https://doi.org/10.1016/j.petlm.2016.11.002
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Regeneration energy is mainly a combination of three parameters as seen in Eq. (21) [156].(21)Qreg=[Qdes]+[Qsen]+[Qvap]where; Qreg (GJ/tonne CO2) is the energy of regeneration, Qdes (GJ/tonne CO2) is the heat of desorption required to break the CO2 carrying species (i.e. carbamates, bicarbonates, carbonates) formed during the amine–CO2 reactions (believed to have the same value as heat of absorption, ΔHabs), Qsen (GJ/tonne CO2) is the sensible heat that must be provided to raise the temperature of the CO2 rich amine solution to the regeneration temperature while Qvap (GJ/tonne CO2) is the latent heat of vaporization of the volatile components in the amine solution (usually the vaporization heat of water).
A simple correlation for calculating the sensible heat and heat of vaporization of amine solutions is shown in Eqs. (22), (23), (24) [169], [170], [171].(22)Qsen=ρCΔT[(αCO2rich−αCO2lean)Camine]MCO2(23)Qsen=[C(TR−TF)ΔαCO2×MsolMCO2×1Xsol](24)Qvap=ΔHvap_H2OPH2OPCO21MCO2where; C is the specific heat capacity of the CO2 loaded amine solution (kJ/kg. oC), ΔT is the temperature difference between the absorption and regeneration (°C), MCO2 is the molecular weight of CO2 (44 g/mol), TR is the regeneration temperature (°C), TF is the temperature of the rich aqueous amine solution at stripper inlet (°C), ΔαCO2 is the cyclic loading of the rich aqueous amine solution (kg CO2/kg amine), Msol is the amine solution molecular weight (g/mol), XH2O is the mole fraction of water in the rich aqueous amine solution and ΔHvap,H2O is the latent heat of water vaporization, PCO2 and PH2O are the CO2 and water partial pressures at regeneration temperatures.
Considering that the specific heat capacity of amine solvents does not differ significantly among themselves [158], the sensible heat of any amine solution can be believed to be influenced by their cyclic loading, amine concentration and density as seen in Eqs. (22), (23).
Heat of vaporization is the final parameter that sums up the regeneration energy. This is the amount of energy required to vaporize the water in the CO2 rich amine solution in order to produce the stripping vapor which largely depends on the amount of water present in the solution [158]. Thus, compared to a less concentrated amine, a highly concentrated aqueous amine solution will benefit from having a smaller water concentration which will only require less latent heat of water vaporization. Therefore, 50 wt% TEA (triethanolamine) will possess lower heat of vaporization compared to 30 wt% MEA [158]. It is also important to highlight that the heat of vaporization will also greatly depend on the regeneration temperature. At same water concentration aqueous amine solutions will consume more vaporization heat at higher temperature (120 °C) than at lower temperature (85 °C). Usually low temperature here is referring to temperatures below boiling point of water. The contribution of vaporization heat towards the regeneration energy will then depend on the water concentration (related to the amine concentration) and temperature of regeneration process.
The energy required for amine solution regeneration is seen as one of the most important parameter that needs to be addressed in the search of potential solvents for CO2 capture. This is because, it accounts for as high as 70–80% of the plant operational cost [84], [85]. In terms of regeneration energy (heat duty) of 5 kmol/m3 MEA, several authors stated that this could be as high as 3.3–4.4 GJ/ton CO2 which is extremely high and very expensive for post–combustion CO2 capture applications [7], [8], [18], [80], [83], [149], [172], [173]. For CO2 capture process to be economically practical in a coal-fired power plant, 0.72 GJ/ton CO2 of reboiler heat duty is required which must be achieved from both process configuration and chemical solvent improvements [76]. From solvent optimization alone (e.g. blended amines), Mangalapally and Hasse reported 20% reduction in reboiler heat duty during the pilot plant test run using CESAR1 (AMP–PZ blend) solvent [127]. Another pilot plant run reported 35% reduction in regeneration energy using their new solvent (i.e. CANSOLV DC-201) [82]. Singh et al. studied new solvents in pilot scale operation which were found to reduce the energy of regeneration of 5 kmol/m3 MEA from 4.33 GJ/ton CO2 to 2.26 GJ/ton CO2 [174]. Fig. 17 shows the current values of the regeneration energy of regular and recent amine solvents. The ideal solvent as well, is shown in the plot to indicate the desired regeneration energy which will most likely to be achieved by blending amine solvents.
