Discussion of total equivalent work

“As mentioned previously, most studies have focused on reducing the regeneration energy as they stated that its reduction would greatly reduce the CO2 capture plant operating cost [18][76][82][84][100][172]. The typical correlation for determining regeneration energy is shown in Eq. (30).(30)Qreg=HinputrCO2_prod=msteamCpsteamΔTrCO2_prodwhere; Hinput is the heat input from the reboiler (GJ/hr), msteam is the mass flow rate of the steam (kg/hr), Cpsteam is the specific heat capacity of the steam (kJ/kg.°C), while ΔT is the temperature difference between the steam inlet and condensate outlet (°C). The heating medium can either be steam or hot oil as the case may be.

However, other studies suggested that incorporating the energy spent on pumping and compression shown as total equivalent work in Eq. (31) would provide a better idea on the overall energy savings or penalties of the CO2 capture plant [233][234][235][236]. In Eq. (31), it is assumed that the temperature of the steam in the boiler is 10 K higher than the reboiler temperature.


where; Weq is the total equivalent work (GJ/tonne CO2), Treb is the steam temperature in the reboiler (K), Wcomp is the compressor work of the produced CO2 (GJ/tonne CO2), Wpump is the pump work (GJ/tonne CO2), and n is the turbine efficiency to produce electricity from steam (0.75).

In addition to the total equivalent work which includes the regeneration energy, pump work and compression work, it is important to add fourth and fifth energy terms namely the energy required for condenser energy (Qcond) and reclaiming amine (Qrec). Therefore, Eq. (31) is modified to Eq. (32);


where; Qrec is the amine reclaiming energy (GJ/tonne CO2) with respect to the amount of produced CO2 while Qcond is the energy required to condense the vapor phase leaving the top of the regenerator (GJ/tonne CO2).

The energy required for the condenser and reclaimer is detailed in Eq. (33).


where; Qcond is the energy required to condense all condensable species from the regenerator top outlet (GJ/tonne CO2), mcool is the mass flow rate of the cooling medium that will aid in achieving produced CO2 purity of 99% (kg/hr), Cpcool is the specific heat capacity of the cooling medium (kJ/kg.°C), while ΔT is the temperature difference between the cooling medium inlet and outlet (°C).

The condenser energy can also be said to equal to the heat of vaporization, because the energy used for vaporization in the reboiler is also needed to condense the vaporized water and amine in the condenser (refluxed back to the regenerator). This implies that an amine solution that has high heat of vaporization will require the same heat for condensation in the condenser which is a higher ‘double energy penalty’. This also highlights the merits of ‘low temperature regeneration’ process because of the hugely reduced vaporization heat. Therefore breaking down the contribution of absorption heat, sensible heat and heat of vaporization towards regeneration energy is very important.”

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