Kiln Burner Technical Session: Complete Guide

Welcome to the burner technical session, the training program that takes the kiln operator, the combustion engineer, the production manager and the maintenance specialist through the complete science and practice of the rotary kiln burner. The burner is the instrument that shapes the flame, and the flame is the heart of the pyroprocess: its shape, its length, its temperature profile and its position in the kiln determine the clinker quality, the fuel consumption, the refractory life, the NOx emissions and the safety of the whole kiln system. This session covers the fundamentals of flame aerodynamics, the role of primary air and the critical concept of flame momentum, the design of the burner nozzle and its annuli, the practical art of flame tuning, the use of alternative fuels in the burner, and the safety systems and the operating discipline that protect the plant and the people. Written as a complete technical course, the session moves from the theory of the combustion air streams, through the hardware of the burner and the adjustments of the flame, to the operational practice that the burner operator applies every day on the kiln. By the end of the session, the participant understands why the burner is called the most influential single instrument in the pyroprocess, and how to use it to run the kiln at its best.

The Role of the Burner in the Kiln

The burner is the device that delivers the fuel and the combustion air to the kiln and creates the flame that provides the heat for the clinkering process. In a modern kiln the burner is mounted at the kiln hood and extends into the kiln inlet, discharging a high-velocity jet of fuel and air into the burning zone. The flame produced by the burner is the source of the gas temperature that drives the clinker reactions, and its radiative heat transfer to the charge and to the refractory is the mechanism by which the clinker is heated to its sintering temperature.

The burner is also the instrument that shapes the entire thermal environment of the kiln. The flame length sets the position and the extent of the burning zone, the flame shape sets the radiative heat flux distribution along the burning zone, and the flame temperature sets the peak gas temperature that the charge and the refractory experience. A burner that produces a long, lazy flame moves the heat release downstream, cools the kiln inlet and leaves the burning zone short; a burner that produces a short, intense flame concentrates the heat release at the kiln nose and can overheat the refractory and the coating.

The burner is therefore the control point for the balance between the kiln throughput, the clinker quality, the fuel efficiency and the refractory life. The kiln operator who masters the burner controls the process to a degree that no other single adjustment can match, and the burner technical session is the training that develops that mastery. The session is structured so that each concept builds on the previous one: the aerodynamics explain the air streams, the air streams explain the momentum, the momentum explains the flame shape, and the flame shape explains everything that the operator observes in the burning zone.

Combustion Air Streams

The combustion air for the kiln is delivered through three distinct streams: the primary air, the secondary air and the tertiary air. The primary air is the air that is delivered through the burner itself, together with the fuel, and it provides the initial momentum that shapes the flame. The secondary air is the hot air drawn into the kiln through the hood from the cooler, and it provides the bulk of the oxygen for the combustion. The tertiary air is the air ducted to the calciner, where it supports the combustion of the calciner fuel, and it does not enter the kiln burning zone directly.

The secondary air is the dominant stream in terms of oxygen supply. For a typical coal-fired kiln, the secondary air provides on the order of 70 to 85 percent of the total combustion air, and its temperature, in the range of 800 to 1100 degrees Celsius, is a major contributor to the thermal efficiency of the flame. The secondary air enters the kiln through the annular space around the burner or through the hood openings, and its flow is driven by the kiln’s natural draft and by the pressure control of the hood.

The primary air is the minority stream in terms of volume, typically in the range of 8 to 15 percent of the total combustion air, but it is the stream that determines the aerodynamics of the flame. The primary air is delivered through the burner at a high velocity, and its momentum creates the recirculation zone that stabilises the flame and entrains the hot secondary air into the combustion zone. The balance between the primary air momentum and the secondary air flow is the fundamental control of the flame shape.

