EMISSION OF DIOXINS FROM INDUSTRIAL THERMAL AND
METALLURGICAL PROCESSES AND ITS IMPACT : AN OVERVIEW
BY
PROJJAL BASU1,*, WONG POH WEI2, AZMAR BIN AHMAD1
SYNOPSIS
Dioxin is a group of chlorinated compounds consisting over 400 types of compositions.
Out of which, polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated
dibenzofurans (PCDF) are extremely toxic, in particular. They affect the living organisms
including human being. As the compounds possess different toxicity levels, an
equivalent toxicity is usually calculated for the entire gas mixture and even a very low
dosage can be fatal. Organic materials, with chlorine present in them, release these
compounds during heating. Municipal incinerators, cement kilns, metallurgical plants,
etc, employing different industrial heating processes, are often held responsible as the
major contributors for this. Even though dioxin emission standards have been set for a
few industries in select countries, the same for metallurgical industries is yet to be
formulated. However, considering its impact even in minute quantities, this imminent
and deadly evil must be managed and controlled before it takes its toll. This paper
presents an overview of the characteristics, generation, impact on environment and
health, abatement, and legislation regarding dioxins emanating from industries dealing
with high temperatures.
Keywords : Dioxin, Furan, PCDD, PCDF, I-TEQ, Toxic Emissions
1,*QATD
Department, Southern Steel, 13600 Penang, Malaysia. 2School of Materials
and Mineral Resources Engineering, USM Engineering Campus, 14300 Penang,
Malaysia. *Corresponding author Email : projjal.basu@southsteel.com Phone : (+60) 16
432 8874
BACKGROUND
"Dioxin" is a shortened version of the technical chemical name given to some of the
family member compounds. These compounds contain two oxygen atoms in their
chemical structure, hence, "di" that refers to "two" and "ox" refering oxygen. They can
travel long distances from the emission source and bio-accumulate in food chains.
Human exposure occurs mainly through consumption of contaminated food. The effects
are most sensitive with children, particularly breastfed infants. Their impact can be well
realized from a series of books.( 1-5)
Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are semi-volatile
organic compounds and known as persistent organic pollutants (POPs). One of their
congeners, the 2,3,7,8-TCDD, e.g., was classified by the International Agency for
Research on Cancer (IARC) as a “known human carcinogen”(6). This led to the intensive
investigations in recent years related to their formation, emission, transport, abatement,
risk assessment, etc. Three mechanisms are known to be associated with PCDD/F
formation(6) :
1. The high temperature gas phase formation (300-600oC)
2. The transformation of precursors with structures similar to PCDD/Fs, such as
chlorophenols, polychlorinated diphenyl ethers, and chlorobenzenes, and
3. The “de novo” synthesis (meaning formation of complex molecules from relatively
simple molecules) occurring at low temperature post combustion zones (200400oC) via interfacial catalysts on metals.
The general structure of PCDD can be as follows (Fig.1).
(a)
(b)
Fig.1 (a) PCDD general structure (n, m = 0 – 4) (a) The skeletal formula and substituent
numbering scheme of dibenzo-1,4-dioxin, the parent compound of PCDD.
Toxicity of the various congeners, however, varies widely. On the other hand, since they
always occur together, it is necessary to evaluate a combined toxicity factor for the
entire mixture. International Toxic Equivalent Factor (I-TEF) represents the effective
combined toxicity and is widely accepted now (Table 1)(7).
