This article was inspired by comments made by a representative of a major cement group during the Global CemFuels Conference & Exhibition in Prague, Czechia in February 2016 that can be best summarised as: ‘There is no perfect rotary kiln burner on the market.’ This may well be true, but there are some very good ones available these days for those that want to optimise their production process and understand that burners are not just pieces of steel, bought by the kilo...
The purpose of this article is to help kiln engineers and production managers select the burner that is most suited to their current and future needs. It is also to assist the end users of the burners in asking pertinent questions to the OEMs and looks at some of the burners’ characteristics that are often overlooked. I draw on my 30 years of experience with rotary kiln burner OEMs, with which I have consulted for the purpose of writing this article. This article does not seek to critique or recommend any particiular OEM. Some of the cement producers I approached, however, were less than forthcoming, as if the ‘pyro-processing book’ really did contain the secret of how to design the perfect burner.
As with modern cars, we can start by saying that, in 2017, there is no ‘bad’ burner made by the major OEMs. However, we can say that the demands placed on the burner vary depending on the type of process, plant configuration, the available fuels and a host of other parameters, just as different drivers may want a car that delivers speed, economy or a large capacity.
As ever, many cement producers would prefer an ‘okay’ burner with great service than the ‘best’ burner with poor service. One must consider the voids between the potential sophistication of the burner designs and the time available by the operators and kiln engineers to optimise every last detail. In general, success for a cement plant burner project can be achieved by involving both parties at the design stage. This is made easier when the burner design is flexible enough to match the specifics of the kiln specifics. Flexibility, as ever, is the key word.
Types of burner
A new burner is usually bought because of: Changes to the fuel mix; Excessive wear; Irreconcilable problems with the existing burner; When satellite coolers are replaced with grate coolers; To increase production and; To reduce NOx emissions.
I have categorised the main burners available on the market, in what is a slightly artificial classification but nevertheless one that can be used as a first point of differentiation. There are three groups, based on the way that the primary air (PA) streams are distributed and adjusted. The main differences are where and how the axial portion of the PA is injected in relation to the fuels, the radial (swirl) PA and the secondary air (SA).
Category 1: Fixed multi-channel burners that have two to four independent air channels, not counting the central/cooling/recirculation channel. The Primary Air (PA) is divided at the burner inlet into axial, radial, swirl, dispersion (and/or other) channels. The amount of air going through the respective channels is adjusted by valves, fan dampers or VDFs and sometimes by using multiple fans or blowers.
In this category one could place the Greco Flexiflame, FCT Turbo-Jet, Dynamis D-Flame, Polysius Polflame and KHD Pyro-Jet. The Fives Pillard Rotaflam also fits into this category. It was the leading burner for many years, but is no longer the company’s main offering.
Important parameters in this first category include: The number of PA jets and their ‘quality.’ By this, I mean are the values quoted real, i.e.: the ratio between the length and the diameter of the air nozzle? Also, how many axial and radial PA channels are there, and what are their respective
locations within the burner?
Traditionally the ‘3-channel’ burners of the 1980s saw the coal channel sandwiched between the axial air (outside) and radial/swirl air (inside). In the 1990s when the trend toward low-NOx burners began the two air channels were located outside the coal. This delayed mixing with the secondary air and thus lowered the temperature of the flame (and hence NOx).
Category 2: Burners that mix the axial (A) and radial (R) PA inside the burner in a chamber located toward the burner tip. The respective amount of A-PA and R-PA or their respective intensity can be adjusted from the burner floor by sliding channels. Thus there is only a single PA exit. Inversely there could be a single PA entrance to the burner with two separate exits after an adjustable split within the burner. In this category we find the FLSmidth Duoflex and the Pillard Novaflam. The Novaflam has one common inlet and two separate outlets, the ratio of which (% axial versus % radial) is fixed at the design stage. The swirl effect is given by vanes with variable pitch which makes it possible to modify the angle of exit of the radial air jet from 5° to 35°. The older Duoflex design saw the two air flows mix before being injected through the conical air nozzle. This internal mixing requires channels to translate one into another, which requires a mechanism at the inlet side of the burner.
