Page 249 - Synthetic Fuels Handbook

FUELS FROM BIOMASS 235

Ash content (which represents in inorganic content) of biomass is the inorganic oxides
that remain after complete combustion of the feedstock. The amount of ash between dif-
ferent types of feedstocks differs widely (0.1 percent for wood up to 15 percent for some
agricultural products) and influences the design of the reactor, particularly the ash removal
system. The chemical composition of the ash is also important because it affects the melting
behavior of the ash. Ash melting can cause slagging and channel formation in the reactor.
Slags can ultimately block the entire reactor.
The elemental composition of the biomass feedstock is important with regard to the
heating value of the gas and to the emission levels. The production of nitrogen and sulfur
compounds is generally small in biomass gasification because of the low nitrogen and
sulfur content in biomass.
The bulk density refers to the weight of material per unit of volume and differs widely
between different types of biomass. Together with the heating value, it determines the
energy density of the gasifier feedstock, that is, the potential energy available per unit
volume of the feedstock. Biomass of low bulk density is expensive to handle, transport,
and store. Apart from handling and storing behavior, the bulk density is important for the
performance of the biomass as a fuel inside the reactor: a high void space tends to result in
channeling, bridging, incomplete conversion, and a decrease in the capacity of the gasifier.
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The bulk density varies widely (100–1000 kg/m ) between different biomass feedstocks
not only because of the character of the biomass but also as a result of the way the biomass
comes available (chips, loose, baled).
The amount of volatile products (volatile matter content) has a major impact on the tar
production levels in gasifiers. Depending on the gasifier design, the volatile matter leaves
the reactor at low temperatures (updraft gasifiers) or pass through a hot incandescent oxida-
tion zone (downdraft gasifiers) where they are thermally cracked. For biomass materials the
volatile matter content varies between 50 and 80 percent.
Feedstock preparation is required for almost all types of biomass materials because of
large variety in physical, chemical, and morphological characteristics. The degree to which
any specific pretreatment is desirable will depend upon the gasifier. For example, capacity
and type of reactor (downdraft gasifiers are more strict to uniform fuel specifications than
updraft gasifiers) are important aspects of biomass gasification.
Sizing of the feedstock may be necessary as different sizes are specified for different
types of gasifiers. For small scale fixed bed gasifiers, cutting and/or sawing of wood blocks
is the preferred form of fuel preparation. Chipped wood is preferred for larger scale applica-
tions. The size range of chips can be chosen by screening such that the fuel is acceptable for
a specific gasifier type. For medium- and large-scale fixed bed gasifiers wood chips from
forestry or wood processing industries are produced by crushers, hammer mills, shredders,
and/or mobile chippers (particularly for thinnings from landscape conservation activities).
Most of these sizing apparatus are provided with a screen. Dependent on the feedstock
morphology and characteristics (hardness) the throughput or capacity varies considerably.
Drying of the biomass fuel is advisable if fresh wet materials (moisture content 50–60
percent on wet basis) are to be gasified. Lowering the moisture content of the feedstock is
associated with a better performance of the gasifier. Utilizing the exhaust gases from an
internal combustion engine is a very efficient way to do so. The sensible heat in engine
exhaust is sufficient to dry biomass from 70 percent down to 10 percent. Rotary kilns are
the most applied dryers. The energy costs of drying are high, but these can be outweighed
by the lower downstream gas-cleaning requirements for dried feed.
Densification, such as briquetting or pelletization, are important techniques to densify
biomass materials for increasing the particle size and bulk density. The reasons for biomass
include: (a) densified biomass is less expensive to transport, (b) densified products are
easier to store and handle, and (c) densification enables certain biomass feedstocks to be
gasified in a specific gasifier.