close

Вход

Забыли?

вход по аккаунту

?

Speciation of Most Volatile Toxic Trace Elements during Coal Combustion.

код для вставкиСкачать
Dev. Chem. Eng. Mineral Process., 11(3/4), pp. 381-394, 2003.
Speciation of Most Volatile Toxic Trace
Elements during Coal Combustion
Liu Yinghui*, Zheng Chuguang and Wang Quanhai
National Laboratory of Coal Combustion
Huazhong University of Science and Technology
Wuhan, Hubei 430074, ??R. China
Different trace elements have different physical and chemical properties,
Understanding the speciation of trace elements in coal combustionjlue gas is very
helpful in order to evaluate their partitioning behavior, emission control, atmospheric
transport, and their effects on the environment and human health. This work used a
chemical-thermal dynamic equilibrium analytical method to study the speciation of
the most volatile toxic trace elements in coal, namely mercury, arsenic and selenium,
during coal combustion. In our work we established two types of thermal models.
The first model is a simple one in which we only consider one type of trace element
and do not consider the interaction between trace elements. The other model is a
complex model in which we consider the interaction between trace elements, and the
interaction between trace elements and major mineral matter in coal. From the
results predicted by these two models in the temperature range between 400 K and
2000 K, and in oxidizing and reducing atmospheres, we concluded that the volatility
of three elements is successively in the order: mercury selenium > arsenic, Chlorine
can greatly enhance the evaporation ability of mercury but has very little influence on
the evaporation of selenium and arsenic. The interaction between trace elements, and
between trace elements and other elements in coal, will greatly affect the predicted
results.
Keywords: Coal; trace elements; combustion; equilibrium analysis.
Introduction
For more than 150 years, coal has been an important source of energy for both
developing and industrial societies. Projections for the next several decades call for
sustained use of coal in most industrial countries, and for increased coal use in many
developing countries such as China and India. While use of this relatively inexpensive
fuel can strengthen economies, coal use can also bring with it many problems
concerned with the environment and human health 111.
Author for correspondence (Iiuyh730214@163.net).
381
Liu yinghui, Zheng Chuguang, and Wang Quanhai
According to their concentration in coal, the elements can be divided into three
categories: the main coal elements (C, H, 0, N and S whose contents are usually more
than 1000 ppm) which build up the organic matrix of coal; the minor coal elements
(Si, Al, Ca, Mg, K, Na, Fe, Mn, P, and Ti having contents generally between 100 and
1000 ppm) and constitute the inorganic mineral matter of coal; and thirdly the
halogens (C1, Br, and F usually having the same concentrations in coal as the minor
elements). In addition, almost all the other elements in the Chemical Periodic Table
may be present in coal in very low amounts (< 100 ppm), and they are called trace
elements [2].
Trace elements are important because of their association with environmental
issues their effect on the health of plants, animals and humans. Many of the trace
elements are toxic even in very small quantities. Trace element in the coal released
during coal combustion can have serious environmental and human health effects.
Environmental legislation in some countries (notably Germany and the USA) already
specifies limits on the emission of certain trace elements to the atmosphere. It is
expected that future regulations will be more stringent and cover a wider range of
trace substances. The Clean Air Act Amendments (USA, 1990) have identified 189
substances as potentially hazardous air pollutants (PHAPs) of particular concern to
human health and the environment [3]. Among these 189 substances, there are 11
inorganic elements and their compounds that are commonly found in coal. These
elements are antimony, arsenic, beryllium, cadmium, chromium, cobalt, lead,
manganese, mercury, nickel, and selenium. Coal combustion is an important
anthropogenic source for many of these elements, and they have been detected by the
Electric Power Research Institute (EPRI) in the flue gas from pulverized coal-fired
boilers [4].
According to their partitioning behavior during coal combustion, trace elements
can be classified into three broad groups [ 5 ] . Group 1 elements are concentrated in
coarse residues, or equally partitioned between coarse residues and finer particles.
Group 2 elements are volatilized in the combustor zone but condense downstream in
the flue gases. They are concentrated in the finer particles that may escape 6-om
particulate control systems such as electrostatic precipitators (ESP). Many studies
have shown enrichment of Group 2 elements with decreasing particle size. The
observed behavior may be explained by a volatilization-condensation mechanism.
