Assessment of ChlorineAlkaliMineral Interactions during Co

Institute of Process Engineering and Power Plant Technology (IVD), University of Stuttgart,. Pfaffenwaldring 23, D-70569, Stuttgart, Germany. Received...
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Energy & Fuels 2002, 16, 1095-1108

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Assessment of Chlorine-Alkali-Mineral Interactions during Co-Combustion of Coal and Straw Xiaolin Wei,* Christian Lopez, Thore von Puttkamer, Uwe Schnell, Sven Unterberger, and Klaus R. G. Hein Institute of Process Engineering and Power Plant Technology (IVD), University of Stuttgart, Pfaffenwaldring 23, D-70569, Stuttgart, Germany Received December 13, 2001

During co-combustion of coal and straw, ash deposition and pollutants (HCl, aerosols, etc.) formation is significantly influenced by the behavior of chlorine and alkali metals. On the bases of measurement data, the effect of inherent minerals in blended fuels on the release and retention of chlorine and alkali metals is investigated through the equilibrium analysis tool FACT-Win. Si, Al, Ca, Mg, and S may greatly affect the behavior of Cl, K, and Na, where the other minor elements, such as Fe, Ti, and Mn have not much influence. According to measurement data, very low amounts of chlorine and sulfur are retained in bottom, air preheater, and cyclone ash in a pulverized fuel combustor. But in filter ash, the chlorine and sulfur contents are relatively high and the ratio (K + Na)/(Cl + 2S) is close to 1. According to equilibrium calculations, in hard coal co-combustion with less than 50% straw, most of the potassium is combined with aluminosilicates to form KAlSi2O6(s,s2), etc., which can prevent ash depositing on the furnace surface. Besides the formation of NaAlSi3O8(s2), some sodium is released as NaCl(g) and NaOH(g). In brown coal co-combustion, most of the potassium is released as KCl(g), KOH(g), and K2SO4(g) because aluminum can be combined in the Ca and Mg compounds, such as Ca3Al2O6(s), CaAl2O4(s), and MgAl2O4(s). Increasing the straw fraction reduces KAlSi2O6(s2) and increases K2Si4O9(liq). Most of the sodium is released as NaOH(g), NaCl(g) and Na2SO4(g). Na2SO4(liq) is formed between 1200 and 1400 K for conditions with less than 50% straw. In pure straw combustion, KCl(g) and K2Si4O9(liq) are the main species. During the gas cooling, KCl(g) and KOH(g) may react with SO2(g) and H2O(g) to form K2SO4(g) and then a lot of aerosols are formed. Finally, the possible reactions involving chlorine and alkali metals are analyzed.

Introduction The thermal utilization of biomass can contribute to the reduction of CO2 emissions since the same amount of CO2 released by combustion is extracted from the air during the growth period of the plants. As one possibility for biomass utilization, co-combustion with coal in existing large-scale pulverized fuel firing systems offers several advantages, e.g., the feasibility to utilize a large quantity of biomass in power plants, or to have lower investment costs as opposed to systems fired exclusively with biomass. Recently, studies were reported about cofiring biomass, such as wood, straw, and switch grass, at heat and power generation stations in Europe and U. S.1-5 They not only decrease CO2 emissions from power plants but also reduce a limited amount of NOx and SOx emissions.6-7 Compared with coal, straw has a high amount of potassium and chlorine, as straw typically has a content * Corresponding author. Institute of Process Engineering and Power Plant Technology (IVD), University of Stuttgart, Pfaffenwaldring 23, D-70569, Stuttgart, Germany. Tel: +49-711-685 7804. Fax: +49-711685 3491. E-mail: [email protected]. (1) Hein, K. R. G.; Bemtgen, J. M. EU Clean Coal TechnologysCoCombustion of Coal and Biomass. Fuel Process. Technol. 1998, 54, 159169. (2) Ru¨diger, H.; Kicherer, A.; Greul, U.; Spliethoff, H.; Hein, K. R. G. Investigation in Combined Combustion of Biomass and Coal in Power Plant Technology. Energy Fuels 1996, 10, 789-796.

of potassium (0.5-1.3%), chlorine (0.2-0.7%), and silicon (0.3-1.0%) on dry straw base as well as minor amounts of Ca, Mg, Al, Na, Fe, S, and P, etc.8 During straw combustion or pyrolysis, significant amounts of alkali metals and chlorine are released into the gas phase, such as KCl, KOH, and HCl.9-12 Equilibrium calculations indicated that the chlorine distribution in (3) Hansen, P. F. B.; Andersen K. H.; Wieck-Hansen K.; Overgaard P.; Rasmussen I.; Frandsen F. J.; Hansen L. A.; Dam-Johansen, K. Co-Firing straw and coal in a 150-MWe Utility Boiler: In Situ Measurements. Fuel Process. Technol. 1998, 54, 207-225. (4) Gold, B. A.; Tillman, D. A. Wood Cofiring Evaluation at TVA Power Plants. Biomass Bioenergy 1996, 10, 71-78. (5) Hunt, E. F.; Prinzing, D. E.; Battista, J. J.; Hughes, E. The Shawville Coal/Biomass Cofiring Test: A Coal/Power Industry Cooperative Test of Direct Fossil-Fuel CO2 Mitigation. Energy Conserv. Manage. 1997, 38, S551-S556. (6) Spliethoff, H.; Hein, K. R. G. Effect of Co-Combustion of Biomass on Emissions in Pulverized Fuel Furnaces. Fuel Process. Technol. 1998, 54, 189-205. (7) Pedersen, L. S.; Morgan, D. J.; van de Kamp W. L.; Christensen, J.; Jespersen, P.; Dam-Johansen K. Effects on SOx and NOx Emissions by Co-Firing Straw and Pulverized Coal. Energy Fuels 1997, 11, 439446. (8) Jensen, P. A.; Stenholm, M.; Hald, P. Deposition Investigation in Straw-fired Boilers. Energy Fuels 1997, 11, 1048-1055. (9) Dayton, D. C.; French, R. J.; Milne, T. A. Direct Observation of Alkali Vapor Release during Biomass Combustion and Gasification. 1. Application of Molecular Beam/Mass Spectrometry to Switch Grass Combustion. Energy Fuels 1995, 9, 855-865. (10) Olsson, J. O.; Ja¨glid, U.; Pettersson, J. B. C.; Hald, P. Alkali Metal Emission during Pyrolysis of Biomass. Energy Fuels 1997, 11, 779-784.

