In mass-burning municipal solid waste incinerators, overfire air injection plays a key role in the improvement of mixing and reaction between oxygen and incomplete combustion products and/or pollutants. However, the design parameters of overfire air nozzles are not well understood and sometimes confusing. In this paper, major design parameters concerning cross jet air nozzles are discussed along with flow simulation results for simplified furnace geometry. The overall performance of jet air mixing and the effects of design parameters are quantitatively evaluated. The flow simulation results are interpreted in terms of the penetration depth of the jet into the main flow, the size of the recirculation zone and the ratio of the unmixed portion of the gas flow. The 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. As the inter-nozzle distance S (in non-dimensional form) decreases, the penetration depth decreases but the size of the recirculation zone increases and the resultant mixing deteriorates. The degree of mixing of the jet with the cross gas stream is evaluated in terms of the mass-averaged 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 evaluation also provides a basis for similarity considerations in cold flow model tests and the validity of the two-dimensional idealization. © 1997 by John Wiley & Sons, Ltd.