Biomass

Energy & Fuels 2006, 20, 1329-1340

1329

Studying the Melting Behavior of Coal, Biomass, and Coal/Biomass Ash Using Viscosity and Heated Stage XRD Data S. Arvelakis,*,† B. Folkedahl,‡ K. Dam-Johansen,† and J. Hurley‡ CHEC, Department of Chemical Engineering, Technical UniVersity of Denmark, Building 229, DK-2800 Lyngby, Denmark, and Energy and EnVironmental Research Center, UniVersity of North Dakota, 15 North 23rd Street, Grand Forks, North Dakota 58203 ReceiVed June 8, 2005. ReVised Manuscript ReceiVed January 25, 2006

The use of biomass for power generation can result in significant economical and environmental benefits. The greenhouse emissions can be reduced as well as the cost of the produced electricity. However, ash-related problems, including slagging, agglomeration, and corrosion, can cause frequent unscheduled shutdowns, decreasing the availability and increasing the cost of the produced power. In addition, the fouling of the heat exchange surfaces reduces the system efficiency. In this work the melting and rheological properties of various biomass and biomass/coal ash samples were studied by using a high-temperature rotational viscometer and a hot stage XRD. The produced data were used to calculate the operating temperature of a pilot-scale entrained flow reactor during the cocombustion of biomass/coal samples in order to ensure the slag flow and to avoid corrosion of the walls due to liquid slag/metal interaction. Biomass ash proved to have significantly different melting behavior compared to that of the coal ash. Furthermore, the addition of biomass to coal ash led to lower viscosity and subsequently to higher stickiness of the produced ash particles. The melting behavior of the slag generated by the cocombustion tests appeared to be somewhat different compared to that of the laboratory-prepared ash samples. The heated stage XRD data provide useful information regarding the reactions among the various ash compounds and the phase transformations during the heating and cooling of the ash samples and helped the explanation of the produced viscosity curves.

1. Introduction The use of biomass for power generation can result in significant economical and environmental benefits. Biomass is considered to be a CO2 neutral and also low-cost fuel, and thus, its widespread use can reduce greenhouse emissions as well as the cost of the produced electricity. However, the use of biomass alone or in combination with coal for power production in conventional coal-fired units is associated with a number of problems mainly created by the extremely reactive nature of its ash. Ash-related problems, including slagging, agglomeration, corrosion, and erosion, can cause frequent unscheduled plant shutdowns, decreasing the availability and increasing the cost of the produced power. In addition, the fouling of the heat exchange surfaces reduces the system efficiency. The biggest problems are associated with the use of agricultural waste materials such as corn stalks, various kinds of straws, olive pits, etc. that contain large amounts of potassium, sodium, calcium, silicon, and chlorine and can also contain relatively medium to high amounts of sulfur. Also, biomass fuels resulting from woody biomass such as bark and tree trimmings can be a problem since they might contain high amounts of sulfur and calcium and also low amounts of alkali metals.1-5 * Corresponding author. Phone: +45 45252835. Fax: +45 45882258. E-mail: [email protected]. † Technical University of Denmark. ‡ University of North Dakota. (1) Morris, G. EnVironmental Costs and Benefits of Biomass Energy Use in California; NREL/SR-430-22765; Future Resources Associates, Inc. Eds.; Berkeley, CA, 1997. (2) Miles, T. R., Sr.; Miles, T. R., Jr. EnVironmental Implications of Increased Biomass Energy Use; Research Report NREL/TP-230-4633, 1; 1992.

The use even of small amounts (up to 20% on a thermal basis) of such biomass fuels in combination with coal in conventional pulverized fuel (pf) boilers, or grate-fired boilers can lead to significant deposition and corrosion problems compared to the coal firing.6-8 Problems appear in both the heat transfer surfaces and the main reactor body. The high amounts of alkali metals and chlorine present in the biomass ash lead to the formation of alkali salt-rich layers on the surface of the fly ash particles increasing their stickiness and therefore their tendency to deposit and create deposition and corrosion problems.9-12 Furthermore, the alkali metals react with the aluminosilicate particles of the fly ash and produce alkali and alkali-calcium aluminosilicates with melting points that are well below the normal operating (3) Miles, P. E.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Alkali deposits found in biomass power plants, Volumes I, II. Summary Report for the National Renewable Energy Laboratory; NREL Subcontract TZ-2-11226-1; 1995. (4) Baxter, L. L. Biomass Bioenergy 1993, 4 (2), 85-105. (5) Arvelakis, S.; Koukios, E. G. Biomass Bioenergy 2002, 22 (5), 331348. (6) Hansen, L. A.; Frandsen, F. J.; Sørensen, H. S.; Rosenberg, P.; Hjuler, K.; Dam-Johansen, K. Impact of Mineral Impurities in Solid Fuel Combustion; 1999; pp 342-356. (7) Mann, M. D.; Galbreath, K. C. Proceedings of the 5th Engineering Foundation Conference on Inorganic Transformations and Ash Deposition During Combustion, 1991. (8) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel Process. Technol. 1998, 54, 17. (9) Skrifvars, B.; Backman, R.; Hupa, M.; Sfiris, G.; Abyhammar, T.; Lyngfelt, A. Fuel 1998, 77, 65. (10) Skrifvars, B.; Backman, R.; Hupa, M. Fuel Process. Technol. 1998, 56, 55. (11) Lin, W.; Dam-Johansen, K. Proceedings of the 15th International Conference on Fluidized Bed Combustion, 1999; p 1188. (12) Ohman, M.; Nordin, A.; Skrifvars, B.; Backman, R.; Hupa, M.; Energy Fuels 2000, 14, 169.

