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ME 416/516
ME 416/516
Motivation
ƒ Boilers, were a major part of the Industrial Revolution
beginning about 1700. They are major consumers of
industry and building energy consumption today.
ƒ Industry: boilers are used for power generation,
process heat (e.g., refineries, petrochemicals, paper
mills, tire manufacturing, etc.) and heating.
ƒ In buildings, boilers are used for steam primary heat,
terminal reheat systems, water heating and absorption
chillers.
ME 416/516
ME 416/516
Introduction
ƒ There is a tremendous variation in boiler design and
size- ranging from home heating size of capacity less
than 100 Ibm/hr of steam to utility boilers in excess of
10 million Ibm/hr.
ƒ The focus here will be on boilers sized in between
these two extremes, which is also where there is the
greatest diversity of design.
ƒ In order of increasing capacity, three boiler types are
the fire-tube, water-tube and waterwall boilers.
ME 416/516
Fire-Tube Boiler (~1800)
ƒ The fire tube boiler, the oldest design, is made
so the products of combustion pass through
tubes surrounded by water in a shell.
ƒ The furnace/flame volume can either be inside
or external to the shell that contains the water.
ƒ The upper steam capacity of fire tube boilers is
about 20,000 Ibm/hr, and the peak pressure
obtainable is limited by their large shells to
about 300 psi.
ƒ Fire-tube boilers are used for heating systems.
ME 416/516
Cleaver-Brooks
Horizontal, four-pass, forced-draft fire-tube boiler
ME 416/516
Water-Tube Boiler (1867)
ƒ A water-tube boiler is one in which the products of
combustion pass around the outside and heat
tubes containing the water.
ƒ The water tube diameter is much smaller than the
shell diameter of a fire-tube boiler, so much higher
pressures can be obtained, well over 2000 psi.
ƒ The furnace and boiler tube area must be
surrounded by a heavily insulated refractory wall
to prevent heat transfer through the boiler walls.
The refractory lining is a high maintenance item.
ME 416/516
Type FM integral-furnace package boiler
Babcock and Wilcox
ME 416/516
“D-type” water-tube boiler
“Flex-tube” water-tube
boiler
English Boiler Co.
Steam Drum
Mud Drum
Water Tubes
Furnace
ME 416/516
“D-type” water-tube boiler
ABB
ME 416/516
Bryan
Exterior of small water-tube package boiler
ME 416/516
Package Boiler
ƒ All but the largest boilers used for heating and
industrial purposes are packaged boilers.
ƒ They are factory-built and shipped whole or in
modular components to the customer.
ƒ Many are constructed in an elongated shape that
will fit through large building doors with minimal field
adjustment required.
ME 416/516
Water-tube package boiler under construction
Riley
ME 416/516
Bryan
Large package boiler installation
ME 416/516
Nebraska Boilers
Large package boiler
ME 416/516
Waterwall Boiler
ƒ All large and many intermediate-sized boilers
are water-tube boiler with a boiler section that
consists of closely-spaced water tubes covering
the furnace wall.
ƒ The waterwall boiler design allows much lighter,
less expensive walls by having the waterwalls
form an integral part of the boiler wall so that the
wall is water cooled.
ƒ If so equipped, the superheater and reheater
are separate sections hanging above the main
furnace volume.
ME 416/516
Membrane wall of waterwall boiler
Membrane
Bar
Wall
Tube
Insulation
Metal
Lagging
ME 416/516
Waterwall under
construction
ME 416/516
Steam Drum
Waterwalls
Aux. Burners
Spreader Stoker
Superheater
Cyclone
Scrubbers
Stirling Boiler- A
small waterwall
boiler for burning
solid material
such as bark,
chips or coal.
ME 416/516
Combustion
in Boilers
ƒ There are four important
factors that control com-
bustion in boiler furnace:
1.Air supply- Need adequate air for complete
combustion.
ƒ The rating (capacity) of a boiler can be
increased by supplying additional air (think of
the effect of bellows on a small fire).
ƒ Too much air can result in excessive stack
losses.
ME 416/516
Combustion Factors
2.Mixing of fuel and air- fuel and air molecules
must be brought into close proximity in order for
combustion to occur.
ƒ The larger the fuel "particles" the greater the
difficulty in achieving good mixing-
• easiest for gaseous fuels,
• more difficult for liquid fuels and pulverized
solids,
• most difficult for stoker coal, bark or large
trash clumps.
ME 416/516
Combustion Factors
3.Temperature- all combustion reactions proceed
exponentially more rapidly with increasing T
ƒ Temperatures too low:
• incomplete combustion, waste fuel
• unburned hydrocarbons and soot emissions
greatly increased
ƒ Temperatures too high:
• equipment failure, metal strength drops off
quickly at high T
• NOx emissions greatly increased.
ME 416/516
Combustion Factors
4.Combustion time- fuel "particles" must be given
sufficient time (residence time) in the furnace to
achieve complete combustion.
