ARTICLE pubs.acs.org/EF
An Approach for Stability Measurement of Wood-Based Fast Pyrolysis Bio-Oils Anja Oasmaa,* Jaana Korhonen, and Eeva Kuoppala VTT Technical Research Centre of Finland, Post Office Box 1000, 02044 VTT, Espoo, Finland ABSTRACT: This paper presents a simple stability test method based on changes in carbonyl compounds. Whereas the method of identifying carbonyl groups by means of titration is in itself well-known, its use in determining the stability of fast pyrolysis bio-oils has previously not been established. Pyrolysis oil stability is typically measured as viscosity increase under specified conditions. The increase in viscosity occurs due to an increase in the amount and molecular weight of water insolubles present in the oil. The change in carbonyl compounds is due to reactions of aldehydes and ketones during storage, the reaction products of which end up in the water-insoluble fraction. This paper shows that the carbonyl titration method, based on changes in aldehydes and ketones, correlates with the viscosity increase-based stability test and that, due to this clear correlation, both methods can be used and comparison between the methods is possible.
’ INTRODUCTION Fast pyrolysis liquids (bio-oils) are not as chemically or thermally stable as conventional petroleum fuels due to their high content of reactive oxygen-containing compounds and lowboiling volatiles. The instability of pyrolysis liquids can be observed as increased viscosity over time, that is, “aging”, particularly when heated. The most significant reactions of pyrolysis liquids take place immediately after quenching of the liquid and cease during the first 3 months of storage.1,2 The principal changes during aging (Figure 1) include a reduction in carbonyl compounds, aldehydes and ketones, and an increase in the heavy water-insoluble fraction.2,3 The volatile acid content remains unchanged.4 The main chemical reactions reported are polymerization of double-bonded compounds, condensation reactions, and etherification and esterification occurring between hydroxyl and carbonyl components5 in which water is formed as a byproduct. The water-insoluble content increases, in turn increasing MW2,3 and viscosity. In addition, the water content increases and the volatility of the oil decreases.2,6,7 A comprehensive overview of the stability of pyrolysis liquids is given by Diebold.1 According to this work,1 the aging rate of softwood bio-oil is about the same as for hardwood bio-oils at 20 °C, with some possible differences at lower storage temperatures. However, the viscosity change during aging is very small (below 20 °C), making low-temperature aging rates subject to measurement errors. According to Czernik,9 the molecular weight of bio-oils correlates very well with viscosity during aging at 37, 60, and 90 °C. During aging,1 chemical reactions, which apparently increase the average molecular weight, take place in bio-oil. On the basis of the good correlation for the aging data,9 relatively similar chemical reactions appear to occur over this temperature range. This is the basis for conducting accelerated aging research at elevated temperatures and then applying the results to predict storage of bio-oils at lower temperatures. The advantage of accelerated aging tests is the short time required to demonstrate the aging properties of a particular bio-oil. Bio-oil contains1 a large number of oxygenated organic compounds with r 2011 American Chemical Society
