Synergistic and Antagonistic Interactions during ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Synergistic and Antagonistic Interactions during Hydrothermal Liquefaction of Soybean Oil, Soy Protein, Cellulose, Xylose, and Lignin Jianwen Lu,†,‡ Zhidan Liu,† Yuanhui Zhang,†,§ and Phillip E. Savage*,‡

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†

Laboratory of Environment-Enhancing Energy (E2E), and Key Laboratory of Agricultural Engineering in Structure and Environment, Ministry of Agriculture, College of Water Resources and Civil Engineering, China Agricultural University, Qinghua Donglu 17, Beijing 100083, China ‡ Department of Chemical Engineering, The Pennsylvania State University, 119 Greenberg Complex, University Park, Pennsylvania 16802, United States § Department of Agricultural and Biological Engineering, University of Illinois at UrbanaChampaign, 1304 West Pennsylvania Avenue, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: We conducted hydrothermal liquefaction (HTL) of soybean oil, soy protein, microcrystalline cellulose, xylose, and lignin as individual compounds and binary, ternary, quaternary, and quinary mixtures at 350 °C for 30 min. The 34.5 wt % biocrude yield from HTL of the quinary mixture, which mimics the biochemical composition of swine manure, is much higher than the 21.5 wt % yield calculated from the weighted average yields from HTL of the individual components. HTL of binary mixtures of protein and cellulose, protein and xylose, cellulose and lignin, and xylose and lignin revealed synergistic effects on biocrude yield. On the other hand, HTL of soybean oil and lignin together exhibited an antagonistic effect on biocrude yield. These results from individual compounds and binary mixtures lead to a new model that can predict the yield, higher heating value, and C, H, and N content of biocrude from HTL of ternary, quaternary, and quinary mixtures of the biomolecules used in this study as well as in biocrude from HTL of different manures, algae, and lignocellulosic materials. The synergies identified in this work provide insights into strategies that could be employed in feedstock blending to improve biocrude yields and feedstock energy recovery from HTL of biomass resources. KEYWORDS: Hydrothermal liquefaction, Biochemical composition, Biocrude, Model, Interactions

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INTRODUCTION Hydrothermal liquefaction (HTL) produces renewable, crude bio-oil from biomass. The process takes place via thermal and hydrolytic cleavage of bonds in biomolecules in the aqueous phase around 200−380 °C and 5−28 MPa.1 A wide variety of biomass, including algae, straws, manures, sewage sludge, food waste, and wood, can be used as the HTL feedstock. HTL provides direct liquefaction of wet biomass without drying, which can reduce time, cost, and energy inputs relative to conversion processes that require dry biomass. Additionally, HTL converts the whole biomass and thus does not require extraction or pretreatment to isolate, for example, triglycerides to make biodiesel or polysaccharides to make ethanol. Moreover, the temperatures used during HTL provide sterile products that are free of biologically active materials such as bacteria, viruses, or prion proteins.2 The biocrude yield from HTL is often closely related to the biochemical composition of the feedstock, with lipid giving © XXXX American Chemical Society

higher yields than protein, which gives higher yields than carbohydrate. There have been several models that correlate and/or predict biocrude yields from HTL of wet biomass and mixtures of model biomolecules using the biochemical composition (lipids, protein, polysaccharide) as input. Some models apply to one specific set of HTL conditions and are based on, for example, mass-fraction averages of experimental results from individual components at that set of HTL conditions.3−9 Other models are based on reaction networks and kinetics and can predict biocrude yields over a wide range of HTL conditions.10−12 A few models included interactions between lipid, protein, and C6 polysaccharide biochemical classes and interaction terms improved the predictive ability of one model but not the others.3,7,10,13 Received: July 3, 2018 Revised: August 5, 2018 Published: September 11, 2018 A

