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Pyrolysis-oil Multiphase Behavior and Phase Stability: A Review Anja Oasmaa, Isabel Fonts, Manuel Raul Pelaez-Samaniego, Martha Estrella García-Pérez, and Manuel Garcia-Perez Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01287 • Publication Date (Web): 21 Jul 2016 Downloaded from http://pubs.acs.org on July 25, 2016

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Pyrolysis-Oil Multiphase Behavior and Phase Stability: A Review Anja Oasmaa1, Isabel Fonts2,3, Manuel Raul Pelaez-Samaniego4, Martha Estrella Garcia-Perez5, Manuel Garcia-Perez*6 1 2 3

Centro Universitario de la Defensa – AGM, Ctra. Huesca s/n, 50091, Zaragoza, Spain

Thermochemical Processes Group (GTP), Aragon Institute of Engineering Research (I3A), 4

5

VTT Technical Research Centre of Finland, Espoo, Finland

Faculty of Chemical Sciences, Universidad de Cuenca, Cuenca-Ecuador,

Facultad de Quimico Farmacobiologia, Universidad Michoacana de San Nicolas de Hidalgo, Morelia, Mich., Mexico

6

Department of Biological Systems Engineering, Washington State University, WA 99164, USA

(Paper submitted to: Energy and Fuels) Abstract: This paper reviews the literature related to the complex chemical composition and multiphase nature of bio-oils and their practical implications. Over time bio-oil forms separated phases due to purely physical phenomena (phase stability) or chemical composition changes in storage (aging reactions). Bio-oil multiphase behavior and the formation of separated phases are controlled by the complex chemical composition of these oils. Fast pyrolysis oils from woody biomass are typically observed in a single phase. However, feedstocks with high extractives content and/or high ash content commonly produce oils with more than one phase (an aqueous phase, an upper layer and a decanted heavy oily phase). The first part of this manuscript focuses on the effect of feedstock composition, particle size, type of pyrolysis reactor and condensation systems on bio-oil chemical composition and their impact on stable oils production. The second section reviews our current understanding of fresh bio-oil multiphase behavior and the effect of aging reactions. The use of phase diagrams as a tool to predict bio-oil phase stability is discussed. The third section focuses on bio-oil upgrading strategies based on the use of solvents and the production of emulsions. In this section we discuss the factors affecting phase equilibrium. This review highlights the importance of developing systematic studies to better understand bio-oil liquid-liquid phase equilibrium and the advantages of using phase diagrams. This understanding could have significant impact on the development of new bio-oil separation 1 ACS Paragon Plus Environment

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processes, on the development of new tools to produce stable bio-oils, as well as on the production of bio-oil derived fuels. Understanding the complex nature of bio-oil multi-phase behavior has been progressing over the years, however more work is still needed to control these phenomena.

Keywords: Pyrolysis oil, Physico-chemical properties, aging, solubility, phase stability, triangular diagrams

Corresponding author: *Manuel Garcia-Perez Associate Professor, Department of Biological Systems Engineering Washington State University e-mail: [email protected] Phone: 509-335-7758 Fax:

509-335-2722

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1. Introduction Biomass fast pyrolysis is a thermochemical process typically carried out between 350 and 550 o

C, in the absence of oxygen, to convert lignocellulosic materials into gases, solids, and liquids

(also known as bio-oils)1-4 . The main product of this process is bio-oil, which accounts for 60 to 75 wt. % of the original dry biomass. Fast pyrolysis is one of the most promising technologies for the production of second generation transportation fuels (gasoline, jet fuel, and diesel)1-6, a replacement of fuel oil for heating 7and electricity8. This technology is gradually reaching commercial status and continues to be the subject of very intense research activity6,

9-11

.

Examples of recent successes in bio-oil utilization are the combustion of 1360 t of Ensyn oil per year in New Hampshire hospitals, and industrial combustion tests in Finland by Fortum/Valmet/UPM/VTT

6, 7

. Along with the commercialization efforts, the business and

research community has also been progressing in bio-oil standardization8, 9, 12, 13. During fast pyrolysis 8 to 15 wt. % of the original biomass (dry basis) is converted into small organic compounds (mostly, hydroxyacetaldehyde, acetol, acetic acid, formic acid and methanol); 5 to 10 wt. % into mono phenols and furans; 6 to 15 wt. % into hydrolysable sugars; 6 to 15 wt. % into lignin oligomers; 10 to 15 wt. % into water; and close to 20 wt. % into an unknown water soluble fraction likely derived from cellulose14. Bio-oil complex multiphase behavior is derived from the differences in polarity between these groups of compounds and by their relative concentrations in the oil. Bio-oil multiphase phenomena manifests by the the presence of separated phases (char particles, waxy materials, aqueous droplets, micelles)15-20 and are responsible for many of the problems encountered during bio-oil production, storage and utilization (e.g., formation of separate phases, clogging of lines, changing of properties over time).

Although excellent reviews have been published on the current status of biomass pyrolysis 11, 21-27

1-4, 6,

, on bio-oil fuel properties7, 8, 28, 29, on bio-oil upgrading22, 25, 30-34 and on bio-oil product

development35, 36 we were not able to find any review on the complex multiphase behavior of bio-oils and its practical implications. Thus, the main goal of this paper is to conduct a critical


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literature review on the factors affecting the formation of separated phases during handling, storage and uses of bio-oils. 2. Factors affecting chemical composition of fresh oils

The multiphase behavior of bio-oil results from its complex and ever changing chemical composition. In this section, the factors affecting bio-oil chemical composition, with special attention to references on their potential effect on bio-oil phase stability, are reviewed.

2.1 Biomass feedstocks: Bio-oil chemical composition is highly influenced by the composition of the raw material processed. Biomass is composed primarily of cellulose, hemicelluloses and lignin with a small fraction of extractives, which include fatty acids, fatty hydroxyacids, fatty alcohols, dicarboxylic acids, fatty alcohols, resin acids, light aliphatics and aromatics, among others15, and mineral matter (known as ash). Table 1 shows the content of these pseudo components in some of the feedstocks studied for bio-oil production.


