Biodegradability Of Pyrolysis Oil

Available online at www.sciencedirect.com

Fuel 86 (2007) 2679–2686 www.fuelfirst.com

Biodegradability of biomass pyrolysis oils: Comparison to conventional petroleum fuels and alternatives fuels in current use Joe¨l Blin a

a,*

, Ghislaine Volle a, Philippe Girard a, Tony Bridgwater b, Dietrich Meier

c

CIRAD Forestry Department, International Research Centre for Agricultural and Development, UPR Biomass Energy, TA 10/16, 73, Avenue J.-F. Breton, 34398 Montpellier, Cedex 5, France b Aston University, Aston Triangle, B4 7ET, Birmingham, United Kingdom c BFH-Institute for Wood Chemistry, Leuschnerstrasse 91, D-21031 Hamburg, Germany Received 29 November 2006; received in revised form 19 March 2007; accepted 20 March 2007 Available online 18 April 2007

Abstract Concern with environmental issues such as global climate change has stimulated research into the development of more environmentally friendly technologies and energy sources. One critical area of our economy is liquid fuels. Fast pyrolysis of lignocellulosic biomass for liquids production is of particular concern, as it is one of the most interesting ways to produce renewable liquid fuel for transport and heat and power production. The aerobic biodegradability of various pyrolysis oils from different origins and of a EN 590 diesel sample was examined using the Modified Sturm (OECD 301B). The results demonstrate that all fast pyrolysis oils assessed are biodegradable with similar shaped curves with 41–50% biodegradation after 28 days, whereas the diesel sample reached only 24% biodegradation. Since pyrolysis oils achieved biodegradability over 20% these are classified as inherently biodegradable. Modelling of biodegradation processes was successfully performed with a first-order chemical reaction. The biodegradability results obtained for biomass pyrolysis oils are compared to those of conventional and alternative fuels.  2007 Elsevier Ltd. All rights reserved. Keywords: Biodegradation; Fast pyrolysis oil; Biofuel; Biomass

1. Introduction 1.1. Fast pyrolysis A wide variety of technologies are available to produce energy from biomass. Biomass is the only renewable resource that can be directly converted into liquid fuel, thus providing a competitively priced fuel for transport, heat and power production. Fast pyrolysis of biomass for liquids production is of particular interest, as it is the only thermal process that directly produces useable liquid product from lignocellulosic biomass [1]. Literally, pyrolysis

*

Corresponding author. Fax: +33 4 67 61 65 15. E-mail address: [email protected] (J. Blin).

0016-2361/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.03.033

corresponds to the thermal conversion occurring in the absence of oxygen. Biomass pyrolysis results in the production of three products: gas, pyrolysis oil and charcoal [2,3]. The relative proportions of these products depend very much on the pyrolysis method, the characteristics of the biomass and the reaction parameters. Temperature, heating rate and vapour residence time can be adjusted to favour charcoal or bio-oil productions [4]. Conventional carbonization or slow pyrolysis has slow heating rates and long vapour residence time and is used to maximise charcoal yields at around 35–40%, together with around 30% liquid. In fast pyrolysis processes, very high heating rate and short hot vapour residence time are used to maximise the liquid product at around 75 wt% on dry feed when the biomass is wood [5]. In both cases, the thermo-decomposition of the biomass generates vapours

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and aerosols [6,7]. After cooling, these condense to give a dark brown mobile liquid composed of a very complex mixture of oxygenated hydrocarbons with some water [8–10]. While fast pyrolysis oils are single phase, slow pyrolysis liquids are composed of two phase: an organic fraction and an aqueous fraction. Fast pyrolysis oil, commonly named bio-oil, has a heating value about half that of conventional fuel oil [11]. It can substitute for fuel oil or diesel in many heat and power applications [11–13]. Almost any form of biomass can be considered for fast pyrolysis from agricultural wastes and residues to annual and perennial energy crops.

biodegradability of pyrolysis oils in case of accidental discharges. Results and conclusions are very interesting, but as they used a non-standardized experimental method data cannot be considered for the elaboration of MSDS. In order to assess bio-oil biodegradability and to be able to set up recommendations and MSDS, the biodegradability of nine different fast pyrolysis bio-oils and one slow pyrolysis oil was measured using the Modified Sturm test 301B [20,21] which is the conventional test used to assess oil biodegradation [22–24]. The biodegradability results obtained are discussed and compared with those of conventional petroleum fuels and alternatives fuels in current use.

