Co-combustion of Animal Waste, Peat, Waste Wood, Forest Residues

Article pubs.acs.org/EF

Co-combustion of Animal Waste, Peat, Waste Wood, Forest Residues, and Industrial Sludge in a 50 MWth Circulating Fluidized-Bed Boiler: Ash Transformation, Ash/Deposit Characteristics, and Boiler Failures H. Hagman,* R. Backman, and D. Boström Department of Applied Physics and Electronics, Thermochemical Energy Conversion Laboratory, Umeå University, SE-901 87 Umeå, Sweden ABSTRACT: In strive to lower the energy conversion cost and CO2 net emission, more complex biofuels are used. The combustion of these fuels often creates aggressive and problematic fireside environments in boilers, resulting in reduced availability, which, in turn, may lead to increased usage of fossil fuel in backup boilers. The objective of the present work was to contribute to the efforts of maximizing the availability of a 50 MWth circulating fluidized-bed (CFB) boiler firing complex fuels with high amounts of P, Ca, S, Cl, N, K, and Na. In the present work, ash and deposit samples collected from the flue gas system of a CFB boiler were further analyzed with X-ray powder diffraction, complementing earlier analysis made on the same sample set with scanning electron microscopy equipped with energy-dispersive spectrometry. Thermodynamic calculations were also made. The results clarify details about the ash speciation and transformation as well as effects on boiler operation. A suggestion of a control strategy to minimize corrosion rates in superheaters and SO2 emission to downstream cleaning equipment in full-scale industrial boilers is made. An equation for rough estimation of fuel mix corrosion tendencies is also presented.

1. INTRODUCTION In strive to lower the energy conversion cost and CO2 net emission, more complex biofuels are used. The combustion of these fuels often creates aggressive and problematic fireside environments in boilers, resulting in reduced availability, which, in turn, may lead to increased usage of fossil fuels in backup boilers. The objective of the present work was to contribute to the efforts of maximizing the availability of a 50 MWth circulating fluidized-bed (CFB) boiler located in Perstorp, Sweden, firing complex fuel with high amounts of P, Ca, S, Cl, N, K, and Na and, thus, minimizing the use of fossil fuels in backup boilers. The present paper is the second of two and continues the work (10.1021/ef4004522) to elucidate the boiler effects and ash transformation of the fuel mix. The work includes a thorough analysis of the flue gas system as a whole, emanating from boiler failure and preventive maintenance statistics, which identify the primary areas of interest of the CFB and an analysis of the elemental composition and mass flow of fuels, flue gas, deposits, and ash (10.1021/ ef4004522). The previous paper (10.1021/ef4004522) also includes details of fuel, ash, and deposit sampling and sample preparation. Previously, extensive studies have been made on local mechanisms or limited chains of mechanisms in fluidized-bed boilers. However, there are yet only limited attempts to develop more comprehensive models involving complex fuel combustion studies for large-scale combustion of fuels rich in alkali, Cl, and to some extent P in CFB boilers.1−3 In the present work, ash and deposit samples collected in the flue gas system of the CFB (14 fractions) were further analyzed with X-ray powder diffraction (XRD), complementing earlier analysis made with scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM−EDS), to identify and quantify crystalline © XXXX American Chemical Society

phases. Taking also considerations from thermochemical calculations into account, the results clarify details about the ash speciation and transformation as well as effects on boiler operation and contribute to the existing platform of knowledge, aiming at the creation of a relevant (chemical and physical) model for safe operation of the studied CFB combustion process. From the results, an equation for rough evaluation of fuel mix corrosion and deposit tendencies is suggested. In addition, a suggestion of a control strategy to minimize corrosion rates and SO2 emission to downstream cleaning equipment in full-scale industrial boilers is made.

