What s in the Exhaust?

Transcription

1 Burn It, Breathe It

2

3 What s in the Exhaust? For most hydrocarbon fuels burned efficiently: Water vapor (H 2 O) Carbon dioxide (CO 2 ) Inefficient combustion can also produce: Carbon monoxide (CO) Nitrogen-oxygen compounds (NO X ) Soot (C)

4 Brain Teaser: What s Your Guess? Rank the following by how much CO 2 you guess is released during combustion, 1 = most, 5 = least. Assume you re burning amounts of each that release equivalent energy. Fuel CO 2 Ranking Gasoline Methane Biodiesel Coal Uranium Want to know? Do the math!

5 Carbon Puts the Carbon in Hydrocarbon Methane (CH 4 ) Propane (C 3 H 8 ) Octane (C 8 H 18 ) Cetane (C 16 H 34 ) 16 g/mole 44 g/mole 114 g/mole 226 g/mole

6 Carbon in Alcohols and Biodiesel Methanol (CH 3 OH) Ethanol (C 2 H 5 OH) Isopropanol (C 3 H 7 OH) Biodiesel (C 19 H 36 O 2 ) 32 g/mole 46 g/mole 60 g/mole 296 g/mole

7 Balanced Combustion Reactions Hydrogen: 2H 2 + O 2 --> 2H 2 O Methane: CH 4 + 2O 2 --> CO 2 + 2H 2 O Gasoline: 2C 8 H O 2 --> 16CO H 2 O Biodiesel: C 19 H 36 O O 2 --> 19CO 2 +18H 2 O Methanol: 2CH 3 OH + 3O 2 --> 2CO 2 + 4H 2 O Ethanol: C 2 H 5 OH + 3O 2 --> 2CO 2 + 3H 2 O Isopropanol: 2C 3 H 7 OH + 9O 2 --> 6CO 2 + 8H 2 O Coal: C + O 2 --> CO 2

8 How Much Carbon? Comparing the combustion of gasoline and ethanol: Gasoline: 2C 8 H O 2 > 16CO H 2 O 1 mole Combustion!? 8 moles Each molecule (or mole) of gasoline produces 8 CO 2 molecules, and each molecule of ethanol produces 2 CO 2 molecules. Ethanol: C 2 H 5 OH + 3O 2 > 2CO 2 + 3H 2 O 1 mole Combustion!? Is burning ethanol, then, greener than burning gasoline? Is this a fair comparison? 2 moles

9 How Much Carbon? How much energy is released by burning a certain amount of fuel? (weight, volume, number of molecules) To release equivalent amounts of energy, what quantity of different types of fuel do you burn? How much CO 2 is released by burning those amounts of fuel?

10 Energy-Equivalent Volumes of Fuels Fuel Hydrogen gas (compressed at 700 bars) Kilojoules per cc* 7.61 Gasoline Biodiesel oil Methanol Ethanol Isopropanol Coal (Anthracite) Uranium ,000,000 Comparing two liquid fuels, gasoline packs kj of energy per cc of volume, while ethanol stores kj/cc. So gasoline contains / = 1.5 times the energy of ethanol per unit volume so in comparing gasoline and ethanol, you need to burn 1.5 times the volume of ethanol to release the same energy as gasoline. Comparing energy density of fuels by volume is useful because it tells you the relative volumes of liquid storage tanks that would store equivalent amounts of fuel energy. *Unless otherwise noted, the value for energy density is the lower heating value (LHV) that only accounts for energy released as direct heat, and not energy stored as latent heat in water vapor. Note: 1 kilojoule = kwh

11 How Much Carbon is Released in a Fair Fuel Contest? Let s burn some fuel and find out!

12 How Much Carbon Should Have Fuel Hydrogen gas (compressed at 700 bars) Been Released? Kilojoules per cc* Kilojoules per gram* Molar Mass (grams/mole) Kilojoules per mole* Gasoline Biodiesel oil Methanol Ethanol Isopropanol Coal (Anthracite) Uranium ,000,000 20,000,000 (fission) 238 4,760,000,000 *Unless otherwise noted, the value for energy density is the lower heating value (LHV) that only accounts for energy released as direct heat, and not energy stored as latent heat in water vapor. Note: 1 kilojoule = kwh

13 Doing The Math Fuel Kilojoules per mole Energy Equivalent Amount CO 2 Output Ratio CO 2 Released (moles) Gasoline Ethanol To get an equivalent amount of energy from ethanol and gasoline, you need to burn 5062 / 1240 = 4.08 times as many ethanol molecules So burning 1 mole of gasoline and 4.08 moles of ethanol produces: From gasoline: (1 x 8) --> 8 moles of CO 2 From ethanol: (4.08 x 2) --> 8.16 moles of CO 2

14 Doing The Math Fuel Kilojoules per mole Energy Equivalent Amount CO 2 Output Ratio CO 2 Released (moles) Gasoline Ethanol Methanol Isopropanol Biodiesel Coal Methane

15 What s Your Guess? Rank the following by how much CO 2 you guess is released during combustion, from most to least. Assume you re burning amounts of each that release equivalent energy. By my calculations Fuel CO 2 Ranking Gasoline Methane Biodiesel Coal Uranium (Even if you could combust uranium, as it contains no carbon, it wouldn t produce any CO 2 )