Simultaneous Thermal Analysis (STA) on Ash from High-Alkali

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Energy & Fuels 2004, 18, 1066-1076

Simultaneous Thermal Analysis (STA) on Ash from High-Alkali Biomass Stelios Arvelakis,* Peter Arendt Jensen, and Kim Dam-Johansen CHEC, Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark Received September 26, 2003. Revised Manuscript Received April 5, 2004

Improved knowledge of the melting and gas-phase release behavior of ash from biomass fuels is of premium importance, to reduce ash-related problems in straw-fired boilers. In this study, a simultaneous thermal analysis (STA) instrument was used to characterize the behavior of 12 significant different annual biomass ash samples, and several model compounds, such as SiO2, KCl, K2SO4, K2CO3, CaSO4, CaCO3, and some of their mixtures. The STA analyses provided information on mass loss and exothermic/endothermic reactions as a function of sample temperature. The mass loss curves of the biomass ashes during STA heating could, in most cases, be explained from the chemical composition of the ash. Below 800 °C, weight loss is caused by the release of CO2 from CaCO3. At temperatures of 850-1150 °C, KCl evaporates and K2CO3 reacts with SiO2 and CO2 is released. Above 1150 °C and up to 1450 °C, a weight loss from the ash samples may be caused by the release of K2O produced from the K2CO3 decomposition or from silicates or the release from sulfates. However, even at 1450 °C, a large fraction of the potassium derived from carbonates remains in the condensed phase in the form of silicates.

1. Introduction The threat of increasing global warming is causing widespread concern and pressure to reduce emissions of greenhouse gases such as CO2, which is emitted especially from the use of fossil fuels to generate power. This need to develop a viable strategy for initial stabilization and future decrease of CO2 emissions has led to an increased use of renewable and sustainable energy sources such as biomass fuels that are used for power generation.1-3 Biomass is considered to be a CO2 neutral fuel, especially in the form of agricultural and/ or agro-industrial wastes and residues, because biomass growth accumulates an amount of CO2, via photosynthesis, that is equal to the amount of CO2 released during its thermochemical conversion. Furthermore, increasing the use of biomass wastes and byproducts from agricultural and agro-industrial processes will provide an additional source of income for farmers. Despite the mentioned environmental benefits, the use of annual biomass (such as straw) in boilers involves several technical difficulties, which are mainly related to the inorganic constituents of biomass. Specifically, the use of straw in grate boilers that are generating heat and power has resulted in severe ash deposit formation and high-temperature corrosion, leading to costly replacements of steam tubes as well as other problems, such as the emission of harmful gases such as SO2 and * Author to whom correspondence should be addressed. Phone: +353-61-234169. Fax: +353-61-213529. E-mail address: [email protected], [email protected]. (1) Dayton, D. C.; Belle-Oudry, D.; Nordin, A. Energy Fuels 1999, 13, 1203-1211. (2) Brus, E.; Nordin, A.; Kassman, H.; Kallner, P. In Power Production in the 21st Century: Impact of Fuel Quality and Operations; United Engineering Foundation: New York, 2001. (3) Sander, B. Biomass Bioenergy 1997, 12, 177-183.

