Utilization of fly ash from coal-fired power plants in China

Cao et al. /J Zhejiang Univ ScA 2008 9(5): 681-687CIENCEASN 1673-565X(Print ) ISSN 1862-1775(Online)www.zju.edu.cmyZus:www.spningerink.comZUSUtilization of fly ash from coal-fired power plants in ChinaDa-zuo CAO Eva SELIc. Jan-Dirk HERBELLUniversity of Duisburg-Essen, Duisburg 47057. Germany)E-mail: eva selic(@uni-duedeReceived July 20, 2007; revision accepted Jan. 14, 2008; published online Mar 27, 2008Abstract: The rapidly increasing demand for energy in China leads to the construction of new power plants all over the countryCoal, as the main fuel resource of those power plants, results in increasing problems with the disposal of solid residues fromombustion and off gas cleaning. This investigation describes chances for the utilization of fly ash from coal-fired power plants inChina. After briefly comparing the situation in China and Germany, the status of aluminum recycling from fly ash and the adantages for using fly ash in concrete products are introduced. Chemical and physical analyses of Chinese fly ash samples, e.gX-ray diffraction(XRD), ICP(Inductive Coupled Plasma)and particle size analysis, water requirement, etc are presented. Reasonable amounts of aluminum were detected in the samples under investigation, but for recovery only sophisticated procedures areavailable up to now. Therefore, simpler techniques are suggested for the first steps in the utilization of Chinese fly ash.Key words: Fly ash utilization, Aluminum recycling, Concrete, Chemical and physical analysesdoi: 10. 1631/jzus. A072163Document code: ACLC number: X7power plants. It is the largest amount of industrial in construction and building material o2aINTRODUCTIONmethods underlocal conditions andFour fly ash samples from a power plantFly ash is the finely dispersed mineral residue China have been analyzed by chemical and physicalresulting from the combustion of pulverised coal in analyses, with respect to compositiond utilizationwaste in the world. The main components of fly ashare a-quartz(SiO2), mullite(3Al2O32SiO2), hematiteALUMINUM RECYCLING FROM FLY ASH(Fe2O3), magnetite( Fe3O4), lime(CaO), and gypsum(CaSO4'2H2O)mainly in the form of sphericalThe mass proportion of alumina(AlO3)in flyparticles(White and Case, 1990; Giere et al., 2003). ash ranges from 20% to 40%. Since long time ago,Even though many attempts have been made to scientists have been focusing on aluminum recyclingfind new application fields for fly ash, for example, from fly ash due to the increasing costs of the primarysuch as environmental and agronomic amendment metal caused by high energy consumption for its(Zhang et al., 2004; Garg et al, 2005), the production production. Many possible approaches were devel-of inorganic polymers(Steveson and Sagoe-Crentsil, oped, for example, Sintering-Alkalization Method2005), the filler in fly ash polymer composites( Chand (Xie and Tang, 1996), NH4F-Solubilization-Acidifiand Vashishtha, 2000), and the production of cation Method with Lime (Liu and Li, 2006), Polynanostructure materials(Paul et al., 2007), the acrylamide Dispersant Method to produce Al(OH)utilization in concrete and cement is still the most (Gui and Fang, 2004), and Solubilization by micro-effective one, both from economic and ecologic point wave中国煤化工blems as chinaThe aim of this work is to investigate the qualiCNMHGounts of fly ash,ties of Chinese fly ash for developing reutilization is deeply researching on alumina recyclingCao et al. /J Zhejiang Univ SciA 2008 9(5): 681-687Balasubramanian et al, 2004; Bhattacharya et al., sumption necessary for the process. The replacement2004). In contrast, Europe has lower interest in alu- of l t of portland cement reduces the overall COzmina recycling from fly ash, due to own bauxite emissions by approximately 1 t(Ehrenberg andsources and high energy consumption for the recovery Geisler, 1997). Additionally, natural resources such asprocesses. Energy consumption is also the reason that gravel and sand are saved.the above-mentioned methods on alumina recoveryare only developed on lab scale. At the moment, themost promising method seems to be the substitution of FLY ASH PRODUCTION AND UTILIZATION INaluminum by fly ash instead of its recycling, for ex- CHINAample by"application in synthesizing low costs metalmatrix composites