Flame Aerodynamics

The aerodynamics of the flame are governed by the interaction of the primary air jet with the secondary air stream and the combustion products. The primary air, carrying the fuel, leaves the burner nozzle at a velocity that is typically many times the velocity of the secondary air, and the high-speed jet creates a low-pressure zone at its boundary that draws the surrounding gas inward. This entrainment is the mechanism by which the flame mixes the fuel with the oxygen and by which the hot combustion products are recirculated back toward the burner face.

The recirculation zone at the burner face is the key to flame stability. The hot combustion products, drawn back toward the nozzle by the entrainment, provide the continuous ignition source that keeps the flame attached to the burner. If the recirculation is too weak, the flame lifts off the burner and becomes unstable; if the recirculation is too strong, the flame can flash back into the burner and damage the nozzle. The design of the burner and the setting of the primary air are arranged to create the stable recirculation zone at the correct position.

The aerodynamics also determine the mixing of the fuel with the air, and hence the rate of combustion and the flame length. A high-velocity primary air jet with a strong turbulence gives rapid mixing, a fast combustion and a short, intense flame. A low-velocity, low-turbulence stream gives slow mixing, a slow combustion and a long, lazy flame. The burner technical session teaches the operator to read the flame shape as the visible expression of the aerodynamics, and to adjust the primary air and the burner settings to achieve the desired combustion behaviour.

Primary Air and Its Role

The primary air is the air delivered through the burner with the fuel, and it serves three distinct purposes. The first purpose is the transport of the fuel: the fuel, whether pulverised coal, alternative fuel particles or gas, is carried into the kiln by the primary air stream. The second purpose is the provision of a portion of the combustion oxygen, particularly for the ignition zone at the burner face. The third purpose, and the most important in modern practice, is the creation of the aerodynamic momentum that shapes the flame.

The primary air is usually a mixture of the carrier air, which transports the fuel, and the swirl air, which imparts a rotational component to the jet. The carrier air is the small stream that conveys the pulverised fuel from the mill to the burner, and the swirl air is an additional stream introduced through the burner annuli to create the swirling motion that enhances the mixing and the recirculation. The proportion and the velocity of the two streams are adjustable, and the adjustments are the principal tools of the flame tuning.

The primary air rate is a critical economic parameter. The primary air is delivered at ambient temperature, and every kilogram of primary air that enters the kiln must be heated to the flame temperature, which consumes energy that would otherwise be available for the clinkering. The modern burners are designed to operate with a low primary air rate, typically in the range of 6 to 12 percent of the total combustion air, and the low primary air designs use the momentum of the fuel carrier air and the secondary air entrainment to maintain the flame stability without the high primary air volumes of the older burners.

Flame Momentum and Its Importance

Flame momentum is the product of the mass flow and the velocity of the burner jet, and it is the single most important concept in modern burner design and operation. The momentum of the primary air jet, and its interaction with the secondary air, determines the entrainment, the mixing and the shape of the flame. The modern burner engineering is based on the optimisation of the flame momentum, and the modern burner design with the adjustable annuli is essentially a device for setting the momentum.

The momentum is quantified in terms of the jet momentum flux, typically expressed as the product of the gas density, the square of the velocity and the flow area, and it is normalised against the kiln diameter and the secondary air flow in the burner design calculations. A high momentum produces a short, intense, compact flame with a strong recirculation and a rapid mixing, which is used for the high-throughput kilns and for the kilns burning the alternative fuels that require the intensive mixing. A low momentum produces a long, diffuse flame, which is used where a gentle heat release is required to protect the refractory.

The setting of the momentum is the central adjustment of the burner. The operator changes the momentum by changing the primary air velocity, by changing the primary air rate, by adjusting the swirl, or by changing the burner position in the kiln, and each adjustment has a predictable effect on the flame. The burner technical session trains the operator to use the momentum concept to reason about the flame: to know, when the flame is too long or too short, whether the correction is the velocity, the rate, the swirl or the position, and to make the correction with the minimum trial and error.

