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ISSN# 1946-0198

Volume# 5 (2013)

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Suggested Citation format for this article:

Behera, Snehasis, Sahu, A.K., Das, Sudipta, Senapatil, P.K., Mishra, S.K., 2013, Scale-Up Design and Erosion Studies of Bottom Ash in Pneumatic Conveying System. Coal Combustion and Gasification Products 5, 1-8, doi: 10.4177/CCGP-D-12-00007.1

http://www.coalcgp-journal.org/

I S S N 1 9 4 6 - 0 1 9 8

j o u r n a l h o m e p a g e : w w w . c o a l c g p - j o u r n a l . o r g

Scale-Up Design and Erosion Studies of Bottom Ash in Pneumatic Conveying System

Snehasis Behera*, A.K. Sahu, Sudipta Das, P.K. Senapati, S.K. Mishra

Institute of Minerals and Materials Technology (CSIR), Bhubaneswar-751013, Odisha, India

A B S T R A C T

Pneumatic conveying characteristics and scale-up studies of the bottom ash from three thermal power plants, i.e., M/s Orissa

Power Generation Corporation (M/s OPGC), M/s Indian Metals & Ferro Alloys Ltd. (M/s IMFA), and M/s Jindal Stainless Ltd.

(M/s JSL), were carried out in a pneumatic conveying test rig at the Institute of Minerals and Materials Technology,

Bhubaneswar, Odisha, India. A minimum conveying line inlet air velocity of approximately 18–25 m/s was required for M/s

OPGC, M/s IMFA, and M/s JSL bottom ash. The erosion rate of M/s OPGC and M/s JSL bottom ash was less in cast-iron bend

compared with mild-steel bend, but for M/s IMFA the erosion rate was high and similar for both types of bends. Scale-up

design was used to determine the variation in pressure drop and phase density at constant mass production flow rate in plant

scale. Particle degradation size distribution studies also were carried out after conveying 2 t of bottom ash out of a total

conveying of 16 t for each source of bottom ash.

f 2013 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association

All rights reserved.

A R T I C L E I N F O

Article history: Received 18 May 2012; Received in revised form 30 November 2012; Accepted 8 December 2012

Keywords: bottom ash; pressure drop; velocity; phase density; erosion; degradation

1. Introduction

In thermal power plants in India, 593 Mt of coal is used to

produce 96.74 GW of power, and approximately 207 Mt of total

ash is generated per year (Shah et al., 2005). Because bottom ash

generation is 20% of total ash, approximately 41 Mt of bottom ash

is generated per year. This ash is generally collected in a water-

impounded hopper and sluiced into a storage lagoon in slurry

form. It can be conveyed pneumatically to the intermediate silo

after crushing to ,45 mm and then sent to the user industries for

making building materials and for other construction purposes

(Shah et al., 2005). Erosion of the plant by the conveyed product is

the first problem during installation of a pneumatic conveying

system. To minimize plant erosion, the abrasive products, such as

silica, sand, fly ash, bottom ash, and alumina, must be conveyed at

low velocity. Because the conveying air velocity increases along

the length of a pipeline, the bends at the end of the pipeline are

likely to fail first. While transporting bottom ash pneumatically

through the pipeline, particle degradation takes place when

particles hit the walls. Particle degradation that occurs during

particle–wall collision is dependent upon particle impact velocity,

impact angle, number of impacts, size, the form and material of the

particle, thickness, and deformation of the pipeline walls (Salman

et al., 1992).

This article presents the results of conveying and scale-up

studies of the bottom ash from three power plants in Bhubaneswar,

Odisha, India: M/s Orissa Power Generation Corporation (M/s OPGC),

Jharsuguda District; M/s Indian Metals & Ferro Alloys Ltd. (M/s

IMFA), Choudwar District; and M/s Jindal Stainless Ltd. (M/s JSL),

Jajpur District. Before conveying of the bottom ash, characterization

studies were conducted to examine particle size, particle density,

bulk density, and moisture content. The scale-up studies were carried

out from the results of conveying characteristics of the bottom ash.

