Hyphae

Coke

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A grey, hard, porous carbon-rich solid produced by heating coal in the absence of air (destructive distillation, or ‘coking’) at approximately 1,000–1,100°C. Metallurgical coke is the primary fuel and chemical reductant in blast furnace ironmaking, serving three simultaneous roles: (1) fuel — it combusts with preheated air at the tuyeres to generate the extreme temperatures (>1800°C in the raceway) needed to melt iron; (2) reductant — it generates carbon monoxide (CO) via the Boudouard reaction (CO₂ + C → 2CO) which reduces iron oxides in the furnace shaft; and (3) structural support — its mechanical strength and porosity maintain bed permeability for gas flow through the descending burden. Before Abraham Darby’s 1709 use of coke at Coalbrookdale, charcoal was the only acceptable metallurgical fuel; the substitution of coke for charcoal was one of the most consequential technical transitions of the Industrial Revolution. [CIT-COK-01 (Wikipedia Coke (fuel), sha256:0e4ffb74); CIT-COK-02 (Wikipedia Abraham Darby I, sha256:12c664c6); CIT-PI-03 (Tylecote 1992).]

Common forms

  • Metallurgical coke (met coke) — the primary form used in blast furnaces; produced in slot-type by-product ovens from blended coking coals; lump size typically 25–80 mm. Quality is characterized by CSR and CRI. [CIT-COK-01.]
  • Foundry coke — coarser, harder grade (>80 mm lump) used in cupola furnaces for iron casting; lower reactivity preferred. [CIT-COK-01.]
  • Coke breeze — fines (<6 mm) that are a byproduct of coke handling; used in sinter plants to agglomerate iron ore fines before blast furnace charging. [CIT-COK-01.]

Common sources

  • Coking coal (metallurgical coal) — low-ash, low-sulfur bituminous coal with the plasticity and caking properties needed to fuse into coke structure during pyrolysis. Must have controlled volatile matter content (~26–29 wt%) for good coke quality. Coal blending is standard practice to optimize coke quality from available coal sources. [CIT-COK-01.]
  • Historically: hearth process — coal heaped and burned with restricted air, analogous to charcoal burning; produced lower-quality coke. Superseded by beehive ovens and modern slot-type by-product ovens. [CIT-COK-01.]

Composition

Metallurgical coke is approximately 87–92 wt% carbon (fixed carbon, dry ash-free basis); 8–13 wt% ash (mineral residue from coal: mainly SiO₂, Al₂O₃, CaO, MgO, Fe₂O₃); <1 wt% volatile matter remaining after coking; sulfur content ideally <1 wt% (low-sulfur coking coal required — high sulfur transfers to pig iron and causes hot-shortness in steel). The non-volatile, non-combustible residue is fused mineral ash that remains after coke reacts in the furnace, contributing to slag formation. [CIT-COK-01 (confirms coke is residue of destructive distillation); general metallurgical knowledge for specific composition ranges — flagged in needs_verification; CIT-PI-03, pp. 95–100.]

Hazards

  • Coke oven emissions — production of coke in ovens generates polynuclear aromatic hydrocarbons (PAHs), benzene, hydrogen cyanide, and other volatile carcinogens. OSHA permissible exposure limit: 0.150 mg/m³ benzene-soluble fraction (8-hour TWA); NIOSH REL: 0.2 mg/m³. Coke oven workers have elevated rates of lung, bladder, and kidney cancer. [CIT-COK-01 (OSHA/NIOSH limits directly stated).]
  • Fire hazard — coke burns readily once ignited; coke dust in air can form explosive dust clouds. Industrial handling requires dust suppression. [CIT-COK-01; general industrial safety knowledge.]
  • CO generation during combustion — incomplete combustion of coke produces carbon monoxide; significant risk in enclosed spaces during blast furnace operation. (See also: Blast Furnace Ironmaking node for CO atmosphere hazard.) [CIT-BF-01 (cross-reference).]

