Solid Fuels (Combustion of Fuel)

1. Solid Fuels
Combustion of Coal

2. Combustion of Coal
• When a solid fuel particle is exposed to a hot
gas flowing stream it undergoes three stages
of mass loss
i. Drying
ii. Devolatilization
iii. Char combustion
The relative significance of these three is
indicated by proximate analysis of coal

3. Combustion of Coal
i. Drying
 The combustible material generally constitutes water e.g. lignites up to 40 %
 Upon entry into the gas stream, heat is convected and radiated to the particle surface
and conducted into the particle
 The drying time of a small pulverized particle is the time required to heat up the
particle to the vaporization point and drive off the water
ii DEVOLATILIZATION
 When the drying of a solid fuel particle is complete, the temperature rises and the
solid fuel begins to decompose
 Devolatilization or pyrolysis is the process where a wide range of gaseous products
are released through the decomposition of fuel.
 The volatile matter (VM) comprises a number of hydrocarbons, which are released
in steps
 Since the volatiles flow out of the solid through the pores, external oxygen cannot
penetrate into the particle, hence the devolatilization is referred to as the pyrolysis
stage

4. Combustion of Coal
• DEVOLATILIZATION
• The rate of devolatilization and the pyrolysis products depend on the temperature
and the type of the fuel
• The pyrolysis products ignite and form an attached flame around the particle as
oxygen diffuses into the products
• While water vapour is flowing out of the pores, the flame temperature will be low
• For lignite coals, pyrolysis begins at 300-400 oC releasing CO and CO2
• Ignition of the volatiles occurs at 400-600 oC
• CO, CO2, chemically formed water, hydrocarbon vapours, tars and hydrogen are
produced as the temperature reaches 700-900 oC
• Above 900 oC pyrolysis is essentially complete and the char (fixed carbon) and
ash remain

5. Combustion of Coal
• DEVOLATILIZATION
• For other types of coal, devolatilization proceeds differently
• Although the proximate analysis provides an estimate of the VM, the actual yield
of VM and its composition may be affected by a number of factors like:
• Rate of heating
• Initial and final temperature
• Exposure time at the final temperatures
• Particle size
• Type of fuel
• Pressure

6. Combustion of Coal
• CHAR COMBUSTION
• The devolatilized fuel, known as char, burns rather slowly.
• For example, it would take 50–150 sec for a char of size less than 0.2 mm to burn out
• Since it takes this long to burn completely, some of the particles may not burn out in
the bed before leaving.
• The elutriation of these unburnt, fine char particles results in combustion losses.
• The combustion of a char particle generally starts after the evolution of volatiles from
the parent fuel particle, but sometimes the two processes overlap.
• The char, being a highly porous substance, has a large number of internal pores of
varying size

7. Combustion of Coal
• Surface areas of the pore walls are several orders of magnitude greater than the
external surface area of the char.
• Oxygen diffuses into the pores and oxidizes the carbon on the inner walls of the pores.
• During the combustion of a char particle, oxygen from the bulk stream is transported to
the surface of the particle.
• The oxygen then undergoes an oxidation reaction with the carbon on the char surface
to produce CO.
• The CO then reacts outside the particle to form CO2.
• The mechanism of combustion of char is fairly complex.
• Some factors which effect the burning rate are:
i. Oxygen concentration
ii. Gas temperature
iii. Reynolds number
iv. Char size and porosity

8. Combustion Systems for solid Fuels
i. Fixed-bed combustion
ii. Fluidized-bed combustion
iii. Suspension Firing/Pulverized Coal combustion
Fixed-bed Combustion:
• Fixed-bed systems require least fuel size reduction compared to other two
systems mentioned above
• Crushed coal up to 4 cm in size is used
• Solid fuel handling and feeding are the focus of much effort compared with
gas or liquid fuels
• A stoker type of boiler is an example of shallow fixed- bed combustion
system

9. Combustion Systems for solid Fuels
Fixed-bed Combustion:
• A continuous fuel feed system is referred to as a stoker
• Air flows up through the grate and through the bed of ash, char and fuel
• Since the bed is thin, the pressure drop is less and the blower costs are
reduced
• There are two classes of stoker, which are distinguished by the direction
of fuel feed: •
Overfeed stokers There are two general types of overfeed stokers, which are
distinguished by the relative direction of fuel and air flow, as well as by the
manner of fuel feed.
a. Cross-feed stokers
b. Spreader stokers
Overfeed—The fuel is fed onto the top of the bed and flows down as it is consumed
while combustion air flows up through successive layers of ash, incandescent coke,
and fresh coal;
Underfeed—The flows of coal and combustion are parallel and usually upward;

