Energy-intensive industry: Engineering methods

Eight processes that swallow most industrial energy, compared

What this tab covers The engineering methods that can do this conversion, compared on efficiency, scale, economics, limitations and reliability.

Eight processes account for most industrial energy. They differ in whether the energy goes into heat, chemical reduction or electrochemistry.

Comparison

Method	Efficiency	Scale	Economics	Limitations	Reliability
Blast furnace and basic oxygen furnace (BF-BOF)¹²	70-80% of energy from coal	69.4% of world steel in 2025	Cheapest route where coking coal is available; very high capital cost per site	CO2 is inherent in carbon reduction	Runs continuously for multi-year campaigns between relines
Electric arc furnace (EAF) with scrap¹²³	About 0.67 MWh per tonne for 100% scrap; electricity is 50-70% of energy input	30.3% of world steel in 2025	Low capital cost; cost follows scrap and electricity prices	Limited by scrap supply and copper or tin contamination	Batch process that can stop and start, which suits flexible power use
Hydrogen direct reduction (H2-DRI) with EAF³⁴¹⁶¹⁷	3.48 MWh per tonne of liquid steel	All DRI totalled 140.8 Mt in 2024; hydrogen plants of about 2-2.5 Mt a year are being built	Competitive at about 40 EUR/MWh with a carbon price of 34-68 EUR/t	Needs high-grade pellets and large amounts of cheap clean electricity	Shaft furnaces are mature on natural gas; pure hydrogen at scale is new
Aluminium smelting (Hall-Heroult)⁵⁶	World average about 14.4 kWh/kg (calculated); best pilot 12.27 kWh/kg	Hundreds of cells in series per potline	Energy about 30% of cost	Anodes release CO2; potlines freeze if power is cut for hours	Cells run continuously for years
Cement kiln (preheater and precalciner)⁷⁸	3.6 GJ per tonne of clinker (2022)	World cement output 4,160 Mt in 2022	Fuel and electricity are major costs; plants are local because cement is heavy and cheap	Calcination gives 53% of emissions whatever the fuel	Kilns run continuously apart from planned refractory repairs
Haber-Bosch ammonia⁹	28 GJ/t net (best) to 41 GJ/t (average); about 1.8 t CO2/t with best gas reforming	185 Mt in 2020; about 2% of world final energy	Gas is the dominant cost	Near-zero-emission routes typically cost 10-100% more	Continuous plants running for years between turnarounds
Chlor-alkali electrolysis⁶¹⁴	About 2,130 kWh per tonne of caustic soda; voltage efficiency about 71% (calculated)	Euro Chlor members alone have 11 Mt of chlorine capacity	Electricity is the main operating cost	Chlorine and caustic soda come in a fixed ratio	Membranes are replaced every few years; plants can modulate load
Industrial motors and compressed air¹²¹³	Compressed air about 10% wire to work; pneumatic tools need 7-8 times the energy of electric tools	Motor systems used 53% of world electricity in 2023 and 72% of industrial electricity	Over 80% of compressor input becomes heat that can be recovered	Oversized motors, throttled flow and air leaks waste much of the input	Motors are very reliable; bearings and windings are the usual failure points
Industrial combined heat and power (CHP)²²	Typically 65-80% total, against 50-55% for separate heat and grid power	From a few MW to hundreds of MW	Pays when a site needs steady heat and power together	Usually burns fossil fuel; output tied to heat demand	Mature turbines and engines

Electrochemical routes (aluminium, chlor-alkali, hydrogen DRI) turn electricity directly into chemical change but need cheap, steady power. Carbon-based routes (blast furnace, cement kiln, gas ammonia) are cheaper today but emit CO2 by chemistry, not only by combustion; cement is hardest because about half its CO2 comes from limestone. Motors, compressed air and CHP are the easiest wins because their losses lie in system design rather than physics.

How each method works

Blast furnace and basic oxygen furnace (BF-BOF)

Coke and iron ore are charged into a tall shaft furnace with hot air blown in at the bottom; carbon monoxide strips oxygen from the ore and liquid iron collects at the base. A converter then blows oxygen through the iron to burn off excess carbon.

Electric arc furnace (EAF) with scrap

Graphite electrodes strike arcs to scrap, melting it batch by batch; oxygen and carbon injection add chemical energy.

Hydrogen direct reduction (H2-DRI) with EAF

Hydrogen from electrolysers flows up through a shaft of iron ore pellets at about 800 C and removes their oxygen as water vapour; the sponge iron is melted in an EAF.

Aluminium smelting (Hall-Heroult)

Alumina dissolved in molten cryolite at about 960 C is electrolysed with carbon anodes, which burn to CO₂; liquid aluminium collects at the bottom.

Cement kiln (preheater and precalciner)

Raw meal falls through cyclones heated by kiln gas, a precalciner drives out most CO₂, and a rotary kiln at 1,400-1,500 C fuses it into clinker, later ground with gypsum.

Haber-Bosch ammonia

Steam reforming of natural gas makes hydrogen, air supplies nitrogen, and the mixture reacts at high pressure over an iron catalyst; ammonia is condensed out and unreacted gas recycled.

Chlor-alkali electrolysis

Brine is split in cells: chlorine forms at the anode, hydrogen and caustic soda at the cathode, separated by an ion-exchange membrane (older plants use diaphragms or mercury).

Industrial motors and compressed air

Electric motors drive pumps, fans, compressors and conveyors; compressors pressurise air for tools and controls.

Industrial combined heat and power (CHP)

An on-site gas turbine or engine makes electricity and its exhaust heat raises steam for the process.

Sources

Energy use in the steel industry (fact sheet) World Steel Association, 2025
World Steel in Figures 2026 World Steel Association, 2026
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
World DRI production reaches 140.8 Mt in 2024 Midrex Technologies, 2025
Hydro Karmoy technology pilot successfully verified Norsk Hydro, 2020
Electrochemical Manufacturing in the Chemical Industry (Botte, The Electrochemical Society Interface, Fall 2014) The Electrochemical Society, 2014
Cement (tracking report) IEA, 2023
Decarbonising hard-to-abate sectors with renewables: Cement IRENA, 2024
Ammonia Technology Roadmap IEA, 2021
Policy brief: Electric motor systems, why are they important? IEA 4E Electric Motor Systems Annex (EMSA), 2025
Improving Compressed Air System Performance: A Sourcebook for Industry (third edition) US Department of Energy and Compressed Air Challenge, 2016
Our industry Euro Chlor, 2026
thyssenkrupp Steel signed a contract with SMS group for the construction of the DRI plant GMK Center, 2023
Dillinger and ROGESA choose Primetals Technologies as supplier for major decarbonisation project Primetals Technologies (company press release), 2024
CHP benefits US EPA Combined Heat and Power Partnership, 2025