Lignin-based jet fuel and its blending effect with conventional jet fuel

Highlights

• A new LJF is reported primarily composed of C6-C18 mono-, di-, and tri-cycloalkanes.

• The LJF exhibited complementary material compatibility to existing SAF pathways.

• Bulk properties of LJF 10% blend with a conventional fuel are reported.

• LJF 10 vol% blend meets the ASTM D7566 Table 1 required properties.

• Removal of any C17+ carbon molecules by distillation would achieve higher blend ratios.

Abstract

Sustainable aviation fuels (SAFs) must demonstrate specific physical and chemical properties as well as material compatibility (i.e., seal swell) to be used as aviation turbine fuels. Several alternative jet fuels incorporated in ASTM D7566 are comprised mainly of n/iso-alkanes and can only be blended up to 50 vol% due to material compatibility and density issues. Prior work illustrated the ability of cycloalkanes to replace the swelling potential of aromatics required for material compatibility. Here, we report the first archival documentation of a feedstock and chemical process to yield a product composition that could complement 5 existing SAF ASTM D7566 annexes. A lignin-based jet fuel (LJF) blend component is generated and composed of mostly C6–C18 mono, di, and tri-cycloalkanes. The neat LJF was blended with conventional jet fuel at 10 vol% (LJF blend) to simulate a potential qualification goal. Fuel properties critical to engine operability (ATSM D4054 Tier 3 & 4) were either predicted or experimentally tested based on the volume availability. All LJF-blended operability properties fall within the experience range of conventional jet fuel, with neat o-ring swelling exceeding the typical range of conventional fuels. These results support the potential use of this LJF pathway to complement other SAF pathways and achieve 100% drop-in SAF.

Introduction

Minimizing the impact of economic activities and growth on human health and the environment is a growing concern in aviation, with CO₂ emissions historically being the focus [1], [2], [3]. Recently, this focus has shifted to include non-volatile particulate matter [4] and contrail formation [5]. The interest in averting these symptoms has yielded a diverse array of possible solutions. Battery, sustainable aviation fuel (SAF), hybrid, and hydrogen technologies have all been proposed as potential solutions [6], [7], [8]. However, for ∼73% of CO₂ emissions (medium and long-haul flights), battery electrification and fuel cell technologies currently appear to be improbable solutions [9].

Additionally, for these applications, it remains unclear what impact hydrogen turbine propulsion will have on human health [10] via increased NOx emissions [11] and contrail formation, which is the predominant radiative forcing term in aviation transportation [5]. SAFs, however, have shown the ability to reduce contrail formation [12], while significantly reducing the carbon intensity to low [13] and negative values [14]. Still, the potential lower limit of contrail formation with drop-in SAF remains to be determined. Nevertheless, one thing appears to be clear from these initial studies, reducing aromatics and sooting potential of SAFs can simultaneously reduce contrails and carcinogenic non-volatile particulate matter emissions.

In response to these environmental and national security concerns, the commercial aviation community has developed standards for the evaluation (ASTM D4054) and specification (ASTM D7566) of drop-in fully fungible alternative aviation fuels. Drop-in fuels offer another advantage over alternative solutions to minimize aviation's environmental impact, reducing infrastructure costs and maintaining current safety. At present and in part, commercial SAF usage is limited by the number of approved SAF specifications, limiting feedstock and conversion options. Unfortunately, the evaluation and qualification of SAFs is arduous as safety is valued above all other value propositions. The qualification process requires 100 gallons of neat candidate fuel for consideration, followed by four tiers of testing, two research reports, Original Equipment Manufacturer (OEM) review, and formal ASTM balloting. In total, tens of thousands of gallons of the neat sample can be consumed, which can take years. After completing this process, a fuel becomes an approved ASTM D7566 annex, with current blend ratios in conventional fuels limited to 50 vol%. More recently, ASTM has developed a Fast-Track qualification process, limiting approved blending to 10 vol%, only requiring 100 gallons for full consideration, and completed in one year [15]. While the Fast-Track process will facilitate the rapid approval of future SAFs, 100 gallons of material for consideration remains a prohibitively high barrier for many burgeoning technologies.

