Understanding the Influence of the Alkaline Cation K+ or Na+ in

the Regeneration Efficiency of a Biogas Upgrading Unit.

Francisco M. Baena-Moreno a,b *, Mónica Rodríguez-Galán a, Fernando Vega a, T. R.

Reina b, Luis F. Vilches a, Benito Navarrete a.

a Chemical and Environmental Engineering Department, Technical School of

Engineering, University of Seville, C/ Camino de los Descubrimientos s/n, Sevilla

41092, Spain

b Department of Chemical and Process Engineering, University of Surrey, GU2 7XH

Guildford, United Kingdom

*Corresponding author.

E-mail address: fbaena2@us.es (Francisco M. Baena-Moreno)

Abstract

This paper reveals a regeneration method for a carbonate compound after carbon

dioxide (CO2) absorption in a biogas upgrading unit run with caustic mixtures, obtaining

precipitated calcium carbonate (PCC) as valuable by-product. This process arises as

an alternative to physical regeneration, which is highly energy intensive. This work

provides novel insights on the regeneration efficiency of carbonates to hydroxides while

also studying the influence of K+ or Na+ in the caustic CO2-trapping solution. The

compared parameters were the reaction time, temperature and molar ratio. Moreover,

psychochemical characterization of solids was obtained by means of Fourier-transform

infrared spectroscopy (FTIR), Raman spectroscopy, X-ray powder diffraction (XRD)

and Scanning Electron Microscopy (SEM) images. The results indicate that

regeneration efficiencies are slightly lower when potassium is used instead of sodium,

but quite acceptable for both of them. The chemical characterization experiments

showed the predominance of calcium carbonate. Overall, the results obtained in this

1

study proved that this process is feasible to upgrade biogas through PCC precipitation,

which appears to be a promising economically viable process to synergise CCS and

CCU.

Keywords

Carbon Capture and Utilization; Biogas Upgrading; Precipitated Calcium Carbonate;

Caustic Absorption; Biomethane Production;

1. Introduction

Our environment is seriously affected by climate change, carried out largely by human

action. In addition, the future shortage of conventional energy sources requires greater

use of renewable energy [1,2]. Within this context, biogas is one of the most promising

sources of renewable energy, having increased the existing number of biogas

processing plants in recent years considerably (Figure 1). Biogas is obtained through

the anaerobic digestion of biomass [3]. Regarding biogas composition, it is mainly

composed by methane (CH4) and carbon dioxide (CO2), in an approximate percentage

of 60% and 40% respectively [4–6]. After a complete upgrading for CO2 removal,

biomethane is obtained from biogas as a valuable product [7]. Biomethane uses

include heat or electricity production, natural gas substitute, compressed natural gas

and diesel replacement alongside liquid natural gas after a compression stage [8–10].

Due to this variety of applications, biogas upgrading technologies have been studied by

multiple researchers, chemical absorption being one of the most propitious due to the

high CO2 capture efficiency of this process. Solvents typically employed for this

process are monoethanolamine (MEA), piperazine (PZ), sodium hydroxide (NaOH) or

potassium hydroxide (KOH) [11–14]. Even though promising results have been found

2

by the employment of amines, nowadays there are some disadvantages discussed by

the scientific community. Some examples are the toxicity produced by the amine

degradation through the formation of nitrosamine as a potential carcinogenic

compound and the high regeneration cost in the stripping step [12,15–17]. Also the use

of caustic solvents requires an elevated energy consumption to regenerate the solvent

via temperature increase [12,17,18]. However, NaOH and KOH present advantages of

being cheaper, having a greater theoretical CO2 capture capacity and their availability

in the industry market [19].

0

4000

8000

12000

16000

20000

Number of plants

2010 2011 2012 2013 2014 2015 20160

2000

4000

6000

8000

10000

Year

Total installed capacity (MW)

Figure 1. Number of biogas processing plants and total installed capacity [8,20].

