Material Insights · Understanding the Material World

Recuperators and regenerators: Engineering and integration

Why these devices suit big furnaces that never stop

What this tab covers Engineering and operating integration: how tightly it is coupled to the host process, how steady the waste stream is, and how hard it is to retrofit.

These devices sit inside the combustion loop, so they are tightly coupled to the furnace. They suit large, continuously fired furnaces best and are hardest to fit to small or batch units.

Process interdependence (coupling risk)

Mixed

The recuperator carries all the combustion air, so a cracked tube, a plugged passage or a stuck reversing valve on a regenerative burner pair directly limits firing. Designs therefore include a cold-air bypass or dilution air to protect the exchanger when exhaust runs too hot, and burners must be rated for hot air. Recuperative burners, which build the exchanger into the burner, are limited to small burners of under about 0.3 MW (1 MMBtu/h). Dirty or corrosive exhaust can plug or attack the exchanger, so its composition needs to be known before a design is chosen. Rotary air heaters on power boilers leak air into the flue gas through their seals: Ljungstrom units typically leak 8 to 12% by weight and Rothemuhle units 25 to 30%, and raising leakage from 6% to 16% increases auxiliary (station service) power by about 11%.54210

Ljungstrom air heater leakage about 8 to 12% by weight; rising with wear10 Recuperative burner size limit under about 0.3 MW (1 MMBtu/h)5 Regenerative burner switching interval several tens of seconds4

Load factor and intermittency

Favourable

Savings scale with operating hours and exhaust temperature. The DOE worked example assumes 8,000 hours a year, far above the 3,000 to 4,000 hour floor investors look for. Continuous glass and steel reheating furnaces fit well. Batch furnaces that cool between charges lose much of the benefit, because the exchanger and any brick regenerator must reheat each cycle.23

Operating hours in DOE example 8,000 h/yr2 Typical energy saving from furnace heat recovery 5 to 30%3

Footprint and retrofit complexity

Mixed

A metallic recuperator in the flue is a moderate retrofit, but raising preheat usually also means new burners, insulated hot-air ducts and controls, so it is often timed to a furnace rebuild. Regenerators are large and cost more than recuperators; a draft 2015 DOE assessment judges stationary regenerators worth their cost only above about 1,090 C and about 15 MW of firing. In glass, the regenerator chambers are part of the furnace structure and are replaced only during a full rebuild.251

System cost, 2.9 MW (10 MMBtu/h) furnace example USD 200,000 to 250,0002 Stationary regenerator threshold above about 1,090 C and 15 MW firing5

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. Install Waste Heat Recovery Systems for Fuel-Fired Furnaces, Process Heating Tip Sheet #8 US DOE Industrial Technologies Program, 2005
  4. Industrial furnaces, burners: regenerative burners (manufacturer R&D page; company claims, undated, about 2023) Daigas Group (Osaka Gas), 2023
  5. Quadrennial Technology Review 2015, Chapter 8 technology assessment: Waste Heat Recovery (draft, pre-decisional) US Department of Energy, 2015
  6. Power 101: Improving the Performance of Boiler Auxiliaries, Part I (Nenad Sarunac) POWER magazine, 2010