DOI: 10.1021/acssuschemeng.8b03156 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

proportions of the components match those in the quinary mixture. For example, equal amounts of cellulose and xylose were added when studying that mixture, and the ratio of protein to lignin was 6:1 when studying that mixture. The reaction conditions were chosen as 350 °C for a holding time of 30 min, which is an approximate optimal condition for biocrude yield from HTL of manures.16,17 We made proxy reactors that are similar to the HTL reactors but also contain an Omega TJ-36 series thermocouple. The proxy reactors are loaded with the same amount of raw materials and DI water, and they provide the temperature inside the reactor during HTL. The temperature profile of the proxy reactors (see Figure S1) was recorded using an Omega HH309A digital thermometer/data logger. The reactors were heated in a fluidized sand bath (IFB-51, Techne) preheated to 350 °C before starting the experiments. The reactors remained in the sand bath for 30 min once they reached 350 °C, which required about 5 min. Then they were removed from the sand bath, quenched in an ice−water bath for 30 min, placed on paper towels, and dried at room temperature. We slowly opened the reactors to release the gases. Afterward, the reactor contents were poured into a 30 mL glass syringe fitted with a steel filter holder (Sartorius Stedim). The aqueous phase passed through a paper filter and was collected. We then used DCM to recover biocrude from the reactor internals, caps, and the solid products on the filter paper, and transferred this solution into a conical tube. We put the conical tube into an ultrasonic bath (CPX1800, Fisher Scientific) for about 30 min to promote the dissolution of the biocrude in the DCM. The contents in the tube were then filtered to remove any remaining solids, and the organic solution was collected in round-bottom tubes. The filter papers with the solid-phase products were dried for ∼12 h in an oven at 105 °C. Finally, the biocrude was recovered by flowing N2 at 40 °C over the DCM solution in an evaporator (Labconco Rapidvap Vertex) for over 5 h until the tube mass changed by less than 0.5 mg. We calculated the yield of biocrude as its dry mass divided by the feedstock mass loaded into the reactor. The HTL experiments were done in triplicate for each condition, and we use the standard deviation to express the uncertainty in measured values. Analytical Methods. The biocrude was dissolved into hexane and then analyzed by gas chromatography with mass spectrometric detection. We used hexane as it would dissolve the lighter components likely to elute from the GC whereas DCM would dissolve the heavier components as well. The GC-MS (QP-2010, Shimadzu) was equipped with an Agilent DB-5MS column (30 m × 0.25 mm × 0.25 μm). A 2 μL portion of the sample was injected with a split ratio of 20:1. The injection temperature and the interface temperature were both set at 300 °C, separately. The oven temperature was programmed from an initial value of 40 °C (hold for 2 min) to 240 °C (hold for 2 min) with a heating rate of 8 °C/ min, followed by a ramp at 15 °C/min to 300 °C (hold for 3 min). The identification of biocrude constituents was facilitated by comparing the spectra of sample components with those in the mass spectrum library from NIST. We tentatively identified the 30 peaks with the largest peak areas for each biocrude, except for that from HTL of lipid, where there were only five large peaks in the total ion chromatogram. The compounds were placed into one of seven groups on the basis of their structures. When a compound could be placed in multiple groups, we assigned it according to the following priority sequence: nitrogenous compounds > phenols > acids and esters > ketones and aldehydes > alcohols > ethers. The functional groups in the whole biocrudes were identified by using attenuated total reflectance Fourier transform infrared (ATRFT-IR) spectroscopy. The measurements were carried out through a Vertex 70 spectrometer (Bruker optics) equipped with a liquidnitrogen-cooled mercury cadmium telluride detector and highintensity water-cooled Globar source. Spectra were collected at 4 cm−1 resolution at an average of 200 scans using a MVP-pro diamond ATR accessory (Harrick Sci.) set at a fixed incident angle of 45°. Each sample was referenced to a clean diamond ATR crystal, and the crystal was cleaned with 2-butanone (spectroscopy grade) between each sample.

