PURPOSE: To prevent the occurrence of choking with combustible particle, by a method wherein an external and an internal pipe are installed relatively movably, and a guide projection is provided on each of the two pipes.

CONSTITUTION: The base part 2b of an internal pipe 2 is formed in diameter larger than that of other part, and slidably and rotatively engages the inner peripheral surface of a base part 1b of an external pipe 1. In a spot in the vicinity of a jet nozzle 7 of a pulverized coal feed passage 3, outer guide projections 13 and inner guide projections 14 are symmetrically formed at a given angle and given intervals on the inner peripheral surface of the external pipe 1 and the outer peripheral surface of the internal pipe 2. By repeatedly moving forward and backward said internal pipe 2 along the direction of a center line lof a burner B, a massive material adhered to a guide surface 13b or 14b is crushed and removed. This always permits to maintain an optimum combustion condition depending upon the type of combustible particle as choking with combustible particle is effectively prevented from occurring.