From tires and sludge to refuse-derived fuel, alternative fuels are moving from niche to normal in clinker kilns, cutting costs and carbon while forcing smart tweaks to burners, cyclones, and controls.
Cement is among heavy industry’s biggest energy hogs — 3–4 GJ per tonne of clinker — and it’s blamed for roughly 5% of global CO₂. With world cement production now above 4 Gt/yr and ~2.2 Gt CO₂ in 2016, the incentive to displace coal and petcoke is obvious (cementequipment.org) (cementequipment.org).
Enter alternative fuels (AF): biomass (straw, wood chips, husks, nutshells, sewage/agro‑industrial sludge, manure), refuse‑derived fuels (RDF) made from municipal solid waste, tyre‑derived fuel (TDF), plastics, spent solvents, and used oils (IntechOpen) (cementequipment.org). Dry biomass clocks ~15–20 MJ/kg versus coal’s ~24 MJ/kg, while scrap tires hit ~30–36 MJ/kg but carry ~20% Zn and other metals (IntechOpen) (IntechOpen).
The economics are leaning in: AF are often cheaper than coal — sometimes so attractive that plants are paid gate fees to take waste — with one study citing ~€10.9/t for solid waste fuel and €15/t for RDF, delivering fuel cost savings up to ~€7.8 per GJ versus coal (cementequipment.org) (MDPI).
Adoption rates and regional benchmarks
In Europe, AF displace ~2.5 Mt of coal annually, with some countries — France ~52%, Switzerland 25% — using ≥15%–50% AF in clinker kilns (IntechOpen) (IntechOpen). Leaders like Poland reach 60–85% AF share (The Jakarta Post) (cementequipment.org).
Indonesia is moving: PT Semen Indonesia Group used 0.5 Mt of AF in 2024, lifting its thermal substitution rate (TSR, the share of total heat from AF) to 7.56% from 7.27% in 2023 (Antara News). Indocement boosted AF from 2.3% (2015) to 11.7% by mid‑2021 (The Jakarta Post).
Combustion profile and moisture burden
AF alter combustion. Many wastes arrive wetter and with higher volatiles. Moisture matters: every 1 t of water in fuel must be evaporated in the system, consuming ~2.26 MJ (latent heat) and swelling exhaust volumes; high‑moisture fuels demand greater induced‑draft (ID) fan capacity and can quench the flame — ID fan turn rate is often the bottleneck, and flame temperatures drop as moisture rises (MDPI) (MDPI). Many plants dry biomass using surplus heat or prefer low‑moisture streams.
High‑volatiles in the main burner lengthen the flame and suppress its peak temperature, nudging the hottest zone downstream and enlarging the pre‑cooling zone, which can trim burnout efficiency unless air/fuel is retuned. AMP tests show kilns must retune burner air/fuel ratios; operators often add secondary air or deploy multi‑olet burners to keep AF in the flame core and prevent quenching (cementequipment.org) (cementequipment.org).
Fuel variability and dosing control
AF streams are heterogeneous: heat content, particle size, and composition vary batch‑to‑batch, so many kilns co‑fire a steady portion of coal or fuel oil as a buffer (MDPI). Mass flowmeters and gravimetric feeders — screw conveyors on load cells — tighten bulk AF control (MDPI).
Pneumatic lines should be short and straight to avoid blockages, with convey velocities of 25–40 m/s and load below ~4 kg fuel/kg air; feed points are typically near the burner or in the precalciner to minimize air leakage and fuel lag (MDPI) (MDPI) (MDPI).
Heat split and calciner strategy
Fuel placement shapes the temperature profile. Low‑calorific wastes favor the calciner; higher‑energy wastes (tyres, used oils) can replace coal at the main burner. Many modern kilns already burn up to ~60% of total heat in the calciner, which runs at ~800–900 °C and lights high‑volatile fuels quickly (PubMed) (MDPI). High‑volatile AF often go to the calciner or tertiary air, while kiln inlets must stay hot to avoid cold rings. Adding a 5th cyclone stage can reduce fuel consumption by ~200 kJ/kg clinker (500 kcal/t) by improving heat recovery (MDPI).
Emission outcomes under high-temperature burn
Under hot (≥1450 °C) and oxidizing kiln conditions, AF achieve burnout comparable to coal when air/fuel is tuned. Plants report CO, NOₓ, SO₂, and dioxins within limits, with NOₓ often lower due to “reburn” in the calciner and reduced flame temperature (cementequipment.org) (IntechOpen). Tests with RDF show little change in NOₓ/CO₂ and minimal dioxins thanks to ≥850 °C and >2 s residence time enforced by waste rules, per the EU Waste Incineration Directive (cementequipment.org) (MDPI).
Incomplete combustion can lift particulates or hydrocarbons, so baghouse efficiency must be maintained or improved. Volatile heavy metals — Hg in solvents, Cd/Pb in sludge — can partly escape; modern bag filters and, where needed, Hg capture systems address this (MDPI). Many plants inject powdered sorbents such as activated carbon when mercury control is required, consistent with the paper’s note that “many plants also inject activated carbon to scrub Hg” (MDPI).
CO₂, NOₓ, SO₂ and metals in the stack
Biomass‑derived fuels are treated as carbon‑neutral in most accounting frameworks; swapping fossil fuels for biomass thus cuts net CO₂ by roughly the fuel portion. Analyses suggest each 1% of fossil fuel replaced by AF trims ≈2–3 kg CO₂ per tonne of cement (cementequipment.org) (cementequipment.org). Semen Indonesia’s 7.56% TSR corresponded roughly to a 38% lower CO₂ intensity — 494 kg CO₂/t versus 800 kg/t — though part of that reduction also comes from higher clinker substitution (e.g., fly ash) (Antara News) (Antara News). Poland’s 60–85% AF users see commensurate CO₂ savings (The Jakarta Post).
