OPTIMUM DESIGN OF HIGH CONCENTRATION FLY ASH SLURRY DISPOSAL PIPELINE

Deo Raj Kaushal, Navneet Kumar
Department of Civil Engineering, Indian Institute of Technology Delhi, India

ABSTRACT

Rheological characteristics of fly ash slurries were determined experimentally at higher concentrations. The pressure gradient was calculated using CFD modeling with input parameters obtained on the basis of rheological and other bench scale laboratory tests conducted on fly ash slurries. CFD modeling results are validated using experimental data collected in the present study on pilot plant test loop. Optimum design methodology for high concentration fly ash slurry disposal pipeline has been proposed on the basis of Specific Energy Consumption concept.

Key words: optimum design, HCSD pipeline, CFD, rheology, fly ash.

INTRODUCTION

Thermal power plants generate more than half of the world’s electric power by burning millions of tonnes of coal and simultaneously produce large quantity of coal ash. Slurry pipelines are commonly used across the world for the transportation of coal ash (fly ash and bottom ash) from the plant to the ash ponds in thermal power plants. Most of the pipelines operating today in thermal power plants transport ash at low or medium solid concentrations over short as well as medium distances. These systems are very energy intensive and also lead to excessive wear of pipeline and wastage of water. Further, the present enhanced consciousness towards the imbalance in the eco-system and related stringent government policies are forcing the thermal power plants to adopt environment friendly transportation systems. Thus, high concentration slurry disposal (HCSD) system has emerged as preferred option to transport coal ash in thermal power plants as it is economical and environment friendly. Kaushal et al. [2–5] have used CFD modeling to simulate pipeline and open channel flows transporting particles.

BENCH SCALE TESTS

The various properties of the solids, carrier fluid and slurry as tabulated in Table 1 and Figure 2 are required as input to a slurry pipeline design. Standard bench scale tests were carried out to determine these properties. The rheological testing of slurry is briefly described in this section.

Rheological Testing of Slurries
The rheological experiments were conducted using Antoon Paar RheolabQC (Fig. 1). This rheometer uses vane technique which has become a simple and effective method for direct measurement of the yield stress properties of minerals slurries and other suspensions as it eliminates serious wall-slip effects. In this study a RheolabQC with measuring cup CC27 and a sensor system ST22-4V-40 having 4-bladed vane geometry was used. The diameter and length of the vane rotor was 22 and 40 mm respectively. This rheometer can also measure the plastic viscosity and yield points. The vane geometry and the measuring cup were cleaned and air dried. About 50 ml of slurry was prepared for each sample and the rheological measurements of slurries at different solid concentration were carried out. The shear rate was applied for a period of 2 min to measure the corresponding viscosity and shear stress under controlling shear rate. Steady shear measurements were performed at room temperature. All rheological measurements were repeated for each suspension to minimize and avoid experimental error during rheological tests. For each oven-dried sample, different suspensions were prepared with distilled water at different solid concentrations. To avoid the undesirable influence from particle settling, the measurement of every sample started from highest shear rate 300 to 20 s^-1, which is of the same order of magnitude expected in slurry pipeline system. The results of rheological testing are tabulated in Table 1(d) and drawn graphically in Figures 3 to 6.

PILOT PLANT LOOP TESTING OF SLURRIES

The schematic layout of the pilot plant test loop is shown in Figure 7. The major components are mixing tank, slurry pumps, and two pipe loops of 60 m length with bypass line and measuring tank. The test loop is horizontally laid in the Fluid Mechanics Laboratory at I.I.T. Delhi.

The mixing tank, a hopper shaped container, was used for preparing the slurry. The tank is fabricated from 4mm thick mild steel sheets and has an overall height of 2.4 m. It has a square shape at the top (1.25 m × 1.25 m) and has an overall capacity of 1.65 m³. This tank is also provided with a stirrer arrangement which kept the slurry well mixed during experimentation. The slurry from this tank was pumped in either of the two pipe loops namely 105 mm (100 mm NB) loop and 55 mm diameter (50 mm NB) loop by separate slurry pumps. The slurry in 105 mm loops were pumped by a slurry pump with Ni-hard casing (WILFLEY, Model 100 K, Manufactured by M/s Dorr-Oliver Ltd.) driven by a 50 H.P. motor through a V-belt drive system. The flow in the 55 mm loop was achieved by a similar type of pump having 30 H.P. motor. The pumps had the capacity to achieve the desired head and discharge needed for simulating the conditions in the prototype pipeline. A bypass line with a valve is provided in each loop which allowed better control over the flow rates in the main pipe loops while at the same time ensuring that the deposition did not occur in the suction pipes while conducting experiments at low velocities in the pipe loop. The flow through bypass line also ensured thorough mixing of the slurry in the mixing tank especially when the flow rate through the pipe loop was small.

