Metal Partitioning in Combustion Processes
WILLIAM P. L1NAK
Air Pollution Prevention and Control Division. MD-65
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711 USA
EPA/600/A-97/085
Fourth International Conference on
Technologies and Combustion for a Clean Environment
July 7-10, 1997
Lisboa, Portugal
Abstract -• This article summarizes ongoing research efforts at the
National Risk Management Research Laboratory of the U.S. Environmental
Protection Agency examining [high temperature) metal behavior within
combustion environments. The partitioning of non-volatile (Cr and Ni),
semi-volatile (Cd and Pb), and volatile (Hg) metals in combustion systems
was investigated theoretically and experimentally. Theoretical predictions
were based on chemical equilibrium and suggested that such calculations
can be useful in predicting relative volatility and speciaticm trends, and to
direct experimental efforts. Equilibrium calculations, however, are not
sufficient to quantitatively predict the behavior of metals even in simple
combustion environments. Experimental studies employing a 59 kW
laboratory scale combustor examined the behavior (volatility, particle size,
and specialion) of metal vapors and particles produced by aqueous mclal
solutions sprayed through a swirling natural gas diffusion flame. These
experiments were designed to study metal transformation mechanisms in a
relatively simple combustion environment without the complex effects of
additional species. Further experiments examined the potential use of
common inorganic sorbents (kaolinite, bauxite, and hydrated lime) to
adsorb metal vapor, offering a potential means of metal emissions control.
its spcciation and partitioning between the submicron (<1 nm in diameter)
and supermicron (>1 jim in diameter) aerosol in the exhaust. Of particular
interest are methods of affecting metal speciation and particle size to
maximize control and minimize toxicity and risk.
While much previous research on mechanisms governing the
transformations of metal compounds in combustion systems has focused on
coal combustion systems (Flagan and Fricdlander, 1978; Nettlelon, 1979;
Smith, 1980; Haynes rial., 1982; Neville and Sarofim, 1982), relatively
little is available on the fate of single metal compounds, introduced one at a
time, in the absence of other major ash constituents, as they might be during
incineration trial burns. The coal data suggest that trace metals are often
enriched in the submicron particle size fraction, and this has been explained
by mechanisms involving metal vaporization and subsequent condensation
or surface reaction. There are complicated exceptions, including cases
where a volatile metal, such as sodium (Na), may be scavenged by alumino-
silicates displacing other bound metals. Therefore, experiments involving
relatively simple combustion environments and pure metal compounds
(such as those presented here) are useful to test theoretical hypotheses and
help isolate mechanisms.
Key Words:
Trace Metals; Partitioning; Volatility; Sorbent Interactions;
Combustion
INTRODUCTION
Metal compounds are present in the stack effluents of many combustion
processes. As health and environmental studies further identify the scope
and magnitude of their adverse effects, the release of metals from boilers,
furnaces, and incinerators into the environment is coming under increasing
regulatory scrutiny in a number of countries. In the U.S., metal air
emissions from hazardous waste incinerators (HWis) and boilers and
industrial furnaces (BIFs) which destroy hazardous waste are regulated by
the Resource Conservation and Recovery Act (RCRA, 1986). Current air
emission limits are based on risk assessment arguments which limit the
ground level concentrations that may be inhaled by the "maximum exposed
individual* Metals regulated by RCRA include a set of carcinogenic metals
[arsenic (As), beryllium (Be), cadmium (Cd), and chromium (Cr)] and a set
of noncarcinogemc metals [antimony (Sb), barium (Ba), lead (Pb), mercury
(Hg), nickel (Ni), selenium (Se), silver (Ag). and thallium (Tl)]. Metal air
emissions from many other combustion sources are regulated under Title
ffl. Section 112 of the Clean Air Act Amendments (CAAA, 1990). Metals
regulated by Section 112 include Sb, As, Be, Cd, Cr, cobalt (Co), Pb,
manganese (Mn), Hg, Ni, and Se. While municipal waste incinerators
(MWIs) are also regulated under Title III, these units are covered by their
own set of metal emission regulations (Section 129), which are likely to
place limits on the emissions of Cd, Pb, and Hg.
This paper, which presents results from theoretical and experimental
studies conducted by EPA's National Risk Management Research
Laboratory, is concerned with the partitioning of metals in combustion
processes. Under well defined conditions, metals can be segregated
according to their relative volatilities. Rizeq et nl (1994) group mcials into
three volatility classes. Metals, such as Hg and Se, tend to be volatile, even
at moderate stack temperatures. Others, such as Sb, As, Cd, Pb, and Tl,
are semi-volatile, and have the potential of vaporizing at the high
temperatures in the flame zone. Finally, a third group including Ba, Be, Cr,
and Ni, are considered to be refractory (non-volatile) over the entire range
of combustion temperatures usually encountered. Of interest arc the
physical and chemical transformations from the initial form of the metal to
Mechanism Overview
Metals may be contained in solid or liquid fuels, chemically bound to the
organic fuel matrix (inherent mineral mailer), dispersed within the solid fuel
as mineral crystallites (included mineral matter), or completely extraneous to
the fuel particle (excluded mineral matter). Metals may be chemically bound
within organomctallic compounds such as chelates or physically mixed, as
in paints, pigments, and solvents! They may enter combustion processes
with other inorganic clays and soils, as during the thermal treatment of
contaminated soils (Eddings and Lijhty, 1992), or they may be contained in
aqueous solutions and sludges. Metals may be introduced into a
' combustion environment continuously through atomizers (Bhalia and
Sirignano, 1991), lances, or screw feeders, or through entrance chutes in a
batch mode as solids or contained liquids. They may be introduced as
single salts or individual compounds, or they may enter as mixtures, to
fact, a critical issue in designing test burns for incinerators is whether the
form in which the metal is introduced during the trial burn is representative
of the behavior of other forms of that metal that may be introduced during
routine operation. The left-hand side of Figure 1 represents some of the
various forms in which a metal may be introduced into a combuslor, and
how this physical state can influence the ultimate fate of that metal.
Upon entry into a combustion environment, the metals contained in a fuel
or waste stream can be transformed into various physical forms. Dissolved
metal salts, such as nitrates or pyrites, may form reactive metal compounds
which may decompose violently at elevated temperatures. They may also
decompose as viscous melts to form cenospheres which can burst into
submicron fragments (Mulholland and Sarofim, 1991). Alternatively, the
metal compound may be confined within a porous char matrix. This will
happen, for example, to inherent (organically bound) metals in heavy oil or
both inherent and included mineral matter in pulverized coal. The metal
must then be released either within the matrix, subsequently to diffuse
through it, or it will enter the gas phase as the char matrix itself is oxidized.
