PCDD and PCDF are not produced intentionally (other than for research). They are formed as by-products in some chemical processes, and in almost every combustion process where carbon, oxygen, hydrogen and chlorine atoms are present (Ministry for the Environment, 2000). They have low vapour pressure and mainly adsorb on to particles.
3.1 PCDD and PCDF formation mechanisms
PCDD and PCDF can be released from combustion processes through three mechanisms (Charles E Napier Ltd, 2000; US EPA 1997):
- incomplete destruction of PCDD and PCDF present in feedstock materials
- 'in-furnace' formation, such as via condensation of chemically related precursor compounds
- low-temperature downstream formation from simple carbon and inorganic chlorine sources (de novo synthesis).
Each of these mechanisms can apply to secondary metals processes.
PCDD and PCDF cannot be made without chlorine, but Charles E Napier (2000) noted that effects other than chlorine concentration dominate PCDD and PCDF formation, based on studies of the chlorine present in commercial combustion systems. Good combustion virtually eliminates furnace carry-over and flame formation of PCDD and PCDF.
3.1.1 Incomplete destruction
If combustion is inefficient, PCDD and PCDF present in materials being combusted can be emitted. Good combustion conditions rely on sufficient temperature, time and turbulence (oxygen) to ensure that any of the compounds present are completely destroyed.
3.1.2 In-furnace formation
During combustion, ring-structured hydrocarbon species (known as precursors) are formed as intermediate combustion products. If chlorine is present, these can react to form PCDD and PCDF. Precursors include chlorobenzene, chlorophenols and chlorinated biphenyls. In-furnace formation has been linked to upset conditions where incomplete combustion of fuels occurs.
3.1.3 De novo formation
De novo synthesis has been described as the "oxidative breakdown and transformation of macromolecule carbon structures to aromatic compounds" (Buekens et al, 2001). It generally occurs in the temperature range of 250 to 400°C, but some researchers have suggested it can occur up to 1000°C (Charles E Napier Ltd, 2000). Studies have shown that oxygen in the gas stream is essential for dioxin formation by this route. The formation mechanism also relies on the presence of degenerate graphitic carbon structures such as coal, charcoal and soot. Copper and other metals can have strong catalytic effects, while rapid cooling and certain additives can inhibit formation.
In de novo synthesis PCDD and PCDF are formed from chemical compounds that have little resemblance to the PCDD and PCDF molecules (i.e. from non-precursor substances). The features of de novo synthesis have been identified as follows:
- The carbon source of the PCDD and PCDF is likely to originate from the solid carbon matrix of fly ash.
- Divalent copper ions have been found to have a strong catalytic effect on PCDD and PCDF formation, while divalent iron, lead, and zinc appear to have only minor effects.
- The presence of molecular oxygen in the gas stream is essential for de novo formation, and the rate increases with the concentration of oxygen.
- Gaseous forms of chlorine such as HCl or Cl2 appear to have little or no influence on the de novo synthesis reactions at concentrations found in municipal solid waste incinerator flue gases. It has been concluded that the chlorine, and also the hydrogen, found in PCDD and PCDF are likely to originate from inorganic compounds associated with carbon particles.
3.2 Sources of PCDD and PCDF in secondary metals processes
PCDD and PCDF emissions have the potential to occur from heat pre-treatment of scrap, melting, refining and casting operations. PCDD and PCDF may be released as a fugitive emission in addition to furnaces and thermal pre-treatment. However, there is little information on fugitive emissions [High-volume sampling for Site L undertaken as part of this study provides an indication of casting emissions, and these appear to be low (see section 5).] and it is assumed that the main sources of PCDD and PCDF are the furnace and thermal pre-treatment operations.
Formation or poor destruction of PCDD and PCDF are more significant during combustion upsets or when mixing of air and combustible hydrocarbon is poor, since higher levels of organic compounds can escape furnaces at these times. When combustion conditions are optimal, PCDD and PCDF are generally effectively destroyed. Downstream formation or de novo synthesis is likely to be the more significant mechanism in a well-operated furnace.
3.3 Process variables affecting PCDD and PCDF emissions
There are many variables in PCDD and PCDF formation and release. Consequently, there is still a relatively low level of understanding of the PCDD and PCDF formation mechanisms, particularly for the metals industry, to the extent that the prediction of emissions is difficult. Process factors such as scrap quality, metal type, process temperatures, residence time of the exit gas in the de novo range and the presence of chlorine, carbon and particulate matter affect the potential for PCDD and PCDF formation. Chlorine may be present from contaminants in scrap, or from process additives such as salt fluxes.
The furnace and metal type appear to be factors, as indicated in the following examples.
The European Commission (2003) reviewed emission data from cupolas, rotary furnaces and induction furnaces with no specific air pollution controls. PCDD and PCDF levels were very low (<0.05 ng/Nm3) for aluminium melting, induction melting of iron and EAF melting of steel. Results were in a wide range (<0.01-3 ng/Nm3) for cupola melting and rotary melting of iron.
Copper processes have a higher potential for formation of PCDD and PCDF due to the metal's catalytic effect on the PCDD and PCDF formation mechanism. Clean raw materials, the use of electric furnaces and gas quenching all minimise the potential for PCDD and PCDF formation.
Furnace operating temperatures in ferrous foundries are high compared to those in non-ferrous foundries. This increases the potential for de novo formation. For non-ferrous foundries, when ingots or clean scrap are used the potential for PCDD and PCDF formation is very low because both carbon and chlorine are absent.
A study of steel plants re-melting scrap in EAFs (Öberg and Allhammar, 1989) found substantial variation in emissions due to both process variation and different efficiencies in flue-gas cleaning. Slow and continuous charging was found to optimise combustion and resulted in lower emissions. Processes free from chlorine contaminants in the input materials were still found to emit chlorinated aromatics, including PCDD and PCDF.
Furnace design is critical: small-scale, indirect-heated furnaces using clean, raw materials will have low potential for PCDD and PCDF emissions. Direct-fired furnaces, in particular using diesel or fuel oil, may result in carbonaceous particulate matter that can catalyse the formation of PCDD and PCDF, particularly in the presence of copper. Secondary measures (i.e. the use of abatement technologies) are more likely to be needed in these circumstances.
Afterburning (thermal incineration) of flue gases and fabric filtration are the main air pollution controls available for PCDD and PCDF removal.