Numerical Simulation Of SCRF System For Diesel Exhaust Gases Aftertreatment

Selective catalytic reduction (SCR) and diesel particulate filter (DPF) are effective technologies applied to reduce NOx and PM emission from diesel engine exhaust gases, respectively. As emissions regulations are becoming increasingly stringent worldwide, multiple exhaust aftertreatment devices are needed in order to minimize diesel engine tailpipe emissions. SCRF has recently been extensively studied due to complex structure and deterioration in diesel engine performance of the traditional aftertreatment system. SCRF incorporates SCR catalyst into a wall-flow filter as a single device, which can remove PM and NOx from diesel engine exhaust gases simultaneously. In this paper, the mathematical formulas which govern the physical and chemical phenomena in the SCRF were derived and a numerical simulation model was established based on these mathematical formulas. The model was verified with experimental data in a systematic manner.The SCRF pressure drop model was established and validated by engine bench test data. The pressure drop characteristics of asymmetric cell structure and cell density were investigated. Results show that asymmetric cell structure can effectively improve soot loading limit, but the asymmetric ratio would have a negative effect on the pressure drop when it is too small or too large. Under the premise of reasonable structure, the monolith with higher cell density exhibits better pressure drop characteristics.The kinetic model of wall-flow SCR was established, and the adsorption and desorption model of NH3 were considered. Results show that the single site adsorption model based on Eley-Rideal mechanism can reproduce the adsorption and desorption process well. The simulated steady-state NOx conversion is in good agreement with experiment data.As NO2 is an active soot oxidizer, the model should capture the concentration of NO2 inside SCRF for better prediction of soot oxidation characteristics. The NO oxidation and NO2 dissociation tests confirm that the model can capture the NO2 conversion with good accuracy. The established dual layer soot oxidation model based on the wall-flow SCR model can capture the regeneration process.Then the interaction effect of soot deposit, soot oxidation and NOx reduction process in SCRF was analyzed. Soot deposits inside the filter wall work as a barrier for mass transfer from gas stream to catalytic sites. Soot oxidation reaction consumes significant amounts of NO2 in soot cake layer, which forms a competitive relationship with SCR reactions.