Article pubs.acs.org/EF
Results of the IEA Round Robin on Viscosity and Stability of Fast Pyrolysis Bio-oils Douglas C. Elliott* Pacific Northwest National Laboratory, Richland, Washington
Anja Oasmaa Technical Research Center of Finland, Espoo, Finland
Fernando Preto CanmetENERGY, Ottawa, Ontario, Canada
Dietrich Meier vTI-Institute of Wood Technology and Biology, Hamburg, Germany
Anthony V. Bridgwater Aston University, Birmingham, U.K. ABSTRACT: An international round robin study of the stability of fast pyrolysis bio-oil was undertaken. Fifteen laboratories in five different countries contributed. Two bio-oil samples were distributed to the laboratories for stability testing and further analysis. The stability test was defined in a method provided with the bio-oil samples. Viscosity measurement was a key input. The change in viscosity of a sealed sample of bio-oil held for 24 h at 80 °C was the defining element of stability. Subsequent analyses included ultimate analysis, density, moisture, ash, filterable solids, and TAN/pH determination, and gel permeation chromatography. The results showed that kinematic viscosity measurement was more generally conducted and more reproducibly performed versus dynamic viscosity measurement. The variation in the results of the stability test was great and a number of reasons for the variation were identified. The subsequent analyses proved to be at the level of reproducibility, as found in earlier round robins on bio-oil analysis. Clearly, the analyses were more straightforward and reproducible with a bio-oil sample low in filterable solids (0.2%), compared to one with a higher (2%) solids loading. These results can be helpful in setting standards for use of bio-oil, which is just coming into the marketplace.
1. INTRODUCTION Bio-oils from fast pyrolysis of lignocellulosic biomass (CAS #1207435-39-9) are totally different from mineral oils (Table 1), and hence, the standard fuel oil analyses cannot always be used, as such, for fast pyrolysis bio-oils. The standard analyses have been systematically tested for bio-oils,1−5 and modifications have been suggested when needed. Because of the significant differences between bio-oils and mineral oils, some new methods, such as solids content analysis, have also been developed.5 A first set of burner fuel specifications has been accepted for fast pyrolysis bio-oil as ASTM D7544.6 Related to the acceptance of this standard, the methods specified should be validated. The first standard method, ASTM D7579, was obtained for determination of the insoluble solids content. The ASTM method includes the validation results of a twolaboratory test over 10 successive days, as represented by the © 2012 American Chemical Society
repeatability measurement. This paper is focused on validating the bio-oil aging test based on viscosity increase. Fast pyrolysis bio-oils are not chemically and thermally as stable as conventional petroleum fuels due to their high content of reactive oxygen-containing compounds1−13 and low-boiling volatiles. The instability of pyrolysis liquids can be observed as increased viscosity over time (i.e. “aging”), particularly when heated. The aging reactions are enhanced by elevated temperature forming higher molecular weight compounds and leading first to separation of a heavier, less hydrophilic fraction. If the temperature is raised above 200 °C, the separated viscous oil polymerizes into gummy-like and finally char-like material.2−5 The stability of bio-oils has to be improved before commercial energy use is possible.14 Common practice is to Received: February 2, 2012 Published: May 15, 2012 3769
dx.doi.org/10.1021/ef300384t | Energy Fuels 2012, 26, 3769−3776
Energy & Fuels
Article
