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A precombustor having a chamber having a first end and a second end, and an inlet configured to deliver a substantially parallel flow of solid fuel surrounded by oxygen at the first end. The first end further includes a recirculation step and the second end fluidly communicates with a furnace. The i

A precombustor having a chamber having a first end and a second end, and an inlet configured to deliver a substantially parallel flow of solid fuel surrounded by oxygen at the first end. The first end further includes a recirculation step and the second end fluidly communicates with a furnace. The inlet and the recirculation step are arranged to form a recirculation zone of at least a portion of the solid fuel and the oxygen. A combustion system and method for operating a combustion system are also disclosed.

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1. A precombustor comprising: a chamber bounded by a first end, outer walls, and a second end; andan inlet configured to deliver a substantially parallel flow of solid fuel surrounded by oxygen through the first end of the chamber,wherein the first end of the chamber includes a recirculation step im

1. A precombustor comprising: a chamber bounded by a first end, outer walls, and a second end; andan inlet configured to deliver a substantially parallel flow of solid fuel surrounded by oxygen through the first end of the chamber,wherein the first end of the chamber includes a recirculation step immediately downstream of the inlet and the second end of the chamber fluidly communicates with a furnace;wherein the inlet and the recirculation step are arranged to form a recirculation zone within the chamber of at least a portion of the solid fuel and the oxygen exiting the inlet, the recirculation step having a step height h from an edge of the inlet to an edge of the chamber defined by the outer walls;wherein the inlet is further configured to provide recycled flue gas; andwherein the inlet is further configured to provide recycled flue gas downstream of the recirculation step. 2. The precombustor of claim 1, wherein the inlet includes an inlet port configured to provide the solid fuel. 3. The precombustor of claim 1, wherein the precombustor is configured such that: 1.2≤Dpd≤2.4wherein Dp is the chamber diameter and d is the inlet diameter. 4. The precombustor of claim 1, wherein the chamber has length (L) and that is greater than or equal to about 5 times the recirculation step height (h). 5. The precombustor of claim 1, wherein the precombustor is configured such that: L≤7.4*d+15.7*h-9.7*h2dwherein L is precombustor length and d is the inlet diameter. 6. The precombustor of claim 1, wherein the precombustor is configured such that: 0.1*d≦h≦0.7*d wherein d is the inlet diameter. 7. The precombustor of claim 1, wherein the precombustor includes a circular cross-sectional geometry. 8. The precombustor of claim 1, wherein the precombustor includes a non-circular cross-sectional geometry. 9. The precombustor of claim 1, wherein the inlet is configured to deliver a second oxygen stream which is at least partially surrounded by fuel. 10. A combustion system comprising: a furnace; andat least one precombustor, the at least one precombustor having a chamber bounded by a first end, outer walls, and a second end, and an inlet configured to deliver a substantially parallel flow of solid fuel surrounded by oxygen through the first end of the chamber, the first end further including a recirculation step immediately downstream of the inlet, the second end fluidly communicating with the furnace;wherein the inlet and the recirculation step are arranged to form a recirculation zone within the chamber of at least a portion of the solid fuel and the oxygen exiting the inlet, the recirculation step having a step height from an edge of the inlet to an edge of the chamber defined by the outer walls;wherein the inlet is further configured to provide recycled flue gas; andwherein the inlet is further configured to provide recycled flue gas downstream of the recirculation step. 11. The system of claim 10, wherein the inlet includes a inlet port configured to provide the solid fuel. 12. The system of claim 10, wherein the precombustor is configured such that: 1.2≤Dpd≤2.4wherein Dp is the chamber diameter and d is the inlet diameter. 13. The system of claim 10, wherein the chamber has length (L) and that is greater than or equal to about 5 times the recirculation step height (h). 14. The system of claim 10, wherein the precombustor is configured such that: L≤7.4*d+15.7*h-9.7*h2dwherein L is precombustor length and d is the inlet diameter. 15. The system of claim 10, wherein the precombustor is configured such that: 0.1*d<=h<=0.7*d wherein d is the inlet diameter. 16. The system of claim 10, wherein the precombustor includes a circular cross-sectional geometry. 17. The system of claim 10, wherein the precombustor includes a non-circular cross-sectional geometry. 18. The system of claim 10, wherein the inlet is configured to deliver a second oxygen stream which is at least partially surrounded by fuel. 19. A method for operating a combustion system comprising: providing a furnace;providing at least one precombustor, the precombustor having a chamber bounded by a first end, outer walls, and a second end, and an inlet configured to deliver a substantially parallel flow of solid fuel surrounded by oxygen through the first end of the chamber, wherein the concentration of O2 is at least 35%, the first end further including a recirculation step immediately downstream of the inlet, the second end fluidly communicating with the furnace; anddelivering the solid fuel and the oxygen to the inlet to form a recirculation zone within the chamber of at least a portion of the solid fuel and the oxygen exiting the inlet, the recirculation step having a step height h from an edge of the inlet to an edge of the chamber defined by the outer walls; wherein the length of the precombustor is sufficient to prevent a flame generated by the system from contacting the precombustor;wherein the inlet provides recycled flue gas; andwherein the inlet provides recycled flue gas downstream of the recirculation step. 20. The method of claim 19, further comprising delivering a second oxygen stream which is at least partially surrounded by fuel. 21. The method of claim 19 wherein the recirculation zone is located within the chamber of the precombustor.