Multiphase Flow Simulation of a Simplified Coal Pulverizer

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Multiphase flow simulation of a simplified

coal pulveriser

H.B. Vuthaluru*, V.K. Pareek, R. Vuthaluru

Department of Chemical Engineering, Curtin University of Technology, GPO Box 1987, Perth, Western Australia 6845, Australia

Received 8 October 2004; received in revised form 9 November 2004; accepted 2 December 2004

Abstract

In coal-fired power plants, the first major component is pulveriser, whose performance dictates

the total power station efficiency. Pulveriser is employed to grind the lumped coal and transport the

fine coal powder to furnace chamber for an efficient combustion. In this study, we have simulated

motion of air and coal particles inside a commercial-scale pulveriser. Multiphase flow simulation of a

simplified pulveriser was carried out using a granular Eulerian–Eulerian approach. Due to inclinedair-distributor vanes, the flow field within pulveriser was slightly asymmetric. Regions of

exceptionally high velocities were predicted close to the outer walls of the pulveriser, indicating a

strong probability of particles carryover within these regions. The 100 Am coal particles qualitatively

followed the air path-lines. However, the velocity vectors for 500 Am particle deviated significantly

from those of airflow. The results presented in the paper provide impetus for the development of a

complex pulveriser model, which further could prove valuable to designers for optimisation of

components within the mill.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Pulveriser; Turbulent flow; Multiphase flow; Grid generation; CFD; Particle trajectories

0378-3820/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.fuproc.2004.12.003

* Corresponding author. Tel.: +61 8 9266 4685; fax: +61 8 9266 2681.

E-mail address: [email protected] (H.B. Vuthaluru).

Fuel Processing Technology 86 (2005) 1195– 1205www.elsevier.com/locate/fuproc


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1. Introduction

Pulverisers play dominant role in dictating the performance and efficiency of

combustion systems in power station boilers. Pulveriser is employed to grind the lumped

coal and transport the fine coal powder to furnace chamber for an efficient combustion.

The development towards the fine grinding techniques are described and reviewed in the

literature [1]. It has been noticed that the mill throughput are reduced by high grinding

requirements.

Coal particles rely largely on a uniform airflow distribution to entrain the different sizes

of pulverised coal particles. If the airflow is unevenly distributed as it enters the wear track

region, several different problems can occur. One problem is the existence of excess air in

some regions of the mill. These regions are more likely to pick up large coal particles that

have not had a chance to get exposed to the grinding action. This can result in choking of

the mill at the classifier, reducing the mill total output and possibly lowering the combustionefficiency if the larger particles are transported to the furnace. Another more serious

problem can exist when the coal particles in the lower sections of the mill are confined in

the low velocity regions. The resulting coal build-up can lead to hazardous conditions such

as mill fire or explosion. Basic studies conducted at different pulveriser manufacturers [2,3]

across the world have shown that there were unequal velocities in the flow at classifier, due

to which, repeated grinding causes concern for rollers’ lifespan.

To ensure a proper air distribution, to optimise the performance of the mill and to

increase the thermal efficiency of a boiler, it is essential to generate parametric behaviour

through numerical simulations. A better understanding can be developed through computer

simulations that can predict the flow field in the mill. The present paper deals with a

preliminary 3-D CFD simulation of a commercial-scale pulveriser. A granular Eulerian– Eulerian model was used to simulate the trajectories of particles with two different sizes.

2. Description of pulveriser

Coal pulverisers or mills grind coal typically from 10 to 50 mm size coal lumps to provide

fine coal dust particles usually less than a micron up to several microns in size (with at least

70% by weight not exceeding 75 Am). These fine coal dust particles are used as a feed to

combustion furnaces. The grinding is accomplished by multiple grinding or pulverizing

rollers rotating about their own axes and crushing the coal against a rotating table driven by a

motor through a speed reducer. The rotation of the table induces rotation of the rollers which

are pressed downwards either by springs or by hydraulic or pneumatic means towards therotating table to enhance the coal crushing and pulverizing action. The raw coal feed enters

the mill vertically by gravity and the ground-pulverised coal is carried from the mill by air

entrainment upwards through a classifier section to external burners from combustion. The

mill classifier allows the fine enough particles to pass on to the external burners, while the

coarser size particles are returned to the mill rotating table for further grinding and size

reduction. Fig. 1 shows a typical design configuration of the pulveriser.

