Atmospheric Emissions of Arsenic, Cadmium, Lead and Mercury From High Temperature Processes in Power Generation and Industry
Atmospheric Emissions of Arsenic, Cadmium, Lead and Mercury From High Temperature Processes in Power Generation and Industry
Atmospheric Emissions of Arsenic, Cadmium, Lead and Mercury From High Temperature Processes in Power Generation and Industry
CHAPTER 7
ABSTRACT
69
70 Lead, Mercury, Cadmium and Arsenic in the Environment
produce fine fraction particles with a significant portion of particles below 2 JIm in
diameter. These particles can penetrate through emission control devices, enter the
atmosphere, and travel for long distances. Penetration of As, Cd and Pb through
the boilers of two coal-fired power plants in Poland. equipped with electrostatic
precipitators of 99.5% efficiency was between 2 to 3%. More than 95% of Hg was
released in a vapour phase. Penetration rates 2-3 times lower for the above elements
were measured through wet scrubbers.
Atmospheric fluxes of As, Cd, Pb and Hg were calculated on global, regional,
and local scales. Estimates in this chapter are then compared with emission surveys
by other authors.
Finally, recommendations for further efforts are presented emphasizing the role
of emission surveys for planning control strategies to achieve ambient air quality
goals. Gaps in the knowledge and uncertainties of emission assessments have been
also taken into account.
INTRODUCTION
employed, and (4) the efficiency and type of emission control devices uti-
lized. Special consideration is given to metal releases from power generation
and non-ferrous metal production. The atmospheric transport of As, Cd, Pb
and Hg is then discussed considering the information on their chemical na-
ture and tendency to concentrate on fine particles.
Coal and oil are the main fossil fuels used to produce electricity. Apart
from many unquestionable advantages, such as the relatively low cost of
energy generation and easy availability of the fuel required, coal- and oil-
power plants are known to create serious environmental hazards due to the
emission of such pollutants as arsenic, cadmium, lead and mercury.
The trace element behaviour during combustion of fuels depends mainly
on: (1) affinity of elements for pure coal and mineral matter, (2) physical-
chemical properties of elements and their concentrations in coal and oil, and
(3) combustion conditions. Kuhn et at. (1980), in their thorough review of
the abundance of trace and minor elements in organic and mineral fractions
of coal, found As, Cd, Pb and Hg as the sulphide-forming elements. They
concluded that information on the affinity of trace elements for pure coal
and mineral matter could be used, in conjunction with concentrations, for
determining the chemical forms of elements, for estimating the theoretical
percentage of an element that can be removed by coal cleaning, and for
predicting material balances in the coal products and wastes. The needed
information on trace element concentrations in coal has been collected by
this author (Pacyna, 1986a). The As, Cd, Pb and Hg concentrations in coals,
taken from this review, are presented in Table 7.1, together with the metal
concentrations in crude oils. As can be seen, there are large differences
between metal concentrations in coals from several producing fields of the
world. As a result, it is difficult to generalize on impurities in coal. However,
the literature (Gluskoter et at., 1977; US EP A, 1980; Pacyna, 1980) suggests
that lignite and sub-bituminous coals are less contaminated by these metals
than medium-, low-, and high-volatility bituminous coals. As an example,
coal from the Western Basin in the United States is much less contaminated
than the coals from Eastern and Illinois basins. In Europe, German brown
coal (lignite) seems to be cleaner than bituminous and sub-bituminous coals
from Czechoslovakia and Poland (Pacyna, 1980; Heinrichs, 1982). On the
other hand, the production of a certain amount of energy (i.e. electricity)
requires a lignite charge almost two times as high as that of bituminous coal
(Dvorak and Lewis, 1978),resulting in enhanced emissions of As, Cd, Pb
and Hg.
