Thermofluids Project: Fly-Ash Slurry Transportation System Design

This project focuses on designing an efficient disposal and transportation system for fly ash slurry, a byproduct of coal-fired power plants. The assignment delves into the challenges of handling fly ash due to its high moisture content and explores various transport methods, including hydraulic syste...Read More

Mechanical Engg.
Master of Engineering
Transportation of Fly-Ash Slurry

Contents
Introduction...........................................................................................................................................1
Literature Survey...................................................................................................................................2
Pump and slurry piping system..............................................................................................................4
Heat exchange analysis/calculation.......................................................................................................5
Rheometer theory/design concept........................................................................................................7
Alternate transportation concept/analysis............................................................................................9
Environmental issues and recycling.......................................................................................................9
References.............................................................................................................................................9

Introduction
The goal of the project is the designing of an efficient disposing and transporting system for
the fly ash slurry. Fly ash is generated when pulverized coal is burnt in the power plants
which use coal. It consists the finer particles that are generated along with the flu gases. They
can be retained using electrostatic precipitator or particle flirtation device. It is not a useful
product and is problematic for environment. It has big moisture content that makes it tough to
handle and dispose off.
Some ways to transport it are : to mix it to water and transport it using hydraulic method,
transport in a state that is not totally liquid on road (environment problems and high cost) or
discharge it using pneumatics. Tailing dams can also be used but they consume more space
area and need more water. It can be made to recycle also like for producing mortar as well as
bricks and for land filling and re vegetating purpose.
Literature Survey
The main concern here is the design of transport system for fly ash slurry. On mixing with
water, the fly ash behavior becomes non-Newtonian. The viscosity becomes variable. In
designing a pipeline transportation system, the method followed must show rhelogical (flow
of fluid and deformation) behavior for the heavy fly ash slurry. Rheologic characterization is
carried out by the use of a pipe based or capillary based rheometer. Some parameters like
particle concentration, temperature as well as the velocity gradient change the characteristics
of the slurry (like viscosity and density).
Two systems have been studied here - the slurry transport mechanism and the pneumatic or
mechanical mechanism. A quality comparison is carried out for both. Here, the theory based
thermo – fluid knowledge and skills are applied for practical system design.
Nowadays, there is stress on producing clean coal. So, the slurry output has increased
tremendously. It consists of finer coal particles, clay particles and water which has to be
removed. If the water is removed and solid material is made to rest on land, it settles under
gravity but the process is very slow and takes so much space. This is also not good for the
environment. Proper method needs to be followed for solid material disposal. To dewater the
coal mine, high speed centrifuges and belt press filters can be used.
Pump and slurry pipe system
Study of the composition and rheological features of the fly ash slurry
Details of Slurry material

The slurry consists of finer coal particles and suspended clay particles in liquor with pH value
of 7.8. The density of slurry is 1.8 g/cc. Other chemical impurities are also present like
flocculant and gypsum ( due to previous stages) and also due to mine impurities. The
particles present have a size from 0.4 to 700 μm. 10 % of the particles’ size is not more than
1.97 μm. 19 % solids are present but after filtration process 40 % solids are obtained. A
homogeneous suspension produced is made to settle. Capillary rheometer can be used to
measure properties of suspension.
The flow index ( n ) is less than 1. It decreases if value of φ decreases from 0.35 to 0.1.
N = slope of shear stress ( τ ) v/s apparent shear rate ( 8 V/D ) log – log plot.
Internal diameter of pipe is taken as 0.1524. Rate of disposal is 65 tpd.
Another method which can be used to find viscosity of slurry in the dispersed condition is by
finding maximum packing volume fraction. Hydraulic transportation is discussed here.
Designing the hydraulic pipeline transport system for fly ash slurry
The rate of flow in a slurry pipeline at a fixed pressure drop is given by
dP/dx = 2 τwall / R
τ = shear stress
γ = shear rate
The test rig uses stainless tube with inner diameter = 50 mm
Concentration range is 70 – 75 % by weight
The various design parameters are :
The rate of discharge of the fly ash is 88.9 tonnes per hour or 640,000 tonnes per year. The
concentration of the slurry is 70 % by weight. The horizontal length of pipeline is 10 km and
inner diameter of pipe is 125 mm. The total pressure drop is predicted as 11.5 MPa, the
power consumed is 500 kW and the pump efficiency is 50 %. The rate of flow is 0.022 m3/s
and the average velocity is 1.8 m/s.
Fly ash characteristics used in the design are as follows:
Major constitution: Silicon Dioxide (Si O2), Calcium oxide (CaO), Aluminium Oxide (Al2
O3) and Iron Oxide (Fe2 O3), density: 1,220 - 1,350 kg/m3 and size of the particles : mainly
sphere shaped and 20% < 7 micron and else lie in between 7 and 15 micron.
The flowing method depends on the velocity of fly ash slurry, that is non Newtonian. The
rheological features and data of slurry are got from the published material. To simplify the
calculations, the value of viscosity is taken nearly constant. (a small range for transportation).

