| Energy crisis and environmental protection are currently the two most significant topics in the world and are two topics that are closely related to the sustainable development of human society and natural resources. It is essential to develop and transform renewable abundant and non-food, biomass resources into high-performance and low-cost biofuels. Microalgae have a great potential for biofuels production due to their higher photosynthetic efficiencies compared with terrestrial lignocellulosic biomass. They are universally viewed as suitable feedstocks for the next-generation biofuels and chemicals. Previous studies proved that low-lipid microalgae could be successfully converted to bio-crude oils by hydrothermal liquefaction. However, the reaction mechanisms for the thermo-chemical conversion process have not been well acknowledged, which is not benefited for the industrial application. As fast-growing and low-lipid microalga species, Chlorella pyrenoidosa (CP) and Spirulina platensis (SP) are converted to bio-crude oils via a hydrothermal liquefaction process. The experimental characteristics and reaction mechanism of this conversion process are investigated by kinetic analysis, response surface analysis and mechanism analysis.The thermal decomposition behavior of two microalgae, Chlorella pyrenoidosa (CP) and Spirulina platensis (SP), were investigated on a thermogravimetric analyzer under non-isothermal conditions. Based on the TG-DTG curves obtained, iso-conversional Vyazovkin approach was used to calculate the kinetic parameters. The universial integral method was applied to evaluate the most probable mechanisms for thermal degradation of the two feedstocks. Reaction order, autocatalytic and diffusional mechanisms were found to well describe the thermal decomposition of CP and SP since the average values of apparent activation energy of the three kinds of mechanism were close to the mean values that obtained by Vyazovkin method. Reaction order models should be the most probable mechanism as they have the best fitting accuracy in addition to the closest mean values. Among the four models of reaction order, the F3model should be the most suitable mechanism function describing the kinetic process for the thermal decomposition of CP. F2model is the most probable mechanism function for the thermal decomposition of SP. Based on the kinetic parameters and reaction mechanism functions deduced, the numerical solution to differential equations for CP and SP were plotted and compared with the experimental data. F3and F2models could reasonably describe the major thermal decomposition process of CP and SP, respectively. Functional group characteristics of CP and SP were investigated by FTIR analysis and topography of CP and SP were investigated by SEM analysis.Based on the kinetic analysis results, CP was converted into bio-crude oils via hydrothermal liquefaction (HTL) process. Response surface methodology (RSM) was applied to investigate the interactions of operating conditions including reaction temperature, retention time and total solid ratio, on the qualities of bio-crude oils including the yield, higher heating value (HHV), carbon recovery (CR) and nitrogen recovery (NR), as well as the qualities of aqueous phase products, including the chemical oxygen demand (COD), total nitrogen (TN), total phosphate (TP) and total suspended solid (TSS). A face-centered central composite design (FCCCD) was employed to design the experiment.CP and SP were converted into bio-crude oils via hydrothermal liquefaction. The levels of retention time and total solid ratio were selected as60min and25wt.%, based on the response surface analysis of HTL conversion of CP. The reaction temperatures were selected as200℃,220℃,240℃,260℃,280℃,300℃and320℃. Then the bio-crude oils and aqueous phase products obtained from HTL of CP and SP were analyzed using spectra analysis, in terms of GC-MS, FT-IR and1H NMR. The reaction mechanism of hydrothermal liquefaction of CP and SP were deduced based on the spectral analysis. In the temperature range of0-100℃, protein is hydrolyzed to produce amino acids. Lipid is hydrolyzed to produce fatty acids, and carbohydrate is hydrolyzed to produce sugars. In the temperature range of100-200℃, amino acids, fatty acids and sugars will undergo further decomposition. Amino compounds are produced due to the decarboxylation of amino acids, and carbon dioxide is produced from the carboxyl group during this process to remove the oxygen from the microalgae. A certain amount of amino acids will undergo deamination reaction to produce carboxylic acids, and ammonia is produced from the amine group during this process to remove the nitrogen from the microalgae. Besides, some alkanes and alkenes are produced from the decarboxylation of long-chain fatty acids from hydrolysis of the lipids and amino acids from hydrolysis of the proteins, and cyclic oxygenates are produced from the reducing sugars at the same temperature range. Above the reaction temperature of200℃, the carboxyl groups in long-chain fatty acids from hydrolysis of lipids react with ammonia from deamination of amino acids from hydrolysis of proteins to produce aliphatic amine compounds. A certain amount of long-chain fatty acids react with the alcohols from the reduction of amino acids after deamination to produce esters. N&O-heterocyclic compounds are produced from the Maillard reaction between amino acids from hydrolysis of the proteins and reducing sugars from hydrolysis of carbohydrates, in terms of pyrrole, pyrrolidinone, pyrrolidinedione, pyridine, thiazole, imidazole and their derivatives. In addition, some N&O-heterocyclic compounds react with long-chain fatty acids to produce pyrrolyl fatty acids. |
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