Optimization of microalgal hydrothermal carbonization parameters using the response surface method for biochar applications in blast furnaces to reduce carbon emissions

Utilizing biochar in blast furnace (BF) ironmaking is a feasible and promising approach to reducing carbon emissions. Microalgae are one of the promising feedstocks for biochar production; however, research on producing high-performance microalgal hydrochars suitable for BF applications remains limited. Herein, we optimized the reaction parameters of the microalgal hydrothermal carbonization process by the response surface method for applying biochar in BF. Then, the properties of hydrochars produced under different conditions were characterized, including microstructure, surface chemical properties, composition, and combustion performance. The results indicated that hydrothermal temperature and time significantly affected hydrochars properties. The hydrochar obtained under the optimized conditions exhibited a high carbonization degree (H/C: 1.06, and O/C: 0.34), with a notable average removal rate of harmful metal elements (about 78 %) and excellent combustion performance (comprehensive combustion index = 11.98). Moreover, we assessed the CO₂ emission of the optimized hydrochar as a partial replacement for injection coal in BF. The results indicated about 30 % reduction in CO2 emissions associated with the coal acquisition process for BF coal injection, reflecting an environmental benefit. The study provides insights into producing high-performance hydrochars suitable for use as BF injection fuel to reduce CO₂ emissions.