a wide range of molecular weights, typically in small percentages. During storage, the chemical composition of the bio-oil changes toward thermodynamic equilibrium under storage conditions, resulting in changes in the viscosity, molecular weight, and cosolubility of its many compounds. There is no standardized method for measuring the stability of pyrolysis liquids. A simple test has, however, been developed8 for quick comparison of the stability of different pyrolysis liquids. Czernik et al.5,9 suggested on the basis of correlations that the reactions of pyrolysis liquids are quite similar over the 3790 °C temperature range. In the test, the pyrolysis liquid is kept at a fixed temperature for a set time (in the VTT test, 80 °C for 24 h) and the increase in viscosity is measured.2,7,10,11 For pyrolysis liquids having a water content of about 25 wt %, the increase in viscosity under test conditions for 24 h at 80 °C correlates with the viscosity increase after 1 year of storage at room temperature.2,3 The test method has since been further refined7 following a round-robin test campaign conducted in 2005,12 which showed variation in the test’s results. The reason for the variation in round robin is most probably lack of experience. At VTT the test has been used for over 15 years without any problems. The accuracy of the test method is 5%.7 Hilten and Das13 compared the viscosity change-based stability test with two standard methods, ASTM D 5304 and E 2009, for a slow pyrolysis oil. ASTM D 5304 determines the storage stability of middle distillate fuels, that is, fuels that fast pyrolysis liquids are intended to replace or supplement. Stability by this method is ranked on the basis of formation of insoluble material after an accelerated aging procedure under pressure (800 kPa absolute) with oxygen heating to 90 °C in a forced-air oven for 16 h. The sample is filtered before the test and the amount of insoluble material formed during the test is measured. However, filtration of fast pyrolysis liquids is known to be difficult and unreliable. The oxidation onset test following ASTM E 2009, Received: May 2, 2011 Revised: June 9, 2011 Published: June 09, 2011 3307
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typically used for edible oils and fats, lubricants, greases, and polyolefins, was also tested with slow pyrolysis oil.13 The test uses differential scanning calorimetry (DSC) analysis to determine the temperature at which oxidation (combustion) begins, that is, the oxidation onset temperature (OOT). As stated in the ASTM method, samples with a higher OOT
are more stable. The stability assessment methods ranked the bio-oils similarly. They concluded that OOT has potential to be used for quick stability determination for slow pyrolysis oils. In their study on pyrolytic lignins, Scholze et al.14 proposed FTIR (Fourier transform infrared spectroscopy) analysis as a fast analytical method to elucidate aging processes of fast pyrolysis liquid. FTIR data indicate that a changing oxygen content affects the intensity of carbonyl absorption bands. FTIR results showed a correlation between carbonyl absorption bands and oxygen content as well as carbon content. Scholze and Meier15 also measured carbonyl groups of pyrolytic lignin using a wet chemical method (oximation) and found16 that the amount of carbonyl groups decreases during storage of fast pyrolysis liquid (VTT pine pyrolysis liquid). The stability of pyrolysis liquids has to be improved before commercial use for energy is possible. Common practice is to preheat fuel oils before combustion to lower their viscosity for better atomization.1 For example, with diesel engines, the fuel is pumped through a preheater to the injector, where only a small fraction of the fuel is injected into the engine. The remainder of the hot fuel is normally recirculated back to the pump. This is problematic because of growth of particulates due to polymerization and/or physical agglomeration of micelles. The change in properties would require changes in fuel feeding system and/or adjustments in combustion conditions, which is not acceptable. The properties of the liquid have to be constant during the typical storage, at least six months at 15 °C. There are various methods
Figure 1. Main changes in forestry residue pyrolysis liquids during storage.3 Presented on dry basis.
Table 1. Chemical Composition of Pine 1 Fast Pyrolysis Liquid component
