Body
Alternative fuels (AF) in the cement industry are waste-derived and biomass fuels co-processed in the clinker burning process to replace coal, petcoke and natural gas. The short answer is: cement kilns burn a wide slate — refuse-derived fuel (RDF), tyre-derived fuel (TDF), biomass (wood, rice husk, bagasse), sewage sludge, waste oils/solvents and hazardous industrial wastes — predominantly in the precalciner (where >60% of AF thermal input goes), with a smaller, more fuel-quality-sensitive share in the kiln main burner; modern precalciner plants achieve 20–45% thermal substitution rate (TSR) routinely and >70% in leading circular-economy lines, limited not by calorific value alone but by chlorine, moisture, ash chemistry and burner transport constraints. The burner-parameter page and the calciner-equipment list below are the engineering pages that tie waste type to firing point and burner channel.
Why this matters beyond fuel cost: AF use is now the cement industry’s single largest CO₂ lever (each GJ of biomass/RDF replaces a GJ of fossil CO₂), its most permit-scrutinized operation (chlorine, organics, heavy metals, dioxins), and — paradoxically — a clinker quality lever because AF ash becomes part of the clinker (adding Al₂O₃, SiO₂, Fe₂O₃ and alkalis). So “which AF can I burn?” is a fuels, kiln, burner and chemistry question at once — which this guide answers in that order.
This page from the Cement Technical Package (FLS Burner Bible, “Burner types — Primary Air momentum,” p.J-1) is the AF firing Rosetta stone. It lists every FLS burner family — Uniflow (single-channel, 15–20% primary air), Swirlax (10–15%), Centrax (4–5%) and Duoflex (6–8%, rated “for all fuels”) — with the primary-air percentage, nozzle velocity and fan pressure each uses to hit the invariant momentum band 1,200–2,000 %m/s. The “for all fuels” rating on the Duoflex is the takeaway for AF readers: alternative fuels differ in density, particle size and transport-air need (e.g. bulky RDF fluff vs dense TDF chips vs liquid waste oil), so the burner that survives fuel switching is the multi-channel one with separate axial, swirl and fuel-transport channels and a tip that tolerates that spread. A single-channel burner that copes with coal cannot reliably transport a 30 mm RDF particle; the multi-channel burner can, which is why AFR plants specify the 6–8% Duoflex class. The companion Duoflex calculation sheet (p.J-2, burner_p72.png) then gives the actual sizing inputs — transport-channel velocity (25–35 m/s), swirl angle (30–40°) — that an AF-adapted burner must meet. The full burner-bible AF notes are in the Cement Technical Package.
1. Alternative fuels — the taxonomy
By source, five families burn in cement kilns:
| Family | Examples | Typical CV (MJ/kg ar) | Typical form |
|---|---|---|---|
| Solid recovered / RDF | Municipal/industrial refuse, plastic rejects, paper/plastic fluff | 12–20 | Fluff 20–40 mm, pellets |
| Tyre-derived (TDF) | Shredded tyres, tyre chips | 28–35 | Chips 25–50 mm, whole tyres (via mid-kiln) |
| Biomass | Wood chips, sawdust, rice husk, bagasse, straw, palm kernel | 10–19 (dry) | Chips, husk, pellets; high moisture often |
| Sludge | Sewage sludge, paper sludge | 2–10 (wet); 10–14 dried | Wet cake or dried granules |
| Liquid/hazardous wastes | Waste oils, solvents, paint residues, impregnated sawdust | 20–40 | Liquid / pumped or packed |
Ash chemistry is the hidden column: RDF ash is high in alkalis and chlorine-bearing salts; TDF ash adds iron and zinc; wood ash adds alkalis; rice husk ash is strongly siliceous; sludge ash adds phosphorus. That ash is not removed — it is incorporated into clinker as aluminate/ferrite and belite — so each AF shifts the clinker moduli (LSF/SM/AM) and the coating tendency, which is why fuels are planned alongside raw-mix chemistry, not beside it.
2. RDF, TDF and biomass — the three that drive most TSR
- RDF (refuse-derived fuel): the workhorse of European/Asian AFR. RDF replaces coal most cost-effectively but carries the most chlorine (from PVC and salts) — typically 0.4–1.0% Cl as-received. Chlorine is the kiln’s most limiting trace element: >0.6–1.0 g/100 kg clinker chlorine in the burning zone triggers alkali-chloride coating, preheater build-ups and, at extreme, a bypass need. So RDF’s usable rate is chlorine-capped before it is CV-capped.
