Pulverized Fuel Fired Systems

The pulverization system is a combination of equipment in

which solid fuel is ground, dried and transferred to the
burners of a boiler furnace.
By the method of delivery of pulverized fuel to the furnaces,
pulverization systems can be classified into central and
individual types. In the former case, the system is arranged
in a separate building (central coal pulverizing plant) where
coal is pulverized on a centralized basis and then distributed
through pipelines between the boilers of the station. In the
later case, each boiler is provided with its own pulverizing
equipment, with certain provision being made to transfer the
pulverized coal to the neighbouring boilers so as to increase
the reliability of the fuel supply.
Centralized pulverization system turns to be more
economical, especially when moist brown coal is pulverized
but the equipment is more intricate and expensive. In
addition to this, they are not sufficiently reliable in operation.
Individual systems are simpler and more reliable and are
widely employed in power stations.
Individual systems may be of closed or open types. This
classification is determined by the way in which the drying
agent is utilized upon fuel drying. In a closed system, it is
directed into the furnace together with dried pulverized fuel.
In an open system, the drying agent is carefully cleaned from
fuel lines and ejected into the stack, bypassing the boiler
furnaces.
Moisture content is an important characteristic of pulverized
fuel. An increase in moisture content above the
recommended level may result in lower boiler productivity
and involve difficulties in dust transport as the dust looses
fluidity and slumps in bunkers, clogs feeders, chutes etc. On
the other hand, over dried brown coal dust is liable to self
ignite in place where it is stored.
When coal dust suspended in air is confined in a closed
volume, it will explode more intensively if its unit surface
area is larger (if it contains more fine particles) and if it has
a higher yield of volatiles. The ignition of an air-dust mixture
in a closed volume results in a sharp rise in temperature and
pressure. The pressure may rise well above the safety limit
of the pulverizing equipment. Damage to the power plants
by explosions is prevented by installing safety valves which
discharge part of the mixture from the system should the
pressure rise excessively. The concentration limit of oxygen
in the drying agent, i.e. the concentration below which fuel
dust can not explode is about 19% for coals. The
concentration of oxygen can be decreased by drying
pulverized fuel with a mixture of hot air and combustion
products. The probability of explosion is lower in fuels with
a lower yield of volatiles. When volatiles are less than 8%,
fuel is explosion safe. A supply of cold mill ventilation air is
maintained to avoid the rise in temperature in the mill to
prevent explosion..

Closed Pulverization System

In this system, a portion of the hot air (called primary air)

coming out from the airpreheater is used for drying as well
as for transporting the pulverized coal. The quantity of the
primary air is determined on the basis of moisture content of
the coal and the amount of coal. The amount of primary air
is usually 30% - 50% of the total consumption of air and it
is increased with the moisture content.
This type of direct blowing system has certain advantages: It
is simple; The pulverizing equipment is compact;
Consumption of electrical energy for dust transport is low;
The fuel supply can easily be controlled.

Raw Coal
Boiler
Bunker
Coal
Feeder Coal Air
Dust Burner Pre-heater
Separator
Secondary
Mill air (SA) Cold air for mill FD Fan
ventilation
Primary air (PA )
(250oC – 400oC)

Closed Type Individual System

In another version of the closed type individual system, the

pulverized fuel is separated from the transporting air in a
cyclone. The dust is directed to an intermediate bunker from
which it is fed to the by special feeders into pulverized fuel
pipelines. The moistened air at the exit of cyclone has a
temperature of 80oC – 100oC and contains 10% - 15% of the
finest coal dust. This air can not be discharged through the
stack and for this reason it is blown by the mill exhauster into
the primary air duct to be distributed among the pulverized
fuel pipelines. Due to the provision of the intermediate
bunker, there is no need to match the productivity of the mill
with that of the boiler. A disadvantage of the intermediate
bunker system is that its equipment is too intricate and bulky.
Furthermore, the system has an elevated hydraulic resistance
which increases the consumption of electrical energy for
dust transport. The storage of a large mass of dry dust
increases fire and explosion hazard. Nonetheless, the system
can reliably supply steam boilers with pulverized coal and
for that reason has found wide application.

