numerical prediction of erosion zones in the aero-mixture ...numerical prediction of erosion zones...

Contemporary Engineering Sciences, Vol. 11, 2018, no. 48, 2357 - 2369

HIKARI Ltd, www.m-hikari.com

https://doi.org/10.12988/ces.2018.85248

Numerical Prediction of Erosion Zones

in the Aero-Mixture Channel

Amel Mešić

EPBIH Concern, Thermal Power Plant Tuzla, 21. april br.4

Tuzla, Bosnia & Herzegovina

Izudin Delić

Faculty of Mechanical Engineering, University of Tuzla

Univerzitetska br.4, Tuzla, Bosnia & Herzegovina

Nedim Ganibegović

EPBIH Concern, Thermal Power Plant Tuzla, 21. april br.4

Tuzla, Bosnia & Herzegovina

Midhat Osmić



Sead Delalić



Copyright © 2018 Amel Mešić et al. This article is distributed under the Creative Commons

Attribution License, which permits unrestricted use, distribution, and reproduction in any medium,

provided the original work is properly cited.

Abstract

Velocity of solid particles in aero-mixture channel has been recognized as the

most significant factor that has an influence on erosion. In this paper, numerical

study of particle erosion in aero-mixture channel was conducted due to the main

parameters that have the greatest impact. Pulverised coal boiler OB650 is supplied with an aero-mixture (pulverised coal and flue gases) through distribution channels,

2358 Amel Mešić et al.

where the distribution of the same aero-mixture is regulated with a tilt of

regulation dumper. These types of regulation cause the problems that have greater

impact on the internal structure of the same distribution channels. It is concluded

that the best way to determinate that problems is with CFD tool. In this case CFD

tool was used to simulate the flow field inside the channel, using Euler-

Lagrangian mathematical approach.

Keywords: CFD analysis, pulverised coal particles, erosion, aero-mixture

channels

1 Introduction

Pneumatic transport of solid particles represents significant type of multiphase

flow in industrial applications. Majority of these systems was affected with

erosion problems [1-3] and these problems are even more expressed in systems

that include more curved sections. Since the early 1990s, computational fluid

dynamics has been widely used for solid particle erosion prediction in curved

pipes and ducts [4-6], with various analytical, semi-empirical and empirical

models which have been developed. Meng and Ludema [7] provided a critical

review of some of the erosion models that has been developed and found, and

they concluded that there were 28 models that were specifically made for solid

particle–wall erosion. The authors reported that 33 parameters were used in these

models, with an average number of five parameters per model that were used for

description of erosion process and erosion mechanism. Erosion of metals occurs

when solid particles are carried by a stream of gas or liquid impinge against the

metal surface and removed microscopic particles from its surface. As a result of

such process, destruction and failure of industrial equipment have occurred.

Therefore, present study is focused on the erosion analysis of aero-mixture

channel walls. From previous papers, it can easily be seen how CFD is actually

very useful in determination and prediction of various variables of multiphase

flow, which cannot be determinate and predicted with classical methods.

The pulverized coal fired boiler unit type OB 650 was selected as the subject

of investigation. Due to the fact that pulverized coal fired boilers require

homogeneous coal mass distribution between burners and sufficiently low particle

moisture content, in order to maintain plant performance and operating efficiency,

pulverised coal boiler has to have good coal preparation system.

The preparation system was constructed from three main elements low-

emission burners, aero-mixture channels and ventilation mills (type S-36.50),

which are used for drying, milling and spraying of coal dust particles. The feed

rate of coal, amount of flue warm gasses and temperature are variable and depend

from boiler load. The raw coal is fed into ventilation mill parallel with warm flue

gasses recirculated from the outlet of the furnace. As the coal gets milled and

dried, ventilation mill supresses that aero-mixture through aero-mixture channel

and low emission burner to be used as fuel. Under normal operating conditions,

Numerical prediction of erosion zones 2359

the heat in the combustion zone will cause ignition of the all incoming fuel. To

achieve better understanding of distribution of the coal and gas mixture through

complicated mill-ducts, various experimental and numerical research have been

conducted (see, for example, Shah et al. [8], Dodds et al. [9], Vuthaluru et al.

[10,11] and Arakaki et al. [12]). Previous papers have shown that CFD analysis

was used for analysing flow profiles and for designs or redesigns of mill-ducts

with aim to improve the overall performance.

