**3. Biochar systems**

Biochar is the porous solid carbonaceous material derived from the thermochemical conversion of biomass in an oxygen-limited environment [9]. It can be produced from various biomass feedstocks, including wood, wood waste, agricultural wastes (e.g., straw, rice husk), wastewater sludge, and food waste [8]. The most commonly used thermochemical conversion process for biochar production is pyrolysis, though other processes, such as gasification, torrefaction, and hydrothermal conversion, can also be used [34]. Pyrolysis is the thermochemical decomposition of biomass into condensable liquids, non-condensable gases, and biochar in the absence of oxygen [35]. The distribution of these end products and their properties depends on the process conditions (i.e., temperature, heating rate, and residence time) and the type of biomass feedstock [36]. Based on the process conditions, pyrolysis is classified as slow, fast, rapid, or flash, with slow pyrolysis being more appropriate for a biochar targeted product [37].

Biochar systems using pyrolysis can be deployed at different scales (small-, mediumand large–scale) and can perform multiple functions, as they can be used for biowaste treatment and bioenergy generation, along with biochar production and use [38]. Bioenergy can be produced through the combustion of the pyrolytic gas and oil products, known as syngas and bio-oil (or bio-tar), respectively. Moreover, bioenergy can be produced by using the produced biochar as solid fuel [9]. In addition to bioenergy production, biochar can be used for a variety of applications, mainly because of its versatile physicochemical properties [8, 10].

### *The Role of Biochar Systems in the Circular Economy: Biomass Waste Valorization and Soil… DOI: http://dx.doi.org/10.5772/intechopen.104389*

The most prominent application of biochar is probably its application to soils. Biochar can be used as a soil amendment for agricultural soils, as it can improve their physicochemical properties and structure, increasing soil fertility and crop productivity [34, 37, 39]. At the same time, the production of biochar and its incorporation into soils sequesters carbon. More specifically, the thermo-chemical conversion of biomass into biochar increases the recalcitrance of carbon, enhancing its resistance to chemical and biological degradation [34]. Thus, when biochar is incorporated into the soil, the return of biomass carbon to the atmosphere as CO2 is impeded [11, 40]. In this way, biochar can act as a carbon sink, thereby contributing to climate change mitigation, and for that reason, the production of biochar with its incorporation in soils has been recognized as a carbon dioxide–removal (CDR) technology [41].

Besides soil amendment and carbon sequestration, biochar has numerous applications across various sectors. Biochar can be used as an additive for production of cement [13], cement mortar [42] and concrete [43], adsorbent for wastewater treatment and water purification, coke replacement in metallurgical processes, raw material for the manufacture of activated carbon, and novel specialty materials for electronic devices, such as carbon nanotubes and nanosheets [10], and platform material for energy storage and conversion, including hydrogen storage and production, fuel cells and lithium/sodium-ion batteries [44]. It can also be used as a feed supplement for poultry or ruminants to improve the health and productivity of the animals, reduce odors and nutrient losses from the manure, and serve, in combination with the manure, as a slow-release fertilizer [45]. Moreover, the sorption properties of biochar have sparked an interest in the use of biochar for remediating soils contaminated with organic and/or inorganic pollutants [9, 46, 47].
