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Pyrolysis of pre-treated tannins obtained from radiata pine bark
Journal of Analytical and Applied Pyrolysis 107 (2014) 250–255
Contents lists available at ScienceDirect
Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Pyrolysis of pre-treated tannins obtained from radiata pine bark Paige A. Case a , Constanza Bizama b , Cristina Segura b , M. Clayton Wheeler a,c , Alex Berg b , William J. DeSisto a,c,∗ a b c
Department of Chemical and Biological Engineering, University of Maine, USA Unidad de Desarrollo Tecnológico, Concepción, Chile Forest Bioproducts Research Institute, University of Maine, USA
a r t i c l e
i n f o
Article history: Received 13 December 2013 Accepted 25 March 2014 Available online 18 April 2014 Keywords: Pyrolysis Tannins Pre-treatment
a b s t r a c t Water soluble and insoluble fractions of tannins obtained from radiata pine bark were pyrolyzed at the bench-scale in a continuous entrained flow pyrolysis reactor at 500 ◦ C. The tannin fractions were pre-treated with both calcium hydroxide and calcium formate to assist in continuous feeding and promote deoxygenation during pyrolysis. Oil yields and qualitative GC–MS were used to characterize the oil products. Pre-treatment assisted in continuous feeding, ameliorating some issues associated with the thermoplastic properties of tannins. A comparison of the pyrolysis of tannins with and without pretreatment is presented. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Biomass and its residues are being researched extensively for use as feedstock for renewable fuels and chemicals. Of the various types of residues worldwide, tree bark is one of the most significant. Approximately one tenth of the mass of trees harvested is bark. Currently, this bark is removed before mechanical processing and is burned for process energy. Because of the unique properties of the bark, however, it may have higher-value use as a chemical precursor or as a refined fuel. Bark from softwood trees, such as Pinus radiata contains up to 60% polyphenolic compounds, of which 8–12% are water soluble and 15–20% are soluble in methanol [1], depending on the age of the tree, the time passed since harvesting, and the particle size, among many other factors. Pine bark polyphenols are condensed tannins, which consist of flavonoid units with varying degrees of condensation [2]. While the traditional use of water soluble tannins was for treatment of animal hides, they have also been examined for medicinal and agricultural applications as well as in manufacture of ink, rubber and plastics and for water treatment [3]. Currently, soluble tannins are being extracted for use as adhesives in wood based panels [4]. These adhesive resins require a very strong crosslinking of the tannin molecules with the bridging agent, formaldehyde. This reaction can only occur if both reactants, tannins and formaldehyde, are
∗ Corresponding author at: Department of Chemical and Biological Engineering, University of Maine, USA. Tel.: +1 207 581 2291. E-mail address: [email protected] (W.J. DeSisto). http://dx.doi.org/10.1016/j.jaap.2014.03.009 0165-2370/© 2014 Elsevier B.V. All rights reserved.
completely solubilized in water. If insoluble tannin fractions are included, a brittle resin will be obtained. Therefore, the insoluble fraction of the tannins remains unused. Tannins, like other biomass sources such as lignin, have the potential to replace a portion of the phenolics used in the manufacture of industrial products, such as resins, which are currently derived from petroleum products [5–7]. Thermal conversion of whole biomass and lignin fractions has been studied for this application, but implementation of the technology has been hindered by several issues. One of the most significant issues is the large number of compounds present in the pyrolysis liquid. When whole biomass is used, many other compounds, such as sugar derivatives, acids, and furans are produced and must be separated from the phenols. Another issue, especially when using lignin, is that the phenolics produced generally are highly substituted and do not have the same properties as phenol [8]. Condensed tannins have an advantage over other types of biomass because of their unique structure. Condensed tannins consist of linked catechins, making them a more uniform polymer than lignin [9]. Micropyrolysis experiments have shown that catechol, pyrogallol and resorcinol are the major thermal decomposition products from condensed tannins, with smaller amounts of methyl catechols also produced [10–12]. Pyrolysis of condensed tannins has previously been limited to micro-scale experiments, most likely because tannins have difficult handling properties. Condensed tannins have a low melting point and swell upon heating, creating challenges with continuously feeding the material into a hot pyrolysis reactor. Similar issues associated with continuous feeding are observed with pyrolysis of lignin [13,14]. One method to overcome these
P.A. Case et al. / Journal of Analytical and Applied Pyrolysis 107 (2014) 250–255 Table 1 Elemental analysis of extracted tannins.
