Modeled economic potential for Eucalyptus spp. production for jet fuel additives in the United States

Highlights

• Economic potential for Eucalyptus spp. to fulfill US terpene and cellulosic fuel markets.

• Annual coppice potential to produce 14.4 dry Mg ha⁻¹ yr⁻¹.

• Production at 204 million L yr⁻¹, valued at $500 million (USD), in 10 years.

Abstract

Feedstock price and availability remain a barrier to adoption of cellulosic biofuels. Eucalyptus spp., can produce an energy-dense terpene suitable for high-density synthetic hydrocarbon-type fuel (grade JP-10) production in addition to cellulosic-based feedstock for traditional jet fuels (e.g., grade Jet A) and gasoline. This study modeled economic potential for Eucalyptus to fulfill US fuel markets. Cold-tolerant Eucalyptus was simulated in an annual coppice system for maximized leaf production. Results of the lowest simulated price ($110 t⁻¹) show that within 10 years, there is potential to produce 204 million L yr⁻¹ of fuel, including 51 million L yr⁻¹ of JP-10-type fuel, 75 million L yr⁻¹ of Jet A type fuel, and 77 million L yr⁻¹ of gasoline. These quantities of fuel could be valued at approximately $500 million (USD), with feedstock costs totaling approximately $100 million (USD). Longer-term markets (to 20 years) or higher priced (to $220 t⁻¹) scenarios show potential for more production. Research to determine potential for genetic improvement, delivered fuel costs, and biorefinery siting near existing infrastructure is recommended.

Introduction

Low-carbon fuels (i.e., fuels with low or no net CO₂ emissions) have reduced global warming potential [1] and could help the aviation industry reduce the 20 million t of carbon currently emitted per month [2]. Low-carbon fuels also have lower sulfur and aromatic compound content [3] and some performance benefits [4]. The potential market for replacement of fossil-based jet fuel is estimated at 35% of current consumption, or 29 billion L of the 83 billion L the United States produces annually [5]. However, overcoming feedstock price and availability is a significant barrier for biomass-derived jet fuel [6].

Feedstocks, as well as production techniques, vary according to the type of fuel being produced and there are several standards of jet fuel including those used for civilian airline (e.g., Jet A), and their military substitutes (e.g., JP-8), as well as high-performance tactical missile fuels (e.g., JP-10, RJ-5) [[7], [8], [9]]. Key differences between these fuels include density, net heat of combustion (NHOC), viscosity, flash point, and freeze point. Higher density and NHOC typically translate into increased range and/or payload, but come at a higher cost of production. JP-10 has much higher density and NHOC than Jet A, Jet A-1 or JP-8 and is synthesized in small quantities. Terpene feedstocks can be used to produce JP-10 type fuels, whereas lignocellulosic feedstocks typically target Jet A type fuels.

The technical viability of aviation biofuels has been demonstrated [10] and biofuels are now a key component of the decarbonization strategy for the aviation industry, albeit with some persistent concerns around safety and sustainability of the alternative fuels [11,12]. Biomass residues and municipal solid waste (MSW) have been considered a cheap (sometimes free) feedstock for jet fuel and therefore preferred to purpose-grown feedstocks [13]. However, these feedstocks require significant cleanup post gasification to be fit for use in the aviation industry. Purpose-grown feedstocks present opportunities to increase production while controlling feedstock quantity and quality. Lignocellulosic biomass can be converted to a Jet A compatible biofuel and is often called biojet. However, the density of biojet (0.78 g L⁻¹) does not compare to that of JP-10 (0.94 g L⁻¹) [14] and the viscosity and freezing point of bio-based (1,8 cineole) fuel is sufficient to fully replace the petroleum based JP10.

