Indian Jou l11al of Engineering & Materials Sciences Vol. 7, Febl1lary 2000, pp. 1-7

Characteristics of coal ash and their role in hydraulic design of ash disposal pipelines

A Biswas", B K Gandhib, S N Singh" & V Seshad ri"

"Department of Applied Mcchanics, Ind ian Institute of Technology, New Delhi I 10 a 16, Indi a hMechani cal Engineering Department , SGSITS, Indore 452 003, India

Received 3 .II/lie 1999; accepted 10 Jalluary 2000

The hydrauli c design of ash di sposal pipelines is strongly dependent on the properties of ash as well as the rheological behaviour of the ash slurries. The va ri ous solid properties of Il y ash and bed ash coll ected from di ffe rent power pl ants spread across the country have becn in vesti gated. Further, the rheological behaviour of the va ri ous samples has been qu anti­lied over a wide range of solid concent rat ion. The present in ves ti gations show that the properties of different ash sampl cs (both Ily as h and bed as h) vary over a wide range. Similarly, the rheological parameters (yield stress and relative viscosity) of the slu rries at different concent rations ex hibit a wide variation. Thus, the design of ash di sposa l pipeline has to take into account the wide va ri ation in some of the rheo logical parameters in order to opt imise the energy and water consumpt ion.

Power generation in India is, at present , primaril y coal based . It is expected that India alone will be producing approximate ly 100 million tonnes of ash by the turn of the century. At present, bulk of the ash is being transported by slurry pipe lines from the thermal power plants to as h ponds, spread over large areas in the vicinity or the plants. Currently , the desi gn of the ash di sposal pipelines is based on the criteria of reliabi lity with little or no emphasis on the optimisation of the va rious parameters like water and the energy consumpt ion, economi c considerations, etc. The present design procedure. result in excessive wastage of both water and energy. It has become imperative to optimi se the des ign methodology of these pipelines as th is is go ing to remain the pri mary meth od for disposa l of ash for years to come. Hence, it is importan t to ha ve basic knowledge about the mechani sm of transportat ion . In order to optimi se the mechani sm of transportati on system, a detail ed knowledge of the characteri sti cs of fl y ash and bed as h slurries being produced in the country is essenti al. This study aims to generate dat a on the properti es of fl y ash and bed ash, and their lurries over a wide range of so lid concent rat ion so

as to establi sh the feas ibility of transport ing as h at higher concentrati on.

The bas ic parameters for the des ign of any slurry pipeline are the hydrau li c parameters which in clude properties o/" carrier fluid , optimum partic le size, optimum solid concentrati on in the slurry, rheology of the slurry, specifi c grav ity of solids, etc . Rheology of

the slurry has been identified by the researchers as a critical parameter for establishing the pressure drop requirements in slurry pipe lines . Einstein l was the first one to propose a theoretical correlation for predicting the viscosity of equi-sized particles under dilute suspension. Thomas2 has proposed a correlat ion for the es timation of viscosity fo r Newtonian suspensions. Gay ef at. ' have proposed a correlati on fo r pred icti ng viscosity of non-Newtonian fluid s based on pseudo-plastic models. Saraf and Khullar.j have studied the effect of surface characteri stics , namely electro-viscous effect and inter-particle fri ction . Gahlot ef a [. 5 have establi shed the effect of scalping of particles on the rheology of coal-water and zinc tai lings-water slurries. They also observed that the rate of settling was a functi on of particle size, and the static settled concentrat ion increases with the increase in the biggest particle size. Panda ef a l .

6 have established the pressure loss in horizontal pipes for transportation of fl y ash up to 60% concentrati on of solids by weight by correlating it to the rheologica l behav iou r of the slurri es. They have reported that the head loss could be estimated reasonabl y well us ing pressure loss models developed for Newtonian fluids in the range of 20-50% so li d concentration by weight. At higher concentrati ons, the model proposed by Dodge and Metzener given in Govi er and Azi z7, was fOllnd to predic t the head loss reasonably close to the measured va lues. They have also reported that tra nsporti ng fl y as h at higher concentrations is economica l.

