In cement, the burning‑zone temperature (BZT) isn’t just a number — it’s the primary indicator of clinker quality. Plants are turning to online analyzers and advanced control to hold the kiln “always hot,” trim fuel, and keep free lime in check.
Ask any kiln operator what makes or breaks clinker and they’ll point to the burning zone. The BZT (burning‑zone/sintering temperature) is the primary indicator of clinker quality, according to plant operation manuals and instrumentation suppliers (cemmentequipment.org; randci.co.za). The key strength mineral, C₃S (alite), forms only above roughly 1300–1450 °C, which is why operators lean toward the upper end — near ~1450 °C — to fully convert C₂S (belite) and minimize free CaO (randci.co.za).
When the burn is right, a typical well‑clinkered discharge shows just ~0.5–1.5% uncombined CaO, with liter weight in the 1250–1350 g/L range — classic signs of complete and uniform sintering (cemmentequipment.org). Higher free CaO flags underburnt clinker, with downstream strength and shrinkage headaches.
Burning‑zone temperature as primary lever
Plants steer BZT mainly with fuel feed rate (primary and secondary fuels), kiln draft (air inflow), and kiln speed (residence time) (cemmentequipment.org). Around the flame, other instruments matter: kiln outlet oxygen (O₂) and CO indicate combustion efficiency; the back‑end (cooler throat) temperature reflects complete calcination; and in preheater/precalciner lines, both calciner flame temperature and calciner‑exit O₂ are critical checks. On grate coolers, clinker bed depth or grate speed is regulated to keep the bed fluidized and cooling correctly.
In short, the kiln control focus is on (1) burning‑zone temperature, (2) burnout indicators (free CaO, clinker moduli), (3) combustion indicators (O₂, CO, NOₓ), and (4) mass balance (fuel and feed rates) (cemmentequipment.org). Typical kiln‑exit oxygen is held around 2–3% (air‑excess) for complete combustion.
Combustion and raw‑mix interaction
Together, these variables determine clinker chemistry and burn completeness. One source [10] reports that adjusting BZT in response to raw‑mix variability — lime saturation factor, LSF (a ratio targeting CaO balance), silica modulus, SM (SiO₂/Al₂O₃ balance), plus magnesia, alkalis — is periodically required to keep free CaO on target. If BZT drifts low (e.g., by –100 °C), free CaO rises sharply, degrading cement performance (cemmentequipment.org; MDPI). Conversely, turning the flame hotter elevates the liquid phase and can flash undesirable compounds, so the setpoint must balance mineralogy and energy use. The practical aim: a stable, high‑enough BZT so the kiln is “always hot,” limiting upsets and ring formation (cemmentequipment.org; MDPI).
Stability at high heat
Maintaining a stable, sufficiently high burning‑zone temperature is crucial because this is where final calcination and phase formation occur. Empirical data show that C₃S forms only above ~1300–1450 °C (randci.co.za). If the burning zone cools unexpectedly (coating collapse, fuel outage), clinker leaves the zone incompletely burned, with high free CaO and weak clinkers (cemmentequipment.org; MDPI).
With a temperature‑stabilized burning zone (near its design setpoint), ≥98% of CaO is combined into silicates. As a rule of thumb, clinker leaving the burning zone should contain no more than ~1.5% free lime; beyond that, unsoundness risk rises (cemmentequipment.org). Clinker samples showing <25% C₂S imply thorough conversion to C₃S — attainable only with a hot enough flame. In the words of plant guides, BZT “determines how well the clinker is burned and how complete the transformation from C₂S to C₃S is” (cemmentequipment.org).
Vendor literature puts it plainly: hold BZT within 1300–1450 °C, because “the stability of burning‑zone temperatures directly determines the quality of the clinker” (randci.co.za). Even a few degree‑C shift in average BZT can swing free lime. In one case, an online control loop raised effective flame temperature via tertiary air by ~4.7% on average, increasing combustion completeness and delivering higher, more uniform clinker temperatures downstream — with a marked reduction in free‑lime variability and lower fuel use (MDPI).
Advanced process control and emissions impact
Stabilizing the flame also shows up in emissions. A hotter, well‑oxygenated flame burns fuel more completely, reducing NOₓ precursors. In practical implementations, activating kiln APC (advanced process control, i.e., coordinated multivariable loops) reduced kiln NOₓ by ~15% on average (MDPI). Because NOₓ formation is highly sensitive to flame temperature, a controlled hot flame can both optimize clinker and meet limits.