Erosion rate studies of mild-steel and cast-iron pipe bends were

conducted at the last bend of the conveying pipeline. Approximate

design equations and their comparison with scale-up design

studies and particle degradation size analysis were made by* Corresponding author. Tel.: +91674 2379359. E-mail: snehasis_behera@

yahoo.com, [email protected]

doi: 10.4177/CCGP-D-12-00007.1

f 2013 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association. All rights reserved.

conveying 16 t of materials for each source of bottom ash to see

how much particle degradation had taken place after 2 t of

conveying.

2. Material Characteristics

2.1. Chemical analysis of bottom ash samples

X-ray fluorescence was used to determine the elemental and

chemical composition of the samples (Table 1). The main chemical

components of a bottom ash were silica (65–68%) and alumina

(24–27%), with lesser amounts of oxides of P, S, Ti, Fe, Ca, Mg, Na,

and K. Other minor elements, such as V, Cr, Ba, Zr, Ni, Sr, Mn, Rb,

and Y, were present in parts per million (Table 1).

2.2. Description of power plants’ ash generation and particle shape

The M/s OPGC, M/s IMFA, and M/s JSL power plants have a total

installed capacity of 420, 108, and 250 MW, respectively. These

plants generate bottom ash rates of 320, 760, and 90 t/day,

respectively. Bottom ash was conveyed in lean slurry form of

approximately 40% concentration by weight in a 150–200-mm-

diameter pipeline, except at M/s IMFA where the ash was piled up in

stockyard and later taken by truck to fill a lowland area. Because the

Table 1

Chemical analysis of bottom ash samples (%ash)

Chemical compound M/s JSL M/s OPGC M/s IMFA

%Vol

SiO2 68.471 64.65 65.784

Al2O3 24.717 25.587 27.536

TiO2 1.281 1.389 1.56

Fe2O3 2.408 4.831 2.218

K2O 0.943 0.859 1.317

P2O5 0.562 0.689 0.798

MgO 0.424 0.306 0.012

Na2O 0.149 0.106 0.094

SO3 0.089 0 0.421

ppm

V2O5 0 1770 0

Cr2O3 0 180 240

BaO 290 0 270

ZrO2 250 130 120

NiO 70 100 120

SrO 100 50 140

MnO 390 400 120

Rb2O 60 0 80

Y2O3 60 0 30

Fig. 1. Particle size distribution of all bottom ash.

Fig. 2. Schematic view of the test rig at Institute of Minerals and Materials Technology, Bhubaneswar, India.

2 Behera et al. / Coal Combustion and Gasification Products 5 (2013)

boiler is stoker coal fired at the M/s IMFA power plant, the generation

of bottom ash was 90% and the remaining 10% was fly ash.

2.3. Moisture content, particle size, and particle and bulk density

All bottom ash samples were dried to bring down the moisture

content to ,1% before charging into the hopper for pneumatic

conveying studies. The particle size distributions of M/s JSL and

M/s OPGC were determined using a particle size analyzer (Malvern,

Worcestershire, UK), and the M/s IMFA bottom ash size distribution

was determined by manual sieving. Figure 1 shows the particle size

distribution of all bottom ash samples.

The particle densities of M/s OPGC, M/s IMFA, and M/s JSL

bottom ash samples were 1734, 1667, and 2086 kg/m3 and the bulk

densities were 803, 750, and 978 kg/m3, respectively.

3. Pneumatic Conveying Test Facility at Institute of Minerals

and Materials Technology (IMMT)

A pneumatic conveying test rig (Figure 2) with a screw air

compressor (0.12 m3/s, 700-kPa maximum capacity) was used for

pneumatic conveying trials of different bottom ash samples. The

rig has a 49-mm bore pipeline, 56-m horizontal length, and 1.79-m

vertical length, and six bends. The test rig is instrumented with

Table 2

Results of all bottom ash carried out in the test rig facility at Institute of Minerals and Materials Technology, Bhubaneswar, India

Bottom ash

source

Pressure drop

(bar)

Air flow rate

(kg/s)

