Pyrolytic Fractionation: A Promising Thermochemical Technique for

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Pyrolytic fractionation: A promising thermo-chemical technique for processing oleaginous (algal) biomass Balakrishna Maddi, Sridhar Viamajala, and Sasidhar Varanasi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02309 • Publication Date (Web): 26 Nov 2017 Downloaded from http://pubs.acs.org on November 26, 2017

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ACS Sustainable Chemistry & Engineering

Pyrolytic fractionation: A promising thermo-chemical technique for processing oleaginous (algal) biomass Balakrishna Maddi1, Sridhar Viamajala2*, Sasidhar Varanasi2

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Pacific Northwest National Laboratory, POB 999, Richland, WA 99352 USA Department of Chemical & Environmental Engineering, The University of Toledo, 2801 W. Bancroft St., Toledo, OH 43606, USA

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*Corresponding Author: Phone: 419-530-8094 Email: [email protected]

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Abstract:

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We report the development of a two-step pyrolytic fractionation approach that is especially

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applicable to processing oleaginous algae feed stocks. The first step is a low-temperature pyrolysis (T

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~ 300-320 °C) to produce bio-oils from degradation of protein and carbohydrate fractions. Solid

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residues left behind can subsequently be subjected to a second higher temperature pyrolysis (T ~

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420-430 °C) to volatilize and/or degrade triglycerides to produce fatty acids, their derivatives and

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long chain hydrocarbons. Thus, pyrolytic fractionation can be used to “fractionate” oleaginous

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biomass and separately recover triglyceride degradation products. Proof-of-concept micro-pyrolyzer

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and subsequent lab-scale fixed-bed experiments were performed using oleaginous Chlorella sp. and

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Scenedesmus sp. to demonstrate the pyrolytic fractionation technique and determine bio-oil yields.

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As expected, triglyceride-specific bio-oils were rich in hydrocarbons and free fatty acids, were nearly

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free of water, short-chain organic acids and other carbohydrate degradation products and had low N-

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content. Due to production of “high quality” triglyceride-specific bio-oil vapors, pyrolytic

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fractionation would allow product upgrading via in situ gas-phase catalytic processes to generate

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drop-in fuels (hydrocarbons) or specialty chemicals (e.g. fatty amides), without the need to condense

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the vapors. A conceptual process design is developed and energy requirements for pyrolytic

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fractionation are assessed and discussed.

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Keywords: Algae, pyrolysis, pyrolytic fractionation, differential scanning calorimetry,

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thermogravimetry, pyroprobe-GC/MS.

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Introduction

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There is a growing interest towards development of renewable fuels as a result of

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increasing global energy consumption, finite petroleum resources and global warming concerns.

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Non-food biomass materials, such as microalgae, could be viable feedstocks for environmentally

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sustainable biofuels. In general, microalgae have greater areal productivity than terrestrial plants,

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can be grown on non-agricultural and marginal lands and can use low quality water and nutrients

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from waste streams.1-3 Several strains are known to accumulate triglycerides – a platform

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chemical that is in current use for production of biodiesel as well as high value oleochemicals.1, 4-

9

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However, a key bottleneck in the commercial development of algal bio-refineries is a lack of

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scalable and viable conversion processes that can produce fuels as well as value-added

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chemicals.2, 5, 7

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The most common downstream processing approach suggested in the literature involves

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extraction of triglycerides from algal cells using organic solvents (such as chloroform and

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hexane) or through in situ transesterification where oleaginous biomass is directly reacted with a

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mixture of methanol and catalyst without prior solvent extraction.8-9 After extraction, the solvent

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and/or methanol must be separated, usually through evaporation, to recover the triglycerides. The

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recovered triglycerides may then be further converted to hydrocarbon fuels via thermo-catalytic

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de-carboxylation or hydro-cracking.10 In methods involving solvent use, the post-extraction solid

