ME 326 Thermal Power Engineering

Topic : Fuels and Combustion

Dr. Arvind Pattamatta

Heat Transfer and Thermal power Lab
Department of Mechanical Engineering
Indian Institute of Technology Madras

Coal Firing
Since the old days of feeding coal into a furnace by hand,
several major advances have been made to improve the
combustion efficiency.
Types of coal firing:
1. mechanical Stoker firing
2. Pulverized firing (1920s:represented a major increase in
combustion rates over mechanical stokers)
3. Cyclone Firing (1940s)
4. Fluidized bed firing (1950s)

Proper Size of Coal for Various Types of Firing

System:
S. No.
1.

Types of Firing System

Size (in mm)

Hand Firing
(a) Natural draft
(b) Forced draft

25-75
25-40

Stoker Firing
(a) Chain grate
i) Natural draft
ii) Forced draft
(b) Spreader Stoker

25-40
15-25
15-25

3.

Pulverized Fuel Fired

75% below 75 micron*

4.

Fluidized bed boiler

< 10 mm

2.

Mechanical Stoker Firing

A stoker is a power operated fuel feeding mechanism and grate.
Automatic stokers are classified as
1. Overfeed stokers.
2. Underfeed stokers.
The overfeed stokers are mainly classified into two types.
1. Travelling grate stoker
a) Chain grate stoker
b) Bar grate stoker
2. Spreader stoker.

Over feed Vs Under feed stoker

Differences between overfeed and underfeed

stokers:
Sl.No

Overfeed stokers

Underfeed stokers

Suitable for boiler installation where the

coal is burnt with pulverisation.

Suitable for semi-bituminous and

bituminous coals with high
volatile matter.

The volatile matter requires longer time

for complete burning and results in
the formation of smoke.

The volatile matter is at higher

temperature before entering the
furnace and hence burns quickly
when mixed with secondary air.

The ash is comparatively at lower

temperature.

The ash left at the bottom of the

stoker is at high temperature.

The coal is fed into the grate above the

point of air admission.

The coal is admitted into the furnace

below the point of air admission.

Both coal and air moves in the opposite

direction.

Both coal and air moves in the same

direction.

Travelling Grate Stoker

The speed of the stoker is 15 cm to 50 cm per minute.

non-caking coals are best suited for chain grate stokers.

The rate of burning with this stoker is 200 to 300 kg per m2 per hour

Stoker system characteristics

All kinds of coal can be fired on stokers.

Efficiency is Low.
Stoker firing is limited to low capacities (12.6 kg/s of steam)
These capacities are the result of the practical limitations of stoker physical sizes
and relatively low burning rates which require a large furnace width for a given
steam output.

Spreader Stoker
Most widely used for stem capacities of 9.5 to 50 kg/s.
It can burn a wide variety of coals from high-rank bituminous
to Lignite.

Pulverized Coal Firing

To prepare coal for use in pulverized firing, it is crushed and
then ground to such a fine powder that approx 70 % of it will
pass a 200-mesh (0.074 mm) sieve.
Advantages of pulverized coal firing are:
The ability to use any type of coal
A lower requirement for excess air for combustion, resulting in
lower fan-power consumption.
Lower carbon loss
Higher combustion temperatures and improved thermal
efficiency
Lower operating and maintenance cost

Pulverized Coal Firing

The mechanism of crushing and pulverizing has not been well
understood.
Mot accepted law (Rittingers Law) states that the work
needed to reduce a material of given size to smaller size is
proportional to the surface area of the reduced size.
To burn pulverized coal in a furnace, two requirements have to
be met:
1. the existence of large quantities of very fine particles of
coal, usually those that would pass a 200-mesh screen, to
ensure ready ignition because of their large surface to volume
ratios.
2. The existence of a minimum quantity of coarser particles to
ensure high combustion efficiency.