Open Pulverization System

Open pulverization systems are employed for fuels having

high moisture content (which can not be dried using hot air
only). Such fuels are dried up using combustion products at
400 oC - 450 oC, which are taken off at an amount of 6% -
10% of the gas volume from the gas duct, downstream of
economizer. The worked off drying agent from the cyclone
together with some finest fuel fractions (which could not be
separated) is fed into the second stage of dust collection. The
separated dust flows by gravity through chutes into an
intermediate dust bunker, while the drying agent after the
dust collectors is discharged into the electrostatic
precipitator. The open system of fuel drying substantially
improves the quality of fuel and increases the efficiency of
fuel combustion. The volume of combustion products in the
boiler flue ducts diminishes, which results in lower
aerodynamic resistance and a lower waste gas temperature.
An essential drawback of the open system is that some fuel
fractions are lost with the discharged drying agent. Another
drawback is elevated energy consumption for separation and
purification of the moist drying agent. Despite the
complicated system of dust collection, around 1% - 2% of
the fuel is lost and discharged to the atmosphere resulting in
air-pollution. For these reasons, application of the open
system is limited only to cases of rather moist fuels which
can not be burnt efficiently by conventional methods.

Dust
Collector Separated gas

Cyclone

Raw Coal
SA Boiler
Bunker
Coal
Feeder Air
Pre-heater

PA
Grinding FD Fan
Mill
Dust PF Feeder
Bunker
Hot gas

Open Type Individual System

Burners and Their Arrangement

Burners do not ignite fuel. Their function is to prepare two
individual flows, a dustprimary air mixture and secondary
air, for ignition and active burning in the furnace space. To
achieve this, part of the hot furnace gases should be sucked
into the fresh dustprimary air jet to preheat it and the ignited
fuel should be mixed at proper time with the secondary air.
For this purpose, secondary air and dust-primary air flows
are introduced into the furnace space at different speeds and
with different degrees of turbulization. There are two main
types of burners: Turbulent or Vortex burners and Straight
Flow burners. In a vortex burner, dust-air mixture and
secondary air are fed as whirled jets. Vortex burners have
circular cross section. In straight flow burners, the dust-air
mixture and secondary air are blown in as parallel jets.
Burners of this type may be either circular or rectangular in
cross section.
Turbulent Burners

These burners are broadly classified into (a) Two Scroll

Burners – in which two scrolls are provided for whirling the
dust-air mixture and secondary air, and (b) Single Scroll
Burners – in which the dust-air mixture is supplied in a
straight flow and spread by a dissector and the secondary air
is whirled in a scroll.
 Fuel-oil Fuel-air
burner mixture
Secondary
air

Two Scroll Burner

Turbulent burners have a throughput capacity ranging from

1 kg to 3.8 kg of fuel per second with their power ranging
from 25 MW to 100MW. The principal aerodynamic
characteristic of a burner is the vorticity parameter (n). The
vorticity parameter is defined as: n = 4Vt / Va, where Vt is the
maximum tangential component of the flow velocity at the
burner exit and Va is the axial velocity component. An
increase in the value of ‘n’ results in greater turbulization of
the jet, more intense entrainment of the surrounding gases
into it and a wider expansion angle. For the turbulent
burners, n ranges from 1.5 to 5. Burners with an elevated
value of n are employed for combustion of low reactive,
poorly ignitable fuels (with a relatively low yield of
volatiles). The completeness of fuel burning depends heavily
on the ratio of axial velocities of primary and secondary air
flows in a burner. The velocity of the primary flow (dust-air
mixture) is usually between 16 – 25 m/sec, higher values
being typical of powerful burners. The optimal velocity of
the secondary air is around 1.3 to 1.4 times that of the
primary air velocity.
 Front Firing Opposed Firing Opposed Firing on
Sidewalls

The vortex burners are normally arranged as the following:

• Front firing
• Double-front firing or Opposed firing
• Side firing
• Double-side firing or Opposed firing on sidewalls
In vortex burners, the flame is shorter and wider compared
to the straight flow burners. The swirling motion ensures the
intense mixing of fuel and air in case of the vortex burners.

Straight Flow Burners

Fuel-air
mixture
 Secondary
air
Straight Flow Burner

Burners of this type turbulize the air flows less substantially

than the turbulent burners and produce a long ranging jet
with a low expansion angle and weak intermixing of the
primary and secondary air flow. Efficient combustion is
achieved by making the jets from various burners to interact
with one another in the furnace space. Straight flow burners
may be either fixed or tiltable which facilitates combustion
control. Burners of rectangular cross section are
characterized by a high injection of the surrounding gaseous
medium into the jet sides. Straight flow burners are mainly
employed with high reactive fuels. The velocity of the dust-
air mixture at the burner outlet is 20-28 m/sec and the
optimal velocity of the secondary air is 1.5 to 1.7 times the
velocity of the dust-air mixture.

