Energy Fuels 2010, 24, 1559–1564 Published on Web 12/30/2009
: DOI:10.1021/ef901188v
Consequences of Unwanted Local Reducing Conditions in Biomass-Fired Boilers on Chemistry and Operation: A Thermodynamic Evaluation Micha€el Becidan* and Lars Sørum SINTEF Energy Research, Energy Processes, NO-7465 Trondheim, Norway Received October 19, 2009. Revised Manuscript Received December 4, 2009
The case studies investigated are focusing on uncontrolled local reducing conditions and their effects on alkali and trace metal chemistry in biomass-fired boilers. The chemical trends obtained by thermodynamic equilibrium were interpreted in terms of practical outcomes for thermal systems. There is a lack of knowledge concerning the fate of Na and K in reducing atmospheres. This study shows the following: (1) Corrosion is expected to be more severe at reducing than oxidizing conditions; however, the corrosive species and therewith the corrosion mechanisms are changing with the oxygen levels. The main corrosive species under pyrolytic conditions are Na2S (s), NaCl (g), and KCl (g), while the main corrosive species at mildly reducing conditions are H2S (g), NaCl (g), and KCl (g). Under oxidizing conditions, alkali chlorides are the main alkali species involved in corrosion. (2) Some chemical elements have stable chemistries; i.e., their speciation and/or phase distribution are little affected by oxygen levels, while (3) other elements are very sensible to changing oxygen availability.
This theoretical study focuses on the fate of the major players in corrosion, i.e., Na, K, S, Cl, Pb, and Zn at reducing conditions alongside a comparison to chemistry at oxidizing conditions using thermodynamic equilibrium. Little attention has been given to alkalis and reducing conditions in thermal systems, except for a few theoretical results on straw combustion,7,8 but no experimental data were found in the literature. Trace metals have been studied to a larger extent4,9-13 mainly because of environmental issues or to discuss new air distribution methods to affect the distribution of heavy metals.6,14 During combustion, an array of transformations is taking place involving ash compounds,15 and reducing conditions may affect these processes, especially speciation and phase distribution. This investigation was therefore endeavored as a first stage to gain better knowledge of the chemical compounds involved in corrosion when uncontrolled reducing conditions take place compared to normal oxidizing conditions. The experimental conditions (operating conditions and fuel properties) are based on two existing Vattenfall Nordic Heat (VNH) biomass-fired boilers as they are encountering
1. Introduction The consequences of local reducing conditions in furnaces have seldom been addressed because these conditions are often considered to be very limited in space and time. However, extensive experimental investigations1-3 have shown that fluctuations between oxidizing and reducing conditions occur in industrial combustion systems and that they can actually be of considerable magnitude and duration, leading to higher than expected in-bed and/or superheater corrosion issues. The main factors that could cause local reducing conditions in thermal systems include:1 fuel properties, especially high volatility influencing therewith fuel/air mixing, fluctuations in fuel properties, particularly with heterogeneous fuels, such as municipal solid waste (MSW) causing unstable operation, and imperfections in the air-feed or fuelfeed system. Other issues associated with reducing conditions (not all specific to reducing conditions) include:1,2 fouling,2 slagging, agglomeration of bed material, and defluidization (for fluidized beds),4 ash sintering (heat exchangers and grate),5 ash properties,6 pressure fluctuations,1 and burn-up issues.1
(7) Nielsen, H. P. Deposition and high-temperature corrosion in biomass-fired boilers. Ph.D. Thesis, Department of Chemical Engineering, Technical University of Denmark, Lyngby, Denmark, 1998. (8) Knudsen, J. N.; Jensen, P. A.; Lin, W.; Frandsen, F. J.; Dam-Johansen, K. Energy Fuels 2004, 18, 810–819. (9) Sørum, L.; Frandsen, F. J.; Hustad, J. E. Fuel 2003, 82, 2273–2283. € (10) Boman, C.; Ohman, M.; Nordin, A. Energy Fuels 2006, 20, 993– 1000. ˚ (11) Elled, A.-L.; Amand, L.-E.; Eskilsson, D. Energy Fuels 2008, 22, 1519–1526. (12) Abanades, S.; Flamant, G.; Gagnepain, B.; Gauthier, D. Waste Manage. Res. 2002, 20, 55–68. (13) Van Lith, S. Release of inorganic elements during wood-firing on a grate. Ph.D. Thesis, CHEC Research Centre, Department of Chemical Engineering, Technical University of Denmark, Lyngby, Denmark, 2005. (14) European Commission. Reference document on the best available techniques for waste incineration (BREF WI), 2006. (15) Obernberger, I. Ash related problems in biomass combustion plants. Inaugural Lecture, Technische Universiteit Eindhoven, Eindhoven, The Netherlands, May 20, 2005.
