In-Line Calciner (ILC) Kiln Systems: Guide

The in-line calciner (ILC) kiln system is the technological backbone of the modern cement plant. Since the introduction of the precalciner concept in the 1970s, the dry-process preheater kiln with an in-line calciner has become the standard for large-scale clinker production, and today nearly all new kilns and most major upgrades are built around it. The in-line calciner moves 55 to 65% of the total fuel combustion out of the rotary kiln and into a calcination vessel in the preheater tower, where the raw meal is calcined at a moderate temperature before it enters the kiln. The result is a kiln that operates with a much lower thermal load, a much higher production rate per unit of kiln volume, and a much lower NOx emission than the preheater-only kilns of the previous generation. This article is a complete technical study of ILC kiln systems, written for process engineers, production managers, commissioning teams and graduate engineers. It covers the chemistry of calcination, the architecture of the ILC system, the role of the tertiary air, the cyclone preheater, the operation and control, the startup and shutdown, the common problems, the upgrades and the modern developments. The objective is to provide a working understanding of how an ILC kiln system is designed, operated and optimized, so that the reader can apply it to the kilns they operate, audit or commission.

From Preheater Kiln to Precalciner Kiln

The evolution of the cement kiln is the history of moving the thermal work out of the rotary kiln. The wet-process kiln and the long dry kiln performed all the work, from the evaporation of water to the clinkering, inside the rotating shell, which limited the production rate to what the kiln volume and the heat transfer could deliver. The suspension preheater, introduced in the 1950s, moved the drying and most of the heating and calcination into a tower of cyclones, where the raw meal is suspended in the hot gas stream and heat is transferred in seconds rather than in minutes. The preheater kiln reduced the fuel consumption dramatically, but it left the calcination, which is the most endothermic part of the process, to be completed in the kiln itself, which still limited the kiln production and created a long burning zone with a high thermal load.

The decisive step was the addition of a calcination vessel to the preheater, with its own fuel and its own combustion air supply, the tertiary air drawn from the clinker cooler. The first commercial installations appeared in Japan in the 1970s, and the concept spread rapidly because it delivered exactly what the industry needed: a kiln system whose production was no longer limited by the kiln’s ability to calcine, because the calcination was already 90 to 95% complete when the meal entered the kiln. The kiln was freed to do what it does best, the clinkering, and the production rate could be increased dramatically without enlarging the kiln itself. The precalciner concept also opened the door to the use of difficult fuels, because the calciner burns its fuel at a moderate temperature with a long residence time, and it created the conditions for the low-NOx operation that the emission regulations of the following decades demanded.

The precalciner systems are classified by the position and the gas path of the calcination vessel. In the in-line calciner (ILC), the calciner is placed in the main gas path between the kiln inlet and the preheater cyclones, and the kiln gas passes through it together with the tertiary air. In the separate-line calciner (SLC), the calciner has its own cyclone system and its own gas path, separate from the kiln gas path, which allows a different fuel split and an independent operation. The ILC is the simpler and the more common arrangement, particularly where the calciner fuel share is moderate, and this article focuses on the ILC while noting the differences where they matter.

System	Calcination location	Fuel split (kiln/calciner)	Typical production	Heat consumption	NOx level
Wet process	All in the kiln	100 / 0	0.5–1.5 tpd per m3 kiln volume	5000–6000 kJ/kg	High
Long dry kiln	All in the kiln	100 / 0	1.5–2.5 tpd per m3	3800–4500 kJ/kg	High
Preheater kiln (no calciner)	Completed in the kiln	100 / 0	2.5–4 tpd per m3	3100–3500 kJ/kg	High
ILC in-line calciner	85–95% in the calciner	35–45 / 55–65	4.5–6.5 tpd per m3	3000–3400 kJ/kg	Moderate to low
SLC separate-line calciner	85–95% in the calciner	25–40 / 60–75	4.5–6.5 tpd per m3	3000–3400 kJ/kg	Low

The ILC design is offered by all the major suppliers, with their proprietary calciner geometries, but the principles are common: a vessel with the meal feed, the fuel injection, the tertiary air inlet, a strong internal mixing, and an outlet to the bottom cyclone.