Conveying air

velocity (m/s)Bottom ash

flow rate (t/h) Power (kW)

Volumetric flow

rate (m3/s)

Energy

consumption

(kWh/t)Inlet Exit Solids loading ratio

M/s OPGC 0.8 0.067 17.00 32.1 6.6 1.6 6.46 0.0549 4.04

M/s IMFA 0.4 0.056 18.00 25.1 6.5 1.3 3.05 0.0453 2.34

M/s JSL 0.7 0.098 25.40 42.9 7.37 2.6 8.49 0.0800 3.26

Fig. 3. Erosion of mild-steel bends by pneumatic conveying of OPGC, JSL, and IMFA bottom ash.

Behera et al. / Coal Combustion and Gasification Products 5 (2013) 3

Fig. 4. Erosion of cast-iron bends in pneumatic conveying of OPGC, JSL, and IMFA bottom ash.

Fig. 5. Particle size degradation of M/s IMFA bottom ash. Fig. 6. Particle size degradation of M/s OPGC bottom ash.


pressure gauges, differential pressure transducers, temperature

gauge, rotameters, digitizer, and air flowmeter.

4. Results and Discussion of Pneumatic Conveying of Bottom Ash

Testing of erosion and particle degradation of bottom ash from

M/s OPGC, M/s IMFA, and M/s JSL were carried out in the test rig

at IMMT; results are presented in Table 2. All types of bottom ash

can be conveyed in dilute phase only, where the solids loading

ratio (phase density) falls between 6 and 8. Conveying line inlet air

velocity for M/s OPGC and M/s IMFA was ,18 m/s and 25 m/s for

M/s JSL. Mass product flow was highest for M/s JSL at 2.6 t/h.

Power (3.05 kW) and energy consumption (2.34 kWh/t) were lowest

for M/s IMFA.

4.1. Erosion studies of bottom ash

Erosion in pneumatic conveying bends are dependent on

variables such as conveying velocity, particle concentration,

particle size, particle shape, and bend geometry (Deng et al.,

2005; Mazumder et al., 2008). Erosion studies of all bottom ash

were carried out by measuring the loss of erosion in grams per

tonne of conveying in two types of bend materials, mild steel and

cast iron, and these bends were located in the last part of the

conveying pipeline.

4.2.1. Erosion studies of bottom ash using mild-steel and

cast-iron bends

M/s OPGC, M/s IMFA, and M/s JSL bottom ash were conveyed in

the test rig by using mild-steel and cast-iron bends incorporated at

the last section of the conveying line. Figures 3 and 4 show total

erosion in grams after 2 t of conveying, for a total of 8 t of

conveying of all bottom ash materials at different air velocities,

pressure drops, phase densities, and mass product flow rates for

both cases of cast-iron and mild-steel bends. Erosion in the cast-

iron bend was less than in the mild-steel bend, particularly for M/s

OPGC. For M/s IMFA, maximum erosion occurred in both cast-iron

Fig. 7. Particle size degradation of M/s JSL bottom ash.

Table 3

Scale-up design results of all bottom ash

Bottom ash

source

Pressure drop

(bar)

Air flow rate

(kg/s)

Conveying air

velocity m/sBottom ash

flow rate (t/h) Power (kW)

Volumetric flow

rate (m3/s)