10.1021/ef0102925 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/13/2002

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the gas species greatly vary with an increasing cofired straw fraction.13 When the gas is cooled later in the boiler, potassium condenses mainly as sulfate and chloride, and ash deposits form on the heat transfer surfaces.8,14-16 The lower melting point for some potassium species (e.g., KCl melts at 1044 K) puts a high risk of formation of hard deposits on furnace walls and convection tubes. Due to the interaction of chlorine and sulfur in ash, complex metal/ash reactions occur, and this results in a severe corrosion problem of the heat transfer tubes.17 Due to the great difference of the elemental composition of coal and straw, the behavior of chlorine and alkali metals during co-combustion will be very different from pure coal or straw combustion because of the complicated interactions between the volatile elements (Cl, K, and Na) and the other mineral elements. It was found that the major ash-forming elements (Al and Si) had a significant influence on this behavior.18-19 The experimental results of the effect of coal minerals showed that the amounts of KCl(g) and NaCl(g) detected during co-combustion were lower than those expected on the bases of the combustion results for the pure fuels.20 The amounts of condensed alkali species were also significantly higher with increasing silicon and aluminum contents of blended fuels, usually in the form of Sanidine (K2O‚Al2O3‚6SiO2) and Albite (Na2O‚Al2O3‚ 6SiO2). On the bases of the reaction mechanisms between alkali chlorides and aluminum silicates, some mineral additives have been used to raise the ash melting temperatures, thus preventing ash fouling in the furnace or bed agglomeration in FBC.21-23 It was found that when kaolin (Al2O3‚2SiO2‚2H2O(s)) or bauxite (Al2O3(s)) is added the fuels, a high amount of alkalies (11) Jensen, A.; Dam-Johansen, K.; Wo´jtowicz, M. A.; Serio, M. A. TG-FTIR Study of the Influence of Potassium Chloride on Wheat Straw Pyrolysis. Energy Fuels 1998, 12, 929-938. (12) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sander, B. Experimental Investigation of the Transformation and Release to Gas Phase of Potassium and Chlorine during Straw Pyrolysis. Energy Fuels 2000, 14, 1280-1285. (13) Wei, X.; Lopez, Ch.; Puttkamer, T.; Schnell, U.; Hein, K. R. G. Release of Chlorine and Its Retention in Ash during Co-combustion of Biomass and Coal in a Pulverized Fuel Combustor. In Proceedings of the Sixth International Conference on Technologies and Combustion for a Clean Environment, Porto, Portugal, July 9-12, 2001; pp 541550. (14) Heinzel, T.; Siegle, V.; Spliethoff, H.; Hein, K. R. G. Investigation of Slagging in Pulverized Fuel Co-Combustion of Biomass and Coal at a Pilot-Scale Test Facility. Fuel Process. Technol. 1998, 54, 109-125. (15) Valmari, T.; Lind, T. M.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Field Study on Ash Behaviour during Circulating Fluidized-Bed Combustion of Biomass. 2. Ash Deposition and Alkali Vapor Condensation. Energy Fuels 1999, 13, 390-395. (16) Heinzel, T.; Maier, J.; Baum, J.; Spliethoff, H.; Hein, K. R. G. Slagging and Fouling in Dry and Molten Ash PFC. Joule III Programme Clean Coal Technology R&D, Vol. V; European Commission, 1999; pp 1-63. (17) Nielsen, N. P.; Frandsen, F. J.; Dam-Johansen, K.; Baxter, L. L. The Implications of Chlorine-Associated Corrosion on the Operation of Biomass-Fired Boilers. Prog. Energy Combust. Sci. 2000, 26, 283298. (18) Hansen, L. A.; Frandsen, F. J.; Dam-Johansen, K.; Sørensen, H. S.; Skrifvars, B.-J. Characterization of Ashes and Deposits from High-Temperature Coal-Straw Co-Firing. Energy Fuels 1999, 13, 803816. (19) Andersen, K. H.; Frandsen, F. J.; Hansen, P. F. B.; WieckHansen, K.; Rasmussen, I.; Overgaard, P.; Dam-Johansen, K. Deposit Formation in a 150 MWe Utility PF-Boiler during Co-Combustion of Coal and Straw. Energy Fuels 2000, 14, 765-780. (20) Dayton, D. C.; Belle-Oudry, D.; Nordin, A. Effect of Coal Minerals on Chlorine and Alkali Metals Released during Biomass/Coal Cofiring. Energy Fuels 1999, 13, 1203-1211.

