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High-Temperature Melting Behavior ofUrban Wood Fuel Ash

P. Thy,*,† B. M. Jenkins,‡ and C. E. Lesher†

Department of Geology and Department of Biological and Agricultural Engineering,One Shields Avenue, University of California, Davis, California 95616

Received November 10, 1998. Revised Manuscript Received March 30, 1999

Ashes from commercial wood fuel blends show wide compositional ranges that in part can berelated to an admixed soil component. To evaluate agglomeration potentials and to formulatepredictive models, an experimental study was conducted of an urban wood waste fuel ash froman operating power plant in California. The melting relations were investigated from the liquidusat 1276 °C to the near solidus at about 1130 °C. The liquidus phase is melilite composed of asolid solution dominated by åkermanite and sodium melilite. A phosphate phase appears at 1207°C and can be described as a solid solution of calcium phosphate, sodium phosphate, and calciumdisilicate. Garnet appears at 1159 °C and is composed of an andradite, pyrope, and grossularsolid solution. The alkali metals are partially lost to the atmosphere during the experimentalduration. Sodium loss is moderate and dependent on the liquid remaining. Potassium showsvery low partitioning into the solid phases and is strongly lost from the slag. The experimentalresults have been used to formulate a calculation procedure for evaluating the high-temperaturecompositional behavior of urban wood fuel slag. The results predict strong potassium volatilizationtendencies for relatively pure wood fuel ashes with less than about 47 wt % SiO2. Because theSiO2 content increases with increasing soil component, the effect of soil is to retain potassium inthe solid residue. In addition, the effect of an admixed soil component on wood fuel slag behavioris principally to increase viscosity and has little effects on the surface tension. Because of theinverse effect of viscosity on agglomeration, the result is that an admixed soil to wood fuel willreduce alkali losses and agglomeration potential.

Introduction

Slag and ash deposits in combustion furnaces andboilers fueled by biomass cause operating problems andreduction in efficiency and steam generation.1 Potas-sium and silica are present in large concentrations inmost annual growth plant materials and are directlylinked to the formation of tenacious surface deposits onfiresides and heat transfer surfaces. During combustion,the organic structure of plant material is decomposedand the inorganic material is released and transportedeither in the form of solid particles or as vapor speciesin the combustion gas. Particulate adventitious materi-als undergo phase transformations and reactions withother inorganic components. Ash particles principallycomposed of refractory silicate minerals and liquids landon fireside surfaces and also agglomerate and sinter toform slag. Entrained particulate matter and potassiumvapor species are carried to the low-temperature partsof the boiler were they may react with other combustiongas components and result in deposits composed ofmixtures of refractory particles and potassium sulfates,carbonates, hydroxides, and chlorides.1-3 Such fouling

of heat exchangers prevents the use of some commontypes of biomass at many power plant facilities.4

While the major mechanisms leading to fouling havebeen investigated,1,5-7 our ability to predict and limitfouling and slag deposition is empirical and at bestunreliable. The nature of the condensed phase dictatesthe severity of fouling, the tenacity of the deposits, andthe ease of removal. Although predictive models utiliz-ing equilibrium between gas and the condensed phaseshave been developed,8,9 little is known about the high-temperature phase relations and physical and chemicalproperties of the condensed phases.10-12 Empirical as-

* Author to whom correspondence should be addressed.† Department of Geology.‡ Department of Biological and Agricultural Engineering.(1) Baxter, L. L. Biomass Bioenergy 1993, 4, 85-102.(2) Rindt, D. K.; Jones, M. L.; Schobert, H. H. In Fouling and

Slagging Resulting from Impurities in Combustion Gases; Bryers, R.W., Ed.; Energy Foundation: New York, 1982; pp 17-35.

(3) Raask, E. Mineral Impurities in Coal Combustion; HemispherePublishing: Washington, D.C., 1985.

(4) Turnbull, J. H. Biomass Bioenergy 1993, 4, 75-84.(5) French, R. J.; Milne, T. A. Biomass Bioenergy 1994, 7, 1-6.(6) Jenkins, B. M.; Bakker, R. R.; Wei, J. B. Biomass Bioenergy 1996,

10, 177-200.(7) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.;

Jenkins, B. M.; Oden, L. L. Biomass Bioenergy 1996, 10, 125-138.(8) Hastie, J. W.; Plante, E. R.; Bonnell, D. W. In Metal Bonding

and Interactions in High-Temperature Systems with Emphasis onAlkali Metals; Cole, J. L.; Stwalley, W. C., Eds.; American ChemicalSociety, Symposium Series 179: Washington, D.C., 1982; pp 543-600.

(9) Cook, L. P.; Hastie, J. W. In Mineral Matter and Ash in Coal;Vorres, K. S., Ed.; American Chemical Society, Symposium Series301: Washington, D. C., 1986; pp 277-286.

(10) Huffman, G. P.; Huggins, F. E. In Mineral Matter and Ash inCoal; Vorres, K. S., Ed.; American Chemical Society, Symposium Series301: Washington, D.C., 1986; pp 100-113.

(11) Raask, E. In Mineral Matter and Ash in Coal; Vorres, K. S.,Ed.; American Chemical Society, Symposium Series 301: Washington,D.C., 1986; pp 138-155.

839Energy & Fuels 1999, 13, 839-850

10.1021/ef980249q CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 05/13/1999

sessment techniques, such as the standard ASTM conetest for determining fusibility of ash,13 are unsuccessfulin predicting the initial melting properties of biomassash or the tenacity of deposits upon formation. Attemptshave been made to use ternary equilibrium phasediagrams to predict melting behavior,14 but their sig-nificance is uncertain because of the multicomponentnature of ashes. A better approach is offered by directexperimental melting studies of natural ashes and hasled to a more reliable understanding of the phaseequilibria and melting behavior of coal slags.11,15-17

These studies have been extended to biomass fuelsystems, but so far generally below melting such as byMisra et al.18 and Olanders and Steenari.12

The principal reason for performing high-temperaturemelting experiments on coal and biomass ashes is thedesire to apply the results to ashes for which only thecomposition is known and to be able to predict theirmelting and slagging behavior. This paper is the firstin a series of experimental determinations of phaseequilibria and phase compositions of typical biomassashes as a function of temperature. Here we report thehigh-temperature experimental characterizations of anurban wood fuel ash selected from considerations of thecompositional variation of ashes of urban wood wastefuel typically received at commercial power plant facili-ties. The composition of approximately 60 ashes19 isillustrated in Figure 1 on binary variation diagramswith SiO2 (wt %) as the abscissa. The observed variationis ascribed to binary mixing between a high CaO andlow Al2O3 and SiO2 component and a low CaO and highAl2O3 and SiO2 component. This trend reflects admis-sion of a soil component high in Si and Al to the fuelmixture as is typical for commercial fuels. The fuelselected as starting material for the melting experi-ments is a relatively pure low SiO2 and high CaO urbanwood fuel (Figure 1; Table 1) obtained from an operatingpower plant in California (Woodland Biomass, Wood-land, California). The results are used to develop andevaluate predictive models for slag formation and itsphysical and compositional properties. These models aretested on the commercial range of urban wood fuel ashesshown in Figure 1.

