Volatility and chemistry of trace elements in a coal combustor

Fuel 80 (2001) 2217±2226

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Volatility and chemistry of trace elements in a coal combustor Rong Yan, Daniel Gauthier*, Gilles Flamant Institut de Science et de GeÂnie des MateÂriaux et ProceÂdeÂs, IMP-CNRS, BP 5 Odeillo, F-66125 Font-Romeu, France Received 29 September 2000; accepted 21 February 2001

Abstract The volatility of 16 trace elements (TEs) (As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Se, Sn, Te, Tl, V, Zn) during coal combustion has been studied depending on the combustion conditions (reducing or oxidizing) and type of coal (high- or low-ash coal), together with their af®nities for several active gaseous atoms: Cl, F, H, O, and S. The elements can be divided into three groups according to their tendencies to appear either in the ¯ue gases or in the ¯y ashes from a coal combustor: Group 1: Hg and Tl, which are volatile and emitted almost totally in the vapor phase. Group 2: As, Cd, Cu, Pb and Zn, which are vaporized at intermediate temperature and are emitted mostly in ¯y ashes. Group 3: Co, Cr, Mn and V, which are hardly vaporized and so are equally distributed between bottom ashes and ¯y ashes. In addition, Sb, Sn, Se and Te may be located between Groups 1 and 2, and Ni between 2 and 3. At 400 and 1200 K, the 16 TEs behave differently in competitive reactions with Cl, F, H, O and S in a coal combustor. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Trace elements; Volatilization; Chemical af®nity; Thermodynamics; Coal combustion

1. Introduction The trace elements (TEs) which are released during coal combustion may cause an environmental and human health risk. Extensive studies have been developed with respect to their abundance, their physico-chemical form, their toxicity and their partitioning behavior in the combustion/environmental control systems. Many papers [1±9] show that in a combustion process, the speciation of a TE and its content in the ¯ue gas or in the ash varies a lot depending on the coal composition and the combustion conditions. It is thus of great interest to study their behavior in the various processes involved in combustion in order to understand their fate, especially in ¯ue gases. During coal combustion, several parameters related to the TE in¯uence their transformation, they are as follows: their volatilization tendency, the kinetics of their release from particles, possible interactions, their competitive af®nity to active atoms (like halogens, H, O and S), etc. Almost each TE has its own particular behavior in these processes. * Corresponding author. Tel.: 133-4-68-30-77-57; fax: 133-4-68-30-2940. E-mail address: [email protected] (D. Gauthier).

Therefore, in a multicomponent and multiphase combustion system involving many TEs, their chemical transformations are complex. It is especially true when considering all coexisting coal major (C, H, O, N, S) and minor elements (Si, Al, Ca, Mg, K, Na, Fe, Mn, Ti, P, and halogens), which are expected to have a great in¯uence on the TE behavior. Tendencies related to the TE behavior can hopefully be drawn from thermodynamic calculations, although great dif®culties exist in dealing with such a complex system. Thermodynamic analysis [1,10±17], using the principle of `minimizing the total Gibbs free energy of the system', is a powerful tool to predict the elemental chemistry in a multicomponent and multiphase system, with respect to the elements interactions, dominant species and competitive af®nity. Although the kinetics and mass transfer limitations associated with metal/particle reactions cannot be explained by a thermodynamic equilibrium analysis, this method can provide useful information on the most favored and stable product distributions at various temperature ranges [14]. In this study, one of the most complex thermodynamic systems considered so far concerning coal combustion is computed by gemini software [18] and involves 54 elements and 3200 species. Among these 54 elements, 16 TEs (As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Se, Sn, Te, Tl,

0016-2361/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0016- 236 1( 01) 00105- 3

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Table 1 Composition (wt%) of the low-ash coal. (Original analyses were given in dry basis; they were transferred into wet basis for simplifying thermodynamic calculations)

