Results of the IEA Round Robin on Viscosity and Aging of Fast

Article pubs.acs.org/EF

Results of the IEA Round Robin on Viscosity and Aging of Fast Pyrolysis Bio-oils: Long-Term Tests and Repeatability Douglas C. Elliott,*,† Anja Oasmaa,‡ Dietrich Meier,§ Fernando Preto,∥ and Anthony V. Bridgwater⊥ †

Pacific Northwest National Laboratory (PNNL), Richland, Washington 99354, United States Technical Research Center of Finland (VTT), FIN-02044 Espoo, Finland § Thünen Institute of Wood Research (vTI), D-21031 Hamburg, Germany ∥ CanmetENERGY, Ottawa, Ontario K1A 1M1, Canada ⊥ Aston University, Birmingham B4 7ET, United Kingdom ‡

ABSTRACT: An international round robin study of the viscosity measurements and aging of fast pyrolysis bio-oil has been undertaken recently, and this work is an outgrowth from that effort. Two bio-oil samples were distributed to two laboratories for accelerated aging tests and to three laboratories of long-term aging studies. The accelerated aging test was defined as the change in viscosity of a sealed sample of bio-oil held for 24 h at 80 °C. The test was repeated 10 times over consecutive days to determine the intra-laboratory repeatability of the method. Other bio-oil samples were placed in storage at three temperatures, 21, 5, and −17 °C, for a period of up to 1 year to evaluate the change in viscosity. The variation in the results of the accelerated aging test was shown to be low within a given laboratory. The long-term aging studies showed that storage of a filtered bio-oil under refrigeration can minimize the amount of change in viscosity. The accelerated aging test gave a measure of change similar to that of 6−12 months of storage at room temperature for a filtered bio-oil. Filtration of solids was identified as a key contributor to improving the stability of the bio-oil as expressed by the viscosity based on results of the accelerated aging tests as well as long-term aging studies. Only the filtered bio-oil consistently gave useful results in the accelerated aging and long-term aging studies. The inconsistency suggests that better protocols need to be developed for sampling bio-oils. 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 Registry Number 1207435-39-9) are chemically different from mineral oils (as shown in Table 1), and hence, the standard fuel oil analyses cannot always be used, as such, for fast pyrolysis bio-oils.1 The standard analyses have been systematically tested for bio-oils,2−5 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 [American Society for Testing and Materials (ASTM) D7579] have also been developed. A first set of burner fuel specifications has been accepted for fast pyrolysis bio-oil as ASTM D7544. 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 repeatability measurement. This paper is focused on validating the bio-oil accelerated aging test based on a viscosity increase over a similar 10 day period. Fast pyrolysis bio-oils are not chemically and thermally as stable as conventional petroleum fuels because of their high content of reactive oxygen-containing compounds6−12 and lowboiling 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

Table 1. Physical Properties of Typical Fast Pyrolysis Biooils and Mineral Oils analysis water (wt %) solids (wt %) ash (wt %) nitrogen (wt %) sulfur (wt %) viscosity at 40 °C (cSt) density at 40 °C (g/mL) pour point (°C) flash point (°C) LHV (MJ/kg) pH distillability stability

fast pyrolysis bio-oil

light fuel oil

20−30 0.01−1 0.01−0.2 0−0.4 0−0.05 15−35

0.025 0 0.01 0 0.2 3.0−7.5

1.1−1.3

0.89

from −9 to −36 does not sustain combustion 13−18 >2.5 not distillable viscosity can increase and can phase-separate

−15 60 40.3 neutral 160−400 °C stable

© 2012 American Chemical Society

Received: October 1, 2012 Revised: October 31, 2012 Published: November 6, 2012 7362

dx.doi.org/10.1021/ef301607v | Energy Fuels 2012, 26, 7362−7366

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room temperature (average, 20.6 °C; standard deviation, 1.1), refrigerator temperature (average, 5.3 °C; standard deviation, 0.2), and freezer temperature (average, −17.0 °C; standard deviation, 0.3).

The stability of bio-oils needs to be considered during commercial energy use.13 Common practice is to preheat fuel oils before combustion to lower their viscosity for better atomization. If a part of the fuel is recirculated back to the pump, such 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 the fuel feeding system and/or adjustments in combustion conditions, which are not acceptable. In addition, the properties of the liquid have to be constant during the typical storage of at least 6 months at 15 °C.14,15 The IEA Bioenergy association has provided the mechanism for round robin analysis for bio-oil analysis and method validation. There have been several over the years evaluating ultimate analysis and physical properties,16−18,21 as well as lignin pyrolysis.19 The most recent example was a round robin in 2011 to evaluate viscosity measurement and its use in the accelerated aging.20 The present round robin is focused on testing repeatability of the accelerated aging method and the change in viscosity (aging) of bio-oil at a range of temperatures over an extended period of time.

