Combustion Engineering for Cement Kilns: Guide

Combustion is the energy engine of the cement plant: the kiln and the calciner burn fuels at the rate of hundreds of tonnes per day, and the combustion process — its stoichiometry, its air supply, its flame, its heat release and its emissions — determines the clinker quality, the fuel consumption, the refractory life and the environmental performance of the entire line. The combustion engineer of the cement plant works with the chemistry of the air and the fuel, the physics of the flame and the gas flow, and the instruments that measure the oxygen, the carbon monoxide, the temperature and the nitrogen oxides. This article is the complete engineering treatment of combustion for the cement process: the stoichiometry and the combustion calculations, the air supply and the excess air, the flame and the burners, the heat release and the temperature, the nitrogen oxide chemistry, the false air and its investigation, the gas analysis and the measurements, the alternative fuels, and the control of the combustion for the stable and the efficient operation.

1. The Role of Combustion in the Cement Process

The cement process is fundamentally a thermal process: the raw meal is dried, calcined and sintered at temperatures up to 1,450 degrees Celsius, and the heat comes from the combustion of the fuels in the kiln burner and the calciner. The clinker is formed in the kiln’s burning zone at the temperature where the alite forms and the liquid phase develops, and the combustion must deliver that heat at the right place, in the right flame shape, with the right temperature and the right atmosphere. The calciner completes the decarbonation of the meal at 850 to 900 degrees Celsius, and its combustion is a separate, high-intensity process with its own air and fuel supply. The combustion is the plant’s largest energy and cost center: the fuel is 30 to 40 percent of the production cost, and the combustion efficiency is the difference between the competitive and the marginal plant.

The combustion engineering of the cement plant is therefore the management of the chemical energy: the fuel is selected and blended, the air is supplied and distributed, the flame is shaped and positioned, the heat is released and transferred, and the products — the CO2, the water, the nitrogen and the pollutants — are measured and controlled. The engineering tools are the combustion calculations, the gas analysis, the temperature and the pressure measurement, the flame observation and the process control, and the engineering decisions are the fuel selection, the air setting, the burner adjustment and the emission management. The sections below treat each element in turn, from the chemistry of the combustion to the practice of the operating plant.

2. The Stoichiometry of Combustion

The stoichiometry of combustion is the quantitative relationship between the fuel, the oxygen and the products. The combustion of a hydrocarbon fuel with the air produces the carbon dioxide and the water, and the chemical equation for the complete combustion of a simple fuel, methane, is CH4 + 2 O2 producing CO2 + 2 H2O. The stoichiometric oxygen requirement is the oxygen needed for the complete combustion of the fuel, and the stoichiometric air requirement is that oxygen in the air, noting that the air is approximately 21 percent oxygen and 79 percent nitrogen by volume. For the practical fuels of the cement plant — the coal, the petcoke, the fuel oil and the alternative fuels — the stoichiometric air is calculated from the fuel’s ultimate analysis: the carbon, the hydrogen, the sulfur, the oxygen, the nitrogen and the moisture content.

The calculation proceeds through the combustion equations for each element. The carbon burns to the carbon dioxide, the hydrogen burns to the water, the sulfur burns to the sulfur dioxide, and the fuel’s own oxygen reduces the external oxygen requirement. The stoichiometric air per kilogram of the fuel is the standard combustion engineering calculation, and for the typical coal it is approximately 7 to 8 kilograms of air per kilogram of fuel, while for the natural gas it is approximately 15 to 17 kilograms per kilogram. The products of the complete combustion are the CO2, the water vapor, the nitrogen from the air and the fuel, and the SO2 from the sulfur. The combustion calculation also handles the incomplete combustion products — the carbon monoxide, the hydrogen and the soot — and the excess air, which appears in the products as the unreacted oxygen. The mastery of the stoichiometric calculation is the foundation of the combustion engineering, because every air setting, every gas analysis and every efficiency figure builds on it.