. 2 INDIAN J ENG. MATER. SCI., FEBRUAR 2000

It is well known that the properties of the produced ash depend on various factors such as properties of coal combustion process and its efficiency, methodology of ash collection, etc. Thus, fly ash and bed ash from different thermal power plants differ greatly in their characteristics and hence the design of ash disposal pipelines has to take into account these variations. It is therefore necessary to have knowledge of the extent of variation in the properties of fly ash/bed ash and their slurries . Hence in the present studies, ash samples have been collected from different thermal power plants spread across the country and their properties have been analysed so as to establish the extent of variation and their effect on ash disposal systems.

Tests Conducted on Ash Samples

During the sample collection, sufficient care was taken to ensure representative nature of the samples [fly ash (FA) and bed ash (BA)] which were obtained from the following thermal power plants.

(i) Rajghat Thermal Power Station - unit II, Delhi (FA-I and BA-I)

(ii) Indraprastha Thermal Power Station - unit II, Delhi (FA-2 and BA-2)

(ii i) Korba Thermal Power Station (FA-3 and BA-3)

(iv) Dadri Thermal Power Station (FA-4 and BA-4)

(v) Ramagundam Thermal Power Station (FA-5 and BA-5)

(vi) Rihand Nagar Thermal Power Station (FA-6 and BA-6)

(vii) Talchar Thermal Power Station (FA-7)

The samples were transported in leak-proof bags and the following properties of the materials were measured in the laboratory.

(i) Specific gravity

The density of solid particles was determined using the standard pyknometer method.

(ii) Settling characteristics and static settled concentration

The static settled concentration is defined as the maximum achievable concentration of solids due to settling of solids in the slurry under gravity . As the concentration of solids approaches the static settled value, viscosity and hence the specific energy consumption for transportation increases disproportionately. For multi-sized slurries, it is quite impossible to accurately evaluate them analytically.

Many researchers, who have developed correlations for determination of various slurry properties, have used static settled concentration as a reference parameter. Fabien et al.8 have shown that the static settled concentration is a very weak function of initial concentration and is nearly constant beyond 30% initial concentration (by weight). Hence in the present investigations, the static settled concentration has been determined by preparing slurry of intermediate concentration (= 30% by weight) and allowing it to settle in a graduated measuring jar till the level of the settled solids becomes constant. The value of solid concentration in the settled slurry is known as the final static settled concentration. The observations of the slurry level recorded at regular intervals of time after preparing the slurry are used to determine the settling rate of the slurry .

(iii) Particle size distribution (PSO)

The total particle size distribution in the solid material is obtained by two methods namely, sieve analysis and hydrometer analy is . Hydrometer analysis was used to determine the distribution of finer particles (below 75 flm). For a multi-sized sample, both the methods are used in respective size ranges. In the present work, a representative sample of ash is taken and washed with water over a BS 200 mesh (=75 flm) . The washed material is dried in an oven and sieving is done with a set of sieves. Special care is taken to ensure that the sample is properly dried. The sample retained on each sieve is collected and its percentage is calculated following the standard procedure. The particle size distribution of the finer particles « 75 fl m) is determined using the standard hydrometer analysis9

. This method was repeated and the variation in PSD was within ± 2%.

(iv) Rheological behavior of the solid-liquid mixture Viscosity of the carrier fluid is affected

considerably by the presence of solid particles and normally results in higher viscosity for the suspension . Addition of solids in the carrier fluid beyond a certain proportion can make the mixture non-Newtonian . To establish the exact nature of the mixture, the variation of shear stress with shear-rate has been measured over a wide range. The detailed procedure for measurement of rheological properties of the slurries is:

(a) Preparation of the slurry sample-To determine the rheological parameters of slurry at different concentrations, 50 mL of the suspension is prepared by mixing the required quantity of solid and

BISW AS et at. : HYDRAULIC DESIGN OF ASH DISPOSAL PIPELINES 3

distilled water. The solid material is accurately weighed in an electronic type single pan balance having least count of 0 .1 mg. The suspension is mixed gently by a glass rod taking care to avoid attrition of the particles . The suspension is allowed to wet for one hour with intermittent mixing before conducting the tests. This eliminates the possibility of air entrapment in the slurry.