10.1021/ef050168b CCC: $33.50 © 2006 American Chemical Society Published on Web 03/15/2006

1330 Energy & Fuels, Vol. 20, No. 3, 2006

temperatures in the boiler’s main body for the typical pf and even the grate-fired boilers. As a result, a running slag is produced that is coming into contact with the walls of the boiler corroding the refractory lining and the internal metal surface. Although in some cases the formation of a running slag is a design parameter for some boilers13-15 in order to remove the produced molten ash from the main reactor body, most pf-fired boilers do not react well to running slag. To achieve formation of a running slag the flow characteristics of the produced slag must be known as a function of temperature to accurately control the wall temperature that will allow the formation of a solid deposit layer between the wall and the running slag avoiding the corrosion of the refractory and the metal surface. The flow characteristics of the generated slag material depend on its viscosity. Viscosity is the primary physical property in the hightemperature regions of a boiler, T > 1093 °C, affecting deposit strength development and slag flow behavior. The viscosity of ash particles and slag materials is dependent upon chemical composition, oxygen level, and temperature. In systems where slag flow must be maintained, the temperatures of the slag must be high enough to produce a lowviscosity slag that will flow. The viscosity system operating conditions relationships currently used to determine ash/slag effects in slagging systems include the temperature of critical viscosity (Tcv) and temperature where the slag has a viscosity of T250. The Tcv is the temperature at which the slag begins to solidify as a result of crystallization. The Tcv is defined as the temperature where the flow behavior moves from a Newtonian, where the shear stress is independent of the shear rate, to nonNewtonian flow, where the shear stress is dependent on the shear rate.16,17 Operating at temperatures near or below Tcv can cause rapid increase in viscosity and solidification of the slag. The T250 is the temperature at which the viscosity of the slag is 250 poise. Maintaining viscosity at or below this value allows for easy slag flow. Determination of the viscosity of slags can be done by using various methods experimentally and also computationally using models based on slag composition and temperature.17-24 In this study, a high-temperature rotational viscometer is used to study the viscosity characteristics of various coal, biomass, and coal/biomass mixtures. The mineralogical and phase transformations as well as the crystallization behavior of the ash samples upon heating and cooling was measured by heated stage XRD (HSXRD). The produced data were used to calculate the operating temperature of a pilot-scale slagging furnace during the cocombustion of biomass/coal samples in order to ensure the slag flow and to avoid corrosion of the walls due to liquid slag/metal interaction. The produced laboratory data were proven (13) Heinzel, T.; Siegle, V.; Spliethoff, H.; Hein, K. R. G. Fuel Process. Technol. 1998, 54, 109-125. (14) Russel, N. V.; Wigley, F.; Williamson, J. Fuel 2002, 81, 673-681. (15) Rawers, J.; Iverson, L.; Collins, K. J. Mater. Sci. 2002, 37 (3), 531538. (16) Benson, S. A. Ash formation and behavior in utility boilers. http:// www.microbeam.com (accessed 2004). (17) Vargas, S.; Frandsen, F. J.; Dam-Johansen, K. Prog. Energy Combust. 2001, 27, 237-429. (18) Nowok, J. W.; Hurley, J. P.; Bieber, J. A. J. Mater. Sci. 1995, 30, 361-364. (19) Senior, C. L.; Srinivasachar, S. Energy Fuels 1995, 9, 277-283. (20) Stanmore, B. R.; Budd, S. Fuel 1996, 12, 1476-1479. (21) Hurst, H. J.; Nowak, F.; Patterson, J. H. Fuel 1999, 78, 439-444. (22) Hurst, H. J.; Nowak, F.; Patterson, J. H. Fuel 1999, 78, 18311840. (23) Nowok, J. W.; Benson, S. A.; Steadman, E. N.; Brekke, D. W. Fuel 1993, 72, 1055. (24) Folkedahl, B. C.; Schobert, H. H. Energy Fuels 2005, 19, 208215.

ArVelakis et al. Table 1. Analysis and Characterization of Coal and Biomass Samples proximate analysis (% as det)

antelope coal

hog fuel

corn stalks

switchgrass

moisture ash volatiles fixed carbon

14.00 4.1 40.66 41.24

9.72 3.64 75.21 11.43

10.82 5.79 69.19 14.21

6.9 5.4 74.1 13.6

1.05 58.44 5.55 0.21 nda 30.66 10239

0.56 43.66 6.28 0.14 nda 45.72 7298

0.9 38.09 5.92 0.08 nda 49.22 6711

1.2 42.4 6.0 0.13 nda 44.9 7402

ultimate analysis (% as det) nitrogen carbon hydrogen sulfur chlorine oxygen GCV (btu/lb) a

nd: not determined.

