Impact of Aviation Non-CO2 Combustion Effects on the Environmental Feasibility of Alternative Jet Fuels

Published: November 22, 2011

r 2011 American Chemical Society 10736 dx.doi.org/10.1021/es2017522 | Environ. Sci. Technol. 2011, 45, 10736–10743

ARTICLE

pubs.acs.org/est

Impact of AviationNon-CO2 Combustion Effects on the EnvironmentalFeasibility of Alternative Jet FuelsRussell W. Stratton, Philip J. Wolfe, and James I. Hileman*

Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts, 02139

bS Supporting Information

ABSTRACT:

Alternative fuels represent a potential option for reducing the climate impacts of the aviation sector. The climate impacts ofalternatives fuel are traditionally considered as a ratio of life cycle greenhouse gas (GHG) emissions to those of the displacedpetroleum product; however, this ignores the climate impacts of the non-CO2 combustion effects from aircraft in the upperatmosphere. The results of this study show that including non-CO2 combustion emissions and effects in the life cycle of a SyntheticParaffinic Kerosene (SPK) fuel can lead to a decrease in the relative merit of the SPK fuel relative to conventional jet fuel. Forexample, 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 assumption set outlined herein.Therefore, climate change mitigation policies for aviation that rely exclusively on relative well-to-wake life cycle GHG emissions as aproxy for aviation climate impact may overestimate the benefit of alternative fuel use on the global climate system.

1. INTRODUCTION

Aviation plays a key role in the global economy and transpor-tation systems. Projections indicate that over the next 2 decades,the demand for aviation within the U.S. will grow at roughly 2%per annum.1 Mitigating climate change from the aviation sectorcan be simplified to consuming less energy, through improve-ments in aircraft technology or operational efficiency, andreducing the climate impacts of the energy source, through theuse of alternative fuels. Synthetic Paraffinic Kerosene, as definedherein, has been certified for jet aircraft use up to a blend ratio of50% and can be created via (1) Gasification of coal, natural gas,or biomass to form synthesis gas, which is processed usingFischer�Tropsch (F�T) synthesis, and subsequently upgradedto a product slate that includes a synthetic jet fuel; and (2)Hydroprocessing of renewable oils, such as those created fromjatropha, camelina, and algae, among many others, to create ahydroprocessed renewable jet (HRJ) fuel.2,3 These fuels, andothers being considered for aviation, differ from conventional jetfuel in that they are comprised solely of paraffinic hydrocarbonsand contain neither aromatic compounds nor sulfur 4,5

The potential of a particular fuel to reduce greenhouse gasemissions (GHG) is generally assessed through a comparison ofthe life cycle GHG emissions inventory of the alternative fuelwith that of the fuel it is intended to displace.2,6 A life cycle GHG

emissions inventory encompasses emissions from recovery andtransportation of the feedstock to the production facility, proces-sing of these materials into fuels, transportation and distributionof the fuel to the aircraft tank, and finally, the combustion of thefuel in the aircraft. The term “well-to-wake” is used to describethe life cycle GHG inventory of aviation fuels. Life cycle analysesof biobased, ground transportation fuels assume that the emis-sions from fuel combustion are equal and opposite to theemissions absorbed from the atmosphere during growth of thefeedstock.2,5�8 However, this approach neglects non-CO2 com-bustion emissions and effects, namely, soot and sulfate aerosols,water vapor, and NOx. Aviation also causes contrails and inducedcirrus clouds called contrail cirrus. Such products will exist evenif the net GHG emissions from the fuel life cycle are zero.Figure 1 schematically demonstrates the life cycle impacts path-way of aviation related climate change using a biobased fuelstarting with emissions from fuel production and fuel combus-tion in the engine and culminating in societal impacts.