The Chemistry of Calcination

The calcination reaction is the decomposition of calcium carbonate into calcium oxide and carbon dioxide: CaCO3 is heated and dissociates to CaO plus CO2, with a heat demand of approximately 1.79 MJ per kg of CaCO3, or about 1.9 MJ per kg of CO2 released. The reaction is reversible, and its equilibrium depends on the temperature and the partial pressure of CO2 in the surrounding gas: at atmospheric pressure in a kiln gas containing 25 to 35% CO2, the decomposition begins at about 700°C and becomes rapid above 800°C, and in a pure CO2 atmosphere the equilibrium temperature is about 895°C. The raw meal in the preheater begins to calcine in the lower cyclones, where the gas temperature is 850 to 900°C, and the calcination is completed in the calciner, where the residence time of a few seconds at 850 to 900°C with a vigorous mixing brings the calcination degree to 85 to 95% by the time the meal leaves the bottom cyclone. The remaining calcination is completed in the kiln, where the meal is heated from the kiln gas and the flame, and the calcination degree of the kiln feed is one of the key operating parameters of the system.

The heat demand of the calcination is the largest single item of the kiln system’s heat balance, roughly 55 to 65% of the total thermal energy, and it is the reason that the calciner must burn more than half of the fuel. The temperature of the calcination must be kept in the range 820 to 900°C, because at higher temperatures the meal and the gas become too hot for the cyclones and the ducts, and at lower temperatures the reaction rate falls and the calcination degree drops. The control of the calciner temperature is the control of the calcination degree, and the calcination degree, in turn, is the control of the kiln’s thermal load: a low calcination degree forces the kiln to do the calcination, which raises the kiln exit temperature, lengthens the burning zone and may cause the ring and the coating problems, while a high calcination degree lightens the kiln load and allows a higher production.

The calcination is also the source of the majority of the process CO2 emissions, and the chemistry has important consequences for the modern decarbonization agenda. Approximately 60% of the CO2 of a cement plant comes from the calcination of the limestone, and it is released as a concentrated CO2 stream in the preheater gas. The carbon capture systems of the future, the oxyfuel and the calcium looping processes, are built around the calciner: the oxyfuel process burns the fuel in oxygen in the calciner and the kiln, producing a gas stream of nearly pure CO2 that can be captured and stored, and the calcium looping process uses the meal itself as a CO2 carrier. The ILC system is therefore not only the standard of today but also the platform on which the carbon capture of tomorrow is being developed, which makes the understanding of its chemistry and its gas flows more valuable than ever.

The Architecture of the ILC System

The ILC kiln system is composed of the raw meal feeding system, the preheater tower, the calciner, the rotary kiln, the clinker cooler and the gas handling system, and the gas flows connect them in a defined circuit. The raw meal, from the homogenizing silo, is lifted to the top of the tower by the bucket elevator or the pneumatic conveying, and it enters the preheater at the top, where it meets the hot gas rising from below. The preheater is a tower of 4 to 6 stages of cyclones, each stage consisting of one or two cyclones and the connecting ducts, in which the meal is dispersed into the gas, heated and separated, passing from the top stage down to the bottom stage as the gas rises from the kiln and the calciner. The number of stages is an economic optimization: the 5-stage tower is the current standard, with a preheater exit temperature of 300 to 340°C, while the 6-stage tower recovers more heat at the cost of a taller structure and a higher pressure drop.

The calciner is located between the bottom cyclone stage and the kiln inlet, in the main gas path. The kiln gas, at 1000 to 1100°C, enters the calciner from below, together with the tertiary air from the cooler, and the meal from the bottom cyclone stage is fed into the calciner at one or more injection points. The fuel for the calciner is injected through the lances at the calciner inlet, and the combustion is completed in the vessel as the gas and the meal rise together through the calciner body and into the bottom cyclone, where the meal is separated and fed to the kiln. The gas, now at 850 to 900°C, continues up through the lower cyclone stages, preheating the incoming meal. The ILC calciner is a large vessel, 6 to 10 m in diameter and 15 to 30 m tall for a 10,000 tpd line, and its internal geometry, with the swirl inlets and the mixing baffles, is engineered to give the residence time and the mixing that the calcination and the combustion need.