Volatile elements preferentially condense on the surface of smaller particles in the flue
gases as cooling occurs. These fmer particles have a greater ratio of surface area to
volume. Group 3 elements are the most volatile elements (such as Hg and the
halogens) and may remain in the gas phase during passage through the plant. There is
considerable overlap between the groups, certain elements showing volatile behavior
in some studies but partitioning into solid residues in others. This may be attributed to
wide variations in the operating conditions, especially temperature, that control the
volatility of the element.
The most volatile trace elements in coal are Hg, Se, As, and the halogens, and they
tend to vaporize during combustion. Subsequently the flue gas cools as it flows to the
stack, these elements then partially condense on the surface of fly ash particles, while
remaining mainly in the vapor phase, and are emitted 6-om the stack mostly or
partially in gaseous forms. The percentage of the total in-stack concentrations for
these elements in the vapor phase have been reported to be: chlorine up to 99% as
382
Speciation of Most Volatile Toxic Trace Elements during Coal Combustion
HCI; fluorine up to 90%as HF; bromine fiom 25-98%as HBr, iodine fiom 90-99%as
HI; mercury up to 98% as Hg, HgC12, HgO and CH3Hg; selenium up to 59% as Se
and Se02;and arsenic fiom 0.7-52%as As203 and As0 [6].
Most early research work [6-121 concerning the main volatile trace elements in
coal focused on their emissions resulting fiom coal combustion and the partitioning
behavior among the various effluents of coal combustion systems, and did not
consider the speciation in combustion flue gases. Only very recently did researchers
begin to quantify the speciation of the most volatile elements in flue gases in
combustion systems. Speciation describes the range of physicochemical forms of an
element that collectively comprises its total concentration in a sample. The
identification and quantification of the forms of an individual element is imperative
for addressing questions concerning their emission control, toxicity, mobility,
bio-accumulation, and atmospheric fate and transport, because each has distinctive
physical, chemical, and biological properties. Here we will give a specific example:
many forms of Hg(I1) are highly water soluble, for example HgClz has a water
solubility greater than 6.9 X 10'og/l; However, Hgo possesses a relatively high vapor
pressure, 2.46 X 10' Pa, and low water solubility, 6 X lom5g/Lat 25% [ 131.
Some researchers [2,9, 14-16] have tried to use a modeling method to predict the
speciation of these volatile trace elements in flue gases. Thermodynamic equilibrium
calculations based on the principle of "minimizing the total Gibbs fiee energy of the
system" can identify the dominant species of each element in a multicomponent and
multiphase system when it is in an equilibrium state. If the kinetic model and data are
not available, then the thermodynamic equilibrium method is the best choice for
modeling predictions. However, we should be careful when applying the prediction
results to a real situation due to the limitations of the thermodynamic method.
Linak and Wendt [9] performed thermodynamic studies on the fate of 9 trace
elements during coal combustion. Their objective was to identify the dominant
chemical compounds of the selected elements in order to obtain equilibrium in a
complex multicomponent system.
Frandsen et al. [14] investigated the equilibrium distributions of 18 trace elements
fiom combustion and gasification, for both reducing and oxidizing atmospheres. Only
simple systems containing one type of trace element were presented in their study.
Frandsen compared their equilibrium result with results fiom the literature. Their
paper also described the so-called GFEDBASE (Gibbs Free Energy DataBASE)
database which includes 33 trace elements, and contains thermal-chemical data for
approximately 800 chemical species.
Wu et al. [15] carried out equilibrium analysis to determine the speciation of
arsenic, cadmium, chromium, lead, mercury and tin in an incinerator. They considered
the effect of temperature and chlorine content on the speciation of trace elements. The
model prediction results show that Hgo and HgC12 are the main forms of mercury in
the flue gas of incinerator.
Joseph [161 performed both thermodynamic equilibrium calculations and an
experimental study to investigate the partitioning behavior of the volatile elements
downstream of atmospheric pressure entrained-flow gasification. The results indicated
that most of the mercury, selenium and arsenic are evaporated during gasification, and
remain in the vapor phase in reducing flue gases for a temperature range of 773-873 K,
which is the operating range of most hot-gas removal systems.