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residues, generally rich in protein, may also need extensive treatment for solvent removal before

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use as animal feed or fertilizer.11

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As an alternative to solvent extraction, thermochemical conversion processes such as

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pyrolysis and hydrothermal liquefaction can be employed to obtain bio-oil or bio-crude for

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subsequent conversion to liquid fuels and value-added chemicals.12-31 Pyrolysis is the thermal 2 ACS Paragon Plus Environment


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degradation of organic matter and is performed in the temperature interval of 450-600 °C at 1 bar

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without oxygen.32-33 Hydrothermal liquefaction is thermal degradation of organic matter in the

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presence of water usually over the temperature interval of 350-450 °C and at an elevated

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pressure of 100-250 bar.28 These thermo-chemical methods can produce fuel/chemical precursors

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from even the non-triglyceride portions of algal cells (e.g. carbohydrates, other lipids and

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proteins). However, thermochemical processes, as traditionally applied, produce bio-oils/bio-

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crude that contains a complex and highly heterogeneous mixture of chemical compounds – long

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chain fatty acids from degradation of triglycerides and other cellular lipids, short chain

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oxygenates (e.g. aldehydes, ketones, organic acids, water and alcohols) from degradation of

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carbohydrates and N-compounds from protein degradation.4, 20, 34-40 Oxygenates in bio-oil/bio-

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crude lower its heating value and degrade/polymerize over time to produce humins or char.32 In

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addition, algal bio-oil/bio-crude obtained from traditional thermochemical processes would

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consist of a broad molecular weight distribution of chemical species – longer chain products

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from triglyceride degradation and lower molecular weight compounds from degradation of

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carbohydrate and protein – that would necessitate further distillation into suitable fuel fractions

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and result in additional energy inputs for fuel production.41

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In this study, we have developed a “pyrolytic fractionation” approach whereby products

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from pyrolysis of triglycerides - the highest energy component of oleaginous microalgae - are

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obtained as homogenous bio-oils. Pyrolytic fractionation approach facilitates the recovery of

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triglyceride degradation products – primarily consisting of long chain fatty acids and hence can

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be integrated with existing industrial catalytic or non-catalytic processes for production of drop-

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in hydrocarbon fuels or specialty oleo-chemicals.42 In contrast, bio-oils from conventional algal

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pyrolysis contain water, organic acids, and oxygenated compounds20, in addition to triglyceride


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degradation products, and pose storage issues as well as challenges in downstream separation

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operations.33 Further, the operating temperature for pyrolytic fractionation approach is ≤ 420 °C

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which is lower than temperatures employed for conventional algal pyrolysis (performed at 500-

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600 °C).13, 43-44 Thus, the presented approach ”fractionates” products during the pyrolysis step

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and allows the recovery of oxygenates and N-compounds from degradation of carbohydrate and

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protein components as a bio-oil fraction that can be separately collected from the high energy-

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density triglyceride biooils.

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Experimental Feedstocks and chemicals Oleaginous Chlorella sp. (a natural isolate) and Scenedesmus sp. were cultivated using

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previously described culture conditions.45 Stationary phase cultures that were rich in triglycerides

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were centrifuged (2500×g), washed with de-ionized water and freeze-dried (Labconco Freezone

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2.5 L bench-top freeze drying system, Kansas City, MO) to obtain feedstocks used in this study.

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Soy oil was purchased from Spectrum Naturals (Boulder, CO) and used as received. GC-grade

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1,3-diolein, palmitic acid, oleic acid, stearic acid, oleic amide and triolein were purchased from

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Sigma-Aldrich (St. Louis, MO).