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Table 1. Content of cellulose, hemicellulose, lignin and extractives of feedstocks typically used for the production of bio-oil [wt. %] (dry basis) Biomass Pine

Hemicellulose 27.1-29

Cellulose 35-40.8

Lignin 28-28.1

Extractives* 3.0-4.0

References 15, 16, 37, 38, 39, 40, 41 Sawdust 21.4 47.4 24.6 n.r. 42 Spruce wood 20.7-27.3 42.7-49.8 27.0-28.2 0.8-2.5 16, 43 Forest residues 20.5 37.1 25.7 5-10 41 Eucalyptus 13.2 43.0 23.2 n.r. 44 Hybrid Poplar 12.7 50.0 21.28 n.r. 45 Poplar 22.9 47.4 31.9 n.r. 46 Red oak chips 19.5 37.9 29.0 n.r. 47 Beech wood 31.2 45.3 21.9 1.6 43 Mallee wood 42.4 23.8 24.7 9.1 48 Spruce needles 25.4 28.2 35.3 6.1 16 Spruce bark 20.3 35.9 35.9 3.8 16 Pine needles 24.9 29.1 28.1 12.6 16 Pine bark 26.2 36.2 28.4 4.5 16 Corncob 31 50.5 15 3.5 43 Corn stover 20.7-24.6 35.1-39.5 11.0-19.1 n.r. 49 Cotton stalk 31 11 30 n.r. 49 Switchgrass 25-30 35-40 15-20 n.r. 49 Sorghum straw 24-27 32-35 15-21 n.r. 49 Rice Husk 19.7 25.2 34.8 9.8 50 Sugarcane bagasse 43.1 35.8 21.1 n.r. 51 Wheat straw 39.4 28.8 18.6 n.r. 43 Hazelnut shell 30.4 26.8 42.9 3.3 43 Walnut shell 22.7 25.6 52.3 2.8 43 Almond shell 28.9 50.7 20.4 2.5 43 Sunflower shell 34.6 48.4 17 2.7 44 *Calculation of extractives depended on the solvents used for their determination (see nature of solvents used in references) n.r.: not reported

Some lignocellulosic materials have significant content of extractives (e.g. spruce needles, mallee wood, stored or fresh forest residues have 6.1, 9.1 and 5-10 wt. % respectively) as seen in Table 1. The content of extractives varies for different biomass sources, e.g. hardwood (1-5 wt % of dry wood), softwood (1-4 wt% of dry wood) and bark (4-5 times higher than bark free wood)15. Extractives have lower oxygen content than cellulose, hemicellulose and lignin. Consequently, a separated upper phase with higher viscosity and heating value is formed in biooil produced from feedstocks with high extractive content15,

52-55

. The upper layer rich in

compounds derived from extractives often contains waxy materials that melt at relatively low temperatures (close to 40 oC)15,

17

. The water content of the upper layer rich in compounds


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derived from extractives is considerably lower than the water content of bio-oils derived from cellulose, hemicellulose and lignin 17, 55.

The quantification of the content of volatiles, fixed carbon and ash (proximate analysis) and the content of C, H, N, S and O (elemental composition) is also commonly used to describe the chemical composition of lignocellulosic materials. Table 2 shows the proximate and ultimate analyses of some of the feedstocks used for bio-oil production. Table 2. Elemental and proximate analyses of some of the lignocellulosic materials that could be processed by fast pyrolysis.

Biomass Pine Eucalyptus Hybrid Poplar Corn stover Poplar Rice husk Sugarcane bagasse Peach pit Switchgrass Read oak wood Wheat straw Beech wood Spruce wood Forest residues

Proximate analyses (dry basis) (wt. %) Ash Vol. Fixed carbon 0.2-0.5 80.3 19.5 0.5 81.9 17.6 5.1 84.0 10.9 1.3 16.4 22.6 61.0 16.7 11.3 15.0

Ultimate analysis (dry basis) (wt. %) References

0.2 0.001 0.01

O (by diff.) 43.3-47 45.2 40.7 42.6 39.6 38.9 39.6

C

H

N

S

46.6-50.3 48.4 50.6 42.5 48.4 47.8 44.8

6.0-6.3 6.3 6.1 5.0 5.9 5.1 5.4

0.04-0.1 0.1 0.6 0.8 0.4 0.1 0.4

14, 37, 56, 14 57 43 43 43 43, 51

1.0 8.9 0.5

76.7 77.6

19.9 14.4 21.9

53.0 46.7 50.0

5.9 5.9 4.0

0.3 0.8 1.2

0.05 0.19 -

39.1 37.4 42.4

43 43 43

13.7 0.5 1.7 2.1-3.8

66.3 82.5 80.2 73.276.7

21.4 17.0 18.1 -

41.8 49.5 51.9 51.1-51.4

5.5 6.2 6.1 5.9-6.0

0.7 0.3 0.3 0.5

-

35.5 41.2 40.9 42-32

43 43 43 15, 58


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2.2. Effect of ash: Compared to woody biomass (pine, spruce, eucalyptus, poplar), the grasses (corn stover, rice husk, sugarcane bagasse, wheat straw) are high in ash (see Table 2). Although, biomass ash composition may vary depending on biomass type, it generally has high mass fractions of alkaline and alkaline earth metals (AAEM), silicon and, in some cases, Fe and Al48, 59, 60

. AAEM species are indigenous to all biomasses and are usually present as potassium or

calcium but with minor amounts of sodium, magnesium and other cations59, 60. Biomass ash content has a great influence on product yields20, 41, 48, 59-64. Organic liquid yields are highest for woody biomass having low ash contents and lowest for agro-biomass containing a high amount of ash and alkali metals41. Moreover, water production increases in the presence of alkalines in some cases leading to the formation of separated phases20. Historical data of pyrolysis liquid yields obtained from different lignocellulosic biomass in the 20 kg/h PDU (process development unit) at VTT; (Finland) have been plotted versus the ash content of biomass. The effect of biomass ash content in the bio-oil phase separation is shown in Figure 1 20

. Bio-oil collected from the fast pyrolysis of materials with low ash content is typically

homogeneous20,

40, 65

. Feedstocks with high ash content typically result in bio-oils with two

separated phases (an aqueous phase and a decanted oily phase rich in lignin derived products).

Demineralization can be an option to reduce ash content in biomass. It can be carried out by water and/or acid washing. Pyrolysis research conducted with demineralized biomass66 have shown that AAEM species are chemically or physically bonded to biomass. According to results obtained by different researchers, divalent AAEM, specially Ca, are the most resistant AAEM to be removed by acid washing. However, alkaline metals can be convineniently extracted from the biomass by washing with water48 or with dilute acid65. Washing at high temperatures with a strong acid could help to remove the alkalines but would cause significant changes in biomass biochemical constituents such as cellulose and hemicellulose 40, 65, 67.


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Figure 1. Effect of feedstock ash on phase separation tendency of bio-oil (Reprinted with Permission of the American Chemical Society ((Copyright 2015)20). Most literature demonstrates that bio-oil yield decreases with an increase in ash content29, 59, 60, 65. The removal of very small quantities of AAEM may not always result in yield changes, but might result in modifications of bio-oil composition48. The pyrolysis of demineralized biomass yields lower light oxygenates, mono-phenols and water, and higher anhydrosugars and lignin oligomers

40, 65, 68

. The catalytic effect of ash can be passivated with the addition of small

quantities of strong acids69, 70.

2.2 Effect of Pyrolysis Temperature: There are several papers discussing the effect of pyrolysis temperature on the yield and composition of fast pyrolysis oils14, 37, 71-74. The maximum bio-oil yield in fluidized bed reactors is typically achieved between 450 and 550ºC (see Figure 2).


