BIOCHAR - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 97

Similar to compost, biochar is a process-defined material, i.e. biochar is defined by the
process in which it is developed and produced rather than by a specific property, e.g.
chemical composition, molecular formulation, or mineral structure. Hence, biochar properties
encompass a wide range of characteristics, depending on feedstock material and pyrolysis
conditions.
Analogous to compost characterization specifications and certification, guidelines and
specifications to meet a set of minimal criteria were developed to serve as guidelines for
standardization of biochar product and use as soil amendment (IBI, 2012). The objective of
this chapter is to provide the framework for understanding the effect of pyrolysis conditions,
feedstock source, and composition on biochar characteristics, and to synthesize existing
knowledge and discuss the effect of biochar application on soil fertility.
E FFECT OF F EEDSTOCK AND P YROLYSIS C ONDITIONS
ON B IOCHAR P ROPERTIES
Common feedstock to produce biochar include organic waste (e.g. green and animal
waste), crop and timber residues (e.g. corn [Zea mays L.] and sorghum [Sorghum bicolor]
stover, sawdust), as well as dedicated cellulosic energy crops (e.g. switchgrass [Panicum
virgatum] and miscanthus [Miscanthus giganteus] ). Biochar is commonly produced at
temperatures ranging from 350 to 700 °C and its yield increases with (1) decrease in pyrolysis
temperature and (2) increase in pressure, feedstock density and particle size, ash, lignin, and
alkali and alkaline earth metal content (Antal et al., 1990; Richards & Zhang, 1991; Wornat et
al., 1992; Antal & Gronli, 2003; Demirbas, 2004; Yang et al., 2007; Mayer et al., 2012; Lee
et al., 2013).
Biochar porosity and surface properties have a marked impact on its ability to interact
with and retain water and nutrients in soil. Biochar is an amphoteric material with pH usually
above neutral; and as pyrolysis temperature and duration increase, surface area, pH, Lewis
base and ash, and C content also increase (Ramon et al., 1999; Rutherford et al., 2004; Yang
et al., 2007; Kwapinski et al., 2010; Tsai et al., 2012). As pyrolysis temperature increases and
cellulose and hemicellulose decompose, the aliphatic carbon content in the biochar decreases
while aromatic carbon content increases (Figure 1), leading to an increase in hydrophobicity
associated with basic groups in the aromatic structures (Chun et al., 2004; Demirbas, 2004;
Rutherford et al., 2004; Kwapinski et al., 2010).
Evaluating the behavior of different organic material during pyrolysis, Yang et al. (2007)
found that losses of hemicellulose occurred at low temperatures (220 to 300 °C) and was
associated with high CO 2 losses (attributed to losses of carboxyl groups), cellulose rapid
losses occurred at temperature between 300 to 400 °C with high CO release (attributed to
losses of carbonyl groups). Lignin losses occurred throughout the temperature range (160 to
900 °C) and were associated with high H 2 and CH 4 release from the thermal cracking of
methoxyl and aromatic C and H of the highly aromatic lignin structure. Fused aromatic ring
structures developed with increasing pyrolysis reaction times and/or peak temperature,
providing the matrix in which microporosity develops, with pores <2 nm in diameter
(Rutherford et al., 2004; Wu et al., 2012).