Study On Carbon Fixation And Upgrading Characteristics Of Biogas By Halophilic And Alkaliphilic Microalgae

In recent years,the excessive utilization of fossil energy has led to serious energy shortage and environmental pollution.Biogas production from crop straw,animal manure and other wastes by microbial anaerobic fermentation has significant energy and environmental benefits.Biogas can be further purified and decarbonized upgrading to obtain high quality biomethane,as an important supplement to conventional natural gas.Biogas upgrading is a key step to obtain biomethane.Traditional biogas upgrading technologies,such as water scrubbing,have many problems,such as high operating conditions,complex equipment structure,high-energy consumption,and lack of effective utilization of the separated CO2.The photosynthetic biogas upgrading can solve the problems of lack of efficient utilization of CO₂ after separation and the high-energy consumption of absorbent recycling and regeneration in the traditional process.Meanwhile,high value-added products such as microalgae biomass can be obtained,which has a broad development prospect.In this paper,based on the working principle of photosynthetic biogas upgrading,halophilic and alkaliphilic microalgae that can tolerate high salt and high alkaline environment were screened,namely Spirulina platensis FACHB-439 as the experimental microalgae species.In order to improve the operating efficiency of biogas upgrading and microalgae cultivation,the growth and carbon fixation characteristics of halophilic and alkaliphilic microalgae under different light intensities and NaHCO₃ concentrations were studied.It revealed the light attenuation and HCO₃^-transformation characteristics of S.platensis.The mechanism and optimization method of the spiral-flow reactor structure and hydraulic operating conditions on carbon fixation of microalgae growth were obtained.Moreover,the method for mixing disturbance of microalgae suspension was proposed to enhance mass transfer in the reactor.On this basis,a two-step coupling system of photosynthetic carbon fixation and biogas upgrading of halophilic and alkaliphilic microalgae was established,and the performance improvement method of biogas carbon fixation upgrading system was obtained.According to the existing research results,the design and analysis of the photosynthetic biogas upgrading system were carried out,which provided guidance for the practical application of the system.The main conclusions are as follows:(1)The biomass yield and carbon fixation efficiency of microalgae were improved by regulating and optimizing the cultivation conditions of microalgae,including light intensity and NaHCO₃ concentration.S.platensis had a serious light attenuation phenomenon,and high light intensity up to 210?mol/(m²·s)was conducive to its growth and carbon fixation,and promoted microalgae biomass concentration and photosynthetic pigment accumulation.NaHCO₃ concentration significantly affected the biomass concentration and pigment content.S.platensis could tolerate 0.6 M NaHCO₃ and had good salinity and alkalinity resistance.A maximum biomass concentration of 1.46 g L^-1was achieved under an optimal growth condition of 0.1 M NaHCO₃,which was 65.9%higher than 0.05 M NaHCO₃.Meanwhile,the carbon utilization efficiency reached 42.0%,which could realize the efficient regeneration of carbonate absorbent.However,the carbon outgasing in the traditional magnetic agitation culture system was serious,accounted for 20.0%.(2)The tangential central spiral-flow formed in the spiral-flow photobioreactor could enhance the mixing disturbance of microalgae suspension,prevent cell deposition,alter light/dark cycle frequency,enhance mass transfer and increase biomass yield.The mixing disturbance of microalgae suspension in the double tangent circle form photobioreactor with higher circulating rate and smaller jet angle was sufficient to promote the biomass concentration,pigment content and carbon utilization efficiency.The maximum biomass concentration at circulating rate of 240 L/h was 1.22 g/L,which was 62.2%higher than that of static condition and the carbon utilization efficiency was 40.2%.Moreover,the biomass yield of spiral-flow reactor could reach the same level as that of the magnetic stirring device(300 r/min),and the proportion of carbon outgassing was less(6.9%-8.0%),which was conducive to the absorbent recycling and had more potential of large-scale application.(3)A two-step coupling system of photosynthetic carbon fixation and biogas upgrading of halophilic and alkaliphilic microalgae was established,and the effect of system operation parameters on the performance of carbon fixation upgrading was studied.The removal of CO₂ in the biogas upgrading mainly depended on the Na₂CO₃chemical absorption,while the CO₂dissolution had little effect on it.Under the same aeration flow rate,the CO₂ dissolution efficiency and volume mass transfer coefficient were the highest in the 0.05 M Na₂CO₃ absorbent.However,under the same Na₂CO₃concentration,a larger aeration flow rate would increase the CO₂ dissolution efficiency and volume mass transfer coefficient.When the dissolved CO₂concentration increased rapidly,the CO₂ removal efficiency decreased sharply.The cycle experiment of microalgae photosynthetic biogas upgrading system was carried out,and the CO₂ removal efficiency was?95%when the aeration time was 5 min,and about 90%when the aeration time was 20 min in four cycles,which verified the feasibility of photosynthetic biogas upgrading.In addition,a photosynthetic biogas upgrading system with a treatment capacity of 800 m³/h was designed for a certain biomethane project.The microalgae biomass yield was 344 kg/h,equivalent to 5614 MJ/h of energy and 281 m³/h of biogas,which has potential economic benefits and industrial application capacity.