-- --pO. U'- - -
During combustion, the volatile species in the coal evaporate in the boiler
and recondense as submicrometre aerosol particles, or on the surfaces of ash
particles as the flue gas cools in the convective sections. The concentrations
of arsenic, cadmium and lead increase markedly with decreasing particle size
from bottom ash through fly ash from control devices to stack fly ash (stack
dust) emitted into the atmosphere. Apparently, more than 90% of mercury
in coal is released as vapor (Billings et at., 1973; Kaakinen et at., 1975; Pa-
cyna, 1980). Combustion temperature in the boiler is one of the key parame-
ters affecting the amounts of metals released. The higher the temperature in
a boiler, the larger the discharges of volatile elements. Thus, larger amounts
of As, Cd and Pb are emitted into the atmosphere from conventionally-fired
boiler systems (e.g. stoker-boiler or cyclone-boiler) burning fuel at temper-
atures higher than 1650 K, as compared with fluidized-bed systems with
temperatures between 1100 and 1200 K. The latter systems are now being
employed in electric utilities together with cyclone furnaces. The stoker-
type boiler is the dominant unit used in industrial plants and in atypically
small 40 MW) power plants. The type of fly ash control system and its
efficiency also influence the As, Cd and Pb emissions. Electrostatic precip-
itators (ESP), wet scrubbers, mechanical collectors and fabric 'bag-house'
filters are the most commonly used systems. The two former are installed
in coal-fired power plants. Generally, a venturi wet scrubber system is more
efficient in removing As, Cd and Pb from a flue gas stream than electro-
static precipitators. Ondov et at. (1979) have studied the penetration of
several elements contained in particles from coal-fired plants equipped with
both control systems. They calculated the following values for penetration
through ESP: 4.3-11.5% for As, 3.3-8.8% for Cd and 2.2-5.5% for Pb. The
arsenic penetration through a venturi wet scrubber ranged from 2.5-7.5%.
No information was presented for cadmium and lead.
Oil-fired power plants are also important sources of As, Cd, Pb and Hg
emissions into the atmosphere. During refining of oil, these metals concen-
trate in the heavy distillate residuals, such as residual fuel oils, asphalts, and
in the liquid and solid waste streams. The limited information available in-
dicates that probably 30% of the metals in crude oil are retained in residual
fuel oil and asphalt (Smith et at., 1975). The amounts of As, Cd, Pb and
Hg discharged during combustion of residual fuel oil in power plants and
Atmospheric Emissions from High Temperature Processes 73
Source As Cd Pb Hg
Coal combustion
power plants 205 64 733 85.5
industry 240 77 870 135.0*
commercial and 16 5 73
residential units
Oil combustion
power plants 79 37 450 v.I.
industry and 138 73 709 v.1.
commercial and
residential units
Total 678 256 2835 221
furnaces. The emissions of As, Cd, Pb and Hg for Europe are presented in
Table 7.2. The Hg emissions in Table 7.2 include both the metal releases
in the vapor phase and as particles. Oil firing contributes insignificantly to
the mercury emissions from fossil fuel combustion. The metal emissions
due to burning of fossil fuels in the United States were the following: 497
tons of Cd in 1979 (US EPA, 1981), 120 tons of Hg in 1975\ (Watson,
1979), and 650 tons of As in 1974 (NAS, 1977). Fuel combustion includes
not only stationary combustion but also mobile sources, particularly gasoline
combustion engines. The fate of lead in gasoline has been studied extensively,
and is not reviewed here in detail. For statistical purposes only, the author's
calculations of Pb emissions from gasoline combustion in Europe give a
value of 74300 tons in 1979, compared with 128000 tons in the United
States in 1975 (US EPA, 1977).
in Europe in 1979. The results are listed in Table 7.3. Similar inventories
have been prepared for the US emissions, indicating releases of about 1000
tons/year for Cd (several US EPA reports), 70 tons/year for Hg (Watson,
1979), and 4800 tons/year for As from copper smelting only (NAS, 1977).
