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Measurement of the Viscosity of Coal-Derived Slag Using Thermomechanical Analysis Bart J. P. Buhre,* Gregory J. Browning, Rajender P. Gupta, and Terry F. Wall Cooperative Research Centre for Coal in Sustainable Development, Chemical Engineering, The University of Newcastle, Callaghan NSW 2308, Australia Received October 25, 2004. Revised Manuscript Received March 2, 2005
This study describes an experimental technique to determine the viscosity of high-temperature ash samples, using a thermomechanical analysis (TMA) apparatus. The experimental technique was validated at low temperature by measuring the viscosity of a synthetic oil and comparing its viscosity with the calibrated value. Validation at high temperatures was achieved by comparing the measured viscosity of ash samples with the viscosity measurements obtained using a conventional high-temperature rotating bob viscometer. The results indicate that the technique can rapidly provide an indication of the viscosity of slags at high temperatures and could prove to be an alternative, cost-effective technique to current high-temperature ash sample viscosity measurement techniques.
Introduction In an integrated gasification combined cycle (IGCC), pulverized coal is gasified and the gas obtained from gasification is used in a gas turbine to generate heat to produce steam for power generation. In entrained flow gasifiers, coal is typically gasified at high temperatures (>1400 °C) and pressures (20-30 atm).1 During gasification, ash is produced from the inorganic matter that is present in the coal. A proportion of the ash deposits on the gasifier wall and forms a slag layer, which flows down the refractory wall under the force of gravity and out of the bottom of the gasifier into a water quenching system. When the ash sample is cooled and solidifies, crystals are formed and the viscosity becomes nonNewtonian at a temperature that is known as the temperature of critical viscosity (Tcv). At Tcv, the solids in the slag increase the viscosity substantially. When the temperature is above Tcv, coal-ash slags behave as Newtonian fluids, which makes the viscosity independent of the shear rate. As a rule of thumb, the desirable slag viscosity at the tapping temperature (∼1400-1500 °C) should be ∼15-25 Pa s for entrained flow gasifiers and 5 Pa s for fixed-bed gasifiers.2,3 Entrained flow gasifiers have limited refractory life, and certain types of gasifiers rely on a minimum slag thickness on the refractory wall to protect it from the aggressive environment.4 This minimum layer thickness results in specific requirements for the ash content and/ or slag viscosity. The constraints on the behavior of the inorganic matter in coal determine, in part, the suc* Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Innes, K. Coal Quality Handbook for IGCC, Technology Assessment Report 8, Cooperative Research Centre for Black Coal Utilisation, Newcastle, NSW, Australia, 1999, p 62. (2) Corey, R. C. Measurement and Significance of the Flow Properties of Coal-Ash Slag. Bull.sU.S. Bur. Mines 1964, 618, 64. (3) Harris, D. J.; Patterson, J. H. Use of Australian Bituminous Coals in IGCC Power Generation Technologies. Aust. Inst. Energy News 1995, 13 (1), 22-32.
cessful operation of IGCC plants and result in a need to experimentally determine the ash sample viscosities at temperatures relevant to gasification. Experimental Techniques to Measure Ash Sample Viscosity Currently, there are several high-temperature viscometers that can be used to measure the viscosity of ash samples. An extensive literature review prepared by Vargas et al. provides detailed descriptions and experimental results obtained using these devices.5 The current study does not attempt to provide a review of experimental techniques; it only highlights the technique most commonly used, which is the rotating bob viscometer. Figure 1 provides a schematic of the rotating bob viscometer. The cup, cylindrical bob, and ash are heated in a furnace until the ash is liquid, and the viscosity of the slag is calculated from the torque required to rotate the bob, the dimensions of the bob and crucible, and the angular velocity.5 In a typical experiment, coal ash is melted at a temperature of ∼100 °C above its liquidus temperature and is allowed to equilibrate with its gaseous environment for ∼30 min.6 The viscosity is determined at progressively lower temperatures, which allows time for equilibration before each measurement. This procedure is repeated to obtain a viscosity curve for the slag. The procedure involves several steps and is rather difficult. However, accurate measurements can be obtained using this technique,5 and the experimental (4) Ploeg, J. E. G. New Shell Coal Gasification ProjectssImpact of Feed Composition and Syngas Application on Plant Design. Presented at the Eighteenth Annual International Pittsburgh Coal Conference, Newcastle, NSW, Australia, 2001. (5) Vargas, S.; Frandsen, F. J.; Dam-Johansen, K.; Rheological Properties of High-Temperature Melts of Coal Ashes and Other Silicates. Prog. Energy Combust. Sci. 2001, 27, 237-429. (6) Harris, D. J.; Novak, F.; Patterson, J. H. Viscosity Measurements and Empirical Predictions for Some Model Gasifier Slags. Fuel 1999, 78, 439-444.
