This invention is directed to an arrangement and a device for distribution of oxygen-containing gas (air) in a furnace, into which fuel is supplied as solid or fluid particles (1). The fuel consists of e.g. spent liquor from the pulp industry. Said liquor burns partly as char (2) on the floor (3), and partly as suspended particles and as volatiles. Horizontal rows of gas jets (4) activate the char burning on the floor. Vertically extended configuration of gas jets (5) higher up induces strong horizontal gas circulation but reduces vertical flow extremes. The improved horizontal mixing increases burning stability, capacity and energy efficiency, but reduces emission of SO.sub.x, NO.sub.x and TRS. Lowered vertical recirculation permits better concentration of burning in the lower furnace and less carry-over of fuel particles.