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Experimental design

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A schematic diagram that includes the experimental and analytical procedures used in

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this study is shown in the Supporting Information, Figure S1. Thermo-gravimetric analysis of

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oleaginous algal feedstocks (Chlorella sp. and Scenedesmus sp.) and model constituent

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biopolymers was initially performed under inert atmosphere to establish the parameters of the

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pyrolytic fractionation approach. Micro-scale pyrolytic fractionation experiments were

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performed next using a PyroprobeTM-GC/MS system to establish the preliminary validity of 4 ACS Paragon Plus Environment


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pyrolytic fractionation. Thereafter, fixed bed experiments were performed to validate actual

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product distribution via comprehensive mass balances. Finally, differential scanning calorimetry

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experiments were performed to determine the theoretical energy requirements for pyrolytic

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fractionation.

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CHN analysis

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Elemental analysis of feedstocks and products collected from pyrolytic fractionation was

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performed using a Perkin-Elmer 2400 Series II CHN Elemental Analyzer (Waltham, MA).

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Calorific values of feedstocks and products collected from fixed-bed pyrolytic fractionation

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experiments were calculated based on CHN analysis results as described previously in the

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literature.20 Protein content of algal feedstock is estimated by multiplying nitrogen content by the

11

factor 6.25.46

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Thermo-gravimetric (TG) analysis

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TG and Differential Scanning Calorimetry (DSC) analyses were performed on a TA

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Instruments SDT Q600 series analyzer (Schaumburg, IL) that provides simultaneous

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measurement of weight change and differential heat flow on a single sample. For these

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measurements, 10-15 mg of biomass was loaded into one alumina (Al2O3) crucible while a

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second identical crucible served as a reference. N2 was used as the carrier gas and also to

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maintain an inert atmosphere. The flow rate of N2 was kept at 100 mL/min.

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Overall thermal degradation behavior of biomass feedstocks was determined by heating

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the samples from room temperature to 600 °C at a constant ramp rate of 20 °C/min under N2

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atmosphere.

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To simulate pyrolytic fractionation, a two-stage heating protocol was used where samples were successively heated to 320 °C and 420 °C and maintained isothermal for 15 min at each of 5 ACS Paragon Plus Environment

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these temperatures. The 15 min isothermal incubation time at each stage was chosen since little,

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if any, weight loss was detected after this period. Inter-stage heating rate was 20 °C/min.

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To estimate energy requirements for pyrolytic fractionation, differential heat flux data for

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biomass samples (normalized by subtracting differential flux values of empty pans) were

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integrated over time (Eq.1).

=

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(1)

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Data obtained over the temperature interval of 230 °C to 420 °C were integrated to

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discount heat of vaporization of bound-moisture associated with biomass samples at lower

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temperatures. Numerical integration was performed by the trapezoidal rule using the in-built

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“cumtrapz” function in MatlabTM.

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In situ transesterification for fatty acid methyl ester (FAME) content

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FAME content was assessed after in situ transesterification of biomass samples.9, 47-48 Dry

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biomass samples (30 mg) were weighed into clean, 2 mL crimp-top GC vials to which 1 mL of a

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solution of 5 % sulfuric acid in methanol (v/v) was added. These vials were sealed with teflon-

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lined caps and placed in a heating block set at 90 °C for 2 h. The vials were removed every 10

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min and vortexed to ensure adequate mixing. After 2 h, the vials were cooled to room

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temperature and then centrifuged to separate the biomass residue. The liquid phase was

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transferred to 15 mL serum bottles and 5 mL of hexane was added to perform liquid-liquid

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extraction for recovery of FAMEs from the acidified methanol solution into hexane. The serum

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bottles were crimp-sealed with Teflon-lined aluminum foil caps to avoid leakage and placed in a

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90 °C water bath for 20 min. Thereafter, the serum bottles were removed and cooled to room

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temperature and an aliquot was recovered from the hexane phase and analyzed using a gas



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chromatograph equipped with a flame ionization detector (GC-FID). FAME standards purchased

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from Sigma-Aldrich were used to obtain calibration curves.