Though including interactions in models did not always improve predictions of biocrude yields, interactions between different biochemical classes certainly occur during HTL. Teri et al. showed that HTL of a binary mixture of protein and a C6 polysaccharide at 350 °C gave a biocrude yield that exceeded the mass-fraction-averaged yield calculated from the individual compound results.3 Zhang et al. found that glucose could enhance the biocrude yield from HTL of protein, presumably via the Maillard reaction.14 Another study showed synergistic interactions for biocrude yield during HTL of the binary mixtures of glucose−glutamic acid, glucose−linoleic acid, and glutamic acid−linoleic acid. Antagonistic effects occurred with the binary mixtures of glucose−guaiacol, glutamic acid− guaiacol, and guaiacol−linoleic acid.15 Most previous studies aiming to model HTL and elucidate potential synergies chose hexose-based saccharides (e.g., glucose, cellulose, starch) as the model carbohydrate.3,7,8 Of course, wet biomass feedstocks (e.g., macroalgae, manures, lignocellulosic biomass) can also contain pentose-based saccharides (hemicellulose) and lignin, and the potential interactions of these biomolecules with others that can be present during HTL of wet biomass has received scant attention. To the best of our knowledge, an article published while this Article was under review is the only other previous work. This very recent article examined HTL of xylan, cellulose, soy protein, soybean oil, and alkaline lignin that was slowly heated (over 35 min) to 290 °C and held there for 10 min.13 In the present work, we performed HTL on a model lipid (soybean oil), protein (soy protein isolate), lignin, C6 polysaccharide (cellulose), and C5 sugar (xylose) individually and in binary, ternary, quaternary, and quinary mixtures. The compositions chosen reflect the biochemical composition of manure, an abundant wet biomass resource. We quantified the yields of biocrude and identified synergistic and antagonistic interactions between the components. Finally, we report models to predict the biocrude yield and its elemental content and higher heating value (HHV) based solely on the feedstock biochemical composition.

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MATERIALS AND METHODS

Soybean oil, (100%, purchased at a local grocery store), soy protein isolate (MP Biomedicals, LLC), cellulose (microcrystalline, Alfa Aesar), xylose (98%, Alfa Aesar), lignin (alkali; water-soluble with a pH of 10.5 at 3 wt %; CAS, 8068-05-1; Aldrich), and dichloromethane (DCM) (99.9%, EMD Millipore) were used as received. Deionized (DI) water was obtained in-house from a water purifier (Direct-Q 3 UV-R, EMD Millipore). HTL Procedure and Product Separation. The HTL experiments were performed in stainless steel batch reactors (∼4.1 mL internal volume), which were assembled from one Swagelok port connector (1/2 in.) and two Swagelok caps. We loaded 0.5 g of feedstock and 2 g of DI water into the reactor for each run. The composition of the biomolecule mixture is illustrated in Table 1. The quinary mixture represents a typical biochemical composition of swine manure.16 For binary mixtures of the compounds, the relative

Table 1. Composition of Biomolecule Mixture (wt %) soybean oil ternary mixture ternary mixture quaternary mixture quinary mixture

15.0

soy protein

cellulose

xylose

lignin

37.5

31.3 45.5 29.4 25.0

31.3 45.5 29.4 25.0

9.1 5.9 5.0

35.3 30.0

B


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ACS Sustainable Chemistry & Engineering The C, H, and N contents of the feedstock biomolecules and the biocrudes were measured using an elemental analyzer (CE Instruments EA 1110). The HHV of the sample was calculated using the Dulong−Berthelot correlation, but excluding sulfur (eq 1).18 The energy recovery was calculated as the fraction of the HHV in the feedstock that was recovered in the biocrude.19

yields from HTL of the lignocellulose model compounds are much lower. They are 4.6, 6.6, and 1.4 wt % for cellulose, xylose, and lignin, respectively. These results are consistent with previous findings.15,21,22 We used alkali lignin in this study, and lignin hydrolysis could be catalyzed by the alkaline pH,27 to produce primarily water-soluble products. This effect could contribute to the low biocrude yields observed. The biocrude yield from HTL of the quinary mixture is 34.5 wt %, which is similar to the biocrude yield from HTL of swine manure (30.8%, dry, ash-free basis),16 but it is much higher than the 21.5 wt % yield calculated from the mass-fraction weighted average of the biocrude yields from HTL of the individual components. This result shows that synergistic interactions occurred during HTL of the mixture. This demonstration of synergy in the quinary mixture motivated much of the balance of the research in the subsequent sections of this article. Figure 2a shows the relative amounts of the different classes of organic compounds in the hexane-soluble fraction of the biocrudes from HTL of the individual biomolecules and the quinary mixture. This GC-MS analysis provides information only about the biocrude molecules that are soluble in hexane and light enough to be analyzed by GC. The results are not necessarily representative of the entire biocrude. The compounds tentatively identified in the biocrude from lipid are fatty acids, such as 9-octadecenoic acid, octadecanoic acid, and n-hexadecanoic acid (Table S1). These arise from the hydrolysis of the triglycerides in soybean oil.23 The main components in the biocrude from soy protein are nitrogenous compounds (e.g., indole, pyrrolidinone, pyrrolidinedione derivatives, and amides) and phenols (Table S2). Glutamic acid from hydrolysis of protein can be further hydrolyzed to produce pidolic acid, and 2-pyrrolidinone could then be formed by decarboxylation of pidolic acid.23 Phenols in protein biocrude could be from condensation and cyclization of side chains of amino acids.24 Biocrudes from HTL of cellulose and xylose have compositions similar to one another, and both have phenols, ketones and aldehydes, hydrocarbons, alcohols, and acids and esters (Tables S3 and S4). Some glucose from