*To whom correspondence should be addressed. Telephone: (þ47) 7359-29-11. Fax: (þ47) 73-59-28-89. E-mail: michael.becidansintef.no. (1) Johansson, A.; Johnsson, F.; Niklasson, F.; A˚mand, L.-E. Chem. Eng. Sci. 2007, 62, 550–560. (2) Uusitalo, M. A.; Vuoristo, P. M. J.; M€antyl€a, T. A. Surf. Coat. Technol. 2002, 161, 275–285. (3) Niklasson, F.; Johnsson, F.; Leckner, B. Chem. Eng. J. 2003, 96, 145–155. (4) Elled, A.-L.; A˚mand, L.-E.; Leckner, B.; Andersson, B.-A˚. Fuel 2007, 86, 843–852. (5) Fern andez Llorente, M. J.; Escalada Cuadrado, R.; Murillo Laplaza, J. M.; Carrasco Garcı´ a, J. E. Fuel 2006, 85, 2081–2092. (6) Dahl, J.; Obernberger, I.; Brunner, T.; Biedermann, F. Results and evaluation of a new heavy metal fractionation technology in gratefired biomass combustion plants as a basis for an improved ash utilisation. Proceedings of the 12th European Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection, Amsterdam, The Netherlands, June 17-21, 2002. r 2009 American Chemical Society
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Energy Fuels 2010, 24, 1559–1564
: DOI:10.1021/ef901188v
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Table 1. Main Features of the Installations location technology fuel bed/furnace temperature (°C) energy production (MW) problems encountered
case 1
case 2
Nyk€ oping, Sweden bubbling fluidized bed demolition waste wood ∼850 90 furnace wall and superheaters corrosion slagging and fouling
Uppsala, Sweden wall-burner furnace peat and sawdust 1000-1300 400 furnace corrosion slagging
Table 2. Fuel Composition (1 kg Dry)
corrosion and slagging issues, which are, at least partly, due to unwanted local reducing conditions because of operation at low air/fuel ratios (λ). Special attention is also given to the consequences of reducing conditions on two trace metals: Cr and As. These trace metals were selected because they are found at relatively high concentrations in the studied fuels. They are also found in the “European Dirty Dozen” (EDD) metals list and are highly toxic at the valences CrVI and AsIII. To optimize plant operation and develop combustion models, it is important to understand the fluid dynamics and combustion process. A first beneficial step to better grasp the latter is the use of thermodynamic equilibrium together with clever methodologies and careful interpretation. Thermodynamic equilibrium calculations have widely proven their relevance in a variety of thermal treatment applications.9,16-19 Despite known limitations,16 they provide valuable information on chemical trends. Such calculations are a powerful tool and, at present, represent the only computational possibility for investigating the elemental chemistry in a multicomponent and multi-phase complex thermal system. Furthermore, it is difficult to perform experiments at reducing conditions in real systems, further increasing the relevance of thermodynamic equilibrium calculations. The approach chosen here is to use thermodynamic equilibrium to conceptualize “local conditions equilibrium”, i.e., to investigate the consequences of local and uncontrolled reducing conditions around specific chemical elements in biomass furnaces. This study is an important theoretical step in assessing the thermodynamically stable compounds and chemical trends at varying oxygen levels (both reducing and oxidizing conditions) with focus on corrosion main players during biomass combustion.
