Kiln, considered as heart of the cement plant needs to be fed with consistent raw meal at uniform feed rate. No mater how much consistent and perfect is your kiln feed quality from blending silo, variation in feed rate can disturb badly kiln operation stability, clinker quality and heat consumption. Such a potential risk associated with kiln feed system justifies a separate mention in cement manufacturing process, in order to understand its various components. The main objective of a kiln feed system is to feed the kiln at any desired rate set by operator (from CCR) and to maintain it with a close degree accuracy (± 2 %) throughout the feed rate range
A fraction (5-12%) of kiln feed gets lost with preheater exhaust gas. As the chemistry of this dust is normally different from kiln feed composition, therefore kiln feed must be adjusted/biased to yield the desired clinker composition. Major portion of this dust is taken to raw mill and coal mill with hot gases and a part of it is directly precipitated in bag filter or electrostatic precipitator before flue gases are vent out. In commonly existing configuration, this dust is transported to blending silo with raw meal when raw mill is running and when raw is stopped for more than two hours the dust is transported to a surge bin. From surge bin the kiln dust can be either transported to silo or fed carefully at controlled feed rate with kiln feed to kiln. In order to minimises re-entrainment of the fines, kiln dust may be fed by insufflation at the hood or at the feed-end of the kiln.
Kiln feed is monitored by analysing composite sample from auto sampler or grab sample in XRF or wet analysis to determine physical and chemical characteristics and statistical variations (standard deviation). Commonly a variation is monitored statistically in terms of C3S or LSF and sometimes with Cao or total carbonates as well
Components of kiln feed system. Important components are briefly discussed here as:
Kiln Feed Bin: A mixing bin with a capacity of half to one-hour kiln feed, mounted on load cells, facilitated with level switches, serving a purpose of maintaining a constant head (pressure/level) of raw meal inside bin to ensure that there are no fluctuations in the rate of discharge due to change of head/level of material. In addition to provide a constant head and a mixing purpose, kiln feed bin mounted on load cells is of pivotal importance for calibration of metering system for kiln feed rate (t/h) to monitor and record production counters with accuracy.
Kiln feed metering: Almost all modern cement plants have adopted an indirect weighing system like solid flow meter, rota-scale etc. with a dosing gate as a prefeeder to feed kiln within a tolerance range of ± 2 %. However standalone volumetric system (like screw feeder) and gravimetric system (like weigh feeder) feedings are still existing in some of the plants as well. Metering systems are briefly described here as:
- In volumetric systems of metering, feed rate is assessed by volume of raw meal delivered than its actual weight. For example, screw conveyors and rotary vane feeders with close tolerances between casing and screw with rpm control by variable speed drive(s). Kiln feed bin mounted on load cells can help here to calculate feed rates similar to Loss in Weight metering system
- In gravimetric systems of metering, weight of raw meal is actually measured as it is fed to kiln. For example, weigh feeder with a prefeeder to control material flow from kiln feed bin
- In indirect Systems (most commonly used) of metering, impact of falling material on a curved plate is translated into feed rate in tph. For example, solids flow meter with a controlling/dosing gate to control feed rate from kiln feed bin. One more common example of indirectly metering system (used commonly in standby feeding provision) is air lift feeding with a dosing/controlling gate, where air pressure in the air chamber is monitored and translated into kiln feed rate in tph
Feed Elevator: In almost all new plants, kiln feed is normally conveyed by bucket elevator to the top of the preheater to minimise power consumption. Previously ‘air lift’ was commonly used for conveying kiln feed to preheater top, this system needs de-aeration before injection, otherwise the air entrained consumes kiln ID fan load capacity and may reduce kiln capacity significantly.
Rotary Feeder: A rotary feeder passing kiln feed to a chute connected to preheater is installed to avoid cold air leakages into the preheater tower. A slide gate, pneumatically/electrically operated is always provided to isolate preheater (Hot gases) when feed is stopped or diverted to recirculation.
It is commonly existing in cement plants and is generally recommended to have a standby feeding provision consisting of metering and conveying system to consider for the period of maintenance or trouble shooting in any components of main feeding system.