Energy Fuels 2009, 23, 4540–4546 Published on Web 08/11/2009
: DOI:10.1021/ef900449v
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 gasification techniques. To develop integrated gasification combined cycles that incorporate hot syngas cleaning facilities, detailed knowledge of the thermophysical properties of coal ashes is imperative. Currently, the surface tension of liquid coal ash slags in a reducing environment was studied by means of the sessile drop method. Three different algorithms were employed to analyze the acquired drop images. The slags under consideration were obtained from black and brown coals as well as from an experimental gasification reactor. Typically, a sharp surface tension decrease with temperature was found in the melting interval of the ashes. This was followed by a temperature range of smooth drop contours during which a slight rise of the surface tension could mostly be observed. Bubbles at the circumference of the drops started to appear when approaching the measurement temperature limit of 1550 °C. With regard to the temperature regime of uncorrugated drop profiles, coal ash slags exhibited surface tension values between 400 and 700 mN/m.
solidification of the entrained slag. The glassy particles can then be withdrawn easily from the process. Unfortunately, this approach provokes efficiency losses as the highest possible syngas temperature is desired for the gas turbine cycle. Hot gas cleaning facilities must be developed aimed at the reduction of heat losses in IGCCs.5 One such technique that already is proved to work is pressurized pulverised coal combustion (PPCC), which makes use of ceramic balls being integrated into the flow path of syngas.6 The assembly of balls acts as a kind of filter on which the slag particles deposit and then drain down by the force of gravity. As a result, the coal ash can be removed from the process in liquid form and the syngas can be fed to subsequent steps at unaltered temperatures. The successful design of such hot gas cleaning installations depends on the availability of thermophysical data of coal ash slags. Viscosity and surface tension are only two properties that are relevant to fluid flow of slags and slag ceramic interactions (wetting). In published literature, surface tension data of coal ash slags are limited. Raask7 found a value of 320 mN/m for a relevant slag in the temperature range from 1300 to 1400 °C. Various American coals were investigated by Falcone,8 who found that surface tensions increased from 364 to 1489 mN/m in the temperature interval from 1225 to 1285 °C. This author assumed a pronounced influence of sodium such that a tripling of the sodium content led to a doubling of the surface tension value. In addition, Falcone pointed out that systematic studies of model systems are essential for interpreting surface tensions. Mills and Rhine9
1. Introduction The global demand for a reduction of CO2 emissions resulted in the development of new power plant technologies that permit a controlled removal of CO2 from the process rather than release it along with other exhaust gases in an unrestricted fashion. One technology allowing for the separation of CO2 is discussed in literature under the name of integrated gasification combined cycle (IGCC).1-4 This process unites the well-established combined cycle power plant (consisting of a gas turbine, a heat recovery boiler, and a steam turbine) and a coal gasification facility in order to generate electricity. At its end, the gasification procedure produces pure hydrogen that can be fed 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 separation unit can be installed downstream of a CO shift reactor. The step of separating carbon dioxide from hydrogen must then be followed by facilities for compression and safe storage of CO2. One crucial challenge in this kind of process is to achieve a high cleanliness of the syngas. Because of temperatures rising up to 1800 °C in the gasifier, the ash contained in the coal occurs in liquid form (slag). Particles of this highly corrosive slag will be entrained by the syngas and therefore present a danger to down-end equipment such as coolers, shift reactors, and gas turbines. Up to now, coal gasifiers rely on water quenching installations in order to cool down the syngas, resulting in a *To whom correspondence should be addressed. E-mail: t.melchior@ fz-juelich.de. (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. r 2009 American Chemical Society
(5) Muller, M.; Pavone, D.; Rieger, M.; Abraham, R. Fourth International 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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used vitreous carbon as the substrate material when measuring the surface tension of a gasifier slag according to the sessile drop method. They found a value of 430 mN/m for a temperature of 1350 °C. Nowok10,11 also used the sessile drop technique to study coal ashes and model glasses. The author published surface tension data ranging from 190 to 960 mN/m for different temperature intervals and gas atmospheres. Real coal ash slags as well as synthetic model systems must be studied to extend property databases. The surface tension data for real slags presented within this article can be used for the validation of property prediction tools derived from systematic measurements of synthetic slags. With regard to process design, a surface tension of slag may finally limit the usage of the corresponding coal in the gasification process. A detailed understanding of the dependence of the surface tension on coal constituents will widen the range of applicable coals through the employment of additives.
