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switchgrass blends for the production of bio-oil enriched with crotonic acid
Journal of Analytical and Applied Pyrolysis 107 (2014) 40–45
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Mild pyrolysis of P3HB/switchgrass blends for the production of bio-oil enriched with crotonic acid夽 Charles A. Mullen a,∗ , Akwasi A. Boateng a , Dirk Schweitzer b , Kevin Sparks b , Kristi D. Snell b a Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 600 E. Mermaid Lane, Wyndmoor, PA 19038, United States b Metabolix Inc., 21 Erie Street, Cambridge, MA 02139, United States
a r t i c l e
i n f o
Article history: Received 24 September 2013 Accepted 31 January 2014 Available online 7 February 2014 Keywords: P3HB Crotonic acid Pyrolysis Switchgrass
a b s t r a c t The mild pyrolysis of switchgrass/poly-3-hydroxybutyrate (P3HB) blends that mimic P3HB-producing switchgrass lines was studied in a pilot scale fluidized bed reactor with the goal of simultaneously producing crotonic acid, a switchgrass-based bio-oil, and bio-char. Factors such as pyrolysis temperature, reactor residence time, flow rate and particle size of the P3HB were studied to determine their effects on the recovery of crotonic acid as a component of the pyrolysis oil produced from the mixture. Crotonic acid yields were maximized at 45 wt% of the input P3HB by using small P3HB particles and a pyrolysis temperature of 375 ◦ C. The remaining components of the liquid product were similar to those produced via fast pyrolysis of switchgrass alone. Fractional collection within the condensation system of the pyrolysis process development unit (PDU) did not significantly fractionate crotonic acid more than the total liquids collected. Concentrations of 6 to 10 wt% crotonic acid in the liquids were found in all fractions and crotonic acid was effectively collected by both condensation and electrostatic precipitation suggesting that pyrolysis of P3HB produces crotonic acid in both gas and aerosol phases. Published by Elsevier B.V.
1. Introduction Decreases in petroleum resources and increases in costs have put pressure on markets for products produced from petroleum including fuels, chemicals and plastics. These forces combined with environmental issues related to non-biodegradable petroleum-based plastics have spurred research into bio-based alternatives. Polyhydroxyalkanoates (PHAs) are a broad family of naturally occurring polyesters that possess physical properties making them suitable replacements for many petroleum derived plastics. They are produced by some microorganisms as a method of carbon and energy storage [1,2]. The production of of poly-3-hydroxybutyrate (P3HB), the simplest member of the PHA family, can be achieved in non-native bacterial producers
夽 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 Agriculture. USDA is an equal opportunity provider and employer. ∗ Corresponding author. Tel.: +1 215 836 6916; fax: +1 215 233 6559. E-mail address: [email protected] (C.A. Mullen). 0165-2370/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jaap.2014.01.022
as well as plants by expressing bacterial genes encoding the P3HB biosynthetic enzymes. Recent advances have led to the production of P3HB in genetically modified switchgrass at levels up to 7.7% dwt in sections of leaf tissue [3]. In addition to being utilized as a renewable, biodegradable material, P3HB can be thermally degraded to crotonic acid in high yield via pyrolysis [4–8].
The selective production of crotonic acid from P3HB using a mild pyrolysis process has been proposed to occur via a random chainscission beta elimination reaction at temperatures above 200 ◦ C [7] to produce cis-crotonic acid which then isomerizes to the more thermodynamically stable trans-crotonic acid.
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Crotonic acid can be used as a platform chemical providing an alternative route to high-valued industrial and fine chemicals. For example, it can be reduced to n-butanol for chemical or fuel applications or oxidized to maleic anhydride, a precursor of unsaturated polyesters. Other targets available through simple transformations include propylene, the precursor to polypropylene, and acrylic acid, a chemical used to produce superabsorbent polymers. Pyrolysis has also been demonstrated as a very efficient liquefaction method for biomass such as switchgrass producing bio-oil or biocrude [10–18]. Pyrolysis liquids are potential intermediates for the production of renewable hydrocarbon fuels and renewable chemicals. Pyrolysis of P3HB-producing switchgrass is thus a potentially attractive process that could simultaneously produce crotonic acid, bio-oil, and bio-char. Previously, the co-pyrolysis of willow biomass and P3HB at 450 ◦ C was reported by Cornelissen et al. [19,20], with the goal of utilizing high levels of P3HB as a co-reactant to improve the quality of pyrolysis oil. We report in this contribution on the mild pyrolysis of switchgrass/P3HB blends with the goal of maximizing crotonic acid recovery while also producing pyrolysis oil and bio-char. To study the potential of a mild pyrolysis process, switchgrass/P3HB blends that mimic P3HB-producing switchgrass lines were subjected to pyrolysis in a pilot scale fluidized bed reactor. Factors such as pyrolysis temperature, residence time, flow rates and particle size of the P3HB were studied for their effects on the recovery of crotonic acid as a component of the pyrolysis oil. Effects of the presence of P3HB and crotonic acid on the chemical composition of the switchgrass bio-oil were also examined. 2. Methods and materials 2.1. Materials Switchgrass was provided by the McDonnell Farm in East Greenville, PA. The switchgrass was ground and sieved to 2 mm
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using a Wiley mill and dried in an oven overnight at 80 ◦ C before use. After drying the switchgrass moisture content was 2 to 3 wt%. P3HB polymer and lime (CaO) were provided by Metabolix, Inc. Two particle sizes of P3HB were used: a fine powder (particle size <0.15 mm in diameter) and a coarser material (particle size ranged from 0.177 to 0.354 mm in diameter). Blends of switchgrass, P3HB and CaO (where applicable) were prepared by physically mixing them in a V-mixer.
2.2. Pyrolysis experiments Pyrolysis experiments were performed using the fluidized bed pyrolysis process development unit (PDU) at USDA’s Eastern Regional Research Center (ERRC). The reactor system has been well described by Boateng et al. [10,21]. A schematic of the PDU is provided in Fig. 1. The reactor bed consisted of a 7.6 cm diameter pipe which was filled to a depth of 20 cm with 800 m silica sand serving as a fluidizing medium. For blends of switchgrass and P3HB the pyrolysis temperature was 350 to 375 ◦ C. Bio-char removal from the vapor stream was accomplished by cyclone separation. Pyrolysis liquids were collected in five fractions via four condensers cooled with ∼4 ◦ C water in series followed by an electrostatic precipitator (ESP). Pyrolysis product yield distribution was determined gravimetrically and corrected for material imbalance caused by product deposition in the system by using a nonlinear programming optimization model. This was previously developed to adjust the experimental data to achieve closed balances without losing the overall representation of the pyrolysis process while keeping within the law of conservation of mass [22]. Non-condensable gas (NCG) composition was measured online using an Agilent 3000 MicroGC.
Fig. 1. Schematic of ERRC pyrolysis PDU.
