Structural behavior of self-reducing briquettes containing biochar pyrolyzed at different temperatures

This study investigated the structural evolution at different reduction stages of self-reducing briquettes produced from pellet feed and 15% by mass of biochar derived from sugarcane bagasse pyrolyzed at 250, 400, and 550 °C. Interrupted tests conducted at 950 and 1250 °C, combined with XRD, SEM, and micro-CT characterization techniques, enabled a comprehensive analysis of the reduction process, as well as swelling and shrinkage behavior, establishing a direct correlation between structural transformations and metallurgical and mechanical performance of the briquettes. XRD analysis revealed the presence of metallic iron in all briquettes from 950 °C onward, with B550 presenting the highest reduction performance. At 950 °C, the briquettes exhibited low mechanical strength, mainly associated with intensified self-reduction reactions and swelling in this temperature range, strongly influenced by the formation of metallic iron whiskers. The more pronounced swelling observed in B550 can also be attributed to the combined effects of increased internal pressure due to intense gas generation, a higher reduction degree, and the elevated ash content of the biochar after pyrolysis at 550 °C. In contrast, B250 maintained its physical and geometrical integrity. After heating to 1250 °C, all briquettes exhibited volumetric shrinkage and increased mechanical strength, ascribed to the sintering of metallic iron particles and the formation of a liquid slag phase. This shrinkage was more pronounced in B400, due to enhanced metallic iron sintering and a more uniform reduction of the briquettes, which promoted a more consolidated structure and resulted in the highest mechanical strength among the evaluated samples.