Influence of Sewage Sludge Addition on Coal Ash Fusion Temperatures

The ash fusibility temperatures of samples in oxidizing atmosphere were measured, ... (one for each coal), the relationships between ash fusibility te...
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Energy & Fuels 2005, 19, 2562-2570

Influence of Sewage Sludge Addition on Coal Ash Fusion Temperatures M. Bele´n Folgueras,*,† R. Marı´a Dı´az,† Jorge Xiberta,† M. Purificacio´n Garcı´a,‡ and J. Juan Pis§ Department of Energy and Department of Materials Science, University of Oviedo, Independencia 13, 33004 Oviedo, Spain, and Instituto Nacional del Carbo´ n, 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 sewage sludge (W), and the corresponding coal-sewage sludge blends (10 and 50 wt % of sludge) were studied. The ash fusibility temperatures of samples in oxidizing atmosphere were measured, and their chemical and mineralogical compositions were determined. The addition of sludge to coal in certain proportions produces blends whose ashes have lower fusibility temperatures than those of coal and sludge. This is related to the differences in chemical composition and modes of elemental combination in both types of materials. The main differences are associated to the elements P, Fe, and Ca. As the sludge is much richer in Ca than the coals, the compositions of the blend ashes pass through low-temperature eutectic regions of the ternary phase diagrams SiO2-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 of sludge 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 urban wastewater treatment process, can be blended with coal to remove this dangerous waste by co-combustion in a boiler pulverized coal and also to provide a lower-cost fuel that allows us to recover its energetic potential. However, the inorganic composition of this blend can be very different from that of the individual coal component, producing serious operational problems associated with combustion, such as an increase of boiler deposits due to fouling and slagging, as well as some trace element emissions. To understand the interaction between both types of materials, several studies have been done.1-5 * Corresponding author. Tel.: +34-98-5104333. Fax: +34-985104322. E-mail: [email protected]. † Department of Energy, University of Oviedo. ‡ Department of Materials Science, University of Oviedo. § Instituto Nacional del Carbo ´ n. CSIC. (1) Folgueras, M. B.; Dı´az, R. M.; Xiberta, J.; Prieto, I. Volatilisation of trace elements for coal-sewage sludge blends during their combustion. Fuel 2003, 82, 1939-1948. (2) Folgueras, M. B.; Dı´az, R. M.; Xiberta, J.; Prieto, I. Thermogravimetric analysis of the co-combustion of coal and sewage sludge. Fuel 2003, 82, 2051-2055. (3) Folgueras, M. B.; Dı´az, R. M.; Xiberta, J. Sulphur retention during co-combustion of coal and sewage sludge. Fuel 2004, 83, 13151322. (4) Ninomiya, Y.; Zhang, L.; Sakano, T.; Kanaoka, Ch.; Masui, M. Transformation of mineral and emission of particulate matter during co-combustion of coal with sewage sludge. Fuel 2004, 83, 751-64. (5) Miller, B. B.; Kandiyoti, R.; Dugwell, D. R. Trace element behavior during co-combustion of sewage sludge with Polish coal. Energy Fuels 2004, 18 (4), 1093-1103.

The determination of ash fusion temperatures is one of the tools used for determining coal and coal blends behavior and to know whether ash deposit problems will be found during combustion. These temperatures are described by initial deformation temperature (IT), softening temperature (ST), hemispherical temperature (HT), and fluid temperature (FT). Although the knowledge of these temperatures is not the only factor that must be considered to predict ash behavior, it is the easiest parameter to determine the effect of the sludge addition on the coal ash fusion temperatures. Also both chemical and mineral compositions of ashes have been widely used to express ash fusibility of coals and their blends, although these attempts have generally yielded different results.6,7 The base-to-acid ratio (B/A) is one of the first indexes used to predict ash fusibility. This index relates the ash basic compounds (Fe2O3, CaO, MgO, Na2O, K2O) that reduce ash fusion temperatures (B) to the ash acidic compounds (SiO2, Al2O3, TiO2) that increase them (A). Together with this type of indexes there are correlations that predict ash fusion temperatures from ash chemical composition. Among these, Bryers and Taylor’ correlations must be pointed out.6 Accordingly, the softening temperature under reducing conditions depends on the sum of basic (6) Bryers, R. W. Fireside slagging, fouling, and high-temperature corrosion of heat-transfer surface due to impurities in steam-raising fuels. Prog. Energy Combust. Sci. 1996, 22, 29-120. (7) Seggiani, M. Empirical correlations of the ash fusion temperatures and temperature of critical viscosity for coal and biomass ashes. Fuel 1999, 78, 1121-1125.