Category 3: Burners with adjustable axial/radial PA jets in a single channel, with or without a small swirl component. This category hosts the Unitherm Mono Airduct System (MAS); the Polysius Polflame VN and the new FLSmidth Jetflex, as well as the now discontinued KHD Pyrostream. All of these come in adjustable and fixed versions with the exception of the Unitherm MAS. Other differences include the location of the PA jets in relation to the coal channel and the presence or absence of an additional (fixed angle) radial channel.
Although OEMs offer various models and have designs that move back and forth within these classifications, these categories broadly reflect the evolution of the market from enhancing coal/petcoke firing with the advance of indirect firing, to low, low-low and ultra-low NOx types, to high momentum designs for alternative fuel firing, then all-purpose designs and, finally, back to more basic, yet flexible designs. The categories are not absolute, each borrowing some features from the others, and have been disputed by the OEMs.
Deeper into the numbers
To further compare each burner design the following parameters must be considered:
- PA flow and exit velocity (pressure);
- Number, adjustability and angles of PA outlets;
- Coal injection angle (concentric, excentric, convergent, divergent);
- Relative positions of all air and fuel pipes/channels;
- The central/cooling/recirculation PA portion of the burner. This ranges from non-existent to simple perforated plates to meshes. Although some suppliers insist it is essential to ensure proper recirculation of the combustion gases into the flame, in reality its influence is unproven.
The above design parameters are calculated by OEMs using various dimensionless numbers and indexes, including turbulence index, tangential number, impulse and swirl number. These are usually proprietary information from the OEM and shouldn’t be used by plants for comparison between manufacturers. For example, Dynamis uses a dimensionless geometrical index to assess the flux of secondary air entrainment into the fuel injection regions per opening interval. FCT uses the Craya-Curtet parameter in burner design, effectively connecting burner momentum to secondary air momentum rather than looking at burner momentum in isolation.
When classifying and assessing the quality of a burner offer, peripherals and accessories also have to be considered, including the flame scanner, igniter, monitoring instruments, safety controls, emergency cooling air fans and others.
Finally, the physical limitations of the burner floor, the positions of the necessary connections, and the burner’s weight are very important. The picture below left shows the tip of a burner equipped with an air canon system. The air tank/blaster is mounted on the rear of the burner, the design of which is often problematic. If the exhaust slot is too far from the burner tip, the blast will be too weak and if too close then it will only partially remove the rhino horn.
Modern burner developments
It is very interesting to read various articles and sales leaflets produced by the OEMs and see the evolution of the ‘message du jour’ over time. New burner concepts and designs are born from a combination of changing cement sector needs (due to external factors) and pure marketing by OEMs that seek to distinguish themselves from the competition. Indeed, I know of new designs that arose almost entirely from a desire to come up with a product that was different from the competition, which were later rationalised as ‘technological innovations.’
The trend toward ultra low-NOx burners ended in the late 1990s as the main adjustable parameter (PA mass flow) had reached its limit in terms of flame (and hence clinker) quality. Selective catalytic and non-catalytic reduction systems also offered a new way to reduce NOx without weakening the flame. That said, it is still important to control NOx formation as much as possible as the NOx level directly impacts upon the size of the de-NOx systems and the amount of urea or ammonia they require.
Besides optimising the amount of primary air, some plants have also looked at flue gas recirculation (FGR) to reduce NOx. One was the Gargenville plant in France, when it was operated by Ciment Français (now HeidelbergCement). The system was developed around a GRECO burner design at the end of the 1990s. Flue gas was captured at the ESP exit (100°C, 10% O2, high moisture) and was injected at the inlet of one of the PA blower inlets. This test was unsuccessful and short-lived. It reduced the NOx level from 1000mg/Nm3 to 900mg/Nm3. The same plant was able to use water injection inside the kiln burner to lower NOx to around 800mg/Nm3, although an SNCR system was
The use of oxygen enrichment, staged combustion and water injection have also been used by some plants as a complement to low-NOx burners. This put an end to the endless redesign of ultra-low-NOx / ultra-low-PA burners.
