OCR Text |
Show Differences in pyrolysis behaviour with coal type are related not only to the total volatile matter contained in the coal, but also to the actual pyrolysis products. The early temperatures and burnout levels measured for the four coals as plotted in figs. 1C and ID appear to be in general accord with the results of pyrolysis studies which have examined the nature of volatile products with coal type. In fig. 7 the tar and gas yields from various coals under pyrolysis conditions are plotted versus proximate volatile matter. The position of the four coals used in this trial are also indicated on the horizontal axis. The general trend in these data is that tar yields (heavy hydrocarbons) have a maximum value somewhere in the high volatile bituminous coal range. Once the proximate volatile matter exceeds the point of maximum tar yield the pyrolysis products consist more of gaseous material. The exact position, as a function of proximate volatile matter, resulting in the maximum tar yield is not clear from these data, perhaps due to the different devices and experimental conditions used in the two separate investigations. However, the Elk Creek coal appears to belong to a group of. coals that produce a very high amount of tar during devolatilization, while the Armco coal should produce more gaseous material. The specific volatile products from a particular coal, particularly in the locally fuel rich conditions in a blast furnace tuyere, will influence the secondary cracking reactions of volatile matter into solid material. In experiments by other investigations [7, 8j large amounts (up to 20% of the input coal) of condensed material from the volatiles of certain coals have been observed in the form of soot-like structures. The amount of volatile matter condensed is probably related to the amount of heavy hydrocarbon material (tar) released by the coal during pyrolysis. This is clearly a detrimental behaviour for a coal used in blast furnace injection, where problems of raceway blockage and soot collection in gas cleaning equipment have been encountered during heavy fuel oil firing. The burnout results for the Elk Creek coal indicate that it is a type of coal that does generate large amounts of soot under certain combustion conditions. In fig. 2 the burnout, Tx, the oxygen concentration, and the C/H mass ratio of the sampled solids, are all plotted versus axial distance for two Elk Creek flames F-25 and F-26. The C/H mass ratio is plotted here 10-14 |