1. ME326 COMBUSTION AND HEAT
TRANSFER
Combustion Equipment and Power Cycles
Dr. Kihedu, J.
2. COMBUSTION OF COAL
Combustion Applications
• There are various types of burners, available in two
categories that depend on particle size;
Large particles on fuel beds, and
Small particles in pulverized form.
Coal Burning Equipment
• Over Feed Stokers
• Traveling-grate or Chain-grate Stoker
• Under Feed or Retort Stokers
• Pulverized Coal Burners
• Cyclone Furnace
3. Coal Burning Equipment
Over Feed Stokers
• The majority of burners operate on over feed principle as
fresh coal is dropped onto the fuel bed.
• The fuel bed thickness varies from 10 to 30 cm and can be
divided in four zones.
– The topmost zone is formed as the distillation zone.
– Then reduction zone (endothermic), oxidation zone
(exothermic) and ash zone.
• Air for combustion enters from below the fuel bed through
the grate and gets heated in turn cooling the ash.
• The oxygen reacts with part of the carbon coke bed.
• When the oxygen becomes depleted, the coke reduces the
CO2 back to CO in the reduction zone.
5. Coal Burning Equipment
Traveling-grate or Chain-grate Stoker
• Popular type of mechanical firing device in which coal is
fed from a hopper onto a grate moving through the
combustion chamber.
• The grate consists of a number of cast iron bars
interlocked to form a grate. The traveling-grate stoker has
more closely interlocking bars.
• The coal enters at one end and by the time it reaches the
other end, the combustion is complete and the ash falls
into the ash pit.
7. Coal Burning Equipment
Under Feed or Retort Stokers
• Coal is forced into the fire from below by a power ram or
screw and moves out to the sides as it burns.
• The main advantage - volatile matter passes through the
oxidation zone, thus ensuring its complete combustion.
• The grate is usually inclined and thus the burned fuel and
the ash move outwards as the fresh fuel is supplied.
• The air for combustion is supplied through tuyeres.
• Disadvantages;
– Some of the unburnt fuel may pass through the grate.
– Fusion of ash or clinker formation may result in an
uneven distribution of air.
9. Coal Burning Equipment
Pulverized Coal Burners
• In pulverized coal burners, more than 85% of the coal
particles should have diameter less than 0.063 mm.
• These finely ground particles are blown into the
combustion chamber by the hot primary air.
• This cloud of coal then burns inside the combustion
chamber in a manner similar to that of a droplet liquid fuel.
• Advantages are: high efficiency, greater flexibility in their
control and operation, flexibility in the quality of coal to be
used, and easy design of burners.
• Disadvantages are high cost of pulverizing the fuel and that
most of the ash is carried along with the exhaust.
11. Coal Burning Equipment
Cyclone Furnace
• Small coal articles usually with a diameter less than 6mm
are burned in suspension with air.
• The fuel swirls forward into the main chamber where it
meets the high speed tangential stream of secondary air.
• Some tertiary air is supplied at the axis of the chamber to
ensure the burning of any fine coal particles.
• Temperature is high therefore ash melt away - ash
globules are carried to the wall by centrifugal force as the
furnace is inclined to permit molten ash to flow down.
• Optimization for smaller furnace is to be done.
13. Fuel Oil Burners
Fuel oils have domestic and industrial use;
• Kerosene is used for illumination and heating
• Types of kerosene lamps
– Yellow flame wick lamp - incomplete combustion,
incandescent carbon particles in the flame radiate light
– Wick fed mantle lamp - mantle increases illuminating
power, kerosene vapours are supplied and burned inside
mantle
– Pressure fed mantle lamp – kerosene is supplied under
pressure through a nozzle rather than a mere wick.
14. Fuel Oil Burners
Types of domestic stoves;
• Wick type - kerosene rises through number of wicks in
concentric cylinders. Cylinders are perforated and heated
by the burner flame itself to vaporize the fuel to give a
slow smokeless flame.
• Pressure type – kerosene reservoir is pressurized by a
small hand pump to rise the kerosene through a tube to
the burner head where it gets vaporized. Out of a small
jet, vapours mix with air to give a turbulent blue flame.
