International Journal of Coal Geology 87 (2011) 112–120

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International Journal of Coal Geology

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A comparative evaluation of minerals and trace elements in the ashes from lignite,coal refuse, and biomass fired power plants

Smriti Singh, Lal C. Ram ⁎, Reginald E. Masto, Santosh K. VermaEnvironmental Management Division, Central Institute of Mining and Fuel Research (Digwadih Campus), P.O.-F.R.I (828108), Dhanbad, Jharkhand, India

⁎ Corresponding author. Tel.: +91 326 2388357; fax:E-mail address: lcramcimfr.dc@gmail.com (L.C. Ram

0166-5162/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.coal.2011.05.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 September 2010Received in revised form 17 May 2011Accepted 18 May 2011Available online 25 May 2011

Keywords:LigniteCoal refuseBiomassFly ashMineralsTrace elements

Coal being a limited source of energy, extraction of energy from other sources like lignite, coal-refuse, andbiomass is being attempted worldwide. The minerals and inorganic elements present in fuel feeds posedifferent technological and environmental concerns. Lignite ash, refuse ash, and biomass ash collected fromIndian power plants burning lignite, coal-refuse, and mustard stalk, respectively, were analyzed for physico-chemical characteristics and trace elements. The lignite ash has high SiO2, CaO, MgO, Al2O3, and SO3; the refuseash has high SiO2 and Fe2O3, but low SO3; the biomass ash has high SiO2 (but low Al2O3), and high CaO, MgO,K2O, Na2O, SO3, and P2O5. A substantial presence of chloride (2.1%) was observed in the biomass ash. Quartz isthe most abundant mineral species. Other minerals are mullite, hematite, gehlenite, anhydrite, and calcite inthe lignite ash; orthoclase in the refuse ash; albite, sanidine, gehlenite, anhydrite, and calcite in the biomass ash.Asheswithhighconcentrations (N100 mg/kg) of trace elements are: lignite ash (VbLabMnbCrbNibNdbBabCe,ZnbSr); refuse ash (CrbCebVbRbbMnbSr, ZnbBa); biomass ash (CubZnbBa, Sr). Based on Earth crustnormalization, Co, Ni, As, Se, Mo, Zn, Pb, U, and REEs (except Pr and Er) are enriched in the lignite ash;molybdenum, Zn, Cs, Pb, Th, U, La, Ce, and Lu in the refuse ash; and Mo, Zn, Sr, Cs, Pb, and Lu in the biomass ash.Elements As, Zn, Mo, Ni, Pb, Rb, Cr, V, Ba, Sr, and REEs are correlated with Al, indicating the possibilities of theirassociation with aluminum silicates minerals. Similarly, barium, Cs, Th, and U are correlated with iron oxides;molybdenum and Srmay also be associatedwith sulfates and chlorides. Due to the alkaline nature of these ashes,the high concentrations of As and Se in the lignite ash; molybdenum in the biomass ash; and Se in the refuse ashmay pose environmental concerns.

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1. Introduction

Although coal is the predominant source of energy for globalpower generation, high-grade coal supplies are becoming limited.Meeting this energy requirement from low grade coals, coal refuse,and biomass is therefore being attempted worldwide. In India, theenergy requirement is projected to increase from 2.4% to 4.5% by 2020,and the use of poor quality, high-ash coals, due to fast depletion ofbetter quality coal, is expected to be higher in the future. The practiceof meeting the energy demand from coal/lignite (presently 70%) willcontinue for some time, in view of the 287.0×109 t of estimated totalavailable coal reserves (Ram and Masto, 2010). Additionally, manymore non-coking coal washeries will be established (Chikkatur et al.,2009), which will produce more washery rejects with high ash yields.These rejects, representing waste with significant amounts of energy,need to be utilized through appropriate technology, such as fluidizedbed combustion (FBC). Concomitantly, energy extraction frombiomass, a low-ash, CO2 neutral fuel with relatively less nett emissions

than fossil fuel, is likely to be expanded significantly (Bartle and Abadi,2010; Khan et al., 2009), with grate-firing systems as the preferredtechnology. India ranks fourth in producing power through biomass,with an installed capacity of up to 8 MW.

Present generation of fly ash (FA) from coal combustion in thermalpower plants (TPPs) in India is about 160 million tonnes per annum,and is expected to increase to 300 million tonnes by 2016–17. Thisgeneration of FA is likely to increase more and more due to thecombustion of several other feedstocks. There may also be an increasein the need for safe handling and disposal, as the additional ash maycontain increased levels of toxic or health-hazardous elements. Studiesof the characteristics of fuel feeds and corresponding ashes areimportant for evaluating the potential behavior of different feedstocks.Similarly the study of trace elements in the ashes is significant inconsiderations of their hazards (Dai et al., 2011; Fedjea et al., 2010;Finkelman et al., 2002; Goodarzi et al., 2008; Haley, 1991; Huggins andGoodarzi, 2009; Meij and Winkel, 2009) and requirements prior totheir utilization.

The concentration of elements in FAs depends on several factors,including the type of fuel feed, the feed source(s), the occurrence ofsignificant elements and their association with the inorganic andorganic components of the fuel, the combustion conditions,

Table 1Characteristics of lignite, coal refuse, and mustard stalk.

