Impact of Municipal Solid Waste (MSW) Quality on the Behavior of

Energy Fuels 2010, 24, 3446–3455 Published on Web 05/21/2010

: DOI:10.1021/ef901144u

Impact of Municipal Solid Waste (MSW) Quality on the Behavior of Alkali Metals and Trace Elements during Combustion: A Thermodynamic Equilibrium Analysis Micha€el Becidan,* Lars Sørum, and Daniel Lindberg Energy Processes, SINTEF Energy Research, NO-7465 Trondheim, Norway. Received October 8, 2009. Revised Manuscript Received May 5, 2010

Municipal solid waste (MSW) is a complex fuel. To study the impact of its composition on speciation during incineration, a novel approach consisting of studying separate waste fractions with the help of thermodynamic equilibrium calculations is used. The focus is on the behavior of alkalis, Pb, Zn, trace elements (As, Cr, and Cd), Cl, and S. The calculations show that (i) the concentration and thermodynamic properties of a given chemical element impact its speciation differently in different waste fractions, (ii) the chemistry of ash and trace elements in different fuels can be similar regarding behavior and speciation, (iii) ratios related to input parameters allow for a better understanding of elemental chemistry, (iv) some chemical elements have a versatile chemistry, i.e., shifting widely with fuel composition, such as Zn, while some others do not (Cr), and (v) S and Cl chemistries are strongly linked. Taking into account interactions between two waste fractions during incineration may have negative consequences (such as increased corrosion) but may also produce synergies mitigating operational challenges.

an example, paper is subdivided into newsprint, high-grade office paper, magazines/catalogues (glossy paper), uncoated paper, coated paper, boxboard, and mixed paper. The mixed paper category illustrates further the difficulty of fully and accurately defining waste. Selected subcategories representing all of the main combustible MSW fractions and posing specific challenges to incineration are investigated in this study: Glossy Paper. As all paper products, the main chemical structure to be found in glossy paper is cellulose, a polymer made of glucose units.3 However, it also contains significant inorganic compounds (mainly calcium carbonate and kaolin), which may provoke an array of operational challenges (corrosion and fouling) during incineration. These compounds are used as fillers, pigments, and binders. Polyvinyl Chloride (PVC). Plastics are synthetic polymerization products; PVC, (C2H3Cl)n, is of particular concern because it has a very high concentration of Cl that may induce corrosion in waste-energy installations but also high HCl and dioxin/furan emissions.4 Various additives are mixed with the PVC to improve its mechanical properties; in the selected item, a sewage pipe, Pb is the main additive and used as a stabilizer. Pb (as chloride) may facilitate corrosive reactions by forming low-temperature eutectics with alkali chlorides. Meat. Meat is mainly made of water (∼75 wt %), protein (polymers of amino acids), fat (carboxylic acids with long aliphatic chains), and minerals. This means that meat (on a dry basis) has high concentrations of C and H but also N (the average N content in protein is 14 wt %), possibly causing problematic NOx and N2O emissions. Furthermore, food

1. Introduction The operational problems (corrosion, slagging, fouling, emissions, etc.) encountered during the thermal treatment of municipal solid waste (MSW) are originating from its chemical and physical properties, especially heterogeneity, which makes stable operation difficult. MSW can be defined as the waste generated by households, commercial activities, and small non-process industries in urban areas. However, it is challenging to define MSW because various countries do not collect waste similarly (MSW may be collected with industrial waste) and do not report MSW data the same way. MSW is very heterogeneous, a complex mixture of numerous materials when it comes to their nature, origin, composition, and chemical and physical properties. Furthermore, modifications in MSW composition may be expected, with factors such as wealth, season, or habits of consumers further complicating waste management. A possible approach to this intricate mixture is by digging step by step into the nature of MSW: first the main materials, second the specific subcategories within these materials, then the associated macromolecules, and finally the elemental composition with proximate and ultimate analyses. The main materials found in MSW may be classified as such:1 (1) paper, cardboard, and carton, (2) glass, (3) plastic, (4) metal, (5) waste electrical and electronic equipment (WEEE), (6) woody biomass, (7) organic waste, (8) yard waste, (9) textile, (10) hazardous waste, and (11) other waste. Each of these categories is itself diverse and includes substances with different properties. A MSW survey from Minnesota2 reveals the numerous subcategories present. As *To whom correspondence should be addressed. Telephone: (þ47) 73-59-29-11. Fax: (þ47) 73-59-28-89. E-mail: michael.becidan@ sintef.no. (1) Statistics Norway website. www.ssb.no. (2) Minnesota Pollution Control Agency (MPCA). Statewide MSW composition study. A study of discards in the state of Minnesota. Final report, March 2000 (available online). r 2010 American Chemical Society

(3) Becidan, M. Experimental studies on municipal solid waste and biomass pyrolysis. Doctoral Dissertation, Norwegian University of Science and Technology (NTNU), Trondheim, Norway, 2007. (4) Saeed, L. Experimental assessment of two-stage combustion of high PVC solid waste with HCl recovery. Doctoral Dissertation, Helsinki University of Technology, Helsinki, Finland, 2004.

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: DOI:10.1021/ef901144u

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Table 1. Composition of the Selected Waste Fractions and MSW Base Case on a Dry Basis glossy paper moisture (as received)

na

a

PVC pipe 0

meat

tire

75

yard waste

leather

MSW

0.62

na

13.3

25

67.06 4.81 28.13

66.04 20.37 13.59

76.5 5.25 18.2

na 26.5 na

Proximate VM (wt %) ash (wt %) fix C (wt %)

na 34.5 na

81.4b 4.2b 14.4b

na na na

C (wt %) H (wt %) O (wt %) N (wt %) S (wt %) Cl (wt %)

27.4 3.5 34.5 0.065 0.02 0.061

37.89 4.82 na na na 52.28

42.51 7.03 30.20 13.08 0.81 2.74c

84.39 7.13 2.19 0.24 1.24 0.150

41.54 4.79 31.91 0.85 0.24 þ 0.20 0.30

54.9 5.1 19.2 14.1 1.4 0.8

39.5 5 26.5 1.5 0.3 0.7

Unitsd Si Ca Al Mg Fe Cr Mn Zn Cd Pb P K Na Ti As MSW category source and comments

mg/kg na 137000 2860 1400 1080 na 172 3.97 na 1.31 37.4 139 1415 77.5 35.9 paper own analysis

