Theory of Cost Effective NOx Control

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NOx is one of the primary air pollutants emitted from combustion processes and environmental regulations are the only driver forcing industry to install NOx control systems. In the Houston Galveston area alone, there is over 6,000 point sources that would need to be controlled to comply with the new State Implementation Plan adopted by the TNRCC. Many in the industry are in the process of evaluating control techniques to comply with the new regulations. Before evaluating NOx control strategies, it is important to establish the present baseline emission levels and to understand the NOx formation mechanism. Most of the NOx formed during combustion of gas and light oil is from high temperature oxidation (or “fixation”) of atmospheric nitrogen and is referred to as Thermal NOx. NO is the major constituent of thermal NOx and its formation can be modeled by the Zeldovich equation: [NO] = k 1 •exp (-k2/T) • [N2] •[O2]1/2 • t where, [ ] = mole fraction, k’s = constants, T = temperature, and t = residence time. The equation indicates that NOx formation is an exponential function of temperature and a square root function of oxygen concentration. Thus, by manipulating the temperature or oxygen concentration, the formation of thermal NOx can be controlled. Systems manipulating the oxygen concentration are referred to as stoichiometry-based combustion control techniques (e.g. Low NOx Burners or LNBs) and those reducing the temperature are referred to as dilution-based combustion control techniques (e.g. Flue Gas Recirculation or FGR). LNBs control NOx emissions by providing air staging to create an initial, fuel-rich zone (partial combustion zone) followed by an air-rich zone to complete the combustion process. Some burner designs incorporate fuel staging that results in lower levels of NOx. Since NOx formation is a square root function of oxygen concentration, the reduction capability of stoichiometry-based technologies is limited. According to the theory, NOx formation should increase with oxygen concentration or with the amount of excess air.

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