We take fly ash and store it in shaded area and then mix it to water at some percentage in a
tank. A mechanical based stirring device is used to mix. We can then pump the fly ash slurry
from the tank at a chosen rate of flow. It is then moved towards the tail dam or the land fill
(as selected) and a part is moved towards a pilot project for research purpose. An amount of
50 ton to 100 ton (only 1 value) of fly ash is transported in 1 day at a temperature of about
28-300 C and the slurry transport system is a discontinuous system.
In winter, the ambient temperature < advised operation temperature and a heat exchanger is
used to increase the temperature of the slurry to reduce the value of friction for the pipe. The
distance between the tank and the disposal point (tail dam or landfilling) is about 1.5 km (a
larger distance may be selected). The viscosity of the fly ash (slurry) needs to be recorded.
The on-line pipe (or capillary) rheometer is required for measuring the parameters.
Heat exchange analysis/calculation
Heat exchanger: The pump and piping system is shown in Figure 1. It gives 0.0785 m3/s
(minimum) of water at 340 C from a tank to a heat exchanger (K=10) and again towards the
tank. The water in the tank is kept at the required temperature using solar heat. The velocity
of the water is from 1.3 m/s to 3 m/s and the pipe diameter is less than or equal to 85 mm.
Figure 1

Rheometer designing
The tube rheometer can be used to characterise some fluids having low viscosity. As a low
value of viscosity is taken, the method used cannot be the application of pressure or the
piston method. The power relation given in Figure shows the relation between shear rate and
shear stress.
. t = k ( dv / dr)n
For low value of viscosity, the above relation is not followed. Hence, a capillary tube
rheometer is designed. The components needed are : a capillary tube, a pressure difference
application, a pressure difference measurement, flow rate measure and a temperature
controller.
8 V / D = 32 Q / ∏ D3
Shear stress, tw = D ∆p / 4L
The shear rate and shear stress can be drawn on a logarithmic graph which is a straight line.
Its slope gives ‘n’ and intercept on y axis gives ‘k’ ( shear rate ). If n is less than 1, its a shear
thinning fluid and if n is more than 1, its a dilatants fluid. If n is equal to 1, its a Newtonian
fluid.
In this case, n < 1.
Apparent viscosity = γcorrec = ( 3 n + 1 / 4 n ) 8 V / D
Μ = tw / ( 3 n + 1 ) 8 V / 4 n D

Figure 2
Figure 3 Capillary Type Rheometer – Schematic diagram (Courtesy - Sultan)
Alternate transport idea
Here, the fly ash to be dry or aerated by the use of pneumatic (vacuum/pressure) or a
mechanical method. This design needs a schematic diagram and operating principles

explanation and calculations required to compare qualitatively the cost per ton per kilometer
of transport for the designed system and slurry transportation system.
Environmental Problems and recycling
Health problems and environment related problems ( related to revegetation ):
The toxic substances present in it can be harmful for human body. It can affect the parts like
brain, kidney, lungs heart etc. There are various toxins like – lead, cadmium, chromium,
arsenic, mercury, boron, molybdenum, thallium, silica etc. as per Manas ( 2011 ).
Research on recycle and important uses of fly ash:
Recycling can help the environment as well the economy. But health problems may still
occur as the toxins may run into streams and reach ground water and pollute it.
References
Manas Ranjan Senapati, (2011), “Fly ash from thermal power plants-waste management and
overview", current Science, Vol. 100, No. 12, Pages 1791-1793.