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

摘要

The reaction kinetics of the oxidation and gasification of four types of model and real diese) 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 percent O2, 0-800 ppm NO_2, 0-250 ppm NO_2 0-8 percent H_2O, 303-773 K, space velocities 1.3 x 10~4-5x 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 nonlinear 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.