Kiln Fuel & Firing Systems: Complete Guide

The firing system of a cement plant is the assembly of equipment that converts fuel energy into the thermal energy required to produce clinker, and it is one of the most consequential systems in the entire plant. It includes the fuel storage and handling, the fuel preparation, the fuel feeding and transport, the burners of the kiln and the calciner, the primary air and combustion air systems, and the instrumentation and safety systems that supervise the whole process. Fuel represents between 25% and 40% of the production cost of cement, and the efficiency with which the firing system converts fuel into heat, and the price at which the fuel is purchased, are two of the largest levers on the plant’s profitability. This article provides a complete technical overview of fuel and firing systems in the cement industry, written for process and mechanical engineers, production managers, fuel procurement specialists and graduate engineers. It covers the fuels themselves, their properties and their selection; the preparation of solid fuels; the design and operation of kiln and calciner burners; the control of combustion; the impact of alternative fuels; and the safety and environmental framework. The objective is to give the reader a working understanding of how a modern firing system is designed, operated, optimized and maintained, so that the technical decisions in their own plant are made on a sound basis.

The Role of Fuel in the Cement Process

Clinker production is an energy-intensive process because the chemical reactions of clinkering are strongly endothermic. The decomposition of calcium carbonate into calcium oxide and carbon dioxide requires approximately 1.79 MJ per kg of CaCO3, and the reactions that form the clinker minerals require additional energy. In a modern dry-process plant, the total thermal energy demand is 3000–3600 kJ per kg of clinker, of which roughly 55–65% is consumed in the calciner and the preheater and 35–45% in the kiln itself. The energy is supplied by the combustion of fuel in two locations: the kiln burner at the discharge end of the rotary kiln, which fires the burning zone and provides the peak temperature for the clinkering reactions, and the calciner burner in the preheater tower, which provides the energy for the calcination of the raw meal. In a precalciner kiln system, the calciner consumes 55–65% of the total fuel and the kiln the remainder, a split that is chosen to keep the kiln thermal load manageable and to stabilize the kiln operation.

The fuel also influences the process through the combustion products. The flue gas volume, which is largely determined by the fuel composition and the excess air, sets the gas velocities in the preheater cyclones and hence their separation efficiency and their pressure drop. The sulfur content of the fuel circulates in the kiln system as SO2, sulfates and alkali sulfates, and it participates in the build-ups and blockages that afflict many plants. The ash of the fuel, which for coal and petcoke is 5–15% of the fuel mass, is absorbed into the clinker, where it modifies the chemistry: a high-ash coal changes the silica and alumina content of the clinker and must be compensated in the raw mix design. The moisture of the fuel adds water vapor to the flue gas, which increases the gas volume and the heat loss in the exit gas. For all of these reasons, the selection and the management of the fuel are intimately connected to the chemistry, the thermal performance and the operability of the entire kiln system, and no plant can be optimized without a complete understanding of its fuel.

Finally, the fuel is the main driver of the plant’s emissions of carbon dioxide. Approximately 60% of the CO2 emitted by a cement plant comes from the calcination of limestone, and approximately 40% from the combustion of fuel. The decarbonization of the industry therefore has two levers: the reduction of the clinker factor of cement, and the replacement of fossil fuels by fuels with a lower carbon footprint, including biomass and waste-derived fuels. The firing system is the technology that makes the second lever possible, and its design must accommodate an increasing variety of fuels with very different properties. This article therefore treats the firing system as a flexible, multi-fuel installation that must be engineered for the fuels of today and the fuels of tomorrow.

Fuel Types Used in the Cement Industry

The cement industry is unusual in the breadth of fuels it can consume. The principal fossil fuels are coal, petroleum coke, fuel oil and natural gas, and the alternative fuels include solid recovered fuel (SRF), refuse-derived fuel (RDF), plastic waste, waste tyres, biomass of many kinds, sewage sludge, waste solvents, used oils and, increasingly, hydrogen-rich industrial gases. The selection of the fuel mix is an economic decision made against the constraints of availability, price, handling, emissions and process compatibility, and it differs from region to region according to the local energy markets and the waste management policies.

Coal has historically been the dominant fuel because of its availability and its favorable combustion characteristics. The coals used in cement plants are generally bituminous and sub-bituminous coals with a net calorific value of 22–30 MJ/kg, an ash content of 5–20%, a volatile content of 20–40% and a sulfur content of 0.5–3%. The high volatile content gives a stable flame that ignites readily, and the ash is absorbed into the clinker. Petroleum coke, or petcoke, is a by-product of the oil refining industry with a net calorific value of 30–34 MJ/kg, a low volatile content of 8–14% and a sulfur content of 3–7%. Petcoke is often cheaper than coal on an energy basis, but it burns with difficulty, requiring fine grinding and a burner flame designed for slow-burning particles, and its high sulfur content increases the SO2 emissions and the sulfur circulation in the kiln.