(b) Rheological test set-up and measurements­The Weissenberg rheogoniometer (Model R 18, Make: M/S. Sangamo Control Ltd, U.K.) has been used for the determination of the rheological characteristics of the slurries. The Weissenberg rheogoniometer is equipped with a motor and gearbox drive unit. The drive between the gearbox to the rheogoniometer is by a shaft with universal coupling. The used motor is a 3-phase synchronous motor having a rating of I HP at 415 V running at 1500 revolutions per minute. The gearbox coupled with the motor can be set to run at 60 rotational speeds in equal ratio steps of I : 1.259.

For tests with ash slurry, concentric cylinder type platens have been used for the determination of rheological characteristics of the slurry. These cylindrical platens were 5 cm in diameter and 5 cm long having an annular gap of 2.5 mm. The cup is rotated by a worm and gear assembly. Due to the rotation of the platen , slurry undergoes shearing action. A LVDT type transducer measures the torsional displacement of the bar. Before conducting tests on rheogoniometer, the bob and cup assembly is thoroughly cleaned. Compressed air is passed through

the air bearing of the torsion bar at the required pressure. The reading from the torsion bar transducer is adjusted to zero by suitably operating the zero setting. The gear ratio required to give the desired angular speed of cup is selected at the gearbox of the driving motor. The torsion bar assembly is calibrated using water as the calibrating fluid. The prepared sample of slurry (50 mL) at a particular concentration is mixed and poured into the cup. The bob is then quickly lowered into the cup until the slurry level touches the upper edge of the bob and then the cup is rotated. The slurry inside the cup is subjected to shearing action. The torque transducer unit indicates the torque experienced by the torsion bar holding the bob . Measurements are made at different speeds of rotation of cup. Temperature of the slurry is also recorded for each sample before and after the tests. The room temperature is kept constant during the tests. The sample is stirred between each reading to prevent the effect of settling of solids. Measurements are repeated to ensure the repeatability/accuracy. The maximum variation was observed to be of the order of ±2%. The shear stress ("C) and shear rate (r ) for each

set of measurements are calculated using standard relationships. The range of shear strain rates covered for each sample is 26 -136 S· I. From the above data, the rheological parameters of the slurry are evaluated using the standard procedure. The constituti ve equation for the slurry is assumed to be of the form (Bingham),

.. . ( \)

Table I-Comparison of physical properties of various fly ash samples

SI.. No. Property FA-I FA-2 FA-3 FA-4 FA-5 FA-6 FA-7

I Specific gravity 2.1 8 2.30 1.95 1.99 2.24 2.3 1 1.95

2 Particle size distribution

d~5 98.0 89.0 160.0 150.0 170.0 160.0 150.0

dXIl 48.0 50.0 110.0 104.7 85.5 75.0 42 .9

d' il 23.0 17.5 70.0 60.0 22.0 40.0 15.0

d25 17.3 14.3 30.8 38.3 18.8 13.0 13.8

d lO 14.0 12.0 18.5 22.0 15.0 9.8 12.0

d5 10.8 5.3 16.0 16.8 4.9 7.8 10.1

dW Il 62.1 67.0 91.7 86.4 71.3 98 .7 85.3

3a Static sett led concentration (% by 54.83 58.78 58.69 65.96 56.40 58.40 54.24 weight)

3b Static sett led concentration (% by 35.76 38.27 38.50 49.33 36.6 1 37.80 37.80 volume)

4 INDIAN J ENG. MATER. SCI. , FEBRUARY 2000

where, 1 y = yield shear stress of the Bingham fluid and 11 = plast ic viscos ity of the Bingham fluid.

The va lues of 1 y and 11 are evaluated by the method of least squares using the data of 1 and y. The

relat ive vi scosity 11 R =11/11 w is al so calculated for each sa mple. If 1) = 0, then the slurry is Newtonian in character. Bingham model was found to fit the rheological behaviour of all suspensions at all the tested concentrations.

Results and Discussion

The physical and rheological properties of al l the fly ash and bed ash samples are given in Tables I to 4.

