Understanding the effect of Ca and Mg ions from wastes in the solvent regeneration stage of a biogas upgrading unit

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ “This is an Accepted Manuscript of an article published by Elsevier in Science of the Total Environment on 15 November 2019, available at: https://doi.org/10.1016/j.scitotenv.2019.07.135 .” 1 Understanding the effect of Ca and Mg ions from wastes in the 1 solvent regeneration stage of a biogas upgrading unit. 2 Francisco M. Baena-Moreno a,b *, Mónica Rodríguez-Galán a, T. R. Reina b, Zhien 3 Zhang c, Luis F. Vilches a, Benito Navarrete a. 4 a Departamento de Ingeniería Química y Ambiental, Escuela Técnica Superior de 5 Ingeniería, Universidad de Sevilla, C/ Camino de los Descubrimientos s/n, Sevilla 41092, 6 Spain 7 b Department of Chemical and Process Engineering, University of Surrey, GU2 7XH 8 Guildford, United Kingdom 9 c William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio 10 State University, Columbus, Ohio 43210, United States. 11 *Corresponding author. 12 E-mail address: [email protected] (Francisco M. Baena-Moreno) 13 Abstract 14 This paper reveals the effect of calcium and magnesium ions in carbonation experiments 15 carried out to regenerate sodium hydroxide from a biogas upgrading unit. This novel 16 study arises as an alternative to standard physical process whose elevated energy 17 consumption imposes economic restrictions. Previous works employed alkaline waste to 18 turn them into value added product. Nevertheless, no attractive economical results were 19 obtained due to the low regeneration efficiencies. Our hypothesis is that both calcium 20 and magnesium waste composition percentages have an impact in the result, hence this 21 work propose an isolated study aiming to determine the of each one in the global 22 performance. To this end, the operational parameters (reaction time, reaction 23 temperature and molar ratio) were tuned as well as physicochemical properties of the 24 2 final solid samples were analysed by several techniques. The results indicate that 25 calcium is much more prone than magnesium to reach high efficiencies in aqueous 26 carbonation experiments. Additionally, higher quality products were achieved with 27 calcium. The results of this study suppose an important step for understanding the 28 aqueous carbonation through waste in the path to achieve a more sustainable city and 29 society. 30 Keywords 31 Carbon Capture and Utilization; Biogas upgrading; Aqueous carbonation; Waste 32 utilization; Sustainable development 33 34 1. Introduction 35 Combating climate change has become a matter of urgency in the last decades as a 36 consequence of the Greenhouse Gases (GHG) accumulation in the atmosphere 37 (Alonso-Moreno and García-Yuste, 2016; Zhang et al., 2019). Carbon dioxide (CO2) is 38 the most important anthropogenic GHG and a plethora of studies have been done to 39 reduce the emissions of this contaminant to the atmosphere (Baena-moreno et al., 2018; 40 Cuéllar-Franca and Azapagic, 2015; Styles et al., 2016; Zhang, 2016). Furthermore, 41 renewable energies are called to play an important role in the future energy paradigm to 42 contribute with CO2 mitigation and achieve the transition towards low-carbon societies 43 (Damanabi et al., 2019; Pires, 2019). Among the different renewable energies, biogas 44 produced during biomass anaerobic digestion is a promising bioenergy resource. Biogas 45 is obtained from organic wastes and its further utilization as an energy vector fulfills the 46 spirit of a circular economy strategy (Bauer et al., 2013). Biogas is composed mainly by 47 CO2 (40%) and CH4 (60%) hence an upgrading stage (i.e. CO2 clean-up) is needed prior 48 to its utilization (González-Kunz et al., 2017; Ryckebosch et al., 2011). Several biogas 49 upgrading technologies are available to achieve 95% of CH4 (biomethane), which can 50 3 be employed as transport fuel or natural gas substitute (Bauer et al., 2013; Ruiz et al., 51 2018; Ullah Khan et al., 2017). The standard technologies for biogas upgrading rely on 52 pressure swing adsorption, high pressure water scrubbing and chemical absorption 53 scrubbing (Baena-Moreno et al., 2019; Muñoz et al., 2015; Patrizio et al., 2015; Zhang 54 et al., 2014). The later technique seems interesting when employing caustic chemicals 55 as CO2 capture solvents due to the high capture efficiency obtained (equation (1)) 56 (Baciocchi et al., 2011; Baena-Moreno et al., 2018). 