Ultimate analysis (% daf) Lignite Coal refuse Mustard stalk

C 60.8 76.8 44.7H 4.4 5.1 8.3N 0.9 2.0 0.4S 2.7 1.1 0.2O 31.3 15.0 46.4Cl – – 0.06

Proximate analysisMoisture (% ad) 8.2 1.4 4.9Ash (% ad) 10.4 54.7 6.94Volatile-matter (% daf) 56 43 86Fixed carbon (% daf) 44 57 14Calorific value (kcal/kg daf) 5870 6378 4083

ad—air dried basis; daf—dry and ash free basis.

113S. Singh et al. / International Journal of Coal Geology 87 (2011) 112–120

volatilization–condensationmechanisms, and theparticle sizeof the ash(Frandsen, 2009; Goodarzi et al., 2008; Hower and Knowles, 2006,Jiménez et al., 2008; Mardon et al., 2008; Querol et al., 1995; Valentimand Hower, 2010; Vassilev et al., 2005; Ward and French, 2006). Themode of occurrence of the elements in coal is expected to affect theirvolatilization during combustion, with corresponding impact on theirconcentration/distribution in the ash. Elements associatedwith inherentminerals in coal have high volatility; elements associated withextraneous minerals have less volatility (Miller et al., 1998). Elementsassociatedwith clay,mica, feldspars, Fe andTi oxides, apatite, zircon, andother inert minerals are either non-volatile or slightly volatile (Vassilevet al., 2010). Several studies on these aspects of the ashes derived fromcombustion of fossil and biomass fuels have been published (Bakisgan etal., 2009; Koukouzas et al., 2009a; Querol et al., 1995; Umamaheswaranand Batra, 2008; Vassilev and Vassileva, 2009). However, studies of thecomparative characteristics of ashes derived from combustion of lignite,coal refuse, andbiomass are few; hence the aimof the present study is toprovide a comparative evaluation of the ashes produced from thesedifferent feedstock types.

2. Materials and methods

2.1. Sample collection

Representative feed and ash samples were collected from TPP-I ofthe Neyveli Lignite Corporation (NLC), Tamilnadu (pulverized fuel); acaptive power plant of Tata Steel, Jamadoba (Dhanbad), Jharkhand(fluidized bed combustion); and the plant of Kalptaru Power Transmis-sion Limited (KTPL), Ganganagar, Rajasthan (grate-combustion plant).The lignite fly ash, refuse ash, and biomass ash were basically thecombustion residues of lignite from the pulverized fuel combustion(PFC)plant, coalwashery rejects fromthe FBCpowerplant, andmustardstalk from the grate-combustion plant, respectively. Representative fuelfeed samples from the respective power plants were collected fromdifferent points of the respective feed stockpiles and thoroughly mixedafter milling to 3 cm size. The required amounts of sample for theanalysis program were prepared by coning and quartering. The ligniteand coal refuse sampleswere passed through 72mesh, and themustardstalk was passed through a 1 mm sieve. The fly ash samples werecollected from the ESP of the relevant plant at hourly intervals for aperiod of 8 h, when the sampled feedstocks were being used forcombustion. Lignite in PFC power plant is burned as a suspension at1400 °C. The FBC system for coal washery rejects uses a continuous airstream to create turbulence in a mixed bed of washery rejects, inertmaterial, and coarse fuel ash particles at 800–900 °C. For mustard stalkcombustion, a stationary grate-fired boiler system (temperature about900 °C) is used, which involves steps such as drying, de-volatilization,gasification, char combustion, and gas phase reactions.

2.2. Methods of analysis

Proximate analysis of the lignite, coal refuse, and mustard stalk wasperformed following Indian Standard Methods (IS 1350: Part I, 1984).Calorific value, sulfur, carbon–hydrogen, and nitrogen were alsoanalyzed by following Indian Standard Methods (IS 1350: Part II:1970, Part III: 1969, Part IV: Sec 1, Sec 2, 1975). The Cl content wasdetermined by using ASTM E776. The particle size of the FAs wasdetermined using a laser-based particle size analyzer (Fritsch analys-sette-22) in an aqueous medium. The pH of the FAs was determinedusing a pH meter (ash: water ratio, 1:2.5), and the Cl− and NO3

−

concentrations in the ash were determined in the water extract (ash:water ratio, 1:2.5) of the ash (Tandon, 1995). Chemical analysis (majorelement oxides) of the ashes was obtained after removing the un-burntC in the fly ashes at 900±15 °C for one hour, and followed IndianStandardMethod IS 1355, 1984. For trace element analysis, the sampleswere digested in PTFE Teflon beakers using an acidmixture (containing

