1. EGEE 451 Energy
Conversion Processes
Lecture 2 – Stationary Combustion Processes
9/07/12

2. Stationary Combustion
 First system to evaluate
 Pulverized coal combustion for electricity generation
 Reasons for doing this:
 1. Dominant technology domestic use of coal.
 85-90% of coal used goes to electric power generation
 2. A majority of electricity generated in US comes from PC
fired power plants
 A couple of years ago, >50% of electricity was generated from
coal
 Changed recently – only about 40% currently due to increased use
of natural gas
 3. Sort of “state of the art” large scale electricity generation
 Base case to compare to other technologies

3. Stationary Combustion
 Begin by looking at  For various coals
overview of technology  Lignite and subbituminous
 Will then “dissect” overall coal – size spec 70-75%
plant into smaller “boxes” -200 mesh (≤ 74 μm)
 Try to see where  Bituminous coal – size
inefficiencies in energy are spec usually 80-85% -200
 Where can improvements mesh
be made  Anthracite can be used for
PC combustion, but little
 First step market for anthracite
 Pulverize coal currently (high carbon
content)

4. Stationary Combustion
 Pulverization means the coal  Boiler usually a rectangular
will undergo one or more size steel box.
reduction operations
 Will ignore these for now  For now, can ignore:
 But need to recognize that  1. how burners are designed
crushing or grinding operations  2. the array of the total number
are energy intensive of burners
 Done onsite represents parasitic
energy losses and reduces
electricity out of plant
 Often, last stage of grinding is
done in mill just ahead of feed
to burners
 Mills can be swept with hot
gases to remove moisture
 Pulverized coal is blown with air
through burners into the boiler
Coal & Air Coal & Air

5. Stationary Combustion
 As coal injected into boiler,  Combustion occurs in 2
pulverized coal ignites and stages
burns in a large, hot  1. Volatiles are driven out
turbulent flame of the coal (thermally),
ignite and burn in gas
phase
 2. the residual solid char
(i.e., fixed carbon) – ignites
and burns as a process of
heterogeneous combustion
called char burnout
Coal & Air Coal & Air

6.  First major energy conversion  Hot combustion gases
 CHEMICAL TO THERMAL proceed through a flue
 Chemical – enthalpy of
combustion of the fuel (chimney) as they exit
 ΔHcombs boiler
 Additional tubes/pipes are
 Generation of heat is to get mounted in the flue as well
water to boil
 One major wall of boiler is made  Here dominant mechanism
of tubes/pipes through which is convection. Region in
water circulates – water wall boiler is sometimes called
 At this point dominant heat convection section or
transfer mechanism is radiation
convection pass
 Sometimes called the radiant
section of boiler
Hot Gasses

7. Electricity Generation
 Follow the steam path and  Turbine is coupled directly
consider environmental to rotary generator.
issues  Third major energy
conversion
 High-pressure, high-
 MECHANICAL TO
temperature steam fed to ELECTRICAL
turbine
 Second major energy  Therefore, net conversion
conversion to plant is
 THERMAL TO  CHEMICAL TO
MECHANICAL ELECTRICAL
 Enthalpy in steam  Efficiency combined,
converted to rotary roughly –
mechanical work in turbine  eC = 33%
 Exact number varies
with age of plant, how
well it’s run, parasitic
energy losses, etc.

8. Steam
 Steam exits turbine and is  Condenser heat is transferred
condensed back to water. from steam (including heat &
condensation) to condenser
 Typically condenser is heat water
exchanger that uses natural
water source as working fluid.  Therefore water leaving
condenser will be hot or warm
 Why many power plants are
located along rivers or on  If dumped directly into water
lakes source and hot, will alter
microclimate and local
 Condensate is returned to the ecology
boiler
 Called thermal pollution
 Water must be extremely pure
 Cooling towers used to cool
 Avoid corrosion in boilers condenser effluent
tubes and/or turbine blades
 Can be stricter than for
drinking water