Opposed Corner Firing with Corner Firing with Vertical Firing

Displaced Firing Encountering Jets Tangential Jets

A single straight flow burner can not produce intense mixing

of fuel and air, as in case of vortex burners, because it does
not have any swirl motion. While using straight flow
burners, normally the mixing of fuel and air is performed by
the interaction of flows from more than one burner. So,
burner arrangement is very critical in case of straight flow
burners to create intense mixture of fuel and air. There are
mainly four types of burner arrangements for straight flow
burners, namely,
• Opposed displaced (offset) firing
• Corner firing with encountering jets
• Corner firing with tangential jets
• Vertical firing or downshot firing
For opposed displaced firing, the burners are located on the
opposite walls. But, they are not placed in line; rather the
burners on one wall are offset compared to the opposite
burners. When, the two flows from opposite burners are
interacting, mixing is created. The flame of a straight flow
burner is longer in size than vortex burner. When the burners
are placed on walls, it must be noted that the flame should
not touch the opposite waterwall. This is to prevent
overheating of the water-wall surfaces.
The most popular firing arrangement for straight flow burner
is corner firing. Here, burners are placed at the four corners
of the furnace. The mounting angles of the burners are such
adjusted that either they can form encountering jets or
tangential jets as shown in figure.
In another arrangement, burners are throwing the flow in
downward direction from the roof of a step made on furnace
walls (as shown in figure). So, more than one jets are coming
in downward direction first and then moving in upward
direction to interact with others. This type of arrangement is
called downshot or vertical firing.
Dry Bottom Furnace

The most common type of pulverized coal-burning furnace

is the dry bottom furnace. When pulverized coal is burned in
a dry bottom boiler, more than 80 percent of the unburned
material or ash is entrained in the flue gas and is captured
and recovered as fly ash. The remaining 20 percent of the ash
is dry bottom ash, a dark gray, granular, porous,
predominantly sand size less than ½ in material that is
collected in a water-filled hopper at the bottom of the
furnace. When a sufficient amount of bottom ash drops into
the hopper, it is removed by means of high-pressure water
jets or screw conveyer and conveyed by sluiceways either to
a disposal pond or to a decant basin for dewatering, crushing,
and stockpiling for disposal or use.
In a dry bottom furnace, the bottom ash is collected in solid
form. The flue gas temperature must be less than the ash
fusion/melting temperature at the exit of the furnace (at the
entry of the convective duct). But, the temperature of the
lower portion of the furnace is such that ash is found in
molten condition at this region. In a dry bottom furnace, the
furnace bottom is cooled to bring down the temperature
below the solidification temperature of ash. At this
temperature, ash normally solidifies and forms clinkers too.
The ash is collected in an ash hopper at the bottom of the
furnace. The slope of the hopper walls should be greater than
the angle of repose (40o – 50o) so that no ash particle gets
stuck on this. The lower part of the hopper is submerged in a
water tank as shown in figure. This water lowers the
temperature of the lower part of the furnace and also acts as
a sealant for ambient cold air to enter into the furnace.

Furnace
Burner

Ash hopper Water tank

Dry Bottom Furnace

Wet Bottom Furnace

In a wet bottom furnace, 405 to 50% of the ash moves with

the flue gas. The rest is collected at bottom ash in molten
condition. The temperature of the lower portion of the
furnace should be higher than the melting temperature of ash
to keep it in liquid condition and to maintain its fluidity. So,
the flames in this type of furnace are placed near the furnace
bottom. The molten ash is poured into a slag tank containing
water. This tank is connected to the furnace bottom as shown
in the figure. The molten ash is quenched in water inside the
slag tank and solidifies in granular form. Water walls of these
boilers are cladded with a refractory coating to protect the
tubes. Usually coals having high ash content and low volatile
content are used in this type of furnaces.