HHV (MJ/kg) = 0.3414C (wt %) + 1.4445H (wt %) −

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N (wt %) + O (wt %) − 1 8

(1)

RESULTS AND DISCUSSION HTL of the Quinary Mixture and Individual Biomolecules. Figure 1 shows that the biocrude yields from HTL

Figure 1. Biocrude yield (wt %) from HTL of individual components and a quinary mixture at 350 °C for 30 min.

of lipid and protein are 82.0 and 21.1 wt %, respectively. These values are similar to the biocrude yields reported from HTL of castor oil (∼85%), sunflower oil (∼85%), casein (∼20%), and soy protein (∼25%) at similar conditions.3,5,20 The biocrude

Figure 2. Characterization of the biocrude from HTL of individual components and a quinary mixture at 350 °C for 30 min. (a) GC-MS analysis. (b) FT-IR spectra. C


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for HTL of a binary mixture of guaiacol and linoleic acid, simple molecules often used as model compounds for lignin and triglycerides, respectively.15 We note, however, that this previous study may not be directly comparable to the present work as it used ethyl acetate to recover biocrude, and we used DCM. The solvent used to recover biocrude can influence biocrude yields and composition.31−34 The experimental biocrude yields from HTL of protein and cellulose, protein and xylose, cellulose and lignin, and xylose and lignin are higher than the yields calculated from the pure component results. These binary mixtures exhibited synergistic effects. The literature provides some insights into potential causes of synergy. One report shows that the biocrude yield can be enhanced through Maillard reactions during HTL of protein and C6 saccharides.14 Additionally, a reason that cellulose and lignin, and xylose and lignin, have synergistic effects on biocrude yield could be the introduction of base from the alkali lignin. As previously reported, the biocrude yield from cellulose at alkali condition was higher than that at neutral condition.21 The very recent work from Yang et al.13 also reports experimental results for HTL of binary mixtures of the same types of compounds used in the present study, albeit at a lower temperature and in different proportions than those used herein. Only two of their binary mixtures (protein and hemicellulose, lipid and hemicellulose) showed experimental biocrude yields that differed by more than 5 wt % from the weight-fraction-averaged yields of the pure components. The synergy they reported for HTL of protein and hemicellulose together is consistent with the synergy observed for the xylose−protein binary mixture in the present results. We did not observe appreciable synergy for the lipid−xylose binary mixture. This difference might be due to the different ratios used for the two binary mixtures or the different reaction temperatures and heating profiles employed. Additional work is needed to test these hypotheses and better identify the regions where synergy can occur. The compound classes in the biocrude from HTL of binary mixtures are shown in Figure 4a. For the biocrude from HTL of the mixture of lipid and protein, nitrogenous compounds were dominant. Some nitrogenous compounds not detected in the biocrude from HTL of protein alone were present. For example, 9-octadecenamide, hexadecanamide, and 9-octadecenamide N,N-dimethyl (Table S7) were probably formed via reaction of fatty acids from the triglyceride with ammonia from deamination of amino acids in the protein.23,35 The main fatty acids and ketones in the biocrude from lipid and cellulose are the same as those from pure lipid and pure cellulose, respectively (Table S8). The fatty acids in the biocrude from HTL of lipid and xylose are also consistent with those in the lipid biocrude, while the tentative identities of some ketones are unlike those in the xylose biocrude. These are ketones such as 2(3H)-furanone, dihydro-5-tetradecyl, and 2H-pyran-2-one, tetrahydro-6-tridecyl (Table S9), which seemingly arose from the reaction of fatty acids from lipid with ketones from xylose. The long chain hydrocarbons (3-heptadecene, 8-heptadecene, etc.) detected in biocrude from HTL of lipid with cellulose or xylose or lignin probably arose from decarboxylation of fatty acids, a pathway that seemed much less important for HTL of lipid alone. Indeed, the components of biocrude from HTL of vegetable oil are mainly fatty acids.3,5,8 One study found that an effective route for the deoxygenation of vegetable oil was to process it via HTL along with soybean straw. One hypothesis for this effect is that free radicals derived from the soybean