383
Liu Yinghui, Zheng Chuguang, and Wang Quanhai
For thermodynamic equilibrium studies, most researchers usually consider a very
simple system containing only one trace element, and neglecting both the interaction
between the trace elements and between trace elements and minor elements (Si, Al,
Ca, Mg, Fe). Due to the lack of thermodynamic data for the slag formed by minor
elements in coal, which is an unrealistic solution, almost all researchers ignored the
interactions between mineral matter and trace elements in their system. Improvements
in both chemical thermodynamic models of multi-oxide component systems and the
computational methods have made predictions for systems containing slag possible.
Kalmanovitch et al. [17] proposed a technique to predict the slagging propensity
of the ash in coal-fired boiler furnaces based on the CaO-FeO-AI203-Si02 system. In
this paper, ash in coal is normalized to these four oxides and plotted on the
appropriate phase diagrams. The liquidus temperature and the nature of the
crystallization process to the point of complete solidification are used to predict the
slagging propensity. The technique has been corroborated by comparison with field
experience in six utility units, operating with wide range of coal ashes.
Fan et al. [ 181 used Ca0-AI2O3-SiO2and Fe0-AI2O3-SiO2ternary phase diagrams
to predict coal ash propensity. Experimental results show that the effect of CaO
additive on the coal ash fusion temperature is in agreement with the result predicted
by the ternary phase diagram.
Jak et al. [ 191 used the AI2O3-Si02-Ca0-Fe0-Fe2O3
five-component system to
predict slagging propensity in an integrated gasification coal combustion (IGCC)
system. Results show that this method can be applied by practicing combustion
engineers, production and marketing personnel,
In this paper we report a new complex system that can consider the interaction
between trace elements and the interactions between trace elements and minor
elements. The calculation conditions are less than standard atmospheric pressure,
reducing and oxidizing atmospheres with excess air coefficients of 0.8 and 1.2
respectively, and a temperature range of 400-2000 K.
Coal Sample Used in Equilibrium Analysis
The analysis of the coal (proximate, ultimate, trace elements and ash composition)
used for the calculations is shown in Table 1. Only elements used in the input file are
shown. All calculations were performed assuming 1 kg of coal.
Calculation Procedure
The chemical equilibrium analysis method is based on the principle that chemical
equilibrium is achieved when the total Gibbs fiee energy is at a minimum in
isothermal and isobaric systems. At a specified temperature and pressure the most
stable products are those associated with the lowest Gibbs energy. Using the Gibbs
energy minimization principle, we can identify the stable species in the gas-phase flue
gases produced during coal combustion under specific thermal conditions (i.e.
temperature, pressure and composition).
384
Speciation of Most Volatile Toxic Trace Elements during Coal Combustion
Table 1. Analysis results of coal sample [2OJ
Proximate DB (wt.%)
Ash
Volatiles
Fixed carbon
8.6
33.0
58.4
Ash analysis (?A)
SiOz
A1203
Fe
CaO
MgO
0.24
Na,O
0.10
0.05
0.15
Trace elements ppm
As
Se
Hg
60
2
0.2
KZ0
Ultimate DB (wt.%)
Carbon
Hydrogen
Nitrogen
Sulfur
Oxygen
Chlorine
76.2
4.9
1.5
2.7
6.1
2800ppm
1.59
3.12
1.87
We have used the major elements in coal (carbons, hydrogen, oxygen, nitrogen,
and sulfur) to represent the coal matrix. We only considered the three most volatile
trace elements in coal, namely mercury, arsenic and selenium. In our complex system
we considered five minor elements (Si, Al, Ca, Fe and Mg) to represent the mineral
substances in the form of the Ca0-Fe0-A1~0~-SiOZ-MgO
slag system.
The input condition under different atmospheres used in thermal equilibrium
analysis was calculated. We assumed the composition of air as 79% Nz and 21%02.
The access air number ALFA = VNO, where the value of V" is determined from the
relationship: Vo= 0.0889 ( C + 0.375 S ) + 0.265 H - 0.0333 0. The input file was
based on the chemical analysis of the organic and mineral matter in the coal. Thus,
amounts of the elements of interest in 1 kg of coal were converted to number of moles.