% carbon % hydrogen % oxygen
251
2.3. Materials and feedstock preparation
Water soluble tannins
Water insoluble tannins
54.4 4.8 40.8
55.6 5.0 39.4
issues is the incorporation of calcium hydroxide or calcium formate with lignin before pyrolysis [15]. This not only improves the feeding of the lignin but also has a significant impact on the oil produced during the reaction, reducing the oxygen content and increasing the overall yield of the oil. In this work, we present results for pyrolysis of radiata pine bark derived tannins with and without calcium hydroxide and calcium formate pretreatment. Similar to our lignin work [15], this pretreatment aided in the continuous feeding of the tannins into the bench-scale pyrolysis reactor. Pyrolysis products were collected and analyzed for yield, elemental composition and the qualitative distribution of chemical compounds. A comparison of the different feed pretreatments is presented. 2. Experimental 2.1. Extraction of tannins The tannins used in these experiments were isolated from the bark of Pinus radiata using methanol extraction. The bark was obtained from 15 year old trees and dried to 12% water content, then ground in a hammer mill with 1.20 mm diameter slots. The process of extracting the tannins was performed in the Technology Development Unit of the University of Concepción, Chile in a 4 m3 pressurized pilot plant with recirculation and indirect heating. The bark was mixed in a batch reactor in a 1:5 mass/volume ratio in order to completely cover the bark with the extracting liquor, a 75 wt% methanol/water solution. The mixture is then heated to a temperature of 120 ◦ C within 90 min. After 120 min at 120 ◦ C, the solution was cooled to ambient temperature, and discharged from the extractor. The dilute tannin solution was concentrated to 20% solids, completely evaporating the methanol and separating the water soluble and the water insoluble fractions. Both tannin fractions were then dried. The procedure corresponds to the operation of the pilot plant, which essentially includes a conical extractor with a volume of 4.0 L, a circulation pump and a heat exchanger. The methanol concentration of 75 wt.% and the extracting temperature of 120 ◦ C are the optimal conditions for extracting the tannins. The heating up time, on the other hand, is specific for the pilot plant used. Further details about this procedure can be found in reference [1] (Table 1).
The water soluble and water insoluble tannin fractions were pyrolyzed in a bench-scale fluidized bed reactor. In addition to pyrolysis of pure tannins, pretreatment of tannins was performed to improve pyrolysis. Pretreatment was performed by mixing each tannin fraction with calcium hydroxide and calcium formate, respectively, in an aqueous solution. The loading of the calcium compounds was 0.43 g calcium/g tannins. Both the untreated and pre-treated tannins were dried in the oven for at least 12 h before pyrolysis to remove excess water. The six different resulting materials were ground and sieved to <2 mm. While no molecular interaction was expected between the tannins and the calcium compounds used for pretreatment, a small temperature increase was observed during the initial mixing. This may be due to the presence of dangling hydroxyl groups which may interact with the cation. Furthermore, this effect may be increased for the soluble tannins, as increased solubility may make these groups more available to the cation. 2.4. Bench-scale pyrolysis Fast pyrolysis experiments were carried out in a bubbling bed reactor that measured 3.75 cm in inner diameter × 30 cm in length, with 40–60 mesh silica beads as a heat transfer medium. The temperature in the reactor was monitored using two K-type thermocouples located on the vertical axis measured 3.0 and 9.5 in. from the top of the reactor. Runs were carried out at 500 ◦ C. The mixtures were metered through a screw feeder and fed into the reactor pneumatically, using a nitrogen flow rate of 6 L/min. Between 100 and 150 g of feed was pyrolyzed in each experiment at a feed rate of ∼1–2 g/min. Char was separated using a hot-gas filter immediately after the reactor at 500 ◦ C. After passing through the hot gas filter, liquid was collected in a condenser operated at 3 ◦ C followed by an electrostatic precipitator (ESP). A diagram of the pyrolysis system is shown in Fig. 1. More detailed information about this system can be found in literature [16]. 2.5. Characterization 13 C NMR was performed on a Varian Unity 9.4T instrument at 29 ◦ C using the method outlined by Joseph et al. [17]. Gas chromatography–mass spectrometry (GC/MS) was conducted using a Shimadzu QP2010 instrument described previously. Elemental analysis was performed on a Thermo Scientific Flash 2000 CHNS/O analyzer.
3. Results and discussion
2.2. Micropyrolysis-GC/MS
3.1. Micropyrolysis
Tannins were analyzed by Py-GC/MS over a range of temperatures between 300 ◦ C and 700 ◦ C to obtain fundamental information on their structures and to determine the optimal pyrolysis temperature. The samples were prepared for the micropyrolysis experiments by filling 1/3 of the quartz tube with quartz wool, adding 200–800 mg of sample and then adding more quartz wool on top of the sample. Prepared samples were pyrolyzed for 25 s using a CDS Analytical Pyroprobe 5200 interfaced with a Shimadzu QP2010 gas chromatograph–mass spectrometer. The GC column was an Rxi-5 ms capillary column (30 m × 0.25 mm and 0.250 m film thickness and a 5% diphenyl and 95% dimethylpolysiloxane stationary phase). The injection temperature was 250 ◦ C with a split ratio of 80:1. Peak identification was made by interpretation of mass spectral information.
Experiments were conducted using Py-GC/MS to determine the major pyrolysis products and the optimal temperature for benchscale experiments. Five temperatures were explored for both water soluble and water insoluble tannins. It was found that negligible volatilization occurred at or below 300 ◦ C. The effect of pretreatment on micropyrolysis of tannins was not explored because of issues with sample inhomogeneity at the scale required for the Py-GC/MS (Figs. 2 and 3). Major products identified during the micropyrolysis experiments include phenol, catechol and methyl catechol. Additionally, a significant amount of carbon dioxide was observed, indicating that cracking is likely taking place. The ratio of carbon dioxide to major products increases with increasing temperature, which is consistent with cracking activity.
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Fig. 1. Diagram of the bench-scale fast pyrolysis system.
The products obtained at each given temperature were similar for both the water soluble and water insoluble tannins. The soluble tannins show more selectivity toward catechol, whereas the insoluble tannins produced more methyl catechol. This is likely due to differences in the chemical structure of the starting materials.
