https://doi.org/10.1016/j.petlm.2016.11.002
“The cost of using an amine solvent for 90% CO2 capture is another challenge for CO2 capture plants because they impact additional cost for CO2 capture. Rubin and Rao [241] stated that the cost of amine solution (MEA) can account for about 10% of the CO2 capture cost.
Nwaoha et al. proposed a parameter known as amine–CO2 ratio shown in Eq. (42) which compared amine solvents in terms of the amount of amine required to capture CO2 [97]. This parameter can be used to compare amine solvents at similar CO2 capture efficiency (e.g. 90% CO2 capture) and/or at similar amount of CO2 produced/desorbed. Hence, the actual cost of the amine solvent can be accurately estimated which is also related to parameters such as emissions, amine make–up and regeneration energy. For instance, an amine can be more expensive than MEA but during 90% CO2 capture it requires less amine circulation than MEA. This means that the overall cost of the amine solution for CO2 capture might be less when compared to MEA. This could also translate to reduced emissions and amine make–up, less pumping and regeneration energy (if the amine does not degrade faster than MEA). Such scenario has been reported in a tri–solvent blend containing specifically AMP–MDEA–DETA which was more expensive than MEA but required lower amine circulation rate as estimated, but higher CO2 production leading to cheaper overall amine cost [97].(42)Amine−CO2Ratio=Famine_solventrCO2_prodwhere; Amine–CO2 Ratio is the ratio of the amine solvent circulation rate to the average CO2 desorbed or produced (l–amine solvent/tonne CO2 or kg–amine solvent/tonne CO2), rCO2_prod is the rate of CO2 desorbed or produced (tonne CO2/hr or kg CO2/hr) while Famine_solvent is the amine solvent circulation rate without considering the water in the solution (l–amine solvent/hr or kg–amine solvent/hr). This is the actual amine content in the aqueous solution.
In order to estimate the cost of the amine solvent for CO2 capture Eq. (42) is modified to Eq. (43).
(43)Aminecost=Famine_solventrCO2_prod(Costamine)
where; Aminecost is the cost of the amine solvent for CO2 capture (US$ amine/tonne CO2), Costamine is the actual cost of the amine solvent(s) in the aqueous amine solution (US$/kg amine). The Costamine can be retrieved from the quotation of the amine supplier which is often available online.
From Fig. 22 it can be seen that lower amine–CO2 ratio might not necessarily translate to a lower amine cost (US$ amine/tonne CO2) because the cost of the amine solvent (US$/kg amine) plays an important role. Looking at the two hypothetical amine solvents, Amine A has higher amine–CO2 ratio but lower cost of amine when compared to Amine B which led to the aminecost to follow the trend Amine A (US$ 280/kg CO2) < Amine B (US$ 315/kg CO2). Similar trend was reported by Nwaoha et al. when AMP–MDEA–DETA blends were compared to single solvent MEA. Fig. 22 also depicts the region for the ideal amine solvent for CO2 capture which should possess low amine–CO2 ratio and low cost.
Fig. 22. Relationship of cost of amine and amine–CO2 ratio towards amine cost.
Eqs. (42), (43) can be easily applied when pilot plants and/or bench scale pilot plants are used for studying amine solvents because amine flow rates are available. However, when semi–batch experimental set–up is used for amine investigation the amine solvent flow rate can be estimated [97].
3.9.4. Amine make–up cost
Amine make–up cost is another parasitic cost for CO2 capture using amine solvents. When amine is lost due to either or both degradation and vaporization the rate of amine make–up will increase hence increased amine make–up cost. Amine degradation and vaporization losses can be easily analysed from both pilot plant studies and semi–batch laboratory experimental set–up [118], [196], [199], [200], [201], [202], [216], [218], [220], [221], [222], [223], [224], [229]. Results from these studies should also be translated to cost of amine make–up which has not been done. It is practical to believe that amine losses during CO2 capture is proportional to amine make–up because more amine losses will lead to increased amine make–up in order to maintain the desired amine concentration.
This review paper is also proposing a new parameter that will account for amine losses due to emissions induced by amine volatility and degradation per amount of CO2 produced and/or CO2 capture efficiency as shown in Eq. (44).
(44)Amineloss=ramine_degrCO2_prod+ramine_vaprCO2_prod
where; Amineloss is the total amount of amine loss during CO2 capture with respect to the amount of CO2 produced (l–amine solvent/tonne CO2 or kg–amine solvent/tonne CO2), ramine_deg is the rate of amine degradation (l–amine solvent/hr or kg–amine solvent/hr), while ramine_vap is the rate of amine losses through emissions or vaporization (l–amine solvent/hr or kg–amine solvent/hr).
For blended amine solvents, it is also suggested that aminedeg and amineemit should be quantified based on the individual amines in the blend. Eq. (44) can also be used for economic and environmental analysis of amine solvents as well as for design purposes during scaling up.
For simplicity amine loss can be quantified as amine disappearance per amount of CO2 produced/desorbed as shown in Eq. (45). This is applicable both in pilot plant studies and in semi–batch laboratory analysis. Eq. (46) can then be used to quantify the actual cost associated with amine make–up.
(45)Amineloss=ramine_disrCO2_prod(46)Aminemake−up_cost=ramine_disrCO2_prod(Costamine)
where; Aminemake-up_cost is the cost required for amine make–up (US$/tonne CO2).
It is important to state that the rate of amine disappearance (amine loss) should be divided by the rate of CO2 produced as shown in Eq. (45). This parameter will depict the actual amine loss per CO2 produced which is a true representation of amine loss or amine disappearance.
In order to quantify the contribution of amine losses through degradation and vaporization, Eq. (44) can be broken down to Eqs. (47), (48).
(47)Aminedeg_cost=(ramine_disrCO2_prod−ramine_vaprCO2_prod)Costamine
(48)Aminevap_cost=(ramine_vaprCO2_prod)Costamine
where; Aminedeg_cost is the cost associated with amine degradation (US$/tonne CO2) while Aminevap_cost is the cost penalty due to amine losses through vaporization/emissions (US$/tonne CO2).
Fig. 23 displays the effect of amine loss and rate of CO2 produced for hypothetical amine solvents. From Fig. 23 it can be seen that at same amine loss (Amine A and Amine B) the amine with higher CO2 produced (Amine B) will be more beneficial, though the ideal amine is desired to have very low amine loss and a very high CO2 production rate. However, if only the amine loss is considered then hypothetical Amine C will be said to not be a good amine solvent, but if the rate of CO2 produced is taken into account Amine C will be preferred when compared to both Amine A and Amine B. This is because the degradation rate with respect to CO2 production rate of the hypothetical amines followed the trend Amine C (0.7 kg amine/kg CO2) < Amine B (0.8 kg amine/kg CO2) < Amine A (1.3 kg amine/kg CO2). This trend is also applicable when amine loss due to either degradation or vaporization is considered.
Fig. 23. Effect of rate of CO2 produced to rate of degradation of hypothetical amine solvents.
All the cost and energy indices described and suggested above will be a good guide towards comparing the efficiency and acceptability of an amine solvent for CO2 capture when compared based on the same CO2 capture efficiency (90% CO2 capture) or/and based on the same amount of CO2 produced. These indices can also be used for scaling up design purposes.”