7:3:1 HF-HNO3-HClO4) at ~200 °C for about 1 h, followed by evapora-tion to dryness, so that the fluorides were removed. The remainingresidues were then dissolved using 10 ml of 1:1 HNO3. Trace elementssuch as Ba, Co, Cr, Cs, Cu, Ga, Hf, Mn, Ni, Pb, Rb, Sc, Sr, Ta, Th, U, V, Y, andZn; and Rare Earth Elements (REEs) such as Ce, Dy, Er, Eu, Gd, Ho, La, Lu,Nd, Pr, Sm, Tb, Tm, and Yb were analyzed using ICP-MS (Perkin ElmerSCIEX, Model ELAN® DRC II). Arsenic and Hg were determined with aUnicom SP-2900 atomic absorption spectrometer by the hydride coldvapor generationmethod. The EDXRF system at the Institute of Physics,Bhubaneswar (India) was used for analysis of Mo and Se. This systemincorporates a low power air-cooled X-ray tube (50 W) as an excitationsource with tri-axial geometry. The X-ray tube is operated at 25 kV and0.6 mA. The X-rays from the tube were exposed on a molybdenumsecondary target and the characteristic K X-rays of molybdenum wereused to excite the characteristic X-rays of elements present in thesamples, which were then detected using a Si (Li) detector and thesignals were amplified and processed (Ashok et al., 2003). X raydiffractograms of the ashes were obtained using a D8 ADVANCE XRDsystem (Bruker AXS, Germany) with parallel beam geometry throughCu-Kα radiation at 40 kV/40 mA.

3. Results and discussion

3.1. Characteristics of fuel feeds

The lignite, coal refuse, and mustard stalk have ash yields of 6.9%,54.7%, and 10.4%, respectively on air dried basis (ad) (Table 1). The ashyield of the refuse forming the feed for the FBC plant is high because itrepresents the rejects from a coal washery and is therefore rich inmineral matter. The volatile matter (VM) for the respective fuel feedsis 56% and 43%, and 86% on dry ash free basis (daf). The highproportion of VM in themustard stalk is attributable to the release of amajor part of the biomass during de-volatilization as reported byQuaak et al. (1999); biomass decomposes into volatiles and charduring its combustion. The VM released from biomass, usuallyconsisting of light hydrocarbons, CO, CO2, H2, moisture, and tars, hasbeen reported as typically being up to 90% (Demirbas, 2004). Thelignite has a higher moisture content (8.2%, ad) than the other fuels(Table 1), which may be significant as low moisture (b2.0%) is said tobe desirable for thermal conversion processes (Varma and Behera,2003). Based on dry and ash free basis (daf), calorific value of themustard stalk (4083 kcal/kg) is inferior to both the lignite (5870 kcal/kg) and the coal refuse (6378 kcal/kg). The amount of heat generatedby a fuel is a function of its chemical composition, where thequantitative conversion of C and H present in the fuel to carbondioxide and water is crucial (Senelwa and Sims, 1999). The C contents(daf) of the lignite, coal refuse, andmustard stalk are 60.8%, 76.8%, and

114 S. Singh et al. / International Journal of Coal Geology 87 (2011) 112–120

44.7%, respectively, and the respective H contents (daf) are 4.4%, 5.1%,and 8.3%. Nitrogen and S contents are highest in the coal refuse andlignite and lowest in the mustard stalk. The mustard stalk contains0.06% Cl (daf).

3.2. Characteristics of the ashes

The lignite ash, refuse ash, and biomass ash have 90% of theirparticles with sizes of less than 36 μm, 182 μm, and 135 μm,respectively (Table 2). The abundance of fine particles in the ligniteash is attributable to the fact that lower rank coals typically containsubstantial proportions of organically bound inorganic elements, suchas Ca, Mg, and Na, which are likely to form finer sized ash particles(Neville and Sarofim, 1985). The lignite ash had the highest pH (11.0)in water, followed by the biomass ash (9.2) and the refuse ash (8.6).The high pH observed for these ashes is probably due to the highproportions of alkali and alkaline Earth metal oxides such as CaO,MgO, K2O, and Na2O (Table 2). The slightly lower pH (8.6) in the caseof the refuse ash is associated with a lower CaO percentage or a lowerCaO/SO4 ratio (Mattigod et al., 1990; Querol et al., 2000). In general,the pH of coal FA varies from 4.5 to 12.0, but the majority of the FAsproduced globally, including those in India, are alkaline (Ram andMasto, 2010).

Maximum levels of un-burnt carbon (Table 2) are encountered inthe refuse ash (10.9%) followed by the biomass ash (8.5%) and thelignite ash (5.7%). The chloride content of the biomass ash (2.1%) ishigher as compared to both the lignite and refuse ashes (0.04%). Highchlorine in ash may cause ash deposition problems at high ormoderate combustion temperatures (Baxter et al., 1998). Though theCl content is high in biomass ash, chlorine bearing mineral specieswere not identified in the XRD patterns (Fig. 1), probably due to thehigh proportions of Si, Al, Ca, S, and P in the ash, which may preventthe formation of alkali chlorides. Overall the concentration of NO3

− in

Table 2Particle size distribution, chemical and XRD analyses of the lignite, refuse, and biomassfly ashes.

