Key outcome

  • Safe maintenance conditions identified
  • Risk mitigated

Industrial stack emission regulations are primarily based off ground level concentrations, assessing peak and long term average values at surrounding sites. Provided the stack output conditions are well known, these values can be quickly estimated using the standard regulatory model, AERMOD, or similar models like its Australian predecessor, AUSPLUME.  However, in special circumstances operators may need to know concentrations closer to the stack.

For points in close proximity to the stack, a model needs much finer resolution than Gaussian models like AERMOD are capable of delivering. In this case study, Synergetics was requested to perform 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, and required a safety assessment for a range of atmospheric conditions, in order to determine under what conditions the work could proceed.

Carbon dioxide concentrations in a stack plume

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 and so form the basis of the sensitivity study. These weather conditions, together with onsite stack measurements provided the input conditions for a high resolution CFD study of the plume dispersion near the stack exit. 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, and appropriate safety margins were integrated into the CFD results to determine what conditions would be suitable for maintenance operations. These results were then used to provide a set of optimal weather conditions under which the maintenance could be safely performed.