These days the consensus among most burner OEMs for a kiln burner that is firing 20 - 40% coal/petcoke, 50 - 60% solid alternative fuels and some liquid alternative fuels is that the total PA (not counting conveying air) should be around 10% of stoichiometric air. With the end of the focus on PA flow consideration, which marks the end of looking only at primary measures to reduce NOx emissions, and with the advance in solid AF substitution, a new criteria emerged.
The ‘M’ word
The ‘M’ word is ‘Momentum.’ It was all the rage several years ago as a reaction to the ultra-low NOx burners of the late 1990s. The burner has to do the job and not focus solely on NOx reduction. After all, to get zero NOx we have to turn the flame off!
Momentum (given in N/MW) is simply the sum of all primary air mass flows multiplied by their respective absolute ejection velocities (at the burner tip) divided by the burner thermal power output. The question of whether or not the momentum should include the conveying air flows led to many unnecessary discussions between plants and suppliers.
The industry became obsessed with momentum, which became, on occasion, the only criteria by which burners were being compared. Some leading OEMs came up with higher and higher values, reaching about 13N/MW. This forced them to use PA blowers instead of fans, sometimes one blower per air channel. To control the escalation toward ‘the bigger the number, the better burner’ some OEMs came up with the notion of ‘useful momentum.’ For example, Pillard focussed on impulse efficiency, which characterises the ratio between the relative amount of secondary air that is absorbed within the first 2m of the flame (kg/s) and the axial momentum (N/MW).
Towards 100% solid alternative fuels
The discussion about momentum slightly preceded and then accompanied the rise of (predominantly solid) alternative fuels (AF) in the main burner. Of course, physically speaking, the kiln burners can easily be designed to accommodate 100% solid alternative fuels. It is purely a question of optimising the size of the AF pipes, followed by the PA channels and the overall burner diameter.
However, depending on the type of solid AF, it would partially burn on the clinker bed creating localised reducing zone and ‘brown’ clinker. Designing 100% alternative fuels into the main burner thus generates new problems. The amount that can be burned depends predominantly on the suspension time of the particulates inside the flame. Smaller, lighter particles are generally consumed rapidly and do not hit the clinker bed but larger, heavier pieces cause problems. In addition to this many plants want to have a 100% fossil fuel capability back-up.
There are two main issues to solve: 1. How can we bring enough PA and SA into contact with the various fuels so that there is no delayed or incomplete combustion and that the flame shape and intensity are commensurate with the kiln process? and; 2. How do we keep the AF in suspension?
How do various OEMs deal with these issues? It is commonly agreed that the ratio between the burner outer diameter and the kiln’s internal diameter (not counting the concrete and refractory) should not exceed 12 - 15%. The reason is that larger diameters reduce the burner’s capability to bring sufficient SA in contact with all the fuels, especially the solid AF that is usually located inside the burner. The first thing to do is ensure that this is the case.
In addition, OEMs have: Added PA channels closer to the AF pipes (Greco); Injected extra AF above the burner through a separate pipe (Polysius); Increased the efficiency of the axial PA jets in drawing SA toward the flame, and; Designed central air channels for a true recirculation effect. A solution that can be applied to all burner designs is oxygen injection. The idea is to bring localised oxygen close to the AF exit in addition to PA and SA flows. This helps burn the difficult fuel, while not increasing the total flue gas volume and, if properly injected, it can also reduce NOx emissions. However the cost of O2 has to be balanced against the benefits.
