The potential for damage from flue gas emissions can be avoided by dispersion and dilution from a tall chimney.
Pollutants emitted from a chimney must first clear the area of turbulent air created by the wind around the chimney top and any turbulent areas caused by the wind over adjacent structures.
The function of the chimney is to discharge flue gases to the atmosphere at such a height and velocity that the concentration of pollutants, such as sulfur dioxide, is kept within acceptable limits at ground level.
After leaving the top of the chimney, the gases are carried higher by their own buoyancy compared to the surrounding air and the momentum of the flue gases emitted.
For large power station plants, flue gas is typically at about 120°C for a coal-fired plant and 150°C for oil-fired applications, and contains appreciable quantities of SO2 for oil and SO2 and HCl for coal. Although these efflux temperatures are set to avoid dew point acid condensation within the duct work, downwash within the flue and heat loss from the stack cause acid deposition within the flues themselves.
For a new plant, chimney heights are determined from a mathematical model calculating ground level concentration of pertinent contaminants, usually SO2 or NOx. The function of the chimney is to reduce the resulting ground level concentration of each constituent of the emission so that it is below the threshold which gives rise to a health hazard or nuisance.
The calculation of this concentration is technically complex and takes into account the topography surrounding the power station, the local meteorology, the presence of tall buildings and other emission sources. In some cases, wind tunnel investigations are required.
In many applications, the height may be estimated with reference to empirical formulas dependent on the height of surrounding buildings and levels of emissions.
Under ideal conditions, and so far as gases are concerned, the ground level concentrations from a chimney discharge vary directly with the mass rate of emission, and inversely with wind speed and the square of the effective chimney height. Maximum ground level concentrations occur at about 10 to 15 times the effective chimneys' height distance down wind. For large power station chimneys in the UK, the design efflux velocity should not be less than 15 m/s at maximum continuous rating.
Brickwork has, in the past, made a suitable structure for freestanding chimneys up to about 60 m high. For taller chimneys, the overturning moment due to increased wind load can be more economically resisted by a reinforced concrete shaft. Due to the need to deal with acid condensates within the flue, it is necessary to provide a lining to protect the concrete shaft internally from heat and acid attack. This lining is most often constructed from free-standing, acid-resisting brickwork about 100 mm thick. In some cases a separate concrete lining is provided, itself protected with a suitable chemical liner, for example, a synthetic resin. An acid brick lining is self-supporting up to a maximum height of 10 m. Consequently, the lining is built as a series of truncated cones carried on corbels inside the concrete windshield at 10 m intervals.
There is typically a cavity 50 mm wide between the concrete shaft and the brickwork lining, which may be filled with an insulating material or left as an air gap.
The lining is usually specified as dense acid-resisting brick laid in potassium silicate mortar.
The basic parameters for flue design are the height of the flue, the temperature, the efflux velocity and the rate of emission of the gases. The diameter at the top of the flue will be determined from the rate of emission and the efflux velocity; the latter being kept as high as practicable to minimize downwash and to enhance dispersion. In order to maintain velocities as high as possible, it is common to provide multiflues within a common windshield on larger installations.
The pressure head which causes the flow of gases up the flue is the result of the difference in density between the flue gases and external atmosphere. It is good practice to maintain a slight negative pressure inside the flue to reduce gas leakage; therefore, a balance must be maintained between the head available through density difference and the losses at entry and exit and by friction in the flue.
In the UK, multiflue chimneys for large power stations have been constructed up to 260 m in height. For a 2000 MW station of 4 × 500 MW units, flues about 200 m high, 6 m in diameter and efflux velocities of 23 m/s have been constructed.
National Society for Clean Air and Environmental Protection, 1994 Pollution Handbooks.
Modern Power Station Practice. Third Edition.
British Electricity International, London, Pergamon Press.