Sample D10 (μm) D50 (μm) D90 (μm)

Lignite ash 4 19 36Refuse ash 6 79 182Biomass ash 8 62.0 135

Chemical analysis(%, except for pH)

Lignite ash Refuse ash Biomass ash Coal ash*1 pH 11.0 8.6 9.2 6.0–11.02 SiO2 48.4±0.99 50.4±0.48 54.4±0.5 38.0–63.03 Al2O3 29.8±0.81 20.3±0.68 12.4±0.5 27.0–44.04 Fe2O3 5.4±0.68 19.3±0.56 2.2±0.23 3.3–6.45 CaO 7.9±0.53 3.3±0.24 9.2±0.34 0.2–0.86 MgO 2.6±0.30 1.9±0.02 5.2±0.27 0.01–0.57 K2O 0.2±0.02 0.9±0.02 8.3±0.27 0.04–0.98 Na2O 0.40±03 0.2±0.01 2.2±0.18 0.07–0.439 SO3 2.8±0.22 0.9±0.03 4.0±0.09 NA10 P2O5 0.4±0.03 1.0±0.01 2.2±0.17 NA11 TiO2 1.4±0.09 1.9±0.07 0.1±0.01 0.4–1.812 LOI 5.7±0.22 10.9±0.48 8.5±0.42 0.2–3.413 Cl− 0.04 0.04 2.10 NA14 NO3− 0.004 0.010 0.010 NA

X-ray diffraction analysisLignite ash Quartz (SiO2), mullite (Al6Si2O13), hematite (Fe2O3), gehlenite

(Ca2Al2SiO7), anhydrite (CaSO4), calcite (CaCO3)Refuse ash Quartz (SiO2), orthoclase (KAlSi3O8)Biomass ash Quartz (SiO2), albite (NaAlSi3O8), sanidine (KAlSi3O8), gehlenite

(Ca2Al2SiO7), anhydrite (CaSO4), calcite (CaCO3)

*range values for Indian coal ash; Source: Ram and Masto (2010); NA, compiled datanot available

the three ashes is low (b0.01%), although it is slightly higher in thebiomass and refuse ashes as compared to the lignite ash. This isprobably attributable to the lower combustion temperature of thefluidized bed and grate systems of combustion for the refuse andbiomass fuels, where the volatilization loss of N is relatively less(Jenkins et al., 1998).

The lignite ash has high proportions of SiO2, CaO, MgO, Al2O3, andSO3 (Table 2). XRD analysis (Fig. 1) also shows the presence of quartz(SiO2), mullite (Al6Si2O13), hematite (Fe2O3), gehlenite (Ca2Al2SiO7),anhydrite (CaSO4), and calcite (CaCO3) in the lignite ash. The refuse ashhas high SiO2 and Fe2O3 but low SO3 (Table 2). XRD analysis shows thepresence of quartz (SiO2) and orthoclase (KAlSi3O8) in this ash, butFe-bearing minerals were not detected. The biomass ash contains highSiO2 (but lowAl2O3), and high proportions of CaO,MgO, K2O, Na2O, SO3,and P2O5 (Table 2). In line with the chemical analysis, the XRD patternfor biomass ash shows the presence of quartz (SiO2), albite (NaAlSi3O8),sanidine (KAlSi3O8), gehlenite (Ca2Al2SiO7), anhydrite (CaSO4), andcalcite (CaCO3). In so far as the class of the ashes is concerned, the ligniteash and refuse ash are Class F (SiO2+Al2O3+Fe2O3N70%), whereas thebiomass ash is a Class C fly ash (SiO2+Al2O3+Fe2O3 in the range50–70%).

Quartz and aluminosilicates are found in all these ashes. The quartzmay represent fragmentsof thatmaterial in theoriginal feedstockwhichpassed without reaction through the respective combustion systems.The aluminosilicates (mullite and gehlenite) were probably formedduring combustion, possibly from kaolinite commonly present in rawcoals and lignites (Ram, 1992), via a meta-stable phase such asmetakaolin or sillimanite. The formation of Al-silicates from the reactionof γ-Al2O3 with free silica (SiO2) is also possible, but as γ-Al2O3 is notvery common in coal, this possibly happens via the formation of anintermediate product (Al2Si2O5) (van Dyk, 2006). The formation ofAl-silicates, especially gehlenite in the lignite ash, orthoclase in therefuse ash, and albite, sanidine and gehlenite in the biomass ashmay bedue to the solid phase reactions between quartz and liberated oxides ofK/Na/Ca via the formation of silicates phases and their transformation tospecific Al-silicates at high temperatures (Vassileva and Vassilev, 2006).

The presence of iron oxideminerals in both the refuse and biomassashes cannot be ruled out, especially since high proportions of Fe2O3

(19.3% and 2.2%, respectively) are present in these ashes (Table 2).However, a high background level in the XRD pattern of the refuse andbiomass ash, along with the possible existence of Fe as quite smallcrystallites or in an amorphous phase, may be the reason for lack of aclear identification of hematite (Olanders and Steenari, 1995).