for automotive and other applica-In China, the reutilization rate of fly ash is in-tions", where aluminum is substituted by fly ash up to creasing, but still lower than 70%. Besides the reuse10%(Rohatgi et al., 2006)in concrete, Chinese industry is interested in recyclingNevertheless, such special application will not be the high alumina content from fly ash. This belongs tothe solution to reduce the increasingly high amounts of the fact that 60% of the industrial Al2O3 needs to beChinese fly ash. For an effective solution, the most imported which is about 10x10 t annually(Yang andreasonable way to recycle fly ash will be applied in Zhang, 2006)construction and building materials industryFrom early 1950s, China has pursued a policy ofash utilization technology, and supported research anddevelopment activities. The utilization rate of the ashADVANTAGES OF USING FLY ASH IN CEMENT remained on a low level at around 10% until theAND CONCRETE1980s. However, during the 1990s, the utilization rategrew significantly and reached more than 53%, andFly ash has a successful history of use in concrete according to the government statistics, the total asharound the world for over 50 years. In the United production in China in 2002 was 150x10 t, of whichStates of America more than 6x10"t, and in Europe about 100x10 t was utilized. The predicted amountsmore than 9x10 t are used annually in cement and of coal-generated fly ash in 2010 and 2020 will beconcrete(lan and lindon,200). Fly ash is used in all320×100-380×10°tand570×10-610×10°t, respecsectors of the concrete industry, covering ready-mixed, tively(lan and Lindon, 2004)precast, and on-site applications due to many advanIn some developed areas, the situation is alreadytages summarized below (Lutze and vom Berg, 2004): better In Nanjing City of Jiangsu Province, the utili-(1)Improvement of long-term strength per- zation rate of fly ash was 100% in the past five yearsformance and durabilityIn Shanghai City since 1997 the produced fly ash was(2)Reduction of permeability, which reduces also 100% reused, mostly in earthwork of road con-shrinkage, creep and gives greater resistance to chlo- struction and wallboard materials (Wu and Zhang,ide infiltration and sulphate attack.2005). In developing areas, the problem is also going(3)Risk minimization of alkali silica reaction. to be solved. In Henan Province, the accumulated(4)Reduction of temperature rise in thick sec- piled fly ash was 130x10 t until 2005, of whichtions of construction elements(bulk concrete)34 10 t were produced in 2005. The reused rate of()Increase of cohesion in concrete leading to 2005 was 80%, and the half was used in cementreduced bleeding rate, easier compaction, better production(CUBN, 2006)pumping properties, and improved surface finishDue to new laws, clay solid bricks will be for-Besides the technical advantages, the reuse of fly bidden in most of Chinese cities from 2007 on, in allash in cement industry has also ecological benefits urban area from 2011, and all over the countrylike an efficient reduction of1996:2003).In2004, the totalsubstitution of Portland cement by fly ash reduces not pro中国煤化工 n pieces, of whichonly the CO2 emission being generated during the 500CNMH GIcks(Wei, 2006)production of clinker but also the high energy con- The igy valuing ue use of solid clayCao et al. /J Zhejiang Univ Sci A 2008 9(5): 681-687bricks will be a newchance for solving fly ash dis- the bvK counted that in future a further stabilposal problems. a sharp increase in demand for ma- of their sales will be obtained by purposeful utilizationsonry materials and other construction units like of the various material properties of their productspavement bricks containing fly ash is expectedClose cooperation between producers and marketersas well as the continuously operated extension ofplications with strong focus on the changing demandsCURRENt UTILIZATION OF FLY ASH in of the market has been proved as substantial successGERMANYfactors for the utilization in high-quality constructionand building materials guaranteeing high recyclingOn average, Germany produces 4.3x10 t of fly rates for fly ashash a year with increasing tendency. BundesverbandKraftwerksnebenprodukte eV(BVK) is the federalassociation for marketing the by-products from power EXPERIMENTALlants in germany. the recent data