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Table 1 Experimental conditions and yields for pyrolysis of switchgrass/P3HB blends. Experiment number
SG FPa
SG MPb
1
2
3
4
5
6
7
8
Reactor temperature % feed switchgrass % feed P3HB % feed CaO P3HB particle sizec N2 flow Liquid yield Char yield Gas yield Concentration crotonic acid in liquid (wt%)d Crotonic acid yield (wt%)e
480 100 0 0 N/A 75 62.3 [69.5] 9.8 [15.6] 11.6 [14.9] 0
375 100 0 0 N/A 75 36.1 [57.5] 29.5 [30.3] [12.2] 0
350 90 10 0 Fine 75 40.1 [42.0] 17.6 [24.9] [33.2] 8.5
375 90 10 0 Fine 75 48.4 [50.1] 15.2 [20.5] [29.5] 9.3
375 85.5 9.5 5.0 Fine 75 16.8 19.9 nd 0.5
375 90 10 0 Coarse 70 41.1 [49.5] 9.6 [20.7] [29.8] 6.3
350 90 10 0 Coarse 60 45.8 [46.9] 19.6 [20.7] [27.5] 8.9
375 90 10 0 Coarse 60 47.4 [40.5] 24.4 [21.0] [30.2] 8.4
375 90 10 0 Fine 60 45.2 [46.3] 13.8 [21.9] [31.9] 9.6
375 75 25 0 Fine 60 47.8 [58.0] 14.6 [17.8] [24.3] 22.1
N/A
N/A
34.3 [35.8]
45.2 [46.8]
0.9
25.9 [31.4]
40.6 [41.5]
39.7 [40.6]
43.5 [44.6]
42.2 [51.2]
a b c d e
Switchgrass fast pyrolysis control. Switchgrass mild pyrolysis control. Coarse means most particles between 0.177 and 0.354 mm. Composite of all liquid fractions. Values in brackets are based on corrected yields via mass balance optimization model [16]. Mass percentage of P3HB collected as crotonic acid in the liquid products.
2.3. Product characterization Water content was measured using Karl–Fischer titration in methanol with Hydranal Karl-Fischer Composite 5 (Fluka) used as titrant. GC with mass spectrometry (MS) detection analysis of pyrolysis oil was performed on a Shimadzu GCMS QC-2010 and was used to quantify several pyrolysis oil components including crotonic acid. The column used was a DB-1701, 60 m × 0.25 mm, 0.25 m 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. The flow rate of the He carrier gas was 1 mL/min. The pyrolysis oil samples for GC analysis were prepared as 3 ± 1 wt% solutions in acetone which were filtered through 0.45 m polytetrafluoroethylene (PTFE) filters prior to injection. For quantification of individual pyrolysis oil compounds, response factors relative to the internal standard, fluroanthene, were determined using authentic compounds [11]. 3. Results and discussion Blends of switchgrass and P3HB were pyrolyzed in a series of several experiments that are summarized in Table 1. In most experiments the blends contained 90% switchgrass and 10% P3HB, with the exception of experiment 8 where the proportion of P3HB was increased to 25%. Also, in one instance, lime (CaO) was added as a 5 wt% loading to the mixture to explore its potential in assisting P3HB thermal decomposition (experiment 3). The blends were pyrolyzed at temperatures of 350 to 375 ◦ C. Two particle size cuts (fine and coarse) of the P3HB particles were used along with three different carrier gas flow rates. The latter enabled the testing of the effect of the residence time of the P3HB particles in the reactor, with the lower flow rate providing more residence time. Residence times are estimated to be 0.12 s at 60 L/min N2 flow and 0.09 s at 75 L/min [10]. For comparison, two control experiments with switchgrass alone were also included, one at traditional fast pyrolysis conditions (480 ◦ C) and one at the mild pyrolysis conditions (375 ◦ C) employed for the P3HB/switchgrass blends. The distribution of the post-pyrolysis material in solid, liquid and non-condensable gas fractions is also provided in Table 1. Actual recoveries are provided along with values corrected for material retained in the system by a non-linear optimization mass balance model, as described in Section 2 [22]. Compared with the fast pyrolysis of switchgrass (480 ◦ C) which produced 62.3% liquid, the yield of liquid (bio-oil) in all of the mild pyrolysis experiments was found to be lower (Table 1). For the mild, lime-free pyrolysis of
switchgrass/P3HB blends, greater liquid yields (40 to 48 wt%) were achieved compared with the mild pyrolysis control experiment containing switchgrass alone (36%). Conversely, the yield of solid products (bio-char) from the switchgrass/P3HB blends (9.6 to 24.4%) was increased compared to the fast pyrolysis of switchgrass alone (9.8%) but decreased relative to the mild pyrolysis of switchgrass alone (∼30%). These results suggest that both the use of lower temperatures, which are necessary to prevent further degradation of crotonic acid, and the presence of P3HB and/or crotonic acid in the reaction mixture influence the product distribution. Use of pyrolysis temperatures lower than 450 ◦ C for biomass pyrolysis has previously been shown to result in decreased bio-oil and increased bio-char production [12]. The total yield of gaseous products is calculated to be higher in the experiments with the switchgrass/P3HB blends, compared to the mild and fast pyrolysis control experiments. This could be due to the increased acidity of the environment resulting in increased decarboxylation and decarbonylation type reactions of the biomass derived materials. The composition of the gases produced with the P3HB/switchgrass blends showed an increase in the ratio of CO2 /CO compared with the control fast pyrolysis of switchgrass (Table 2) but similar to the mild pyrolysis control. Decomposition of crotonic acid into CO2 and propylene [23] could also contribute to the increased gas yield, but propylene was not observed. However, its concentration may be too small to detect by the GC method employed since it is diluted with the N2 fluidizing gas. Crotonic acid decomposition could however not produce enough gas to explain the difference between the gas yield in the switchgrass fast pyrolysis control experiment and the experiments with the P3HB-switchgrass blends suggesting that indeed the increased acidity of the environment does help to convert some condensable range products to permanent gases. The concentration of crotonic acid in the liquid pyrolysis oil products ranged from 6.3 wt% to 9.6 wt% for the lime-free pyrolysis of the 9/1 switchgrass/P3HB blends, corresponding to the yields of crotonic acid in the liquid phase ranging from 25.9 wt% to 45.2 wt% Table 2 Composition of non-condensable gas (vol%) for pyrolysis of switchgrass/P3HB blends, selected experiments. Experiment
SG FPa
SG MPb
4
5
7
H2 CO CO2 CH4
5.1 57.6 29.5 7.8
1.5 44.4 48.4 5.6
1.0 44.7 48.7 5.5
1.5 44.4 48.5 5.6
0.2 36.5 59.6 3.7
a b
Switchgrass fast pyrolysis control (480 ◦ C). Switchgrass mild pyrolysis control (375 ◦ C).
C.A. Mullen et al. / Journal of Analytical and Applied Pyrolysis 107 (2014) 40–45
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Table 3 Concentration of pertinent switchgrass derived pyrolysis oil components (GC/MS, wt%), total liquids. Experiment
SG FPa
SG MPb
1
2
4
5
6
7
8
Water Acetic acid Acetol Levoglucosan Phenol Cresols Guaiacol 4-Methyl guaiacol Syringol
13.9 4.29 2.35 4.33 0.24 0.24 0.18 0.07 0.20
12.3 6.68 2.18 4.03 0.09 0.08 0.10 0.12 0.11
24.3 3.85 2.73 4.41 0.09 0.09 0.11 0.11 0.09
22.4 3.85 2.48 4.98 0.09 0.11 0.09 0.09 0.13
28.6 3.77 2.96 3.20 0.14 0.13 0.25 0.22 0.22
14.6 4.01 2.19 2.95 0.09 0.16 0.16 0.10 0.11
18.6 4.12 1.97 2.65 0.07 0.08 0.13 0.13 0.11
10.2 5.85 2.66 2.30 0.11 0.09 0.18 0.18 0.16
13.1 3.51 1.26 2.41 0.09 0.12 0.07 0.08 0.07
a b
Switchgrass fast pyrolysis control (480 ◦ C). Switchgrass mild pyrolysis control (375 ◦ C).
