Evolution of Raman signal during lignin pyrolysis and its correlation with the binding mechanism in anthracite briquettes

Abstract As alternative fuel for the partial replacement of coke in foundry operations, our research team has engineered anthracite briquettes that use lignin in its binder formulation. To understand the mechanism by which lignin provides strength to the briquettes, in this study we characterized the progressive increase in mechanical strength of lignin-containing anthracite briquettes following progressively longer pyrolysis. We compared these results with the progressive change of the Raman signal of lignin that was pyrolized under the same conditions. The rate of strength gain by the anthracite briquettes increased linearly with the pyrolysis time up to a point, and this rate is described by an Arrhenius-like plot, which exhibited an apparent activation energy of 33.6 kJ/mol. We analyze the D and G bands from the Raman signal of pyrolyzed lignin, after isolating these bands via deconvolution. The D band area increased and the G band area decreased with the pyrolysis time up to a point. We correlated both the rate of D-band growth and G-band diminishment with pyrolysis time in Arrhenius-like plots; and these two bands exhibited apparent activation energies of 51.9 and 43.2 kJ/mol respectively. In light of the similar activating energy values obtained during the increase of strength of the anthracite briquette and the formation of a polyaromatic structure using Raman signal, we discerned that the mechanism by which the pyrolyzed lignin strengthened the briquettes was via the creation of poly aromatic structures that connected the grains of anthracite into this rigid network. This network provided resistance to fracture when the briquettes were subjected to the unconfined compressive strength (UCS) test.