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Bio-oil

Organics

Water Pyrolytic lignin

Figure 2. Effect of reactor temperature on the yield of water, pyrolytic lignin, total organics organics and total bio-oil for woody biomass (Built with data from 37, 14, 73).

Figure 2 shows that while the water yield does not change much with pyrolysis temperature, the maximum organics yield is achieved at the same temperature at which the yield of lignin oligomers is higher. At temperatures above 550 oC, secondary phase reactions seem to be accelerated, resulting in cracking of oligomers and extra gas formation61,

75

. Hoekstra et al.61

studied the influence of heterogeneous and homogeneous vapor-phase reactions on yield and biooil composition. Gas residence time from 1.1 s to 15.3 s and temperatures of 400, 500 and 550ºC were investigated61. The water production (close to 10 wt. %) was not affected by the homogeneous gas phase reactions within the studied range (400 – 550 °C, 1-15 s). This result confirms Mamleev’s view that cellulose dehydration reactions should only happen in liquid intermediates where acid catalysts for oxygen protonation may be formed76. Both average molecular weight and pyrolytic lignin content drop further upon increasing temperature from 500 and 550 ºC. According to Hoekstra et al (2012)61, cracking reactions are dominant over polymerization reactions at 400 – 550 °C and residence times between 1 and 15 s. It was not possible to find any report on the effect of pyrolysis temperature on bio-oil phase stability. The use of hot filters can accelerate secondary cracking reactions and consequently resulting in lower bio-oil yields77. The authors did not find any study on the effect of hot filtration on the multiphase nature and phase stability of resulting oils. 9 ACS Paragon Plus Environment

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2.3 Effect of Biomass Particle Size: Particle size impacts the microstructure of biomass particles. It plays a determinant role in the heat and mass transfer processes during fast pyrolysis78-80. Biooil yield decreases as the particle diameter increases, although not following a linear trend. An important drop of bio-oil and oligomers yield occurs when the particle size increases from less than 0.1 mm to 1 mm 78, 79. This bio-oil yield reduction happens at the same conditions where an increase in the yield of char and gas is observed. According to Wang et al.73, in the case of fluidized bed reactors particle sizes from 0.7 mm to 17 mm have only a minor effect on the total liquid yield. Heating rate of biomass particles falls from approximately 1000 ºC/s in particles of about

0.7 mm to 1.5 ºC/s in particles close to 17 mm73. The experimental data of total

conversion time vs. particle size reported by Wang et al.73 suggests that the system is in a kinetically controlled regime for particles with diameters below 2 mm. The decrease in lignin oligomers yield observed by some authors78, 79, resulted mainly from the retention of oligomeric products inside the biomass particles which are released by thermal ejection81. Depending on the particle size, two pyrolysis regimes were suggested38, 79, 82. The first one occurs when very small particles (formed mostly by cell walls) are pyrolyzed allowing the release of the aerosols. In the second regime, the thermally ejected oligomers are trapped inside the cell cavities leading to the formation of extra-char and bio-oils with lower content of oligomers82. This outcome is supported by the increase in the char yield and the changes occurring in bio-oil chemical composition (See Figure 3)78, 79.


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Bio-oil Organics

Water Pyrolytic lignin

Figure 3. Effect of particle size on the yield of water, pyrolytic lignin and total organics for woody biomass (Built with data from78, 79).

2.4 Effect of pyrolysis reactor type: The main reactors used for bio-oil production are fluid beds (bubbling, circulating, transported and spouted), ablative reactors (vortex reactor, rotating cone and plate type), vacuum and auger reactors4, 6, 23, 24, 83. In bubbling fluidized beds and circulating beds, the heat is transferred mainly by convection (9-19%) and conduction (80-90%), although some radiation (1%) occurs83. When using particles with diameters below 2 mm, these reactors operate under a kinetically controlled regime73. Gas and solid residence times are controlled by the fluidizing gas. Substantial amount of carrier gas is used to fluidize or transport the bed and also to obtain low gas residence times. High bio-oil yields at temperatures over 500 oC are achieved if the residence time of vapors inside the reactor is limited to less than 2 s. If pyrolysis is conducted at temperatures below 400 oC longer residence time of the vapors (more than 15 s) may be tolerated without a decrease in bio-oil yield61.

Some tail pyrolysis gas can be

recirculated in order to improve the economics of the process71. When processing woody biomass with low moisture content in these reactors, a homogeneous liquid is typically obtained in single step condensers.

The conical spouted bed reactor (CSBR) has lower operational and construction cost than the fluidized beds84-86. These reactors have lower volume requirements, can operate with larger 11 ACS Paragon Plus Environment

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biomass particles, and do not require distributor plates. In spouted bed reactors the gas raises through the spout and annulus and the particles descend countracourrent through the annular region enhancing heat and mass transfer rates

84-86

. Heat supply to the pyrolysis reactor through

the walls can be avoided by adding small oxygen quantities converting the bed into autothermal conditions87. Bench scale studied at 500 oC and adding 15 % stoichiometric oxygen show an increase in the yield of oil mostly by the formation of extra water without much change in the organics. A 25 kg/h pilot plant was built at the IK4-Ikerlan Research Center in collaboration of the University of the Basque Country (UPV/EHU)88.

In ablative-based pyrolysis reactors, biomass is pressed against a hot surface. In these reactors, the biomass melts on the heated surface leaving an oil film behind which evaporates or is thermally ejected. The main mechanism of heat transfer from the hot surface to the biomass is by conduction (95%)83. This process may use larger particles compared to fluidized beds. The hot surface abrades the char product formed, exposing fresh biomass for reaction83. This reactor is compact but its processing capacity is surface area limited and has mechanical problems associated to moving parts at high temperature. Homogeneous liquids can be obtained when using woody biomass with low moisture content.

In vacuum pyrolysis reactors it is possible to achieve low residence time of the vapors inside the reactor without the use of carrier gases89, 90. These reactors can be operated with larger particles but the heat transfer rate is slower than in fluidized bed and ablative reactors22. Inside the vacuum chamber, the biomass particles are moved by a transport belt, the biomass is heated when it comes in contact with a hot surface internally heated by molten salts90. Although the overall yield of liquid obtained is comparable with fluidized bed reactors, the yield of water is much higher (close to 20 wt. %). The high water yields result from the low heating rates achieved when large particles and vacuum are used. Consequently a decanted oil and a separated aqueous phase are typically obtained51. Homogeneous oils can be obtained when using fractional condensation systems operating between 50 and 80 oC. Another system studied to produce bio-oil is the auger reactor72, 91. Although this reactor does not require a carrier gas to move the biomass, high yields of oil can only be obtained if the 12 ACS Paragon Plus Environment

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residence time is decreased by use of a carrier gas72. The heat transfer in these reactors takes place when biomass particles come in contact with the hot surface of the tube; consequently, the heat transfer rate is controlled by the renovation of the surface23. Yields of water close to 10 wt. % have been reported in these reactors72. Homogeneous oils are obtained when processing small woody biomass particles72.