57 2𝑁𝑎𝑂𝐻/𝐾𝑂𝐻(𝑎𝑞) + 𝐶𝑂2(𝑔)→ +𝑁𝑎2𝐶𝑂3/𝐾2𝐶𝑂3 + 𝐻2𝑂 (1) 58 However, using caustic solvents such as sodium hydroxide (NaOH) or potassium 59 hydroxide (KOH) causes a high energy penalty in the regeneration stage by means of 60 physical methods (Leonzio, 2016). This step is critical in the overall performance of the 61 process to make it economically viable. Previous investigations studied the solvent 62 regeneration – CO2 precipitation by chemical reaction with high Ca or Mg content wastes 63 according to equations (2) and (3), which represent the so-called aqueous carbonation 64 (Baciocchi et al., 2010; Gao et al., 2018; Morone et al., 2014). Figure 1 depicts the typical 65 diagram of this process. Additionally the final solid carbonate product can be employed 66 in many different applications such as an aggregate for civil engineering or in 67 construction, sand replacer or gravel fraction in concrete bricks or as secondary raw 68 construction material (González-Kunz et al., 2017; Wu et al., 2018). These actions 69 compile with the CO2 emissions reduction strategies adopted by the European Union 70 (Cheng et al., 2019; Li and Zhou, 2019). 71 𝑁𝑎2𝐶𝑂3 /𝐾2𝐶𝑂3(𝑎𝑞)+𝐶𝑎(𝑂𝐻)2(𝑠) → 2𝑁𝑎𝑂𝐻/𝐾𝑂𝐻(𝑎𝑞)+ 𝐶𝑎𝐶𝑂3(𝑠) (2) 72 𝑁𝑎2𝐶𝑂3 /𝐾2𝐶𝑂3(𝑎𝑞)+𝑀𝑔(𝑂𝐻)2(𝑠) → 2𝑁𝑎𝑂𝐻/𝐾𝑂𝐻(𝑎𝑞)+ 𝑀𝑔𝐶𝑂3(𝑠) (3) 73 4 74 Figure 1. Regeneration stage through aqueous carbonation with waste as precipitating 75 agent in a biogas upgrading plant. 76 In this aqueous carbonation reaction, industrial wastes have been employed as for 77 example steel slags (Eloneva et al., 2012; Said et al., 2013) and air pollution control 78 residues (Baciocchi et al., 2013b; Starr et al., 2012). A snapshot of the utilization of 79 residues in previous studies is presented in Table 1. Stainless steel slag sampled at from 80 an argon oxygen decarburization unit and waste incineration bottom ash were proposed 81 as waste materials to use in aqueous carbonation (Baciocchi et al., 2011). However, any 82 of them achieved a competitive efficiency in comparison with other technologies (15% 83 and 46% respectively). Air pollution control residues were employed previously in a pilot 84 scale plant which was thought to be promising due to its high content of calcium 85 hydroxide. Nevertheless, the results obtained did not get over 50-60% of efficiency 86 5 (Baciocchi et al., 2013a). Said et al. (2013), proposed producing precipitated calcium 87 carbonate from steelmaking slag to contribute to the reduction of CO2 emissions from 88 iron and steel industries. As the results of directly carbonation of steelmaking slag were 89 very poor, they decided to extract calcium from the slags in a preliminary step, which 90 increased the overall cost and decrease the competitiveness of the process (Said et al., 91 2013). Despite the added value of re-suing industrial waste to capture CO2, the poor 92 regeneration efficiencies exhibited by these precipitating agents, typically around 50% to 93 60%, hinders the economic viability of the process (Pan et al., 2012). As we will address 94 later, no studies were done to evaluate which chemical composition is affecting the most 95 to the overall efficiency, as we include in our manuscript. 96 Table 1. Chemical composition of waste employed for CO2 precipitation. 97 Waste type Ca-phase composition (%) Mg-phase composition (%) Efficiency (g CO2 / kg waste) Reference Steel Slag 31.7 6.0 250 (Huijgen et al., 2005) Stainless Steel Slag 45.0 3.4 130 (Baciocchi et al., 2009b) Blended Hydraulic Cement Slag 52.8 4.7 286 (Chang et al., 2011) Air Pollution Control Residues 35.0 - 250 (Baciocchi et al., 2009a) Cement Kiln Dust 34.5 2.1 191 (Huntzinger et al., 2009) Oil Shale Ash 50.8 15.2 167 (Uibu and Kuusik, 2009) 98 As can be seen in Table 1, very different efficiencies in terms of CO2 grams precipitated 99 per waste kilogram (equivalent to solvent regeneration per stoichiometry) are obtained 100 depending on the composition in Ca or Mg contained in the sample. In fact, generally it 101 seems that when the Ca content increases also the efficiency does, while a different 102 behavior is shown by Mg content. Air pollution control residues employed by Baciocchi 103 et al. (2009) shown no composition of Mg and these authors