilers being brought online, a greater emphasis has been placed onenhanced beneficial useof ash thanin thepast. Studies have shown thatthe environmentalimpactfrom CFBashes islessthan those from p.c. ashesand shouldnot limittheirutilizationas marketable by-productsConn and

8、Sellakumar, 1997; Young, 1996.Traditionally, p.c. fly ash has often been sold for use as anadmixture in the production of Portland cement. The utilization optionsfor CFB ashes are somewhat more diverse than p.c. ash, due to the effectof sorbent calcium on the overall ash chemistry. These options inc

9、ludeagriculturalapplications,constructionapplicationsandwastetreatment.Beneficialuse forconstructionpurposesisone ofthemost commonmarkets for CFB ash. These uses include soil stabilization, road base,structuralfills,and syntheticaggregate. To qualify forthese uses, theash must have specialproperties

10、and pass certainASTMtests.Compressivestrengthisone of themost importantphysicalpropertiesa materialmustpossess when being considered for different construction applications.Depending upon the specificapplication,differentdegrees of compressivestrengthare required.In thisstudy,the unconfinedcompressi

11、ve strengthwas measured for hydratedCFB fly ashes from boilers firinga wide rangeof fuels These results provided a indication of potential constructionuses for fly ashes with very different compositions.The specific objectives of this work were:CHARACTERIZATION OF FLY ASHESNine fly ashes evaluated i

12、n this study were obtained from CFBboilersfiringdiversefuelssuch as bituminousgob 0.5% S, low volatilebituminouscoal0.3% S, highsulfur 4.7% bituminouscoal,petroleumcoke5.0% S, and RDF0.3% SThe flyashes had significantlydifferentchemicalcompositions as would be expected considering the types of fuels

13、 beingfired.The bituminousgob flyash was composed primarilyof coalash sinceitwas taken from a boilerthatdoes not use limestoneinjectionforsulfurcapture.The bituminouscoalflyash samples containedboth coalash andsorbent,withrelativelyhighamounts of freelime.Fly ash from petroleumcoke was composed main

14、lyof sorbent compoundsdue to thelow ash contentof the fuel. Finally, the RDF fly ash had a composition fairly similartothatof a bituminous coal.However, most of thecalcium in theflyashwas inherentin theRDFand notderived from sorbent.SomecalciumintheRDF fly ash did originate from the semi-dry scrubbe

15、r used to remove SO2and HCl from the flue gas.Phase analyses of the ashes by x-ray diffraction XRD are shown inTable 2. The coal fly ashes were composed primarily of anhydrite CaSO4,hematite Fe2O3, silica SiO2. Anhydrite was not found in the bituminousgob flyash,sinceitdidnotcontainsorbent.The petro

16、leum coke ash wascomposed principally of sorbent derived compounds and minor amount ofsilica3.0%The flyashes were allrelativelyfinewithgreater than80%passing a 200-mesh screen 74_m. As a result, these ashes can readily bemade into cement-type pastes without further milling. The poured bulkdensityof

17、the flyashes ranged from about 34 to 57 lb/ft3385 to 913 kg/m3;the compacted bulkdensity of the flyashes were slightlyhigherand rangedfrom 53 to 74 lb/ft3 849 to 1186 kg/m3. The specific gravity ranged from1.8 to 3.0 forthe flyashes. The RDFflyash had a relativelylow specificgravitycomparedtotheothe

18、rashes,probablysinceitcontained adifferenttype of inorganics.This ash was derivedfrom fineinorganicsinRDF,not limestonesorbentorcoalminerals.Moisturewas generallylessthan1.0%, except for the RDF fly ash, which contained 1.6% moisture.EXPERIMENTAL PROCEDURESUnconfinedcompressivestrengthoftheflyashesw

19、as measuredsimilarto ASTMC-109.A pastewas preparedby mixingabout35%by weightwater and 65%flyash toform 0.75 in.1.91cm pelletsina plasticmold.The bulkdensity of the ash inthese pelletswas about 60 lb/ft3 960 kg/m3.For soil stabilization tests, flyash 15% by weight wasmixed with claysto forma pellet.T

20、hese samples were curedundersaturatedconditionsat23oC for3, 7 and 28 days. The compressivestrengthof the hydratedsampleswas then measured using a compressivetestingmachine Thisprocedurewasintendedto simulate theactualconstructionuses inwhich cement pasteswould be madefromflyashes. Considerablyless w