In this case aero-mixture distribution in upper and lower part of burner was

regulated with the tilt of the regulation damper, which allows distribution of aero-

mixture in required ratio. Regulation damper moves manually and fixes at certain

position. The whole system has 21 position. The distance between two positions is

3.14°, so the possible tilt of dumper is 33° on both sides of aero-mixture channel

from the axis (Figure 1). The entire channel, above the separator is divided by a

partition that is located exactly in the middle of the channel. All parts of the

channel are made of steel sheets of the thickness 16mm, but in the place where the

aero-mixture changes its flow, the thickness is adopted to 32mm. The boiler has

six lowemission burners and every burner is consisted of two aero-mixture jets

and three jets of secondary air. Every aero-mixture jets has thirteen tubes of core

air that are sorted like a cross. This type of construction allows deep reduction of

NOx and better regulation and distribution of air and pulverised coal. It is

important to emphasize that plant uses brown coal as a fuel, with the

characteristics given in Table 1. Coal granulation, speed, mass and volume flows

of individual phase’s data, that were used for numerical simulations, were taken

from the submitted Report of the optimization of the boiler unit.

Table 1 - Basic characteristics of used coal.

Characteristics of coal Project values Limit values

Low. heating value 15.000 [kJ/kg] 14.000-18.000 [kJ/kg]

Ash content 25 [%] 15-23.8 [%]

Moisture content 20 [%] 14-22 [%]

Figure 1 shows better the construction of air and aero-mixture channels with

the position of regulation damper. Due to the this kind of regulation, the observed

system has a problem with erosion of the internal structure, and the size of this

problem is generally best explained with the situation in Figure 2, that is taken

during the control of the aero-mixture channels in one of the boilers in Thermal

Power Plant.


Fig.1: Secondary air and aero-mixture channels with regulation damper.

Velocity of the solid particles has been recognized by researchers as the

significant factor that has influence on erosion. Experimental results have shown

that the erosion rate is exponentially proportional to the velocity of the solid

particles or the velocity of the carrier gas that surround the particles [13].

Therefore special attention in this paper is given to this parameters.

Fig. 2: The consequences of the erosive effect in the aeromixture channals.

2 Numerical modeling

Modelling of the geometry for discretization process represents very important

and first step in whole process of numerical modeling and this step isn’t simple at

all. During this process, it is very important to realize the construction from every

unimportant constituents of the construction. Of course, this step requires special

knowledge about that area, and if every important part isn’t considered properly,

as a result these different situations may appear:

₋ Too simplified geometric configuration, where excessive simplifying can undermined the validity of the results

₋ Extremely complex geometric configuration that requires extremely large computing resources for the simulation process.


Of course optimal geometric configuration is the target that will allow us good

discretization process and further analysis. In this case, the whole geometry was

modeled (Figure 5) in CAD package Catia V5, and as it can be seen that the

whole geometry isn’t very demanding in terms of complexity, but the overall

dimensions of the geometry are problem. Best convergence in this case can be

achieved with tetrahedral discretisation, so entire area was discretised with

1.522.007 tetrahedral cells, in case of existing dumper and 1.522.007 tehtrahedral

cells in case of modified dumper. Important thing is that volume discretization of

domain for every simulation is done with same initial conditions and that for

reference basic value is taken 0.05 [m].

Fig. 3: 3D model, mesh and boundary condition of aero mixture channel

Better view of fluid and discretized domain with inlet and outlet regions is

shown in figure 3. The observed problematics is solved with Euler-Lagrangian

approach, in which primary phase is solved by applying the Eulerian model and

secondary or dispersed phase by applying the Lagrangian model. In this case

Eulerian equations are solved for the fluid phase (the carrier gas – in this case air)

and Newtonian equations of motions are solved for dispersed phase (pulverised

coal particles). Each particle can have different properties. Mathematical

definition of the particle size as the injected material in the continuous phase has

an important role in this process due to the fact that velocity of every reaction

depends from the surface contact, so several assumptions needs to be adopted. It

is assumed that all injected particles are correct geometrical bodies of the

spherical shapes and that particle sizes in the injector area are generated with

cumulative distribution function (CDF). In this case Rosin-Rammler's distribution

function is used for description of granuleometric characteristics of pulverised

coal [Eq.1], where the whole particle size interval is divided into the set particles

of certain dimensions.

n

d

dR exp100 (1)

Where is:

n of uniformity, for dust this value ranges between 0.4 and 1.2.

d a representative diameter is defined as the size at which 63.2% of particles


are less then representative diameter.

