18-01-2013, 01:52 PM
GUIDELINES FOR ESTIMATING CHIMNEY HEIGHTS FOR SMALL TO MEDIUM SIZE FUEL BURNING EQUIPMENT
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INTRODUCTION
Organisations making application to the Environment Protection Authority (EPA) for
approval to install fuel-burning equipment at their premises need to demonstrate that the
installation will meet recognised air quality goals. The chimney to be used must be of
adequate height to sufficiently disperse the pollutants before they reach ground level.
The formulae which follow apply to relatively small installations where the only emissions
of concern are sulphur oxides, nitrogen oxides and fluorides. Larger installations, those in
sensitive residential areas, industrial processes which produce pollutants other than the
three mentioned above and cases requiring simulation of local meteorology would need the
application of more complex computer modelling. The chimney heights determined by
these formulae are not necessarily final and the EPA may vary the height in individual
cases if the circumstances warrant it.
HILLY TERRAIN
In the unusual case that the chimney has no buildings around it and is surrounded
by flat terrain, hu is the final height of the chimney. If the chimney is in flat terrain
and there are buildings present, go to Section 3.
If the chimney is in hilly terrain, care must be exercised. This terrain correction
procedure is not applicable to complex situations. In such situations wind tunnel
studies may be required.
The simple terrain correction is undertaken by
adding to the isolated chimney (hu) half the
maximum increase in the height of hills or
rising terrain (ht) within a radius of ten chimney
heights from the location of the chimney. This
is called the corrected chimney height (hc).
WORKED EXAMPLE
A new coal-fired boiler designed to use 20,000 kg/h of coal with a sulphur content
of 0.5% is housed in a square building 35m high and 35m wide with a rise in terrain
of 6m, 300m south of the building.
BACKGROUND NOTES FOR USE WITH THE GUIDELINES
GUIDELINES APPLICABILITY
The simplified chimney height determination method used by the EPA is based on a
regression curve fitted to several empirical and theoretical chimney height curves. This
method is not appropriate for mass emissions of sulphur dioxide in excess of 300 kg/h or
nitrogen oxides in excess of 100 kg/h. The other assumptions in the regression formulae
are a plume rise consistent with an exhaust gas temperature of 165oC and an exhaust gas
velocity of 15 m/s with a wind speed at chimney height of 6 m/s (see Fig 1). Rain caps if
fitted should be of a type that do not impede the vertical discharge of gases.
Where new fuel burning equipment is to be installed in a premises which already contains
sources of air pollutants, the existing air quality should be assessed. As maximum
pollutant ground level concentrations are additive, the sum total of all maximum ground
level concentrations should not exceed the ambient criteria. In situations where two or
more chimneys are to be located in close proximity such that their separation is less than
twice the uncorrected heights, they may be regarded as a single source with mass emission
rate equal to the sum of the individual sources.
Any change in the terrain surrounding the chimney or any nearby building within a radius
of ten isolated chimney heights needs careful evaluation. The presence of large obstacles
such as hills or buildings near a chimney will normally reduce the effective height of that
chimney. When compared to the isolated chimney, maximum pollutant ground level
concentrations are higher and appear at distances nearer the source. The greatest effect
occurs when the chimney is attached to a building and the aerodynamic influence becomes
less significant the further away the building is from the chimney. For equivalent
distances, a building causes greater downwash when it is upwind of the chimney. In
addition, the taller the chimney in relation the nearby building, the less significant the
building effect. If the chimney height to building height ratio is greater than 3 to 1 the
building effect is negligible. The building effect formula in the guidelines represents the
worse-case situation where the chimney is attached to the building. Therefore, the
application of the formula to all cases will yield conservative results.