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Influence of Particle Size Distribution and Temperatureon Rheological Behavior of Coal SlurryMani Kanwar Singha, Dwarikanath Rathab, Satish Kumarc & Deepak Kumarda Research Scholar, Department of Civil Engineering, Thapar University Patiala, Punjabb Assistant Professor, Department of Civil Engineering, Thapar University Patiala, Punjabc Assistant Professor, Department of Mechanical Engineering, Thapar University Patiala,Punjabd Assistant Professor, Department of Mechanical Engineering, K C Institute of Engg andTechnology, Pandoga, HPAccepted author version posted online: 18 May 2015.

To cite this article: Mani Kanwar Singh, Dwarikanath Ratha, Satish Kumar & Deepak Kumar (2015): Influence of Particle SizeDistribution and Temperature on Rheological Behavior of Coal Slurry, International Journal of Coal Preparation and Utilization,DOI: 10.1080/19392699.2015.1049265

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1

Influence of Particle Size Distribution and Temperature on Rheological Behavior of

Coal Slurry

Mani Kanwar Singh1, Dwarikanath Ratha

2,, Satish Kumar

3, Deepak Kumar

4

1Research Scholar, Department of Civil Engineering, Thapar University Patiala, Punjab

2Assistant Professor, Department of Civil Engineering, Thapar University Patiala, Punjab

3Assistant Professor, Department of Mechanical Engineering, Thapar University Patiala,

Punjab

4Assistant Professor, Department of Mechanical Engineering, K C Institute of Engg and

Technology, Pandoga, HP

Corresponding author: Dwarikanath Ratha E-mail: dnath.ratha@thapar.edu

Abstract

The rheological behavior of coal water slurry of Indian coal is studied using rheometer.

The effect of particle size, solid concentration and temperature on rheology of the coal

water slurry has been investigated. The settlement analysis of various size ranges of coal

particles has been carried out. The particle size less than 75 m is used for analysis of

rheological behavior of coal slurry and it is found that the increase in solid concentration

cause the increase in apparent viscosity of coal-water mixture. Also it is observed that as

the solid concentration increases the coal water mixture converts to non Newtonian fluid.

The rheological behavior of slurry is also analysed by blending the coal samples with

mixture of coarse and fine particles and hence making a bimodal particle size

distribution. The slurry having the bimodal particle size distribution is prepared by

blending the fine particles of 53-75 m with the coarse particles of 106-150 m or 150-

250 m with various proportions. The optimum fraction obtained in bimodal slurry

sample is 30% at which the apparent viscosity is minimum. The change in the flow

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behavior of coal slurries are fitted to rheological models and the model parameters are

calculated.

KEYWORDS: Coal water slurry, Rheology, Non Newtonian fluid, Pseudoplastic fluid,

Herschel-Bulkley model

INTRODUCTION

The efficient utilization of fuel to generate the power at the most economic level became

one of the most significant scientific challenges in the world. The crisis of oil force to

replace the oil by coal as a source of energy and the extensive studies have been started

particularly on coal gasification, liquefaction and combustion. In addition to these

existing extensive studies, research on coal-water mixture preparation and utilization

processes have received much attention due to the use of coal as an energy source. The

concentrated coal water slurry contains 60-70% coal powder which is used as a liquid

fuel for the replacement of oil as a fuel. The concentration of solid particles in slurry

affects the flow behavior of slurry in a pipe line. The maximum concentration of slurry

with appreciable viscosity increases the efficiency of transportation of coal water slurry

in pipe for fuel generation. So it is necessary to study the rheological behavior of coal

water slurry to determine the various rheological parameters. Viscosity is one of the most

important rheological properties of coal water slurry and it is desired to be as low as

possible [2]. So a good understanding of rheological properties of coal water slurry is

essential for design and optimization of the processes and process equipment employed

[13]. But the viscosity of coal water slurry increases with solid concentration in the slurry

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and the stability of the suspension become poor if the viscosity is reduced [8, 14, 12].