(or 31.4–46.8 wt% when using modified liquid yields). The highest concentration of crotonic acid in the liquid products and the highest yields were obtained when finer sized P3HB particles were used and the pyrolysis was performed at 375 ◦ C (experiments 2 and 7). The difference between these two experiments was the N2 flow rate; the one with a lower flow rate of 60 L/min (experiment 7) resulted in a longer residence time in the reactor than the one employing a higher carrier gas rate of 75 L/min, however, little difference was seen in the yield of crotonic acid. The fluidizing gas flow rate and thus the residence time made a more significant difference when the coarser P3HB particles were employed. In these cases with the coarse P3HB particles the yield of crotonic acid in the liquids increased from about 26 wt% to about 40 wt% when the flow rate of N2 was decreased from 70 L/min (experiment 4) to 60 L/min (experiments 5 and 6), suggesting that the larger particles do not fully depolymerize under the short residence time conditions. In the experiment incorporating lime (CaO) into the feed (experiment 3), the observed yield of crotonic acid was nearly zero, likely due to its decarboxylation [23]. The presence of lime also had the undesired effect of fouling the reactor by fusing the sand particles in the bed, resulting in a loss of fluidization. When the level of P3HB included in the blend was increased to 25%, the concentration of crotonic acid found in the liquid phase products increased, as expected. However, the yield of crotonic acid remained at about the same level of 42% (51% of corrected liquid yield). In each case, the biochar was analyzed for crotonic acid and residual P3HB; only ppm levels were detected which did not significantly affect the overall recoveries. The components other than crotonic acid of the very complex liquid mixture are those that result from the breakdown of the other biopolymers present in switchgrass: cellulose, hemicellulose and lignin. Table 3 provides a comparison of the concentrations of some of the most abundant of the hundreds of components that make up switchgrass pyrolysis bio-oils for the experiments done here and those done under traditional fast pyrolysis conditions (control). The major components of the pyrolysis oil from the switchgrass/P3HB blend were the same as the control indicating that the presence of P3HB does not result in any major change in the composition of the bio-oil, other than the presence of crotonic acid. There were some minor differences in the concentrations of some individual components observed. The concentration of acetol was mostly similar to those found in fast pyrolysis oils of switchgrass, but the acetic acid concentrations were decreased compared with the mild pyrolysis control experiment, perhaps suggesting that there is some interaction between these two carboxylic acids (acetic- and crotonic acid) in the product. The levoglucosan concentration was lower in experiments 4–8, where the lower rates of N2 carrier gas flow were used, but was similar to the control experiments with the higher gas flow rate. This could be a small effect of the presence of the crotonic acid combined with the longer residence time associated with the lower N2 flow rates, but residence time effects alone cannot be
ruled out. The phenolic compounds (e.g. phenol, cresols, guaiacols) derived from lignin are found in lower concentrations in all of the mild pyrolysis experiments compared to the fast pyrolysis control, but likely due to the less effective conversion of lignin at lower temperatures. As shown in Fig. 1, the ERRC pyrolysis PDU fractionates condensable vapors in its bio-oil recovery system. Liquid products are therefore collected as five fractions. Table 4 presents the distribution of the pyrolysis liquids into the five collection fractions, the concentration of crotonic acid in each fraction and the fractionation of the crotonic acid produced. In most cases, there was not much variation in the concentration of crotonic acid in the collected fractions. For the experiments done with a 10% blend of P3HB, the concentration of crotonic acid in the individual fractions ranged from 6 to 11 wt%, with the exception of experiment 4 which had a lower crotonic acid yield and its fractions had crotonic acid concentrations of 4 to 8 wt%. Because the concentration of crotonic acid varies only to a small extent, its fractionation of closely tracks with how the overall liquid fractionates. In most cases, the largest fractions collected are those at the beginning and the end of the condenser train, which are the first condenser and the ESP. The liquid collected via the first condenser accounted for ∼30 wt% of the total in each of the experiments with the 10% P3HB blend (experiments 1–2 and 4–7), and the fraction of the total crotonic acid produced collected at the first condenser ranged from 26 to 46%. The ESP collected similar amounts, with the exception of experiments 5 and 6 where the larger particle P3HB was used in conjunction with the lower flow rate. In these experiments, very little of the pyrolysis liquids or the crotonic acid were collected via ESP, and larger fractions were collected via the third and fourth condensers. The lower flow rates lead to a longer residence time in the chilled zone of each condenser likely resulting in an increase in the amounts collected in the condensers. However, why the trend does not hold when the finer P3HB is used at the low flow rate (experiment 7), even for switchgrass derived products, is unclear. When the amount of P3HB in the blend was increased to 25% (experiment 8), the first condenser and ESP still collected the most crotonic acid, but the remaining portion was more evenly distributed among the second, third and fourth condenser fractions. While the vapor temperature is decreased along the condensation train, pyrolysis liquids collected do not follow a strict boiling point distribution among the fractions. This is due to a variety of reasons including the fact that the pyrolysis vapors consist of both true gases (not solids or liquids entrained in the vapor stream) which are condensed and aerosols which are best collected by the electrostatic precipitator [24]. Solubility interactions among the components also play a role in how pyrolysis oils fractionate. Fig. 2 compares the concentration of crotonic acid in each fraction with some of the major components of the pyrolysis liquids derived from switchgrass: water, acetic acid, acetol and levoglucosan. Each of these components is water soluble. The concentration of acetic
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Table 4 Fractionation of crotonic acid (CA) in pyrolysis liquids. Experiment
Cond. 1
Cond. 2
1
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (%) Fraction of P3HB collected as CA (%)
34.9 11.4 46.6 16.0
19.7 7.1 16.4 5.6
Cond. 3 7.2 6.7 5.7 2.0
Cond. 4 3.2 6.3 2.4 0.8
ESP 34.9 7.1 29.0 9.9
2
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (wt%) Fraction of P3HB collected as CA (%)
29.0 11.2 34.8 15.7
14.5 7.5 11.6 5.2
5.6 6.9 4.2 1.9
25.0 9.5 25.5 11.5
25.8 8.7 23.9 10.8
4
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (wt%) Fraction of P3HB collected as CA (%)
29.7 6.0 28.2 7.3
15.8 4.0 10.1 2.6
3.2 5.9 3.0 0.8
18.4 6.7 19.5 5.1
32.9 7.5 39.2 10.2
5
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (wt%) Fraction of P3HB collected as CA (%)
30.3 7.7 26.3 10.7
27.0 10.5 31.9 13.0
11.2 9.0 11.4 4.6
28.3 8.3 26.5 10.8
3.3 10.7 4.0 1.6
6
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (wt%) Fraction of P3HB collected as CA (%)
29.4 8.3 29.1 11.6
14.3 10.0 17.1 6.8
23.8 9.1 26.0 10.3
29.4 6.9 24.3 9.7
3.2 9.4 3.6 1.4
7
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (wt%) Fraction of P3HB collected as CA (%)
30.6 10.2 32.5 14.1
15.0 7.3 11.3 4.9
6.3 7.9 5.2 2.3
9.2 11.4 10.9 4.7
38.8 10.0 40.1 17.4
8
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (wt%) Fraction of P3HB collected as CA (%)
25.5 24.8 28.7 12.1
14.8 21.3 14.2 6.0
10.7 20.7 10.1 4.3
10.7 24.0 11.6 4.9
38.3 20.4 35.4 14.9
Concentration of CA is wt% of CA in liquid collected in each condensation point measured by GC/MS. Fraction of CA collected is the percentage CA condensed in that fraction. Fraction of P3HB collected as CA is calculated by the line above multiplied by CA yield from Table 1.
