Surface Tension Measurements of Coal Ash Slags under Reducing Conditions at Atmospheric Pressure

4540r 2009 American Chemical Society pubs.acs.org/EF

Energy Fuels 2009, 23, 4540–4546 : DOI:10.1021/ef900449vPublished on Web 08/11/2009

Surface Tension Measurements of Coal Ash Slags under Reducing Conditions at

Atmospheric Pressure

Tobias Melchior,* Gunther Putz, and Michael Muller

Institute of Energy Research, Forschungszentrum Julich GmbH, Julich, Germany

Received May 13, 2009. Revised Manuscript Received July 16, 2009

The global demand for reduced CO2 emission from power plants can be answered by coal gasificationtechniques. To develop integrated gasification combined cycles that incorporate hot syngas cleaningfacilities, detailed knowledge of the thermophysical properties of coal ashes is imperative. Currently, thesurface tension of liquid coal ash slags in a reducing environment was studied by means of the sessile dropmethod. Three different algorithms were employed to analyze the acquired drop images. The slags underconsideration were obtained from black and brown coals as well as from an experimental gasificationreactor. Typically, a sharp surface tension decrease with temperature was found in the melting interval ofthe ashes. This was followed by a temperature range of smooth drop contours during which a slight rise ofthe surface tension could mostly be observed. Bubbles at the circumference of the drops started to appearwhen approaching the measurement temperature limit of 1550 �C.With regard to the temperature regimeof uncorrugated drop profiles, coal ash slags exhibited surface tension values between 400 and 700 mN/m.

1. Introduction

Theglobal demand for a reductionofCO2emissions resultedin the development of new power plant technologies thatpermit a controlled removal of CO2 from the process ratherthan release it alongwith other exhaust gases in an unrestrictedfashion. One technology allowing for the separation of CO2 isdiscussed in literature under the nameof integrated gasificationcombined cycle (IGCC).1-4 This process unites the well-estab-lished combined cycle power plant (consisting of a gas turbine,a heat recovery boiler, and a steam turbine) and a coalgasification facility in order to generate electricity. At its end,the gasification procedure produces pure hydrogen that can befed to the gas turbine instead of natural gas or oil.

As the synthesis gas (syngas) that consists mainly of CO,H2, and H2O leaves the gasification vessel, a CO2 separationunit can be installed downstream of a CO shift reactor. Thestep of separating carbon dioxide fromhydrogenmust thenbefollowedby facilities for compression and safe storage ofCO2.One crucial challenge in this kind of process is to achieve ahigh cleanliness of the syngas. Because of temperatures risingup to 1800 �C in the gasifier, the ash contained in the coaloccurs in liquid form (slag). Particles of this highly corrosiveslag will be entrained by the syngas and therefore present adanger to down-end equipment such as coolers, shift reactors,and gas turbines.

Up to now, coal gasifiers rely on water quenching installa-tions in order to cool down the syngas, resulting in a

solidification of the entrained slag. The glassy particles canthen be withdrawn easily from the process. Unfortunately,this approach provokes efficiency losses as the highest possi-ble syngas temperature is desired for the gas turbine cycle.Hotgas cleaning facilities must be developed aimed at the reduc-tion of heat losses in IGCCs.5One such technique that alreadyis proved to work is pressurized pulverised coal combustion(PPCC), which makes use of ceramic balls being integratedinto the flow path of syngas.6 The assembly of balls acts as akind of filter onwhich the slag particles deposit and then draindown by the force of gravity. As a result, the coal ash can beremoved from the process in liquid formand the syngas canbefed to subsequent steps at unaltered temperatures.

The successful design of such hot gas cleaning installationsdepends on the availability of thermophysical data of coal ashslags. Viscosity and surface tension are only two propertiesthat are relevant to fluid flow of slags and slag ceramicinteractions (wetting). In published literature, surface tensiondata of coal ash slags are limited. Raask7 found a value of320 mN/m for a relevant slag in the temperature range from1300 to 1400 �C.VariousAmerican coals were investigated byFalcone,8 who found that surface tensions increased from364 to 1489 mN/m in the temperature interval from 1225 to1285 �C. This author assumed a pronounced influence ofsodium such that a tripling of the sodium content led to adoubling of the surface tension value. In addition, Falconepointed out that systematic studies of model systems areessential for interpreting surface tensions. Mills and Rhine9

*To whom correspondence should be addressed. E-mail: [email protected].(1) Kanniche, M.; Bouallou, C. Appl. Therm. Eng. 2007, 27, 2693–

2702.(2) Pruschek,R.; Oeljeklaus,G.; Brand,V.;Haupt,G.; Zimmermann,

G.; Ribberink, J. S. Energy Convers. Manage. 1995, 36, 797–800.(3) Kwong, K. S.; Petty, A.; Bennett, J. P.; Krabbe, R.; Thomas, H.

