Occurrence Factor of Free-board Combustion in Pressurized Fluidized Bed Combustion Boiler.

Relationship between the percent of combustion in free board and the conditions of pressurized fluidized bed combustion (PFBC), including the characteristics of coal water paste (CWP), has been studied using 2MWth and 4MWth-PFBC test plant. The 2MWth-PFBC test plant which bed height is 4m and cross sectional area of fluidized bed is 0.28m2, was operated at the bed temperature of 865°C under 0.95MPa pressure in full load condition. The 4MWth-PFBC test plant has two beds which are the same dimensions of 2MWth-PFBC's bed. Six kinds of coal were fed as a CWP. The percent of combustion in free board decreased with increase of bed height, bed temperature and air ratio, also with decrease of superficial gas velocity. The fraction of the fine coal under 0.25mm diameter in CWP affects to the free board combustion. Therefore, it is considered that the free board combustion is due to the decrease of combustion efficiency in the bed, and blowing up of the unburned char from the bed to freeboard. Furthermore, in order to demonstrate the experimental results, a simple calculation model for the free-board combustion was developed. The calculation results have been in good agreement with the experimental results.