collection fromBVK is presented in Fig. 1, showing the total amountTo investigate the utilization possibilities ofand different utilisations in construction and building Chinese fly ash, four fly ash samples from a powermaterials of marketed fly ash within Germany during plant in Northem China have been analysed.the period of 1997-2005(BVK, 2006). The figureX-ray diffraction(XRD)and Inductive Coupledindicates that in the last 8 years almost 100%of fly Plasma(ICP)analyses were done for checking theash has been recycled with only small changes in the possibility of Al-recycling. XRD has been done onapplication areas.one fly ash sample using spectrometer SPECTROMore than 50% of the reused fly ash is utilized XEPOS. Aluminium content has been quantified byfor ready-mix concrete. The second largest applica- ICP spectrometry (ICP-OES Vista AX,"Variantion area with approximate 17% is mining and dry Deutschland gmbH")construction materials, followed by precast units withTo prove the application possibilities in theconcrete and cement domain, several parameters havea Ready-ImIs concrete D Mining and dry construction materialsbeen determined according to European standards asa Cement prouts Road construction and ground workfollows: Loss on Ignition(LoI) and free Cao have口 Precast uitsbeen determined according to en 196-2(2005)andEN 451-1(2005), respectively; Particle size has beenanalysed by air jet sieving(HOSOKAWAALPINEWater requirement property(EN 450), slump(EN 196-3), and particle density(En 196-6)havebeen measured as well as compressive and bendingstrength after 28 d from mortar mixes of Chinese fly199719981992000200120022032)4205ash and cement. Bending and compressive strengthhave been tested with strength testing apparatus fromFig 1 Utilization rates of fly ash in different application "Toni Technik Baustoffprufsysteme GmbHareas from 1997-2005In addition, pavement bricks speThe driving force for the high recycling quote is from Chinese fly ash have been prepared andthe German"Act for Promoting Closed Substance compressive strength has been measuredCycle Waste Management and Ensuring Environ-mentally Compatible Waste Disposal"(KrW-/AbfG,1994). German fly ash fulfills strong quality criteria RESULTS AND DISCUSSIONguaranteed by extensive technology. Cements with flyashes are classified according to European standardV凵中国煤化工 wws that the mainEN 197-1(2000), allowing fly ash contents up to 35%. crysCNMH Gullite and quartz.Altogether the organized marketers and producers in Due to the rapid cooling at high temperature of fly ash,ao et al. /ZHejiang Univ SciA 2008 9(5): 681-687Chinese Standards GB 1596-79(1979). Therefore,harmful Ettringite (CaAl2(SO4)3(0H)12: 26H20)formulation( Richard, 1987) is minimizedThe results of the loi and free Cao measure-ments as well as the particle size distribution deter-mined by sieving are listed in Table 2.Table 2 LOl, free Cao and particle size distribution120304050607080of nly ash samples from China(unit: %Fig 2 X-ray diffraction pattern of one Chinese fly ashSample LOI Free Cao20m<40m>200m1199<0.01031.250.5glass accounted for a large proportion. The broad2124<0.10041.2593hump in the region between 100 and 25 indicates the312.8<0.10039462.1presence of glassy phases. The structure of Al-O-Si is 4 12.8<0.100 39.457.3very tight in glassy phases. Regarding the idea ofAl-recycling, high amount of energy would be nec-essary for breaking the SiO2-Al2O3 bondLoI is a parameter that when done with fly ashIn Table I the average main composition of all mainly describes the carbon content in the substancefly ash samples determined by ICP analysis is pre-( Richard, 1987). Carbon has a low density and cansented. The high alumina content of 44%of the ana- absorb significant amounts of water. This means thatlyzed fly ash may give reasons for thinking of feasible the maximum dry density and optimum moisturerecycling. Similar to aluminum recovery, alumina content of fly ash are influenced by the LOl. Higherycling out of an AlO3-SiOz-matrix like that in fly LOl ash is lower in density, but has higher optimumash did not proceed beyond lab scale up to now for moisture contents. Generally, the lower the carboneconomic reasons. At present, Chinese government content and the finer the ash particles, the betterand industry should focus on using fly ash in the prospects for