Thatta Cement Company Limited Main: Complete Guide & Downloa

Main Burner Unitherm M.A.S. — Design, Installation and Pre-Commissioning Alignment (Part 1)

The main burner is the single most important combustion device on a cement rotary kiln, because it creates the flame that provides virtually all the heat for the clinkering process, and its design, installation and alignment determine the production rate, the fuel consumption, the clinker quality, the refractory life and the emissions of the whole kiln line. The Unitherm-type multi-channel burner, a family of high-performance kiln burners developed for the cement industry, is designed to produce a compact, intensely mixed, adjustable flame that can burn a wide range of fuels, from natural gas and heavy fuel oil to pulverised coal, petcoke and alternative fuels, while operating reliably at the high temperatures of the kiln firing zone. This article, the first of a two-part technical package accompanying the main burner alignment documentation, expands the generic Unitherm-type main burner into a complete technical reference covering the function of the burner in the kiln process, the construction and components of the multi-channel burner, the aerodynamics of the flame and its heat transfer, the design principles that make the burner flexible for different fuels, the mechanical installation of the burner in the kiln hood, the alignment requirements that ensure the flame is positioned correctly in the kiln, and the pre-commissioning checks that must be completed before the burner is fired for the first time. All content is written in fully generic engineering terms and applies to Unitherm-type and similar multi-channel kiln burners as a class.

1. The Role of the Main Burner in the Kiln Process

The main burner of a rotary kiln performs a deceptively simple task: it must convert the chemical energy of the fuel into a stable, high-temperature flame that radiates heat to the kiln charge and to the refractory, and it must do this for the life of the kiln campaign under continuously varying conditions. The flame is located in the burning zone of the kiln, normally starting a few metres from the kiln discharge end, and it must be long enough to heat the material to the clinkering temperature of about 1450 degrees Celsius over the required length of the burning zone, yet compact enough that the heat is concentrated where it is needed and the refractory at the nose ring and in the cooler inlet is not overheated. The flame must be stable over the full range of kiln loads, from startup to full production, and it must be adjustable so that the operator can correct for changes in raw meal chemistry, kiln speed, fuel quality and alternative fuel substitution.

The heat transfer from the flame to the material occurs by three mechanisms. Radiant heat transfer dominates, because the flame and the kiln gas are at temperatures above 1500 degrees Celsius and radiate strongly to the material bed and the refractory; convective heat transfer transfers heat from the gas to the material surface where the gas sweeps the bed; and conduction transfers heat within the material bed itself. The design of the burner flame therefore controls the heat transfer profile along the kiln: a long, lazy flame spreads the heat over a longer zone and gives a softer burning process, while a short, intense flame concentrates the heat near the burner tip and gives a sharper, hotter burning zone. The ability of the multi-channel burner to adjust the flame shape, through the distribution and velocity of the primary air jets, is precisely the flexibility that the kiln operator needs to match the flame to the process, and it is the central design feature that distinguishes the modern multi-channel burner from the old single-channel burners with their fixed, uncontrollable flame.

The burner also performs a vital secondary function in the process: the momentum of its primary air jets establishes the recirculation of the kiln gas near the burner tip, which draws hot combustion products back towards the flame root and stabilises the ignition of the fuel. Without this recirculation, the flame would be unstable, the ignition of the fuel would be delayed, and the flame would be blown downstream by the secondary air entering the kiln from the cooler. The design of the multi-channel burner, with its carefully proportioned jets of primary air around a central fuel nozzle, is engineered to create exactly the right recirculation pattern for stable ignition across the whole fuel range, and the correct installation and alignment of the burner, the subject of the alignment documentation that accompanies this article, is what guarantees that the flame is positioned centrally in the kiln where the recirculation and the heat transfer work as designed.

