(351d) Impact of Flaring Emissions On Regional Air Quality Associated With An Ethylene Plant Shutdown Through Camx Simulations

Flaring is a common industrial practice designed to eliminate the accumulation of un-reacted feed gases or off-specification product gases through stack burning to ease the strain on a process unit especially during plant startups, plant shutdowns and operational upsets. Although the practice is necessary and crucial to chemical plant safety, excessive flaring represents the loss of precious feed materials and also results in the generation of extra amounts of undesired emissions including VOCs and NO_X, both of which are ozone precursors. Especially, the shutdown emission data provided by plant has no high resolution such as hourly-specific emission. With an even distribution of emission over the reported shutdown period, the impact of shutdown emission on regional air quality (e.g., ozone increment) is unclear.

In this study, the air-quality effects of flaring emissions during an ethylene plant shutdown with various shutdown scenarios on the increment of regional ozone concentrations were investigated through multi-scale simulations. In the practice, dynamic process simulations are performed to quantify and characterize flaring emission source during an ethylene plant shutdown. The obtained flaring emissions were then incorporated into a CAMx air quality simulation model to simulate the effect of these extra VOCs and NO_X emissions on the regional ozone concentrations. In the CAMx simulations, the ethylene plant was selected to be located in the Houston area and the August 2006 ozone episode was selected to represent the meteorological conditions. The effects of shutdown starting time, plant location, and episode day on ozone increments with various shutdown scenarios were investigated and scientific insights were provided. The results have indicated that, depending on the local meteorological conditions and the plant location, a well-planned ethylene plant shutdown may effectively increase the ozone concentration both on a high ozone day and on a low ozone day. The increments in ozone concentration are expected to be high if the plant is located in the VOC-limited ozone formation zone. The shutdown starting time and duration may affect the increments in ozone concentration due to the dynamic nature of flaring emissions. The environmentally conscious shutdown schedule is the one that avoids having the peak flaring emissions occur near the time of the highest solar intensity to minimize the impact of the shutdown process on the regional ozone concentrations.