Solid, liquid and gas product yields from bench-scale pyrolysis are given in Table 2. The liquid and solid yields were measured by
weight. Char yields were calculated by subtracting the amount of calcium added (assuming all calcium ended up in the solid product). Gas yields were calculated by difference. No bench-scale yield data was collected for untreated water soluble tannins. This is because during feeding, the tannins swelled and solidified at the inlet of the reactor, causing plugging and a rapid pressure increase. In the case of the water insoluble tannins, however, the addition of calcium hydroxide and calcium formate resulted in a decrease in overall liquid yield accompanied by an increase in both char and gas yield. The increase in char yield for
Fig. 2. Comparison of micropyrolysis-GC/MS chromatograms for water soluble tannins at various pyrolysis temperatures.
Fig. 3. Comparison of micropyrolysis-GC/MS chromatograms for water insoluble tannins at various pyrolysis temperatures.
3.2. Bench-scale results
P.A. Case et al. / Journal of Analytical and Applied Pyrolysis 107 (2014) 250–255
253
Table 2 Mass balances based on grams of reactant/product per 100 g of tannins fed. Insoluble tannins Pretreatment Ca(OH)2 added (g/100 g tannins) Ca(COOH)2 added (g/100 g tannins) Product yields Oil product (ESP) (g/100 g tannins) Aqueous product (Condenser) (g/100 g tannins) Char producta (g/100 g tannins) Gas product (g/100 g tannins) a
0 0
8.25 19.3 43.3 29.2
Ca(OH)2 treated insoluble tannins
Ca(OH)2 treated soluble tannins
Ca(COOH)2 treated insoluble tannins
Ca(COOH)2 treated soluble tannins
79 0
79 0
0 140
0 140
1.54 22.5 59.2 95.8
2.01 17.8 81.0 78.2
1.59 17.8 93.1 128
6.2 14.5 109.6 109.7
On a calcium-free basis.
Table 3 Properties of pyrolysis products.
% C in ESP oila % H in ESP oila % C in char % H in char % Ca in char % C in aqueous % H in aqueous % O in aqueous a
Insoluble tannins
Ca(OH)2 treated insoluble tannins
Ca(OH)2 treated soluble tannins
Ca(COOH)2 treated insoluble tannins
Ca(COOH)2 treated soluble tannins
72.4 8.00 70.5 0 0 21.3 8.69 70.0
90.9 10.8 25.1 0.04 41.9 32.3 12.0 55.8
82.6 8.10 22.5 0.15 34.5 29.2 11.8 59.0
86.2 8.50 26.2 0.14 31.6 23.0 8.3 68.8
80.8 7.70 32.1 0.39 28.2 33.0 12.0 55.0
Values may sum to greater than 100% due to uncertainty in measurement (±1% of value).
the formate treated tannins is partially due to the formation of calcium carbonate with the decomposition of calcium formate. In both cases, the calcium may also be catalyzing polymerization reactions that form char [18]. Since the reactor has two types of liquid collectors in series, a cold water condenser and an ESP, yields are reported for each. The ESP product is essentially water free and has high carbon content. This is considered the “oil”. The cold water condenser contains both aqueous and oil phases, and these phases are mixed in order to calculate the carbon content. As shown in Table 3, the carbon content for the condenser oil is significantly lower than ESP oil because it has high water content. The physical properties of the pyrolysis oil varied significantly. The product from pyrolysis of pure water insoluble tannins that was collected downstream of the condenser was extremely viscous and tar-like. This product collected at the entrance to the ESP. The oil from pyrolysis of the calcium treated samples, however, was less viscous and condensed after passing through the ESP. Fig. 4 provides visual comparison of the oils produced from pyrolysis of untreated and pretreated tannins. The elemental analysis (C, H, and O) of the ESP oil and the filter char for each of the pyrolysis experiments is given in Table 3. In all cases, pyrolysis produced a liquid/tar product that had lower oxygen to carbon ratio than the starting material. Pretreatment of the tannins decreased this ratio further, with calcium hydroxide producing a greater deoxygenating effect than calcium formate. The carbon is low in the char from all of the pretreated samples because of the high inorganic content, and the hydrogen is extremely low in all char samples. The calcium content in the char is calculated based on the assumption that all of the calcium introduced to the feedstock will remain in the solid product after pyrolysis. Table 4 gives the integrated areas for 13 C NMR analysis of the oils [17]. The oil from the insoluble tannins is not included because it was not soluble in any of the solvents typically used in our group for NMR analysis. From these results, it is seen that the majority of the carbon in the pretreated oils is in aromatic and alkyl hydrocarbon functionalities. Based on the feedstock composition and structure, this is an expected result. It is important to note that any aromatic species with attached hydroxyl groups also have chemical shifts
in the aromatic/alkene region of 13 C NMR, so from this technique alone, the amount of oxygen in phenol and catechol compounds is impossible to determine. Analysis of the ESP oil from each bench-scale pyrolysis experiment was performed using GC–MS (Figs. 6 and 7). This included the ESP oil from pyrolysis of untreated water soluble tannins, because although not enough sample was pyrolyzed to get a reliable mass balance, sufficient amount ESP oil was collected for GC–MS analysis. The samples were extracted using ethyl acetate as a solvent. It is important to note that while the oil collected from pyrolysis of pretreated samples was fully soluble in ethyl acetate, the tar from the pyrolysis of untreated tannins was only partially soluble. The ethyl acetate insoluble fraction was about 15% of the oil weight and was soluble in methylene chloride, but showed no response when analyzed using GC–MS. This could indicate the presence of higher molecular weight polar compounds in the oil (Fig. 5).