Experimental and Analytical Methods

The ash content of the wood fuel is determined over a rangeof temperatures from 575 to 1000 °C and is constant at about3.2% (575 °C, 3.39%; 750 °C, 3.02%; 900° C, 3.09%; 1000 °C,3.16%). The ash produced at 575 °C was used in this study.The ash was ground in an agate mortar, mixed with an organicbinder (PVA), and formed into a pellet. The pellet was brokeninto 20-30 mg pieces that were sintered to 4 mil platinum

wire loops20 by heating in a H2-O2 gas torch. The experimentalcharges were suspended in an isothermally controlled verticalquench furnace21 in air at atmospheric pressure. The furnacetemperature was kept constant during the duration of theexperiments and was monitored by a Pt100/Pt90Rh10 (S type)thermocouple calibrated against the melting point of gold.22

The experimental temperatures were chosen from above theliquidus to the near solidus in approximate steps of 10 °C, andduration was varied from 50 min near the liquidus to 5500min near the solidus. All experiments were terminated byquenching in air. The experimental conditions are summarizedin Table 2.

The experimental products were mounted in epoxy, sec-tioned, and surface polished to allow examination with re-flected light and scanning electron microscopes. The glass andcoexisting mineral phases were analyzed using a CAMECASX50 electron microprobe operated at 15 kV voltage, a beamcurrent of 10 nA, and counting times between 10 and 30 s.Natural minerals and synthetic oxides were used as stand-ards. The potential volatile elements, Na and K, were analyzedfirst with a 10 µm beam size and counting times of 10 s inorder to minimize volatilization.23,24 A fused international rock

(12) Olanders, B.; Steenari, B.-M. Biomass Bioenergy 1995, 8, 105-115.

(13) ASTM. Fusibility of coal and coke ash. D1857-68, 1986.(14) Huggins, F. E.; Kosmack, D. A.; Huffman, G. P. Fuel 1981, 60,

577-584.(15) Kalmanovitch, D. P.; Williamson, J. In Mineral Matter and Ash

in Coal; Vorres, K. S., Ed.; American Chemical Society, SymposiumSeries 301: Washington, D.C., 1986; pp 234-255.

(16) Huffman, G. P.; Huggins, F. E.; Dunmyre, G. R. Fuel 1981, 60,585-597.

(17) Biggs, D. L.; Lindsay, C. G. In Mineral Matter and Ash in Coal;Vorres, K. S., Ed.; American Chemical Society, Symposium Series301: Washington, D.C., 1986; pp 128-137.

(18) Misra, M. K.; Ragland, K. W.; Baker, A. J. Biomass Bioenergy1993, 4, 103-116.

(19) B. M. Jenkins, unpublished compilation, 1998.

(20) Donaldson, C. H.; Williams, R. J.; Lofgren, G. Am. Mineral.1975, 60, 324-326.

(21) Williams, R. J.; Mullins, O. NASA Technical Memorandum58234, 1981.

(22) Biggar, G. M. Mineral. Mag. 1972, 38, 741-770.(23) Spray, J. G.; Rae, D. A. Can. Mineral. 1995, 33, 323-332.(24) Reed, S. J. B. Electron Microprobe Analysis, 2nd ed.; Cambridge

University Press: Cambridge, 1996.

Figure 1. The compositional variation of CaO, Al2O3, and SiO2

in a compilation of urban wood waste fuel ashes obtained fromvarious sources.19 All analyses have been normalized to 100%SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, andP2O5 (wt %). All iron is calculated as Fe2O3. The compositionof the superliquidus liquid from Table 4 is indicated by boxedsymbols.

Table 1. Wood Ash Composition

SiO2 30.18 Na2O 3.58TiO2 0.72 K2O 5.90Al2O3 7.47 P2O5 2.42Fe2O3 4.68 SO3 3.26MnO 0.26 Cl 0.74MgO 4.97 LOIa 17.12CaO 19.23 total 100.45

a LOI represents loss on ignition after burning in air at 575 °C.

840 Energy & Fuels, Vol. 13, No. 4, 1999 Thy et al.

standard25 and a natural volcanic glass26 with comparablealkali concentrations were analyzed intermittently and con-sistently showed (Table 3) that the alkali elements were notlost during analysis. The precision of the electron microprobeanalyses is generally within 1-3% for major elements and5% or above for minor elements. The accuracy of the analy-ses can be evaluated by comparing the measured values withthe recommended values for the two standard glasses assummarized in Table 3. The analytical data are reported inTable 4 as averages and one standard deviation of replicate(<13) analyses (all given as wt % oxides). Despite significantamounts of Cl and SO3 in the starting material (Table 1),these components are below detection limits for all glasses(Table 4).

The phase proportions for each melting experiment wereestimated by weighted least-squares mass balance calculationsusing the phase compositions given in Table 4.27 All oxideswere assigned a weighting factor of 1.0, except SiO2 and Al2O3,which were weighted by 0.4 and 0.5, respectively. The extentof elemental losses to furnace gas during melting was esti-mated by including the oxides Na2O, K2O, and P2O5 as phasesin the mass balance calculations. The losses estimated in thismanner are tabulated in Table 2.

Experimental Results

Phase Appearance and Proportions. The lastphase to disappear in the melting range of the wood ashat 1276 ( 5 °C is a melilite (disilicate of the generalformula (Ca,Na)2(Mg,Al,Si)3O7) that occurs as large,typically above 200 µm diameter, equant to prismaticgrains with sharp grain boundaries (euhedral) (Figure2A). The average grain size of melilite decreases mark-edly with decreasing temperature to about 10-30 µmfor experiments closer to the solidus temperature (Fig-ure 2B). The second phase is a phosphate phase at 1203( 7 °C that occurs as small 10-20 µm sized roundedanhedral grains (Figure 2B). The third phase appearsat 1159 °C and is a garnet (orthosilicate of the generalformula (Ca,Mg)3(Fe,Al)2Si3O12) that occurs as largeeuhedral, several hundred micrometer large interstitialgrains containing abundant inclusions of melilite andphosphate grains (Figure 2C). The volumetric amountof the liquidus phase, melilite, increases linearly withdecreasing liquid until about 30 wt % liquid remaining(Figure 3B). Phosphate appears at about 55% liquidremaining and then increases in amount systematicallyuntil the appearance of garnet between 35 and 15%liquid remaining.

The solidus has not been directly determined becausethe amount of liquid remaining below 1139 °C is too low(<14%) to be analyzed. Therefore, the precise solidustemperature and solidus mineral assemblage are notknown. Since the starting material is high in potassium,it is plausible that potassium-bearing silicate phases(orthoclase, kalsilite, or potassium tetrasilicate) mayappear at the solidus. If the temperature is extrapolatedto zero percent liquid, a minimum solidus temperatureof about 1130 °C can be estimated (Figure 3A). There-fore, the examined wood ash solidifies over a tempera-ture range of about 150 °C and shows the followingequilibrium crystallization order: melilite, phosphate,garnet, prior to reaching the solidus. The extrapolationto the solidus (Figure 3B) further suggests a phaseassemblage dominated by melilite, followed by garnet,and then phosphate; however, undetermined additionalphases are likely to be present at the solidus.

Alkali Metal Losses. The alkali metal oxides, K2Oand Na2O, are lost significantly during the experiments.

(25) Govindaraju, K. Geostand. Newsl. 1989, 13, 1-113.(26) Jarosewich, E.; Nelen, J. A.; Norberg, J. A. Geostand. Newsl.

1980, 4, 43-47.(27) Bryan, W. B.; Finger, L. W.; Chayes, F. Science 1969, 163, 926-

927.

