Effects of Various Reactive Gas Atmospheres on ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Effects of Various Reactive Gas Atmospheres on the Properties of Bio-Oils Produced Using Microwave Pyrolysis Paul C. Tarves, Charles A. Mullen,* and Akwasi A. Boateng Eastern Regional Research Center, USDA-ARS, 600 E. Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States S Supporting Information *

ABSTRACT: Fast pyrolysis of lignocellulosic biomass produces organic liquids (bio-oil), biochar, water, and noncondensable gases. The noncondensable gas component typically contains syngas (H2, CO, and CO2) as well as small hydrocarbons (CH4, C2H6, and C3H8). To understand the influence of reactive gas in various pyrolysis processes, we have employed a laboratory scale microwave reactor and performed pyrolysis of switchgrass under varying gaseous atmospheres and characterized the bio-oils obtained. The batch (100 g of biomass) microwave pyrolysis was performed at 900−1000 W over the course of 7 min in the presence of a microwave absorber (10 g of activated charcoal). The products formed were quantified and the bio-oils were characterized by GC−MS, elemental analysis, Karl Fischer and TAN titrations, bomb calorimetry, and 13C NMR spectroscopy. Pyrolysis experiments performed under a N2 atmosphere were used as the control and then compared to experiments performed under various reactive gases (CO, H2, and CH4) and a model pyrolysis gas mixture (“PyGas”). The use of a CO atmosphere had a negligible effect on the quantity and quality of bio-oils produced, whereas the use of H2, CH4, and PyGas atmospheres each provided more deoxygenated products (i.e., BTEX, naphthalenes, etc.) and lower oxygen content. The use of different particle sizes also displayed a pronounced effect on the product distribution and the composition of the bio-oils obtained. KEYWORDS: Microwave, Pyrolysis, Lignocellulosic biomass, Bio-oil, Deoxygenation

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produced.2−6 However, the effectiveness of this process is limited by loss of H content via aromatization which leads to coke formation and catalyst deactivation.7 The deactivated catalyst must then be regenerated, which adds complexity to the required design. Furthermore, alkali metals contained within the biomass can cause permanent catalyst deactivation.8 Therefore, alternative technologies to produce deoxygenated bio-oil are desirable. In 2012, Mante et al. reported the catalytic fast pyrolysis of hybrid poplar wood in a fluidized bed reactor while recycling the noncondensable gases back into the reactor.9 The pyrolysis gas mixture was composed of CO, H2, CO2, CH4, and other C1−C5 hydrocarbons produced during pyrolysis as well as the initial fluidizing gas, N2. These recycle conditions led to a modest decrease in O content (∼15%) for the pyrolysis oils obtained. Recently, USDA-ARS has developed a fast pyrolysis process in a fluidized bed reactor using a similar product gas recycling process, but without the use of externally added catalyst.10,11 In this process called tail gas reactive pyrolysis (TGRP), the reducing environment provided by the recycle process led to a much larger decrease in O content (∼60%)

INTRODUCTION Lignocellulosic biomass is a readily available, inexpensive, renewable carbon source that can potentially be used to produce renewable biofuels, commodity and/or specialty chemicals. The development of energy efficient methods for the conversion of biomass to more energy dense bio-oils is necessary to reduce transportation and storage related costs.1 The bio-oils may be used directly in boilers/burners and/or upgraded to transportations fuels and chemicals. Fast pyrolysis is a method that has garnered much attention because of its ability to provide liquid products from lignocellulosic biomass. Fast pyrolysis is the rapid thermal decomposition of organic material at high temperature (∼450−600 °C) under an inert atmosphere (most often N2). The process produces, condensed organic vapors (referred to as bio-oils or pyrolysis oils), water, biochar, and noncondensable gases. Unfortunately, pyrolysis oils are generally too acidic and contain a high concentration of reactive oxygenated species for long-term storage. Therefore, the goal has become to develop processes that provide less acidic, more deoxygenated oils that will, in turn, be more compatible with current refineries. The most common approach is to utilize a catalyst to promote deoxygenation reactions. Solid acid catalysts (i.e., zeolites) that can induce dehydration reactions and produce aromatics/olefins and decrease the O content of the oils This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society

