Influence of Sewage Sludge Addition on Coal Ash Fusion Temperatures

Influence of Sewage Sludge Addition on Coal Ash FusionTemperatures

M. Belen Folgueras,*,† R. Marıa Dıaz,† Jorge Xiberta,†M. Purificacion Garcıa,‡ and J. Juan Pis§

Department of Energy and Department of Materials Science, University of Oviedo,Independencia 13, 33004 Oviedo, Spain, and Instituto Nacional del Carbon, CSIC,

Francisco Pintado Fe 26, 33011 Oviedo, Spain

Received January 12, 2005. Revised Manuscript Received July 25, 2005

The ash fusion characteristics of three types of bituminous coal (A, B, and C), one type of sewagesludge (W), and the corresponding coal-sewage sludge blends (10 and 50 wt % of sludge) werestudied. The ash fusibility temperatures of samples in oxidizing atmosphere were measured,and their chemical and mineralogical compositions were determined. The addition of sludge tocoal in certain proportions produces blends whose ashes have lower fusibility temperatures thanthose of coal and sludge. This is related to the differences in chemical composition and modes ofelemental combination in both types of materials. The main differences are associated to theelements P, Fe, and Ca. As the sludge is much richer in Ca than the coals, the compositions ofthe blend ashes pass through low-temperature eutectic regions of the ternary phase diagramsSiO2-CaO-Al2O3 and SiO2-CaO-Fe2O3. As a result, for the sludge-coal blend ashes series(one for each coal), the relationships between ash fusibility temperatures and the percentage ofsludge ash in blend ashes fit to second-order polynomial functions. The minima of these functions,as well as some sludge-coal blend ashes, are located in the above-mentioned low fusion regions.Differing from coal ashes, in the sludge and 50 wt % blend ashes, the minerals calcium ferrite,larnite, and chloroapatite were found.

1. Introduction

Sewage sludge, the waste product of the urbanwastewater treatment process, can be blended with coalto remove this dangerous waste by co-combustion in aboiler pulverized coal and also to provide a lower-costfuel that allows us to recover its energetic potential.However, the inorganic composition of this blend canbe very different from that of the individual coalcomponent, producing serious operational problemsassociated with combustion, such as an increase of boilerdeposits due to fouling and slagging, as well as sometrace element emissions. To understand the interactionbetween both types of materials, several studies havebeen done.1-5

The determination of ash fusion temperatures is oneof the tools used for determining coal and coal blendsbehavior and to know whether ash deposit problems willbe found during combustion. These temperatures aredescribed by initial deformation temperature (IT), soft-ening temperature (ST), hemispherical temperature(HT), and fluid temperature (FT). Although the knowl-edge of these temperatures is not the only factor thatmust be considered to predict ash behavior, it is theeasiest parameter to determine the effect of the sludgeaddition on the coal ash fusion temperatures.

Also both chemical and mineral compositions of asheshave been widely used to express ash fusibility of coalsand their blends, although these attempts have gener-ally yielded different results.6,7 The base-to-acid ratio(B/A) is one of the first indexes used to predict ashfusibility. This index relates the ash basic compounds(Fe2O3, CaO, MgO, Na2O, K2O) that reduce ash fusiontemperatures (B) to the ash acidic compounds (SiO2,Al2O3, TiO2) that increase them (A). Together with thistype of indexes there are correlations that predict ashfusion temperatures from ash chemical composition.Among these, Bryers and Taylor’ correlations must bepointed out.6 Accordingly, the softening temperatureunder reducing conditions depends on the sum of basic

* Corresponding author. Tel.: +34-98-5104333. Fax: +34-98-5104322. E-mail: [email protected].

† Department of Energy, University of Oviedo.‡ Department of Materials Science, University of Oviedo.§ Instituto Nacional del Carbon. CSIC.(1) Folgueras, M. B.; Dıaz, R. M.; Xiberta, J.; Prieto, I. Volatilisation

of trace elements for coal-sewage sludge blends during their combus-tion. Fuel 2003, 82, 1939-1948.

(2) Folgueras, M. B.; Dıaz, R. M.; Xiberta, J.; Prieto, I. Thermo-gravimetric analysis of the co-combustion of coal and sewage sludge.Fuel 2003, 82, 2051-2055.

(3) Folgueras, M. B.; Dıaz, R. M.; Xiberta, J. Sulphur retentionduring co-combustion of coal and sewage sludge. Fuel 2004, 83, 1315-1322.

(4) Ninomiya, Y.; Zhang, L.; Sakano, T.; Kanaoka, Ch.; Masui, M.Transformation of mineral and emission of particulate matter duringco-combustion of coal with sewage sludge. Fuel 2004, 83, 751-64.

(5) Miller, B. B.; Kandiyoti, R.; Dugwell, D. R. Trace elementbehavior during co-combustion of sewage sludge with Polish coal.Energy Fuels 2004, 18 (4), 1093-1103.

(6) Bryers, R. W. Fireside slagging, fouling, and high-temperaturecorrosion of heat-transfer surface due to impurities in steam-raisingfuels. Prog. Energy Combust. Sci. 1996, 22, 29-120.

(7) Seggiani, M. Empirical correlations of the ash fusion tempera-tures and temperature of critical viscosity for coal and biomass ashes.Fuel 1999, 78, 1121-1125.

2562 Energy & Fuels 2005, 19, 2562-2570

10.1021/ef058005a CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 10/06/2005

oxides, adjusting to a second-order polynomial functionfor SiO2/Al2O3 ≈ 1 or SiO2/Al2O3 . 1.