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Triglyceride quantification

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10-15 mg of freeze dried algae was added to 1 mL of chloroform in 2 mL stainless steel

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bead beating vials capped with polypropylene plugs (BioSpec Products, Bartlesville, OK). As

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previously described,49 a Mini-Beadbeater-1 (BioSpec Products, Bartlesville, OK) was used to

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agitate the stainless steel vials. Each vial was agitated for 20 s at 2500 oscillations per minute

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and then cooled in an ice bath for 1 min. Total bead beating time was 45 min (equal to

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approximately 30 bead beating cycles). No lipid loss was observed after 45 min of bead beating

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(Supplementary Information Table S1). The organic phase in stainless steel vials was transferred

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to 5 mL glass vial. Stainless steel vials were then rinsed with 1 mL of chloroform and added to

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the organic phase collected. This organic phase was filtered and transferred to GC vials for

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quantification of triglycerides, mono-glycerides, di-glycerides and fatty acids using GC-FID.

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Micro-pyrolysis experiments

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Pyrolysis experiments were performed on a CDS PyroprobeTM 5200 unit (CDS

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Analytical, Oxford, PA) connected to a Bruker 450 gas chromatograph (GC) equipped with a

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300 series mass spectrometer (MS) (Billerica, MA). An open-ended quartz tube (1" long) served

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as a micro-pyrolysis reactor in the PyroprobeTM system. The reactor temperature was set and

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maintained using a resistively heated a platinum element coiled around the tube. Vapors from

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pyrolysis were routed through a gas trap packed with Tenax® adsorbent material. After pyrolysis,

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the volatiles from the trap were desorbed and sent to the GC-MS for analysis. A heated transfer

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line connected the trap to the GC injector.


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Before the start of the experiment, approximately 1 mg of biomass sample was placed

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into the quartz tube for pyrolysis. During the experiment, the system environment (including

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reactor and trap) was kept inert by applying a continuous helium purge (50 mL/min). To simulate

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pyrolytic fractionation, a three stage heating protocol was used similar to the TG-DSC

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experiments described above. At each stage, the pyrolysis reactor was heated to the desired set

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point (320 and 420 °C) and maintained isothermal for 15 min. Similar to the TG-DSC

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experiments, inter-stage heating rate was kept at 20 °C/min. The vapors generated during

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pyrolysis were adsorbed in the gas trap that was held at a much lower temperature of 50 °C to

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facilitate better retention of the volatiles. Following the completion of each pyrolysis stage, the

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reactor was allowed to cool down to room temperature while the trap was heated for 7 min to

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desorb the volatiles for GC analysis. Helium was used as the purge gas (100 mL/min). For the

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first two desorption stages, the trap temperature matched the pyrolysis temperature. However,

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the trap was heated only up to 350 °C in the third desorption stage since Tenax® degrades above

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this temperature. Desorbed volatiles were routed to the GC injector via a transfer line that was

15

also maintained at the same temperature as the trap.

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GC-MS analysis was synchronized with the desorption steps. An Agilent DB-5MS fused

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silica capillary column (30 m × 0.25 mm × 0.25 µm film thickness, Agilent Technologies, Santa

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Clara, CA) was used in the GC. The injector temperature was 300 °C and a split ratio of 1:100

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was maintained. Helium, used to purge the trap, also served as the carrier gas (1.0 mL/min) in

20

the GC column. The temperature program of GC column was as follows: constant temperature of

21

50 °C for 7 min (to match the time for desorption of volatiles from the trap) followed by

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temperature ramp to 300 °C at 10 °C/min and finally a constant temperature of 300 °C for 3 min.

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The MS source was maintained at 150 °C. The transfer line (between GC and MS) stayed at 300



1

°C. Chemical compounds corresponding to chromatogram peaks were identified using the

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NIST2008 mass spectral database. Only compounds with a “confidence” value above 600 are

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reported.

