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Enhancing efficiency

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World Cement,

Sergio Morlacchi, EXERGY, explains how the integration of waste heat recovery systems into cement plants could help enhance the overall energy efficiency of the cement manufacturing process, while helping to lower emissions.

Despite the effect of the COVID-19 pandemic on economies all over the world and the consequent dramatic contraction of demand in the cement sector, cement production reached 4.2 billion t in 2020 globally, maintaining almost the same levels as the year before. Even with the pandemic continuing to affect global economies, cement demand is projected to increase by 2.45% in 2022, and reach a production increase of 5% in 2030, as reported in the IEA’s World Energy Outlook.

The cement sector is a significant contributor to global annual energy consumption and carbon emissions.

The direct CO2 intensity of cement production grew at a rate of 1.8% per year during 2015 – 2020, despite needing to reach a decline of 3% annually to meet the target of Net Zero by 2050. Therefore, the sector is facing the difficult challenge of meeting growing demand while limiting CO2 emissions.

One key lever for reaching the goal of carbon emissions reduction is improving energy efficiency in the cement manufacturing process. Progress in thermal and electrical energy consumption to produce clinker has been enormous in recent decades due to the continuous modernisation of kilns, with dry-process kilns replacing wet-process kilns, as well as the implementation of modern technologies in new installations. To accelerate on this road towards energy efficiency, an effective and market-ready solution for thermal energy utilisation should be introduced. This is the integration of Waste Heat Recovery (WHR) systems in cement plants. These can contribute to enhance the overall energy efficiency of cement manufacturing while also helping to lower emissions deriving from electrical energy consumption.

Improving ORC

The cement manufacturing process uses a high amount of thermal energy in the kiln and raw material preheating and burning process. A huge part of this heat energy, as much as 40 – 45%, is lost mainly as waste gases. This waste heat can become the resource used to feed a WHR system and generate electricity. The commonly employed technologies for a WHR system in a cement plant are the Steam Rankine cycle or Organic Rankine Cycle (ORC). These methods differ in their choice of working fluid – the steam cycle uses water and ORC uses an organic fluid, typically a hydrocarbon. Steam Rankine cycle systems have been the traditional and more widely employed solution in previous decades, but in recent years ORC applications have experienced a rapid increase in adoption for small-to-medium size and temperature applications, due to some advantages over steam cycle systems:

  • Modular and compact design, easily customisable, with low associated cost for foundation and assembly and reduced footprint.
  • Automated solution with no need for operators, thus leading to lower running costs.
  • High efficiency of the cycle at a variety of operating temperatures and loads.
  • No water treatment plant or make up water required.
  • Well suited for cycling environments and where fast starts/stops are required.
  • Easy maintenance leading to lower maintenance costs.

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