Below na 0.04 wt % na na na na na 36 mg/kg na 4 wt % na na na na na plastic 4, sewage pipe

wt % na 0.055 na 0.099 0.022 na 0.0004 0.018 na na 0.782 1.6 2.03g na na organic waste 10, table salt (1 wt % on a wet basis)

wt % na 0.380 na na 0.323 0.0098 na 1.53 0.0006 0.0065 na na na na na other waste 11, waste tire

wt % 12.15e 4.84e 0.62e 0.44e 0.40e na na na na na 0.40e 0.60e 0.20e 0.07e na yard waste 12

wt % na na na na na 3.2 na na na na na na na na na textile 5, footwear shavings

Below nif ni ni ni ni 120 mg/kg ni 0.03 wt % 8 mg/kg 0.03 wt % ni 0.25 wt % 0.25 wt % ni ni MSW 3 and 13

Ultimate

a

na = not available. b PVC only. c As table salt. d Elements in bold are included in the calculations. e Oxides. f ni = not included. g 1.76 as table salt.

waste contains table salt, added either during cooking, eating, or processing; the amount considered here is 1 wt % on a wet basis. This NaCl may be involved in corrosion during combustion in waste-to-energy installations. Scrap Rubber Tire. Automobile tires are made of natural and synthetic rubber materials, which are hydrocarbon polymers. A large array of chemical substances is also added to tire rubber (pigments, reinforcing agents, processing aids, protective agents, etc.). Typical additives found at high concentrations are S and Zn (1.5 wt %). It can be foreseen that corrosion (Zn chloride forming low-temperature eutectics, similarly as Pb chloride) and maybe Zn emissions are the main challenges. Yard Waste. Yard waste is made of grass, leaves, and branches, i.e., mainly herbaceous biomass. Biomass macromolecules are cellulose (see glossy paper), hemicellulose, and lignin.3 This fraction might not be particularly problematic but is interesting to investigate because biomass is a significant part of the biogenic fraction of MSW. Leather. Leather is made from animal skin and therefore water, proteins, and lipids. Additives and processing agents are added during the tanning process. Light, thermal- and bacterial-resistant, and inexpensive leathers, especially in the footwear industry, are obtained by the Cr tanning method. This treatment method uses chromium sulfate and leads to high concentrations of Cr and S in the leather (3.2 and 1.4 wt %, respectively5). The rest is mainly protein (with 14 wt % N). The main concerns will therefore be connected to the fates of Cr and N.

It is interesting to notice that the problematic chemical elements are often due to added substances that are not present in the pure material. The objective of this study is to investigate the impact of waste composition on the behavior of alkalis, Pb, Zn, selected trace metals, Cl, and S during combustion and the resulting practical consequences on waste-to-energy facilities operation. The novel approach used to achieve this goal is the study of waste fractions one by one using thermodynamic equilibrium calculations. This approach gives an improved and more realistic picture of MSW combustion than the traditional use of equilibrium calculations, where an average MSW composition is used as input. Few experimental or computational investigations have been made using single waste fractions6,7 (see section 3.4 for further discussion), and there is therefore a need for more knowledge in this field. 2. Methodology Thermodynamic equilibrium calculations are also known as global equilibrium analysis. Elemental composition is the essential input for thermodynamic equilibrium calculations. The limitations of this method are well-known;8 however, clever and carefully designed methodologies can bring valuable information on overall stabilities and speciation trends. The original approach used here can be referred to as local conditions equilibrium analysis, where local conditions are related to the chemistry of separate fuel fractions, i.e., conditions within the complex waste (6) Pedersen, A. J.; Frandsen, F. J.; Riber, C.; Astrup, T.; Thomsen, S. N.; Lundtorp, K.; Mortensen, L. F. Energy Fuels 2009, 23, 3475–3489. (7) Poole, D.; Argent, B. B.; Sharifi, V. N.; Swithenbank, J. Fuel 2008, 87, 1318–1333. (8) Becidan, M.; Sørum, L.; Frandsen, F.; Pedersen, A. J. Fuel 2009, 88, 595–604.

(5) Bahillo, A.; Armesto, L.; Cabanillas, A.; Otero, J. Waste Manage. 2000, 24, 935–944.

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Table 2. Main Na Speciesa (Two Maximum Per Temperature Subrange) in the Waste Fractions and the MSW Base Case

Table 3. Main K Speciesa (Two Maximum Per Temperature Subrange) in the Waste Fractions and the MSW Base Case

temperature (°C)

600-800

800-1000

1000-1200

temperature (°C)

600-800

800-1000

1000-1200

glossy paper

yard waste

Na2CO3(s), Na2SO4(s) Na2SO4(s), NaCl(s) Na2SO4(s)

KCl(g), KCl(s) KCl(s), KCl(g) K2SO4(s) K2SO4(s), KCl(g)

KCl(g), KOH(g) KCl(g), KOH(g) K2SO4(s), KCl(g) KCl(g), K2SO4(s)

KOH(g), KCl(g) KCl(g), KOH(g) KCl(g), K2SO4(l) KCl(g), KOH(g)

Na2SO4(s)

NaOH(g), NaCl(g) Na2SO4(l), NaCl(g) Na2SO4(l), NaCl(g) NaCl(g), Na2SO4(l)

glossy paper meat yard waste MSW

MSW

Na2CO3(l), NaCl(g) Na2SO4(l), NaCl(g) Na2SO4(l), Na2SO4(s) Na2SO4(l), NaCl(g)

meat

a Bold font, the compound contains 50.0% or more of the specified chemical element; italic font, the compound contains between 25.0 and 49.9% of the specified chemical element; normal font, the compound contains between 1.0 and 25.0% of the specified chemical element. Compounds representing between 0.1 and 1.0% of the specified chemical element are not reported in any case.