In the event of the former process, there is the opportunity for the metal to
react with included silicates to form stable compounds which fail to
vaporize. Alternatively, the metal which has been released may diffuse back
into the remaining char matrix, to react with included silicates situated there,
or it may react with excluded silicate particles in the disperse phase.
Finally, one metal (e.g., Na) may displace another metal [e.g., potassium
(K)J which would otherwise be immobile and bound in a stable mineral
-------
Submicron Aerosol
( Aqueous Solutions )
NucIeatiofVCondensation/
Distillate Oil and
Organ ometallics
Dissolved/\
Inherent,
Reactive Metal
X '
Porous^ "^
Submicron Inclusions,
Residual Particles.
Secondary Atomization
Heavy Oil and
Coal
ndMalal \Cenospheric
Particle
I Solids and
^Contaminated Soils
Inorganic Mixture
Supermicron (Collectable)
Aerosol
FIGURE 1 Possible controlling mechanisms for particle formation in
combustion systems.
form, such as illite. Mineral inclusions may also coalesce as the carbon
matrix recedes lo form panicles larger lhan the individual inclusions. The
physicochemical processes involved in the release of metals may thus be
quite complex. They may be kinetically controlled, and the overall amount
of a metal released may, under certain circumstances, have little to do with
equilibrium.
When a metal contained in contaminated soils, sludges, or slurries is
introduced into an incineration environment, an inorganic mixture
containing both the metal and potential scavenging agents, such as clays and
glasses, is formed. It is not surprising, therefore, that (upon heating) much
of the metal may react with the clay and only a little will be released to form
a condensable vapor. This has been experimentally confirmed for
Pb/montmorillonite clay mixtures by Eddings and Lighiy (1992). Their
study showed large deviations from simple equilibria based on selected pure
condensed phases, during the heating of inorganic mixtures. The work of
Queneau et at (1991) addresses the thermodynamics of the vitrified
mixtures likely 10 be formed under these conditions, and is useful where
inorganic mixtures are formed, as shown in Figun: 1.
The primary physical forms of the meuls outlined above (reactive metal
compounds, porous chars, and inorganic mixtures as shown in Figure 1)
can then undergo further transformations to other physical forms including
metal vapors, porous metal ash particles, cenospheric (hollow) ash
particles, or dense ash particles. Upon cooling, the supersaturated vapor
may condense on the surfaces of existing particles, or if sufficient surface
area is not available, homogeneously nucleate to form tiny particles. These
particles will subsequently collide, coagulate, and agglomerate.
Alternatively, there is evidence that a metal vapor may react on the surface
of existing particles or sorbents. In contrast to the first two processes
(heterogeneous condensation and homogeneous nucleation), surface
reaction does not require the metals' partial pressures to exceed their vapor
pressures. These mechanisms and processes strongly influence the
chemical and physical form in which the metal under consideration enters
the environment. They are likely to depend strongly on the combustion
environment and temperature history experienced by the metal compound,
as well as on the initial form in which it is introduced into the combustion
chamber, and on the presence or absence of other species in the mixture.
Panicle Growth via Coagulation
In addition to the heterogeneous condensation of vapor-phase species,
particle growth occurs through panicle coagulation and agglomeration.
Figure 2 illustrates the predicted evolution of an aerosol due to coagulation
only. The MAEROS code (Gelbard and Seinfeld, 1980) was used in which
the particle size domain [0.001 to 20.0 )lm particle diameter (Dp)] was
divided into 13 geometrically equal sections or bins. Coagulation was the
only mechanism considered; all other mechanisms (condensation,
nucleation, deposition, etc.) were disabled. At time zero, an initial mass of
was assigned to section 2 (0.0021 - 0.0046 \im diameter) to
simulate the nucleation of a submicron fume. This aerosol was assumed to
have the properties of Pb-oxide, producing a number concentration of
4.0x10" /mj. An additional 500 rng/m' of mass was assigned to section
11 (2.0-4.4 urn diameter) to simulate the presence of flyash or sorbent,
System pressure and temperature were maintained at l.OlxlO5 Pa (1 aun)
and 810 K (1000 °F) to simulate post-flame conditions. Following the
initial distribution. Figure 2 presents six mass distributions which follow
the evolving aerosol through six orders in lime (t=0.1, 1.0, 10, 100, 1000,
10000 s). Note that coagulation does not change the total aerosol mass and
that the areas under all seven curves represent HX50 mg/mA Number
concentrations, however, are greatly affected, and Figure 2 shows that at
0.1 s the number concentration has fallen approximately 2 orders of
magnitude (2.3x10" /m>), and the average nuclei size has grown to
approximately 0.03 fim. At 1.0 and 10.0 s the distributions have grown
only slightly fanner into the 0.01 to 1.0 fim diameter range. This is
important, as these times represent a range of typical residence times within
combustion/incineration systems. In fact, even after 10000 s (2.S hours)
the average Dp is only approximately 1.0 pm with a number concentration
of 3.3x10" /m}. Thus, as can be seen from Figure 2, aerosol nuclei tend
to coagulate very quickly at small times, due to the large number
concentrations of nuclei particles, and then, at larger limes, as number
concentrations fall, coagulation slows considerably causing the aerosol to
accumulate into a mode approximately between 0.1 and 1.5 jlm diameter.
Panicles in this size range, posessing neither high diffusivities nor high
momentum, exhibit minimum collection efficiencies in most air pollution
control devices. This characteristic distribution of a coagulating aerosol has
been termed the accumulation mode. Note that the coagulation mechanism
does not include the effect of differing fractal properties of the agglomerate
formed, as developed by Matsoukas and Fricdlander (1991). It has been
assumed here that only spheres result from the coagulation process.
Also evident from Figure 2-is that coagulation between the evolving
nuclei and the sorbent is very slow, and the small particles grow as if the
sorbent were not present. Again, this is due in part to the large differences
in number concentrations. At time zero, nuclei are present in concentrations
of approximately 4.0xl018 /m'. Sorbent particles are present in
concentrations of 4.7x109 /m'. This difference encourages nuclei-nuclei
coagulation. Thus, it would seem that the use of a sorbent to scavenge
submicron metal particles through coagulation is not possible in the times
available. However, as has been described previously, mechanisms other
than coagulation may allow sorbents to be utilized to remove aerosol mass
from the submicron fraction. Again, these results assume a fractal
dimension of 3 (i.e., spheres) for the resulting agglomerates. Even though
the simulation presented in Figure 2 shows very little nuelei-sorbent
interaction, fine particle coagulation and diffusion to the coarse mode are
competitive processes. Friedlander el at, (1991) reason that, since diffusion
and coagulation are first and second order with respect to particle number
concentration, respectively, one should expect the following qualitative
behavior. If, as presented in Figure 2, particle nucleation results in a large
initial number concentration, then coaRulalion will dominate, causing the
-------
600
400-
200-
mitial distribulion
t = 0s
number/m3 = 4.0x1018
t = 0.1 s
number/m3 = 2.3x1016
t= 1.0s
number/m3= 1.5x1015
t=10s
number/m3? 1.2x1014
t=100s
number/m3 = 1.2x1013
I= 1000 s
number/m3 = 1.7x1012
t = 10000 S
number/m3 = 3.3x1011
0.001 0.01 0,1 1
100
Dp (urn)
FIGURE 2 Predicted evolution of Pb-oxidc nuclei panicles in the presence
of a simulated sorhenl via coagulation in a post-flame comhusiion
envifonmenl.