The rotary kiln of an ILC system is a conventional kiln, typically 4 to 6 m in diameter and 55 to 80 m long, but its thermal load is much lighter than that of a preheater-only kiln, because the meal arrives 90% calcined and the kiln has only to finish the calcination and perform the clinkering. The kiln production rate of an ILC system is 4.5 to 6.5 tpd per cubic meter of kiln volume, compared with 3 to 4 for the preheater-only kilns, and the kiln flame is shorter and more intense, because the heat is concentrated on the clinkering. The clinker cooler, a grate cooler in the modern systems, receives the clinker at 1400°C, cools it to 60 to 100°C above ambient, and recovers the heat in the secondary air to the kiln, the tertiary air to the calciner and the cooler exhaust. The gas handling system, with the induced draft fan, the dust collection and the bypass, completes the circuit and maintains the pressures and the flows that the process requires.

The Tertiary Air System

The tertiary air is the combustion air of the calciner, and its supply is the feature that distinguishes the precalciner system from the preheater system. The tertiary air is drawn from the clinker cooler, where the hot clinker heats the air to 750 to 950°C, and it is ducted to the calciner through the tertiary air duct, a large refractory-lined pipe that runs from the cooler, through the kiln floor or alongside the kiln, to the calciner inlet. The flow of the tertiary air is controlled by the damper at the duct outlet, and it is measured by the flow meters or calculated from the pressure drop. The division of the cooler air between the kiln (secondary air) and the calciner (tertiary air) is a key operating balance: the kiln receives 25 to 35% of the combustion air, and the calciner 65 to 75%, in proportion to their fuel shares.

The design of the tertiary air duct must manage several conflicting requirements. The duct must recover the heat of the cooler air with a minimum pressure drop, which favors a large duct with smooth bends, but it must also be resistant to the dust, which settles in the low-velocity areas and forms the blockages that plague many plants. The duct must be refractory-lined to keep the air hot, and it must be positioned so that it does not interfere with the kiln access and the maintenance. The velocity of the tertiary air is typically 10 to 20 m/s, high enough to carry the dust, and the duct is provided with the inspection openings and the cleaning facilities. The blockages of the tertiary air duct, caused by the dust settling at the bends and the low points, are a classic operating problem, and their prevention is a matter of the design velocity, the smoothness of the bends and the regular cleaning.

The tertiary air temperature and flow are among the most important process variables of the system. The temperature of the tertiary air affects the calciner temperature and the fuel combustion: the air is the main heat source of the calciner flame besides the fuel, and a drop in the tertiary air temperature, caused by a cooler problem or a cooler kiln, must be compensated by an increase in the calciner fuel. The flow of the tertiary air, and its balance with the kiln secondary air, sets the gas velocities in the two paths and hence the heat transfer and the pressure drops. The control of the tertiary air is therefore part of the integrated control of the cooler, the kiln and the calciner, and the modern systems include the tertiary air measurement and the automatic damper control as standard. The health of the tertiary air system, the duct, the damper and the measurement, is a condition of the stable operation of the whole line.

The Cyclone Preheater

The cyclone preheater performs the heat exchange between the hot gas and the cold meal, and its performance decides the thermal efficiency of the system. Each stage consists of the riser duct, in which the meal is dispersed into the gas and heated by the direct contact, and the cyclone, in which the meal is separated from the gas and fed to the next stage. The gas velocities in the ducts are 15 to 25 m/s, sufficient to carry the meal, and the cyclone separation efficiency is 90 to 96% per stage, with the escaped dust carried to the next stage or to the fan. The overall heat exchange is extremely effective, because the surface area of the dispersed meal is enormous, and the gas is cooled from 850–900°C at the bottom to 300–340°C at the top in a matter of seconds. The pressure drop of the preheater, which is the sum of the duct losses and the cyclone losses, is typically 4000 to 6000 Pa for a 5-stage system, and it is the main load of the induced draft fan.

