Influence of additives on energy loss in pipeline flow

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Influence of additives on energy loss in pipeline flow of

flyash mixtures

A. Thomas, Slurry Systems Pty Limited, Australia J. Sobota, Agricultural University, Poland

Abstract

disposal of chemical wastes which are difficult to store on the surface.

This paper presents the results of pipeloop tests on flyash mixtures with additives of brine and acid. Other pipeloop tests on tailings plus flyash, flyash, and flyash with coarse particles are also presented.

1. INTRODUCTION

Huge quantities of flyash from coal burning power stations are produced throughout the world each year. Some flyash is used in industries such as the cement industry but much must be disposed of. Flyash is generally mixed with water and pumped to a disposal area near the power station, generally as a low density slurry (Wright and Brown (1)) or more recently as a high density slurry (Verkerk (2), Venton et al (3), Heywood et al (4)). The latter reference provides a good summary of previous work on both low and high concentration flyash slurries. Another alternative is to dispose of the flyash in mined out voids, either on the surface from open cut mining, or in disused underground mines. The flyash can be mixed with other waste materials. Verkerk (5), Bunn (6) and Verkerk and Lorenzato (7) report on flyash and bottom ash mixtures. Moore (8) investigated mixtures of flyash, gypsum and waste water sludge. Chadra et al (9) and Havlik et al (10) studied the flow of flyash and gypsum mixtures.

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Pipeloop test results for flyash with acidic waste, flyash with brine, and copper tailings with flyash are also reported.

Test data for various slurries are presented to illustrate the pipe flow properties of

1 indicates the range of data presented.

Table 1 Summary of Test Loop Data Slurry Description Pipe Size Densities (kg/m') and Combinations

(mm) Tested

Australian flyash 105 Densities 1532 & 1584 Polish flyash 100 Densities 1213,1408 & 1522 Polish flyash 207 Densities 1213, 1408 & 1522 4 Polish flyash in brine 31 Densities 1254 & 1501 5 Australian flyash + furnace 105 Flyash 90%, furnace ash 10% ash Densities 1454, 1524 & 1574 Polish flyash plus grit Flyash/Grit 100/0, 90/10, 80/20

Densities 1424, 1536 & 1668

7 Polish flyash plus slag 50 0% slag to 12% slag

Densities 1227 to 1365

8 Polish tailings plus flyash 51 Tailings/flyash 100/0, 95/5, 85/15

Densities 1610 & 1620

9 Polish tailings plus flyash 51 9% ash, 6% acid waste plus acid waste Densities 1420, 1500 & 1600

2. PIPELINE FLOW OF FLYASH SLURRIES

2.1 Illustration of typical flyash slurry behaviour

Table 2 Flyash particle size

[ Particle Size (microns) 21.5 10.1 Cumulative % Passing 100.0 | 99.2 96.6 79.5 54.4 | 37.7 | 18.6 In Figure 1 data are shown for a slurry density of 1584 kg/m (66.1% concentration by weight) and 1532 kg/m' (62.3% concentration). Rheology data at these tw concentrations were obtained using a Contraves Rheomat 115 rotational viscometer a shear rates between 56 and 661s! '. Over this shear rate range the data closely

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followed a straight line on a linear plot and were therefore fitted by a Bingham Plastic model with the resulting Bingham parameters given in Table 3.

Table 3 Flyash slurry Bingham Plastic parameters Concentration Slurry Density Yield Stress Plastic Viscosity (Wt%) (kg/m") (Pa) (mPas) 66.1 1584 2.69 46.5 62.7 1532 1.77 22.6

10000

Pressure Gradient (Pa/m) 1000

100 0.1 10 Velocity (m/s)

1584 kg/cu.m 1532 kg/cu.m.... Water - Deposition Locus - Prediction 1584 kg/cu.m —— Prediction 1532 kg/cu.m

Figure 1 Flyash slurry in 105 mm pipe The lower dashed line in Figure 1 is for water. The two full lines through the two sets of slurry data represent predictions assuming homogeneous flow. Turbulent flow predictions use the method of Wilson and Thomas (11) and laminar flow predictions are based on the Buckingham equation (12). Transition between laminar and turbulen flow is assumed to occur at the intersection of the two curves. The test loop included transparent viewing spool through which a stationary bed of solids could be seen at