1.2. Health and safety

2. Materials and methods

The scale up of biomass pyrolysis technology and the opportunity liquid may offers as concentrated bioenergy carrier for large scale application, would expose a large numbers of persons to potentials hazards. Diebold [14] published the most extensive review on the toxicity of biomass pyrolysis liquids. This paper reported on a significant number of works done on hazards of acute and long term exposure to ‘‘wood smoke and pyrolysis vapour’’ but none on their eco-toxicity. However, with the increasing interest in bio-oil, the health and safety aspects are of utmost importance, including determination of their environmental impacts in the event of inadvertent problems or leaks during transport, storage or processing. As biodegradation is the dominant pathway for the environmental transformation of most chemicals, biodegradability properties of bio-oils should be known to assess the impact of accidental release [15,16]. Information on the kinetics of the degradation of a chemical substance in the environment is essential for proper risk assessment and of primary importance to apprehend the long-term effects of this substance. Moreover such data can also provide information on the product behaviour for measures to adopt in case of large quantities of spillage, in order to minimise adverse effect on human health or ecosystem.

2.1. Pyrolysis oils

1.3. Biodegradability of bio-oil To date, only one study on bio-oil biodegradation has been published by Piskorz and Radlein [17]. They used a non-standardized experimental method with a respirometer to monitor the bio-oil biochemical oxygen demand in a closed reactor over 5 days. The protocol they used, which was developed for biochemical studies in activated sludge process, is similar to the 5-days biochemical oxygen demand (BOD5) norm [18] but their methods were further developed in order to compare biodegradation kinetics of wastes in water rather than to measure the ultimate biodegradability in controlled and standardized conditions [19]. In their study, Piskorz and all found that bio-oils are biodegradable, with biodegradation rates and patterns similar to, but substantially higher than those for diesel fuel. The aim of Piskorz and all was to assess the ultimate aerobic

Lignocellulosic biomass is a complex mixture of macromolecules or polymers (hemicellulose, cellulose, lignin) and minor amounts of other organics which each pyrolyse or degrade at different temperatures and by different mechanisms and pathways [3,25,26]. The rate and extent of decomposition of each of these components depends on the process parameters (reactor temperature, biomass heating rate and pressure) [5]. Pyrolysis has received considerable creativity and innovation in design reactor systems that provide the essential ingredients of high heating rates, moderate reaction temperatures and short hot vapour product residence times for liquids. As a result, a large variety of reactor configurations have been developed and are used by bio-oil producers [5,27,28]. Bio-oils collected to carry out this study were provided by companies and research laboratories. These were selected in order to study the influence of the process parameters, such as the reaction temperature and the feedstock, on the biodegradability of the oil. The production parameters and characteristics of these oils are given in Table 1. Nine fast pyrolysis oils (FPO) were produced by the three main processes of fast pyrolysis: fluidised bed, circulating fluidised bed and ablative reactors. The characteristics of these reactors are described by Bridgwater et al. [27,28]. Biodegradation of slow pyrolysis oil (SPO) was also studied. This sample, which was produced in a fixed bed reactor, was biphasic with 77 wt% aqueous fraction and 23 wt% organic fraction. In a first step the biodegradability of the whole oil was studied. It was introduced into the inoculated medium as an emulsion after being vigorously stirred. In a second step, biodegradation of both the organic (SPO-Org) and aqueous (SPO-Aq) fractions were determined separately after decantation. The water content of all samples was measured using the standard test method ASTM D1744 by Karl Fisher titration; and elemental CHN composition was determined using ASTM D529192. The results of these measurements are presented in

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Table 1 Production parameters and characteristics of pyrolysis oils Bio-oils

