Green Hydrogen Production: A Work in Progress

Green Hydrogen Production: A Work in Progress

Publication Date:01-April-2006
09:30 AM US Eastern Timezone
Source: Parkinson, Gerald-Chemical Engineering Progress



For decades, hydrogen has been touted as a "clean" fuel that could be
produced around the world from renewable resources. But the dream of
its promoters has remained unfulfilled for lack of competitive
technology. Today, however, the outlook for extracting hydrogen from
sustainable feedstocks as a viable alternative to petroleum and
natural-gas fuels seems much brighter.* Prodded by the prospect of
diminishing petroleum supplies, the U.S. and other countries have been
making heavy investments in "green" hydrogen production technologies,
with the overarching goal to accelerate the adoption of what is
commonly called a hydrogen economy. President Bush, who expressed his
commitment to hydrogen his State of the Union Address on Jan. 31, 2006,
has asked Congress for $289 million in fiscal year (FY) 2007 (up $53
million from FY 2006. ending Sept. 30) to support the Hydrogen Fuel
Initiative (HFI). Managed by the U.S. Dept. of Energy (Washington.
D.C.: www.doe.gov). this Initiative is the host of extensive R&D
efforts currently underway at various national laboratories and scores
of companies and universities For its part, industry is investing more
than $1 billion/yr. estimates Steven Chalk, DOE's hydrogen program
manager.

The Initiative calls for an investment of $ 1.2 billion over five
years, from 2004 to 2008. A basic goal is to develop technology by 2015
that could deliver hydrogen to the market for $2-3/gasoline gallon
equivalent (gge: untuxed, in 2005 dollars), independent of the pathway
used to produce and deliver the gas. One gge is roughly equivalent to
one kilogram of H^sub 2^.

"A primary objective of the program is to substitute hydrogen for
gasoline in vehicles using onboard proton-exchange-membrane (PEM) fuel
cells as the enabling technology," says Chalk. In the current budget
(FY 2006), $73 million has been allocated for H, production and
delivery. $44 million for fuel-cell research, $40 million for fuel-cell
vehicle demonstrations and fueling infrastructure. $32.5 million for
basic research, and $34 million for H^sub 2^ storage. Storage, of
course, is vital to the success of the program (box, p. 10).

At present, about 95% of the H^sub 2^ used in the world is made by
steam-reforming natural gas. The remaining 5% is high-purity H^sub 2^
produced by electrolysis, an energy-intensive route for splitting water
into H^sub 2^ and O2. One of DOE's basic goals is to reduce the cost of
extracting H^sub 2^ from water, either by improving the economics of
electrolysis or by employing other water- splitting techniques. "We are
looking for diversity in processes, and we are indifferent as to how
hydrogen is made, as long as the technology is domestic, carbonneutral
and economically competitive," says Chalk.

High-temperature electrolysis

Conventional electrolysis yields high-purity H^sub 2^, but it is
energy-intensive and too expensive for mass production. A promising
alternative is high-temperature or steam electrolysis, which operates
at a minimum temperature of approximately 800C. Heat - preferably from
an inexpensive source - is added to reduce the electricity requirement
for H^sub 2^ production. Solar energy and heat from nuclear power
plants are the principal sources under investigation. While systems
that use solar energy have already been tested, the nuclear option is
still years away, pending the development of high- temperature
reactors. Six types of reactors are being developed through the
Generation IV International Forum, a DOE initiative that coordinates
the R&D efforts of 10 nations and the European Commission. Up to three
would use the reactor's heat, or heat and electricity, to make H^sub
2^. explains David Henderson. program manager for nuclear hydrogen
research in DOE's Office of Nuclear Energy Science and Technology
(Germantown. MD). DOE's efforts are focused on the Very High
Temperature Reactor (VHTR). cooled by helium, under development by
Idaho National Laboratory (INL: Idaho Falls: www.inl.gov). For the
longer term, Oak Ridge National Laboratory (Oak Ridge. TN: www.oml.gov)
is working on a variation of the VHTR. the Advanced High Temperature
Reactor, which will use a liquid-salt coolant. Both would operate at
800-1,000C. versus about 300C for today's commercial reactors, says
Henderson. He notes that both Japan and China already have reactors
that operate at 700-850C.