V, Zn) are considered, which are all limited by the French Draft Regulation. The system considered here differs from previous ones since it considers simultaneously all possible interactions. In a complex system where many elements co-exist, the TE behaviors are greatly in¯uenced not only by their interactions, but also by their competitive af®nities for the active gaseous atoms (i.e. Cl, F, H, O, S) in the system. This in¯uences the formation of stable TE oxides, sul®des, hydrides and halides; it may also explain the existence of

some particular dominant species under given conditions, especially the differences found when solving a complex thermodynamic system or a simple one. Moreover, the TE volatilization properties will in¯uence their behavior during coal combustion; volatile elements may leave the combustion system totally (or partly) in the vapor phase, causing a serious environmental harm; other hardly vaporized elements tend to distribute in bottom ashes and/or ¯y ashes. The various distributions of the 16 TEs in gas, ¯y ash and bottom ash can be predicted by comparing their

Table 2 Composition (wt%) of the high-ash coal. (Original analyses were given in dry basis; they were transferred into wet basis for simplifying thermodynamic calculations)

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Table 3 Initial mole number of elements in the thermodynamic calculations

volatilization tendencies. Moreover, the af®nities to several active atoms will be a useful tool for simulating the combustion and the condensation zones. 2. Equilibrium calculation methodology The computation method and the system considered have been described before [19], including the software (gemini) and database (coach), the system (54 elements and about 3200 species), etc. The method for determination of the air excess number (l ) was described previously when dealing with simple thermodynamic systems [20,21]. l values for combustion under reducing and oxidizing atmospheres are set, respectively, to 0.6 and 1.2 in this work. For the study of the volatilization tendency, two coals (low-ash coal and high-ash coal) are considered under oxidizing and reducing conditions. Tables 1 and 2 list the coal composition and the 16 TE contents in the low-ash coal and in the high-ash coal, respectively, whereas the initial total mole numbers of the 54 elements in the respective

systems are listed in Table 3. In all cases, the temperature ranges from 300 to 1800 K and the total pressure is 1 atm. However, only low-ash coal has been assessed in the af®nity study. A simpler system, which contains all TEs, the major elements of coal but mineral elements, plus all halogens, was considered. The important difference with previous systems is that all molar numbers for the 16 TEs are set to 0.001 mol (that is to say, 5:5 £ 1029 mol fraction under oxidizing conditions and 9:7 £ 1029 mol fraction under reducing conditions) in order to compare their af®nities to the active gaseous atoms. Other elements are in the same content as they were in the complex system, as described elsewhere [19]. Two temperatures (400 and 1200 K) are considered in order to simulate, respectively, the cooled ¯ue gases and the combustion zone situation. Both oxidizing and reducing conditions are considered. It should be kept in mind that the tendencies obtained from thermodynamic calculations are theoretical. In the real situation, TEs exist in the coal in compounds, which affect their volatilization signi®cantly. Those associated

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Fig. 1. TE volatilization percentages during low-ash coal combustion under oxidizing conditions.

with the organic and mineral sul®de fractions are volatilized and easily adsorbed on the ®ne particles. Very volatile ones (like Hg, Se, As) even release directly in gaseous forms. The less volatile species do not volatilize appreciably during combustion and are of little environmental concern. 3. Results and discussion The 16 TE behaviors are described and compared in this section, with respect to their volatilization tendencies versus temperature, and their af®nities to Cl, F, H, O and S versus the increase of these active atoms in the system. Several changing tendencies concerning the various behaviors of the 16 TEs were obtained, the results are discussed hereafter and compared with previous ones. 3.1. TE volatilization trends during coal combustion The 16 TE volatilization percentages at given temperatures are calculated (gemini code) concerning the complex systems by the following equation, in which Mg represents the total gaseous molar number for a speci®c TE including all its gaseous species at given temperature: TE volatilization percentage at given temperature ˆ 100 £

TE gaseous molar number …Mg † TE total molar number …Mt †

…1†

Their volatilization percentages are studied during coal combustion, depending on both the combustion conditions and the coal type. The results are displayed in four ®gures corresponding respectively to low-ash coal and oxidizing conditions (Fig. 1), low-ash coal and reducing conditions (Fig. 2), high-ash coal and oxidizing conditions (Fig. 3), and high-ash coal and reducing conditions (Fig. 4). HAC