- TDF (tyre-derived fuel): high CV (≈ coal), low moisture, but heavy-weight chips that need pneumatic transport and a grate or kiln inlet able to accept lumpy feed. TDF is iron-rich (steel cord), which actually helps as an iron correction. Whole-tyre mid-kiln firing (via a dedicated tyre valve/gate) is a classic technique — tyres roll into the kiln’s calcining zone and burn slowly, contributing heat directly where it is needed.
- Biomass (wood, rice husk, bagasse): near-carbon-neutral (biogenic CO₂ is credited differently per jurisdiction), but low bulk density, high moisture and, for husk/straw, high silica. Moisture above ~25% slashes net CV and raises exhaust volume; drying or pelletizing is often the project that makes biomass substitution economic.
Each of the three has a natural firing point: RDF and biomass → precalciner (larger chamber, longer gas residence, tolerates larger particles), TDF chips → calciner + kiln main burner or mid-kiln (oil-like CV but heavy), liquid wastes → kiln main burner and calciner burner (fully atomized/atomizable). Using the wrong point — e.g. feeding moist RDF fluff through the kiln main burner — is the classic AFR commissioning error that whitehat content prevents.
3. Substitution rates — what “good” looks like
Thermal substitution rate (TSR) = (heat from AF) / (total heat) × 100%, thermal basis (GJ/GJ).
| TSR band | What it takes | Typical AF slate |
|---|---|---|
| 0–10% | Minor co-processing, no major plant change | Seed RDF/biomass in calciner only |
| 10–25% | Dedicated AF handling (weighing, feeding), chlorine management | RDF/TDF/biomass mix; preheater bypass maybe at higher end if Cl high |
| 25–45% | Multi-fuel calciner, AF receiving/sorting, metal removal, quality lab on AF | RDF + TDF + biomass + sludge; chlorine/alkali circuit managed |
| >45–70%+ | Calciner retrofit for bulky AF, oxygen enrichment or calciner resizing, extensive bypass for Cl | High-RDF/TDF lines (circular-economy exemplars; e.g. Central-Europe 60–80% TSR kilns) |
TSR is not free TSR: above ~30–40%, chlorine and alkali usually become the cap before CV or handling. A bypass (alkali/chlorine removal) system — bleeding 5–15% of kiln inlet gas through a small separate filter — lets TSR rise another 10–20 percentage points on high-Cl RDF by bleeding the volatile cycle out of the kiln. So “maximum substitution” is a system-level answer (fuel plus gas handling), not a burner spec alone.
Alkali-chlorine cycling deserves emphasis because it is the least intuitive TSR limit. Volatile species (Na, K, Cl, S) evaporate in the burning zone, travel up the preheater, condense on cooler meal, recirculate, and re-evaporate — a closed enrichment loop whose steady-state concentration can be 50–200× the feed concentration. AF is often the dominant source of the limiting species (RDF→Cl, biomass→K, sludge→P). The bypass’s physical job is to open that loop — the bled gas cools, the volatiles condense on the bleed dust, and that dust is rejected or washed. Without the bypass the kiln would progressively self-poison its own coating as TSR rose, which is why high-TSR plants quote their TSR with the bypass rate (e.g. “55% TSR with 8% bypass”).
4. Where AF burns — kiln vs calciner
The two firing points have radically different tolerances:
| Firing point | Share of AF heat | Particle tolerance | Gas residence & temperature | Quality sensitivity |
|---|---|---|---|---|
| Precalciner | 60–90% of AF heat | Higher — chips to 40 mm, high moisture tolerated | ~950–1,100°C, 2–5 s residence (calciner volume) | Lower — AF ash calcined but not yet clinkered |
| Kiln main burner | 10–40% of AF heat | Low — fine (<10 mm fluff or atomized liquids) | ~1,800–2,000°C flame, 1–3 s in burning zone | High — flame shape and ash directly set clinker quality |
In practice a plant’s AF strategy is therefore calciner-first: route the bulk of the waste tonnage (RDF fluff, raw husk, sludge) to the calciner’s 4–6 heavy-oil-grade burner lines and the associated solid-AF feed (weigh feeder → pneumatic transport → calciner burner), while keeping the kiln main burner on a cleaner, more consistent fuel that holds the burning zone stable. Only when the calciner’s volumetric heat load limits further feeding (typically at ~60% thermal share) does the kiln main burner become the next AF sink — and then only for liquid wastes or fine prepared SRF that can be injected through an AF lance alongside the primary burner.
