1. Introduction
Global warming and climate change are being caused by the increase in greenhouse gases, primarily carbon dioxide (CO
2), in the atmosphere. CO
2 emissions were estimated to be more than 33 Gtons in 2018 [
1]. In order to achieve the “2 °C target” proposed by the Intergovernmental Panel on Climate Change (IPCC), CO
2 may need to be captured at a rate of 25 Gt/yr in 2050 [
2]. About half of these emissions are due to diffuse sources, belonging to the transport sector and small sources (homes and offices) [
3].
CO
2 can be captured by using a negative emission technology such as Direct Air Capture (DAC), proposed first as a mitigation option by Lackner in 1999 [
4,
5,
6]. By using this technology, the captured CO
2 can be verifiably removed from the atmosphere. Potential technologies include Direct Air Carbon Capture and Storage (DACCS) processes, or the CO
2 could potentially be used in a Direct Air Carbon Capture and Utilization (DACCU) system [
7,
8,
9].
Direct Air Capture technologies might include absorption, adsorption, mineral carbonation, membrane, photocalysis, cryogenic separation, electrochemical approaches, and electrodialysis [
4,
10,
11,
12,
13,
14,
15]. However, among these, absorption and adsorption are the most mature and most investigated in the literature [
4].
For defined advantages of adsorption system, potential applications for CO
2 capture using this technology are growing significantly and this work is focused on this technology addressing the relative merits of different sorbents (among physisorbents and chemisorbents), from a performance point of view, regarding capacity, energy efficiency, and economic viability [
16,
17].
For physisorption, only a physical interaction occurs between CO
2 and surface sorbent, weak Van Der Waals forces are established, and low CO
2 adsorption capacities are exhibited, owing to the relatively low adsorption heat [
18]. Potential sorbents include zeolites, activated carbons, metal-organic frameworks (MOFs), and other new materials (e.g., boron nitride nanosheets and nanotubes) [
10,
19].
Particular interest has recently been paid towards MOFs, as underlined by publications growing rapidly in the last decade [
20,
21,
22,
23,
24,
25,
26]. In fact, MOFs have a specific surface area much higher than other porous materials such as activated carbons and zeolites with values up to 7140 m
2/g [
27,
28].
Chemisorption involves a chemical reaction between CO
2 and surface sorbent, for example, an amine functionalized sorbent, making the CO
2 capture capacity larger [
29,
30]. Amine-functionalized sorbents are divided in three different classes [
4]. In class I, there is a physical impregnation of amines on a porous support. In class II, amines are covalently bonded on the walls of porous support, while in class III, amine monomers are polymerized in situ, resulting in a polyamine structure bonded to the walls of support.
Chemisorbents are investigated in more detail than MOFs for air capture, due to their high uptake capacity and selectivity, resilience to humidity (which has a positive effect on CO
2 capture), and the possibility of regeneration under relatively mild conditions [
3,
10].
Generally, research in this field has explored and optimized adsorption capacities, kinetics and regenerability for amine-functionalized sorbents individually [
31,
32,
33,
34,
35,
36] and this is also the case for MOFs [
37,
38,
39,
40,
41].
Experimentally, as shown in
Table 1, it has been found that for amine-based sorbents of class I, CO
2 uptakes can be up to 3.36 molCO
2/kg of sorbent for a polyethylenimine (PEI) functionalized hierarchical bimodal meso/microporous silica support in the presence of 19% of relative humidity [
32]. A mixture of PEI and poly(ethylene glycol) (PEG) impregnated on a fumed silica support has been reported in Meth et al. [
42] with an adsorption capacity of 6.8 × 10
−1 molCO
2/kg. Brilman and Veneman [
43] considered a mixture of tetraethylenepentamine (TEPA) and PEI, obtaining a CO
2 uptake of 2.5 molCO
2/kg, using temperature swing adsorption for regeneration. Other amines which have been investigated are: poly(propylenimine) (PPI) [
33,
44] and poly(allylamine) (PAA) [
45].
