Pushing Preheater Potential

In modern dry-process cement production, the preheater tower is essential for maximising heat exchange and optimising fuel utilisation in the pyro process. Each cyclone stage plays a distinct role, with upper-stage cyclones typically designed for high separation efficiency to ensure minimal raw meal losses while lower-stage cyclones are focused on effective heat transfer. The overall preheater performance is a function of each single cyclone stage’s performance and design; if one or more stages do not operate as intended, the remaining cyclones must compensate, leading to higher energy consumption in terms of both heat input and power.

Within the cyclones, the dip tube, also known as a thimble or vortex finder, plays a crucial role in defining the flow field and preventing the entering particles from shortcutting the cyclone. The dip tube is an integral part of the cyclone separation design, ensuring maximum separation efficiency of raw meal particles. The goal is that they are appropriately preheated before entering the kiln, contributing to the overall thermal efficiency and operational stability of the preheater system.

A missing or damaged dip tube significantly reduces separation efficiency leading to diminished quality due to unstable operation. Consequently, the heat recovery efficiency of the entire preheater tower is reduced, as a considerable portion of the preheated raw material particles circulates to the upper cyclone stages, prolonging the time before they enter the kiln.

Dip tubes help maintain optimal operating temperatures and system pressure balance, ultimately lowering the overall energy demand on the ID-fan. The dip tube influences the overall system pressure drop in two opposing ways. While they increase local pressure loss within the cyclone – leading to higher ID-fan power demand – their presence also lowers the mean operating temperature in each stage, significantly reducing gas volume flow. This reduction in volume flow decreases the energy required to transport the hot gases through the preheater, ultimately outweighing the additional pressure drop.

Accurately quantifying the energy savings of operating with an optimally designed dip tube is complex due to the varying system conditions and designs. However, based on customer feedback and internal calculations, operating without a dip tube in the lowest-stage cyclone typically increases heat input by 2 – 3% and ID-fan power consumption by 5 – 7%, highlighting the importance of maintaining a properly functioning dip tube for energy efficiency.

Evolving fuel mix

Traditional preheater designs were developed when cement plants primarily relied on coal, petcoke, and other homogenous fossil fuels. However, the cement industry’s shift toward more sustainable production has led to a significant increase in alternative fuel (AF) utilisation. Global players such as Cemex, CRH, Heidelberg Materials, and Holcim, report thermal substitution rates (TSR) approaching 40%,while the European cement industry reached 53% AF usage in 2021, with CEMBUREAU targeting 60% by 2030 and 95% by 2050. It is not uncommon to encounter certain individual plants today that can exceed 90% AF substitution.

AFs are primarily introduced in the calciner section of the preheater, where they bring new operational challenges. Unlike conventional fuels, AFs vary significantly in size, heat value, and combustion characteristics. This leads to temperature and pressure fluctuations, incomplete combustion, and higher peak temperatures throughout the preheater cyclones. In lower-stage cyclones, peak temperatures may occasionally exceed 1150°C, placing extreme stress on refractory linings and components such as dip tubes.

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