2. The Unitherm-Type Multi-Channel Burner: Design Concept

The Unitherm-type burner is a multi-channel burner in which the fuel is delivered through a central channel and the primary air is delivered through a set of concentric annular channels or a pattern of individual jets arranged around the central channel. The fundamental design idea is the separation of the air streams: instead of a single stream of primary air that carries the fuel and provides the combustion air, the multi-channel burner divides the primary air into several channels, each with a different velocity and a different swirl, so that the mixing of the fuel and air can be controlled independently and the flame shape can be adjusted by changing the air distribution between the channels. In the typical configuration, the central channel carries the fuel, the intermediate channels carry the high-velocity primary air jets that atomise, transport and mix the fuel, and the outer channel or channels carry the swirl air that shapes the flame and controls the recirculation of the hot gas.

The primary air fraction of a modern multi-channel burner is low, typically 6 to 12 percent of the stoichiometric air requirement of the fuel, compared to the 25 to 35 percent of older single-channel burners. This low primary air fraction is one of the great advantages of the multi-channel design, because less cold primary air means more of the combustion air is hot secondary air from the cooler, which raises the flame temperature, improves the fuel efficiency and reduces the specific heat consumption of the kiln. The primary air is supplied by a dedicated combustion air fan at a pressure of several hundred millibar, and the air is distributed through the burner channels at velocities ranging from about 80 to 300 metres per second, with the highest velocities in the fuel transport channel, where the air must atomise and carry the fuel, and the lowest velocities in the swirl channel, where the air must create the flame shaping recirculation without simply pushing the flame sideways.

The burner is designed as a package with the fuel preparation and delivery systems. For solid fuels, the burner is supplied from the coal mill or petcoke mill through a fuel pipe with a dosing system, and the burner tip is equipped with a fuel nozzle designed for the specific fuel, with an adjustable swirler or deflector to control the spread of the fuel jet. For liquid fuels, the burner is equipped with an atomising nozzle that breaks the fuel into fine droplets, using either the pressure of the fuel itself or the kinetic energy of an atomising medium such as steam or compressed air. For gaseous fuels, the burner is equipped with a gas nozzle ring with multiple orifices that distribute the gas into the primary air stream. Many modern multi-channel burners are designed to burn combinations of fuels simultaneously, with solid fuel through the central channel and gas or liquid fuel through auxiliary channels, giving the plant the flexibility to switch between fuels, and to substitute alternative fuels, without stopping the kiln.

Channel	Function	Typical Medium	Velocity Range
Central fuel channel	Delivers the primary fuel to the flame	Pulverised coal, petcoke, gas, liquid fuel	20–40 m/s (solid fuel transport)
Primary air transport channels	Atomise, transport and mix the fuel	Primary air from the combustion air fan	150–300 m/s
Swirl air channel	Shape the flame and control recirculation	Primary air with adjustable swirl	80–150 m/s
Auxiliary fuel channels	Deliver secondary fuels or support fuel	Gas, liquid or alternative fuel	Fuel dependent

3. Flame Aerodynamics: How the Multi-Channel Flame Works

The aerodynamics of the multi-channel burner flame are determined by the interaction of the fuel jet, the primary air jets and the surrounding secondary air from the cooler. When the burner is fired, the fuel leaves the central channel as a jet or a spray, the high-velocity primary air jets entrain the fuel and begin the mixing process, and the swirl air creates a rotating motion in the outer part of the jet that expands the flame and establishes the recirculation zone at the flame root. The recirculation zone, an internal region of reverse flow immediately downstream of the burner tip, is the heart of the flame stability: the hot combustion products that recirculate back to the flame root provide the continuous source of ignition energy for the incoming fuel, and without this internal recirculation the flame cannot be maintained at the low primary air fractions that make the multi-channel burner so efficient.

The shape and length of the flame are controlled by the momentum of the primary air jets and the swirl number of the outer channel. A high-velocity, low-swirl setting produces a long, narrow, jet-like flame that concentrates the heat at a distance from the burner tip and is used when the burning zone needs to be extended or when the refractory at the kiln inlet needs protection. A lower-velocity, higher-swirl setting produces a short, wide, bushy flame with intense mixing close to the burner tip, which raises the peak flame temperature and is used when the material needs a hotter burning zone or when the fuel is difficult to ignite. The operator adjusts the flame by changing the air distribution between the channels, and the correct setting is found by observing the flame shape through the kiln hood, by measuring the kiln shell temperature profile along the burning zone, and by monitoring the clinker quality, the free lime content and the nitrogen oxide emissions.