Received: September 3, 2015 Revised: December 18, 2015

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DOI: 10.1021/acssuschemeng.5b01016 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering

Research Article

Figure 1. Schematic diagram of microwave reactor and product collection system.

oils have been isolated, characterized, and compared across all gaseous atmospheres tested.

than what had been previously reported. However, it is important to note the observation of diminished deoxygenation behavior (∼35% decrease in O content) at high concentrations of pyrolysis gas (≥90%) in the reaction atmosphere. The bio-oil produced by the TRGP process was rich in aromatic hydrocarbons such as benzene, toluene, ethylbenzene and xylenes (BTEX) and naphthalenes. Both reports highlight the reducing power of the pyrolysis gas mixture, which may allow for the production of more stable pyrolysis oils without the use of a catalyst. Optimization of the aforementioned TGRP process and other noninert thermochemical processes requires a greater understanding of how the individual components within the pyrolysis gas mixture affect the product yields and composition. Zhang et al. published the first example of biomass (corncob) fast pyrolysis under varying gas atmospheres (N2, CO2, CO, CH4, and H2) using a fluidized bed reactor.12 They observed a small decrease in pyrolysis oil yield (∼5%) under CO and H2 atmospheres. However, CO and H2 atmospheres also led to the largest increases in HHV (∼6 MJ/kg), which one may expect based upon the reducing nature of the gases. In 2011, Meesuk et al. reported the fast pyrolysis of rice husk under N2 and H2 using a fluidized bed reactor.13 They observed a decrease in pyrolysis oil yield (∼5%) and a minor decrease in O content (∼10%) under the H2 atmosphere. In 2013, Pilon and Lavoie reported the fast pyrolysis of switchgrass under N2 and CO2 in a fixed bed reactor.14 They report no real difference in pyrolysis oil yield or HHV under the two atmospheres at 500 °C. The literature precedent suggests that N2 and CO2 provide inert atmospheres while H2, CO, and light hydrocarbons (i.e., CH4) provide reactive atmospheres. In recent years, microwave reactors have become increasingly popular for use in fast pyrolysis of biomass and waste feedstocks.15 Microwave irradiation provides rapid and efficient volumetric heating which is ideal for the pyrolysis process. Microwave reactors also allow for convenient testing of a variety of pyrolysis conditions in a laboratory scale batch process on the benchtop. In order to further investigate the potential trends in pyrolysis oil yield and composition under different atmospheres, a series of laboratory scale microwave pyrolysis (MWP) experiments have been performed under varying reactive gaseous atmospheres. The resulting pyrolysis

