Technical considerations for pipeline transport of lump coal

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International Conference on Bulk Materials Handling and Transportation; Symposium on Freight Pipelines Wollongong, Australia 6-8 July 1992

Technical Considerations for

Pipeline Transport of Lump Coal

A.D. THOMAS, PhD Manager, N.T. COWPER, BE Managing Director,

SUMMARY Slurry Systems Pty Ltd, Sydney, interest Australia, being the efficient transport of export size cal. largest exporter of cal in the world, has considerable This has prampted • a number of research projects over the past fifteen years and Australia has leac the world in research into the pumping of lump coal. The aim of these projects was to achieve reliable transport of cal in buried pipelines with low pressure gradients and low pipe wear. Technologies for pipeline transport of coal are considered. A new system using a high density Special Vehicle Slurry is summarised and shown to be economically competitive with either road or rail alternatives.

1. INTRODUCTION

Although Australia is the largest exporter of coal in. water or oil at the the world it is facing increasing competition fran pressure gradient should not exceed 2 to 3 times that countries such as South Africa and Indonesia. The of water headloss. recent political changes in the Soviet Union may also mean Japan obtaining increasing amounts of coal fran that country. In Australia, transport charges from Internal pipe wear is a major consideration in slurry pipelines. In steel pipelines the oxide layer which third of overall production costs. Any reduction in the mine to the port are typically one quarter to one pipe by inhibiting the corrosion process. When the develops when steel corrodes serves to protect the export coal price. transport costs has a significant impact on the pipeline transports abrasive slurry, the oxide layer may be continuously removed by the slurry flow which Envirormental considerations are also becoming accelerates the corrosive process. most environmentally attractive solution to overland increasingly important. A buried pipeline offers the erosive pipe Providing the particle settling predilection is low, • wear can be kept within acceptable coal transport. limits. This, together with corrosion control

measures, allows use of ordinary steel pipe. If wear

Long distance transport of coal has been a reality resistant linings must be used the capital cost can Ior over twenty years since the Black Mesa pipeline be doubled. began operation in 1970 in the U.S.A. The Black Mesa pipeline transports 5 Mtpy of coal over a distance of 3. TRANSPORT HECHANTSHS 450 kms but is limited to fine coal with a top size of only 1 mm. Coal exported fran Australia is There are two mechanisms available for transporting typically 50 mm top size and to be carmercially successful in Australia a pipeline systen mist solids in pipes. transport lump coal. 3.1 Turbulent Suspension methods Australia is at the forefront of research into of pumping lump coal and a number of small the particles are maintained in suspension by If the particle settling tendency is sufficiently alternative technologies have been investigated. The the turbulent eddies. The solid particles follow the present paper considers the various technologies for fluid motion. This form of transport results in low the flow mechanisms involved. The paper summarises transport of carse cal with particular emphasis or pressure gradient and minimum pipe wear, especially if the velocity is low. A necessary requirement is the benefits of a new technology using a special turbulent flow. vehicle slurry as a transport media slurry.

2. REQUIREMERIS

There are two major requirements of a successful lump coal pipeline system. The operating velocity should approximates pressure a straight line of slope approaching gradient velocity be in the range 1 to 2 m/s and pumping pressures mist 1.7-2, paralleling the water curve. be low to moderate. A third requirement is the pipeline wear must be minimal. This is automatically satisfied if the first two requirements are met. There are thousands of kilametres of water and oil the pipe invert as with heterogeneous thing in camon: The flow velocity is in the range 1 pipelines throughout the world, and they all have one to 2 m/s. This velocity range results in minimum 3.2 Sliding Bed Transport total transport costs taking into account capital cost (pump station and pipeline) (which decreases Particles too large to be suspended by turbulence are with increasing velocity due to the smaller pipe dragged along by the drag force exerted by fluid diameter increases with increasing velocity due to higher required) and operating costs (which flowing around them. The particles congregate near the bottan of the pipe and move as a saltating or pressure gradient). Io be competitive a coal pipeline sliding bed. Hence the Black Mesa pipeline operates at 1.5 m/s. should also operate in this same velocity range. The flow can be analysed as a sliding bed (1). The

bed of solids is driven along by the axial force due

Not only should the velocity be in this range but the shear force exerted by the fluid flowing across the to the pressure difference across the ends and by the

