Usually, due to the larger contacting area of unit mass of the coal of small particle size with oxygen,  and under the condition of the same fluidization velocity, its particles movement is in more lively degree, forming more intense heat transfer and mass transfer process, gaining much more opportunity with the hot material for heat exchanging than that of the coal of large particle size,  thus easily have rapid temperature rise. Therefore, generally speaking the coal of small particle size is easily to be burnt than the coal of larger particle size.   (lsh)