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low velocities. The thick, near-vertical line in the low velocity region of Figure 1, represents the locus of incipient stationary deposition. Consider the lower density data in Figure 1. The predicted curve fits the turbulent flow data reasonably well at moderate velocities but there is about 10% over prediction at high velocities. approximately coincides with the observed deposition velocity of 0.87 m/s. However at lower concentrations, down to 44%, for which the laminar-turbulent transition velocity was very much lower, the observed deposition velocity was still around 0.80 m/s. This suggests that at a density of 1532 kg/m deposition is marginally influenced by transition to laminar flow. The higher density data in Figure 1 are predicted reasonably well in the turbulent regime. Laminar-turbulent transition is predicted to occur at 1.1 m/s. The three lowest velocity data points suggest possible transition around 1.3 m/s with the data below this velocity appearing to parallel the predicted laminar flow curve. However the higher than predicted pressure gradient in the laminar flow region could also be due to heterogeneous effects in the laminar regime. At velocities below transition there are no turbulent eddies to support the coarsest particles and there is some sliding bed type behaviour with the attendant increased pressure gradient. Deposition was observed under laminar flow conditions at 0.73 m/s.

The flyash slurry behaviour illustrated in Figure 1 is typical of flyash flow behaviour.

above about 1 m/s, homogeneous type flow

behaviour occurs with the slurry curve essentially paralleling the water curve on a logarithmic plot. Deposition, which occurs at around 0.8 m/s in the 105 mm diameter pipe, will occur at higher velocities as the pipe size is increased. However as predicted by Thomas (13), the deposit velocity for this fine particle slurry should only increase approximately as D°.15, where D is the pipe diameter.

As the concentration is increased eventually the laminar-turbulent transition velocity

rises above 1 m/s and laminar flow effects start to influence flow behaviour. Once laminar flow occurs the pressure gradient rises above that expected for homogeneous turbulent flow as illustrated in Figure 1. The increased pressure gradient is due not only to pure laminar flow behaviour but also due to increased sliding bed behaviour in the absence of turbulent support. Predictions of laminar flow behaviour of flyash slurries at high concentrations is not yet fully defined. As outlined first by Thomas (14) the pressure gradient and deposition velocity under laminar flow conditions depend on pipe size. Laminar flow at a particular velocity in one pipe size does not guarantee that laminar flow without deposition can be achieved with the same slurry in a larger pipe size. Pipeline length is also a factor. Under laminar flow operation slow settling of coarser particles with distance along a pipeline will result in a slow increase in pressure gradient as discussed by Aude et al (15). For these reasons high concentration flyash pipelines such as the Bayswater pipeline in Australia (Venton et al (3)) are designed to operate in the turbulent flow regime.

2.2 Additional flyash data

Many different flyash mixtures have been tested at the Agricultural University, Wroclaw, e.g. Sobota and Jodlowski (16), Sobota and Czaban (17), and Sobota et al (18). Figure 2 shows test loop data for a Polish flyash in a 100 mm diameter pipe.

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The behaviour is similar to that measured with the Australian flyash in Figure 1 in a similar sized pipe. The highest density data in Figure 2 (1521.8 kg/m') is of similar density as the lowest density (1537 kg/m') in Figure 1 and their behaviour is similar.

10000

Prossure Gradient (Pa/ m) 1000 100 a4 44x%0. ***" × d

10 0.1 1.0 10.0 Velocity (m/ s) -water O1213 kg/cu.m ×1408 kg/cu.m 4 1522 kg/cu.m

Figure 2 Flyash slurry in 100 mm pipe For example at a velocity of 2 m/s Figure 2 indicates a pressure gradient around 750 Pa/m compared with about 780 Pa/m in Figure 1. At 1 m/s the pressure gradient is around 200 Pa/m in both figures. Laminar-turbulent transition is indicated around 1 m/s in Figure 2, similar to the 0.9 m/s predicted in Figure 1. Figure 3 shows test loop data for the same Polish flyash in a 207 mm diameter pipe.

At the two lowest densities the data show no evidence of deposition at velocities down to 1 m/s. This is consistent with the comments above regarding deposition under turbulent flow conditions. At first sight the highest density data suggest laminar-turbulent transition at around 1.3 m/s. However the transition velocity for this density slurry was around 1 m/s in the smaller 100 mm diameter pipe. The transition velocity should not vary much between these two pipe sizes. It is more likely that the change in behaviour evident below 1.3 m/s in Figure 3 reflects increased heterogeneous behaviour as laminar-turbulent transition influences begin to prevail.

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1000

100

10.0

Velocity (m/ s) -water 01213 kg/cu.m ×1408 kg/cu.m 1522 ka/cu.m Figure 3 Flyash slurry in 207 mm pipe ne pressure ar Venton et al (3) in the 200 mm diameter Bayswater high density ash pipeline in New Wales, Australia, for a somewhat higher density (1640 kg/m') 69.7% concentration flyash slurry. Thus comparisons between Figures 1, 2 and 3 and data from Venton et al (3) indicate that the pressure gradient in the turbulent regime is similar for these flyash slurries from Australia and Poland. Also, at low densities the deposit velocity under turbulent flow conditions is less than 1 m/s, as expected.

