Cement makers are zeroing in on a simple truth: keep the burning zone hot and stable, watch the chemistry live, and let advanced controls do the rest. The payoff is tighter free‑lime, stronger alite, and fuel use down about 4.6%.
Clinker quality is a chemistry-and-combustion problem solved in real time. Plants that hit their raw mix targets and keep the burning zone steady at roughly 1400–1450 °C are the ones that land the desired mineral phases—especially alite (C3S, the main strength‑builder)—with minimal free CaO (free lime), according to industry guides and field results from automated control rollouts (scirp.org; cementequipment.org; cementequipment.org; mdpi.com).
The rulebook is getting sharper: on‑line XRF, gas analyzers, infrared pyrometers, and model‑predictive control (MPC) systems are now orchestrating fuel, air, feed, and cooling so the kiln stays in the sweet spot—producing clinker with target free CaO around 0.5–1.5% (many plants aim ~0.8%) and energy savings that have clocked in near 4–5% (randci.co.za; researchgate.net; mdpi.com).
Raw mix chemistry and phase targets
Plants steer by the Lime Saturation Factor (LSF, a proxy for how much CaO is available to form silicates), the Silica Modulus (SM) and Alumina Modulus (AM, ratios describing the balance of silica, alumina, and iron), alongside the clinker phase contents C3S (alite), C2S (belite), C3A, and C4AF (scirp.org).
In practice, the four key oxides—CaO, SiO2, Al2O3, Fe2O3—are kept on target by continuous blending and feeder trims driven by on‑line XRF checks every few minutes instead of hourly lab tests (cementequipment.org). Plants typically run LSF near 0.95–1.00; if LSF or alkalis drift, burnability suffers and clinker can turn unsound (cementequipment.org). High magnesia or alkalis tend to “ball” in the burning zone and yield unsound clinker unless burnt very hot and quenched rapidly; potassium oxides can also lead to troublesome buildup (cementequipment.org; cementequipment.org).
Operators also watch indirect kiln health signals—kiln shell draft, motor power/torque, and O2, CO, CO2, NOx at cyclones and fan inlets—to triangulate burn quality and stability (cementequipment.org).
Burning‑zone temperature stability
Alite (C3S) forms above roughly 1300–1450 °C, with peak liquid phase around 1450 °C; modern kilns therefore aim for burning‑zone temperatures in excess of ≈1400 °C and, crucially, keep them stable across load changes (randci.co.za). If the flame cools or fluctuates, parts of the charge underburn and free CaO rises.
The target free CaO window is tight: around 0.5–1.5% (most plants try to hit ~0.8%); values above ~2% signal underburn, while below ~0.5% indicates overburn and unnecessary fuel use (researchgate.net; cementequipment.org). One industry note advises that if free CaO drifts out of the 0.4–1.2% range, first suspect a mix problem or falling flame temperature (cementequipment.org). Each 0.1% excess free CaO translates into measurable loss of reactivity and extra grinding energy; each 0.1% below optimum signals fuel waste (researchgate.net; cementequipment.org).
There are efficiency dividends here too. One account reports careful flame control cutting kiln shell heat loss by 30–50% by shaving shell temperature by tens of degrees (globalcement.com). Another shows advanced control raising tertiary‑air temperature by 4.7% while smoothing variance (mdpi.com).
Fuel, air, and residence‑time control
Fuel quality and distribution matter as much as chemistry. Poor fuel or insufficient secondary air leads to underburning and high free CaO; too much fuel risks overheating and ring formation. Operators adjust fuel flow and secondary/tertiary air as actuators—if free CaO rises, the control system typically boosts fuel/air to lift temperature. Plants also infer burn‑out from gas composition: CO levels and calciner exit CO2 indicate calcination degree. If feed is over‑calcined (CO2 too low entering the kiln), fuel is shifted from precalciner to kiln, and vice versa (cementequipment.org).
Mechanical settings close the loop. Kiln speed (rotation) and retention time directly affect reaction completion: faster speed reduces residence time and risks incomplete reaction and high free CaO; slower speed increases output but may overheat the clinker. Plants also manage kiln charge/filling—bed depth and burning‑zone thickness—so the zone is neither “choked” nor too thin (cementequipment.org).
Cooler performance and quenching control
Cooling locks in mineralogy. Plants hold clinker exit temperature around ~80–100 °C by modulating cooler fan flows. Good cooling prevents re‑melting and keeps phase composition uniform. In one model‑predictive setup, automatic cooler‑fan modulation tracked kiln feed to hold exit temperature within spec and prevent “burnback” that could recrystallize alite and degrade strength (mdpi.com).
On‑line analyzers, pyrometry, and APC
Advanced plants now enforce quality with on‑line instruments and automatic control. Raw‑mix analyzers (often XRF) sample the blended feed every 1–5 minutes, feeding the distributed control system (DCS) to trim weigh‑feeders and keep LSF and moduli in range—without lab delays (cementequipment.org). On‑line free‑CaO analyzers can sample clinker every ~10 minutes; with that feedback, firing conditions can be corrected automatically to maintain the ~0.8% free‑lime target and avoid cement with disappointing early strength (cementequipment.org).
Combustion is kept honest by two‑color infrared pyrometers—optical thermometers with water/air cooling and purge—that look into the flame, “measure closer to the highest temperature,” and are not fooled by dust in the harsh view path (randci.co.za). Around the kiln, gas analyzers continuously track O2, CO, CO2, and NOx. One APC deployment prioritized “cyclone oxygen” control; after implementation the mean kiln O2 rose ~3% and its variability fell 39%, while NOx mean and variance dropped ~15% and ~32% (mdpi.com).
The capstone is advanced process control (APC), often model‑predictive control (MPC), which coordinates kiln feed, fuel, dampers, and fans to meet multiple targets (free lime, temperatures, O2) at minimum fuel. In one field case, a kiln+cooler MPC cut specific fuel by about 4.6% (March–Dec 2020), maintained clinker bed depth and exit temperature within tighter limits, and raised tertiary‑air temperature by 4.7% while trimming cooler‑fan power ~10% (mdpi.com; mdpi.com). The controller used “sporadic feedback” from on‑line free‑CaO analyzers; if free lime ticked up, it immediately raised coal or slowed feed—no manual intervention required (mdpi.com; cementequipment.org). Some plants augment these loops with machine‑learning “soft sensors” to predict quality drift (mdpi.com).
Quality targets and measured outcomes
What success looks like: target free CaO ideally ~0.8%; stable O2/CO2; minimal variation in NOx and kiln draft; and clinker that meets phase ratios without over/under‑burning. One APC implementation cut kiln fuel use by ~4.6% and stabilized NOx 15–32% lower while raising kiln air flow; another saw cyclone‑O2 standard deviation down 39% and NOx variance down ~32% (mdpi.com). Across deployments, these instrumentation‑driven controls routinely deliver ~4–5% energy efficiency gains in practice (mdpi.com; mdpi.com).
Snapshot summary
- Feed chemistry control via fast XRF analyzers keeps LSF, SM, AM on target and the raw mix stable (cementequipment.org).
- Burning‑zone temperature and burn‑out are held near ~1400–1450 °C with pyrometers and gas probes, steering free CaO to ~0.5–1.5% (randci.co.za; researchgate.net).
- Closed‑loop APC adjusts fuel, air, and feed based on on‑line composition and kiln atmosphere to maintain quality and trim fuel (~4.6% reduction shown) (mdpi.com).
Sources: randci.co.za; researchgate.net; cementequipment.org; mdpi.com; cementequipment.org.
