Deposit Formation in SCR-Systems - Optical Investigations

Abstract

<div class="section abstract"><div class="htmlview paragraph">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.</div><div class="htmlview paragraph">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.</div><div class="htmlview paragraph">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.</div><div class="htmlview paragraph">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.</div></div>

Urea Conversion for Low‐Temperature Selective Catalytic Reduction in a Swirled Diesel Exhaust Gas Configuration

“…It should be stated that this strategy to produce a swirl for eliably reliably converting urea is novel and has been practically elaborated in this work for the first time. By contrast, in the literature, mixers with internal swirling devices have primarily been reported so far [15][16][17][18][19][20][21].…”

“…It should be stated that the described modeling strategy is a compromise between accuracy and computational effort to evaluate the flow and spray dynamics, which is the focus of the numerical investigation in this work. More advanced modeling approaches would be needed to describe the detailed mechanism of deposit formation on the surfaces as demonstrated in the literature 27, 28.…”

“…A review on the state-of-the art of modeling of AdBlue spray, impingement, wall wetting, and deposit formation in mixers was recently published [16]. Additionally, it should be men-tioned that starting from original impact mixers more advanced types such as in-pipe swirl mixers and body swirl mixers were increasingly engineered for various exhaust pipe geometries depending on dosing strategies and specific vehicle requirements [17][18][19]. Exemplarily, a mixing unit with an interior swirl formation was suggested.…”

CFD Simulation of SCR Systems Using a Mass-Fraction-Based Impingement Model

“…Consequently, surface cooling due to intermittent spray injection over several injection cycles was used as a reference. The experimental results used for comparison were recorded during an earlier project published in [41]. In the exhaust line of a heavy-duty diesel engine, an optically accessible box was integrated.…”

“…The plate's temperature was measured at the black-varnished bottom side through zinc-selenide (ZnSe) windows using infrared (IR) thermography. More details regarding the test bench setup can be found in [41]. The recorded IR images, as shown in Figure 11 at the bottom left, are the experimental reference data.…”