2. MATERIALS AND METHODS 2.1. Boiler Description. The boiler consists of an 18.5 m high furnace with a rectangular base 3 × 6.5 m. Combustion air is forced through primary air nozzles (PANs) at the very bottom of the furnace (see Figure 1). Just above the PANs, recirculated flue gas is forced through nozzles in the furnace walls. At 2.5 m above the PANs, secondary air is added through secondary air nozzles (SANs) positioned in the boiler walls and the conditions in the furnace volume above become globally oxidizing. Between the two stages where air is injected, fuel, bed material, and additives are added and recycled bed material reenters the furnace. In the top of the furnace, flue gas exits through two rectangular ports and enters two parallel cyclones. The mean crosssectional flue gas velocity varies between 3.8 and 6.5 m/s in the furnace, resulting in a furnace residential time between 2.5 and 4.2 s, calculated from the secondary air nozzles to the furnace exit. The mean velocity of the flue gas is 4.9 m/s. The temperature in the lower part of the bed Special Issue: Impacts of Fuel Quality on Power Production and the Environment Received: March 27, 2013 Revised: July 9, 2013

A

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varies mainly within the interval of 840−870°C, and the temperature in the top of the furnace varies mainly between 870 and 920°C. Vaporized ammonia is injected into the flue gas in the furnace exit for reduction of nitrogen oxides (NOx). The flue gas passes through the cyclone exits (CEs), together with particles 0 (°C)

0.5Na2SO4 + 1.5K2SO4 → K3Na(SO4)2 2Na2SO4 + 3KCl → K3Na(SO4)2 + 3NaCl 2K2SO4 + NaCl → K3Na(SO4)2 + KCl

−4652 −1553 −5685

930 540 1100

paths are inhibited. One can see that, of the three alternatives, the reaction from just sulfates or K2SO4 and NaCl is the most likely to occur from a thermodynamical point of view. The Gibbs energy of the reactions also implies that deposit buildup and formation of aphtitalite can occur not only by initial condensation of alkali chlorides, followed by separate sulfation and a secondary reaction between sulfates, but also directly through chemical reaction between alkali chlorides in gaseous or solid phase and sulfates, giving that the temperature of the deposit surface is low enough (below 470°C, see Figure 4). The inconsistency of the amount of aphtitalite in the SSH sample compared to the samples from the proximity of the TSH and SSH tube protection (Table 3) might be a result of a major disturbance in the flue gas flow pattern because of the extreme deposit formation in the TSH directly upstream of the sample position in the SSH. Na2SO4, KNa(SO4)2, and K2SO4 were found in the PAN deposit. NaCl was found in low concentrations, and KCl was found at even lower concentrations. A rough estimation gives at hand that the surface temperature of an undeposited metal nozzle lies between 300 and 550°C. The deposition on these nozzles does not have any direct effect of the boiler availability, but might be used as a lead to identify various mechanisms and reactions in the boiler. The low presence of aphtitalite in FA and bottom ash and high presence on cooled surfaces makes it likely that it forms on the surfaces. According to equilibrium calculations, NH4Cl (salmiac) should not exist at temperatures as high as 140°C but should be stable only at temperatures as low as 120°C and below. However, salmiak is found both in the flue gas downstream of the baghouse filter, on duct walls close to thermal bridges and flue gas stagnation points, and in the FA, indicating flaws in the thermodynamic data or the presence of some unknown intermediate that transforms to salmiak when the temperature is lowered. 3.1.4. Oxides. The iron oxides possibly act as a catalyst increasing the level of SO3 in the flue gas,23 increasing the rate of sulfation. Approximately 70% of the fuel Fe originates from peat. 3.1.5. Carbonates. Calcium carbonate found in low-temperature accumulated ash and FA is the only carbonate found in the process. According to equilibrium calculations, it cannot exist during the prevailing conditions in the furnace or upper part of the draft. Either the carbonate is formed in the low-temperature region of the process, where the temperatures are about 200°C and below, or is a result of limestone particle disintegration, resulting in fine particles that are transported from the hightemperature regions before equilibrium can be reached. The existence of CaCO3 in FA (CO32− having in this context a relatively weak affinity to the ash-forming metals) shows that not all Ca interacts with P, S, or Si in the high-temperature part of the process. CaCO3 found in the FA fraction equals approximately 4% of Ca added to the process. 3.2. Limestone Test. The evaluation was made during a period when the HCl concentration was higher at the same time G

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Figure 6. HCl, SO2, and limestone dosage levels during the limestone test, with data recorded every 3 min.