The performance of the cyclones is characterized by their separation efficiency curve and by the dust loss, which is the meal that escapes with the gas. The escaped dust is returned to the process by the dust collection system, the bag filter or the ESP, and the dust circulation adds to the heat losses and the pressure drops. The cyclone design parameters are the diameter, the inlet velocity, the vortex finder and the cone angle, and the modern designs are optimized by the CFD simulation for the efficiency and the pressure drop. The operational problems of the preheater are the blockages and the build-ups, caused by the condensation of the volatile components, the alkali sulfates and the chlorides, and by the mechanical bridging of the material; their management is described later in this article.

The operation of the preheater is the control of the temperatures and the pressure drops at each stage. The temperature profile through the tower, measured at each stage, is the map of the heat exchange: a stage that is too hot or too cold indicates a flow or a distribution problem, and the comparison of the actual profile with the design profile is the standard diagnostic. The pressure drops of the stages, which are proportional to the gas flows, are the indicators of the flow distribution and the incipient blockages: a rising pressure drop in a stage signals the build-up that must be cleaned before it becomes a blockage. The modern systems are equipped with the temperature and the pressure instruments at every stage, and the trends are displayed in the control room, because the preheater is the instrument panel of the thermal process, and its readings are the first evidence of everything that happens in the kiln system.

Fuel Firing in the ILC System

The fuel split between the kiln and the calciner is the defining operating parameter of the ILC system. In the typical operation, the calciner receives 55 to 65% of the total fuel and the kiln 35 to 45%, with the exact split chosen to balance the kiln thermal load, the calcination degree, the NOx and the clinker quality. The calciner fuel is fired through the lances at the calciner inlet, into a gas stream that contains the kiln gas at 1000–1100°C and the tertiary air at 800–950°C, with the meal in suspension. The fuel ignites rapidly in this environment, and the combustion is completed within the calciner body, with the gas residence time of 2 to 4 seconds. The calciner is designed to burn the fuel completely before the gas leaves the vessel, because the unburned fuel would pass into the cyclones, where it would cause the CO emissions and the deposits.

The calciner is the natural home of the alternative fuels. Its moderate temperature and long residence time can accommodate coarse particles, high-moisture materials and slow-burning wastes that the kiln burner cannot handle, and the injection of the alternative fuels into the calciner is the standard practice of the plants with high substitution rates. The alternative fuel injection is distributed between the calciner and the kiln according to the fuel properties: the fine, dry, high-calorific fuels can be fired at the kiln burner, while the coarse, moist fuels are fired in the calciner. The calciner fuel quality requirements are defined by the combustion: the particles must burn within the residence time, and the chlorine and the heavy metals of the wastes must be managed in the system chemistry, as described later.

The firing system of the calciner includes the fuel feeding, the lances and the safety devices. The fuel is fed through the rotary valves and the screw conveyors to the lances, each with its own flow control and its pressure monitoring, and the lances are water-cooled where the heat is intense. The flame in the calciner is not a visible flame in the usual sense, because the meal and the gas are in a dense suspension, but the combustion is intense and its control is the control of the calciner temperature. The safety system of the calciner firing includes the flame monitoring or the temperature monitoring, the fuel shut-off valves and the interlocks with the gas flows, because the calciner, like the kiln, must never operate with the fuel and without the combustion air. The calciner firing system is the less visible half of the firing system, but it is the half that carries the majority of the energy, and its reliability and its control are as important as those of the kiln burner.

