Controlling the Phase Stability of Biomass Fast Pyrolysis Bio-oils

Article pubs.acs.org/EF

Controlling the Phase Stability of Biomass Fast Pyrolysis Bio-oils Anja Oasmaa,*,† Tom Sundqvist,† Eeva Kuoppala,† Manuel Garcia-Perez,‡ Yrjö Solantausta,† Christian Lindfors,† and Ville Paasikallio† †

VTT Technical Research Centre of Finland, Limited, Biologinkuja 5, 02044 Espoo, Finland Biological Systems Engineering, Washington State University, Pullman, Washington 99164, United States

‡

ABSTRACT: Fast pyrolysis bio-oil oil is a promising alternative to fossil fuels and is currently entering the heating oil market. However, there is a lack of available information about the phase stability of bio-oil. The water-soluble and water-insoluble compounds in bio-oil can either be in one homogeneous phase or form two individual phases, to which we refer to as phase separation. Phase separation can occur immediately after condensation of the pyrolysis vapors to bio-oil because of certain pyrolysis conditions or type of raw material or after years of aging because of changes in composition caused by repolymerization reactions. We present how the phase separation of bio-oils is related to the chemical composition and show that the probability of phase separation can be predicted with a numerical stability index based on the chemical composition. The chemical composition of the bio-oils studied was characterized using a solvent extraction scheme that describes the composition of bio-oil as a blend of three macro fractions: C1−C6 oxygenated molecules (named co-solvents), water-insoluble molecules, and watersoluble polar molecules (including water but excluding the co-solvents), e.g., anhydrosugars. The results show that the required amount of co-solvent to dissolve both fractions and keep the bio-oil homogeneous varies depending upon the chemical composition. The minimum amount of co-solvent for homogeneous bio-oils was observed to be from 15 to 30 wt %. The correlation between the chemical composition and homogeneity of fresh and aged bio-oils is shown in ternary-phase diagrams. Addition experiments were made with model compounds to cover a larger part of the phase diagram.

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INTRODUCTION Fast pyrolysis bio-oil (FPBO) is starting to enter the heating oil market in an attempt to replace fossil fuel oils. Commercialscale pyrolysis plants are in startup at several locations, notably Joensuu, Finland, commissioned by Fortum, and BTG-BTL in the Netherlands.1−4 Large-scale combustion of FPBO has been demonstrated, 5−7 and standards and norms are being prepared.8 Two ASTM boiler fuel standards (ASTM D7544) are already available, and CEN standardization is in progress.8,9 Information on critical fuel oil properties related to bio-oil combustion is available.5,6,8,10,11 However, there is insufficient data on the multi-phase structure of FPBO, phase stability, and especially, separation of the lignin-rich phase observed during storage and handling. Although the phase separation phenomenon has been observed by many researchers, such as Piskorz et al.,12,13,32 today, it is not possible to predict which bio-oils are more likely to form separated phases during storage. It is important to control these bio-oil properties because phase-separated bio-oil in storage tanks will cause pumping problems, blockages, and irregular combustion. New phases can be formed during the storage of bio-oils because of the changes in bio-oil composition. Even short times at a high temperature can lead to irreversible changes in bio-oil to form separated phases. The phase stability of FPBO is closely related to the chemical composition and solubility distributed between its various constituents. Unlike heavy and light fossil fuel oils, FPBOs are highly soluble in polar organic solvents, such as alcohols (methanol, ethanol, and isopropanol), ketones (acetone), and organic acids (acetic acid). Because of the polar nature of biooils, they are insoluble in hydrocarbon solvents.12,13 FPBOs are comprised of 70−80 wt % water-soluble (WS) oxygenated © 2015 American Chemical Society