Table 7. Average Values before, during, and after an Increased Feeding Rate of Limestone, Together with the Levels of HCl and SO2 in the Flue Gas Compared to the Period before the Increased Dosage (Columns Diff HCl and Diff SO2), in dg Values limestone (kg/h)

period (h)

HCl (mg/Nm3 at 11% O2 dg)

diff HCl (%)

SO2 (mg/Nm3 at 11% O2 dg)

diff SO2 (%)

36 72 36 after 72

2 2 1.5

430 380 405

100 88 94

99 71 80

100 72 81

Table 8. Occurrence of Non-metals at an Average Combustion Temperature of 890°C Based on XRD Quantification (Table 3) occurence 1 2 3 4 a

Si P S Cl

mol %a

1

2

products

23 14 5 4

Ca and Al Ca Ca and K Na and K

Mg, K, and Na Mg, Fe, and K Na

Ca2SiO4, MgCaSi2O6, Ca2Al(AlSiO7), Ca3Mg(SiO4)2, KAlSiO4, KAlSi3O8, and NaAlSi3O8 Ca5(PO4)3OH and Ca19(Mg1.5Fe0.5)(PO4)14b CaSO4, K2Ca2(SO4)3, KNaSO4, K2SO4, and Na2SO4 NaCl and KCl

The mol % of the ash-forming elements from fuel and additives (excluding N). bSeveral modifications are possibly containing Mg, Fe, and/or K.

metals bound to sulfates, enabling more alkali chlorides to form. An effect of the latter was observed in terms of reduced levels of HCl. The fact that limestone affects the alkali metal availability to sulfur is noteworthy, because according to equilibrium calculations, both Na and K should have a stronger affinity to sulfate than Ca. However, the reaction might be possible as a result of the abundance of Ca present as refractory burnt lime, which may give it some advantage over the available but more volatile K or Na species. The sulfur capture with limestone is also affected by the periodically oxidizing−reducing environment in the bed.24,25 The amount of Ca found as CaCO3 in the FA fraction likely increases with an increased use of limestone. A closer look at the emission levels during the evaluation period shows a clear covariation of SO2 and HCl not connected to the altered level of limestone. This is interesting because the fuel containing the major part of the fuel mix sulfur (peat) is not the same as the fuels containing the major part of chlorine (residue wood, animal waste, and wastewater treatment sludge), making it unlikely that the covariation is due only to a simultaneous increase/decrease of sulfur and chlorine in the fuels. A number of chemical interactions affecting the SO2 and HCl levels in a way similar to the way described in the example with limestone is more likely.

that the SO2 concentration was somewhat lower compared to average levels in the flue gas. However, for biomass boilers, the flue gas composition ratio HCl/SO2 > 4 is not unusual.22 With regard to the fuel and additive elemental contribution to the process of the total amount of ash-forming elements, Ca from limestone makes only 2 mol % and the molar contributions to ash-forming elements from fuels are 27 mol % Ca, 3 mol % K, 7 mol % Na, 5 mol % S, and 4 mol % Cl. The levels of these constituents should be kept in mind when the effect of limestone is discussed. As the limestone dosage increases, it starts to accumulate in the bed and furnace, increasing the amount of available Ca. The bed and furnace seem to reach a new steady state at 50−60 min (Figure 6) after the increased dosage was initiated. Table 7 presents HCl and SO2 levels before, during, and after the increased dosage of limestone. Both absolute and relative values are presented, showing a clear connection between the rate of added limestone and both SO2 and HCl levels. As the amount of Ca increases, more sulfur is captured through the formation of Ca2K2(SO4)3 in the furnace and bed and probably to some extent CaSO4 found in the FA, decreasing the amount of SO2 and SO3 in the flue gas and, thus, the sulfation potential. A decrease of the sulfation potential in the flue gas decreases the amount of alkali H

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SA = S − 0.96Ca − 0.5K − 0.5Na + 1.6P + 0.4Si