case 1
case 2
demolition wood
peat and sawdust
element
input (g)
element
input (g)
C O H N Pb Zn S Cl Na K Cr As
493 395 60 5.9 0.365 2.810 1.1 2.6 3.900 3.275 0.058 0.034
C O H N Pb Zn S Cl Na2O K2O Cr As
537 333.4 56.6 17.2 0.04041 1.7214 2.3 0.1 5.5 1.48 0.08203 0.02555
with a furnace temperature of 1000-1300 °C. The steam data are similar to those of Nyk€ oping. The fuel used is peat, typically 70-90 wt %, with the rest being sawdust. Both installations are running at low air/fuel ratios, resulting in O2 concentrations in the flue gas of 4 and 1.5% (wet basis), respectively. Both installations have been confronted with several challenges (see Table 1). Comprehensive experimental investigations strongly indicate that these issues are, at least partly, due to local reducing conditions. Table 1 summarizes the main features of the investigated installations. 2.2. Calculations and Input. The thermodynamic calculations are carried out with the FactSage 5.5 software package20 and a custom-made database based on the Scientific Group Thermodata Europe (SGTE) database for pure substances (see the Supporting Information). The selected phases are the gas phase and stoichiometric liquid and solid phases. No liquid or solid solutions were considered. Complex organic C-H-O species were not included because they are not stable at the investigated conditions. In addition, interactions between the trace elements or with the alkali metals were not considered. Interactions between different trace elements are very unlikely because of the low concentration. In addition, other ash-forming elements, such as Ca, Fe, Mg, and Si, were not considered. The calculations are performed at a single temperature representative of the incinerators: for the Uppsala plant (case 2), the temperature of the calculations is 1300 °C (furnace temperature), while for the Nyk€ oping fluidized bed (case 1), it is 850 °C (bed temperature). The compositions of the fuels used in the calculations are presented in Table 2. The values used for case 2 are typical values. On the other hand, the values selected for the major corrosion players (Na, K, Pb, Zn, S, and Cl) for case 1 are maximum values to portray extreme scenarios. The bed material (case 1) is considered inert. Three oxygen levels are used in the calculations: two to illustrate reducing conditions and one to illustrate oxidizing conditions because thermodynamic calculations are optimally used for trends determination rather than to obtain quantitative evaluations. The two reducing environments are fixed at
2. Methodology 2.1. Case Studies. Two VNH full-scale installations were used as the basis for the calculations input concerning operating conditions as well as fuel elemental composition. The first installation (referred to as case 1) is a 90 MW bubbling fluidized bed located in Nyk€ oping, Sweden, with a 65 ton sand bed kept at 850 °C. Steam is produced at 535 °C and 140 bar. The fuel is waste wood chips. The second installation (referred to as case 2) is a 400 MW wall-burner furnace, located in Uppsala, Sweden, (16) Becidan, M.; Sørum, L.; Frandsen, F.; Pedersen, A. J. Fuel 2009, 88, 595–604. (17) Frandsen, F. Trace elements from coal combustion. Ph.D. Thesis. Department of Chemical Engineering, Technical University of Denmark, Lyngby, Denmark, 1995. (18) Konttinen, J.; Backman, R.; Hupa, M.; Moilanen, A.; Kurkela, E. Trace element behavior in the fluidized bed gasification of solid recovered fuels;A thermodynamic study. Report 05-02, Process Chemistry Centre, Faculty of Chemical Engineering, A˚bo Akademi University, Turku, Finland, 2005. (19) Lindberg, D. Thermochemistry and melting properties of alkali salt mixtures in black liquor conversion processes. Ph.D. Thesis, Laboratory of Inorganic Chemistry, Process Chemistry Centre, Faculty of Technology, A˚bo Akademi University, Turku, Finland, 2007.
(20) Bale, C. W.; Belisle, E.; Chartrand, P.; Decterov, S. A.; Eriksson, G.; Hack, K.; Jung, I.-H.; Kang, Y.-B.; Melanc-on, J.; Pelton, A. D.; Robelin, C.; Petersen, S. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2009, 33, 295–311.
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: DOI:10.1021/ef901188v
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Figure 1. Case 1: Na speciation (mol % Na). Fuel = demolition waste wood. Nyk€ oping plant. Pyrolysis = no external O added; 0.5 and 1.3 refer to air/fuel ratios. Temperature = 850 °C.