Table 4. Main S Speciesa (Two Maximum Per Temperature Subrange) in the Waste Fractions and the MSW Base Case temperature (°C)

fuel. Local conditions do not refer to the air/fuel ratio. It is less systematic but more realistic and concrete than a parametric investigation because it is focusing on plausible or existing situations. Calculations are made using the FactSage 6.0 software package9 with a custom-selected 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 are considered in the present study. Complex organic C-H-O species are not included because they are not stable at combustion conditions. Interactions between trace elements or with alkali metals are not considered. Such interactions are unlikely because of low concentrations. In addition, other ash-forming elements, such as Ca, Fe, Mg, and Si, are not considered in the present study. The selected waste items are representing the major combustible fractions found in MSW, i.e. paper, plastic, textile, and biogenic materials (both food and biomass), but also the other waste fraction (a mixed and poorly defined fraction). The studied items can be considered problematic because of their elemental composition, possibly causing an array of challenges during incineration. The elemental and proximate analyses of the selected items are summarized in Table 1. Furthermore, calculations depicting twofraction mixtures are carried out; in these calculations, two of the selected items are burnt together, i.e., an intimate mixture is assumed, leading to extensive interactions between two items. This is aimed at evaluating the negative effects and/or synergies on operation. This study is focusing on several categories of chemical elements: (1) minor elements (S and Cl), (2) alkali (Na and K), and (3) trace elements (Pb, Zn, Cr, Cd, and As). This study is focused solely on grate incineration of MSW and the behavior of selected chemical elements (Na, K, Pb, Zn, As, Cd, Cr, Cl, and S) during combustion of one or two waste fractions and the resulting implications for operation. Calculations are implemented with 1 kg of dry material and an air excess ratio (λ) of 1.3.14 The temperature is a key parameter. Because the focus is on the grate section, temperatures of 600-1200 °C are selected for the calculations (with one calculation point every 50 °C). Because

600-800

800-1000

1000-1200

SO2(g), SO3(g)

Na2SO4(l), Na2SO4(s) Na2SO4(l), Na2SO4(s) SO2(g), SO3(g)

K2SO4(s), Na2SO4(s) SO2(g), SO3(g)

SO2(g), K2SO4(g) SO2(g), SO3(g)

Na2SO4(s), K2SO4(s)

SO2(g), Na2SO4(l)

Na2SO4(l), Na2SO4(g) Na2SO4(l), SO2(g) SO2(g), SO3(g) SO2(g), Na2SO4(l) SO2(g), SO3(g) SO2(g), SO3(g)

glossy paper

Na2SO4(s)

meat

Na2SO4(s)

tire yard waste leather MSW


the impact of fuel composition on speciation and not temperature dependency is the focus of this study, the calculations results discussed are averaged results over the selected temperature range. This approach is an efficient way of taking into account the fact that chemistry takes place over a wide range of temperatures on the grate. It is true that part of the information provided by the calculations is not used, but it is necessary to be able to present the results in a clear, understandable, and concise manner. Most importantly, it does not prevent from identifying strong underlying chemical trends.

3. Results and Discussion 3.1. General Results. The main Na, K, Cl, and S species are given in Tables 2-5 for the six waste fractions and the MSW base case. To allow for concise and clear presentation of a large amount of data, the temperature range studied is separated into three sections (600-800, 800-1000, and 1000-1200 °C) and the number of species is limited to two per temperature section. The important overall comments concerning speciation and temperature are as follows: (a) in 86% of the cases studied (fuel and temperature combinations), 50% or more of Na, K, S, and Cl is to be found in a single compound; (b) increasing temperatures lead to increasing amounts of gaseous alkali chlorides and decreasing amounts of solid alkali sulfates (none present above 1000 °C), indicating increasingly corrosive flue gas; (c) alkali carbonates are not stable above 1000 °C; and (d) no alkali hydroxides are formed under 800 °C. 3.2. Impact of Fuel Composition on Speciation. As discussed in the Methodology, when studying the impact of fuel composition on speciation, the calculated results are

(9) 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. (10) Webb, E. C.; Casey, N. H.; Simela, L. Small Ruminant Res. 2005, 60, 153–166. (11) Kim, B. Y.; Vigil, S. A. A case study of waste tire incineration at a biomass power plant. Rubber Manufacturer Association (RMA), Washington, D.C., 1999; TDF-038 (available online). (12) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel Process. Technol. 1998, 54, 17–46. (13) European Commission. Reference document on the best available techniques for waste incineration (BREF WI), Aug 2006. (14) Sørum, L.; Frandsen, F. J.; Hustad, J. E. Fuel 2003, 82, 2273– 2283.

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Table 5. Main Cl Speciesa (Two Maximum Per Temperature Subrange) in the Waste Fractions and the MSW Base Case temperature (°C)

600-800

800-1000

1000-1200

glossy paper PVC pipe meat tire yard waste leather MSW

NaCl(s), NaCl(g) HCl(g), Cl2(g) NaCl(s), KCl(s) HCl(g), ZnCl2(g) HCl(g) HCl(g) HCl(g), ZnCl2(g)

NaCl(g), KCl(g) HCl(g), Cl2(g) KCl(g), NaCl(g) HCl(g), ZnCl2(g) HCl(g), KCl(g) HCl(g) HCl(g), KCl(g)

NaCl(g), KCl(g) HCl(g), PbCl2(g) NaCl(g), KCl(g) HCl(g), ZnCl2(g) KCl(g), NaCl(g) HCl(g) NaCl(g), KCl(g)


Figure 1. (Relative) Na speciation. Average values are in the temperature range of 600-1200 °C, with one calculation point every 50 °C.

averaged over the whole temperature range. Thermodynamic equilibrium calculations are very efficient for the determination of trends and for understanding underlying chemical mechanisms rather than providing quantitative values. General Remark Concerning the Figures. For a given chemical compound, all of the phases (g, gas; l, liquid; s, solid) are presented together to ensure better readability. Calculations results including phase distribution are to be found in the Supporting Information. 3.2.1. Sodium (Na). The MSW fractions having significant Na and K concentrations are all constituents of the biogenic fraction of waste. Figure 1 presents the speciation and distribution of Na (mol %) for three waste fractions as well as the MSW base case. The values presented are average values are in the temperature range of 600-1200 °C, with one calculation point every 50 °C. To be able to compare the results from the various fractions (mainly with the MSW base case but also with one another) in a sound way, it is necessary not only to look at the absolute input values (i.e., different chemical compositions) observed in the various fractions but also to examine key ratios, which may assist in revealing more clearly underlying mechanisms. Some ratios are compiled and listed in Table 6. The MSW base case results can be summarized as follows (Figure 1): about 40 mol % of Na is to be found as gaseous chloride, while the rest is in the form of sulfates (solid and liquid phases). The main difference between yard waste and the base case is that yard waste produces twice as much Na2SO4(l), instead of forming NaCl(g). This is related to the fact that the yard waste contains much less Cl than MSW base case (Table 1), and the S/Cl ratio is also considerably lower (Table 6). In absolute terms, meat contains the highest Cl amount as well as the highest alkali amount (see Table 1). However, the distribution of Na between respectively chlorides (all phases) and sulfates (all phases) is very similar to the base case (MSW, 61/39; meat, 57/42; see Figure 1). This likeness can be explained by the fact that the (S/Cl) ratios are relatively close, i.e., both under 0.5 (see Table 6). Glossy paper has a notably different Na speciation than the other fractions, and the base case as NaOH and Na2CO3 represents 56 mol % Na, while these compounds are almost completely absent of the other investigated fractions (see Figure 1). The remaining Na is distributed almost evenly between sulfates (20 mol %) and chlorides (24 mol %).