Coal Sieve Analysis

Coal Crushers
If the coal is too large, it must go through crushers for being
broken into required size (about 3cm), which are part of the
coal-handling system.
To prepare coal for pulverization, the following crushers are
preferred:
Ring Crusher
Hammer-mill
BradFord Breaker
Roll crusher

Ring type coal crusher

Hammer-mill coal crusher

Bradford Breaker

Pulverizers
Pulverizing process is composed of the following stages:
Feeding system
Which automatically controls the fuel-feed rate according to
the boiler demand and the air rates required for drying and
transporting fuel to the burner.
Drying
Removal of moisture from coal
Pulverizer or Grinding Mill
Grinding is accomplished by impaction, attrition, crushing, or
combination of these.
Classifier
Separates oversized coal and returns it to the grinders to
maintain the proper fineness for the particular application.

The Pulverized coal system

It comprises of
Pulverizing
Delivery and
Burning equipment.
Classification:
The bin or storage system
The direct-firing system.

Pulverized coal bin system

The bin system is essentially a batch system

The coal is pneumatically conveyed through pipelines to utilization bins near the
furnace.
Used mainly in older coal fired plants.

Pulverized coal direct firing system

It has greater simplicity, greater safety, lower space requirement, and lower
operating cost.
It continuously processes coal from the storage receiving bunker through a feeder,
pulverizer, and primary air fan to the furnace burners.
Large steam generators are provided with more than one pulverizer system, each
feeding a number of burners, so that a wide control range is possible by varying the
load on each.

Pulverized coal burner

Similar to an oil burner

It receives dried pulverized coal in suspension in the primary air and mixes with the
main combustion air from the steam generator air preheater.
Initial ignition of the burner is accomplished by light-fuel oil jet, spark-ignition.

Excess air requirement

The total air-fuel ratio is greater than stoichiometric but just
enough to ensure complete combustion without wasting energy
by adding too much sensible heat to the air.

Cyclone Furnace(1940s)

It is widely used to burn poorer grades of coal that contain a high ash content (6-25
%), and a high volatile matter (>15%) to obtain the necessary high rates of
combustion.
Tangential injection of primary and secondary air to impart a centrifugal motion to
the coal.
Tertiary air admitted at the center
The whirling motion of air and coal results in large heat rate volumetric densities
(4.7-8.3 MW/m3) and high combustion temperatures ( > 1650 deg C)

Cyclone Furnace

Advantages:
The removal of much of the ash, about 60 %, as molten slag is collected on cyclone
walls by centrifugal action and drained off the bottom.
Only 40 % ash leaves with the flue gases, compared with 80 % for pulverized coal
firing.
Only crushed coal is used and no pulverization equipment is needed and hence
boiler size is reduced.
Limitations:
Formation of relatively more Nox in the combustion process.
High forced draft fan pressure and therefore higher power requirements.

Fluidized-bed combustion
It has been under development since 1950s.
In a fluidized bed the turbulent state increases heat and mass
transfer and reduces time of reaction, plant size and power
requirement.
Fluidized bed combustion results in high combustion
efficiency and low combustion temperatures.
It occurs at lower temperatures, resulting in lower production
of Nox as well as the avoidance of slagging problems.
It differs from the cyclone furnace in that sulfur is removed
during the combustion process.

Advantages of FB combustion

Major advantage of CFBC is the concurrent removal of SO2.

Desulfurization is accomplished by the addition of limestone directly to the bed
together with the crushed coal.
Limestone absorbs the SO2 with the help of some O2 from the excess air

1
CaCO3 + SO2 + O2 CaSO4 + CO2
2
The rate of this reaction is max. at bed temp between 815 to 870 deg C though a
practical range of operation of fluidized beds of 750 deg C to 950 deg C is
common.
Other advantages are:
Complete and efficient combustion
Low emissions
Favorable Ash property
Low operating costs and Maintenance

Fluidization

A fluidized bed is a bed of solid particles which are set into motion by blowing a
gas stream upward through the bed at a sufficient velocity to suspend the particles.
The bed appears like a boiling liquid.
The fluidization occurs when the drag force on the particles in the bed due to the
upward flowing gas just equals the weight of the bed.

