Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 277

9.3 SOLAR-BASED CO 2 RECYCLINGWITH HYDROGEN

As presented previously, solar-based hydrogen production is not only a substitute for
fossil fuels in the future, it is also a necessity for CO 2 recycling and hydrogenation in
synfuel production. With the addition of hydrogen to CO 2 , methanol and its derivatives
can be produced. This will significantly increase the sustainability of our limited fossil
fuel resources. Currently, there are several methods of adding hydrogen to CO 2 .A
technology is methane-assisted processes utilizing the hydrogen in methane to convert
CO 2 to carbon-based fuels, for example (Von Zedtwitz-Nikulshyn, 2009),

CH 4 (g) + CO 2 (g) = 2H 2 (g) + 2CO (g) (9.3.1)

The CO 2 present in Equation (9.3.1) can also be captured by a CaO-based cycle and
then used in the following reaction to synthesize fuels:

CaO (s) + CO 2 (g) = CaCO 3 (g) (9.3.2)

CaCO 3 (s) + CH 4 (g) = CaO (g) + 2CO (g) + 2H 2 (g) (9.3.3)

However, this technology is not strictly renewable because methane is used in the
processes. So it won't be further discussed in detail in this section.
A renewable option is to use H 2 and CO 2 to synthesize methanol catalytically
(Fornero et al., 2011; Olah et al., 2009):

H 2 O (g), 260 ◦ C, H ◦ =− 3H 2 (g) + CO 2 (g) = CH 3 OH (g) + 49 . 7kJ / mol

(9.3.4)

The enthalpy change of the preheating process of the reactants is:

CO 2 (g)] of 20 ◦ C CO 2 (g)] of 260 ◦ C, H ◦ = [3H 2 (g) + = [3H 2 (g) + 30 . 6kJ / mol

(9.3.5)

The enthalpy change of Reaction 9.3.4 is a negative value, indicating an exothermic
reaction. As the preheating of 3 moles of H 2 and 1 mole of CO 2 from 20 ◦ Cto260 ◦ C
requires 30.6 kJ, which is smaller than the heat released from Reaction 9.3.4, the
methanol production process can be assumed as a self-sustained process if the heat
losses and the heat recovered from the cooling of the products can offset the electricity
requirement. Then it can be approximated that the energy consumption for the CO 2
recycling is mainly established by the H 2 production and CO 2 capture, which will be
examined in the following sections.
Table 9.3.1 lists the energy requirements of hydrogen production with the solar-
based conventional water electrolysis and hybrid Cu-Cl thermochemical cycle. The
energy requirement of the hybrid Cu-Cl cycle for hydrogen production is 222 MJ/kg
H 2 of solar thermal energy and 32 MJ/kg of solar electricity (Wang et al., 2010).
The energy requirements of CO 2 recycling for synfuel production include the cap-
ture and purification of CO 2 from industrial emissions and ambient air. A challenge in
the industrial design of CO 2 recycling is the major energy requirements of CO 2 cap-
ture, which corresponds to a high energy cost. The energy requirements are influenced