Nuclear Production of Hydrogen, Fourth Information Exchange ...

TRANSIENT MODELLING OF S-I CYCLE THERMOCHEMICAL HYDROGEN GENERATION COUPLED TO PEBBLE BED MODULAR REACTOR
Introduction
Hydrogen is a candidate for an all purpose energy carrier. Since the American oil crises of the 1970s,
hydrogen has been regarded as an attractive supplement or replacement for fossil fuels as an energy
carrier. The recent rapid development in the area of fuel cells and fuel cell powered vehicles yields
additional impetus to the development of hydrogen as an energy carrier. Hydrogen has an energy
density from combustion of around 120 MJ/kg, in comparison natural gas has an energy density of
43 MJ/kg, gasoline has an energy density of 44.4 MJ/kg, and ethanol has an energy density of 26.8 MJ/kg
(Thomas, 2001). The waste product of hydrogen combustion is water, which is a great environmental
advantage of hydrogen compared to the fossil fuels. Hydrogen is the most abundant element in the
universe. Hydrogen is also very chemically reactive, and thus it is not found on Earth in its elemental
form. The major challenges in shifting to a hydrogen economy involve generation, storage and
transportation of hydrogen.
The most common current method of hydrogen generation is hydrocarbon extraction, typically
from natural gas (Rostrup-Nielsen, 2005). The fundamental problem with this method is that it retains
an inherent reliance on fossil fuels and emits greenhouse gases. Thus, neither the problem of energy
reserves nor the problem of carbon emission is solved. A carbon-free method of hydrogen generation
that is independent of fossil fuel supplies is required for a sustainable hydrogen economy.
Water covers more than two-thirds the surface of the Earth, however directly splitting water into
hydrogen and oxygen requires temperatures of above 2 200 K (Kogan, 2000). There are few terrestrial
heat sources capable of providing such temperatures on a large scale. In addition to direct splitting of
hydrogen, there is also the option of a thermochemical cycle. A thermochemical cycle consists of a
series of chemical reactions performed in parallel, of which the overall reaction is the splitting of
water. Thermochemical cycles still require high temperatures, on the order of 1 100 K (Brown, 2003).
There are several heat sources under consideration for driving thermochemical cycles, namely nuclear
energy. Two of the primary candidates for use as thermochemical water-splitting cycles are the
sulphur-iodine cycle and the Westinghouse hybrid sulphur cycle. A potential driving scheme for a
thermochemical cycle is shown in Figure 1.
Figure 1: Nuclear hydrogen generation scheme
The sulphur-iodine (S-I) cycle consists of three chemical reaction steps expressed as sections:
• Section 1 (Bunsen reaction, 393 K): I 2 + SO 2 + 2H 2 O → 2HI + H 2 SO 4
• Section 2 (sulphuric acid decomposition, 1 123 K): H 2 SO 4 → H 2 O + SO 2 + 1/2O 2
• Section 3 (hydrogen iodide decomposition, 773 K): 2HI → H 2 + I 2
364 NUCLEAR PRODUCTION OF HYDROGEN – © OECD/NEA 2010