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Influence of potassium carbonate addition on the condensable species released during wood torrefaction
Fuel Processing Technology 169 (2018) 248–257
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Research article
Influence of potassium carbonate addition on the condensable species released during wood torrefaction
MARK
Lucélia Alves de Macedoa,b,c, Jean-Michel Commandréb,⁎, Patrick Roussetb,d, Jérémy Valetteb, Mathieu Pétrissansc a
Laboratório de Produtos Florestais, Serviço Florestal Brasileiro, Brasília, 70818900, Brazil CIRAD, UPR BioWooEB, F-34398 Montpellier, France c Université de Lorraine, Inra, LERMaB, F88000 Epinal, France d Joint Graduate School of Energy and Environment, Centre of Excellence on Energy Technology and Environment, King Mongkut's University of Technology Thonburi, Bangkok, Thailand b
A R T I C L E I N F O
A B S T R A C T
Keywords: Torrefaction Potassium Thermogravimetric analysis Lignin derivatives
In order to investigate the effect of potassium addition on the composition of torrefaction condensates, two demineralized wood species were impregnated with different concentrations of K2CO3 and then torrefied at 275 °C up to an anhydrous weight loss (AWL) of 25%. Torrefaction was carried out in both a thermogravimetric analysis (TGA) instrument and a laboratory fixed-bed reactor. Condensates from the fixed bed reactor were collected and analyzed by Gas Chromatography-Mass Spectroscopy (GC–MS). TGA of raw and K2CO3-impregnated biopolymers (cellulose, xylan and lignin) were performed to facilitate interpretation of the results. TGA showed that when potassium content increased in the biomass, shorter torrefaction times were sufficient to obtain the targeted AWL. GC–MS showed, for both wood species, that potassium promotes the formation of acetol and slightly enhances acetic acid yield. The amount of some lignin derivatives (guaiacol, syringol, 4vinylguaiacol) also rose with potassium addition. Yields of levoglucosan, LAC (1-hydroxy-(1R)-3,6-dioxabicyclo [3.2.1]octan-2-one) and DGP (1,4:3,6-dianhydro-α-D-glucopyranose), as well as furfural and 5-hydroxymethylfurfural, decreased drastically in the presence of potassium. In conclusion, small additions of potassium carbonate deeply affected thermal degradation of wood species and the speciation of torrefaction condensates.
1. Introduction Torrefaction is a mild pyrolysis process usually carried out between 200 and 300 °C in the absence of oxygen, and is used to increase the hydrophobicity, energy content and grindability of biomass [1]. Torrefaction, which is regarded as a biomass pretreatment method, has been studied for gasification [2–5] and combustion applications [6–8]. In a conventional torrefaction process, the solid product amounts to about 70% of the initial dry weight of the biomass and the remainder is released as condensable and non-condensable gases [9], which are still under-exploited. Recent studies have suggested that condensable gas could be potentially utilized as bio-sourced chemicals [10,11] and should be considered for enhancing economic viability in torrefaction plants. Efforts have been made to identify and quantify condensable species released during biomass torrefaction and more than 85 condensable products have been reported, in addition to discovering the strong influence of temperature and biomass composition in their
formation [11]. During thermal degradation, mineral matter may affect the behavior of biomass components, altering the distribution and chemical speciation of the pyrolysis products. The mineral matter content of raw biomass may vary from less than 1 wt% to more than 25 wt% depending on the type of biomass, mainly consisting of Al, Ca, Fe, K, Mg, Na and Si, with smaller amounts of S, P, Cl and Mn [12]. Among the metals present in biomass, potassium appears to have the greatest influence on thermal degradation mechanisms [13] and generally its content in biomass is much higher than other alkali metals [14]. The influence of alkali metals on biomass pyrolysis has been widely studied and the catalytic role of potassium in char formation at the expense of pyrolysis liquid yield has been reported [15–17]. Contrary to what occurs under pyrolysis temperatures, an increasing mass loss with potassium addition has been observed during torrefaction, which might allow the use of shorter residence times as well as lower temperatures to obtain the target solid yield [18,19]. These torrefaction studies, however, did not
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Corresponding author at: Laboratório de Produtos Florestais, Serviço Florestal Brasileiro, Brasília 70818900, Brazil. E-mail addresses: lucelia.macedo@florestal.gov.br (L.A.d. Macedo), [email protected] (J.-M. Commandré), [email protected] (P. Rousset), [email protected] (J. Valette), [email protected] (M. Pétrissans). http://dx.doi.org/10.1016/j.fuproc.2017.10.012 Received 23 June 2017; Received in revised form 13 October 2017; Accepted 14 October 2017 0378-3820/ © 2017 Elsevier B.V. All rights reserved.
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analyze the effect of potassium addition on the composition and yield of the condensable species. Increasing potassium content may enhance lignin derivative yield during pyrolysis [20,21]. Some of these pyrolytic compounds, such as guaiacol, vynil-guaiacol, syringol, syringaldehyde, eugenol, isoeugenol and vanillin are recognized as valuable chemicals and could be considered for future biorefinery approaches. Hence, understanding the catalytic effect of potassium during biomass torrefaction may be an important step towards chemical utilization of condensable gases in future torrefaction plants, in addition to torrefied biomass. Consequently, to analyze the influence of potassium content on condensable species released during wood torrefaction, two demineralized wood species were impregnated with three different concentrations of K2CO3 and then torrefied at 275 °C up to an anhydrous weight loss (AWL) of 25%. In order to gain a better understanding of the role of potassium in thermal degradation, cellulose, lignin and xylan (representing hemicelluloses) were also impregnated with K2CO3 and studied by thermogravimetric analysis (TGA). Torrefaction of the wood species was carried out in both a TGA instrument and a laboratory-scale fixed-bed reactor. Condensates from the fixed-bed reactor were collected and analyzed by GC–MS.
Table 1 K, Na, P and Ca content in control, raw and impregnated biomass analyzed by ICP-AES. ⁎ DL: detection limit.
2. Material and methods
wt%, dry basis
K
Na
P
Ca
Eucalyptus Control Raw 0.003 M K2CO3 0.006 M K2CO3 0.009 M K2CO3
0.009 0.027 0.149 0.253 0.387
0.005 0.012 0.003 0.004 0.004
0.0028 0.0030 0.0065 < 0.0028 0.0095
< DL* < DL < DL < DL < DL
Amapaí Control Raw 0.003 M K2CO3 0.006 M K2CO3 0.009 M K2CO3
0.011 0.086 0.229 0.314 0.527
0.005 0.026 0.004 0.004 0.006
0.0081 0.0140 0.0099 0.0050 0.0091
0.008 0.179 0.054 < DL 0.005
Cellulose Raw 0.003 M K2CO3
0.018 0.096
0.004 0.008
0.0046 < 0.0028
< DL < DL
Lignin Raw 0.003 M K2CO3
0.114 0.230
0.531 0.186
< 0.0028 0.003
< DL < DL
Xylan Raw 0.003 M K2CO3
0.223 0.614
1.043 1.080
0.0132 0.0142
0.600 0.634
2.1. Biomass feedstock and sample preparation 0.009 M K2CO3 aqueous solution. Cellulose, lignin and xylan were impregnated in the 0.003 M K2CO3 solution only. The suspension was stirred at 250 rpm for 60 min, and then filtered through a filter paper (Rundfilter MN615, Macherey-Nagel) in a Buchner funnel. Given the difficulty of filtering the suspension of xylan in water, freeze drying was used instead of filtration and consequently all of the K2CO3 in the solution was retained in xylan, and thus, the K content was higher than that observed in the cellulose and lignin samples. The impregnated samples were then oven dried for 24 h at 105 °C. These samples were labeled according to the K2CO3 concentration of the impregnation solution in which they were immersed (0.003 M K2CO3, 0.006 M K2CO3 and 0.009 M K2CO3). Demineralized samples without addition of salt served as the control. After demineralization and impregnation, the samples were analyzed for inorganic elements by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) (Table 1). The K content of the samples varied from 0.009 to 0.614 wt %, corresponding to the range in which the effect of K in torrefaction mass loss is more pronounced [18].