obtained one of the best 104 6 precipitation efficiencies. Oil shale ash studied by Uibu and Kuusik (2009) contained high 105 composition both of Ca and Mg. Nevertheless, the precipitation efficiency that they found 106 was not as promising as they expected. Hence, the disparity of results found in literature 107 opens up a debate on the influence of Ca and Mg on the precipitation efficiency. In 108 agreement with Table 1, it can be suspected a negative influence in the precipitation 109 efficiency in those studies with high Mg content. Nevertheless it seems that Ca content 110 can affect positively to the overall performance. In this context, a systematic study of the 111 influence of the individual cation (Mg and Ca) has been set in this work as a proposal for 112 analyzing the effect of both cations in the precipitation efficiency. The precipitation tests 113 were carried out by swinging different reaction parameters such as reaction temperature, 114 reaction time and molar ratio molar between precipitating agent and carbonate solvent 115 solution, as they have been proved to have a marked impact on the overall regeneration 116 efficiency (Baciocchi et al., 2012; Baena-Moreno et al., 2018; Baena-Moreno et al., 117 2019a). The purpose of our work was not only to characterize the regeneration efficiency 118 under the variation of the parameters mentioned above for both precipitating agents, but 119 also to provide an exhaustive physicochemical characterization of the final solid product 120 obtained. Yet, our study aims to be serve as a guide for future works in which potential 121 application could be developed via industrial waste utilization for aqueous carbonation 122 regeneration. Implementing this novel idea in biogas upgrading could be an appealing 123 strategy to synergize industrial waste recyclability with bio-methane generation as green 124 fuel thus improving the carbon fingerprint of chemical plants. 125 2. Materials and Methods 126 2.1 Materials 127 Chemical compounds used in the experiments (Ca(OH)2, Mg(OH)2, Na2CO3) were 128 provided by PanReac-AppliChem (pure-grade or pharma-grade, 99% purity). 129 2.2 Precipitation experiments 130 7 The methodology proposed for lab scale experiments as well as the necessary devices 131 for controlling the selected parameters are widely explained in this section. Both the 132 precipitating agent (Ca(OH)2, Mg(OH)2) and the carbonate aqueous solution (Na2CO3) 133 were prepared with the specifications required for each test which are collected in Table 134 2. Na2CO3 aqueous solution was selected as the carbonate source since NaOH has 135 been widely studied for several authors and composition data of the carbonate aqueous 136 solution result are available (Baciocchi et al., 2012, 2013a; Lombardi et al., 2011). 137 Moreover Na2CO3 was chosen before K2CO3 from background of our previous work 138 (Baena-Moreno et al., 2019a). As reported in the previous literature, NaOH as solvent 139 provides 90% of CO2 capture efficiencies (Lombardi et al., 2011; Mahmoudkhani and 140 Keith, 2009; Yoo et al., 2013). In our experiments, the composition of this aqueous 141 solution was set at 20 g/100 mL in agreement to these typical values expected after the 142 absorption step. The precipitating agent solution composition was varied according to 143 the molar ratio value selected for each test. Forthwith the solutions of both reactants 144 were prepared, the instruments needed for the precipitation reaction were tuned and 145 afterwards the reaction was produced. Once the reaction time selected for each 146 experiment was over, immediately the products were filtered and separated for 147 analyzing. In order to study the effect of each parameter on the precipitation efficiency, 148 standards values were selected and then varied one by one according with previous 149 studies (Baciocchi et al., 2012; Baena-Moreno et al., 2018; Baena-Moreno et al., 2019b; 150 Baena-Moreno et al., 2019c). The standard value for temperature reaction was set at 151 50°C, molar ratio at 1.2 mol Ca-Mg per mol Na2CO3 and reaction time at 30 minutes. The 152 reaction time was tested from 15 minutes to 60 minutes, the reaction temperature was 153 varied between 30°C and 70°C and the molar ratio between 1 and 1.5 mol Ca-Mg per 154 mol Na2CO3. Table 2 collects the main data selected for each test. The experiments were 155 conducted in a 600 mL beaker placed in a water bath to control temperature test which 156 was measured by a Trison Instrument and continuously recorded in a data logger. During 157 each whole experiment time the solution was stirred by an electromagnetic magnet at a 158 8 constant speed of 1000 rpm. Reproducibility was checked by repeating every test and 159 analysis at least twice, resulting in an overall experimental error of ±2%. 