# Boiler heat recovery: Science and thermodynamics

Why condensing the flue gas pushes efficiency past 90%

Source page: https://www.materialinsights.org/energy/e_boiler_recovery/rc-sci/

What this tab covers The thermodynamics: the grade (temperature or pressure) of the energy being recovered, energy versus exergy efficiency, and how harsh the waste stream is on equipment.

Boiler heat recovery captures three leaks around a steam or hot-water boiler: hot flue gas, hot blowdown water and hot condensate. The heat is medium to low grade, but it has an ideal user right next to it, the cold water feeding the boiler.

#### Grade of energy
Favourable

Boiler flue gas leaves at about 230 to 480 C before recovery and 70 to 230 C after it, so the stream is medium grade at the start and low grade by the end. Stacks often run 55 to 85 C hotter than the steam they make. Returned condensate is typically 55 to 107 C, and blowdown from a 10 bar (150 psig) boiler is about 186 C. All of these can heat feedwater or makeup water arriving at 10 to 15 C, so a use is always on site.^[6](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf)^[1](https://www.energy.gov/sites/prod/files/2014/05/f16/steam3_recovery.pdf)^[5](https://www.energy.gov/sites/prod/files/2014/05/f16/steam8_boiler.pdf)^[4](https://nrel.gov/docs/fy12osti/52761.pdf)
 Boiler exhaust before recovery 230 to 480 C^[6](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf) Boiler exhaust after recovery devices 70 to 230 C^[6](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf) Returned condensate 55 to 107 C^[5](https://www.energy.gov/sites/prod/files/2014/05/f16/steam8_boiler.pdf) Blowdown at 10 bar (150 psig) about 186 C^[4](https://nrel.gov/docs/fy12osti/52761.pdf)

#### Exergy versus energy efficiency
Favourable

A useful rule: boiler efficiency rises about 1 percentage point for every 22 C (40 F) drop in flue gas temperature. A feedwater economiser typically cuts fuel 5 to 10%. Going further and condensing the water vapour in the flue gas recovers its latent heat: a condensing economiser can lift overall efficiency above 90% and raise system efficiency by up to 10%, and ORNL reports that cooling flue gas from about 230 C to 41 C can take a boiler from 80% to 95% efficiency on a higher heating value basis. The exergy of 60 C heat is only about 10% of its energy (Carnot factor, calculated), but because it warms water that would otherwise be heated by fuel, the low quality is well matched to the job.^[1](https://www.energy.gov/sites/prod/files/2014/05/f16/steam3_recovery.pdf)^[2](https://www.energy.gov/sites/prod/files/2014/05/f16/steam26b_condensing.pdf)^[7](https://info.ornl.gov/sites/publications/Files/Pub164247.pdf)
 Efficiency gain per 22 C flue gas drop about 1 percentage point^[1](https://www.energy.gov/sites/prod/files/2014/05/f16/steam3_recovery.pdf) Feedwater economiser fuel saving 5 to 10%^[1](https://www.energy.gov/sites/prod/files/2014/05/f16/steam3_recovery.pdf) Condensing economiser overall efficiency above 90%^[2](https://www.energy.gov/sites/prod/files/2014/05/f16/steam26b_condensing.pdf) Carnot factor of 60 C heat about 0.10 (calculated)^[2](https://www.energy.gov/sites/prod/files/2014/05/f16/steam26b_condensing.pdf)

#### Media compatibility and corrosiveness
Mixed

Cooling flue gas below its acid dew point condenses corrosive acid on the tubes. Standard economisers therefore keep exit gas above about 121 C for natural gas, 149 C for coal and low-sulphur oil and 177 C for high-sulphur oil. Condensing units deliberately go below this and need stainless steel (316-type tubes, 304-type shells) or coatings; the condensed water is acidic and may need treatment before reuse or discharge. Plastic exchangers resist corrosion but are limited to about 104 C and 10 bar (150 psi). Natural gas is the easiest fuel; sulphur-bearing coal and oil, common in Indian boilers, limit how far heat can be recovered.^[1](https://www.energy.gov/sites/prod/files/2014/05/f16/steam3_recovery.pdf)^[7](https://info.ornl.gov/sites/publications/Files/Pub164247.pdf)
 Minimum stack temperature, natural gas about 121 C (250 F)^[1](https://www.energy.gov/sites/prod/files/2014/05/f16/steam3_recovery.pdf) Minimum stack temperature, high-sulphur oil about 177 C (350 F)^[1](https://www.energy.gov/sites/prod/files/2014/05/f16/steam3_recovery.pdf) Plastic exchanger limit about 104 C and 150 psi^[7](https://info.ornl.gov/sites/publications/Files/Pub164247.pdf)