The woody biomass consisted of a eucalyptus hybrid clone (Eucalyptus urophylla x E. camaldulensis) and amapaí (Brosimum potabile Ducke), a tropical wood species. Biomass was knife-milled down to a particle size of < 1 mm and then dried for 24 h at 105 °C in a forceddraft oven prior to demineralization, impregnation and torrefaction experiments. Microcrystalline cellulose (Avicel PH105; 20 μm; product code 14205) and xylan from beech wood (product code 38500) were purchased from Serva Electrophoresis. Lignin kraft (product code 370959) was purchased from Sigma Aldrich. The biomass components were dried as received and they were not demineralized prior to impregnation and TGA. 2.2. Demineralization and impregnation of samples Wood samples were washed with 1% acetic acid (Sigma Aldrich, purity 99.7%) in accordance with the procedure used by Wigley et al. [22]. We decided to work with an organic acid instead of the mineral acids usually employed, to avoid major modifications in biomass polymer composition induced by acid leaching, and to gain a better understanding of the effects of mineral matter removal and potassium addition. First, 50 g of dried wood was added to 500 mL of leaching solution using a 1 L beaker glass. The beaker was then heated to 30 °C for 4 h using magnetic hotplates with a stirring speed of 250 rpm. Lastly, the samples were rinsed with deionized water in a vacuum using a Buchner funnel with filter paper (Rundfilter MN615, MachereyNagel). The acid-washed samples were then oven-dried for 24 h at 105 °C. Even though the chosen acid-washing method does not change significantly the polymer composition of wood samples, it was not employed for isolated polymers (cellulose, xylan and lignin). These polymers, when arranged together in the wood fiber structure, are expected to be more resistant to changes induced by acid leaching than their isolated form. For instance, the presence of xylan-lignin and xylancellulose bonds may impact xylan hydrolysis [23]. In this same sense, the amount of hydroxyl groups in isolated lignin may be reduced by acid washing [24]. Demineralized wood samples were impregnated with three different concentrations of K2CO3 (Sigma Aldrich, purity 99.99%) based on the procedure employed by Shoulaifar, et al. [18]. Thirty grams of ovendried woody samples was immersed in 1 L of 0.003 M, 0.006 M and
2.3. Thermogravimetric analysis (TGA) The behavior of raw, control and potassium-impregnated biomass samples was studied under an inert nitrogen atmosphere in a thermogravimetric analyzer. A constant flow rate of 0.5 L/min was applied. Each of the biomass samples (100 ± 15 mg) in an alumina pan was heated up from room temperature to 105 °C and kept isothermally at that temperature for 30 min to remove the moisture. The samples were then heated to 275 °C with a heating rate of 10 °C/min and held at that temperature isothermally for 50 min. All the TGA experiments were replicated twice. The repeatability of the TGA experiments was good, with a relative standard deviation between two replications of less than 1%. 2.4. Torrefaction experiments The fixed-bed reactor consisted of an external stainless steel tube (500 mm high, 36 mm ID) and an internal quartz tube reactor (410 mm high, 26 mm ID), equipped with a fixed porous bed on the top to hold the biomass. The external tube was closed at the top and heated by a 4 kW furnace. A constant N2 flow rate of 0.55 L/min was applied. The 249
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Fig. 1. Schematic diagram of the reactor assembly.
carrier gas entered through the bottom of the device, was preheated between the two tubes, and then passed through the bed of biomass, transporting torrefaction gas to the outlet of the reactor. The reactor outlet was connected to two glass condensers in series cooled at −20 °C and then to a micro-GC (Agilent Technologies, Varian CP-4900). The non-condensable gases were then analyzed online and the condensable species were recovered in the two condensers and analyzed later by GC–MS. A schematic diagram of the reactor assembly is showed in Fig. 1. The furnace was pre-heated to 140 °C prior to each torrefaction treatment. Around 2.5 g ( ± 0.005 g) of each dried sample was placed in the quartz tube reactor, heated from 140 °C to 275 °C at a heating rate of 10 °C/min and kept isothermally at that temperature up to an anhydrous weight loss (AWL) of 25%. We fixed an AWL (25%) instead of the usual holding time to gain a better understanding of the role played by K2CO3 in the composition of the torrefaction liquid phase. This method seemed useful for investigating the effects of different potassium concentrations since it avoided modifications caused by different degrees of thermal degradation. The experiment time (i.e., total experiment time counted from the initial temperature of 140 °C) was adjusted to achieve 25% of AWL for each sample (Table 2). Each experiment was conducted in duplicate.
Table 2 Experiment times (min) for achieving the AWL of 25% in the laboratory fixed-bed reactor.
Eucalyptus Amapaí
Control
0.003 M K2CO3
0.006 M K2CO3
0.009 M K2CO3
85 125
69 60
56 50
48 45
condensers and the pipes connecting these components before and after each experiment. Non-condensable gas yields were calculated using nitrogen as the tracer gas. A known and constant flow of nitrogen was injected during the experiment, enabling quantification of the noncondensable gas masses. The yields of solid, condensable products and gas are defined as the mass formed divided by the dry mass of the initial biomass sample. Since we worked with an AWL on a dry basis only, we decided to express the yields of all products on a dry basis too, instead of a dry and ash free basis. The ash content of samples ranged from 0.01 to 0.70% and 0.24 to 0.86% for the eucalyptus and amapaí samples, respectively, and we verified that its effect on the TG curves was insubstantial. The condensable system (condensers and pipes) and the quartz tube were washed with acetone (Honeywell, purity 99.5%), to remove the condensable species, and stored in a freezer for further GC–MS analysis.
2.5. Overall mass balance and product recovery 2.6. GC–MS analysis Yields of torrefied biomass and condensable gases were determined gravimetrically by weighing the biomass, quartz reactor tube,
GC–MS analysis was performed on an Agilent 6890 gas 250
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of cellulose and hemicellulose (xylan), adding that salt to biomass in different weight ratios (0, 0.01, 0.05, 0.10, corresponding to 0, 0.56, 2.84, 5.60 K wt%, respectively) and observed different DTG patterns for cellulose depending on the temperature range (220–315 °C or 315–390 °C). For the 220–315 °C range, the mass loss increased with the increase in K content, while the opposite was observed in the 315–390 range. They also observed that the mass loss of xylan at 220–315 °C did not significantly change with increasing K2CO3 content. This result was in accordance with the TG profile for xylan observed in our study (Fig. 3). The TG curves for xylan and impregnated xylan almost overlapped with each other, even though we found a slight shift of the DTG peak from 20 to 24 min (240 to 262 °C). Jensen et al. [17] and Nowakowski and Jones [25] did also not observe any influence of potassium addition on the solid yield of xylan under pyrolysis conditions. These authors speculated that due to the significant alkali metal content of raw xylan, the impact of further addition of potassium is small. Another reason for xylan insensitivity to potassium addition may be that single polymers are not affected in the same way as in the whole biomass, where potassium could affect interactions between the biopolymers [17]. Shoulaifar et al. [18] noticed a stronger effect on mass loss with increasing K2CO3 for hemicellulose and cellulose during torrefaction, but the difference between their results and ours was most likely due to the fact that we used different types of biomass components. We used xylan as a representative of hemicelluloses and crystalline cellulose and the other authors used galactoglucomannan as the hemicellulose representative and Whatman ash-free filter paper as the cellulose sample. Eucalyptus and amapaí samples followed the same trends of increasing weight loss with K2CO3 addition as the cellulose studied by Yang et al. [26] at 220–315 °C. As found by those authors, adding K2CO3 shifted cellulose degradation to a lower temperature, which might lead to decomposition into gas, liquid and solid products instead of just decreasing its crystallinity as expected at low temperatures. The effect of potassium in shifting cellulose pyrolysis to lower temperatures was also reported by Nowakowski and Jones [25] and Jensen et al. [17] under pyrolysis conditions. Likewise, as shown in Fig. 3, the peak of the mass loss rate for cellulose shifted from 44 to 24 min (from 275 to 262 °C), in addition to a reduction in the peak value. Hence, the DTG peaks for the woody species shown in Fig. 1 may represent xylan degradation with partial contribution of cellulose decomposition, which is enhanced by the presence of potassium. The addition of K2CO3 did not show any influence on the lignin TG profile (data not shown). Possibly, if there is an effect of K2CO3 on lignin degradation at torrefaction temperatures, it cannot be detected without considering interactions with other biomass components. Based on the thermogravimetric analysis, we could speculate that the shorter torrefaction times required for obtaining the target AWL (25%) with increasing K content might most likely be attributed to changes in cellulose decomposition induced by the presence of K2CO3.
