Acid and Alkali Catalyzed Hydrothermal Liquefaction of Dairy Manure

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Acid and alkali catalyzed hydrothermal liquefaction of dairy manure digestate and food waste Roy Posmanik, Celia M. Martinez, Borja Cantero-Tubilla, Danilo Cantero, Deborah Sills, Maria Jose Cocero, and Jefferson W. Tester ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04359 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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ACS Sustainable Chemistry & Engineering

Acid and alkali catalyzed hydrothermal liquefaction of dairy manure digestate and food waste

R. Posmanik

1,2*

, C. M. Martinez

2,3

, B. Cantero-Tubilla 1,2, D. A. Cantero

1,2

, D. L. Sills 4, M. J

Cocero 3 and J. W. Tester 1,2

1. School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, United States. 2. Cornell Energy Institute, Cornell University, Ithaca, NY 14853, United States. 3. Department of Chemical Engineering and Environmental Technology, University of Valladolid, Valladolid 47011, Spain. 4. Department of Civil and Environmental Engineering, Bucknell University, Lewisburg, PA 17837, United States.

*

Corresponding author.

Tel.: 607-254-4785 Fax: 607-255-8313 Email: [email protected]. [email protected] (R. Posmanik)

1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The objective of this study was to elucidate the effect of adding acid and alkali to hydrothermal liquefaction (HTL) of two waste biomass feedstocks—manure digestate and carbohydrate-rich food waste. HTL reactions were conducted at 300 °C for 60 min, with and without the addition of acid or base. We measured the quantity and characterized the quality of the three main HTL products: oil, aqueous, and hydro char. For both feedstocks, carbon recovery distributions had wide ranges among 1) bio-crude oil (26–61 wt%); 2) aqueous product (9–49 wt%); and 3) hydro-char (1–36 wt%). The addition of acid affected HTL reactions for manure more than for food waste. For the aqueous phase, the addition of acid decreased the recovery of C1–4 carboxylic acids and increased the production of cyclic furan compounds. GC-MS analysis of the bio-crude oil suggested that dehydration reactions were enhanced by adding acid to the HTL media. FTIR spectroscopy coupled with principal component analysis showed that hydro-char samples cluster according to acidmodified and base-modified reactions, based on distinct chemical structures. This study clarifies the role of pH during HTL and its effect on chemical pathways and carbon distribution among products.

Keywords: Waste valorization, Catalytic HTL, HTL phase distribution, Bio-crude oil, Hydro-char, HTL aqueous phase


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INTRODUCTION As global population grows and the availability of natural resources shrinks, society faces a grand challenge of sustainable food supply. Increases in productions of animal manure and food wastes under intensified agricultural systems represent a major challenge worldwide. In the U.S., for example, dairy and food industries generate approximately 19 and 36 million tons of waste per year, respectively.1,2 The majority of these residues, rich in organic carbon and valuable nutrients, are currently disposed in landfills. Recycling these waste streams, however, may lower the use of energy-intensive resources and reduce “end-of pipe” pollution such as greenhouse gases, and soil and water pollutants.3 Waste to energy strategies for manure and food waste are primarily based on biological processes, which convert organic wastes into biogas using anaerobic digestion. However, incomplete conversion of the waste by anaerobic microorganisms limits biogas yields to 40–50% and generates large volumes of a secondary liquid effluent, containing an aqueous mixture of carbohydrates, proteins, lipids, minerals and nutrients.4,5 This organic matter should be further processed to maximize its economic value and reduce environmental impacts if discharged untreated.6 Hydrothermal conversion of biomass using suband supercritical water represents a promising technology for both energy and nutrient recovery from waste streams with high water contents. Hydrothermal liquefaction (HTL) of organic wastes is based on fast hydrolysis reactions, followed by dehydration and condensation of sugars, lipids, proteins, and their degradation products, using subcritical water.7 The optimum processing temperature ranges from 250 to 350



○

C, and HTL reactions are usually completed within 20–60 min.8,9 HTL generates four products:

bio-crude oil, hydro-char, an aqueous product and a gas stream. The relative amounts of the four products depends on reaction conditions (e.g. time and temperature), as well as the presence of catalysts.10 The main product of HTL, bio-crude oil, can be used as a precursor for liquid biofuels.10 HTL can reduce the oxygen content of biomass to approximately 10% in the biocrude by dehydration and decarboxylation reactions.8 Furthermore, process conditions (reaction time, temperature, pressure) may be manipulated to adapt to feed compositions to maximize conversion yields and minimize residual oxygen in the bio-crude oil. As a consequence of the hydrolysis and condensation reactions carried out in a liquid phase, an aqueous phase is produced, containing soluble carbon that should be utilized. HTL benefits from changes in the chemical behavior of water as reaction conditions approach its critical point (374 ○C and 22.1 MPa).7 When increasing the temperature up to ~ 300 °C at liquid state, water becomes less polar, its dielectric constant drops and its ionic product (Kw) increases up to three orders of magnitude.4 The high Kw favors ionic reactions and therefore increases activities of both acid and base catalyzed reactions.7 For example, water at 300 ○C under elevated pressure (9–25 MPa) has a low dielectric constant (20.3–21.6 compared to 80 at room temperature), which leads to higher solubility of organic compounds. Moreover, at these conditions, water has the highest value of ionic product. The approximate pKw of water at these conditions is 11.1–11.3 (compared to 14 at room temperature), thus creating a medium with high concentrations of H+ and OH−, which favors acid or base catalyzed reactions. For example, under acidic conditions, glucose is converted primarily to 5-hydroxymethyl-furfural (HMF), whereas under alkaline conditions glucose degradation shifts to produce short-chain carboxylic