Int. J. of Appl. Ceram.Technol. 2007, 4, 503–513.(4) Bennett, J. P.; Powell, C. Molten 2009 - Proceedings of the VIII

International Conference on Molten Slags, Fluxes and Salts, Santiago,Chile, 2009; pp 1323-1333.

(5) Muller, M.; Pavone, D.; Rieger, M.; Abraham, R. Fourth Inter-national Conference on Clean Coal Technologies, Dresden, Germany,May 2009.

(6) Forster, M.; Hannes, K.; Teloken, R. VGB PowerTech 2001, 81,30–35.

(7) Raask, E. J. Eng. Power 1966, 88, 40–44.(8) Falcone, S. K.Ash and Slag Characterization . Final Report for the

Period Ending March 31, 1986, Grand Forks, North Dakota, USA,June 1986.

(9) Mills, K. C.; Rhine, J. M. Fuel 1989, 68, 193–200.

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Energy Fuels 2009, 23, 4540–4546 : DOI:10.1021/ef900449v Melchior et al.

used vitreous carbon as the substrate material when measur-ing the surface tension of a gasifier slag according to the sessiledrop method. They found a value of 430 mN/m for atemperature of 1350 �C.Nowok10,11 also used the sessile droptechnique to study coal ashes and model glasses. The authorpublished surface tension data ranging from190 to 960mN/mfor different temperature intervals and gas atmospheres.

Real coal ash slags as well as synthetic model systems mustbe studied to extend property databases. The surface tensiondata for real slags presented within this article can be used forthe validation of property prediction tools derived fromsystematic measurements of synthetic slags. With regard toprocess design, a surface tension of slag may finally limit theusage of the corresponding coal in the gasification process.A detailed understanding of the dependence of the surfacetension on coal constituents will widen the range of applicablecoals through the employment of additives.

2. Experimental Section

2.1. Measurement Fundamentals and Setup. The facility usedto perform the current surface tension measurements fullycomplies with sessile drop arrangements frequently cited inliterature.10,12-16 In these arrangements, a drop of the liquidunder consideration rests on a certain substrate material andallows for a surface tension calculation by analyzing its shape.This method is based on the Young-Laplace equation (eq 1)that describes a surface contour of liquid in fully generalizedform. In this equation σ refers to the surface tension; R1 and R2

are the main radii of curvature of the surface; F1 and Fg denotethe density of the liquid and the density of the surrounding gasphase, respectively; g depicts the (constant) gravitational accel-eration; and z is a height coordinate measured from a referenceplane.

σ1

R1þ 1

R2

� �¼ ðFl -FgÞgz ð1Þ

When using the sessile drop technique, it is assumed that thedrop has axial symmetry. Respecting this limitation, Bashforthand Adams17 as well as Hartland and Hartley18 numericallyintegrated the Young-Laplace equation to which no analyticalsolution exists and tabulated plenty of drop parameters. Theformer approaches to axisymmetric drop shape analysis (ADSA)aimed at manually finding the coordinates of drop contourpoints from the pictures of a drop of liquid. The numerical tablesthen allowed for a determination of the liquid’s surface tension.Nowadays, computer algorithms perform an analysis of dropimages, integrate the Young-Laplace equation, and fit it to thedetected drop profile. The computer codes finally provide valuesfor the capillary constant c or the dimensionless shape parameterβ as defined by eqs 2 and 3, respectively. In eq 3, r denotes the

curvature radius at the drop’s apex.

c ¼ ðFl -FgÞgσ

ð2Þ

β ¼ ðFl -FgÞgr2σ

ð3Þ

As the ADSA algorithms additionally provide results for theradius r and the drop volume, the surface tension σ can bederived if the sample mass is known. The density of thesurrounding gas phase was neglected when analyzing the dropimages obtained during current experiments because it is severalorders of magnitude lower than the slag density. It should benoted that calculating the slag density from an average samplemass (taken before and after the measurement), as presentlydone, leads to an accumulation of algorithm errors.