For this reason, previously an alternative path for solvent regeneration was proposed

by our group [13], which resulted to be much less energy intensive and economically

attractive due to the production of a valuable by-product. A general scheme of the

process can be seen in Figure 2. In brief, NaOH was employed as a CO2 capture agent

to form sodium carbonate (Na2CO3), according to reaction (1). In order to make the

process economically appealing, a regeneration of Na2CO3 via chemical reaction with

3

calcium hydroxide (Ca(OH)2) was chosen to generate precipitated calcium carbonate

(PCC) (reaction (2)). PCC is demanded as raw material in several applications, such as

for example in paper industry, polymers applications and as a neutralizer in healthcare

[21–23].

2NaOH ( aq )+CO2 (g )→+Na2CO3+H 2O (1)

Na2CO3 ( aq )+Ca (OH )2(s)→2NaOH ( aq )+CaCO3(s) (2)

High regeneration efficiencies as promising results for further investigations were

achieved. Since KOH was proven to have improved CO2 absorption compared to

NaOH [24,25], in this paper a characterization of regeneration efficiencies in a

precipitation reactor for this novel method is presented. Hence, in this work novelty

data regarding the comparison between both cations is presented, which have not

been found in the literature. For the collection of these inputs, the purpose of this study

was to analyze the differences between regeneration efficiencies and psychochemical

composition of both NaOH and KOH, with the intention to investigate the effect of

potassium ion in comparison with sodium. Potassium ion may have an enhancement

effect into the regeneration efficiencies, as well as in the purity of final PCC. The

chemical reaction carried out if reflected by reaction (3).

K2CO3 (aq )+Ca (OH )2(s)→2KOH (aq )+CaCO3(s) (3)

4

Figure 2. Bio-waste to PCC production and biogas upgrading process.

The key variables studied were the reaction time, the reaction temperature, and the

molar ratio between Ca(OH)2 and K2CO3 (R), since these variables were proved to

have an effect on the reaction rate [13,26,27]. Inasmuch as in the previous work [13] an

exhaustive characterization of these reaction parameters was done. However, in this

paper a selected number of tests were chosen to investigate the potassium ion effect.

2. Materials and Methods

2.1 Materials

PanReac-AppliChem supplied the reagents employed in this work (Ca(OH)2, Na2CO3,

K2CO3, CaCO3) in a pure-pharma grade (99% purity).

2.2 Regeneration experiments

5

The methodology was explained in detail in [13]. Firstly, both the carbonate aqueous

solution and hydroxide slurry were prepared. Secondly, a 600 mL beaker was chosen

as reactor to carry out the lab scale batch precipitation experiments, under the

conditions that can be found in Table 1. Once the reaction time was finished, the

solution was quickly filtered and separated for chemical analysis. The solid obtained

was dried during one day at 105°C. After this time, the samples were weighed to obtain

the PCC grams precipitated. The main result were considered on the KOH

regeneration efficiency, which is defined as follows:

KOH regeneration efficiency (%)=PCC obtained (g)×

2×PM KOH

PMCaCO3

Maximum KOH ¿regenerate(g)¿×100

The maximum KOH to be regenerated can be obtained stochiometrically from K2CO3

initial concentration. The reaction carried out at 50°C, with a molar ratio Ca/K2CO3 of

1.2 mol and a reaction time of 30 minutes was selected as a reference experiment

[13,27]. The concentration of the aqueous solution was fixed in 20g/100mL since it is

an expectable value after chemical absorption of KOH [27,28]. On the other hand the

Ca(OH)2 solution concentration was calculated stoichiometrically for each experiment

accordingly with the molar ratio.

Table 1. Matrix of experiments done in this study.

EXPERIMENT CARBONATE TO REGENERATE TIME (MIN) TEMPERATURE (ºC) MOLAR RATIO (R)1 Na2CO3 30 50 1.22 Na2CO3 15 50 1.23 Na2CO3 60 50 1.24 Na2CO3 30 30 1.25 Na2CO3 30 50 1.26 Na2CO3 30 70 1.27 Na2CO3 30 50 18 Na2CO3 30 50 1.29 Na2CO3 30 50 1.510 K2CO3 30 50 1.211 K2CO3 15 50 1.212 K2CO3 60 50 1.213 K2CO3 30 30 1.214 K2CO3 30 50 1.215 K2CO3 30 70 1.216 K2CO3 30 50 117 K2CO3 30 50 1.218 K2CO3 30 50 1.5

