ARTICLE pubs.acs.org/EF
Catalytic Fast Pyrolysis of White Oak Wood in a Bubbling Fluidized Bed Charles A. Mullen, Akwasi A. Boateng,* David J. Mihalcik, and Neil M. Goldberg USDA-ARS, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States ABSTRACT: Catalytic fast pyrolysis was performed on white oak wood using two zeolite-type catalysts as bed material in a bubbling fluidized bed reactor. The two catalysts chosen, on the basis of a previous screening study, were Ca2+ exchanged Y zeolite and a proprietary β-zeolite type catalyst (catalyst M) both supplied by UOP. Each catalyst proved effective at partially deoxygenating the oak wood pyrolysis vapors during the initial pyrolysis process and adding aromatic hydrocarbons to the liquid product mixture. However, each incurred a penalty of reduced liquid yield and catalyst deactivation due to coke formation on the catalysts’ surfaces. The coking on the Ca Y zeolite catalyst was relatively less severe because the deoxygenation process followed decarbonylation and decarboxylation reaction pathways more compared to the dehydration and dehydrogenation pathways for catalyst M, although evidence that both catalysts were active for all the reaction mechanisms exists. The severe coking problem on catalyst M on catalyst activity was mitigated by successfully regenerating the catalyst in situ, resulting in effective production of partially deoxygenated pyrolysis oils over extended periods of time and concomitantly improving the C/O ratio of the upgraded pyrolysis oils from 1.8/1 to 5.9/1 at best. However, the severe coking does limit the overall carbon conversation efficiency from biomass to catalytic pyrolysis oil. Nonetheless, this demonstrates the potential of producing partially deoxygenated and stable fuel intermediates by in situ catalytic fast pyrolysis for ready use as refinery fuel blendstock using the bubbling fluidized bed technology.
’ INTRODUCTION Fast pyrolysis has been proven to be one of the most efficient methods for the liquefaction of lignocellulosic biomass into potential renewable transportation fuel intermediates.1,2 However, the utilization of fast pyrolysis oils as such intermediates has been inhibited by some undesirable properties of the liquids. These properties include their tendency to partially repolymerize during storage, resulting in viscosity increases that cause piping and pumping problems. Raw biomass pyrolysis oil can also be corrosive and heterogeneous, further exacerbating such problems. Each of these problems can be directly related to the high oxygen content of biomass pyrolysis oil. Unlike petroleum, which consists nearly entirely of hydrocarbons, biomass pyrolysis oil is a complex mixture of oxygen functionalized hydrocarbons, including carboxylic acids, aldehydes, ketones, cyclic ethers, and phenolics.1 3 The high concentration of these reactive compounds gives rise to problems noted above and serves as a major barrier to overcome before its widespread use in existing oil refinery infrastructure is possible. Because of these issues, there is a need to produce a higher quality, stable pyrolysis oil that can be more easily handled by a petroleum refinery or other upgrading facility. One potential method for achieving this is to partially deoxygenate the biomass during the initial pyrolysis process, using catalytic approaches. The goal of the catalytic process is to decrease the concentration of the reactive oxygenated species and increase the fraction of hydrocarbons in the liquid pyrolyzate, which should result in pyrolysis oil that has better storage stability and is less corrosive. In addition to containing highly oxygenated compounds, biomass pyrolysis oils also differ from petroleum because they are deficient in hydrogen. This means that, to utilize all of the carbon in biomass, hydrogen will have to be added to create needed compounds, such as alkanes. Therefore, the goal of partial deoxygenation of pyrolysis vapors is further challenged by removing oxygen while losing the minimal amount of both carbon and hydrogen.4,5 Catalytic pyrolysis is best thought of as a two stage process, where the first step involves the thermal decomposition of the This article not subject to U.S. Copyright. Published 2011 by the American Chemical Society
biomass into pyrolysis vapors to render the compounds available for secondary reactions over the catalyst. The catalytic processing step is similar to the catalytic cracking process used in the petroleum industry for the manufacture of gasoline. In that industry, catalytic cracking uses zeolites and related aluminosilicate catalysts, which have both Brønsted and Lewis acidic active sites. In hydrocarbon cracking, the acid catalysts work by formation of carbocations by either protonation of olefins (eq 1.1) in the feedstock or hydride abstraction (eq 1.2) from saturated hydrocarbons. This leads to isomerization, cyclization, and aromatization of the hydrocarbons.