For amine-based sorbents of class II, CO
2 uptakes of up to 2.13 molCO
2/kg and 3.89 molCO
2/kg have been obtained, respectively for 3-aminopropylmethyldiethoxysilane (APDES) grafted onto nanofibrillated cellulose (NFC) [
46] and hydrazine (H
2N
4) onto Mg
2(dobpdc) [
47]. Good adsorption capacities are also reported in the work of Belmabkhout et al. [
30], Ng et al. [
48], and Choi et al. [
40], respectively of 1.4 molCO
2/kg for triaminesilane (TRI) onto MCM-41, 1.39 molCO
2/kg for N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (AEAPDMS) onto NFC and 1.5 for ethylenediamine (ED) grafted onto Mg/DOBDC.
Lower CO
2 uptakes are measured for amine-functionalized sorbents of class III. In total, 1.78 molCO
2/kg and 1.68 molCO
2/kg are shown in the work of Choi et al. [
49] and Abhilash et al. [
36]. In the first case, aziridine is considered with silica support, while in the second case, 3-aminopropyl triethoxysilane (APTMS) is evaluated in hybrid silica material.
Similar experimental analyses have been carried out for MOFs, showing adsorption capacities to be lower compared to those of amine-based sorbents, as in
Table 2. Capacities of 0.18 molCO
2/kg for SIFSIX-3-Ni [
50], 0.05 molCO
2/kg for HKUST-1 [
39], and 0.14 for Mg-MOF-74 [
51] have been measured.
These experimental analyses are complemented by mathematical models, which are able to replicate the process cycles, explore operating conditions not investigated in experimental work, and save time and cost for screening, development, and optimization [
52]. More importantly, modeling can carry out economic and energetic analysis to evaluate costs and energy consumption, including the thermal energy required for the desorption stage and the electrical energy for fans blowing air through the air sorbent contactor.
Moreover, other key performance indicators (KPIs) can also be evaluated by using mathematical models, with the aim to compare, screen, and select the best scenarios.
Estimates of costs and energy consumptions for main DAC technologies, and in particular for adsorption systems are reported by Fasihi et al. [
53]. Here, electricity requirements can range between 150 and 300 kWh
el/tonneCO
2 for amine-based sorbents, and up to 1420 kWh
el/tonneCO
2 for MOFs. On the other hand, thermal energy of between 1170 and 2000 kWh
th/tonneCO
2 is required. Regarding costs for adsorption, as reported in
Table 3, Fasihi et al. [
53] suggest a value between 63 and 86 USD/tonneCO
2 obtainable by 2040.
The expected cost for the technology in the future is lower than those evaluated by others in the literature for the present day. Kulkarni and Sholl [
54] report a cost of CO
2 capture of 43–494 USD/tonneCO
2, while Zhang et al. [
18] suggest a cost between 91 and 227 USD/tonneCO
2 (not including capital costs for equipment). Sinha et al. [
55] estimate a cost of capture of 60–190 USD/tonneCO
2 (without discounting cash flows, and assuming a self-contained sorbent). However, these analyses reported in the literature are conducted with different assumptions for energy inputs, different output conditions (pressure and CO
2 purity), and without transparency [
53].
Estimates provided by companies are also present in the literature. Generally, these companies suggest a low cost, but such estimates need to be considered in the context that the companies are in the process of commercializing the technology [
2]. Climeworks is expecting to reduce the cost from 600 USD/tonneCO
2 to 100 USD/tonneCO
2 by 2030, while a more optimistic forecast is proposed by Global Thermostat, with suggested costs of 15–50 USD/tonneCO
2 for the near future [
53]. Such costs appear to be unrealistic.
From the considered literature analysis, it is evident that there is a gap to conduct an independent and complete analysis to evaluate some important KPIs, such as costs (operating and capital) and energy consumption of a number of important DAC technologies, including chemisorption and physisorption, based on the design of DAC process. Of the two, physisorption has hitherto received less attention from the research community. The work of Sabatino et al. [
58] reports a comparison between some MOFs and amine-functionalized sorbents to evaluate costs and energy consumptions; however, their research is not based on design calculation but multi-objective optimization without any correlation between design parameters. Other research on adsorption technology are concerned with the sensitivity analysis of operating parameters [
59,
60], regeneration methodology [
61], new adsorbent bed [
62], and thermodynamics [
63].
This proposed work, following a design procedure, compares the theoretical performance and techno-economics of two amine-functionalized sorbents, SI-AEATPMS ([N-(2-aminoethyl)-3-aminopropyl]trimethoxysilane (AEATPMS) grafted on silica gel) and APDES-NFC-FD (3-aminopropylmethyldiethoxysilane (APDES) on nanofibrillated cellulose (NFC)), in addition to three different MOFs (MIL-101, MOF-177, MOF-5), with the aim to capture CO2 from dry air, by using an adsorption system based on temperature swing for regeneration.
