Energy & Fuels 1992,6, 387-398
387
Improved Prediction of Coal Ash Slag Viscosity by Thermodynamic Modeling of Liquid-Phase Composition Bongjin Jungt and Harold H. Schobert* Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received November 26, 1991. Revised Manuscript Received April 20,1992 The calculation of the viscosity of a coal ash slag as a function of temperature is of interest both to obviate the difficult experimental measurement of viscosity and for mathematical modeling of ash behavior. Traditionally, the composition of ash prepared in the laboratory has been used as the data for calculation of the viscosity. This approach has inherent inaccuracies. The viscosity of the liquid will be dependent on its composition. For situations involving partial melting of the ash, as in viscous flow sintering, the composition of the melt phase will be that of the low-melting-point constituents and not that of the ash as a whole. For slags well above the original ashing temperature, high-temperaturephysical or chemical processes could ale0 make the composition of the liquid different from the original ash. In the present work, we demonstrate that prediction of the liquid-phase composition at the temperature of interest, and subsequently using that predicted composition for calculation of viscosity, substantially improves the accuracy of prediction in the Newtonian flow range. Classification criteria based on ash composition, base-to-acid ratio, silica ratio, and lignite factor are used to select the most useful viscosity calculation model for a particular ash. For a set of 33 coal ashes, prepared from coals ranging from brown coal to bituminous in rank, the average deviation of predicted viscosities from experimental measurements were within 30% (a criterion considered to represent acceptable prediction) for 29 of the 33 cases.
Introduction The viscosity of a coal ash slag as a function of temperature, and the dependence of the viscositytemperature behavior on slag composition, are important parameters in the successful operation of coal utilization equipment, such as the slag tap boiler, the cyclone combustor, and the slagging gasifier. The viscosity of coal ash slag and its relationship to temperature and composition provide an evaluation of the suitability of coals for use in such devices. Apparatus for the measurement of slag viscosities at high temperatures is not commonly found in coal laboratories, and the experimental determination of slag viscosity over a range of temperature is a difficult undertaking. Hence it is helpful to be able to calculate the viscosity behavior of a coal ash slag from some more easily measured parameter, such as ash composition. Increasing sophistication of models for predicting boiler or gasifier performance makes it helpful to be able to calculate the effects of changing temperature, composition, or both, on slag flow behavior without necessarily having a large base of experimental data on hand. An accurate method of predicting slag flow characteristicsfrom chemical composition of the ash would provide a useful tool in boiler or gasifier design and modeling calculations. Recent work has demonstrated the importance of the viscous flow of a melt phase in the growth of strength of ash deposits and in the sintering of ash particles. Benson demonstrated that the strength of deposits produced in the drop-tube furnace combustion of low-rank coals was inversely proportional to the calculated viscosity of the liquid phase that would have been present as the deposit formed.' He further showed that this behavior is consistent with the mechanism of development of deposit strength proposed by Raask.2 In subsequent work, the present authors found that the growth of strength in sin'Present address: h a r c h Institute of Industrial Science and Technology, Pohang, Kyungbuk 790-600, Korea.
tered samples of low-rank and bituminous coal ashes was due to the formation of calcium aluminosilicate "glue" bonding unmelted ash particles together? in accord with the Frenkel model of viscous flow sintering! Further, for ashes sintered at reaction times of -1 s in a drop-tube furnace, a correlation of the onset of sintering with sodium content of the ash was shown to relate to the formation of low-melting nepheline, NaA1Si04, and its subsequent fluxing action.6 We also found agreement, for agglomerates produced in fluidized bed combustion of bituminous coal, between the viscosity of a melt phase calculated from ita composition and that predicted from the Frenkel model, suggesting the potential extension of the viscous flow modeling approach to studies of agglomeration in fluidized beds! Because experimental viscosity measurements may require tens of grams of sample, and since there is no practical way of separating this quantity of the oncemolten material from a sample of ash deposit or bed agglomerate, continued studies of viscous flow sintering of ashes must continue to rely on calculations of viscosity. The development of procedures for calculating the viscosity of coal ash slags as function of temperature and composition has been a continual exercise for at least a half-~entury?-'~Traditionally, slag viscosity, v, is shown (1) Benson, S.A. Ph.D. Dissertation, The Pennsylvania State University, 1987. (2) Raaek, E. Mineral Impurities in Coal Combustion; Hemisphere: Washington,DC, 1985. (3) Jung, B.; Schobert, H. H. Energy Fuels 1991,5,555. (4) Frenkel, J. J. J . Phys. (Moscow) 1945, 9,385. (5)Jung, B.; Schobert, H. H. Energy Fuels, in press. (6) Schobert, H. H.; Conn, R. E.; Jung, B. In Fourth Annual Pittsburgh Coal Conference, Proceedings; University of Pittsburgh Pittsburgh, 1987; p 423. (7) Reid, W. T.;Cohen, P. Trans. ASME 1944,66,83. (8) Hoy, H.R.; Roberts, A. G.; Williams, D. M. J . Inst. Gaa Eng. 1965, 5,444. (9) Watt, J. D.;Fereday, F. J. J. Inst. Fuel 1969, 42, 99. (10) Bomkamp, D.National Bureau of Standards Report, E(49-18)1230, 1976. (11) Urbain, G.;Cambier,F.; Deletter, M.; Anseau, M. R. Trans. J . Br. Ceram. SOC.1981,80, 139.
0887-0624/92/2506-0387$03.00/00 1992 American Chemical Society
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388 Energy & Fuels, Vol. 6, No. 4, 1992
as a function of temperature on a semilog plot (e.g., Figurea 2-12). Such curves usually have three main features. At high temperatures log 9 vs temperature is linear with a small negative slope. The slag is considered to have Newtonian flow characteristics in this region. At the low-temperatureend of the Newtonian flow region, most curves will exhibit a pronounced change of slope. The temperature at which this slope change occurs is the temperature of critical viscosity, T,. Below T, the curve generally has a much more negative slope and may or may not be linear. In this region the slag is non-Newtonian. Ideally, a comprehensive model of slag viscosity would accept as input data some readily measured parameters (such as ash composition, mentioned above) and calculate the Newtonian and non-Newtonian portions of the viscosity curve over the temperature range of interest, as well as predicting T,. Furthermore, the ideal model should be applicable to a wide range of slag compositions. To our knowledge, such a model does not yet exist; ita development is a long-term activity in this laboratory. In this paper we present one step along the way to the development of a general model of slag viscosity: a method for substantially improving the accuracy of calculating the Newtonian portion of the viscosity curve. Two complications arise in using the analysis of ash as an indication of the composition of slag. First, ash is normally prepared in the laboratory following ASTM pr0~edures.l~ However, conversion of the laboratoryprepared ash to a slag involves further heating, in some cases to -1600 "C. This heating can cause both mineralogical phase changes and changes in elemental composition (as,for example, by vaporization of alkalies or sulfur). A comparison of the compositions of ASTM ash, hightemperature ash, and the solidified slag from a viscosity test shows significant differences for some elements.15 In addition to the decrease in sulfur content, there is usually a loss of iron and volatile elements such as sodium, phosphorus, and potassium. Consequently, the composition of the liquid viscosity sample may be different from that of the ash from which the liquid was prepared. Second, situations of ash sintering, bed agglomeration, or ash deposit growth involve a liquid formed by partial melting of the ash. If only a portion of the ash has melted, it is very likely that the composition of the melt phase will not be the same as that of the total ash sample. In either case, for accurate modeling some method is needed to calculate the composition of the liquid actually present in the sample of interest from the composition of the laboratory-prepared ash. These complications are addressed in the present work by using a computer program, SOLGASMIX, which determines the composition of phases in a system by minimization of free energy.lglS The first extensive application of SOLGASMIX to problems of coal slag behavior was the work of Hastie et al., who investigated loss of alkali seed and slag deposition in magnetohydrodynamic (12) Riboud, P. V.; Roux, Y.;Lucas, L. D.; Gaye, H. Fachber. Huttenprax. MetalZ. 1981, 19, 859. (13) Schobert, H. H.: Streeter, R. C.; Diehl, E. K. Fuel 1985.64,1611. (14) American Society for Testing and Materials. Annual Book of ASTM Standards; ASTM Philadelphia, 1991; Volum 05.05. (15) Falcone, S. K.; Schobert, H. H. In Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; American Chemical Society: Washington DC, 1986; Chapter 9. (16) Eriksaon, G . Acta Chem. Scand. 1971, 25, 2651. (17) Eriksson, G.; Rosen, E. Chem. Scr. 1973,4, 193. (18) Eriksson, G. Chem. Scr. 1975, 8, 100. (19) Hastie, J. W.; Bonnell, D. W.; Plante, E. R.; Horton,
W. S. In Thermochemistry To&y and its Role in the Immediate Future; dasilva, A. V. R., Ed.; Reidel: Boston, 1983. (20) Hastie, J. W.; Bonnell, D. W. High Temp. Sci. 1984, 19, 275.