10.1021/ef0497311 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/05/2005
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Figure 1. Schematic drawing of a rotating cylinder viscometer.
uncertainty for viscosity measurement has been reported to be (10%, as determined using careful calibration and molybdenum components.7 Although accurate results can be attained, the experiment is time-consuming. The experiments also require significant amounts of slag; typically, some 100 g of slag is required for each experiment. This study proposes the use of thermomechanical analysis (TMA) equipment to determine the slag viscosity at a particular temperature more rapidly, more cost effectively, and with less sample required. Thermomechanical Analysis (TMA) as a Tool To Determine Ash Heating Behavior TMA has been used in the past as a tool to determine the heating characteristics of ash and, in particular, the fusibility of coal ash samples (see, for example, the work of Bryant and co-workers8,9 and Wall et al.10). The TMA apparatus has been described in previous studies.9 The apparatus is capable of applying pressure on a ram into a sample holder and accurately determining its displacement, as a function of time, as it penetrates the sample. The TMA apparatus is capable of accomplishing this feat at temperatures up to 2400 °C, which is sufficient for measurement at temperatures that are experienced inside typical gasifiers. The basic principle of the technique is that, as a ram is forced into the slag, the slag is forced out of the bottom of the assembly into the annular region between the ram and the crucible wall. The viscosity of the slag can be calculated from a measurement of the ram velocity and the length and thickness of the annular region. A schematic diagram of the sample assembly is given in Figure 2. As the ram penetrates, the volumetric flow rate out of the crucible (QT) is given by
QT ) π(κR)2u
Figure 2. Schematic diagram of the sample assembly.
where κ is the ratio of the radius of the ram to the internal radius of the crucible, R is the internal radius of the crucible (given in meters), and u is the velocity of the ram into the sample (in units of m/s). The flow rate of a viscous flow through an annulus (QA) has been wellestablished and can be calculated using11
QA )
]
(2)
where µ is the viscosity of the slag (in units of Pa s), L is the length of the annular region (given in meters), and ∆P is the pressure drop over the length of the annulus, which is given by
∆P )
mg - FgL π(κR)2
(3)
where m is the mass applied to the ram (given in kilograms), g is the acceleration due to gravity (in units of m/s2), and F is the density of the slag (in units of kg/ m3). The value of FgL is negligible, compared to the pressure exerted by the ram; thus, ∆P becomes
∆P ≈
mg π(κR)2
(4)
As the ram penetrates into the sample, the length of the annulus (L) is given by the initial length (L0) plus the displacement of the ram (d), or
(1)
(7) Slag Atlas; Verein Deutscher Eisenhu¨ttenleute, Verlag Stahleisen: Du¨sseldorf, Germany, 1995. (8) Bryant, G. W.; Lucas, J. A.; Gupta, S. K.; Wall, T. F. Use of Thermomechanical Analysis to Quantify the Flux Additions Necessary for Slag Flow in Slagging Gasifiers Fired with Coal. Energy Fuels 1998, 12, 257-261. (9) Bryant, G. W.; Browning, G. J.; Emanuel, H.; Gupta, S. K.; Gupta, R. P.; Lucas, J. A.; Wall, T. F. The Fusibility of Blended Coal Ash. Energy Fuels 2000, 14, 316-325. (10) Wall, T. F.; Gupta, S. K.; Gupta, R. P.; Sanders, R. H.; Creelman, R. A.; Bryant, G. W. False Deformation Temperatures for Ash Fusibility Associated with the Conditions for Ash Preparation. Fuel 1999, 78, 1057-1063.