Coal pulveriser is a device used for separating large and small particles by relying, in

general, on centrifugal forces to cause different size particles to follow different

H.B. Vuthaluru et al. / Fuel Processing Technology 86 (2005) 1195–12051196


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trajectories. By means of bowl rotation and due to centrifugal forces, the coal particles are

moved away to clearance, where the pressure-rollers crush them to fine powder. The

uniform airflow enters tangentially, through angular vanes (distributors) fixed to inlet duct

and lifts the fines via classifier outlets. The coarse particles, which do not flow along withair towards outlet, fall back on the bowl for re-grinding. Very often the pulverisers

experience unequal flow velocities and repeated grinding. The unequal flow distribution in

the air housing has an effect up to the classifier. Furthermore, it has become apparent that

the different components fitted in the grinding chamber cause secondary and tertiary swirl

motions in different forms and intensities. The application of multiphase simulation to

Fig. 1. General arrangement showing all mill features.

Table 1

Specifications of the simplified pulveriser considered for the present study

Parameter Value

Outer diameter

Bottom 3.0 m

Top 4.0 m

Inner diameter at top 3.0 m

Height 4.0 m

Distributor

Number of holes 12

Inclination 208

Total area 0.72 m2

Airflow rate 31.5 kg s1

Solid flow rate 20 tons h1

H.B. Vuthaluru et al. / Fuel Processing Technology 86 (2005) 1195–1205 1197


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Fig. 2. Geometry of pulveriser.

Fig. 3. Air velocity vectors for an inlet air flow rate of 35 m 3 s1.



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such flows is often motivated by the excessive cost associated with or the technical

difficulties in undertaking direct full- and pilot-scale measurements.

The pulveriser under consideration is one of the three identical units being used in a 330

MW power station boiler and is experiencing high coal reject rates and wear particularly

on air swept components. Details of the simplified pulveriser unit considered for the

Radial position (m)

(m/s)Velocity

Axial

1.510.50-0.5-1-1.5

30.0

25.0

20.0

15.0

10.0

5.0

0.0

-5.0

-10.0

z=3.0mz=2.0m

z=1.0mz=0.5m

Fig. 4. Axial velocity (m s1) of air on a plane x =0, inlet airflow rate=35 m3 s1.

Fig. 5. Coal particle velocity vectors, inlet airflow rate=35 m3 s1, inlet particle flow rate for each of the sizes=2.8

kg s1: (a) 100 Am particles; (b) 500 Am particles.



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present study are reported in Table 1. The simulation work carried out in the present work

is targeted at evaluating the flow field for an optimum mill performance.

3. Model development

3.1. Geometry creation

In the numerical modelling of CFD problems, geometry modelling and grid generation

takes up the significant amount of project time (typically close to 60–70%). To study air

and particle flow distributions in the pulveriser based on design data input, the step-by-

step approach consisted of three aspects, viz., geometry creation, grid generation, and flow

Radial Position (m)

(m/s)Velocity

Axial

1.510.50-0.5-1-1.5

20.0

15.0

10.0

5.0

0.0

-5.0

-10.0

z=3.0mz=2.0mz=1.0mz=0.5m

Radial Position (m)

(m/s)Velocity

Axial

1.510.50-0.5-1-1.5

8.0

6.0

4.0

2.0

0.0

-2.0

-4.0

-6.0

z=3.0mz=2.0mz=1.0mz=0.5m

(a)

(b)

Fig. 6. Coal z -velocity (m s1) on a plane x=0, inlet airflow rate=35 m3 s1, inlet particle flow rate for each of the

sizes=2.8 kg s1: (a) 100 Am particles; (b) 500 Am particles.



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simulation for a given tolerance. Different components fitted in the grinding chamber and

the classifiers have not been considered in the present model. However, the effect of

inclined distributor holes has been taken into account. The three-dimensional space was

discretized in tet rahedral grids. An isotropic view of the geometry and grid system is

shown in Fig. 2. The meshed geometry contained 73,663 nodes and 387,553 cells of

tetrahedral grids. GAMBIT–a FLUENT pre-processor [6] –was used for the geometry

creation and meshing.

