Environmental Engineering Reference
In-Depth Information
Figure 11.17. Schematic of the life cycle of a first-generation biofuel.
benefits that are created during the processing step, and they add value in the overall life cycle
assessment. Inputs that are consumed and waste that is generated yield a value that represents
their environmental cost. Co-products thus offset some of the net environmental cost, so if the
inputs and waste are expressed as positive values, then the co-products are given negative values.
After refinement, the aviation fuel is transported to the airport, where it is stored and eventually
some proportion of that fuel is loaded onto an aircraft. When the aircraft takes off, the fuel is con-
sumed and the combustion products are exhausted into the atmosphere. Our “system boundaries”
in this LCA then extend from the oil well where the crude was extracted to the exhaust gases left
behind in the wake of the aircraft, leading to the widely used description of this sort of LCA as
“well to wake”.
A similar LCA for an alternative fuel, such as castor-derived Hydrotreated Esters and Fatty
Acids (HEFA) fuel, might look quite similar to Figure 11.16, but it requires additional steps to
describe it. In Figure 11.17, we have added a couple of steps at the front end to represent the
production of the castor feedstock. The first box represents the growth of the castor in the fields,
harvesting, and the separation of seeds from the rest of the plant material. Inputs might include
the environmental costs of seeds, fertilizer, herbicides and pesticides, irrigation, gasoline and
electricity for farm machinery, and packaging materials. Output waste might consist of gaseous
emissions, fertilizer runoff and plant debris. However, perhaps the plant debris left over on the
farm could be transformed into a useful co-product by running it through an anaerobic digester
to create methane fuel or turned into compost to improve soil quality. In that case, some of the
waste stream might be turned back in toward the crop production process and reduce the amount
of input energy or fertilizers needed to produce the crop.
Once the castor seeds are delivered to the processing plant, depicted in the third box of Figure
11.17, they could be crushed to extract vegetable oil. The debris left over from the crushing
process could be used for animal feed, so this would be tallied as a co-product. The vegetable
oil must be further processed and blended to yield qualified aviation fuel, generally done in a
different facility, so it travels on to the crude oil refinery. Beyond this point, the fuel's journey
has the same stages as that of petroleum-based jet fuel.
LCAs are meant to systematically evaluate processes, and since processes change over time,
the LCAs must also be capable of evolving over time. These changes might include new farming
techniques for feedstocks that can increase yield or reduce fertilizer or irrigation requirements,
more efficient equipment, development of markets for co-products, changes in water utilization,
land use, and waste management. Clearly, there is some degree of subjectivity in defining the
components of an LCA, and published studies may not always agree because they include different
sets of processes, or different inputs and outputs, or they may not assign consistent values to each
component. To address this issue, the inputs and outputs to the LCA can be considered part of a
Life Cycle Inventory (LCI), which is a database that provides the numbers that are assigned to each
constituent process/material/energy usage in the functional units of interest. Standardized LCIs
have become available; some are free (such as the European Reference Life Cycle Database
1
)
1
See http://lca.jrc.ec.europa.eu/lcainfohub/datasetArea.vm
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