This unique situation is clearly due to the very high (Na þ K)/(2S þ Cl) ratio (see Table 6). This ratio is an indicator of the relative availability of alkali to form other components than sulfates and chlorides; in glossy paper, excess alkali is available to form hydroxide and carbonate as the most thermodynamically stable compounds. The aforementioned results can be condensed as follows: (i) When enough S and Cl are available (i.e., (Na þ K)/(2S þ Cl) , 1), Na will almost solely form sulfates and chlorides, with the shares of each class of compounds related to the amounts of S and Cl; in this situation, K chemistry does not affect Na chemistry. (ii) When not enough S and Cl are available for Na to form only sulfate and chloride, the excess Na will form NaOH and Na2CO3. The share of hydroxide and carbonate will be related to the amount of S and Cl, with K chemistry also influencing Na speciation. These results are valid for oxidizing conditions but the situation may be significantly different at reducing conditions.15 It appears that other (possible) Na components (gaseous Na, oxides, oxy-hydroxides, etc.) are not predicted to form in noticeable amounts, i.e., are not thermodynamically stable enough at the studied conditions. An overall noteworthy result is that rather different fractions may have similar results. 3.2.2. Potassium (K). As well as with Na, only waste fractions belonging to the biogenic fraction of MSW have significant levels of alkalis. Results concerning K are summarized in Figure 2. K is almost equally distributed between gaseous chloride and solid sulfate in the MSW base case. The results exhibited by the three waste fractions are all significantly different from the base case, clearly showing the heterogeneous situations and the complex and contrasting circumstances occurring in different waste fractions. Yard waste simultaneously contains more S and less Cl than MSW, logically leading to more sulfate and less chloride. On the contrary, meat contains much more Cl and alkalis than MSW, and KCl (gas and solid) represents about 97 mol % K (see Figure 2). The third fraction investigated, glossy paper, is of particular interest because it contains high levels of alkali (in comparison to the base case) but very low concentrations of S and Cl [(Na þ K)/(2S þ Cl) ∼ 2.2]. There is not sufficient S and Cl for Na and K to react. About 63 mol % of K is forming chloride, and the remaining part constitutes KOH(g) (see Figure 2). The chemistry of K is relatively similar to Na; more than a third (molar basis) of the alkalis is found as non-sulfate (15) Becidan, M.; Sørum, L. Energy Fuels 2010, 24, 1559–1564.

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Table 6. Molar Input and Ratios for the Studied Waste Fractions glossy paper PVC pipe meat tire yard waste leather MSW a

S

Cl

K

Na

0.0063

0.0172 14.7460 0.7730 0.0423 0.0846 0.2257 0.1970

0.0036

0.0615

0.2530 0.3840 0.1370 0.4370 0.0936

Pba

Zna

Cra

S/Cl

(Na þ K)/(2S þ Cl)

(Na þ K)/2S

(Na þ K)/Cl

0.3634

2.1902

0.0002

3.782

0.3273 9.0780 1.6194 1.9362 0.4751

1.0102

0.1634

1.671

0.5340

0.0131

2.264

0.4503

0.0081

0.878

0.1930 0.4090

0.8830 0.2340

0.1270

0.0645 0.6150

0.0640

0.1090

Specified only if high concentrations.

Table 7. Gaseous Alkali Chlorides Amount (mol) in Waste Fractions, with 1 kg of Dry Fuel As Inputa

fuel fraction b

glossy paper meatb yard waste MSW base case

Cl input

NaCl

KCl

sum

percentage of Cl as gaseous alkali chlorides

0.017 0.773 0.085 0.197

0.012 0.240 0.010 0.042

0.002 0.283 0.027 0.035

0.014 0.523 0.037 0.077

82 68 44 39

Average values are in the temperature range of 600-1200 °C, with one calculation point every 50 °C. b Presence of liquid and/or solid alkali chloride also predicted. a

Meat is clearly the most problematic MSW fraction when it comes to the formation of corrosive alkali salts. Glossy paper and yard waste exhibit lower alkali chloride amounts than the MSW base case. A challenge to mention concerning food (meat) is its very high moisture content (typically 6075 wt %), which further complicates combustion. Corrosion issues will therefore be especially acute for food-rich waste, for example, for installations receiving large quantities of waste from restaurants and large kitchens. Despite the obvious mitigation effect of properly mixing the waste in the bunker using a crane, another way of reducing corrosion when burning MSW could be by sorting out food waste and pretreating them by thermal or non-thermal treatments. Because relative results are employed to understand speciation, absolute results from thermodynamic equilibrium can also be useful for the determination of trends. 3.2.4. Zn and Pb. These two trace (or sometimes minor) elements (Zn and Pb) are investigated because they are important in the promotion of Cl-corrosion mechanisms. Zn and Pb chlorides promote the formation of low-temperature eutectics and, therefore, facilitate corrosive reactions.8,16 The volatilization characteristics of Pb and Zn are markedly different in the various MSW fractions, thereby (1) exposing the varying local situations that can be encountered in separate waste fractions and (2) explaining the extreme situations that might occur in installations burning single fraction-rich MSW, as might be the case in very heterogeneous and/or poorly mixed waste. The relative speciation of Zn is presented in Figure 3. About 2/3 of Zn is found as ZnCl2(g) in the MSW base case, which is a significantly high fraction that is only surpassed by the PVC pipe results, where all Zn is found as chlorides. It is important to keep in mind that the PVC pipe is a very extreme case because it contains 10 times less Zn and 75 times more Cl than the base case (see Table 1). On the other hand, tire and glossy paper have both ZnO(s) as their main Zn compound because, in both fuels, too little Cl is available for Zn to form chlorides. It is very interesting to notice that, even

Figure 2. (Relative) K speciation. Average values are in the temperature range of 600-1200 °C, with one calculation point every 50 °C.