wet, wt %
dry, wt %
whole oil
23.9
0
water
23.9
0
acids
4.3
5.6
formic acid
1.5
acetic acid
3.4
propionic acid glycolic acid alcohols
2.2
N, %
O, %
53.3
6.5
0.08
40
40.0
6.7
0
53.3
0.6 2.9
37.5
12.5
0
50.0
40.0
6.7
0
53.3
48.6
8.11
0
43.2
44.1
6.6
0.1
49.2
68
6.7
0.1
25.2
0.3
methanol
2.6 15.4
20.3
nonaromatic aldehydes
9.72
aromatic aldehydes
0.009
nonaromatic ketones
5.36
furans pyrans
3.37 1.10
sugars
H, %
0.2
ethylene glycol aldehydes, ketones, furans, pyrans
C, %
34.4
1,5-anhydro-β-D-arabinofuranose
45.3 0.27
anhydro-β-D-glucopyranose (levoglucosan)
4.01
1,4:3,6-dianhydro-R-D-glucopyranose
0.17
hydroxy, sugar acids LMM lignin
13.4
catechols lignin-derived phenols
17.7 0.06 0.09
guaiacols (methoxyphenols)
3.82
HMM lignin
1.95
2.6
63.5
5.9
0.3
30.3
extractives
4.35
5.7
75.4
9.0
0.2
15.4
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Table 2. Repeatability of Carbonyl Determination sample, g
mL
mL
carbonyl, mmol
carbonyl content, mmol/g
unreacted hydroxylamine, %
4.9375
7.0801
4.5713
6.5009
12.7612
7.7
3.58
44.5
12.3873
16.3
3.56
5.0184
7.0882
47.5
12.5854
17.7
3.53
43.7
5.015
3.53
43.4
5.2003
3.48
40.9
5.5566
3.41
38.9
5.009
3.50
42.4
4.9090 4.8631
3.47 3.49
45.4 46.1
5.112
3.49
44.4
4.8301
3.52
44.5
5.1596
3.46
41.8
average
3.50
std dev
0.046
for stability improvements, including dilution with alcohols,1,20 reduced pressure distillation,21 and hydrotreatment. Carbonyl groups are known to be one reason for the instability of pyrolysis liquids. The increase in viscosity is due to increase in water insolubles.3 As shown in Figure 1, it seems that, besides condensation, reactions yielding watercarbonyl compounds cause an increase in water-insoluble fraction. Acids do not react in aging.4 Most of the aging reactions1 are not well-known. Aldehydes can react with each other to form polyacetal oligomers and polymers. The poly(oxymethylene) polymer has limited solubility in water.1 This indicates a correlation between decrease in carbonyl compounds and increase in water insolubles. No test method based on changes in carbonyls has previously been defined, nor has a correlation between change in viscosity and carbonyl content been suggested. This paper compares the stability test method based on viscosity change with a test method based on change in carbonyl content.
’ EXPERIMENTAL SECTION Samples. The pyrolysis liquids studied were produced at VTT’s Process Development Unit (PDU), which employs a transport bed reactor.17 The pyrolysis temperature was around 480520 °C, and the residence time for pyrolysis vapors was around 0.52 s. The majority of char particles and heat transfer sand were removed by cyclones from the hot stream of product gases and vapors before it entered liquid scrubbers. The product vapors were condensed in the liquid scrubbers, in which the product liquid was used as a cooling agent. Typical product yields (weight percent of dry ash-free feedstock) from pine wood are 64 wt % organic liquids, 12 wt % product water (chemically dissolved in organic liquids), 12 wt % char, and 12 wt % noncondensable gases. Separation of the top phase (11 wt % for pine pyrolysis liquid in Table 1) from the pine/forest residue product liquid was carried out at 35 °C over 24 h by the method described earlier.18 The bottom phase was subjected to analysis. Stability Test Based on Viscosity Change. The new bottles are heated at 80 °C for a few hours before use to remove moisture. The pyrolysis liquid sample is mixed thoroughly and left to stand until free of air bubbles. Next, 45 mL of the sample is poured into 50 mL tight glass bottles. The bottles are then firmly closed and preweighed before being placed in a heating oven at 80 °C ((1 °C) for exactly 24 h. It is recommended that the same heating oven and preferably the same number of bottles are used each time. At VTT, a maximum of five bottles
