abdullah, husain, pong - 2003 - analysis of cold flow fluidization test results for various biomass...

8/12/2019 Abdullah, Husain, Pong - 2003 - Analysis of Cold Flow Fluidization Test Results for Various Biomass Fuels

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Biomass and Bioenergy 24 (2003) 487494

Analysis of cold ow uidization test results for variousbiomass fuels

M.Z. Abdullah, Z. Husain, S.L. Yin Pong

School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

Received 11 July 2001; received in revised form 3 October 2002; accepted 16 October 2002

Abstract

A systematic theoretical and experimental study was conducted to obtain hydrodynamic properties such as particle size

diameter, bulk density, uidizing velocity, etc. for locally available biomass residue fuels in Malaysia like rice husk, sawdust,

peanut shell, coconut shell, palm ber as well as coal and bottom ash. The tests were carried out in a cold ow uidization

bed chamber of internal diameter 60 mm with air as uidizing medium. The height of the chamber could be raised up to

630 mm by ve separate cylindrical rings. Bed-pressure drop was measured as a function of supercial air velocity over a

range of bed heights for each individual type of particle. The data were used to determine minimum uidization velocity,

which could be used to compare with theoretical values. The particle size of biomass residue fuel was classied according to

Gildarts distribution diagram. The results show that Gildarts particle size (B) for sawdust, coal bottom ash, coconut shell

have good uidizing properties compared to rice husk, type (D) or palm ber, type (A). The bulk density and voidage are

found to be main factors contributing to uidizing quality of the bed.? 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Biomass residue fuel; Coal bottom ash; Fluidizing velocity

1. Introduction

Feasibility studies on the ecient combustion by

existing combustors for low-grade fuels having high

ash content, high moisture content and low heating

value have not been encouraging and have indicated

the need for development of suitable uidizing bed

chamber systems [1]. Fluidization bed technology is

a method to achieve higher combustion eciency

of 9698% and about 6080 times higher heat

Corresponding author. Tel.: +604-593-7788x6314; fax: +60-

4-594-1025.

E-mail address: zoeb [email protected] (Z. Husain).

transfer rate in steam boiler rings than those ob-

tained by submerging the heat exchanger in combus-

tion gases of the fuels [2]. It gives a boiler eciency

of 80 85% with added advantage of low combustion

temperature (800900

C) and thus low emission ofNOx. The uidized combustion of low grade fuels

is now a well-recognized technology well adapted in

power plants. The uidizing medium can be air/gas

[3]. The combustion zone in uidized bed boilers op-

erate under special hydrodynamic conditions essential

for good combustion process.

Prachir Barce [4] proposed that laboratory model

uidized bed can be used to simulate the process

and eventually a full-sized boiler can be constructed.

Among the various parameters and properties of the

0961-9534/03/$- see front matter? 2003 Elsevier Science Ltd. All rights reserved.

P I I : S 0 9 6 1 - 9 5 3 4 ( 0 2 ) 0 0 1 5 0 - 2
mailto:[email protected]:[email protected]


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488 M.Z. Abdullah et al./ Biomass and Bioenergy 24 (2003) 487 494

Nomenclature

A cross-sectional area of the bed (m2)

Ar Archimedes no.

di arithmetic mean of the aperture (opening)

of adjacent sieves (m)

dm mean diameter of particle (m)

D diameter of the bed

G1, G2 weights of lled-in and empty cylinders

(kg)

M mean of particles (kg)

Remf Reynolds no. at minimum uidized con-

dition

U supercial gas velocity (m/s)

Umf minimum uidized velocity

xi weight fraction of samples collectedbetween two sieves

void fraction

mf voidage at minimum uidization ve-

locity

viscosity of uidizing medium

(N s=m2)

b bulk density (kg=m3)

f uidizing gas density (kg=m3)

p particle density (kg=m3)

s solid density (kg=m3)

P pressure drop across bed (Pascal)

material that are to be taken into account are particle

size, uidizing gas/air velocity, bulk density which in-

uences the bed behaviour [5,6]. It has been estimated

that 23% of total power generated in this country

can be produced by burning biomass residue as fuel.

Biomass residues such as rice husk, palm ber, co-

conut shell are available in plenty from various mills

and they can be used as fuels more eciently through

uidized bed systems.

2. Theoretical analysis

2.1. Bulk density

Bulk density is the overall density of loose material

including interparticle distance separation. It is dened

as overall mass of material/unit volume. It is measured

simply by pouring weighed quantity of sample of par-

ticles through a funnel into a graduated cylinder and

volume occupied determines the bulk density:

Bulk density b= G1 G2=Vi;

where G1and G2are the weights of lled in and empty

cylinders and Vi internal volume of cylinder (Vi =

1000 ml).

2.2. Solid density

Solid density is dened as density of particles in-

cluding the voids within the individual particle. It is

dened as the weight of the particle divided by the vol-

ume. Solid fuels have to be converted into loose par-

ticle form before making into pellets using a press to

determine solid density. In some cases the material is

poured into a 30 mm diameter female mold and man-

ually compressed to 6 MPa by mechanical pressure.

