Precalinig: Complete Technical Guide

The precalciner is the vessel that changed the cement process: a combustion chamber installed between the preheater tower and the rotary kiln, in which 55 to 65 percent of the total fuel is burned and most of the calcination reaction is finished before the meal enters the kiln: the machine of the modern dry process: the reason the kiln itself is shorter, the production per kiln volume is doubled, and the temperature profile of the system is controlled stage by stage.

The Complete Cement Technical Package (931 files including the books, the courses, the Excel tools and the presentations: $249.99 one-time: instant download via the PayPal payment) holds the dedicated documents of the precalciner systems: the design guides, the process technology series of the industry, the cyclone design chapters, the operation manuals and the troubleshooting pages: this article walks the reader along the same path: the chemistry of the calcination, the types of the calciner, the air splits, the operating values and the daily problems: the complete map of the precalcination.

The precalciner is frequently mistaken for a simple heater stage: it is not: it is a furnace in its own right, with its own burners, its own oxygen supply and its own balance of fuel and meal: the calcination degree achieved in the vessel determines the heat load of the kiln burning zone, the refractory campaign and the stability of the whole line: this article reads the precalciner the way the process engineers read it: the reaction first, the hardware second, the operation always.

1. The Calcination Reaction: The Chemistry the Precalciner Performs

Calcination is the thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide: the reaction, simply written, is:

CaCO3 → CaO + CO2

The reaction is strongly endothermic: it absorbs roughly 1780 kilojoules per kilogram of CaCO3, the largest single energy demand in the whole clinker production: this is the reason the process engineers moved the reaction out of the kiln, where everything is expensive, into the precalciner, where the fuel burns directly against the meal cloud:

• The temperature window: the decomposition begins around 600 to 700 degrees and proceeds rapidly above 850: the practical calcining temperature of the industrial vessels is 850 to 900 degrees Celsius;
• The equilibrium pressure: the CO2 pressure of the decomposition rises steeply with the temperature: at 900 degrees the equilibrium pressure exceeds the partial pressure of CO2 in the surrounding gas, so the reaction runs continuously to the right;
• The kinetics: the rate depends on the particle size and the time at temperature: the small meal particles of 30 to 100 microns calcine in seconds: the coarse particles lag and may reach the kiln partially calcined;
• The degree of calcination: the exit degree of 90 to 95 percent is the design target of the modern vessel: the remaining 5 to 10 percent is finished inside the rotary kiln;
• The self-limitation: the calcination absorbs heat, so the gas leaving the vessel is cooled by the reaction itself: the gas temperature in the chamber holds near 850 to 900 degrees even when the flame above it is much hotter: the elegant trick of the process: the meal is the coolant;

The calcination is the master event of the tower: when the meal is fully calcined at the tower exit, the kiln only performs the combination and the sintering: the kiln length is sized for that work alone, and the production capacity of the whole line grows with the vessel: the chemistry of the carbonate explains the equipment: the reaction first.

2. The Position of the Precalciner in the Flow Sheet

The physical arrangement of the modern precalcining line follows a fixed order that the reader of the process drawings recognizes at once, from the raw meal silo to the cooler:

• The top of the preheater: the raw meal is fed into the topmost cyclones and falls downward, stage by stage, against the rising gas of the tower;
• The kiln riser duct: the hot kiln exit gases, about 900 to 1050 degrees, rise through the riser duct toward the lowermost cyclone and mix with the meal;
• The calciner vessel: where the riser widens into the reaction chamber, the fuel and the tertiary air are introduced: the combustion and the calcination proceed in suspension, the meal carried entirely by the gas: the dust cloud burns with the fuel;
• The exit cyclone: the calcined meal is separated in the following cyclone and drops into the kiln feed pipe, or through the kiln feed flap, into the kiln inlet;
• The exhaust path: the gas continues upward through the remaining tower stages to the raw mill and the baghouse, driven by the main exhaust fan of the system;

The precalciner sits in the riser duct between the kiln outlet and the lowermost cyclone: the section of the drawings where the riser widens into the calcining vessel is the most detailed page of the process book: the path of the gas and the meal around the vessel gives the modern cement plant its distinctive profile: the tower and the furnace above the kiln.