and LAC are used hereafter to indicate the high-ash coal and low-ash coal, respectively. Fig. 1 shows the volatilization trends of the 16 elements considered during LAC combustion and under oxidizing conditions. They are in the order: (Hg, Sn) …below 300 K† . …Se; Tl† …400±600 K† . Te …600±800 K† . …Cd; As† …800±1000 K† . …Sb; Pb†…900± 1100 K† . Cu …1000±1300 K† . …Ni; Zn† …1200±1500 K† . …some Cr; V† …1300±1600 K† . most Cr …1600± 1800 K† . Co …1700±1800 K† . Mn (above 1800 K). Notice in Fig. 1 that Sn is vaporized at quite a low temperature (300 K), whereas its volatilization percentage decreases between 1100 and 1200 K, due to solid SnO2 formation in this temperature range. Similar situations are found concerning Pb at low temperature (450±500 K) and Cr at high temperature (1600 K). Similarly, Fig. 2 presents the volatilization tendencies for LAC under reducing conditions; the TE volatilization order is: (Sb, Te) …below 300 K† . Hg …300±350 K† . Tl …400± 500 K† . Se …400±700 K† . Cd …600±700 K† . Sn …600±800 K† . Pb …700±900 K† . Zn …800±1000 K† . As …800±1100 K† . …Cu; Ni† …1100±1500 K† . Co …1300±1600 K† . …Mn; V† …1500±1800 K† . Cr (above 1800 K). The TE volatilization tendencies concerning HAC under oxidizing conditions are plotted in Fig. 3, and their order is listed as below: Hg …below 300 K† . …some Te; Sb; Tl† …300±500 K† . …Cd; Se; most Te† …550±800 K† . …Pb; Sn; half As† …700± 900 K† . …Zn; half As† …900±1200 K† . …Cu; Ni† …1200±

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Fig. 2. TE volatilization percentages during low-ash coal combustion under reducing conditions.

1500 K† . Co …1400±1700 K† . V …1500±1800 K† . Mn …1500±above 1800 K† . Cr (above 1800 K). Reducing conditions for HAC are illustrated in Fig. 4, and the TE volatilization trend is (Hg, Sb, some Te) …below 300 K† . Tl …400±550 K† . …Cd; Se; most Te† …600±800 K† . …Pb; Sn; Zn† …700± 1000 K† . As …1000±1300 K† . Cu …1200±1500 K† . …some Ni; Co; Mn; most Ni† …1400±1700 K† . V …1600± above 1800 K† . Cr (above 1800 K). Several general conclusions can be drawn: ² Reducing conditions do not always enhance the TE volatilization depending on both the TE and type of coal.

Previous studies [21] showed that reducing conditions increase the volatilization of mineral matters in coal. However, in a more complex analysis, different behaviors are found: 1. Reducing conditions enhance the volatilization of the following elements: Mn, Sb and Zn whatever the coal type, but Cd, Co, Pb, Te and Tl in the low-ash coal case only. 2. On the contrary, reducing conditions reduce the volatilization of the following elements: As, Sn and V whatever the coal type; Cr, Cu, Hg and Se in the low-ash coal case only; Ni in the high-ash coal case only. ² TE volatilization trends during coal combustion depend greatly on the coal considered.

Fig. 3. TE volatilization percentages during high-ash coal combustion under oxidizing conditions.

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Fig. 4. TE volatilization percentages during high-ash coal combustion under reducing conditions.

1. Under oxidizing conditions, As, Cr, Cu, Se, Sn and V in the LAC case vaporize completely at relatively lower temperatures than they do in the HAC case; whereas Cd, Co, Mn, Pb, Sb, Te and Zn behave contrarily. Only three elements (Hg, Ni, Tl) are hardly affected by the type of coal. 2. Under reducing conditions, As, Cd, Co, Cu, Ni, Se, Sn, Te and V contained in low-ash coal vaporize completely at relatively lower temperatures than they do in high-ash coal; whereas Hg and Mn behave contrarily. A few elements (Cr, Pb, Tl, Zn) are not affected by the type of coal. So, this study shows that As, Cu, Se, Sn and V vaporize more easily when they exist in low-ash coal than when they are in high-ash coal, whatever the conditions of combustion, whereas Mn behaves contrarily and Tl volatilization does not depend on the type of coal. Thermodynamic calculations predict: 1. On the one hand, several TEs (Co, Cr, Mn, V) do not volatilize, even at temperature higher than 12008C and whatever the type of coal and combustion conditions. They will most probably remain in the bottom ash. 2. On the other hand, other TEs volatilize more or less completely below 12008C. Some of them remain vaporized and are emitted with exhaust gases, others are condensed onto ¯y ash. 3.2. TE af®nities to chemically active atoms The TE af®nities to several active gas-contained atoms (Cl, F, H, S, and O) were investigated in this complex system, under both oxidizing and reducing conditions, and at 1200 and 400 K to simulate both the zones of combustion and condensation. Several general tendencies were found;