2.3. Voidage

A mass of material has particles resting on each

other due to force of gravity to form a packed bed.

Depending on the shape of particles and packing char-

acteristics, however, a certain volume of space in be-

tween the particles remains unoccupied, such space is

called voidage. It is dened as

= voidage volume

volume of particles + voids:

voidage is related to solid and bulk density given

by [7]

= 1 b=s: (1)

2.4. Classication of particle size

All types of particles cannot be uidized satisfac-

torily. The particle size is an important parameter for

uidization. The particles can be classied according

to [8]classication based on density and particle size.

Category (C) whose particle size diameter is less


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Fig. 1. Schematic diagram of uidized bed apparatus.

The air velocity in the cylinder of internal diameter

D= 60 mm.

U= Q

4 D2: (9)

The pressure drop in pascal across the bed given by

P= g[H2(m=f 1)] + Z2 Z1; (10)

wherem= 1000 kg=m3 for water.

4. Material preparation and properties

The materials are grouped as conventional solid fu-

els such as coal and non-conventional biomass residue

fuels such as coconut shell, palm bre, peanut shell,

etc. The coal bottom ash is unburnt particles of coal

collected from pulverized coal-red boiler furnace.

Sawdust specimen is collected from logging opera-

tions in Malaysia. The mean diameter is obtained from

the data collected from sieving the sawdust particles.

The mean diameter

dm= 1

xi=di=

1

1:275= 0:786 mm:

Peanut shell in its natural form is not used to carry

out the tests. A grinder is used to produce peanut

shell particles. The mean diameter is calculated as per

Table1.

Coconut shell is also not used in the natural formand same procedure used for peanut shell. Palm ber

is obtained from palm oil mills after removing oil from

the fruit. It is dried and shredded into particles size

and a sieve shaker used to determine the mean diam-

eter. The coal specimen is collected from the thermal

power station at Kapar in Malaysia. It is used as pul-

verized fuel in boilers and specimen collected from the

ball mill. The mean diameter is obtained from sieving

process. The mean diameter of coal bottom ash is also

obtained from sieving process.


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M.Z. Abdullah et al./ Biomass and Bioenergy 24 (2003) 487 494 491

Table 1

Determination of mean diameter of sawdust

Diameter range (mm) Arithmetic mean of Weight collected (g) Weight fractionxi xi=di

aperture di (mm)

0 0.15 0.075 4.5 4:5=340:2 = 0 :0132 0.17630.15 0.30 0.225 2.4 2:4=340:2 = 0 :0071 0.0352

0.30 0.50 0.400 3.2 3:2=340:2 = 0 :0094 0.02500.50 0.85 0.675 25.1 25:1=340:2 = 0 :0738 0.122

0.85 1.00 0.925 180.1 180:1=340:2 = 0 :5291 0.6188

1.00 1.50 1.250 125.1 125:1=340:2 = 0 :3674 0.2939

W= 340:2

xi=di= 1 :275

Table 2

Hydrodynamic properties of biomass residue fuels

No. Name of specimen Mean diameter Bulk density Solid density Voidage HHV Gildartdm (m) (kg=m3) (kg=m3) () (kJ=kg) classication

1. Sawdust 786.5 241 570.3 0.5770 12987.2 B2. Rice husk 1500 129 630.1 0.800 10298.5 D

3. Peanut shell 613.4 250 566.8 0.5590 12004.3 B4. Coconut shell 987.4 430 547.92 0.2152 17289.3 B

5. Palm ber 600 73 407.36 0.8200 14105.9 A

6. Coal 518.8 945 1450 0.3483 32635.8 B7. Bottom ash 475 1188 1400 0.1514 2265.9 B

5. Solid and bulk density of particles

Sawdust is turned into pellets by using a hydraulic

press under a manual pressure of 6 MPa and solid

density determined as the mean of specimen divided

by volume. The solid density of other biomass residue

fuels is calculated in a similar manner. Coal and coal

bottom ash solid densities are calculated by displace-

ment of water using a measuring cylinder. The bulk

densities of the materials are calculated by the method

outlined earlier. The caloric values of the materials

are obtained by a bomb calorimeter. The voidage ofthe materials is calculated from Eq. (1). The results

are given in Table2.

6. Results and discussion

Sawdust of mean particle size 0:786 mm classied

as Gildart B is used in this study. The plots of pres-

sure drop P across the chamber against supercial

velocity gas velocity U is shown in Fig. 2. The in-

tersection of slope of xed bed of voidage mf andhorizontal constant uidized pressure drop is taken as

a measure of minimum uidized velocity Umf. The

value of Umf from graph is 0:22 m=s. The pressure

drop (W=A) = 212 Pa for sawdust for a bed height of

90 mm almost coincides with the experimental value.

Rice husk of mean diameter particle size 1:5 mm

classied as Gildart D is used in the study. The plot of

pressure drop Pacross the chamber against super-

cial gas velocity is shown in Fig. 3. The minimum

uidized velocityUmf= 0:37 m=s. The static pressure

drop P= W=A = 155 Pa for a bed height of 120 mmhas a value slightly lower than the experimental result.