3.3. Evaluation of Fuel Mix Corrosion Tendency. The information in Table 3 can be organized by the occurrence of non-metals and corresponding metals (see Table 8). All metals occurring together with the non-metal independent of other metals to a significant degree are given a place in column 1. Metals occurring in small amounts or only as associated with other metals are placed in column 2. Si dominates the non-metals, placing Si on row 1 of Table 8. Si always occurs together with either Ca and/or Al, placing them in column 1. Some of the silicates also include K, Na, Fe, and Mg but not independent of Ca and Al, rendering them a place in column 2. After Si, P is the most abundant non-metal, always occurring together with Ca, and in the case of the whitlockite, also together with Mg, Fe, and/or K, placing Ca in column 1 and Mg, Fe, and K in column 2. S occurs in the bulk fractions (bottom ash and FA) primarily as CaSO4, K2Ca2(SO4)3, and KNaSO4, placing Ca and K in column 1. The limited occurrence of Na together with K in KNaSO4 in the FA and deposits and Na2SO4 in deposits places Na in column 2 (because of the small presence of sodium sulfate). Cl occurs mainly as NaCl, which may be a result of the formation of K2Ca2(SO4)3 decreasing the amount of K available for reaction to KCl, but because KCl also occurs in the bulk fraction FA, both Na and K are given a place in column 1. This organization presented in Table 8, together with the phase quantification data presented in Table 3, can be used to roughly formulate the overall fuel ash reaction stoichiometry, including the effects from all major constituents. Historically, the fuel composition ratio S/Cl has been used and misused to evaluate the corrosion and deposit potential of a fuel mix. Salmenoja stated that it is not the content of fuel sulfur that is important but the concentration of available sulfur in the flue gas.22 This is well in line with other findings, all verifying that alkali chlorides reaching boiler tubes frequently are at the heart of deposit buildup and boiler tube corrosion,8,10,14,26 and several others also stress the role of sulfation to reduce the negative effect of alkali chlorides12,14,15,19. One can say that the sulfur content of the fuel mix must be high enough to create a sulfation potential in the furnace and draft that counterbalances the chloridization potential of the flue gas and that sulfur must be present in such excess that SO2/SO3 can rapidly sulfurize the alkali chlorides to some extent that they will always reach the superheater boiler tubes. With this said, considering the XRD data set (Tables 3 and 8), Ca, K, Na, P, and Si contents of the fuel mix all influence the availability of sulfur in flue gas directly or indirectly. Via the stoichiometry of the quantified phases and the mass contribution of each phase, one can formulate an expression roughly describing the availability of sulfur in flue gas SA (eq 1). The equation describes that Ca, K, and Na capture S via the formation of sulfates, as long as P and Si do not make these metals unavailable to sulfur capture via the formation of phosphates or silicates (see Table 8), with the net of the formula being the “excess” sulfur available for sulfation. There is no need to include Al, Fe, and Mg in the formula, because these elements never occur without the metals included in the formula (Table 8) and, therefore, do not have any direct impact on the stoichiometry ruling the availability of sulfur. Cl is not included because of the weaker affinity between metals and Cl compared to metals and S. The equation assumes that enough Ca is present to bind P to apatite and whitlockite and that the amount of K in whitlockite is negligible. With a lack of Ca in the fuel mix, P will instead react with K, Na, and Si, creating an altogether different scenario27,28 for which eq 1 does not apply.

(1)