15. Fuel Oil Burners
• Industrial burners and furnaces normally operate with cheap
heavier oils.
• The combustion of such oils requires vaporization or at least
atomization into droplets.
• These vapours or droplets need to be thoroughly mixed with
air to give a stable flame.
• The finer the atomization, the more rapid will be the
evaporation, resulting in more rapid and efficient combustion.
• Typical types include; Vaporizing burner, Rotating cup burner,
Mechanical or oil-pressure atomizing burner, Steam or high-
pressure air atomizing burner and Low-pressure air atomizing
burner.
16. Fuel Oil Burners
Vaporizing Burners
• These are similar to the wick and pressure stoves.
• The oil is fed by gravity to the bottom of a pot by a pipe where
fuel is evaporated by the radiant heat from the flame and the
nearby heated surface.
• The vapours rise in the pot and mix with the primary air. The
fuel-air mixture near the bottom of the pot is too rich to
support combustion.
• Consequently, the flame rises to a position just above the rim
where enough air is available to give a good burning mixture.
• Some soot formation is inevitable in such burners which
necessitate periodical cleaning.
17. Fuel Oil Burners
Rotating cup burner
• Used in steam boilers, capable of using variety of oils without any
major modifications in their design.
• Oil flows through a tube into the cup rotated at speeds of 3,500
to 10,000 rpm therefore centrifugal force spreads oil into a thin
film on the inside walls of the cup.
• About 10 to 15% of the theoretical air is supplied as primary air.
• The angle at which the air hits the fine oil mist may be adjusted
by regulating the relative position of the cup and the air cone.
• Additional atomizing effect is obtained as air blasts the fuel mist.
• The shape of the flame is controlled by the shape of the cup and
the position of the air nozzle.
• Secondary air is usually supplied by a natural draft through air
shutters in the furnace wall.
19. Fuel Oil Burners
Mechanical or oil-pressure atomizing burner
• Oldest and commonly used burners for land and marine boilers.
• Atomization by fluid pressure and released through an orifice.
• Oil is preheated to attain viscosity of 10 to 30 centistokes and fed
tangentially under high pressure into a conical swirl chamber.
• Half of the oil pressure is consumed in generating rotational
energy in the liquid, which then flows out from the orifice at high
velocity in the form of fine droplets forming a cone of oil mist.
• For large capacity boilers, a greater number of, boilers, a great
number of burners are employed instead of a single large capacity
burner as it gives better atomization at lower pressure.
21. Fuel Oil Burners
Steam or high-pressure air atomizing burner
• Operates like a "scent spray".
• For heavier oils or for boilers, steam is preferred as it also
preheats the oil and is available at high pressure.
• However, compressed air gives better mixing of air and fuels and
combustion.
• As with most steam atomizers, the steam and oil flow side by
side, thereby preheating the oil so that the viscosity of the oil is
reduced, resulting in smaller oil droplets.
• The pressure of the air or steam required for such atomizers is
usually greater than 1 kg/cm2 and may be as high as 7 kg/cm2,
depending upon the viscosity of the oil.
• Air and fuel may either mix inside the burner or totally outside it,
i.e., inside the combustion chamber
23. Fuel Oil Burners
Low-pressure air atomizing burner.
• The principle is the same, the only difference is that the air
pressure is low, about 0.035 to 0.15 kg/cm2.
• Such burners are more suitable for lighter, less viscous oils,
such as kerosene.
• The primary air required for atomization is comparatively
higher, of about 20% or more.
• Design of almost all the burners is as a result of the long
experience gained in the use of fuel oils.
• Lot of research work is in progress on the mechanism of spray
combustion too, but the gap between the work done so far, and
that required for the direct application, is still wide.
24. Gas Burners
• Gas burners are mainly used for purposes of cooking and
heating in homes, and for ovens and furnaces in
industries. Three main classes:
– Non-aerated burners,
– Aerated burners, and
– Surface combustion burners.