6

FTIR, Raman spectroscopy. XRD and SEM technique were employed for the

characterization of the solid samples. A Perkin Elmer FTIR BX spectrometer was used

to perform the attenuated total reflection Fourier transform infrared spectroscopy in the

powders (ATR-FTIR). Background subtracted spectra of the raw and the treated

samples were collected at room temperature by co-adding 32 scans at 4 cm−1

resolution in transmittance mode. Data were baseline corrected using Spectrum 5™

software. XRD analysis was completed by an X’Pert Pro PAN analytical instrument.

The 2θ angle was increased by 0.05o, with a 450 time per step over a range of 10-90o.

Diffraction patterns were then recorded at 40 mA and 45 kV, using Cu Kα radiation

(λ=0.154 nm).

Raman measurements of the powders samples were recorded using a Thermo DXR2

spectrometer equipped with a Leica DMLM microscope. A diffraction grating of 600

grooves per mm, a CCD detector, a green laser with a wavelength of 532.14 nm

(maximum power 20 mW), and a 50x objective were used. The morphology of the

samples were studied using SEM. This was carried out on a JSM6400 equipped with

an Energy Dispersive X-ray Spectroscope (EDS) analyzer (Oxford Link) and operated

at 20 KV. Reproducibility of the experiments were conducted giving as result ±2% of

error.

3. Results

Herein regeneration results are presented and discussed. Firstly, both NaOH and KOH

regeneration efficiencies are compared under the reaction conditions indicated above.

Secondly, chemical comparison between both carbonates obtained are shown by

means of Raman, FTIR and XRD measurements as well as SEM images.

3.1 Regeneration efficiencies results

7

Figures 3, 4 and 5 reveal the curves of NaOH and KOH resulting from the precipitation

experiments previously identified in section 2. The left hand side of the Figures

correspond to the net regeneration efficiency obtained with both caustic solutions, while

the right hand side indicates the regeneration efficiency differences between NaOH

and KOH.

10 15 20 25 30 35 40 45 50 55 60 65

60

65

70

75

80

85

90

95

100

Reg

ener

atio

n ef

ficie

ncy

(%)

Reaction time (min)

KOH NaOH

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5

Differences

Reg

ener

atio

n ef

ficie

ncy

diffe

renc

es

Figure 3. Comparison of NaOH and KOH regeneration with time at T=50oC and R=1.2.

3.1.1 Reaction Time Variation

The evolution of regeneration efficiencies during the reaction time is shown in Figure 3.

Curiously, KOH regeneration efficiencies are to be lower than those of NaOH. This

result is in good agreement with separated previous studies [29] and may be caused by

the higher stability of K2CO3 in comparison to Na2CO3. Given these circumstances, it

may be better to employ NaOH instead of KOH. However, as discussed previously,

KOH was proven to be a better solvent for CO2 removal [12,30] and the differences in

the regeneration stage can be overcome by increasing the reaction time. At 60

8

minutes, the differences between NaOH and KOH regeneration efficiencies are much

lower than that at 15 minutes (1.27% vs 3.99%). In any case, an almost 4% of

difference in regeneration efficiency could be industrially assumed as long as the

overall economic balance results positive.

25 30 35 40 45 50 55 60 65 70 7555

60

65

70

75

80

85

90

95

100

Reg

ener

atio

n ef

ficie

ncy

(%)

Temperature (°C)

KOH NaOH

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5

Differences

Reg

ener

atio

n ef

ficie

ncy

diffe

renc

es

Figure 4. Influence of temperature variation at R=1.2 and t=30min.