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combine with aluminosilicates in the ash, and thus much chlorine is released into the gas phase.23 A few results have been reported about the effect of Ca and Mg on the behavior of chlorine and alkali metals.13,23 It was found that, during brown coal combustion, due to the lower content of aluminum, Al more likely combines with Ca and Mg, so only a low content of potassium aluminosilicate is formed. Thus a high amount of gaseous alkali chloride forms, which may be captured by the fly ash or deposits as KCl(s) and NaCl(s) during the gas cooling.13 On the other hand, the influence of limestone addition on the retention of chlorine has been reported and most studies have focused on the impact of limestone on the dechlorination and desulfurization efficiency.24-32 Experimental results showed that the concentration of chlorine in fly ash tended to increase, and the HCl concentration was found to decrease with increasing limestone dosage.23 Limestone was proposed to remove gaseous Cl through the reaction between CaO and HCl at low temperatures (823-973 K) in the flue gas path.24 Under the fluidized-bed conditions (temperature and gaseous species concentration), limestone is not likely to act as a significant insitu capture means for HCl. The formed CaCl2 will tend to release HCl as the solid circulates in the fluidized bed.32 In addition, according to the analysis of the ratios (Na + K)/(Cl + 2S) and (Na + K)/2S in fine fly ash, it was suggested that limestone provokes a shift from alkali chlorides to alkali sulfates.23 Up to now, although some reports have been published about the influence of Al, Si, Ca, etc., on the behavior of chlorine and alkali metals, the results were related to co-combustion of straw and coal only for a limited range of the straw fraction. Few studies have analyzed the influence of fuel minerals on this behavior considering all of the minor elements (Al, Si, K, Na, Ca, Mg, Fe, Ti, Mn, S, Cl, P, etc.). In this paper, on the bases (21) Kyi, S.; Chadwick, B. L. Screening of Potential Mineral Additives for Use as Fouling Preventatives in Victorian Brown Coal Combustion. Fuel 1999, 78, 845-855. (22) O ¨ hman, M.; Nordin, A. The Role of Kaolin in Prevention of Bed Agglomeration during Fluidized Bed Combustion of Biomass Fuels. Energy Fuels 2000, 14, 618-624. (23) Coda, B.; Aho, M.; Berger, R.; Hein, K. R. G. Behaviour of Chlorine and Enrichment of Risky Elements in Bubbling Bed Combustion of Biomass and Waste Assisted by Additives. Energy Fuels 2001, 15, 680-690. (24) Xie, W.; Pan, W.-P.; Riley, J. T. Behavior of Chloride during Coal Combustion in an AFBC System. Energy Fuels 1999, 13, 585591. (25) Daoudi, M.; Walters, J. K. A Thermogravimetric Study of the Reaction of Hydrogen Chloride Gas with Calcined Limestone: Determination of Kinetic Parameters. Chem. Eng. J. 1991, 47, 1-9. (26) Liang, D. T.; Anthony, E. J.; Loewen, B. K.; Yates, D. J. Halogen Capture by Limestone during Fluidized Bed Combustion. In Proceedings of 11th International Conference on FBC 1991; Vol. 2, pp 917922. (27) Weinell, C. E.; Jensen, P. I.; Dam-Johansen, K.; Livbjerg, H. Hydrogen Chloride Reaction with Lime and Limestone: Kinetics and Sorption Capacity. Ind. Eng. Chem. Res. 1992, 31, 164-171. (28) Mura, G.; Lallai, A. Reaction Kinetics of Gas Hydrogen Chloride and Limestone. Chem. Eng. Sci. 1994, 49, 4491-4500. (29) Wang, W.; Ye, Z.; Bjerle, I. The Kinetics of the Reaction of Hydrogen Chloride with Fresh and Spent Ca-based Desulfurization Sorbents. Fuel 1996, 75, 207-212. (30) Matsukata, M.; Takeda, K.; Miyatani, T.; Ueyama, K. Simultaneous Chlorination and Sulphation of Calcined Limestone. Chem. Eng. Sci. 1996, 51, 2529-2534. (31) Lawrence, A. D.; Bu, J.; Gokulakrishnan, P. The Interactions between SO2, NOx, HCl and Ca in a Bench-Scaled Fluidized Bed Combustor. J. Inst. Energy 1999, 72, 34-40. (32) Lawrence, A. D.; Bu, J. The Reactions between Ca-based Solids and Gases Representative of Those Found in a Fluidized-Bed Incinerator. Chem. Eng. Sci. 2000, 55, 6129-6137.

Co-Combustion of Coal and Straw

Figure 1. 0.5 MW pulverized fuel combustion test rig.

of measurement data, the influence of inherent minerals in blended fuels on the behavior of chlorine and alkali metals is investigated by means of the equilibrium analysis tool FACT-Win for co-combustion in the range of straw fractions from 0% to 100%. Experimental Section Experimental Facility. The 0.5 MW pulverized fuel combustion test rig at IVD allows various burner configurations for fuels, such as gas, coal, and biomass (see Figure 1). The facility includes different milling, dosing, and blending devices for the various fuels. The vertical combustion chamber, with a total length of 7 m, is constructed in a way that the temperature and residence time conditions during combustion of fuel particles correspond to the characteristics of industrial power plants. The first section, 4 m in length and 0.75 m in diameter, refractory-lined, simulates the furnace of a boiler, whereas the second section, 0.85 m in diameter, water-cooled, simulates the convective part. This section is followed by the flue gas path with air preheater, cyclone, bag house filter, ID fan, and stack. Three rows of accesses at the vertical chamber, each staggered by about 90°, allow the use of probes and optical measurement techniques. The main burner is installed in the center of the furnace top together with the flame detector and the ignition burner. Fuel and staged air may be fed to the reactor by additional nozzles. The secondary air is heated to 573 K by the preheater and passes a swirl generator (movable block) before injection. Flame temperature profiles are measured in all flames along the combustion zone down to convective sections. The composition of the flue gas is analyzed for all flames by continuous measurement of O2, CO2, CO, NOx, and SO2. The suction probe is located at the end of the furnace section, at a temperature level of approximately 973 K. Below this port, fly ash is sampled by an uncooled suction probe. The second location for ash sampling is the bottom slag/ash hopper at the end of the top-fired vertical furnace. The third location is the ash removal box of the air preheater at a temperature of approximately 773 K. Following the air preheater, the cyclone is the main ash discharge, at temperatures of approximately 623 K,

Energy & Fuels, Vol. 16, No. 5, 2002 1097 separating particles in the range of 5-100 µm. The final gas cleaning step is the bag house cloth filter, operated at 473 K, separating particles in the range from 10 µm down to submicrons. The amounts of different elements in various sample ashes, such as S, Cl, P, Al, Si, Ca, Mg, K, Na, Fe, Ti, and Mn, are analyzed. During the experiments, ash is collected in four locations, i.e., bottom ash hopper, air preheater, cyclone and bag filter (see Figure 1). The weight fractions of various ashes in the tests are shown in Table 1. Fuel Composition. The fuels in this investigation are two coals and two types of straw (see Table 2). Raw fuel is milled and samples of it are taken from the dosing system. “Dry” means calculated on a water-free basis. One coal is a typical bituminous German hard coal, “Go¨ttelborn”, with a relatively high amount of chlorine and alkali metals (0.22% Cl in fuel and 2.91% K2O and 1.51% Na2O in fuel ash). Another coal is a German lignite, “brown coal”, from the Rhenish area, with very high amounts of Ca, Mg and low amounts of Al, Si in fuel ash (37.95% CaO, 15.51% MgO, 4.29% Al2O3, and 6.99% SiO2 in ash). Straw 1, “Biopack”, with 0.29% Cl in fuel, cofired with hard coal, is a regular German wheat straw from the interior of the country. Straw 2, “DK chlorine-rich”, with 0.53% Cl in fuel, cofired with brown coal, was selected for its higher chlorine content and delivered by the suppliers of the Studstrup Power Plant on the Danish coast. Experiments are carried out for various straw fractions, such as 0, 12.5%, 25%, 50%, and 100% on the base of thermal energy. The thermal input is 400 kW. The target air ratio is set at 1.2, corresponding to an oxygen content in the flue gas of 3.5%. Actually, the straw fraction in large p.f. co-combustion power stations (>50 MWe) is normally less than 25%19,33 because of the characteristics of its preparation, the slagging, fouling, and corrosion hazard during combustion, and the ash utilization limit. However, all the tests with a wide range of straw fractions will still be useful to evaluate the possibility of co-combustion technology. The ash elementary compositions of blended fuels are shown in Table 2. Increasing the straw fraction increases Cl, K, and Si and reduces S, Al, and Fe in the fuel ash. In the case of hard coal co-combustion, the percentages of Si and Al in the ash are high, whereas in the case of brown coal co-combustion, the content of Al in ash is low and the contents of Ca and Mg are relatively high.