Solar heat

Solar heat has boosted the H^sub 2^ output of an electrolyzer by more
than 45% in a process developed by Solar Systems Pty Ltd. (Melbourne.
Australia; www.solarsystems.com.au), a manufacturer of commercial solar
equipment. More significantly, the process promises to achieve 45%
efficiency for H^sub 2^ production by solar energy - more than five
times greater than what hhas been achieved with conventional solar
panels and electrolyzers, says John Lasich, technical director.

In Solar's system, a concentrator delivers high-intensity sunlight to a
spectrum splitter that divides the radiation into short and infrared
(IR) fractions (Figure 1). Short wavelengths are channeled to gallium
arsenide-based, multijunction solar cells that convert light energy to
electricity with an efficiency of 30%, says Lasich. The IR radiation
delivers heat energy to an yttria- stabilized zirconia electrolyzer,
increasing the electrolyzer's operating temperature to 850C or more.

Initial tests indicate that the addition of the heat can increase the
electrolyzer's hydrogen yield to 1.47 W per watt of electrical input.
Lasich notes that a conventional electrolyzer operated at room
temperature has an efficiency of about 70%, yielding 0.7 W of H^sub 2^
per watt of electricity.

The process has been tested at a scale of only a few watts, but Lasich
says the technology is "readily scalable." A "several- hundred-watt"
system is slated for testing later this year at the National Renewable
Energy Laboratory (NREL; Golden, CO; www.nrel.gov). If the expected
performance is confirmed, Lasich estimates that a commercial plant
using a 10-MW electrolyzer could produce 10,000 m.t./yr of H^sub 2^ at
a cost of $2.48/kg.

A key element in steam electrolysis is the availability of high-
temperature solid-oxide electrolysis cells (SOECs), notes Robert
McConnell, a senior project leader at NREL. Initially developed for
fuel cells, a solid-oxide electrolyzer is "basically a fuel cell
running backwards," he says.

Meanwhile, Ceramatec, Inc. (Salt Lake City, UT; www.ceramatec.com) has
tapped its fuel cell expertise to develop SOECs of yttria-stabilized
zirconia and other materials. The company makes flat sheets that are
stacked in series (Figure 2). Joseph Hartvigsen, a senior engineer with
Ceramatec, explains that in electrolysis, the H^sub 2^ production rate
is stoichiometric with the current, so the efficiency of the cell
depends only on its operating potential, or voltage. At higher
temperatures, part of the total energy is supplied as thermal energy,
thereby permitting lower voltages.

So far, Ceramatec has built cells of 10 10-cm and is developing 20
20-cm cells for high-temperature electrolysis. A 25-cell stack was
commissioned in January at INL with an initial H, output of 200 normal
L/h. After one month of continuous operation, the H, production rate
dropped to 165 L/h, but "the cell has otherwise performed well, with no
leaks," according to Stephen Herring, technical director for
hightemperature electrolysis at INL. Investigation into this slow
degradation is ongoing. Next year, INL plans to start testing a 15-kW
unit, Herring says.

A novel steam-electrolysis system is being developed by SRI
International (Menlo Park, CA; www.sri.com). Carbon monoxide is fed to
the anode side of a solid-oxide cell and depolarizes the anode, thereby
decreasing the high-temperature electrolysis voltage from 1 V to
0.2-0.3 V. The use of CO as a depolarizer also generates heat at the
anode, says Iouri Balachov, a senior research engineer at SRI, and this
would be enough to sustain the reaction temperature of 800-850C in a
5-10-kW electrolyzer.

Although the process has been tested only in the laboratory, SRI
expects that the combination of anodic depolarization with steam
electrolysis will reduce electricity consumption to about 20-30% of the
value associated with conventional electrolysis, while generating pure
H^sub 2^ at a cost of $2-3/gge, vs. more than $4/ gge for conventional
electrolysis. SRI believes the process would fit well within or near a
chemical process plant or integrated coal- gasification, combined-cycle
plant, where CO is readily available and the process could serve as an
alternative to a water-gas shift reaction.