3.1.2 Temperature Influence

Figure 4 reflects the strong impact of temperature in the regeneration efficiencies. It

seems clear that in the low temperature range both solutions behave differently while at

the high temperature window similar behavior was identified. In particular, at 30oC a

6.88% of regeneration efficiency difference is obtained, whereas at 70oC this value

drops to 0.86%. This again can be ascribed to the greater stability of K2CO3 compared

to Na2CO3. In a hypothetical industrial application, in the case of choosing a

temperature around 50°C, a high regeneration efficiency would be achieved for both

hydroxides, without significant discrepancies between them. This is a remarkable

9

finding that deserves to be featured, since a process temperature as low as 50°C is

feasible to achieve by the employ of renewable energy, being an important advance

from an energy consumption perspective.

1.0 1.2 1.4 1.6

79

80

81

82

83

84

85

86

87

Reg

ener

atio

n ef

ficie

ncy

(%)

R

KOH NaOH

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5

Differences

Reg

ener

atio

n ef

ficie

ncy

diffe

renc

esFigure 5. Regeneration experiments for R values at t=30min and 50oC.

3.1.3 Molar Ratio Influence

The impact of precipitant/reactive molar ratio is depicted in Figure 5. As shown in the

plot, both NaOH and KOH regeneration are favored when the molar ratio increases.

Nevertheless, NaOH regeneration efficiency is always higher than KOH regeneration

capacity at every studied value of R. In addition, the differences in regeneration

efficiencies remains consistently steady across the studied R range. Contrary to the

observations for the other reaction parameters, it seems that the impact of R in the

performance is rather constant and the two families of solvent do not display similar

behavior when tuning this parameter. Assuming 80% of regeneration efficiency as an

acceptable result in an industrial case, an R value of 1 would be required to meet this

10

value employing Na2CO3. However, a R=1.2 would be necessary for K2CO3, what

promotes a higher amount of free calcium ions that need to be eliminated before

recirculating the resulting aqueous KOH solution in order to prevent fouling in the

absorption tower. In other words, it can speculated that KOH is more prone to produce

solid fouling in the absorption column thus leading to higher operational cost in terms of

shut-down / clean-up /star-up cycles.

3.2 Physicochemical Comparison for both PCC Obtained

Once regeneration efficiencies differences have been compared, the purpose of this

section is to contrast both PCC obtained from K2CO3 and Na2CO3. The analyses were

carried out over the tests number 1 and 10 (T=50oC, R=1.2, t=30 min), which were

selected as standard tests in a previous study [13]. At the same time, these two

samples were compared with commercial Ca(OH)2 and CaCO3. Figure 6 shows the

representative FTIR spectra of the studied samples for sake of comparison. CaCO3 is

characteristic of monoclinic structure inside P21/c group [31,32]. As can be seen in

Figure 6, both PCC represented are quite similar to commercial CaCO3, suggesting an

almost pure calcium carbonate was obtained. A strong Characteristic peak of CaCO3

polymorphs is shown at 874 cm-1 wavenumber [33]. This peak is presented by both

PCCs, so it could be an indication of CaCO3 purity.

11

Figure 6. FTIR spectra for selected samples.

500 1000 1500

Ram

an in

tens

ity (a

.u.)

PCC from K2CO3

PCC from Na2CO3

Commercial Ca(OH)2

Raman shift (cm-1)

Commercial CaCO3

12

Figure 7. Raman spectra for PCCs obtained (t=30min, T=50°C, R=1.2) in comparison

with commercial Ca(OH)2 and CaCO3.

In order to investigate the purity of the selected samples, Raman spectra are

represented in Figure 7. In this analysis, it is easier to identify a weak peak of

remaining Ca(OH)2 at around 300 cm-1 in both PCCs spectra. This result agrees with

those discussed previously in section 3.1, where 100% of regeneration efficiency is

never achieved indicating some leftovers of Ca(OH)2 in the obtained solids. In fact, the

high intensity Raman mode of the PCCs spectra at around 1150 cm-1 again reveals the

major contribution of CaCO3 in the obtained solid. To corroborate Raman experiments

and further gather information regarding the crystalline structure of the solids, Figure 7

shows XRD patterns of the studied samples. Herein it can be Clearly observed that

some contamination of Ca(OH)2 remains in both PCCs, denoted by the diffraction peak

at 34 degrees. Nevertheless, as announced before, this phase is present in very small

concentrations in comparison with CaCO3 as intended from the low intensity peak

demonstrated by PCCs. Overall, the two XRD patterns of the obtained PCCs are quite

similar to that of the commercial CaCO3 further confirming the predominance of this

crystalline phase in the solid composition. Concretely, the XRD pattern exhibited by our