ash utilization, principally in ceconstruction and building materials industry until new ment-based formulations. When the ash is used inmethods open more economic ways for recycling brick manufacturing, higher LOI is acceptable, oraluminum/alumina from fly ash in futureeven welcomed since the carbon in the ash could beTable 1 Average main composition of Chinese fly ashburnt during the calcination process, therebysamples determined by ICP analysisenergyContent(%)The LOi analysis of Sample 1 is about 2%; thisALO,result is very good for its utilization in cement re-0.9ferred to EN 450(2005). But the other three samplesshow an LOI of about 12%, which is too high to beused in cement or concrete production Only ChineseStandards(GB 1596-79, 1979)still allows utilizationNa,O03in the lowest Class iilFree Cao is a recognized cause of unsoundnessfactor for concrete(Richard, 1987). The values of freeCaO are all far less than 1%, which corresponds withThe high silicate content of the investigatedFineness has long been recognized as one of thesamples of approximate 44% yields a good quality for most important characteristics of fly ash. The meas-puzzolanic reaction. The content of SO3 in each ured中国煤化工: Chinese fly ashsample is far lower than the upper limit value of 3% sampCNMH PArticles >45from both European Standards EN 450(2005)and accords w EI 4Jv (vuJ), uut corresponds toCao et al. /ZHejiang Univ SciA 2008 9(5): 681-687requirements of Chinese Standards GB 1596-79The results indicate that the investigated Chi-nese fly ash cannot reach enough quality level toTable 3 shows the results of measurements in provide advantages in concrete fabrication accordingwater requirement, particle density, slump and to European standards, even though the compressivecompressive and bending strength after 28 d of and bending strengths are slightly better than thecement-fly ash mortar mixtures with a ratio of reference.75%: 25% and of 100% cement as reference. theThe compressive strengths for the preparedamount of water necessary for complete moistening pavement brick specimen using Chinese fly ash areof the cement, and a workable mixture with definite presented in Table 4.slump is reflected by the water requirement. LowReferring to the Chinese Standards JC446-91water requirement is preferred yielding solid concrete (1991)the pavement bricks specimens have a gooddue to low water/cement ratiosquality in pressure strength and fulfill the require-Plasticity of fresh batches of concrete is ments of Class I for sidewalk and Class Il fordescribed by slump test. High slump increases the driveway. However, the low frost resistance causedworkability and is desired if the water/cement ratio by high Lol of the investigated fly ash must be takenremains low. Normally, the admixing of fly ash with into account.applicable properties leads to reduced water requirement and increased slump, due to a fluidizingeffect caused by the spherical form of the fly ash CONCLUSIONparticles(ball-bearing effect)The water requirement of approximately 70% of Besides an already high reutilization rate of flythe Chinese fly ash mortar mixtures is much higher ash in general, China is actually interested in recyclingthan the typical required value of 20%-40% in Ger- the aluminum content in fly ash from coal-fired powermany. A reason for this is the absorption of water by plants. ICP analysis of fly ash samples from powerporous carbon particles as is indicated by high LoI plants in Northem China, showing an alumina contentvalues(Table 2). As a result, the slump testing value, of more than 40% supports this idea. However, actualin average 121 mm, is rather low compared to the aluminium recycling from fly ash is limited to labreference cement sample whose value is 173 mm. In scale due to uneconomic energy consumption andaddition, the particle density of Chinese fly ash mor- technical restrictions. Thus, the suggestion is to focustar mixtures being 2453 kg/mon average is higher on using fly ash in construction and building materialthan the typical value of 2250 kg/m. Fly ash with industry until there is a promising and more economiclower particle density is preferred for usage in con- way for recycling of the aluminum content in futureInvestigations on recycling potential of ChineseTable 3 Construction and building material properties of