HCN oxidation in an O 2/CO 2 atmosphere: An experimental and kinetic modeling study

Abstract

HCN is one of the most important intermediates in NO x chemistry including formation and removal processes and the knowledge of HCN oxidation is thus very important to minimize NO x emissions. The present work aims to evaluate the oxidation behavior of HCN in an O 2/CO 2 atmosphere, due to the lack of studies at these specific operating conditions and the increasing importance of the oxy-fuel combustion processes, characterized by an O 2/CO 2 combustion atmosphere instead of air. With this purpose, a flow reactor experimental and kinetic modeling study of the oxidation of HCN under CO 2 diluted conditions, in the 900–1450 K temperature range and for different stoichiometries, ranging from very reducing to oxidizing conditions, has been performed. The large experimental differences observed in the O 2/CO 2 atmosphere in comparison to traditional air-fired combustion corroborate the importance of the HCN oxidation study in an O 2/CO 2 environment. The presence of high CO 2 concentration levels clearly inhibits HCN oxidation, since CO 2 competes with O 2 for atomic hydrogen through the CO 2 + H ⇌ CO + OH reaction. The experimental results show the oxidation regime of HCN for different stoichiometries, analyzing the formation of the main products of the process: CO, NO, N 2, N 2O and HNCO. The higher availability of oxygen increases the HCN conversion, even though the onset temperature for reaction is almost similar at any stoichiometry studied. The mechanism used for calculations was that developed by Dagaut et al. [P. Dagaut, P. Glarborg, M.U. Alzueta, Prog. Energy Combust. Sci. 34 (2008) 1–46] for the oxidation of HCN in air combustion, updated in the present work to take into account the presence of an O 2/CO 2 combustion atmosphere. In general, the modified model gives a reasonably good description of the experiments performed.