• Non aerated burners are used where long, lazy flames are
required, e.g., in baking furnaces. In these burners, the
fuel enters from a pin hole jet or a slot and all the oxygen
is supplied by the air around the flame
• The air is supplied by natural convection only in the case
of small burners or by forced convection in the large one.
• A stable flame is obtained irrespective of wide pressure
or velocity fluctuations.
25. Gas Burners
• Aerated burners are the most widely used type of gas burners
for domestic and industrial uses. These are based on the
famous Bunsen burner principle.
• The fuel enters a tube through a jet. The suction induced by
the jet of gas draws the primary air.
• The primary fuel-air mixture flows through the tube to the
burner top or port where the flame is stabilized. The
secondary air is supplied from the atmosphere by
entrainment, through the outer envelope of the flame.
• If the primary air supply is insufficient, the flame becomes
long, slightly smoky, and luminous; and if the primary air
supply is increased, the flame becomes short and non-
luminous.
• Depending upon the pressure of the gas admitted to the
burner, the aerated burners may be either the atmospheric or
high pressure type
27. Gas Burners
Atmospheric gas burner have the following characteristics:
• Controllable over a wide range of turn down without
flashback,
• Provide uniform heat distribution over the heated area,
• Capable of completely burning the gas,
• Provide ready ignition with the flame traveling rapidly
from port to port,
• Operate quietly during ignition, burning, and extinction
• Withstand severe heating and cooling during the life of
the appliance.
29. Gas Burners
Pressure type aerated burners
• Concentric primary air and gas jets under pressure are used in
place of the simple gas jet.
• Separate jets can be used to induce secondary air, or they can
be first fed into a mixing chamber.
• Large furnaces may be heated by multiple gas jets set in a
common head, each jet being surrounded by compressed air,
with a concentric orifice for the supply of air.
• The premixed stoichiometric proportions of gas and air can be
burned in a tube or a narrow tunnel.
• These burners can support a laminar or turbulent flame,
depending upon the heat release rate required. Turbulence
helps in the proper mixing of fuel and air.
30. Gas Burners
• In surface combustion phenomenon gas premixed with
more than 100% primary air is fired tangential to the
incandescent porous refractory surface of the furnace.
• The incandescent surface apparently has a catalytic and
radiant effect, which promotes very rapid and complete
combustion even at high burning rates.
• Most gas burners can be used for different gases by
minor adjustment in the fuel jet and/or burner head.
• As a result of the systematic research carried out in the
field of combustion, it has become possible to design a
gas burner which is efficient and stable in operation
33. Thermal Power Plants (System A)
1 2tW m h h
Turbine
2 3outQ m h h
Condenser
4 3pW m h h
Pump
1 4inQ m h h
Boiler
5
6
w
6 5
6 5
out w
out w p
Q m h h
Q m c T T
34. Boiler
• Steam is used in
vapor power
cycles
– Low cost,
– Availability, and
– High enthalpy
of vaporization
• Heat transfer to
be discussed
later…
35. Power Cycles
• The model cycle for vapor power cycles is the
Rankine cycle which is composed of four internally
reversible processes:
– Constant-pressure heat addition in a boiler,
– Isentropic expansion in a turbine,
– Constant-pressure heat rejection in a
condenser,
– Isentropic compression in a pump.
• Steam leaves the condenser as a saturated liquid
at the condenser pressure.
36. Carnot vs Rankine Cycles
• Carnot cycle is the most efficient
power cycle operating between two
specified temperature limits.
– NOT suitable for actual power
cycles due to impracticalities which
can be eliminated by:
• Superheating the steam,
• Completely condensing steam.
• The modified Carnot cycle is called the
Rankine cycle, where the isothermal
processes are replaced with constant
pressure processes.
38. Vapor Power Deviation and Pump and
Turbine Irreversibilities
(a) Deviation of actual vapor power cycle from the ideal Rankine cycle.
(b) The effect of pump and turbine irreversibilities on the ideal cycle.
40. Increasing thermal efficiency of the
Rankine cycle
• Increasing the average temperature at which heat is added to
the working fluid and/or by
– The average temperature during heat addition can be
increased by raising the boiler pressure, or
– Superheating the fluid to high temperatures.