HCl. The relatively high alkali and chlorine content of herbaceous biomass fuels has been reported as one of the main reasons for increased slagging, fouling, and corrosion in boilers.1,2,4-16 The large ash deposit formation is caused by the relatively large gas-phase concentration of alkali-containing species such as KCl and K2SO4, and the low melting temperature of many alkalicontaining ash components. Gas-phase KCl and K2SO4 generate deposits by condensation on boiler heattransfer surfaces and the partially melted deposits act as efficient glue for silicate-rich ash particles. If the alkali-containing ash components in a grate-fired boiler are contained in the condensed phase, they will primarily leave the boiler chamber as bottom ash and thereby never come into contact with the superheaters, where the most severe ash deposit formation is observed. The (4) Davidsson, K. O.; Engvall, K.; Hagstrom, M.; Korsgren, J. G.; Lonn, B.; Pettersson J. B. C. Energy Fuels 2002, 16, 1369-1377. (5) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel Process. Technol. 1998, 54, 17-46. (6) Dayton, D. C.; Jenkins, B. M.; Turn, S. Q.; Bakker, R. R.; Williams, R. B.; Belle-Oudry, D.; Hill, L. M. Energy Fuels 1999, 13, 860-870. (7) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 9, 855-865. (8) Olsson, J. G.; Ja¨glid, U.; Pettersson, J. B. C. Energy Fuels 1997, 11, 779-748. (9) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D. C.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol. 1998, 54, 47-78. (10) Bjo¨rkman, E.; Stro¨mberg, B. Energy Fuels 1997, 11, 1026-1032. (11) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sanders, B. Energy Fuels 2000, 14, 1280-1285. (12) Wibberly, L. J.; Wall, T. F. Fuel 1982, 61, 87-95. (13) Steenari, B. M.; Lindqvist, O. Biomass Bioenergy 1998, 14, 67. (14) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen; K. Energy Fuels 1999, 13, 1114-1121. (15) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Baxter L. L. Prog. Energy Combust. Sci. 2000, 26, 283-298. (16) Wornat, M. J.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 131-143.

10.1021/ef034065+ CCC: $27.50 © 2004 American Chemical Society Published on Web 06/09/2004

Thermal Analysis of Ash from High-Alkali Biomass

determination of the melting and gas-phase release behavior of biomass fuel ashes is of premium importance, and, in the present study, thermal analysis was applied to characterize the ash of different herbaceous biomasses. Thermal analysis is widely used in combustion research for the determination and understanding of various properties and the behavior of different fuels under a wide range of conditions and applications.17-22 It has been reported that thermal analysis methods including thermogravimetric analysis/differential thermal analysis (TGA/DTA), thermogravimetric analysis/ differential scanning calorimetry (TGA/DSC), thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA) can be used to provide an estimation regarding the ash-melting characteristics of various coal and biomass samples.18,23-28 In this work, a TGA/DSC instrument that is also called a simultaneous thermal analysis (STA) instrument is used to characterize the ash behavior of 12 annual biomasses. The study has been focused on obtaining information on both ash melting and the ash gas-phase release as a function of temperature, whereby the melting and gas-phase release behavior of the samples could be related to the chemical composition of the biomasses. The thermal analysis investigation was expanded to include the melting and evaporating characteristics of various inorganic model compounds such as SiO2, KCl, K2SO4, K2CO3, CaSO4, CaCO3, and their mixtures. These compounds constitute the majority of the ash of the studied biomass fuels, and their study provides valuable information toward the explanation of the behavior of the biomass ashes. 2. Experimental Section 2.1. Experimental Setup. A Netzsch model STA 409 C thermal analyzer was used in the present investigation. STA enables DSC to be performed simultaneously with TGA. During the experiments, the sample material is placed in the sample crucible and a reference material is placed in the reference crucible on top of the sample carrier. A thermocouple is mounted to the bottom of each crucible, allowing for continuous measurement of the temperature difference between the sample and the reference material. The sample carrier shown in Figure 1 is located on top of a highly sensitive analytical balance, which is located in a vacuum-tight casing. The high-temperature furnace is heated by tubular SiC (17) Kok, M. V. J. Therm. Anal. Calorim. 2002, 68, 1061-1077. (18) Stenseng, M.; Zolin, A.; Cenni, R.; Frandsen, F.; Jensen, A.; Dam-Johansen, K. J. Therm. Anal. Calorim. 2001, 64, 1325-1334. (19) Liang, X. H.; Kozinski, J. A. Fuel 2000, 79, 1477-1486. (20) Otero, M.; Diez, C.; Calvo, L. F.; Garcia, A. I.; Moran, A. Biomass Bioenergy 2002, 22, 319-329. (21) Bassilakis, R.; Carangelo, R. M.; Wojtowicz, M. A. Fuel 2001, 80, 1765-1786. (22) Jensen, A.; Dam-Johansen, K.; Wo´jtowicz, M. A.; Serio, M. A. Energy Fuels 1998, 12, 929-938. (23) Gupta, S. K.; Wall, T. F.; Crelman, R. A.; Gupta, R. P. Fuel Process. Technol. 1998, 56, 33-43. (24) He, Y. Fuel Process. Technol. 1999, 60, 69-79. (25) Acosta, A.; Iglesias, I.; Aineto, M.; Romero, M.; Rincon, J. M. J. Therm. Anal. Calorim. 2002, 67, 249-255. (26) Hansen, L. A.; Frandsen, F. J.; Dam-Johansen, K.; Sorensen, S. Thermochim. Acta 1999, 326, 105. (27) Stenseng, M.; Hansen, L. A. Practical Experience Obtained Using Netzsch STA 409 C. CHEC Report No. 9822, Department of Chemical Engineering, Technical University of Denmark, Lyngby, Denmark, 1998. (28) Partanen, J.; Backman P.; Huppa, M. Combust. Flame 2002, 130, 376-380.

