Amongst the well-known carbonization processes, pyrolysis, gasification, hydrothermal carbonization (HTC), and torrefaction are generally employed to obtain biochar from several raw materials and for various types of applications.

Slow pyrolysis is a thermal conversion technology conducted at temperatures between 300–800 °C, aiming to maximize biochar yield. The process is performed at atmospheric pressure, and it is characterized by a relatively long residence time and low heating rates ^[1][8]. Different types of reactors have been used for biochar production via slow pyrolysis, such as agitated drum sand rotating kilns, wagon reactors, and paddle pyrolysis kilns. Moreover, in this process, high biochar yields are favored when using feedstocks with high lignin and ash content, along with large particle sizes. These characteristics improve biochar yield by increasing cracking reactions that reduce the amount of bio-oil (liquid products). On the other hand, fast pyrolysis offers particularly promising advantages in maximizing bio-oil yield (up to 75 wt.%), basically due to the very significant heating rates (over 200 °C/min) and shorter residence times ^[2][9].

Unlike pyrolysis, the gasification process is carried out in the presence of an oxidizing agent, and it is primarily used for syngas production (i.e., H₂, CO, CO₂, CH₄). As a result, biochar is considered a byproduct, and yields are low (<25%), resulting in limited research on the feasibility of biochar production ^[3][4][3,10].

Besides pyrolysis and gasification, torrefaction is an emerging approach for biochar production. In this process, moisture, CO₂, and O₂ contained in biomass are removed under inert conditions at 200–300 °C and long polysaccharide chains are depolymerized to produce a hydrophobic solid product with a low O/C ratio ^[5][11]. This process is generally operated with a slow heating rate; hence, it is also known as mild pyrolysis. Nonetheless, torrefaction is not considered a promising technique for biochar production, regardless of the higher product yields (70–80 wt.%), because torrefied biomass still contains a significant fraction of volatile components from raw biomass and the physical-chemical properties do not meet biochar requirements (e.g., O/C > 0.4). As a result, torrefaction is often used as a biomass pre-treatment process for moisture removal, feedstock densification, and increased brittleness ^[6][12].

Opposite to pyrolysis and torrefaction, which are carried out under a dry atmosphere, HTC proceeds in wet conditions and can also be referred to as wet pyrolysis or wet torrefaction. This process is performed in a biomass-water solution at temperatures of 180–300 °C and autogenous pressure (subcritical conditions) for several hours. Similar to pyrolysis, HTC presents significant biochar yields (50–80 wt.%), but also a liquid fraction composed of a bio-oil and water mixture (5–20 wt.%), and a gas phase that mainly includes CO₂ (2–5 wt.%) ^[7][13]. The great interest in HTC for biochar production is that the process can avoid the energy-intensive drying step that is usually required for conventional pyrolysis, and thus minimize operational costs. Also, HTC can convert feedstocks having >75 wt.% moisture content (diversifying feedstock options for biochar production) and decrease the leaching of salts and minerals, yielding biochars (or hydrochars) with reduced ash content ^[8][14].

Overall, slow pyrolysis is the preferred process for biochar production. The technology can be applied to almost all types of biomass feedstocks and the slow heating rates, coupled with low temperatures and long residence times, are appropriate for the formation of stable carbonaceous solid materials ^[9][15]. Moreover, it should be highlighted that for the above-mentioned processes, particularly pyrolysis, torrefaction, and HTC, there are other products of interest, such as bio-oil, which can be further processed into drop-in liquid biofuels; wood vinegar, which can be applied as a biopesticide; or HTC process water, which shows potential to be used in hydrothermal gasification for producing renewable gases or synthetic liquid biofuels. Addressing the application of these by-products is of extreme relevance to achieving circularity and, consequently, increased sustainability in biochar production.

Table 1 summarizes and compares the typical operating conditions and biochar yields of the described biochar production processes.

18].

^3][45][3,50]. Table 4 presents studies focused on possible biochar uses for GHG mitigation.

Although the use of biochar has demonstrated promising results for CO₂ capture contained in flue gases and GHG mitigation when applied to agricultural soils, results are strongly dependent on operational or application conditions. According to these studies, parameters like temperature and pressure significantly influenced CO₂-capture processes, while crop type and cultivation period affected GHG production during crop cultivation. Therefore, optimal conditions must be defined through field tests before establishing the best biochar for market purposes and intended applications.

