Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 264

To syngas cooler
Gas-Solid
disengager
Cyclone
Riser gasifier
Maxing zone
Loopscal
Feed
Air
Steam
Standpipe
Air, Oxygen, Steam
J-Value
FIGURE 8.11 A sketch of a typical transport fluidized-bed gasifier.
exothermic combustion reaction necessary for a self-sustained gasifier. To
avoid the dilution, oxygen could be used instead, but oxygen gasification
is expensive and highly energy intensive (see Example 8.5). A twin reactor
(e.g., a dual fluidized bed) overcomes this problem by separating the combus-
tion reactor from the gasification reactor such that the nitrogen released in the
air combustion does not dilute the product gas. Twin reactor systems are used
for coal and biomass. They are either externally or internally circulating.
This type of system has some limitations; for example, Corella et al.
(2007) identified two major design issues with the twin or dual fluidized-bed
system:
1. Biomass contains less char than coal contains; however, if this char is
used for gasification, the amount of char available may not be sufficient
to provide the required endothermic heat to the gasifier reactor to main-
tain a temperature above 900 C. This external heating may be necessary.
2. Though the gasifier runs on steam, only a small fraction ( , 10%) of the
steam participates in the gasification reaction; the rest of it simply leaves
the gasifier, consuming a large amount of heat and diluting the product gas.
The Technical University of Vienna used an externally circulating system
to gasify various types of biomass in an industrial plant in Gussing, Austria.
The system is comprised of a bubbling fluidized-bed gasifier and a CFB
combustor ( Figure 8.12 ). The riser in a CFB that is fluidized by air operates