Preheating combustion air
Combustion air preheating recovers heat from a furnace or boiler exhaust and uses it to warm the incoming combustion air before it reaches the burner. Hotter combustion air carries energy into the flame that the fuel no longer has to supply, so preheating directly raises fuel efficiency on high-temperature fired processes.
What it is
Air drawn into a burner is normally at ambient temperature and must be heated by the flame itself. An air preheater — a recuperator or regenerator placed in the exhaust path — transfers waste exhaust heat into that incoming air. The practice is most powerful on high-temperature furnaces, where exhaust gases are very hot and a large fraction of the heat can be recycled into the air stream.
Why it is done
On a hot furnace the flue gases leave at extreme temperatures, carrying away energy that scales with how hot the process runs. Returning that heat through the combustion air reduces the fuel needed to reach flame temperature and lifts the flame temperature achievable, which can also raise throughput. It is a cornerstone efficiency measure for furnaces, kilns and high-temperature reactors.
How it is done
Exhaust temperature, flow and dust loading are characterised, and a preheater type is chosen — recuperative for continuous duty, regenerative for the highest temperatures. The preheater is integrated into the exhaust and air paths with attention to the higher metallurgical demands of hot air ducting and burners. Burner settings are re-tuned for the preheated air, since hotter air changes the air-fuel relationship, and fuel saving is verified against baseline.
- Characterise exhaust
- Select preheater type
- Integrate hot-air ducting
- Upgrade burner materials
- Re-tune air-fuel ratio
- Verify fuel saving
What to watch for
Preheated air burns hotter, which can raise nitrogen-oxide emissions and overheat burner components not rated for it — both must be designed for. Dust and fouling in the exhaust quickly clog a recuperator if cleaning provision is omitted.
Related practices
Running a compressed-air leak survey programme
Retrofitting waste-heat recovery
Retrofitting variable-speed drives
Related topics
Waste Heat Recovery in Industry: Methods and Where It Pays · Flue Gas Loss · Heat Exchanger · Specific Energy Consumption (SEC)
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