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EXPERIMENTAL SECTION

Microwave Pyrolysis of Biomass. Switchgrass pellets (5 × 10 mm) were provided by the McDonnell Farm (East Greenville, PA, U.S.A.). The model pyrolysis gas mixture (“PyGas”) was purchased from Air Liquide (Plumsteadville, PA, U.S.A.). Microwave pyrolysis experiments were performed using a bench scale Milestone RotoSYNTH microwave reactor equipped with an infrared temperature sensor and a 2 L quartz reaction vessel. To the reaction vessel was added 100 g of switchgrass and 10 g of activated charcoal (Darco, 20−40 mesh particle size, granular). The vessel was then evacuated and backfilled three times with the desired gas to properly deoxygenate the system prior to pyrolysis. The microwave power was set to 900 W for 1 min and then increased to 1000 W for 6 min. During pyrolysis, the products were collected using a condenser, cold water/liquid nitrogen traps, and a gas reservoir (Figure 1). The solid (biochar) and liquid (pyrolysis oil and water) yields were quantified gravimetrically. The volume of noncondensable gas produced was measured by displacement using a gas reservoir. Liquid products were isolated from the condenser and cold water/liquid nitrogen traps using organic solvent (acetone or methanol). Pyrolysis oil and water yields were determined using a Karl Fisher titrator. Experiments were performed in triplicate and the average product yields and distributions are reported. Characterization of Pyrolysis Oils. Elemental analysis of oils was performed on a Thermo EA1112 CHNS/O analyzer and oxygen content was calculated by difference. Higher heating values (HHV) of oils were determined using a Leco AC600 bomb calorimeter. Total acid number (TAN) was measured using a Mettler T70 titrator, 0.1 M KOH in 2-propanol as the titrant, and wet ethanol as the solvent. Water content was measured via Karl Fisher titration in methanol with Hydranal Karl Fisher Composite 5 as the titrant. Gas chromatography with mass spectroscopy (GC−MS) analysis of liquid products was performed on a Shimadzu GCMS QC-2010. The column used was a DB-1701, 60 m × 0.25 mm, 0.25 μm film thickness. The oven temperature was programmed to hold at 45 °C for 4 min, ramp at 3 °C/min to 280 °C, and hold at 280 °C for 20 min. The injector temperature was 250 °C, and the injector split ratio was set to 30:1. Helium carrier gas flowed at 1 mL/min. The GC−MS samples of pyrolysis oils were prepared in acetone and filtered through a 0.45 m PTFE filter prior to injection. Quantification of individual compounds was determined using response factors for authentic standards relative to an internal standard (fluoranthene). B



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RESULTS AND DISCUSSION Product Yield and Distribution. The goal of this work was to study the possible effects of varying gaseous atmospheres (N2, CH4, CO, H2, and a PyGas mixture shown in Table 1) on the yields and composition of pyrolysis products

observation of more pronounced effects on the products obtained. Pyrolysis Oil Composition. The composition of the pyrolysis oils obtained under the different gaseous atmospheres was characterized using a variety of analytical techniques. The reaction set up (Figure 1) allowed for the collection of two pyrolysis oil fractions. The first fraction was collected using a cold water (CW oils) condenser and contained the majority of the water (Figure 2) produced during pyrolysis as well as the

Table 1. Composition of Pyrolysis Gas (PyGas) Mixture Used N2

CO

H2

CO2

CH4

C2H4

30%

22%

20%

16%

10%

2%

obtained from microwave pyrolysis of switchgrass. The products have been categorized as biochar, organic liquids (bio-oil), water, or noncondensable gases. The biochar, bio-oil, and water have been directly quantified. The yields of noncondensable gases have been calculated using measured volumes and an estimated average molecular weight (MW = 24.2 g/mol). As shown in Table 2, microwave pyrolysis of switchgrass formed into 5 × 10 mm pellets provided consistent yields of all pyrolysis products for all atmospheres tested. However, differences in yield were obtained when using fine ground (2 mm) switchgrass pellets under identical conditions. Compared with using pelletized switchgrass, the yield of organic liquid decreased (∼5%) and the noncondensable gases increased (∼10%), whereas the yields of biochar and water remain unchanged under an N2 atmosphere. For the H2 atmosphere, large differences in yield were observed compared with the pelletized switchgrass. The organic liquid yield decreased significantly (by ∼10%) and the noncondensable gases increased dramatically (by ∼15−20%). Both the biochar and water yields decreased slightly (∼2−3%) as well. Differences in product distribution were also observed when comparing the yields of the pyrolysis of the ground pellets under N2 and H2 atmospheres (Table 2). Under a H2 atmosphere, the yield of organic liquids decreased (∼5%) and the noncondensable gases increased (∼5−10%) compared with the N2 atmosphere. A slight decrease (∼2−3%) in the yield of biochar and water was also observed. These results suggest that the particle size of the biomass is an important factor influencing the product yields. Furthermore, the particle size influences the role the atmosphere plays in the product distribution of microwave pyrolysis of switchgrass, with the ground material having a larger response to the variation in atmosphere, especially when going from N2 to H2. Lei et al. observed no significant effect on product yield or composition of microwave pyrolysis oils from corn stover when comparing particles sizes ranging from 0.5−4 mm.16 However, the comparison between the pelletized switchgrass (5 × 10 mm) and the fine ground pellets (2 mm) used in the present work provides a larger difference in particle size which has led to the

