2.2. Factors affecting biochar properties
Table 2 summarizes the important key parameters collected from recently published studies on biochar production and their characteristics. A number of feedstocks including crop residues, wood biomass, animal litter, and solid waste have been utilized to produce biochar via slow to intermediate pyrolysis processes. The pyrolysis temperature in these studies varied from 100 to 900 °C with heating rates ranging from 2.5 to 20 °C min−1. Biochar yield was dependent on the feedstock type, pyrolysis temperature, and heating rate. Generally, animal litter and solid waste generated a high yield of biochar compared to that from crop residues and wood biomasses (Enders et al., 2012). The high yield is related to the higher inorganic constituents of the feedstock materials, as indicated by their relatively high ash content. Cantrell et al. (2012) suggested that various inherent metals in animal litter may protect against the loss of volatile material by changing the bond dissociation energies of organic and inorganic C bonds. This finding was supported by Raveendran et al. (1995) who reported a high biochar yield from rice husk, groundnut shells, coir pith, and wheat straw due to the higher levels of K and Zn. Generally, biomass with high lignin content results in high biochar yields (Sohi et al., 2010). Heating rate was the least effective factor in determining biochar yield. Karaosmanoğlu et al. (2000) reported a slight decrease in biochar yield with an increase of the heating rate from 5 to 15 °C min−1.