Environmental Engineering Reference
In-Depth Information
11.3.3.2 Mixed Wood Waste .............................................................................. 270
11.3.3.3 Hybrid Poplar ........................................................................................ 271
11.3.3.4 Carbon Sequestration ............................................................................ 272
11.4 Land-Use Impacts ............................................................................................................... 272
11.4.1 Introduction .......................................................................................................... 272
11.4.2 Life-Cycle GHG Emissions .................................................................................. 273
11.4.3 Life-Cycle Land-Use Metrics ............................................................................... 276
11.5 Overall Conclusions ............................................................................................................ 277
References ...................................................................................................................................... 279
11.1
the crItIcal role oF lIFe-cycle assessment
11.1.1 i
ntroduction
Biofuels have caused a great deal of controversy in recent years. Opponents point to studies indi-
cating that biofuels have increased global food prices and can cause adverse land-use changes,
whereas proponents point out domestic economic benefits and increased energy security. Policy-
makers are responsible for passing laws that will guide society either toward or away from biofuels.
Ideally, the direction would be chosen with complete understanding of the economic, social, and
environmental impacts associated with biofuel production. Similarly, biomass electricity genera-
tion has the potential to scale significantly and its impacts must be understood. Life-cycle assess-
ment (LCA) provides a comprehensive framework for evaluating the environmental impacts of
biofuels and biomass electricity.
LCA is a systematic method for evaluating the environmental performance of products and
processes by tracking the energy and material flows related to their production and consumption
activities. These energy and material flows generate environmental, economic, and social impacts,
both positive and negative. This chapter focuses on the environmental impacts, particularly energy
and greenhouse gas emissions, from the life-cycles of liquid biofuels, biomass electricity, and
comparable conventional fuels such as petroleum, coal, and natural gas. Through examples and
a review of the literature, the most important concepts, methods, and challenges in LCA will be
discussed.
LCA has four basic components (Keoleian and Spitzley 2006): goal and scope definition, a
life-cycle inventory (LCI), life-cycle impact assessment, and interpretation. Goal definition and
scoping is essentially a roadmap for how the life-cycle model will be constructed and executed.
The system boundaries, allocation method, metrics, data characteristics, and functional unit are
all established in this phase. Once these parameters are defined and a model of the life-cycle
system is constructed, an LCI can be generated. The inventory is a quantification of system
inputs and outputs, including energy, material, air and water pollutant emissions, and wastes.
Life-cycle impact assessment can then be used to translate the energy, emissions, and material
flows catalogued in the inventory into environmental impacts. Results from LCI analysis and
impact assessment studies are interpreted to inform policy, engineering design decisions, and
consumer choice.
11.1.2 f
functional
u
nit
The functional unit is the fundamental basis upon which life-cycle model results are calculated.
Critically, if different life-cycle studies are to be compared, the functional unit must allow for com-
parison on an equivalent basis. For example, directly comparing the life-cycle impacts of 1 gallon
of ethanol fuel to 1 gallon of gasoline is not appropriate because the fuels have different energy
contents by volume. A functional unit of 1 mile driven on each of the fuels (in a single, well-defined
