Background related to heat of absorption using ammonia based CO2 capture

https://doi.org/10.1039/C9RA00164F

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In view of the fact that ammonia escape appears to be the greatest concern to the industry, the chilled ammonia process (CAP) has being developed to address this problem.²² In a CAP process, CO₂ is absorbed at low temperatures in the range of 2–20 °C to minimize the volatilization of ammonia. The CO₂-enriched solution is then regenerated at 100–150 °C and 2–136 atm. Bak et al.²³ pointed out that, when the absorber operated at a feed gas temperature of 10 °C and lean solution at a temperature of 7 °C, the CO₂ absorption efficiency could reach more than 85% with ammonia loss less than 8%.

However, there is limited information on the contribution of each individual reaction occurring during CO₂ absorption by NH₃ to the overall heat of CO₂ absorption in CAP. In addition, conditions for the formation of solid ammonium bicarbonate, NH₄HCO₃(s), must be well understood. Since the temperatures in CAP are low in general, solid may precipitate in the absorber. Yu et al. analyzed the solid composition in the absorber by XRD, the result suggested that the pilot plant samples were predominantly NH₄HCO₃(s).²⁴ Besides, Diao et al. studied the crystalline solids by FT-IR analysis, the FT-IR patterns of the crystalline solids were compared to standard ammonium bicarbonate powders. They found that ammonium bicarbonate was the main product.²⁵ NH₄HCO₃(s) formation would dramatically change the heat of CO₂ absorption of the NH₃–CO₂–H₂O system, because of the exothermic property of NH₄HCO₃(s) formation.²⁶ The heat of CO₂ absorption is an important thermodynamic property, as a higher heat of CO₂ absorption means more energy required in solvent regeneration. The detailed thermodynamic analysis for the contribution of each individual reaction to the overall heat of absorption is one of the key ways to clarify the reaction mechanism and process optimization. According to the exothermic/endothermic characteristics of each individual reaction, the operating parameters such as CO₂ loading and temperature, can be adjusted to optimize system energy consumption. Therefore, some researchers studied the heat of absorption for each individual reaction in amine-based capture system²⁷ and ammonia-based system,²⁸ but temperatures ranged from 40 to 80 °C, which were much higher than those encountered in CAP; in addition, at those higher temperatures solid precipitation was not observed and not considered an issue. Energy consumption in CAP has been evaluated by thermodynamic models,^29,30 but they all focused on the whole process rather than analyzed the heat change caused by each individual chemical reaction in the absorber. Although Jilvero et al.³¹ and Kurz et al.³² reported phase equilibrium experimental data for the NH₃–CO₂–H₂O system at temperatures in the range 10–80 °C, the effect of solid formation on heat of absorption was not reported in their studies. The contribution of each individual reaction to the overall heat of CO₂ absorption in CAP is a gap, which is very important to understand the absorption mechanism and control the system absorption heat. The various contributions can be controlled by adjusting the operation parameters, such as CO₂ loading and temperature, to optimize overall heat of absorption.

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