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Figure 1. Schematic representation of the STA measuring system used in the present work. Table 1. Biomass Samples Studied Using STA sample

code name

origination

rice oats wheat (Marius) rape winter barley wheat (Soisson) Carinata spring barley maize wheat (2000) wheat (2001) barley (2001)

HIAL 1 HIAL 2 HIAL 3 HIAL 4 HIAL 5 HIAL 6 HIAL 7 HIAL 8 HIAL 9 HIAL 10 HIAL 11 HIAL 12

Spain Spain Spain Spain Spain Spain Spain Spain Spain Denmark Denmark Denmark

heating elements and operates at temperatures from 25 °C to 1550 °C, with possible heating rates of 0.1-50 °C/min. The output information from each experiment is temperature, sample mass (TGA), and heat flow (DSC), as a function of time. The DSC signal describes the heat flow required to keep the sample and reference at the same temperature. A difference in heat flow corresponds to exothermic or endothermic reactions inside the sample, e.g., changes of phases, changes of state (such as melting or evaporation), and reactions and differences in mass and thermophysical properties (heat capacity and thermal conductivity) of the sample and reference material. The operation procedure applied during the specific STA measurements can be divided in the following steps: (1) Into a 30-mL platinum/rhodium crucible is placed 19.520.5 mg of the biomass ash sample, or 10-20 mg in the case of the model compounds. Between the platinum crucible and the platinum sample carrier, a disk of alumina is placed, to prevent the two parts from sticking to each other during the experiments. (2) Lids cover the crucible, to obtain a more uniform temperature distribution inside the crucible. A hole in the lids allows the evaporation products to escape. A second crucible (the reference) contains ∼19.5-20.5 mg of R-Al2O3. (3) A flow rate of pure nitrogen (100 mL/min) maintains inert conditions in the oven and removes any gaseous products that may be released. (4) The sample is heated at a heating rate of 10 °C/min to 1500 °C, where the experiment ends. The temperatures of the sample and reference are recorded by a platinum/rhodium thermocouple (type S), and a high-precision balance registers the potential weight loss due to evaporation/decomposition of the sample. Measurements on the majority of the samples were repeated twice, to test the sample’s homogenization and also the reproducibility of the instrument. Furthermore, after the end of each STA test, the sample crucible was visually inspected, to define the final condition of the sample material and correlate it with the results derived from the generated STA curves. 2.2. Samples. The STA measurements were performed on the biomass samples presented in Table 1 and on selected inorganic model compounds and their mixtures. The biomass

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Arvelakis et al.

