Pseudo-Source Parameters for Flare Stacks

摘要

Although the regulatory air dispersion model AERMOD is now run on computers that were once equivalent to super-computers, many basic input components have retained the roots of simplified models and their implied assumptions. Perhaps one of the most influential components of the air dispersion model to ground-level concentration predictions is the calculation of plume rise. This paper derives the plume buoyancy flux in terms of combustion variables readily known or calculated without simplifying assumptions so that the fundamental theory can be used for hot plumes from flares burning gas of varying composition. Prediction sensitivity to flared gas and ambient temperature are now correctly handled by a new equation. The flare pseudo-parameters are based on both the buoyancy and momentum flux, thus conserving momentum rather than arbitrarily assigning the flare a velocity and thus a momentum flux. The plume buoyancy flux increases compared to the classic simplified approach, but reduces to the existing US EPA approach when the simplifying assumptions of the plume having the same molar mass and specific heat of the ambient air are made. An approach is demonstrated to calculate pseudo-source parameters for elevated flares during varying meteorological conditions and is compared to the US EPA approach. The effective stack height for a flare allows for combustion as the flared gas exits the tip. Instead of a single source for all meteorological conditions, multiple co-located sources with varying effective stack height and diameter are used. AERMOD is run with NOSTD option as flare stack tip downwash is accounted for in the effective stack heights rather than the AERMOD model calculating the downwash incorrectly using the pseudo-source parameters. The modelling approaches are compared for an example flare. Maximum ground level predictions change, generally increasing near the source and decreasing further away, with the new flare pseudo-source parameters and the dispersion conditions the maximums occur under change.