Hyphae

Cokemaking

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The industrial process of heating coking coal (metallurgical coal) in the absence of air at temperatures of 900–1,100 °C to drive off volatile matter, leaving behind metallurgical coke — a hard, porous, carbon-rich solid. The dominant modern method uses horizontal slot-type by-product recovery ovens arranged in batteries of 20–100 chambers; each coking cycle takes approximately 15–18 hours. Older beehive ovens required 48–72 hours. Byproducts recovered from by-product ovens include coke oven gas (COG), coal tar, ammonia, benzene, and other aromatic compounds, which have commercial value as fuels and chemical feedstocks. Cokemaking is the necessary upstream process that converts raw coking coal into the metallurgical coke required by blast furnaces for pig iron production.

Conditions

Oven temperature: 900–1,100 °C (coking zone); up to 2,000 °C at flue walls. Oven atmosphere: reducing (COG atmosphere maintained by positive back pressure). Charging coal moisture: typically 8–12 wt%; pre-drying or stamp-charging used in some operations to improve bulk density and coke quality.

Duration

Modern by-product slot ovens: 15–18 hours per coking cycle (extendable to 24 hours at low demand). Beehive ovens: 48–72 hours. Oven batteries operate continuously; individual ovens are charged and pushed in sequence to maintain steady COG flow.

Equipment

  • Slot-type by-product coke oven battery — the dominant modern cokemaking technology; a battery of 20–100 horizontal slot chambers (~15–20 m long × 6–9 m high × 500–600 mm wide), alternating with heating flues. Chambers are heated externally through the flues by burning COG; ovens are pushed sequentially. First built ~1856; basic design completed by 1940s.
  • Beehive coke oven — older batch design; domed firebrick chamber ~4 m wide × 2.5 m high; coal charged through top hole; volatile matter burns inside chamber providing heat without byproduct recovery. Still used in some regions for non-recovery cokemaking.
  • Non-recovery / heat-recovery coke oven — modern alternative to by-product ovens; volatile matter burned on-site with heat recovered as steam; no chemical byproduct recovery. Used where byproduct markets are unfavorable.
  • Charging car — overhead rail vehicle that loads crushed coal into oven chambers from the top.
  • Pusher machine — large ram that pushes the finished coke cake out of the oven chamber at the end of the coking cycle.
  • Quench car and quench tower — transport and water-quench the incandescent coke emerging from the oven.
  • Byproduct recovery plant — series of scrubbers, absorbers, and distillation columns downstream of the collecting main that extract tar, ammonia, benzene, and other chemicals from raw COG.

Hazards

  • Coke oven emissions (PAHs, benzene, HCN) — carbonization generates polynuclear aromatic hydrocarbons (PAHs), benzene, hydrogen cyanide, and other volatile carcinogens; classified as Group 1 human carcinogens by IARC. OSHA PEL: 0.150 mg/m³ benzene-soluble fraction (8-hr TWA); NIOSH REL: 0.2 mg/m³. Coke oven workers have elevated rates of lung, bladder, and kidney cancer. Key exposure points: charging, leveling, pushing, door removal, topside work.
  • Coke oven gas (COG) explosion and asphyxiation — raw COG is ~50% hydrogen; highly flammable and explosive. Purified COG distributed throughout the steel plant is a constant explosion/asphyxiation risk. Positive back pressure maintained in ovens to prevent air ingress; fixed and personal CO/H₂ monitors required.
  • Radiant heat and burns — pushed coke is ~1,000 °C; pushing and quenching operations expose workers to intense radiant heat and possible steam burns. Full protective gear required.
  • Coal dust explosion — crushed coal handling generates fine dust that can form explosive dust clouds. Dust suppression and ventilation required in coal handling areas.

Inputs

  • Coking coal (metallurgical coal) — low-ash, low-sulfur bituminous coal with plasticity and caking properties; volatile matter 26–29 wt% in the blend. Primary raw material, consumed in the process.
  • Energy for oven heating — in by-product recovery ovens, the ovens are heated by burning a portion of the COG produced; makeup fuel (natural gas or blast furnace gas) used at startup.

Outputs

  • Metallurgical coke — hard, porous, carbon-rich solid (~87–92 wt% fixed carbon dry ash-free); lump size 25–80 mm for BF use; characterized by CSR and CRI. Primary product.
  • Coke oven gas (COG) — hydrogen-rich fuel gas (~50–55 vol% H₂, ~25–30 vol% CH₄, ~5–7 vol% CO); recovered from by-product ovens; high calorific value (~17–19 MJ/m³); used as plant fuel.
  • Coal tar — heavy viscous liquid; feedstock for tar chemicals (anthracene, pyrene, pitch for electrodes), carbon black, and road materials.
  • Ammonia (NH₃) — recovered as ammonium sulfate or anhydrous ammonia; used in fertilizer manufacture.
  • Light oil (benzene, toluene, xylene) — aromatic chemical feedstocks recovered by wash-oil scrubbing of COG.
  • Coke breeze — fines (<6 mm) separated during coke screening; used in iron ore sintering.