3. FLYASH IN BRINE

Saline water is often present in Polish coal and copper mines. Discharge of the water

into rivers is undesireable. If a power station is nearby the brine and flyash can be

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mixed and disposed underground. This solves the problem of river pollution, overcomes the need for surface ash dams, and strengthens the underground voids. Data obtained in a 51.2 mm diameter test loop with mixtures of brine plus flyash are presented in Figure 4. The brine contains 70g NaC1 per litre and has a density of 1021 • The two slurry densities shown represent flyash to brine ratios of 0.5:1 and 1.5:1 and volume concentrations of 0.188 and 0.366.

10.0

Pressure Gradient (k Pa/m) 1.0

0.1

Velocity (m/s)

• Water • Brine 1021 kg/ cum A Ash+Brine 1254 kg/ cum • Ash+Brine 1501 kg/ cum

Figure 4 Flyash in brine in 51.2 mm pipe

The flyash has a solids density of 2367 kg/m". The particle size is summarised in

Table 4.

Table 4 Flyash particle size

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observations. The viscosity of the brine is only about 1.5 times that of water so it should not increase the transition to this degree. Also heterogeneous behaviour woul not be expected in this size pipe at around 2 m/s for this particle size flyash

4. FLYASH WITH COARSE PARTICLE ADDITION

4.1 Flyash plus furnace ash Furnace or bottom ash is generally disposed separately to flyash. One alternative is to sh. The flyash has properties previously summarised in Tables 1 and 2. The particl ize of the furnace ash is given in Table S

Table 5 Furnace ash particle size

[Particle Size (mm) 2.36 0.60 0.30 0.106 0.045

Cumulative % Passing 99.82 98.62 96.72| 93.82 85.62 60.82|16.92

The solids density of the furnace ash was 2340 kg/m giving an overall mixture solids density of 2268 kg/m'. Rheology of the mixture is summarised in Table 6.

Flyash plus furnace ash slurry Bingham Plastic parameters

Concentration Slurry Density Yield Stress Plastic Viscosity

(Wt%) (kg/m') (Pa) (mPas) 65.27 1574 1.98 56.0 55.87 61.48 1524 1454 0.99 0.41 23.4 11.2 Figure 5 shows plots of pressure gradient versus velocity for the three mixture

laminar flow region is about 15% higher than predicted but the data follow the predicted slope. Deposition was observed under laminar flow conditions below 0.7

Comparisons between results for flyash alone (Figure 1) and flyash plus 10% furnace ash (Figure 5) indicate similar behaviour. This is not surprising given the low

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proportion of furnace ash in the mixture and the relatively small percentage (6.18%) of the furnace ash which is coarser than the 300 micron top size of the flyash.

10000

1000

100

10 0.1 1 10 Velocity (m/s) 1574 kg/cu.m 1524 kg/cum 1454 kg/cu.m... Water -Prediction 1574 kg/cu.m - Prediction 1524 kg/cu.m - Prediction 1454 kg/cu.m — Deposition Locus Figure 5 Flyash plus crushed furnace ash in 105 mm pipe 4.2 Flyash plus grit Figure 6 shows pressure gradient data obtained in a 51.2mm diameter test loop for flyash alone and flyash with 10% and 20% grit component. The grit is crushed reject rock from a coal mine and has a particle size from 2 to 5 mm. Solids densities are 2310 kg/m? for the flyash and 2600 kg/m? for the grit. The particle size of flyash is

Table 7 Particle size of flyash

article Size (microns) 250 50 imulative % Passing | 100 he three sets of data each appear to indicate turbulent flow at high velocities ar minar flow at low velocities. The transition velocities deduced from the plot a

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around 1.6 m/s for the flyash alone, around 1.8 m/s for the 10% grit mixture and around 2.5 m/s for the 20% grit mixture. Of course, the fact that laminar flow was obtained in the 51.2mm diameter pipe, does not necessarily mean that laminar flow could be obtained in larger size pipes, as explained by Thomas (14).

