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The droplet combustion and thermal characteristics of pinewood bio-oil from slow pyrolysis
Accepted Manuscript The droplet combustion and thermal characteristics of pinewood bio-oil from slow pyrolysis S.I. Yang, M.S. Wu PII:
S0360-5442(17)31977-1
DOI:
10.1016/j.energy.2017.11.119
Reference:
EGY 11907
To appear in:
Energy
Received Date: 13 July 2017 Revised Date:
24 October 2017
Accepted Date: 20 November 2017
Please cite this article as: Yang SI, Wu MS, The droplet combustion and thermal characteristics of pinewood bio-oil from slow pyrolysis, Energy (2017), doi: 10.1016/j.energy.2017.11.119. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The Droplet Combustion and Thermal Characteristics of Pinewood bio-oil from Slow Pyrolysis
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Prof. Shou Yin Yang Department of Power Mechanical Engineering National Formosa University Yunlin 63202, Taiwan Tel: 886-5-631-5432 Fax: 886-5-631-2110 E-mail: [email protected]
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* Corresponding author:
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Department of Power Mechanical Engineering National Formosa University Yunlin County, 63202 Taiwan
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S. I. Yang*, M. S. Wu
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1. Introduction Biomass fuels can be used to produce energy through thermal, biological, or physical processes,
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all of which rely on technical ingenuity, and the applications of such alternative energy sources to
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conducting demonstrations or establishing commercial facilities have attracted the attention of
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researchers. Combustion, gasification, and pyrolysis are the three prevailing types of thermochemical
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process used to convert biomass into profitable energy. Of these three, gasification is recognized as
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the relatively more attractive option, particularly when compared with combustion, as it is highly
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efficient and clean. By contrast, fast pyrolysis has been long been, and can result in products with
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different characteristics when using varied reactors or source materials [1]. As bio-oils produced
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through fast pyrolysis tend to have considerable affect on power equipment (e.g. pH value, viscosity,
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and moisture of bio-fuel), various studies have been conducted to determine how their undesirable
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effect can be decreased; the most notable solutions proposed include physical, catalytic, and
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chemical upgrading [2,3]. Although bio-oils generally have very different properties from petroleum
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fuels, most commercially available bio-fuels are fairly similar to them, and thus, are easily applied to
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existing power equipment. As existent burner designs are sensitive to the quality of bio-oil used,
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which can make burners prone to ignition, flame detection, and flame stabilization problems, an
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adequate bio-oil burner design, and its corresponding bio-oil composition, have become the focus of
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numerous studies [4-10]. Therefore, many studies have used different biomass material sources to
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produce oil by pyrolysis, and discussed the characteristics of bio-oil produced through different
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processes. For example, Abdelrahman et al. [6,7] used apricot and fish waste to produce bio-oil. Hu
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et al. [8] used blue-green algae blooms. Biradar et al. [9] used Jatropha Curcas de-oiled seed cake for
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testing. Chen and Lin [10] used oil palm fiber for oil-producing testing, hoping to understand the
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correlation between different oil-producing material sources and bio-oil combustion characteristics.
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In addition, biomass butyl alcohol is a novel biofuel. As a substitute fuel source, butyl alcohol
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has higher energy and volatility than methanol and ethanol. As butyl alcohol may be used in engines, 2
ACCEPTED MANUSCRIPT some studies have discussed the application of butyl alcohol to engines [11,12]. Some studies
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showed that the butyl alcohol diesel fuel blend, as used in diesel engines, could improve the
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emissions of soot and CO without a strong impact on engine performance [13-15]. Mixtures of bio-
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oil and petroleum fuels have become a prevalent approach for mitigating the negative effects of bio-
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oil, and their mixture and combustion properties can be improved by either improving the quality of
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the bio-oil used in the mixture or using a suitable petroleum fuel. Butanol has a short carbon chain
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structure (with four carbon atoms) that enables the emulsification of certain substances when added
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to bio-oil, which improves the mixing of substances by preventing stratification, reduces a mixture’s
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viscosity and improves its thermal mass, thus, butanol is often used as an additive in bio-
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oil/petroleum fuel mixtures. Kim et al. [16] used 10%, 20%, and 30% bio-oil in butanol as a fuel for
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a diesel engine, and verified that the presence of butanol made the fuel more likely to be auto-ignited,
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effectively suppressed bio-oil polymerization, and reduced gummy polymer generation. The
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combustion properties of a fuel can also be changed when the mixture is altered, which is due to the
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entirely different chemicals and physical properties of the mixture, such as its composition, volatility,
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boiling point, latent heat of evaporation, and specific heat capacity [17-19]. These considerations
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have prompted a large number of studies to explore the combustion properties of multicomponent
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droplets with varied droplet composition. For example, Kitano et al. [20] used numerical simulations
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to investigate the evaporation of single and multicomponent fuel droplets under various ambient
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pressures, gas temperatures, and combustion reactions. Liu et al [21] compared the isolated droplet
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burning characteristics of butanol isomers using detailed numerical modeling and demonstrated that
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the transport coefficients of isomer-specific species altered the fuel’s combustion properties. Hoxie
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et al. [22] investigated the combustion properties of droplets of soybean oil mixed with butanol, and
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determined that the two-component droplets, whose components had different volatilities, induced
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flame shrinkage due to diffusion-limited gasification. When butanol constituted 40% of the droplet,
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intense micro-explosions were induced, which indicated that the droplet had reached its superheat
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ethanol (ABE) and diesel in a high-temperature environment, and revealed that the ABE–diesel
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mixture expanded substantially at 823 K, which was caused by the rapid expansion and shrinkage of
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droplets. These rapid changes were due to the large amount of heat in the droplets, which caused the
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rapid formation and disintegration of air bubbles within them. The components of the ABE–diesel
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mixture had varied degrees of volatility, which could cause micro-explosions and change the
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mixture’s combustion properties.
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To this end, our previous study [24] used fast pyrolysis to decompose various biomass residues
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and investigated the bio-oil properties under varied operating parameters, which was followed by a
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diesel engine test using a diesel fuel diluted with various concentrations of bio-oil [25]. The results
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indicated that the addition of bio-oil both enhanced the diesel’s combustion efficiency and reduced
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pollutant emissions; however, as the bio-oil concentration and operating time were increased, the
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efficiency of the diesel engine decreased, and slagging was produced in its cylinders. Therefore, bio-
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oil in varying concentrations was subsequently added to kerosene for spray combustion testing
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through actual measurement [26] and numerical simulation [27]. The results indicated that the
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presence of bio-oil not only changed the properties of the spray, it also shortened the length of the
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flame produced, which is mainly caused by the premature ignition of the volatile substances in the
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bio-oil. To verify the added bio-oil’s effect on spray combustion, this study conducts combustion
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testing on the multicomponent droplets of a bio-oil/kerosene mixture [28] and identifies numerous
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phenomena, including micro-explosions, puffing, and slagging, which could be attributed to the
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composition of the bio-oil induced combustion behavior and speed, which are different from those of
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single-component droplets. With the objective of producing bio-oil of high quality and quantity using
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pyrolysis, this study further investigates the effect of fluidization velocity on the yield [24] and
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quality [29] of the produced bio-oil by adopting various fluidization velocities in a fluidized bed
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reactor. Emulsification technology, which enables the adequate mixing of bio-oil and petroleum fuels,
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is essential for this purpose [30,31]. From the aforementioned studies, the spray combustion properties of bio-oil are clearly affected
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by the varied pyrolysis processes that the biomass is subjected to, as well as the varied petroleum
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fuels that the bio-oil is mixed with. If the difference between the different bio-oil/petroleum fuel
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mixtures used for spray combustion are to be investigated, the magnitude of combustion is a critical
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factor that substantially affects the spray combustion achieved. To alleviate problems concerning
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bio-oil combustion, this study mixed butanol with bio-oil produced through the slow pyrolysis of
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pinewood. The burning rate and thermochemical properties of the resulting mixtures are determined,
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and the properties of bio-oil created through the pyrolysis of biomass are elucidated, as well as the
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combustion properties of the mixture’s multicomponent droplets.
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2. Approaches
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This study used slow pyrolysis to produce bio-oil from pinewood. Thermo-gravimetric analysis
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(TGA) and a droplet combustion test platform are then used to investigate the thermochemical
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characteristics of the bio-oil/butanol mixture. 2.1 Pyrolysis system
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A biomass pyrolysis reactor is used to produce bio-oil from pinewood. The reactor (as
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illustrated in Figure 1), has an inner volume of 0.125 m3 (0.5 × 0.5 × 0.5 m3), which is wrapped
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outwardly by a layer of thermal insulation material, heated by a set of high power heating wires, and
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temperature is controlled using a thermocouple. The pinewood is shredded into small pieces less than
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2 mm in size and then dried at 80 °C for 2 h to reduce their moisture content. Next, 1000 g of the
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dried pinewood is placed in the reactor, which is preheated to 300 °C. Subsequently, CO2 preheated
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to 300 °C is introduced into the reactor as a carrier gas at a flow rate of 8–12 cm3/s for 1 h, during
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which pyrolysis occurred. Afterwards, the vaporized pinewood oil, as produced through high-
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temperature pyrolysis is guided though a heat exchanger for condensation and collection (Figure 1).
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2.2 Droplet combustion equipment Figure 2 illustrates the test platform used in the droplet combustion tests, which consists
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primarily of a droplet generator, a combustion chamber, an igniter, and an image capturing system.