Representative diameter and parameter of distribution were determinated by

data taken from the sieve analysis (80μm, 200μm, 500μm) of coal dust of the mill

with seventy working hours (Table 2).

Table 2 - Calculated parameter of distribution and representative diameter

Number of working

hours of the coal mill [h] Parameter of

propagation n [/] Representative

diameter d̅ [μm]

70 1,4544 154,554

Diagram on Figure 4 was constructed from sieve analysis of the coal that was

taken after mills with different working hours.

Fig. 4: Sieve analysis for different working hours of ventilation mill [14]

From the Figure 4 it is indicative that deterioration process of striking plates in

ventilation mills have a large impact to the quality of the milling process, and so

on the calculated parameters in Table 3. Boundary conditions for this generated

domain were determinated from the experimental data, and overview of all

essential boundary conditions is given in Table 3.

Table 3 - Overview of boundary conditions

Type

magnitude

Aer

o-m

ixtu

re v

elo

city

[m/s

]

Pa

rtic

le v

elo

city

[m

/s]

Ma

ss f

low

of

pa

rtic

les[

kg

/s]

Av

era

ge

dia

met

er o

f

pa

rtic

les

[μm

]

Pa

ram

eter

dis

trib

uti

on

of

pa

rtic

les

[/]

Inle

t fl

uid

tem

per

atu

re

[oC

]

Ta

ng

enti

al

coef

fici

ent.

rest

itu

tio

n o

f th

e

pa

rtic

les

[/]

No

rmal

coef

fici

ent

of

rest

itu

tio

n p

art

icle

s [/

]

Inlet bound. 23 23 6,83 154,554 1,4544 195 / / Wall bound. / / / / / / 0,9 1


3 Results and discussion

In this part of the paper, simulation results of the flow aero-mixture through the

channel are shown in the two different views. Secondary phase (solid) is shown

in the form of a 3D trajectory, describing the coal pulverised particles in the feed

channel, while the primary phase (fluid) is shown in the form of the scalar

velocity distribution in the plane of the longitudinal section channel (Fig. 5).

Fig. 5: Flow of primary and secondary phases in the channel

Vector velocity field (Figure 6) shows the impact of the regulation dumper on

the aero-mixture dispersion in the channel. The regulation dumper separates the

flow into two parts where a larger proportion of 70% is in the upper part, and less

than about 30% in the lower part of the channel. By increasing the inclination of

the regulation dumper, the flow through the channel is reduced, and the fluid flow

is concentrated along the channel walls. This increases the flow rate and the

particles are directed towards the zone along the channel wall.

Fig. 6: Distribution of the vector field velocity in the channel


Figure 7 shows particle trajectories of a smaller diameter and the particles of a

larger diameter. Different diameters represent different efficiency of mill

operation, the time after the overhaul and the time before the overhaul of the mill.

During the exploration the efficiency of coal pulveriser in the mill decreases,

Figure 2.

Fig. 7: Particle trajectories in the channel (fine granulation and coarse granulation)

It can be noticed that the flow of the particles with a smaller diameter (Dp =

103μm) has a tendency to follow the dominant flow of fluid that follow the

curvature of the channel, while the particles of a larger diameter (Dp = 310μm)

particles do not follow the dominant fluid flow and strike the channel wall. So, by

the increase in particle diameter, the concentration of particles, that have more

aggressive contact with the wall, increases. Further movement in the inclined part

of the particle takes different positions. Fine particles continue to follow the

geometry of the channel, while particles with a larger diameter due to the impact

on the wall of the channel change the direction of movement. From the aspect of

the durability of the channel construction, this effect is negative due to the angle

under which the particles strike from the walls of the channel. From the enclosed

pictures, two distinct types of erosion can be clearly seen; it is impact (acute angle

erosion) and abrasive (erosion of obtuse angle) erosion.

The size of the erosive damage depends on the influence parameters and the

mechanical characteristics of the channel wall material. Influential parameters are

primarily angle of impact particles on the wall, particle velocity, particle size as

well as the shape and density of the particles.