We are supposing conditions without the relative humidity of air due to limitations in the literature data for the selected sorbents. In any case, this assumption reduces the energy consumption of the desorption stage for chemisorbents and to ensure a higher capture efficiency for physisorbents.
SI-AEATPMS and APDES-NFC-FD are considered because they were suggested by the Climeworks company for DAC systems. MOF-5, MOF-177, and MIL-101 are taken into account because they have already been used in the literature for carbon dioxide capture with good performances and advantages compared to other sorbents (high adsorption capacity, thermally stable, and good strength to withstand pressures) [
64,
65]. As the most investigated in the literature and in real applications for CO
2 capture, these sorbents can provide a good representation of the whole families of sorbents. The chemisorbents and physisorbents are analyzed and compared in terms of the previously mentioned KPIs (electricity and thermal energy consumption, costs and amount of CO
2 captured or CO
2 productivity), and against data provided by the Climeworks company for the first “commercial” plant in the world, used as a case study. The defined KIPs are chosen because they suggest the techno-economic feasibility of the process. A mathematical model describing the adsorption and desorption stage is developed, to allow an independent analysis.
3. Results and Discussion
Firstly, the model was validated by using the experimental data for an adsorber at the lab scale reported in the work of Wurzbacher [
74], with the experimental setup as in
Figure 2. In this work, CO
2 is captured from air (CO
2 concentration between 400 and 440 ppm) with the SI-AEATPMS sorbent (50 cm
3) at 298 K. The air flow rate is of 2 × 10
−5 m
3/s, with 0–40% of relative humidity. Adsorber diameter and height are 40 mm, ensuring small pressure drops. The steel adsorbent bed is contained in a water bath for heating and cooling, during adsorption and desorption stages. Hot water at 75–95 °C is used for the regeneration stage, while cooling water at 20 °C is used for the cooling stage. All operating conditions are regulated by electronic controllers. At the equilibrium condition, adsorption and desorption times are 24 h and 2 h, respectively.
This means that the entire bed is in equilibrium with the feed in about 24 h, as obtained by our mathematical model and shown in the breakthrough curve of
Figure S1 in the Supporting Information. This demonstrates good agreement between experimental and simulated results, validating our mathematical model.
3.1. Adsorption and Desorption Curves of DAC Systems
The mathematical model provides the breakthrough curves for the adsorption of CO
2, for the considered chemisorbents and physisorbents. As already mentioned, these plots show the ratio between CO
2 concentration at the bed outlet and the CO
2 feed concentration, as a function of time.
Figure 3 shows these trends for MOFs.
Overall, in each Figure, three different sections can be identified as a function of time. At first, the concentration is zero because the step function of CO
2 has not passed the end of the air contactor. After a defined time (the breakthrough time, where the outlet CO
2 concentration rises to 5% of the inlet CO
2 concentration), the concentration increases because CO
2 reaches the end of the adsorber, and the sorbent continues to capture CO
2 until it is in equilibrium with the feed. An adsorption front moves along the bed with time, defining a mass transfer zone. Finally, the sorbent does not capture any more CO
2, because the entire bed is in equilibrium with the CO
2 in the feed. No additional mass transfer occurs between gas and solid phases. This is defined here as the equilibration time (outlet concentration is 95% of the inlet CO
2 concentration) [
98]. The difference between breakthrough and equilibration times defines the length of the mass transfer zone. Generally, this value depends on the value of the mass transfer rate: a higher mass transfer rate creates a shorter mass transfer zone [
98]. The higher mass transfer rates imply lower mass transfer resistance, and a higher rate of adsorption [
99]. Our results show that the shortest mass transfer zone of 146.2 s is present for MOF-177 with the highest mass transfer rate of 1.49 × 10
−1 s
−1. The greatest mass transfer zone of 703.5 s is present for MOF-5 due to the lowest mass transfer rate, of 1.9 × 10
−2 s
−1. On the other hand, for MIL-101 a mass transfer zone of 331.2 s is obtained, for which the value of k
l is of 3.3 × 10
−2 s
−1.
