Renewable aqueous ammonia from biogas slurry for carbon capture: chemical composition and CO₂ absorption rate

Renewable aqueous ammonia (RAA) can be recovered from biogas slurry by vacuum membrane distillation (VMD). Bio-natural gas can be produced from biogas after removing its CO₂ using RAA as the CO₂ absorbent. However, RAA contains some impurities, e.g. organic carbon (mainly the volatile fatty acids, VFAs) and inorganic carbon (mainly the dissolved CO₂) that may influence its CO₂ absorption performance. In this study, RAA was firstly recovered by VMD to investigate the concentrations of free ammonia and impurities in RAA changed with the initial total ammonia nitrogen (TAN) concentration. Then CO₂ absorption performance of RAA was investigated in a wetted-wall column, and compared with the unloaded aqueous ammonia (AA). Through VMD, higher TAN concentration of RAA up to 2.4 mol-N/L can be achieved. Due to higher pH value (10.7–11.5), higher than 99% of N element is formed as free ammonia. Additionally, CO₂ and some VFAs mainly including ethanol, acetic acid, propionic acid and butyric acid can be extracted from biogas slurry and transferred into RAA. However, their concentrations are much lower than free ammonia in RAA. Furthermore, at 2 mol-N/L, the second-order reaction constant (k₂) of RAA is about 4306 L/mol s at 298 K, which is slightly lower than that of AA (4772 L/mol s) due to the negative effect of VFAs. The average overall mass transfer coefficients of RAA are lower by 4.67%–17.31% than that of unloaded aqueous ammonia. Furthermore, increasing the concentration of VFAs and CO₂ loading in RAA leads to a rapid decline of the overall mass transfer coefficient of CO₂. Butyric acid has the greatest negative impact on CO₂ absorption, followed by acetic acid, ethanol and then CO₂. However, the increase of TAN concentration in RAA can minimize the negative influence of impurities on CO₂ absorption performance.