K.N. Rao writes on the importance of waste heat recovery (WHR) plants in energy conservation and why they will become a feature in this fast expanding cement market
Waste heat recovery plants offer a reliable supplement to captive power generation in an energy-intensive industry like cement, particularly in an energy-deficient country like India. In 2014, ACC Ltd launched its first WHR system at the Gagal (Himachal Pradesh) cement plant, and currently this practice has been replicated across all ACC cement plants.
The WHR system harnesses waste heat discharged in the cement manufacturing process as exhaust gases, channeling them into a boiler that runs a steam turbine and converts it into useful electrical energy. The new WHR project generates electricity at a cost that is significantly lower than that of a captive power plant and only a fraction of the cost of grid power. ACC sees the project as an important step in energy conservation and is exploring the possibility of installing similar systems at a few of its other cement plants.
Heat recovery for power generation
The cement manufacturing process is energy intensive, requiring very high temperature in the order of 1,400 C in the kilns. Thermal energy is also used in other stages of the process, including the pre-heater, during grinding in the coal mill and raw mill and for drying additives such as flyash and slag. Significant amounts of heat energy are released as exhaust streams in different stages of the cement manufacturing process, chiefly from the kiln exhaust streams, clinker cooler, kiln pre-heater and pre-calciner.
Waste heat to electricity
Waste heat generated in cement manufacture has proven to be amenable to conversion into electrical energy, provided it is tapped in adequate measure and the temperature is sufficiently high to make the project viable. In a typical Indian cement plant, the potential generation of power from waste heat is estimated at roughly 20- 25 kWh/t of clinker.
Sustainability and other issues
This system plays a vital role in energy conservation as they use waste heat and do not need any additional fuels to generate electricity. They help conserve fuel and reduce overall carbon emissions. Where they substitute power from an external grid or a captive power plant, there is an additional advantage of reduced fuel consumption and lower CO2 emissions. Since it is based on waste heat, the energy produced is green.
India’s cement sector already has several working WHR plants, and such plants will undoubtedly become a feature in this fast expanding market. Waste heat recovery can comprise an economical, reliable supplement to captive power generation. In addition, this practice comprises energy conservation and efficiency that helps the cement industry meet its low carbon technology roadmap for the future.