Background of ammonia based CO2 absorption

https://doi.org/10.1016/j.seppur.2021.118959

“As a consequence of the high volatility of NH₃, NH₃-based CO₂ capture processes are usually criticized in the literature due to their complexity [6], [7], [17], [22]. Indeed, they require specific units not only related to the NH₃ abatement from the CO₂-depleted FG, but also associated with the removal of NH₃ from the CO₂ gas stream and with the minimization of aqueous NH₃ solution make-up. As a consequence of the FG post-treatment, the temperature of the FG exiting the NH₃ absorber is generally lower than the temperature of the FG entering the CO₂ absorber. Therefore, a purge stream is required to avoid water building up within the solvent circulating in the CO₂ capture loop. NH₃ and CO₂ contained in the purge stream are recuperated using thermal energy in a solvent recovery section and recycled to the capture process in order to minimize the CO₂ losses and the NH₃ make-up [14]. In any case, a small NH₃ make-up is always required to maintain the NH₃ concentration in the solvent circulating in the CO₂ capture loop in order to counteract the NH₃ losses in the vented FG and in the CO₂ stream. Such NH₃ make-up is introduced by means of a fresh aqueous NH₃ solution whose water content also affects the water balance in the process. Therefore, the solvent recovery section is not only required in the CAP, where low temperature is used as one of the strategies for NH₃ abatement [14], but also in mild-temperature NH₃-based capture processes, specially if the temperature of the gas stream is not controlled properly. An example of the latter situation is the advanced, NH₃-based combined NO_x/SO_x/CO₂ emission control process mentioned above [20], where the temperature of the vented gas sent for NH₃ removal is lower than the temperature of the FG entering the absorber. Another example is the mild-temperature NH₃-based capture process developed at the Tunghai University, where the water accumulation in the solvent can be calculated from mass balances around the battery limits of the capture process using the data of the reported stream tables [23], [24]. Therefore, even mild-temperature NH₃-based capture processes require to purge a fraction of liquid solvent, thus should include a process section for NH₃ (and CO₂) recuperation in order to achieve stable process operation and to minimize the NH₃ make-up. Alternatively, the aforementioned CSIRO’s mild-temperature NH₃-based capture process [2] includes a water recovery section that does not only consist of a solvent recovery unit, but also includes a cooling tower. The cooling tower decreases the temperature of the FG that enters the CO₂ absorber to values similar to those of the FG exiting the CO₂ capture section in order to avoid the accumulation of water in the solvent.”