Received: May 23, 2011Accepted: October 3, 2011Revised: September 25, 2011

10737 dx.doi.org/10.1021/es2017522 |Environ. Sci. Technol. 2011, 45, 10736–10743

Environmental Science & Technology ARTICLE

Soot and sulfate aerosols generate atmospheric warming andcooling, respectively, and have lifetimes on the order of days toweeks.10 Contrails and contrail cirrus sustain only for hours todays and cause atmospheric warming;9,11 however, their impact isthe most uncertain of all aviation induced climate forcing.9 NOx

results in both short-term warming and long-term cooling. In themonths following a pulse of NOx in the upper atmosphere, ozoneproduction is stimulated causing a short-term warming. NOx

emissions also stimulate the production of additional OH, actingas a sink for CH4. The corresponding reduction in CH4, which isan important ozone precursor, leads to a long-term reduction inozone. Both the long-term reduction in CH4 and ozone cool theatmosphere and decay with a lifetime of approximately 11 years.12

The purely paraffinic nature and lack of sulfur present in SPKfuels has been shown to cause changes in the combustion emissionsfrom gas turbine engines;13�16 hence, the purpose of this paper is2-fold: (1) develop ratios by which the CO2 from combustioncan 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 thefuel life cycle changes the merit of alternative jet fuels relative toconventional jet fuel from the perspective of climate change.Select alternative jet fuel life cycle GHG inventories, as devel-oped by Stratton et al.2 and documented in the SupportingInformation (SI), have been leveraged herein as examples of hownon-CO2 combustion effects change the climate change mitiga-tion potential of alternative fuel options. The SI also containsadditional details on the underlying methodology adopted in thecreation of these life cycle GHG inventories; however, theconclusions of this work are independent of the life cycle GHGinventories to which the climate forcing from non-CO2 combus-tion effects are added.

2. MATERIALS AND METHODS

This paper implements a modified version of the climateimpacts module of the Aviation Portfolio Management Tool(APMT) to establish a “basis of equivalence” between emissions ofdifferent species, such that the climate impacts of non-CO2 com-bustion emissions and effects can be related to those of CO2. Thisprocess is described in further detail in Sections 2.1 through 2.4.2.1. Modeling Framework. The APMT climate module has

been documented and tested in the literature.17�19 The model isbased on the Bern carbon-cycle impulse response function with asimplified analytical temperature change model to estimate cli-mate impacts for aviation CO2 and non-CO2 effects. The temporalresolution is limited to one year while the spatial resolution is anaggregated global mean level.Inputs to APMTClimate are a background emissions scenario,

a demand scenario for aviation fuel burn and corresponding emissionsinventories for CO2 and NOx. Radiative forcing estimates forNOx were obtained by linearly scaling radiative forcing estimatesfrom the literature based on NOx emissions because the short-lived nature of the species inhibits a well-defined gas-cycle likethat for carbon.12,20,21 All short-lived effects (aerosols, H2O, con-trails and contrail cirrus) are scaled linearly with fuel burn levelsbased on radiative forcing estimates from the literature.22�24

Outputs from the model are temporal profiles of RF and changein global mean average temperature from each forcing agent.It is important to note the treatment of climate forcing from

aircraft induced cloudiness (AIC). AIC encompasses both con-trails and contrail cirrus and its treatment is based on results fromLee et al.22 Instead of having distinct contrail and contrail cirrusclimate forcings, results are given for contrails and for AIC. Thistreatment stems from a coupling between the uncertainty levelsof contrails and total AIC.

Figure 1. Aviation climate change impacts pathway (adapted from ref 9).