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First, a digital delay generator sends a pulsating direct current to activate the piezoelectric chips, and
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their deformation would (in a technique similar to ink-jet printing) squeeze the fuel in the tank into
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the droplet generator. The fuel mixture is ejected from the glass nozzle in the form of a droplet
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approximately 0.9 mm in diameter, which is then caught and fixed in place by crossed (90°) low-
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thermal-conductive ceramic fibers in the combustion chamber. A glow plug igniter (operated at 12 V,
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7 A, and 84 W) is subsequently extended beneath the droplet for 0.3–0.8 ± 0.3 s, and then quickly
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withdrawn to ignite the droplet. The resultant combustion was then captured using a Phantom v7.3
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high-speed camera using backlighting method. The high-speed camera operated at 1000 fps, has a
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resolution of 384 × 384 pixels, spatial resolution of 0.022 mm/pixel, and exposure time of 1 µs. A
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Phantom M310 color high-speed camera with a 4× optical lens was also used to capture magnified
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images at 2000 fps, with a resolution of 512 × 512 pixels and exposure time of 100 µs. Use such
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shooting conditions, in the image of each droplet, the droplet diameter has ±0.044 mm/pixel error.
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3. Results and Discussions
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This study produces bio-oil through the pyrolysis of pinewood, and measured the characteristics
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of bio-oil using various methods, such as ultimate and proximate analyses. The bio-oil is mixed with
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butanol, and TGA, differential scanning calorimetry (DSC), and droplet combustion tests are
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performed to determine the bio-oils properties and explore the relationship between the evaporation
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and combustion behavior of bio-oil droplet.
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3.1 Fuel Preparation
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is added to bio-oil, the properties of which are listed in Table 1. Specifically, while the bio-oil had
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relatively low moisture content (8.1 wt.%), it has high heat value (LHV, 17.12 MJ/kg) and viscosity
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(7600 cP). The bio-oil had a pH of approximately 3, indicating that it would corrode mechanical
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components and cause malfunctions if used alone. Certain properties of butanol nullify the need for
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an emulsifier when mixed with the bio-oil, as together they increased pH of a bio-oil/butanol mixture,
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as compared with pure bio-oil, which indicates that a bio-oil/butanol mixture would be suitable for
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fueling power equipment.
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Gas chromatography–mass spectrometry (GC-MS) is used to analyze the organic compounds in
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the pinewood bio-oil, as produced at a pyrolysis temperature of 500 °C and interface temperature of
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300 °C. The GC-MS system (Hewlett-Packard HP 6890 GC equipped with an HP 5973 mass
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selective detector) is directly connected to a Double-shot PY-2020D (Frontier Lab) pyrolyzer. The
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resultant GC-MS chromatogram is presented in Figure 3; however, because of the lack of a standard
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mixture for the calibration of the MS detector, this study could not conduct in-depth investigation
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into the large number of unknown and complicated organic compounds observed in the figure. Table
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2 presents a tentative list of compounds in the pinewood bio-oil, as well as the areas under their
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peaks; the compounds are all index compounds in the MS search file. Large amounts of aromatic and
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oxygenated compounds are identified in the pinewood bio-oil, including carboxylic acids, phenols,
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and ketones, as previously determined for other bio-oil types [24]. However, contrary to the results
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of another study [24] that investigated the oily phases of cedar sawdust, coffee bean residue, and rice
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straw, the pyrolysis of the pinewood bio-oil in this study reveals that the primary components are
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able to completely burned off were petroselinic acid (a nonsaturated aliphatic acid) and palmitic and
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stearic acids (saturated aliphatic acids), with a combined under-peak area of 35.34%. In addition, the
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remainder of the constituents were aromatics that easily produce soot or residues during combustion
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(including isoengenol, vanillic aldehyde, 2,6-dimethoxyphenol, 4-ethylguaiacol, 2-methoxy-4-
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Furthermore, the pinewood bio-oil consists of phenols with oxygen functional groups, for example
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2,6-Dimethoxyphenol, which is an aromatic compound that absorbs water in an alkaline solution.
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Finally, this study discovers that bio-oil feature notable organic acids contents, such as 4-hydroxy-3-
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methoxyphenylacetic acid, palmitic acid, petroselinic acid, stearic acid, dehydroabietic acid, and 7-
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Oxodehydroabietic acid. The presence of acidic compounds makes the bio-oil hydrophilic and easily
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mixed with alkaline alcohols, which can neutralize the acids in the bio-oil, thus, decreasing the
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amount of corrosion it causes. As is evident from Table 2, the compounds in the bio-oil are all
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macromolecules, which are not prone to evaporation or decomposition when heated, due to their
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relatively high boiling points (205–362 °C). The presence of these macromolecular compounds in
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bio-oil/butanol droplets will also affect the droplets’ properties by retaining volatile matter in the
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mixture, thus, lowering the droplets’ activation energy. The C18 fatty acid in the bio-oil would
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polymerize under intense heat [33] and cause residue production.
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The aforementioned results all indicated that the bio-oil produced through slow pyrolysis of
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pinewood relatively complicated composition and, compared with ordinary petroleum fuels, contain
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a large amount of oxygen-containing reactive functional groups, rendering it unstable.
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3.2 Thermogravimetric Analysis for the bio-oil/butanol fuel
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This study mixes bio-oil and butanol in various concentrations and performed thermal analysis
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on the thermal decomposition behavior of the mixtures through TGA and DSC. As the combustion of
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these mixtures would be affected by their properties, such as volatility and boiling point, which in
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turn depended on the concentrations of both constituents, a PerkinElmer STA8000 thermal analyzer
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is used to investigate the thermal behavior of the mixtures at high temperatures. The results are
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presented in TGA-DSC curves in Figure 4, with 12 mg of fuel sample, 80 mL/min air flow, and 30
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°C/min heating rate used. Four mixture of bio-oil are employed: 0%, 10%, 30%, and 50% (labelled
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BO0, BO10, BO30, and BO50, respectively). When the temperature is increased from 30 and 160 °C
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observed. As butanol is highly volatile and likely to absorb heat and evaporate, the heat of reaction
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(∆H) during this phase transition is 592 kJ/kg. A two-stage weight change is observed in BO10
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(Figure 4b), with the first and primary stage being a rapid decrease of 92.15%, while the second
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stage, occurs when the temperature is increased to 450 °C and beyond, being 3.92%. This
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phenomenon is attributed to the heavy fraction in the bio-oil, which contains many macromolecular
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compounds, and thus, has a high boiling point, requiring substantial heat to break up the compounds.
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The heat absorbed during the evaporation is relatively lower at 565 kJ/kg, as compared with that in
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sample BO0. The second-stage weight loss occurrs between 450 and 650 °C and is caused by
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combustion of the residues that formed from the reactions between the macromolecules in the bio-oil;
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the ∆H is −735 kJ/kg, indicating an exothermic process. As the concentration of the bio-oil rise to
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30% and then 50% (Figures 4c & 4d), the weight loss occurs more slowly; the heat transfers are
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gradually decreased in the endothermic reaction, but are increased in the exothermic reaction. The
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reduction of butanol in the mixture makes it more difficult to evaporate, and the residue content is
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increased with the heavy fraction. Overall, the results indicate that multicomponent fuels exhibit
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distinct reaction mechanisms during different stages of combustion, and that the addition of bio-oil to
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a petroleum fuel alters its thermal decomposition behavior. During the first-stage weight change, the
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added bio-oil enables the fuel to evaporate and release combustible fuel gas, which is primarily
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composed of non-saturated and saturated aliphatic acids from the bio-oil. However, a further increase
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in temperature induces the aromatics in the bio-oil to polymerize, and when the temperature reached
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450 °C, residues are from the burned-up polymerized aromatics. Thus, the residues can be
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considered a precursor of soot.
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3.3 DSC results for the bio-oil/butanol mixtures
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The Coats-Redfern integral method [34] is used to further investigate the thermal decomposition kinetics of the mixed fuel through the following equation: 9
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] =
−
(1)
where α denotes the time duration of the fuel’s biomass conversion, A represents the pre-exponential
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factor (min−1), β is the heating rate (K/min), R denotes the gas constant (8.314 kJ mol−1K−1), T
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pertains to the reaction temperature (K), and E represents the activation energy (J mol−1). Determined
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from Figure 5, the temperature at which BO0, BO10, BO30, and BO50 lost weight at their highest
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rate was 117, 116, 105, and 97 °C, respectively, indicating that the maximum weight loss rate
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temperature gradually decreased as more bio-oil is added, and the weight loss rate also gradually
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decreases. BO0 consists solely of butanol, which is prone to evaporation, and thus its temperature at
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maximum weight loss rate is close to the boiling point of butanol. Using 50% bio-oil in the sample
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changed its thermal decomposition properties, with the evaporation of the light fraction decreasing
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the temperature at maximum weight loss rate; however, such heavy fraction formed compounds that
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are difficult to break up, thus decreasing the weight loss rate and resulting in an increase in residue.
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Overall, as the concentration of bio-oil increased, the activation energy of the mixed fuel in the
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evaporation stage gradually decreases, as does the residue’s activation energy in the burning stage,
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which both reflect a tendency toward increased volatility. The activation energy of BO50 is
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determined to be 40.50 kJ/mol at 30–140 °C and 5.45 kJ/mol at 400–480 °C; the thermal
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decomposition kinetic parameters of the various bio-oil/butanol mixtures are listed in Table 3.