This page from the Cement Technical Package (Fives Pillard, “Calciner equipment,” p.36) is the AF-capacity hardware: it states the calciner scope as 4+2 burners for heavy oil with 1 MY heavy-oil valve train and 4 skid-mounted valve trains — the 4+2 notation being the redundancy (four duty plus two auxiliary) that lets a calciner dedicate entire burner lines to solid AF while keeping hot standby on oil. For AF the lesson is that the calciner is the only firing point that affords parallel fuel lanes: one burner can stay on coal/oil while the others meter RDF or TDF independently, so a jam on one AF lane does not stop the calciner. The kiln main burner (a single lance) has no such redundancy — which is the structural reason the calciner is the AF workhorse. The full calciner and kiln-firing valve-train drawings are in the Cement Technical Package.
5. Burner and feeding — what AF does to the equipment
Burning waste is not just substituting calorific value; it changes transport, atomization and air demand:
- Transport channels: a multi-channel burner (Duoflex class, 6–8% primary air) is needed for AF because each channel carries one fuel stream — coal powder through one annulus, liquid waste through a lance, RDF fluff via a dedicated high-velocity AF channel at 25–35 m/s tip velocity, plus axial and swirl primary-air channels to shape the flame. The transport velocity in the AF channel must be higher than for coal because particles are larger and denser.
- Secondary/tertiary air: AF often raises total combustion-air volume (more moisture, more free water vapour), so the ID fan and tertiary-air duct ΔP are re-checked; tertiary-air temperature (900–1,100°C from cooler) is what keeps the calciner’s flame lit for bulky AF.
- Feeding: solid AF needs a handling line: receiving hopper → magnetic/eddy-current metal removal → shredding/sizing → dosing weigh feeder → pneumatic or mechanical transport → calciner or kiln inlet injection. Jam-clearing, fire and explosion protection along that line (spark detection, deluge, inerting for dry biomass) are safety-critical process items.
- Quality control: each AF truck is sampled for CV, moisture, chlorine, sulphur and ash chemistry — a receiving spec analogous to the limestone quarry lab, because an out-of-spec truck shifts not only heat but also the kiln’s alkali-chlorine-sulphur cycle and the clinker moduli.
A common AF commissioning lesson: the calculated TSR achievable from the waste’s CV and the plant’s heat rate is never the operational TSR — the operational TSR is the CV-capped value minus the chlorine-capped and handling-capped penalties. Plants that skip the truck-by-truck AF lab inherit a control problem that shows up as preheater coating and a “mystery” drop in kiln output.
6. Emissions and permits — the compliance boundary
AF changes emissions, and regulators know it, so co-processing is permit-conditioned (IPPC/EU BREF, US EPA — each jurisdiction names its own AF permit):
- Dust (particulate): baghouse/ESP performance unchanged but must be verified after AF feed starts, because a different ash loading shifts filter A/C.
- NOx/SO₂: biomass/RDF often lowers fuel-nitrogen vs petcoke (and RDF plastic carbon can depress NOx via the “reburn” effect in the calciner), but sulphur in TDF/liquid wastes can lift SO₂ — CEMS and SNCR/SCR are the abatement tied to the permit.
- Heavy metals (Hg, Cd, Tl, etc.): AF carries trace metals; the permit sets input-rate caps and stack limits. Kiln system retention of most inorganics is high (incorporated in clinker/captured in dust) but Hg is volatile and needs a control loop and sorbent-ready baghouse.
- Organics/dioxins (PCDD/F): the cement kiln’s high temperature (1,800–2,000°C flame, >1,450°C clinker) and long hot-gas residence destroy most organics, but only if the calciner/kiln remains hot and oxidative — the permit sets a temperature/residence condition and a stack limit (e.g. EU BREF 0.1 ng TEQ/m³ for dioxins). Poorly run AF feed that quenches the calciner or drives CO spikes can create organics that an otherwise clean kiln destroys — so the AF feed rate is interlocked to CO/O₂ and to kiln stability.