[
(1 - κ2)2 π∆PR4 (1 - κ4) 8µL ln(1/κ)
L ) L0 + d
(5)
Substituting eqs 4 and 5 into eq 2 gives the relation
QA )
[
]
(1 - κ2)2 mgR2 4 (1 κ ) ln(1/κ) 8κ2µ(L0 + d)
(6)
In addition, the drag effect of the ram on the liquid must be taken into consideration. To determine this (11) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena; Wiley: New York, 1960.
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contribution, the equation for annular flow with an inner cylinder moving axially is used:11
QB )
[
]
πR2u 1 - κ2 - 2κ2 2 ln(1/κ)
(7)
This flow, which is induced by the drag of the ram (eq 7) is in the opposite direction to that described by eq 6; therefore, the total flow out of the crucible can be calculated using
Q T ) QA - QB
(8)
Substitution of eqs 1, 6, and 7 into eq 8 gives
π(κR)2u )
[
]
(1 - κ2)2 mgR2 4 (1 κ ) ln(1/κ) 8κ2µ(L0 + d)
[
]
2 πR2u (1 - κ ) - 2κ2 (9) 2 ln(1/κ)
By rearranging for µ, eq 9 reduces to the following relation:
µ)
[
]
ln(1/κ)(1 + κ2) - (1 - κ2) mg 4πu(L0 + d) κ2
(10)
This results in the calculation of the viscosity of the sample as a function of the pressure applied to the ram and the dimensions of the sample holder and the ram. By heating the sample in the sample holder and measuring its penetration rate, the sample viscosity can be calculated. Because the sample holder and ram are small (the ram diameter and penetration length are typically 6 and 11 mm, respectively), the sample can be heated rapidly and is able to quickly equilibrate with its gaseous environment. Experimental Section Experiments were performed in a Setaram model TMA92 thermomechanical analyzer. Two sets of experiments were performed: (i) low-temperature experiments, to determine the accuracy of the technique using an oil with known viscosity, and (ii) high-temperature experiments, in which the slag viscosities were measured and compared with the viscosities determined using a conventional rotating bob viscometer.6 During the low-temperature experiments, the crucible was filled with ∼50 mg of Brookfield viscosity standard oil, with a viscosity of 97.280 Pa s at 25 °C. For the high-temperature experiments, the crucibles were filled with ∼140 mg of slag powder. Molybdenum was selected as the material for the crucibles and rams, because of its high melting point (2610 °C), and previous studies have indicated that the material is suitable for experiments of this type in an inert environment.12,13 The ram was inserted into the crucible, and the sample was compacted by applying a mass of 750 g onto the ram. After compaction, the entire assembly was placed into the TMA apparatus. After the apparatus was purged with argon, a load of 1 g was applied to the ram, to hold the assembly in place. The assembly was heated at 50 (12) Bryant, G. W.; Browning, G. J.; Gupta, S. K.; Lucas, J. A.; Gupta, R. P.; Wall, T. F. Thermomechanical Analysis of Coal Ash: The Influence of the Material for the Sample Assembly. Energy Fuels 2000, 14, 326-335. (13) Browning, G. J. Measurement and Prediction of the Viscosity of Slag from Coal Ash, Chemical Engineering Ph.D. Thesis, The University of Newcastle, Newcastle, Australia, 2002, p 181.
Figure 3. Typical viscosity plot, as a function of time, using a viscosity standard oil at ambient temperature. °C/min to the desired test temperature and maintained at that temperature for some time, to equilibrate the sample with the gas (high-purity argon) and ensure that a homogeneous molten slag has been obtained. A time period of 1 h seemed sufficient to eliminate any fluctuations resulting from thermal expansion. A load of 50 g was then applied, and the displacement and time was recorded in intervals of 0.4 s. The viscosity of the slags at high temperature was obtained as described by Hurst et al.6 Their experimental procedure and setup can be briefly described as follows. Viscosity measurements were made under mildly reducing conditions, with a Haake model 1700 high-temperature rotational viscometer, using molybdenum rotors and crucibles. A sacrificial graphite holder was used to ensure the removal of any oxygen in the nitrogen gas stream. The melts were heated to a temperature ∼100 °C above the previously determined melt temperature, and viscosity measurements were taken at intervals of 30 °C during stepwise cooling until the melt recrystallized. An equilibration time period of ∼30 min was allowed at each temperature. The rotor speed and the torque on the rotor, were recorded and processed using the Haake Rotovisco software.
