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Using CFD modeling to improve combustion in rotary kilns and precalciners Dr Patrick MUSCAT, Doctor-Eng. of Marseilles University, dipl. engineer of ENSAE (Sup Aéro) Manager of CFD Department, Fives Pillard, Marseille (France)

PLANTS CONCERNED AND TYPES OF CFD SOFTWARE USED The kilns studied are: rotary kilns for the production of grey and white cement, rotary kilns for lime, nickel. The cement precalciners studied are suspension preheater type, cyclone type. The CFD software used is i s FLUENT® of ANSYS. Two types of CFD simulations are carried out: Cold CFD simulations (flow patterns): They allow to improve the combustion air flow pattern to the burners in order to smooth the combustion and render the flame more symmetrical. Inside the rotary kiln, it appears also possible to simulate the introduction of hot secondary air into the flame, which influences the flame temperature and radiation onto the calcinated bed. CFD simulations in combustion: They allow to compare the influence of different burner designs on flame shape on the radiation received by the material bed to be calcinated, to understand the influence of several inlet parameters concerning either the fuel used, or the kiln geometrical dimensions, the position of the burner in the kiln, the various velocities. 

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Since the studies are essentially comparative, the necessity to master absolute values is not essential. However, it is possible to tune the model by taking an example of a kiln in operation (for which we dispose of kiln shell scanner recordings as well as all the geometrical dimensions and inlet data parameters), and adapt the model so that it gives a similar simil ar temperature profile on the kiln shell to the one measured, thus ensuring that the simulated flame pattern is comparable to the experimental one.

EXAMPLE OF IMPROVEMENTS ACHIEVED THANKS TO THE USE OF CFD Rotary kilns for white cement The user intended to reduce NOx N Ox emissions with a new burner, but wanted to be sure that with the new burner the radiati on received by clinker bed would beat least maintained or, if possible, improved. First of all the simulations showed that the influence of combustion air distribution was very large and it was absolutely necessary to be improved. The result of the simulations shows that with the new generation of Fives Pillard burner and an optimisation of the combustion air flow distribution, the radiation received by the clinker cli nker bed was improved. The success at the commissioning site confirms the CFD conclusions and the validity of the CFD model used.

Temperature profile (old burner)

Temperature profile (new burner)

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Radiation received by clinker bed

Rotary kiln for nickel In the plant concerned, a problem of flame shape and stability was encountered. CFD modeling was used to understand, then correct the problem:

Geometrical model with details of the kiln hood

The analysis showed that a bad combustion air velocity distri bution in the kiln hood was probably the cause of the problem:

Visualisation of flow in the kiln hood

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Several simulations have been carried out to define the shape of devices necessary to optimize air flow:

Detail of the mechanical element of the flow correction device

We can see that once the air distribution has been deeply modified in the kiln hood, then the problem of the uncontrolled flame air root disappeared and the temperature profile improves:

Before optimisation

 After optimisation

Temperature profiles before and a fter optimisation

The commissioning results confirmed very good kiln operation.

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Studies on cement precalciner a) SNCR system on a rising duct downstream a rotary kiln The CFD is an essential tool for the engineering phase of a SNCR (NOx reduction) system applied to existing plants. Such an engineering process can be broken down as follows: First, further to site temperature measurements, we determine the appropriate reaction zone. Then, we use CFD for calculating the flue gas balance and simulate velocities and temperature profiles in the reaction zone:

Temperature and velocity profiles at kiln outlet (Fives Pillard Neutrinox® SNCR system)

The second phase consists in modelling the droplets of (urea/water) or (ammonia/water) solution, and their vaporizing in the flue gas stream. The number of injectors, their location point and their droplet velocities may be optimized:

First level: main injection (Neutrinox®)

Second level: optimization (Neutrinox®)

First and second level injectors (Neutrinox®)

The target consists in covering the entire cross section of the flue gas duct with the injected urea / ammonia spray. 4/9

Profiles of Mass fraction of NH 3 (Neutrinox®)

Higher profiles

Depending on the CFD modelling conclusions, the urea (or ammonia) / water solution is injected through 6-10 atomizing guns installed on 1, 2, or 3 levels in the calciner. Usually two levels are sufficient. The first level aims at covering the maximum surface with the longest resident time, while the second level aims at optimizing the reaction and efficiency. The injectors optimization includes the choice of emulsion velocity (an atomizing fluid is used, in general compressed air), the choice of orifices, number and angle. The special atomizer i s patented (n° FR 0605355 of 06/15/2006). b) Another example of SNCR system on suspension preheaters application concerns a cement plant located in Vietnam:

Hon Chong Cement Vietnam

Due to the high capacity of the cement line (5,000 tpd), it has been installed on two levels with six injectors each. Another example in Italy shows that data collected during the first commissioning period obliged us to reconsider the hypothesis taken at the project stage, thus modifying the position of the injectors. They were moved downwards in the calciner chamber:

