Discussion of a recent publication on Life Cycle Assessment
(LCA) of biodiesel from microalgae may be instructive (Lardon
et al., 2009). The authors state quite precisely that the potential
environmental impacts are investigated via LCA. The inventory is
compiled after defining the production system reaching from algae
culture to the use of diesel in an engine. Bench scale research and
other extrapolations are employed since industrial operation to allow
an inventory does not exist at this time. The difference of this
LCA to the mass balance approach proposed here is immediately
obvious in the production system schematic: no materials are
actually ‘‘cycled”. The schematic does not indicate quantitative or
even qualitative tracking and reconciliation of any mass flows to
allow a test for (reasonable) closure of mass balances.
After a number of assumptions and extrapolations are reasonably
made, the impact of streams to/from the production system
is quantified based on established weighing factors. Essentially,
one would assign a certain factor to, say, a kg CO2 emitted, etc. Impact
on human health, ecosystems, and resources is assigned and
then normalized so all impacts are shown on the same scale to
identify major contributions. While this may be called ‘‘LCA proper”
the authors attempt in the discussion to expand the analysis to
energy balances and this cannot succeed because a first law of
thermodynamics analysis does not suffice. This type of extension
of LCA away from the environmental impact is often attempted
and leads to wildly different results due to the absence of a proper
scientific foundation. This is perhaps demonstrated by the ongoing
debates about the net energy contributions of bio-ethanol production
from corn.
While LCA is concerned with the environmental impact of a given
processing system, it is often used, as by Lardon et al. (2009) to
prove or disprove the usefulness of a given bio-energy approach.
An (often partial) first law of thermodynamics energy balance is
developed along the LCA results, essentially asking the question
‘‘How many joules are used to produce one joule of the target
fuel?” This can be deceiving since it only takes in account the quantity
(first law of thermodynamics) but not the quality of energy. A
joule of lower heating value from coal is thermodynamically and
economically much less valuable than a joule as electricity. Lardon
et al., for example disregard the influx of solar energy to the system
and show a range from 2.6 MJ lost to +105 MJ gained per kg of algae
biodiesel produced. These values may, for example, all become
negative if the input of solar energy is counted. However, this does
by no means invalidate all algae-based diesel concepts.
The simple mass balance approach limited for example to the
critical element carbon for liquid transportation fuels shows a necessary
but not sufficient condition of sustainability. However, it
will allow to decide early on if a given concept has any hope of
operating sustainably, and where the most serious issues reside
(for example Pfromm et al., 2010). If the carbon mass balance appears
promising, a complete mass balance will show environmental
compatibility since for example the unit operation
‘‘atmosphere” may not be enriched or depleted over time to maintain
steady state and achieve sustainability. It is acknowledged
that a unit operation such as ‘‘atmosphere” is exceptionally complex
and that our knowledge is in flux, but the mass balance approach
is amenable to handle very high levels of complexity in
an adaptable mathematical fashion.