10000

Pressure Gradient (Pa/ m) 1000 -*-

100 0.1 1.0 10.0 Velocity (m/ s) A Flyash 100% 1424 kg/ cu.m

Flyash 90% Grit 10% 1536 kg/cu.m • Flyash 80% Grit 20% 1668 kg/ cu.m Figure 6 Flyash plus Grit in 51.2 mm Pipe 4.3 Flyash plus slag Figure 7 shows pressure gradient data obtained in a 51.2 mm diameter test loop for lyash alone and flyash plus slag. The slag comes from a power station and has a maximum particle size of 3 mm and solids density of 2690 kg/m'. The particle size of the flyash is summarised in Table 8. No details of the particle size below 60 microns

is available. Deposition was observed at the lowest velocity for each density.

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Table 8 Particle size of flyash

Particle Size (microns) 500 250 120 80 60 Cumulative % Passing 99.9 99.7 94.1 85.9 75.2

Pressure Gradient (Pa/ m) 1000

•

100 0.1 1.0 10.0 Velocity (m/s) c/cu.m 9% Sia 21330 kg/co% 33 * 1365 kg/ cum 12% Siag Figure 7 Flyash plus Slag in 50 mm Pipe

5. FLYASH pH EFFECTS

When water is added to dry flyash the resulting slurry exhibits a pH value determined by the chemical nature of the flyash and water. The resulting slurry pH can vary widely between flyash slurries from different sources. Tests on an Australian flyash resulting from burning low rank coal indicated an initial low slurry pH immediately after water addition of around pH 2.5. The slurry pH increased slightly with time reaching a pH around 3.75 after 24 hours. This sligh increase in pH is accompanied by an increase in viscosity. For example the yiel

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stress of a 70% concentration slurry sample increased by about 50% from 5.5 Pa to

8.7 Pa. The plastic viscosity increased 2.5 times from 79 mPas to 202 mPas. Lime is added to this flyash slurry prior to disposal to achieve a neutral pH. Addition of lime causes an additional increase in viscosity. For example the yield stress of another 70% concentration flyash mixture increased from 4.9 Pa to 13.3 Pa after lime was added to raise the pH to 7.3. The plastic viscosity increased marginally from 93

In contrast to the Australian flyash discussed above, another Australian flyash from Bayswater power station, exhibits a pH of around pH 5 immediately after water addition but the pH then increases rapidly to around pH 11 after about 2 hours with a subsequent slower increase to pH 12 after about 16 hours and beyond (Venton et al (3)). During the first 2 hours there is little change in rheology but thereafter the viscosity increases significantly. For example for a 72% concentration flyash slurry the yield stress increased from 5 Pa after 2 hours to 13 Pa after 48 hours. During the same period the plastic viscosity increased tenfold from 100 mPas to 1000 mPas.

6. TAILINGS PLUS FLYASH MIXTURES

6.1 Tailings plus Ash Mine tailings may be mixed with flyash to assist disposal. Figure 8 shows test results obtained in a 51.2 mm diameter test loop with mixtures of tailings from a copper mine and flyash.

1.5

Pressure Gradient [kPa/m} 0.5 2 1 /..... Velocity [m/s] 1 Water 2 3

- woda • 1610 kg/m3 × 1610 kg/m3 (5%) • 1620 kg/m3 (15%)

Figure 8 Tailings plus Flyash (5% and 15%) in 51.2 mm Pipe

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are 2760 kg/m are summarised in Table 9.

Table 9 Particle size of tailings and ash

Particle Size (microns) 1000[ 500 | 250| 100 Cumulative % Passing Flyash 100 96 87 67 50 15 Tailings 100 100 99 85 50 28 Increasing the proportion of coarser flyash results in a higher pressure gradient. The total solids concentrations of the three slurries are similar, ranging from 59.4% for the

6.2 Tailings plus ash plus acid waste Disposal of acid wastes from industry is a problem. One solution is to add the waste to mine tailings with flyash added to raise the pH to a neutral level. Figure 9 shows esults obtained in a 51.2 mm diameter test loop for a slurry consisting of tailing nixed with 9% ash plus 6% acid waste at three different slurry densitie

.....

Pressure Gradient [kPa/m] 1.5 0..5 119/1114 Water

1 2.5 - woda • 1420 kg/m3 (9%+6%) × 1500 kg/m3 (9% +6%) • 1600 kg/m3 (9% +6%)

Figure 9 Tailings plus 9% Ash plus 6% Acid Waste in 51.2 mm Pipe

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The tailings and ash are the same as discussed in Section 6.1. Increasing the density

from 1500 to 1600 kg/m' results in a large increase in the pressure gradient. This must

be due to the viscosity increasing sufficiently to result in laminar flow since this increase in density would only result in a moderate increase in pressure gradient if turbulent flow prevailed.