Fluidization

The total pressure drop in a fluidized bed is composed of

P = Pw + Ps + Pf
Pw pressure drop due to friction at the wall
Ps pressure drop due to static weight of solids in bed
Pf pressure drop due to static weight of fluid in bed.
Fluidized beds usually have large wall diameter, so Pw is relatively small.
The average gas density of hot gaseous products is much smaller than that of the
solids and Pf therefore is also relatively small

P = Ps = H(1-)s g

Where = average porosity or void fraction of bed

m b
vol. of bed vol. of solids
=1- ( s
)
vol. of bed
s m b
Since the voids may be regarded as empty spaces, ms=mb,
=

Where b is the bulk density of the bed.

= 1-(b/s)

H0
1
=
10
H

Where 0 is porosity in collapsed state = 0.4 for randomly packed beds.

Fluidization

The minimum fluid velocity necessary for fluidization may be calculated by

equating the drag force on a particle due to the motion of the fluid to the weight of
the particle.
2

C D A f v s
2

= V s g

CD Drag coefficient, a function of shape and Re

A Cross sectional area of the particle
f Density of the fluid
s Density of the solid particle
V volume of the particle
Vs velocity of the fluid
g gravitational acceleration

For a spherical particle

8 s
vs =
rp g
3C D f

where rp is the radius of the particle

Bed pressure drop

Regimes of Fluidization
1. Packed bed (Stoker)
2. Bubbling Fluidized Bed
3. Turbulent Bed
4. Fast Fluidized bed (Circulating Fluidized)
5. Pneumatic Transport (Pulverized)

Regimes of Fluidization

Regimes of Fluidization

Regimes of Fluidization

Packed bed

A packed bed consists of a bed of stationary particles on a perforated grid through

which a gas is flowing.
The pressure drop per unit height of a packed bed of uniformly sized particles is
given by Erguns equation
2

v
p
(1 )
v
1 f
= 150
+ 1.75 3
3
2
H

(d p )
d p
3

Where is the viscosity and s is the density of the gas, dp is the diameter of the
particles and is the sphericity of particles

surface area of a sphere of the same volume as the particle

surface area of the particle

Bubbling Fluidized Bed

When the superficial velocity of gas flow through a fixed bed reaches the minimum
fluidization velocity, vs, the fixed bed transforms into an incipiently fluidized bed
and the bed starts behaving as a liquid.
The pressure drop across the bed is equal to the weight of the bed, the fluid drag is
given as

FD = P A = A H(1-) (s f )g
Also P /H = (1-) (s f )g
The minimum superficial velocity, vmf, may be given in terms of Reynolds
number:

Re mf =

g d p vmf

= [C12 + C 2 Ar ]0.5 C1

Ar = Archimedes number =
C1 = 27.2 and C 2 = 0.0408

f ( s f ) gd p 3
f2

Turbulent Bed

As the velocity of gas through a bubbling fluidized bed is increased, the bed
expands, and a point is eventually reached when the bubbles constantly collapse
and reform resulting in a violently active bed.
The bed surface is highly diffused and particles are thrown into the free board
above.
The pressure drop fluctuates rapidly.
The amplitude of pressure fluctuation reaches a peak and reduces to a steady state
value.

Fast Fluidized Bed

Also referred to as the Circulating Fluidized Bed.

Defined by Basu and Fraser (1991) as follows:

High slip velocity (Ug-Us) between gas and solid, formation and
disintegration of particle agglomerates, and a very good gas-solid mixing
are the characteristic features of this regime.
The main difference between bubbling beds and CFB lies in the gas
velocity used. While bubbling beds normally operate at gas velocities of
around 1-3 m/s, CFB typically runs at 5-10 m/s

Fast Fluidized Bed (CFB)

Combustion of fuel particles in a fluidized bed

Fluidized bed combustion of solid fuels

A type of furnace or reactor in which fuel particles are
combusted while suspended in a stream of hot gas.
Coal size used 6 to 20 mm.
Types of FBC:
Bubbling fluidized bed combustion
1) Atmospheric
2) Pressurized
Circulating fluidized bed combustion
1) Atmospheric
2) Pressurized

Bubbling fluidized bed (BFB)

A bubbling fluidized bed boiler comprises a fluidizing grate

through which primary combustion air passes and a containing
vessel, which is either made of (lined with) refractory or heatabsorbing tubes.
The vessel would generally hold bed materials. The open
space above this bed, known as freeboard, is enclosed by heatabsorbing tubes.
The secondary combustion air is injected into this section
The boiler can be divided into three sections:
1. Bed
2. Freeboard
3. Back-pass or convective section.

