The depletion of the reserves of fossil fuel promotes the search for sustainablerenewable sources of energy. Due to their similarities with the petroleumproducts, the biofuels represent a better alternative than other clean energies.The conversion process greatly depends on the biomass composition, which isa topic of debate, especially when it increases the price of the food. Therefore,third generation biofuels, derived from algae, are better accepted by societythan using other raw materials. Due to the way of cultivating and the highmoisture content of microalgae, hydrothermal liquefaction (HTL) is thetransformation technology most suitable for this type of feedstock.Even when the application of HTL to microalgae is quite recent, a lot ofresearch is being done because of the good expectations for biofuel production.In this way, the best operating conditions have already been determined, theyare the critical point of water (374 ˚C and 221 bar), and the research isfocussing now in the improvement of the quality of the products and theupscaling to a continuous process. The most important product is the biocrude,which has a high content of oxygen and nitrogen. The use of hydrogen for theremoval of these heteroatoms is one of the most investigated techniques.Especially important is the presence of nitrogen, as the large production of NOxupon combustion is banned.The use of ultrasound as a pretreatment technology before the HTL of themicroalgae slurry is intended to increase the yield of the biocrude whilereducing the severity of the operating conditions. Also, milder conditions resultin lower nitrogen content of the biocrude.This thesis shows the results of the use of a sonication bath to disrupt themicroalgae cells before being liquefied. Three different microalgae species wereconsidered: Nannochloropsis gaditana (N. gad.), Scenedesmus almeriensis (S.alm.), and Tetraselmis suecica (T. suec.). The experiments were carried out in atubular batch reactor without stirrer and with an electric heater. It was found that the ultrasonic pretreatment does not affect the performance ofthe HTL of microalgae. The lack of influence of the pretreatment on the quantityand quality of the liquefaction products could be related to the fact that themicroalgae samples were already disrupted during the drying process toprepare the powder biomass. Moreover, the poor performance of the sonicationbath, in terms of achieving the microalgae cell breakage, is also considered asexplanation. Therefore, the analysis of the ultrasonic equipment was done tounderstand the reason for its poor operation. The characteristics of thesonication equipment (configuration, ultrasonic power output, and energyfrequency) were defined for future experiments.
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