Abstract
Over the past decades, selective catalytic reduction (SCR) using aqueous urea sprays as ammonia precursor has become the prevalent technique for NOx emission control in mobile applications. Preparation of ammonia from urea water sprays still represents a challenge in aftertreatment engineering as complex interactions of multi-phase physics and chemical reactions have to be handled. Increasingly stringent emission legislations and the ongoing development of fuel-efficient engines and close-coupled aftertreatment systems raise high demands to SCR systems. Due to highly transient conditions and short mixing lengths, incomplete spray evaporation can result in liquid/wall contact and formation of solid urea deposits lowering ammonia selectivity and homogeneity. This article reviews the ongoing development of SCR systems with focus on the efficient evaporation and decomposition of the injected spray for a homogeneous ammonia distribution in front of the SCR catalyst. Critical aspects of spray evaporation and impingement, liquid film and deposit formation are pointed out and potentials for system optimization are discussed.
Highlights
Over the past decades, selective catalytic reduction (SCR) using aqueous urea sprays as ammonia precursor has become the prevalent technique for NOx emission control in mobile applications
Since the presence of a liquid film for a certain residence time inevitably results in the formation of solid deposits, spray/wall interaction and droplet/wall interaction represents a critical phenomenon in the preparation of ammonia from urea water solu tion (UWS)
Efficient NOx removal by urea SCR systems demands a complete conversion of adequately dosed urea to ammonia and its homogeneous distribution over the catalyst cross section without system degradation by solid by-product deposits
Summary
Internal combustion engines always produce nitrogen oxides to a certain extent due to the use of air as oxidizing agent. Regarding particulate matter (PM) and NOx emis sions from diesel engines, a trade-off occurs, which can be managed by motor operation Another trade-off, in particular for NOx emissions, has often to be considered in terms of fuel consumption and exhaust pollutant concentration. Selective catalytic reduc tion of nitrogen oxide emissions describes the conversion of NOx to ni trogen (N2) and water on a catalyst using ammonia as reducing agent. Ammonia can be produced on the NSC, which is stored in the SCR catalyst to be released again under lean conditions for NOx reduction Another approach is the usage of two consecutive SCR cata lysts as described by Balland et al [15]. With regard to SCR systems, higher temperatures in this region promote UWS spray conversion and increase the catalytic activity for NOx removal. A possible increase of backpressure by the SCR coating and its thermal stability has to be accounted for [7]
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