Environmental Engineering Reference
In-Depth Information
the SilvaGas process, which was initially developed at the Battelle Columbus labs
(United States) and is characterized by two interconnected CFBs of which one is a
steam-blown biomass gasifier and the second a char combustor storing heat into
bed material that is circulated to the gasifier reactor. Finally, there is the MILENA
concept (van der Meijden et al., 2009), developed at ECN (the Netherlands), in which
the central part is a riser in which steam-blown biomass gasification takes place; the
surrounding annular space has been designed for combusting the char from the core
part. Table 10.4 shows an overview of large-scale fluidized bed biomass gasification
demonstrations.
Factors playing decisive roles in the selection of employing a certain reactor design
for biomass gasification are:
Scale of the energy conversion process
Feedstock flexibility (particle sizes and fuel composition)
Sensitivity to the amount of ash and its composition
Tar generation characteristics
The scale of operation is most probably the primary criterion. Small, decentralized
systems will benefit from a simple and cheap reactor that is easy to control and main-
tain. Here, the feedstock is probably well defined based on local conditions. On the
other hand, a biomass-to-liquid (BTL) plant or maybe even a biorefinery, where the
gasifier is only one of the units of operation, benefits from the larger scale of the reac-
tor in terms of its thermal efficiency and economies of scale.
The feedstock flexibility is another aspect. The structural appearance of biomass is
often fibrous and tough, and consequently, it is difficult to cut or pulverize. Therefore,
it is not desirable to reduce the biomass in size too much, because of the adverse effect
on the energy efficiency of the whole process. Additionally, raw biomass is not dry,
but contains varying amounts of moisture. Considering the preceding text, the gasi-
fication reactor should be able to cope with the changes in fuel supply characteristics,
which are both physical and chemical in nature.
Next to the moisture and volatile fraction, biomass also contains inorganic matter,
usually referred to as ash. This ash-forming matter can be variable in composition
(see, for instance, Chapter 2). In general, wood has low ash contents with calcium
and silica as the main constituents, but agricultural residues may contain appreciable
amounts of alkali metals (potassium, sodium) and chlorine, which may pose chal-
lenges to fluidized bed reactors (agglomeration) as well as EF gasifiers (slagging
and fouling).
While ash-related issues may lead to difficulties in process operation and unsched-
uled maintenance stops, the tar produced in the gasifier may affect the downstream
equipment in a negative way, resulting in the need for extensive downstream gas treat-
ment and upgrading.
is an umbrella term for various kinds of larger hydrocar-
bons produced during gasification. There are diverse definitions of the term, but there
is a broadening global consensus in defining biomass tar as organic contaminants with
“
Tar
”
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