Inserting the elemental composition of the fuel mix into eq 1, one realizes that the SO2 + SO3 concentration should be 132 mg/ Nm3 at 3.5% O2 dg, while the measured average level of the system of SO2 was in fact 182 mg/Nm3 at 3.5% O2 dg. This estimate has a surprisingly high precision if one regards the low amount of sulfur in the process compared to the other elements. However, it deviates with 50 mg/Nm3 (assuming that the amount of SO3 ≪ SO2). The strong ability of Si as a glass former, especially together with K and Na, makes it likely that Si captures more alkali than indicated with XRD. The Si factor in eq 1 can be slightly adjusted from 0.4 to 0.42 to compensate for an assumed higher amorphous fraction, resulting in eq 2. The fuel elemental composition inserted in this modified equation gives a concentration of SO2 + SO3 in the flue gas of 174 mg/Nm3 at 11% O2 dg. This formula of course needs further evaluation but might be one step in the direction of creating an easy to use method for rough estimation of complex fuel mixes. The complex role of Si in the ash transformation process represents the most challenging part when it comes to formulating an accurate and general equation estimating the amount of available sulfur in a wide range of biomass fuel mixes and various additives. If, for example, large amounts of kaolin [Al2Si2O5(OH)4] are added, kaolin will react with alkali metals, resulting in the formation of NaAlSiO4 and KAlSiO4. Consequently, the factor 0.42Si in eq 2 should be increased toward 0.5Si. On the other hand, if the fuel is contaminated with a lot of relatively inert SiO2, the same factor needs to be decreased. Also, the factor of Ca, 0.96, will need to be lowered if the limestone to the furnace is added in excess or in a more unavailable form than in the Perstorp CFB boiler (see 10.1021/ef4004522), making it difficult for all Ca added with the limestone to react. Because the equations are based on “real boiler findings”, the stoichiometric factors in the equations are also affected by the temperature and availability issues as well as different kinetic aspects of the process. If a different boiler design or significantly different temperature level is used, the formula would need to be adjusted. However, we think the tendency of the chemical sulfur availability as we have found in the boiler studied and summarized in eq 2 can be useful as a guideline also in other fluidized-bed multi-fuel boilers. SA = S − 0.96Ca − 0.5K − 0.5Na + 1.6P + 0.42Si

(2)

When the result from eq 2 is divided by the fuel amount of Cl (which is volatile enough to without exception occur in the gas phase), one obtains an adapted molar gas-phase availability ratio “S/Cl”, which can be used to roughly evaluate the corrosion tendency of new complex fuel mixes. If one calculates the sulfur availability according to eq 2 for the relatively problem-free operation with 36 kg/h of limestone added, the adapted gasphase availability ratio (S/Cl) equals around 0.6. The same ratio calculated for the problematic increased rate of limestone of 72 kg/h equals 0.01. Pyykönen and Jokiniemi conclude that sulfation occurs mainly close to the deposit surface in solid deposits.17 Skrifvars et al. confirm that already low concentrations of chloride near the tube surface in the deposits can result in major corrosion of the tubes.29 Kassman et al. evaluated co-combustion of K- and Clrich biomass with peat in a CFB boiler and concluded that the peat did not have any major effect on the amount of KCl in the flue gas but greatly reduced the concentration of Cl in the deposit.30 From these observations together with the observations made by Piotrowska et al.1 and observations from the I