acid tracks very closely with the amount of water in each fraction while that of crotonic acid, although also a carboxylic acid, does not track as closely with the water content. In most cases, the water and acetic acid concentrations are lowest in the ESP fraction, which is
the last collection point. In contrast the crotonic acid concentration is higher in the ESP fraction than it is in the condenser fractions in several cases despite its lower volatility (crotonic acid boiling point (bp) = 185 ◦ C, acetic acid bp = 118 ◦ C). Acetol (bp = 145 ◦ C)
30
20
15
10
5
Experiment 1
Experiment 2
Experiment 4 Cond 1
Cond 2
Experiment 5 Cond 3
Experiment 6 Cond 4
Experiment 7
ESP
Fig. 2. Concentration of some pyrolysis oil components by fraction. CA = crotonic acid, AA = acetic acid, LG = levoglucosan.
Experiment 8
LG
AA
Acetol
CA
H20
LG
AA
Acetol
H20
LG
CA
AA
Acetol
H20
LG
CA
AA
Acetol
H20
LG
CA
Acetol
AA
H20
LG
CA
Acetol
AA
CA
H20
LG
AA
Acetol
CA
0 H20
Concentration in Liquid (wt%)
25
C.A. Mullen et al. / Journal of Analytical and Applied Pyrolysis 107 (2014) 40–45
concentrations do not track as closely with water as acetic acid does, but unlike crotonic acid its concentration generally decreases along the condensation train. The compound with a condensation (or deposition if present as an aerosol) behavior closest to crotonic acid is levoglucosan, which also is collected in a higher concentration at the end of the condensation train by the ESP despite its high boiling point (>350 ◦ C). Levoglucosan is a six carbon molecule (dehydrated glucose) which is formed from cellulose thermal decomposition and occurs in both a gas and an aerosol form in pyrolysis vapors. Its entrained aerosols escape the impinging condensers and are collected best electrostatically. The similar behavior of crotonic acid vapors may suggest that it, too, is present as both a gas and an aerosol. Utilizing these observations to optimize the in situ separation of crotonic acid from the biomass derived components as well as developing methods for post-production isolation of crotonic acid from pyrolysis liquids are the subjects of ongoing research. 4. Conclusions Blends of switchgrass and P3HB can be converted to bio-oil enriched with crotonic acid utilizing mild pyrolysis conditions with temperatures in the range of 350 to 375 ◦ C. In blends consisting of 10% P3HB and 90% switchgrass, recovery of crotonic acid was maximized by utilizing fine P3HB over coarse P3HB powder. Crotonic acid yield was maximized at about 45 wt% of the input P3HB, using the fine P3HB particles at a pyrolysis temperature of 375 ◦ C and a pyrolysis PDU that fractionates pyrolysis liquids. Pyrolysis liquids were collected via four condensers followed by an electrostatic precipitator, and crotonic acid was found in significant concentrations in all fractions. Crotonic acid collection did not follow a pattern based on water solubility or vapor temperature, suggesting that it is present as both a gas and an aerosol in the vapor stream. Acknowledgements The authors thank Dr. Christina Dorado, Craig Einfeldt and Michelle Hall of ERRC for technical assistance. This research was supported by a grant from The US Department of Energy’s (DOE) Office of Energy Efficiency and Renewable Energy (EERE) to Metabolix (grant DE-EE0004943). References [1] P. Suriyamongkol, R. Weselake, S. Narine, M. Moloney, S. Shah, Biotechnological approaches for the production of polyhydroxyalkanoates in microorganisms and plants – a review, Biotechnol. Adv. 25 (2007) 148–175. [2] F. Gironi, V. Piemonte, Bioplastics and petroleum-based plastics: strengths and weaknesses, Energy Sources A 21 (2011) 1949–1959.
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[3] M.N. Somleva, O.P. Peoples, K.D. Snell, PHA bioplastics, biochemicals, and energy from crops, Plant Biotechnol. J. 11 (2013) 233–252. [4] F.D. Kopinke, M. Remmler, K. Mackenzie, Thermal decomposition of biodegradable polyesters. 1. Poly(beta-hydroxybutyric acid), Polym. Degrad. Stabil. 52 (1996) 25–38. [5] F.D. Kopinke, K. Mackenzie, Mechanistic aspects of the thermal degradation of poly(lactic acid) and poly(beta-hydroxybutyric acid), J. Anal. Appl. Pyrol. 40 (1997) 43–53. [6] Y. Aoyagi, K. Yamashita, Y. Doi, Thermal degradation of poly(R)-3hydroxybutyrate, poly epsilon-caprolactone, and poly(S)-lactide, Polym. Degrad. Stabil. 76 (2002) 53–59. [7] N.C. Billingham, T.J. Henman, P.A. Holmes, Degradation and stabilisation of polyesters of biological and synthetic origin, Dev. Polym. Degrad. 7 (1987) 81–121. [8] J. van Walsem, E. Anderson, J. Licata, K.A. Sparks, C. Mirley, M.S. Sivasubramanian, Process for producing a monomer component from a genetically modified polyhydroxyalkanoate (PHA) biomass, wherein the biomass is heated in the presence of a catalyst to release a monomer component from the PHA. International Patent Application WO 20120315681. [10] A.A. Boateng, D.E. Daugaard, N.M. Goldberg, K.B. Hicks, Bench-scale fluidizedbed pyrolysis of switchgrass for bio-oil production, Ind. Eng. Chem. Res. 46 (2007) 1891–1897. [11] C.A. Mullen, A.A. Boateng, Chemical composition of bio-oils produced by fast pyrolysis of two energy crops, Energy Fuels 22 (2008) 2104–2109. [12] D. Mohan, C.U. Pittman Jr., P.H. Steele, Pyrolysis of wood/biomass for bio-oil: a critical review, Energy Fuels 20 (2006) 848–889. [13] R. He, X.P. Ye, B.C. English, J.A. Satrio, Influence of pyrolysis condition on switchgrass bio-oil yield and physicochemical properties, Bioresour. Technol. 100 (2009) 5305–5311. [14] D. Vamvuka, V. Topouzi, S. Sfakiotakis, Evaluation of production yield and thermal processing of switchgrass as a bio-energy crop for the Mediterranean region, Fuel Process. Technol. 91 (2010) 988–996. [15] P.R. Adler, M.A. Sanderson, A.A. Boateng, P.J. Weimer, H.-J.G. Jung, Biomass yield and biofuel quality of switchgrass harvested in fall or spring, Agron. J. 98 (2006) 1518–1525. [16] A.A. Boateng, K.B. Hicks, K.P. Vogel, Pyrolysis of switchgrass (Panicum virgatum) harvested at several stages of maturity, J. Anal. Appl. Pyrol. 75 (2006) 55–64. [17] R. Fahmi, A.V. Bridgwater, L.I. Darvell, J.M. Jones, N. Yates, S. Thain, I.S. Donnison, The effect of alkali metals on combustion and pyrolysis of Lolium and Festuca grasses, switchgrass and willow, Fuel 86 (2007) 1560–1569. [18] T. Imam, S. Capareda, Characterization of bio-oil, syn-gas and bio-char from switchgrass pyrolysis at various temperatures, J. Anal. Appl. Pyrol. 93 (2012) 170–177. [19] T. Cornelissen, M. Jans, J. Yperman, G. Reggers, S. Schreurs, R. Carleer, Flash co-pyrolysis of biomass with polyhydroxybutyrate. Part 1: Influence on bio-oil yield, water content, heating value and the production of chemicals, Fuel 87 (2008) 2523–2532. [20] T. Cornelissen, M. Jans, M. Stals, T. Kuppens, T. Thewys, G.K. Janssens, H. Pastijn, J. Yperman, G. Reggers, S. Schreurs, R. Carleer, Flash co-pyrolysis of biomass: the influence of biopolymers, J. Anal. Appl. Pyrol. 85 (2009) 87–97. [21] A.A. Boateng, C.A. Mullen, N.M. Goldberg, K.B. Hicks, H.G. Jung, J.F.S. Lamb, Production of bio-oil from alfalfa stems by fluidized bed fast pyrolysis, Ind. Eng. Chem. Res. 47 (2008) 4115–4122. [22] A.A. Boateng, C.A. Mullen, L. Osgood-Jacobs, P. Carlson, N. Macken, Mass balance, energy and exergy analysis of bio-oil production by fast pyrolysis, J. Energy Res. Technol. 134 (2012), 04200-1–042001-9. [23] D.B. Bigley, M.J. Clarke, Studies in decarboxylation. Part 14. The gas-phase decarboxylation of but-3-enoic acid and the intermediacy of isocrotonic (cisbut-2-enoic) acid and its isomerization to crotonic (trans-but-2-enoic) acid, Perkins Trans. II 2 (1982) 1–6. [24] A.S. Pollard, M.R. Rover, R.C. Brown, Characterization of bio-oil recovered as stage fractions with unique chemical and physical properties, J. Anal. Appl. Pyrol. 93 (2012) 129–138.