Table 1 : List of individual PCDD/F congeners and their respective I-TEF (International
Toxic Equivalent Factor) values (as per EN-1948 EU standard)(7)
PCDD i-congener
2,3,7,8-TCDD
1,2,3,7,8-P5CDD
1,2,3,4,7,8-H6CDD
1,2,3,6,7,8-H6CDD
1,2,3,7,8,9-H6CDD
1,2,3,4,6,7,8-H7CDD
OCDD
I-TEF
1
0.5
0.1
0.1
0.1
0.01
0.001
PCDF i-congener
2,3,4,7,8-P5CDF
2,3,4,6,7,8-H6CDF
2,3,7,8-TCDF
1,2,3,4,7,8-H6CDF
1,2,3,6,7,8-H6CDF
1,2,3,7,8,9-H6CDF
1,2,3,7,8-P5CDF
I-TEF
0.5
0.1
0.1
0.1
0.1
0.1
0.05
PCDF i-congener
1,2,3,4,6,7,8-H7CDF
1,2,3,4,6,7,8-H7CDF
OCDF
I-TEF
0.01
0.01
0.001
EXPOSURE AND EFFECTS ON HUMAN HEALTH
In the general population, more than 95% of human exposure to PCDD/F and dioxin like
PCBs typically occurs via food(8). Short term exposure to high levels of dioxins may
cause persistent skin lesions known as chloracne and prolonged exposure causes a
range of toxicity, including immunotoxicity, neurodevelopmental effects, and effects on
thyoroid and reproductive function. Dioxins has a high solubility in fatty tissues and
hence, once ingested, remains in the body with long half life period, often as long as
seven years.
In the year 2002, the Joint Food and Agricultural Organization of the United Nations
(FAO)/WHO Expert Committee on Food Additives (JECFA) established a provisional
tolerable intake of 70pg/kg body weight/month for PCDD/Fs expressed as TEFs.
However, no standard has been set for water because of their very poor water solubility.
Guideline for air quality in terms of dioxin has also not been set because it constitutes
only 1-2% of the total exposure.(9)
The UK government has adopted a tolerable daily intake (TDI) of 10pg/kg-bodyweight/day, calculated on the basis of toxic equivalent concentrations of PCDD/PCDFs
and PCBs in order to protect the human health(10). Minh et al(11) presented a comparison
of PCDD/PCDFs in soils from various countries (Table 2).
Attempts have been made to assess and correlate the PCDD/Fs level in blood samples
for municipal waste incinerators with the service conditions but no concrete pattern has
been obtained.(12)
In general, sources with tall stacks and/or high plume rise (e.g. copper smelter, cement
kiln, sinter plant), only a small fraction (typically <10%) of PCDD/Fs is deposited locally
(within 100km radius).(13)
GENERATION AND EMISSION STANDARDS
Dioxins (polychlorinated dibenzo-p-dioxin [PCDD] and polychlorinated dibenzofurans
[PCDF] are not intentionally manufactured except in small quantities for research and as
pure standard for chemical analysis. They are formed as trace by-products from
chemical processes such as chloro-chemical industries, paper and pulp industry,
chlorophenol usage, accidents such as Seveso, and in high temperature processes
such as combustion.(14)
Table 2 : PCDD/PCDFs concentration in soils from few selected countries(11)
Country
Year
Philippines
Cambodia
India
Vietnam
Hanoi
Ho Chi Minh
Brazil
1999
1999
2000
2000
Greece (Crete)
Spain (Barcelona)
USA
Ohio
Michigan
Indiana
Japan
Hong Kong
Germany
UK
Norway
1999
1997
1996
1999
1995/6
1998
1996
1997/8
1996
1996/7
1990s
1997
PCDD/PCDF concentration,
pg/g dry wt (av)
Contaminated General
site
soil
61,000
57
30,000
130
7,400
32
I-TEQ, pg/g dry wt
Contaminated
site
546
402
52
General
soil
6,100
370
13,900
102
2.7
1.1
1.27
370
190
1.9
0.22
20
37,000
700
410
11.85
15,700
458
230
2,000
3700
6,100
649
4,660 urban
324 rural
130
42.8
21 urban
1.9 rural
The annual bonfire night in UK on 5th of November every year releases about 30g I-TEQ
dioxin which is comparable to the annual total release from five sinter plants in UK (38g
I-TEQ) with the mean waste gas PCDD/F concentration at 1.2ng I-TEQ (1998 data).(14)
In a recent study in Taiwan, dioxin emission from secondary aluminum smelters was
found to be far more potent than the same from EAFs even though the total annual
release from these two sources were similar. Interestingly, the contributions from waste
incinerators were found significantly lower (Table 3)(15). In a separate study conducted in
Taiwan(16), it has been reported that total PCDD/F I-TEQ concentrations (ng I-TEQ/Nm3)
in the stack gases from various sources are : municipal solid waste incinerator (3.352),
crematories (3.003), medical waste incinerators (0.168), cement kilns (0.062), industrial
waste incinerators (0.030 with bag house, 0.137 with cyclone and ESP).