2.2. Model Uncertainty.Monte Carlo methods were used tocapture uncertainties in inputs and model parameters. Thisrequires expressing inputs and parameters as distributions wherepossible. As described by Mahashabde et al.,18 in order to extractmeaningful insights about the possible costs and benefits of apolicy, it is helpful if the analysis options are synthesized into a setof predefined combinations of inputs and assumptions. Thesecombinations of inputs and model parameters each describe aparticular point of view or perspective on analysis. Each of thesecombinations is designated as a lens as it symbolizes a particularviewpoint through which one can assess environmental andeconomic impacts. There are currently three lenses implementedin the APMT climate module; namely, low impacts, nominalimpacts and high impacts. Each lens corresponds to the use ofdifferent values or distributions for the most influential param-eters in the climate module. Parameters captured in the lenses arethe projected growth of aviation, climate forcing of each non-CO2 effect, climate sensitivity and a climate damage coefficient.While APMT would normally allow the background CO2 con-centration to vary between lenses, this work uniformly adopted aconstant background CO2 concentration of 378 ppm tomaintainconsistency with existing IPCC GWP calculations (even thoughthe current atmospheric CO2 concentration has subsequentlyrisen to 390 ppm)In a manner that parallels the lenses described above, Stratton

et al.2,6 developed a new methodological approach for con-structing life cycle GHG inventories of transportation fuels. Init, key parameters were identified through examination of theGHG emissions resulting from each life cycle step. Optimistic,nominal and pessimistic sets of these key parameters weredeveloped and used to formulate corresponding low GHGinventories, baseline or nominal GHG inventories, and highGHG inventories using attributional life cycle analysis (LCA).The low lens, nominal lens and high lens of APMT, which wereused to assess tank-to-wake combustion emissions, mirror theformulation of the low, baseline and high well-to-tank GHGemissions inventories.2.3. Combustion of SPK Fuel Compared to Conventional

Jet Fuel. The purely paraffinic nature and lack of sulfur in SPKfuels result in increased specific energy, decreased energy densityand changes to the emissions characteristics of CO2, H2O, soot,sulfates andNOx.

4,13�16 Therefore, independent non-CO2 ratiosare required for conventional jet fuel and SPK fuel. The changesin combustion properties between conventional jet fuel and SPKfuel are summarized in Table 1 and were implemented into theAPMT climate impacts module.

A detailed characterization of conventional and synthetic jetfuel by Hileman et al.4 was used to determine changes in specificenergy, energy density and CO2 emissions. Water vapor emis-sions were modified based on the carbon to hydrogen ratio of JetA and SPK fuel. Synthetic fuels contain negligible quantities ofsulfur so all sulfate emissions were eliminated. Changes in sootand NOX emissions were represented as probabilistic distribu-tions to compliment the existing lens framework of APMT andreflect reduced certainty. The formation of contrails and contrailcirrus were assumed unchanged by the use of SPK fuel. Furthercharacterization of combustion emissions and effects of SPK fuelrelative to conventional jet fuel is available in the SI.2.4. Climate Metrics and Functional Unit. Well-to-wake

GHG emissions are presented per unit of energy (lower heatingvalue) consumed by the aircraft. The life cycle emissions of a fuelpathway can be presented either with or without the inclusion ofclimate impacts from non-CO2 combustion emissions and effects.When non-CO2 combustion emissions and effects are ignored,the emissions inventory is a pure GHG emissions inventorycomposed of CO2, CH4 and N2O. Merging non-CO2 combus-tion emissions and effects into a fuel life cycle GHG inventoryrequires them to be presented per megajoule (LHV) of fuelconsumed by the aircraft. This work developed non-CO2 ratiosto scale the CO2 from combustion to account for the climateforcing from non-CO2 combustion emissions. This approachdraws from the process and results developed for conventional jetfuel by Dorbian et al.25 Although non-CO2 combustion emissionsand effects have climate impacts that have been represented interms of CO2, they are not themselves greenhouse gases (withthe exception of water vapor). As such, integrating the non-CO2

combustion emissions and effects into aGHG life cycle inventoryresults in a combination of a GHG inventory and an impactanalysis. The terminology “well-to-wake (+)” is introduced hereto identify the combination of CO2 and non-CO2 effects fromfuel combustion in aircraft.Global warming potentials (GWP) are commonly used to

express the impact of long-lived gases such as CH4 and N2O interms of a carbon dioxide equivalent.24,26,27 The time window over