Almost all sulfur in any fuel becomes SO₂, but many wastes are low‑sulfur, and the alkaline kiln charge captures SO₂ and HCl; chloride and sulfate mostly remain bound in clinker or dust (cementequipment.org). NOₓ can drop via reburn or lower flame temperatures. One sludge co‑incineration study found increased PAH and heavy‑metal emissions (especially Cd/Pb in fly ash) and elevated downwind ambient levels — evidence that such wastes demand rigorous flue‑gas cleaning and monitoring; many cement plants meet incinerator‑level standards with advanced bagfilters, scrubbers, or SNCR/SCR for NOₓ (MDPI). Typical dust emissions remain below ~20 mg/Nm³ because ESPs/baghouses capture >99%; metals largely follow fly ash into filters, with Hg partly escaping — typical stack Hg ~1–2 µg/Nm³ — hence the need for sorbent injection (MDPI).
Clinker chemistry under ash loading
More than 95% of fuel ash goes into the clinker or dust, so ash chemistry matters. Woody biomass ash (rich in Ca, K) can lift alkalis; excessive alkalis (>0.6–1.0% K₂O+Na₂O in the raw meal) or chlorine (>0.1%) risks kiln build‑ups or cement durability issues (IntechOpen) (IntechOpen) (cementequipment.org). Operators limit total chloride input; empirical guidance: install a bypass if total Cl >0.3–0.4 kg/t clinker (≈50% AF @ 0.5% Cl) to prevent preheater rings, and manage alkali balance by bleeding high‑alkali dust or purging fuel ash (MDPI).
Sewage sludge introduces P and S (P₂O₅ ~4–5%), while plastics/tyres bring Cl and Zn. Trace additions like ZnO and F act as mineralizers/fluxes; up to ~0.2–0.5% ZnO is tolerable, and many cement standards cap ZnO ≤0.5%. Scrap tires — ≈20% ZnO in ash — are used sparingly for this reason (IntechOpen). AF firing often yields slightly larger C₃S (alite) crystals with a more glassy matrix, marginally slowing early strength gain and making clinker a bit harder to grind; experienced plants tweak raw mix accordingly, and trials keep compressive strength and set within spec (cementequipment.org) (cementequipment.org) (cementequipment.org). Notably, Indonesian PCC cement made with AF was measured at 494 kg CO₂/t — far below conventional cement at 800 kg/t (Antara News).
Burner, calciner, and feeder modifications
Making AF work usually means hardware changes. Fuel feeding needs enclosed conveyors, hoppers, gravimetric screw weigh feeders (on load cells), and sealed rotary airlocks; 3‑chamber rotary or double‑pendulum valves are common to limit air inleakage (MDPI) (MDPI).
Burners are adapted or replaced with AF‑capable designs (e.g., Pillard Rotaflam, M.A.S., KHD Pyrojet). The AF injection pipe should enter straight into the burner so fuel hits the core flame; the dosing station sits close to the throat to prevent blockages, and conveying air is kept minimal to avoid cooling. Plants use co‑annular or multi‑olet burner heads, typically staying under ~4 kg AF per kg burner air to avoid pulsations, while retuning primary/secondary air splits and adding swirl/staging to complete combustion (MDPI) (MDPI) (MDPI).
High‑TSR setups rely on an in‑line precalciner. Adding a calciner burner downstream of the 4th cyclone allows >50% of heat to fire at ~800–900 °C, ideal for high‑volatile AF. Calciner feeds run either pneumatic (15–20 m/s in high‑temperature steel lines) or gravity via enclosed conveyors; emergency shutdown cuts fuel instantly and keeps the fan on briefly to purge lines (MDPI) (MDPI) (MDPI). Where chloride input runs high, an alkali bypass from the cyclone to the stack prevents rings; many operators also inject limestone or lime in the calciner to neutralize chlorine and sulfur (MDPI).
Draft, cyclones, and energy recovery
More moisture and fines call for stronger draft: plants commonly uprate ID fans or add auxiliaries. Cyclone internals may be revised to handle RDF particulates. Moving from 4 to 5–6 cyclone stages lowers exit gas temperatures and improves energy recovery; one modernization case cut heat use ~200 kJ/kg clinker with an added 5th stage (MDPI).
Controls, monitoring, and NOₓ reagents
AF co‑processing leans on analytics: many systems use real‑time scanning (XRF) of incoming RDF to track chloride, moisture, and other setpoints, and Continuous Emission Monitoring Systems (CEMS) on stacks are standard (cementequipment.org) (MDPI). For NOₓ, plants apply SNCR/SCR as needed; reagent skids often rely on precise metering with dosing pumps to deliver ammonia or urea solutions to the right temperature window (no new performance claims added; aligns with the paper’s SNCR/SCR note).
Costs, carbon, and a cautious path forward
Studies consistently find that, with burner/feeder/calciner design and allowances for moisture and chloride bypass, co‑firing AF reduces net fuel cost and CO₂ with little adverse effect on normal stack emissions (cementequipment.org) (cementequipment.org). Vietnamese or Indonesian firms considering AF should analyze local waste availability (palm‑kernel shells, rice husks, MSW), invest in preprocessing (shredding/drying), and upgrade kiln systems accordingly — the same best‑practice pathway seen in Europe and early Indonesian adopters (MDPI).
Source links
Sources: Peer‑reviewed and industry data as cited above (cementequipment.org) (IntechOpen) (Antara News) (The Jakarta Post) (cementequipment.org) (MDPI) (MDPI) (MDPI) (cementequipment.org), plus regulatory and company reports (The Jakarta Post).