Fig. 4. Oil from pyrolysis of untreated water insoluble tannins (top) does not flow and has a tar-like consistency; while the oil produced from pyrolysis of calcium formate pretreated tannins (bottom) flows easily and has a significantly lower viscosity.
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Table 4 13 C NMR integrated areas. Type of carbon
Chemical shifts
Ca(OH)2 treated insoluble tannins (%)
Ca(OH)2 treated soluble tannins (%)
Ca(COOH)2 treated insoluble tannins (%)
Ca(COOH)2 treated insoluble tannins (%)
Alkyl Methoxy/hydroxy Aromatic/alkene Carbonyl
0–54 54–84 110–163 163–215
44.37 0 54.95 2.19
38.06 0 55.25 7.09
29.48 0 68.91 2.60
23.33 0 74.13 2.70
Fig. 5. Ethyl acetate insoluble residue of the oil produced from pyrolysis of untreated water insoluble tannins obtained after dissolving the oil in ethyl acetate and filtering the solution.
The GC–MS analysis of the ethyl acetate soluble ESP oil from untreated tannins shows similar product distribution as the micropyrolysis/GC–MS experiments. Major products include phenol, catechol and methyl catechol. The addition of calcium
Fig. 6. Comparison of chromatograms of ESP oil from pyrolysis of untreated and pre-treated water soluble tannins.
Fig. 7. Comparison of chromatograms of ESP oil from pyrolysis of untreated and pre-treated water insoluble tannins.
formate and calcium hydroxide also has a significant effect on the composition of the oils. Pretreatment with calcium formate resulted in an increase in phenol, cresol and polyaromatic hydrocarbons (PAH) relative to the amount of catechol obtained as well as formation of new compounds such as methyl-cyclopentenone and dimethyl phenol. Pretreatment with calcium hydroxide resulted in very few phenolics, indicating that calcium hydroxide may suppress formation of primary pyrolysis products. The presence of phenolics with alkyl substituents for each sample is consistent with the NMR results showing signals in the aromatic and alkyl regions only. The shift from catechol-type to phenol-type compounds and the increased formation of PAHs that occurs with pretreatment also confirms the deoxygenation shown in elemental analysis of the oils. The results presented here demonstrate that pretreatment of tannins with calcium compounds has a significant effect on the pyrolysis yield and composition of the liquid product. Without pretreatment, the selectivity of pyrolysis toward catechol and methyl catechol is higher. For the production of renewable chemicals, this is favorable, but the physical properties of the oil cause issues with processing. Pretreatment does have a positive effect on the viscosity and solubility of the oil, but changes the product distribution. The types of compounds produced during pyrolysis of the pretreated tannins have a lower O:C ratio than those produced without pretreatment, which may be favorable for fuels production. However, the formation of polyaromatic hydrocarbons upon pretreatment is not a desirable result. It should be emphasized that the water insoluble tannin fraction is more suitable for pyrolysis, because it can be more easily fed into the reactor relative to the water soluble tannin fraction. Only the water soluble tannin fraction can be used for adhesive production or other known applications, making the thermochemical
P.A. Case et al. / Journal of Analytical and Applied Pyrolysis 107 (2014) 250–255
decomposition of water insoluble tannins a good complement for using the polyphenolic bark extracts as a whole. 4. Conclusions Pyrolysis of untreated and pre-treated tannins obtained from Pinus radiata bark was studied and the resulting products were analyzed. Tannins present unique challenges for fast pyrolysis because their thermophysical properties cause agglomeration issues during feeding. Pretreatment of tannins with calcium hydroxide and calcium formate enabled continuous feeding and allowed for the quantification of product yields at the bench scale. In addition, the pretreatment changed the chemical composition of the liquid product. This bench-scale work demonstrates the viability of producing renewable chemicals from tannin feedstocks. Acknowledgements This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DE-FG0207ER46373. Additional support from the Technology Development Unit of Universidad de Concepción (Base Project PFB-27) for the tannins production is acknowledged. The authors also acknowledge technical support from Nick and Ken Hill and Chemical Engineer Héctor Grandón. Constanza Bizama acknowledges support from the National Science Foundation Research Experience for Undgraduates grant EED-1063007.
