Coal-Based Sponge Iron Production Explained
Coal-based sponge iron production uses a rotary kiln, non-coking coal, and iron ore to make DRI. Process, chemistry, and why kiln sealing matters.
Coal-based sponge iron production is the direct reduction of iron ore using non-coking coal as both reductant and fuel inside a rotary kiln, carried out in the solid state at 1,000-1,100°C without any melting. The product, called sponge iron or direct reduced iron (DRI), is the principal steelmaking feed for India's electric arc furnace sector. In financial year 2024, India produced 54.7 million tonnes of DRI -- the largest national output in the world -- with coal-based rotary kilns accounting for roughly 80% of that total [1][2].
Direct reduced iron (DRI) / sponge iron: metallic iron produced by removing oxygen from iron ore in the solid state, below the melting point of iron. The porous, honeycombed structure that remains after oxygen removal gives the product its colloquial name. See the sponge iron production process for the full cross-route comparison.
What is coal-based sponge iron and how does it differ from gas-based DRI?
Coal-based DRI uses a rotary kiln and non-coking coal; gas-based DRI uses a shaft furnace and reformed natural gas (H2+CO). Both routes produce the same end product -- metallic iron with 88-94% metallization -- but the equipment, feed materials, and operating conditions differ substantially. Coal-based is the dominant route in India because the country has abundant reserves of non-coking coal and historically limited domestic natural gas. Gas-based dominates the Middle East and North Africa. There are 285 DRI plants in India, the majority operating coal-based rotary kilns [3].
Feed materials and their roles
The three feed materials are iron ore (lump ore or pellets), non-coking coal (reductant and heat source), and dolomite (sulphur absorber and de-fluxing agent).
| Material | Typical specification | Role in process |
|---|---|---|
| Iron ore (lump) | 65-67% Fe, 10-30 mm size | Iron source; higher Fe reduces gangue and improves metallization |
| Non-coking coal | 35-45% fixed carbon, <0.5% S, 25-30% volatile matter | Reductant (provides CO) and fuel (combusts to supply heat) |
| Dolomite | CaCO3.MgCO3, low silica | Absorbs sulphur from coal combustion; moderates ash fusion behavior |
Source: TERI Energy-Efficient Technology Options for DRI, 2021 [3]; IspatGuru coal-based DRI overview [4].
Non-coking coal is the pivotal input. The blast furnace route requires coking coal, which must be imported at a premium by most Indian steelmakers. Non-coking coal, available domestically at lower cost, is what makes coal-based DRI economically viable for Indian producers. Dolomite is added at roughly 5-8% of iron ore weight; its primary function is sulphur capture, but it also shifts the melting point of coal ash upward, reducing accretion risk.
The rotary kiln: zones and temperature profile
The rotary kiln is divided into three functional zones: a preheating zone (ambient to approximately 800°C), a reduction zone (800-1,050°C), and a transition/discharge zone where peak temperatures reach 1,000-1,100°C.
| Zone | Temperature range | Main reactions |
|---|---|---|
| Preheating | Ambient to ~800°C | Moisture evaporation; coal devolatilisation; partial combustion of volatiles |
| Reduction | 800-1,050°C | Indirect reduction: Fe2O3 + 3CO → 2Fe + 3CO2; Boudouard: CO2 + C → 2CO |
| Discharge transition | ~1,000-1,100°C (peak) | Final reduction; controlled cooling begins after kiln exit |
Source: IspatGuru [4]; TERI report [3].
Kiln geometry for a typical Indian plant: 40-80 m in length, 3-5 m in diameter, inclined at 2-3 degrees, rotating at 0.3-1.0 rpm [3][4]. Feed enters the elevated (inlet) end; DRI discharges at the lower (outlet) end. Residence time is 6-10 hours for kilns in the 100-500 TPD capacity range [3].
The key chemistry is the Boudouard reaction and indirect reduction:
Fe2O3 + 3CO → 2Fe + 3CO2 (indirect reduction, dominant)
CO2 + C → 2CO (Boudouard equilibrium, regenerates reductant)
Where:
- Fe2O3 = iron oxide (haematite, principal iron ore mineral)
- CO = carbon monoxide (reducing agent, generated from coal)
- C = fixed carbon in non-coking coal
- Boudouard reaction is endothermic and favoured above ~700°C
Controlled air injection through shell-mounted air tubes manages the CO/CO2 ratio and sustains the temperature in the reduction zone. The air injection rate is the primary process-control variable in a coal-based kiln.