Biochars can be considered a new low-cost alternative to commercial activated carbon applied in water disinfection and wastewater remediation processes. Batch adsorption studies have shown that biochars have significant adsorption capacities for contaminants present in real wastewaters, which is justified by their macroporous surface structure. These materials are therefore capable of remediating complex wastewaters while avoiding premature pore-clogging. The lower cost and history of land application combined with the need to remove new pollutants (e.g., antibiotics) has led to an increased interest in exploring biochars for new remediation solutions ^[50][55].

Conventional remediation strategies include, for instance, reverse osmosis, chemical oxidation or reduction, and precipitation. The use of biochars to adsorb aqueous contaminants presents important advantages over the aforementioned treatments, namely lower costs and the minimization of secondary by-products (e.g., sludges) [3]. Furthermore, biochar’s surface characteristics may be enhanced through activation methods to reach a higher degree of porosity and density of functional groups, enabling their application in the removal of aqueous organic and inorganic pollutants. These activation processes can be categorized into physical or chemical activation, including ball milling, acid-base modification, clay mineral modification, or metal oxide modification ^[51][56]. Activation treatments can further develop biochar’s pore structure and allow the development of functional groups (e.g., -COOH, -OH, and -CHO) that promote the capture of cationic and anionic inorganic contaminants, as well as organic pollutants (e.g., phenolic compounds and pesticides). Table 5 shows studies focused on biochar applications for wastewater treatment and general pollutant removal.

In line with the chosen production process, the physical-chemical properties of biochar are very important to define its final application. Biochar characteristics and yields are highly dependent on feedstock and operation parameters, particularly temperature. Ippolito et al. (2020) studied the influence of feedstock choice and process parameters on main biochar properties through a meta-analysis. The authors assessed that process type plays a minor role in biochar’s physical-chemical properties, whereas temperature is the dominating parameter. Higher process temperatures can be responsible for increased carbon content and specific surface area (SSA) properties that promote soil improvement when using biochar. The authors also stated that feedstock choice has the largest influence on biochar properties, with wood-based feedstocks presenting higher SSA and crop- and grass-based biochars showing increased cation exchange capacities (CEC). The overall results of the study, including temperature and feedstock variations, are represented in Table 2 ^[12][

21]. Regarding biochar yields and feedstock variability and composition, in general, higher biochar yields can be obtained from feedstocks with higher ash contents, but the effect is less pronounced for ash contents >5% ^[16][22]. According to different studies, cellulose and hemicelluloses are the most promising components in producing volatile products via thermochemical conversion (e.g., pyrolysis) because these two compounds have a lower molecular weight than lignin and are easily released as pyrolytic gas. On the other hand, lignin is the main component responsible for biochar production due to its resistance to thermal degradation; as such, feedstocks with higher lignin contents generally lead to higher biochar yields ^[13][14][19,20].

Temperature is considered the most important parameter in controlling carbonization reaction mechanisms. This property influences the characteristics and yield of biochar to a greater extent when compared with residence time, heating rate, or feedstock particle size ^[1][8]. In general, process temperature affects SSA, pH, carbon content, stability, volatile fraction, and other biochar physical-chemical properties. Biochar produced at low temperatures can present high acidity, polarity, and low aromatic content, as well as low hydrophobicity. When process temperature is increased, acid functional groups (e.g., hydroxyl or carboxyl) and mass yields are reduced, meaning that alkaline functional groups increase along with pH and ash content. In addition, as a consequence of higher process temperature, volatile compounds are further released, resulting in larger SSA values and a more developed pore structure (increased PV) ^[2][9]. These features of high-temperature biochars indicate that their most suitable applications are related to the sorption or retention of nutrients and contaminants (organic and inorganic), while PV is assumed to affect water availability and soil aeration. Some authors have been emphasizing that biochar particle size can affect plant nutrient content, nutrient availability in growing media or soils, and PAH content ^[17][18][19][23,24,25]. The addition of biochar particles of different sizes can directly affect biochar-soil interactions, causing changes in the soil’s physical properties. The smaller the biochar particles, the better the mixing and interaction with soil particles ^[20][26]. Given that biochar’s characteristics are influenced by several parameters, the corresponding biochar properties also vary widely. This fact relates to arguably the most prominent aspect of biochar as a marketable product: the ability to be “tailor-made”. Since biochar is becoming increasingly used in several areas, standardization before its final use is extremely important to generalize and predict its performance in different applications.

In the following subchapters, a description of potential biochar applications and related studies is presented to provide an idea regarding market diversity for these materials.

