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Article

Sustainable Aviation Fuel from Hydrothermal Liquefaction of Wet Wastes

by
Dylan J. Cronin
1,
Senthil Subramaniam
1,
Casper Brady
1,
Alan Cooper
1,
Zhibin Yang
2,
Joshua Heyne
2,
Corinne Drennan
1,
Karthikeyan K. Ramasamy
1 and
Michael R. Thorson
1,*
1
Pacific Northwest National Laboratory (PNNL), Richland, WA 99352, USA
2
Department of Mechanical and Aerospace Engineering, School of Engineering, Kettering Laboratories, University of Dayton (UD), Dayton, OH 45469, USA
*
Author to whom correspondence should be addressed.
Energies 2022, 15(4), 1306; https://doi.org/10.3390/en15041306
Submission received: 21 January 2022 / Revised: 8 February 2022 / Accepted: 8 February 2022 / Published: 11 February 2022
(This article belongs to the Special Issue Hydrothermal Liquefaction for Aviation and Maritime Sustainable Fuels)
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Abstract

:
Hydrothermal liquefaction (HTL) uses heat and pressure to liquefy the organic matter in biomass/waste feedstocks to produce biocrude. When hydrotreated the biocrude is converted into transportation fuels including sustainable aviation fuel (SAF). Further, by liquifying the organic matter in wet wastes such as sewage sludge, manure, and food waste, HTL can prevent landfilling or other disposal methods such as anerobic digestion, or incineration. A significant roadblock to the development of a new route for SAF is the strict approval process, and the large volumes required (>400 L) for testing. Tier α and β testing can predict some of the properties required for ASTM testing with <400 mL samples. The current study is the first to investigate the potential for utilizing wet-waste HTL biocrude (WWHTLB) as an SAF feedstock. Herein, several WWHTLB samples were produced from food waste, sewage sludge, and fats, oils, and grease, and subsequently hydrotreated and distilled to produce SAF samples. The fuels (both undistilled and distilled samples) were analyzed via elemental and 2D-GC-MS. Herein, we report the Tier α and β analysis of an SAF sample derived originally from a WWHTLB. The results of this work indicate that the upgraded WWHTLB material exhibits key fuel properties, including carbon number distribution, distillation profile, surface tension, density, viscosity, heat of combustion, and flash point, which all fall within the required range for aviation fuel. WWHTLB has therefore been shown to be a promising candidate feedstock for the production of SAF.

