Article pubs.acs.org/EF
Characteristics of Particles in Pyrolysis Oil Roger Molinder,* Linda Sandström, and Henrik Wiinikka SP Energy Technology Center, Box 726, SE-941 28 Piteå, Sweden S Supporting Information *
ABSTRACT: Particles filtered out of pyrolysis oil produced through fast pyrolysis of stem wood, willow, reed canary grass, bark, and forest residue were characterized using scanning electron microscopy and energy-dispersive spectroscopy with the aim of identifying particle categories and discussing transport mechanisms of particles and inorganics into the oil. Particles filtered out of both the condensed and the aerosol fractions of the oil displayed three types of morphology: (i) char-like structures (1−15 μm), (ii) spheres (100 nm to 1 μm), and (iii) irregularly shaped residue (50−500 nm). The char-like structures were identified as char. The spheres and irregularly shaped residue shared morphology and composition with tar balls and organic particles with inorganic inclusions. These particles could have formed either during the fast pyrolysis stage or through precipitation from the oil during storage. All particles consisted mainly of C and O but also small amounts of inorganics. The particles from the aerosol fraction of the oil had higher inorganics content than the particles from the condensed fraction. The results were discussed, and suggested transport mechanisms of inorganics into particles were presented.
1. INTRODUCTION The European parliament and the Council of the European Union (EU) has through directive 2009/28/EC set a target for the EU member states of a 20% share in renewable energy in total energy consumption1 and has since then proposed a new target of at least 27% by 2030.2 Directive 2009/28/EC also sets a target of a 10% share of renewable energy in all forms of transport by 2020. However, a midterm assessment of the progress of the EU and its member states toward this target revealed that the progress has been slow, partly due to a lack of commercial availability of alternative second-generation biofuels.3 The assessment concluded that a breakthrough in advanced biofuels is key to achieving the target. Bio-based raw material can be converted to pyrolysis oil (also referred to as bio-oil) which has a higher energy density than the original raw material as well as a wider field of application. Pyrolysis oil can be produced from a wide variety of low grade bio-based materials4,5 and can be used as a substitute for heating oil and diesel in boilers, engines, and gas turbines for the production of heat and power6 and has the potential to be upgraded to a drop-in transportation fuel.7 Pyrolysis oil could, therefore, be used to increase the share of renewable energy in both total energy consumption and transport. Pyrolysis oil is mainly produced through fast pyrolysis which, in short, involves heating the raw material at a high heating rate, thus producing pyrolysis gas, vapors, and char. The char is separated from the gas and vapors using cyclones and sometimes also hot filters before the gas and vapors are cooled to produce oil as they condensate out of the gaseous phase into a liquid phase. In some processes, a second separation step is used to produce oil from aerosols. There are several commercial and semicommercial technologies available for pyrolysis oil production, and a review of combustion tests on those oils carried out by Lehto et al. have shown that the combustion technologies work well.8 However, incombustible solids present in the oil can cause problems with fouling of heat transfer surfaces in boilers and erosion in nozzles, valves, and © XXXX American Chemical Society
pumps. Lehto et al., therefore, recommended reductions in the solids content of the oil to 1 μm spheres in the condensed fraction were bigger than previously reported spherical soot particles and tar balls. Trubetskaya et al. have, however, observed similar spheres (produced during fast pyrolysis of wheat straw at 1500 °C) that were larger than 1 μm,25 but they contained for the most part O, Ca, and Si (in descending order) as well as smaller amounts of Al, Fe, and Mg. This indicated that the spheres observed by Trubetskaya et al.25 consisted of metal- and silicon oxides, which is inconsistent with the spheres presented here, which were composed mainly of C and O (ses section 3.2.1). The irregularly shaped residue consisted of particles that were much smaller than the char and the majority of the spheres (Figures 2 and 3). They were 50−500 nm in diameter (Figure 5) and displayed the same size range in both the
particle size between the condensed fraction and the aerosol fraction, but the particles observed using light microscopy were much larger than the ones observed using electron microscopy. This is attributed to agglomeration of particles into larger aggregates during storage, which has been reported previously.12 Such aggregates would not be visible during SEM analysis due to the sonication step carried out as part of the sample preparation which was used to break up aggregates. Char-like structures were also found in Cyclone 2 (Figure 4). Most of them were 10−15 μm in diameter, while some were as
Figure 4. SEM images of char-like structures recovered from Cyclone 2 after fast pyrolysis of stem wood.