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Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Mild pyrolysis of P3HB/switchgrass blends for the production of bio-oil enriched with crotonic acid夽 Charles A. Mullen a,∗ , Akwasi A. Boateng a , Dirk Schweitzer b , Kevin Sparks b , Kristi D. Snell b a Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 600 E. Mermaid Lane, Wyndmoor, PA 19038, United States b Metabolix Inc., 21 Erie Street, Cambridge, MA 02139, United States
a r t i c l e
i n f o
Article history: Received 24 September 2013 Accepted 31 January 2014 Available online 7 February 2014 Keywords: P3HB Crotonic acid Pyrolysis Switchgrass
a b s t r a c t The mild pyrolysis of switchgrass/poly-3-hydroxybutyrate (P3HB) blends that mimic P3HB-producing switchgrass lines was studied in a pilot scale fluidized bed reactor with the goal of simultaneously producing crotonic acid, a switchgrass-based bio-oil, and bio-char. Factors such as pyrolysis temperature, reactor residence time, flow rate and particle size of the P3HB were studied to determine their effects on the recovery of crotonic acid as a component of the pyrolysis oil produced from the mixture. Crotonic acid yields were maximized at 45 wt% of the input P3HB by using small P3HB particles and a pyrolysis temperature of 375 ◦ C. The remaining components of the liquid product were similar to those produced via fast pyrolysis of switchgrass alone. Fractional collection within the condensation system of the pyrolysis process development unit (PDU) did not significantly fractionate crotonic acid more than the total liquids collected. Concentrations of 6 to 10 wt% crotonic acid in the liquids were found in all fractions and crotonic acid was effectively collected by both condensation and electrostatic precipitation suggesting that pyrolysis of P3HB produces crotonic acid in both gas and aerosol phases. Published by Elsevier B.V.
1. Introduction Decreases in petroleum resources and increases in costs have put pressure on markets for products produced from petroleum including fuels, chemicals and plastics. These forces combined with environmental issues related to non-biodegradable petroleum-based plastics have spurred research into bio-based alternatives. Polyhydroxyalkanoates (PHAs) are a broad family of naturally occurring polyesters that possess physical properties making them suitable replacements for many petroleum derived plastics. They are produced by some microorganisms as a method of carbon and energy storage [1,2]. The production of of poly-3-hydroxybutyrate (P3HB), the simplest member of the PHA family, can be achieved in non-native bacterial producers
夽 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 Agriculture. USDA is an equal opportunity provider and employer. ∗ Corresponding author. Tel.: +1 215 836 6916; fax: +1 215 233 6559. E-mail address: [email protected] (C.A. Mullen). 0165-2370/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jaap.2014.01.022
as well as plants by expressing bacterial genes encoding the P3HB biosynthetic enzymes. Recent advances have led to the production of P3HB in genetically modified switchgrass at levels up to 7.7% dwt in sections of leaf tissue [3]. In addition to being utilized as a renewable, biodegradable material, P3HB can be thermally degraded to crotonic acid in high yield via pyrolysis [4–8].
The selective production of crotonic acid from P3HB using a mild pyrolysis process has been proposed to occur via a random chainscission beta elimination reaction at temperatures above 200 ◦ C [7] to produce cis-crotonic acid which then isomerizes to the more thermodynamically stable trans-crotonic acid.
C.A. Mullen et al. / Journal of Analytical and Applied Pyrolysis 107 (2014) 40–45
Crotonic acid can be used as a platform chemical providing an alternative route to high-valued industrial and fine chemicals. For example, it can be reduced to n-butanol for chemical or fuel applications or oxidized to maleic anhydride, a precursor of unsaturated polyesters. Other targets available through simple transformations include propylene, the precursor to polypropylene, and acrylic acid, a chemical used to produce superabsorbent polymers. Pyrolysis has also been demonstrated as a very efficient liquefaction method for biomass such as switchgrass producing bio-oil or biocrude [10–18]. Pyrolysis liquids are potential intermediates for the production of renewable hydrocarbon fuels and renewable chemicals. Pyrolysis of P3HB-producing switchgrass is thus a potentially attractive process that could simultaneously produce crotonic acid, bio-oil, and bio-char. Previously, the co-pyrolysis of willow biomass and P3HB at 450 ◦ C was reported by Cornelissen et al. [19,20], with the goal of utilizing high levels of P3HB as a co-reactant to improve the quality of pyrolysis oil. We report in this contribution on the mild pyrolysis of switchgrass/P3HB blends with the goal of maximizing crotonic acid recovery while also producing pyrolysis oil and bio-char. To study the potential of a mild pyrolysis process, switchgrass/P3HB blends that mimic P3HB-producing switchgrass lines were subjected to pyrolysis in a pilot scale fluidized bed reactor. Factors such as pyrolysis temperature, residence time, flow rates and particle size of the P3HB were studied for their effects on the recovery of crotonic acid as a component of the pyrolysis oil. Effects of the presence of P3HB and crotonic acid on the chemical composition of the switchgrass bio-oil were also examined. 2. Methods and materials 2.1. Materials Switchgrass was provided by the McDonnell Farm in East Greenville, PA. The switchgrass was ground and sieved to 2 mm
41
using a Wiley mill and dried in an oven overnight at 80 ◦ C before use. After drying the switchgrass moisture content was 2 to 3 wt%. P3HB polymer and lime (CaO) were provided by Metabolix, Inc. Two particle sizes of P3HB were used: a fine powder (particle size <0.15 mm in diameter) and a coarser material (particle size ranged from 0.177 to 0.354 mm in diameter). Blends of switchgrass, P3HB and CaO (where applicable) were prepared by physically mixing them in a V-mixer.
