Environmental Engineering Reference
In-Depth Information
The first process step comprises the carbonization of relatively dry biomass (moisture
contents varying up to 15
−
20 wt%) using partial oxidation with air/oxygen at mild
500
C. In this stage, volatiles and char are formed as products.
The second process step comprises the understoichiometric combustion of the gener-
ated volatiles with oxygen and/or air in a combustion chamber that operates above the
melting point of the fuel
temperatures of 400
−
s ash to turn it into a hot gasification medium. The third stage,
finally, consists of char grinding to a pulverized fuel that is blown into the hot gas-
ification medium created in the EF reactor of the bottom part of the second stage.
The pulverized fuel here reacts in an endothermic process that generates a raw syngas.
Further treatment by cooling and cleaning follows. The syngas can be used for differ-
ent purposes; nowadays, the focus is on diesel fuel production, called SunDiesel.
'
10.3.3 Large-Scale Gasifiers: Fluidized Bed Gasifiers
The fluidized bed reactor has been one of the workhorses for large-scale coal conver-
sion since it was first patented by Fritz Winkler in 1922 and commercialized in 1926
(see, e.g., Howard (1983) and Kunii and Levenspiel (1991)). A number of basic
designs of fluidized bed gasifiers have been developed since these early days. Since
the oil crises in the 1970s, biomass use in such reactors has been investigated. Such
biomass-based fluidized bed reactors have been targeted at the midscale thermal capa-
cities of 10 MW
th
to large-scale capacities of more than 100 MW
th
.
An inert or catalytic bed material of small solid particles is used to facilitate heat
and mass transfer throughout the reactor. The bed is kept in a fluidized state by blow-
ing a gasification agent through it, lifting the bed against gravity by drag force. Here-
with, the turbulence in the bed creates an even temperature distribution in the bed.
Thus, no different distinct reaction zones appear in the bed, like in the case of fixed
bed gasifiers. The prevailing temperatures in a fluidized bed are usually in the range of
700
900
C, and the reactors are operated with gauge pressures between 0 and 7.0
MPa. Even within the relatively low operating temperature window, bed sintering
is a common problem when biomass with a high ash content is used. The alkali com-
ponents in the ash, facilitated by chlorine, show the tendency to form low-melting
eutectics with silica sand, which is the most common bed material. These eutectics
cause bed agglomeration and bed sintering, which can lead to loss of fluidization
(Bartels, 2008; Fryda et al., 2008).
The most common types of fluidized beds are the BFB and the CFB. Also, hybrid
forms or interconnected fluidized bed reactors are encountered in modern designs.
Figure 10.9 shows the basic reactor configurations (Olofsson et al., 2005).
In a BFB, the gasification agent is blown through the bed in such a way that it forms
bubbles within the bed zone. The gas velocity is significantly above the minimum
fluidization velocity and below the maximum terminal velocity (typical values range
from
-
s
−1
), so that the bed material largely remains in the reactor.
In the BFB design configuration, the majority of the gasification reactions take
place in the dense fluidized bed part. Some reactions, especially thermal cracking
and reforming reactions, the water
0.5 to 2 m
gas shift reaction, and the gasification of entrained
small particles, continue in the freeboard above the bed. The carbon conversion in the
-
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