big as 20−25 μm in diameter. This indicates that Cyclone 2 efficiently removed all particles larger than ≈20 μm from the gas, while a portion of the particles that were smaller than ≈20 μm passed through it. Note that the particles shown in Figure 4 were produced during fast pyrolysis of stem wood and were considered representative of all particles collected in Cyclone 2 during fast pyrolysis of all fuels investigated here. Previous work on the same pilot-scale pyrolyzer has shown that the size range of particles collected from the storage tank underneath the ablative cyclone after fast pyrolysis of stem wood was 40−1000 μm with the bulk of the particles being in the 250−500 μm size range.23 This means that the ablative cyclone, Cyclone 2, and the water cooled condenser removed progressively smaller particles from the gas phase. The spheres stood out from the other particles because of their regular shapes and smooth surfaces (Figures 2 and 3). They resembled tar balls collected from aged smoke of a smoldering fire.29 Tar balls are spherical and amorphous particles containing mostly C with smaller amounts of O and trace amounts of K, S, Cl, and Si.29 They are likely formed when low-volatility organic gases such as lignin pyrolysis products convert into aerosol particles to form hydrophilic compounds which polymerize in water droplets.30 As they grow with further polymerization, they become more hydrophobic, and when the water droplet evaporates, a spherical tar ball remains. The spheres also resembled spherical soot particles formed by fast pyrolysis of beech wood and by gasification of fir and spruce wood.28,31 However, the temperatures used in those experiments were much higher (1130−1400 °C) than the temperature used in this work (wall temperature 750 °C), so no conclusions can be drawn from this comparison other than the similarity in morphology. Note that the actual pyrolysis temperature was lower than the wall temperature. The spheres varied considerably in size from 100 to 200 nm to more than 1 μm. In the condensed fraction, spheres in this entire size range were observed, but in the aerosol fraction, the largest spheres only reached ≈0.5 μm. The size range of the soot particles formed by fast pyrolysis of beech wood and by gasification of fir and spruce wood28,31 was a few nm to a few
Figure 5. SEM image of irregularly shaped residue filtered out of oil produced through fast pyrolysis of forest residue.
condensed fraction and the aerosol fraction. They resembled organic particles with inorganic inclusions found in smoke plumes from biomass fires.29 They also resembled irregularly shaped soot particles formed during fast pyrolysis of pine wood at 1400 °C26 and during gasification of pine and spruce wood at 1260−1460 °C.33 However, as with the similarity between the spheres observed here and previously reported spherical soot particles, the temperatures used in those fast pyrolysis and gasification experiments were significantly higher than the temperature used in this work. So again, no conclusions can be drawn from this comparison other than the similarity in morphology. The vast majority of the residue was spread across the surfaces of char-like particles. Smaller particles spread across larger char particles produced during fast pyrolysis of sweet sorghum bagasse have been reported previously.21 However, those particles were found to comprise mainly K and Cl, and it was concluded that they had formed through crystallization. The smaller spheres and the irregularly shaped residue could be expected to have similar aerodynamics as the carrier gas and could, therefore, be expected to have passed through the heat exchanger without collecting in the condensed fraction, as discussed above. Therefore, the smaller spheres and irregularly shaped residue in the condensed fraction had likely been transported there by being attached to char-like particles. D
DOI: 10.1021/acs.energyfuels.6b01726 Energy Fuels XXXX, XXX, XXX−XXX
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reactions.35 Spherical and irregularly shaped particles containing oxygen and inorganics could form this way if inorganic salts present in the oil are removed from the liquid phase to the particles during precipitation. 3.2.2. Inorganics. The initial inorganics content of the fuels used to produce the oils varied significantly from 0.1 wt % for the stem wood to 1.2 wt % for the reed canary grass,4 while the char-like structures contained 1.2−5.5 wt %, the spheres 1.0− 6.6. wt %, and the irregularly shaped residue 0.5−3.6 wt % inorganics (Table 1). Note that the inorganics contents for the
3.2. Composition. 3.2.1. Major Components. The major components of all particles were C and O, but they also contained small amounts of inorganics (Figure 6). There were
Table 1. Inorganics Content of Char-like Structures, Spheres, and Irregularly Shaped Residue from the Condensed Fraction and the Aerosol Fraction of the Oils (unit: wt %) condensed fraction stem wood willow reed canary grass bark forest residue
Figure 6. Elemental composition of char-like structures filtered out of the condensed fraction of oil produced during fast pyrolysis of stem wood, willow, reed canary grass (RCG), bark, and forest residue.