Subsequently, Benson pioneered the use of SOLGASMIX in problems relevant to pulverized coal combustion. He used this program to predict the quantity of liquid phase that would be present in an ash deposit, and showed a trend of increasing deposit strength with increasing quantity of liquid phase.' The present authors then demonstrated the utility of SOLGASMIX in calculating the low-melting sodium-containing phases in the sintering of ashes in a drop tube f ~ r n a c e .In ~ the present work, we have used SOLGASMIXto calculate the composition of liquid phase at high temperatures from the composition of the ash prepared in the laboratory. The calculated liquid-phase compositions were compared with those determined by analysis of slags after experimental viscosity measurements and were also used as the input data to test various published methods for the calculation of viscosity from composition. By comparing the calculated viscosities with the experimental values, it was possible to develop guidelines for the selection of the most accurate viscusity calculation methods for liquids of given compositions. Clearly one should prefer experimental measurementa, if available, to results calculated from a model. We do not mean to imply in this paper that calculation of liquid-phase composition by SOLGASMIX (or other models, for that matter) is preferable to actual analysis of liquid, or onceliquid, phases. Our purpose in relying on a large body of extant viscosity data and its supplemental postmeasurement analyses of slags is to establish confidence in the combination of SOLGASMIX with viscosity calculation models, 80 that the combined approach can be applied with confidence to situations, such as viscous flow sintering or ash deposition, in which the recovery of samples for composition determination is impractical.
Experimental Procedures Viscosity Measurement. The experimental apparatus and procedures used in this study were generally similar to those used in other laboratories and described in detail The rotating bob viscometer is a Haake RV-2 Rotovisco unit. The speed of bob rotation was 64 rpm. Viscosity tests were conducted in an oxidizing (air) atmosphere. The air was injected into the furnace from the bottom through an alumina tube a t a flow rate of approximately 350 cm3/min. For operation in an oxidizing atmosphere, the viscometer bob was fabricated from 12.5" titanium bar stock. The bob was approximately 23 mm long with a 30° angle taper machined on both ends. The top of the bob terminates in a 15 mm long X 4 mm diameter shaft which is tapered to accommodate a 5 mm diameter X 421 mm long alumina tube. The top of the alumina tube was connected with a 3 mm diameter, 15 mm long steel shaft. The bob was attached to the viscometer measuring head by a rigid shaft containing a set screw to hold the stem. During a viscosity test, the bob is immersed until the slag just covers its top. The data profides are displayed and stored on an IBM PC which is interfaced to the slag viscosity apparatus through an Omega WB-ASC-B interface card. Three coals were studid Robinson (Montana) subbituminous, Eagle Butte (Wyoming)subbitiminous, and Beulah (North Dakota) lignite. Each sample was pulverized to -60 mesh (250 pm) and ashed according to the ASTM (750 O C ) ashing procedures. The resulting ash was sieved a t 60 mesh and then compressed into 10 mm diameter, 10 mm long pellets weighing about 1.2 g each. The ash pellets were dropped into a heated crucible. A typical coal ash charge was about 70-80 g. The crucible is composed of high-purity alumina measuring 25 X 50 mm. The sample (21) Hastie, J. W.; Bonnell, D. W.; Plante, E. R. Twenty-third SEAM Conj., h o c . 1985; Paper 3. (22) Streeter, R. C.; Diehl, E. K.; Schobert, H. H. In The Chemistry of Low-Rank Coals; Schobert, H. H., Ed.; American Chemical Society:
Washington DC; 1984; Chapter 12. (23) Streeter, R. C. U.S. Department of Energy Report, DOE/FC/ 10159-1, 1984.
Prediction of Coal Ash Slag Viscosity crucible is contained in a larger refractory brick "guard crucible" (which serve8 to protect the furnace lining and heating elements in the event that the eample crucible cracked or overflowed), with the annular space between the two crucibles packed with glass fiber. In this study, the instrument factor was determined by testing National Bureau of Standards glass viscosity standard No. 711, the viecosities of which are precisely d e f i e d and similar to those of coal ash slags over the temperature range of interest. The instrument factor is related to the viscosity by the equation viscosity = (instrument factor x torque reading)/bob speed In the present work, the bob speed was 64 rpm. The torque reading was taken from a Haake RV-2 Rotovisco unit. Measurements of the glass standard were made over a range of temperatures similar to those for which the slag viscosities were m e a s 4 the logarithm of the instrument factor i n d linearly with decreesing temperature over this temperature range. J3ecauae the effect of temperature on instrument factor was observed in the range of interest, no attempt was made to obtain room-temperature data using standard viscosity oils. During a viscosity test, the sample was held for 30 min a t the desired temperature before the viscosity measurements were started. Measurements were normally started at the highest temperature a t which an on-scale reading could be obtained with a bob speed of 64 rpm. The temperature was then slowly decreased a t a rate of about 0.6 "C/min until the next temperature was reached for which a measurement waa deaired. This procedure was repeated over a decreasing sequence of temperatures until the viscosity of the melt waa too high to obtain reliable data, After each coal ash slag viscosity test, the solidified slag samples are analyzed to determine chemical composition by energy-dispersive X-ray fluorescence spectrometry (Kevex) and to identify crystalline phases by X-ray diffraction (Rigaku MEBOODX). Viscosity D a t a Base. In order to have a large base of viscosity-composition-temperature data to evaluate the SOLGASMIX-ViSCOeity model, we have made considerable use of data already in the literature. Most were drawn from the extensive study by Streeter and colleague^,'^**^^^^ which focused largely, though not exclusively, on low-rank coal ash slags. Lesser amounts of data were taken from the compilations of Hoy et aLa and Boow.= The complete data base included coals ranging in rank from brown coal through bituminous, and coals from four nations. Thirtythree materials available from the literature, including high- and low-rank coal ashes and slags, were chosen. The criterion for evaluating the goodness of agreement between predicted and experimental viscosities follows the earlier work of Watt and Fereday, who considered that an average deviation between calculatedand experimental data in the Newtonian region of 30% is considered "acceptable"? As will be discuesed below, acceptable predictions of the Newtonian viscosity-temperature behavior are achievable for slags in the composition range 25 ISiOz I55%.
Results and Discussion Framework of the Combined soLGAsMIx-Viscosity Program. The prediction of liquid composition is done using the SOLGASMIX program, which minimizes the total free energy of the system. SOLGASMIX is based on the standard equation for the total free energy of a system G = &(pio + RT In ai) 1
where n is the number of moles, p o the standard chemical potential, R is the gas constant, T is the temperature in kelvins, and ai is the activity of the ith component. The origins of this equation are treated in standard texts.26 The thermodynamic data base is the most important part of the equilibrium calculation. Thermochemical data (24)Hoy, H.R.;Roberta, A. G.;Williams, D. M. J . Zmt. Fuel 1958, 429. (25)Boow, J. J. Zmt. Fuel 1965,3. (26)Nordstrom, D. K.;Munoz, J. L. Geochemical Thermodynamics; Blackwelk Palo Alto, CA, 1986,Chapter 6.