Production parameters

Chemical characteristics (on wet basis) %H

%N

% Oa

41.67 43.13 43.66 39.37 45.10

7.87 8.14 7.67 7.64 7.35

0.00 0.22 0.00 0.37 0.38

50.46 48.51 48.67 52.62 47.17

28.50 29.10

39.45 39.44

7.96 8.01

0.01 0.30

52.58 52.25

Pine Pine

24.90 37.00

41.27 32.64

7.79 8.30

0.01 0.35

50.93 58.71

Spruce

33.36 28.60 67.8

25.56 62.54 14.88

10.8 7.59 11.46

0.3 0.00 0.4

63.34 29.87 73.26

Reactor

Reactor T C

Biomass

Water (% m/m)

%C

FPO-1 FPO-2 FPO-3 FPO-4 FPO-5

Fluid bed

500 500 500 425 600

Beech Miscanthus Spruce Spruce Spruce

26.80 24.60 22.40 26.70 20.30

FPO-6 FPO-7

CFB

500 500

Beech Forest residue

FPO-8 FPO-9

Ablative

480 560

SPO SPO-Org SPO-Aq

Fixed bed

500

CFB: circulating fluidised bed, FPO: fast pyrolysis oil, SPO: slow pyrolysis oil, Org: organic fraction, Aq: aqueous fraction. a By difference.

Table 1. The water contents varied between 20 and 37 wt% which is due to the disparity in the moisture content of the feedstocks used and the different technologies. As pyrolysis oils can be used as a renewable energy source to substitute for conventional fossil fuel in static engine application, the aerobic biodegradation of a EN 590 diesel fuel was also studied. The diesel sample contained 86.5 wt% carbon, 13.5 wt% hydrogen, and 0.005 wt% nitrogen.

where Bmaterial is the biodegradation of the tested material, (CO2)material is the amount of CO2 measured in the test vessel containing the material and (CO2)blank in blank control. ThCO2 represents the amount of carbon dioxide formed by complete oxidation of the organic carbon. If mmaterial is the amount of the sample introduced in the tests vessel, C% is the percentage of carbon in the sample, ThCO2 is calculated according to Eq. (2) [33]. 44 : ð2Þ 12 Because of the nature of the biodegradation process and of the bacterial populations used as inoculum, all tests were duplicated. To test one oil, five flasks were necessary: two flasks containing the oil to be tested in duplicate, two ‘‘blank’’ flasks to characterize the inoculum respiration and one reference flask containing sodium acetate to act as a control. The experimental set-up consisted of 11 flasks to test four different oils in parallel. The inoculum source used for tests was activated sludge freshly collected from the aeration tank of the sewage treatment plant in St Gely du Fesc close to Montpellier (France). This treatment plant receives predominantly freshly collected domestic sewage and the activated sludge has a concentration of 3.2 g l1 of suspended solids as recommended in the OECD guideline [21,34]. Fresh activated sludge was collected one day prior to test initiation, and homogenised in a blender for 30 min by stirring at medium speed. After the sample settled the homogenized mass was decanted, avoiding solids losses. A 10 ml l1 activated sludge material was added to the test medium. Sodium acetate was used as the reference compound to check the procedure. It was tested in parallel in an appropriate vessel at the same time as the other test runs. A test is considered valid if the percentage of degradation of the reference compound reached 60% biodegradation by day 14. ThCO2 ¼ mmaterial  C% 

2.2. Biodegradability test The method used to evaluate the aerobic biodegradability of pyrolysis oil was the OECD 301B Modified Sturm Test for ready biodegradability [21,23]. This 28-day ultimate aerobic biodegradation test was established for poorly water-soluble compounds and it is the commonly test selected to assess the biodegradation of petroleum products [22,29,30]. The principle of the method involves addition of a test material to a chemically defined liquid medium inoculated with wastewater micro organisms and aerated at 20–25 C. The OECD 301B Modified Sturm Test is the more common test used to assess oil biodegradation [22,24,31,32]. The biodegradation is quantified by trapping CO2 formed in the vessel during biodegradation. Carbon dioxide-free air is passed through the test solution, and the carbon dioxide released from bio-oxidation of the test material, is reacted quantitatively in a solution of barium hydroxide. The biodegradation (B) is calculated and expressed as a percentage of the measured carbon dioxide, after subtracting blank values, compared to the theoretical carbon dioxide (ThCO2) that would be generated if the tested material were completely mineralized: Bmaterial ¼