3. The Fuel Split and the Heat Distribution of the System

The modern process fires the fuel in two separate points, and the distribution between the two burners is the first control decision of the designer and the operator alike:

• The kiln burner share: 35 to 45 percent of the total heat is burned in the kiln, creating the flame and the burning zone for the sintering of the clinker;
• The precalciner share: 55 to 65 percent of the total heat is burned in the vessel, providing the energy of the calcination and the heating of the meal;
• The benefit of the split: the majority of the enthalpy is released where the heat is absorbed immediately by the calcination: less fuel in the kiln means a lower flame temperature, a thinner burning zone and a cleaner NOx;
• The calciner burner design: the fuel is fired at the top, the side or the bottom of the vessel; the burner geometry and the mixing of the tertiary air decide the completion of the combustion within the residence time of the gas;
• The flame inside the dust cloud: the meal particles damp the flame and absorb the radiation: the combustion in the vessel is cool and safe: the flame of the precalciner is a calm, invisible distributed burn;

The 60/40 split is the signature of the precalcination: the kiln burning zone is relieved, the vessel carries the bulk of the chemistry, and the system produces far more clinker from the same shell: the split is controlled by the fuel rates of the two burners, and everything downstream, the emissions, the refractory life, the kiln control, follows from these two numbers: the balance of the fires.

4. Types of the Precalciner: In-line and Separate Line

The industrial precalciners classify according to the path of the combustion air and the position of the vessel, and the design choice shapes the whole tower:

Type	Air path	Advantages	Considerations
In-line (flash) calciner	The tertiary air duct joins the kiln gases in the same riser vessel	The simple layout, the compact footprint, the good mixing	The vessel temperature follows the kiln conditions
Separate-line calciner	The tertiary air is ducted directly to the vessel; the kiln gas takes its own path	The stable vessel temperature, the low NOx	More ducting and an extra cyclone arrangement
Swirl calciner	The tertiary air enters tangentially and creates the swirl zone	The high utilization of the vessel volume from the mixing	The higher pressure drop and the tuning effort
Staged combustion calciner	The fuel fired in stages with the oxygen-controlled zones	The very low NOx, the tolerance of the alternative fuels	More control degrees and the complex combustion logic

Every vendor of the industry, the FLS, the KHD, the Polysius, the CB, the IKN and the others, has its own calciner name and shape, but the physics is common: the vessel must mix the fuel, the air and the meal in the smallest possible volume and hold the temperature by the dust: the operators of the different plants transfer their skills across the types because the chemistry is identical: the variants of the same furnace.

5. The Tertiary Air and the Combustion Air Split

The precalciner fires with its own oxygen, and delivering that oxygen without disturbing the kiln burning zone is the business of the tertiary air duct:

• The source: the tertiary air is drawn from the clinker cooler exhaust, still at 700 to 900 degrees Celsius, and routed to the precalciner vessel and to the riser;
• The duct: a long refractory-lined pipe, often routed beside or below the kiln, carrying the hot air: the dampers in the duct divide the flow between the calciner path and the kiln path;
• The air follow the fuel: the total combustion air of the system splits in proportion to the two fires: the kiln receives its secondary air through the firing hood, the vessel is oxygen for the burning; the air of third kind, hence the name;
• The oxygen inventory: the vessel exit oxygen of 1 to 2.5 percent signals the complete combustion: the tower exhaust oxygen of 2.5 to 4 percent includes all the paths and the false air;
• The heat recuperation: the tertiary duct returns the cooler heat to the vessel instead of venting it: it raises the vessel temperature and improves the overall efficiency of the heat bill;

The tertiary duct is not minor piping: it is the oxygen management of the whole plant: a small adjustment of the tertiary damper changes the kiln burning, the calcination, the NOx and the clinker quality at once: the process engineer must think of the oxygen pathway of the plant as a whole, not of the equipment items in isolation: the split of the air, the split of the heat.

