Energy & Fuels 2008, 22, 1519–1526
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Fate of Zinc during Combustion of Demolition Wood in a Fluidized Bed Boiler Anna-Lena Elled,*,†,‡ Lars-Erik Åmand,† and David Eskilsson§ Department of Energy and EnVironment, Chalmers UniVersity of Technology, SE-412 96 Göteborg, Sweden; UniVersity College of Borås, SE-501 90 Borås, Sweden; and Department of Energy Technology, SP Swedish National Testing and Research Institute, SE-501 15 Borås, Sweden ReceiVed May 10, 2007. ReVised Manuscript ReceiVed January 6, 2008
Demolition wood can be used as a fuel in heat and power plants. However, it may contain elevated amounts of zinc, originating from white paint, which can cause problems related to deposit formation and corrosion on heat transfer surfaces. In this work, combustion tests with zinc addition were carried out in a fluidized bed boiler to investigate its effect on deposit formation. Thermodynamic equilibrium calculations were performed to complement the experimental data. The results show that combustion of demolition wood only contaminated with zinc generates a modest amount of deposit. Combustion of demolition wood contaminated with both zinc and chlorine promotes the deposit formation due to the increased amount of submicron particles in the flue gas. The thermodynamic equilibrium analyses show further that reducing conditions increase the release of zinc to the flue gas. On the other hand, in the case of oxidizing conditions, the retention of zinc in the ash is strong. Zinc, in combination with chlorine, gives rise to formation of zinc chloride in the flue gas. The formation is, at reducing conditions, thermodynamically favored between 450 and 850 °C. At oxidizing conditions, the formation is initiated at 400 °C and gradually increased with the temperature.
Introduction Demolition of old buildings generates waste material of various kinds that has to be sorted and recycled. The remainder, called demolition wood, can with favor be used for energy generation since it is considered CO2 neutral, has high heating value, and is easily accessed on the market at a low price. Energy recovery from wastes and reduction of material put on refuse dump are further motives to make use of demolition wood as fuel. However, demolition wood is more or less contaminated from paint, wood preservatives, plastics, and metals that may cause operational problems during combustion.1 Boiler operators firing demolition wood experience problematic ash quality and increased rate of deposition on heat transfer surfaces. The deposits seem, in addition, to be more corrosive over a broader temperature interval compared to deposits formed during ordinary biomass combustion.2 This work aims to improve the knowledge on the fate of zinc during combustion of demolition wood in order to facilitate energy recovery from waste materials. The focus is directed to the effect of elevated amounts of zinc in the fuel on deposit formation and the influence of parameters such as presence of oxygen, chlorine, and sulfur in the flue gas. Background The composition of deposits formed on heat transfer surfaces in biomass combustors can roughly be approximated by the fly * Corresponding author. Telephone: +46-33-4354644. E-mail:
[email protected]. † Chalmers University of Technology. ‡ University College of Borås. § SP Swedish National Testing and Research Institute. (1) Jermer, J.; Ekvall, A.; Tullin, C. InVentory of contaminants in waste wood, Report 732 (F9-820) (in Swedish with English summary), Värmeforsk, ISSN 0282-3772, Stockholm, 2001. (2) Andersson, C.; Högberg, J. Fouling and slagging problems at recoVered wood fuel combustion, Report 733 (F9-821) (in Swedish with English summary), Värmeforsk, ISSN 0282-3772, Stockholm, 2001.
