Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 279

Table 9.3.2 Heat requirements of some typical CO 2 capture cycles.

Process enthalpy

H (a) ,

Cycle Function Processes T , ◦ C kJ/mol

Na 2 CO 3 -based Absorption Na 2 CO 3 (s) + H 2 O (g) + CO 2 (g) = 2NaHCO 3 (s) 20-60 − 135.5

Desorption 2NaHCO 3 (s) = Na 2 CO 3 (s) + H 2 O (g) + CO 2 (g) 120-180 135.5

K 2 CO 3 -based Absorption K 2 CO 3 (s) + H 2 O (g) + CO 2 (g) = 2KHCO 3 (s) 20-60 − 140.9

Desorption 2KHCO 3 (s) = K 2 CO 3 (s) + H 2 O (g) + CO 2 (g) 120-180 140.9

CaO-based Regeneration CaO (s) + H 2 O (l) = Ca (OH) 2 (s) 100 − 65.3

Absorption Ca (OH) 2 (aq) + CO 2 (g) = CaCO 3 (s) + H 2 O (g) 100 − 69.8

Desorption CaCO 3 (s) = CaO (s) + CO 2 (g) 900 179.2

CaO-NaOH- Absorption 2NaOH (s) + CO 2 (g) = Na 2 CO 3 (s) + H 2 O (g) 20-60 − 127.2

based Precipitation Na 2 CO 3 (s) + Ca (OH) 2 (aq) = CaCO 3 (s) + 20-60 − 5.3

2NaOH (aq)
Desorption CaCO 3 (s) = CaO (s) + CO 2 (g) 900 179.2

Alkalization CaO (s) + H 2 O (l) = Ca (OH) 2 (s) 100 − 65.3

MEA-based (b) Absorption RNH 2 + H 2 O (l) + CO 2 (g) = RNH 3 + HCO 3 38 − 72.0

RNH 3

+ HCO 3

Desorption

= RNH 2 + H 2 O (g) + CO 2 (g)

120 165.0

(a) H is the process enthalpy change. A positive value for H means endothermic (requiring heat), otherwise
exothermic (releasing heat).
(b) MEA, also ETA, is monoethanolamine, which is often denoted by RNH 2 , where R is “OH (CH 2 ) 2 '' [Ali 2004].

capture from flue gases rather than air is assumed to be 50%, which is an average value
of the 40%-60% range reported by investigators for various CO 2 capture methods
(Tzimas, 2009; Von Zedtwitz-Nikulshyna, 2009; David et al., 2000). Therefore, the
thermal energy for CO 2 capture from flue gases is approximately 270-360 kJ/mol CO 2 .
Other energy requirements in the CO 2 capture process include at least three por-
tions: (i) work to transport the flue gas to the CO 2 capture process for the separation
of CO 2 and other gases; (ii) work to compress the concentrated CO 2 to the reservoir
pressure, and (iii) work to move the compressed CO 2 into a distant storage location
including a storage tank or geologic formation. It can be shown that the lower bound
of the total work with ideal Second-Law efficiencies for these three portions is about
9, 13 and 2 kJ/mol CO 2 , respectively, assuming the flue gas comprises 78% N 2 from
the atmosphere, 15% CO 2 from the oxidation of the carbon in the hydrocarbon,
7% steam, reservoir pressure of 70 bars, and the ground water depth is only 2 km.
Assuming further the isothermal compression efficiency is 65%, then the total elec-
tricity requirement to complete the above three steps is approximately 37 kJ/mol CO 2
(House et al., 2009). Taking the value of 45% as the conversion efficiency for the solar
thermal energy conversion to electricity, then the primary solar thermal energy is about
82 kJ/mol CO 2 . Consequently, the total thermal energy requirement for CO 2 capture
and storage lies in the range of 352-442 kJ/mol CO 2 .
Table 9.3.3 summarizes the energy requirements of the H 2 production, CO 2 cap-
ture and compression for methanol synthesis for the production of 1 mole of methanol
with Reaction 9.3.4. Figure 9.3.1 shows the percentage of CO 2 capture in the synthesis
of methanol production based on the data of Table 9.3.3. It can be concluded that a
key to CO 2 recycling is an economic hydrogen source, because the energy required