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solid/solid friction between the bed of solids and top of the bed. Movement of the bed is resisted by FIG. 1 COAL IN WATER

the pipe wall. The force balance between the driving found that a similar analysis applies even when the and resisting forces determines the flow. Wilson PRESSURE GRADIENT (Pa/m) solids are saltating (bouncing) along the bottan 10000 rather than sliding. PIPE DIAM. 300 mm I'he driving force required, and hence pressure gradient required, depends on the sliding friction

1000 50 mm

pressure gradient/velocity plot tends to be near horizontal, except for particles in the transitior size between suspended and sliding bed transport, in which case the pressure gradient/velocity plot has a 2.0 mm pronounced negative slope.

1.5 mm

For lump coal transported as a sliding bed in water the pressure gradients are high as is the pipe wear rate. Pipe wear is concentrated in the bottan zone of 100 1.0 mm: the pipe.

The above discussion concerns sliding bed transport

transported by sliding bed mechanism under laminar where the fluid flow is turbulent. Lump coal is also + Water

flow conditions (2). This occurs if the there is 0.5 mm vehicle slurry. high proportion of fine coal forming a viscous 10

0.1 10

4. FACIORS AFFECTING IRANSPORT MECHANISM

4.1 Coal in Water Export size coal of 50 mm top size (10 mm median exhibits the characteristic flat pressure For cal particles in water the changeover fran bed flow, with the attendant likelihood of pipeline gradient curve indicative of sliding bed, or fixec flow occurs at quite small particle size. Figure 1 suspension type flow to saltating-sliding bed type plugs. In the target velocity range of 1 to 2 m/s the predicted behaviour

Even wear resistant lined pipe lasts only a few

below which a stationary bed of solids forms in the bottan of the pipe. It is desirable to 4.2 Effect of Increased Viscosity slightly above this velocity, typically 0.3 m/s

Figure 1 applies to coal in water. The fine particle of a slurry results in an

Using the theory of Wilson (1) as modified by Thomas it is possible to calculate the proportion of viscosity. Typically a slurry viscosity of around 15

mPas is about the highest which can be achieved before flow tends from turbulent to laminar flow.

suspended by turbulence. At 1.5 m/s the predicter water. Figure 2 shows predicted behaviour at this This is a 15 fold increase above the viscosity of 0.00000068 for 0.1 mn particles, 0.448 for 0.2 mn, percentage of particles travelling as bed load is viscosity. Comparison with Figure 1 indicates the proportion travelling as bed load changes fran being 5.58 for 0.3 mm, and 148 for 0.4 mn. i.e. particle size increases from 0.1 mm to 0.3 m the increased vinod as el ta by raile of 3 to a topsize of 4.5 mn. Whilst this is an insignificant to significant. improvement it is still well below the desired 50 mm The increase in bed load between 0.1 mm and 0.3 mm results in a marked change in behaviour as seen i Comparison between Figures 1 and 2 indicates little Figure 1. The flow behaviour changes fran one benefit results from the higher viscosity when paralleling water behaviour for 0.1 m particle size pumping 50mm top size cal. to one experiencing obvious sliding bed influence at 0.3 mn. Allowing for an operating velocity margin 1.3 Effect of Increased Slurry Density above deposition the preferred operating velocity range of between 1 and 2 m/s is only possible up to about 0.3 mm particle size. The optimm pressure gradient limit of about double that of water places a similar upper limit on particle size. Figure 1 applies to mono-sized particles but also approximates the behaviour of a wide particle size analysis the viscosity of the vehicle slurry is distribution by substituting the median size (so) • For crushed coal the top size (d) is typically about 100 mPas case indicated. assumed constant at 15 mPas for all except the single 5 times the median size, indicating the change in behaviour occurs between top sizes of 0.5 mm and 1.5 A progressive increase in vehicle density fran 1000 mm. The 1 mm top size of the Black Mesa cal falls in kg/nỉ (water) to 1390 kg/n? results in near this range. hamogeneous type flow, as evidenced by the close