Natural gas and fuel oil are used where they are available and economic. Natural gas has a net calorific value of approximately 34–36 MJ/Nm3, produces no ash and no sulfur emissions, and is the cleanest of the fossil fuels, but its price is often too high for large-scale use except in regions with abundant supply. Fuel oil, with a net calorific value of 38–42 MJ/kg, was the dominant fuel in the 1970s and retains a role as a start-up fuel and as a support fuel for alternative fuel firing. The alternative fuels, which are treated in detail later in this article, now provide 30–90% of the thermal energy in many European plants, and their role is growing worldwide as the industry seeks both to reduce its fuel costs and to contribute to the circular economy by co-processing waste.

Fuel Properties and Their Significance

The properties of a fuel define its behavior in storage, handling, preparation, combustion and emissions, and they must be measured regularly and accurately. The most important properties are the calorific value, the proximate analysis (moisture, volatile matter, fixed carbon and ash), the ultimate analysis (carbon, hydrogen, nitrogen, sulfur, oxygen), the ash composition, and the grindability. The net calorific value, which is the heat released per unit mass when the water in the combustion products remains vapor, is the basis of the fuel cost comparison and the plant’s specific heat consumption. The moisture content affects the calorific value, the mill performance, the flame and the gas volume; the volatile content affects the ignitability and the flame stability; the ash content and composition affect the clinker chemistry and the emissions; the sulfur content affects the SO2 emissions and the circulation phenomena; and the grindability, measured by the Hardgrove index for coal, sets the mill power and the achievable fineness.

For alternative fuels, additional properties become significant. The chlorine content is critical, because chlorine circulates in the kiln system as potassium chloride and sodium chloride, condenses in the preheater and can cause severe blockages in the ducts and the cyclones. Many plants therefore impose a chlorine limit of 0.05–0.1% on the mixed fuel input. The heavy metal content of the waste, especially mercury, thallium and cadmium, is regulated because these metals are volatile and concentrate in the dust. The physical form of the fuel, its particle size and its bulk density, determines the handling equipment and the injection system. The variability of the alternative fuels, both between batches and within batches, requires continuous monitoring and a blending strategy, because the burner and the process are sensitive to the fluctuations of the energy input.

Combustion calculations use the ultimate analysis to determine the stoichiometric air requirement and the flue gas volume. For a typical coal, the stoichiometric air requirement is approximately 8.5–9.5 kg per kg of fuel, and the flue gas volume is approximately 8–9 Nm3 per kg of fuel at the actual excess air. These figures are the basis for the sizing of the fans, the ducts and the dust collection system, and they are also the basis of the daily energy accounting of the plant, which compares the fuel consumption with the production and the clinker quality. A plant that does not know its fuel composition precisely cannot close its heat balance, and a plant that cannot close its heat balance cannot optimize its firing system. The analytical laboratory of the plant therefore has a direct role in the firing system, and the sampling and analysis frequency of the fuel should match its variability. The following table summarizes the typical properties of the main fuels used in cement plants, which are the reference data for the fuel selection and the process calculations:

Fuel	Net calorific value	Volatile matter	Ash	Sulfur	Typical use
Bituminous coal	22–30 MJ/kg	20–40%	5–20%	0.5–3%	Base fuel of most plants
Petroleum coke	30–34 MJ/kg	8–14%	0.2–1%	3–7%	Low-cost base fuel, hard to burn
Natural gas	34–36 MJ/Nm3	—	nil	nil	Clean fuel, start-up and co-firing
Fuel oil	38–42 MJ/kg	—	0.1–1%	0.5–4%	Start-up and support fuel
RDF / SRF	10–18 MJ/kg	high	10–25%	0.2–1%	Alternative fuel, variable quality
Plastic waste	25–35 MJ/kg	very high	1–5%	low	Alternative fuel, high chlorine risk
Biomass (wood, husks)	8–17 MJ/kg	high	1–5%	low	Alternative fuel, high moisture
Waste tyres	28–33 MJ/kg	medium	5–15%	1–2%	Alternative fuel, coarse particles

Fuel Selection and Procurement Strategy

The fuel mix of a plant is chosen by an optimization that weighs the delivered energy price of each candidate fuel, its process penalties, its handling costs, its emission costs and its supply risk. The delivered energy price is the price per tonne divided by the net calorific value, expressed in USD per GJ or per MWh, and it is the primary ranking criterion. The process penalties are the costs of the effects of the fuel on the process: the grinding energy for a hard fuel, the limestone compensation for a high-ash fuel, the SO2 abatement for a high-sulfur fuel, the refractory and build-up risk for a high-chlorine fuel, and the efficiency loss for a high-moisture fuel. The handling costs include the receiving, storage, reclaiming and feeding equipment, and the emission costs include the permits, the monitoring and the abatement. The supply risk includes the volatility of the market price, the reliability of the supplier and the logistics of the delivery.

The result of this optimization is usually a portfolio of fuels rather than a single fuel. A typical modern plant operates with a base fuel, normally coal or petcoke, and a variable fraction of alternative fuels, purchased on long-term contracts from waste management companies. The portfolio is reviewed regularly, because the relative prices of the fuels change with the energy markets and the waste markets. The procurement strategy must also include the logistics: the storage capacity for each fuel, the unloading rates, the buffer stocks against supply interruptions, and the quality control at receipt, because a plant that accepts a fuel of variable quality without testing pays for the variability in its process. Many plants operate a fuel laboratory at the receiving station, with rapid analysis of the calorific value, moisture, ash, sulfur and chlorine, so that each delivery is accepted or rejected on objective data.