Table 2. Experimental Conditions and Resultsa

losses (wt % oxides)

run no. temp. (°C) duration (min) glass melilite phosphate garnet Na2O K2O P2O5 ∑R2

36 1299 50 1.00028 1289 1560 1.000 2.20 4.00 1.00 0.61727 1280 1560 1.000 2.00 3.90 0.70 0.49526 1271 1375 0.907 0.093 1.70 3.60 0.50 0.24725 1258 1435 0.892 0.108 1.80 3.50 0.50 0.27224 1249 1487 0.734 0.266 1.20 2.60 0.50 0.19323 1239 1480 0.794 0.206 1.50 3.60 0.50 0.33815 1228 1415 0.745 0.255 1.30 3.70 0.20 0.32717 1210 1255 0.753 0.247 1.20 3.20 0.2825 1197 1340 0.609 0.370 0.021 3.40 0.2234 1187 1430 0.532 0.443 0.025 0.50 3.10 0.1113 1178 1295 0.461 0.485 0.054 0.70 2.20 0.2531 1168 1460 0.367 0.566 0.067 0.50 3.30 0.4058 1149 4210 0.145 0.621 0.093 0.143 0.30 4.40 0.4199 1139 5555 0.143 0.652 0.091 0.114 0.40 4.40 0.276

a Phase proportions (wt %) have been estimated by least-squares mixing calculations and the determined phase compositions of Table4. Na2O, K2O, and P2O5 losses have been estimated by including these oxides in the mixing calculations. ∑R2 is the sum of the square ofthe residuals of the mixing calculations. The calculations use interpolated phosphate compositions for run nos. 3 and 5 since this phasewas not analyzed for these.

Table 3. Analytical Precision and Accuracya

rhyolite Glass basaltic Glass

N ) 10 1 SD rec. N ) 50 1 SD rec.

SiO2 76.26 0.30 76.71 52.55 0.37 52.46TiO2 0.09 0.04 0.12 1.31 0.19 1.07Al2O3 12.18 0.13 12.06 15.03 0.20 15.00Fe2O3 1.30 0.11 1.37 11.76 0.36 11.18MnO 0.02 0.02 0.03 0.15 0.05 0.17MgO 0.03 0.02 <0.10 6.52 0.09 6.62CaO 0.43 0.03 0.50 10.76 0.13 11.00Na2O 3.70 0.07 3.75 2.23 0.07 2.16K2O 4.63 0.14 4.89 0.62 0.03 0.64P2O5 0.00 0.00 <0.01 0.22 0.03 0.13total 98.64 99.52 101.15 100.43

a N is numbers of analyses used for calculating each averageand one standard deviation (1 SD). Rec. is the reported recom-mended values. Rhyolite glass is a natural Yellowstone NationalPark rhyolite glass standard (USNM 72854; Jarosewich etal.26). Basaltic glass is a fused rock standard W-1 from Govin-daraju.25

High-Temperature Melting Behavior of Urban Wood Fuel Ash Energy & Fuels, Vol. 13, No. 4, 1999 841

Tab

le4.

An

alyt

ical

Res

ult

s(w

t%

Oxi

des

)

wt

%ox

ides

1S

.D.

run

no.

tem

p.(°

C)

dura

tion

(min

)N

SiO

2T

iO2

Al 2

O3

Fe 2

O3

Mn

OM

gOC

aON

a 2O

K2O

P2O

5S

iO2

TiO

2A

l 2O

3F

e 2O

3M

nO

MgO

CaO

Na 2

OK

2OP

2O5

glas

s36

1299

5010

35.1

70.

868.

614.

980.

446.

7731

.29

4.05

4.80

4.29

0.26

0.03

0.13

0.06

0.14

0.13

0.75

0.14

0.08

0.10

2812

8915

6010

39.6

90.

879.

284.

560.

346.

8034

.12

1.96

0.81

3.54

0.37

0.03

0.08

0.09

0.04

0.13

0.88

0.06

0.06

0.09

2712

8015

609

39.2

80.

829.

084.

600.

396.

8633

.62

2.22

0.92

3.83

0.33

0.03

0.09

0.14

0.10

0.13

0.73

0.08

0.03

0.11

2612

7113

755

38.0

70.

889.

434.

930.

406.

5632

.84

2.58

1.41

4.46

0.32

0.09

0.08

0.14

0.05

0.05

0.91

0.05

0.06

0.17

2512

5814

357

38.6

71.

009.

605.

050.

416.

3332

.92

2.52

1.50

4.55

0.60

0.09

0.07

0.20

0.10

0.10

0.48

0.08

0.04

0.11

2412

4914

8710

36.0

61.

289.

866.

150.

455.

9730

.07

3.61

2.94

5.28

0.33

0.07

0.14

0.14

0.05

0.14

0.62

0.12

0.10

0.15

2312

3914

808

37.1

10.

979.

535.

410.

455.

8931

.02

2.91

1.52

4.92

0.15

0.07

0.09

0.19

0.07

0.10

0.59

0.14

0.05

0.11

1512

2814

1510

36.7

91.

249.

755.

670.

505.

7530

.80

3.18

1.46

5.64

0.25

0.12

0.08

0.15

0.05

0.07

0.43

0.08

0.08

0.06

1712

1012

5511

36.1

91.

179.

625.

260.

496.

3230

.89

3.39

2.18

6.35

0.16

0.12

0.09

0.15

0.05

0.10

0.94

0.06

0.09

0.21

511

9713

407

35.0

41.

4110

.48

6.30

0.57

5.24

27.2

54.

792.

115.

520.

700.

050.

250.

210.

100.

080.

420.

120.

070.

204

1187

1430

635

.29

1.56

10.7

87.

140.

535.

1226

.26

4.30

2.95

5.49

0.47

0.10

0.41

0.29

0.10

0.06

0.38

0.18

0.07

0.12

311

7812

955

34.5

31.

9612

.03

7.74

0.59

4.58

23.3

14.

125.

024.

740.

760.

120.

520.

180.

060.

110.

500.

190.

180.

121

1168

1460

637

.50

1.89

11.0

18.

000.

604.

5922

.63

4.34

3.40

4.78

0.93

0.12

0.14

0.17

0.06

0.07

0.40

0.19

0.15

0.13

811

4942

106

41.1

32.

3813

.07

6.59

0.70

4.08

20.2

65.

961.

983.

790.

810.

030.

400.

130.

070.

130.

580.

220.

070.

189

1139

5555

539

.88

2.13

13.8

86.

810.

653.

9520

.73

5.82

2.21

3.84

0.62

0.07

0.11

0.18

0.13

0.07

0.23

0.09

0.07

0.17

mel

ilit

e26

1271

1375

542

.55

0.05

5.60

1.07

0.11

10.3

841

.22

1.73

0.22

0.49

0.16

0.03

0.17

0.11

0.01

0.18

0.63

0.09

0.03

0.02

2512

5814

3513

42.2

20.

025.

411.

160.

0510

.43

40.6

31.

490.

270.

480.

330.

020.

210.

180.

050.

231.

100.

140.

040.

0424

1249

1487

741

.85

0.03

4.77

1.24

0.12

10.7

139

.88

1.24

0.33

0.47

0.40

0.03

0.36

0.10

0.08

0.21

0.73

0.06

0.05

0.04

2312

3914

806

42.3

30.

035.

271.

250.

1110

.43

40.8

61.

530.

270.

510.

550.

030.

280.

150.

080.

161.

280.

090.

090.

1115

1228

1415

642

.62

0.03

5.25

1.52

0.19

10.1

739

.19

2.00

0.19

0.46

0.30

0.03

0.75

0.21

0.05

0.42

0.36

0.28

0.05

0.03

1712

1012

554

42.2

90.

035.

681.

860.

119.

9841

.27

1.85

0.23

0.48

0.20

0.03

0.77

0.20

0.04

0.46

0.59

0.42

0.01

0.04

511

9713

405

39.2

90.

027.

252.

250.

229.

