1

DIOXFREE – POWDERED ACTIVATED CARBON INJECTION (ACI)

SYSTEM TO CONTROL EAF'S DIOXIN AND MERCURY EMISSION

BY

ALDO GIACHERO

SILVIA TOSATO

ABSTRACT

Mainly depending on the charged scrap quality, Electric Arc Furnace (EAF) steelmaking plants can

be a source of dioxins and mercury emission to air.

Emission limits at the stack and pollution prevention practices for the control of these pollutants are

currently governing steel plants in Europe and USA, and are expected to become more restrictive,

as general awareness increases and BATs become available.

Polychlorinated-dibenzo-p-dioxins (PCDD) and polychlorinated-dibenzofurans (PCDF), commonly

referred to as “dioxins”, are a family of chlorinated hydrocarbon compounds, some of which are

classified as toxic for humans by the United Nations Environment Programme (UNEP).

Mercury is a heavy metal defined by UNEP as a global threat to human and environmental health.

Dioxins are unintentionally formed in the EAF Steelmaking process and emitted to the air through

the stack.

The most common source of mercury in the EAF is the contaminated scrap.

Today new emission control technologies are available for the control of dioxins and mercury:

DIOXFREE is a powdered Activated Carbon Injection (ACI) system, installed in some European

EAF plants, to remove, by means of the adsorption process, dioxins and mercury from the flue

gasses, ensuring the compliance with the most severe emission limits that cannot be respected only

by implementing pollution prevention practices.

KEYWORDS: Air, Emission, Mercury, Hg, Dioxin, PCDD, PCDF, Powdered Activated Carbon,

PAC, ACI, Injection, Adsorption, EAF, Fume, Dedusting, Steelmaking

ALDO GIACHERO Product Manager – TTF, Genova, Italy

SILVIA TOSATO Environment Regional Specialist – Dalmine SpA, Dalmine (Bergamo), Italy

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1 INTRODUCTION

PCDD and PCDF are two of the twelve Persistent Organic Pollutants (POPs) included in the

Stockholm Convention [14] by the United Nations Environment Programme (UNEP), as they

persist in the environment, accumulating in the soil through atmospheric deposition and resisting to

all forms of environmental degradation.

In the Steel Industry, the Electric Arc Furnace (EAF) plants are considered to be a significant

contributor to such pollutant emission due to the use of steel scrap potentially contaminated with

organic substances (chlorinated plastics, paints, oil etc).

During the steel melting process, dioxins/furans can be formed in the EAF and in the fume

treatment plant from related chlorinated precursors or via De Novo Synthesis then released into the

atmosphere with the treated fume.

Mercury is still used today in a wide range of products, including batteries, paints, switches,

electrical and electronic devices, thermometers, blood-pressure gauges, fluorescent and energy-

saving lamps, pesticides, fungicides, medicines, and cosmetics. As reported in the Global Mercury

Assessment 2013 by the United Nations [1], once used, many of the products and the mercury they

contain enter waste streams. While mercury in landfills may slowly become re-mobilized to the

environment, waste that is incinerated – or melted, in the case of scrap – can be a major source of

atmospheric mercury.

Steel plants and in particular the electric arc furnaces are recognized as a significant source of

mercury emissions right after the Coal-fired Power Plants, which represent the biggest source of

mercury emissions in the atmosphere.

The EU Member State Authorities are prescribing increasingly more stringent standards emission

limits for the Steelmaking Plants: this called the main steel producers to find technical solutions to

control the emission of these kinds of pollutants in their EAF mills.

2 PCDD – PCDF

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs),

commonly referred to as Dioxin/Furans, are tricyclic, aromatic compounds formed by two benzene

rings connected by two oxygen atoms in polychlorinated dibenzo-p-dioxins and by one oxygen

atom and one carbon-carbon bond in polychlorinated dibenzofurans and the hydrogen atoms of

which may be replaced by up to eight chlorine atoms.

There are in total 75 PCDD congeners and 135 PCDF congeners. The physical–chemical properties

of these compounds are affected by their chlorination level.

Fig. 1: Structural Formula of 2,3,7,8-Tetrachlorodibenzodioxin

PCDD and PCDF are almost insoluble and have a low volatility under normal air pressure. They are

lipophilic and concentrate in animal and human adipose tissues. Fat solubility as well as vapour

pressure have the following attitudes that depend on the rate of chlorination:

- The fat solubility increases at a higher rate of chlorination;

- The vapour pressure of PCDD and PCDF decreases at a higher rate of chlorination.

