OCR Text |
Show as an indicator of condensed material. The C/H mass ratio of soot formed from the rapid pyrolysis of a bituminous coal has been measured to be about 300 [7]. The C/H mass ratio of the Elk Creek char produced in these flames appears to be between 80 and 100. This number has been obtained by measuring the C/H ratios of solids collected at 5 m axial distance for the high stoichiometric rate Elk Creek flames. For these flames any soot particles should be preferentially oxidized before 5 m axial distance because of their small size, and any solid material collected at that position should be predominantly composed of the much larger Elk Creek char particles. Char is defined here as the solid portion of the original coal particle that remains after devolatilization, including any volatile material that has cracked inside the original particle. The soot-like material refers to any volatile material either gaseous or liquid that has undergone cracking reactions resulting in the formation of solid material after it has been expelled from the coal particle. If large amounts of soot are collected with the char in a solid sample, the C/H ratio should be increased to a value above 80. The stoichiometric ratio one case re- . tains a high C/H ratio throughout the length of the furnace. For flame F-25, high coal rate, the on axis oxygen concentration remains low and the soot-like material apparently remains throughout the furnace during the heterogeneous oxidation period. F-26, low coal rate, demonstrates a different behaviour. When the oxygen concentration is low on axis, X < 0.75 m, the C/H ratio is high and the burnout remains relatively constant. However, when the O2 concentration increases rapidly to 10%, the C/H ratio quickly drops and the burnout Tx quickly rises. As surmised, it appears that for high oxygen concentrations, the soot particles are burned very rapidly, while for low oxygen concentrations, F-25, they persist throughout the length of the furnace. In fig. 3 the reduced blast temperature Elk Creek flames (1030°C) are compared in a similar way. The trends observed for the 1200°C blast flames reappear in this figure with an important exception. With a high coal input rate, F-22, the C/H ratio is initially low, indicating a smaller amount of condensed material than for the 1200°C blast temperature flame F-25. This helps explain the initially higher measured burnout for F-22 when compared to F-25. 10-16 |