Table 1. Normalized Emissions Characteristics of SPK FuelRelative to Conventional Jet Fuel. All Are on a Mass Basisexcept Energy Density, Which Is on a Volumetric Basis

fuel characteristics SPK relative to conventional jet fuel

specific energy 1.023

energy density 0.963

combustion CO2 0.98

water vapor 1.11

sulfate aerosols 0.0

soot aerosols 0.05�0.4

nitrogen oxides 0.9�1.0

contrails

AIC 1.0

Figure 2. Non-CO2 ratios disaggregated by species and time windowfor conventional jet fuel and SPK fuel.



which the radiative forcing is integrated is a value judgmentrather than a matter of science, although 100 years is commonlychosen. Other metrics are also available to relate the climateforcing of one substance to another.26,27 Clearly stating thechosen metric and time window is essential in all situations. Thisis particularly important when assessing the climate impacts ofnon-CO2 combustion emissions and effects within a greenhousegas inventory that includes CH4 and N2O. For example, the useof 100-year global warming potentials to express the CH4 andN2O in terms of CO2 equivalent should require the use of 100-year integrated radiative forcing to assess the non-CO2 combus-tion emissions effects.

The IPCC provides GWP values for CH4 and N2O overtime windows of 20 years, 100 years and 500 years.24 Main-taining consistency with the treatment of life cycle CH4 andN2O limits the assessment of non-CO2 combustion emissionsand effects in this work to integrated-radiative forcing over20 year, 100 year, and 500 year time windows. The non-CO2

ratio for a given time window, Δt, and its integration witha fuel life cycle GHG inventory to form a well-to-wake (+)inventory is given by the equations below. Each RF element isthe aggregated radiative forcing from all direct and indirectmechanisms through which that species or effect influencesthe climate system.

ðCO2eÞwell-to-wake ¼ ðCO2 þ CH4 3GWPCH4 þ N2O 3GWPN2OÞFuel Production þ ðCO2ÞCombustionðCO2eÞwell-to-wakeðþÞ ¼ ðCO2 þ CH4 3GWPCH4 þ N2O 3GWPN2OÞFuel Production þ ðCO2ÞCombustion 3 ðnon-CO2 ratioÞ

non-CO2 ratio ¼

Z t0 þ Δt

t0

½RFCO2ðtÞ þ RFNOxðtÞ þ RFsoot þ RFsulf atesðtÞ þ RFAICðtÞ þ RFH2OðtÞ�dtZ t0 þ Δt

t0

½RFCO2ðtÞ�dt

The challenge in treating non-CO2 combustion emissions andeffects lies in reconciling the wide range of atmospheric lifetimesranging from centuries for CO2 to hours for contrails. Long livedgases becomewell mixed in the atmosphere; however, short-livedemissions can remain concentrated near flight routes, mainly inthe northern midlatitudes; hence, these emissions can lead toregional perturbations to radiative forcing.27 The impact ofaircraft on regional climate could be important, but is currentlybeyond the capability of the models used in this work; hence, theresults from APMT Climate may not be applied to individualflights. Despite these limitations, assessing short-lived effects on aglobally averaged basis gives an indication of the contributionfrom non-CO2 forcing agents to climate change for the purposesof policy making.24

3. RESULTS

3.1. Non-CO2 Ratios for Conventional and SPK Fuel. Thenon-CO2 ratios derived for conventional jet fuel and SPK fuel aregiven in Figure 2 for time windows of 500 years, 100 years, and20 years. Each time window has ratios derived using the low,nominal, and high impact lens to capture climate-modelinguncertainties of the APMT climate model. The bars correspondto results using the nominal lens while the whiskers show theresults using the low and high lens. Shorter time windows em-phasize the climate forcing of short-lived effects.Note that total non-CO2 ratios are given for a case where only

contrails are included as well as a case where total AIC is included(recall that contrails and AIC are mutually exclusive within theformulation of APMT). All calculations subsequently presentedherein will be using the non-CO2 ratio including total AIC.Although these results indicate that both the time horizon and