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2,3,7,8 TCDD 1 2,3,7,8 tetraCDF 0,1

1,2,3,7,8 PCDD 0,5 2,3,4,7,8 PCDF 0,5

1,2,3,4,7,8 HxCDD 0,1 1,2,3,7,8 PCDF 0,05

1,2,3,7,8,9 HxCDD 0,1 1,2,3,4,7,8 HxCDF 0,1

1,2,3,6,7,8 HxCDD 0,1 1,2,3,7,8,9 HxCDF 0,1

1,2,3,4,6,7,8 HpCDD 0,01 1,2,3,6,7,8 HxCDF 0,1

OctaCDD 0,001 2,3,4,6,7,8 HxCDF 0,1

1,2,3,4,6,7,8 HpCDF 0,01

1,2,3,4,7,8,9 HpCDF 0,01

OctaCDF 0,001

DIOXIN CONGENERS

PCDD

NATO 1988

TEF

FURAN CONGENERS

PCDF

NATO 1988

TEF

Congeners with chlorine substitutes in the 2,3,7,8-position, 7 of 75 PCDDs and 10 of 135 PCDFs,

are classified as toxic for humans, each one with an assigned toxicity level.

The Toxic Equivalency Factor (TEF) shows the toxicity of a special compound in relation to the

most toxic substance. Tetrachlorodibenzo-p-dioxin (TCDD) is the reference compound to assign the

toxicity equivalent factor for related congeners.

Fig. 2: Dioxin and Furan Congeners TEFs (NATO 1988 TEF values).

3 PCDD/F FORMATION MECHANISM

Dioxin/Furan formation pathways are influenced by different factors, as temperature, the presence

of organic precursors, as PCB, PCP, polychlorinated benzenes and diphenylethers, and metal

catalysts as copper.

A brief description of the main PCDD/PCDF formation mechanisms is synthesized below.

Condensation

The condensation reaction consists in the condensation of two chlorophenol molecules.

Phenolic compounds adsorbed on the dust particle surface are chlorinated to form the precursor, and

the dioxins/furans are formed from the breakdown and molecular rearrangement of the precursor.

This reaction mostly occurs at temperatures lower than 350°C.

Substitution

Polychlorinated dibenzodioxins/furans can be formed from none or single halogenated

dibenzodioxins and dibenzofurans, by means of the substitution of hydrogen by chlorine in the

2,3,7,8-position with the presence of a metal catalyst.

Radical

Dioxin/Furan formation involves a radical reaction between simple carbon radicals and chloride

radicals under high temperature conditions (Huang & Buekens, 1995). In this reaction organic

precursors combust with chlorine compounds at 300°C to 600°C.

De Novo synthesis

The De Novo Synthesis occurs at a temperature of 250÷450 °C as the formation of dioxins and

furans compounds from non-chlorinated materials, with the presence of chlorine compounds and

carbon, supported by catalytic reactions with metal. The important start reaction is the formation of

chlorine from copper and other metal chlorides with oxygen. The de novo reaction is characterized

by its long reaction time.

Above 800°C, the pyrolysis (thermal decomposition) and the reaction with oxygen start. At the cool

down process dioxins and furans can be reformed by the de novo reaction. These opposite reactions

lead to the typical dependence of PCDDs/Fs on the temperature.

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4 PCDD/F EMISSIONS TO AIR IN EAF STEELMAKING

The range of dioxin/furan emission from EAF steelmaking process is very wide, from 0,04 to 6 μg-

ITEQ/ton liquid steel [5]. These differences in the dioxin/furan emission depend on different types

of scrap charge, varying conditions into the EAF resulting from changes in EAF operating practices

that could change from heat to heat and plant to plant, different fume cooling and dedusting

systems, and different bag filter efficiencies. Correspondingly, concentrations at stack between 0,02

and 9,2 ng-ITEQ/Nm³ have been measured [5].

4.1 Formation in the EAF

Dioxins/Furans appear to be formed in the EAF process from related chlorinated precursors and via

De Novo Synthesis from chemically unrelated compounds such as polyvinyl chloride (PVC) and

other chlorocarbons, i.e. by the combustion of non-chlorinated organic matter such as polystyrene,

coal and particulate carbon in the presence of chlorine donors.