curvature radius at the drop’s apex. ðFl - Fg Þg c ¼ σ β ¼
ðFl - Fg Þgr2 σ
ð2Þ ð3Þ
As the ADSA algorithms additionally provide results for the radius r and the drop volume, the surface tension σ can be derived if the sample mass is known. The density of the surrounding gas phase was neglected when analyzing the drop images obtained during current experiments because it is several orders of magnitude lower than the slag density. It should be noted that calculating the slag density from an average sample mass (taken before and after the measurement), as presently done, leads to an accumulation of algorithm errors. Currently, three different software packages were employed to analyze all acquired drop images. Details on those codes are given in the following section. Apart from finding capillary constants or shape parameters, values for the contact angle between the circumference of a drop and the substrate line are provided by the algorithms. When the sessile drop method is applied to measure surface tensions at room temperature, syringe systems can be used to exactly dose an amount of liquid onto a substrate material. This drop creation technique is not straightforward for the analysis of high melting substances. To perform the present experiments, all coal ashes to be studied were therefore pressed into ash pellets of 5 mm diameter and of approximately 5 mm height. A pellet pressing 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 graphite substrate. A CCD camera attached to a zoom lens is directed into this furnace and connected to a framegrabber in an analysis computer. Most ashes under investigation start to form liquid drops at temperatures of about 1250 °C, with heat radiation playing an important role. Background illumination is consequently not needed during the experiment, but it helps to align the sample inside the furnace at room temperature. A clear view of the drop can be achieved by combining an infrared cutoff filter (reduction of blurred contours) and a polarizing filter (reduction of reflections) in front of the zoom lens. The safety glass of the furnace mainly filters out ultraviolet radiation according to the manufacturer. Adjusting the aperture of the zoom lens to its maximally closed setting yields best results. A schematic of the measurement setup is provided in Figure 1.
2. Experimental Section 2.1. Measurement Fundamentals and Setup. The facility used to perform the current surface tension measurements fully complies with sessile drop arrangements frequently cited in literature.10,12-16 In these arrangements, a drop of the liquid under consideration rests on a certain substrate material and allows 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 generalized form. In this equation σ refers to the surface tension; R1 and R2 are the main radii of curvature of the surface; F1 and Fg denote the density of the liquid and the density of the surrounding gas phase, respectively; g depicts the (constant) gravitational acceleration; and z is a height coordinate measured from a reference plane. 1 1 σ ¼ ðFl - Fg Þgz þ ð1Þ R1 R2 When using the sessile drop technique, it is assumed that the drop has axial symmetry. Respecting this limitation, Bashforth and Adams17 as well as Hartland and Hartley18 numerically integrated the Young-Laplace equation to which no analytical solution exists and tabulated plenty of drop parameters. The former approaches to axisymmetric drop shape analysis (ADSA) aimed at manually finding the coordinates of drop contour points from the pictures of a drop of liquid. The numerical tables then allowed for a determination of the liquid’s surface tension. Nowadays, computer algorithms perform an analysis of drop images, integrate the Young-Laplace equation, and fit it to the detected drop profile. The computer codes finally provide values for the capillary constant c or the dimensionless shape parameter β as defined by eqs 2 and 3, respectively. In eq 3, r denotes the (10) Nowok, J. W.; Bieber, A. J.; Benson, A. S.; Jones, M. L. Fuel 1991, 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 Temperature Glass Melt Property Database for Process Modeling; 2005; pp 119-130. (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 of Drops of Fluid; Cambridge University Press: Cambridge, 1883. (18) Hartland, S.; Hartley, R. W. Axisymmetric Fluid-Liquid Interfaces; Elsevier Scientific Publishing Company: Amsterdam, Oxford, New York, 1976.