Figure 2. Change in viscosity of a pine pyrolysis liquid under different storage conditions. are placed in an approximately 10 dm3 heating oven. A reference sample of a known pyrolysis liquid is included in the series. The bottles are retightened after 10 min. After a set time the closed sample bottles are cooled at room temperature for 1.5 h, weighed, and analyzed. The weighing of the sample bottles is important, as no evaporation of the sample can be allowed. The samples are mixed and measured for viscosity and water. The viscosity of the liquid at 40 °C is measured as kinematic viscosity using a standard method (ASTM D 445). The water content is analyzed by Karl Fischer titration according to ASTM D 1744.7 Both are expressed as percentages. ν2 ν1 Δviscosity@40°C ¼ ν1 Δwater ¼
ω2 ω1 ω1
where ν1 = viscosity of the original sample, measured at 40 °C, in centistokes; ν2 = viscosity of the aged sample, measured at 40 °C, in centitokes; ω1 = water content of the original sample (weight percent); and ω2 = water content of the aged sample (weight percent). •Note 1: The test is recommended for use in internal comparisons of stability of pyrolysis liquids from a single process. The test is more reliable if the initial viscosities of the tested samples are similar. 3309
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Table 3. Change in GC-Eluted Compounds of Pine 1 Liquid by GC/FID over 4 Years at Various Temperatures fresh, compd
sample [16.5 °C, 4 years], sample [+9 °C, 4 years], sample [+22 °C, 4 years], sample [+80 °C, 24 h],
wt %
wt %
wt %
wt %
wt %
acetaldehyde
0.15
0.17
0.06
0.03
0.09
furan
0.03
0.04
0.01
0.01
0.02
2-propanone (acetone)
0.06
0.07
0.06
0.05
0.06
methanol
0.49
0.49
0.18
0.28
0.45
2-butanone
0.04
0.04
0.03
0.04
0.04
2-propanol
1.81
1.86
1.36
1.52
1.80
ethanol 2-pentanone
0.00 0.05
0.00 0.06
0.03 0.04
0.00 0.04
0.00 0.05
n-propanol
0.00
0.00
0.00
0.00
0.00
n-butanol, ISTD
0.00
0.00
0.00
0.00
0.00
1 -hydroxy-2-propanone
2.08
2.06
1.72
1.10
1.83
glycolaldehyde (2-hydroxyacetaldehyde)
7.00
7.12
5.48
3.71
4.70
acetic acid
2.61
2.50
2.53
2.74
2.68
furfural
0.22
0.22
0.14
0.10
0.22
2-furyl methyl ketone (2-acetylfuran) propionic acid
0.02 0.19
0.02 0.18
0.02 0.17
0.03 0.17
0.03 0.18
isobutanoic acid (2-methylpropanoic acid)
0.05
0.05
0.02
0.02
0.04
5-methylfurfural
0.03
0.03
0.01
0.01
0.03
butanoic acid
0.06
0.02
0.05
0.05
0.06
valeric acid (n-pentanoic acid)
0.01
0.02
0.02
0.01
0.01
n-hexanoic acid (caproic acid)
0.05
0.05
0.04
0.04
0.06
guaiacol (2-methoxyphenol) or vanillic acid
0.24
0.23
0.16
0.17
0.23
3-methylguaiacol (3-methoxy-4-methylphenol) o-cresol
0.27 0.02
0.24 0.01
0.15 0.01
0.16 0.01
0.24 0.01
phenol
0.02
0.02
0.02
0.02
0.03
4-ethylguaiacol
0.03
0.02
0.02
0.03
0.03
m-cresol
0.10
0.10
0.06
0.08
0.08
2-propylphenol
0.01
0.02
0.01
0.01
0.01
4-ethylphenol
0.05
0.05
0.04
0.06
0.06
syringol or 4-propyphenol
0.03
0.03
0.02
0.02
0.03
4-methylsyringol 5-(hydroxymethyl)furfural
0.03 0.45
0.03 0.45
0.01 0.27
0.01 0.14
0.02 0.39
catechol
0.02
0.01
0.01
0.02
0.02
4-methylcatechol
0.00
0.00
0.00
0.03
0.01
4-ethylcatechol
0.03
0.04
0.00
0.01
0.00
sum of aldehydes and ketones
10.1
10.3
7.8
5.3
7.4
alcohols
2.3
2.3
1.6
1.8
2.2
phenols
0.8
0.8
0.5
0.6
0.8
3.0 16.3
2.8 16.2
2.8 12.7
3.0 10.7
3.0 13.5
acids total sum
•Note 2: Any difference in weight before and after the test is an indication of leakage. The test should be repeated if the net weight loss is above 0.1 wt % of the original weight. •Note 3: The reference sample is a good-quality pyrolysis liquid that has been freshly divided into sample bottles and stored in a freezer below 9 °C. Carbonyl Titration. The method uses the reaction between hydroxylamine hydrochloride and pyridine to determine more than 30 aliphatic, alicyclic, and aromatic aldehydes and ketones (eq 1). Carboxylic acids or esters do not react. The purpose of the pyridine is to ensure complete oxygenation. The acid liberated as pyridine hydrochloride
acid is determined by titration. The test result is the amount of carbonyl groups in the sample.
