Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 298

From knowledge of the required solid residence time, θ , then, the bed

volume, V bed , is:

F ½ C θ

V bed 5 (8.38)

ð 1

2 εÞρ s x char

where F[C] is the char feed rate into the gasifier and ρ s is the density of the

bed solids. In a typical bubbling bed, the bed voidage is 0.7. The bed gen-

B

erally contains 5 8% (by weight) of reacting char (x char ); the remaining

solids are inert bed materials.
The bed height, H bed , is known by dividing bed volume by the bed area,
A b , which is known from chosen superficial velocity.

V bed
A b

H bed 5 (8.39)

, of test coals for different feed conver-
sions and S/C or O/C ratios are given in the Coal Conversion Systems
Technical Data Book (U.S. DOE, 1978). The residence time may be adjusted
for the reactivity of the char in question and for the reactivity of its partial
gasification before it enters the gasifier.

Design charts for residence time, θ

Other Considerations
Although virgin biomass contains little or no sulfur, some waste biomass
fuels do. For these, limestone is fed into the fluidized-bed gasifier for in-bed
sulfur removal. The height of the gasifier (freeboard and bed) should be ade-
quate to allow the residence time needed for the desired sulfur capture.
The tar produced should be thermally cracked inside the gasifier as far as
possible. Therefore, the depth of the gasifier should be such that the gas resi-
dence time is adequate for the desired tar conversion/cracking.
The deeper the bed, the higher the pressure drop across it and the higher
the pumping cost of air. Because bubble size increases with bed height,
a deeper bed gives solid mixing.

Furthermore, if the bubble size becomes comparable to the smallest dimen-
sion of the bed cross-section, a highly undesirable slugging condition is
reached. This imposes another limit on how deep the dense section of a flu-
idized bed can be.
Some biomass char, like that from wood, is fine and easily undergoes
attrition in a fluidized bed. In such cases, a deeper bed may not guarantee a
longer residence time (Barea, 2009). Here, special attention must be paid to
capturing the char and either combusting it in a separate chamber to provide
heat required by the gasifier or re-injecting it at an appropriate point in the
bed where solids are descending.

larger bubbles with reduced gas