Effect of Rapid Preheating on the Coking Behavior of Bituminous Coals: Retrogressive Reactions and Extract Yields

Abstract

Mechanisms underlying observed changes in the plastic properties and swelling ability of weakly coking coals, induced by rapid heating to 400−450 °C, have been investigated. The enhanced softening would allow the shifting of coke blend compositions toward weakly coking coals. A surge in extractables content was observed at heating rates of >500 °C/s. After rapid heating, 25%−28% more of the mass of the weakly coking coal could be extracted. At these low temperatures (∼400−450 °C), repolymerization rates of solvent-extractables seem to be negligible. Increases in the extractables content that were determined by rapid heating clearly improve coke strengths via the related increase in thermoplasticity. However, when a “prime coking coal” was tested under similar conditions, no measurable effect of heating rate on the extractable contents was observed. A third “coking” coal showed behavior that was intermediate between these two cases. Taken together, the data strongly suggest the sequential occurrence of (i) fast retrogressive recombinations between reactive free radicals formed during thermally induced covalent bond cleavage reactions during the heat-up process, and (ii) slower repolymerization/recombination reactions occurring between the more-stable free radicals, observable at >450 °C. The example of the prime coking coal suggests that more of the reactive free radicals might be stabilized (quenched) by locally available hydrogen, irrespective of the heating rate. Liquefaction in the presence of hydrogen donors shows that high conversions to extractables are possible during slow heating. However, for transitional coals (between weakly coking coal and prime coking coals), pyrolysis yields are clearly sensitive to changes in heating rate. Transitional coals thus seem to be those that are marginally deficient in donatable hydrogen.