https://doi.org/10.2516/ogst/2013160
“Table 2 lists the solvent regeneration energies reported in the literature. The regeneration energies refer to the heat
requirement for regeneration of solvents (reboiler duty) in the stripper. The values are quite different and vary from less than 1 000 KJ/kg CO2 to more than 4 200 KJ/kg CO2. The significant discrepancy is due to under-estimation of heat of desorption in the previous studies and the large variation in operational conditions in particular the solvent concentrations. During the solvent regeneration, heat is required for:
– CO2 desorption reaction;
– steam regeneration;
– sensible heat.
Due to the different process conditions, the contribution of the above three parts of energy is various, resulting in the various heat requirements in the CO2 capture process. During the stripper process, CO2 can be desorbed via many pathways including the following overall reactions (Jilvero et al., 2012).”
“The previous research reported a low regeneration energy requirement based on the assumption that the regeneration only involves the decompose reaction of ammonia bicarbonate to CO2 and ammonia carbonate (R1) and the heat of reaction for this reaction is low (DH = 26.88 kJ/mol). The equilibrium based modeling suggested that one cannot single out only one particular pathway for the solvent regeneration. The experimental and modeling investigations show that the heat of absorption is a function of CO2 loading and a weak function of temperature and ammonia concentration.
Figure 7 shows the experimental and predicted heat of absorption of CO2 in 5% ammonia solution at the temperature of 40C (Qi et al., 2013). The predicted results were obtained using the thermodynamic model for CO2-NH3-H2O developed by Aspentech (Que and Chen, 2011). Over the large CO2 loading range (0-0.6), the heat of absorption is more than 60 kJ/mol, suggesting that the overall absorption/decomposition pathway is mainly via the formation of ammonium bicarbonate (R2) and carbamate (R3) which is consistent with the conclusion drawn by Jilvero et al. (2012). Through the thermodynamic analysis of the speciation distribution
in both lean and rich solvent at NH3 concentrations 5 and 15% and lean loading of 0.2 to 0.5, it has been found that a large proportion of the CO2 is absorbed through reactions with free ammonia. As shown in Figure 8, for lean and rich solvents, there are negligible differences with respect to the fractions of carbamate and the carbonate, while the level of free ammonia is decreasing and the amount of bicarbonate is increasing. This suggests that the main reaction pathway for the absorption of CO2 was identified as the formation of bicarbonate through the reaction of ammonia with CO2. In other words, the decomposition of bicarbonate to ammonia and CO2 is the main reaction for the CO2 desorption.
The heat of reaction required to desorb CO2 of ammonia was similar to that required for MonoEthanolAmine (MEA). The simulation results show that the heat requirement of ammonia regeneration could be less than 2 500 kJ/kg CO2 captured. (Jilvero et al., 2012). The main reason for the lower heat requirement for ammonia compared to MEA is that the thermodynamic properties of the NH3-CO2-H2O system allow for pressurized regeneration, which results in significantly lower water vaporization.
All investigations based on equilibrium-based thermodynamic process modeling are in agreement regarding the potential to achieve a regeneration heat requirement that ranges between 2 000 and 3 000 kJ/kg CO2 for an aqueous ammonia process. The recent publication from Alstom shows a heat requirement of 2 200 KJ/kg CO2 captured for the chilled ammonia process (Jo¨nsson and Telikapalli, 2012). This result is in line with the modeling prediction.
These recent studies show that the aqueous ammonia based CO2 capture process requires much less heat for the solvent regeneration than MEA based capture processes (Jo¨nsson and Telikapalli, 2012; Jilvero et al., 2012; Valenti et al., 2012; Darde et al., 2012). The pilot plant trials conducted by CSIRO and Delta Electricity at Delta Electricity’s Munmorah power station showed a much higher heat requirement (Yu et al., 2011b). One reason for the high heat requirement is that a diluted ammonia solution was used and the amount of CO2 captured/released is low. As a result, more than 50% of energy is used to heat up the solvent (sensible heat). The results also show the heat of desorption is around 70-80 kJ/mol, suggesting under the pilot plant conditions where CO2 loading in stripper varied between 0.4 and 0.1, the major pathway is the formation of carbamate via the reaction of CO2 with free ammonia.”