Energy & Fuels, Vol. 6, No. 4, 1992 389
Initial Ash Composition (weight 5% of oxides)
YI INITMOL Program
Number of Moles of Each Element
SOLGASMIX Program
l-5 Conversion
4 Weight % of Oxides in Liquid Species
I VISCOSITY Program
w Rediction of Viscosity
Selection of Best
Figure 1. Flow chart for prediction of coal ash slag viscosity.
can be found in a number of source^.^^-^^ A major limitation in the accuracy of determining equilibrium composition is the uncertainty in the thermochemical data. For cases where no literature data were available for the liquids, the thermodynamic functions were estimated in the manner described by Hastie and co-workemm Data available for the corresponding solid phase were usually converted to liquid functions by using the melting enthalpy or entropy. The selection of liquids and solids to be considered was based on information from liquidaolid phase diagrams.31 Sulfur oxide and sulfate compounds were considered as the major gaseous species because most of weight losses of ash during melting were primarily a result of volatilization of sulfur compounds.23 In this study, 70 chemical species containing gaseous, liquid, and solid species and 10 major elements (Si-Al-Ti-Fe-Ca-MgNa-K-S-O) incorporated in the coal ash were considered in the equilibrium calculation^.^^ All of the thermodynamic data were obtained from JANAF2' and from h b i e and co-workers28at 1400 K, which is in the middle of the 0 in the present temperature range of interest ( 1 ~ 1 8 0 K) work. Because the constituent oxides in coal ash are customarily reported as weight percents, these data are converted to the number of moles of each element in the ash by the INITMOL program. The numbers of moles of each element are used as input data in the SOLGASMIX program. For the conversion of the liquid composition determined by SOLGASMIX to the form used as input to the six major viscosity models, multiple oxides and silicates were assumed to be equivalent to the same amount of their constituent components. For example, anorthite (CBA12Si207,or CaO. A1203.2Si02)was "decomposed" to CaO plus A 1 2 0 3 plus 2 mol of Si02. The INITMOL, SOLCASMIX, and VISCOSITY (27)Stull, D. R.; Prophet, H. JANAF Thermochemical Tables; U.S. Department of Commerce: Washington, DC, 1971. (28)Robie, R. A.; Hemingway, B. S.; Fisher, J. R. US. Geological Survey Bulletin, 1452,1978. (29)Kelley, K. K. U.S.Bureau of Mines Bulletin, 601,1962. (30)Hastie, J. W.; Horton, W. S.; Plante, E. R.; Bonnell, D. W. High
Temp. High Press. 1982,14,669. (31)Levin, E.M.;Robbins, C. R.; McCurdie, H. F. Phase Diagrams for Ceramists; American Ceramic Society: Columbus, OH, 1964. (32)Jung, B. Ph.D. Dissertation, The Pennsylvania State University, 1990.
390 Energy & Fuels, Vol. 6,No. 4, 1992
programs are together used to predict the viscosity of coal ash slag. A flow chart for the calculation procedure is shown in Figure 1. The variety of viscosity models available, and the conflicting results occasionally obtained from viscosity models, led us to develop a master program which incorporates six of the major viscosity models, and which calculates the viscosity of a given liquid by all six models: Hay: WattFereday: modified Watt-Fereday,'O Urbain," IRSID,12 and modified Urbain.13 This approach allows an immediate comparison of the calculated results with the experimental data and was of assistance in assessing the applicability of the various viscosity models in the program. The input data for the SOLGASMIX program were the compositions determined by X-ray fluorescence analysis of ashes produced by the ASTM or high-temperature ashing (lo00 OC) procedures. The liquid-phase composition was calculated at 25 "C intervals in the temperature range 1150-1700 "C. At each temperature interval, the liquidphase composition calculated for that temperature was then used as the input data for the viscosity The mineral matter in coal is converted, during ashing, to a variety of silicates, aluminosilicates, and other new phases, but is has long been customary to express ash composition on the basis of oxides: SO2, A1203,TiOz, Fe203,CaO, MgO, Na20, K20, and SO3. Much of the SO3 is an artifact of the laboratory ashing procedure, in which SO3 is captured by the alkaline earths and the alkaliesa3* Because the alkali metal and alkaline earth metal sulfates are decompsed above about 980 "C, little or no SO3 is found in most slags that have been heated to much higher temperatures.34 To determine the ash composition data that is most useful as a predictor of viscosity, four ways of expressing ash compositions were used in the initial evaluation of the method. That is, the composition of ash prepared by standard ASTM methods,14 which we will hereafter refer to as "ASTM ash"; ash prepared from the coal at 1000 "C in air, which we call "high-temperature ash"; the ASTM ash composition expressed on an S03-free basis; and the high-temperature ash composition also on an S03-freebasis were used as input data to the SOLGASMM program. In addition, it was important to verify that the use of SOLGASMIX actually provided an improvement relative to predictions that could be obtained by inserting the ash or slag compositions directly into the six viscosity models."13 Those calculations were made by running the model without using the SOLGASMIX program to predict liquid-phase compositions.
Results and Discussion Predicted Liquid-PhaseComposition. The first test of the SOLGASMIX-viscosity model is the ability of the SOLGASMIX program to predict accurately the compositions of the liquid phase. The predicted liquid-phase compositions are based on the ASTM ash (SO3-free basis) or high-temperature ash (also on an SO3-freebasis) composition because sulfates tend to decompose above about 980 "C,and little, if any, SO3is found in slags. The predicted liquid-phase composition of samples in each group changed slightly as a function of temperature in the range of interest for which we compared predicted and experimental viscosity data. However, it can be seen from Tables 11-VI1 that the agreements between the SOLGASMIX predictions and the X-ray fluorescence analysis of the solidified slag (33) Schobert, H. H.; J u g , B. Prepr. Pap.-Am. Chem. Soc.,Diu. Fuel Chem. 1988, 33 (2), 28. (34) Reid, W. T. In Chemistry of Coal Utilization, Second Suppl. Vol.; Elliott, M. A., Ed.; Wiley: New York, 1981; Chapter 21.
