An equilibrium analysis to determine the speciation of metals in an incinerator

C O M B U S T I O N A N D F L A M E 93:31-40 (1993)

31

An Equilibrium Analysis to Determine the Speciation of Metals in an Incinerator CHANG Y. WU and PRATIM BISWAS* Aerosol Research Laboratory, Department of Civil and Environmental Engineering, Universityof Cincinnati, Cincinnati, OH 45221-0071 An equilibrium analysis is carried out to determine the speciation of metals among their various forms in an incinerator. The Gibbs free energy of the system is minimized by the method of element chemical potentials combined with atom population constraints using a thermodynamic equilibrium computer code. Ninety-one compounds of six metallic species--arsenic, cadmium, chromium, lead, mercury, and t i n - - a r e analyzed in this study. The effect of temperature and chlorine content on the speciation is established. The effect of combustion of hydrocarbon fuels on the equilibrium calculations is also evaluated.

* To whom correspondence should be addressed.

ature history, chlorine content, residence time, and cooling rates downstream. Pilot scale studies by Waterland et al. [8] support the findings of other researchers [6, 7]. Thermodynamic equilibrium approaches have been used by Lee [9] to predict the form of the metallic species in the combustion chamber. The calculations were done with a limited set of compounds and the phase of metallic species was used to infer the degree of volatility. Fernandez et al. [10] examined the behavior of heavy metals in the combustion gases of urban waste incinerators and categorized them into three stability classes based on a thermodynamic analysis. Rizeq et al. [11] have also carried out similar thermodynamic equilibrium calculations and inferred the partitioning of metals among the various phases. An equilibrium analysis cannot determine the partitioning because the solid phase matter may also be in the gas stream in the form of a submicron aerosol. However, in a system of multiple metals, the analysis may identify the dominant forms of the metallic and radical species, which can provide useful information in the development of more accurate kinetic models [6]. In this article, a thermodynamic analysis is carried out to determine the speciation of a metal among its various forms. The major forms are identified under different operating conditions and the relative affinities of the metal to chlorine are established.

Copyright © 1993 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc.

0010-2180/93/$6.00

INTRODUCTION Incineration is an effective waste management option for destruction and volume reduction of waste materials. Because of the energy recovery options offered by incineration, and the increasing cost and decreasing availability of landfilling, incineration is expected to become a popular and viable waste management technique. It is important, however, that incinerators be well designed and operated to minimize the risk associated with emissions [1]. Of concern are emissions of toxic metal compounds from the incineration of hazardous wastes, municipal wastes, and industrial sludges [1, 2]. Several studies conducted on metals emissions from incinerators indicate an enrichment of the volatile metal species in the submicrometer-sized particles [3, 4]. Controlled laboratory scale studies on metallic aerosol formation in high-temperature systems have identified some of the mechanisms of particle formation and growth [5, 6]. The ultimate size distribution of the aerosols formed depends on the properties of the various metallic species, the relative rates of reaction and the rates of nucleation and condensation [6]. Barton et al. [7] have indicated that the partitioning of metals in incinerators is dependent on the temper-

32 EQUILIBRIUM CALCULATION METHODOLOGY The method of element potentials combined with the atom population constraints is used to minimize the Gibbs energy of the system. The computer code, STANJAN [12], was used to implement the calculations and obtain the equilibrium composition at different temperatures. The thermodynamic data for the various species were obtained from the literature [13-18]. Six metals were analyzed in this study --arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), lead (Pb), and tin (Sn). These are generally observed in wastes that are incinerated and are among a list of toxic metals established by the EPA [1]. The different species that were included in the analysis are listed in Table 1 along with the source of their thermodynamic data. A computation methodology to determine thermodynamic data for gas-phase chromium hydroxide species has been provided by Ebbinghaus [16], and these data were also used in our simulation. Table 2 lists the different conditions at which the simulations were performed. The initial calculations were done for a single metal-oxygen and a single metal-chlorine system followed by computations for a single metal-oxygen-chlorine system. These calculations were carried out to establish the major species for the particular metal under consideration. The ratio of metal to oxygen was selected for typical incineration conditions [9]. Chlorine has been identified to play an important role in metals emissions [7, 8], and hence its concentration was varied to establish trends. The temperature range selected was that observed in a typical rotary kiln incinerator system [19]. The final calculation was done to simulate an incinerator using methane as a fuel combusting in 100% excess air (rather than oxygen). As discussed later, this is extremely important in establishing trends as the hydrogen also competes for the chlorine in conjunction with the metals. RESULTS AND DISCUSSION The simulations were carried out for the various conditions listed in Table 2 and are discussed by category.