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acids.11 Since HMF is an important building block for the production of dimethyl-furan (DMF) as well as other biofuel precursors,12 its production in subcritical water may positively affect the HTL process and increase biomass conversion into bio-crude oil.13 Previous studies, focused on model compounds, illustrate the important role of pH in the formation of HTL bio-crude oil. However, complex biomasses such as manure and food waste likely complicate the process, due to cross-interactions among carbohydrates proteins and lipids as well as with their degradation products.8 This complexity may result in high fractions of organic carbon in the solid and aqueous products that should be further utilized.14 Consequently, the objective of the current study was to elucidate the role of acidity and alkalinity of the hydrothermal media on bio-crude oil production from manure digestate and carbohydrate-rich food waste.

In addition, to improve our understanding of oil formation and valorization

opportunities for aqueous and solid materials, we characterized the chemical compositions of the oil, aqueous, and solid HTL products.

EXPER IMENTAL SECT ION Materials Digested cattle manure and carbohydrate-rich food waste were used as feedstocks. Digested cattle manure (herein referred to as ‘manure’) was taken from an anaerobic digester located on a dairy farm (Sunnyside farm, Scipio Center, NY). Carbohydrate-rich food waste (herein referred to as ‘food waste’) was collected from Cornell University dining halls (Ithaca, NY) and characterized in the lab (fruits 15 wt%, vegetables, 47 wt%, grains and breads, 38 wt%). Manure



and food waste had average solids contents of 8 and 10 wt%, respectively. Milli-Q water was used as reaction medium for all experiments. Phosphoric acid (H3PO4) and sodium hydroxide (NaOH) were selected as acid and base catalysts, respectively. For liquid-liquid separation, dichloromethane (DCM, CH2Cl2), ethyl acetate (EA, C4H8O2) and acetone (C3H6O) were used as solvents. For high-performance liquid chromatography (HPLC) analysis, the following standards were used: glucose, fructose, arabinose, xylose, succinic acid, glycolaldehyde, lactic acid, formic acid, acetic acid and HMF. All chemicals (≥ 99% purity) were purchased from Sigma Aldrich. Experimental setup The HTL batch reactor used in this study, described in detail in a previous study,8 is shown in Fig. S1 (supporting information). Briefly, a 500 mL stainless steel vessel (Model 4575 Parr Instruments Co.) was loaded with 200 mL of feed solution. The initial solids concentrations loaded to the reactor for all experiments were 4 wt% for manure and 5 wt% for food waste. Once the biomass was loaded, the reactor was closed and the system was purged with nitrogen (N2) and pre pressurized up to a pressure of 2.5 MPa. For acidic and alkaline experiments, a high pressure pump (Varian, PrepStar, SD1 system, Agilent Technologies) was used to inject the acid or base to the reactor. 15 mL of 5M H3PO4 and 25 mL of 1M NaOH were added for acidic and alkaline runs, respectively. The contents of the reactor were stirred (100 rpm) during the reaction using a magnetic agitator. The temperature was set to 300 ○C and the reaction time for all runs was set to 60 min. The reaction time started the instant temperature reached 80 ○C and ended when the product was collected. The first step of the temperature profile for the reaction (shown in Fig. S2 (supporting information)), consisted of a heating ramp from 80 to 300 ○C, which took


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approximately 20 min. Then, the reactor temperature was maintained at 300 ○C for additional 40 min. Finally, samples were collected, within 3–5 min. All experiments were run in triplicate. The liquid product from the reactor was collected through a heat exchanger connected to the reactor’s outlet to rapidly quench the liquid effluent. To recover remaining product, the reactor was washed with 100 mL acetone, followed by 100 mL deionized water. As a result of the washing procedure, a wash sample was collected for further separation. After collecting all samples, the heater was turned off and the cooling loop was opened. After completely cooling the reactor, the remaining solids were collected and added to the wash sample. Phase separation was conducted following the procedure reported previously,8 and is outlined in Fig. S3 (supporting information). Analysis The C, H and N contents of dried materials (i.e., raw biomass, hydro-char and bio-crude oil fractions) were measured using a CE440 elemental analyzer (Exeter Analytical, North Chelmsford, MA). The ash content was determined following 4 h of combustion at 550 ○C (ASTM E1755-01). By calculating the difference between the wt% of residual (difference to 100% of the sum percentage of C, H, and N) and the wt% of ash,15 the oxygen content was estimated, assuming the sulfur concentration is negligible.16 Based on the elemental composition, the higher heating value (HHV) of each feedstock and bio-crude oil products was calculated using Dulong’s formula (Eq. 1).17

