Physicochemical comparison of precipitated calcium carbonate for different configurations of a biogas upgrading unit

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/jctb.6013 Physicochemical Comparison of Precipitated Calcium Carbonate for Different Configurations of a Biogas Upgrading Unit. Francisco M. Baena-Moreno a,b *, Cameron Alexander Hurd Price b, Estelle le Saché b , Laura Pastor-Pérez b,c , Daniel Sebastia-Saez b, T. R. Reina b. 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 c Laboratorio de Materiales Avanzados, Departamento de Química Inorgánica Instituto Universitario de Materiales de Alicante Universidad de Alicante, Apartado 99, E03080 Alicante, Spain *Corresponding author. E-mail address: [email protected] (Francisco M. Baena-Moreno) Abstract BACKGROUND: This paper presents a physicochemical comparison of the solid products obtained from two alternative processes that recycle waste sodium carbonate This article is protected by copyright. All rights reserved. (Na2CO3) solution, which is produced following the absorption of CO2 in a biogasupgrading unit. Chemical regeneration processes offer an attractive alternative to the energetically demanding standard physical methods. In the first process, sodium hydroxide (NaOH) is regenerated as a precipitate from the chemical reaction of Na2CO3 with calcium hydroxide (Ca(OH)2). The second process shows a path to obtain a valuable sodium chloride (NaCl) and Calcium carbonate (CaCO3) rich brine from calcium chloride (CaCl2) acting as a precipitant agent. In both processes, Precipitated Calcium Carbonate (PCC) is obtained as the most valuable by-product, but with varying properties due to the different origin. RESULTS: The purpose of this work is to analyse physicochemically both variations of PCCs obtained and examining the differences between these solid samples in order to determine which method produces more desirable characteristics in the final product. To this end, FTIR, Raman, XRD and SEM were employed as characterization methods. The results reflect that both PCCs have a calcite crystal structure, or morph, being as both PCC products originate from CaCl2 that is more similar to commercial calcium carbonate calcite. CONCLUSION: These results confirmed that a pure CaCO3 valuable by-product can be obtained from a biogas upgrading unit with several industrial applications. Keywords: Biogas; Carbon Capture; Carbon Utilisation; Chemical Analysis; Green Chemistry; Precipitation 1. Introduction This article is protected by copyright. All rights reserved. The need to combine renewable energies with current fossil fuels to reduce pollution is one of the topics most discussed by the scientific community 1–3. Biogas, mainly composed of carbon dioxide (CO2) and methane (CH4), is one of the most studied sources of renewable energy 4–8. Biogas comes from the anaerobic digestion of different types of biomass. To be used as a substitute for natural gas, biogas should be upgraded to obtain biomethane, which can be injected into the natural gas distribution network. Additionally, biomethane can be used as fuel or as a base material for the synthesis of methanol 9–11. The major contaminant within biogas that is formed during its synthesis, is CO2. As such, the removal of this contaminant and the resultant upgrading of biogas is a topic of considerable research by several authors 12–14. Among these techniques, CO2 absorption seems to be the most promising since high capture efficiencies can be achieved 15,16. For instance, some authors employed monoethanolamine (MEA) or piperazine (PZ) as solvents, achieving 90% CO2 capture efficiency 17–21. On the other hand, caustics solvents such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) were employed towards carbon sequestration showing similar results 22–25. NaOH does not present degradation as amines do, which can produce emissions with an elevated toxicity. Moreover the formation and harmful effects of nitrosamines and nitramines known as potential carcinogenic compounds is characteristic of amines, which does not occur by the employed of caustic solvents 26. Furthermore, caustic solvents are cheaper and more abundant than MEA or PZ, but the main problem with caustic solvents compared to these traditional solvents is the high energy penalty in the regeneration stage via physical methods 11,27. For this reason, many researchers investigate the regeneration via chemical reaction in a recyclable process, as shown in Figure 1, in order to balance the energy penalty with This article is protected by copyright. All rights reserved. favorable cost 13,25,28–32. However, these processes did not achieve technical feasibility due to the relatively low regeneration efficiencies (50-60%) seen by the use of waste solutions with high Ca(OH)2-Mg(OH)2 content 13,22,25,33. In our previous work 12 a method for biogas upgrading and precipitated calcium carbonate (PCC) production was investigated, obtaining promising results for further research. Figure 1. Process for biogas upgrading and PCC production. In this work, CO2 was removed from biogas through sodium hydroxide (NaOH) absorption in a packed tower, according to equation (1). In order to regenerate the NaOH and make the process economically viable, a precipitation step was undertaken to produce PCC (equation (2)), employing calcium hydroxide (Ca(OH)2) as a precipitating agent, as an improved method against weaker hydroxide waste solutions, that were employed previously. 