Valentina Bordei, thyssenkrupp, discusses the optimisation of clinker coolers to meet industry demands for energy efficiency and low maintenance.
In every cement plant, the cooler operation has a crucial impact on operating performance, clinker quality and overall productivity. The development of clinker coolers in recent times has been guided by market demands for maximised availability, reduced maintenance and rapid conversion with minimum production downtime, accompanied by the over-arching requirements to reduce energy consumption and minimise the impact on the environment. thyssenkrupp has responded to these demands with its new generation of the polytrack® clinker cooler.
Since its market launch in 2001, this system has been installed more than 130 times in cement plants worldwide. Based on the experience collected over almost 20 years of product placement, thyssenkrupp devised and carried out a design upgrade programme to align the cooler to the ever-more stringent market demands for energy efficiency and low maintenance. Several key design aspects were targeted, in order to maximise the performance of the cooler under ever more taxing operating conditions.
Aeration floor design
The first improvement tackled the power consumption of the cooler. The key feature of the product is the separation of aeration and clinker conveying, which allows for high cooling performance over time, and lends a long service life to the aeration elements. This is due to the fact that the stationary aeration floor is covered with a static, cold layer of clinker, which acts as autogenous wear protection. The tracks move above the aeration floor and convey the clinker using a reciprocating movement.
The electrical energy consumption of the cooler is the sum of the power consumption of the aeration cooling fans, the hydraulic drive powering the clinker conveying system, and the clinker crusher. Aeration accounts for by far the largest consumption and therefore has the highest savings potential. The ideal power expenditure of a fan is, if losses are to be neglected, a function of the total pressure increase in the fan and the volume flow delivered by the fan.The pressure increase is required to overcome the three sources of flow resistance and pressure drop in the cooler aeration:
- The pressure drop of the aeration floor itself, due to the flow through the narrow aeration slits.
- That of the static, protective clinker layer.
- That of the active clinker bed to be cooled.
Only the fraction overcoming the flow resistance of the active clinker bed is useful work. The focus of the in-house research activities was on minimising the other two components.
The new design of the aeration elements is a labyrinth design, like the previous one. The labyrinth-like flow path of the cooling air prevents clinker from falling through at the cost of a certain pressure drop. The new element configuration reduces this pressure drop. It increases the total air outflow area of each unit by increasing the number of labyrinths. This in turn reduces the air outflow velocity through the aeration slits, effectively reducing the pressure drop of the aeration floor. Another benefit of the updated labyrinth design of the aeration units is a decreased flow resistance in the protective clinker layer on top of the aeration floor. The previous design featured pockets in which the clinker settled between the aeration channels. Due to the area restriction of the pockets, the air velocity in this area was higher than in the rest of the static layer, resulting in an increased pressure drop. The new design reduces this effect, leading to additional power savings. In total, the improved velocity profile of the cooling air in the aeration elements and the static clinker layer leads to a saving of more than 10 mbar pressure drop, which reduces the power consumption of the fans at a given flow of cooling air. These improved aeration elements can also be retrofitted to older coolers. Combined with new conveying elements with reduced height, which decrease the overall height of the static clinker layer, the combined power savings of the new design can reach up to 1 kWh per t of clinker (based on measurements carried out in several plants).
The tendency in the cement industry is towards simplified, fast, predictable maintenance of the key equipment. Although the number of components requiring maintenance in a polytrack is quite small, the goal was to enhance the ease of maintenance operations. As part of the upgrade, the entire subgrate compartment of the cooler and the sealing were optimised. The final result brought greater advantages than just easier maintenance.
One target was to update the cooler sealings. The sealings make up the juncture between the aeration floor units and conveying elements, the latter being connected to the hydraulic system in the subgrate compartment. They ensure that no clinker falls under the grate at the interface between the static aeration floor and the moving tracks. While the original flat guide sealings proved to be very effective in many plants, due to a shift towards fuels with an increased sulfur content, the clinker size decreased significantly in some kiln lines. In these plants, increased maintenance, especially for the spring system, was required to keep the sealings spillage free. The spring system is needed to keep the horizontal sealing aligned and tight against the structures above the grate, and to prevent clinker from trickling under the grate into the aeration chambers. It requires regular maintenance, especially in plants with very fine clinker, to ensure spillage-free operation.
A simplified sealing was designed to tackle this challenge. The new sealing arrangement was tested successfully under challenging operating conditions. It is a vertical, continuous sealing, without springs, made out of hardened steel to withstand the friction resulting from the movement of the tracks. It has fewer components, and it is faster and easier to install.
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