The Burner Nozzle and Its Annuli

The burner nozzle is the terminal section of the burner that discharges the fuel and the air into the kiln, and it is engineered as a set of concentric annuli that deliver the various streams. The central tube or the central annulus delivers the main fuel stream with its carrier air, and the surrounding annuli deliver the swirl air, the auxiliary air and, in the multi-channel burners, the individual streams of the alternative fuels. The geometry of the annuli, their areas, their angles and their positions, sets the aerodynamics of the jet.

The modern burner is the multi-channel design, with typically three to six channels arranged concentrically. The central channel delivers the main fuel, the surrounding channel delivers the swirl air with the angled vanes that create the rotation, and the outer channels deliver the additional air streams that shape the flame. The channels are adjustable in their settings, and the adjustments change the velocity, the swirl and the position of the individual streams, giving the operator a wide range of flame shapes from a single burner.

The design of the nozzle is protected by the wear and the temperature: the nozzle operates at the very front of the kiln in the region of the highest heat flux, and it is made of the high-temperature alloys and is protected by the refractory blocks at the burner pipe entry. The nozzle is inspected at every shutdown for the erosion, the distortion and the wear, and the damaged nozzles are replaced, because the geometry of the nozzle is the geometry of the flame, and a worn nozzle produces a distorted flame.

The Burner Pipe and the Hood

The burner pipe is the water-cooled or air-cooled pipe that carries the burner into the kiln, and it is mounted through the kiln hood and the nose ring. The burner pipe is positioned in the kiln so that the nozzle sits at the correct distance from the kiln inlet, typically at a set distance inside the refractory face, and the position is adjustable to allow the flame to be placed at the optimum point in the burning zone. The axial and the radial position of the burner are part of the flame tuning.

The hood is the structure that encloses the kiln inlet and houses the burner, the secondary air inlet and the observation ports. The hood is refractory-lined and is kept under a slight negative pressure to contain the dust and the hot gas, and the hood pressure is one of the control parameters that the operator monitors. The hood also carries the tertiary air duct take-off in the calciner systems, and the balance of the secondary air and the tertiary air is managed through the hood pressure and the duct dampers.

The cooling of the burner pipe is essential for its life. The burner tip, exposed to the radiant heat of the flame and the hot gas, would overheat without the cooling, and the water-cooled and the air-cooled designs remove the heat continuously. The cooling system is monitored, and the loss of the cooling water flow or the cooling air flow is a trip signal that protects the burner from the meltdown. The maintenance of the burner pipe, the seals, the cooling system and the refractory protection is part of the kiln shutdown scope.

The Swirl and the Flame Shape

The swirl is the rotational component imparted to the burner jet by the angled vanes or the tangential injection in the swirl channel, and it is the principal control of the flame shape. The swirl creates a centrifugal motion in the jet that widens the mixing zone, enhances the entrainment and strengthens the recirculation. A high swirl produces a short, wide, bushy flame with a strong recirculation, while a low swirl produces a longer, narrower, more axial flame.

The swirl number is the dimensionless parameter that quantifies the swirl: it is the ratio of the tangential momentum flux to the axial momentum flux times the nozzle radius. A swirl number below approximately 0.6 gives a jet that is still essentially axial, while a swirl number above approximately 0.6 gives the flow transition to the strong recirculation that characterises the short flames. The burner design sets the swirl number of each channel, and the adjustments of the swirl air allow the operator to move the flame between the long and the short forms.

The flame shape is observed directly through the kiln observation ports, and the operator correlates the visual appearance with the process response: the burning zone temperature, the kiln drive torque, the NOx emissions and the refractory temperatures. The burner technical session teaches the systematic observation of the flame: the length, the width, the position, the colour and the attachment to the burner, and the correlation of each visual characteristic with the burner settings and the process response.

