Heat recovery in the cement industry
Background
According to a World Bank report, waste heat recovery can reduce the operating costs and improve EBITDA margins of cement factories by about 10 to 15 percent. This represents a relatively significant advantage when competing on price with other cement manufacturers.
Accordingly, the cement industry has a large number of heat recovery installations in use around the world. Probably the most common form of heat recovery at cement plants is to use the Clinker and/or Cooler heat to generate electricity with a steam or organic rankine cycle turbine. (World Bank)
Benefits of waste heat recovery projects at cement facilities:
- Reduces purchased power consumption (or reduces reliance on fossil-fuel-based captive power plants)
- Mitigates the impact of future electric price increases
- Enhances plant power reliability
- Improves plant competitive position in the market
Heat Sources
Typical sources of waste heat for both old and small-sized shaft furnace types of units and plants that use the dry process are as follows:
- Clinker cooling air from cooling bed: The air temperature varies from 350°–900°F (180°-480°C). Waste heat recovery technologies used are preheating air from coal- and fuel-fired boilers to raise steam for power generation and for other auxiliary heating.
- Exhaust gases from the system, usually from the preheaters: These gases contain a high percentage of CO2 generated from the calcining reaction and products of combustion from fuels used in the kiln and precalciner. The gas temperature can vary from 300°–500°F (150°-260°C). In most cases, the exhaust gases are mixed with clinker cooling air and the heat is recovered in boilers or other heat recovery units.
- Hot shell or surfaces of kilns and precalciner: The surface temperature can vary from 500°–800°F (260°-430°C). There is no practical way to recover this heat yet. The best solution is to redesign the insulation-refractory system to reduce the kiln shell temperature. (US ORNL)
Economics
As an example, for a typical cement plant producing 5,000 tons per day, a waste heat recovery system can reliably supply about a third of the plant’s power needs. Based on an average electric grid price of 0.12 USD/kWh and a cost of operating a cogeneration unit at 0.01 USD/kWh, the plant stands to save a massive USD 6 million a year. (FL Smidth)
Examples
- 1,500kW heat to power facility at a cement plant in Germany [link]