The role of carbon monoxide in NO2 plume formation

Through a series of computational studies, carbon monoxide has been identif ied as an important promoter of NO oxidation to NO2 in combustion turbine e xhaust gas at intermediate temperatures (450 to 750 degreesC). NO2 formatio n is accompanied by enhanced CO burnout at these temperatures. Perfectly st irred reactor and plug flow reactor calculations indicate that concentratio ns of CO as low as 50 ppmv in exhaust gas containing 25 ppmv NO can result in the conversion of 50 percent of the NO to NO2 in less than 1 s. NO2 conc entrations as low as 15 ppmv can result in visible, yellow-brown plumes fro m large diameter exhaust stacks. If NO2 plumes are to be prevented, then de signers of gas turbines and heat recovery steam generators need to be aware of the relationships between time. temperature, and composition which caus e NO2 to form in exhaust gas. Reaction path analysis indicates that the mut ually promoted oxidation of CO and NO occurs through a self-propagating, th ree-step chain reaction mechanism. CO is oxidized by OH (CO + OH --> CO2 H), while NO is oxidized by HO2: NO + HO2 --> NO2 + OH. In a narrow tempera ture range, the H-atom produced by the first reaction can react with O-2 in a three body reaction to yield the hydroperoxy radical needed in the secon d reaction: H + O-2 + M --> HO2 + M, where M is any third body. The observe d net reaction is CO + O-2 + NO --> CO2 + NO2, which occurs stoichiometrica lly at temperatures below about 550 degreesC. As the temperature increases, additional reaction pathways become available for H, HO2, and OH which rem ove these radicals from the chain and eventually completely decouple the ox idation of CO from NO. An abbreviated set of elementary chemical reactions, including 15 species and 33 reactions, has been developed to model reactio n 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.