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Judging from these results, adding bio-oil to butanol slightly lowers its activation energy, which has
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little effect on its evaporation; however, as temperature and bio-oil concentration are increased, the
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fuel ceases to evaporate completely and the aromatics in the bio-oil began to polymerize, leaving a
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high amount of residue for combustion.
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3.4 Photographic of bio-oil/butanol droplet combustion
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Figure 6 illustrates the combustion of the bio-oil/butanol droplets, and its composition listed in
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Table 3. The diameters of the droplets are between 0.92 and 0.97 mm. As the igniter approached the
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droplet, the droplet suddenly burst into flames because of the intense heat, and its diameter gradually 10
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equal amount of time to ignite; however, the droplet’s shape become irregular at 0.98 s/mm2, which
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clearly indicates the combustion of the residue. Increasing the bio-oil concentration further lengthens
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the time needed for the droplet to ignite, as well as the duration of its combustion. Puffing occurs to
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the BO30 droplet as early as 1.15 s/mm2. When the concentration of bio-oil is 50%, the time required
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for the droplet to ignite is extended, and the droplet expands dramatically. Residue in BO50 appeared
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by 2.02 s/mm2, after which the combustion continued. The duration of residue combustion is
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increased when the concentration of bio-oil is increased. The addition of bio-oil thus raised the
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boiling point of the droplet, lengthens its combustion duration, and induces the formation of residue,
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as the bio-oil makes the droplet less prone to evaporation and decomposition. Comparison of the
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droplet combustion images with the mixtures’ GC-MS, TGA, and DSC mixture results reveal that,
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heat during the early stages of combustion will evaporate the non-saturated and saturated aliphatic
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acids in the bio-oil in much the same manner as those in the butanol, and the resultant vapor, when
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mixed with the oxidants in the environment, will burn. As the bio-oil also contains some water,
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combustion results in micro-explosions and puffing in the droplets. With an increase in bio-oil, the
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polymerization of aromatics in the bio-oil will also increas the amount of residue, which in turn,
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leads to noticeable residue combustion.
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3.5 Change in droplet diameters of the various mixtures
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The images in Figure 6 reveal that in some circumstances, the droplet does not shrink with
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combustion time, and in at least one instance, it expands noticeably. Figure 7 records the changes in
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the diameter of the various droplets from first ignition at 723 K to their gradual shrinkage, which is
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due to evaporation caused by combustion. The BO0, BO10, BO30, and BO50 droplets all begin with
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an initial diameter of 0.94 ± 0.03 mm. The combustion process occurs in four distinct stages: the
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heating stage (I), steady burning stage (II), swelling stage (III), and residue burning stage (IV).
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According to Figure 7a, the stage I of BO0 is fairly short, which can be attributed to the high
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approached (50 µs), after which the droplet burned steadily until it was consumed. While stage I
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(heating) of BO10 is longer than for BO0, but stage II is nearly the same length, and a substantial
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stage III (expansion and puffing) occurs from 1.22 s/mm2 onward, caused by the heavy fraction in
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the bio-oil, which changed the boiling point of the butanol (Figure 7b). The different compositions
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and volatilities of the droplet are also supported by the data in Figure 6, where the droplet is
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observed to leave a residue after burning. Stage I for the BO50 droplet is notably longer than those of
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the other droplets, with ignition only occurring after heated for 1400 µs. Furthermore, as the puffing
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and expansion in stage III become more intense, and the amount of residue increased. The diameter
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of the residue is also increased when higher concentrations of bio-oil are used. Therefore, as the bio-
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oil concentration increases, stages I, II, and IV are gradually lengthened, while stage III is gradually
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shortened. The relatively low volatility of bio-oil also increases the droplet’s duration of combustion,
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due to its extended evaporation time, and the extensive expansion of the droplet occurs as soon as the
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boiling point of the bio-oil is reached.
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These results suggest that the water, volatiles, non-saturated and saturated aliphatic acids, and
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aromatics in the bio-oil all had a different effect on the combustion of the droplet, as does the
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concentration of bio-oil in the mixture. The droplets evaporate and burn mostly because of the non-
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saturated and saturated aliphatic acids and aromatics in the bio-oil. Micro-explosions are mostly
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caused by moisture, however, some of the aromatics that polymerize during evaporation lead to
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residue formation and subsequent combustion. Aside from the effect on droplet composition,
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different concentrations of bio-oil will change the internal and external heat transfer characteristics
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of the droplet, which is also a crucial factor in droplet combustion.
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3.6 Internal convection of droplets with varying bio-oil concentrations
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Figure 8 displays the combustion of a droplet observed through the 4× lens of a high-speed
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camera. The internal heat transfer mechanism, altered by the mixture’s differences in volatility, can 12
ACCEPTED MANUSCRIPT plainly be discerned from the images. The droplet diameter is slightly decreased upon heating as the
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light fraction of the droplet (i.e., butanol) evaporated first. Stage I is extended when the butanol
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percentage in the droplet is decreased. The droplet is ignited after 1.42 s, and burns steadily due to
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the high volatility of butanol. By the time the droplet is heated beyond boiling point of butanol, the
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boiling inside the droplet had caused some constituents in the bio-oil to reach their superheating
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limits and vaporize, as indicated by the bloated droplet at 1.55 s. In this particular image, the droplet
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is significantly deformed by the increasing vapor within it, which is prevented from escaping by the
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droplet’s high viscosity. As the droplet contains a large amount of macromolecular compounds that
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have a high boiling point and are difficult to decompose, continued heating eventually causes the
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liquid phase cracking of these compounds, which results in the formation of residue and marks the
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beginning of stage IV, as suggested in a previous study [21]. These images clearly suggest that the
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forming of bubbles within the droplet, as well as the expansion, rotating convection, and bursting of
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the droplet, affects the droplet’s combustion mechanism and changes its combustion efficiency.
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These phenomena require further practical and theoretical investigation in order to understand of the
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mass/heat transfer in the combustion of a droplet.
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3.7 Burning rate of the droplet with varying bio-oil concentration The burning rate of a droplet is prone to be affected by the volatility of its constituents, and the
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combustion characteristics of a droplet are closely associated with its constituent compounds. The
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burning rates of the droplets with varying concentration of bio-oil are plotted in Figure 9. The
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equation for the burning rate is as follows:
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d2 = d02 − Kt
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where d, d0, t, and K are the droplet diameter (mm), initial droplet diameter (mm), time (s), and
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burning rate (mm2/s), respectively. Therefore, the error shown in Figure 9 is the propably existent
314
slight error in each image, and as the K value is calculated from Eq.2, these error will not influence
315
calculation, thus, they are not drawn in Figure 9. Given the same ignition temperature of 723 K, the
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ACCEPTED MANUSCRIPT steady burning stage (II), swelling stage affected by bio-oil constituents (III), and residue burning
317
stage (IV) are investigated with the following results: Pure butanol has the highest burning rate (1.03
318
mm2/s), and the rate is decreased as the concentration of bio-oil increased. For a concentration of
319
50%, the burning rate is at its lowest point of 0.713 mm2/s. The combustion of droplets with mid-to-
320
high bio-oil concentration leads to extensive droplet swelling, as caused by the trapping of
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macromolecular vapor inside the droplet due to the droplet’s high viscosity. The last stage of
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combustion is the combustion of the residue, where the burning rate exhibits a trend inverse to that of
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the droplet; specifically, the residue burning rate is increased when the bio-oil concentration is
324
increased. This can be explained by the smaller gaseous molecules created by the macromolecules
325
decomposed during the residue combustion, which in turn will release large amounts of volatiles to
326
increase the burning rate. These results suggest that the complex composition of the bio-oil affected
327
the processes of evaporation and combustion. When mixed with butanol, the non-saturated and
328
saturated aliphatic acids burned with the butanol, and the heat released during this burning heated the
329
droplet. However, the moisture and aromatics in the droplet, however, caused micro-explosions,
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which leads to the formation of surface residue; the former increased the burning rate, whereas the
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latter decreased it and produced soot. Consequently, while the burning rate of the droplet is
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decreased with the increase in bio-oil concentration, the residue burning rate is increased.
333
Conclusions
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To understand the combustion properties of bio-oil, this study used slow pyrolysis to produce
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pinewood bio-oil, and then, investigated its composition and characteristics through ultimate and
336
proximate analysis and GC-MS. Subsequently, TGA and DSC were used to further investigate the
337
effect of varied environmental temperatures. Additionally, droplet combustion test was performed to
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determine the combustion and soot formation processes of butanol/bio-oil mixtures in varying
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proportions. The following conclusions were drawn from the results.
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ACCEPTED MANUSCRIPT 1. Compared with bio-oil acquired through fast pyrolysis, the pinewood bio-oil acquired through
341
slow pyrolysis had low moisture content, but high heat value. However, the high viscosity of the
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bio-oil made it unsuitable to be used alone, thus, it should be mixed with petroleum fuel. GC-MS
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results indicated that this type of bio-oil is rich in non-saturated aliphatic and saturated aliphatic
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acids and contains relatively few aromatics, which means that it is less likely to produce soot after
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combustion.
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2. TGA revealed a two-stage weight change in fuel mixed with the bio-oil. The first stage occurred
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at 30–160 °C and corresponded to the endothermic evaporation of the light fraction of the
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compound. The second stage occurred at 450–650 °C, indicating the pyrolytic exothermic
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reaction of residue combustion fed by the macromolecular residue of the interactions between the
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constituents of the bio-oil.
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3. The activation energy of the mixed fuel gradually decreased in the evaporation stage. Likewise,
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the activation energy of the residue, during residue combustion, was also gradually decreased.
