Read part one of System for the Use of Alternative Fuels in Cement Production here.
New fuel handling system
In 2012, Vecoplan FuelTrack was contracted to develop a solution to enable the use of the broadest possible range of present and future solid recovered fuels, such as tyre chips, RDF and biomass, in the calciner.
The day-usage quantity of tyre chips from the main store are fed onto a modern moving-floor system, which doses a set amount of material onto a broad chain belt conveyor. The container unit of the moving floor system is designed to permit future enlargement for a higher feed volume and capacity. Together with the fitting of longer moving-floor push rods, the enlarged container would then be able to buffer store up to 300 m3 instead of the current 100 m3.
After discharge onto the chain belt conveyor, the material undergoes a safety check for unacceptable particle sizes, using a disc screen. This ensures that the material supplied to the calciner complies 100% with the specification and will not cause any disturbances in the further course of the process.
After dosing by the weighbelt feeder, the material is fed onto the 240 m long VecoBelt, the centrepiece of the system. The VecoBelt conveyor bridges the distance of 240 m between the tyre chip store and the preheater tower with very low power consumption and minimal maintenance requirement. The VecoBelt is a completely enclosed conveying system in which a conveyor belt is pulled through a tube. Instead of running on conventional belt idlers, the feed belt is supported by an air cushion. This air cushion beneath the conveyor belt is generated by small fans and reduces the kinetic friction to a minimum, enabling large quantities of AFR/RDF material to be transported over long distances at flexible speeds and with minimal power consumption.
A special feature of the VecoBelt is its simple and low maintenance design, due to the fact that no mechanical belt-supporting idlers are required along the entire conveying distance. Moreover, there is no need for maintenance platforms along the conveyor, which often cost more than the conveyor system itself. Thanks to the optimised construction, significantly less steel substructure is needed than is the case with conventional belt conveyor systems. The lack of rotating idler systems, which cause noise and also represent a fire hazard in the case of lengthy failure, is a further positive aspect.
In the preheater tower, the VecoBelt system feeds the material via a trough chain conveyor to a specially-designed screw conveyor, which (equivalent to a dosing device) stuffs the material into the fall shaft. The screw conveyor, with its storage bin, simultaneously forms the airlock between material feed and system, minimising the intake of false air. The gravimetric feeding unit of the weighbelt feeder is synchronised with the volumetric feeding of the screw conveyor in order to ensure constant compliance with the set material supply rate.
To break up any adhesive clumps of material before they enter the calciner chamber, the material first passes through the VecoJector zone.The VecoJector device is very efficient, achieving a tangential infeed swirl for optimised combustion, which is particularly important if fluff particles are later used as fuel. The VecoJector system, consisting of a vertical feed shaft, feeding screw and swirl chamber, enables consistently uniform and continuous feeding of solid lumps of alternative material into the calciner. Conventional flap valve systems (double/triple) operate on a cyclical basis and can cause pulsations and restrictions in the production process, particularly if large feed rates are being handled.
The simple but effective VecoJector system was specially developed for the cement manufacturing process and permits AFR/CDR materials with particle sizes of up to 150 mm and densities of up to 0.6 t/m3 to be continuously fed into the process. The use of adjustable nozzles for the swirl air ensures that agglomerations can be broken up and the material fed into the calciner in a swirl of air for optimum combustion without introducing the large cold air volume of a pneumatic conveying system. If the material is loose and free-flowing, its gravitational energy is sufficient and the amount of swirl air can be reduced. A hot gas damper installed downstream of the VecoJector allows the system to be isolated, especially in the case of sudden operating trouble and overpressure in the kiln system.
The future lies in flexible usage of the various substitute fuels. The described material handling concept is forward-looking in allowing other substitute fuels to be fed into the kiln system either in combination with tyre chips or as an individual stream of material. From the outset, the system conceived by FuelTrack was designed to permit the use of other types of solid fuel without requiring any conversion measures. In doing so, the company implemented a modular, future-oriented concept.
Meanwhile, the described first stage of the system has been successfully put into operation and is continuously supplying up to 3.5 tph of scrap tyre chips with a lump size of up to 150 mm. This corresponds to a 20% thermal substitution rate for the kiln system. In the future, the installed system will achieve considerably higher substitution rates (especially when lighter RDF materials are introduced).
Written by Wolfram Zschiesche and Karl Menzel, Vecoplan FuelTrack. This is an abridged version of the full article, which appeared in the February 2014 issue of World Cement. Subscribers can view the full article by logging in.