A computational study of chemical reactions occurring in the exhaust system of natural gas engines has been conducted, emphasizing the formation and destruction of formaldehyde. The modeling was based on a detailed reaction mechanism, developed for describing oxidation of C{sub}1-C{sub}2 hydrocarbons and formaldehyde. The mechanism was validated against data from laboratory flow reactors and from the exhaust system of a full-scale gas engine. A parametric study of the exhaust system chemistry was performed, investigating the effect of temperature, stoichiometry, pressure, and exhaust gas composition. The results indicate a complex interaction between unburned hydrocarbons (UHC), formaldehyde, and nitrogen oxides. Above 850 K, partial oxidation of unburned hydrocarbons may occur, resulting in net formation or net destruction of CH{sub}2O depending on the unburned hydrocarbons/CH{sub}2O ratio and the reaction conditions. At the typical unburned hydrocarbons/CH{sub}2O ratio of 1.0-1.5 for gas engines, net formaldehyde formation may occur in the exhaust system if temperatures above 850 K are reached.
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