Table 3 GC–MS analytical method conditions. Capillary column
Agilent DB1701, 60 m × 0.25 mm × 0.25 μm, 14% cyanopropyl-phenyl 86% PDMS
Carrier gas Injection volume Injection temperature Oven, two injections: Split mode (ratio 1:10) Splitless mode
Helium, 1.9 mL·min− 1 1 μL 250 °C
Transfer line temperature Ionisation mode Ionisation energy Ion source temperature Quadrupole temperature
45 °C (4 min) → 120 °C at 3 °C/min → 270 °C at 20 °C/ min 45 °C (4 min) → 250 °C at 3 °C/min → 270 °C at 20 °C/ min held for 60 min 270 °C Electronic impact 70 eV 230 °C 150 °C
chromatograph coupled to an Agilent 5975 mass spectrometer. Table 3 shows the analytical conditions used in the GC–MS analysis of condensable gases. Prior to analysis, a 2 mL sample of condensable species was filtered with 0.45 μm nylon microfilter (Agilent). Then, a sample volume of 1 mL was transferred to a vial and mixed with a known concentration of four deuterated compounds used as internal standards (acetic acid‑d4, phenol-d6, toluene‑d8 and phenanthrene-d10) for quantification. 1 μL was injected and analyzed by GC–MS. The compounds were identified by comparing their spectrum with those in the NIST database. The concentration of the compounds (mg/L) was determined from the associated internal standard, whose concentration is known. This concentration is used to calculate the yield of the compounds (mg/g biomass), by making the ratio between the mass of the compound and the dry mass of the initial biomass sample used to produce it by torrefaction. 3. Results and discussion 3.1. Effects of potassium addition on thermal degradation TG profiles for torrefaction conducted at 275 °C for the raw, control and impregnated samples are showed in Fig. 2. The impact of the K content on thermal degradation of both wood species was clear and became more pronounced as the torrefaction time progressed. The solid yield significantly decreased with increasing potassium content. For instance, for the control sample of eucalyptus, the AWL after 40 min was about 24% and when increasing the K content to 0.387% (0.009 M K2CO3), the AWL increased to 34%. This trend was also observed by Shoulaifar et al. and Saleh et al. [18,19] in their experiments operated in torrefaction conditions at temperature below 300 °C. However, Shoulaifar et al. reported a mass loss enhancement with increasing K content up to a certain level above which further increase in K did not accelerate the mass loss any further. They assumed that this limit might be associated with the fact that K is bonded to carboxylic groups (existing mainly in hemicelluloses), which degrade during torrefaction. This interpretation was supported by the fact that in the samples with high K concentration, the K added is bonded to carboxylic groups and also to phenolic groups in lignin, whose degradation is less intense during torrefaction. Conversely, results from studies at typical pyrolysis temperatures have shown an increasing solid yield with an increasing biomass potassium content [15,16,25]. This opposite behavior may have been due to changes in the chemical structure of hemicelluloses or the decomposition steps of cellulose, which vary depending on the temperature range [26]. These last authors thoroughly investigated the influence of K2CO3 on the pyrolysis
3.2. Overall mass balance The overall mass balance of the torrefaction experiments was close to or above 94.5% for both species (Fig. 4). As torrefied solid and gas can be easily and precisely quantified, the losses were assumed to arise from the recovery of condensable gases. The mass balances are between 97.8% and 98.7% for the control samples (Fig. 4) and decrease for the K loaded samples. As we did not notice solid deposit in the pipe connected to the μGC nor on the μGC filter, we can suppose that the condensable species composition evolves towards more volatiles species that are difficult to recover in a condensation device, and could explain why the mass balance is reduced for the K-impregnated samples. The gas yields, consisting of CO and CO2, are shown in Fig. 5 and their changes during the torrefaction experiments are displayed in Fig. 6. Only traces of H2, CH4, C2H4 and C2H6 were detected, and their low amounts do not allow to analyze the effect of potassium addition 251
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Fig. 2. TG and DTG curves for raw, control and impregnated samples of the eucalyptus and amapaí. Db: dry basis. Fig. 3. TG and DTG profiles for cellulose, xylan, K2CO3 impregnated cellulose and K2CO3 impregnated xylan. Db: dry basis.
much higher than that which could be attributed to decarboxylation of acid groups in hemicelluloses and speculated that cellulose might partially contribute to its formation, since its degradation was promoted by K addition. CO formation may be attributed to secondary reactions between CO2 and water with the char [30], as well as by other reactions that are catalyzed by the presence of metals [31]. However, Pan et al. [27] speculated that CO formation at low temperatures cannot be explained by secondary gasification reactions with char, since they found similar CO yields for demineralized and calcium-exchanged samples, a metal previously identified by them as an effective gasification catalyst [32]. Conversely, for potassium-exchanged samples, Pan et al. [27] found ten times more CO than in the demineralized samples, peaking at about 340 °C. They thus concluded that CO is a primary product from wood pyrolysis, which appears to be produced largely from cellulose, by unknown mechanisms catalyzed by potassium. The increase in CO2 and CO yields reported during cellulose
and cannot explain the decrease of mass balance in the presence of potassium. CO and CO2 yields increased in the presence of K2CO3, although this influence became less evident above a certain K level. This effect is easily observed in Fig. 6. For the eucalyptus samples, for both CO and CO2, the yield peaks increased with increasing K content. These results are consistent with those reported by Pan et al. [27], who analyzed the formation of CO2 from K-impregnated wood in isothermal experiments at 250 °C. For the amapaí samples, we found that above 0.23 wt% K (0.003 M), there was no apparent effect of K in increasing CO2 yield, since the yield peaks corresponding to the 0.003, 0.006 and 0.009 M samples were almost overlapped. CO2 formation at low temperatures can be attributed to decarboxylation of uronic acids in hemicelluloses and pectins [28] and is increased in the presence of potassium, in addition to reducing the peak production temperature [29]. These last authors analyzed the effect of K addition during pyrolysis and found that the total CO2 yield was
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temperatures. 3.3. Effect of K2CO3 addition on the condensable species The results of the GC–MS analysis of condensates showed some effects of potassium addition on the yields of cellulose, hemicellulose and lignin derivatives for both wood species. Fig. 7 represents cellulose and hemicellulose derivatives and Fig. 8 represents lignin derivatives. The error bars in the Figs. 7 and 8 indicate the minimum and the maximum yields obtained from two independently conducted torrefaction experiments. As shown in Fig. 7, the production of levoglucosan dramatically dropped in the presence of potassium. When increasing the potassium content from 0.009 wt% (control) to 0.149% (0.003 M) and from 0.011 wt% (control) to 0.527 wt% (0.009 M), the amount of levoglucosan decreased from 3.48 to 0.1 mg/g and from 2.89 to 0.1 mg/g for the eucalyptus and amapaí samples, respectively. The yields of the anhydrosugars, LAC (1-hydroxy-(1R)-3,6-dioxabicyclo[3.2.1]octan-2one) and DGP (1,4:3,6-dianhydro-α-D-glucopyranose), also decreased sharply when potassium content increased. Levoglucosan suppression by the presence of K is commonly observed under pyrolysis conditions [15,20,21,34]. Accordingly, substantial increases in DGP yield were reported after biomass demineralization by Eom et al. [35], which corroborates that inorganic species play an important role in suppressing anhydrosugar formation. Levoglucosan is an anhydrosugar produced from pure cellulose depolymerization through glycosidic linkage cleavage by intramolecular transglycosylation reactions during pyrolysis [36]. The mechanism of these reactions involves intramolecular substitution of the glycosidic linkage by one of the free eOH groups in cellulose [37]. According to the mechanism proposed by Nishimura et al. [33], in the presence of K2CO3, glycosidic linkages are cleaved by the attack of K+ and/or CO32– instead of by the intramolecular attack by eOH that produces levoglucosan. These last authors observed a decrease in tar yield with increasing K2CO3 addition at pyrolysis temperatures and attributed this fact to levoglucosan suppression. However, the assumption that the secondary decomposition of levoglucosan is the main source of chemical species during pyrolysis has been discussed with the hypothesis of direct formation of low molecular weight species from cellulose. Patwardhan et al. [38] analyzed product distribution from the fast pyrolysis of glucose-based carbohydrates and levoglucosan and found that the latter was much more volatile than the former, being evaporated easily before any decomposition, even when pyrolyzed at high temperatures (400, 500 and 600 °C). These authors thus concluded that levoglucosan and low molecular weight compounds, such as glycolaldehyde and acetol, are produced through competitive pyrolysis reactions rather than secondary decomposition of levoglucosan. In a subsequent study, Patwardhan et al. [39] proposed that mineral salts present in biomass promote the direct formation of low molecular weight species from cellulose, by lowering the activation energy of reactions required to open the pyranose-ring, instead of promoting its formation through secondary reactions of levoglucosan decomposition. Likewise, Yang et al. [36] earlier suggested a mechanism for slow pyrolysis in which levoglucosan and compounds such as CO2, glycolaldehyde, acetic acid, etc., are produced through two parallel and competitive reactions. One is catalyzed by metal ions, promoting the homolytic cleavage of pyranose rings in cellulose leading to the formation of different compounds depending on the cleavage position (glycolaldehyde, for example, should be formed by the scission of C3–C4 and C1–C2 bonds and acetol by the scission of C5–O5 and C3–C4) and the other by transglycosylation, producing levoglucosan. In accordance with that competitive nature of pyrolysis reactions, our results showed a decrease in anhydrosugar yields accompanied by a substantial increase in acetol yield for both wood species and an increase in glycolaldehyde yield for eucalyptus samples from 1.89 to 3.33 mg/g when the K content changed from 0.009 wt% (control) to
Fig. 4. Average overall mass balance of the torrefaction experiments. The error bars indicate the minimum and the maximum values for two independently conducted torrefaction experiments.
