Environmental Engineering Reference
In-Depth Information
where (g) means gaseous phase and (l) means liquid phase. The minus sign indicates that energy is
liberated; that is, the reaction is exothermic, with 171 kJ evolved per mole of CO
2
reacted. However,
the reaction as written requires three moles of hydrogen for each mole of CO
2
to produce one mole
of CH
3
OH(l). The production of one mole of hydrogen from dissociation of water requires 286 kJ
of energy; three moles require 858 kJ. Thus, the energy balance of equation (10.6) is actually
negative, requiring 687 kJ mol
−
1
CH
3
OH produced. The production of methanol from CO
2
and H
2
would only make sense if that hydrogen were derived from nonfossil energy, such as solar (e.g.,
photodissociation of water) or nuclear (e.g., electrolysis of water using nuclear electricity). Even if
hydrogen is derived from nonfossil sources, the question is whether it should not be utilized directly,
such as in fuel cells, rather than producing methanol. Furthermore, when methanol is burned in a
heat engine, carbon is reoxidized to CO
2
, and nothing has been gained in terms of global warming
mitigation.
Another example is the production of urea from CO
2
. Urea is an important industrial chemical,
because it is used in chemical fertilizers, in polyurethane foam production, and as an intermediate
in a host of other chemicals. The production of urea can be written in the following simplified
reaction:
632 kJ mol
−
1
CO
2
(
g
)
+
3H
2
(
g
)
+
N
2
(
g
)
→
NH
2
CONH
2
(
s
)
+
H
2
O
(
l
)
+
(10.7)
where (s) means solid phase. This reaction is highly endothermic, with 632 kJ of energy required
per mole of solid urea produced, including the 858 kJ of energy necessary for the production of three
moles of hydrogen. This example shows again that unless hydrogen is produced from nonfossil
energy sources, the utilization of CO
2
for converting to other chemicals makes no sense.
Nature uses CO
2
as a raw material for producing biomass by means of photosynthesis:
ν
→
h
5 kJ mol
−
1
n
CO
2
+
C
n
(
)
m
+
O
2
+
.
m
H
2
O
H
2
O
570
(10.8)
where
h
)
m
is a basic building block of biomass. This
reaction is endothermic; it requires 570.5 kJ of energy per mole of CH
2
O(s) produced. That energy
comes from the sun. All the vegetation and phytoplankton in the world are based on reaction (10.8).
Because plants and planktons are at the bottom of the food chain, all animals and mankind are
dependent on reaction (10.8) for their sustenance. In fact, all fossil fuels were created over the eons
by geochemical conversion of biomass to coal, oil and natural gas.
The utilization of biomass for fuel in boilers or heat engines is CO
2
neutral. Every carbon
atom burnt or utilized from biomass is reabsorbed in the next generation of plants or planktons.
The utilization of biomass as a renewable energy source is discussed in Section 7.3.
Here we should also mention that fostering biomass growth without its utilization would have
a positive effect on reducing CO
2
concentrations in the atmosphere. It is estimated that typical
coniferous and tropical forests absorb between 6 and 10 metric tons of carbon per hectare per year
(t C ha
−
1
y
−
1
). Planting new forests, without using the wood until atmospheric CO
2
concentrations
start to decline because of depletion of fossil fuels or other CO
2
emission abatement measures, would
absorb part of the anthropogenic emissions of CO
2
due to the photosynthesis reaction (10.8). Indeed,
the Kyoto Convention of 1997 and the subsequent Buenos Aires Conference in 1998 envisaged
that part of the greenhouse effect mitigation efforts will come by afforestation. Large CO
2
-emitting
countries are encouraged to reduce global concentrations of CO
2
by investing in other countries
ν
represents a solar photon, and C
n
(
H
2
O
