Levels Of Pollutants In The Air

COULD STACK EMISSIONS AFFECT HEALTH? Levels of pollutants in the air can be defined as low, moderate or high. The Department of Health has said that, at low pollutant levels, nobody is likely to experience adverse health effects. It is these low pollutant concentrations that form the proposed UK Air Quality Standard as shown in Table 2. So, in setting limits for stack emissions from power plant, air quality standards should be met. Most people are not affected by the levels of the various substances found in the general environment if the standards are met. For

example, the Department of Health states that, if the NOx limit is met, “it is very unlikely that anyone will experience any adverse effects”. It is worth noting that, as shown in Table 2, concentrations of pollutants in the home can often exceed those outside. They can also exceed the proposed Air Quality Standard. Pollutant levels resulting from wood-fired plant will be small compared with existing outdoor levels, and also less than those often found in homes.

Table 2 Air quality: proposed standards and existing levels

Substance

Averaging timea

Proposed UK Air Quality Standard

Air quality in Central London for 1998b

Air quality in West Wales for 1998b

Air quality in homes with gas cookersc

Concentration

Concentration

Concentration

Concentration

3

3

Carbon monoxide

8 hours

11.6 mg/m

1–13 mg/m

not available

up to 25 mg/m3

Nitrogen dioxide

1 hour

200 µg/m3

12–312 µg/m3

2–65 µg/m3

up to 1115 µg/m3

Particulates (<10µm)

24 hours

50 µg/m3

12–67 µg/m3

4–52 µg/m3

35–95 µg/m3

Sulphur dioxide

15 minutes

266 µg/m3

3–203 µg/m3

1–178 µg/m3

a

Averaging time is the period of time over which the given measurements of a particular emission are averaged. The shorter the recommended averaging time, the more important short-term peaks are for human health. The averaging times shown apply to measured figures for all locations. The exception is carbon monoxide where the measured figures are based on hourly averages rather than on the eight hourly average specified by the proposed Air Quality Standard.

b

The air quality figures come from monitoring station records (Marylebone Road in Central London as an example of urban air quality near a busy road, and Pembrokeshire in West Wales as an example of rural air quality). Monitoring station records for sites throughout the UK can be found on the UK National Air Quality Information Archive (www.aeat.co.uk/netcen/airqual)

c

The indoor air quality figures come from reports by the Medical Research Council Institute for Environment and Health at the University of Leicester.

For further information on issues raised in this leaflet, and on other new and renewable energy matters, contact: New & Renewable Energy Enquiries Bureau ETSU Harwell Didcot Oxfordshire OX11 0RA Tel: 01235 432450 Fax: 01235 433066 E-mail: [email protected]

Figure 3 Estimated pollution levels from the stack emissions of a wood-fired electricity generating plant as a percentage of the relevant air quality standards

DTI New & Renewable Energy Web Site: www.dti.gov.uk/renewable/ index.html First published March 2000

Effects of Renewable Energy

ERE-WF3

Wood Fuel for Electricity & Heat Stack Emissions, Air Quality and Health WHY USE WOOD FOR FUEL? The Government, as part of its overall strategy for improving sustainability and reducing the impact of climate change, supports the use of renewable energy sources for generating electricity and heat. Wood is an important renewable energy source, and around 900,000 tonnes of wood are already used for fuel each year in the UK, mainly for domestic and commercial heating. There are sound environmental and economic reasons for using wood as fuel:

o

Wood is a renewable and sustainable resource that can be grown and managed as part of existing forestry or agricultural systems.

o Using wood instead of fossil fuels helps to reduce carbon dioxide (CO2) and other potentially damaging emissions. When wood is burned, it only puts back into the atmosphere the CO2 that it absorbed from the atmosphere while it was growing. CO2 is one of the main greenhouse gases contributing to climate change.

o A demand for wood fuel may encourage farmers to plant short rotation coppice crops – an important opportunity for diversification, which could help to supplement farm incomes.

o A demand for wood fuel will encourage the use of forestry residues – the branches and thinnings often left on site by modern forestry practice.

o Building and operating wood-fired heat or power plant can create jobs and increase demand for local goods and services in rural communities.

Using wood to produce electricity and/or heat is technically proven. Wood can be burned and the heat used to provide hot water or to raise steam for direct industrial use or for electricity generation. Alternatively, wood can be subjected to modern gasification or pyrolysis techniques: the resulting gases or liquids can be burned in an engine or turbine to generate electricity, or in a boiler to produce heat. Wood-chip-fired furnace

AIM OF THIS LEAFLET The aim of this leaflet is to describe the stack (chimney) emissions associated with a woodfired plant for generating electricity and/or heat, and to assess the possible impact on air quality and human health. It is designed to be of interest to the general public, members of local authority planning committees, organisations with a specific concern for the environment, and others involved in the planning process.

This leaflet has been endorsed by: British BioGen, English Nature, Forestry Commission, Game Conservancy Trust, Ministry of Agriculture, Fisheries and Food, National Farmers’ Union, Woodland Trust and World Wide Fund for Nature.