3. The Air Supply and the Excess Air

The air supply to the combustion is always greater than the stoichiometric requirement, and the difference is the excess air. The excess air serves three purposes: it ensures the complete combustion of the fuel, it provides the oxygen for the completion of the reactions in the turbulent flame, and it carries the heat and the flue gas through the system. The excess air is expressed as a percentage of the stoichiometric air or as the oxygen content of the flue gas, and the two are related: for the typical fuels, the flue gas oxygen of 1 percent corresponds to approximately 5 percent excess air, and the flue gas oxygen of 3 percent corresponds to approximately 15 to 16 percent excess air.

The management of the excess air is the central combustion control. Too little air leaves the combustion incomplete: the carbon monoxide rises, the fuel burns inefficiently, the reducing atmosphere attacks the refractory and the volatile sulfur cycle is fed, and the unburned fuel is wasted. Too much air dilutes the flue gas and the flame: the flame temperature falls, the heat recovery is lost to the excess nitrogen heating, the fan power rises and the NOx can increase with the higher oxygen availability. The optimum excess air is the minimum that gives the complete combustion with the stable flame, and for the kiln and the calciner of the cement plant the typical oxygen settings are 1.5 to 3 percent at the kiln inlet and 2 to 4 percent at the calciner exit, depending on the fuel and the system. The optimum is found by the systematic testing: the oxygen is stepped down, the CO and the unburned indicators are watched, and the setting is established at the point where the CO just begins to rise.

Excess Air (%)	Typical Flue Gas O2 (%)	Effect on Combustion	Effect on Efficiency
0	0	Stoichiometric, incomplete in practice	Theoretical maximum, unachievable
5	~1.0	Near-complete, sensitive to mixing	Very high, tight control required
10	~2.0	Complete with margin	High, the typical optimum band
15	~3.0	Complete, stable	Moderate, excess nitrogen heated
25	~4.5	Complete, dilute flame	Lower, flame temperature reduced
50	~7	Complete, cold and dilute	Poor, large stack loss

4. The Combustion Calculations in Practice

The practical combustion calculations of the plant are the toolset that converts the fuel and the gas measurements into the operating quantities: the required air volume, the exhaust gas flow and the amount of the false air. The required air volume is calculated from the fuel rate, the fuel’s stoichiometric air requirement and the excess air setting, and the result is the fan and the duct sizing basis and the operating setpoint. The exhaust gas flow is calculated from the fuel rate, the combustion products and the process gas — the CO2 from the raw meal calcination, the water from the meal and the kiln feed — and the result is the basis of the fan capacity, the heat balance and the emission calculations.

The practical calculations are made in the standard combustion table or the spreadsheet: the fuel analysis, the combustion equations, the air and the gas volumes and the flue gas composition are organized in the rows and the columns, and the plant’s engineers use the table for the daily settings and the investigations. The fuel analysis is the input: the proximate and the ultimate analysis of the fuel, with the moisture, the ash, the volatiles, the fixed carbon, the calorific value and the elemental composition. The calculations are the verification tools as well: the measured flue gas composition is compared with the calculated composition, and the differences indicate the false air, the incomplete combustion or the measurement problems. The combustion calculation is also the basis of the thermal efficiency: the heat of the fuel is accounted against the heat of the products, the losses and the useful heat, and the efficiency of the combustion is quantified.

5. The False Air and Its Investigation

The false air is the air that enters the system through the leaks rather than through the controlled inlets, and its investigation is one of the standard exercises of the combustion engineer. The false air dilutes the flue gas, reduces the oxygen concentration that the instruments see, increases the gas volume and the fan power, cools the gas and disturbs the heat balance, and its presence is a silent and continuous loss. The false air enters through the kiln seals, the tower and the duct flanges, the inspection doors, the air slides, the expansion joints and the damaged casings, and its quantity is often 5 to 15 percent of the total gas flow in the older and the poorly maintained plants.