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Both exhibited a tendency toward high volatility. BO50 had relatively lower activation energy of
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40.50 kJ/mol at 30–140 °C, as compared with the other mixtures, and its activation energy was
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5.45 kJ/mol at 400–480 °C.
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4. An increase in bio-oil concentration extended the combustion duration of the droplet. When 10%
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of bio-oil was added (BO10), irregularity in droplet shape occurred at 0.98 s/mm2, accompanied
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by the formation of considerable residue. When a concentration of 50% was used (BO50), the
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droplet expanded dramatically during combustion, and residue was formed at 2.02 s/mm2, which
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was subsequently combusted until consumed.
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5. The droplet burning rate decreased when the bio-oil concentration was increased. Contrarily, the residue burning rate increased with an increase in bio-oil concentration.
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Acknowledgments
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The authors thank the Ministry of Science and Technology, Taiwan, for financially supporting
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this research under MOST 106-3113-E-008 -004, MOST 105-2221-E-150 -061 and MOST 106-
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2923-E-006 -003 -MY3.
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characteristics of a palm methyl ester droplet at high ambient temperatures, Fuel 2015;143:
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Figure Captions Figure 1. Biomass pyrolysis fixed bed reactor.
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Figure 2. Droplet combustion equipment and the High speed camera image capture system.
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Figure 3.GC-MS chromatogram of the pyrolysis oil from pinewood.
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Figure 4. TGA and DSC results for various bio-oil/butanol mixtures: (a) BO0; (b) BO10; (c) BO30;
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and (d) BO50.
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Figure 5. Effect of bio-oil concentrations on DSC results.
458
Figure 6. Direct image of single droplet with varying bio-oil concentrations.
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Figure 7. Droplet diameters with varying bio-oil concentrations.
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Figure 8. Direct color image of single droplet combustion with internal convection effect.
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Figure 9. Droplet burning rates with varying concentrations of bio-oil.
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Table Captions
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Table 1. Bio-oil properties.
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Table 2. List of GC-MS area percentages of compounds in pyrolysis oil.
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Table 3. Thermal decomposition kinetic parameters of the various bio-oil/butanol mixtures.
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84
ASTM 7544[35]
ASTM D5291
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Flash point, C
Test method Brookfield(USA), LVT, UL-adopter Micro-compter pH-Vision Datalogger 6091 Karl-Fischer Moisture Titrator MKS-500
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Pinewood 7600 3 8.1 52.1 6.6 0.5 17.12
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Feedstock Viscosity, cP pH Moisture, wt.% Carbon, wt.% Hydrogen, wt.% Nitrogen, wt.% LHV, MJ/kg
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ACCEPTED MANUSCRIPT A -1 (min ) 1.58E+07 ─ 3.87E+05 1.86E+07 1.48E+04 1.79E+07 1.55E+02 1.22E+07
R
2
0.981 ─ 0.99 0.999 0.989 0.992 0.989 0.991
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E (KJ/mol) 76.02 ─ 61.06 8.69 53.78 7.13 40.5 5.45
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Heating rate：30(℃/min) Temperature range (℃) 30～137 BO0 ─ 30 138 BO10 450 480 30 140 BO30 310 335 30 140 BO50 400 480
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Table
List of GC-MS area percentages of compounds in pyrolysis oil. (min)
Component
Area, %
MF
1 2 3
9.7 10.6 11.2
2-Methoxyphenol 2-Methoxy-4-methylphenol 4-Ethylguaiacol
0.81% 1.93% 1.49%
C7H8O2
4 5 6 7 8 9
11.8 12.1 12.4 12.7 13.6 14.2
2,6-Dimethoxyphenol Vanillic aldehyde Isoeugenol
1.16% 2.96% 1.84% 4.46% 1.10% 1.44%
C8H10O3 C8H8O3 C10H12O2
13.29% 19.05% 3.00% 22.25% 13.47%
C16H32O2 C18H34O2 C18H36O2 C20H28O2 C21H28O3
C8H10O2 C9H12O2
C9H10O3 C9H10O4 C10H10O3
285 266 263-265
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10 15.3 Palmitic acid 11 16.1 Petroselinic acid 12 16.2 Stearic acid 13 17.6 Dehydroabietic acid 14 18.2 7-Oxodehydroabietic acid *MF: Molecular Formula; Tb: Boiling point temperature
205 221-222 234-236 261
175 351 361 -
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4 -Hydroxy-3 -methoxyacetophenone 4-Hydroxy-3-methoxyphenylacetic acid 4-Hydroxy-3-methoxycinnamaldehyde
Tb,℃
90-05-1 93-51-6 2785-89-9 91-10-1 121-33-5 5932-68-3
C7H8O2
498-02-02
C9H10O3
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Peak no.
ݐோ
C8H10O2
C9H12O2 C8H10O3 C8H8O3 C10H12O2
306-08-1 458-36-6 57-10-3 593-39-5 57-11-4
C9H10O4
1740-19-8
C20H28O2
C10H10O3 C16H32O2 C18H34O2 C18H36O2
110936-78-2 C21H28O3
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gas outlet
condenser
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thermal insulation material
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water in
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heating wire
water out
thermocouple
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Bio-oil
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after pyrolysis
Bio-oil
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Ceramic fiber
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Droplet
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Droplet generator Mixing chamber
Bio-oil
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2.0x10
13 10 11
+06
1.0x10
Pine Bio-oil
14
+00
0.0x10
1
0
5
2 34
10
12 8 9
15
Time(min)
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Abundance
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25
30
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100 0
40
-100
wight change~99.06%
20
residues wight~0.64%
-200
150
300
450
600
750
BO30-TG BO30-DSC
80 wight change~86.11%
60 40 residues wight ~2.84% wight change~%
150
Temperature(°C)
100 0
wight change~92.18%
20
wight change~
-100
residues wight ~3.83% -200
%
-300
0 150
300
450
600
750
-400
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200
100 80
Weight (%)
Weight (%)
60
300
450
600
750
(d)
Heating rate: 30°C/min
BO50-TG BO50-DSC
wight change~81.64%
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Heat flow (mW/mg)
(b)
Heating rate: 30°C/min
40
300
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400
80
100
-200 -300 -400
Temperature(°C)
120 100
200
-100
0
-400
300
0
20
-300
0
(c)
Heating rate: 30°C/min
60
300 200 100 0
40
-100 residues wight ~2.24%
20
wight change~%
-200 -300
0
150
400
Heat flow (mW/mg)
Weight (%)
60
200
100
Heat flow (mW/mg)
BO0-TG BO0-DSC
80
300
400
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Heating rate: 30°C/min
Weight (%)
100
Heat flow (mW/mg)
120
300
450
600
750
-400
Temperature(°C)
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100
Heating rate: 30°C/min
BO0 BO10 BO30 BO50
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60 40 20 0 150
300
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BO0 (D0~0.93mm)
droplet
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droplet combustion
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0.00s/mm2 0.14s/mm2 0.28s/mm2 0.42s/mm2 0.66s/mm2 0.70s/mm2 0.84s/mm2 0.98s/mm2 1.12s/mm2 1.26s/mm2
BO10 droplet combustion
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residue combustion
0.00s/mm2 0.14s/mm2 0.28s/mm2 0.42s/mm2 0.66s/mm2 0.70s/mm2 0.84s/mm2 0.98s/mm2 1.12s/mm2 1.26s/mm2
BO30 droplet combustion
puffing
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residue combustion
BO50 droplet combustion
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0.00s/mm2 0.28s/mm2 0.43s/mm2 0.72s/mm2 1.01s/mm2 1.15s/mm2 1.30s/mm2 1.59s/mm2 1.73s/mm2 1.83s/mm2
expansion
residue combustion
0.00s/mm2 0.31s/mm2 0.63s/mm2 1.57s/mm2 1.61s/mm2 1.66s/mm2 2.02s/mm2 2.12s/mm2 2.21s/mm2 2.36s/mm2
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(a)BO0 (D0~0.93mm)
1.5
II
1
I : heating zone II : steady burning zone III : swelling zone IV : residue burning zone
0.5
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(d/d0)
2
I
Tignition=723K
0
I
1 0.5 0
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(c)BO30 (D0~0.95mm)
2
I
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Tignition=723K
IV
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1.5
0
(d)BO50 (D0~0.97mm)
Tignition=723K II
I
III
IV
(d/d0)
2
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0.5 0
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2.4
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II：steady burning
I：heating
III：swelling
droplet
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BO30, (D0~0.95mm) puffing
1.55s
1.73s
1.46s
1.42s
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residue increase 2.24s
2.34s
2.42s
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Tignition=723K
KII,III KIV
1.25 -->liquid phase to vapor phase -->heavy fraction increase -->K BO0>KBO10>KBO30>KBO50
1
2
1.5
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0.75
1
0.5
-->solid phase to vapor phase -->volatiles increase -->K BO50 > KBO30 > KBO10
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droplet burning zone
0.25
0.5
residue burning zone
0
10
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Highlights In droplet combustion, the blended bio-oil shows puffing and micro-explosion.
The components of bio-oil influence the different states of droplet combustion.
Multicomponent droplets result in different phases of combustion.
Low blending ratio bio-oil can increase combustion efficiency.