Flame Length and the Burning Zone

The flame length determines the position and the extent of the burning zone, which is the region of the kiln where the clinker reaches its maximum temperature. A short flame releases the heat over a short length near the kiln nose, concentrating the heat flux and raising the peak clinker temperature; a long flame releases the heat over a longer length, spreading the heat flux and lowering the peak temperature. The optimum flame length is the one that gives the clinker its target burning temperature with the minimum fuel and the maximum refractory life.

The burning zone temperature, typically in the range of 1400 to 1500 degrees Celsius for the clinker, is set by the balance between the heat release and the heat demand. A flame that is too short overheats the refractory and the coating at the nose, a flame that is too long leaves the clinker under-burned, and both conditions are visible in the clinker quality and the process data. The operator uses the flame length to control the peak temperature and the heat distribution.

The flame length is controlled by the momentum and the swirl, and by the burner position. A burner positioned further into the kiln moves the flame and the burning zone downstream; a burner withdrawn toward the hood moves them upstream. The combination of the flame length and the burner position sets the location of the maximum heat flux, and the operator adjusts both to keep the burning zone at the correct position relative to the coating and the refractory.

Fuel Types and Fuel Injection

The kiln burner is fuelled by a range of materials, from the fossil fuels to the alternative fuels. The principal fossil fuels are the pulverised coal, the petroleum coke and the natural gas, and the principal alternative fuels are the shredded tyres, the waste-derived fuels, the meat and bone meal, the plastics, the solvents and the biomass. Each fuel has its own combustion characteristics: its heating value, its moisture, its ash content, its particle size and its volatility, and the burner must deliver and ignite each fuel reliably.

The pulverised coal and the petroleum coke are injected as a suspension in the carrier air, with the particle size typically in the range of 1 to 3 percent residue on the 90 micron sieve, and the ignition of the coal particles depends on the flame temperature and the particle residence time. The alternative fuels are injected through the separate channels of the multi-channel burner, often as coarse particles or as shredded pieces, and they require a high-momentum flame with a strong recirculation to ignite and to burn completely within the kiln.

The injection of the alternative fuels is one of the principal challenges of the modern burner. The alternative fuel particles are larger and more heterogeneous than the coal, and they require a longer residence time and a more intensive mixing to burn out completely. The modern burners are designed with the dedicated alternative fuel channels, positioned so that the coarse particles are injected into the core of the high-temperature flame, and the burner is tuned to give the alternative fuel its required residence time while maintaining the main flame shape.

Combustion and the Flame Temperature

The combustion in the kiln flame is a high-temperature, gas-phase and particle-phase reaction that converts the fuel and the oxygen into the combustion products and the heat. The theoretical flame temperature, the temperature that the products would reach with the complete combustion and no heat loss, is determined by the fuel composition, the air-to-fuel ratio and the preheat temperature of the air. The secondary air, preheated to 800 to 1100 degrees Celsius in the cooler, is the principal contributor to the high flame temperature.

The actual flame temperature is lower than the theoretical value because of the heat losses: the radiative heat transfer to the charge and the refractory, the heat carried by the combustion products, and the incomplete mixing and combustion. The flame temperature profile along the kiln is the result of the balance between the heat release and the heat transfer, and it is the temperature profile that drives the clinker reactions and the refractory loading.

The air-to-fuel ratio is the fundamental control of the combustion efficiency. Too little air gives the incomplete combustion with the carbon monoxide in the kiln gas, the rising fuel consumption and the reducing atmosphere that damages the clinker quality and the refractory. Too much air gives the excess oxygen, the heat carried away in the excess air and the rising NOx. The kiln is operated with a slight excess of oxygen, typically 1.0 to 2.5 percent at the kiln exit, and the operator controls the air-to-fuel ratio through the fuel rate and the kiln draught.