= 0.338 × C + 1.428 (H − )

(1)



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Where HHV is the higher heating value (MJ kg−1), and C, H, and O are the mass percentages of carbon, hydrogen, and oxygen in the dried sample, respectively. The carbon in the aqueous product was analyzed by total carbon analysis (TOC) using a TOCVcph analyzer (Shimadzu Corporation, Kyoto, Japan). Consequently, the carbon distribution among all three HTL phases (oil, aqueous and hydro-char) was calculated. Carbon recovery was defined as the total amount of carbon in each phase relative to the carbon in the feed and was calculated with Eq. (2).

=

×

(2)

×

where CR is carbon recovery (wt%); ‘i’ is the phase (oil, aqueous or hydro-char); ‘Mi’ is the mass (g) of product ‘i’; ‘Ci’ is the carbon concentration (wt%) in product ‘i’; ‘M feed’ is the mass (g) of feedstock; and ‘C feed’ is the carbon concentration (wt%) of the feedstock. Monosaccharides and carboxylic acids in HTL aqueous samples were measured with high performance liquid chromatography (HPLC). A Shimadzu HPLC equipped with an Aminex HPX-87H column and UV and refractive index detectors were used. The mobile phase was 5mM sulfuric acid with a flow of 0.6 mL min−1, and the column was held at 65 ºC. The volume of sample injected was 20 µL and the run time was 40 min. A change index (CI), was calculated for each detectable product to evaluate the effect of acid or base on the aqueous product composition (monosaccharides and carboxylic acids) (Eq. 3). The CI compares the compositions of the aqueous products that were generated with and without acid or base. A positive or negative CIi value indicates that the aqueous phase from the acid or base modified reaction had a higher or lower amount of product (i) compared to the aqueous phase from the non-modified reaction,


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respectively. A CIi value equal to zero indicates that the acid or base had no effect on the production of the specific compound.

=

, !" #!$ %, !"&'! #!$ , !"&'! #!$

(3)

Where ‘CI’ is change index; ‘i’ is the specific product; ‘Ci, with additive’ is concentration of ‘i’ in the HTL product following the addition of acid or base; and ‘Ci, without additive’ is concentration of the product ‘i’ following HTL reaction without additive. Chemical composition of the bio-crude oil was analyzed with gas chromatography mass spectrometry (GC-MS) consisting of an Agilent 7890A gas chromatograph, 5975C VL MSD mass selective detector, and a 19091S-433 column (30 m × 250 µm × 0.25 µm) (Agilent Inc, Palo Alto, CA) using Helium as a carrier gas. Bio-crude oil samples were dissolved in DCM, injected with a split ratio 10:1 into an injection port at 310 ○C. The temperature program consisted of 4 min at 50 °C, followed by a heating ramp to 110 ○C at 2 ○C min−1, hold at 110 ○C for 3 min, and heating up to 300 ○C at 2 ○C min−1 with a final hold at 300 ○C for additional 3 min. A mass spectral library was used to identify the separated compounds. The solid hydro-char was analyzed with Fourier transform infrared spectroscopy (FTIR) to identify specific chemical functional groups present in hydro-char obtained from each feedstock at each reaction condition. Prior to FTIR measurement dried hydro-char samples were ground with a mortar and pestle. FTIR spectra were collected in triplicate for wavenumbers between 7000 and 400 cm−1 using a Bruker Vertex 70 FTIR spectrometer (Bruker Optics, Ettlingen, Germany) in transmission mode coupled with an attenuated total reflectance (ATR) sampling accessory (MIRacle) equipped with a ZnSe crystal (PIKE Technologies, Madison, WI).



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Spectra were transformed to absorption mode, ATR corrected, cut above 4000 cm−1 and below 620 cm−1 (to avoid spectral regions with high absorption from ATR crystal), baseline corrected, and vector normalized using the OPUS software package. To compare functional groups among hydro-char generated under alternative HTL conditions, FTIR spectra were analyzed using principal component analysis (PCA). PCA was performed with the fingerprint region of the spectra (1800–620 cm−1),18 using software package Rstudio (RStudio, Inc., Boston, MA). Additional details about the FTIR–PCA analysis are described elsewhere.19

RESULTS AND DISCUSSION Conversion yields Acidic and basic conditions affected the carbon yields in the oil, aqueous, and solid fractions produced by HTL (Fig. 1). The effect of acid and base on the HTL reaction was more significant for manure (Fig. 1a) than for food waste (Fig. 1b). For manure, the bio-crude oil yield with the addition of acid was 58±2 wt% (carbon basis), higher by 59% than the yield without acid. Under alkali conditions, the bio-crude oil yield from manure was 42±4 wt% (carbon basis), higher by only 15% than the yield without base. Studies by other researchers, on the conversion of cattle manure in sub-critical water, showed similar bio-crude oil yields at 260–340 ○C.15,20