160 Table 2. Matrix of the experiments carried out. 161 Test Precipitating agent Time (min) Temperature (ºC) Molar ratio 1 Ca(OH)2 30 50 1.2 2 Ca(OH)2 30 30 1.2 3 Ca(OH)2 30 70 1.2 4 Ca(OH)2 30 50 1 5 Ca(OH)2 30 50 1.5 6 Ca(OH)2 15 50 1.2 7 Ca(OH)2 60 50 1.2 8 Mg(OH)2 30 50 1.2 9 Mg(OH)2 30 30 1.2 10 Mg(OH)2 30 70 1.2 11 Mg(OH)2 30 50 1 12 Mg(OH)2 30 50 1.5 13 Mg(OH)2 15 50 1.2 14 Mg(OH)2 60 50 1.2 162 2.3 Physicochemical analysis 163 The solid obtained by filtration was dried at 105°C, and to study its physicochemical 164 properties Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray 165 Powder Diffraction (XRD) and Scanning Electron Microscopy (SEM) were employed. The 166 final solution was measured in order to obtain data about the hydroxide composition by 167 means of inductively coupled plasma atomic emission spectroscopy. 168 A Perkin Elmer FTIR BX spectrometer was used for performing the attenuated total 169 reflection Fourier transform infrared spectroscopy in the powders (ATR-FTIR). 170 Background subtracted spectra of the raw and the treated samples were collected at 171 room temperature by co-adding 32 scans at 4 cm−1 resolution in transmittance mode. 172 Data were baseline corrected using Spectrum 5™ software. Raman measurements of 173 the powders samples were recorded using a Thermo DXR2 spectrometer equipped with 174 a Leica DMLM microscope. The wavelength of applied excitation line was 532 nm ion 175 15 272 Figure 7. Temperature effect on solvent regeneration for both precipitating agents. 273 Experiments carried out at 1.2 molar ratio and 30 min. 274 275 Temperature influence seems to have a higher impact in the reaction with Ca(OH)2 as 276 the precipitating agent. Not only the differences between regeneration efficiencies are 277 considerable at room temperature but also this difference is lofty when increasing the 278 temperature. In fact, in the best case scenario at 70oC this value reached 63% of 279 regeneration efficiency with Mg(OH)2 as precipitating agent whereas 68% of 280 regeneration efficiency can be obtained with Ca(OH)2 at 30oC. This means that the 281 differences existing between both precipitating agents cannot be overcome by increasing 282 the reaction temperature. Furthermore, the employment of Ca(OH)2 slurry as 283 precipitating agent resulted in a really high NaOH recovery at 70oC (97%), which means 284 that is possible to close the proposed process with energy savings in comparison with 285 the traditional physical methods. Indeed, an acceptable regeneration efficiency is 286 obtained at 50°C, which should be highlighted from an energy consumption perspective, 287 as this temperature could be easily achieved through renewable energy. Overall, it can 288 30 40 50 60 70 45 50 55 60 65 70 75 80 85 90 95 100 With Ca(OH)2 With Mg(OH)2 Solvent regeneration efficiency (%) Temperature (°C) 16 be concluded from Figure 7 that Ca seems to play a more important role than Mg in 289 carbonation reactions as previously commented. Nevertheless, these big differences 290 between the products from both precipitating agents might be offset by increasing the 291 reaction time or the molar ratio. 292 293 Figure 8. Molar ratio effect on solvent regeneration for both precipitating agents. 294 Experiments carried out at 50oC and 30 min. 295 Regarding the effect of molar ratio on solvent regeneration Figure 8 reveals that it is 296 lightly higher in the product obtained from Mg than from Ca. Nevertheless, the overall 297 impact from 1.2 molar ratio to 1.5 molar ratio reached only a 6% of improvement in the 298 regeneration efficiency. Furthermore, the addition of a bigger amount of precipitating 299 agent would suppose a higher quantity of Mg/Ca ions which need to be removed from 300 the resulting solution before recirculating to the beginning of the process. This would fall 301 into a higher operational costs or in fouling phenomenon in the packed tower in case of 302 not removing the ions. The differences between the regeneration efficiencies obtained 303 with Ca(OH)2 and Mg(OH)2 as precipitating agents are almost 30%. This result further 304 confirms our previous hypothesis and suggest that future researches should be focused 305 on finding waste with high Ca content. 