The behavior of single and multiple horizontal gas jet injections into a rectangular bubbling fluidized bed are studied with three-dimensional numerical simulations. The jet penetrations as well as the interactions between the jet and the surrounding gas, solids, bubbles, and other jets are investigated. As far as the average jet penetration length is concerned, limited influence of each jet on the others is observed in multiple jet injections until the jets start to overlap. The effect of the secondary gas injection on the flow hydrodynamics in the bed is examined for multiple jet injections with different jetting velocities and arrangements. It is found that the secondary gas injection mainly affects the hydrodynamics of the upper section above the injection level, while its effect is nearly negligible below the injection. Above the injection level, more particles are brought upward and the circulation of solids is promoted. Furthermore, the tendency of slugging inside the bed is observed at high secondary injection flow rates. (C) 2009 Elsevier Ltd. All rights reserved.