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Fixed-bed pyrolysis experiments

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Fixed-bed pyrolytic fractionation experiments were conducted in a quartz tubular reactor

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(L = 43 cm, OD = 2.54 cm) placed in a horizontal split shell electric furnace (Applied Test

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Systems Inc., Butler, PA) as previously described.20 A K-type thermocouple remained in contact

8

with the biomass during the experiments to directly measure the temperature inside the pyrolysis

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chamber. The outlet of the reactor was connected to two glass condensers (connected in series)

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that had a continuous flow of ethylene glycol (maintained at -15 °C using dry ice) as coolant. N2

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was continuously passed through the reactor during pyrolysis to maintain oxygen-free

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conditions. The flow rate of N2 was maintained at 100 mL min-1 using mass flow controllers

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(model 316L MCS, Alicat Scientific, Tucson, AZ). The reactor and glass condenser were

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connected by ¼″ stainless steel tubing that was maintained at pyrolysis temperature using

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heating tape to prevent in-line condensation. ⅛″ stainless steel tubing was used for all other

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connecting lines. Lines carrying N2 was first routed through the pyrolysis furnace to preheat the

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gases before entering the reactor.

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Before the start of the experiment, 9.33 g of dry biomass was placed in the tubular reactor

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using quartz wool as a support and the reactor was purged with N2 (100 or 1000 mL/min) for 15

20

min to remove air from the system. Thereafter, the pyrolysis furnace was heated to set-point

21

temperature (i.e. 320 and 420 °C) at a ramp rate of 30 °C/min (verified through monitoring the

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thermocouple readout). We observed that all biomass samples in the pyrolysis reactor reached

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reaction temperatures within 20 min. After reaching set-point, the reactor was maintained at that


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temperature for 10 min. Based on the dimensions of pyrolysis reactor and N2 flow rates used in

2

our studies, the vapor residence time in the reactor was calculated to be ~2s (for 1000 mL/min at

3

420 °C.

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At the end of the experiment, the pyrolysis reactor was cooled to room temperature and

5

bio-oils (collected in the condenser) were weighed and stored at -20 °C for subsequent analyses

6

as described below. Solids were also recovered after this step and a portion was retained for

7

measurement of lipid content and elemental analysis (described below). The remaining solid

8

residues from the 320 °C step were pyrolyzed again at 420 °C to produce char and triglyceride-

9

based bio-oil. Since the tristearate, stearic acid and nitrile of stearic acid are in solids at room

10

temperature, a portion of bio-oils formed at 420 °C solidified along the walls of the condenser.

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Hence for complete recovery of these solid products obtained, chloroform was used to dissolve

12

the products and quantify in GC or GC-MS. Bio-oils collected at 320 °C has both aqueous and

13

organic phase and were therefore dissolved in methanol for GC analysis.

14

Bio-oil analysis by GC-MS

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Chemical compounds present in the bio-oils collected from lab-scale pyrolytic

16

fractionation experiments were identified using a Bruker 450 gas chromatograph (GC) equipped

17

with a 300 Electron impact (EI) ionization mass spectrometry (MS). Analytes were separated on

18

an Agilent (Agilent Technologies Santa Clara, CA) DB-5MS (30m ×0.25mm×0.25µm film

19

thickness) fused silica capillary column. The GC injector was maintained at 300 °C and its split

20

ratio was 100. Helium, maintained at flow rate of 1.0 mL/min, was used as the GC column

21

carrier gas. The temperature program of GC column was as follows: constant temperature of 60

22

°C for 3 min followed by temperature ramp to 300 °C at 10 °C/min and finally a constant

23

temperature of 300 °C for 3 min. The transfer line that connects GC and MS was maintained at



1

300 °C. The MS source was maintained at 150 °C. A mass spectrum obtained for a peak in GC

2

chromatograms was identified using NIST2008 mass spectral database.