Many of these substances can be contained in trace concentrations in the steel scrap or are process

raw materials such as injected coal. The graphite electrodes represent another source of carbon.

The environment inside the EAF is constantly varying and can produce conditions that are

favourable for dioxin/furan formation. The organic compounds contained in the scrap may be

vaporized, cracked, partially or completely combusted. It may be possible to have formation of

dioxins/furans in one area of the EAF, while thermal decomposition is taking place in another area,

depending on the conditions in the furnace or parts of the furnace during or after charging.

The increase in the oxygen concentrations promotes PCDD/F formation.

Since the gas contained in the furnace is not homogeneously mixed, not all the PCDD/F formed in

the low temperature areas can be thermally decomposed and a portion of them is expected to leave

the EAF in the off-gas.

4.2 Formation in the Fume Dedusting System

The EAF Fume Dedusting System operational conditions may be favourable for De Novo Synthesis

formation of dioxins/furans: the thermal profile of the fume treating process strongly affects the

generation of these organic compounds.

The EAF off-gas is cooled down to reach the required filter inlet temperature, which usually must

be lower than 130 °C; part of the dioxins/furans contained in the fume condenses and is adsorbed by

dust particles that are separated in the bag filter.

Condensation starts in the 125÷60 °C range with the higher chlorinated dioxins and increases very

rapidly as the temperature drops. The lower chlorinated furans are the last to condense, which

explains why they often constitute the majority of the congeners observed in EAF emission tests.

Several variations of the fume temperature at the filter inlet, typically in the 50÷125 °C range,

occurring during each tap-to-tap time, lead to changes in the PCDD/F adsorption/desorption

equilibrium. When the temperature rises, the dioxin/furan vapor phase fraction in the fumes gets

higher resulting in higher concentrations at the stack.

The PCDD/F formation, their vapor/solid phase ratio and their adsorption/desorption equilibrium

are strongly affected by the temperature; this must be carefully taken into account in the choice of

the system to be installed for the dioxins and furans emission control in EAF plants.

It is insufficient to provide the Fume Dedusting System with a fast quenching unit in order to

minimize the residence time of the fume in the De Novo Synthesis temperature range: the low

temperatures of the EAF off-gas (150÷500 °C) during some phases of the process, e.g. during scrap

charging or first step of melting, lead to the formation of dioxins and furans in the system upstream

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of the quenching section: these formed PCDDs/Fs flow in the fume stream without encountering the

condition for the thermal decomposition.

To solve the above mentioned problems, a post combustion should be provided to maintain the

fume temperature upstream of the fast-quenching above 850 °C, in order to perform the PCDD/F

thermal decomposition and avoid reformation.

This solution is confirmed to be very expensive in terms of energy consumption and cannot

guarantee the complete compliance with the emission limits required by the Authorities.

The experience of several European steel companies shows that a completely reliable control on the

dioxin and furan emission is achievable only by means of the installation of a system suitable to

operate efficiently during all the EAF plant process phases.

5 MERCURY EMISSIONS TO AIR IN EAF STEELMAKING

In the EAF steelmaking, different grades of scrap metal are recycled: this material is contaminated

by the presence of chemical substances and thus various root sources of emissions are plausible.

The most common source of mercury comes from scrap obtained by old motor vehicles containing

mercury switches. Nonetheless, other sources have to be considered too: mercury coming from non-

automobile scrap can be a significant portion of total mercury present in the charge, and it is not

removed by the switch removal program. Scrap quality is the most influencing factor for mercury

emission from EAF facilities, but also other raw material should be considered as mercury sources:

according to a study by the Swedish Environmental Research Institute, fluorspar (CaF₂) contains

1,1 ppm of mercury [11]. Other raw materials contain up to 0.2 ppm of mercury, as per a more

recent investigation by the Norwegian steel plant of Mo i Rana in 2004. Based on an addition of

150 kg of fluorspar per heat, the concentration of 0.2 ppm of mercury from the raw material

becomes 0.3 gram of mercury per heat [4].

The following diagram shows the mercury content of various raw materials involved in the EAF

steel melting process.

Fig.3: Parts per million of mercury in EAF-related materials [4]

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In the Nordic steel mills continuous measurement of mercury emissions was implemented in the

years following these studies, but a consistent correlation between raw materials input and emission

peaks has not yet been demonstrated.

