---
title: "Alternative fuel (AFR)"
description: "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."
canonical_url: "https://sylio.co/glossary/alternative-fuel"
last_updated: "2026-06-28T02:29:26.671Z"
---

**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](/glossary/rotary-kiln) destroy organic contaminants, and the alkaline raw materials neutralise acidic combustion products.

## Common AFR streams

- [RDF](/glossary/rdf-srf-tdf) — refuse-derived fuel
- [SRF](/glossary/rdf-srf-tdf) — solid recovered fuel (higher-spec RDF)
- [TDF](/glossary/rdf-srf-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](/glossary/sulphur-cycle-chloride-cycle-alkali-cycle)
- More [kiln-inlet build-up](/glossary/kiln-inlet-ring-snowman) and [preheater coatings](/glossary/build-up-coating-accretion)
- More frequent [chloride bypass](/glossary/chloride-bypass) operation
- More demanding [calciner](/glossary/calciner) burner control

[Sonic horns](/glossary/sonic-horn) and [air cannons](/glossary/air-cannon-air-blaster) on the preheater and kiln inlet become more important as TSR rises.

## Related terms

- [RDF / SRF / TDF](/glossary/rdf-srf-tdf)
- [Thermal substitution rate (TSR)](/glossary/thermal-substitution-rate)
- [Calciner](/glossary/calciner)
- [Chloride bypass](/glossary/chloride-bypass)
- [Sulphur / chloride / alkali cycles](/glossary/sulphur-cycle-chloride-cycle-alkali-cycle)