Enhancing biochar quality for the steel industry via Hydrothermal Pretreatment-Steam Explosion and pyrolysis

Biochar has potential applications in steelmaking processes, but faces technical challenges such as low material density, high alkali content, and high reactivity compared to coal. This study explores converting the solid residue, following hydrothermal pretreatment-steam explosion (HTP-SE) of Miscanthus and other biomass feedstocks, into biochar to facilitate the replacement of coal in blast furnace and electric arc furnace operations. It is the first to demonstrate the enhanced combustion characteristics of pretreated fibre and the compatibility of the biochar for use in steelmaking. Biomass from birch, miscanthus, wheat straw, both untreated and pretreated, was evaluated. HTP-SE was conducted at 192 °C and 1.3 MPa, conditions aligned with hemicellulose extraction for application in biobased products. Biochars were produced at temperatures ranging from 300 °C to 550 °C. HTP-SE increased the carbon, hydrogen, and energy content by approximately 10%, 8%, and up to 5 MJ/kg, respectively, while reducing ash quantity by up to 45%. In addition, it reduced the alkali and phosphorus content from the solid fraction into aqueous phase. Gas analysis indicated that HTP-SE enhanced the energy content of pyrolysis syngas. Thermogravimetric studies revealed that pretreated biochars exhibited significantly lower reactivity with carbon dioxide compared to untreated counterparts, approaching the reactivity of coal. This was attributed to increased aromaticity, C=C bonding, cross-linkages enriching lignin and by the removal of hemicellulose through HTP-SE. Overall, the upgraded biochar addresses key limitations of conventional biochar and shows strong potential as a substitute to replace injection coal entirely in both blast and electric arc furnaces.