Reactive Absorption of NO2 by Water in the NOx Measurement System

Abstract

In this study, firstly it was demonstrated that the absorption of NO2 by condensed water in the sampling system might lead to considerable errors in the measurement of NOx for combustion gases containing relatively high concentration of NO2. Secondly, the loss of NO2 into water was systematically measured using bubblers in series or an NO2 absorption tube (a horizontal tube partly full of water). NO2/air mixtures were passed through the bubblers or the NO2 absorption tube and then analyzed by a chemiluminescent NOx analyzer. The initial NO2 concentration and water temperature were varied in the ranges 20-300 ppm and 0-30°C, respectively. The ratio of the decrease in the concentration of NO2 to the initial value Δ [NO2] / [NO2] 0 is found to be proportional to the surface area of water and inversely proportional to the volume flow rate of the mixture but practically independent of the residence time in the NO2 absorption tube. Also, Δ [NO2] / [NO2] 0, gradually increases with increasing the initial concentration of NO2 but it is independent of those of NO and O2. The temperature dependence of Δ [NO2] / [NO2] 0 is found to be negative and of an Arrhenius relationship. The species balance between the gaseous and liquid phases was also verified. Finally, based on a simple model and the present experimental data, an equation for estimating the magnitude of the loss of NO2 was developed.