Youhong Xiao

Study on the mixing performance of static mixers in selective catalytic reduction (SCR) systems

Selective catalytic reduction (SCR) is a promising technique for reducing nitrogen oxide (NOx) emissions from diesel engines. Static mixers are widely used in SCR systems before reactors to promote the mixing of ammonia and exhaust streams. This work aims to investigate the effects of the location of static mixers and the volume ratio of two species on mixing quality using the computational fluid dynamics (CFD) method. The simulation results show that a more homogenous ammonia distribution can be achieved at the exit of the pipe if static mixers are placed close to the ammonia injection point or if more ammonia is injected. Another phenomenon found in the study is that the mixing performance of an identical static mixer may behave discrepantly under different flow conditions if using B and C as the evaluating indexes for mixing homogenization.

Numerical Simulation of Urea Aqueous Solution Injection Process in a Selective Catalytic Reduction System

Selective catalytic reduction (SCR) technique has been used to reduce NOx emission from engines for decades. The concentration and distribution of ammonia before SCR converter can affect the conversion efficiency of NOx in SCR system. In this paper, the injection process of urea aqueous solution has been simulated by the CFD software Fluent to obtain the distribution of ammonia mass fraction in the exhaust pipe, and the factors which can affect the distribution of ammonia have been analyzed.

Simulation of Flow Field in an SCR Converter

SCR converters have been used in ships for several years and various types of SCR are available. Although an SCR is very effective for NOx emission, due to progressively stricter emission regulations, the catalytic converter design and performance need to be continuously improved. The design improvement efforts require broadening of fundamental understanding of various physiochemical processes that occur in a catalytic converter. The complexity of modern systems and the resulting flow dynamics, thermal and chemical mechanisms have increased the difficulty in assessing and optimizing system operational performance. Due to overall complexity and increased costs associated with these factors, modeling and simulation continue to be pursued as a method of obtaining valuable information supporting the design and development process associated with SCR optimization. Detailed flow velocity, temperature and species concentrations in a catalyst are often difficult to measure experimentally, thus, simulation can provide additional information that may be unobtainable from other ways. In this paper, a theoretical investigation has been performed on the distribution of flow, uniformity index and pressure loss in a diesel engine catalyst converter. The numerical simulation of the flow field in a catalytic converter with different diffusers was carried out. The effects of different diffusers on the flow pattern, velocity distribution, uniformity index, temperature field, conversion efficiency and pressure loss in a catalytic converter were studied.

An investigation into catalysts to improve low temperature performance in the selective catalytic reduction of NO with NH3

Selective catalytic reduction with NH3 is considered one of the most effective technologies controlling NOx emission. Metal Fe-based catalysts were used in the investigation to improve low temperature performance of NOx conversion. The temperature range studied was between 150°C and 350°C in increments of 50°C. The honeycomb catalysts were prepared by an impregnation method. The study also included characterisation of catalysts by BET, XRD, H2-TPR and XPS methods. It was found that an increase in metal Fe content from 2 to 6% wt offered an improvement in the catalytic performance. However, a further increase in Fe content resulted in a decrease in its performance. More than 90% NOx conversion rate could be achieved over the Fe-based honeycomb catalyst at a low temperature by doping with different weights of Ni and Zr metals. Amongst all the catalysts studied, the mixed metal catalyst of Fe-Ni-Zr was the one with most potential. This was because of its higher NOx conversion rate at a low temperature and also because of its wider operating temperature window. The effect of gas hourly space velocity (GHSV) was also investigated and the results showed that as GHSV increased, the reduction of NOx decreased.