ash composition with a slight enrichment of potassium and chlorine.3 Combustion of demolition wood contaminated with zinc gives rise to deposits with similar composition as when “clean” wood is used as fuel apart from the elevated content of zinc.4,5 The presence of zinc may decrease the ash melting temperature, cause sticky melts,6 and increase the deposit formation rate.7 Metal salts are, in addition, known to increase the corrosion in the boiler2 since it interferes with the oxide normally protecting the heat transfer surfaces. Extensive research has been carried out on deposition phenomena describing the release of mineral matter from fuels, formation of ash in flue gas, and mechanisms controlling the transport of particles to a surface. The size distribution of ash has been found to be bimodal, with a lower peak in the submicron range and a main peak in the micrometer range.8 The submicron particles (aerosols) originate from nucleation and condensation of vaporized inorganic matter, and the micrometer-sized particles are formed by char and mineral fragmentation and agglomeration.8–11 Calcium, magnesium, and silica are examples of elements relevant for formation of coarse particles. The total amount of chlorine, potassium, sodium, (3) Frandsen, J. F. Fuel, 2005, 84, 1277–1294. (4) Åmand, L.-E.; Leckner, B.; Eskilsson, D.; Tullin, C. Energy Fuels, 2006, 20, 1001–1007. (5) Niemi J.; Enestam S.; Makela, K. In Proceedings of the 19th International Conference on Fluidized Bed Combustion; Vienna, 2006. (6) Sjöblom, R. Hypotheses and mechanisms for deVelopment of deposits containing zinc and lead in conjugation with combustion of wood waste, Report 734 (F9-810) (in Swedish with English summary), Värmeforsk, ISSN 0282-3772, Stockholm, 2001. (7) Backman, R.; Hupa, M.; Hiltunen, M.; Peltola, K. In Proceedings of the 18th International Conference on Fluidized Bed Combustion; American Society of Mechanical Engineers: Toronto, 2005; Paper FBC200578074. (8) Sarofim, A. F.; Helble, J. J. In The Impact of Ash Deposition on Coal Fired Plants; Williamson, J., Wigley, F., Eds.; Taylor & Francis: Washington, DC, 1993; pp 567–582..
10.1021/ef700234c CCC: $40.75 2008 American Chemical Society Published on Web 02/29/2008
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Table 1. Influencing Parameters on Zinc Behavior in Combustion Processes oxidizing atmosphere
Zinc is, in absence of chlorine, retained in the ash under oxidizing conditions within the temperature range usually encountered in solid waste combustion4 due to formation of stable solid compounds.7
reducing atmosphere
Limited access of oxygen promotes the volatility of zinc20 and increases the release of metallic zinc7 to the flue gas which may contribute to deposit formation.8
chlorine
Increase of available chlorine is an effective means to increase zinc volatilization;20 presence of hydrogen chloride in the flue gas rends formation of zinc chloride possible which is more volatile compared to the elemental and oxide form of zinc.7
sulfur
Sulfur may form alkali sulfates and inhibit formation of alkali chlorides, and thus allow chloride to form hydrogen chloride in the flue gas which promotes the formation of zinc chloride.7
water vapor
Presence of water vapor has a decreasing influence on the zinc volatility.20
alkali metals
Alkali metals confiscate available chlorine by formation of alkali chlorides, and decrease the partial pressure of hydrogen chloride in the flue gas which constrains the formation of zinc chloride.7
sorbents
Magnesium silicate hydroxide hydrate (Mg4Si6O15(OH)2 · 6H2O)18 zinc aluminum oxide (ZnAl2O4)17–20 zinc ferrite (ZnFe2O4)20 zinc silicate (Zn2SiO4)15,17,20 fly ash with calcium (at coal gasification conditions).19
sulfur, and heavy metals are of importance for the aerosol formation.11,12 Depending on the size of ash particles, different modes apply for their transportation to a surface. Particles larger than 10 µm are transported by inertial impaction, whereas particles between 1 and 10 µm are transported by thermophoresis and eddy diffusion.9 Submicron particles are mainly transported by vapor and molecular diffusion, Brownian motion, and boundary layer effects.9 Further mechanisms to consider are condensation and chemical reactions.13 Condensation includes all vapors that pass and subsequently deposit on a surface, and chemical reactions involve sulfation of alkali in the deposits, combustion of residual carbon, and absorption of alkali on silicates.13 Frandsen et al.3 summarized results obtained from a number of full scale measuring campaigns conducted in order to investigate fundamental mechanisms of fly ash and deposit formation