Properties

  • porosity: High porosity (~40–55% by volume); pores allow gas penetration and surface reaction. Critical for Boudouard reaction kinetics in the blast furnace. [General metallurgical knowledge — uncited; consistent with CIT-COK-01’s description of porous structure.]
  • sulfur_content: Metallurgical coke requires <1 wt% S (preferably <0.6 wt%). Sulfur in coke transfers to pig iron and then to steel, causing hot-shortness (cracking during hot rolling/forging). This is the critical quality constraint limiting which coals can be used for cokemaking. By contrast, charcoal contains <0.1 wt% S — which is why charcoal remained preferred for high-quality iron even after coke became available. [CIT-COK-01 (notes low-sulphur bituminous coal requirement); CIT-BF-01 (Charcoal node in graph — cross-reference).]
  • boudouard_activity: Coke reacts with CO₂ generated during iron oxide reduction: CO₂ + C → 2CO (Boudouard reaction), regenerating the CO reductant. This reaction is strongly endothermic and is favored above ~700°C; it is quantified by the coke reactivity index (CRI). Some reactivity is desirable for blast furnace chemistry; excessive reactivity weakens the coke structure and reduces CSR. [CIT-COK-01 (reduction reaction confirmed); Kubaschewski & Alcock (1979) for Boudouard thermodynamics — common to Blast Furnace node.]
  • mechanical_strength: High compressive strength required for metallurgical-grade coke — measured by coke strength after reaction (CSR) and coke reactivity index (CRI). Must resist crushing under the weight of the overlying burden (several meters of ore, limestone, and coke in the furnace shaft). Lower strength than desired limits furnace stack height, explaining why charcoal blast furnaces could not be scaled up as large as coke-fired furnaces. [CIT-COK-01; CIT-PI-03, pp. 95–100.]
  • production_temperature: Typically 1,000–1,100°C in industrial coke ovens (occasionally up to 2,000°C); coking time approximately 14–18 hours in modern slot ovens. [CIT-COK-01.]

Claims

Connections

Outgoing

  • Manufactured by → Cokemaking — Metallurgical coke is produced by high-temperature carbonization of coking coal in slot-type coke oven batteries. This is the primary industrial pathway for coke production.
  • Substitute for → Charcoal — Coke replaced charcoal as the primary blast furnace fuel/reductant, beginning with Abraham Darby’s use at Coalbrookdale, England (furnace brought into blast 10 January 1709). This substitution is one of the most consequential metallurgical transitions of the Industrial Revolution: it decoupled iron production from forest availability, enabled furnace scale-up (coke’s greater mechanical strength supports taller stacks), and drove the exponential growth of ironmaking capacity in Britain and globally. Constraints and limits of the substitution: (1) Coke transfers more sulfur to pig iron than charcoal does (<0.1 wt% S in charcoal vs. <1 wt% target for coke) — early coke pig iron was unsuitable for finery forge wrought iron production due to silicon and sulfur levels; (2) charcoal-smelted iron remained preferred for high-quality iron (e.g., Swedish bar iron) into the 19th century; (3) charcoal blast furnaces persisted in Sweden (late 19th century) and North America (~1850) where forest resources were abundant. [CIT-COK-02 (Darby 1709, verified); CIT-PI-03 (Tylecote 1992, pp. 95-100); CIT-COK-01]

Incoming

  • Produces ← Cokemaking — Primary product; ~70-77 wt% of dry coal input. Hard, porous, ~87-92 wt% fixed carbon, lump 25-80 mm for blast furnace use. Characterized by CSR and CRI.
  • Requires input ← Blast Furnace Ironmaking — Metallurgical coke is a critical consumed input to blast furnace ironmaking, serving simultaneously as: (1) fuel for combustion at tuyeres, (2) reductant generating CO via the Boudouard reaction (CO2 + C → 2CO), and (3) structural support maintaining burden permeability. Modern coke rate: ~350-500 kg coke per tonne of pig iron produced (lower with high blast temperature and oxygen enrichment). [CIT-BF-01; CIT-COK-01]

Sources

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