Kiln Chemistry Course 1: Combustion Module

The flame of the cement kiln is the most powerful chemical instrument of the plant: a stream of the pulverised coal, the primary air and the secondary air from the cooler, burning at temperatures towards 1800 to 2000 degrees Celsius, delivering the heat that the calcination demands and the sintering zone disposes: the flame is also the chemist of the system: it sets the atmosphere of the charge, the fate of the sulphate and the alkalis, the formation of the NOx and the stability of the coating: the combustion chemistry, the subject of the last lesson of this kiln chemistry course, is the base on which every other lesson stands: no chemistry of the burning zone happens without the flame.

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) includes the combustion chapter of the kiln chemistry course, the burner and the flame calculation tools and the fuel reference tables: this article is the lesson: the fuel chemistry, the combustion reactions, the stages of the coal burning, the air, the flame, the NOx and the practice of the firing, from the fuel reception to the flame shape.

The combustion lesson closes the course, and the reader should feel the loop close: the course began with the introduction of the reactions, climbed through the burnability, the variability, the calcination, the sintering, the mineralisation and the cycles, and arrives at last at the fire that drove every reaction: the flame is not the first subject of the cement science, it is the last, because it can be understood only after everything that it serves.

1. The Fuels of the Cement Kiln: The Chemistry of the Coal, the Coke and the Alternatives

The fuel of the kiln is chosen on the boiler of the chemistry, not on the price alone:

• The coal: the classical fuel, burned as the pulverised powder of the fineness comparable to the raw meal: the bituminous coals carry 20 to 35% of the volatile matter, 40 to 70% of the fixed carbon, 1 to 3% of the sulphur and 5 to 15% of the ash: the net calorific value of 24 to 30 MJ/kg: the volatile matter is the quality that the burners care about most, because it decides the ignition behaviour of the flame;
• The petroleum coke: the petcoke of the delayed cokers: the unprepossessing king of the calorific values, 30 to 34 MJ/kg, with the very low volatile matter (8 to 12%), the sulphur of 3 to 7% and the ash poor in the fluxing oxides: the petcoke burns slowly, needs the higher fineness and the longer flame, and pays its sulphur forward to the sulphate chemistry of the previous lesson;
• The alternative fuels: the tyres, the plastics and the refuse-derived fuels, the sludges and the solvents: each brings its own chemistry: the tyres with the steel and the sulphur, the plastics with the chloride, the biomass with the alkali: the alternative fuels are not substitutes, they are new reagents, and the reception laboratory of the modern plant analyses every lot against the element limits of the kiln chemistry;
• The fuel analysis form: the ultimate analysis (the carbon, the hydrogen, the sulphur, the nitrogen, the oxygen, the ash, the moisture) and the proximate analysis (the volatile matter, the fixed carbon, the ash, the moisture) are the two documents of the fuel chemistry, and the burner settings are computed from them: the fuel that arrives with the variable analysis is the variability problem of the flame, the mirror of the raw meal variability of the earlier lesson;

The fuel section is the input side of the combustion: the flame is the fuel chemistry read at 2000 degrees Celsius, and nothing about the flame can be understood without the proximate and the ultimate analyses of what is being burned: the engineers of the firing systems carry the fuel certificates the way the quality engineers carry the raw meal analyses.

2. The Combustion Reactions: The Chemistry of the Burning

The combustion is a family of exothermic oxidations, and the heat account of the flame is the account of these reactions:

• The carbon combustion: C + O2 → CO2, releasing about 32.8 MJ per kg of carbon: the principal reaction of the char: the complete combustion of the carbon is the goal of the whole firing system, and the CO in the kiln gas is the ledger entry of the carbon that did not complete the reaction;
• The hydrogen combustion: 2H2 + O2 → 2H2O, releasing about 120 MJ per kg of hydrogen: the hydrogen of the volatiles burns first and fast, carries the ignition, and its water vapour enters the gas: the hydrogen is the reason the volatile-rich fuels ignite easily;
• The sulphur combustion: S + O2 → SO2: the sulphur of the fuel burns to the dioxide, and the downstream oxidation to the trioxide and the sulphate retention belong to the sulphate lesson: the combustion reaction itself is the quick step;
• The heat of the flame: the sum of the reaction heats, corrected by the sensible heat of the reactants and the products, gives the flame temperature: the adiabatic temperature of the coal flame reaches 1800 to 2000 degrees Celsius with the preheated air, and the measured gas temperature in the kiln runs 900 to 1700 degrees Celsius along the shell: the difference between the adiabatic and the measured values is the heat delivered to the charge and the shell; the flame is a heat exchanger as much as a reactor;

