THE EARLY STAGES OF THE COMBUSTION OF PULVERIZED COAL AT HIGH-TEMPERATURES .2. MEASUREMENTS OF THE SIZE AND COMPOSITION OF THE RESULTING CHAR

Fine particles (50-60 μm) of four different coals have been added to the burner supplies of four premixed flames of H2 + O2 + N2. These are good approximations to one-dimensional flames, are at atmospheric pressure, have final temperatures in the range 1850-2160 K, and vary in stoichiometry. While the particles traverse through the flame front and pass along the burned gases, they heat up rapidly and then undergo devolatilization. These follows some combustion of the resulting char, depending on the residence time of the particles in the hot gases. The particles leaving these flames were collected, after being in them for 31 ms, and then chemically analyzed. The composition of the char was then contrasted with that of the parent coal. In addition, the size distributions of the char particles were compared with those of the original coal fed to the flame. The following conclusions were made. No swelling of the particles occurs during pyrolysis under these conditions, but fine particles ( < 20 μm) are produced. These are either tiny soot particles, formed in the fuel-rich region around each devolatilizing particle, or other particles resulting from fragmentation of the original coal. The effect of flame conditions and coal rank on these processes has been investigated. The volatiles are found to have a C H ratio that increases linearly as devolatilization proceeds. However, the loss of nitrogen from coal undergoing this pyrolysis closely follows the loss of mass. There appears to be no significant difference between the weight percentages of nitrogen in the volatiles and in the resulting char particles. © 1990.