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Waste Heat Recovery for a Cleaner Climate

Published by , Editorial Assistant
World Cement,


Energy efficiency is one of the most effective measures to reduce the energy intensity of the cement process and curb carbon emissions. One area of intervention for energy efficiency improvement is related to thermal energy harvesting.

Waste Heat Recovery for a Cleaner Climate

The cement manufacturing process requires a substantial amount of heat for its processes, but due to inefficiencies in energy conversion, almost 40% of it becomes exhaust heat left unexploited. This represents a significant opportunity for waste heat recovery (WHR), which can greatly enhance overall efficiency. According to an analysis by Persson, et al. reported in KC ORC study on European energy-intensive industries, thermal energy is utilised for only about 25% of the total energy input, meaning that 75% of the thermal energy obtained from primary fuels is currently wasted.1 The analysis identifies 1175 European industrial sites with waste heat potential exceeding 50 MWth each. By recovering the estimated waste heat in the cement plants mapped in this study, approximately 447.3 MW of electrical power could be generated using Organic Rankine Cycle (ORC) technology.

ORC waste heat recovery systems versus Steam Rankine Cycle

In cement plants, waste heat can be recovered from the exhaust flue gases of the kiln preheater and the clinker cooler using two primary technologies: the Steam Rankine Cycle (SRC) and the ORC.

The SRC utilises water as its working fluid, while the ORC employs organic fluids, such as hydrocarbons (mainly cyclopentane) or refrigerants. These organic fluids are vaporised and expanded in a turbine to generate electricity. One of the key advantages of ORC is that its working fluids have lower boiling points and higher vapour pressures compared to water, allowing for more efficient electricity production from low to medium-high temperature heat sources. While the SRC has been the traditional choice for WHR in cement plants and still accounts for the majority of WHR installations globally, ORCs have been gaining traction in recent years. In Table 1, the characteristics of both systems are compared.

Innovation in ORC systems

Exergy International has innovated in the field of ORC systems through the use of its Radial Outflow Turbine (ROT). This turbine is the first-of-its-kind to be used in an ORC system. The ROT is highly effective at converting the energy contained in the working fluid into mechanical power with higher efficiency than conventional axial and radial inflow turbines.

The use of ROT technology in ORC systems for WHR applications offers several distinct advantages:

  • Flexible and efficient design: its radial outflow arrangement allows for a broader range of applicable fluid conditions.
  • Lower noise and vibrations: due to the straight blades and radial configuration and the direct-drive, low-speed turbine.
  • Simplified maintenance: the built-in mechanical group – including bearings, oil lubrication systems, and seals – can be easily removed without draining the organic fluid from the cycle. The entire operation maintenance on the turbine can be completed in less than one day.
  • Reduced plant downtime compared to other technologies.
  • High reliability: the turbines are designed with a standard mechanical design for each turbine frame, ensuring optimal performance.
  • Performance and competitiveness: the unique multiple pressure admissions on a single disk turbine enable a favourable trade-off between performance and competitiveness, while a high number of stages on a single disk maximises efficiency.

How ORC waste heat recovery systems work

ORC WHR systems for the cement sector are typically designed with an indirect heat exchange using an intermediate loop that employs a heat transfer fluid (HTF), such as thermal oil, pressurised water, or saturated steam.

The WHR extracts thermal power from the exhaust gases in the preheater and clinker cooler, using a thermal oil loop to connect the recovery system to the ORC module. In the ORC closed circuit, the organic fluid in liquid form is pressurised by a pump and then heated and evaporated in the preheater and evaporator, respectively, receiving heat from the thermal transfer fluid. The resulting vapour is then expanded in a turbine to generate electricity. After expansion, the exhaust vapour is condensed in the regenerator and cooled in the condensing system before the working cycle restarts. Typically, an air-cooled condenser (ACC) is used to avoid water consumption, making it a more sustainable solution. However, when a stable water source is available, a water-cooled condenser (WCC) can also be employed.

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Read the article online at: https://www.worldcement.com/special-reports/31122024/waste-heat-recovery-for-a-cleaner-climate/

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