Material Insights · Understanding the Material World

Energy-intensive industry: Science and limits

Why chemistry sets an energy floor nobody can dig under

What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.

Heavy industry uses energy for two jobs: supplying heat (to melt steel or fire cement clinker) and driving chemical reactions that remove oxygen or other elements from ores and molecules (iron ore to iron, alumina to aluminium, brine to chlorine, nitrogen to ammonia). Chemistry sets a hard minimum for the second job, and the best plants now use about 1.5 to 2.5 times that minimum.

The conversion, step by step

Blast furnace and oxygen converter
Chemical (coke and coal) → Heat plus chemical reduction (iron oxide to liquid iron), then steel · Steelmaking averaged 20.95 GJ per tonne of steel in 2024 (all routes together, down from about 50 GJ in the 1960s); coal supplies 70-80% of BF-BOF energy1Electric arc furnace: arcs melt scrap or DRI
Electrical (plus some chemical energy from oxygen and carbon) → Heat (molten steel) · About 0.67 MWh of electricity per tonne for 100% scrap against a melting minimum of roughly 0.38 MWh, so about 57% (calculated)3Hydrogen DRI: electrolyser, shaft furnace, then EAF
Electrical to chemical (hydrogen) → Chemical reduction (iron ore to solid iron), then heat · About 3.5 MWh of electricity per tonne of liquid steel, most of it for the electrolyser3Aluminium smelting (Hall-Heroult cell)
Electrical (DC) → Chemical (alumina to aluminium) plus heat · Cell at about 4.7 V against a 1.7 V decomposition voltage (about 36%, calculated); a 14.4 kWh/kg world average against the 6.16 kWh/kg theoretical minimum is about 43% (calculated)56Cement kiln: fuel calcines limestone and sinters clinker
Chemical (fuel) → Heat plus chemical change (CaCO3 to CaO plus CO2, then clinker minerals) · 3.6 GJ per tonne of clinker (world, 2022) against a chemical minimum of roughly 1.7-2.0 GJ, so about 50-55% (calculated); grinding adds about 91 kWh of electricity per tonne of cement (2014)715Ammonia: gas reforming plus Haber-Bosch loop
Chemical (natural gas) plus mechanical (compression) → Chemical (ammonia) · Best plants 28 GJ per tonne net against a minimum of 18.6 GJ, about 66%; world average 41 GJ, about 45% (calculated)9Chlor-alkali membrane cell
Electrical (DC) → Chemical (chlorine, caustic soda, hydrogen) · Cell at about 3.1 V against a 2.2 V decomposition voltage, about 71% (calculated)6Motor, compressor, then air tool
Electrical to mechanical to pneumatic → Mechanical work at the tool · Wire-to-work efficiency around 10%13

The science

Most metals occur as oxides because oxygen binds to them strongly. Breaking that bond takes a fixed amount of energy set by the reaction's enthalpy (heat content) and Gibbs free energy (the part that must be supplied as work, for example as electricity). In a blast furnace carbon from coke takes the oxygen and leaves as CO2; in hydrogen direct reduction hydrogen takes it and leaves as water vapour. Electrolysis does the job with electrons: Faraday's law fixes the charge per tonne (three electrons per aluminium atom), so energy use depends on cell voltage, and any voltage above the decomposition voltage becomes heat. Cement is different: most of its fuel drives calcination, which releases CO2 from limestone whatever fuel is used. Ammonia synthesis itself releases a little heat; the energy goes mainly into making hydrogen from natural gas and compressing the gases to 100-300 bar.

Limits set by science

LimitValueWhy it existsBest today
Minimum energy to reduce iron ore with hydrogenAbout 54 kg H2 per tonne of iron, about 1.8 MWh of hydrogen energy, plus about 0.9 GJ of reaction heat (calculated from Fe2O3 + 3H2 → 2Fe + 3H2O, deltaH about +99.5 kJ/mol)3The hydrogen demand is fixed by chemistry; making it in a 70% efficient electrolyser takes about 2.6 MWh (calculated, assumed efficiency).Modelled plant: 3.48 MWh of electricity per tonne of liquid steel in total
Faraday limit for aluminiumAbout 2,980 ampere-hours per kg of aluminium (calculated: 3 x 96,485 C/mol x 37.1 mol/kg); theoretical minimum energy in a carbon-anode cell 6.16 kWh/kg56Bath resistance, gas bubbles and electrodes push real cells to about 4.7 V, and the extra voltage ends up as heat.Hydro's Karmoy pilot cells reached 12.27 kWh/kg against a target set about 15% below the world average
Minimum heat for clinkerRoughly 1.7-2.0 GJ per tonne of clinker (calculated: calcining about 1.16 t of limestone absorbs about 2.1 GJ, partly offset by heat released as clinker minerals form)78Calcination also releases about 0.51 t of CO2 per tonne of clinker from the rock itself (calculated), and gives 53% of cement-making emissions.World average 3.6 GJ/t clinker in 2022; IEA net-zero milestone 3.4 GJ/t by 2030
Theoretical minimum for ammonia from methane18.6 GJ per tonne net (20.9 GJ gross)9Set by the methane that must become hydrogen plus the reaction energies; the rest of today's use is heat lost in reforming, compression and separation.Best gas reforming 28 GJ/t net; coal gasification 36 GJ/t net
Decomposition voltage for brine electrolysisAbout 2.2 V6Every volt above this is lost as heat. At 3.1 V a membrane cell needs about 2,340 kWh of DC per tonne of chlorine at full current efficiency (calculated from Faraday's law).Membrane cells about 3.1 V and 2,130 kWh per tonne of caustic soda

Sources

  1. Energy use in the steel industry (fact sheet) World Steel Association, 2025
  2. Assessment of hydrogen direct reduction for fossil-free steelmaking (Vogl, Ahman and Nilsson, Journal of Cleaner Production 203, 736-745) Elsevier (open copy on Zenodo: zenodo.org/record/2669111), 2018
  3. Hydro Karmoy technology pilot successfully verified Norsk Hydro, 2020
  4. Electrochemical Manufacturing in the Chemical Industry (Botte, The Electrochemical Society Interface, Fall 2014) The Electrochemical Society, 2014
  5. Cement (tracking report) IEA, 2023
  6. Decarbonising hard-to-abate sectors with renewables: Cement IRENA, 2024
  7. Ammonia Technology Roadmap IEA, 2021
  8. Improving Compressed Air System Performance: A Sourcebook for Industry (third edition) US Department of Energy and Compressed Air Challenge, 2016
  9. Technology Roadmap: Low-Carbon Transition in the Cement Industry IEA and WBCSD Cement Sustainability Initiative, 2018