during combustion of a wide range of fuels. It was concluded that the mass of aerosols formed during combustion and the chemical composition of the particles almost exclusively depend on the chemical composition of the fuel used and the release of chlorine, sulfur, and inorganic metals. Nucleation and condensation of alkali salts and subsequent coagulation of particles were suggested to be the dominating aerosol formation mechanisms during combustion of pure wood. During combustion of waste wood, it was suggested that the nucleation of alkali salts was suppressed by condensation on the large specific surface area provided by a high number of zinc oxide nucleus.3 Zinc is considered as volatile at reducing conditions in combustion processes which means that the element vaporizes in the combustion chamber. The species may, on the other hand, form submicron particles, and condense on fine fly ash particles or other available surfaces.14,15 Important general factors influencing the fate of zinc during combustion are the concentrations and the elemental modes of occurrence in the fuel, temperature, pressure, oxidizing, or reducing conditions and the presence of ash-forming elements.16 Struis et al.17 investigated the speciation and evaporation rate of zinc in fly ashes from a municipal solid waste incinerator. The temperature of the ash was increased linearly in an oven. Zinc vapors were detected online while the ash samples were collected at different heat treatment stages. At reducing conditions it was shown that zinc sulfide was formed from 400 °C, at the expense of zinc oxides, and beyond 745 °C gradually decomposed into volatile zinc species. Zinc silicate and zinc aluminum oxide were found in the fly ash samples and their fractions remained practically constant during the experiment. The analysis at oxidizing conditions resulted in an ash composed of zinc aluminum oxide,
zinc oxide, and zinc silicate. Abanades et al.18 performed experiments in a fluidized bed to investigate the influence of operating parameters on the extent of heavy metal release from municipal solid waste incinerators. Zinc was to a great extent retained in the ash and it was particularly stabilized in mineral matrices. Since the volatility of zinc from zinc aluminum oxide was found to be negligible, the substance was suggested to be an important actor in the retention of zinc in ash. Magnesium silicate hydroxide hydrate showed ability to retain zinc as well but the release was larger compared to aluminum oxide, especially in the presence of chlorine. Diaz-Somoano and Martinez-Tarazona19 made similar conclusions on aluminum oxide but in the case of coal gasification. Fly ash with high calcium content also showed ability to capture zinc but aluminum oxide maintained better sorbent properties in presence of hydrogen chlorine than fly ash.19 Table 1 gives an overview of influencing parameters on zinc behavior in combustion processes. Experimental Section This work is a continuation of a study on ash deposition on heat transfer tubes during combustion of demolition wood.4 Here, the experimental investigation is complemented with thermodynamic equilibrium analysis to support the interpretation of the measured data. The following section summarizes the implementation of the experiments; a more detailed description is found in Åmand et al.4 Boiler. The 12 MWth circulating fluidized bed (CFB) boiler located at Chalmers University of Technology was used for the (9) Benson, S. A.; Steadman, E. N.; Zygerlicke, C. J.; Erickson, T. A. In Application of AdVanced Technology to Ash-Related Problems in Boilers; Baxter L. L., DeSollar R. W., Eds.; Plenum Press: New York, 1996; pp 1–15.. (10) Lind, T.; Valmari, T.; Kauppinen, E.; Nilsson, K.; Sfiris, G.; Maenhaut, W. Proc.Combust. Inst. 2000, 28, 2287–2295. (11) Brunner, T.; Joeller, M.; Obernberger, I.; Frandsen, F. In Proceedings of the European Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection, WIP: Amsterdam, 2002. (12) Valmari, T.; Lind, T. M.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Energy Fuels 1999, 13, 379–389. (13) Baxter, L. L. Biomass Bioenergy 1993, 4, 85–102. (14) Belevi, H.; Moench, H. EnViron. Sci. Technol. 2000, 34, 2501– 2506. (15) Belevi, H.; Langmeier, M. EnViron. Sci. Technol. 2000, 34, 2507– 2512. (16) Miller, B. B.; Kandiyoti, R.; Dugwell, D. R. Energy Fuels 2002, 16, 956–963. (17) Struis, R. P. W. J.; Ludwig, C.; Lutz, H.; Scheidegger, A. M. EnViron. Sci. Technol. 2004, 38, 3760–3767. (18) Abanades, S.; Flamant, G.; Gauthier, D. EnViron. Sci. Technol. 2002, 36, 3879–3884. (19) Diaz-Somoano, M.; Martinez-Tarazona, M. R. Energy Fuels 2005, 19, 442–446.
