| Resource exhaustion and global warming call for the development of clean energy,and the capture,utilization and storage of carbon dioxide.Microalgal cultivation can capture,utilize and store the carbon dioxide,meanwhile microalgae biomass produced from cultivation is considered as an ideal feedstock for biofuel production.Thus,the research and development of microalgal energy conversion are significant in the alleviation of energy and environmental crises.Anaerobic digestion is a promising approach for biogas production from microalgae biomass.However,the microalgae biomass possess a low utilization and degradation,obtaining a poor methane yield during anaerobic digestion,because of the compact structure of microalgal cell wall.Hydrothermal pretreatment is an efficient method for destroying microalgal cell wall,hydrolyzing organic matters and further improving methane yield.Nevertheless,the lack of effective hydrolytic kinetics description of microalgae biomass makes the performance enhancement of microalgal hydrolysis difficult,and the energy intensity of hydrothermal pretreatment is adverse to its industrial application.Herein,the hydrolytic reaction kinetics of microalgae biomass were conducted,and a solar-driven hydrothermal pretreatment system was proposed on this basis.Firstly,the hydrolytic kinetics of microalgae biomass during hydrothermal pretreatment were conducted.The hydrolytic characteristics of microalgae biomass during hydrothermal pretreatment,as well as the hydrolytic kinetics equations of carbohydrates and proteins from microalgae biomass,were obtained in this study.Secondly,a solar-driven hydrothermal pretreatment system of microalgae biomass,which was integrated with parabolic solar collectors,was designed and constructed.The microalgal hydrolytic characteristics in the solar-driven hydrothermal pretreatment system were conducted,and the effects of direct normal irradiation,flow rate and concentration of microalgae slurry on the yield of carbohydrates and proteins were obtained.The biochemical methane potential test of hydrolysates from microalgae biomass with solar-driven hydrothermal pretreatment were conducted,to demonstrate the feasibility of improving biogas production by the solar-driven hydrothermal pretreatment.The exergy analysis of biogas production from microalgae biomass with solar-driven hydrothermal pretreatment via anaerobic digestion was carried out,providing guidances for improving the exergy efficiency of the biogas production.Thirdly,the flow,heat transfer and conversion model of microalgae slurry under non-uniform heat flux was built,acquiring the flow,heat transfer and conversion characteristics of microalgae slurry in the solar-driven hydrothermal pretreatment system.Additionally,the model was expanded to the conditions of practical applications(e.g.,with a large flow rate of microalgae slurry and large collector area).Finally,the life cycle and economic assessments of the integrated system for biogas production from microalgae biomass with solar-driven hydrothermal pretreatment via anaerobic digestion were investigated,obtaining the environmental and economic efficiencies of the integrated system.The main conclusions of this study were as follows:(1)The effect of hydrothermal pretreatment conditions on depolymerization of organic matters within microalgae biomass were investigated,and hydrolytic kinetics equations of carbohydrates and proteins were obtained.The results showed that the increase of temperature and retention time of hydrothermal pretreatment could improve the hydrolysis of carbohydrates and proteins in microalgae biomass.However,the solube carbohydrates were degraded at the temperature of 180°C,therefore,the hydrolytic temperature of microalgae slurry should be below 180°C.The hydrolysis of carbohydrates and proteins from microalgae biomass during hydrothermal pretreatment were all first-order reactions,and the hydrolytic activation energy of carbohydrates and proteins from microalgae biomass were 29.14 and 37.49 k J/mol,respectively.Additionally,the organic matters with low thermal stability within solid residue were gradually released into the solution,with the increase of retention time from 0 to 40 min at the pretreatment temperature of 160°C.The lipid in microalgae biomass were decomposed with the increase of pretreatment temperature from 120 to 180°C at the retention time of 20 min.The research on hydrolytic kinetics of microalgae biomass during hydrothermal pretreatment provides theoretical guidances for the design and optimization of hydrolytic reaction,the control of hydrolytic process and the prediction of hydrolytic production.(2)A solar-driven hydrothermal pretreatment system was design and constructed.The effect of direct normal irradiation,flow rate and concentration of microalgae slurry on hydrolytic characteristics of microalgae biomass in the solar-driven hydrothermal pretreatment system were conducted.The results showed that the direct normal irradiation,flow rate and concentration of microalgae slurry affected the hydrolytic temperature of microalgae slurry,making the yield of solube carbohydrates and proteins changed.The yield of solube carbohydrates from microalgae biomass firstly increased and then decreased,while the yield of solube proteins from microalgae biomass increased,due to the increase of average temperature of microalgae slurry,with the increase of direct normal irradiation from 604 to 823 W/m2 at the flow rate of microalgae slurry of 40 L/h.The yield of solube carbohydrates from microalgae biomass firstly increased and then decreased,while the yield of solube proteins from microalgae biomass decreased,due to the decrease of average temperature of microalgae slurry,with the increasing flow rate of microalgae slurry from 30 to 60 L/h at the direct normal irradiation of 735 W/m2.The yield of solube carbohydrates and proteins from microalgae biomass decreased due to the increasing temperature difference of microalgae slurry between near wall and main flow,with the increase of concentration of microalgae slurry from 1%to 7%at the direct normal irradiation of 770 W/m2 and the flow rate of microalgae slurry of 40 L/h.Additionally,the methane yield from microalgae biomass with the solar-driven hydrothermal pretreatment via anaerobic digestion was 347 ml/g(volatile solid),increased by 57%compared with that without pretreatment.The exergy efficiency of biogas production from microalgae biomass with the solar-driven hydrothermal pretreatment via anaerobic digestion was 39.23%,increased 52.8%and 13.9%,respectively,compared with that without pretreatment and with hydrothermal pretreatment.The exergy efficiency of the biogas production can be further improved by utilizing the exergy from biogas residue.The solar-driven hydrothermal pretreatment system could improve the biogas yield and the exergy efficiency of biogas production from microalgae biomass via anaerobic digestion.