7. CONCLUSIONS

Disposal of flyash from coal burning power stations is an environmental problem

throughout the world. Disposal into mined out voids, left over from either open cut

mining or underground mining, offers an attractive solution. Addition of other wastes to the flyash slurry can solve two disposal problems together. This paper has shown that the pipeflow behaviour of the tested flyash slurries from Australia and Poland are similar. The laminar-turbulent transition velocity rises above 1 m/s once the slurry density approaches 1500 kg/m'. The turbulent flow pressure gradient is predicted adequately by Wilson and Thomas (11) based on the Bingham Plastic parameters determined in a rotational viscometer. The laminar flow pressure gradient is generally higher than predicted by the Buckingham (12) equation. This is most likely due to heterogeneous effects which, as discussed by Thomas (13), will have increasingly greater influence as the pipe size is increased. Slurries composed of flyash in brine exhibit similar behaviour but with a higher transition velocity around 2 m/s at 1500 kg/m density. Flyash-water mixture is a good carrier liquid for grit above 3.5 m/s velocity in a 50 mm pipe and for a slag above 3 m/s velocity because a pressure gradient is the same as for flyash-water mixture. In Polish mine backfilling installations a velocity of order 3 or 3.5 m/s is low. Addition of 15% flyash to tailings increases the pressure gradient by about 100%.

Do paried raine ailings can used a he matisen tient of all beasts Pipe no ala.

for these materials are given.

8. REFERENCES

3. Venton, P.B, Wortley, M.G., and Ward, P. The Bayswater high density fly ash. Heywood, N.I., Mehta, K.B., Poplar, D. and Moore, C.D. Assessment of Flo haracteristics of Pulverised Fuel Ash Slurries at High Concentration, Pro Hydrotransport 12, Brugge, Belgium, Sept 1993, pp353-376. 5. Verkerk, C.G., The transport of flyash and bottom ash in a slurry form, Mech. Eng.

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ash. Proc. 3'd Intl Conf. on Bulk Materials, Storage, Handling and Transportation. 6. Bunn, T.F. Dense phase hydraulic conveying of power station flyash and bottom June 1989, Newcastle, Australia, pp 250-255. 7. Verkerk, C.G. and Lorenzato, A. The mixed regime PFA and BBA slurry disposal system at Malta power station, Proc. Hydrotransport 12, Sept 1993, Brugge, Belgium. 8. Moore, C.D. An analysis of hydraulic transportation of power station residues to disposal sites, Proc. 3d Int Conf on Desulphurisation, I. Chem. E, Rugby, U.K., 9. Chadra, Z., Severa, M., Vlasak, P, Havlik, V, Vyital, J. Flow of dense kaolin and ash slurries, Proc. Hydrotransport 13, Sept 1996, Johannesburg, South Africa. 10. Havlik, V, Vesela, J., Chara, Z, and Vlasak, P. Hydraulic transport and disposal of ash-gypsum slurries, Proc. Hydrotransport 13, Sept 1996, Johannesburg, South 11. Wilson, K.C. and Thomas, A.D. A new analysis of the turbulent flow of non- Newtonian fluids, Can. Jnl Chem. Engng, Vol 63, Aug 1985, pp 539-546. 12. Buckingham, E., A.S.T.M. Proc., Vol 21, 1921, p 1154. 13. Thomas, A.D., Predicting the deposit velocity for horizontal turbulent pipe flow of slurries, Int. J Multiphase Flow, Vol. 5, 1979, pp 113-129.| 14. Thomas, A.D., Pipelining of coarse coal as a stabilized slurry-another viewpoint, Proc. 4" Int. Tech. Conf. on Slurry Transportation, Las Vegas, 1979, pp 196-205. 15. Aude, T.C., Derammelaere, R.H., and Wasp, E.J., Instability of laminar flow in long distance pipelines and solutions, 21" Int Tech Conf. on Coal Utilization & Fuel

Systems, March 1996, Clearwater, Florida. Sb d.P. The studies of the global and local characteristics (

ater-ash mixture flow in horizontal pipe, Archiwum Hydrotechniki, tom XXXI zeszyt 3-4, 1987, pp.277-284. (in Polish).

non-Neoto, ia, Czaban, S. y debeie Transportatio nal Esy Ash ilo iuzoniam and

Hydromechanisierung im Bergbau und im Bauwesen, 4-6.09.1991, Varna, Bulgarien

18. Sonic Joum al or Polish gril aid al al charie nici ormet nieurlo piens,

Volume 3, Issue 2, 2000. 19. Thomas, A.D. The influence of coarse particles on the rheology of fine particle slurries, Proceedings Rheology in the Mineral Industry Conf., Hawaii, March 1999, United Engineering Foundation, N.Y.