Jung and Schobert
recovered after viscosity tests were generally quite good. They sometimes showed some difference between the predicted and measured amounts of A1203. One of the reasons for this might be dissolution of A 1 2 0 3 into the slag from the alumina crucible. In one case, Gascoyne lignite (Table V), the amount of Fe203showed a much lower value in the slag than in the predicted liquid-phase composition, likely due to reduction of Fe203 to Fe by the carbon crucible. This behavior has been noted consistently in other work in carbon crucibles.22 In a previous paper we have shown that SOLGASMIX can also provide excellent predictions of the actual chemical species present in the liquid phase.33 There we demonstrated correct prediction of four of the five principal species in a slag produced from Martin Lake lignite. (The slag had been quenched to ambient temperature and the resulting solid had been analyzed by semiquantitative X-ray diffraction analysis.) Consequently, it appears that SOLGASMIX can be used with some confidence to predict liquid-phase compositions of the ashes in the ash composition range covered in the present work. Viscosity Predictions by the Combined SOLGASMMVISCOSITY Program. To develop the improved method to predict the viscosity of coal ash slag, 33 samples, including low- and high-rank coal ashes and slags, were tested in the combined SOLGASMIX-VISCOSITY program. Not surprisingly, none of the six viscosity models tested provided the most accurate prediction (based on a comparison of the calculated viscosity with the experimental measurement) for the entire data set of 33 slags. To establish criteria for selection of the most accurate of the six predictive models for a given slag, the data base was subdivided into seven groups, based on the traditional parameters for classification of coal ashes, such as the lignite factor,%silica ratio,' base-to-acid and percentages of Si02,Fe203,and Na20, based on the ASTM (S03-free basis) ash or hightemperature (SO3-free basis) ash composition. (When using the SO3-freebasis to express ash compositions, the content of SO3 used as input data for the computer calculations was arbitrarily assigned to be 0.1 % because on some occasions the SOLGASMIX calculations do not converge when the SO3 content is set exactly equal to zero. The reason for nonconvergence is not presently known or understood.) Following criteria established by other workersg the results of applying each of the six viscosity models to a given slag were then classified as "acceptable" (within 30% average deviation from experimental data) or "marginally acceptable" (within 60 7% average deviation from experimental data). The deviation from experimental data was calculated as 100[(SOLGASMIX-VISCOSITY prediction) (exptl viscosity)] /exptl viscosity The deviation of predicted viscosity from the experimental value was calculated using this formula for each temperature a t which a viscosity prediction was made. The average of each of these individual results was then calculated, and that average is referred to in this paper as the "average deviation from experimental data." The most satisfactory of the correlative equations for the prediction of viscosity for any one of the seven groups of slags was then determined. The results of this extensive series of calculations are summarized in Table I. They show that it is possible to use these classifications to predict with reasonable accuracy the viscosity of coal ash slag using the (35) Attig, R. C.; Duzy, A. F. h o c . Am. Power Conf. 1969, 31, 290. (36) Sage, W. L.; McIlroy, J. B. J.Eng. Power; Trans. ASME, Ser. A 1960, 82, 145.
Energy & Fuels, Vol. 6, No. 4, 1992 391
Prediction of Coal Ash Slag Viscosity 150
I
Watt
Mo. Watt
HOY
SOLGASMIX + Mo.Watt I Mo.Watt, Slag
1000 1
100:
50
n d,
B
1200
1300
1400
0
1500
Tem perature,OC Figure 2. Comparisons of experimental viscosity (ref 22) and predicted results for Martin Lake plus 15% limestone, showing improved predictions from modeling liquid composition with
0 SOLGASMIX
1 SLAG
a
Figure 3. Average deviation of predicted from experimental viscosity data for Martin Lake lignite + 15% limestone in the Newtonian region.
SOLGASMIX.
combined SOLGASMIX-VISCOSITY program, provided that the appropriate %election rulesn are used to classify the slag of interest into one of the seven groups and hence select the most accurate of the various calculational models. Test results for examples of each of the seven groups, and the appropriate classification criteria for each group, are discussed in more detail in the sections which follow. Group I. The classification criteria for group I are the following: 1 C lignite factor 5 3,42 5 Si02% < 55, and Na20% I 5. (The group numbers are used only for convenience and have no other significance.) The compositions of the ash, slag, and predicted liquid phase for group I samples are shown in Table 11. The Si02and Na20 are taken on an SO3-free basis. If the compositions of both the ASTM ash and high-temperature ash are available, the first priority of classification is based on the composition of the high-temperature ash. The modified Watt-Fereday equation is of best general applicability to group I slags. In five of the seven cases,this equation provided acceptable agreement between prediction and experimental results (average deviation from experimental data was within 30%) and in the remaining two cases the results were within 60% average deviation from experimental data. which is considered marginally acceptable. To illustrate the results obtained for the group I slags, we discuss the case of Martin Lake lignite high-temperature ash fluxed with 15% limestone. The viscosity of slag from the mixture of Martin Lake high-temperature ash and 15% limestone was determined at BCR National Laboratory (BCRNL) under a reducing atmosphere (20% H2 and 80% N2) in an alumina crucible. The predicted viscosity-temperature curve was obtained using the composition of the mixture of Martin Lake high-temperature ash and limestone (S03-free basis) as input data to the combined SOLGASMIX-VISCOSITY program. The results are displayed in Figures 2 and 3. (In Figure 3, SOL GAS MIX^ means the average deviation of data predicted by the combined SOLGASMIX-VISCOSITY program from experimental viscosity data, SLAGbmeans the average deviation of data predicted using only the experimentally measured slag composition without the SOLGASMIX calculation from experimental viscosity data, and the horizontal line marked
l
o
-
11111
1000 7 v)
a
m o Experimental SOLGASMIX + Hoy Hoy, Slag
m
.-0
v)
n
.-j: w
100:
v)
0 0
.-> v)
10:
1 : 1300
I
i
1400
1500
Temper at ure,OC
Figure 4. Comparisons of experimental viscosity (ref 22) and predicted results for Colstrip, comparing results obtained with and without SOLGASMIX prediction of liquid viscosity.
"c" indicates acceptable (within 30% average deviation) correlations to experimental viscosity data.) The modified Watt-Fereday equation, which provides the best predictions for the group I slags taken as a whole, gives a 10.4% average deviation from experimental data when using the combined SOLGASMIX-VISCOSITY program. Group 11. The classification criteria for group I1 are 1 < lignite factor I 3,34 ISi02% < 42, and Na20% I 5. The compositions of the ashes, slags, and predicted liquid phases for group I1 samples are shown in Table 111. Si02and Na20 are taken on an SO3-freebasis. The Hoy equation showed acceptable correlation (within 30% average deviation from experimental data) in the Newtonian region in all six of the trials, based on the high-temperature ash (SO3-freebasis) composition. The results obtained for Colstrip (Montana) subbituminous coal ash are discussed as an example. The viscosity of the slag was determined at BCRNL in a 20% H2:80%N2atmosphere in an alumina crucible. Results from the combined SOLGASMIX-VISC~~ITY
Jung and Schobert
392 Energy & Fuels, Vol. 6, No. 4, 1992 10000
,
1
-
--'u--'
-- ---
Experimental SOLGASMIX + IRSlD IRSID.Slag
.-0
.-aa0l
1000
u)
1200
1300
1000:
n
2.-
.-i
CI
c.
u)
u)
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0
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.->01
Experimental SOLGASMIX + Mo.Wan Mo. Wan. Slag
u)
cn 0
0
.-> u)
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10 1100
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.1200
I
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1300
Temperaturelac
Figure 5. Comparisons of experimental viscosity (ref 22) and predicted reault.9 for Decker, showing improved predictions from modeling liquid composition with SOLGASMIX. program are shown in Figure 4 and Table 111. The Hoy equation, in combination with SOLGASMIX prediction, provided a 15.5% average deviation from experimental data. We show the results for Colstrip because it represents an example of an uncommon instance in which the SOLGASMIX-VISCOSITYprediction, while providing acceptable accuracy, is nevertheless slightly less accurate than the prediction obtained by using the Hoy equation without SOLGASMIX. The average deviation in the latter case was only 10.6%. Group 111. The ash compositions of Decker subbituminous coal and Beulah low-sodium lignite showed the following characteristics: 1 < lignite factor 5 3, 34 I SO2% < 42, and Na20% > 5. These two coals are classified as Group 111. Si02and Na20 are on an S03-free basis. The compositions of the ash, slag, and predicted liquid phase for group 111samples are shown in Table IV. The IRSID equation gave a reasonably good fit of the experimental data in the Newtonian region for both group I11 coals. However, more experimental viscosity data for group I11 coals are needed in the future to check the selection of the IRSID equation as the most appropriate, because of the limited amount of existing data. Decker (Montana) subbituminous coal ash is an example of group I11 slags. The viscosity was determined at BCRNL in a 20% Hz:80% N2 atmosphere and an alumina crucible. Results from the combined SOLGASMIX-VISCOSITY program are shown in Figure 5 and Table IV. The IRSID equation provided a 14.8% average deviation from experimental data. Group IV. The ash compositions of Big Horn subbituminous coal, Eagle Butte subbituminous coal, and Beulah lignite showed the following characteristics: 1 < lignite factor 5 3 and SiOz 5 34% (S03-free basis), and are classified as group IV. The compositions of the ash, slag,and predicted liquid phases for group IV samples are shown in Table IV. The modified Watt-Fereday equation gave an acceptably good fit of the experimental data in the Newtonian region for group IV coals, based on the ASTM ash (S03-free basis) composition. The viscosity of slag from Big Horn (Wyoming) subbituminous coal ash was determined at BCRNL in 20% Hz:80% N2 using an alumina crucible. Results from the combined SOLGASMIXVISCOSITY program are shown in Figure 6 and Table IV.
