Gas turbine unit comprising: a combustion chamber provided with a plurality of burner assemblies, each burner assembly comprising a respective lance injector (20; 120); and a supply system, configured to supply fuel to the burner assemblies; wherein each lance injector (20; 120) has a plurality of first nozzles (60; 160) and a first internal fluid line (58; 158) fluidically coupled with the first nozzles (60; 160); wherein the supply system comprises a first fuel supply line and an inert supply line, configured to respectively supply a primary fuel and an inert fluid to the first nozzles (60; 160) through the first internal fluid line (58; 158); wherein in each burner assembly the lance injector (20; 120) comprises: an outer tubular body extending along an axis and an inner tubular body housed in an axial through cavity of the tubular body 50; one or more second nozzles; and a second internal fluid line (52, 53) fluidically coupled with the one or more second nozzles; and wherein the supply system comprises a second fuel supply line, configured to supply a secondary fuel to the one or more second nozzles through the second internal fluid line (52, 53); characterized in that the second internal fluid line (52, 53) comprises: a delivery line, defined by the inner tubular body; a return line, defined by an annular air space between the outer tubular body and the inner tubular body; a terminal element, placed at an injection tip of the lance injector, forming a chamber that connects the delivery line and the return line and provided with said one or more second nozzles, wherein the delivery line and the return line are coaxial; and wherein the first internal fluid line is housed in the outer tubular body concentrically outside the second internal fluid line (52, 53). a combustion chamber provided with a plurality of burner assemblies, each burner assembly comprising a respective lance injector (20; 120); and a supply system, configured to supply fuel to the burner assemblies; wherein each lance injector (20; 120) has a plurality of first nozzles (60; 160) and a first internal fluid line (58; 158) fluidically coupled with the first nozzles (60; 160); wherein the supply system comprises a first fuel supply line and an inert supply line, configured to respectively supply a primary fuel and an inert fluid to the first nozzles (60; 160) through the first internal fluid line (58; 158); wherein in each burner assembly the lance injector (20; 120) comprises: an outer tubular body extending along an axis and an inner tubular body housed in an axial through cavity of the tubular body 50; one or more second nozzles; and a second internal fluid line (52, 53) fluidically coupled with the one or more second nozzles; an outer tubular body extending along an axis and an inner tubular body housed in an axial through cavity of the tubular body 50; one or more second nozzles; and a second internal fluid line (52, 53) fluidically coupled with the one or more second nozzles; and wherein the supply system comprises a second fuel supply line, configured to supply a secondary fuel to the one or more second nozzles through the second internal fluid line (52, 53); characterized in that the second internal fluid line (52, 53) comprises: a delivery line, defined by the inner tubular body; a return line, defined by an annular air space between the outer tubular body and the inner tubular body; a terminal element, placed at an injection tip of the lance injector, forming a chamber that connects the delivery line and the return line and provided with said one or more second nozzles, wherein the delivery line and the return line are coaxial; and wherein the first internal fluid line is housed in the outer tubular body concentrically outside the second internal fluid line (52, 53). a delivery line, defined by the inner tubular body; a return line, defined by an annular air space between the outer tubular body and the inner tubular body; a terminal element, placed at an injection tip of the lance injector, forming a chamber that connects the delivery line and the return line and provided with said one or more second nozzles, wherein the delivery line and the return line are coaxial; and wherein the first internal fluid line is housed in the outer tubular body concentrically outside the second internal fluid line (52, 53).