The pipe loops started just down-stream of the pump and were provided with AUDCO valve (plug type) V-1 for controlling the flow rate. This pipe loops contained bends and observation chamber. At the exit of the pipe loop, a diverter has been provided which facilitates the diversion of the flow to the measuring tank. The measuring tank is again in the shape of a hopper. The height of the tank is 1.5 m with the top being a square of 1 m × 1 m with total volume capacity of 1.25 m³. This tank is also connected to the suction of the pump through a 50 mm plug valve to facilitate the transfer of slurry from the measuring tank to the mixing tank. The measuring tank is also provided with a stirrer arrangement. The diverter system at the end of the pipe loop facilitated the diversion of slurry flowing out of the pipeline into the measuring tank for any given length of time. The duration for which the flow is diverted is measured by an electronic stop watch having a resolution of 0.01 second. The volume of slurry collected during this interval is measured by noting the rise in the level of the slurry accurately after allowing sufficient time for the level to stabilise. The area of cross-section of the measuring tank was accurately determined and from this data the flow rate of slurry could be determined to accuracy better than ±1.0%. This method was used to calibrate the magnetic flow meter installed in the pipeline for regular monitoring of the flow rate.

The pipe loops is instrumented with different instruments to carry out different measurements. Several pressure taps with separation chambers have been provided at different locations in the straight section of the pipeline. In the return straight section of the pipeline, a sampling probe is provided for measuring the concentration profile in the straight pipe. At the end of the pipe loop a sampling point is provided in the vertical portions for collecting the efflux sample. In the straight pipeline, a small length of Perspex pipe was provided to establish the deposition velocity of the slurry in the pipeline by observing the motion of the particles at the bottom of the pipeline.

MATHEMATICAL MODEL

The use of a specific multiphase model (the discrete phase, mixture, Eulerian model) to characterize momentum transfer depends on the volume fraction of solid particles and on the fulfillment of the requirements which enable the selection of a given model. In practice, slurry flow through pipeline is not a diluted system, therefore the discrete phase model cannot be used to simulate its flow, but both the mixture model and the Eulerian model are appropriate in this case. Further, out of two versions of Eulerian model, granular version will be appropriate in the present case. The reason for choosing the granular in favour of the simpler non-granular multi-fluid model is that the non-granular model does not include models for taking friction and collisions between particles into account which is believed to be of importance in the slurry flow. The non-granular model also lacks possibilities to set a maximum packing limit which makes it less suitable for modeling flows with particulate secondary phase in the present case. Lun [6] and Gidaspow [1] proposed such a model for gas-solid flows. Slurry flow may be considered as gas-solid (pneumatic) flow by replacing the gas phase by water and maximum packing concentration by static settled concentration. Furthermore, few forces acting on solid phase may be prominent in case of slurry flow, which may be neglected in case of pneumatic flow. In the present study slurry pipeline bend is modeled using granular-Eulerian model as described below.

Eulerian two-phase model assumes that the slurry flow consists of solid “s” and fluid “f” phases, which are separate, yet they form interpenetrating continua, so that α_f + α_s = 1.0, where α_f  and α_s are the volumetric concentrations of fluid and solid phase, respectively. The laws for the conservation of mass and momentum are satisfied by each phase individually. Coupling is achieved by pressure and inter-phase exchange coefficients.

The forces acting on a single particle in the fluid are:

1. Static pressure gradient, P.
2. Solid pressure gradient or the inertial force due to particle interactions,P_s.
3. Drag force caused by the velocity difference between two phases, K_rf(_r-_f), where K_sf is the inter-phase drag coefficient, _s and _f are velocity of solid and fluid phase, respectively.
4. Viscous forces, ·_f , where _f  is the stress tensor for fluid.
5. Body forces, ρ, where ρ is the density and g is the acceleration due to gravity.
6. Virtual mass force, C_vmα_sρ_f(_f·_f - _s·_s), where C_vm is the coefficient of virtual mass force and is taken as 0.5 in the present study.
7. Lift force, C_Lα_sρ_f(_f - _s) × (× _f), where C_L is the lift coefficient taken as 0.5 in the present study as similar value is considered in earlier studies.

