Biochar Production and Properties: Comparison
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Biochar (BC) is the solid residue recovered from the thermal cracking of biomasses in an oxygen-poor atmosphere. BC has been increasingly explored as a sustainable, inexpensive, and viable alternative to traditional carbonaceous fillers for the development of polymer-based composites. In fact, BC exhibits high thermal stability, high surface area, and electrical conductivity; moreover, its main properties can be properly tuned by controlling the conditions of the production process.

  • biochar
  • zero-waste approach
  • circular economy

1. Introduction

Environmental safety and the progressive depletion of fossil fuel-based sources are currently a great concern for both academic and industrial research. As a result, there is an increasing interest in sustainable manufacturing [1,2,3][1][2][3]. The investigation of new eco-sustainable and bio-based composites has gained great attention, especially concerning eco-friendly systems derived from waste and renewable resources [4]. Accordingly, a promising alternative to conventional carbonaceous fillers is biochar (BC), a carbonaceous and renewable material produced by the thermo-chemical conversion of biomasses in an oxygen-limited environment [5,6,7][5][6][7]. Unlike other carbon-based materials, BC is derived from sustainable biomass resources and possesses high thermal stability and hardness, high surface area, good chemical stability, and electrical conductivity [8,9,10,11][8][9][10][11]. Up to now, BC has been widely investigated for environmental remediation [12[12][13][14],13,14], as catalyst support [15], and for energy storage applications [16]. Nevertheless, BC-based composites still require optimization to reach performance comparable to traditional carbon-based fillers such as graphene and carbon nanotubes. These materials can be used to reach great composites performance, but they are costly. In 2020, the price of single layer graphene was higher than USD 230/cm2, while for graphene oxide the price was USD 140/kg. Conversely, carbon black was sold for around USD 1.2/kg [8]. Compared to a cheap carbon filler such as carbon black, BC has a lower cost and is derived from biomass. In this context, investigations dealing with the formulation of BC-containing polymeric systems based either on thermoplastic or thermosetting matrices have been increasing exponentially in recent years. Considering its intriguing characteristics, along with the possibility to tailor its structure and functionalization, BC represents an attractive alternative to traditional carbonaceous fillers for improving the mechanical, electrical, and physical properties of polymer-based composites [4,17,18][4][17][18]. Nevertheless, the relation between the properties of BC and those of its composites is hard to establish, mainly due to the great BC variability [19]. The spread of the use of BC for the production of polymeric composites brings about the need of a reference point for both the specialists and the newcomers in the field.

2. A Brief Overview of BC Production and Properties

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][21][23][24][25][26] and plant design [7,27][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][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].

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