The Role of Carbon Monoxide in NO2 Plume Formation

Abstract

Through a series of computational studies, carbon monoxide has been identified as an important promoter of NO oxidation to NO2 in combustion turbine exhaust gas at intermediate temperatures (450 to 750°C). NO2 formation is accompanied by enhanced CO burnout at these temperatures. Perfectly stirred reactor and plug flow reactor calculations indicate that concentrations of CO as low as 50 ppmv in exhaust gas containing 25 ppmv NO can result in the conversion of 50% of the NO to NO2 in less than 1 second. NO2 concentrations as low as 15 ppmv can result in visible, yellow-brown plumes from large diameter exhaust stacks. If NO2 plumes are to be prevented, then designers of gas turbines and heat recovery steam generators need to be aware of the relationships between time, temperature, and composition which cause NO2 to form in exhaust gas. Reaction path analysis indicates that the mutually promoted oxidation of CO and NO occurs through a self-propagating, three-step chain reaction mechanism. CO is oxidized by OH, (R2)CO+OH→CO2+Hwhile NO is oxidized by HO2:(R23)NO+HO2→NO2+OHIn a narrow temperature range, the H-atom produced by R2 can react with O2 in a three body reaction to yield the hydroperoxy radical needed in R23:(R9)H+O2+M→HO2+Mwhere M is any third body. The observed net reaction isCO+O2+NO→CO2+NO2which occurs stoichiometrically at temperatures below about 550°C. As the temperature increases, additional reaction pathways become available for H, HO2, and OH which remove these radicals from the chain and eventually completely decouple the oxidation of CO from NO. An abbreviated set of elementary chemical reactions, including 15 species and 33 reactions, has been developed to model CO-enhanced oxidation of NO to NO2. This reaction set was derived from a larger reaction set with more than 50 species and 230 elementary chemical reactions, and was validated by comparison of PSR and PFR calculations using the two sets.