Use of Nitric Acid to Control the NO ₂ :NO _X Ratio within the Exhaust Composition Transient Operation Laboratory Exhaust Stream

The Exhaust Composition Transient Operation Laboratory^TM (ECTO-Lab^TM) is a burner system developed at Southwest Research Institute (SwRI) for simulation of IC engine exhaust. The current system design requires metering and combustion of nitromethane in conjunction with the primary fuel source as the means of NO_X generation. While this method affords highly tunable NO_X concentrations even over transient cycles, no method is currently in place for dictating the speciation of nitric oxide (NO) and nitrogen dioxide (NO₂) that constitute the NO_X mixture. NO_X generated through combustion of nitromethane is dominated by NO, and generally results in an NO₂:NO_X ratio of < 5 %. Generation of any appreciable quantities of NO₂ is therefore dependent on an oxidation catalyst to oxidize a fraction of the NO to NO₂. Presented within this manuscript is a method for precise control of the NO₂:NO_X ratio within the ECTO-Lab exhaust stream by using nitric acid as the NO_X precursor molecule in lieu of nitromethane. While decomposition of nitromethane generates NO as the dominate component of the NO_X mixture, nitric acid decomposition produces primarily NO₂. The method described herein entails metering a 70 % aqueous nitric acid solution into an upstream decomposition furnace prior to injection into the ECTO-lab exhaust. By use of nitric acid as the precursor NO_X molecule, and precisely controlling the conditions at which decomposition of nitric acid occurs, the ECTO-Lab may be used as a platform to evaluate catalytic activity in absence of a diesel oxidation DOC, coming one step closer to a full engine simulator. In this study, concentrations of NO₂ well in excess of the theoretical thermodynamic equilibrium value were achieved by specifying the residence time and temperature of the nitric acid within the decomposition oven.