Energy

consumption

(kWh/t)Inlet Exit Solids loading ratio

M/s OPGC 0.8 0.665 18.00 32.1 3.05 7.29 63.83 0.543 8.75

M/s IMFA 0.4 0.519 18.00 25.1 2.94 5.48 28.45 0.423 5.19

M/s JSL 0.7 0.918 25.40 42.9 1.35 4.46 79.45 0.749 17.82

Table 4

Input data for bottom ash

Summary of data Symbol Parameter

Bottom ash

UnitM/s OPGC M/s IMFA M/s JSL

Dp Estimate total pressure drop 0.653 0.46 0.713 bar

Gas T Air temperature 301.00 301.00 301.00 K

R Characteristics gas constant 287.10 287.10 287.10 J/kgK

m Viscosity of air 0.00001846 0.00001846 0.00001846 kg/ms

Pipe Lh Horizontal length 155.00 155.00 155.00 m

Lv Vertical length 25.00 25.00 25.00 m

Nb No. of bends 8.00 8.00 8.00

k Bend loss coefficient 0.15 0.15 0.15

e Pipe surface roughness 0.15 0.15 0.15 mm

d Diameter of pipeline 0.15 0.15 0.15 m

Flow mp Product mass flow rate 7.29 5.48 4.46 t/h

C1 Estimate inlet air velocity 18.00 18.00 25.40 m/s

p2 Exit pressure 1.01325 1.01325 1.01325 barab Sum of entry, exit and bend loss 2.7 2.70000 2.70000

Material rs Particle density 1734.00 1667.00 2086.00 kg/m3

ds Particle diameter 212 3930 242 mm


and mild-steel bends. The erosion rate in the case of the cast-iron

bend for M/s JSL was 2.1 g/t, for M/s OPGC it was 1.0 g/t, and for M/s

IMFA it was 4.8 g/t. These values are less than those for the mild-

steel bend, which were 3.7 g/t for M/s JSL, 3.7 g/t for M/s OPGC, and

5.5 g/t for M/s IMFA. At an average product flow rate of 1.6 t/h and

conveying air velocity of 20 m/s, as were achieved in this pipeline,

the bends would last for 12–16 hours.

4.2. Particle degradation of bottom ash

Sixteen tonnes of bottom ash from each source (M/s OPGC, M/s

IMFA, and M/s JSL) was conveyed pneumatically in the test rig.

Particle degradation size analyses were carried out after conveying

2 t of the total 16 t of bottom ash. The mean particle size of bottom

ash from M/s IMFA (Figure 5), M/s OPGC (Figure 6), and M/s JSL

(Figure 7) was reduced from 3.93 to 0.267 mm, 212 to 127.48 mm,

and 242 to 93.66 mm, respectively. Severe particle wall erosion

took place in M/s IMFA bottom ash.

5. Scale-Up Design

The pipeline transfers the bottom ash from intermediate reception

hoppers to delivery silos. The reception hoppers are generally

vacuum loaded from the duct hopper by a negative-pressure

pneumatic conveying system. In total, the plant pipeline consists of

155 m of horizontal pipelines, 25 m of vertical pipeline, and eight

90u bends. The scaling parameters, i.e., horizontal distance, vertical

lift, number and geometry of bends, and pipeline bore, were taken

into account (see the Appendix for scaling equations). The scaling

process was carried out in two stages. In the first stage, the scaling

was done for pipeline geometry and included pipeline routing and

bends that take into account the relative horizontal and vertical

distances and the number of bends. In the second stage, scaling was

in terms of pipeline bore to give the desired bottom ash flow rate

(Pan and Wypych, 1992; Behera et al., 2000).

5.1. Summary of scale-up design for all experiments at M/s OPGC,

M/s IMFA, and M/s JSL

Table 3 shows the scale-up design results for a plant pipeline of

150-mm bore pipeline, 155-m horizontal length, 25-m vertical

length, and eight bends by using the data obtained in the test

facility at IMMT for M/s OPGC, M/s IMFA, and M/s JSL bottom ash

samples (Table 2).

6. First Approximation Design Methods

The first approximation method for pneumatic conveying

system design is based on air only pressure drop data. These

equations (Mills, 1990; also see the Appendix) are used to find out

the pressure drop and air mass flow rate value by taking into

account the mass product flow rate at 7.30, 5.48, and 4.46 t/h for

M/s OPGC, M/s IMFA, and M/s JSL, respectively, for plant design

calculations. These figures of mass product flow rate have been

taken from scale-up design results. Excel programming input and

output data for all bottom ash are shown in Tables 4 and 5.

7. Comparison between First Approximate Equations and

Scale-Up Design Equations

Figure 8 shows the pressure drop and phase density values of all

bottom ash. The trend lies close to the scale-up design data, but

prediction of phase density or mass product flow rate values was

high compared with scale-up design data because the conveying

air inlet value is same for both the cases.