PCCs is characteristic of calcite phase. Comparing both PCCs obtained from K2CO3

and Na2CO3, the first one presents a different peak at around 32 degrees. This seems

to be a minor contribution of vaterite phase formed in good agreement with previous

reports [34]. It is relevant to mention that such a secondary phase – vaterite – is only

formed when KOH is used as a capturing solution in the tower what points out the

impact of the alkaline cation not only in the regeneration efficiency of the upgrading

process but also in the structure of the obtained solid.

13

20 25 30 35 40 45 50 55 60

PCC from K2CO3

PCC from Na2CO3

VateriteCalcite

Ca(OH)2

Commercial Ca(OH)2

Commercial CaCO3

2q (degree)

Inte

nsity

(a.u

.)

Figure 8. XRD bands for selected samples.

Finally, SEM images were taken to get further insights into the precipitated carbonates

morphology (Figure 9-10). PCC from Na2CO3, represented in Figure 9, seems to have

a polymorphous calcite structure, as previously reported in a recent study [13]. On the

other hand, PCC from K2CO3, present a morphology dominated by calcite polymorphs.

One main conclusion from the SEM images is that no significant morphology

differences exist between both products.

14

Figure 9. PCC SEM from Na2CO3.

Figure 10. PCC SEM from K2CO3.

4. Conclusions

15

The present paper shows an interesting method for biogas upgrading from an

economical point of view. With respect to other commercial methods for biogas

upgrading, this process generates bio-methane and PCC as valuable products. The

process performance results are quite promising exhibiting high regeneration

efficiencies (60-95%) for both tested carbonates. Generally and comparing with other

previous studies in which waste were chosen as precipitating agents [27,29], our

results are enhanced regarding regeneration efficiency (60-95% vs 50-60%). Thus, our

process seems to be more viable from an industrial point of view.

KOH regeneration efficiencies are always lower than those of NaOH, probably due to

the greater stability of K2CO3. However, these are subtle differences that might be

subjugated by the enhancement of operation parameters, as long as the overall

economics of the process is improved. Regarding the effects of the different

parameters studied, it seems that temperature is the most influential factor indeed, an

increase in its value from 30oC to 50oC has caused an almost 20% of improvement in

the overall regeneration performance. Nevertheless, reaction time could also play a key

role in the hypothetical case of an industrial plant to enhance the regeneration

efficiencies.

FTIR and XRD studies confirm the purity of the carbonate phase obtained in the

precipitation experiments, whereas no significant differences in the morphology were

detected by the employment of SEM. This however could open new research avenues

since other precipitating materials could be tested to get a more valuable product.

Overall, the technical feasibility of this process was demonstrated for both solvents,

indicating that further research on this process is worth considering in the context of

carbon capture/utilization.

Future works will deal with finding new precipitating materials that can produce a more

valuable final product, in order to improve the overall economy of the process.

Furthermore, preliminary studies for scaling up the lab-scale work presented in this

16

paper to a bench-scale unit will be carried out, where the economy of the process could

be examined more accurately. Potentially our work could be integrated with other

industrial processes in the future. For instance, there is a possibility to use the

carbonate solution obtained after the absorption step as draw solution in forward

osmosis and then precipitating CO2 as valuable by-product [35].

Acknowledgments and Funding

This work was supported by University of Seville through its V PPIT-U. Financial

support for this work was also provided by the EPSRC grant EP/R512904/1 as well as

the Royal Society Research Grant RSGR1180353. This work was also partially

sponsored by the CO2Chem UK through the EPSRC grant EP/P026435/1.

Furthermore this work was supported by EMASESA through NURECCO2 project and

Corporación Tecnológica de Andalucía (CTA).

References

1. IPCC (2018) Global Warming of 1.5°C. Summary for Policymakers.

2. BP Energy Economics (2018) BP Energy Outlook 2018. 2018 BP Energy

Outlook.