Calculating Method A global equilibrium analysis (GEA) has been performed to predict thermodynamics stable chemical and physical forms as functions of temperature, pressure, and total elementary composition in the combustion system. The composition of the system at given temperature and pressure is calculated by minimizing the total Gibbs free energy of the system.34,35 When the total Gibbs free energy is at minimum, the system is in a thermodynamic equilibrium state where all possible reactionshomogeneous and heterogeneous have reached equilibrium. A lot of software have been developed to predict the compositions of chemical system through global equilibrium analysis, e.g., HSC Chemistry, FACT-Sage, and GEA software in DTU. In this paper, the equilibrium analysis tool FACTWin is used to predict the co-combustion system because its database includes many chemical species and components. In (33) Spliethoff, H.; Unterberger, S.; Hein, K. R. G. Status of Cocombustion of Coal and Biomass in Europe. In Proceedings of the Sixth International Conference on Technologies and Combustion for a Clean Environment, Porto, Portugal, July 9-12, 2001; pp 575-584. (34) Smith, W. R.; Missen, R. W. Chemical Reaction Equilibrium Analysis: Theory and Algorithms; John Wiley & Sons: New York, 1982. (35) Frandsen, F.; Dam-Johansen, K.; Rasmussen, P. Trace Elements from Combustion and Gasification of Coals An Equilibrium Approach. Prog. Energy Combust. Sci. 1994, 20, 115-138.

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Table 1. Weight Shares of Various Ashes during Co-Combustion16 co-combustion of hard coal with straw 1 biomass fraction (%) bottom ash (%) preheater ash (%) cyclone ash (%) filter ash (%)

0 13.5 11 58 17.5

12.5 21 9 48 22

25 27 8 42 23

50 40.5 6.5 27 26

co-combustion of brown coal with straw 2 100 62 7 8 23

0 13 8 42 37

12.5 41.5 8.5 8.5 41.5

25 52.5 12.5 7 28

50 56 11 10 23

100 48 13 6 33

Table 2. Composition of Fuels chemical analysis (%) dry

hard coal mean

brown coal mean

Straw 1 mean

Straw 2 mean

ash analysis (%)

hard coal mean

brown coal mean

Straw 1 mean

Straw 2 mean

moisture volatile ash fixed C C N S H O (diff) Cl LHV (MJ/kg)

2.03 33.16 9.30 56.24 73.52 1.48 0.88 4.26 8.98 0.22 30.89

10.48 53.47 4.28 42.24 65.24 0.75 0.37 4.69 24.82 0.02 24.51

9.65 78.69 6.36 14.85 46.24 0.65 0.07 5.46 40.82 0.29 17.44

11.71 71.32 13.42 15.26 41.43 1.09 0.10 4.18 38.94 0.53 16.01

Al2O3 CaO Fe2O3 K2O MgO Na2O SiO2 SO3 TiO2 P2O5 MnO2a

23.57 5.48 13.27 2.91 2.92 1.51 40.09 7.40 0.95 0.19 0.02

4.29 37.95 14.52 0.63 15.51 2.73 6.99 15.28 0.23 0.03 0.02

1.02 6.42 0.56 10.60 1.42 1.97 70.43 2.38 0.07 2.71 0.02

0.36 4.34 0.26 14.79 0.87 0.35 68.59 1.93 0.02 1.93 0.02

a

Data estimated. Table 3. Main Species Obtained from the Thermodynamic Equilibrium Calculation category combustion gas products