– There is a limit to the degree of superheating, however,
since the fluid temperature is not allowed to exceed a
metallurgically safe value.
• Decreasing the average temperature at which heat is
rejected to the cooling medium.
– The average temperature during heat rejection can be
decreased by lowering the turbine exit pressure.
– Consequently, the condenser pressure of most vapor
power plants is well below the atmospheric pressure.
42. Increasing efficiency (Superheating)
• Superheating has the added advantage of decreasing the
moisture content of the steam at the turbine exit.
– Lowering the exhaust pressure or raising the boiler
pressure, however, increases the moisture content.
• To take advantage of improved efficiencies at higher boiler
pressures and lower condenser pressures, steam is
reheated after expanding in the high-pressure turbine.
– This is done by extracting the steam after partial
extraction in the high-pressure turbine, sending it back to
the boiler where it is reheated at constant pressure, and
returning it to the low-pressure turbine for complete
expansion to the condenser pressure.
• The average temperature during the reheat process, and
thus the thermal efficiency of the cycle, can be increased by
increasing the number of expansion and reheat stages.
44. Increasing efficiency (Re-generation)
• Another way of increasing the thermal efficiency of
the Rankine cycle is by re-generation.
• During a regeneration process, liquid water
(feedwater) leaving the pump is heated by some
steam bled off the turbine at some intermediate
pressure in devices called feedwater heaters.
• The two streams are mixed in open feedwater
heaters, and the mixture leaves as a saturated
liquid at the heater pressure.
• In closed feedwater heaters, heat is transferred
from the steam to the feedwater without mixing.
46. Increasing efficiency - Co-generation
• The production of more than one useful form of
energy (such as process heat and electric power)
from the same energy source is called co-generation.
• Co-generation plants produce electric power while
meeting the process heat requirements of certain
industrial processes.
• This way, more of the energy transferred to the fluid
in the boiler is utilized for a useful purpose.
• The faction of energy that is used for either process
heat or power generation is called the utilization
factor of the cogeneration plant.
47. Increasing efficiency (Binary Cycle
and Combined Cycle)
• The overall thermal efficiency of a power plant can be
increased by using binary cycles or combined cycles.
• A binary cycle is composed of two separate cycles, one
at high temperatures (topping cycle) and the other at
relatively low temperatures.
• The most common combined cycle is the gas-steam
combined cycle where a gas-turbine cycle operates at
the high-temperature range and a steam-turbine cycle at
the low-temperature range.
• Steam is heated by the high-temperature exhaust gases
leaving the gas turbine.
• Combined cycles have a higher thermal efficiency than
the steam- or gas-turbine cycles operating alone.
50. 50
Flow in Steam Power Plant
Path of the steam
• Steam is produced at high pressure from a boiler.
• The steam from boiler first goes to super-heater
• The super heated steam next goes to turbine.
– High pressure steam rushes through the blades of
the turbine.
– The momentum of steam is transferred to the
turbine.
• The turbine is coupled to the generator.
• The turbine transfers its momentum to the generator
to produce electricity.
51. 51
Flow in Steam Power Plant (Cont.)
Path of condensed steam/feed water
• Low pressure steam is condensed and exhausted to the
condenser.
• Condensed steam I hot water is pumped the LP
heater.
• Next, hot water is pumped to the HP heater. (Feed
pump supplies additional water for steam and
water leakages).
• Next, hot water from HP heater goes to economizer.
• Next, hot water from economizer goes to the boiler.
52. 52
Cooling Water in Steam Power Plant
Open Circuit Systems
• Wet steam comes to the condenser.
• Wet steam is condensed when it comes in contact with the cool
water tubes of condenser.
• The cool water after receiving heat from wet steam is taken to
river, lake etc. and discharged
• Fresh water from river or lake is taken back to the condenser.
Closed Circuit Systems
• Wet steam comes to the condenser.
• Wet steam is condensed when it comes in contact with the cool
water tubes of condenser.
• The cool water receiving heat from wet steam is taken to the
cooling tower and cooled again.
• Cool water from tower is re-circulated back to the condenser.