hydrolysis of cellulose might degrade to furfurals, which can be condensed to phenols or dehydrated to acids.25 The cyclic ketones were perhaps produced via dehydration, isomerization, and cyclization of the monosaccharides.24 Phenols were the dominant component of biocrude from HTL of lignin (Table S5), which is consistent with recent work where phenols have been reported as favored monomeric products from lignin HTL.26 The phenols and methoxy phenols could be formed by hydrolysis of ether linkages in lignin and these products could degrade further through hydrolysis of methoxy groups.27 Since GC-MS only analyzed the volatile compounds in the biocrude, we used FT-IR as a complementary technique that could characterize the entire biocrude. Figure 2b shows the results. The peaks at 3300−3400 cm−1 suggest the presence of hydrogen-bonded OH groups in alcohols, phenols, and organic acids, as well as hydrogen-bonded NH groups.28 The intense peaks in 2850−3000 cm−1 can be attributed to CH stretching vibrations in aliphatic structures. Together with the peaks at 1453 and 1377 cm−1, which correspond to CH3 and CH2 bending modes, these features are consistent with the presence of aliphatic moieties in the biocrude.29 The CO vibrations at 1650−1750 cm−1 indicate the presence of ketones, aldehydes, and/or carboxylic acids. The bands at 950−1300 cm−1 arise from the CO groups, consistent with the presence of alcohols, ethers, and acids. Some other absorbance peaks appearing at 650−900 cm−1 are ascribed to the CH bending vibrations from aromatics.30 Overall, the results of FT-IR are consistent with those from the GC-MS. HTL of Binary Mixtures. HTL of binary mixtures was performed to identify any synergistic or antagonistic effects on biocrude production during HTL of the biomolecules. Figure 3

Figure 3. Biocrude yields (wt %) from HTL of binary mixtures at 350 °C for 30 min. Calculated yields are the mass-fraction weighted average, based on results from HTL of the individual components.

shows the results. The yields of biocrude from HTL of binary mixtures of lipid and protein, lipid and cellulose, lipid and xylose, protein and lignin, and cellulose and xylose are similar to those calculated as the weight-fraction-averaged yields from HTL for the individual compounds. The biocrude yield from the lipid and lignin mixture is lower than the estimated yield, indicating that this mixture had an antagonistic effect on biocrude production. This negative synergy was also reported D


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Figure 4. Characterization of the biocrude from HTL of binary mixtures at 350 °C for 30 min. (a) GC-MS analysis. (b) FT-IR spectra.

straw stimulate the cleavage of CC and (CO)O bonds in vegetable oil.36 It is conceivable that something similar occurred during HTL of lipid along with cellulose, xylose, or lignin. That is, the lignocellulose components promoted the decarboxylation of fatty acids. The main fatty acids and phenols in biocrude from HTL of lipid and lignin together are identical to those in biocrude from HTL of lipid and lignin alone (Table S10), but some new alkylbenzenes appear in the biocrude from HTL of the lipid and lignin mixture. These hydrocarbons might be produced from deoxygenation reactions, as the oxygen content in the lipid−lignin biocrude is lower than that in the lipid biocrude or lignin biocrude (Table 2). Oxygen could be removed via the formation of H2O through dehydration or CO2 through decarboxylation.36−38 Additionally, loss of O in the mixture HTL is consistent with the antagonistic interactions between lipid and lignin on biocrude yield. Some tentatively identified nitrogenous compounds, like ethylpyrazine and 3-isobutyl-2,5-piperazinedione, appear in the biocrude from protein and cellulose but were not observed in the biocrude from HTL of protein (Table S11). These compounds might be formed from glucose, derived from cellulose, reacting with amino acids, derived from protein, via Maillard reactions.14,23,24 Similar compounds were also found in the biocrude from protein and xylose (Table S12). The compounds in the biocrude from HTL of protein and lignin together were analogous to those in biocrude from HTL of protein and lignin individually (Table S13). Biocrude from HTL of cellulose and xylose together had a composition similar to the biocrude from HTL of the individual compounds (Tables S14−S16). The FT-IR results for the biocrudes from these combinations were consistent with the results from GCMS analysis (Figure 4b). Table 2 provides the elemental content, HHV, and energy recovery for the various biocrudes. HTL produced biocrude with much lower heteroatom content than the feedstock for protein, cellulose, xylose, and lignin. The HHVs of the biocrudes from HTL of the lignocellulosic model compounds were lower than those for the biocrude from HTL of lipid and protein. The HHVs of lipid biocrude, protein biocrude, and

