Page 289 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 8 Design of Biomass Gasifiers 265

Flue gas with
nitrogen

Product gas
Cyclone

Riser
combustor
BFB gasifier

Biomass
feed

Solid

Char
Steam Air

FIGURE 8.12 Twin reactor (dual fluidized-bed) gasifier.

as a combustor; the bubbling fluidized bed in the return leg operates as a
gasifier. Pyrolysis and gasification take place in the bubbling fluidized bed,
which is fluidized by superheated steam. Unconverted char and tar move to
the riser through a nonmechanical valve.
Tar and gas produced during pyrolysis are combusted in the riser’s com-
bustion zone. Heat generated by combustion raises the temperature of the

inert bed material to around 900 C. This material leaves the riser and is cap-
tured by the cyclone at the riser exit. The collected solids drop into a stand-
pipe and are then circulated into the bubbling fluidized-bed reactor to supply
heat for its endothermic reactions. The char is gasified in the bubbling bed in
the presence of steam, producing the product gas. This system overcomes the
problem of tar by burning it in the combustor. In this way, a product gas rel-
atively free of tar can be obtained.
Rentech-Silvagas process is another technology based on the externally
circulating principle. Here, both the combustor and the gasifier work on CFB
principles.
Besides these, there is an internally circulating system. In the internally
circulating design, the fluidized-bed reactor is divided into two chambers