The result for peanut shell is shown in Fig. 4.The

value ofUmf= 0:29 m=s. The value of pressure drop

P= (W=A) = 295 Pa for the bed height of 120 mm,

which is slightly higher than experimental results. It

has been observed that no uidization occurs for palm

ber classied as Gildart A type.

Big voids are immediately noticeable as soon as gas

allows to ow across the bed; the chamber bed begins

to expand. The particles are in the form of powder,


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0

50

100

150

200

250

300

350

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

Gas Velocity (m/s)

PressureDrop

(Pa)

0.22

Fig. 2. Pressure drop vs. supercial velocity for a bed height of 900 mm for sawdust; umf = 0:22 m=s.

0

100

200

300

400

500

600

700

800

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

Gas Velocity (m/s)

PressureDrop

(Pa)

0.37

Fig. 3. Pressure drop vs. gas velocity for a bed height of 120 mm for rice husk; umf = 0:37 m=s.

0

50

100

150

200

250

300

350

400

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

Gas Velocity (m/s)

PressureDrop

(Pa)

0.29

Fig. 4. Pressure drop vs. supercial velocity for a bed height of 120 mm for peanut shell; umf = 0:29 m=s.

interparticle forces are known to be high and impact

of gravitational forces on particles is weak due to its

light weight.

Coal is the conventional fuel used in boilers as fuel

in power house. The mean particle size of 0:518 mm

classied as Gildart B type. The plot of pressure drop

P against the supercial velocity U is shown in

Fig.5.The value ofUmf = 0:51 m=s and static pres-

sure P= W=A =833 Pa for the bed height of 90 mm

which is lower than the experimental value.

Coal bottom ash has a mean diameter particle

size of 0:475 mm and is classied as Gildart B type.


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M.Z. Abdullah et al./ Biomass and Bioenergy 24 (2003) 487 494 493

0

500

1000

1500

2000

2500

3000

0.00 0.20 0.40 0.60 0.80 1.00 1.20

Gas Velocity (m/s)

PressureDrop(P

a)

0.51

Fig. 5. Pressure drop vs. supercial velocity for a bed height of 900 mm for coal; umf = 0:51 m=s.

0

500

1000

1500

2000

2500

0.00 0.20 0.40 0.60 0.80 1.00 1.20

Gas Velocity (m/s)

PressureDrop

(Pa)

0.58

Fig. 6. Pressure drop vs. gas velocity for a bed height of 900 mm for coal bottom ash; umf = 0:58 m=s.

The plots of pressure drop P against supercial

velocity are shown in Fig. 6. The value of Umffrom graph is 0:58 m=s and value of pressure drop

P= (W=A) = 1047 Pa for a bed height of 90 mm

and is the same as obtained from experiment.

7. Conclusion

The results show that Gildarts type B particles

(sawdust, peanut shell, coconut shell, coal and bottom

ash) have good uidizing behavior and good bubbling

bed. It has been found that Gildarts type D (rice husk)

and type A (palm ber) particles have poor uidiza-

tion behavior.

Bulk density and voidage are found to be two main

factors contributing to uidizing quality of a bed. The

larger the bulk density, the better is the uidizing qual-

ity of bed. Voidage gives adverse eect to uidization.

Particles in the form of powder is poor as uidizing

material and gives rise to expanding bed chamber. The

experimental uidized velocity does not coincide with

theoretical values for most of biomass residue fuels

except rice husk.

References

[1] Van Swaaij WPM, Afgan NH. Heat and mass transfer in xed

and uidized bed. Washington DC: Hemisphere Publishing

Corporation, 1986. p. 21929, 33351, 37180.

[2] Harrison D. Fluidization. Science Progress, in Oxford

1974;61:191217.

[3] Delgado J, Aznar MP, Corella J. Biomass gasication with

steam in uidized bed: eectiveness of CaO, MgO and

CaOMgO for hot raw gas cleaning. Ind. Eng. Chem. Res.

1997;36:153543.


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[4] Basu P, Fraser SA. Circulating uidized bed boilers design

and operations. Stoneham, MA: Butterworth-Heinemann,

1991. p. 153, 95, 121, 20116.

[5] Mchirgui A, Tadrist L, Pantaloni J. Inuence of particle-size

distribution on entrainment solid rate in uidized beds.

Powder Technology 1993;77(2):17799.

[6] Howard JR. Fluidized bed technologyprinciples and

applications. Bristol, New York: Adam Hilger, 1989. p. 1 69.

[7] Cheremisino NP, Cheremisino PN. Hydrodynamics of

gassolids uidization. Honston: Gulf Publishing Company,

1984. p. 210, 13761.

[8] Geldart D, Abrahemsen AR. Fluidization of ne porous

powders. AICHE Symposium Series 1981;77:205.

[9] Ergun S. Fluid ow through packed columns. Chemical

Engineering Progress 1952;48:8994.

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