Some investigators have shown that ash fusion tem-peratures do not depend only on chemical compositionbut also on mineral composition.8-11 Thus, the modesof elemental combination (minerals and phases) in coaland coal ashes and their behavior during heating areof great importance.9 Refractory minerals (quartz, meta-kaolinite, mullite, rutile, etc.) rise ash fusion tempera-tures, while fluxing minerals (anhydrite, calcium sili-cates, hematite, etc.) reduce them.9

For coals and coal blends, the slagging propensity hasalso been related to the low-temperature eutecticsformation, the molar ratio Fe2O3/CaO being a key fortheir prediction.6,12

However, little has been done on the prediction ofslagging tendency of sewage sludge-coal blends. Thatis why an attempt was made to correlate ash fusiontemperatures with both the blending ratio and the ashblend composition and to predict the possible low-temperature eutectics formation. To this end, an ex-perimental survey was carried out on three bituminouscoals and dried sewage sludge. Ash fusion temperatureswere determined under oxidizing conditions. Moreover,the main minerals of samples used were also studied.

2. Experimental Section

Three types of bituminous coals with different ash yieldsranged from 11 to 53 wt % were used, corresponding tobituminous coals from the Asturias Central Basin (A coal, Bcoal, and C coal). Moreover, the sewage sludge (W sludge) froman urban wastewater treatment plant situated in Asturias wasalso used. In the wastewater treatment plant, FeCl3 and limeare used for sludge conditioning, while lime is used for itsstabilization. From these materials, six sludge-coal blendswere prepared by adding dry sewage sludge to coals, obtainingtwo blends whose sewage sludge contents were 10 wt % (WA1blend, WB1 blend, and WC1 blend for A, B, and C coals,respectively) and 50 wt % (WA2 blend, WB2 blend, and WC2blend for each coal). Different samples from these types ofmaterials were used in previous works1-3 where characteriza-tion analyses are described. The results of the characterizationof materials are given in Table 1.

The ashing process was carried out according to ASTMStandard D 3174-89, with the final temperature being main-tained for 2 h at 800 °C. The ashes obtained from the abovematerials and their blends were used for determining both thechemical and the mineral compositions as well as their fusiontemperatures. The ash chemical composition was establishedby inductively coupled plasma-atomic emission spectroscopy(ICP-AES), samples being prepared by LiBO2 fusion. Theminimum detection limits of the technique applied were asfollows: (a) 0.001 wt % for Cr2O3; (b) 0.01 wt % for MgO, CaO,Na2O, TiO2, P2O5, and MnO; (c) 0.02 wt % for SiO2; (d) 0.03wt % for Al2O3; and (e) 0.04 wt % for Fe2O3 and K2O. Thedetermination of sulfur was carried out with a LECO SC-32analyzer (minimum detection limit 0.01 wt %). The fusibility

temperatures in oxidizing atmosphere were determined by aLECO AF-600 analyzer according to test method ASTMD1857-87D. For each fusibility temperature, the values re-ported are the average of the results obtained in two differentassays. Moreover, two ash cones of the same sample weresimultaneously used in the above two assays. For ashes of bothcoals and sludge-coal blends, the difference for each fusibilitytemperature between two separate runs was lower than 30°C. However, the sludge ashes did not have the necessaryconsistency to establish a precise temperature for each ashfusibility temperature; this is why the highest ranges oftemperatures observed for them were recorded. In Tables 2and 3, major and minor elements contents (expressed asoxides) of ashes and their fusibility temperatures are shown,respectively.

The minerals of the materials and their blends as well astheir ashes at 800 °C were established (qualitative analyses)by X-ray diffraction (XRD). The diffractograms were madeusing a Phillips PW 1710 X-ray powder diffractometer and CuKR radiation (using a graphite monochromator). Diffractionintensities were recorded in the 2Θ range of 5-65°. To explainash fusibility behavior, the ashes of materials and their blendsobtained at 800 °C (ground at <75 µm) were gradually heatedat a rate of 10 °C/min in an oxidizing atmosphere in thetemperature range from room temperature to 1100 °C, thistemperature being maintained for less than 1 min. The ashesobtained at 1100 °C were dampened with water to avoidchanges in their mineral composition, and their crystallineminerals were also identified by XRD.

3. Results and Discussions

3.1. Chemical Composition and Mineralogy ofMaterials and Their Blends. 3.1.1. Chemical Com-position of Ashes (Materials and Their Blends).Sewage sludge has variable characteristics that differfrom bituminous coals, which is seen in the results ofproximate and ultimate analyses, as well as highercalorific values, of materials given in Table 1.

The compositions of the sludge ash, the three coalashes, and the ashes of the binary blends obtained arelisted in Table 2. The main ash elements (expressed asoxides) in these Asturian coals are SiO2, Al2O3, Fe2O3,and CaO (cited in decreasing order), while the main ashcomponents of the sewage sludge are CaO, SiO2, Fe2O3,Al2O3, and P2O5. The CaO and P2O5 percentages ofsludge are much higher than those obtained in coals,while those of alkaline oxides and TiO2 are lower (Table2).

For ashes, the oxide compositions (SiO2, Al2O3, Fe2O3,MgO, CaO, TiO2, P2O5, and MnO) for each type of blendseries were plotted against the calculated percentageof sludge ash in the total ashes, and in each case theplots were linear. Moreover, although a slight volatiliza-

(8) Couch, G. Understanding Slagging and Fouling in pf Combus-tion; IEA Coal Research: London, UK, 1994.