Figure 2. Case 1: K speciation (mol % K). Fuel = demolition wood. Nyk€ oping plant. Pyrolysis = no external O added; 0.5 and 1.3 refer to air/fuel ratios. Temperature = 850 °C.
distribution, K is much more volatile than Na at all O levels. K is solely found in the gas phase at reducing conditions but forms some condensed sulfates at oxidizing conditions. The secondary compounds at reducing conditions are KOH (g) (increasing with increasing O levels), K (g) (decreasing with increasing O level), KOH (g), and K2SO4 (s) at oxidizing conditions. The rather similar KCl (g) levels would suggest that KCl- (and thereby K-) induced corrosion is not particularly affected by oxygen concentrations. The combined alkali-induced corrosion risk (out of the furnace) may be evaluated by the sum of Na2S (s) þ NaCl (g) þ KCl (g). It is clear that the reactivities as well as the corrosion mechanisms in which these compounds are involved are different and changing, but it can be considered as a rough approach to corrosion trends. By doing so, it can be said that alkali-induced corrosion is at its most severe at pyrolytic conditions and at its mildest at mildly reducing conditions. In other words, it appears that alkali-induced corrosion is more important at oxidizing than mildly reducing conditions. However, to evaluate the overall corrosion risk, another compound should be considered, namely, H2S (g). S forms mainly H2S (g), a very corrosive compound, at mildly reducing conditions, while it forms sulfates at oxidizing conditions (less corrosive). Consequently, approximately the same amounts of corrosive species are produced at both reducing conditions, but they are different, implying, as mentioned, different reactivities and corrosion mechanisms; however, it is reasonable to conclude that reducing conditions lead to a more corrosive environment than oxidizing ones. The complexity and multiplicity of corrosion species and reactions show the difficulty of understanding the processes taking place in a multi-element system and the need for careful interpretation. No experimental or theoretical data discussing Na chemistry under reducing conditions during biomass combustion were found in the literature; therefore, this theoretical study provides important knowledge and is an important first step. A few thermodynamic calculations have been performed at reducing conditions7,8 for straw with K as the sole alkali, and a comparison to the present results is therefore difficult. Nevertheless, a difference observed between oxidizing and reducing conditions is common to the various works; potassium sulfate is not stable under reducing conditions. 3.2. Case 2 (Wall-Burner Furnace, Uppsala): Alkali (Na and K) Chemistry. In this case (Figure 3), Na (g) is the central Na species at reducing conditions. Contrary to the previous case, Na2S and NaCl are not present in noteworthy quantities, most likely because of low S and Cl concentrations. This “lack” of S and Cl is also clear at oxidizing conditions because
(1) pyrolysis, i.e., no external oxygen is added to the fuel, and (2) a λ (air/fuel ratio) of 0.5, to illustrate severely and moderately reducing conditions. The oxidizing environment is depicted by a λ of 1.3. The air/fuel ratio describes the amount of O necessary to convert the fuel C and fuel H to CO2 and H2O. A ratio of 1 therefore describes stoichiometric combustion. When calculating λ, fuel O is included. The total amount of added gas was kept constant by adding inert gas (Ar) at the reducing conditions. The two installations represent two different categories: Case 2 represents high-temperature, high S/Cl molar ratio plants with a fuel having a typical elemental composition, while case 1 illustrates relatively low-temperature, low S/Cl molar ratio installations with a fuel having high concentrations of corrosion players.