no significant differences in major composition between particles of different morphology. In other words, char-like structures, spheres, and residue all contained, for the most part, C and O with smaller amounts of inorganics. Note that the EDS analyses carried out here suffer from analytical uncertainties due to, e.g., matrix effects, meaning that the uncertainties in the quantification can be significant. Therefore, this section will focus on identification of major components and their relative quantities as opposed to absolute amounts. Only the elemental compositions of char-like structures filtered out of the condensed fraction are shown in Figure 6 for brevity as their compositions are representative of all particles. Particles from the condensed fraction consisted of 75−88 wt % C, 10− 21 wt % O, and 1−7 wt % inorganics, while particles from the aerosol fraction consisted of 68−91 wt % C, 8−27 wt % O, and 0.5−6 wt % inorganics. This is in agreement with previous work with the same pilot-scale pyrolyzer where it was reported that the solid phase of the oil consisted mostly of combustible material.23 Previously reported C and O compositions of particles produced during fast pyrolysis vary between 40−78 wt % and 12−68 wt % for C and O, respectively.17,19−22 The C and O compositions in the fuels used to produce the oils were 49.5−53.5 wt % and 37.7−42.0 wt %, respectively. The elemental composition of the char-like structures is in agreement with previously reported elemental compositions of char,17,19−22 which is further evidence that they are in fact char. The elemental compositions of the spheres and the irregularly shaped residue are in agreement with previously reported elemental compositions of tar balls and organic particles with inorganic inclusions.29,30 The amounts of O and inorganics in the spheres and the irregularly shaped residue are much higher than classical soot particles which consist of pure graphite.29,34 The spheres and irregularly shaped residue could, therefore, be tar balls and organic particles with inorganic inclusions that formed during the fast pyrolysis process inside the pilot-scale pyrolyzer. However, it is also possible that the particles formed from the oil during storage. Both formation of particles35 and reduction of inorganic content12 during storage of pyrolysis oil have previously been observed. It has been suggested that particles formed by precipitation following condensation
aerosol fraction
char
spheres
residue
char
spheres
residue
2.2 1.9 4.3 1.2 3.6
2.1 1.4 6.6 1.0 1.0
2.8 3.3 2.9 1.4 1.1
2.6 5.5 1.1 3.1 5.2
2.5 2.1
3.4 2.5 0.5 1.5 3.6
1.8 1.9
fuels cited above are the sums of the contents of Si, Al, Ca, Fe, K, Mg, Mn, Na, and P in the fuels listed in ref 4. The inorganics contents for the char, the spheres, and the irregularly shaped residue are the sums of Cu, Na, Ca, Si, K, Fe, Cr, Ni, and Mg identified and quantified using EDS. The Quantax 70 EDS system was used on the particles filtered out of oil produced from stem wood while the 80 mm2 X-Max Silicon Drift Detector was used on all the other particles. No data was collected from a RCG sphere from the aerosol fraction. Note that not all of the elements listed above were found in all of the particles. For example, only four of these elements (Cu, Na, Ca, Si) were identified in the stem wood spheres from the condensed fraction, whereas the reed canary grass spheres contained eight of them (all except Mg). The variations in inorganics content of the particles can be partly explained by the variations in the inorganics content in the raw material used to produce the oils. Such variations can be found in previously published work. For example, particles collected during fast pyrolysis of pine and sweet sorghum bagasse contained 1.820 and 6.3 wt %21 inorganics, respectively. There are also challenges related to data collection, and examples of this can also be found in previously published work. Jendoubi et al., for example, reported big uncertainties in the reproducibility of inorganic values for particles collected in cyclones during pyrolysis oil production21 and attributed them to heterogeneous sampling issues. Leijenhorst et al.14 concluded after a literature review on the topic of inorganics composition in oil that relative uncertainty in the data is quite high and attributed this to the heterogeneous nature of biomass combined with the low absolute concentrations of inorganics. Because of this and the analytical uncertainties in the EDS analyses, this section will focus on general trends in the data on inorganics composition as opposed to absolute values. With the exception of the particles filtered out of oil produced from reed canary grass and the irregularly shaped residue from oil produced from willow the particles from the aerosol fraction had higher inorganics content than the particles from the condensed fraction (Table 1). Previous work has shown that the majority of inorganics in pyrolysis oil are E