C.Y. WU AND P. BISWAS

Single Metal-Oxygen System The results of this analysis are summarized and presented in Fig. 1. For all metals other than Hg and Cd, the oxides are the dominant species. The oxides of cadmium and mercury, both Group IIB elements, are unstable at elevated temperatures (500 K for Hg and 900 K for Cd) and the elemental forms are the major species. At higher temperatures, the oxides tend to be in the gas phase. PbOcg ) is the major lead species at the higher temperatures ( > 1000 K). The polymers of lead oxide, (PbO) n, are present at the higher temperatures but account for less than 12% of the lead and are not shown in Fig. 1 in order to maintain clarity.

Single Metal-Chlorine System The results are summarized in Fig. 2. Other than cadmium, all the metals have a strong affinity for chlorine and the chlorides are the major species. With the exception of chromium, these chlorides are typically in the vapor phase. Cadmium chloride is the major species of cadmium at temperatures below 1200 K, above which elemental cadmium is the dominant species.

Single Metal-Chlorine-Oxygen System The above analysis (Fig. 2) yields information on trends and identifies the major species. However, oxygen is prevalent in all incinerators and the calculations are therefore carded out with the metal in the presence of both chlorine and oxygen. The metal may preferentially form either the oxide or chloride and the results are summarized in Fig. 3. At higher, temperatures, the chlorides of As, Cr(oxychloride), Pb, and Sn are the dominant species. Contrary to experimental observations [20], lead tetrachloride is the predicted dominant chloride species indicating that kinetic limitations may be important. At temperatures lower than 800 K, SnO 2 is the dominant species of tin. Cadmium chloride is the major species at temperatures lower than 1000 K, with the elemental form being dominant at higher temperatures. Mercury exhibits a similar behavior, the

SPECIATION OF METALS IN AN INCINERATOR

33

TABLE 1 Different Species of the Six Metals Used in the Thermodynamic Analyses

Ref.

As

Cd

[13]

Cr Cr(g) Cr(t) Cr(~) CrO(g) Cr 203(1) Cr 20 3( ) CrO2(g) CrO~g) s

[141

[15]

AS(g) As(~) As 2(g) As3(g) As 4(g) AsCl3~g) AsCl3(t) As203(t) As203(s) As2Os(s)

Cd(t ) Cd(s ) CdO(s) Cd3As2(s) CdCI 2(t) CdCI2(s)

Hg(g) Hg(t) HgCI(g) Hg2CI 2(s) HgCI 2(g) HgC12(1) HgCl2(s) HgO(g) HgO(s)

Pb

CdO(g) CdCO3(s) Cd3(AsO4)2(s) Cd(OH)2(s)

Sn

Pb(g) Pb(t) Pb(s ) Pb2(g) PbCl(g) PbC12(g) PbC12(t) PbCl2(s) PbC14(g) PbOtg) PbO(t) PbO(r) PbO(y) PbaO4(s) PbOe(s)

CrCI2(/) CrCl2(s) CrC13(s) CrO2C12(g) CrO 2(s) CrsO12(s) CrsO21(s) CrO3(i) CrO3(s)

SnO(g) SnO(s ) SnC12(g) SrlCI2(/) SnCI 2(s) SnC14(g) SnO2(s)

Cd(g)

[161

[171

Hg

Sn(g) Sn(t) Sn(s) SnO(t) SnO2~t) CrOzOH(g) CrO2(OH)2(g) CrO(OH)3(g) CrO(OH)~g) CrCl4(g) CrAsO4(s) Cr3(AsO4)2(s)

118]

elemental form being predominant at temperatures greater than 1500 K. An interesting feature is noted in the Hg-O2-C12 and Cd-O2-C12 system. Calculations in the Hg-CI 2 system (Fig. 2) indicate that HgC12 is the dominant species at 1600 K; in the Hg-O2-CI 2 system, however, Hg is dominant. This is because the excess oxygen alters the equilibrium and causes the elemental form to be the dominant species. In the case of cadmium, the temperature above which