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The Vani

FIG. 2 EFFECT OF HIGHER VISCOSITY 5. REVIEW OF PIPELINE TRANSPORT METHODS

10000 PRESSURE GRADIENT (Pa/m) 5.1 Lump Coal in Water (Brute Force Pumping) PIPE DIAM. 300mm within coal washeries and has been used to transport It is technically feasible to pump lump coal at relatively low concentrations in water. It is camon VISCOSITY 15 mPas illustrated in Figure 1, it requires high velocities coal over a few kilometres (4). However, as is

and high pressure gradients.

1000 50 mm Tha high velocity and sliding bed flow mechanism causes very high pipe steel wear rates. The pipe is 5.0 mm typically only lasts a few years. 4.5 mm The high pressure gradients mean a centrifugal pump 100 3.0 mm lock hopper distance. Even a high pressure positive displacement is required about every 500 to 700 m pumping

pressure will only pump about 10 kms type pump of say 10 MPa discharg

1.5 mm Water. 5.2 Stabilised Slurry Transport 10 0.1 1 first reported by Elliot and Glidden (5). Pertuit et The concept of transporting lump coal in a fine cal vehicle slurry under laminar flow conditions was VELOCITY (m/s) 10 al (6) gave the concept the name Stabflo and carried investigations were conducted by Thomas (2). further tests. subsequent

is reduced to less than 0.5 m/s. At a suitable paralleling of the water curve. The deposit velocity operating velocity of 1.8 m/s the pressure gradient develop stabilised slurry transport in 1978. CSIRO instigated a major loop testing program pressure gradient between 2 and 3 times water are twin goals of operation between 1 and 2 m/s at a is 210 Pa/m, some 2.5 times that of water alone. The with water. At suitable concentrations and ratios of The stabilised flow concept involves pumping the lump coal with fine coal at relatively high concentration If the vehicle slurry density is made exactly equal to the density of the lump coal there is no settling properties of the vehicle sher testa homogeneous Like mixture. In particular, in static conditions, the lump coal is locked in the Bingham suspension plastic a operate in hamogeneous flow. tendency and the coarse cal/vehicle slurry will slurry. This is a desirable property. without settling by the yield stress of the vehicle FIG. 3 EFFECT OF SLURRY DENSITY 50mm TOP SIZE, D = 300 mm supports the lumps and they drop to the bottan of the sheared in pipeline flow the yield stress no longer However, Thomas (2) showed that once the mixture is 10000 PRESSURE GRADIENT (Pa/m) coal in water although reduced samewhat because of pipe and are transported by a sliding bed mechanism. This means the pressure gradients are similar to lung VOLUME CONCENTRATION 40% viscosity of the vehicle. the buoyancy effect of the higher density and 1000 100 mPas 708 with fine coal making up approximately 40% of the solids. The resulting vehicle density is around 1180 Typically the total solids concentration is around 1000 predictions for this vehicle density and viscosities kg/cu.m. The dashed curves shown in Figure 3 are 1180the plastic viscosities reported by Duckworth et al of 15 and 100 mPas. The latter value is typical of 1300 that for the same coal in water. However it is still (7). The pressure gradient is reduced to about half 100 1350 + significant sliding bed component suggesting a pipe Furthermore the flat shape of the curves indicates about an order of magnitude higher than water alone.

wear problem.

