Cogeneration Of Hydrogen And Methane From Microalgae Biomass Through Dark-fermentation,Hoto-fermentation,and Methanogenesis

The extensive utilization of fossil fuels has resulted in serious energy crisis and environmental pollution. Hydrogen is considered as an ideal carbon-free secondary energy carrier with high energy density and clean combustion product. Hydrogen production from renewable biomass through fermentation is increasingly attracting worldwide attention. Microalgae biomass is a potential feedstock for fermentative hydrogen production because of its high photosynthetic efficiency, fast growth, and global distribution. In this study, microalgae biomass was used as feedstock to cogenerate hydrogen and methane through a novel three-stage method comprising dark-fermentation, photo-fermentation, and methanogenesis. The components in microalgae biomass were efficiently used through the three-stage method, therefore hydrogen yield and energy conversion efficiency (ECE) were significantly increased.Glutamic acid, a typical amino acid degraded from protein components in microalgae biomass, was used as feedstock to investigate the feasibility of cogeneration of hydrogen and methane through the three-stage method comprising dark-fermentation, photo-fermentation, and methanogenesis. Hydrogen-producing bacteria (HPB), photosynthetic bacteria (PSB), and methane-producing bacteria (MPB) were used as the inocula during dark-fermentation, photo-fermentation, and methanogenesis, respectively. HPB can efficiently ferment glutamic acid to abundant soluble metabolite products (SMPs) and little hydrogen during dark-fermentation. The residual solution of dark-fermentation mainly contained acetate, butyrate, and ammonium. Because high concentration of ammonium (36.7mM) in the residual solution of dark-fermentation can significantly inhibit the activities of PSB in sequential photo-fermentation, a modified zeolite were used to extract ammonium by ion exchange to reduce the ammonium concentration to3.2mM (91.1%of ammonium removal efficiency). After ammonium removal, the treated solution was inoculated with PSB, exhibiting the maximum hydrogen yield of292.9ml H2/g glutamic acid during photo-fermentation. The residual solution from photo-fermentation was reused by MPB to produce the maximum methane yield of102.7ml CH4/g glutamic acid. The ECE from glutamic acid to gas fuels significantly increased from18.9%in hydrogen fermentation to40.9%in combined hydrogen fermentation and methanogenesis. Trehalose, a typical carbohydrate component in microalgae biomass, was used as feedstock to investigate the feasibility of cogeneration of hydrogen and methane through the three-stage method comprising dark-fermentation, photo-fermentation, and methanogenesis. As a stable non-reducing sugar, trehalose was not easily used by HPB for efficient hydrogen production. Trehalose was first pretreated by microwave heating with dilute acid, and then was inoculated with HPB to produce hydrogen during dark-fermentation. The residual solution of dark-fermentation was reused by PSB during photo-fermentation. The residual solution of photo-fermentation was reused by MPB during methanogenesis. Overall, the maximum hydrogen yield of731.3ml H2/g trehalose and methane yield of116.9ml CH4/g trehalose were achieved. The sequential generation of hydrogen and methane from trehalose remarkably enhanced the ECE from47.2%in hydrogen fermentation to72.2%in combined hydrogen fermentation and methanogenesis.Hydrogen production from Arthrospira platensis biomass through dark-heterofermentation by the [FeFe] hydrogenase of HPB and dark-auto fermentation by the [NiFe] hydrogenase of A. platensis was discussed. A. platensis biomass pretreated by ultrasonication and enzymatic hydrolysis was inoculated with HPB to produce hydrogen during dark-heterofermentation. The maximum hydrogen yield of92.0ml H2/g dry weight (DW) was obtained at20g/l of A. platensis biomass. In dark-autofermentation, hydrogen yield decreased from51.4ml H2/g DW to11.0ml H2/g DW with increasing substrate concentration from1g/1to20g/1. The hydrogen production peak rate and maximum hydrogen yield at20g/1of A. platensis biomass in dark-heterofermentation showed110.0-and8.4-fold increases, respectively, relative to those in dark-autofementation. Therefore, dark-heterofermentation was selected for the further investigation of fermentative hydrogen production from microalgae biomass. A. platensis biomass was pretreated by microwave heating with dilute acid to improve saccharification during enzymatic hydrolysis and hydrogen production during dark-fermentation. The residual solution of dark-fermentation was treated by zeolite to reduce ammonium concentration before photo-fermentation. The maximum hydrogen yield from A. platensis biomass was significantly increased to337.0ml H2/g DW through combined dark-fermentation and photo-fermentation.Three methods for hydrogen and methane production from Nannochloropsis oceanica biomass were discussed as the following:(1) three-stage method comprising dark-fermentation. photo-fermentation, and methanogenesis;(2) two-stage comprising dark-fermentation and methanogenesis;(3) single-stage methanogenesis. N. oceanica pretreated by microwave heating with dilute acid was inoculated with HPB to produce hydrogen during dark-fermentation.The consumption time of most amino acids was about2times as long as that of most reducing sugars during dark-fermentation. The total ECE from N. oceanica biomass to gas fuels through the three-stage method showed1.7-and1.3-fold increases, respectively, compared with those through the two-stage and single-stage methods.Effects of pretreatment methods on biomass saccharification and hydrogen fermentation from Chlorella pyrenoidosa were investigated. The steam heating with dilute acid and microwave heating with dilute acid can remarkably enhance the biomass hydrolysis and hydrogen fermentation. The maximum hydrogen yield of198.3H2ml/g total volatile solids (TVS) and methane yield of186.2ml H2/g TVS were achieved through the three-stage method comprising dark-fermentation, photo-fermentation, and methanogenesis. Semi-continuous fermentation of C. pyrenoidosa biomass was carried out based on batch fermentation. Compared with the simple microbial community formed at earlier stages of fermentation, the complex microbial community formed at later stages of fermentation was more adaptable to C. pyrenoidosa biomass and can utilize C. pyrenoidosa biomass more efficiently, thereby resulting in efficient and stable hydrogen fermentation. In order to enhance the ECE from C. pyrenoidosa biomass, cassava starch was mixed with C. pyrenoidosa biomass to optimize the carbon/nitrogen (C/N) molar ratio for efficient dark-fermentation. The maximum dark hydrogen yield of276.2ml H2/g TVS from the mixed biomass at C/N molar ratio of25.3showed3.7-and1.8-fold increases, respectively, compared with those from only C. pyrenoidosa biomass and only cassava starch. The maximum hydrogen yield of664.2H2ml/g TVS and methane yield of126.0ml H2/g TVS corresponding to the total ECE of67.2%were achieved through the three-stage method comprising dark-fermentation, photo-fermentation, and methanogenesis.