Operation and Control of the ILC System

The operation of the ILC system is the management of the flows and the temperatures around a stable balance point. The production rate is set by the raw meal feed, and the fuel is set to match the feed at the target calcination degree and the target burning conditions. The primary control variables are the kiln fuel, the calciner fuel, the kiln speed, the tertiary air damper, the ID fan and the cooler grate speed, and the primary measured variables are the calciner exit temperature, the kiln exit gas temperature, the kiln exit oxygen and NOx, the preheater temperatures, and the cooler parameters. The control strategy, in its classical form, holds the calciner exit temperature at 850–880°C by the calciner fuel, holds the kiln exit oxygen at 1.5–3.5% by the ID fan, and holds the kiln burning conditions by the kiln fuel and the speed, with the tertiary air balanced between the kiln and the calciner. The modern systems add the advanced control: the model-based optimization that predicts the burning zone temperature, the coating condition and the quality from the measured variables, and the expert systems that replicate the best operators’ behavior.

The stability of the system is the key to its performance, and the instabilities of the ILC system have characteristic signatures. A drop in the raw meal feed, whether from the silo or the mill problems, must be matched by a reduction of the fuel, or the system overheats: the calciner exit temperature rises, the kiln exit temperature rises, and the coating and the bricks suffer. The feed fluctuations are buffered by the control, but the best buffer is the stability of the upstream: the raw mill, the silo and the feeding equipment must deliver the feed at a constant rate and quality. The fuel fluctuations, from the coal mill or the alternative fuel feeding, must be absorbed by the same control, and the instruments that measure the fuel flows and the gas composition are the senses of the system. The stability of the cooler is the third leg: the clinker bed on the grate, its level and its movement, determine the secondary and the tertiary air temperatures, and the cooler control keeps the bed stable so that the kiln and the calciner receive their air at the design temperature.

The quality control closes the loop. The clinker is sampled at the cooler, and its analysis, the free lime, the C3S, the C2S, the liquid phase and the microscopy, is the confirmation that the thermal operation produces the required clinker. The free lime is the fast indicator: a rising free lime with a stable feed means the burning is insufficient, and the response is the increase of the kiln fuel or the burning zone temperature, while a falling free lime with an overburned clinker means the burning is too intense, with the risks of the refractory damage and the coating melt. The target free lime of a modern plant is 0.5 to 2.5%, depending on the product mix, and the control keeps the free lime in its band by the continuous adjustment of the firing. The operation of the ILC system is therefore a closed loop of the measurements, the controls and the quality, and the skill of the operators is the management of this loop under the varying conditions of the feed, the fuel and the weather.

Startup and Shutdown of the ILC System

The startup of an ILC kiln system is a carefully sequenced operation that follows the defined procedures. The preparation includes the inspection of the kiln, the preheater, the cooler and the firing equipment, the establishment of the services, and the pre-start checks of the instruments and the interlocks. The kiln is then rotated slowly, with the auxiliary drive or the main drive at the crawl speed, while the preheater and the ducts are checked for the blockages and the debris. The heating is started with the preheater or the kiln burner at a low firing rate, and the temperature of the system is raised at the controlled rate, typically 50–100°C per hour, to protect the refractory from the thermal shock and to dry the new or the repaired lining. The heating is monitored by the thermocouples and the shell temperature scans, and the refractory behavior is observed through the sequence: the water vapor, then the smoke of the drying, then the stable heating.

The feed is introduced when the system has reached the operating temperature and the calcination degree has reached the level that the calciner can sustain. The feed is started at a low rate and ramped up gradually, with the fuel following the feed and the temperatures held at the targets. The first clinker appears at the cooler several hours after the feed start, and the production is ramped to the target over the following hours or days, with the burning conditions and the quality monitored continuously. The start-up is the period of the highest risk: the refractories are being stressed, the processes are being established, and the operators are handling the transitions, and the procedures and the training are the protection against the start-up problems, which range from the cracked brick to the blocked preheater.