Graphical Abstract

1. Introduction

Hydrothermal liquefaction (HTL) of municipal and agricultural wastes combined with hydrotreating can produce sustainable and cost-effective fuel blendstocks, including gasoline, diesel, marine, and, most significantly, sustainable aviation fuel (SAF) [1,2]. Commercial aviation has always been reliant on liquid hydrocarbons, and it will continue to mostly rely on them for long-haul flights that are difficult to electrify [3]. HTL is a hydrothermal process that utilizes high pressure (~50–250 bar) and moderate temperature (~250–400 °C) to emulate the natural processes involved in crude oil production, albeit in a matter of minutes rather than millennia. HTL biocrude, as practiced at Pacific Northwest National Laboratory (PNNL), is produced from the solids content of wet wastes without the need for energy-intensive drying processes (required for alternative technologies such as pyrolysis and gasification), expensive catalysts, or solvents [4,5,6,7,8].
By utilizing wet wastes such as sewage sludge, HTL ensures a stable supply of feedstock, and one which is obtained at a significant negative cost. The cost to urban municipalities for disposing of sewage sludge is USD 200–600 per dry ton (or USD 40–120 per wet ton, given an assumed solids content of 20 wt.%), so by using this material as a feedstock, the cost of fuels derived therefrom can potentially be offset by ~USD 1.5–7/gallon for disposal avoidance [9,10,11]. Still, without the avoided disposal fee, the modeled minimum fuel selling price (MFSP) of wet-waste HTL derived fuels, projected by PNNL, is just over USD 3 per gallon of gasoline-equivalent (GGE) in the 2021 state of technology (SOT) [9].
HTL produces a thermally stable, biocrude product with a high C content (~70–80 wt.%) and a heat of combustion value (~35–40 MJ.kg−1) close to that for crude oil (45 MJ.kg−1) in a process with an overall carbon yield of (~60–70 C% [12]). Like traditional crude oil, biocrude requires hydrotreating to be utilized as a drop-in fuel [9]. Doing so allows for the reduction of the heteroatom content (predominantly oxygen and nitrogen) of wet-waste HTL biocrude (WWHTLB), which typically ranges from 10 to 15 wt.% [12]. Compared to traditional crude oils, HTL biocrudes tend to have a higher metal content (Na, K, Ca, Si, and Fe) [13,14] and oxygen content in the form of fatty acids, resulting in a high total acid number (TAN) [15].
Several groups have demonstrated the efficacy of hydrotreating HTL biocrude at moderate pressure (~30–170 bar) and temperature (~350–450 °C) in a trickle bed reactor with traditional hydrotreating catalysts (sulfided CoMo and NiMo bimetal catalysts) [16,17,18,19,20,21]. Within the field of sustainable drop-in fuels development, the area that presents both the greatest challenges and possibly the biggest opportunities is that of sustainable aviation fuel (SAF). The aviation industry contributes a relatively low quantity of our total greenhouse gas emissions and crude oil consumption, at 2.5 and 3%, respectively [22]. However, the energy intensity consumed per passenger is three times that of alternative means of transport [23]. Unlike cars, busses, and trains, many medium and long-haul flights, responsible for ~73% of CO2, are estimated to need SAF for net-zero emissions by 2050. The current production rates of approved SAF processes meet only a fraction of the demand, and the only commercially available SAF product is feedstock limited to 1.1 billion gallons per year (BGPY) in 2017 for both diesel and jet fuel or ~1.5% of the total jet fuel market for the same year [24]. The development of numerous different SAF technologies is being investigated by commercial and research institutions worldwide. Identifying advanced sustainable solutions, which maximize fuel yield, optimize fuel properties, and achieve cost parity with fossil-based fuels, is the focus of developing HTL-based SAF technology [1].
Aviation safety is built upon redundancy for many critical systems. There is, however, only one energy source available for generating thrust on an aircraft. Hence, the inherent difficulty associated with the development of SAF is that these materials are held to more stringent standards and specifications than alternative applications, such as sustainable gasoline, diesel, and marine fuels. Four hydrocarbon families compose an acceptable alternative to conventional jet fuel: aromatics, cycloalkanes, iso-alkanes, and n-alkanes. Other molecular families—oxygenated molecules, heteroatom-containing molecules (N, S), unsaturated hydrocarbons (olefins), and metals—are unacceptable for various reasons, including poor thermal stability, freeze point, and specific energy properties [25,26]. Additionally, fuel properties are strongly influenced by the specific quantities of molecule classes (iso-, n-, and cyclic alkanes and aromatics), and the specific isomers within those classes [2,27,28]. The SAF qualification process (ASTM D4054 [29]) and eventual approval in an ASTM annex (ASTM D7566 [25]) is strict and rigorous [2,30]. From several to several hundred liters of a candidate SAF is needed to initiate the qualification process, and additional volumes (tens of thousands of liters) are required to complete Tier 3 and 4 tests, which are historically required for blending beyond 10% volume [29,30]. A prescreening methodology has been proposed to mitigate the volume requirements for ASTM D4054 and enable producers to hone methods by testing critical properties at the lowest possible volumes (Tier β ) and to predict other properties (Tier α ) from compositional information when requisite volumes are unavailable [2].
The current study investigates the potential for upgraded WWHTLB to be utilized as an SAF. Most prior and recent biocrude upgrading work has focused primarily on producing sustainable diesel and gasoline [31,32,33,34]. In these instances, the fuel specification is far more lenient with respect to heteroatom content (e.g., N, S, O), compared to that of aviation fuel [2,30,35,36]. This work is focused on both expanding the potential of WWHTLBs and furthering our understanding of nitrogen removal from biocrude products via HDN, a field where limited information is currently available.
In this work, a selection of five different WWHTLB samples (derived from wet-waste materials comprised of food waste and sewage sludge) were utilized as feeds for hydrotreatment upgrading with either a CoMo or a NiMo catalyst. The upgraded products obtained were then characterized via elemental analysis and 2D-GC-MS. A jet-fuel cut distillate of one of the upgraded products was then subjected to Tier α and β testing [2]. The results obtained over the range of experiments were then used to evaluate the efficacy of the upgrading process and the potential for application of upgraded WWHTLB as SAF.

2. Materials and Methods

2.1. WWHTLB Preparation

The 5 biocrude samples utilized in this study were obtained via the HTL of food waste, sewage sludge, and a blend of food waste and fats, oils, and grease (FOG). The origin and description of these HTL feedstocks is detailed in Table 1 and Table 2. The HTL process was performed at PNNL, using bench-scale, continuous-flow process equipment described previously [37,38]. The biocrude samples were recovered as a separate phase from the aqueous product by direct gravity separation (no solvents) and subsequently upgraded, as described below.

2.2. Hydrotreating and Distillation

The WWHTLB samples were upgraded via hydrotreatment in a continuous trickle-bed reactor, described [39] and utilized [20,21,40,41,42] in previous studies. On the basis of previous HTL biocrude upgrading work conducted at PNNL, CoMo and NiMo catalysts were applied in the hydrotreatment process [20,21,40,41,43]. The upgraded WWHTLB material was distilled into 3 fractions (gasoline: 0–150 °C, jet: 150–250 °C, and heavier than jet: >250 °C) via vacuum distillation.

2.3. Elemental Analysis

Elemental analysis was conducted using an Elementar Vario Macro Cube. Combustion and reduction tubes were packed accordingly to analyze carbon, nitrogen, sulfur, and hydrogen. The combustion tube was heated to 1150 °C and the reduction tube to 850 °C. Helium was used as the carrier gas. Typical sample sizes range from 10–30 μL. Oxygen analysis was conducted independently using an Elementar Rapid Oxy-Cube. Combustion tube was packed according to manufacturer specifications. The combustion tube was heated to 1450 °C. Helium was used as the carrier gas. Sample size should not exceed 20 μL.

2.4. Heat of Combustion (HOC) Determination

The heat of combustion (HOC) was estimated for each of the WWHTLB feed and upgraded samples using the obtained elemental data and the equation determined by Channiwala and Parikh for the computation of higher heating value [44].