choice of lens play an important role in assessing the climateimpacts of non-CO2 combustion emissions and effects, Figure 2does not convey that the use of different background emissionsscenarios has an important influence on the resulting non-CO2

ratios. The default background scenarios used in the APMTclimate lenses are inconsistent with the assumptions used by theIPCC in calculating GWP values. Specifically, the low, nominal

and high lenses typically assume SRES B2, SRES A1B, and SRESA2 background scenarios, respectively, whereas the IPCC adoptsa constant background CO2 concentration of 378 ppm in allGWP calculations. While a constant CO2 background concen-tration assumption is unrealistic, it was adopted in this work forconsistency with the IPCC treatment of CH4 and N2O. Forexample, the 100-year non-CO2 ratios for conventional jet fuelcalculated using the APMT prescribed low, nominal and highlens SRES background scenarios are 37% lower, 11% higher and24% higher, respectively, than those calculated using the constantbackground scenario. These differences are amplified underlonger timewindows (i.e., 500 year), while reduced under shortertime windows (i.e., 20 year).The combustion of conventional jet fuel emits 73.2 gCO2/MJ.

In the nominal case (100-year time window and nominal lens),the results from Figure 2 show that accounting for the climateimpacts of all combustion products from aircraft is equivalent toemitting 2.07 times the amount of CO2 actually produced fromthe combustion process, or 151.3 gCO2e/MJ instead of 73.2gCO2/MJ.Similarly, the combustion of SPK fuel emits 70.4 g CO2/MJ.

When compared to conventional jet fuel, the elimination ofsulfates and the increase in water vapor lead to warming while thereduction in NOx and soot lead to cooling. Using the low lensunder the 100-year and 500-year time windows, the reduction inNOx instead leads to a small warming effect. In the nominal case,Figure 2 shows that accounting for the climate impacts of theactual combustion products from aircraft consuming SPK fuel isequivalent to emitting 2.22 times the amount of CO2 actuallyproduced from the combustion process, or 156.1 gCO2e/MJinstead of 70.4 gCO2/MJ.The contribution of contrails and contrail cirrus to the total

non-CO2 ratio is sufficiently large that even a small change ofeither is more significant than a large change in other species;hence, the total non-CO2 ratios are most sensitive to error inthese two forcing agents. The reader is reminded that theradiative forcing from contrails and contrail cirrus were assumedunchanged per unit mass of fuel burned. As discussed in the SI,more detailed models and empirical measurements are needed to



determine what, if any, changes may actually occur to contrailsand contrail-cirrus with a change in fuel composition.3.2. Well to Wake (+) Emissions Inventories. The results

from Figure 2 can be combined with well-to-wake GHG emis-sions inventories to complete the framework required to includenon-CO2 combustion emissions and effects in a fuel life cycle.The non-CO2 ratios developed using the low, nominal and highimpact lenses of the APMT climatemodel were paired with selectlife cycle GHG inventories developed by Stratton et al.2 TheseGHG inventories were created by identifying key parametersthrough examination of the GHG emissions resulting from eachlife cycle step. Optimistic, nominal and pessimistic sets of thesekey parameters formed the basis for corresponding low GHGinventories, baseline or nominal GHG inventories, and highGHG inventories. The select fuel pathways used herein areconventional jet fuel, Fischer�Tropsch jet fuel from switchgrass,Fischer�Tropsch jet fuel from coal without carbon capture andsequestration (CCS), and hydroprocessed jet fuel from rapeseedoil. These fuel pathways were chosen to span a wide range of totalGHG emissions and also the relative fractions of methane andnitrous oxide. Details regarding the analysis of these fuels aregiven in the SI. All results and discussion in this section arelimited to a 100-year time window because of its prevailing use inthe scientific community for global warming potentials. Thesensitivity of results to choice of time window is addressed inSection 3.3.Life cycle analysis is fundamentally a comparative tool; hence,