10. A . Cross-feed stokers In cross-feed stokers, the fuel is dumped by gravity
from a hopper onto one end of a moving grate, which carries the fuel into
the furnace and down its length. The fuel moves horizontally and the
combustion air moves upward at right angles to the fuel.
Cross-feed stoker boilers are also termed travelling- grate stoker boilers. In
the cross-feed stoker the fuel flows at right angles to the air flow. Cross-
feed (travelling-grate) boilers are more commonly used with smaller scale
industrial boilers
B. Spreader stokers In spreader stokers, the fuel is propelled into the
furnace. A portion of the fuel burns in suspension while the rest burns on
the grate. In most units, the fuel is pushed off a plate under the storage
hopper onto revolving paddles (either overthrow or underthrow), which
distribute the fuel on the grate. The largest fuel particles travel the furthest,
while the smallest become partially consumed during their trajectory

11. Spreader stokers
Uses both suspension and grate
burning
Coal fed continuously over
burning coal bed
Coal fines burn in suspension
and larger coal pieces burn on
grate
Good flexibility to meet
changing load requirements
Preferred over other type of
stokers in industrial application
Spreader stoker with travelling grate

14. Combustion Systems for solid Fuels
Overfeed stokers:
• The flow of fuel and air is counter current. Fuel is fed onto the top of the
bed and moves downward as it is consumed
• Air flows up through the layers of ash, char and fresh fuel
• Volatile gases burn above the bed and some fine fuel particles burn above
the bed
• Overfire air is supplied to complete the combustion
• Ash is removed by dumping, shaking, vibrating or continuously moving the
grate
• The bed is usually 10-20 cm deep
• Fresh fuel is heated by the upward moving gases and by radiation from
the flame above the bed
• The speed of the grate is adjusted so that the coal burns out before it
reaches the edge of the grate and the ash dumps nto the ash pit below

15. Combustion Systems for solid Fuels
Underfeed stokers:
• The flow of fuel and air is upward
• The evolved moisture, volatile matter and air pass through the burning fuel
layer
• The bed is up to 1 m deep near the centre.
• Fresh fuel is forced from below by a screw conveyor
• The grate is usually inclined so that the ash automatically moves outwards
as the fresh fuel is forced from below

16. Combustion Systems for solid Fuels
Crossfeed stokers:
• An older type of stoker and grate arrangement
• This type of system is often used for hard-to-feed fuels such as
unprocessed refuse, bagasse, lignite, wood pulp etc
• The fresh fuel is moved to a horizontal platform where it ignites
• When the next charge of the fuel enters, the ignited fuel moves across a
sloping vibrating grate
• The air flows upward through the grate
• Such stokers operate with high excess air and considerable fuel loss in
the ash pit

18. Fluidized-bed combustion
• 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.
• There are two principal types of fluidized
bed boilers:
• 1. Bubbling fluidized bed (BFB)
• 2. Circulating fluidized bed (CFB)

20. Quality of fluidization

21. Fluidized-bed combustion
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 heat-absorbing tubes.
• The vessel would generally hold bed materials. The
open space above this bed, known as freeboard, is
enclosed by heat-absorbing 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.

22. Fluidized-bed combustion
Bubbling fluidized bed (BFB)
• As the velocity is increased above minimum fluidization,
bubbles are formed
• The bubbles are referred to as the dilute phase
• The size of the bubbles depend upon the type of the
distributor plate
• A plate with a few large orifices inlets will have larger
bubbles while a plate with many small inlets will have
many bubbles near the plate
• In fluidized bed combustion the bed temperature is
maintained well below the melting point of the ash
• To capture SO2 in the bed, limestone CaCO3 which is
calcined in the bed to form CaO
• The optimum temperature for CaO reaction with SO2 to
form CaSO4 at atmospheric pressure is 815-900 C

23. Pressurised Fluidised Bed Combustion System (PFBC).
Pressurised Fluidised Bed Combustion (PFBC) is a variation of fluid bed
technology that is meant for large-scale coal burning applications. In
PFBC, the bed vessel is operated at pressure up to 16 ata ( 16 kg/cm2).
The off-gas from the fluidized bed combustor drives the gas turbine. The
steam turbine is driven by steam raised in tubes immersed in the fluidized
bed. The condensate from the steam turbine is pre-heated using waste
heat from gas turbine exhaust and is then taken as feed water for steam
generation.
The PFBC system can be used for cogeneration or combined cycle power
generation. By combining the gas and steam turbines in this way,
electricity is generated more efficiently than in conventional system. The
overall conversion efficiency is higher by 5% to 8%. .
At elevated pressure, the potential reduction in boiler size is considerable
due to increased amount of combustion in pressurized mode and high
heat flux through in-bed tubes.