Viscosity that affects the fluidity of the coal water slurry primarily depends on the coal

properties [3] and there is a delicate balance between the viscosity, fluidity and the

settlement rate of particles [2]. So it is required to find out the optimum concentration of

coal slurry which can be transported with low cost. Indian coals are low in rank, have fine

grained mineral matter and are also high in ash [10]. Rheological studies of Indian coal

with high ash content are almost non existant [12]. The rheological properties of coal

water slurry depend on type of coal, its concentration, particle size distribution,

temperature etc. Several studies reported by the researchers on the rheological behavior

of coal slurry [1, 5, 11, 16, 20, 22, 24, 25]. Also a number of studies have been conducted

on the effect of particle size distribution on rheology of coal water slurry [4, 16, 17, 19,

21, 23]. The research work has been carried out on rheological properties of South

Australian coal water slurry and conclude that there is an optimum ratio of coarse to fine

particles exists at which the slurry viscosity is minimum [13]. The rheological behavior

of low rank coal water slurry prepared by using normal and hot water dried coal shows

the pseudoplastic behavior in both the cases. Also the apparent viscosity is increasing

with the increase in solid loading in both the cases but the increasing the apparent

viscosity is slow in case of hot water dried coal slurry [9]. The study on coal samples

having different rank shows the volume fraction of heterogeneous mixture is higher

compared to homogeneous mixture and the apparent viscosity is found to decrease from

low to high rank of coal [4]. A very few works has been carried out to determine the

rheological behavior of bimodal coal slurry [18, 15]. Some of the researchers developed

the different rheological models [7, 26] to fit the experimental data with the rheological

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model. Herschel-Bulkley model has the maximum correlation coefficient in comparison

to other rheological models such as power law, Bingham plastic, Casson and Siscom

models respectively [26].

But in spite of all these efforts, there is a definite dearth of information regarding the

rheological behavior of coal-water slurry of Indian coal with high ash content. Very few

literatures are available regarding the analysis of rheological behavior of slurry of Indian

coal by considering all the factors. Almost no work has been carried out to determine the

optimum concentration required for the transportation in pipe line for Indian coal. So the

present work is carried out to add the existing knowledge of rheological behavior of

slurry of Indian coal. The objective of this study is to examine the effect of particle size,

solid concentration and unimodal and bimodal particle size distribution on rheology of

coal water slurry.

EXPERIMENTAL METHODS

The coal is collected from mining of Assam which is used for industry. The coal

particles are crushed with a laboratory size ball mill to the different size fractions. Those

size fractions are used for preparing the samples for experiment. Also the proximate and

ultimate analysis of collected coal sample is done. Table 1 shows the summary of raw

coal analysis.

Particle Size Distribution:

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Since the rheological properties of coal water slurry depend on its concentration as well

as the particle size distribution, the particle-size analysis is essential to determine the

approximate size range of particles present in the slurry. A known weight of coal sample

is taken and washed over a B.S. 200 mesh. Then the coal particles are sieved through a

set of British Standard sieves. The weight of the coal sample retained in each sieve is

measured and the percentage retained is calculated. Fig. 1 shows the particle size

distribution curve of coal sample and it is found that all the coal particles are finer than

500m, 79.64% particles are finer than 150 m and 38.39% particles are finer than 75m.

Experiment On Settlement Of Coal Particles:

It is expected that the settlement of coal particles in coal water slurry should not occur

during the static as well as the dynamic condition. The objective of the static settled

concentration is to find out the maximum achievable concentration by gravity effect.

During transportation of slurry if the concentration of solid approaches to the static

settled concentration, the viscosity and hence the specific energy consumption for

transportation increases. So the settlement analysis is necessary to determine the time

taken by a particular size range of coal particle to settle. The experiments are performed

in a 500 ml graduated cylinder having diameter 50 mm. The coal slurry of 30-50%

concentrations is used and the experiment is conducted for different size range of coal

particles. The interface readings between the settled slurry and free water at the top are

taken for 16 hours. The maximum settled concentration CW max is found by using Eq. 1

maxS

W

S W

WC

W W (1)

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Where WS is the weight of solids in the settled mass and WW is the weight of water

present in the settled mass. There are three different size ranges of coal particle, i.e 53-75

m, 75-150 m, and 150-250 m is considered for settlement analysis. Table 2 shows the

maximum static settled concentration of various size coal particles at various

concentrations. It is found that the fine particle have higher maximum settled

concentration in comparison to the coarse particles. It is also found that the settlement is

almost negligible beyond 10 hours for 53-75 m and for 150-250 m the settlement is

almost negligible beyond 5 hours.

Rheology Experiment

The present study considers two different cases for the study of rheological behavior of

coal water slurry. The first case is the unimodal particle size distribution which considers

the size of the particle between 75-53 m and the second case is the bimodal particle size

distribution which considers the certain percentage of particle of size between 75-53 m

and certain percentage of particle between 106-150 m or 150-250 m. The rheological

properties of coal water slurry are measured using Anton Paar Rheolab QC rheometer.

