Formation of PCDDs and PCDFs in the torrefaction of biomass with different chemical composition

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Formation of PCDDs and PCDFs in the torrefaction of biomass with different chemical composition Qiuju Gao a , Mar Edo a,b , Sylvia H. Larsson c , Elena Collina d , Magnus Rudolfsson c , Marta Gallina d , Ibukun Oluwoye e , Mohammednoor Altarawneh e , Bogdan Z. Dlugogorski e , Stina Jansson a,∗ a

Umeå University, Department of Chemistry, SE -901 87, Sweden Umeå University, Industrial Doctoral School, SE-901 87, Sweden c Swedish University of Agricultural Sciences, Department of Forest Biomaterials and Technology, SE-90183 Umeå, Sweden d University of Milano-Bicocca, Department of Earth and Environmental Sciences, Italy e Murdoch University, School of Engineering and Information Technology, Murdoch WA 6150, Australia b

a r t i c l e

i n f o

Article history: Received 6 October 2016 Received in revised form 13 December 2016 Accepted 17 December 2016 Available online xxx Keywords: Polychlorinated dibenzo-p-dioxin Polychlorinated dibenzofuran Persistent organic pollutant Thermochemical conversion Lignocellulosic biomass

a b s t r a c t Torrefaction is a thermal pre-treatment technology used to refine biomass, mainly for energy production purposes. However, there is currently a lack of information on the potential formation of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) in the torrefaction process. In this study, torrefaction was conducted using five different types of feedstock: stemwood, bark, wood from a discarded telephone pole, cassava stems and particle board. The feedstock as well as the torrefied biomass (chars) and the volatiles (non-condensable and condensable) generated during torrefaction were analyzed for PCDDs and PCDFs. PCDD concentrations in the torrefaction products were about 2–5 fold of those in the feedstocks. Torrefaction of particle board resulted in extensive formation of PCDDs (7200 ng kg−1 ) compared to the other four feedstocks (13–27 ng kg−1 ). Examination of the homologue profiles suggested that the observed PCDDs in the torrefaction products partly originated from new formation and partly physical transformation from volatilization and re-condensation of PCDDs present in the feedstock. Dechlorination of highly chlorinated compounds (HpCDD and OCDD) in the feedstock to form less chlorinated PCDDs was also observed. Compared to PCDDs, the net formation of PCDFs in the torrefaction process was low, except for the telephone pole sample, for which a dramatic increase (44-fold) of PCDFs was observed. PCDDs and PCDFs were mainly retained in the chars, accounting for 76–96% and 39–74% of the total concentration, respectively. It was also found that the highly chlorinated congeners tended to be retained in the chars, whereas the less chlorinated ones were predominantly volatilized into the gas phase. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Torrefaction is a mild pyrolysis process operated at low temperature (200–350 ◦ C) under oxygen-deficient conditions to upgrade biomass to an energy carrier with improved fuel properties [1]. The process is initiated with a moisture evaporation step followed by partial devolatilization; meanwhile, the reactive components of biomass, such as hemicellulose and extractives, are partly decomposed [2]. The devolatilization of components with high oxygen content generates a material with higher heating value compared to untreated feedstock. Torrefaction does not only increase the

∗ Corresponding author. E-mail address: [email protected] (S. Jansson).

energy density, but the thermal treatment also lowers hydrophilicity, improves grindability and reduces susceptibility to microbial degradation [2]. Torrefaction has been increasingly utilized for feedstock pre-treatment, particularly for fuel supply to entrained flow gasification, small-scale combustion and co-firing in pulverized coal-fired power stations [3]. The selection of biomass feedstock for torrefaction depends largely on resource availability and economic considerations. Biomass used for energy recovery generally comprises stemwood-based assortments from conventional forestry (e.g., pine, spruce and other softwood species), fast-growing energy crops with short rotation and residues from agricultural production (i.e., corn stover and straw) [4]. Under the temperature conditions employed in torrefaction (200–350 ◦ C), persistent organic pollutants (POPs) such as polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs)

http://dx.doi.org/10.1016/j.jaap.2016.12.015 0165-2370/© 2016 Elsevier B.V. All rights reserved.