Table I gives the compari son of all the fly ash samples in terms of specific gravity, PSD and static sett led concentration. The variation in spec ific gravity is seen to be from 1.95 to 2.31. The normal va lue of specific gravity of pure fly ash is ex pected to be around 1.6 which implies that the fl y ash produced in our country is relatively much heav ier than the fly ash produced in other countries . For comparison of the PSD, few representative particle sizes, namely d ll with

II = 95, 80, 50, 25, 10, 5 (where Il represents the percentage by weight of the particles which are finer than this size) and dwn (weighted mean diameter) are presented. A comparison of larger size particles (d9S )

shows a vari ation from 89 !-tm to 270 iJ.m where as the smaller size particle (ds) shows a variat ion from 4.9 f..lm to 16.8 f..lm . The median diameter and weighted mean diameter for these samples also vary between 15 -70!-tm and 62. 1 - 98 .7 f..lm respec ti vely. The wide variation in the particle sizes suggests that the design analysis of pipeline for each sample has to be done individuall y to arrive at an optimum design. The variation in static sett led concentration is al so seen to be from 54.24% to 65 .96% by weight. The comparison of dso and ds of di fferent samples with their static settled concentration shows that the coarse sized particu late slurry (FA-5) has hi ghest value of static settled concentration where as a fine particulate slurry (FA-3) has the lowest value. The importance of static settled concentration is that th is is the maximum achievable concentration by gravi ty effects and the optimum so lid concentration (on the bas is of spec ific energy consumption) is usually 5 - 10% lower than this va lue.

Table 2 - Comparison or rheol ogical properti es of various Il y ash samples in terms of relat ive viscosity and yield stress

SI No. Sol id concentral ion FA- I FA-2 FA-3 FA-4 FA-5 FA-6 FA-7 (% by weight)

10 1.2 1 (0) 1.35(0) 1.14(0) 1.1 3(0) 1.3 1 (0) 1.32(0) 1. 81 (0)

2 20 1.80 (0) 2.49(0) 1.42(0) 1.32(0) 1.64(0) 1.60(0) 2.29(0)

3 30 3. 10(0.48) 4 . 20(O . ~6) 1.76(0.05) 1.87(0) 2.28(0. 13) 2.57(0.25) 3.56(0. 35)

4 40 6.07 ( 1.39) 11 .33(7.94) 3.79(0.11 ) 4. I I (0. 10) 4.25(0.38) 4.75 (0.89) 7.92(2.0)

5 50 23.07 ( 14) 33.78(22.4) I 1.86(085) 16.33 (0.93) I 0.45( 1.75) 19.78(2.5) 30.95( 16.9)

Numbers indicate the rel at i ve viscosit y ('lR) and the numbers in parenthesis indi ca te the yield stress (1y) in dyne/mlll1

35

30

Solid cor,centrotion, ~ by weight

Fig. I - Vari:lIioll of relati ve viscos ity with cOllcentration (by weigh t) ror Il y ash

25,-------------------------------,

.... 20

5 ';;-~ 15

"

o

~ ~~I~~~~::~O °'1; Cl~h IH+H Ta!chor fly Clsh ........ Korbe fly o!!h f.++++ Dodri fl y ash - Romcgun:lom fly ash - - Rihond nagol fly I'lsh

10 20 30 40 So!id concentrat ion. :i6 by weigr.t

50 60

Fig. 2 - Variati on or yield stress with concentrati on (by weight ) for Il y ash

· BI SWAS e l al. : HYDR AU LI C DES IGN OF AS H DISPOSAL PIPELI NES 5

The rheo logica l parameters of the fl y ash samples are compared in Table 2 using values of relati ve viscosity and yield shear stress at various solid concentration ranging from 10% to 50% by weight. It is seen that at low concentrat ions, suspension of all these samples behave as Newtoni an fluid with viscos ity increasi ng with increase in concentrati on. For example, at 10% so l id concentration (by we ight), the relati ve viscosity va ries from 1.13 to 1.8 1 and the same vari ati on at 20% so lid concentration (by weight) is from 1.32 to 2.49. At 30% solid concentrati on (by

weight), all the suspensions start behaving as Bingham fluid except the sample FA-5 which has coarse sized distribution of the particles. The relati ve viscosity and the yield stress values for all the suspensions increase sharpl y after 30% concentration (by weight) and the ri se in these values is more steep beyond 40% solid concent rati on (by weight) except for yield st ress of FA-4 and FA-5 samples as these samples are having more coarse sized particles (dso =

70 f..lm and 60 f..lm respecti vely) compared to other samples. They exhibit nearl y Newtonian behaviour

