Page 223 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

200 Biomass Gasification, Pyrolysis and Torrefaction

into gases or liquids; it can also add hydrogen to the product. The use of a
medium is essential for the gasification process, which is not the case for
pyrolysis or torrefaction.

7.2.1 Gasifying Medium
Gasifying medium (also called “agent”) reacts with solid carbon and heavier
hydrocarbons to convert them into low-molecular-weight gases like CO and
H 2 . The main gasifying agents used for gasification are as follows:
Oxygen
Steam
Air
Oxygen is a popular gasifying medium though it is primarily used for the
combustion or the partial gasification in a gasifier. It may be supplied to a
gasifier either in pure form or via air. The heating value and the composition
of the gas produced in a gasifier are strong functions of the nature and
amount of the gasifying agent used. A ternary diagram (Figure 3.12) of car-
bon, hydrogen, and oxygen demonstrates the conversion paths toward the
formation of different products in a gasifier.
If oxygen is used as the gasifying agent, the conversion path moves
toward the oxygen corner. Its products include CO for low amount of oxygen
and CO 2 for high oxygen. When the amount of oxygen exceeds a certain
(stoichiometric) amount, the process moves from gasification to combustion,
and the product is “flue gas” instead of “fuel gas.” The flue gas or the com-
bustion product contains no residual heating value. A move toward the oxy-
gen corner of the ternary diagram in a gasification process (Figure 3.12)
results in low hydrogen content and an increase in carbon-based compounds
such as CO and CO 2 in the product gas.
If steam is used as the gasification agent, the process moves upward
toward the hydrogen corner in Figure 3.12. Then, the product gas contains
more hydrogen per unit of carbon, resulting in a higher H/C ratio.
The choice of gasifying agent affects the heating value of the product gas
as well. [For example, if air is used instead of oxygen, the nitrogen in it
would dilute the product reducing the heating value of the product gas.]
From Table 7.1, we can see that oxygen gasification has the highest heating
value followed by steam and air gasification. Air, as the gasification
medium, results in the lowest heating value in the product gas primarily due
to the dilution effect of nitrogen.

7.3 THE GASIFICATION PROCESS

A typical gasification process generally follows the sequence of steps listed
below (illustrated schematically in Figure 7.1).