Results and Discussion Low-Temperature Experiments. The low-temperature experiments were performed at ambient temperature and were repeated several times. Figure 3 displays the results of a successful experiment, displaying a constant viscosity during the experiment. The first portion of the experiment is discarded, because only a small load (1 g) is applied to the assembly and the displacement is too small to determine the viscosity accurately. The dark line in the figure indicates the average viscosity measured, whereas the dotted lines provide the standard deviations of the measurement over the time period following the start of the experiment. This experiment is repeated several times, and Table 1 provides the average measured viscosities and their
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Energy & Fuels, Vol. 19, No. 3, 2005 1081
Table 1. Experimentally Determined Viscosity of a Viscosity Standard Oil with a Calibrated Viscosity of 97.280 Pa s experiment number I II III IV V VI VII average
measured viscosity (Pa s)
standard deviation (Pa s)
115.3 96.9 109.5 134.1 73.6 86.5 64.8
21.8 15.9 16.2 18.6 20.6 7.2 14.1
97.2
16.4
standard deviations, which are calculated using the relation
standard deviation )
x
∑ x - xj n-1
(11)
The viscosity measurements obtained during the lowtemperature experiments are in good agreement with the expected values. During viscosity measurements of the standard oil at ambient temperature, considerable scatter in the data is noted; the measured viscosities vary over a range of 65-134 Pa s. Despite this variation, the average value of the seven repeat experiments is 97.2 Pa s, which is almost exactly the same as the calibrated viscosity (97.280 Pa s). This indicates that the measurement technique is correct; however, care should be taken when interpreting the data, to eliminate incorrect measurements. High-Temperature Experiments. To determine the suitability of the technique for viscosity measurement at high temperatures, the viscosities of seven slags were determined at several temperatures and compared to the viscosities measured using a high-temperature rotating bob viscometer. Figure 4 displays a typical plot of the measured viscosity, as a function of the experiment time at high temperatures. At the start of the test, the annular region of the sample assembly contains no slag. This causes the initial data to be unreliable as the annular region fills with slag. For the time period of 100-150 s, a sharp decrease in the measured viscosity is evident. The likely cause of this decrease is the presence of an air bubble moving through the annular region, producing a lower apparent viscosity. A region of stable measurement is identified to determine the viscosity of the slag. Table 2 shows the results of all measurements and the comparison with the results obtained using the high-temperature rotating bob viscometer. The experiments that were conducted at low temperatures indicate that a single experiment on a slag is unlikely to provide an accurate estimation of the viscosity. To average the viscosity measurements from different temperatures from the same slag, the T-shift model has been used. The T-shift model has been proposed by Browning et al.14 and is based on the ash sample viscosity data for 117 compositions. The T-shift model is based on the (14) Browning, G. J.; Bryant, G. W.; Hurst, J. H. J.; Lucas, A.; Wall, T. F. An Empirical Method for the Prediction of Coal Ash Slag Viscosity. Energy Fuels 2003, 17, 731-737.