Combustion of Demolition Wood experiments. The boiler produces heat for the campus and is similar to conventional commercial units. It is provided with advanced measurement equipment specially adapted for research purposes and the facilities are described in, for instance, Åmand et al.21 Test Program. Load and operating data employed were typical for a commercial CFB boiler. The bed material was silica sand and no limestone was added for sulfur capture. Wood pellets (WP) were used as base fuel and high-quality waste wood from the demolition industry (WW) as additional fuel. Both fuels are low ash wood fuels, quite similar to each other except for the higher moisture content in the waste wood. Zinc oxide was added to the fuel before reaching the boiler in two of the tests to represent pigment of white color. Hydrogen chloride was introduced directly into the furnace through a spray nozzle in one test, together with zinc oxide, to simulate contamination of plastic fractions in the demolition wood which is a common experience in plants using this waste material. The amount of zinc injected was such that it corresponded to 125 and 105 g/kg dry ash, respectively. In the latter test, chlorine was added and the amount corresponded to an increase in the fuel from 0.02 to 0.16% based on dry and ash free mass (daf). Detailed information on the fuel composition and the operating data is given by Åmand et al.4 Sampling. An air-cooled probe, equipped with a removable ring, was inserted horizontally into the center of the flue gas pass, downstream of the primary cyclone, to collect deposit samples. The tube was exposed to flue gases of 825 °C, for 8–12 h, and its surface temperature was maintained at 500 °C. Fly ash particles were isokinetically sampled by a heated probe (kept at 120 °C) downstream of the convective heat exchangers at a flue gas temperature of 270 °C. The particle mass size distribution was determined by a Dekati low-pressure impactor (DLPI). The gas concentrations were measured at the same position where the deposit probe was inserted and in the stack using conventional gas analyzers and a FTIR (Fourier transform infrared) analyzer. Analyses. The contents of moisture, combustibles, and ash in the fuels were analyzed by a MAC 400 Proximate Analyzer 785700 system. The main components and the content of zinc were analyzed by ICP-OES (inductive coupled plasma–optical emission spectrometry). The ash sample composition was analyzed by ion chromatography, ICP-OES (inductive coupled plasma–optical emission spectrometry) or ICP-MS (inductive coupled plasma–mass spectrometry). The relative concentration of selected compounds in the fly ash samples was determined using a TOF-SIMS (timeof-flight second ion mass spectrometer) instrument. This instrument carries out both surface and depth analyses on amorphous and nonamorphous materials. It was calibrated against standard solutions of KCl, K2CO3, K2SO4, NaCl, ZnCl2, ZnO, and ZnSO4. The TOFSIMS instrument has many advantages compared to other analysis techniques but also limitations. It gives the chemical composition of the sample surface, thus not of the entire sample, and requires standards and therefore prior knowledge of the material. Here it is used to confirm the presence of certain compounds in the ash and indicate trends.
Theoretical Section Thermodynamic equilibrium studies have earlier been proven useful in gaining fundamental understanding of the chemistry in combustion processes. The computer program FactSage, version 5.4.1, with the module EQUILIB was used in this study, and the thermodynamic data was collected from the database F*A*C*T.22 The conditions were similar to those applied in the experiments. The temperature range between 400 and 1200 °C was investigated and the total pressure set to 1 atm. The (20) Verhulst, D.; Buekens, A.; Spencer, P. J.; Eriksson, G. EnViron. Sci. Technol. 1996, 30, 50–56. (21) Åmand, L.-E.; Leckner, B. Fuel 2004, 83, 1803–1821. (22) Bale, C. W.; Chartrand, P.; Degterov, S. A.; Eriksson, G.; Hack, K.; Mahfoud, B. R.; Melancon, J.; Pelton, A. D.; Petersen, S. Calphad 2002, 26, 189–228.
Energy & Fuels, Vol. 22, No. 3, 2008 1521 Table 2. Case Matrix for the Thermodynamic Equilibrium Calculations
(WP (WP (WP (WP
+ + + +
WW) WW + Zn) WW + Zn + Cl) WW + Zn + Cl + S)
Zn g/kg dry ash
Cl% by mass (daf)
S% by mass (daf)
5 5 to 172 172 172
0.02 0.02 0.02 to 0.65 0.65
0.01 0.01 0.01 0.01 to 1.2
equilibrium model contained 15 elements (Al, C, Ca, Cl, Fe, H, K, Mg, N, Na, O, P, S, Si, and Zn) and gas, pure solids, and pure liquids were considered. Solutions were not taken into account. Particular attention was paid to the effect of reducing and oxidizing conditions and various concentrations of zinc, chlorine, and sulfur in the fuel on the formation of zinc compounds. The fuel mixture employed in the initial case illustrated a moderate contaminated fuel composed by equal shares of wood pellets and waste wood based on respective energy value. The content of zinc in the ash corresponded to 5 g/kg dry ash and the content of chlorine in the fuel to 0.02% (on dry and ash-free mass). The calculations were conducted on the initial case with two different air supply strategies termed reducing (Re) and oxidizing conditions (Ox). In the case of reducing conditions, no air was added to the equilibrium reactor and the only oxygen available for reaction was that already contained in the fuel. In the case of oxidizing conditions an excess of air was used such that the product gas contained 