Lignin is a natural resource which exists in woody materials, agricultural residues, and other plant materials (so-called lignocellulosic materials). Lignocellulosic materials consist of 10–30% lignin by weight and 40% by energy. However, it has mainly been used as an energy source in combustion processes, and less than 5% lignin has been used for other purposes nowadays.
Because of its high energy content and polymer structure, lignin is considered as a potentially renewable resource of chemicals and fuels especially in the condition of escalating petroleum price and renewable energy demand. Lignin depolymerization is the very promising process which can generate value-added products from lignin raw materials. The primary purpose of lignin depolymerization is to convert the complex lignin compound into small molecules for fuels and basic chemicals or oligomers for further application.
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A considerable amount of research has been done to convert lignin into renewable fuels and chemicals using pyrolysis and gasification methods. Pyrolysis refers to the thermal treatment of the biomass or lignin in the absence of oxygen, with or without any catalyst usually at the temperature between 300 and 600°C. The cleavage of OH functional group linked to an aliphatic side chain, the breaking of alkyl side chain, aryl ether, and linkage between aromatic rings occur when temperature increases, forming a mixture of phenol, guaiacol, syringol, and catechol’s. Moreover, the aromatic ring cracking occurs at the temperature above 500°C. However, the process is highly complex and is affected by several factors, including feedstock type, heating rate, and reaction temperature.