DOI: 10.1021/acs.energyfuels.6b01726 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels associated with particles.12−14 Jendoubi et al. have also shown that >60 wt % of inorganics and 41−54 wt % of particles in total oil are in the aerosol fraction.13 Jendoubi et al. discussed several possible pathways that could explain the higher particle and inorganics content in aerosols. Char particles could for example serve as condensation nuclei during formation of aerosols in the condensation step and inorganics in the form of salts could dissolve in aerosols formed in the pyrolyzer and then be transported through the condenser. However, while this helps explain the higher inorganics and particle content of the aerosol fraction it does not explain the higher inorganics content of the particles in the aerosol fraction compared with the particles in the condensed fraction. Differences in water content between the two oil fractions could however explain this. The same pyrolysis oils from which particles have been filtered out for this work have been characterized in previously published work.4 The characterization showed that the condensed oil fractions had a higher water content (26−37 and 6−7 wt % for condensed and aerosol fractions respectively). Water stabilizes inorganic salts and it is, therefore, suggested that a higher water content results in a lower amount of inorganic salts being removed from the liquid phase to particles during precipitation. As discussed above, the condensed fraction has previously been found to contain larger visible particles (≈100−200 μm), whereas no visible particles were observed in the aerosol fraction.23 This implies that particles in the condensed fraction have a higher tendency for agglomeration than particles in the aerosol fraction. The differences in inorganics content could explain the differences in the tendency to agglomerate. Wigley et al.36 pretreated pine wood through leaching to reduce the amount of inorganics before producing pyrolysis oil by fast pyrolysis with both pretreated and untreated wood. They reported that the particles produced during fast pyrolysis of the pretreated wood (containing lower amounts of inorganics) had a higher tendency to agglomerate. This indicates that the higher agglomeration tendency of particles in the condensed fraction compared to the particles in the aerosol fraction was due to their lower inorganics content. In the literature on pyrolysis oil production, all particles formed in the process are commonly referred to as char. The definition of char has, for example, been (i) all solid material left in cyclones, on reactor walls, and in bed material13 and (ii) carbonaceous solid residue left after the pyrolysis (including the ash).12 The results presented here show that particles in pyrolysis oil consist not only of char but also of spheres and irregularly shaped residue as well, which could either be tar balls and organic particles with inorganic inclusions formed during pyrolysis or be the result of precipitation during storage.
also possible that they precipitated out from the oil during storage. The particles from the aerosol fraction had higher inorganics content than the particles from the condensed fraction. It was suggested that the higher water content in the condensed fraction resulted in less inorganic salts being removed from the liquid phase to particles during precipitation.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01726. Details on how the rodlike structures in the SEM images were identified as fibers from the filter papers (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +46 (0)705 532 387. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was made possible by funding from the Swedish Energy Agency, Bio4Energy, Sveaskog, Smurfit Kappa, Framtidens bioraffinaderi and the County Board of Norrbotten. Calle Ylipäa,̈ Mathias Lundgren, Daniel Svensson, AnnChristine Johansson, and Jimmy Narvesjö are also gratefully acknowledged for their invaluable contribution to this work through the commissioning and the operating of the pilot-scale pyrolyzer.
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REFERENCES
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4. CONCLUSIONS Particles filtered out of both the condensed and the aerosol fractions of pyrolysis oils produced from stem wood, willow, reed canary grass, bark, and forest residue displayed three types of morphology: (i) char-like structures (1−15 μm), (ii) spheres (100 nm to 1 μm), and (iii) irregularly shaped residue (50−500 nm). The major components of all particles were C and O with small amounts of inorganics, which, in the case of the char-like structures, leads to the conclusion that they were char particles. The spheres and irregularly shaped residue shared both morphology and composition with tar balls and organic particles with inorganic inclusions, respectively, that would have formed during the fast pyrolysis process. However, it is F
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