Molten slag Furnace

Burner

Slag tank/Water tank

Wet Bottom Furnace

Advantages and Disadvantages of Pulverized Fuel

Firing

The advantages that pulverized coal firing offers are:

1. The removal of capacity limitations imposed by
stokers
2. Improvement is response to load fluctuations
3. The ability to burn all ranks of coal from lignite to
anthracite
4. Ease of combination firing of oil and gas with coal
 5. An increase in thermal efficiency because of lower
excess air for combustion and lower carbon loss than that
with stoker firing The limitations and undesirable features
are:
1. Large power consumption requirement for driving
pulverizers
2. High maintenance cost for pulverizers
3. Excessive fly-ash discharge through stack (without dust
collectors)
4. Erosion of boiler pressure parts by fly-ash entrained in
the flue gases (unless low gas velocities are maintained)
5. Erosion of ID fan blades and scrolls, even when the fans
are located after dust collectors
6. Relatively large furnace volume required for good
combustion
For understanding the last item, a brief review of the
combustion and the functions of a furnace may be helpful.
Combustion of coal is a chemical reaction in which carbon
combines with oxygen to form CO2. This gas tends to
blanket the coal particles and retard further combustion. To
maintain rapid combustion, this blanketing CO2 must be
scrubbed away and every particle of coal should be brought
rapidly into intimate contact with additional oxygen.
Pulverized coal firing requires that the coal be reduced to
from -2 inch diameter lumps to a powder, so fine that
approximately 70% will pass through a 200 mesh screen,
which requires a large amount of powder. The finely
pulverized coal is then very intimately mixed with
combustion air in the burner. However, after this initial
mixing the tiny coal particles are merely carried along in the
air stream and very little additional scrubbing by the air
occurs. Thus further contact of oxygen with coal must be
largely by diffusion. The furnace consequently has to be
relatively large to give necessary retention time for oxygen
to diffuse through the blanketing CO2 layer to reach the coal
particles and at the same time temperature must be
sufficiently high to complete combustion. After combustion,
since the residual ash particles are much smaller than the
original tiny coal particles, the former are easily carried
along the flue gases from the furnace.
At the same time, the pulverized coal fired boiler furnace
also has the function of cooling the combustion gases, so that
when they enter convection surfaces, they are below the
temperature at which slagging (melting of ash) occurs. This
function conflicts with that of maintaining high temperatures
necessary for to complete combustion.
It would therefore be preferable to separate these functions
and use the boiler furnace for cooling only. This would
require a separate small combustion chamber where high
turbulence and temperature may be maintained. Cyclone
furnace is an outgrowth of efforts to meet these needs.
Cyclone Furnace

The cyclone furnace is a water-cooled horizontal cylinder in

which fuel is fired, heat is released at extremely high rates
and combustion is completed. Its water cooled surfaces are
studded and covered with refractory chrome ore. Coal
crushed in a simple crusher, so that approximately 95% will
pass through a 4-mesh screen, is introduced at the end of the
cyclone. About 20% of the combustion air also tangentially
enters the burner and imparts a whirling motion to the
incoming coal. Secondary air with a velocity of around 300
ft/sec is admitted in the same direction tangentially at the
roof of the main barrel of cyclone and imparts a further
whirling or centrifugal action to the coal particles. The
incoming coal particles (except for a few very fine particles
that burn in suspension) are thrown to the walls by
centrifugal force, held in the slag and scrubbed by the high
velocity tangential secondary air. Thus the air required to
burn the coal is quickly supplied and the products of
combustion are rapidly removed.
Secondary
Cooling
water

Molten slag

Air + Fuel
air

Schematic Diagram of
a Cyclone Furnace

Even though the release of heat per unit volume in the

cyclone furnace is very high, the total amount of heat
actually absorbed is relatively low. This is because of small
amount of surface in the cyclone and the insulating
properties of the covering slag layer. This combination of
high heat release rate and low heat absorption insures the
high temperatures necessary to complete combustion and to
provide the desired liquid slag covering the surface. The
results of this method of combustion are that the fuel is burnt
quickly and completely in the small cyclone chamber and the
boiler furnace is used only for cooling the flue gases. Most
of the ash is retained as a liquid slag and tapped into the slag
tank under the boiler furnace. Thus the quantity of fly-ash is
low and its particle size is so fine that erosion of boiler
heating surfaces has never been experienced even at high
volatiles. The amount of bottom ash is very high (70% -
80%) and comes out of the furnace in molten or slag form.
The major drawback for this system is a very high heat loss
through the molten slag, which is poured from the cyclone
to the bottom of the furnace.