Figure 2. Fractionation of pyrolysis products for switchgrass pellets.

some of the less volatile or highly water-soluble products (i.e., phenols/cresols and acetic acid). The second fraction was collected using a double liquid nitrogen (LN2 oils) trap, which allowed for the condensation of volatile organic compounds (i.e., BTEX-type products). The most informative methods of characterization were elemental analysis and NMR spectroscopy because of the ability of these methods to represent the bulk material isolated. Elemental Analysis. The elemental analysis of the LN2 oils obtained from the pelletized switchgrass provided a readily observable trend in H and O content (Table 3). The H content increased from 5.58 under N2 atmosphere to 7.56 wt % under H2. The trend in observed H content was N2 < CO < CH4 < PyGas < H2. The ∼35% increase in H content led to an increase in the H/C ratio from 1.16 to 1.44. The H2 and PyGas atmospheres provided very similar H content and H/C ratios, which suggests that H2 was the most reactive component within the PyGas gas mixture. In addition, higher H content (and lower O content) was observed under CH4 than CO. Previous work by Zhang et al. using a fluidized bed reactor found that with CH4 as the fluidizing gas the pyrolysis oil obtained had a similar HHV value (17.2 MJ/kg) to that of the N2 control (17.8

Table 2. Product Yields and Distributions under Various Gas Atmospheres particle size

fine ground pellets

switchgrass pellets

gas atmosphere

N2

CO

CH4

H2

PyGas

N2

H2

PyGas

max. temp. (C) % biochar % organic liquids % water % NCG’s % mass balance

610 25.2 20.9 24.2 19.7 90.0

565 26.2 22.1 24.3 13.6 86.2

595 25.7 21.5 24.6 13.4 85.2

575 25.8 21.7 24.7 17.6 89.8

570 25.9 21.1 25.0 15.4 87.4

470 25.5 16.0 23.5 27.0 92.0

460 24.1 11.2 21.6 ≥33.1 ≥90.0

515 24.8 16.0 23.7 26.2 90.7

C



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Table 3. Elemental Analysis, Energy Content (HHV), and Total Acid Number (TAN) Data for Liquid Nitrogen Trap (LN2) Oils under Various Gas Atmospheres particle size


switchgrass pellets

gas atmosphere

N2

CO

CH4

H2

PyGas

N2

H2

PyGas

C (wt %, db) H (wt %, db) N (wt %, db) O (wt %, db) O/C H/C HHV (MJ/kg) TAN (mg KOH/g)

57.89 5.58 3.06 33.46 0.43 1.16 24.6 112

57.59 5.96 3.56 32.89 0.43 1.24 24.8 99

62.50 6.68 3.39 27.43 0.33 1.28 26.4 105

63.19 7.56 3.87 25.38 0.30 1.44 28.2 120

63.75 7.43 3.99 24.84 0.29 1.40 28.7 141

70.53 6.58 5.29 17.61 0.19 1.12 30.5 131

68.48 6.95 5.99 18.59 0.20 1.22 27.5 106

67.28 6.78 5.63 20.31 0.23 1.21 26.7 117

Table 4. 13C NMR Analysis of LN2 Oils from Switchgrass Pellets under Various Gas Atmospheres particle size


switchgrass pellets

gas atmosphere

N2

CO

CH4

H2

PyGas

N2

H2

PyGas

0−55 ppm (aliphatics) 55−95 ppm (alcohols/sugars) 95−165 ppm (aromatics) 165−180 ppm (acids/esters) 180−215 ppm (ketones/aldehydes)

