Cement being a continuous industry, the pyro section that runs 24/7 is the process part which is a non-stop operation. Though the cooler appears to be fairly simple but the operational part is more difficult when the capacity of a plant goes up.
It is a common practice in the industry to enhance the capacity of the kiln after a certain period of time when the plant stabilises. The problem arises when the engineers try to augment the capacity of a cooler with that of enhanced capacity of a kiln. The job becomes even more difficult when the space is limited. However with the support from cooler suppliers and after the selection of a right consultant, the job can be done. Retrofitting of cooler is quite often undertaken at a number of plants. (Refer to the article by ERCOM Engineers in this feature) Few leading manufacturers of coolers are FLSmidth, IKN (Ingenieurbnro Knhlerbau Neustadt), CemProTec, Claudius Peters, KHD Humboldt Wedag, ThyssenKrupp Polysius AG and FONS Technology. Major differences lie in the manner in which material is conveyed over the grates, the mode of aeration and the solid-gas heat exchange, the type and presence of self-regulating valves for airflow and the hydraulic system for moving parts.
Jayant Saha, a consultant having a vast experience with coolers, says, "The clinker cooler becomes a major bottleneck to support capacity upgradation. Almost all capacity upgradation projects involve increment in the capacity of existing coolers. Even with some extent of compromise with clinker temperature increase at cooler outlet, extra grate area requirement comes out to approximately 50-60 per cent for upgradation in existing calciner kiln system. For upgradation of older SP plants, the cooler grate area increment goes to even 150 per cent. Grate area can be increased only if free area is available within the cooler itself or by extending cooler longitudinally or by both methods. In most of the cases, the existing layout limits the cooler expansion and thus limits the upgradation scope. It is always advisable to keep a provision in the layout for cooler expansion, by keeping a little longer horizontal length for clinker pan conveyor."
Saha further says that though there are many types of coolers for cooling the clinker, the state-of-the-art coolers are either grate coolers or cross bar coolers. As cooling is a heat-transmission process, cooling efficiency is greatly dependent on the temperature difference between the two media, i.e., air and clinker.
In the course of rapid development in coolers in recent years, new generations of grate coolers have come into the market. In fact, it is little difficult to keep a track of the current cooler generation. However, little improvements have been made with regards to cooler efficiency and the cooling principle. At the same time, the grate load has typically been increased from approximately 40 to 50 t/d clinker per m2, considering the same clinker temperature, though the widely accepted value is about 45.
However good choice you make to erect a good cooler, the way in which it is operated is equally important. Here a process engineer plays a very important role. Snow man, Red River and jamming are some of the problems faced in cooler operation. These are process-related issues for which a good process engineering knowledge is necessary. The raw meal design has its impact on cooler operation.
Cooler optimisation can be carried out by maintaining:
-Maximum possible clinker residence time in the cooler to achieve the best possible distribution of clinker and cooling air. The clinker retention time in the cooler is determined by the grate surface loading and clinker bed depth;
-Proper cooling air flow distribution;
-Proper tuned PID controller (grate speed vis-a-vis under grate pressure) plays an important role.
Suchismita of ERCOM says that the efficiency of the cooler depends on its ability to recover the heat from the clinker entering the cooler, and cooling the clinker. The cooler also fulfills the key role of transporting the clinker away from the kiln in a reliable manner.
Saha writes, the cooler also provides hot air to be used in waste heat recovery system (WHRS). The steam turbine-based waste heat recovery boilers (WHRB) are placed between the cooler and the de-dusting system. Depending on situations/requirements, various configurations are considered. If no modification is made to the cooler and the excess gas is ducted from the cooler exit to the WHRB, then the vent fan should have enough margins to take care of the pressure drop in the heat recovery boiler.
The upgradation of cooler is a purely an engineering job requiring correct inputs about the existing status and the expected results.