2.5 Condensation Systems: Bio-oils can be condensed in one or multiple steps. Fractional condensation systems are a useful tool to control the composition and phase stability of bio-oils. These systems were extensively used by the old wood distillation industry more than a century ago92, 93 for separating the tars from the aqueous phase (at the time referred to as pyroligneous water). Study of bio-oil condensation systems within the biomass thermochemical conversion research community is relatively new38, 94-105.

Fractional condensers are typically formed by condensers operating in series at different temperatures38, 94-104, 106. The first step is typically operated between 40 and 90 oC and the second between 20 and 30 oC38, 39. Fractional condensers offer an inexpensive way to separate large biooil molecules (precursors of transportation fuels) from small C1-C4 oxygenated molecules and water allowing for control of bio-oil multi-phase behavior. The first condenser is typically empty (does not contain packing) to avoid intense polymerization and clogging. As temperature in the first condenser increases, the liquid collected downstream will likely be in aqueous phase with high water content. It is very important to differenciate the oils and aqueous phases obtained by conditions that result in high water yields (example, slow pyrolysis) collected in single condensation stage (composition depends on liquid-liquid equilibrium); and the oil and aqueous phases obtained in fractional condensation systems. While the composition of the first group of oils (decanted oil) and the pyroligneous water is controlled by the liquid-liquid equilibrium that establishes in the storage tank, the composition of the second group (heavy oil and aqueous phase) is controlled by the liquid-vapor equilibrium in the hot condensers. The first oils are very rich in non polar compounds (lignin derived compounds) and are often refered to as “decanted oil”. The corresponding water rich phase is rich in polar compouds and refered to as called “pyroligneous water.” This phase contains most of the very light oxygenated molecules and the heavy anhydrosugars. The oil obtained in the hot stages of fractional condensation systems is 13 ACS Paragon Plus Environment

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called “heavy oil” and contains the heavy polar and non-polar fractions. The water rich phases collected downstream in cold condensation stages are refered to as the “aqueous phase” and present high content of the light organics (acetic acid, hydroxyacetaldehyde, acetol)38, 39, 96, 97, 100. The separated oily phase typically observed on the top of oils derived from extractive rich lignocellulosic materials is called “top oil”. Table 3 shows some of the condensers used by the the old wood distillation industry and by the scientific community and their main design and operational parameters. A scheme of some of these condensers is shown in Figure 4. In all these cases the pyrolysis vapors get in contact with a hot bio-oil liquid. In the case of the Meyer and Barbet tab condensers the pyrolysis vapors bubble through a hot layer of pyrolysis oils. In the

case of the scrubbers the pyrolysis vapor gets in contact with bio-oil droplets from a liquid spray. Results of key studies conducted on the use of fractional condensation systems for bio-oil collection are summarized in Table 4.

Table 3. Main direct and indirect contact condensers used by the old and current biomass pyrolysis industry. Condenser type Meyer condenser

Main characteristics It is formed by a layer of condensed bio-oil through which vapors are bubbled. Gases ascent from the lower compartment thorugh a perforated dome. Temperature is kept high enough to minimize condensation of acetic acid and methy alcohol, and to accelerate reactions of aldehydes and phenols which further produce tarry substances. Barbet tab separator Similar to the Meyer condenser but the pyrolysis vapors heat a lower chamber full of oil before bubbling through the plates. Scrubbers Pyrolysis vapors are put in contact with a liquid spray (an immiscible hydrocarbon, bio-oil or the aqueous phase). Since clogging in the first condensation tower is very intense, it is important to avoid packing. If the temperature in the first condenser is maintained at 80 oC, acetic acid can be almost completely removed. Reactive scrubbers Scrubber in which a chemical reactant (typically an alcohol) is added as cooling liquid to stabilize bio-oil. Reactive species concentrations are reduced. Indirect Contact Heat exchangers Tube and shell The pyrolytic gases and vapors pass through an easily cleanable section given the tendency of pyrolysis vapors to form incrustations during the condensation process. The length of these exchangers should be limited, keeping the cooling path relatively short. Coil condenser The pyrolysis vapors pass through a single tube cooled by an internal coil or a number of inclined and superimposed straight tubes connected by bends or elbows to facilitate cleaning.


References 93

93 39, 92-96, 106

94

92, 93, 107

92

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Meyer bubbling condenser

Barbet tab condenser

Two steps scrubber

Cooler Pyrolysis vapors

Pyrolysis vapors

Pyrolysis vapors

Heavy Oil

Heavy Oil

Cooler

Heavy Oil

Figure 4. Scheme of direct bio-oil condensation systems


Aqueous Phase Pump

Pump

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Table 4. Summary of the main findings on bio-oil fractional condensation systems Pyrolysis conditions and condensation system Pine pellets, 1 kg/h auger pyrolysis reactor coupled with several condensation steps. The first step is a scrubber.

20 kg/h transport bed reactor, feedstock: < 5 mm forest residues. Condensation system formed by two scrubbers and one cooler.

Condensation Conditions

Yield of bio-oil

Major findings

Ref.

Cooling and reactive agent: atomized ethanol. Temperature of first step varied between 20 and 120 oC Condenser operated at temperature from 36 to 66 oC

Oil formed by two phases. Overall liquid yield: 55 - 62 wt. % (aqueous phase: 80-90 wt. %)

Maximum conversion of 40 % acetic acid to ethyl acetate. The conversion increased with temperature and concentation of ethanol.

94

The water content in the bio-oil decreased from 24 to 7 wt. %. Organics from the scrubbers to the cooler was 15 wt. %. Most of these organics were light volatile compounds. Overall pyrolysis oil yield: 70 wt. %, Water and light molcules content decreased when the flow of carrier gas and condensation temperature in first stage increased. Overall pyrolysis oil yield: close to 60 wt. %. The water content for the oil was between 9 and 25 wt. %. Bio-oil collected in the first condenser has higher water content than liquid collected in the second step.

An important fraction of the water and the light organic molecules were removed.

104

Predictions of equilibrium model are in good agreements with results. The concentration of water and light molecules decrease while the concentration of heavy molecules increases.

39, 96

The oils collected in the different condensers were combined. No analysis was done on the effect of condensation temperature.

97,

Most of the water is collected in the first condenser.

98

Each fraction of bio-oil has a distinct appearance. oil 1 and 2 were black and extremely viscous, oil 3 flows like honey, oil 4 and 5 had lower viscosity.

Five stages for bio-oil recovery were employed to separately collecting both the vapors and aerosols of the oligomeric rich “heavy ends”, a middle cut of monomeric phenols and furans and an aqueous phase containing the light oxygenates. The effect of pyrolysis temperature on the composition of each of the fractions was studied.

