Purdue study projects that under likely adoption rates, use of biojet fuel alone will not meet aviation emissions reduction targets for 2050; the need to go above 50% blends
|Uncertainty range of the aviation GHG emissions under the High Oil price scenario (the most optimistic for biojet adoption), given in a box plot depicting the minimum, quartile, and maximum values. Credit: ACS, Agusdinata et al. Click to enlarge.|
A study by a team from Purdue University has found that, at what it determined as likely adoption rates, the use of drop-in biojet fuel (produced from US feedstocks) at up to a 50:50 blend with petrojet fuel alone would not be sufficient to achieve the aviation emissions reduction target of 50% below 2005 levels by 2050.
In a paper published in the ACS journal Environmental Science & Technology, they report finding that in 2050, under a high oil price scenario assumption, GHG emissions can be reduced to a level ranging from 55 to 92%, with a median value of 74%, compared to the 2005 baseline level. The study combines lifecycle analysis of different biojet pathways with a model of the supply and demand chain of biojet involving farmers, biorefineries, airlines, and policymaker, considering the factors that drive the decisions of actors (i.e., decision-makers and stakeholders) in the lifecycle stages.
Although life cycle assessment (LCA) provides a sound basis to evaluate the overall environmental impacts (including GHG emissions) of biofuels, traditionally LCA studies have focused mostly on the environmental performance of technology options and largely left out the economic aspect of the system in question or at most include economic performance as a separate part.
To reveal the achievable environmental benefits of emerging technologies such as biofuels, the economic motives of actors (i.e., decision-makers and stakeholders) involved along the life cycle stages have to be considered along with technical advances. There have been increasing interests and efforts on developing LCA methodology along this line, but to our best knowledge there has been no study conducted which focuses on the GHG emissions reduction potential of biojet.
The objective of this paper is to reveal the extent to which biojet can reduce US aviation GHG emissions through the consideration of the role and perspective of relevant actors. Decisions made by actors based on their motives, interests, and responses to incentives determine whether a policy objective can be achieved.—Agusdinata
This study only examines the GHG emissions within the US domestic context. It explores two major biojet production technologies available: (1) hydrotreating/hydrocracking process which uses vegetable oil as the feedstock, and (2) gasification followed by Fischer-Tropsch synthesis and syncrude upgrading, which uses lignocellulosic feedstocks.
For feedstock options, the Purdue team considered: (1) camelina (as representative of low-input oilseeds) and algae, and (2) lignocellulosic biomass: short rotation woody crops (SRWCs), corn stover (as representative of agriculture residue), and switchgrass (as representative of herbaceous energy crops). They excluded soybean out of concern to the disruption on food production and effects of land-use change.
The biojet produced by current processes does not contain aromatic compounds, which can account for up to 25% of petrojet by volume and are needed for proper lubrication and sealing. This, along with the requirement to meet aviation fuel density specifications, requires that biojet be blended with petrojet. The Purdue team uses the current 50:50 blend ratio approved by ASTM as the norm and as the upper threshold for blending in their study.
To evaluate actors’ decisions and their impacts on achievable GHG emissions reduction, the team developed a forecast that presents simplified biojet supply and demand logic influenced by actors’ decisions, regulatory and land constraints, as well as the cost, technology, and dynamics. The model uses three price scenarios: low oil, reference and high oil.
They found that with a business-as-usual case, the emission trajectory generated without biojet use and a 2% annual demand growth grows to up to 128% of 2005 levels by 2050; emissions grow at an annual average rate of around 0.67% due to the continuous improvement in the aircraft payload fuel energy efficiency (PFEE).
With biojet options, under the high oil price scenario (the most optimistic for biojet adoption), the median (i.e., 50th percentile, Q2) of the emissions in 2050 is about 74% of the 2005 baseline level. In this scenario, the lowest emission level attainable (i.e., minimum value of 55% of the 2005 baseline level) is still above the 2050 reduction target. The condition is much worse in the reference oil and low oil scenarios, in which the minimum emission level is 120% and 128% of the 2005 baseline level, respectively.
The Purdue team made a number of observations based on the work:
Feedstock viability is conditional on two major factors: oil price and land availability. Lignocellulosic biomass-based biojet only becomes viable when the oil price is high. In this condition, its supply potential is more than four times larger than that of oil-producing feedstock (i.e., camelina and algae).
When the oil price is lower, camelina is viable but its supply is constrained by suitability of land on which it can grow. Because policy makers may not intend to favor certain feedstock prematurely, they will need to consider the likelihood of oil price evolution, the team suggested.
A policy aiming to improve the productivity of marginal lands—through a development of special feedstock variety, for example—should become a priority.
As biojet alone appears insufficient to achieve the 2050 GHG emission reduction target, other measures are needed, including a steeper improvement in the fuel efficiency of the US aircraft fleet than the current trend shows and more fuel efficient operational procedures.
The result shows that the 50:50 blend requirement can significantly hamper the attainment of the policy goal. The relaxation of this requirement will be enabled by improvements in areas such as development of additives for improving biojet density and improving aircraft fuel tank to prevent leakage due to lack of aromatics compounds in biojet.
The scope of the work may underestimate the amount of potential biojet supply as well as feedstock mix and hence GHG emissions impact. This work focuses only on US production capacity and is limited to the consideration of marginal lands. It therefore excludes supply that may come from countries such as Canada and Brazil and a possibility that farmers may actually cultivate crop lands, resulting in higher yields. First generation feedstocks such as soybean cannot be ruled out completely. Due to its importance, different oil price scenarios such as price spikes and oscillations may result in different actor decision behaviors. Further work will need to address these issues. It should also include a further study of different incentive schemes that can be targeted to actors and feedstocks.
The methodology presented in this paper can inform the design of incentives that are more aligned with actors’ interest. Also, the questions of “who should pay” and “how the costs and payoﬀs should be shared” encapsulate the central policy problem. This equity issue needs to be addressed with vigor commensurate with the technical evaluation.—Agusdinata et al.
Datu B. Agusdinata, Fu Zhao, Klein Ileleji and Dan DeLaurentis (2011) Life Cycle Assessment of Potential Biojet Fuel Production in the United States. Environmental Science & Technology DOI: 10.1021/es202148g