Figure 1. Schematic of the measurement setup.
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Figure 2. Different photos taken of a sessile drop at a time.
and fits a numerically integrated set of Young-Laplace equations to the drop profile found in an image. The position of the substrate line needs to be given manually to the software because there is no visible reflection of the drop shape into the substrate in the currently acquired photos. Another drop shape analysis code, “SCA20”, is comparable to ADSA since it is commercially sold, performs a numerical integration of the Young-Laplace equation, and requires user interaction to correctly find the substrate line. SCA20 was developed by DataPhysics in Germany23 and accompanies contact angle measuring devices sold by this firm. Apart from using SCA20 to communicate with such instruments, it performs an automatic drop shape analysis if the corresponding images are assembled into an AVI video file. When processing the output of SCA20, the surface tension needs to be calculated from the shape parameter β (eq 3), whereas the other algorithms rely on the capillary constant c (eq 2). Contrary to both codes presented so far, “LBADSA” uses the calculation results of one drop image to initialize the iteration on the subsequent picture. By doing so it does not implement the exact Young-Laplace equation as realized by ADSA and SCA20, instead it fits an approximate solution obtained by small perturbation theory24 to the drop profile. The adaption is performed by minimizing an energy function that is known from active contours or snakes.25,26 LBADSA can be found freely on the Internet in a compiled version.27 It comes along as a Java plugin for the image processing toolbox “ImageJ”28 and is not designed to process multiple drop images in its original form. Through cooperation with the author of LBADSA it was possible to modify the source code in order to perform an automated 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. If a certain drop image led to an inconsistent calculation, the data set of the corresponding algorithm was fully neglected. The check for physical plausibility includes filtering out of data sets in which negative capillary constants or shape parameters occurred. Furthermore, negative drop volumes and contact angles lower than 0° or larger than 180° are reasons for omission. The calculated surface tension is additionally limited to values below 1500 mN/m, as erroneously small drop volumes lead to extremely high slag densities that boost σ according to eqs 2 or 3. 2.3. Ashes. The substances under investigation can be divided into three categories: black coal ashes, brown coal ashes, and
To perform an automated drop image acquisition, a software running on the analysis computer was developed. This measurement program communicates with the furnace temperature controller as well as with the pressure controller. By doing so, the conditions under which drop images are recorded can be saved automatically. During a sessile drop measurement the pressure inside the furnace was always kept at a constant level. A flow of 100 mL/min of argon hydrogen gas (4 vol % H2 in Ar) assured reducing atmospheric conditions. At the beginning of each experiment, the furnace was heated at a rate of 10 °C/min until a temperature of about 50 °C below the melting point of the sample was reached. From this point onward the heating rate was decreased to a constant value of 2 °C/min being retained up to Tmax. At the time of heating rate reduction, the image acquisition process was initiated. The computer utility is designed to store pictures at temperature intervals of 1 °C in an uncompressed bitmap format. Upon arrival of a new temperature interval, the software waits ten seconds before snapping four different drop images shortly one after another. These photos differ with respect to the contrast and brightness settings of the framegrabber. Generally spoken, the so-called “Slave2” image is richest in contrast whereas the “Master” and “Slave1” images become increasingly darker. By way of example, Figure 2 shows all three drop images for a slag at 1347 °C. A common feature of the mentioned photos is that they are corrected for optical distortion. Prior to performing surface tension measurements, a highly accurate dot grid was therefore used to obtain calibration information on the optical system. The fourth image being named “UncalibratedMaster” exhibits the same contrast and brightness settings as Master but has no calibration information applied to it. All photos have a resolution of 768 576 pixels, and a higher number of image points is not thought to increase accuracy considerably.12 2.2. Drop Shape Analysis Algorithms. Axisymmetric drop shape analysis is a challenging approach for measuring surface tensions from a numerical point of view,12,19 and thus three different algorithms were used to calculate surface tension values from the acquired drop images. All programs performed an analysis of all four image series (Master, Slave1, Slave2, UncalibratedMaster) produced during every measurement. Because the codes sometimes failed to analyze certain drop images, the number of results generated for each experiment differed from algorithm to algorithm. Surface tension values shown within this article represent an arithmetic average of the image series’ surface tensions grouped by temperature interval. An averaging of the individual computing software’s results was not performed as their outputs occasionally differed too much. The first algorithm, being named “ADSA” like the procedure itself, is commercially sold and frequently discussed in literature.12,16,19-22 This software follows the classical approach
(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 Polytechnique Federale de Lausanne, Drop Shape Analysis . Free Software for high precision 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).
