DESIGN CONSIDERATION FOR CROSS JET AIR MIXING IN MUNICIPAL SOLID-WASTE INCINERATORS

In mass-burning municipal solid waste incinerators, overfire air injec tion plays a key role in the improvement of mixing and reaction betwee n oxygen and incomplete combustion products and/or pollutants. However , the design parameters of overfire air nozzles are not well understoo d and sometimes confusing. In this paper, major design parameters conc erning cross jet air nozzles are discussed along with flow simulation results for simplified furnace geometry. The overall performance of je t air mixing and the effects of design parameters are quantitatively e valuated. The flow simulation results are interpreted in terms of the penetration depth of the jet into the main flow, the size of the recir culation zone and the ratio of the unmixed portion of the gas flow. Th e momentum flux ratio J of the jet to the cross flow strongly affects the penetration depth of the jet and the mixing of two flow streams. A s the inter-nozzle distance S (in non-dimensional form) decreases, the penetration depth decreases but the size of the recirculation zone in creases and the resultant mixing deteriorates. The degree of mixing of the jet with the cross gas stream is evaluated in terms of the mass-a veraged probability distribution of the relative concentration. Fresh air disperses more efficiently into the gas stream as J and S increase . The momentum flux ratio and the inter-nozzle distance are considered as important design parameters, and optimum values of these variables can be chosen for the given furnace conditions. This numerical evalua tion also provides a basis for similarity considerations in cold flow model tests and the validity of the two-dimensional idealization. (C) 1997 by John Wiley & Sons, Ltd.