Hg3(AsO4)2(s)

Pb3(AsO4)2(s)

(PbO),,(g) n = 2to6 PbO2(s) Pb203(s)

the elemental form becomes the dominant species over the chloride is lowered (to 1000 K) for the Cd-O2-CI 2 system in comparison (to 1200 K) to the Cd-C12 system. Cd and Hg are in the same family (Group IIB) in the periodic table and exhibit this similar behavior. All Metals-Oxygen-Chlorine System The previous analysis was done with a single metallic species and the major species were

34

C . Y . W U A N D P. B I S W A S TABLE

2

Simulation Conditions for the Different Analyses (Numbers Denote Amount of Species in Moles) Single MetalOxygen Pressure T(°K) Metals O2 C12 CH4 N2

Single MetalChlorine

Single MetalOxygen-Chlorine

All MetalsOxygen-Chlorine

1 atmosphere 500, 600, 800, 1000, 1100, 1200, 1300, 1400, 1500, 1600

1 atmosphere 1100 and 1500

1

1

1

1

106 0 0 0

0 104 0 0

106 104 0 0

106 2to 106 0 0

I

I

I

I

All MetalsCH4-Air-Chlorine

7x10

7

4 (in air) 2.4 X 10 -2 1 15 (in air)

I

1.0

"

[

Cd(g)

Cd0(s ) 0.8 t~

Asz05(~)

0.6

-6

As203(g)

0.4 0.2

(a) I 0.0 1.0 - (c)

I

I

I

I

....

0.8 - '~CrO2(s)

I

'

0.6

!.g%)~~__:

i

-5

I',,.

0.4 0.2

:

CrOa(g)/'i

~ . ~

0.0 • r'-, ~1.0 - .... PbOz(s) , / ~ :! I

0.8

II

I

:

I

/

!i

•

tl

(d)

%

i

- . HgO(g)

"

.~.

_

.....

I~'1

SnOz(s)

PbO(g)

N,

0.6 0

I

-, 0.4 0.2

304(s) .~,/,

0.0

2oo

400

~

600

i:

, • 800

1000

1200

1400

SnO,~ g ),

(f) . .

(e) 1600

200

400

600

800

1000

1200

tl

1400

600

T e m p e r a t u r e (K) T e m p e r o t u r e (K) Fig. 1. Moles of major species for single metal-oxygen system as a function of temperature: (a) arsenic, (b) cadmium, (c) chromium, (d) mercury, (e) lead, and (f) tin.

S P E C I A T I O N O F M E T A L S IN AN I N C I N E R A T O R 1

I

I

I

I

35

I

1.0

AsCI3(g)

CdC12(c)

-~

Cd(g)

0.8 0.6 -6 0.4

0.2

t (a)

(b)

0.0

I

I

I

I

I

I

~

t

10 CrC13(c)

-6

HgCla(~)

\

0.6

z(c/

0.8

\

~

().4 0.2

(c)

@.0 10

CrC1 I

~

(d) --~

Hg(g)

I

PbCl4(g )

SnCl4(g )

0.8 01

0.6

0

0.4 0.2 (e) 0.0 200

(f) I

I

400

600

i 800

i

i

1000

1200

i 1400

1600

200

400

600

800

1000

1200

1400

]600

Ternperoture (K) Temperoture (K) Fig. 2. Moles of major species for single metal-chlorine system as a function of temperature: (a) arsenic, (b) cadmium, (c) chromium, (d) mercury, (e) lead, and (f) tin.

identified to be chlorides in most cases. The chlorides have different properties in comparison to the oxides, and hence the aerosol formation characteristics will be different [6]. Also, the toxicities of these species are very different. When the metals are all present together, it is expected that there will be a scale of affinities for the chlorine to preferentially form the chloride. This becomes important especially if chlorine is not in excess concentrations. Hence, the calculations were carried out with all metals present together with oxygen

and varying amounts of chlorine as indicated in Table 2. The results of the simulation at temperatures of 1100 and 1500 K and chlorine varying from 2 to 106 mol (Table 2) are plotted in Figs. 4a and 4b, respectively. Only the major chloride species of the metals are plotted, for purposes of clarity. At 1100 K, when the ratio of chlorine to metal is 4 or higher, nearly all the lead is in the form of lead chloride. On further increasing the amount of chlorine, the order in which the metals react with chlorine to form