While investigating the PCDD/F release from various combustion and metallurgical
industrial and thermal processes, Du et al(17) found that the TEQ concentration and
mass concentration from metallurgical processes were 0.14-1.5ng/Nm3 and 0.565.8ng/Nm3 respectively compared to the same from combustion processes (0.0100.054ng/Nm3 and 0.025-0.15ng/Nm3).
Table 3 : Dioxin release from incinerators and typical metallurgical industries, Taiwan(15)
Source
Secondary aluminum smelters
EAF
Sinter plants
Medical waste incinerators, MWI
Municipal solid waste incinerators,
MSWI
Release/Nm3,
ng I-TEQ
3.3
0.28
Annual total release,
g I-TEQ
18
20
45
0.37
0.74
Relative
contribution
49
54
122
1
2
Chang et al(18) evaluated the PCDD/F emission from two EAFs in Taiwan as PCDD/F
concentration in the stack gas (4.39 and 2.20 ng/Nm 3) and Toxicity (0.35 and 0.14 ITEQ/Nm3). More than 90% of the PCDD/Fs were found in the particulates and the
overall concentration was found to be marginally more during the oxidation stage of
EAF steel making. In another study in Poland (2004), maximum dioxin release from
EAFs was found at 0.62μg I-TEQ/ton of steel produced whereas the same for iron ore
sintering plant was found to be up to 1.47 μg I-TEQ/ton of sinter.(7)
Taking as an example, the coastal city of Taranto in southern Italy has an alarmingly
high level of dioxin. 92% of Italy's dioxin is produced in Taranto which is also 8.8% of
the dioxin in Europe. In ten years, leukemia, myeloma and lymphoma increased by 30–
40%. Furthermore, dioxin accumulates over the years and so far at least 9kg of dioxin
have been discharged into Taranto's air by its factories, i.e. three times the quantity
discharged in the Seveso disaster (the one in 1976 where the Italian city Seveso was
contaminated by dioxin)(19). The annual global emission of PCDD/PCDF puts Japan and
USA far ahead of others.(14)
Table 4 : Annual PCDD/PCDF fluxes for various countries (reference year 1995)(14)
Country
Japan
USA
France
Belgium
Emission (g I-TEQ)
3981
2744
873
661
Country
UK
Netherland
Canada
Hungary
Emission (g I-TEQ)
569
486
290
112
In another study(20), it was estimated that the total dioxin emission in Japan in 1998 was
2900-2940g I-TEQ/year, of which 960g I-TEQ/year came from the industrial waste
incineration, and 1340g I-TEQ/year contributed by the municipal solid waste
incineration.
Huang and Buekens(10) proposed that “de novo synthesis” can produce PCDD/Fs with
PCDF/PCDD ratio >1, while “precursor formation” produces the same with the ratio <<1.
They estimated the potential release of PCDD/Fs to land and water from various
sources in UK(Table 5). The inventory puts total quantified releases to land at between
1500-12000g I-TEQ/year, significantly more than the releases to air (estimated at 1000g
I-TEQ/year) or water. This is as expected, considering the nature of the processes that
form PCDD/Fs and the propensity of PCDD/Fs to bind tightly to solid materials. In
another assessment made in South Korea (21), secondary copper production has been
found to be the largest contributor (Table 6).