The investigation of the false air is a systematic survey. The oxygen and the temperature are measured at the successive points along the gas path — the kiln inlet, the tower stages, the duct sections, the fans — and the rise of the oxygen and the fall of the temperature between the points indicate the leaks. The oxygen balance, computed from the measured oxygen and the theoretical combustion, quantifies the leak between the two measuring points. The positive methods — the smoke testing, the tracer gas — localize the individual leaks, and the infrared imaging finds the cold spots of the leaking flanges and the damaged insulation. The investigation’s output is the leak register with the quantities and the locations, and the repair program — the seal replacement, the flange gasketing, the casing repair — is the corrective action. The false air investigation is repeated at the intervals and after every major repair, and the plant that masters its false air recovers its fan power, its temperatures and its efficiency.

6. The Flame and the Burners

The flame of the kiln burner is the most important single element of the kiln’s thermal process: its shape, its position, its length, its luminosity and its momentum set the heat transfer, the coating, the clinker formation and the refractory life. The kiln flame is a turbulent, diffusion flame in which the fuel and the primary air are injected through the burner and the combustion is completed with the secondary air drawn into the kiln from the cooler. The flame shape is set by the burner design and the settings: the primary air momentum, the swirl, the fuel injection velocity and the burner position. The long, lazy flame heats a long zone gently; the short, intense flame concentrates the heat at the burning zone; and the optimum flame is the one that delivers the heat at the clinker formation zone with the well-formed coating and the stable operation.

The burner pipe alignment and the flame adjustment are the operating controls of the flame. The burner is centered and aligned in the kiln axis so that the flame does not lick the material or the walls: the flame that licks the material creates the local reducing conditions and the sulfate volatilization, and the flame that licks the wall destroys the coating and the refractory. The flame adjustment tools are the primary air flow, the axial and the swirl air distribution, the fuel rate and the burner position, and the observations — the flame shape through the sight glass, the burning zone temperature, the kiln shell temperature profile, the clinker quality — verify the adjustment. The flame and the burner engineering is treated in detail in its own article in this library, and the combustion engineer must master both the flame physics and the burner mechanics.

7. The Heat Release, the Flame Temperature and the Heat Transfer

The heat release of the combustion is the rate at which the fuel’s chemical energy is converted to the thermal energy, and the flame temperature is the temperature of the combustion products at the completion of the reaction. The theoretical flame temperature is calculated from the fuel’s calorific value, the air-to-fuel ratio and the heat capacities of the products, and for the coal and the petcoke flames with the preheated secondary air the theoretical temperature reaches 1,900 to 2,100 degrees Celsius. The actual flame temperature is lower, because of the heat losses to the surroundings, the incomplete mixing and the radiation, and the actual gas temperature at the burning zone is 1,700 to 2,000 degrees Celsius, enough to hold the clinker bed at 1,450 degrees.

The heat transfer from the flame to the material is the kiln’s thermal mechanism: the radiation from the flame and the kiln surfaces, the convection from the gas and the conduction in the material. The radiation dominates at the burning zone temperatures, and its rate depends on the fourth power of the temperature and the emissivity of the flame. The luminous flame — with the soot particles and the ash radiating — transfers the heat more effectively than the non-luminous flame, and the flame management is therefore the management of the heat transfer. The heat is delivered to the material in the stages: the preheating, the calcination and the sintering, and the temperature profile along the kiln is the result of the combustion and the material reactions. The combustion engineer’s thermal tools — the fuel rate, the air setting, the flame shape and the secondary air temperature — are the controls of the heat release and the temperature profile.

8. The Combustion and the Clinker Quality

The combustion conditions set the clinker quality through the temperature, the atmosphere and the residence time. The burning zone temperature determines the extent of the alite formation and the free lime: the well-burned clinker has the free lime below 1.5 percent and the full alite development, while the underburning leaves the free lime high and the clinker weak. The atmosphere determines the chemistry: the reducing conditions in the burning zone cause the decomposition of the sulfates and the volatilization of the sulfur and the alkalis, disturbing the volatile cycle and the clinker composition, while the oxidizing conditions preserve the sulfate and the alkali in the clinker. The residence time at the temperature determines the completion of the reactions, and the kiln speed and the material load set the residence time.