Flame Tuning

Flame tuning is the practical art of adjusting the burner settings to achieve the target flame, and it is the central skill that the burner technical session develops. The tuning procedure is systematic: the operator observes the flame through the observation port, reads the process data, makes a single adjustment, observes the response, and repeats until the flame and the process are at the target. The adjustments available are the primary air rate, the primary air velocity, the swirl air, the burner position, and, in the multi-fuel burners, the individual fuel and air streams.

The objective of the tuning is the flame that gives the optimum balance of the process parameters: the burning zone temperature, the kiln power draw, the specific fuel consumption, the NOx emissions, the refractory temperatures and the clinker quality. The tuning is a multi-variable optimisation, and the experienced operator learns the trade-offs: the short flame that raises the burning zone temperature but risks the refractory, the high momentum that improves the alternative fuel burn-out but increases the NOx, and the adjustments that move the process in the desired direction with the minimum side effects.

The tuning is also a response to the changing conditions. The fuel quality changes, the alternative fuel substitution rate changes, the kiln production changes and the cooler performance changes, and each change requires a re-tuning of the burner. The disciplined plant documents the burner settings for each operating condition, so that the operator can return the burner to a known good configuration quickly, and the burner technical session trains the operator to record and to reproduce the settings systematically.

Observing the Flame

The observation of the flame through the kiln observation ports is the operator’s primary tool for the direct assessment of the combustion. The flame is observed for its length, its width, its shape, its position relative to the burner and the nose ring, its colour and its brightness, and each characteristic carries information. A short, bright, compact flame with a well-defined shape indicates a high momentum and an intensive combustion; a long, dull, diffuse flame indicates a low momentum and a slow combustion; a dark or smoky flame indicates the incomplete combustion or the excess fuel.

The colour of the flame is an indicator of the flame temperature. The flame at a temperature above approximately 1300 degrees Celsius appears bright yellow-white, and as the temperature rises the flame becomes whiter and brighter. A flame that is orange or red indicates a lower temperature, and a flame with dark regions indicates the local fuel-rich zones where the combustion is incomplete. The experienced operator correlates the colour and the brightness with the thermocouple and the pyrometer readings to build a consistent picture of the flame temperature.

The observation is also used for the detection of the problems. A flame that lifts off the burner indicates the low recirculation and the instability, a flame that impinges on the charge indicates the wrong position or the wrong shape, a flame that is asymmetric indicates the blockage or the wear of a burner channel, and a flame with the soot or the smoke indicates the poor mixing or the fuel problem. The operator responds to each observation with the appropriate correction, and the systematic observation is the foundation of the flame management.

The Primary Air Rate and the Specific Heat Consumption

The primary air rate has a direct effect on the specific heat consumption of the kiln, and the minimisation of the primary air is a principal objective of the modern burner operation. The primary air enters the kiln at ambient temperature, and the heating of this air from the ambient temperature to the flame temperature consumes energy that is not available for the clinkering. The modern low-primary-air burners operate with the primary air rates in the range of 6 to 12 percent, compared with the rates of 20 to 30 percent used by the older high-momentum burners.

The relationship between the primary air and the heat consumption is significant. The reduction of the primary air rate by a few percentage points of the total combustion air reduces the specific heat consumption by a measurable amount, and the cumulative saving over the kiln’s life is substantial. The low-primary-air operation is achieved by the use of the fuel carrier air and the secondary air entrainment to provide the flame momentum, and by the careful tuning of the burner channels.

The minimisation of the primary air is constrained by the flame stability and the combustion requirements. A primary air rate that is too low gives a flame that is unstable, a lift-off or a poor ignition of the alternative fuels, and the operator must find the minimum primary air that gives the stable, complete combustion. The burner technical session trains the operator to optimise the primary air against the flame quality, the specific heat consumption and the process stability, rather than to operate at the fixed settings.

Alternative Fuels in the Burner

The use of the alternative fuels is one of the dominant trends in the cement industry, driven by the economics and the decarbonisation targets, and the burner is the key to the reliable and the safe use of the alternative fuels. The alternative fuels are injected through the dedicated channels of the burner, and the burner is tuned to give the alternative fuel its required residence time, temperature and oxygen for the complete combustion. The substitution rate, the proportion of the heat input from the alternative fuels, has risen to 60 to 90 percent in the leading plants.

