NOx Reduction in Fluidized Bed Combustors

A fundamental research program has been undertaken to improve combustion efficiency and emission levels in a coal water slurry fuelled fluidized bed combustor. The effect of freeboard hydrodynamics on particulate emissions, i.e. elutriation; and gaseous emissions, such as carbon monoxide, sulphur oxides and nitrogen oxides, is being investigated. Previous studies indicated that due to the bubbling nature of fluidized bed combustors, gas-gas mixing in the freeboard region is limited by the action of large scale eddies in the form of isolated gas pockets. It has also been shown that in the combustion of coal or char additional NOx reduction reactions are possible provided that a reducing zone is available. Thus, to improve the mixing in the freeboard and create a reducing environment, a secondary air injector was installed in the freeboard. The injection of swirling secondary air introduces a tangential component to the velocity vector, thus develops a shear flow between the isolated air pockets and the surrounding air flow. It also creates a cyclonic air motion along with char particles. The net result is increased turbulence, increased solids residence time distribution and reduction in NOx emissions due to additional reactions of NOx with char particles and unburnt volatiles. The work is carried out in two separate phases; cold hydrodynamics study and actual hot experiments. The results from both studies are presented