13901 *water Until recently the researchers of the stabilised carse coal technology were convinced that scale up

pipelines was based on a Bingham plastic model and fran loop test data to larger diameter commercial

10 0.1 10 eliminated pipe wear. slurry vehicle created a fluid boundary layer that that pipe wear was not a problem since the fine VELOCITY (m/s) Scale up based on a Bingham plastic model meant that

pipeline friction loss was inversely related to pipe 87

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diameter. The larger the pipe diameter the lower the pressure drop and hence, the perceived commercial density to near that of the lump cal, i.e. achieve near neutral buoyancy. attraction of the stabilised concept. The required per tonne of cal transport was quoted as All previous technologies have failed because they less than the Black Mesa Pipeline. The basic premise of maintaining coarse coal particles in suspension by have not used high density vehicles. They have either relied on high viscosity (stabilised slurry) or have fine slurry yield stress has proven not valid. ignored the penalties of sliding bed operation (lump Although the predictions of Figure 3 are for cal in water at both high and low concentrations). turbulent flow experience has shown that the head loss is similar stabilised cannercial Pending) utilises a special high density slurry to The SVS (Special Vehicle Slurry) systen (Patent headlosses. This is to be expected since few of the achieve the desired vehicle density of around 1400 particles are supported by turbulence so the sliding kg/cu.m. bed component is similar in laminar flow where there is no turbulent support. The predicted pressure gradient in the velocity range 1 to 2 m/s for a The SVS slurry is specifically formulated to create properties suited for long distance pipelining. The mPas is similar to the measured pressure gradients of vehicle density of 1180 kg/cu.m and viscosity of 100 of 1.35 to 1.45 for neutral buoyancy, low rheology, properties include, slurry specific gravity in range Duckworth et al (7) for stabilised flow operation in this size pipe. shutdown/restart operability characteristics, and non contamination of lump coal. The SVS slurry is One advantage of the Stabflo concept is pipeline in the line and readily restarted without plugging. operability, the pipeline can be shutdown with slurry carmonly used in coal washeries. Magnetic techniques a high density (SG 4.9) material In summary, although stabilised laminar flow can recycling purposes. However even ultrafine magnetite can be used to separate it fran the slurry for too high for economic long distance operation. The result in reduced pressure gradients they are still (95 to 988 minus 53 microns) exhibits some settling tendency in pipeline flow at moderate velocities, transported by a sliding bed mechanism indicating a operating velocity can be low but the lump coal is pipe. This means the required near neutral buoyancy i.e. the magnetite migrates towards the bottan of the the high proportion of very fine coal (408) required. potential pipe wear problem. A major disadvantage is is not achieved over the whole pipe cross section.

6.2 Flow Behaviour

5.3 High Concentration Coarse Coal in Water Recently the pumping of lump coal with minimm fines, vehicle slurry creates a mixture which flows in a The near neutral buoyancy of the lump coal in the in Australia (8). The granular mixture is packed into i.e. typical export coal, has been extensively tested similar manner as fine coal in water. Figure 4 shows the pipe at concentrations approaching the maximm data obtained in a 105 mm diameter pipe. The first data set is for fine cal of top size 0.5 mm at 448 packing density (70 to 758 by weight). The solids are concentration. Also shown is data set for lump coal packed so tightly that a separate stationary bed of solids cannot for and the whole mixture can be purped at very low velocities, less than 1 m/s. The concentrations of 148 and 22%. Above 1.5 m/s the lump researchers in this technology also considered flow coal exhibits similar pressure gradient as the fine coal confirming the similar flow behaviour. This on a Bingham Plastic model. means the 20 years experience with the Black Mesa Because of the low velocity and tight packing of fine coal pipeline is applicable to the SVS lump coal transport system. particles there is negligible turbulent support of The pressure gradients are consequently very high, in particles and the flow mechanism is sliding bed type. FIG. 4 LUMP COAL IN SVS excess of 1000 Pa/m (C.f. Figure 3). Because of the 105mm PIPE capability of operating at low velocity, pipe wear is not as severe as for conventional turbulent flow of PRESSURE GRADIENT (Pa/m) coal. Nevertheless it is still too high to 10000 permit use of buried unlined steel pipe. operability. additional negative on this technology is concentration coarse coal and water batches breaks up the coarse coal leading to pipeline plugs durina and restart. Because of these 1000 problems; high pressure gradient, unacceptable pipe wear rates and inoperability, this technology has LUMP COALIN SVS. effectively been abandoned. SG 1.37 SG 1.39