and non-chloride (hydroxide for K and hydroxide and carbonate for Na). However, while K solely forms chloride, Na forms both sulfate and chloride. This translates the influence of the different thermodynamic properties of the alkalis, and K appears to display a higher affinity to Cl than Na, as previously observed.8 The speciation of a chemical element is governed by its thermodynamic properties and fuel composition. Most often, a combination of both parameters will be the driver of speciation, and it will be utterly difficult to separate their effects and evaluate their relative importance. However, in some cases, one parameter dominates. For example (as discussed in section 3.2.1), when (Na þ K)/(2S þ Cl) , 1, Na and K chemistry will be overwhelmingly controlled by their respective thermodynamic properties. 3.2.3. Practical Considerations Concerning Corrosion and Alkalis. Apart from these chemical and mechanistic considerations, a fruitful manner of employing thermodynamic equilibrium is to translate the calculations into practical considerations for full-scale MSW incinerators. Presently, the focus is corrosion. A practical yet efficient way to quantify the relative corrosion potential of the gas phase evolving from the combustion chamber can be evaluated by the amount (moles) of gaseous alkali (Na and K) chlorides formed because the main corrosion mechanism is Cl-induced corrosion.8 The higher the amount of these gaseous (alkali) chlorides, the more corrosive the gas phase is considered to be. Table 7 summarizes the results for the four waste fractions containing alkalis. It is clear that increasing Cl and alkali concentrations will lead to more alkali chlorides. However, as previously shown,8 increasing Na and K concentrations will significantly affect gaseous alkali chloride formation only at high Cl levels. Furthermore, fuels with (Na þ K)/(2S þ Cl) > 1 and the lowest S/Cl (paper and meat) have a Cl conversion to gaseous alkali chlorides superior to 2:3, while fuels with (Na þ K)/(2S þ Cl) < 1 (yard waste and MSW) exhibit a Cl conversion to gaseous alkali chlorides smaller than 1:2.

(16) Bankiewicz, D.; Yrjas, P.; Hupa, M. Energy Fuels 2009, 23, 3469– 3474.

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Figure 3. (Relative) Zn speciation. Average values are in the temperature range of 600-1200 °C, with one calculation point every 50 °C.

Figure 4. (Relative) Pb speciation. Average values are in the temperature range of 600-1200 °C, with one calculation point every 50 °C.

though Zn relative speciation is quite similar in these two fractions, a tire contains almost 4000 times more Zn than glossy paper (see Table 1). It can be observed that meat has a high Cl concentration (second highest in the fractions selected), but Zn forms mostly ZnO(s) and no chloride, because almost all Cl is in the form of alkali chlorides. This can be expected from the fact that the molar ratio of (Na þ K)/(2S þ Cl) is above 1, meaning that there is excess alkali to react with Cl in the fuel, suppressing the formation of ZnCl2. This speciation is quite similar to a tire, even though its elemental composition is rather different. In summary, the speciation and distribution phase of Zn is changing drastically depending upon the MSW fractions: from over 90 mol % solid ZnO to 100 mol % gaseous ZnCl2. Furthermore, Zn speciation is apparently not always directly related to the fuel composition, as observed with meat. This versatile Zn chemistry is also observed with changing atmospheres.15 The relative speciation of Pb is presented in Figure 4. Contrary to Zn, Pb has a similar speciation in tire and MSW base case: PbO(g), PbCl(g), PbCl2(g), and PbSO4(s). The only major difference is that MSW exhibits a larger share of PbCl2(g), mostly at the expense of PbSO4(s). This difference directly reflects the differences in composition of the compounds associated with Pb, i.e., S, Cl, and O. Once again, PVC pipe has a rather extreme speciation because of the fact that Cl is the major element of this waste fraction (52.28 wt %); Pb is solely found as PbCl2(g), just like Zn is only found as ZnCl2(g). However, a noticeable fact is that Pb is found at a very high concentration (4 wt %), in excess of 100-fold more than in the MSW base case. It can therefore be said that PVC pipe may have dramatic consequences on corrosion in MSW incinerators. Glossy paper is different from the two previous fractions because little S and Cl are available to Pb, leading to the formation of PbO(g) as the major compound. One noteworthy feature common to all of the waste fractions investigated is that Pb is predicted to be overwhelmingly found in the gas phase (78-100 mol %; see Figure 4). Practically, tire and glossy paper appear to be less problematic than the MSW base case when it comes to corrosion, while PVC pipe clearly shows the possible risk encountered if Pb and Cl are able to react together; Pb will solely form PbCl2(g), which may in turn strongly enhance corrosion. 3.2.5. Selected Trace Elements (As, Cd, and Cr). These compounds do not pose corrosion risks as Na and K, conveyors of Cl, or Pb and Zn, facilitators of Cl-induced

corrosion, but have harmful effects to humans and the environment.17 They are particularly of interest when it comes to (1) (bottom and fly) ash properties, e.g., composition, toxicity, and leaching, which are central to determine the further treatment and/or use of this solid fractions, and (2) volatile emissions and subsequent release. The fractions presented in this section are the fractions where the concentrations of the trace elements are significant and/or where data are reliable. The calculations are carried out for the base case (all three elements) and glossy paper (As), tire (Cd and Cr), and leather (Cr). Even though the MSW base case contains more S and Cl than glossy paper, the speciation of As is predicted to be similar in these two fuels, with AsO(g) representing more than 90 mol % of As. Considering practical implications for real installations, As appears to be very volatile and may therefore be found in the flue gases and eventually in the environment if it is not captured by the APC system. Ca3(AsO4)2(s) is often predicted to be thermodynamically stable in combustion systems14 but is not considered in this study because Ca is not included. When included in the glossy paper calculations, almost all Ca is predicted to form CaO(s) and, hence, does not affect S, Cl, Pb, Zn, and alkali speciations. The two fractions containing Cd (tire and MSW) have very different compositions (see Table 1), with the most significant disparities being (i) a very high C content in tire, rather similar to fossil fuels actually, (ii) a much higher S content in tire compared to the base case, (iii) a much lower Cl content in tire compared to the base case, and (iv) a very high Zn content in tire compared to the base case. Despite these differences, Cd exhibits very similar speciation both qualitatively and quantitatively in the two fuels investigated. The main compounds predicted are Cd(g) and CdCl2(g), representing in both cases more than 90 mol % Cd. The main difference is that, in MSW, CdCl2(g) is about 15 mol % higher than in tire, while this 15 mol % is equally distributed between Cd(g) and CdSO4(s) in tire. This reflects the aforementioned disparities in fuel compositions, but it is interesting to note that the overall impact of very varying inputs is rather minor. As for As, the vast majority of Cd is found to be in the gas phase at the studied temperatures, and there is therefore a risk that it escapes waste-energy installations with the flue gases. Cr2O3(s) is almost the sole component in all fractions studied (tire, leather, and MSW base case), implying that (17) European Commission. Heavy metals in waste. Final report, Feb 2002.