Currently, three different software packages were employedto analyze all acquired drop images. Details on those codes aregiven in the following section. Apart from finding capillaryconstants or shape parameters, values for the contact anglebetween the circumference of a drop and the substrate line areprovided by the algorithms.

When the sessile drop method is applied to measure surfacetensions at room temperature, syringe systems can be used toexactly dose an amount of liquid onto a substrate material. Thisdrop creation technique is not straightforward for the analysisof highmelting substances. To perform the present experiments,all coal ashes to be studiedwere therefore pressed into ash pelletsof 5 mm diameter and of approximately 5 mm height. A pelletpressing force of 1kN leads to the smoothest drop contours.

These pellets were placed into a high temperature (Tmax=1550 �C), high pressure (pmax=20 bar) furnace on a graphitesubstrate. A CCD camera attached to a zoom lens is directedinto this furnace and connected to a framegrabber in an analysiscomputer. Most ashes under investigation start to form liquiddrops at temperatures of about 1250 �C, with heat radiationplaying an important role. Background illumination is conse-quently not needed during the experiment, but it helps to alignthe sample inside the furnace at room temperature. A clear viewof the drop can be achieved by combining an infrared cutofffilter (reduction of blurred contours) and a polarizing filter(reduction of reflections) in front of the zoom lens. The safetyglass of the furnace mainly filters out ultraviolet radiationaccording to the manufacturer. Adjusting the aperture of thezoom lens to its maximally closed setting yields best results.A schematic of the measurement setup is provided in Figure 1.

Figure 1. Schematic of the measurement setup.

(10) Nowok, J. W.; Bieber, A. J.; Benson, A. S.; Jones, M. L. Fuel1991, 70, 951–956.(11) Nowok, J. W.; Hurley, J. P.; Bieber, A. J. J. Mater. Sci. 1995, 30,

361–364.(12) Hoorfar, M.; Neumann, A. W. Adv. Colloid Interface Sci. 2006,

121, 25–49.(13) Mehta, A. S.; Sahajwalla, V. Scand. J. Metall. 2000, 29, 17–29.(14) Tanaka, T.; Nakamoto, M.; Oguni, R.; Lee, J.; Hara, S. Z.

Metallkd. 2004, 95, 818–822.(15) Clare, A. G.; Kucuk, A.; Wing, D. R.; Jones, L. E.High Tem-

perature Glass Melt Property Database for Process Modeling; 2005; pp119-130.(16) Applied Surface Thermodynamics; Neumann, A. W., Spelt, J. K.,

Eds.; Marcel Dekker: NewYork, 1996; Vol. 63.(17) Bashforth, F.; Adams, C. J. An Attempt to Test the Theories of

Capillary Action: by Comparing the Theoretical and Measured Forms ofDrops of Fluid; Cambridge University Press: Cambridge, 1883.(18) Hartland, S.; Hartley, R. W. Axisymmetric Fluid-Liquid Inter-

faces; Elsevier Scientific Publishing Company: Amsterdam, Oxford,New York, 1976.

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To perform an automated drop image acquisition, a softwarerunning on the analysis computer was developed. This measure-ment program communicates with the furnace temperaturecontroller as well as with the pressure controller. By doing so,the conditions under which drop images are recorded can besaved automatically. During a sessile drop measurement thepressure inside the furnace was always kept at a constant level.A flow of 100mL/min of argon hydrogen gas (4 vol%H2 in Ar)assured reducing atmospheric conditions. At the beginning ofeach experiment, the furnace was heated at a rate of 10 �C/minuntil a temperature of about 50 �C below the melting point ofthe sample was reached. From this point onward the heatingrate was decreased to a constant value of 2 �C/min beingretained up to Tmax.