To increase the time that AF spends in suspension, one could use mechanical dispersion devices. However, these would wear or plug quickly. Therefore, the trend is to use various air assisted techniques. The Unitherm pneumo-deflector injects air underneath the SAF and lifts it up by a few degrees from the burner axis, enough to increase the suspension time by 50% and thus ensuring complete combustion. It requires a jacket-pipe, through which the required air (about 30% of AF conveying air at around 250mBar) is injected. This air connects to the primary air pipe at the back of the burner or to a separate air fan.
The Greco Flexiflame uses air jets around the exit of the solid AF pipe as well as the dispersion air channel around the centre of the burner. FCT’s Lofting Air, for Air Assisted Dispersion of RDF and other similar fuels performs a similar task.
Meanwhile the FLSmidth JetFlex withdraws the solid AF pipes inside the burner to create a small expansion chamber. The centre pipes can be retracted up to 200mm and the solid AF particles are stopped and then accelerated again. The retraction in combination with the axial air nozzles enables a significant drop in fuel velocity in front of the burner. This feature strongly increases the fuel retention time in the flame and enables early ignition of low grade AF.
There are also novel approaches devised by some OEMs. These include injecting the AF through a channel (as opposed to a pipe) as it is done for coal and petcoke. The benefit is faster ignition as the secondary air comes into contact with the AF at an earlier point. However it requires fine AF particles, otherwise the channel will plug quickly at the location of the spacers. Some types of AF, such as dried sludges and even meat and bone meal (MBM) can be mixed with the coal/petcoke and thus injected through a channel instead of a central pipe.
The Greco Flexiflame EcoPro® burner also injects the fluff into the flame root through a ring channel. To the inside and outside there is swirl flow induced by swirl air channels. This way the turbulence experienced by the fluff is increased, which leads to faster ignition and better burnout. The basic idea is to provide high temperature and high oxygen content to the particles. However, such an approach does require the particle size distribution to be below the current standard for RDF. A special mill
It may be possible to mechanically disperse the AF stream, although one would have to consider the likely fast erosion of such a system. Another way would be to incline the AF pipe upward inside the burner by a few degrees, although the drawback would be that the inclination could only be corrected by modifying the burner. One can also adjust the amount and pressure of the conveying air to match the characteristics of the AF. The apparent easier method, one that was used at the very beginning of solid AF injection, is to inject the lower calorific value and larger volume AF in a separate pipe or pipes above the main burner. This solution was advanced by Polysius and has the big advantage of needing a smaller diameter main burner. Different types of AF can also be mixed into a single injection pipe or channel. This is an area where more developments are under way.
Overlooked and practical issues
As seen above, it can be easy for the cement sector and OEMs to become ‘obsessed’ with various burner parameters, like low NOx or high momentum. This however, can lead them to neglect other important areas that should be looked at in more detail. I have encountered many of these over the decades.
Many plants struggled for years with such issues as ring and ball formation, plugging, premature refractory failures and other problems. Sometimes, a burner that is poorly designed or that doesn’t fit the plant is the cause. However, I have also seen plants replace their burners to no avail. The following are additional points of consideration when a burner has been selected and doesn’t appear to perform as expected.
1. Particulate size (distribution): Getting the correct particle size distribution is easier said than done, especially when AF suppliers have the upper hand. However, even with coal and petcoke some issues such as recurrent ring formation can be solved with a different coal fineness. For instance making the coal coarser can slightly delay combustion.
2. Secondary air temperature is of great importance to the quality and shape of the flame, as well as the overall efficiency of the kiln.
3. Burner position inside the kiln: This is an important parameter that is not used to its full extent. It sets the start of the cooling zone but it also affects the AF flow pattern around the burner and thus the flame shape and the impact on the kiln refractory.