Fig. 5. Non-condensable gas yield of the torrefaction experiments. Db: dry basis.
pyrolysis with K2CO3 [33] coupled with the fact that this salt lowers the pyrolysis temperature of cellulose, could support the assumption that cellulose degradation contributes to CO2 and CO yields at torrefaction 253
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Fig. 6. Changes in CO2 and CO yields during the torrefaction experiments for the eucalyptus and amapaí samples. Db: dry basis.
Fig. 7. Effect of K addition on yields of cellulose/hemicellulose derivatives during the torrefaction experiments. The error bars indicate the minimum and the maximum yields for two independently conducted torrefaction experiments. Db: dry basis.
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Fig. 8. Effect of K addition on yields of lignin derivatives during the torrefaction experiments. The error bars indicate the minimum and the maximum yields for two independently conducted torrefaction experiments. Db: dry basis.
0.527 wt% (0.009 M). Despite the absence of potassium in the control samples, we found the acetic acid yield to be higher than that generated from K-impregnated samples, which may indicate two different formation routes for acetic acid depending on the presence or absence of potassium at torrefaction temperatures. The typical source of acetic acid is the hydrolysis of the acetyl ester groups of the 4-O-methylglucuronoxylans [28]. Nevertheless, acetic acid may also be formed from cellulose impregnated with K [25] and some authors have found significant acetic acid production from lignin depolymerization at pyrolysis temperatures [20]. Hence, we speculate that the acetic acid formed from the control samples in our study may most probably be attributed to the obvious source of acetic acid, which is the hydrolysis of acetyl groups in the xylan. In the presence of K2CO3, as shown in section 3.1, K inhibited xylan degradation and enhanced that of cellulose (DTG peak for xylan shifted from 238 to 262 °C and that for cellulose shifted from 275 to 262 °C). Consequently, in the K-impregnated samples, it seems that cellulose degradation, which is enhanced in the presence of K, might partially contribute to acetic acid formation, in addition to the contribution from xylan. It is interesting to note that, for both species, the acetic acid yield was only the same as that in the control samples above 0.3 wt% K (0.006 M for amapaí and 0.009 M for eucalyptus), which may mean that at torrefaction temperatures, above a certain level of K only, the catalytic effect of K in increasing acetic acid yield can be equivalent to the primary formation route. As shown in Fig. 8, potassium addition increased the formation of some lignin derivatives such as syringol, guaiacol and 4-vinylguaiacol for both wood species. For example, the amount of syringol for the
0.149 wt% (0.003 M). Further increases in K2CO3 concentration led to a slight reduction in glycoaldehyde yield. These results tally with those obtained by Eom et al. [21] under pyrolysis conditions using KCl-impregnated samples. The amapaí samples displayed a downward trend for glycolaldehyde yield when the K content rose from 0.011 wt% (control) to 0.229 wt% (0.003 M). This tendency tallies with the findings of Mahadevan et al. [20] in fast pyrolysis experiments using KOHimpregnated samples. These authors speculated that scission at the C2 or C4 position was not favored, resulting in a reduction in glycolaldehyde yield. As shown in Fig. 7, K sharply reduced the formation of furfural and 5-hydroxymethylfurfural. These results tally with those observed by Eom et al. [21] under pyrolysis conditions, who found a moderate decrease in furan yields in the presence of K and suggested that its formation competes with the formation of glycolaldehyde, acetic acid, acetol and butanedial. A strong reduction in furfural formation with K addition during pyrolysis was also reported by Nowakowski et al. [16]. Mahadevan et al. [20] also found a decrease in 5-hydroxymethylfurfural in the presence of K, but no significant effects in furfural formation. Regarding acetic acid yield, for the eucalyptus samples, with a K content increasing from 0.149 (0.003 M) to 0.387 wt% (0.009 M), the yield gradually increased from 12.48 to 16.44 mg/g. This trend is consistent with results reported in pyrolysis experiments [19,20,34]. For the amapaí samples, with a K content increasing from 0.229 (0.003 M) to 0.314 wt% (0.006 M), the acetic acid yield increased from 11.14 to 14.08 mg/g and remained constant when the K content rose to
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species. TGA showed that AWL increased with increasing potassium content and consequently shorter torrefaction times were sufficient to obtain the targeted AWL. This behavior seems to be most likely a result of changes in cellulose decomposition induced by the presence of K2CO3. The results of the GC–MS analysis showed that K2CO3 has a notable catalytic effect on decreasing anhydrosugar (levoglucosan, LAC and DGP) and furan yields, corroborating results obtained under pyrolysis conditions. This suppression was accompanied by a substantial increase in acetol yield for both wood species. The amount of some lignin derivatives, such as guaiacol, syringol and 4-vinylguaiacol, increased with potassium addition. These results suggest that low additions of K could lead to the desired solid yield being obtained with shorter torrefaction times, simultaneously improving the yields of valuable compounds such as guaiacol and syringol.
eucalyptus samples increased from 0.63 mg/g in the control sample to 2.01 mg/g in the 0.003 M K2CO3 impregnated sample. Similar increases for guaiacol and 4-vinylguaiacol can be seen in Fig. 8 for both wood species. This evident increase in phenol derivative yields with increasing potassium content has also been reported under pyrolysis conditions [16,20,21,40]. For instance, the results obtained by Eom et al. [21] demonstrated that an increasing K content in biomass enhanced the yield of guaiacol from 0.21 (control) to 0.58 wt% (1.0-KCl) and syringol from 0.51 wt% (control) to 1.45 wt% (1.0-KCl). Increases in guaiacol and syringol yields were also reported by Peng et al. during pyrolysis of lignin with carbonate additives (K2CO3 and Na2CO3) [41]. Lignin consists of monomeric phenylpropane units with 0–2 methoxyl group(s) which are connected by CeC and ether (CeOeC) linkages [41]. Among the different types of interunit linkages, α-O-4 and β-O-4 aryl ether bonds are the most important ones, accounting for approximately 60% of the total linkages of lignin [42]. The phenylpropane units are known as syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) [43] and are differentiated by the number of methoxyl groups on the benzene ring [44]. Shen et al. [42], who investigated the mechanism of lignin pyrolysis, suggested that guaiacol-type and siryngol-type compounds are mostly produced by the cracking of side-chain CeC bonds of the lignin monomeric units as well as by cleavage of α-O-4 or β-O-4 aryl ether linkages. As regards to guaiacol and syringol specifically, they speculated that the cleavage of the aryl–alkyl–aryl linkage (β-1 carbon‑carbon linkage) as well as the elimination of the side-chain of the lignin monomeric units are two possible formation routes for these compounds. These mechanisms seem to start at low temperatures, as reported by Wen et al. [45], who studied the chemical and structural transformations of lignin during torrefaction by nuclear magnetic resonance (NMR). Their results indicated that aryl-ether bonds (β-O-4) in lignin were cleaved during the torrefaction process, beginning to degrade at 225 °C, which was accompanied by fragmentation, depolymerization, condensation and demethoxylation reactions. Similarly to our findings, Mahadevan et al. [20] observed that K led to an increase in the yield of phenol derivatives during fast biomass pyrolysis and suggested that alkali metals, in the same way as for cellulose and hemicelluloses, promote the cleavage of intermolecular linkages in lignin, enhancing its depolymerization. Contrary to what occurred with guaiacol and syringol, the yield of phenol-2-methoxy-4-methyl (4-methylguaiacol) decreased with increasing K content for both wood species (Fig. 8). Phenol-2-methoxy-4methyl is probably formed by the cleavage of the Cα-Cβ linkage in the lignin side-chain [46], while guaiacol and syringol are believed to be originated from the cleavage of the β-1 linkage and side-chain elimination [42]. Therefore, our findings may suggest that the cleavages of β-1 and C1-Cα linkages were favored rather than that of Cα-Cβ which delivers phenol-2-methoxy-4-methyl. Regarding eugenol and isoeugenol yields, the wood species displayed opposite effects with increasing K content. For the amapaí samples, isoeugenol and eugenol yields decreased with increasing K content, as reported by Mahandevan et al. [20] in pyrolysis experiments. For the eucalyptus samples, the K effect was less obvious with a slight upward trend for isoeugenol and eugenol yields, with minimal K addition (0.003 M), followed by a decrease with further additions. Hwang et al. [40] also reported an increase in isoeugenol yield during fast pyrolysis of KCl-impregnated poplar wood, whose concentration varied from 0.39 mg/g (control sample) to 0.90 mg/g biomass (1.0 wt% KCl), followed by a reduction with further increases of the salt. Since only a few studies have focused on how K addition affects eugenol and isoeugenol yields, no clear trend could be suggested for the formation of these compounds.