results are normalized by the life cycle GHG emissions of thebaseline, conventional jet fuel. The range in life cycle GHGemissions of conventional jet fuel is primarily driven by the originof the crude oil and the refining process. Specifically, the low,baseline and high GHG emissions results of conventional jet fuelcorrespond to straight run jet fuel from domestically sourcedcrude oil, the 2008 average U.S. refining efficiency with a weightedaverage of all crude oil fed into U.S. refineries, excluding Canadian

oil sands, and hydroprocessed jet fuel from Nigerian crude oil,respectively.2

As noted in Table 1, the specific energy of SPK is 2.3% higherthan that of conventional jet fuel. Therefore, the use of SPK leadsto a reduced takeoff weight and hence a reduced quantity of fuelneeded to complete a given mission. Hileman et al.4 quantifiedthis effect on a fleet wide basis, finding that a 0.3% reduction infleet wide fuel energy use would result from the use of all SPK;therefore, a 0.3% improvement in energy efficiency was appliedto SPK fuels when normalizing by conventional jet fuel.Figure 3 compares the well-to-wake and well-to-wake (+) emis-

sions inventories normalized by the corresponding baseline ofconventional jet fuel. The upper half shows only the well-to-wakeinventories as presented by Stratton et al.2 and documented inthe SI. The lower half shows the well-to-wake (+) version of thesame GHG inventories. The solid error bars, which have lines ontheir ends, correspond to combining the 100-year nominal non-CO2 ratios with the well-to-wake low, baseline and high GHGemissions scenarios. The dashed error bars, which have circleson their ends, correspond to pairing the 100-year low impact,nominal and high impact non-CO2 ratios with the well-to-wakelow, baseline and high GHG emissions scenarios, respectively.Presenting the results in this manner separates the variabilityintroduced by the well-to-tank GHG inventories from the climatemodeling uncertainty of the non-CO2 ratios. The uncertainty ofthe non-CO2 ratios has a larger influence than the internal variabilityof the GHG inventories on the range of normalized well-to-wake(+) emissions for each fuel pathway.Relative to the normalized well-to-wake GHG inventories

shown in the upper half of Figure 3, intrapathway variability isincreased with the inclusion of non-CO2 combustion emissionsand effects while interpathway variability in normalized well-to-wake (+) emissions is reduced. For these pathways, when onlyGHG emissions are considered, the range in baseline life cycleGHG emissions is 0.2�2.2 times those of baseline conventional

Figure 3. Well-to-wake (+) emissions for select alternative jet fuel pathways, normalized by conventional jet fuel.



jet fuel. When GHG emissions and non-CO2 combustion emis-sions and effects are included, the range in baseline well-to-wake(+) emissions is reduced to 0.6�1.7 times those of baselineconventional jet fuel. Hence, the inclusion of non-CO2 combus-tion emissions and effects in the fuel life cycle increases theabsolute emissions range of each fuel pathway but reduces theoverall range in the life cycle emissions of alternative fuels relativeto conventional jet fuel.Interest is primarily focused on fuel options with well-to-wake

GHG emissions lower than those of conventional jet fuel.Because of the similar magnitude of the non-CO2 combustionemissions and effects of SPK and conventional jet fuel, thenormalized well-to-wake (+) emissions of fuels in this categoryare higher than their normalized well-to-wake GHG emissions.Hence, a percentage reduction in life cycle GHG emissions of thejet fuel mix is less than the actual percentage reduction in aviationrelated climate impacts. For example, an SPK fuel option withzero life cycle GHG emissions would offer a 100% reduction inGHG emissions but only a 48% reduction in actual climateimpact using a 100-year time window and the nominal lens.Therefore, aviation GHG reduction scenarios (e.g., emissionswedge charts) that rely exclusively on relative changes in GHGemissions may overestimate the benefit of alternative fuel use onthe global climate system. Only a percentage reduction in fuelburn is equivalent to the same percentage reduction in aviationrelated climate forcing. The degree of overestimation is depen-dent on the assumptions used for the climate impact analysis ofnon-CO2 combustion emissions and effects. Conversely, aviationGHG reduction scenarios that rely on absolute changes in GHG

