# Waste-heat power (steam Rankine): Science and thermodynamics

Why steam cycles capture only part of the heat's potential

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

Why steam cycles capture only part of the heat's potential 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.

Steam Rankine recovery works best on hot, steady exhaust above roughly 230 C, such as gas turbine exhaust, cement kiln preheater and cooler gas, and large engine exhaust. Steam cycles capture only a fraction of the theoretical maximum, and dusty or acidic gas sets the design.

#### Grade of energy
Favourable

The US DOE counts industrial waste-heat-to-power potential only for streams above 450 F (about 230 C), because cooler heat is hard to turn into power with steam. Indian cement kilns release two medium-grade streams: preheater exit gas at about 290-320 C on a five-stage preheater and clinker cooler exhaust at about 280-350 C. Gas engine exhaust is hotter, at 700-1,000 F (about 370-540 C), while the jacket water and oil coolers on the same engine run below 300 F (about 150 C) and are too cool for steam; Korlak's marine engine study likewise finds exhaust is the only engine stream hot enough for a steam cycle. Gas turbine exhaust feeding a heat recovery steam generator (HRSG) is the classic high-grade case.^[1](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/Waste_Heat_to_Power_Fact_Sheet.pdf)^[2](https://www.greenbusinesscentre.com/doc/resources/publicationsbl/23.pdf)^[13](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/CHP_Reciprocating_Engines.pdf)^[14](https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-756de647-bc5e-4a75-80b1-3c95ad960df5/c/Korlak_Comparative_2_2023.pdf)
 Minimum stream temperature counted by DOE above 450 F (about 230 C)^[1](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/Waste_Heat_to_Power_Fact_Sheet.pdf) Cement preheater exit gas (5-stage) about 290-320 C^[2](https://www.greenbusinesscentre.com/doc/resources/publicationsbl/23.pdf) Cement cooler exhaust about 280-350 C^[2](https://www.greenbusinesscentre.com/doc/resources/publicationsbl/23.pdf) Gas engine exhaust 700-1,000 F (about 370-540 C)^[13](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/CHP_Reciprocating_Engines.pdf)

#### Exergy versus energy efficiency
Favourable

Energy efficiency counts every kWh of heat; exergy counts only the share that could ever become work, which is set by the Carnot limit between source and surroundings. DOE says real Rankine cycles reach 30-50% of Carnot: for a 900 F (about 480 C) source, Carnot is 60% and actual conversion is 18-30%. For cement exhaust near 300 C rejecting to 30 C, the Carnot limit is about 47% (calculated), so a steam cycle would turn roughly 14-24% of the recovered heat into power (calculated). In a combined cycle, the HRSG and steam turbine lift a 7HA gas turbine from 42.6-43.3% alone to 63-64% net, so the steam side supplies about a third of plant output (calculated).^[1](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/Waste_Heat_to_Power_Fact_Sheet.pdf)^[5](https://www.gevernova.com/gas-power/products/gas-turbines/7ha)
 Rankine efficiency relative to Carnot 30-50%^[1](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/Waste_Heat_to_Power_Fact_Sheet.pdf) Actual conversion, 900 F source 18-30%^[1](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/Waste_Heat_to_Power_Fact_Sheet.pdf) 7HA simple cycle vs combined cycle 42.6-43.3% vs 63-64% net^[5](https://www.gevernova.com/gas-power/products/gas-turbines/7ha) Steam share of combined-cycle output about one third (calculated)^[5](https://www.gevernova.com/gas-power/products/gas-turbines/7ha)