1 )O
Temperaturelac
Figure 6. Comparisons of experimental viscosity (ref 22) and predicted results for Big Horn,showing improved predictions from modeling liquid composition with SOLGASMIX. loOOo
0 - Experimental
-
-----
SOLGASMIX + Wan Watt, Slag
cn
.-$0
1000:
n
.-i cn c)
0 0
.> u)
100:
10 1100
1200
1300
1400
Temperature,"C
Figure 7. Comparisons of experimental viscosity (ref 22) and predicted resulta for Gaecoyne, showing slightly improved pre-
diction by modeling liquid composition with SOLGASMIX.
The modified Watt-Fereday equation gave a 22.1% average deviation from experimental data. Group V. The ashes of Gatxoyne lignite,Medicine Bow No. 11 and No. 12 subbituminous coals, and Rosebud subbituminous coal had the following characteristics: lignite factor > 3 and Si02 > 40%. These four ashes are classified as group V. Compositions of the ash, slag, and predicted liquid phase for Group V samples are shown in Table V (S03-free basis). The Watt-Fereday equation provided an acceptable correlation in all four cases. The viscosity of slag from Gascoyne (North Dakota) lignite ash was determined at BCRNL in 20% Hz:80% N2 and a carbon crucible. Results are shown in Figure 7 and Table V. The Watt-Fereday equation provided a 15.3% average deviation from experimental data. Group VI. The ash compositions of Greta bituminous coal, an Auetrian brown coal slag, the (otherwise unidentified) No. 34 bituminous coal, Bulli bituminous coal, and
Energy & Fuels, Vol. 6, No. 4, 1992 393
Prediction of Coal Ash Slag Viscosity
Table I. Criteria for the Classification of Ash Compositions for Application of the Combined SOLGASMIX-VISCOSITY Program correlative eqsd input data PUPS parameters test samples and av dev: % (S03-free) ref Mo. Watts 17.7 Martin Lake lignite HTA 22 1 66 B/Ac < 0.35 Fe203fI15
Northern N.S.W. Greta bituminous Austrian Brown coal slag Sample No. 34 bituminous Southern N.S.W. Bulli bituminous Cronton No. 2273-B bituminous
VI1
LF < 1 SR I 6 6 B/A > 0.35 FezOd > 15
Rossington bituminous Queensland Garrick bituminous Cronton No. 2273-A bituminous Leigh Creek Coal slag No. 16418 Leigh Creek Coal slag No. 16419 Choctaw lignite
Hoy8
15.5 9.1 5.4 18.8 8.6 13.2
HTA HTA HTA HTA HTA ASTM
22 22 22 22 23 32
HTA HTA
22 22
ASTM ASTM ASTM
22 32 32
ASTM HTA HTA HTA
22 22 22 37
ASTM HTA ASTM ASTM ASTM
25 22 36 25 24
HTA ASTM ASTM HTA HTA HTA
22 25 24 22 22 23
IRSIDe 14.8 26.5
Mo. Watt8
+
22.1 14.6 20.3
Watt8 15.3 7.7 9.3 17.8
Mo. Watt8 8.6 2.8 3.2 2.0 29.8
Watt8
+
20.0 13.0 17.7 78.7 47.3 16.6
+
+
+
+
OLignite factor = (CaO MgO)/FezOB. bSilica ratio = 100 SiOz/(SiOz “FeZO3” CaO MgO), where “Fe203” = Fez03 1.11FeO 1.43Fe. ‘Base/acid ratio = (Fez03 CaO MgO NazO KpO)/(Si02 AZO3 + TiOz). dAcceptable equations (within 30%) and marginally acceptable equations (within 60%). e Average deviation (%) from experimental viscosity data. f u s e high-temperature ash (SO3
+
+
+
+
+
free basis) data if available otherwise ASTM ash (SO3free basis) data. #Best correlative equation.
Cronton No. 2273-B bituminous coal showed the following conditions: lignite factor < 1,silica ratio > 66,base/acid ratio C0.35, and Fez03 I15%; these coals are classified as group VI. The compositions of the ash, slag, and predicted liquid phase for group VI samples are shown in Table VI. Fe203 is expressed on an SO3-freebasis. The modified Watt-Fereday equation provided acceptable fits of experimental data in all five casea. The viscosity of slag from Greta (New South Wales, Australia) bituminous coal ash was determinedz2in a nitrogen atmosphere using an alumina crucible. Results are shown in Figure 8 and Table VI. In this case, the modified Watt-Fereday equation showed a 8.6% average deviation from experimental data. Group VII. Ashes of the Rossington, Garrick, and Cronton No. 2273-A bituminous coals, Leigh Creek subbituminous coal samples No. 16418 and 16419; and Choctaw lignite had lignite factor < 1, silica ratio I66, base/acid ratio > 0.35, and Fe203> 15%. These six coals are classified as group VII. Composition of the ash, slag, and predicted liquid phase for group VI1 samples are shown in Table VII. Fe203is used on an SO3-freebasis. The Watt-Fereday equation showed that in four of the six
tests the average deviation was w i t h 30% of experimental results, and in a fifth case the deviation was within 60%. The viscosity of slag from Rossington (U.K.) bituminous coal ash was determined at BCRNL in a 20% H2:80%N2 atmosphere in an alumina crucible. For this slag WattFereday equation showed a 20.0% average deviation from experimental data (Figure 9). Measurement and Prediction of Slag Viscosity for New Samples. The development of the SOLGASMIX-VISCOSITY model as described in the previous section was based on data already available in the literature. Further, we relied upon the analysis of that data, when tested against all six viscosity equations, for the development of the criteria for classification of slags into one of the seven groups. We considered it critical to test the applicability of the model to newly acquired data not part of the initial data base used for development of the model. That is, if the model is to have any real utility as a predictive tool, it must be able to provide acceptably accurate predictions of viscosity behavior for slags for which experimental data are not already in hand. Slags produced from three coals ashes (Robinson and Eagle Butte subbituminous coals and
394 Energy & Fuels, Vol. 6,No. 4, 1992
-
1woo
loooo
Jung and Schobert
I
-
-
--*
I -
Exmrimental SOLGASMIX + Mo.Wan Mo.Wan.Shg
m 8
.-m0
Experimental SOLGASMIX + Hoy Hoy, Slag
1000
eL
.-2; m L.
0
0
.->m
10 1300
1400
1500
1600
Temperature,"C Figure 8. Comparisons of experimental viscosity (ref 26) and predicted results for Greta, showing results obtained with and without SOLGASMIX prediction of liquid viscosity.
100
10 1100
1200
1300
1400
Temperature,% Figure 10. Comparisons of newly determined experimental viscoeity (ref 32) and predicted results for Robinson subbituminous
coal, showing improved prediction with SOLGASMIX.