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References [1] A. Berg, P. Navarrete, L. Olave, Biochemicals and standardized solid fuels from Radiata Pine Bark, in: 15th European Biomass Conference & Exhibition, Berlin, Germany, 2007, p. 2198. [2] L.B. Pepino, Paulo, Caldeira, Fernando, Extraction of pine bark tannins in aqueous-organic systems, 2nd International Conference on Biopolymer Technology, Ischia, Italy, 2000, p. 36. [3] A.G. Austin, Shreve’s Chemical Process Industries, McGraw Hill, New York, 1994. [4] J. Li, F. Maplesden, Trans. Inst. Prof. Eng. N. Z.: Electr./Mech./Chem. Eng. Sect. 25 (1998) 46. [5] Q. Bu, H.W. Lei, S.J. Ren, L. Wang, J. Holladay, Q. Zhang, J.M. Tang, R. Ruan, Bioresour. Technol. 102 (2011) 7004. [6] C. Amen-Chen, B. Riedl, X.M. Wang, C. Roy, Holzforschung 56 (2002) 167. [7] C. Amen-Chen, B. Riedl, C. Roy, Holzforschung 56 (2002) 273. [8] A. Effendi, H. Gerhauser, A.V. Bridgwater, Renew. Sustain. Energy Rev. 12 (2008) 2092. [9] K. Khanbabaee, T. van Ree, Nat. Prod. Rep. 18 (2001) 641. [10] G.C. Galletti, J.B. Reeves, Org. Mass Spectrom. 27 (1992) 226. [11] S. Ohara, Y. Yasuta, H. Ohi, Holzforschung (2003) 145. [12] J. Kaal, K.G.J. Nierop, P. Kraal, C.M. Preston, Org. Geochem. 47 (2012) 99. [13] P. de Wild, R. Van der Laan, A. Kloekhorst, E. Heeres, Environ. Prog. Sustain. Energy 28 (2009) 461. [14] S.H. Beis, S. Mukkamala, N. Hill, J. Joseph, C. Baker, B. Jensen, E.A. Stemmler, M.C. Wheeler, B.G. Frederick, A. van Heiningen, A.G. Berg, W.J. DeSisto, Bioresources 5 (2010) 1408. [15] S. Mukkamala, M.C. Wheeler, A.R.P. van Heiningen, W.J. DeSisto, Energy Fuels 26 (2012) 1380. [16] W.J. DeSisto, N. Hill, S.H. Beis, S. Mukkamala, J. Joseph, C. Baker, T.H. Ong, E.A. Stemmler, M.C. Wheeler, B.G. Frederick, A. van Heiningen, Energy Fuels 24 (2010) 2642. [17] J. Joseph, C. Baker, S. Mukkamala, S.H. Beis, M.C. Wheeler, W.J. DeSisto, B.L. Jensen, B.G. Frederick, Energy Fuels 24 (2010) 5153. [18] H. Kawamoto, D. Yamamoto, S. Saka, J. Wood Sci. 54 (2008) 242.
Contents lists available at ScienceDirect
Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Pyrolysis of pre-treated tannins obtained from radiata pine bark Paige A. Case a , Constanza Bizama b , Cristina Segura b , M. Clayton Wheeler a,c , Alex Berg b , William J. DeSisto a,c,∗ a b c
Department of Chemical and Biological Engineering, University of Maine, USA Unidad de Desarrollo Tecnológico, Concepción, Chile Forest Bioproducts Research Institute, University of Maine, USA
a r t i c l e
i n f o
Article history: Received 13 December 2013 Accepted 25 March 2014 Available online 18 April 2014 Keywords: Pyrolysis Tannins Pre-treatment
a b s t r a c t Water soluble and insoluble fractions of tannins obtained from radiata pine bark were pyrolyzed at the bench-scale in a continuous entrained flow pyrolysis reactor at 500 ◦ C. The tannin fractions were pre-treated with both calcium hydroxide and calcium formate to assist in continuous feeding and promote deoxygenation during pyrolysis. Oil yields and qualitative GC–MS were used to characterize the oil products. Pre-treatment assisted in continuous feeding, ameliorating some issues associated with the thermoplastic properties of tannins. A comparison of the pyrolysis of tannins with and without pretreatment is presented. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Biomass and its residues are being researched extensively for use as feedstock for renewable fuels and chemicals. Of the various types of residues worldwide, tree bark is one of the most significant. Approximately one tenth of the mass of trees harvested is bark. Currently, this bark is removed before mechanical processing and is burned for process energy. Because of the unique properties of the bark, however, it may have higher-value use as a chemical precursor or as a refined fuel. Bark from softwood trees, such as Pinus radiata contains up to 60% polyphenolic compounds, of which 8–12% are water soluble and 15–20% are soluble in methanol [1], depending on the age of the tree, the time passed since harvesting, and the particle size, among many other factors. Pine bark polyphenols are condensed tannins, which consist of flavonoid units with varying degrees of condensation [2]. While the traditional use of water soluble tannins was for treatment of animal hides, they have also been examined for medicinal and agricultural applications as well as in manufacture of ink, rubber and plastics and for water treatment [3]. Currently, soluble tannins are being extracted for use as adhesives in wood based panels [4]. These adhesive resins require a very strong crosslinking of the tannin molecules with the bridging agent, formaldehyde. This reaction can only occur if both reactants, tannins and formaldehyde, are
∗ Corresponding author at: Department of Chemical and Biological Engineering, University of Maine, USA. Tel.: +1 207 581 2291. E-mail address: [email protected] (W.J. DeSisto). http://dx.doi.org/10.1016/j.jaap.2014.03.009 0165-2370/© 2014 Elsevier B.V. All rights reserved.
completely solubilized in water. If insoluble tannin fractions are included, a brittle resin will be obtained. Therefore, the insoluble fraction of the tannins remains unused. Tannins, like other biomass sources such as lignin, have the potential to replace a portion of the phenolics used in the manufacture of industrial products, such as resins, which are currently derived from petroleum products [5–7]. Thermal conversion of whole biomass and lignin fractions has been studied for this application, but implementation of the technology has been hindered by several issues. One of the most significant issues is the large number of compounds present in the pyrolysis liquid. When whole biomass is used, many other compounds, such as sugar derivatives, acids, and furans are produced and must be separated from the phenols. Another issue, especially when using lignin, is that the phenolics produced generally are highly substituted and do not have the same properties as phenol [8]. Condensed tannins have an advantage over other types of biomass because of their unique structure. Condensed tannins consist of linked catechins, making them a more uniform polymer than lignin [9]. Micropyrolysis experiments have shown that catechol, pyrogallol and resorcinol are the major thermal decomposition products from condensed tannins, with smaller amounts of methyl catechols also produced [10–12]. Pyrolysis of condensed tannins has previously been limited to micro-scale experiments, most likely because tannins have difficult handling properties. Condensed tannins have a low melting point and swell upon heating, creating challenges with continuously feeding the material into a hot pyrolysis reactor. Similar issues associated with continuous feeding are observed with pyrolysis of lignin [13,14]. One method to overcome these
P.A. Case et al. / Journal of Analytical and Applied Pyrolysis 107 (2014) 250–255 Table 1 Elemental analysis of extracted tannins.
