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How does one turn coal into gasoline? Write a single chemical equation that balances for the reaction you use as an example. Gasoline is typically C 10 H 22 . Inputs are only carbon and water. The C and H do not directly match the gasoline molecule, so something

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slide1

How does one turn coal into gasoline? Write a

single chemical equation that balances for the reaction

you use as an example. Gasoline is typically C10H22.

Inputs are only carbon and water. The C and H do not

directly match the gasoline molecule, so something

else is left over on the right side. Take it to be CO2.

X C + Y H2O C10H22 + Z CO2

So there are three unknowns X, Y, and Z.

Start with hydrogen, since H appears only once on

each side. 2Y=22, so Y=11.

Then balance oxygen, once on each side. Y=2Z, or

since Y=11, so Z=5.5.

Now do carbon X=10 + Z, so X=15.5.

Our equation is then 15.5 C + 11 H2O C10H22 + 5.5 CO2

Or 31 C + 22 H2O 2 C10H22 +11 CO2.

slide2

27 Quads = 27 x 1015 Btu. I will do this solution in Btu

language, with the given 50,000 Btu/kg of C10H22.

We will thus need 27 x 1015 Btu / 50,000 Btu/kg

= 0.00054 x 1015 kg=5.4x1011 kg of gasoline.

15.5 x 12 =186 kg of carbon will make 10 x 12 + 22 x 1

= 142 kg of gasoline, from the masses in the equation.

So we have the ratio

186 :142 = XX : 5.4x1011 kg

So XX=7.07 x 1011 kg of carbon, which is 14.1 x 1011 kg of

coal (at the given ratio). At 1000 kg per metric tonne,

we will need to dig 1.41 x 109 metric tonnes,

in addition to the 109 or so tonnes we dig anyway.

We will also need even more coal to run the reaction above.

This would be a very, very big deal! Feel free to edit your

answer to part b) in relation to this number.

heat engines

Heat engines

Use heat energy to do work.

Chapter 3

slide4

Think about a water mill.

Water at height Hhigh

* work

Falls to Hlow

CHANGE in potential energy = m g (Hhigh-Hlow)

With g =9.8 m/sec2

reminder

Reminder!

Be orthodox in your units. If m is in kg, heights in meters, and g=9.8 m/sec2, then the energy will be in Joules.

because water falls down spontaneously

Because water falls down spontaneously.

And we can use that fall to do useful work.

ditto for heat
Ditto for heat
  • And heat (a form of energy) falls spontaneously from high temperature to low.
  • As water falling, we may extract useful energy from this process.
  • Called a ‘heat engine’. Forget the details.
slide9
Laws
  • Energy is conserved

Heat in = heat out + work done

Qin = Qout + W

Or---QHot = QCold + W

  • Efficiency = e = Work done/Heat energy in

(careful! Heat in calories or Btu, work in Joules or kW-hr)

example
Example

I burn one ton of coal to generate 2.66 x 107 Btu, and use this energy to do 10 10 Joules of useful work.

A) how much heat was exhausted?

Qout = Qin – W = 2.66 x 107 Btu – 1010 J

= 2.81 x 1010 J – 1 x 1010 J= 1.81 x 1010 J

B) What is the efficiency of my engine?

e=W/Qin = 1010 J / 2.66 x 107 Btu

=1010 J / 2.81 x 1010 J = 0.3559 = 35.59%.

ditto for the rate of doing work
Ditto for the RATE of doing work

Heat Power in

= Heat power out +(work) Power out.

Efficiency

=work power out/Rate of heat energy in.

slide12

I burn one bbl of oil per hour at 25% efficiency. How many kW can I make?

I bbl/hr=6.12x109 J/3600 sec

=1.7x106 J/sec

=1.7 x 106 watts = 1700 kW

Useful power = 0.25 x 1700 kW= 425 kW

slide13

Or--

One bbl/hr

=1700kWh/hour=1700 kW

At 25%425 kW

how many btu must i dump in that hour
How many Btu must I dump in that hour?
  • The other 75%,

or 0.75 x 1700 kWh=1275 kWh

  • 1 kwh=3413 Btu
  • So I must dump 1275 kWh x 3413 Btu/kwh

=4351575=4.35 million Btu

into a cooling stream or the air.

two ways to specify power
Two ways to specify power
  • Power plants may be rated by their thermal power GWt
  • Or their electrical power output GWe

with GWe = efficiency x GWt

  • A typical thermal power plant has e = 30-35%.
  • The rest of the thermal energy is dumped somewhere.
  • Why so poor?
there is a limit to the efficiency of any heat engine
There is a limit to the efficiency of any heat engine.
  • Since we cannot extract all the heat energy, since to do so would have to exhaust at absolute zero, equal to -273 deg C.
  • But– we can do work to pump heat energy up hill (from cold to hot).
  • Friday
slide18

2. (3) New materials may allow us to burn a fire at 600

degrees C. If a power plant at this temperature is next

to a cold lake, just at freezing, to dump the waste heat,

what is the best efficiency possible for this plant to make

salable energy?

3.(3) Here’s another idea, with free !! energy. Cold, deep

ocean water is at 4 deg C, while surface water in the tropics

can be at 30 deg. C. What is the best efficiency you could

obtain with some sort of heat engine operating between

these temperatures?

slide19

4. (2) Draft a one-sentence (!!!) question (on energy and the environment only!!)you would like to ask of your representative in Washington. Then, add a brief answer that you would hope to hear.

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