nuclei lo grow inlo panicles with low dift'usivities and little possibility to be
scavenged by the coarse mode panicles. If. on the other hand, nucleation
results in a small initial nuclei number concentration, nuclei-nuclei
coagulation rales will be small, the nuclei mode will not grow substantially
but, provided sufficient lesidence time is available, will diffuse to, and be
scavenged by, the coarse mode particles. Thus, using ihe self preserving
size distribution theory, Friedlander et al. (1991) argue that significant
scavenging of nuclei by coarse mode (sorbent) particles is possible if: the
diffusion time (see Hagan and Friedlander, (1978); Friedlander et al,
(1991)3 *s small compared to the residence lime; and the mass of the line
mode is below a threshold value for a given coarse mode aerosol.
EXPERIMENTAL APPROACH AND PROCEDURE
Laboratory Swirl Flame Combuxtor
Experiments were performed using the laboratory scale 59 kW (actual), 82
kW (maximum rated) horizontal tunnel comhustor presented in Figure 3.
This rttfraciory-lined research cornbustor was designed to simulate the
time/lernperature and mixing characteristics of practical industrial liquid and
gas combustion systems. Natural gas fuel, aqueous metal solutions, gas
dopants, and combustion air were introduced into the burner section
through an International Flame Research Foundation (1FRF) moveable-
block variable-air swirl burner. This burner incorporates an interchangeable
injector positioned along its center axis. Swirling air passes through the
annuliis around ihe fuel injector promoting flame stability and attachment to
the water-cooled quarl. A high swirl (IFRF type 2) flame with internal
recirculaiion (Swirl No.=1.48) was used. Gaseous and aerosol samples
were taken from a stack location 5,9 m from the burner quarl. The
temperature at this location was approximately 670 K (745 °F). Further
details regarding die experimental combuslor can he found elsewhere (Unak
eta!., 1994, 1995, 1996, 1997).
Metal Syslenis Investigated
To dale, experimental investigations examining four metals (Cd, Cr, Pb,
and Ni) have been conducted and published (Linak et al., 1994, 1995,
1996, 1997). Additional studies focusing on Hg, vanadium (V), and zinc
(Zn) are on-going. Typically, water soluble metal nitrates or oxides were
introduced as aqueous solutions through a special fuel/waste injector which
incorporated a small air atomizing system down the center of a standard
natural gas injector. The resulting droplet size distribution was relatively
narrow wiih z mean droplet diameter of approximately 50-80 Jim (Linak et
al., 1994). Diatomic chlorine (C\i'l or sulfur dioxide (SOj) dopants were
introduced, separately from the metal solutions, with the (secondary)
combustion air. Thus, the metal and chlorine or sulfur, were not mixed
prior to their introduction into the cornbustor. All interactions between the
components were dependent upon normal turbulent mixing patterns.
Aqueous solutions typically containing 1.5% metal (by weight) were
used. Solution flow rates were maintained so as to produce stack gas
concentrations of approximately 100 ppm metal (by volume). Metal feed
rales corresponded to constant molar feed rates of approximately 0.005 g-
moles/min. Experimental programs included evaluating Ihe effects of
chlorine and sulfur on metal partitioning. For these experiments, Cl| or
SOj were introduced at different molar ratios of chlorine or sulfur to metal.
These feed rates and resulting stack concentrations varied from
substoichiometric with respect to the metal concentration lo excess
concentrations approaching 10,000 ppm (stack). Typically, chlorine and
sulfur slack concentrations of 10(X) ppm were examined. Excess air was
maintained at 20%. No air preheat was employed.
Aerosol Particle Size Distribution: Sampling ami Analysis
Particle size distribution (PSD) measurements were taken from the slack
localion using three techniques. Extractive samples were taken for
collection by inertia! impaction and electrical mobility analyses using an
isokinetic aerosol sampling system based on the modified designs of Scotio
etal. (1992) and Linak aaL (1994). In order to minimize in-probe gas and
aerosol kinelics. the sampling system dilutes and cools the aerosol sample
using filtered nitrogen and air immediately after sampling. Dilution ratios
arc measured directly for each experiment and verified independently by the
measurement of a nitric oxide tracer gas.
Extracted samples were directed 10 an Andersen Inc. eighi-stage, 28.3
L/min. (I W/min.), atmospheric pressure cascade impaetor and a Thermo-
Systems Inc. scanning mobility particle sizer (SMPS). The cascade
impaclor is designed to collect physical samples less than approximately 10
Jim diameter (for subsequent gravimetric and/or chemical analysis) on nine
stages (including the after filler). The SMPS classifies and counts particles
within a working range of 0.01 lo 1.0 pm diameter using principles of
charged particle mobility through an electric field. The SMPS, used for
later experiments, is an upgraded version of the differential mobility particle
sizer (DMPS), used during early experiments. The SMPS upgrade allows
for improved PSD resolution and shorter sampling limes. The SMPS and
DMPS were configured to yield 54 and 27 channels, respectively, evenly
spaced (logarithmically) over the O.U! to 1.0 (am diameter range.
In addition to the inertia! impaction and electrical mobility devices which
require an extracted sample, in-situ light scattering PSDs were taken using
an Insitec Inc. laser doppler voloeimeter. This instrument determines
particle size by measuring the light scatiering intensity of particles which
pass through a sampling volume- established within the cornbustor stack by
a laser focused through a set of quartz optical access ports. The working
range of this device was approximately 0.3 lt> 30 jim diameter which
slightly overlapped and extended tins PSD data collected by the SMPS.
In addition to the three PSD instruments, samples were collected on
silver fillers and analyzed using a field emission scanning electron
microscope (SEM) equipped with an energy dispersive x-ray (EDX)
spectrometer. This provided morphological .information as well as
qualitative chemical analysis of individual particles.
