Effect Of Exogenous Absorbents Addition On CO₂ Absorption Performance And Ecotoxicity Of Biogas Slurry

After removal of CO₂ and other impurities from biogas,biogas can be upgraded to the so-called bio-natural gas in which CH4 concentration is equal to that in natural gas.Clearly,bio-natural gas is a renewable and high-grade energy,which can be used to alleviate the gap between natural gas production and requirement in China if large-scale applications.Additionally,bio-natural gas can achieve near-zero emissions of CO₂ during energy utilization.One of the key problems to produce bio-natural gas through biogas upgrading is CO₂ separation from biogas with low cost.Among so many kinds of CO₂ separation technologies from biogas,CO₂ chemical absorption method has the advantages of high CO₂ separation efficiency,low CH4 loss,high CH4 purity and easy operation,making it as one of the most promosing in the near future.The main issue of CO₂ chemical absorption from biogas is its high separation cost,especially the high CO₂ regeneration heat requirement accounting for more than 60% of the total energy consumption.Therefore,the key to reduce the CO₂ separation cost from biogas is to reduce the regenerative energy consumption of CO₂-rich absorbent solution.It is possible to reduce the cost of chemical absorption greatly by only retaining the CO₂ absorption?non-recycling process?stage,but it may meet the problem of large consumption of CO₂ chemical absorbent.Therefore,the screening of a suitable absorbent is the key to its successful application.As a byproduct of biogas production from biomass anaerobic fermentation,biogas slurry is weakly alkaline and rich in beneficial ingredients for stimulating plant growth,and theoretically it can be used as a renewable absorbent for CO₂ absorption and biofixation,which basically meet the requirements of CO₂ absorption non-recycling process.Its primary problem is determining how to improve the CO₂ absorption performance of biogas slurry.The addition of exogenous CO₂ absorbent may help enhance the CO₂ absorption performance of biogas slurry,but also affect the phytotoxicity of biogas slurry.Based on these assumptions,the effects of six typical exogenous CO₂ absorbents on the CO₂ uptake and ecotoxicity of biogas slurry were investigated in this study.The effects of concentrating biogas slurry and the amount of exogenous absorbent on CO₂ absorption performance of biogas slurry and agricultural application potential of CO₂-rich biogas slurry in terms of phytotoxicity and total phosphorus content were considered as well.Finally,the feasibility of the real agricultural applications of biogas slurry was studied through the hydrophonic lettuce in biogas slurry.The results showed that:?1?Compared to the raw biogas slurry,CO₂ uptake of biogas slurry increases by about 71.4% when it was concentrated by about 5 times.In contrast,CO₂ absorption rate decreases due to the increase of viscosity after concentrating the biogas slurry.The removal rate of ammonia nitrogen in the 5 times concentrated biogas slurry was 87.69%.The phosphorus concentration in the biogas slurry decreases when biogas slurry was concentrated or exogenous CO₂ absorbents were added,which significantly reduces the environmental hazards of biogas slurry applications.?2?Compared to the direct addition of absorbent with low concentration into the raw biogas slurry,CO₂ uptake per volume of biogas slurry greatly increases by adding a high concentration of absorbent into the concentrated biogas slurry.CO₂ loading of the 5 times concentrated biogas slurry after adding monoethanolamine?MEA?at 35? reaches about 0.40 mol/L,which is 14.8 times higher than that of 5 times concentrated biogas slurry without adding any absorbents.And biogas slurry can be CO₂ saturated after 10 min absorption.Adding potassium L-Argininate?PA?with 0.5mol/L into the 5 times concentrated biogas slurry has the highest CO₂ loading with about 0.68mol/L.?3?Although PA has some certain advantages over high absorption loading,the phytotoxicity of amino acid absorbents is much higher than that of alkanolamines.If CO₂-rich biogas slurries are used to germinate the Chinese cabbage seeds,the concentration of PA in the diluted biogas slurry is suggested to less than 0.035 mol/L.The concentration of MEA adding into biogas slurry is required to not higher than 0.1 mol/L,in which it does not affect the seed germination when this biogas slurry was used in seed germination after dilution.Additionally,the biodegradability of MEA in biogas slurry has obvious advantages over other absorbents,and the biodegradation rate of MEA in the raw biogas slurry can reach about 55% after 5 days.Therefore,MEA has a comprehensive advantage among the high absorption loading,high absorption rate and low plant phytotoxicity.?4?The ammonia nitrogen content in the biogas slurry is the key factor affecting the phytotoxicity of the plants or crops.The removal rate of ammonia nitrogen in the biogas slurry can reach about 92.2% and 88% through deamination treatment or 5 times concentration,respectively.The dilution ratio of CO₂-rich biogas slurry after ammonia removal is only 5 to achieve a good appetite for lettuce.The relative growth of lettuce which was planted in biogas slurries with the dilution ratio from 5 to 30 times is higher than that in the nutrient solution.Especially for the CO₂-rich biogas slurry with 5 times dilution ratio,the relative growth rate of lettuce can reach 150%.CO₂-rich biogas slurry which was concentrated by 5 times can be used as the hydroponic culture solution of lettuce after being diluted by about 20 times?equivalent to the original biogas slurry diluted more than 4 times?.Compared to the chemical nutrient solution,the quality of hydroponic lettuce cultured by biogas slurry improves in terms of nitrate content,and the content of NO3-N is only 13 mg/kg in 5 times dilution of biogas slurry after ammonia removal,which is only 4.6% of that in nutrient solution.?5?The lettuce has a good growth performance when the concentration of MEA is less than 0.001 mol/L.However,the lettuce is completely decayed when the concentration of MEA is higher than 0.002 mol/L.The relative growth of lettuce was higher than 100% when the concentration of MEA is 0.001 mol/L.When the concentration of MEA is 0.0005 mol/L,the relative growth of lettuce reached the peak.In summary,the process of "concentration-addition of absorbent-dilution application" can increase the amount of exogenous absorbent added into biogas slurry,while it can reduce the phytotoxicity of biogas slurry.The introduction of the appropriate concentration of MEA into the biogas slurry not only greatly increases its CO₂ absorption performance,but also does not increase the phytotoxicity of biogas slurry,and even has a positive effect on plant growth.