The contractual specification of the fuel is a critical document. It defines the ranges of the calorific value, the moisture, the ash, the sulfur, the chlorine, the particle size and the contaminants, together with the sampling and analysis protocol and the acceptance criteria. The specification protects the plant from the worst batches and gives the operations team the basis for the blending plan. For alternative fuels, the specification also includes the legal classification of the waste, the documentation of its origin, and the limits on the heavy metals and the other pollutants. The experience of the industry is that the fuel procurement function is a technical function as much as a commercial function, and the best plants integrate the procurement, the laboratory, the process engineering and the operations into a single team that manages the fuel portfolio as an operational resource.

Storage and Handling of Solid Fuels

Solid fuels are stored in open stockpiles, covered storage or silos, depending on the fuel and the climate. Coal is commonly stored in open or covered stockpiles with a capacity of 1–4 weeks of consumption, reclaimed by stackers and reclaimers. The key risks of coal storage are spontaneous combustion and dust explosion: coal piles must be managed to avoid internal heating, which is monitored by temperature probes, and the coal dust must be suppressed by water sprays and contained by the housekeeping. Petcoke is stored similarly, with an additional care because of its very low volatile content, which makes it less prone to spontaneous heating but more prone to dust generation. The alternative fuels are stored according to their properties: shredded solids in covered halls or bunkers, liquids in bunded tanks, and biomass with particular attention to self-heating and dust.

The handling equipment includes belt conveyors, bucket elevators, screw conveyors and pneumatic conveying lines. The design of the conveying system must avoid dust emissions, accumulation points and ignition sources, and it must be equipped with the magnetic separators and metal detectors that protect the fuel preparation equipment from tramp metal, which is common in waste-derived fuels. The feed to the fuel mill is controlled by the mill feed system, which typically includes a bunker with a discharge gate, a weigh feeder and, for alternative fuels, a shredder or a pre-crusher that ensures the particle size is within the specification. The reliability of this chain is a production issue: a failure in the fuel handling is a failure of the entire kiln system, and the availability of the handling line is therefore managed with the same rigor as the availability of the kiln itself.

Safety in fuel handling is a legal and moral obligation. The fuel storage and handling areas are classified for explosion risk, and the electrical equipment is selected accordingly. The dust collection in the transfer points and the silos must be designed to prevent the accumulation of dust, and the silos are equipped with level instruments, flow aids and explosion relief. The fire protection of the fuel storage includes the detection and the suppression systems, and the procedures cover the response to a fire in a bunker or a pile, which is a serious incident because it can burn for weeks. The training of the operators in the hazards of the fuel handling is part of the plant’s safety management system, and the audits of the fuel handling are part of the plant’s loss prevention program.

Coal and Petcoke Grinding

The preparation of pulverized coal or petcoke for the burners is performed by a fuel grinding mill, either a vertical roller mill or a ball mill, which grinds, dries and classifies the fuel in one operation. The mill receives the raw fuel from the storage, dries it with hot gas drawn from the kiln system or a dedicated hot gas generator, grinds it in a grinding table or a ball charge, and classifies the ground material in a dynamic classifier so that only the material finer than the target is carried to the product. The target fineness for kiln firing is 1–3% residue on 90 micron for coal and 1–2% for petcoke, with a proportion of coarse particles on 212 micron below 0.5%, because the coarse particles are the ones that survive into the kiln and create combustion problems.

The grinding process consumes between 20 and 35 kWh per tonne of fuel, which is a significant cost, and the mill performance is affected by the moisture, the grindability and the fineness requirement. A coal with a Hardgrove index of 50 grinds much harder than one with an index of 70, and the mill capacity must be derated accordingly. The drying requires hot gas at 300–450°C at the mill inlet, and the mill exit temperature is typically 75–95°C, below the ignition temperature of the dust. The mill is operated under a slight negative pressure to prevent dust leakage, and it is protected against explosions by the inerting and the suppression systems. The mill’s safety is one of the most demanding areas of the plant, because pulverized coal dust is explosive, and the industry has developed strict standards for the design and operation of coal mills, including the emergency shut-off valves in the fuel lines to the kiln and the calciner.

The operation of the fuel mill is tied to the operation of the kiln system. The mill must deliver the fuel at the rate required by the process, with a stability that matches the combustion demand, and it must be able to follow the load changes of the kiln. The mill control system adjusts the feed rate, the hot gas flow and the classifier speed, and it includes the interlocks that stop the mill on the alarms of temperature, pressure and bearing vibration. The mill’s availability is a production factor: a coal mill failure forces the kiln to operate on the reserve fuel or to stop, and many plants therefore install the fuel grinding capacity with a margin, or two mills for one kiln, so that a single failure does not stop the production. The maintenance of the mill, including the grinding elements and the classifier, is planned to coincide with the kiln maintenance stops, and the wear parts are managed as critical spares.