The combustion control is therefore the clinker quality control. The burning zone temperature is held in its window by the fuel rate and the flame adjustment, and the free lime is the confirming measurement. The reducing conditions are watched through the CO and the O2 at the kiln inlet, and the operation is held oxidizing by the excess air and the flame management. The fuel’s ash and its composition enter the clinker: the coal ash contributes the silica, the alumina and the iron, and the fuel selection is coordinated with the raw mix design. The combustion engineer and the quality engineer work from the same data — the temperatures, the gas analysis and the clinker chemistry — and the clinker quality is the daily report card of the combustion.

9. The Nitrogen Oxide Chemistry

The nitrogen oxides are the combustion emissions of the greatest regulatory and engineering importance in the cement plant. The NOx forms by three mechanisms: the thermal NOx, formed at the high flame temperatures by the combination of the nitrogen and the oxygen in the air; the fuel NOx, formed from the nitrogen in the fuel; and the prompt NOx, formed by the radical reactions in the flame front. In the cement kiln, the thermal NOx dominates because of the high flame temperatures, and the NOx concentration at the kiln exit is typically 400 to 1,200 mg per cubic meter, depending on the fuel, the temperature and the system.

The NOx chemistry drives the abatement strategy. The primary measures reduce the formation at the source: the lower flame temperature, the reduced excess air at the flame, the staged combustion and the flame shaping. The secondary measures remove the formed NOx: the selective non-catalytic reduction (SNCR) with the ammonia or the urea injection at the tower’s temperature window of 850 to 1,050 degrees Celsius, and the selective catalytic reduction (SCR) with the catalyst. The cement kiln’s NOx control is a system: the process optimization holds the NOx at its natural minimum, the SNCR trims the peaks and the modern plants meet the strict limits with the combination. The NOx measurement at the stack and the tower, the oxygen and the temperature correlation, and the injection control are the combustion engineer’s daily NOx tools, and the understanding of the formation chemistry is the basis of the cost-effective abatement.

10. The Gas Analysis and the Measurement Systems

The gas analysis is the combustion engineer’s primary measurement: the oxygen, the carbon monoxide, the carbon dioxide, the nitrogen oxides and the sulfur dioxide are measured at the strategic points of the system, and the readings drive the combustion control and the emission reporting. The measuring points are the kiln inlet (the oxygen and the CO, the window of the reducing conditions), the tower exit and the calciner (the oxygen and the temperature), and the stack (the full emission set). The measurement technologies are the extractive analyzers with the heated sampling and the conditioning, and the in-situ analyzers that measure in the duct. The sampling system is the engineering challenge: the hot, dusty gas requires the probe, the filter, the heating and the conditioning, and the plugging and the dilution of the sample are the chronic maintenance problems.

The quality of the gas analysis is the quality of the combustion control. The analyzers are calibrated against the standard gases at the scheduled intervals, and the calibration records are the audit trail. The sampling probes are cleaned and the filters are replaced at the scheduled intervals, and the response time of the sample system is verified. The data is validated against the mass balance: the measured oxygen and the CO2 are checked against the combustion calculations, and the inconsistent readings are investigated before they mislead the control. The modern plants run the redundant and the cross-checked analyzers, and the combustion engineer’s skill includes the diagnosis of the analyzer problems as much as the interpretation of the readings.

11. The Fuel Management and the Fuel Selection

The fuel is the combustion engineer’s raw material, and its management covers the selection, the blending, the handling and the analysis. The cement fuels are the coal, the petcoke, the fuel oil, the natural gas and the growing share of the alternative fuels from the waste streams. The selection criteria are the calorific value, the volatile content, the ash content and composition, the sulfur content, the moisture, the grindability and the price, and the selection is the optimization of the cost against the process impact. The petcoke, with its high calorific value and its low price but its high sulfur and its low volatiles, requires the special combustion design; the alternative fuels bring the chlorine, the sulfur, the phosphorus and the volatile metals that challenge the process and the emissions.