36.9% 18.1% 36.4% 6.1% 2.6%

43.8% 8.9% 41.7% 4.9% 0.8%

35.2% 9.9% 45.0% 6.6% 3.3%

47.1% 10.1% 37.6% 4.9% 0.3%

36.4% 9.9% 47.3% 4.2% 2.1%

32.3% 7.3% 53.1% 6.0% 1.2%

31.0% 4.4% 59.0% 4.3% 1.2%

36.3% 6.9% 51.1% 4.2% 1.5%

Table 5. GC−MS Data for Liquid Nitrogen (LN2) Trap Oils particle size


switchgrass pellets

gas atmosphere

N2

CO

CH4

H2

PyGas

N2

H2

PyGas

BTEX (wt %) naphthalenes (wt %) phenol (wt %) cresols (wt %) acetic acid (wt %) levoglucosan (wt %)

2.22 0.21 0.59 0.55 1.10 1.22

2.73 0.09 0.37 0.44 0.97 1.43

2.83 0.09 0.35 0.35 1.40 2.08

2.54 0.08 0.37 0.38 1.81 2.36

2.50 0.11 0.40 0.43 0.83 1.10

6.13 0.42 0.67 0.66 1.20 0.48

9.41 0.91 0.90 0.94 0.88 0.37

5.32 0.35 0.44 0.48 1.64 0.66

suggest a dehydration-type mechanism. The ∼5−15% increase in C content (depending on atmosphere) also led to lower H/ C ratios (1.20 ± 0.05), which were most similar to those obtained from pelletized switchgrass under an N2 atmosphere. The results of the elemental analysis (and bomb calorimetry) clearly displays the effects of varying the gaseous atmosphere and particle size on the overall composition of oils obtained via microwave pyrolysis. 13 C NMR Spectroscopy. 13C NMR analyses of the LN2 oils provided information about the relative amounts of various types of compounds present (Table 4). The oils obtained from pelletized switchgrass under a N2 atmosphere contained the lowest percentage of aliphatic (36.9%) and aromatic (36.4%) compounds, but the greatest percentage of alcohols/sugars (18.1%), which is consistent with elemental analysis results of the highest O content and lowest H content. The H2 and CO atmospheres produced oils with the greatest percentages of aliphatic compounds (47.1 and 43.8%, respectively) and the lowest percentages of ketones/aldehydes (0.3 and 0.8%, respectively) which is consistent with the higher H content found for the H2 case. The PyGas and CH4 atmospheres produced oils with the greatest percentage of aromatic compounds (47.3 and 45.0%, respectively), but lower percentages of aliphatic compounds (36.4 and 35.2%, respectively). With the exception of the oils produced under the N2 atmosphere, all of the oils contained approximately the same percentage of alcohols and sugars (9.7 ± 0.5%). In addition, all oils produced contained approximately the same

MJ/kg), whereas the use of CO and H2 provided oils with greater HHV values (23.7 and 24.4 MJ/kg, respectively).12 However, CH4 has been shown to possess modest hydrogenation activity when applied to the pyrolysis and liquefaction of coal in a small batch bomb reactor.17 Concurrent to the increase in H content, a corresponding decrease in O content was also observed when varying the gaseous atmosphere. The O content decreased from 33.46 wt % under N2 atmosphere to 24.84 wt % under the PyGas mixture. The trend in observed O content (N2 > CO > CH4 > H2 ≥ PyGas) displays an inverse relationship to the aforementioned trend in H content. The ∼25% decrease in O content has led to a decrease in the O/C ratio from 0.43 to 0.29, which is consistent with a hydrogenation-type process. Once again, the H2 and PyGas atmospheres provided very similar O content and O/C ratio, which further supports the prevalent reactivity of H2 within the PyGas mixture. The improved deoxygenation under the H2 and PyGas atmospheres was further supported by increased experimental HHV values (28.2 and 28.7 MJ/kg, respectively) relative to the N2 control (24.6 MJ/kg). The LN2 oils obtained from the ground switchgrass pellets displayed large differences in O and C content in comparison to the corresponding pelletized switchgrass elemental analysis results discussed above. A ∼20−40% decrease in O content (depending on atmosphere) was obtained for the LN2 oils from ground pellets, which led to the lowest O/C ratios (0.22 ± 0.01) in the present work. However, the decrease in O content was not accompanied by an increase in H content, which may D