% carbon % hydrogen % oxygen
251
2.3. Materials and feedstock preparation
Water soluble tannins
Water insoluble tannins
54.4 4.8 40.8
55.6 5.0 39.4
issues is the incorporation of calcium hydroxide or calcium formate with lignin before pyrolysis [15]. This not only improves the feeding of the lignin but also has a significant impact on the oil produced during the reaction, reducing the oxygen content and increasing the overall yield of the oil. In this work, we present results for pyrolysis of radiata pine bark derived tannins with and without calcium hydroxide and calcium formate pretreatment. Similar to our lignin work [15], this pretreatment aided in the continuous feeding of the tannins into the bench-scale pyrolysis reactor. Pyrolysis products were collected and analyzed for yield, elemental composition and the qualitative distribution of chemical compounds. A comparison of the different feed pretreatments is presented. 2. Experimental 2.1. Extraction of tannins The tannins used in these experiments were isolated from the bark of Pinus radiata using methanol extraction. The bark was obtained from 15 year old trees and dried to 12% water content, then ground in a hammer mill with 1.20 mm diameter slots. The process of extracting the tannins was performed in the Technology Development Unit of the University of Concepción, Chile in a 4 m3 pressurized pilot plant with recirculation and indirect heating. The bark was mixed in a batch reactor in a 1:5 mass/volume ratio in order to completely cover the bark with the extracting liquor, a 75 wt% methanol/water solution. The mixture is then heated to a temperature of 120 ◦ C within 90 min. After 120 min at 120 ◦ C, the solution was cooled to ambient temperature, and discharged from the extractor. The dilute tannin solution was concentrated to 20% solids, completely evaporating the methanol and separating the water soluble and the water insoluble fractions. Both tannin fractions were then dried. The procedure corresponds to the operation of the pilot plant, which essentially includes a conical extractor with a volume of 4.0 L, a circulation pump and a heat exchanger. The methanol concentration of 75 wt.% and the extracting temperature of 120 ◦ C are the optimal conditions for extracting the tannins. The heating up time, on the other hand, is specific for the pilot plant used. Further details about this procedure can be found in reference [1] (Table 1).
The water soluble and water insoluble tannin fractions were pyrolyzed in a bench-scale fluidized bed reactor. In addition to pyrolysis of pure tannins, pretreatment of tannins was performed to improve pyrolysis. Pretreatment was performed by mixing each tannin fraction with calcium hydroxide and calcium formate, respectively, in an aqueous solution. The loading of the calcium compounds was 0.43 g calcium/g tannins. Both the untreated and pre-treated tannins were dried in the oven for at least 12 h before pyrolysis to remove excess water. The six different resulting materials were ground and sieved to <2 mm. While no molecular interaction was expected between the tannins and the calcium compounds used for pretreatment, a small temperature increase was observed during the initial mixing. This may be due to the presence of dangling hydroxyl groups which may interact with the cation. Furthermore, this effect may be increased for the soluble tannins, as increased solubility may make these groups more available to the cation. 2.4. Bench-scale pyrolysis Fast pyrolysis experiments were carried out in a bubbling bed reactor that measured 3.75 cm in inner diameter × 30 cm in length, with 40–60 mesh silica beads as a heat transfer medium. The temperature in the reactor was monitored using two K-type thermocouples located on the vertical axis measured 3.0 and 9.5 in. from the top of the reactor. Runs were carried out at 500 ◦ C. The mixtures were metered through a screw feeder and fed into the reactor pneumatically, using a nitrogen flow rate of 6 L/min. Between 100 and 150 g of feed was pyrolyzed in each experiment at a feed rate of ∼1–2 g/min. Char was separated using a hot-gas filter immediately after the reactor at 500 ◦ C. After passing through the hot gas filter, liquid was collected in a condenser operated at 3 ◦ C followed by an electrostatic precipitator (ESP). A diagram of the pyrolysis system is shown in Fig. 1. More detailed information about this system can be found in literature [16]. 2.5. Characterization 13 C NMR was performed on a Varian Unity 9.4T instrument at 29 ◦ C using the method outlined by Joseph et al. [17]. Gas chromatography–mass spectrometry (GC/MS) was conducted using a Shimadzu QP2010 instrument described previously. Elemental analysis was performed on a Thermo Scientific Flash 2000 CHNS/O analyzer.
3. Results and discussion
2.2. Micropyrolysis-GC/MS
3.1. Micropyrolysis
Tannins were analyzed by Py-GC/MS over a range of temperatures between 300 ◦ C and 700 ◦ C to obtain fundamental information on their structures and to determine the optimal pyrolysis temperature. The samples were prepared for the micropyrolysis experiments by filling 1/3 of the quartz tube with quartz wool, adding 200–800 mg of sample and then adding more quartz wool on top of the sample. Prepared samples were pyrolyzed for 25 s using a CDS Analytical Pyroprobe 5200 interfaced with a Shimadzu QP2010 gas chromatograph–mass spectrometer. The GC column was an Rxi-5 ms capillary column (30 m × 0.25 mm and 0.250 m film thickness and a 5% diphenyl and 95% dimethylpolysiloxane stationary phase). The injection temperature was 250 ◦ C with a split ratio of 80:1. Peak identification was made by interpretation of mass spectral information.