24. Fluidized-bed combustion
Circulating Fluidized Bed Boiler
• 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 gas–solid 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.

25. Bubbling Fluidized Bed (BFB)
FACTORS AFFECTING COMBUSTION EFFICIENCY
• The combustion efficiency of a bubbling fluidized bed (BFB) boiler is
typically up to 90% without fly-ash recirculation and could increase to
98–99% with recirculation .
• The efficiency of a circulating fluidized bed (CFB) boiler is generally
higher due to its tall furnace and large internal solid recirculation. The
efficiency depends to a great extent on the physical and chemical
characteristics of the fuel as well as on the operating condition of the
furnace.
Factors affecting the combustion efficiency can be classified into three
categories:
• 1. Fuel characteristics
• 2. Operational parameters
• 3. Design parameters

26. EFFECT OF Fuel characteristics ON COMBUSTION
EFFICIENCY
• The fuel ratio of a fuel is the ratio of fixed carbon (FC) and VM
contents of the fuel.
• This ratio has an important effect on the combustion efficiency of
coal in a CFB boiler
• Higher ratios possibly leading to lower combustion efficiencies
• A high rank fuel like anthracite has a higher fuel ratio than a low rank
fuel like lignite.
• For this reason low-rank fuels (or low fuel ratio) like lignite and
• bituminous have higher efficiencies than anthracite.
• The fuel ratio is easily computed from the proximate analysis of a
fuel

27. Effect of Operational parameters ON COMBUSTION EFFICIENCY
Fluidizing Velocity
• The combustion efficiency generally decreases with increasing fluidizing velocity due to higher
• entrainment of the unburnt fines and oxygen by-passing.
Excess Air
• The mixing between fuel and air is never perfect. Some areas will be oxygen-deficient and some
• areas even oxygen-starved.
• Ultimately, all fuel particles must have the necessary oxygen to complete their burning; thus, extra
oxygen is always provided in FB boilers in the form of excess air.
• The combustion efficiency improves with excess air, but this improvement is less significant
above an excess air of 20%.
• Bubbling bed boilers may need a slightly higher amount of excess airthan CFB boilers.
Combustion Temperature
• The combustion efficiency generally increases with bed temperature because the carbon fines
• burn faster at high temperatures.
• The effect of temperature is especially important for less
• reactive particles, which burn under kinetic-controlled regimes.

28. EFFECT OF DESIGN PARAMETERS ON COMBUSTION
EFFICIENCY
• The combustion efficiency of bubbling FBs
is affected by several design parameters :
• Bed height
• Freeboard height
• Recirculation of unburnt solids
• Fuel feeding
• Secondary air injection

29. Bed Height
A deeper bubbling bed would give higher combustion efficiency as it provides longer
residence time for combustion, but it increases the fan power requirement and entrainment
rate of solids.
Recirculation of Unburnt Solids
The recirculation of fly ash has a positive influence on the combustion efficiency.
Freeboard Height
The freeboard height increases the combustion efficiency as it allows longer time for
combustion. A heavily cooled freeboard, however, may not be as effective.
Fuel-Feeding
The fuel ratio, defined earlier, is an important parameter affecting the combustion efficiency.
A low fuel-ratio is often responsible for low combustion efficiency especially in a BFB
furnace, but under bed feeding gives higher efficiency for low fuel-ratio. Over-bed feeding is
more effective for higher fuel ratio feed stock.
Secondary Air Injection
Secondary air is more commonly used in bubbling bed boilers, especially for biomass fuels.
It helps burn the volatiles and also destroys NOx more effectively. For low VM fuels,
secondary air may not improve the combustion efficiency (Oka, 2004), but it could reduce
the requirement of in-bed tubes, which are prone to erosion.

