Also, the tar removal efficiency was sharply increased from 87.3% (Run 8) to 97.1% (Run 6).
A high reaction temperature of the fixed bed reactor would
enhance the thermal and catalytic cracking of the tar adsorbed on
the activated carbon. As the result of enhanced tar cracking at the
higher temperature, the H2 production in Run 6 was sharply
increased. The effect of the temperature of the fluidized bed gasifier
(Runs 4, 9, and 10) on the tar reduction did not seem to be crucial.
The tar content in producer gas and the tar removal efficiency were
not significantly variable according to the temperature of the fluidized
bed gasifier. This may have resulted from the fact that a
considerable amount of tar could still be produced by the fluidized
bed gasifier itself and then a large part of the tar could escape from
the fluidized bed gasifier without any cracking due to the short
residence time in the fluidized bed gasifier. However, the fluidized
bed gasifier temperature had a significant influence on the producer
gas composition; with increasing temperature, the H2, CO,
and CH4 contents clearly increased. In particular, the H2 concentration
was significantly increased from 19.6 (Run 10: 636 C) to
28.6 vol% (Run 4: 823 C). The higher temperature in the fluidized
bed gasifier for Run 4 enhanced cracking reactions to promote the
formation of H2 and CH4. On the other hand, the increase in the
temperature shifted the equilibrium for some endothermic reactions,
such as that below (Eqs (7) and (8)), toward the formation
of the products, and for an exothermic reaction such as in Eq (9),
toward the formation of the reactants (Le Chatelier's law).