100, 103

1 kg/h fluidized bed reactor, 1 mm pine wood. Two step scrubbers.

Condensation temperature studied: 15-90 oC. The first condenser was operated at higher temperatures.

1-1.6 kg/h fluidized bed of quartz sand. Raw materials were corn cob and corn stover.

Four condenser canisters cooled by circulating water maintained at about 4 oC Oil condensed in four condensers in series operating between 22 and 42 o C.

0.2-0.6 mm pine particles were pyrolyzed in a 1-5 kg/h bench-scale fluidized bed reactor. Red oak pyrolyzed in a 8 kg/h fluidized bed pyrolyzer

Five-stage bio-oil recovery system. First stage operated between 342 and 102 oC. Second state was an electrostatic precipitator at 129 o C. Third stage was a heat exchanger between 129 and o 77 C. Fourth


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stage: 77 oC. Fifth stage: 18 oC

Figure 5 shows the effect of first condenser temperature on the composition of bio-oils. The results obtained by Westerhof et al.39, 96 clearly indicate that the mass fractions of water and light molecules decrease as the pyrolysis temperature increase. Most of these species are collected in downstream condensers operating at lower temperatures. The authors39, 96 did not find evidence of significant reactions of acetol, acetic acid and monophenols when the first condenser was operated at high temperature. Hydroxyacetaldehyde seems to react when the first condenser was operated at temperatures over 45 oC. A mathematical model based on vapor-liquid phase equilibrium was developed and validated with experimental data39,

96

. The reduction in the

content of the light molecules (water, acetic acid, acetol, hydroxyacetaldehyde and phenols) and the increase in the content of the heavy oligomeric fractions, have an important impact on the multiphase behavior and phase stability of the resulting bio-oils. With the use of fractional condensation systems it is possible to obtain homogeneous oils even when high water yields are produced.

Water

Monophenols Acetol Acetic acid

Figure 5. Effect of first condenser temperature on bio-oil composition (Pyrolysis temperature 480 oC) (Built with data from39, 96).

3. Bio-oil Chemical Composition


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Excellent reviews on the chemical composition of bio-oils and on the analytical methods used for their characterization exist

108-111

. Excellent reviews on bio-oil physico-chemical properties

(viscosity, density, flash point, etc) have been published112-113. Due to bio-oil’s diverse chemical functionalities and wide range of molecular weights and boiling points, a combination of several analytical techniques is needed for the measurement of its composition. Table 5 shows a list of some of the techniques used for bio-oil characterization reported in the literature. Table 5. Methods used to study bio-oil chemical composition Property measured Volatile fraction (classification of volatile fraction in chemical families) Elemental Analysis (CHN-O) Molecular weight distribution Molecular weight distribution, overall content and composition of heavy different fractions, nature of chemical compounds Analysis of bio-oil in chemical families Water content Mono phenols and furans Carboxylic, fatty and resin acids Aromatic hydrocarbons Ligin oligomers (pyrolytic lignin) Chemical structure of lignin oligomers

Methoxy groups in pyrolytic lignin OH groups (phenolic, aliphatic, carboxyl, H2O) Hydrolyzable sugars soluble in water Total content of sugars Sulfuric acid assay for total carbohydrates Overall functional groups Overall functional groups Carbonyl, aromatic, carbohydrate, methoxy, hydroxyl Aliphatic, aldehydes, ketone, carboxyl, methoxyl

Analytical method or standard used GCxGC-TOF-MS; GC-APCI-FT-ICR-MS

Ref. 114-118

ASTM D 5291 GPC, MALDI-TOF-MS, LDI-ToF-MS, PyFIMS Negative Ion EI FT-ICR-MS, Orbitrap, QTOF

119-121 54, 122-127

Decovolution of DTG curves KF-Titration (ASTM 203, 1744) GC/MS previous fractionation by minicolumn liquid chromatography GC/MS, HPLC GC/ITD* Cold water precipitation

54 54, 119 133-135

Titration with tri-sulfate, FTIR, SEC, MALDI-TOF-MS, LADO-TOF-MS, and PyFIMS Titration with trisulfate Derivatization with TMDP and analysis of 31 P-NMR Hydrolysis of water soluble fraction followed by HPLC Brix determination (Anton Paar DIMA 4500 hydrometer) The phenolic-sulfuric acid method Quantitative 2D-NMR (HSQC) FTIR 13 C-NMR 1

H-NMR

Total Acid Number Total Phenols and Carboxilic acids Total carbonyl Groups Total phenols

Titration Titration Non-aqueous titration Folin-Ciocalteu

Semi-quantitative estimation of molecular weight of phenols *ITD: Ion Trap Detector

UV-Fluorescence


128-132

134-140 141 124, 126, 127, 142-148 127, 142

142 149 23, 150 110 151-153 154-159 142, 121, 160 91, 161, 162 91, 120, 161, 163 29, 164 165 166 154, 167, 168 14

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In spite of the number of analytical techniques in use, some bio-oil fractions are still very poorly characterized. Garcia-Perez et al.54 proposed to characterize bio-oils in chemical families as a mixture of: (1) very volatile organic compounds (mainly hydroxyacetaldehyde, acetol, acetic acid and methanol), (2) water, (3) monolignols (4) polar compounds with moderate volatility (furans), (4) anhydro-sugars (mainly levoglucosan), (5) extractive derived compounds, (6) heavy polar compounds (likely oligomers derived from cellulose and hemicellulose), (7) heavy nonpolar compounds (lignin oligomers) and (8) MeOH-Toluene insoluble (char). The content of some of the most important compounds and fractions is presented in Table 6. The composition of a typical oil referrers to fast pyrolysis oils obtained from lignocellulosic materials with relatively low ash content. Table 6. Typical bio-oil composition [wt. % dry basis, except water] 16, 20 Compound/fraction Amount Water 25 Acids 3-7 Acetic 2-3 Formic 1-2 Alcohols (ethylene glycol, methanol) 15 wt. %). This behaviour could be similar to that of bituminous petroleum171. Oasmaa et al.20 conceptualized bio-oil as a system in which the polar fraction (water and sugars) are cosolubilized with the non polar fraction water insoluble (also known as lignin oligomers) by the aid of solvents (mono-phenols and light oxygenated compounds) and used this vision to create a triangular phase diagram, as shown in Figure 9. This diagram describes the bio-oil compositions resulting in a single or two phases. There is clearly a region formed by a single phase and a region where more than one phase coexists. The line that separates these two regions was used for the formulation of a phase stability index for bio-oils 20.


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Figure 9. Ternary-phase diagram of phase stability including addition experiments with water, a water insoluble (WIS) fraction and a model solvent mixture (Reprinted with Permission of the American Chemical Society ((Copyright 2015)20.