3. Results and Discussion Nomenclature. (g) gas, (l) liquid, (s) solid, and (c) condensed compound, i.e., (l) or (s). For case 1 and 2 characteristics, see Table 1. 3.1. Case 1 (BFB, Nyk€ oping): Alkali (Na and K) Chemistry. At oxidizing conditions, Na (Figure 1) is found as sulfate (s), carbonate (s), and chloride (g). As the O levels decrease, the average degree of oxidation of Na decreases with the disappearance (i.e., non-formation) of first sulfate and then carbonate. When it comes to phase distribution, Na is found in the condensed phase at about 90 mol % at mildly reducing and oxidizing conditions but only 40 mol % at pyrolytic conditions. The evolution of NaCl (g) (and with it NaCl-induced corrosion) is not linearly correlated to O levels; it is decreasing from about 11 mol % Na at oxidizing conditions to 6 mol % at mildly reducing conditions before increasing back to about 12 mol % under pyrolytic conditions. It implies that NaClinduced corrosion is similar and worst at a pyrolytic and oxidizing atmosphere than at mildly reducing conditions. Under pyrolytic conditions, another alkali corrosive compound is appearing, namely, Na2S (s). At reducing conditions, in real installations, metal surfaces are not protected by an oxide layer and the presence of solid sulfides may be disastrous in terms of corrosion (and slagging) in the furnace2 because they are produced as condensed compounds but also at other locations (heat-exchange surfaces especially) in the plant if entrained as solid particles. Concerning K (Figure 2), neither carbonates nor sulfides are formed. KCl (g) is the predominant potassium compound at reducing as well as oxidizing conditions, representing from about 2/3 to 3/4 of all K. When it comes to phase 1561
Energy Fuels 2010, 24, 1559–1564
: DOI:10.1021/ef901188v
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Figure 5. Case 2: Pb speciation (mol % Pb). Fuel = peat and sawdust. Uppsala plant. Pyrolysis = no external O added; 0.5 and 1.3 refer to air/fuel ratios. Temperature = 1300 °C.
Figure 3. Case 2: Na speciation (mol % Na). Fuel = peat and sawdust. Uppsala plant. Pyrolysis = no external O added; 0.5 and 1.3 refer to air/fuel ratios. Temperature = 1300 °C.
to metal surfaces, Pb and Zn chlorides are facilitators of corrosive reactions by forming mixtures with lowmelting points on metal surfaces. These mixtures, called eutectics, accelerate corrosion even when found at low amounts, because reactions are faster in even partly melt phases.21 In case 1, the speciation is clearly different at oxidizing and reducing conditions; while Pb (g) represents more than 80 mol % Pb at reducing conditions [and is complemeted by PbS (g)], PbO (g) is the sole Pb compound at oxidizing conditions. In case 2 (Figure 5), at reducing conditions, Pb (g) is also the most important lead compound and PbS (g) is present at significant levels. Similar to case 1, PbO (g) is predicted to be the main lead compound accompanied by some Pb (g) in oxidizing atmospheres. These results clearly show that reducing conditions could provoke sulfide-induced corrosion in combustion plants. Despite the rather different fuel chemical compositions (especially when it comes to minor and trace compounds) and operating conditions (temperature) in the two installations, the speciation as well as the phase distribution of Pb is little affected. The results obtained in this study are consistent with other studies in that Pb is solely found in the gas phase4,9-12 at the given conditions. However, other studies usually predict the formation of chlorides, especially at oxidizing conditions.9,12,16 This was not the case here because of, respectively, high levels of alkali in demolition wood (case 1) and a very low level of Cl in the peat and sawdust mixture (case 2), showing the importance of fuel properties in the overall chemistry. Contrary to the behavior of Pb, where only speciation is affected, the change in atmosphere conditions has a radical outcome on both phase distribution and speciation of Zn in case 1. Under reducing conditions, Zn is to be found solely as Zn (g), while under oxidizing conditions, the most thermodynamically stable Zn compound is ZnO (s). These results are in accordance with previous studies,12 which are including other chemical elements, such as Al and Si. Experimental results also confirm that ZnO (s) is a key compound for waste wood; alkali and trace metal vapors are thereafter expected to condense onto ZnO (s).15 The main consequence is that fly ash will be enriched in alkali and trace metals. The overall chemistry of Zn is strongly affected by changing atmospheres.
Figure 4. Case 2: K speciation (mol % K). Fuel = peat and sawdust. Uppsala plant. Pyrolysis = no external O added; 0.5 and 1.3 refer to air/fuel ratios. Temperature =1300 °C.