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rapeseed cake in circulating fluidized bed boiler Part 1: Cocombustion with wood. Energy Fuels 2010, 24, 333−345. (2) Skrifvars, B. J.; Backman, R.; Hupa, M.; Sfiris, G.; Abyhammar, T.; Lyngfelt, A. Ash behaviour in a CFB boiler during combustion of coal, peat or wood. Fuel 1998, 77, 65−70. (3) Davidsson, K. O.; Åmand, L. E.; Steenari, B. M.; Elled, A. L.; Eskilsson, D.; Leckner, B. Countermeasures against alkali-related problems during combustion of biomass in a circulating fluidized bed boiler. Chem. Eng. Sci. 2008, 63, 5314−5329. (4) The International Centre for Diffraction Data (ICDD). The Powder Diffraction File, PDF-2; ICDD: Newtown Square, PA, 2004. (5) Institute of Standards and Technology. Inorganic Crystal Structure Database (ICSD); Institute of Standards and Technology: Karlsruche, Germany, 1978. (6) Bale, C. W.; Bélisle, E.; Chartrand, P.; Decterov, S. A.; Eriksson, G.; Hack, K.; Jung, I.-H.; Kang, Y.-B.; Melançon, J.; Pelton, A. D.; Robelin, C.; Petersen, S. FactSage thermochemical software and databases Recent developments . CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2009, 33, 295−311. (7) Bale, C. W.; Chartrand, P.; Degterov, S. A.; Eriksson, G.; Hack, K.; Ben Mahfoud, R.; Melancon, J.; Pelton, A. D.; Petersen, S. FactSage thermochemical software and databases. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2002, 26, 189−228. (8) Grabke, H. J. Fundamental Mechanisms of the Attack of Chlorine, HCl and Chlorides on Steels and High Temperature Aloys in the Temperature Range 400°C to 900°C; Hemisphere Publishing Corporation: Washington, D.C., 1991; pp 161−176. (9) Lai, G. Y. High-temperaure corrosionIssues in alloy selection. JOM 1991, 43, 54−60. (10) Warnqvist, B.; Norrstrom, H. Chlorides in recovery boiler and a mechanism for chloride removal. Tappi 1976, 59, 89−91. (11) Etok, S. E.; Valsami-Jones, E.; Wess, T. J.; Hiller, J. C.; Maxwell, C. A.; Rogers, K. D.; Manning, D. A. C.; White, M. L.; Lopez-Capel, E.; Collins, M. J.; Buckley, M.; Penkman, K. E. H.; Woodgate, S. L. Structural and chemical changes of thermally treated bone apatite. J. Mater. Sci. 2007, 42, 9807−9816. (12) Salmenoja, K.; Makela, K.; Hupa, M.; Backman, R. Superheater corrosion in environments containing potassium and chlorine. J. Inst. Energy 1996, 69, 155−162. (13) Skrifvars, B. J.; Lauren, T.; Backman, R.; Hupa, M. The role of alkali sulfates and chlorides in post cyclone deposits from circulating fluidized bed boilers firing biomass and coal . Impact Miner. Impurities Solid Fuel Combust., [Proc. Eng. Found. Conf. Miner. Matter Fuels] 1997, 525−539. (14) Iisa, K.; Lu, Y.; Salmenoja, K. Sulfation of potassium chloride at combustion conditions. Energy Fuels 1999, 13, 1184−1190. (15) Glarborg, P.; Marshall, P. Mechanism and modeling of the formation of gaseous alkali sulfates. Combust. Flame 2005, 141, 22−39. (16) Spiegel, M.; Zahs, A.; Grabke, H. J. Fundamental aspects of chlorine induced corrosion in power plants. Mater. High Temp. 2003, 20, 153−159. (17) Pyykönen, J.; Jokiniemi, J. Modelling alkali chloride superheater deposition, and its implications. Fuel Process. Technol. 2003, 80, 225− 262. (18) Kassman, H.; Brostrom, M.; Berg, M.; Amand, L. E. Measures to reduce chlorine in deposits: Application in a large-scale circulating fluidised bed boiler firing biomass. Fuel 2011, 90, 1325−1334. (19) Broström, M.; Kassman, H.; Helgesson, A.; Berg, M.; Andersson, C.; Backman, R.; Nordin, A. Sulfation of corrosive alkali chlorides by ammonium sulfate in a biomass fired CFB boiler. Fuel Process. Technol. 2007, 88, 1171−1177. (20) Zbogar, A.; Frandsen, F. J.; Jensen, P. A.; Glarborg, P. Heat transfer in ash deposits: A modelling tool-box. Prog. Energy Combust. Sci. 2005, 31, 371−421. (21) Broström, M. Aspects of alkali chloride chemistry on deposit formation and high temperature corrosion in biomass and waste fired boilers. Academic Dissertation, Energy Technology and Thermal Process Chemistry, Umeå University, Umeå, Sweden, 2010.

Perstorp CFB boiler (this work), the conclusion is made that it is of outermost importance that the sulfation potential is kept sufficiently high, at all times, and that an increase in available sulfur through co-combustion with peat is enough to reduce the corrosion potential of the fuel mix but not necessarily to have any greater impact on the rate of deposit buildup. To reach the requirement of a constantly sufficient high sulfation potential during the combustion of heterogeneous fuels, online measurement of HCl and SO2 is required to control the furnace chemistry. Hence, the co-combustion of peat together with the addition of limestone can be used as cost-effective means in fullscale biomass boilers to control the sulfation potential and obtain an optimal level regarding high-temperature corrosion and SO2 emission from the furnace with respect to the chloridization potential of the system.