they are detailed in Ref. [22], and the discussion hereafter is focused on the results at 1200 K. Two ®gures (Figs. 5 and 6) illustrate the results; they correspond to, respectively, the TEs af®nity to Cl under oxidizing conditions, and their af®nity to S under reducing conditions. 3.2.1. TE af®nity to chlorine Chlorine molar number varies between 0.063 and 3.15 mol, corresponding to Cl content in coal ranging from 0.001 to 0.05 wt%. Due to the varying air excess number, these ®gures correspond to a chlorine mole fraction ranging from 3:47 £ 1027 to 1:73 £ 1025 under oxidizing conditions, and from 6:1 £ 1027 to 3:05 £ 1025 under reducing conditions. Oxidizing conditions: The molar numbers of the dominant TE chlorides at 1200 K under oxidizing conditions are plotted in Fig. 5 versus the mole fraction of chlorine in the system. Chlorides can be divided into two groups according to their content levels: on the one hand Cu, Sn, Tl, and Zn chlorides which are the dominant forms (.80% mol/mol), especially when the chlorine content increases. On the other hand Cd, Co, Mn, Ni, Pb, Sb chlorides which represent less than 30% of the TE total number (0.001 mol). In this case, the af®nity order to Cl is: Cu . Tl . …Sn; Zn† . …Pb; Cd; Ni; Co; Sb; Mn† . other TEs if focusing on the chlorine average content in coal: about 0.01±0.02 ppmw, i.e. 0.63±1.26 mol of chlorine in the system (mole fraction 3:47 £ 1026 ±6:94 £ 1026 ). Reducing conditions: The TE af®nities to Cl change between oxidizing and reducing conditions. In the latter case, the af®nity order to Cl is: Ni . Tl . Cu . Mn . …Co; Ni; Pb; Sn† . other TEs. Usually, chlorine has a signi®cant impact on the TE

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Fig. 5. Af®nities of the TEs to chlorine under oxidizing conditions at 1200 K (all TEs 0.001 mol, i.e. 5:5 £ 1029 mol fraction).

behavior, through the formation of volatile TE chlorides. Wey et al. [8] studied the Cr, Pb and Cd behavior during waste incineration in a ¯uidized bed under various chlorine additives. They found organic chlorine (PVC) induces more TE emission than inorganic chlorine (NaCl), and the proportion of the three TEs in the ¯y ash increases by 5±15% in the presence of organic chlorine. Several thermodynamic studies [10,13,20] investigated the effects of the chlorine presence and content on TE speciations. Among them, Wu and Biswas [13] compared the relative af®nity of six TEs (As, Cd, Cr, Hg, Pb, Sn) to chlorine in the all metals± O2 ±Cl2 system and all metals±CH4 ±O2 ±Cl2 system, respectively, at 1100 and 1500 K. Their results are mostly opposite to ours, due to the signi®cant difference between the ther-

modynamic systems considered. Nevertheless, it is found in both studies that Pb has a stronger af®nity to Cl than Cd. 3.2.2. TE af®nity to ¯uorine Fluorine molar number is varied in the simple system between 0.1179 and 5.895 mol, corresponding to F content in coal ranging from 0.002 to 0.1 wt%. Because of the varying air excess number, these ®gures correspond to a ¯uorine mole fraction ranging from 6:49 £ 1027 to 3:24 £ 1025 under oxidizing conditions, and from 1:14 £ 1026 to 5:71 £ 1025 under reducing conditions. Oxidizing conditions: At 1200 K, TlF(g) is the only major ¯uoride, other TEs ¯uorides are negligible. Reducing conditions: The situation is almost the same as

Fig. 6. Af®nities of the TEs to sulfur under reducing conditions at 1200 K (all TEs 0.001 mol, i.e. 9:7 £ 1029 mol fraction).