ƒ Like fuel/air mixing, the required residence time
is least for gases and most for large solid fuels:
• Gases and fine liquid sprays- 10 - 20 ms
burnout
• Pulverized fuel (coal, sawdust)- 1 s burnout
• Stoker coal, bark, wood waste, trash- 10’s of
minutes
ME 416/516
Fuel Considerations
ƒ Natural gas and fuel oil burners. The fuel is
brought to a burner at elevated pressure and
jetted (gas) or sprayed (oil) into the furnace.
Relatively simple and low cost.
ME 416/516
Large Oil Burner
ABB
ME 416/516
Coal/Solid
Fuel Firing
ƒ There are a considerable
number of ways to feed
coal in use, including,
hand-fired boilers, chain
or traveling grate stokers,
vibrating grate stokers,
underfeed stokers,
spreader stoker,
pulverized coal boilers,
cyclone boilers and
fluidized bed boilers.
Tangential-fired coal
furnace
ME 416/516
Stoker Boilers
ƒ The term stoker implies a boiler that
automatically feeds (or " stokes”) the boiler.
ƒ Stoker coal size is typically 1.25 inches
maximum with less than 30% under 0.25 inches.
ME 416/516
Traveling or Chain Grate
Stokers
ƒ Traveling or chain grate stokers feed coal out
onto a rotating metal belt that supports the fire.
ƒ Coal is fed from a hopper.
ƒ Grate speed is automatically controlled to
maintain desired steam pressure.
ƒ Burning progresses as the belt moves from front
to back of furnace.
ƒ Combustion is essentially complete at the back
end of belt, and ash is dumped off into an ashpit
there.
ME 416/516
Traveling
Grate
Water-Cooled
Grate Elements
ME 416/516
Vibrating Grate Stoker
ƒ Vibrating grate stoker is similar to a traveling
grate, except that instead of being on a
continuous loop, grate sections are sloped
downward and periodically vibrate to cause fuel
particle movement from front to back.
ƒ Vibration frequency is controlled to obtain
desired steam pressure/ heat output.
ME 416/516
Water-Cooled Vibrating Grate Boiler
Coal
hopper
Overfire
air nozzles
Vibration
generator
Underfire
air supply
To ashpit
Waterwalls
Vibrating
grate
Fuel
ME 416/516
Underfeed Stokers
ƒ So named because they use rams to force the
coal up underneath the burning fuel bed.
ƒ Grates are designed to flex up and downs to
break up fuel bed and prevent "clinker" formation.
ƒ Action of feed rams and fuel bed flexing cause
fuel to move from front to back of furnace.
ƒ Underfeed stokers range in size from small home
heating boilers to large industrial size.
ƒ Underfeed stokers are very good at burning high
volatile coal with a high turn-down ratio.
ME 416/516
Underfeed Stoker Boiler
Coal
hopper
Feeder rams
and actuator
Fuel
ME 416/516
Spreader Stokers
ƒ Fed by a rotating bladed wheel that throws the
coal out over the grate.
ƒ Spreaders stokers are more expensive than other
stokers in small sizes, and are more expensive
than pulverized coal boilers in large sizes (over
500,000 Ibm/hr of steam) but are very common in
the intermediate (large industrial) size range.
ƒ Compared to previous stokers, more fuel burning
occurs in suspension- that is, in the air as the fine
particles are slung out over the fuel bed.
ME 416/516
Spreader Stokers (Cont’d)
ƒ Because of feeding method, more small particles
and fly ash are carried up with the exhaust.
ƒ Particles trapped up in boiler, economizer, air
preheater and dust collectors are recycled for better
combustion efficiency and reduced particulate
emissions.
ƒ Grates are of several types. Some are traveling or
vibrating to move fire from back to front of furnace
and dump ash over the front into an ashpit. Others
grates periodically are turned over to collect ash.
ME 416/516
Spreader Stoker Boiler
Airborne
fuel
Fuel
hopper
Fuel
feeder
ME 416/516
Adjustable
spill plate
Coal
hopper
Reciprocating
feed plate
Spreader rotor
Reciprocating Coal Feeder
ME 416/516
Coal Feeders
ME 416/516
Pulverized Fuel Boilers
ƒ Pulverized coal boilers fire finely powdered coal,
typically with an average particle size of about 25
µm (0.001 in). Coal burns in suspension, like the
combustion in an oil- or gas-fired boiler.
ƒ Coal is pulverized in some type of large mill
ƒ Pulverized coal is fired out into the furnace
volume using burners that look somewhat like oil
or gas burners.
