CFD-simulations of urea-SNCR for NOx-reduction in combustion

Abstract

Urea-SNCR is a technology that can reduce NOx emissions from biomass combustion. In order to have good results of NOx reduction in a certain urea-SNCR system, simulations are advised to be conducted, it can be used to optimize the system design, which also significantly reduces the time and costs of experimental optimization in the real plant. In the simulation, urea evaporation and decomposition and NOx reduction are two important processes that needs to be simulated. In this thesis work, a CFD model (k-epsilon model, chemical-turbulence interaction model and discrete random walk model) is used to simulate the urea evaporation and decomposition. A CSTR (Continuous Stirred-Tank Reactor) model, a PFR (Plugged Flow Reactor) model and the CFD model are all evaluated in simulations of the NOx reduction process. The CSTR, PFR, and CFD models are tested at different conditions (temperature, geometry, retention time, injection points, etc.) for a complete urea-SNCR process. Under the given conditions, the effects of turbulent velocity fluctuations on the urea spray, the effects of mixing and chemical kinetics on each reaction, the effects of temperature and retention time on the NOx reduction and the reaction selectivities are studied. Then the CFD model is validated against experimental data from a power plant in Rorvik, which shows the CFD model is suitable for the simulation. Based on the CFD simulations, a new injection strategy for urea-water mixture is evaluated.