Thermal substitution rate (TSR) - alternative-fuel share in cement

Thermal substitution rate (TSR) is the percentage of total thermal energy in a cement kiln system supplied by alternative fuels instead of conventional fossil fuels such as coal, petcoke or gas. If a kiln gets 60 percent of its heat input from RDF, SRF, tyres, biomass residues or other alternative fuels, its TSR is 60 percent.

TSR is a central cement decarbonisation and cost metric, but it is also a combustion-stability and fouling metric. Higher TSR changes fuel moisture, particle size, ash chemistry, chlorine, sulphur, alkali input and where heat is released in the kiln system.

How TSR is calculated

TSR is based on energy, not mass. A tonne of wet biomass and a tonne of petcoke contribute very different heat inputs. Reliable TSR therefore requires fuel feed measurement, lower heating value, moisture correction and a clear boundary around which burners and calciner feeds are included.

Plants may report total TSR for the kiln line, biomass TSR separately, or fossil-fuel replacement by fuel class. Comparisons between plants should check the reporting basis.

Operational effects

Higher TSR can increase:

Build-up in preheaters, calciners and tertiary air ducts
Chlorine, sulphur and alkali cycling
CO excursions if fuel burnout is poor
Dust loading and ash variability
Demand on bypass, ID fan and gas-cleaning systems
Hopper and silo flow problems from heterogeneous fuels

High TSR plants usually need stronger fuel preparation, dosing control, burner tuning, bypass strategy and cleaning access than low-TSR plants.

Why it drives acoustic-cleaning demand

Alternative fuels often create more variable and stickier dust loads in ducts, filters, raw mills, cyclones and storage systems. Sonic horns are useful where the resulting deposits are dry and friable enough to move before they harden. They do not solve poor combustion, excessive chlorine input or hard kiln rings. In Sylio-style cement surveys, TSR is used as a warning that fuel chemistry and operating variability must be part of the cleaning design.

Operating meaning

Thermal substitution rate is more than a fuel-purchasing metric. In cement kilns and some industrial furnaces it changes flame shape, oxygen demand, moisture input, ash chemistry, chlorine input and the distribution of heat between main burner and calciner. A high TSR can be beneficial economically and for waste recovery, but only if combustion stability, product quality and emissions remain controlled.

The relevant variables include alternative-fuel calorific value, particle size, moisture, ash content, chlorine, sulphur, alkalis, biogenic carbon fraction, feeding point, residence time and how quickly the control system can respond to fuel variability. A tonne of prepared solid recovered fuel does not behave like a tonne of pulverised coal. Feed surges can create CO peaks, reducing conditions, incomplete burnout and kiln upsets even when the average TSR looks acceptable.

Fouling and maintenance implications

Higher alternative-fuel rates often change the deposit regime. More chlorine and alkali can increase preheater build-up, kiln inlet rings, sticky dust in riser ducts, baghouse loads and corrosion risk. More moisture can lower local temperatures and increase condensation problems. Maintenance teams should track TSR against pressure drop, build-up cleaning frequency, bypass dust chemistry, preheater cyclone stability, clinker quality and stack emissions.

Acoustic cleaning can support high-TSR operation where the limiting issue is dry dust accumulation in ducts, hoppers, preheater areas or filters. It cannot solve poor fuel preparation, inadequate residence time, chloride cycling or combustion instability. A good specification connects the cleaning system to observable symptoms such as riser-duct build-up or hopper bridging rather than to TSR alone.

Evidence for cleaning design

The useful TSR record links fuel data to plant symptoms. Operators should keep the fuel mix, chlorine input, bypass rate, calciner temperature, CO excursions, preheater pressure drop, baghouse pressure drop, hopper discharge and outage deposit samples in the same review. That makes it possible to tell whether a cleaning problem is driven by higher dust loading, a stickier ash chemistry, a feeding upset or an unsuitable access arrangement. Without that evidence, a high TSR number can be blamed for every deposit problem even when the actual cause is local gas velocity, poor fuel preparation or inadequate evacuation of loosened dust.