The flame is also influenced by the secondary air flow from the cooler, which enters the kiln at the discharge end and flows towards the flame. The secondary air is hot, typically 800 to 1000 degrees Celsius, and it provides the bulk of the combustion air for the flame, so the mixing of the primary air jets with the secondary air determines how much of the fuel burns near the tip and how much burns further down the kiln. The design of the burner tip, the hood and the nose ring is such that the secondary air approaches the flame in a controlled, symmetric pattern, and the correct alignment of the burner is what ensures that the flame sits centrally in the secondary air stream, so that the mixing is symmetric and the flame is straight. A misaligned burner produces an asymmetric flame, with richer conditions on one side and leaner conditions on the other, which causes uneven refractory wear, uneven heat transfer to the charge, ring formation on one side of the kiln, and elevated emissions, all of which are the symptoms that a misaligned burner produces and that the alignment documentation is designed to prevent.

4. Fuel Flexibility: Combustion of Coal, Petcoke, Oil, Gas and Alternative Fuels

The multi-channel burner is engineered for fuel flexibility because the economic operation of a modern cement plant depends on the ability to use the cheapest available fuels. The fuels differ in their volatile content, calorific value, ash content, moisture content and grindability, and each requires a different combustion strategy. Pulverised coal and petcoke are the dominant solid fuels: the coal is ground in a coal mill to a fineness typically in the range of 1 to 5 percent residue on a 90 micron sieve, depending on the volatile content, and it is transported to the burner in a stream of air or inert gas. Petcoke, with its very low volatile content, is more difficult to ignite and requires a longer retention time and a higher flame temperature, so the burner must be set with a stronger recirculation and a hotter flame root, and the petcoke is ground to a finer size than coal to compensate for its lower reactivity.

Liquid fuels are atomised by the burner nozzle into fine droplets of 50 to 150 microns, which evaporate and burn rapidly, and the atomising medium, fuel pressure and nozzle design are selected to give the correct droplet size distribution for the flame. Gaseous fuels are simply distributed into the primary air stream through the gas nozzle ring, where they mix and ignite readily, and the flame of a gas-fired burner is typically shorter and more radiant than the flame of an oil burner. Alternative fuels, including refuse-derived fuel, tyre chips, plastics, biomass and dried sewage sludge, are increasingly burned in the main burner, and they are either fed through the main fuel channel after suitable grinding and drying, or introduced through separate injection points where the burner design allows. Each alternative fuel has its own combustion characteristics, its own ash chemistry that must be compatible with the clinker, and its own volatile emissions, and the flexibility of the multi-channel burner, combined with the careful setting of the air distribution and the recirculation, is what makes the substitution of these fuels possible without sacrificing the stability of the flame or the quality of the clinker.

The design of the burner for fuel flexibility imposes specific engineering requirements. The fuel channel must be sized for the maximum fuel flow of the design fuel, with sufficient capacity margin for the heavier, lower-calorific-value fuels; the burner tip and the fuel nozzle must be interchangeable, so that the correct nozzle for the current fuel is installed and the change between fuels is a planned operation rather than a compromise; the atomising air and fuel lines must be equipped with the control valves, flow meters and safety interlocks that allow the fuel switching without interruption of the flame; and the refractory of the kiln inlet and the nose ring must be designed for the heat release pattern of the full fuel range. The commissioning of a burner on a new fuel is always a stepwise process, in which the flame is observed, the shell temperature profile is measured and the clinker quality is verified at increasing substitution rates, so that the combustion settings are proven before the fuel is accepted for routine operation.