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under oxidizing conditions: TlF(g) is the only signi®cant species, although its amount is less. Normally, TE ¯uorides rarely exist in coal combustion systems since HF is the strongest acid. Very few reports (for example, Ref. [10]) dealing with ¯uorine effects on the TE speciation can be found in the literature. 3.2.3. TE af®nity to hydrogen Oxidizing conditions: Hydrogen molar number in this case varies between 1128.312 and 56415.6 mol, corresponding to H content in coal ranging from about 0.288 to 22.43 wt%, and hydrogen mole fraction ranging from 6:21 £ 1023 to 0.311. When the total H molar number is less than 20 000 (corresponding to H content in coal about 5%, and mole fraction less than 0.11), besides a very few gaseous PbH and NiH, the only signi®cant H-combined compound is Ni(OH)2(g). Then, when the total H molar number is increased, Cu, Se and Te hydrides are present signi®cantly and they are in the following af®nity order to H: Se . Cu . Te: Reducing conditions: Hydrogen molar number varies in this case between 1049.9 and 52496 mol, corresponding to H content in coal varying from about 0.29 to 22.43 wt%, and hydrogen mole fraction ranging from 1:02 £ 1022 to 0.509. More H-combined species appear than under oxidizing conditions, among which three are signi®cant: H2Se(g), CuH(g) and H2Te(g). In this case the af®nity order to hydrogen is: Se . Cu . Te . …Ni; Pb; As†: The in¯uence of hydrogen on the TE behavior was discussed previously [13,20], with respect to its signi®cant impact on the formation of TE chloride, since hydrogen picks up most of the chlorine in the system to form HCl, thus altering strongly the TEs speciations. Moreover, Wey and Fang [23] indicated that the emitted HCl changes significantly with the source of hydrogen (e.g. H2O, sawdust, polyethylene) and the source of chlorine (PVC and NaCl). However, the direct in¯uence of the hydrogen presence to TE speciations, through the formation of hydrated salts and/ or hydrides, was rarely studied before. 3.2.4. TE af®nity to sulfur The TE af®nity to sulfur was studied previously [21], but in the case of a simpler chemical system than in the present study. The molar number of sulfur is varied in the system between 3.74 and 186.9 mol, corresponding to S content in coal ranging from 0.053 to 2.67 wt%. Because of the varying air excess number, these ®gures correspond to a mole fraction of sulfur ranging from 2:05 £ 1025 to 1:03 £ 1023 under oxidizing conditions, and from 3:62 £ 1025 to 1:81 £ 1023 under reducing conditions. Oxidizing conditions: At 1200 K, the only signi®cant Scontaining species is SbS(g) whose molar number increases gradually to 0.001 mol (i.e. mole fraction increases to 5:5 £ 1029 ) as S increases, whereas at low temperature sulfates are dominant.

Reducing conditions: Fig. 6 shows the TE af®nities to sulfur under reducing conditions. Many metallic sulfur species exist in signi®cant amounts, they are all TE sul®des. Notice the temperature has no effect on this phenomenon. All sul®des are gaseous at 1200 K, except for Mn and Cu whose sul®des are in condensed phases. The order of af®nity to sulfur is in this case: Sb . Sn . Mn . Pb . Cu . Se @ …As; Co; Cu; Ni†: Linak and Wendt [1] reported an equilibrium calculation on the condensed TE species as a function of temperature, depending on the presence of sulfur. If sulfur is removed, TEs are more likely to form chlorides, which are generally more volatile, especially in Cr, Cd and Pb cases. When considering the TE af®nities to Cl, S, O or H in a simple thermodynamic system, the possible interactions between these major elements and their speci®c concentrations will strongly affect the TE speciation. Missing out a species which contains these major elements will induce calculation errors; for example, Wu and Biswas [13] noticed that the inclusion of hydrocarbon fuel in the analysis is essential to evaluate the TE af®nities to chlorine, because HCl is then predominant and minimizes chlorine impact. In our complex system, because almost all coal elements and their possible interactions are considered, there should be less risk of such an incorrect evaluation of the TE competitive reactions for the co-existing active atoms. 3.2.5. TE af®nity to oxygen The amount of oxygen in the thermodynamic system (equal to that in the coal plus that in the combustion air) affects greatly the TE behavior during coal combustion. As shown before [22], many O-combined species exist especially for high oxygen concentrations. Most compounds are oxides (for Co, Cu, Hg, Mn, Pb, Sb, Sn, Se, Te, V), except for some Sn as Sn±Cl-oxides, for most Ni as Ni(OH)2(g) and for Zn as chromate. 4. Summary and conclusion Figs. 7±9 summarize the calculation results concerning a complex thermodynamic system containing 54 elements and 3200 species, with respect to the TE volatilization tendencies, and their af®nities to Cl, F, H, O, S. Results at low temperature (400 K) describe the TE behavior in the condensing zones, whereas results at high temperature (1200 K) describe the volatilization tendencies. 4.1. Volatilization tendencies of the trace elements in a coal combustor The TE volatilization trends during coal combustion vary depending on both the combustion conditions and type of coal. However, the 16 TEs can be roughly classi®ed into three groups according to their volatilization tendencies, thus to their possible distribution in the