Table 2. Elemental Content (wt %), HHV, and Energy Recovery in the Biocrude from HTL of Different Feedstocks at 350 °C for 30 min C (%) lipid (soybean oil) protein (soy protein isolate) cellulose xylose lignin

77.3 48.0

H (%)

N (%)

Feedstocks 10.9 0.09 7.3 13.4

Oa (%)

HHV (MJ/kg)

11.7 31.3

40.8 21.4

42.6 6.6 0.04 50.8 17.8 40.3 7.2 0.02 52.5 17.7 47.5 5.0 0.10 47.4 17.6 Biocrude from HTL of Model Biomolecules lipid 75.6 11.7 0.22 12.5 41.2 protein 74.3 9.4 6.3 10.0 37.0 cellulose 72.8 6.8 0.23 20.2 32.3 xylose 73.2 6.5 0.12 20.2 31.9 lignin 70.1 7.5 0.29 22.1 32.1 lipid and protein 75.0 10.9 4.1 10.0 39.6 lipid and cellulose 75.4 10.3 0.24 14.1 39.0 lipid and xylose 75.4 10.5 0.16 13.9 39.2 lipid and lignin 75.9 11.6 0.18 12.3 41.3 protein and 75.8 8.4 6.8 9.0 36.1 cellulose protein and xylose 75.9 8.4 6.8 8.9 36.2 protein and lignin 74.2 8.9 6.3 10.6 36.2 cellulose and 75.1 7.2 0.25 17.5 33.9 xylose cellulose and lignin 75.8 7.2 0.17 16.8 34.3 xylose and lignin 74.5 7.1 0.12 18.3 33.4 protein, cellulose, 75.3 8.1 6.7 9.9 35.5 and xylose 74.3 6.8 0.12 18.8 33.0 cellulose, xylose, and lignin protein, cellulose, 75.4 8.0 6.0 10.6 35.4 xylose, and lignin lipid, protein, 75.4 10.1 4.1 10.4 38.7 cellulose, xylose, and lignin a

E

energy recovery (%)

82.7 36.4 8.3 11.9 2.5 57.5 46.9 56.4 55.4 46.4 47.6 41.3 11.9 37.5 28.6 42.6 24.7 43.5 59.9

Calculated by difference.


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are all more than twice the yields expected based on the massfraction weighted average of yields from HTL of the compounds individually. Interestingly, the biocrude yield from protein, cellulose, and xylose is lower than those from HTL of the binary mixtures of protein and cellulose or protein and xylose. The ternary mixture of cellulose, xylose, and lignin behaved similarly. Thus, adding xylose or cellulose to protein or to lignin (binary mixtures) had a greater impact on the biocrude yield than did adding an equal amount of the other saccharide as was done in the ternary mixtures. That is, the biocrude yield was higher with a 5:1 mass ratio of saccharide (cellulose or xylose) to lignin (binary mixtures) than with a 10:1 mass ratio (ternary mixture of cellulose, xylose, and lignin). This result suggests that the extent of synergy is related to the relative amounts of the different biomolecules present during HTL. The literature provides some support for this hypothesis. The biocrude yield from HTL of protein and glucose increased from 4.0% to 22% when their mass ratio increased from 0.5:1 to 4:1, and the biocrude yield decreased to 19% when the mass ratio increased further to 5:1.14 The GC-MS results for the biocrudes from HTL of ternary and quaternary mixtures are illustrated in Figure 6a. The biocrude from protein, cellulose, and xylose was primarily composed of nitrogenous compounds, ketones, alcohols, hydrocarbons, and phenols while the biocrude from cellulose, xylose, and lignin mainly comprised phenols and ketones. Nitrogenous compounds, phenols, ketones, hydrocarbons, and ethers were tentatively identified in biocrude from protein, cellulose, xylose, and lignin. FT-IR analysis showed that the functional groups of these three kinds of biocrudes were similar (Figure 6b). Prediction of Biocrude Yield, Elemental Content, and HHV. The literature reports several algebraic models aiming to correlate and/or predict the biocrude yield from HTL of a complex feedstock at one specific condition based on its biochemical composition.3,4,6−9,13 The results in the present study permit development of a new and more versatile model that accounts separately for C5 and C6 carbohydrates and also accounts for lignin. We also account for the synergistic or antagonistic interactions observed during HTL of the binary mixtures of the biomolecules. The model equation appears

