Effect of Crosswind On Flaring Combustion Efficiency and Subsequent Increase in 8-Hr Ozone Concentration in the Houston-Galveston Ozone-Nonattainment Area

Emissions from industrial flares have been identified as a significant VOC emission source in a region heavily concentrated with chemical plants, e.g., the Houston-Galveston (HG) Region in Texas. The amount of flaring emissions from each flare depends directly on a flare’s combustion efficiency (CE). Several studies have shown that high crosswinds may reduce the flaring CE to a level well below the currently assumed CE of 98% and increase the amount of flaring emissions.  A decrease of CE from 98% to 96 % would effectively double the currently estimated flaring emissions based on the assumed CE of 98%.  The impact of crosswind, therefore, can be significant in the increase of regional ozone concentration due to the increase of flaring emissions.  The objective of this study was to perform CAMx simulations to quantify the effect of crosswind in the increase of regional 8-hr ozone in the HG Region during the 2006 ozone episode from May 31 through June 15, 2006.

In this study, a correlation between the crosswind speed and the flaring CE was first established based on the experimental obtained CEs available in the literature and the computer simulated CEs predicted from a computational fluid dynamics (CFD) model. The derived correlation equation was then used to generate dynamic flaring emissions for each CAMx grid with wind speeds provided from the CAMx meteorological file. The generated wind speed-induced flaring emissions were then incorporated into the total emissions for CAMx simulations. The CAMx simulation results with and without the wind speed-induced flaring emissions were then compared to quantify the effect of crosswind on the increase of 8-hr ozone concentration. The results have indicated that the flaring CE may range from 98% to 91% during the episode depending on the specific wind speed at the location and time of simulation and the observed 8-hr ozone concentration increase may range from 0.3 to 2.2 ppb depending on the overall meteorological conditions and the dynamic distribution of NOx and VOCs in the region. It is worth reporting that the ozone increase has been observed to occur predominately in a VOC-controlled area during the simulation.  More detailed simulation results will be reported at the meeting.