to be very helpful allowing the accurate determination of the boiler’s working temperature in order to avoid corrosion problems and also to ensure the high efficiency of the system during the cocombustion tests. 2. Experimental Section The materials used in this study were a coal sample and three different biomass samples. The coal was a North Antelope subbituminous coal. The biomass samples were a woody (hog) fuel, which contains bark and tree trimmings, corn stalks from the Northern American plains region, and switchgrass from energy plantations. The coal/biomass ash mixtures were formed by mixing ash from the biomass fuels with coal ash on a basis of 20% thermal input of the correspondent coal/biomass blends. Furthermore, in the case of the hog fuel one additional mixture was formed on the basis of 10% thermal input. The ashing of the coal and biomass fuels was performed according to the ASTM D3174-03 standard method. Table 1 presents the results from the proximate and ultimate analysis of the coal and biomass fuels, while Table 2 presents the results regarding the elemental analysis of the coal, biomass, and coal/biomass ash samples studied. Table 3 gives the results regarding the elemental analysis of the various coal, biomass, and coal/biomass slags used in the viscosity studies. The Antelope coal has low ash content. The coal heating value is approximately 30% higher compared to the heating value of the biomass fuels. The hog fuel contains the highest amount of volatiles and has the lowest ash content of all the four fuels. The corn stalks biomass has the highest ash and the lowest sulfur content, while it is high in nitrogen and oxygen compared to the hog fuel. Switchgrass appears to have high volatiles and ash content, while it also has the highest caloric value of all the biomass fuels. The amounts of carbon, hydrogen, and sulfur appear to be similar to those of the hog fuel. Coal ash contains large amounts of silica, aluminum, and calcium, while significant amounts of iron, magnesium, and sulfur are also present. The presence of alkali metals and other elements such as titanium and phosphorus is limited. Hog fuel ash contains mainly calcium as the dominant element. The amount of silica and alkali metals is rather low, while the amounts of elements such as iron, aluminum, magnesium, titanium, and sulfur is below 2% on an ash basis. The poor closure of the elemental analysis results is suspected to be due to the high amount of calcium carbonates being present in the ash material. Corn stalks ash has basically a silicate structure with silica accounting for almost half of the ash material. It also contains medium amounts of calcium and alkali metals, while the amount of magnesium appears to be among the highest observed in biomass samples reaching 11% on an ash basis. Some enrichment of the specific sample in magnesium due to extraneous factors is suspected. Finally, the switchgrass ash is dominated by the presence of silica up to 70% on an ash basis, while alkali metals, calcium, magnesium, phosphorus, and sulfur can be found in concentrations

Melting BehaVior of Coal/Biomass Ash

Energy & Fuels, Vol. 20, No. 3, 2006 1331

Table 2. Ash Elemental Analysis of Coal, Biomass, and Coal/Biomass Samples ash basis %

K2O

Na2O

CaO

MgO

SiO2

Al2O3

Fe2O3

TiO2

SO3

P2O5

antelope coal hog fuel corn stalks coal/hog fuel (6/1) coal/hog fuel (3.33/1) coal/corn stalks (1.86/1) switchgrass coal/switchgrass (2.11/1)

0.32 4.83 9.39 1.11

0.95 2.45 0.57 1.08

23.2 30.5 9.7 25.3

6.02 3.04 10.51 5.82

27.9 9.9 45.6 21.6

16.4 1.2 3.0 13.2

7.35 1.10 1.89 5.09

1.30 0.10 0.23 1.03

7.66 0.66 1.99 10.63

1.36 1.58 3.08 1.39

1.61

1.18

26.1

5.91

24.5

12.2

4.43

0.97

9.21

1.33

3.39

0.80

19.7

7.79

32.6

12.3

5.58

0.99

9.74

1.82

7.0 2.5

0.03 0.65

9.4 18.8

4.9 5.6

70.3 41.5

0.0 11.15

0.25 5.1

0.03 0.89

3.3 6.3

4.8 2.5

Table 3. Elemental Analysis of Coal, Biomass, and Coal/Biomass Slag Samples ash basis %

K2O

Na2O

Fe2O3

TiO2

SO3

P2O5

coal slag hog fuel slag hog fuel slaga coal/hog fuel (6/1) slag coal/hog fuel (3.33/1) slag coal/hog fuel slagb corn stalks slaga coal/corn stalks (1.86/1) slag coal/corn stalks (1.86/1) slaga coal/corn stalks slagb switchgrass slag coal/switchgrass (2.11/1) slag coal/switchgrass (2.11/1) slaga

0.3 5 5.3 3 0.3 1.0 3

1.58 3.54 1.03 1.7

26 41.5 45.1 28

5.46 3.63 3.58 5.5

35.3 25 28.6 32.9

17.7 2.9 3.1 15.8

6.74 2.18 2.59 6.55

1.35 0.23 0.28 1.23

0.0 1.1 6 0.0 0.0

1.13 1.73 1.92 1.24

1.8 4

2.17

30.6 5

4.91

32.2 1

13.2 6

5.37

1.01

0.3 5

1.31

1.1 9.2 7 2.8 5

1.2 1.98 1.24

28 11.4 17.1

4.4 11.5 8 6.21

37.7 51.7 36.7

20.1 3.1 11.5

5.4 2.63 4.72

1.1 0.37 0.9

0.0 0.0 3.1 2

0.9 2.92 1.43

2.5 3

1.11

16.9

6.46

42

12.9

5.25

0.99

0.0

1.4

1.4 8.4 9 2.5 3

0.9 0.13 1.44

20.3 11.6 7 18.5

5.9 3.29 4.61

45.5 69.8 7 45.3

18.6 0.15 13.5

5.3 1.04 6.02

1.1 0.17 1.03

0.0 0.1 2 0.0 0

1.0 5.01 1.88

1.5

0.8

18.9

5.0

51.0

15.2

5.4

1.0

0.0

1.1

a

CaO

MgO

SiO2

Al2O3

Slag after the viscosity measurement. b Slag from the cocombustion tests.

