**2. Lignin valorization processes**

 To describe how to use lignin as a valuable resource (see **Figure 1** ), this chapter initially describes the hydrogenolysis of lignin. Afterward, the pyrolysis and gasification are described that lignin might be used as an energy source. Material collection on a wet basis is discussed in the chapter on hydrothermal liquefaction (HTL). This chapter closes with a discussion of hydrothermal carbonization (HTC) of lignin.

## **2.1 Lignin hydrogenolysis**

 Lignin hydrogenolysis is a three-step process. First, lignin macromolecule functional groups are reduced without breaking the main structure. Second, macromolecules are broken into phenolics and arenes. Third, further reduction of second-step molecules into alkanes takes place.

## **2.2 Lignin pyrolysis and gasification**

 Lignin pyrolysis is an oxygen-free thermochemical conversion process. It aims to recover energy and other materials under a temperature ranging from 300 to 800°C [ 16 ]. Typical products of pyrolysis are biochar, bio-oil, and syngas. The quality of these products is highly dependent on temperature and dwell time. Longer residence time, or slower heating rate, reduces the bio-oil yield and boosts syngas content due to secondary cracking [ 17 ]. Lignin molecular weight is another important factor. Low molecular weight leads to the formation of CH 4 , CO, and CO 2 from the methoxy group, whereas high molecular weight yields guaiacol and alkyl guaiacol [ 18 ].

 While lignin pyrolysis is working in an oxygen-free environment for the production of syn-gas as the main product, which could also be of interest for future industries, as over 50% of syn-gas production is still petro-based and is based on the injection of pure oxygen or steam for stoichiometric balance, crucial for the

 **Figure 1.**  *Lignin valorization process.* 

production of higher quality syn-gas from biomass. The constitution of syn-gas is highly dependent on the biomass utilized, gasifier type, and agent, as well as operational conditions. It should be noted that lignin in itself is limited in hydrogen that is covalently bound to the structure ranging in the region of 5−6 wt%, which would be the maximum theoretically achievable ceiling, with respect to the energetic input needed for extraction. With gasification utilizing stream, the hydrogen output at higher temperatures is influenced due to the reaction between water and free carbon in the reaction chamber. Currently, the gasification of wood as the main feedstock is fairly common. However, the true interesting contender is biomass based on agricultural residues, such as straw, husks, shells, and more, which exhibit comparable values in syngas composition from the current benchmark in lab scale [19, 20].
