Page 235 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

212 Biomass Gasification, Pyrolysis and Torrefaction

methane (Sutton et al., 2001). In postgasification reactions, catalysts are
placed in a secondary reactor downstream of the gasifier to convert the tar
and methane formed. This has the additional advantage of being independent
of the gasifier operating condition. The second reactor can be operated at
temperatures optimum for the reforming reaction.
The catalysts in biomass gasification are divided into three groups:

1. Earth metal catalysts: Dolomite (CaCO 3 MgCO 3 ) is very effective for
disposal of tar, and it is inexpensive and widely available, obviating the
need for catalyst regeneration. It can be used as a primary catalyst by
mixing it with the biomass or as a secondary catalyst in a reformer down-
stream, which is also called a guard bed. Calcined dolomite is signifi-
cantly more effective than raw dolomite (Sutton et al., 2001). Neither,
however, is very useful for methane conversion. The rate of the reforming
reaction is higher with carbon dioxide than with steam.
1. Alkali metal catalysts: Potassium carbonate and sodium carbonate are
important in biomass gasification as primary catalysts. K 2 CO 3 is more
effective than Na 2 CO 3 . Unlike dolomite, they can reduce methane in the
product gas through a reforming reaction. Many biomass types have
inherent potassium in their ash, so they can benefit from the catalytic
action of the potassium with reduced tar production. However, potassium
is notorious for agglomerating in fluidized beds, which offsets its cata-
lytic benefit.
1. Ni-based catalyst: Nickel is highly effective as a reforming catalyst for
reduction of tar as well as for adjustment of the CO/H 2 ratio through
methane conversion. It performs the best when used downstream of the
gasifier in a secondary bed, typically at 780 C (Sutton et al., 2001).

Deactivation of the catalyst with carbon deposits is an issue. Nickel is rel-
atively inexpensive and commercially available, though not as cheap as
dolomite. Appropriate catalyst support is important for optimum
performance.

7.3.6 Gasification Processes in Reactors
The sequence of gasification reactions depends to some extent on the type of
gas solid contacting reactors used. A brief description of this process as it
occurs in some principal reactor types are discussed in the following sections.

7.3.6.1 Moving-Bed Reactor
To explain the reaction process in moving-bed gasifiers, we take the example
of a simple updraft gasifier reactor (Figure 7.4).
In a typical updraft gasifier, fuel is fed from the top; the product gas
leaves from the top as well. The gasifying agent (air, oxygen, steam, or their
mixture) is preheated and fed into the gasifier through a grid at the bottom.