21、aterisused intheASTMC-109 procedure compared to the hydration technique used in this study.In addition, the ash bulk density was less than that typically used forASTM C-109. As a result, the compressive strengths may differ somewhatfrom those obtained by the ASTM test.Inmost cases no other materials

22、were mixed withthe flyashes exceptwater.Strengthdevelopment resultedsolelyfromthe self-cementingpropertiesoftheashes.No concrete-typemixtures incorporatingsand oraggregatewere evaluatedin thisstudy.The finesizedistributionofthefly ashes makes them ideal candidates for producing pastes simply withthe

23、 additionofwater.Bottomashes mayalsobesuitableforsomeconstruction applications, but could require milling to a desired, muchfiner size distribution.CONSTRUCTION USES FOR CFB ASHESLaboratorytests were performedto address theuse ofdifferentflyashesinanumberofconstructionapplicationsincluding1 cementre

24、placementandmanufacturing,2structuralfills,3roadbase, 4syntheticaggregate,and 5soilstabilizationCONCRETEAND CEMENTPRODUCTIONThepotentialalso existsforusing CFB ash forregulatingthesettime of Portland cement, instead of conventionally used gypsum calciumsulfatedihydrate.Tests were conductedwithpetrol

25、eumcoke flyash thatcontainedhighconcentrationsofCaOandSO3calciumsulfate.Quantitative XRD analysis showed that this fly ash contained 66% CaSO4and 30% CaO. To compare performance of cements with the fly ash and withcommercial grade gypsum, three samples were prepared with aType I cement clinker inclu

26、ding:94.5% clinker, 4.6% gypsum;_94.5% clinker, 2.3% gypsum, 2.8% fly ash; and_94.5% clinker, 5.5% gypsum.Thecements weregroundinabatchballmilland testedforcompressive strength and time of set according to ASTM standards C-109and C-191, respectively.The resultsin Table6 confirmedthe strength charact

27、eristicsofthethree cements exceeded the standard specifications of ASTM C-150. Thecements using the petroleum coke flyash slightlyoutperformedthe controlcement with conventional gypsum in 28-day strength tests. Setting timewas shorterforthe experimentalcements, but remained comfortablywithinstandard

28、limits.Testresultswouldbe expectedtovaryforcementclinkers of different compositionsSTRUCTURAL FILLSNaturalsoilborrow,granularfill,boilerslag,and otherembankmentor structuralfillmaterialsare typicallytestedto determinetheirshearstrengthBrendaletal,1997. Cementitiousmaterialssuch asfly ash, however, a

29、re more appropriately evaluated by the unconfinedcompressivestrengthtestThe twomajortypesofstructuralfillmaterials are 1 flowable or excavatable and 2 compacted or embankment.Flowablefillisusuallymixed in a ready-mixconcretetruck,withmixingcontinuing during transport to prevent segregation. Although

30、 flowablefill may be designedforuse under highloads,thismaterialis typicallydesigned for a compressive strength of 50 to 150 psi 345 to 1035 kP at28 days. Note that this strength may continue to increase with time.Strengthslowerthan50 psi345kP are insufficientfor use as a structuralfill. Strengths h

31、igher than 150 psi 1035 kP at 28 days could result infill materials which would not allow excavation.Compactedfillsand embankments requirematerialswithhighstrength for supporting heavy loads and should be considered permanent.These materials should not be considered for use around pipes, utilityline

32、s, or other locations that may need to be accessedCompressivestrengthresultsshow that onlythe RDFflyash would qualifyas a flowablefillsince its28-daystrengthwas 145 psi100 kP. Itshouldalsobe notedthatthisashshowed considerablerapidexpansionuponhydration,resulting in a very porous material. In fact,

33、the hydrated ash pelletsgrew in volume by 50% in only ten minutes. The reason for this expansionisuncertain,butmay be due toreactionoffinealuminum metaland CaOH2inthe ash withwater resultinginevolutionof hydrogen gas. Thisreactionis similar to that used for autoclave cellular concrete ACC.Thehigh-su

34、lfurbituminouscoal flyash would qualifyas a permanentcompacted filland had a relativelyhigh 28-day strengthofnear 1500 psi10.3MP. Thishighstrengthisnotsurprisingsincetheashnearlyqualifiesas a Class C pozzolan or self-cementingmaterial.As a result,itiscurrentlybeingmarketed as a component inpermanent

35、 fillmaterialsFree lime,particularlyincombinationwithFAScomponents, is one ofthekey ash components that influence the strength of hydrated ashes. Thecompressivestrengthdidcorrelatewiththefree lime contentofmost ofthebituminouscoalflyashes.Freelime,oncehydratedtocalciumhydroxide,would be expected to