The fuel management in operation is the blending and the feeding: the fuels are blended to the target composition, the feed rate is controlled to the kiln demand, and the analysis of the delivered fuel verifies the composition. The fuel changes are the combustion events: the switch from the coal to the petcoke or to a new alternative fuel changes the stoichiometric air, the flame, the heat release and the emissions, and the change is planned and executed with the combustion calculations and the process monitoring. The fuel analysis — the proximate and the ultimate analysis at the delivery and the frequency — is the combustion engineer’s quality control of the fuel, and the fuel register with the composition and the performance of each source is the knowledge base of the selection.

12. The Alternative Fuels and the Combustion

The alternative fuels are the modern direction of the cement combustion: the waste-derived fuels — the refuse-derived fuel, the sewage sludge, the tyres, the solvents and the plastics — replace the fossil fuels at substitution rates from a few percent to nearly 100 percent. The combustion engineering of the alternative fuels addresses the fuel preparation, the feeding, the combustion and the emissions. The alternative fuels vary widely in the calorific value, the moisture, the particle size and the chemical composition, and their preparation — the sorting, the shredding, the drying and the homogenization — is the first combustion control. The feeding of the alternative fuels to the calciner and the kiln is engineered for the reliable, the dust-free and the complete combustion, with the fuel-specific feed systems.

The combustion of the alternative fuels affects the flame, the temperature and the emissions. The low-calorific, high-moisture fuels cool the flame and require the additional process heat, the high-volatile fuels change the flame character, and the chlorine and the alkali of the fuels feed the volatile cycles and the build-ups. The emissions are the challenge: the chlorine forms the hydrogen chloride and the dioxins, the sulfur forms the SO2, the metals — the mercury, the thallium, the lead — concentrate and must be managed, and the combustion control — the temperature, the residence time, the oxygen and the quench — is the emission control. The modern alternative fuel combustion is engineered with the process modeling, the emission monitoring and the careful control, and the combustion engineer of the modern plant is as much an environmental engineer as a thermal engineer.

13. The Combustion Control and the Automation

The combustion control of the modern plant is automated: the distributed control system regulates the fuel rate, the air flow, the kiln speed and the temperatures against the measured process variables, and the combustion engineer supervises the control loops and handles the disturbances. The primary loops are the fuel feed against the burning zone temperature or the kiln drive, the air flow against the oxygen at the kiln inlet, the kiln speed against the free lime and the material load, and the calciner fuel and the air against the calcination temperature. The control is supported by the advanced systems: the model-predictive controllers, the soft sensors that estimate the free lime, and the optimization that finds the operating point of the minimum fuel and the maximum production.

The automation does not remove the combustion engineer: it raises the level of the work. The engineer validates the instruments, sets the control limits and the envelopes, analyzes the disturbances and the upsets, investigates the process changes and the fuel changes, and reviews the performance against the targets. The shift-to-shift handover covers the combustion state, the fuel situation, the instrument health and the pending issues, and the control system’s logs and the trends are the combustion record. The combustion control is the combination of the automation and the expertise, and the plants with the best combustion have both.

14. The Combustion and the Emissions Control

The combustion is the source of the plant’s gaseous emissions, and the emissions control is the combustion engineer’s environmental responsibility. The regulated pollutants are the particulate matter, the nitrogen oxides, the sulfur dioxide, the carbon monoxide, the volatile organic compounds, the dioxins and furans, the metals and the acid gases, and the combustion conditions — the temperature, the oxygen, the residence time and the fuel composition — determine their formation. The control strategy is the hierarchy of the process measures, the abatement equipment and the monitoring. The process measures reduce the formation: the combustion optimization for the NOx and the CO, the fuel selection and the blending for the sulfur and the chlorine, and the temperature and the residence time control for the dioxins. The abatement equipment removes the formed pollutants: the bag filters and the electrostatic precipitators for the dust, the SNCR for the NOx, the dry or the wet scrubbers for the SO2 and the acid gases, and the activated carbon for the mercury and the dioxins.