3

Bio-oil analysis by GC-FID

4

A Shimadzu 2010 GC equipped with a Restek (Bellefonte, PA) Rtx Biodiesel column (15

5

m × 0.32 mm ID × 0.1 µm) and flame ionization detector (FID) was used to quantify fatty acids,

6

hydrocarbons, mono-, di- and tri-glycerides. This method was previously described in the

7

literature.49 The GC oven was programmed to initially hold a temperature of 60 °C for 1 min

8

followed by a steady increase to 370 °C at a ramp rate of 10 °C/min. Finally, the oven

9

temperature was maintained at 370 °C for 6 min. Helium was used as a carrier gas with a

10

constant linear velocity of 50 cm/s in the column. The injector and FID temperatures were both

11

maintained at 370 °C.

12 13

Results and Discussion Thermogravimetry (TG) studies to identify biopolymer degradation temperatures

14

TG analysis was performed to obtain an a priori estimate of pyrolysis behavior of

15

Chlorella sp. and Scenedesmus sp. feedstocks. Figures 1a and 1b show two well-separated

16

derivative weight loss peaks at 320 °C and 420 °C for both of these oleaginous algal samples.

17

Based on previous studies, the derivative weight loss peak at 320 °C likely resulted from

18

degradation of algal proteins and carbohydrates,20, 50-52 while the derivative weight loss peak at

19

420 °C could be attributed to volatilization of the triglyceride fraction of algal samples. TG

20

analysis of soy oil (Figure S1, supplementary information) confirmed that the peak at 420 °C

21

was associated with pyrolysis of triglycerides.


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Since the protein and carbohydrate fractions degrade at lower temperatures (280-350 °C)

2

than triglycerides in oleaginous feedstocks (370-450 °C), it was expected that a sequential

3

exposure of biomass to these temperature intervals would first result in pyrolysis of both proteins

4

and carbohydrates followed by triglycerides pyrolysis. Further, when condensed separately, this

5

“pyrolytic fractionation” method would produce triglyceride-specific bio-oils with low

6

contamination by small molecular weight N- and O- compounds produced from pyrolysis of

7

carbohydrate and protein. This hypothesis of pyrolytic fractionation was first tested in a micro-

8

pyrolysis system and subsequently demonstrated using a bench-scale fixed-bed pyrolysis reactor.

9

One additional observation from these experiments is that triglyceride pyrolysis did not

10

produce any measurable residues (Supplementary information, Figure S2) suggesting that high

11

product yields from triglycerides could be expected upon implementation of pyrolytic

12

fractionation.

13

Simulation of pyrolytic fractionation by TG

14

In these studies, pyrolytic fractionation was simulated on the TG analyzer by step-wise

15

heating. The dotted arrows in Figure 2a indicate the temperature path implemented for TG

16

experiments. Samples were first heated to 320 °C and then incubated isothermally (yellow

17

arrows) to pyrolyze both protein as well as carbohydrate fractions. After 10 min at this

18

temperature, further weight loss was not significant (Supplementary information, Figure S3)

19

suggesting that pyrolysis of the protein and carbohydrate fractions was substantially complete

20

during this period. Thereafter, when the samples were cooled back to 100 °C and reheated (green

21

arrows in Figure 2a), the peak at 320 °C was absent from the thermogram (green differential

22

weight loss profile in Figures 2a) confirming the removal of thermally labile protein and

23

carbohydrate fractions. Further heating resulted in triglycerides pyrolysis at 420 °C (green



1

derivative weight loss curves in Figure 2a). Similar results were obtained with Scenedesmus sp.

2

(Figure 2b), when subjected to an identical thermal treatment for volatilization of protein and

3

carbohydrate fractions. These observations suggest that prolonged exposure at 320 °C during

4

pyrolytic fractionation did not alter the thermal degradation characteristics of triglyceride

5

fraction in the tested biomass samples.

6

Finally, 10 min of isothermal incubation at 420 °C was sufficient to pyrolyze triglyceride

7

fraction from biomass (supplementary information, Figure S4) that had been previously

8

subjected to pyrolytic fractionation for thermal degradation of proteins and carbohydrates.