Precalciner (Italy): CFD model and location of urea injection levels 5/9

The first injection level position corresponded to a too low residence time which was inducing low SNCR efficiency. The CFD helped to redefine the new injection level position with higher residence time leading to an increase of urea consumption efficiency: The analysis of flue gas flow shape in the precalcinator showed that -as a matter of fact- a big recirculation zone was increasing the mixing time in a convenient temperature window:

Temperature profile

Velocity profile

Visualisation of urea mixing

With such a modification, an increase of about 10% of deNOx reaction efficiency was reached, corresponding to a 10% urea consumption saving. c) Studies on precalciner burners A comparative study of the effect of primary air used on precalciner burners has been carried out , and allowed us to conclude that the primary air induces a reducing combustion zone around the burners which has an big effect on the temperature profile:

Effect of precalciner burner primary air on temperature profile

Further studies on precalciners are pending; they aim at looking deeper into the position of burner air inlets and to try to understand whether the precalciner’s operation can be improved in the future by simple modifications. 6/9

Rotary cement kilns: comparison of flame shapes CFD simulations can be used intensively to analyse flow patterns around the burner. Cold CFD simulations allow to visualize and compare flow patterns. A first example concerns the comparison of secondary air flow shape as a function of the burner tip position inside the kiln. It confirms the site results i.e. the penetration of secondary air inside the flame depends on the burner tip position:

Secondary air flow shapes as a function of burner position inside the kiln

This study (also) allows to prove that there is no recirculation zone around the burner flame: it is clear that no recirculation eddy can exist around the flame:

Secondary air flow shape

Another example concerns the effect of swirl with 2 d ifferent burner configurations (single annular outlet or double outlet). The comparison of the velocity profile at the burner outlet clearly shows that the double air outlet is better than a single air outlet since the flame stays under control and there no risk of flame opening (wide flame) l eading to kiln shell overheating as well as sulphur volatilization.

Single air outlet burner: effect of increasing swirl

Double outlet burner: effect of increasing swirl 7/9

The CFD modelling allows also to analyze the loss of velocities at the burner outlet. The simulations have been carried out with the same air fan pressure (250 mbar). It clearly appears that with a single primary air outlet there is a loss of velocity: The useful velocities at burner outlet are about 100 m/s, due to the enlargement of the burner cross section, while it remains at about 250 m/s when a separated second “slot-designed”:  yellow is # 250 m/s

green is # 100 m/s

Velocities at single air outlet burner

Velocities at double air outlet burner

Cold modelling of different burner configurations (axial primary air tip) show that in increasing the burner “slot effect” promotes hot secondary air suction into the flame, thus intensifying the flame: AXIAL TIP WITHOUT “SLOTS” fully open

closed

AXIAL TIP WITH “SLOTS” fully open

closed

 

Visualisation of the “slot” effect on air flow patterns (velocities)

Visualisation of secondary air introduction into the flame (the colour represents the temperature of the mix)

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In order to confirm the above described “cold modelling ”, a complete modelling in combustion has been achieved. The geometric size of the kiln is represented below:

Rotary kiln model

Only the first 50 m from the burner have been modelled due to computational considerations. The kiln rotation is t aken into account. The details of the burner geometry have been taken into account. The fuel is coal. The secondary air temperature has been chosen at 800°C. The clinker has been modelled as a solid entity in motion, and the heat exchange has been modelled. The simulations concerning the combustion of coal particles include various axial primary air injections for the same burner, thus several different “SLOT EFFECTS” for same operational conditions. The conclusion appears as follows:

Radiation on clinker bed with different burner axial air injections

CONCLUSION The belief that CFD modelling cannot bring a real progress in combustion for rotary kilns or cement precalciners now appears completely obsolete. It is clear that comparisons based on different “inlet” [combustion/kiln fuel] parameters give precise data and allow to choose which configuration can improve a real situation. The site results obtained allowed to confirm that the provisions were sufficiently reliable. Simulations with liquid fluid flows do not hold any real interest since CFD modelling is now an efficient tool and the time available for modelling work is decreasing rapidly. Such a tool allows to design a combustion system associated with a real kiln which, as a matter of fact, may allow to simplify adjustments at site. Even more interesting is the fact that such a design can be “tuned” to the priority targets of the user, which may differ from one kiln to another (for example : “clinker reactivity”, “high LSF raw material”, “limited NOx emission”, “high percentage of alternative solid fuels”, “use high ash low volatile coal”, etc...). Such CFD studies are now a common way of working, and bring feedback from experience every day. This is why we hope that , thanks to such new tool, real progress on the calcined product quality or on the polluants emission (NOx, CO, and CO2….) will be more easily achievable. 9/9