(9) Vassilev, S. V.; Kitano, K.; Takeda, S.; Tsurue, T. Influence ofmineral and chemical composition of coal ashes on their fusibility. FuelProcess Technol. 1995, 45, 27-51.

(10) Quiu, J. R.; Li, F.; Zheng, Y.; Zheng, C. G.; Zhou, H. C. Theinfluences of mineral behavior on blended coal ash fusion character-istics. Fuel 1999, 78, 963-969.

(11) Kalmanovitch, D. P. Ph.D. Thesis, University of London, 1983.(12) Su, S.; Pohl, J. H.; Holcombe, D.; Hart, J. A. Slagging propensi-

ties of blended coals. Fuel 2001, 80, 1351-1360.

Table 1. Technological Properties of Materials

samples A coal B coal C coal W sludge

Proximate Analysis (wt %, db)volatile matter 34.8 24.7 23.8 54.9ash yield 11.0 22.3 52.6 42.3fixed carbon 54.2 53.0 23.6 2.8

Higher Calorific Value (MJ/kg)HCV 30.50 25.25 11.04 12.60

Ultimate Analysis (wt %, db)C 72.42 66.65 36.88 28.18H 4.50 4.02 2.60 4.56N 1.43 1.03 0.87 2.87S 0.53 0.94 1.06 0.58Cl 0.9

Sewage Sludge Influence on Coal Ash Energy & Fuels, Vol. 19, No. 6, 2005 2563

tion of both Na and K was observed3 at 800 °C, linearrelationships were also obtained between the ashes ofmaterials and their blends for both oxides. Accordingto this, the following general equation was derived foreach oxide:

with YM being the oxide percentage in blend ashes, XSSbeing the above-mentioned percentage of sludge ashesin blend ashes, and m and n being parameters of thelinear equation that depend on both the concentrationof corresponding oxide and the ash yield of coal. InFigure 1, these plottings can be seen for ashes fromsludge-C coal blends, which show straight lines withcorrelation coefficients g0.98. As a consequence of thelatter together with the fact that the pseudo-ternarycompositions (SiO2, Al2O3, CaO) of the three coal ashesare close in the corresponding triangular diagram, theplots of the three series of blend ashes lie in a narrowbundle of three straight lines in the above diagram. Forthe three coals and the sludge, the SiO2/Al2O3 ratioranges from 1.4 to 2.4.

3.1.2. Mineralogy of Materials and Their Ashes.XRD was used for detecting the crystalline phases ofboth types of materials and the ashes obtained fromthese and their blends at 800 °C, as well as the possiblechemical reactions that took place among the severalmineral matter components. The main minerals andreactions among the components of inorganic matter arethe following:

For coals, the results indicated that coal samples (A,B, and C coals) exhibit a similar mineralogical composi-tion, which agrees with the results obtained by other

authors for Asturian bituminous coals.13 The majorminerals found were clay minerals (kaolinite and illite),quartz (SiO2), carbonates (calcite), and pyrite (FeS2).

Mineralogical composition of sewage sludge differsfrom coals since it contains mainly calcite and quartz.Calcite found in the sewage sludge sample is mostly dueto chemisorption of CO2 by the Ca(OH)2 added to sewagesludge, according to reaction 2:

In the case of the ashes obtained from these materialsat 800 °C (A, B, C, and W ash samples), new anddifferent minerals were identified due to the varioustransformations that took place during the ashing

(13) Martınez-Tarazona, M. R.; Spears, D. A.; Palacios, J. M.;Martınez-Alonso, A.; Tascon, J. M. D. Mineral matter in coals ofdifferent rank from the Asturian Central Basin. Fuel 1988, 71, 367-372.

Table 2. Ash Chemical Composition (wt %) of Materials and Their Blends

ash samples SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO Cr2O3 S

Coal AshesA 28.77 20.95 21.18 3.20 10.76 0.37 2.6 0.63 0.28 0.21 0.012 3.95B 36.88 23.80 21.91 2.94 6.28 0.37 2.7 0.80 0.17 0.22 0.024 1.14C 43.26 21.9 21.10 2.64 3.38 0.39 3.4 0.86 0.18 0.25 0.010 1.36

Sewage Sludge AshesW 15.51 6.45 14.41 3.67 49.94 0.07 0.3 0.42 5.77 0.11 0.003 1.30

Sewage Sludge-Coal Blend AshesWA1 26.85 17.83 20.62 3.70 24.10 0.34 1.6 0.62 2.08 0.19 0.011 3.74WA2 18.67 9.66 15.97 3.61 42.46 0.20 0.8 0.48 4.66 0.13 0.007 2.08WB1 32.98 19.52 19.36 2.69 12.64 0.29 2.4 0.66 1.03 0.17 0.01 3.66WB2 23.27 12.85 16.37 3.32 35.25 0.24 1.3 0.56 3.87 0.13 0.01 2.58WC1 41.17 20.60 21.18 2.80 7.03 0.37 2.9 0.83 0.63 0.24 0.013 1.69WC2 31.23 15.00 18.41 3.19 23.88 0.29 2.1 0.67 2.71 0.19 0.013 1.65

Table 3. Ash Fusibility Temperatures of Materials andTheir Blends

ash sample IT (°C) ST (°C) HT (°C) FT (°C)

Coal AshesA 1332 1343 1350 1361B 1334 1350 1382 1407C 1316 1352 1383 1409

Sewage Sludge AshesW 1419-1494 1447-1495 1484-1502 1497-1504

Sludge-Coal Blend AshesWA1 1235 1246 1268 1298WA2 1363 1374 1382 1385WB1 1319 1328 1339 1364WB2 1294 1303 1316 1329WC1 1305 1318 1329 1348WC2 1244 1254 1263 1278

YM ) mXSS + n (1)

Figure 1. Relationship between ash composition and sludgeash content in sludge-C coal blend ashes.