Thermochemical cycles

Solar and nuclear energy are also the major sources of heat for the
thermochemical routes to H^sub 2^. A water-splitting thermochemical
cycle that uses concentrated solar energy to reduce zinc oxide (ZnO)
particles to metallic Zn, then reacts Zn with H2O to produce H^sub 2^
is being developed by the ETH-Swiss Federal Institute of Energy
Technology Zurich (ETH; www.p\re.ethz.ch) and the Paul Scherrer
Institute (Zurich; www.psi.ch). ZnO dissociates at above 2,000C, but
the researchers have lowered the reaction temperature to 1,000-1,300C
by adding a reducing agent, such as biomass-based charcoal.

A 300-kW prototype reactor for the carbothermic reduction was tested at
the solar tower facility of the Weizmann Institute of Science (Rehovot,
Israel; www.weizmann.ac.il) and produced 50 kg/h of Zn, with a
solar-to-fuel energy conversion efficiency of 30%, according to ETH
professor Aldo Steinfeld. "A commercial-scale plant of 10 MW should
achieve 50% energy-conversion efficiency," he adds.

As for the second step, ETH has developed a process in which Zn
nanoparticles are formed and immediately hydrolyzed in situ for H^sub
2^ production. "The high specific surface area of the nanoparticles
enhances the reaction kinetics, and heat and mass transfer," says
Steinfeld. "This results in a residence time of less than 1 second."
The process has been demonstrated at the laboratory scale in a
continuous, tubular aerosol flow reactor.

The nuclear option

In DOE's nuclear hydrogen program, thermochemical research and
development is focused on sulfur-iodine and hybrid sulfur cycles. "Of
about 100 thermochemical cycles that had been proposed in the past,
these are the most mature technologies," says Henderson. Both processes
are expected to meet DOE's goal of making H^sub 2^ for under $2.00/kg
at a nuclear plant site. In the S-I cycle, H^sub 2^ is formed by the
decomposition of hydriodic acid (HI) at 400-500C. Iodine, the
co-product, is reacted with SO^sub 2^ and H2O at around 120C to produce
HI plus H^sub 2^SO^sub 4^ (this is known as the Bunsen reaction).
SO^sub 2^ is regenerated by decomposing the acid at temperatures up to
900C.

The development of the S-I cycle, a cooperative effort between DOE and
France's Atomic Energy Commission (CEA; Paris; www.cea.fr), is headed
by Paul Pickard of Sundia National Laboratories (Albuquerque. NM;
www.sandia.gov). CEA is working on the Bunsen reaction. Sandia on H^sub
2^SO^sub 4^ decomposition, and General Atomics (San Diego. CA;
www.ga.com) on HI decomposition. An integrated laboratory-scale unit
demonstrating the closedloop operation of the entire cycle is slated to
start up at the end of 2007.

Finding materials (e.g., to make heat exchangers and catalysts) that
can withstand the hot, corrosive environments of the S-I cycle is "a
significant challenge." says Pickard. Another problem is that the
Bunsen reaction requires excess I, to promote a phase separation of the
HI from the H^sub 2^SO^sub 4^. Exacerbating the issue is the presence
of excess water, which increases the amount of energy needed to pump
and heat up the process liquid. Pickard notes. INL is developing a
membrane system to remove water - a step that would also make I^sub 2^
less soluble in the acid.

The hybrid sulfur (HyS) thermochemical process, which is being
developed at the Savannah River National Laboratory (Aiken, SC;
www.srnl.doe.gov), combines H^sub 2^SO^sub 4^ decomposition with
electrolysis. SO^sub 2^ from the decomposition step is mixed with
dilute H^sub 2^SO^sub 4^ and fed to the anode of an electrolyzer. At
25C, the presence of SO^sub 2^ decreases the reversible cell potential
from 1.23 V, for conventional water electrolysis, to 0.17 V, says
William Summers, program manager for energy security at Savannah River.
H^sub 2^ evolves from the cathode, while concentrated H^sub 2^SO^sub 4^
is generated at the anode and is recycled for decomposition.

