Promoting hydrocarbon-SCR of NOx in diesel engine exhaust by hydrogen and fuel reforming

Abstract

A hydrocarbon-selective catalytic reduction (HC-SCR) silver–alumina monolith catalyst has been prepared and tested for NOx emissions control in a diesel engine. The work is based on ongoing laboratory experiments, catalyst research, and process development. Hydrogen and actual reformate (i.e. H2 and hydrocarbon species produced in a partial and exhaust gas fuel reformer) significantly improved the passive control (i.e. no externally added hydrocarbons) NOx reduction activity over the SCR catalyst using the whole engine exhaust gas from a single-cylinder diesel engine. Optimisation of the reforming process is required for various engine conditions in order to maximise H2 production and minimise fuel penalty. When diesel fuel partial oxidation and exhaust gas reforming for SCR were implemented, the calculated fuel penalty was in the range of 5–10%, which is relatively high, as both reformers were not optimised yet. During HC-SCR of NOx over silver–alumina, the known promoting effect of H2 has been found to be sensitive to various factors, especially the engine exhaust gas temperature, H2 concentration, HC concentration, HC:NOx ratio, and space velocity. Under active control (i.e. hydrocarbon injection) SCR operation, powdered Ag–Al2O3 catalysts gave significantly higher initial NOx reduction, but the catalyst activity deteriorated rapidly with time due to poisoning species adsorption (e.g. HCs, nitrates, particulate matter (PM), etc.), whilst for the Ag–Al2O3-coated monolithic catalysts, NOx reduction activity was lower but remained constant for the duration of the tests. The improved physical (mass transfer, filtering of C-containing species) and chemical (reaction kinetics) processes during HC-SCR over powders compared to monoliths led to better initial catalyst activity, but it also accelerated catalyst deactivation which led to increased diffusion limitations.