Research Article

Table 6. GC−MS Data for Cold Water (CW) Trap Oils particle size


switchgrass pellets

gas atmosphere

N2

CO

CH4

H2

PyGas

N2

H2

PyGas

BTEX (wt %) naphthalenes (wt %) phenol (wt %) cresols (wt %) acetic acid (wt %) levoglucosan (wt %)

0.19 0.13 0.88 0.84 1.46 0.36

0.10 0.05 0.64 0.72 2.29 0.38

0.12 0.06 0.85 0.86 2.91 0.45

0.16 0.08 0.77 0.78 2.50 0.52

0.09 0.05 0.69 0.74 1.97 0.32

0.21 0.19 0.59 0.52 3.17 0.63

0.19 1.11 1.59 1.51 1.01 0.06

0.17 0.11 0.36 0.37 3.32 0.64

percentage of acids and esters (5.3 ± 1.0%), which correlates well to the corresponding TAN measurements. The oils obtained from the ground switchgrass pellets under a N2 atmosphere contained a similar percentage of aliphatic compounds (32.3%) as the LN2 oils obtained from the pelletized switchgrass (36.9%). However, the percentage of aromatic compounds contained in the oils from the ground switchgrass pellets under a N2 atmosphere increased significantly (∼45% increase). The oils obtained from ground switchgrass pellets under a H2 atmosphere contained the highest percentage of aromatic compounds (59.0%) and the lowest percentage of aliphatic compounds (31.0%). On the other hand, the oils obtained from the pelletized switchgrass under a H2 atm contained the highest percentage of aliphatic compounds (47.1%) and a lower percentage of aromatic compounds (37.6%). The differences observed between the particle sizes suggest the possibility of different deoxygenation mechanisms. GC−MS Analysis. GC−MS methods were used to quantify select compounds (i.e., BTEX, naphthalenes, phenols, acetic acid, and levoglucosan) in the two oil fractions (Tables 5 and 6). The concentration of BTEX compounds in the LN2 oils obtained from pelletized switchgrass was found to be 2.56 (±0.24) wt % depending on the atmosphere (Table 5), whereas the concentration in the CW oils was found to be 0.13 (±0.04) wt % (Table 6). The 25-fold difference between the two oil fractions illustrates the ability of the collection system to provide the necessary fractionation. The concentration of BTEX compounds in the LN2 oils obtained from the ground switchgrass pellets under a N2 atmosphere was found to be 6.13 wt %. Despite the lower oil yield using the ground pellets, the 2-fold increase in concentration still provides ∼80% increase in the yield of BTEX compounds. The concentration of BTEX compounds in LN2 oils obtained from ground pellets under a H2 atmosphere was found to be 9.41 wt %, which is ∼4-fold increase compared to results with the pelletized switchgrass under the same atmosphere. The BTEX concentration is also ∼55% greater than the N2 control with the ground pellets. The concentration of BTEX compounds in LN2 oils obtained from ground pellets under the PyGas atmosphere was found to be 5.32 wt %, which is similar to the results obtained under an N2 atmosphere. The concentration of BTEX in the CW oils obtained from the ground pellets was found to be 0.19 (±0.02) wt % under N2, H2, and PyGas atmospheres, which is slightly higher than what was observed for the CW oils from the pelletized switchgrass. The concentration of naphthalenes (naphthalene, 1-methylnaphthalene, and 2-methylnaphthalene) in both the LN2 and CW oils obtained from pelletized switchgrass was found to be 0.10 (±0.05) wt % depending on the atmosphere. However, the concentration of naphthalenes from ground pellets under a N2 atmosphere was found to be 0.42 wt % in the LN2 oils and 0.19