2.2. Pyrolysis experiments Pyrolysis experiments were performed using the fluidized bed pyrolysis process development unit (PDU) at USDA’s Eastern Regional Research Center (ERRC). The reactor system has been well described by Boateng et al. [10,21]. A schematic of the PDU is provided in Fig. 1. The reactor bed consisted of a 7.6 cm diameter pipe which was filled to a depth of 20 cm with 800 m silica sand serving as a fluidizing medium. For blends of switchgrass and P3HB the pyrolysis temperature was 350 to 375 ◦ C. Bio-char removal from the vapor stream was accomplished by cyclone separation. Pyrolysis liquids were collected in five fractions via four condensers cooled with ∼4 ◦ C water in series followed by an electrostatic precipitator (ESP). Pyrolysis product yield distribution was determined gravimetrically and corrected for material imbalance caused by product deposition in the system by using a nonlinear programming optimization model. This was previously developed to adjust the experimental data to achieve closed balances without losing the overall representation of the pyrolysis process while keeping within the law of conservation of mass [22]. Non-condensable gas (NCG) composition was measured online using an Agilent 3000 MicroGC.
Fig. 1. Schematic of ERRC pyrolysis PDU.
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Table 1 Experimental conditions and yields for pyrolysis of switchgrass/P3HB blends. Experiment number
SG FPa
SG MPb
1
2
3
4
5
6
7
8
Reactor temperature % feed switchgrass % feed P3HB % feed CaO P3HB particle sizec N2 flow Liquid yield Char yield Gas yield Concentration crotonic acid in liquid (wt%)d Crotonic acid yield (wt%)e
480 100 0 0 N/A 75 62.3 [69.5] 9.8 [15.6] 11.6 [14.9] 0
375 100 0 0 N/A 75 36.1 [57.5] 29.5 [30.3] [12.2] 0
350 90 10 0 Fine 75 40.1 [42.0] 17.6 [24.9] [33.2] 8.5
375 90 10 0 Fine 75 48.4 [50.1] 15.2 [20.5] [29.5] 9.3
375 85.5 9.5 5.0 Fine 75 16.8 19.9 nd 0.5
375 90 10 0 Coarse 70 41.1 [49.5] 9.6 [20.7] [29.8] 6.3
350 90 10 0 Coarse 60 45.8 [46.9] 19.6 [20.7] [27.5] 8.9
375 90 10 0 Coarse 60 47.4 [40.5] 24.4 [21.0] [30.2] 8.4
375 90 10 0 Fine 60 45.2 [46.3] 13.8 [21.9] [31.9] 9.6
375 75 25 0 Fine 60 47.8 [58.0] 14.6 [17.8] [24.3] 22.1
N/A
N/A
34.3 [35.8]
45.2 [46.8]
0.9
25.9 [31.4]
40.6 [41.5]
39.7 [40.6]
43.5 [44.6]
42.2 [51.2]
a b c d e
Switchgrass fast pyrolysis control. Switchgrass mild pyrolysis control. Coarse means most particles between 0.177 and 0.354 mm. Composite of all liquid fractions. Values in brackets are based on corrected yields via mass balance optimization model [16]. Mass percentage of P3HB collected as crotonic acid in the liquid products.
2.3. Product characterization Water content was measured using Karl–Fischer titration in methanol with Hydranal Karl-Fischer Composite 5 (Fluka) used as titrant. GC with mass spectrometry (MS) detection analysis of pyrolysis oil was performed on a Shimadzu GCMS QC-2010 and was used to quantify several pyrolysis oil components including crotonic acid. The column used was a DB-1701, 60 m × 0.25 mm, 0.25 m 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. The flow rate of the He carrier gas was 1 mL/min. The pyrolysis oil samples for GC analysis were prepared as 3 ± 1 wt% solutions in acetone which were filtered through 0.45 m polytetrafluoroethylene (PTFE) filters prior to injection. For quantification of individual pyrolysis oil compounds, response factors relative to the internal standard, fluroanthene, were determined using authentic compounds [11]. 3. Results and discussion Blends of switchgrass and P3HB were pyrolyzed in a series of several experiments that are summarized in Table 1. In most experiments the blends contained 90% switchgrass and 10% P3HB, with the exception of experiment 8 where the proportion of P3HB was increased to 25%. Also, in one instance, lime (CaO) was added as a 5 wt% loading to the mixture to explore its potential in assisting P3HB thermal decomposition (experiment 3). The blends were pyrolyzed at temperatures of 350 to 375 ◦ C. Two particle size cuts (fine and coarse) of the P3HB particles were used along with three different carrier gas flow rates. The latter enabled the testing of the effect of the residence time of the P3HB particles in the reactor, with the lower flow rate providing more residence time. Residence times are estimated to be 0.12 s at 60 L/min N2 flow and 0.09 s at 75 L/min [10]. For comparison, two control experiments with switchgrass alone were also included, one at traditional fast pyrolysis conditions (480 ◦ C) and one at the mild pyrolysis conditions (375 ◦ C) employed for the P3HB/switchgrass blends. The distribution of the post-pyrolysis material in solid, liquid and non-condensable gas fractions is also provided in Table 1. Actual recoveries are provided along with values corrected for material retained in the system by a non-linear optimization mass balance model, as described in Section 2 [22]. Compared with the fast pyrolysis of switchgrass (480 ◦ C) which produced 62.3% liquid, the yield of liquid (bio-oil) in all of the mild pyrolysis experiments was found to be lower (Table 1). For the mild, lime-free pyrolysis of
switchgrass/P3HB blends, greater liquid yields (40 to 48 wt%) were achieved compared with the mild pyrolysis control experiment containing switchgrass alone (36%). Conversely, the yield of solid products (bio-char) from the switchgrass/P3HB blends (9.6 to 24.4%) was increased compared to the fast pyrolysis of switchgrass alone (9.8%) but decreased relative to the mild pyrolysis of switchgrass alone (∼30%). These results suggest that both the use of lower temperatures, which are necessary to prevent further degradation of crotonic acid, and the presence of P3HB and/or crotonic acid in the reaction mixture influence the product distribution. Use of pyrolysis temperatures lower than 450 ◦ C for biomass pyrolysis has previously been shown to result in decreased bio-oil and increased bio-char production [12]. The total yield of gaseous products is calculated to be higher in the experiments with the switchgrass/P3HB blends, compared to the mild and fast pyrolysis control experiments. This could be due to the increased acidity of the environment resulting in increased decarboxylation and decarbonylation type reactions of the biomass derived materials. The composition of the gases produced with the P3HB/switchgrass blends showed an increase in the ratio of CO2 /CO compared with the control fast pyrolysis of switchgrass (Table 2) but similar to the mild pyrolysis control. Decomposition of crotonic acid into CO2 and propylene [23] could also contribute to the increased gas yield, but propylene was not observed. However, its concentration may be too small to detect by the GC method employed since it is diluted with the N2 fluidizing gas. Crotonic acid decomposition could however not produce enough gas to explain the difference between the gas yield in the switchgrass fast pyrolysis control experiment and the experiments with the P3HB-switchgrass blends suggesting that indeed the increased acidity of the environment does help to convert some condensable range products to permanent gases. The concentration of crotonic acid in the liquid pyrolysis oil products ranged from 6.3 wt% to 9.6 wt% for the lime-free pyrolysis of the 9/1 switchgrass/P3HB blends, corresponding to the yields of crotonic acid in the liquid phase ranging from 25.9 wt% to 45.2 wt% Table 2 Composition of non-condensable gas (vol%) for pyrolysis of switchgrass/P3HB blends, selected experiments. Experiment
SG FPa
SG MPb
4
5
7
H2 CO CO2 CH4
5.1 57.6 29.5 7.8
1.5 44.4 48.4 5.6
1.0 44.7 48.7 5.5
1.5 44.4 48.5 5.6
0.2 36.5 59.6 3.7
a b
Switchgrass fast pyrolysis control (480 ◦ C). Switchgrass mild pyrolysis control (375 ◦ C).