The input file contains the mole number of all elements in coal, and the molar amount
of air required for combustion.
A list of all possible compounds in the database is then produced automatically.
The database at McGill University containing about 5000 species and compounds can
be accessed via a personal computer. The database of the FACT software package
contains standard enthalpies of formation, absolute entropies at 298.15 K of
stoichiometric compounds, and heat capacity expressions for over 5000 elements and
stoichiometric compounds. For most condensed phases, density data are also included.
For compounds with more than one phase, enthalpies and temperatures of transition
may be stored instead of DH298 and S298. The majority of the data are taken from
the most recent standard compilations, but for several hundred compounds the data
have been critically evaluated and optimized at the CRCT. For most substances, Cp
expressions for a phase outside its temperature range of stability have been checked to
give reasonable extrapolations.The Gibbs energy expression is:
'
G,,,
0 = H: -TS: = Hm(298)
+ CpdT -T S:(298)+
[
298.15
] [
ICP
dT]
(1)
298.15
385
Liu Knghui, Zheng Chuguang, and Wang Quanhai
The EQUILIB program in the FACT software package was used to determine the
concentration of chemical species when elements or compounds specified reacted to
reach an equilibrium state. Only reactants need to be specified; products are not
specified and only their temperature and pressure are given.
In our calculations we assumed that the gas phase in the system is a perfect gas,
and all the condensed phases are pure phases. The species included are the specific
trace elements considered in our equilibrium analysis as shown in Table 2. Solid,
liquid and gas phases are represented by [S], EL], and [GI, respectively.
The output file included the chemical composition of the gas phase produced, and
the solid phase of ash or slag. From the output file, we can determine the distribution
of nitrogen, s u l k and chlorine, as well as the trace elements such as Hg, As and Se in
the flue gas emissions.
Our aim was to simulate trace element evaporation behavior during combustion in
the high-temperature furnace zone, and post-combustion behavior during cooling of
the gas and fly ash. The calculations were performed in the temperature range
between 400 and 2000 K, and considered both oxidizing and reducing atmospheres
with excess air coefficients of 1.2 and 0.8 respectively. It was assumed that 85% of
the mineral matter ends up as fly ash and 15% as bottom ash. The fly ash is carried
out and simultaneously cooled with combustion gases. It was assumed that the
amount of trace elements and minor elements that disappeared from the vapor phase
upon cooling either deposited on fly ash particles or solidified as very fine particles.
Table 2. Trace elements species considered in equilibrium analysis.
Element
Hg
As
Se
All
386
Speciation of Most Volatile Toxic Trace Elements during Coal Combustion
Results and Discussion
(0 Singie trace eiement/coafchiorinesystem
Our objective was to estimate the distribution of toxic trace emissions during coal
combustion. After setting the final conditions (i.e. temperature and pressure), the
EQULIB program automatically searched in the FACTBASE database for the thermal
dynamic property data, which includes 5000 types of chemical compounds and our
trace elements of mercury, selenium, and arsenic.
Simple systems contain only coal and single trace elements, we specifically
included the interaction between single trace elements and the halogen element
(mainly chlorine). A complex system also contains minor elements and we also
considered the interaction between three types of the most volatile trace elements in
coal, namely mercury, selenium and arsenic, and the interaction between trace
elements and between minor elements. In this paper we have adopted the designation:
trace element namekoalkhlorine to identify the different systems.
Mercury is the easiest trace element in coal to volatilize. The equilibrium
distribution of the Hg/coaVCl system under an oxidizing atmosphere is shown in
Figure 1. When the temperature is higher than 800 K, elemental mercury Hgo(G) is
the main form of mercury. For temperatures lower than 600 K, chlorinized mercury
HgC12(G) becomes the main form of mercury. Temperatures in the range 600-1000 K
show the smallest amount (< 5%) of oxidized mercury HgO(G) produced. Under
reducing atmospheres, only the elemental mercury exists in the flue gas.
1.oo
8
F
2
L
0.75-
! 0.50-
8!
0.00
400
600
800
1000
-/
-
1800
2000
Temperature/K
Figure 1. Hg speciation in an oxidizing atmosphere.