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Research article
Influence of potassium carbonate addition on the condensable species released during wood torrefaction
MARK
Lucélia Alves de Macedoa,b,c, Jean-Michel Commandréb,⁎, Patrick Roussetb,d, Jérémy Valetteb, Mathieu Pétrissansc a
Laboratório de Produtos Florestais, Serviço Florestal Brasileiro, Brasília, 70818900, Brazil CIRAD, UPR BioWooEB, F-34398 Montpellier, France c Université de Lorraine, Inra, LERMaB, F88000 Epinal, France d Joint Graduate School of Energy and Environment, Centre of Excellence on Energy Technology and Environment, King Mongkut's University of Technology Thonburi, Bangkok, Thailand b
A R T I C L E I N F O
A B S T R A C T
Keywords: Torrefaction Potassium Thermogravimetric analysis Lignin derivatives
In order to investigate the effect of potassium addition on the composition of torrefaction condensates, two demineralized wood species were impregnated with different concentrations of K2CO3 and then torrefied at 275 °C up to an anhydrous weight loss (AWL) of 25%. Torrefaction was carried out in both a thermogravimetric analysis (TGA) instrument and a laboratory fixed-bed reactor. Condensates from the fixed bed reactor were collected and analyzed by Gas Chromatography-Mass Spectroscopy (GC–MS). TGA of raw and K2CO3-impregnated biopolymers (cellulose, xylan and lignin) were performed to facilitate interpretation of the results. TGA showed that when potassium content increased in the biomass, shorter torrefaction times were sufficient to obtain the targeted AWL. GC–MS showed, for both wood species, that potassium promotes the formation of acetol and slightly enhances acetic acid yield. The amount of some lignin derivatives (guaiacol, syringol, 4vinylguaiacol) also rose with potassium addition. Yields of levoglucosan, LAC (1-hydroxy-(1R)-3,6-dioxabicyclo [3.2.1]octan-2-one) and DGP (1,4:3,6-dianhydro-α-D-glucopyranose), as well as furfural and 5-hydroxymethylfurfural, decreased drastically in the presence of potassium. In conclusion, small additions of potassium carbonate deeply affected thermal degradation of wood species and the speciation of torrefaction condensates.
1. Introduction Torrefaction is a mild pyrolysis process usually carried out between 200 and 300 °C in the absence of oxygen, and is used to increase the hydrophobicity, energy content and grindability of biomass [1]. Torrefaction, which is regarded as a biomass pretreatment method, has been studied for gasification [2–5] and combustion applications [6–8]. In a conventional torrefaction process, the solid product amounts to about 70% of the initial dry weight of the biomass and the remainder is released as condensable and non-condensable gases [9], which are still under-exploited. Recent studies have suggested that condensable gas could be potentially utilized as bio-sourced chemicals [10,11] and should be considered for enhancing economic viability in torrefaction plants. Efforts have been made to identify and quantify condensable species released during biomass torrefaction and more than 85 condensable products have been reported, in addition to discovering the strong influence of temperature and biomass composition in their
formation [11]. During thermal degradation, mineral matter may affect the behavior of biomass components, altering the distribution and chemical speciation of the pyrolysis products. The mineral matter content of raw biomass may vary from less than 1 wt% to more than 25 wt% depending on the type of biomass, mainly consisting of Al, Ca, Fe, K, Mg, Na and Si, with smaller amounts of S, P, Cl and Mn [12]. Among the metals present in biomass, potassium appears to have the greatest influence on thermal degradation mechanisms [13] and generally its content in biomass is much higher than other alkali metals [14]. The influence of alkali metals on biomass pyrolysis has been widely studied and the catalytic role of potassium in char formation at the expense of pyrolysis liquid yield has been reported [15–17]. Contrary to what occurs under pyrolysis temperatures, an increasing mass loss with potassium addition has been observed during torrefaction, which might allow the use of shorter residence times as well as lower temperatures to obtain the target solid yield [18,19]. These torrefaction studies, however, did not
⁎
Corresponding author at: Laboratório de Produtos Florestais, Serviço Florestal Brasileiro, Brasília 70818900, Brazil. E-mail addresses: lucelia.macedo@florestal.gov.br (L.A.d. Macedo), [email protected] (J.-M. Commandré), [email protected] (P. Rousset), [email protected] (J. Valette), [email protected] (M. Pétrissans). http://dx.doi.org/10.1016/j.fuproc.2017.10.012 Received 23 June 2017; Received in revised form 13 October 2017; Accepted 14 October 2017 0378-3820/ © 2017 Elsevier B.V. All rights reserved.
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analyze the effect of potassium addition on the composition and yield of the condensable species. Increasing potassium content may enhance lignin derivative yield during pyrolysis [20,21]. Some of these pyrolytic compounds, such as guaiacol, vynil-guaiacol, syringol, syringaldehyde, eugenol, isoeugenol and vanillin are recognized as valuable chemicals and could be considered for future biorefinery approaches. Hence, understanding the catalytic effect of potassium during biomass torrefaction may be an important step towards chemical utilization of condensable gases in future torrefaction plants, in addition to torrefied biomass. Consequently, to analyze the influence of potassium content on condensable species released during wood torrefaction, two demineralized wood species were impregnated with three different concentrations of K2CO3 and then torrefied at 275 °C up to an anhydrous weight loss (AWL) of 25%. In order to gain a better understanding of the role of potassium in thermal degradation, cellulose, lignin and xylan (representing hemicelluloses) were also impregnated with K2CO3 and studied by thermogravimetric analysis (TGA). Torrefaction of the wood species was carried out in both a TGA instrument and a laboratory-scale fixed-bed reactor. Condensates from the fixed-bed reactor were collected and analyzed by GC–MS.
Table 1 K, Na, P and Ca content in control, raw and impregnated biomass analyzed by ICP-AES. ⁎ DL: detection limit.
2. Material and methods
wt%, dry basis
K
Na
P
Ca
Eucalyptus Control Raw 0.003 M K2CO3 0.006 M K2CO3 0.009 M K2CO3
0.009 0.027 0.149 0.253 0.387
0.005 0.012 0.003 0.004 0.004
0.0028 0.0030 0.0065 < 0.0028 0.0095
< DL* < DL < DL < DL < DL
Amapaí Control Raw 0.003 M K2CO3 0.006 M K2CO3 0.009 M K2CO3
0.011 0.086 0.229 0.314 0.527
0.005 0.026 0.004 0.004 0.006
0.0081 0.0140 0.0099 0.0050 0.0091
0.008 0.179 0.054 < DL 0.005
Cellulose Raw 0.003 M K2CO3
0.018 0.096
0.004 0.008
0.0046 < 0.0028
< DL < DL
Lignin Raw 0.003 M K2CO3
0.114 0.230
0.531 0.186
< 0.0028 0.003
< DL < DL
Xylan Raw 0.003 M K2CO3
0.223 0.614
1.043 1.080
0.0132 0.0142
0.600 0.634
2.1. Biomass feedstock and sample preparation 0.009 M K2CO3 aqueous solution. Cellulose, lignin and xylan were impregnated in the 0.003 M K2CO3 solution only. The suspension was stirred at 250 rpm for 60 min, and then filtered through a filter paper (Rundfilter MN615, Macherey-Nagel) in a Buchner funnel. Given the difficulty of filtering the suspension of xylan in water, freeze drying was used instead of filtration and consequently all of the K2CO3 in the solution was retained in xylan, and thus, the K content was higher than that observed in the cellulose and lignin samples. The impregnated samples were then oven dried for 24 h at 105 °C. These samples were labeled according to the K2CO3 concentration of the impregnation solution in which they were immersed (0.003 M K2CO3, 0.006 M K2CO3 and 0.009 M K2CO3). Demineralized samples without addition of salt served as the control. After demineralization and impregnation, the samples were analyzed for inorganic elements by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) (Table 1). The K content of the samples varied from 0.009 to 0.614 wt %, corresponding to the range in which the effect of K in torrefaction mass loss is more pronounced [18].