The shutdown is the mirror operation. The feed is reduced and stopped, the fuel is reduced and the kiln is burned down, and the cooler is emptied of the clinker. The kiln rotation is maintained during the cooling, with the periodic movement that prevents the shell distortion and the charge freezing, and the system is cooled at the controlled rate. The maintenance stop follows the shutdown, with the inspections of the refractory, the preheater internals, the cooler, the burners and the instrumentation, and the planning of the next campaign. The startup and the shutdown procedures are documented, reviewed after each event and kept current, because the events are the times when the system’s design, its condition and its operation are tested, and the records of the events are the memory that improves the next ones.

NOx Emissions and the Low-NOx ILC Operation

The NOx emission of an ILC system is fundamentally lower than that of a preheater-only kiln, because the majority of the fuel is burned in the calciner at 850–900°C, where the thermal NOx formation is negligible. The NOx of the system comes from the kiln flame, where the temperature reaches 1800–2000°C, and from the fuel-bound nitrogen of the fuels. The typical NOx concentration of an ILC system is 300–700 mg/Nm3 as NO2, and the modern plants meet the limits of 200–500 mg/Nm3 by a combination of the process measures: the low-NOx burner, the staged combustion in the calciner, the control of the kiln excess oxygen and the kiln exit temperature, and, where required, the SNCR system at the preheater.

The staged combustion in the calciner is the most important of the NOx reduction measures available in the ILC system. In the staged calciner, the fuel is introduced in two stages: the first stage receives a substoichiometric amount of air, creating a reducing zone in which the nitrogen compounds are reduced to molecular nitrogen, and the second stage completes the combustion with the remaining air. The reducing zone also acts on the NOx that the kiln gas carries: the NOx from the kiln flame passes through the calciner, and the reducing conditions destroy a significant fraction of it, an effect that is measured as the NOx reduction across the calciner. The staged combustion can reduce the system NOx by 30 to 50% compared with the unstaged operation, and it is a standard feature of the modern ILC designs.

The SNCR system, which injects the urea or the ammonia solution into the preheater gas at the temperature window of 850–1000°C, achieves an additional 30 to 60% reduction, bringing the system NOx to the compliance level with an ammonia slip that is kept below the permit value. The combination of the low-NOx burner, the staged calciner and the SNCR is the standard configuration of the plants in the strictest jurisdictions, and the optimization of the combination is a matter of the operating cost: the SNCR reagent is a significant cost, and the plants minimize it by tuning the burner and the calciner to reduce the NOx at the source before they spend on the reagent. The NOx instrumentation, with the analyzers at the preheater exit and the intermediate points, is the evidence base of this optimization, and the NOx trends are part of the daily operating review.

Circulation Phenomena and Preheater Blockages

The volatile components of the raw materials and the fuels, the alkalis, the sulfur and the chlorine, circulate in the kiln system and create the phenomena that threaten the operation. The alkali oxides and the sulfates volatilize in the kiln at the high temperatures, condense in the preheater at the lower temperatures, and return with the meal to the kiln, creating a circulation loop in which the concentrations in the system are many times higher than the concentrations in the feed. The chlorine, which volatilizes almost completely in the kiln, condenses in the upper preheater, where the potassium chloride and the sodium chloride form the sticky deposits that block the cyclones and the ducts. The sulfur, present as the alkali sulfates and the calcium sulfate, is less volatile, but its circulation contributes to the build-ups at the kiln inlet and in the lower preheater.

The management of the circulation is the management of the volatile load, the system temperatures and the dust extraction. The first measure is the control of the inputs: the raw materials and the fuels are selected and blended to keep the chlorine, the alkalis and the sulfur within the plant’s design limits. The second measure is the temperature control: the gas and the meal temperatures in the preheater are kept within the bands that limit the condensation, and the calciner and the kiln operations are tuned to avoid the local conditions that favor the build-ups. The third measure is the extraction: the kiln gas bypass, which diverts a part of the kiln gas around the preheater and removes the volatile components with the bypass dust, is the most effective tool against the chlorine and the alkali circulation, and its capacity, typically 3 to 10% of the kiln gas, is sized by the volatile balance of the plant.