Fig. 17. Desired regeneration energy of the ideal amine solution.
Most often, regeneration energy is only reported based on the amount of energy required to strip an amount of CO2 (GJ/tonne CO2). However, as pointed out by Idem et al. this could be misleading because the temperature at which regeneration was carried out is a big factor and should be reported [175]. For instance from Fig. 18, two hypothetical amine solvents (Amine A and Amine B) can have same regeneration energy but Amine A was regenerated at a lower temperature (e.g. 90 °C) while Amine B was regenerated at higher temperature (120 °C). Such scenario reveals that Amine A is easier to regenerate and will be preferred for CO2 capture applications when compared to Amine B. In addition, Amine A will not require steam for regeneration process which will maintain the efficiency of the power plant. Nwaoha et al. suggested several benefits of low temperature regeneration to include reduced amine degradation, low emissions and possibility of not consuming steam needed for power generation [97].
Fig. 18. Desired regeneration temperature and energy of the ideal amine solvent.
For performance screening purposes (to compare several amine solutions), regeneration energy can also be determined in a small scale semi–batch process in the laboratory [68], [97], [111], [176]. Nwaoha et al. reported that both highly concentrated AMP–PZ–MEA tri–solvent blends (6 kmol/m3) and AMP–MDEA–DETA blends reduced regeneration energy by half compared to 5 kmol/m3 MEA at 90 °C regeneration temperature [97], [111]. For blended amine solution, careful selection of the amine components in the blends will go a long way towards reducing the regeneration energy compared to the single solvent MEA [97], [111]. From previous studies as discussed in this paper, blended amine solution should contain a tertiary amine or sterically hindered amine solvent due to their formation of HCO3− ions in the solution.
Shi et al. investigated the role and contribution of HCO3− in reducing regeneration energy [176]. It was discovered that HCO3− in the CO2 loaded amine solution will both facilitate the deprotonation of protonated amine (AmineH+) to release free amine and also act as a proton (H+) acceptor to directly liberate CO2 as shown in Eqs. (25), (26). Hence, the more HCO3− in the amine solution the faster the reactions are. This deprotonation can also be done by H2O, but H2O being less basic than HCO3− (pH = 8.5) makes its contribution less and slow as seen in Eq. (27). Another big disadvantage of the amine deprotonation by H2O is that it is strongly endothermic. Also, the pathway for amine deprotonation by H2O only follows a single route and does not liberate CO2 directly. This phenomenon has also been used by Nwaoha et al. in explaining the low regeneration energy of tri–solvent blends containing HCO3− forming amine solvents [97], [111].(25)AmineH++HCO3−↔Amine+H2CO3(26)H2CO3↔CO2+H2O(27)AmineH++H2O↔Amine+H3O+
Additionally, previous studies have reported regeneration energy reduction due to improvements and modifications in amine solvents and process configuration respectively [119], [127], [177], [178], [179], [180]. In order to greatly reduce this energy a combination of these will go a long way. Recent experimental and bench–scale pilot plant results has shown that employing solid catalysts (HZSM–5 and γ–Al2O3) in the regenerator reduces the regeneration energy and also lead to the use of hot water instead of steam as the heating medium [176], [181]. They reported that HZSM–5 donates H+ to AmineCOO– (carbamate) which then breaks it down to free amine and CO2 while the γ–Al2O3 deprotonates the AmineH+ hence releasing free amine. Both reaction routes by the aid of the catalysts reduce the thermal energy required to perform similar tasks in the regenerator.
This technology has now introduced an additional route towards reducing regeneration energy at lower regeneration temperatures.
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