The temperature of the fluid is maintained constant to 250c during the experiment using a

thermostatically controlled water bath which is connected to viscometer. The coal water

slurry sample is prepared by mixing known amount of coal of size less than 75m with

the deionized water to obtain the required slurry concentration. The slurry is continuously

stirred for 5 to 10 minutes by a glass rod to ensure the homogenization of coal water

slurry and a proper care is taken to avoid attrition and spillage of coal water slurry. The

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required weight of the coal is measured in an electronic balance with least count

0.0001g.

RESULTS AND DISCUSSION:

Effect Of Concentration On Rheology:

The rheological experiments are conducted with solid weight concentration of 30%, 40%,

50% and 60% by weight and the shear rate is applied from 0-600 s-1

for time interval of 2

minutes to measure the apparent viscosity and shear stress at a given shear rate. The flow

curves obtained by the rheological experimentation revealed that the rheological behavior

of coal water slurry is greatly affected by the variation in concentration of solids in the

slurry. Fig. 2 shows the variation of shear stress with shear strain for different

concentration of coal slurry and Fig. 3 shows the variation of apparent viscosity with

shear rate. From Fig. 2 & 3, it is found that the solids concentration had a considerable

effect on rheological behavior of coal-water mixture. As solid concentration is increased,

the apparent viscosity of coal-water mixture increased. The increase in the apparent

viscosity with concentration may be due to increase in particle to particle shear

interactions increase the resistance. The frictional force between the particles becoming

significant and the accompanying resistance is reflected in the increase in viscosity [6,

12].

Fig. 2 also shows that the shear stress is independent to shear rate for 30% concentration

of coal slurry but the apparent viscosity is decreased with increase in shear rate beyond

30% concentration of coal water slurry. This shows the pseudo plastic behavior of fluid at

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higher concentration. It is also found that the decreasing trend of apparent viscosity is

upto the shear rate of 100 s-1

for 40% concentrations and 300 s-1

for 50% and 60%

concentrations and there is almost negligible effect on apparent viscosity for further

increasing the shear rate.

The pseudoplastic behaviour of coal water slurries at higher solids concentration

indicated by the rheograms and viscosity curves may be due to the breakdown of the

slurry structure which is in similar to behavior reported by Mishra et al. [12].

3.2 Effect Of Bimodal Particle Size Distribution On Rheology

Since the objective of slurry transportation is also to transport the maximum

concentration of solid with low viscosity, it is required to find out the optimum

concentration of coal slurry which can be transported with low cost. So the experiment is

conducted to determine the rheological behavior of slurry by blending the coal samples

with mixture of coarse and fine particles and hence making a bimodal particle size

distribution. The bimodal slurry sample is prepared by mixing the coal particle of size 53-

75 m with 106-150 m as well as the coal particle of size 53-75 m with 150-250 m

with various proportions and the experiment is conducted to study the rheological

behavior of coal slurry having over all concentration ranging from 30% to 60%.

Fig. 4(a-d) shows the variation of apparent viscosity with different shear rate for bimodal

slurry having particle size 53-75 m and 106-150 m for various concentrations ranging

from 30% to 60%. Similarly Fig. 5(a-d) shows the variation of apparent viscosity with

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different shear rate for bimodal slurry having particle size 53-75 m and 150-250 m for

various concentrations ranging from 30% to 60%. In both the cases, it is found that the

apparent viscosity of the slurry is decreased with increase in percentage of coarse particle

up to a certain fraction and then increases with increase in percentage of coarse particles.

The optimum fraction obtained from both the cases for various concentrations is 30% up

to which the apparent viscosity decreases. This indicates the bimodal slurry sample

having 30% coarse particle is preferable for transportation in a pipe line.

3.3 Effect Of Temperature On Rheology

The experiment is also conducted to determine the effect of temperature on the apparent

viscosity of coal water slurry. Fig. 6 and 7 shows the variation of shear stress and

apparent viscosity with shear strain for 40% concentration of coal water slurry at

different temperatures. Similarly Fig. 8 and 9 shows the variation of shear stress and

apparent viscosity with shear strain for 60% concentration of coal water slurry at

different temperatures. In both the cases it is found that as the temperature increases the

viscosity of slurry decreases due to increase in kinetic energy of particles as well as

decrease in cohesive force between the liquid molecules.

3.4 Fitting Into Rheological Model

The change in the flow behavior of coal slurries may be fitted to rheological models to

calculate the model parameter. Power law model can be fitted having no yield stress but

Herschel-Bulkley model can be fitted having yield stress in the flow behavior.