3.2. Granular Eulerian–Eulerian multiphase model

Multiphase flow in pulveriser can be modelled using either of the two approaches—

Eulerian–Lagrangian and Eulerian–Eulerian [4,5]. In the Eulerian–Lagrangian approach,

the continuous phase is solved in the Eulerian frame of reference in the same manner as

for single phase and individual particle or bubbles of the discrete phase are trackedtherein. The advantage of this approach is that the dynamics of individual particles

can be assessed. However, with the high concentration of particle and for the large

vessels the tracking process becomes highly memory intensive. In the Eulerian–

Eulerian approach (also called two-fluid model), both discrete and continuous phases

are treated as a continuum and the ensemble averaged equations for individual phases

are solved in the Eulerian frame of reference. Any interaction between the inter-

penetrating phases is accounted for using closure laws (or jump conditions). This

approach does not provide information about hydrodynamics of individual particles

Fig. 7. Coal particle velocity vectors on the walls of pulveriser, with an inlet airflow rate of 35 m 3 s1 and inlet

particle flow rate for each of the sizes of 2.8 kg s1: (a) 100 Am particles; (b) 500 Am particles.



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but it requires significantly less computational resources, which makes it more

effective in simulating large industrial reaction systems. As one of the phases is in

particulate (granular) form, implementation of an Eulerian–Eulerian model needs some

special considerations. Therefore, in this study, a granular Eulerian–Eulerian approach

is used to simulate the multiphase flow hydrodynamics in the pulveriser.

4. Results and discussion

Simulations were carried out for two different coal particle sizes, viz., 100 and 500

Am. It was assumed that turbulent intensity and length scales of order and of airflow are

present at the inlet. The solution was marched on each grid point in the computational

domain, till the maximum residuals for each scalar mass, momentum, turbulence

intensity [4], and length scales were reached to an order 1E

04. All the calculationswere performed using a FLUENT solver [6]. Turbulence was modelled using standard

k – e model for the individual phases. The typical Reynolds numbers used in the

simulation were in the range of 50,000–100,000.

4.1. Air velocity field

Fig. 3 shows air velocity vectors on an arbitrary plane parallel to and passing through

the axis of pulveriser. Because of the inclined nature of the distributors, there was slight

asymmetry in the air velocity vectors. The region between gas and solid inlets was clearly

dominated by two air-circulation patterns on the either side of the pulveriser axis.

However, in the annular region above the solid inlet, the velocity vectors mainly pointedupwards. It is clear from the figure that, except for few isolated regions, the simplified

pulveriser had a good fluid mobility.

While the air velocity vectors in Fig. 3 show a qualitative nature of the flow field,

the axial velocities plotted in Fig. 4 are mainly responsible for the particle carryover.

Axial velocities within the radial region r b1.0 m were predominantly negative, which is

a desired effect as the intention is to expose as many particles to the grinder (located at

the bottom—but not accounted in the current geometry) as possible. However,

exceptionally high velocities in the region 1.0br b1.5 m may be harmful as it may

carryover heavy particles with it without exposing them to grinding action.

Quantitatively, axial air velocities in some regions were as high as 27 m s1 and as

low as 7 m s1 in others.

4.2. Coal particle velocity field

Two different particle sizes (viz. 100 and 500 Am) were simulated using a granular

Eulerian–Eulerian approach. Since the total feed to the simplified pulveriser was about 0.8

Fig. 8. Comparison of measurements with model predictions. (a) Observed actual flow pattern as air emerges from

nozzle rings. Arrows indicate the trajectory of dust laden air streams. (b) Velocity vectors for 100 Am. (c) Velocity

vectors for 500 Am particle streams.



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tons h1, 0.4 tons h1 of each of the categories was introduced from the solid inlet.

Density of the simulated coal particles was 1800 kg m3.

Fig. 5a shows velocity vectors for 100 Am coal particles. A comparison between Figs. 3

and 5a reveals that 100 Am particles closely followed the gas path lines. With this pattern,

it is clear that many of the particles will be carried over with air without being exposed to

the grinder. Corresponding velocity vectors for 500 Am are shown in Fig. 5 b. The flow

pattern for this particle size was significantly different from that of the gas flow. Because

of increased gravitational effect, the velocity vectors within the region of solid entrance

were predominantly pointing downwards. An additional circulation pattern was created in

the left-hand side of plane, which decreased the probability of particle carryover. However,

the upward pointing velocity vectors on the right-hand side are not a good sign, as particles

of this size are highly undesired in the upstream furnace.

