9.1 Characteristics of the New Zealand industry
Key findings from the survey and test programme are as follows.
- Eleven of 14 large and medium sites have fabric filtration (80%) compared to only 10 of 68 for small-to-medium and small sites (15%). Overall, the rate of control appears low at just over 25% in terms of the number of sites, but in terms of metal melted over 80% is controlled by fabric filtration.
- Five of the 82 (6%) sites have afterburners, which represents 12% in terms of total metal melted.
- Raw materials were reported as comprising generally clean/internally generated scrap or ingots for the majority of sites; most contaminated scrap is processed at a few large sites.
- There is a high level of use of electrical induction furnaces and crucibles, which are generally externally heated with gas or electricity.
- Only two or three sites have thermal scrap pre-treatment processes (pyrolysis, combustion, drying).
- Large sources are responsible for the great majority of the production (71%) and the emissions.
- There are a very small number of sites with relatively high discharges of PCDD and PCDF.
- Emissions factors compare well with emission factors for Europe for highly controlled sites.
9.2 The need for further controls in New Zealand
Given that this study has characterised the New Zealand secondary metal industry as a relatively low emitter of PCDD and PCDF compared to findings for other countries, further controls do not appear to be warranted.
Reductions for the principal emission sources could be considered. Control of PCDD and PCDF is complex - it is not simply a matter of clean scrap, fabric filtration and afterburning, but also involves process control, particularly combustion control. It may be necessary to consider reductions via both primary and secondary measures. Primary measures prevent the formation of PCDD and PCDF and secondary measures prevent the compounds from entering the environment.
Primary measures include:
- selection of clean scrap
- substitution of process raw materials
- improving combustion conditions
- avoiding slow pre-heat of raw materials under conditions that can result in formation of PCDD and PCDF emissions
- quenching the off gases from 500°C to below 200°C
- inhibition of formation by urea or strong basic raw materials (Buekens, 2002).
Secondary measures to remove PCDD and PCDF include:
- the use of afterburners to destroy organic pollutants
- high-efficiency fabric filtration
- catalytic filtration systems which destroy PCDD and PCDF
- adsorption on active carbon or coke
- dry scrubbing with lime and active carbon followed by fabric filtration
- rapid quench to minimise de novo formation (650-200°C).
Therefore, given the complexity, potential range of control options and relatively small number of significant sources, the most effective approach to further reductions of PCDD and PCDF emissions from New Zealand's secondary metals industry may be by addressing individual sites on a case-by-case basis. The logical forum for this is through the resource consent process as administered by regional councils. However, it is important for regional decision-makers to recognise the national and global significance of PCDD and PCDF emissions and not just focus on the potential effects on local environments.