Why kiln sealing is critical in DRI production
The coal-based DRI process depends on a controlled reducing atmosphere inside the kiln: any ingress of ambient air converts CO and H2 back to CO2 and H2O, reverting already-reduced iron back to iron oxides.
In cement kilns, false air ingress is primarily a fuel and fan-power penalty. In DRI kilns, false air directly damages product quality: re-oxidation of metallic iron lowers the metallization rate (target: greater than 88-92%) and raises FeO content in the finished sponge iron. A lower metallization rate translates to a lower-grade product commanding a lower market price, in addition to the energy penalty.
The two primary seal interfaces are the kiln inlet (feed end) and kiln outlet (discharge end). Both are rotating-to-stationary junctions operating in a hot, dusty, reducing-atmosphere environment -- exactly the conditions for which Oswal's duplex kiln sealing system is designed. For the full technical case on DRI-plant sealing requirements, see kiln sealing in DRI plants.
Accretion: the defining operational challenge
Accretion is the buildup of semi-molten material on the kiln refractory lining; it is the most disruptive operational challenge in coal-based DRI kilns [5][6].
Accretion forms when low-melting complex compounds from coal ash, iron ore gangue, and dolomite fuse under reducing conditions at operating temperature and adhere to the refractory wall. Contributing factors include excess temperature in the reduction zone, high ash content in coal, elevated fines content in ore feed, and a low carbon/iron ratio at the discharge end. As accretion builds, kiln cross-section narrows, capacity drops, and -- if left unchecked -- the kiln must be shut down for manual removal.
Tata Sponge Iron Limited (Odisha, India) published a peer-reviewed case study on accretion control at its coal-based DRI plant, demonstrating that a combination of tighter temperature management, improved coal quality specification, and optimised air-tube scheduling reduced accretion frequency and improved kiln availability [6].
Control measures in practice:
- Maintain peak reduction-zone temperature below the ash fusion temperature of the coal used
- Specify coal with ash fusion temperature above 1,250°C where possible
- Maintain adequate dolomite dosing to raise the effective fusion temperature of the ash-gangue mix
- Adjust air-tube injection to prevent localised hot spots
Key operating parameters at a glance
| Parameter | Typical range | Note |
|---|---|---|
| Kiln temperature, peak reduction zone | 1,000-1,100°C | Higher end risks accretion; lower end risks incomplete reduction |
| Residence time | 6-10 hours | For 100-500 TPD kilns; scales with kiln length [3] |
| Metallization rate (product target) | >88-92% | Higher is better; re-oxidation and low reduction temperature reduce this |
| Installed capacity range, India | 50-500 TPD per kiln | 285 plants in India, mostly coal-based [3] |
| Non-coking coal consumption | ~1.0-1.3 t per t DRI | Varies with coal quality and kiln efficiency [3][4] |
| Dolomite addition | ~5-8% of iron ore weight | Source: TERI [3] |
Common questions about this topic
Coal-based DRI uses a rotary kiln and non-coking coal as the reductant; [gas-based DRI](/en/blog/gas-based-dri-direct-reduction) uses a shaft furnace and H2+CO reducing gas reformed from natural gas. Coal-based dominates in India because non-coking coal is abundant and low-cost domestically. Gas-based dominates in the Middle East, where cheap natural gas is available. Both routes produce equivalent metallized iron, but gas-based typically achieves higher metallization rates and generates lower CO2 per tonne of DRI.
Non-coking coal with approximately 35-45% fixed carbon, less than 0.5% sulphur, and 25-30% volatile matter [3][4]. Coking coal is not required, which is the key economic advantage of coal-based DRI over the blast furnace route. High volatile matter contributes to early gas generation and aids temperature control; high fixed carbon drives the Boudouard equilibrium toward CO generation. Sulphur must be low because it transfers to the product and degrades downstream steel quality; dolomite addition absorbs residual sulphur in the kiln.
False air ingress at the kiln inlet or outlet re-oxidises metallic iron that has already been reduced, lowering the metallization rate and degrading product quality -- a quality impact that does not occur in cement kilns where the product is unaffected by minor oxygen exposure at operating temperature. For [metallurgical industry](/en/industries/metallurgical) operators, a 2-3 percentage-point drop in metallization from poor sealing can shift a full heat of DRI from a premium to a discount grade. See [kiln sealing in DRI plants](/en/blog/kiln-sealing-dri-plants) for the engineering detail.
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