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. 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 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.

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.

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.

Category 3: Burners with adjustable axial/radial PA jets in a single channel, with or without a small swirl component.

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.

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.’

Low NOx

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 later installed.

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.’

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.

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?

Conclusions

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.

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!