These results suggest that the dynamic capacity of the adsorbent bed with MOF-177 is higher than other sorbents. Reducing the dynamic mass transfer zone increases the dynamic capacity of the bed [
100]. Higher values of k
l, as for MOF-177, allow a steeper curve compared to others, suggesting that the bed is being utilized more efficiently [
101].
Figure 3 also suggests that the bed with MIL-101 sorbent has a very low residence time (the time that the air has spent inside the adsorber), as also reported in the work of Sinha et al. [
55] for a MIL-101-based sorbent.
Breakthrough curves for amine-based sorbents are shown in
Figure 4.
The same three sections are present. However, a greater mass transfer zone is present for these chemisorbents compared the previous sorbents, due to a lower mass rate constant of 4 × 10
−4 s
−1 for the silica gel-based sorbent, and of 3.8 × 10
−3 s
−1 for the cellulose-based sorbent. In fact, the mass transfer zone for the SI-AEATPMS is around 14,380 s, while for APDES-NFC-FD it is 1915 s. A higher mass transfer resistance is present in chemisorbents. A steeper breakthrough curve in these last sorbents is due to a higher bed porosity, the effect of which is studied in the work of Papurello et al. [
97]. However, even though chemisorbents are characterized by lower adsorption time constants, the breakthrough curve is shifted to later times, suggesting a stronger affinity between the adsorbent and CO
2 [
102]. Since a chemical reaction occurs, a greater adsorption capacity is present, as shown in
Figure 5 and
Figure 6, reporting, respectively, the trend of loading for physisorbents and chemisorbents as a function of time and length of adsorbent bed. The loading is defined as the adsorbed CO
2 and sorbent mass ratio, then it is depending on the adsorbent length.
Lower values of equilibrium loading are apparent for MOFs. For MOF-177, MOF-5, and MIL-101 this parameter is, respectively, 0.00043 mol/kg, 0.00093 mol/kg, and 0.00039 mol/kg. On the other hand, for SI-AEATPMS and APDES-NFC-FD the equilibrium loading is, respectively, 0.4 mol/kg and 0.85 mol/kg.
The above discussion is related to the adsorption stage, when CO
2 is captured by sorbent. Heating is provided for desorption, as in a TSA process.
Figure 7 shows CO
2 concentration as a function of time and length for MOFs, while
Figure 8 reports the same parameter for chemisorbents.
It is possible to see that the CO
2 concentration starts from a value corresponding to the inlet concentration of 0.016 mol/m
3, has a maximum close to the inlet of bed and after that, it decreases towards zero, as also reported in the work of Morales–Ospino et al. [
82]. The maximum value is due to the impact of heating when the bed is still saturated with CO
2. The loading starts from the value at the inlet condition and has a decreasing trend.
3.2. KPI: CO2 Productivity of DAC Systems
From the previously discussed curves, related to the adsorption and desorption stages, it is possible to find adsorption and desorption times, the full cycle time, and then the CO
2 productivity (amount of CO
2 captured). The adsorption and desorption times are for a 90% approach to equilibrium and are in agreement with data reported in the literature [
17,
74].
The required mass of the sorbent is calculated knowing the volume and porosity of bed and particle density [
103]. With all of this information, it is possible to evaluate the average single pass recovery, Rc, and the amount of captured CO
2. The average single pass recovery is defined as the ratio between the difference of CO
2 at the inlet and outlet of the bed and CO
2 at the inlet (in terms of mole). The adsorption time can also be calculated as the ratio between the 90% of equilibrium loading and mass sorbent product and the average single pass recovery and CO
2 mole in the feed product [
104]. Then, the value of Rc can be obtained by iteration to a self-consistent value.
The full cycle time (including adsorption and desorption times), average single pass recovery, the mass of the sorbent, and captured CO
2 are evaluated for each sorbent as reported in
Table 10.
Due to a higher adsorption capacity, as discussed above, chemisorbents are able to capture a greater amount of CO2: 6.43 kgCO2/h and 6.66 kgCO2/h, respectively, for the silica gel-based sorbent and the cellulose-based sorbent, compared to physisorbents. The MOFs MIL-101, MOF-177, and MOF-5 capture, respectively, 3.96 kg CO2/h, 4.5 kgCO2/h, and 4 kgCO2/h. The best performances of MOF-177 and APDES-NFC-FD among the physisorbents and chemisorbents are due to a lower mass transfer zone, and are underlined by a greater amount of captured CO2, and then by a higher value of the average single pass recovery, which is 0.89 for the cellulose-based sorbent and 0.59 for MOF-177.
