1. Introduction

BC is produced through thermochemical cracking of biomasses following three main routes named hydrothermal liquefaction, pyrolysis, and gasification.

Hydrothermal liquefaction is a thermochemical conversion operating in a temperature range up to 350 °C, in a water medium, and under moderate pressure. This procedure promotes advanced depolymerization of biomass giving rise to highly functionalized BC named hydrochar [20].

Proper pyrolytic processes take place at temperatures above 400 °C in an oxygen-limited [21] or inert atmosphere [7]. By using the pyrolytic approach, it is possible to achieve a fast and advanced cracking process of each biomass component (lignin, cellulose, and hemicellulose) with the simultaneous production of BC, bio-oils, and non-condensable gases [22] with a wide variation in fraction yields based on heating technologies [21,23,24,25,26] and plant design [7,27].

Gasification is the other route for BC production that is run in an oxidant atmosphere by using air [28], oxygen, or even steam [29] with temperatures higher than 800 °C. The combination of high temperature and oxidant atmosphere induces the conversion of biomass into a gas mixture mainly composed of hydrogen, methane, carbon dioxide, carbon monoxide, and steam. The solid output of gasification is a BC with a very high ash content and low carbon percentage. In a common pyrolytic process, biomass undergoes proper carbonization at temperatures ranging from 300 °C to 400 °C with cracking of its components through complex reaction routes and forming. In this stage, BC is massively tailored with oxygen-based functionalities (i.e., hydroxyl, carbonyl, and carboxylic residues) and displays a highly defective carbon structure. By increasing the temperature from 600 °C to 800 °C, the aromatic structures further condense, forming proper graphite-like domains, still highly disordered but with less residual groups. These materials are commonly classified as hard carbon due to their high mechanical hardness [30]. Further temperature increments lead to a progressive enlargement and ordering process of graphitic domains through turbostatical rearrangement [31] that ends at about 3000 °C when the maximum graphitization degree is reached [32]. As widely discussed by Weber et al. [33], the properties of BC (i.e., surface area, porosity, grindability, etc.) originated from a complex combination of interactions due to the morphology and chemical composition of the feedstock and can be tailored by post-treatments such as surface tailoring or activation [34].

The technology chosen for BC production plays a crucial role in determining the final properties of the filler, and it is related to a complex combination of economic and strategic features. All properties of BC are simultaneously affected by all the selected process parameters such as production temperature, reactor design, and feedstock used [35], and it is hard to establish systematic and general rules for their simultaneous optimization. Nevertheless, the quality of BC can be ensured for large scale-production over time [36].

As far as the influence of feedstock is concerned, it has been demonstrated that wood-derived BC exhibits a highly volatile content compared to materials obtained from non-woody sources. Furthermore, depending on the type of feedstock, a variation in the quality and amount of heteroatoms and metal elements incorporated into BC has been observed [37].

In general, an increase in pyrolysis temperature, apart from the already discussed structural modifications, causes a decrease in the content of functional groups, thus affecting the affinity of the obtained BC toward polar moieties. In particular, a concurrent decrease of the O/C and N/C ratios is usually observed in BC pyrolyzed at high temperatures due to the occurrence of temperature-induced dehydration and decarboxylation processes [38,39]. Furthermore, pyrolysis processes performed at high temperatures induce an improvement in the solvent absorption capability of BC, because of the formation of nonporous structures [40].

Finally, it has been shown that the reactors employed for BC production have a marginal effect on the elemental carbon content, surface functionalities, and thermal degradation of BC. Interestingly, Das et al. reported a significant effect of the pyrolysis reactor on the fire resistance of the resulting BC. In particular, they showed that BC obtained in a hydrothermal reactor exhibits high fire resistance due to the presence of tarry volatiles, which are able to seal water molecules within the BC pores, thus hindering the material combustion [41].