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The power law model can be represented as in Eq 2 where as Eq. 3 represents the

Herschel-Bulkley model

nk (2)

n

y k (3)

Where n and k are the model parameters known as consistency index and flow behavior

index respectively and y is the yield stress. The model parameters are determined after

fitting into the rheological model. Table 3 shows the values of the model parameters of

the rheological curves. It is found that Herschel-Bulkley model is fitted well to the slurry

having concentration more than 30%.

CONCLUSIONS

The present paper analysed the impact of particle size and solid concentration on

transportation of coal slurry in pipeline. The rheological behavior of coal water slurry is

found to be shear thinning above 30 % solids concentration with viscosity decreasing

with an increasing shear rate. The rheological study is also conducted to investigate the

effect of solids concentration, fraction of coarse particles in fine particles and

temperature. It is found that the slurry viscosity increased with an increase in solids

concentration and the apparent viscosity of the slurry decreases with increase in

temperature. In order to determine the effect of addition of fraction of coarse particles,

the coarse particles are also added to relatively finer coal particles of size less than 75 m

and it is observed that the slurry viscosity decreased with the addition of coarse particles

until an optimum ratio of coarse to fine particles. The optimum fraction is obtained is

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30% up to which the apparent viscosity decreases. An increase in slurry viscosity is

found beyond the optimum ratio of coarse to fine particles. It is also found that with

increase in temperature the apparent viscosity decreases due to increase in kinetic energy

of particles as well as decrease in cohesive force between the liquid molecules.

The energy required during the transportation of slurry should be minimum and it is

possible when the viscosity of the slurry is minimum. It is found that the bimodal slurry

sample having 30% coarse particle at wide range of concentration possess the minimum

viscosity and is preferable for transportation in a pipe line. Also the settlement of the

particle may be delayed due to presence of coarse particles so that the concentration of

solid will never approach to the static settled concentration and hence viscosity of slurry

will not increase during transportation.

The shear stress-strain rate data are also fitted into the rheological models. Herschel-

Bulkley model is found to be fitted suitably to the shear stress-strain rate data for the

slurry concentration more than 30%.

ACKNOWLEDGEMENT

The author gratefully acknowledges the University Grant commission, Government of

India for providing the financial support for this study.

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Table 1: Proximate and ultimate analysis of coal sample

Proximate Analysis Ultimate Analysis

Parameters Weight % Parameters Weight %

Moisture 3.52 C 44.37

Ash 38.30 O 51.16

Volatile matter 16.60 S 0.391

Fixed carbon 41.58 H 3.028

Mineral matter 42.13 N 1.05

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Table 2: Maximum settled concentration for different size of coal particles at various

concentrations

Size of coal

particles

(m)

d50 (m) Maximum settled concentration

30%

Concentration

40%

Concentration

50%

Concentration

53-75 65 65.8 67.0 70.0

75-150 98 64.22 66.1 69.0

150-250 200 63.2 65.1 68.4

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Table 3: Fitting parameters of the Rheological curves

Solid concentration Name of model

used

Flow behavior index

(n)

Type of flow

30% Power law 1 Newtonian

40% Herschel-Bulkley 0.92 Pseudoplastic

50% Herschel-Bulkley 0.9 Pseudoplastic

60% Herschel-Bulkley 0.8 Pseudoplastic

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Fig. 1. Particle size distribution of coal.

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Fig. 2 Rheogram of coal water slurry at different concentrations.

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Fig. 3 Variation of apparent viscosity with shear rate for different concentration of coal

water slurry.

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Fig. 4 Effect of bimodal particle size distribution of size 53-75 m and 106-150 m on

apparent viscosity of coal water slurry having a) 30 % Concentration b) 40 %

concentration c) 50 % Concentration d) 60 % concentration.

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Fig. 5 Effect of bimodal particle size distribution of size 53-75 m and 150-250 m on

apparent viscosity of coal water slurry having a) 30 % Concentration b) 40 %

concentration c) 50 % concentration d) 60 % concentration.

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Fig. 6 Rheogram of coal water slurry at different temperature for 40% concentration of

coal water slurry.

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Fig 7 Effect of temperature on apparent viscosity of slurry having 40% concentration.

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Fig. 8 Rheogram of coal water slurry at different temperature for 60% concentration of

coal water slurry.

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Fig 9 Effect of temperature on apparent viscosity of slurry having 60% concentration.

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