aluminosilicate

Si

Al SO4, PO4 Cl O, CO3

species Main Gas Species (71, Obtained from 143 Gas Species) N2, CO2, H2O, O2, SO2, HCl, NO, SO3, NaCl, OH, KCl, Cl, NO2, CO, H2, O, NaOH, HOO, N2O, MnCl2, HOCl, KOH, ClO, Cl2, HONO(g,g2), (NaCl)2, Fe(OH)2, Na2SO4, FeCl2, HOOH, O2S(OH)2, ONCl, H, (KCl)2, K2SO4, Mg(OH)2, Na, SO, MgCl2, HNO, CaCl2, NaO, K, FeCl3, Ca(OH)2, PO2, KO, FeO, OAlOH, (NaOH)2, (KOH)2, FeCl, MgCl, NaFeCl4, COCl2, HCOOH, NH3, COCl, TiOCl2, (FeCl2)2, HNCO, SOCl2, NH2, N, OAlCl, HNNH, NH, H2CO, HCO, HCN Main Condensed Species (86, Obtained from 94 Liquid and 374 Solid Species) Al2SiO5(s,s2,s3) (Al2O3‚SiO2(andalusite, sillimanite, kyanite)), Al6Si2O13(s) (3Al2O3‚2SiO2(mullite)), NaAlSiO4(s3,s4) (Na2O‚Al2O3‚2SiO2(carnegieite)), NaAlSi3O8(s2) (Na2O‚Al2O3‚6SiO2(s2)), KAlSi2O6(s,s2) (K2O‚Al2O3‚4SiO2(leucite(rhf)-a, b)), KAlSi3O8(s2) (K2O‚Al2O3‚6SiO2(k-feldspar)), CaAl2Si2O8(s2) (CaO‚Al2O3‚2SiO2(anorthite)), Ca3Al2Si3O12(s) (3CaO‚Al2O3‚3SiO2(grossular), Mg2Al4Si5O18(s) (2MgO‚2Al2O3‚5SiO2(s)), Mg5Al2Si3O10(OH)8(s), KAl3Si3O10(OH)2(s) (muscovite), KMg3AlSi3O10(OH)2(s) (phlogopite) K2Si4O9(liq) (K2O‚4SiO2(liq)), MgO‚CaO‚2SiO2(liq), CaSiTiO5(liq) (CaO‚SiO2‚TiO2(liq)), SiO2 (quartz(l), quartz(h), tridymite(h), cristobalite(h)), MgSiO3(s,s2,s3) (MgO‚SiO2(low-clinoenst, high-t-clinoe, orthoenstatit)), Mg2SiO4(s) (2MgO‚SiO2(s)), K2Si2O5(s,s2) (K2O‚2SiO2(s,s2)), K2Si4O9(s,s2) (K2O‚4SiO2(s,s2)), CaSiO3(s,s2) (CaO‚SiO2(wollastonite, pseudowollastonite)), Ca2SiO4(s,s2) (2CaO‚SiO2(solid-beta, alpha-pri)), Ca3SiO5(s) (3CaO‚SiO2(hatrurite)), Na2Mg2Si6O15(s) (Na2O‚2MgO‚6SiO2(s)), Na2Ca2Si3O9(s) (Na2O‚2CaO‚3SiO2(s)), Na2Ca3Si6O16(s) (Na2O‚3CaO‚6SiO2(s)), MgO‚CaO‚SiO2(s) (monticellite), MgO‚CaO‚2SiO2(s) (diopside(clino)), MgO‚2CaO‚2SiO2(s) (akermanite), MgO‚3CaO‚2SiO2(s) (merwinite), Mg3Si4O10(OH)2(s) Al2O3(H2O)(s), MgAl2O4(s) (MgO‚Al2O3(s)), CaAl2O4(s) (CaO‚Al2O3(s)), Ca3Al2O6(s) (3CaO‚Al2O3(s)), KAl9O14(s) (K2O‚9Al2O3 (k-beta-alumina)) Na2SO4(liq), Na2SO4(solid-a, b), K2SO4(solid alpha, beta), KAl(SO4)2(s), CaSO4(s), MgSO4(s), MgSO4(H2O)(s) (kieserite), Ca3(PO4)2 (whit.kite, solid-b), Ca5HO13P3(s), Na3(PO4)(s), Mg3(PO4)2(s), AlPO4(solid-a, b, c) NaCl(s), KCl(s), CaCl2(s) MgO(s), CaO(s), CaCO3(calcite), CaMg(CO3)2(dolomite), TiO2(s), Fe2O3(hematite), Fe3O4(magnetite), MgO‚Fe2O3(s), Ca2Fe2O5(s) (2CaO‚Fe2O3(s)), CaTiO3(perovskite-a, b), Ca3Fe2Si3O12(s) (3CaO‚Fe2O3‚3SiO2(andradite)), Fe2O5Ti(s), MgTiO3(s), CaTiO3(s,s2) (perovskite-a,b), CaSiTiO5(s) (CaO‚SiO2‚TiO2(s))

the calculation, besides the major elements, all of the minor elements in the fuels have been considered to investigate the effect of mineral elements in ash on the behavior of chlorine and alkali metals. When the parameters, such as the elementary composition of the fuel and the air, temperature and pressure have been entered, FACT-Win will automatically search the species involving using elements from the database. Then the thermodynamic equilibrium calculation is conducted for the system with these species. In this paper, for the system including the elements C, H, O, N, S, Cl, Si, P, Ca, K, Na, Mg, Al, Fe, Ti, and Mn between 400 and 1800 K, there are 611 species (143 gaseous, 94 liquid, and 374 solid species) selected.

The main gaseous and condensed species from the results of the equilibrium calculation are shown in Table 3. Although the equilibrium analysis is a powerful tool to predict the stable species during the chemical process, there is some drawback for the application to the combustion case.35,36 Either the temperature must be high enough or the species residence time long enough in order to reach the thermodynamic equilibrium. For the coal-fired system, in lowtemperature (T < 800 K) zones, the reaction rates may be too (36) Yan, R.; Gauthier, D.; Flamant, G.; Badie, J. M. Thermodynamics Study of the Behavior of Minor Coal Elements and Their Affinities to Sulphur during Coal Combustion. Fuel 1999, 78, 1817-1829.

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Figure 2. Ash elementary compositions of blended fuels.

Figure 3. The mole elementary compositions for various ashes during co-combustion of hard coal and Straw 1. low to reach equilibrium. In addition, temperature and composition gradients have not been taken into account in the equilibrium analysis, thus the mixing phenomena cannot be calculated. Physical processes, such as particle nucleation, agglomeration and adsorption in the gas, are not considered in equilibrium calculations. In general, the global equilibrium analysis can only be used to give the equilibrium distribution of elements and the reaction mechanism of various species. Thus, the difficulties will arise when the results of the equilibrium analysis are compared with of real industrial cocombustion systems.

Results Contents of Chlorine and Alkali Metals in Ash: Measurement Data. Figure 3 shows the mole elementary compositions for various ashes during co-combustion of hard coal and Straw 1. Very low amounts of chlorine, sulfur, and sodium are captured or retained in bottom, air preheater, and cyclone ash. The contents of potassium in these combustion ashes are similar to contents in blended fuel ash. Because of the high contents of Si and Al in ash, potassium aluminosilicates are likely compounds and only low contents of KCl(s) and K2SO4(s) may develop in these ashes.