So far, the HyS process has been tested at the single-cell scale at
ambient temperature, using a polymer electrolyte membrane (PEM) similar
to that used in fuel cells for vehicles. Next year, the laboratory
plans to build a cell stack to produce about 120 L/h H^sub 2^ at 80C
and 90 psig.

Increasing the pressure and temperature improves the kinetics and
lowers the voltage requirements of the process, says Summers. A
commercial plant is expected to operate at 300 psig, with a cell
voltage of 0.6 V - about one third that of a conventional electrolyzer.


Hydrogen from biomass

A one-step process to produce H^sub 2^ from biomass-derived oxygenated
compounds, such as glycerol, glucose and sugar-alcohols, has been
developed by Virent Energy Systems (Madison, WI), a spinoff from the
Univ. of Wisconsin (www.uwisc.edu). Virent's aqueous-phase reformation
(APR) uses a precious-metal-based catalyst at 210-250C and 300-500 psia
to convert the sugars to a mixture of H^sub 2^, methane, ethane,
propane, CO2 and steam. The H2O is removed by condensation, then H^sub
2^ of 99.999% purity is recovered through a palladium membrane filter
or by pressure-swing absorption (Figure 3).

Eric Apfelbach, Virent's CEO points out that APR is simpler than the
conventional alternatives, such as producing, then reforming ethanol to
obtain H^sub 2^, or gasifying biomass and recovering H^sub 2^ from the
resultant syngas. A test system is producing 450 g/ h of H^sub 2^
(about 6 m^sup 3^/h) - enough to power a 5-7-kW fuel cell.

Next year, Virent is scheduled to start up a 50-kg/d unit for DOE.
Feedstock will be supplied by Archer Daniels Midland (ADM; Decatur, IL;
www.adm.com). a major corn processor and Virent's partner in the DOE
project. Apfelbach says the process could produce H^sub 2^ for $2-3/gge
from sugar streams generated in corn wet- milling.

In a related project. United Technologies Research Center (East
Hartford. CT: www.utrc.utc.com) is working on a process to hydrolyze
ground-up poplar wood in sulfuric acid, then reform the hydrolyzed
biomass at 200-300C to obtain H^sub 2^. The process has not yet been
tested, says staff engineer Scan Emerson, but researchers have
identified some candidate precious-metal catalysts and have modeled the
process. "We expect to meet the 2010 efficiency target of 50% (energy
input vs. H^sub 2^ output) at a cost of $ 1.75/kg H^sub 2^," Emerson
says.

Using sunlight and water

Photoelectrochemical systems eliminate most of the cost of an
electrolyzer by using sunlight to split H2O via semiconductors that are
immersed in the water. Also, they achieve H^sub 2^ production at about
1.35 V, for an electrolysis efficiency of 91%, notes John Turner, a
principal scientist with NREL.

However, developing materials that have long-term corrosion resistance
is challenging Intematix Coip. (Fremont, CA; www.intematix.com) says it
has developed tungsten-based metal-oxide photo-electrode cells that are
not corroded by H2O.

A photochemical system that is said to yield more than 80% H^sub 2^,
vs. 12% for TiO^sub 2^, has been developed by Coastal Hydrogen Energy,
Inc. (Stillwater, OK). Water vapor at 115-125C flows through a cylinder
that is coated with a proprietary catalyst. Simultaneously, the vapor
is irradiated by an ultraviolet lamp and radiofrequency energy is
passed through the vapor, splitting it into H^sub 2^ and O2. According
to Gary Austin, chief science officer, the cost of H^sub 2^ production
is below $1/kg of H^sub 2^.

* For further coverage of the hydrogen economy in CEP, see Dec. 2005,
pp. 20-22 and Nov. 2004, pp. 4-6.

GERALD PARKINSON is a contributing editor with over 25 years of
experience writing about the chemical process industries.

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