grey2green, green cement, green polysius cement plant, polysius fuel substitution

prepol® SC – step combustor for waste utilization in cement plants

The prepol® SC is an add-on for the prepol® precalciner. On this simple combustion grate, waste can burn for more than 1,000 seconds at high temperatures. Compared to ordinary calciner technology with only up to 7 seconds residence time the prepol® SC opens a new dimension in burning: It provides a high flexibility for a wide variety of wastes that leads to a new level of fuel-cost savings – and paves the way for a more sustainable cement production.

The prepol® SC is a very simple combustion grate, made of several static, non-moving refractory steps, giving the system its name: Step combustor

Samuel Zühlsdorf, Senior Engineer Products & Quotation, thyssenkrupp Industrial Solutions AG

Waste management in cement industry – the path to alternative fuels

In 2025, according to UN figures, more than 8.1 billion people will live on the earth. The cement industry can be a huge leaver for a greener future and to manage upcoming waste challenges regarding resources and waste management: Through the use of the prepol® SC coal, gas and oil as primary energy sources can be replaced 100 percent by alternative fuels. This contributes clearly to our vision of the green polysius® cement plant. We are convinced that this is a transformation journey - from #grey2green.

prepol® SC - advantages at a glance

  • incineration of even large amounts of very coarse and wet alternative fuels of up to 300 mm

  • Patented air blast transporting, with no moving parts in the hot zone

  • Highest fuel flexibility thanks to the fuel separation effect on the grate – “burn what you get”

  • Constant agitation of the fuel for a superior oxygen supply and fewest CO peaks

  • Complete and safe operation with lowest emissions due to most suitable combustion conditions

  • Maximum reliability, proven in six years of operation with lowest maintenance costs and several step combustors in operation

prepol® SC – the design of the step combustor

The prepol® SC is designed with innovative features to meet the customer demands. As the combustor is connected to the tertiary air channel, the hot air can deliver what is needed for a complete and successful combustion: Heat for drying and ignition, and oxygen for safe and complete combustion.

Modular design, simple maintenance and high availability

The design is modular which leads to an easy integration in nearly every existing plant. Moreover, a simple maintenance and a high availability of the kiln operation is ensured as there are no moving parts in the combustion chamber.

The step-shaped grate can be connected to any kind of precalciner loop and is nothing more than a tube-like extension: small in size but with huge potential in terms of combustion! The grate area of the unit is big enough to offer waste a residence time of more than 1,000 seconds.

The benefits of our step combustor – the prepol® SC

The thermal process always takes place with an air excess. It is therefore impossible for dangerous carbonisation gases such as CO, H2 and Cx Hn to form. Forced feeding of the fuel seals the system off from the environment. In an overpressure situation (e.g. failure of the system fans), ignition of the fuel in the conveyors can be prevented. During a stoppage, the combustor is completely isolated from the conveyors by pneumatically-operated airlock slide valves.

prepol® SC – the working principle of the step combustor

Especially for existing plants that face operation problems when burning alternative fuels,  the “suspension” of fuel particles in the gas stream as well as the extension of residence time can be reached with minimum effort: Compared to an extension of the calciner loop, a step combustor offers highest residence times in a significant smaller volume. This is possible as the fuel lies on the grate instead of following the gas stream in the loop. The small volume and the resulting lower weight of the combustion chamber mean that less civil construction reinforcement is required.

The refractory steps of the grate are equipped with nozzles which are connected to air-blast cannons. The air blasts agitate the fuel up and progressively convey it along the grate. Moreover, the swirling-up constantly turns over the fuel and assists the combustion process, ensuring that a very high throughput rate is achieved per unit area of grate. A further advantage is that the light, already-converted char of the fuel particles is quickly blown out of the combustor and entrained in the suspension gas stream of the calciner, while the heavier particles remain on the grate for further thermal processing. The fuel transformation is thus positively assisted by the active fuel conveyance system throughout the entire fuel retention time.

One step in preparation, several steps in combustion. With direct calciner feeding, refuse-derived secondary fuels can only be successfully used if the waste has passed through a multistage mechanical processing plant. The secondary fuel has to be in such small pieces that it can be suspended in the gas stream and totally burns out within a retention time of approx. 5 seconds in the calciner. This demands an edge length of about 50–100 mm. Using the advanced prepol® SC technology, even heavy waste fuels can be used that are non-flyable and that have an edge length of up to 300 mm. Waste preparation is only needed to adjust the heating value and to control contaminations, inert constitutants and harmful substances. Instead of multiple shredding and sorting steps in fuel preparation, the fuel is treated on the multiple steps of the combustion grate.

In order to ensure the best combustion performance, the step combustor is equipped with a robust and reliable feeding system. The material is fed by two screw conveyors which push the fuel very defined onto the first step of the grate. The cold fuel protects the conveyors from direct contact with the fire. The first step of the combustion grate is the platform-like ignition table, on which the incoming fuel piles up. The heap of fuel that acts as a seal against false air and prevents hot gas from flowing back into the screw conveyors and avoids overheating. The fuel remains on the ignition table for a few minutes where it is slowly dried, pyrolysed and ignited by contact with the hot air. The heap of fuel buffers also fluctuations in the feed rateand the calorific value of the alternative fuel.

A new type of step combustor – the prepol® SC-S

If boundary conditions of projects require a small chamber size, e.g. due to little available space in the preheater tower or availability of higher quality alternative fuels, it is worth thinking about the short version of the step combustor, the prepol®-SC-S. In this version, the combustor only consists of the ignition table, the first three steps of the grate together with the screw feeding system. The retention time is smaller compared to the prepol® SC but big compared to ordinary calciner loops: about 150-300 seconds can be achieved – a valuable add- on to boost existing installations. The combustor is integrated into the pre- calciner and does not require a separate tertiary air or meal connection – the radiation heat pyrolizes the alternative fuel on the grate   before the gases and size reduced particles enter the precalciner atmosphere. This makes retrofitting of the system even easier.