The phase stability of bio-oils and their solubility in solvents is governed by a number of molecular interactions203-206. The Teas graph is a useful tool to study these interactions. Teas graph is a triangular diagram used to plot the solubility of different solvents with respect to dispersion force, the hydrogen bonding force and the polar force207.

Depending on the

compound classes of solvents, they will occupy one zone or another within the Teas graph. Using this graph, it is possible to find out the zone of solvents that solubilize a material (solubility window). For this, the material is tested in several solvents. The authors were not able to find any reference to the use of Teas diagrams or other theoretical tools used to study liquidliquid phase equilibrium in the study of bio-oil phase stability.

6.

Bio-oil upgrading strategies with the use of solvents

Most of the literature on bio-oil upgrading focuses on hydroprocessing, however there are also opportunities to improve bio-oil properties by blending with other fuels36 and/or by using 29 ACS Paragon Plus Environment

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solvents. The main bio-oil fuel properties typically targeted with such upgrading strategies are thermal stability, ignition delay, acidity, low thermal stability, high viscosity, poor lubrication and formation of engine deposits7.

Use of organic solvents: Based on Figure 9, it is clear that solvent addition is a viable way to increase bio-oil phase stability. The effect of organic solvent addition on bio-oil physio-chemical properties has been studied by several authors

53, 95, 172, 197, 208-211

. Many of these investigations

focused on increasing the homogeneity by using simple dilution effects. Others targeted improving bio-oil’s thermal stability by reacting reactive species (typically an aldehyde, a ketone or an acid) with the stabilizing agent (typically an alcohol). The potential formation of separated phases and their equilibrium are important elements that need to be taken into account when using organic solvents to upgrade bio-oils. Figure 9 shows a tertiary phase diagram used by Zhang and Wu212 to support the formulation of bio-oil/glycerol/methanol blends. The effect of adding different solvents on bio-oil phase stability is discussed in Table 10.

Figure 10. Phase diagram of bio-oil/glycerol/methanol ternary system (based on wt. %) ((Reprinted with Permission of the American Chemical Society ((Copyright 2015)212


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Table 10. Summary of the effect of adding Solvents on Phase stability Bio-oil origin/type Whole oil from forest residue

Solvent Added

Observation

Alcohols

Wood oil

Ethyl acetate, methyl isobutyl ketone and methanol, acetone, methanol, methanol and acetone, and ethanol Methanol

Did not achieve the complete co-solvency of all the compounds that form both phases, although the top phase was concentrated at less than 10 wt %. Solvents were added to the whole pyrolysis liquids for stabilisation during long-term storage periods.

pyrolysis

Bio-oils from the vacuum pyrolysis of softwood residues.

Pyrolysis liquids from forestry residue

Isopropanol

Microwave pyrolysis liquids from corn stover

Methanol and ethanol

BTG oil

Various alcohol

Not specified commercial supplier. Oil produced from the fast pyrolysis of malee eucalyptus wood

Glycerol and methanol

Oil produced by pyrolysis of wood chips in a circulating-bed unit

Methanol in the presence of acid catalyst (Amberlyst-70) between 70 and 170 oC

Methanol, ethanol in the presence of Amberlyst 15 and Amberlite IR-120 at 50 and 70 oC.

Reference 172

172

Investigated the properties of bio-oils in an aqueous phase (25 wt. %) and an oily pyrolytic phase (75 wt. %). The size of the water drops increased with the quantity of the pyrolytic aqueous phase. The diameter of the drops decreased when methanol was added to the mixture, and the dispersion of the drops in the matrix also improved. Adding alcohols to the whole pyrolysis oil enhanced the solubility of poorly water-soluble compounds (lignin dimers and extractives) into the matrix, reducing the quantity of the top phase and concentrating the extractives and the solids in the top phase. The addition of alcohols had a positive effect on the stability of pyrolysis oils. Solvents reduced the size of the aqueous phase droplets, and drop diameter decreased when the quantity of solvents increased. Catalytic esterification at 60ºC using an alcohol, in the presence of an acid catalyst and a dehydrating agent. The viscosity and acidity of the oil decreased. Water content increased during aging.

197, 209

Observed formation of esters (methyl formate, methyl acetate, methyl propionate) and ethers (dimethoxymethane, 1,1,2-trimethoxyethane). These reactions decreased the coking propensity of bio-oil The acid number decreased with time. The viscosity of the oils decreased and the thermal stability improved significantly. No phase separation during aging. Bio-oil was less corrosive.

213, 214

53, 95

208

210, 211

212

215, 216

The addition of solvents, especially alcohols, improves pyrolysis bio-oil viscosity, stability and homogeneity (see Table 11). Considering the simplicity of the solvent addition and its positive effect on pyrolysis liquid properties, this method can be one of the most practical approaches to upgrade pyrolysis oil quality. Nevertheless, the use of organic solvents to improve bio-oil fuel properties is limited by the decrease of the flash point and the cost of the solvents. 31 ACS Paragon Plus Environment

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Table 11. Effect of solvents on the physico-chemical properties of bio-oils Solvent used Isopropanol, ethanol, methanol. Addition of alcohols, in the presence of an acid catalyst Methanol, ethanol and isopropanol Methanol Methanol, isopropanol

ethanol

and

Addition of alcohols, in the presence of an acid catalyst Organic bases (cyclohexylamine and nbutylamine) and alcohols (ethanol) Catalytic treatments Ethyl Acetate, methyl isobutyl ketone, methanol, acetone, ethanol, isopropanol Addition of alcohols in presence of acid catalyst Addition of alcohols in the presence of acid catalysts

Impact on bio-oil physico-chemical properties The viscosity of the pyrolysis oils is reduced to a significant degree when solvent is added. Reduction of the pyrolysis oil viscosity by means of the catalytic upgrading of the pyrolysis oil The density decreased when solvents were added. The addition of 10 wt% of alcohol decreased the flash point from approximately 65ºC to 30ºC. There were some discrepancies on how the addition of alcohols affected the heating value of pyrolysis oils. Some studies report increase others reduction The heating value of pyrolysis oils increased after the upgrading treatment pH increased and a single homogeneous liquid was obtained.