mortar mixes from Chinese fly ash and cementSampleWater requirement Slump Particle density Compressive strength for Bending strength for(%)(kg/m)mortar testing by 28d mortar testing by 28 dCement 100%83(100.0%)Cement: fly ash65.6567(1029%)93(112.0%Cement: fly ash72.0245060.0(1089%)Cement: fly ash71.811047054.7(993%)94(1133%Table 4 Compressive strength of pavement bricks prepared from Chinese fly ashPressure areaSpecimenHeight (mm) Mass(kg) Density(kgload nn Compressive strength中国煤化工m278501402233570.5CNMHG56.27850233450.7Cao et al. /ZHejiang Univ SciA 2008 9(5): 681-68fly ash in cement show a promising high silicate(Ecological Properties of Blast-fumace Cement--part ofcontent of more than 40% yielding good puzzolanicLife Cycle: Production). Beton-informationen( Concretereactions. Unfortunately, high LOI values of threeformation ), Nr. 4(in German)from four samples, which all exceed 12%, restrict theEN 196-2, 3, 6, 2005. Methods of Testing Cement. GermanDIN-EN Standard Beuth Verlag GmbH, Berlin.chances of application. Moreover, the fineness was EN 197-1, 2000. Cement-Part 1: Composition, Specifica-found to be quite coarse and just in the range of the tions and Conformity Criteria for Common Cementsacceptable limit. Most European standards for usingGerman DIN-EN Standard. Beuth Verlag GmbH, Berlin.fly ash in cement and concrete industry are not ful- EN 450, 2005.Fly Ash for Concrete German DIN-EN Stanfilleddard. Beuth Verlag gmbH, BerlinEN451-1,2005sting Method foAsh. GermanChinese standards still support the utilization ofDIN-EN Standard Beuth Verlag gmbH, Berlin.fly ash in cement, concrete in Class lll, and in wallGarg,RN,Pathak,H, Das,DK,Tomar,RK,2005.Use ofbricks in Class II or III. However, with respect to thely ash and biogas slurry for improving wheat yield andenvironment and regarding the fast increasing Chiphysical properties of soil. Enm Monitoring and Assenese economy and globalisation, Chinese legislationsmen,107(1-3)1-9.|o:10.1007/s106610052021-xand standards are proposed to be, and should be, moreere,R, Carleton, L E, Lumpkin, GR, 2003. Micro-andstringent in order to correspond with the standards innano-chemistry of fly ash from coal-fired power planAm. mineral, 88: 1853developed countriesGui, Q, Fang, R, 2004. Production of nano-Al(OH) by usingBesides legislation, the operation of powerfly ash Fly Ash, 2: 20-22plants in China should improve to actual state-of- lan, B, Lindon, S, 2004. Ash Utilization from Coal-Basedthe-art. Careful input control, efficient burn out, andPower Plants. Report No. COAL, R274DTI/Pub, URNan additional feed of suitable components to improve04/1915Http://www.dti.gov.uk/energy/sources/renewables/publications/page 19184. htmla high quality of fly ash with low LOl will ensure KrW-/AbfG(Kreislaufwirtschafts-und Abfallgesetz),1994higher standards in the utilization of Chinese fly ashAct for Promoting Closed Substance Cycle WasteFor the time being, the best chance is to replace solidManagement and Ensuring Environmentally Compatibleclay bricks which become banned from 2007 onWaste DisposalLiu,Y, Li, L, 2006. Progress in research of alumina recyclingReferencesfrom fly ash. Light Metal, 5: 20-23(in Chinese ).Balasubramanian, G, Nimje, M. T, Kutumbarao, VV, 2004SCNPC(the Standing Committee of the National People'sRecycling of Aluminum Industrial Waste into Alumino-Congress), 1996. Law on Preventing and Control of En-borosilicate Glass-ceramic Tiles. 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Inorganic Industry, 2: 47-49Protection and Utilization of the Resources, p. 2(inEditor-in-Chief: Wei YANGISSN1673-565X(Print); ISSN 1862-1775(Online), monthlyJournal of zhejiang UniversityScienCeawww.zju.edu.cn/jzus;www.springerlink.comjus@zju.edu.cnJZUS.A focuses on"Applied Physics EngineeringOnlinesubmissionhttp://www.editorialmanager.com/zusa/JZUS.A has been covered by SCl-E since 2007i Welcome Your Contributions to JZUS-AJournal of Zhejiang University SCIENCE A warmly and sincerely welcomes scientists all overi the world to contribute Reviews, Articles and Science Letters focused on Applied Physics Engii neering. Especially, Science Letters(3 4 pages)would be published as soon as about 30 days(Notei detailed research articles can still be published in the professional journals in the future after ScienceLetters is published by JZUS-A)中国煤化工CNMHG