2𝑁𝑎𝑂𝐻(𝑎𝑞) + 𝐶𝑂2(𝑔)→ +𝑁𝑎2𝐶𝑂3 + 𝐻2𝑂 (1) 𝑁𝑎2𝐶𝑂3 (𝑎𝑞)+ 𝐶𝑎(𝑂𝐻)2(𝑠)→ 2𝑁𝑎𝑂𝐻(𝑎𝑞)+𝐶𝑎𝐶𝑂3 (𝑠) (2) The aim of our previous study was to optimize the reaction parameters to achieve a reasonable NaOH regeneration efficiency, as well as to obtain a high product quality, which could be sold to improve the overall economic performance of the process. Nevertheless, after studying the psychochemical characteristics of PCC, it was concluded that an impure material was obtained under the conditions imposed. However, to achieve higher purity PCC from the chosen concentration of precipitating agent required higher temperature or pressure that would in turn increase the energy penalty of the whole process. Therefore manipulation of the precipitating agent This article is protected by copyright. All rights reserved. presents a much more viable solution. In this sense, calcium chloride (CaCl2) is an attractive candidate towards improving PCC purity, since it presents a higher solubility in water than Ca(OH)2. The benefit of this higher solubility is that the co-precipitation of CaCl2 with PCC as it remains in the aqueous solution. The reaction between Na2CO3 and CaCl2 is defined by equation (3). 𝑁𝑎2𝐶𝑂3 (𝑎𝑞)+ 𝐶𝑎𝐶𝑙2(𝑠)→ 2𝑁𝑎𝐶𝑙(𝑎𝑞)+𝐶𝑎𝐶𝑂3 (𝑠) (3) However, the use of CaCl2 produces NaCl rather than the desired regeneration of NaOH. Since the feed of fresh solvent is crucial to maintain the economy of the process, it could be obtained from caustic waste solutions by means of Bipolar Membrane Electrodialysis (BMED). This technique has been widely studied, reporting promising results; as well as benefitting from a cost effective economic balance and positive life cycle assessment when employing renewable energy 34–38. Figure 2 represents a schema of the proposed innovative process. In this way, high-purity PCC and sodium chloride (NaCl) brine are obtained as valuable products instead of NaOH, making this a potential process for closing the CO2 capture and utilization cycle. Figure 2. Innovative proposed process for biogas upgrading and PCC production. Therefore, the purpose of this new work was to establish a physicochemical comparison for PCC production between calcium chloride (CaCl2) and/or Ca(OH)2 as precipitants to evaluate if the product obtained meets our hypothesis towards high purity PCC production. To define the physicochemical properties of both types of formed carbonates, Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray Diffraction (XRD) and Scanning electron microscopy (SEM) were employed. Should the results prove the production of high purity PCC, this process This article is protected by copyright. All rights reserved. would allow further research to focus on a potential industrial application of this innovative process. 2. Materials and Methods 2.1 Materials Chemicals used in the experiments (Ca(OH)2, Na2CO3, CaCl2, CaCO3) were provided by PanReac-AppliChem (pure-grade or pharma-grade, 99% purity, Barcelona, Spain). Table 1. Specifications of the chemicals. 2.2 Process conditions and Physicochemical characterization measurements The process description was previously addressed in our previous work 39. In brief, first the reactant solutions and equipment were prepared. After these steps, the precipitation reaction was carried out and once finished the solid products obtained were filtered and separated for analyzing. The reaction conditions were set at 50°C, 1.2 mol Ca per mol Na2CO3 and 30 minutes for reaction time. After the precipitation experiments, the solid obtained by filtration was dried at 105°C overnight in order to characterize the samples by means of FTIR, Raman, XRD and SEM. A Perkin Elmer FTIR BX spectrometer (Waltham, Massachusetts, United States) was used for performing the attenuated total reflection Fourier transform infrared spectroscopy in the powders (ATR-FTIR, uncertainty 2%). Background subtracted spectra of the raw and the treated samples were collected at room temperature by coadding 32 scans at 4 cm−1 resolution in transmittance mode. Data were baseline corrected using Spectrum 5™ software. Raman measurements of the powders samples were recorded using a Thermo DXR2 spectrometer (Waltham, This article is protected by copyright. All rights reserved. Massachusetts, United States) equipped with a Leica DMLM microscope (uncertainty 1%). The wavelength of applied excitation line was 532 nm ion laser and 50x objective of 8-mm optical was used to focus the depolarized laser beam on a sport of about 3 µm in diameter. XRD analysis was completed using an X’Pert Pro PANalytical instrument (Malvern, United Kingdom) using a step size 0.05o,450 time per step over a range of 10-90o at 40 mA and 45 kV using Cu Kα radiation (λ=0.154 nm, uncertainty 1%). A JEOL JSM6400 (Akishima, Tokyo, Japan) operated at 20 KV equipped with energy dispersive X-ray spectroscopy (EDX) and a wavelength dispersive X-ray spectroscopy (WDS) system was used for the microstructural/chemical characterization (SEM with EDS and WDS). 