8. Design Calculations of a Typical Bottom Ash Collection and

Conveying System

A schematic of the entire ash collection and conveying system is

shown in Figure 9. Bottom ash from the boiler is collected in three

hoppers. Ash outlet is through a set of four grated doors. A single roll

crusher below this gate crushes the ash to reduce the particle size to

Table 5

Output data for bottom ash

Symbol Parameter

Bottom ash

UnitM/s OPGC M/s IMFA M/s JSL

ma Air mass flow rate 0.6133 0.5423 0.8966 kg/s

a Pressure loss factor 3.30172 2.80709 1.38173

Re Reynold’s no. 282015 249350 412285

f Pipeline friction coefficient 0.00532 0.00534 0.00525

Leq Equivalent length of pipe 224.05 223.95 224.28 m

DPa Air only pressure drop 0.152 0.121 0.300 bar

Dp Pressure drop 0.65290 0.46025 0.71535 bar

w Phase density 3.3 2.8 1.4 –

C1 Inlet air velocity 18.0 18.0 25.3 m/s

C2 Outlet air velocity 29.6 26.1 43.2 m/s

Fig. 8. Comparison of pressure drop and phase density between scale-up and

design equations in bottom ash conveying.


9 mm for pneumatic conveying. Crushed ash is pushed to a screw

feeder that controls ash feed into the vacuum conveyor (Rastogi,

1997; Shah et al., 2005). The pneumatic conveyor consists of 250-

mm pipe through which materials are transferred under vacuum

to a silo approximately 180 m away. The results of vacuum

conveying 9-mm bottom ash where an ash transfer truck is

interposed in the pipe approximately 30 m from the feed hopper

are given in Figure 9. The truck has a collection tank and a pair of

pipe headers (supply and return) above it. When docked, it

becomes part of the conveying path and collects coarse ash (most

of the total ash), whereas fine ash continues with the return air

passing on to the silo. The 45-mm fine ash design results are

mentioned in Figure 9.

On the top of the silo is a filter/separator that separates the fine

ash and transfers it into the silo. Clean air passes on to the vacuum

source, a twin lobe type mechanical exhauster. Ash from the silo is

taken by trucks to a landfill site on the power plant premises.

9. Conclusions

Bottom ash with a mean particle size of 212 mm, 3.93 mm, and

242 mm from M/s OPGC, M/s IMFA, and M/s JSL thermal power

plants, respectively, can be conveyed in a dilute phase mode

positive-pressure pneumatic conveying system with 50-mm-

diameter pipeline. These bottom ashes were successfully conveyed

with product flow rates of up to 2 t/h at a low conveying line

pressure drop up to 80 kPa; a minimum conveying line inlet air

velocity of 17 m/s is required.

The erosion rate of M/s IMFA bottom ash is very high for both mild-

steel and cast-iron bends. In other cases, cast-iron bends erode less

than mild-steel bends. So, all bends must be reinforced with wear-

resistant materials and, even then, wear can be expected at some bends.

The scale-up design studies show that this product cannot be

conveyed in dense phase. The solids loading ratio value decreases

by increasing conveying distance. For the plant pipeline the

maximum phase density that can be expected with a conventional

blow tank system is approximately 3.

Acknowledgments

The authors thank Prof. B.K. Mishra, Director, Institute of

Minerals and Materials Technology, Bhubaneswar, India, for

permission to publish this paper. We also thank Dr. Vimal Kumar,

Head Flyash Mission, DST, Flyash Utilization Unit, Government of

India, for funding to study the behavior of bottom ash.

References

Behera, S., Das, S., Jones, M.G., Mohanty, R.C., 2000. Scaling up of conveyingparameters using computer aided design from test rig data to commercialdesign parameters for crushed bath. In: International Conference on Powder &Bulk Solids Handling. IMechE, London, U.K.