NON-VOLATILE METALS
Volatile and semi-volatile metals have been of particular interest because
they remain as vapors within combustion and Hue gas cleaning systems,
resulting in poor removal efficiencies, or tend to form fumes of submicron
particles resulting from nuclealion and condensation of metal vapor. These
mechanisms lead to substantial enrichment of these metals on submicron
particles which are often difficult to collect in paniculate pollution control
equipment. It has been shown (Scotto et til., 1992; Wendt, 1994; Linak et
al., 1995) that some meial vapors can also be reacted at high temperatures
above their dewpoints with sorbent substrates to form environmentally
benign water insoluble products. Thus, the emissions of some of these
volatile and semi-volatile metals can be managed because they form a vapor
which can be scavenged and reacted. However, several researchers
-------
J To air pollution control system
Gas sample port
In-situ light scattering particle sizer
-Aerosol sample port
(electrical mobility & impaction particle sizers)
- Cooling water
Moveable-block
burner
Natural gas
• ^ Aqueous metal
I solutions
I—Atomizing air
CI2 or S02
Combustion air
Transport air
FIGURE 3 EPA horizontal lunnd combustor.
(Davisonera/.. 1974; Klein et aL, 1975; Markowski etal.. 1980; Quann
and Sarofim, 1982) have also noted enrichment of the refractory metals Ni
and Cr in submicron panicles, even though it is not obvious that these
metals necessarily vaporize in the combustion process. The mechanisms by
which this occurs are not readily apparent. It is appropriate, therefore, to
examine refractory metals, and to determine if their partitioning among
various particle sizes can be predicted and controlled.
Among possible refractory metals we focused on Ni and Cr. Ni is only
slightly less volatile than Cr, and appears in both oil and coal tlyash. Ni
and Cr (metal) have boiling points of 3003 and 2945 K. respectively. Cr is
of particular interest because it commonly exists in two forms in the
environment (Goyer, 1991; Seigneur and Constantinou, 1995), as either
trivalent Cr(Ill) or hexavalent Cr(Vl), as in the chromate artion CrOv2(Vl)
or the compound CrOj(VI). CrfVT) has the lowest risk specific dose for all
carcinogenic metals (0.00083 ng/m') (Fed. Reg., 1991). while Cr(lII) is
not considered particularly hazardous. However, from a regulatory
viewpoint, all Cr must be considered to be Cr(Vl) unless difficult silo
specific speciation is performed. Therefore, it is important to determine not
only how Cr is chemically partitioned among valency states in the exhaust.
but also on how this can be manipulated through the addition of sulfur and
chlorine compounds, and how the physical partitioning of Cr among
various particle sizes occurs in a practical combustion configuration.
Equilibrium Predictions
Multicomponent equilibrium calculations can provide insight into which
species are thermodynamically stable at flame and Hue gas temperatures.
However, the accuracy of equilibrium results depends on the accuracy of
the ihermodynamic data available, and on the availability of thermodynamic
data for all important species containing the elements in question. In
addition, equilibrium calculations do not take into account kinetic or mixing
limitations and, therefore, represent an idealized solution that may not be
realized in practical systems. Thermochemical predictions were determined
using the CET89 computer code for calculating complex chemical
equilibrium compositions (Gordon and McBride, 1986). Twenty-six Ni
species and physical states and 48 Cr species and physical slates are
considered in these calculations (see Linak etal. 1996, 1997). Of interest is
the thermodynamic partitioning between vapor and condensed phases, as
well as the partitioning between various species. Also, of special interest
for Cr is the partitioning between Cr(Vl) and other Cr valent species. For
both Ni and Cr, the influences of chlorine, sulfur, and both chlorine and
sulfur were investigated.
Equilibrium predictions (Figure 4a) indicate that both metals are
refractory, with dewpoinls of 2000 and 1900 K for 100 ppm Ni and Cr,
respectively. Sulfur addition (Figure 4b) has little effect on either metals'
dewpoint. Chlorine addition (Figure 4c) lowers the Ni dewpoint by
approximately 200 K and moves the Ni curve from the right to the left of the
Cr curve, which shews no significant effect of chlorine. The tact that
chlorine is more likely to devolatilize Ni than Cr, at high temperatures,
suggests that, in the presence of chlorine, high temperature sorbents might
be able to capture Ni, but are less likely to capture Cr.
At low temperatures, Ni and chlorine are predicted to form a condensed
Ni-chloride salt, thus rendering the residue water soluble. However,
chlcrine's predicted effect on Cr at low temperatures is profound. Not only
is Cr predicted to form vapor-phase Cr-chlorides, but it is also predicted to
form the hexavalent Cr-chlorides, CrOCl4(VI) and CiCl6(Vl), as shown on
Figure 4c. The presence of sulfur (Figure 4d) completely eliminates the
chlorine enhanced formation of low temperature Cr(Vl) species, but has
(d) Cl = 2500 ppm. S = 2500 ppm
0.0
300 600 900 1200 1500
Temperature (K)
1800 2100
RGURE 4 Ni and Cr equilibrium predictions for four conditions: (a) 0
ppm chlorine, 0 ppm sulfur; (b) 0 ppm chlorine, 2500 ppm sulfur; (c) 2500
ppm chlorine, 0 ppm sulfur; and (d) 25(K) ppm chlorine, 2500 ppm sulfur.
-------
little effect on the high temperature Cr(VI) species (not shown on Figure 4,
see Linak eiaL, 1996). This is because sulfur lies up Cr to form trivalent
Cr-sulfate, but only at low temperatures.
The effect of sulfur on the Ni/chlorine mixture is predictable. It has no
effect at high temperatures on the devolatilization of Ni, sinee Ni-sulfales
arc unstable there, while its effect at low temperatures is merely to replace a
solid Ni-chloride by a solid Ni-sulfale. Additional calculations exploring
the equilibrium effects of calcium addition showed that calcium, even at
concentrations in excess wilh respect to sulfur, displaced neither the Ni- nor
the Cr-sulfate and. therefore, had no appreciable effect on Ni or Cr
partitioning.
Particle Sue Distributions without Chlorine
Figure 5 presents the PSDs for Ni [injected as Ni(NOj)j], for Cr(IlI)
[injected as Cr(N03)j(III)], and forCr(Vl) [injected as CrOj(VI)]. These
data (open symbols) were obtained using the SMPS for particles in the 0.01
to 1.0 urn diameter size range and the in-siiu light scattering panicle sizer
for particles the 0.3 to >10 |am diameter size range. Each panel also shows
the effect of chlorine addition (solid symbols), where the chlorine was
added as Clj gas, with the secondary combustion air. While
complementary impactor samples were taken, those data are not presented
here but may be found elsewhere (Linak et al., 1997).
It should be noted that all three methods of panicle collection and sizing
produced consistent results that supported each other. Data from the in-situ
light scattering panicle sizer slightly overlapped and extended the range of
the SMPS for the sampled panicles. This suggests that the isokinelic
dilution sampling procedure used maintained aerosol size integrity.
