Glossary
Cement
Alternative fuel
Also known as AFR, alternative fuels, secondary fuel, waste-derived fuel.
Alternative fuel (AFR) — sometimes secondary fuel or waste-derived fuel — refers to non-fossil energy sources used to replace coal, petcoke and natural gas in cement-kiln combustion. The cement industry is the largest single user of AFR worldwide because the high temperatures and long residence times in a rotary kiln destroy organic contaminants, and the alkaline raw materials neutralise acidic combustion products.
Common AFR streams
- RDF — refuse-derived fuel
- SRF — solid recovered fuel (higher-spec RDF)
- TDF — tyre-derived fuel
- Sewage sludge (dried)
- Animal-meal residues
- Agricultural residues
- Used solvents and waste oils
- Plastic and paper fractions
Drivers
- CO₂ reduction — biomass fractions reduce net carbon emissions
- Waste-disposal economics — gate fees offset fuel cost
- EU ETS pressure — carbon prices punish fossil-fuel firing
- Regional waste-management policies
Operational consequences
AFR firing typically intensifies several existing operational problems:
- More chlorine and sulphur in the sulphur and chloride cycles
- More kiln-inlet build-up and preheater coatings
- More frequent chloride bypass operation
- More demanding calciner burner control
Sonic horns and air cannons on the preheater and kiln inlet become more important as TSR rises.
Related terms
Related terms
- RDF, SRF and TDFRDF (refuse-derived fuel), SRF (solid recovered fuel, higher spec) and TDF (tyre-derived fuel) are the three dominant waste-derived alternative fuels for cement kilns and WtE boilers.
- Thermal substitution rateTSR is the percentage of total kiln-energy input supplied by alternative fuels rather than fossil fuel. The headline AFR adoption metric for cement-industry decarbonisation.
- CalcinerA calciner is a combustion chamber in the cement preheater tower where raw meal is pre-calcined (CaCO3 → CaO) before entering the rotary kiln. Common site for AFR firing.
- Chloride bypassA chloride bypass extracts a slipstream of kiln gas before the preheater to remove chlorine from the recirculating Cl cycle. Essential at high TSR; the bypass duct itself fouls heavily.
- Sulphur, chloride and alkali cyclesSulphur, chloride and alkali cycles describe how volatile species evaporate from the kiln burning zone, condense in the cooler preheater, and recirculate. Their build-up drives kiln-stop fouling.