(19) Hoorfar, M.; Neumann, A. W. J. Adhes. 2004, 80, 727–743. (20) del Rio, 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.
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Table 1. Ash/Slag Compositions in Equivalent Oxides (wt %) Al2O3
BaO
CaO
Fe2O3
K2O
MgO
Mn2O3
Na2O
SiO2
TiO2
ST-D-1 ST-D-2 ST-D-3 ST-D-4 ST-D-5 ST-D-6 ST-N-1 ST-N-2 ST-P-1 K2-4
26.26 20.03 26.83 25.70 23.43 29.29 13.42 20.22 24.00 23.62
0.09 0.21 0.12 0.11 0.18 0.19 0.28 0.31 0.25 0.31
2.94 7.00 4.06 3.50 3.64 3.50 11.05 9.74 6.80 3.01
10.72 9.58 8.58 7.58 7.44 20.45 9.58 10.31 7.99 7.48
2.53 2.17 3.37 3.49 3.98 1.69 1.45 2.78 2.96 3.00
2.32 3.98 2.32 2.82 2.82 1.53 3.81 2.67 4.26 2.01
0.16 0.23 0.09 0.12 0.11 0.13 0.04 0.02 0.14 0.05
1.04 1.35 1.24 0.80 1.00 2.02 2.83 5.47 2.16 3.63
43.86 40.43 42.79 47.92 49.85 39.15 37.22 41.93 46.85 56.27
1.00 0.80 0.98 0.95 0.92 0.53 0.85 1.03 1.14 0.92
HKT K2-1 K3-1
17.01 2.12 3.10
0.12 0.39 0.09
12.03 24.07 7.07
2.15 9.58 13.07
0.63 0.15 0.55
4.98 7.84 2.26
0.04 0.16 0.07
2.29 1.90 0.08
44.50 38.08 56.91
1.10 0.43 0.23
S1-1 S1-2 S1-4
3.02 24.75 12.28
0.15 0.67 0.34
19.45 12.62 15.11
8.02 4.86 7.62
0.27 0.93 1.49
5.64 2.34 4.31
0.14 0.08 0.08
1.52 1.78 4.77
60.54 48.14 52.41
0.30 0.48 0.52
slags from an experimental gasification facility. Table 1 provides the compositions of the studied systems in weight percentages of equivalent oxides calculated from elementary analysis. The black 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 in Germany. Except for HKT and the slags (S1-1, S1-2, S1-4), all substances were ashed at 815 °C in air. The HKT brown coal was handled in the same way, but the temperature was reduced to 450 °C. All slags were received as medium-sized, glassy pieces that were milled down to powder form. The resulting ashes as well as the slag powder were sent for elementary analysis and finally pressed into pellets before performing sessile drop experiments.
