Optimized Coal Efficiency in Blast Furnaces

Overview

This lecture covers direct and indirect reduction processes in blast furnaces, emphasizing how combining both methods optimizes carbon efficiency and CO gas utilization. At 54% direct reduction and 46% indirect reduction, carbon consumption reaches minimum while CO utilization maximizes at 85%.

Direct Reduction Mechanism

• Represented as FeO + C → Fe + CO, where carbon directly reacts with iron oxide.
• Although appears solid-solid, reaction occurs via gaseous intermediates (CO and CO₂).
• Two-step process: FeO + CO → Fe + CO₂, then CO₂ + C → 2CO (carbon gasification).
• CO₂ from reduction gasifies carbon in-situ, generating fresh CO for continued reduction.
• Endothermic process requiring temperatures above 900°C for carbon gasification.
• Occurs in lower furnace zones where temperature exceeds 900°C.
• Economical: one mole carbon removes one mole oxygen from ore.

Indirect Reduction Mechanism

• Sequential reactions: Fe₂O₃ + CO → Fe₃O₄ + CO₂, then Fe₃O₄ + CO → FeO + CO₂.
• Carbon does not directly participate; CO generated elsewhere in furnace performs reduction.
• Occurs in upper furnace where temperature falls below 900°C.
• Carbon gasification (C + CO₂ → 2CO) impossible at these lower temperatures.
• CO must be supplied from lower furnace regions to sustain reduction.
• Exothermic reactions take place in upper furnace zones.
• Expensive: requires 3.3 moles CO to remove one mole oxygen from FeO.

Temperature Requirements and Furnace Zones

• Below 900°C: only indirect reduction occurs; no in-situ CO generation possible.
• Above 900°C: direct reduction proceeds via carbon gasification in lower furnace.
• Blast furnace operates as counter-current gas-solid reactor with gas moving upward.
• Ascending CO gas first encounters FeO (wüstite), then Fe₃O₄ (magnetite), finally Fe₂O₃ (hematite).
• Fe₂O₃ least stable, highest oxidation potential; reduces at furnace top with lowest gas reduction potential.
• FeO most stable among iron oxides; requires highest gas reduction potential, reduces in lower zones.

Carbon Efficiency Comparison

• Fe₂O₃ contains 429 kg oxygen per tonne iron produced.
• Fe₃O₄ contains 381 kg oxygen per tonne iron.
• FeO (wüstite) contains approximately 302 kg oxygen per tonne iron (non-stoichiometric, Fe:O ratio 1:1.06).
• Removing Fe₂O₃ → Fe₃O₄ requires extracting 48 kg oxygen.
• Removing Fe₃O₄ → FeO requires extracting 79 kg oxygen.
• FeO → Fe reduction removes 302 kg oxygen; most oxygen associated with wüstite stage.
• Wüstite reduction can proceed via both direct and indirect routes, enabling optimization.

Optimum Partitioning Calculations

• Let y kg wüstite oxygen be removed by direct reduction; (302 - y) kg removed indirectly.
• Direct reduction generates y/16 kg-mole CO.
• Indirect reduction requires (302 - y)/16 × 3.3 kg-mole CO.
• Optimum condition: CO generated by DR equals CO required by IDR.
• Solving yields y = 232 kg, representing 54% of total removable oxygen (429 kg).
• At optimum, 232 kg oxygen removed directly, 197 kg removed indirectly.
• Indirect reduction produces 12.3 kg-mole CO₂ from 14.5 kg-mole CO generated by direct reduction.
• CO utilization efficiency reaches 85% (12.3/14.5 × 100).
• If only FeO reduced indirectly, efficiency drops to merely 30%.
• Carbon requirement at optimum: 172 kg per tonne iron (14.5 kg-mole × 12 kg/mole).
• After wüstite indirect reduction, 10.06 kg-mole CO remains to reduce higher oxides.
• This remaining CO satisfies requirements for Fe₃O₄ (needing 9.25 kg-mole) and Fe₂O₃ reductions.

Performance Metrics

• 100% direct reduction yields zero CO utilization (product gas entirely CO, no CO₂).
• 100% indirect reduction produces highest carbon consumption (747 kg/tonne) with 43% CO efficiency.
• Minimum carbon consumption (172 kg/tonne) occurs at 54% direct, 46% indirect reduction.
• Maximum CO utilization (85%) also achieved at 54% direct, 46% indirect balance.

Hydrogen as Alternative Reductant

• Reaction: FeO + H₂ → Fe + H₂O compared to FeO + CO → Fe + CO₂.
• At 21°C, both η(CO) and η(H₂) efficiencies are equal.
• At higher temperatures (especially 900°C where wüstite reduction occurs), η(H₂) exceeds η(CO).
• H₂ thermodynamically more efficient than CO at elevated temperatures typical for indirect reduction.
• Hydrogen reduction reactions are endothermic, unlike some CO-based reductions.
• H₂ efficiency increases with temperature, while CO efficiency for FeO reduction decreases.

Key Conclusions

• Fe₂O₃ reduced at furnace top, Fe₃O₄ at intermediate zones, FeO in lower sections due to stability differences.
• Below 900°C: oxygen potential rises monotonically as only indirect reduction occurs, generating CO₂.
• Above 900°C: oxygen potential fluctuates as gas alternates between coke/ore layers, enabling CO generation.
• Indirect wüstite reduction highly carbon-intensive; requires ~70% CO in equilibrium, yielding only 30% utilization.
• Combining 54% direct and 46% indirect reduction achieves 85% CO efficiency and 172 kg carbon consumption.
• This optimization based on material balance; heat balance considerations increase indirect reduction percentage slightly.
• Additional carbon must be burned to meet furnace heat demands beyond reduction requirements.