ME 416/516 B&W
Pyrites
trap
Windbox
Classifier
Raw coal
feeder
Driving
mechanism
Balls
(~18-in)
Ball Mill
Coal
Pulverizer
ME 416/516
Grinding table
Grinding roller
Nozzle ring
Coal feed inlet
PC outlet
Primary air inlet
Rejects hopper Planetary gear
drive
Classifier housing
DB Riley
Roller Mill
Coal
Pulverizer
ME 416/516
Oil/gas lighter
Waterwall
Coal impeller
Refractory throat with studded tubes
Windbox
Secondary air
register door
Pulverized
Coal Burner
ME 416/516
Water-
wall
With
Four
Burners
ME 416/516
Low-NOx Pulverized
Coal Burner
DB Riley
ME 416/516
PC vs. Stoker Boilers:
Advantages/Disadvantages
ƒ Advantages of PC vs. stoker boilers:
• much quicker response to changing loads
• lower excess air/higher efficiency
• easily adaptable to automatic control
• can burn wide variety of coals
ƒ Disadvantages of PC vs. stoker boilers:
• more expensive (at least for smaller capacities)
• require more skilled personnel
• require better emission control (particulates)
• require more energy to pulverize fuel
ME 416/516
Coal Combustion
ƒ We have examined the
combustion of fuels for
which we have a molecular
formula, e.g., C3H8 or CH4
ƒ Coal is characterized by a
mass based formula
resulting from an ultimate
analysis.
ƒ Ultimate analysis gives the
elemental composition
ME 416/516
Ultimate Analysis
ƒ The ultimate analysis is a measurement of a coal
sample that yields the mass percent of each
element tested, plus the percent of ash in the
coal.
ƒ The primary elements are C, H, O, N and S.
ƒ The ash is the noncombustible portion of the coal.
ƒ There are other elements present in lesser
quantities, some of which are hazardous, such as
Cl, V, and Hg, but the “CHONS” are the important
elements for combustion calculations.
ME 416/516
Conversion from a Mass to a Mole
Basis
ƒ The secret to doing coal combustion calculations
is to calculate the chemical formula from the
ultimate analysis data
ƒ One standard method is to start with the
assumption of 100 lbm of coal
ƒ Recall that the relationship between mass, m,
and number of moles, n, is given by the
molecular weight, M, where:
M = m/n or n = m/M
ME 416/516
Example: mass to mole basis
ƒ If a coal has an ultimate analysis of H = 5%, C =
90% and Ash = 5%, find its molecular formula
ƒ Assume a total mass of 100 lbm of coal
ƒ The mass of carbon out of the total 100 lbm of
coal is: mC = 0.9 * 100 = 90 lbm carbon
ƒ Number of moles of carbon is nC = mC/MC, where
MC = 12, so nC = 90 lbm/12 lbm/lbmol = 7.5 lbmol
ƒ Similarly, mH = 5 lbm, nH = mH/MH = 5/1 = 5 lbmol
ƒ We ignore the ash because it does not burn
ME 416/516
Coal Combustion Chemistry
ƒ Once we have the number of moles of each fuel
component, we can calculate the moles of air
needed for complete combustion (stoichiometric
reaction) just as we did earlier
ƒ Once we obtain the moles of air required, we
can convert that to mass of air required and
calculate the air-to-fuel ratio, A/F
ƒ A/F is defined as the mass of air per mass of
fuel for a reaction
ME 416/516
Stoichiometry Terms
ƒ The stoichiometric A/F is the A/F obtained for the
stoichiometric reaction (no excess air)
ƒ The actual A/F is higher than the A/Fstoich to
insure complete fuel burnout
ƒ The stoichiometric ratio is defined:
ƒ The fuel equivalence ratio is Φ = 1/SR
ƒ Percent excess air = 100% * (SR - 1)
ƒ Percent theoretical air = 100% * SR
stoich
act
F
A
F
A
SR =
ME 416/516
Estimating Coal HHV
ƒ A handy rule of thumb formula for estimating the
higher heating value of coal is provided by the
Dulong formula:
HHV = 14,600·C + 62,000·(H – O/8) + 4050·S
where HHV is the higher heating value in
Btu/lbm, and C, H, O and S are the coal mass
fractions of carbon, hydrogen, oxygen and
sulfur, respectively, from the ultimate analysis of
the coal.
ME 416/516
Coal HHV Example
PROBLEM: A coal has an ultimate analysis of
78% carbon, 4% hydrogen, 3% oxygen, 6%
sulfur, and 9% ash. Estimate the HHV of the
coal.
SOLUTION: Use the Dulong formula:
HHV ≅ 14,600 * 0.78 + 62,000 * (0.04 – 0.03/8) +
4050 * 0.06)
HHV ≅ 13,900 Btu/lbm
ME 416/516
Coal Combustion Example
A coal has an ultimate analysis of C = 84%, H =
4.5%, S = 2.5% and Ash = 9%. The coal is to be
burned in a boiler to raise 150,000 lbm/hr of
steam using inlet water at 46
ME 416/516
Coal Combustion Example
A coal has an ultimate analysis of C = 84%, H =
4.5%, S = 2.5% and Ash = 9%. The coal is to be
burned in a boiler at 12% excess air to raise
150,000 lbm/hr of 120 psia steam using inlet
water at 46°F. If the boiler efficiency is 83%,
find: (a) HHV, (b) coal firing rate in lbm/hr, and
(c) the air flow rate in cfm entering the boiler at
55°F.
ME 416/516
Boiler Efficiency
ME 416/516
Introduction
ƒ One of the "usual suspects" in looking for
efficiency improvements is the boiler.