Table 5 : Release of PCDD/PCDFs to land and water from metal processes, UK(10)
Processes
Metal
Iron ore sintering
EAF
Primary aluminum
Secondary aluminum
Secondary magnesium
Copper
Secondary lead
Mineral
Cement
Lime
Other processes
Chemical
Chlorine production
PVC/EDC production
Trichloroethylene production
Pesticide production
Waste incineration
MSW (old plants)
MSW (new plants)
Chemical waste
Clinical waste
Sweage sludge
Release to land,
g TEQ/year
Potential release to water,
g TEQ/year (*)
0.020-0.060
59
0.082
29-230
0.38-3.20
24
95-220
N
N
L
L/M
L/M
L/M
L/M
0.00040-12
0.000060-1.8
Not quantified
L
L
6.0
25-80
350-630
8.9-2000
L
M (0.07-0.40)
M (0.07--.40)
H (0.009-2.0)
510-2400
14-38
0.0058-2.0
12-37
0.98
M
L
H (0.018-1.1)
M
L (0.0020)
(*) N = Negligible, L = Low, M = Medium
The Waelz process is a proven method for recovering zinc from EAF dust and
generates significant quantities of PCDD/F (22). In the flue gas downstream from the dust
settling chamber (DSC), PCDD/F concentration was measured at 1223ng TEQ/Nm 3. In
addition, the cyclone and bag filter can only remove 51.3% and 69.4% respectively of
the PCDD/F in the flue gas, resulting in a high PCDD/F concentration of 145ng
TEQ/Nm3 in the stack gas(22). The total PCDD/F discharge (stack gas emission plus ash
discharge) while treating 1ton of EAF dust was found as 840ng TEQ/Nm 3. The PCDD/F
TEQ flow in the Waelz plant investigated is shown in Fig. 2.(22)
Yu et al(23) showed that the distribution of PCDD/Fs in the point source (stack) and area
source (plant fugitives) of an EAF dust treatment plant were 2360 and 1080 ng I-TEQ h1 respectively. The sources had nearly the same extent of effect on the surrounding
environment.
Table 6 : PCDD/PCDF emission from major metallurgical industries, South korea (21)
Metallurgical processes
Ferrous industries
Pig iron foundries
Steel foundries
Copper melting industries
Primary production
Secondary melting
Lead production
Zinc production
Aluminum production
Total emission
Emission, ng TEQ/ton
Emission, g TEQ/year
0.088
194
110
31.706
14
24,451
3140
166
1240
0.170
0.017
0.384
35.259
Fig.2 : PCDD/F TEQ flow in
a Waelz plant.(22)
Studies conducted at a sintering plant in UK showed that PCDFs are present in
significantly larger amounts in both the dust and stack emissions and are the main
contributors to the I-TEQ as shown in Table 7 below.(24)
Table 7 : Total concentrations and I-TEQ of PCDD/Fs in ESP dust and main
stack of a sinter plant(24)
Total
PCDD
PCDF
PCDD/PCDF
Concentration
Main stack, ng/Nm3 ESP dust,
ng/kg
I-TEQ
Main stack, ng/Nm3
42.9
4.3
38.6
0.111
1.026
0.134
0.892
0.150
8634
1774
6857
0.259
ESP dust,
ng/kg
253.2
25.5
237.8
0.107
Buekens et al(25) hypothesized the de novo formation at 250-400oC as the most
plausible PCDD/Fs formation mechanism in thermal and metallurgical processes and
proposed the estimated amount as : (565) x (u) x (v) x (w) g-PCDD/F/h (where, u = dust
loading (g/Nm3), v = residence time (s), w = gas flow rate (Nm3/h).