Table 3 - Comparison of physical properties of various bed ash samples

SI No. Property BA- I BA-2 BA-3 BAA BA-5 BA-6

Specific gravi ty 1.86 2.02 1.88 2.02 1.92 2.36

2 Partic le size distribution

d~5 3 10.0 400.0 420.0 570.0 1000.0 550.0

dxo 200.0 255 .3 233 .3 360.0 300.0 280.6

dso 80.0 120.0 80.0 160.0 150.0 140.0

d2S 44.5 73.6 40.0 108. 1 90.5 99.4

dlO 25 .0 24.0 26.0 29.0 33 .0 37.0

ds 22.3 21.3 22.9 8.5 27.5 20.0

dwn 147.5 146.2 140.3 284.0 280.0 294.1

3a Stati c settled concent ration (% by weight) 45 .13 47.89 46.64 49.4 1 50.00 58.68

3b Stati c sett led concentration (% by volume) 30.66 3 1.27 31.7 1 32.59 34.23 37.53

Table 4 - Comparison of rheo logical properti es of various bed ash samples in terms of re lative viscosi ty and yield stress

SI No. Sol id concentration(% by weight) BA-I BA-2 BA-3 BA-4 BA-5 BA-6

10 1.09(0) I. I 1(0) 1.30(0) I. 13(0) I. 15(0) 1.29(0)

2 20 1.20(0) 1.24(0) 1.45(0) 1.32(0) 1.20(0) 1.35(0)

3 30 269(0) 2.36(0) 1.65 (0) 1.99(0) 1.43(0) 2.21 (03)

Numbers indicate the re lati ve viscosi ty (ll R) and the numbers in parenthesis ind icate the yie ld stress (' y) in dyne/m m"

35~--------------------________ --,

30

25

£ ~ 20 . ., j 15 ~

-.; '" 10

5

=~ ~~ohpCr~:t~o o~l~ otlh ~ Teicher fly u:;h ............ Korba fly ash ++-t .... + Dodd fly ash

::::: ~l:~3\J~~~~~ ;~ ~:~

0.20 0.40 060 0 .80 1.00

C Fig. 3 - Variation of re lati ve viscosi ty with ---'- for Ily ash

(C,)"

::f ~

~10

5

c~~,~~~~~~~~~~~~~~ 0.00 0 .20 0.40 0 .60 0.80 1.00

C, i(C.)",

Fi g. 4 - Variation of yield stress with ~ for ny ash (C,.)"

6 INDIAN J ENG. MATER. SCI., FEBRUARY 2000

3.0

2.5

,.,2:0 ~ 0 g

1.5 '> u

~ 1.0 c;

'" 0 .5

0.0

0 10 20 30 40 Solid concentration, ,. by .weight

Fig. 5 - VaIiation of relative viscosity with concentration (by weight) for bed ash

even up to solid concentration of as high as 50% (by weight). The variation of relative viscosity and yield stress are also presented graphically as a function of concentration in Figs I and 2 which shows that the increase in relative viscosity and yield stress for different samples is not identical. Thus, these properties could be a function of other properties like PSD, particle shape, etc. Gahlot et aL. S have shown that in multi-sized particulate slurries, the ratio CJ(Cv)ss (Cv = solid concentration of the slurry by volume and (Cv)ss = static settled concentration of solids by volume) is a useful parameter in predicting rheological parameters. (Cv)ss is dependent on properties of solid particles like, PSD, particle shape and specific gravity, and hence the ratio CJ(Cv)ss incorporates the effect of these parameters . Gahlot et ars observed that both llR and 'ty show more or less a unique dependence on CJ(Cv)ss for various slurries. Hence, the measured values of 'ty and llR for various fly ash samples are plotted in Figs 3 and 4 as a function of this ratio. It is observed from these figures that the variation of relative viscosity and yield stress wi th CJ(Cv)ss is also not unique and hence there must. be some additional factors which affect these properties.

Similar comparative study for the bed ash samples is given in Tables 3 and 4. The specific gravity shows a variation from 1.86 to 2.36 for all the samples . The bigger size particles (d9s) vary from 310 mfl to 1000 mfl and the smaller size particles (d5) vary between 8.5 . - 7.5 flm. The median and weighted mean diameter show a variation between 80 - 160 flm and 140.3- 294.1 flm respectively. The overall size of the particles in the bottom ash samples is much higher than that for fly ash samples for the same power plant.