INTRODUCTION All processes that involve combustion, gasification or pyrolysis (for example central heating systems, car engines and power stations) give rise to emissions to the air. It is therefore important to consider stack emissions produced by a wood-fired plant in the existing environmental context. At the local level, this means comparing them with other sources of emissions and with current air quality. In the broader context, it means comparing the stack emissions from a woodfired electricity generating plant with those from a power station fuelled by coal, oil or gas. To assess the effects on air quality and health, it is necessary to answer a series of questions, for example:

o What stack emissions are produced, and in what quantities?

o How can stack emissions be controlled? o What effects are these stack emissions likely

Controlling the emissions from a wood-fired electricity generating plant will normally be achieved by:

o operating the plant correctly and efficiently o installing pollution control equipment where this proves necessary

o making sure that all staff involved in emissions control are properly trained.

The Environment Agency, or, for smaller plant, the local authority, will specify the stack emission limits for a particular plant in order to meet UK and European standards. These standards are designed to protect the environment and human health by maintaining acceptable local air quality. Emissions will have to be monitored to ensure that the limits are being met.

WHAT LEVEL OF STACK EMISSIONS WILL BE PRODUCED?

to have?

WHAT STACK EMISSIONS WILL BE PRODUCED? The main stack emissions from a wood-fired plant burning “clean”, non-treated wood from forests or coppice will consist largely of water vapour and CO2 (plus nitrogen and oxygen from the combustion air). The emissions will also contain traces of carbon monoxide (CO), oxides of nitrogen (NOx), particulates (small particles of dust) and volatile organic compounds (VOCs). The same pollutants are produced when fuels are burned elsewhere, eg in domestic cooking and heating appliances, in vehicles and in fossil-fuel-fired power stations. Some sulphur dioxide (SO2) will also be produced but, as there is little sulphur in wood compared with coal, levels will be relatively low.

1

HOW CAN STACK EMISSIONS BE CONTROLLED?

Wood that is not contaminated with paint, preservatives etc.

The level of stack emissions will vary with plant type, size and method of operation. Table 1 shows typical figures for a 10 megawatt electricity (MWe) generating station fuelled with clean wood.1 Wood-fired plant will typically have a capacity of 0.25–30MWe, much smaller than a typical fossil-fuelfired power station with a capacity of 500–2000MWe. This means that total stack emissions from a properly operated wood-fired unit will be very much lower than those produced by a large-scale fossil-fuel-fired power station, and consequently the effect on local air quality should be much smaller. Because the plant referred to above vary so much in size, a more useful way of looking at stack emission levels is to examine life-cycle emissions, ie the quantities emitted over the lifetime of the plant per unit (kWh) of electricity generated. Figure 1 clearly shows that emissions per kWh from properly designed and operated wood-fired power plant are lower than those from coal- or oil-fired plant. The best wood-fired power plant should have emissions close to those from natural gas combustion.

Table 1 Typical stack emission concentrations from a wood-fired electricity generating plant (mg/m3)

a

Generation technology

Plant size

SO2

NOx

Particulates

VOCs

CO

Combustion

10MWe

50

300

100

50

200

Gasification

10MWe

30

100

<50

20

400

Pyrolysis

10MWe

20

200

<50

not available

150

a

Milligrams per cubic metre of exhaust gas

Source: Combustion emissions based on operating plant data. Pyrolysis and gasification emissions based on estimates for plant under development.

Figure 2 The dispersion process

Figure 1 Comparison of the life-cycle emissions of various electricity generation technologies

WHAT HAPPENS TO THESE STACK EMISSIONS? The relatively concentrated emissions leaving a power station stack become increasingly diluted as they mix with the surrounding air. This is called dispersion (Figure 2). When considering a power station’s impact on human health, it is the ground-level concentration of pollutants that is important, ie the concentration in the air people breathe. Ground-level concentrations will depend on a number of factors:

o the height of the chimney – the higher the stack, the more the emissions products are dispersed and the lower the ground-level concentrations will be at any given point

Ground-level concentrations can be predicted using “dispersion modelling”. Various dispersion models have been developed to take into account the factors that affect how stack emissions are dispersed into the atmosphere. They use information on emission sources and dispersion characteristics to simulate, using computers, the downwind dispersion of the emissions. Estimates can then be derived of the concentration of pollutants in the air in places where people may be exposed to power plant emissions.

WHAT EFFECT WILL STACK EMISSIONS HAVE ON LOCAL AIR QUALITY? A broad idea of the possible effect of stack emissions on air quality can be obtained by comparing pollutant levels predicted by dispersion models with UK air quality standards and existing air quality. Table 2 summarises relevant air quality standards and gives examples of current air quality in the UK. Figure 3 shows the estimated effects of NOx, SO2 and particulate emissions (from the stack of a wood-fired electricity generating plant) on air quality, given as a percentage of the relevant air quality standards. It indicates the likely impact of such a plant on local air quality over a oneyear period.

o the emission rate, which is determined by plant type, size and operation

o weather conditions around the power station – wind speed and direction, turbulence, atmospheric conditions etc

o existing local air quality, which depends on other

sources of pollutants such as existing traffic, other industrial processes and domestic heating systems

o building and landscape features, through their influence on air movement and hence on the dispersion of pollutants.

Units used in this leaflet Milligrams (mg)

Thousandths of a gram. A drop of water with a volume of 1mm3 weighs 1mg. Micrograms (µg) Millionths of a gram. Megawatts (MWe) Millions of electrical watts. A watt is a measure of power. Kilowatt hour (kWh) Thousands of watt hours. A watt hour is the amount of energy used when a power of 1 watt is provided for 1 hour; for example, running a 60 watt light bulb for 5 hours uses 60 x 5 = 300 watt hours. Generally, electricity companies charge consumers for each kWh they consume.