Pulverized Fuel Transport and Injection

The ground fuel is transported from the mill to the burners by pneumatic conveying. The mill product is collected in a bag filter or a cyclone, from which it falls into a small buffer silo, and the feed to the conveying line is controlled by a loss-in-weight feeder or a volumetric feeder with a rotary valve. The transport air, supplied by a roots blower or a centrifugal fan, carries the fuel through the line at a velocity of 18–25 m/s, which keeps the particles in suspension and prevents the settling and the plugging of the line. The line pressure is monitored continuously, and a rising pressure indicates a developing blockage, which is cleared by reducing the feed and increasing the air. The line is provided with the shut-off valves at the burner and the emergency flap valves that isolate the kiln from the fuel system on a flame failure or a trip.

The fuel is injected into the kiln through the fuel channel of the burner, and into the calciner through the calciner burner lances or the fuel nozzles mounted in the calciner walls. The injection velocity and the distribution of the fuel at the injection point are critical: the fuel must be mixed with the combustion air within the flame envelope, and the injection momentum must be matched to the local conditions. In the kiln, the fuel channel velocity is typically 25–45 m/s, which combined with the primary air momentum gives the jet the momentum it needs to entrain the secondary air. In the calciner, the fuel is injected into a hot suspension of meal and gas at 850–900°C, where it ignites rapidly, and the injection must distribute the fuel evenly so that all the meal is heated uniformly and no local reducing zones form.

The measurement of the fuel flow is the basis of the process control. The loss-in-weight feeder measures the fuel mass flow directly, and the fuel energy input is calculated by multiplying the flow by the calorific value, which is updated from the laboratory analyses. The ratio of the fuel to the raw meal feed is the primary control of the burning conditions, and it is adjusted by the operators to maintain the kiln exit temperature and the quality parameters. The accuracy of the fuel metering is therefore a process issue, not just a commercial one, and the calibration of the feeders is a routine maintenance item. Modern plants also measure the solids concentration in the transport line with microwave or capacitance devices, which gives a faster response than the loss-in-weight signal and improves the stability of the fuel feed.

Kiln Burner Design and Operation

The kiln burner is the equipment that shapes the flame in the burning zone, and its design determines the thermal profile of the kiln. Modern kiln burners are multichannel burners, of which the Fives Pillard Rotaflam and the FLS IKN Jetflex are well-known examples, in which the primary air is divided into axial, swirl and radial channels that are adjusted independently. The fuel channel is at the center, surrounded by the air channels, and the burner tip is designed so that the fuel jet, the axial jets, the swirl jets and the radial jets interact to create a flame of the desired length, width and intensity. The primary air represents 8–15% of the stoichiometric air, and the balance, the hot secondary air from the cooler, is drawn into the flame by the momentum of the jets.

The operation of the kiln burner is the daily craft of the kiln operator. The operator adjusts the axial and the swirl air to control the flame length: more swirl gives a shorter, hotter flame, and more axial air gives a longer, cooler flame. The radial air stabilizes the ignition near the tip, and its setting affects the recirculation and the early mixing. The operator uses the burner to manage the burning zone temperature, the coating stability, the NOx, the clinker quality and the refractory temperature, and the adjustments are made against the indications of the kiln exit temperature, the NOx analyzer, the shell temperature scan and the laboratory results. The skill is to make small, deliberate changes and to observe their effects, because the burner is a powerful instrument and its misuse is quickly visible in the process.

The burner is also the primary control of the thermal NOx formation. A flame with a high peak temperature generates more NOx, and the operator can trade flame intensity against NOx by lengthening the flame, accepting a slightly higher heat consumption for a lower emission. The design of the burner includes the low-NOx features: the staged mixing of the fuel and the air, the recirculation zones that dilute the flame and the momentum distribution that spreads the heat release. In combination with the low-NOx calciner and, where required, the SNCR system, the burner allows the plant to meet the emission limits of 200–500 mg/Nm3 NOx that are now common. The burner is therefore not only a combustion device but also an emission control device, and its tuning is part of the plant’s environmental compliance program.

Calciner Firing Systems

The calciner is the reactor in the preheater tower in which 55–65% of the fuel is burned and in which the calcination of the raw meal is completed. The calciner receives the preheated meal from the top cyclones, the hot gas from the kiln, the tertiary air from the cooler, and the fuel, and it discharges into the bottom cyclones with a meal calcination degree of 85–95%. The combustion in the calciner takes place at 850–900°C, well below the temperature of the kiln flame, which is the key to the low NOx of the precalciner process: at these temperatures the thermal NOx formation is negligible. The calciner is a large reactor, with a gas residence time of 2–4 seconds, and its design must provide the mixing of the fuel, the tertiary air and the meal that ensures complete combustion and complete calcination.

The firing of the calciner is distributed across several injection points. The fuel is injected through lances mounted in the calciner walls or through a central lance from below, and the tertiary air is introduced through a duct that creates a strong swirl in the calciner body. The meal is fed from the preheater into the top of the calciner, and the mixing of the three streams is engineered so that the fuel burns completely before the gas leaves the calciner, because unburned fuel in the gas causes the CO emissions and the deposits in the ducts. The fuel for the calciner is often of lower quality than the kiln fuel, because the combustion conditions are more forgiving: the calciner can burn coarse alternative fuels, high-ash wastes and fuels that the kiln burner cannot handle. This is the reason that many plants fire the majority of their alternative fuels in the calciner.

