Environmental Engineering Reference
In-Depth Information
fermentation, and fortunately, there is no way to prevent
biosphere-wide bacterial denitrification (if there were, the
biosphere would run out of nitrogen).
In the short run it should be possible to moderate the
rate of greenhouse gas emissions (the aim of the Kyoto
Protocol). But the effort would be truly effective only if
many options were pursued vigorously and simultane-
ously with effective international cooperation and virtu-
ally universal commitment. This seems unlikely during
the coming one or two generations, and that is why most
strategies aimed at moderating carbon emissions (Pacala
and Socolow 2004) will fall short of their goals. In the
long run only substantially reduced rates of fossil fuel
combustion (stabilization would not suffice) and limits
on deforestation, fertilizer applications, and ruminant
livestock can break the secular trend of steady increases
in greenhouse gas emissions. There is no shortage of
bold proposals aimed at making the use of fossil fuels
more efficient and their environmental impacts less intru-
sive, but even the most likely combination of these tech-
niques will not prevent further substantial
of 71.4 MPa), putting away just 10% of its global flux
would require annual handling of a volume equivalent
to the worldwide extraction of crude oil.
Nor is there any early possibility of a hydrogen-based
system. Undeniably, energy transitions have been steadily
decarbonizing the global supply as average atomic H/C
ratios rose from 0.1 for wood to 1 for coal, 2 for crude
oil, and 4 for methane. As a result, a logistic growth
process points to a methane-dominated global economy
after 2030, but a hydrogen-dominated economy, requir-
ing production of large volumes of the gas without fossil
energy, could take shape only during the closing decades
of the twenty-first century (Ausubel 1996). Mass produc-
tion of hydrogen could eventually take place in facilities
energized by advanced forms of nuclear fission or effi-
cient photovoltaics. But hopes for an early reliance on
hydrogen are just that (Mazza and Hammerschlag
2004). There is no inexpensive way to produce this high-
energy density carrier and no realistic prospects for a
hydrogen economy to materialize for decades (Service
2004). A methanol economy may be a better, although
also very uncertain, alternative (Olah, Goeppert, and Pra-
kash 2006). There will also be no rapid and massive
adoption of fuel cell vehicles because they do not offer
any significant efficiency advantage over hybrid cars in
city driving (Demird¨ven and Deutch 2004).
Prospects for more efficient fission designs remain
highly uncertain. Public acceptance of nuclear generation
and final disposal of radioactive wastes are the key
obstacles to massive expansion. And it is extremely un-
likely that nuclear fusion can be part of a solution before
2050, if at all. U.S. spending on fusion has averaged
about $250 million a year for the past 50 years with
nothing practical to show for it, and the engineering
increases in
emissions.
Suggested innovations range from high-efficiency,
zero-emissions coal-fired power plants to large-scale se-
questration of CO
2
. Expensive alterations can make
coal-fired plants much cleaner, but contrary to a new-
found enthusiasm for CO
2
capture and storage (IPCC
2006), any realistic assessment must see carbon seques-
tration as nothing but a marginal effort. The scaling chal-
lenge is immense: in 2005 the annual storage of the three
experimental projects in oil and gas fields rated 1-2 Mt
CO
2
, and fossil fuel combustion generated more than
7GtCO
2
. Even if the gas were stored entirely in the
supercritical form (CO
2
density 0.468 g/mL at pressure