1737

.84

2.80

0.18

0.56

0.49

0.03

0.81

0.15

0.07

0.26

0.56

0.13

0.02

0.10

411

8714

308

39.5

60.

095.

692.

130.

259.

6038

.35

2.71

0.26

1.34

0.96

0.05

0.95

0.28

0.08

0.46

0.89

0.20

0.02

1.11

311

7812

955

41.2

90.

104.

802.

450.

2210

.01

38.1

92.

670.

530.

570.

650.

090.

900.

210.

070.

380.

880.

370.

240.

241

1168

1460

640

.02

0.10

6.53

2.88

0.18

9.06

35.8

82.

960.

300.

570.

740.

070.

270.

130.

040.

220.

360.

150.

070.

308

1149

4210

642

.32

0.14

7.21

2.33

0.19

8.54

35.7

03.

890.

120.

560.

590.

050.

810.

300.

060.

230.

410.

270.

030.

239

1139

5555

541

.56

0.03

7.72

2.64

0.15

8.32

34.3

83.

700.

110.

460.

760.

030.

900.

170.

060.

210.

100.

150.

020.

14ph

osph

ate

411

8714

302

6.01

0.03

0.05

0.33

0.15

0.15

50.7

24.

600.

7335

.69

0.17

0.04

0.04

0.11

0.04

0.11

0.41

0.12

0.01

0.16

111

6814

602

5.46

0.04

0.36

0.39

0.12

1.35

47.8

24.

820.

7235

.03

0.88

0.04

0.06

0.15

0.10

0.15

0.59

0.05

0.08

0.73

811

4942

102

4.44

0.03

0.20

0.29

0.24

1.23

48.0

25.

910.

2239

.02

0.43

0.00

0.29

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The extent of this loss is estimated by mass balancecalculations tabulated in Table 2. It is seen that Na2Oshows losses that are dependent on the liquid remain-ing, but that losses only become significant after 75%liquid remaining (Figure 4). In contrast, K2O shows highand constant losses that are relatively independent ofliquid proportion. These differences are illustrated interms of solid-liquid and liquid-vapor distributioncoefficients in Figure 4. The oxide Na2O is moderatelypartitioned into both melilite and phosphate phaseswith a Na2Osolid/Na2Oliquid partition coefficient of 0.50-0.70, while K2O is nearly completely partitioned into

the liquid with a K2Osolid/K2Oliquid coefficient of less than0.02 (Figure 4B). The distribution for K2O betweenliquid and vapor phases varies from 0.4 to 1.0 K2Oliquid/K2Ovapor (Figure 4A), reflecting that this oxide is ap-proximately evenly distributed between the liquid andvapor phase at near solidus conditions. On the otherhand, Na2O is preferentially partitioned into the liquidphase at all temperatures with Na2Oliquid/Na2Ovapor

varying from 1.4 to 20.0 for near solidus conditions(Figure 4B).

These experimental results indicate that potassiumis poorly partitioned into the crystalline solid phases.The activities and concentrations of alumina and silicain the liquid are insufficient to form potassium alumi-nosilicates before perhaps relatively late in the crystal-lization sequence. In contrast, sodium is partitioned intomelilite and phosphate phases throughout the meltinginterval. The result is an enhanced partitioning ofpotassium over sodium into liquid and vapor phases.

Figure 2. Backscattered electron images of experimental products. (A) Equant melilite (dark) in glass (gray) (Run no. 25, 1258°C). (B) Phosphate (light), melilite, and matrix glass (gray) (Run no. 1, 1168 °C). (C) Interstitial garnet, melilite, phosphate, andglass (Run no. 9, 1139 °C)

Figure 3. Experimental results as a function of liquidproportions (wt basis). (A) Temperature (°C); the melting ofgarnet, phosphate, and melilite is indicated; the liquidus wasdetermined to occur at 1276 °C, and the solidus has beenextrapolated to ∼1130 °C. (B) The phase and liquid proportions(wt) calculated by least-squares mixing approximations.

Figure 4. The distribution coefficients for the alkali metaloxides (wt %) as a function of experimental temperature (°C).(A) Liquid/vapor distribution. The concentration of the oxidesin the liquid is given in Table 4. The concentration of the oxidesin the vapor phase has been estimated (Table 2). (B) Solid/liquid distribution. The solid concentration has been calculatedfrom the mineral compositions in Table 4 and the modes inTable 2. The liquid concentration is from Table 4.


Phase Chemistry and Mineral-Liquid Rela-tions. The compositions of the quenched liquids (glasses)in the experimental products are summarized in Table4 and in Figure 5, which shows oxide concentrations asa function of CaO content. Most oxides change system-atically with decreasing CaO (and temperature) and canbe related to the coexisting solid phases. MgO and CaOare strongly partitioned into melilite and systematicallydecrease in the liquid with extent of crystallization. SiO2initially decreases with decreasing CaO until the ap-pearance of phosphate, and then increases since thephosphate contains little SiO2. Similarly, Al2O3, Na2O,and TiO2 show systematic increases throughout theinvestigated melting interval because these oxides arepartitioned only to a moderate extent into the solidphases. Except for garnet, TiO2 is nearly completelyexcluded from the solid phases and, therefore, providesan indirect measure of the amount of liquid (Figure 6).

The melilite is described as a solid solution28 ofåkermanite (Ca2MgSi2O7) and gehlenite (Ca2Al2SiO7)

with minor amounts of soda melilite (NaCaAlSi2O7) andferri-gehlenite (Ca2Fe3+AlSiO7) (Figure 7). The melilitecrystallizing from the wood ash is dominated by aker-manite with the amount of akermanite (0.78-0.57)decreasing in the solid solution and the amounts ofsodium melilite (0.12-0.34) increasing with decreasingtemperature (Figure 7). The analysis of the partitioningof the alkali metals between melilite and liquid iscomplicated because of the alkali losses. If melilite isnot in equilibrium with the coexisting liquid, but inequilibrium with the initial bulk composition, then thedistribution coefficient for sodium (Na2Omel/Na2Oliq, wt%) increases systematically as a function of both de-creasing akermanite content and temperature (“Lossadded”, Figure 8). If it is assumed that melilite equili-brates with the liquid as sodium is partially lost, theNa2Omel/Na2Oliq ratio is relatively independent of com-position and temperature (0.67 ( 0.10). The distributionof Na2O for the latter set of calculations is consistentwith the results of Pan and Longhi29 (0.66-0.53) andwith the low range of Becket and Stolper30 (0.59-1.52).

(28) Deer, W. A.; Howie, R. A.; Zussman, J. Rock-Forming Minerals,Vol. 1B, 2nd ed., Disilicates and Ring Silicates; Longman: Harlow,1986.

(29) Pan, V.; Longhi, J. Contrib. Mineral. Petrol. 1990, 105, 569-584.

(30) Beckett, J. R.; Stolper, E. M. Lunar Planet. Sci. 1994, 25, 79-80.

Figure 5. Glass (liquid) composition shown as a function ofCaO content (wt % oxides). All iron has been calculated asFe2O3 and the analytical results have been normalized to 100.The melting-out of the garnet, phosphate, and melilite areindicated on the MgO and Fc2O3 variation curves.

Figure 6. Liquid proportions of experiments (wt) as a functionof TiO2 concentrations of liquid phase (glass) (wt % oxide). Themelting of garnet, phosphate, and melilite are indicated.

Figure 7. The solid solution of melilite calculated in termsof åkermanite (Ca2MgSi2O7), gehlenite (Ca2Al2SiO7), ferri-gehlenite (Ca2Fe3+AlSiO7), and soda melilite (NaCaAlSi2O7)as molecular proportions. The range in melting temperaturesis 1139-1276 °C.