The challenges of the alternative fuel combustion are the particle size, the heterogeneity, the moisture and the volatile content. The coarse alternative fuel particles require a longer residence time in the hot zone to burn out completely, and the incomplete burn-out produces the coarse ash, the reducing zones and the rising CO emissions. The high-moisture fuels reduce the flame temperature locally, and the high-volatile fuels can release the volatiles rapidly and create the local fuel-rich zones.

The burner operation with the alternative fuels requires the intensive mixing and the high momentum. The modern burners are designed with the high-momentum jets that entrain the secondary air strongly, creating the turbulent recirculation that keeps the coarse particles in the hot zone and provides the oxygen for the complete combustion. The operator manages the alternative fuel feed against the kiln conditions, the flame quality and the gas analysis, and the burner technical session includes the specific training for the alternative fuel operation.

NOx Control and the Burner

The nitrogen oxide emissions from the kiln are formed principally by the thermal mechanism, in which the nitrogen in the combustion air reacts with the oxygen at the flame temperatures above approximately 1400 degrees Celsius, and the burner has a major influence on the NOx formation. The flame temperature, the oxygen concentration in the flame and the residence time at the high temperature are the three factors that set the thermal NOx, and all three are controlled by the burner tuning.

The low-NOx burner strategies reduce the flame temperature and the oxygen availability in the zones where the NOx is formed. The use of a longer flame reduces the peak flame temperature and the NOx, the staging of the air reduces the oxygen concentration in the primary flame zone, and the reduction of the excess oxygen reduces the NOx at the kiln exit. The strategies are balanced against the clinker quality and the fuel consumption, because the measures that reduce the NOx can also reduce the flame temperature and the burning zone heat.

The plant-level NOx control combines the burner tuning with the process measures: the stabilisation of the kiln operation, the control of the excess oxygen, the use of the alternative fuels with the low nitrogen content and the selective non-catalytic reduction in the preheater. The burner technical session presents the NOx formation mechanisms and the burner measures, so that the operator understands the effect of each adjustment on the emissions and can operate the kiln at the optimum between the emissions and the efficiency.

Burner Safety Systems

The burner and the fuel system are protected by a comprehensive set of safety systems, and the safety discipline is a non-negotiable part of the burner operation. The safety systems protect against the fuel leakages, the flame failure, the high temperatures, the over-pressures and the unburned fuel accumulations, and they are designed so that any dangerous condition trips the fuel and the burner to a safe state.

The principal safety systems are the flame detection, the fuel shut-off valves, the inert gas purging, the pressure and the temperature interlocks and the ventilation interlocks. The flame detector monitors the flame continuously, and the loss of the flame signal closes the fuel valves within seconds. The fuel shut-off valves are the double-block-and-bleed arrangements that isolate the fuel completely when closed. The purging system removes the unburned fuel and the flammable gases from the kiln before the ignition, and the interlocks prevent the burner start unless the purging has been completed.

The start-up sequence of the burner is a safety-critical procedure. The kiln is purged with the air for a defined period to remove any accumulated fuel, the draught is established, the pilot flame or the ignition source is verified, and only then is the main fuel admitted. The sequence is enforced by the safety logic of the control system, and the operator cannot bypass the sequence. The burner technical session includes the complete training on the safety systems, the start-up and the shut-down sequences, and the response to the trips and the emergencies.

Start-Up and Shut-Down of the Burner

The start-up of the burner is carried out to a strict procedure that protects the plant and the personnel. The sequence begins with the visual inspection of the burner and the fuel system, the verification of the safety interlocks and the purging of the kiln. The draught is established, the pilot is lit and verified, and the main fuel is admitted at a low rate and is increased gradually as the flame stabilises. The operator watches the flame, the temperatures and the gas analysis continuously during the start-up, and any abnormality stops the sequence.