-
-
#-
I I
__
J
Experimental SOLGASMIX + Wan Wan, Slag
-
#--
1
1000 :
Experimental SOLGASMIX + Mo.Wan Wan*Siag
100:
10
10 1150
1250
1350
-
1450
Temperature,"C Figure 9. Comparisons of experimental viscosity (ref 22) and predicted results for Roesington. In this specific instance the application of SOLGASMIX provided no improvement in prediction. Beulah lignite) were studied in a rotating bob viscometer. These newly measured experimental viscosities were then compared with those predicted from the combined SOLGASMIX-VISCOSITY program. The predicted liquid-phase compositions of three samples, baaed on the ASTM ash composition (S03-freebasis), changed slightly with temperature in the range for which we compared predicted and experimental viscosity data. In addition, the comparisons the SOLGASMIX predictions and analyses of the solidified slag remaining after viscosity testa showed some differences (Tables I11 and IV). The most likely reason is partial dissolution into the slag of A1203 and TiOz from the ah"crucible and by oxidation and melting of the titanium bob, respectively. (The titanium bob was severely attacked above 1350 "Cin the testa of these three samples. Titanium may not be suitable for constructing viscometer bobs even though it is relatively cheap compared to such alternatives as molybdenum and
1 1 1000
.
, 1100
.
, 1200
.
, 1300
.
, 1400
. 1500
Temperaturelac Figure 11. Comparisons of newly determined experimental viscosity (ref 32) and predicted results for Eagle Butte subbituminous coal, showing improved prediction with SOLGASMIX. platinum. The effect of dissolved titanium oxide on the viscosity behavior of the coal ash slags is not known,and indeed the behavior of titanium in silicate melts is quite complicated. However, it is generally presumed that Ti4+ enhances polymerization of silicate structures,3* so that additions of titanium to the melt would likely increase viscosity.) The viscosity of slag from Robinson subbituminous coal ash was measured over the range 1150-1400 "C in an oxidizing (air)atmosphere and an alumina crucible. The ash sample that was melted to produce the slag was prepared by the standard ASTM ashing method. Ash, slag, and (37) Schobert, H.H.; Witthoeft, C . FLlel Process Technol.1981,6,157. (38) Mysen, B.0. Structure a d Properties of Silicate Melts; Elasvier: Amsterdam, 1988; Chapter 5.
Prediction of Coal Ash Slag Viscosity
I I
I
1
I
C
3
i
n ~
I
6
. .I c
,
5 E
Energy & Fuels, Vol. 6, No. 4,1992 396
Jung and Schobert
396 Energy & Fuels, Vol. 6, No. 4, 1992
Table IV. Ash, Slag, and Predicted Liquid Compositions (wt %) of Group 111 and IV Samples group I11 group IV
composn Si02 A1203 Ti02 Fe203 CaO MgO Na,O K20
SO, B/Ae SRf SAg LFh
HTA" 37.4 18.2 2.0 9.3 19.2 4.1 9.4 0.3 0.1d
Decker sl& 1250 OC 38.9 36.3 18.8 23.1 2.0 2.1 8.8 9.5 18.2 19.7 3.8 4.2 7.5 6.5 0.3 0.2 0.0 0.1
Beulah Low Sodium Big Horn Eagle Butte HTA" sl& 1325 OC' ASTM" ala$ 1300 OC' ASTMk slag' 1375 "0 40.1 34.3 30.5 32.6 30.4 26.1 33.5 38.5 35.8 22.1 25.0 23.8 17.7 11.3 27.5 18.2 10.7 18.3 1.4 1.5 1.4 1.8 1.6 0.9 1.4 7.3 0.9 14.0 13.8 14.8 13.2 11.7 14.1 13.6 4.5 12.0 22.7 20.7 23.6 15.5 18.3 19.0 26.0 25.3 26.8 6.6 7.1 5.5 6.4 5.8 7.5 6.4 5.5 6.6 5.9 4.2 1.9 4.8 4.5 4.0 4.5 3.5 1.8 0.7 0.5 0.4 O.ld 0.2 0.1 0.2 0.1 0.2 O.ld 0.1 0.1 0.0 0.1 O.ld 0.1 O.ld 0.1
0.67 53.8 2.07 2.52
0.73 0.63 53.4 54.0 2.06 1.57 2.51 2.50
0.97 0.68 47.1 56.7 3.60 1.95 2.06 5.07
0.89 47.1 3.57 2.04
0.73 48.4 1.55 1.61
0.74 44.8 1.22 1.73
0.79 43.4 1.28 2.00
0.95 42.5 1.84 2.75
0.64 42.5 0.95 6.84
ASTMk 32.2 18.4 0.8 12.6 23.1 6.3 6.4 O.ld O.ld
0.90 42.5 1.84 2.77
0.94 43.4 1.75 2.33
Beulah slag' 1375 "CY 21.3 33.9 28.6 19.4 18.6 0.8 1.7 13.3 20.1 24.3 5.0 6.6 4.5 1.5 0.1 0.1 0.1 0.1 0.46 44.3 0.75 14.77
0.85 43.4 1.75 2.33
"Reference 22 (based on SO3-freebasis). bReference 22. 'Predicted liquid composition based on HTA (SO,-free). dAssume contents of SO8 and K 2 0 are 0.1 in ash because so,metimesSOLGASMIX does not converge using SO3 and K 2 0 of 0.0. 'Base/acid ratio. fSilica ratio. 8Si02/A120, ratio. hLignite factor. I Predicted liquid composition based on ASTM (SO,-free) ash. kReference 32 (SO,-free basis). 'Reference 32. Table V. Ash, Slag, and Predicted Liquid Compositions (wt % ) of Group V Samples
composn Si02 Alz03 Ti02 Fez03 CaO MgO Na20 K20
so3
B/A' SRf SA8 LFh
ASTM" 42.6 13.2 1.0 9.0 21.0 8.1 4.7 0.3 O.ld
Gascope sl& 1350 OCC 42.6 40.8 14.5 14.9 1.4 1.2 3.1 7.8 25.8 24.3 8.2 8.2 3.7 2.9 0.1 0.2 0.2 0.1
0.76 52.8 3.23 3.23
0.69 53.5 2.86 10.97
Medicine Bow No. 11 HTA" slag 1450 OC' 46.7 k 46.7 16.1 k 16.1 0.6 k 0.6 7.5 k 7.5 23.6 k 23.6 3.7 k 3.7 k 1.4 1.4 0.3 k 0.3 O.ld
0.58 57.3 2.90 3.64
0.77 50.3 2.81 4.20
k k k k k
0.1
0.58 57.3 2.90 3.64
Medicine Bow No. 12 HTA' slag 1475 OC' 51.9 k 51.9 16.6 k 16.6 0.5 k 0.5 6.6 k 6.6 19.0 k 19.0 2.9 k 2.9 0.7 k 0.7 1.7 k 1.7 O.ld k 0.1 0.45 64.6 3.13 3.32
k k k k
0.45 64.6 3.13 3.32
HTAj 44.6 20.9 1.1
6.0 21.3 5.4 0.2 0.6 O.ld
Rosebud slag 1475 OC' k 44.5 k 20.9 k 1.1 k 6.0 k 21.2 k 5.4 k 0.2 k 0.6 k 0.1
0.50 57.7 2.13 4.45
0.50 57.7 2.13 4.45
k k k k
