Atmospheric Dispersion

The traditional approach to the dispersion of pollutants in process exhaust gases is to emit them at as high a velocity as possible (say up to 15m/s) from a stack built fairly tall to improve the dispersion of the pollutants before they return to ground level. This approach can, in theory, reduce downwind ground level concentrations to any specified level although, in practice, physical and economic constraints upon stack height and the power costs associated with high exit velocities provide a limit to what can be achieved. Hence it is vitally important to have techniques for the prediction of downwind ground level concentrations in order to ensure that the proposed combination of stack height and exit velocity will achieve a low enough downwind ground level concentration under all likely wind speeds, directions and prevailing weather conditions.

The course starts with the simple text-book approach to modelling of stack emissions and provides examples of its use in a simple spreadsheet for predicting ground-level downstream concentrations at varying emission rates, exhaust velocities, temperatures and wind and weather conditions. Such a simplified approach is often very useful for screening studies to identify worst-case conditions and to identify where a more rigorous approach is required. This is followed by a survey of available computer programs, which allow the user to relax some of the assumptions of the simplified approach. This gives the attendee a range of techniques for tackling the problem.

One important fact which is often forgotten is that the likely downwind concentration, at any point and for any wind conditions, is the sum of the background concentration of the pollutant ignoring the emissions from the stack and the ground level concentration predicted by the model as being produced at the point and at the prevailing wind conditions due to the emissions from the stack. Hence, if the pollutant is likely to have any significant background level without the stack operating, then this background level will need to be known or measured. IPC/IPPC authorisations may need some measurements to be taken of the background level.