Since the major changes in aging happen in carbonyl and water insoluble fractions, the changes in these product groups could be used as stability indicators. The changes in carbonyls can be measured by GC analyzing aldehydes and ketones,11,13 carbonyl titration,16 or FTIR.12 The changes in water-insolubles can be measured by changes in viscosity2−6,8−10 or molecular weight.4,13 The change in water-insoluble content correlates with increased molecular weight distribution and viscosity.4,13 The viscosity increase-based stability test (80 °C4,13 or 90 °C8−10 for 24 h) measures the change in viscosity of the pyrolysis liquid. The change in carbonyl content16 of bio-oil correlates with the change in viscosity measured by stability testing conducted at 80 °C over 24 h.4,13 Due to the clear correlation between the methods, comparison of results from different laboratories should be straightforward. The first IEA (International Energy Agency) round robin was carried out by Elliott, McKinley, and Overend.17 It was found out that xylene distillation EN 95, which is used for mineral oils, cannot be used for the determination of water content of fast pyrolysis bio-oils because bio-oils contain a significant amount of volatile water-soluble compounds that end up being counted as part of the water fraction by this method. Karl Fischer titration was recommended as a suitable method for fast pyrolysis bio-oils. Two separate round robin tests were initiated in 1997: one within EU PyNe (pyrolysis network)18 and the other within IEA PYRA (pyrolysis activity).19 From both of them, it was concluded that the precision of carbon, hydrogen, density, and water by Karl Fischer titration was good. High variations were obtained for nitrogen, viscosity, pH, and solids. The conclusion was also that clear instructions for analyses are needed. In 2001 a round robin20 was organized within the IEA-EU PyNe cooperation. Analyses were carried out by 12 laboratories for four different fast pyrolysis bio-oils (originating from pine, spruce, hardwood mix, and bark) produced by different largescale pyrolysis processes. Water, solids content, pH, viscosity, stability test, and CHN determinations were included. In general, the accuracy of physical analyses, except the stability test, was good. The present round robin is focused on testing viscosity and the stability method as an accelerated aging test at 80 °C for 24 h.5
Table 1. Physical Properties of Pyrolysis Liquids (Bio-oils) and Mineral Oils analysis
pyrolysis liquids
light fuel oil (Tempera 15)
water, wt % solids, wt % ash, wt % nitrogen, wt % sulfur, wt % stability viscosity (40 °C), cSt density (15 °C), kg/dm3 flash point, °C pour point, °C LHV, MJ/kg pH distillability
20−30 0.01−1 0.01−0.2 0−0.4 0−0.05 unstable 15−35 1.10−1.30 40−110 −9 to −36 13−18 2−3 not distillable
0.025 0 0.01 0 0.2 stable 3.0−7.5 0.89 60 −15 40.3 neutral 160−400 °C
preheat fuel oils before combustion to lower their viscosity for better atomization. If a part of the fuel is recirculated back to the pump, as in diesel engines, polymerization and condensation reactions in bio-oil will cause an increase in viscosity. 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 of at least six months at 15 °C. The most significant reactions of pyrolysis liquids take place immediately after quenching of the liquid and cease during the first three months of storage.4,5,8−13 The principal changes during aging (Figure 1) include a reduction in carbonyl
2. MATERIALS AND METHODS The research was carried out within the IEA Bioenergy Agreement Pyrolysis Task 34. This paper summarizes results from the tests performed in the participating laboratories. Fifteen laboratories from the five participating countries in Task 34 agreed to contribute to this round robin, and each was supplied with samples of two bio-oils. The participants included:
Figure 1. Main changes in softwood (forestry residue, FR, and pine saw dust) pyrolysis liquids during storage.7 Presented on dry basis. Ranges are based on observing the changes for four FR liquids and two pine saw dust liquids. The continuous lines have been drawn for the same green FR liquid.