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Table 3 Concentration of pertinent switchgrass derived pyrolysis oil components (GC/MS, wt%), total liquids. Experiment
SG FPa
SG MPb
1
2
4
5
6
7
8
Water Acetic acid Acetol Levoglucosan Phenol Cresols Guaiacol 4-Methyl guaiacol Syringol
13.9 4.29 2.35 4.33 0.24 0.24 0.18 0.07 0.20
12.3 6.68 2.18 4.03 0.09 0.08 0.10 0.12 0.11
24.3 3.85 2.73 4.41 0.09 0.09 0.11 0.11 0.09
22.4 3.85 2.48 4.98 0.09 0.11 0.09 0.09 0.13
28.6 3.77 2.96 3.20 0.14 0.13 0.25 0.22 0.22
14.6 4.01 2.19 2.95 0.09 0.16 0.16 0.10 0.11
18.6 4.12 1.97 2.65 0.07 0.08 0.13 0.13 0.11
10.2 5.85 2.66 2.30 0.11 0.09 0.18 0.18 0.16
13.1 3.51 1.26 2.41 0.09 0.12 0.07 0.08 0.07
a b
Switchgrass fast pyrolysis control (480 ◦ C). Switchgrass mild pyrolysis control (375 ◦ C).
(or 31.4–46.8 wt% when using modified liquid yields). The highest concentration of crotonic acid in the liquid products and the highest yields were obtained when finer sized P3HB particles were used and the pyrolysis was performed at 375 ◦ C (experiments 2 and 7). The difference between these two experiments was the N2 flow rate; the one with a lower flow rate of 60 L/min (experiment 7) resulted in a longer residence time in the reactor than the one employing a higher carrier gas rate of 75 L/min, however, little difference was seen in the yield of crotonic acid. The fluidizing gas flow rate and thus the residence time made a more significant difference when the coarser P3HB particles were employed. In these cases with the coarse P3HB particles the yield of crotonic acid in the liquids increased from about 26 wt% to about 40 wt% when the flow rate of N2 was decreased from 70 L/min (experiment 4) to 60 L/min (experiments 5 and 6), suggesting that the larger particles do not fully depolymerize under the short residence time conditions. In the experiment incorporating lime (CaO) into the feed (experiment 3), the observed yield of crotonic acid was nearly zero, likely due to its decarboxylation [23]. The presence of lime also had the undesired effect of fouling the reactor by fusing the sand particles in the bed, resulting in a loss of fluidization. When the level of P3HB included in the blend was increased to 25%, the concentration of crotonic acid found in the liquid phase products increased, as expected. However, the yield of crotonic acid remained at about the same level of 42% (51% of corrected liquid yield). In each case, the biochar was analyzed for crotonic acid and residual P3HB; only ppm levels were detected which did not significantly affect the overall recoveries. The components other than crotonic acid of the very complex liquid mixture are those that result from the breakdown of the other biopolymers present in switchgrass: cellulose, hemicellulose and lignin. Table 3 provides a comparison of the concentrations of some of the most abundant of the hundreds of components that make up switchgrass pyrolysis bio-oils for the experiments done here and those done under traditional fast pyrolysis conditions (control). The major components of the pyrolysis oil from the switchgrass/P3HB blend were the same as the control indicating that the presence of P3HB does not result in any major change in the composition of the bio-oil, other than the presence of crotonic acid. There were some minor differences in the concentrations of some individual components observed. The concentration of acetol was mostly similar to those found in fast pyrolysis oils of switchgrass, but the acetic acid concentrations were decreased compared with the mild pyrolysis control experiment, perhaps suggesting that there is some interaction between these two carboxylic acids (acetic- and crotonic acid) in the product. The levoglucosan concentration was lower in experiments 4–8, where the lower rates of N2 carrier gas flow were used, but was similar to the control experiments with the higher gas flow rate. This could be a small effect of the presence of the crotonic acid combined with the longer residence time associated with the lower N2 flow rates, but residence time effects alone cannot be
ruled out. The phenolic compounds (e.g. phenol, cresols, guaiacols) derived from lignin are found in lower concentrations in all of the mild pyrolysis experiments compared to the fast pyrolysis control, but likely due to the less effective conversion of lignin at lower temperatures. As shown in Fig. 1, the ERRC pyrolysis PDU fractionates condensable vapors in its bio-oil recovery system. Liquid products are therefore collected as five fractions. Table 4 presents the distribution of the pyrolysis liquids into the five collection fractions, the concentration of crotonic acid in each fraction and the fractionation of the crotonic acid produced. In most cases, there was not much variation in the concentration of crotonic acid in the collected fractions. For the experiments done with a 10% blend of P3HB, the concentration of crotonic acid in the individual fractions ranged from 6 to 11 wt%, with the exception of experiment 4 which had a lower crotonic acid yield and its fractions had crotonic acid concentrations of 4 to 8 wt%. Because the concentration of crotonic acid varies only to a small extent, its fractionation of closely tracks with how the overall liquid fractionates. In most cases, the largest fractions collected are those at the beginning and the end of the condenser train, which are the first condenser and the ESP. The liquid collected via the first condenser accounted for ∼30 wt% of the total in each of the experiments with the 10% P3HB blend (experiments 1–2 and 4–7), and the fraction of the total crotonic acid produced collected at the first condenser ranged from 26 to 46%. The ESP collected similar amounts, with the exception of experiments 5 and 6 where the larger particle P3HB was used in conjunction with the lower flow rate. In these experiments, very little of the pyrolysis liquids or the crotonic acid were collected via ESP, and larger fractions were collected via the third and fourth condensers. The lower flow rates lead to a longer residence time in the chilled zone of each condenser likely resulting in an increase in the amounts collected in the condensers. However, why the trend does not hold when the finer P3HB is used at the low flow rate (experiment 7), even for switchgrass derived products, is unclear. When the amount of P3HB in the blend was increased to 25% (experiment 8), the first condenser and ESP still collected the most crotonic acid, but the remaining portion was more evenly distributed among the second, third and fourth condenser fractions. While the vapor temperature is decreased along the condensation train, pyrolysis liquids collected do not follow a strict boiling point distribution among the fractions. This is due to a variety of reasons including the fact that the pyrolysis vapors consist of both true gases (not solids or liquids entrained in the vapor stream) which are condensed and aerosols which are best collected by the electrostatic precipitator [24]. Solubility interactions among the components also play a role in how pyrolysis oils fractionate. Fig. 2 compares the concentration of crotonic acid in each fraction with some of the major components of the pyrolysis liquids derived from switchgrass: water, acetic acid, acetol and levoglucosan. Each of these components is water soluble. The concentration of acetic