compounds, mainly aldehydes, ketones, carboxylic acids, and carbohydrate (“sugar”-type) compounds. The organic part of the water-insoluble (WIS) fraction of a fresh bio-oil is for the most part lignin-derived oligomers. Table 1 details the chemical composition of pine and forest residue FPBOs. The compound concentration in the bio-oil highly depends upon the feedstock but also the pyrolysis process, product condensation, and storage conditions. It has been suggested14 that, as a rule of thumb, the ratio of water, WS, and WIS should be around 25:50:25 to avoid phase separation. There are two main types of phase separations related to the type of feedstock used to produce the FPBO. Figure 1 shows separation of whole forest residue bio-oil into a bottom phase and an extractive-rich top phase. Further separation of the bottom phase into the aqueous phase and lignin-derived phase occurs as well. The former happens relatively fast, in about 1 day at room temperature, but the latter requires typically more than 1 year of aging at room temperature or several weeks at higher temperatures. Phase separation can also happen when water or pyrolytic lignin is added to the oil.31 The first phase stability mechanism resulting in the formation of a waxy hydrophobic upper phase can only be observed in FPBOs produced from forest residues or some softwood saw dust with relatively high contents of extractives.15−18 This mechanism can be easily recognized by the low water content (typically close to 5 wt %) of the upper phase. When frozen, this upper phase tends to form a semi-solid because of the presence of high contents of waxy materials. An oily phase can Received: March 26, 2015 Revised: June 5, 2015 Published: June 5, 2015 4373

DOI: 10.1021/acs.energyfuels.5b00607 Energy Fuels 2015, 29, 4373−4381

Article

Energy & Fuels Table 1. Chemical Composition of a Fresh Pine1 and Forest Residue FPBOs Produced at VTT’s 20 kg/h Unita pine

forest residue

bottom 97% FPBOs water acids formic acid acetic acid propionic acid glycolic acid alcohols methanol ethylene glycol aldehydes, ketones, furans, and pyrans acetaldehyde, hydroxypropionaldehyde, 3-hydroxy hydroxypropanone (acetol) butanone, 1-hydroxy-2butandial or propanal cyclopentene-1-one, 2-hydroxy-2cyclopentene-3-one, 2-hydroxy-1-methyl-1furanone, 2(5H)furaldehyde, 2furaldehyde, 5-(hydroxymethyl)-, 2pyran-4-one, 3-hydroxy-5,6-dihydro-, (4H)pyran-4-one, 2-hydroxymethyl-5-hydroxy-2,3-dihydro-, (4H)sugar type compounds anhydro-β-D-arabino-furanose, 1,5anhydro-β-D-xylofuranose, 1,5anhydro-β-D-glucopyranose(levoglucosan) dianhydro-α-D-glucopyranose, 1,4:3,6cellobiosan cellotriosan LMM lignin catechols lignin-derived phenols guaiacol guaiacol, 4-methylguaiacol, 4-propenyl-(trans) (isoeugenol) vanillin homovanillin (phenylacetaldehyde, 4-hydroxy-3-methoxy-) acetoguaiacon (phenylethanone, 4-hydroxy-3-methoxy-) coniferylaldehyde syringol syringol, 4-methylsyringol, 4-allylsyringol, 4-(1-propenyl)-, trans syringaldehyde sinapaldehyde (trans) HMM lignin extractivesb solids

wet (wt %) 23.9 4.3

0.23

17.4

34.4

13.4

1.95 4.35 0.011

dry (wt %) 0 5.6 1.51 3.38 0.20 0.55 0.93 0.63 0.30 22.3 8.93 0.75 2.84 0.23 0.29 0.84 0.53 0.69 0.54 1.14 0.72 0.39 45.3 0.27 0 4.01 0.17 1.3 0.1 17.7 0.06 0.09 0.52 0.49 0.40 0.50 0.27 0.22 0.26 0 0 0 0 0 0 2.6 5.7 0.014

bottom 89% wet (wt %) 24.4 3.3

0

20.4

dry (wt %)

wet (wt %)

dry (wt %)

0 4.4 1.46 7.35 0.18 0.33 0 0 0 27.0 8.66 1.17 2.55 0.22

19.5 6.4

0 7.9

0.73

28.8

12.0

4.3 2.8 0.040

top 11%

6.01 0.18 0.16

12.8

0.20 0 0.20 15.9 8.94 1.02 2.25 0.20

0.64 0.21 0.50 0.73 0.67 0.44 1.38 0.14 38.1 0.17 0.33 3.48 0.15 NA NA 15.8 0.09 0.22 0.28 0.15 0.12 0.23 0.09 0.09 0.05 0.40 0.29 0.28 0.28 0.41 0.57 5.6 3.7 0.053

