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Abstract

The waste heat recovery (HR) systems are employed to increase the overall thermal efficiency of electric generation units (EGUs). Although the emission factors (in terms of gram of pollutants per unit of thermal energy consumed or electric energy generated) generally decreases after installing HR systems, the emission rates in terms of grams of pollutants per unit of time remain unchanged. However, HR systems reduce stack exit temperature, resulting in lower effective emission heights, which lead to higher near-source ground level concentrations (GLCs) of air pollutants. In order to comprehensively evaluate the near-source air quality impact from deploying HR systems, we proposed a new modeling framework by integrating a computationally efficient Gaussian-based dispersion model (AERMOD) and a (relatively) more accurate computational fluid dynamics (CFD) model. As a demonstration of the proposed framework, we investigated the HR impact on NOx concentrations near a simple cycle gas turbine located in Brentwood, NY. Specifically, we applied the AERMOD modeling system to screen the hourly GLCs over five years, and highest values (and the corresponding hours) were shown to cluster into two main meteorological conditions: the stable atmospheric boundary layer with relatively high wind speed (Stable, HW) and the unstable atmospheric boundary layer with relatively low wind speed (Unstable, LW). These two conditions were further simulated using a CFD model that have been extensively evaluated previously for detailed analysis. By setting different stack exit temperatures, the near-source air quality impact of different waste heat conversion rates was evaluated. We introduced a concept called the heat recovery amplified factor (HRAF), defined as the ratio between the maximum GLC with HR system and that without HR system, as an indicator of HR impact. HRAF was shown to be much more sensitive to temperature in the Unstable, LW condition than in the Stable, HW condition. Although the results were limited to a specific simple cycle gas turbine, the proposed modeling framework and HRAF can be used for evaluating the HR systems impact for other emission sources.