Received: August 26, 2011 Revised: October 13, 2011 Published: October 20, 2011 5444
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Table 1. White Oak Wood Feedstock Elemental Composition (wt %) C
H
N
S
O
H2O
ash
as is
50.12
6.29
0.51
0.00
40.93
1.6
0.59
dry-ash free
51.23
6.43
0.52
0.00
41.83
Unlike petroleum feedstocks, however, pyrolysis vapors are highly oxygenated and oxygen groups are more reactive toward both Brønsted and Lewis acid activation. This makes the acid catalyst well suited for cracking pyrolysis vapors. However, the oxygen groups also make them more likely to strongly bind to an acid catalyst, and therefore, they tend to deactivate it. Furthermore, protonation of hydroxyl groups followed by dehydration to form carbocations removes hydrogen from the already hydrogen deficient molecules. Along with the ability to dehydrogenate, coke formation is prevalent because of the lack of available hydrogen to support hydrocarbon formation. Additionally, biomass pyrolysis vapors do not tend to have long carbon chains, as in the above example, which makes formation of small olefins and intermolecular recombination a likely, more prevalent mechanism in cracking and production of hydrocarbons from these oxygenates (eq 2.1).
Previous work on catalytic pyrolysis has shown that zeolites can, in fact, be effective materials to incorporate into the pyrolysis reactor to reduce oxygen content while adding aromatic hydrocarbons to the liquid product. Several reports identified HZSM-5 as an effective catalyst for this process;7 12 however, as is the case for most zeolite type catalysts, the deoxygenation was usually accompanied by a dramatic loss in yield and carbon efficiency as a result of the deposition of coke on the catalyst and an increase in the production of permanent gases including carbon oxides. This also causes the effective lifetime of the catalyst to be relatively short.7,9 As part of a project aimed at producing a stable pyrolysis oil, we screened several proprietary catalysts using analytical pyrolysis (py-GC/MS) and a small bench scale reactor in the pyrolysis of white oak wood.13 From these studies, two catalysts were chosen for scaled up studies in the ARS bench scale fluidized-bed pyrolysis reactor, by changing the heat transfer medium from sand to a catalytic material in a bubbling fluidized bed.14 The heterogeneous catalyst materials chosen were Ca Y zeolite and a proprietary catalyst related to β-zeolite, which we will label catalyst M. Ca Y zeolite was chosen because, in the initial small-scale studies, it was shown to significantly reduce the production of acetic acid by py-GC/MS, and catalyst M was chosen because it produced a high level of aromatic hydrocarbons and nearly eliminated detectable oxygenates.13 Scaling these studies up to the bench scale allowed for the production of an amount of oil sufficient for stability testing, elemental analysis, total acid number (TAN) determination, and GC/MS compositional analysis; some of which are discussed herein. It also allowed the study of catalyst coking and the approaches for its regeneration.
’ EXPERIMENTAL SECTION General Pyrolysis Methods. The pyrolysis feedstock was white oak wood. It was received as pellets and ground and sieved through a 2 mm screen using a Wiley mill. Analysis of the as-used white oak wood feed is provided in Table 1. Pyrolysis was carried out in a fluidized bed reactor at approximately 500 °C in a medium of silica sand or catalyst pellets and was fluidized with N2. Catalysts were supplied by UOP, LLC as ∼5 mm long cylindrical extrudates. The reactor bed consisted of a 7.6 cm diameter pipe filled to a depth of 20.3 cm (8 in.) with the fluidizing medium. For both catalysts, CaY zeolite and M, this translates to ∼800 g of catalyst. The reactor system has been well described by Boateng et al.14,15 The input N2 flow rate to fluidize the catalytic bed was 85 L/min. Char removal is accomplished by twin cyclones, and pyrolysis oil is collected at four stations in a condensation train connected in series, followed by a series of electrostatic precipitators (ESPs). For catalytic pyrolysis experiments without regeneration, samples were collected from each of these collection points at timed intervals, every 15 min for Ca Y zeolites experiments and 20 min for catalyst M experiments. For experiments with in situ regeneration, the reactor was fed for the designated period (5 or 10 min), at which point the feed was stopped and air was added in place of the N2. The oxygen in the air at this point allowed for combustion of carbonaceous materials in the reactor bed, raising its temperature in excess of 650 °C and providing additional heat to supplement the heaters for the endothermic pyrolysis reactions. The hot combustion gas was quickly cooled