The operating mantra that follows is straightforward: kiln firing should be hot and smooth. Short‑term BZT dips or cycling ramp‑downs often translate into lost quality and broken refractory, so operators aim to avoid any “real cool‑down” of the burning zone (MDPI; cemmentequipment.org). Automated control systems and safe spare‑fuel flames (“maintenance flame”) are used to hold the burning zone steady at ~1450 °C, producing chloride‑free, stresses‑resistant clinkers, maximizing throughput, and minimizing shutdowns (cemmentequipment.org).
Real‑time analyzers and moduli control
Modern lines increasingly deploy real‑time analyzers and closed‑loop control to stabilize composition. Online XRF (X‑ray fluorescence) or PGNAA (prompt gamma neutron activation analysis) systems scan raw mix or kiln feed on belts, continuously measuring elemental oxides — CaO, SiO₂, Al₂O₃, Fe₂O₃, alkalis, SO₃, and more (Thermo Fisher; Thermo Fisher). Signals, typically every 1–5 minutes, feed the DCS, which automatically adjusts raw‑material proportions or fuel rates to counter swings. If SiO₂ spikes, limestone or sand feed is increased to hold down the silica modulus, SM. Closing the loop on LSF (lime saturation), SM, and AM (alumina modulus) keeps clinker chemistry on target at all times (Thermo Fisher).
One belt PGNAA case study reported a dramatic drop in raw‑meal variability after real‑time data and adaptive tuning went live: LSF standard deviation fell by ~70%, SM by 50%, and AM by ~33% (Thermo Fisher). Consequently, clinker became more consistent too: the standard deviation of clinker free CaO dropped from 0.72→0.37 (≈50% reduction) (Thermo Fisher) (clinker gy increased). In plain terms, analyzing 100% of lumber feed, rather than spot lab samples, virtually eliminated sampling error and control lag; the eal time loop removed a ~90 min feedback delay (Thermo Fisher).
Energy, stability, and APC payback
Lower variability pays back in fuel and uptime. With smoothed chemistry, kilns avoid burning extra fuel to accommodate “runs” of rich or lean mix, and process upsets are avoided (Thermo Fisher). As that source notes, “using an online analyzer to minimize chemistry variation, fuel and energy consumption can be reduced and process upset conditions avoided” (Thermo Fisher). Reduced free‑lime/moduli scatter also lessens the need to over‑“clinkerize,” saving ~2–5% fuel on large kilns.
These analyzers increasingly feed coordinated, multivariable APC modules — for example, ABB’s ACS/800 and FLSmidth’s ECS — that balance BZT, O₂, CO, and composition targets in tandem. In an industrial APC program in Italy (2019–2020), deploying such controllers across kiln and cooler delivered ~4.6% cumulative energy savings (fuel reduction) over 9 months and significantly tightened oxygen and NOₓ variation (lower standard deviations) after APC went live (MDPI; MDPI). With smoother operation, kiln specific energy dropped and the plant gained energy‑efficiency certificates for the improvement (Italian acronym TEE) (MDPI).
Adoption cases and operating outcomes
Online analyzers are spreading, including in emerging markets. Southern Province Cement (Saudi Arabia) recently ordered belt and airslide analyzers for a new 5000 t/d clinker line (Global Cement). Such systems continuously track raw‑mix CaO, SiO₂, and more, feeding a raw‑mix optimizer. Similar technology is used in Indonesia’s modern plants (e.g., Semen Indonesia’s lines use FLS/ECS control) to meet strict cement standards.
By automatically tuning limestone/callback proportions and fuel rates, these controls hold clinker chemistry to target with minimal human intervention. The measurable outcomes align with case studies: beyond energy savings, plants report higher yield and product quality; with raw variability cut, clinker composition drifts are reduced to within specification bands nearly 100% of the time. Stable composition lets the grinding plant run at optimum Blaine, and cement meets strength classes on the first test.
Maintenance gains follow from stability: as noted above, a steadier flame extends refractory life (fewer partial ring burns) and reduces kiln thermal stress (Thermo Fisher; MDPI).
System view: sensing to closed‑loop
The throughline is clear: modern kiln firing hinges on robust sensing and closed‑loop adjustment. Critical parameters — BZT, O₂, draft, feed, and composition — are instrumented and tied into APC so that any drift in raw feed or fuel quality is instantly compensated via fuel/air and feed‑rate changes, keeping clinker phases (C₃S, C₂S) on specification. The result is consistent, high‑quality clinker with quantifiable gains: on‑center moduli (LSF, SM, AM), minimal free CaO (<1.5%), and fuel savings on the order of a few percent (MDPI; Thermo Fisher).