3. Zhang, T., Yang, Y., and Xie, D. (2015) Insights into the production potential and

trends of China’s rural biogas. Int. J. Energy Res.

4. Singhal, S., Agarwal, S., Arora, S., Sharma, P., and Singhal, N. (2017)

Upgrading techniques for transformation of biogas to bio-CNG: a review. Int. J.

Energy Res.

5. Feroldi, M., Neves, A.C., Bach, V.R., and Alves, H.J. (2016) Adsorption

technology for the storage of natural gas and biomethane from biogas. Int. J.

Energy Res.

17

6. Kerroum, D., Mossaab, B.L., and Hassen, M.A. (2014) Production of bio-energy

from organic waste: Effect of temperature and substrate composition. Int. J.

Energy Res.

7. Demirel, Y., Matzen, M., Winters, C., and Gao, X. (2015) Capturing and using

CO2as feedstock with chemical looping and hydrothermal technologies. Int. J.

Energy Res.

8. Kadam, R., and Panwar, N.L. (2017) Recent advancement in biogas enrichment

and its applications. Renew. Sustain. Energy Rev.

9. Heyne, S., and Harvey, S. (2014) Impact of choice of CO2separation technology

on thermo-economic performance of Bio-SNG production processes. Int. J.

Energy Res.

10. Walker, S.B., Sun, D., Kidon, D., Siddiqui, A., Kuner, A., Fowler, M., and

Simakov, D.S.A. (2018) Upgrading biogas produced at dairy farms into

renewable natural gas by methanation. Int. J. Energy Res.

11. Vega, F., Cano, M., Gallego, M., Camino, S., Camino, J.A., and Navarrete, B.

(2017) Evaluation of MEA 5 M performance at different CO2 concentrations of

flue gas tested at a CO2 capture lab-scale plant. Energy Procedia, 114 (114),

6222–6228.

12. Leonzio, G. (2016) Upgrading of biogas to bio-methane with chemical absorption

process: Simulation and environmental impact. J. Clean. Prod.

13. Baena-Moreno, F.M., Rodríguez-Galán, M., Vega, F., Reina, T.R., Vilches, L.F.,

and Navarrete, B. (2018) Regeneration of Sodium Hydroxide from a Biogas

Upgrading Unit through the Synthesis of Precipitated Calcium Carbonate: An

Experimental Influence Study of Reaction Parameters. Processes, 6 (205).

14. Zhang, F., Zhou, Y., Sun, W., Hou, S., and Yu, L. (2018) CO2 capture from

18

reheating furnace based on the sensible heat of continuous casting slabs. Int. J.

Energy Res., 42 (6), 2273–2283.

15. Zhang, Y., Zhang, L., Wang, Y., Wang, M., Wang, Y., and Ren, S. (2015)

Dissolution of surfactants in supercritical CO2with co-solvents. Chem. Eng. Res.

Des., 94, 624–631.

16. Zhu, L., Chen, H., Fan, J., and Jiang, P. (2016) Thermo-economic investigation:

an insight tool to analyze NGCC with calcium-looping process and with

chemical-looping combustion for CO2 capture. Int. J. Energy Res., 40 (14),

1908–1924.

17. He, Q., Yu, G., Wang, W., Yan, S., Zhang, Y., and Zhao, S. (2017) Once-

through CO2absorption for simultaneous biogas upgrading and fertilizer

production. Fuel Process. Technol.

18. Jande, Y.A.C., Asif, M., Shim, S.M., and Kim, W.S. (2014) Energy minimization

in monoethanolamine-based CO2capture using capacitive deionization. Int. J.

Energy Res.

19. Abdeen, F.R.H., Mel, M., Jami, M.S., Ihsan, S.I., and Ismail, A.F. (2016) A

review of chemical absorption of carbon dioxide for biogas upgrading. Chinese

J. Chem. Eng., 24 (6), 693–702.

20. Dareioti, M.A., and Kornaros, M. (2014) Effect of hydraulic retention time (HRT)

on the anaerobic co-digestion of agro-industrial wastes in a two-stage CSTR

system. Bioresour. Technol., 167, 407–415.