The Ni volume PSD without chlorine (Figure 5a) showed a maximum at
about 0.3 to 0.4 Jim diameter. ForCr(lII) without chlorine, a single panicle
size mode peaking between 1.0 and 10 |am diameter is shown by both the
in-siiu light scattering particle sizer data (Figure 5h) and the impactor data
(data not shown, see Linak et aL, 1997). For Cr(Vl), two mode;, are
apparent from the SMPS and the in-situ light scattering panicle sizer, wilh a
dominant mode peaking at about 0.1 to 0.3 |Jm diameter (Figure 5c). This
is verified by the impaetor results (dala noi shown), which also show a
dominant mode at ahoul 0.2 lo 0.3 |Jm diameter. These results suggest that
CrOj(VI) vaporized, while Cr(NOj).i(IIl) did not. The difference in
volatilization behavior of the iwo Cr compounds is, of course, in conlrasl to
10s.
10V
10".
10*.
10
10°.
10-'
(a)
o Ni(N03)2
• Ni(NO3)2w/CI
• Ni(NO3)2 w/CI and kaolinite
+ kaolinite only
ng 10s
£ 10'
I lOV
O 10*.
en
i 10'.
1 10°
10-'
(b)
Cr(N03)3(lll)
Cr(NO3)3(lll) w/CI
106
10s.
10V
10'.
10*.
10'.
10°.
10-'.
(c)
0.01
0.1
10
100
Dp (urn)
FIGURE 5 Particle volume distributions measured by electrical mobility
and light scattering for. (a) Ni(NOj)2; (b) CrCNOjbqil); and (c) CrO,(Vl)
aqueous solution feeds wilh and withoul kaolinite and chlorine.
equilibrium predictions, which aie independent of the inilial Cr speciation.
Effect nf Chlorine
Chlorine has a significant effect on the Ni number and volume PSDs. The
maximum number conccntralion now occurs al 0.03 nm diameler (data not
shown), while the maximum volume concentralion has shifted to particle
diameters less lhan 0.1 pm (Figure 5a). These PSDs are consislenl with a
nucleation/vaporization mechanism for Ni in ihe presence of chlorine. The
Cr results, by conlrast. show no effect on the stack PSD by chlorine, as
illustrated by comparison of open and solid symbols on Figures 5b and 5c.
In the case of Cr(NOj).i(lH), the chlorine did not facilitate vaporization,
while in the case of CrOj(VI), 'which vaporized without chlorine, no
difference in PSD was noted. The Ni and Cr results arc qualitatively
consistent with the equilibrium predictions of Figure 4, which show the
effecl of chlorine to he that of moving the Ni dewpoint from above thai of
Cr lo below lhat of Cr. According lo both iheory and experiment, chlorine
facilities Ni volatilization at high temperalures, hul has little effect on Cr.
Note, however, lhat results are inconsistent with the low lemperature
equilibrium predictions of Cr with chlorine, since the Cr was found to
condense at low lemperature (i.e., the predicted equilibrium yield of almost
10073(VI) underwent coagulation over a longer period of time than
did Ni with chlorine, presumably because it had a higher effeclive
-------
dewpoint. Hydrated liifie also had negligible effect on ihc PSDs from
Cr(NOj)3(lII) and CrOj(VI), and so it can he concluded that lime is an
ineffective sorben; for all forms of Cr tested, whether vaporization occurred
or not.
Cr Spec/alien
In addition to factors influencing PSDs, chemical speciation is also of
paramount importance, especially as far as Cr is concerned. Equilibrium
predictions of Cr speeiaiion (sec Linak ti til., 1996) suggest that, in the
absence of chlorine, the fraction of Cr(VI) is small and appears only at the
higher temperatures. When chlorine is added, two additional Cr(VI) species
are predicted to appear at lower temperatures [CrOCU(Vl) and CrCloO/I)].
EDX analysts of collected particles shows that chlorine is found only on the
fused spherical particles, and not on the angular crystalline panicles. One
might speculate that the fused particle may contain Cr compounds including
CrOCUCVl) and/or CrClc(Vl). However, since the equilibrium calculations
at low temperatures do not predict the existence of a solid species containing
Cr, one might conclude that kinetic limilalions prevent significant Cr(VI)
formation. Whatever the formation route to chlorinated Cr in the exhaust,
sulfur is predicted by equilibrium (Figure 4) to eliminate those species and,
thus, (potentially) eliminate one source of Cr(VI).
Figure 6 depicts the overall partitioning between Cr(VI) and total Cr
(Linak el al, 1996). !n the upper panel (Figure 6a), Cr partitioning resulting
from the introduction of Cr(NO->}j(IH) is presented. With neither chlorine
nor sulfur present, approximately 2% of the total Cr in the slack gas effluent
is hexavalcnt. Addition of 1000 ppm (low) chlorine (stack) increased the
percent Cr(VI) in the exhaust slightly to 2.5%. Addition of 6,700 ppm
(high) chlorine (stack) raised the Cr(Vl) percentage in the exhaust to
approximately 8%. The addition of sulfur (no chlorine present) sharply
diminished the emission of Cr(Vl). In fact, with a high concentration of
sulfur (7,900 ppm, slack), the Cr(VI) percentage was reduced to near
detection levels.
The trends exhibited in Figure 6a, are consistent wiih equilibrium
predictions, although the absolute values are not. In the absence of both
chlorine and sulfur, some Cr(Vl) which is stable at higher temperatures
appears to persist through to the lower temperature regime, even though
equilibrium would not predict its presence there. High chlorine
concentrations sharply enhanced Cv(Vl) emissions, possibly due to CrCI^
which was predicted to be stable at low temperatures. The effect of sulfur is
consistent with equilibrium if it is assumed that conversion of Cr to
Cr2(S04)j is rapid. It is interesting that equilibrium predicts that even
relatively small quantities of sulfur can counteract the Cr(Vl) formation
tendencies of chlorine. This has been verified experimentally, where even
stoichiometric quantities of sulfur were able to prevent the formation of
measurable quantities of Cr(VI) (Linak rial., 1997). Both theory and
experiment suggest that sulfur is effective in eliminating Cr(VI) because it
displaces chlorine from the Cr(Vl) compounds otherwise formed without
sulfur. Both theory and experiment suggest lhal only a very small amount
of sulfur, determined hy the Cr/sulfur stoichiomeiry. is sufficient to
suppress chlorinated Cr(Vl) compounds.
In the lower panel (6h). analogous results arc presented for CrOi(VI)
feed. It is significant that Uiey are very similar to the results presented in the
top panel (6a) with Cr(ll() feed. The partitioning of Cr in a combustor thus
seems to tie independent of the initial valence of Cr waste feed. Most of the
CrCVI) that entered was converted to Cr(III), The similarity in partitioning
between the upper and lower panels of Figure 6, and the fact that the final
Cr(VI)/lotal Cr partitioning is independent of initial speciation, suggests that
prior to sampling, some type of equilibrium controlled mechanism, with
neither kinetic nor mixing limitations, is operable. However, the low
conversion to Cr(Vl) in the presence of chlorine, and the finite conversion
in the absence of chlorine and sulfur, suggest that this equilibrium is
"frozen" at a temperature higher than the sampling or exhaust temperature.
