It is hard to think of a building material that is as ubiquitous, or as useful, as cement. It is literally the glue that holds everything together. This potent blend of rock-derived minerals (principally calcium, silicon, aluminium and iron) is the basic stuff of buildings: mixed with sand it makes mortar; add gravel and it creates concrete. Without cement the construction industry could construct nothing.
In one form or another it has always been around. The ancient Romans exploited the bonding properties of volcanic ash to raise their mighty monuments; and extensive use was made of oyster shells as a lime resource in the Middle Ages. The standard cement of our own times is Portland cement (so named because it has the look of Portland stone): its combination of versatility and strength are preferred over any other version and its raw materials (chiefly limestone and clay) are cheap and easily accessed.
Its manufacture, on the other hand, is hard work.
The rotary kiln (patented in the UK in 1885) was the great technological breakthrough for industrial cement production – and these gigantic, revolving, cylinder-shaped ovens are now at the heart of every cement works. They are not only extremely hot (around 1400°C), so that they transform the raw material that works its way down them into a new substance (called clinker), but they rotate on an incline so that the clinker forms evenly and flows continuously – in true production-line style.
Elsewhere in the production line are the crushers and grinders that at the beginning of the process break the quarried rock down to a form suitable for the kilns, and the mills that later on powderise and mix the clinker material into a final product. Conveyor belts link process to process throughout.
Motors keep all this machinery in motion; and here, as in all industrial operations, variable speed drives make motor behaviour more reliable, easier to control precisely, easier to look after and more efficient to run.
The kilns’ rotational mechanism, for example, benefits greatly from gradual acceleration and continuously adjustable speed. The enormous dimensions of the kilns themselves, combined with typically eccentric load distribution, mean they must overcome significant inertia when starting up. They can achieve this, without the mechanical stress caused by a sudden inrush of current, thanks to variable speed drives supplying motorised rollers with an appropriately high starting torque (250% of full load torque is normal) which then smoothly tails off as speed picks up. Rotational speed is thereafter directly proportional to the speed of material moving through the kiln.
Air flow must also be controlled. Induced draft fans are responsible for this, driving the air necessary for combustion through the kiln and drawing off the great volumes of exhaust gas. The process is critical to the regulation of temperature, which must be kept high enough to deliver thorough incineration, but not so high as to risk damaging the kiln shell. Moreover, for the sake of the system’s thermal and chemical stability, the work the fans do needs to be in proportion to the workload.
Using variable speed drives to adjust air flow – in response to such factors as air inlet temperature and the exact nature of the raw matter being processed – delivers this. And it allows systems to dispense with the miscellaneous louvres, vanes and dampers formerly used to modify the impact of fans running at constant speed.
Separate cooling fans quickly take the temperature of the fresh clinker down to around 100°C. Again, temperature control is vital: without rapid cooling, clinker can lose mineral content essential to the quality of the final cement. Variable speed drives, keeping the cool air flow in close proportion with the activity of the kiln, therefore work well here too.
The efficiency gains that variable speed drives bring to fan systems are well-known to engineers: thanks to the unique relationship between speed and power created by the Affinity Laws in physics, a reduction in fan speed equates to a greater than equal reduction in power consumed – 10% less speed, for example, implying an energy saving of around 30%.
The significance of the equation for industrial cement manufacture is considerable: fans figure prominently throughout the plant, as important to the milling operations that both precede and follow the clinker production as they are to the kilns themselves, and serving vital exhaust and de-dusting functions throughout.
Cement plants have much in common with mines, and their machinery tends to benefit from drive motor control for the same reasons. For example, nothing could be more stressful on conveyor systems than random-sized heaps of rocks. And both cement works and mines tend to have very long, very hard-working conveyor belts. Wear and tear on these belts, caused by elasticity and tension issues, can be combatted through the use of multiple motors that have been behaviourally synchronised by drives with load sharing functionality. (The same drive function, again to improve smoothness of motion, can be brought to rotary kilns that have dual motors.)
In both mines and cement works, too, the rock-laden conveyors usually cover long distances downhill. Not only do belt motors here benefit from the accurate torque and speed control necessary for efficient continuous braking, but the braking energy can be fed back to variable speed drives and thus returned to the plant’s power network.
There is no aspect of cement manufacturing that is not energy-intensive, and in overall terms it is one of the most energy-intensive industries of all. It is also, inevitably, one of the most polluting. Every year it puts billions of tons of CO2 into the atmosphere – nearly 10% of total global output.
Much of this comes from the burning gases in the clinker kilns. And research into clinker-free possibilities for cement is currently well under way. For as long as the existing industrial model stands, though, manufacturers are under pressure to make it as sustainable as they can. Energy-efficient technologies in general, and variable speed drives in particular, are proving central to their response.