Abstract
AbstractSelective catalytic reduction (SCR) systems are the state-of-the-art technology to reduce nitrogen oxide emissions (NOx) of modern diesel engines. The system behaviour is well understood in the common temperature working area. However, the system properties below light-off temperature are less well known and offer a wide scope for further investigations. Vehicle measurements show that under specific conditions during cold start, NOx can be partially stored and converted on on-filter and flow-through SCR catalysts. The purpose of this work was in a first step to analyse the main influence parameters on the NOx storage behaviour. Therefore, synthetic gas test bench measurements have been carried out, varying the gas concentrations, temperature, and gas hourly space velocity (GHSV). These investigations showed that the NOx storage effect strongly depends on the NH3 level stored in the catalyst, GHSV, the adsorbed water (H2O) on the catalyst, and the temperature of the catalyst. Further influence parameters such as the gas composition with focus on carbon monoxide (CO), short-chain hydrocarbons and long-chain hydrocarbons have been analysed on a synthetic gas test bench. Depending on operating conditions, a significant amount of NOx can be stored on a dry catalyst during the cold start phase. The water vapor from the combustion condenses on the cold exhaust pipe during the first seconds, or up to a few minutes after a cold start. As the water vapor reaches the surface of the catalyst, it condenses and adsorbs onto it, leading to a sudden temperature rise. This exothermal reaction causes the stored NOx to be desorbed, and furthermore it is partially reduced by the NH3 stored in the catalyst.
Highlights
Even though the market share of electrically powered passenger cars has increased in the European market over the last few years, the share of diesel engines in new passenger cars was still 30.5% in the year 2018
Main targets are NOx and particulate matter emissions, causing a need for a complex exhaust aftertreatment system for modern diesel vehicles. These systems typically consist of a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF) with SCR coating, a selective catalytic reduction catalyst (SCR), and eventually an NH3 slip catalyst (ASC) [3,4,5]
This sharp NOx-emissions increase is caused by the temperature rise due to water adsorption in the catalyst, as it can be seen in various literature [4, 25, 27]
Summary
Even though the market share of electrically powered passenger cars has increased in the European market over the last few years, the share of diesel engines in new passenger cars was still 30.5% in the year 2018. Main targets are NOx and particulate matter emissions, causing a need for a complex exhaust aftertreatment system for modern diesel vehicles. These systems typically consist of a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF) with SCR coating, a selective catalytic reduction catalyst (SCR), and eventually an NH3 slip catalyst (ASC) [3,4,5]. The DPF with SCR coating has major advantages compared to two separate systems, such as a better packaging, as well as an improved heating up of both subsystems due to the possible closed-couple position These facts lead to an improved cold start emission performance. The SCR on filter catalyst is a state-of-the-art technology for NOx and particulate matter emission reduction in passenger vehicles [6,7,8]
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