ðCO2 Þmaterial  ðCO2 Þblank ; ThCO2

ð1Þ

J. Blin et al. / Fuel 86 (2007) 2679–2686

To estimate the biodegradability, the amount of oil introduced into the flasks has to be precisely known, in order to have around 15 mg l1 of dissolved organic compounds (DOC) in the flask. Therefore, based on the oils’ elemental analysis, all tests were performed by introducing the equivalent of 45 mg of carbon equivalent in the liquid in 3 l of inoculum/mineral solution. As pyrolysis oils are partially insoluble in water and sticky, the sample to be introduced into the mineral medium cannot be precisely weighted without losses. As recommended by Morisier et al. [35] for this kind of substance, a solid carrier was used. Pyrolysis oils were deposited and weighed on non-biodegradable glass filters, before introduction in the mineral medium. After 2 or 3 days stirring in the inoculated medium, the filter breaks up into the original filament form and the entire oil sample is then released into the solution. The CO2 emitted from the 11 flasks was periodically and individually measured during the 28 days period. On the 28th day, 1 ml of concentrated hydrochloric acid was added to each vessel to drive off any remaining carbon dioxide present in the test suspensions. On day 29, the last measurement of carbon dioxide production was taken.

50

biodegradation rate [%]

2682

40

30

20

10

0 0

5

10

15

20

25

30

time [d] Fig. 1. Biodegradation curves of fast pyrolysis oils: (m) FPO-1; (d) FPO2; (j) FPO-3; () FPO-4; (n) FPO-5; (s) FPO-6; (h) FPO-7; (e) FPO-8; (·) FPO-9.

90 80

2.3. Results biodegradation rate [%]

70

All oils were tested in duplicate as reported above except SPO which was tested in four replicates because of its inhomogeneity. The sodium acetate used as the control, degraded for all runs to approximately 69% (69.2%; 70.1%; 69.2%; 68.6%) by day 14, which validated the tests and confirmed the viability of the microbial inoculums. The reproducibility of the measurements is satisfactory since the standard deviation between the four references is 7% of the biodegradation. The average across the two replicates cumulative biodegradation measured for each sample as a function of time are plotted in Figs. 1 and 2. As can be seen, all the pyrolysis oils tested were biodegradable as CO2 release was observed in every flask. As shown on Fig. 1, all the fast pyrolysis oils tested have very similar biodegradation rates, with similar shaped curves and biodegradation between 41% and 50% after 28 days. Surprisingly, despite fast pyrolysis oils being produced using different conversion technologies and feedstocks, and consequently different chemical compositions [36], these all have very similar biodegradation behaviour. Biodegradation curves show high degradation rates during the first eight days followed by a second phase where the production of CO2 is reduced. Tests were stopped after 28 days as recommended in the OECD guideline, but it is clear that biodegradation was not completed as the curves had not reached a plateau. It would be interesting to continue the tests for a longer period. The slow pyrolysis sample achieves 62% biodegradation and is more easily mineralized by the bacteria than fast pyrolysis oils (Fig. 2). The two slow pyrolysis oil fractions have very different biodegradability: the SPO aqueous

60 50 40 30 20 10 0 0

5

10

15

20

25

30

time [d] Fig. 2. Biodegradation curves of slow pyrolysis oil, references and diesel: (·) sodium acetate; (d) SPO-Aq; () SPO; (j) SPO-Org; (n) diesel.

phase reached 74% whereas the organic phase is only mineralized to 31%. Taking account of the relative proportions of the organic and aqueous fractions (respectively 23 wt% and 77 wt%), the biodegradability of SPO is equal to the sum of the biodegradabilities of the two phases as reported in Eq. (3). BðSPOÞ ¼

