Modeling of selective non-catalytic reduction system using detailed chemistry and a jet-entrainment model

Selective Non-Catalytic Reduction is a secondary mean for abating NO_x emissions in FBC boilers. This paper focus on ammonium-based SNCR solutions, where carrier air is used for introducing NH₃ into the upper freeboard. The SNCR process is studied using a detailed description of the chemistry, combined with a simplified description of the fluid dynamics. This description is based on a plug-flow reactor concept extended with a continuous entrainment of flue gas, able to quantitatively model the flow in a jet. Both SNCR solutions based on NH₃ injection in a gaseous form as well as using an aqueous solution is investigated. Additional parameters are the temperature of the flue gas entrained into the jet, the composition of the flue gas as well as the ratio between NH₃ and NO_x. The numerical study shows that using small air jets to inject NH₃ into the upper free board results in that most of the NH₃ reacts in a region where the NH₃/NO ratio is high. Consequently, the reduction efficiency is not sensitive to the initial NH₃ level. Tests using a carrier air based SNCR system in a biomass fired FBC partly supports the findings from the numerical study. These data shows that a similar SNCR efficiency can be obtained using a large variation of NH₃/NO ratios. On the other hand, in the experimental results also higher efficiency was observed, which cannot be explained by the model, partly due to large uncertainties in the boundary conditions.