For

temperatures below 250 ○C, the main product from hydrothermal conversion of manure was reported as hydro-char.21 In the present study, the use of acid or base decreased the production hydro-char from manure at 300 ○C (Fig. 1a). Hydro-char yields from manure were 22±3 and 18±3 wt% (carbon basis) under acidic and alkali conditions, respectively, lower by 10% and 25%


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compared to the yield without acid or base added, respectively. The aqueous product yield from HTL of manure was 12±3 wt% (carbon basis) under acidic conditions, lower by 35% compared to the yield without acid added (Fig. 1a). On the other hand, the addition of base, increased the aqueous product yield to 27±8 wt%, higher by 39% compared to the yield without base. For food waste, the addition of acid to the HTL media, increased the oil yield to 43±7 wt% (carbon basis), higher only by 18% than the yield without acid added (Fig. 1b). The use of base resulted in a slightly lower oil yield of 33±4 wt% (carbon basis), compared to the use of acid. Hydro-char yield from food waste with no acid or base addition was 11±2 wt%, considerably lower than hydro-char yield from manure. Interestingly, the addition of acid to food waste increased the hydro-char yield to 15±1 wt% while the addition of base decreased the yield to 3±1 wt%. The aqueous phase was the dominant product for food waste, with yields of 28±3 wt %, 34±3 wt% and 41±4 wt% (carbon basis) for HTL reactions with acid, base, and no additive, respectively (Fig. 1b). Different distribution of carbon among the HTL products (oil, aqueous, and hydrochar) from food waste and manure can be explain by the different reaction chemistry, due to the nature of these feedstocks. Food waste had low alkalinity, thus the reaction media without acid or base addition had a lower pH compared to manure (4.0±0.1), because of sugar degradation to short carboxylic acids during HTL.8 This explains the lack of major differences in oil yields with or without the addition of acid (Fig. 1b). Manure has higher alkalinity than food waste, due to high ammonia content.22 Therefore, the reaction media without additive had a higher pH (9.1±0.3), and the addition of acid had a notable effect on bio-crude oil yield (Fig. 1a). For all treatments except alkaline food waste, 81–92% of the initial carbon was recovered in the three



products (oil, aqueous and char) (Fig. 1), suggesting that the amount of gasified material was lower than 20%. In addition, the chemical compositions and structures of these feedstocks may explain changes in chemical pathways. Manure is a lignocellulosic biomass (i.e., rich in cellulose, hemicellulose and lignin).23 The food waste that was used here is carbohydrate-rich and contains mostly fruit and vegetable waste, as well as breads and grains with only small amounts of lipids. Therefore, fruit and vegetable sourced fibers (i.e., cellulose, hemicellulose and pectin) were a dominant component of this feedstock.24 Cellulose, the major structural component of plant cell walls, is a polysaccharide consisting of a linear chain of glucose units. Hemicellulose, the second major component of lignocellulosic biomass, is a hetero-polymer formed by pentoses (xylose and arabinose) and hexoses (glucose, galactose and mannose). Lignin is a highly amorphous polymer formed by phenolic units in a complex structure. Pectin is a structural heteropolysaccharide, rich in galacturonic acid, contained in the primary cell walls of fruit and vegetables. Structural differences among these polymers may explain changes in chemical pathways as their primary hydrolysis products are different. Cellulose and hemicellulose are hydrolyzed mainly to hexoses and pentoses; lignin forms mainly phenols, alcohols, aldehydes, catechols, and organic acids; and pectin is hydrolyzed mainly to galacturonic acid.24,25 Bio-crude oil composition The primary product of HTL, bio-crude oil, is rich in carbon (70–79 wt%) and hydrogen (7– 10%) and contains minimal amounts of oxygen (12–18 wt%) and nitrogen (1–5 wt%). Negligible amounts of ash (0–0.5 wt%) and sulfur (0–04 wt%) were detected in bio-crude oil samples from both feedstocks (Table S1). Manure, a feedstock with 34 wt% carbon and 26 wt% oxygen