36

C.Y. WU AND P. BISWAS I

1.0

I

I

I

AsClz(g)

CdCl2(c)

:1

Cd(g)

0.8 0.6

0

-ASzOs(s)

0.4 0.2 (a)

0.0 1.0

I

I

I

I

t

i

CrO2Cl2(g)

t

(b)

HgC12(g)

0.8

\.. 0.6

0 0.4 0.2 0.0 1.0

(s) I""

i

(c) I

I

I

-

X

I

PbCl4(g)

Sn0z(s ) " x\

0.8 tO

0.6

-6

"l

0.4

1

II\

0.2

SnCl2(g~,

(e) 0.0 200

I

I

i

400

600

800

I

i

i

1000 1200 1400 1600

Temperature (K)

200

400

600

800

1000

1200

1400

1600

Temperature (K)

Fig. 3. Moles of major species for single metal-oxygen-chlorine system as a function of temperature: (a) arsenic, (b) cadmium, (c) chromium, (d) mercury, (e) lead, and (f) tin. chloride is Cr, Hg, As, Sn, and Cd (Fig. 4a). The relative affinity of Hg and As to chlorine changes at a higher chlorine concentration. For chlorine-to-metal ratios greater than 5 × 102 , the chlorides are the dominant forms of the metallic species, other than Sn and Cd. The dominant tin species is SnO 2 and the cadmium remains primarily in elemental form. When the temperature is increased to 1500 K (Fig. 4b) a different behavior is observed. At higher temperatures, more of the chlorine remains in the radical form (CI) rather than reacting with the metals to form the chlorides.

For the chloride species to be dominant, a higher amount of chlorine needs to be present in the system. The relative affinities of the metals for chlorine are altered at 1500 K and the results are summarized in Table 3. Hg and Cd have the least affinity for chlorine and remain primarily in the elemental form even at high chlorine-metal ratios ( ~ 104). All Metals-CH

4 (Fuel)-Alr-Chlorine

System

In a real incinerator, typically hydrocarbon fuels are combusted in excess air. The simula-

SPECIATION OF METALS IN A N INCINERATOR

I

1.0

I

"PTc,?

I

I

.-.

t

./

l: .I//HICl=

/

J/;

0.8 in

37

0.6

Cr0~Cl2 I'

0

I

0.4

i

•

/i

/$nCI x

"/, '

:

0.2

_~'/:1"/#" 0.0 10 0

i

# '/isCl

/ '

I

101

""i ""''"':_

......

10 2

03

l

I CdCI= 10 5

10 4

I I < )<;

R a t i o o f CI to M e t a l (a) I

I

I

1.0

t'

I

I'

# 0.6

!

I

-6

I

0.4

i

i

/

I_.*

101

/

I

I

10 3

10 4

10 5

I

l" /

s. •

.

/HgCI=

..

/ CrOzCl÷/ Z

/

•"

i #

PbCl./

:"SnClx

I

#

10 0

I

f

/

0.8

0.0

I

f

/

0.2

I

~"

=.,ft..*

10 2

Ratio

CdCl= I I0 b

of Cl to Metal (b)

Fig. 4. Moles of major species for all metals-oxygen-chlorine system as a function of moles of chlorine in the system at two temperatures: (a) 1100 K, (b) 1500 K.

TABLE 3 Relative Affinity of Metals to Chlorine Temperature (K) All Metals-O2-C12 System All Metals-O2-Cl 2 System All Metals-CH4-Air-C12 System All Metals-CH4-Air-C12 System

1100 K 1500 K 1100 K 1500 K

Relative Affinity Pb > Cr > H g / A s > Sn > Cd Pb > Cr > As > Sn > Hg > Cd H > Pb > Cr > Hg > As > Sn > Cd H > Pb > Sn > Hg > Cr > AS > Cd

38

C.Y. WU AND P. BISWAS TABLE 4

Moles of Major Species of the Different Metals for the All M e t a l s - C H 4 - A l r - C h l o r i n e System Temperature = 1100 K

Moles As(g) AsCl3(g ) As203(/) Cd(g) Hgtg) HgC12(g) HgO(g) PbCI(g) PbCl2(g ) PbCl~g) PbO(g) SnC12(g) SnO(g~ SnO2~,) CI C12