77  BðSPO-AqÞ þ 23  BðSPO-OrgaÞ : 100

ð3Þ

The difference between the biodegradability rates of fast and slow pyrolysis oils can be explained by the different chemical nature of the two kinds of oil. Even if the same chemical families of compounds are present in both oils,

J. Blin et al. / Fuel 86 (2007) 2679–2686

2.4. Modelling degradation curves In order to better characterize and quantify the ultimate aerobic degradation of pyrolysis oils, the experimental cumulative curves were simulated with a first-order kinetics equation (4) [39–41]. Carbon dioxide productions were regression-analysed by the least squares method, minimisation of target function applying the Solver subprogram of Microsoft Excel 2002 SP3 [42]
BCO2 ¼ Bmax 1  ekðttlag Þ ; ð4Þ where BCO2 is the percentage biodegradation at time t (days), Bmax is the maximum percentage biodegradation, k is the rate constant (day1) and tlag is the lag phase of biodegradation (day). The half-life time of CO2 production T1/2 is obtained by T 1=2 ðdaysÞ ¼ tlag

ln 2 : k

ð5Þ

Fig. 3 shows that biodegradation of pyrolysis oils can be very well described by a first-order kinetic equation. The kinetics parameters of the first-order models are numerically presented in Table 2. The curves in Fig. 1, and the calculated kinetics data in Table 2 show very low lag phases for all pyrolysis oils (tlag  1 day). This corresponds to the inoculation period at the beginning of the tests until biodegradation starts [21]. For the Diesel sample a lag phase of 3–4 days was

60

biodegradation rate [%]

theirs concentrations are significantly different. In slow pyrolysis processes, the heating rate and flow of gas produced are low, so that the residence times of pyrolysis vapours in the reactor are high. This encourages secondary reactions (cracking and recombination) with production of both light functionalised compounds and more stable heavy organic compounds. Light functionalised compounds are predominantly soluble in water whereas heavy organic compounds are not, which results in phase separation. In the fast pyrolysis process, the residence times of vapours are very short, minimising secondary reactions [5]. Fast pyrolysis oils are mainly composed of functionalised organic compounds close to monomers of cellulose and hemicelluloses together with cracked lignin polymers known as pyrolytic lignin. It has been established that high molecular weight, low water solubility and the presence of several aromatic rings increases resistance to biodegradation [37,38]. This explains the different biodegradation of the materials studied. It has been found that bacteria more easily degrade light highly functionalised compounds of SPO-Aq, than the organic partially water soluble constituents of FPO and the more stable heavy organic compounds of SPO-Org. All pyrolysis oils, both slow and fast, are more biodegradable than the Diesel sample (24%), which is mainly composed of branched chain alkanes, cycloalkanes, aromatic hydrocarbons and mixed aromatic cycloalkanes.

2683

50

40

30

20

10

0 0

5

10

15

20

25

30

time [d] Fig. 3. Biodegradability of (n) FPO-2 and (s) SPO. Experimental values (points) and biodegradation curves are modelled following Eq. (3).

Table 2 Measured biodegradation and kinetics constants of first-order equation describing biodegradation of pyrolysis oils and diesel Oils

FPO-1 FPO-2 FPO-3 FPO-4 FPO-5 FPO-6 FPO-7 FPO-8 FPO-9 SPO SPO-Org SPO-Aq Diesel

Measured

Calculated

Bday

Bmax (%)

k (day1)

tlag (days)

T1/2 (days)

46.15 43.86 42.23 49.27 40.33 44.75 43.34 51.13 49.14 62.08 30.73 73.93 26.61

0.14 0.14 0.15 0.15 0.14 0.15 0.15 0.13 0.17 0.18 0.13 0.16 0.10

0.91 1.09 0.94 1.16 1.20 1.03 1.47 1.04 1.66 0.74 1.13 1.53 4.03

5.06 5.00 4.66 4.59 4.88 4.69 4.60 5.17 4.02 3.87 5.20 4.25 7.22

29

45.90 44.25 42.77 49.72 40.78 44.64 43.29 50.11 49.50 62.25 30.63 74.02 24.33

(%)