The calciner firing control is part of the overall kiln control. The calciner fuel flow is the primary control of the meal calcination degree and hence of the kiln thermal load, and it is adjusted to hold the calciner exit temperature at 850–880°C and the kiln exit gas temperature at 1000–1100°C. The tertiary air flow is controlled by the damper in the tertiary air duct, which balances the air between the kiln and the calciner. The fuel split between the kiln and the calciner is a strategic operating variable: increasing the calciner share reduces the kiln thermal load and the kiln flame intensity, which lowers the NOx, while increasing the kiln share raises the burning zone temperature and the clinker quality for hard-burned clinkers. The optimum split is found by tuning, typically in the range of 55–65% to the calciner, and it is one of the most important settings in the whole firing system.

Combustion Control and Instrumentation

The control of combustion rests on the measurement of the combustion gases and the temperatures. The kiln exit gas analysis measures the oxygen, the carbon monoxide and the nitrogen oxides at the kiln inlet, and it is the primary instrument for the combustion control. The oxygen is the direct measure of the excess air, and it is controlled in the range 1.5–3.5% for the kiln and 2–4% for the calciner exit, depending on the plant and the fuels. The CO is the alarm for incomplete combustion, and the NOx is the emission and the process indicator. The gas analyzers are installed at the kiln inlet and at the calciner or the bottom preheater, and they must be maintained rigorously, because the analysis is the basis of the control and of the emission reporting. The sampling system of the kiln inlet gas analyzer is a demanding application, with the hot, dusty gas requiring a water-cooled probe and a conditioning system, and its reliability is a plant-wide issue.

The combustion control loops manage the fuel flow, the primary air, the tertiary air and the kiln speed. The simplest and most effective strategy is the ratio control: the fuel flow is set by the operator, the primary air follows the fuel at the ratio set by the burner settings, the tertiary air is held at the setpoint by its damper, and the kiln exit oxygen is controlled by the total airflow via the main induced draft fan. The modern control includes the advanced loops, such as the automatic control of the calciner fuel by the calciner exit temperature, the automatic correction of the fuel ratio by the oxygen and the model-based optimization of the kiln by the soft sensors that estimate the burning zone temperature and the coating condition. The control system is the layer that turns the burner hardware into a process, and its design must include the fail-safe logic that trips the fuel on the loss of the flame, the draught or the feed.

The instrumentation of the firing system also includes the temperature measurements of the shell, the secondary air, the kiln inlet and the calciner, the pressure measurements of the air legs and the fuel lines, and the level and the flow measurements of the fuel systems. The shell temperature scanning system, which measures the temperature of the kiln shell along its full length and around its circumference, is the plant’s window into the refractory condition and the coating profile, and it is used both for the daily operation and for the refractory management. The calibration and the maintenance of this instrumentation is a scheduled activity, and the replacement of a faulty instrument in the firing system must be treated with the priority that the process impact demands.

Alternative Fuels: Types, Preparation and Feeding

The alternative fuels used by the cement industry are the residues of the urban, commercial and industrial economy. The most important are solid recovered fuel (SRF) and refuse-derived fuel (RDF), produced by the mechanical treatment of municipal waste, with a calorific value of 10–18 MJ/kg, a moisture of 10–25% and a chlorine of 0.3–1%; plastic waste, with a calorific value of 25–35 MJ/kg and a high chlorine risk; waste tyres, whole or shredded, with a calorific value of 28–33 MJ/kg; biomass, including wood chips, bark, sawdust, rice husks and agricultural residues, with a calorific value of 8–17 MJ/kg and a moisture of 20–60%; sewage sludge, dried or thermally treated; waste solvents and oils; and animal meal. The common feature is the variability of the properties and the need for a controlled feed to the process.

The preparation of the solid alternative fuels includes the sorting, the shredding, the drying, the metal removal and the quality control. The target particle size is typically below 50 mm for kiln injection and below 20–30 mm for calciner injection, and the shredding stage is followed by the magnetic and eddy-current separation of the metals, which would damage the feeding and the firing equipment. The drying is required for the fuels with a high moisture, because the water in the fuel reduces the flame temperature and increases the gas volume; the drying is often performed by the process waste heat. The quality control includes the sampling and the analysis of each batch for the calorific value, the moisture, the chlorine and the heavy metals, and the blending of the batches to keep the fuel quality within the process tolerance. Many plants operate a fully automated alternative fuel preparation plant with the control of every stage in the DCS.

The feeding of the alternative fuels to the kiln and the calciner is engineered for the specific fuel. The solids are fed through a chain of screw conveyors, rotary valves and airlocks that maintain the pressure difference between the process and the atmosphere, and they are injected into the kiln through a dedicated channel in the burner or through a separate lance, and into the calciner through dedicated nozzles. The injection velocity is matched to the particle size: coarse particles need a high velocity to stay suspended in the flame and to achieve a complete burnout. The liquids are pumped from the storage tanks through filters and atomizing lances, and the gases are metered through the gas train with the safety interlocks. The whole alternative fuel chain is monitored for the flow, the pressure and the blockages, and its design includes the cleaning and the maintenance access, because the blockages in the alternative fuel systems are a common source of downtime.

