Mechanism of aliphatic hydrogen on the caking property of 1/3 coking coal during rapid preheating

摘要

It is of significance to reveal the mechanism of rapid preheating to enhance the caking property of coking coal and expand high-quality coking coal resources. In this study, a 1/3 coking coal was preheated at two low and one rapid rates, then raw coal and heat-treated samples were comprehensively analyzed. The results revealed that the aliphatic H in coal formed volatiles at the early stage of pyrolysis (<= 380 degrees C) under low-rate preheating. In the thermoplastic stage, the bonding phase was confirmed to be formed by a part of aliphatic H. The decreases amount of volatile and hydrogen contents and H/C and O/C atomic ratios of rapidly preheated coals were less than those of low-rate preheated coals, indicating that its thermal decomposition reaction degree was lower. This was considered to be the basis for keeping G values of rapidly preheated coals from decreasing. The aliphatic H concentration and CH2/CH3 ratios of rapidly preheated coals were higher and lower than those of raw coal respectively, which contributed to the fact that their G values were basically close to or higher (similar to 349-401 degrees C) than that of raw coal. Rapid preheating weakened the consumption of aliphatic H by volatiles formed during lowrate heating. Moreover, it forced weak bonds in the molecular of coal to break, such as bridge bonds and intermolecular forces that connect structural units. The breaking of bridge bonds between alkyl side chains or cycloalkanes and aromatic groups enhanced hydrogen supply. The weakening of intermolecular force resulted in loose macromolecular network structure and enhanced molecular dynamic properties. These evolutions of coal molecular structure strengthened the combination of hydrogen and free radicals and generated more bonding phases. Finally, a mechanism which provided a new explanation for the rapid preheating to alter the molecules structure and enhance the caking property of coking coal was proposed, and it will be significant valuable to enlarge the utilization of weakly-caking coal in coking process.