| With the economic development,the countries’ demand for energy continues to increase.Now,energy depletion and environmental pollution caused by fossil fuels affect the country’s development seriously.Energy shortages and environmental pollution have become the primary problem affecting the country economy development.Renewable energy technologies to develop biomass energy are urgent.As a third generation of biofuels,microalgae biomass energy is more advantageous than the first generation(ethanol produced by grain fermentation),and the second generation(rural and urban waste fiber),it is gaining more attention.Fermentation of microalgae biomass into gas fuel(H2,CH4,etc.),with a mild fermentation conditions is the most promising method utilizing microalgae biomass.It can produce high purity gas in simple process,and it costs lowly,what’s more it does not pollute the environment.Fermentation of microalgae biomass can maximum use microalgae biomass(including carbohydrates,protein,lipid,etc.).Due to the complex cell structure of microalgae and cell wall,the effect of microalgae fermentation is not ideal.Therefore,it is necessary to use high-efficiency pretreatment method to rapidly break the multi-layered three-dimensional structure of cell wall fibrous tissue in order to realize the efficient energy utilization of subsequent fermentation.At present,hydrothermal hydrolysis pretreatment method gaining more attention because of its better treatment,low energy consumption,and no pollution.However,there are still a lot of imperfections in the mechanism of micro-algae hydrothermal hydrolysis pretreatment,which is not conducive to the commercialization of microalgae hydrothermal hydrolysis pretreatment combined with bacterial fermentation into gas biofuels.Chlorella pyrenoidosa(CP)was used in this paper.To investigate the mechanism of substance released and reaction during the hydrothermal pretreatment of microalgae.The research studied morphological changes of microalgae in the process of low temperature hydrothermal process.And also investigated the effects of single factor and multi-factor synergy on substance releasing and hydrolysis during microalgae hydrothermal.Kinetic and thermodynamic analysis of the precipitated substances and hydrolyzed low temperature hydrothermal microalgae.Finally,the low temperature hydrothermal hydrolysis of microalgae was regulated by the product which matched the fermentation environment.The microstructures of microalgae cells were analyzed by scanning electron microscopy and transmission electron microscopy.It was found that as the reaction temperature and residence time increased,the particle size of microalgae decreased,the more severe the destruction,the more the dissolution and precipitation of the internal organelles,the larger the area of the cells.The low temperature hydrothermal hydrolysis characteristics of microalgae were studied by single factor variable method.The reaction process was characterized by the degradation of carbohydrate,protein,reducing sugar and hydrolysis.The effects of reaction temperature,residence time,algae concentration and initial pressure on the hydrothermal hydrolysis pretreatment of microalgae were studied.The results showed that the yield of total carbohydrate and protein was increased and then decreased with the increase of temperature,but the optimum temperature was not consistent.The optimum temperature of carbohydrate and reducing sugar is between 160-170 °C and about 259.55 mg/g dry biomass.The optimum temperature of the protein is 368.75 mg/g dry biomass at 190 °C.Protein and carbohydrate production increased with the residence time,the first increase after the basic remained unchanged,hydrolysis of reduced sugar yield with the increase in residence time to increase first and then continue to reduce.With the increase of algae concentration,protein and reducing sugar production continued to decrease,carbohydrate production increased first and then decreased.With the initial pressure increases,carbohydrates,protein precipitation dissolved up and down fluctuations,basically remain unchanged.The initial pressure increases,the hydrolysis of reduced sugar yield fluctuations,the basic remain unchanged.The reaction temperature and residence time of hydrothermal hydrolysis have a significant effect on the microalgae cell structure.The kinetics of precipitation and release of carbohydrates and proteins in microalgae was studied by using the two-site model for solid-liquid two-phase extraction.Then the thermodynamics of the carbohydrate and protein precipitation process in the microalgae was studied by the second order reaction model and the Van’t Hoff formula under isothermal.Based on the cell structure of microalgae,the two-site precipitation kinetics was used to analyze the whole process and the reaction process of the carbohydrate and protein in the microalgae.Regardless of the specific chemical reaction,the microalgae