A combustion process comprising: providing burner assembly including: (i) a central flame holder having inlet means for an oxidant gas, inlet means for a primary fuel, a combustion region (17, 19, 21) for combusting the oxidant gas and the primary fuel, and an outlet for discharging a primary effluent from the flame holder; (ii) a plurality of secondary fuel injector nozzles surrounding the outlet of the central flame holder, wherein each secondary fuel injector nozzle comprises (a) a nozzle body having an inlet face, an outlet face, and an inlet flow axis passing through the inlet and outlet faces; and (b) one or more slots (207, 209, 211, 213, 215) extending through the nozzle body from the inlet face to the outlet face, each slot having a slot axis and a slot center plane; and (iii) one or more igniters associated with the plurality of secondary fuel injector nozzles; introducing the primary fuel and the oxidant gas into the central flame holder, combusting the primary fuel with a portion of the oxidant gas in the combustion region (17, 19, 21) of the flame holder, and discharging a primary effluent containing combustion products and excess oxidant gas from the outlet of the flame holder; injecting the secondary fuel (35, 37) through the secondary fuel injector nozzles into the primary effluent from the outlet of the flame holder; and operating the one or more igniters and igniting the fuel from the secondary fuel injector nozzles to cause combustion of the fuel with the excess oxidant in the combustion products. providing burner assembly including: (i) a central flame holder having inlet means for an oxidant gas, inlet means for a primary fuel, a combustion region (17, 19, 21) for combusting the oxidant gas and the primary fuel, and an outlet for discharging a primary effluent from the flame holder; (ii) a plurality of secondary fuel injector nozzles surrounding the outlet of the central flame holder, wherein each secondary fuel injector nozzle comprises (a) a nozzle body having an inlet face, an outlet face, and an inlet flow axis passing through the inlet and outlet faces; and (b) one or more slots (207, 209, 211, 213, 215) extending through the nozzle body from the inlet face to the outlet face, each slot having a slot axis and a slot center plane; and (iii) one or more igniters associated with the plurality of secondary fuel injector nozzles; (i) a central flame holder having inlet means for an oxidant gas, inlet means for a primary fuel, a combustion region (17, 19, 21) for combusting the oxidant gas and the primary fuel, and an outlet for discharging a primary effluent from the flame holder; (ii) a plurality of secondary fuel injector nozzles surrounding the outlet of the central flame holder, wherein each secondary fuel injector nozzle comprises (a) a nozzle body having an inlet face, an outlet face, and an inlet flow axis passing through the inlet and outlet faces; and (b) one or more slots (207, 209, 211, 213, 215) extending through the nozzle body from the inlet face to the outlet face, each slot having a slot axis and a slot center plane; and (a) a nozzle body having an inlet face, an outlet face, and an inlet flow axis passing through the inlet and outlet faces; and (b) one or more slots (207, 209, 211, 213, 215) extending through the nozzle body from the inlet face to the outlet face, each slot having a slot axis and a slot center plane; and (iii) one or more igniters associated with the plurality of secondary fuel injector nozzles; introducing the primary fuel and the oxidant gas into the central flame holder, combusting the primary fuel with a portion of the oxidant gas in the combustion region (17, 19, 21) of the flame holder, and discharging a primary effluent containing combustion products and excess oxidant gas from the outlet of the flame holder; injecting the secondary fuel (35, 37) through the secondary fuel injector nozzles into the primary effluent from the outlet of the flame holder; and operating the one or more igniters and igniting the fuel from the secondary fuel injector nozzles to cause combustion of the fuel with the excess oxidant in the combustion products.