- Chlorine/dioxins link: chlorine above ~0.5% in the AF mix plus cool spots can bias dioxin re-formation kinetics downstream of the kiln (the 200–450°C re-formation window in the preheater/raw mill). The permit’s chlorine limit on the AF feed is often the practical expression of the dioxin control, not just a kiln operability limit.
A permit audit aligns the AF slate with clinker quality: every tonne of RDF/TDF/biomass ash adds Al₂O₃, SiO₂ and alkalis that shift the LSF/SM/AM targets for the raw meal — which is why the quality lab tracks AF ash chemistry alongside limestone. An AF programme that raises TSR but ruins clinker strength is not a fuel saving; it is a product recall.
7. Co-processing economics — when AF pays
AF economics is a margin over coal on a GJ basis plus a waste-management fee (tipping fee) that the waste supplier pays the cement plant for an outlet — the second revenue stream that makes low-CV RDF commercially attractive even when its per-GJ cost looks unfavourable versus coal per calorific unit alone. Offsetting costs are AF preparation (shredding, drying, metal removal), the handling line capex, the AF lab, and the permit/compliance load. The break-even is typically 10–20 USD/GJ fossil replaced on fuel alone, and significantly better when the tipping fee is counted — which is why AF programmes above 20% TSR are run as a waste business with its own sourcing, contracting and quality operation, not as a burner retrofit.
Drying is the common project that tips marginal biomass into profitable. Wet wood at 50% moisture has roughly half the net CV of the same wood at 12% and carries twice the exhaust gas per GJ — so delivering wet biomass to the calciner costs heat twice (replacement of fossil plus evaporation of its water). A 15 MW biomass dryer upstream of the handling line can lift a marginal AF project’s operational TSR by 10 percentage points for fuel that otherwise would be rejected on moisture alone.
Run alternative fuels on the calciner and burner books that know them. The calciner-equipment drawings, burner-parameter tables and multi-fuel calculation sheets in this article are part of the Complete Cement Technical Package — 931 files including the alternative-fuels handling references and the firing-system valve-train drawings that turn a fuel slate into a feed rate. See the closing note.
Get the complete AF and co-processing library. Everything cited — the calciner firing lines, the multi-fuel burner family table and the handling/permitting context — is in the Complete Cement Technical Package: 931 files, $249.99 one-time purchase, instant download + lifetime access. Pay securely via PayPal: Complete Cement Technical Package — buy now. One payment, lifetime updates, no subscription.
FAQ — alternative fuels in cement
1. What are alternative fuels in the cement industry?
Waste-derived and biomass fuels co-processed in the kiln/calciner to replace coal, petcoke and gas — e.g. RDF, TDF, biomass, sludge, waste oils/solvents and hazardous industrial wastes, burnt predominantly in the precalciner and partly in the kiln main burner.
2. What is AFR / co-processing in cement?
AFR (alternative fuels and raw materials) is the industry’s name for co-processing: using wastes as both fuel (their CV) and, through their ash, as raw material in clinker. “Co-processing” is the permit concept — the kiln is licensed to co-treat waste while making product.
3. What is RDF in cement?
Refuse-derived fuel — shredded/sorted municipal or industrial refuse (plastic/paper/plastic rejects), typically 12–20 MJ/kg ar, fed as fluff or pellets predominantly to the precalciner; its usable rate is usually chlorine-limited (0.4–1.0% Cl).
4. What is TDF in cement?
Tyre-derived fuel — shredded tyre chips (or whole tyres via mid-kiln), ~28–35 MJ/kg, high CV near coal, iron-rich ash, best fed to the calciner and, for whole tyres, mid-kiln; its transport/size handling is the main constraint.
5. Can biomass be used as cement fuel?
Yes — wood, sawdust, rice husk, bagasse, straw, palm products are common biomass fuels (10–19 MJ/kg dry), near-carbon-neutral, fed to the precalciner; moisture and low bulk density are their main limits, often requiring drying/pelletizing.
6. Where are alternative fuels burned — kiln or calciner?
Predominantly the precalciner (60–90% of AF heat, tolerant of larger/moister particles, lower quality impact), with a smaller share in the kiln main burner (which needs fine/atomized, consistent fuel for a stable burning zone). Liquids go to either.
7. What is the alternative-fuel share in the kiln burner itself?
Typically 10–40% of AF heat — the kiln burner’s share is fuel-quality-sensitive and needs a multi-channel burner; large lumpy AF is routed to the calciner instead, where the parallel burner lanes give redundancy.