From the thermal equilibrium analysis results, we can infer the mercury behaviour
during coal combustion. In the high temperature zone of the furnace chamber, almost
all the mercury evaporates into elemental mercury and exists in the gas phase. When
the flue gases flow to the stack, the flue gas temperature will decrease fiom nearly
2000 K to 400 K. During this process, evaporated elemental mercury will react with
other species in the flue gases and be oxidized into HgClzor HgO. Prestbo and Bloom
387
Liu yinghui, Zheng Chuguang, and Wang Quanhai
[21Jindicate that in a coal-fired power station stack, flue-gas elemental mercury and
Hg + make up approx. 6-60% and 40-94% of the total mercury respectively. The
amounts depend upon coal type, boiler configuration and operating conditions. Carpi
[22] states that nearly 20-50% of total mercury exists in the form of elemental
mercury, and 5040% of mercury exists in the form of Hg2+. When the chlorine
content in the coal increases, then Hg2+also increases.
The equilibrium chemistry of mercury in oxidizing atmospheres can be explained
by the reactions:
Equation (2) indicates that when the temperature is higher than 650 K, then
elemental mercury is oxidized to oxidized mercury in progressive steps. For
temperatures above 800 K, the balance in Equation (3) shifts to the right as the
temperature rises.
g
2
1.oo
0.75
Q)
2
0.50
L
$ 0.25
L
0.00
1
400
.
1
500
I
1
600
700
.
800
1
900
f G 0 0
Te m pe rature/K
Figure 2. Hg speciation at diflerent C1 content.
Figure 2 shows the speciation of Hg at three different C1 contents in the flue gas.
As C1 content in the flue gas increases, then the temperature range where HgCh(G)
exists will be wider.
Chemical-thermal equilibrium results under reducing and oxidizing atmospheres
for the As/coaYCI system are shown in Figures 3 and 4 respectively. Under the
reducing atmosphere for temperatures under 500 K, arsenic mainly exists in the form
of As&(S). In the temperature range 500-700 K, arsenic mainly exists in the form of
388
Speciation of Most Volatile Toxic Trace Elements during Coal Combustion
ASS@). For temperatures above 800 K the arsenic is mainly in the form of AsO(G),
and Asz(G) and As4(G) will appear at temperatures between 700-800 K.
Under oxidizing atmospheres below 800 K, the main form of arsenic is As20s(S).
For temperatures near 800 K, AsO(G), AszOs(S), and A&Os(G)coexist, and As40s(G)
is the main form. When temperatures are higher than 1000 K, only AsO(G) exists in
the flue gas.
1.oo
ff 0.75
z
OSO
5- 0.25
(II
=
0
0.00
400
600
800
Tern perature/K
18002000
Figure 3. As speciation in a reducing atmosphere.
1.oo J-&A4-LL+.3
/o-=-
w3w
/&--I
400
600
800
18002000
Tern peraturelK
Figure 4. As speciation in an oxidizing atmosphere.
Chemical equilibrium distributions of a SekoaVCI system under reducing and
oxidizing atmospheres are shown in Figures 5 and 6 respectively. Under reducing
atmospheres, for temperature at 300 K, or between 350-1000K, or 1200-1600K,then
the main forms of selenium are elemental selenium, HzSe(G) and SeS(G) respectively.
When temperatures are above 1600 K, then SeS(G) will be oxidized to gaseous
elemental selenium Se(G) and oxidized selenium SeO(G).
Under oxidizing atmospheres, the main forms of selenium are various types of
oxidized selenium. When the temperature is 300 K, the main form of selenium is
oxidized selenium SeOz(S).For temperatures in the range 350-1600K, the main form
389
Liu Yinghui, Zheng Chuguang, and Wang Quanhai
of selenium is gaseous oxidized selenium Se02(G). At temperatures above 1600 K,
the amount of gaseous Se02(G) decreases as SeO(G) increases due to the
decomposition of SeOz to SeO. When the temperature is 2000 K, the amount of
SeO(G) is almost equal to that of Se02(G), and there is a small amount of elemental
selenium Se(G) produced at this condition.
3 0.25L
5
'
0.00I
300
k
350
.
800
T
'
1200
r
1600
'
I
2000
TernperaturdK
Figure 5. Se speciation in a reducing atmosphere.