2.5. Two-Dimensional Gas-Chromatography Mass-Spectrometry (2D-GC-MS) Analysis

The Tier α 2D-GC-MS testing performed by the University of Dayton (UD) has been described in detail in the previous publication by Yang et al. [28] Separately, 2D-GC-MS was also used in this study to determine the nitrogenate group-type distribution of WWHTLB samples, both prior to and after upgrading. Like the Tier α test, this analysis benefited from requiring minimal sample material (<1 mL). The 2D-GC-MS experiment was performed with a LECO Pegasus 4D instrument, comprising an Agilent 5975 GC-MS system in conjunction with a dual-stage modulator and a time-of-flight (TOF) mass spectrometer. The primary column was a 60 m Rxi-1 0.25 mm inner diameter column, and the modulated secondary column was a 1.0 m Rxi-17 0.25 mm inner diameter. The system utilized a 0.50 mL.min−1 flowrate of helium. The primary GC oven profile applied a starting temperature of 60 °C, heating at 2.0 °C.min−1 to a temperature of 235 °C. The secondary (modulator) oven utilized a modulation period of 15.0 s, a hot pulse time of 1.0 s, and a cool time of 6.5 s. The total runtime of the method was 8862 s (~2.5 h). The MS acquisition rate was 100 spectra.s−1, scanning a mass to charge ratio range of 19 to 334. The ChromaTOF software package was used (version 4.72.0.0).
Compounds were identified individually to determine the distribution of elemental nitrogen as accurately as possible. The GC-GC process attempts to separate the vast quantity of unique compounds present in the fuel mixture by both volatility and polarity via the first and second columns, respectively. The range of observed nitrogenates was classified into several groups: pyrazines, pyrroles, long-chain amides, indoles, pyrimidines, pyridines, N-containing phenols benzenamines, imidazoles, and pyrrolidines. In some instances, such as in the case of long-chain amides and indoles, these nitrogenate classes were able to be eluted within their own unique region of the GC-GC chromatogram. In other cases, such as for pyrazines, pyridines, pyrimidines, and imidazoles, these compounds were too similar in size and polarity to be separated into distinct elution periods. Regardless, the nitrogenate species present were identified and the relative concentrations of each class calculated. The relative concentration of each nitrogenate peak was determined by multiplying its area by the approximate elemental nitrogen ratio of the compound’s identity ( N   # N # + C # ). The values obtained were summed for each nitrogenate class and expresses as a percentage of the total for all nitrogenates.

2.6. Simulated Distillation (SIMDIS) Analysis

Simulated distillation samples were analyzed using an Agilent 6890N gas chromatograph (GC) equipped with a flame ionization detector (FID) and an AC Analytical direct injection programmable temperature injector cooled with compressed air at 40 psi. The column used was an SGE Analytical Science BPX1 column, 6 m × 0.53 mm and 2.65 µm film thickness. Sample analysis is based on ASTM D2887. The GC is calibrated using a standard mixture of n-alkanes (n-paraffins) from C5 to C44. The retention times of the n-paraffins are calibrated with respect to their reported boiling point in °C. The simulated distillation software used was by AC Analytical Controls Inc., Revision 6.6. The simulation distillation software integrates the chromatograms by area slices rather than peak integration. The area slices are quantified for mass % with respect to the total area summation of the chromatogram.

3. Results and Discussion

3.1. Analysis of WWHTLB

Five distinct wet-waste HTL biocrude samples were prepared, and their elemental compositions were determined to illuminate the impact of wet-waste feedstock type on the properties of the upgraded products (see Table 3).
The variation observed in the elemental composition of each of the biocrudes analyzed is relatively low and far less significant than the variation between traditional petroleum crude (PC) composition. Most significant is the variation in oxygen and nitrogen content between biocrudes and PC. In the case of oxygen content, the biocrudes range from 8.2–13.3 wt.%, compared to only 0.05–1.5 wt.% in the case of PC [45]. Similarly, the nitrogen content of the biocrudes exhibited a range of 3.6–5.6, far in excess of PC at 0.1–2 wt.%.
The reason for the significantly higher heteroatom content of HTL biocrudes compared to traditional crude oil is that HTL biocrude is produced from a broad spectrum of carbohydrates, lipids, and proteins [48], all compounds high in both nitrogen and oxygen content. A recent study by Jarvis et al. [46] compared the chemical composition of PC to that of several biocrudes, including a sample produced via wet-waste HTL of sewage sludge. The study demonstrated that ~50% of the chemical species present in the petroleum material contained either no heteroatoms or a single nitrogen atom per molecule. Comparatively, for the three biocrudes analyzed, these two classes of compounds contributed 0–2% cumulatively. HTL biocrudes were observed to contain compounds that included >15 heteroatoms per molecule [46].
The high heteroatom content of HTL biocrudes also results in a significant reduction in the heat of combustion (HOC) of these mixtures. The range observed for the biocrudes utilized in this study was 35.4–37.4 MJ.kg−1, representing a reduction from that of PC of 42–47 MJ.kg−1. The function of the heterogeneous catalyst(s) in biocrude upgrading is primarily to achieve hydrodeoxygenation (HDO), hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and the removal of trace metal content. In doing so, the upgraded material’s carbon content, HOC, and aviation applicability are increased.