For filter ash, the chlorine content is still low except in the case of pure straw combustion. The potassium content in filter ash is higher than that in blended fuel ash. The sodium content in filter ash is close to that in blended fuel ash. This can be explained by the formation of KCl(s) and K2SO4(s) in filter ash. In general, during the gas cooling, gaseous KCl(g) may condense on the surface of ash particles (especially on small particles) or react with SO2 to form K2SO4(g,s). Jensen reported that a very high amount of submicro aerosols may form during straw combustion.8 The particles consisted almost completely of K, Cl, and S. The mean diameter of the particles was 0.3 µm. Most of these small particles were captured by the bag filter in the experiments. If K, Na, Cl, and S are all present as KCl(s), NaCl(s), K2SO4(s), and Na2SO4(s) in ash, the ratio (K + Na)/(Cl + 2S) should be approximately 1. This ratio for filter ash is shown in Table 4. Obviously, it is close to 1 for hard coal co-combustion. Figure 4 shows the mole elementary compositions of various ashes during co-combustion of brown coal and Straw 2. Very small amounts of chlorine and sulfur are retained in bottom, air preheater and cyclone ash. The

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Table 4. Ratio (K + Na)/(Cl + 2S) for Filter Ash biomass fraction (%)

0

12.5

25

50

100

hard coal + straw 1 brown coal + straw 2

0.727 0.874

1.44 0.825

0.912 0.881

1.10 1.19

1.18 1.08

contents of potassium and sodium are lower than those in blended fuel ash. There are high amounts of calcium and magnesium in ash. Increasing the straw fraction increases the content of silicon and reduces the content of aluminum. Due to the very small amount of aluminum in ash, besides potassium aluminosilicate, K most likely combines with SiO2(s) to form potassium silicate. For filter ash, the chlorine and sulfur contents are relatively high. The contents of potassium and sodium are higher than those in blended fuel ash. The ratio (K + Na)/(Cl + 2S) for brown coal co-combustion is also close to 1 (see Table 4). During straw combustion, a lot of aerosols are formed along with the enrichment of K, Cl, and S. In cocombustion, decreasing the straw ratio reduces the formation of aerosols. Most of them can be collected in the bag filter, and some may be released into the air from the stack. In addition, the distribution of chlorine and alkali in various ashes (bottom, air preheater, cyclone, and filter ash) is significantly different. Obviously, the formation of aerosols and varying ash compositions will bring difficulties on the comparison of equilibrium predictions and measurement data. Behavior of Chlorine, Potassium, and Sodium: Equilibrium Analysis. Figure 5 shows the behavior of the K-Cl-S system during co-combustion of hard coal and Straw 1. The results of conditions with 12.5% straw are similar to those for hard coal and have been neglected in Figure 5, but the condition with 75% straw is added. Potassium may be formed as gaseous species, such as KCl(g) and KOH(g) and ash components, such as KAlSi2O6(s,s2) (K2O‚Al2O3‚4SiO2(leucite(rhf)-a,b)), KAlSi3O8(s2) (K2O‚Al2O3‚6SiO2(k-feldspar)), KAl(SO4)2(s), K2SO4(s,s2), K2Si4O9(liq,s2) (K2O‚4SiO2(liq,s2)), and KCl(s). At high temperatures (>1000 K), for hard coal cocombustion with less than 50% Straw 1, most of the chlorine and sulfur are released as HCl(g), SO2(g), and SO3(g). Less than 10% of the potassium is released as KCl(g) and KOH(g) because most of the potassium is combined with aluminosilicate (KAlSi2O6(s2)). In the medium-temperature range (800-1000 K), KAlSi2O6(s2) is converted to KAlSi2O6(s). Because KAlSi2O6(s,s2) exists in the form of solid minerals at combustion temperatures, these solids can prevent ash depositing on the furnace surface. Increasing the straw fraction reduces the formation of KAlSi2O6(s,s2) due to the lower content of aluminum in straw. For the condition with 75% straw, KAlSi2O6(s,s2) takes only half of the potassium, and KCl(g) (>1000 K) and K2SO4(s2) (800-1200 K) significantly increase. During pure straw combustion, KCl(g) is still a major species and more K2Si4O9(liq) forms instead of KAlSi2O6(s2) because of the higher silicon content in straw. Obviously, a liquid mineral, K2Si4O9(liq) is prone to stick on the furnace wall. Therefore, very large straw fractions might result in the slagging of ash in the furnace. At lower temperatures (1400 K), some gaseous chlorides also may react with H2O to form KOH or NaOH (reactions 8 and 9). After the process of volatilization of chlorine and alkali metals, the capture and condensation of gaseous chlorides occurs during the gas cooling. It should be noted that the possible reactions involving the chlorine and alkali metals during combustion may be as many

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Figure 7. The behavior of the K-Cl-S system during co-combustion of brown coal and Straw 2.

Figure 8. The behavior of the Na-Cl-S system during co-combustion of brown coal and Straw 2.

as described in the works of Kyi et al.21 However, we will only consider the reactions involving the compounds

and species under the chemical equilibrium states which may occur during co-combustion of coal with straw,

Co-Combustion of Coal and Straw

Energy & Fuels, Vol. 16, No. 5, 2002 1105

Figure 9. The aluminum behavior during co-combustion of hard coal and Straw 1.

Figure 10. The aluminum behavior during co-combustion of brown coal and Straw 2.

especially for the German hard coal and brown coal. Nevertheless, for other coals or straws, these reactions

are still of importance for the analysis of the behavior of chlorine and alkali metals.

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Table 5. Reactions of Volatilization Gibbs free energy (kJ/mol-HCl)

temperature range (K)

no.

reaction

1 2 3 4 5

Cl, K, Na in fuels f HCl(g), Cl2(g), KCl(g), NaCl(g), KOH(g), NaOH(g), K2SO4(g), Na2SO4(g), K(g), Na(g) Cl2(g) + H2O(g) f 2HCl(g) + 0.5O2(g) 2KCl(liq) + SO2(g) + H2O(g) + 0.5O2(g) f K2SO4(g) + 2HCl(g) 2NaCl(liq) + SO2(g) + H2O(g) + 0.5O2(g) f Na2SO4(g) + 2HCl(g) KCl(liq) f KCl(g)

∆G ) 29.76 - 0.03429T ∆G ) -101.3 + 0.07390T ∆G ) -109.4 + 0.07537T ∆G ) 170.5 - 0.09786T

900-1800 1044-1300 1074-1400 1044-1800

6

NaCl(liq) f NaCl(g)

∆G ) 178.0 - 0.09944T

1074-1800

7 8 9

2KCl(liq) + 4SiO2(s2,s4) + H2O(g) f K2O‚4SiO2(liq) + 2HCl(g) KCl(g) + H2O(g) f KOH(g) + HCl(g) NaCl(g) + H2O(g) f NaOH(g) + HCl(g)

∆G ) 100.8 - 0.05133T ∆G ) 140.1 - 7.784× 10-3T ∆G ) 141.9 - 9 .361× 10-3T

1044-1600 1400-1800 1400-1800

comment

400-1800 ∆G < 0 ∆G < 0 ∆G < 0 ∆G > 0 (T < 1740 K) ∆G > 0 (T < 1790 K) ∆G > 0 ∆G > 0 ∆G > 0

Table 6. Reactions of Capture of Gas Chlorides Gibbs free energy (kJ/mol-HCl)

temperature range (K)