To study the rheological properties of the bed ash samples, the representative samples have been used only up to solid concentrations of 30% (by weight). For concentrations higher than 30% (by weight), samples were sieved over BS52 mesh (300 flm) and the scalped samples (solid particles < 300 flm) were used. This has been done to prevent migration of the larger particles towards the wall of the cylinder of the viscometer during measurements. Gahlot et ai.s have demonstrated that this procedure does not affect the rheological properties significantly. It is observed from Table 4 that the relative viscosity of each sample increases with increase in solid concentration by weight and all the bed ash slurries behave as Newtonian fluid . However, there is a significant variation in relative viscosity with solid concentration (by weight) as depicted in Fig. 5. It is observed that the variation is non-uniform and the rel ative viscosity of these samples may also be dependent on shape and size distribution of the solid particles.

The comparison of some of the physical properties of fly ash and bed ash samples collected from various thermal power plants shows a large variation in specific gravity, PSD and static settled concentration . The values of rheological parameters at any given concentration also vary over a wide range. This may be attributed to differences in the coal being utilised, the combustion efficiency and collection procedure existing in thermal power plant. It is also seen that the rheology of a sample is a function of solid concentration as well as PSD in the slurry . A fine particulate slurry shows more viscous nature and behaves as Bingham fluid at even low solid concentration (Cw = 30% by weight) compared to a coarse sized particulate slurry which may remain Newtonian till higher concentrations (Cw = 50% by weight). However, due to very complex dependence of rheological behaviour of the suspensions on PSD, it has not been possible to develop unique correlation for its prediction.

Conclusions

The analysis of the properties of fly ash and bed ash from various power plants spread over the country has shown that their physical properties are not identical and their rheology also can not be correlated to a single parameter. A representative rheological model is essential to predict the pressure drop in the pipeline . In the absence of such a model, due to variation in properties of ash produced, most of the designs of ash disposal pipelines are highly

BISWAS et al. : HYDRAULIC DESIGN OF ASH DISPOSAL PIPELINES 7

conservative which leads to excessive energy and water consumption. Currently, most of the power plants are transporting ash at concentrations less than 10% (by weight). The same pipeline is being used for transportation of both fly ash and bed ash. The transportation velocity is relatively higher for bed ash as if is coarse material leading to higher erosion . It is known from literature that at higher concentration, the solid particles are more homogeneously distributed and hence need lower transportation velocity . Therefore, it is feasible to upgrade the current designs without much alteration to transport the ash at concentrations as high as 30% by weight, based on the increase in relative viscosity of the slurry. This will lead to substantial saving in water and energy consumption and also lead to reduced plant maintenance cost. It is well known that the minimum transportation velocity and pressure drop requirements in ash di sposal pipelines are strongly dependent on solid r.roperties and rheological parameters of the slurry .7. Hence, the design of ash disposal pipelines has to take into account the actual values of these parameters in any specific case and use them to optimise the design . The present study

establishes the extent of variation of these parameters in the case ofIndian coal ash.

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2 Thomas D G, J Colloid Sci, 20 (1965) 267.

3 Gay E C, Nelson P A & Armstrong W P , AlChE J, 15 (6) (1969) 815.

4 Saraf D N & KuHar S D, Can J Chem Eng, 53 ( 1975).

5 Gahlot V, Seshadri V & Malhotra R C, A Method for the Ex­perimental Determination of the Rheological Parameters of Multi-sized Coarse Particulate Slurries, International Sympo­sium on Hydraulic Transportation of Coal and Other Minerals (ISHT-88) , Regional Research Laboratory, Bhubneshwar, January 1988.

6 Parida A, Panda D, Mishra P K, Senapati P K & Murthy J S, in Ash ponds and ash disposal systems, edited by V S Raju, V Seshadri , V K Agrawal & Vimal Kumar (Narosa Publi shing House), 1996.

7 Govier G W & Azi z K, The jlow of complex mixtures in pipes (Van Nostrand Reinhold Co., Toronto), 1972.

8 Singh S N, Seshadri V , Charu Fabien & Mi shra R , J Powder Handl Process, 11 (4) (1999) , in press.

9 BS : 1377, Methods of Test fo r Soils for Civil Engineering Purposes, Gr 10 (British Standards Institution, London), 1975 ,39-42.