Higher proportions of CaO and SO3 in lignite ash are probably dueto the marine influence of the respective lignite origin. It is assumedthat the main lignite seam was formed from in situ mangrove-mixedmoist tropical forests vegetation (Singh et al., 1992). Furthermore, thehigh Ca in lignite might have sequestered the S during combustion(Koukouzas et al., 2009b). Higher proportions of K2O, CaO,MgO, Na2O,SO3, and P2O5 are observed in the biomass ash than both the ligniteash and the refuse ash. Since they are essential plant nutrients, higherproportions of P, K, Ca, Mg, and S would be expected in the mustardstalk (feed), and they are converted into their respective oxidesduring combustion. These elements may be associated with theorganic molecules in the feed as they are taken from the soil by theplant roots in ionic form, although some authigenic mineral formationinside the plant is also possible. Thus the specific mineral formation(albite, sanidine, gehlenite, anhydrite, and calcite) involving theseelements must have occurred during combustion. Significantly lowerlevels, particularly of Al and Ti oxides, observed in the biomass ash areattributable to the relatively low uptake of these elements from thesoil during plant growth.

As indicated in Table 2, the Fe2O3 content of the refuse ash is 3–4times higher than other typical fly ashes. This may be due to thepreferential concentration of dense iron-bearing minerals into therefuse during coal washing. The biomass ash has significantly lower

(a) Lignite ash

0

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2-Theta - Scale10 20 30 4 50 60 70

M

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(b) Refuse ash

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0100200300400500600700800900

1000110012001300140015001600

2-Theta -Scale10 20 30 40 50 60 70

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AAn A AnA Q Q

Fig. 1. XRD analysis of (a) lignite, (b) refuse, and (c) biomass ash [Q: Quartz(SiO2); A: Albite (NaAlSi3O8);C: Calcite(CaCO3); H:Hematite(Fe2O3); Or: Orthoclase(KAlSi3O8);G:Gehlenite(Ca2Al2SiO7); An: Anhydrite(CaCO3); M: Mullite(Al6Si2O13)].

115S. Singh et al. / International Journal of Coal Geology 87 (2011) 112–120

proportions of Al2O3 and Fe2O3 than the other ashes, probably due tothe lesser proportion of admixed quartz and clay-rich sediment. Mostof the mineral species in lignite and coal are derived from extraneousmaterials (detrital sediment and authigenic minerals) added duringtheir formation. The Ca and Mg oxides in the lignite, refuse andbiomass ashes of the present study are significantly higher than thevalues reported for coal ash generally (Table 2). Oxides of plant

nutrient elements like P, K, Ca, Mg, Na, are significantly higher in thebiomass ash as compared with typical coal fly ashes.

Themineralogical constituents in the feedstock play amajor role inslagging and ash deposition during combustion (Zhang et al., 2010).The acid-base reactions among inorganic constituents account forsuch deposit formation to a great extent (Henry et al., 2004). Thecorrelation between the high temperature melting acid group (SiO2,

116 S. Singh et al. / International Journal of Coal Geology 87 (2011) 112–120

Al2O3, and TiO2) (A) and the low melting basic group (Fe2O3, CaO,MgO, K2O, and Na2O) (B) can be expressed by the base to acid ratio [B/A=(Fe2O3+CaO+MgO+K2O+Na2O) / (SiO2+Al2O3+TiO2)](Wang et al., 2008). The B/A values for the lignite, refuse and biomassashes are 0.20, 0.35, and 0.40, respectively, which suggest that thelignite is less prone to slagging than the other feedstocks. In addition,very high chloride content observed in the biomass ash (2.1%) may beassociated with ash deposition problems (Baxter et al., 1998).

3.3. Trace elements in the ashes

The concentrations of 39 elements analyzed in the ashes (As, Ba,Co, Cr, Cs, Cu, Ga, Hf, Hg, Mn, Mo, Nb, Ni, Pb, Rb, Se, Sr, Ta, Th, U, V, Zn,Zr, Sc, Y, Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sm, Tb, Tm, and Yb) arepresented in Table 3. Overall, the concentration of the elements inthese ashes follows the order: Sr N Ba N Zn N Ce NMn N Nd N

Ni N Cr N La N V N Rb N Cu N Co N Pb N Sm N Sc N Dy N Th N Gd N Cs NMo N Yb N Eu N U N Ho N Tb N As N Hf N Tm N Se N Pr N Lu N Er N Y N

Nb N Ta N Ga N Zr. Excluding the high concentration of Ba (721 mg/kg)in the refuse ash and Sr (885 mg/kg) in the biomass ash, the range ofvalues for the lignite, refuse, and biomass ashes (0.19 to 391 mg/kg,0.31 to 722 mg/kg, and 0.14 to 885 mg/kg, respectively) is reduced to0.14–391 mg/kg. Mercury is below detection limit (b0.1 mg/kg) in allthe ashes; arsenic and Se are also b0.1 mg/kg in the biomass ash.

The concentrations of Sc, V, Cr, Co, Ni, As, Hf, Pb, and the REEsare significantly higher in the lignite ash. Scandium, Se, Mn, Zn,Rb, Cs, B, Pb, Th, and U are higher in the refuse ash, whereas Mo,

Table 3Trace elements in lignite, refuse, and biomass fly ashes.

