Effect of temperature and CO2 ratio of stripping biogas on biogas slurry stripping of ammonia

Abstract: Ammonia stripping is an effective treatment technology for recovery of nitrogen from the anaerobic digestate (biogas slurry). Biogas stripping of ammonia from anaerobic digestate can realize biogas purification and ammonia removal coupling through recycled of biogas, which is a new research direction of ammonia stripping. However, the main difficulty is that the ammonia stripping performance is negatively correlated to the CO2 ratio in the stripping gas. In this study, the effect of temperature (70、80、90 ℃) and the ratio of CO2 (10%、20%、40%) in biogas on the removal efficiency of ammonia were studied. The ammonia removal rate (η) and kinetics constant (k) were adopted to evaluate the ammonia removal performance. The results indicated that the ammonia removal rate from the biogas slurry were 96.81%、98.65%、99.28%, respectively at 70 ℃, 80 ℃ and 90 ℃when the ratio of CO2 in the stripping gas was 10%, and ammonia concentration was lower than 60 mg/L after biogas slurry stripping. The ammonia removal rates were 72.81%、84.00% and 88.39%, respectively at 70 ℃, 80 ℃ and 90 ℃ when the ratio of CO2 in stripping gas was 20%. While CO2 ratio in stripping gas reached 40%, the ammonia removal rate were 62.94%、75.23% and 83.29%, respectively at different stripping temperatures. As the temperature of stripping gas increase, the ammonia nitrogen rate increased, however as the ratio of CO2 in the stripping gas increase, the ammonia removal rate decreased. Increase of temperature is beneficial to reduce inhibitory effect of CO2 on ammonia removal efficiency in biogas. And the ammonia removal rate of biogas slurry reached 90% within 40 min at 90 ℃ while the maximum removal rate of ammonia were close to 2 h and 3 h at 80 ℃ and 70 ℃, respectively. Ammonia stripping can raised pH value to above 8.6 relying on the removal of acidic substances in the biogas slurry by mixed gas of CH4 and CO2. Theoretical maximum ammonia removal rate in biogas slurry can be calculated according to the maximum pH value in biogas slurry during the ammonia stripping process. And the result showed that when the temperature was 90 ℃, even the ratio of CO2 in the stripping gas was 40%, and the removal time can be extended to make the ammonia removal rate in the biogas slurry reach 92.15%. At 80 ℃ and 70 ℃, when ratio of CO2 in stripping gas was 10%, the maximum ammonia removal rate can reach 90% by extending stripping time. The calculation result of kinetic constant k indicates when the CO2 ratio was 40%, the k value of the ammonia removal reaction of the biogas slurry at different temperatures was more than 0.075 h(1. The water quality analysis showed that the COD removal rate reached 23.53%-43.2%, a total phosphorus removal rate reached 15.85%-32.97%, and a turbidity removal rate reached 20.79%-29.74%. The results provide a reference for the use of biogas on biogas slurry stripping of ammonia.