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Key outcome

  • Safe maintenance conditions identified
  • Risk mitigated

The regulations for industrial stack emission impacts rely on peak and long-term average concentrations ground level concentrations at surrounding sites. These values can be rapidly estimated using standard regulatory models, such as AERMOD, or similar models like AUSPLUME, provided the stack emissions are well understood. However, in special circumstances operators may need to know concentrations closer to the stack where effects like downwash are critical.

For points in close proximity to the stack, a model needs much finer resolution than Gaussian models like AERMOD are capable of delivering. Synergetics performed a sensitivity study on a stack under a continuous operation cycle. The facility operator wished to undertake maintenance work on the stack, without shutting down the plant. This required a safety assessment for a range of atmospheric conditions, in order to determine safe work conditions.

Contours of concentrations from a tall stack during moderate winds. The contours immediately drop below the stack tip level due to downwash.

Figure 1: Modelled plume concentration of carbon dioxide under moderate wind speeds on a cool day. A small amount of tip downwash is observed.
Synergetics assessed meteorological conditions at the site to provide a typical range of weather conditions for a sensitivity study. These weather conditions, together with onsite stack measurements provided the input conditions for a high resolution CFD study of the plume dispersion. Results from the simulation were assessed to determine what conditions led to stack tip downwash, where the plume is recirculated behind the stack. Concentrations of all relevant toxic compounds in the plume were assessed against workplace safety limits. Appropriate safety margins were integrated into the CFD results to determine what conditions would be suitable for maintenance operations. Based on these findings, a set of optimal weather conditions were determined for safe execution of the maintenance tasks.

For further stack emissions modelling see our air quality page.