6. The Cyclone Stages: The Heat Exchange Above the Calciner

Above the precalciner the tower stacks the cyclone stages, and each cyclone is a heat exchanger made of the casing, the gas inlet, the vortex finder and the collection cone:

• The mixing duct: the falling meal is dispersed into the hot gas stream before the separator: the fine particles accelerate toward the gas temperature in fractions of a second: the principal heat exchange happens in the duct between the cyclones, not inside them;
• The stage count: the preheater of the calcining line runs four to five cyclone stages: the lowest stage receives the meal from the vessel, the stages above preheat the incoming meal toward the 800 degrees;
• The cyclone efficiency: a well-designed cyclone separates 90 to 98 percent of the incoming meal: the carry-over after the separation becomes the circulating dust of the tower, the hidden load of the fans;
• The short residence: the meal stays suspended in each stage for only a few seconds: the total residence of the meal in the tower is 30 to 60 seconds, and the reaction time is precious;
• The build-up risk: the cold meal near the wall, the condensing sulfates and the alkalis at the colder surfaces: the scaling of the cyclones and the ducts is the classic operational disease of the tower;

The tower above the vessel is a counter-flow heat recovery machine: each stage pulls the heat out of the gas and passes it to the descending meal: the stage temperatures of a typical tower run roughly 320, 470, 620, 740 and 860 degrees from the top downward: the temperature gaps between the stages are the map of the heat exchange, and the operator reads them daily: the health of the tower, the fuel bill of the plant.

7. The Temperatures, Pressures and Setpoints of the Calcining System

The control room of the precalcining line watches a defined set of numbers, and the table below is the condensed notation of the plant operation manual:

Point	Typical value	Significance
Vessel exit gas temperature	850 – 880 °C	The calcination temperature: the heart of the vessel
Kiln riser duct temperature	900 – 1050 °C	The kiln exit gas heat delivered to the tower
Oxygen at the vessel exit	1 – 2.5 %	The combustion completeness in the vessel
Oxygen at the tower exhaust	2.5 – 4 %	The total airflow and the false air inventory
Degree of calcination at tower exit	90 – 95 %	The meal quality leaving the vessel
Top to bottom stage temperatures	~320 / 470 / 620 / 740 / 860 °C	The profile of the heat exchange health
Fuel split to the vessel	55 – 65 % of the total	The heat share burned in the calciner

The control philosophy holds the vessel exit temperature as the primary loop: the fuel is trimmed against this temperature with the oxygen and CO as the constraints: the upper tower temperatures are the slower indications: the daily operation is the balancing of the fuel, the feed and the air, with one central target: the 90 to 95 percent calcination at the tower exit: the values in the table are the operating room of the tower.

8. The Effects of the Precalciner on the Kiln Operation

The insertion of the precalciner converted the kiln into a smaller specialist that turns the calcined meal into clinker: the consequences for the kiln operation are the reason every modern plant is built this way:

• The shorter kiln: the kiln no longer owns the long calcining zone: the length-diameter ratio of the calcining kilns falls toward 10 to 14, and the capital and the refractory savings follow;
• The lower exit gas temperature: the calcination has already left the kiln, so the kiln exit gas is 900 to 1050 degrees instead of the old 1100 to 1200: the exhaust heat of the kiln falls;
• The concentrated burning zone: the production per kiln volume rises, and the process concentrates in a shorter zone: the refractory of the zone works harder, and the coating management is the campaign discipline;
• The gentle heat profile: the meal arrives hot and calcined, so the kiln no longer suffers the thermal shock of the undigested feed: the temperature gradients of the shell are gentler and the flexing less;
• The two-fire control: the fuel has two hearths: the vessel gas controls the calcination, the kiln fuel controls the burning zone: two levers instead of one: both the strength and the discipline of the system;

The kiln of the precalcining line is not the kiln of the old monographs: the machine of the final combination, relieved of the weakest work: the designers say the kiln runs better because it receives the better-prepared meal: the clinker quality starts at the calcination degree at the tower exit: the partnership of the vessel and the kiln.

9. The Bypass and the Volatile Recirculation: Alkalis, Sulfur, Chlorine

Not everything in the raw meal is welcome: the volatiles, the alkalis, the sulfur and the chlorine circulate between the kiln and the tower, condense on the cooler surfaces and may close the system with the build-ups: the precalcining lines of the modern plant apply the known counter-measures:

• The alkali cycles: the alkalis evaporate in the hot kiln zones and condense in the colder tower: the cycles build to the levels that choke the ducts and the cones of the lower stages;
• The kiln gas bypass: a few percent of the kiln exit gas is drawn around the tower through the bypass duct, cooled and filtered, and the volatile-laden dust leaves the system: the bypass ratio commonly 3 to 10 percent;
• The chlorine tolerance: the chlorine condenses at the highest temperatures and is the most aggressive: the chlorine input, from the raw material or the alternative fuels, sets the required bypass rate;
• The sulfur to alkali balance: the SO3 condenses with the alkalis: the ratio of the sulfate to the oxide alkali in the raw mix decides which species dominate the build-ups and the sticking:
• The build-up weapons: the air cannons, the soaking grids and the refractory profiles keep the vessel and the cones free: the temperature windows and the angles of the cones are the design against the scaling;

