Environmental Engineering Reference
In-Depth Information
energy cost of the Haber
-
Bosch process is high, since it involves four passes through
a bed of catalysts, at about 400
C, each time releasing pressure from 100 atm and
then repressurizing.)
Hydrogen for ammonia production comes from steam reforming of methane
(natural gas). Therefore, (nonrenewable) hydrogen production for ammonia alone,
worldwide, can be obtained from the ammonia output multiplied by the fraction of
the mass of anhydrous ammonia represented by hydrogen. This fraction is
(3
0.1776, where the mass of nitrogen is
14.0067 in atomic mass units. Therefore, the worldwide production of hydrogen
(speci
cally the part associated with production of ammonia) in 2004 was 109million
metric tons
1.008)/[(3
1.008)
þ
14.0067]
¼
¼
19.35 million metric tons. (The proportion of this produced
in the United States was 8.17%, or 1.58 million metric tons of hydrogen.) Com-
mercial hydrogen production is also done from town gas, methane plus carbon
monoxide, which is a result of gasification of coal. All of these are carbon-dependent
(nonrenewable) processes that release greenhouse gas. An independent estimate is
world production of hydrogen (2004) is 50 million tons, about half for ammonia and
about half for petroleum industry; hydro-cracking to produce light hydrocarbons
from heavy hydrocarbons.
0.1776
9.8.2
Hydrogen as Potential Intermediate in U.S. Electricity Distribution
As an upper limit on hydrogen that might be needed, we can calculate the hydrogen
production equivalent in energy to the U.S. electricity usage per year, which was
460 GW in 2004 [122]. The total electric energy in 2004, then, taking a year as
3.154
10
7
s, was 1.45
10
19
J. If the energy carried per H
2
is 1.23 eV
¼
1.968
10
19
J, and the mass per H
2
is 2
10
27
kg, then the equivalent
1.008
1.6605
10
8
metric tons, or 247 million
metric tons. (Another estimate [109] is that the total transportation energy need of the
United States could be met (2004) by 150 million tons of hydrogen.)
This estimate of 247 million metric tons of hydrogen is 12.8 times the worldwide
production of hydrogen in the preparation of ammonia for fertilizer. The price per
metric ton of ammonia onMay 15, 2008was quoted as $550, or $0.55/kg. If the cost of
ammonia is attributed entirely to its hydrogen, which is 17.8% of its weight, then the
cost of hydrogen is $0.55/(0.178)
10
11
kg
mass of hydrogen per year is 2.47
¼
2.47
$3.09/kg. It is clear that this is an overestimate
because the costs associated with fractionating air to get nitrogen, and with theHaber
reaction to make ammonia, would be avoided in the pure hydrogen production. An
independent estimate of cost of hydrogen from natural gas is $2.70/kg. It is also
stated that retail prices per kg for hydrogen at
filling stations in California and in the
Washington DC area range from $5 to $6 per kg. Again, according to Figure 1 in
Turner [109], electrolysis of water would match $2.70/kg for producing hydrogen at
electricity costs in the vicinity of 4 c/kWh. It is also stated that the lowest price
hydrogen is produced by electrolysis at wind farms.
As the price of natural gas rises, it might occur that ammonia (fertilizer)
productionwould represent amarket for renewably produced hydrogen, for example,
¼