Assuming that potassium behaves in a manner similarto that of sodium, the K2Omel/K2Oliq ratio varies from0.20 to 0.06 as temperature decreases (Figure 8). Theimplication is that the slag equilibrates to the changingcomposition resulting from alkali losses during theduration of the individual experiments.

The second phase to melt is a calcium phosphate thatcan be calculated as a solid solution of Ca3P2O8 (72-80%), Na6P2O8 (8-10%), and Ca4Si2O8 (10-17%); thesecomponents show little variation with temperature.There is no analytical indication of the presence of OH-

and Cl- anions in the phosphate structure and this,therefore, is not apatitic.

The first phase detected to melt is a garnet withthe structural formula (Ca4.0Mg1.7)(Fe3+

2.1Al1.7Ti4+0.3)-

(Si5.6Al0.4)O24 and with small amount of Na (0.1 cationsto 24 O). The calculation of the formula indicates thatthe majority of iron occurs as Fe3+. This garnet can bedescribed as a solid solution of approximately 51%andradite, 31% pyrope, and 18% grossular.31

Discussion

Crystallization Model. The results of the meltingexperiments on the wood ash have been used to formu-

late a crystallization model that can forecast solid andsemisolid slag mineralogy and alkali loss from anunknown ash for which the major element compositionis known. To forecast the mineralogy of the slag graphi-cally, the 10 oxide components of the ash (e.g., Table 4)are combined into more complex components, therebyreducing the total number of components of the system.As a first step, the oxides have been transformed into aset of ideal and simplified normative minerals modeledafter the results of the melting experiments. The orderof calculation of the simplified normative componentsis based on the observed partitioning of the variouselements and is not necessarily identical to the crystal-lization order. The normative minerals and their cal-culation sequence are (1) rutile TiO2, (2) phosphateCa3P2O8, (3) soda melilite NaCaAlSi2O7, (4) andraditeCa3Fe2Si3O12 (Fe as Fe3+), (5) pyrope Mg3Al2Si3O12, (6)akermanite Ca2MgSi2O7, (7) rankinite (3C2S) Ca3Si2O7,(8) orthoclase KAlSi3O8, (9) potassium tetrasilicate(K4S) K2Si4O9, (10) K2O, and (11) silica SiO2. All ironis calculated as Fe2O3, MnO is added to MgO, and theresults are expressed on a molecular basis. Details ofthe calculation procedure are given in the Appendix.This type of normative presentations of chemical analy-sis has been widely used in igneous petrology, and thecalculation procedure adopted here is tailored afterprocedures developed for silicate rocks (often referredto as the molecular Barth-Niggli norm32-35). Thecalculated mode gives a reasonable representation,based on the present experimental results, of themineralogy of a completely crystalline slag near itssolidus temperature.

The normative mineralogy can be further reduced.Melilite can be expressed as a solid solution of sodamelilite, akermanite, and rankinite by adding thesecomponents together. This procedure reduces the ex-perimental melilite to two main components and ignoresminor gehlenitic components. Rankinite was added tothe melilite solid solution as a hypothetical “Mg-freemelilitic” component. Rankinite was not observed in theexperiments, but proved necessary for balancing thecalculations. Similarly, garnet is calculated as a solidsolution of pyrope and andradite and ignores a minorgrossularitic component of the experimental garnet solidsolution. The ideal phosphate used in the calculationsignores significant Na and Si components in the solidsolution of the experimental phosphate. The “residual”Si-K silicates (orthoclase, potassium tetrasilicate, andsilica) are added together since these components areenriched in the residual liquid and only form late in thecrystallization sequence. Because crystalline potassiumsilicates and quartz are not encountered in this study,future work will likely require that this step in thenormative procedure be modified. Orthoclase and tet-rasilicate have been chosen as the potassium-bearingcomponents because these phases follow quartz crystal-lization at the orthoclase-quartz-tetrasilicate eutecticin the simplified K2O-SiO2-Al2O3 system.36 Since K2O

(31) Deer, W. A.; Howie, R. A.; Zussman, J. Rock-Forming Minerals.Vol. 1A, 2nd ed., Orthosilicates; Longman: Harlow, 1982.

(32) Johannesen, A. A Descriptive Petrography of the Igneous Rocks.Vol. 1; University of Chicago Press: Chicago, 1931.

(33) Niggli, P. Rocks and Mineral Deposits; Freeman: San Francisco,1954.

(34) Barth, T. F. W. Theoretical Petrology, 2nd ed.; John Wiley: NewYork, 1962.

(35) Ragland, P. C. Basic Analytical Petrology; Oxford UniversityPress: Oxford, 1989.

Figure 8. The partitioning of minor elements Fe2O3, Na2O,K2O between melilite and liquid expressed as the distributioncoefficient calculated by dividing the concentration of the oxidein melilite by the concentration in liquid (e.g., Na2Omel/Na2Oliq).The curves labeled “loss-added” are the expected results if noalkali loss had occurred (see text for discussion).


is strongly partitioned into the volatile phase, thisresidual component can be ignored. Rutile also isignored for the sake of approximating the mineralogyof the condensed phase.

This procedure reduces the wood ash to four “compo-nents” (phosphate, melilite, garnet, and residual sili-cates) that can be illustrated in a tetrahedron or asprojections on triangular diagrams. The normativecomponents account for 96% of liquid, 95% of melilite,88% of phosphate, and 93% of garnet compositions. Theresidual K-Si silicate component is typically very lowor zero, except for the garnet-saturated liquids for whichit may reach 9% of the total, mainly because of apotassium tetrasilicate component. These “reduced”components are plotted (large symbols) in Figure 9 asprojections on the triangular diagrams melilite-garnet-phosphate and phosphate-melilite-residual silicate.The first observation is that the experimental mineralphases, as expected, all plot slightly off the ideal norma-tive compositions. The second observation is that mostexperimental liquids plot on the plane defined by theexperimental mineral phases (melilite, garnet, phos-phate) (Figure 9A). Only three low-temperature liquidsplot significantly off this plane (Figure 9B; one liquidsaturated with melilite and phosphate, and two liquidsin addition saturated with garnet). Consequently, theinvestigated wood ash appears to be located near apseudoquaternary thermal divide defined by melilite,garnet, and phosphate. With a high extent of crystal-lization (and decreasing temperature), the residualliquids evolve along a pseudounivariant melilite-garnet-phosphate cotectic toward increasing silica content.

The general validity and the extent to which theresults from the wood ash can be applied to other

biomass ashes can only be tested by future work.Experimental results on coal ashes have suggested thatthese are relatively well represented by the quaternaryCaO-FeO-Al2O3-SiO2 system.15 However, the resultfrom the present study of wood ash suggests thatmelting of wood ash is controlled by equilibria involvinga phosphate and silicates that cannot adequately bemodeled by known ternary and quaternary phasediagrams.37 A possible solution to this problem, and theapproach taken in this paper, is to conduct meltingexperiments on ashes of direct relevance to commonbiomass fuels. Such results may then be used toconstruct simplified diagrams and algorithms and mayallow the compositional evolution of solid and liquidphases in a specific slag to be predicted with a reason-able confidence. The results presented here are merelya first step toward this goal.

