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

2. Outline Combustion: Energy Generation and Pollutants
1st Law of Thermodynamics
2nd Law of Thermodynamics
Efficiency

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

4. What happens in combustion?
Fuel + oxidizer -> 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

5. C6H12O6 + O2  CO2 + H2O C6H12O6 + O2  6CO2 + H2O
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

6. 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?

7. Remember your atoms C = 6 p + 6 n = 12 O = 8 p + 8 n = 16
(atomic mass unit, OR
1 mole of C is 12 grams)

8. 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 ??

9. Simple combustion equation, but put it in air
Example: methane reacts with air
CH4 + (O N2) -> CO2 + H2O + N2
(this is termed the ‘unbalanced’ version)
CH4 + 2(O N2) -> CO2 + 2H2O N2
(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

10. 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. ...

11. 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

12. 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

13. 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.

14. National Ambient Air Quality Standards

15. Criteria Pollutants & the Clean Air Act
POLLUTANT Emissions Air Concentration
‘83 - ‘ ‘93 - ‘02 ‘83 - ‘02 ‘93 - ‘02
CO % % - 65% - 42%
Pb % % - 94% - 57%
O3 [1 hr] % % - 22% - 2%
PM % % %
PM % %
SO % % -34% - 39%
NOx % % - 2% %

17. Announcements 2/16 No Class THIS Friday (note the change)
Homework #3 posted today on D2L, due
next Monday

18. POWER PLANTS are Heat Engines Using Heat to Do Work (a bit of Thermodynamics)

19. 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

20. 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?

21. Coal Fired Steam Power Plant

22. 1st Law of Thermodynamics
Water in
Water out
Imagine you have a bathtub – water coming in is the Ein, water going out the drain is Eout, if Ein is greater than Eout, then the bathtub will fill up, if Eout is greater than Ein, then the bathtub will empty.
What is Ein ?– Fuel, Sunlight, energy stored in chemical bonds, energy as photons in sunlight, electrons in a electrical device.

23. 1st Law of Thermodynamics
Conservation of Energy Principle
Esystem = Ein - Eout
Ein
Esystem
Eout
Imagine you have a bathtub – water coming in is the Ein, water going out the drain is Eout, if Ein is greater than Eout, then the bathtub will fill up, if Eout is greater than Ein, then the bathtub will empty.
What is Ein ?– Fuel, Sunlight, energy stored in chemical bonds, energy as photons in sunlight, electrons in a electrical device.
What is Eout? Heat, Other chemicals (like in the case of combustion),
Energy balance can be used on almost anything: coffee cup, car engine, later in the class you will use the first law of thermo to analyze renewable energy systems (solar, hydro, wind), also refrigeration systems and to analyze the energy efficiency of buildings – SO it’s very important. Although the math is not very difficult – conceptually it takes practice to understand how to account for the energy.
But, let’s look at an energy balance for the entire earth

24. Earth Energy Balance Energy from Sun EARTH Energy Stored
Let’s do an energy balance about the earth. Where do we get our energy? The Sun.
So our Ein is the energy we get from the sun.
We store energy on the earth in a lot of ways (heat in the atmosphere, kinetic energy of wind, plants store energy, etc.)
The Elost: is energy radiated into space.
Average energy/time from sun = 350 W/m^2
That’s 20e16 W to the earth
Assuming an average power plant ~ 1000MW – that’s 200million power plants
Etotalearth = 18e16W
Ereflectedspace = 53e15W
Eabs in atm = 44e15W
Energy Radiated to Space

25. Earth Energy Balance
Energy from Sun
EARTH
Energy
Stored
Let’s do an energy balance about the earth. Where do we get our energy? The Sun.
So our Ein is the energy we get from the sun.
We store energy on the earth in a lot of ways (heat in the atmosphere, kinetic energy of wind, plants store energy, etc.)
The Elost: is energy radiated into space.
Average energy/time from sun = 350 W/m^2
That’s 20e16 W to the earth
Assuming an average power plant ~ 1000MW – that’s 200million power plants
Etotalearth = 18e16W
Ereflectedspace = 53e15W
Eabs in atm = 44e15W
Energy Radiated to Space
Estored = Ewind + Eplants + Eheat + etc. = Esun - Elost

26. 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
Or, the water stays at the same level in the tub.
The systems discussed in this class will be steady-state systems.
Examples are: Natural gas power plant, Coal fired power plant, nuclear power plant
Example of a non-steady device: Battery – if you are using a battery in calculator or Cd player, you have an Eout, but no Ein (while you are using it) – then Esystem, or the energy stored in the battery, would reduce.