High Substitution Rates: Process Integration

Thermal substitution rates above 50% require more than the feeding equipment; they require a process integration that manages the effects of the alternative fuels on the whole kiln system. The first effect is the variability of the energy input, which must be buffered by a responsive control system and by a stable base fuel, because the flame cannot be stabilized by a fuel that arrives in batches of very different quality. The second effect is the changed flame: an alternative fuel with a high moisture or a coarse particle size produces a cooler, longer flame, and the kiln burner settings must be adjusted to compensate. The third effect is the chemistry: the chlorine, the alkalis and the sulfur of the wastes circulate in the kiln system, and their concentrations must be managed by the control of the fuel mix, the gas bypass and the dust purging, or the plant will suffer from the preheater blockages and the kiln rings.

The chlorine circulation is the most serious limitation on the substitution rate in many plants. Chlorine enters the system with the fuels and the raw materials, volatilizes in the kiln and condenses in the preheater, where it forms the sticky deposits of potassium and sodium chloride that block the cyclones and the ducts. The management measures include the limitation of the chlorine input by the fuel selection and the blending, the extraction of the chlorine through a kiln gas bypass, which diverts 3–10% of the kiln gas around the preheater and removes the volatile compounds with the bypass dust, and the regular cleaning of the preheater. The bypass is an expensive solution, both in capital and in the thermal efficiency, and its sizing must be based on the chlorine balance of the fuels and the raw materials.

The second limitation is the alkali and the sulfate circulation, which cause the build-ups in the preheater and the kiln inlet, and the third is the emission of the trace components of the waste, including the mercury and the dioxins. The management of these effects is a continuous engineering task, supported by the process measurements of the bypass gas, the dust composition and the circulation indices. The plants with the highest substitution rates operate with a dedicated alternative fuel team that manages the fuel portfolio, the process chemistry and the emissions as one system, and they demonstrate that substitution rates of 70–90% are compatible with a stable process, a good clinker quality and the compliance with the emission limits, provided that the engineering is thorough.

Fuel Switching and Multi-Fuel Operation

The flexibility to switch between fuels, or to operate with a changing mix, is now a design requirement of the firing system. The switching may be planned, when a cheaper fuel becomes available or a supplier changes, or unplanned, when a fuel supply fails. The firing system must therefore be able to change its fuel without stopping the kiln, and the control system must handle the changeover smoothly. The changeover from coal to petcoke, for example, involves the change of the mill settings, the fineness target and the burner settings, and it is performed over several hours with the process parameters monitored continuously. The changeover to a high alternative fuel rate is slower, because the feeding systems and the process chemistry adapt gradually, and it is usually performed over days.

The design of the multi-fuel firing system includes the common elements that all fuels share and the dedicated elements for each fuel. The common elements are the combustion air system, the kiln hood, the instrumentation and the control; the dedicated elements are the fuel channels, the feed systems and the safety devices. The burner tip is the interface where the fuels meet, and its design must avoid the interaction problems: a channel for a coarse solid fuel must not interfere with the swirl of the primary air, and the cooling of the tip must be adequate for the most intense fuel in the portfolio. The engineering of the multi-fuel system is a specialist activity, and the major burner manufacturers supply the complete system, including the fuel channels, the lances, the instrumentation and the control, so that the interactions are resolved in the design rather than in the operation.

The operating procedures of the multi-fuel plant include the documented fuel changeover plans, the matrix of the burner settings for each fuel mix, and the training of the operators in the specific behavior of each fuel. The changeover plans define the sequence of the adjustments, the hold points at which the process is stabilized, and the alarms that indicate a problem during the change. The burner settings matrix is established by the tuning campaigns for the main fuel mixes, and it is the reference for the operators in their daily decisions. The training covers the combustion characteristics of each fuel, the expected process responses and the troubleshooting of the fuel-specific problems. The result of this preparation is that the fuel switching becomes a routine operation rather than a risky event, which is what the market requires of a modern plant.

Emissions from the Firing System

The combustion of the fuels generates the emissions of carbon dioxide, nitrogen oxides, sulfur dioxide, carbon monoxide, dust, and, for the waste-derived fuels, the trace pollutants including the heavy metals, the dioxins and the furans. The CO2 is the dominant emission by mass, and its reduction is the strategic objective of the industry, pursued through the fuel substitution, the alternative fuels and, eventually, the carbon capture. The NOx is controlled by the burner design, the calciner design and the SNCR, as described above. The SO2 comes from the sulfur in the fuels and the raw materials, and it is controlled by the fuel selection, by the absorption of the SO2 on the raw meal in the preheater, which is the natural scrubber of the process, and, where required, by a wet or dry SO2 abatement. The CO is the indicator of the incomplete combustion, and its concentration is kept low by the combustion control and the fuel fineness.

