Briquetes autorredutores de pellet feed e biochar de bagaço de cana-de-açúcar para uso em altos-fornos: desempenho de redução e caracterização estrutural avançada

Abstract

The use of biomass in self-reducing briquettes constitutes a promising alternative to reduce fossil fuel consumption and CO2 emissions in the iron and steel industry. This Thesis aims to investigate the reduction and mechanical strength performance of self-reducing briquettes containing pellet feed and sugarcane bagasse biochar for application as a complementary burden in blast furnaces. In Article 1, the influence of biochar pyrolysis temperature (250, 400, and 550 °C) on the behavior of laboratory-scale briquettes was evaluated, while Article 2 investigated the structural evolution of these agglomerates, expanding the understanding of the phenomena occurring during reduction. Characterization of the raw biomass and the resulting biochars revealed an increase in surface area and porosity with increasing pyrolysis temperature, as well as decomposition of the main biomass components and progressive degradation of the carbonized structure. Based on interrupted experiments, all briquettes exhibited high reduction performance up to 950 °C, although the use of biochar pyrolyzed at 550 °C (B550) resulted in a higher metallization degree, reaching 86% at 1250 °C compared to 28% for B250. The mechanical strength of the briquettes decreased with increasing heating temperature, reaching minimum values at 950 °C after the isothermal stage. Above this temperature, a catastrophic swelling behavior was observed in B400 and B550, strongly influenced by the formation of iron whiskers, and was more pronounced in briquettes produced with biochars obtained at higher pyrolysis temperatures. The volumetric shrinkage observed in all briquettes at 1250 °C was associated with the sintering of metallic iron particles and slag formation, so that the more uniform internal reduction observed in B400 contributed to higher strength at elevated temperatures. These results were supported by combined SEM and micro-CT analyses. In Article 3, briquettes containing 5%, 10%, and 15% by mass of biochar pyrolyzed at 400 °C were produced at pilot scale and tested under different temperature and atmospheric conditions representative of the blast furnace granular zone. Metallic iron was detected at 950 °C, with metallization intensifying as temperature and biochar content increased. The external reducing atmosphere favored iron formation in the agglomerates; however, higher reduction efficiency was predominantly driven by the solid carbon from the biochar. Compared with conventional burdens such as sinter and pellets, the briquettes exhibited considerably superior reduction performance. Briquettes containing 10% biochar (B10) showed the highest mechanical strength at elevated temperatures, providing the best balance between reduction efficiency and mechanical integrity. The lower strength observed in B05 and B15 was associated, respectively, with low metallization and with the presence of residual carbon combined with gas accumulation within the briquettes. This Thesis demonstrates that the biochar pyrolysis conditions and its proportion in the briquettes significantly affect the metallurgical and mechanical performance of these agglomerates, contributing to the advancement of knowledge related to the use of biochar as a reducing agent and to the development of more sustainable steelmaking routes.