Prerequisites

  • Coal characterization and quality assessment — coking suitability requires assay for volatile matter, ash, sulfur, plasticity (Gieseler or Audibert-Arnu plastometer), and dilatometry. Blending metallurgy is a specialist skill.
  • Refractory design and oven maintenance — coke oven silica refractory must withstand repeated thermal cycling; oven rebuilding requires specialized expertise.
  • Byproduct chemistry — effective byproduct recovery requires knowledge of coal tar chemistry, ammonia absorption, and benzene scrubbing unit operations.

Steps

  1. Coal selection and blending
    • description: Coking coal (low-ash, low-sulfur bituminous coal with plastic/caking properties) is selected and blended to achieve a target volatile matter content of approximately 26–29 wt%. Different coal types with varying maceral composition and rheological properties are blended to optimize coke strength (CSR) while avoiding excessive wall pressure from coal swelling during coking. Coal must be crushed to approximately 80% minus 3 mm with a top size of ~15 mm before charging.
  2. Charging the ovens
    • description: Crushed, blended coal is loaded along the top of the oven chambers using a charging car on rails. In modern slot ovens, each chamber is approximately 15–20 m long, 6–9 m high, and 500–600 mm wide. Coal is leveled by a retractable bar to form an even charge. In beehive ovens (older design), coal is dropped through a top hole to form a layer 600–900 mm deep and is leveled through a side door.
  3. High-temperature carbonization
    • description: The oven is heated externally (in by-product ovens) through heating flues between chambers. Temperatures reach 900–1,100 °C (occasionally up to 2,000 °C in oven walls); typical BF-coke production uses the higher end of the 900–1,100 °C range. Carbonization proceeds as heat transfers through the coal mass: volatile matter distils out as raw coke oven gas (COG) through collecting mains. A positive back pressure is maintained to prevent air ingress. In beehive ovens, the volatile matter burns inside the partially-closed side door, providing heat but sacrificing byproduct recovery. Coking time in modern by-product slot ovens: 15–18 hours for BF coke; can be extended to 24 hours during low-demand periods. Beehive ovens require 48–72 hours.
  4. Byproduct recovery (by-product ovens only)
    • description: Raw COG exiting the ovens passes through the collecting main into a byproduct recovery plant where it is cooled and cleaned in stages: (a) flushing liquor scrubbing removes ammonia and coal tar; (b) electrostatic precipitators remove tar mist; (c) ammonia absorption/stripping; (d) benzene scrubbing (wash oil absorber). Recovered products include coal tar (feedstock for tar chemicals, carbon black, electrode pitch), ammonia (for fertilizer or ammonium sulfate), light oils including benzene and toluene (chemical feedstocks), and purified COG (used as fuel for heating the ovens themselves and for other plant energy needs). Non-recovery/heat-recovery ovens burn all volatiles to generate steam rather than recovering individual chemicals.
  5. Pushing and quenching
    • description: At the end of the coking cycle, the incandescent coke mass (~1,000 °C) is pushed from the oven by a ram on the pusher machine and falls into a quench car. The coke is transported to the quench tower where it is wet-quenched with water (rapid cooling) or dry-quenched (CDQ — coke dry quenching — using inert gas to recover heat). Wet quenching produces steam and surface water; CDQ recovers significant heat energy as high-pressure steam and produces a more uniformly quenched coke with better CSR.
  6. Screening and sizing
    • description: Quenched coke is screened to remove fines (coke breeze, <6 mm) and sized for blast furnace use (typically 25–80 mm lump). Coke breeze is diverted to the sinter plant. Oversized material is crushed and re-screened. Coke quality is characterized by coke reactivity index (CRI) and coke strength after reaction (CSR), measured by standardized laboratory tests.

Variants

  1. By-product recovery (slot oven) — dominant modern method
    • description: Horizontal slot-type oven batteries with external heating through flues. Volatiles collected and separated into commercial products (COG, tar, ammonia, benzene). First built ~1856; design matured by 1940s. Produces highest-quality metallurgical coke with best process control.
  2. Beehive oven — historical / developing-world
    • description: Domed firebrick batch ovens. Volatile matter burned inside chamber; no byproduct recovery. Simple construction. Coking time 48–72 hours. Still operated in some regions (notably India) due to lower capital cost, despite worse environmental performance.
  3. Non-recovery / heat-recovery oven
    • description: Volatiles combusted on-site with heat recovered as steam for power generation. No chemical byproducts sold. Lower capital cost than by-product ovens; better environmental profile than beehive (contained combustion). Used where byproduct markets do not justify recovery plant investment.
  4. Stamp-charging
    • description: Instead of gravity-charging loose coal, coal is compacted (‘stamped’) outside the oven into a dense cake that is then pushed into the chamber. Increases bulk density, enables use of weakly-coking coals, and can improve CSR. Common in Germany and India.

Yield

Approximately 70–77 wt% of dry coal input reports as coke; 23–30 wt% is driven off as volatile matter (recovered as COG, tar, ammonia, light oil, and water). Specific yield depends on coal volatile matter content and oven conditions.

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