Temperature. The temperature variation in themelting experiments is well defined, and Figure 10shows this as a function of CaO content of the liquid.The results define the melilite liquidus surface fromabout 1300 to 1200 °C. We can compare the meliliteliquidus to the congruent melting point of pure aker-manite (∼40 wt % CaO, 1454 °C38). Adding FeO, Fe2O3,Na2O, and small amounts of Al2O3 to a pure åkermanitesystem will have the effect of depressing the tempera-ture for the appearance of melilite39 as reflected byFigure 10. A marked kink in the liquidus defines theonset of crystallization of phosphate at about 1200 °C,after which the liquid moves along the melilite-phosphate cotectic until reaching garnet saturation at1159 °C. This saturation temperature is significantlyhigher than would have been predicted from ternary

(36) Osborn, E. F.; Muan, A. Phase Equilibria Diagrams of OxideSystems; American Ceramic Society: Columbus, Ohio, 1960.

(37) Pierre, P. D. S. St. J. Am. Ceram. Soc. 1956, 39, 147-150.(38) Osborn, E. F.; Schairer, J. F. Am. J. Sci. 1941, 239, 715-763.(39) Schairer, J. F.; Yoder, H. S.; Tilley, C. E. Carnegie Inst. Year

Book 1965, 64, 95-100.

Figure 9. The compositions of the experimental liquids calculated in molecular proportions in terms of four components (melilite,garnet, phosphate, and “residuals”). The calculation procedure is described in the text and detailed in the Appendix. Experimentalphases are shown as large symbols, where unfilled symbols are the glasses (indicating the phase assemblage) and filled symbolsare the coexisting mineral phases. The small unfilled symbols are urban wood ashes (see Figure 1). Although drawn as projectionsof a quaternary phase diagram, the system is not and cannot be treated as a quaternary phase diagram. It merely serves as aconvenient reference frame for discussion and further work. (A) Projected on the melilite-garnet-phosphate triangle. Tie-linesare drawn between coexisting solid phases for garnet saturated assemblages. The full drawn lines with arrows are the inferredpositions of the piercing garnet-melilite, melilite-phosphate, and garnet-phosphate cosaturated relations. The direction of thearrows indicate the inferred directions of decreasing temperature. (B) Projected on the phosphate-mellilite-K4S+Or+Q triangle.Symbols as for Figure 9A. The line with arrow is the inferred four-phase cotectic where liquid coexist with melilite, garnet, andphosphate with down temperature indicated by the arrow.


eutectic relations in simplified potassium-containingsystems (710-760 °C36,40,41), but it corresponds betterto the temperatures inferred from the potassium-free,CaO-SiO2-Al2O3 ternary system (1170 °C36). Relevantexperimental data on cosaturation of phosphate andmelilite are not available. A study of the anorthite-phosphate-silica ternary of the CaO-SiO2-Al2O3-P2O5 system37 shows a cotectic between phosphate andanorthite from 1430 to 1300 °C and an eutectic below1300 °C, where silica is added to the assemblage. Thesystem CaO-MgO-SiO2-P2O5 contains an eutecticslightly below 1300 °C between phosphate, diopside, andquartz.42 No information on the effect of substitutingCaO in these systems with Na2O and K2O is available,but such substitutions are likely to significantly lowermelting temperatures. This brief discussion shows thatmelting intervals inferred from known ternary andquaternary phase diagrams cannot replace direct ex-perimental determinations.

Physical Properties. The high-temperature physi-cal properties of ashes are important variables forunderstanding the behavior of slags and for optimizingfurnace operations.3,43 Here we evaluate the density,viscosity, and surface tension of the experimental liquidsproduced from melting wood ash. The liquid density canbe estimated from partial molar volumes.43-46 Theviscosity is calculated using additive viscosity compo-nents.43,47,48 The surface tension has been estimated onthe basis of additive component relations developed byBoni and Derge49 and Mills.43 The results of thesecalculations are reported in Figure 11.

After saturation of phosphate at 1203 °C, the inter-stitial liquid in the wood ash slag shows significantdecreases in density and increases in viscosity withdecreasing temperature (Figure 11, parts A and B). Thecalculated high-temperature viscosity is sufficiently low(∼4 poise) for flow to occur on inclined or verticalfurnace walls. We can use the viscosity (η) and density(F) estimates to evaluate possible flow velocities of liquidwood ash under gravity along an inclined furnace wall.The flow velocity is inversely proportional to the kine-matic viscosity, ν ) η/F (e.g., Philpotts,50 p 17). Forreasonable slopes of furnace walls (45°-90°) and as-suming Newtonian laminar flow behavior, the surfaceflow of a 2 cm thick liquid deposition at 1300 °C can beestimated to be between 0.9 and 1.3 m/s. These veryhigh flow velocities will show little variation withtemperature down to 1200 °C because of the relativeconstancy of the densities and viscosities (Figure 11).However, since wall temperatures in most biomassfurnaces, except in refractory-lined bottom sections, are

(40) Morey, G. W.; Kracek, F. C.; Bowen, N. L. J. Soc. Glass Technol.1930, 14, 149-187.

(41) Roedder, E. Am. J. Sci. 1952, Bowen Vol., 435-456.(42) Levin, E. M.; Robbins, C. R.; McMurdie, H. F. Phase Diagrams

for Ceramists; American Chemical Society: Columbus, Ohio, 1964.(43) Mills, K. C. In Mineral Matter and Ash in Coal; Vorres, K. S.,

Ed.; American Chemical Society, Symposium Series 301: Washington,D.C., 1986; pp 195-214.

(44) Bottinga, Y.; Weill, D. F. Am. J. Sci. 1970, 269, 169-182.(45) Bottinga, Y.; Weill, D. F.; Richet, P. Geochim. Cosmochim. Acta

1982, 46, 438-475.(46) Lange, R. A.; Carmichael, I. S. E. Geochim. Cosmochim. Acta

1987, 51, 2931-2946.(47) Bottinga, Y.; Weill, D. F. Am. J. Sci. 1972, 272, 438-475.(48) Shaw, H. R. Am. J. Sci. 1972, 272, 879-893.(49) Boni, R. E.; Derge, G. J. Metals 1956, 8, 53-59.

(50) Philpotts, A. R. Principles of Igneous and Metamorphic Petrol-ogy; Prentice Hall: Englewood Cliffs, 1990.

Figure 10. Melting temperature (°C) as a function of CaOconcentrations of the experimental liquids. The curve formelilite saturation is fitted to the congruent melting point ofpure åkermanite (1454 °C). The break in the curve is definedby the melting of phosphate.

Figure 11. Estimated physical properties of the interstitialexperimental liquid as a function of melting temperature (°C).See text for calculations. (A) Density (g/cm3). (B) Viscosity (logpoise). The melting-out of melilite, phosphate, and garnet isindicated. (C) Surface tension (dyn/cm). The calculation schemeof Boni and Derge49 is based on alkali metal and phosphorus-free systems. The calculation scheme of Mills43 includes theseelements. The temperature-adjusted curve has been calculatedfrom the 1500 oC used in the calibration of Mills43 to 1300 °Cby a δγ/δT of -0.15.


kept below 1200 °C, surface melt-flow velocities aresubstantially reduced.

The crystalline load of the slag increases with de-creasing temperature below the liquidus. This willincrease the “effective” viscosity. Calculation of effectiveviscosities for crystal-laden lava flows has been at-tempted51 and, if applicable to wood ash slag, wouldsuggest an effective viscosity at 1200 °C of 100-4000poise showing, if nothing else, that the viscosity in-creases by several orders of magnitude as the solid loadof the melt increases.3 At lower temperatures and lowfractions of liquid, the movement of the interstitialliquid in the partially molten slag will be determinedby the surface tension and the porosity and permeabilityof the slag.