At the time of heating rate reduction, the image acquisitionprocess was initiated. The computer utility is designed to storepictures at temperature intervals of 1 �C in an uncompressedbitmap format. Upon arrival of a new temperature interval, thesoftware waits ten seconds before snapping four different dropimages shortly one after another. These photos differ withrespect to the contrast and brightness settings of the framegrab-ber. Generally spoken, the so-called “Slave2” image is richest incontrast whereas the “Master” and “Slave1” images becomeincreasingly darker. By way of example, Figure 2 shows all threedrop images for a slag at 1347 �C. A common feature of thementioned photos is that they are corrected for optical distor-tion. Prior to performing surface tension measurements, ahighly accurate dot grid was therefore used to obtain calibrationinformation on the optical system. The fourth image beingnamed “UncalibratedMaster” exhibits the same contrast andbrightness settings asMaster but has no calibration informationapplied to it. All photos have a resolution of 768� 576 pixels,and a higher number of image points is not thought to increaseaccuracy considerably.12

2.2. Drop Shape Analysis Algorithms. Axisymmetric dropshape analysis is a challenging approach for measuring surfacetensions from a numerical point of view,12,19 and thus threedifferent algorithms were used to calculate surface tensionvalues from the acquired drop images. All programs performedan analysis of all four image series (Master, Slave1, Slave2,UncalibratedMaster) produced during every measurement. Be-cause the codes sometimes failed to analyze certain drop images,the number of results generated for each experiment differedfrom algorithm to algorithm. Surface tension values shownwithin this article represent an arithmetic average of the imageseries’ surface tensions grouped by temperature interval. Anaveraging of the individual computing software’s results was notperformed as their outputs occasionally differed too much.

The first algorithm, being named “ADSA” like the proce-dure itself, is commercially sold and frequently discussed inliterature.12,16,19-22 This software follows the classical approach

and fits a numerically integrated set of Young-Laplace equa-tions to the drop profile found in an image. The position of thesubstrate line needs to be givenmanually to the software becausethere is no visible reflection of the drop shape into the substratein the currently acquired photos.

Another drop shape analysis code, “SCA20”, is comparableto ADSA since it is commercially sold, performs a numericalintegration of the Young-Laplace equation, and requires userinteraction to correctly find the substrate line. SCA20 wasdeveloped by DataPhysics in Germany23 and accompaniescontact angle measuring devices sold by this firm. Apart fromusing SCA20 to communicate with such instruments, it per-forms an automatic drop shape analysis if the correspondingimages are assembled into an AVI video file. When processingthe output of SCA20, the surface tension needs to be calculatedfrom the shape parameter β (eq 3), whereas the other algorithmsrely on the capillary constant c (eq 2).

Contrary to both codes presented so far, “LBADSA” uses thecalculation results of one drop image to initialize the iteration onthe subsequent picture. By doing so it does not implement theexact Young-Laplace equation as realized by ADSA andSCA20, instead it fits an approximate solution obtained bysmall perturbation theory24 to the drop profile. The adaption isperformed byminimizing an energy function that is known fromactive contours or snakes.25,26 LBADSA can be found freely onthe Internet in a compiled version.27 It comes along as a Javaplugin for the image processing toolbox “ImageJ”28 and is notdesigned to process multiple drop images in its original form.Through cooperation with the author of LBADSA it waspossible to modify the source code in order to perform anautomated analysis of image series.

As none of the presented algorithms produce errorless results,it is necessary to check the output data for physical plausibility.Detailed explanation of the program failures cannot be given. Ifa certain drop image led to an inconsistent calculation, the dataset of the correspondingalgorithmwas fully neglected.The checkfor physical plausibility includes filtering out of data sets inwhich negative capillary constants or shape parameters oc-curred. Furthermore, negative drop volumes and contact angleslower than 0� or larger than 180� are reasons for omission. Thecalculated surface tension is additionally limited to values below1500 mN/m, as erroneously small drop volumes lead to extre-mely high slag densities that boost σ according to eqs 2 or 3.

2.3. Ashes. The substances under investigation can be dividedinto three categories: black coal ashes, brown coal ashes, and

Figure 2. Different photos taken of a sessile drop at a time.

(19) Hoorfar, M.; Neumann, A. W. J. Adhes. 2004, 80, 727–743.(20) delRio, O. I.; Neumann,A.W. J. Colloid Interface Sci. 1997, 196,

136–147.(21) Cheng, P.; Li, D.; Boruvka, L.; Rotenberg, Y.; Neumann, A. W.

Colloids Surf. 1990, 43, 151–167.(22) Cheng, P.; Neumann, A. W. Colloids Surf. 1992, 62, 297–305.

