Background and challenges of ammonia based CO2 capture

https://doi.org/10.1016/j.jcou.2022.102085

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Compared with the ethanolamine techniques, the ammonia solution approaches are characterized by a higher CO₂ capture capacity, lower price, and non-degradation in the presence of O₂ and SO₂ [20]. For example, the absorption capacity of ammonia is 1.2 kg CO₂/kg NH₃, while in the case of MEA it is 0.4 kg CO₂/kg MEA [20]. Moreover, ammonia solution have the potential to capture many acid gases besides CO₂ present in exhaust gases, i.e., SO_x and NO_x. More importantly, processes using NH₃ solutions have low energy demand [21], [22]. Despite the many advantages, the biggest problem with using NH₃ for CO₂ absorption is its high volatility [23]. The intrinsically high volatility of ammonia causes its escape from the aqueous solution, lowering its concentration in the solution and thus reducing the efficiency of the CO₂ capture process. Consequently, this increases the amount of NH₃ needed to absorb a given amount of CO₂ [23].

Many methods have been proposed to prevent the escape of ammonia from the solution. One of them is the addition of the so-called ammonia escape inhibitors to the solution [24], [25], which is a promising solution for capturing CO₂ in the CaCO₃ carbonation process. Our previous research compared organic and inorganic ammonia escape inhibitors and determined their influence on the characteristics of the obtained calcium carbonate [26]. None of the tested inorganic inhibitors (e.g., ZnCl₂, CoCl₂, and CuCl₂) was found to show any improvement in CO₂ sequestration. Moreover, the precipitated CaCO₃ particles often contained colored ammonia complexes of the metals used when inorganic ammonia escape inhibitors were added to the solution [26]. Another important disadvantage of applying inorganic inhibitors is the potential precipitation of hydroxide or carbonate of the added divalent cations in an alkaline solution [23]. Thus, it is obvious that organic substances are more suitable for this type of application. One such group of organic inhibitors are alcohols and polyols due to the presence of hydroxyl groups in their molecules capable of forming hydrogen bonds with ammonia molecules in an aqueous solution [27]. There are reports in the literature on the use of ethanol [28], [29], glycerol [30], [31], and ethylene glycol [30], [32] as ammonia escape inhibitors for the purpose for CO₂ capture. Furthermore, the alcohol-water mixtures are popular additives in the CaCO₃ precipitation process, because the addition of alcohols may have a chemical effect on the aqueous solution by decreasing the solubility of CaCO₃ and thus increasing its supersaturation tendency [33]. The use of a small (several percent) addition of alcohols or polyols also has advantages from an economic point of view. Due to the low prices of this type of solvents (e.g. ethylene glycol 1.5 $/kg, glycerol 1 $/kg, methanol and isopropanol 3–5 $/kg), such a solution practically does not increase the cost of the process. Moreover, the use of expensive equipment is not required, and the introduction of such a solution allows the use of existing equipment without costly modifications. Finally, while the addition of an alcohol or polyol greatly improves the selectivity of the absorbent, it thereby reduces the amount of solvent circulation and energy consumption, and then lowers the operating costs [23].

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