These pollutants usually have some form of discharge licence limits imposed on their concentrations. These licence limits are usually:

1. Concentration limits at the point of discharge which have been set regardless of the amount of the pollutant or the location of the point of discharge; and/or

2. Concentration limits at one or more points away from the point of discharge.

Equipment can be installed to clean the flue gas so that it complies with the discharge point concentration limits. This equipment is discussed below.

Compliance with concentration limits away from the point of discharge usually is brought about by economic and performance optimisations of several factors:

1. The height of the chimneys - height is usually an advantage, but more costly;

2. The buoyancy of the flue gas leaving the chimneys - hotter gas has more buoyancy (i.e. has a lower density), but may reduce the thermal efficiency of the generating plant;

3. The discharge velocity of the gas from the chimneys - higher velocities are an advantage, but may require additional fan power; and

4. The location of the chimneys in relation to each other, the power station's buildings and cooling towers, buildings outside of the power station boundaries and weather conditions, particularly the strength and direction of the wind.
These factors will not be discussed further in this information sheet, except where they (mainly buoyancy and discharge velocity) are influenced by the flue gas cleaning equipment.

The equipment to clean the flue gas can be divided into several groups:

1. Equipment to remove solid particles (generally called "particulates") - the two main types of equipment for this duty are electrostatic precipitators and fabric filters;

2. Equipment to remove gaseous impurities such as oxides of sulphur and oxides of nitrogen; and

3. Equipment to remove other gases for which there are not licence limits but which could have economic implications, such as carbon dioxide if carbon taxes or carbon trading are implemented.