Urea Decomposition at Low Temperature in SCR Systems for Diesel Engines 2014-01-2808
Selective catalytic reduction (SCR) has been demonstrated as one of the most promising technologies to reduce NOx emissions from heavy-duty diesel engines. To meet the Euro VI regulations, the SCR system should achieve high NOx reduction efficiency even at low temperature. In the SCR system, NH3 is usually supplied by the injection of urea water solution (UWS), therefore it is important to improve the evaporation and decomposition efficiency of UWS at low temperature and minimize urea deposits.
In this study, the UWS spray, urea decomposition, and the UWS impingement on pipe wall at low temperature were investigated based on an engine test bench and computational fluid dynamics (CFD) code. The decomposition of urea and deposits was analyzed using thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and fourier transform infrared spectroscopy (FTIR).
The TGA experiment shows that urea decomposition started at about 150°C, and exhibited two stages of rapid mass loss. The off-gas in each stage was mainly NH3 and HNCO respectively.
In tailpipes the decomposition rate of the injected urea was significantly influenced by the exhaust temperature. At 185°C about 20% urea decomposed and produced NH3. The UWS wall film had formed at low temperature and resulted in urea deposits. The TGA and DSC experiments showed that most of the deposits formed and sampled below 180°C were urea crystal, and the urea deposits in the tailpipes could decompose gradually at high exhaust temperatures. The deposits formed after several WHTC tests at the inlet of the SCR catalyst was also sampled and analyzed by TGA and DSC, which proposed to be cyanuric (CYA) acid, and started to decompose at about 300°C.