5. Burner Construction: Materials, Cooling and the Burner Tip

The mechanical construction of the main burner must survive the most hostile environment in the plant: the burner tip operates in the flame at temperatures above 1500 degrees Celsius, in the presence of the hot gas and the clinker dust that returns from the kiln, and it is exposed to the radiant heat of the flame and the kiln charge. The burner is therefore built from heat-resistant materials and is cooled to keep its temperature within the safe working range of its materials. The burner pipe is manufactured from heat-resistant steel, and the burner tip, the section closest to the flame, is manufactured from a more highly alloyed heat-resistant material or is protected by a ceramic coating or a replaceable tip casting, because it is the first component to show the effects of overheating and erosion.

The cooling of the burner tip is a critical design feature. The most common cooling system is the air-cooled design, in which a portion of the primary air is directed over the tip surface before it enters the flame, or in which a separate cooling air stream is passed through a jacket around the tip. Water-cooled designs are also used, particularly for burners firing high-ash fuels or for burners where the tip duty is extreme, but they add the complexity of a water circuit, with its own pumps, instrumentation and risk of leakage. The cooling design must keep the tip temperature below the level at which the material creeps and the tip distorts, because a distorted tip changes the air distribution and the flame shape, and a tip that has been overheated once is weakened and fails early. The inspection of the burner tip at every maintenance stop, checking for distortion, cracking, erosion and the correct condition of the cooling passages, is therefore a mandatory part of the burner maintenance programme.

The internal components of the burner, the swirlers, the vanes, the fuel nozzle and the wear sleeves, are manufactured from abrasion-resistant and heat-resistant materials, and they are designed as replaceable wear parts with defined inspection and replacement intervals. The fuel nozzle of a solid-fuel burner carries the abrasive pulverised fuel at high velocity, and its internal bore is protected by a wear-resistant insert; the swirl vanes are exposed to the primary air and to the recirculated hot gas, and they are cast in heat-resistant alloy; and the replaceable tip section is designed so that the highest-wear part of the burner can be renewed without dismantling the whole burner assembly. The engineering of these components follows the same logic as the rest of the plant’s wear protection: the correct material, in the correct place, with a defined inspection and replacement programme, so that the burner operates for the full campaign with planned maintenance rather than emergency repair.

6. Burner Installation in the Kiln Hood

The installation of the burner in the kiln hood is the mechanical foundation of the whole combustion system, because the burner must be supported so that it points into the kiln at the correct position and angle, and it must be movable so that it can be withdrawn for maintenance and adjusted during operation. The burner is mounted on a burner carriage or a support trolley that runs on a track or rails at the kiln hood level, and the carriage is equipped with the adjustment mechanisms that move the burner laterally, vertically and in the axial direction. The burner passes through a seal in the kiln hood, the burner opening, which is fitted with a sealing arrangement that prevents the leakage of hot gas and dust around the burner pipe while allowing the burner to move within its adjustment range.

The alignment of the burner in the kiln hood is established by the installation survey, which determines the position of the kiln axis, the burner centreline and the relationship between them. The kiln axis is established by the survey of the kiln shell and the support stations, and the burner centreline is established by the survey of the burner mounting surfaces. The design requirement is that the burner centreline, when projected into the kiln, falls at the designed position relative to the kiln axis, normally coincident with the kiln axis or offset by a small, defined amount in the direction that compensates for the kiln rotation and the material bed, and that the burner is set at the correct axial position so that the burner tip protrudes into the kiln by the designed amount. The installation survey therefore fixes the burner in three dimensions: the vertical and horizontal position of the burner tip in the kiln cross-section, and the axial position of the tip relative to the kiln nose ring.

The adjustment mechanisms of the burner carriage give the operator the ability to fine-tune the burner position after the installation survey. The burner can be moved up and down, left and right, and in and out, within the limits of the adjustment range and the hood seal, and the position is locked after adjustment and recorded. The axial adjustment is particularly important because it controls the protrusion of the burner tip into the kiln: a burner tip that protrudes too far into the kiln is exposed to the full heat of the flame and the radiation from the charge, while a tip that is withdrawn too far sits inside the hood, where the flame root is disturbed by the hood geometry and the combustion is destabilised. The correct protrusion, set by the installation and verified during commissioning, places the tip just inside the kiln, where the flame root is established in the open kiln cross-section and the tip is protected by the cooling design of the burner itself.

