Flames Burners: Complete Technical Guide

Somewhere in every cement plant there is a stable, quiet flame: the heart of the pyro-process. The flame in the rotary kiln burns the fuel, delivers the heat to the clinker bed at 1,450 C and draws the whole process behind it: the preheater, the calciner, the cooler, the fuel bill and the quality of the clinker. Yet the ballet of the flame is the least understood machine in the plant. The burner is one of the smallest physical devices of the burning line – the nozzle of a few meters – and one of the largest thermal effects.

This file is the complete industrial reference about the flames and the burners of the cement kiln: the physics of the flame, the types of the burners (the multichannel, the strong, the standard), the firing systems for the coal, the petcoke, the gas and the alternative fuels, the flame shape and the burn-out, the NOx, the control and the optimization, the burner pipe and the maintenance. Every paragraph is practical, every number is an operating figure, every table is a real plant comparison. The file is part of the Complete Cement Technical Package (931 files) and is addressed to the process engineers, the kiln operators and the graduates of the cement industry.

1. The Flame in the Kiln: The Physics of the Combustion

The flame of the rotary kiln is the product of three inputs – the fuel, the air and the mixing – and the cement engineers steer them to hold the flame in the exact place of the burning zone:

• The fuel and the air: the fuel (coal, coke, gas, oil, the alternative) enters the burner with the primary air and the kiln air (the oxygen of the secondary air drawn through the cooler): the total air is only 2-5% more than the stoichiometric – the kiln runs with the low excess oxygen (the typical O2 in the kiln gas: 1.5-3.5%):
• The mixing: the injected fuel and the hot secondary air mix at the burner nozzle: the faster the swirl and the higher the jet momentum, the shorter and the hotter the flame: the momentum (the N-force) defines the flame length: the low-impulse and the high-impulse burners are the two families of the market:
• The combustion stages: the inert heating of the fuel jet, the ignition at 400-600 C (the volatiles and then the char), the volatile combustion in the flame core, the char burn-out in the longer tail: the total time of the combustion of the coal particle in the kiln: 0.5-3 seconds at the 1,800-2,000 C gas peak:
• The radiation: the flame radiates at the 1,500-2,000 C of the gas: the kiln gas temperature at the burning zone 1,700-1,900 C: the radiation transfers the heat to the clinker bed and the brick: the radiation is the effective transport of the kiln – the flame must be as bright as the soot particles allow:

The physics of the file proves that every plant decision – the fuel type, the burner, the air flow, even the kiln speed – changes the flame: the flame changes the heat transfer; the heat transfer changes the clinker. The operator’s first skill is the reading of the flame: the reading of the flame is the reading of the whole burning zone.

2. The Types of the Burners: From the Old Pipe to the Multichannel

The burner has evolved in the generations, and the modern plant uses two main families:

• The multichannel burner: the standard of the modern kilns (the Pyrojet, the Jetflex, the Pillard, the IKN): 4-7 channels of air and fuel: the axial air, the radial (swirl) air, the central gas or the liquid fuel gun, the external pulverized coal channel: the controls permit the flame shape adjustment during the running: the impulse 8-30 N per MW depending on the brand:
• The single channel burner: the pipe in the pipe: the fuel plus the primary air: the flame is set by the velocity only: still found in the gas and the small plants: the fewer adjustments in the running day:
• The pulverized coal channel: the coal enters through the annular channel, the conveying air from the mill: the 2-8% moisture coal ground to the 2-5% residue of 90 microns, the swirl air for the flame spreading at the base:
• The gas and the liquid guns: the gas (natural, the biogas, the syngas) through the central gun, the liquid (the heavy fuel oil) via the atomization nozzle with the steam or the compressed air:

The burner family	The channels	The impulse (N/MW)	The flame control
The old single pipe	1-2	3-6	The velocity only
The first multichannel	3-4	8-15	The axial vs the swirl
The modern multichannel	4-7	12-30	The full shape control in the hour

The file details the multichannel burner, the one that the plant uses: the arrangement of the sections, the air velocities (the axial 200-450 m/s, the swirl 80-150), the swirl numbers, the channel functions and the setting procedures: also the selection chapter compares the burner brands by the impulse, the flexibility and the maintenance, and gives the technical evaluation for a burner purchase decision.

