Introduction - Biomass Gasification, Pyrolysis and Torrefaction - page 7

plants (Zeng et al., 2004). Biochar produced from biomass provides viable
and less expensive means of sequestration of carbon dioxide. Biochar, pro-
duced through pyrolysis, can provide long-term sink for storage as atmo-
spheric carbon dioxide in terrestrial ecosystems. Besides this it also helps in
soil fertility and increased crop production (Lehmann et al., 2006). Thus bio-
char can retain the carbon naturally buried in ground instead of releasing it
as CO 2 to the atmosphere. The potential annual biochar production from
agricultural waste materials such as forest residues and urban wastes is
0.162 Pg/year (Lehmann et al., 2006). Life cycle analysis for stover and yard
waste shows a negative CO 2 emission exceeding 800 kg/CO 2 equivalent per
ton of dry feedstock (Roberts et al., 2010).

1.2 BIOMASS CONVERSION
Bulkiness, low energy density, and inconvenient form of biomass are major
barriers to a rapid transition from fossil to biomass fuels. Unlike gas or liq-
uid, biomass cannot be handled, stored, or transported easily. This provides a
major motivation for the conversion of solid biomass into liquid and gaseous
fuels, which are more energy dense and can be handled and stored with rela-
tive ease. This conversion can be achieved through one of two major routes
( Figure 1.3 ): biochemical conversion (fermentation) and thermochemical
conversion (pyrolysis, gasification). The inconvenience of bulkiness and
other shortcomings of solid biomass are overcome to some extent through
the production of more convenient cleaner solid fuel through carbonization
and torrefaction.
Biochemical conversion is perhaps the most ancient means of biomass gasifi-
cation. India and China produced methane gas for local energy needs by anaero-
bic microbial digestion of animal wastes. In modern times, most of the ethanol
for automotive fuels is produced from corn using fermentation. Thermochemical
conversion of biomass into gases came much later. Large-scale use of small bio-
mass gasifiers began during the Second World War, when more than a million
units were in use ( Figure 1.4 ).

Biomass conversion

Biomchemical route Thermochemical route

Digestion Fermentation Pyrolysis Gasification Liquifaction Combustion

Anaerobic Aerobic

(Composting)

Supercritical
water

Air/Oxygen Steam

FIGURE 1.3 Different options for conversion of biomass into fuel gases or chemicals.