Environ. Sci. Technol. 1996, 30, 50-56
Thermodynamic Behavior of Metal Chlorides and Sulfates under the Conditions of Incineration Furnaces D I R K V E R H U L S T * ,† A N D ALFONS BUEKENS Department of Industrial Chemistry, Free University of Brussels (VUB), Pleinlaan 2, 1050 Brussels, Belgium
PHILIP J. SPENCER AND GUNNAR ERIKSSON Lehrstuhl fu ¨ r Theoretische Hu ¨ ttenkunde, Technical University (RWTH), Kopernikusstr. 16, D-52074 Aachen, Germany
Thermodynamic equilibrium calculations performed with the program ChemSage reproduce the observed behavior of metals in refuse incineration furnaces and reveal general trends. Presentation of the results as a series of small graphs allows a quick overall view of the significant species. The calculations predict that all of the Hg present and most of the Cd and Pb volatilize, as is observed in practice. The amounts of Zn and Cu volatilized depend on the reduction conditions and on the amount of Cl present. As and Sb also volatilize, but Mg, Ca, Fe, and Ni do not volatilize under the prevailing oxidizing conditions. The presence of sulfur stabilizes many of the metals in sulfate phases at low temperature but has little influence above 800 °C. Under locally reducing conditions, it may favor the volatilization of Sn as SnS. More detailed studies, in which comparison is made with data from industrial furnaces with measurements carried out under well-defined conditions, are required to answer specific questions.
Introduction Removal of heavy metals from the gas stream of municipal incineration furnaces is necessary to achieve strict emission standards. The formation of dioxins and furans has also been connected to the presence of metal species (1). Calculation of thermodynamic equilibria provides an improved understanding of the factors that influence the behavior of the metals during incineration processes, and thermodynamic considerations alone have been previously shown to yield a useful model for the similar case of the fuming of zinc (2). Mojtahedi and Larjava have published calculated data on the volatilization of Hg, Pb, Zn, and to a limited extent Cd for standard incineration furnace conditions (3). † Present address: The Minerals Laboratory, BHP Minerals Inc., 204 Edison Way, Reno, Nevada 89502.
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TABLE 1
Data from Literature on Combustion of Waste in Incineration Furnace, Considering Primary Air + Incomplete Combustiona species
kg
Nm3
kmol
44 464 35571 1588 N2 7814 348.9 O2 (from air) 11 164 1 591 49.7 O2 (in feed) 412 204 H2 (in feed) 3 600 200 H2O (in feed + air) C 2 080 173.2 Cl 60 1.69 S 26 8.11 × 10-1 Hg 0.026 1.30 × 10-4 Zn 21.5 3.29 × 10-1 Cd 0.11 9.78 × 10-4 Pb 15.7 7.58 × 10-2 Cu 5.5 8.60 × 10-2 Ni 1.1 1.88 × 10-2 Fe 480 8.595 Sn 9 7.6 × 10-2 Mn 0.1 Mg 0.1 Al 0.1 Ti 0.1 As 0.05 0.7 × 10-3 Sb 0.6 4.9 × 10-3
literature ref 7 7 7 7 7 7 8 8 8 8 8 8 8 8 8 9 b b b b 9 9
a Values for 1-h operation and 10 t of feed. b Values chosen arbitrarily for the sake of the calculation. The amount involved is not critical since oxides are always the stable form in the conditions of practice.
Mathews also made calculations on Hg, Pb, and Cl species (4). The present study considers the behavior of a greater number of elements, including Sn, Cu, As, and Sb; discusses the behavior of nonvolatile species; and provides further practical comments. Calculations were carried out with the program ChemSage (5). All species present in the SGTE (6) data base (update 1994) for the elements O, H, N, Cl, C, Hg, Cd, Zn, Pb, Cu, Ni, Fe, Mn, Mg, Sn, Al, Ti, As, and Sb have been considered. The total is close to 350 species.