3. RESULTS In the first set of experiments, the bio-oil accelerated aging test was evaluated for intra-laboratory repeatability over an extended period. Two laboratories repeated the accelerated aging tests over consecutive working days. As each test extended over 2 days, the total length of time for the 10−12 tests was 3−4 weeks. The results are given in the provided tables: Table 2 for the filtered bio-oil sample and Table 3 for the unfiltered sample. Table 2. Ten-Test Repeatability of Accelerated Aging a Filtered Bio-oil PNNL

2. EXPERIMENTAL SECTION The research was carried out within the IEA Bioenergy Agreement Pyrolysis Task 34. This paper summarizes results from the tests performed in the three participating laboratories, subsequent to the 2011 round robin.20 Each was supplied with samples of two bio-oils, one unfiltered (2.4 wt % filterable solids) and one filtered (0.06 wt % filterable solids). The two bio-oil samples differed in that one was an “as-produced” bio-oil, while the other had been filtered to remove solid char/fluid-bed carryover particles. After receipt of the bio-oil samples, they were stored in a refrigerator (5 °C) until analyzed. The participants included (i) Technical Research Centre of Finland (VTT), Finland, (ii) Institute of Wood Technology and Wood Biology (vTI), Germany, and (iii) Pacific Northwest National Laboratory (PNNL), U.S.A. Instructions for the analyses were provided with the bio-oil samples. The accelerated aging test was described as follows: The pyrolysis liquid sample is mixed properly and left to stand until the air bubbles are removed. A total of 50 mL of air-tight 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 glass bottles. The bottles are firmly closed and pre-weighed 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 a ∼10 dm3 heating oven. The bottles are re-tightened after 10 min. After the specified 24 h, the closed sample bottles are cooled at room temperature for 1.5 h, weighed, and analysed. The samples are mixed and measured for viscosity at 40 °C. Note that 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. Viscosity measurements were performed by standard methods and reported as centistokes at 40 °C. Both methods (kinematic at VTT and vTI, while dynamic at PNNL) were used in these tests to track the viscosity of bio-oil over extended periods of storage at a range of temperatures. The aging index was calculated as the percentage of increase in viscosity divided by the starting viscosity. In the long-term aging studies, the bio-oil viscosity was measured directly without an accelerated aging step. The bulk bio-oil samples were well-stirred before the individual samples were drawn for the aging studies. Individual samples for each measurement were placed in a separate jar and stored in the dark. The viscosity of the bio-oil samples was measured after being stored for extended periods of time at a range of temperatures. The three temperatures evaluated were

VTT, start, 31.0 cSt

days from start

viscosity, start (cSt)

viscosity, aged (cSt)

aging index (%)

days from start


0 1 2 8 9 10 13 14 15 16

30.41 30.72 30.39 30.64 30.93 30.70 30.67 30.75 30.53 30.70

51.57 46.20 46.90 50.23 49.70 49.34 52.43 51.55 51.14 51.44 average standard deviation

70 50 54 64 61 61 71 68 67 68 63 6.8

0 1 5 6 7 13 14 18 19 21 27 28

53.77 57.19 50.80 50.71 54.74 54.54 54.73 52.63 52.92 53.65 51.49 53.10

aging index (%) 74 85 64 64 77 76 77 70 71 74 66 72

average standard deviation

72 6.0

Table 3. Ten-Test Repeatability of Accelerated Aging an Unfiltered Bio-oil at PNNL days from start

viscosity, start (cSt)


aging index

0 1 2 8 9 10 13 14 15 16

45.07 45.33 44.53 45.02 45.98 45.73 45.56 45.55 45.79 45.53

76.42 68.00 66.67 68.94 70.03 70.64 70.50 70.37 70.37 67.38 average standard deviation

70 50 50 53 52 54 55 55 54 48 54 5.9

With the filtered bio-oil, VTT only reported a starting viscosity for measurement for calculating the aging index. At PNNL, the starting viscosity was measured for each sample being placed into the oven for aging. In the case of the unfiltered bio-oil, no aging index results were possible from VTT because all of the aged samples had phase-separated and no viscosity could be measured. In the second set of experiments for long-term aging studies, the bio-oil viscosity was measured directly without an 7363


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accelerated aging step. The viscosity of the bio-oil samples was measured after being stored for extended periods of time in a range of temperatures. The results are given in the provided tables: Table 4 for the filtered bio-oil sample and Table 5 for the unfiltered sample.

dramatically. Storage at freezer temperatures reduced the viscosity increase to an even lower level. In the case of the unfiltered bio-oil, the trends are much less consistent, as is also the extent recorded in the different laboratories as well as within each laboratory.