Figure 1. Experimental setup: (a) test rig, (b) test components.

varying from 3% to 10% on an ash basis. The amount of iron in the switchgrass ash appears to be the lowest among all the selected biomass and coal samples as well as the amount of alumina. Mixing of the coal ash with the biomass ashes lowers the amounts of silica, iron, and aluminum and increases the amounts of calcium, alkali metals, sulfur, and phosphorus compared to the coal ash in the case of the coal/hog fuel mixture. In the case of the coal/corn stalks mixture the amount of magnesium also increases significantly together with the amounts of silica and alkali metals, while the amounts of calcium, aluminum, and iron decreases. The viscosity measurements were performed using a Haake hightemperature rotational viscometer. The viscometer shown in Figure 1 is composed of three different parts. A high-temperature furnace up to 1600 °C, a temperature control box, and a viscometer VT550 system, with a measuring head, which is a rotating bob viscometer. The viscometer furnace is a box type furnace with cylindrical holes aligned from the top to the bottom of the furnace. An alumina tube is inserted through both holes with the ends of the tube extending outside the furnace. The tube exiting out the bottom of the furnace is sealed with a cap. This is designed to prevent outside gases from entering, as well as minimizing the test gas mixture from seeping out of, the test chamber environment. In addition, an electric steam generator is used to regulate the moisture output into the test gas

mixture. Inside the alumina tube there is a cylindrical alumina pedestal shown in Figure 1b having a diameter of 38.6 mm and length of 152.4 mm, where the crucible used for the test is standing during the measurement. Before the start of the measurement the laboratory-prepared ash material was preheated to 900 °C in a porcelain dish to burn any residual carbon that may have been left from the ashing process. The preheated ash samples as well as the slag samples that resulted from the pilot-scale combustion tests were then premelted in a PtRh (80/20 wt %) dish using a muffle furnace for at least 2 h. During the premelting process the ash material is transformed to a glassy or nonglassy slag, while the melting temperature of the slag material and subsequently the working temperature of the viscometer is also determined. After the premelting stage the material was cooled, crushed with a hammer, and used to fill the crucible used in the viscosity measurements. During the viscosity measurement the slag is heated to 50 °C higher than its liquidus temperature, and then the viscometer bob is submerged into the slag until the slag just covers its top, and then it is rotated at 45.3 rpm. Testing in a reducing environment requires the use of a spindle-shaft and bob fabricated from a molybdenum alloy. When testing is performed in an oxidizing atmosphere, such as this one, the spindle and bob are made of 90% platinum and 10% rhodium. The bob is 9 mm in diameter and 20 mm long and has a 45° taper at each end. The torque applied to the viscometer is read and converted to an electrical signal that is then sent to a computer with a data acquisition program that determines the viscosity of the slag. The viscosity is subsequently measured as the temperature is dropped in intervals of 50 °C. The slag is kept at each new temperature for 1 h before the new viscosity is determined. This time is considered to be enough for the slag to reach equilibrium as also seen from previous studies.18,23-25 The viscosity of the slags are measured (25) Vuthaluru, H. B.; Domazetis, G.; Wall, T. F.; Vleeskens, J. M. Fuel Process. Technol. 1996, 46, 117-132.

1332 Energy & Fuels, Vol. 20, No. 3, 2006 over the range of 10 to as high as over 100 000 poise, while the torque ranges from 0% to 100%. Normally, the test was continued until the torque reached a value close to 90%, which indicated that the material had been almost completely solidified and/or crystallized, at which point the measurements were terminated. The high-temperature XRD measurements were performed using a Philips X’prt diffractometer. The diffractometer has a copper source for X-ray generation and is equipped with a hot stage configuration that can be used to determine phase changes and mineralogical transformations from ambient temperature to temperatures up to 1600 °C. Unfortunately, in our study the hot stage configuration was able to function only up to 1250 °C, and so our tests were limited to this temperature. The ash material was first mixed with ethanol to form slurry and then was sprayed on the top of a Pt/Au (90/10 wt %) sample holder to form a uniform layer. Then the sample was heated to the maximum temperature of 1250 °C starting from the temperature of 600 °C at intervals of 50 °C. At each interval temperature an XRD scan was taken from 5° to 75°, and the scan speed was 0.02°/s. After reaching the Tmax the ash sample was cooled, and new XRD measurements were performed under the same conditions. In this way information on the changes and transformations during both the heating and the cooling stages was obtained for each ash sample.

3. Results and Discussion 3.1. Coal. Figure 2a presents the results of the viscosity measurements performed using the coal ash. The coal ash is seen to melt around 1250 °C during the premelting stage, while it forms two separate phases, a glassy dark phase and a yellow nonglassy phase on the top of the glassy phase consisting of alkali and calcium sulfates as the XRF analysis shows. The analysis of the coal glassy phase presented in Table 3 shows that it is enriched in elements such as Si, Al, Ca, K, Na, etc., while the sulfur has been removed completely from the system. The first two viscosity tests presented in Figure 2a are repetitions using the same rotational speed 45.3 rpm, while the third was performed using a higher rotational speed of 107.8 rpm. As seen from the first two viscosity curves, coal ash melt shows Newtonian behavior until the temperature of 1200 °C. Below that temperature the coal ash melt is characterized by the presence of a gradually increasing number of crystals, and its behavior starts to change to non-Newtonian. The melt appears to reach its Tcv at 1190 °C where it starts to solidify rapidly as seen from the rapid increase in viscosity from the viscosity curve. The viscosity curve in the second test differs compared to the one in the first test in both the temperature segments where Newtonian and non-Newtonian behavior of the melt is observed. This is probably due to the presence of small nuclei in the melt even after the reheating of the slag back to the initial temperature that now tend to increase in size rapidly forming solids inside the melt causing an increase of the observed viscosity and leading to a more rapid solidification of the melt. As seen from the third test the use of higher rotational speed (107.8 rpm) leads to higher viscosity values, and the viscosity curve appears to shift to slightly higher temperatures regarding the formation of the first crystals in the melt as well as reaching of the Tcv temperature. Figure 2b presents the heated stage XRD data for the coal ash while heating to a melt. It shows that a substantial amount of quartz and anhydrite (CaSO4) exists in the coal ash, along with small amounts of brownmillerite (Ca2Al2O5) initially. These largely react between 950 and 1000 °C, along with some of the aluminosilicate glass material, to form gehlenite (Ca2Al2SiO7), which slowly dissolves at higher temperatures. Figure 2c shows the heated stage XRD data for the coal slag while cooling. It shows that there is a small amount of gehlenite in the molten slag even at 1250 °C, which increases as the slag