21.9

15.5

7.6 16.4 2.90

0.19 0.41 0.65 0.55 0.38 1.18 0.12 27.1 0.12 0.31 3.31 0.14 NA NA 19.2 0.08 0.19 0.25 0.14 0.13 0.21 0.09 0.08 0.04 0.36 0.26 0.06 0.27 0.37 0.54 9.5 20.4 3.60

a Modified from ref 24. Quantitative gas chromatography−mass spectrometry (GC−MS) analyses were made at Thünen Insitute (TI, Germany). WIS = LMM + HMM + extractives. bContains triglycerides, resin and fatty acids, fatty alcohols, and sterols.

The second phase stability mechanism can be observed in all lignocellulosic oils with high water contents and can be easily recognized by the formation of an upper layer with a high water content (typically close to 40−50 wt %) and a heavy tar-like phase at the bottom with a lower water content (typically less than 25 wt %). This tar-like phase is rich in lignin-derived oligomers.31

easily be observed when punching this semi-solid waxy layer. The top phase is very rich in extractive compounds and contains solids (char) and small quantities of other bio-oil components,18 such as carbohydrates and lighter phenolics, depending upon the condensation conditions (i.e., temperature and time). The stratification to top and bottom phases17 is slow (about 24 h at 35 °C) and can be avoided by constant mixing. 4374


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Figure 1. Phase separation of forest residue FPBO (VTT data).

FPBOs contain insoluble solid particles (char including ash and sand), which settle in time to the bottom or rise up to the surface depending upon the density and extractive content of the bio-oil.17 In practice, after bio-oil condensation, solid char particles adhere to the sticky extractives and rise up together to the surface of the bio-oil.17 Behavior of the biochar particles depends upon the type of bio-oil produced and their feedstock. In the case of bio-oils derived from materials with a very low content of extractives, the biochar tends to settle. In the case of bio-oils with an upper layer derived from extractives, the biochar has affinity for this layer and can be typically recovered as a fraction of this phase. The main goal of this paper is to present and discuss new results on how the phase stability of FPBOs is dependent upon bio-oil chemical composition. A chemical-composition-based phase stability diagram was used for identifying how close a FPBO is to phase separation. Furthermore, a new phase stability index is also proposed with associated suggested methods to prevent phase separation. Results are derived from fresh and aged bio-oils produced between 2002 and 2013 at VTT. Additional experiments with water, a WIS fraction, and a model solvent mixture are included and discussed.

Phase separation of bio-oil through the second mechanism can happen immediately after pyrolysis vapors have condensed to bio-oil or during storage because of changes in the chemical composition of the oil. There are two general reasons for phase separation:1 a high water content (>30 wt %) of bio-oil2 and a high amount of high-molecular-mass compounds. The water content in fresh bio-oil can be high because of high moisture (>about 10 wt %) or ash content (>about 2 wt %) in the feedstock (ash catalyzes dehydration reactions). Other influences on bio-oil−water content include low volatile content (about 550 °C), which can increase the fragmentation of primary products. Fragmentation increases the yield of gases and reduces the content of organic compounds in the oil, which causes the organic phase to become more hydrophobic because of the deoxygenation reactions.12,29 Before and after the phase separation happens, the change in chemical composition increases the average molecular weight, viscosity, WIS fraction, and water content and decreases the amount of carbonyl compounds.12,19−28,30 The addition of solvent (mainly alcohol) is a common method to prevent phase separation or redissolve the sediment lignin and aqueous fraction.12,23,25,26 Figure 2 shows how the

