Abstract
Selective catalytic reduction (SCR) is one of the potential methods to reduce NOx emissions from mobile and stationary power plants. An experimental study on a urea- SCR system with the porous media coated by the MnO2-V2O5-WO3/TiO2 catalyst in diesel combustors is being performed by authors. A three-dimensional model is developed to simulate an SCR system with urea gas as a reducing agent. The transport coefficients exist as parameters in each of the conservation equations for momentum, mass continuity, energy and species. The viscosity coefficient is needed to correlate a velocity gradient in a fluid to the force required to maintain the gradient. The thermal conductivity is required to correlate the temperature gradient across a fluid to the energy flux, that is, Fourier’s law of heat conduction. The diffusion coefficient is required to correlate the gradient in species concentration to the mass flux. Finally, the thermodynamic quantity of specific heat is required to calculate the energy equation in a chemically reacting flow. The purpose of this study is to investigate the influence of the temperature-dependent gas transport properties on the results of simulation. The flow field and chemical reactions inside the reactor are calculated simultaneously by a porous medium approach. The process of NOx reduction is predicted by the modification of source terms in a commercial CFD package. The estimated parameters from packed bed at wet conditions with the V2O5-WO3/TiO2 catalyst are selected for the processes of NOx reduction and NH3 oxidation. A steady-state calculation with SIMPLE algorithm is performed in this study. The temperature-dependent gas transport properties of the pure species viscosities, thermal conductivities and pairs of binary diffusion coefficients as well as specific heats are fitted with the polynomial equations. The results of the simulation using these temperature-dependent gas transport properties are then compared with the constant ones. It is shown that the temperature-dependent gas transport properties should not be neglected especially in a high temperature range of exhaust gas simulation with a chemically reacting flow in a urea-SCR system.
Keywords: NOx emissions, Urea-SCR system, Porous medium approach, Chemical reaction, Temperature-dependent gas transport properties