Page 241 - Synthetic Fuels Handbook

FUELS FROM BIOMASS 227

Underutilized wood species include Southern red oak, poplar, and various small diameter
hardwood species. Unharvested dead and diseased trees can comprise a major resource in
some regions. When such timber has accumulated in abundance, it comprises a fire hazard
and must be removed. Such low grade wood generally has little value and is often removed
by prescribed burns in order to reduce the risk of wildfires.
Waste streams can also be exploited for ethanol production. They are often inexpensive
to obtain, and in many instances they have a negative value attributable to current disposal
costs. Some principal waste streams currently under consideration include mixed paper from
municipal solid waste, cellulosic fiber fines from recycled paper mills, bagasse from sugar
manufacture, corn fiber, potato waste, and citrus waste, sulfite waste liquors, and hydrolysis
streams from fiber board manufacture. Each waste stream has its own unique characteristics,
and they generally vary from one source or time to another. Waste streams with lower lignin
contents and smaller particle sizes are easier to deal with than those with higher lignin con-
tents and larger particle sizes. Waste paper that has been treated by a chemical pulping process
is much more readily converted than is native wood or herbaceous residue.

8.1.2 Feedstock Properties
The components of biomass include cellulose, hemicelluloses, lignin, lipids, proteins,
simple sugars, starches, water, hydrocarbons, ash-forming constituents, and extractable
compounds. These constituents influence the properties of biomass and, in turn, have a
significant bearing on the thermal conversion of biomass. In addition, the high moisture,
oxygen content, hydrogen content, and volatile matter content, and low energy density also
influence biomass conversion.
The high oxygen and hydrogen contents account for the high proportion of volatile mat-
ter and consequent high yields of gases and liquids on pyrolysis. A relatively high water
yield results from the high oxygen concentration in biomass, and which consumes con-
siderable hydrogen. Consequently the advantages of the high hydrogen-to-carbon (H/C)
ratio associated with biomass are not reflected in the products to the extent that might be
expected. In fact, pyrolysis gases can be deficient in pure hydrogen and pyrolysis liquids
are highly oxygenated, viscous tars.
An additional and significant source of water vapor in biomass gases is the high mois-
ture content of the source materials. In countercurrent flow schemes such as the Lurgi
moving bed gasifier, this water is evolved in the relatively low temperature drying and
pyrolysis zones and does not partake in gas phase or carbon-steam gasification reactions.
On the other hand, in fluidized bed systems the moisture is evolved in the high temperature
well-mixed reaction zone and therefore does participate in the reactions. If the system is
directly heated and air blown, the additional heat required to evaporate the water will result
in more nitrogen being introduced, and more carbon dioxide being produced, reducing the
calorific value of the product gas. As the gas from air-blown processes is, in any case, a low-
calorific value product, this factor is probably of little consequence other than with very wet
feedstock. In oxygen-blown systems, however, the additional pure oxygen is required and
higher carbon dioxide content of the medium calorific value off-gas may be of sufficient
impact to dictate some degree of drying as a pretreatment.
Apart from drying, additional beneficiation may be undertaken to yield a resource of
higher energy density. These operations will normally be undertaken at the source, so trans-
port and subsequent storage costs may be reduced as well. Beneficiation steps include size
reduction and densification. Waste heat, if available, may be used for drying, while size
reduction and compression to form pellets or briquettes is estimated to require less than
2 percent of the energy in the dry biomass. Nevertheless, these operations are time consum-
ing, and can be either labor or capital intensive.