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EXPERIMENTAL SECTION

FPBOs. Bio-oil is produced in VTT’s 20 kg/h process development unit (PDU) using a transport bed reactor. The feedstock has been pine sawdust and various forest residues containing mainly softwood. The feedstock is fed to the reactor by a screw feeder. The pyrolysis temperature is in the range between 480 and 520 °C, and the residence time for pyrolysis vapors is about 0.5−2 s. The main part of the char particles as well as heat-transfer sand is removed by cyclones from a hot stream of product gases and vapors before entering liquid scrubbers. The product vapors are condensed in liquid scrubbers, where the product liquid is used as a cooling agent. Typical product yields from pinewood are 64 wt % organic liquids, 12 wt % reaction water, 12 wt % char, and 12 wt % non-condensable gases. Separation of the top phase from the pine/forest residue liquid product is performed at 35 °C within 24 h by the standard method described earlier.17 Characterization. Feedstock analyses are carried out employing standard methods: moisture content according to DIN 51718 and ash content according to EN 7 (at 815 °C). Fuel analyses for bio-oils are carried out using standard methods.10 Chemical composition of the

Figure 2. Effect of water and alcohol addition. Pine pyrolysis bio-oil1 (left) having 24 wt % water, (middle) after 10 wt % water addition, and (right) after 10 wt % addition of isopropanol into the former phase-separated oil (VTT data).

addition of water (10 wt %) to fresh FPBO forces lignin and aqueous phases to separate but the addition of isopropanol (10 wt %) redissolves the mixture. The separation of lignin and the aqueous fraction can be slow or happen instantly, depending upon the composition of FPBO. 4375


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Energy & Fuels bio-oils studied is analyzed with solvent fractionation (Figure 3), as described in detail in earlier papers.18,24,29 In the method, bio-oil is

Homogeneity of the mixture was determined with optical microscopy. Figure 4 shows an example of homogeneous and phase-separated

Figure 3. Solvent fractionation scheme. divided into WS and WIS fractions by water extraction. The “sugars” are obtained from the WS fraction, with extraction of diethyl ether as an ether-insoluble (EIS) fraction. The water content is analyzed with Karl Fischer titration according to ASTM E203. “Co-solvents” are obtained by subtracting “sugars” and water from the WS fraction. Phase Stability Diagram. For the construction of the phase diagram, the bio-oil is considered a blend of three pseudo-components. The polar fraction is formed by the water and sugars. The nonpolar phase is associated with the lignin oligomers (WIS). The amount of co-solvents is calculated by subtracting the amount of the other two fractions from whole bio-oil. Addition Experiments. To create a phase stability indicator and to cover larger parts of the phase diagram, various solvent/water/biooil fraction addition experiments were carried out. Three components were used: water, a WIS fraction isolated from the bottom phase of bio-oil, and a model solvent mixture. The model solvent mixture was made by blending five solvents that are commonly found in wood FPBOs. The solvents were acetic acid (40 wt %), hydroxyacetone (25 wt %), furfural (25 wt %), phenol (5 wt %), and methanol (5 wt %). Each solvent represents a certain functionality, and their amount corresponds to typical proportions of the ether-soluble (ES) fraction. Table 2 shows a list of the addition experiments and the added amount of water, WIS, and solvent. The composition (water and

Figure 4. Phase separation of lignin and aqueous fractions of bio-oil: (A) whole bio-oil, (B) phase-separated whole bio-oil, (C) bottom phase of bio-oil, and (D) phase-separated bottom phase.

whole bio-oil and the bottom phase of bio-oil. Whole bio-oil (A) contains a greater amount of insoluble solids (dark asymmetric pieces) and poorly soluble extractives (light spots) than the bottom phase (C), but eventually, both of them separated in a similar fashion. Darker areas of the phase-separated bio-oils (B and D) are the lignin phase, and lighter areas are the aqueous phase. The chemical composition of the mixtures from the addition experiments was calculated, as opposed to the chemical composition of the fresh and aged bio-oils that was analyzed by solvent fractionation. The mass percentage of each fraction in a mixture can be calculated if the chemical composition of the starting bio-oil and the added amount of water, WIS, and/or solvent are known. The following equation gives the mass percentage of one fraction in the mixture after the addition. The water content of the isolated WIS fraction was relatively high (16 wt %) and is taken into account in the equation by subtracting it from the WIS fraction and adding it to the polar fraction.

