306
Water for Energy and Fuel Production
While a single-step thermolysis is conceptually simple, its realization is very
challenging since it needs a high-temperature heat source above 2200°C for achieving a reasonable degree of dissociation and an effective technique to separate hydrogen and oxygen to avoid explosive mixture. The ideas proposed to separate hydrogen
from the products include effusion separation and electrolytic separation. Membranes
made of zirconia and other ceramics can withstand such high temperatures, but they
fail to absorb severe thermal shocks that often occur when working under high-flux
solar radiation. Other techniques that have been evaluated are rapid quench by injecting a cold gas, expansion in a nozzle, or submerging a solar-irradiated target in liquid
water. The last technique is workable and simple, but a quench introduces a significant
drop in energy efficiency and produces an explosive gas mixture. The efficiency can
also be further decreased by reradiation, and the type of temperature (e.g., 2725°C for
64% dissociation at atmospheric pressure) required creates material limitations [1,11].
One of the problems for thermal dissociation of water is the materials that can
stand temperatures in the excess of at least 2200°C–2500°C. Several materials such
as tantalum boride, tantalum carbide, tungsten, and graphite are possible. However,
at these temperatures, only oxides are stable. Graphite is chemically unstable in the
presence of hydrogen and oxygen at these high temperatures. Tungsten and tungsten
carbide get oxidized at these temperatures. The effect of hydrogen on oxide catalysts
at these temperatures is not known. Ceramic materials such as boron nitride can
also be useful if its oxidation can be controlled. Recent studies have shown that a
low amount of dissociation is possible [11]. The separation of oxygen and hydrogen can be carried out in a semipermeable membrane of palladium or ZrO 2 –CeO 2 –
Y 2 O 3 , which removes oxygen preferentially. Lede et al. [126,127] used a ZrO 2 nozzle
through which steam is forced into a thermal stream and decomposed and unreacted
water is quenched suddenly to remove water and oxygen. The resulting gas contained
only a small amount (about 1.2 mol%) of hydrogen. Another possible solution is the
use of heat-resistant membrane made of Pd or ZrO 2 , both of which selectively permeate hydrogen. The gas can also be separated using a magnetic field. The source of
heat is also an issue. Solar or nuclear sources are possibilities. They are, although at
the early stages of development and at the present time, only possible on a smaller
scale [11,135–139].
In the recent years, thermal dissociation of water is achieved using nuclear
[90,91,93,94] (Funk, 2011, pers. comm.; Bamberger, 2011, pers. comm.) and solar energy
[1,92,108–110]. Some prototype generation IV reactors operate at 850°C–1000°C, a
temperature considerably higher than the existing commercial nuclear power plants.
General Atomics predicts that hydrogen cost using HTGR would cost $1.53/kg, a
cost that compares well with $1.40/kg costing by steam reforming mechanism. One
advantage of nuclear reactor producing both electricity and hydrogen is that it can
shift production between the two. For example, plant can produce electricity during
the day and hydrogen during night by matching the variations in electricity demand.
Thus, hydrogen can act as a storage unit from which electricity can be generated
when needed. The peak demand of electricity can be handled by the energy stored
in hydrogen.
The high temperature needed to split the water can also be provided by solar
energy. In Spain, a 100-kW HYDROSOL II pilot plant is operated at the Plataforma
Water for Energy and Fuel Production
While a single-step thermolysis is conceptually simple, its realization is very
challenging since it needs a high-temperature heat source above 2200°C for achieving a reasonable degree of dissociation and an effective technique to separate hydrogen and oxygen to avoid explosive mixture. The ideas proposed to separate hydrogen
from the products include effusion separation and electrolytic separation. Membranes
made of zirconia and other ceramics can withstand such high temperatures, but they
fail to absorb severe thermal shocks that often occur when working under high-flux
solar radiation. Other techniques that have been evaluated are rapid quench by injecting a cold gas, expansion in a nozzle, or submerging a solar-irradiated target in liquid
water. The last technique is workable and simple, but a quench introduces a significant
drop in energy efficiency and produces an explosive gas mixture. The efficiency can
also be further decreased by reradiation, and the type of temperature (e.g., 2725°C for
64% dissociation at atmospheric pressure) required creates material limitations [1,11].
One of the problems for thermal dissociation of water is the materials that can
stand temperatures in the excess of at least 2200°C–2500°C. Several materials such
as tantalum boride, tantalum carbide, tungsten, and graphite are possible. However,
at these temperatures, only oxides are stable. Graphite is chemically unstable in the
presence of hydrogen and oxygen at these high temperatures. Tungsten and tungsten
carbide get oxidized at these temperatures. The effect of hydrogen on oxide catalysts
at these temperatures is not known. Ceramic materials such as boron nitride can
also be useful if its oxidation can be controlled. Recent studies have shown that a
low amount of dissociation is possible [11]. The separation of oxygen and hydrogen can be carried out in a semipermeable membrane of palladium or ZrO 2 –CeO 2 –
Y 2 O 3 , which removes oxygen preferentially. Lede et al. [126,127] used a ZrO 2 nozzle
through which steam is forced into a thermal stream and decomposed and unreacted
water is quenched suddenly to remove water and oxygen. The resulting gas contained
only a small amount (about 1.2 mol%) of hydrogen. Another possible solution is the
use of heat-resistant membrane made of Pd or ZrO 2 , both of which selectively permeate hydrogen. The gas can also be separated using a magnetic field. The source of
heat is also an issue. Solar or nuclear sources are possibilities. They are, although at
the early stages of development and at the present time, only possible on a smaller
scale [11,135–139].
In the recent years, thermal dissociation of water is achieved using nuclear
[90,91,93,94] (Funk, 2011, pers. comm.; Bamberger, 2011, pers. comm.) and solar energy
[1,92,108–110]. Some prototype generation IV reactors operate at 850°C–1000°C, a
temperature considerably higher than the existing commercial nuclear power plants.
General Atomics predicts that hydrogen cost using HTGR would cost $1.53/kg, a
cost that compares well with $1.40/kg costing by steam reforming mechanism. One
advantage of nuclear reactor producing both electricity and hydrogen is that it can
shift production between the two. For example, plant can produce electricity during
the day and hydrogen during night by matching the variations in electricity demand.
Thus, hydrogen can act as a storage unit from which electricity can be generated
when needed. The peak demand of electricity can be handled by the energy stored
in hydrogen.
The high temperature needed to split the water can also be provided by solar
energy. In Spain, a 100-kW HYDROSOL II pilot plant is operated at the Plataforma