The surface tension (γ) of the liquid shows markeddecreases with decreasing temperature (Figure 11C).The surface tension model of Boni and Derge49 did notinclude the alkali metals and phosphorus. These ele-ments are surface active and will tend to migrate towardliquid surfaces and cause marked drops in the surfaceenergy for even small concentrations. If K2O, Na2O, andP2O5 are included in the surface tension estimates,43 theresult, as expected, shows a significant lowering insurface tension relative to the estimates based on Boniand Derge49 (Figure 11C). Since both sets of estimatesare isothermal, the positive slopes with temperature inFigure 10C are due solely to compositional effects. Theeffect of temperature on the surface tension of multi-component silicate liquids is uncertain, but is generallyassumed to be small.52,53 Mills43 suggested a negativetemperature coefficient (∂γ/∂T ) -0.15 dyne cm-1 K-1)that would imply a decrease in surface tension withincreasing temperature. He bases this value on simplebinary systems.52 Results of Walker and Mullins53 onmulticomponent natural silicate melts would implysmall positive slopes (∂γ/∂T ) 0.02-0.04 dyne cm-1

°C-1). The temperature correction applied to the iso-thermal curve in Figure 10C is based on Mill;43 however,a small positive correction53 would only result in minordifferences.

The working and pouring characteristics of slags withlow viscosities are controlled by their surface tension.Liquid slags with low surface tension pour freely.49 Theestimated relatively low surface tension of the intersti-tial liquids in the experimental slags and their lowviscosity points to significant spreading and pouringeffects of the liquid on furnace walls. Such liquids wouldalso have strong wetting capabilities that will result incorrosion and adhesion to refractories.3,54 Agglomerationis principally governed and enhanced by low surfacetension that results in strong adhesion and sintering.54

It is also possible that the liquid in a semi-molten slagmay be able to migrate downward in a permeable slag.

Application to Urban Wood Fuel Ash

The commercial urban wood fuel ashes shown inFigure 1 are used to test the proposed crystallization

model and to evaluate the potential for high-tempera-ture potassium volatilization. We calculate the norma-tive mineralogy of the fuel ashes using the sameprocedure as developed for the experimental glasses(Appendix). This method calculates simplified mineralsmelilite, phosphate, and garnet. The oxide K2O isassumed to form orthoclase or potassium tetrasilicatedependent on the availability of Al2O3 and SiO2. Theexcess K2O and SiO2 resulting from these calculationsare illustrated in Figure 12 and show that significantamounts of these oxides remain after the main cationshave been assigned likely structural components. Anyexcess silica will appear as tridymite or quartz. Thepresent experimental results for a low SiO2 wood fuelash show that the excess potassium is preferentially lostto the gas phase. As shown by Figure 12, this partition-ing of potassium into the vapor phase may occur for fuelashes with less than 47% SiO2. In addition, it is shownthat a silica phase is likely to be stable for ashes withSiO2 above 47%. This analysis provides a convenientfirst-order evaluation of the severity of potassiumevaporation from high-temperature wood ash slags.

The high-temperature physical properties can beevaluated for the wood fuel ashes using the sametechniques as for the experimental liquids. The proper-ties of most practical concern (density, viscosity, andsurface tension) are functions of temperature. An esti-mate of melting temperature, therefore, would be ad-vantageous. However, as shown above, there is atpresent no reliable method for estimating meltingtemperatures of biomass slags. A reasonable approxi-mation for liquidus temperatures for ashes with morethan 29 wt % CaO may be obtained from the liquiduscurve for melilite saturation shown in Figure 10. Reli-able estimates, however, cannot be obtained for asheswith less than 29% CaO. It is true that if the liquidussurface for melilite and phosphate saturation in Figure10 is extrapolated to low CaO contents, reasonablemelting temperature estimates for potassium and silica-rich soil compositions can be obtained (about 1000 °C).Nevertheless, it cannot be assumed that intermediatephases and thermal divides do not appear, in particular,since the modeling of the solidus mineralogy indicates

(51) McBirney, A. R.; Murase, T. Annu. Rev. Earth Planet. Sci. 1984,12, 337-357

(52) King, T. B. In Physical Chemistry of Melts; Institute of Miningand Metallurgy: London, 1953.

(53) Walker, D.; Mullins, O. Contrib. Mineral. Petrol. 1981, 76, 455-462.

(54) Miller, S. F.; Kalmanovitch, D. P. American Chemical Society,Division of Fuel Chemistry Preprints 1988, 33, 42-49.

Figure 12. The excess oxides K2O and SiO2 (mol %; calculatedas described in text and Appendix) plotted against SiO2 contentof wood fuel ash (wt %; Figure 1). Potassium is dominantlypartitioned into the vapor phase, while SiO2 is assumed toappear as a silica phase. The implication is that wood fuelashes with less than 47 wt % SiO2 may show significantvolatilization of potassium.


potassium-silicate phases (orthoclase and potassiumtetrasilicate). For this reason, all physical propertycalculations will be referenced to a temperature of 1300°C selected to be slightly above the liquidus temperaturefor the investigated wood ash with a CaO content ofabout 30 wt %.

The 1300 °C density estimate calculated from partialmolar volumes of the oxide components (Figure 13A)shows a well-defined variation from about 2.8 to 2.4g/cm3 caused mainly by the CaO variation. This calcula-tion is based on the method of Bottinga et al.,45 usingthe most recent partial molar volume estimates.46 Thecalculated densities have been compared to those ob-tained from the relatively similar method of Mills43

intended for coal fuel ashes (but with K2O added). Onlyfor high CaO ashes do the two methods give reasonablysimilar results, but deviate by up to 4% for high SiO2ashes, with the higher values calculated by the methodof Mills.43

The viscosity of a silicate liquid is commonly calcu-lated by adding the contributions of the individualtemperature-dependent viscosities of melt componentsor by empirical expressions relating viscosity to com-position and temperature.3,43,47,48,55-57 Here we evaluatethe results obtained using the methods of Shaw48 andof Urbain et al.56 as modified later by Kalmanovitch and

Frank.57 The calculated results are shown in Figure 13Bfor a liquid temperature of 1300 °C. There is a goodcorrespondence for SiO2-rich liquids indicating viscosi-ties of about 3 × 103 poise, but the correspondenceworsens significantly for CaO-rich liquids for whichviscosities are calculated to be below 1 poise based onthe method of Shaw48 and about 10 poise according tothe method of Urbain et al.56 It is unclear what causesthese differences. For both methods, the main limita-tions in the experimental viscosity data are the effectsof K2O and Fe2O3. The method of Urbain et al.56,57 iscalibrated on experimental viscosity data in the CaO-MgO-Al2O3-SiO2 system. In contrast, the model ofBottinga and Weill47 and Shaw48 includes a much largerand compositional varied database, but, admittedly, wasonly thoroughly tested for natural silicate melts usingmelt components that may not be valid for very CaO-rich wood ash compositions. Despite the uncertaintyconcerning the absolute values of the viscosity of thelow SiO2 melts, the general variation is consistentbetween the two calculation methods.

The surface tension at 1500 °C has been calculatedaccording to Mills43 and is shown in Figure 13C alongwith the 1300 °C surface tension calculated using anegative ∂γ/∂T ratio. The results suggest a minimumsurface tension ∼200 dyn/cm at a silica content of about45 wt %. The sintering potential of silicate liquids isinversely proportional to the viscosity and proportionalto the surface tension.3,54 The ratio between surfacetension (γ) and viscosity (η) can be used as a measureof the severity of sintering and agglomeration of liquidash particles on furnace surfaces.54 Ashes with low γ/ηratios are predicted to show low extents of agglomera-tion. Ashes with high γ/η ratios can be expected to resultin significant agglomeration on boiler surfaces. This isillustrated in Figure 14, where it can be seen that thehigh SiO2 wood fuel ashes predict little agglomerationpotential, while the high Ca wood ashes may causeagglomeration.