Definition of the Conditions We define standard conditions for the calculations based on literature data for the material balance and flue gas composition of an incineration furnace (7, 8). To simulate the conditions during burning on the grate of the furnace, we consider that primary air represents 72.5% of the total air injected (average of the literature values of 65-80%) and that contact between the air and the charge is not perfect, i.e., only 80% of the oxygen is effectively in contact with the charge. Even with these limiting assumptions, there is always excess air available for complete combustion on the grate (λ ) oxygen available/oxygen needed for complete combustion ) 1.45). The data given in Table 1 are for 1 h of operation of a furnace treating 10 t/h waste. For Sn, Sb, and As, order of magnitude values were obtained by comparing the data in Table 1 of ref 9 with ref 8.
Equilibria in the System C H O Cl N with No Metals Present Since excess air is available, substantially all carbon is converted to CO2, and CO remains negligible over the entire
0013-936X/96/0930-0050$12.00/0
1995 American Chemical Society
FIGURE 1. Equilibrium amounts of Hg, Cd, Pb and Zn species with no sulfur present.
temperature range. The ratio CO/CO2 goes from < 10-30 at 100 °C to 2.7 × 10-6 at 1100 °C. HCl is the major Cl species. Cl2 is thermodynamically stable below 150 °C, but formation of Cl2 from HCl does not take place in practice. At higher temperatures, gaseous Cl may play a role in chlorination reactions. The Cl/HCl ratio is greater than 10-3 above 800 °C. HOCl also exists, with HOCl/HCl on the order of 0.5 × 10-3 above 300 °C. Small amounts of NO are stable at the higher temperatures, up to 0.6% NO at 1100 °C.
Behavior of Each Metal and Metal Chloride To simplify interpretation of the results, calculations have first been carried out for the reaction of each metal in turn with the amounts of N2, O2, H2, C, H2O, and Cl given in Table 1 (without sulfur). Diagrams illustrating the resulting behavior of the metals are presented in condensed form in Figures 1 and 2. Hg. Hg volatilizes completely at and above 100 °C, as HgCl2(g) up to approximately 600 °C, mostly as Hg(g) above 700 °C. HgO(g) is also present at higher temperatures. Cd. Volatilization of Cd as CdCl2(g) starts above 300 °C and is complete at 400 °C. Above 1000 °C, Cd(g) is the major gas species. Zn. This is the volatile species present in the largest amount. ZnCl2(s), the stable phase at low temperature, converts to ZnO(s) around 280 °C, while ZnCl2(g) also starts to form. The amount of ZnCl2(g) increases slowly with temperature. Twenty percent of the Zn is volatilized at 800 °C.
Pb. Pb has volatile chlorides and oxides. For the conditions chosen here, Pb starts to volatilize at about 300 °C as PbCl2(g). The volatilization is complete at 430 °C. Above 800 °C, PbO(g) and, to a somewhat smaller extent, PbCl(g) also play significant roles, while PbCl2(g) slowly decomposes. A small amount of PbCl4(g) is present between 200 and 400 °C, and some Pb(g) is present above 1000 °C. Cu. CuCl2 is stable up to 200 °C, and CuO is stable between 100 and 700 °C. At higher temperature, Cu3Cl3(g) between 700 and 900 °C and CuCl(g) above 900 °C are the predominant species. Ni. NiCl2 is the stable phase at low temperature, but it is replaced by NiO above about 250 °C. Small quantities of NiCl2(g), Ni(OH)2(g), and NiCl(g) appear in the higher temperature range. As. As stays in the condensed state as As2O5 up to about 500 °C. It volatilizes mainly as As4O6(g) between 500 and 1000 °C. Above 1000 °C, AsO(g) is predominant. AsCl3(g) is present in small amounts with a maximum around 500 °C. Sb. Sb2O5 is stable up to 630 °C, and SbO2 is stable from 630 to 800 °C. Above this temperature, Sb volatilizes as Sb4O6(g). SbCl(g) is also present and becomes the predominant species above 1080 °C. Small amounts of SbCl3(g) also exist. No figures have been drawn for the remaining metals. Fe. Although the boiling point of FeCl3 is approximately 650 °C, the iron oxides are very stable, and for the amounts of oxygen and Cl given in Table 1, no significant amount of Fe is volatilized, even at 1100 °C. Fe2O3 is the stable Fe phase under the given conditions.