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(Table 1), was valorized via HTL, by producing a bio-crude oil with a carbon content higher by 100% and oxygen content lower by more than 40%, compared to the feedstock. This was also demonstrated by the increased HHV of the bio-crude oil, 140% higher than the raw feedstock. Food waste had a higher carbon content as well as higher oxygen content than manure. Interestingly, the elemental characteristics of bio-crude oil from food waste were comparable to manure (Table 1). In other words, the elemental composition of the produced bio-crude oil demonstrates that HTL valorizes food waste but to a lesser extent than for manure. Our results suggest that HTL can generate a bio-crude oil with a HHV of 32–36 MJ kg−1 from both manure and food waste, similar to HHVs from HTL of woody biomass,10 algae,26 and anaerobic sludge.16 The removal of oxygen in HTL occurs by decarboxylation, which removes oxygen in the form of carbon dioxide, and by dehydration, which removes oxygen in the form of water.7 A decrease in the atomic oxygen to carbon ratio (O/C) indicates decarboxylation, whereas a decrease in hydrogen to carbon ratio (H/C) indicates dehydration reactions. For both manure and food waste, the atomic oxygen to carbon ratio was reduced from 0.5–0.6 to 0.1–0.2 after HTL (Table 1). The hydrogen content was increased from 4 wt% in manure and 7 wt% in food waste to 7.6–9.2 wt% in the biocrude oil. This led to similar H/C ratios for manure feedstock and its oil product and slightly lower H/C ratio for food waste feedstock its oil product. These data suggest that for manure, decarboxylation was the main mechanism for oxygen removal, whereas for food waste, both decarboxylation and dehydration contributed to oxygen removal. A decrease in the O/C ratio (i.e., enhanced decarboxylation) with less reductions in the H/C (i.e., reduced dehydration) ratio represents an advantage with respect to the oil quality and its downstream upgrading to fuel.



Adding acid or base to hydrothermal media may enhance or suppress both decarboxylation and dehydration reactions.7 For manure, decarboxylation was enhanced by either acid or base addition, as both bio-crude oils had lower O/C ratios than for bio-crude oil produced by HTL without additives. Food waste behaved differently, with a lower O/C ratio for the base-modified bio-crude oil (compared to the acid-modified). In addition, for food waste dehydration reactions were probably enhanced by acid conditions, as both acid modified and non-modified HTL generated bio-crude oil with lower H/C ratios (compared to the feedstock), suggesting oxygen removal in the form of H2O. Another important characteristic of the bio-crude oil is its nitrogen content, which for both feedstocks remained around 3 wt%, suggesting that nitrogen removal will be required during oil upgrading. Still with the increase of the carbon content in the oil, the atomic N/C was reduced to below 0.04, similar to values reported by others.27

Interestingly, for both feedstocks, alkaline

conditions resulted in lower nitrogen contents in the bio-crude oils (Table 1). A possible explanation of this pattern may be a volatilization of NH3; the exhaust gas composition was beyond the scope of this study. A qualitative analysis of the bio-crude oil composition was conducted with GC-MS (Table 2 and Table S2). For each bio-crude oil sample, approximately 80% of the sum of GCMS peak areas was identified and products were categorized into several groups, such as cyclic hydrocarbons, phenol derivatives, furans, fatty acids and straight amide derivatives. For manure, the majority (~60%) of the sum of the GC-MS peak areas for bio-crude oil produced by nonmodified HTL was associated with compounds that were categorized as cyclic hydrocarbons while the rest were categorized as N heterocyclic compounds, phenols and alkenes. Relative presence of compounds in the bio-oil can be linked to dehydration and decarboxylation reactions


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in the HTL media. Fatty acids, for example may decarboxylate into hydrocarbons and carbon dioxide. However, these reaction may be suppressed or enhanced under different pH conditions. For example, the addition of acid to the HTL media of manure shifted the HTL oil products towards straight fatty acids (68%) and amides (6%) (Table 2). The addition of base had less effect on the oil composition (compared to the non-modified HTL), having both cyclic and straight compounds. Food waste showed a different pattern compared to manure, where the nonmodified bio-crude oil contained more compounds that were categorized as straight fatty acids (26%), cyclic hydrocarbons (20%) and N heterocyclic compounds (18%) (Table 2 and Table S3). The addition of acid to food waste shifted the bio-crude oil composition to furan compounds, suggesting enhanced dehydration processes. On the other hand, the addition of base to food waste favored the production of phenols as well as straight fatty acids. Subcritical water may act both as an acid or base catalyst due to the higher value of the water ionization constant (Kw) under these conditions (up to three order of magnitude higher than for ambient conditions).7 For example, hydrothermal conversion of polysaccharides, enhanced at temperatures of 280–300 ºC,13 lead to formation of HMF (for hexoses), and furfural (for pentoses), which are further converted to furans.12 Therefore, the formation of furans observed in the present study was probably due to the dehydration of hexoses and pentoses – both acidcatalyzed processes.13 The use of alkali catalysts in HTL may increase hydrolysis of macromolecules, as well as decrease char formation, and, subsequently, stabilize the bio-oil).28 Moreover, bio-crude oil generated by base-catalyzed HTL of woody biomass contained mainly phenolic compounds and phenol derivatives — the primary degradation products of lignin.29 In our study, the presence of phenolic compounds in bio-crude oil from food waste may also be



