4. Biochar productionBiochar is pro

4. Biochar productionBiochar is produced by the thermal conversion of biomass. Although biochar and compost are both produced from organic wastes, they do not compete for the same feedstocks. An ideal feedstock for biochar production has a low moisture level (10%–20%) and a high lignin content, such as field residues or woody wastes (Fig. 2). In contrast, composting is more suitable for wastes that have a high moisture content (60%–70%) and a high nutrient value, such as sewage sludge, livestock manure, or animal mortalities (IBI, 2015; Steiner et al., 2015). Biochar is mainly produced by pyrolysis, which involves the thermochemical conversion of dry biomass at elevated temperatures (300–650 _C) in the absence of oxygen. The process results in the formation of a carbon-rich solid product (biochar), a liquid phase (bio-oil), and non-condensable gases (CO, CO2, CH4, and H2). Among the various techniques employed (Table 1), slow pyrolysis (heating rates ranging from 10 to 30 _C/min) is considered as the main process for biochar production (Cha et al., 2016; Kambo and Dutta, 2015). The reaction time, temperature, heating rates, and initial moisture content of the biomass are the key parameters affecting the physicochemical properties of the biochar produced and percentage yield (Kambo and Dutta, 2015; Xiao et al., 2017). Hydrothermal carbonization (HTC), is performed over a temperature range of 180–260 _C under autogenic pressure (2–6 MPa). The advantage of the HTC process is that it eliminates the predrying procedure required for wet biomass. It is suitable for biomass with a high moisture content since, during the process, water approaches its supercritical condition and serves as both a reactant and a catalyst (Peterson et al., 2008). It also has a higher carbon efficiency. Although the HTC process has certain advantages over pyrolysis, hydrochars have been reported to have lower surface areas and porosities compared with biochars. When they are applied to the soil, they decompose rapidly regardless of solid type (50% in 100 days), indicating a lower carbon stability and carbon sequestration potential compared with most pyrolysis-derived biochars (Kambo and Dutta, 2015). Torrefaction, also known as mild pyrolysis, is a process during which biomass is heated at temperatures of approximately 200–300 _C. Technically, torrefied biomass cannot be considered as biochar because it has properties intermediate between those of raw biomass and biochar (Kambo and Dutta, 2015). The torrefaction process is used in the field of bioenergy as an important preprocessing step for improving the properties of biomass for combustion (da Silva et al., 2018). Gasification is a heating process in which biomass is converted to gases and ash at high temperatures (600–900 _C) and short residence times (10–20 s). A small amount of biochar (