The volatile equilibria of the tower are the hidden chemistry of the availability: a plant can lose dozens of stops a year to the build-up blocking of the vessels and the cyclones: the engineering of the bypass is the engineering of the up-time: the equilibrium models of the package show how much sulfur and chlorine the system can hold before the blockage: the bypass of the plant, the accounting of the cycles.

10. The Combustion in the Vessel: Mixing and the CO Control

The dense suspension in the vessel must be mixed aggressively so the fuel finds the oxygen within the short residence time: the quality of the combustion inside the calc is judged by the gas analysis and the exit temperature:

• The residence time of the gas: the vessel gas dwells for 3 to 5 seconds, long enough for the dust-rich combustion at moderate temperature, short enough to avoid the NOx:
• The mixing problem: the swirl of the vessel, the deflecting bales, the diffusers, all create the intimacy of the fuel, the air and the meal: poor mixing shows up as the CO peaks and the unburned fuel in the discharge cyclone;
• The CO control: the CO at the vessel exit above a few hundred parts per million means the mixing is inadequate or the oxygen is short: the CO is the direct loss of the heat and the danger of the dust explosion;
• The excess air policy: the vessel exit oxygen of 1 to 2.5 percent balances the complete combustion against the fuel consumption and the NOx: every extra percent of the dry oxygen carries the heat out of the system;
• The alternative fuels: the waste-derived fuels, the tires, the plastics, burn preferably in the precalc: the low temperature and the long residence of the solids burn the coarse particles: the vessel is the alternative fuel engine of the plant;

The good combustion of the vessel is invisible and silent: the only proof is the flat temperature and the absence of the CO: the operators trust the design of the swirl, the momentum of the jet and the correct nozzle: the vessel stays clean and the measurement stays smooth: the combustion of the precalc: the complete and the calm burn.

11. The Troubleshooting of the Precalciner and the Tower

The failure patterns of the calcining system are well archived in the industry, and the troubleshooting chart of the package orders them by the symptom:

Symptom	The probable cause	The first counter-action
Vessel exit temperature falls	The fuel starvation, the meal overfeed, the poor mixing	Check the fuel rate and its density, verify the feed, adjust the oxygen
CO peaks at the tower sample point	Degraded mixing, the blocked tertiary duct, low oxygen	Raise the fan flow, inspect the dampers, the vessel inspection
The build-up closing the vessel	The alkalis, the sulfur cycles, the chlorine inputs	Increase the bypass, the air cannons, tighten the raw chemistry
The tower temperatures jump and dump	A blocked cyclone, the meal flushing, the false air	Check the cone aerators, the stage pressures, unblock the cone
Low calcination degree at the kiln feed	Low vessel temperature, the coarse meal, the wrong fuel split	Verify the fuel split, raise the setpoint, improve the mill fineness
The tower pressure swings	A blocked cyclone, the duct scaling, the fan instability	Read the stage pressures individually and resolve the culprit cone

The troubleshooting tables are the memory of the shift engineers: the temperature, the pressure and the gas analysis, the three readings that tell the story: half of the problems of the tower are the heat transport, a quarter the air, a quarter the raw: the disciplined reading of the three lines keeps the tower open and the vessel at temperature: the diagnosis list of the blueprint page: the symptom, the cause, the fix.