attributed to condensation or cyclization of the cellulose and hemicellulose-derived carbohydrates— an additional base catalyzed process.30 HTL aqueous product composition The effect of acid and base on HTL of manure and food waste can be further elucidated by the composition of the aqueous product (Table S4). For manure, the dominant forms of dissolved organic carbon were lactic and acetic acids. The recovery of lactic acid (C3O3H5) from manure with no additive was 26±4 mg C per g of C in the feed. The addition of acid decreased this value by 85%, whereas the addition of base increased this value by 88% as demonstrated by the CI values for lactic acid (Fig. 2a). Lactic acid is likely the main product from alkaline degradation of sugars. Moreover, under hydrothermal conditions, lactic acid was the main degradation product of sugars even without the addition of an alkaline catalyst, due to the base catalytic role of subcritical water.13 It was also suggested that the conversion of sugars in subcritical water is similar to the mechanism in alkaline solutions, i.e., via the benzilic acid rearrangement of pyruvaldehyde to lactic acid.30 On the other hand, researchers reported that the addition of acid to the hydrothermal media decreased the yields of lactic acid and pyruvaldehyde from sugars,7 suggesting that this pathway is base catalyzed. A similar trend was observed for acetic acid (C2O2H4). HTL without acid or base resulted in an acetic acid recovery of 38±4 mg C per g of C in manure (Table S4) with a negative CI of 44% for acid and a positive CI of 176% for alkali (Fig. 2a). Small concentrations of succinic acid (C4H6O4) and formic acid (CH2O2) were also found in the HTL aqueous product of manure (Table S4). Succinic acid had a negative CI of 36% when acid was added and a positive CI of 72% following a base-modified reaction (Fig. 2a). Formic acid had negative CIs of 31 and 23%


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for acid and base, respectively (Fig. 2a). The production of short chain organic acids (C1–4) in the aqueous phase of HTL was also reported for cellulose,11,31 feedstocks.33

rice straw,32 and algal

Moreover, studies on cellulose, confirm our observations regarding higher

production of short chain organic acids under alkali conditions,11,31 which may explain the lower bio-crude oil yield from manure under these conditions (Fig. 1a). The recovery of monosaccharides (i.e., glucose, fructose, xylose and arabinose) from manure was very small for all reaction conditions, less than 5 mg C per g of C in manure feed (Table S4). Still, the recovery of monosaccharides for the acid-modified reaction (< 2 mg C per g C in manure) was less than one half of that for the base-modified and non-modified reactions (~ 4.3–4.4 mg C per g C in manure). HMF (C6H6O3), a degradation products of glucose and fructose,12 showed an opposite trend to lactic and acetic acids with a positive CI for acid and a negative CI for base (Fig. 2a). In other words, acidic conditions induced the production of HMF by 85%, while the alkali conditions suppressed it by 62% (Fig. 2a). These trends have been reported by other researchers looking at hydrothermal conversion of cellulose.11,31 The positive CI of HMF for the acidic condition (Fig. 2) together with the higher oil yield (Fig. 1) suggest that adding acid to HTL media enhances the production of bio-crude oil from manure through the production of HMF. For food waste, lactic acid recoveries were in the range of 70–120 mg C per g of C in the feed (Table S4) — significantly higher than for manure. The addition of acid suppressed the formation of lactic acid by 26% while alkali conditions increased its formation by 34% (Fig. 2b). Lactic acid is a valuable co-product as a precursor for bio-based chemicals, such as ethyl lactate and poly-l-lactic acid — biodegradable and environmental friendly solvents.34 Lactic acid was



reported to be stable at hydrothermal conditions below 240 ○C, while above this point it is gradually degraded 35. Since our experiments were conducted at 300 ○C, some of the lactic acid was probably degraded. Yet, future work aimed to selectively recover lactic acid from food waste at alkali sub-critical water should be considered. Acetic and succinic acids were also generated from food waste. Each one of these two products had negative CI of 49% for the acidmodified reaction and positive CIs of 26 and 13%, respectively, for the base-modified reaction (Fig. 2b). Acetic acid was reported as a stable intermediate product in HTL of almost all food wastes.36,37 Acetic and succunic acids are also important resuable organic chemicals. One of the industrial uses of acetic acid, for example, is to produce calcium magnesium acetate — a noncorrosive road deicer.34 Compared to manure, the recovery of monosaccharides from food waste was remarkably high — 55±3 mg C per g of C in food waste feed with no additive (Table S4). This may have resulted from the presence of pectin in food waste (composed primarily of fruit and vegetables),22 compared to manure — a lignocellulosic rich biomass.23 The addition of acid to the hydrothermal media decreased the recovery of monosaccharides to 18±0.8 mg C per g of C in food waste (Table S4) with a negative CI of 67% (Fig. 2b). On the other hand, the small CI for the alkaline condition suggests no effect on the recovery of monosaccharides from food waste (Fig. 2b), similar to manure (Fig. 2a). HMF formation from food waste, in general was lower than for manure (Table S4), yet, the same trends were observed with a positive (45%) CI for acid and a negative (57%) CI for the alkaline condition (Fig 2b). Dehydration reactions, probably enhanced by the addition of the acid as discussed in section 3.2, were probably leading to the production of HMF,34 as demonstrated by its higher CI.


