An an example, they point out that an SPK fuel option with zero life cycle GHG emissions would offer a 100% reduction in GHG emissions but only a 48% reduction in actual climate impact using a 100-year time window and the nominal climate modeling assumptions in the paper.

The term “well-to-wake” is used to describe the life cycle GHG inventory of aviation fuels. Life cycle analyses of biobased, ground transportation fuels assume that the emissions from fuel combustion are equal and opposite to the emissions absorbed from the atmosphere during growth of the feedstock. However, this approach neglects non-CO₂ combustion emissions and effects, namely, soot and sulfate aerosols, water vapor, and NO_x. Aviation also causes contrails and induced cirrus clouds called contrail cirrus. Such products will exist even if the net GHG emissions from the fuel life cycle are zero.

...Soot and sulfate aerosols generate atmospheric warming and cooling, respectively, and have lifetimes on the order of days to weeks. Contrails and contrail cirrus sustain only for hours to days and cause atmospheric warming; however, their impact is the most uncertain of all aviation induced climate forcing. NO_x results in both short-term warming and long-term cooling. In the months following a pulse of NO_x in the upper atmosphere, ozone production is stimulated causing a short-term warming. NO_x emissions also stimulate the production of additional OH, acting as a sink for CH₄. The corresponding reduction in CH₄, which is an important ozone precursor, leads to a long-term reduction in ozone. Both the long-term reduction in CH₄ and ozone cool the atmosphere and decay with a lifetime of approximately 11 years.

—Stratton et al.

SPK fuels have been certified for jet aircraft use in blends up to 50%, and can be created via two primary pathways:

1. Gas-to-liquids (GTL). Gasification of coal, natural gas, or biomass followed by Fischer-Tropsch (F-T) synthesis of the resultant syngas, with subsequent upgrading to a product slate that includes a synthetic jet fuel.

2. Hydroprocessing of renewable oils to create a hydroprocessed renewable jet (HRJ) fuel.

The SPK fuels, and others being considered for aviation, differ from conventional petroleum-derived jet fuel in that they consist solely of paraffinic hydrocarbons and contain neither aromatic compounds nor sulfur.

The purely paraffinic nature and lack of sulfur in SPK fuels results in increased specific energy, decreased energy density and changes to the emissions characteristics of CO₂, H₂O, soot, sulfates and NO_x.

The MIT team sought to (1) develop ratios by which the CO₂ from combustion can be scaled to include the climate forcing from non-CO₂ combustion effects of conventional jet fuel and SPK, and (2) quantify how including non-CO₂ combustion species within the fuel life cycle changes the merit of alternative jet fuels relative to conventional jet fuel from the perspective of climate change.

The researchers used a modified version of the climate impacts module of the Aviation Portfolio Management Tool (APMT) to establish a “basis of equivalence” between emissions of different species to be able to compare the climate impacts of non-CO₂ combustion emissions and effects can be related to those of CO₂.

Well-to-wake (+) emissions for select alternative jet fuel pathways, normalized by conventional jet fuel. Credit: ACS, Stratton et al. Click to enlarge.

Overall, they found that the decrease in relative environmental merit of SPK fuels upon inclusion of the non-CO₂ combustion effects was mainly due to contrails and contrail-cirrus clouds. These dominate the climate impact of the non-CO₂ effects from conventional fuel and the MIT team assumed that SPK fuel use would not change the prevalence or impact of these effects. The team does note that additional work should be done to better understand how changes in fuel composition affect the formation of contrails and contrail-cirrus clouds as their impact may be reduced with a change in fuel composition.

The decrease in relative merit of SPK fuels means that methods of tracking climate change mitigation that rely exclusively on relative well-to-wake life cycle GHG emissions as a proxy for aviation climate impact may overestimate the impact of alternative fuel use on the global climate system. Furthermore, the variability introduced into the results by including non-CO₂ combustion emissions and effects highlights the broad challenges faced in collapsing multiple attributes into a single metric for comparison when assessing any new energy technology options.

Determining an absolute “better or worse” requires an evaluation system, which is usually accomplished through a weighting scheme, monetization or time windowing with one or more metrics. Greenhouse gases are a convenient metric of comparison because their cause and environmental effect are both important and readily quantified. Many other factors have less quantifiable impacts. The need for absolute comparisons requires defining a “basis of equivalence” that introduces significant variability into the result.

This work indicates that aviation has an opportunity space extending beyond improving fuel efficiency and burning alternative fuels to reduce its climate impact. Technologies that reduce GHG emissions from fuel production, combustion CO₂ emissions, and non-CO₂ combustion emissions and effects can all be considered simultaneously. Currently, these areas are largely examined in isolation. A holistic analysis framework is needed that examines alternative fuels for reduced well-to-wake (+) emissions, aircraft design and operations for reduced fuel consumption, and changes to operational procedures for reduced contrails and contrail-cirrus impacts; however, an equitable comparison of the climate mitigation options for aviation requires consistent accounting of the climate impacts of non-CO₂ combustion emissions and effects.

—Stratton et al.

Resources

• Russell W. Stratton, Philip J. Wolfe, and James I. Hileman (2011) Impact of Aviation Non-CO₂ Combustion Effects on the Environmental Feasibility of Alternative Jet Fuels. Environmental Science & Technology doi: 10.1021/es2017522