Well-to-wake analysis of ethanol-to-jet and sugar-to-jet pathways

Jet fuel consumption in the US has been estimated at 3.0 trillion MJ in 2015, accounting for 10.1% of energy supplied to the US transportation sector, and this consumption is projected to steadily increase to 3.7 trillion MJ in 2040 [1]. Greenhouse gas (GHG) emissions from jet fuel combustion in the US were 149 million ton CO₂e in 2014, accounting for 8.5% of total GHG emissions by the US transportation sector [2]. Globally, jet fuel consumption has been estimated at 377 billion liters or 13.1 trillion MJ in 2012 [3]. Moreover, air traffic is expected to grow steadily: the US Energy Information Administration projected revenue passenger miles in the US will increase from 4.0 trillion miles in 2015 to 9.6 trillion miles in 2040 [1]. In response to growing environmental concerns, the aviation industry is exploring environmentally, economically, and socially sustainable solutions to reduce fuel consumption and GHG emissions for the sustainable growth of air traffic [4]. While fuel consumption can be reduced by the development and use of more efficient aircraft, shorter routing, and optimized flight management and planning, it is also beneficial to displace fossil jet fuels with low-carbon bio-based jet fuels to reduce GHG emissions significantly.

To promote bio-based jet fuel deployment, several organizations (e.g., the US Federal Aviation Administration, the US Air Force, the US Navy, the International Civil Aviation Organization, and the European Union) have committed to using bio-based jet fuels. For example, the US Department of Defense purchased about 7.6 million liters of alternative fuels between fiscal years 2007 and 2014 for testing purposes [5]. The purchased alternative fuels include largely renewable jet and diesel from hydroprocessed ester and fatty acids (HEFA) and Fischer–Tropsch jet (FTJ) along with a smaller volume of alcohol-to-jet (ATJ), synthetic iso-paraffins produced via direct sugar-to-hydrocarbon technology, and Fischer–Tropsch diesel [6]. Renewable Jet from HEFA, also known as hydroprocessed renewable jet (HRJ), is produced through hydroprocessing of fatty acids from hydrogenation of vegetable, algae, or waste oil, while FTJ is produced from gasification of natural gas (NG), coal, and biomass and with a subsequent Fischer–Tropsch synthesis. In the current ATJ process, alcohol (e.g., ethanol, methanol, or iso- or normal-butanol) is first dehydrated and converted into linear olefins via catalytic oligomerization. Then, the olefinic double-bonds are saturated via a hydrotreating process to make ATJ. For commercial aviation uses, the American Society for Testing and Materials (ASTM) International has certified HRJ, FTJ (such as Fischer–Tropsch synthetic paraffinic kerosene and Fischer–Tropsch synthetic kerosene with aromatics), synthetic iso-paraffins produced via direct sugar-to-hydrocarbon, and butanol-to-jet technologies. Other production pathways undergoing certification processes include other ATJ pathways, pyrolysis-based hydrotreated depolymerized cellulosic jet, other sugar-to-jet (STJ) pathways, and catalytic hydrothermolysis jet [7].