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S0360-5442(17)31977-1
DOI:
10.1016/j.energy.2017.11.119
Reference:
EGY 11907
To appear in:
Energy
Received Date: 13 July 2017 Revised Date:
24 October 2017
Accepted Date: 20 November 2017
Please cite this article as: Yang SI, Wu MS, The droplet combustion and thermal characteristics of pinewood bio-oil from slow pyrolysis, Energy (2017), doi: 10.1016/j.energy.2017.11.119. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1 2 3
The Droplet Combustion and Thermal Characteristics of Pinewood bio-oil from Slow Pyrolysis
4
Prof. Shou Yin Yang Department of Power Mechanical Engineering National Formosa University Yunlin 63202, Taiwan Tel: 886-5-631-5432 Fax: 886-5-631-2110 E-mail: [email protected]
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* Corresponding author:
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Department of Power Mechanical Engineering National Formosa University Yunlin County, 63202 Taiwan
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S. I. Yang*, M. S. Wu
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1. Introduction Biomass fuels can be used to produce energy through thermal, biological, or physical processes,
22
all of which rely on technical ingenuity, and the applications of such alternative energy sources to
23
conducting demonstrations or establishing commercial facilities have attracted the attention of
24
researchers. Combustion, gasification, and pyrolysis are the three prevailing types of thermochemical
25
process used to convert biomass into profitable energy. Of these three, gasification is recognized as
26
the relatively more attractive option, particularly when compared with combustion, as it is highly
27
efficient and clean. By contrast, fast pyrolysis has been long been, and can result in products with
28
different characteristics when using varied reactors or source materials [1]. As bio-oils produced
29
through fast pyrolysis tend to have considerable affect on power equipment (e.g. pH value, viscosity,
30
and moisture of bio-fuel), various studies have been conducted to determine how their undesirable
31
effect can be decreased; the most notable solutions proposed include physical, catalytic, and
32
chemical upgrading [2,3]. Although bio-oils generally have very different properties from petroleum
33
fuels, most commercially available bio-fuels are fairly similar to them, and thus, are easily applied to
34
existing power equipment. As existent burner designs are sensitive to the quality of bio-oil used,
35
which can make burners prone to ignition, flame detection, and flame stabilization problems, an
36
adequate bio-oil burner design, and its corresponding bio-oil composition, have become the focus of
37
numerous studies [4-10]. Therefore, many studies have used different biomass material sources to
38
produce oil by pyrolysis, and discussed the characteristics of bio-oil produced through different
39
processes. For example, Abdelrahman et al. [6,7] used apricot and fish waste to produce bio-oil. Hu
40
et al. [8] used blue-green algae blooms. Biradar et al. [9] used Jatropha Curcas de-oiled seed cake for
41
testing. Chen and Lin [10] used oil palm fiber for oil-producing testing, hoping to understand the
42
correlation between different oil-producing material sources and bio-oil combustion characteristics.
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In addition, biomass butyl alcohol is a novel biofuel. As a substitute fuel source, butyl alcohol
44
has higher energy and volatility than methanol and ethanol. As butyl alcohol may be used in engines, 2
ACCEPTED MANUSCRIPT some studies have discussed the application of butyl alcohol to engines [11,12]. Some studies
46
showed that the butyl alcohol diesel fuel blend, as used in diesel engines, could improve the
47
emissions of soot and CO without a strong impact on engine performance [13-15]. Mixtures of bio-
48
oil and petroleum fuels have become a prevalent approach for mitigating the negative effects of bio-
49
oil, and their mixture and combustion properties can be improved by either improving the quality of
50
the bio-oil used in the mixture or using a suitable petroleum fuel. Butanol has a short carbon chain
51
structure (with four carbon atoms) that enables the emulsification of certain substances when added
52
to bio-oil, which improves the mixing of substances by preventing stratification, reduces a mixture’s
53
viscosity and improves its thermal mass, thus, butanol is often used as an additive in bio-
54
oil/petroleum fuel mixtures. Kim et al. [16] used 10%, 20%, and 30% bio-oil in butanol as a fuel for
55
a diesel engine, and verified that the presence of butanol made the fuel more likely to be auto-ignited,
56
effectively suppressed bio-oil polymerization, and reduced gummy polymer generation. The
57
combustion properties of a fuel can also be changed when the mixture is altered, which is due to the
58
entirely different chemicals and physical properties of the mixture, such as its composition, volatility,
59
boiling point, latent heat of evaporation, and specific heat capacity [17-19]. These considerations
60
have prompted a large number of studies to explore the combustion properties of multicomponent
61
droplets with varied droplet composition. For example, Kitano et al. [20] used numerical simulations
62
to investigate the evaporation of single and multicomponent fuel droplets under various ambient
63
pressures, gas temperatures, and combustion reactions. Liu et al [21] compared the isolated droplet
64
burning characteristics of butanol isomers using detailed numerical modeling and demonstrated that
65
the transport coefficients of isomer-specific species altered the fuel’s combustion properties. Hoxie
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et al. [22] investigated the combustion properties of droplets of soybean oil mixed with butanol, and
67
determined that the two-component droplets, whose components had different volatilities, induced
68
flame shrinkage due to diffusion-limited gasification. When butanol constituted 40% of the droplet,
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intense micro-explosions were induced, which indicated that the droplet had reached its superheat
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ACCEPTED MANUSCRIPT limit. Ma et al. [23] investigated the evaporation properties of the mixture of acetone–butanol–
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ethanol (ABE) and diesel in a high-temperature environment, and revealed that the ABE–diesel
72
mixture expanded substantially at 823 K, which was caused by the rapid expansion and shrinkage of
73
droplets. These rapid changes were due to the large amount of heat in the droplets, which caused the
74
rapid formation and disintegration of air bubbles within them. The components of the ABE–diesel
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mixture had varied degrees of volatility, which could cause micro-explosions and change the
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mixture’s combustion properties.
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To this end, our previous study [24] used fast pyrolysis to decompose various biomass residues
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and investigated the bio-oil properties under varied operating parameters, which was followed by a
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diesel engine test using a diesel fuel diluted with various concentrations of bio-oil [25]. The results
80
indicated that the addition of bio-oil both enhanced the diesel’s combustion efficiency and reduced
81
pollutant emissions; however, as the bio-oil concentration and operating time were increased, the
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efficiency of the diesel engine decreased, and slagging was produced in its cylinders. Therefore, bio-
83
oil in varying concentrations was subsequently added to kerosene for spray combustion testing
84
through actual measurement [26] and numerical simulation [27]. The results indicated that the
85
presence of bio-oil not only changed the properties of the spray, it also shortened the length of the
86
flame produced, which is mainly caused by the premature ignition of the volatile substances in the
87
bio-oil. To verify the added bio-oil’s effect on spray combustion, this study conducts combustion
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testing on the multicomponent droplets of a bio-oil/kerosene mixture [28] and identifies numerous
89
phenomena, including micro-explosions, puffing, and slagging, which could be attributed to the
90
composition of the bio-oil induced combustion behavior and speed, which are different from those of
91
single-component droplets. With the objective of producing bio-oil of high quality and quantity using
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pyrolysis, this study further investigates the effect of fluidization velocity on the yield [24] and
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quality [29] of the produced bio-oil by adopting various fluidization velocities in a fluidized bed
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reactor. Emulsification technology, which enables the adequate mixing of bio-oil and petroleum fuels,
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is essential for this purpose [30,31]. From the aforementioned studies, the spray combustion properties of bio-oil are clearly affected
97
by the varied pyrolysis processes that the biomass is subjected to, as well as the varied petroleum
98
fuels that the bio-oil is mixed with. If the difference between the different bio-oil/petroleum fuel
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mixtures used for spray combustion are to be investigated, the magnitude of combustion is a critical
100
factor that substantially affects the spray combustion achieved. To alleviate problems concerning
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bio-oil combustion, this study mixed butanol with bio-oil produced through the slow pyrolysis of
102
pinewood. The burning rate and thermochemical properties of the resulting mixtures are determined,
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and the properties of bio-oil created through the pyrolysis of biomass are elucidated, as well as the
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combustion properties of the mixture’s multicomponent droplets.
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2. Approaches
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This study used slow pyrolysis to produce bio-oil from pinewood. Thermo-gravimetric analysis
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(TGA) and a droplet combustion test platform are then used to investigate the thermochemical
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characteristics of the bio-oil/butanol mixture. 2.1 Pyrolysis system
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A biomass pyrolysis reactor is used to produce bio-oil from pinewood. The reactor (as
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illustrated in Figure 1), has an inner volume of 0.125 m3 (0.5 × 0.5 × 0.5 m3), which is wrapped
112
outwardly by a layer of thermal insulation material, heated by a set of high power heating wires, and
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temperature is controlled using a thermocouple. The pinewood is shredded into small pieces less than
114
2 mm in size and then dried at 80 °C for 2 h to reduce their moisture content. Next, 1000 g of the
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dried pinewood is placed in the reactor, which is preheated to 300 °C. Subsequently, CO2 preheated
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to 300 °C is introduced into the reactor as a carrier gas at a flow rate of 8–12 cm3/s for 1 h, during
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which pyrolysis occurred. Afterwards, the vaporized pinewood oil, as produced through high-
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temperature pyrolysis is guided though a heat exchanger for condensation and collection (Figure 1).
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2.2 Droplet combustion equipment Figure 2 illustrates the test platform used in the droplet combustion tests, which consists
121
primarily of a droplet generator, a combustion chamber, an igniter, and an image capturing system.
