A preliminary study on the quality evaluation of coking coal from its structure thermal transformation: Applications of fluidity and swelling indices

Evaluating the coking coal quality from coal structure thermal transformation was crucial to control the coking process. The coal quality indices of ash composition, volatile matter, element distribution and caking indices were redefined based on the coal structural characteristics. Five structure parameters derived from volatile matter and element distribution, including carbon aromaticity (fa), average number of condensation rings (NCR), ring condensation index (2(NCR - 1)/C), aliphatic hydrogen content (AlH) and number of bridge bonds (AlB), were proposed to evaluate the comprehensive structure properties of coals. The rationality of these parameters was verified through 13C NMR analysis. The superiority of fluidity and swelling indices in reflecting the structure transformation of individual and blended coals was clarified. In addition, a novel mothed for measuring swelling pressure was developed, taking into account the impact of heat and mass transfer on the coking process. The results show that the bridge bonds content (AlB) affects the coal particle softening which attributes to the thermal cleavage of Cal-Cal/O and Car-O. The high aliphatic hydrogen content (AlH) relating to the stabilization of pyrolytic fragments decreases maximum fluidity temperature. The formula of F(MD) captures the relevance among maximum dilatation, volatile matter, caking and plastic layer indices, which aids in comprehending the swelling mechanism based on the balance between gas release and permeability evolution. The fluidity and swelling indices serve as indicators of the structural transformation during individual coal coking, contributing to the quality evaluation of blended coals with similar coke performance. The self-designed device provides a more accurate prediction of pushing current based on the maximum swelling pressure, compared to the Audibert-Arnu dilatation. This work contributes to the scientific evaluation of coking coals and the precise control of the coking process.