when passing through the condensation train. When the bed temperature again dropped below 550 °C, the air was again replaced with N2; then, the white oak wood feed was restarted for the designated period, and the cycle repeated. In this case all pyrolysis products were collected at the end of the experiment. Yields of pyrolysis oils and char were determined gravimetrically and non-condensable gas (NCG) yields were measured using a Metris gas meter. NCG composition was measured online using an Agilent 3000 MicroGC. Pyrolysis Oil and Catalyst Characterization. The elemental analysis of feedstock and product streams was carried out using a Thermo EA1112 CHNS/O analyzer. TAN was measured using a Mettler T70 automatic titrator using 0.1 M KOH in isopropanol as titrant and a 1:1 mixture of toluene and wet methanol as the titration solvent. The pH of the pyrolysis oil was measured with a Thomas Scientific 675 pH/ISE meter at ambient temperature conditions. Accelerated aging tests were modified from the protocol established by Oasmaa and Meir.16 The protocol involves heating the pyrolysis oil at 80 °C for 24 h, which is equivalent to one year storage at room temperature conditions. The method was modified to use 90 °C to conform to the standards in the DOE pyrolysis oil stabilization program. The pyrolysis oil was placed in a sealed vial and heated in an oven at 90 °C for 8 and 24 h, followed by viscosity determination using a Grabner Minivis II viscometer. GC/MS analysis was done according to Mullen and Boateng3 using ∼5 wt % solutions of pyrolysis oil samples in acetone, with fluoroanthene used as an internal standard for quantification. Calibration curves for each quantified compound were made from authentic samples purchased from Sigma-Aldrich and used as received. The GC column used was a DB-1701 60 m 0.25 mm, 0.25 μm film thickness. The oven was programmed to hold at 45 °C for 4 min, ramp at 3 °C/min to 280 °C, and hold there for 20 min. The injector temperature was 250 °C, and the injector split ratio was set to 30:1. The flow rate was 1 mL/min of the He carrier gas. The bio-oil samples were prepared as ∼6% solutions in acetone which were filtered through a 0.45 μm PTFE filter prior to injection. Brunauer Emmett Teller (BET) surface area measurements of catalysts were determined using a NOVA 2220e Surface Area Analyzer. After heating to 250 °C under vacuum for 3 h, the surface area of fresh and spent catalyst was determined using the BET method, which 5445
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Figure 1. Production of relevant compounds by Py-GC/MS of white oak wood in the presence and absence of catalysts.13
assumes monolayer coverage of the adsorbent gas, in this case nitrogen. The BET method requires a linear plot of 1/[W(P0/P) 1] vs P/P0, where W is the weight of gas adsorbed and P/P0 is in the range of 0.05 to 0.35, which is the effective pressure range for the multipoint BET method.
Table 2. Pyrolysis Product Recoveries (wt %) for Catalytic Pyrolysis Experiments without Regeneration catalyst
none
total pyrolysis oil
62
water
’ RESULTS AND DISCUSSION Replacement of Sand with Ca Exchanged Y Zeolite. Ion
exchange of zeolite and related aluminosilicates with alkaline earth metals removes the Brønsted acid function from the porous active sites of the catalyst. However, the catalyst still has acid function at the surface Lewis acid sites. In hydrocarbon processing, studies comparing the cracking of n-heptane on H- and Ca2+exchanged ferrierite revealed that the Ca2+ ions did not prevent the cracking of the heptane to olefins, but it did prevent further dehydrogenation reactions and therefore improved the selectivity of the cracking reaction for olefins.17 For biomass derived, hydrogen-poor oxygenate vapors, suppression of a final dehydrogenation step could be important in reducing coke formation. Additionally, Ca2+, in the form of CaO has been shown to act as a decarboxylation catalyst (eq 3.1) and decreases the concentration of organic acids in pyrolysis oils.18,19
This is consistent with the py-GC/MS results where >50% reduction in the production of acetic acid on white oak wood pyrolysis was noted with the addition of Ca-Y zeolite (Figure 1).13 Also observed in the py-GC/MS was a small amount of aromatic hydrocarbon production, which was not observed without catalyst. To test the effects of Ca-Y zeolite on a large scale, extrudates of the material were used as the bed material in place of sand. Samples were collected from each of the collection points at
catalyst M
30
29
13
18
organics
54
17
11
char total non-condensable gases
18 13
13 27
24 14
CO2
5.9
5.0
3.5
CO
6.0
16.5
8.3
CH4
1.0
4.5
1.5
H2
0.2
0.7
total mass recoverya a
7.8
CaY
93
70
0.7 67
Not including deposits on catalysts or material stuck in the system.