ƒ In smaller businesses or institutions there may
be no employees adequately trained to operate
and maintain boilers for efficient operation.
ƒ In larger firms or institutions (e.g., UA?), the
people who are trained are overextended and
lack time for optimal boiler maintenance and
improvement.
ME 416/516
Simplified Boiler Efficiency Analysis
ƒ The following section provides a
simplified analysis method to
determine the boiler efficiency.
ƒ This method works reasonably well
for fossil fuel fired boilers.
ME 416/516
Boiler Efficiency & Losses
ƒ Boiler efficiency, hb, is the fraction of energy
input that actually goes into raising steam.
ƒ The remainder of the input energy is the boiler
losses, which have the following components.
ƒ Carbon Loss or Refuse Loss results from the
presence of unburned combustible materials,
mostly carbon, in the "refuse" or "bottom ash".
ƒ Heat Transfer Losses result from convective
and radiative losses from hot boiler exterior
surfaces to the surroundings.
ME 416/516
Blowdown Loss
ƒ Most boilers circulate water from a steam drum
above the boiler to the bottom of the boiler
through unheated downcomers and back up to
the steam drum via risers, which are waterwall
and other water heating tubes.
ƒ The “mud drum” lies at the lowest point in the
circulation and is designed to collect sediment.
ƒ The mud drum is periodically discharged to
remove the collected sediment, a process called
"blowdown". The energy content of the water
blown out is lost and reduces ηb.
ME 416/516
Stack Losses
ƒ Vapor Loss- H2O in exhaust leaves as vapor rather
than liquid, so the heat of vaporization is lost as
useful energy.
ƒ H2O that enters with combustion air is already in
vapor form and can be neglected.
ƒ The other two sources of H2O are the moisture
content of the fuel (from the proximate analysis) and
the H2O formed from fuel hydrogen combustion.
ƒ CO Loss is the unused fuel energy of exhaust,
primarily from unburned CO.
ME 416/516
Stack Losses (Cont’d)
ƒ Sensible Loss is the loss of
energy from the exhaust.
ƒ Ideally, the products of combustion could be
cooled to the surroundings temperature and the
fire would give up the maximum possible heat.
ƒ For practical reasons the exhaust leaves the
boiler considerably hotter than Tsurr, so the
sensible energy of the hot exhaust in excess of
ambient temperature is lost.
ƒ The sensible loss is typically the largest single
boiler loss component.
T
c
m p ∆
⋅
⋅
&
ME 416/516
Losses from Non-Coal Boilers
ƒ These losses apply equally well to other fossil-
fuel-fired boilers, including oil or natural gas,
except that there may not be a refuse/carbon
loss if there is negligible ash.
ƒ Only heat transfer losses will apply directly to
electric boilers. However, significant indirect
losses have already occurred at the powerplant
where the electricity was generated (~85% of
electricity generated by Rankine cycle with
steam generator).
ME 416/516
Method for Calculating Boiler
Efficiency
ƒ Equations are presented in following slides that
can be used to determine the boiler losses as
Btu per Ibm of coal that is fired.
ƒ The boiler efficiency is given by:
Efficiency (fractional) =
1 - (Σ Boiler Losses in Btu/lbm)/HHV
ME 416/516
Relations for Individual Boiler
Losses
ƒ Carbon Loss:
qcarb = 14,540 * ab/[100 * (100 - b)]
ƒ a is the % of ash in the fuel
ƒ b is the % of combustible (carbon) in dry refuse
(measured). “Refuse” is the bottom ash.
ƒ 14,540 is the heating value of carbon in Btu/Ibm
ME 416/516
f
4
4
h
8
m
)/
T
(T
*
A
*
10
0.1714 &
∞
−
−
×
qrad =
ƒ 0.1714 x 10-8 is the Stefan-Boltzmann constant,
σ, in Btu/hr-ft2-R4
ƒ A is the area of the surface from which heat is
transferred
ƒ Th is the hot boiler surface temperature (in
degrees R)
ƒ T∞ is the ambient temperature (in degrees R)
ƒ is the mass flow rate of fuel in Ibm/hr
f
m
&
ME 416/516
ƒ 0.18 * (Th - T∞)1/3 Btu/hr-ft2-F is a natural
convection heat transfer coefficient for air
ƒ is the mass flow rate of blowdown water
ƒ hb is the enthalpy of water bled from mud drum
ƒ hc is enthalpy of water fed to the boiler (hc ≅ hf at
boiler feedwater inlet temperature).
f
3
4
h
conv m
/
)
T
(T
*
A
*
0.18
q &
∞
−
=
f
c
b
b
blow m
)/
h
(h
*
m
q &
& −
=
b
m
&
ME 416/516
qvap = 0.9 * H * (100 – M) + 10 * M
ƒ H is % hydrogen in fuel from ultimate analysis
ƒ M is % moisture in fuel from proximate analysis
qsens = 0.24 * (1 + A/F) * (Texh – Tin)
ƒ 0.24 is cp of air in Btu/Ibm-°F
ƒ A/F is actual air/fuel ratio
ƒ Texh is exhaust T leaving boiler into stack.
