In certain embodiments, a feed injector system includes an inner channel configured to convey at least one of a solid fuel feed or a liquid reactant or moderator to a reaction zone. A first oxidizer channel extends around the inner channel, wherein the first oxidizer channel is configured to convey a first oxidizer stream to the reaction zone. A second oxidizer channel extends around the first oxidizer channel, wherein the second oxidizer channel is configured to convey a second oxidizer stream to the reaction zone. Additionally, a third channel extends around the inner channel and the first and second oxidizer channels, wherein the third channel is configured to convey at least one of the solid fuel feed or the liquid reactant or moderator to the reaction zone.