Table 7.3 Emissions of As, Cd, Pb and Hg from non-ferrous
metal production in Europe in 1979 (in tons/year)
Source As Cd Pb Hg
Primary non-ferrous*
metal production
copper-nickel 4500 600 9250 61
zinc-cadmium 900 1050 7880 13
lead 300 8 10450 3
Secondary non-ferrous
metal production
copper v.1. 55
lead v.1. 400
Mining v.1. 1090
Total 5700 1658 29125 77
Source As Cd Pb Hg
It should be noted that for some sources, as well as for certain coun-
tries in Europe, the information necessary for calculations of trace element
emissions was difficult to obtain or was simply lacking. The uncertainties
included data on: (1) metal concentrations in fuels and other raw materials,
(2) industrial technologies, and (3) the efficiencies of control devices used.
A simple trajectory model has been used to verify the above estimates for
As, Cd and Pb emissionsfrom anthropogenic sources in Europe. The metal
concentrations calculated by the model were compared with measured daily
78 Lead, Mercury, Cadmium and Arsenic in the Environment
5
..
4 ',',
..
',',
3 ',',
..
..
2 ..
:::
:::: "
..
0 .. 1'/1 ..
As Cd Hg Pb Pb
(x103) Ix103) (x102) ('10') ('10')
(without gasoline)
Figure 7.1 Total atmospheric emissions of As, Cd, Hg, and Pb in Europe, USA,
USSR, and from natural sources
As Cd Hg Pb m EUROPE Pb
100~.,. (without gaaollne)
(jUSA
0 USSR
60
60
40
20
0
II III II III II III II III II III IV
Figure 7.2 Atmospheric emissions of As, Cd, Hg, and Pb from fossil fuel combus-
tion (I), non-ferrous metal production (II), other anthropogenic sources (III), and
gasoline combustion (IV) in Europe, USA. and USSR
inorganic forms dominate in the air over emission areas. Major chemical
species of arsenic and of the other elements discussed which evolve dur-
ing high-temperature processes are presented in Table 7.5. Trivalent arsenic
is the most common form of the metal. Air samples containing arsenic, of
either smelter or coal-fired power plant origin, consist largely of trivalent ar-
senic in both vapour and particulate states. This is very important, because,
as a general rule, inorganic arsenicals exhibit greater toxicity than organic
arsenicals, and the trivalent state more so than the other states. Methylated
forms of arsenic are probably of minor significance.
Table 7.5 Major chemical species evolved during fossil fuel combustion and in-
dustrial processes
Process As Cd Pb Hg
Coal combustion As(O), ASZ03 Cd(O), CdO PbClz, PbO, Gaseous Hg,
ASZS3 CdS PbS,Pb
Oil combustion As(O), AsZ03 Cd(O), CdO PbO Gaseous Hg
Organic arsines
Non-ferrous metal ASZ03 CdO, CdS PbO, PbS04,
production PbO.PbS04
Iron and steel CdO PbO
manufacturing
Refuse incineration As(O), ASZ03, Cd(O), CdO, Pb(O), PbO, Gaseous Hg
AsCI3 CdClz PbCI2
Cadmium and its oxide are the predominant chemical forms of the metal
emitted from all the sources considered in this chapter. These two forms
seem to be the most toxic cadmium speciations, together with cadmium
chloride (found in releases from refuse incineration). With respect to the
chemical forms of lead from sources considered here, it can be suggested
that inorganic forms are the most widely released, particularly lead oxide
and lead chloride. The mercury from industries and power plants is emitted
primarily as mercury vapour. This vapor consists mainly of elemental mer-
cury and dimethyl mercury. It is difficult to say which volatile compound
dominates the discharge process. Mercury species other than elemental Hg
and (CH3hHg can also contribute. Seiler (in Lindqvist el aI., 1983) sug-
gests that most mercury is emitted as dimethyl mercury with a relatively
fast degradation to elemental mercury taking place in the air. Brosset (1981)
concludes that Hg(O) is mobilized to the atmosphere where it is subjected to
atmospheric oxidation processes to yield water soluble forms, subsequently
scavenged by wet or dry deposition.