cellulose biocrude are 40.8, 36.4, and 32.3 MJ/kg, respectively, which were close to those reported in previous studies.3,8 The energy recovery in the biocrudes from HTL of binary mixtures of protein, cellulose, xylose, or lignin were all higher than that from HTL of either of the constituent compounds. This improved energy recovery is largely due to the synergy that produced higher biocrude yields for these mixtures, though the cellulose, lignin, and xylose mixtures did also generate biocrude with greater HHV than did HTL of any component alone. When lignin was added to cellulose or xylose, it increased the carbon and hydrogen content, and hence energy recovery, of the biocrude. This outcome again shows the synergy that takes place during HTL of lignin, cellulose, and xylose. HTL of Ternary and Quaternary Mixtures. Figure 5 shows results from HTL of ternary and quaternary mixtures of

Figure 5. Biocrude yields (wt %) from HTL of ternary and quaternary mixtures at 350 °C for 30 min. Calculated yields are the mass-fraction weighted average, based on the biocrude yields from HTL of the individual components.

compounds that had exhibited synergy in the binary mixture experiments. The biocrude yields from HTL of these mixtures

Figure 6. Characterization of the biocrude from HTL of ternary and quaternary mixtures at 350 °C for 30 min. (a) GC-MS analysis. (b) FT-IR spectra. F


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ACS Sustainable Chemistry & Engineering below (eq 2). The first term accounts for contributions from lipid, protein, cellulose (C6 sugars or polysaccharides), hemicellulose (C5 sugars or polysaccharides), and lignin individually. The second term accounts for the synergy and antagonism observed from HTL of the binary mixtures. Y=

∑ aiXi + ∑ ∑ aijXiXj i

i

(2)

j

Y represents the quantity of interest (wt % yield of biocrude, wt % C, H, or N in the biocrude). Xi is the mass fraction of biochemical constituent i (e.g., lipid, protein, C5 saccharides, lignin) in the feedstock, on a dry, ash-free basis. The values of the ai coefficients come directly from the HTL experiments with the individual compounds, and the values of the aij coefficients were obtained by considering additionally the results from HTL of the binary mixtures. For binary mixtures that showed little synergy or antagonism (experimental and calculated biocrude yield, carbon, hydrogen, or nitrogen content differed by less than 3%, 1.5%, 0.5%, and 0.1%, respectively), we took the value of aij to be zero. Table 3 lists the parameter values for predicting different quantities of interest.

Figure 7. Predictions of biocrude yield, elemental content, and HHV from HTL of different biomass feedstocks. Details are in the Supporting Information (Table S20).

would fall on the diagonal line. For most cases, the new model makes good predictions for the biocrude yield, HHV, and C, H, and N content. Instances where the prediction of biocrude yield is less accurate might be due to an influence from inorganic material (ash) in these biomass feedstocks (e.g., manures). Elucidating the influence of ash on biocrude yields is a good direction for future studies. Though models to predict the elemental content in the biocrude were proposed before, they required as input information about the feedstock composition beyond protein, lipid, and carbohydrate content.4,6,9 The current model can predict the biocrude yield and its C, H, and N content based solely on the feedstock’s biochemical composition. The model had especially good performance for the prediction of biocrude yield for lignocellulosic feedstocks like cornstalk, Cunninghamia lanceolata, and Pinus massoniana Lamb. The sum of squared errors for the biocrude yield predictions in Figure 7 was 471 for the present model but 2869 if we excluded the interaction terms. It is clear that the interaction terms improved the predictions. By way of comparison, the sum of squared errors for the biocrude yield predictions was 1458 and 1652 for the models of Teri et al.3 and 2800 for the model of Biller and Ross.8