Table 2. Addition Experiments with Water, a WIS Fraction, and a Model Solvent Mixturea addition (wt %) sample number 1 2 3 4 5 6 7 8 9 10

water

WISb solvent

10 20

20 20 20 30 20

10 20 30 20 30 40 30 10

m(xm) =

addition (wt %) sample number 11 12 13 14 15 16 17 18 19 20

water

WIS

solvent

30 30 40 40 50

20 20 20 30 60 10 20 10 20 30

20 40 30 70 110

m(x i) + m(x) ± wwm(WIS) × 100% 100 + m(total)

(1)

where x is the polar, WIS, or solvent fraction, m(xm) is the mass of fraction x in the mixture (wt %), m(xi) is the mass of fraction x in the original bio-oil (wt %), m(x) is the mass of addition of fraction x (wt %), m(WIS) is the mass of added WIS, ww is the water proportion of isolated WIS (0.16), and m(total) is the mass of added WIS, water, and solvent (wt %).

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RESULTS AND DISCUSSION Phase Separation Because of High Water or Ash Contents in the Feedstock. A high ash content of agrobiomass leads to bio-oils with a higher water content and lower oil yield. This is because ash will catalyze pyrolysis reactions, increasing water and gas yields and decreasing bio-oil yield.29 Figure 5 shows the effect of the feedstock ash content on the yield of organics and phase separation tendency of FPBOs. The figure is based on over 20 years of experiments with bio-oils at VTT. Figure 6 presents a correlation between feedstock and bio-oil−water contents for VTT’s PDU at pyrolysis temperatures around 500 °C. Typically, fresh bio-oils are prone to phase separation at a water content above 30 wt %.

a

The composition (water and sugars/WIS/solvents) of the base oil was 55:20:25 (wt %). bThe water content of added WIS was 16 wt %.

sugars/WIS/solvents) of the base oil was 55:20:25 (wt %), which is very typical for wood-based FPBOs. The amounts were chosen on the basis of the need to cover certain areas of the phase diagram that were obtained from analysis of fresh and aged bio-oils. One or two components were added at a time to fresh bio-oils, and the mixture was shaken for 1 h at room temperature and left to stand for 2 days. 4376


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Figure 7. Changes in compound groups of a pine bio-oil1 during storage at room temperature. There is more inaccuracy in results of 24−96 month samples because of non-homogeneity.

Figure 5. Effect of feedstock ash on phase separation tendency of fresh bio-oil. The curve is based on numerous experimental data points with wood and agrobiomass obtained by VTT’s 20 kg/h pyrolyzer.

Figure 6. Estimated correlation between feedstock and bio-oil−water content, with input data from the PDU at VTT, with the reaction temperature around 500 °C. Condensation of recycle gas has not been included.

Figure 8. Change in the carbonyl content of a pine bio-oil1 by time at room temperature. The trend for carbonyl is clearer because the carbonyl titration (±0.1 mol/kg) is more accurate than the WIS determination (±1.5 wt %). db = dry basis.

Phase Separation Because of the Increase in HighMolecular-Mass Compounds in Long-Term Aging. All bio-oils phase separate at some point. The change in a goodquality pine FPBO1 was followed over 8 years. The phase separation took place after 2 years of storage at room temperature, even though the water content of the bio-oil was only 26 wt % (Figure 7). However, there was a significant increase in the high-molecular-weight WIS fraction and a major decrease in the ES “co-solvents”. In Figure 8, it can be seen that there is a gradual decrease in the carbonyl content and the WS/WIS ratio in the course of time. The trend for carbonyl is clearer because the carbonyl titration (±0.1 mol/kg) is more accurate than the WIS determination (±1.5 wt %). However, the decrease in WS/ WIS seems to follow the decrease in the carbonyl content. This is quite logical because, in earlier research,24 it has been shown that the decrease in carbonyl compounds (included in WS) happens almost at the same rate as the increase in WIS. It has earlier12,19 been presented that the changes at room temperature correlate with the changes in accelerated aging test at ≤90 °C. For example, changes in 1 year of storage at room temperature correlate roughly with changes in 24 h at 80 °C.19 Figure 9 shows the change in the carbonyl content of the pine pyrolysis bio-oil1 at 80 °C and the phase separation zone.

