Energy, Society, and the Environment

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Energy, Society,and the Environment
Unit IV How Do Power Plants Work?
Combustion and Heat Engines
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Outline

• Combustion Energy Generation and Pollutants
• 1st Law of Thermodynamics
• 2nd Law of Thermodynamics
• Efficiency

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Combustion

• What do we do with fossil fuels burn them
• Combustion impacts
• Fuels
• Balancing combustion chemical equations
• Combustion products

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What happens in combustion?

• Fuel oxidizer -gt Products light heat
• Combustion, in its simplest form, e.g methane
• CH4 2O2 ? CO2 2H20
• A clean reaction, except for the issue of carbon
  dioxide and the global climate
• This idealized reaction takes place in an
  atmosphere (oxygen) free of impurities

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How much CO2 is produced when 1 ton of cellulose
(C6H12O6) is burned? (Cellulose is the
primary component of plant matter, wood
etc) Write the equation C6H12O6 O2 ? CO2
H2O Balance first the carbon C6H12O6 O2 ?
6CO2 H2O Then the hydrogen C6H12O6 O2 ?
6CO2 6H2O Last, the oxygen (because you can
change the oxygen without altering other
elements) C6H12O6 O2 ? 6CO2 6H2O, So, 6
x 18 ? x 12, or 6 O2 The balanced equation
is C6H12O6 6O2 ? 6CO2 6H2O
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How much CO2 is produced when 1 metric ton of
wood is burned, continued ...
1 molecule of cellulose produces 6 molecules of
CO2
How much do these weigh?
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Remember your atoms
O 8 p 8 n 16
C 6 p 6 n 12
(atomic mass unit, OR 1 mole of C is 12 grams)
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How much CO2 is produced when 1 metric ton of
wood is burned, continued ...
1 molecule of cellulose produces 6 molecules of
CO2
How much do these weigh? C6H12O6 (6 x 12)
(12 x 1) (6 x16) 180 grams (in 1 mole) 6 CO2
6 x (1x12) (2 x16) 264 grams (from 1 mole
of cellulose)
So, from C6H12O6 ? 6CO2 6H2O, we see that
180 grams cellulose ? 264 grams CO2 106 grams
cellulose ? ?? grams CO2
??
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Simple combustion equation,but put it in air

• Example methane reacts with air
• CH4 (O2 3. 76N2) -gt CO2 H2O N2
• (this is termed the unbalanced version)
• CH4 2(O2 3. 76N2) -gt CO2 2H2O 7.52N2
• (balanced exactly the correct amount of oxidizer
  to convert all C to CO2, and all H to H2O)
• Note Air is 78 nitrogen, 21 oxygen, other
  stuff

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Real Combustion

• If combustion occurs without complete oxidation,
  we get instead
• CH4 O2 N2 ? mostly (CO2 2H20 N2)
• traces (CO HC NO...)
• This can occur when
• temperature too low
• insufficient O
• combustion too rapid
• poor mixing of fuel and air, etc. ...

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Real, Real Combustion

• At higher temperatures, N reacts with O
• air(N2 O2) heat ? NOx (nitrogen oxides, x
  can be 1 or 2)
• So much for pure fuels, now add impurities
• enter N, S, metals and ash (non-combustibles)
• What we really get
• Fuel (C, H, N, S, ash) air (N2 O2) ?
• (CO2, H2O, CO, NOx, SOx, VOCS, particulates)
  ash
• Volatile Organic Compounds VOCs

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Real, Real Combustion and Emissions

• NOx VOCs SMOG
• CO2, NOx, SOx oxidants water ACID
  RAIN

Particulates (especially ultrafine) Create
inflammatory response Affect heart rate
variability Inducing cellular damage May be
associated with premature death
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Emissions Controls

• Clean Air Act requires EPA to set National
  Ambient Air Quality Standards (NAAQS) for seven
  pollutants (referred to as criteria pollutants)
  carbon monoxide (CO), lead (Pb), ozone (ground
  level), nitrogen oxides (NOx), particulate matter
  (PM2.5 and PM10), sulfur oxides (SO2). Last
  amended in 1990.