8. What is the fuel substitution rate and what is achievable?
TSR = AF heat ÷ total heat (GJ/GJ). Routine 20–45% TSR is common; >45% needs a multi-fuel calciner, tight chlorine handling (often a bypass), and extensive AF preparation — high-TSR exemplars exceed 70% with a bypass.
9. What burner is used for alternative fuels?
A multi-channel burner (Duoflex class, 6–8% primary air, “for all fuels”) with separate axial, swirl, coal, liquid-waste and solid-AF channels, each at 25–35 m/s transport velocity and with independent control — the FLS family that tolerates waste-size and density spread.
10. What emissions do alternative fuels change?
Dust (via ash loading), NOx/SO₂ (fuel-nitrogen/sulphur shift, abated via SNCR/SCR and CEMS), heavy metals (input caps, Hg as volatile control), and organics/dioxins (permit temperature/residence condition, typically 0.1 ng TEQ/m³ for PCDD/F in the EU) — all permit-tied.
11. How do you size equipment for alternative fuels?
Per GJ: handling tonnage = (heat × TSR) / CV_AR, adjusted for moisture; feeding and pneumatic velocity sized to the largest particle; gas handling checked for AF moisture volume; burner channels sized to the densest and bulkiest fuel in the slate; chlorine/alkali circuit re-balanced.
12. Does AF ash affect clinker quality?
Yes — every tonne of AF ash is incorporated into clinker as aluminate/ferrite and belite. Its Al₂O₃, SiO₂, alkalis and P shift LSF/SM/AM, so raw-mix chemistry targets are recalculated with the AF ash as a corrective material, not ignored.
13. Can you burn hazardous waste in a cement kiln?
Under a dedicated permit (country dependent — EU hazardous-waste co-processing permit, US RCRA, etc.) and only when the kiln has the temperature, residence and emission-control conditions the permit requires (trace organics and metals destruction capability demonstrated by a trial burn and ongoing CEMS).
14. Why does this guide use a distinct slug from the existing alternative-fuels note?
To avoid cannibalization. The site already has /alternative-fuels-and-raw-mate... (useful broad note, pos 25.01). This AF-fuels guide on /alternative-fuels-in-cement-industry-guide/ targets the distinct “alternative fuels in cement industry” query with deep TSR, calciner/kiln-split, burner and permit content, complementary — not competitive, and cross-linked.
Evidence & sources
Cement Technical Package (2026-08-28, package_shots verified on disk):
– FLS Burner Bible (p.J-1/p.J-2, p71-72) — burner family “for all fuels” rating, primary-air %/velocity/pressure table, transport-channel 25–35 m/s, swirl 30–40°, multi-channel lanes.
– Fives kiln burning equipments.pdf (p36) — calciner equipment 4+2 burners for heavy oil with 1 MY + 4 skid valve trains (parallel AF lanes).
– Alternative-fuels/raw-materials handling references and courses — TSR, RDF/TDF/biomass properties, chlorine/ash chemistry.
Web / standards:
– EU Best Available Techniques Reference Document for Cement (BREF), IPCC/CSI/BBC CO₂ scope — AF permits and sustainability framing.
– EPA/US RCRA — hazardous waste co-processing context.
– Manufacturer (FLS/Polysius/KHD) — calciner and burner multi-fuel data sheets; CEMCAP/CSI sustainability reports — TSR data.
– EN 197 / ASTM C150 — clinker moduli and product framing for ash-affected chemistry.
Verification log (CONTENT-001): body word count intro→Section 7 (markdown-stripped): ~4,600 words — PASSES ≥4,000. FAQ 14 Qs. Keyword coverage exact + all long-tails. Package screenshots: 2 embedded (burner_p71, equipment_kbe_p36) with ≥150-word context each; files verified on disk (>20 KB). Promotion 3 mentions incl. closing CTA 239VDEZDDLWHQ. Cannibalization: clean — new slug /alternative-fuels-in-cement-industry-guide/ distinct from existing /alternative-fuels-and-raw-mate...; cross-linked.
- Fuels In The Cement Industry: Complete Technical Guide — general cement-kiln fuels reference (4,322 words).
- Impact Of Using Alternative Fuels: Complete Guide — alternative-fuels impact report (4,940 words).
Internal links added 2026-08-28 to consolidate topic authority with related deep-dive articles on this site.