Executive summary
The Scottish Government encourages the adoption of biomass combustion in order to reduce emissions of greenhouse gases, mitigate against climate change effects and improve energy security and rural development. In addition, as part of the Renewable Energy Framework, The Scottish Government is committed to the growth of the biomass market, particularly in areas not connected to the gas grid network. However, combustion of biomass, along with many other industrial, commercial and transport activities, leads to emissions of air pollutant species that are potentially harmful to human health. Within Scotland (and the remainder of the UK) there is relatively little knowledge or understanding of the possible scale of and impact of pollutant emissions from biomass combustion.
As part of the UK Air Quality Strategy, Scotland has adopted challenging Air Quality Objectives for particulate matter in the atmosphere in order to provide enhanced protection of human health. However, in several Scottish cities the Objectives set in the Air Quality Strategy are already closely approached or exceeded. This is the case for Dundee and Edinburgh and these cities were therefore selected for this detailed study to evaluate the potential cumulative impact of biomass boilers on particle concentrations in urban areas.
A key component of this study was the inclusion of specific detailed measurements of particle emissions from a range of typical small-scale biomass boilers installed and operational in urban areas throughout Scotland. In total, 6 boilers were tested to determine emissions of PM 10 and PM 2.5 particle size fractions 1. The boilers chosen for the test programme cover a range of manufacturers, sizes and fuel types. The results of these tests and a review of available literature indicated a wide range of emission factors. Based on these measurements, and the literature review, two emission factors 2 of 20g/ GJ and 60g/ GJ were selected as representative of the range of boilers tested. These values were also found to be generally consistent with the biomass emission factors reported elsewhere. However, it is recognised that this is a relatively small sample and it is restricted to a specific boiler capacity range and, hence, extrapolation of the results to other cities with potentially different biomass boiler installations needs to be undertaken with caution. Lower emission factors can be achieved by means of more effective abatement technologies, but there is currently no requirement to apply these technologies. Unabated emissions from some plant could be greater.
In this study a number of important assumptions relating to the likely profile of typical biomass boiler installations have been made. In particular that the maximum local contribution to annual mean particulate matter concentrations from each individual plant operating at capacity is limited to less than 1µg m -3. (In this study, it has been assumed that this will be achieved by use of an appropriate chimney height. However, other measures, such as, additional emission abatement control systems, fuel type and boiler selection could be used to achieve the same outcome.)
Information from Dundee City Council and The City of Edinburgh Council was used to prepare scenarios for possible biomass implementation in 2010 and 2020. The 2010 scenario was developed using details of existing planning applications for biomass boilers in both cities. It was assumed that all of these boilers would be granted planning permission and be installed by 2010. In order to assess the potential impact of biomass combustion on air quality in Dundee and Edinburgh in 2020, it was necessary to estimate where the biomass combustion would occur and the quantities of heat to be provided. Potential biomass installations were identified from local development plans together with assumed property replacement and renovation rates 3. This provided an estimate of the distribution of biomass combustion sources throughout both cities.
Air quality modelling, using recognised and validated air quality models, was then undertaken for each scenario in each city using the two emission factors derived from the emission monitoring programme.
The modelling of particulate concentrations for 2010 shows that the potential impact of the current proposed biomass installations for both Dundee and Edinburgh for both the 20g/ GJ and the 60g/ GJ emission factor cases is likely to be less than 0.1µg m -3 except in the immediate vicinity of the proposed installations. The modelled PM 10 concentration arising from all other sources in 2010 is in the range 14 to 20µg m -3 in these city centre areas and hence, the biomass contribution is in the range 0.5 -0.7%.
The model for 2020, using the 20g/ GJ emission factor, shows that the effect of biomass combustion is likely to increase annual mean PM 10 concentrations across much of the city centres for both Dundee and Edinburgh by 0.2-0.5µg m -3. For an emission factor of 60g/ GJ the model shows increases in particulate concentrations of 0.5-1.0µg m -3 across large parts of both cities. The modelled PM 10 concentration arising from all other sources in 2020 is in the range 14 to 20µg m -3 in these city centre areas and hence, the biomass contribution is in the range 1 - 7%.
The Scottish Air Quality Objective for annual mean PM 2.5 is not predicted to be exceeded at any background locations for any scenario in either Dundee or Edinburgh. The UK has also set a PM 2.5 exposure reduction target of 15% by 2020 in urban background areas. The business as usual scenario without biomass installations indicates this target will not be achieved. The combined impact of large-scale uptake of biomass installations, under the conditions assumed in this study could increase the difficulty in achieving this target. Additional controls on emissions from individual boilers could be explored to minimise this impact.
The modelling study demonstrates that biomass boilers will not be the major source of PM 10 or PM 2.5 in urban areas. However, in areas that are already close to PM 10 Air Quality Objectives the additional contribution of biomass may lead to an exceedence at some city background locations. Note that this result applies to urban background concentrations and higher particle concentrations may be seen in areas close to other specific sources.
As part of this study screening tools have been developed to assist Local Authorities to assess the impact of both individual and multiple boiler applications. The individual installation tool will allow Authorities to make informed judgements on the impact of biomass combustion on air quality and the potential need to specify control measures. Emissions from individual boilers can be controlled by boiler design, specification and rating, fuel type and quality, emission abatement equipment and/or chimney height specification.
The combined impact tool will help to identify high-density housing or industrial areas where single large district or community heating schemes may be more appropriate, and have less impact on air quality, than many individual smaller boilers. For example, at one large proposed housing development in Edinburgh, this study shows that use of a small number of centralised biomass boilers may contribute 0.5-1µg m -3 to PM 10 and PM 2.5 concentrations, compared to a contribution of 2-5µg m -3 for individual heating systems.
Currently, the Clean Air Act is the main legislative instrument for the control of emissions from small and medium scale boilers. However this Act was developed primarily to control emissions from coal combustion and is not entirely appropriate to biomass combustion in modern appliances. The Act focuses on visible smoke and larger particle emissions rather that the smaller particle size fractions considered in this report. The Act may therefore need to be revised to provide greater consistency with current Air Quality Objectives. In the light of the findings of this study there is a need to review the provisions of the Clean Air Act and to consider the way the planning system operates in practice, so as to take better account of the potential cumulative air quality impacts of district level biomass boilers in urban areas to help ensure that fine particulate levels do not exceed national and EU limit values. In addition, potential costs and benefits of emission abatement equipment, such as particulate filters, to reduce PM 10 and PM 2.5 emissions could usefully be explored for certain boilers, especially in urban areas where levels are close to EU or national objectives for air quality. However, investigation of specific changes to the Clean Air Act, planning guidance and cost benefit analysis are beyond the scope of this report.
This study has focused on Dundee and Edinburgh. For other areas, the screening tools will allow Local Authorities to take account of the likely different background particle concentration arising from other sources in these areas.