Page 41 - Materials Chemistry, Second Edition

Key Issues in Conducting Life Cycle Assessment 27

feedstocks in this category may include perennial plants grown on degraded lands,
crop residues, sustainably harvested wood and forest residues, double crops and
mixed cropping systems, municipal and industrial wastes. These various feed-
stocks and bioenergy products in LCA should be treated with proper allocation and
recycling procedures to attribute environmental burden of multi-functional pro-
cesses to their input or output flows.
A multi-functional process is a unit process, yielding more than one functional
flow including co-production, combined waste processing, and recycling. Co-
production is a multi-functional process having more than one functional outflow and
no functional inflow. Recycling is a multi-functional process having one or more
functional outflows and one or more functional inflows. Combined waste processing
is a multi-functional process having no functional outflow and more than one
functional inflow. The most relevant multi-functional processes in bioenergy
systems with reference to the types of input and output inventory are the first two
cases as illustrated in Fig. 4. Guinée (2002) distinguishes two steps in solving the
multi-functionality problem. The first concerns avoiding burden allocation in
accordance with the ISO preference. This is done by specifying the system boundary
to a unit operation level (e.g., individual machines) to reduce the number of multi-
functional processes or by system expansion. It is accomplished by extending the
analyzed product system to include additional functions related to the co-products or
recycled wastes. The system then includes more than one functional unit. The term
system expansion is sometimes used to refer to the substitution method. The second
step concerns solving the remaining multi-functionality problems by allocation on
the basis of mass, energy, or economic values. Further discussion on the procedure to
deal with allocation procedures and system expansion can be found in Tillman et al.
(1994) and Heijungs and Guinée (2007).
If some waste streams from agriculture are used to make bioenergy products,
how the waste was produced is not included in the inventory. It is assumed that its
production is free of environmental burden. This, however, requires a clear dis-
tinction between products and wastes. To distinguish products from wastes, the
economic value of flows can be used as the determining factor. A product is a flow
between two processes with a positive economic value, whereas a waste is a flow
between two processes with a negative economic value (Guinée et al. 2009).
However, there are quite a few cases where we do not know for certain if the price
of an agricultural residue is positive or negative, especially when it remains within

Bioenergy product1*
Biomass product Multi-output process Bioenergy product2*
Other product (co-product)
Waste

Biomass waste* Input-output process Bioenergy product*
Other product (recycling) Other waste

Fig. 4 Relevant multi-functional processes in bioenergy systems (*= functional flows)