3.2. Production and Analysis of Upgraded WWHTLB

A range of hydrotreatment experiments were conducted, which utilized each of the five different wet-waste HTL biocrudes and either a sulfided CoMo or a NiMo catalyst. A set of 11 different upgraded samples was collected for analysis, beginning with their respective elemental compositions (see Table 4).
In general, the hydrotreatment process effectively reduced the heteroatom content and increased the HOC of WWHTLBs. The carbon content of each upgraded sample was observed to increase (>11%) from the biocrude feed, bringing each sample to 84–85 wt.% of carbon. The oxygen content of the biocrudes was consistently reduced by >90%, reducing the range to 0.09–1.4 wt.%. Similarly, the sulfur content underwent consistent reduction to leave only 0.02–0.04 wt.%; however, the observed range of reduction in nitrogen content was less consistent at 63–87%, giving a range of 0.49–1.34 wt.%. The cumulative reduction in oxygen, nitrogen, and sulfur content for the samples ranged from 85.1–96.5%, which increased the calculated HOC values to a range of 45.4–47.0 MJ.kg−1.
The results of this work were also grouped to determine the impact of the catalyst or feed type or both on individual heteroatom content, and the respective degree of reduction observed in the upgraded product (see Table 5). Based on the average values obtained, there appears to be no significant contribution imparted by the choice of catalyst. However, some evidence suggests that upgraded WWHTLB produced from sewage sludge contains a lower heteroatom content than that produced from food waste. The same trend is unobserved in the HTL feed themselves (see Table 2), suggesting that the HDO and HDN processes were more effective for WWHTLBs produced from sewage sludge than they were in the case of food waste.

3.3. Jet Fuel Cut of Upgraded WWHTLB

For SAF usage, the WWHTLB will require distillation to 150–250 °C. Jet fuel cut (JFC) samples were prepared via fractional distillation of two whole upgraded WWHTLB products. The upgraded WWHTLBs utilized to prepare the jet fuel cut samples were those prepared from sewage sludge (S1) hydrotreated with NiMO (NiMo:S1(JFC)) and food waste (FW2) treated with CoMo (CoMo:FW2(JFC)).
Regarding the JFC samples produced in the current study, the variation observed between the heteroatom content of the whole upgraded WWHTLB and that of the JFC fraction was not consistent (see Table 4). Therefore, it is unclear what impact fractionation may have on the heteroatom contents of the various fractions. However, the nature and properties of the heteroatom-containing species present in the JFC samples are likely to vary significantly from the whole sample [49].

3.4. Tier α and β Testing of Jet Fuel Cut of Upgraded Wet-Waste HTL Biocrude

The Tier α test developed by the team at the University of Dayton (UD) utilizes two-dimensional gas-chromatography mass spectrometry (2D-GC-MS) to comprehensively analyze the composition of candidate fuels in a process referred to as hydrocarbon type analysis (HTA) [28]. The reason behind this approach is that few hydrocarbon compositional restraints are placed on aviation fuels, with ASTM standards instead relying on a strict set of property limits. However, there is a strong structure–property relationship between the chemical composition of a candidate aviation fuel and its performance. The Tier α method development involved the analysis of a range of ASTM-D7566-approved synthetic fuels and 57 conventional fuels [50]. The analysis of these fuels allowed for the determination of key characteristics such as the carbon number (C#) and hydrocarbon type distribution of aviation fuels that have passed, or are likely to pass, ASTM qualification. Not only does the Tier α test provide the C# and hydrocarbon type distribution, but it allows for the prediction of the distillation curve, derived cetane number (DCN), and HOC. Additionally, simulated distillation testing was also conducted on the sample. Tier β then involves the determination of surface tension, density, viscosity (at −20 and −40 °C), flash point, and freeze point, through physical testing. The HOC and DCN can also be physically measured as part of the Tier β testing (in addition to being predicted through the Tier α test). However, the DCN requires ~150 mL of sample, and HOC determination methods are an active area of research in the aviation community [51,52]. Additionally, some evidence suggests that the HOC prediction method used here is more precise.
Where Tier α is an early-stage prediction of critical properties based on 2D-GC-MS, Tier β is the direct measurement of these properties to guide fuel processing development. These critical properties that affect ASTM D4054 Tier 3 and Tier 4 testing or so-called combustor figure of merit operability limit were the main focus of the National Jet Fuel Combustion Program (NJFCP). The overarching results of the NJFCP work imply that nearly all observed combustor operability variance is captured by a few critical bulk physical and chemical properties (i.e., viscosity, density, surface tension, DCN, etc.). In other words, deleterious operability behavior can be captured by bounding a jet fuel’s properties within the typical experience range of conventional fuels [53].
The JFC of upgraded WWHTLB produced in this study via the hydrotreatment of sewage sludge HTL biocrude with NiMo (sample NiMo:S1(JFC)) was characterized according to the Tier α and β testing at the University of Dayton (UD). These tests were very promising for the potential application of WWHTLB as an SAF and will now be explored in further detail.
Figure 1a compares the hydrocarbon type and C# distribution of the candidate fuel produced in this study to the C# distribution of an average conventional jet fuel (Jet A, POSF 10325). The green region depicts the C# distribution of the jet fuel as a reference, where the dark green line represents the average C# of 11.4. The candidate fuel is plotted over the same C# distribution; however, the data is further split into hydrocarbon types. The average C# for the candidate fuel is 11.2, and the hydrocarbon types present and quantified are aromatics, n-alkanes, iso-alkanes, monocycloalkanes, and dicycloalkanes.
Figure 1c depicts the distillation curve of the candidate fuel, with comparison to results range obtained through the collective analysis of three ASTM-approved jet fuels (POSF 10325, 10264, and 10289). It is observed that each of the data points obtained for the candidate fuel lie within the specified jet fuel limits, and all but the 100% distillation temperature lie within the conventional jet fuel experience range, which could be addressed with improved distillation.
Figure 1b presents the range of physical properties determined for the candidate fuel, through both Tier α prediction and Tier β measurement. It is observed that the measured surface tension (σ), density (ρ), viscosities (v), and flash point all fall within the ideal conventional jet fuel range. Similarly, the predicted HOC and DCN also fall within their respective ideal comparison ranges. The only property observed to be slightly outside that of the typical range (−50 to −52 °C) is freeze point, with a value of −49 °C. Nonetheless, the sample is within the specification of Jet A-1 with a freeze point <−47   ° C. A tighter distillation (FBP < 260   ° C), although not required, would likely lower the freeze point.
In summary, the Tier α and β results for the analyzed SAF sample do not identify any major fuel quality issues for the bulk properties. Not only is the average C# of the candidate fuel (11.2) nearly equivalent to a typical Jet A (11.4), but the mixture also exhibits a high cycloalkane and aromatic content, known to be essential for energy density and seal swelling properties, respectively. In almost all cases, the upgraded WWHTLB candidate fuel lies within the ideal range of the property under consideration.