ƒ Tin is temperature of air entering boiler (usu.
outside air temp.)
ME 416/516
qCO = [1.02 * (100 – M) * C * CO]/(CO + CO2)
ƒ C is % of carbon in fuel from ultimate analysis
ƒ CO is volume % of CO in exhaust from stack gas
analyzer
ƒ CO2 is volume % of CO2 in exhaust from stack
gas analyzer

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Thermal Engineering - II

  • 2. ME 416/516 Motivation ƒ Boilers, were a major part of the Industrial Revolution beginning about 1700. They are major consumers of industry and building energy consumption today. ƒ Industry: boilers are used for power generation, process heat (e.g., refineries, petrochemicals, paper mills, tire manufacturing, etc.) and heating. ƒ In buildings, boilers are used for steam primary heat, terminal reheat systems, water heating and absorption chillers.
  • 4. ME 416/516 Introduction ƒ There is a tremendous variation in boiler design and size- ranging from home heating size of capacity less than 100 Ibm/hr of steam to utility boilers in excess of 10 million Ibm/hr. ƒ The focus here will be on boilers sized in between these two extremes, which is also where there is the greatest diversity of design. ƒ In order of increasing capacity, three boiler types are the fire-tube, water-tube and waterwall boilers.
  • 5. ME 416/516 Fire-Tube Boiler (~1800) ƒ The fire tube boiler, the oldest design, is made so the products of combustion pass through tubes surrounded by water in a shell. ƒ The furnace/flame volume can either be inside or external to the shell that contains the water. ƒ The upper steam capacity of fire tube boilers is about 20,000 Ibm/hr, and the peak pressure obtainable is limited by their large shells to about 300 psi. ƒ Fire-tube boilers are used for heating systems.
  • 6. ME 416/516 Cleaver-Brooks Horizontal, four-pass, forced-draft fire-tube boiler
  • 7. ME 416/516 Water-Tube Boiler (1867) ƒ A water-tube boiler is one in which the products of combustion pass around the outside and heat tubes containing the water. ƒ The water tube diameter is much smaller than the shell diameter of a fire-tube boiler, so much higher pressures can be obtained, well over 2000 psi. ƒ The furnace and boiler tube area must be surrounded by a heavily insulated refractory wall to prevent heat transfer through the boiler walls. The refractory lining is a high maintenance item.
  • 8. ME 416/516 Type FM integral-furnace package boiler Babcock and Wilcox
  • 9. ME 416/516 “D-type” water-tube boiler “Flex-tube” water-tube boiler English Boiler Co. Steam Drum Mud Drum Water Tubes Furnace
  • 11. ME 416/516 Bryan Exterior of small water-tube package boiler
  • 12. ME 416/516 Package Boiler ƒ All but the largest boilers used for heating and industrial purposes are packaged boilers. ƒ They are factory-built and shipped whole or in modular components to the customer. ƒ Many are constructed in an elongated shape that will fit through large building doors with minimal field adjustment required.
  • 13. ME 416/516 Water-tube package boiler under construction Riley
  • 14. ME 416/516 Bryan Large package boiler installation
  • 16. ME 416/516 Waterwall Boiler ƒ All large and many intermediate-sized boilers are water-tube boiler with a boiler section that consists of closely-spaced water tubes covering the furnace wall. ƒ The waterwall boiler design allows much lighter, less expensive walls by having the waterwalls form an integral part of the boiler wall so that the wall is water cooled. ƒ If so equipped, the superheater and reheater are separate sections hanging above the main furnace volume.
  • 17. ME 416/516 Membrane wall of waterwall boiler Membrane Bar Wall Tube Insulation Metal Lagging
  • 19. ME 416/516 Steam Drum Waterwalls Aux. Burners Spreader Stoker Superheater Cyclone Scrubbers Stirling Boiler- A small waterwall boiler for burning solid material such as bark, chips or coal.
  • 20. ME 416/516 Combustion in Boilers ƒ There are four important factors that control com- bustion in boiler furnace: 1.Air supply- Need adequate air for complete combustion. ƒ The rating (capacity) of a boiler can be increased by supplying additional air (think of the effect of bellows on a small fire). ƒ Too much air can result in excessive stack losses.
  • 21. ME 416/516 Combustion Factors 2.Mixing of fuel and air- fuel and air molecules must be brought into close proximity in order for combustion to occur. ƒ The larger the fuel "particles" the greater the difficulty in achieving good mixing- • easiest for gaseous fuels, • more difficult for liquid fuels and pulverized solids, • most difficult for stoker coal, bark or large trash clumps.
  • 22. ME 416/516 Combustion Factors 3.Temperature- all combustion reactions proceed exponentially more rapidly with increasing T ƒ Temperatures too low: • incomplete combustion, waste fuel • unburned hydrocarbons and soot emissions greatly increased ƒ Temperatures too high: • equipment failure, metal strength drops off quickly at high T • NOx emissions greatly increased.