Elements Lignite ash (mg/kg) Refuse ash(mg/kg) Biomass ash (mg/kg)

Sc 28 297 10.0V 131 120 59Cr 137 102 61Co 77 15 7Ni 140 49 35As 1.2 0.4 b0.1Se 2.4 2.3 b0.1Mo 3.0 3.3 10Hg b 0.1 b0.1 b0.1Mn 135 226 77Cu 64 86 113Zn 298 377 161Ga 0.5 0.5 0.2Rb 4.5 129 87Sr 391 258 885Y 1.2 0.3 0.2Zr 0.4 0.3 0.2Nb 0.5 0.9 0.3Cs 0.2 13 4.5Ba 210 722 376Hf 2.8 2.2 0.9Ta 0.7 0.4 0.4Pb 40 36 26Th 9.1 22 11U 3.5 5.3 1.7La 134 58 32Ce 295 114 61Pr 2 0.54 0.30Nd 152 43 25Sm 33 9.3 5.2Eu 8.1 1.8 1Gd 20.2 5.8 3.2Tb 3.7 1 0.6Dy 22.8 6.2 3.3Ho 4.6 1.2 0.6Er 1.3 0.3 0.2Tm 2.6 0.7 0.4Yb 9.4 2.6 1.2Lu 1.5 0.4 0.2

Cu, Zn, Rb, Sr, and Ba are higher in the biomass ash. Asheswith high concentrations (N100 mg/kg) of trace elements are:lignite ash (VbLabMnbCrbNibNdbBabCebZnbSr); refuse ash(CrbCebVbRbbMnbSrbZnbBa); and biomass ash (CubZnbBabSr).This shows that the lignite ash has the highest total concentrationof trace elements followed by the refuse ash and the biomass ash(Table 3).

The total content of REEs (∑REE) in the lignite ash is the highest(690 mg/kg), followed by the refuse ash (244 mg/kg), and thebiomass ash (133 mg/kg). The REE content of the biomass ash isrelatively low compared with the other ashes (Table 3). For example,ΣREE for the biomass ash is equivalent to 19.3% and 54.6% of those forthe lignite and refuse ashes, respectively. The ∑LREE/∑HREE ratiofor these ashes varies from 9.4 to 13, which suggests that light rareearth elements (LREE) are more abundant in these ashes.

The lower concentration of the elements in the biomass ash than inthe lignite or refuse ash is ascribable to their relatively low concentra-tion in themustard stalk feed than in the lignite or coal refuse (Liao et al.,2004). In general, the uptake of elements in plants from soil is a smallfraction of their total content in the soil. In agricultural crops, theconcentrations of As and Se are 0.1–70 μg/kg and 1–100 μg/kg,respectively and that of most REEs in the range 0.01–2000 μg/kg(Ericksson, 2001; Kabata-Pendias and Mukherjee, 2007). This may be apossible reason for their lower content in the biomass ash. Concentra-tions of most REEs in the above-ground biomass of vascular plants areusually quite low as compared to the soil.

The high concentrations of Cu, Mo, and Sr in biomass ash aresomewhat baffling. As a possibility, strontium enrichment may be dueto contamination of mustard stalk with soil/dust during harvesting,transport, storage, and handling. (Van and Koppejan, 2007); soil maycontain up to 1000 mg/kg Sr. Copper and Mo, being the essentialelements for plant growth, would be expected to be presentappreciably in mustard stalk (5–30 mg/kg and 0.2–5.0 mg/kg,respectively) (Kabata-Pendias, 2001), and hence the higher contentin biomass ash due to enrichment on burning.

3.3.1. Enrichment factorsEarth crust-normalized trace element patterns (Clarke and Sloss,

1992) were used to calculate the enrichment factors of elements inthe ashes (Fig. 2). Trace elements including Co, Se, Mo, Ni, Zn, Pb, U,and the REEs (except Pr and Er) are enriched in lignite ash; selenium,Mo, Zn, Cs, Pb, Th, U, La, Ce, and Lu in the refuse ash; andMo, Zn, Sr, Cs,Pb, and Lu in the biomass ash. In particular, selenium is predominantlyenriched (~70 times) in the lignite and refuse ashes, whereas in caseof the other elements the enrichment is within 10 times. Beingvolatile, selenium is likely to be enriched in fly ash due tocondensation and absorption processes during the removal ofparticulates and flue gas. Lignite combustion at higher temperature(~1400 °C) probably favored the volatilization of Se and its conden-sation on finer particles (90% ash particles b35.69 μm) being capturedin the ESP at lower temperature (~140 °C), which led to itsenrichment (Dai et al., 2010; Levandowski and Kalkreuth, 2009; Liet al., 2007; Mardon et al., 2008, Miller et al., 1998; Querol et al., 1995,Vassilev et al., 2005; Xu et al., 2003). Other elements enriched in theashes are Mo, Zn, Ni, Pb, Sr and U; Co, Cs, Th, and the REEs, althoughenriched, are less volatile or non-volatile. The enrichment of the non-volatile elements may be due to their higher content in the respectivefeedstock.