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First, a digital delay generator sends a pulsating direct current to activate the piezoelectric chips, and
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their deformation would (in a technique similar to ink-jet printing) squeeze the fuel in the tank into
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the droplet generator. The fuel mixture is ejected from the glass nozzle in the form of a droplet
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approximately 0.9 mm in diameter, which is then caught and fixed in place by crossed (90°) low-
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thermal-conductive ceramic fibers in the combustion chamber. A glow plug igniter (operated at 12 V,
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7 A, and 84 W) is subsequently extended beneath the droplet for 0.3–0.8 ± 0.3 s, and then quickly
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withdrawn to ignite the droplet. The resultant combustion was then captured using a Phantom v7.3
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high-speed camera using backlighting method. The high-speed camera operated at 1000 fps, has a
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resolution of 384 × 384 pixels, spatial resolution of 0.022 mm/pixel, and exposure time of 1 µs. A
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Phantom M310 color high-speed camera with a 4× optical lens was also used to capture magnified
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images at 2000 fps, with a resolution of 512 × 512 pixels and exposure time of 100 µs. Use such
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shooting conditions, in the image of each droplet, the droplet diameter has ±0.044 mm/pixel error.
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3. Results and Discussions
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This study produces bio-oil through the pyrolysis of pinewood, and measured the characteristics
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of bio-oil using various methods, such as ultimate and proximate analyses. The bio-oil is mixed with
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butanol, and TGA, differential scanning calorimetry (DSC), and droplet combustion tests are
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performed to determine the bio-oils properties and explore the relationship between the evaporation
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and combustion behavior of bio-oil droplet.
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3.1 Fuel Preparation
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is added to bio-oil, the properties of which are listed in Table 1. Specifically, while the bio-oil had
143
relatively low moisture content (8.1 wt.%), it has high heat value (LHV, 17.12 MJ/kg) and viscosity
144
(7600 cP). The bio-oil had a pH of approximately 3, indicating that it would corrode mechanical
145
components and cause malfunctions if used alone. Certain properties of butanol nullify the need for
146
an emulsifier when mixed with the bio-oil, as together they increased pH of a bio-oil/butanol mixture,
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as compared with pure bio-oil, which indicates that a bio-oil/butanol mixture would be suitable for
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fueling power equipment.
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Gas chromatography–mass spectrometry (GC-MS) is used to analyze the organic compounds in
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the pinewood bio-oil, as produced at a pyrolysis temperature of 500 °C and interface temperature of
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300 °C. The GC-MS system (Hewlett-Packard HP 6890 GC equipped with an HP 5973 mass
152
selective detector) is directly connected to a Double-shot PY-2020D (Frontier Lab) pyrolyzer. The
153
resultant GC-MS chromatogram is presented in Figure 3; however, because of the lack of a standard
154
mixture for the calibration of the MS detector, this study could not conduct in-depth investigation
155
into the large number of unknown and complicated organic compounds observed in the figure. Table
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2 presents a tentative list of compounds in the pinewood bio-oil, as well as the areas under their
157
peaks; the compounds are all index compounds in the MS search file. Large amounts of aromatic and
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oxygenated compounds are identified in the pinewood bio-oil, including carboxylic acids, phenols,
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and ketones, as previously determined for other bio-oil types [24]. However, contrary to the results
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of another study [24] that investigated the oily phases of cedar sawdust, coffee bean residue, and rice
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straw, the pyrolysis of the pinewood bio-oil in this study reveals that the primary components are
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able to completely burned off were petroselinic acid (a nonsaturated aliphatic acid) and palmitic and
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stearic acids (saturated aliphatic acids), with a combined under-peak area of 35.34%. In addition, the
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remainder of the constituents were aromatics that easily produce soot or residues during combustion
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(including isoengenol, vanillic aldehyde, 2,6-dimethoxyphenol, 4-ethylguaiacol, 2-methoxy-4-
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ACCEPTED MANUSCRIPT methylphenol, 2-methoxyphenol, dehydroabietic acid, and 7-oxodehydroabietic acid) [32].
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Furthermore, the pinewood bio-oil consists of phenols with oxygen functional groups, for example
168
2,6-Dimethoxyphenol, which is an aromatic compound that absorbs water in an alkaline solution.
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Finally, this study discovers that bio-oil feature notable organic acids contents, such as 4-hydroxy-3-
170
methoxyphenylacetic acid, palmitic acid, petroselinic acid, stearic acid, dehydroabietic acid, and 7-
171
Oxodehydroabietic acid. The presence of acidic compounds makes the bio-oil hydrophilic and easily
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mixed with alkaline alcohols, which can neutralize the acids in the bio-oil, thus, decreasing the
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amount of corrosion it causes. As is evident from Table 2, the compounds in the bio-oil are all
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macromolecules, which are not prone to evaporation or decomposition when heated, due to their
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relatively high boiling points (205–362 °C). The presence of these macromolecular compounds in
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bio-oil/butanol droplets will also affect the droplets’ properties by retaining volatile matter in the
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mixture, thus, lowering the droplets’ activation energy. The C18 fatty acid in the bio-oil would
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polymerize under intense heat [33] and cause residue production.
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The aforementioned results all indicated that the bio-oil produced through slow pyrolysis of
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pinewood relatively complicated composition and, compared with ordinary petroleum fuels, contain
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a large amount of oxygen-containing reactive functional groups, rendering it unstable.
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3.2 Thermogravimetric Analysis for the bio-oil/butanol fuel
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This study mixes bio-oil and butanol in various concentrations and performed thermal analysis
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on the thermal decomposition behavior of the mixtures through TGA and DSC. As the combustion of
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these mixtures would be affected by their properties, such as volatility and boiling point, which in
186
turn depended on the concentrations of both constituents, a PerkinElmer STA8000 thermal analyzer
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is used to investigate the thermal behavior of the mixtures at high temperatures. The results are
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presented in TGA-DSC curves in Figure 4, with 12 mg of fuel sample, 80 mL/min air flow, and 30
189
°C/min heating rate used. Four mixture of bio-oil are employed: 0%, 10%, 30%, and 50% (labelled
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BO0, BO10, BO30, and BO50, respectively). When the temperature is increased from 30 and 160 °C
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ACCEPTED MANUSCRIPT (Figure 4a), the weight of sample BO0 is decreased by 99.6%, and a notable endothermic reaction is
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observed. As butanol is highly volatile and likely to absorb heat and evaporate, the heat of reaction
193
(∆H) during this phase transition is 592 kJ/kg. A two-stage weight change is observed in BO10
194
(Figure 4b), with the first and primary stage being a rapid decrease of 92.15%, while the second
195
stage, occurs when the temperature is increased to 450 °C and beyond, being 3.92%. This
196
phenomenon is attributed to the heavy fraction in the bio-oil, which contains many macromolecular
197
compounds, and thus, has a high boiling point, requiring substantial heat to break up the compounds.
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The heat absorbed during the evaporation is relatively lower at 565 kJ/kg, as compared with that in
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sample BO0. The second-stage weight loss occurrs between 450 and 650 °C and is caused by
200
combustion of the residues that formed from the reactions between the macromolecules in the bio-oil;
201
the ∆H is −735 kJ/kg, indicating an exothermic process. As the concentration of the bio-oil rise to
202
30% and then 50% (Figures 4c & 4d), the weight loss occurs more slowly; the heat transfers are
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gradually decreased in the endothermic reaction, but are increased in the exothermic reaction. The
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reduction of butanol in the mixture makes it more difficult to evaporate, and the residue content is
205
increased with the heavy fraction. Overall, the results indicate that multicomponent fuels exhibit
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distinct reaction mechanisms during different stages of combustion, and that the addition of bio-oil to
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a petroleum fuel alters its thermal decomposition behavior. During the first-stage weight change, the
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added bio-oil enables the fuel to evaporate and release combustible fuel gas, which is primarily
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composed of non-saturated and saturated aliphatic acids from the bio-oil. However, a further increase
210
in temperature induces the aromatics in the bio-oil to polymerize, and when the temperature reached
211
450 °C, residues are from the burned-up polymerized aromatics. Thus, the residues can be
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considered a precursor of soot.
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3.3 DSC results for the bio-oil/butanol mixtures
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The Coats-Redfern integral method [34] is used to further investigate the thermal decomposition kinetics of the mixed fuel through the following equation: 9
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] =
−
(1)
where α denotes the time duration of the fuel’s biomass conversion, A represents the pre-exponential
218
factor (min−1), β is the heating rate (K/min), R denotes the gas constant (8.314 kJ mol−1K−1), T
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pertains to the reaction temperature (K), and E represents the activation energy (J mol−1). Determined
220
from Figure 5, the temperature at which BO0, BO10, BO30, and BO50 lost weight at their highest
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rate was 117, 116, 105, and 97 °C, respectively, indicating that the maximum weight loss rate
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temperature gradually decreased as more bio-oil is added, and the weight loss rate also gradually
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decreases. BO0 consists solely of butanol, which is prone to evaporation, and thus its temperature at
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maximum weight loss rate is close to the boiling point of butanol. Using 50% bio-oil in the sample
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changed its thermal decomposition properties, with the evaporation of the light fraction decreasing
226
the temperature at maximum weight loss rate; however, such heavy fraction formed compounds that
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are difficult to break up, thus decreasing the weight loss rate and resulting in an increase in residue.
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Overall, as the concentration of bio-oil increased, the activation energy of the mixed fuel in the
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evaporation stage gradually decreases, as does the residue’s activation energy in the burning stage,
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which both reflect a tendency toward increased volatility. The activation energy of BO50 is
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determined to be 40.50 kJ/mol at 30–140 °C and 5.45 kJ/mol at 400–480 °C; the thermal
232
decomposition kinetic parameters of the various bio-oil/butanol mixtures are listed in Table 3.