The monitoring completes the system: the continuous emission monitoring at the stack reports the pollutants to the authority and to the control room, the periodic manual measurements verify the monitors, and the emission records are the plant’s compliance evidence. The combustion engineer’s role is the integration: the emissions are minimized within the process constraints, the abatement equipment is operated and maintained at its design, and the monitoring is validated. The modern plant runs the emissions as an operating variable, and the combustion engineering of the emissions is the engineering of the plant’s environmental license.

15. The Combustion Troubleshooting

The combustion troubleshooting is the systematic diagnosis of the combustion problems: the incomplete combustion, the unstable flame, the flame shape problems, the high fuel consumption, the high emissions and the process upsets. The incomplete combustion shows as the high CO, the smoke, the reducing atmosphere and the wasted fuel, and its causes are the low air, the poor mixing, the large fuel particles, the burner problems and the flame impingement. The unstable flame shows as the flame fluctuations, the blow-off and the pulsation, and its causes are the fuel quality changes, the air disturbances, the burner wear and the draft problems. The high fuel consumption shows against the heat balance, and its causes are the process inefficiencies, the false air, the heat losses and the low thermal efficiency.

The troubleshooting method is the standard engineering investigation: the symptoms are defined, the data — the fuel, the air, the gas analysis, the temperatures, the pressures and the clinker quality — is collected, the hypotheses are ranked and tested, and the corrections are verified. The combustion engineer’s tools for the diagnosis are the gas analyzers, the temperature measurements, the flame observation and the heat and the mass balances, and the findings are recorded in the operating notes and the investigation reports. The combustion troubleshooting is the daily work of the process engineer, and the mastery of the diagnosis is the mastery of the combustion.

16. The Heat and the Mass Balance of the Combustion

The heat and the mass balance of the kiln system is the accounting of the energy and the material, and it is the combustion engineer’s most powerful audit tool. The heat balance accounts the heat input — the fuel, the sensible heat of the feed and the air, the exothermic reactions — against the heat output — the clinker, the exhaust gas, the kiln shell losses, the cooler losses and the radiation — and the closure of the balance verifies the measurements and the losses. The mass balance accounts the material — the feed, the fuel, the air, the product and the dust — and the closure verifies the flows and the losses. The balances are computed from the combustion calculations, the fuel analysis, the gas analysis, the temperatures and the flows, and the results are the plant’s thermal efficiency and the loss breakdown.

The balance-based analysis finds the improvement opportunities: the exhaust gas loss, the largest loss, is minimized by the low excess air and the heat recovery; the shell loss is minimized by the refractory and the insulation; and the cooler loss is minimized by the good air distribution and the heat recovery. The balance is computed at the commissioning, at the performance tests and at the intervals, and the comparison with the baseline tracks the plant’s efficiency. The modern plant runs the continuous energy monitoring, and the heat and the mass balance is the framework of the data. The combustion engineer’s balance is the truth about the plant’s energy, and its accuracy depends on the quality of the measurements that the engineer maintains.

17. The Combustion Training and the Knowledge

The combustion engineering is a discipline of the calculation and the experience, and the plant’s competence is built through the training and the knowledge management. The training program covers the combustion chemistry and the stoichiometry, the calculations, the flame and the burner physics, the gas analysis and the instrumentation, the control and the troubleshooting, and the emissions. The training combines the classroom theory with the plant practice: the engineers calculate the combustion of the plant’s fuels, analyze the plant’s gas data, observe the plant’s flames and investigate the plant’s problems. The standard courses, such as the technical development programs of the industry’s major producers, structure the curriculum of the combustion for the process engineers, and the plant’s own training adapts the standard to its fuels and its equipment.

