Mixing effects in the selective noncatalytic reduction of NO

An engineering model that combines a simple mixing model and a detailed rea ction mechanism has been used to investigate selective noncatalytic reducti on (SNCR) of NO. In this process a jet of NH3 is injected at high temperatu res into a flue gas containing NO and O-2. The mixing model used is based o n the "maximum mixedness" model proposed by Zwietering. The chemical kineti c model of Miller and Glarborg was validated against experimental data obta ined in a flow reactor over a range of NH3/NO/O-2 compositions correspondin g to conditions ranging from early jet entrainment to full mixing between r eactants. The effect of mixing was investigated on three different experime ntal scales: A laboratory-scale diffusion-mixing reactor, a bench-scale set up, and a full-scale grate-fired furnace. The results show that finite rate mixing affects the SNCR process efficiency at high temperatures where it m ay cause a narrowing or a widening of the temperature window, depending on the NO concentration. The effect of mixing on the selectivity for NO reduct ion could be modeled qualitatively in all scales with the proposed model, u sing mixing times estimated from simple jet correlations. Calculations for a full-scale wood-fired grate fumace indicate that for this system with NO concentrations of 50-100 ppm the initial segregation of reactants may enhan ce the process efficiency. In systems with higher NO levels finite rate mix ing may have an adverse effect on the SNCR process.