BIOCHAR - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 96

I NTRODUCTION
Biochar is a carbonaceous solid material composed of charred and partially carbonized
biomass (charcoal) used as soil amendment. Use of material from the continuum of fresh
organic residue and/or raw organic waste to their post-combustion ash as soil amendment is
known to be practiced for thousands of years. The historic use of combustible and/or partially
combustible organic residue and their long lasting effect of improving soil organic carbon
content and soil fertility are documented on lowland humid tropical soils in the Amazonian
basin (Glaser et al., 2001) and in similar climatic zones in West Africa (Fairhead and Leach,
2009). Moreover, the high productivity of USA Midwest prairie soils has been attributed in
part to the soils high black carbon content - a buildup of charred material over centuries of
spontaneous prairie fires (Laird et al., 2009). Charred material contains recalcitrant organic
matter and carbonized moieties of slower decomposition rates than the original feedstock
material. Amid the rapid increase of CO 2 in the atmosphere and the opportunity of building
soil organic matter, sustaining and improving soil fertility, and increasing carbon
sequestration by incorporating rather recalcitrant organic residue in soil sprung the promotion
of the use of this material as soil amendment (Lehmann et al., 2006). The product is produced
from similar feedstock and under the same process as charcoal; yet, promoting its use needed
a sharp distinction from charcoal in order to distance it from the impact of fossil fuels on
carbon emission, which is associated with ‗coal'. Thus, the term ‗biochar' was devised and is
currently used to indicate of a charred material that is associated with sustainable use of
biomass - with cyclical, if not reverse, impact on carbon emission and with a target use as soil
amendment (Lehmann and Joseph, 2009).
Similar to charcoal, biochar is produced by thermal decomposition of organic matter in
oxygen-limited environment. Biochar can be produced in a very low-cost and crude
fabricated devise or in more sophisticated and regulated pyrolysis chamber. Depending on
production objectives and target product, pyrolysis is conducted at a wide range of
temperatures, bordering torrefaction at low temperatures (250 to 350 °C) and gasification at
high temperatures (> 750 °C). During pyrolysis, cellulose and hemicellulose are decomposed
at lower temperatures producing volatile compounds, while lignin decomposes at much
higher temperature. The three main products of pyrolysis include char, organic vapor and
steam, and gasses (mainly CO 2 , CO, CH 4 , and H 2 ). High heating rates (>500 °C s -1 ) and short
vapor residence time (<4 s) under moderate temperatures (350 to 450 °C) favor yield of liquid
(bio-oil), while longer vapor residence time and lower temperatures (300 to 400 °C) favor
char formation. High temperature (800 °C) and long vapor residence time favor gas
formation. There are two main biochar production methodologies, (1) ‗fast' pyrolysis, with
reaction times of millisecond to few seconds and (2) ‗slow' pyrolysis, with reaction times
greater than 30 minutes. In the former, pyrolysis is used for thermal conversion of cellulosic
biomass to produce bio-oil with biochar being generated as a byproduct where pyrolysis
conditions are optimized to maximize bio-oil yield and quality. In the slow pyrolysis, the
main product is biochar and hence conditions are optimized to maximize biochar production
and quality. Both pyrolysis conditions and feedstock origin have a profound impact on
biochar properties. Hence, biochar characteristics and its effects on soil properties and fertility
are both process- and feedstock-dependent.