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POSSIBILITY OF ACTIVE CARBON RECYCLE ENERGY SYSTEM
Enthalpy evaluation of ACRES
Practical hydrocarbons are examined those availability in ACRES by an enthalpy balance evaluation.
Availability of ACRES with methane
Methane as a basic hydrocarbon was discussed firstly. The structure of an ACRES in which methane
was recycled is shown in Figure 4. With a methane process, methane combustion [Eq. (2)] produces
heat, methane steam reforming [Eq. (3)] produces hydrogen, and polymerisation of methane [Eq. (4)]
produces polymeric materials:
CH 4 + 2O 2 → CO 2 + 4H 2 O (2)
CH 4 + H 2 O → 4H 2 + CO 2 (3)
xCH 4 → (-CH 2 -) x + x/2H 2 (4)
CO 2 is recoverable by physical adsorptions of active carbons or zeolites, or a chemical sorption by
carbonation of calcium oxide (CaO) at the CO 2 recovery and separation process. CaO is absorbable
chemically CO 2 at temperatures of 500-800°C [Eq. (5)]. CaO can remove CO 2 from hydrocarbon reaction
system at the reaction temperature for CO 2 production with small sensible heat loss, and also enhance
reaction rate and yield of the CO 2 production reaction (Kato, 2003).
CaO + CO 2 → CaCO 3 (5)
In a process of methane regeneration form CO 2 , a two-step reaction of hydrogen production by
water electrolysis and methanation of CO 2 with the hydrogen [Eqs. (6) and (7)] is available:
4H 2
O → 4H 2
+2O 2
(6)
CO 2
+4H 2
→ CH 4
+2H 2
O (7)
Figure 4: Structure of ACRES with methane
Recovery and
separation
Physical and chemical sorptions
Recov./Sep. energy
Effluent CO 2
Usage
4H 2 O
Materials
Energy
CH 4
CO 2
Regen. energy
4H 2 O
Regeneration
2H 2 O+2O 2
The enthalpy balance of ACRES for methane is shown in Figure 5. Required enthalpies per one
molecule of methane for the processes of usage and regeneration are depicted in high heat value
(HHV). The recovery and separation process needs relatively smaller enthalpy than that of other
processes, and is capable to be driven by waste heat at relatively lower temperature (less than 100°C).
Because enthalpy evaluation for the recovery and separation process had uncertainties, the process
was not accounted for in this system evaluation. In the regeneration process it was assumed that
hydrogen was used for hydrocarbon regeneration by the two-step reaction in Eqs. (6) and (7). Production
of H 2
of 4 mol requires an enthalpy of 967 kJ/mol-CH 4 . Methanation of CO 2 with H 2 leads to an
exothermic reaction of 165 kJ/mol-CH 4 . Regenerated methane has a reaction enthalpy of 802 kJ/mol-CH 4 .
A circulation rate (η) which is a formation enthalpy ratio between regenerated hydrocarbon and
required hydrogen is defined in Eq. (8).
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