Table 2. Analysis and Characterization of the Biomass Samples HIAL 1 HIAL 2 HIAL 3 HIAL 4 HIAL 5 HIAL 6 HIAL 7 HIAL 8 HIAL 9 HIAL 10 HIAL 11 HIAL 12 moisture (%) composition (% dry) ash C H N S Cl Si Al Fe Ca Mg Na K P LHV dry (MJ/kg) a

13.6 7.4 44 5.9 0.63 0.13 0.32 1.9 0.017 0.01 0.47 0.24 0.028 0.77 0.067 15.8

7.8 3.8 48 6.3 0.93 0.14 0.05 0.27 0.006 0.003 0.72 0.074 0.13 0.55 0.11 17.4

12.4 5.7 46 6.1 0.28 0.06 0.27 1.7 0.009 0.006 0.22 0.06 0.004 0.80 0.04 16.3

8.4

8.5

12.4

2.7 46 6.1 0.46 0.058 0.022 0.03 0.06 0.03 0.94 0.05 0.005 0.34 0.017 17.0

6.9 45 6.0 0.76 0.20 0.79 0.81 0.008 0.004 0.34 0.091 0.26 2.3 0.055 16.5

7.3

4.9 47 6.2 0.39 0.07 0.10 1.5 0.02 0.02 0.39 0.05 0.005 0.37 0.05 17.1

12.2

4.9 45 6.0 1.1 0.26 0.05 0.5 mm was used for the analysis. (3) The biomass samples were ashed using a high-temperature muffle furnace that was supplied with air. Small amounts of the biomass samples (3-6 g) were put in porcelain crucibles, each with a volume of 100 mL, and heated to 550 °C using low heating rates (2 °C/min). After 24 h at 550 °C, the produced ash samples were removed from the furnace and ground in a mortar, to ensure the homogeneity of the samples used for the STA tests. The produced ash samples were seen to have a white to gray color, with the particles keeping the shape and structure of the initial biomass particles before the grinding treatment. To achieve a deeper understanding and to explain the STA tests of the various straw samples, several experiments were conducted using model compounds. We refer to simple inorganic salts and oxides that are considered to represent the

main inorganic elements that are present in the ash of the biomass materials after initial combustion as being model compounds. The STA tests using the model compounds were divided into two categories. The first category includes the STA tests conducted using pure substances such as KCl, K2SO4, K2CO3, CaSO4, and CaCO3, and their mixtures are described in Table 4, to study their thermal behavior and mass loss during heating. The second category includes STA tests that were performed using pure SiO2 and two (30%/70% weight basis) mixtures of SiO2-KCl and SiO2-K2CO3. Pure chemical compounds such as SiO2, KCl, K2SO4, K2CO3, CaSO4, and CaCO3 were purchased from Merck and SigmaAldrich, whereas the studied inorganic mixtures were artificially prepared in the laboratory by mixing the necessary pure components and grinding them in a mortar.

3. Results The results produced by the STA measurements with the 12 different biomass ash samples, and the model compounds, are presented in Figures 2-5, whereas Tables 4 and 5 depict a comparison among the final condition of the sample and the main findings from the STA results in all cases. As shown in Figures 4c, 4e, 4f, 5c, and 5e, which depict the results produced after several test runs with some of the investigated biomass ash samples, the results of the STA measurements appear to be reproducible TG curves (percentage of the initial weight), showing an identical match. The DSC curves (enthalpy change, in units of mw/mg) shows, in most cases, a reproduction of endothermic peaks but a drift of the curve at high temperatures. The difficulties with drift



Table 4. Results from the STA Runs with the Model Compounds and Their Mixtures sample KCl K2CO3 K2SO4 CaSO4 CaCO3 KCl + K2SO4 (50%/50% w.b.)b KCl+CaSO4 (50%/50% w.b.)b KCl+CaCO3 (50%/50% w.b.)b SiO2 SiO2+KCl (30%/70% w.b.)b SiO2+ K2CO3 (30%/70% w.b.)b a

melting peak(s) (°C) 770 904 1072 1010, 1209 nda 683 688 770 nda 762 863

final sample condition

Mass Loss during Various Temperature Segments (%) total mass 400-850 °C 850-1150 °C 1150-1450 °C loss (%)

white particles white particles white crystals white particles white particles

2.6 0.6 0.8 0.9 44.3

96.4 2.8 0.7 0.7 0.3

1.4 73.1 4.5 36.5 0.5

100 76.6 6.3 38.6 45.2

white flakes white particles white particles brown sintered deposit white glassy deposit dark glassy gel

