Heavy metal emission estimates

Introduction

The NAEI currently reports emissions of ten heavy metals. These are:

• Arsenic · Mercury
• Cadmium · Nickel
• Chromium · Selenium
• Copper · Vanadium
• Lead · Zinc

Emissions inventories for all those except selenium and vanadium were reported by Leech (1993), Gillham et al (1994) and Couling et al (1994). Emissions of all ten metals were reported by Salway et al (1996, 1996a, 1997, 1999) and Goodwin et al (1999, 2000).

Heavy metal emissions arise from a number of different sources, but in general fuel combustion and certain industrial processes which produce dust are the main contributors. The metal emissions arise from the trace concentrations in the fuels or in the case of industrial processes, the raw materials. In the case of combustion, metals are emitted either as vapour or particulate or both. Volatile metals such as mercury and selenium are mostly emitted as vapour. Metals such as cadmium and lead are emitted as both with some of the vapour condensing onto the ash particles. Other metals such as chromium do not vaporise and are emitted in the ash particles.

The inventories are compiled based on the list of source categories given by van der Most (1992) and EMEP/CORINAIR(1996). Most of the emissions are estimated using emission factors applied to fuel consumption statistics (DTI, 1999) and, in the case of industrial processes, production statistics.

UK data is used for the metal contents of coal and fuel oils where available. Emissions from the combustion of liquid fuels are based on data reported by Wood (1996) and other sources in the literature (Sullivan, 1992; Lloyds 1995). Lead emissions from petrol combustion are based on detailed data on the lead content of petrol published by the Institute of Petroleum (1999). The emissions from coal and oil fired power stations are based on estimates reported in the Pollution Inventory (Environment Agency, 1999) or the operators’ annual reports. Emissions from other coal combustion sources follow the PARCOM methodology (van der Most, 1992) but use data based on UK coal (Smith, 1987). Many of the emission factors for industrial processes such as iron & steel, primary lead/zinc manufacture, lead-glass and have been based on a study sponsored by DETR (now DEFRA) which reported emissions based on Environment Agency data (Smyllie, 1996). Emissions from other non-ferrous metal processes, cement production and coke ovens are based on data reported in the Pollution Inventory (Environment Agency, 1999). Emissions from the chloralkali industry are based on manufacturers estimates (Ratcliffe, 1999). Emissions from waste incineration are based on measured data for historic emissions (Clayton et al, 1991) and Environment Agency (1999) data for current estimates. Where UK specific emission factor data were unavailable, other sources of emission factor data were taken from EMEP/CORINAIR(1996), van der Most (1992) and Jockel and Hartje (1991).

Heavy metal emissions can be reduced using gas cleaning equipment which removes particulates from waste gases. This abatement equipment can be fitted to large coal-fired industrial boilers and power station boilers and also industrial processes which produce large amounts of dust. Hence, when estimating emission factors it is often necessary to assume some efficiency of abatement.

The majority of the emission factors used in generating emission estimates are based on the mass of metal emitted per unit mass of fuel burnt, or mass of metal emitted per unit mass of product for processes. These emission factors are assumed not to vary with time for many of the sources considered. This is assumed as there is usually insufficient information to estimate any temporal variation of the emission factor. However, for sources such as road transport, chlorine production, waste incineration and public power generation, there is sufficient information to allow time dependent emission factors to be estimated.

At the end of 1996 all municipal solid waste and clinical incinerators had to comply with new emission standards (see also Section 6.2). As a result, a number of old incinerators have closed, whilst some have been renovated and some new ones opened. Hence there have been significant reductions in emissions from waste incineration. Data is available for most metals for the new plant (Environment Agency, 1999).