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FIGURE 2. Equilibrium amounts of Cu, Ni, As and Sb species with no sulfur present.
Mn. Little volatilization occurs: roughly 1% of the total amount of Mn in the system is present as gaseous MnCl2 at 1000 °C. Mg, Al, and Ti. Oxides are the stable phases at the temperatures where the chlorides could volatilize. AlCl3 boils below 450 °C, but Al2O3 is very stable and no volatilization occurs. Sn. Sn does not volatilize significantly under the given conditions and remains present as SnO2 over the whole range of temperatures. When all metals are taken together, with the same amount of total Cl, the results do not change significantly, since there is more Cl present than the amount needed to react with all metals that can be chlorinated in the present circumstances. The small differences observed are negligible compared to the inaccuracies related to mixing of condensed phases and knowledge of activity coefficients, which will be discussed below.
Effect of Changes in the Input Conditions
FIGURE 3. Effect of the amount of carbon present on the equilibrium amounts of tin species. No sulfur present.
Oxidation Level (Amount of Oxygen/(C + H)). Under the given oxidizing conditions, only Hg(g) and Cd(g) exist as metallic gaseous species, the latter only above 1000 °C. A small amount of Zn(g) (about 0.3% of the total) also forms at 1100 °C. In practice, the feed of a furnace is not homogeneous. Zn and Sn are present as bulk metals, which do not readily oxidize, or are mixed with plastics, paper, etc., which will create a reducing atmosphere locally. To simulate this situation, we recalculate the equilibrium with increasing amounts of carbon in the system.
Sn. If the amount of C available locally is twice the average amount, all Sn will be volatilized at 750 °C, mostly as SnCl2(g). At higher temperature, SnCl(g) and SnO(g) become predominant. Figure 3 shows the effect of gradually increasing the amount of carbon at 800 °C. As soon as the conditions become reducing (λ < 1, amount of carbon >297 kmol), significant amounts of SnCl2(g) are formed with complete volatilization at 340 kmol of C present (original amount of carbon multiplied by 1.96). The CO/CO2 ratio, which is 5
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only at lower levels of Cl or lower temperatures, and the amount of Cl available is not expected to limit volatilization of these metals in practice. Cl has virtually no effect on As and Sb since chlorides are not the major gaseous species (except for Sb above 1080 °C). Influence of Water in Feed. The presence of H2O also decreases Zn volatilization, by shifting the equilibrium
ZnO + 2HCl(g) f ZnCl2(g) + H2O(g) to the left. When the amount of H2O as such in the feed is reduced to 1/10 of the original amount, zinc volatilization in the range 800-1100 °C is multiplied by about 1.6. This effect can be of importance in practice. It should be noted that even if the feed is completely dried, H2O is still formed by the burning of hydrocarbons. FIGURE 4. Equilibrium amounts of copper species, under reducing conditions (amount of carbon × 2) with no sulfur present.
FIGURE 5. Influence of the amount of chlorine on Zn and Cu volatilization at 800 °C.