30. Advantages of Fluidised Bed Combustion Boilers
1. High Efficiency
FBC boilers can burn fuel with a combustion efficiency of over 95% irrespective of ash
content.
FBC boilers can operate with overall efficiency of 84% (plus or minus 2%).
2. Reduction in Boiler Size
High heat transfer rate over a small heat transfer area immersed in the bed result in overall
size reduction of the boiler.
3. Fuel Flexibility
FBC boilers can be operated efficiently with a variety of fuels. Even fuels like washer
rejects and agro waste can be burnt efficiently. These can be fed either independently or in
combination with coal into the same furnace.
4. Ability to Burn Low Grade Fuel
FBC boilers would give the rated output even with inferior quality fuel. The boilers can fire
coals with ash content as high as 62% and having calorific value as low as 2,500 kCal/kg.
Even carbon content of only 1% by weight can sustain the fluidised bed combustion.
5. Ability to Burn Fines
Coal containing fines below 6 mm can be burnt efficiently in FBC boiler, which is very
difficult to achieve in conventional firing system.
6. Pollution Control
SO2 formation can be greatly minimized by addition of limestone or dolomite for high
sulphur coals. 3% limestone is required for every 1% sulphur in the coal feed. Low
combustion temperature eliminates NOx formation.

31. 7. Low Corrosion and Erosion
The corrosion and erosion effects are less due to lower combustion temperature,
softness of ash and low particle velocity (of the order of 1 m/sec).
8. Easier Ash Removal – No Clinker Formation
Since the temperature of the furnace is in the range of 750 – 900 °C in FBC boilers, even
coal of low ash fusion temperature can be burnt without clinker formation. Ash removal is
easier as the ash flows like liquid from the combustion chamber. Hence less manpower is
required for ash handling.
9. Less Excess Air – Higher CO2 in Flue Gas
The CO2 in the flue gases will be of the order of 14 – 15% at full load. Hence, the FBC -
boiler can operate at low excess air - only 20 - 25%.
10. Simple Operation, Quick Start-Up
High turbulence of the bed facilitates quick start up and shut down. Full automation of
start up and operation using reliable equipment is possible.
11. Fast Response to Load Fluctuations
Inherent high thermal storage characteristics can easily absorb fluctuation in
fuel feed rates.
Response to changing load is comparable to that of oil fired boilers.
12. No Slagging in the Furnace–No Soot Blowing
In FBC boilers, volatilisation of alkali components in ash does not take place
and the ash is non sticky. This means that there is no slagging or soot blowing.

32. 13 Provisions of Automatic Coal and Ash Handling System
Automatic systems for coal and ash handling can be incorporated, making the plant easy to
operate comparable to oil or gas fired installation.
14 Provision of Automatic Ignition System
Control systems using micro-processors and automatic ignition equipment give excellent
control with minimum manual supervision.
15 High Reliability
The absence of moving parts in the combustion zone results in a high degree of reliability
and low maintenance costs.
16 Reduced Maintenance
Routine overhauls are infrequent and high efficiency is maintained for long periods.
17 Quick Responses to Changing Demand
A fluidised bed combustor can respond to changing heat demands more easily than stoker
fired systems. This makes it very suitable for applications such as thermal fluid heaters,
which require rapid responses.
18 High Efficiency of Power Generation
By operating the fluidised bed at elevated pressure, it can be used to generate hot
pressurized
gases to power a gas turbine. This can be combined with a conventional steam turbine
to improve the efficiency of electricity generation and give a potential fuel savings of at
least 4%.

33. Pulverized Fuel Combustion
Pulverized coal powder blown with
combustion air into boiler through
burner nozzles.
•Combustion temperature at 1300 -1700 °C
•Benefits: varying coal quality coal, quick
response to load changes and high pre-heat air
temperatures
Coal is pulverized to a fine powder, so that less than 2% is +300
microns, and 70-75% is below 75 microns.
Coal is blown with part of the combustion air into the boiler plant
through a series of burner nozzles.
The particle residence time in
the boiler is typically 2 to 5
seconds and the particles
has to be small enough to be
completely combusted during
this time period.

34. Advantages
Its ability to burn all ranks of coal from anthracitic to
lignitic, and it permits combination firing (i.e., can use
coal, oil and gas in same burner). Because of these
advantages, there is widespread use of pulverized coal
furnaces.
Disadvantages
High power demand for pulverizing
Requires more maintenance, flyash erosion and
pollution complicate unit operation
Pulverized Fuel Boiler (Contd..)