Bday 29: percentage biodegradation measured after 29 days, Bmax: calculated maximum percentage biodegradation, k: rate constant (day1), tlag: lag phase of biodegradation (day), T1/2: half-life time of biodegradation (day).

observed. This time lag corresponds to the acclimatization period for the bacteria fauna to develop competent degrader for effective degradation [24,31,43,44]. The fast biological response factors towards pyrolysis oils can be explained by the solubility of functionalised compounds in the aqueous medium, which are quickly degraded by bacteria from conventional inoculum sources. Low lag phases for pyrolysis oil biodegradation, also show that despite use of a glass filter as solid carrier, there were no mass transfer limitations in the inoculated solution. Oils were rapidly dispersed in the bacteria medium [40]. The CO2 produced was also rapidly released from flasks and was not dissolved in the aqueous solution due to the acidity of oils [43]. This was confirmed on day 29 when the addition of 1 ml of the acid solution released a large amount of dissolved CO2 from only the reference

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and diesel samples (which are not acidic) but not for the bio-oil samples. To be classified as readily biodegradable as in the OECD protocol, a compound must achieve at least 60% of the ThCO2 within 28 days, and should reach that within 10 days of achieving the first 10% of the ThCO2. Based on this definition, only the aqueous fraction of the SPO can be classified as readily biodegradable. The slow pyrolysis sample can be classified as readily biodegradable but without meeting the 10-day window criteria [23,24,34] and fast pyrolysis oils can be classified as inherently biodegradable [45] since these oils achieved biodegradability over 20% based upon CO2 evolution within the 28 days. 3. Comparison of the biodegradability of pyrolysis oils to conventional petroleum fuels and alternatives fuels in current use As pyrolysis oils can be used as a renewable energy source to substitute conventional fossil fuels in burner or heavy fuel in combustion engines, it is worthwhile to compare their biodegradation to those of petroleum products and others biofuels (Table 3). As confirmed by the American Petroleum Institute [45] very little data is available on the behaviour of fossil fuel oils in standard tests for biodegradability. Much of what is known is based on information gained from testing hydrocarbon mixtures of other petroleum products. The only values available in the literature are presented in Table 3. Heavy fuel oils are blended products based on the residues from various refinery distillation and cracking processes. This fuel has low biodegradation of 11%, in Table 3 Biodegradability data of fossils and bio fuels Fuels products Fossil fuels Gas oil Commercial

Degradation (28 days)

References

24–36%

Fischer Tropsch

60%

Present study [22,30,37,44,49] [30]

Gasoline

91 octane

28%

[37]

Heavy fuel

Bunker C oil

11%

[46,47]

Fast pyrolysis oils

41–50%

Present study

Vegetable oils

Refined rapeseed oil Refined soybean oil

83% 78% 76%

[49] [44] [44]

Methyl esters

Rapeseed oil methyl ester Sunflower oil methyl ester Soybean oil methyl ester

87% 88% 90%

[49] [44] [44]

85.4%

[49]