Table 4. Long-Term Aging of a Filtered Bio-oil in a Range of Temperatures

4. DISCUSSION 4.1. Viscosity Measurement. In the 2005 round robin,21 the use of different viscosity measurement methods, dynamic versus kinematic, was found to compare favorably to proper conversion between the units of measurement, centipoises and centistokes. It was also suggested that dynamic measurement might be better for use with bio-oils that are more heterogeneous, as with higher levels of solids. In the report of the 2011 round robin,20 both the method and the quality of the bio-oil, whether filtered or not, impacted the consistency of the data. Those laboratories using the standard method of kinematic viscosity measurement provided a consistent set of data for the filtered bio-oil but somewhat less consistent for the unfiltered sample. Those laboratories using the dynamic measurement systems, whether rotational spindle, falling ball, or rotational cylinder, are more subject to variability in the results, and the results often did not align with the kinematic results. Both methods (kinematic at VTT and vTI, while dynamic at PNNL) were used in these tests to track the viscosity of bio-oil over extended periods of storage at a range of temperatures. Both methods were found to provide consistent results with a filtered bio-oil product with less than 0.1 wt % filterable solids. 4.2. Accelerated Aging Test. A 2005 round robin21 reported results from the accelerated aging test that were considered unacceptable because the standard deviation of the results were on par or greater than the average of the measurements (for three laboratories). The conclusion was that the methods had been imprecisely defined, and therefore, variations in the method in practice gave the highly variable results. In the 2011 round robin,20 the viscosity increase for the

sample

0 months

3 months

6 months

9 months

12 months

32.6

41.2 26 42.4 33 NAa

53.7 65 49.3 54 NA

62.9 93 60.6 90 NA

71.6 120 63.2 98 73.5 137

34.3 5 35.3 11 NA

36.5 12 38.2 19 NA

36.6 12 38.0 19 NA

37.7 16 38.7 21 41.7 45

33.3 2 33.9 6 NA

32.9 1 34.0 6 NA

33.1 2 34.3 7 NA

36.6 12 34.1 7 32.1 4

21 °C/PNNL (cSt) aging index (%) 21 °C/vTI (cSt) aging index (%) 21 °C/VTT (cSt) aging index (%)

a

32.0 31.0


32.6

−17 °C/PNNL (cSt) aging index (%) −17 °C/vTI (cSt) aging index (%) −17 °C/VTT (cSt) aging index (%)

32.6

32.0 31.0

32.0 31.0

NA = not analyzed.

With the filtered bio-oil, all three laboratories generated similar results over the 1 year period. A viscosity increase was shown to be significant at room temperature, but the refrigeration of the samples reduced the viscosity increase

Table 5. Long-Term Aging of an Unfiltered Bio-oil in a Range of Temperatures sample 21 °C/PNNL (cSt) aging index (%) 21 °C/vTI (cSt) aging index (%) 21 °C/VTT (cSt) aging index (%)

a

0 months

3 months

6 months

9 months

12 months

47.8

49.4 3 64.4 38 NAa

57.2 20 76.4 64 NA

83.1 74 92.5 99 NA

70.0 phase separated 105.4 127 phase separated

39.2 ?? 53.9 16 NA

43.4 ?? 55.5 19 NA

63.1 32 57.5 24 NA


74.6 56 53.9 16 NA

98.4 106 52.2 12 NA

144.0 201 52.4 13 NA


46.5 49.8


47.8

−17 °C/PNNL (cSt) aging index (%) −17 °C/vTI (cSt) aging index (%) −17 °C/VTT (cSt) aging index (%)