9. Steam Flow
 Steam flow and
High P Steam
,T condensing water flow
Turbine complex
Boiler Low P T Steam
,  Also have to consider
environmental issues
Water Condenser
Water
Pump
Water
Air Reservoir
Water
Cooling Tower
Air

10. Environmental Issues
 Ash  Ash partitions between
Fly ash (PM) material falling to the bottom
SOX of the boiler and fine
NOX particles entrained in the hot
CO2 combustion gases
 Sulfur undergoes conversion
to SO2 and SO3, or SOX
 Small amount of NOX comes
from nitrogen in coal (fuel
NOX)
 Most comes from nitrogen in
air at high temperatures of
combustion system (thermal
NOX)
Bottom ash  N2 + O2 2NO
 N2 + 2O2 2NO2

11. Pollutant Clean Up
 Fly ash
 Typically dealt with in one of two technologies
 Electrostatic precipitator
 Baghouse filtration
 SOX is commonly treate in scrubbers where it
reacts with aqueous slurry of lime
 Ca(OH)2 + SO2 + ½ O2 CaSO4 + H2O
 Ca(OH)2 + SO3 CaSO4 + H2O
 Precipitated CaSO4 called scrubber sludge
 Need to dispose of
 ~25% is used in sheetrock (wallboard)

12. Pollutant Clean Up
 NOX can be treated by reduction with ammonia
 6NO + 4NH3 5N2 + 6H2O
 6NO + 8NH3 7N2 + 12H2O
 Or urea
 6NO + 2 CO(NH2)2 5N2 + 4H2O + 2CO2
 6NO + 4 CO(NH2)2 7N2 + 5H2O + 4CO2
 Alternative technologies involve fuel gas
recirculation or staged combustion (e.g., overfire air
or low-NOX burners)

13. Pollutant Clean Up
 Environmental  Whole operation is
technologies represent complex plant
parasitic energy losses
 Several factors impact eC
 Anything done to cool  Incomplete combustion
inside of boiler (to combat  Ineffective heat transfer
thermal NOX formation)
 Heat losses
reduces steam temp, which
 Inefficiencies in turbine
will affect efficiencies in
the turbine  Inefficiencies in generator
 Parasitic energy losses
 Also CO2 production
 Problem with putting CCS  Next lecture will begin to
on power plant stem partly examine these effects
from CO2 concentration in
flue gas being ~10-15%
 Makes effective carbon
capture difficult to do

14. Stationary Combustion
 Electricity production in PC-fired power plant involved 3 major
energy conversion processes
 1. Chemical to thermal – enthalpy of comb of coal enthalpy in steam
 2. Thermal to mechanical – enthalpy in steam rotation of turbine/generator
 3. Mechanical to electrical – rotation of generator electrical energy
 And with these energy conversions, if draw “box” around whole
process (eC or “big box” conversion), value of eC = 33%
 Not particularly good. If viewed another way, two out of three tons
of coal is wasted.
 Want to determine
 1. where the inefficiencies are and
 2. what, if anything, can be done about them.
 Therefore, useful to divide “big box” into three smaller “boxes”,
corresponding to one of three energy conversion processes

15. Stationary Combustion
Chemical Energy
 Will concentrate on boiler “box” today
 Effective energy output going to be energy input minus
the losses. So we can look at these different items as
“little” boxes.
 Major energy input will be enthalpy of combustion of
the fuel
 As noted previously,
 Fuel from coal is pulverized to 75-85% that is ≤ 74 μm
 Typically, last stage of pulverization is effected by
pulverizers directly upstream of the burners
 Often pulverizer output is swept directly into the burners

16. Stationary Combustion
Chemical Energy
 Combustion occurs in two steps
 Volatiles from coal ignite and burn in homogenous gas-
phase combustion
 Char ignites and burns out in heterogeneous gas-solid
combustion
 Time for combustion of a coal particle is 0.25-1 sec
 Important to assure that abundant oxygen is available
for complete combustion
 If reaction 2C + O2 2CO occurs to any extent
 Less heat is evolved than for C + O2 CO2
 Incomplete combustion (or non-combustion) leaving
unburned carbon can lead to smoke and soot emission in
addition to being wasteful of energy.
 Boilers are then run with 20-30% excess air