The acidity of the oil was reduced The additives reduced the aging rate (measured by the change in viscosity) by a factor of 1 to18 compared with the original pyrolysis Improves the chemical stability of pyrolysis oils. Aldehydes, ketones and carboxylic acids are converted to acetals, ketals and esters (respectively). The pungent odour of sewage sludge pyrolysis oils was reduced as a result of the generation of esters

References 53, 172, 197, 208, 217 210, 211 15, 53, 197, 209 53 53, 208, 209

210, 211 146

211 15, 16, 52, 53, 172, 197, 208 210, 211

210

There are several published studies on spray and combustion of raw pyrolysis oil218-220. Some combustion tests of pyrolysis oils with organic solvents have been reported by Bertoli et al.221, and Lopez-Juste and Salva-Monfort222. Combustion of bio-oil blends with dyglyme have been conducted by the Instituto Motori-CNR in Italy221. Pyrolysis oils from VTT were mixed with dyglyme at 15.8, 30, 44.1 and 56.8 wt. %, and tested in an 11 kW diesel engine. Lopez-Juste and Salva-Morfort222 used a blend of biomass pyrolysis bio-oil (80 wt. %) and ethanol (20 wt. %) to fuel a gas turbine for electricity generation. The high viscosity of pyrolysis oils posed problems in the injector system, but these were solved by ethanol addition. These authors compared the injection, ignition and combustion characteristics of the pyrolysis-oil ethanol blend with those of a standard petroleum fuel (JP-4). The sprays obtained with bio-oil/ethanol blends had similar characteristics to those obtained with JP-4, but the spray cone angles were slightly smaller than the nominal ones.


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Use of inorganic solvents: Addition of inorganic solvents to pyrolysis oils has been conducted for two purposes: first to upgrade their acidic character and second, to isolate some of their fractions according to their acid-basic character. Adjaye and Bakhshi146 studied the effect of adding solid NaOH and KOH on the pH of bio-oil. After two months, the pellets had barely dissolved in the bio-oil. They concluded that the best way to add inorganic basis was in the form of a solution. Consequently, the authors added saturated solutions of three bases (NaOH at concentration of 48.7 wt. %, KOH at concentration of 69.3 wt. % and ammonia at concentration of 29.6 wt %) via titration. When the NaOH solution was added, a single phase bio-oil product was obtained with a pH of up to 3.53 (0.4 g of NaOH, 0.7 ml of solution). When the pH exceeded this threshold, two different phases appeared: a very viscous organic fraction and a less viscous fraction contained mainly water. About 6.0 g of NaOH (12.2 ml of solution) was needed per 100 g bio-oil to reach neutrality. When the KOH solution was added, phase separation occurred beyond a pH of 3.69 (1.2 g of KOH, 1.8 ml of solution). The bio-oil became neutral when 9.9 g of KOH (14.2 ml of solution) were added. When ammonia was used, phase separation occurred beyond a pH of 3.62 (1.2 g of KOH, 1.4 ml of solution) and 18.6 g of ammonia (21.4 ml of solution) were needed to reach neutrality.

6.2. Production of bio-oil blends with other fuels: Although it is known that pure pyrolysis oils have very limited solubility in mineral oils due to high water and oxygen content, it is possible to produce miscible blends if co-solvents are used. Adjaye and Bakhshi146,

147

investigated the

solubility of pyrolysis liquids with diesel, using different solvents (methanol, ethanol and acetone). They studied the influence of the mixing temperature at 23ºC and at 45ºC. The same experiments were repeated with the water-insoluble fraction of the pyrolysis liquids. The pyrolysis liquids were miscible with diesel only at low concentrations (up to 9:1 pyrolysis liquid:diesel), but after 4 h, a layer of diesel was visibly separated from the blend. The time of stability of the sample decreased as the diesel portion was increased. The authors also investigated the miscibility of mixtures of pyrolysis liquids and organic solvents with diesel. They found that the blends of pyrolysis liquid and methanol only formed stable blends at low diesel concentrations. The stability time of the sample decreased as the proportion of diesel rose. It was concluded that the mixture containing methanol in a proportion of 9:1 (pyrolysis liquid/methanol) had the best miscibility with diesel, with a ratio of 7:3 (pyrolysis liquid/diesel). 33 ACS Paragon Plus Environment

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In another work, Bakhshi and Adjaye223, blended Bunker C (No. 6 fuel oil) with pyrolysis biooil. The authors determined that acetone was a better solvent than methanol and ethanol for that mixture of pyrolysis oil and mineral oil. Blends of bio-oil, acetone and Bunker C were stable for up to 24 hours (Bio-oil/acetone/Bunker C ratio, 9:1:1). Weeracahnci et al.224 studied phase behaviors and basic fuel properties of palm kernel derived bio-oil/diesel/alcohol (ethanol and butanol) blends. The authors also used triangular phase diagrams to identify the zones of complete solubility (See Figure 11). Butanol was a better cosolvent but large amounts of alcohols are needed to obtain good phase stability. Blended fuels had superior density, viscosity, carbon residue, ash, pouring point and heating value.

Figure 11. Phase equilibrium diagrams for blends of Palm kernel bio-oil, diesel and an alcohol (ethanol and butanol). (Source: 224) Garcia-Perez et al.56,

225, 226

studied the production of blends of pyrolysis oils and bio-diesel

without the use of co-colvents. Blends of oils from the pyrolysis of pine chips and pine pellets produced in batch and auger slow-pyrolysis reactors were prepared56. The bio-oils consisted of two phases: an oily bottom phase and an aqueous top phase. The authors removed the water and


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the volatile compounds from the aqueous phase, obtaining a second oily phase, referred to as "polar oil." They then attempted to mix this polar oil and the oily bottom phase with the biodiesel. Several blends were prepared with bio-oil concentrations of 10, 20, 40, 50 wt. %. The blends were stirred while being heated at 60ºC. After that, they were cooled to 25ºC, and two different phases appeared: a bio-oil rich phase and a bio-diesel rich phase. The oily bottom phases (rich in lignin) were more soluble in bio-diesel than the polar oil phases (rich in anhydrosugars). Biodiesel-rich phases with up to 34 wt. % of bio-oils were obtained when equal amounts of pine chip bottom phase bio-oils were blended with bio-diesel. The most soluble compound groups of the bio-oils were monolignols, furans, extractive derived compounds and dimers. The density, the viscosity and the water content of the bio-diesel rich phase increased with the addition of bio-oils. The pH of bio-diesel was reduced. Addition of bio-oils did not effect the heating value of the bio-diesel rich phase. The bio-oil fractions solubilized in biodiesel acted as antioxidants and slightly improved biodiesel cold flow properties225, 226. Researchers from Aston University227 prepared fuel blends with a pyrolysis oil (produced from anerobic digestion digestate), waste cooking oil and butanol (10, 20 and 30 vol. %). The physical and chemical properties of the blend were measured and compared with thise of diesel and waste cooking oil. The blends were combusted in a multi-cylinder indirect injection compression ignition engine. The ignition delay period of the blend was longer. The total burning duration of the 20 and 30 % blends decreased by 12 and 3 % compared with diesel.

The utilization of pyrolysis oils and diesel fuels blended immediately prior combustion was carried out successfully by PYTEC for a Mercedes Benz diesel engine228. A mixture of pyrolysis oil produced in the Finnish ForesteraTM process developed by Fortum (96 vol %) and diesel oil (4 vol. %) was fed into a 12-cylinder high-pressure single-injection engine. The pumps and the injector were redesigned and their materials carefully selected. Twelve hours of efficient operation were reported.