(23) Homepage of DataPhysics Instruments GmbH. URI: http://www.dataphysics.de (Accessed: April 24, 2009).

(24) Mathematical Tools for Physicists; Trigg, G. L., Ed.; Wiley-VCH:Weinheim, 2005.

(25) Stalder, A. F.; Kulik, G.; Sage, D.; Barbieri, L.; Hoffmann, P.Colloids Surf., A 2006, 286, 92–103.

(26) Jacob, M.; Blu, T.; Unser, M. IEEE Trans. Image Process 2004,13, 1231–1244.

(27) Stalder, A. F.; Biomedical Imaging Group of �Ecole Polytechni-que F�ed�erale de Lausanne,Drop Shape Analysis . Free Software for highprecision contact angle measurement. URI: http://bigwww.epfl.ch/demo/dropanalysis (Accessed: April 24, 2009).

(28) ImageJ . Image Processing and Analysis in Java. URI: http://rsbweb.nih.gov/ij (Accessed: April 24, 2009).

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slags froman experimental gasification facility. Table 1 providesthe compositions of the studied systems in weight percentages ofequivalent oxides calculated from elementary analysis. Theblack coals originate from mines in Germany (ST-D), Norway(ST-N), Poland (ST-P), and Columbia (K2-4). All brown coals(HKT, K2-1, K3-1) can be found in opencast mines inGermany.

Except for HKT and the slags (S1-1, S1-2, S1-4), allsubstances were ashed at 815 �C in air. The HKT brown coalwas handled in the same way, but the temperature was reducedto 450 �C. All slags were received as medium-sized, glassy piecesthat were milled down to powder form. The resulting ashes aswell as the slag powder were sent for elementary analysis andfinally pressed into pellets before performing sessile drop ex-periments.

3. Results

To judge the capability of the setup (hardware and soft-ware) to produce reliable data, gold was chosen as referencematerial. Melting of a pure substance allows for a verificationof the furnace’s temperature display in addition to comparingmeasured surface tension data to literature values. Gold’smeltingpoint of 1065 �C29,30 permits a surface tension analysisin the entire temperature interval relevant to coal ash slags.Figure 3 presents the measurement results of all three algo-rithms in the form of linear functions fitted to the data points.In addition, the linear dependence of the surface tension ofgold on temperature suggested by Keene30 is provided in thediagram. A discussion of the results of this referencemeasure-ment is given below.

During heating of the furnace, most of the ash pellets underconsideration show shrinking before finally transforming intoliquid phase. The resulting drops can then be analyzed usingall three algorithms, which leads to diagrams like that inFigure 4. It was found that SCA20s data typically scatters alot,whereasLBADSAandADSAare in goodagreementwitheach other. Looking at the nonscattered values in Figure 4, acommon progression of surface tension with temperaturebecomes obvious. Largely negative gradients occur at lowertemperatures and are followed by a range of constant (about430 mN/m) or slightly rising surface tensions. This behaviorcan be observed for nearly all substances studied so far.

Asa consequenceof the characteristic surface tension trend,eq 4 was used to fit the measurement data. T therein refers tothe temperature in degrees Celsius, and a-e denote the fittingparameters. The adaptation procedure was performed by aLevenberg-Marquardtmethod for nonlinear optimization.31

SCA20s output needed to be excluded from this fit because ofheavy scattering. For ADSA, making use of the exactYoung-Laplace equation, its results were given a weight of60%, while LBADSA had a weight of 40%.

σðTÞ ¼a þ b 1000

T

� �þ c 1000

T

� �2

1 þ d 1000T

� �þ e 1000

T

� �2ð4Þ

Figure 5 shows the temperature dependence of the surfacetension for German black coal ashes obtained by processingthemeasurement data in theway just described. Except for the

Table 1. Ash/Slag Compositions in Equivalent Oxides (wt %)

