Page 39 - Materials Chemistry, Second Edition

Key Issues in Conducting Life Cycle Assessment 25

introduced are involving significant amounts of fossil fuel and agricultural input
(scenario 2). In this case, the bioenergy system may emit more total carbon than
the sequestration capacity of trees, resulting in net accumulation of CO 2 in the
atmosphere. On the other hand, carbon emissions from fossil fuel combustion are
considered as an irreversible one-way process (scenario 3). It transfers geological
carbon, locked underground, over long-term geological time into the atmosphere.
This process increases atmospheric carbon levels with time. Therefore, to properly
assess the benefit of the bioenergy system over fossil fuel systems, it is necessary
to account for all relevant input–output flows in the inventory phase of LCA
studies, including carbon sequestration and carbon emissions of both biogenic and
geologic sources. From various options provided in LCA studies, the bioenergy
product with the larger GHG saving, among other criteria, would be the preferred
energy system.
There are at least two points to make with respect to carbon neutrality of
bioenergy. First, if there is anything neutral, it is LCA, as an analytical tool, that
makes the conclusion. If biofuels are carbon neutral, this will result from the LCA
study instead of being a starting point of the LCA study. Second, there are several
situations where the carbon neutrality of bioenergy is challenged. One of these
occurs in the situation when some of the CO 2 absorbed is not released as CO 2 ,
instead, as CH 4 , a greenhouse gas that is much stronger than CO 2 . This may
happen, for instance, when the biotic feedstock is subject to a process of incom-
plete burning or anaerobic decomposition with leakages occurring along the way.
Another case is a plantation with two co-products (e.g., palm oil and palm kernel
oil) where part of the absorbed CO 2 is allocated to each of the co-products. In
chains with only one product, exclusion of biogenic carbon can result in the same
outcome as long as the issue of CH 4 does not arise. However, in cases of chains
with co-products, it makes a difference. Allocation may place the credits for
extracted CO 2 in a different part of the multi-product chain, while ignoring bio-
genic CO 2 would not have this effect (van der Voet et al. 2010). A recent review
indicated that carbon sequestration, if included at the biomass generation stage,
can offset the GHG emissions from all parts of the life cycle chains at a high
ethanol percentage (C85 %) (Wiloso et al. 2012). A final example challenging the
bioenergy neutrality is the fact that there is a time difference between CO 2 fixation
and release. A specific dynamic LCA method has been developed to account for
such situations.
The most appropriate way to treat carbon cycles is to view them as genuine cycles.
During tree growth, a certain amount of atmospheric CO 2 is fixed but is ultimately
released as CO 2 or CH 4 when the wood is landfilled, is incinerated, or decays nat-
urally.At the systems’ level, the fixation of CO 2 during tree growth is subtracted from
the CO 2 emitted during waste treatment of discarded wood (Guinée et al. 2009). For
fossil fuels, carbon fixation has taken place as a natural process millions of years ago,
but carbon emissions occur immediately when these fuels are burned.
The rationale behind different treatments between biogenic carbon and geologic
carbon is because, for example, forestry (the process that fixates the CO 2 )is
considered as a unit process. It is an intentional activity, controlled by humans,