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In addition to short-chain organic acids, monosaccharides and HMF, the aqueous product from HTL at these conditions may contain other soluble compounds, such as levulinic acid,11 amino acids and their derivatives, as well as nitrogen heterocyclic compounds.14,33 For example, pyroglutamic acid (C5H7NO3), a dehydration product of glutamic acid (C5H9NO4)35 — one of the main amino acids available in soybean protein — was found in the HTL aqueous product following the conversion of protein-rich biomass at 300 ○C.14 There are several opportunities for further valorization of the HTL aqueous products such as employing catalytic hydrothermal gasification,38,39 or anaerobic digestion.14,32,33 In addition, internal combined heat and power recovery can be designed as part of a process integration system to reduce external energy demands and to lower environmental and operating costs.26 Nitrogen and phosphorous in the HTL aqueous product may represent additional opportunities for valorization, e.g. using it for direct fertilization.21

Furthermore, catalytic hydrothermal

gasification may allow the recovery of those nutrients as salts by precipitation and filtration.38 Hydro-char characterization The addition of acid or base to the reaction media affects the nature of the hydro-char produced from both feedstocks as demonstrated by the FTIR spectra (Figs. S4a and S5a). For all hydrochars, the FTIR spectra suggest a complete disappearance of hydrogen bonds (3700–3000 cm−1). 40

Less hydrogen bonding is consistent with the depletion of oxygen in the hydro-char measured

by elemental analysis (data not shown). For manure, Hydro-char generated under acid conditions (Fig. S4a) showed a significantly different FTIR spectra than under alkaline conditions and with no additive, with a complete disappearance of the 1400 and 870 cm−1 bands that represent the carboxylate group stretching,41 and the glyosidic linkage for cellulose and hemicellulose,42



respectively. Furthermore, the PCA bi-plot shows three distinct clusters for hydro-char generated from manure (Fig. S4b), mostly due to the PC1 axis (71.5% of variation). This suggests that hydro-char generated from manure via HTL with no additive or with base additive may share similar functional groups, which are different from those in hydro-char generated with acid additive. The loading plot for manure hydro-chars (Fig. S4c) demonstrates that the major contribution for the clustering along PC1 results from differences absorbance at 1637, 1258, 1028, 970 and 868 cm−1. Interestingly, other researchers reported that the bands at 1028 and 970 cm−1 refer to mineral phosphate residues in bio-chars 43. Since phosphoric acid was used as the acid additive in our experiments, this evidence is a further validation for the FTIR-PCA approach. Variation among hydro-chars, due to the other wavenumbers (i.e., 1637, 1258 and 868 cm−1) were related mostly to changes of chemical bonds in lignin (Table 3).44–47 For hydro-char generated from food waste, there were decreases in absorbance at 2920 and 2850 cm−1, which represent the asymmetrical and symmetrical C-H stretching vibrations for aliphatic groups (sp3 carbon).40 This phenomenon was also observed in bio-char produced from 500 °C pyrolysis of biomass.48 In addition, hydro-char generated with alkali additive showed a significantly different FTIR spectra compared to the hydro-char generated with acid additive and with no additive. This was mostly due the bands at 1030–1015 cm−1, which represent C-O stretching in primary alcohols. The PCA bi-plot showed two clusters for hydro-char generated from food waste (Fig. S5b). The hydro-char from HTL without additive and with acid additive overlap in the PCA bi-plot at negative values of PC1. On the other hand, hydro-char from HTL with alkaline additive appear at positive values of PC1. Most of the variation (91.2%) was explained by PC1. The loading plot (Fig. S5c) shows that differences in FTIR spectra at 1150,


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1105, 976, 905 and 638 cm−1 contributed to PC1, and these wavenumbers represent modifications mostly the structure of cellulose, hemicellulose and pectin (Table 3).42,44,45,49–52 Since food waste is acidic, both non-modified and acid-modified reactions occur in a low pH media (2.0–4.1), suggesting similar conditions and therefore, similar FTIR spectra. Manure, on the other hand, is basic, thus both non-modified and base-modified reactions occur in higher pH (8.8–10), generating hydro-chars with similar FTIR spectra. In summary, the FTIR-PCA analysis demonstrates that the pH conditions in HTL of manure and food waste, generate hydrochars with different chemical structures.

Those differences result primarily from reaction

phenomena associated with decomposition of the fibers in the raw biomass— lignin in manure and pectin in food waste (Table 3). This information is important for understanding the HTL reaction mechanisms for manure and food waste. Moreover, improved characterization of hydrochar could expand the range of its utilization opportunities such as soil amendment, nanostructured materials, adsorbent materials, wastewater purification, energy production, carbon sequestration and reduction of greenhouse gas emissions.21,53,45 Implications for future research This study explains the role of acid and base addition on HTL of complex biomass such as manure digestate and carbohydrate-rich food waste. Our experimental data showed that increased bio-crude oil yield is associated with alternative chemical pathways such as dehydration reactions that were enhanced by the acidification of the reaction media. In addition, we characterized the aqueous and solid phase products and provided information necessary to advance the utilization of HTL co-products. These valorization alternatives include selective recovery of bio-based chemicals, water recovery from the aqueous phase, direct use of hydro-



char in agricultural applications and as packing material for adsorption columns. Increasing the fundamental understanding of the catalytic effect for acid and base on chemical pathways to yield higher recovery of quality bio-crude, as well as the properties of HTL co-products generated, is essential for adjusting the conditions for valorization strategies.

