Читать книгу Handbook of Biomass Valorization for Industrial Applications - Группа авторов - Страница 54

Pyrolysis is one of the important processes for the transformation of biomass to energy. During the pyrolysis the biomass is heated at high temperature with a higher heating rate and cleaves the chemical bonds present in the components of cellulose, hemicellulose and lignin. The products of pyrolysis are oxygenates which are formed from the depolymerization of the biomass made up of phenol, furan and aromatic derivatives. Oxygenates also known as bio-oils can be produced from other liquefaction technologies such as hydrothermal and solvothermal liquefaction, which is usually carried out under high pressure and low temperatures. Depending on the process involved bio-oils obtained have different properties with different oxygen content, carbon percentage, water content and viscosity. The bio-oils are oxygen rich; it is required to selectively reduce the oxygen content for the application as the transportation fuel [15].

Upgradation of bio-oils can be performed by catalytic transformation. Pyrolysis can be classified on slow pyrolysis and fast pyrolysis depending on the heating rate. Slow pyrolysis is carried out at temperatures 400–600 °C @ 0.1–1 °C/s. Fast Pyrolysis is carried out at 400–650 °C @ 10–100°C/s with a residence time of seconds only. Fast pyrolysis results in biphasic liquid with bio-oils and aqueous phase, gases and chars. Single phase bio-oils can achieve by carrying out pyrolysis at lower temperature or with catalytic system. Different reactors such as fluidized and spouted bed, ablative, and vacuum pyrolizers have been assessed for the pyrolysis [16]. Pyrolysis mechanism consists of different reactions such as cracking, decarboxylation, hydrocracking, decarbonylation, hydrodeoxygenation, and hydrogenation. Bio-oils obtained from the pyrolysis are rich in oxygen and have low energy density. The catalytic upgradation is carried out to reduce the oxygen content as water and carbon dioxide. The products obtained with catalytic pyrolysis have similar structure as that of the petrochemical products. The nature of bio-oils depends on the properties of the catalyst such acidity, metal composition, pore size [17, 18]. Pyrolysis is carried out using different alkali metals (NaCl, KCl), alkaline earth metals (MgCl2, CaCl₂), metal oxides (CaO and La₂O₃), and transition elements. The effect of transition metal (Fe & Ni) was studied by Collard et al. [19]. The catalyst containing Fe results in increased production of char with low tar formation, whereas catalysts with Ni metal catalyzed the depolymerization of amorphous xylan with high yields of furans, aromatics and hydrogen. Pd, Ru, Ni showed the selectivity towards the aromatics and showed high potential to convert the oxygenates to hydrocarbons [20]. Apart from metal catalyzed pyrolysis, zeolites are also reported in various literatures. HZSM-5, H-beta, H-Y. The acidity in zeolites favors the formation of aromatics, Ga supported on ZSM-5 increase the selectivity towards the aromatics and reduce the char formation. The desilication of zeolites studies during the fast pyrolysis of lignocellulosic biomass showed enhanced selectivity towards the aromatics with less char formation [21–23]. Lu et al. reported the K₃PO₄ catalyzed biomass pyrolysis that results in high yields of phenolics derivatives. The selectivity increased with the catalyst loading first then decreased on further increasing the catalyst loading [24]. Hussain carried out the pyrolysis using cement as the catalyst to yield 7% oil, 14% water, 29% fuel gases and 50% char. Apart from the oil, it also contains acids, esters, ketones and aldehydes [25]. Shadangi et al. used alumina, CaO and Kaolin as the catalyst for the pyrolysis of biomass. Catalytic pyrolysis enhanced the quality of the oil but decreased the yield of the oil, whereas CaO decreased the viscosity of the pyrolytic oil. Catalysts such as silica with weak acidity and moderate pore size (16 nm) found to be highly efficient in deoxygenation and increased the hydrocarbon content [26, 27].