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Judging from these results, adding bio-oil to butanol slightly lowers its activation energy, which has
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little effect on its evaporation; however, as temperature and bio-oil concentration are increased, the
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fuel ceases to evaporate completely and the aromatics in the bio-oil began to polymerize, leaving a
236
high amount of residue for combustion.
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3.4 Photographic of bio-oil/butanol droplet combustion
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Figure 6 illustrates the combustion of the bio-oil/butanol droplets, and its composition listed in
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Table 3. The diameters of the droplets are between 0.92 and 0.97 mm. As the igniter approached the
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droplet, the droplet suddenly burst into flames because of the intense heat, and its diameter gradually 10
ACCEPTED MANUSCRIPT decreased as it burned. BO0 exhibits steady combustion. Compared with BO0, BO10 took almost an
242
equal amount of time to ignite; however, the droplet’s shape become irregular at 0.98 s/mm2, which
243
clearly indicates the combustion of the residue. Increasing the bio-oil concentration further lengthens
244
the time needed for the droplet to ignite, as well as the duration of its combustion. Puffing occurs to
245
the BO30 droplet as early as 1.15 s/mm2. When the concentration of bio-oil is 50%, the time required
246
for the droplet to ignite is extended, and the droplet expands dramatically. Residue in BO50 appeared
247
by 2.02 s/mm2, after which the combustion continued. The duration of residue combustion is
248
increased when the concentration of bio-oil is increased. The addition of bio-oil thus raised the
249
boiling point of the droplet, lengthens its combustion duration, and induces the formation of residue,
250
as the bio-oil makes the droplet less prone to evaporation and decomposition. Comparison of the
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droplet combustion images with the mixtures’ GC-MS, TGA, and DSC mixture results reveal that,
252
heat during the early stages of combustion will evaporate the non-saturated and saturated aliphatic
253
acids in the bio-oil in much the same manner as those in the butanol, and the resultant vapor, when
254
mixed with the oxidants in the environment, will burn. As the bio-oil also contains some water,
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combustion results in micro-explosions and puffing in the droplets. With an increase in bio-oil, the
256
polymerization of aromatics in the bio-oil will also increas the amount of residue, which in turn,
257
leads to noticeable residue combustion.
258
3.5 Change in droplet diameters of the various mixtures
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The images in Figure 6 reveal that in some circumstances, the droplet does not shrink with
260
combustion time, and in at least one instance, it expands noticeably. Figure 7 records the changes in
261
the diameter of the various droplets from first ignition at 723 K to their gradual shrinkage, which is
262
due to evaporation caused by combustion. The BO0, BO10, BO30, and BO50 droplets all begin with
263
an initial diameter of 0.94 ± 0.03 mm. The combustion process occurs in four distinct stages: the
264
heating stage (I), steady burning stage (II), swelling stage (III), and residue burning stage (IV).
265
According to Figure 7a, the stage I of BO0 is fairly short, which can be attributed to the high
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ACCEPTED MANUSCRIPT volatility and low boiling point of butanol; the droplet must have been ignited as soon as the igniter
267
approached (50 µs), after which the droplet burned steadily until it was consumed. While stage I
268
(heating) of BO10 is longer than for BO0, but stage II is nearly the same length, and a substantial
269
stage III (expansion and puffing) occurs from 1.22 s/mm2 onward, caused by the heavy fraction in
270
the bio-oil, which changed the boiling point of the butanol (Figure 7b). The different compositions
271
and volatilities of the droplet are also supported by the data in Figure 6, where the droplet is
272
observed to leave a residue after burning. Stage I for the BO50 droplet is notably longer than those of
273
the other droplets, with ignition only occurring after heated for 1400 µs. Furthermore, as the puffing
274
and expansion in stage III become more intense, and the amount of residue increased. The diameter
275
of the residue is also increased when higher concentrations of bio-oil are used. Therefore, as the bio-
276
oil concentration increases, stages I, II, and IV are gradually lengthened, while stage III is gradually
277
shortened. The relatively low volatility of bio-oil also increases the droplet’s duration of combustion,
278
due to its extended evaporation time, and the extensive expansion of the droplet occurs as soon as the
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boiling point of the bio-oil is reached.
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These results suggest that the water, volatiles, non-saturated and saturated aliphatic acids, and
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aromatics in the bio-oil all had a different effect on the combustion of the droplet, as does the
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concentration of bio-oil in the mixture. The droplets evaporate and burn mostly because of the non-
283
saturated and saturated aliphatic acids and aromatics in the bio-oil. Micro-explosions are mostly
284
caused by moisture, however, some of the aromatics that polymerize during evaporation lead to
285
residue formation and subsequent combustion. Aside from the effect on droplet composition,
286
different concentrations of bio-oil will change the internal and external heat transfer characteristics
287
of the droplet, which is also a crucial factor in droplet combustion.
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3.6 Internal convection of droplets with varying bio-oil concentrations
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Figure 8 displays the combustion of a droplet observed through the 4× lens of a high-speed
290
camera. The internal heat transfer mechanism, altered by the mixture’s differences in volatility, can 12
ACCEPTED MANUSCRIPT plainly be discerned from the images. The droplet diameter is slightly decreased upon heating as the
292
light fraction of the droplet (i.e., butanol) evaporated first. Stage I is extended when the butanol
293
percentage in the droplet is decreased. The droplet is ignited after 1.42 s, and burns steadily due to
294
the high volatility of butanol. By the time the droplet is heated beyond boiling point of butanol, the
295
boiling inside the droplet had caused some constituents in the bio-oil to reach their superheating
296
limits and vaporize, as indicated by the bloated droplet at 1.55 s. In this particular image, the droplet
297
is significantly deformed by the increasing vapor within it, which is prevented from escaping by the
298
droplet’s high viscosity. As the droplet contains a large amount of macromolecular compounds that
299
have a high boiling point and are difficult to decompose, continued heating eventually causes the
300
liquid phase cracking of these compounds, which results in the formation of residue and marks the
301
beginning of stage IV, as suggested in a previous study [21]. These images clearly suggest that the
302
forming of bubbles within the droplet, as well as the expansion, rotating convection, and bursting of
303
the droplet, affects the droplet’s combustion mechanism and changes its combustion efficiency.
304
These phenomena require further practical and theoretical investigation in order to understand of the
305
mass/heat transfer in the combustion of a droplet.
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3.7 Burning rate of the droplet with varying bio-oil concentration The burning rate of a droplet is prone to be affected by the volatility of its constituents, and the
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combustion characteristics of a droplet are closely associated with its constituent compounds. The
309
burning rates of the droplets with varying concentration of bio-oil are plotted in Figure 9. The
310
equation for the burning rate is as follows:
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d2 = d02 − Kt
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where d, d0, t, and K are the droplet diameter (mm), initial droplet diameter (mm), time (s), and
313
burning rate (mm2/s), respectively. Therefore, the error shown in Figure 9 is the propably existent
314
slight error in each image, and as the K value is calculated from Eq.2, these error will not influence
315
calculation, thus, they are not drawn in Figure 9. Given the same ignition temperature of 723 K, the
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ACCEPTED MANUSCRIPT steady burning stage (II), swelling stage affected by bio-oil constituents (III), and residue burning
317
stage (IV) are investigated with the following results: Pure butanol has the highest burning rate (1.03
318
mm2/s), and the rate is decreased as the concentration of bio-oil increased. For a concentration of
319
50%, the burning rate is at its lowest point of 0.713 mm2/s. The combustion of droplets with mid-to-
320
high bio-oil concentration leads to extensive droplet swelling, as caused by the trapping of
321
macromolecular vapor inside the droplet due to the droplet’s high viscosity. The last stage of
322
combustion is the combustion of the residue, where the burning rate exhibits a trend inverse to that of
323
the droplet; specifically, the residue burning rate is increased when the bio-oil concentration is
324
increased. This can be explained by the smaller gaseous molecules created by the macromolecules
325
decomposed during the residue combustion, which in turn will release large amounts of volatiles to
326
increase the burning rate. These results suggest that the complex composition of the bio-oil affected
327
the processes of evaporation and combustion. When mixed with butanol, the non-saturated and
328
saturated aliphatic acids burned with the butanol, and the heat released during this burning heated the
329
droplet. However, the moisture and aromatics in the droplet, however, caused micro-explosions,
330
which leads to the formation of surface residue; the former increased the burning rate, whereas the
331
latter decreased it and produced soot. Consequently, while the burning rate of the droplet is
332
decreased with the increase in bio-oil concentration, the residue burning rate is increased.
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Conclusions
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To understand the combustion properties of bio-oil, this study used slow pyrolysis to produce
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pinewood bio-oil, and then, investigated its composition and characteristics through ultimate and
336
proximate analysis and GC-MS. Subsequently, TGA and DSC were used to further investigate the
337
effect of varied environmental temperatures. Additionally, droplet combustion test was performed to
338
determine the combustion and soot formation processes of butanol/bio-oil mixtures in varying
339
proportions. The following conclusions were drawn from the results.
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ACCEPTED MANUSCRIPT 1. Compared with bio-oil acquired through fast pyrolysis, the pinewood bio-oil acquired through
341
slow pyrolysis had low moisture content, but high heat value. However, the high viscosity of the
342
bio-oil made it unsuitable to be used alone, thus, it should be mixed with petroleum fuel. GC-MS
343
results indicated that this type of bio-oil is rich in non-saturated aliphatic and saturated aliphatic
344
acids and contains relatively few aromatics, which means that it is less likely to produce soot after
345
combustion.