7. Alignment Fundamentals: Why Burner Alignment Matters

The alignment of the main burner is the position and direction of the burner centreline relative to the kiln axis, and it is one of the most important, and most frequently underestimated, factors in the performance of the kiln combustion system. A correctly aligned burner produces a straight, symmetric flame that sits on the kiln axis and burns the fuel uniformly, while a misaligned burner produces an asymmetric flame that is pushed to one side of the kiln, with a cascade of negative consequences. The heat transfer to the charge becomes uneven, so the clinker is over-burned on the side near the flame and under-burned on the far side, and the free lime content of the clinker rises, forcing the operator to raise the burning zone temperature to compensate, which consumes extra fuel. The refractory on the hot side is overheated and wears rapidly, shortening the refractory campaign, while the cooler side of the kiln develops thick rings of adhering material that restrict the material flow and disturb the kiln operation.

The misaligned flame also affects the emissions and the safety of the operation. The asymmetric combustion produces local zones of high temperature where the thermal nitrogen oxide formation is increased, and it can produce incomplete combustion on the fuel-rich side, which raises the carbon monoxide level and creates the risk of a CO explosion if the hot reducing gas accumulates and is suddenly mixed with air. The fuel impingement on the refractory, which occurs when the flame or the unburned fuel stream touches the kiln lining, damages the refractory, and the impact of the fuel jet on the material bed can disturb the bed, increase the dust carry-over and reduce the effective residence time of the material in the burning zone. Every one of these effects is a direct consequence of the alignment, which is why the alignment documentation that accompanies the main burner is not a formality but a critical engineering deliverable that must be performed, verified and recorded to the same standard as the mechanical installation itself.

The alignment requirement is defined in three elements: the parallel alignment, which requires the burner centreline to be parallel to the kiln axis in both the vertical and the horizontal plane; the offset, which is the lateral and vertical distance between the burner centreline and the kiln axis, normally set to zero or to a small designed value; and the axial position, which is the protrusion of the burner tip into the kiln. The alignment is set during installation on the cold kiln, when the kiln shell is at ambient temperature, and it must account for the fact that the kiln axis changes when the kiln is hot, because the shell expands, the tyres and rollers settle, and the support structure takes up the operating loads. The cold alignment values are therefore set to achieve the correct hot alignment, using the thermal growth data of the kiln, the burner and the support structure, and the verification of the hot alignment, the subject of the second part of this documentation package, confirms that the design intent has been achieved.

8. Pre-Commissioning Checks Before First Firing

Before the burner is fired for the first time, a systematic sequence of pre-commissioning checks verifies that the burner and its systems are complete, correctly installed and safe to operate. The checks cover the mechanical condition of the burner, the alignment and position settings, the fuel systems, the air systems, the instrumentation, and the safety interlocks, and they are recorded on a commissioning checklist that is signed off step by step. The mechanical checks verify that the burner pipe is securely mounted, that the carriage and the adjustment mechanisms operate smoothly and lock correctly, that the tip and the internal components are clean and correctly assembled, and that the burner opening seal is in good condition and adjusted to the burner position.

The fuel system checks verify the integrity and readiness of each fuel line. The solid fuel line is checked for cleanliness, for the correct installation of the dosing and transport components, and for the calibration of the fuel flow measurement; the liquid fuel system is checked for the pressure, temperature and atomising medium readiness; and the gas system is checked for the pressure and the leak tightness of the connection. The air system checks verify the readiness of the combustion air fan, the control valves, the flow measurements and the pressure measurements for each burner channel, and the setting of the air distribution to the preliminary values for the startup fuel. The instrumentation checks verify the operation of the flame detection, the temperature and pressure instruments, the burner management system and the interlocks that link the burner to the kiln operation.