  • 23. ME 416/516 Combustion Factors 4.Combustion time- fuel "particles" must be given sufficient time (residence time) in the furnace to achieve complete combustion. ƒ Like fuel/air mixing, the required residence time is least for gases and most for large solid fuels: • Gases and fine liquid sprays- 10 - 20 ms burnout • Pulverized fuel (coal, sawdust)- 1 s burnout • Stoker coal, bark, wood waste, trash- 10’s of minutes
  • 24. ME 416/516 Fuel Considerations ƒ Natural gas and fuel oil burners. The fuel is brought to a burner at elevated pressure and jetted (gas) or sprayed (oil) into the furnace. Relatively simple and low cost.
  • 25. ME 416/516 Large Oil Burner ABB
  • 26. ME 416/516 Coal/Solid Fuel Firing ƒ There are a considerable number of ways to feed coal in use, including, hand-fired boilers, chain or traveling grate stokers, vibrating grate stokers, underfeed stokers, spreader stoker, pulverized coal boilers, cyclone boilers and fluidized bed boilers. Tangential-fired coal furnace
  • 27. ME 416/516 Stoker Boilers ƒ The term stoker implies a boiler that automatically feeds (or " stokes”) the boiler. ƒ Stoker coal size is typically 1.25 inches maximum with less than 30% under 0.25 inches.
  • 28. ME 416/516 Traveling or Chain Grate Stokers ƒ Traveling or chain grate stokers feed coal out onto a rotating metal belt that supports the fire. ƒ Coal is fed from a hopper. ƒ Grate speed is automatically controlled to maintain desired steam pressure. ƒ Burning progresses as the belt moves from front to back of furnace. ƒ Combustion is essentially complete at the back end of belt, and ash is dumped off into an ashpit there.
  • 30. ME 416/516 Vibrating Grate Stoker ƒ Vibrating grate stoker is similar to a traveling grate, except that instead of being on a continuous loop, grate sections are sloped downward and periodically vibrate to cause fuel particle movement from front to back. ƒ Vibration frequency is controlled to obtain desired steam pressure/ heat output.
  • 31. ME 416/516 Water-Cooled Vibrating Grate Boiler Coal hopper Overfire air nozzles Vibration generator Underfire air supply To ashpit Waterwalls Vibrating grate Fuel
  • 32. ME 416/516 Underfeed Stokers ƒ So named because they use rams to force the coal up underneath the burning fuel bed. ƒ Grates are designed to flex up and downs to break up fuel bed and prevent "clinker" formation. ƒ Action of feed rams and fuel bed flexing cause fuel to move from front to back of furnace. ƒ Underfeed stokers range in size from small home heating boilers to large industrial size. ƒ Underfeed stokers are very good at burning high volatile coal with a high turn-down ratio.
  • 33. ME 416/516 Underfeed Stoker Boiler Coal hopper Feeder rams and actuator Fuel
  • 34. ME 416/516 Spreader Stokers ƒ Fed by a rotating bladed wheel that throws the coal out over the grate. ƒ Spreaders stokers are more expensive than other stokers in small sizes, and are more expensive than pulverized coal boilers in large sizes (over 500,000 Ibm/hr of steam) but are very common in the intermediate (large industrial) size range. ƒ Compared to previous stokers, more fuel burning occurs in suspension- that is, in the air as the fine particles are slung out over the fuel bed.
  • 35. ME 416/516 Spreader Stokers (Cont’d) ƒ Because of feeding method, more small particles and fly ash are carried up with the exhaust. ƒ Particles trapped up in boiler, economizer, air preheater and dust collectors are recycled for better combustion efficiency and reduced particulate emissions. ƒ Grates are of several types. Some are traveling or vibrating to move fire from back to front of furnace and dump ash over the front into an ashpit. Others grates periodically are turned over to collect ash.
  • 36. ME 416/516 Spreader Stoker Boiler Airborne fuel Fuel hopper Fuel feeder
  • 37. ME 416/516 Adjustable spill plate Coal hopper Reciprocating feed plate Spreader rotor Reciprocating Coal Feeder
  • 39. ME 416/516 Pulverized Fuel Boilers ƒ Pulverized coal boilers fire finely powdered coal, typically with an average particle size of about 25 µm (0.001 in). Coal burns in suspension, like the combustion in an oil- or gas-fired boiler. ƒ Coal is pulverized in some type of large mill ƒ Pulverized coal is fired out into the furnace volume using burners that look somewhat like oil or gas burners.