306 1.0 1.1 1.2 1.3 1.4 1.5 45 50 55 60 65 70 75 80 85 90 95 100 Molar ratio Solvent regeneration efficiency (%) With Ca(OH)2 With Mg(OH)2 17 307 Figure 9. Solvent regeneration with time for both precipitating agents. Experiments 308 carried out at 50oC and 1.2 molar ratio. 309 As for the evolution of the solvent regeneration with the reaction time, Figure 9 shows 310 that very similar behave are presented by both precipitating agents from 15 min to 30 311 min, while on the contrary the differences get higher from 30 min to 60 min. This means 312 than a higher reaction time cannot overcome the lack of efficiency provoked by the 313 employ of Mg(OH)2 as precipitating agent, which can be translated to the same effect in 314 residues with high Mg content. Moreover this would mean in a hypothetical industrial 315 reactor a duplication of its volume to achieve worse regeneration efficiencies than 316 employing a high Ca content waste. The fact that generally Mg(OH)2 get lower 317 regeneration efficiencies is a phenomenon which need some explanation. This fact is 318 probably owing to the dehydroxilation phenomenon which occurs in Mg(OH)2 and does 319 not take place in Ca(OH)2 carbonation reaction because of the different solubility of both 320 species (Béarat et al., 2004; Zevenhoven and Teir, 2004). Dehydroxilation phenomenon 321 of Mg(OH)2 consist on the liberation of hydroxide agents (water release) to produce MgO 322 and therefore to react with CO2 to produce MgCO3. It seems that in the present of an 323 aqueous solution, this phenomenon is diminished. Nevertheless, at the temperature and 324 10 20 30 40 50 60 45 50 55 60 65 70 75 80 85 90 95 100 Solvent regeneration efficiency (%) Time (min) With Ca(OH)2 With Mg(OH)2 18 pressure conditions tested in this work, the phenomenon cannot be completely 325 overcome. The increase of temperature – pressure conditions will lead to higher 326 operational cost in a hypothetic industrial application, which will result in less 327 competitiveness of this technique. Therefore and in agreement with the results presented 328 in this study, Ca(OH)2 should be employed instead Mg(OH)2 to overthrow this 329 phenomenon. 330 4. Conclusions and future remarks 331 The results obtained from this lab scale work confirm our previous hypothesis of the 332 effect of Ca and Mg ions on the regeneration efficiency in a biogas upgrading plant. In 333 general, the high content of Ca is reflected in better precipitation – regeneration 334 efficiencies, whereas the presence of Mg seems to be less effective. This statement was 335 confirmed by both physicochemical characterization methods and solvent regeneration 336 tests. The physicochemical characterization of the solid samples obtained was analyzed 337 by means of FTIR, Raman, XRD and SEM. The results revealed polymorph calcite in the 338 case of the product from Ca(OH)2 as precipitating agent and a mixture between 339 unreacted Mg(OH)2 and MgCO3 in the case of the second precipitating agent tested. As 340 in the regeneration experiments carried out in this paper, noticeable results were 341 obtained when employing Ca(OH)2 as precipitating agent. In fact, high regeneration 342 efficiency (97%) was reached with 70ºC as reaction temperature. Nevertheless, 343 appreciable differences were observed with Mg(OH)2 as precipitating agent under the 344 same operational conditions. Indeed, in our work the variation of the reaction parameters 345 did not show to improve significantly the results obtained with Mg species. 346 Overall, this paper proposes an innovative study to serve as a guide for future works in 347 the field of aqueous carbonation experiments. Future work in this area should focus on 348 finding unstudied waste following the insights provided in this paper. Furthermore, this 349 paves the way for further by-product valorization in order to achieve more economical 350 19 and sustainable biogas upgrading plants. 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