Ca(OH)2 + CO2 f CaCO3 + H2O (2)

2564 Energy & Fuels, Vol. 19, No. 6, 2005 Folgueras et al.

process. For coal ashes, anhydrite (CaSO4) and hematite(Fe2O3) were mainly identified. However, for sewagesludge ashes, clinker minerals such as calcium ferrite(Ca2Fe2O5) and larnite (Ca2SiO4) together with mineralsof apatite group were mainly obtained.

For coal, the ashing global reactions have been widelystudied14,15 and are the following:

Pyrite decomposes and oxidizes to form hematite,magnetite, and SO2 (reaction 3). Calcite decomposeslosing CO2 at about 800 °C to form lime (CaO) (reaction4). Anhydrite is produced by reaction of free calciumoxide and SO2 (reaction 5) from decomposition of sulfideminerals and from organic sulfur. The kaolinite andillite proportion is considerably diminished because ofits transformation to metakaollinite and other phases.

The minerals of coal ashes are in different proportionsdepending on the coal used. Thus, the highest proportionof anhydrite is in A coal ashes, while the lowest one isin the C coal ashes, which agrees with the concentrationof sulfur in these coals. In the case of quartz, the highestproportion of quartz was obtained in C coal ashes, whilethe lowest one was in A coal ashes. Figure 2a showsXRD diffractogram of B coal ashes obtained at 800 °C.

During the ashing process of sewage sludge, calcitedecomposes to form CaO at about 700-750 °C. Also,

there exist reactions among the minerals of sludge ashesbeside decomposition. For example, ferrite and larniteare produced by the reaction of Fe2O3 and SiO2 withexcess free CaO according to reactions 6 and 7:

Fe2O3 comes mainly from Fe(OH)3 that is formed in theconditioning stage of the sludge in the wastewatertreatment process. In this conditioning stage, FeCl3(compound added to the sludge) reacts with Ca(HCO3)2to form Fe(OH)3 according to reaction 8:

Other minerals found in sludge ashes were quartz andchloroapatite (Ca5Cl(PO4)3) and anhydrite (Figure 2b).The presence of chloroapatite is related to the highcontents of phosphate and chloride in the sludge.Chloride comes from FeCl3 added to the sludge, whilethe addition of Ca(OH)2 to the sludge for its conditioningproduces the precipitation of calcium phosphate. XRDdiffractograms show that chloroapatite is present in thesludge ashes obtained at 800 °C. However, it was notdetected in sludge ashes obtained by heating at 200 °Cduring 4 h.

3.1.3. Effect of Blending Ratio on Mineralogy ofAshes. For blend ashes at 800 °C, some differences werefound depending on the sludge-coal blending ratio. Themain differences observed were the following:

The DRXs of 10 wt % blend ashes show the sameminerals as the ones found in coal ash samples (quartz,hematite, illite, and anhydrite). This is why a lowproportion of sludge does not seem to affect mineralcomposition, although it lowers ash fusibility temper-

(14) Demir, I.; Hughes, R. E.; DeMaris, P. J. Formation and use ofcoal combustion residues from three types of power plants burningIllinois coals. Fuel 2001, 80, 1659-1673.

(15) Tomeczek, J.; Palugniok, H. Kinetics of mineral matter trans-formation during coal combustion. Fuel 2002, 81, 1251-1258.

Figure 2. XRD diffractograms of B coal ashes, sludge ashes, and their sludge-coal blend ashes at 800 °C.

5FeS2 + 27/2O2 f Fe2O3 + Fe3O4 + 10SO2 (3)

CaCO3 f CO2 + CaO (4)

CaO + SO2 + 1/2O2 f CaSO4 (5)

Fe2O3 + 2CaO f Ca2Fe2O5 (6)

SiO2 + 2CaO f Ca2SiO4 (7)

2FeCl3 + 3Ca(HCO3)2 f 2Fe(OH)3 + 3CaCl2 + 6CO2(8)


atures (Table 3), which shows the influence of theamorphous phase on the above temperatures. On thecontrary, the addition of higher amounts of sludge (50wt % blend) yields, apart from the minerals of coal, someclinker minerals that had been detected in the sludgeashes. The minerals formed in the samples used dependon the ratio sludge ashes/coal ashes. Thus, the clinkerminerals found in the 50 wt % blends were the following:

In WA2 blend ashes (with 79.5 wt % of sludge ash intotal ash), calcium ferrite and chloroapatite were alsofound.

In WC2 blend ashes (with 44.6 wt % of sludge ash intotal ash), larnite was detected but not calcium ferriteor any mineral of apatite group.

In WB2 ashes (with 65.5 wt % of sludge ash percent-ages in total ash), calcium ferrite was found, butminerals of apatite group were not detected (Figure2c,d).

3.2. Ash Fusion Temperatures and Their Rela-tionships with both the Sludge Ash Content inBlend Ashes and the Ash Chemical Composition.As can be seen in Table 3, the ashes of bituminous coalssoften at ST 1343-1352 °C and fluidize at FT 1361-1409 °C. It can be observed that A coal ashes show anarrow range of fusion temperatures of 29 °C. Toanalyze the fusibility data, the average value of theintervals of temperatures of sludge ashes has beentaken (ST ) 1471 °C and FT ) 1501 °C). It should benoted that the temperatures for the whole interval ofash fusibility for sludge are higher than the tempera-tures obtained for the three coals.