The formed oxime has the general formula R1R2CdNOH, where R1 is an organic side chain and R2 may be hydrogen, forming an aldoxime, or another organic group, forming a ketoxime. The liberated HCl is titrated with a base. The titration solvent is c(NaOH) = 1.0 mol/L Merck 1.09956.0001. For preparation of the 3310
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Energy & Fuels hydroxylamine hydrochloride solution, 8.75 g of hydroxylamine hydrochloride (Merck 1.04616.0250) is dissolved in 40 mL of ion-exchanged water and diluted up to 250 mL with ethanol (g99%). For preparation of the pyridine solution, 10 mL of pyridine (Merck 7463.0500) is added to a 500 mL measuring bottle and diluted with ethanol. Buffer solutions of pH 4 and 7 are used. The method was tested with acetone, propanal, furfural, and 2,3-butanedione. The sample is weighed in a sealed bottle. The sample size (approximately 25 g) is chosen so that the amount of unreacted hydroxyl amine is 3350% but never below 33%. Hydroxylamine hydrochloride solution (50 mL) and pyridine solution (100 mL) are added to bottles, which are closed tightly. The sample solutions are then mixed for a minimum of 10 h to complete the oxygenation reactions. The sample solution is then washed in a 250 mL sample bottle with ethanol. A 100 mL sample is then taken and titrated with 1 N NaOH solution. The first end point is at pH 4.48 and the second is at pH 9. At the end point, the tangent shows the largest change. The repeatability of the method is presented in Table 2. In the literature, Bryant et al.22 have claimed that the presence of acid compounds in the sample during titration leads to erroneous results. They had different type of liquids, but to be sure, this was tested. Increasing amounts (0, 3, and 6 wt %) of acetic acid, the main acid in fast pyrolysis liquids, was added in one pine pyrolysis liquid and carbonyl content was measured. The results were the same (4.34.4 mmol/g), and it is concluded that with fast pyrolysis liquids the amount of acid does not affect the carbonyl content. Gas Chromatograph/Flame Ionization Detector. Quantitative composition by GC/FID (Gas Chromatograph/Flame Ionization Detector), column: HP-Innowax (Cross-linked Polyethylene Glycol) 60 m, 0.25 mm, 0.25 μm. Aqueous phase preparation by extracting the sample with water (sample:water =1:20). N-buthanol is added to the aqueous extract as an internal standard. A 1 μL sample is injected (split injection, ratio 20:1) into the oven. Temperature program of the oven: 60 °C for 1 min, heating rate 3 °C/min up to 230 °C, hold time 30 min. Identification and quantification of compounds using 36 reference compounds. Solvent Fractionation. Liquid products were characterized using a solvent fractionation scheme. The following fractions are obtained by this procedure: a water-insoluble (WIS) fraction, which was further divided into a DCM (dichloromethane)-soluble (low-molecular-mass, LMM lignin, extractives present in this fraction) and a DCM-insoluble (high-molecular-mass, HMM lignin) fraction; and a water-soluble (WS) fraction, which was further extracted into an ether soluble (ES) and ether insoluble (EIS, ‘sugars’) fraction.2,19
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Figure 3. Stability (viscosity change at 80 °C over 24 h) of VTT PDU (20 kg/h) pyrolysis liquids from various softwoods (pine, spruce, forest residues) as a function of water.
Figure 4. Viscosity over time at 80 °C for two softwood pyrolysis liquids. Viscosity was measured at 40 °C.
’ RESULTS AND DISCUSSION Changes in Viscosity during Aging. Figure 2 shows the change in viscosity at various storage conditions for a pine pyrolysis liquid (Pine 1). The change in carbonyl content for the same samples is shown in Figure 6. The viscosity increase in the viscosity increase based stability test (24 h at 80 °C) varies considerably depending on the chemical composition, mainly amount of water, and aldehydes and ketones (Table 3), of the pyrolysis liquid. Figure 3 shows the effect of water content on pyrolysis liquid stability. The samples were produced by VTT’s Process Development Unit (VTT PDU, 20 kg/h) from various softwoods (pine sawdust, forest residues) under similar reaction conditions. As can be seen from Figure 4, under the stability test conditions the viscosity increase over time for the two pine pyrolysis liquids is very steady.