3.5. 2D-GC-MS Analysis of the HDN Efficacy of WWHTLB Upgrading

At this point in the development of WWHTLB as a potential feedstock for SAF production, the most significant obstacle may be deep HDN. As previously discussed, the nitrogen content of WWHTLB is significantly higher than that of PC (see Table 3). The hydrotreatment upgrading of WWHTLB with CoMo and NiMo investigated in this study was effective in increasing carbon content and HOC of the crude material. However, the HDN may still be insufficient considering the trace quantities present in currently approved aviation fuels. Jet A fuel is reported to contain <10 ppm nitrogen [55], and both the ASTM D7566 specification and the Department of Defense MILDTL-83133H specification for SAF specify that nitrogen be present at ≤2 ppm and sulfur at ≤15 ppm [56]. The sulfur contents of the upgraded products reported in this study ranged from 100–400 ppm, which evidently requires further reduction to meet SAF standards. However, the more pressing concern is the nitrogen content, which was determined to be in the range of 4900–13,400 ppm (see Table 4).
To expand the current understanding of N-containing species present in WWHTLB and those which are most recalcitrant towards HDN, 2D-GC-MS was used to characterize the range of crude feed samples further, as well as several upgraded samples prepared in this study. Towards this aim, a 2D-GC-MS method was established to achieve adequate separation of the hundreds to thousands of different chemical species present in the WWHTLB and upgraded samples, with a focus on nitrogenates (see Figure 2). The results of this work determined that the most prevalent nitrogenates in the four WWHTLBs were consistently pyrazines, pyrroles, and amides. Also present were varying quantities of indoles, pyrimidines, pyridines, N-containing phenols, benzenamines, imidazoles, and pyrrolidines. Each of these classes generally accounted for less than 5% of the nitrogen present in the biocrude (see Figure 3).
Upon analyzing the upgraded samples in the same manner as the feeds, several trends were evident (see Figure 4). Firstly, pyrazines, the most prevalent type of nitrogenate in each of the feeds, were completely removed in the majority of instances (and contributed <2% of the total nitrogen in the three cases in which they were observed). Similarly, amides were only in 2 of the 10 samples analyzed. Pyrroles were the only nitrogenate class that was present at high concentration in all WWHTLB and upgraded products, contributing more than half of the total nitrogen present in four of the samples. Additional classes of nitrogenates which exhibited recalcitrance towards HDN were indoles, pyrimidines, pyridines, and imidazoles.
Pyrrolidines were often observed to be present at higher concentrations than was expected, given their low concentration (~0–5 N%) in each of the WWHTLB feeds. Considering the higher prevalence of pyrroles in the feeds and the fact that pyrrolidines are the hydrogen-saturated form of pyrroles, it is very likely that pyrrolidines are being formed in the hydrotreatment process. The presence of these species is an example of successful hydrogenation but incomplete HDN.
As for why particular classes of nitrogenates were observed to be more recalcitrant towards HDN than others, the trends observed can be partially inferred from structural stability; however, further investigation is required to understand these circumstances completely. It is expected for example that aliphatic amides and six-membered aromatic rings will be more easily cleaved, and ultimately lost in the form of ammonia, than five-membered rings (due to their increased structural rigidity). For the most part this trend is observed in the current study; however, it appears that the NiMo catalyst is less effective in the removal of pyridines and pyrimidines (both six-membered aromatics) than CoMo. This result is evidence that the reaction mechanisms associated with each catalyst differ. A previous study on the HDN of biocrude generated from model carbohydrate/protein mixtures also reported remnant pyridines, pyrroles, pyrrolidines, and indoles. The authors suggest that their presence may be partially explained by these species playing a greater role in catalyst deactivation [57].
Wet-waste feedstocks did not impact the nitrogenate class distribution in products from subsequent hydrotreatment with sulfided NiMo or CoMo catalysts. Limited evidence was observed to suggest a significant variation in nitrogenate distribution of the upgraded product as a result of catalyst choice. Regarding the impact of fractional distillation of the upgraded product on nitrogenate distribution, slight variation was observed between the distribution of the whole mixture and that of the JFC.

