Target Fineness and Analytical Requirements for Refused Derived Fuels

The target fineness of refused derived fuels should always be aligned with the analytical method:

• Calorific value determination often requires only moderate fineness
• Ash, LOI, and elemental analyses benefit from consistent particle size and careful splitting
• XRF analysis on pressed pellets typically requires finer grinding to minimise particle‑size effects and improve comparability

Because alternative fuels also introduce mineral components into the kiln system, analytical results are directly relevant for process stability. Chlorine can promote buildups and internal cycles, while sulfur and alkalis influence clinker chemistry and emissions control. Documented and standardised preparation parameters - including splitting schemes, mill and sieve selection, rotor speed, and cyclone operation - are essential for reproducible data. Retaining reference sub samples and routinely running duplicates supports long term quality control and reliable refused derive fuel qualification.

Material‑Specific Preparation Considerations

Plastic Residues and Tyre‑Derived Fuels

Plastic‑rich, film‑like, or tyre‑derived refused derived fuels require careful temperature control during grinding. Cyclone operation supports cooling and rapid discharge, while dry‑ice embrittlement can prevent smearing and enable efficient size reduction. A combination of cutting mills for pre‑crushing and the Ultra Centrifugal Mill ZM 300 for final grinding is a proven approach for homogenising these materials. Depending on the sample properties (like bigger, more compact plastic blocks) the use of the 6-disc rotor in the pre-cutting step instead of the standard parallel section rotor might be beneficial.

Secondary fuels

Biomass and Wood Residues

Biomass‑based alternative fuels such as wood chips, bark, or demolition wood can be efficiently homogenised when the sample is representative and pre‑sorted. Metal contaminants (e.g. nails, screws, staples) must be removed prior to comminution to protect cutting tools and prevent analytical bias. Cutting mills are typically used for coarse size reduction, followed by further homogenisation and milling in the ZM 300 using an appropriate sieve. As for plastic samples, depending on the sample properties (like bigger twigs or more compact wooden blocks) the use of the more robust 6-disc rotor in the pre-cutting step instead of the standard parallel section rotor might be beneficial. A steady feed and optional cyclone operation in both, pre-crushing and fine- grinding step, help ensure stable and reproducible processing.

Used Paper and Paper Rejects

Used paper and paper rejects often show strong variability in fillers, coatings, and moisture content, making thorough homogenisation essential. Cutting mills are well suited for the initial shredding step. Using a V‑rotor and pre‑crumpling the material into smaller portions improves cutting efficiency. After mixing and splitting, the analytical portion can be milled in the ZM 300 to meet the requirements for ash, elemental analysis, or XRF.

Sewage Sludge as Refused Dervied Fuel

Sewage sludge is typically wet and prone to agglomeration, so a defined drying step (e.g. using the TG 200) is required before reproducible homogenisation is possible. After drying, the material may be pre‑comminuted in a cutting mill or jaw crusher, then thoroughly homogenised to minimise local concentration effects of ash‑forming minerals and trace elements. For fine grinding and maximum homogeneity, planetary ball mills such as the PM 400 are recommended. Milling parameters should achieve the required fineness while limiting excessive heat build‑up. A standardised drying and milling protocol ensures comparable analytical results of this refused derived fuel over time.

Refused Derived Fuels as a Reliable Alternative to Fossil Energy

When properly sampled, prepared, and analysed, refused derived fuels and other alternative fuels offer a reliable and sustainable pathway to reduce fossil fuel consumption in cement manufacturing. Standardised laboratory workflows, robust homogenisation strategies, and material‑specific preparation techniques are the foundation for consistent kiln performance, regulatory compliance, and long‑term process optimisation.