Biomedical Engineering Reference
In-Depth Information
8.10.1 Position of Biomass-Feeding Position
The feed points for the biomass should be such that entrainment of any parti-
cles in the product gas is avoided. This can happen when the feed points are
located too close to the expanded bed surface of a bubbling fluidized bed. If
they are in close proximity to the distributor plate, excessive combustion of
the volatiles in the fluidizing air produced can occur. To avoid this, they
should be some distance further above the grate.
Nascent tar is released close to the feed point, so tar cracking can be
important for some designs. If tar is a major concern, the feed port should be
close to the bottom of the gasifier so that the tar has adequate residence time
to crack (Barea, 2009).
8.10.2 Distributor Plate
The distributor plate of a fluidized bed supports the bed materials. It is no dif-
ferent from that used for a fluidized combustor or boiler. The ratio of pressure
drop across the bed and that across the distributor plate must be chosen appro-
priately to arrive at the plate design. More details are available in topics on
distributor plate design, including Basu (2006, chapter 11). The typical open
area in the air distributor grate is only a few percentage points.
8.10.3 Bed Materials
For the process design of a fluidized-bed gasifier, the choice of bed materials
is crucial. These comprise mostly granular
inorganic solids and some
(
10%) fuel particles. For biomass, sand or other materials are used (as
explained next); coal gasification requires granular ash produced from the
gasification process. Sometimes limestone is added with coal particles to
remove sulfur. At different stages of calcination and sulfurization, the lime-
stone can also form a part of the bed material.
Biomass has very little ash (less than 1% for wood), so silica sand is nor-
mally used as the inert bed material. This is a natural choice because silica is
inexpensive and the most readily available granular solid. One major prob-
lem with silica sand is that it can react with the potassium and sodium com-
ponents of the biomass to form eutectic mixtures having low melting points,
thereby causing severe agglomeration. To avoid this, the following alterna-
tive materials can be used:
,
Alumina (Al
2
O
3
)
Magnesite (MgCO
3
)
Feldspar (a major component of Earth's crust)
Dolomite (CaCO
3
.
MgCO
3
)
Ferric oxide (Fe
2
O
3
)
Limestone (CaCO
3
)
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