ArVelakis et al.

cools. However, there is not a strong increase in peak area on cooling, indicating that most of the solidification of the slag is simple hardening of the amorphous glass phase and not the formation of large quantities of solid crystals. 3.2. Hog Fuel. The premelting of the hog fuel results in a nonglassy slag with no flow characteristics even at temperatures higher than 1500 °C. The heated stage XRD data shown in Figure 3b indicate that the initial ash contains high levels of calcite (CaCO3) and quartz, which appear to decompose and react to form calcium- and magnesium-rich silicates such as calcium silicate (Ca2SiO4), merwinite (Ca3Mg(SiO4)2, and akermanite (Ca2MgSi2O7), undoubtedly also along with calcium aluminosilicate glass (which cannot be detected by XRD because it is not crystalline). The apparent disappearance of crystalline forms above 1050 °C in the data is most likely due to an experimental problem, such as the slag flowing out of the sample holder, or to the formation of an amorphous phase, rather than a true melting of the crystals since their melting points are in the 1450-1575 °C range and because no crystallization occurred during cooling. It is likely that high concentrations of these minerals are present at 1500 °C, which is why the slag does not flow well at that temperature. Figure 3a shows that the hog fuel slag shows mainly non-Newtonian behavior during the viscosity measurements. The measured viscosity is characterized by large variations probably due to the periodic formation and dissolution of crystals in the melt. The results from the two different measurements differ substantially between them though they were both conducted under the same experimental conditions. The viscosity curves appear to follow the same pattern only at the first steps of the tests down to 1400 °C. After that point the observed differences are very big and cannot be explained. During the first test the material appears to reach its Tcv at the level of 1130 °C, where the slag solidifies rapidly. However, in the second test the Tcv appears to shift down to temperatures below 1100 °C. As seen from Table 3, the slag after the end of the viscosity measurements appears to be depleted in alkali metals and sulfur, while the amounts of calcium, silicon, aluminum, as well as of the other elements increase compared to those of the premelted slag. The low amounts of alkali metals, silicon, and aluminum in the slag and the high amounts of Ca and Mg seem to prevent the formation of a silicate network and, thus, the solidification of the slag at high temperatures. However, as seen by the heated stage XRD data, there is probably very little liquid material present in the slag. The low amounts of alkali metals, silicon, and aluminum in the slag and the high amounts of calcium and magnesium lead to the formation of Ca- and Mg-rich silicates that are crystalline even at 1500 °C. This would lead to a great deal of difficulty in operating a slagging system firing only the hog fuel but may simplify the operation of a pc-fired boiler because the ash would be relatively nonsticky. Figure 4, parts a and b, presents the results from the viscosity measurements performed using the two coal/hog fuel ash mixtures prepared in the laboratory as well as the slag generated from the cocombustion test of the coal/hog fuel mixture in the pilot-scale slagging combustor. The addition of biomass ash in the coal ash appears to have some very interesting consequences on the melting characteristics of the produced mixture. As seen in Figure 4a the mixing of hog fuel ash in a ratio of 1/6 with the coal ash leads to an ash that melts at approximately the same temperature as the coal ash. Curve 1 depicts the results from the measurement performed using the standard rotational speed of 45.3 rpm, while curve 2 shows the results from the measurement performed using a higher rotational speed of 107.8 rpm in order to study the effect of rotational speed to the



Figure 2. Viscosity and heated stage XRD data for the coal slag: (a) viscosity measurements, (b) XRD during heating to a melt, (c) XRD during cooling from a melt.


ArVelakis et al.

Figure 3. Viscosity and heated stage XRD data for the hog fuel ash: (a) viscosity measurements, (b) XRD during cooling from a melt.

viscosity characteristics of the melt. As seen from the second curve, the curve follows the same pattern as the viscosity curve of the first test, but the viscosity values appear to be significantly lower now. The specific pattern is observed in both the Newtonian and the non-Newtonian flow parts of the curve. The solidification process now seems to proceed in a slower mode and at lower temperatures compared to those of the first test. The coal/hog fuel ash slag shows Newtonian behavior in a larger temperature segment compared to that of coal ash slag, and its behavior changes to non-Newtonian at approximately the temperature segment of 1144-1149 °C as seen from Figure 4a and curves 1 and 2. The solidification process of the specific coal/hog fuel slag is seen to proceed in a much slower mode compared to that of the coal ash slag. The lower viscosity of the slag is likely due to the increased calcium content, due to the addition of the hog fuel that breaks the silicate networks. The increase of the hog fuel ash in the coal/hog fuel ash mixture leads to somewhat different results. Figure 4a shows that the 70/30 wt % coal/hog fuel ash mixture melts at 1400 °C and its flow characteristics follow the Newtonian pattern until