emissions (e.g., mass of CO2e/MJ) will yield similar results forthe impact of alternative fuel use on the global climate systemregardless of whether well-to-wake or well-to-wake (+) emissionsare used. The discrepancy between conventional jet fuel and SPKfuels caused by combustion CO2 emissions and the non-CO2

ratios is small by comparison to the variability in well-to-tankGHG emissions among the fuel options of Figure 3.While the results of this work were developed using the best

currently available data, climate forcing from contrails and con-trail cirrus remains uncertain, especially for SPK fuel; therefore,these results should be used with caution until further research isavailable on how SPK fuel use changes their impacts.3.3. Sensitivity of Results to Time Window. The results of

Figure 3 were created using a 100-year time window to express allspecies and effects in terms of carbon dioxide equivalent; however,other time windows (i.e., 500-year and 20-year) could have beenchosen for the non-CO2 ratios and GWP values of CH4 andN2O. The sensitivity of the well-to-wake and well-to-wake (+)emissions inventories to the choice of time window was exam-ined through the use of alternative jet fuel case studies. Specifi-cally, the well-to-wake GHG emissions inventories of baselineconventional jet fuel from U.S. crude oil, baseline conventionaljet fuel from Nigerian crude oil and baseline rapeseed oil to HRJwere chosen to span a fuel pathway dominated by CO2, by CH4,and by N2O. Methane and nitrous oxide have atmospheric pertur-bation lifetimes of 12 and 114 years, respectively; therefore, a 20-year time windowmore heavily weights methane while a 100 yeartime window more heavily weights nitrous oxide. Additionally,the CO2 and non-CO2 combustion emissions and effects of

Figure 4. Sensitivity to time window of well-to-wake GHG emissions inventories of baseline conventional jet fuel from U.S. crude oil, baselineconventional jet fuel from Nigerian crude oil, baseline rapeseed oil to HRJ and the combustion emissions and effects of conventional jet fuel. Timewindows of 500 years, 100 years, and 20 years were considered using the nominal lens.



conventional jet fuel were evaluated using the nominal lens with500-year, 100-year, and 20-year time windows. The sensitivity ofeach GHG inventory and the combustion emissions and effectsto time window selection is shown in Figure 4.The life cycle GHG inventory of conventional jet fuel from

U.S. crude oil is largely composed of CO2 emissions. Only asmall fraction is from CH4 or N2O; therefore, this inventory isinsensitive to the choice of time window chosen to representCH4 and N2O in terms of CO2. The choice of time windowcauses only a 1.3 gCO2e/MJ change in the well-to-wake GHGemissions the fuel.Substantial methane emissions result from the venting pro-

cesses used in Nigeria for crude oil extraction. The global warmingpotential of CH4 varies by approximately an order of magnitudewhen evaluated using a 20-year time window versus a 500-yeartime window. This variation is carried through to the life cycleGHG inventories of fuels where CH4 is an important contributorto the total. Choosing different time windows to assess the lifecycle GHG emissions of baseline conventional jet fuel fromNigerian crude oil results in a range of 33.6 gCO2e/MJ.The life cycle GHG inventory of HRJ from rapeseed oil is

strongly influenced by N2O emissions from direct and indirectconversion of synthetic nitrogen applied to the field and nitrogenrich crop residues. The global warming potential for N2O is lesssensitive to time window than that of CH4; however, it still variesby approximately a factor of 2 and this variation is carried throughto the life cycle GHG inventories of fuels where N2O is animportant contributor. Baseline HRJ from rapeseed oil (assumingno GHG emissions from land use change) is subject to a range of13.5 gCO2e/MJwith the use of 500-year and 20-year timewindows.While both the time horizon and choice of lens are important