3% O2 (by volume). To investigate the effect of zinc contamination, calculations were conducted on the initial case with a gradually increased quantity of zinc, from 5 to 172 g/kg dry ash. The effect of chlorine was investigated by calculations performed on the initial case with a constant high quantity of zinc (172 g/kg dry ash) and a gradually increased quantity of chlorine. The highest content of chlorine reached 0.65%. The effect of increased amount of sulfur on the equilibrium composition in the case of high contamination of zinc and chlorine was investigated as well. Table 2 summarizes the parameter study, and Table 3 shows selected molar ratios of the elemental composition in the cases. Chlorides and sulfates of alkali and zinc are of importance concerning deposit formation, and thus the relative concentrations of involved elements of interest. For instance, the low content of zinc in the initial case is demonstrated by a high Cl/Zn ratio. The concentration of chlorine is low relative the content of alkali unless it is added in form of contaminations such as plastic. Also, the limited amount of sulfur in wood is illustrated which suggests a restrained formation of alkali sulfates. The (Cl-K-Na)/Zn ratio demonstrates the access of chlorine available for reaction with zinc even though potassium and sodium chlorides are more stable compounds compared to ZnCl2. Results and Discussion Deposits. The samples collected during the tests should be regarded as initial deposits since the exposure time was relatively short. The layer of solids formed on the tube surface was composed of fine particles, evenly distributed around the sample ring. Table 4 shows the deposit formation and the amount of relevant elements found in the deposits. These data have been published earlier.4 Substitution of wood pellets with high-quality waste wood did not have any great influence on the deposit formation rate since the two fuels, essentially, were quite similar. The formation rate was increased in case of zinc addition but still small. The elevated amount of both zinc and chlorine to the boiler resulted in an increased deposit formation rate by 3
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Table 3. Molar Ratios in the Calculated Cases Cl/(K + Na)
Cl/Zn WP WP WP WP
+ + + +
WW WW + Zn WW + Zn + Cl WW + Zn + Cl + S
11 11 to 0.3 0.3–11 11
0.3 0.3 0.3–10 10
Table 4. Deposit Formation and Elements on the Test Tube test build up rate (mg/(m2 h)) elemental deposit rate (mg/(m2 h)) Al Ca Cl K Na S Zn
WP
WP + WW
WP + WW + Zn
429
425
781
9.6 107 0.27 180 5.3 86.1 2.0
24.5 89.7 0.29 221 48.1 8.9 1.3
11.3 108 1.3 394 19.5 151 32.8
WP + WW + Zn + Cl 1231
18.7 109 245 593 73.9 130 18.9
times compared to the test with no additives at all. Table 4 shows further that the main causes for the increased formation rate in the test with zinc addition were potassium, sulfur and, to some extent, also zinc. Elevated amounts of chlorine, potassium, sodium, and sulfur were found in the deposits collected during the test with both zinc and chlorine addition. The amount of zinc and sulfur was increased compared to the tests with wood only, but decreased compared to the tests with zinc addition. Fly Ash. Figure 1 shows the mass distribution of particles and some selected elements (Cl, K, Na, S, and Zn) in the convection pass as a function of aerodynamic particle diameter. Figure 1a shows results obtained from the test with only wood (WP + WW). Figure 1b shows results from the test with zinc addition (WP + WW + Zn) and Figure 1c results from the test where both zinc and chlorine were added to the boiler (WP + WW + Zn + Cl). The total particle mass distribution was scarcely affected by zinc addition. Contemporary addition of zinc and chlorine increased the mass of submicron particles. The figure reveals that a large part of the particles below 1 µm contain chlorine, potassium, and sodium since their curve shapes (650 °C) zinc is vaporized and found in the flue gas as metallic zinc. The retention of zinc in ash is strong at oxidizing conditions. The formation of zinc aluminum oxide is in particular favored by the presence of oxygen and predicted to be the main zinc compound in the ash. Increase of Zinc. The result presented below was obtained from calculations conducted on the initial case with gradually increased quantity of zinc. The initial vaporization temperature is not affected by the higher zinc concentration in the fuel. The actual amount of zinc released to the flue gas is increased with the zinc feed even though the share of zinc released is slightly decreased. Zinc is completely volatilized and found in the flue gas as metallic zinc above 750 °C at reducing conditions. The zinc retention in the ash is again strong during oxidizing conditions. Above 1000 °C zinc starts to volatilize as metallic zinc. This temperature exceeds the average temperature in a fluidized bed boiler but may occur locally in the furnace. Increase of Chlorine with a Constant High Quantity of Zinc. To investigate the effect of chlorine available for reaction on the fate of zinc, calculations were conducted on the initial case with constant high quantity of zinc and gradually increased quantity of chlorine. The calculations resulted in six cases named by the molar ratios of chlorine and zinc: 0.3, 0.7, 1.4, 2.8, 5.6, and 11. Zinc is predicted to be volatilized in all cases at reducing
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Figure 4. Release of zinc at reducing and oxidizing conditions (Cl/Zn ) 11).