The woody biomass consisted of a eucalyptus hybrid clone (Eucalyptus urophylla x E. camaldulensis) and amapaí (Brosimum potabile Ducke), a tropical wood species. Biomass was knife-milled down to a particle size of < 1 mm and then dried for 24 h at 105 °C in a forceddraft oven prior to demineralization, impregnation and torrefaction experiments. Microcrystalline cellulose (Avicel PH105; 20 μm; product code 14205) and xylan from beech wood (product code 38500) were purchased from Serva Electrophoresis. Lignin kraft (product code 370959) was purchased from Sigma Aldrich. The biomass components were dried as received and they were not demineralized prior to impregnation and TGA. 2.2. Demineralization and impregnation of samples Wood samples were washed with 1% acetic acid (Sigma Aldrich, purity 99.7%) in accordance with the procedure used by Wigley et al. [22]. We decided to work with an organic acid instead of the mineral acids usually employed, to avoid major modifications in biomass polymer composition induced by acid leaching, and to gain a better understanding of the effects of mineral matter removal and potassium addition. First, 50 g of dried wood was added to 500 mL of leaching solution using a 1 L beaker glass. The beaker was then heated to 30 °C for 4 h using magnetic hotplates with a stirring speed of 250 rpm. Lastly, the samples were rinsed with deionized water in a vacuum using a Buchner funnel with filter paper (Rundfilter MN615, MachereyNagel). The acid-washed samples were then oven-dried for 24 h at 105 °C. Even though the chosen acid-washing method does not change significantly the polymer composition of wood samples, it was not employed for isolated polymers (cellulose, xylan and lignin). These polymers, when arranged together in the wood fiber structure, are expected to be more resistant to changes induced by acid leaching than their isolated form. For instance, the presence of xylan-lignin and xylancellulose bonds may impact xylan hydrolysis [23]. In this same sense, the amount of hydroxyl groups in isolated lignin may be reduced by acid washing [24]. Demineralized wood samples were impregnated with three different concentrations of K2CO3 (Sigma Aldrich, purity 99.99%) based on the procedure employed by Shoulaifar, et al. [18]. Thirty grams of ovendried woody samples was immersed in 1 L of 0.003 M, 0.006 M and
2.3. Thermogravimetric analysis (TGA) The behavior of raw, control and potassium-impregnated biomass samples was studied under an inert nitrogen atmosphere in a thermogravimetric analyzer. A constant flow rate of 0.5 L/min was applied. Each of the biomass samples (100 ± 15 mg) in an alumina pan was heated up from room temperature to 105 °C and kept isothermally at that temperature for 30 min to remove the moisture. The samples were then heated to 275 °C with a heating rate of 10 °C/min and held at that temperature isothermally for 50 min. All the TGA experiments were replicated twice. The repeatability of the TGA experiments was good, with a relative standard deviation between two replications of less than 1%. 2.4. Torrefaction experiments The fixed-bed reactor consisted of an external stainless steel tube (500 mm high, 36 mm ID) and an internal quartz tube reactor (410 mm high, 26 mm ID), equipped with a fixed porous bed on the top to hold the biomass. The external tube was closed at the top and heated by a 4 kW furnace. A constant N2 flow rate of 0.55 L/min was applied. The 249
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Fig. 1. Schematic diagram of the reactor assembly.
carrier gas entered through the bottom of the device, was preheated between the two tubes, and then passed through the bed of biomass, transporting torrefaction gas to the outlet of the reactor. The reactor outlet was connected to two glass condensers in series cooled at −20 °C and then to a micro-GC (Agilent Technologies, Varian CP-4900). The non-condensable gases were then analyzed online and the condensable species were recovered in the two condensers and analyzed later by GC–MS. A schematic diagram of the reactor assembly is showed in Fig. 1. The furnace was pre-heated to 140 °C prior to each torrefaction treatment. Around 2.5 g ( ± 0.005 g) of each dried sample was placed in the quartz tube reactor, heated from 140 °C to 275 °C at a heating rate of 10 °C/min and kept isothermally at that temperature up to an anhydrous weight loss (AWL) of 25%. We fixed an AWL (25%) instead of the usual holding time to gain a better understanding of the role played by K2CO3 in the composition of the torrefaction liquid phase. This method seemed useful for investigating the effects of different potassium concentrations since it avoided modifications caused by different degrees of thermal degradation. The experiment time (i.e., total experiment time counted from the initial temperature of 140 °C) was adjusted to achieve 25% of AWL for each sample (Table 2). Each experiment was conducted in duplicate.
Table 2 Experiment times (min) for achieving the AWL of 25% in the laboratory fixed-bed reactor.
Eucalyptus Amapaí
Control
0.003 M K2CO3
0.006 M K2CO3
0.009 M K2CO3
85 125
69 60
56 50
48 45
condensers and the pipes connecting these components before and after each experiment. Non-condensable gas yields were calculated using nitrogen as the tracer gas. A known and constant flow of nitrogen was injected during the experiment, enabling quantification of the noncondensable gas masses. The yields of solid, condensable products and gas are defined as the mass formed divided by the dry mass of the initial biomass sample. Since we worked with an AWL on a dry basis only, we decided to express the yields of all products on a dry basis too, instead of a dry and ash free basis. The ash content of samples ranged from 0.01 to 0.70% and 0.24 to 0.86% for the eucalyptus and amapaí samples, respectively, and we verified that its effect on the TG curves was insubstantial. The condensable system (condensers and pipes) and the quartz tube were washed with acetone (Honeywell, purity 99.5%), to remove the condensable species, and stored in a freezer for further GC–MS analysis.
2.5. Overall mass balance and product recovery 2.6. GC–MS analysis Yields of torrefied biomass and condensable gases were determined gravimetrically by weighing the biomass, quartz reactor tube,
GC–MS analysis was performed on an Agilent 6890 gas 250
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of cellulose and hemicellulose (xylan), adding that salt to biomass in different weight ratios (0, 0.01, 0.05, 0.10, corresponding to 0, 0.56, 2.84, 5.60 K wt%, respectively) and observed different DTG patterns for cellulose depending on the temperature range (220–315 °C or 315–390 °C). For the 220–315 °C range, the mass loss increased with the increase in K content, while the opposite was observed in the 315–390 range. They also observed that the mass loss of xylan at 220–315 °C did not significantly change with increasing K2CO3 content. This result was in accordance with the TG profile for xylan observed in our study (Fig. 3). The TG curves for xylan and impregnated xylan almost overlapped with each other, even though we found a slight shift of the DTG peak from 20 to 24 min (240 to 262 °C). Jensen et al. [17] and Nowakowski and Jones [25] did also not observe any influence of potassium addition on the solid yield of xylan under pyrolysis conditions. These authors speculated that due to the significant alkali metal content of raw xylan, the impact of further addition of potassium is small. Another reason for xylan insensitivity to potassium addition may be that single polymers are not affected in the same way as in the whole biomass, where potassium could affect interactions between the biopolymers [17]. Shoulaifar et al. [18] noticed a stronger effect on mass loss with increasing K2CO3 for hemicellulose and cellulose during torrefaction, but the difference between their results and ours was most likely due to the fact that we used different types of biomass components. We used xylan as a representative of hemicelluloses and crystalline cellulose and the other authors used galactoglucomannan as the hemicellulose representative and Whatman ash-free filter paper as the cellulose sample. Eucalyptus and amapaí samples followed the same trends of increasing weight loss with K2CO3 addition as the cellulose studied by Yang et al. [26] at 220–315 °C. As found by those authors, adding K2CO3 shifted cellulose degradation to a lower temperature, which might lead to decomposition into gas, liquid and solid products instead of just decreasing its crystallinity as expected at low temperatures. The effect of potassium in shifting cellulose pyrolysis to lower temperatures was also reported by Nowakowski and Jones [25] and Jensen et al. [17] under pyrolysis conditions. Likewise, as shown in Fig. 3, the peak of the mass loss rate for cellulose shifted from 44 to 24 min (from 275 to 262 °C), in addition to a reduction in the peak value. Hence, the DTG peaks for the woody species shown in Fig. 1 may represent xylan degradation with partial contribution of cellulose decomposition, which is enhanced by the presence of potassium. The addition of K2CO3 did not show any influence on the lignin TG profile (data not shown). Possibly, if there is an effect of K2CO3 on lignin degradation at torrefaction temperatures, it cannot be detected without considering interactions with other biomass components. Based on the thermogravimetric analysis, we could speculate that the shorter torrefaction times required for obtaining the target AWL (25%) with increasing K content might most likely be attributed to changes in cellulose decomposition induced by the presence of K2CO3.