1 .oo
s?
{
0.75-
2
!a, 0.50:
a
s- 0.25lu
0
=
0.00-
3iO
'
350
.
ii"o0
1600
2000
Tern perature/K
Figure 6. Se speciation in an oxidizing atmosphere.
The equilibrium chemistry of selenium under oxidizing atmospheres can be
explained by the reactions:
seo2 (s)
390
seo2( g >
(4)
Speciation of Most Volatile Toxic Trace Elements during Coal Combustion
(ii) (Hg+As+Se)/coaVsiag/chlorine complex system
When three types of the most volatile trace elements, namely mercury, arsenic and
selenium, coexist in the flue gas they may interact with each other and produce new
chemical compounds such as HgSe, HgSe03, Hg2Se03,AsSe and As2Se3.
These trace elements may also react with major mineral matter elements in the
coal to produce compounds such as Ca3(As04)2. To accurately predict the speciation
of trace elements in flue gas, we also need to consider minor elements in our complex
system. We always used several oxides of aluminum, silicon, calcium, iron,
magnesium, sodium and potassium to represent the minor elements in coal. These
materials at high temperatures will form ash or slag, which are a type of mixture
behaving quiet differently from ideal liquids. When heated, coal ash/slag has no single
liquidus and it melts over a wide temperature range. In this paper, the five main
oxides of aluminum, silicon, calcium, iron and magnesium are chosen to represent the
coal ash system. In an actual slag system we would need to take into account more
elements in the coal ash, such as the alkali metals, in order to get more accurate
results.
The development of the chemical thermodynamic model involves bringing
together available thermodynamic data and phase equilibrium data for the system.
These data are then evaluated simultaneously to obtain one set of model equations for
the Gibbs energies of all phases, as functions of temperature and composition. In this
simultaneous data evaluation process, conflicts between the data sets resulting from
inaccurate or inconsistent measurements can be resolved, thus resulting in an
optimized model of the system. In this way, all the data are rendered self-consistent
and also consistent with thermodynamic principles. From the model equations, all of
the thermodynamic properties and the phase diagram within the system can be
recalculated and checked.
For the molten-slag phase, a modified quasi-chemical model has been used. A
description of the model and its application to various systems has been reported
previously [23]. The polynomial and sub-lattice models were used for the solid
solutions in this study.
The five-component system of Ca0-Fe0-A1203-Si02-Mg0contains ten binary,
ten ternary, and five quaternary sub-systems. The usual method of constructing the
slag system is to start from binary systems, then proceed to ternary, and so on.
Nevertheless, this advance from low- to high-order systems has to be modified
because for a number of low-order systems, there are insufficient experimental data to
characterize the thermodynamic properties of all the phases. In that case experimental
data for higher-order systems are used to select the parameters of the models for the
low-order systems.
Thermodynamic optimizations using the FACT package with many of the
sub-systems, including CaO-Si02, AI2O3-Ca0, A1203-Si02, A1203-CaO-SiO~and
391
Liu Yinghui, Zheng Chuguang, and Wang Quanhai
CaO-Fe0-Fe2O3-SiO2, have been reported previously [23]. Thermodynamic and
phase equilibrium data for these multicomponent systems were used to derive the
parameters of the models describing the thermodynamic properties of all phases. As a
result, a self-consistent database has been developed for the five-component system
Ca0-Fe0-A1203-Si02-Mg0using the FACT computer package.
Under reducing atmospheres, the speciation distribution of mercury in the
(mercury + arsenic + selenium)/coaYslag/Cl complex system is shown in Figure 7.
When the temperature is 300 K, solid state HgSe(S) is the main form of mercury. For
temperatures between 300-400 K, gaseous HgSe(G) is the main form of mercury. At
temperatures above 400 K, elemental mercury Hg(G) is the main form of mercury.
These results differ widely from those of the Hg/coaYchlorine system under reducing
atmospheres.
Under oxidizing and reducing atmospheres, the speciation distribution of arsenic
and selenium in (Hg+As+Se)/coaVCl system are the same as for the preceding simple
systems. This is because of the competition between many different types of species
and compounds. If we ignore the existence of HCI in the flue gas, a small amount of
chlorinized arsenic will be produced, however HC1 has no influence on the speciation
distribution of selenium. That is to say, C1 has a very weak affinity to Se.
