Page 74 - Materials Chemistry, Second Edition

56 LIFE CYCLE ASSESSMENT HANDBOOK

of a product system, but also the energy associated with the material content of
the product. Feedstock energy refers to the energy value of resources extracted
from nature that are used as material feedstocks for product systems. For
example, crude oil and natural gas are material feedstocks for production of
traditional plastic resins, trees are harvested as feedstock for paper and lumber
products, and palm oil extracted from palm fresh fruit bunches is used as a
feedstock for surfactants and detergents.
Franklin Associates, the original LCA firm in North America, has tradition-
ally used the term "energy of material resource" (EMR) to refer to feedstock
energy. Originally, EMR was used by Franklin Associates to track the energy
value of fossil fuel resources that were diverted from their primary use as
energy resources for use as material inputs. EMR was not tracked for biomass
materials, for two reasons: (1) biomass materials were primarily used as mate-
rial feedstocks or food sources rather than fuel resources, and (2) biomass is a
renewable energy source, so that use of biomass for energy did not result in
a net depletion of finite fuel reserves. As biomass materials are increasingly
being utilized for energy purposes, the energy value of biomass materials is
becoming an increasingly important issue. In addition, including feedstock
energy for all types of resources, whether fossil or renewable, provides a more
consistent and complete energy accounting approach. Therefore, it is good
practice to track feedstock energy for all material inputs, while continuing to
distinguish between renewable and non-renewable feedstocks.
Different bases can be used for assigning and tracking feedstock energy.
Feedstock energy is usually based on the higher or lower heating value of the
material at the point of extraction from nature. The amount of energy remain-
ing in the finished product is less than the total energy content of the materi-
als extracted from nature, since there are losses during the steps required to
convert the raw material into a finished product. In contrast to process and
transportation energy that is irretrievably expended when fuel is combusted,
the feedstock energy embodied in the finished product represents potentially
recoverable energy. The energy remains embodied in the material as long as
the material remains in use, in the original product or in recycled applications.
Energy can be recovered from the material through end of life management
processes, such as waste-to-energy combustion.
If biomass feedstock energy is assigned to the material at the point of removal
from nature, there can be additional challenges in tracking feedstock energy
through subsequent processing steps. Consider the example of trees harvested
as feedstock for paper production. The mass of roundwood logs brought to the
mill includes the moisture content of the wood as well as the weight of bark.
Bark and wood chips generated during subsequent processing of the logs are
used as an energy source at the mill, and black liquor that is burned to provide
energy at the mill contains lignin extracted from the wood fiber during the
chemical pulping process. If feedstock energy is assigned to the total weight
of wood entering the mill, then one must be careful not to double count the
process energy from bark, chips, and black liquor derived from the same mass
of incoming wood.