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Figure 5. (Relative) Cl speciation. Average values are in the temperature range of 600-1200 °C, with one calculation point every 50 °C.

Figure 6. S speciation. Average values are in the temperature range of 600-1200 °C, with one calculation point every 50 °C.

the majority of Cr is to be found in the bottom ash. It is also reported15 that Cr is expected to be found as Cr2O3(s) at both mildly and very reducing conditions. It appears that Cr has a very stable behavior, almost completely unaffected by its environment; whether Cr is to be found as a trace element or at high concentrations (3.2 wt % Cr in leather) in the various fractions, as long as enough oxygen is available, Cr will form Cr2O3(s). 3.2.6. Cl Speciation. Looking at the results from another perspective may also help understand the intricate chemical processes taking place. Figure 5 presents the speciation of Cl in the six studied waste fractions, as well as in the MSW base case. Results may be categorized into three groups: (1) fractions where above 90 mol % of Cl is found as HCl(g): PVC pipe, tire, and leather, (2) fractions where no HCl(g) is formed: glossy paper and meat, and (3) fractions where about half of Cl (actually 55 mol %) is found as HCl(g): MSW base case and yard waste. HCl(g) is the main Cl compound [i.e., HCl(g) represents more than 50 mol % Cl] in four of the six waste fractions studied, as well as in the MSW base case. It is important to remember that the various fractions contain very different Cl amounts (see Table 1). The various speciations may be explained as follows: “Half HCl” Group. (1) MSW: Na, K, and Zn at typical (i.e., as minor and trace components) concentrations react with a significant part of the available Cl. (2) Yard waste: very similar to the MSW base case; differences in output reflect the differences in input values. “More than 90 mol % HCl” Group. (3) Leather: no alkali is present, and Cr does not react with Cl at all. (4) Tire: low Cl, no alkali, and Zn is reacting to a little extent with Cl. (5) PVC pipe: a very high amount of Cl but little Na, K, Pb, and Zn to react with it. However, Pb reacts fully with Cl. This is the only case where a significant amount of Cl2 is formed. Cl2 is a very corrosive compound, contrary to HCl in the flue gas, further emphasizing the specific high corrosion risk posed by PVC. “No HCl” Group. (6) Glossy paper: both alkalis and Cl concentrations are small, but still (Na þ K)/(2S þ Cl) > 2 (highest of the fractions studied), which explains why no HCl is formed. (7) Meat: as with the previous case, (Na þ K)/ (2S þ Cl) > 1. Even though, in real systems, some HCl(g) may be formed, it shows that the (Na þ K)/(2S þ Cl) ratio is an important key to evaluate Cl speciation and, therefore, corrosion. 3.2.7. S Speciation. Figure 6 presents S speciation for the various waste fractions (except the PVC pipe, where no S is

present). Similarly to Cl, three groups may be defined: (i) fractions in which more than 80 mol % of S is found as SO2(g): leather and tire, (ii) fractions in which 40-50 mol % of S is found as SO2(g): yard waste and MSW, and (iii) fractions in which (almost) no SO2(g) is formed: glossy paper and meat. These three groups can be put in parallel to the three Cl speciation groups: the waste fractions having more than 80 mol % S as SO2(g) are the same ones having above 90 mol % Cl as HCl(g); the waste fractions having about 40-50 mol % S as SO2(g) are the same ones having about 55 mol % of Cl as HCl(g); and the waste fractions having (almost) no SO2(g) are the same ones having no HCl(g). Alkalis, S, and Cl chemistries are closely related; when alkalis and/or trace elements react to a large extent with S, they do the same with Cl. Even though the reasons for this might at first appear straightforward (alkalis react with all available Cl and S when (Na þ K)/(2S þ Cl) . 1 and do not affect Cl and S speciation significantly when (Na þ K)/(2S þ Cl) , 1, these explanations cannot clarify the behavior of the intermediate group of fuels (yard waste and MSW). This shows the complex processes and intertwined effects involved in overall chemistry in a multi-element system. A further comment should be made: glossy paper and meat are rather different but are still belonging to the same Cl and S speciation category, most likely because of their similar (Na þ K)/(2S þ Cl) ratio (see Table 6). This further emphasizes the already mentioned point that fuels might be different in their composition but alike in their speciation. The corrosivity of the predicted S-containing species is expected to be rather low compared to the predicted chloride species. S-Associated corrosion is usually related to reduced S-containing species, such as S2, H2S, and metal sulfides, but these are not stable at combustion conditions with air/fuel ratios above 1. Acidic sulfates, such as NaHSO4 or Na2S2O7, can form at oxidizing conditions and are highly corrosive in a molten state. However, high SO2 and SO3 levels are required for their formation, and they decompose to more benign forms of sulfate below the temperatures that are studied here. 3.3. Comparison of Calculated Results to Experimental Data. Few experimental studies investigating the behavior of alkali, trace metals, Cl, and S during combustion, either at laboratory- or full-scale incinerators, have been carried out on MSW and/or separate waste fractions. A comparison of the calculations with the literature is further complicated by the limitations of both types of studies: calculations do not take into account physical processes (heterogeneous condensation of vapors for example), temperature or concentration 3452

