Innovative methods with low energy consumption for efficient CO2 capture and its re-use as a building block for the synthesis of useful chemicals

The purpose of this research activity has been the development of new and efficient systems for the capture of CO2 from a gas stream in a sustainable way from an energetic, economic and environmental point of view. The chemical absorption by aqueous alkanolamines is considered the most efficient and mature technique for the CO2 capture and separation. Alkanolamines are widely used due to the fast reaction with CO2 and to their solubility in water. In particular aqueous 2-amine ethanol (MEA) has a long story as efficient systems for CO2 separation in ammonia and hydrogen plants, natural gas extraction and gas refinery. Recently, these aqueous sorbents have been also studied for application on CO2 removing from industrial exhaust streams. However, the high operating costs associated to the thermal regeneration of the sorbents because of the particularly high evaporation enthalpy and heat capacity of water, are the major obstacles to extensive application to large scale commercial plants. In addition, the higher is the desorption temperature, the greater are the amine decomposition and evaporation, as well as equipment corrosion, thus increasing the maintenance costs of the CCS process. With the aim of taking advantages of the high efficiency of conventional aqueous alkanolamines yet reducing their disadvantages, we have been pursuing a lab-scale research on alternative absorbent formulations aimed at reducing the energy of the absorbent regeneration and the amine degradation, yet maximising the CO2 capture. With the objective of reducing the energy required by the desorption process, in this thesis has been devised the strategy to avoid water. The replacement of water by organic solvents, or the absence of any solvents, may redirect the reaction of amines with CO2 towards less stable species which, consequently, require lower stripping temperatures at room pressure. The study has been focused on amines which combine high reaction rate with CO2 in a non-aqueous environment, with lower reaction enthalpy (in absolute value) and therefore these absorbents require lower regeneration temperatures compared to those of the aqueous solutions (75-95 °C at room pressure instead of 120-140 °C under pressure). Furthermore, the lower operating temperatures reduce the decomposition rate and the loss of the amines. As additional, not negligible benefit, the absence of water strongly reduces the equipment corrosion and avoids foaming problems. A first stage of this study has been devoted to solutions of alkanolamines in organic solvents. Replacing water with organic solvents may provide significant advantages in regard to the reduced absorbent decomposition and to the energy saving in the regeneration step due to the lower heat capacity (about half), the lower heat of vaporization of organic solvents and the higher boiling temperature compared to water. Furthermore, the use of organic solvents does not require major changes to the equipment which works with aqueous solutions. The sorbents formulated comprised some single or blended alkanolamines and alcohol mixtures containing ethylene glycol. 13C NMR analysis indicates that CO2 is reversibly captured in solution as either monoalkyl carbonate (R-OCO2−) and amine carbamate. Due to the lower stability of monoalkyl carbonates with respect to HCO3− and to the carbamates which are formed in the aqueous solutions, stripping temperatures of 75–90°C at room pressure are sufficient to attain absorption efficiencies greater than 90%. Between different formulation designed, one of the best performing was tested on the pilot plant of the ENEL coal-fired power plant located in Brindisi. Even if the replacement of water by organic solvents may reduce the costs of the stripping process, a lot of energy is wasted by the organic solvent heating from the absorption to the desorption temperatures. Moreover, the solvent, either water or alcohols account for about 70% of the absorbent and therefore requires very large equipments. The avoidance of any solvent, should be a decisive improvement towards the ideal absorbent. Therefore, the study has been focused on amines capable of absorbing CO2 without any dilution provided they are liquid before and after the carbon dioxide uptake. A technique of reversible CO2 capture that does not require absorbent dilution would have beneficial effects over those based on diluted absorbents, in that it avoids the sensible heat and vaporization heat of the solvent that contribute to the overall reboiler duty, as long as the operational conditions and the efficiency of the two techniques were comparable. Further potential benefits would be the reduced mass of the absorbent (water accounts for 70 wt % of the mass of aqueous MEA) and, consequently, an appreciable reduction of the plant size. In this study has been formulated two different classes of solvent-free single-component absorbents based on inexpensive and commercially available amine, in particular some secondary amines and some secondary alkanolamines. CO2 is captured with high efficiency (over 90%) as amine carbamate and amine-carbamic acid. In another part of the study has been devised and developed the use of biphasic sorbents. In such technique, the absorbent solution, after the reaction with CO2, separates into two phases (liquid/liquid or liquid/solid) that comprise the solvent and, separately, the carbonatated species. In this way it is possible to desorb the sole carbonatated phase, preventing the energy wasted by solvent heating. After its thermal regeneration, the absorbent is mixed again with the solvent to get back the starting absorbent solution. This biphasic technique combines the advantages of both organic solvents and solvent–free sorbents previously studied. A first series of experiments, which involves sodium and potassium carbonates and some amines (piperazine and AMP), has been completed with good results. Another series of tests very promising, using alkanolamines in low viscous solvents, is still under development. In these three years, in our laboratory, has been also developed a new concept of CO2 capture technology which combines the CO2 abatement with the production of commercially valuable products. Turning carbon dioxide into useful chemicals in relatively mild conditions circumvent most of the drawbacks of the energy consuming steps of CO2 desorption, absorbent regeneration as well as of CO2 transporting and ultimate disposal. It has been verified the efficient absorption of CO2 by water-ethanol ammonia and by some non-aqueous amines. These experiments were addressed to recover the captured CO2 as solid ammonium carbamate or carbamates of the protonated amines. By heating the solid ammonium carbonate and bicarbonate mixtures or the amine carbamates in a closed reactor, we obtained their conversion into urea and, respectively, 1,3-disubstituted ureas with reasonable yields (30-50%). Urea is the world's most used fertilizer and it is produced in large quantities, while 1,3-disubstituted ureas are valuable products with a wide range of application as intermediates in agrochemical, pharmaceutical, dye chemicals and raw materials of polyurethanes. In addition, it has been studied the chemistry of the CO2 uptake by resorcinol (1,3-dihydroxy benzene) in alkaline aqueous and water/glycerol solutions under different experimental conditions, with the purpose of unveiling the reaction mechanism and maximizing the resorcinol conversion into β-resorcylic acid (2,4-dihydroxybenzoic acid).