Table 3 GC–MS analytical method conditions. Capillary column
Agilent DB1701, 60 m × 0.25 mm × 0.25 μm, 14% cyanopropyl-phenyl 86% PDMS
Carrier gas Injection volume Injection temperature Oven, two injections: Split mode (ratio 1:10) Splitless mode
Helium, 1.9 mL·min− 1 1 μL 250 °C
Transfer line temperature Ionisation mode Ionisation energy Ion source temperature Quadrupole temperature
45 °C (4 min) → 120 °C at 3 °C/min → 270 °C at 20 °C/ min 45 °C (4 min) → 250 °C at 3 °C/min → 270 °C at 20 °C/ min held for 60 min 270 °C Electronic impact 70 eV 230 °C 150 °C
chromatograph coupled to an Agilent 5975 mass spectrometer. Table 3 shows the analytical conditions used in the GC–MS analysis of condensable gases. Prior to analysis, a 2 mL sample of condensable species was filtered with 0.45 μm nylon microfilter (Agilent). Then, a sample volume of 1 mL was transferred to a vial and mixed with a known concentration of four deuterated compounds used as internal standards (acetic acid‑d4, phenol-d6, toluene‑d8 and phenanthrene-d10) for quantification. 1 μL was injected and analyzed by GC–MS. The compounds were identified by comparing their spectrum with those in the NIST database. The concentration of the compounds (mg/L) was determined from the associated internal standard, whose concentration is known. This concentration is used to calculate the yield of the compounds (mg/g biomass), by making the ratio between the mass of the compound and the dry mass of the initial biomass sample used to produce it by torrefaction. 3. Results and discussion 3.1. Effects of potassium addition on thermal degradation TG profiles for torrefaction conducted at 275 °C for the raw, control and impregnated samples are showed in Fig. 2. The impact of the K content on thermal degradation of both wood species was clear and became more pronounced as the torrefaction time progressed. The solid yield significantly decreased with increasing potassium content. For instance, for the control sample of eucalyptus, the AWL after 40 min was about 24% and when increasing the K content to 0.387% (0.009 M K2CO3), the AWL increased to 34%. This trend was also observed by Shoulaifar et al. and Saleh et al. [18,19] in their experiments operated in torrefaction conditions at temperature below 300 °C. However, Shoulaifar et al. reported a mass loss enhancement with increasing K content up to a certain level above which further increase in K did not accelerate the mass loss any further. They assumed that this limit might be associated with the fact that K is bonded to carboxylic groups (existing mainly in hemicelluloses), which degrade during torrefaction. This interpretation was supported by the fact that in the samples with high K concentration, the K added is bonded to carboxylic groups and also to phenolic groups in lignin, whose degradation is less intense during torrefaction. Conversely, results from studies at typical pyrolysis temperatures have shown an increasing solid yield with an increasing biomass potassium content [15,16,25]. This opposite behavior may have been due to changes in the chemical structure of hemicelluloses or the decomposition steps of cellulose, which vary depending on the temperature range [26]. These last authors thoroughly investigated the influence of K2CO3 on the pyrolysis
3.2. Overall mass balance The overall mass balance of the torrefaction experiments was close to or above 94.5% for both species (Fig. 4). As torrefied solid and gas can be easily and precisely quantified, the losses were assumed to arise from the recovery of condensable gases. The mass balances are between 97.8% and 98.7% for the control samples (Fig. 4) and decrease for the K loaded samples. As we did not notice solid deposit in the pipe connected to the μGC nor on the μGC filter, we can suppose that the condensable species composition evolves towards more volatiles species that are difficult to recover in a condensation device, and could explain why the mass balance is reduced for the K-impregnated samples. The gas yields, consisting of CO and CO2, are shown in Fig. 5 and their changes during the torrefaction experiments are displayed in Fig. 6. Only traces of H2, CH4, C2H4 and C2H6 were detected, and their low amounts do not allow to analyze the effect of potassium addition 251
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Fig. 2. TG and DTG curves for raw, control and impregnated samples of the eucalyptus and amapaí. Db: dry basis. Fig. 3. TG and DTG profiles for cellulose, xylan, K2CO3 impregnated cellulose and K2CO3 impregnated xylan. Db: dry basis.
much higher than that which could be attributed to decarboxylation of acid groups in hemicelluloses and speculated that cellulose might partially contribute to its formation, since its degradation was promoted by K addition. CO formation may be attributed to secondary reactions between CO2 and water with the char [30], as well as by other reactions that are catalyzed by the presence of metals [31]. However, Pan et al. [27] speculated that CO formation at low temperatures cannot be explained by secondary gasification reactions with char, since they found similar CO yields for demineralized and calcium-exchanged samples, a metal previously identified by them as an effective gasification catalyst [32]. Conversely, for potassium-exchanged samples, Pan et al. [27] found ten times more CO than in the demineralized samples, peaking at about 340 °C. They thus concluded that CO is a primary product from wood pyrolysis, which appears to be produced largely from cellulose, by unknown mechanisms catalyzed by potassium. The increase in CO2 and CO yields reported during cellulose
and cannot explain the decrease of mass balance in the presence of potassium. CO and CO2 yields increased in the presence of K2CO3, although this influence became less evident above a certain K level. This effect is easily observed in Fig. 6. For the eucalyptus samples, for both CO and CO2, the yield peaks increased with increasing K content. These results are consistent with those reported by Pan et al. [27], who analyzed the formation of CO2 from K-impregnated wood in isothermal experiments at 250 °C. For the amapaí samples, we found that above 0.23 wt% K (0.003 M), there was no apparent effect of K in increasing CO2 yield, since the yield peaks corresponding to the 0.003, 0.006 and 0.009 M samples were almost overlapped. CO2 formation at low temperatures can be attributed to decarboxylation of uronic acids in hemicelluloses and pectins [28] and is increased in the presence of potassium, in addition to reducing the peak production temperature [29]. These last authors analyzed the effect of K addition during pyrolysis and found that the total CO2 yield was
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temperatures. 3.3. Effect of K2CO3 addition on the condensable species The results of the GC–MS analysis of condensates showed some effects of potassium addition on the yields of cellulose, hemicellulose and lignin derivatives for both wood species. Fig. 7 represents cellulose and hemicellulose derivatives and Fig. 8 represents lignin derivatives. The error bars in the Figs. 7 and 8 indicate the minimum and the maximum yields obtained from two independently conducted torrefaction experiments. As shown in Fig. 7, the production of levoglucosan dramatically dropped in the presence of potassium. When increasing the potassium content from 0.009 wt% (control) to 0.149% (0.003 M) and from 0.011 wt% (control) to 0.527 wt% (0.009 M), the amount of levoglucosan decreased from 3.48 to 0.1 mg/g and from 2.89 to 0.1 mg/g for the eucalyptus and amapaí samples, respectively. The yields of the anhydrosugars, LAC (1-hydroxy-(1R)-3,6-dioxabicyclo[3.2.1]octan-2one) and DGP (1,4:3,6-dianhydro-α-D-glucopyranose), also decreased sharply when potassium content increased. Levoglucosan suppression by the presence of K is commonly observed under pyrolysis conditions [15,20,21,34]. Accordingly, substantial increases in DGP yield were reported after biomass demineralization by Eom et al. [35], which corroborates that inorganic species play an important role in suppressing anhydrosugar formation. Levoglucosan is an anhydrosugar produced from pure cellulose depolymerization through glycosidic linkage cleavage by intramolecular transglycosylation reactions during pyrolysis [36]. The mechanism of these reactions involves intramolecular substitution of the glycosidic linkage by one of the free eOH groups in cellulose [37]. According to the mechanism proposed by Nishimura et al. [33], in the presence of K2CO3, glycosidic linkages are cleaved by the attack of K+ and/or CO32– instead of by the intramolecular attack by eOH that produces levoglucosan. These last authors observed a decrease in tar yield with increasing K2CO3 addition at pyrolysis temperatures and attributed this fact to levoglucosan suppression. However, the assumption that the secondary decomposition of levoglucosan is the main source of chemical species during pyrolysis has been discussed with the hypothesis of direct formation of low molecular weight species from cellulose. Patwardhan et al. [38] analyzed product distribution from the fast pyrolysis of glucose-based carbohydrates and levoglucosan and found that the latter was much more volatile than the former, being evaporated easily before any decomposition, even when pyrolyzed at high temperatures (400, 500 and 600 °C). These authors thus concluded that levoglucosan and low molecular weight compounds, such as glycolaldehyde and acetol, are produced through competitive pyrolysis reactions rather than secondary decomposition of levoglucosan. In a subsequent study, Patwardhan et al. [39] proposed that mineral salts present in biomass promote the direct formation of low molecular weight species from cellulose, by lowering the activation energy of reactions required to open the pyranose-ring, instead of promoting its formation through secondary reactions of levoglucosan decomposition. Likewise, Yang et al. [36] earlier suggested a mechanism for slow pyrolysis in which levoglucosan and compounds such as CO2, glycolaldehyde, acetic acid, etc., are produced through two parallel and competitive reactions. One is catalyzed by metal ions, promoting the homolytic cleavage of pyranose rings in cellulose leading to the formation of different compounds depending on the cleavage position (glycolaldehyde, for example, should be formed by the scission of C3–C4 and C1–C2 bonds and acetol by the scission of C5–O5 and C3–C4) and the other by transglycosylation, producing levoglucosan. In accordance with that competitive nature of pyrolysis reactions, our results showed a decrease in anhydrosugar yields accompanied by a substantial increase in acetol yield for both wood species and an increase in glycolaldehyde yield for eucalyptus samples from 1.89 to 3.33 mg/g when the K content changed from 0.009 wt% (control) to
Fig. 4. Average overall mass balance of the torrefaction experiments. The error bars indicate the minimum and the maximum values for two independently conducted torrefaction experiments.
