Heavy metal emission estimates
Introduction

The NAEI currently reports emissions of thirteen metals. These are:



· Arsenic · Mercury

· Beryllium · Nickel

· Cadmium · Selenium

· Chromium · Tin

· Copper · Vanadium

· Lead · Zinc

· Manganese



Emissions inventories for all those except beryllium, manganese, selenium, tin, and vanadium were reported by Leech (1993), Gillham et al (1994) and Couling et al (1994). Emissions of all metals except beryllium, manganese and tin were reported by Salway et al (1996, 1996a, 1997, 1999) and Goodwin et al (1999, 2000). Emission estimates for beryllium, manganese, and tin are reported here for the first time.



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. 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 matter 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 may be emitted in the ash particles.



Emission estimates for combustion sources are generally based on emission factors developed from fuel composition data, applied to fuel consumption statistics (DTI, 2000). Emission estimates for industrial processes are generally based on data taken from the Pollution Inventory or based on the use of emission factors and activity data taken from the literature. The methodology for industrial process emissions has recently been reviewed (Passant et al, 2002) and numerous changes have been made. A similar review of the methodology for combustion related sources is currently being undertaken and may lead to revisions to the 2001 version of the NAEI.



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, 2001) 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, secondary copper and cement manufacture are based on data given in the Pollution Inventory, although literature-based emission factors are also used (sources include Clayton et al, 1991, EMEP/CORINAIR(1996), van der Most (1992), Jockel and Hartje (1991), and Smyllie, 1996). Details of the methodology are given in Passant et al, 2002. Emissions from the chloralkali industry are based on manufacturers estimates (Ratcliffe, 1999).



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 6.2.2). 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, 2001).