Deng, T., Li, J., Chaudhry, A.R., Patel, M., Hutchings, I., Bradley, M.S.A., 2005.Comparison between weight loss of bends in a pneumatic conveyor anderosion rate obtained in a centrifugal erosion tester for the same materials.Wear 258, 402–411. doi: 10.1016/j.wear.2004.02.012

Mazumder, Q.H., Shirazi, S.A., McLaury, B., 2008. Experimental investigation ofthe location of maximum erosive wear damage in elbows. Journal of PressureVessel Technology 130, 113031–113038. doi: 10.1115/1.2826426

Mills, D., 1990. Pneumatic Conveying Design Guide. Butterworths, London, U.K.Pan, R., Wypych, P., 1992. Scale-up procedures for pneumatic conveying design.

Powder Handling and Processing 4, 167–172.Rastogi, S., 1997. Pneumatic conveying of bottom ash. Powder Handling and

Processing 9, 370–373.Salman, A.D., Verba, A., Mills, D., 1992. Particle degradation in dilute phase

pneumatic conveying systems. In: Proceedings of the 18th Powder and BulkSolids Conference, Chicago, IL, pp. 337–346.

Shah, S.R., Rastogi, S., Mathis, O., 2005. Applications of dry bottom ash removaland transport for utilization. Fly Ash India, New Delhi, India. http://en.wikipedia.org/wiki/Electricity_sector_in_India, accessed June 2012.

Fig. 9. Design results of the bottom ash collection and conveying system.


Appendix

See Nomenclature table for an explanation of equation symbols.

Scaling equations

C1~_mmatf T1

2:737d2p1m=s ð1Þ

Le~Lhz 2|Lvð ÞzNbLeb m ð2Þ

_mmppp~ _mmptf |Dppp

Dptf

|Letf

Lepp

ð3Þ

_mmppp~ _mmptf

|Dppp

Dptf

|dpp

dtf

� �2

ð4Þ

_mmapp~ _mmatf

|dpp

dtf

� �2

kg=s ð5Þ

P~202 _VV 0 lnp1

p0

� �kW ð6Þ

Dpa~ 1:0z1:34 Y _mm2

atf

d4|105

� �0:5

{1:0

" #|100 kPa ð7Þ

w~m:

ptf

3:6 _mmatf

ð8Þ

y~4 f L

d

� �zX

k

� �ð9Þ

_VV 0~ma

_RR T0

p0m3�s ð10Þ

First approximation design equations

f ~0:001375 1z 20,000e

dz

106

Re

� �13

" #ð11Þ

Leq~Lhz2Lvzbd

4fð12Þ

ma~pd2p1C1

4RTð13Þ

Dpa~ p12z

16

p2|

4fLeqma2RT

d5

� �12

{p2|100 ð14Þ

Dp~Dpa(1za) ð15Þ

Re~4ma

pdmð16Þ

a~w ð17Þ

m~0:006515:

T

273:15

� �1:5

(Tz105:65)ð18Þ

Nomenclature

Symbol (unit) Parameter

C (m/s) Conveying air velocity

d (m) Pipeline bore

D (m) Pipe bend diameter

f Pipeline friction

k Bend loss coefficient (for smooth pipe and 90u bends

with D/d 5 10, k 5 0.1)

L (m) Pipeline length

ma (kg/s) Air mass flow rate

mp (t/h) Mass product flow rate

N No. of bends

p (kPa) Air pressure

P (kW) Power

t (uC) Actual temperature

T (K) Absolute temperature 5 (tuC +273)

V̇ (m3/s) Volumetric flow rate of air

Greek

a Pressure loss factor

b Sum of entry, exit, and bend loss

Y Pipeline friction loss coefficient

D Difference

w Phase density

m (Pa?s) Viscosity of air

e Pipe surface roughness (0.015)

gk Bend losses 5 Nb?k

Subscripts

1 Pipeline inlet, material feed point

2 Pipeline outlet, material discharge point

a Air only

b Bends

e Equivalent value

h Horizontal

min Minimum value

0 Free air conditions

P0 5 101.3 kN/m2

T0 5 288 K

pp Plant pipeline

tf Test facility

v Vertically up


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