SEMI-VOLATILE METALS
While non-volatile metals may not readily vaporize during combustion
processes and are often reported to contribute preferentially to bottom ash
and collectable portions of the PSDs, semi-volatile metals are often easily
vaporized at combustion temperatures. These vapor-phase species will
become supersaturated and subsequently undergo homogeneous nucleation
or heterogeneous condensation at the lower temperatures downstream to
contribute to the submicron PSD. These particles, because of their small
size, are difficult to collect in pollution control systems. However, their
propensity to vaporize also allows the possibility of control through
interactions with injected sorbents. Therefore, this research investigated
sorbent injection processes in which the high temperatures of practical
combustion environments might be exploited to transform semi-volatile
metals into constituents that are both, more easily collected, and more
environmentally benign, than metal effluents in the absence of combustion
modifications. Semi-volatile metals of interest were Cd and Pb, and
kaolinile, bauxite, and hydrated lime were chosen as representative
sorbents.
Using a downflow laboratory combustor, Scolto et al. (1992) found that
Pb could be reactively scavenged, in-situ, by kaolinite powder which was
injected into ihe post flame. Reactive scavenging (chemisorption) of a metal
occurs at temperatures above the metal vapor dewpoint. This group also
"O
0)
3
CO
-------
0.8.
0.6.
0.4.
0.2'.
ed species
p ^ p
OS O O
c. 0.6.
m
|0.4.
« 0.2.
£ 0.0.
•D 1.0
•5 °-8-
H 0.6
o
§ 0.4
| 0.2
I 1-°'
0.8
0.6
0.4
0.2.
0.0
\\ (a) Cl = 0 ppm, S = 0 ppm
\ \
Pb\ \Cd
i *
\ \
\ \
,
"V (b) Cl = 0 ppm, S = 2500 ppm
V"
\
\ \ (c) Cl = 2500 ppm, S = 0 ppm
\ \
\ Pb ', Cd
\ \
\ \
\ \(d) Cl = 2500 ppm, S = 2500 ppm
\ \
\ \
Pbl 'tCd
\
\ \
, _, , . , . , J , > , . . , , , , T
300 600 900 1200 1500
Temperature (K)
1800
2100
FIGURE 7 Cd and Pb equilibrium predictions for four conditions: (a) 0
ppm chlorine, 0 ppm sulfur; (b) 0 ppm chlorine, 2500 ppm sulfur; (c) 2500
ppm chlorine, 0 ppm sulfur; and (d) 2500 ppm chlorine, 2500 ppm sulfur;
dewpoinL For Cd, CdO(s} is formed at 1300 K and CdSO«(s) is preferred
over CdCl2(s) forming at 1000 K.
Pb/Kaolinite/Chlorine System
In contrast to Ni (Figure 5a), the Pb data (Figure 8a), without chlorine,
indicate the presence of a distinct submicron mode with a mean particle
diameter between O.I and 0.2 urn. This behavior is consistent with Pb
vaporization followed by subsequent aerosol formation and growth and is
consistent with the known volatilities of elemental Pb and Pb-oxide. With
chlorine added, this mode is shifted towards even smaller panicle sizes
(between 0.03 and 0.1 nm) possibly indicating delayed nucleation and a
less mature aerosol at the sampling location. There is also evidence of
bimodal behavior in the presence of chlorine which may indicate the
formation of at least two Pb species with different nucleation characteristics.
The impactor data (Figure 8b) indicate that between 80 and 82% of the
measured Pb is associated with particles smaller than 1.1 (im.
With the addition of kaolinite, boih the DMPS and impactor data indicate
substantial reductions in the submicron aerosol volume and Pb mass
fraction (72 and 98%, respectively) compared to the corresponding PSDs
without chlorine. Similar reductions are also evident comparing the
distributions with chlorine (49 and 86%). Morphological observations
indicated that much of the kaolinite melted, both with and without Pb
present. These results are consistent with those of Scotto et a!. (.1992),
where high uptakes of Pb on sorbent particles were associated with
formation of melts on sorbent surfaces.
Cd Interactions with Sorbenfs
Cd baseline and Cd/chlorine data (without kaolinite) are similar to
corresponding Pb data presented above. Elemental Cd, CdO, and CdCb
vapor pressures are similar to those for elemental Pb, PbO, and PbClj, all
of which are notably high at the peak temperatures seen in the combustor.
As with the Pb system, the Cd behavior is indicative of particle formation
via a vaporization mechanism. The impactor data (Figure 9a) show that 88
and 85% of the Cd mass are associated with particles less than I.I u.m for
the Cd baseline and Cd/chlorine experiments, respectively. Also consistent
with the Pb data, the Cd data show that the addition of kaolinite causes
substantial decreases in both the DMPS submicron volume concentration
(61%-data not shown) and the <1.1 ujn impactor Cd mass fraction (97%-
Figure 9a).
These results differ from those of Uberoi and Shadman (1991) in two
important respects: (1) the amount of Cd removed here (97%), in a time-
scale of seconds, is far higher than the 5% removed by kaolinite in their
moderate temperature bench-scale studies, and (2), the sorbent particles that
removed Cd here were melted, with no observable (by XRD) Cd-rclated
crystalline structure, while in the bench-scale studies they remained
crystalline. The melt appeared to avoid limitations of pore blockage by
reaction products, as identified by Uberoi and Shadman (1991).
With the addition of bauxite, the DMPS distributions (Cd baseline and
Cd/bauxite) illustrate significant removal of particles <0.2 |lm (distributions
not shown). The impactor data (Figure 9b) indicate that 97% of the Cd
originally associated with particles < 1.1 (im in diameter was removed from
that particle size range through the addition of bauxite. These results are in
agreement with those of Uberoi and Shadman (1991) which suggest bauxite
to be an exceptional sorbent for use with Cd. Furthermore, in both this
combustor study and in the previous bench-scale studies, the sorbent
panicles remained unmelted and crystalline. Therefore, sorbents that do not
melt can also be effective in reactively scavenging vapor-phase metals, if
pore blockage is not a factor.
As with the two other sorbents, hydrated lime acts as an effective agent
to scavenge Cd which would otherwise contribute to the submicron aerosol
fraction (Figure 9c). However, these results are in contrast to those of
Uberoi and Shadman (1991), and one would not expect reactive scavenging
to occur. It is interesting to note that hydrated lime seems to be particularly
effective even in the presence of chlorine. The 99% reductions in both
submicron volume (DMPS data not shown) and Cd submicron mass
fraction (impactor) with chlorine present (Figure 9c), represent the greatest
measured removals seen for any chlorinated system examined here. In the
absence of Cd, the lime sorbent particles were crystalline, angular, and had
not melted. With the addition of Cd, the calcium-rich sorbent particles
melted. Calcium oxide, which is basic, is known to enhance formation of
eutectic melts with acidic metal oxides.
