Variable momentum burner assembly for separately injecting fuel gas and oxidant in a combustion zone, comprising at least two fuel gas canals, at least one oxidant canal and a fuel gas distributor, whereby the at least two fuel gas canals comprise an inner fuel-conducting passage forming an inner fuel gas outlet and an outer fuel-conducting passage forming an outer fuel gas outlet, said inner and said outer fuel-conducting passages being coaxially arranged; and whereby said fuel gas distributor comprises a first fuel gas chamber in fluid connexion with the inner fuel-conducting passage of the at least two fuel gas canals and a second fuel gas chamber in fluid connexion with the outer fuel-conducting passage of the at least two fuel gas canals.