Experiments were conducted using Py-GC/MS to determine the major pyrolysis products and the optimal temperature for benchscale experiments. Five temperatures were explored for both water soluble and water insoluble tannins. It was found that negligible volatilization occurred at or below 300 ◦ C. The effect of pretreatment on micropyrolysis of tannins was not explored because of issues with sample inhomogeneity at the scale required for the Py-GC/MS (Figs. 2 and 3). Major products identified during the micropyrolysis experiments include phenol, catechol and methyl catechol. Additionally, a significant amount of carbon dioxide was observed, indicating that cracking is likely taking place. The ratio of carbon dioxide to major products increases with increasing temperature, which is consistent with cracking activity.
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Fig. 1. Diagram of the bench-scale fast pyrolysis system.
The products obtained at each given temperature were similar for both the water soluble and water insoluble tannins. The soluble tannins show more selectivity toward catechol, whereas the insoluble tannins produced more methyl catechol. This is likely due to differences in the chemical structure of the starting materials.
Solid, liquid and gas product yields from bench-scale pyrolysis are given in Table 2. The liquid and solid yields were measured by
weight. Char yields were calculated by subtracting the amount of calcium added (assuming all calcium ended up in the solid product). Gas yields were calculated by difference. No bench-scale yield data was collected for untreated water soluble tannins. This is because during feeding, the tannins swelled and solidified at the inlet of the reactor, causing plugging and a rapid pressure increase. In the case of the water insoluble tannins, however, the addition of calcium hydroxide and calcium formate resulted in a decrease in overall liquid yield accompanied by an increase in both char and gas yield. The increase in char yield for
Fig. 2. Comparison of micropyrolysis-GC/MS chromatograms for water soluble tannins at various pyrolysis temperatures.
Fig. 3. Comparison of micropyrolysis-GC/MS chromatograms for water insoluble tannins at various pyrolysis temperatures.
3.2. Bench-scale results
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Table 2 Mass balances based on grams of reactant/product per 100 g of tannins fed. Insoluble tannins Pretreatment Ca(OH)2 added (g/100 g tannins) Ca(COOH)2 added (g/100 g tannins) Product yields Oil product (ESP) (g/100 g tannins) Aqueous product (Condenser) (g/100 g tannins) Char producta (g/100 g tannins) Gas product (g/100 g tannins) a
0 0
8.25 19.3 43.3 29.2
Ca(OH)2 treated insoluble tannins
Ca(OH)2 treated soluble tannins
Ca(COOH)2 treated insoluble tannins
Ca(COOH)2 treated soluble tannins
79 0
79 0
0 140
0 140
1.54 22.5 59.2 95.8
2.01 17.8 81.0 78.2
1.59 17.8 93.1 128
6.2 14.5 109.6 109.7
On a calcium-free basis.
Table 3 Properties of pyrolysis products.
% C in ESP oila % H in ESP oila % C in char % H in char % Ca in char % C in aqueous % H in aqueous % O in aqueous a
Insoluble tannins
Ca(OH)2 treated insoluble tannins
Ca(OH)2 treated soluble tannins
Ca(COOH)2 treated insoluble tannins
Ca(COOH)2 treated soluble tannins
72.4 8.00 70.5 0 0 21.3 8.69 70.0
90.9 10.8 25.1 0.04 41.9 32.3 12.0 55.8
82.6 8.10 22.5 0.15 34.5 29.2 11.8 59.0
86.2 8.50 26.2 0.14 31.6 23.0 8.3 68.8
80.8 7.70 32.1 0.39 28.2 33.0 12.0 55.0
Values may sum to greater than 100% due to uncertainty in measurement (±1% of value).
the formate treated tannins is partially due to the formation of calcium carbonate with the decomposition of calcium formate. In both cases, the calcium may also be catalyzing polymerization reactions that form char [18]. Since the reactor has two types of liquid collectors in series, a cold water condenser and an ESP, yields are reported for each. The ESP product is essentially water free and has high carbon content. This is considered the “oil”. The cold water condenser contains both aqueous and oil phases, and these phases are mixed in order to calculate the carbon content. As shown in Table 3, the carbon content for the condenser oil is significantly lower than ESP oil because it has high water content. The physical properties of the pyrolysis oil varied significantly. The product from pyrolysis of pure water insoluble tannins that was collected downstream of the condenser was extremely viscous and tar-like. This product collected at the entrance to the ESP. The oil from pyrolysis of the calcium treated samples, however, was less viscous and condensed after passing through the ESP. Fig. 4 provides visual comparison of the oils produced from pyrolysis of untreated and pretreated tannins. The elemental analysis (C, H, and O) of the ESP oil and the filter char for each of the pyrolysis experiments is given in Table 3. In all cases, pyrolysis produced a liquid/tar product that had lower oxygen to carbon ratio than the starting material. Pretreatment of the tannins decreased this ratio further, with calcium hydroxide producing a greater deoxygenating effect than calcium formate. The carbon is low in the char from all of the pretreated samples because of the high inorganic content, and the hydrogen is extremely low in all char samples. The calcium content in the char is calculated based on the assumption that all of the calcium introduced to the feedstock will remain in the solid product after pyrolysis. Table 4 gives the integrated areas for 13 C NMR analysis of the oils [17]. The oil from the insoluble tannins is not included because it was not soluble in any of the solvents typically used in our group for NMR analysis. From these results, it is seen that the majority of the carbon in the pretreated oils is in aromatic and alkyl hydrocarbon functionalities. Based on the feedstock composition and structure, this is an expected result. It is important to note that any aromatic species with attached hydroxyl groups also have chemical shifts
in the aromatic/alkene region of 13 C NMR, so from this technique alone, the amount of oxygen in phenol and catechol compounds is impossible to determine. Analysis of the ESP oil from each bench-scale pyrolysis experiment was performed using GC–MS (Figs. 6 and 7). This included the ESP oil from pyrolysis of untreated water soluble tannins, because although not enough sample was pyrolyzed to get a reliable mass balance, sufficient amount ESP oil was collected for GC–MS analysis. The samples were extracted using ethyl acetate as a solvent. It is important to note that while the oil collected from pyrolysis of pretreated samples was fully soluble in ethyl acetate, the tar from the pyrolysis of untreated tannins was only partially soluble. The ethyl acetate insoluble fraction was about 15% of the oil weight and was soluble in methylene chloride, but showed no response when analyzed using GC–MS. This could indicate the presence of higher molecular weight polar compounds in the oil (Fig. 5).
