Page 226 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 7 Gasification Theory 203

evaporates the moisture in it. Above 100 C, the loosely bound water that is
in the biomass is irreversibly removed. As the temperature rises further, the
low-molecular-weight extractives start volatilizing. This process continues
until a temperature of approximately 200 C is reached.

7.3.2 Pyrolysis

In pyrolysis, no external agent is needed. As per the ternary diagram
(Figure 3.12), a slow pyrolysis or torrefaction process moves the solid prod-
uct toward the carbon corner, and thus more char is formed. The fast pyroly-
sis process, on the other hand, moves the product toward the C H axis
opposite to the oxygen corner (Figure 3.12). The oxygen is thereby largely
diminished producing more liquid hydrocarbon.
As detailed in Chapter 5, pyrolysis involves the thermal breakdown of
larger hydrocarbon molecules of biomass into smaller gas molecules (con-
densable and noncondensable) with no major chemical reaction with air, gas,
or any other gasifying medium. This reaction generally precedes the gasifica-
tion step.
One important product of pyrolysis is tar formed through condensation of
the condensable vapor produced in the process. Being a sticky liquid, tar cre-
ates a great deal of difficulty in industrial use of the gasification product. A
discussion of tar formation and ways of cracking or reforming it into useful
noncondensable gases is presented in Chapter 6.

7.3.3 Char Gasification Reactions
The gasification step involves chemical reactions among the hydrocarbons in
fuel, steam, carbon dioxide, oxygen, and hydrogen in the reactor, as well as
chemical reactions among the evolved gases. Of these, char gasification is
the most important. The biomass char produced through pyrolysis of biomass
is not necessarily pure carbon. It contains a certain amount of hydrocarbon
comprising hydrogen and oxygen.
Biomass char is generally more porous and reactive than coke produced
through high temperature carbonization of coal. The porosity of biomass
char is in the range of 40 50% while that of coal char is 2 18%. The
pores of biomass char are much larger (20 30 μm) than those of coal char
˚
(B5A) (Encinar et al., 2001). Thus, its reaction behavior is different from
that of chars derived from coal, lignite, or peat. For example, the reactivity
of peat char decreases with conversion or time, while the reactivity of
biomass char increases with conversion (Figure 7.2). This reverse trend
can be attributed to the increasing catalytic activity of the biomass char’s
alkali metal constituents (Risnes et al., 2001).