The key advantages of the alternative jet fuels (AJFs) over petroleum jet fuel are potential reductions in petroleum consumption and GHG emissions, which need to be evaluated on a life-cycle basis. Several life-cycle analyses of AJFs have been published. Using HEFA production details provided by UOP, Shonnard et al. [8] and Fan et al. [9] estimated the well-to-wake (WTWa) GHG emissions associated with camelina- and pennycress-based HRJ using an energy-based allocation method, with results of 22 and 33 g CO₂e/MJ, respectively. These studies assumed little land use change (LUC) impact of these fuels because the feedstocks are rotational crops. Ukaew et al. [10] investigated soil organic carbon impacts of rapeseed cultivated in inter-year rotation with wheat (wheat–wheat-rapeseed rotation) as compared to the reference wheat–wheat-fallow rotation. They modeled the top five wheat-producing counties in ten different states in the US, and demonstrated large variations in soil organic carbon changes (?0.22 to 0.32 Mg C/ha/year) incurred by rapeseed cultivation in rotation with wheat, depending on location and farming practices. The soil organic carbon changes resulted in direct LUC impacts estimated to range from ?43 to 31 g CO₂e/MJ HRJ. Ukaew et al. [11] further examined the impact of crop prices on LUC estimates for HRJ from canola produced in North Dakota, and showed a strong correlation between canola price and LUC. Bailis and Baka [12] estimated WTWa GHG emissions from jatropha-based HRJ to be 40 g CO2e/MJ without LUC, and estimated that direct LUC GHG emissions would range from ?27 to 101 g CO₂e/MJ, depending on the soil type. In addition, Seber et al. [13] discussed the GHG emissions from waste oil- and tallow-based HRJ, which depend highly on the system boundary for the waste feedstock. Other studies examined the GHG emissions of HRJ from camelina, algae, and jatropha with various farming and fuel production assumptions [14, 15]. Hydrothermal liquefaction, using algae as the feedstock, has also been examined for AJF production [16, 17]. On the other hand, Skone and Harrison [18] investigated FTJ production from coal and biomass using a process engineering model. The study estimated the FTJ’s WTWa GHG emissions to range from 55 to 98 g CO₂e/MJ, depending on biomass type and share, catalyst type, carbon management strategy, and co-product handling method. Lastly, the GHG emissions associated with jet fuel obtained from mallee via pyrolysis was estimated at 49 g CO₂e/MJ [19].

Since these studies were conducted with different assumptions and life-cycle analysis (LCA) approaches, efforts were made to compare these different AJFs on a consistent basis. Stratton et al. [20] compared the GHG emissions associated with FTJ from NG, coal, and biomass and HRJ from several oil crops and algae with those from petroleum jet fuel. They showed that FTJ from biomass and HRJ from vegetable oil and algae have potentials to reduce GHG emissions up to 102 and 66%, respectively, relative to petroleum jet depending on process assumptions and LUC emissions. These authors further discussed the impact of variation in several parameters and key LCA issues (e.g., co-product handling method and LUC) on the GHG emissions of FTJ and HRJ [21]. Elgowainy et al. [22] expanded the AJF options by adding pyrolysis jet fuel derived from corn stover, and updated key parameters for FTJ and HRJ as well as petroleum jet fuel. Han et al. [23] refined HRJ production process assumptions on the basis of fatty acid profiles of oil seeds, and showed that WTWa GHG emissions can be reduced by 41–63% (for HRJ), 68–76% (for pyrolysis jet fuel), and 89% (for FTJ from corn stover) relative to petroleum jet fuel. Agusdinata et al. [24] conducted WTWa analyses of bio-based jet fuel from non-food crops (e.g., camelina, algae, corn stover, switchgrass, and woody biomass), and projected a substantial GHG emissions reduction in 2050 under several economic and policy assumptions.

Compared to HRJ and FTJ, only a few WTWa studies on ATJ and STJ are available as summarized in Table 1. Cox et al. [25] evaluated the STJ from sugarcane molasses, and estimated its GHG emissions at 80 g CO₂e/MJ, using a system expansion method. On the other hand, Moreira et al. [26] estimated the GHG emissions of STJ from sugarcane at 8.5 g CO₂e/MJ, using a system expansion method. The large difference in the GHG emissions between these two studies stemmed from differing approaches to estimating indirect effects. Cox et al. [25] assumed that sorghum production will increase as sugarcane is used as a jet fuel feedstock, resulting in LUC GHG emissions of over 100 g CO₂e/MJ from the increased sorghum production. Moreira et al. [26], on the other hand, used the Global Trade Analysis Project model to estimate the LUC, and reported subsequent LUC GHG emissions of 12 g CO₂e/MJ. Staples et al. [27] examined nine advanced fermentation pathways from sugarcane, corn, and switchgrass (including both ATJ and STJ), and showed that the WTWa GHG emissions of jet fuels from these three feedstocks varied significantly depending on the feedstock-to-fuel conversion routes and the co-product handling method: ?27 to 20 g CO₂e/MJ for sugarcane, 48 to 118 g CO₂e/MJ for corn, and 12 to 90 g CO₂e/MJ for switchgrass without LUC. Additionally, they investigated the direct LUC effects for three cases (low, baseline, and high emissions), and reported estimated LUC GHG results of 20–47 g CO₂e/MJ for sugarcane, 38–101 g CO₂e/MJ for corn, and 1–12 g CO₂e/MJ for switchgrass. Recently, Budsberg et al. [28] examined the WTWa GHG emissions and fossil fuel use of ATJ from poplar. They investigated two options for H₂ production: NG steam methane reforming and lignin gasification resulted in 60–66 and 32–73 gCO₂e/MJ, respectively.