• • • • •
compounds, aldehydes, and ketones and an increase in the heavy water-insoluble fraction.4,5 The volatile acids content remains unchanged.4,5,15 Aldehydes can react with each other to form polyacetal oligomers and polymers. The poly(oxymethylene) polymer has limited solubility in water.10 In addition, the water content increases and the volatility of the oil decreases. A comprehensive overview of the stability of pyrolysis liquids is given by Diebold.10
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National Research Council of Canada, Canada Ensyn, Canada CanmetENERGY, Canada VTT, Technical Research Centre of Finland, Finland Fraunhofer Institute-Environmental, Safety, and Energy Technology, Germany Karlsruhe Institute of Technology, Germany vTI-Institute of Wood Technology and Wood Biology, Germany Aston University, U.K. University of Leeds, U.K. dx.doi.org/10.1021/ef300384t | Energy Fuels 2012, 26, 3769−3776
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• • • • • •
3. RESULTS The two bio-oil samples differed in that one was an “asproduced” bio-oil while the other had been filtered to remove solid char/fluid-bed carryover particles. Eight laboratories reported solids measurement on the bio-oil samples. The results are in Table 2. Solids were determined per the ASTM
University of York, U.K. Eastern Regional Research Center (USDA), U.S.A. Iowa State University, U.S.A. Mississippi State University, U.S.A. National Renewable Energy Laboratory, U.S.A. Pacific Northwest National Laboratory, U.S.A. Two wood fast pyrolysis oils (1000 mL each) were provided. One of the oils was filtered. Some instructions for the analyses were provided with the bio-oil samples, and a spreadsheet was provided for reporting the results. It was requested that the participants add the receiving date of the samples to the spreadsheet. In addition, it was requested that the biooil samples be stored under refrigeration between +5 and −10 °C. A suggestion for the sample bottles for the stability test was given: 50 mL SCHOTT: bottle, DURAN, with screw cap. SCHOTT number: 218011753. https://www.vwrcanlab.com/ The analyses requested included:
Table 2. Insoluble Solids Measurements in Two Bio-oil Samples
• Stability test at 80 °C for 24 h • Water content by Karl Fischer titration according to ASTM E203 or equivalent, with at least triplicates
a
• Viscosity at 40 °C as kinematic viscosity (cSt) or dynamic viscosity, in which case the density at the same temperature should be provided (to allow conversion to kinematic viscosity). The stability test was described as follows: The pyrolysis liquid sample is mixed properly and left to stand until the air bubbles are removed. New, empty 50 mL airtight bottles are treated at 80 °C for a few hours before use to remove moisture. Next, 45 mL of the sample is poured into 50 mL tight glass bottles. The bottles are 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 every time. Typically a maximum of five bottles are placed in an ∼10 dm3 heating oven. The bottles are retightened after 10 min. After the specified 24 h, the closed sample bottles are cooled at room temperature for 1.5 h, weighed and analyzed. The samples are mixed and measured for viscosity and water. The viscosity of the liquid at 40 °C is measured as kinematic viscosity by a standard method (e.g. ASTM D 445). The water content is analyzed by Karl Fischer titration according to ASTM D 1744. Δ viscosity@40°C [%] = Δ water [%] =
lab
bio-oil #1, wt % (avg)
bio-oil #2, wt % (avg)
4 5 6 7 9 14 15 16
0.045 0.076 0.07 0.013 0.02 0.21a 0.080 0.14
3.80 2.70 2.41 2.44 2.15 0.88a 0.78 2.34
Not microfiber filtered.
D7579-09 method for pyrolysis solids content in pyrolysis liquids by filtration of solids in methanol. Bio-oil #1 is the filtered bio-oil, and the average value for the seven laboratories that strictly followed the procedure is 0.06 wt %. Clearly, the use of a nonstandard filter element, having also a very large (20 μm) pore size, as done by lab #14, gives a different result. The unfiltered bio-oil #2 had an average solids content of 2.37%, for those laboratories adhering to the standard method. All 15 laboratories reported viscosity measurements with the filtered bio-oil sample, see Table 3. Eight laboratories (shown Table 3. Viscosity/Aging Measurements in Bio-oil #1a
ν2 − ν1 ν1
ω2 − ω1 ω1
ν1 = viscosity of the original sample, measured at 40 °C, cSt ν2 = viscosity of the aged sample, measured at 40 °C, cSt ω1 = water content of the original sample, wt % ω2 = water content of the aged sample, wt % Note 1. The test is recommended for use in internal comparisons of liquid stability for pyrolysis liquids from one process. The test is more reliable if the initial viscosities of the tested samples are similar. Note 2. The possible difference in weights 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. If possible, the tests and analyses were repeated 10 days after the first analyses. If laboratories were willing, more comparison could be done by carrying out optional analyses: density, solids as insoluble in a specific solvent, CHN, S, Cl, ash, total acid number, TAN, GPC. For reporting results, an Excel spreadsheet was provided. Besides analytical results, the methods used and dates of analyses were requested.
a
Avg of 3, when std dev is listed. NA = not analyzed.