36、undergo pozzolanicreactionswithferricoxide, aluminum oxide and silicon oxideThe low-volatilebituminous fly ashdid not develop very highstrength despite its moderate free lime content of 12.5%. The cause oflow strength development is unclear. The high carbon content of the ashLOI18.9%mayhavebeenrespo

37、nsibleforlimitingitsstrengthdevelopment. On the other hand, the lower CaSO4 content of this ash mayhave limitedtheformationofetrringiteorgypsum. Thereis conflictingevidenceastotheeffectofhighcarboncontentonthestrengthdevelopment of CFBC ashes.Figure 1 also shows compressive strength data for a low v

38、olatilebituminouscoalflyash,which also had a relativelyhighLOI of12%. Thisfly ash was obtained from a boiler firing a 4.5% S lowvolatile coal withsignificant inert carbon content. Although this fly ash had 12% LOI, itdevelopeda 28-day strengthof1620 psi11.2MP, possiblydue toitshighfreelimecontent of

39、22.5%.Consequently,itappears thathighLOI may notlimit the strength of hydrated ash, provided itcontains sufficient freelimeand FAS toform pozzolanicreactions or solublealuminaand calciumsulfate to from ettringite or gypsum.The RDFash was also very high in freelime content16.3% and almostqualifies as

40、 a Class C pozzolan. This ash developed low strength eventhoughitwouldbeexpectedtohaveconsiderableself-cementingproperties. This low strength was a result of the formation of a poroushydrated ash as mentioned earlier.The petroleum coke fly ash listed in Table 1 had high free-limecontent,yet moderate

41、 compressive strength520 psi 3.6 MPafter28 days.The petroleum coke ash developed thismoderate strengthdue to hydrationreactions of lime and calcium sulfate, not pozzolanic reactions:CaO + H2O _ CaOH2 calcium hydroxide 5CaSO4 + 2H2O _ CaSO4_2H2O gypsum slow 6Insignificantpozzolanicreactionswould be e

42、xpected with thisashsince it contains only minor amounts of FAS components 3% SiO2. Anotherpetroleum coke flyash see Figure 1 developed considerablyhigher strength820 psi/5.7 MP but contained only 8.6% free lime. This strength wouldnearlyqualifythe ash as a suitablecompacted fill,since italmost meet

43、sthatrequiredby the ASTMC-109 test.As a result,calcium sulfatecontentmay be a better indication of strength development than free lime forhydrated petroleum coke ashes, since it may be the principal bondingmechanism.Asshown in Table 4, the bituminousgob flyash did not develop anysignificantstrengths

44、inceitcontainedlittlefreelimenoself-cementingproperties.The effectoflimeadditionon ash compressivestrengthwas investigated.As shown inFigure2,additionofonly10%limeby weight roughly doubled the 28-day strength 93 psi/641 kP of the flyash making itsuitableforexcavatableflowablefilluse. Additionof25%li

45、me increased the 28-day compressive strength to 750 psi 5.2MP. Slightly higher amounts oflimeadditionshouldmake themixturesuitable for use as a compacted fill.The effect of different additives on the strength of hydratedpetroleum coke fly ash was also investigated. As shown in Figure 3,addition of P

46、ortland cement and coal fly ash/Portland cement raised thecompressive strengthafter28 days togreaterthan1500 psi10.3MP. Thisadditionalstrength was partiallya resultof pozzolanicreactionsoffreelime with FAS components in the cement or fly ash. These mixtures wouldhavesufficientstrengthtoqualifyas pot

47、entialcompactedfills.Addition of blast furnace slag to the petroleum coke ash did not resultin as high as strength,possibly due to its lower FAS content.Compressive strength is only one of the physical properties thatfillmaterialsmust meet. Othergeotechnicaltestsmust also be met suchas expansion, sw

48、ell and permeability. The expansion test is defined byspecificASTM standardsC-157.Expansionofthefillmaterialisundesirable and often occurs in hydrated coal ashes due to formation ofettringite.However, with coalashes,the expansiongenerallyoccurs overa longerperiodoftimeup to sixmonths compared tothat

49、 mentioned earlierfor the RDF ash.The permeabilityof an ash is a measure ofthe rate atwhich a fluidpasses through a material and, along with leachate data, may be used toestimatepossibleimpacts on groundwater quality.For comparison purposes,a permeability coefficientof 1 x 10-7 cm/sec orlower is often requiredfor clay liners i