× 10-9 at 800 °C in the standard conditions, rises rapidly when the stoichiometric amount of carbon is reached to 8 × 10-3 for 300 kmol of C added and to 0.125 for 340 kmol of C. Zn. Adding more carbon allows the formation of Zn(g) when λ < 1. By doubling the amount of carbon, complete volatilization as Zn(g) is achieved at 800 °C. Cu. Reducing conditions decrease the volatilization of Cu, Figure 4 shows that by doubling the amount of carbon present one obtains a stable Cu(metal) phase extending to the higher temperatures with the amounts of Cu3Cl3(g) and CuCl(g) very much reduced. Amount of Chlorine Present (Figure 5). Increasing the amount of available chlorine is an effective means of increasing zinc volatilization. For the standard conditions corresponding to Table 1, doubling the original amount of Cl increases Zn volatilization from 20 to 70% at 800 °C. For Cu, volatilization at 800 °C is nearly complete under the standard conditions (1.69 kmol of Cl present). When the amount of Cl is decreased, Cu volatilization decreases rapidly. The effect of Cl on other metals is less significant. For Pb, volatilization starts to decrease if the amount of Cl is halved. For Hg and Cd, a reduction in volatilization appears
Effect of the Presence of Sulfur At low temperature, the presence of sulfur leads to the formation of stable metal sulfates that displace the chlorides and sometimes the oxides. At high temperature, volatile sulfides may appear, but in the oxidizing conditions that prevail in incineration furnaces, they do not play a significant role. Figure 6 gives an overall picture of the effect of S on Cd, Zn, Pb, and Cu. Sulfur stabilizes the condensed phase in several cases, and volatilization may occur first up to 300 °C higher than when no S is present. (a) Fe forms Fe2(SO4)3 up to about 450 °C. The total amount of iron is larger than the amount needed to react with the available S, and more than 90% of the Fe remains as oxide. (b) For Ni, the only difference is a stability region for NiSO4, which replaces NiO and NiCl2 below 650 °C. (c) Volatilization of Sn remains negligible under the standard (oxidizing) conditions, but with the assumption of locally reducing conditions, SnCl2(g) as well as SnS(g) appear to play an important role (Figure 7). The presence of S would have a positive effect on the volatilization of Sn. (d) AsS and SbS compounds are not stable here. The diagrams in the presence of sulfur are the same as without sulfur. Also for Hg, except for a possible small region of stability for HgSO4 around 100 °C, the diagram remains the same as in Figure 1. In practice, metal oxides such as CaO, MgO, Na2O, and K2O, which have not been introduced in the present calculations, are typically present in larger amounts than that necessary to convert all S to stable sulfates (9), but lack of homogeneity of the feed and the fact that the oxides are often present in a coarse form will limit the degree of conversion. We can, however, state that the amount of sulfur actually available for reaction with the volatile metals will be smaller than the value given in Table 1. Table 2 gives volatilization temperatures of the metals with and without considering sulfate formation. Actual behavior in an industrial furnace is expected to lie somewhere in between these two cases.
Comparison with Data from Industrial Practice The comparison with two different sets of data from the literature appears in Table 3. For Zn, Cd, and Pb, the differences between the two sources of experimental data are large. One should know more about the exact conditions under which both sets of data were obtained to comment on them. The present calculations suggest that
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FIGURE 6. Equilibrium amounts in the presence of sulfur for those elements presenting significant differences with Figures 1 and 2. TABLE 2
Volatilization Temperatures of the Different Metals with S in system (sulfate formation)
no S in system
FIGURE 7. Equilibrium amounts of tin gaseous species and condensed phases in reducing conditions (amount of carbon × 2) in the presence of sulfur.
differences in the amount of Cl or reducing agent present could explain the discrepancies. The volatilization of Sn can be explained by locally reducing conditions. Mn, Fe, Ni, Mg, and Al do not volatilize in theory, although 1-3% of these metals volatilize in practice, compatible with mechanical entrainment. For Cu, practice shows that volatilization indeed occurs, but in smaller proportions than for Zn, Cd, and Pb. Locally reducing conditions could explain the more limited volatilization. The formation of complex oxides, discussed in
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metal
beginning of volatilization (°C)
Hg Cd Zn Pb Cu Ni Fe Sn Mn Mg Al Ti As Sb
300 300 350 600 1000 >1100 >1100 >1100 >1100 >1100 >1100 500 800
Zn Sn Cu
300 400 900
end of volatilization (°C)
beginning of volatilization (°C)
1100 500 800 >1100
550 850
500 600 700 600 1000 >1100 >1100 >1100 >1100 >1100 >1100 500 800
Reducing Conditions (C × 2) 800 300 800 350 >1100 900
end of volatilization (°C) 1100 800 800 >1100
550 850 800 400 (SnS) >1100
the next paragraph, also plays an important role in this case.