3. Results To judge the capability of the setup (hardware and software) to produce reliable data, gold was chosen as reference material. Melting of a pure substance allows for a verification of the furnace’s temperature display in addition to comparing measured surface tension data to literature values. Gold’s melting point of 1065 °C29,30 permits a surface tension analysis in the entire temperature interval relevant to coal ash slags. Figure 3 presents the measurement results of all three algorithms in the form of linear functions fitted to the data points. In addition, the linear dependence of the surface tension of gold on temperature suggested by Keene30 is provided in the diagram. A discussion of the results of this reference measurement is given below. During heating of the furnace, most of the ash pellets under consideration show shrinking before finally transforming into liquid phase. The resulting drops can then be analyzed using all three algorithms, which leads to diagrams like that in Figure 4. It was found that SCA20s data typically scatters a lot, whereas LBADSA and ADSA are in good agreement with each other. Looking at the nonscattered values in Figure 4, a common progression of surface tension with temperature becomes obvious. Largely negative gradients occur at lower temperatures and are followed by a range of constant (about 430 mN/m) or slightly rising surface tensions. This behavior can be observed for nearly all substances studied so far.
Figure 3. Measured surface tension of pure gold in comparison to literature.
As a consequence of the characteristic surface tension trend, eq 4 was used to fit the measurement data. T therein refers to the temperature in degrees Celsius, and a-e denote the fitting parameters. The adaptation procedure was performed by a Levenberg-Marquardt method for nonlinear optimization.31 SCA20s output needed to be excluded from this fit because of heavy scattering. For ADSA, making use of the exact Young-Laplace equation, its results were given a weight of 60%, while LBADSA had a weight of 40%. 2 1000 a þ b 1000 þ c T T ð4Þ σðTÞ ¼ 2 1000 1000 1þd T þe T Figure 5 shows the temperature dependence of the surface tension for German black coal ashes obtained by processing the measurement data in the way just described. Except for the (31) Press, W. H.; Teukolsky, S. A.; Vetterling,W. T.; Flannery, B. P. Numerical Recipes: The Art of Scientific Computing, Third ed.; Cambridge University Press: Cambridge, 2007.
(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.
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Figure 4. Results of different algorithms for ST-D-1 coal ash.
Figure 6. Results for foreign black coal ashes.
than for German ones. Surface tension values vary from 400 to 700 mN/m with respect to the major measurement intervals, whereas 500 mN/m represents the corresponding upper limit for German black coal ashes. The Colombian K2-4 ash fully disobeys the trend of decreasing surface tension while melting and reveals the lowest σ values in all performed experiments (215 mN/m). This behavior is connected with an inflating ash pellet, forming a very unbalanced large “drop” (beyond the field of view of the camera) that collapsed and stabilized at 1378 °C. Astonishingly, the outcomes for Norwegian ashes ST-N-1 and ST-N-2 deviate a lot from each other, although both samples were taken from the same pit (Spitsbergen). It has to be noted that the specimens were mined at different times (years), which may account for a change in composition and the alternating surface tension behavior. In the case of the ST-N-1 ash, the sample pellet almost instantly turned into a stable sessile drop. In contrast, the ST-N-2 pellet took much more time for this conversion. The German brown coal ashes under investigation generally revealed higher surface tensions (up to 700 mN/m apart from melting) than those found for German black coal ashes according to Figure 7. HKT and K3-1 fully correspond to the functional relationship described above, in contrast, the K2-1 sample behaves like the ST-D-5 black coal ash in Figure 5. Like in the ST-D-5 case, a locally corrugated drop profile can be observed and is responsible for the deviating calculation results. Despite these problematic characteristics of the K2-1 ash, the presented fit model was applied successfully again. Calculating the regression curve for measurement data for HKT required neglecting values beyond 1420 °C as the slag shows better wetting with the graphite from this temperature onward. Improved wetting directly leads to lower contact angles between the drop and the substrate, which in turn flatten the droplet shape. In agreement with literature,12,19,32 flat sessile drops pose difficulties for all kinds
Figure 5. Results for German black coal ashes.