2.8 3.1 20.0 2.0 1.6 6.3

49.5 47.3 44.5 1.4 69.9 15.4

34.3 25.7 10.8 0.7 0.5 5.6

86.8 76.4 75.4 4.1 72.1 27.3

Not determined. bThe term “w.b.” denotes weight basis.

Figure 2. STA measurements using various pure model compounds: (a) KCl, (b) K2CO3, (c) K2SO4, (d) CaSO4, (e) CaCO3, and (f) SiO2.

of the DSC curve have also been observed in previous studies.29 Generally, the drift of the DSC curve observed

in all cases is attributed to the change in the physical properties of the ash material during the STA run.


Arvelakis et al.

Figure 3. STA measurements using various mixtures of pure model compounds: (a) KCl + K2SO4 (50%/50% w.b.), (b) KCl + CaSO4 (50%/50% w.b.), (c) KCl + CaCO3 (50%/50% w.b.), (d) SiO2 + K2CO3 (30%/70% w.b.), and (e) SiO2 + KCl (30%/70% w.b.). (Note: The term “w.b.” represents weight basis.)

Specifically, during swelling or shrinkage of the sample, the thermal conductivity and the physical contact between the sample and the crucible will change. Both phenomena will lead to changes in heat uptake efficiency and, thus, to a change in baseline position. It is not possible to give a general description of the effects of the swelling/shrinkage on the DSC baseline. One just has to be aware of the fact that effects will occur. The ash-melting studies indicate that, when the ashes shrunk during melting, the DSC baseline was shifted upward.28 The drift of the DSC curve is also caused by the movement of the sample holder from the center of the STA furnace during the setting and the removal of the sample and the reference crucibles during the STA measurements. (29) Arvelakis, S.; Sotiriou, C.; Moutsatsou, A.; Koukios, E. G. J. Therm. Anal. Calorim. 1999, 56, 1271.

3.1. STA Measurements on Inorganic Compounds and Their Mixtures. Figures 2 and 3 and Table 4 present the measurements from the STA tests using the model compounds and their mixtures. The most inorganic mixtures have been formed in a 50%/ 50% weight basis, as shown in the corresponding figures (Figures 2 and 3). All measurements were performed in a N2 atmosphere, except for the experiments with SiO2, where an air atmosphere was used. The reason for that was that, as other researchers have also found, the reaction among silica and alkali metals cannot proceed without the presence of air or water vapor.10,26 The STA tests on the model compounds were conducted to facilitate the interpretation of the biomass ash tests. Although the melting temperatures of the tested compounds can be found in property tables,30 the gas-phase release is, to some extent, influenced by



Figure 4. STA measurements using various straw samples: (a) HIAL3, (b) HIAL6, (c) HIAL10, (d) HIAL11, (e) HIAL5, and (f) HIAL8.

equipment design such as lid and crucible geometry and gas type and velocity. For example, the gas-phase release of KCl from the crucible is influenced by the saturation pressure at the KCl surface, the diffusion through the crucible and the lid hole, and the removal of KCl gas in the boundary layer above the lid. To make a basis for the understanding of the STA biomass ash mass loss curves, it is necessary to have available mass loss data of relevant ash compounds on the actually used STA experimental configuration. All the pure compounds tested showed a melting behavior that is consistent with the experimental data found in handbooks,30 showing melting peaks starting as early as at 770 °C in the case of pure KCl. In some cases, a mass loss is observed at 1150 °C versus calculated potassium content in the ash as K2O.

