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POSSIBILITY OF ACTIVE CARBON RECYCLE ENERGY SYSTEM
Figure 5: Enthalpy balance of ACRES with methane
CO 2 +4H 2 , +2O 2
Metanation
-165
Thermally reversible
CH 4 +2H 2 O, +2O 2
Electricity
+967
Water
decompo
sition
CO 2 +4H 2 O(g)
Work/heat
-802
Oxidation
[HHV-kJ/mol-CO 2 ]
Formation enthalpy of regenerated hydrocrabon
η =
(8)
Formation enthalpy of hydrogen of required amount for hydrocarbon regeneration
The η of methane ACRES was 83%. This means that enthalpy loss generates in methanation
process. However, when the same amount of enthalpy is stored in methane or H 2 , methane storage
pressure is reduced to one-third for H 2 storage. Compression work can be reduced also to one-third for
methane storage in comparison with H 2 storage. For comparison of energy efficiency between ACRES
and H 2 system, a comprehensive discussion is needed. A chemical reaction equilibrium for CO 2 /methane
system is shown in Figure 6. It was assumed in the evaluation that the reaction proceeded at an
equivalent ratio under a pressure of 100 kPa. A CO 2 /methane system is reversible around 500°C.
Exothermic heat of methanation is recoverable by other endothermic reactions like steam reforming.
Enthalpy loss of methanation can be minimised by coupling with other endothermic reaction processes.
Figure 6: Chemical reaction equilibrium for CO 2 /methane system
0.7
CH 4 rich
H 2 rich
Partial pressure in equilibrium
0.6
H 2 O H 2
0.5
4H 2 +CO 2 CH 4 +2H 2 O
0.4
P ini,CO2 =20 kPa
CH 4 P ini,H2 =80 kPa
0.3
Total P=100 kPa
0.2
0.1
CO
CO 2
0
0 500 1000 1500
Reaction temperature [ o C]
The η of some hydrocarbons were calculated:
CO (104%) > methanol (93%) > ethanol (88%) > methane (83%) (9)
A higher η means that enthalpy loss for regeneration of the hydrocarbon becomes smaller.
Methanol and ethanol are generally in liquid phase, easy in terms of transportation and storage, and
applicable to vehicle. Carbon monoxide (CO) has the highest η in the hydrocarbons.
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