Terpenoids, the largest class of plant secondary metabolites, offer a promising renewable feedstock for upgrade to biofuels with JP-10 characteristics [15]. Eucalyptus is largely undeveloped as an energy crop in the United States. Some Eucalyptus species (e.g., oil mallees) produce large quantities of foliar terpene-based oil and rapidly accumulate lignocellulosic biomass on short-coppice rotation basis. This ability to provide both lignocellulosic and terpene feedstocks makes a Eucalyptus system interesting for biofuel production due to the potential to produce high-powered and high-value JP-10 class fuel from specific terpenes as well as Jet A type fuels from the lignocellulosic biomass. Eucalyptus is also a candidate feedstock considered in the U.S. Department of Energy's (DOE) Co-Optimization Biomass to Energy studies [16]. Fiscal year (FY) 2019 Military Budget Estimates (USDOD, 2018) show planned consumption of 159,000 L of JP-10 type fuel at $7.27 L^-1. This study therefore analyzed economic potential for Eucalyptus to fulfill U.S. biojet (Sustainable Aviation Fuel (SAF)) and military-grade fuel markets in the near term (20 years). A secondary objective of this study was to analyze the potential for producing a coproduct, Jet A or the military-grade equivalent fuels.

Eucalyptus’ desirable leaf oil, 1,8-cineole has an existing international market primarily as a pharmaceutical product, and an emerging market for use as an industrial solvent. Depending on the type and quality of oil, as well as market dynamics, prices can fluctuate between $2 and $10 kg⁻¹ of oil [17]. This oil is sometimes traded commercially as “eucalyptol” and is a cyclic ether with the empirical formula C₁₀H₁₈O and systematic name 1,3,3-trimethyl-2-oxabicyclo octane. 1,8-cineole is a stable monoterpene with low chemical reactivity, and contains two or more isoprene (C5H8) units, volatile organic compounds, hydrocarbons, alcohols, aldehydes, ketones, acids, ethers and esters. Unlike other terpenoid compounds, 1,8-cineole is resistant to oxidation, polymerization and thermal decomposition [17]. High-powered fuels can be produced from various monoterpene inputs. 1,8-cineole is oxygenated, which can make it challenging for fuel use, but production of high-density biofuels has been shown to be possible using acid-catalysts [18] and by using a biphasic tandem catalytic process [19]. When used as an additive to ethanol-gasoline fuel blends, 1,8-cineole can prevent phase separation that can occur with water contamination [20].

Eucalyptus mallees (E. polybractea, E. kochii, and E. loxaphleba) produce and store large quantities of oil containing 70–95% 1,8-cineole [21] plus smaller proportions of other monoterpenes (C10) and sesquiterpenes (C15). Eucalyptus polybractea (blue mallee), one of the highest-yielding species for 1,8-cineole, often has individuals with oil occupying over 10% of leaf dry mass [22]. However, there is enormous genetic diversity in mallees, where terpene profiles vary within species, and individuals trees can produce very different terpene profiles [22]. Terpene yields in E. polybractea can be accurately predicted from genetic markers using genomic prediction models, which has benefits for breeding elite lines for fuel plantations [23]. A kg of oil from genetically modified mallees grown for pharmaceutical oil will be 90–95% 1,8-cineole. If other terpenes are desired, then different species/breeding would be required. For example, engineered lines could alter the genetic profile of terpene synthase genes which are ultimately responsible for the ratios of different terpenes in the oil.

We do not find estimates of US potential production of Eucalptus and aim to assess this potential in this research. In Australia an estimated 800,000 Mg yr⁻¹ of 1,8 cineole could be produced, with total global production potentially exceeding several million Mg yr⁻¹ [17,[24], [25], [26], [27]]. Blue mallees were trialed in Australia at a 2–3 year short rotation system [28] and have even shown sustained yield of cineole and leaf biomass with annual coppicing [29]. This approach, designed to maximize leaf production, would yield the highest annual biomass production under a high-density planting of >2300 individuals ha⁻¹ in a narrow belt consisting of two rows spaced 4 m or less apart [30]. Pasture and crop land could be converted in strips rather than blocks and could therefore fit well with an integrated landscape management approach. For example, a 5 m wide strip of land at a length of 2 km could equate to 1 ha (ha) of block planting [17], allowing this potential energy crop to be grown in smaller strips of cropland (e.g., marginal land) rather than vast dedicated fields of energy crops that are associated with concerns about land use change.

