A preliminary dispersion modeling analysis of emissions from gas-fired engines at a pipeline compressor station reveals that ambient air exceedences occur outside the station's property line. To obtain an air quality permit for the station, the modeling analysis must show no modeled exceedences. What would be the best alternative for eliminating the modeled exceedances?

A preliminary dispersion modeling analysis of emissions from gas-fired engines at a pipeline compressor station reveals that ambient air exceedences occur outside the station's property line. To obtain an air quality permit for the station, the modeling analysis must show no modeled exceedences. What would be the best alternative for eliminating the modeled exceedances?

• A. Increase exhaust stack heights up to BEP
• B. Quench the discharge
• C. Increase the engine exhaust stack diameter
• D. Reduce the air/fuel ratio of engines

Answer: (A)

The key idea is that how high the emission plume rises determines how much it will dilute before reaching ground level receptors. When a dispersion model shows exceedances off the site, making the plume rise higher often reduces ground-level concentrations enough to meet the permit requirements. Increasing the exhaust stack height to the regulatory BEP height enhances dilution and mixing in the atmosphere, lowering the potential for downwind exceedances without changing how much pollutant is emitted. Quenching the discharge would make the plume less buoyant and likely lower, not higher, which would worsen near-ground concentrations. Increasing the stack diameter reduces exit velocity and momentum, which can also reduce plume rise and may not reduce downwind concentrations. Reducing the air/fuel ratio lowers emissions but changes combustion and may not address dispersion as effectively as increasing height. Therefore, raising the stack height to achieve adequate dilution is the best path to eliminate the modeled exceedances.

The key idea is that how high the emission plume rises determines how much it will dilute before reaching ground level receptors. When a dispersion model shows exceedances off the site, making the plume rise higher often reduces ground-level concentrations enough to meet the permit requirements. Increasing the exhaust stack height to the regulatory BEP height enhances dilution and mixing in the atmosphere, lowering the potential for downwind exceedances without changing how much pollutant is emitted.

Quenching the discharge would make the plume less buoyant and likely lower, not higher, which would worsen near-ground concentrations. Increasing the stack diameter reduces exit velocity and momentum, which can also reduce plume rise and may not reduce downwind concentrations. Reducing the air/fuel ratio lowers emissions but changes combustion and may not address dispersion as effectively as increasing height. Therefore, raising the stack height to achieve adequate dilution is the best path to eliminate the modeled exceedances.