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2. TGA revealed a two-stage weight change in fuel mixed with the bio-oil. The first stage occurred
347
at 30–160 °C and corresponded to the endothermic evaporation of the light fraction of the
348
compound. The second stage occurred at 450–650 °C, indicating the pyrolytic exothermic
349
reaction of residue combustion fed by the macromolecular residue of the interactions between the
350
constituents of the bio-oil.
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3. The activation energy of the mixed fuel gradually decreased in the evaporation stage. Likewise,
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the activation energy of the residue, during residue combustion, was also gradually decreased.
353
Both exhibited a tendency toward high volatility. BO50 had relatively lower activation energy of
354
40.50 kJ/mol at 30–140 °C, as compared with the other mixtures, and its activation energy was
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5.45 kJ/mol at 400–480 °C.
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4. An increase in bio-oil concentration extended the combustion duration of the droplet. When 10%
357
of bio-oil was added (BO10), irregularity in droplet shape occurred at 0.98 s/mm2, accompanied
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by the formation of considerable residue. When a concentration of 50% was used (BO50), the
359
droplet expanded dramatically during combustion, and residue was formed at 2.02 s/mm2, which
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was subsequently combusted until consumed.
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5. The droplet burning rate decreased when the bio-oil concentration was increased. Contrarily, the residue burning rate increased with an increase in bio-oil concentration.
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Acknowledgments
364
The authors thank the Ministry of Science and Technology, Taiwan, for financially supporting
365
this research under MOST 106-3113-E-008 -004, MOST 105-2221-E-150 -061 and MOST 106-
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2923-E-006 -003 -MY3.
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thermochemical biomass conversion 2001: 977-997.
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Figure Captions Figure 1. Biomass pyrolysis fixed bed reactor.
453
Figure 2. Droplet combustion equipment and the High speed camera image capture system.
454
Figure 3.GC-MS chromatogram of the pyrolysis oil from pinewood.
455
Figure 4. TGA and DSC results for various bio-oil/butanol mixtures: (a) BO0; (b) BO10; (c) BO30;
456
and (d) BO50.
457
Figure 5. Effect of bio-oil concentrations on DSC results.
458
Figure 6. Direct image of single droplet with varying bio-oil concentrations.
459
Figure 7. Droplet diameters with varying bio-oil concentrations.
460
Figure 8. Direct color image of single droplet combustion with internal convection effect.
461
Figure 9. Droplet burning rates with varying concentrations of bio-oil.
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Table Captions
466
Table 1. Bio-oil properties.
467
Table 2. List of GC-MS area percentages of compounds in pyrolysis oil.
468
Table 3. Thermal decomposition kinetic parameters of the various bio-oil/butanol mixtures.
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21
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ASTM 7544[35]
ASTM D5291
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Flash point, C
Test method Brookfield(USA), LVT, UL-adopter Micro-compter pH-Vision Datalogger 6091 Karl-Fischer Moisture Titrator MKS-500
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Pinewood 7600 3 8.1 52.1 6.6 0.5 17.12
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Feedstock Viscosity, cP pH Moisture, wt.% Carbon, wt.% Hydrogen, wt.% Nitrogen, wt.% LHV, MJ/kg
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ACCEPTED MANUSCRIPT A -1 (min ) 1.58E+07 ─ 3.87E+05 1.86E+07 1.48E+04 1.79E+07 1.55E+02 1.22E+07
R
2
0.981 ─ 0.99 0.999 0.989 0.992 0.989 0.991
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E (KJ/mol) 76.02 ─ 61.06 8.69 53.78 7.13 40.5 5.45
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Heating rate：30(℃/min) Temperature range (℃) 30～137 BO0 ─ 30 138 BO10 450 480 30 140 BO30 310 335 30 140 BO50 400 480
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Table
List of GC-MS area percentages of compounds in pyrolysis oil. (min)
Component
Area, %
MF
1 2 3
9.7 10.6 11.2
2-Methoxyphenol 2-Methoxy-4-methylphenol 4-Ethylguaiacol
0.81% 1.93% 1.49%
C7H8O2
4 5 6 7 8 9
11.8 12.1 12.4 12.7 13.6 14.2
2,6-Dimethoxyphenol Vanillic aldehyde Isoeugenol
1.16% 2.96% 1.84% 4.46% 1.10% 1.44%
C8H10O3 C8H8O3 C10H12O2
13.29% 19.05% 3.00% 22.25% 13.47%
C16H32O2 C18H34O2 C18H36O2 C20H28O2 C21H28O3
C8H10O2 C9H12O2
C9H10O3 C9H10O4 C10H10O3
285 266 263-265
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10 15.3 Palmitic acid 11 16.1 Petroselinic acid 12 16.2 Stearic acid 13 17.6 Dehydroabietic acid 14 18.2 7-Oxodehydroabietic acid *MF: Molecular Formula; Tb: Boiling point temperature
205 221-222 234-236 261
175 351 361 -
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4 -Hydroxy-3 -methoxyacetophenone 4-Hydroxy-3-methoxyphenylacetic acid 4-Hydroxy-3-methoxycinnamaldehyde
Tb,℃
90-05-1 93-51-6 2785-89-9 91-10-1 121-33-5 5932-68-3
C7H8O2
498-02-02
C9H10O3
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ݐோ
C8H10O2
C9H12O2 C8H10O3 C8H8O3 C10H12O2
306-08-1 458-36-6 57-10-3 593-39-5 57-11-4
C9H10O4
1740-19-8
C20H28O2
C10H10O3 C16H32O2 C18H34O2 C18H36O2
110936-78-2 C21H28O3
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gas outlet
condenser
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thermal insulation material
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water in
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heating wire
water out
thermocouple
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after dried
after pyrolysis
Bio-oil
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Ceramic fiber
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Droplet
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Butanol
Droplet generator Mixing chamber
Bio-oil
ACCEPTED MANUSCRIPT Solvent
2.0x10
13 10 11
+06
1.0x10
Pine Bio-oil
14
+00
0.0x10
1
0
5
2 34
10
12 8 9
15
Time(min)
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30
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100 0
40
-100
wight change~99.06%
20
residues wight~0.64%
-200
150
300
450
600
750
BO30-TG BO30-DSC
80 wight change~86.11%
60 40 residues wight ~2.84% wight change~%
150
Temperature(°C)
100 0
wight change~92.18%
20
wight change~
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residues wight ~3.83% -200
%
-300
0 150
300
450
600
750
-400
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Weight (%)
Weight (%)
60
300
450
600
750
(d)
Heating rate: 30°C/min
BO50-TG BO50-DSC
wight change~81.64%
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Heat flow (mW/mg)
(b)
Heating rate: 30°C/min
40
300
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400
80
100
-200 -300 -400
Temperature(°C)
120 100
200
-100
0
-400
300
0
20
-300
0
(c)
Heating rate: 30°C/min
60
300 200 100 0
40
-100 residues wight ~2.24%
20
wight change~%
-200 -300
0
150
400
Heat flow (mW/mg)
Weight (%)
60
200
100
Heat flow (mW/mg)
BO0-TG BO0-DSC
80
300
400
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Weight (%)
100
Heat flow (mW/mg)
120
300
450
600
750
-400
Temperature(°C)
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100
Heating rate: 30°C/min
BO0 BO10 BO30 BO50
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60 40 20 0 150
300
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BO0 (D0~0.93mm)
droplet
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droplet combustion
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0.00s/mm2 0.14s/mm2 0.28s/mm2 0.42s/mm2 0.66s/mm2 0.70s/mm2 0.84s/mm2 0.98s/mm2 1.12s/mm2 1.26s/mm2
BO10 droplet combustion
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residue combustion
0.00s/mm2 0.14s/mm2 0.28s/mm2 0.42s/mm2 0.66s/mm2 0.70s/mm2 0.84s/mm2 0.98s/mm2 1.12s/mm2 1.26s/mm2
BO30 droplet combustion
puffing
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residue combustion
BO50 droplet combustion
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0.00s/mm2 0.28s/mm2 0.43s/mm2 0.72s/mm2 1.01s/mm2 1.15s/mm2 1.30s/mm2 1.59s/mm2 1.73s/mm2 1.83s/mm2
expansion
residue combustion
0.00s/mm2 0.31s/mm2 0.63s/mm2 1.57s/mm2 1.61s/mm2 1.66s/mm2 2.02s/mm2 2.12s/mm2 2.21s/mm2 2.36s/mm2
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(a)BO0 (D0~0.93mm)
1.5
II
1
I : heating zone II : steady burning zone III : swelling zone IV : residue burning zone
0.5
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Tignition=723K
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1 0.5 0
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(c)BO30 (D0~0.95mm)
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(d)BO50 (D0~0.97mm)
Tignition=723K II
I
III
IV
(d/d0)
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0.5 0
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t/d0 (s/mm )
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II：steady burning
I：heating
III：swelling
droplet
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1.55s
1.73s
1.46s
1.42s
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2.34s
2.42s
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Tignition=723K
KII,III KIV
1.25 -->liquid phase to vapor phase -->heavy fraction increase -->K BO0>KBO10>KBO30>KBO50
1
2
1.5
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droplet burning zone
0.25
0.5
residue burning zone
0
10
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Highlights In droplet combustion, the blended bio-oil shows puffing and micro-explosion.
The components of bio-oil influence the different states of droplet combustion.
Multicomponent droplets result in different phases of combustion.
Low blending ratio bio-oil can increase combustion efficiency.
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