The blockages of the preheater, whatever their cause, are treated by a combination of the prevention and the response. The prevention includes the design of the cyclone inlets and the ducts with the anti-build-up geometry, the provision of the cleaning openings and the air cannons, and the regular inspection of the tower at the kiln stops. The response to a developing build-up is the use of the air cannons and the cleaning equipment, the reduction of the feed and the fuel to lower the temperature, and, in the extreme case, the shutdown and the manual cleaning. The monitoring of the pressure drops and the temperatures at each stage gives the early warning of the build-ups, and the plants that monitor their tower continuously detect the build-ups at the stage where the cleaning is simple. The circulation phenomena are the most frequent cause of the unplanned kiln stops in the precalciner plants, and their management is one of the most valuable skills of the kiln operation team.

Kiln Operation with the ILC System

The kiln of an ILC system is operated with a different philosophy than the kiln of a preheater-only system, because its feed is 90% calcined and its task is concentrated on the clinkering. The burning zone of the ILC kiln is shorter and hotter, and the flame must be intense enough to reach the clinkering temperature over a length of perhaps 8 to 15 m. The kiln speed is higher, typically 3.5 to 4.5 rpm for a modern kiln, because the material passes more quickly through the kiln, and the residence time in the kiln is 20 to 35 minutes. The coating and the refractory management are the same as in any kiln, but the thermal shocks are different: the kiln of an ILC system is sensitive to the changes in the calcination degree, because a drop in the calcination shifts the thermal load from the calciner to the kiln, raising the kiln exit temperature and the shell temperatures in the transition zone.

The burning zone temperature is the central control variable of the kiln operation, and the modern kilns estimate it from the kiln shell temperature scans, the NOx, the free lime and the model-based soft sensors. The operator manages the burning zone by the kiln fuel, the speed and the burner settings, with the objectives of the clinker quality, the coating stability and the refractory life. The kiln feed end conditions, the exit gas temperature and the oxygen, are the links to the preheater: the exit gas must be hot enough to sustain the preheater operation but not so hot that the heat is wasted, and the exit oxygen must be high enough for the complete combustion but low enough for the efficiency. The cooler is operated to recover the heat efficiently and to deliver the secondary and the tertiary air at the design temperatures, and the cooler problems, the snowmen, the clinker pile and the grate failures, are the classic disturbances of the kiln operation.

The interaction of the kiln and the calciner is the defining feature of the ILC operation. The kiln and the calciner share the gas path and the fuel balance, and a change in one affects the other: an increase in the calciner fuel raises the calciner temperature and the calcination degree, which lightens the kiln and allows an increase in the feed, while a decrease in the kiln fuel, to control the NOx, must be compensated by the calciner or by the acceptance of a lower burning temperature. The modern control systems handle these interactions automatically, but the operators must understand them to manage the abnormal situations: the loss of the calciner fuel, the failure of the tertiary air and the blocked cyclone each have their own signature and their own response. The training of the operators in the ILC system is therefore a training in the interactions, and the best operators are the ones who anticipate the effects of their actions on the whole system.

Upgrades and Modern Developments

The ILC system of today is the result of decades of incremental development, and the upgrades of the existing systems follow the same path. The most common upgrades are the addition of a cyclone stage, which reduces the preheater exit temperature and the heat loss by 20–40 kJ per kg of clinker per stage; the replacement of the cyclones by the more efficient low-pressure designs, which reduce the pressure drop and the fan power; the modification of the calciner for the staged combustion and the higher alternative fuel rates; the installation of the low-NOx burner; the bypass installation or expansion for the volatile control; and the modernization of the control system with the advanced process control. Each upgrade is justified by the measured baseline and the projected benefits, and each is followed by the re-optimization of the system.

