Alternative fuel

An alternative fuel is a non-traditional or waste-derived fuel used to replace part of the fossil fuel in a cement kiln, calciner or industrial furnace. In cement, the term is often paired with alternative raw materials as AFR. Typical fuels include RDF, SRF, tyres, waste wood, plastics, solvents, sewage sludge, biomass residues and selected industrial wastes.

The benefit is reduced fossil-fuel demand and, where biogenic carbon is present, lower reported fossil CO2. The difficulty is variability. Alternative fuels can add moisture, chlorine, sulphur, alkalis, metals, ash and coarse fragments that change kiln stability and gas-path fouling.

Where it appears

In cement plants, alternative fuel is fed at the main burner, calciner, riser duct or separate combustion chamber depending on particle size, calorific value and burnout time. High substitution rates usually require improved dosing, chloride bypass capacity, combustion control, oxygen management and stronger build-up control in the preheater.

Failure modes

Poorly controlled alternative fuel can cause CO spikes, reducing conditions, rings, snowmen, cyclone blockages, chloride cycles, mercury recirculation, unstable free lime and high dust loading. Wet or coarse fuel can burn late, shifting heat release into the wrong part of the system. Chlorine and alkali enrichment are central drivers of sticky build-up.

Acoustic-cleaning relevance

Sonic horns are relevant where alternative fuels increase dry powder build-up in preheater cyclones, riser ducts, calciner areas, ID-fan ducts and dust hoppers. They are preventive tools: they keep early deposits moving before they mature into hard coating. They do not replace process controls for fuel preparation, dosing stability, bypass operation or kiln chemistry.

Plant variables

Alternative fuels are judged by more than calorific value. Operators need moisture, particle size, ash content, chlorine, sulphur, alkali metals, heavy metals, volatile matter, biogenic fraction, storage behaviour and feed consistency. A fuel that burns acceptably in a trial can still create long-term problems if seasonal composition, supplier sorting or shredding quality changes.

In cement kilns, biomass boilers, waste-to-energy plants and industrial furnaces, the fuel affects flame shape, oxygen demand, residence time, ash stickiness and emissions-control load. High moisture can reduce flame temperature and raise gas volumes. Chlorine and alkalis can increase build-up in preheaters or backpasses. Metals and fine ash can change dust resistivity or catalyst fouling. These changes often show up first as unstable draft, rising pressure drop, more frequent cleaning or deposits in previously clean areas.

Handling and safety

Storage and conveying are central to alternative-fuel reliability. RDF, SRF, biomass chips, pellets, tyre-derived fuel and sludges have different risks: bridging, self-heating, odour, dust explosion, biological degradation, tramp metal, variable bulk density and poor metering. Feed systems need fire detection, explosion protection, isolation, housekeeping and maintenance access that reflect the actual material, not a generic fuel label.

Acoustic-cleaning relevance

Acoustic cleaning can help manage the extra fouling created by variable ash, especially in ducts, preheater vessels, economisers, air heaters, hoppers and filters. It is not a licence to ignore fuel specification. The cleaning plan should be reviewed when the fuel blend changes because a horn placement that works for dry friable ash may struggle with wet chlorine-rich deposits or molten alkali salts. Trend data before and after fuel changes is often the best way to prove whether the fouling regime has changed.

Monitoring context

Alternative-fuel trials should include a fouling and cleaning baseline. Useful records include fuel analysis, blend percentage, kiln or boiler load, pressure-drop slope, deposit photographs, bypass dust chemistry, emissions and cleaning-system activity. Without that baseline, a deposit increase can be wrongly blamed on the cleaning system or missed until production is constrained.

Plants should also check whether the alternative fuel changes explosion, odour or biological hazards in storage. A cleaning device fitted to a fuel bin may need a new hazardous-area or fire-risk assessment after the material changes.

Acceptance checks

A fuel substitution should not be judged only by stable flame or heat input. The trial should also check deposit return rate, hopper flow, filter pressure drop, reagent use and ash disposal quality. These secondary effects often decide whether an alternative fuel is operable at scale.

Related terms

• Build-up, coating and accretion
• RDF, SRF and TDF
• Sulphur, chloride and alkali cycles