Several studies have reported that the use of biochar for soil amendment improves soil physical properties, hydrological characteristics, water content, and water use efficiency, as well as soil fertility and crop yields ^[21][27]. Mixing biochar with decomposed manures, composts, and crop residues also improves nutrient use efficiency.

Soil application methods are heavily influenced by farming system type, labor availability, and power machinery available ^[22][28]. In Portugal, soils have very little carbon content. Thus, “tailor-made” biochars can be developed for particular soils and crops to achieve specific outcomes ^[23][29]. Despite these benefits, the feasibility of using biomass wastes to produce biochar for subsequent use in agriculture depends on its environmental and economic performance. Limitations exist since farmers are often risk-averse and have less investment capacity than other potential users, and there is still an enormous variability in the predictability of biochar impacts ^[24][25][30,31]. Agricultural biochar markets are also very seasonal, requiring producers to store large quantities of biochar or find alternative markets. The European biochar market has been mostly focused on livestock, with 90% of the biochar produced being used in livestock farming, whether mixed with feed, added to litter, or used in the treatment of slurries. This situation may be mainly due to the lack of regulation regarding the application of biochar as a soil amendment ^[26][32]. Therefore, in terms of marketability, it is important to understand which benefits matter the most to each farmer and which specific product biochar can potentially replace. Furthermore, the cost of biochar is critical for determining livestock pricing ^[27][33]. Some published cases of biochar use in agriculture are summarized in Table 3.

As seen in Table 2, different feedstocks show different properties that affect biochar mass and energy yields and their designated applications ^[1][13][8,19]. Feedstocks rich in nutrients, such as manures and biosolids, produce biochar with high nutrient content, which is reflected in their values of CEC, AEC, CCE, and ash content. Moreover, wood-based biochar presents increased values of SSA and PV, meaning that these biochars have very significant potential for the removal of organic pollutants, carbon sequestration, and amending soil pH ^[14][20]. The pore size may vary between 2–18 nm (mesopore range) when the biochar is obtained from the pyrolysis of rice straw and tends to decrease with the process temperature ^[15][

Currently, 60% of the global warming effect is caused by CO₂ emissions, meaning that new strategies must be implemented to control carbon dioxide levels in the atmosphere. Biochar has an interesting ability to retain significant amounts of carbon for longer periods that may range from decades to thousands of years. In particular, biochar can be used for carbon sequestration by retaining CO₂ captured by the vegetable feedstock used for its production. When applied as a soil amendment, biochar contributes to climate change mitigation by fixing carbon in stable aromatic bonds that are resistant to microbial degradation. This stability reduces immediate labile carbon release into the atmosphere. Moreover, other GHG emissions such as N₂O and CH₄ are significantly minimized, depending on soil type, with reductions that may achieve more than 50%, considering the introduction of biochar amounts equivalent to 10% of soil mass and 20 t ha⁻¹. Conversion of animal or vegetable feedstocks into biochars also minimizes GHG emissions through the natural decomposition of such feedstocks ^[3][43][44][3,4,49].

A different carbon sequestration method involves the use of biochars to adsorb the CO₂ contained in industrial flue gases as a replacement for other high-cost materials (e.g., zeolites, porous polymers, and metal oxides). This process takes advantage of the good properties of biochar in terms of porosity and surface area (0.4–0.9 cm³ g⁻¹ and 1000–2000 m² g⁻¹, respectively), but requires a chemical activation post-treatment using KOH or sodium amide. Activated biochars produced from biomass feedstocks like hazelnuts, garlic peels, and olive oil wastes have shown adequate properties to adsorb CO₂, with retention efficiencies between 3.5–6.2 mmol g⁻¹. As an alternative, flue gases may be conducted through a bed of biochar heated at high temperatures and in the absence of oxygen to convert CO₂ into CO that may be employed for subsequent energy applications ^[

This literature survey demonstrated the large spectrum of contaminants that may be removed with biochars, as well as the diversity of effluents that may be remediated considering different adsorption techniques. In fact, the adsorption performances obtained in most studies were considered sufficiently good even without any biochar activation of physical or chemical nature, which represents a significant advantage in terms of lower energy demands, investment, and by-product generation during biochar preparation. Other benefits include plant biomass development when biochars are applied in constructed wetlands while performing wastewater remediation. These applications suggest that environmental remediation may be a promising strategy for biochar valorization in the near future with the emergence of new pollutants generated by households, rural activities, and industry.