^aLUC GHG was estimated at 12 g CO₂e/MJ

^bLUC GHG was estimated at 20–47 g CO₂e/MJ

^cLUC GHG was estimated at 38–101 g CO₂e/MJ

^dLUC GHG was estimated at 1–12 g CO₂e/MJ

^eThe jet production process used in Moreira et al. [26] recovers and export yeast as a co-product

Cox et al. [25] and Moreira et al. [26], however, examined only STJ produced via biological conversion from sugarcane, which is not widely available for fuel production outside Brazil. Staples et al. [27] included corn and corn stover, which are more relevant to the US biofuel industry. However, Staples et al. [27] divided the production process into four stages (pretreatment, fermentation, extraction, and upgrading), and employed process assumptions for each stage (such as efficiency, energy, and mass balances) from various literature sources to estimate energy consumption in each fuel production route rather than developing a conversion process as an integrated plant. Also, the efficiencies and process energy requirements of certain processes (such as fermentation and ETJ processes) were based on theoretical maximum and expert opinions while other processes (e.g., pretreatment) were from previous techno-economic analyses (TEA) of other biofuel production (such as ethanol). Thus, assumptions (e.g., plant scale) might be inconsistent among stages and processes that might not be well-integrated. Moreover, STJ produced via catalytic conversion is yet to be investigated.

To conduct WTWa analysis on emerging ATJ and STJ from the feedstocks relevant to the US using well-integrated process assumptions, the present study incorporated the results from three TEAs into the Greenhouse gases, Regulated Emissions and Energy use in Transportation (GREET^®) model and systematically estimated WTWa GHG emissions reductions as well as fossil fuel use and water consumption by the use of these new AJFs relative to petroleum jet fuel [29]. The three TEA studies include ethanol-to-jet (ETJ) production [30], STJ production via biological conversion [31], and STJ via catalytic conversion [32]. Note that ETJ is a subset of ATJ processes using ethanol as an intermediate. Key advantages of ETJ pathways over other ATJ or alternative fuel pathways include the large feedstock availability (both sugar/starch and lignocellulosic biomass) and the technological maturity of fuel ethanol conversion, especially with starch and sugar feedstocks. Currently in the US, ethanol is largely used as a fuel additive in E10 gasoline. The Renewable Fuels Association estimated the US ethanol production at 55.6 billion liters in 2015, while the US gasoline consumption was 553 billion liters in 2015 and is expected to be reduced in the future [1, 33]. Thus, with the 10% “blend wall,” ethanol production could potentially surpass consumption in the US E10 market, which would create opportunities for ETJ pathways.

This study presents the baseline LCA results of corn-based ETJ (using integrated and distributed plants), corn stover-based ETJ (using integrated and distributed plants), and corn stover-based STJ (via biological and catalytic conversions) as compared to conventional petroleum jet using the GREET model. The GREET model is an attributional LCA model while LUC impacts are estimated via a consequential analysis. The STJ pathway via catalytic conversion uses H₂ from external source. After describing the baseline results, we assess the key drivers for the GHG reductions through sensitivity analyses that examine the influence of the following: ethanol production pathways for ETJ with a distributed ETJ production, H₂ sources for STJ produced via catalytic conversion, and co-product handling methods. Also, sensitivity analyses on key parametric assumptions are provided to show the impact of these parameters on the WTWa results. Lastly, GHG emissions for different jet fuel production pathways using one metric ton of corn stover as a uniform feedstock are presented to examine the impact of liquid fuel yields and GHG intensities of AJFs on the total GHG emissions.