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National Ambient Air Quality Standards
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Criteria Pollutants the Clean Air Act
POLLUTANT Emissions Air Concentration
83 - 02 93 - 02 83 - 02 93 -
02 CO - 41 - 21 - 65 - 42 Pb
- 93 - 5 - 94 - 57 O3 1 hr
- 40 - 25 - 22 - 2 PM10 - 34
- 22 - - 10 PM2.5 -
- 17 - - 8 SO2 - 33
- 31 -34 - 39 NOx - 15
- 9 - 2
.5
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(No Transcript)
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Announcements 2/16

• No Class THIS Friday (note the change)
• Homework 3 posted today on D2L, due
• next Monday

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POWER PLANTS areHeat EnginesUsing Heat to Do
Work(a bit of Thermodynamics)
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What use is thermodynamics?

• For different energy sources compare
• Efficiency
• Amount of fuel needed
• Pollution produced
• Use as a tool for improving energy systems
• Analyze each part of a power plant (pumps,
  turbine, heat exchangers, etc.)
• Analyze alternative energy scenarios
• Ethanol, biodiesel, hydrogen

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Some Questions That Can Be Answered

• How many fewer power plants would need to be
  built in Arizona if we increased our efficiency
  by 10 by 2020?
• How much could SO2, CO2, and other pollutants be
  reduced by that efficiency increase?

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Coal Fired Steam Power Plant
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1st Law of Thermodynamics
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1st Law of Thermodynamics

• Conservation of Energy Principle
• Esystem Ein - Eout

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Earth Energy Balance
Energy from Sun
EARTH Energy Stored
Energy Radiated to Space
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Earth Energy Balance
Energy from Sun
EARTH Energy Stored
Energy Radiated to Space
Estored Ewind Eplants Eheat etc. Esun -
Elost
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Steady-State Condition

• Under steady-state conditions, there is no change
  in the stored energy of the system.
• ?Esystem 0 Ein Eout
• or Ein Eout

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Energy Balance for a Power Plant
Efuel 1000 MJ
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Energy Balance for a Power Plant
Efuel 1000 MJ Euseful 350 MJ
Efuel Euseful Ewaste
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Energy Balance for a Power Plant
Efuel 1000 MJ Euseful 350 MJ Ewaste 650 MJ
Efuel Euseful Ewaste
1000 MJ 350 MJ 650 MJ
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Power Balance for a Power Plant
1 W 1 J/s
Pin Pout Pout Pout 1 P out 2 Pfuel
Puseful Pwaste
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Power Balance for a Power Plant
Pin 1000 MW Pout 350 MW 650MW
Pin Pout
Pfuel Puseful Pwaste
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Power Plant
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Parts of a power plant

• Energy Source
• Combustion Coal, natural gas, oil, biomass,
  garbage
• Heat Generated
• Boiler Coal burned, heats water, creates steam
• Combustion Chamber CH4 burned, hot gases
• Work Produced
• High pressure steam or hot gases turn turbine
  blades
• Wasted Heat Removed/Exhaust Treatment
• hot water dumped into a body of water or cooling
  tower
• exhaust gases dumped into atmosphere
• Some waste heat can be recovered

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Turbine
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Turbine Blades
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Cooling Tower
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Connection to Environment

• Fuel Side Mining, drilling, transporting
• Waste Heat Side
• Increase temperature of body of water
• Affect fish, algae blooms, etc.
• Pollutants in waste heat stream
• Air pollutants
• Pollutants in waste water stream
• Environmental justice
• Location, impact management of power plants

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Heat Engine

• A device that converts heat into mechanical
  energy
• Used to approximate thermal systems

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Heat Engine
High Temp. Source of Heat
Qhot
Heat Engine
Wnet
Qcold
Low Temp. Sink of Waste Heat
Energy Balance Qhot Wnet Qcold
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Definitions

• High Temp Source of Heat This is the source of
  energy that drives the power plant (heat of
  combustion, geothermal heat source, nuclear
  reactor, etc.).
• Qhot This is the heat transferred from the hot
  source.
• Heat Engine This includes the working parts of
  the power plant
• (including pumps, turbines, heat exchangers,
  condensers, etc.).
• Wnet This is the net amount of work that exits
  the power plant. A turbine generates energy, but
  the pumps and compressors use energy.
• Qcold This is the rejected or waste heat, which
  is dumped to a cold source (i.e. river,
  atmospheric air, lake, etc.).
• Low Temp Sink of Waste Heat This is the
  reservoir (river, air, lake, etc.) that the waste
  heat is dumped into (often goes through a cooling
  tower).