conditions and completely present in the flue gas above 750 °C, mainly as metallic zinc. Zinc is, at oxidizing conditions, to a great extent retained in the ash. Figure 4 shows the zinc release at equilibrium in the case of gasification (Re) and combustion (Ox) of a fuel heavy contaminated with both zinc and chlorine (case 11). The strong impact of oxygen available for reaction on the zinc volatility is clearly demonstrated in the figure. Frandsen et al.3 pointed out the importance of zinc oxide formation during combustion of demolition wood and Figure 5 shows the temperature range at which the compound is thermodynamically stable in the six cases. Zinc oxide is stable at both reducing and oxidizing conditions but oxidizing conditions allow the formation also at higher temperatures. The presence of chlorine inhibits the formation of zinc oxide at both reducing and oxidizing conditions. An increased volatility is seen in case 2.8, 5.6, and 11 and Figure 6 reveals the cause. If a sufficient amount of chlorine is present, it reacts with zinc and forms zinc chloride. Matching formation is seen in the fly ash samples from the combustion test with zinc and chlorine addition presented in Figure 2. In case 2.8 there is an incipient formation of zinc chloride but the concentration of chlorine is to low to have a strong impact. The formation of zinc chloride is promoted between 450 and 850 °C at reducing conditions. This temperature interval coincides with temperatures normally reached on heat transfer surfaces in the furnace. The formation of zinc chloride during oxidizing conditions and high levels of chlorine present causes a zinc release already at 400 °C which slowly increases with the temperature.
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It is well-known that chlorine, together with potassium and to some extent sodium, plays an important part in the formation of deposits on heat transfer surfaces during combustion of wood.3 It is, for that reason, of great interest to comprehend the mechanisms involved. The following section depicts the results achieved in the thermodynamic equilibrium analyses on the fate of these elements. When chlorine is present at lower concentrations than potassium, it reacts with potassium and forms solid potassium chloride which starts to volatilize at 500 °C. At 700 °C potassium chloride is completely volatilized and present in the flue gas. At higher temperatures (>800 °C) sodium chloride and some hydrogen chloride are present in the flue gas as well. Zinc chloride is formed first in case 1.4 (high amount of zinc and intermediate amount of chlorine). This is the case at which the fuel composition molar ratio (Cl-K-Na)/ Zn turns from a negative to a positive value. The result indicates that potassium and sodium chlorides are more stable than ZnCl2. In case all potassium and sodium are present as chlorides, further increase of chlorine provides conditions for increased formation of, in first hand, hydrogen chloride but also of zinc chloride. Figure 7 shows chloride compounds formed in the case with the highest amount of zinc and chlorine (case 11) at reducing and oxidizing conditions. The prevailing (Cl-K-Na)/Zn molar ratio in this case was as high as 10 and the amount of zinc chloride formed in the same range as the amount of alkali chlorides. The remaining share of chlorine (80–90%) was present in the flue gas mainly as hydrogen chloride. Sulfur has a tendency to react with alkali metals and form alkali sulfates during combustion. The thermodynamic analyses show that sulfur forms mainly hydrogen sulfide in the flue gas at reducing conditions and low contamination of zinc and chlorine. An increased amount of available zinc at reducing conditions promotes the formation of solid zinc sulfide at temperatures below 700 °C. The equilibrium composition of sulfur compounds is more or less unaffected by the addition of zinc at oxidizing conditions. Solid potassium sulfate is stable below 1000 °C and above this temperature sulfur is mainly found in the flue gas as sulfur dioxide. Figure 8 shows the equilibrium composition of sulfur compounds in case 0.3 (high amount of zinc and low amount of chlorine) and 11 (high amount of zinc and chlorine) at oxidizing conditions. The increased amount of chlorine reacts with available alkali and the sulfur released forms calcium sulfate. The formation of calcium sulfate occurs already at rather low chlorine concentrations and the stability range increases as the chlorine contamination increases. Increase of Sulfur with a Constant High Quantity of
Figure 5. Percent of total Zn as ZnO(s) at reducing and oxidizing conditions and gradually increasing Cl/Zn ratios from 0.3 to 11.