timed intervals to study any deactivation of the catalyst over time. Overall, the recovery of pyrolysis oil on a mass basis averaged 30% (Table 2), but this does not account for unrecovered material stuck in the reactor system which can be as much as 40%, based on previous experience .14,15,19 The liquid produced in the presence of the catalyst contained a much higher proportion of water than the pyrolysis oil produced with sand only, with water constituting 43% of the liquid collected at the first collection time (15 min into run), compared with only 15% in the case of sand. Some of this increased water content may come from water produced from deoxygenation of the pyrolysis vapors over the catalyst via the dehydration pathway (and lower organic yield due to coking), but some of it is likely attributable to a higher collection efficiency for water compared with light organics, especially at small reaction volumes. The organic portion of the liquid collected was slightly deoxygenated, with the molar C/O ratio increasing from 1.7/1 over sand to 2.6/1 for the liquid collected at the first time interval over Ca Y zeolite (Table 3). This constituted a 17% increase in carbon content and 22% 5446
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Table 3. Elemental Analysis of Pyrolysis Oil Produced over Catalysts without Regeneration catalyst
a
none as is
none dry basis
Ca Y as isa
CaY dry basisa
43.8
M as isa
M dry basisa
water (wt %)
15.1
C (wt %)
44.33
52.46
34.72
61.75
57.6 23.0
54.40
H (wt %)b
4.65
5.48
3.17
5.62
2.31
5.45
N (wt %)
0.41
0.47
0.455
0.80
0.07
0.16
S (wt %)
0.00
0.00
0.00
0.00
0.00
0.00
O (wt %)b
33.79
41.01
17.9
31.84
17.0
40.0
C/O (mol)b
1.7/1
1.7/1
2.6/1
2.6/1
1.8/1
1.8/1
H/C (mol)b HHV (MJ/kg)
1.3/1 19.4
1.3/1 22.9
1.1/1 14.2
1.1/1 25.1
1.2/1 10.0
1.2/1 23.6
TAN (mg KOH/g)
126
88 112
124
pH
2.5
2.9
2.6
Combined fractions of pyrolysis oil collected during 1st collection of run. b Hydrogen and oxygen values are organic only.
decrease in oxygen content. GC/MS analysis (Table 4) indicated that this deoxygenation was the result of decreased concentration of highly oxygenated species, such as acetic acid, levoglucosan and acetol, consistent with the results of the py-GC/MS study (Figure 1). The overall presence of acids was decreased, as measured by a drop in the total acid number (TAN) from 128 for liquids produced over sand to the range of 88 112 for liquids in the first collection produced over Ca-Y zeolite (Table 3). Concurrent with the decrease in oxygenates is an increase in overall non-condensable gas production. In agreement with results from the small scale fixed bed batch reactor,21 the non-condensable gas is more rich in CO, CH4, and H2 than is the gas mixture produced over sand, which contained a higher proportion of CO2 (Table 2). The increased production of H2 suggests mechanisms similar to eq 1.2 and 2.1 are most responsible for the presence of aromatics, rather than direct deoxygenation of lignin derived phenolics. The high level of CO produced suggests that, in addition to decarboxylation of acids to generate CO2, decarbonylation of other carbonyl groups, such as ketones and aldehydes, (eq 3.2) could be an active pathway.22 This is further supported by the observation in the reduction in the level of acetol compared with the liquid produced over sand. Furfural, an aldehyde, was not decreased but that could be due to a concurrent increase in its formation by dehydration reaction of carbohydrate derivatives, such as levoglucosan, which was also reduced in concentration compared with the non-catalytic pyrolysis oil.
