Burner assembly comprising (a) an elongated body having a periphery, a discharge end adjacent a combustion zone, and an axis, wherein the axis extends into the combustion zone; (b) one or more oxidant nozzles disposed at the discharge end of the elongated body and adapted to discharge a gaseous oxidant into the combustion zone; and (c) one or more fuel nozzles disposed at the discharge end of the elongated body and adapted to discharge a fuel into the combustion zone. At least one of the oxidant and fuel nozzles is characterized by a shape factor, σ, that is greater than about 10, wherein σ is a dimensionless parameter defined as σ=P2/2A where P is the perimeter dimension of the discharge opening and A is the area of the discharge opening.