Moles

Percentage of total for the specific metal

1.66 6.53 2.34 7.00 6.10 7.53 1.47 6.63 1.98 7.00 7.80 1.07 2.17 6.99 5.63 6.50 4.78

x x x X × X X X X x × x x x × x X

10 -16 10 -7 10 - s 10- 7 10 -7 10 -8 10 - s 10-15 10-11 10 -7 10-16 10-1o 10-13 10 -7 10 -5 10 -6 10 -2

0.0 93.3 6.7 100.0 87.1 10.8 2.1 0.0 0.0 100.0 0.0 0.0 0.0 99.9 0.12 0.27 99.60

2.59 4.64 1.05 7.00 6.91 3.79 9.15 8.37 4.34 5.97 0.20 6.01 6.21 1.85 8.87 1.11 4.71

X X X × X X x x x x × x x x x x ×

10-8 10-7 10-7 10- 7 10 -7 10 T M 10 -9 10- s 10- 7 10 -8 10-7 10 -8 10 -7 10 -8 10 -4 10 -5 10-2

3.7 66.3 30.0 100.0 98.7 0.0 1.3 12.0 62.0 8.5 17.2 8.6 88.8 2.6 1.85 0.05 98.10

3.05 6.82 7.63 9.18

× x × X

10 -15 10- 7 10-11 10 -9

0.0 97.4 0.0 2.6

5.55 4.07 2.77 4.82

X X X X

10 -9 10- 7 10- 7 10- 9

0.8 58.2 39.6 1.4

20.5 0.1 77.3

7.72 X 10 -9 1.04 X 10- 7 5.23 X 10-7

1.1 14.8 74.8

HCI Not including chromium hydroxide species CrO2~g) CrO2Cl2(g ) CrO3(g ) Cr203(s)

Percentage of total for the specific metal

Temperature = 1500 K

Including chromium hydroxide species CrO2Cl2(g ) CrO2(OH)(g ) CrO2(OH)z(g )

1.43 × 10 .7 5.09 × 10-1° 5.41 x 10 -7

tion was done by considering combustion of methane (CH a) in 100% excess air ( O 2 : N 2 = 1:3.78). The relative compositions are listed in Table 2 (last column). The adiabatic flame temperature for this system of reactants is close to 1500 K, and the simulation was carried out at 1100 and 1500 K, with the results listed in Table 4. Several interesting features can be noted. First, the presence of H in the system, alters the affinities of metals to form chlorides. The relative affinities are tabulated in Table 3. Due to the H being in excess relative to C1, it reacts with the majority of chlorine in the system to form HC1. Hg and Cd primarily remain in the elemental form and Sn is in the form of oxide. Pb and As still form the chlorides in spite of the presence of excess H in the system. Data for the hydroxide species of chromium were available from Ebbinghaus [16]. Carrying out the analysis with these species, it was found

that the oxychloride (CrO2C12) is a major species only at temperatures below 1000 K, above which CrO2(OH) 2 becomes the dominant species. On not including the chromium hydroxide species in the analysis, the results indicate that the major species is CrOEC12 (Table 4). At a higher temperature (1500 K), the chlorides are no more the only dominant species. Most of the chlorine (98%) ends up as HCI due to the excess hydrogen. However, the metals are distributed among the chlorides and oxides. Hg and Cd indicate a similar behavior, with the elemental forms being the dominant species. SnO is the major species for Sn compounds. CrOE(OH) 2 is the major species at 1500 K with CrO2(OH) being the species with the next highest concentration. The relative affinities for chlorine are altered and listed in Table 3. The affinities of the metals to form chlorides

SPECIATION OF METALS IN AN I N C I N E R A T O R

39

10 - 6

o 10 - 7 10 - 8 PbCl x 10 -9

.-'""

10 - 1 0 10 -11 o "5

10 -12

~ 10 - 1 3

j

f" J J,./.' f

.t..._-""

CrOzClz

10 - 1 4 10 - 1 5

I

."

,'"

101

10 o

."

i'"'" X , •

i0 -16

..-"

**-"

.-

..""

.''