6KCl(g) + 3Al2O3‚2SiO2(s) + 10SiO2(s2,s4) + 3H2O(g) T 3(K2O‚Al2O3‚4SiO2(s2)) + 6HCl(g) 6NaCl(g) + 3Al2O3‚2SiO2(s) + 16SiO2(s2,s4) + 3H2O(g) T 3(Na2O-Al2O3‚6SiO2)(s2) + 6HCl(g) 2NaCl(g) + 2(MgO‚SiO2(s2,s3)) + 4SiO2(s2,s4) + H2O(g) T Na2O‚2MgO‚6SiO2(s) + 2HCl(g) 2NaCl(g) + 3(CaO‚SiO2(s1,s2)) + 3SiO2(s2,s4) + H2O(g) T Na2O‚3CaO‚6SiO2(s) + 2HCl(g) 2KCl(g) + 4SiO2(s2,s4) + H2O(g) T K2O‚4SiO2(liq) + 2HCl(g) 2KCl(g) + SO2(g) + H2O(g) + 0.5O2(g) T K2SO4(g) + 2HCl(g) 2KCl(g) + SO2(g) + H2O(g) + 0.5O2(g) T K2SO4(s,s2) + 2HCl(g) 2NaCl(g) + SO2(g) + H2O(g) + 0.5O2(g) T Na2SO4(g) + 2HCl(g) 2NaCl(g) + SO2(g) + H2O(g) + 0.5O2(g) T Na2SO4(liq) + 2HCl(g) 2KOH(g) + SO2(g) + 0.5O2(g) T K2SO4(g) + H2O(g) 2KOH(g) + SO2(g) + 0.5O2(g) T K2SO4(s2) + H2O(g) 2NaOH(g) + SO2(g) + 0.5O2(g) T Na2SO4(g) + H2O(g) 2NaOH(g) + SO2(g) + 0.5O2(g) T Na2SO4(liq) + H2O(g) 6KOH(g) + 3Al2O3-2SiO2(s) + 10SiO2(s4,s6) T 3(K2O‚Al2O3‚4SiO2(s2)) + 3H2O(g) 6NaOH(g) + 3Al2O3-2SiO2(s) + 4SiO2(s4,s6) T 3(Na2O‚Al2O3‚2SiO2(s3)) + 3H2O(g) 2NaOH(g) + 2(MgO-SiO2(s3)) + 4SiO2(s4,s6) T Na2O‚2MgO‚6SiO2(s) + H2O(g) 2KOH(g) + 4SiO2(s4,s6) T K2O‚4SiO2(liq) + H2O(g)

∆G ) -131.4 + 0.04524T

1000-1800

∆G ) -142.8 + 0.06527T

1000-1800

∆G ) -69.69 + 0.03140T

900-1800

∆G ) -230.5 + 0.1578T

400-1400

∆G ) -65.51 + 0.04550T

1044-1800

∆G ) -144.7 + 0.1243T

1100-1500

∆G ) -302.0 + 0.1956T

400-1300

∆G ) -149.2 + 0.1264T

1000-1500

∆G ) -299.9 + 0.1865T

1100-1500

∆G ) -283.2 + 0.1312T ∆G ) -434.3 + 0.1948T ∆G ) -288.2 + 0.1337T ∆G ) -428.2 + 0.1834T ∆G ) -271.3 + 0.05288T

1200-1800 800-1800 1200-1800 1157-1800 1400-1800

∆G < 0, Figure 6, Figure 8 ∆G < 0, Figure 6, Figure 8 ∆G < 0 (T < 1440 K), Figure 5, Figure 7 ∆G < 0 (T < 1160 K), Figure 7 ∆G < 0, Figure 5, Figure 7 ∆G < 0 (T < 1180 K), Figure 8 ∆G < 0, Figure 7, Figure 8 ∆G < 0, Figure 7 ∆G < 0, Figure 7 ∆G < 0, Figure 8 ∆G < 0, Figure 8 ∆G < 0, Figure 7

∆G ) -254.2 + 0.06176T

1400-1800

∆G < 0, Figure 8

∆G ) -211.9 + 0.04092T

1400-1800

∆G ) -195.0 + 0.04557T

1400-1800

KCl(g) f KCl(liq) KCl(g) f KCl(s) NaCl(g) f NaCl(liq) NaCl(g) f NaCl(s) K2SO4(g) f K2SO4(liq) K2SO4(g) f K2SO4(s2) Na2SO4(g) f Na2SO4(liq) Na2SO4(g) f Na2SO4(s2)

∆G ) -170.5 + 0.09786T ∆G ) -210.1 + 0.1358T ∆G ) -178.0 + 0.09944T ∆G ) -218.3 + 0.1375T ∆G ) -254.2 + 0.09368T ∆G ) -304.7 + 0.1336T ∆G ) -278.4 + 0.09860T ∆G ) -327.6 + 0.1425T

1044-1300 400-1044 1074-1300 400-1074 1342-1500 856-1342 1157-1500 514-1157

∆G < 0, Figure 6, Figure 8 ∆G < 0, Figure 5, Figure 7 ∆G < 0

no.

reaction

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

During co-combustion of hard coal and Straw 1, with less than 50% straw fractions, KCl(g) or NaCl(g) might react with aluminum silicon minerals to release HCl under high temperatures (Table 6, reactions 10 and 11). With an increasing straw fraction, the aluminum content in fuel blends decreases and NaCl(g) might react with solid MgSiO3 or CaSiO3 to release HCl (reactions 12 and 13). In pure straw combustion, KCl(g) might react with SiO2 to form K2Si4O9(liq) and release HCl (reaction 14) under high-temperature conditions. SO2(g) will play an important part in reactions with KCl(g) or NaCl(g) to form K2SO4(s,s2,g), Na2SO4(s2,liq,g), and HCl(g), (Table 6, reactions 15-18). Although the sulfation reaction of gas chlorides may form K2SO4(g)

comment ∆G < 0, Figure 5, Figure 7 ∆G < 0, Figure 6

∆G < 0 ∆G < 0 ∆G < 0

or K2SO4(s,s2,liq), due to the very low vapor pressure of K2SO4(g),37 the more likely products may be solid sulfates, corresponding to the results of equilibrium calculations. In the cooling process of the flue gases, K2SO4(g) quickly condenses as K2SO4(liq,s). This process may result in a great number of aerosols formed in the gas. In addition, KOH(g) and NaOH(g) are also easily sulfated at high temperatures (reactions 19-22) or combined with aluminum or magnesium silicon minerals retaining alkali metals in ash (reactions 23-26). In the actual combustion process, besides the reactions of gaseous chlorides with gaseous or solid species, (37) Iisa, K.; Lu, Y.; Salmenoja, K. Sulfation of Potassium Chloride at Combustion Conditions. Energy Fuels 1999, 13, 1184-1190.