6.3. Emulsions of bio-oil with other fuels: Although pyrolysis oils and mineral oils are not completely soluble, they can form emulsions. Emulsions of pyrolysis oils with mineral oils have


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been studied as a promising path for introduction of pyrolysis liquids into liquid fuel markets 229233

.

Falcon et al.234 presented a pioneering work on emulsions of pyrolysis bio-oil with mineral oils. However, they did not use the pyrolysis liquids as the water phase of the emulsion; instead, they used some pyrolysis liquid compounds as surfactants. More recently, other authors

230, 231, 235, 236

have successfully produced emulsions of pyrolysis oils with mineral oils. The use of these emulsions is of particular interest due to possible improvement of pyrolysis liquid properties (compared to pure pyrolysis liquids) such as ignition, viscosity and stability. Prakash et al.237 studied the production and combustion of emulsions between a jatropha bio-diesel with pyrolysis oil in the presence of a surfactant. The authors observed an increase in NO emissions and decrease in volatile organic compounds, CO emissions and smoke opacity. Ikura et al.230,

231

studied the emulsification of pyrolysis oils in No. 2 diesel fuel. Before the

emulsification process, the heaviest fractions of the pyrolysis liquids were removed by centrifugation. The operational conditions used in the emulsion production process were optimized (temperature, residence time, bio-oil concentration, surfactant concentration and power input per unit volume) towards emulsion stability and the processing costs. The authors found that in their process, only the bio-oil concentration, the surfactant concentration, and the power input had a significant effect on the emulsion stability. The authors used bio-oil concentrations of up to 10, 20 and 30%. The amount of surfactant ranged from 0.8 to 1.5 wt. % of the total emulsion, although the most stable emulsions were produced with the highest surfactant concentration. Chiaramonti et al.236 designed a procedure to prepare emulsions of pyrolysis liquids and diesel fuels. In their work, the authors studied approximately one hundred types of surfactants, and a consistent number of mixtures of them. Four pyrolysis liquids obtained from different feedstocks (oak, beech, pine) were used along with three different pyrolysis technologies (transporting bed, bubbling fluid bed, and rotating cone reactor). Three emulsions were prepared as follows: (1) 25% pyrolysis liquids, 74% diesel oil, 1% additive (w/w), (2) 50% pyrolysis bio-oil, 49% diesel oil, 1% additive (w/w), and (3) 75% pyrolysis liquids, 24% diesel oil, 1% additive (w/w). The 36 ACS Paragon Plus Environment

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emulsions were prepared at 60 ºC and 65ºC to guarantee stability. In addition to temperature, Chiaramonti et al.236 studied the phase diagram of the system pyrolysis liquid/diesel fuel/emulsifier to find the best proportions for better homogeneity, stability, engine performance, and operational costs. According to their results, large quantities of diesel fuel are required to meet most fuel standards and expensive sufactants are typically needed. The emulsions prepared by Chiaramonti et al.236 were tested in four different diesel engines without making significant engine modifications237. The authors focused on the injector, since it is the most important component in direct contact with emulsions. Preliminary tests were performed on a low-capacity high-speed motor using different ratios of bio-oil-diesel (25/75, 50/50 and 75/25% w/w), showing major damage to the injector holes and the fuel pump, especially when emulsions with low pyrolysis liquid content were used. The main problem was derived form the increase of the injector channel diameter, which led to higher CO emissions (due to the bad atomisation), although lower combustion temperature and therefore, lower NOx emissions.

Bio-oil utilization is still in its infancy. The mild upgrading of bio-oil with solvents to produce fuel oil and its further refining to obtain high value products are promising paths for its commercialization. The use of bio-oils in boilers has been demonstrated7 and there are ASTM standards and within 2016 EN standards9 for using of fast pyrolysis bio-oils as fuel in these systems. Addition of solvents, like alcohols improve the storage properties of these bio-oils. The development of bio-oil derived fuels and bio-oil separation strategies (solvent extraction, membranes, distillation) for bio-refineries will require a better understanding of the phase equilibrium phenomena with the environment relevant to these practical applications. More work is needed to develop predictive models capable of predicting the relationship between feedstock composition, pyrolysis and condensation conditions and the composition and multiphase properties of bio-oils. We need to advance our understanding on how each of the bio-oil fractions behave during storage, handling and utilization. 7. Conclusions


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The relative content of polar (water, and hydrocarbon derived sugars), non polar (lignin oligomers) and the co-solvent organics (small oxygenated molecules) are the most important factors controlling bio-oil multiphase behavior. The critical ratio at which phase separation occurs and how far an oil is from forming separated phases can be visualized with the aid of triangular phase diagrams. For a typical fast pyrolysis oil, phase separation (formation of an aqueous phase and a heavy oil) occurs when the biomass water content exceeds 25 wt. %. Controlling biomass composition (chiefly ash content), fast pyrolysis parameters (type of reactor, particle size, residence time of solid and vapor, pyrolysis temperature) and condensation conditions (design, number of steps, condensation temperatures) are critical to ensure the production of homogeneous oils. Current knowledge on biomass pyrolysis reactions does not allow us to recommend bio-oil production conditions that will result in the formation of homogeneous liquids. The reactivity of the oils associates to their content of carbonyl and carboxyl functional groups. The content of these functional groups together with the storage conditions (mainly temperature and time) impact the rate at which bio-oil properties change with time. Gradual reduction of low molecular weight very reactive compounds that act as co-colvents and the increase in the content of water and oligomeric materials can lead to phase separation during aging. Low molecular weight alcohol addition to fresh pyrolysis bio-oil is a promissing solution to improve phase and aging stability. The addition of alcohols to bio-oil, improves its viscosity, stability, heating value and homogeneity.

Blends of pyrolysis oils with organic solvents, biodiesel, and petroleum derived fuels have been prepared and tested in some combustion devices (e.g., diesel engines) with promising results. The emulsification of pyrolysis oils with mineral oils can produce stable emulsions with higher quality than the original pyrolysis oils, because the emulsion properties are a combination of the properties of the two fuels used. The emulsions produced by this upgrading method appear promising, based on results of previous experiments in diesel engines. However, further research is necessary to control the erosion that certain parts of the diesel engine suffers during the tests. More studies on the multiphase nature of bio-oil are needed to solve key problems encountered during bio-oil storage, feeding and combustion.


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Acknowledgements:

Dr. Garcia-Perez thanks the Washington State Agricultural Research

Center (NIFA-Hatch-WNP00701) and the National Science Foundation (CBET-1434073, CAREER CBET-1150430) for their financial support. Dr. Gracia-Perez wants to thank very specially Dr. Abdelkader Chaala for introducing him to bio-oil research during his graduate studies. Dr. Chaala is not any more with us, but his work and ideas greatly influenced this review paper and his teachings and kindness had a lasting impact on Dr. Garcia-Perez’s professional development.

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