Similarly to the coals, the blend ashes show a singlevalue for each fusibility temperature. The blend ashessoften at ST 1246-1374 °C and fluidize at FT 1278-1385 °C, with the result that the addition of sewagesludge to the coal reduces the blend fusibility intervalas compared to coal with the exception of the 10 wt %sludge-A coal blend ashes (WA1 ashes). The compari-son of the ash melting data of blends and their compo-nents reveals that in general sticky particles and slagsare expected in a wider temperature range for blendsand therefore a higher slagging propensity. The 50 wt% sludge-A coal blend ashes (WA2 ashes) constitutean exception, since it shows higher fusion temperaturesthan the corresponding coal.

3.2.1. Effect of Sludge Addition on Ash FusibilityTemperatures. To determine the influence of theaddition of sludge to each coal on the ash fusibility, theash fusion temperatures were plotted against the con-tent of the sewage sludge ash in the total ashes,expressed in weight percentage (Figure 3).

Figure 3 shows that there is no linear relationshipbetween the above variables. For coal blends, otherauthors10,11 have also found that ash fusion tempera-tures of coal ashes are generally nonlinear with respectto the blend proportion. On the contrary, the data fittedreasonably to second-order equations, the best fittingbeing obtained for C coal blends (r2 > 0.990). The aboveequations can be expressed as follows:

where T is the fusibility temperature chosen; XSS is thecontent of sewage sludge ash in the total ashes (ex-pressed in weight percent); and p, q, and r are the

equation parameters. The correlations obtained by theleast-squares method as well as the correspondingcorrelation coefficients are given in Table 4.

The calculation of the minima of the second-orderlines revealed that they are produced to very similarproportions of sludge ashes in the total ashes for boththe blends associated with every type of coal and everyfusibility temperature, the minimum temperature val-ues being listed in Table 4. In the case of the minimumash softening temperatures, the values calculated were1256 °C (range of ash fusion temperatures 1246-1294°C) for sewage sludge-A coal blends, 1283 °C (range1275-1310 °C) for sewage sludge-B coal blends, and1253 °C (range 1245-1275 °C) for sewage sludge-C coalblends.

For every minimum fusibility temperature, the aver-age sludge ash percentages calculated were 40.5 ( 2.5wt % for both B and C coals and 37.1 ( 0.5 wt % for Acoal. The 50 wt % of sludge with C coal blend yielding44.6 wt % of sludge ash is sited approximately at theminimum of the curve; therefore, it shows the lowestfusion temperatures of the sludge-C coal series. Con-sequently, for the three coals, with a proportion of about40 wt % of sludge ashes in total ashes the blends,minimum fusibility temperatures are obtained. Thispercentage in the ashes corresponds to different propor-tions of sludge in the each blend coal-sludge, since the

T ) pXSS2 + qXSS + r (9)

Figure 3. Relationship between ash fusion temperatures andthe percentage of sludge ashes in blend ashes for the threeblend series.


ash yield of every coal is different. These equations alsopermit one to predict the minimum proportion of sludgein sludge-coal blends that increases the fusibilitytemperatures compared to coal. This value can bedetermined when T > T′, T′ being the values of ashfusibility temperatures for coals. The data obtained werehigher than 74, 81, and 84 wt % of sludge ash in totalashes for the blends with A, B, and C coals, respectively.

Although, in general, ash fusion temperatures fittedrather well to a second order function, the range offusion temperatures for each blend is only reasonablypredictable. The highest fusibility temperature devia-tions were found for WB1 blend ashes ranging between1.1 and 1.7%.

3.2.2. Relationships between Ash Fusibility Tem-peratures and Ash Chemical Composition. Bysubstituting eq 1 into eq 9, a new parabolic function isobtained for each type of oxide:

whose parameters are given by the following expres-sions:

The minimum of this parabolic function can beexpressed by

Equation 10 implies that the relation existing betweenash fusion temperatures and chemical compositionobeys the same type of function found for ash fusiontemperatures-sludge ash in total ashes (eq 9), obtainingtherefore a new second order function for each oxide orgroup of oxides. According to these results, correlatingash fusion temperatures with chemical composition

seems possible, although to do so there must be arelationship between the chemical compositions ofsamples.

According to eq 1, the linear parameters m and n canbe also expressed considering only the chemical com-position of both coal and sewage sludge ashes:

where YSS and YC are the oxide content of sludge andcoal ashes, respectively.

The expressions (eqs 15 and 16) permit us to obtainthe parameters of eq 10 by using only the oxidecompositions of materials (sludge and coal ashes).However, to minimize possible chemical analysis errors,m and n values from the ashes of materials and theirblends, obtained by linear regression (according to eq1), are preferred.