(7) It was assumed that, in the initial low-temperature ashes, potassium is more likely present as K2SO4 than as KCl and the residual potassium appears as K2CO3. (8) As a general simplification, the behavior of minor elements such as magnesium, sodium, and phosphorus is not taken into consideration. The assumptions regarding the low-temperature release of chlorine and sulfur (assumptions 1 and 2) are reasonable levels, according to a previously performed experimental fixed-bed study of the release of chlorine and sulfur.32 Assumptions 3-5 are based on the results of the model compound tests. Sulfur appears probably as a mixture of potassium and calcium sulfates, possibly in the form of K2Ca2(SO4)3, as suggested by other researchers.33 However, to simplify the calculations, all sulfur was assumed to be present as K2SO4 (assumption 6). The distribution of potassium in different species (assumption 7) is in accordance with previously performed thermodynamic calculations.34 Figure 6a presents the comparison between the calculated and measured mass loss in the temperature (32) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K. Laboratory Investigation of the Release to Gas Phase of Potassium, Sulphur and Chlorine at Grate Combustion Conditions. Presented at the First Biennial Meeting Organized by The Scandinavian-Nordic Section of the Combustion Institute, April 10-20, 2001. (33) Ollanders, B.; Steenari B.-M. Biomass Bioenergy 1995, 8, 105. (34) Nielsen, H. P. Ph.D. Dissertation, Department of Chemical Engineering, Technical University of Denmark (DTU), Lyngby, Denmark, 1998.

segment between 400 °C and 850 °C for the STA tests of the ash samples. The calculated mass loss between 400 °C and 850 °C is based on the ash composition. The mass loss is calculated by assuming that it is caused by the release of CO2 from CaCO3. As shown in Figure 6a, the measured mass loss seems to be less than the calculated mass loss for most of the HIAL samples. A significant amount of the calcium is present as CaO and not as CaCO3 in some of the ash samples. This is probably caused by the fact that the formation of CaCO3 or CaO, which happens during the ashing procedure of the initial straw sample, can occur, to a variable degree. Figure 6b presents the results from the comparison between the calculated KCl ash content, which is assumed to be totally released to the gas phase in the temperature segment between 850 °C and 1150 °C, and the measured mass loss in the temperature segment between 850 °C and 1150 °C for the STA tests. As shown in Figure 6b, the mass losses generally are in good agreement with what can be accounted for by the ash KCl content. Exceptions appear in the case of the HIAL 2, 5, 7, and 9 ash samples, where the measured mass loss is substantially higher, compared to the KCl content in the ash. This is probably caused by the large amounts of alkali carbonates present in some of the ash samples. Figure 6c presents also results from the comparison between the calculated and the measured mass loss in the temperature segment between 850 °C and 1150 °C. The calculated mass loss is, in this case, based on the