Comments on the Assumptions The calculations describe the overall thermodynamic equilibrium of a system composed of all the fuel, water,
FIGURE 8. Equilibrium amounts of Zn and Cu, including some complex metal oxides, with all metals and sulfur present. TABLE 3
Comparison with Data from Industrial Practice calculated volatilization (%) at 800 °C all metals separately, no complexes
all metals oxidizing reducing and complexes, Plant data element λ ) 1.45 λ ) 0.89, C × 2 oxidizing ref 10 ref 11 Hg Zn Cd Pb Cu Ni Fe Sn Mn Mg Al As Sb
100 20 100 100 98 0.25 0 0 0.2 0 0 100 2.4
100 100 100 100 0.2 100
100 100
100 1 100 100 3 0 0 0 0.2 0 0 100 2.4
96 46 50 59 16 2 2.3 44 2.6 2.6 1.1
95 27 90 33 7 7
Cu and Zn, for which there is a significant difference in volatilization compared to Figure 6, have been represented. The region of stability of the sulfates is reduced. Volatilization is hampered by the formation of ZnO‚Fe2O3, CuO‚Al2O3, and CuO‚Fe2O3. Only 1% of the zinc and 3% of the Cu (instead of 20 and 98%, respectively) are now predicted to go into the gas phase at 800 °C. These values have also been added in Table 3. Other elements that have not been considered here can also play a role: silicates of Zn, Pb, Cu, and Cd as well as aluminates and silicoaluminates exist and could also decrease volatilization. A more comprehensive approach to the problem of complex solid compounds is presently under study.
Conclusions 75
oxygen, and hydrogen introduced into the incineration furnace together with the metals, the chlorine and the sulfur known to be present. The whole system is assumed to be homogeneous. A source of uncertainty is the state of the condensed phases. In the present study, metal oxides, chlorides, and sulfates have been assumed to be present as separate phases. In practice, some of these phases may be soluble in each other. As a consequence, condensed compounds could exist at an activity of less than 1, and they would be stable over a wider temperature range. When one tries to take into account these solubility effects, one quickly gets into unknown territory: thermodynamic values describing the mixing properties of different types of phases are often missing, and the reaction kinetics between condensed phases are probably often slow. To give an idea of the possible effects of the formation of complex condensed phases, we have recalculated the equilibria with all complex oxides/sulfates/chlorides of the elements we consider that are available in the data base. For Fe, for instance, ZnO‚Fe2O3, MgO‚Fe2O3, CuO‚Fe2O3, CdO‚Fe2O3, and NiO‚Fe2O3 all have regions of stability above 600 °C. Figure 8 shows the effect of taking into account these compounds as well as other interactions arising for instance from the fact that the different metals now have to compete for the amount of available Cl. The elements
Consideration of the global thermodynamic equilibrium gives an overall picture of the behavior of metals in an incineration furnace and assists an understanding of the factors involved. The following points summarize the findings: (a) Hg will volatilize completely under all circumstances above 100 °C as HgCl2(g) or Hg(g). (b) Cd and Pb are also easily volatilized as chlorides even under oxidizing conditions. (c) Zn will volatilize partially as ZnCl2(g). Zn is generally present in larger quantities and has a more stable oxide. To volatilize Zn completely, an excess of Cl or more reducing conditions are necessary. The formation of stable spinels (ZnO‚Fe2O3) and probably also of silicates or silicoaluminates can greatly decrease Zn volatilization. (d) Sn can only volatilize in reducing circumstances. The degree of volatilization will depend on the amount of carbon and sulfur available at the site of the reaction. (e) Thermodynamically, total volatilization of Cu as CuCl(g) and Cu3Cl3(g) occurs in oxidizing conditions when no complex oxides are formed. In practice, the amount vaporized is much smaller. Formation of stable Cu metal in locally reducing circumstances or of the double oxides CuO‚Fe2O3 and CuO‚Al2O3, or silicates, could explain that Cu volatilization remains very limited. (f) As and Sb do not need Cl to go into the gas phase. They are completely volatilized above 650 or 850 °C, respectively. (g) Other metals do not volatilize significantly under the conditions of the incineration furnaces.
