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Steel plant energy recovery: Science and thermodynamics

Red-hot coke, pressurised gas and molten slag compared

What this tab covers The thermodynamics: the grade (temperature or pressure) of the energy being recovered, energy versus exergy efficiency, and how harsh the waste stream is on equipment.

An integrated blast furnace steelworks throws off unusually high-grade waste: red-hot coke, hot sinter, molten slag, pressurised top gas and three combustible gases. That makes it among the easiest places in industry to recover energy.

Grade of energy

Favourable

The streams are mostly far above the 100 C threshold where recovery gets hard. Coke leaves the ovens at about 1,000 to 1,200 C, depending on the source, and molten slag is tapped at around 1,500 C. Blast furnace top gas leaves at only about 200 C, but it carries useful pressure of 1.6 to 3 bar above atmospheric. The by-product gases are fuels, not heat. Coke oven gas carries about 5 kWh per normal cubic metre (Nm3, a cubic metre at standard temperature and pressure), converter gas about 3 kWh/Nm3, and blast furnace gas less than 1 kWh/Nm3.237811

Coke discharge temperature about 1,000-1,100 C (IspatGuru); about 1,200 C (CTCN)211 Molten slag temperature about 1,500 C7 BF top gas pressure about 1.6-3 kg/cm2 gauge, about 200 C3 Gas heating values coke oven about 5, converter about 3, blast furnace about 0.9 kWh/Nm38

Exergy versus energy efficiency

Favourable

Coke dry quenching (CDQ) cools the coke with circulating inert gas instead of a water spray, and a boiler turns the heat into steam. CTCN reports about 150 GWh a year of power for a plant handling 1 Mt of coke a year, about 150 kWh per tonne (calculated). Nippon Steel Engineering quotes about 18 MW for a 100 t/h unit, about 180 kWh per tonne (calculated; vendor figure). A top-pressure recovery turbine (TRT) converts pressure, a high-quality (high-exergy) form of energy, straight into power. A practitioner source gives 15 to 60 kWh per tonne of hot metal. Sinter cooler recovery is lower grade and yields about 22 kWh per tonne of sinter from about 60,000 kcal of recovered heat per tonne. Slag holds about 1.8 GJ per tonne, about 500 kWh (calculated), but none of it is yet captured at commercial scale.3471112

CDQ power, CTCN about 150 GWh/y per 1 Mt/y coke, about 150 kWh/t (calculated)11 CDQ power, vendor about 18 MW per 100 t/h, about 180 kWh/t (calculated)12 TRT output 15-60 kWh/t hot metal (second-hand)3 Sinter cooler power 22.1 kWh/t sinter4

Media compatibility and corrosiveness

Mixed

Every stream here is dirty. Raw top gas carries about 5 g of dust per Nm3, and cleaning must bring it below 5 mg before the turbine. Dry gas cleaning leaves the gas hotter than wet scrubbing, so dry-type TRTs make more power. Kawasaki lists dry-type units at 8 to 31.5 MW against 5 to 14 MW for its wet-type range. CDQ handles abrasive coke dust inside a sealed loop. Converter gas is 60 to 65% carbon monoxide, so it is toxic and needs safety systems wherever it is handled. Slag recovery fails mainly on materials: molten slag must be broken up and solidified fast so it stays glassy and can be sold to cement makers.37814

BF top gas dust about 5 g/Nm3 raw, under 5 mg/Nm3 cleaned3 TRT unit range, dry vs wet (vendor) 8-31.5 MW dry vs 5-14 MW wet14 Converter gas CO content about 60-65%8

Recovery routes compared3478111214

RouteSource streamTypical yieldStatus
Coke dry quenchingCoke at about 1,000-1,200 CAbout 150-180 kWh/t coke (calculated)Mature; 24 Japanese and 102 overseas units by one vendor (2018); about 40% of Chinese steel-plant coke ovens (CTCN, data from about 2006)
Sinter cooler heatHot sinter cooling air22.1 kWh/t sinterCommercial, used in India
Top-pressure turbineBF top gas, 1.6-3 bar gauge15-60 kWh/t hot metalMature; units up to about 35 MW
Converter (BOF) gasAbout 50 Nm3/t steel at about 3 kWh/Nm3About 55 kWh/t steel as power (calculated, at 37% engine efficiency)Mature
Dry slag granulationSlag at about 1,500 C, 1.8 GJ/tNot yet commercial20 t/h demonstration in China, 2019

Sources

  1. Dry cooling of coke (practitioner article, second-hand) IspatGuru (Satyendra Kumar Sarna)
  2. Blast furnace gas top pressure recovery turbine (practitioner article, second-hand) IspatGuru (Satyendra Kumar Sarna)
  3. Sinter plant heat recovery (power generation from sinter cooler waste heat) UNFCCC Climate Technology Centre and Network (CTCN)
  4. Dry slag granulation CSIRO
  5. Steel production gases (application note; vendor claims) Clarke Energy
  6. Coke dry quenching for iron and steel sector UNFCCC Climate Technology Centre and Network (CTCN)
  7. Coke Dry Quenching (CDQ) technology sheet (vendor claims) Nippon Steel Engineering, via JASE-W (Japanese Business Alliance for Smart Energy Worldwide), 2018
  8. Top pressure recovery turbines (product page; vendor claims) Kawasaki Heavy Industries