  • 40. ME 416/516 B&W Pyrites trap Windbox Classifier Raw coal feeder Driving mechanism Balls (~18-in) Ball Mill Coal Pulverizer
  • 41. ME 416/516 Grinding table Grinding roller Nozzle ring Coal feed inlet PC outlet Primary air inlet Rejects hopper Planetary gear drive Classifier housing DB Riley Roller Mill Coal Pulverizer
  • 42. ME 416/516 Oil/gas lighter Waterwall Coal impeller Refractory throat with studded tubes Windbox Secondary air register door Pulverized Coal Burner
  • 45. ME 416/516 PC vs. Stoker Boilers: Advantages/Disadvantages ƒ Advantages of PC vs. stoker boilers: • much quicker response to changing loads • lower excess air/higher efficiency • easily adaptable to automatic control • can burn wide variety of coals ƒ Disadvantages of PC vs. stoker boilers: • more expensive (at least for smaller capacities) • require more skilled personnel • require better emission control (particulates) • require more energy to pulverize fuel
  • 46. ME 416/516 Coal Combustion ƒ We have examined the combustion of fuels for which we have a molecular formula, e.g., C3H8 or CH4 ƒ Coal is characterized by a mass based formula resulting from an ultimate analysis. ƒ Ultimate analysis gives the elemental composition
  • 47. ME 416/516 Ultimate Analysis ƒ The ultimate analysis is a measurement of a coal sample that yields the mass percent of each element tested, plus the percent of ash in the coal. ƒ The primary elements are C, H, O, N and S. ƒ The ash is the noncombustible portion of the coal. ƒ There are other elements present in lesser quantities, some of which are hazardous, such as Cl, V, and Hg, but the “CHONS” are the important elements for combustion calculations.
  • 48. ME 416/516 Conversion from a Mass to a Mole Basis ƒ The secret to doing coal combustion calculations is to calculate the chemical formula from the ultimate analysis data ƒ One standard method is to start with the assumption of 100 lbm of coal ƒ Recall that the relationship between mass, m, and number of moles, n, is given by the molecular weight, M, where: M = m/n or n = m/M
  • 49. ME 416/516 Example: mass to mole basis ƒ If a coal has an ultimate analysis of H = 5%, C = 90% and Ash = 5%, find its molecular formula ƒ Assume a total mass of 100 lbm of coal ƒ The mass of carbon out of the total 100 lbm of coal is: mC = 0.9 * 100 = 90 lbm carbon ƒ Number of moles of carbon is nC = mC/MC, where MC = 12, so nC = 90 lbm/12 lbm/lbmol = 7.5 lbmol ƒ Similarly, mH = 5 lbm, nH = mH/MH = 5/1 = 5 lbmol ƒ We ignore the ash because it does not burn
  • 50. ME 416/516 Coal Combustion Chemistry ƒ Once we have the number of moles of each fuel component, we can calculate the moles of air needed for complete combustion (stoichiometric reaction) just as we did earlier ƒ Once we obtain the moles of air required, we can convert that to mass of air required and calculate the air-to-fuel ratio, A/F ƒ A/F is defined as the mass of air per mass of fuel for a reaction
  • 51. ME 416/516 Stoichiometry Terms ƒ The stoichiometric A/F is the A/F obtained for the stoichiometric reaction (no excess air) ƒ The actual A/F is higher than the A/Fstoich to insure complete fuel burnout ƒ The stoichiometric ratio is defined: ƒ The fuel equivalence ratio is Φ = 1/SR ƒ Percent excess air = 100% * (SR - 1) ƒ Percent theoretical air = 100% * SR stoich act F A F A SR =
  • 52. ME 416/516 Estimating Coal HHV ƒ A handy rule of thumb formula for estimating the higher heating value of coal is provided by the Dulong formula: HHV = 14,600·C + 62,000·(H – O/8) + 4050·S where HHV is the higher heating value in Btu/lbm, and C, H, O and S are the coal mass fractions of carbon, hydrogen, oxygen and sulfur, respectively, from the ultimate analysis of the coal.
  • 53. ME 416/516 Coal HHV Example PROBLEM: A coal has an ultimate analysis of 78% carbon, 4% hydrogen, 3% oxygen, 6% sulfur, and 9% ash. Estimate the HHV of the coal. SOLUTION: Use the Dulong formula: HHV ≅ 14,600 * 0.78 + 62,000 * (0.04 – 0.03/8) + 4050 * 0.06) HHV ≅ 13,900 Btu/lbm
  • 54. ME 416/516 Coal Combustion Example A coal has an ultimate analysis of C = 84%, H = 4.5%, S = 2.5% and Ash = 9%. The coal is to be burned in a boiler to raise 150,000 lbm/hr of steam using inlet water at 46
  • 55. ME 416/516 Coal Combustion Example A coal has an ultimate analysis of C = 84%, H = 4.5%, S = 2.5% and Ash = 9%. The coal is to be burned in a boiler at 12% excess air to raise 150,000 lbm/hr of 120 psia steam using inlet water at 46°F. If the boiler efficiency is 83%, find: (a) HHV, (b) coal firing rate in lbm/hr, and (c) the air flow rate in cfm entering the boiler at 55°F.
  • 57. ME 416/516 Introduction ƒ One of the "usual suspects" in looking for efficiency improvements is the boiler. ƒ In smaller businesses or institutions there may be no employees adequately trained to operate and maintain boilers for efficient operation. ƒ In larger firms or institutions (e.g., UA?), the people who are trained are overextended and lack time for optimal boiler maintenance and improvement.
