A burner for combusting solid fuel, comprising: a first annular conduit in fluid flow communication with a supply of an annular gas comprising one of oxygen and a gaseous fuel, the first annular conduit being operationally configured to discharge the annular gas at an annular gas velocity, the first annular conduit having a first inner wall and a first outer wall ; a central conduit located within the first annular conduit and being in fluid flow communication with a supply of a central gas comprising the other of oxygen and a gaseous fuel, the supply of central gas and the central conduit being operationally configured to discharge the central gas at a central gas velocity of at least 50 meters per second, the central conduit having a central outer wall ; and a second annular conduit in fluid flow communication with a supply of a transport gas mixture comprising a solid fuel and a transport gas, the second annular conduit surrounding the first annular conduit , the supply of a transport gas mixture and the second annular conduit being operationally configured to discharge the transport gas mixture at a solid fuel velocity, wherein the central gas velocity is at least about 50 meters per second; and wherein the solid fuel velocity being less than the central gas velocity, further comprising a cavity formed in a cavity-containing wall, the cavity-containing wall being at least one of the central outer wall , the first inner wall , and the first outer wall ; wherein the central conduit has a downstream end defining an outlet plane ; and wherein the cavity has a downstream wall positioned proximal to and a non-zero distance upstream from the outlet plane . a first annular conduit in fluid flow communication with a supply of an annular gas comprising one of oxygen and a gaseous fuel, the first annular conduit being operationally configured to discharge the annular gas at an annular gas velocity, the first annular conduit having a first inner wall and a first outer wall ; a central conduit located within the first annular conduit and being in fluid flow communication with a supply of a central gas comprising the other of oxygen and a gaseous fuel, the supply of central gas and the central conduit being operationally configured to discharge the central gas at a central gas velocity of at least 50 meters per second, the central conduit having a central outer wall ; and a second annular conduit in fluid flow communication with a supply of a transport gas mixture comprising a solid fuel and a transport gas, the second annular conduit surrounding the first annular conduit , the supply of a transport gas mixture and the second annular conduit being operationally configured to discharge the transport gas mixture at a solid fuel velocity, wherein the central gas velocity is at least about 50 meters per second; and wherein the solid fuel velocity being less than the central gas velocity, further comprising a cavity formed in a cavity-containing wall, the cavity-containing wall being at least one of the central outer wall , the first inner wall , and the first outer wall ; wherein the central conduit has a downstream end defining an outlet plane ; and wherein the cavity has a downstream wall positioned proximal to and a non-zero distance upstream from the outlet plane . <EOS>