9

Micro-pyrolysis experiments

10

To assess the degradation products formed during pyrolytic fractionation of algal biomass

11

at the temperatures predicted during TG studies, further experiments were performed on a

12

PyroprobeTM micro-pyrolysis system. Products obtained were analyzed using GC-MS and the

13

product identities were used to verify the source biopolymer. While py-GC-MS (pyrolysis probe

14

coupled with GC-MS) rapidly provides reliable information on the class of compounds

15

produced, it is not a convenient tool for quantification.53-54 However, the py-GC-MS is

16

commonly practiced as a screening method to determine optimal operating conditions for

17

pyrolysis and preliminary data obtained through py-GC-MS can be corroborated through larger-

18

scale fixed/fluidized bed experiments.55

19

The first step of pyrolytic fractionation was implemented by heating and incubating the

20

samples at 320 °C to degrade the protein and carbohydrate fractions of algal biomass and the GC

21

chromatograms for the resulting products are shown in Figure 3 (confidence levels of identified

22

products are shown in Supplementary Information, Table S2). At this temperature, pyrolysis

23

products obtained from both Chlorella sp. and Scenedesmus sp. contained oxygenated


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compounds such as acetic acid, furfural, 2-furanmethanol, 2-(5H)-furanone, 2-hydroxy-2-

2

cyclopenten-1-one, cyclopropyl carbinol, glucopyranose, levoglucosans and maltols, which are

3

typically form from volatilization of carbohydrate 41. N-compounds were also present in

4

pyrolysis products from both algae species. However, N-products were more diverse from

5

Chlorella sp. pyrolysis (such as amino-2-oxazolidinone, 3-butoxy propanenitrile, 1-amino-2,6-

6

dimethylpiperidine and indole) (Figure 3a) than Scenedesmus sp. (only 3-amino-2-oxazolidinone

7

was identified) (Figure 3b), possibly due to the lower protein content of Scenedesmus sp. It is

8

also possible that some N-compounds (e.g. NH3 and NOx) might not have adsorbed on the

9

Tenax® trap of PyroprobeTM.

10

During the pyrolysis step at 320 °C intended for protein and carbohydrate pyrolysis,

11

some C14-C18 hydrocarbons and fatty acids (e.g. octadecanoic acid) were also identified among

12

the products. While cellular free fatty acids could have volatilized at this temperature (normal

13

boiling points of C14-C18 fatty acids are in the range 250-300 °C 56), limited breakdown of other

14

lipids including glycerides or phospholipids could also have occurred. Indeed, when di- and tri-

15

glycerides were pyrolyzed at 320 °C, small amounts of fatty acids and fatty acid anhydrides were

16

observed (Figures S5 and S6, supplementary information). Diglycerides underwent more thermal

17

degradation than triglycerides, as is evident by the significantly higher concentrations of

18

products obtained from diglyceride pyrolysis at 320 °C (note the order of magnitude difference

19

in the y-axis scales between Figures S5 and S6) suggesting that smaller molecular weight lipids

20

(possibly including monoglycerides and other short-chain lipids) are more thermally labile than

21

triglycerides.

22

Most triglyceride pyrolysis however occurred at much higher temperatures (>380 °C,

23

Figures 1 and S1). When pyrolyzed at 420 °C, soy oil (assessed as a model triglyceride-only



1

feed) produced large quantities of C16 and C18 fatty acids as well as hydrocarbons such as

2

tetradecane, pentadecane, 8-heptadecene, heptadecane, 9,12-octadecadien-1-ol (see

3

supplementary information, Figure S7) confirming the extensive cleavage of the glyceride ester

4

bonds as well as partial decarboxylation during the thermal degradation process.4

5

Accordingly, in the second step of pyrolytic fractionation, solid residues from the algae

6

samples left behind from previous step (at 320 °C) were heated at 420 °C. Similar to soy oil

7

pyrolysis products, the primary products for all the biomass samples were observed to be C16-

8

C18 fatty acids and alkanes such as tetradecane, pentadecane, 8-heptadecene, heptadecane

9

(Figure 4; confidence levels of identified products are shown in Supplementary Information

10

Table S3). A small C16 fatty nitrile peak was seen in the Chlorella sp. chromatogram but no N-

11

compounds were discernible in the products from Scenedesmus sp. samples pyrolyzed at 420 °C.