Figure 9. Phase stability of a pine bio-oil1 at 80 °C.

Phase Diagram for Describing the Phase Stability of FPBO. To better understand the phase stability of FPBOs, a 4377


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Figure 10. Ternary-phase diagram of phase stability based on chemical composition and homogeneity of fresh and aged bio-oils produced at VTT’s PDU.

separated to two phases. Figure 10 shows the correlation between the chemical composition (the above-mentioned three fractions) and the homogeneity of fresh and aged bio-oils in a ternary-phase diagram. Blue circles and red squares represent one- or two-phase bio-oils, respectively. Their positions in the diagram show their chemical composition. Small arrows attached to the axes point to the directions of how to read the diagram. The composition of normal fresh bio-oils was typically 55:20:25 (weight ratio of water and sugars/WIS/co-solvents). After 1 year of aging, their composition changed to approximately 55:25:20, but they remained homogeneous. After 4−8 years of aging, the WIS content increased and the solvent content decreased to as low as 55:40:5 and the bio-oils separated to two phases. The change in chemical composition during aging is highlighted with the blue dashed arrow in the diagram. Aged bio-oils appear in the bottom of the diagram because the amount of WIS content increases and the amount of C1−C6 oxygenated compounds decreases during repolymerization. The composition of fresh dry bio-oils (water content less than 10 wt %) was also homogeneous, but their position was slightly further away from the polar bottom left corner compared to wet bio-oils having an average composition of about 45:30:25. Although the water content was lower, the solvent fraction was similar to wet bio-oils. Addition Experiments with Model Compounds. The main objective of this study was to create an index that could predict how soon any given bio-oil would go through phase separation or explain why a certain bio-oil is not homogeneous. Although typical fresh and aged bio-oils are located in specific areas of the phase diagram, there are times when the chemical

phase diagram was developed on the basis of the fact that the bio-oil is formed by three pseudo-components. The chemical composition of the bio-oil was described as a blend of a very hydrophilic fraction, a very hydrophobic fraction, and a fraction that acts as a co-solvent. In this study, it was considered that the very hydrophilic fraction was formed by water and WS−EIS compounds (“sugars”). The very hydrophobic fraction was associated with the WIS compounds. The co-solvent fraction was associated with the WS−ES compounds. The fractionation procedure is described in Figure 3. The hydrophilic fraction consists of water and the most polar organic compounds, such as anhydrosugars, polyols, and other compounds with many hydroxyl groups. The second is the WIS fraction, which is mainly composed of lignin degradation products and some dissolved wood extractives (e.g., fatty acids and resin acids). The third fraction consists of C1−C6 oxygenated molecules that include aliphatic and aromatic acids, aldehydes, ketones, alcohols, and phenols. We assumed that the first and second fractions are poorly soluble to each other, but the third fraction, i.e., the C1−C6 oxygenated compounds, acts as a co-solvent for the first two. Bio-oil was considered homogeneous when no separation of lignin and aqueous phase (Figure 1) could be determined visually or with a microscope (Figure 4). Several single- and two-phase bio-oils were selected for the determination of correlation between phase stability and chemical composition. The bio-oils were produced at 20 kg h−1 in the PDU during the last 12 years. Fresh bio-oils were typically homogeneous, unless the moisture content of the feed was very high. However, after prolonged aging at room temperature or accelerated aging at 80 °C, they eventually 4378


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Figure 11. Ternary-phase diagram of phase stability including addition experiments with water, a WIS fraction, and a model solvent mixture.