From the speciation results of the complex system, we cannot find any obvious
interaction between As and Ca as was mentioned by other researchers [lo]. Perhaps
this is because the amount of Ca is lower in our system.
I
,
300
,
,
400
.
,
so0
600
//-?-.-l
18002000
Tempemture/K
Figure 7. Hg speciation in reducing atmospheres in a complex system.
From the results of chemical-thermal equilibrium analyses for the simple system
and complex system above, we can determine whether it is necessary to consider the
interaction between trace elements. In addition, whether it is necessary to consider all
possible products, and if they will have a significant effect on the final modeling
prediction results.
The volatility of an element depends upon the temperature at which the gaseous
species were originally produced. From the chemical equilibrium analysis above, we
can infer that the volatility of these three types of trace elements is of the order:
mercury > selenium > arsenic. This conclusion is in agreement with the result
obtained fiom an experimental study by other researchers [2].
392
Speciation of Most Volatile Toxic Trace Elements during Coal Combustion
In general, the saturation vapor pressure of the chlorides of trace element is lower
than that of the sulfides and sulfates. The chlorine element can significantly
strengthen the evaporation of some types of trace elements. From the chemical
equilibrium analysis of the (Hg+As+Se)/coaVslag/CI system we found that chloride
has far greater affinity with mercury than with arsenic and selenium.
The HCI content in the flue gas is very important to the speciation distribution of
trace elements. If we do not consider the HCI in the flue gas, the original temperature
that mercury will evaporate into elemental mercury will rise. In the arsenic/coaVCl
system and selenium/coal/Cl system, most of the chlorine combined with hydrogen
ions to form HCI. In the mercury /coaVchlorine system most of mercury combined
with chlorine and formed HgCI2. Hence we can conclude that the chlorine element
can greatly strengthen the evaporation of mercury, but has little effect on the
evaporation of selenium and arsenic.
Conclusions
From the chemical equilibrium analysis described above we can draw the following
conclusions.
1. Using a chemical-thermal equilibrium model to predict the speciation of trace
elements, whether considering the interaction between the trace elements or
whether considering all the possible products, will greatly influence the model
prediction results. In our complex thermal model, Hg, As and Se will react with
each other at lower temperatures under a reducing atmosphere. Although we have
not found obvious interactions between the most volatile trace elements and
minor elements in our work, it is appropriate to consider the interactions between
them when building a complex thermal equilibrium model to consider other
non-volatile trace elements.
2. When the temperature is about 450 K, under oxidizing atmospheres, mercury,
selenium and arsenic will exist in the flue gas in the gas phase and will cause
problems for their emissions control. While under reducing atmospheres, these
trace elements show quiet different speciation distribution. Due to the different
physical and chemical properties, speciation of these trace elements in flue gas is
very useful for the sampling and control of trace elements emissions from coal
combustion sources.
3. The volatility of these elements is in the order: mercury > selenium > arsenic, and
this conclusion is in agreement with the experimental results of other researchers.
4. Chlorine has a strong affinity with mercury, but a weak affinity with arsenic and
selenium. The chlorine element can greatly strengthen the evaporation of mercury,
but has little effect with selenium and arsenic. The more CI present in the flue gas,
then the wider range of temperature over which HgCl,(G) exists. Therefore, C1
will greatly affect the Hg emissions control fiom coal combustion sources.
Acknowledgements
This research was subsidized by the Special Funds for Major State Basic Research
Projects (No. 199902212), and National Nature Science Foundation of China
(No.59876013). We would like to thank them for their supervision and support.
393
Liu Hnghui, Zheng Chuguang, and Wang Quanhai
References
I.
2.
3.
4.
5.
6.
I.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
Finkelman, R.B., and Gross, P.M.K. 1999. The types of data needed for assessing the environmental
and human health impacts ofcoal, lnternational Journal of Coal Geology, 40,91-101.
Yan, R., Gauthier, D., and Flamant, G 2000. Possible interactions between As, Se, and Hg during
coal combustion, Combustion and Flame, 120,49-60.