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

Sewage Sludge Influence on Coal Ash

oxides, adjusting to a second-order polynomial function for SiO2/Al2O3 ≈ 1 or SiO2/Al2O3 . 1. Some investigators have shown that ash fusion temperatures do not depend only on chemical composition but also on mineral composition.8-11 Thus, the modes of elemental combination (minerals and phases) in coal and coal ashes and their behavior during heating are of great importance.9 Refractory minerals (quartz, metakaolinite, mullite, rutile, etc.) rise ash fusion temperatures, while fluxing minerals (anhydrite, calcium silicates, hematite, etc.) reduce them.9 For coals and coal blends, the slagging propensity has also been related to the low-temperature eutectics formation, the molar ratio Fe2O3/CaO being a key for their prediction.6,12 However, little has been done on the prediction of slagging tendency of sewage sludge-coal blends. That is why an attempt was made to correlate ash fusion temperatures with both the blending ratio and the ash blend composition and to predict the possible lowtemperature eutectics formation. To this end, an experimental survey was carried out on three bituminous coals and dried sewage sludge. Ash fusion temperatures were 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 yields ranged from 11 to 53 wt % were used, corresponding to bituminous coals from the Asturias Central Basin (A coal, B coal, and C coal). Moreover, the sewage sludge (W sludge) from an urban wastewater treatment plant situated in Asturias was also used. In the wastewater treatment plant, FeCl3 and lime are used for sludge conditioning, while lime is used for its stabilization. From these materials, six sludge-coal blends were prepared by adding dry sewage sludge to coals, obtaining two blends whose sewage sludge contents were 10 wt % (WA1 blend, WB1 blend, and WC1 blend for A, B, and C coals, respectively) and 50 wt % (WA2 blend, WB2 blend, and WC2 blend for each coal). Different samples from these types of materials were used in previous works1-3 where characterization analyses are described. The results of the characterization of materials are given in Table 1. The ashing process was carried out according to ASTM Standard D 3174-89, with the final temperature being maintained for 2 h at 800 °C. The ashes obtained from the above materials and their blends were used for determining both the chemical and the mineral compositions as well as their fusion temperatures. The ash chemical composition was established by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), samples being prepared by LiBO2 fusion. The minimum detection limits of the technique applied were as follows: (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) 0.03 wt % for Al2O3; and (e) 0.04 wt % for Fe2O3 and K2O. The determination of sulfur was carried out with a LECO SC-32 analyzer (minimum detection limit 0.01 wt %). The fusibility (8) Couch, G. Understanding Slagging and Fouling in pf Combustion; IEA Coal Research: London, UK, 1994. (9) Vassilev, S. V.; Kitano, K.; Takeda, S.; Tsurue, T. Influence of mineral and chemical composition of coal ashes on their fusibility. Fuel Process Technol. 1995, 45, 27-51. (10) Quiu, J. R.; Li, F.; Zheng, Y.; Zheng, C. G.; Zhou, H. C. The influences of mineral behavior on blended coal ash fusion characteristics. 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 propensities of blended coals. Fuel 2001, 80, 1351-1360.