wt % in the CW oils. The concentration of naphthalenes in oils obtained from ground pellets increased to 0.91 wt % in the LN2 oils and 1.11 wt % in the CW oils under an H2 atmosphere. This result suggests that the H2 atmosphere has a larger effect on the naphthalenes produced than the change in particle size. The concentration of phenol/cresols in oils obtained from pelletized switchgrass was found to be 0.84 (±0.17) wt % in the LN2 oils and 1.56 (±0.16) wt % in the CW oils, depending on the atmosphere used. The concentration of phenol/cresols from ground pellets under a N2 atmosphere was found to be 1.33 wt % in the LN2 oils and 1.11 wt % in the CW oils. The small observed variation between the pellets and the ground pellets was within error in this particular case. However, the phenol/cresols concentration obtained from ground pellets under a H2 atmosphere was found to be 1.84 wt % in the LN2 oils and 3.10 wt % in the CW oils, which represents a significant increase when compared to the ground pellets under a N2 atmosphere. This observation combined with the increase in BTEX yield suggest that a similar aromatization (dehydrogenation) type mechanism to that observed in TGRP may be operative for the MW pyrolysis of the small particle material. The concentration of acetic acid and levoglucosan in the LN2 and CW oils from the pelletized switchgrass does not vary much when using different atmospheres. However, differences are observed using the ground pellets. The concentration of levoglucosan in the CW oils (0.06 wt %) from the ground pellets under an H2 atmosphere is much lower than any other tested conditions. In addition, the concentration of acetic acid in the CW oils (3.24 ± 0.11 wt %) produced under N2 and PyGas atmospheres was significantly higher than other tested conditions. Regardless of conditions, there was very little variation observed in the total acid number (TAN) of the pyrolysis oils obtained (116 ± 14 mg KOH/g). The results discussed above help to illustrate the effects of both the reaction atmosphere and the particle size on the yields and composition of pyrolysis oils. On one hand, the reaction atmosphere appears to have only a small effect on the yield and product distribution for the microwave pyrolysis of switchgrass pellets. On the other hand, particle size (pelletized vs ground pellets) appears to have a pronounced effect on the yields of organic liquids and product distribution. The greater external surface area of the ground switchgrass could allow for improved contact with the exogenous microwave absorber (activated charcoal), which may lead to an increase the rate of heat transfer and subsequently the overall heating rate.18 This proposed increase in heating rate is consistent with the observed increase in both hydrocarbon and noncondensable gas yield. The composition of the pyrolysis oil was effected by both the reaction atmosphere and the particle size. The elemental composition of the oils obtained under reactive gas atmospheres display trends consistent with improved deoxygeE



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Figure 3. Particle size dependent deoxygenation pathways.

(which contains 20% H2) atmospheres provided the most deoxygenated pyrolysis oils (∼25% decrease in O content), which one would expect given the reductive nature of the gases. However, the CH4 atmosphere also produced oils with decreased O content (27.43%) relative to the N2 control (33.46%), whereas the CO atmosphere (32.89%) was observed to provide negligible deoxygenating effects. The lower external surface area of the pelletized switchgrass led to similar results under both the H2 atmosphere and the PyGas (20% H2) atmosphere, which suggests that the surface area may be the limiting factor in the deoxygenation reaction. On the other hand, the ground switchgrass provides increased external surface area and displays greater deoxygenation under all atmospheres tested (N2, H2, and PyGas). Although the H2 atmosphere affected the deoxygenation of both particle sizes, it did so by different pathways. For the pelletized switchgrass, the deoxygenation was aided by a hydrogenation-type mechanism that produced more aliphatic groups and an increase in the H/ C ratio of the bio-oil produced. Conversely, the ground switchgrass response to microwave pyrolysis under an H2 atmosphere was to produce more aromatic compounds and a negligible change in the H/C ratio, which may suggest a dehydration and aromatization type mechanism. These results suggest that the preferred deoxygenation pathway is governed by the surface area of the feedstock and/or the heating rate of pyrolysis.