21. Munawaroh, F., Muharrami, L.K., Triwikantoro, T., and Arifin, Z. (2018) The

effect of CO2 gas flow rate on precipitated CaCO3 formed at room temperature.

AIP Conf. Proc., 2014.

22. Liu, X., Jia, H.H., Zhou, J., Yong, X.Y., Wei, P., and Jiang, M. (2016) Application

19

of a Gas-Liquid Mixing Pump in Biogas Purification and the Co-production of

Nano Calcium Carbonate. Energy and Fuels, 30 (11), 9947–9952.

23. Liu, X., Zhou, J., Zhang, Y., Liu, X., Chen, Y., Yong, X., Wang, S., Zheng, T.,

and Yuan, H. (2015) Continuous process of biogas purification and co-

production of nano calcium carbonate in multistage membrane reactors. Chem.

Eng. J., 271, 223–231.

24. Mahmoudkhani, M., Heidel, K.R., Ferreira, J.C., Keith, D.W., and Cherry, R.S.

(2009) Low energy packed tower and caustic recovery for direct capture of CO2

from air. Energy Procedia, 1 (1), 1535–1542.

25. He, Q., Yu, G., Yan, S., Dumée, L.F., Zhang, Y., Strezov, V., and Zhao, S.

(2018) Renewable CO2 absorbent for carbon capture and biogas upgrading by

membrane contactor. Sep. Purif. Technol., 194, 207–215.

26. Baciocchi, R., Costa, G., Gavasci, R., Lombardi, L., and Zingaretti, D. (2012)

Regeneration of a spent alkaline solution from a biogas upgrading unit by

carbonation of APC residues. Chem. Eng. J.

27. Baciocchi, R., Carnevale, E., Corti, A., Costa, G., Lombardi, L., Olivieri, T.,

Zanchi, L., and Zingaretti, D. (2013) Innovative process for biogas upgrading

with CO2 storage: Results from pilot plant operation. Biomass and Bioenergy.

28. Baciocchi, R., Carnevale, E., Costa, G., Gavasci, R., Lombardi, L., Olivieri, T.,

Zanchi, L., and Zingaretti, D. (2013) Performance of a biogas upgrading process

based on alkali absorption with regeneration using air pollution control residues.

Waste Manag., 33 (12), 2694–2705.

29. Lombardi, L., Baciocchi, R., Carnevale, E., and Corti, A. (2012) Investigation of

an innovative process for biogas up-grading – pilot plant preliminary results.

Proc. ECOS 2012- 25th Int. Conf. Effic. cost, Optim. Simul. Environ. impact

20

energy Syst. June 26-29, Perugia, Italy, 1–12.

30. Mahmoudkhani, M., and Keith, D.W. (2009) Low-energy sodium hydroxide

recovery for CO2 capture from atmospheric air-Thermodynamic analysis. Int. J.

Greenh. Gas Control, 3 (4), 376–384.

31. Altiner, M., and Yildirim, M. (2017) Production of precipitated calcium carbonate

particles with different morphologies from dolomite ore in the presence of various

hydroxide additives. Physicochem. Probl. Miner. Process.

32. Dandeu, A., Humbert, B., Carteret, C., Muhr, H., Plasari, E., and Bossoutrot,

J.M. (2006) Raman spectroscopy - A powerful tool for the quantitative

determination of the composition of polymorph mixtures: Application to CaCO3

polymorph mixtures. Chem. Eng. Technol.

33. Ni, M., and Ratner, B.D. (2008) Differentiating calcium carbonate polymorphs by

surface analysis techniques - An XPS and TOF-SIMS study. Surf. Interface

Anal.

34. Kontoyannis, C.G., and Vagenas, N. V. (2000) Calcium carbonate phase

analysis using XRD and FT-Raman spectroscopy. Analyst.

35. Nematzadeh, M., Samimi, A., and Shokrollahzadeh, S. (2016) Application of

sodium bicarbonate as draw solution in forward osmosis desalination: influence

of temperature and linear flow velocity. Desalin. Water Treat., 57 (44), 20784–

20791.

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