Table 3. Parameter Values for Predicting the Yield and Elemental Composition of Biocrude from HTL of Wet Biomass at 350 °C for 30 min parameter

yield (wt %)

carbon (wt %)

hydrogen (wt %)

nitrogen (wt %)

alipid aprotein acellulose ahemiellulose alignin alip−pro alip−cell alip−hemi alip−lig apro−cell apro−hemi apro−lig acell−hemi acell−lig ahemilig

82.0 21.1 4.57 6.57 1.39 0 0 0 −73.9 47.9 46.2 45.6 0 111.1 67.5

75.6 74.3 72.8 73.2 70.1 0 6.5 0 9.24 9.01 8.63 0 8.49 24.9 13.1

11.7 9.41 6.84 6.48 7.48 3.11 7.57 9.25 4.45 0 0 0 2.33 0 0

0.22 6.34 0.23 0.12 0.29 0 0 0 0 14.5 14.9 3.5 0 0 0

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CONCLUSIONS Performing HTL at 350 °C for 30 min on a quinary mixture that mimicked the biochemical composition of swine manure provided a biocrude yield similar to that observed from HTL of the actual manure feedstock. We hypothesize that working with this and other model mixtures can give new insights into the chemistry taking place during HTL of more complex wet biomass resources. HTL of the model mixture gave a biocrude yield of 34.5%, which is much higher than the mass-fraction-weighted yield (21.5%) calculated from HTL results with the individual biochemical components. This synergy in the quinary mixture was traced to synergy during HTL of protein or lignin with C5 or C6 saccharides. There was also an antagonistic interaction between lipid and lignin, which reduced biocrude yield, but it was more than offset by the multiple combinations of components that produced synergies. A new model based on the results from HTL of five individual components and their binary mixtures provided

This model is versatile in that, in addition to predicting the biocrude yield from HTL, it can also predict the C, H, and N wt % in that biocrude and hence higher HHV. Thus, the model can predict both biocrude quantity and biocrude quality for HTL of wet biomass. As this model accounts explicitly for lignin and accounts separately for C5 and C6 polysaccharides, we anticipate it will be useful for HTL of lignocellulosic biomass in addition to microalgae and food waste. We used the results from the ternary, quaternary, and quinary mixtures of model compounds reported herein, and literature results from HTL of different biomass resources to test the predictive ability of the model. Table S20 provides the details for the specific biomass feedstocks in view, which include manure, algae, cornstalk, and some other lignocellulosic materials. Figure 7 is a parity plot with the experimental and predicted results from HTL experiments around 350 °C for times around 30 min. If all of the predictions were perfect, the data points G


Research Article


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good prediction of the yield, elemental composition, and HHV of the biocrude from HTL of ternary, quaternary, and quinary mixtures, as well as biocrude from HTL of microalgae, manures, and lignocellulosic biomass. With this model, biocrude yields and compositions from HTL at conditions near 350 °C and 30 min can be predicted simply from knowledge of the lipid, protein, lignin, and C5 and C6 polysaccharide content of the biomass feedstock of interest. The synergies identified in this work provide insights into strategies that could be employed in feedstock blending to improve biocrude yields and feedstock energy recovery from HTL. For example, blending lignocellulosic biomass with protein-containing biomass such as microalgae, sewage sludge, or food waste could lead to higher biocrude yields than could be achieved by HTL of the feedstocks separately.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b03156. Tentative identities of compounds in the biocrude from HTL of the model compounds; predicted and experimental biocrude yield, elemental content, and HHV for HTL of different feedstocks; and temperature profile of the proxy reactor when the sand bath was 350 °C (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jianwen Lu: 0000-0002-6020-0798 Zhidan Liu: 0000-0002-4411-7644 Phillip E. Savage: 0000-0002-7902-3744 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2016YFD0501402), the National Natural Science Foundation of China (U1562107), and the China Scholarship Council. We thank Yang Guo, Jimeng Jiang, James D. Sheehan, Akhila Gollakota, and Azin Padash at Pennsylvania State University for their experimental assistance.

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REFERENCES

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