Energy & Fuels, Vol. 19, No. 6, 2005 2563 Table 1. Technological Properties of Materials samples

A coal

B coal

C coal

W sludge

volatile matter ash yield fixed carbon

Proximate Analysis (wt %, db) 34.8 24.7 23.8 11.0 22.3 52.6 54.2 53.0 23.6

54.9 42.3 2.8

HCV

Higher Calorific Value (MJ/kg) 30.50 25.25 11.04

12.60

C H N S Cl

Ultimate Analysis (wt %, db) 72.42 66.65 36.88 4.50 4.02 2.60 1.43 1.03 0.87 0.53 0.94 1.06

28.18 4.56 2.87 0.58 0.9

temperatures in oxidizing atmosphere were determined by a LECO AF-600 analyzer according to test method ASTM D1857-87D. For each fusibility temperature, the values reported are the average of the results obtained in two different assays. Moreover, two ash cones of the same sample were simultaneously used in the above two assays. For ashes of both coals and sludge-coal blends, the difference for each fusibility temperature between two separate runs was lower than 30 °C. However, the sludge ashes did not have the necessary consistency to establish a precise temperature for each ash fusibility temperature; this is why the highest ranges of temperatures observed for them were recorded. In Tables 2 and 3, major and minor elements contents (expressed as oxides) of ashes and their fusibility temperatures are shown, respectively. The minerals of the materials and their blends as well as their ashes at 800 °C were established (qualitative analyses) by X-ray diffraction (XRD). The diffractograms were made using a Phillips PW 1710 X-ray powder diffractometer and Cu KR radiation (using a graphite monochromator). Diffraction intensities were recorded in the 2Θ range of 5-65°. To explain ash fusibility behavior, the ashes of materials and their blends obtained at 800 °C (ground at 0.990). The above equations can be expressed as follows:

T ) pXSS2 + qXSS + r

(9)

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

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

equation parameters. The correlations obtained by the least-squares method as well as the corresponding correlation coefficients are given in Table 4. The calculation of the minima of the second-order lines revealed that they are produced to very similar proportions of sludge ashes in the total ashes for both the blends associated with every type of coal and every fusibility temperature, the minimum temperature values being listed in Table 4. In the case of the minimum ash softening temperatures, the values calculated were 1256 °C (range of ash fusion temperatures 1246-1294 °C) for sewage sludge-A coal blends, 1283 °C (range 1275-1310 °C) for sewage sludge-B coal blends, and 1253 °C (range 1245-1275 °C) for sewage sludge-C coal blends. For every minimum fusibility temperature, the average sludge ash percentages calculated were 40.5 ( 2.5 wt % for both B and C coals and 37.1 ( 0.5 wt % for A coal. The 50 wt % of sludge with C coal blend yielding 44.6 wt % of sludge ash is sited approximately at the minimum of the curve; therefore, it shows the lowest fusion temperatures of the sludge-C coal series. Consequently, for the three coals, with a proportion of about 40 wt % of sludge ashes in total ashes the blends, minimum fusibility temperatures are obtained. This percentage in the ashes corresponds to different proportions of sludge in the each blend coal-sludge, since the

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Table 4. Sewage Sludge-Coal Blend Ashes Correlations for Fusion Temperatures temperature range (°C)

equation

r2

minimum temperature (°C)

IT (1235-1494) ST (1246-1495) HT (1268-1502) FT (1298-1504)

Ashes of Sludge-A Coal Blends (WA) IT (°C) ) 0.0568XSS2 - 4.2754XSS + 1326 ST (°C) ) 0.0577XSS2 - 4.3379XSS + 1337.4 HT (°C) ) 0.0565XSS2 - 4.1485XSS + 1347.5 FT (°C) ) 0.051XSS2 - 3.7303XSS + 1361.9

0.9743 0.9777 0.9957 0.9993

1246 1256 1271 1294

IT (1294-1494) ST (1303-1495) HT (1316-1502) FT (1329-1504)

Ashes of Sludge-B Coal Blends (WB) IT (°C) ) 0.0475XSS2 - 3.7132XSS + 1347.3 ST (°C) ) 0.0506XSS2 - 4.0223XSS + 1362.4 HT (°C) ) 0.0577XSS2 - 4.7815XSS + 1390.6 FT (°C) ) 0.0575XSS2 - 4.9478XSS + 1416.7