the temperature of 1222 °C. Below that temperature the melt shows non-Newtonian behavior, and the solidification of the slag progress faster now compared to the case of the 1/6 coal/ hog fuel ash slag. Repeating the viscosity test with the 70/30 wt % coal/hog fuel ash mixture results in a different viscosity curve compared to that of the first measurement. The viscosity curve is seen to have the same shape as in the previous measurement, but the measured viscosity is seen to be significantly lower now. The Tcv appears at approximately 1215 °C, and the slag solidifies in a slower mode now. The viscosity values are seen to be average even at temperatures approaching 1160 °C. Heated stage XRD data shows that it is principally akermanite (Ca2MgSi2O7), which crystallizes out of the slag upon cooling, and the rest of the material is amorphous glass that hardens upon cooling. Apparently, when this amount of hog fuel is added to the coal/hog fuel mixture the presence of the solid akermanite crystals in the slag increases its viscosity more than the breakup of silicate networks can decrease it. Figure 4b presents the results from the viscosity measurements performed using the slag generated after the combustion


Figure 4. Viscosity characteristics of coal and hog fuel ash and slag samples: (a) coal/hog fuel ash, (b) coal/hog fuel slag.

test using the 20% thermal input hog fuel coal/hog fuel mixture. The generated slag appears to be a dark glass as in the case of the coal and the synthetics coal/hog fuel slags. As seen from the viscosity curves shown in Figure 4b the slag melts at approximately 1300 °C and has a low viscosity and flows well until approximately 1250 °C. In both measurements the shape of the viscosity curve makes it difficult to distinguish between the Newtonian and the non-Newtonian flow behavior when the viscosity is given in logarithmic scale as now. As can be seen from Figure 4b the slag shows Newtonian behavior until 1220 °C where some solids can be seen starting to form as the light upward shifting of the viscosity curve indicates. These values appear to be in the same area as the values indicated by the viscosity measurements with the 70/30 wt % synthetic coal/ hog fuel slag mixture. The solidification of the melt after reaching its Tcv proceeds rapidly now as in the case of the coal slag and the second 70/ 30 wt % synthetic slag mixture. The second viscosity curve appears to be in good agreement with the first, though the formation of solids in the melt and the change of behavior from Newtonian to non-Newtonian appears to proceed in a somewhat faster rate now probably due to the fact that when the slag is reheated back to its initial melting temperature nuclei remain in the melt and lead to a faster solidification during cooling. However, in this case the viscosity curves show a very good match before as well as after the Tcv point compared to the case of the synthetic coal/hog fuel slag mixtures. The comparison of the elemental composition of the synthetic coal/hog fuel ash slags and the slag resulting from the cocombustion test shows that the cocombustion slag appears to be enriched in silicon and aluminum and slightly depleted in alkali metals, iron, magnesium, titanium, sulfur, and phosphorus compared to the artificial slags. This observation appears to be in good agreement with the results from the viscosity measure-


ments and explains the shifting of the viscosity curves to higher temperatures in the case of the cocombustion slag. 3.3. Corn Stalks. Figure 5a presents the results from the viscosity measurement using the corn stalks ash sample. The corn stalks ash contains the highest amount of alkali metals compared to all the biomass and the coal samples, while it also contains high amounts of silica and substantial amounts of calcium, magnesium, phosphorus, and sulfur. The formation of a yellow 1 mm thick sulfate layer on the top of the slag material was observed during the ash premelting at 1400 °C. Heated stage XRD data, shown in Figure 5b, indicate that the ash contains primarily quartz with some hannerbachite (CaSO3) initially, which melts together on heating, most likely with the loss of sulfur from the melt. Some periclase (MgO) crystallizes out of the melt during heating but then redissolves above 1050 °C. Unlike the heated stage XRD of the hog fuel ash, there are crystalline phases seen in the cooling curve for the corn stalks slag. Figure 5c shows the heated stage XRD data for the slag as it cools, indicating the formation of diopside (CaMg(SiO3)2) crystals in the slag between 1150 and 1100 °C as the slag begins to solidify. According to Figure 5a the corn stalks slag shows mainly Newtonian behavior over a wider temperature segment compared to the cases of the other slags tested. The viscosity curve shows some variations in the beginning of the measurement probably due to instrumental fault. A faster increase of viscosity is seen mainly below 1200 °C. The slag reaches its Tcv at 1129 °C, where the viscosity increases sharply and the slag solidifies rapidly. Repetition of the viscosity measurement using higher rotational speed (107.8 rpm) now shows general agreement with the lower rotational rate but with a slightly higher Tcv of 1150 °C. As seen from Table 3 the elemental analysis of the corn slag after the end of the viscosity tests shows that the slag contains large amounts of alkali metals, and magnesium, in combination with low amounts of calcium and aluminum and large amounts of silicon. This results in the formation of low melting point alkali silicates during the viscosity measurement and as a result to a low melting point slag that appears to solidify now at very low temperature compared to that of the other biomass, coal, and coal/biomass slags. Figures 6a and 7 present the results from the viscosity tests with the coal/corn ash artificial mixture as well as the coal/ corn slag generated after the performed cocombustion test in the pilot-scale combustor. Figure 6b shows the heated stage XRD data for the synthetic mixture of coal and corn ash while heating. It indicates that the anhydrite in the coal ash reacts with the silicates in the ashes to form akermanite (Ca2MgSi2O7) and diopside (CaMg(SiO3)2) at 700 °C, then completely melting between 850 and 900 °C. Heated stage XRD data show that crystals do not form in the melt during the cooling process until the temperature is lowered to 950 °C, although the peaks could not be identified with the data reduction software. As seen from Figure 6a the synthetic coal/corn slag melts at approximately 1350 °C. The viscosity of the slag appears to be constant during the first steps of the measurement down to the temperature of 1250 °C where it is seen to gradually increase following Newtonian behavior. The addition of the corn ash to the coal ash appears to shift the Tcv of the slag to a lower temperature 1176.6 versus 1190 °C compared to the case of the coal slag. This is due to the formation of alkali silicates with low solidification points. The crystallization of the melt appears to progress gradually in the first steps after the Tcv point until the temperature of 1150 °C, where the slope of the viscosity curve


ArVelakis et al.