The decarbonization agenda is driving the next generation of the ILC developments. The oxygen-enriched combustion and the oxyfuel process, which burn the fuel in a mixture of the oxygen and the recycled CO2, produce a preheater gas that is nearly pure CO2, ready for the compression and the storage; the demonstration plants have shown that the kiln system can operate with the oxyfuel combustion with the adjustments of the temperatures and the gas flows. The hydrogen and the ammonia co-firing are being tested as the means to reduce the fossil fuel use; the calciner, with its moderate temperature, is the natural first place for the hydrogen combustion. The electrification of the calcination, using the electric heating of the meal in a separate reactor, is under development for the plants with access to the low-carbon electricity. The ILC system, which has been the workhorse of the industry for fifty years, is thus at the center of its next transformation, and the engineers who understand its flows, its chemistry and its limits will be the ones who carry the transformation through.

The digitalization of the ILC operation is the other major development. The modern plants collect the data of the whole system at the minute level, and the machine learning models now predict the quality, the coating and the risks from these data, giving the operators the early warning and the decision support that were previously the preserve of the most experienced engineers. The digital twins of the kiln system, which simulate the process in real time, are used for the training, the optimization and the testing of the control strategies. The development of the digital tools does not reduce the value of the fundamental understanding; it increases it, because the models must be built on the physics, and the operators must judge the recommendations of the models. The ILC system, in its technology and in its operation, remains a system that rewards the mastery of the fundamentals, which is what this article has set out to provide.

Frequently Asked Questions about ILC Kiln Systems

What does ILC mean in a kiln system?

ILC stands for in-line calciner, a calcination vessel placed in the main gas path between the kiln inlet and the preheater cyclones. The kiln gas passes through the calciner together with the tertiary air, and the raw meal is calcined to 85–95% before it enters the kiln.

Why is more than half of the fuel fired in the calciner?

The calcination of the raw meal is the largest heat demand of the process, roughly 55–65% of the total thermal energy. Firing that proportion of the fuel in the calciner provides the heat where it is needed and keeps the kiln thermal load moderate, which allows the high production rates and the low NOx.

What is the function of the tertiary air?

The tertiary air is the combustion air of the calciner, drawn from the clinker cooler at 750–950°C and ducted to the calciner. It supplies the oxygen for the calciner fuel and the heat that sustains the calcination temperature.

How is the calcination degree controlled?

By the calciner exit temperature, which is controlled by the calciner fuel. The target is typically 850–880°C, which gives a calcination degree of 85–95% at the kiln feed, and the balance of the calcination is completed in the kiln.

Why is the NOx of an ILC system lower than that of a preheater kiln?

Because the majority of the fuel is burned in the calciner at 850–900°C, where the thermal NOx formation is negligible. The kiln flame, which is the main NOx source, is further controlled by the low-NOx burner and the staged combustion.

What causes the preheater blockages in an ILC system?

The condensation of the volatile components, especially the chlorides, the alkali sulfates and the sulfur compounds, which form the sticky deposits on the cyclone walls and the ducts. The management is the control of the volatile inputs, the temperatures and, where required, the kiln gas bypass.

Summary and Final Recommendations

The in-line calciner kiln system is the standard technology of the modern cement industry, and its mastery is the core of the kiln engineering profession. This article has covered the chemistry of the calcination, the architecture of the ILC system, the tertiary air, the preheater, the firing, the operation and control, the startup and shutdown, the NOx and the circulation phenomena, the upgrades and the future developments. The recommendations for the plant are these: keep the calcination degree at the target, because it is the balance point of the whole system; manage the tertiary air, because its temperature and flow connect the cooler to the calciner; monitor the preheater temperatures and pressures, because the build-ups are detected early or not at all; understand the kiln-calciner interaction, because the system is operated as a whole; control the volatile inputs, because the chlorine and the alkalis are the masters of the tower; and apply the modern control and the digital tools, because the ILC system rewards the plants that measure and optimize continuously. The ILC system is a mature technology, but its operation remains a craft that combines the science of the chemistry and the flows with the art of the operators, and this article has aimed to equip its readers with both, so that the kilns they operate produce the best clinker, at the lowest cost, with the lowest emissions and the fewest surprises.

This file is part of the Complete Cement Technical Package (931 files) available from cementequipment.org. Respective rights holders; library copy for the licensed single user.