Continuation of the experiment past the first collection points illustrates the continual deactivation of the catalyst. As the collection of the liquid product continued, its analysis revealed
Table 4. Pertinent Pyrolysis Oil Components (wt %) in Pyrolysis Oil Produced over Catalyst without Regenerationa catalyst
a
none
CaY
M
acetic acid
9.6 (12.4)
7.2 (10.2)
5.5 (12.9)
furfural
0.6 (0.8)
0.6 (0.9)
0.3 (0.8)
acetol
4.6 (6.0)
2.3 (3.2)
1.5 (3.5)
levoglucosan
9.3 (12.0)
4.2 (8.0)
2.6 (6.3)
phenol
0.2 (0.2)
0.4 (0.5)
0.2 (0.5)
cresols guaiacol
0.3 (0.4) 0.3 (0.4)
0.6 (0.8) 0.2 (0.3)
0.3 (0.6) 0.1 (0.2)
syringol
0.7 (0.9)
0.2 (0.3)
0.2 (0.4)
toluene
trace
trace
trace
xylenes
trace
trace
trace
naphthalene
trace
trace
trace
Dry basis values in parentheses.
that it continually became more like the pyrolysis oil produced over sand, indicating catalyst poisoning or deactivation. Figure 2 illustrates how, over time, the deoxygenation decreases as the concentration of the oxygenates, such as acetic acid, acetol, and levoglucosan, increases as the catalyst is exposed to greater amounts of biomass. The short-term mechanism for this deactivation is coke-like carbon deposits blocking the active sites of the catalysts as evidenced by its BET surface area, which decreased from 627 m2/g to 118 m2/g over the course of the experiment. Replacement of Sand with Catalyst M. Similar bench scale experiments where a proprietary catalyst related to a β-zeoiltes and referred to as catalyst M was used as the medium for the fluidized bed were performed. Samples were collected from each of the pyrolysis oil collection points every 20 min to monitor catalyst activity with time. Previous small scale experiments with catalyst M had shown a dramatic decrease in condensable range oxygenates and a large increase in the concentration of aromatic hydrocarbons (Figure 1).13 However, the organic liquid collected at each time point in these experiments was, on an elemental dry basis, very similar to that produced over sand (Table 3). Similar to the experiments with Ca Y zeolite, the liquid contained a higher proportion of water, but again, that can be attributed to its relative ease of condensation. GC/MS revealed that the concentration of acetic acid (dry basis) was similar to that in the 5447
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Figure 2. Concentration of oxygenates in pyrolysis oil produced over CaY zeolite at different collection times.
pyrolysis oil produced over sand and the TAN was similar. The only effect the catalyst seemed to have was to suppress levoglucosan formation, but its production has been shown to be very sensitive to the presence of any inorganic material (excluding silica sand), including biomass ash components.23 Because catalyst M was shown to be effective at producing deoxygenated pyrolysis vapors, as shown by the py-GC/MS and batch reactor systems, but not so in a continuous system, it was likely that the catalyst was rapidly deactivated by carbonaceous deposits, and therefore, the resulting products were produced under non-catalytic conditions for most of the experiment. Ca Y zeolite, while less effective than catalyst M at producing deoxygenated vapors in the screening, appeared to be deactivated less quickly. This could be attributed to the attenuation of the Brønsted acid sites on the Y zeolite because of the Ca2+, leading to fewer dehydrogenation reactions and therefore less coke production than is the case for the fully acidic catalyst M. Replacement of Sand with Catalyst M and in Situ Regeneration. Because it appeared that catalyst M was deactivated by carbon deposits very quickly, experiments were designed to attempt to regenerate the catalyst in situ, so that a sample of pyrolysis oil produced over active catalyst M could be produced. To do this, the pyrolysis reactor (fluidized and carried with N2) was fed with white oak wood for a short period of time (5 or 10 min, corresponding to approximately 175 and 350 g of oak per cycle); then, the feed was stopped, and an air stream was added (Figure 3). This resulted in the combustion of the carbon deposits on the catalyst, reactivating the catalyst. When the temperature increase that resulted from the exothermic combustion reaction stopped and the catalyst bed temperature returned to e550 °C, the feed was restarted, and this procedure was repeated several times. This procedure allowed for a small scale mimic of a potential commercial process similar to FCC (fluid catalytic cracking), where large catalyst/feed ratios are used and continual recycling of the catalyst occurs. Using this in situ regeneration process, in both 5 and 10 min pyrolysis cycles, pyrolysis oil (total liquid) yield with catalyst M ranged from 25 to 28% of the input biomass; a large amount of this liquid was water (Table 5). The condensation configuration of the pyrolysis system allows for separation of a large amount of this water in the condenser fraction and collection of a mostly organic fraction at the ESP. Because the condenser fractions were largely composed of water (>65 wt % H2O) the ESP
