| With the gradual depletion of traditional fossil energy sources and the increasing demand for the sustainable development of society, the development of new, renewable, alternative energy sources has become a general trend. Biomass is a clean, renewable energy source. In recent years, algal biomass has become the focus of biomass energy for such advantages as large quantity, fast propagation, and lack of occupied farmland. The thermochemical conversion technology for the transformation of microalgae into bio-oil as an alternative fuel for engines has become a research frontier and is the focus of this study.Most freshwater lakes in China contain two kinds of typical microalgae biomass:Chlorella (commonly known as "green algae") and Spirulina (commonly known as "blue algae"). However, the systemic thermal liquefaction of these microalgae remains at the exploratory stage. In addition, the performance of microalgae bio-oil, including its tribological properties and upgrading methods, should be further explored because the lubricative efficiency and service life of engines are significantly affected by the tribological behavior of bio-oil.In this paper, related studies on the preparation, upgrading, and tribological properties of the bio-oil derived from Chlorella and Spirulina via thermochemical liquefaction are conducted. This study aims to establish an experimental basis for the development of a new generation of biomass liquid fuels and to promote the application of microalgae bio-oil in internal combustion engines. The specific research topics include the following aspects:First, the basic physical properties including the components of Chlorella and Spirulina were studied. The rare earth-loaded HZSM-5catalysts were prepared via catalytic pyrolysis of the microalgae, and the pyrolysis kinetic behavior was investigated. The results indicate that compared with HZSM-5, except for La(Ⅱ)/HZSM-5, the load of rare-earth catalysts such as Ce(I)/HZSM-5, Ce(Ⅱ)/HZSM-5, Pr-Nd/HZSM-5, and La(Ⅰ)/HZSM-5can lower the catalytic pyrolysis activation energy of Chlorella and Spirulina. Ce(Ⅰ)/HZSM-5has the best catalystic effect for Chlorella, whereas Ce(Ⅱ)/HZSM-5has the best catalytic effect for Spirulina. Pyrolysis activation energy decreased by47.1%and43.1%for Chlorella and Spirulina, respectively. The results show the efficiency of the rare-earth modified catalysts and provide a reference for algae biomass catalytic liquefaction.Second, the catalytic liquefaction preparation and mechanisms of bio-oil from Chlorella were studied systematically. The effects of the catalyst, liquefaction conditions, and other factors on the liquefaction behavior of the microalgae biomass were analyzed. The basic physical properties and combustion performance of bio-oil were tested. Emulsion technology was used to upgrade the Chlorella bio-oil. The piston ring-cylinder friction of the engine as well as wear experiments were used to simulate the changes of wear when fuel is injected into the cylinder wall in internal combustion engines and to analyze the mechanism of friction. The results show that the use of Ce(Ⅰ)/HZSM-5as a catalyst for liquefying Chlorella not only increases the liquefaction yield but also changes the molecular composition of the liquefied products. Moreover, the use of this catalyst can increase the hydrogen-to-carbon (H/C) ratio, reduce the oxygen-to-carbon (O/C) ratio, and increase the hydrocarbon content of the liquefied products. The optimal reaction conditions include: the selection of5wt%Ce(I)/HZSM-5as catalyst, Chlorella-to-solvent volume ratio of1:10g-mL"1, and reaction at300℃for20min. The maximum liquefaction yield reached39.87%, and the heating value of final bio-fuel reached26.09MJ-kg-1. The main components of the bio-fuel from Chlorella are alcohol, ester derivatives, and a number of hydrocarbons. The basic properties, calorific value, corrosion, and wear performance of Chlorella bio-oil improved after emulsion. A better lubricity of the upgrading bio-oil was attributed to organics during oil adsorption on the friction surface to form a lubricant film while the corrosion components in the oil were diluted.Third, the catalytic liquefaction regularity of bio-oil from Spirulina was investigated systematically. Moreover, the basic physiochemical properties and combustion performance of the Spirulina bio-oil were analyzed. Catalytic esterification technologies were used to upgrade the Spirulina bio-oil. The piston ring-cylinder friction of the engine as well as wear experiments were used to test the lubricant performance of the fuels, and the friction mechanism was also investigated. The results indicate that the optimal liquefaction catalyst for Spirulina is Ce(Ⅱ)/HZSM-5at5wt%. The maximum liquefaction yield can reach49.71%. The main components of Spirulina bio-oil are carboxylic acids, ketones, olefins, amides, ethers, esters, and a number of ring compounds that contain N. Spirulina bio-oil has a high acid value of approximately21.79mg KOH·g-1. The acid components of Spirulina bio-oil decreased, whereas the ester components increased evidently after catalytic esterification. Moreover, the basic physical properties of the bio-oil improved; the H/C ratio increased, the O/C ratio decreased, and the calorific value improved significantly. The lubricity of bio-oil after esterification significantly improved. The average coefficient of friction of the esterified fuel such as AEO, HEO, AMO, and HMO were decreased by22.52%,9.91%,21.64%, and11.41%, respectively, and the wear amount decreased as well. Energy dispersive spectroscopy and X-ray photoelectron spectroscopy showed that the adsorption and extrusion of organics on the surface of the friction pairs to form a lubricant film and a tribochemical reaction film such as Fe2O3, especially the ester (-COOR) and alkyl groups of esterified bio-oil, were deposited onto the friction surface, all of which play an antifriction and wear reduction roles.Finally, hydrothermal liquefaction and supercritical fluid liquefaction methods were employed to study the behavior and performance of the co-liquefaction of bio-oil obtained from Chlorella and Spirulina. The studies show the synergistic effects of the co-liquefaction when the quality of Chlorella and Spirulina are close at the hydrothermal liquefaction process. La2—3is an efficient liquefaction catalyst of hydrothermal liquefaction. Super/sub-critical methanol and alcohol can remarkably enhance the co-liquefaction yield of microalgae up to approximately74.71%and64.43%, respectively. These values are approximately two to three times of the maximum yield of hydrothermal liquefaction. The main components of co-liquefaction bio-oil are complex mixtures including alcohols, ethers, hydrocarbons, aromatics, esters, ketones, acids, aldehydes, and a number of nitrogen-containing compounds. Alcohols not only serve as a liquefaction solvent but also act as a reactant and an esterification modifier in a supercritical fluid environment. Compared with the case wherein water is used as a solvent without La2O3as catalyst or the case wherein a super/sub-critical alcohol system is adopted for co-liquefaction of bio-oil from microalgae, the H/C ratio and calorific value of alcohol increased, whereas the O/C ratio and the acid value decreased. The comprehensive performance improved significantly. Four-ball tribometer results show that the co-liquefaction bio-oil has efficient tribological effects because of the reduction in the friction coefficient and wear volume, in which the maximum decreasing range is approximately61.8%and32.2%, respectively, when10wt%bio-oil is added to the15W-40diesel engine oil. The results show that organic groups such as C-C, C-OH, C=O, and-COOR are adsorbed on the friction surface and react with the Fe of the steel substrate to form the lubricant film that also contains Fe2O3. The N-containing compounds deposited on the friction surface in the form of C-NH2and FeN tribochemical reaction films altogether play a lubrication role, which shows good application potential. |
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