47.8

46.5 49.8

46.5 49.8

NA = not analyzed. 7364


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filtered bio-oil ranged from 40 to 88%, with an average of 71% increase in 24 h at 80 °C. For the unfiltered bio-oil, the viscosity increase ranged from 48 to 536%. The very large range of results reflects the tendency of the unfiltered bio-oil to phase separate and the difficulty to consistently sample the whole biooil after the phase separation that occurred in the accelerated aging test. In addition, the high solids content of the unfiltered sample can cause erroneous results when viscosity is determined as kinematic viscosity using capillary tubes. The intra-laboratory assessment of the method, reported here, provided a much better result. The standard deviation of the viscosity measurements was quite good within each laboratory, and as a result, the viscosity increase in the accelerated aging test was also consistently measured. Using filtered bio-oil, the average increase in viscosity in the accelerated aging test was 63%, with a standard deviation of 6.8%. Similarly, in the second laboratory using filtered bio-oil, the average increase in viscosity in the accelerated aging test was 73%, with a standard deviation of 6.0%. Using unfiltered bio-oil, the average increase in viscosity in the accelerated aging test was 54%, with a standard deviation of 5.9%. The second laboratory reported that, in all of the aging tests with the unfiltered bio-oil, the aged bio-oil had phaseseparated and could not generate an aging result. The coefficients of variation for these data sets is nearly acceptable at about 10%. 4.3. Long-Term Aging Studies. The effect of the temperature on viscosity change during extended storage time was obvious when considering the difference between the three temperatures used in the aging studies with the filtered bio-oil. The effect of reducing the storage temperature further from 21 to 4 to −17 °C was significant in all cases. While at room temperature the viscosity increased about 100% in 1 year’s time, it only increased 10% in a freezer and about 20% at the in between temperature of a refrigerator. The increase in viscosity determined in the accelerated aging test of 24 h at 80 °C was shown to be about equivalent to aging a filtered bio-oil at room temperature for 6 months. The storage of the high solids bio-oil resulted in widely varying viscosity results. One of the three laboratories (vTI) reported results suggesting the effect of improved storage at lower temperatures. The results from PNNL showed an inconsistent increase in viscosity and a surprisingly large increase in viscosity of the samples held at the lowest temperature. VTT reported phase separation at all three temperatures after 1 year of storage. Although the exact mechanisms are not addressed in this research, it is clear that the higher solids loading in the bio-oil caused much variation in the viscosity measurement results. A contributing factor to the inconsistency of the analytical results for this round robin may be the difficulty in consistent sampling of bio-oil by the participants and the production of comparable samples for the participating laboratories.

From the results of this round robin, still using ovens, we conclude that the accelerated aging test still needs further refinement to provide consistent results. The intra-laboratory repeatability was found to be much higher in this work than the inter-laboratory reproducibility reported previously.20 Clearly, the difference in the implementation of the method among the participating laboratories makes the reproducibility of the accelerated aging test quite low and not the method itself. The effect of the temperature on the viscosity change during extended storage of filtered bio-oil was obvious when considering the difference between 21, 5, and −17 °C. The lower temperatures helped limit the increase in viscosity of the bio-oils. The increase in viscosity when aging at room temperature for 6 months was confirmed to be similar to the increase determined in the accelerated aging test of 24 h at 80 °C. Consistent results were achieved only with the filtered bio-oil containing less than 0.1% filterable solids. For an unfiltered biooil, containing more than 2% solids, the viscosity change with aging appeared to be higher, in most cases, but the results were inconsistent. The accelerated aging test results were more inconsistent. Storage for 1 year’s time, even in refrigerated storage, resulted in phase separation in 2 of 3 laboratories. Overall, the use of round robins is useful for validating analytical methods. A contributing factor to the inconsistency of the analytical results for this round robin may be the difficulty in consistent sampling of bio-oil by the participants and the production of comparable samples for the participating laboratories. It is worth repeating that fast pyrolysis bio-oils are chemically different from mineral oils or biodiesels. Special care has to be used in the proper handling and sampling of these bio-oils to ensure the homogeneity of the bio-oil.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the operations within their respective laboratories and their attentive equipment operators: Todd R. Hart at PNNL, Silke Radtke at vTI, and Sirpa Lehtinen, Jaana Korhonen, and Elina Paasonen at VTT. The authors also acknowledge the financial support from the funding agencies from each of the five countries that participated in the IEA Bioenergy Task 34 Round Robin: Natural Resources, Canada, the U.S. Department of Energy, TEKES, Finland, Fachagentur Nachwachsende Rohstoffe e.V., Germany, and the Department of Energy and Climate Change, U.K.

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REFERENCES

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5. CONCLUSION In the 2011 round robin,20 the accelerated aging test was more clearly described in the written method provided to the participants. Variations in the methods used by the participants were still discernible in debriefing after the completion of the tests. The consistency of the oven specification and operation was identified as a key point for improvement. For example, it may be more useful to prescribe the use of a heating bath rather than an oven, to have better temperature control. 7365


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Results of the IEA Round Robin on Viscosity and Aging of Fast

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