Al2O3 BaO CaO Fe2O3 K2O MgO Mn2O3 Na2O SiO2 TiO2

ST-D-1 26.26 0.09 2.94 10.72 2.53 2.32 0.16 1.04 43.86 1.00ST-D-2 20.03 0.21 7.00 9.58 2.17 3.98 0.23 1.35 40.43 0.80ST-D-3 26.83 0.12 4.06 8.58 3.37 2.32 0.09 1.24 42.79 0.98ST-D-4 25.70 0.11 3.50 7.58 3.49 2.82 0.12 0.80 47.92 0.95ST-D-5 23.43 0.18 3.64 7.44 3.98 2.82 0.11 1.00 49.85 0.92ST-D-6 29.29 0.19 3.50 20.45 1.69 1.53 0.13 2.02 39.15 0.53ST-N-1 13.42 0.28 11.05 9.58 1.45 3.81 0.04 2.83 37.22 0.85ST-N-2 20.22 0.31 9.74 10.31 2.78 2.67 0.02 5.47 41.93 1.03ST-P-1 24.00 0.25 6.80 7.99 2.96 4.26 0.14 2.16 46.85 1.14K2-4 23.62 0.31 3.01 7.48 3.00 2.01 0.05 3.63 56.27 0.92

HKT 17.01 0.12 12.03 2.15 0.63 4.98 0.04 2.29 44.50 1.10K2-1 2.12 0.39 24.07 9.58 0.15 7.84 0.16 1.90 38.08 0.43K3-1 3.10 0.09 7.07 13.07 0.55 2.26 0.07 0.08 56.91 0.23

S1-1 3.02 0.15 19.45 8.02 0.27 5.64 0.14 1.52 60.54 0.30S1-2 24.75 0.67 12.62 4.86 0.93 2.34 0.08 1.78 48.14 0.48S1-4 12.28 0.34 15.11 7.62 1.49 4.31 0.08 4.77 52.41 0.52

Figure 3. Measured surface tension of pure gold in comparison toliterature.

(29) Egry, I.; Schwartz, E.; Szekely, J.; Jacobs, G.; Lohoefer, G.;Neuhaus, P. Metall. Mater. Trans.B 1998, 29B, 1031–1035.(30) Keene, B. J. Int. Mater. Rev. 1993, 38, 157–192.

(31) Press, W. H.; Teukolsky, S. A.; Vetterling,W. T.; Flannery, B. P.Numerical Recipes: TheArt of Scientific Computing, Third ed.; CambridgeUniversity Press: Cambridge, 2007.

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ST-D-5 ash, all samples follow sharply decreasing and slightlyincreasing surface tensions. The peak at about 1480 �C in theST-D-5 measurement can be attributed to bubble formationat the drop circumference as well as to an obvious loss of axialsymmetry.Measurement values around this temperature havea high uncertainty. Nevertheless, the corresponding data canalso be represented by eq 4. Likewise, the fit of ST-D-6 resultshad to be aborted at 1340 �C because the drop continuouslyinflated and collapsed from this point onward. In the case ofthe ST-D-3 investigation, a lack of data prevails as the sessiledrop slipped off the graphite substrate at 1420 �C. The lastsurface tension value found for this ash is 451 mN/m.

Looking at the findings for foreign black coal ashes inFigure 6,melting point differences aremuchmore pronounced

than for German ones. Surface tension values vary from 400to 700mN/mwith respect to themajormeasurement intervals,whereas 500 mN/m represents the corresponding upper limitfor German black coal ashes. The Colombian K2-4 ash fullydisobeys the trend of decreasing surface tension while meltingand reveals the lowest σ values in all performed experiments(215 mN/m). This behavior is connected with an inflatingash pellet, forming a very unbalanced large “drop” (beyondthe field of view of the camera) that collapsed and stabilizedat 1378 �C. Astonishingly, the outcomes for Norwegian ashesST-N-1 and ST-N-2 deviate a lot from each other, althoughboth samples were taken from the same pit (Spitsbergen). Ithas to be noted that the specimens were mined at differenttimes (years), which may account for a change in compositionand the alternating surface tension behavior. In the case of theST-N-1 ash, the sample pellet almost instantly turned into astable sessile drop. In contrast, the ST-N-2 pellet took muchmore time for this conversion.

The German brown coal ashes under investigation gener-ally revealed higher surface tensions (up to 700 mN/m apartfrom melting) than those found for German black coal ashesaccording to Figure 7. HKT and K3-1 fully correspond tothe functional relationship described above, in contrast, theK2-1 sample behaves like the ST-D-5 black coal ash inFigure 5. Like in the ST-D-5 case, a locally corrugated dropprofile can be observed and is responsible for the deviatingcalculation results. Despite these problematic characteristicsof the K2-1 ash, the presented fit model was applied success-fully again. Calculating the regression curve for measurementdata for HKT required neglecting values beyond 1420 �C asthe slag shows better wetting with the graphite from thistemperature onward. Improved wetting directly leads tolower contact angles between the drop and the substrate,which in turn flatten the droplet shape. In agreement withliterature,12,19,32 flat sessile drops pose difficulties for all kinds

Figure 4. Results of different algorithms for ST-D-1 coal ash.

