Thermochemical Water-Splitting Cycles

Abstract

This chapter presents and analyzes thermochemical cycles, which are promising methods of nuclear produced hydrogen at a large scale. The introduction presents the origins of concepts and a historical perspective on the technology development. In the first part, the most important aspects and fundamental concepts for cycle modeling and synthesis are introduced and detailed. The discussion proceeds from single-step thermochemical water-splitting processes, to two-step and multi-step processes, followed by a presentation of hybrid cycles. Relevant analysis methods are introduced in the context of each type of cycle presentation. These concepts include chemical equilibrium, chemical kinetics, reaction rate and yield, and others. Analysis of the practicality of chemical reactions is established based on their yield. A large number of reactions and thermochemical cycles are compiled, categorized, and discussed. In total, the chapter presents 122 thermochemical cycles, 25 hybrid cycles, and six special cycles (assisted with photonic or nuclear radiation).

The most important reactions, encountered in pure and hybrid cycles, are analyzed in detail. For example, both the Deacon reaction and H₂SO₄ decomposition methods are the most encountered oxygen-evolving reactions. Hydrogen iodide decomposition has a major role as a hydrogen-evolving reaction. The Bunsen reaction is also significant. In thermochemical cycle synthesis and assessment, it is important to account for the energy associated with chemical separation, chemical recycling, and material transport; this is explained and exemplified in the chapter. Another discussion involves cycle synthesis and a down selection process, a methodology that systematically leads to identification of the most promising cycles. A comparative assessment of cycles is presented and the use of exergy as a potential analysis tool is introduced. The final part of the chapter refers to three thermochemical plants which are considered as the most promising. These are plants based on the sulfur–iodine cycle, the hybrid sulfur cycle, and the hybrid copper–chlorine cycle. Some bench-scale or pilot plants exist for the sulfur–iodine and hybrid sulfur plants and they are in the course of development for the copper–chlorine cycle.

Nomenclature

$\it{A}$ :

Pre-exponential factor

$c$ :

Molar concentration, kmol/m³

$\mathrm{ ex}$ :

Specific molar exergy, kJ/mol

Ex:

Exergy, kJ

$G$ :

Molar Gibbs free energy, kJ/mol

H :

Molar enthalpy, kJ/mol

HHV:

Molar based higher heating value, kJ/mol

IP:

Improvement potential, kJ

$\it k$ :

Rate constant, ${{\mathrm{ s}}^{-1 }}$

${K_{\mathrm{ eq}}}$ :

Equilibrium constant

m :

Mass, kg

n :

Number of moles

$\dot{n}$ :

Molar flow rate, mol/s

$P$ :

Pressure, Pa

$Q$ :

Heat flux, kJ

$\dot{Q}$ :

Heat flux, kW

r :

Recycling ratio

$R$ :

Universal gas constant, J/mol K

$S$ :

Molar entropy, kJ/mol K

SI:

Sustainability index

$T$ :

Temperature, K

$\it{v}$ :

Molar volume, m³/kmol

W :

Work, kJ

$y$ :

Molar fraction

5.1.1 Greek Letters

$\eta$ :

Energy efficiency

$\mu$ :

Chemical potential, kJ/mol

$\psi$ :

Exergy efficiency

$\xi$ :

Extent of reaction

5.1.2 Subscripts

0:

Reference state

act:

Activation

aux:

Auxiliary

b:

Backward

d:

Destruction

el:

Electric

elchem:

Electrochemical

eq:

Equivalent

f:

Forward, formation

gen:

Generation

in:

Inlet, input

loss:

Losses

out:

Output

P:

Products

R:

Reactants

sep:

Separation

tresh:

Threshold

5.1.3 Superscripts

0:

Reference state

circ:

Circulation

ch:

Chemical

Q:

Heat