Fig. 4. Oil from pyrolysis of untreated water insoluble tannins (top) does not flow and has a tar-like consistency; while the oil produced from pyrolysis of calcium formate pretreated tannins (bottom) flows easily and has a significantly lower viscosity.
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Table 4 13 C NMR integrated areas. Type of carbon
Chemical shifts
Ca(OH)2 treated insoluble tannins (%)
Ca(OH)2 treated soluble tannins (%)
Ca(COOH)2 treated insoluble tannins (%)
Ca(COOH)2 treated insoluble tannins (%)
Alkyl Methoxy/hydroxy Aromatic/alkene Carbonyl
0–54 54–84 110–163 163–215
44.37 0 54.95 2.19
38.06 0 55.25 7.09
29.48 0 68.91 2.60
23.33 0 74.13 2.70
Fig. 5. Ethyl acetate insoluble residue of the oil produced from pyrolysis of untreated water insoluble tannins obtained after dissolving the oil in ethyl acetate and filtering the solution.
The GC–MS analysis of the ethyl acetate soluble ESP oil from untreated tannins shows similar product distribution as the micropyrolysis/GC–MS experiments. Major products include phenol, catechol and methyl catechol. The addition of calcium
Fig. 6. Comparison of chromatograms of ESP oil from pyrolysis of untreated and pre-treated water soluble tannins.
Fig. 7. Comparison of chromatograms of ESP oil from pyrolysis of untreated and pre-treated water insoluble tannins.
formate and calcium hydroxide also has a significant effect on the composition of the oils. Pretreatment with calcium formate resulted in an increase in phenol, cresol and polyaromatic hydrocarbons (PAH) relative to the amount of catechol obtained as well as formation of new compounds such as methyl-cyclopentenone and dimethyl phenol. Pretreatment with calcium hydroxide resulted in very few phenolics, indicating that calcium hydroxide may suppress formation of primary pyrolysis products. The presence of phenolics with alkyl substituents for each sample is consistent with the NMR results showing signals in the aromatic and alkyl regions only. The shift from catechol-type to phenol-type compounds and the increased formation of PAHs that occurs with pretreatment also confirms the deoxygenation shown in elemental analysis of the oils. The results presented here demonstrate that pretreatment of tannins with calcium compounds has a significant effect on the pyrolysis yield and composition of the liquid product. Without pretreatment, the selectivity of pyrolysis toward catechol and methyl catechol is higher. For the production of renewable chemicals, this is favorable, but the physical properties of the oil cause issues with processing. Pretreatment does have a positive effect on the viscosity and solubility of the oil, but changes the product distribution. The types of compounds produced during pyrolysis of the pretreated tannins have a lower O:C ratio than those produced without pretreatment, which may be favorable for fuels production. However, the formation of polyaromatic hydrocarbons upon pretreatment is not a desirable result. It should be emphasized that the water insoluble tannin fraction is more suitable for pyrolysis, because it can be more easily fed into the reactor relative to the water soluble tannin fraction. Only the water soluble tannin fraction can be used for adhesive production or other known applications, making the thermochemical
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decomposition of water insoluble tannins a good complement for using the polyphenolic bark extracts as a whole. 4. Conclusions Pyrolysis of untreated and pre-treated tannins obtained from Pinus radiata bark was studied and the resulting products were analyzed. Tannins present unique challenges for fast pyrolysis because their thermophysical properties cause agglomeration issues during feeding. Pretreatment of tannins with calcium hydroxide and calcium formate enabled continuous feeding and allowed for the quantification of product yields at the bench scale. In addition, the pretreatment changed the chemical composition of the liquid product. This bench-scale work demonstrates the viability of producing renewable chemicals from tannin feedstocks. Acknowledgements This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DE-FG0207ER46373. Additional support from the Technology Development Unit of Universidad de Concepción (Base Project PFB-27) for the tannins production is acknowledged. The authors also acknowledge technical support from Nick and Ken Hill and Chemical Engineer Héctor Grandón. Constanza Bizama acknowledges support from the National Science Foundation Research Experience for Undgraduates grant EED-1063007.
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