In a separate study by Fiedler(26), plants with emission far below 0.1ng I-TEQ/Nm3 were
found to include blast furnaces, aluminum melting plants, zinc melting plants, vinyl
chloride plant, etc. Total PCDD/F emission from Taiwanese EAFs was found to be 138g
I-TEQ/year that was significantly higher than many other countries. Accordingly, 5.0ng ITEQ/Nm3 PCDD/F emission limit has been set for the existing EAFs from year 2004 and
0.5ng I-TEQ/Nm3 in effect from year 2007. PCDD/F emission limit for newly built EAFs
is set at 0.5ng I-TEQ/Nm3 from year 2002(18). The emission limit set by the government
of Taiwan in 2006 for PCDD/Fs from existing Waelz plants(27) is 1.0ng I-TEQ/Nm3 which
is also the set limit for total PCDD/F content in secondary building materials (e.g. ash) in
Taiwan(28). European Union directive for the emission limit for PCDD/Fs is at 0.1ng ITEQ/Nm3.(29) The Federal Environmental Agency in Germany is considering to bring
down the tolerable daily intake (TDI) of PCDD/Fs recommended by WHO from its
present value of 70pg I-TEQ/kg body weight/month to 30pg I-TEQ/kg body
weight/month, taking into account that the present value is too high for children in the
pre and post-natal stage.(30)
ABATEMENT
While treating EAF dust using the Waelz process, powder activated carbon (PAC) was
used in exhaust gas to control PCDD/Fs. With PAC addition of 0.155%, 0.310%, and
0.387% of the total input raw materials (EAF dust, coke, sand), effective removal of
PCDD/F could be achieved even though below 0.155% PAC addition, the removal
efficiency was found rather poor (Table 8).(31)
Table 8 : Effectiveness of addition of powder activated carbon (PAC)
on the removal efficiency of PCDD/Fs from Waelz emissions(31)
PAC addition
No treatment
0.155%
0.310%
0.387%
ng I-TEQ/Nm3
181.000
25.250
7.725
4.935
PCDD/F removal efficiency, %
-86.05
95.73
97.27
In a separate study involving processing of EAF dust in a Waelz plant, the atmospheric
PCDD/F concentration in the vicinity of the plant has been brought down to 48.9-130fg
I-TEQ/Nm3 from 568-1465fg I-TEQ/Nm3 by employing suitable abatement methods.
This well meets the emission limit set by the government of Taiwan for PCDD/Fs from
existing Waelz plants (1.0ng I-TEQ/Nm3).(27)
The operating conditions in an iron ore sintering plant in Taiwan were optimized from
the present values (water content W c = 6.0-7.0%, suction pressure Ps = 10001400mmH2O, bed height Hb = 500-600mm) to Wc = 6.5%, Ps = 1000mmH2O, Hb =
500mm using Taguchi Experimental Technique that led to an effective decrease of total
PCDD/F up to 62.8%.(32)
As secondary aluminum smelters (SAS) release much higher PCDD/Fs compared to
aluminum ingot smelters since they deal with a high amount of waste/recycled
aluminum, powdered activated carbon injected at 110mg/Nm 3 in the emitted gases from
a SAS was found effective to reduce PCDD/F by 70%.(33) The avoidance of chlorine
contamination and the temperature range feasible for de novo formation have been
found very effective to bring down the overall PCDD/F level.
Selective Catalytic Reduction (SCR) using V2O5-TiO2-based catalysts was found
effective towards the abatement of PCDD/Fs emission from sinter plants even though
gas velocity was found to affect the removal efficiency strongly.(34)
Table 9 : Effect of temperature on the PCDD/F removal efficiency
using selective catalytic reduction method(34)
Catalyst
Pt supported
V2O5-WO3-TiO2
V2O5-WO3-TiO2
V2O5-WO3-TiO2
V2O5-WO3-TiO2
V2O5-WO3-TiO2
Reactor temperature, oC
300-400
150
210
230
280
300
Removal, %
66.7
99.9
90.0
99.9
97.0
>98.0
In large scale thermal processes, the PCDD/F concentrations have been found to
increase with temperature in the range up to 280oC. The ESP temperature should thus
be kept preferably within 180-200oC, where de novo synthesis is reduced and where
PCDD/Fs are increasingly adsorbed on the fly ash.(35)
CONCLUSION
Efficacy of the evil effects of dioxin and dioxin like compounds has been well
documented in the recent times. Inconsistent and often discrete efforts have also been
put in various countries towards understanding the extent of generation, distribution,
mechanism, abatement, etc. However, it is imminent to make a comprehensive and
holistic analysis on broader aspects of this issue, e.g., inventory, extent of generation
with actual plant operating conditions, economic and effective abatement methods,
health effects, etc. It is necessary to estimate the inventory and establish emission
standards on a global scale for various groups of industries particularly in the thermal
and metallurgical sectors. The present paper aims towards a compilation to generate
awareness for these extremely toxic POPs.
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