In a paper published in the ACS journal Environmental Science & Technology, researchers from MIT conclude that it is “paramount” that decision makers and the general public be given the range of LC-GHG emissions that could result from the production and use of a given bio or synthetic fuel, due to high variability within pathways. Among their findings in the study are that subjective choices such as coproduct usage and allocation methodology can be more important sources of variability in the life cycle greenhouse gas (LC-GHG) inventory of a fuel option than the process and energy use of fuel production.
Variability in lifecycle analysis (LCA), Stratton et al. say, must be distinguished from uncertainty. Variability—which is inherent due to both inexact LCA procedures and variation of numerical inputs—is a dispersion of discrete results, each of which has been measured or calculated with an inherent uncertainty in the result. The authors group the sources of variability into three categories: pathway-specific variability; coproduct usage and allocation; and land use change (LUC).
In their paper, they used specific examples from the production of diesel and jet fuels from 14 different feedstocks to demonstrate general trends in the types and magnitudes of variability present in life cycle greenhouse gas (LC-GHG) inventories of middle distillate fuels.
To understand how variability impacts LC-GHG inventories of transportation fuels, a new methodological approach was developed using screening level LCAs. Screening level analyses provide preliminary assessments of technology alternatives with the intent of informing research funding and decision makers.
A requirement of screening level LCAs is to identify the pivotal factors defining the LC-GHG emission profiles of fuel production for each LC step and each feedstock. Optimistic, nominal,and pessimistic sets of these key parameters were developed to formulate corresponding low LC-GHG emissions, baseline or nominal LC-GHG emissions, and high LC-GHG emissions scenarios for each feedstock-to-fuel pathway; hence, results for each feedstock-to-fuel pathway are a range of possible LC-GHG inventories intended to demonstrate variability in fuel production processes.
A requirement of screening level LCAs is to identify the pivotal factors defining the LC-GHG emission profiles of fuel production for each LC step and each feedstock. Optimistic, nominal, and pessimistic sets of these key parameters were developed to formulate corresponding low LC-GHG emissions, baseline or nominal LC-GHG emissions, and high LC-GHG emissions scenarios for each feedstock-to-fuel pathway; hence, results for each feedstock-to-fuel pathway are a range of possible LC-GHG inventories intended to demonstrate variability in fuel production processes.—Stratton et al.
Transportation fuel pathways often result in coproducts; to allocate emissions among products, a usage must first be defined for the coproduct. In the study, the team examined four allocation methods to assign LC-GHG emissions between the primary fuel product and any coproducts: mass allocation; energy allocation; market-value allocation; and displacement (a.k.a., system expansion).
The choice of coproduct usage and allocation method may significantly affect the final results of the LCA. Several studies in the literature have acknowledged the variability introduced to LCA by different allocation methods. Three examples were chosen herein to demonstrate the variability introduced by coproduct treatment: (1) the oil and biomass coproduct system of soybeans where the biomass has an existing market as an animal feed; (2) the oil and biomass coproduct system of jatropha capsules where the biomass coproducts have a variety of potential uses; and (3) the liquid fuel product slate from a coal and biomass fed F-T facility. All three examples show a general shortcoming in the displacement approach. When the coproduct creation is large relative to the primary product, the LC-GHG inventory of the primary product depends more strongly on the LC-GHG inventory of the displaced product than the processes and energy flows of the product being examined.—Stratton et al.
The team found that all of the biofuel options examined in their study could either potentially be produced with lower LC-GHG emissions than conventional diesel, or with LC-GHG emissions that exceed those of conventional diesel. The difference is due to the LC-GHG intensity of the processes and the emissions resulting from LUC.
For this reason, it is critical to emphasize that the use of renewable resources as feedstock does not guarantee an environmentally beneficial fuel. Knowledge of specific production details is required for any definitive conclusions to be drawn. This constitutes a strong argument for LC-GHG inventories of transportation fuels to be presented as a range.
...Three key conclusions can be drawn from the potentially dominating influence of variability due to coproduct usage and allocation and LUC assumptions: 1) minimizing variability across LCA results by maximizing methodological consistency is essential to making useful comparisons between fuel options; 2) the absolute result from attributional LCAs have a diluted physical meaning and are most effectively used as a comparative tool, given the condition from the first key conclusion; and 3) it is paramount that decision makers and the general public be given the range of LC-GHG emissions that could result from the production and use of these fuels.
Such an approach emphasizes the importance of understanding the key aspects that determine the LC-GHG emissions from fuel production and use. Furthermore, it can do so before any production actually occurs. Such knowledge would help to develop technologies and policies that diversify our energy supplies and stimulate economic development while mitigating the LC-GHG emissions from transportation.—Stratton et al.
Russell W. Stratton, Hsin Min Wong, James I. Hileman (2011) Quantifying Variability in Life Cycle Greenhouse Gas Inventories of Alternative Middle Distillate Transportation Fuels. Environmental Science & Technology Article ASAP doi: 10.1021/es102597f