3. The Firing Position: The Flame in the Kiln Geometry

The flame position in the kiln is the product of the burner alignment and the operator intent:

• The burner pipe position: the burner pipe enters the kiln through the hood with the setting degrees from the kiln axis: the horizontal alignment with the kiln axis: the height and the angle at the tip set the flame direction over the clinker bed:
• The flame zones: the desired: the flame starts shortly after the nose, the burning zone 15-25 m length at the 4,500-5,500 t/d kiln, the diameter 4.8 m: the flame must not hit the material at the nose (the premature burning, the ring) and must not hit the brick wall (the local overheating, the brick spalling):
• The flame length: the L/D (the flame length per burner kiln diameter) of the design: 8-20 D: the short flame for the highest temperature, the long flame for the soft zone or when the kiln builds the rings: the burner mechanics: the axial vs the swirl vs the fuel distribution:
• The kiln hood constraints: the secondary air enters at the kiln nose: its temperature and the velocity (5-15 m/s at the hood) shape the flame root: the hood vents and the leaks: the disciplines of the plant seal the nose:

The “nose of the flame” and the “flame hold” are the operator concepts: the flame root at the hot spot 30-60 cm from the burner tip: the root must be stable: the dark space at the kiln nose means the fuel ignites late: the flame is dragged, the temperature is depressed, the clinker loses the quality. The chapter gives the visual mapping: what the good flame looks like through the camera, and what problems each shape means.

4. The Fuels: From the Coal to the Alternative Fuels

The fuel of the kiln is the economics of the plant, and the burner is the birthplace of the fuel change:

• The coal: the classic: the volatiles 25-40%, the calorific value 25-30 MJ/kg: the ash goes into the clinker (the fuel ash adjustment in the raw mix), the moisture dried in the mill to under 1-2%: the grind fineness: the 2-5% residue on 90 microns: the flame speed rises with the fineness:
• The petcoke: the cheap: the volatiles 10-12%, hard to ignite, the high sulfur: the calcined coke requires the highest flame temperature and the strong swirl to hold the root: the fineness 1-2% on 90 microns: the kiln combustion margins of the petcoke are small:
• The natural gas: the volatiles 100%: the easy ignition: the NOx penalty (the flame temperature is high), the expensive in the regions: the flame very short, the heat flux peaked at the base:
• The alternative fuels (AF/RDF): the shredded tyres, the plastics pellets, the solid recovered fuel, the meat and the bone meal: each has the ash, the volatiles, the heavy metals, the salt: the modern burners fire the alternative fuels at 30-80% of the thermal input with the clinker quality and the emission controls:

The fuel: the volatiles, the calorific value, the ash composition and the oxidation behavior define the burner settings: the fuel mixtures (the coal and the coke, the gas and the alternative) are fired in the one burner with the flame adjustments per the ratio: the file includes the fuel selection tables and the calculation of the specific heat consumption per the clinker at the fuel ratio of the plant.

5. The Flame Shape, the Momentum and the Burn-out

The shape of the flame is the score of the burning process: the long, lazy flame and the short, fierce flame are two completely different kilns:

• The flame length: the flame length of the design: 10-18 times the kiln diameter: the long flame covers the soft calcination zone, but spreads the heat and lowers the flame temperature: the short flame concentrates the heat on the clinker in the burning zone, raises the temperature 50-100 C, but risks the brick and the instability:
• The momentum (the impulse): the product of the jet velocity and the mass flow: the momentum of the modern burner 12-30 N per MW: the higher momentum pulls the secondary air into the flame (the recirculation), shortens the flame and improves the combustion: the momentum table per the kiln size in the file:
• The burn-out: the char must burn out inside the kiln before the gas leaves: the unburned carbon in the hot meal = the heat losses of the kiln, the CO in the exhaust: the green shots (the unburned coal with the dark core) identify the poor combustion: the burn-out at the 99%+ is the goal:
• The flame temperature: the adiabatic flame temperature of the coal-air 2,000-2,200 C, the actual gas 1,700-1,900: the operator in the field sees the yellow-white flame: the white = the hot and the hard (the risk), the orange = the soft and possibly the long: the cameras of the hood see the difference:

The kiln diameter	The typical flame length	The momentum range	The kiln speed
4.2 m	20-45 m	12-22 N	2.8-3.6 rpm
4.8 m	25-55 m	15-28 N	3.0-4.0 rpm
5.6 m	30-65 m	18-30 N	3.5-4.5 rpm

The file’s practice: the flame adjusted by the operator in steps of 2-5% of the flow, the effect watched in the kiln camera, the kiln temperature, the NOx and the CO: every adjustment is logged in the shift book: the plant that logs the flame experiments doubles the speed of the learning of the fire.

6. The Air System of the Burner and the Secondary Air

The air side of the burner is as important as the fuel side: the primary air (the carrying air of the burner) and the secondary air (the combustion air from the cooler) play different roles:

• The primary air: the air through the burner channels: 6-15% of the total combustion air in the modern multichannel (vs. the 25-40% in the old): the low primary: the cool air not needed in the flame: the remaining 85-94% is the hot secondary air at 800-1,100 C: the high secondary = the high efficiency of the flame:
• The split of the primary: the axial (the straight momentum), the swirl (the rotation for the recirculation) and the center (the fuel carrier + the atomization): the ratio of the axial to the swirl is the flame shape control: the typical swirl 0.5-2.0 (the swirl number):
• The velocities: the axial 100-450 m/s, the swirl 50-200, the fuel carrier 20-30: the velocity is the momentum divided by the density: the density of the primary air is the cold (the 1.2 kg/m3), and of the hot secondary the 0.3-0.4: the momentum of the two mixes is calculated in the file’s sheet:
• The O2 of the combustion: the kiln oxygen target 1.5-3.5% at the kiln back: the insufficient O2: the CO, the kiln smoke, the unstable: the excess O2: the energy loss of the excess air, the NOx rise, the O2 optimizer of the plant:

The complete air balance of the kiln (the primary, the secondary, the tertiary, the calciner air, the false air) is a table of the file: the plant with the balanced air has the flame in its place: the file walks through the “air audit” of a kiln: the measurement, the leaks, the effects, the fixing: the air is the fuel of the flame bill.

7. The Flame and the NOx: The Environmental Response

The modern kiln: the flame is also an emission control device: the NOx of the kiln is born in the flame and dies in the preheater:

• The NOx formation: the thermal NOx (the peak temperatures above 1,400 C, the nitrogen), the fuel NOx (the fuel nitrogen), the prompt NOx: in the kiln flame the thermal NOx dominates: the longer, the softer the flame: the lower the NOx: the short flame peak temperature: the high NOx:
• The flame and NOx correlation: the kiln NOx 300-1,200 mg/Nm3 (as NO2 at the 10% O2): the standard targets 200-500 in the permit: the flame shape and the secondary air temperature: the trends: the longer flame, the staged air, the lower heat flux edge: the every 1% O2: +10-20% the NOx:
• The measures: the kiln (the SNCR: NH3/urea into the preheater area at 850-1,050 C, the SCR catalyst at the tail), the process measures: the lower flame temperature, the staged combustion, the Nwuffle: the NOx reduction 30-60% by the SNCR: the file quantities the aqueous systems:
• The balance: the NOx control costs the fuel: the SNCR doses the NH3, the ammonia slip below 5-10 mg: the file is the optimizer of the NOx vs. the fuel bill vs. the whiteness of the stone: the KPI of the environmental compliance:

The NOx chapter is not a policy chapter: it is the process chapter: the operator who understands the flame understands the NOx: the file gives the full map of the interactions and the optimization of the operating cost of the NOx while the clinker quality is held.