5.1 Mercury in the EAF’s Flue Gas

In the case of Mercury, in general, things get more complicated than for PCDD-F case, and for the

EAF very few full-scale tests results are available. Based on the theory and on tests carried out for

Coal Fired Power Plants we can assume the following:

There are 3 forms of Mercury-based pollutants: elemental Hg (Hg⁰), particulate Hg (Hgp) and

reactive (divalent) gaseous mercury (Hg2+

) [2-3], each one with different chemical characteristics

and behavior. The determination of these three forms of mercury in the flue gas is called the

speciation of mercury.

Emissions from steelmaking have historically been believed to be comprised of approximately 80 %

Hg0, 5 % HgP and 15 % Hg

2+ [4].

For this reason, injecting a sorbent material upstream of the Bag Filter, efficient in capturing

gaseous mercury in the Hg⁰ form at lower temperatures (50-120 °C), is the most effective way to

achieve the required mercury abatement.

The following table provides an overview of the specific emission factors that can be used as

reference for the emission floor assessment in USA and Europe:

SOURCE EMISSION FACTOR NOTES

EPA: AP42 – 2009 0,000110 [lb/ton] 55 [mg/tls] (*)

IPPC: EUR 25521 EN 2013 0,000004 [lb/ton] 2 [mg/tls] Minimum value

0,000400 [lb/ton] 200 [mg/tls] Maximum value

(*) “tls”: metric tons of liquid steel

Fig.4: EU – US Mercury Emission Factors Comparison

6 RULES

Mercury and dioxin emission limits at the stack are currently governing steel plants in Europe, but

not steel plants in the US. Yet in Europe those limits are expected to become more restrictive as

general awareness increases and BATs (Best Available Techniques) become available to the

industry [5].

It has to be noted that the U.S. Environmental Protection Agency (EPA) rule in force today is based

on pollution prevention [6-7]. Such rule requires eliminating mercury at the source: scrap

proceeding from motor vehicles has to be decontaminated from mercury switches before it can be

melted in EAF facilities. Future revisions could lead to mercury stack emission limit and continuous

emission monitoring requirements in the US.

European Best Available Techniques for EAF Steelmaking Facilities are:

Mercury

• BAT for the electric arc furnace (EAF) process is to prevent mercury emissions by avoiding, as

much as possible, raw materials and auxiliaries which contain mercury.

• The BAT-associated emission level for mercury is < 0,05 mg/Nm³.

Dioxin (PCDD-PCDF)

• The BAT-associated emission level for Polychlorinated Dibenzodioxins/Furans (PCDD/F) is

<0.1 ngI-TEQ/Nm³.

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7 DIOXFREE: POWDERED ACTIVATED CARBON INJECTION SYSTEM

DIOXFREE is a Powdered Activated Carbon (PAC) injection system, with process specificities and

design features leading to the achievement of the required abatement efficiency together with the

minimization of the PAC injection.

7.1 PROCESS DESCRIPTION

The basic principle under which the PCDD/F control is performed is the adsorption: this term

means the attachment of molecules (in this case PCDDs/Fs, Mercury, PAH and other pollutants) to

the surface of a solid.

To obtain an efficient adsorption, the following requirements need to be satisfied:

- Sorbent must have large surface areas;

- Sorbent must have micropores;

- Sorbent chemical characteristics must be considered especially for the case of mercury control;

- Good contact time for an efficient separation (a good distribution in the flow is mandatory);

- The sorbent dosage must be regulated according to the fumes variable conditions.

To reach the required results the following factors have been considered in the design: the system

operates using PAC as adsorbent material; each PAC particle has an extremely large surface area

(surface to weight ratio > 400 m²/g) with micropores, which helps it to easily adsorb gaseous

pollutants. Different kinds of PAC are available on the market, so the choice must be done

considering the process characteristics.

Chemically embedded activated carbon (specifically sulphur, chlorine, bromine) enhances the

uptake of mercury: this solution is sometimes adopted in coal combustion facilities, depending by

the chemical composition of the flue-gas.

The contact between PAC and gaseous PCDDs/Fs is very strong when the fumes pass through the

dust cake accumulated on the fabric bags, so it is very important to ensure a good PAC distribution

on the whole filtering surface. A CFD model is usually developed to improve the distribution of

activated carbon taking into account the plant peculiarities.