Figure 5. Results for German black coal ashes.

Figure 6. Results for foreign black coal ashes.

(32) Melchior, T.; Muller, M.Molten 2009 - Proceedings of the VIIIInternational Conference on Molten Slags, Fluxes and Salts, Santiago,Chile, 2009; pp 161-170.

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of ADSA algorithms, and heavy scatter in the output is self-evident under such circumstances.

One of the three slags originating from an experimentalgasification facility does not show the distribution of surfacetension found formost of the specimens (Figure 8). This S1-4slag is characterized by a largely inflating drop during themelting temperature interval, followed by a period of decreasein drop size at which the presented measurement data wascalculated.At about 1340 �C the sample starts to inflate again,which is accompanied by an improved wetting of the sub-strate. Due to the corresponding scattering of algorithmoutput, the regression curve is not defined for higher tem-peratures. A more general view of the results in Figure 8suggests that the slag surface tensions range from 440 to

540 mN/m (melting excluded), which is in accordance withthose found for black and brown coal ashes.

4. Discussion

The surface tension model derived by fitting eq 4 to themeasurement data can be divided into three temperatureintervals as shown in Figure 9. Because the behavior of thedrops in the particular segments is comparable for differentslags, most of the substances investigated so far follow theillustrated trend of surface tension.

Interval I is characterized by a shrinking ash pellet thatfinallymelts to form a drop of liquid slag. The drop formationstarts with rounding of the pellet top edges and a constrictionof its bottom diameter. Consequently, a drawn-out dropshape can be observed that flattens and becomes increasinglyround with rising temperature. This evolution of the dropleads to highly negative surface tension gradients in the firstinterval. The position of the corresponding branch of thefitting curve must be regarded as an indicator for the slagmelting temperature range. As drawn-out contours are notcommon for sessile drop experiments, the remarkably highsurface tensions may also result from the inability of theanalysis algorithms to deal with such images. Furthermore,a lack of axial symmetry partly observed in the first intervalcan be responsible for the tendency of the codes to computehigh surface tension values. The very first image to benumerically processed was always selected based on the factthat all pellet edges showed evidence of rounding.

The transition to interval II is marked by a stabilization ofthe drop’s contour and low surface tension gradients. A well-deformed drop in terms of literature12,14,19,33,34 can be ob-served in this period of the measurement. Such drop shapesallow for the calculation of very reliable results using ADSA.With regard to Figure 4, in which an almost constant surfacetension of about 430 mN/m between 1300 and 1450 �C can be

Figure 7. Results for German brown coal ashes.

Figure 8. Results for slags from experimental gasification facility.

Figure 9. Surface tension model for coal ash slags in stylized form.

(33) Lee, J.; Kiyose, A.; Nakatsuka, S.; Nakamoto, M.; Tanaka, T.ISIJ Int. 2004, 44, 1793–1799.

(34) Jimbo, I.; Cramb, A. W. ISIJ Int. 1992, 32, 26–35.

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noticed, positive gradients in the second interval might beoverestimated by the fitting model. A rise of the surfacetension in the subsequent temperature interval (which maybe caused by scattered calculation results) pulls the regressioncurve toward higher values in interval II as well. Nevertheless,data for the surface tensionof coal ash slags under gasificationconditions should primarily be taken from this part of thegraph. The current results are supported by the findings ofNowok10,11 that also suggest a surface tension increase withtemperature. Measurement values published by this authorare of the same magnitude as the results presented within thisarticle. From the current point of view it is therefore ambig-uous whether the assumption of rising surface tensions ispreferable to constant values in interval II.