US Statutes At Large. 1990.Public Law 101-549.Provisions For Attainment And Maintenance Of
National Ambient Air Quality Standards. lOlst Congress, 2nd Session, 104,Part 4,2353-3358.
Chow, W., Miller, M.J., and Tonens, 1.M. 1994.Pathways of trace elements in power plants: interim
research results and implications, Fuel ProcessingTechnology, 39,520.
Clarke, L.B. 1993.The fate of tracer elements during coal combustion and gasification: an overview,
Fuel; 72(6), 731-736.
Ratafa-Brown, J.A. 1994.Overview of trace element partitioning in flames and furnaces of utility
coal-fired boilers, Fuel Processing Technology; 39, 139-157.
Germani, M.S.,and Zoller, W.H. 1998.Vapor-phase concentrations of arsenic, selenium, bromine,
iodine, and mercury in the stack of a coal-tired power plant, Environ. Sci. Technol., 22(9),
1079-1085.
Swaine, D.J. 1994. Trace elements in coal and their dispersal during combustion, Fuel Processing
Technology, 39, 121-137.
Linak. W.P., and Wendt, J.O.L. 1994. Trace metal transformation mechanisms during coal
combustion, Fuel Processing Technology,39,173-198.
Meij, R. 1994,Trace element behavior in coal-fired power plants, Fuel Processing Technology, 39,
199-217.
Richaud, R.., Lachas, H., and Collot, A.G, 1998. Trace mercury concentrations in coals and
coal-derived material determined by atomic absorption spectrophotometry,Fuel, 77(5), 359-368.
Queml, X. 1995. Trace elements in coal and their behavior during combustion in a large power
station, Fuel, 74(3), 331-343.
Galbreath, K.C., and Zygarlicke, C.J.1996.Mercury speciation in coal combustion and gasification
flue gases, Environ. Sci. Technol., 30(8), 2421-2426.
Frandsen, F., Dom-Johansen, K., and Rasmussen, P. 1994. Trace elements from combustion and
gasification of coal: an equilibrium approach, Progress in Energy and Combustion Science, 20,
115-138.
Wu, C.Y., and Biswas, P.1993.An equilibrium analysis to determine the speciation of metals in an
incinerator, Combustion and Flame, 93,3140.
Helble, J.J., Mojtahedi, W., and Lyyranen, J. 1996. Trace element partitioning during coal
gasification, Fuel, 75(8), 931-939.
Kalmanovitch, D.P., Sanyal, A., and Williamson, J.. 1986.Slagging in boiler furnaces: a prediction
technique based on high-temperature phase equibibria, J. Inst. Energy, 20,ZO-23.
Fan, L., Qiu, J., Zheng, C. 1996.Phase diagram analysis of the effect of mineral matter in coal to
fusion temperature,ACTA Huazhong University (in Chinese), 24(10), 96-99.
Jak, E., Degterov, S., and Pelton, A.D. 1997. Thermodynamic modelling of the system
AIzO~-S~OZ-C~O-F~~-F~ZO~
to characterize coal ash slags. Impact of mineral Impurities in solid fuel
combustion,An Engineering Foundation Conference, 2-7November, Kona, Hawaii.
Furimsky, E. 2000.Characterization of trace element emissions from coal combustion by equilibrium
calculations, Fuel Processing Technology,63,29-44.
Prestbo, E.M., and Bloom, N.S. 1995. Mercury speciation adsorption (mesa) method for combustion
flue gas: methodology, artifacts, intercomparison, and atmospheric implications, Water, Air and Soil
Pollution, 80,145-158.
Carpi, A. 1997. Mercury from combustion sources: a review of the chemical species emitted and
their transport in the atmosphere, Water, Air and Soil Pollution, 98,241-254.
Pelton, A.D., Thompson, W.T., Bale, C.W. 1998.Phase equilibrium calculations in multicomponent
systems, Advances in Phase Transitions, Embury, J.D., and Prudy, GR., (eds), Pergamon Press, New
York, pp.52-67.
Received: 3 1 January 2002; Accepted after revision: 1 July 2002.
394
Документ
Категория
Без категории
Просмотров
2
Размер файла
809 Кб
Теги
elements, volatile, coal, trace, speciation, combustion, toxic
1/--страниц
Пожаловаться на содержимое документа