As expected, the ash fusion temperatures for eachseries of blends were plotted against their contents ofeach oxide (expressed in wt %) and in each case theplottings obtained fitted to a parabolic function (seeFigure 4 for WC blends). The parameters in theseequations obtained by least-squares method are givenin Table 5 for ST. The oxide contents fitted to the abovesecond-order equations with correlation coefficientsg0.8, except MnO which yields a lower value (0.6438)in the case of blends with B coal. The parameterscalculated by using m and n from eqs 15 and 16 are in

Table 4. Sewage Sludge-Coal Blend Ashes Correlations for Fusion Temperatures

temperaturerange (°C) equation r2

minimumtemperature (°C)

Ashes of Sludge-A Coal Blends (WA)IT (1235-1494) IT (°C) ) 0.0568XSS

2 - 4.2754XSS + 1326 0.9743 1246ST (1246-1495) ST (°C) ) 0.0577XSS

2 - 4.3379XSS + 1337.4 0.9777 1256HT (1268-1502) HT (°C) ) 0.0565XSS

2 - 4.1485XSS + 1347.5 0.9957 1271FT (1298-1504) FT (°C) ) 0.051XSS

2 - 3.7303XSS + 1361.9 0.9993 1294

Ashes of Sludge-B Coal Blends (WB)IT (1294-1494) IT (°C) ) 0.0475XSS

2 - 3.7132XSS + 1347.3 0.9436 1275ST (1303-1495) ST (°C) ) 0.0506XSS

2 - 4.0223XSS + 1362.4 0.9543 1283HT (1316-1502) HT (°C) ) 0.0577XSS

2 - 4.7815XSS + 1390.6 0.9801 1292FT (1329-1504) FT (°C) ) 0.0575XSS

2 - 4.9478XSS + 1416.7 0.9714 1310

Ashes of Sludge-C Coal Blends (WC)IT (1244-1494) IT (°C) ) 0.0549XSS

2 - 4.1734XSS + 1324.4 0.9914 1245ST (1318-1495) ST (°C) ) 0.0612XSS

2 - 4.9400XSS + 1353.0 0.9999 1253HT (1263-1502) HT (°C) ) 0.0682XSS

2 - 5.6623XSS + 1377.8 0.9972 1260FT (1278-1504) FT (°C) ) 0.0692XSS

2 - 5.9263XSS + 1401.7 0.9941 1275

T ) p′YM2 + q′YM + r′ (10)

p′ ) pm2

(11)

q′ ) qm

- 2npm2

(12)

r′ ) p( nm)2

- qnm

+ r (13)

(YM)min ) -qm + 2np2p

(14)

m )YSS - YC

100(15)

n ) YC (16)

Figure 4. Relationship between softening temperature (ST)and the MexOy contents of WC blend ashes.


accordance with those obtained by using eq 1 (Table 5).The percentage deviations of the minimum valuescalculated show an average deviation of 2.3%. Thehighest deviations were for Na2O (27.8% in A coalblends and 7.4% in B coal blends), TiO2 (6.8% in B coalblends), and MgO (7.0% in B coal blends). The resultsare slightly improved by using m and n obtained fromlinear correlations according to eq 1. Also, in Table 5, itcan be seen that the oxide compositions of WC2 blend

(Table 2) are nearly coincident with those obtained forthe minimum of the parabolic function of WC blends.The highest deviations were for CaO (7.3%) and Na2Oand P2O5 (10 wt %). The remaining oxides show devia-tions e4%.

As it has been previously mentioned, for coal ashes,Bryers and Taylor found a parabolic function thatprovides softening temperature from the sum of basicoxides. From eq 10, the Bryers-Taylor correlations canbe derived. This is due to the two facts that the ashcompositions of the coals used obey SiO2/Al2O3 ≈ 1 (orSiO2/Al2O3 . 1) and that the coal ash compositions lieon a straight line that passes through the low-temper-ature eutectic region in the triangular diagram SiO2-Al2O3-CaO (colored gray in Figure 5). Figure 5 showsthe ternary phase diagram SiO2-Al2O3-CaO. In thisdiagram, the compositions of the coal ashes that fit tothe Bryers-Taylor’s correlation (SiO2/Al2O3 ) 1) arerepresented by a discontinuous line.

3.3. Some Considerations about the Effect ofTemperature on Ash Mineral Behavior and ItsRelationship with Sludge/Coal Ash Fusion Char-acteristics. Ternary phase diagrams such as CaO-SiO2-Al2O3 and CaO-SiO2-Fe2O3 have shown theirusefulness in predicting ash fusibility.11 However, insome of the sludge-coal blend ashes studied, thepresence of minerals, such as chloroapatite, differentfrom those of the above diagrams reduces the usefulnessof these diagrams. Due to this fact, only some data ofSiO2-Al2O3-CaO and SiO2-Fe2O3-CaO systems wereused to explain the minimum fusion temperatures found(both experimental and calculated ones) and to clarify

Table 5. Parameters of Parabolic Equations Softening Temperature-MexOy

correlation parameters (eq 10) calculated parameters (eqs 10-16)

oxide p′ q′ r′ R2 Ymin p′ q′ r′ Ymin

Ashes of Sewage Sludge-A Coal Blends (WA)SiO2 2.93 -142.08 2982.6 0.9085 24.25 3.28 -156.11 3112.46 23.79Al2O3 2.68 -83.92 1913 0.9508 15.67 2.74 -85.07 1915.15 15.51Fe2O3 10.48 -398.21 5044.1 0.8745 19.01 12.59 -469.20 5627.71 18.64MgO 2974.9 -20175.0 35441 0.9984 3.39 2612.04 -17640.0 31038.17 3.38CaO 0.38 -19.70 1507.3 0.9879 25.73 0.38 -19.16 1500.05 25.49Na2O 2059.2 -1479.50 1556 0.7874 0.36 6411.11 -3298.26 1680.07 0.26K2O 100.43 -346.99 1563.3 0.9473 1.73 105.38 -360.47 1564.14 1.71TiO2 6970.5 -8159.10 3673.6 0.8317 0.59 13083.90 -14420.1 5229.03 0.55P2O5 19.15 -90.08 1362.1 0.9812 2.35 19.14 -89.74 1361.02 2.34MnO 60625 -20906.0 3048.4 0.9626 0.17 57700.00 -19896.1 2971.01 0.17