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Table 4 Fractionation of crotonic acid (CA) in pyrolysis liquids. Experiment
Cond. 1
Cond. 2
1
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (%) Fraction of P3HB collected as CA (%)
34.9 11.4 46.6 16.0
19.7 7.1 16.4 5.6
Cond. 3 7.2 6.7 5.7 2.0
Cond. 4 3.2 6.3 2.4 0.8
ESP 34.9 7.1 29.0 9.9
2
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (wt%) Fraction of P3HB collected as CA (%)
29.0 11.2 34.8 15.7
14.5 7.5 11.6 5.2
5.6 6.9 4.2 1.9
25.0 9.5 25.5 11.5
25.8 8.7 23.9 10.8
4
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (wt%) Fraction of P3HB collected as CA (%)
29.7 6.0 28.2 7.3
15.8 4.0 10.1 2.6
3.2 5.9 3.0 0.8
18.4 6.7 19.5 5.1
32.9 7.5 39.2 10.2
5
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (wt%) Fraction of P3HB collected as CA (%)
30.3 7.7 26.3 10.7
27.0 10.5 31.9 13.0
11.2 9.0 11.4 4.6
28.3 8.3 26.5 10.8
3.3 10.7 4.0 1.6
6
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (wt%) Fraction of P3HB collected as CA (%)
29.4 8.3 29.1 11.6
14.3 10.0 17.1 6.8
23.8 9.1 26.0 10.3
29.4 6.9 24.3 9.7
3.2 9.4 3.6 1.4
7
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (wt%) Fraction of P3HB collected as CA (%)
30.6 10.2 32.5 14.1
15.0 7.3 11.3 4.9
6.3 7.9 5.2 2.3
9.2 11.4 10.9 4.7
38.8 10.0 40.1 17.4
8
Fraction of total liquid (wt%) Concentration CA (wt%) Fraction of CA collected (wt%) Fraction of P3HB collected as CA (%)
25.5 24.8 28.7 12.1
14.8 21.3 14.2 6.0
10.7 20.7 10.1 4.3
10.7 24.0 11.6 4.9
38.3 20.4 35.4 14.9
Concentration of CA is wt% of CA in liquid collected in each condensation point measured by GC/MS. Fraction of CA collected is the percentage CA condensed in that fraction. Fraction of P3HB collected as CA is calculated by the line above multiplied by CA yield from Table 1.
acid tracks very closely with the amount of water in each fraction while that of crotonic acid, although also a carboxylic acid, does not track as closely with the water content. In most cases, the water and acetic acid concentrations are lowest in the ESP fraction, which is
the last collection point. In contrast the crotonic acid concentration is higher in the ESP fraction than it is in the condenser fractions in several cases despite its lower volatility (crotonic acid boiling point (bp) = 185 ◦ C, acetic acid bp = 118 ◦ C). Acetol (bp = 145 ◦ C)
30
20
15
10
5
Experiment 1
Experiment 2
Experiment 4 Cond 1
Cond 2
Experiment 5 Cond 3
Experiment 6 Cond 4
Experiment 7
ESP
Fig. 2. Concentration of some pyrolysis oil components by fraction. CA = crotonic acid, AA = acetic acid, LG = levoglucosan.
Experiment 8
LG
AA
Acetol
CA
H20
LG
AA
Acetol
H20
LG
CA
AA
Acetol
H20
LG
CA
AA
Acetol
H20
LG
CA
Acetol
AA
H20
LG
CA
Acetol
AA
CA
H20
LG
AA
Acetol
CA
0 H20
Concentration in Liquid (wt%)
25
C.A. Mullen et al. / Journal of Analytical and Applied Pyrolysis 107 (2014) 40–45
concentrations do not track as closely with water as acetic acid does, but unlike crotonic acid its concentration generally decreases along the condensation train. The compound with a condensation (or deposition if present as an aerosol) behavior closest to crotonic acid is levoglucosan, which also is collected in a higher concentration at the end of the condensation train by the ESP despite its high boiling point (>350 ◦ C). Levoglucosan is a six carbon molecule (dehydrated glucose) which is formed from cellulose thermal decomposition and occurs in both a gas and an aerosol form in pyrolysis vapors. Its entrained aerosols escape the impinging condensers and are collected best electrostatically. The similar behavior of crotonic acid vapors may suggest that it, too, is present as both a gas and an aerosol. Utilizing these observations to optimize the in situ separation of crotonic acid from the biomass derived components as well as developing methods for post-production isolation of crotonic acid from pyrolysis liquids are the subjects of ongoing research. 4. Conclusions Blends of switchgrass and P3HB can be converted to bio-oil enriched with crotonic acid utilizing mild pyrolysis conditions with temperatures in the range of 350 to 375 ◦ C. In blends consisting of 10% P3HB and 90% switchgrass, recovery of crotonic acid was maximized by utilizing fine P3HB over coarse P3HB powder. Crotonic acid yield was maximized at about 45 wt% of the input P3HB, using the fine P3HB particles at a pyrolysis temperature of 375 ◦ C and a pyrolysis PDU that fractionates pyrolysis liquids. Pyrolysis liquids were collected via four condensers followed by an electrostatic precipitator, and crotonic acid was found in significant concentrations in all fractions. Crotonic acid collection did not follow a pattern based on water solubility or vapor temperature, suggesting that it is present as both a gas and an aerosol in the vapor stream. Acknowledgements The authors thank Dr. Christina Dorado, Craig Einfeldt and Michelle Hall of ERRC for technical assistance. This research was supported by a grant from The US Department of Energy’s (DOE) Office of Energy Efficiency and Renewable Energy (EERE) to Metabolix (grant DE-EE0004943). References [1] P. Suriyamongkol, R. Weselake, S. Narine, M. Moloney, S. Shah, Biotechnological approaches for the production of polyhydroxyalkanoates in microorganisms and plants – a review, Biotechnol. Adv. 25 (2007) 148–175. [2] F. Gironi, V. Piemonte, Bioplastics and petroleum-based plastics: strengths and weaknesses, Energy Sources A 21 (2011) 1949–1959.
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