ST-D-5 ash, all samples follow sharply decreasing and slightly increasing surface tensions. The peak at about 1480 °C in the ST-D-5 measurement can be attributed to bubble formation at the drop circumference as well as to an obvious loss of axial symmetry. Measurement values around this temperature have a high uncertainty. Nevertheless, the corresponding data can also be represented by eq 4. Likewise, the fit of ST-D-6 results had to be aborted at 1340 °C because the drop continuously inflated and collapsed from this point onward. In the case of the ST-D-3 investigation, a lack of data prevails as the sessile drop slipped off the graphite substrate at 1420 °C. The last surface tension value found for this ash is 451 mN/m. Looking at the findings for foreign black coal ashes in Figure 6, melting point differences are much more pronounced
(32) Melchior, T.; Muller, M. Molten 2009 - Proceedings of the VIII International Conference on Molten Slags, Fluxes and Salts, Santiago, Chile, 2009; pp 161-170.
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Figure 7. Results for German brown coal ashes.
Figure 9. Surface tension model for coal ash slags in stylized form.
540 mN/m (melting excluded), which is in accordance with those found for black and brown coal ashes. 4. Discussion The surface tension model derived by fitting eq 4 to the measurement data can be divided into three temperature intervals as shown in Figure 9. Because the behavior of the drops in the particular segments is comparable for different slags, most of the substances investigated so far follow the illustrated trend of surface tension. Interval I is characterized by a shrinking ash pellet that finally melts to form a drop of liquid slag. The drop formation starts with rounding of the pellet top edges and a constriction of its bottom diameter. Consequently, a drawn-out drop shape can be observed that flattens and becomes increasingly round with rising temperature. This evolution of the drop leads to highly negative surface tension gradients in the first interval. The position of the corresponding branch of the fitting curve must be regarded as an indicator for the slag melting temperature range. As drawn-out contours are not common for sessile drop experiments, the remarkably high surface tensions may also result from the inability of the analysis algorithms to deal with such images. Furthermore, a lack of axial symmetry partly observed in the first interval can be responsible for the tendency of the codes to compute high surface tension values. The very first image to be numerically processed was always selected based on the fact that all pellet edges showed evidence of rounding. The transition to interval II is marked by a stabilization of the drop’s contour and low surface tension gradients. A welldeformed drop in terms of literature12,14,19,33,34 can be observed in this period of the measurement. Such drop shapes allow for the calculation of very reliable results using ADSA. With regard to Figure 4, in which an almost constant surface tension of about 430 mN/m between 1300 and 1450 °C can be
Figure 8. Results for slags from experimental gasification facility.
of ADSA algorithms, and heavy scatter in the output is selfevident under such circumstances. One of the three slags originating from an experimental gasification facility does not show the distribution of surface tension found for most of the specimens (Figure 8). This S1-4 slag is characterized by a largely inflating drop during the melting temperature interval, followed by a period of decrease in drop size at which the presented measurement data was calculated. At about 1340 °C the sample starts to inflate again, which is accompanied by an improved wetting of the substrate. Due to the corresponding scattering of algorithm output, the regression curve is not defined for higher temperatures. A more general view of the results in Figure 8 suggests that the slag surface tensions range from 440 to
(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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of error is given by the alignment of the substrate line. A deviation of the graphite substrate from its horizontal position inside the furnace influences the ADSA as the algorithms currently do not correct such a tilt.34 Further measurement errors may arise from visual distortion that is not fully removed by the calibration procedure. Movements of the sample in the course of the experiment can lead to a location of the drop in undesired flats along the optical axis. Despite the application of optical filters and due to heat radiation, contours sometimes appear blurred, which is thought to account for imperfect drop profile detection. The liquefaction of gold additionally revealed a temperature discrepancy of 20 °C. As the position of the thermocouple and the location of the sample differed by some millimeters, the furnace controller overestimates the drop temperature. In Figure 3 this deviation is corrected by shifting the measurement data to lower temperatures whereas all other results diagrams within this article still exhibit the variation. Temperatures above the melting point of gold may lead to differences higher than 20 °C that cannot be uncovered by a single reference substance. Therefore, the surface tension data presented within this article may exhibit minor errors in the temperature correlation.