27. Energy Balance for a Power Plant
1000 MJ
Energy from Fuel
Efuel = 1000 MJ
This 1000MJ coming in could be as coal, uranium, natural gas, etc.
How much fuel is 1000 MJ = 10E3 x 10E6 = 10E9 J
Energy in gasoline = 115,000 BTU/Gal = E8J So 1000 MJ = 8 gals gas (1/2 tank full of a car).

28. Energy Balance for a Power Plant
1000MJ
350MJ
Energy from Fuel
Useful Energy (Work)
Efuel = 1000 MJ
Euseful = 350 MJ
As I said, we will be dealing with steady-state systems: therefore, if 350MJ of the total 1000MJ comes out as useful work --- then there must be some left over (650MJ)
Efuel = Euseful + Ewaste

29. Energy Balance for a Power Plant
Useful Energy (Work)
Power
Plant
1000MJ
350MJ
650MJ
Energy from Fuel
Wasted Energy
Efuel = 1000 MJ
Euseful = 350 MJ
Ewaste = 650 MJ
Efuel = Euseful + Ewaste
1000 MJ = 350 MJ MJ

30. Power Balance for a Power Plant
1 W = 1 J/s
Power
Plant
1000MW
350MW
650MW
Pin = Pout
Pout = Pout 1 + P out 2
Pfuel = Puseful + Pwaste

31. Power Balance for a Power Plant
Pin = 1000 MW
Pout = 350 MW + 650MW
Power
Plant
1000MW
350MW
650MW
Pin = Pout
Pfuel = Puseful + Pwaste

32. Power Plant

33. Parts of a power plant Energy Source Heat Generated Work Produced
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
The energy balance can be used for any system (combustion, nuclear, renewables)

34. Turbine:

35. Turbine: Blades

36. Cooling Tower

37. 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

38. Heat Engine A device that converts heat into mechanical energy
Used to approximate thermal systems

39. Heat Engine Energy Balance: Qhot = Wnet + Qcold
High Temp. Source of Heat
Qhot
Heat
Engine
Wnet
Qcold
Low Temp. Sink of Waste Heat
Energy Balance: Qhot = Wnet + Qcold

40. 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).
Low Temp Sink is where most of our problems lie (although there are definitely problems with drilling, mining, etc. for fuels).

41. Efficiency (Several names: I =1st Law, Actual, or Thermal 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)
Qhot
 =
Wnet =
Qhot-Qcold

42. Energy Balance for a Power Plant
1000MJ
350MJ
650MJ
Energy from Fuel
Useful Energy (Work)
Wasted Energy
Ein=1000 MJ from fuel
Eout=350 MJ useful energy
+650 MJ wasted energy
 = Wnet/Qhot
 = 350 MJ / 1000 MJ = 0.35 = 35%
The thermal efficiency is a great way to compare various systems – it’s not the only way (economics, pollution, land space needed, etc.). For example, thinking about other thermal systems – the diesel engine is more efficient than a regulate gasoline engine. Therefore, they put emit less CO2/mile than gasoline powered cars do – so, why don’t we use diesels exclusively then? They emit about 100times the soot particles (black stuff coming out of old buses and semi-trucks) and much more NOx (which causes photochemical smog). But, they do use less fuel – which means less drilling.
Normal power plants have efficiencies of around 30%. With some extra work and money, that efficiency can be increased to 40%. So, is 50, 60 , 70 % possible. Can we get 100% efficiency out of a power plant – but we’re just to money grubbing to do it?
Note: Qhot = Qin = Qfuel

43. 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)

44. 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

45. 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 < Qhot ALWAYS
In other words, efficiency is always less than 100%.
Well, how much less?

46. 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)

47. 2nd Law of Thermodynamics and Carnot Efficiency
Important: Temperatures must be in units of Kelvin.
K = ºC
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 = 67%
If this plant were an ideal heat engine, 33% of the energy would
be lost (to nearby water or to atmosphere via cooling tower)

48. Efficiencies of the Power Plant
Ein=1000 MJ from fuel
Eout=350 MJ useful energy
+650 MJ wasted energy
Power
Plant
1000MJ
350MJ
650MJ
Energy from Fuel
Useful Energy (Work)
Wasted Energy
Thigh=900 K
Tlow =300 K
I= Wnet/Qhigh=350 MJ / 1000 MJ = 0.35 = 35% Reality
 c = 1 - Tcold / Thot = 1 - (300/900) = 0.67 = 67% Ideal

49. Comparison of Efficiencies
Type
Th (K)
Tc (K)
Carnot
Eff.
Actual
Coal
800
300
62.5%
35%
Nuclear
1200
75%
Geo-
Thermal
525
350
33%
16%

50. Waste Energy
Note: When the working fluid reaches Tlow no more energy can be used – although the working fluid still contains energy
(typically > 60% of Efuel)

51. 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.

52. 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)

53. 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 > 30%
Less CO emission, more NOx and particulate emissions