carbohydrate and protein precipitation process is simplified,as a solid and water solution solution into the solution.The second order reaction model was used to analyze the dissolution reaction of carbohydrate and protein.On this basis,the microalgae carbohydrate and protein precipitation process for thermal analysis.The experimental data and application model of microalgae carbohydrate and protein precipitation process are well fitted.The main influencing factors of carbohydrate and protein precipitation during the hydrothermal hydrolysis of microalgae are not intermolecular forces and hydrogen bonds.The main factors may be the energy required for macromolecule carbohydrates and proteins to hydrolyze into small molecules of sugar and proteins,namely activation energy of glycosidic bonds and peptide bonds.The reaction kinetics and thermodynamics of the hydrolysis of cellulose,starch,protein,glucose and amino acid during the hydrothermal hydrolysis of microalgae were simulated by Guassion software and density functional theory(DFT).The activation energy,enthalpy and free energy of cellulose,starch,protein hydrolysis,glucose and amino acid were analyzed at different temperatures.Temperature for carbohydrate,protein hydrolysis,glucose and amino acid dehydration condensation reaction have a significant impact.The thermogravimetric analysis shows that the hydrolysis of the glycosidic bond in the carbohydrate increases and the heat release increases with the increase of the temperature.The maximum heat release is 2.91 kJ/mol at 473 K and the conversion rate increases with the increase of the temperature.The kinetic analysis shows that the hydrolysis of glycosidic bonds in carbohydrates requires more than 264.39 kJ/mol of energy barrier.As the temperature increases,the higher the energy of the reactants,the more likely to cross the barrier.In the thermodynamic analysis,the hydrolysis of the protein peptide bond at room temperature belongs to the endothermic reaction,and the hydrolysis reaction at 373 K and above is an exothermic reaction.The maximum heat release rate is 0.96 kJ/mol at 473 K,and the conversion rate increases with the temperature increase.Kinetic analysis shows that the hydrolysis of peptide bonds in the protein requires more than 185.05 kJ/mol of energy barrier.Thermodynamic analysis showed that glucose and glycine dehydration condensation to form Schiff base heat absorption,and the amount of heat up with the increase in temperature,the maximum endotherm of 473 K when 4.67 kJ/mol.The kinetic molecules show that the dehydration condensation reaction of glucose and glycine needs to pass over the energy barrier of 270.98 kJ/mol.As the temperature increases,the higher the energy of the reactants,the easier it is to cross the barrier and react.The synergistic effect and effect of reaction temperature,residence time and initial pressure on the degradation of carbohydrate and protein in algal cells were studied by using the response surface method.It was found that the interaction of reaction temperature,initial pressure and reaction residence time was very important to the precipitation of microalgae hydrothermal hydrolyzate.For carbohydrate precipitation,the interaction between reaction temperature and reaction time has a significant effect on the precipitation of carbohydrates,followed by the interaction between temperature and initial pressure.The effect of the interaction between reaction temperature and reaction time is more pronounced for protein precipitation.The effect of the reaction temperature and the initial pressure on the COD removal in the aqueous solution after the reaction is more significant.On the basis of this,the product with the content of reducing sugar as the main index was matched with the fermentation environment by using low content of acid/alkali.Low content of acid/alkali assisted for the hydrothermal conditions,microalgae internal material hydrolysis of the process of reducing sugar are significantly improved role.However,the addition of alkali catalyst also greatly enhanced the side reaction intensity during the hydrolysis of microalgae under hydrothermal condition.The chromaticity of the solution after the reaction was more than 30 times higher than that of the non-added catalyst,which was not conducive to the later fermentation.Compared with the alkali-assisted,low-content acid-assisted effect is similar to that of the reducing sugar hydrolysis,and there is no obvious improvement for the side reaction.And low content of formic acid auxiliary,can make the secondary reaction intensity relative to the sulfuric acid auxiliary to further reduce the late formic acid can also be effectively fermented by bacteria,low-content formic acid-assisted directional control microalgae in the water to produce reducing sugar as the main indicator Fermentation environment to match the product,the effect is very good. |
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