SUPPORT ING INFORMAT ION The Supporting Information is available on the ACS Publications website. ACKNOWLEDGEMENTS The authors thank Prof. Lars Angenent, Cornell University for providing the feedstocks and Monica Hoover, Bucknell University, for her analytical assistance. The research was supported by a BARD postdoctoral award, the academic venture fund of Cornell’s Atkinson Center for Sustainable Future, Junta de Castilla y León, University of Valladolid, the Fundacion La Caixa Fellowship, Cornell College of Engineering and the Cornell Energy Institute.


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TOC/ABSTRACT GRAPHIC



SYNOPSIS The addition of acid to hydrothermal media enhances the conversion of manure and food waste to bio-based valuable products.


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Table 1. Elemental composition of the bio-crude oil. Carbon, nitrogen, hydrogen, and oxygen contents are given as percentage on a dry weight basis, H/C and O/C are the atomic ratios of elements and higher heating values (HHV) are given as MJ kg−1. All values represent mean values of three replicate experiments ± standard errors. Manure

Food waste

Feed

No additive

Acid

Alkaline

Feed

No additive Acid

C

34.3

73.0±2.1

73.9±1.4 75.9±2.8

49.0

73.3±1.7

70.6±0.4 74.1±2.0

H

3.7

8.0±0.6

8.7±1.1

7.6±0.3

6.9

8.9±0.6

8.1±0.8

9.2±0.2

N

2.3

3.3±0.8

3.8±1.5

2.6±1.5

3.5

4.2±1.1

3.4±1.2

2.8±0.8

O*

25.9

15.8±2.3

13.6±1.3 14.0±1.4

34.9

13.4±1.5

17.8±0.6 13.9±1.3

H/C

1.3

1.3±0.1

1.4±0.2

1.2±0.1

1.7

1.4±0.1

1.4±0.1

1.5±0.0

O/C

0.6

0.2±0.0

0.1±0.0

0.1±0.0

0.5

0.1±0.0

0.2±0.0

0.1±0.0

33.4±0.5

34.8±1.8 34.0±1.1

20.2

34.8±1.3

32.5±0.7 35.4±1.0

HHV 13.3

*Oxygen content was calculated by difference (including the ash content).


Alkaline

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Table 2. Bio-crude oil composition from hydrothermal liquefaction of manure digestate and carbohydrate-rich food waste with and without the addition of acid or base. All components were characterized by GC–MS and categorized into several groups. Values represent % of total relative peak area. Manure

Acid N.D

Alkaline 4.6

No additive 2.6

Acid N.D

Alkaline N.D

N heterocyclic compounds 6.5

2.8

4.6

18.1

N.D

4.2

Cyclic hydrocarbons

59.4

6.3

20.8

19.9

10.3

2.8

Phenols

3.7

4.3

4.7

9.9

8.3

11.4

Furans

N.D

N.D

N.D

1.9

45.6

0.0

Long fatty acids

N.D

67.5

38.0

26.4

15.0

64.8

Straight amides

N.D

5.8

N.D

1.0

N.D

1.0

Total:

73.1

86.7

72.7

79.8

79.3

84.2

Group Alkenes

No additive 3.4

Food waste

N.D, not detected



Page 34 of 36

Table 3. Assignment of the specific FTIR wavenumbers detected to contribute to variation among hydro-char products with their functional group and composition they represent, according to cited literature.

Wavenumber

Functional group

Suggested composition

References

1637

C=O stretching

Carbonyl (lignin)

40,41

1258

C-O stretching

Guaiacyl (lignin)

42,43

1028, 970

Mineral phosphate residues

868

Aromatic C-H deformation

Guaiacylpropane (lignin)

41,42

1150

Symmetric C-O stretching

Hemicellulose, Pectin

41,50

1105

Ring deformation

Pectin

46,50

976

Aromatic C-H deformation

Cellulose, Pectin

46,47,50

905

Ring deformation

Cellulose

38,40

638

Aromatic C-O deformation

Cellulose

48

(cm−1) Manure:

39

Food waste:


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Figure 1. Distribution of the carbon (g in product per g in feed) among the main three products (oil, aqueous and char) from the hydrothermal liquefaction of manure (a) and food waste (b) at the three different HTL conditions (no additive, acid and alkaline). 150x170mm (150 x 150 DPI)

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Figure 2. Change indexes (CI) of aqueous products following hydrothermal conversion of (a) manure; and (b) food waste under acid and alkaline addition (indexes were calculated based on the comparison to the non-modified product). Bars represent mean values of three replicate experiments ± standard deviations. *Monosaccharides represent the sum of glucose, fructose, xylose and arabinose; **Total C1–4 acids represent the sum of formic, acetic, lactic and succinic acids.

131x144mm (150 x 150 DPI)

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Acid and Alkali Catalyzed Hydrothermal Liquefaction of Dairy Manure

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