•

'"

X

,I"

,~'"

I

10 2

10 `3

10 4

R a t i o o f CI t o

,

10 5

Metal

(a) 10 - 6

I

I

I

I

. - "

10 - 7

.-"

__~ f" J ,-"

10 - 8 10 - 9

PbCl

-"

.-:

10 - 1 0

.-'"

10 -11 O 2~

/

,.- ' ' '

10 - 1 2

."" " " 10 -13

.-"

SnClx

10 - 1 4 i0 -15 10 - 1 6 lO 0

°°'°'"

~, / .,

,~-

J ~,,

. . . . . . " ' ~ . . i . ~ s "~ ~ "'

....~"~.."J~ Cl •

~

101

"

I

I

10 2

10 5

Rotio

of

Cl t o

I

04

c

i0'

Motel

(b)

Fig. 5. Moles of total chloride concentrations of the various metallic species as a function of chlorine concentration in the system for the all metals-CHn-air-chlorine system at two temperatures (a) 1100 K and (b) 1500 K.

are greatly affected in the presence of a hydrocarbon fuel. H has the greatest affinity for C1 as it is typically in excess with respect to the chlorine and the metals, and picks up most of the chlorine. The number of moles of the chlorides of the different metals for the all metals-CH4-air-chlorine system are plotted in Fig. 5 for varying chlorine concentrations. At low chlorine concentrations, arsenic does not form the chloride; however, when the ratio

of C1 to metal exceeds approximately 5000 (at 1100 K) and 10000 (at 1500 K), arsenic has a greater tendency to form the chloride relative to the other metals (with the exception of Pb). At higher temperatures, tin has a greater affinity for chlorine. Several fuels used in incinerators could have varying ratios of H to C. A calculation was also carried out for H / C ratio of 1. The results showed similar trends as for the case with a

40 H / C ratio of 4. As the H concentration is lower, more C1 is available for the reaction with metals, and hence the concentration of the metal-chlorides increase. At 1100 K, nearly 99% of arsenic forms arsenic trichloride, 45% of the mercury forms mercuric chloride (in comparison to 10.8% for H / C = 4). CONCLUSIONS A detailed equilibrium analysis of the speciation of six metals among its various forms in an incinerator system was performed. The role of temperature and chlorine content in the speciation has been established. The relative affinities of the metals to form chlorides/oxides were established. The inclusion of hydrocarbon fuel in the analysis is essential as the hydrogen picks up most of the chlorine (to form HC1) and this alters the speciation. Though the results are dependent on the specific conditions in the incinerator, a few general trends can be identified. For typical incineration conditions, mercury and cadmium can be expected to remain in their elemental forms. Chromium primarily forms the hydroxide species, with the fraction of the 5 + species increasing at higher temperatures. Lead forms the chloride species (primarily 4 + ) with the 2 + chloride and oxide fractions increasing at higher temperatures. Arsenic preferentially forms the chloride, with the oxide fraction increasing at higher temperatures. Tin preferentially forms the oxide with the 2 + oxide being more prevalent at higher temperatures. The analysis also indicates that it is important to include as many species as possible (for which reliable data are available). Detailed equilibrium analyses provide indication and trends of the presence of major species. As the chemical, physical, and toxicological properties of these species are different, the speciation has important implications in the particle formation characteristics, emissions and ultimate environmental risks. The results may indicate trends as to which of the species would probably form the aerosol phase. A species in the condensed phase (as per the equilibrium analysis) may form particulate matter and be in the form of an aerosol and be emitted in conjunction with the aerosol that is formed from the gas phase metallic compounds. However, to accurately estimate the

C.Y. WU AND P. BISWAS emissions of these species from the incinerator, kinetic analyses in conjunction with aerosol formation and growth models [6] have to be used.

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15. Cox, J. D., Wagman, D. D., and Medvedev, V. A., CODATA Key Values For Thermodynamics, Hemisphere, New York, 1989. 16. Ebbinghaus, B. B., Proc. 1992 Incineration Conference, Albuguerque, NM, May 1992, p. 599. 17. Barin, I., Thermochemical Data o f Pure Substances, VCH, Germany, 1989. 18. Lamoreaux, R. H., Hildenbrand, D. L., and Brewer, L., J. Phys. Chem. Ref. Data 16(3):419-444 (1987). 19. Stewart, G. G., in Thermal Processes (H. M. Freeman, Ed.), Technomic, Lancaster, PA, 1990, vol. 1, p. 59. 20. Scotto, M. V., Peterson, T. W,, and Wendt, J. O. L., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, in press. Received 16 July 1992; revised 12 October 1992