Bio fuel Pyrolysis oils

28 day laboratory studies, due to its higher proportion of high molecular weight aromatics [46,47]. Diesel and gasoline, which are light crude oil derived fuels, are more biodegradable and achieve up to 24–36% and 28% respectively. Zhang et al. [44] used the test method EPA 560/6-82-003 which is equivalent to the OECD 301B [45]. Morgan et al. [30] compared the biodegradability of a synthetic GTL diesel (gas-to-liquids) obtained by Fischer Tropsch conversion of natural gas, and a crude oil derived diesel fuel. Using the OECD 301B test the synthetic GTL diesel from Sasol reached 60% biodegradation in around 28 days whereas the crude oil derived diesel only degraded by 34%. The relatively good biodegradability of the synthetic diesel is due to the low aromatic content in the GTL fuel. Vegetables oils and their derived methyl esters have been used as a diesel fuel substitute or additive for many years and previous work has demonstrated that these are rapidly degraded to reach biodegradation of between 76% and 90% [44,48,49]. In their studies Zhang et al. [44] have shown that vegetables oils are slightly less degraded than their modified methyl ester. This has been attributed to the higher viscosity of the vegetable oils and their limited solubility, which limit their biodegradability. The results in Table 3 show that biodegradation of fast and slow pyrolysis oils is more effective and faster than that of fossil fuel but not as good as that of vegetable oils. Oil institutes generally agree that petroleum hydrocarbon biodegradability is governed by the molecular structure of individual hydrocarbons [50]. As explained before, the simpler molecules are most readily degraded, while increased molecular weight, branching compounds and aromatic structures tend to decrease the biodegradation rate and sometimes the extent of biodegradation of hydrocarbons of the same carbon number [16]. Linear aliphatic alkanes of Fischer–Tropsch diesel and linear fatty acid in vegetables oils or oil methyl ester are quite extensively biodegraded, while branching and aromatic compounds of fossil fuels are more stable and less biodegradable. For a biochemical process to occur rapidly, appropriate enzymes must be available [15]. Vegetable oil consists of pure fatty acids; the enzymes responsible for their breakdown exist naturally. Fatty acids are hydrocarbon chain in ester form, with oxygen atoms attached which makes them very biologically active. The enzymes responsible for the dehydrogenation/oxidation reactions that occur in the process of degradation, recognize oxygen atoms and attack them immediately [44]. Moreover in many fossil fuels the contents of heavy metals decreases microbial activities and thus decreases their biodegradability, which is not the case for biofuel [51]. 4. Conclusions The study of the biodegradability of biomass pyrolysis oils, show that this biofuel is biodegradable. According

J. Blin et al. / Fuel 86 (2007) 2679–2686

minor adaptation the OECD 301B test method appears to be suitable to assess the aerobic ultimate biodegradability of these oils in fresh water. Despite being produced using different conversion technologies and feedstocks, nine tested fast pyrolysis oils have very similar biodegradation rates, with similar shaped curves and biodegradation between 41% and 50% after 28 days. These can be classified as inherently biodegradable. Biodegradation curves show high degradation rates during the first 8 days followed by a second phase where the production of CO2 is reduced. Modelling of the biodegradation of biomass pyrolysis oils show that these can be very well described by a firstorder kinetic equation. Biodegradation starts immediately with very short lag phases at the beginning of tests. This indicates a very fast biological response and also reflects a rapid mass transfer of oil from the glass filters to the solution, which validates the method used to introduce oil samples in test medium. It would be very interesting to repeat these tests with a more specialised source of inoculums. It is known that inoculums from industrial wastewater treatment plants generally show a higher biodegradation potential for the chemical substances tested than inoculums from municipal wastewater treatment plants. Inoculums from wastewater treatment from oil industries or refinery plants should be tested, as they have developed bacteria that have specially adapted to break up cyclic and aromatic organics compounds similar to those in bio-oils. The biodegradability value of fast pyrolysis oils show that in case of accidental spillages this fuel would be biodegraded better than all fossil fuels, but not as well as vegetables oils. Acknowledgements The experimental tests were carried out in the framework of an R&D project partially funded by the European Commission through the Pyrolysis Network PyNe. We appreciate the help of the fast pyrolysis liquids producers. References [1] Demirbas A. The influence of temperature on the yields of compounds existing in bio-oils obtained from biomass samples via pyrolysis. Fuel Process Technol, in press. [2] Garcia-Perez M, Chaala A, Pakdel H, Kretschmer D, Roy C. Characterization of bio-oils in chemical families. Biomass Bioenergy 2007;31:222–42. [3] Yaman S. Pyrolysis of biomass to produce fuels and chemical feedstocks. Energy Convers Manage 2004;45:651–71. [4] Horne PA, Williams PT. Influence of temperature on the products from the flash pyrolysis of biomass. Fuel 1996;75:1051–9. [5] Lede J. Solar thermochemical conversion of biomass. Solar Energy 1999;65:3–13. [6] Bridgwater AV, Peacocke GVC. Fast pyrolysis processes for biomass. Renew Sustain Energy Rev 2000;4:1–73.

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