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A guide to waste heat recovery – Part One

World Cement,

This is part one of a three-part article written by Peter Beleznay, Heatcatcher,for World Cement’s August issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch upon-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the August 2015 issue here.

In recent years the cement industry across Europe has gone through a transformation in terms of kiln fuel efficiency. Increasingly more heat has been utilised from the kiln exhausts to pre-heat feed stock and fuels plus the optimisation of the clinker coolers to recover more heat back to the kiln, all resulting in reduced fuel consumption. The thermal efficiency improvements leave the remaining available heat to be considered “low grade” as exhaust gas temperatures are typically lower than 300°C, additional electrical energy is consumed by fans to further reduce the temperature of this low grade heat before it enters the dust filtration systems.

With stakeholder pressure on the cement industry to reduce carbon emissions, the recovery of this low grade waste heat by integrating Waste Heat Recovery (WHR) systems to generate low carbon electricity is increasingly being evaluated against the return on investment.

WHR technologies are developing to lower the installed costs per kW of electrical generating capacity, and with the added benefit of reducing electrical energy consumed by filter cooling fans, the returns on investment are improving. This article introduces the reader to the integration challenges of WHR and the best available technologies and techniques that can provide up to 30% electrical operational savings for a kiln.

WHR Integration positions

There are 3 main positions in the process to recover the waste heat from:

  • Pre Heat (PH) from the exhaust of the pre-heater tower after the rotary kiln. The exhaust gas from the kiln exits at a high temperature, and passes through different stages of cyclones to preheat the raw material. The majority of plants are equipped with a Gas Conditioning Tower (GCT), where the exhaust gases are cooled down to required temperatures prior to recovery to a raw mill or coal mill, and to meet the inlet conditions of the dust filtration system. This is where the greatest amount of heat energy can be recovered.
  • Tail End (TE) exhaust gas flow after the Bag Filter or ESP (Electrostatic Precipitator) provides a low temperature heat source, but the gas has a very low dust content compared to other points in the process. Recovering this heat comes with low technological risk and lower electricity generation.
  • Air Quench Cooler (AQC). The clinker leaving the kiln is passed through the clinker cooler for cooling. The exhaust gasses extracted from the cooler provide a heat source in the form of hot dry air, which can be recovered and used to generate electricity with WHR technology. Typical temperature of the hot air exiting from the clinker is around 300°C. Normally the hot air is further cooled by direct or indirect cooling method to avoid high temperature at the inlet of Electrostatic Precipitators (ESP) or bag filters. Recovering waste heat prior to the ESP/bag filter, can be used for electricity generation but also adds additional benefit to the plant by reducing the electricity consumed by the air heat exchanger fans and/or the main filter (Induced Draft) fan.

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