The dust emissions are controlled by the dust collection system, the bag filter or the ESP, which treats the preheater exit gas, the bypass gas and the vent gases of the mills and the silos. The dust concentration in the stack gas of a modern plant is below 10–30 mg/Nm3, and the continuous emission monitoring reports the dust, the NOx, the SO2 and the CO to the authorities. The trace pollutants of the alternative fuels are managed by the process conditions: the temperature and the residence time in the preheater, which must be sufficient to destroy the organic pollutants; the control of the chlorine and the metals in the fuel mix; and the capture of the volatile metals in the dust. The mercury is the most problematic, because it is highly volatile and its concentration in the stack depends on the temperature and the dust recirculation, and the modern plants add an activated carbon injection or a mercury-dedicated removal system where the limits are strict.

The emission monitoring and the reporting are now an integral part of the firing system operation. The continuous emission monitoring systems are calibrated and tested against the reference methods, and their data are used both for the compliance and for the process control. The annual emission reports, the permit reviews and the audits of the monitoring systems are managed by the environmental function of the plant, but the firing system operators are the first line of defense, because the emissions are the direct result of the combustion. The relationship between the firing conditions and the emissions is a central part of the operator training, and the operators are trained to recognize the emission consequences of their decisions: an increase in the swirl to raise the burning zone temperature raises the NOx, an increase in the alternative fuel rate with a poor fuel quality raises the CO, and so on.

Safety Systems of the Firing System

The safety of the firing system is engineered at three levels: the design, the protection and the procedures. The design includes the selection of the materials for the temperatures and the abrasion, the explosion relief and the inerting of the fuel handling and the grinding, the flame-proof classification of the electrical equipment, and the lay-out that separates the hazardous areas. The protection includes the fire and the gas detection, the suppression systems, the flame scanners of the burners, and the emergency shut-off valves of the fuel lines. The procedures include the permit-to-work systems for the maintenance in the hazardous areas, the confined space entry procedures, and the emergency response plans for the fire, the explosion and the fuel spill.

The flame scanner and the burner management system are the core of the firing safety. The flame scanner detects the flame in the kiln and the calciner, and the burner management system monitors the flame, the fuel pressures and the interlocks, and it performs the controlled start-up and the emergency shutdown of the firing. The trip logic ensures that the fuel is shut off if the flame is lost, if the draught is lost, if the feed is lost or if the key temperatures exceed the limits, and the trip is designed so that a single failure cannot prevent the shutdown. The testing of the burner management system is a scheduled task, and the results are recorded, because the system protects the plant from the most serious accidents, the explosions and the fires.

The procedures of the firing system include the safe start-up, the safe shutdown, the fuel changeover, the hot and the cold maintenance, and the emergency response. The start-up procedure begins with the preheating of the kiln with a burner flame established at a low firing rate, and it includes the purge sequences of the fuel lines that remove the explosive mixtures before the ignition. The shutdown procedure includes the burning down of the kiln, the emptying of the fuel lines and the isolation of the fuel systems. The maintenance procedures include the work on the fuel mills and the burners, where the dust and the fuel residues must be removed and the hot work permits are required. The training and the drills of the operators and the maintenance crews complete the safety system, and the audit of the firing safety is part of the plant’s overall safety program.

Energy Efficiency of the Firing System

The energy efficiency of the firing system is measured by the specific heat consumption of the kiln system, typically 3000–3400 kJ per kg of clinker for a modern dry process plant, and the main losses are the exit gas heat, the cooler exhaust heat, the radiation and the convection from the kiln shell, and the heat carried by the clinker and the dust. The firing system influences the efficiency through the excess air, the combustion completeness, the secondary air temperature and the flame shape. The excess air determines the gas volume and hence the exit gas loss: each percentage point of excess oxygen adds approximately 1–2% to the fuel consumption. The combustion completeness determines the combustibles in the clinker and the CO in the gas: each percentage point of unburned fuel is a direct loss. The secondary air temperature, which depends on the cooler performance and the kiln hood sealing, determines the flame temperature: each 100°C of secondary air temperature is worth roughly 30–50 kJ per kg of clinker in the flame energy.

The flame shape is the subtle lever. A flame that is too short produces a local overheating, which increases the NOx and the refractory wear without a gain in the efficiency; a flame that is too long leaves the burning zone underfired and forces the operator to increase the fuel to compensate. The optimum flame is the shortest flame that produces the required clinker quality at the target excess air, and its maintenance is the daily objective of the burner tuning. The kiln exit temperature, which should be 950–1100°C, is the indicator of the heat transferred to the preheater: a low exit temperature means the heat is being lost in the cooler exhaust or the radiation, and a high exit temperature means the preheater is overloaded. The optimization of the firing system is therefore an exercise in balancing the temperatures and the losses, and it is supported by the heat balance, which quantifies the losses and identifies the improvement potential.

The specific heat consumption is also influenced by the fuel quality. A high-moisture fuel increases the gas volume and the exit gas loss; a high-ash fuel requires more limestone compensation and carries more heat out with the clinker; a fine fuel burns more completely and allows a lower excess air. The fuel quality is a procurement and a preparation issue, but its effect is measured in the process: the plant’s energy accounting, which compares the fuel input, the production and the quality daily, is the instrument that ties the fuel management to the plant performance. The best plants close their energy balance monthly, review the trend of the specific heat consumption against the target, and investigate every deviation, because the fuel is 25–40% of the production cost and the firing system is where that cost is controlled.