composition of a batch differs from the typical composition. To cover a larger area of the diagram and draw borderlines, addition experiments were performed with model compounds. The representatives for polar, WIS, and solvent fractions were water, isolated WIS, and a mixture of solvents, respectively. The results of the addition experiments together with previously shown results of fresh and aged bio-oils are gathered in the ternary-phase diagram in Figure 11. The phase diagram shows how much co-solvent was needed to homogenize a certain amount of polar and WIS fractions of bio-oil. About 15−20 wt % of co-solvent was the minimum requirement for a homogeneous bio-oil. However, when the relative amount of the polar fraction (water and “sugars”) increased, more solvent was needed to homogenize the mixture. For example, when the amount of the polar fraction increased to around 60 wt %, up to 30 wt % of co-solvent was needed or the mixture phase separated. The phase diagram shown in Figure 11 was used for developing a phase stability index for bio-oils. To build a regression model, 10 data points were obtained directly from the equilibrium line. Each data point, therefore, contained compositional information, i.e., content of WIS, sugars + water, and solvents, for the hypothetical bio-oils, which would be situated directly on the equilibrium line. Because the sum of the aforementioned pseudo-components always adds up to 1, only two of the variables were truly independent. Thus, the task to formulate the stability index consisted of finding an equation for correlating two of the compositional parameters from the equilibrium line data points. After several models and regression strategies (linear, exponential, logarithmic, etc.) were tried, it was concluded that the best regression was obtained using the equation shown in Figure 12. To establish a

Figure 12. Correlation between the content of WIS and the WIS/ (sugars + H2O) ratio.

clear-cut numerical value for the stability index, both terms of the equation were divided. The idea is that, if the system is close to phase separation, then the stability index should be close to 1. In contrast to this, a very high value of the stability index suggests that the system is very unlikely to form separated phases. Figure 12 confirms that, as the content of WIS increases, the ratio WIS/(sugars + H2O) has to increase exponentially mainly because of the need to replace part of the sugars + H2O by a cosolvent capable of solubilizing the WIS fraction. The proposed index is shown in eq 2 stability index = 4379

10.66WIS × e(−0.0611WIS) water + sugars

(2)


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Energy & Fuels

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where WIS is the content of the water-insoluble fraction (wt %) and water + sugars is the content of water and sugar (wt %). This index can be used to predict the expected phase stability of bio-oils if the contents of water, sugars, and WIS fractions are known. A stability index value close to 1 indicates that the oil is very close to phase equilibrium. Consequently, very small amounts of water formed during aging can result in the formation of two phases. A very high stability index indicates that the oil is unlikely to form separate phases during storage. A stability index below 1 is indicative of oils that will form at least two phases upon storage.

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CONCLUSION In this paper, we presented how the phase separation of bio-oils (separation of the lignin-rich phase from the aqueous phase) can be explained by the chemical composition of the oils. A chemical-composition-based phase stability diagram shows how close to phase separation any given FPBO is. A phase diagram was drawn using data from numerous fresh and aged bio-oils and addition experiments with water, a WIS fraction, and a model solvent mixture. The phase diagram was used to propose a stability index that represents the storage stability of bio-oils in the homogeneous phase. Oils with high stability index values are likely to take longer storage times for phase separation to occur. A homogeneous fresh FPBO consists typically of 55 wt % polar compounds (water and “sugars”), 20 wt % WIS compounds (lignin-derived material, extractives, and solids), and 25 wt % “co-solvent” compounds (light aliphatic and aromatic acids, aldehydes, ketones, alcohols, and monophenols). An increase in the relative amount of the polar fraction above 60 wt % or a WIS fraction above 35 wt % (increase in polymerization products) or a decrease in the amount of co-solvents to below 15 wt % (reactions of carbonyl compounds leading to WIS material) may lead to phase separation. An oversized polar fraction may result from moist feedstock or high ash content of feedstock that decreases the organic yield and increases the amount of pyrolysis water. Phase separation can be hindered by specifying the maximum moisture and ash contents of feedstock, adding co-solvents, such as alcohols, and preventing the aging reactions by keeping the storage temperature as low as practically possible. More work is needed for in-depth analysis of phase separation, including the effect of molecular weights and solubility parameters.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: anja.oasmaa@vtt.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Jaana Korhonen, Sirpa Lehtinen, and Elina Paasonen are acknowledged at VTT for the analytical assistance. REFERENCES

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Controlling the Phase Stability of Biomass Fast Pyrolysis Bio-oils

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