# Technology-Economy-Policy: Biochar in the Low-Carbon Energy Transition--A Review
## 4 Technologies: Biochar Production
### Pyrolysis  
Pyrolysis is a process in which biomass undergoes thermal decomposition in the absence of oxygen. The process of biomass pyrolysis is used on an industrial scale [64, 65, 66, 67]. The products of the process are carbonizate, which is a highly carbonized solid, bio-oil, also known as pyrolytic oil, and gas [64, 68]. There are two stages in the process, i.e., the primary and secondary reactions. The primary stage includes the processes of dehydration, dehydrogenation, and decarboxylation [69]. Once the primary reaction is complete, a secondary reaction begins, which includes the breaking of large molecules and the transformation of solids into biochar and gases [63]. Based on the process parameters used, a distinction can be made between slow (biocarbonization), moderate, fast, and flash pyrolysis [8]. Slow pyrolysis involves heating at a lower rate (0.1 to 1 \({}^{\circ}\)C s\({}^{-1}\)) for a long time (hours to days) at a temperature of 300-900 \({}^{\circ}\)C. The slow pyrolysis process provides a favorable environment for secondary processes and results in the increased production of biochar. In fast pyrolysis, biomass is heated at a higher temperature (300-1000 \({}^{\circ}\)C) and a high heating rate (10-1000 \({}^{\circ}\)C s\({}^{-1}\)) for a short time (0.5-2 s) [70]. The main products produced during the pyrolysis process include a solid (biochar), a liquid (bio-oil), and a gas (syngas). Fast pyrolysis (temperature of 500 \({}^{\circ}\)C, residence time at final temperature of 1 s) yields about 12% of biochar [50]. It should be noted that fast pyrolysis generally focuses on the production of bio-oil, while slow pyrolysis seeks to generate more biochar [71]. Moderate pyrolysis (temperature 400 \({}^{\circ}\)C, residence time at final temperature 10-20s) yields about 20 to 25% of biochar. The highest percentage of biochar obtained at about 35% can be achieved using slow pyrolysis (temperature 400-500 \({}^{\circ}\)C, residence time at final temperature of 5-30 min) [72; 73]. Flash pyrolysis is a rapid thermal decomposition process typically occurring at temperatures between 400 \({}^{\circ}\)C and 700 \({}^{\circ}\)C, with heating rates exceeding 1000 \({}^{\circ}\)C/min and very short residence times of 0.5 to 10 s, aimed at maximizing the production of bio-oil and gases while minimizing biochar yield. Biochar is a solid carbon material that can be used as a catalyst, adsorbent, and fuel. Syngas consists of CH\({}_{4}\), CO\({}_{2}\), H\({}_{2}\), CO, and other low-molecular-weight gases, which can be used in gas engines after processing. Bio-oil consists of water, alcohol, phenolic compounds, aliphatic and aromatic hydrocarbons, and nitrogenous compounds (pyrazines, pyridines, andamines, etc.), which can be used in boilers to generate heat [63; 74].  
The pyrolysis temperature affects the properties of the biochar, such as the surface area, functional groups, and porosity. The increase in temperature causes an increase in the surface area and porosity due to the breakdown of the aliphatic alkyl and ester groups present in organic compounds, leading to the removal of pore-blocking substances. Biochar produced at a lower temperature is hydrophilic and simulates the structure of graphene, with fewer functional groups on its surface. In contrast, biochar produced at higher temperatures is hydrophobic; the functional groups are reshuffled and new groups are introduced, such as carboxyl, lactone, phenol, and pyridine, etc., which can act as electron donors and acceptors [75; 76]. The temperature of pyrolysis and the types of raw materials are the most important factors in determining the properties of carbonates and thus their application [77]. The types of biomass feedstock affect the yield and quality of biochar. Pyrolysis of forest plants yields 30% of biochar compared to 45.69% of biochar produced from lignin, indicating that biochar yields depend on the lignin content [78; 79].