Energy, Society,and

the Environment

Unit IV: How Do Power Plants Work? Combustion and Heat Engines

Outline

• Combustion: Energy Generation and Pollutants

• 1st Law of Thermodynamics

• 2nd Law of Thermodynamics

• Efficiency

Combustion

• What do we do with fossil fuels: burn them – Combustion: impacts

• Fuels

• Balancing combustion chemical equations

• Combustion products

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

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

How much CO2 is produced when 1 metric ton of wood is burned,continued ...

How much do these weigh?

1 molecule of cellulose produces 6 molecules of CO2

Remember your atoms

C = 6 p + 6 n = 12 O = 8 p + 8 n = 16

(atomic mass unit, OR1 mole of C is 12 grams)

How much CO2 is produced when 1 metric ton of wood is burned,continued ...

So, from C6H12O6 6CO2 + 6H2O, we see that:

€

=264 10g x 6g

180g=1.47x 106 g =1.47 tons 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)

1 molecule of cellulose produces 6 molecules of CO2

180 grams cellulose 264 grams CO2

106 grams cellulose ?? grams CO2

??

Simple combustion equation,but put it in air

• Example: methane reacts with airCH4 + (O2 + 3. 76N2) -> CO2 + H2O + N2

(this is termed the ‘unbalanced’ version)

CH4 + 2(O2 + 3. 76N2) -> 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

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. ...

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

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

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.

National Ambient Air Quality Standards

Pollutant Metric Fed. Std. CA Std. Effects

CO 8 hr 10 mg/m39 ppm Angina, pre-natal

asthmaNO2 Ann. mean 40 mg/m3 20 ppm Respiratory dis.

PM10 Ann. Mean 50 μ / 3g m 30 / 3mg m . , Resp disease

2.5PM . Ann Mean 15 / 3mg m in review . , Resp diseaseSO2 . Ann Mean 80 μ / 3g m in review , .wheez plant dam

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%

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Announcements 2/16

• No Class THIS Friday (note the change)

• Homework #3 posted today on D2L, due

next Monday

POWER PLANTS areHeat Engines

Using Heat to Do Work(a bit of Thermodynamics)

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

• 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?

Some Questions That Can Be Answered

Coal Fired Steam Power Plant

1st Law of Thermodynamics

Water in

Water out

1st Law of Thermodynamics

• Conservation of Energy Principle

Esystem = Ein - Eout

EinEsystem Eout

Earth Energy Balance

EARTHEnergyStored

Energy from Sun

Energy Radiated to Space

Earth Energy Balance

EARTHEnergyStored

Energy from Sun

Energy Radiated to Space

Estored = Ewind + Eplants + Eheat + etc. = Esun - Elost

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

Energy Balance for a Power Plant

PowerPlant

1000 MJ

Energy from Fuel

Efuel = 1000 MJ

Energy Balance for a Power Plant

PowerPlant

1000MJ 350MJ

Energy from FuelUseful Energy (Work)

Efuel = 1000 MJ

Euseful = 350 MJ

Efuel = Euseful + Ewaste

Energy Balance for a Power Plant

Useful Energy (Work)

PowerPlant

1000MJ 350MJ

650MJ

Energy from Fuel

Wasted Energy

Efuel = Euseful + Ewaste

1000 MJ = 350 MJ + 650 MJ

Efuel = 1000 MJ

Euseful = 350 MJ

Ewaste = 650 MJ

Power Balance for a Power Plant

PowerPlant

1000MW 350MW

650MW

Pin = Pout

Pout = Pout 1 + P out 2

Pfuel = Puseful + Pwaste

1 W = 1 J/s

Power Balance for a Power Plant

PowerPlant

1000MW 350MW

650MW

Pin = 1000 MW

Pout = 350 MW + 650MW

Pfuel = Puseful + Pwaste

Pin = Pout

Power Plant

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

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

Heat Engine

• A device that converts heat into mechanical energy

• Used to approximate thermal systems

Heat Engine

HeatEngine

Wnet

Qcold

Qhot

High Temp. Source of Heat

Low Temp. Sink of Waste Heat

Energy Balance: Qhot = Wnet + Qcold

DefinitionsHigh 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).

Efficiency

(Several names:

I =1st Law, Actual, or Thermal Efficiency)

Efficiency = what you want

what you pay for

work done

energy put into the system=

Qhot

=Wnet

Qhot

=Qhot-Qcold

Ein=1000 MJ from fuel

Eout=350 MJ useful energy

+650 MJ wasted energy

= Wnet/Qhot

= 350 MJ / 1000 MJ = 0.35 = 35%

Energy Balance for a Power Plant

PowerPlant

1000MJ 350MJ

650MJ

Energy from Fuel Useful Energy (Work)

Wasted Energy

Note: Qhot = Qin = Qfuel

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)

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

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?

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.

€

Qcold

Qhot

=Tcold

Thot

c =Qhot - Qcold

Qhot

= 1 - Qcold

Qhot

= 1 - Tcold

Thot

This is the best you can achieve (under ideal conditions)

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 = 67%

If this plant were an ideal heat engine, 33% of the energy wouldbe lost (to nearby water or to atmosphere via cooling tower)

Ein=1000 MJ from fuel

Eout=350 MJ useful energy

+650 MJ wasted energy

Efficiencies of the Power Plant

PowerPlant

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

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%

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)

Heat PollutionThe 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.

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)

Gasoline and Diesel EnginesGasoline 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 rateTypically 25% efficiency (mechanical energy/combustion energy), lots of nasty pollutants

Diesel engine also an internal combustion engineFuel and air mixed differently than gasoline engineAir compressed for heating, no electric sparkCombustion temperature even higher than gasoline engineEfficiency > 30%Less CO emission, more NOx and particulate emissions