Considering the volatility of As, Cd, Pb and Hg compounds in Table
7.5, elemental arsenic (As(O)), AS203, AsCI3, elemental Cd (Cd(O)), and
Atmospheric Emissions from High Temperature Processes 81
CdCh are volatile, PbCh is intermediate, and CdO, elemental lead (Pb(O
and PbO are non-volatile (Gerst Ie and Albrinck, 1982). The volatile species
would be in the vapor state during the high-temperature processes, whereas
the non-volatile compounds would continue to be emitted largely as fly ash,
even at the higher temperature.
ATMOSPHERIC TRANSPORT
Volatile elements evaporated in a furnace condense on small particles in the
flue gas stream. The relationship between volatile metal concentration and
particle size during high-temperature processes has already been mentioned,
when discussing emission from coal combustion. The condensation of trace
elements continues in the atmosphere. The outermost layer on the surface of
emitted ashes is often composed of H2SO4, which permits surface condensa-
tion of several elements including As, Cd, Pb and partly Hg. The condensa-
tion takes place preferentially on fine particles in the 0.1 to 1.0 p.m diameter
range. Additionally, the presence of sulphuric acid on some particle surfaces
may give rise to post-combustion crystal formation of metal and/or ammo-
nium sulphates, but how this affects the physico-chemical form of trace
elements has not been studied. Small particles from high-temperature pro-
cesses are very easily transported by air masses over long distances. Gaseous
mercury behaves in a similar manner to As, Cd and Pb on fine particles.
The concentrations of As, Cd and Pb on the larger particles will be low and
decrease rapidly with distance from a power plant or smelter. The results
from several studies seem to indicate that at a distance of 30 km from the
high-temperature source the As, Cd and Pb concentrations reach the back-
ground level (e.g. Rozenshtein, 1970; Yankel et al., 1977; Glowiak et al.,
1977; Pacyna, 1980). Lead concentrations around power plants, smelters or
other industrial plants are often enhanced by emissions from gasoline com-
bustion. Several studies in the surroundings of copper and lead smelters in
Poland (Glowiak el al., 1977; Kubacka et al., 1977; Pacyna et al., 1981) have
been carried out to assess the 'local deposition' of trace elements. It was
found that about 11% of the mass of trace element is deposited in the same
area as it is released. Pacyna et at. (1984) have studied long range trans-
port of several trace elements in Europe. Based on the literature sources
(OECD, 1979), they have inferred local deposition to be 5% of the emis-
sions.
A major portion of the arsenic, cadmium, and lead emissions from high
temperature sources with tall stacks is, however, transported over long dis-
tances. The behaviour of these elements during transport depends on their
physical-chemical properties, the particle size-distribution, and meteoro-
logical conditions, such as the rate of turbulent vertical air exchangeand
wind speed. The two former parameters seem to dominate the long range
82 Lead, Mercury, Cadmium and Arsenic in the Environment
Th
C Sm Eu
Cs Ce Ti
Sb Fe Lo Sc
Ag In 80 Cr Co Co
Pb Hg N H As Sn Zn Go Cd K CI AI Dy
Figure 7.3 Mass median diameter of trace elements. Mean values (after Rahn,
1976)
CONCLUSION
A number of studies and reviews cited in this chapter demonstrate the
progress in evaluation of trace element emissions. Emission surveys of As,
Cd, Pb and Hg from anthropogenic sources are presented in the literature,
with measurements and calculations done either for a certain country or
for an individual source. Fuel combustion and metal smelting are the most
investigated types of emission sources. However, a number of uncertainties
have emerged during these investigations concerning the content of the met-
als in fuelsand other raw materials,the efficienciesof controldevices,and
the uses of different technologies to produce industrial goods.
84 Lead, Mercury, Cadmium and Arsenic in the Environment
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