By analyzing the trajectories of particles in the previous figures we can deduce

where zones of erosion will appear. These are primarily zones with a pronounced

erosion speed as shown in Figure 8. The highest level of erosion is expected in the

area of the highest speed, right after the dumper on the outer wall of the channel

where his geometry changes, which is the location of the bends. Some of the most


important influential parameters for the formation of erosion are the angle of the

impact, changes geometry, high speed and large diameter particles. The smaller

diameter particles are carried by fluid flow and follow the channel geometry

predominantly without touching the wall. The main ways of reducing erosive

wear are: eliminating particles with a larger diameter, changing the angle of

impact on the surface, reducing relative fluid velocity, choosing a suitable

material, and changing the surface of the material in the form of improving its

characteristics.

Fig. 8: Areas with possible erosive action Fig. 9: Modified geometry of the

of the particles regulation dumper

By changing the size and the position of the regulation dumper can be

influenced to the main parameters, what causes material erosion. Primarily, it was

considered to change the angle of impact of the particle on the surface and reduce

the relative velocity of the fluid. Geometric modification (Figure 9) is performed

under the condition that the ratio of the particle distribution to the upper and lower

channels remains as in the first case. Increasing the flow section to the upper

channel would reduce the local resistance generated by turning the manual flap, so

that it would significantly improve the current image. The most obvious influence

of the proposed modification of the control surfaces is obtained by analyzing the

velocity field of particles, with speeds greater than 30 m/s (Figure 10), where

there is an evident reduction in the zone of the increased speeds.

This increases the number of particles without the impact of the blade and

decreases the number of those that hit the channel wall. It allows keeping the

speed below the critical speed level to a much greater extent. In this way, zones

with high speeds are reduced or completely prevented.


Fig. 10: Impact of changing the geometry of the control blade to the fluid flow

Figure 11 better represents velocity values on the cross sectional axis immediately

after regulation blade, before and after modification. In the first case, the

maximum value of the speed is 43.2 m/s along the outer wall of the channel, while

after the change the maximum speed was reduced to 33.1 m/s along the inner part

of the channel.

Fig. 11: Distribution of the velocity in the cross-section of the channel before (V1)

and after the modification (V2)

The post-modification state has an area around the exterior wall that is fully

degraded at high speeds. The consequence of this is the smaller possibility of

erosion of particles on the canal wall in this area.

This velocity distribution allows the fluid to be distributed in the horizontal part of

the channel uniformly across the transverse section, primarily in the lower

channel, Figure 12. In the upper channel, the main flow in the center of the channel is formed away from the walls, which significantly reduces the possibility of abrasive erosion in the final part of the channel.


Fig. 12: Distribution of fluid velocity in the cross section of the horizontal part of

the channel before and after modification

4 Conclusion

The erosion of the boiler plant is directly related to the quality of coal burning in

boilers of thermal power plants. The paper presents a CFD analysis of the

possibilitys of reducing particle erosion in the aero-mixture channel. Numerical

simulations represent a fast and cost-effective way of predicting erosion in aero-

mixture channels and could be used before their implementation. The approach

presented in this paper enables the detection of locations most exposed to the

erosive effect of coal particles as well as finding the reason for its occurrence.

Based on this, it was proposed to change the blade geometry in order to reduce the

two main factors (particle velocity and angle of impact of the particle) for erosion.

The numerical solution to the problem clearly indicated the significant

improvement of the flow conditions in aero-mixture channels. Once again,it is

proved that the numerical simulation can now be considered as a standard,

modern method for solving problems of this type and that the numerical

simulation does not represent a one-way process but, alternating and iterative. On

the one hand, the conducted numerical analysis clearly showed the extent and

complexity of the analyzed phenomena, on the other hand it has also pointed out

to the directions of future research with the aim of innovating and improving the

multiphase flow image and thus the life of the plant. Of course, the analysis of the

different granulometric and chemical composition of coal on erosion in the aero-

mixture channel is particularly emphasized.

Acknowledgements. The research was made with financial support from

Federation Bosnia and Herzegovina represented by Ministry of Education and

Science and Education of The Federation Bosnia and Herzegovina for the purpose

of implementation of the Federal Target Program “Support researches and

development on priority directions of the science for the 2017 year”.


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Received: June 7, 2018; Published: June 27, 2018
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numerical prediction of erosion zones in the aero-mixture ...numerical prediction of erosion zones...

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