Figure 4. Calculated viscosity of slag VI at 1568 °C, as a function of time. Table 2. Comparison of the Viscosity Determined Using Thermomechanical Analysis (TMA) and the Viscosity Determined Using a High-Temperature Rotating Bob Viscometer for Various Slags at Various Temperatures
sample slag I slag I slag I slag I
temp (°C) 1522 1549
TMA viscosity (Pa s)
standard deviation (Pa s)
26.6 39.9 19.8 29.8
4.8 1.3 1.7 9.1
rotating bob viscosity (Pa s) 51.5 40.5
slag III
1447
17.1
1.8
18.8
slag IV slag IV slag IV
1370 1430 1462
33.7 8.3 17.6
8.7 2.8 4.9
38.6 18.0 13.7
slag V slag V
1508 1492
8.4 6.4
0.9 1.0
11.8 10.2
slag VI
1418
23.2
3.5
28.5
1568 1598
26.1 27.2 21.3 13.6
5.3 0.9 1.4 1.1
50.2 37.0
1496 1555 1585
65.6 8.8 13.5
4.3 0.4 1.8
27.0 14.6 11.2
slag VIII slag VIII slag VIII slag VIII slag IX slag IX slag IX
1537
67.0
theory that, if all viscosity-temperature curves for different ash samples are translated along the temperature axis, they will overlay, as proposed by Nicholls and Reid.15 Thus, if a standard viscosity-temperature curve is selected, the distance that the viscosity curve of any slag must be shifted along the temperature axis to overlay the standard curve can be quantified using the following equation:
log
(
)
14788 η ) - 10.931 T - TS T - TS
(12)
Browning and co-workers have proposed and validated a correlation to calculate the temperature shift (TS), based on the chemical composition of the slag. (15) Nicholls, P.; Reid, W. T. Viscosity of Coal Ash Slags. Trans. ASME 1940, 62, 141-153.
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Figure 5. (a) High-temperature thermomechanical analysis (TMA) experimental results, shown together with linearization and Haake experimental results, for (a) slag S1, (b) slag S4, (c) slag S8, and (d) slag S9.
This study has used the T-shift model to calculate the TS value of the slags used for the high-temperature experiments. This TS value provides the slope of the viscosity curve on the log viscosity-log temperature curve. From each slag, the TS value (and, thus, the slope of the log viscosity-log temperature curve) is calculated, the TMA data have been fitted to a line of best fit, using the minimization of the least-squares method. Therefore, using the T-shift model, slag viscosity measurements at various temperatures can be averaged and fitted to a line of best fit. Figure 5 shows the results of the four slags, for which three or more measurements were taken. The experiments conducted at low temperatures indicate that a single experiment on a slag is unlikely to provide an accurate estimation of the viscosity, and, thus, individual experiments should not be directly compared with the results obtained with the rotating bob viscometer. The linearized TMA results shown in Figure 5 show reasonable comparison with the results obtained using the rotating bob viscometer. The hightemperature measurements using slags S4 and S8 show very good comparison with the results obtained using the rotating bob viscometer, whereas the viscosity measurements of slags S1 and S8 underestimate the viscosity, in comparison to the other technique. At ambient temperatures, the average measured viscosity of the repeat experiments resulted in the correct viscosity. However, at high temperatures, not all samples show the same experimental accuracy. The observed difference between the viscosity measured using the high-temperature rotating bob viscometer and the viscosity measured using the TMA could be the result of several factors:
(1) The difference between the estimated slag temperature in the TMA apparatus and/or the rotating bob viscometer. It is impossible to measure the temperature of the slag directly in both experimental techniques, and, although the temperature is measured as close as possible to the slag, differences between the estimated and the real slag temperature could occur, and could result in differences between the measured viscosities. However, it is suggested that the differences are limited to only a few degrees celsius and it is unlikely that this will contribute significantly to the difference between the two experimental setups. (2) Nonhomogeneous mixture, in terms of temperature and slag mixture. During the experiments both in the TMA apparatus and in the rotating bob viscometer, it is assumed that the slag temperature is homogeneous and well-mixed. As opposed to the rotating bob viscometer, samples are not stirred in the TMA experiments, and the homogeneity is assumed by (i) the use of slags and (ii) diffusive transport of slag components through the mixture. Inhomogeneous mixtures could result in differences in measured viscosity. (3) Non-Newtonian flow through the annulus. In the experimental technique, it is assumed that the slag flow through the annulus is Newtonian. If the molten ash would not be completely molten, the liquid could contain solids, which could lead to an incorrect data interpretation. This is more likely a source of errors for liquids that display high viscosities. Most of the measurements presented in this paper are of low viscosity (
