Energy and Human Ambitions on a Finite Planet, 2021a

14 Biological <strong>Energy</strong> 230
biological matter, 12 we would run through all the currently-living
mass—l<strong>and</strong> <strong>and</strong> sea—in a short 15 years!
12: About a quarter of the biomass is “dry”
combustible material, at about 4 kcal/g.
Can you imagine burning through all of Earth’s forests <strong>and</strong> animals
in 15 years? That’s the rate at which we use energy today—illustrating
the disparity between the biological resources on the planet <strong>and</strong>
our staggering energy appetite. We can’t expect to maintain our
pace based on biomass <strong>and</strong> biofuels, <strong>and</strong> still have a vibrant natural
planet.
14.3 Biofuels
Biofuels deserve their own category because the origins <strong>and</strong> end uses are
different enough to warrant distinction. While the biomass sources from
Section 14.2 tend to be in solid form, biofuels—as treated here—are liquid.
Liquid fuels are instantly a big deal because they have the energy density
<strong>and</strong> versatility to be used in transportation applications. An airplane
can’t very well fly on firewood, hydroelectricity, solar, wind, ocean
currents, geothermal, or nuclear energy. 13 matter. Biofuels therefore
occupy a special place in the pantheon of renewable resources as the
most obvious viable replacement for petroleum—the dominant fossil
fuel responsible for 92% 14 of transportation in the U.S.
In the U.S. in 2018, 2.28 qBtu (2.3%; 0.08 TW) came from biofuels [34],
which is very similar to the amount from biomass (wood, waste). Out
of the 11.41 qBtu of all renewables, biofuels account for 20% of the U.S.
renewable budget (Table 10.3; p. 170).
Most prominently, ethanol is the chief biofuel, accounting for about 80%
of the total. It is an alcohol that can be produced by fermenting the
photosynthetically-produced sugars in the plant <strong>and</strong> then distilling the
result. 15 Structurally, ethanol is very similar to ethane 16 except that the
terminating hydrogen on one end of the chain is replaced by a hydroxyl
group (OH; shown in Figure 14.3).
17: Appendix B provides some background
on chemical reactions <strong>and</strong> associated energy.
Though it is not necessary to fully underst<strong>and</strong> the chemistry, 17 combustion
of ethanol—for comparison to the fossil fuel reactions in Eq. 8.1
(p. 121)—goes according to
C 2 H 5 OH + 2O 2 → 2CO 2 + 3H 2 O + 29.7kJ/g. (14.2)
In other words, ethanol combines with oxygen via combustion (burning)
producing carbon dioxide <strong>and</strong> water, also releasing energy. It is almost
like the photosynthesis reaction (Eq. 14.1) in reverse.
The energy density works out to 7.1 kcal/g, which is considerably lower
than octane (representing gasoline) at 11.5 kcal/g (Table 8.2; p. 121). In
terms of CO 2 production, the reaction generates 88 g of CO 2 for each
46 g of ethanol, coming to 1.9 g/g—which is lower than the 3.09 factor
13: See, for instance, Box 13.3 (p. 212) <strong>and</strong>
Box 17.1 (p. 290).
14: Another 5% is from biofuels, usually
blended into gasoline.
[34]: U.S. <strong>Energy</strong> Inform. Administration
(2011), Annual <strong>Energy</strong> Review
15: . . . also how “moonshine” alcohol is
made
16: . . . C 2 H 6 : the second in the alkane sequence
of methane, ethane, propane, butane,
...,octane, etc.
H
H H H
C C O
H H
Figure 14.3: Ethanol is similar to ethane,
but replacing the hydrogen at the end with
hydroxyl (OH).
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.