In most cement manufacturing plants, production of cement is basically integrated on the operational basis of the rotary kiln. Based on the specs from the rotary kiln manufacturers, it could be seen as a large and lengthy cylinder rotating on an axis inclined at an obtuse angle. It is positioned such that it rotates along its axis with a burner integrated at its lower part.
The operational principle is quite simple- the raw feed is fed through the cool end of the rotary kiln while rotating through to the hot end before being passed out to the collector where it cools. While it is a productive method for large-scale processes, it was built such that plants invested in continuous processes in the stead of batch processes are beneficiaries.
Rotary cement kilns, according to rotary kiln manufacturers, were originally operated as ‘wet process kilns’ but the dry process kilns were later on added based on cement manufacturers demands.
Wet Process Kilns
In their most basic forms, wet process kilns are the simplest to operate. The specs given by the rotary kiln manufacturers indicate that the wet process kilns were integrated as a 200m long cylinder sporting a 6m diameter. This specific length is considered ideal for this rotary kiln due to the fact that the heat transfer rate in the kiln is not evenly distributed especially since it involves the evaporation of lots of water.
The feed is fed in as a slurry with about 40% water composition at room temperature or an equivalent temperature to the surrounding (i.e. ambient temperature of say 200C). This process was basically incorporated to overcome the challenges faced by plants with raw materials with specific requirements of being well blended. This is because efficient dry blending is much more difficult than slurry blending.
As a result, a lot turned to wet process kilns for effective results. However, the energy charts, as presented by rotary kiln manufacturers, indicate that more energy is needed for full incorporation of this process. This is because a significant portion of the wet process is aimed at the reduction of the water content of the slurry. As a result, more technology was incorporated into the rotary kilns and while few wet process kilns are operational with operations based on high-tech ends, the dry process kiln becomes more popular.
Dry Process Kilns
While most dry process kilns bear slight semblances to the wet process kilns (i.e. 200m in length and 6m in diameter), they are operationally quite different. As implied by their names, the feed (i.e. raw materials) is fed in with little or no water content. As such, the energy of the kiln is basically focused on getting the feed efficiently processed and not on evaporation.
As indicated by rotary kiln manufacturers, these kilns are built such that the thermal energy used in the process is maximally effective. Therefore, heat transfer in the kiln is evenly distributed.
The dry process inculcates a suspension preheater (i.e. a preheater tower) into its working principle. This preheater tower is basically a heat exchanger integrated with a series of cyclones which employs hot gases from the kiln in keeping the blended feed suspended in air while increasing its temperature via heat transfer from the hot gases.
The blended feed (in powdery form of limestone and shale) enters the kiln via this tower at a high temperature while passing through the series of cyclones (fast rotating hot gases which mixes with the blended feed) with hot air from the clinker cooler of the kiln rising up to meet the feed. Due to the particle size of the feed, a very high surface area is attributed to the feed, and thermal transfer is efficiently done.
The heat transfer is so efficiently incorporated such that about 30-40% of the feed is decarbonated before moving into the kiln. However, with more hot air from the clinker cooler moving up, almost 85-95% of the feed is decarbonated before finally entering the kiln. A more definitive process developed for this technology is the “precalciner cement kiln” which is integrated with an additional burner for more heat production.
The heating up of the feed in the pre-heater tower basically sees the feed go into the tower at about 9000C. This makes the dry process kiln more cost-effective simply because the addition of the pre-heater tower, which ensures that the feed is well-heated, negates the need of a longer rotary kiln. For example, it could be of a 70m length and 6m diameter, and still effectively give the same required output.