3.6. Carbon Balance, Hydrocracking, and Its Impact on SAF Potential from WWHTLB

SIMDIS analysis was conducted on the upgraded WWHTLB sample tested as an SAF candidate in this study (CoMo:S1). The results of this testing indicate that approximately 15% of the upgraded biocrude lies within the desired JFC range of 150–250 °C (see Figure 5). However, this fraction generally accounts for only 15–25% of upgraded WWHTLB [20,21]. Ideally, the majority of the sample would fall in this range to increase the SAF cut, and hence the attractiveness of HTL for SAF. To increase the quantity of material which may contribute to SAF, the heavier than jet cut (HTJC) (55–75%, >250 °C) can be cracked to make additional SAF. Based on similar research conducted on the hydrocracking of green diesel, this process could potentially convert 70% of the HTJC to JFC, thereby increasing the carbon yield of the JFC to approximately 65% (on carbon basis of the WWHTLB, see Figure 6) [58,59,60]. As discussed previously, the nitrogen content of the upgraded WWHTLB remains an issue, and it is therefore likely that a further upgrading HDN process will also be required. Exactly what this process will involve remains to be determined.

4. Conclusions

The current study is the first publication to investigate the potential for upgraded biocrude, produced via the HTL of wet-waste feedstocks, to be used as sustainable aviation fuel. Five different wet-waste hydrothermal liquefaction biocrude samples were hydrotreated with either a sulfided NiMo or a CoMo catalyst for this study. In each case, hydrotreating was effective in reducing the heteroatom content and increasing the heat of combustion of the biocrude material. Subsequent fractionation and testing of a biocrude sample produced from sewage sludge and upgraded via a commercial catalyst indicated that all key Tier α and β properties were in the typical range for conventional aviation fuels. These properties included C# distribution, distillation profile, surface tension, density, viscosity, heat of combustion, flash point, derived cetane number, and freeze point. The most significant obstacle preventing wet-waste hydrothermal liquefaction biocrude as a sustainable aviation fuel feedstock is the much higher nitrogen content than traditional petroleum crude. Even after hydrotreating the sample, the nitrogen content of the product mixtures obtained was found to range from 9600–13,400 ppm (compared to <10 ppm for Jet A Fuel). The distribution of nitrogenates present in both the upgraded and non-upgraded WWHTLB was studied via GC-GC-MS. Pyrazines and amides were generally found to account for the majority of the nitrogen in the biocrude, and were also the nitrogenates most efficiently removed via upgrading. The nitrogenates found to be most recalcitrant towards hydrodenitrogenation were pyrroles, pyrrolidines, indoles, pyrimidines, pyridines, and imidazoles.
Further hydrodenitrogenation of the upgraded biocrude will be required. Additionally, hydrocracking of the heavier than jet fraction (which accounts for up to 75% of the feed) could improve the aviation fuel yield. However, further developments in these two areas should allow for sustainable aviation fuel production from wet-waste hydrothermal liquefaction biocrude.

Author Contributions

Conceptualization, M.R.T. and K.K.R.; methodology, M.R.T. and J.H.; software, C.B., D.J.C., Z.Y. and J.H.; validation, Z.Y. and J.H.; formal analysis, D.J.C. and J.H.; investigation, C.B., D.J.C., S.S., A.C., Z.Y. and J.H.; resources, M.R.T. and J.H.; data curation, D.J.C., C.B., Z.Y., J.H. and M.R.T.; writing—original draft preparation, D.J.C.; writing—review and editing, D.J.C., Z.Y., C.D., J.H., K.K.R. and M.R.T.; supervision, M.R.T. and K.K.R.; project administration, M.R.T.; funding acquisition, C.D., K.K.R., J.H. and M.R.T. All authors have read and agreed to the published version of the manuscript.

Funding

Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC05–76RL01830. Support is acknowledged from the U.S. Department of Energy’s (DOE’s) Bioenergy Technologies Office (BETO) of Energy Efficiency and Renewable Energy. Support for a portion of this work was through DOE BETO subcontract PO 2196073.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This work was financially supported by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies Office (BETO), and was performed at the Pacific Northwest National Laboratory (PNNL) under Contract no. DE-AC05–76RL01830. Support for the work conducted at the University of Dayton (UD) was through DOE BETO subcontract PO-2196073. The authors thank the Great Lakes Water Authority (GLWA) for collaborating and providing wet-waste feedstocks, including sewage sludge. The authors would also like to thank the following institutions for their support and the contribution of feedstock materials to this study: Contra Costa Central Sanitary District (CCCSD), Waste Management and Engineered BioSlurry, and the Ghost Warrior and Courage Inn Restaurants of the Lewis–McCord United States Air Force Base. The views expressed in this article do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, expressed, or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

2D-GC-MS: Two-dimensional gas-chromatography mass-spectrometry; C#, carbon number; DCN, derived cetane number; FOG, fat, oils, and grease; GGE, gasoline gallon equivalent; HDN, hydrodenitrogenation; HDO, hydrodeoxygenation; HDS, hydrodesulfurization; HOC, heat of combustion; HTA, hydrogen type analysis; HTL, hydrothermal liquefaction; JFC, jet fuel cut; MFSP, minimum fuel selling price; PC, petroleum crude; SAF, sustainable aviation fuel; TAN, total acid number; WWHTLB, wet-waste hydrothermal liquefaction biocrude.

