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Use of Nitric Acid to Control the NO2:NOX Ratio Within the Exhaust Composition Transient Operation Laboratory Exhaust Stream
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on April 14, 2020 by SAE International in United States
The Exhaust Composition Transient Operation Laboratory (ECTO-Lab) 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 NOX generation. While this method affords highly tunable NOX 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 NOX mixture. NOX generated through combustion of nitromethane is dominated by NO, and generally results in a NO2:NOX 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:NOX ratio within the ECTO-Lab exhaust stream by using nitric acid as the NOX precursor molecule in lieu of nitromethane. While decomposition of nitromethane generates NO as the dominate component of the NOX 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 NOX molecule, and precisely controlling the conditions at which decomposition of nitric acid occurs, the ECTO-Lab may be used a platform to evaluate the efficiency of the standard, fast, and slow SCR reactions without need of an oxidation catalyst. 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.