Figure 5. Viscosity and heated stage XRD data for corn stalks ash: (a) viscosity measurement, (b) XRD during heating to a melt, (c) XRD during cooling from a melt.



Figure 6. Viscosity and heated stage XRD data for synthetic coal/corn slag: (a) viscosity measurement, (b) XRD during heating to a melt.

Figure 7. Viscosity data for the coal/corn cocombustion slag.

is seen to change rapidly and the melt to solidify. Repetition of the viscosity measurement using higher rotational speed shows it to be in good agreement with the previous curve in most of the temperature segment where the melt behaves as a Newtonian fluid. The Tcv appears again to be 1176.6 °C, while the

solidification of the melt proceeds now faster compared to the measurement using the low rotational speed. The results obtained from the viscosity measurements with the coal/corn slag resulted from the cocombustion test in the slagging combustor appear now to be different compared to the results of the synthetic coal/corn slag. As seen from Figure 7 the cocombustion slag is seen to melt at higher temperatures compared to those of the synthetic coal/corn slag. The viscosity of the specific slag sample appears also to be higher now, while the slag reaches its Tcv point at significantly higher temperature (1235 vs 1176.6 °C) compared to the case of the synthetic slag. The Tcv point appears to be higher now even compared to the case of the synthetic coal slag. The use of higher rotational speed (107.8 rpm) in the viscosity measurement appears to have the same effects as in the case of the synthetic slag measurements. The viscosity of the melt is seen to take somewhat lower values compared to the case of the measurement with the lower rotational speed for the temperature segment where the Newtonian flow prevails. The Tcv also appears now at higher temperature (1252 vs 1235 °C) compared to the measurement using the lower rotational speed. The higher rotational speed is seen to favor the formation of crystals in the melt, and the


ArVelakis et al.

Figure 8. Viscosity and heated stage XRD data for the switchgrass, synthetic coal/switchgrass ash: (a) viscosity measurement, (b) XRD during cooling from a melt.

viscosity values are higher now in the first steps after the Tcv point compared to the case of the viscosity measurement with the low rotational speed. The viscosity curve matches the curve produced at the lower rotational speed gradually. The elemental analysis of both the artificial coal/corn ash slag as well as the coal/corn slag from the cocombustion test shows it to be in good agreement with the results obtained from the viscosity measurements. According to Table 3 the artificial coal/corn ash slag is enriched in alkali metals and depleted in Si and Al even after the end of the viscosity measurement compared to the coal slag suggesting a lower melting as well as solidification point of the specific slag as depicted in the viscosity measurements. Furthermore, the slag resulted from the cocombustion test appears to be substantially depleted in alkali metals and enriched in Ca, Si, and Al compared to the artificial slag, which leads to increased viscosity values and higher solidification points due to the formation of mainly aluminum silicates compared to the

alkali silicates in the case of the synthetic slags as seen from Figure 7.25-27 3.4. Switchgrass. Figure 8a presents the results from the viscosity measurements with the synthetic switchgrass slag and the synthetic coal/switchgrass mixture slag. As seen from Figure 8a the switchgrass slag appears to melt at significantly higher temperature (>1500 °C) compared to all the other biomass and coal/biomass slags. The slag was seen to have white color, while even at 1550 °C it forms a thick viscous melt with limited flow characteristics. This is probably due to the large amount of silica contained in the specific slag sample as well as due to the low amounts of alkali metals, calcium, and alumina. The slag follows Newtonian behavior in a large temperature segment (15001273 °C), while its viscosity is seen to take significantly higher (26) Browning, G. J.; Bryant, G. W.; Hurst, H. J.; Lucas, J. A.; Wall, T. F. Energy Fuels 2003, 17, 731-737. (27) Kim, K.-D., Lee, S.-H. J. Mater. Sci. 1997, 32, 6561-6565.