'Reference 22 (based on SO,-free basis). bReference 22. cPredicted liquid composition based on ASTM (S03-free)ash. dAssume content of SO, is 0.1 in ash because sometimes SOLCASMIX does not converge using SO, of 0.0. eBase/acid ratio. fsilica ratio. gSi02/A1203ratio, hLignite factor. jReference 37 (based on SO,-free basis). Data not available. 'Predicted liquid composition based on HTA (S03-free). Table VI. Ash, Slag, and Predicted Liquid Compositions (wt 70) of Group VI Samples Greta Austrian slag No. 34 Bulli Cronton No. 2273-B composn ASTM" ala$ 1575 "C' HTAj slag 1500 OC' ASTM"' slag 1525 OC' ASTM" slag 1625 OC' ASTM" slag 1600 OCC 41.7 40.9 56.4 k 56.4 48.4 k 48.3 Si02 40.8 49.8 k 49.9 49.6 k 49.7 k 20.6 33.5 20.6 27.6 k 27.5 29.8 k 29.8 28.2 k 28.2 A1203 33.5 33.1 2.1 2.0 0.4 k 0.4 1.3 k 1.3 1.2 TiOz 2.0 k 1.2 0.1 k 0.1 13.0 12.0 k 12.0 k 15.3 12.1 k 12.1 13.5 15.3 12.3 k 12.3 Fe203 13.0 4.1 3.4 6.2 k 6.2 0.7 k 0.7 CaO 3.4 3.3 k 3.3 2.8 k 2.8 k 2.9 2.1 k 2.1 3.7 2.9 1.3 k 1.3 2.1 2.0 k 2.0 MgO 3.7 2.7 2.9 0.7 k 0.7 4.0 k 4.0 NazO 2.9 0.5 k 0.5 0.5 k 0.5 0.7 k 0.7 0.6 k 0.6 1.9 k 1.9 0.6 0.6 4.4 k 4.4 0.6 K20 0.0 0.1 k 0.1 O.ld k 0.2 0.1 O.ld k 0.0 O.ld k 0.0 so3 O.ld
B/A' SRf SA8 LFh
0.31 67.0 1.22 0.55
0.30 61.9 1.26 0.46
0.31 67.0 1.22 0.55
0.29 72.8 2.74 0.76
k k k
k
0.29 12.8
2.74 0.76
0.29 12.8
1.75 0.18
k k k k
0.30 72.7 1.76 0.19
0.24 15.0
1.67 0.38
k k k k
0.24 14.9 1.67 0.39
0.28 74.4 1.76 0.39
k k k k
0.28 74.4 1.76 0.39
" Reference 25 (baaed on SO3-freebasis). Reference 25. e Predicted liquid composition based on ASTM (S03-free)ash. Aeaume content of SO3 is 0.1 in ash because sometimes SOLGASMIX does not converge using the SO,-free basis (SO,of 0.0). eBase/acid ratio. fSilica ratio. gSi02/A1203ratio. Lignite factor. 'Reference 22 (baaed on SO,-free basis). 'Not available. 'Predicted liquid composition based on HTA (SO,-free). "'Reference 36 (based on SO,-free basis). "Reference 24 (based on SO,-free basis). predicted liquid compositions are summarized in Table III. Robinson subbituminous coal ash belongs to group 11. Hence the Hoy equation would be used to calculate viscosity. Results are shown in Figure 10 and Table 111. The Hoy equation showed a 13.2% average deviation, well within the limit considered acceptable, from experimental viscosity data in the Newtonian region.
The Viscosity of Eagle Butte subbituminous coal ash was determined as done for the Robinson sample. Ash, slag, and predicted liquid phase compositions are summarized in Table IV.Eagle Butte ash is classified in group IV. The modified Watt-Fereday equation gave 14.6% average deviation, again well within acceptable limits from experimental data.
Energy & Fuels, Vol. 6, No. 4, 1992 397
Bediction of Coal Ash Slag Viscosity
Table VII. Ash, Slag, and Predicted Liquid Compositions (wt %) of Group VI1 Samples Leigh Creek slag Leigh Creek slag Garrick Cronton No. 2273-A No. 16418 No. 16419 Rossington 1425 1600 1500 1375 1425 composn HTA' el& 'Cc ASTMj slag O C ' ASTMm slag 'C' HTA~ elan 'cC HTAj elan O C ' k 46.1 38.5 36.3 k 36.3 45.7 k 46.3 45.5 k 39.6 Si02 38.4 37.4 39.1 k 24.7 26.7 k 27.4 22.4 k 22.4 25.7 k 26.0 24.4 A1203 27.1 25.1 27.6 1.7 k 1.7 1.4 k 1.4 1.2 k 1.2 0.1 k 0.1 1.2 1.2 1.4 Ti02 k 22.0 k 18.8 21.4 k 38.5 18.1 k 18.4 18.6 38.5 Fez03 20.0 22.2 20.3 4.0 k 4.0 5.0 k 5.1 0.6 k 0.6 3.6 k 3.6 7.5 7.1 CaO 7.0 1.7 k 1.7 1.9 k 2.0 2.4 k 2.4 0.3 k 0.3 1.6 1.6 1.4 MgO 3.7 k 1.5 2.5 k 1.7 0.2 k 0.1 0.8 k 0.4 1.3 2.4 2.4 Na20 1.2 k 0.9 1.2 k 0.6 0.4 k 0.3 3.5 k 2.7 2.3 1.7 K20 2.2 k 0.1 0.4 k 0.4 k 0.3 O.ld 0.4 k 0.4 O.ld 0.1 O.ld 0.3 SO3
B/Ae SRf SA8 LFh
0.50 0.56 0.47 57.3 54.6 57.3 1.42 1.49 1.42 0.43 0.40 0.43
0.67 47.9 1.62 0.02
k k k k
0.67 47.9 1.62 0.02
0.40 65.5 1.78 0.33
k k k k
0.38 0.39 65.5 65.2 1.78 1.87 0.33 0.31
k k k k
0.38 0.50 65.2 57.6 1.87 1.44 0.31 0.32
k k k k
Choctaw HTA" 37.3 15.0 1.0 28.7 13.8 3.0 O.ld
1.0 O.ld
1250 slag" O C C 35.6 36.4 19.9 14.8 0.9 1.0 26.9 30.8 13.0 13.2 2.6 2.9 0.0 0.1 1.1 0.7 0.0 0.1
0.46 0.87 0.77 0.92 57.6 45.0 45.6 43.7 1.44 2.49 1.78 2.46 0.32 0.59 0.78 0.52
'Reference 22 (based on SO3free basis). *F&ference 22. CPredictedliquid composition based on HTA (S03-free). dAssume contents of SO3y d NazO are 0.1in ash because sometimes SOLGASMIX does not converge using SO3and NazO of 0.0. 'Base/acid ratio. [Silica ratio. pSiOz/A1203ratio. hLignite factor. jReference 25 (based on S03-free basis). kData not available. 'Predicted liquid composition based on ASTM (S03-free) ash. "'Reference 24 data (baaed on SO3-freebasis). "Reference 23 (based on SO3-free basis). "Reference 23. loooo
we compared predicted and experimental viscosity data; I however, the agreement between the SOLGASMM predic-
__
-*-
lo
Experimental SOLGASMIX + Mo.Wan Mo.Watt,Slag
1
1 1100
1200
1300
1400
Temper at ure,OC
Figure 12. Comparisons of newly determined experimental viscosity (ref 32)and predicted results for Beulah lignite,showing improved prediction with SOLGASMIX.
The viscosity of Beulah lignite ash slag was determined as in the previous two cases. Ash, slag, and predicted liquid-phase compositions are summarized in Table IV. Beulah lignite ash also is classified as group IV. The results are shown in Figure 12 and Table IV.For this slag, the modified Watt-Fereday equation showed a 20.3 % average deviation from experimental data. In this instance, if the modified Watt-Fereday equation has been used without SOLGASMIX a 91.8% average deviation from experimental data would have been obtained.