-CV-22% CV-14%

6. SVS TRANSPORT

6.1 Basic Concept In Section 4 it was shown how for cal particles in 100 - FINE COAL Water water the demarcation suspended flow and sliding bed behaviour occurs at coal top sizes around 1 mm. The goal is to samenow increase this demarcation to around 50 mm to allow econamic transport of lump coal hanogenously turbulent flow. It was shown how increasing desired low pressure gradient and minimal pipe wear viscosity of the suspending medium cannot achieve the 10 0.1 1 10 rate. The alternative was to increase the vehicle VELOCITY (m/s)

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The Variation of Boundarv Friction for Crannlan

Figure 4 illustrates the pressure gradient for lump coal in SVS is less than twice that of water alone. M. Over half of this ($2.4 M) is power costs. The direct unit transport cost is $1.37/t. 6.3 System Description initial capital cost the overall unit transport cost If the annual cost of capital is assumed to be 208 of The SVS transport system consists of two pipelines, is $4.37/t. a main pipeline transporting the lump coal and a return pipeline for returning the special vehicle 7.2 Comparison with Lump Coal in Water slurry. Both pipelines are unlined steel pipe, fully welded and buried. These figures also illustrate the impracticality of At the preparation facility the lump coal is fed by conveyor into an agitated tank containing vehicle slurry. Fron this tank the mixture is fed to a positive displacement type pump and then to the gradient is about five times higher indicating about Although no return pipeline is involved the pressure available concrete or piston type pumps with slide are modified camercially $30 M for pump station cost. With a pipe wear rate of about 20 mm/y the pipeline, even with periodic see bese stan built lack hopper type pos For distances less than about 70 kms only one pump suitably upgraded for continuous will need to be replaced 10 times which, at the above continuous operation. For a 20 year life the pipeline last about two years under a booster pump station every 70 kms. station is required. Greater distances will require impractical burden on system economics. indicated cost of $10.5 M each time places an

facility. The plus 0.5 um material is lewatered in basket centrifuges and conveyed to It the pipeline terminal the slurry enters the over a series of sprayed screens then period timing or the pana be arised i The overall 2.5 times increase in power will also stockpile. The minus 0.5 mm material comprises the rehicle slurry plus any fine coal product resulting 7.3 Comparison with Truck and Rail Costs ron attrition of the lump coal during transport. The inus 0.5 mm stream is split in a ratio depending on Truck and rail costs will vary considerably between ecovers fine attrited coal fran the special vehicle he proportion of attrition products. A bleed stream between these costs in Australia. Typical trucking different cuntries. Here a comparison is made lurry. The majority of minus 0.5 mm coal is ransported as a fines slurry in batch mode and goes tonne. This cost assumes use of existing public costs to transport cal over 50 kms are $6 to $7 per ortion is sent to magnetic separators where the o a fine cal slurry storage tank. The bleed stream roads. increases in truck licence fees to more fully reflect The government is currently proposing ank. The separated fine coal attrition and fine coal agnetite is removed and sent to the vehicle storage about a 3 cnts per tonne-km increase meaning a the damage they do to the roads. This could result in ambination of screen bowl centrifuges and belt press thickened then dewatered using for initial road upgrading or new roads will also add $1.50/t freight increase over 50 kms. Any requirement watered plus 0.5 mm coal on the stockpile. From the The dewatered fine coal then joins the to the trucking cost. Taking these factors int account the trucking cost is about twice the sv ehicle storage tank the vehicle slurry is pumped in he return pipeline back to the preparation facility pipeline costs. Environmentally a buried pipeline system is immensely superior. or recycling. All equipment used is standard coal ashery equipment. In Australia rail freight charges on existing rail major advantage of the SVS system is that since it lines are generally similar to trucking rates. The true rate is probably less than for trucks but rail ther than that required for the initial fill. ses a recycle pipeline the system uses no water freight charges are used to subsidise less profitable operations., Coal companies are asked to •O TRANSPORT COSTS involves the building of a new rail line. The SVS contribute additional capital when coal haulage •1 SVS System 50% of current Australian rail freight rates. lurry alternative offers unit costs approximatel