values compared to the case of the other biomass and coal/ biomass slags. The rheological characteristics of the specific slag after its Tcv point (1273 °C) appear to be similar to those of the hog fuel slag. The viscosity curve shows large variations probably due to the periodic formation and dissolution of solids in the melt or due to the formation of large particles that move around the melt coming in contact with the spindle at different spots thus increasing or decreasing the measured viscosity during the spindle rotation. However, the non-Newtonian behavior prevails now in a significantly smaller temperature segment compared to the case of the hog fuel slag. Both the hog and the switchgrass slags appear to freeze at approximately the same temperature area (1100 °C). Figure 8b shows the heated stage XRD data for the switchgrass slag as it cools from a melt. The data indicate that in the temperature range where viscosity shows instability cristobalite (a high-temperature version of SiO2), nepheline (NaAlSiO4), and crystals begin to form in the melt. The instability in the viscosity curve that develops when these crystals form is likely due to their segregation within the melt. The addition of switchgrass ash to the coal ash, (32/68 wt %), produces a slag that melts at approximately the same temperature as the coal slag. Two measurements were performed using the specific coal/switchgrass slag under the same conditions, and the results appearing in Figure 8a from both viscosity curves show a perfect match. The viscosity of the slag is seen to be lower, compared to that of the switchgrass slag, but higher compared to that of the coal slag. The specific slag reaches its Tcv at the 1100 °C and is significantly lower compared to the Tcv for both the coal slag as well as the switchgrass slag. The viscosity of the slag appears to increase rapidly after the Tcv point, and the slag solidifies at a fast rate. The slag elemental analysis shows that it is enriched in aluminum and depleted in alkali metals. The silicon content of the slag is also seen to be average as well as the content of calcium. 4. Discussion In this study a high-temperature rotational viscometer and a heated stage XRD (HSXRD) were used in order to study the viscosity characteristics as well as the transformations and phase changes of various coal, biomass, and coal/biomass ash samples. Biomass ash has shown significantly different rheological behavior compared to that of the coal ash. All the biomass ash samples solidify at lower temperatures compared to that of the coal ash even though their initial melting point is higher compared to that of the coal ash. The addition of biomass ash to the coal ash is seen to lower the ash solidification point due to increased amounts of alkali metals, calcium, and magnesium that in combination with low to average amounts of silica do not favor the formation of a constant silicate network. The increased aluminum and the lower alkali contents of the coal/ biomass slags lead to lower melting points but to higher solidification points compared to those of the pure biomass slags.25-27 The results from the hog fuel ash tests show a very strange rheological behavior. The non-Newtonian flow in the case of this biomass ash sample is present in a rather large temperature segment compared to the results from the coal ash as well as the other biomass ash samples. This is believed to be mainly due to the low amounts of silica and the high amount of calcium that result in the formation of solid and liquid phase that cannot react among each other and produce crystals. The final solidification of the ash is mainly due to the solidification and hardening of these two phases. However, the increase of the percent of hog fuel ash to the coal/hog fuel ash mixture shows an increase in the viscosity of the specific sample though


it still remains lower compared to the case of the coal ash. This is probably due to the lower amounts of silica and higher of calcium compared to those of the coal ash that lowers the melting point of the produced ash. In addition, the lower amounts of aluminum and the high amount of calcium as well as the increased amounts of alkali metals in the coal/hog fuel mixture ash favor the formation of higher amounts of alkali and calcium silicates and lead to lower viscosity values and solidification rates compared to those of the coal ash. Corn stalks and switchgrass ash appear to have similar rheological behavior, though the switchgrass ash partly follows the rheological behavior of the hog fuel ash in the temperature segment below 1300 °C. As the heated stage XRD data indicate in this temperature range, where viscosity shows instability, cristobalite (a high-temperature version of SiO2), nepheline (NaAlSiO4), and crystals begin to form in the melt. This is consistent with the ash chemical analysis of switchgrass slag that shows a very large amount of silica, low amounts of alkali metals, and average of calcium. As a result, alkali metals and calcium cannot flux all the silica, and thus, at T < 1300 °C liquid alkali-calcium silicates are existing together with solid silica minerals and crystallized alkali-calcium aluminosilicates creating a two-phase mixture that leads to the specific rheological behavior. The slags that resulted from the cocombustion of the coal/biomass mixtures in the slagging combustor have different viscosity characteristics compared to those of the synthetic ash mixtures. The cocombustion slags are seen to contain higher amounts of silica and alumina and lower amounts of alkali metals compared to the synthetic slags, while the viscosity takes higher values and the solidification of the slags is seen to proceed at higher temperatures. The cocombustion slags appear to solidify at higher temperatures also compared to those of the coal ash, but this is attributed to the fact that the coal ash is also synthetic and thus its viscosity characteristics do not correspond to those of a coal slag produced from a coal combustion test. Generally, the heated stage XRD data correlate fairly well with the viscosity data. Several reasons contribute to this. One is that there usually were no strong slope changes in the viscosity data that one could attribute to sudden onset of crystal formation. Also, the HSXRD measurements were started at 1250 °C on cooling due to instrumental problems, and there were already crystals present at that temperature, or crystals did not form until a much lower temperature than the viscosity could be measured. Supercooling of the slag in the pristine conditions of the HSXRD may be part of the problem. Also, almost all of the phases that crystallized during cooling in the HSXRD were alkaline earth silicatessthere were only small amounts of crystals that contained alumina. Our experience indicates that aluminosilicate crystallization is more common in deposits formed in actual boilers, although there were not substantial amounts of aluminosilicates in the heating HSXRD curves either. 5. Conclusions The determination of the viscosity characteristics of the coal/ biomass ash mixtures using the experimental setup demonstrated into this research study is seen to provide relatively accurate information regarding the melting and solidification temperatures of the produced ashes during the cocombustion tests performed at EERC. The measured viscosity and melt solidification temperatures of the synthetic ash mixtures are seen to be slightly lower compared to those for the slags produced from the cocombustion tests. Thus, the laboratory viscosity tests with the synthetic ash samples can be used to provide a rather accurate


estimation of the boiler working temperature in order to decrease possible ash-related problems. The heated stage XRD data was seen to contribute to the interpretation of the produced viscosity data, but relative experience with the method is required to get valid results. The combined use of these two techniques may be an important tool toward the elimination of deposition and corrosion problems in full-scale boilers by temperature control.

ArVelakis et al. Acknowledgment. This work is part of the CHEC (Combustion and Harmful Emission Control) Research Program, funded by the Technical University of Denmark, Elsam A/S, Energy E2 A/S, PSO funds from Eltra A/S and Elkraft A/S, and the Danish Energy Research Program. It was also sponsored by the US Department of Energy and the Xcel Energy, Renewable Development Fund. EF050168B

Biomass

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