The safety checks are the final gate before firing. The flame detection and the burner management system are tested to confirm that the burner will be cut off automatically on loss of flame, low combustion air flow, low or high fuel pressure, fan trip or the operator’s emergency stop command. The area around the burner is cleared and the communication between the burner operator and the kiln control room is tested. The purge sequence is executed, blowing the kiln and the hood with air to remove any accumulated fuel gas before the igniter is energised. The igniter, the auxiliary burner that lights the main burner, is tested for its flame stability and its position relative to the main burner tip, and the operating procedures for the startup, the warm-up and the ramp-up are reviewed with the operating team. Only when every check on the checklist has been completed and signed is the main burner permitted to be fired, and the pre-commissioning documentation, with the recorded settings and the signed checks, becomes the baseline for the commissioning and tuning that follow.

9. Frequently Asked Questions

What is a Unitherm-type multi-channel main burner?

A Unitherm-type multi-channel burner is a kiln burner design in which the fuel is delivered through a central channel and the primary air is delivered through multiple channels around it, with different velocities and swirl, so that the flame shape, mixing and recirculation can be adjusted independently and a wide range of fuels can be burned efficiently.

Why is the primary air fraction so important?

A low primary air fraction, typically 6 to 12 percent of stoichiometric air, means that most of the combustion air is hot secondary air from the cooler, which raises the flame temperature, reduces the heat consumption and improves the fuel efficiency of the kiln, compared to old single-channel burners with 25 to 35 percent primary air.

What does burner alignment mean?

Burner alignment is the position and direction of the burner centreline relative to the kiln axis, including the parallel alignment, the offset and the axial protrusion of the tip, and it must be correct so that the flame is straight, symmetric and central in the kiln.

Why does a misaligned burner cause problems?

A misaligned burner produces an asymmetric flame that causes uneven heat transfer, over-burned and under-burned clinker, rapid refractory wear on one side, ring formation, elevated nitrogen oxide and carbon monoxide emissions, and the risk of fuel impingement on the refractory and the charge.

How is the burner cooled at the tip?

The burner tip is cooled by air, either by directing part of the primary air over the tip surface or through a separate cooling air jacket, and some designs use water cooling, with the cooling design keeping the tip below its material temperature limit so that it does not distort or creep.

Which fuels can a multi-channel burner fire?

Multi-channel burners are designed for fuel flexibility and can fire pulverised coal, petcoke, heavy fuel oil, natural gas and a range of alternative fuels, individually or in combination, with interchangeable nozzles and adjustable air settings for each fuel.

10. Summary

The main burner is the heart of the kiln combustion system, and the Unitherm-type multi-channel burner design represents the modern engineering answer to the demand for efficient, flexible and stable kiln firing. Its separation of the fuel delivery and the primary air into independent channels gives the operator independent control of the flame mixing, the flame shape and the flame recirculation, which makes it possible to burn the full range of cement plant fuels, from natural gas and heavy fuel oil to pulverised coal, petcoke and alternative fuels, at a low primary air fraction that maximises the use of hot secondary air and minimises the specific heat consumption. The mechanical construction of the burner, with its heat-resistant materials, its cooled tip and its replaceable wear components, is engineered for the extreme environment of the burning zone, and the installation of the burner in the kiln hood, with its support carriage, its adjustment mechanisms and its sealing arrangements, provides the mechanical foundation for the correct positioning of the flame. The alignment of the burner relative to the kiln axis, which this first part of the documentation package establishes for the cold kiln, is the critical geometric condition that ensures the flame is straight and symmetric in operation, and the pre-commissioning checks documented here provide the final verification that the burner is complete, correctly installed and safe to fire. The second part of this documentation package, covering the alignment verification, the commissioning sequence, the operation, the tuning and the maintenance of the burner, completes the technical picture. Together the two parts form a complete reference for the plant engineer who must install, commission, operate and maintain a modern multi-channel main burner, and they show that the difference between a well-run kiln and a problem kiln is very often not the burner design but the discipline with which the burner is installed, aligned and tuned.

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.