Among the depleted elements, manganese, Ga, Y, Zr, Nb, Pr, and Erare common in all the ashes; scandium, V, Cr, Co, Ni, Hf, U and the REEs(except Lu) are in the biomass ash; cobalt, Ni, and Ta are in refuse ash;barium, Th, Rb, and Cs are in lignite ash. The strongest depletionpattern is observed for Ga (~100 times) and Zr (1000 times). A similarZr depletion was observed for lignite fly ash by Pentari et al. (2004).Studies have shown that 10–20% of Ga, Y, or Cs, and 40–60% of Zr canbe released into the atmosphere during combustion (Vassilev and

117S. Singh et al. / International Journal of Coal Geology 87 (2011) 112–120

Vassileva, 1997). Newly formed sub-micron size solid phases contain-ing these elements may also escape the entrapment, hence thedepletion. Slight depletion (up to 10 times) is observed for someelements, where the depletion may also be due to their volatilizationin the combustion chamber and subsequent limited condensationfrom the flue gasses on the ash particles. Despite the variation in thefuel feeds, similar normalization patterns (Fig. 2) are observed formost of the trace elements in all of the ashes.

3.3.2. AssociationThe likely association of the trace elements with the minerals in

ashes was studied based on correlation analysis, involving the datafrom all three ashes (n=9). Based on the correlations, relativecontents of elements in the ashes, the mineral species identifiedthrough XRD and chemical analysis of the particular ash, the possibleassociations are summarized in Table 4. However, further detailedmineralogical studies with many numbers of samples are needed toreinforce the results. A trace element with a significant positivecorrelation with SiO2 is Cu (r=0.768). Elements having correlationswith Al2O3 (Fig. 3) are Sc (r=0.823), V (r=0.890), Cr (r=0.989), Co(r=0.927), Ni (r=0.941), As (r=0.966), Se (r=0.855), Ga(r=0.851), and the REEs (r=0.929–0.970). The above correlationsuggests that these elements may be associated with the Al-silicate

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

Sc V Cr Co Ni As Se Mo Mn Cu Zn G

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

La Ce Pr Nd Sm Eu Gd

Lignite ash

Lo

g10

(ash

/ear

th c

rust

)L

og

10 (a

sh/e

arth

cru

st)

Fig. 2. Earth crust-normalization plots of trace elem

minerals, in particular mullite and gehlenite in lignite ash; orthoclasein refuse ash; and albite, sanidine, and gehlenite in biomass ash(Fig. 1). Although Sr, Mo, and Cu in the biomass ash are not correlatedwith Al2O3, their significant correlation with K2O, CaO, and Na2O(r=0.647–0.99) supports their association with sanidine (KAlSi3O8),gehlenite (Ca2Al2SiO7), and albite (NaAlSi3O8). Elements correlatedwith Fe2O3 are Mn (r=0.967), Zn (r=0.837), Nb (r=0.981), Cs(r=0.877), Ba (r=0.869), Th (r=0.946), and U (r=0.933). Theabove Fe2O3 association is more applicable for the refuse ash, whereFe2O3 (19.3%) and the contents of these elements are higher (Table 3).Trace elements showing significant correlation with un-burnt carbonare Rb (r=0.935), Ba (r=0.939), and Th (r=0.890), particularly forthe refuse ash where un-burnt carbon (10.9%) and these elements arehigher (Table 3). Strontium (r=0.899), and Mo (r=0.772) arecorrelated with sulfate. Accordingly Sr in the lignite ash, and Sr andMo in the biomass ashmay also have such an association, especially asSO3 and the corresponding elements are higher in those ashes.Molybdenum (r=0.994), Sr (r=0.974), and Cu (r=0.829) also showsignificant correlation with chlorides, particularly in case of thebiomass ash, which has a higher chloride content (2.1%). Apart fromthe foregoing discussion, further detailed mineralogical investigationsand sequential fractionation of trace elements are advantageous forbetter understanding of the trace elements association.

a Rb Sr Y Zr Nb Cs Ba Hf Ta Pb Th U

Tb Dy Ho Er Tm Yb Lu

Lignite ash Refuse ashBiomass ash

Refuse ash Biomass ash

ents present in lignite, refuse, and biomass ash.

Sc

Cr

As

Se

Mo

Mn

Cu

Zn

GaRb

Sr

Y

Zr

NbCsBa

Hf

Ta

Pb

Th U

∑REE

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

-1 -0.8 -0.6 -0.4 -0. 0 0.2 0.4 0.6 0.8 1Fe 2O

3

Al2O3

Fig. 3. Correlation coefficients between the trace elements and Fe2O3/Al2O3.

118 S. Singh et al. / International Journal of Coal Geology 87 (2011) 112–120

3.3.3. Environmental concernThe disposal of ash may pose a contamination risk to soil, plants,

and groundwater due to elevated concentrations of potentially toxicheavy metals, soluble salts, acidity/alkalinity and radionuclides (RamandMasto, 2010). Tracemetals (e.g., As, B, Cd, Co, Cr, Cu, Hg, Ni, Pb, Se,V, Zn, U, Th, and Cs), and even the REEs present in fly ash areconsidered as potential elements of environmental concern andpossible health hazards (Finkelman et al., 2002), so that theirconcentration level and mobility decide their safe disposal andutilization. In the three ashes studied, the enriched potentially toxicelements are: As, Se, Mo, and Pb in the lignite ash; Se, Mo, and Pb inthe refuse ash; and Mo, Pb, and Cu in the biomass ash. Lead and Cumay not be of much concern, especially in view of the alkaline nature(pH, 8.6–11.0) of these ashes (Jankowski et al., 2006, Ram et al., 2007).However, due to the alkaline pH of the ashes, anionic elementsincluding As, Mo, and Se in the lignite ash; molybdenum and Se in therefuse ash; andMo in biomass ashmay becomemuchmobile and poseenvironmental concerns. In another possibility, when pH is high,dominant constituents like SiO2, Al2O3, CaO, and SO4

