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Energies 2019, 12, 809 20 of 41 HZSM-5 is considered a good catalyst for bio-oil upgrading through zeolite cracking as it provides high yields of liquid oil and propylene. 5.4. Hydrothermal Liquefaction of Biomass and Upgrading of Bio-Crude Hydrothermal liquefaction (HTL), also known as hydrous pyrolysis or direct liquefaction, is another thermal technology used for the conversion of biomass into a liquid product similar to bio-oil which is usually called bio-crude. During the HTL process the biomass is converted directly into bio-crude and other aqueous, gaseous and solid phase byproducts in presence of a solvent and sometimes of a catalyst. The most common solvent is water but it usually produces low yields of bio-crudes with higher oxygen content and lower heating values. Other organic solvents such as alcohols, phenol, esters, ketones, tetralin and various alcohol-water mixtures can provide higher bio-crude yields with lower oxygen content and can serve also as donors of hydrogen during the process [184]. The catalyst may be used to inhibit the formation of char and tar [185]. This is usually a homogeneous catalyst such as sulfuric acid, oxalic acid, hydrochloric acid, sodium hydroxide or a metallic salt but various heterogeneous catalysts are also under investigation. The HTL process usually takes place at temperatures of 200–400 ◦C and at elevated pressures of 50 to 200 bar with relative long residence times of 10–60 min. The high process pressure sustains the produced solvents in the liquid state and increases the yield of liquid bio-crude but also increases the cost of the HTL technology. The mechanism of the HTL process is complex and still under investigation. It proceeds with the depolymerization of the biomass macromolecules into smaller compounds and with various chemical and thermal decompositions of monomers by cleavage, dehydration, decarboxylation and deamination (removal of aminoacids) which create smaller fragments and remove oxygen from the biomass product. Finally the created fragments are rearranged through condensation, cyclization and repolymerization [184,186]. Ramirez et al. [187] have reported that the chemical composition of bio-crudes varies significantly and contains various groups of chemicals such as phenolics, esters, aromatics and heterocyclics, aldehydes, carboxylic acids, ketones, saturated hydrocarbons and nitrogenates. A general comparison of HTL bio-crudes with the pyrolysis bio-oils shows that bio-crude has a higher carbon content (68–81% vs. 56–66%), lower oxygen content (9–25% vs. 27–38%), lower water content (6–25% vs. 24–52%) and higher heating value equal to about 28–41 MJ/kg [187,188]. On the other hand, HTL bio-crude has a much greater viscosity than bio-oil, petroleum diesel and biodiesel. Although similar to the biomass pyrolysis, the HTL does not need the preliminary drying of the feedstock and therefore wet feeds of lower cost can be exploited such as the microalgae which are cultivated in aqueous media. Dimitriadis and Bezergianni [189] have reviewed the most useful biomass feedstocks studied for HTL including lignocellulosic (woody) biomass, wastes (sewage sludge, solid wastes, swine manure, pulp/paper sludge powder, agricultural residues and food processing wastes) and microalgae. According to their review, the yield of bio-oil from these sources ranges between 15 and 68 wt% depending on the feedstock type and size, the operation parameters, catalyst and solvent type and solvent to biomass mass ratio. Xu et al. [190] have reviewed the catalytic HTL of algae over homogeneous catalysts such as CH3COOH, HCOOH, KOH and Na2CO3 and have reported that Na2CO3 provides the highest bio-crude yields between 20.4 and 51.6% for various algae varieties at 280–360 ◦C and with residence times of 30 and 60 min. The same study also reviewed the HTL of algae over various heterogeneous catalysts such as Pd/C, Pt/C, Ni/SiO2-Al2O3, CoMo/Al2O3, Ce/HZSM-5 and Ru/C and concluded that Ni/SiO2-Al2O3 is the best option in terms of bio-crude yield and cost and Ce/HZSM-5 is the best option in terms of bio-crude quality providing high hydrogen content, high heating value, high yield and low heteroatom content. Due to the similarity of pyrolysis bio-oil and HTL bio-crude, the upgrading of the bio-crude is accomplished with the same hydrotreating and zeolite cracking technologies mentioned before in the Sections 5.2 and 5.3. The hydrotreatment of bio-crude may proceed in two stages as it was discussed also for bio-oil and over the same catalysts. The initial mild hydrotreatment stage is usefulPDF Image | Green Diesel: Biomass Feedstocks, Production Technologies
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