More ways to cut costs with alternative fuels

May 11, 2015 Thomas Jennewein

Combining various fuels has long been seen by the cement industry as a way of controlling costs. Now, it’s also becoming a source of competitive advantage.

More cement plants fire their kilns with low-price materials, such as biomass and tyre chips, instead of conventional fuels, such as oil, natural gas, coal or lignite. Not only does the use of alternative fuels make cement plant operations less vulnerable to energy price fluctuations, but waste and other superfluous biomaterials are often available locally and a lower-cost alternative.

In the cement making process secondary fuel are employed for the calciner process and for kiln feeding

Proven and tested concept
The Pfister® TRW-S/D is a rotor weighfeeder used for dosing rotary kilns and calciner with materials for fuel. It has been used in 240 implementations worldwide, either as a replacement for or together with the Pfister® DRW rotor weighfeeder, which is used for conventional pulverized fuels. The Pfister® multi-fuel dosing concept supports reliable and flexible low-maintenance operations and has been used in 44 countries around the world. 

A view inside the Pfister® TRW-S/D rotor weighfeeder, which is designed for dosing of alternative fuels 

The greatest challenge in dosing alternative fuels is their diversity. Particle size, density and water content are never the same. Refuse-derived fuels (RDF) are fluffy, fibrous or compressible and biomass, and sewage sludge may contain additional moisture. Others, such as bone meal, are powderous, requiring careful distribution management. Locally available materials range from wood dust in Scandinavia to dried sewage sludge and biomass in Egypt to tyre chips and landfill materials in other parts of the world.

The production machinery itself must be able to cope with this diversity of the materials. The Pfister® TRW-S/D has been designed with this in mind. It is built to handle a broad variety of alternative fuel installations, including pneumatic material feeding to kiln burners and mechanical material transport into calciner inlets. 

Another significant challenge is that alternative fuels are never completely homogenous. In an Irish cement plant’s calciner, for example, RDF is combined with wood waste containing wood particles of many different sizes. The Pfister® TRW-S/D can nevertheless still accurately dose the combined fuel. 

Almost all alternative bulk materials contain some foreign bodies, which can complicate pre-preparation of the material. FLSmidth Pfister has carefully tested a wide variety of alternative fuels, including different sizes and bulk densities. These tests demonstrate that the Pfister® TRW-S/D prevents blockages even when the material contains foreign bodies. The biggest foreign body tested was 300 x 300 x 10 mm in size. Performance was not even affected when the material contained tyre shredding with steel wiring, long corn straw or liquid-filled plastic bottles. 

An important feature of the Pfister® TRW-S/D is its ability to dose secondary fuels with a wide density range (70 kg/m3 – 700 kg/m3) and a wide weight range (1:10). Furthermore, the weighfeeder can work with almost any alternative fuel bulk material without needing to be rebuilt.

How it works
The concept of the rotor weighfeeder Pfister® TRW-S/D can be compared to a circular conveyor. It is a fully enclosed gravimetric dosing system that feeds, weighs and discharges kiln fuels in precisely measured amounts, maintaining perfect temperature control in the kiln. The material is transported from the inlet to the outlet, producing a moment around the weighing axis, which is measured by the load cell device. 

The weighing electronics control the motor speed according to the set point and immediately adjust the rotor speed and thus the actual feed rate. Additionally, stable fuel dosage is controlled by the patented proactive control strategy ProsCon® for long-term accuracy and outstanding burning conditions. Very light materials such as plastics with their extremely low gravimetric force are measured in material layers of up to 500 mm in the feeder. 

 Constant material output during operations of the Pfister® TRW S/D is controlled by adjusting the rotor speed according to the set point (right diagram). In contrast, conventional follow-up control systems vary considerably (left diagram) 

In order to ensure optimum feeding control of all fuel materials, the control strategy ProsCon® is preconfigured and adapted by FLSmidth Pfister electronics specialists to the cement plant’s individual requirements. No configuration knowhow is required and all parameters (mass flow, motor speed, etc.) are sent via Profibus directly to the control room. 

Long-term investment
With feed rates of more than 25 tph and great flexibility in material size and density, customers integrating a Pfister® rotor weighfeeder into their plant installation are investing in future-oriented technology. With its modular design concept, it integrates easily in future plant extensions or relocations. Its stand-alone, low-maintenance system requires no additional support and no special or additional tools. All parts that come into contact with the various fuels are made of steel and it contains only one rotating part: the rotor wheel itself. The pre-hopper is equipped with a stirrer, which lifts and homogenises the material by avoiding any compression. 

For the processing of potentially flammable alternative fuels such as sawdust or sewage, the rotor weighfeeder Pfister® TRW-S/D features explosion-proof design and is ATEX-certified.  

Facts & Figures
• The production of one kilogram of clinker consumes anything from 3300 kilojoules (dry process) up to 5000 kilojoules (wet process) of energy. Cutting energy costs in half using secondary fuels increases the profitability and efficiency of clinker production 
• 240 latest-model Pfister® TRW-S/D rotor weighfeeders have been sold in 44 countries 
• Energy derived from refuse (RDF) replaces valuable fossil energy carriers. As a result of continuous technological upgrading, one of the world‘s large cement manufacturers, Cemex, substituted more than 28% fossil fuels with secondary fuels in 2013

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