7.2 PCDD-F MEMORY EFFECT

The high levels of PCDD and PCDF concentrations contained in the fume from the EAF during the

period of operation without proper abatement, previous to the installation of the PAC Injection

System, can be responsible for a contamination of the clean side of the Fume Dedusting System

including all the elements downstream of the fabric bags.

The DIOXFREE installation leads to the abatement of vapour phase dioxins and furans contained in

the fume upstream of the fabric bags.

Downstream of the filtering elements, the adsorption/desorption equilibrium depending on the

temperature variations can often lead to the PCDD/F desorption from the contaminated elements to

the cleaned fume, affecting the expected abatement performances.

This effect is called Memory Effect and should be considered during a short period of regular plant

operation subsequent to the start-up phase, after which the clean side contamination will be

removed.

7.3 SYSTEM DESCRIPTION

Pac Storage Silo

The PAC (Powdered Activated Carbon) storage is performed by means of a 50 m³ capacity silo

complete with hopper, discharge and dosing systems. The PAC is pneumatically unloaded from a

bulk truck transport trailer with the truck’s compressor and hoses and then charged into the silo.

The silo is equipped with fluidization nozzles and a vibrating bottom to prevent powder bridge

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PAC STORAGE SILO

FUME DUCT

INJECTION

SYSTEM

CONTROL

ROOM

formation. A feeder delivers the activated carbon to a rotary valve and the PAC is pneumatically

conveyed as a dilute phase mixture to the injection point.

The silo shall be equipped with pulse jet type vent filter, suitable for air dedusting during filling

operations and all the devices that make its conformity with ATEX standards.

Injection System

The PAC is pneumatically conveyed into the fume duct. The injection system is constituted by the

following items:

- Dosage Regulation Unit, able to perform a multi-way dosage that gives the system the

necessary versatility to achieve the maximum efficiency in all working conditions with the

lowest PAC consumption (operational cost optimization).

- PAC Blowing Unit and conveying system.

- Injection Unit: thanks to the CFD analysis the nozzles system configuration can be customized

on a case-by-case basis in order to obtain the best distribution of the PAC on the filter bags.

- The Carbon content in the dust collected by the bag filter should not exceed 5% in order to

avoid the risk of self-ignition in the dust hoppers.

Control Room

A suitable continuous weighing and feeding system has been adopted to optimize PAC dosage

according to the different process parameters. The dosage can be set to a fixed quantity [kg/h] or

variable according to parameters as fume temperature [°C] and flow-rate [mg/Nm³]. These

functions can be carried out positioning a remote programmable logic controller (PLC) complete

with an interface panel arranged with the needed selections. The system can be easily monitored

remotely to check in real time all process variations and possible unexpected system arrests.

Fig. 5: ACI System Typical Arrangement

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8 TENARIS DALMINE PLANT ACI INSTALLATION

DIOXFREE technology was pioneered by Tenaris Dalmine.

Fig. 6: DIOXFREE Installation

Fig. 7: Tenaris Dalmine EAF Dedusting System: Flow Sheet

The installation of the package for the 140 t/h EAF took a total of five months in which design,

supply and assembly were completed. The system went into operation two years ago, and has been

in continuous service since then.

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Main Plant Data:

Electric Arc Furnace: 140 t/h

Tap to tap: 40 min

Fume Treatment Plant: 2 filtering units (primary and secondary fumes + 2 LF)

Preheating Station: scrap heating provided by primary fumes

Installation: operating for primary fumes plant 750.000 m³/h

Injection: upstream bag-filter

PAC: high surface to weight ratio

Mean PAC Granulometry: 20 µm

PAC Silo: 50 m³ capacity

Injection System: pneumatic conveyor in dilute phase

Dosage: micro-dosage in continuous regulation

PAC Flow Rate: 10 ÷ 100 kg/h

PAC Concentration in fume: 20 ÷ 100 mg/Nm³

Selectable 3-ways Dosage:

1) Fixed PAC concentration in the fumes

2) Variable PAC concentration according to fume temperature

3) Fixed PAC flow rate

Fig. 8: Main Plant Data

Previous to the installation, several PAC injection tests were carried out injecting the activated

carbon into the duct upstream the bag filter with variable dosage, ranging from 40 to 130 mg/Nm³.

