Re: Hydrogen from nuclear heat
- From: Rolf Martens <rolf.martens@xxxxxxxxx>
- Date: Tue, 27 Feb 2007 07:07:08 GMT
Interesting info, Dave. Thank you for this.
Rolf M.
www.rolf-martens.com
In article <1172398467.811888.143100@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
dave.walters@xxxxxxxxxxx says...
Hydrogen from nuclear heat
[full: http://www.uic.com.au/nip73.htm]
Several direct thermochemical processes are being developed for
producing hydrogen from water. For economic production, high
temperatures are required to ensure rapid throughput and high
conversion efficiencies.
In each of the leading thermochemical processes the high-temperature
(800-1000°C), low-pressure endothermic (heat absorbing) decomposition
of sulfuric acid produces oxygen and sulfur dioxide:
H2SO4 ==> H2O + SO2 + 1/2O2
There are then several possibilities. In the iodine-sulfur (IS)
process iodine combines with the SO2 and water to produce hydrogen
iodide which then dissociates to hydrogen and iodine. This is the
Bunsen reaction and is exothermic, occurring at low temperature
(120°C):
I2 + SO2 + 2H2O ==> 2HI + H2SO4
The HI then dissociates to hydrogen and iodine at about 350°C,
endothermically:
2HI ==> H2 + I2
This can deliver hydrogen at high pressure.
The net reaction is then:
H2O ==> H2 + 1/2O2
All the reagents other than water are recycled, there are no
effluents.
The Japan Atomic Energy Authority (JAEA) has demonstrated laboratory-
scale and bench-scale hydrogen production with the IS process, up to
30 litres/hr.
The Sandia National Laboratory in the USA and the French CEA are also
developing the IS process with a view to using high-temperature
reactors for it.
General Atomics' preliminary laboratory work on thermochemical
production should be complete by 2006. A 10MW pilot hydrogen plant
using fossil heat would then be built, followed by nuclear
thermochemical production by 2015.
The economics of hydrogen production depend on the efficiency of the
method used. The IS cycle coupled to a modular high temperature
reactor is expected to produce hydrogen at $1.50 to $2.00 per kg. The
oxygen by-product also has value.
For thermochemical processes an overall efficiency of greater than 50%
is projected. Combined cycle plants producing both H2 and electricity
may reach efficiencies of 60%.
Production reactor requirements
High temperature - 750-1000°C, is required, though at 1000°C the
conversion efficiency is three times that at 750°C. The chemical plant
needs to be isolated from the nearby reactor, for safety reasons,
possibly using an intermediate helium or molten fluoride loop.
Three potentially-suitable reactor concepts have been identified,
though only the first is sufficiently well developed to move forward
with:
High-temperature gas-cooled reactor (HTGR), either the pebble bed or
hexagonal fuel block type. Modules of up to 285 MWe will operate at
950°C but can be hotter.
Advanced high-temperature reactor (AHTR), a modular reactor using a
coated-particle graphite-matrix fuel and with molten fluoride salt as
primary coolant. This is similar to the HTGR but operates at low
pressure (Lead-cooled fast reactor, though these operate at lower
temperatures than the HTGRs - the best developed is the Russian BREST
reactor which runs at only 540°C. A US project is the STAR-H2 which
will deliver 780°C for hydrogen production and lower temperatures for
desalination.
These are described more fully in the Small Nuclear Power Reactors
paper (with coolant characteristics) and the Advanced Reactors paper.
Each 600 MWt module would produce about 200 tonnes of hydrogen per
day, which is well matched to the scale of current industrial demand
for hydrogen.
The Korean Atomic Energy Research Institute (KAERI) has submitted a
Very High Temperature Reactor (VHTR) design to the Generation IV
International Forum with a view to hydrogen production from it. This
is envisaged as 300 MWt modules each producing 30,000 tonnes of
hydrogen per year. KAERI expects the design concept to be ready in
2008, engineering design in 2014, construction start 2016 and
operation in 2020.
KAERI also has a research partnership with China's Tsinghua University
focused on hydrogen production, based on China's HTR-10 reactor. A
South Korea-US Nuclear Hydrogen Joint Development Center involving
General Atomics was set up in 2005.
Molten fluoride salts are a preferred interface fluid between the
nuclear heat source and the chemical plant. The aluminium smelting
industry provides substantial experience in managing them safely. The
hot molten salt can also be used with secondary helium coolant
generating power via the Brayton cycle, with thermal efficiencies of
48% at 750°C to 59% at 1000°C.
Moving forward
A 2004 evaluation by JAEA has indicated that by 2010 it expects to
confirm the safety of high-temperature reactors and establish
operational technology for an IS plant to make hydrogen
thermochemically. In April 2004 a coolant outlet temperature of 950°C
was achieved in its High-Temperature Engineering Test Reactor (HTTR) -
a world first, and opening the way for direct thermochemical hydrogen
production.
Meanwhile a pilot plant test project producing hydrogen at 30 m3/hr
from helium heated with 400 kW is under way to test the engineering
feasibility of the IS process. After 2010 an IS plant producing 1000
m3/hr (90 kg/hr, 2t/day) of hydrogen should be linked to the HTTR to
confirm the performance of an integrated production system, envisaged
for 2020s.
JAEA plans a 600 MW GTHTR300C unit for hydrogen cogeneration using
direct cycle gas turbine for electricity and IS process for hydrogen,
deploying the first units after 2020. This could produce hydrogen at
60,000 m3/hr (130 t/day) - "enough for about a million fuel cell
vehicles" (@ 1 t/day for 7700 cars).
The economics of thermochemical hydrogen production look good. General
Atomics projects US$ 1.53/kg based on a 2400 MWt HTGR operating at
850°C.with 42% ovrall efficiency, and $1.42/kg at 950°C and 52%
efficiency (both 10.5% discount rate). At 2003 prices, steam reforming
of natural gas yields hydrogen at US$ 1.40/kg, and sequestration of
the CO2 would push this to $1.60/kg. Such a plant could produce 800
tonnes of hydrogen per day, "enough for 1.5 million fuel cell
cars" (@1 t/day for 1800 cars).
In the meantime, hydrogen can be produced by electrolysis of water,
using electricity from any source. Non-fossil sources, including
intermittent ones such as wind and solar, are important possibilities
(thereby solving a problem of not being able to store the electricity
from those sources). However, the greater efficiency of electrolysis
at high temperatures favours a nuclear source for both heat and
electricity.
.
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