CO₂ absorption using biogas slurry: recovery of absorption performance through CO₂ vacuum regeneration

As a by-product of anaerobic biogas digestion using biomass as the substrates, raw biogas slurry (RBS) may be a renewable CO₂ solvent to capture CO₂ and fix CO₂ into the crops/plants by forming organic carbon through carbon concentration mechanism (CCM). However its CO₂ absorption performance should be enhanced greatly without deteriorating its inherent low phytotoxicity. Recovery and enhancement of CO₂ absorption performance of RBS with initial total ammonia nitrogen (TAN) concentration (∼0.152 mol-N/L) was investigated through stripping CO₂ saturated in RBS during anaerobic digestion by vacuum regeneration technology. Additionally, phytotoxicity of the regenerated RBS (RegBS) after CO₂ reabsorption was also evaluated in terms of EC₅₀ value which represents the CO₂-rich RegBS application concentration causing 50% inhibition to root elongation of mungbean seeds. Results showed that vacuum regeneration technology is a time-effective method to regenerate CO₂ from RBS. And the activation energy of CO₂ regeneration from RBS under 20 kPa and 35–75 °C is about 57.78 kJ/mol. The main mechanism of CO₂ regeneration from RBS may be the decomposition of ammonium bicarbonate. CO₂ reabsorption capacity of RegBS is enhanced up to 0.125 mol/L at 77 °C and 40 kPa, which is approximately 3.18-fold higher than that of RBS. If RBS with high TAN concentration can be adopted, much higher CO₂ reabsorption capacity may be achieved. CO₂-rich RegBS can obtain the higher EC₅₀ value than RBS, indicating higher application concentrations in agriculture/horticulture, and hence lower phytotoxicities to crops and plants. The maximum EC₅₀ value with 168.8 mL/L can be achieved when using the CO₂-rich RegBS obtained at 47 °C and 10 kPa. 77 °C and 40 kPa may be the optimum conditions to recover CO₂ absorption performance of RBS in terms of higher CO₂ reabsorption performance and lower phytoxicity of CO₂-rich RegBS.