INTEGAIRE case study submission form

1. Topic
		Power plants

2. Introduction
		In 1997 fossil fuel-fired power plants produced around 51% of electricity in the EU-15, i.e. around 1.2 millions GWh. Hard coal and natural gas had the higher share (19.5% and 13.7% respectively) followed by oil (7.7%), lignite (7.6%), biomass (1.1%) and derived gases (1.1%). Combustion of fossil fuels poses major threats to the air quality: power and large combustion plants are the most important non-natural source of SO_x and the second source of NO_x after road traffic. Their impact on the urban air quality crucially depends on their location and on the meteorological features of the site. It is worth noticing that, together with primary pollutants, also secondary pollutants induced by plants emissions, and especially very fine particulate matter (PM_2.5 and PM₁), can constitute an important threat to the public health.

3. Discussion

Pollutants and their origin

Power plants are important sources of many typical urban pollutants.

SO_x(SO₂ with 1-3% of SO₃) are mainly emitted when fossil solid or liquid fuels are employed, but also natural gas can contain sensible amounts of sulphur and in some cases gas desulphurisation can be necessary.

NO_x is generated by means of three mechanisms: the thermal reaction between N and O in the air, the “combustion” of nitrogen contained in the fuel and the so-called prompt NO formation on the flame front because of the presence of intermediating hydrocarbons.

The thermal NO_x formation is strongly reduced if the combustion takes place at temperature lower than around 1000 °C. In this case, the nitrogen content of the fuel is crucial. Coal can reach the highest N share (0.5 – 2%) whereas oil contains around 1% of N, biomass less than 0.5% and Natural gas is virtually free from nitrogen (< 0.1%).

Besides acting directly as pollutants, NO_xhave a crucial role in the production of ozone from oxidation of VOCs (volatile organic compounds). Nevertheless, quantifying the role of a single NO_x emitter in the overall ozone production of a region is quite complex as the ozone production depends on the whole emissions of the area, its meteorological features and also on the composition of air transported from other regions. Complex photochemical modelling is then unavoidable to understand the urban ozone origin and to assist local and regional authorities in making policies for ozone containment.

Particulate Matter derives from the mineral fraction of the solid fuels, with a small fraction coming from condensation of low-vapour pressure compounds volatilised during combustion. When liquid fuels are employed, poor combustion can lead to the formation of soot, that can act as condensation nucleus for sulphur and other pollutants. On the contrary, natural gas is not a significant source of primary particles.

It is worth noticing that particulate matter can form in the atmosphere also by means of the nucleation of ammonium sulphate or nitrate and the condensation of organic compounds. Generally speaking, a straightforward method to evaluate the amount of secondary particulate matter generated by power plants emissions does not exist as nucleation and condensation rates depend on the meteorological and chemical features of the atmosphere around the plant.

Heavy metals (Lead, Cadmium, Arsenic, Mercury and others) are present in trace in fossil fuels and are emitted as a component of primary particulate matter, except for mercury and selenium that can be emitted as vapours. Coal is by orders of magnitude richer in heavy metals than oil and natural gas.

Emissions regulations and abatement

Emissions of Large Combustion Plants (LCP) have been directly limited by Directive 2001/80/EC and their control is also crucial in reaching targets set in the National Emissions Ceiling Directive (2001/81/EC). Furthermore, large power plants are concerned by the Integrated Pollution Prevention and Control (IPPC) directive (96/61/EC).

The LCP Directive focuses on the NO_x and SO_x emissions for existing plants, whereas emission limits are set also for PM in new plants.

In the frame of the IPPC directive, a reference document on Best Available Technologies (BAT) is in preparation (see References). This document will collect and examine cost and benefits of all the technological measures useful for emissions abatements in power plants and will constitute a reference for plants managers. Measures can be taken at different points of the power plant and involve different end points e.g. fuel switch, combustion optimisation, catalytic reduction and so on.

Directives-driven actions, together with measures imposed by national and local administrations are expected to induce a sensible decrease of the power plants emissions. The direct effect of this emission decreasing on the urban air quality is crucially linked to the relative position of power plants, normally located in industrial areas, and the city, but in any case the power plants emissions enter in the background pollution and can be transported for very long distances.

4. Recommendation / Conclusion

From the point of view of city policy makers some few recommendations about air pollution induced by power plants can be set:

· If power plants are located in the city surroundings, it is important to have a careful meteorological characterisation of the site. Such a study should focus on wind patterns at least at the ground level and at the effective pollutants release point. If possible the study should also establish typical seasonal and daily patterns for wind profiles in the Planetary Boundary Layer and for the mixing height.

· To achieve a consistent evaluation of the pollution induced in a town located downwind of a power plant, complex models should be employed taking into account also secondary pollutants (ozone and secondary particulate matter). In this case a careful study of the features of the atmosphere surrounding the plant plume is needed, and all other relevant urban sources of ozone and particles precursors, such as traffic and domestic heating, must be estimated and included in the simulation.

· When evaluating a power plant environmental impact, besides the direct emissions coming from the combustion process, it is important to estimate the effect of other pollution sources induced by the activity, especially road, river or sea traffic due to fuel transportation.

· Power plants are often perceived as a pressure on the environment and a possible threat for public health by the population living in the area surrounding an existing or planned LCP. Local authorities should take into account this sensitivity when planning and locating energy production devices and involve citizens in decisions as much as possible.