53. 53
Gas turbine power plant (GTPP)
Low and high pressure air compressor (LPC & HPC),
High and low pressure gas turbine (HPT & LPT),
Combustion chamber (CC)
54. 54
Operation of a GTPP
1. Atmospheric air is passed through air filter.
2. Purified air is passed to the low-pressure compressor
(LPC) - the air is compressed to a certain extent.
3. Air is passed to the intercooler - temperature of outlet
air from LPC is reduced in inter cooler.
4. Air is passed to the high-pressure compressor (HPC) -
the air is again compressed to high pressure.
5. Air is passed through regenerator where it absorbs heat
from outgoing exhaust gases.
6. High temperature, high pressure air is mixed with fuel
and burnt in combustion chamber (CC).
55. 55
Operation of a GTPP
5. Burnt gas mixture first expands through the high
pressure gas turbine (HPT) and rotates the turbine shaft.
6. Not all the heat and mechanical energy of burnt gas is
utilized in running HPT. Therefore the gas is reheated
again in combustion chamber (CC).
7. Burnt gas is again passed through low pressure turbine
(LPT) where it rotates turbine shaft.
8. Exhaust gas of low-pressure turbine is sent through the
regenerator to atmosphere.
9. In the regenerator the burnt gas heats the incoming air
from HPC.
10.Since the turbines namely HPT and LPT are connected
to load useful work is done.
56. 56
Advantages of Gas Turbine Power Plant
• Natural gas or any poor quality fuel which is widely
available can be used as fuel in gas turbine power plant.
• Gas turbines are widely used in aircrafts, ships where
the weight and size are more important.
• Low initial cost compared to steam power plant.
• Quick starting of the plant.
• Low maintenance cost.
• It does not require heavy foundation and buildings.
• Speed is very high, (40,000 to 100,000 RPM).
57. 57
Disadvantages of Gas Turbine Power Plant
• Net work output is low since a lot of the power is used to
the compressors.
• Special materials are required for the parts of power plant,
since high temperature (2000oC) and high speeds (100000
RPM) are involved.
• Part load efficiency is poor compared to diesel power plant.
• High pitch noise due to very high speed
• Special high temperature alloys are needed in the
combustion chamber and in the turbine to compensate for
the higher operating temperature.
• Large size exhaust duct due to increased requirement of
air for combustion and also for cooling.
58. Reciprocating ICE
• Internal combustion engine
(ICE) operates on a
mechanical cycle because
the piston system goes to
the same initial points.
• But not in thermodynamic
cycle because new air and
fuel enters the engine in
order to initiate the
combustion process.
• Internal cycle:
– Intake stroke
– Compression stroke
– Power stroke
– Exhaust stroke
60. 60
Reciprocating ICE (Cont.)
• Bottom-dead center (BDC) – piston position where volume is
maximum
• Top-dead center (TDC) – piston position where volume is minimum
• Clearance volume – minimum cylinder volume (VTDC)
• Compression ratio (r) - is the ratio of volume at bottom dead center
divided by volume at top dead center
• Displacement volume
• Four-stroke engine - piston executes intake, compression,
expansion, and exhaust while crankshaft completes two revolutions
• Two-stroke engine - piston executes intake, compression,
expansion, and exhaust while crankshaft completes one revolution
TDC
BDC
V
V
V
V
r
min
max
TDCBDCdisp VVV
65. Air Standard Otto Cycle
This is an ideal cycle that assumes that
heat addition occurs instantaneously
while the piston is at TDC.
Process
(1-2) Isentropic Compression
Compression from ν1 => v2
↓ ↓
BDC(β=180º ) TDC (θ=0º)
(2-3) Constant Volume heat input: QH
•While at TDC: umin
•Ignition of fuel
(3-4) Isentropic Expansion
•Power is delivered while s = const.
(4-1) Isentropic Expansion
•QL at umax=constant (BDC, θ
=180º)
66. 66
Otto Cycle Analysis
• Thermal efficiency
• Heat addition (process 2-3, v = const)
• Heat rejection (process 4-1, v = const)
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