12

It is possible that N-derivatives of fatty acids were formed as a result of reactions of the free fatty

13

acids (produced in this step due to breakdown of triglycerides) with polymerized (charred)

14

proteins from the previous pyrolysis steps.57 Since, fatty nitriles have been considered as fuel

15

additives to improve lubricating properties,57-61 the presence of these compounds at low

16

concentrations may, in fact, enhance the fuel value of triglyceride-derived bio-oil from pyrolytic

17

fractionation. Trace amounts of toluene and 4-methyl-phenol were also observed in triglyceride-

18

based bio-oils from both the oleaginous algal feedstocks and could have formed from

19

degradation of recalcitrant or charred proteins.

20

Lab-scale pyrolysis experiments

21

To validate the scalability of observations made from micro-pyrolysis experiments for

22

production of triglyceride-specific bio-oils, larger scale fixed bed pyrolytic fractionation tests

23

were performed using oleaginous Chlorella sp. These experiments were performed under “fast


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1

pyrolysis” mode with a vapor residence time approximately 2 s.32, 62 Biomass was first

2

pyrolyzed at 320 °C (Step 1) using 9.33 g of biomass with a total lipid content of 0.27 g-lipid/g-

3

biomass (measured as total fatty acid methyl ester (FAME)47). The majority of the lipids were

4

measured to be triglycerides (0.23 g-TAG/g-biomass) and the remaining small fraction (0.04 g-

5

lipid/g-biomass) was likely composed of membrane lipids and cellular free fatty acids. Mass

6

balance data for the Step 1 pyrolysis (at 320 °C) is shown in Figure 5. As expected, the bio-oils

7

produced in this step contained several protein and carbohydrate derived products including

8

levoglucosans and N-compounds (see product chromatogram in Figure S8 and identified

9

products in Table S7(a), supplementary information). In addition, a small amount of fatty acids

10

were also produced, likely from the more thermally labile cellular lipids. However, the vast

11

majority of lipids, likely triglycerides, were not pyrolyzed during Step 1 and remained associated

12

with the residue from this step (see lipid balance data in Figure 5 quantified using GC-FID

13

analysis of bio-oil samples and solid residues).

14

The residue from Step 1 was subjected to a second pyrolysis reaction at 420 °C (Step 2).

15

As expected, the second step at 420°C, targeted towards triglyceride pyrolysis, produced bio-oil

16

largely composed of fatty acids and glycerides. In addition, small amounts of hydrocarbons and

17

fatty amides were also produced (see GC chromatograms in Figure S9 and identified products in

18

Table S7(b), supplementary information). Quantification of bio-oil constituents through

19

correlation of GC-FID peak areas with corresponding calibration standards for all detected

20

compounds shows that nearly 95% of the recovered mass could be attributed to lipid-derived

21

products, most likely obtained from thermal degradation of triglycerides (see total mass and

22

lipid-derivative mass data for stream 2b in Figure 6a and mass fraction of bio-oil components in

23

Figure 6b). The lipid mass balance data for step 2 (Figure 6) also shows that nearly 94% of the



1

lipid mass fed to step 2 (steam 2a) was recovered in the bio-oil (stream 2b). Although some N

2

was also present in Step 2 bio-oil, N-balance analysis across both Steps 1 and 2 indicates that

3

Pyrolytic Fractionation: A Promising Thermochemical Technique for

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