sulfates and chlorides are only minor compounds; therefore, Na is combining with compounds available in large quantities, i.e., O and H, to form hydroxide. An important outcome of this chemistry for real systems is that the severity of Na-induced corrosion does not appear to be significantly affected by varying oxygen levels, contrary to case 1. The results for K (Figure 4) show little potassium chloride (less than 1 mol % K) because little Cl is available. At reducing conditions, K (g) becomes the dominant species, while KOH (g) importance is growing to reach about 92 mol % at oxidizing conditions because more oxygen is present in the system, with the rest being mostly gaseous sulfate. On the basis of this case study, it can be said that, practically, a high temperature (1300 °C) does not mean high levels of alkali chlorides (in other words, corrosion) in absolute terms, because very little Cl is present in the fuel. However, in relative terms, KCl (g), at both reducing conditions, contains about 85 mol % of the available Cl against about 50 mol % at oxidizing conditions. The evaluation of the overall corrosion risk must include corrosive S species. Here, no sulfides are formed; therefore, only H2S (g) is of interest. The amounts formed at both reducing conditions are relatively close (0.068 and 0.066 mol, respectively), while no H2S (g) is formed at oxidizing conditions. Consequently, similar to case 1, the corrosion risk is similar at both pyrolysis and milder reducing conditions and higher than at oxidizing conditions, even though the variety of species and corrosion mechanisms involved prevent any further quantification. 3.3. Trace Metals Involved in Cl-Induced Corrosion: Pb and Zn. While alkali chlorides are the main conveyors of Cl
(21) Spiegel, M. Mater. Corros. 2000, 51, 303–312.
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Figure 6. Case 1: As speciation (mol % As). Fuel = demolition wood. Nyk€ oping plant. Pyrolysis = no external O added; 0.5 and 1.3 refer to air/fuel ratios. Temperature = 850 °C.
Figure 8. Case 2: Cr speciation (mol % Cr). Fuel = peat and sawdust. Uppsala plant. Pyrolysis = no external O added; 0.5 and 1.3 refer to air/fuel ratios. Temperature = 1300 °C.
At both reducing and oxidizing conditions, Cr2O3 (s) is the sole Cr compound in case 1. Similar to As, it has been suggested that Cr has a strong affinity to Ca and gives CaO 3 Cr2O3 (c) as the main Cr compound at both reducing and oxidizing conditions at 950-1550 K.9 Even though this speciation is strongly dependent upon the presence of reactive Ca, it does not affect the overall result that Cr will be found in the condensed phase as Cr2O3 (s), eventually associated with calcium oxide if it is largely available. Whatever the atmosphere, it appears that Cr is to be found in the bottom ash. In case 2 (Figure 8), the speciation is rather different than in case 1. At very reducing conditions (near pyrolysis), no Cr2O3 (s) is formed but Cr (g) and CrS (l and g) are formed, clearly shifting speciation of Cr compared to oxidizing conditions but not changing phase distribution because in both cases about 60 mol % Cr is expected to be in the condensed phase. At “mildly” reducing conditions, Cr2O3 (s) becomes the single Cr compound, as in the previous case. As more oxygen is made available at oxidizing conditions, new gaseous species are appearing and reach almost 40 mol % Cr. The gaseous species present at oxidizing conditions are CrO2, CrO3, and CrO3H (and CrO4H2). All of these species have a higher O/Cr molar ratio than Cr2O3, clearly indicating the increasing attraction of Cr for O at increasing O concentrations. However, it appears that these gaseous oxides and hydroxides require high temperatures because they are not found in the first installation where the temperature is 850 °C. This trend is confirmed in other studies9 but to a smaller extent. An important outcome for real systems is that Cr phase distribution is fluctuating with oxygen levels. 3.5. Overview of the Practical Implications of Local Reducing Conditions for Full-Scale Plants and Proposed Abatement Measures. The main practical outcomes can be summarized as follows. Case 1: Nyk€ oping (fuel = demolition waste wood), low temperature, low S/Cl molar ratio, and high concentrations of corrosion players. (i) Globally, corrosion is expected to be more severe at reducing conditions. (ii) At reducing conditions, the presence of sodium sulfides may be disastrous in terms of corrosion and slagging in the furnace. (iii) The corrosion risk associated with the K gas phase is alike at all O levels. (iv) Reducing conditions could provoke leadsulfide-induced corrosion in combustion plants. (v) Fly ash will be enriched in alkali and trace metals at oxidizing conditions, because of ZnO (s). (vi) Reducing conditions may increase the volatility of As, hence reducing its concentration in the bottom ash (both installations). (vii) Whatever the atmosphere, Cr is to be found in the bottom ash.