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Efficiency
what you want
work done
Efficiency

what you pay for
energy put into the system

• (Several names
• ?I 1st Law, Actual, or Thermal Efficiency)

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Energy Balance for a Power Plant

• Ein1000 MJ from fuel
• Eout350 MJ useful energy
• 650 MJ wasted energy
• ? Wnet/Qhot
• ? 350 MJ / 1000 MJ 0.35 35

Note Qhot Qin Qfuel
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Second Law of Thermodynamics

• Order tends to disorder, concentration tends to
  chaos
• No process can occur that only transfers energy
  from a cold body to a hot body (heat must flow
  from hot to cold)
• No process can occur that converts a given
  quantity of thermal energy into an equal quantity
  of mechanical work (always some
  degradation-always some wasted energy)

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2nd Law of Thermodynamics,alternate statements

• states in which direction a process can take
  place
• heat does not flow spontaneously from a cold to a
  hot body
• heat cannot be transformed completely into
  mechanical work
• it is impossible to construct an operational
  perpetual motion machine

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2nd Law of Thermodynamics and Carnot Efficiency

• 2nd Law Heat cannot be converted to work
  without creating some waste
  heat.
• What does this mean for a heat engine?
• Wnet lt Qhot ALWAYS
• In other words, efficiency is always less than
  100.
• Well, how much less?

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2nd Law of Thermodynamics and Carnot Efficiency

• Carnot Efficiency The net work produced and the
  heat into the system only depend on temperatures.
  No thermal system can be more efficient than the
  Carnot efficiency.

Qhot - Qcold
Qcold
Tcold
1 -
1 -
? c
Qhot
Qhot
Thot
This is the best you can achieve (under ideal
conditions)
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2nd Law of Thermodynamics and Carnot Efficiency

• Important Temperatures must be in units of
  Kelvin.

K ºC 273.15 1 ºC 1.8 F
Example In the power plant we considered
earlier, Thot 900 K (very hot steam) and Tcold
300 K (room temperature). What is its Carnot
efficiency?
? c 1 - Tcold / Thot 1 - (300/900) 0.67
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If this plant were an ideal heat engine, 33 of
the energy would be lost (to nearby water or to
atmosphere via cooling tower)
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Efficiencies of the Power Plant

• Ein1000 MJ from fuel
• Eout350 MJ useful energy
• 650 MJ wasted energy

Thigh900 K
Tlow 300 K

• I Wnet/Qhigh350 MJ / 1000 MJ 0.35 35
  Reality
• ? c 1 - Tcold / Thot 1 - (300/900) 0.67
  67 Ideal

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Comparison of Efficiencies
Type Th (K) Tc (K) Carnot Eff. Actual Eff.
Coal 800 300 62.5 35
Nuclear 1200 300 75 35
Geo- Thermal 525 350 33 16
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Waste Energy
Note When the working fluid reaches Tlow no more
energy can be used although the working fluid
still contains energy (typically gt 60 of
Efuel)
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Heat Pollution
The circulation rate of cooling water in a
typical 700 MW coal-fired power plant with a
cooling tower amounts to about 71,600 cubic
metres an hour (315,000 U.S. gallons per minute)
and the circulating water requires a supply
water make-up rate of about 5 percent (i.e.,
3,600 cubic metres an hour). If that same plant
had no cooling tower and used once-through
cooling water, it would require about 100,000
cubic metres an hour and that amount of water
would have to be continuously returned to the
ocean, lake or river from which it was obtained
and continuously re-supplied to the plant.
Discharging such large amounts of hot water may
raise the temperature of the receiving river or
lake to an unacceptable level for the local
ecosystem.
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Cogeneration

• Use waste energy in another application
• e.g., Heaters in cars use waste energy from the
  engine
• Easy uses Space and water heating, especially
  if small
• power plants can be built near urban areas
  increase
• total efficiency from 35 to 70
• Take the Qcold through another cycle with
  Qcolder
• Great potential in reducing fuel use and
  environmental impacts
• (by bringing Qcolder as close to atmospheric
  temperature as
• possible)

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Gasoline and Diesel Engines
Gasoline engines in our cars are also heat
engines called internal combustion engines
Fuel combined with gas in a closed chamber and
ignited combustion proceeds at a very high
temperature and rate Typically 25 efficiency
(mechanical energy/combustion energy), lots
of nasty pollutants Diesel engine also an
internal combustion engine Fuel and air mixed
differently than gasoline engine Air compressed
for heating, no electric spark Combustion
temperature even higher than gasoline
engine Efficiency gt 30 Less CO emission, more
NOx and particulate emissions