8. The Burner Pipe and the Maintenance of the Firing Equipment

The burner is the machine on the hot end: the main components and their care:

• The burner pipe: the steel water-cooled pipe of the burner, the sections: the fuel channel, the annular air channels, the tip: the refractory coating of the tip (the castable) protect the metal at the 1,500 C: the tip wear: the inspection during the kiln steps:
• The burner gallery: the cart carrying the burner, the slide for the position adjustment, the water seals, the flexible hoses: the inspection every week: the gallery must slide freely during the operation:
• The valves: the air valves of the burner channels: the butterfly valves, the control of the axial and the swirl, the sensors of the channels: the leak check, the calibration with the flow meters:
• The igniter: the high-energy igniter (the spark) at the tip: the flame rod/UV scanner detects the flame (the flame failure: the automatic shut-off of the fuel): the time to the detection: < 2-4 s:
• The maintenance: the registers and the charts of the file: the daily (the temperature, the pressure of the burner), the weekly (the gallery, the hoses), the monthly (the valves, the igniter test, the camera clean), the annual (the tip inspection, the cleaning of the channels, the refractory repair):

The file carries the condition-based thermography of the burner: the tip temperature: the early sign of the broken refractory, the missing coal flame-holder: the maintenance of the burner is the cheapest insurance of the combustion stability: the kiln with the clean burner fires better and the file shows the before-after case.

9. The Start-up, the Shutdown and the Flame Failure of the Kiln Burner

• The start-up sequence: the kiln is purged with the air for the fixed time (the volume of the kiln gas exchange, typically 3-5 minutes at the purge rate), the fuel valves are closed during the purge: the igniter is inserted, the spark confirmed (the UV or the flame rod), the fuel valve is slowly opened: the flame established at the pilot: the ramp of the flame and the air to the operating points in steps:
• The shutdown sequence: the fuel ramp down slowly (the flame reduction, the kiln cooling), the air follows, the burner fuel valve closed: the kiln must not be left with the fuel and the heat without the air: the flash risk is the discipline of the order of the closing:
• The flame failure protection: the flame monitoring (the UV scanner, the flame rod) with the proof test: the failure: the fuel valve closed within the seconds, the alarm: the scanner purged and the window clean: the maintenance weekly:
• The kiln camera: the modern kiln the camera in the hood or the nose with the cooling: the operator sees the flame live: the camera is the primary instrument of the burning zone control: its maintenance (the lens purge air, the water cooling) is in the file:

The start-up and the shutdown of the kiln burner are the highest risk phases of the burning line: the file defines the full sequences of the operator with the checks, the limits and the typical pitfalls: the flame advocates: the purge time, the interlock tests, the flame-out drills: the learning from the accidents of the industry is included in the part of the file.

10. The Flame Diagnostics: The Cases of the Plant

The real kilns show the flame problems in the recognizable patterns, and the file presents the case methodology of the diagnosis:

• The very long pale flame: the low momentum, the too much swirl off, the primary too high, the coal too coarse: the consequences: the burning zone moves down the kiln, the heat of the flame wasted, the smoke ring: the correction: the momentum up, the axial air up, the fineness improved:
• The short hot flame with the white: the intense: the consequences: the brick overheat at the nose, the NOx spiked: the correction: the swirl reduced, the flame pushed, the excess O2 to the lower:
• The flame flaring upward: the burner tilted, the kiln load high, the momentum weak against the clinker: the consequences: the brick above the flame, the kiln skin overheating: the corrections: the burner alignment, the kiln speed review:
• The combustion with the unburned: the coal particles in the clinker sieve (the black core): the consequences: the fuel losses, the kiln stack C: the corrections: the mill fineness, the flame temperature, the coal moisture:

The file closes the chapter with the troubleshooting table: the flame symptom, the possible cause, the diagnosis test, the correction: the table is designed for the shift use: the k: the operator prints the table, and the diagnosis of the flame is done in the minutes, not in the days.

11. The Fuel Preparation and the Delivery to the Burner

The flame is born in the mill, not in the burner: the quality of the pulverized fuel decides the fire, and the file covers the complete delivery chain:

• The coal grinding: the ball mill or the vertical mill of the coal: the fineness by the volatile and the reactivity: the 75-90 micron residue 1-4% for the petcoke, 5-12% for the high-volatile coal: the moisture 0.5-2% at the exit of the mill (the drying inside):
• The dosing: the weigh feeders at the coal silo: the flow accuracy 1-2%: the feed to the mill and the fuel flow to the burner: the coal metering errors: the kiln can see 20-30 C per 1% of the fuel the dosing error:
• The mill separators: the blade and the static types: the fineness control point: the separator speed curve: the fineness of the coal = the flame length: coarser = the longer and the cooler, the finer = the short and the hot:
• The conveying to the burner: the air slide, the screw conveyor, the the pneumatic: the coal at the burner pressure: the velocity and the flow the carrier: the distribution uniform in the coal channel: the pulsation of the fuel the flame flutter: the dose uniform:

The chapter includes the mill tests of the fineness (the sieve analysis, the Rosin-Rammlerr curve), the moisture control, the blast of the mill (the temperature, the CO), the safety of the coal plant (the inerting with the N2, the explosion protection): the fuel handling is the front door of the flame, and the file integrates the two into the single chain, from the coal pile to the tip of the burner.

12. The Flame Monitoring, the Instruments and the Control System

The modern control of the flame is a set of instruments that the operator trusts more than the eyes on the hood glass:

The parameter	The instrument	The purpose
The flame shape and the position	the kiln camera (the CCD, the color)	the visual of the burning zone, the flame position
The gas temperature at the nose	the pyrometer (the ratio type), the hood	the burning zone temperature signal
The shell temperature	the IR scanners of the kiln shell	the hot spots, the coating, the flame stray
The O2, the CO, the NOx in the kiln gas	the gas analyzers (the in-situ, the extractive)	the combustion control
The fuel flow, the air flow	the flowmeters of the coal and the primary	the heat input, the air balance

The instrument chain of the file: the signals wired to the process control system (the DCS), the trends the operator runs, the classifications, controls the answers: the advanced plants add the soft sensors (the virtual flame temperature) and the model of the burning zone to the optimizer: the file describes the hierarchy, including the maintenance and the calibration of the pyrometers of the kiln: the accurate measurement is the prerequisite of the flame control: the numbers, the file: the instrument loops of each parameter.

13. The Frequently Asked Questions

What is the ideal flame length of the cement kiln?

The flame length of the kiln flame: 10-15 times the kiln diameter in the classic practice, and the exact is the case-specific: the 4.2 m kiln: 25-45 m flame: the balance: the short flame (the radical) for the high clinker quality with the high temperature of the burning zone, the long flame for the low NOx and the soft coating: the file gives the length by the fuel and the product.

Does the burner matter for the fuel consumption?

Absolutely: the burner velocity, the mixing and the momentum derive the completeness of the combustion and the radiation peak: the field data of the file: with the same fuel the 10-40 kJ/kg cl savings between the poorly-fired and the well-fired kilns: the burner is the primary fuel-saving component of the pyro line after the cooler.

What flame does the very thick ring? The black clinker?

The very thick, fluffy, black clinker: the reducing atmosphere in the burning zone: the flame not enough air: the check the O2 in the kiln back (below the 1.0 beware), the coal feed increased without the air: the correction: the air up, the feed hold: the kiln will flush: the file: the full reducing burning chapter and the recovery of the kiln atmosphere.

Thermal NOx and the SNCR: which?

The NOx of the kiln: the thermal NOx dominates: the measures: the process (the flame, the O2, the temperatures), the SNCR (the urea or the ammonia solution at the preheater, 850-1,050 C window) and the SCR: the file: the costs, the effectiveness and the operating constraints of each tool: the plant often combines the process + the SNCR benefitting the balance.

Single channel vs multichannel burner: which one for me?

The multichannel burner: the flexibility (the AF, the flame shape, the momentum), the low primary air (the efficiency), the higher price: the single channel: the low cost, the simplicity, but the flame fixed and the primary air high: for the stable gas or the coal kilns at the capacity, the single channel still works; for the growth and the fuel diversity, the multichannel wins: the file’s decision matrix.

14. The Conclusion

The flame is the eye of the kiln: the burning zone temperature, the fuel consumption, the clinker quality and the emissions are all written in the flame: the file of the Complete Cement Technical Package (931 files) puts the fire in the hands of the engineer: the physics, the hardware, the operation, the optimization and the troubleshooting: the flame shapes, the fire: from the ignition spark to the 2,000 degrees of the burning zone, the file is the master course of the combustion.

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.