4. What is the real importance of the central/cooling/recirculation air channel? It is supposed to produce an internal recirculation zone (IRZ), assuming that one is designed on purpose and not as an afterthought. In reality, being the most fragile portion of the burner tip, they sometimes plug or burn and operators often don’t see any change. One has to assume that cooling is the only use. This was not always the thinking as earlier designs focused more on providing sufficient air at the root of the flame. Experience and CFD show that the usefulness of central air can’t be proven.
5. Coal injection: nozzles, shapes, and angles. Of course coal is not the ‘topic du jour’ but most of the burners in the world still operate from 20 - 100% on coal or petcoke. Yet how the coal is ejected from the burner doesn’t figure as a key parameter in the OEM’s literature. When you are trying to influence the mixing of 5t/hr of coal with 80,000Nm3/hr of SA using only 8000Nm3/hr of PA, the design of the coal nozzle is important. The impact is important and should be considered more closely. Is it straight, angled towards the axis of the burner (convergent) or divergent, adjustable cross-section? What is the shape and number of the spacers? The impact can be great.
6. On the topic of coal, an often encountered issue is the uneven distribution of the coal at the burner tip, which is often the result of an improper coal inlet injection angle coming to the burner, or an excessively high velocity. It can also be that the coal channel is too narrow. In most cases 20mm is a safe minimum.
Once the burner has been selected there are some practical ‘tricks’ that have been shown to help:
a. Use of a laser pointer and a target to reposition the burner precisely where it was prior to its removal during maintenance. The laser is installed in one of the pipes of the burner and pointed at a target several metres down the kiln. Alternatively the laser can point to a surface on the burner floor.
b. Progress has been made in the concrete casting of the kiln burner outer-pipe as fewer issues are encountered these days and the target of one year life is often met. This is despite the fact that higher levels of AF lead to higher levels of chemical attack. Nevertheless installing thermocouples between the outer steel pipe of the burner and the concrete during pouring will send an alarm to the operator in case of concrete failure to decide if the burner should be pulled out of the kiln immediately.
c. There were previously attempts to improve the concrete life by using special refractory bricks, special anchors and special installation methods. This was a particular topic of interest around 12 years ago with the use of sinter ceramic bricks between ribs welded onto the outer pipe.
d. Dealing with the ‘rhino-horn.’ Some kilns and coolers will produce huge build-up on the top of the burner. Two common solutions are to use a droplet shape casting and/or an air cannon. The air cannon solution is hard to properly design: Too close to the burner tip and it can only handle a portion of the build-up. Too far and it loses its blowing power. The extra concrete of the inverted droplet could work but does add considerable weight.
e. The issue of the automatic adjustment of the burner is an issue that comes back on a regular basis. But is it a valid option?
One may recall that a thermal imaging camera company that claimed to be able to predict, on line, the free lime or NOx emissions, only by viewing and analysing the flame. It was supposed to send signals to the burner to automatically adjust the various PA flows in order to optimise the flame. Other kiln data were also used, including rotation speed, O2 levels and kiln shell temperature.
A way to automatically adjust the main burner settings based on an array of measures with sophisticated control loops has been tried but to my knowledge not very successfully and it is not an option offered by the OEMs these days.
Remote control of the burner setting is another matter that can be easily achieved by investing in extra actuators for the PA air flow valves and adjusting the angle of the nozzles (if applicable) and transmitters for feedback. It is also a matter of philosophy, depending on the plant. Some plants give more or less freedom to their operators. In cases where there is less freedom the valves are held in position using padlocks, the keys for which live in the kiln engineer’s pocket.
On paper, I have designed a revolutionary rotary kiln burner for modern cement plants that can achieve: 100% AF with a large fuel mix flexibility; Unique rhino horn elimination; Automatic flame adjustment relative to process kiln conditions; Fully instrumented for flows, pressures, temperatures, positions and; It operated well over any load conditions. However, reality has other ideas and it never went further than sketches on a paper tablecloth.
