Simulation of the reduction of nitrogen species in air jets

The reduction of nitrogen species to N2 in the air jets in an industrial bubbling fluidized bed boiler was modelled using detailed gas phase kinetics and compared to in-furnace measurement data. The stack measurement of the boiler showed over 90% of reduction of nitrogen species, although only air staging was applied. The measurements revealed that most of the reduction occurred over the secondary air jet level. The reduction of nitrogen species in the secondary and the tertiary air jets were studied using the CHEMKIN software and a detailed reaction mechanism. Different mixing approaches in the modelling were tested: direct mixing, gradual mixing, inverse mixing using PFRs, and also PSRs were used to describe the jets. The simulations with gradual mixing were carried out by using plug flow reactors to describe the jets and the mixing of the bulk gas into the jets. The residence time, the entrainment of gas into the air jet, and the temperature in the different reactors were based on a CFD simulation of a single jet. The residence time in all simulations were based on the CFD simulation. Direct mixing of the air jet and bulk gas using PFRs showed the poorest agreement. The speciation and reduction of reactive nitrogen species was in best agreement with the measurements when using the gradual dilution of the air jet with the bulk flow approach.