Average cineole yield and maximum cineole yield for E. kochii, E. loxophleba (ssp lissophloia), and E. polybractea ranged from 2.24 to 8.33 dry t ha⁻¹ yr⁻¹ in Australian trials [17,30]. Measured E. polybractea saplings on a yearly coppicing schedule were found to produce up to 715.6 g of leaf (dry weight, DW) with 137 g of total oil (19.1% oil concentration DW⁻¹) from a single sapling, resulting in the potential to produce 686 kg of essential oil ha⁻¹ yr⁻¹ from clones of elite lines. Mallee trials in Australia have also shown yields between 150 and 300 kg of essential oil ha⁻¹, with estimates of up to 500 kg oil ha ⁻¹ with increased density.To achieve similar results, arable land with annual precipitation rates at the same rate or even exceeding 600 mm natural rainfall would be needed. High precipitation rates would also be important for a one- to two-year coppice schedule. Further development of cold-hardy varieties of Eucalyptus would be required to extend the range further in the southeastern U.S. [31], though root systems from annual harvests may be less susceptible to frost damage. E. polybractea is frost tolerant but widespread propagation further north in the USA would certainly require investment in engineering to handle freeze conditions. An analysis of the region in which this strain could be commercially viable shows as much as 1.1 million ha of Eucalyptus in the USA in a study area limited to USDA plant hardiness zones 8b and higher, limiting minimum temperature to −9.4° C (USDA 2012). Other restrictions beyond freezing that may limit adoption include sustainability concerns. Pervasive concerns around environmental effects of Eucalyptus in the U.S. persist, without strong empirical evidence for significant invasiveness and in spite of studies showing increased diversity compared to some alternative land uses [32]. Water use concerns are founded in eucalyptus’ high interception and evapotranspiration rates and so restriction to areas with sufficient annual preciption would be important. As with all crops, it is imperative to consider a suite of sustainability criteria [33] when assessing potential production sites, including competition with other crops and land management change considerations.

Currently Eucalyptus is grown commercially in the US Southeast, predominantly in Florida, for pulp and paper markets. Eucalyptus harvest scheduling, designed to maximize profitability based on production of cellulosic biomass, typically calls for multiple years between coppice harvests [[34], [35], [36]]. This analysis of feedstock availability for jet fuels aims to maximize production of terpenes derived from Eucalyptus leaves. Thus, this analysis simulates annual coppice harvesting designed to maximize annual average leaf production ha ⁻¹ and a maximum ratio of leaf-to-stem biomass. With wildfires and hurricanes projected to become more extreme [[37], [38], [39], [40], [41]] an added benefit of annual harvesting is reduced risk exposure to extreme events [42]. Dense stands of 1-year growth stems that are pliable and low to the ground are less exposed to windthrow than conventional multi-year grown stems. Further, annual harvest of growing stock eliminates risk of loss of production more than one year old.

Widespread potential for Eucalyptus production in the U.S. was found in a recent national resource assessment, the 2016 Billion-Ton Report (BT16) [43], but BT16 constrained Eucalyptus by limiting the range to primarily Florida and a few counties bordering the Gulf of Mexico. BT16 used the Policy Analysis System (POLYSYS) to estimate how agricultural producers may respond to changing agricultural markets, including conventional crops and biomass resources. POLYSYS is an economic model that simulates the U.S. agricultural sector through price adjustments in commodity markets to balance supply and demand [44], and has been used to quantify potential biomass resource potential [43,[45], [46], [47], [48]]. POLYSYS fixes its analyses to the USDA-published baseline of price projections, yield, and planted area for the agriculture sector [49] and includes conventional crops: corn, soybeans, grain sorghum, oats, barley, wheat, cotton, rice, and hay. These crops comprise approximately 90% of the U.S. agricultural land area.