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(h) S can, in principle, delay volatilization. If the incineration temperature reaches at least 800 °C, the effect should not be significant. S may help volatilize Sn by forming SnS(g) in reducing conditions. The comparison with a few data from industrial practice shows that the very simplified approach taken here gives a good qualitative view of how the different metals behave. More detailed studies involving comparison with data from industrial furnaces and measurements under better defined conditions as well as a more comprehensive study of the formation of solid complex compounds are desirable in order to answer more specific questions.
Acknowledgments P.J.S. and G.E. acknowledge support of their work by the SCIENCE Program of the European Union.
Literature Cited (1) Hinton, W. S.; Lane, A. M. Characteristics of Municipal Solid Waste Incinerator Fly Ash, Promoting the Formation of Polychlorinated Dioxins. Chemosphere 1991, 21 (5-6), 473-483. (2) Kellogg, H. H. A Computer Model of the Slag Fuming Process for Recovery of Zinc Oxide. Trans. AIME 1967, 239, 1439-49. (3) Mojtahedi, W.; Larjava, K. Fate of Some Trace Elements in Combustion and Gasification Processes. In Proceedings of the Second European Conference on Environmental Technology; Martinus Nijhof: Dordrecht, The Netherlands, 1987; pp 323333.
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(4) Mathews, A. P. Chemical Equilibrium Modeling of Trace Metal Speciation in Hazardous Waste Incinerators. In Hazardous Waste: Detection, Control, Treatment; Abbou, R., Ed.; Elsevier Science: Amsterdam, 1988; pp 593-603. (5) Eriksson, G.; Hack, K. Metall. Trans. B 1990, 21B, 1013-1023. (6) Ansara, I.; Sundman, B. In Computer Handling and Dissemination of Data; Glaeser, P., Ed.; CODATA, Elsevier Science Publishers: Oxford, England, 1987; pp 154-158. (7) Thome´-Kozmiensky, K. J.; Widmer, F. Thermische Behandlung von Haushaltsabfa¨llen, Stand und Tendenzen (Thermal Treatment of Household Refuse, Situation and Tendencies). In Abfallwirtschaftseminar an der T. U. Berlin (Seminar on Waste Management at the Berlin Technical University); Thome´Kozwiensky, K. J., Ed.; Technische Universita¨t Berlin, 1978; p 22. (8) Berwein, H. J. In Mu ¨ llverbrennung und Umwelt (Refuse Incineration and the Environment); Thome´-Kozmiensky, K. J., Ed.; EF-Verlag fu ¨ r Energie und Umwelttechnik: Berlin, 1987; p 685. (9) Born, J.; Mulder, P.; Louw, R. Fly Ash Mediated Reactions of Phenol and Monochlorophenols: Oxychlorination, Deep Oxidation, and Condensation. Environ. Sci. Technol. 1993, 27, 18491863. (10) Buekens, A.; Schoeters, J. Thermal Methods in Waste Disposal; EEC Contract ECI 1011/B7210/83/B; Free University of Brussels: Brussels, 1984; p 442. (11) Borchers, H. W.; Thome´-Kozmiensky, K. J. Abfallwirtsch. J. 1989, 1 (1), 28-31.
Received for review December 28, 1994. Revised manuscript received July 17, 1995. Accepted August 2, 1995.X ES940780+ X
Abstract published in Advance ACS Abstracts, November 1, 1995.