(3)A heat transfer and conversion model of microalgae slurry under non-uniform heat flux was established,and the effect of direct normal irradiation and the concentration of microalgae slurry on the heat transfer and conversion characteristics of microalgae slurry were investigated.The results showed that the microalgae slurry was in a state of laminar natural convection in the solar collector.A non-uniform temperature distribution occurred in the absorber tube due to the non-uniform heat flux outside absorber tube,causing the occurrence of a secondary flow of microalgae slurry on the cross section of absorber tube.The secondary flow of microalgae slurry on the cross section enhanced the heat transfer between absorber tube and microalgae slurry.Additionally,the non-uniform temperature distribution led to a non-uniform viscosity distribution,making the flow and heat transfer characteristics of microalgae slurry complicated.The secondary flow velocity of microalgae slurry on the cross section and local Nu firstly decreased and then increased,while the average wall shear stress firstly increased and then decreased.It was because that the viscosity of microalgae slurry near wall firstly increased and then decreased,along the flow direction at the flow rate of 60 L/h and the concentration of microalgae slurry of 5%with the increase of direct normal irradiation from 650 to 850W/m2.The secondary flow velocity of microalgae slurry on the cross section and local Nu decreased,while the average wall shear stress increased,due to the increasing viscosity of microalgae slurry near wall at the flow rate of 60 L/h and direct normal irradiation of 750 W/m2,with the increase of concentration of microalgae slurry from 5%to 15%.The understanding of heat transfer and conversion characteristics of microalgae slurry in the solar collector will be beneficial to the design and optimization of the solar-driven hydrothermal pretreatment system.(4)Considering the future application of the pretreatment system,the heat transfer and converstion model was expanded to the conditions of practical applications(e.g.,with a large flow rate of microalgae slurry and large collector area).The effect of direct normal irradiation and the concentration of microalgae slurry on the heat transfer and conversion characteristics of microalgae slurry were investigated.The results showed that the microalgae slurry was in a state of laminar mixed convection in the solar collector,and the variation of shear rate and temperature led to the variation of viscosity of microalgae slurry,making the flow and heat transfer characteristics of microalgae slurry complicated.Additionally,the local Nu firstly decreased and then increased,while the average wall shear stress firstly increased and then decreased,because the viscosity of microalgae slurry near wall firstly increased and then decreased,along the flow direction at the flow rate of 600 L/h and concentration of microalgae slurry of 5%,with the increase of direct normal irradiation from 650 to 850 W/m2.The average local Nu decreased,and the average wall shear stress increased at the 0?34.5 m and 45?60 m from the collector inlet(total length of 60 m)due to the increase of average visocity of microalgae slurry near wall at the flow rate of 600 L/h and direct normal irradiation of 750 W/m2 with the increase of concentration of microalgae slurry from 5%to 15%.In addition,the local Nu firstly increased and then decreased at the 34.5?45 m from the collector inlet because the viscosity of microalgae slurry near wall firstly decreased and then increased,while the average wall shear stress increased because of the increasing average wall shear rate at the flow rate of 600 L/h and direct normal irradiation of 750 W/m2,with the increase of concentration of microalgae slurry from 5%to 15%.The understanding of heat transfer and conversion characteristics of microalgae slurry in the industrial system will be beneficial to the design and optimization of the solar-driven hydrothermal pretreatment system for industrial application.(5)The life cycle model of the integrated system for biogas production from microalgae biomass with solar-driven hydrothermal pretreatment via anaerobic digestion was established,and the environmental and economic benefits of the integrated system were investigated.The results showed that the net energy ratio(energy input/energy output),greenhouse gas emissions and levelized cost of energy of the integrated system were 0.69,-166.13 g CO2-eq/k Wh(biogas)and 0.17$/m3(biogas),respectively.The integrated system possessed optimal energy output and economic benefits,compared with that biogas production from microalgae biomass without pretreatment and with hydrothermal pretreatment.Additionally,the biogas yield had great influence on the energy benefits and economic benefits of the integrated system,and the efficiency of nitrogen recovery from the biogas residual had great influence on the greenhouse gas emissions of the integrated system.The life cycle and economic assessments provide therotical guidances for the improvement of environmental and economic benefits of the integrated system for biogas production from microalgae biomass with solar-driven hydrothermal pretreatment via anaerobic digestion. |
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