Material Insights · Understanding the Material World

Recuperators and regenerators: Impact and additionality

Big carbon cuts, with a NOx catch and lock-in risk

What this tab covers The impact and additionality lens: direct emission cuts, side benefits for water and air, and the risk of prolonging the life of a polluting host plant.

Air preheating cuts on-site fuel burn directly, by a fifth to a half on hot furnaces. The main side effect is higher NOx, and the main strategic risk is that it makes fossil-fired furnaces cheaper to keep running.

Direct Scope 1 and Scope 2 abatement

Favourable

Savings are pure Scope 1 (on-site combustion). In the DOE example the 22% cut equals about 17,600 MMBtu of gas a year, or roughly 930 t CO2 a year at 0.0053 t CO2 per therm (calculated). The 2008 DOE report puts the efficiency gain from furnace heat recovery at 10 to as much as 50%. ASME estimated in 1995 that all Ljungstrom air heaters in service had saved fuel equal to about 4.5 billion tonnes of oil. DOE also estimates about 1,500 TBtu (about 440 TWh, calculated) a year of unrecovered exhaust heat in the US processes it studied.2619

CO2 avoided, DOE 2.9 MW furnace example about 930 t/yr (calculated)26 Furnace efficiency gain 10 to 50%1 Unrecovered US exhaust losses (studied processes) about 1,500 TBtu/yr1 Cumulative fuel saved by Ljungstrom air heaters (oil equivalent, 1995 estimate) about 4.5 billion tonnes9

Water and air synergy

Mixed

Burning less fuel shrinks flue gas volume and all combustion pollutants in proportion. Hotter combustion air, however, raises flame temperature and thermal NOx, which is why suppliers sell low-NOx regenerative burners as standard. Air heaters downstream of NOx catalysts suffer ammonium bisulphate deposits. No water is consumed, and recovered heat no longer warms the surroundings.410

Fuel saving that also cuts flue gas volume 35 to 50% (regenerative burners)4

Avoided lock-in

Mixed

Preheating lowers the running cost of gas- or oil-fired furnaces and may delay electric or hydrogen alternatives, particularly in glass where each furnace rebuild commits the plant to one technology for a long campaign. Against that, the capital is small and repaid within two years, so it rarely tips a closure decision. Investors should prefer projects tied to furnaces that are already low-carbon-ready (for example hydrogen-capable burners).2

Typical payback limiting stranded capital 15 to 19 months2

Sources

  1. Waste Heat Recovery: Technology and Opportunities in U.S. Industry (BCS Inc. for US DOE Industrial Technologies Program) US Department of Energy, 2008
  2. Preheated Combustion Air, Process Heating Tip Sheet #1 (DOE/GO-102007-2482) US DOE Industrial Technologies Program, 2007
  3. Industrial furnaces, burners: regenerative burners (manufacturer R&D page; company claims, undated, about 2023) Daigas Group (Osaka Gas), 2023
  4. Not Just Hot Air: Low-Cost Decarbonization through Heat Recovery (Zahlan and Talan, Cadmus), ACEEE Summer Study on Energy Efficiency in Industry ACEEE, 2021
  5. Ljungstrom Air Preheater, Stockholm 1920: International Historic Mechanical Engineering Landmark brochure ASME, 1995
  6. Power 101: Improving the Performance of Boiler Auxiliaries, Part I (Nenad Sarunac) POWER magazine, 2010