17. Stationary Combustion
Chemical Energy
 Two other energy inputs, though neither is as important as the fuel
combustion
 Previously discussed convection section of boiler
 1. At the end of the convection section, before gaseous products of
combustion go to the stack, is a heat exchanger to preheat
combustion air
 Typically combustion air is used at 55-80°C
 Can count the “extra” heat as a contribution to the total energy input
 And,
 2. Small but measureable contribution comes from the fact that air
will be passing through devices like fans, pumps, pulverizers, etc.
These devices will add slight amount of heat to the air
 Where does this heat go? Want it to go to energy in steam
generated

18. Stationary Combustion
Chemical Energy
 Heat transferred to water/  Method of interference
steam by 2 mechanisms  Some ash can adhere to the
 1. Radiation – in furnace tubes in the convection
section of boiler, this is section or on the water wall
dominant heat transfer of the radiant section.
mechanism  Deposition results from
 2. Convection – in flue, hot partially or wholly molten
combustion gases enter, and components of ash
this is the dominant heat impacting one of these heat
transfer mechanism transfer surfaces and
 Each accounts for about sticking there
50% of heat transfer  Continued impacting builds
up sticky layer on steel
surface
 This will trap particles that
are not molten

19. Interference with Ash
 Ash adhering to heat  Reduce heat transfer to the
transfer surfaces is solid, water/steam
problem called ash
deposition or ash fouling  To overcome and maintain
same rate of steam
 If deposits are semi- or fully production (and electricity
molten, they are called slag production) is to increase
deposits temperature in boiler
 Can also be referred to as  Produces vicious cycle of
slagging more fouling or slagging,
which requires still higher
 From perspective of boiler temperatures, causing
efficiency, ash or slag more fouling or slagging….
deposits act as insulators

20. Interference with Ash
 Remedial measures for  Hot combustion gases
fouling/slagging pass a succession of
 Soot blowing steam tubes in the
 Shotgunning convection section
 Dynamiting  To recover as much heat as
possible
 Coming off line for
detailed maintenance  At very end – heat
exchanger to preheat the
 Boiler structure itself is combustion air
extremely hot
 Peak temp of “fireball”  At this point, gases
could be ~1500°C entering stack will be
 Not all heat will be above ambient temp
captured internally – some
heat lost through walls

21. Energy Losses in Boiler
 Energy losses include:  Moisture that formed chemically as a
 Energy in “so-called” dry gas result of combustion of hydrogen in
– sensible heat in the gas fuel
energy is the moisture in gas  4CH0.5 + 4 ½O2 H2O + 4CO2
 Stack gas will be at some  Moisture that came into the system
temp above the dew point, with combustion air
have to consider sensible  All air contains some amount of
heat and latent heat of moisture
moisture
 Other class of loss – Unaccounted
 Where does moisture in Losses
stack gas come from?
 This could be a highly variable
 Moisture present in fuel number
and vaporized during
combustion  However, in practice when boiler
efficiency tests are done, results are
not accepted if losses “unaccounted
for” are > 2%

22. Energy Losses in Boiler
 So in summary, here are  Since EnergyOUT = EnergyIN – Losses
energy inputs and energy
losses, where * denotes the  Efficiency = (EnergyIN – Losses)/
big contributions EnergyIN
 Energy In  The following are quantities of losses
 * Enthalpy of combustion estimated for a boiler running on 25%
fuel excess air
 Preheating combustion air  Stack heat loss = 9%
 Air heating by fans, blowers,  Loss in heat transfer & unaccounted loss
etc. = 6%
 Incomplete combustion = 0.5%
 Losses  Furnace heat loss = 0.5%
 *Stack gas losses
 * Inefficient heat transfer  Therefore boiler efficiency is 84%
and unaccounted loss
 Incomplete combustion
 Long way from combined efficiency of
33%
 Furnace heat loss
 Need to look at efficiency of turbine
and generator