7.4 Other Considerations

is considered."

sile at the tational tata paner egit is and rail haulage. The direct operating cost is very The SVS coal transport system can compete with road 800 kw. a separation relates to capital charges. However similar capita. low in comparison. The majority of the operating cost

costs are involved for new rail links.

le capital cost is: MS A pipeline is particularly attractive when geographic environmental factors are important. Pipelines Preparation FaciLity Pump stations 21.5 transport in the South Coast area of New South Wales is apod a ten stuf beat toe ty transported by trucks Separation Facility TOTAL CAPTIAL COST $42.0 M coal is transported by rail but because of the steep cluding power (assumed a pipeline even more attractive. distance of a more direct pipeline route. This makes terrain the rail routes are sometimes twice the

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8. CONCLUSIONS

Technical considerations for pipeline transport of 3. Thomas, A.D. Unpublished technologies available compared with the physical been examined and the various • Alexander, D.I Transport Loveridge Mine", method of purping lump coal is to use a high density requirements. It is concluded that the only economic Pipelines, Vol. 4, 1984, PP 235-240.

the lumps. vehicle slurry to provide near neutral buoyancy for 5. Transport of coal at High concentration", and Gliddon, B.J. "Hydraulic

vehicle slurry. The result is an operable pipeline The SVS systen uses a specially formulated patented Solids in Pipes, Cranfield, U.R., 1970. BHRA, Proc. 1st Int. Conf. on Hydraulic Transport of Cranfield, U.R., pp G2/25-56. proven fine cal pipelines. The system uses system with pressure gradients similar to existing separate pipeline to return the vehicle slurry. This 6. Pertuit, P., Tennant, J.D., Lawler, H.I., and Cowper, N.T. "Application of Stabilised Flon means following the initial filling the system uses Particle Coal", Proc. 3rd Int Tech. Conf. Concepts of Pipeline Transportation of Large on The lump coal is separated fran the vehicle slurry standard washery technology and the Slurzy Transportation, Las Vegas, USA, 1978. PP 164-176. pipeline product moisture is similar to coal washery: 7. product moisture. Lockyear, uckworth, R.A., Pullun, I., Addie, G.R., anc C.F. "The Pipeline Pransport o 9. REFERENCES Coarse Materials in a Non-Newtonian Carrier 1. Analosis of Solid-Liquid pipeline Transport o. Int. Conf. on Hydraulic Transport of Unified Physically-Based Austria, 1986. pp 69-88. Transport 'luid", Proc. 10th Int. Conf. on Hydrauli Solids in Pipes, Innsbruck Solids in Pipes, Banff, Canada, 1976. BHRA, Cranfield, U.K., pp E2/22-36. hattacharya, A., and Imrie, I. "Developmen of the ASEA Mineral Slurry Transport Syste 2. homas, A.D. "Pipelining of Coarse Coal as: tabilised Slurry - Another Viewpoint", Proc 4th Tech. Conf. on Slurry Transportation, Las Vegas, USA, 1979. pp 196-205.

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