Co-Combustion of Coal and Straw

Energy & Fuels, Vol. 16, No. 5, 2002 1107 Table 7. Reactions of Alkali Species with Deposits

no.

reaction

Gibbs free energy (kJ/mol-HCl)

temperature range (K)

comment

31 32 33 34 35 36

2KCl(liq) + SO2(g) + H2O(g) + 0.5O2(g) f K2SO4(liq) + 2HCl(g) 2KCl(s) + SO2(g) + H2O(g) + 0.5O2(g) f K2SO4(s2) + 2HCl(g) 2NaCl(liq) + SO2(g) + H2O(g) + 0.5O2(g) f Na2SO4(liq) + 2HCl(g) 2NaCl(s) + SO2(g) + H2O(g) + 0.5O2(g) f Na2SO4(s2) + 2HCl(g) K2O‚4SiO2(liq) + 2HCl(g) f 2KCl(liq) + 4SiO2(s,s2,s4) + H2O(g) K2O‚4SiO2(s2) + 2HCl(g) f 2KCl(s) + 4SiO2(s,s2,s4) + H2O(g)

∆G ) -102.2 + 0.07465T ∆G ) -96.6 + 0.06505T ∆G ) -110.1 + 0.0761T ∆G ) -96.0 + 0.0619T ∆G ) -114.1 + 0.0606T ∆G ) -120.9 + 0.0674T

800-1300 800-1300 800-1300 800-1300 800-1300 800-1300

∆G < 0 ∆G < 0 ∆G < 0 ∆G < 0 ∆G < 0 ∆G < 0

they also may condense directly as KCl(s,liq), NaCl(s,liq), K2SO4(s2,liq), or Na2SO4(liq) (Table 6, reactions 27-30). This process has been verified by the experiments. When the deposit sampling probe is inserted into the combustion chamber, it will quickly be covered with a layer of “white” chlorides. In fact, various mechanisms can overlap in various ranges of temperature because the actual combustion conditions cannot reach real equilibrium. For example, reactions 15-18 and 27 and 28 may overlap, i.e., one part of the gaseous chlorides will combine with SO2 to form alkali sulfates and another part can directly condense as chlorides. In addition, the importance of reactions may differ according to the fuel combusted, reaction 10, for instance, is very important for hard coal co-combustion, and reactions 15 and 16 are significant for brown coal co-combustion. Reactions of Chlorine and Alkali Metals in Ash Deposits. During co-combustion, deposition phenomena such as condensation and thermophoresis are of great importance with regard to the deposition of potassium, chlorine, and sulfur.38 After KCl (m.t. 1044 K) or K2SO4 (m.t. 1342 K) have condensed on the wall, some reactions may continue to occur (see Table 7). In co-combustion with less than 50% straw fractions, there is a high partial pressure of SO2 which may influence the relative amounts of KCl and K2SO4 deposited in the superheater tube. Reactions 31-34 show the sulfation of alkali chloride to alkali sulfate, which is likely at medium temperatures (800-1200 K). Because of the low melting point, KCl(s,liq) in deposits is expected to play a major part in the corrosion of the superheater tubes.8,17,38 The sulfation reactions in the deposits are beneficial in preventing the corrosion, slagging, and fouling processes. For co-combustion with a very high straw fraction (>75%), more K2Si4O9(liq,s), i.e., K2O‚4SiO2(liq,s) (m.t. 1044 K), can be found in the deposit, which may react with HCl(g) to form KCl(liq,s) (see reactions 35-36). So more KCl may be found in the deposits for straw combustion. Conclusions The influence of inherent minerals in blended fuels on the behavior of chlorine and alkali metals is investigated through the equilibrium analysis tool FACT-Win for co-combustion of coal with straw in the range of biomass fractions from 0% to 100%. According to measurement data, very low amounts of chlorine and sulfur are retained in bottom, air preheater and cyclone ash in a pulverized fuel combustor. But for bag filter ash, the chlorine and sulfur (38) Nielsen, H. P.; Baxter, L. L.; Sclippab, G.; Morey, C.; Frandsen, F. J. Deposition of Potassium Salts on Heat Transfer Surfaces in StrawFired Boilers: A Pilot-Scale Study. Fuel 2000, 79, 131-139.

contents are relatively high and the ratio (K + Na)/(Cl + 2S) is close to 1. This can be explained by the formation of KCl(s), NaCl(s), K2SO4(s), and Na2SO4(s) in filter ash. According to equilibrium calculations, in hard coal cocombustion with less than 50% straw, most of the potassium combines with aluminosilicate to form stable KAlSi2O6(s,s2), which can prevent ash depositing on the furnace walls. Less than 10% potassium are released as KCl(g) and KOH(g). Increasing the straw fraction reduces KAlSi2O6(s,s2) and increases KCl(g) due to the lower content of aluminum in straw. Besides the formation of NaAlSi3O8(s2), some sodium is released as NaCl(g) and NaOH(g). Increasing the straw fraction results in the formation of Na2Mg2Si6O15(s) and Na2Ca3Si6O16(s). In pure straw combustion, KCl(g) is a main species and more K2Si4O9(liq) is formed, which is prone to stick on the furnace walls inducing slagging. In co-combustion of brown coal with straw, the potassium behavior is very different from that in hard coal co-combustion. Most of the potassium is released as KCl(g), KOH(g), and K2SO4(g). Increasing the straw fraction reduces KAlSi2O6(s2) and increases K2Si4O9(liq). A high amount of K2Si4O9(liq) is likely to induce the slagging of ash on the walls. In addition, gaseous species of potassium (KCl(g) and K2SO4(g)) easily condense on the walls (temperature normally