Table 2. Maximum Relative Errors of Each Image Analysis Algorithm (%) based on data points based on regression
ADSA
LBADSA
SCA20
8.6 4.9
19.4 14.7
54.2 22.4
noticed, positive gradients in the second interval might be overestimated by the fitting model. A rise of the surface tension in the subsequent temperature interval (which may be caused by scattered calculation results) pulls the regression curve toward higher values in interval II as well. Nevertheless, data for the surface tension of coal ash slags under gasification conditions should primarily be taken from this part of the graph. The current results are supported by the findings of Nowok10,11 that also suggest a surface tension increase with temperature. Measurement values published by this author are of the same magnitude as the results presented within this article. From the current point of view it is therefore ambiguous whether the assumption of rising surface tensions is preferable to constant values in interval II. Defining an explicit temperature for the changeover from interval II to interval III is not straightforward because scattering of measurement data only occurs step by step. The strong deviation of surface tension values from a certain trend in interval III is mainly due to bubble formation at the drop’s circumference (corrugated profile) as well as continuous inflation and collapse of the samples (loss of axial symmetry). Gaseous species seem to form and blow up the drop from inside. CO or CO2 may evolve from a reaction between graphite and oxides in the slags.9 The existence of bubbles cannot be traced back to the inclusion of gas during sample preparation (e.g., ashing of coal) because the specimens S1-1 to S1-4 showed exactly the same behavior. Like mentioned in the previous section, the last temperature interval sometimes needs to be excluded from the regression analysis for the described reasons. Apart from interpreting the surface tension results with the help of the stylized diagram in Figure 9, attention must also be paid to possible measurement errors. Determining the surface tension of pure gold (Figure 3) made it obvious that the ADSA algorithm performs best on the drop images. LBADSA also reflects the surface tension decrease with temperature correctly but exhibits larger errors when compared to the reference. The SCA20 code does not even reveal a decrease of the surface tension of gold. This is in accordance with the data presented in Figure 4 and gives further evidence for the exclusion of SCA20 when fitting eq 4. Calculating the relative measurement errors for each algorithm leads to the maximum values presented in Table 2. “Based on data points” therein means that the relative error was derived for each temperature interval (average of four drop images) before choosing the maximum deviation. The quantities for “based on regression” can be obtained by comparing the algorithm’s linear regression curves to the reference one. As regression analysis was also performed for coal ash slags, those values are an indication for the measurement error contained in Figures 5-8. Besides flaws in the computer codes, the hardware setup also influences the measurement results. One possible source
5. Conclusions The surface tension data of coal ash slags are essential for the development of hot gas cleaning facilities for IGCC power plants and thus can be successfully measured by the sessile drop method. Sixteen different substances were melted in a high temperature furnace under reducing conditions, and the corresponding drop images were processed using three different drop shape analysis algorithms. Two of the computer codes produced results that agreed well with each other, whereas the third program’s output was neglected due to heavy scattering. All valid data was fitted to a nonlinear regression curve representing surface tension as a function of temperature. As similar surface tension trends were observed for nearly all of the samples, three characteristic temperature intervals describing the state of the slag could be derived. During melting of the initial ash pellet, largely negative surface tension gradients can be found. Those are followed by almost constant surface tension values that originated from stable, welldeformed drops. When approaching temperatures of up to 1550 °C, drop profiles become more and more corrugated as a result of gas formation inside the slag. Consequently, surface tensions were scattered greatly and exhibited an increasing tendency. Being affected by the lowest uncertainty in measurements, the second temperature interval showed surface tensions ranging from 400 to 700 mN/m for the substances investigated so far. This data can be used to validate models in conjunction with property databases that predict surface tensions from the slag composition. Acknowledgment. The authors thank Bundesministerium fur Wirtschaft und Technologie for supporting this work in the framework of the HotVeGas-EM project (FKZ 0327773C).
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