The dioxin/furan abatement performances resulted in emissions at stack ranging from 0,02 to 0,1

ng-ITEQ/Nm³, depending on the quantity of injected PAC and process conditions.

To ensure the best PAC distribution on the whole filtering surface, a Computational Fluid

Dynamics (CFD) model were developed in order to improve the PAC distribution in the fume

flowing to the bag filter sections: adsorbent particles were tracked using the Discrete Particle

Modelling (DPM) method.

The Bag Filter is divided in two units as represented in Fig. 9: CFD model shows the PAC particles

path to the filter bags in the different configurations. After the analysis the injection system was

optimized according to the configuration leading to the best distribution.

Fig. 9: CFD Model

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8.1 SAFETY

DIOXFREE system has been designed in compliance with the EU directives 1999/92/EC and

94/9/EC, concerning explosion protection measures to guarantee the highest possible level of safety

in areas where combustible dusts or gases are or may be present.

PAC injection system has been conceived to prevent the formation of explosive atmospheres but

considering the possibility of a dust cloud presence, suitable devices have been installed both to

avoid explosion and to contain its effects.

8.2 ABATEMENT PERFORMANCES

After the start-up, several performance tests were carried out in heavy operating conditions of the

EAF with various running conditions of the Fume Cooling and Dedusting plant.

The PAC injection is automatically adjusted according to the fume temperature and performed

values of dioxin/furan concentration at the stack always resulted below the limits required by the

Authorities.

In the following graph the measured values and trends of dioxin/furan concentration at the stack,

with and without PAC injection, are compared.

The blue points represent all the measurements taken after the DIOXFREE start-up. Red points are

related to the previous tests without PAC injection.

Fig. 10: PCDD/F Abatement Performances

In order to represent the real effect of activated carbon injection, the reported values are those

obtained after the increase by 50% of the primary fume filtering capacity, so all the values refer to

comparable plant operating conditions.

Each plotted point represents the PCDD/F average concentration and temperature detected during

an 8 hours sampling time. It is to be noted that, considering the EAF process, an average fume

temperature of 85 °C includes several periods during which the temperature reaches 120 °C.

12

The value higher than 0,1 ng-ITEQ/Nm³ at 85°C was measured less than 2 months after the start-up

of the system, while the value of 0,1 ng-ITEQ/Nm³ at 86°C was measured about one year later. In

both cases the EAF was operating in heavy condition, so the PCDD/F concentration difference

between these two values can be explained as a consequence of the memory effect occurring in the

first period of operation of the system. In all the other cases the dioxin/furan abatement

performances led to concentrations in the 0,02÷0,1 ng-ITEQ/Nm³ range, in line with the more

stringent emission limits that could be imposed in the future.

DIOXFREE is performing PCDD/F abatement leading to emissions at the stack much lower than

the limits required by the Authorities.

An important aspect to be considered is that, as showed in Fig. 7, the Tenaris Dalmine EAF primary

and secondary fume control have two dedicated dedusting units and not only one dedusting system

as usual in the EAF steelmaking plants.

Considering that almost the total amount of generated PCDD/F flows through the primary fume

line, the concentration of PCDD/F referred to the total amount of off-gas captured by the two

dedusting units (and not only to the primary system flowrate) is still lower.

The installation of DIOXFREE system led to the significant reduction of PCDD/F emission, as well

as mercury, PCB and PAH. In the following table the pollutant concentrations at the primary stack

are compared for two years pre and post dedusting plant improvement investments.

Fig. 11: Pollutants concentration pre and post improvement

(*) All concentration values are referred to a common flow rate reference value.

This environmentally-effective solution allows Tenaris Dalmine to meet the highest air emission

standards and to continuously improve the level of environmental protection.

Tenaris Dalmine experience shows that DIOXFREE is an easy-to-use tool, designed to maximize

the PCDD/F abatement with the lowest use of resources (PAC and utilities) and with a plant

operational cost of approximately 0,23 €/tsteel. The full compliance with the more stringent emission

limits required by the Authorities is guaranteed thanks to the great operational flexibility of the

system.

POLLUTANT

PRE

IMPROVEMENTS

YEAR (*)

POST

IMPROVEMENTS

YEAR (*)

PAH (Bohorneff series) [μg/Nm³] 3,76 0,14

Hg [mg/Nm³] 0,0036 0,0016

PCB tot. [μg/Nm³] 0,23 0,01

13

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