Mechanisms and Conclusions
There appear to be two high temperature mechanisms that allow the
scavenging of metals at combustion temperatures above their dewpoints.
The first mechanism involves reaction between metal vapor and a sorbent
100000
0 w/o Cl or kaolinite
D w/ Cl only
» w/ kaolinite only
" w/ Cl and kaolinite
£ 0.0
1 10
Dp (pirn)
100
FIGURE 8 DMPS submicron volume distribution and impactor Pb mass
fraction distribution for the Pb/kaoiinUe/chlorinc system.
-------
° w/o Cl or sorbent
w/ Cl only
w/sorben| only
w/Clandsorbent
0.1
100
FIGURES Interactions hclwccn Cd and three sorbenls. Impactor mass
fraction distributions lor; (a) Cd/kaolinite; (h) Cd/bauxile; and (c) Cd/lttne.
with and without chlorine.
crystalline surface (Uheroi and Shadman, 1991). SEMs provided evidence
of this sorption mechanism for the Cd/hauxile system, which, by exhibiting
a I/Dp dependence of Cd mass fraction on the sorbent, also suggested pore
diffusion or external reaction controlled processes (Linak and Wendt,
1993). The second mechanism allows scavenging of meiul vapor by a
liquid melt on the sorbenl. Melting appears to improve capture. Cd was
scavenged by kaolinile in the combustor used here, because the sorbenl
melted (as depicted in SEMs), bul it was only poorly scavenged at lower
temperatures (Uberoi and Shadinan, 1991) where no melting was observed.
In the Cd/lime system, the melt was created by the very interaction of
dissolution. Melts were also observed alter the scavenging of labile Ni (in
the presence of chlorine). Thus, metal capture by sorbenls may be more
practical in high temperature combustion environments, where melting is
more likely, than was initially suggested by ihe moderate temperature
bench-scale thermogravimeirie reactor studies (Uheroi and Shadinan. 199!),
1991).
The effect of chlorine is to significantly increase the submicrort volume
concentrations and suhmicron metal mass fractions, in the absence of
sorbents, and, for the Pb/kaolinite, Cd/hauxite, and Cd/kaolinite systems, to
diminish sorbent effectiveness when they are present. An explanation for
this behavior is as follows: for Pb and Cd interaction with alumino-silicate
sorbents, the true metal reactant with the substrate is probably the metal
hydroxide or oxide. As chlorine is introduced, equilibrium is shifted away
from these reactive metal species towards unreactive metal chlorides. This
has been shown for the Na/kaolinite system (Mwahe, 1993). and suggested
for Pb (Scotto etui.. 1992). Ni, however, can be vaporized only by
interaction with chlorine, and then (possibly through equilibrium with other
reactive Ni species) scavenged by kaolinite. The capture of Cd by hydrated
lime is through a different (physical) mechanism (suggested by the SEMs)
where the pertinent mechanisms involving chlorine are currently unknown.
VOLATILE METALS
Volatile metals including Hg 'jnd Se can be distinguished from semi-volatile
metals in combustion systems by the fact that they exhibit significant vapor
pressures even at low to moderate temperatures typical of flue gas cleaning
equipment. This typicaliy results in poor emission control of these species.
!n fact, of all the trace metals emitted during fossil fuel combustion and
waste incineration processes, Hg is likely considered the most problematic.
This is not because it is the most toxic or typically present in highest
concentrations, but rather, because current control processes designed for
paniculate, nitrogen oxide (NOX), and acid gas emissions are minimally
effective in controlling vapor-phase Hg species.
Hg exists in two valent states; elemental Hg (Hg°), and oxidized (ionic)
Hg (Hg*2). Effective Hg control using methods designed for paniculate
and acid gas emissions will depend largely on Uw form (vapor or condensed
phase) and specialion (elemental or oxidized) of the Hg in post-combustion
regions prior to air pollution control devices. It has been noted by Senior et
al. (1997) that oxidized Hg*2 js more likely to be captured by residual
carbon or removed by existing flue gas desulfurization units, while
elemental Hg° is more likely to escape the air pollution control devices and
be emitted (o the atmosphere. Several investigations are in progress
examining the potential of carbon and inorganic-based sorbenls for Hg
control at moderate to low temperatures typical of Hue gas cleaning systems
(Miller etal.. 1995; Krishnan «til, 1995, 1997).
Equilibrium Predictions
Figure 10 presents equilibrium predictions of the behavior of Hg within a
combustion environment. Sixteen Hg species and physical states were
considered. These calculations were made in the same manner as those
presented for the non-volatile and semi-volatile metals (Figures 4 and 7),
with the exception that the HE concentration (stack) used was 100 ppb
instead of KK3 ppm. Meihane/air combustion at a stoichiomelric ratio of 1.2
was used. Chlorine and sulfur concentrations (stack) were both 100 ppm.
The top two panels (Figures Kb and Itth) plot condensed tnass fraction of
Hg and oxidized Hg*3 mass fraction as a function of temperature. Four
conditions (with and without chlorine and sulfur) are presented on each
panel. Figure lOa indicates a Hg dewpoint (without chlorine or sulfur)
helween 4(KS and S!X) K through ihe condensation of HgO(s). The effcci of
sulfur is to increase this threshold temperature approximately 100 K, while
the effect of chlorine (even with sulfur present) is 10 decrease (he Hg
dewpoint through the formation of HgCljCs). Note that, in contrast to Cr,
HgSO4(s) is not preferred to HgCI;i(s). Figure lOb indicates thai at high
temperatures (>9(X) K) almost all the Hg exists as Hg". Without chlorine
or sulfur, HgQ and then HgO(s) are predicted with decreasing temperature.
With sulfur (but without chlorine), HgO and then HgS04(s) are predicted.
However, HgO, HgClj, and then HgClj(s) are predicted to be formed
whenever chlorine is available (<9()() K), and vapor-phase HgClj is
predicted lo be present ax the dominant Hg species at temperatures between
900 and 400 K. This may offer the possibility of Hg control through
interactions of oxidized HgClj with sorbents.
It should be re-emphasized that equilibrium calculations can be used only
to determine which species are thermodynamically possible. They do not
include kinetic or mixing considerations which may'severely limit die
attainment of equilibrium in the short times available in practical combustion
systems. Senior et til. (1997) report that, in a survey of 14 coal combustion
systems, oxidized Hg*5 concentrations upstream of the air pollution control
devices (750-900 K) ranged between 3(1 and 95% of the total Hg measured
(averaging 75%). They went on to conclude that equilibrium could not be
used to quantitatively predict Hg specialion in ihe flue gas. However, even
with this liability, equilibrium calculations are useful lo test hypotheses and
suggest experiments.