Atmospheric Fluidized bed combustion

Operating Pressure : 1 atm

Temperature
: 850 deg C
Fluidizing velocity : 2-4 m/s
Avg. bed material size : 1000 micron
Fuels
: Multi fuel
Combustion efficiency : 90 99 %
Pollutant emission control : very good
Application
: industrial boilers, power
generation.

Atmospheric Fluidized bed combustion

Pressurized Fluidized bed combustion

Operating Pressure : up to 16 atm

Temperature
: 850 deg C
Fluidizing velocity : 1-1.5 m/s
Avg. bed material size : 1000 micron
Fuels
: Multi fuel
Combustion efficiency : 99 %
Pollutant emission control : Excellent
Application
: Combine cycle power generation

Pressurized Fluidized bed combustion

Advantages of PFBC over AFBC

Increase in specific power output and hence potential
reduction in capital cost
Increased power generation efficiency
Emissions of oxides of Nitrogen are substantially reduced.

Circulating Fluidized Bed combustion (CFBC)

In a CFB boiler furnace the gas velocity is sufficiently high to blow all the solids
out of the furnace.
The majority of the solids leaving the furnace is captured by a gassolid separator,
and is recirculated back to the base of the furnace.
A CFB boiler is shown schematically in Figure
The primary combustion air (usually substoichiometric in amount) is injected
through the floor or grate of the furnace
The secondary air is injected from the sides at a certain height above the furnace
floor.
Fuel is fed into the lower section of the furnace, where it burns to generate heat.
A fraction of the combustion heat is absorbed by water- or steam-cooled surfaces
located in the furnace, and the rest is absorbed in the convective section located
further downstream, known as the back-pass.

CFBC

Circulating FBC

Operating Pressure : up to 16 atm (if pressurized)

Temperature
: 850 deg C
Fluidizing velocity : 4-8 m/s
Avg. bed material size : 200-350 micron
Fuels
: Multi fuel
Combustion efficiency : 99 %
Pollutant emission control : Excellent
Application
: industrial boilers, combined cycle
power generation power generation

Coal Gasification
The gasification of coal for use as a powerplant fuel is being
considered as the supply of natural gas diminishes.
Coal gasification existed since 1800s !
Intially gas was manufactured from coal and distributed as
town gas for smelting of iron and for burning etc.
The coke was placed in large beds and burned for a period
with less than the stoichiometric quantity of air to give
producer gas

Coal gasification process

Low BTU gas:
2 C + ( O 2 + 3 . 76 N 2 ) 2 CO + 3 . 76 N
coke

Air

producer

gas

When the bed is heated to a high temperature, the flow of air is

replaced by a flow of steam and water gas is produced.
C + H 2 O CO + H
coke

steam

water

gas

The resultant mixture also called as synthesis gas or syn gas

is a lower quality gas with a heating value of about 10 MJ/m3.

Coal gasification

Coal gasification process

Medium BTU Gas:
If the product desired is a medium or high BTU gas, a synthesis gas shift reaction
or conversion is used to produce additional hydrogen by reacting some of the CO
with steam and removing CO2 from the products

CO + H 2O CO2 + H 2
High BTU gas:
The final step in producing a pipeline quality gas is called catalytic methanation.
The products of the above reaction are reacted over a nickel catalyst at a
temperature of 1100 deg C and pressure of 6.8 bar

CO + 3H 2 CH 4 + H 2O

The product gas is of a higher quality with heating value of 37.7 MJ/m3 and is a
direct substitute for natural gas.

Coal gasifier combined cycle power plant