0.9436 0.9543 0.9801 0.9714

1275 1283 1292 1310

IT (1244-1494) ST (1318-1495) HT (1263-1502) FT (1278-1504)

Ashes of Sludge-C Coal Blends (WC) IT (°C) ) 0.0549XSS2 - 4.1734XSS + 1324.4 ST (°C) ) 0.0612XSS2 - 4.9400XSS + 1353.0 HT (°C) ) 0.0682XSS2 - 5.6623XSS + 1377.8 FT (°C) ) 0.0692XSS2 - 5.9263XSS + 1401.7

0.9914 0.9999 0.9972 0.9941

1245 1253 1260 1275

ash yield of every coal is different. These equations also permit one to predict the minimum proportion of sludge in sludge-coal blends that increases the fusibility temperatures compared to coal. This value can be determined when T > T′, T′ being the values of ash fusibility temperatures for coals. The data obtained were higher than 74, 81, and 84 wt % of sludge ash in total ashes for the blends with A, B, and C coals, respectively. Although, in general, ash fusion temperatures fitted rather well to a second order function, the range of fusion temperatures for each blend is only reasonably predictable. The highest fusibility temperature deviations were found for WB1 blend ashes ranging between 1.1 and 1.7%. 3.2.2. Relationships between Ash Fusibility Temperatures and Ash Chemical Composition. By substituting eq 1 into eq 9, a new parabolic function is obtained for each type of oxide:

T ) p′YM2 + q′YM + r′

(10)

whose parameters are given by the following expressions:

p m2

(11)

q 2np - 2 m m

(12)

p′ )

q′ )

r′ ) p

(mn )

2

-

qn +r m

seems possible, although to do so there must be a relationship between the chemical compositions of samples. According to eq 1, the linear parameters m and n can be also expressed considering only the chemical composition of both coal and sewage sludge ashes:

m)

YSS - YC 100

n ) YC

(15) (16)

where YSS and YC are the oxide content of sludge and coal ashes, respectively. The expressions (eqs 15 and 16) permit us to obtain the parameters of eq 10 by using only the oxide compositions of materials (sludge and coal ashes). However, to minimize possible chemical analysis errors, m and n values from the ashes of materials and their blends, obtained by linear regression (according to eq 1), are preferred. As expected, the ash fusion temperatures for each series of blends were plotted against their contents of each oxide (expressed in wt %) and in each case the plottings obtained fitted to a parabolic function (see Figure 4 for WC blends). The parameters in these equations obtained by least-squares method are given in Table 5 for ST. The oxide contents fitted to the above second-order equations with correlation coefficients g0.8, except MnO which yields a lower value (0.6438) in the case of blends with B coal. The parameters calculated by using m and n from eqs 15 and 16 are in

(13)

The minimum of this parabolic function can be expressed by

(YM)min )

-qm + 2np 2p

(14)

Equation 10 implies that the relation existing between ash fusion temperatures and chemical composition obeys the same type of function found for ash fusion temperatures-sludge ash in total ashes (eq 9), obtaining therefore a new second order function for each oxide or group of oxides. According to these results, correlating ash fusion temperatures with chemical composition

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

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Table 5. Parameters of Parabolic Equations Softening Temperature-MexOy correlation parameters (eq 10) oxide

p′

q′

r′

calculated parameters (eqs 10-16) R2

Ymin

p′

q′

r′

Ymin

-156.11 -85.07 -469.20 -17640.0 -19.16 -3298.26 -360.47 -14420.1 -89.74 -19896.1

3112.46 1915.15 5627.71 31038.17 1500.05 1680.07 1564.14 5229.03 1361.02 2971.01

23.79 15.51 18.64 3.38 25.49 0.26 1.71 0.55 2.34 0.17

SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO

2.93 2.68 10.48 2974.9 0.38 2059.2 100.43 6970.5 19.15 60625

-142.08 -83.92 -398.21 -20175.0 -19.70 -1479.50 -346.99 -8159.10 -90.08 -20906.0

Ashes of Sewage Sludge-A Coal Blends (WA) 2982.6 0.9085 24.25 3.28 1913 0.9508 15.67 2.74 5044.1 0.8745 19.01 12.59 35441 0.9984 3.39 2612.04 1507.3 0.9879 25.73 0.38 1556 0.7874 0.36 6411.11 1563.3 0.9473 1.73 105.38 3673.6 0.8317 0.59 13083.90 1362.1 0.9812 2.35 19.14 3048.4 0.9626 0.17 57700.00

SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO

1.07 1.49 7.91 369.63 0.28 4059.60 78.36 2827.40 16.73 34216.00

-60.74 -50.90 -299.55 -2236.00 -13.29 -2172.20 -278.81 -3731.00 -80.64 -12018.0

Ashes of Sewage Sludge-B Coal Blends (WB) 2152.20 0.9571 28.50 1.11 1731.20 0.9318 17.13 1.68 4126.00 0.8011 18.92 9.00 4686.90 0.7929 3.02 949.52 1434.00 0.9674 23.78 0.27 1602.00 0.9783 0.27 5622.22 1535.00 0.9953 1.78 83.61 2532.30 0.9093 0.66 3504.16 1375.10 0.9638 2.41 16.14 2346.50 0.6438 0.18 41818.18

-62.90 -56.83 -340.55 -6134.19 -12.55 -2819.68 -289.68 -4548.15 -77.31 -14743.4

2175.27 1762.79 4505.65 11189.62 1430.73 1636.00 1533.36 2758.26 1375.08 2581.94

28.39 16.90 18.93 3.23 23.63 0.25 1.73 0.65 2.40 0.18

SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO

0.79 2.56 12.35 588.26 0.28 6115.00 57.10 3137.80 19.63 31001.00

-50.77 -80.31 -459.38 -3610.30 -12.47 -3191.50 -243.56 -4290.60 -95.66 -12018.0

Ashes of Sewage Sludge-C Coal Blends (WC) 2068.30 1.0000 32.10 0.79 1882.20 0.9998 15.66 2.56 5525.60 0.9693 18.59 13.67 6795.20 0.974 3.07 576.87 1391.30 1.0000 22.14 0.28 1664.50 0.9995 0.26 5976.56 1526.90 0.9706 2.13 60.90 2719.70 0.9997 0.68 3161.16 1369.40 0.9999 2.44 19.59 2417.90 0.9999 0.19 31224.49

-50.96 -80.32 -503.21 -3525.48 -12.52 -3117.97 -260.74 -4314.46 -95.42 -12083.7

2070.19 1882.42 5882.80 6639.72 1392.09 1659.97 1532.38 2725.45 1369.54 2422.39

32.06 15.66 18.40 3.06 22.17 0.26 2.14 0.68 2.44 0.19

accordance with those obtained by using eq 1 (Table 5). The percentage deviations of the minimum values calculated show an average deviation of 2.3%. The highest deviations were for Na2O (27.8% in A coal blends and 7.4% in B coal blends), TiO2 (6.8% in B coal blends), and MgO (7.0% in B coal blends). The results are slightly improved by using m and n obtained from linear correlations according to eq 1. Also, in Table 5, it can be seen that the oxide compositions of WC2 blend