The estimates are summarised in Tables 6.11 to 6.20 in the UN/ECE/ CORINAIR SNAP 94 reporting format. This differs from the SNAP 90 system used for previous NAEI annual reports (with the exception of Salway et al 1999, Goodwin et al 1999 and 2000). The main differences are that Combustion in Energy Production and Transformation now includes combustion emissions from electricity generation both from fossil fuels and waste incineration together with refineries and other energy industries. These sources were previously reported under industrial

combustion and waste incineration. The waste incineration category now only includes incinerators which do not generate electricity. By 1997 all MSW incinerators in operation generated electricity, hence from 1997 onwards no emissions from MSW incineration are reported under waste. In previous inventories, the emission for road transport was based on total UK petrol and DERV consumption. However, in the new inventory it is assumed that a small proportion of fuel is used by off-road sources such as agricultural vehicles and construction equipment. These emissions are reported under Other Transport and Machinery but are only significant for lead. The changes in the classification have no effect on the total.

Emissions of Arsenic

Table 6.11 and Figure 6.10 summarise the UK emissions of arsenic. Emissions have declined by 82% since 1970. The largest source of emission arises from coal combustion with other sources being very small by comparison. Coal use has declined over the period considered, in favour of natural gas use. The emissions from the industrial sector are large compared with the emissions from public power generation; this is due to the different levels of abatement efficiency that are assumed. New data from the Pollution Inventory has resulted in an increase in emissions from cement kilns since the last inventory and the inclusion of a new source – copper refining. The large reduction in waste emissions is due to improved controls on MSW incinerators from 1997 onwards and their conversion to power generating plant.

Emissions of Cadmium

Table 6.12 and Figure 6.11 summarise the UK emissions of cadmium. Emissions have declined by 76% since 1970. The main sources are non-ferrous metal production and iron and steel manufacture. The former includes a lead-zinc smelting plant and a number of lead battery recycling plants. The estimate for energy production includes a significant proportion from waste combustion and fuel oil combustion for electricity generation. The decline in emissions is a result of the general fall in coal combustion and the decline in fuel oil combustion in power generation. The large reduction in waste emissions is due to improved controls on MSW incinerators from 1997 onwards and their conversion to power generating plant.

1 See Appendix 4 for definition of UN/ECE Categories

Figure 6.11 UK Emissions of Cadmium

 

Emissions of Chromium

Table 6.13 and Figure 6.12 summarise the UK emissions of chromium. Emissions have fallen by 68% since 1970. The largest sources are various forms of coal combustion, iron and steel production processes, and other industrial processes. There have been significant revisions based on data from the Pollution Inventory. In particular, emission estimates have been added for a specialist chemical company manufacturing chromium-based products under a new category ‘chromium chemicals’, which is included under ‘Processes in Industry’. This has led to a substantial improvement in the estimate of emissions of chromium compared with last year.

1 See Appendix 4 for definition of UN/ECE Categories

Emissions of Copper

Table 6.14 and Figure 6.13 summarise the UK emissions of copper. Emissions have declined by 72% since 1970. The main sources are coal combustion, iron and steel manufacture and non-ferrous metals production. Emissions have declined over the period due to the decline in coal combustion and to a lesser extent the combustion of heavy fuel oil. The large reduction in waste emissions is due to improved controls on MSW waste incinerators from 1997 and their conversion to power generating plant. However new emission data reported in the Pollution Inventory has led to emissions being revised downwards for recent years. The main changes in the inventory are a small reduction in coke oven emissions in ‘processes in industry’ and a new large source in ‘other industrial combustion’ – copper refining. In addition, emissions from cement kilns have been estimated for the first time.

1 See Appendix 4 for definition of UN/ECE Categories

Emissions of Lead

Table 6.15 and Figure 6.14 summarise the UK emissions of lead. Emissions have declined by 92% since 1970. The largest source is still from anti-knock lead additives in petrol and it is here where the most significant reductions have been made. The lead content of leaded petrol was reduced from around 0.34 g/l to 0.143 g/l in 1986 and since 1987 sales of unleaded petrol have increased particularly as a result of the increased use of cars fitted with catalytic converters. Leaded petrol was phased out from general sale at the end of 1999, and consequently a dramatic reduction in emissions from the road transport sector is expected for 2000.