References

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Figure 1. Results of SAF candidate testing (NiMo:S1(JFC)): (a) hydrocarbon type analysis result, with comparison to an average conventional jet fuel (green region); (b) critical jet fuel properties as predicted (Tier α) and measured (Tier β), with comparison to the specifications for Jet A–1; (c) distillation curve, with comparison to the collective analysis of 3 aviation fuels (POSF 10325, 10264, and 10289) [54].
Figure 1. Results of SAF candidate testing (NiMo:S1(JFC)): (a) hydrocarbon type analysis result, with comparison to an average conventional jet fuel (green region); (b) critical jet fuel properties as predicted (Tier α) and measured (Tier β), with comparison to the specifications for Jet A–1; (c) distillation curve, with comparison to the collective analysis of 3 aviation fuels (POSF 10325, 10264, and 10289) [54].
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Figure 2. A 2D-GC-MS chromatogram of a WWHTLB sample dissected into several key regions relating to: (a) hydrocarbons; (b) aromatics, pyrroles, and phenols; (c) benzenamines, N-containing phenols, and other oxygenates; (d) long-chain alcohols and carboxylic acids; (e) pyrazines, pyridines, pyrimidines, imidazoles, and cyclic ketones; (f) pyrrolidines; (g) indoles; and (h) long-chain amides.
Figure 2. A 2D-GC-MS chromatogram of a WWHTLB sample dissected into several key regions relating to: (a) hydrocarbons; (b) aromatics, pyrroles, and phenols; (c) benzenamines, N-containing phenols, and other oxygenates; (d) long-chain alcohols and carboxylic acids; (e) pyrazines, pyridines, pyrimidines, imidazoles, and cyclic ketones; (f) pyrrolidines; (g) indoles; and (h) long-chain amides.
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Figure 3. The relative concentrations of the major classes of nitrogenates present in each of the 4 WWHTLBs produced in this study, as determined by 2D-GC-MS.
Figure 3. The relative concentrations of the major classes of nitrogenates present in each of the 4 WWHTLBs produced in this study, as determined by 2D-GC-MS.
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Figure 4. Relative concentrations of classes of nitrogenates present in a selection of upgraded WWHTLBs produced via hydrotreatment with (a) CoMo or (b) NiMo catalyst, as determined by 2D-GC-MS.
Figure 4. Relative concentrations of classes of nitrogenates present in a selection of upgraded WWHTLBs produced via hydrotreatment with (a) CoMo or (b) NiMo catalyst, as determined by 2D-GC-MS.
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Figure 5. Simulated distillation data for a whole upgraded WWHTLB sample (CoMo:S1), as well as those of the gasoline, JFC, and HTJC fractions obtained via fractional distillation, with comparison to the collective analysis of 3 aviation fuels (POSF 10325, 10264, and 10289) [54].
Figure 5. Simulated distillation data for a whole upgraded WWHTLB sample (CoMo:S1), as well as those of the gasoline, JFC, and HTJC fractions obtained via fractional distillation, with comparison to the collective analysis of 3 aviation fuels (POSF 10325, 10264, and 10289) [54].
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Figure 6. Flow diagram of the potential carbon yield of the jet fraction product from WWHTLB, if the HTJC undergoes hydrocracking/isomerization and this product is combined with the JFC prior to HDN upgrading.
Figure 6. Flow diagram of the potential carbon yield of the jet fraction product from WWHTLB, if the HTJC undergoes hydrocracking/isomerization and this product is combined with the JFC prior to HDN upgrading.
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Table
Table 1. Feed type and source utilized in each wet-waste continuous flow HTL run.
Table 1. Feed type and source utilized in each wet-waste continuous flow HTL run.
RunFeed TypeFeed Origin
FW1Food wasteFood waste from Ghost Warrior and Courage Inn Restaurants from the United States Air Force Joint Base Lewis–McChord airbase (Washington)
FW2Food wasteFood waste from Waste Management, Engineered BioSlurry (Boston)
S1Sewage sludgeA 2:1 mixture of primary and secondary sludge from the Great Lakes Water Authority (Michigan)
S2Sewage sludgeAn equal-parts mixture of primary and secondary sludge from the Great Lakes Water Authority (Michigan)
SFOGSewage sludge and FOG blend80% sludge and 20% FOG (fats, oils, and grease) blend from Contra Costa Central Sanitary District (California)
Table
Table 2. Composition of the HTL feed materials processed in the preparation of the biocrude samples utilized in this study.
Table 2. Composition of the HTL feed materials processed in the preparation of the biocrude samples utilized in this study.
