Reaction Behavior of Biochar Composite Briquette Under H2-N2 Atmosphere: Experimental Study

Charging biochar composite briquettes (BCBs) and the injection of hydrogen-rich gas into the blast furnace (BF) are two efficient methods for reducing CO2 emission in BF ironmaking. This study investigated the reaction behavior of BCBs under a hydrogen-rich atmosphere to explore the potential combination of these two methods for enhanced CO2 emission reduction efficiency in the BF. The employed BCB had a chemical composition of 52.57 wt.% Fe3O4, 24.54 wt.% FeO, 0.98 wt.% Fe, 13.16 wt.% C, and 8.75 wt.% gangue. Isothermal BCB reaction tests were conducted using a custom-design thermogravimetric device under temperatures ranging from 1173 K to 1373 K and under an atmosphere of N2-H2 with a H2 content from 25 vol.% to 75 vol.%. A mathematical model was developed for the kinetics of the BCB reaction behavior under the H2-N2 atmosphere. Results showed that the developed model was adequate in predicting the reaction behavior of BCB. Under an atmosphere of 50 vol.% H2-N2, increasing the temperature from 1173 K to 1373 K resulted in a decrease in the fraction of iron-oxide oxygen removed by hydrogen from 62% to 26% and an increase in the fraction removed by biochar from 29% to 72%, indicating that hydrogen is the primary reducing agent under low temperatures, whereas, under high temperatures, biochar plays a more significant role. Under a constant temperature of 1273 K, increasing the H2 content in the atmosphere from 25 vol.% to 75 vol.% led to an increase in the fraction of iron-oxide oxygen removed by hydrogen from 37% to 45%, and a decrease in the fraction removed by biochar from 57% to 53%, suggesting that a higher H2 content enhances the iron oxide reduction by hydrogen but has little impact on the reduction by biochar. In the reaction process, the main products were CO and H2O, the iron oxide reduction occurred more rapidly near the center than near the surface, whereas the gasification of biochar followed the opposite trend. The structural transformation of the BCB progressed from sinter iron oxides into the metallic iron network in the reaction.