Lecture 08

Lecture 8: Blast Furnace Productivity & Modern Practices

1. Productivity: Definition and Optimization

The productivity of a Blast Furnace (BF) is defined as the Rate of Production ($R$), typically measured in Tonnes of Hot Metal (THM) per day.

The Governing Formula

To understand how to maximize production, we express $R$ as:

$R(\text{thm/day})=\frac{\text{Total Volume of Gas Blown in a Day}(m^3/\text{day})}{\text{Specific Blast Rate}(m^3/\text{thm})}$

• Numerator: The total wind/air pumped into the furnace per day.

• Denominator: The amount of air required to produce one tonne of hot metal.

Optimization Strategy

To increase Productivity ($R$), we have two options:

1. Increase the Numerator: Blow more air.

2. Decrease the Denominator: Make the process more efficient so less air is needed per tonne of iron.

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2. Constraints on Gas Flow (The Numerator Limit)

We cannot simply blow infinite air into the furnace. Increasing the gas velocity beyond a critical limit triggers dangerous aerodynamic phenomena.

Aerodynamic Limits (Step-by-Step Failure)

1. Fluidization: High gas velocity causes smaller particles to become suspended/dance in the gas stream.

2. Elutriation: Extremely high velocity carries fine particles out of the furnace top (dust loss).

3. Loading:

   • Occurs in the Cohesive Zone (where solids and liquids coexist).

   • The upward gas drag balances the downward gravity of the liquid (slag/metal).

   • Result: Liquid stops descending and accumulates.

4. Flooding:

   • Gas drag exceeds gravity. Liquid is pushed upward.

   • Liquid enters cooler upper regions, solidifies, and blocks the pores.

5. Hanging:

   • The final catastrophic state.

   • The burden ceases to descend (“hangs” in the shaft).

   • Production stops completely.

Instructor Note: Because of these limits, most furnaces operate at a maximum critical gas velocity (within ~5-10%). Therefore, the only real way to increase productivity is to reduce the Specific Blast Rate (Denominator).

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3. Factors Affecting Specific Blast Rate

The Specific Blast Rate depends heavily on raw material quality and thermal efficiency.

Case Study: High Grade vs. Low Grade Ore

Consider two scenarios:

• Case A: Ore with 80% $F{e}_2{O}_3$ (20% Gangue).

• Case B: Ore with 70% $F{e}_2{O}_3$ (30% Gangue).

Why Case A has higher productivity:

1. Less Slag: Case B has 50% more gangue per tonne of Fe $\to$ Higher slag volume.

2. Less Heat Load: Slag must be melted to 1450°C. More slag = More heat required.

3. Less Coke: More heat needed in Case B means burning more Coke.

4. Less Blast: Burning more Coke requires more Air (Blast).

5. Result: Case B has a higher Specific Blast Rate $\to$ Lower Productivity.

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4. Modern Blast Furnace Practices (The 8 Pillars)

Modern ironmaking has evolved 8 distinct practices aimed specifically at reducing Coke Rate and Specific Blast Rate.

1. Giant Blast Furnaces

• Concept: Move from 1000 $m^3$ to 4000–5000 $m^3$ furnaces.

• Physics: Heat loss is proportional to Surface Area ($A$), while heat generation is proportional to Volume ($V$).

• Benefit: Larger furnaces have a smaller $A/V$ ratio $\to$ Less relative heat loss through walls $\to$ Lower Coke Rate.

2. High Blast Temperature

• Practice: Preheating air to ~1200 K (limit imposed by Stove refractories).

• Benefit: Sensible heat enters with the air, reducing the need to burn Coke for heat.

• Equipment: Modern Stoves or Metallic Blast Heaters.

3. High Top Pressure

• Practice: Operating the furnace top at 1.5–2.5 atm (vs. near atmospheric).

• Benefit:

  • Reduces gas velocity (Volume $\propto$ 1/P).

  • Increases residence time $\to$ Better Gas-Solid heat transfer $\to$ Lower top gas temperature (Waste Heat reduction).

  • Suppresses the Boudouard Reaction ($C+C{O}_2\to 2CO$), which is solution loss (carbon waste).

4. Oxygen Enrichment

• Practice: Increasing $O_2$ in blast from 21% to ~24%.

• Benefit:

  • Reduces Nitrogen volume (which is an inert heat sink).

  • Increases flame temperature and combustion efficiency.

  • Note: We cannot use 100% pure oxygen yet as it would overheat the raceway uncontrollably.

5. Coal Dust Injection (PCI - Pulverized Coal Injection)

• Practice: Injecting non-coking coal dust (cheap) directly into tuyeres.

• Typical Rate: 150–200 kg/thm (Record ~260 kg).

• Benefit: Replaces expensive Metallurgical Coke.

• Challenge: Coal ash adds to slag volume; incomplete combustion can generate soot.

6. Humidification of Blast

• Practice: Injecting Steam ($H_{2}O$) into the blast.

• Reaction: $C+H_{2}O\to CO+H_2$ (Endothermic).

• Role:

  • Generates Hydrogen: $H_2$ is a smaller molecule than $CO$, diffuses faster, and reduces ore more efficiently.

  • Control Tool: Used to fine-tune the RAFT (Raceway Adiabatic Flame Temperature). If the raceway gets too hot (due to $O_2$ enrichment), steam cools it down.

7. Lime Dust Injection

• Practice: Injecting flux powder through tuyeres.

• Benefit:

  • Lime reacts immediately with Ash/Sulfur in the raceway.

  • Removes the thermal burden of calcining limestone inside the stack.

8. Engineered Raw Materials (Burden Preparation)

• Practice: 80-90% Sinter/Pellet burden (vs. Lump Ore).

• Benefit:

  • Channeling Prevention: Uniform size prevents gas from finding “easy paths” (Channeling).

  • Reducibility: Sinter/Pellets have engineered porosity, allowing gas to penetrate and reduce iron oxides faster.

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5. Summary: The Interconnected Goal

All 8 practices aim to lower the Coke Rate ($k{g}_{coke}/thm$).

• Relationship:

  $\downarrow \text{Coke Rate}⟹\downarrow \text{Blast Rate}⟹\uparrow \text{Productivity}$

• Evolution:

  • 1970s: Coke Rate ~800 kg/thm.

  • Today: Coke Rate ~350–400 kg/thm.

  • Productivity: Increased from ~1.5 to 2.5–3.5 thm/day/m³.