Defining an explicit temperature for the changeover frominterval II to interval III is not straightforward becausescattering of measurement data only occurs step by step.The strong deviation of surface tension values from a certaintrend in interval III is mainly due to bubble formation at thedrop’s circumference (corrugated profile) as well as contin-uous inflation and collapse of the samples (loss of axialsymmetry). Gaseous species seem to form and blow up thedrop from inside. CO or CO2 may evolve from a reactionbetween graphite and oxides in the slags.9 The existence ofbubbles cannot be traced back to the inclusion of gas duringsample preparation (e.g., ashing of coal) because the speci-mens S1-1 to S1-4 showed exactly the same behavior. Likementioned in the previous section, the last temperature inter-val sometimes needs to be excluded from the regressionanalysis for the described reasons.

Apart from interpreting the surface tension results with thehelp of the stylized diagram inFigure 9, attentionmust also bepaid to possible measurement errors. Determining the surfacetension of pure gold (Figure 3) made it obvious that theADSA algorithm performs best on the drop images. LBAD-SA also reflects the surface tension decrease with temperaturecorrectly but exhibits larger errors when compared to thereference. The SCA20 code does not even reveal a decrease ofthe surface tension of gold. This is in accordance with the datapresented in Figure 4 and gives further evidence for theexclusion of SCA20 when fitting eq 4. Calculating the relativemeasurement errors for each algorithm leads to themaximumvalues presented in Table 2. “Based on data points” thereinmeans that the relative errorwas derived for each temperatureinterval (average of four drop images) before choosing themaximumdeviation. The quantities for “based on regression”can be obtained by comparing the algorithm’s linear regres-sion curves to the reference one. As regression analysis wasalso performed for coal ash slags, those values are an indica-tion for the measurement error contained in Figures 5-8.

Besides flaws in the computer codes, the hardware setupalso influences the measurement results. One possible source

of error is given by the alignment of the substrate line.A deviation of the graphite substrate from its horizontalposition inside the furnace influences the ADSA as the algo-rithms currently do not correct such a tilt.34 Further measure-ment errors may arise from visual distortion that is not fullyremoved by the calibration procedure. Movements of thesample in the course of the experiment can lead to a locationof the drop in undesired flats along the optical axis. Despitethe application of optical filters and due to heat radiation,contours sometimes appear blurred, which is thought toaccount for imperfect drop profile detection.

The liquefaction of gold additionally revealed a tempera-ture discrepancy of 20 �C.As the position of the thermocoupleand the location of the sample differed by some millimeters,the furnace controller overestimates the drop temperature. InFigure 3 this deviation is corrected by shifting the measure-ment data to lower temperatures whereas all other resultsdiagrams within this article still exhibit the variation. Tem-peratures above the melting point of gold may lead todifferences higher than 20 �C that cannot be uncovered by asingle reference substance. Therefore, the surface tension datapresented within this article may exhibit minor errors in thetemperature correlation.

5. Conclusions

The surface tension data of coal ash slags are essential forthe development of hot gas cleaning facilities for IGCCpowerplants and thus can be successfully measured by the sessiledrop method. Sixteen different substances were melted in ahigh temperature furnace under reducing conditions, and thecorresponding drop images were processed using three differ-ent drop shape analysis algorithms. Two of the computercodes produced results that agreed well with each other,whereas the third program’s output was neglected due toheavy scattering. All valid data was fitted to a nonlinearregression curve representing surface tension as a functionof temperature.

As similar surface tension trends were observed for nearlyall of the samples, three characteristic temperature intervalsdescribing the state of the slag could be derived. Duringmeltingof the initial ashpellet, largelynegative surface tensiongradients can be found. Those are followed by almost con-stant surface tension values that originated from stable, well-deformed drops. When approaching temperatures of up to1550 �C, drop profiles becomemore andmore corrugated as aresult of gas formation inside the slag. Consequently, surfacetensions were scattered greatly and exhibited an increasingtendency.

Being affected by the lowest uncertainty in measurements,the second temperature interval showed surface tensionsranging from400 to 700mN/m for the substances investigatedso far. This data can be used to validatemodels in conjunctionwith property databases that predict surface tensions from theslag composition.

Acknowledgment. The authors thank Bundesministerium furWirtschaft und Technologie for supporting this work in theframework of the HotVeGas-EM project (FKZ 0327773C).

Table 2. Maximum Relative Errors of Each Image Analysis Algo-

rithm (%)

ADSA LBADSA SCA20

based on data points 8.6 19.4 54.2based on regression 4.9 14.7 22.4

Documents

Surface Tension Measurements of Coal Ash Slags under Reducing Conditions at Atmospheric Pressure