Ashes of Sewage Sludge-B Coal Blends (WB)SiO2 1.07 -60.74 2152.20 0.9571 28.50 1.11 -62.90 2175.27 28.39Al2O3 1.49 -50.90 1731.20 0.9318 17.13 1.68 -56.83 1762.79 16.90Fe2O3 7.91 -299.55 4126.00 0.8011 18.92 9.00 -340.55 4505.65 18.93MgO 369.63 -2236.00 4686.90 0.7929 3.02 949.52 -6134.19 11189.62 3.23CaO 0.28 -13.29 1434.00 0.9674 23.78 0.27 -12.55 1430.73 23.63Na2O 4059.60 -2172.20 1602.00 0.9783 0.27 5622.22 -2819.68 1636.00 0.25K2O 78.36 -278.81 1535.00 0.9953 1.78 83.61 -289.68 1533.36 1.73TiO2 2827.40 -3731.00 2532.30 0.9093 0.66 3504.16 -4548.15 2758.26 0.65P2O5 16.73 -80.64 1375.10 0.9638 2.41 16.14 -77.31 1375.08 2.40MnO 34216.00 -12018.0 2346.50 0.6438 0.18 41818.18 -14743.4 2581.94 0.18

Ashes of Sewage Sludge-C Coal Blends (WC)SiO2 0.79 -50.77 2068.30 1.0000 32.10 0.79 -50.96 2070.19 32.06Al2O3 2.56 -80.31 1882.20 0.9998 15.66 2.56 -80.32 1882.42 15.66Fe2O3 12.35 -459.38 5525.60 0.9693 18.59 13.67 -503.21 5882.80 18.40MgO 588.26 -3610.30 6795.20 0.974 3.07 576.87 -3525.48 6639.72 3.06CaO 0.28 -12.47 1391.30 1.0000 22.14 0.28 -12.52 1392.09 22.17Na2O 6115.00 -3191.50 1664.50 0.9995 0.26 5976.56 -3117.97 1659.97 0.26K2O 57.10 -243.56 1526.90 0.9706 2.13 60.90 -260.74 1532.38 2.14TiO2 3137.80 -4290.60 2719.70 0.9997 0.68 3161.16 -4314.46 2725.45 0.68P2O5 19.63 -95.66 1369.40 0.9999 2.44 19.59 -95.42 1369.54 2.44MnO 31001.00 -12018.0 2417.90 0.9999 0.19 31224.49 -12083.7 2422.39 0.19

Figure 5. Ternary plots of the three series of sludge-coalashes in the SiO2-Al2O3-CaO phase diagram.


the effect of temperature on mineral behavior of experi-mental blend ashes. The above diagrams17 show lowfusion regions that lie between eutectics formed at theAl2O3/CaO molar ratios 0.1 and 0.3 for the SiO2-Al2O3-CaO system and at the Fe2O3/CaO molar ratios 0.4 forthe SiO2-Fe2O3-CaO system (Table 6 and Figure 5).In Table 6, the initial ash fusibility temperaturestogether with their corresponding molar ratios for theexperimental blends and the predicted ones that mini-mize IT [(WA)min, (WB)min, and (WC)min] are also shown.It can be seen that the chemical compositions of theblends WA1 and WC2 lie in the low fusion region of thediagram SiO2-Al2O3-CaO (Figure 5).

As a consequence of the heating of coal ashes (A, B,and C ashes) from 800 to 1100 °C, an important effectof temperature was observed in illite and anhydrite. Theformer is clearly diminished mainly due to the formationof amorphous aluminosilicate, while the latter decreasesdue to the CaSO4 decomposition. In the case of sludgeashes, only slight changes were observed, some mineralsidentified by XRD in ashes at 800 °C (calcium ferriteand quartz) being also detected in ashes at 1100 °C.Hydroxylapatite chlorian was also detected in ashes at1100 °C, though not chloroapatite, due probably to thetransformation of chloroapatite in sludge ashes afterthey had been dampened with water.

The ashes of blend series with C coal show differentminerals depending on both the sludge ashes/coal ashesratio and the temperature. During the heating of WC1ashes from 800 to 1100 °C, the behavior of crystallinematter was similar to that of coal ashes. However, forWC2 ashes, the only blend of this series sited in thelow fusion zone of the phase diagram CaO-SiO2-Al2O3and close to the eutectic E1 (Figure 5), the results showsome reactions between ash components. Thus, in theashes at 1100 °C, the diffraction peaks of both illite andlarnite disappear, while the presence of gehlenite(2CaO‚Al2O3‚SiO2) is observed. The formation of thismineral may be associated with the illite and/or theamorphous aluminosilicate reaction with larnite, ac-

cording to the following reaction:

The low fusibility temperatures and narrow fusibilityrange (1245-1275 °C) of WC2 ash sample, as well asthe formation of gehlenite, can be predicted from theternary phase diagram CaO-SiO2-Al2O3 (Figure 5). Onthe other hand, the calculated blend ash compositionthat minimizes fusibility temperatures, which is nearlycoincident to that of WC2 ashes, is also very close tothe eutectic E1 of the phase diagram CaO-SiO2-Al2O3(Table 6). Figure 6, panels a and b, presents the XRDof ashes at 800 °C compared with that obtained at 1100°C for WC2 ashes.