  • 58. ME 416/516 Simplified Boiler Efficiency Analysis ƒ The following section provides a simplified analysis method to determine the boiler efficiency. ƒ This method works reasonably well for fossil fuel fired boilers.
  • 59. ME 416/516 Boiler Efficiency & Losses ƒ Boiler efficiency, hb, is the fraction of energy input that actually goes into raising steam. ƒ The remainder of the input energy is the boiler losses, which have the following components. ƒ Carbon Loss or Refuse Loss results from the presence of unburned combustible materials, mostly carbon, in the "refuse" or "bottom ash". ƒ Heat Transfer Losses result from convective and radiative losses from hot boiler exterior surfaces to the surroundings.
  • 60. ME 416/516 Blowdown Loss ƒ Most boilers circulate water from a steam drum above the boiler to the bottom of the boiler through unheated downcomers and back up to the steam drum via risers, which are waterwall and other water heating tubes. ƒ The “mud drum” lies at the lowest point in the circulation and is designed to collect sediment. ƒ The mud drum is periodically discharged to remove the collected sediment, a process called "blowdown". The energy content of the water blown out is lost and reduces ηb.
  • 61. ME 416/516 Stack Losses ƒ Vapor Loss- H2O in exhaust leaves as vapor rather than liquid, so the heat of vaporization is lost as useful energy. ƒ H2O that enters with combustion air is already in vapor form and can be neglected. ƒ The other two sources of H2O are the moisture content of the fuel (from the proximate analysis) and the H2O formed from fuel hydrogen combustion. ƒ CO Loss is the unused fuel energy of exhaust, primarily from unburned CO.
  • 62. ME 416/516 Stack Losses (Cont’d) ƒ Sensible Loss is the loss of energy from the exhaust. ƒ Ideally, the products of combustion could be cooled to the surroundings temperature and the fire would give up the maximum possible heat. ƒ For practical reasons the exhaust leaves the boiler considerably hotter than Tsurr, so the sensible energy of the hot exhaust in excess of ambient temperature is lost. ƒ The sensible loss is typically the largest single boiler loss component. T c m p ∆ ⋅ ⋅ &
  • 63. ME 416/516 Losses from Non-Coal Boilers ƒ These losses apply equally well to other fossil- fuel-fired boilers, including oil or natural gas, except that there may not be a refuse/carbon loss if there is negligible ash. ƒ Only heat transfer losses will apply directly to electric boilers. However, significant indirect losses have already occurred at the powerplant where the electricity was generated (~85% of electricity generated by Rankine cycle with steam generator).
  • 64. ME 416/516 Method for Calculating Boiler Efficiency ƒ Equations are presented in following slides that can be used to determine the boiler losses as Btu per Ibm of coal that is fired. ƒ The boiler efficiency is given by: Efficiency (fractional) = 1 - (Σ Boiler Losses in Btu/lbm)/HHV
  • 65. ME 416/516 Relations for Individual Boiler Losses ƒ Carbon Loss: qcarb = 14,540 * ab/[100 * (100 - b)] ƒ a is the % of ash in the fuel ƒ b is the % of combustible (carbon) in dry refuse (measured). “Refuse” is the bottom ash. ƒ 14,540 is the heating value of carbon in Btu/Ibm
  • 66. ME 416/516 f 4 4 h 8 m )/ T (T * A * 10 0.1714 & ∞ − − × qrad = ƒ 0.1714 x 10-8 is the Stefan-Boltzmann constant, σ, in Btu/hr-ft2-R4 ƒ A is the area of the surface from which heat is transferred ƒ Th is the hot boiler surface temperature (in degrees R) ƒ T∞ is the ambient temperature (in degrees R) ƒ is the mass flow rate of fuel in Ibm/hr f m &
  • 67. ME 416/516 ƒ 0.18 * (Th - T∞)1/3 Btu/hr-ft2-F is a natural convection heat transfer coefficient for air ƒ is the mass flow rate of blowdown water ƒ hb is the enthalpy of water bled from mud drum ƒ hc is enthalpy of water fed to the boiler (hc ≅ hf at boiler feedwater inlet temperature). f 3 4 h conv m / ) T (T * A * 0.18 q & ∞ − = f c b b blow m )/ h (h * m q & & − = b m &
  • 68. ME 416/516 qvap = 0.9 * H * (100 – M) + 10 * M ƒ H is % hydrogen in fuel from ultimate analysis ƒ M is % moisture in fuel from proximate analysis qsens = 0.24 * (1 + A/F) * (Texh – Tin) ƒ 0.24 is cp of air in Btu/Ibm-°F ƒ A/F is actual air/fuel ratio ƒ Texh is exhaust T leaving boiler into stack. ƒ Tin is temperature of air entering boiler (usu. outside air temp.)
  • 69. ME 416/516 qCO = [1.02 * (100 – M) * C * CO]/(CO + CO2) ƒ C is % of carbon in fuel from ultimate analysis ƒ CO is volume % of CO in exhaust from stack gas analyzer ƒ CO2 is volume % of CO2 in exhaust from stack gas analyzer