The knowledge management captures the plant’s combustion experience: the fuel register, the burner settings and the adjustment history, the gas analysis records, the troubleshooting reports, the heat balance results and the performance reviews. The knowledge is documented and accessible, the new engineers are trained and mentored, and the lessons of the investigations are applied. The combustion engineering of the plant is therefore the combination of the universal science and the plant’s particular practice, and the plants that train their people and keep their knowledge maintain their combustion at the standard of the best.

18. The Future of the Combustion in the Cement Industry

The future of the cement combustion is shaped by the decarbonization: the cement industry targets the net-zero CO2, and the combustion is the largest source of the emissions. The directions are the higher alternative fuel substitution, the hydrogen and the bio-based fuels, the electrification of the process, and the carbon capture from the flue gas. The alternative fuel combustion will grow toward the 100 percent substitution with the engineered fuel preparation and the combustion control. The hydrogen combustion, with its different flame and its NOx challenges, is under the development and the demonstration. The electrification — the electric calcination, the plasma and the electric kiln heating — changes the energy source but keeps the chemical process. And the carbon capture — the oxyfuel combustion, the post-combustion capture and the calcium looping — is the end-of-pipe solution that the industry is developing at the commercial scale.

The combustion engineer of the future works in the new energy landscape: the fuel flexibility, the CO2 measurement and the capture, the process electrification and the digital optimization. The fundamentals of the combustion engineering — the stoichiometry, the air, the flame and the heat — remain the foundation, and the new technologies are built on them. The combustion engineering of the cement industry is therefore a field of the continuity and the change, and the engineers who master the fundamentals are the ones who lead the transition.

Frequently Asked Questions

What is the optimum excess air for the kiln combustion?

The optimum excess air is the minimum that gives the complete combustion with the stable flame: typically 1.5 to 3 percent oxygen at the kiln inlet and 2 to 4 percent at the calciner exit, depending on the fuel and the system. The exact setting is found by the systematic testing of the oxygen against the CO.

How is the false air measured?

The false air is measured by the oxygen survey along the gas path: the oxygen is measured at the successive points, and the rise between the points, converted through the oxygen balance, quantifies the leak. The smoke testing and the tracer gas localize the individual leaks.

Why does the flame shape matter for the clinker quality?

Because the flame shape sets the heat transfer and the burning zone temperature: the long flame heats a long zone, the short flame concentrates the heat, and the misaligned flame licks the material or the wall, creating the reducing conditions, the coating damage and the refractory loss. The flame is aligned and shaped to deliver the heat at the clinker formation zone.

How does the excess air affect the NOx?

The excess air provides the oxygen for the thermal NOx formation at the high flame temperatures, so the higher oxygen at the flame generally increases the NOx. The NOx abatement therefore combines the reduced excess air and the flame temperature control with the SNCR injection.

Why is the CO measured at the kiln inlet?

Because the CO at the kiln inlet is the direct indicator of the incomplete combustion and the reducing conditions, which feed the volatile sulfur cycle, damage the refractory and waste the fuel. The CO is the watchdog of the combustion atmosphere.

Summary

The combustion engineering of the cement plant is the engineering of the chemical energy that drives the clinker formation: the stoichiometry and the combustion calculations set the air and the gas quantities, the excess air management balances the completeness and the efficiency, the false air investigation recovers the wasted fan power and heat, and the flame and the burners deliver the heat at the clinker formation zone. The combustion sets the clinker quality through the temperature and the atmosphere, drives the NOx and the other emissions, and is measured and controlled through the gas analysis, the automation and the heat and the mass balances. The fuel management and the alternative fuels bring the cost and the environmental opportunities, and the troubleshooting and the training build the plant’s competence. The fundamentals of the combustion — the chemistry, the air, the flame and the heat — are the foundation of the cement process, and the engineers who master them run their kilns at the efficiency, the quality and the compliance that the modern industry demands.

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.