3. Results FTIR spectrum The first step of the physicochemical comparison was to corroborate the formation of the carbonate phase in the collected samples. For this purpose, Figure 3 represents both FTIR spectrum for PCC from Ca(OH)2 and CaCl2 as precipitant agents in comparison with a commercial CaCO3 solid sample. Figure 3. FTIR spectra for obtained samples. Typically, a strong band of CaCO3 spectra appears at around 1400-1500 cm-1 as well as a weaker peaks at 876 cm-1 and 715 cm-1 40. These are easily identified in every spectra of Figure 1, so it is possible to conclude that the carbonate phase is presented in both studied samples. According to the vibration bands at 715 cm-1 are indicative This article is protected by copyright. All rights reserved. that calcite is the predominant crystal type in the investigated CaCO3 samples, although it will be confirmed later by XRD 41. Previously, in 40, FTIR spectra for different CaCO3-Ca(OH)2 mixtures were studied, obtaining strong vibrations at around 3600 cm-1 which are characteristic of hydroxyl groups. Indeed, only when pure CaCO3 was measured, the produced spectra did not show this peak, inferring the peak at this value suggests the presence of hydroxide ions from Ca(OH)2. In the Figure 3 inset, the data corroborates that there is no presence of these hydroxides, meaning that the purity of the PCC samples obtained is relatively high. Nevertheless, is not possible by means of this technique to distinguish the complete absence of Ca(OH)2 in the represented spectrum. There may be some remnants of Ca(OH)2 in the samples that would appear also at approximately 1400-1500 cm-1, as in CaCO3 spectra 40. It is also possible to confirm that there is no CaCl2 remaining in the inset of Figure 3, as there is no peak at 3200 cm-1, characteristic of CaCl2. Table 2. FTIR vibration bands for the chemicals studied. Raman measurements To distinguish possible residual Ca(OH)2, Raman spectroscopy was employed. Furthermore, this technique was useful to corroborate a carbonate phase in the collected samples, which can be seen in Figure 4 that represents both the PCC Raman spectrum from Ca(OH)2 and CaCl2 precipitant agents in comparison with a commercial CaCO3 solid sample. Figure 4. Raman spectra for obtained samples. This article is protected by copyright. All rights reserved. CaCO3 shows a monoclinic structure that belongs to the P21/c group 42. The main characteristic band of CaCO3 polymorphs appears at around 1100 cm-1 and ca. 700 cm-1 and is typically ascribed to this monoclinic type of structure 42,43. Figure 4 shows that these two peaks are presented in both PCC spectra for PCC from Ca(OH)2 and CaCl2, confirming the successful precipitation process due to the formation of the carbonate phase. The appearance of a band at 400 cm-1 and 1086 cm-1 are characteristic of Ca(OH)2 that must be due to a small amount of Ca(OH)2 remaining present in PCC from Ca(OH)2 sample as this does not appear in the PCC from CaCl2 spectrum. This was distinguished by the Raman vibration mode at ca. 400 cm-1, which belongs to the most characteristic intense band of commercial Ca(OH)2 as confirmation of our previous hypothesis 44. The absence of this peak in PCC from CaCl2 spectra confirms a pure CaCO3 employing CaCl2 as a precipitant agent, proving that by using this innovative process, it is possible to obtain PCC with a purity of 100% by removing CO2 from biogas streams. Table 3. Raman bands for the chemicals studied. XRD analysis Once CaCO3 was confirmed as the only species for PCC from CaCl2 and predominant species for Ca(OH)2 by both FTIR and Raman measurements, XRD analysis was carried out to distinguish which crystal morph of CaCO3 has been obtained. The future use of the final CaCO3 obtained depends on the morph, so that makes the crystal structure of the final product extremely important for both precipitant agents. There are three types of CaCO3 crystal morphs: calcite, vaterite, and aragonite 45. In terms of thermodynamics, the most stable form is calcite, followed by aragonite with vaterite This article is protected by copyright. All rights reserved. Figure 4. Raman spectra for obtained samples. 300 450 600 750 900 1050 1200 1350 1500 1650 PCC from Ca(OH) 2 Raman Intensity (a.u.) PCC from CaCl 2 Commercial CaCO 3 Raman shift (cm -1 ) This article is protected by copyright. All rights reserved. Figure 5. XRD spectra for obtained samples. 10 15 20 25 30 35 40 45 50 55 60 PCC from Ca(OH) 2 Intensity (a.u.) PCC from CaCl 2 Commercial calcite 2 θ (degree) This article is protected by copyright. All rights reserved. Figure 6. SEM image for PCC from Ca(OH)2. This article is protected by copyright. All rights reserved. Figure 7. 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