Adsorption and desorption times are higher for amine-functionalized sorbents. Additionally, the higher value for the SI-AEATPMS sorbent compared to the other amine-based sorbent is due to the higher amine content of the APDES-NFC-FD sorbent [
105]. In fact, results show that a full cycle for the silica gel-based sorbent is 89.2 h (305,500 s for the adsorption time and 16,650 for the desorption time), while only 9.53 h (32,560 s for the adsorption time and 1845 s for the desorption time) are required for the cellulose-based sorbent. For MOFs, full cycle times are lower than 1 h: the lowest value is for MOF-177 for which this parameter is 0.09 h (214 s for the adsorption time and 106 s for the desorption time). For this reason, the number of cycles per day is higher using physisorbent: MOF-177 conducts 271 cycles/day, while 0.3 cycles/day are needed for SI-AEATPMS sorbent.
Overall, amine-functionalized sorbents capture 152 kgCO2/day (APDES-NFC-FD sorbent) and 147 kgCO2/day (SI-AEATPMS), while MIL-101, MOF-177, and MOF-5 capture, respectively, 73 kgCO2/day, 72 kgCO2/day, and 68 kgCO2/day.
3.3. KPIs: Energy Consumption and Cost of DAC Systems
As explained previously, energy consumption includes the electrical energy for fans blowing air inside the bed, and thermal energy for the regeneration.
Table 11 shows the amount of energy required for each sorbent.
It is possible to underline why physisorbents are characterized by higher values of energy consumption in terms of both electrical and thermal energy. The electrical energy demand is higher for MOFs due to higher pressure drop and lower amount of captured CO
2 per day, compared to systems using amine-functionalized sorbents. The required electricity energy for MOFs and amine-functionalized sorbents is comparable with the values reported by Fasihi et al. [
53] in their work. In our research, the electricity consumption for MIL-101, MOF-177, and MOF-5 are, respectively, 1540 kWh
el/tonneCO
2, 1350 kWh
el/tonneCO
2, and 1520 kWh
el/tonneCO
2. The amine-based sorbents exhibit lower values: 357 kWh
el/tonneCO
2 and 299 kWh
el/tonneCO
2, respectively, for SI-AEATPMS and APDES-NFC-FD.
Particular consideration is required for the thermal energy to regenerate the sorbent. No data of this parameter are present for MOFs capturing CO2 from air in the literature.
For amine-functionalized sorbents, values similar to those reported by Fasihi et al. [
53], Elfving et al. [
106], and Wijesiri et al. [
3] are obtained: 1884 kWh
th/tonneCO
2 for the silica gel-based sorbent and 1427 kWh
th/tonneCO
2 for the cellulose-based sorbent.
However, higher values are obtained for physisorbents, ranging from 494,000 kWhth/tonneCO2 for MOF-5 to 1,300,000 for MOF-177 (due to the sensible heating requirements for the sorbent). These greater values are due to a very low working capacity for MOFs, ranging from 0.00084 mol/kg for MOF-5 to 0.00035 mol/kg for MIL-101 under atmospheric concentration of CO2. Working capacity depends on the equilibrium loading, this is then a critical parameter for the design of adsorption processes. This is an important finding in the context of MOFs.
Moreover, it is evident that DAC facilities using MOFs are impracticable due the huge amount of thermal energy requirement. This is true not only for an energetic point of view but also for an environmental aspect because additional CO2 would be emitted by the MWI to produce the heat energy needed to regenerate the adsorbent.
For DAC processes based on amine-functionalized sorbents, a life cycle assessment analysis is suggested to account for the additional emissions of MWI.
Economic analysis is carried out with the evaluation of CAPEX and OPEX for each process with different sorbents.
Table S3 shows the analysis of capital costs for different adsorbent beds, starting from the evaluation of equipment (adsorber and fan) costs. For the adsorber, the mass of the shell of carbon steel is calculated according to the relation provided by Towler and Sinnott [
107] and equal to 1200 kg. For the fan, the impeller diameter is evaluated to be 0.38 m [
108]. Overall, CAPEX is determined by the sum of working capital and fixed capital investment, working capital assumed to be 15% of total capital investment.