nation as expected. However, the use of a smaller particle size also provided oils with lower O content compared to results with the pelletized material. The main drawback of the smaller particle size seems to be the decrease in yield of organic liquids and the increase in yield of noncondensable gases. However, the use of ground pellets and a reducing atmosphere (H2) provided the highest quality pyrolysis oil, but promising results were also obtained with the pelletized switchgrass under CH4 and the PyGas gas mixture. Interestingly, the effect of the H2 atmospheres appears to promote deoxygenation by different mechanisms for the ground switchgrass compared with the pellets (Figure 3). In the case of the pellets, the H content of the pyrolysis oil increases, as described above, while in the case of the ground material the H2 atmosphere appears to promote aromatization, producing more BTEX compounds and decreasing the H/C ratio. The aromatization path is consistent with the role of the reducing atmosphere during TGRP, where the product is rich in aromatic hydrocarbon and phenols. However, higher concentrations of aromatic compounds were obtained under the H2 than under the PyGas atmosphere, which suggests that the lower H2 concentration in the PyGas mixture (20% H2) may be a limiting factor in the aromatization process when using the ground material under MW conditions. In comparison to the TGRP process,10 microwave pyrolysis provides very different product distributions and oil compositions, as might be expected due to the differences in heating mechanisms. Microwave pyrolysis of switchgrass produced significantly more biochar (∼85% more) and less organic liquid (∼35−50% less) than the TGRP process. The microwave pyrolysis (LN2) oils obtained had lower C content (∼15−20% lower), but higher H content (∼15−30% higher) than the oils obtained from the TGRP process. This leads to a large difference in H/C ratio between the two processes (TGRP = 0.84 and MWP = 1.25 ± 0.12). In addition, the TGRP process produced oils with much lower O content (∼30−50% lower) than those obtained via microwave pyrolysis. However, one similarity was that when lower O content bio-oils were produced by MWP, as was the case for the ground material, it was due to an increase in the aromatic nature of the bio-oil and subsequently an increase in the concentration of BTEX compounds.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01016. Tables of quantitative compound concentrations (GC− MS) for bio-oil products (PDF)

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AUTHOR INFORMATION

Corresponding Author

*C. A. Mullen. Telephone: +1-215-836-6916. E-mail: charles. [email protected].

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Notes

The authors declare no competing financial interest.

CONCLUSIONS Bench scale microwave pyrolysis experiments have been carried out under various reactive gaseous atmospheres in order to further probe the role of reaction atmosphere on noncatalytic pyrolysis of biomass. The reactive gases used in these studies are common gaseous products (CO, CH4, H2, and PyGas gas mixture) found in the noncondensable gas component obtained from fast pyrolysis of lignocellulosic biomass. The particle size of switchgrass used greatly influenced the yield and composition of the bio-oil and also its response to the reactive atmospheres. For pelletized switchgrass, the H2 and PyGas

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ACKNOWLEDGMENTS The authors thank Dr. Gary Strahan for NMR experiments. We also thank Patrick West and Jennifer Gallup, ERRC co-op students from Drexel University for technical assistance. Funding from USDANIFA-BRDI Grant No. 2012-1000820271 is hereby acknowledged. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of F



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Agriculture. USDA is an equal opportunity provider and employer.

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REFERENCES

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