There is one major uncertainty in the estimates of thephysical properties that is of particular concern. Asshown, the alkali metals and phosphorus are partiallylost to the furnace atmosphere. This loss will affect thephysical properties of the slag. We can perform a

(55) Watt, J. D.; Fereday, F. J. Inst. Fuel 1969, 42, 99-103.(56) Urbain, G.; Cambier, F.; Deletter, M.; Anseau, M. R. Trans. J.

Br. Ceram. Soc. 1981, 80, 139-141.(57) Kalmanovitch, D. P.; Frank, M. In Mineral Matter and Ash

Deposition From Coal; Bryers, R. W., Vorres, K. S., Eds.; EngineeringFoundation Conferences, 1990; pp 89-101.

Figure 13. Estimated physical properties of urban wood fuelashes (Figure 1) at 1300 °C as functions of SiO2 (wt %). (A)Density (g/cm3) after Bottinga et al.45 and Mills.43 (B) Viscosity(log poise) after Shaw48 and Urbain et al.56 as modified byKalmanovitch and Frank.57 (C) Surface tension (dyn/cm). Thecalculation scheme of Mills43 has been used and is also shownfor 1500 °C. Dashed lines illustrate the effect of the predictedloss of alkali metals and phosphorus on the 1300 °C model(see text).

Figure 14. The ratio of surface tension divided by viscosityas a function of SiO2 concentration for urban wood fuel ash(Figure 1). This ratio is a measure of the agglomerationpotential.54 The dashed curve illustrates the effect of thepredicted loss of alkali metals and phosphorus (see text).


semiquantitative test of these effects assuming that therelative losses observed for the investigated wood fuelash applies to the high CaO ashes and that these lossesdecrease to zero for a SiO2 content of 47 wt %. Thephysical properties calculated in this way are illustratedin Figure 13 as dashed curves. As expected, the densityfor the low SiO2 ashes increases because of the relativelyhigh partial molar volumes of the alkali metals andphosphorus. The viscosity increases by up to 100% forashes below ∼25 poise. Similarly, the estimated surfacetension increases markedly for the low SiO2 ashesrelated to the loss of surface-active components. Asshown in Figure 14, the effect of alkali volatilizationappears to increase the agglomeration potential for thelow SiO2 ashes. However, more rigorous evaluations,including direct experimental determinations, of theextent to which alkali metal losses occur to the vaporphase for these melt types will be required in order tomore fully understand their high-temperature behavior.

Conclusion

Melting experiments on wood fuel ash have been usedto predict the behavior of alkali metals in commercialfuel slags. The results suggest that potassium is stronglypartitioned into the vapor phase because of excess K2Orelative to Al2O3 and SiO2 during solidification of slags.It is predicted that ashes with less than 47 wt % SiO2will show significant alkali metal losses, while asheswith higher contents of SiO2 will retain alkalis in themelt and crystalline structures. Estimates of density,viscosity, and surface tension of completely molten woodfuel slag can be based on existing information andnormalized to a constant temperature (1300 °C in thisstudy). It is predicted that urban wood fuel slag willshow a marked decrease in density and an increase inviscosity as an admixed soil component is increased. Thesurface tension will show limited variation with SiO2content. The effects of alkali loss to the vapor phase canonly be estimated semiquantitatively by extrapolationof the present experimental information. This extrapo-lation suggests that the alkali losses will have signifi-cant effects on the physical properties and will increasethe agglomeration potential for low SiO2 molten slags.The conclusion is that the presence of a soil componentin wood fuel can be advantageous in reducing high-temperature alkali losses from the slag and may helpto reduce the sintering and agglomeration of ash infurnaces burning wood fuels. However, offsetting thesepossible benefits are the negative effects of soil additionin reducing melting temperatures (and hence increasingthe probability of slag formation) and in increasing thecapacity needed, and hence cost, for ash-handling facili-ties and ash disposal.

Acknowledgment. The study was funded by theUniversity of California Energy Institute’s Energy Sci-ence and Technology Program.

Appendix

Normative Mineral Calculation Procedure forWood Fuel Ash. A molecular based procedure for

calculating a set of normative minerals has been devel-oped to mimic the mineralogy of a completely crystallinewood ash equilibrated at high temperature at its solidustemperature.

(1) Cation percentages are calculated by dividing eachoxide as given in Table 4 by the equivalent molecularweight of the oxide based on a single cation (e.g., SiO2,AlO1.5, FeO1.5, KO0.5, etc.) and normalizing to 100. Thecations are then distributed between a set of simplifiedminerals. An example, orthoclase (K2O‚Al2O3‚6SiO2) iscalculated by assuming that all K enter orthoclase.Hence, provisional orthoclase is equal to the sum of therequired cations (5 K). If Al and/or Si are deficient, theamount of orthoclase must be reduced giving excess K.Therefore, a running balance is required, and for eachstep in the following, the calculations must be adjustedfor negative cation concentrations.

(2) Calculate rutile (TiO2) as all Ti.(3) Calculate phosphate (3CaO‚P2O5) as 2.5 P and

reduce Ca by 1.5 P.(4) Calculate sodium melilite (Na2O‚2CaO‚Al2O3‚

4SiO2) as 5 Na and reduce the remaining Ca by 1 Na,Al by 1 Na, Si by 2 Na.

(5) Calculate andradite (3CaO‚Fe2O3‚3SiO2) as 4 Feand reduce the remaining Ca by 1.5 Fe, Si by 1.5 Fe.

(6) Calculate pyrope (3MgO‚Al2O3‚3SiO2) as 4 Al andreduce the remaining Mg by 1.5 Al, Si by 1.5 Al.

(7) Calculate akermanite (2CaO‚MgO‚2SiO2) as 5 Mgand reduce the remaining Ca by 2 Mg, Si by 2 Mg.

(8) Calculate rankinite (3CaO‚2SiO2) as 1.67 Ca andreduce the remaining Si by 0.67 Ca.

(9) Calculate orthoclase (K2O‚Al2O3‚6SiO2) as 5 Al andreduce the remaining K by 1 Al, Si by 3 Al.

(10) Calculate potassium tetrasilicate (K2O‚4SiO2) as3 K and reduce the remaining Si by 2 K.

(11) Calculate residual K2O as 1 K.(12) Calculate residual SiO2 as 1 Si.The residual concentrations of Al, Ca, Mg, and Na

should be low or zero, and are a measure of the successof the calculation scheme. Si, Ti, Fe, K, and P willalways be zero. The total of calculated normativeminerals including residual oxides should be 100. Anexample of the calculations is given in Table A1.

EF980249Q

Table A1. Normal Mineralogy and Physical Variables fora Fused Urban Wood Fuel Ash (Table 3, Glass Analysis

No. 36)

wt %oxides

normativemineralogy

residualcomponents

SiO2 35.17 rutile 0.58 SiO2 0.00TiO2 0.85 phosphate 8.16 Al2O3 0.00Al2O3 8.61 Na melilite 35.27 K2O 5.50Fe2O3 4.98 andradite 13.47 CaO 3.78MnO 0.44 pyrope 8.25 MgO 1.64MgO 6.77 akermanite 23.35 Na2O 0.00CaO 31.29 rankinite 0.00 total 100.00Na2O 4.05 orthoclase 0.00K2O 4.80 K2O‚4SiO2 0.00 density 2.70 (g/cc)P2O5 4.29 viscosity 0.0898 (log poise)total 101.25 surface tension 320 (dyn/cm)