fraction was used to compare the composition of the organic portions of the pyrolysis oil for these experiments. ESP pyrolysis oil for the experiments with ∼350 g oak wood per cycle had a C/O ratio of 3.1/1 compared with 1.8/1 for noncatalytically produced oil. For experiments with only ∼175 g oak wood per cycle, the C/O ratio further increased to 5.9/1; representing an overall increase in carbon content of 40% and a decrease in oxygen content of 55% (Table 6). GC/MS (Table 7) shows that major oxygenates, such as acetic acid, acetol, and levoglucosan, show the same expected trend, decreasing with greater exposure to active catalyst M. The TAN was also reduced (Table 6). The major deoxygenation pathway for catalyst M appears to follow a protonation/dehydration mechanism similar to that described in eq 2.1, as evidenced by the large amount of reaction water generated. The overall non-condensable gas production was similar to that for the non-catalytic experiment, but the gas was richer in H2, CO, and CH4, with a smaller CO2 concentration (Table 5). This suggests that dehydrogenation and decarbonylation of carbonyls are also operative mechanisms over the acid catalyst (eq 3.2). Dehydration and dehydrogenation contribute to loss of hydrogen; hence, the molar H/C ratios of the pyrolysis oil were reduced from 1.3/1 (non-catalytically produced liquid) to 1.1/1 for the ∼350 g/cycle and to 0.9/1 for the ∼175 g/cycle upgraded liquids, respectively. Therefore, not surprisingly, GC/MS analysis shows the upgraded oil is high in aromatics, including very low H/C compounds, such as naphthalenes (H/C ratio 0.8/1) (Table 7). The GC/MS analysis also reveals that non-methoxylated phenolic compounds, phenol, and alkyl phenols increased in concentration with the shorter regeneration cycle run, producing more quantities. Phenols are known to have a low deoxygenation reactivity on acidic zeolite type catalysts; however, guaiacol has been shown to be partially deoxygenated and then alkylated when co-processed with hydrocarbons over HZSM-5.25 It is therefore logical to assume that a similar mechanism, where guaiacol is demethoxylated to phenol, with transfer for H2 from another molecule in the feed, is responsible for the increases noted in the production over catalyst M (eq 4.1). That phenol can also then be alkylated by reaction with olefins present from cracking of other components (eq 4.2). Furthermore, the fact that the concentration of guaiacols does not appear to decrease in the upgraded liquid produced over the catalyst indicates the likelihood that catalyst M has the concurrent effect of producing further de-polymerization 5448
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Figure 3. Example fluidized bed temperature profile for pyrolysis (blue)/catalyst regeneration (red) cycles. The bottom chart is an close-up of the boxed area in the top chart.
of the lignin in the white oak wood than mere thermal pyrolysis alone.9,25,26
Table 5. Pyrolysis Product Recoveries (%) for Catalytic Pyrolysis Experiments with in Situ Regeneration catalyst
a b
As shown in Table 5, a large amount of coke, about 25% of the total mass of input biomass, was deposited on the catalyst as a result of the loss of hydrogen associated with the dehydration and dehydrogenation occurring during the catalytic pyrolysis over catalyst M. This accounts for about half of the input carbon. So, even though the acid functionalized catalyst was able to deoxygenate the white oak wood pyrolysis vapors, it did
none
M, 350 g cycles
M, 175 g cycles
total pyrolysis oil
62
28
25
water
7.8
14.5
16.5
organics
54
13.5
8.5
char total non-condensable gases
18 13
21 14
16 16
CO2
5.9
2.9
2.6
CO
6.0
9.1
12.0
CH4
1.0
1.3
1.0
H2
0.2
0.6
0.4
total mass recoveryb
93
63
57
coke-like deposits on catalyst
N/A
nd
25a
Elemental analysis of coked catalyst (wt %): C, 5.59; H, 0.465; H/C, 0.6/1. Not including deposits on catalysts or material stuck in the system.
