Reaction behavior of the biochar composite briquette in the blast furnace

Blast furnace (BF) ironmaking is considered to be the most popular technology to meet the increasing steel demand worldwide, but it is responsible for the most CO₂ emissions in the blast furnace-basic oxygen furnace production process. The utilization of biomass/biochar in BF ironmaking is an effective countermeasure to reduce its CO₂ emission, as biomass/biochar is a renewable carbon source and environment neutron. Charging the biochar composite briquette (BCB) is a convenient method to introduce biomass/biochar into BF. The present research investigates the reaction behavior of the BCB in the BF. The BCB for the BF was prepared using cold briquetting followed by low-temperature heat treatment. The BCB was composed of 11.1% carbon, 72.7% magnetite, 11.25% wustite, 0.77% metallic iron, and 4.67% gangue (all in mass fraction). The BCB reaction model in the BF was developed considering the step-wise gaseous reduction of iron-oxide particles, CO₂ gasification of biochar particles, internal gas diffusion in the BCB, and mass transfer between the BCB and the environment. Isothermal BCB reaction tests were conducted for model validation. Using the model, the changes of the BCB iron-oxide reduction fraction and biochar conversion rate and the BCB microstructure evolution under simulated BF conditions were analyzed. The model was also applied to predict the change of the BCB iron-oxide reduction fraction, change of the BCB biochar conversion, change of the BCB CO generating rate, and change of the BCB CO₂ generation rate along a solid flowing path near the mid-radius in an actual BF. Results showed that under simulated BF conditions, the BCB underwent fast self-reduction and structure changes (forming low-melting compounds and transforming from the slag matrix to the iron network) from 60 min (973 K) to 120 min (1273 K). In an actual BF, the BCB reaction route is mainly divided into three stages: (1) reduction by BF gas (473–853 K), (2) reduction by the BF gas and partial self-reduction (853–953 K), and (3) full self-reduction (953–1150 K). In the stages involving BCB self-reduction, the iron oxide in the BCB reduces faster than the sinter, and the biochar gasifies faster than the coke. Moreover, in these stages, the BCB has the functions of increasing the BF gas utilization efficiency and lowering the temperature level of the BF thermal reserve zone.