2− may formvarious secondary minerals like calcium silicate hydrate gel, calciumaluminosilicate hydrate, and ettringite, which may reduce mobilityeither by physically reducing the porosity of the ash or by chemicallybinding the element. However, further leaching studies may be ofsignificant use to understand the potential environmental hazards, ifany, on disposal of these ashes.

Table 4Possible association of trace elements with different mineral species.

Fly ash Significantelements

Possible association as evident fromcorrelations, XRD and chemical analysis

Reference

Ligniteash

As, Zn, Cu, Ni, Pb,Se, Cr, V, REEs

Aluminum silicate minerals(mullite, and gehlenite)

Querol et al. ((2004); Vassil

Sr Sulfates Chatziapostolo

Refuseash

Rb, Cr, V, Cu, Pb,Zn

Orthoclase Querol et al. ((2005); Bahor

Ba, Cs, Th, U Fe oxides Hulett et al. (1

Rb, Ba, Th Unburnt carbon Richaud et al.

Biomassash

Mo, Cu, Sr, Rb, Ba,Zn, Pb

Aluminum silicates (albite, sanidine,gehlenite, anhydrite, calcite)

Vamvuka and

Mo, Cu, Sr Sulfates and chlorides Vassilev and V

Rb, Ba Unburnt carbon Richaud et al.

5. Conclusions

The lignite ash in the present study has high proportions of SiO2,CaO, MgO, Al2O3, and SO3; the refuse ash has high SiO2 and Fe2O3 butlow SO3; the biomass ash contains high SiO2 (but low Al2O3) and highCaO,MgO, K2O, Na2O, SO3, and P2O5. The lignite and refuse ashes fall inClass F, and the biomass ash in Class C. The B/A ratios in the ashessuggest that biomass is more prone to slagging than lignite or coalrefuse. A substantial presence of chloride in the biomass ash (2.1%)may also cause fouling and corrosion in the furnace. As per XRDanalysis, quartz is the common mineral species. Other minerals aremullite, iron oxide, calcium silicates, and sulfates and carbonates of Cain the lignite ash; K-Al-Si species in the refuse ash; K/Ca-Al-Si species,and sulfates and carbonates of Ca in the biomass ash.

Ashes having concentrations (N100 mg/kg) of trace elements are:the lignite ash (VbLabMnbCrbNibNdbBabCe, ZnbSr); the refuseash (Cr bCe bV bRb bMn bSr, Zn bBa); and the biomass ash(CubZnbBa, Sr). Based on correlations, most of the trace elementslike Sc, V, Cr, Co, Ni, As, Se, Ga, and the REEs are associated with Al2O3.Elements correlated with Fe2O3 are Mn, Zn, Nb, Cs, Ba, Th, and U.Unlike the lignite or coal refuse, the biomass is not significantlyadmixed with detrital minerals and its ash has much lower Al2O3,Fe2O3, and trace element contents, except for Sr, Mo, and Cu, whichmay be associated with sulfate or chloride.

Similar Earth crust normalization patterns are observed formost ofthe trace elements in all the ashes. Based on the results it can beconcluded that the ash characteristics in respect of mineral andelemental content are strongly influenced by the feed composition, aswell as the volatility of the element depending on combustiontemperature. Potentially toxic elements enriched in the lignite ash areAs, Se, Mo, and Pb; in the refuse ash are Se, Mo, and Pb; and in thebiomass ash are Mo, Pb, and Cu.

Acknowledgments

The authors are thankful to Dr. Amalendu Sinha Director, CentralInstitute of Mining and Fuel Research, Dhanbad for his kindpermission to publish this research paper. The authors are alsothankful to the authorities of the Power plants of NLC (Neyveli), TataSteel (Jamadoba), and Kalptaru (Ganganagar) for making availablethe FA samples. The instrumental/analytical support extended by Dr.S. Maiti andMr. T. B. Das are gratefully acknowledged. The first authorexpresses her thankfulness to CSIR for the grant of the researchfellowship for carrying out this research work.

1995); Hulett and Weinberger (1980); Hulett et al. (1980); Goodarzi and Hugginsev and Vassileva (2005); Dai et al. (2010; 2011); Zhang et al. (2007)

ua et al. (2006)

1995); Hulett and Weinberger (1980); Hulett et al. (1980) Vassilev and Vassilevaet al. (1981); Llorens et al. (2000), Dai et al. (2010)

980) Vassilev & Vassileva (2005)

(2000), Vassilev and Vassileva (2005)

Zografos (2004)

assileva (2005); Chatziapostoloua et al. (2006)

(2000), Vassilev and Vassileva (2005)

119S. Singh et al. / International Journal of Coal Geology 87 (2011) 112–120

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