when assessing the climate impacts of non-CO2 combustionemissions and effects, the focus of this section is the influenceof the time window; therefore, the nominal lens was used as arepresentative assumptions set. The combustion emissions andeffects from conventional jet fuel vary by 267.9 gCO2e/MJ, from96.6 gCO2e/MJ to 364.5 gCO2e/MJ, with the use of a 500-yearor 20-year time window, respectively. SPK fuels are subject to thesame influence because of the similarity in the non-CO2 ratios ofSPK fuel and conventional jet fuel. As a result, all SPK fuels usedby aviation are affected by the substantial influence of timewindow choice on well-to-wake (+) emissions. The scope is notlimited to fuel pathways that are strong in a particular type ofemissions and the magnitude is several times larger than that ofCH4 or N2O. Despite this undesirable variability, the timewindow used to assess non-CO2 combustion emissions andeffects should be constrained to the same as that used in theglobal warming potentials of well-to-tank CH4 and N2O; hence,the need for consistency serves as a constraint in choosing theappropriate time window.

4. DISCUSSION

This work implemented a modified version of the APMTclimate impacts module to develop ratios to scale the CO2

emissions from fuel combustion to account for the additionalclimate forcing from the non-CO2 combustion emissions andeffects of SPK fuel and conventional jet fuel. The results indicatedthat including non-CO2 combustion effects from SPK fuel usecan lead to an important decrease in the relative merit of the SPKfuel relative to conventional jet fuel. This is because contrails andcontrail-cirrus clouds dominate the climate impact of the non-

CO2 effects from conventional fuel and the analysis hereinassumed that SPK fuel use would not change the prevalence orimpact of these effects. Additional work should be devoted tobetter understanding how changes in fuel composition affect theformation of contrails and contrail-cirrus clouds as their impactmay be reduced with a change in fuel composition.

The decrease in relative merit of SPK fuels means thatmethods of tracking climate change mitigation that rely exclu-sively on relative well-to-wake life cycle GHG emissions as aproxy for aviation climate impact may overestimate the impact ofalternative 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 challengesfaced in collapsing multiple attributes into a single metric forcomparison when assessing any new energy technology options.Determining an absolute “better or worse” requires an evalua-tion system, which is usually accomplished through a weight-ing scheme, monetization or time windowing with one or moremetrics. Greenhouse gases are a convenient metric of comparisonbecause their cause and environmental effect are both importantand readily quantified. Many other factors have less quantifiableimpacts. The need for absolute comparisons requires defining a“basis of equivalence” that introduces significant variability intothe result.

This work indicates that aviation has an opportunity spaceextending beyond improving fuel efficiency and burning alter-native fuels to reduce its climate impact. Technologies thatreduce GHG emissions from fuel production, combustion CO2

emissions, and non-CO2 combustion emissions and effects canall be considered simultaneously. Currently, these areas arelargely examined in isolation. A holistic analysis framework isneeded that examines alternative fuels for reduced well-to-wake(+) emissions, aircraft design and operations for reduced fuelconsumption, and changes to operational procedures for reducedcontrails and contrail-cirrus impacts; however, an equitablecomparison of the climate mitigation options for aviation re-quires consistent accounting of the climate impacts of non-CO2

combustion emissions and effects.

’ASSOCIATED CONTENT

bS Supporting Information. Information on the combus-tion emissions of SPK fuel and the development of the life cycleGHG inventory used here is provided. This material is availablefree of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Phone: 617-452-2879; e-mail: [email protected].

’ACKNOWLEDGMENT

This work was made possible by funding from the FederalAviation Administration and Air Force Research Laboratoriesthrough the PARTNER Center of Excellence under Award Num-ber 06-C-NE-MIT, Amendment Nos. 012 and 021. We thank Mr.Chris Dorbian, Mrs. Hsin Min Wong, Prof. Steven Barrett, Prof.Jessika Trancik, and Prof. Ian Waitz for their help in improving thequality of the work presented herein as well as Warren Gillette andLourdes Maurice, of FAA, and Tim Edwards and Bill Harrison,both of AFRL, for their leadership in managing this project.



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Documents

Impact of Aviation Non-CO2 Combustion Effects on the Environmental Feasibility of Alternative Jet Fuels