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Fig. 7. Volatilization trends of the considered TEs during coal combustion.

¯ue gases or ashes (either bottom ash or ¯y ash) after coal combustion. Group 1: Hg and Tl are volatile below 600 K whatever the coal and combustion conditions; hence they will be emitted almost totally in the vapor phase. Group 2: As, Cd, Cu, Pb and Zn are vaporized at intermediate temperature (600±1400 K) in all cases, so they will be most possibly enriched in submicron particles and emitted mostly in ¯y ash. Group 3: Co, Cr, Mn and V are hardly vaporized as long as the temperature is less than 1400 K. Generally, submicron particles are not enriched in these

elements and they will be most possibly distributed evenly between bottom ash and ¯y ash. Between Groups 1 and 2, there are Sb, Se, Sn and Te which may behave either as Group 1 or as Group 2 metals, depending on the conditions (combustion conditions and type of coal). Between Groups 2 and 3, there is Ni which behaves sometimes as a Group 2 metal and sometimes as a Group 3 metal, depending on the conditions. Rata®a-Brown [6] classi®ed the TEs on the basis of their volatility behavior and their corresponding emissions, and correlated qualitatively the TE class with their boiling points.

Fig. 8. Af®nities of the 16 TEs to Cl, F, H, O and S during coal combustion under oxidizing conditions, at 400 and 1200 K, respectively.

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Fig. 9. Af®nities of the 16 TEs to Cl, F, H, O and S during coal combustion under reducing conditions, at 400 and 1200 K, respectively.

He grouped As, Cd, Hg, Mn, Ni, Pb, Se, and Zn as we do in this work, whereas there is a slight difference concerning Co, Cr, Cu, Sb, Sn, Te and V. For example, Co, Cr and V are located between Groups 2 and 3 in Rata®a-Brown's work, whereas they belong to Group 3 according to our results. There is however a signi®cant difference with regards to Tl: in our work, it is vaporized completely below 600 K whatever the combustion conditions and the type of coal, whereas it is located between Groups 2 and 3 according to Rata®a-Brown. 4.2. Af®nities of the TEs to chemically active atoms in a coal combustor The general trends about the TE af®nities to Cl, F, H, O and S considered separately in a coal combustor were deduced from the thermodynamic study, depending on both the combustion conditions (oxidizing or reducing) and temperature (400 or 1200 K). They can be a useful tool for explaining some competitive reactions between the 16 TEs and the active atoms, and the stable existence of several TE species in the complex systems under given conditions. Compounds involving TEs and O are the most abundant TE-containing compounds, whereas TE±¯uorine combinations are always rather rare in the coal combustion system. TE sulfates exist only at low temperature, whereas sul®des exist at high temperature. Reducing conditions promote TE sul®des formation. Finally, TE chlorides are more abundant at high temperature than at low temperature since they are obviously favored in their gaseous forms.

Acknowledgements This work has been supported by European Community of Steel and Coal (ECSC) through a subcontract between

CERCHAR and CNRS. The authors are grateful to Dr B. Cheynet (THERMODATA) for help with gemini software. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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