Thermal characteristics of lignin residue from industrial processes
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Many pulp mills and biorefineries today are focusing on the utilization of their residual lignin for economic return. Although lignin can be burned to produce energy, it also has the potential for the production of value-added products. Technical lignins have modified structure and contain different impurities, which depend on the original material, as well as the extraction process. Among the various techniques for lignin extraction, kraft and steam explosion processes are the most commonly used in the pulping and biorefinery industries, respectively. The objective of this work was to compare the thermal behavior of industrial lignins produced from kraft pulping and steam explosion, with that of their chemically extracted, purified forms. It was found that the purified lignins have very similar thermal properties to one another, while impurities in the industrial lignins significantly alter their thermal behavior, and hence their potential in value-added applications. The percentage of degradation from 200 to 600 °C and glass transition temperature of original steam-exploded lignin was 68.5% and 149.16 °C, while of original kraft lignin was 26.0% and 109.82 °C. These values were altered after purification to 61.0% and 158.99 °C for steam-exploded lignin; and to 40.0% and 129.82 °C for kraft lignin, respectively.