Refuse-derived fuels (RDF) such as plastic and paper waste were the first non-fossil sources of energy with which Spenner Zement already experimented back in the mid-1990s to feed its main furnace and calciner in partial substitution for lignite, the primary fuel of the time. This was an “experiment” in the true sense of the word as there was little experience in the cement industry at that time on the behaviour and reliable, constant dosing of secondary fuels. On top of this came then, as now, the fact that no two alternative fuels behave alike. Not even secondary fuels of the same type have the same chemical and physical properties consistently. However, density, piece size and moisture content are important factors in fuel dosing for ensuring a constant combustion process.

Volumetric dosing unsuitable for dosing of RDF 

After installing its secondary fuel supply system in 1995 – a tank infeed system with stuffing screw as air seal for pneumatic conveyance to the main furnace – Spenner Zement was confronted with the problem that the feed technology did not work and that its supplier could not provide a solution. The improvised volumetric solution with the already installed silo discharge technology and star feeder developed for powder dosing to charge the rotary furnace proved too inexact and susceptible to faults: the fuel was not weighed during feeding and the conveyor air of the downstream conveyor system counteracted the conveyance of the light material (<0.05 kg/dm³). “A volumetric feeder is not suitable for supplying a rotary furnace with RDF,” concluded Franz- Josef Schäfers, head of Engineering and Maintenance at Spenner Zement, at that time. 

Our main demand on feeder systems for secondary fuels: high reliability with precision dosing of different grades of material.

Franz-Josef SchäfersHead of Engineering and Maintenance, Spenner Zement

Problem meets solution

The moment this problem met its solution was when the German cement feeding specialists FLSmidth Pfister identified the trend towards the use of renewable energies and “wastes” in cement production. At that time it was already testing adaptation of its gravimetric rotor weighfeeder technology for coal dust and raw meal feeders as addition or substitute for the traditional, open belt weighfeeders for feeding of alternative fuels. The completely closed rotor weighfeeder Pfister® TRW-S developed for this is based on a star wheel that rotates horizontally at a slow speed of a few revolutions per minute. The alternative fuel drops into the rotor via 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, based on the nominal rate, by weighing electronics inversely to the measured force. This enables a constant rate of material discharge at the outlet of the rotor weighfeeder. “We then implemented a rotor weighfeeder at the end of 1996 to feed fluff to the main furnace and were immediately able to achieve the performance we wanted in our cement works,” recalled Schäfers. These positive experiences with use of the substitute fuel for the main flame of the rotary furnace resulted a year later in a next step at Spenner Zement in use of fluff in the calciner as well. A rotor weighfeeder Pfister® TRW-S is also used for feeding here. 

The completely enclosed rotor weighfeeder Pfister® TRW-S is based on a slowly with only a few rounds per minute horizontally revolving rotor.

A growing range of alternative fuels: multi-fuel dosing as the solution

Whereas only industrial wastes such as foil, paper and board were used in the initial phase, the fuels became more varied n the following years - unfortunately also lower in energy content. Plastic wastes from refuse collection were also used increasingly. During the BSE crisis, Spenner Zement helped to dispose of animal meal with approval by the authorities. The Pfister® rotor weighfeeders were able to feed this material without problem, also alternately with RDF.

Following use of RDF in other industrial production processes as well, the various materials have found not only a market price in dependence on their calorific values, impurities and process-impacting compositions, but also buyers. Spenner Zement is now also experimenting with materials with lower calorific values and typical impurities (minerals, metals, etc.). It has proved possible to improve Spenner Zement’s fuel quality by installing additional equipment for removal of ferrous and non-ferrous metals as well as coarse minerals. “The rotor weighfeeder for secondary fuels from FLSmidth Pfister is de facto the only system available on the market that can really be called ‘multi-fuel’,” says Thomas Jennewein, responsible at FLSmidth Pfister for dosing of alternative fuels. 

Rotor weighfeeder Pfister® TRW-S dosing secondary fuels to the calcinator at Spenner Cement Germany.

Development of the Pfister® rotor weighfeeder with consideration of every possible flow characteristics of the bulk materials