Figure 5. Carbonyl content over time at 80 °C.
Change in Carbonyl Groups during Aging: Results of Titration. Carbonyl groups were measured (Figure 5) for
pyrolysis liquids aged according to the stability test but for varying lengths of time. Every second sample was analyzed for viscosity change (Figure 4). The remainder of the samples were quantified for individual compounds by GC/FID (Table 3). There was a steady decrease in carbonyls over time. Figure 6 shows the change in carbonyl content over time at various storage temperatures. 3311
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Figure 6. Change in carbonyl content over time at various storage temperatures for a pine pyrolysis liquid (Pine 1). Viscosities of the same samples are shown in Figure 2.
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Figure 8. Correlation between change in carbonyl content and change in viscosity for pine pyrolysis liquid samples kept at 80 °C for up to 60 h.
Figure 9. Correlation between carbonyl content and water-insoluble content for pine pyrolysis liquid (Pine 2) samples kept at 80 °C for up to 60 h. Figure 7. Change in aldehydes and ketones at 80 °C over time for Pine 1 and Pine 2 liquids. Change in carbonyl content for these liquids is shown in Figure 5.
Change in Carbonyl Compounds during Aging: Consumption of Compounds. The change in aldehydes and ketones was
similar to that observed for carbonyls. Figure 7 shows the change in aldehydes and ketones at 80 °C over time for the Pine 2 liquid. The change in carbonyl content for this liquid follows the same pattern (Figure 5). Table 3 shows the change in aldehydes and ketones during 4-year storage at various temperatures for Pine 1. It can be seen that the amount of carbonyl compounds remains unchanged at low temperatures (16.5 °C) but decreases at +9 to +22 °C. This has also been observed in earlier studies.1,2,5,7,11 Figures 2 and 6 show similar trends in the viscosity and carbonyl content of these samples (48 months). Comparison of Stability Tests. There are clear correlations between the changes in carbonyl content and viscosity (Figure 8) and between the changes in carbonyl content and water-insoluble content (Figure 9) of pine pyrolysis liquids. This is also evident from Figure 1, where the change in carbonyl content over time occurs at virtually the same rate as the change in waterinsoluble content. Increased water-insoluble content is known to correlate with viscosity increase.1,2,5 It is known1,5 that the aging proceeds quite similarlyin pyrolysis liquids from softwoods and
hardwoods. Hence, the same correlations may be assumed for other wood pyrolysis liquids.
’ CONCLUSIONS The principal changes involved in the aging of pyrolysis oils are decreased carbonyl compound content paired with an increase in water-insoluble content. The change in water-insoluble content correlates with increased molecular weight distribution and viscosity. The viscosity increase-based stability test (80 °C for 24 h) measures the change in viscosity of the pyrolysis liquid. It was proven in this study that the change in carbonyl content of pyrolysis liquid correlates with the change in viscosity measured by stability testing conducted at 80 °C over 24 h. Therefore, both the viscosity increase-based method and the proposed carbonyl titration method can be used for stability testing for wood pyrolysis liquids. Carbonyl titration may be easier to conduct in the laboratory, as the test can be carried out with standard laboratory glassware. Finally, due to the clear correlation between the methods, comparison of results from different laboratories should be straightforward. ’ AUTHOR INFORMATION Corresponding Author
*E-mail Anja.Oasmaa@vtt.fi. 3312
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’ ACKNOWLEDGMENT Special thanks to Douglas C. Elliott of Pacific Northwest National Laboratory for his valuable comments and to Elina Paasonen, Kaija Luomanper€a, and Sirpa Lehtinen for their analytical assistance. Appreciation also is expressed to Metso, UPM, Fortum, Tekes, and VTT for project funding.
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