so with very low carbon efficiency despite the relatively low production of carbon oxides in the gas. Elemental analysis of the coke deposit reveals a H/C ratio of 0.6/1 suggesting a polyaromatic structure. The stability of the upgraded ESP pyrolysis oils produced over catalyst M with in situ regeneration was studied by accelerated aging and viscosity measurements.15 The oil produced over catalyst M in the 350 g cycles had an initial viscosity similar to that of the pyrolysis oil produced over sand (Figure 4), but the rate of viscosity change was actually observed to be faster than that of the pyrolysis oil produced non-catalytically. The pyrolysis oil produced in the shorter cycles, however, was less viscous 5449
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Table 6. Elemental Analysis of ESP Pyrolysis Oil Produced over Catalyst M with in Situ Regenerationa no catalyst
a
no catalyst dry basis
350 g cycles
350 g cycles dry basis
175 g cycles
8.6
175 g cycles dry basis
water (wt %)
11.7
C (wt %)
48.21
54.59
60.19
65.8
7.7 70.59
76.5
H (wt %)b
5.10
5.78
5.28
5.78
5.28
5.63
N (wt %)
0.14
0.16
0.203
0.22
0.350
0.38
O (wt %)b
34.85
39.48
25.83
28.22
16.08
17.4
C/O (mol)
1.8/1
1.8/1
3.1/1
3.1/1
5.9/1
5.9/1
H/C (mol)
1.3/1
1.3/1
1.1/1
1.1/1
0.9/1
0.9/1
HHV (MJ/kg) TAN (mg KOH/g)
19.9 130
22.5
22.9 112
25.0
29.7 68
32.3
pH
2.5
2.5
2.8
ESP pyrolysis oil collected during 1st collection of run. b Hydrogen and oxygen values are organic only.
Table 7. Pertinent Pyrolysis Oil Components (wt %) in Esp Pyrolysis Oil Produced over Catalyst M with in Situ Regeneration catalyst
none
350 g cycles
175 g cycles
acetic acid
9.5
7.9
4.5
furfural
0.9
0.8
0.2
acetol
2.7
1.5
0.4
levoglucosan
23.5
9.0
3.5
phenol
0.2
0.7
1.0
cresols dimethyl- and ethyl-phenols
0.2 0.3
1.2 0.4
1.5 0.9
guaiacol
0.3
0.2
0.1
4-methyl guaiacol
0.3
0.1
0.2
syringol
0.3
0.2
0.3
toluene
trace
0.1
0.2
xylenes
trace
0.2
0.4
naphthalene
trace
0.6
1.60
1-methyl naphthalene
trace
0.7
2.6
’ CONCLUSIONS Two zeolite-type catalysts, one of which was ion exchanged with Ca2+ and had shown promise for the production of partially deoxygenated pyrolysis oils, were used on the bench scale for catalytic pyrolysis of white oak wood biomass. Each was successful at reducing oxygenates, producing aromatic hydrocarbons, and increasing the overall C/O ratio of the pyrolysis oils. However, both did so with decreased pyrolysis oil yields (e30 wt %). Some portion of the low yield may be due to material stuck in the pyrolysis system, but the main source of lower yield is the result of carbon deposits on the surface of the catalyst. These deposits can account for up to 50% of the overall carbon contained in the biomass. These deposits also deactivated the catalyst. Dehydration and dehydrogenation were the major mechanisms for the deoxygenation over catalyst M. The Ca Y zeolite appeared to be deactivated less quickly, possibly as a result of the presence of Ca2+ ions in place of Brønsted acid sites, which allowed for more oxygen removal following decarboxylation pathways. This allowed for slower coke formation because slightly more hydrogen was conserved in the pyrolysis products. An in situ catalyst regeneration process was designed to combust deactivating coke deposits in the bubbling fluidized bed reactor that mimicked the conditions of a catalytic cracker (i.e., FCC) where large catalyst/feed ratios are used with continual catalyst reactivation. This allowed continuous production of partially deoxygenated pyrolysis oils over catalyst M, which exhibited lower viscosity and slightly enhanced storage stability compared with pyrolysis liquids produced non-catalytically. Ongoing research includes testing process conditions, including new catalysts ratios and temperature effects that may increase the carbon and hydrogen efficiency of the catalytic pyrolysis process. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Figure 4. Comparison of viscosity changes with accelerated aging of pyrolysis oils produced over catalyst M.
initially (Figure 4), and the rate of viscosity increase was slightly lower than that produced non-catalytically. It is, therefore, fair to say that the significant partial deoxygenation was required to reduce the rate of aging, but did not eliminate pyrolysis oil instability.