Fig. 6 shows plots of axial velocities for the above particles. Even though the velocity

vectors of 100 Am particles qualitatively followed the path of gas velocity vectors;quantitatively, the axial velocity varied between a relatively narrow range of 7.0 to 14 m

s1 (compare with 7 to 27 m s1 for air velocity).

Interestingly, 500 Am showed negative axial velocity even in the region of high fluid

mobility. For example, velocities for z =2.0 m were negative in most part of the left-hand

side of the plane x=0. Particles of this size were, in general, much less mobile than 100 Am

particles with axial velocities varying in the range 5 to 5 m s1.

Finally, Fig. 7 shows coal particle velocity vectors on the outer walls of pulveriser.

Because of their relative vicinity to distributor holes, most particles showed an upward

motion. Furthermore, the effect of inclined distributor holes is clearly visible in these

vector plots.

4.3. Comparison with plant data

Pulveriser is one of the most complex units in power utility equipment and

experimental measurements are often difficult mainly due to accessibility problems. To

avoid these problems, some of the indirect observations made during maintenance of the

mills would provide insights into the wear pattern of the mill and associated

experimental evidences. Some of these indirect experimental observations made in the

actual pulveriser for dust laden airflow pattern are presented in Fig. 8a. These

measurements are carried out mainly with the intention of knowing the actual cause for

wear pattern especially in the vicinity of vane discharge regions and slightly above

portions. Measured data indicated that the major cause for wear pattern is mainly

emanating from high velocity air jets with velocities in the range of 28 to 30 m/s. Thevectors shown in figure indicate the direction of air jet streams. The angle of impact of

air stream with vertical direction is in agreement with angle of vanes in the actual

pulveriser. Additionally, the wear pattern can be higher, if the discrete particles such as

silica are picked from rotating table wheel. As air velocity jets are the primary cause for

wear pattern, the predicted velocity vectors from our multiphase flow modelling of

simplified pulveriser are compared with experimental observations. In general, our

preliminary findings from multiphase flow modelling of simplified pulveriser are

showing similar flow patterns especially at the emerging section of the vane rings (Fig.



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8 b and c). However, further work is warranted in complex pulveriser geometry to

substantiate our preliminary findings.

5. Conclusions

Multiphase flow simulation of a simplified pulveriser is carried out using a granular

Eulerian–Eulerian approach. From our preliminary modelling work, the following

conclusions are drawn:

! Flow field: The flow field within pulveriser is slightly asymmetric. This is mainly due

to inclined air-distributor vanes.

! Airflow pattern: Very high velocity regions are predicted close to the outer walls of

pulveriser. This indicates a strong probability of particle carryover close to the wallsurfaces.

! Particle trajectories: The 100 Am coal particles qualitatively followed the air path lines.

However, the velocity vectors for 500 Am particle deviated significantly from those of

airflow.

! Validity of the model: Good qualitative agreement is found between the model

predictions and plant observations for actual flow field within the pulveriser. However,

it is essential to substantiate these findings further using a complex geometry of the

pulveriser.

Overall, the results presented in the paper form a good basis for the development of a

complex pulveriser model, which could pave the way for refinements and design changesin the mill housing. As a future work, its integration with boiler furnace would provide

more effective design strategies.

References

[1] D.H. Scott, Coal Pulverisers Performance and Safety, No: IEACR/79, IEA Research, UK (1979).

[2] K.D. Tigges, W. Bischoff, T. Steinhage, Ring and roller bowl mills as components of modern firing

technology, VGB Power Technol 11 (1998) 34–45.

[3] G.V. Baldin, V.P. Derazhinski, Investigation of influence of wear and of variation of milling elements on

operation of bowl mills, Thermal Engineering 20 (2) (1973) 79–84.

[4] V.V. Ranade, Computational flow modeling for chemical reactor engineering, Academic Press, New York,

2002.[5] C. Bhaskar, Numerical simulation of turbulent flow in complex geometries used in power plants, Advances in

Engineering Software 33 (2002) 71–82.

[6] FLUENT 6.0 Users Guide, Lebanon, USA, 2002.


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Multiphase Flow Simulation of a Simplified Coal Pulverizer