Conclusions The appropriate group classification is the key to successful application of the combined SOLGASMIX-VISCOS~ program to predict the viscosity of coal ash slags or melt phases. At the present stage of model development, the group classification rules are based on the traditional approaches to describe the coal ash characteristics, such as the base-to-acid ratio, silica ratio, lignite factor, and percentages of SO2, Fe203,and Na20. The predicted liquid-phase composition of 33 chosen materials changed slightly as a function of temperature in the range for which
tions and the analyaes of the solidified slag after a viscosity test was generally quite good. For a large number of ashes, excluding high @io2 > 5 5 % ) and very low (Si02 < 25%) silica ashes, it is possible to obtain useful predictions of the liquid-phase composition and viscosity over a range of temperatures in the Newtonian region from the combined SOLGASMIX-VISCOSITY program using only the initial ash composition as the input data. We have demonstrated the ability to obtain acceptably accurate predictions of Newtonian-region viscosity in 29 of 33 trials (that is, in 88% of the cases). The 33 sets of data tested included measurements of ashes from coals of a wide rank range (brown coal to bituminous) and measurements made in five laboratories in four nations, as well as three newly generated measurements. Thus the present combined SOLGASMM-VISCOSITY model should be able to be applied with confidence to coals falling within the ash composition range used here. Nevertheless, we recognize that, at the present state of development, a very powerful predictive method (SOLGASMM) based on thermodynamic principles is coupled with viscosity calculation methods that were largely derived from empirical fitting of data. The group classification rules, which are the essential nexus between the two parta of the combined model, are also empirical. Refinementa of the present model must be directed to eatablishing a f m e r first-principlesfoundation for the viscosity calculation portion of the model. As is the case with virtually all viscosity calculation models, the work we have done so far does not provide a prediction of the temperature of critical viscosity, T,, nor of the viscosity behavior as a function of temperature in the non-Newtonian flow region below Tcv. However, it would seem that the ability to calculate the amount and composition of the liquid and solid phases afforded by SOLGASMIX may provide the key to development of a means for calculating both of these important slag properties.
Acknowledgment. Financial support for various aspects of this study was provided by the Commonwealth of Pennsylvania under the Coal-Water Fuel Project, the U.S. Department of Interior Mineral Institutes program (Grant G1164142) via the Pennsylvania Mining and Mineral Research Institute, and the Pennsylvania State University Research Initiation Grant program. Mr. Carl Martin rendered valuable technical assistance in setting up the viscometer and machining the rotating bobs. The
Energy h Fuels 1992,6,398-406
398
coal samples used for our viscosity determinations were provided by John P. Hurley and Sharon Falcone Miller. Data on the Medicine Bow coals was kindly made available by Robert C. Streeter from work performed at BCR National Laboratory. The sample of Austrian brown coal ash slag and supporting data were provided by Dr. Gernot
Staudinger of the Technische Universitiit Graz. We are grateful for the helpful advice of Karl E. Spear on use and applications of the SOLGASMIXprogram. Registry No. Si02, 7631-86-9;A1203,1344-28-1;TiOz, 13463-67-7; Fe203,1309-37-1; CaO, 1306-78-8;MgO, 1309-48-4; KzO, 12136-45-7. Na20, 1313-59-3;
Studies on the Reduction of Nitric Oxide by Carbon: The NO-Carbon Gasification Reaction Hsisheng Teng,? Eric M. Suuberg,* and Joseph M. Calo Division of Engineering, Brown University, Providence, Rhode Island 02912 Received December 19, 1991. Revised Manuscript Received March 20, 1992
The heterogeneous reduction of NO by carbon was studied in a thermogravimetric analysis system, employing both pseudosteady and transient reaction methods. The reaction was studied at temperatures from near ambient up to 1073 K, and at NO partial pressures in the range 1.01-10.1 kPa. A relatively pure carbon derived from phenolic resin was studied. Gaseous products of rection were measured. The gasification of carbon by NO involves two parallel processes: (1)somewhat slow desorption of relatively stable surface complexes; (2) processes involving NO attack on active unoccupied sites that results in essentially immediate desorption of gaseous products. The first process controls the overall gasification rate at lower temperatures and is governed by a distribution of desorption activation energies, involving mainly surface oxides that yield CO upon desorption. The second process dominates at high temperatures (somewhat arbitrarily defined by T > 650 "C or 923 K)and is suggested to be controlled by the dissociative chemisorption of NO on the carbon surface. 1. Introduction The mechanisms of the reactions of carbons with 02, COP,and H 2 0 are not yet well understood in many respects. The reactions of carbons with NO are even less well characterized. However, the fact that NO formed during combustion can be heterogeneously reduced by carbonaceous residues produced in situ is well-known.' There have been a modest number of studies of various aspecta of the NO-carbon gasification reaction in the past 30 years2-16(see Table I). A separate literature exists on the process of chemisorption of NO by carbons, and this will be reviewed elsewhere.17 Heterogeneous reactions of NO with carbon can reduce NO to N2 and form CO and C02gaseous products. In general, the overall gasification reaction of carbon with NO has been reported to include the following stoichiometric reactions (see studies cited in Table I): C + 2N0 +C02 + N2
C + NO
-
CO +
CO + NO
C02 + Y2N2
The last reaction in the sequence is actually carbon-surface-catalyzed oxidation of CO by NO, based upon the observation that NO reduction by carbons is enhanced in the presence of C0.l2J8 At low temperatures, the process may not involve the release of gaseous carbon oxides, as outlined above; instead, stable surface oxides may be
* Corresponding author. 'Present address: Advanced Fuel Research, 87 Church Street, East Hartford, CT 06108.
formed on the carbon via chemisorption. Chemisorption of NO on carbon has recently been studied by our group.17J920 (1)Pereira, F. J.; Beer, J. M.; Gibbs, B.; Hedley, A. B. 15th Symposium (International)on Combustion; The Combustion Institute: Pittsburgh, 1975;p 1149. (2)Watts, H.Trans. Faraday SOC.1958,54,93. (3)Smith, R. N.;Swinehart, J.; Lesnini, D. J. Phys. Chem. 1969,63, 544. Also: Smith, R. N.; Lesnini, D.; Mooi, J. J. Phys. Chem. 60,1063 (1956). (4)Edwards, H. W. AIChE Symp. Ser. No. 126 1972,68,91. (5)Lai, C.-K. S.;Peters, W. A.; Longwell, J. P. Energy Fuels 1988,2, 586. (6)Degroot, W. F.; Richards, G. N. Carbon 1991,29,179. (7)Gibbs, B. M.; Pereira, F. J.; Beer, J. M. 16th Symposium (Internatronal) on Combustion; The Combustion Institute: Pittsburgh, 1977; p 461. (8)Song, Y. H.; Beer, J. M.; Sarofim, A. F. Combust. Sci. Technol. 1981,25,237. (9)Levy, J.; Chan, L. K.; Sarofii, A. F.; Be&, J. M. 18th Symposium (International)on Combustion; The Combustion Institute: Pittsburgh, 1981;p 111. (10)Schuler, J.; Baumann, H.; Klein, J. ErdoZ Kohle-Erdgas 1988,41, 296. (11)Matos, M. A. A,; Pereira, F. J. M. A.; Ventura, J. M. P. Fuel 1991, 70, 38. (12)Chan, L. K.; Sarofim,A. F.; Beer, J. M. Combust. Flame 1983,52, 37 _ .I145.
(13)Furusawa, T.; Kunii, D.; Osuma, A.; Yamada, N. Kogaku Kogaku 1978,6,562; Znt. Chem. Eng. 1980,20,239. (14)Bedjai, G.;Orbach,H. K.; Reisenfeld, F. C. I d . Eng. Chem. 1958, 50, 1165. (15)Kapteijn, F.; Mierop, A. J. C.; Abbel, G.; Moulijn, J. A. Proc. Carbone 84,Int. Carbon Conf. Bordeaux, Fr. 1984,84. (16)Suuberg, E. M.; Teng, H.; Calo, J. M. 23rd Symposium (Internotional) on Combustion; The Combustion Institute: Pittsburgh, 1991; p 1199. (17)Teng, H.;Suuberg, E. M., to be submitted to Carbon. (18)Shelef, M.; Otto, K. J. ColZoid. Interface Sci. 1969,31,73. (19)Teng, H.; Suuberg, E. M.; Calo, J. M. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1990,35(3),592.
0887-0624/92/2506-0398$03.~0/0 0 1992 American Chemical Society
