Page 236 - Synthetic Fuels Handbook

222 CHAPTER EIGHT

level of organic matter. Thus, a huge renewable resource is not being usefully exploited
since wheat straw contains a range of potentially useful chemicals. These include: (a) cellulose
and related compounds which can be used for the production of paper and/or bioethanol,
(b) silica compounds which can be used as filter materials such as those necessary for water
purification, and (c) long-chain lipids which can be used in cosmetics or for other specialty
chemicals.
However, for the purpose of this chapter, three distinct sources of biomass energy
are: (a) agricultural crops, (b) wood, and (c) municipal and industrial wastes (Chaps. 9, 10,
and 11, respectively). Landfill gas is also included in this work (Chap. 11).
This includes everything from primary sources of crops and residues harvested/collected
directly from the land to secondary sources such as sawmill residuals, to tertiary sources
of postconsumer residuals that often end up in landfills. A fourth source, although not usu-
ally categorized as such, includes the gases that result from anaerobic digestion of animal
manures or organic materials in landfills (Chap. 9) (Wright et al., 2006).
Primary biomass is produced directly by photosynthesis and includes all terrestrial
plants now used for food, feed, fiber, and fuel wood. All plants in natural and conservation
areas (as well as algae and other aquatic plants growing in ponds, lakes, oceans, or artificial
ponds and bioreactors) are also considered primary biomass. However, only a small por-
tion of the primary biomass produced will ever be harvested as feedstock material for the
production of bioenergy and by-products.
More generally, biomass feedstocks are recognized by the specific plant content of the
feedstock or the manner in which the feedstocks is produced.
For example, primary biomass feedstocks are thus primary biomass that is harvested or
collected from the field or forest where it is grown. Examples of primary biomass feedstocks
currently being used for bioenergy include grains and oilseed crops used for transportation
fuel production, plus some crop residues (such as orchard trimmings and nut hulls) and
some residues from logging and forest operations that are currently used for heat and power
production. In the future it is anticipated that a larger proportion of the residues inherently
generated from food crop harvesting, as well as a larger proportion of the residues gener-
ated from ongoing logging and forest operations, will be used for bioenergy (Smith, 2006).
Additionally, as the bioenergy industry develops, both woody and herbaceous perennial
crops will be planted and harvested specifically for bioenergy and product end uses.
Secondary biomass feedstocks differ from primary biomass feedstocks in that the second-
ary feedstocks are a by-product of processing of the primary feedstocks. By processing it is
meant that there is substantial physical or chemical breakdown of the primary biomass and
production of by-products; processors may be factories or animals. Field processes such as
harvesting, bundling, chipping, or pressing do not cause a biomass resource that was pro-
duced by photosynthesis (e.g., tree tops and limbs) to be classified as secondary biomass.
Specific examples of secondary biomass includes sawdust from sawmills, black liquor
(which is a by-product of paper making), and cheese whey (which is a by-product of cheese-
making processes). Manures from concentrated animal-feeding operations are collectable
secondary biomass resources. Vegetable oils used for biodiesel that are derived directly
from the processing of oilseeds for various uses are also a secondary biomass resource
(Wright et al., 2006; Bourne, 2007).
Tertiary biomass feedstock includes postconsumer residues and wastes, such as fats,
greases, oils, construction and demolition wood debris, other wood waste from the urban
environments, as well as packaging wastes, municipal solid wastes, and landfill gases. The
category, other wood waste from the urban environment includes trimmings from urban
trees, which technically fits the definition of primary biomass. However, because this mate-
rial is normally handled as a waste stream along with other postconsumer wastes from
urban environments (and included in those statistics), it makes the most sense to consider
it to be part of the tertiary biomass stream.