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

(Table 2) are nearly coincident with those obtained for the minimum of the parabolic function of WC blends. The highest deviations were for CaO (7.3%) and Na2O and P2O5 (10 wt %). The remaining oxides show deviations e4%. As it has been previously mentioned, for coal ashes, Bryers and Taylor found a parabolic function that provides softening temperature from the sum of basic oxides. From eq 10, the Bryers-Taylor correlations can be derived. This is due to the two facts that the ash compositions of the coals used obey SiO2/Al2O3 ≈ 1 (or SiO2/Al2O3 . 1) and that the coal ash compositions lie on a straight line that passes through the low-temperature eutectic region in the triangular diagram SiO2Al2O3-CaO (colored gray in Figure 5). Figure 5 shows the ternary phase diagram SiO2-Al2O3-CaO. In this diagram, the compositions of the coal ashes that fit to the Bryers-Taylor’s correlation (SiO2/Al2O3 ) 1) are represented by a discontinuous line. 3.3. Some Considerations about the Effect of Temperature on Ash Mineral Behavior and Its Relationship with Sludge/Coal Ash Fusion Characteristics. Ternary phase diagrams such as CaOSiO2-Al2O3 and CaO-SiO2-Fe2O3 have shown their usefulness in predicting ash fusibility.11 However, in some of the sludge-coal blend ashes studied, the presence of minerals, such as chloroapatite, different from those of the above diagrams reduces the usefulness of these diagrams. Due to this fact, only some data of SiO2-Al2O3-CaO and SiO2-Fe2O3-CaO systems were used to explain the minimum fusion temperatures found (both experimental and calculated ones) and to clarify

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Energy & Fuels, Vol. 19, No. 6, 2005 2569

Table 6. Comparison of Ash Initial Temperatures (IT) and Molar Ratios of Blend Ashes with Ternary Eutectics Data SiO2-Al2O3-CaO system blend or eutectic

SiO2/CaO

Al2O3/CaO Experimental Data 0.4 0.1 0.9 0.2 1.6 0.3

T (°C)

SiO2/CaO

Fe2O3/CaO

WA1 WA2 WB1 WB2 WC1 WC2

1235 1363 1319 1294 1305 1244

1.0 0.4 2.4 0.6 5.5 1.2

1235 1363 1319 1294 1305 1244

1.0 0.4 2.4 0.6 5.5 1.2

0.3 0.1 0.5 0.2 1.1 0.3

(WA)mina (WB)mina (WC)mina

1246 1275 1245

Predicted Data from eqs 1 and 10-14 0.9 0.3 1246 1.2 0.4 1275 1.6 0.4 1245

0.9 1.2 1.6

0.3 0.3 0.3

0.7 1.7

0.4 0.4

eutectic 1 (E1) eutectic 2 (E2) eutectic 3 (E3) a

T (°C)

SiO2-Fe2O3-CaO system

1265 1170 ≈1310

1.0 2.5 0.8

Eutectic Points Datab 0.3 0.3 0.1

1214 1204

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

the effect of temperature on mineral behavior of experimental blend ashes. The above diagrams17 show low fusion regions that lie between eutectics formed at the Al2O3/CaO molar ratios 0.1 and 0.3 for the SiO2-Al2O3CaO system and at the Fe2O3/CaO molar ratios 0.4 for the SiO2-Fe2O3-CaO system (Table 6 and Figure 5). In Table 6, the initial ash fusibility temperatures together with their corresponding molar ratios for the experimental blends and the predicted ones that minimize IT [(WA)min, (WB)min, and (WC)min] are also shown. It can be seen that the chemical compositions of the blends WA1 and WC2 lie in the low fusion region of the diagram 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 effect of temperature was observed in illite and anhydrite. The former is clearly diminished mainly due to the formation of amorphous aluminosilicate, while the latter decreases due to the CaSO4 decomposition. In the case of sludge ashes, only slight changes were observed, some minerals identified by XRD in ashes at 800 °C (calcium ferrite and quartz) being also detected in ashes at 1100 °C. Hydroxylapatite chlorian was also detected in ashes at 1100 °C, though not chloroapatite, due probably to the transformation of chloroapatite in sludge ashes after they had been dampened with water. The ashes of blend series with C coal show different minerals depending on both the sludge ashes/coal ashes ratio and the temperature. During the heating of WC1 ashes from 800 to 1100 °C, the behavior of crystalline matter was similar to that of coal ashes. However, for WC2 ashes, the only blend of this series sited in the low fusion zone of the phase diagram CaO-SiO2-Al2O3 and close to the eutectic E1 (Figure 5), the results show some reactions between ash components. Thus, in the ashes at 1100 °C, the diffraction peaks of both illite and larnite disappear, while the presence of gehlenite (2CaO‚Al2O3‚SiO2) is observed. The formation of this mineral may be associated with the illite and/or the amorphous aluminosilicate reaction with larnite, ac(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 coal ash. Energy Fuels 2000, 14 (2), 316-325. (17) Levin, E. M.; Robbins, C. R.; McMurdie, H. F. Phase Diagrams for Ceramists, Vol. 1; The American Ceramic Society: Columbus, OH, 1964; pp 219-228.