Results show that for all sorbents, annualized capital costs are 11500 USD/year, while the CAPEX cost breakdown is reported in
Figure S2. Specific capital costs are lower for amine-functionalized sorbents: 214 USD/tonneCO
2 for the cellulose-based sorbent and 221 USD/tonneCO
2 for the silica gel-based sorbent. For MIL-101, MOF-177, and MOF-5, specific capital costs are, respectively, 446 USD/tonneCO
2, 447 USD/tonneCO
2, and 479 USD/tonneCO
2.
The evaluation of operating costs is reported in
Table S4 while the OPEX breakdown cost in reported in
Figures S3–S7. Here, the OPEX is different for each adsorbent bed, and higher operating costs are required for physisorbents due to higher costs for utilities, especially cooling water (since a greater amount of cooling water is required to remove the greater overall amount of heat that is required for the regeneration step) and thermal energy. Specific OPEX for SI-AEATPMS and APDES-NFC-FD are, respectively 7.756 USD/tonneCO
2 and 5.537 USD/tonneCO
2. On the other hand, for MIL-101, MOF-177, and MOF-5 these costs are 66,100 USD/tonneCO
2, 38,900 USD/tonneCO
2, and 15,800 USD/tonneCO
2.
Overall, total costs are 751 USD/tonneCO2 for the cellulose-based sorbent, 977 USD/tonneCO2 for the silica gel-based sorbent, 66,600 USD/tonneCO2 for the MIL-101 sorbent, 39,400 USD/tonneCO2 for the MOF-177 sorbent, and 16,300 USD/tonneCO2 for the MOF-5 sorbent.
The economic analysis suggests that operating costs have the highest influence on total costs for all investigated systems, especially for those using MOFs.
In particular, the contribution of OPEX and CAPEX on total costs is, respectively, 72% and 28% for the system using the cellulose-based sorbent, 77% and 23% for the system using the silica gel-based sorbent, 99% and 1% for the process using MIL-101 and MOF-177, and 97% and 3% for the system using MOF-5.
For the capture system using the SI-AEATPMS sorbent, the cost of cooling water is significant, suggesting the use of free air for the cooling stage in order to reduce costs. Another significant contribution is from the maintenance and repairs cost, which is evaluated as 2% of the fixed capital investment. This cost could be reduced by using equipment with another material that with a lower carbon content could allow lower costs.
For the capture system using the APDES-NFC-FD sorbent, the maintenance and repair cost is the most significant element OPEX; hence, the use of another material for equipment could help in cost reduction. The cooling water cost has a great influence on OPEX too.
Overall, for processes using amine-functionalized sorbents, it is suggested to use alternative materials of construction and use free air for cooling.
For capture systems based on MOF-5 and MOF-177, the cost for the required thermal energy is the most significant, while for the process capturing CO2 using MIL-101 the cost of cooling water has the highest contribution, although the cost of thermal energy is great too. This suggests that a reduction in costs could be achieved by using air as a cooling medium (as already done in the Antecy Company) and improving the sorbent efficiency of MOFs in terms of loading, which is a critical parameter, as mentioned before. Future research should be focused on improving the efficiency of these new materials, still under study and characterized by unprecedented chemical and structural tunability.
Moreover, from the economic analysis we found that the lowest total cost is for the process using the chemisorbent APDES-NFC-FD, while the highest cost is for the process using the physisorbent MIL-101.
According to the previous considerations (OPEX with the highest influence on total costs with direct production costs as the main contribution as in
Figures S4–S7), we can say that the total costs can range according to the price of significant utilities and raw materials in the market, for the capture system using the silica gel-based sorbent and MOFs.
For the process based on the cellulose sorbent, fixed charges have the highest influence on OPEX so we expect that total costs can range ±20%.
In any case, total costs for amine-based sorbents are in agreement with the order of magnitude estimated by House et al. [
109] for DAC systems up to 1000
$/tonCO
2.
The consideration is for other costs reported in the literature and revised by the work of Fasihi et al. [
53]. These reported costs are of a few hundred dollars per tonne of CO
2 and are based on different assumptions [
3,
98]. In particular, a few hundred dollars per tonne of captured CO
2 are estimated by Kulkarni and Sholl [
54], Zhang et al. [
18], and Sinha et al. [
55]. However, Krekel et al. [
110] report in their work that once capital costs are considered, the total capture cost increases significantly.
