Influence of turbulent Schmidt number on fugitive emissions source quantification

Abstract

Abstract Finding and quantifying unknown (fugitive) releases of gases such as methane from downstream concentration data is a critical environmental problem. Many proposed solutions involve wind and gas dispersion modelling for which an assumed value of the turbulent Schmidt number (Sct), the ratio of the eddy kinematic viscosity to the turbulent diffusivity, is used to scale the estimated diffusivity. This model constant has a range of physically reasonable values. Numerical simulations were performed on multiple test cases to quantify the impact of Sct uncertainty on the ability to locate and quantify fugitive emissions sources using data from a network of gas concentration sensors. The analysis further considers ways to reduce quantification uncertainty, by either specifying detected source locations based on ancillary knowledge, or by using a two-step optimization in which the presented adjoint approach is used to locate sources and simulated annealing is subsequently used to quantify emissions for these specified locations. Multiple test cases considered both real and numerically generated controlled releases using an open-field geometry and a complex bluff-body dominated geometry based on an actual gas plant in the Alberta, Canada oil and gas sector. Results suggest that correct prediction of unknown source locations is minimally affected by Sct, but emission rate quantification can be heavily influenced. The presence of bluff-bodies was found to partially mitigate these effects, such that, if repeated analyses over a range of Sct is not tractable, open field release results (including those presented here) can be used to estimate conservative error bounds on predicted emission rates. Ultimately this paper demonstrates the under-appreciated importance of considering Sct uncertainty when seeking to quantify unknown fugitive sources from downstream concentration data.