Figure 7. Case 2: As speciation (mol % As). Fuel = peat and sawdust. Uppsala plant. Pyrolysis = no external O added; 0.5 and 1.3 refer to air/fuel ratios. Temperature = 1300 °C.
For case 2, thermodynamic equilibrium calculations forecast that Zn (g) is the one and only Zn compound for this installation, no matter the oxygen level. It is rather surprising that no O-containing zinc compound of any type (sulfate, hydroxide, or oxide) appears at oxidizing conditions. No clear explanation can be given for this result. 3.4. Toxic Trace Metals (Not Involved in Corrosion): As and Cr. The toxicity of trace metals and their dispersion in the environment are major concerns. This explains that their fate has been studied under a variety of conditions, including reducing conditions.4,9,10,12 However, the joint interpretation of oxidizing and reducing calculations is rarely performed. Two trace metals found at relatively high levels in the studied biomasses are investigated here. At very reducing conditions (pyrolysis), As (g) represents about 80 mol % arsenic, with the rest being mostly AsO (g) for case 1 (Figure 6). As more oxygen is available, even at still reducing conditions, AsO (g) becomes the exclusive As compound. Ca3(AsO4)2 (c) has been predicted as the most stable arsenic compound at oxidizing conditions;9 however, this is subject to Ca high availability, and it is probable that, in real systems, As will be found as both AsO (g) (as a main component) and some Ca3(AsO4)2 (c), with As (g) only present at very reducing conditions. With case 2 (Figure 7), the situation is very similar to the previous case, despite the different fuels and operating conditions (temperature). This confirms the very strong affinity of As for oxygen no matter the fuel and operating conditions. It can therefore be said that reducing conditions may increase the volatility of As as already observed,2 hence reducing its concentration in the bottom ash in combustion plants. 1563
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Case 2: Uppsala (fuel = peat and sawdust), high temperature, high S/Cl molar ratio (low Cl), and typical concentrations of corrosion players. (i) The severity of (Na)Cl-induced corrosion does not appear to be significantly affected by varying oxygen levels. (ii) The distribution of Na in the various fractions (bottom and fly ash and gas) is unlikely to change with the oxygen level. (iii) Reducing conditions could provoke lead-sulfide-induced corrosion in combustion plants. (iv) Cr phase distribution is fluctuating with oxygen levels. It is clear that local reducing conditions may have deleterious consequences and should be minimized. The main factors causing local reducing conditions have been presented earlier and can be mitigated by promoting fuel homogeneity by various methods, by designing efficient air injection systems or fluidized-bed bottoms (as recently performed in the Nyk€ oping plant) and/or by better monitoring of oxygen levels in the furnace. The monitoring of pressure fluctuations could provide an innovative solution because local pressure increases have been associated with local reducing conditions.1 However, some factors, such as fuel intrinsic properties, are difficult to alter without the introduction of new materials (additives and secondary fuels) to modify the chemistry and/or physical processes taking place.
selected trace metals in two biomass-fired boilers. The overall chemical trends obtained by thermodynamic equilibrium calculations were interpreted in terms of practical outcomes for real thermal systems. There is a lack of knowledge concerning the fate of Na and K in reducing atmospheres. This study provides better insight into alkali chemistry in such environments. The main results may be summarized as follows: (1) Overall corrosion is expected to be more severe at reducing than oxidizing conditions because of the combined generation of alkali chlorides, sulfides, and H2S (g). (2) Some compounds have stable chemistries; i.e., their speciation and/or phase distribution are little affected by varying oxygen levels (such as Na in case 2) or by changing fuel and temperature (such as As), while (3) other compounds (such as Zn) have more versatile chemistries. Acknowledgment. This work is part of the “NextGenBioWaste” project, co-funded by the European Commission under the Sixth Framework Programme. The authors express their gratitude to Vattenfall Nordic Heat for providing data and complementary information. The authors also thank the KRAV project (supported by the Research Council of Norway) for its financial support.
4. Conclusions
Supporting Information Available: Custom-made Factsage database. This material is available free of charge via the Internet at http://pubs.acs.org.
The case studies investigated in this work are focusing on local reducing conditions and their effects on alkali and
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