The ashes of blend series with A coal shows somedifferences with respect to the C coal blend series. Inthe range of temperatures studied (from 800 at 1100

(16) Bryant, G. W.; Browning, G. J.; Emanuel, H.; Gupta, S. K.;Gupta, R. P.; Lucas, J. A.; Wall, T. F. The fusibility of blended coalash. Energy Fuels 2000, 14 (2), 316-325.

(17) Levin, E. M.; Robbins, C. R.; McMurdie, H. F. Phase Diagramsfor Ceramists, Vol. 1; The American Ceramic Society: Columbus, OH,1964; pp 219-228.

Table 6. Comparison of Ash Initial Temperatures (IT) and Molar Ratios of Blend Ashes with Ternary Eutectics Data

SiO2-Al2O3-CaO system SiO2-Fe2O3-CaO system

blend or eutectic T (°C) SiO2/CaO Al2O3/CaO T (°C) SiO2/CaO Fe2O3/CaO

Experimental DataWA1 1235 1.0 0.4 1235 1.0 0.3WA2 1363 0.4 0.1 1363 0.4 0.1WB1 1319 2.4 0.9 1319 2.4 0.5WB2 1294 0.6 0.2 1294 0.6 0.2WC1 1305 5.5 1.6 1305 5.5 1.1WC2 1244 1.2 0.3 1244 1.2 0.3

Predicted Data from eqs 1 and 10-14(WA)min

a 1246 0.9 0.3 1246 0.9 0.3(WB)min

a 1275 1.2 0.4 1275 1.2 0.3(WC)min

a 1245 1.6 0.4 1245 1.6 0.3

Eutectic Points Datab

eutectic 1 (E1) 1265 1.0 0.3 1214 0.7 0.4eutectic 2 (E2) 1170 2.5 0.3 1204 1.7 0.4eutectic 3 (E3) ≈1310 0.8 0.1

a Sludge-coal blend ashes that minimize IT correlations. b Eutectic points from the ternary phase diagrams in ref 17.

Figure 6. Comparison of XRD diffractograms of WC2 blendashes at 800 and 1100 °C.

2CaO + Al2O3 + SiO2 f 2CaO‚Al2O3‚SiO2 (17)


°C), the temperature effect on WA blend ashes was notsignificant. The WA1 blend ash, as in the case of WC2blend ash, is sited in the low fusion temperature regionof SiO2-Al2O3-CaO diagram (close to the eutectic E1)and its composition is very close to the calculated WAblend ashes that minimize IT function (Table 6 andFigure 5), which would explain the low ash fusiontemperatures of this blend (1235-1298 °C). However,changes due to transformation and reaction betweenphases were hardly detected at 1100 °C. The changesproduced by heating were the lack of illite and thedecrease of anhydrite. In the case of the WA2 blend ash,whose composition is nearest to that of sludge ashes,there are scarcely differences with the minerals detectedin the sludge ashes. For this blend ash, the presence ofminerals of the apatite group and ferrite makes theinformation from these diagrams insufficient to predictash fusibility temperatures.

The ashes of blend series with B coal show anintermediate behavior between those of A and C series.In the heating of WB1 blend ashes from 800 to 1100°C, changes in the crystalline matter were not detected.In the case of WB2 blend ashes at 1100 °C, both thepresence of gehlenite and the lack of illite (similarly toWC2 blend ashes) were observed. The presence of bothferrite and gehlenite in this sample indicates that theinformation provided by a single ternary diagram isinadequate to predict its ash fusibility temperature.However, the WB2 ash sample is located in the proxim-ity of the low fusion region of both the SiO2-Al2O3-CaO and the SiO2-Fe2O3-CaO diagrams (Table 6),which agrees with its low ash fusion temperatures(1294-1329 °C) and the presence of gehlenite.

4. Conclusions

The addition of sludge with high contents of CaO tocoal in variable proportions may produce different typesof changes in ash fusion temperatures depending on the

composition of sludge and coal ashes and on sludgeashes/coal ashes ratio.

The addition of low proportions of sludge to the coalsto form 10 wt % blends reduces coal ash fusion temper-atures, although minerals found in the ashes were thesame as those in coal ashes at 800 and 1100 °C. Whenhigher proportions of sludge are added to the coals toform 50 wt % blends, different effects on ash fusiontemperatures depending on the coal ash yield wereobserved. Clinker minerals (calcium ferrite and larnite)were detected in the ashes at 800 °C. Moreover, gehlen-ite was seen, during heating from 800 to 1100 °C, inthe blends with B and C coals (those with higherproportions of SiO2).

The ashes of the sludge used are much richer in CaOthan those of the coals, but the SiO2/Al2O3 ratio isrelatively close. Consequently, some sludge-coal blendashes compositions are located in the low-temperatureeutectic region of the ternary phase diagram SiO2-Al2O3-CaO. Thus, the relationship between ash fus-ibility temperatures of each sludge-coal blend series andthe percentage of sludge ashes in blend ashes fitssatisfactorily to second-order polynomial functions. Ac-cording to these polynomial functions, for each sludge-coal series there exists an ash chemical composition thatminimizes fusibility temperatures. These minimumfusibility temperatures may be approximately predictedby the SiO2-Al2O3-CaO phase diagram.

The above diagram allows us to estimate ash fusibilitytemperatures of the sludge-coal blends rather well,although the presence of minerals in some blends, suchas chloroapatite, lessens the usefulness of the informa-tion given by this diagram.

Acknowledgment. Financial support from the Min-istry of Education and Science in Spain and FEDER(Project EN2004-07282/ALT) is gratefully acknowledged.

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Influence of Sewage Sludge Addition on Coal Ash Fusion Temperatures