Abstract: A feedstream combustion device for combusting a fuel feedstream having one or more burners, where each burner comprises a mixer having a first mixer segment and a second mixer segment that mixes a first air intake of a first air feedstream and a fuel feedstream at a first mixer segment inlet end and mixes a second air intake of a second air feedstream with the first air feedstream and fuel feedstream in an annular space between the external surface of the first mixer segment and the internal surface of the second mixer segment.