Biocrude SampleContent of HTL Feed Material Used to Produce Biocrude (wt.%)
SolidsAshCHNOSHeteroatomsCarbohydratesFatFatty AcidsProtein
FW125.71.113.210.20.8574.90.0675.813.65.14.15.3
FW218.71.69.510.30.5978.30.0578.97.75.24.04.3
S1 and S220.95.68.810.10.8879.90.2180.92.85.42.57.1
SFOG16.82.98.310.40.5378.10.0878.77.72.24.33.5
Table
Table 3. Source type and CHONS content of the various wet-waste HTL biocrudes utilized as hydrotreatment feeds in this study, as well as the approximate range for traditional petroleum crude [45,46,47].
Table 3. Source type and CHONS content of the various wet-waste HTL biocrudes utilized as hydrotreatment feeds in this study, as well as the approximate range for traditional petroleum crude [45,46,47].
Biocrude SampleC Content (wt.%)H Content (wt.%)O Content (wt.%)N Content (wt.%)S Content (wt.%)Approximate Heteroatom Content (wt.%)HOC (MJ.kg−1)
PC83–8710–140.05–1.50.1–2.00.05–6.0<742–47 [47]
FW173.4110.249.155.570.3815.1036.7
FW275.3010.0810.683.620.1114.4137.0
S175.349.9210.524.380.1115.0136.8
S275.799.988.194.690.6413.5237.4
SFOG70.3610.3913.283.790.6317.7035.4
Table
Table 4. Elemental composition of the upgraded wet-waste HTL biocrudes obtained, and the changes observed relative to the starting material.
Table 4. Elemental composition of the upgraded wet-waste HTL biocrudes obtained, and the changes observed relative to the starting material.
Sample CodeCHONSHeteroatomsHOC
Content (wt.%)Change (%)Content (wt.%)Change (%)Content (wt.%)Change (%)Content (wt.%)Change (%)Content (wt.%)Change (%)Content (wt.%)Change (%)Value (MJ.kg−1)Change (%)
Co-Mo:FW2 (a)84.1011.714.0739.50.79−92.61.34−63.00.02−84.32.15−85.145.823.8
Co-Mo:FW2 (JFC)84.9512.814.5043.90.22−98.20.81−77.90.02−81.51.01−93.046.726.2
NiMo:FW1 (a)84.2514.813.7033.70.51−89.20.96−82.80.03−91.51.98−86.945.423.8
NiMo:FW1 (b)84.6315.313.8134.80.65−92.91.07−80.80.02−96.01.73−88.545.724.6
NiMo:S1 (a)84.6612.414.3144.30.29−97.30.64−85.40.04−69.00.96−93.646.425.9
NiMo:S1 (b)85.1013.014.3444.71.40−86.70.71−83.80.03−77.92.13−85.846.526.1
NiMo:S1 (c)85.4213.414.1342.50.24−97.70.88−80.00.02−82.31.35−92.546.526.0
Ni-Mo:S1 (d)85.0915.914.2539.10.26−97.20.75−86.50.03−92.61.03−93.246.526.6
NiMo:S1 (JFC)84.6312.314.0141.30.32−97.01.10−74.90.03−72.61.45−90.346.024.9
Ni-Mo:S284.2911.214.1641.90.46−94.40.96−79.50.03−94.71.45−89.346.123.3
Ni-Mo:SFOG85.3321.314.5940.40.09−99.30.49−87.10.04−93.70.96−96.547.032.5
The sample code suffix (a)/(b)/(c)/(d) indicates a replicate sample, taken from the same hydrotreatment run under identical conditions.
Table
Table 5. Average elemental composition of the upgraded wet-waste HTL biocrudes obtained, and the changes observed relative to the starting material (grouped on variable of catalyst or feed type).
Table 5. Average elemental composition of the upgraded wet-waste HTL biocrudes obtained, and the changes observed relative to the starting material (grouped on variable of catalyst or feed type).
Sample CodeCHONSHeteroatomsHOC
Content (wt.%)Change (%)Content (wt.%)Change (%)Content (wt.%)Change (%)Content (wt.%)Change (%)Content (wt.%)Change (%)Content (wt.%)Change (%)Value (MJ.kg−1)Change (%)
CoMo catalyst84.53 ± 0.4312.3 ± 0.614.29 ± 0.2241.7 ± 2.20.51 ± 0.29−95.4 ± 2.81.08 ± 0.27−70.5 ± 7.40.02 ± 0.00−82.9 ± 1.41.58 ± 0.57−89.1 ± 4.046.3 ± 0.525.0 ± 1.2
NiMo catalyst84.82 ± 0.4014.4 ± 2.814.14 ± 0.2640.3 ± 3.60.47 ± 0.36−94.6 ± 4.00.84 ± 0.19−82.3 ± 3.70.03 ± 0.01−85.6 ± 9.71.45 ± 0.41−90.7 ± 3.346.2 ± 0.526.0 ± 2.5
Food waste feed84.48 ± 0.3313.7 ± 1.514.02 ± 0.3138.0 ± 4.10.54 ± 0.21−93.2 ± 3.21.05 ± 0.19−76.1 ± 7.80.02 ± 0.00−88.3 ± 5.71.72 ± 0.43−88.4 ± 2.945.9 ± 0.524.6 ± 1.0
Sewage sludge feed84.87 ± 0.3713.0 ± 1.514.20 ± 0.1142.3 ± 1.90.50 ± 0.41−95.1 ± 3.90.84 ± 0.16−81.7 ± 4.00.03 ± 0.01−81.5 ± 9.51.40 ± 0.38−90.8 ± 2.746.3 ± 0.225.5 ± 1.1
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MDPI and ACS Style

Cronin, D.J.; Subramaniam, S.; Brady, C.; Cooper, A.; Yang, Z.; Heyne, J.; Drennan, C.; Ramasamy, K.K.; Thorson, M.R. Sustainable Aviation Fuel from Hydrothermal Liquefaction of Wet Wastes. Energies 2022, 15, 1306. https://doi.org/10.3390/en15041306

AMA Style

Cronin DJ, Subramaniam S, Brady C, Cooper A, Yang Z, Heyne J, Drennan C, Ramasamy KK, Thorson MR. Sustainable Aviation Fuel from Hydrothermal Liquefaction of Wet Wastes. Energies. 2022; 15(4):1306. https://doi.org/10.3390/en15041306

Chicago/Turabian Style

Cronin, Dylan J., Senthil Subramaniam, Casper Brady, Alan Cooper, Zhibin Yang, Joshua Heyne, Corinne Drennan, Karthikeyan K. Ramasamy, and Michael R. Thorson. 2022. "Sustainable Aviation Fuel from Hydrothermal Liquefaction of Wet Wastes" Energies 15, no. 4: 1306. https://doi.org/10.3390/en15041306

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