without color in the tables) used the standard kinematic viscosity method (D445), while six measured dynamic viscosity, three (shown in orange in the tables) used a Brookfield rotational unit, two (shown in mauve in the tables) used the Anton Parr automated rotational viscosity method, and two (shown in olive in the tables) used a Minivis II automated falling ball system. Fifteen of the sixteen laboratories were able to report results from the aging test, while one reported that the bio-oil phase separated in the aging test. Thirteen laboratories reported moisture analysis results. As shown in Table 4, 10 of the laboratories repeated the tests after 10 days. Typically, a slight increase in viscosity was 3771
dx.doi.org/10.1021/ef300384t | Energy Fuels 2012, 26, 3769−3776
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Table 4. Viscosity/Moisture Measurements in Bio-oil #1a
a
Table 6. Viscosity/Moisture Measurements in Unfiltered Bio-oil #2a
Avg of 3, when std dev is listed. NA = not analyzed. a
Avg of 3, when std dev is listed. ND = too dark colored to read capillary. NA = not analyzed.
measured, while moisture contents, on average, appeared unchanged, as some increased and some decreased. In the case of the unfiltered bio-oil, see Table 5, one of the laboratories using the kinematic viscosity method could not
the results show a much wider variation than can be explained by the temperature differences.
Table 5. Viscosity/Aging Measurements in Unfiltered Bio-oil #2a
Table 7. Density Measurements in Two Bio-oil Samples lab
bio-oil #1, g/cm3
bio-oil #2, g/cm3
measurement temp., °C
2 4 6 7 9 10 11 14 15 16
1.192 1.214 1.194 1.213 1.151 1.190 1.188 1.166 1.187 1.212
1.195 1.210 1.323 1.212 1.153 1.195 1.184 1.149 1.193 1.215
15 15 40 40 40 40 40 25 40 15
Elemental analyses (Table 8) including carbon, hydrogen, nitrogen, sulfur, and chlorine. Although oxygen is likely the most prominent element in the bio-oil, it was not analyzed by any lab and could only be determined by difference. The nitrogen analysis was typically from a thermo-automated analysis with a sensitivity limit near that of the nitrogen content in these bio-oils. In addition to that method, lab #16 performed nitrogen analysis also by a more sensitive method (D5762, using chemiluminescence) and reported lower numbers, 0.0904% and 0.0839%, for the filtered and unfiltered bio-oils, respectively. Other analyses (Table 9) including ash, TAN (total acid number), pH, and carbonyl content. The numbers in the several categories show a consistency in scale, but they also suggest a large variability in the reproducibility from one lab to another. Gel permeation chromatography (GPC), also known as size exclusion chromatography (SEC), for the two bio-oil samples was reported by three of the participating laboratories. At lab #14, the GPC was performed with a Polymer Laboratories GPC50 using a refractive index (RI) detector. The separation was performed using two columns in series. The mobile phase was tetrahydrofuran (THF, HPLC grade). The flow rate of the mobile phase was 1 mL/min. The GPC was operated at 40 °C. The GPC was calibrated against polystyrene standards provided
a
Avg of 3, when std dev is listed ND = too dark colored to read capillary. NA = not analyzed.
generate useful data, as they could not get an optical reading because of the particulates. Twelve of the other fourteen laboratories were able to generate aging test results; two reported no results due to phase separation, while four others reported some phase separation in the aging test that required careful mixing and sampling in order to generate viscosity measurements but led to inconsistency in the measurement. Fourteen laboratories reported moisture analysis results. As shown in Table 6, ten of the laboratories repeated the tests after 10 days. As was seen for the filtered bio-oil, typically, a slight increase in viscosity was measured, while moisture contents, on average, appeared nearly unchanged as a few increased but most decreased. Other analyses reported included density (Table 7), wherein several temperatures were used by the different participants, but 3772
dx.doi.org/10.1021/ef300384t | Energy Fuels 2012, 26, 3769−3776
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Table 8. Elemental Analyses of Two Bio-oil Samples (wt % of Bio-oil as Received)a bio-oil #1 (filtered)
a
bio-oil #2 (unfiltered)
lab
C
H
N
S
Cl
C
H
N
S
Cl
2 4 5 6 7 11 12 15 16 avg std dev coefficient of variation
42.61 41.22 41.92 40.17 42.35 42.52 40.68 44.92 42.7 42.12 1.39 3.29%
7.50 7.73 6.51 6.04 7.453 7.46 7.58 6.19 7.69 7.13 0.68 9.52%
0.06