sum of KCl in the ash and the contribution from the release of CO2 from K2CO3. The content of K2CO3 is calculated by assuming that all the potassium that does not appear as K2SO4 and KCl as K2CO3. Reasonable agreement is observed between the calculated and measured mass loss data in this case. In Figure 6d, the results from the comparison between the measured mass loss above 1150 °C in the STA tests and the amount of residual potassium in the ash calculated as K2O are shown (residual potassium is defined as potassium that does not appear as K2SO4 or KCl in the low-temperature ash). The K2O content of the ashes is calculated by assuming all K2CO3 have been converted to K2O at temperatures of >1150 °C. The results show an increased mass loss during the STA tests with increased K2O content. It is generally observed that the mass loss is much less than the content of K2O in the samples. This is probably because a large fraction of the potassium is bound as silicates. It is the case for all samples, except HIAL 7 and HIAL 4, that there is sufficient silicon to bind all residual potassium as K2O‚SiO2. A large mass loss is observed for HIAL 4, which does not have sufficient silicon to bind all the potassium. The amount of silicon present in HIAL 7 is also very low, so a large mass loss could be expected. However, the ash has a large phosphorus content and the potassium may appear as K3PO4 that is not released into the gas phase. SEM-EDX analysis of the specific ash sample has shown the existence of a large amount of potassium phosphates after the end of the STA measurement. 5. Conclusions In this experimental study, a simultaneous thermal analysis (STA) instrument was used to characterize the behavior of 12 annual biomass ashes during heating. To make a basis for the interpretation of the STA measurements on the biomass ashes, various inorganic model compounds, such as SiO2, KCl, K2SO4, K2CO3, CaSO4, and CaCO3 and some of their mixtures, were also investigated. The study was focused on obtaining information on both ash melting and ash gas-phase release as a function of temperature, whereby the gasphase release behavior of the samples could be related to the chemical composition of the biomasses. The results from the STA measurements appeared to be reproducible in most cases, verifying the value of the method for studying the transformation of the ash samples. We have not managed to quantify the melted fractions of the biomass ash samples as a function of temperature. However, in all cases, initial low-temperature melting at temperatures of 600-700 °C were observed from ash samples (HIAL 5, HIAL 9, HIAL 10, and HIAL 12) with a high chlorine content (>3 wt % in the ash). The STA mass loss curves could, in most cases, be explained by the chemical analysis of the studied ash samples. During heating of the biomass ash samples in the STA instrument, significant weight losses (and, thereby, gas-phase releases) were observed. The reasonably good agreement between the measured and calculated mass loss in the different temperature segments

Arvelakis et al.

of Figure 6 indicate that the processes responsible for the major mass loss contributions are correctly described. We believe that the mechanisms responsible for most of the weight loss in the different temperature segments are as follows: (1) Below 800 °C, weight loss is caused by the release of CO2 from CaCO3. (2) At temperatures of 850-1150 °C, potassium that appears as KCl evaporates. A large fraction of the amount of potassium that is not present as KCl appears probably as K2CO3 in the ash that is generated at low temperatures. In the temperature segment between 850 °C and 1150 °C, K2CO3 reacts with SiO2 and CO2 is released. (3) Above 1150 °C and up to 1450 °C, a weight loss from the ash samples may be caused by the release of K2O produced from the K2CO3 decomposition or by its release from silicates and sulfates. However, even at 1450 °C, a large fraction of potassium that is derived from carbonates remains in the condensed phase in the form of silicates. Some information on the ash transformations in the boiler chamber of grate boilers that use high-alkali fuels can be provided, based on the present study. However, the STA results regarding ash transformation should be used with some precaution when transferred to fullsize boilers. The combustion process on the grate of a straw-fired boiler differs in several ways, compared to the STA test conditions.35 The heating rate is higher, the ash is in contact with char much of the time, and the flue gas has a large content of CO, CO2, water vapor, and hydrocarbons. The gas-phase release in the STA instrument occurs at higher temperatures than probably would be the case in a boiler chamber. The sample crucibles have a lid on top of them, so the vapor pressures shall be relatively high before significant amounts are released to the gas phase through the hole in the lid. However, with these differences in mind, we believe that some of the trends observed in the STA data also will appear under grate boiler conditions. High chlorine content in the fuel increases the gas-phase release of potassium at relatively low temperatures. In the boiler ash, potassium can be bound to silicates and thereby be present in a condensed phase, even at very high temperatures. This means that the largest fraction of fuel alkali release to the gas phase from the grate combustion zone will originate from fuels with a high chlorine content and low silicon content. 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. This study was sponsored by the European Union, through Contract No. EESD Project NNE5-200100064 (HIAL)sBiofuels for CHP PlantssReduced Emissions and Cost Reduction in the Combustion of High Alkali Biofuels. EF034065+ (35) Van der Lans, R.; Pedersen, L. T.; Jensen, A.; Glarborg, P.; Dam-Johansen, K. Biomass Bioenergy 2000, 19, 199-208.

Simultaneous Thermal Analysis (STA) on Ash from High-Alkali

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