Table of the combustion reactions and their heat (per kg of the element):

Reaction	Heat released	Role in the flame
C + O2 → CO2	about 32.8 MJ/kg C	The main char heat of the flame
2H2 + O2 → 2H2O	about 120 MJ/kg H2	Ignition and the volatile heat
S + O2 → SO2	about 9.2 MJ/kg S	The sulphur heat, small but real
C + 1/2 O2 → CO	about 11 MJ/kg C	The incomplete step: half the heat, CO in the gas

The reaction table is the energy ledger of the flame: the difference between the CO2 line and the CO line is the wasted heat of the incomplete combustion, and the kiln gas analysis (the CO, the O2) is the auditors’ report of the ledger: the operator who holds the table understands why the kiln gas CO is a fuel bill as much as a chemistry alarm.

3. The Stages of the Coal Combustion: From the Particle to the Ash

The pulverised coal particle burns through four stages, and the flame of the kiln is the superposition of millions of these life stories:

THE LIFE OF A COAL PARTICLE IN THE FLAME
----------------------------------------
1. HEATING      the particle in the primary air stream enters the
                flame envelope: heated by the radiation of the hot gas
                and the flame: the moisture evaporates, the particle
                warms to several hundred degrees in milliseconds
2. DEVOLATILISATION  at about 350-600 C the volatile matter escapes:
                the tars and the gases (H2, CH4, CO, the light
                hydrocarbons) leave the particle and burn around it:
                the volatile flame is the visible bright part
3. IGNITION     the volatile-air mixture ignites: the volatiles burn
                fast, releasing their heat, and carry the particle
                temperature to the 1000 C+ needed for the char
4. CHAR BURNOUT the fixed carbon skeleton (the char) burns by the
                surface reaction with the oxygen of the surrounding
                gas: the slowest stage: the char of the petcoke burns
                for many milliseconds to seconds, and the particles
                that finish the burnout late travel with the gas
                towards the charge
5. ASH          the mineral matter of the particle is left behind,
                melted into the slag droplets, and deposited on the
                charge and the coating: the ash of the fuel becomes
                the chemistry of the clinker

Three practical laws follow from the particle life story:

• The ignition law: the volatile matter decides the ignition: the high-volatile coals ignite easily and support the short, stable flame, while the low-volatile petcoke needs the high fineness, the hot air and the long flame to complete its lines: the plants switching their fuel switch their entire firing strategy;
• The fineness law: the char burnout time scales with the particle size, and the pulverised fuel fineness (typically 1 to 3% residual on the 90 micrometer sieve for the coal, 1 to 2.5% on the 90 for the petcoke) is the direct control of the combustion completeness: the coarse particles are the CO generators of the flame, exactly as the coarse quartz is the free lime generator of the meal;
• The ash law: the ash of the fuel enters the clinker as the silica, the alumina, the iron and the alkalis: the high-ash fuels change the modules of the material in the kiln, and the raw mix is corrected against the ash analysis of the fuel contract: the combustion chemistry and the mix chemistry are one bookkeeping, and the ash is the page where they merge;

The particle story is the micro view of the flame, and the plant practice of the fineness and the fuel selection is the macro consequence: the engineer who thinks in particles understands why the mill of the firing system is as important as the burner itself.