12. The Design, the Retrofit and the Economics of the Precalciner

The design of the precalciner starts from the production target and the specific fuels of the plant, and the rules of the dimensioning are well documented in the handbooks of the package:

• The vessel diameter: follows from the gas flow: the space velocity in the vessel of 10 to 14 meters per second: the gas volume at the calcination temperature sets the shell circle of the vessel:
• The vessel height: follows from the residence: three to five seconds of the gas at the operating velocity give the large vessels their vertical dimension of 12 to 20 meters;
• The specific volume: the rule of thumb of the design: 4 to 6 cubic meters of the vessel volume per tonne per day of the clinker production: the compactness of the calciner is one of its selling points;
• The cyclone sizing: the stage cyclones follow the gas flow at the inlet velocity of 16 to 20 meters per second with the pressure drop of 5 to 8 millibars per stage: the separation efficiency of the good design above 96 to 98 percent;
• The retrofit: the conversion of an existing kiln line into the precalcination line: the new vessel, the tertiary duct, the tower stages, the fans and the cooler upgrades: the capacity increase of 40 to 100 percent, depending on the bottleneck analysis;

The economics of the precalcination run through the fuel price, the demand and the emission rules: the package contains the spreadsheets of the retrofit evaluation, the mass and heat balances of the vessel, and the dimensioning tables: the numbers of the design and the retrofit, the decision of the board: the precalciner is the proven path of the expansion: the calculation, the investment, the return.

13. Frequently Asked Questions

What is the calcination degree at the exit of the precalciner?

The modern calc systems run 90 to 95 percent calcination at the tower exit: the remaining 5 to 10 percent is finished inside the kiln: the calcination that remains in the kiln absorbs heat and cools the burning zone, so the vessel is kept at the highest degree of calcination that the tower can stably maintain.

Why does the precalciner need its own separate air?

The tertiary air delivers pure hot oxygen-rich air from the cooler directly to the vessel: the kiln exhaust gases cannot supply enough oxygen at the right temperature: the separate air path lets the vessel fire its fuel independently and keeps the kiln burning zone on its own oxygen band: two furnaces, two air supplies, one balance.

Can an existing kiln be upgraded with a precalciner?

Yes: the precalc retrofit is the classic expansion of the industry: the kiln shell, the drive, the cooler and the fans are rechecked for the new capacity: increases of 40 to 100 percent are historically documented, but every upgrade requires the new vessel, the new tower stages and the new fan capacity: the decision is the bottleneck by bottleneck study.

Why is the temperature of the vessel limited to about 900 degrees?

Because the calcination needs only 850 to 900 degrees and the reaction absorbs everything the fuel releases: the temperature cannot rise further at the same feed: the decomposition of the sulfate and the stickiness of the meal would grow: the window of 850 to 880 is the fastest reaction without the build-up problems: the limit is the chemistry, converted into the design.

What happens if the fuel ratio to the precalciner is too high?

The kiln is underfed: the calcination degree reaches 100 percent, the vessel temperature climbs, the coating of the riser suffers and the meal arrives slagged: the danger of the CO grows: the fuel split is the central equilibrium: the operator restores it by the kiln fuel, the vessel temperature target defined by the quality of the clinker.

Why is a 5-stage tower more efficient than a 3-stage?

Each additional stage recovers more heat from the gas: the gas leaves the tower colder and the meal enters the vessel hotter: the five-stage towers cut the waste heat of the system by roughly 30 to 40 kilowatt-hours per tonne of clinker compared to the three-stage, at the cost of the taller tower and the higher pressure drop: the fuel price decides how many stages the plant can afford.

What is the typical gas velocity and pressure drop inside the cyclone?

The inlet velocity of the industrial cyclone is 16 to 20 meters per second and the pressure drop 5 to 8 millibars per stage: the velocity is the tension between the separation efficiency and the wear: the higher velocities give the finer separation and the higher pressure cost: the balance of the design is the classic trade of the tower.

14. Conclusion

The precalciner took the heaviest reaction of the process, the calcination, out of the kiln and gave it an efficient, controllable furnace of its own: the result: the highest capacities of the industry, the longer refractory campaigns and the cleaner flue gas: the engineer of the modern plant owns the vessel: the 850 degrees, the 60/40 split, the 90 percent calcination: the numbers of the tower govern the kiln, the output and the cost of the cement: the guide, the full precalciner file of the package accompanies this article: the drawings, the operation sheets and the Excel balances of the whole 931-file package: the price: $249.99 one-time: the download: instant: the professional of the tower, the complete map of the precalc.

The Complete Cement Technical Package includes the precalc document set: the vessel designs, the operation manuals, the mass and heat balances and the troubleshooting the operator: the engineer who commands the precalc commands the heart of the modern process: what the guide is to the knowledge, the package is to the career: the 931 files: the calciner of the plant: the cement of the world: the kiln with the vessel: the process made full.

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.