Page 23 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

2 Biomass Gasification, Pyrolysis, and Torrefaction

includes liquid or even gases to produce more useful fuels. For example, par-
tial oxidation of methane gas is widely used in production of synthetic gas,
or syngas, which is a mixture of H 2 and CO.
Torrefaction (Chapter 4) is gaining prominence due to its attractive use in
co-firing biomass (Chapter 10) in existing coal-fired power plants. Pyrolysis
(Chapter 5), the pioneering technique behind the production of the first
transportable clean liquid fuel kerosene, produces liquid fuels from biomass.
In recent times, gasification of heavy oil residues into syngas has gained
popularity for the production of lighter hydrocarbons. Many large gasifica-
tion plants are now dedicated to the production of chemical feedstock from
coal or other hydrocarbons. Hydrogenation, or hydrogasification, which
involves adding hydrogen to the feed to produce fuel with a higher
hydrogen-to-carbon (H/C) ratio, is also gaining popularity. Supercritical gasi-
fication (Chapter 9), a new option for gasification of very wet biomass, also
has much potential.
This chapter introduces the above biomass conversion processes with
a short description of the historical developments of gasification, its
motivation, and its products. It also gives a brief introduction to the
chemical reactions that are involved in important biomass conversion
processes.

1.1 BIOMASS AND ITS PRODUCTS

Biomass is formed from living species like plants and animals—i.e., any-
thing that is now alive or was alive a short time ago. It is formed as soon
as a seed sprouts or an organism is born. Unlike fossil fuels, biomass
does not take millions of years to develop. Plants use sunlight through
photosynthesis to metabolize atmospheric carbon dioxide and water to
grow. Animals in turn grow by taking in food from biomass. Unlike fossil
fuels, biomass can reproduce, and for that reason, it is considered renew-
able. This is one of its major attractions as a source of energy or
chemicals.
Every year, vast amounts of biomass grow through photosynthesis by
absorbing CO 2 from the atmosphere. When it burns, it releases the carbon
dioxide that the plants had absorbed from the atmosphere only recently
(a few years to a few hours). Thus, the burning of biomass does not make
any net addition to the earth’s carbon dioxide levels. Such release also hap-
pens for fossil fuels. So, on a comparative basis, one may consider biomass
“carbon-neutral,” meaning there is no addition to the CO 2 inventory by the
burning of biomass(see Section 1.3.2.1).
Of the vast amount of biomass in the earth, only 5% (13.5 billion metric
tons) can be potentially mobilized to produce energy. Even this amount is