This method is the most traditional form of charcoal production all over the world. It is probably the most common way to produce biochar at slow heating rates, relatively low temperature (from 300 to 600 °C) and long residence time. Slow pyrolysis is recommended for solid production (20-40% of charcoal or biochar), once the operational conditions reduce other fraction such as gas and oil. Typically, one third of each fraction is produced when feedstock is slowly pyrolyzed and the higher the temperature, the lower is the charcoal yield. At low temperatures, lignin is partially degraded and some residual cellulose can still be present. Short residence time can lead to the same result, Which is incomplete carbonization and also tar residue impregnates the resulting charcoal. For energy, the quality of slow pyrolysis charcoal is low, but for biochar, this depends on the desired function of the final product. In general, low temperature and residence time lead to a highly functionalized biochar, since the incomplete degradation and insufficient tar draining preserve part of the original structure of carbohydrates and phennols, maintaining a high content of hydroxyl and carboxyl groups. These characteristics are desirable when the main intended function of biochar is to enhance the soil cationic exchange capacity (CEC).
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