#### Media compatibility and corrosiveness
Mixed

Clean natural gas turbine exhaust is easy on an HRSG. Cement kiln gas is not: dust loads of about 35-125 g per normal cubic metre (depending on cyclone efficiency of 88-97%) mean WHR boilers need vertical tube layouts, hoppers and rapping or soot blowing to stop tubes fouling. Sulphur in fuel or raw meal sets a minimum outlet temperature to avoid acid condensation, which caps how far the gas can be cooled. Engine exhaust from heavy fuel oil carries soot and sulphur that foul economisers. DOE notes that stream composition and variability are a main design factor for any waste-heat-to-power project.^[1](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/Waste_Heat_to_Power_Fact_Sheet.pdf)^[2](https://www.greenbusinesscentre.com/doc/resources/publicationsbl/23.pdf)
 Cement kiln gas dust load about 35-125 g/Nm3^[2](https://www.greenbusinesscentre.com/doc/resources/publicationsbl/23.pdf) Cyclone efficiency range behind that dust load 88-97%^[2](https://www.greenbusinesscentre.com/doc/resources/publicationsbl/23.pdf)

#### Variants compared^[2](https://www.greenbusinesscentre.com/doc/resources/publicationsbl/23.pdf)^[3](https://gccassociation.org/2050-net-zero-roadmap-one-year-on/action-progress-case-study-cnbm/)^[5](https://www.gevernova.com/gas-power/products/gas-turbines/7ha)^[7](https://www.powermag.com/wrtsils-latest-combustion-engine-offers-new-options-for-intermediate-capacity)^[13](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/CHP_Reciprocating_Engines.pdf)^[6](https://wartsila.com/services-catalogue/engine-services-4-stroke/wartsila-flexicycle-upgrade)^[15](https://www.cseindia.org/content/downloadreports/13236)

| Variant | Typical heat source | Scale | What recovery adds |
|---|---|---|---|
| **Combined cycle (HRSG plus steam turbine)** | Gas turbine exhaust | Hundreds of MW per block | Efficiency from about 43% to 63-64% |
| **Cement WHR boiler with steam turbine** | Preheater gas 290-320 C and cooler air 280-350 C | Several MW to about 13 MW per line | 29-30 kWh per tonne of clinker in a new CNBM design; 30-35 kWh/t in Indian plants (CSE) |
| **Steam bottoming of large gas engines (e.g. Wartsila Flexicycle)** | Engine exhaust 370-540 C | 100-500 MW engine plants | Efficiency from better than 48% to better than 50%; vendor claims up to 10% gain |

#### Sources

1. [Waste Heat to Power, CHP Technology Fact Sheet Series (DOE/EE-2347)](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/Waste_Heat_to_Power_Fact_Sheet.pdf) US Department of Energy, 2021
2. [Manual on Waste Heat Recovery in Indian Cement Industry](https://www.greenbusinesscentre.com/doc/resources/publicationsbl/23.pdf) Confederation of Indian Industry, Sohrabji Godrej Green Business Centre, 2009
3. [2050 Net Zero Roadmap one year on, action progress case study: CNBM, using new WHR power generation technology](https://gccassociation.org/2050-net-zero-roadmap-one-year-on/action-progress-case-study-cnbm/) Global Cement and Concrete Association, 2022
4. [7HA gas turbine (product page)](https://www.gevernova.com/gas-power/products/gas-turbines/7ha) GE Vernova, 2026
5. [Wartsila Flexicycle upgrade (service catalogue, company claim)](https://wartsila.com/services-catalogue/engine-services-4-stroke/wartsila-flexicycle-upgrade) Wartsila, 2025
6. [Wartsila's latest combustion engine offers new options for intermediate capacity](https://www.powermag.com/wrtsils-latest-combustion-engine-offers-new-options-for-intermediate-capacity) POWER Magazine, 2012
7. [Reciprocating Engines, CHP Technology Fact Sheet Series (DOE/EE-2764)](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/CHP_Reciprocating_Engines.pdf) US Department of Energy, 2023
8. [Comparative analysis of the heat balance results of the selected Tier III-compliant gas-fuelled two-stroke main engines (Korlak), Combustion Engines 193(2)](https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-756de647-bc5e-4a75-80b1-3c95ad960df5/c/Korlak_Comparative_2_2023.pdf) Combustion Engines (journal), 2023
9. [Cementing the Transition: the role of energy efficiency in decarbonising India's cement sector](https://www.cseindia.org/content/downloadreports/13236) Centre for Science and Environment, 2026

## Other sections

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