Figures lOe and Hid present the results of equilibrium calculations
designed to examine possible interactions between Hg and calcium that
might be present in the ash or introduced as a sorhent. While searching
thermodynamic data bases for data on Hg species, we discovered dala for a
set of ealctum-Hg amalgams (TAPP, 1995), and these "species" were
included in the data base for the calculations presented in Figures lOc and
lOd. Figure lOc presents predicted Hg speciation as a function of
temperature for a fuel-lean (SR=1.2) scenario with calcium. Sulfur was
also included because of its possible interactions with calcium. As expected
calcium oxides, hydroxides, carbonates, and CaSO,t(s) are the preferred
species over all temperatures examined. However, under fuel-rich
conditions, such as those used 10 stage combustion for NO, control, these
oxidized species are not predicted, and Figure lOd indicates that two
calcium-Hg amalgams are ihermodynamically stable within a temperature
window between 900 and 1300 K. This result may be significant because it
represents conditions that may be achieved in practical combustion systems.
Experiments are currently underway to examine these predictions. Other
experiments are examining Hg speciation as functions of combustion
conditions and chlorine and sulfur content, and factors which promote the
formation of Cl? over HCI which may influence concentrations of HgCI 3
(Gullett eiat.. 1990; Senior « al.. 1997).
Preliminary experiments (without chlorine or sulfur addition) indicate
that Hg speciates primarily as elemental Hg° (>95%) and that hydrated lime
(introduced as a sorbenl) is minimally effective in reducing the vapor-phase
oxidized Hg*2 under fuel-lean (SR=1.2) conditions. However, similar
-------
0.0
300 600
900 1200 1500
Temperature (K)
1800 2100
FIGURE 10 Hg equilibrium predictions: (a) condensed fraction with and
without chlorine and sulfur; (h) oxidized fraction with and without chlorine
and sulfur; (c) Hg species with calcium and sulfur - fuel lean (SR=1.2); and
(d) Hg species with calcium and sulfur - fuel rich (SR=0.6).
experiments performed under staged conditions (SRprim;uy=0,8) suggest
that hydrated lime might he an effective sorhent. indicating that vapor-phase
oxidized Hg*J was reduced to less than detection limits. However, these
same experiments also indicated that the elemcnfa! Hg° remained >95% of
the total Hg measured and was unaffected hy the presence of hydrated lime.
Therefore, the locus of ongoing efforts includes strategies to promote the
formation of oxidized Hgc/mo/., submitted.
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Submicron Aerosol Mode in Flue Gas from a Pulverized Coal Utility
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Maisoukas, T. and Friedlander, S.K. (1991). Dynamics of Aerosol
Agglomerate Formation. /. Colloid Interface. Set.. 14C(2), 495-506.
Miller, SJ., Laudal. D.L.. Dunham. G.E., Chang, R., and Bergman, P.O.
"Pilot-Scale Investigation of Mercury Control in Baghouscs," EPRI/DOE
Third International Conference on Managing Hazardous and Paniculate
Air Pollutants. Toronto, Canada, August (1995).
Mulholland, J.A. and Sarofim, A.F, (1991). Mechanisms of Inorganic
Particle Formation Dunne Suspension Heating of Simulated Aqueous
Wastes. Environ. Sci. Techno!.. 25(2). 268-274.
Mulholland, J.A., Sarofim, A.F., and Yue, G. (1991), The Formation of
Inorganic Particles During Suspension Healing of Simulated Wastes.
Environ. Prog.. 10(2), 83-88.
Mwahe, P.O. (1993). Mechanisms Governing Alkali Metal Capture by
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Nettleton, M.A. (1979). Paniculate Formation in Power Stations Boiler
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1440, Comb. Inst., Pittsburgh, PA.
-------
Queneau P.ff.. May, L.D., and Cregar. D.E., "Applicaiioii of Slag
Technology to Recycling of Solid Wastes." 199! Incineration
Conference. 69-85. Knoxville.TN. May 1991.
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Rizeq, E.G., Hansel!. D.W., and Seeker, W.R. (1994). Prediction of
Metals Emissions and Partitioning in Coal-fired Combustion Systems.
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Scotto, M.A., Peterson, T.W., and Wendt, J.O.L. (1992). Hazardous
Waste Incineration: The In-stiu Capiure of Lead by Sorbents in a
Laboratory Down-How Combustor. 24ih Comb. (Ins.) 5ym/;., 1109-
1118, Comb. Insl., Pittsburgh, PA.
Seigneur, C. and Constaminou, E. (1995). Chemical Kinetic Mechanism
for Atmospheric Chromium. Environ, Sci. Ttchnol., 29,222-231.
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Sarofirn, A., Olmez, I., and Zeng, T. "A Fundamenlal Study of Mercury
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10
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NRMRL-RTP-P-250
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before camp
I. REPORT NO.
PA/600/A-97/085
4, TITLE AND SUBTITLE
Metal Partitioning in Combustion Processes
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S>
William P. Linak
8. PERFORMING ORGANIZATION BEPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
10, PROGRAM ELEMENT NO.
See Block 12
11. CONTRACT/GRANT NO.
NA (Inhouse)
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Air Pollution Prevention and Control Division
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Published paper; 10/89-6/97
14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES ^PPCD project officer is William P. Linak. Mail Drop 65, 919/
541-5792. Presented at 4th Int. Conf. on Technologies and Combustion for a Clean
Environment. Lisbon. Portugal.. 7/7-10/97.
16. ABSTRACT ,
The paper summarizes ongoing research efforts at the National Risk Man-
agement Research Laboratory of the U. S. Environmental Protection Agency examin-
ing (high temperature) metal behavior within combustion environments. The parti-
tioning of non-volatile (Cr and Ni), semi-volatile (Cd and Pb), and volatile (Hg) me-
tals in combustion systems was investigated theoretically and experimentally. Theo-
retical predictions were based on chamical equilibrium and suggested that such cal-
culations can be useful in predicting relative volatility and speciation trends, and to
direct experimental efforts. Equilibrium studies employing a 59 kW laboratory scale
eombustor examined the behavior (volatility, particle size, and speciation) of metal
vapors and particles produced by aqueous metal solutions sprayed through a swirling
natural gas diffusion flame. These experiments were designed to study metal trans-
formation mechanisms in a relatively simple combustion environment without the
complex effects of additional species. Further experiments examined the potential
use of common inorganic sorbents (kaolinite, bauxite, and hydrated lime) to adsorb
metal vapor, offering a potential means of metal emissions control.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution
Metals
Emission
Separation
Combustion
Volatility
Sorbents
Pollution Control
Stationary Sources
Trace Metals
Partitioning
13B 11G
11F, 07B
14G
21B
20M
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report}
Unclassified
21. NO. OF PAGES
20, SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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