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

cording to the following reaction:

2CaO + Al2O3 + SiO2 f 2CaO‚Al2O3‚SiO2

(17)

The low fusibility temperatures and narrow fusibility range (1245-1275 °C) of WC2 ash sample, as well as the formation of gehlenite, can be predicted from the ternary phase diagram CaO-SiO2-Al2O3 (Figure 5). On the other hand, the calculated blend ash composition that minimizes fusibility temperatures, which is nearly coincident to that of WC2 ashes, is also very close to the eutectic E1 of the phase diagram CaO-SiO2-Al2O3 (Table 6). Figure 6, panels a and b, presents the XRD of ashes at 800 °C compared with that obtained at 1100 °C for WC2 ashes. The ashes of blend series with A coal shows some differences with respect to the C coal blend series. In the range of temperatures studied (from 800 at 1100

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°C), the temperature effect on WA blend ashes was not significant. The WA1 blend ash, as in the case of WC2 blend ash, is sited in the low fusion temperature region of SiO2-Al2O3-CaO diagram (close to the eutectic E1) and its composition is very close to the calculated WA blend ashes that minimize IT function (Table 6 and Figure 5), which would explain the low ash fusion temperatures of this blend (1235-1298 °C). However, changes due to transformation and reaction between phases were hardly detected at 1100 °C. The changes produced by heating were the lack of illite and the decrease 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 detected in the sludge ashes. For this blend ash, the presence of minerals of the apatite group and ferrite makes the information from these diagrams insufficient to predict ash fusibility temperatures. The ashes of blend series with B coal show an intermediate 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 the presence of gehlenite and the lack of illite (similarly to WC2 blend ashes) were observed. The presence of both ferrite and gehlenite in this sample indicates that the information provided by a single ternary diagram is inadequate to predict its ash fusibility temperature. However, the WB2 ash sample is located in the proximity of the low fusion region of both the SiO2-Al2O3CaO and the SiO2-Fe2O3-CaO diagrams (Table 6), which agrees with its low ash fusion temperatures (1294-1329 °C) and the presence of gehlenite.

composition of sludge and coal ashes and on sludge ashes/coal ashes ratio. The addition of low proportions of sludge to the coals to form 10 wt % blends reduces coal ash fusion temperatures, although minerals found in the ashes were the same as those in coal ashes at 800 and 1100 °C. When higher proportions of sludge are added to the coals to form 50 wt % blends, different effects on ash fusion temperatures depending on the coal ash yield were observed. Clinker minerals (calcium ferrite and larnite) were detected in the ashes at 800 °C. Moreover, gehlenite was seen, during heating from 800 to 1100 °C, in the blends with B and C coals (those with higher proportions of SiO2). The ashes of the sludge used are much richer in CaO than those of the coals, but the SiO2/Al2O3 ratio is relatively close. Consequently, some sludge-coal blend ashes compositions are located in the low-temperature eutectic region of the ternary phase diagram SiO2Al2O3-CaO. Thus, the relationship between ash fusibility temperatures of each sludge-coal blend series and the percentage of sludge ashes in blend ashes fits satisfactorily to second-order polynomial functions. According to these polynomial functions, for each sludgecoal series there exists an ash chemical composition that minimizes fusibility temperatures. These minimum fusibility temperatures may be approximately predicted by the SiO2-Al2O3-CaO phase diagram. The above diagram allows us to estimate ash fusibility temperatures of the sludge-coal blends rather well, although the presence of minerals in some blends, such as chloroapatite, lessens the usefulness of the information given by this diagram.

4. Conclusions

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

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

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