Fig. 5. Non-condensable gas yield of the torrefaction experiments. Db: dry basis.
pyrolysis with K2CO3 [33] coupled with the fact that this salt lowers the pyrolysis temperature of cellulose, could support the assumption that cellulose degradation contributes to CO2 and CO yields at torrefaction 253
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Fig. 6. Changes in CO2 and CO yields during the torrefaction experiments for the eucalyptus and amapaí samples. Db: dry basis.
Fig. 7. Effect of K addition on yields of cellulose/hemicellulose derivatives during the torrefaction experiments. The error bars indicate the minimum and the maximum yields for two independently conducted torrefaction experiments. Db: dry basis.
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Fig. 8. Effect of K addition on yields of lignin derivatives during the torrefaction experiments. The error bars indicate the minimum and the maximum yields for two independently conducted torrefaction experiments. Db: dry basis.
0.527 wt% (0.009 M). Despite the absence of potassium in the control samples, we found the acetic acid yield to be higher than that generated from K-impregnated samples, which may indicate two different formation routes for acetic acid depending on the presence or absence of potassium at torrefaction temperatures. The typical source of acetic acid is the hydrolysis of the acetyl ester groups of the 4-O-methylglucuronoxylans [28]. Nevertheless, acetic acid may also be formed from cellulose impregnated with K [25] and some authors have found significant acetic acid production from lignin depolymerization at pyrolysis temperatures [20]. Hence, we speculate that the acetic acid formed from the control samples in our study may most probably be attributed to the obvious source of acetic acid, which is the hydrolysis of acetyl groups in the xylan. In the presence of K2CO3, as shown in section 3.1, K inhibited xylan degradation and enhanced that of cellulose (DTG peak for xylan shifted from 238 to 262 °C and that for cellulose shifted from 275 to 262 °C). Consequently, in the K-impregnated samples, it seems that cellulose degradation, which is enhanced in the presence of K, might partially contribute to acetic acid formation, in addition to the contribution from xylan. It is interesting to note that, for both species, the acetic acid yield was only the same as that in the control samples above 0.3 wt% K (0.006 M for amapaí and 0.009 M for eucalyptus), which may mean that at torrefaction temperatures, above a certain level of K only, the catalytic effect of K in increasing acetic acid yield can be equivalent to the primary formation route. As shown in Fig. 8, potassium addition increased the formation of some lignin derivatives such as syringol, guaiacol and 4-vinylguaiacol for both wood species. For example, the amount of syringol for the
0.149 wt% (0.003 M). Further increases in K2CO3 concentration led to a slight reduction in glycoaldehyde yield. These results tally with those obtained by Eom et al. [21] under pyrolysis conditions using KCl-impregnated samples. The amapaí samples displayed a downward trend for glycolaldehyde yield when the K content rose from 0.011 wt% (control) to 0.229 wt% (0.003 M). This tendency tallies with the findings of Mahadevan et al. [20] in fast pyrolysis experiments using KOHimpregnated samples. These authors speculated that scission at the C2 or C4 position was not favored, resulting in a reduction in glycolaldehyde yield. As shown in Fig. 7, K sharply reduced the formation of furfural and 5-hydroxymethylfurfural. These results tally with those observed by Eom et al. [21] under pyrolysis conditions, who found a moderate decrease in furan yields in the presence of K and suggested that its formation competes with the formation of glycolaldehyde, acetic acid, acetol and butanedial. A strong reduction in furfural formation with K addition during pyrolysis was also reported by Nowakowski et al. [16]. Mahadevan et al. [20] also found a decrease in 5-hydroxymethylfurfural in the presence of K, but no significant effects in furfural formation. Regarding acetic acid yield, for the eucalyptus samples, with a K content increasing from 0.149 (0.003 M) to 0.387 wt% (0.009 M), the yield gradually increased from 12.48 to 16.44 mg/g. This trend is consistent with results reported in pyrolysis experiments [19,20,34]. For the amapaí samples, with a K content increasing from 0.229 (0.003 M) to 0.314 wt% (0.006 M), the acetic acid yield increased from 11.14 to 14.08 mg/g and remained constant when the K content rose to
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species. TGA showed that AWL increased with increasing potassium content and consequently shorter torrefaction times were sufficient to obtain the targeted AWL. This behavior seems to be most likely a result of changes in cellulose decomposition induced by the presence of K2CO3. The results of the GC–MS analysis showed that K2CO3 has a notable catalytic effect on decreasing anhydrosugar (levoglucosan, LAC and DGP) and furan yields, corroborating results obtained under pyrolysis conditions. This suppression was accompanied by a substantial increase in acetol yield for both wood species. The amount of some lignin derivatives, such as guaiacol, syringol and 4-vinylguaiacol, increased with potassium addition. These results suggest that low additions of K could lead to the desired solid yield being obtained with shorter torrefaction times, simultaneously improving the yields of valuable compounds such as guaiacol and syringol.
eucalyptus samples increased from 0.63 mg/g in the control sample to 2.01 mg/g in the 0.003 M K2CO3 impregnated sample. Similar increases for guaiacol and 4-vinylguaiacol can be seen in Fig. 8 for both wood species. This evident increase in phenol derivative yields with increasing potassium content has also been reported under pyrolysis conditions [16,20,21,40]. For instance, the results obtained by Eom et al. [21] demonstrated that an increasing K content in biomass enhanced the yield of guaiacol from 0.21 (control) to 0.58 wt% (1.0-KCl) and syringol from 0.51 wt% (control) to 1.45 wt% (1.0-KCl). Increases in guaiacol and syringol yields were also reported by Peng et al. during pyrolysis of lignin with carbonate additives (K2CO3 and Na2CO3) [41]. Lignin consists of monomeric phenylpropane units with 0–2 methoxyl group(s) which are connected by CeC and ether (CeOeC) linkages [41]. Among the different types of interunit linkages, α-O-4 and β-O-4 aryl ether bonds are the most important ones, accounting for approximately 60% of the total linkages of lignin [42]. The phenylpropane units are known as syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) [43] and are differentiated by the number of methoxyl groups on the benzene ring [44]. Shen et al. [42], who investigated the mechanism of lignin pyrolysis, suggested that guaiacol-type and siryngol-type compounds are mostly produced by the cracking of side-chain CeC bonds of the lignin monomeric units as well as by cleavage of α-O-4 or β-O-4 aryl ether linkages. As regards to guaiacol and syringol specifically, they speculated that the cleavage of the aryl–alkyl–aryl linkage (β-1 carbon‑carbon linkage) as well as the elimination of the side-chain of the lignin monomeric units are two possible formation routes for these compounds. These mechanisms seem to start at low temperatures, as reported by Wen et al. [45], who studied the chemical and structural transformations of lignin during torrefaction by nuclear magnetic resonance (NMR). Their results indicated that aryl-ether bonds (β-O-4) in lignin were cleaved during the torrefaction process, beginning to degrade at 225 °C, which was accompanied by fragmentation, depolymerization, condensation and demethoxylation reactions. Similarly to our findings, Mahadevan et al. [20] observed that K led to an increase in the yield of phenol derivatives during fast biomass pyrolysis and suggested that alkali metals, in the same way as for cellulose and hemicelluloses, promote the cleavage of intermolecular linkages in lignin, enhancing its depolymerization. Contrary to what occurred with guaiacol and syringol, the yield of phenol-2-methoxy-4-methyl (4-methylguaiacol) decreased with increasing K content for both wood species (Fig. 8). Phenol-2-methoxy-4methyl is probably formed by the cleavage of the Cα-Cβ linkage in the lignin side-chain [46], while guaiacol and syringol are believed to be originated from the cleavage of the β-1 linkage and side-chain elimination [42]. Therefore, our findings may suggest that the cleavages of β-1 and C1-Cα linkages were favored rather than that of Cα-Cβ which delivers phenol-2-methoxy-4-methyl. Regarding eugenol and isoeugenol yields, the wood species displayed opposite effects with increasing K content. For the amapaí samples, isoeugenol and eugenol yields decreased with increasing K content, as reported by Mahandevan et al. [20] in pyrolysis experiments. For the eucalyptus samples, the K effect was less obvious with a slight upward trend for isoeugenol and eugenol yields, with minimal K addition (0.003 M), followed by a decrease with further additions. Hwang et al. [40] also reported an increase in isoeugenol yield during fast pyrolysis of KCl-impregnated poplar wood, whose concentration varied from 0.39 mg/g (control sample) to 0.90 mg/g biomass (1.0 wt% KCl), followed by a reduction with further increases of the salt. Since only a few studies have focused on how K addition affects eugenol and isoeugenol yields, no clear trend could be suggested for the formation of these compounds.
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