Impact of Oxidation Catalysts on Exhaust NO2/NOx Ratio from Lean-Burn Natural Gas Engines

Abstract

Oxides of nitrogen (NO_x) emitted from internal combustion engines are composed primarily of nitric oxide (NO) and nitrogen dioxide (NO₂). Exhaust from most combustion sources contains NO_x composed primarily of NO. There are two important scenarios specific to lean-burn natural gas engines in which the NO₂/NO_x ratio can be significant: (1) when the engine is operated at ultralean conditions and (2) when an oxidation catalyst is used. Large NO₂/NO_x ratios may result in additional uncertainty in NO_x emissions measurements because the most common technique (chemiluminescence) was developed for low NO₂/NO_x ratios. In this work, scenarios are explored in which the NO₂/NO_x ratio can be large. Additionally, three NO_x measurement approaches are compared for exhaust with various NO₂/NO_x ratios. The three measurement approaches are chemiluminescence, chemical cell, and Fourier-transform infrared spectroscopy. A portable analyzer with chemical cell technology was found to be the most accurate for measuring exhaust NO_x with large NO₂/NO_x ratios.