’ DISCLOSURE Disclaimer 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. 5450
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’ ACKNOWLEDGMENT This work was performed under a Cooperative Research and Development Agreement between ARS/USDA and UOP, LLC, a Honeywell Company (ARS Agreement No. 58-3K95-9-1354). Catalysts were provided by UOP, a Honeywell Company. Funding was provided by DOE: Pyrolysis Oil Stabilization Program Award No. DE-FG36-08GO18213. ’ REFERENCES (1) Mohan, D.; Pittman, C. U., Jr.; Steele, P. H. Pyrolysis of Wood/ Biomass for Bio-oil: A Critical Review. Energy Fuels 2006, 20, 848–889. (2) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts and Engineering. Chem. Rev. 2006, 106, 4044–4098. (3) Mullen, C. A.; Boateng, A. A. Chemical Composition of Bio-Oils Produced by Fast Pyrolysis of Two Energy Crops. Energy Fuels 2008, 22, 2104–2109. (4) Huber, G. W.; Corma, A. Synergies between Bio- and Oil Refineries for the Production of Fuels from Biomass. Angew. Chem., Int. Ed. 2007, 46, 7184–7201. (5) Corma, A.; Huber, G. W.; Sauvanaud, L.; O’Connor, P. O. Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst. J. Catal. 2009, 247, 307–327. (6) Speight, J. G. In The Chemistry and Technology of Petroleum; Marcel Dekker, Inc.: New York, 1991; pp 473 497. (7) Carlson, T. R.; Vispute, T. P.; Huber, G. W. Green Gasoline by Catalytic Fast Pyrolysis of Solid Biomass Derived Compounds. ChemSusChem 2008, 1, 397–400. (8) Vispute, T. P.; Zhang, H.; Sanna, A.; Xiao, R.; Huber, G. W. Renewable Chemical Commodity Feedstocks from Integrated Catalytic Processing of Pyrolysis Oils. Science 2010, 330, 1222–1227. (9) Williams, P. T.; Nugranad, N. Comparison of Products from the Pyrolysis and Catalytic Pyrolysis of Rice Husks. Energy 2000, 25, 493–513. (10) Zhang, H.; Xiao, R.; Huang, H.; Xiao, G. Comparison of NonCatalytic and Catalytic Fast Pyrolysis of Corncob in a Fluidized Bed Reactor. Bioresour. Technol. 2009, 100, 1428–1434. (11) Boateng, A. A.; Mullen, C. A.; McMahan, C. M.; Whalen, M. C.; Cornish, K. Guayule (Parthenium argentatum) Pyrolysis and Analysis by PY-GC/MS. J. Anal. Appl. Pyrolysis 2010, 87, 14–23. (12) Mihalcik, D. L.; Mullen, C. A.; Boateng, A. A. Screening Acidic Zeolites for Catalytic Fast Pyrolysis of Biomass and its Components. J. Anal. Appl. Pyrolysis, 2011, In Press. (13) Compton, D. L.; Jackson, M. A.; Mihalcik, D. J.; Mullen, C. A.; Boateng, A .A. Catalytic Pyrolysis of Oak via Pyroprobe and Bench Scale, Packed Bed Pyrolysis Reactors. J. Anal. Appl. Pyrolysis 2011, 90, 174–181. (14) Boateng, A. A.; Daugaard, D. E.; Goldberg, N. M.; Hicks, K. B. Bench-Scale Fluidized-Bed Pyrolysis of Switchgrass for Bio-Oil Production. Ind. Eng. Chem. Res. 2007, 46, 1891–1897. (15) Boateng, A. A.; Mullen, C. A.; Goldberg, N.; Hicks, K. B.; Jung, H. J.; Lamb, J. F. S. Production of Bio-Oil from Alfalfa Stems by Fluidized-Bed Fast Pyrolysis. Ind. Eng. Chem. Res. 2008, 47, 4115–4122. (16) Oasmaa, A.; Kuoppala, E. Fast Pyrolysis of Forestry Residue. 2. Physicochemical Composition of Product Liquid. Energy Fuels 2003, 17, 1075–1084. (17) Komatsu, T.; Ishihara, H.; Fukui, Y.; Yashima, T. Selective Formation of Alkenes through the Cracking of n-heptane on Ca2+ -Exchanged Ferrierite. Appl. Catal., A 2001, 214, 103–109. (18) Bulushev, D. A.; Ross, J. R. H. Catalysis for Conversion of Biomass to Fuels via Pyrolysis and Gasification: A Review. Catal. Today, 2011 In Press. (19) Mullen, C. A.; Boateng, A. A.; Goldberg, N. M.; Lima, I. M.; Laird, D. A.; Hicks, K. B. Bio-Oil and Bio-Char Production from Corn Cobs and Stover by Fast Pyrolysis. Biomass Bioenergy 2010, 34, 67–74.
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