4. The Air of the Flame: The Primary, the Secondary and the Tertiary

The kiln flame breathes through three air streams, and the mass balance of the air is the skeleton of the whole kiln gas system:

• The primary air: the air that carries the fuel into the burner, roughly 5 to 15% of the total combustion air in the modern multichannel burners: the primary air provides the momentum and the swirl that shape the flame: the modern burners minimise the primary air share to conserve the hot secondary air and to lower the NOx, and the “nozzle velocity” of the primary stream (commonly 150 to 300 m/s at the nozzle, with the swirl components) is the first design number of the burners;
• The secondary air: the hot air from the cooler, 700 to 1000 degrees Celsius, drawn into the kiln hood: the secondary air carries 50 to 70% of the combustion oxygen and most of the heat of the flame: the secondary air temperature is the first efficiency number of the kiln, and its drop (the cooler problems, the low clinker bed depth) is a fuel bill in disguise: the flame temperature is set by the secondary air temperature;
• The tertiary air: the third stream, drawn from the cooler to the calciner through the tertiary air duct: the calciner burns its fuel in this stream at 850 to 900 degrees Celsius: the split of the air between the kiln and the calciner is the operatic balance of the precalciner plant, tuned daily through the dampers against the oxygen readings of the two outlets;
• The oxygen balance: the total combustion air against the fuel gives the excess air: the modern kilns run the outlet oxygen of 0.5 to 3.5% (the kiln inlet) and the calciner oxygen of 1 to 3%, with the stoichiometric air demand of the fuel computed from the ultimate analysis: the excess oxygen is the insurance of the complete combustion, and its cost is the heat carried out with the surplus gas: the oxygen target is the compromise that the sulphate lesson of the previous page made famous;

The air section is the macroscopic frame of the flame: the momentum, the swirl, the preheat and the split: the burner is not a nozzle, it is the gate of the three airs, and the flame shape, the heat release profile and the NOx of the kiln are the consequences of the gate settings: the burner tuning is the daily art of the four variables: the primary air rate, the swirl, the axial velocity and the position of the burner in the hood.

5. The Flame and the Charge: The Heat Transfer Chemistry

The flame does not only burn, it radiates, and the transfer of the flame heat to the charge is the physical chemistry that the operator actually controls:

• The radiative transfer: the flame radiates through the hot particles of the char and the soot, the triatomic gas (CO2, H2O) of the products, and the luminous zone: the radiation dominates the heat transfer of the burning zone, and the flame luminosity is the index of the radiative power: the luminous, lazy flame radiates more to the charge, the short, sharp flame concentrates its heat in the front segment;
• The flame shape as the chemistry: the short flame heats the front of the zone violently (the local overheating, the refractory risk and the sulphate release), while the long flame spreads the heat over the shell (the lower peak temperature, the weaker sintering): the flame length and the peak temperature are the two variables that the burner geometry and the primary air tuning set: the plant fixes the flame length at the value that matches the zone position of its chemistry;
• The flame temperature profile: the gas temperature falls from the flame peak (1500 to 1700 degrees Celsius measured) towards the kiln inlet (1000 to 1100 degrees Celsius): the temperature profile of the gas along the shell is the delivery curve of the heat, and the charge follows it with its own lag: the calcination of the remaining meal, the transition zone reactions and the melt growth are the charge’s response to the delivery curve;
• The practice of the position: the burner is positioned in the hood so that the flame lands neither on the charge nor on the brick: the flame impingement on the charge spikes the local temperature and destroys the coating, and the flame impingement on the brick exposes the refractory to the 2000 degrees hot core: the flame position is a mechanical setting with the chemical consequences, and the plants review the shell scanner after every burner adjustment;

The heat transfer paragraph closes the physics of the flame: the combustion chemistry makes the heat, the radiation delivers it, and the geometry positions it: the operator who tunes the flame tunes a heat distribution system whose customer is the sintering chemistry of lesson KC 1.5: the two lessons speak the same language of the temperature and the zone.

6. The NOx Chemistry: The Price of the Flame

The high flame temperature, so valuable for the sintering, pays an environmental toll in the nitrogen oxides, and the NOx chemistry is the modern frontier of the firing practice:

• The thermal NOx (the Zeldovich mechanism): at the temperatures above about 1500 degrees Celsius, the nitrogen of the air reacts with the oxygen: N2 + O → NO + N, followed by the chain: the rate climbs exponentially with the temperature, so the thermal NOx is created in the hot core of the kiln flame: the oxides of the thermal mechanism dominate the kiln NOx;
• The fuel NOx: the nitrogen bound in the fuel (0.5 to 2% of the coal) is released with the volatiles and oxidises partly into the NO: the fuel NOx forms at the lower temperatures and is significant in the calciner firing, where the thermal mechanism is weak;
• The formation numbers: the kiln inlet NOx concentrations of the ordinary coal kilns run 300 to 800 mg/Nm3 (in the modern units, the values scaled to the standard oxygen), and the calciner contributes its share: the total is limited by the emission regulations and optimised against the fuel and the production costs;
• The control chemistry: the first lever is the flame: the low-NOx burners flatten the temperature peak through the staged primary air and the flame shaping; the second lever is the calciner staging: the calciner is run with the zones of the incomplete combustion (the reducing pockets) that reduce the returning NOx: NO + CO → 1/2 N2 + CO2: the third lever is the process: the lowest workable excess oxygen and the stable feed, because the NOx follows the temperature and the oxygen of the process: the NOx control is the discipline of the whole firing system, not the additive of one device;

The NOx section is the environmental conscience of the combustion lesson: the flame that the course has described as the servant of the sintering is also the source of the emission that the modern plants must minimise: the engineering of the flame in the 2020s is the balance of the three objectives: the heat delivery, the stability and the NOx, and the balance is struck with the same instruments: the primary air, the burner geometry, the oxygen and the calciner staging: the combustion chemistry and the environmental chemistry are one science.

7. The Calciner Firing: The Combustion of the Second Flame

The calciner flame is a different animal: it burns in the dust-laden gas at 850 to 900 degrees Celsius, far below the kiln flame, and its chemistry obeys its own rules:

• The low-temperature constraint: the calciner temperature is pinned by the calcination equilibrium (the lesson KC 1.4): the fuel must release its heat into a vessel that can never overheat, and the combustion runs to completion in the gas at the moderate temperature: the volatile-rich fuels thrive in the calciner, and the slow-burning fuels (the coarse petcoke, the low-volatile chars) need the longer residence or the finer grinding;
• The oxygen and the staging: the tertiary air enters the calciner in the staged portions: the first stage burns the fuel in the reduced air (the reducing zone, where the returning NOx is destroyed), and the second stage completes the burnout: the staged calciner is now the standard architecture, and its tuning is the third instrument of the NOx control;
• The fuel flexibility: the calciner accepts the alternative fuels that the kiln flame cannot: the tyres, the plastics and the biomass burn in the moderate temperature of the calciner within the element limits of their chemistry: the calciner is the door through which the waste-derived energy enters the cement process;
• The coupling: the calciner combustion and the kiln combustion share the gas system: the oxygen of the kiln inlet, the CO of the riser duct and the preheater temperatures are the joint telemetry of the two flames: the unbalanced firing (too much fuel in the calciner, the shortage in the kiln) shows in the sintering zone temperature and the free lime of the clinker, and the balance of the split is the daily setpoint pair of the precalciner operation;

The calciner paragraph is the second act of the combustion story: the kiln flame and the calciner flame are two partners with different temperatures, different fuels and different duties, and the chemistry of the course has prepared the reader for both: the calcination of the lesson KC 1.4 explains why the calciner burns cool, and the NOx of this lesson explains why it burns staged.

8. The Flame Diagnostics: The Instruments of the Firing Control

The flame is diagnosed through the instruments of the hood and the kiln, and the diagnostic language is the daily language of the firing control:

• The flame imaging: the camera in the hood watches the flame geometry: the length, the shape, the brightness and the position: the experienced operators read the flame image as the fingerprint of the burner settings, and the flame image archives are compared across the shifts and the fuel lots: the camera is the first instrument of the flame practice;
• The gas analysis: the oxygen, the CO and the NOx at the kiln inlet and the stack: the O2 and the CO are the combustion completion report, the NOx is the environmental ledger, and the two together are the primary feedback of the firing line: the gas sample of the kiln inlet is the chemical signature of the flame moments after it happened;
• The temperature chain: the kiln inlet temperature, the burning zone pyrometers and the shell scanner: the temperature chain reports the heat distribution that the flame delivered, and the shell scanner adds the coating picture: the three reports, the flame image, the gas analysis and the temperature chain, triangulate the state of the firing;
• The fuel and the air meters: the coal feed rate, the mill fineness, the primary air flow and pressure, the secondary air temperature and the tertiary air flow: the input instruments of the combustion balance: the plant that meters its four airs and its fuel continuously holds the complete combustion account, and the account is the base of every tuning decision;

The diagnostics section is the closing of the loop: the combustion chemistry of this lesson predicts, the instruments measure, and the operators correct: the loop of the firing is the same closed loop that the introduction of the course drew for the whole plant: the measurement, the comparison, the intervention: the flame is the last reactor of the course and the first one that the instruments watch in real time.