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gradients, kinetics, or modes of occurrence of the chemical elements in the fuel, while experimental studies are dependent upon reliable analytical methods, good reproducibility, and the difficulty to maintain well-controlled and well-defined conditions. Trends concerning phase distribution and speciation between experimental results from the literature and the results from the present thermodynamic calculations are discussed here. 3.3.1. MSW Base Case. The quality of the calculations concerning MSW is critical because they are used as the central comparison point to assess the impact of the changing waste fraction compositions. Concerning alkali metals in MSW, Pedersen et al.6 presents a full-scale study in a Danish waste incinerator and reports that 82-89% Na and 62-71% K are to be found in the bottom ash, with the rest being in the fly ash. Furthermore, even though no detailed speciation is reported, alkali-Cl bonding is shown to happen. The present calculations predict that 39% Na and 55% K as gaseous alkali chlorides, with the rest being in the condensed phase. This seems to infer that alkali chlorides condense (homogeneously to form aerosols or heterogeneously onto fly ash) after their formation. Concerning Pb and Zn in MSW, experimental results from several studies6,18 indicate that 60-80% Pb is in the bottom ash. The results for Zn are varying from less than 50 to over 80% retained in the bottom ash, with the rest being in the fly ash. It is hypothesized that these spread results indicate that Pb and Zn volatilities are closely linked to the Cl concentration. The present calculated results lead to the more general conclusion that Pb and Zn phase distributions are very dependent upon the waste fraction; while almost 95% of Zn is predicted to be in the condensed phase (bottom ash) in meat, Zn is predicted to be completely vaporized in the PVC pipe. However, Pb volatility is overestimated by calculations in the MSW base case, maybe because of kinetics limitations. Concerning S and Cl, full-scale studies6 show that little Cl and about 20-40% S are found in the bottom ash. These results are in good accordance with our calculated results, which predict that no Cl and about 25% S (as alkali sulfates) are in the condensed phase. Concerning Cd and Cr in MSW incinerators, the experimental trends18 indicate that Cd is volatilized to a large extent, while Cr remains in the bottom ash. It is in very good accordance with the calculations because over 99% Cr is predicted to form solid Cr2O3 and Cd is vaporized as 67.1% CdCl2(g), 29.3% Cd(g), 2% CdOH(g), and 1.6% CdO(g). 3.3.2. Separate Waste Fractions. Even fewer experimental studies have studied the fate of alkalis, trace metals, S, and Cl during combustion of separate waste fractions, except for biomass. Pedersen et al.6 studies six different waste fractions separately under different operational conditions in a full-scale incinerator: NaCl (road salt), household batteries, automotive shredder waste (rubber and plastics), Cu-Cr-As (CCA)-impregnated wood, PVC, and shoes (leather mainly). The focus of the study is the partitioning of Pb, Zn, Cl, S, Na, and K because they are considered critical with respect to deposition and corrosion. It appears of special interest to compare three overall trends associated with the possible influence of feedstock chemical and physical properties: (1)

Table 8. Release of Pb and Zn to the Gas Phase during Combustion of Separate Waste Fractions chemical element

Pb

Zn

c

fuel PVC pipe PVC tire (rubber) shredder (rubber and plastic) PVC pipe PVC tire (rubber) shredder (rubber and plastic)

experimental results (%)a

calculation resultsb (%)

nac ∼67 na ∼53

100 na 0.3 na

na ∼68 na ∼55

100 na 78 na

a Average value for 500-1000 °C. b Average value for 600-1200 °C. na = not applicable.

“There is not necessarily a correlation between the input concentration of an element in the feedstock and the amounts recovered in the fly ash and flue gas fractions”. Pedersen at al.6 interprets this fact by the influence of the feedstock on the release pattern (vaporization and condensation) of chemical elements. Feedstock properties (except for elemental composition) cannot be incorporated into thermodynamic calculations, and experiments are therefore of great interest; however, the interpretation and understanding of feedstock influence are still not clear, and more experimental and modeling work is required. (2) “When firing Cl-rich waste fractions (PVC, salt, and shoes), the partitioning of Pb seemed to shift toward increased vaporization”. This behavior is also observed in the calculations presented here; the higher the input fuel Cl concentration, the higher the partitioning toward volatile Pb chlorides (Figure 4). Pb (100%) in the Cl-rich PVC pipe is forming chloride, while tire and paper are low-Cl waste fractions and exhibit a Pb conversion to chloride of 40 and 25%, respectively. (3) Partitioning may also be affected by the modes of occurrence of the chemical element. This was experimentally observed for Cl. This cannot be asserted with thermodynamic calculations. Frandsen et al.19 presents further experimental results from the same plant as Pedersen et al.6 for PVC, shoes, CCA-impregnated wood, and automobile shredder. The focus is not on speciation but on the release of given chemical elements to the gas phase. Table 8 summarizes the main comparison points. While PVC results are in good accordance (most Pb and Zn are in the gas phase), shredder residue (rubber and plastics) and tire (rubber) exhibit different behaviors, in all probability showing the interactions between rubber and plastics during combustion of the shredder residue (see section 3.4 concerning interactions between waste fractions). Concerning biomass, van Lith et al.20 have developed three separate laboratory methods to investigate the release to the gas phase of Cl, S, K, Na, Pb, and Zn during biomass thermal treatment. Experimental results from fiberboard (mainly wood) and calculations for yard waste are in very good accordance: (a) Cl experimental, 85-100% release to the gas phase from 500 °C; Cl calculations, 100% release to the gas phase at 600-1200 °C; (b) S experimental, 25-55% release from 500 °C; S calculations, 45% release to the gas (19) Frandsen, F. J.; Pedersen, A. J.; Hansen, J.; Madsen, O. H.; Lundtorp, K.; Mortensen, L. Energy Fuels 2009, 23, 3490–3496. (20) van Lith, S. C.; Alonso-Ramı´ rez, V.; Jensen, P. A.; Frandsen, F. J.; Glarborg, P. Energy Fuels 2006, 20, 964–978.

(18) Menard, Y.; Patisson, A. A.; Sessiecq, Ph.; Ablitzer, D. Trans. IChemE, Part B 2006, 84 (4), 290–296.

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Table 9. Gaseous Alkali Chloride Formation in Two-Waste Fractions and Their Mixturea

NaCl (mol) KCl (mol) sum

yard waste (1 kg dry)

leather (1 kg dry)

yard waste þ leather (2 kg dry)

mixture (2 kg dry)

difference (%)

0.010 0.027 0.037

0 0 0

0.010 0.027 0.037

0.029 0.061 0.090

þ190 þ126 þ143

Table 10. Gaseous Alkali Chloride Formation in Two-Waste Fractions and Their Mixturea

NaCl (mol) KCl (mol) sum

meat (1 kg dry)

tire (1 kg dry)

meat þ tire (2 kg dry)

mixture (2 kg dry)

difference (%)

0.240 0.283 0.523

0 0 0

0.240 0.283 0.523

0.152 0.207 0.359

-37 -27 -31

a Average values are in the temperature range of 600-1200 °C, with one calculation point every 50 °C.