The shut-down of the burner is the reverse procedure. The fuel rate is reduced gradually, the main fuel is stopped when the kiln can no longer maintain the combustion, and the kiln is purged to remove the residual fuel and the gases. The shut-down sequence is designed to prevent the accumulation of the unburned fuel in the hot kiln, which could ignite explosively when the kiln is restarted, and the purging is verified before the kiln is left unattended.

The emergency trips interrupt the normal sequence. The loss of the flame, the high pressure, the high temperature or the loss of the draught trips the fuel instantly, and the operator then follows the emergency procedure: the confirmation of the fuel isolation, the purging of the system and the investigation of the cause. The discipline of the emergency response is trained and rehearsed, because the safe handling of a burner emergency is the difference between a minor event and a serious incident.

Burner Maintenance

The burner is a precision instrument, and its maintenance is scheduled as part of the kiln shutdown. The principal maintenance items are the nozzle, the channels, the swirl vanes, the seals, the cooling system and the refractory protection. The nozzle is inspected for the erosion and the distortion, the channels are inspected for the wear and the blockage, and the swirl vanes are inspected for the damage that would change the swirl and the flame.

The burner is also inspected for the internal condition: the fuel and the air channels must be clean and free of the deposits, the pipe must be straight and the seals must be tight. The cooling system is inspected for the scale, the blockages and the leaks, and the refractory protection at the hood is inspected for the damage. The burner settings and the configuration are recorded at the shutdown, so that the burner can be returned to the operating configuration after the maintenance.

The maintenance of the fuel system is equally important. The fuel lines, the valves, the filters and the dosing devices are inspected and serviced, and the safety systems are tested. The testing of the safety interlocks, the flame detector and the shut-off valves is a scheduled activity, and the results are documented. The burner technical session includes the maintenance module, so that the maintenance staff understand the function of each component and the effect of the wear and the damage on the flame.

Key Burner Parameters

The management of the burner is supported by a small set of measurable parameters. The table below summarises the typical values for a modern multi-channel kiln burner and the meaning of a deviation from the normal range.

The value of the table is that it converts the qualitative description of the burner into quantitative limits that the operator can measure, trend and act on, and it provides the framework for the systematic burner management.

Troubleshooting the Flame and the Burner

The troubleshooting of the burner problems follows a systematic method: the observation of the symptom, the identification of the possible causes, the confirmation of the cause with the measurements, and the correction. The common symptoms are the unstable flame, the flame lift-off, the asymmetric flame, the smoky flame, the excessive NOx, the rising CO and the refractory damage at the nose, and each symptom has a defined set of probable causes in the burner settings, the fuel quality, the air streams and the mechanical condition.

The unstable flame and the lift-off are usually caused by the low momentum, the low primary air velocity, the worn nozzle or the incorrect fuel rate, and the correction is the increase of the momentum, the replacement of the nozzle or the correction of the fuel. The asymmetric flame is usually caused by the blocked or the worn channel, the build-up at the nozzle or the misalignment of the burner, and the correction is the cleaning, the replacement or the re-alignment. The smoky flame is caused by the poor mixing, the high fuel rate or the low oxygen, and the correction is the increase of the momentum, the reduction of the fuel or the increase of the air.

The troubleshooting is supported by the measurements: the gas analysis, the temperatures, the pressures and the visual observations. The plant that documents the troubleshooting cases, and that shares the lessons, builds the organisational knowledge that turns the burner problems from the mysteries into the routine. The burner technical session includes the troubleshooting case studies, so that the participants learn the method through the real examples.