Combustion of Demolition Wood
Energy & Fuels, Vol. 22, No. 3, 2008 1525
Figure 6. Percent of total Zn as ZnCl2(g) at reducing and oxidizing conditions and Cl/Zn ratios 2.8, 5.6, and 11.
Figure 7. Chlorine compounds at reducing and oxidizing conditions for the case Cl/Zn ) 11. Remaining chlorine (80%–90%) was found in the flue gas mainly as hydrogen chloride.
Figure 8. Equilibrium composition of sulfur compounds at oxidizing conditions for the cases 0.3 and 11.
Zinc and Chlorine. To investigate the effect of sulfur available for reaction, calculations were conducted on the initial case with constant high quantity of zinc and chlorine and gradually increased quantity of sulfur in the fuel from 0.01% to 1.2% (daf). The formation of chlorine, potassium, and zinc compounds at reducing conditions is more or less the same despite the increased concentration of sulfur. The effect is rather clearer at oxidizing conditions. Zinc sulfate is favored by the increased amount of sulfur and stable at temperatures below 650 °C. Sulfur reacts also with elements such as calcium, potassium, and sodium at low and intermediate temperatures. The effect of
increased amount of sulfur on formation of potassium and zinc chlorides seems to be weak. Conclusions The conclusions presented below are based on results derived from combustion tests and equilibrium analyses at conditions typical for a fluidized bed boiler firing demolition wood. The thermodynamic equilibrium analyses supported the conclusions made in ref4 that the major reason for initial deposit formation during combustion of wood, as well as demolition wood, is potassium chloride. Zinc may be of subordinate significance,
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but still important since the element increases the deposit build up rate and affects the composition of the deposit. The thermodynamic equilibrium analyses showed that reducing conditions promote the release of metallic zinc to the flue gas. The retention of zinc in the ash is strong in case of oxidizing conditions, even though the fuel is contaminated with zinc. The presence of chlorine increases the volatility of zinc at both reducing and oxidizing conditions. Zinc oxide is thermodynamically stable at both reducing and oxidizing conditions but oxidizing conditions allows the compound to be present also at high temperatures. Zinc, in combination with chlorine, gives rise to zinc chloride in the flue gas. The formation is elevated between 450 and 850 °C at reducing conditions and, at oxidizing conditions, initiated at 400 °C and gradually increased with the temperature. Potassium and sodium chlorides seem to be more stable compared to zinc chlorides during combustion. Further, the effect of sulfur on chloride formation in the flue gas seems to be limited. Combustion of demolition wood contaminated with zinc (only) generates a deposit which contains an increased amount of zinc but the total deposit formation during combustion of such a fuel is modest due to lack of chlorine. The submicron particles in the flue gas are mainly caused by nucleation and condensation of alkali chlorides. The increased amount of particles (around 1 µm and larger) origin from zinc which either
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was released in the bed at reducing conditions and again oxidized in the riser, or immediately carried away with the flue gas in the riser and some of them chlorinated into ZnCl2. Zinc was present in the flue gas mainly as zinc oxide but also as sulfates and chlorides. A fuel contaminated with both zinc and chlorine gives rise to more severe deposit formation. The main reason is the elevated amount of chlorine which increases the formation of submicron particles (mainly alkali chlorides but also zinc chlorides) in the flue gas. The sulfate forms of alkali and zinc are suppressed by chlorine and the released sulfur reacts with calcium. The increased deposit formation rate is mainly caused by chlorine, potassium, and sodium but zinc is present in the deposit, mainly in the form of zinc oxide and zinc chloride. Acknowledgment. This work was supported financially by the Swedish National Energy Administration. Operation of the boiler by personnel from Akademiska Hus, support in sampling and recollection of data from the technical staff of the Division of Energy Conversion, and the patient support in carrying out chemical analyses of the particle samples and deposits by Peter Sjövall and Benny Lyvén at the Swedish Testing and Research Institute are gratefully acknowledged. EF700234C
