A parametric model for the efficiency of a flare in crosswind

Abstract

Despite the prevalence of gas flares used throughout the world, there are limited data to address concerns over their efficiencies and emissions. This paper has two main objectives. The first is to present experimental data from diffusion flames in crosswinds to elucidate the relative importance of parameters that affect their carbon conversion efficiency (η). Drawing on these data, the second objective is to develop empirical models to estimate the efficiency of flares. Physical parameters shown to be important are fuel type, wind speed ( U ∞ ), exit velocity ( V j ), stack outsidediameter ( d o ), fraction of inert diluent in the fuel stream, and the specific energy content of the fuel mixture (expressed here as the lower heating value, LHV mass ). Experiments revealed that increasing the amount of diluent in the fuel has a profound effect on inefficiency (1-η) in that reduced energy density flames become much more susceptible to the effects of the crosswind. A model was developed to correlate experimental data spanning 2 m/s ≤ U ∞ ≤17 m/s, 0.5 m/s ≤V j ≤4 m/s, 12.2 mm ≤ d o ≤49.8 mm with fuel dilutions using CO 2 or N 2 of up to 80% (by volume). This model is ( 1 − η ) ⋅ ( LHV mass ) 3 = A ⋅ exp ⁡ ( B U ∞ ( g V j d o ) 1 3 ) where A and B are coefficients and U ∞ /(gV j d o ) 1/3 is a dimensionless parameter based on the Richardson number which emphasizes the importance of the specific energy density of the fuel stream. Separate coefficients were needed to describe natural-gas- and propane- or ethane-based flare streams such that for natural gas, A =156.4 (MJ/kg) 3 and B =0.318, and for propane and ethane, A =32.06 (MJ/kg) 3 and B =0.272. Given that data from natural-gas- and propane- or ethane-based flares did not follow a single correlation, coupled with the fact that the natural gas correlation could be slightly improved with d o 1/2 , suggests that there may be different underlying mechanisms that lead to the inefficiecies for these fuels that need further exploration.