9. The Frequently Asked Questions

What is the difference between the volatile matter and the fixed carbon of the coal?

The volatile matter is the part of the coal that evaporates and burns as the gas when it is heated (the hydrocarbons, the hydrogen, the CO, the tars), while the fixed carbon is the solid residue that burns as the char: the volatile matter decides the ignition behaviour, the fixed carbon the main heat release: the petcoke with the low volatiles is hard to ignite, and the bituminous coal with the high volatiles lights up easily: the two numbers from the proximate analysis shape the whole firing strategy.

Why is the pulverised fuel fineness so important for the flame?

Because the char burnout happens at the particle surface, and the burnout time grows with the particle size: the coarse particles leave the flame unburned, produce the CO, and their ash lands wherever the gas takes it: the fineness of the fuel is the direct control of the combustion completeness and the flame length: the coal mills of the kiln are therefore tuned like the raw mills, with the sieve residues at the target, typically 1 to 3% on the 90 micrometer sieve for the coal and finer for the petcoke.

What is the ideal oxygen at the kiln inlet?

The modern precalciner kilns operate the kiln inlet oxygen in the range of about 0.5 to 3.5%, with the common practice near 1 to 3%: the exact target balances the complete combustion and the sulphate retention (the higher oxygen) against the fuel economy and the NOx (the lower oxygen): the CO must stay below about 0.1% at the same time: the oxygen target is a compromise, reviewed at every fuel change and every burner adjustment.

Does the secondary air temperature really change the flame?

It changes the flame temperature directly: the air from the cooler arrives at 700 to 1000 degrees Celsius and carries the preheat into the flame: the hotter the secondary air, the higher the flame temperature and the faster the ignition: the drop of the secondary air temperature (the cooler problems, the low clinker bed, the air leaks) is seen immediately in the burning zone: the cooler and the flame are one system, and the plants watch the secondary air temperature as the first efficiency gauge of the firing.

How does the flame influence the coating and the refractory?

Through the heat distribution: the short, hot flame concentrates the heat in the front of the zone and overheats the local charge and brick, the long flame spreads the heat and weakens the sintering: the flame impingement destroys the coating and exposes the brick to the hot core: the coating is the product of the moderate, stable heat, and the refractory campaign of the kiln is decided as much by the flame discipline as by the brick quality: the shell scanner shows the consequences of both.

Can the alternative fuels replace the coal completely?

Technically, the kiln flame and the calciner can run on the alternative fuels, but the chemistry sets the practical ceiling: the chloride, the alkali and the heavy metal limits of the alternative fuels (the lessons of the cycles) bind the substitution rate, and the flame properties of the low-volatile and the wet fuels bind the firing system: the plants achieve the substitution rates of 50 to 90% in the calciner and lower in the kiln flame: the complete substitution is rare, and the reasons are the chemistry of this course, not the combustion alone.

10. Conclusion

The combustion lesson closes the kiln chemistry course where every lesson began: at the fire: the fuel chemistry, the reaction heats, the particle life, the air system, the flame shape, the NOx and the calciner: the engineer who has followed the ten lessons can now walk the whole system: the mix and the burnability at the raw mill, the variability at the silo, the calcination in the preheater, the sintering in the burning zone, the mineralisers and the cycles in the chemistry of the minor elements, and the flame that drives it all: the kiln chemistry is one body of knowledge, and the combustion is the beating heart of the body: the course is complete.

The Complete Cement Technical Package includes the combustion chapter of the kiln chemistry course, the burner and the flame calculation tools and the fuel reference tables, together with the full kiln chemistry course and the 931 files of the package: the one-time $249.99: the instant download: the kiln chemistry from the raw meal to the flame: the professional library of the cement process, the chemistry of the kiln, complete: the engineer’s knowledge, by the mechanisms: the flame, mastered: the course, finished: the plant, understood.

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