#### Variants compared^[1](https://www.energy.gov/sites/prod/files/2014/05/f16/steam3_recovery.pdf)^[2](https://www.energy.gov/sites/prod/files/2014/05/f16/steam26b_condensing.pdf)^[7](https://info.ornl.gov/sites/publications/Files/Pub164247.pdf)^[3](https://www.energy.gov/sites/prod/files/2014/05/f16/steam10_boiler_blowdown.pdf)^[4](https://nrel.gov/docs/fy12osti/52761.pdf)^[5](https://www.energy.gov/sites/prod/files/2014/05/f16/steam8_boiler.pdf)

| Variant | What it recovers | Typical gain | Payback |
|---|---|---|---|
| **Feedwater economiser** | Sensible heat in flue gas, heating feedwater | 5 to 10% less fuel | Under 2 years |
| **Condensing economiser (indirect)** | Sensible and latent heat; water heated to about 93 C | Up to 10% more; overall above 90% | 8 months to 2 years |
| **Direct-contact flue-gas condenser** | Latent heat by spraying water through the gas | Boiler efficiency gain up to 20% | 2 to 4 years |
| **Blowdown heat recovery** | Hot water purged to control dissolved solids | About 90% of blowdown energy in DOE example | Good candidate where continuous blowdown exceeds 5% of steam flow |
| **Condensate return** | Hot condensate plus already-treated water | More than 10% of steam energy stays in condensate | Site-specific; often fast |

#### Sources

1. [Use Feedwater Economizers for Waste Heat Recovery, Steam Tip Sheet #3 (DOE/GO-102012-3412)](https://www.energy.gov/sites/prod/files/2014/05/f16/steam3_recovery.pdf) US DOE Advanced Manufacturing Office, 2012
2. [Considerations When Selecting a Condensing Economizer, Steam Tip Sheet #26B (DOE/GO-102012-3397)](https://www.energy.gov/sites/prod/files/2014/05/f16/steam26b_condensing.pdf) US DOE Advanced Manufacturing Office, 2012
3. [Recover Heat from Boiler Blowdown, Steam Tip Sheet #10 (DOE/GO-102012-3408)](https://www.energy.gov/sites/prod/files/2014/05/f16/steam10_boiler_blowdown.pdf) US DOE Advanced Manufacturing Office, 2012
4. [Install an Automatic Blowdown-Control System, Steam Tip Sheet #23 (DOE/GO-102012-3402)](https://nrel.gov/docs/fy12osti/52761.pdf) US DOE Advanced Manufacturing Office, 2012
5. [Return Condensate to the Boiler, Steam Tip Sheet #8 (DOE/GO-102012-3410)](https://www.energy.gov/sites/prod/files/2014/05/f16/steam8_boiler.pdf) US DOE Advanced Manufacturing Office, 2012
6. [Waste Heat Recovery: Technology and Opportunities in U.S. Industry (BCS Inc. for US DOE Industrial Technologies Program)](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf) US Department of Energy, 2008
7. [Technology Assessment on Low-Temperature Waste Heat Recovery in Industry, ORNL/TM-2021/2150 (Thekdi, Nimbalkar et al.)](https://info.ornl.gov/sites/publications/Files/Pub164247.pdf) Oak Ridge National Laboratory, 2021

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_boiler_recovery/index.md)
- [Overview](https://www.materialinsights.org/energy/e_boiler_recovery/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_boiler_recovery/correlation/index.md)
- [Engineering and integration](https://www.materialinsights.org/energy/e_boiler_recovery/rc-eng/index.md)
- [Economics and contracts](https://www.materialinsights.org/energy/e_boiler_recovery/rc-econ/index.md)
- [Impact and additionality](https://www.materialinsights.org/energy/e_boiler_recovery/rc-impact/index.md)
- [What to track](https://www.materialinsights.org/energy/e_boiler_recovery/tracking/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_boiler_recovery/links/index.md)