Prescreening of SAF candidates expedites platform development and lowers the volume requirement for preliminary SAF evaluation to <1 mL [16], [17]. Already, these prescreening methods have facilitated the development of one SAF technology from an idea to requisite testing volumes in less than a year [14]. In prescreening procedures, a candidate fuel is evaluated for key operability properties, which are important for combustion [18], [19], [20], [21], material compatibility [22], [23], and other advantaged properties [24]. Prescreening novel SAFs, for example, can illustrate the strategic benefits of technologies to enable 100% SAF opportunities, which is the high interest in the aviation community [25].

All currently approved SAF routes are limited to 50 vol% in conventional fuel. For some approved pathways (e.g., ASTM D7566 A2), this blend limit is due to two issues: material compatibility and density. For example, a fuel composed of mostly iso-alkanes in the kerosene range cannot meet the density requirements (i.e., density requirement for ASTM D7566 A2 is between 730 to 772 kg/m³, and the density requirement for Jet-A is between 775 to 840 kg/m³ at 15 °C). Further, D7566 A2 blend components have been shown to provide insufficient material compatibility [22], [23]. For conventional fuels, these properties are brought into specification limits with the addition of aromatic hydrocarbons, which have higher densities and associate with nitrile rubber O-rings better than iso-alkanes. More recently, work has shown the possibility of cycloalkanes replacing aromatic compounds. Specifically, cycloalkanes' density and O-ring swelling characteristics can be similar to aromatics. The benefit is that cycloalkanes additionally confer lower sooting propensities (i.e., non-volatile particulate matter) than aromatics compounds with similar molecular weights [26], [27].

Facing the uncertainties and challenges of SAF market deployment, the current vision of SAF produced from renewable feedstocks, such as biomass, has prompted the jet fuel community to develop strategic plans that will significantly reduce cost alongside the adaptation of existing supply chains and infrastructure [28]. It is evident from the prior studies that depolymerizing lignin into low molecular weight fragments, developing catalysts with high selectivity for C–O–C bond cleavage, and obtaining high levels of deoxygenation are all essential to yield lignin jet fuel range hydrocarbons. Our recent results demonstrated the feasibility of converting biomass-derived lignin into C7–C18 jet fuel range hydrocarbons (US patent 9,518,076 B2 and US patent 11,078,432 B2) [28], [29], [30], [31], [32], [33], [34], [35], [36]. In that work, up to 1/3 to 2/3 of the lignin was depolymerized into monomers and dimers through cleavage of C–O–C bonds without disrupting the C–C linkages [29], [30], [31], [32], [33], [34], [35], [36], [37]. Lignin-substructure-based hydrocarbons (C7–C18), primarily C12–C18 cyclic structure hydrocarbons in the jet fuel range, were generated through a catalytic process, involving hydrodeoxygenation (HDO) of lignin catalyzed by the Ru-based bifunctional catalyst Ru/H + -Y and Ru-M/H + -Y (M = Fe, Ni, Cu, Zn) (Fig. 1) [35], [36], [37], [38], [39]. The Lignin Jet Fuel (LJF) is lignin-based fuel, mainly composed of cycloalkane hydrocarbons, including alkyl-substituted mono-, bi-, and tri-cyclohexyl alkanes (with a small amount of acyclic alkanes), and aromatics. Notably, the LJF shows favorable energy density, possible low emissions, and favorable blend characteristics meeting drop-in specifications [40]. Thus, the analytical data and predictions did show that the fuel produced through this process can be an excellent high energy density fuel additive or possible high energy fuel for specific applications although the LJF was not proved to provide conventional standardized aviation fuel out of a ‘one-pot’ reactor,

Overall, the properties of the LJF offer great opportunities for increasing fuel performance, higher fuel efficiency, reduced emission, and lower costs. The fact that these molecules show sealant volume swell comparable with aromatics opens the door to developing jet fuels with virtually no aromatics, very low emissions, and very high-performance characteristics. With multiple iso-alkane routes already approved, a cycloalkane-rich (>90% cycloalkanes) pathway is not yet supported. Although, one such pathway is currently undergoing the ASTM D4054 process. The LJF has high concentrations of naturally occurring cycloalkanes within the jet fuel range. Unfortunately, the properties of cycloalkane molecules like these are not as well established as other molecular classes of interest in jet fuel. Variations of the ring size, alkyl chain length, and isomerization, for example, can have dramatic effects on jet fuel properties such as freezing point, viscosity, sooting propensity, and surface tension. Moreover, processing minutiae for preferred cyclic compounds remain uncertain [40].