Other major sources are non-ferrous metal production and industrial processes. There has been some reduction in emissions from iron and steel production processes due to improved abatement measures. Emissions have also declined as a result of the decreasing use of coal. The large reduction in waste emissions is due to improved controls on MSW incinerators from 1997 onwards and their conversion to power generating plant. There are only relatively minor changes since the last inventory. These are an increase in coke oven emissions in ‘processes in industry’ and a new source in ‘other industrial combustion’ – copper refining. In addition, emissions from cement kilns have been revised downwards

1 See Appendix 4 for definition of UN/ECE Categories

Emissions of Mercury

Table 6.16a and Figure 6.15 summarise the UK emissions of mercury. Emissions have declined by 81% since 1970. The main sources are waste incineration, the manufacture of chlorine in mercury cells, non-ferrous metal production and coal combustion. Emissions have declined as a result of improved controls on mercury cells and their replacement by diaphragm or membrane cells and the decline of coal use. The large reduction in waste emissions is due to improved controls on MSW incinerators from 1997 onwards and their conversion to power generating plant. There are only relatively minor changes since the last inventory. These are an increase in cement kiln emissions in ‘industrial combustion’ and a small revision upwards in MSW incineration emissions.

1 See Appendix 4 for definition of UN/ECE Categories

Speciation of Mercury Emissions

When mercury is emitted to air it occurs in one of several different forms, and the emissions of each of these forms has been estimated individually- the emissions have been "speciated".

Three species of mercury have been considered

• Hg⁰ unreactive gaseous elemental Hg
• Hg-p attached to particulate material
• RGM reactive gaseous mercury (includes both inorganic and organic forms normally

in the Hg²⁺ oxidised form)

The methodology for estimating the emissions of each of these three species is similar to that used for speciating the VOC emissions. Each source of mercury emission is considered individually. A speciation profile identifying the fractional contribution from each species to the mercury emission from that source is then applied. Summing across the individual sources then allows a total for each of the three species to be evaluated.

The data which have been used to generate the speciation profiles for each of the source emissions have come from a number of different reports (Lee et al 2001, Pacyna 1999, Clarke and Sloss 1992, Davidson and Clarke 1996,) and personal communications (Pacyna 2001, Scott and Jones 2001, Fynes and Crowther 2001, Patrick and Snape 2001).

There are a number of areas where the emission inventory may be improved.

• There appear to be differences in the species according to abatement used in power stations burning coal (e.g. Flue Gas Desulphurisation and Electrostatic Precipitation). At present the specific form of abatement which is used has not been included, and therefore these differences have not been accounted for in the emission estimates presented here.

• To date, only a small amount of data on Hg species emitted from domestic coal combustion has been obtained. Additional data are required to give the speciation increased certainty.

• Similarly, there is little information on Hg species emitted from crematoria, and more data are required.

These areas will be investigated for future reports.

Emissions of Nickel

Table 6.17 and Figure 6.16 summarise the UK emissions of nickel. Emissions have declined by 89% since 1970. The main sources of nickel emissions are the combustion of coal and heavy fuel oil. These have declined in use since 1970 in favour of natural gas and are largely responsible for the reduction in total emissions. Since 1989 heavy fuel oil has been replaced by Orimulsion (an emulsion of bitumen in water) in some power stations though this has now been discontinued. The nickel content of Orimulsion was higher than that of heavy fuel oil and resulted in higher emissions in spite of the flue gas cleaning equipment required on these power stations. Emissions from refineries are important because of the large amount of refinery fuel oil and residues burnt. There have been significant revisions based on data from the Pollution Inventory. In particular, a large source pertaining to nickel chromium use has been added under ‘iron & steel processes’ as well as a significant increase in coke oven emissions under ‘processes in industry’. Industrial combustion has been revised upwards due to new cement kiln data. Glass production now includes flat glass as well as container glass.