Particular attention should be placed on the use of MOFs, since due to higher operating costs caused by the very low working capacity, these are very expensive for CO2 capture from air and are not recommended. Future research should be focused on the development of MOFs that ensure higher values of loading. This could be achieved by adding special compounds. This solution could make MOFs suitable for CO2 capture.
3.4. Comparison with Climeworks Data
Climeworks has developed a DAC plant at semi-commercial scale, and uses an amine-functionalized cellulose chemisorbent, as discussed here. The geometry of the adsorbent bed is the same as that used in our calculation. Overall, results show a good agreement between operating conditions and energy consumptions evaluated by us for the APDES-NFC-FD sorbent with those reported proposed by this DAC Company.
The duration of a full cycle is lower for Climeworks and in the range of 4–6 h, because only the adsorption of CO
2 is considered in our model, whereas some humidity is present in the real air flow rate. However, experimental analyses show that after 5 h adsorption time, the CO
2 uptake curves are still on an upwards path, while the corresponding H
2O curves have reached a steady state for all relative humidity values [
105]. This explains why Climeworks utilizes a shorter cycle.
For other operating parameters, no significant differences are present. In fact, for Climeworks the amount of captured CO2 is 135 kgCO2/day, with an average single pass recovery of 0.9. Regarding the energetic consumption, the proposed electrical energy consumption is between 200 kWhel/tonneCO2 and 450 kWhel/tonneCO2. The required thermal energy consumption is 1500–2000 kWhth/tonneCO2.
Regarding the economic analysis, our total cost (751 USD/tonneCO
2) is the same order of magnitude of that proposed by Climework (600 USD/tonneCO
2) [
111].
Moreover, our calculated total cost can be higher than that estimated when humidity is present in the air. In fact, as reported in Wurzbacher et al. [
104], the energy consumption for the regeneration can increase up to 20% when the relative humidity increases from 20% to 80%.
4. Conclusions
An independent analysis is developed in this research work to evaluate some KPIs (energy consumption, CO2 productivity, and costs) of a DAC processes, based on adsorption using different sorbents. In particular, chemisorbents (two amine-functionalized sorbents, SI-AEATPMS and APDES-NFC-FD) and physisorbents (three MOFs: MIL-101, MOF-5, and MOF-177) are compared.
For this purpose, a mathematical model describing adsorption and desorption stages is developed for each sorbent.
Results show that MOFs are characterized by a very low equilibrium loading and hence a very low working capacity in comparison to chemisorbents. For MIL-101, MOF-177, and MOF-5 this last parameter is, respectively, 0.00035 mol/kg, 0.00039 mol/kg, and 0.00084 mol/kg. On the other hand, for SI-AEATPMS and APDES-NFC-FD working capacity is, respectively, 0.367 mol/kg and 0.771 mol/kg.
The equilibrium loading is a critical parameter because it influences the adsorption capacity, energy consumption, and cost. Better results are found for the adsorption processes using amine-functionalized sorbents. For the cellulose-based sorbent, 152 kgCO2/day are captured, consuming 299 kWhel/tonneCO2 of electrical energy and 1427 kWhth/tonneCO2 of thermal energy at an overall total cost of 751 USD/tonneCO2. For MOF-177, the amount of captured CO2, electrical energy and thermal energy consumptions, and total costs are, respectively, 72 kgCO2/day, 1350 kWhel/tonCO2, 1,300,000 kWhth/tonCO2, and 39,400 USD/tonneCO2. It is evident that the huge amount of thermal energy requirement makes the processes based on MOFs impracticable.
This independent analysis shows that costs reported in the literature appear to be with the same order of magnitude of our analysis. A comparison with the Climeworks Company (using the APDES-NFC-FD sorbent) shows a good agreement with KPIs, and total cost. Higher costs are expected when humidity is in the air.
It is important to note that we are assuming here the availability of industrial waste heat—should this not be available, costs will be somewhat higher (22%, when a traditional boiler is used for heating, considering a cost for heat of 0.165 USD/kWh [
112]. Suggestions to reduce costs are provided. It was found that for systems using amine-functionalized sorbents, the use of air for the cooling stage and cheaper materials for equipment could reduce OPEX and therefore overall costs. For processes using MOFs, more research should be focused to improve their loading allowing a lower cost for thermal energy. The use of air for cooling could reduce costs too.