Average values are in the temperature range of 600-1200 °C, with one calculation point every 50 °C. a

3.4.2. Second Mixture: 1 kg of Dry Tire and 1 kg of Dry Meat. The resulting mixture can be said to contain high levels of alkali and S and a low level of Cl. Table 10 summarizes the main results concerning gaseous alkali chlorides (i.e., corrosion). In this mixture, the inclusion of interactions between the two waste fractions appears to significantly mitigate the severity of corrosion, with an expected decrease in gaseous alkali chlorides of 31%. The decrease is mainly due to the influx of additional S from the tire fraction, which leads to partial sulfation of the chlorides for the meat-tire mixture. However, differences in formation (amount) decrease with increasing temperatures in the two cases, and produced amounts are similar from about 1100 °C for NaCl(g) and 1000 °C for KCl(g). This means that the temperature profile in the combustion zone may have a significant effect on the practical importance of eventual interactions. When it comes to trace elements, the following remarks are of interest. (i) Cd and Cr: no major changes observed. (ii) Pb: þ74 mol % PbCl2(g) is predicted in the mixture (average values in the temperature range of 600-1200 °C). This increase is blurring the results when it comes to the corrosion risk. (iii) Zn: some ZnCl2(g) is formed in the mixture, which is not the case for tire and meat. The behaviors of Zn and Pb are due to the Cl made available by the alkalis and which is only partly converted to HCl(g). The two two-fraction mixtures investigated in this section clearly show that the outcome of complex mixtures, such as MSW, is difficult to foresee but also that both deleterious consequences and synergies might take place when various waste items interact on the grate of MSW incinerators. The possibility of synergies shows a potential for improved operation of MSW incinerators: if source-sorting is practiced, carefully selected waste fractions may be mixed, before incineration, to mitigate corrosion or other operational challenges.

phase; (c) K experimental, release to the gas phase is below 20% (500-1150 °C); K calculations, 30% release; and (d) Na experimental, about 26% release (average value for 500-1150 °C); Na calculations, 20% release (average value for 600-1200 °C). Nielsen21 gives details about the speciation of K during the combustion of different biomass samples. The speciation is elucidated using mass spectrometry. Straws and grasses can be described as high-Cl, highK biomass (no Na reported), and KCl(g) is the primary alkali species. For biomass with high-alkali, low-Cl values, KOH(g) becomes the most abundant alkali vapor. For switchgrass, 15-23% K in the fuel is measured to be released as KCl(g) (it is estimated to be a lower limit). Yard waste, investigated in the calculations, can be defined as a highalkali (Na and K), high-Cl fuel. In accordance with the experimental results, KCl is the main gaseous K species and represents 21% K (about 70% of gas-phase K). 3.4. Two-Fraction Mixtures: Two Examples. Studying the behavior of selected chemical elements during the combustion of a single-waste fraction is an efficient way of approaching the influence of MSW heterogeneity as well as describing the diverse situations that might occur on a MSW grate incinerator. However, it is certain that interactions occur between diverse waste fractions, even though their extent is difficult to evaluate and vary with different fractions. It is especially difficult to establish clear relationships between furnace behavior factors and the fuel mixture. Nonlinear relationships are common.22 Furthermore, it is possible to foresee that some of these interactions may have deleterious effects on an installation operation but also that synergies may also appear and thereby reduce operational challenges. Two cases comprising two waste fractions together are investigated to illustrate the possible negative but also positive consequences. However, the overall picture of a given case may be blurred by having simultaneously positive and negative outcomes. The focuses are on alkali compounds and speciation. 3.4.1. First Mixture: 1 kg of Dry Leather and 1 kg of Dry Yard Waste. The resulting mixture can be described as having an average level of Cl, average levels of alkali, and a high level of Cr. Table 9 summarizes the results for gaseous alkali chloride formation; a very significant increase in gaseous alkali chlorides (þ143%), i.e., in corrosion risk, is observed when an intimate mixture and interactions between the two waste fractions are taken into account. The reason for this is that the excess Cl in the leather can react with alkali compounds from the yard waste fraction to form additional alkali chloride. No change in Cr speciation is predicted because only Cr2O3(s) is formed in the mixture.

4. Conclusion The behavior of selected chemical elements (Na, K, Pb, Zn, As, Cd, Cr, Cl, and S) during combustion of six different waste fractions (one by one or two at a time) is investigated by thermodynamic analysis. The fractions are studied one by one but also two at a time to assess the impact of fuel quality. The results pertaining to single-waste fractions show that (i) the concentration and thermodynamic properties of a given chemical element impact its speciation differently in different waste fractions, (ii) the chemistry of ash and trace elements in different fuels can be similar regarding behavior and speciation, (iii) the study of ratios related to input parameters lead to a better understanding of chemistry, (iv) some chemical elements have a versatile chemistry, i.e., shifting widely with fuel composition, such as Zn, while other do not (Cr),

(21) Nielsen, H. P. Deposition and high-temperature corrosion in biomass-fired boilers. Doctoral Dissertation, Technical University of Denmark, Lyngby, Denmark, 1998. (22) Hupa, M. Fuel 2005, 84, 1312–1319.

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therefore complicating waste management as to successfully optimize the combustion process, and (v) both S and Cl chemistries may be divided into three groups: one where almost all S and Cl are to form SO2 and HCl, one where about half of S and Cl are to form SO2 and HCl (the rest being alkali and traceelement compounds), and finally, one where no SO2 and HCl are produced. The linkage observed between S and Cl chemistries is interesting; however, it appears that the explanation is not straightforward because it is most likely involving complex interactions between several chemical elements. The results concerning a two-waste fraction mixture show that (i) taking into account interactions between two-waste fractions during incineration may in fact lead to negative consequences, for example, higher corrosion and trace-element emissions, but also opposite effects, i.e., synergies mitigating operational challenges, and (ii) positive and negative results may occur simultaneously in such mixtures, therefore blurring the results.

A comparison of the results from experimental and thermodynamic studies show good agreement. The current study provides a novel look and better understanding of the behavior of Na, K, Pb, Zn, As, Cd, Cr, S, and Cl during combustion of separate waste fractions, but more experimental laboratoryand pilot-scale studies in well-controlled test conditions are needed to fully understand the mechanisms at work. Acknowledgment. This work has been part of the NextGenBioWaste project, co-funded by the European Commission under the Sixth Framework Programme. The authors also thank the Bioenergy Innovation Centre (CenBio) co-funded by the Research Council of Norway. Supporting Information Available: Custom-made database and calculation results including phase distribution and minor compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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