The Future of the Burner

The future of the kiln burner is being shaped by the decarbonisation, the alternative fuels and the digitalisation. The burners are being designed for the higher alternative fuel substitution rates, with the dedicated channels and the high-momentum jets that can burn the coarsest and the most heterogeneous fuels completely. The burners are also being designed for the oxy-fuel combustion, the technology that uses the oxygen instead of the air and produces a CO2-rich flue gas that can be captured, and for the electrified heating concepts that reduce the fossil fuel dependence.

The digitalisation brings the burner into the plant’s data network. The burner is instrumented with the sensors that measure the fuel flows, the air flows, the pressures and the temperatures, and the advanced control systems adjust the burner settings automatically against the process model. The flame monitoring with the digital cameras and the image analysis provides the continuous, quantitative assessment of the flame shape and the position, and the digital twin of the kiln and the burner predicts the effect of the settings before they are applied.

The fundamentals of the burner will remain: the flame aerodynamics, the primary air, the momentum, the nozzle and the tuning. The burner technical session provides the training that keeps the operator and the engineer at the leading edge, so that the plant extracts the full value of the burner technology, whether the burner is a simple gas lance or the most advanced multi-channel, multi-fuel, digitally controlled machine.

Frequently Asked Questions

What is the function of the kiln burner?

The burner delivers the fuel and the combustion air to the kiln and creates the flame that provides the heat for the clinkering. The flame’s shape, length, temperature and position set the clinker quality, the fuel consumption, the refractory life and the emissions.

What is primary air and why is it important?

The primary air is the air delivered through the burner with the fuel. It transports the fuel, provides a portion of the combustion oxygen and, most importantly, creates the aerodynamic momentum that shapes the flame. Modern burners operate with the primary air in the range of 6 to 12 percent of the combustion air.

What is flame momentum?

Flame momentum is the product of the mass flow and the velocity of the burner jet. It determines the entrainment of the secondary air, the mixing and the flame shape: a high momentum gives a short, intense flame and a low momentum gives a long, diffuse flame.

What is the burner nozzle and its annuli?

The nozzle is the terminal section of the burner that discharges the fuel and the air into the kiln, built as a set of concentric annuli that deliver the fuel, the swirl air and the auxiliary air streams. The geometry and the adjustment of the annuli set the aerodynamics and the flame shape.

How is the flame tuned?

The flame is tuned by the systematic adjustment of the primary air rate and velocity, the swirl, the burner position and the individual fuel and air streams. The operator observes the flame, reads the process data, makes single adjustments and repeats until the flame and the process are at the target.

What safety systems protect the burner?

The burner is protected by the flame detection, the double-block-and-bleed fuel shut-off valves, the inert gas purging, the pressure and temperature interlocks, and the enforced start-up sequence. Any dangerous condition trips the fuel and the burner to a safe state.

Summary

Welcome to the burner technical session, and welcome to the complete picture of the kiln burner. This session has covered the burner from the fundamentals to the practice: the role of the burner in shaping the thermal environment of the kiln, the three combustion air streams and the dominance of the secondary air, the aerodynamics of the flame and the recirculation zone that stabilises it, the three roles of the primary air and its economic significance, the concept of the flame momentum as the central principle of the modern burner, the nozzle and its annuli and the multi-channel design, the swirl and its control of the flame shape, the flame length and the burning zone, the fuels from the coal to the alternative fuels and their injection, the combustion and the flame temperature, the practical art of the flame tuning, the observation of the flame, the relationship of the primary air to the specific heat consumption, the alternative fuels and their challenges, the NOx control, the safety systems and the start-up and shut-down sequences, the maintenance of the burner and the troubleshooting of the flame. The session has presented the burner as the most influential single instrument in the pyroprocess, and it has given the operator, the engineer and the maintenance specialist the concepts, the parameters and the discipline to use it. The burner technical session is a training journey, and the journey is complete: the participant now understands the aerodynamics, the momentum, the nozzle, the tuning and the safety, and is equipped to apply them on the kiln, to produce the best flame, the best clinker and the safest operation.

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.