4. CONCLUSION The predominant phases in the various ash fractions were as follows: (1) bottom ash, 48 mass % SiO2, 16 mass % Ca5(PO4)3OH, and 13 mass % Na(AlSi3)O8; (2) FA, 39 mass % Ca5(PO4)3OH, 8 mass % SiO2, 8 mass % Ca19(Mg1.5Fe0.44)(PO4)14, 7 mass % Na(AlSi3)O8, and 6 mass % K(AlSi3)O8; and (3) superheater deposits (TSH, SSH, and SSH protection shield), 35 mass % Ca5(PO4)3OH, 29 mass % KNaSO4, 7 mass % Ca2SiO4, and 6 mass % Na2SO4. The formation of KNaSO4 (aphtitalite/glaserite) occurs on cooled surfaces, during either operation or shutdown. During operation, it can probably form via reactions between NaCl and K2SO4 or between KCl and Na2SO4 not only by phase transformation from sulfates. On the basis of online measurement of SO2 and HCl in the flue gas, the combustion of peat together with the addition of limestone can be used as cost-effective means in full-scale biomass boilers to control the sulfation potential and obtain an optimal level regarding high-temperature corrosion and SO2 emission from the furnace with respect to the chloridization potential of the system. When the molar elemental composition is inserted in equation SA = S − 0.96Ca − 0.5K − 0.5Na + 1.6P + 0.42Si, the sulfation potential of a fuel mix can be roughly estimated. Together with the chloridization potential of the fuel mix, the corrosion tendency of complex fuel mixes containing high concentrations of P, Ca, K, Na, Al, Si, S, Cl, Mg, and Fe can be evaluated.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support given by Perstorp Specialty Chemicals AB and the National (Swedish) Strategic Research Program Bio4Energy is gratefully acknowledged. The cooperation and support given by friends and colleagues at Thermochemical Energy Conversion Laboratory, Umeå University, and Perstorp Specialty Chemicals AB are deeply appreciated.

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REFERENCES

(1) Piotrowska, P.; Zevenhoven, M.; Davidsson, K.; Hupa, M.; Åmand, L. E.; Barisic, V.; Zabetta, E. C. Fate of alkali metals and phosphorus of J

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(22) Salmenoja, K. Field and laboratory studies on chlorine-induced super heater corrosion in boilers fired with biofuel. Ph.D. Thesis, Åbo Akademi, Turku, Finland, 1999; Report 00-6. (23) Zevenhoven, R.; Kilpinen, P. Control of Pollutants in Flue Gases and Fuel Gases; Helsinki University of Technology: Espoo, Finland, 2001. (24) Hansen, P. F. B.; Dam-Johansen, K.; Ostergaard, K. Hightemperature reaction between sulfur-dioxide and limestoneThe effect of periodically changing oxidizing and reducing conditions. Chem. Eng. Sci. 1993, 48, 1325−1341. (25) Mattisson, T.; Lyngfelt., A. The reaction between limestone and SO2 under periodically changing oxidizing and reducing conditions Effect of temperature and limestone type. Thermochim. Acta 1999, 325, 59−67. (26) Spiegel, M. Corrosion in Advanced Power Plants: Proceedings of the Second International Workshop on Corrosion in Advanced Power Plants; Tampa, FL, March 3−5, 1997; pp 221−226. (27) Piotrowska, P.; Grimm, A.; Skoglund, N.; Boman, C.; Ohman, M.; Zevenhoven, M.; Bostrom, D.; Hupa., M. Fluidized-bed combustion of mixtures of rapeseed cake and bark: The resulting bed agglomeration characteristics. Energy Fuels 2012, 26, 2028−2037. (28) Bostrom, D.; Skoglund, N.; Grimm, A.; Boman, C.; Ohman, M.; Brostrom, M.; Backman., R. Ash transformation chemistry during combustion of biomass. Energy Fuels 2012, 26, 85−93. (29) Skrifvars, B. J.; Backman, R.; Hupa, M.; Salmenoja, K.; Vakkilainen, E. Corrosion of superheater steel materials under alkali salt depositsPart 1: The effect of salt deposit composition and temperature. Corros. Sci. 2008, 50, 1274−1282. (30) Kassman, H.; Pettersson, J.; Steenari, B.-M.; Åmand, L.-E. Two strategies to reduce gaseous KCl and chlorine in deposits during biomass combustionInjection of ammonium sulphate and co-combustion with peat. Fuel Process. Technol. 2013, 105, 170−180.

K

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