A burner for a rotary kiln comprising an elongated tubular body (6) having a longitudinal axis (L) and a discharge end (7) adjacent a combustion zone comprising a flame, at least one fuel supply pipe for transporting and ejecting fuel through a fuel pipe outlet (10) at the discharge end (7), the fuel being alternative fuel or a mixture of alternative fuel and fossil fuel, and at the discharge end (7), a number of high speed primary air jet outlets for ejecting primary air and being arranged, when seen towards the discharge end, along a closed line, such as a circle, outwardly of the fuel outlet (10) and surrounding the fuel outlet, wherein at least one of the primary air outlets and preferably a number of the primary air outlets comprise a single orifice outlet or a multiple orifice outlet forming a flat jet air outlet (11) having a major axis and a minor axis and being configured to eject a flat jet air stream (13) having a flat fan pattern with a predetermined fan angle v.