Comprehensive kinetic characterization of the oxidation and gasification of model and real diesel soot by nitrogen oxides and oxygen under engine exhaust conditions: Measurement, Langmuir–Hinshelwood, and Arrhenius parameters

Abstract

The reaction kinetics of the oxidation and gasification of four types of model and real diesel soot (light and heavy duty vehicle engine soot, graphite spark discharge soot, hexabenzocoronene) by nitrogen oxides and oxygen have been characterized for a wide range of conditions relevant for modern diesel engine exhaust and continuously regenerating particle trapping or filter systems (0–20% O 2, 0–800 ppm NO 2, 0–250 ppm NO, 0–8% H 2O, 303–773 K, space velocities 1.3 × 10 4–5 × 10 5 h −1). Soot oxidation and NO 2 adsorption experiments have been performed in a model catalytic system with temperature controlled flat bed reactors, novel aerosol particle deposition structures, and sensitive multicomponent gas analysis by FTIR spectroscopy. The experimental results have been analyzed and parameterized by means of a simple carbon mass-based pseudo-first-order rate equation, a shrinking core model, oxidant-specific rate coefficients, Langmuir–Hinshelwood formalisms (maximum rate coefficients and effective adsorption equilibrium constants), and Arrhenius equations (effective activation energies and pre-exponential factors), which allow to describe the rate of reaction as a function of carbon mass conversion, oxidant concentrations, and temperature. At temperatures up to 723 K the reaction was driven primarily by NO 2 and enhanced by O 2 and H 2O. Within the technically relevant concentration range the reaction rates were nearly independent of O 2 and H 2O variations, while the NO 2 concentration dependence followed a Langmuir–Hinshelwood mechanism (saturation above ∼200 ppm). Reaction stoichiometry (NO 2 consumption, CO and CO 2 formation) and rate coefficients indicate that the reactions of NO 2 and O 2 with soot proceed in parallel and are additive without significant non-linear interferences. The reactivity of the investigated diesel soot and model substances was positively correlated with their oxygen mass fraction and negatively correlated with their carbon mass fraction.