Increasing the proportion of alternative fuels is one of the most important factors in cutting costs in cement production. German cement manufacturer Spenner Zement has used secondary fuels for more than two decades. Its substitution rate lies on the national average.
Rotor weighfeeder Pfister® TRW-S: As the fuel is transported from the inlet to the outlet in the completely closed rotor weighfeeder, the fuel is weighed and dosed. The pre-hopper is equipped with a stirrer, which keeps light or compressed materials flowable. © FLSmidth Pfister
Spenner Zement began experimenting with alternative fuels (AF) in the mid-1990s, when it substituted lignite with refuse-derived fuel (RDF), such as plastic and paper waste, in its main burner. Prior to this, the cement industry had little experience with secondary fuels. A major challenge is that no two alternative fuels are alike; not even the same type of secondary fuels have consistent chemical and physical properties. Density, piece size and moisture content are important factors to ensure a constant combustion process.
After installing its secondary fuel supply system in 1995 – a tank infeed system using a stuffing screw as air seal for pneumatic conveyance to the main furnace – Spenner Zement’s feed technology did not work. The improvised volumetric solution with the already-installed silo discharge technology and star feeder proved too inexact and susceptible to faults. Franz-Josef Schäfers, Head of Engineering and Maintenance at Spenner Zement, concluded that a volumetric feeder was unsuitable for supplying a rotary kiln with fluff.
Franz-Josef Schäfers realised there were opportunities to use renewable energies and in particular municipal waste. FLSmidth Pfister had already begun testing its gravimetric rotor weighfeeder technology – the rotor weighfeeder Pfister® TRW-S – for coal dust and raw meal feeders instead of the traditional, open belt weighfeeders.
"Our main demand on feeder systems for secondary fuels: High reliability with precision dosing of different grades of material" - Franz-Josef Schäfers, Head of Engineering and Maintenance, Spenner Zement
The rotor weighfeeder is a completely closed system based on a slow-rotating horizontal star wheel. The AF drops into the rotor from a pre-hopper above the rotor weighfeeder and is dosed gravimetrically while being transported from the inlet to the outlet. The speed of the rotors is controlled by weighing electronics inversely to the measured force. This enables a constant rate of material discharge at the outlet of the rotor weighfeeder.
“By the end of 1996 we implemented a Pfister rotor weighfeeder to feed fluff to the main furnace and were immediately able to achieve the performance we wanted in our cement works,” recalls Schäfers.
Whereas industrial waste including foil, paper and cardboard was used in the initial phases, it became more varied, starting to include plastics. However, the increased variety typically produces less energy. Spenner Zement’s fuel quality has been improved by installing additional equipment for removing ferrous and nonferrous metals and coarse minerals.
Expanding the calciner
The plant for secondary fuels at Spenner Zement was expanded in 2003. The storage capacity for fuel was enlarged for additional charging of the calciner. When a further increase in the substitution rate was planned, two proposals to improve the feed quality of the alternative fuels were developed: use of a rotor weighfeeder Pfister TRW-S directly after the hopper screw conveyor or installation of this gravimetric feed system in the calciner tower behind the pipe belt conveyor.
Spenner Zement decided to install this third rotor weighfeeder Pfister TRW-S in the calciner tower. The advantages of such a configuration were:
• The rotor weighfeeder acts as a material buffer directly before the fire
• Variations in pre-feeding and material compaction by the intermediate conveyor can be balanced out by the pre-hopper
• The material buffer in the pre-hopper supports the star feeder in sealing off against the vacuum pressure
One aspect, however, remains a challenge: the abrasiveness of alternative fuels. This increases maintenance costs for the shredding tools and feed systems. There were also costs involved in removing the chlorine introduced into the process by the alternative fuels. To reduce maintenance, all parts subject to wear have been designed for easy replacement or furnished with replaceable wear protection. And, there is only one moving part: the rotor star feeder itself.
Third generation rotor weighfeeder
As the market for alternative fuels has changed continuously over the last two decades and Spenner Zement has adjusted to new requirements, FLSmidth Pfister engineers have continuously developed their rotor weighfeeder technology further. Pfister rotor weighfeeders are now controlled by the third generation of controllers. The new F-Control generation runs completely on ethernet networks and can be operated via an internet browser. Although changes or upgrades to the electronics have not always proceeded without problem, workable solutions have always been found.
Franz-Josef Schäfers says: “Thanks to a cooperative working relationship with the experts from FLSmidth Pfister, we have always been able to sort out teething problems over the last 20 years. We again opted for their latest rotor weighfeeder generation when it became necessary last year to modernise our feeding technology for secondary fuels.”