“Frequent changes in the type of fuel, the particle sizes and compositions are the order of the day when it comes to burning alternative fuels. When an operator of a cement plant wants to have little changeover and adjustment work without downtimes when using such fuels, the feed system must also be able technically to manage this.” Apart from ongoing changes in the composition of the RDF feedstock, the size of the shredded waste material being used at Spenner Zement has, for example, been increased from initially 20 mm to 30 mm over the course of time. The reason for the multi-fuel capability of the rotor weighfeeder Pfister® TRW-S lies in the intensive research and development work conducted by the German-based weighfeeder specialists FLSmidth Pfister. During the development of the rotor weighfeeder Pfister®TRW-S, every possible potential fuel and every possible flow characteristic of the different bulk materials were considered so that the complete feed system can weigh and discharge materials that flow both badly and well both accurately and reliably. Since it is becoming increasingly easier to dose secondary fuels that flow easily, the drives and torque-transmitting components of the Pfister® rotor weighfeeders were designed for substances that flow badly due to their particle sizes and high bulk densities. A pre-hopper mounted directly at the inlet to the actual rotor weighfeeder is equipped with an agitator to prevent compression of the secondary fuel. Use of the agitator also enables discharge of bridgebuilding materials and materials that flow badly. Thanks to constructional elimination of the equipment weight, it is even possible to weigh alternative fuels with a very low bulk density.In addition to this, a Pfister® rotor weighfeeder is able to cover the large range of bulk densities of alternative fuels by enabling a speed variance of up to 1:100. 

The rotor weighfeeder for secondary fuels from FLSmidth Pfister is de facto the only system available on the market that can really be called ‘multi-fuel’.

Thomas Jennewein, Product Specialist in Alternative FuelsFLSmidth Pfister

Poor dosing quality: rotor weighfeeder replaces belt weighfeeder

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. A dosing hopper with mixer arm for homogenisation is filled with a wheel loader. A downstream belt weighfeeder controls the

collecting screw underneath the hopper from where a tubular belt conveyor transports the material to the calciner tower and discharges it via a star feeder. However, due to the compression of the material in the collecting screw, the feed result was unsatisfactory. 

Recurrent topics: Adjustments to new requirements and search for alternative fuels

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 tubular belt conveyor. Spenner Zement decided to install this third rotor weighfeeder Pfister® TRW-S in the calciner tower. The advantages of such a configuration are as follows:

  • 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 for all systems used: the less readily available alternative fuels are extremely abrasive. This increases the maintenance costs for both the shredding tools and the feed systems. In Spenner Zement’s case there are also costs for removal of the chlorine introduced into the process by the alternative fuels.

To reduce maintenance on the rotor weighfeeders caused by the highly abrasive secondary fuels to a minimum, all parts subject to wear have since been designed for easy replacement or furnished with replaceable wear protection. In addition to this, there is only one moving part, the rotor star feeder itself.

As the market for alternative fuels has changed continuously over the last two decades and Spenner Zement has also adjusted to new requirements, FLSmidth Pfister engineers have also continuously developed their rotor weighfeeder technology further.

Pfister® rotor weighfeeders are now controlled by the third generation of controllers since their launch on to the market. The new F-Control generation runs completely on Ethernet networks and can be both integrated in company intranets and telemaintenanced by browsers. This controller can be accessed from any point in the plant. The changes or respective upgrades to the dosing electronics did not always proceed without problem. Franz-Josef Schäfers: “Thanks to a cooperative working relationship with the experts from FLSmidth Pfister, we have also always been able to sort out the teething problems in this regard in the last 20 years. We in any event again opted for the latest rotor weighfeeder generation from this supplier when it became necessary last year to modernise our feeding technology for secondary fuels.”

The search for alternative fuels and their use to increase the substitution rate remain a constant topic of discussion. In addition to licensing aspects, the focus here lies on process and emission requirements. The cost analysis also considers the negative accompanying substances (impurities such as metals, foreign bodies, moisture, chlorine, etc.). This also applies to consideration of the resultant residues and possible reduction of denitrification agents. 

Spenner Zement

Spenner Zement produces and markets cement, lime and dry-building materials. It is the largest family-run business in its segment in Northern Germany. Located in Erwitte, it has a workforce of almost 200, of which around 10 % are apprentices. The raw material needed for production is obtained from quarries in Erwitte and Brilon. Apart from its site in Erwitte, Spenner Zement also runs a granulated slag crushing mill in Duisburg and produces granulated-slag cements (blast furnace cements) there.


FLSmidth Pfister GmbH: decades of dosing experience

Stable and accurate dosing of fuels and materials is one of the key elements required to produce clinker profitably and efficiently. With its state-of-the-art Pfister® rotor weighfeeder concept, FLSmidth Pfister is offering a future oriented technology that is able to dose small to huge numbers of tons per hour. With almost 3,000 installations worldwide, customers of FLSmidth Pfister are feeding millions of tons each and every year. FLSmidth Pfister GmbH does not only supply the single dosing machines. FLSmidth Pfister’s know-how includes the complete setup and surrounding of the installation like silo engineering, intermediate material transport and safety equipment. In addition FLSmidth Pfister also offers customer trainings and seminars. That ensures that customers get all engineering from one experienced partner and one single source.



Thomas Jennewein, Product Specialist in Alternative Fuels, FLSmidth Pfister and
Franz-Josef Schäfers, Head of Engineering and Maintenance, Spenner Zement

Stories from FLSmidth