Cement’s preheater and precalciner decide the heat balance long before the kiln sees the load. Keep the raw‑meal feed steady, lock the temperature profile, and close the loop with online calcination analyzers — or watch energy use and clinker quality swing.
The suspension preheater and precalciner do the heavy lifting in limestone decarbonation (calcination: driving off CO₂ from CaCO₃) before raw meal reaches the rotary kiln. In a typical 4–5 stage tower, raw meal enters near ambient (~50 °C) and exits the bottom cyclones near ~800 °C, while gas co‑flows from roughly ~1100 °C down to ~330 °C (cementequipment.org). Maintaining this designed gradient keeps most calcination in the calciner — not unevenly in upper cyclones.
When that balance slips, costs climb. Overly high calciner temperatures can “excessively decompose” raw meal in the cyclones, driving build‑ups and higher coal use (mdpi.com). Weak gas flow or air‑fuel imbalances tend to deliver under‑calcination or incomplete burning. Stable preheater operation is widely cited as one of the largest levers on thermal efficiency (researchgate.net; cementequipment.org).
Core control parameters and heat balance
Key control parameters are raw‑meal feed rate (mass flow and composition), calciner fuel (solid or liquid), primary/secondary/tertiary air flows, and overall gas flow via ID/FD fans (induced/forced draft). Together they set temperatures at each cyclone and duct. Thermocouples watch each cyclone’s gas and/or meal temperature — typically ~800 °C at the lowest stage declining to ~350 °C at the exhaust (cementequipment.org). Rapid swings anywhere are alarms.
Gas analyzers for O₂/CO₂ (oxygen/carbon dioxide) at the calciner exit track combustion and calcination completeness. Advanced control loops often trim tertiary air or fuel to hold the bottom‑stage cyclone (calciner inlet) on target, a practice that underpins overall efficiency (researchgate.net; cemenequipment.org).
Feed‑rate stability and composition control
A uniform raw‑meal feed is non‑negotiable. Fluctuations in rate or chemistry translate into variable heat release and unstable calcination, often forcing production cuts or excess fuel just to avoid overheating. “Variability in feed composition can lead to inefficient energy use and increased fuel consumption” (cemenequipment.org).
There are measurable gains from tightening variability. In one case study using online analysis, the standard deviation of lime saturation and silica modulus fell 50–70%, while clinker free‑lime variation nearly halved — coinciding with a more stable kiln and lower fuel use (thermofisher.com).
On the hardware side, gravimetric weight feeders keep raw meal at the set tonnage, while online XRF (X‑ray fluorescence) and PGNAA (prompt gamma neutron activation analysis) analyzers at the raw mill or blend silo enable rapid composition correction — with control loops adjusting feed proportions (thermofisher.com; cemenequipment.org). Hourly lab assays simply miss fast swings.
Preheater short‑pass and instability signals
Flow instabilities show up fast. A sudden feed drop can cause a “short pass” — meal bypassing cyclones — and a calciner exhaust temperature spike. In one plant, the calciner outlet jumped from ~870 °C to ≈1100 °C as kiln feed fell from 110 t/h to 40 t/h; calcination at the final cyclone slid to ~60% (versus ~92% normal), yielding dusty clinker (cemnet.com). The lesson was simple: keep feed rate and blend steady to hit design throughput and fuel efficiency (cemenequipment.org; thermofisher.com).
Temperature profile by cyclone stage
Even with constant feed, the temperature profile across cyclones needs tight control. Design practice staggers stages so material leaves at ~800–820 °C (cementl.com; cemenequipment.org). Gas temperatures step down from roughly 1000–1100 °C at the top cyclone to ~330–350 °C at the preheater exit (cemenequipment.org).
Operators commonly target ~350–400 °C at the top‑stage cyclone (C1). Too hot there signals too much coal burning or too little raw feed (risking pre‑calcination upstream). If the lowest cyclone runs cold, the calciner likely isn’t fully converting CaCO₃, shifting load — and fuel — to the kiln.
Heat recovery and stage imbalances
A balanced profile maximizes heat recovery. Under normal four‑stage operation, about 80% of heat transfer occurs in the gas ducts, delivering exit gas near ~330 °C that can dry raw feed (cemenequipment.org). Dry preheater exit gas below <350 °C is typical, and ~800 °C inlet meal yields a kiln‑inlet decarbonization of roughly 20% (cemenequipment.org).
Cold stages (e.g., overcooled by excess tertiary air) under‑transfer heat to the meal. Hot stages (under‑fed or over‑fired) can overshoot calcination or even form clinkers in cyclones, increasing pressure drop and dust. Operators track each cyclone’s gas temperature and static pressure; a higher‑than‑normal C1 temperature with falling pressure drop, for example, can prompt feed redistribution or extra air to re‑balance flow.
Online analyzers at the calciner exit
Because calcination consumes roughly ~45% of Portland cement’s thermal energy budget, plants monitor the degree of calcination just before the kiln. Modern lines deploy on‑line analyzers — often an automatic “ignition‑loss” (LOI) unit — that quench a small sample to freeze reactions, weigh it, then heat it (typically to 975 °C) to measure CO₂ release; the weight loss ties directly to CaCO₃ decomposed (cemenequipment.org).
Industry practice targets ~90–95% calcination in the precalciner, adjusting fuel up if low or down if high. With an automated sampler at the calciner exit chute feeding a lab‑automated LOI tester, plants get updates every 15–30 minutes — values like 92% or 85% — and use the trend for fuel trim. As one source notes, “a more completely calcined kiln feed requires less thermal input in the kiln, and leads to improved production” (cemenequipment.org).
Plants frequently report lower coal consumption and steadier kiln firing after installing these analyzers. One plant said online calcination control with automated fuel trim raised preheater efficiency to design and let clinker output run at the optimum rate without overheating, halving free‑lime excursions (cemenequipment.org; thermofisher.com). The analyzer “allows an adjustment in fuel supply to the precalciner…to optimize efficiency” (cemenequipment.org).
Taken together, continuous calcination monitoring tightens the preheater/calciner loop. The result is stable, near‑complete precalcination that minimizes kiln fuel, avoids free‑CaO spikes in clinker, and sustains smooth, energy‑efficient production.
Sources (cited inline): industry/academic studies and vendor case reports — mdpi.com; thermofisher.com; cemenequipment.org; cemenequipment.org. Specific operational data are drawn from plant case histories and process handbooks — cemenequipment.org; cemnet.com.
