Deposit Formation in SCR-Systems - Optical Investigations
The permanently tightening emission regulations for nitrogen oxides (NOx) pollutants force further development of mobile exhaust aftertreatment systems with selective catalytic reduction (SCR). Of particular interest is the long-term reliability of SCR-systems with regard to unfavorable operating conditions, such as high injection rates of urea water solution (UWS) or low exhaust gas temperatures. Both may lead to the formation of solid deposits which decrease system efficiency by increasing backpressure and impairing ammonia formation. In order to study the most relevant processes of deposit formation, an optical box with heat resistant glass was designed. Three UWS injectors with different spray characteristics were used to study their influence on the deposit formation under a wide range of stationary and transient operating conditions. Infrared thermography was applied to observe spray-induced wall cooling, both below and above the Leidenfrost point. The formation of a liquid fluid film on a hot surface as well as deposit growth and decomposition were monitored by video recording. A chemical analysis of obtained solid deposits complemented the experimental investigations. This paper describes influences on deposit formation and decomposition. A strong impact of spray properties, such as droplet Weber number and spray area load, on the critical wall temperature for film formation was found. Different types of liquid film propagation were observed for different surface temperatures. The impact of operating conditions, such as exhaust gas temperature and gas flow rate, on the amount and the chemical composition of the deposits was determined. The experimental observations revealed different impact factors on the persistence of solid deposits. The obtained experimental results show the complexity of physical and chemical processes leading to formation of solid deposits in SCR systems. Based on that, the requirements of CFD models for modeling of deposit formation and decomposition were defined in terms of physical and chemical properties as well as the necessary simulated time scales.