As environmental, quality, financial and other constraints grow stronger and stronger, ‘perfect’ burner design and kiln/plant conditions are more important than ever. Thankfully, these days all major OEM burners are of high quality. A portion of the arguments they develop is of course more good marketing than proven science, as not everything in the burner can be calculated, especially in relation to the kiln. Plant engineers in their discussions with burner OEMs and with their evaluation should keep an open mind and first see which OEM approach they feel more comfortable with.
Producers of burners with moving parts may assume that one burner can fit all conditions/cases, while the ones with fixed parts will spend more time studying the specifics of each application. The third category, with sliding pipes and channels, appears to be going out of favour, as these adjustments have often proven to be difficult to make under dusty and hot conditions. Parts often stick in place and there has been difficulty in reproducing flame shapes. Furthermore bringing two separate air streams and mixing them inside the burner prior to being ejected as a single stream creates efficiency losses.
As with other products, trends in the burner market are cyclical and burner features are borrowed from one another. For instance, the KHD channel is universally adopted. The Pillard Rotaflam arrangement was the first to break with the established axial air-coal-swirl air arrangement and now OEMs play more freely with the location of channels and pipes. The Unitherm MAS concept of an adjustable single PA channel was the precursor of other designs. FCT’s stance on ‘no moving parts’ is shared by other OEMs such as Greco and Dynamis.
However care should be taken to separate the true benefits from the façade. While this article was not intended as a critique of any design in particular, I question the validity of some of the common arguments. What is the real impact of the shape of the PA jets (square, round, rectangular) on the fuel/air mixing efficiency? How is a large number of PA axial jets different, in reality, from an open channel? (Some OEMs simply add a plate with some holes to give the illusion of jets). Some of the techniques and devices to break or spread the streams of solid AF into the flame are also to be examined closely. When uncontrolled this can have the effect of throwing more RDF on the clinker bed. Finally, be aware that, by trying to keep all the benefits of previous designs, some designs have become overly complicated, producing large burners that do not work well.
At the same time, newer concepts are emerging. If 100% coal or petcoke back-up is no longer required, why not inject the 10 - 30% coal through a pipe rather than a channel? In Europe, with so many types of solid AF that vary greatly in quality and quantity, why not change the configuration of the burner internals to optimise the given situation? Why not mix various AF streams with coal prior to entering the burner? Why not design a smaller, more agile burner and inject some of the solid AF above that burner in separate pipes? If the AF can be ground finely enough, why not inject it through open channels? Why not partition some of the channels? Finally, why not have a few spare configurations of burner internals and replace them as the AF market dictates?
For markets like the US and Egypt that alternate between coal/petcoke and natural gas firing (due to either availability or price), why not have two optimally-designed burners (one for each fuel), instead of a hybrid that is optimised for neither?
Evolution is key...
Above all, the burners of the future will continue to evolve both due to technical advances and good marketing. They have several variations around some company specific ‘principles.’ Versatile design is the main trend, with OEMs offering options around their main platforms in keeping with the analogy of the automobile industry. The difficulty for the plant is that their objectives can be accomplished in different ways and they have to choose!
The March 2017 issue of Global Cement Magazine will include a survey of major burner OEMs and will consider their history, the drivers behind their famous designs, design philosophies and approaches to different fuels.
The author would like to express his thanks to the following:
- Alex Knoch, Product Manager Burners & Firing Systems, KHD Humboldt Wedag, Germany.
- Dipl.-Ing. Stephan Pallmann, Head of Process & Quotation Pyroprocessing thyssenkrupp Industrial Solutions (Polysius), Germany.
- Carsten Damslund Jensen, Regional Product Line Management, Pyro Products, FLSmidth, Denmark.
- Alexander Lederer, Managing Director Unitherm Cemcon, Austria
- Joel Maia, Technical Director FCT, Germany
- Renata Favelli, Commercial & Marketing, Dynamis, Brasil
- Fives Pillard, France
- A TEC GRECO Combustion Systems Europe, Austria