Maintenance of the Firing System

The maintenance of the firing system follows the same principles as the maintenance of the rest of the plant, with the priorities set by the risk: the safety-critical components are maintained preventively with a fixed schedule, the wear components are inspected at the kiln stops, and the process-critical instruments are calibrated on a defined cycle. The burner is inspected at every kiln stop: the tip condition, the nozzle diameters, the swirl vanes and the seals, with the worn components replaced and the dimensions recorded. The fuel mill is maintained on the mill’s own schedule, with the grinding elements, the classifier and the drives inspected and the wear limits recorded. The conveying lines and the feeding systems are inspected for the wear, the blockages and the dust leaks, and the alternative fuel systems are cleaned and checked for the foreign bodies.

The primary air fans, the blowers and the valves of the firing system are maintained with the lubrication, the alignment and the operation checks of their schedules. The gas analyzers are the most maintenance-intensive instruments of the plant, because their sampling systems handle hot, dusty gas, and their maintenance includes the probe cleaning, the filter changes, the calibration gases and the functional tests. The shell temperature scanner, the flame cameras and the burner management system are tested and calibrated, and the test records are kept. The maintenance planning is integrated with the kiln maintenance planning, because the firing system components are accessible only when the kiln is stopped, and the spare parts of the wear components are managed as critical spares with the reorder points set from the consumption history.

The maintenance of the firing system also includes the condition monitoring of the rotating equipment, the thermographic surveys of the electrical equipment and the inspections of the ductwork for the corrosion and the deposits. The deposits in the kiln hood, the ducts and the calciner, which form from the volatile components of the fuels, are removed at the kiln stops, and the severity of the deposits is recorded, because it is the indicator of the circulation phenomena that the fuel chemistry drives. The maintenance history of the firing system, combined with the process data, is the basis of the reliability improvement: the analysis of the failures identifies the weak components and the operating practices that cause them, and the improvement actions, either the design changes or the procedure changes, close the loop. The result is a firing system whose availability and performance are continuously improved, and whose contribution to the plant’s efficiency and safety is fully realized.

Frequently Asked Questions about Fuel and Firing Systems

Which fuel is best for a cement kiln?

There is no single best fuel. The choice is an economic optimization of the delivered energy price, the process effects, the handling costs and the emissions, made against the local market conditions. Most plants use coal or petcoke as the base fuel, supplemented by alternative fuels whose price and availability favor their use.

Why is petcoke harder to burn than coal?

Petcoke has a low volatile content of 8–14%, so it does not release the volatile gases that ignite easily and stabilize the flame. It must be ground finer than coal, typically 1–2% on 90 micron, and it needs a burner flame with strong recirculation and a long residence time for the char particles.

What is the function of the calciner fuel?

The calciner fuel provides the energy for the calcination of the raw meal, typically 55–65% of the total thermal input. Burning the fuel at 850–900°C in the calciner keeps the kiln thermal load moderate and keeps the NOx low, because the thermal NOx formation is negligible at these temperatures.

What limits the thermal substitution rate of alternative fuels?

The main limits are the chlorine circulation, which causes preheater blockages; the alkali and sulfate circulation, which causes build-ups; the variability of the fuel quality, which destabilizes the process; and the emissions of the trace pollutants. Each is managed by engineering: fuel blending, gas bypass, process control and emission control.

How much excess air does a kiln need?

The kiln exit oxygen is typically controlled at 1.5–3.5%, which corresponds to an excess air of roughly 10–20%. The exact value is a compromise between the complete combustion of the fuel and the heat loss in the exit gas.

What is the primary air ratio of a kiln burner?

The primary air of a modern multichannel burner represents 8–15% of the stoichiometric combustion air. The balance, the hot secondary air from the cooler, is entrained by the momentum of the primary air jets and provides the bulk of the oxygen for the combustion.

Summary and Final Recommendations

The fuel and firing system is the energy machine of the cement plant, and its design, operation and optimization determine the plant’s fuel cost, its emissions and its process stability. This article has covered the fuels, their properties and their selection; the storage, handling and preparation of the solid fuels; the design and operation of the kiln and the calciner burners; the control and the instrumentation; the alternative fuels and the high substitution operation; the emissions and the safety; and the maintenance and the efficiency. The recommendations that emerge are practical: know your fuel, because its properties are the foundation of everything; keep the fuel preparation at the design quality, because the flame cannot be better than the fuel; tune the burners deliberately and record the results, because the optimum is found and held by discipline; integrate the alternative fuels with the process chemistry, because the chlorine and the alkalis punish the plants that do not; and maintain the instrumentation and the safety systems, because the firing system is both the plant’s profit center and its highest-risk area. A plant that applies these principles will convert its fuel into clinker at the lowest practical cost, within the emission limits and with a stable, safe and flexible operation, and that is the technical objective that this article has set out to support.

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.