Wall Function
In the region near the wall, the gradient of quantities is high and requires fine grids. This causes the calculation to become more expensive, meaning time-consuming, requiring greater memory and faster processing on the computer, as well as expensive in terms of the complexity of equations. A wall function, which is a collection of semiempirical formulas and functions, provides a cheaper calculation by substituting the fine grids with a set of equations linking the calculated variables at near-wall cells and the corresponding quantities on the wall. The wall function helps in more precise calculation of near-wall shear stresses for both liquid and solid phases.

Geometry for CFD Simulation
The computational grids for the 6.1 m long, 105 mm internal diameter horizontal pipe approximately 3,84,450 cells and 4,20,926 nodes have been generated using GAMBIT software (Fig. 8). The length of pipe is sufficiently long (i.e., more than 50D, where D is the pipe diameter) for fully developed flow. The presence of fully developed flow is confirmed by studying the computational results for pressure drop along the slurry pipeline. It is observed that pressure profile becomes linear, thus making pressure drop constant. This number was generated by applying the same cross-sectional meshes obtained from the optimum cross-sectional meshes of pipe for the single-phase flow. Optimum mesh is independent of grid density. The grid-independence tests were carried out keeping all the solver parameters same for each simulation case. The grid independence tests consisted of refining the initial grid by approximately doubling the number of cells present in the initial grid. To obtain better convergence and accuracy for a long pipe, the hexagonal shape and Cooper type element has been employed. The Cooper type element is a volume meshing type in GAMBIT, which uses an algorithm to sweep the mesh node patterns of specified “source” faces through the volume.

Boundary Conditions
There are three faces bounding the calculation domain, the inlet boundary, the wall boundary and the outlet boundary. Flow velocity and volume fraction of liquid and solid phases were introduced at the inlet condition of this pipe, i.e., V_m = υ_s = υ_f, α_s = C_vf and α_f = 1-C_vf, where V_m is the mean flow velocity which was measured experimentally using electromagnetic flow meter, C_vf is the efflux concentration in the slurry pipeline can be computed using following equation:

(1)

The fully developed flow obtained at the outlet is used as the final results for concentration profiles in the present study.

Modeling Results for Pressure Drop
Comparison between measured and predicted pressure drops in pipeline of 55 mm diameter are presented in Figure 9 at different concentrations and flow velocities for flyash. The critical deposition velocity was observed in the range of 0.8–0.9 m/s for the concentration range of up to 70% by weight and velocity range of 1 to 3 m/s selected in the present study. From these figures, it is observed that Eulerian model gives fairly accurate predictions for pressure drop at all the efflux concentrations and flow velocities considered in the present study.

OPTIMUM DESIGN BASED ON SPECIFIC ENERGY CONSUMPTION

Specific Energy Consumption (SEC) is defined as the energy required to transport one tonne of ash over a distance of one kilometre. Typical results for a 55 mm pipe at two different velocities namely 1 and 2 m/s is shown in Figure 10. It is observed that at any given velocity Specific Energy Consumption reduces up to 65% solid concentration and increases drastically with further increase in solid concentration. This can be attributed to steep increase in the rheological properties of coal ash slurries at higher concentrations.

CONCLUSIONS

Rheological and pressure drop characteristics of fly ash slurries were determined experimentally at higher concentrations upto 70% by weight. The pressure gradient was calculated using CFD modeling with input parameters obtained on the basis of rheological and other bench scale laboratory tests conducted on fly ash slurries. Optimum concentration for fly ash slurry disposal pipeline on the basis of Specific Energy Consumption concept was found to be 65% by weight

REFERENCES

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3. Kaushal D.R., Kumar A., Tomita Y., Kuchii S., Tsukamoto, H. 2013. Flow of Mono-Dispersed Particles through Horizontal Bend, International Journal of Multiphase Flow, Elsevier Publications, Vol 52, 71–91.
4. Kaushal D.R., Thinglas T., Tomita Y., Kuchii S., Tsukamoto H., 2012. Experimental Investigation on Optimization of Invert Trap Configuration for Sewer Solid Management, Powder Technology, Volume 215–216, Issue 1, 1–14.
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6. Lun C.K.K., Savage S.B., Jeffrey D.J., Chepurniy N., 1984. Kinetic theories for granular flow: inelastic particles in couette flow and slightly inelastic particles in a general flow field, Journal of Fluid Mechanics, 140, 223–256.

Accepted for print: 5.11.2013

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