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may be formed, based on previous studies of wood combustion and other thermochemical processes [5,6]. However, there have been no studies to date on their occurrence and characteristics in biomass torrefaction. PCDDs and PCDFs are preferably formed under conditions of inadequate processing temperature and insufficient oxygen supply [7]. In the low temperature range of 200–400 ◦ C, the formation occurs mainly via heterogeneous reactions. Two main pathways have been proposed: a precursor pathway and de novo synthesis from a carbonaceous matrix [8,9]. The presence of chlorine or chlorinated precursors (i.e., chlorinated benzenes and phenols) in the feedstock is necessary for formation of PCDDs and PCDFs [10]. In addition, it is widely accepted that transition-metal species, especially copper compounds, have a strong catalytic effect on the formation of PCDDs and PCDFs via the two heterogeneous pathways [11]. Although the temperatures employed in torrefaction are suitable for PCDD and PCDF formation, it remains unknown whether PCDDs and PCDFs are actually formed during torrefaction. This information is crucial for processing biomass containing different types of contaminants or impurities. Furthermore, it is unclear to what extent formation is affected by the fuel type and torrefaction conditions and how PCDDs and PCDFs are distributed between the gas-phase and solid-phase torrefaction products. These aspects are important for gaining a fundamental understanding of the characteristics of PCDD and PCDF formation in torrefaction as well as for assessing the possible environmental implications of the torrefaction process and utilization of its products. Lack of data on occurrence, profiles and transformation of PCDDs and PCDFs in biomass torrefaction represents a key knowledge gap that need to be filled. In the present study, we carried out the first attempt to investigate formation and transformation characteristics of PCDDs and PCDFs in biomass torrefaction. We conducted torrefaction experiments at bench-scale using various types of biomass and waste wood with different contamination profiles. PCDDs and PCDFs in various torrefaction products (gas, liquid and chars) were quantified and the results were compared with those in feedstock. The main objective was to evaluate the extent of formation of PCDDs and PCDFs during torrefaction; to estimate how PCDDs and PCDFs are distributed between gas-phase and solid products; and to investigate the influence of feedstock composition on the formation of these compounds. The knowledge gained increases understanding of the behavior of PCDDs and PCDFs in torrefaction processes. This information will help in developing strategies to control and minimize formation of organic pollutants in torrefaction and other mild pyrolysis processes, thereby contributing to clean biomass utilization processes and an environmentally sustainable society.

2. Materials and methods 2.1. Feedstock Five types of biomass were used as feedstock for torrefaction: 1) stemwood pellets (Ø 8 mm, 10–20 mm in length), an assortment from Norway spruce and Scots pine (mixed); 2) bark pellets (Ø 8 mm, 10–20 mm in length) from Norway spruce; 3) a discarded telephone pole (shredded to <1 mm), impregnated with organic and metal-based preservatives (herein after referred to as impregnated stemwood); 4) cassava stem pellets (Ø 8 mm, 10–20 mm in length) made from stem residues after cropping of the starchy roots; and 5) particle board (chopped into chips of size 2–3 cm) collected at a waste disposal site in Italy, composed of wood chips, sawmill shavings and sawdust pressed together with synthetic resin or other suitable binder, partly coated by paint and polyvinyl chloride. Similar to common practice in industrial scale torrefaction, all

Fig. 1. Schematic of the experimental torrefaction and sampling apparatus. 1 – furnace; 2 – tubular reactor; 3 – condenser (water-cooled); 4 – liquid collector; 5 – gas sampler; 6 – electromagnetic valve; 7 – vacuum pump.

feedstocks were subjected to pre-drying (90 ◦ C for 8 h) prior to the actual torrefaction. Characterization of the feedstocks and chars, including proximate and ultimate analysis, energy content and chlorine content, was performed by Bränslelaboratoriet Umeå AB. Analysis of the metal content in the feedstocks were performed by ALS Scandinavia AB. All the analyses were performed according to standard methods specified in the Supporting Information (SI). Furthermore, PerkinElmer STA 8000 facilitated simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The method involved approximately 5.5 mg samples subjected to a nominal heating rate of 10 K min−1 from 30 ◦ C to 290 ◦ C, followed by a severe constant-temperature torrefaction step at 290 ◦ C for 1 h, and another temperature ramp (10 K min−1 )–600 ◦ C, under inert argon atmosphere. The continuous flow rate of the purge gas amounted to 80 mL min−1 at standard temperature and pressure (STP). 2.2. Torrefaction experiments Torrefaction experiments were conducted with a bench-scale tubular reactor made of stainless steel (Ø 120 mm; 300 mm in length) placed in a muffle furnace (ELF 11/6, Carbolite). The sampling setup (Fig. 1) for collection of condensable volatiles consisted of a water-cooled condenser. A gas sampler consisting of a glass fiber filter and a polyurethane foam plug (PUF) was connected to the outlet of the liquid collector for collection of non-condensable gases. A partial vacuum was connected to the outlet of the gas sampler to facilitate sampling of volatiles. For each experimental run, about 100–300 g of feedstock, depending on the bulk density of the material, was placed in the tubular reactor and then nitrogen (with purity >99.9%) was flushed through at a flow rate of 3 L min−1 for 20 min in order to provide an inert atmosphere. The reactor was subsequently placed inside a muffle furnace with temperature set to 290 ◦ C, which enabled the temperature inside the reactor to reach approximately 250 ◦ C within 1.5 h (Fig. 2). The duration of each run was 2.5 h including heating-up time. Nitrogen gas was continuously applied during the entire torrefaction process at a flow rate of 0.5 L min−1 . After torrefaction, the reactor was removed from the muffle oven and quenched in water. The temperature in the reactor was continuously monitored using a thermocouple connected to a PicoLog TC-08 data logger (Pico Technology, Cambridgeshire, UK). All experiments were run in triplicate. A field blank was prepared in duplicate. In detail, the gas sampler (containing a PUF and filter) was passively exposed to ambient air at the sampling area for the same sampling duration (2.5 h) as the

Please cite this article in press as: Q. Gao, et al., Formation of PCDDs and PCDFs in the torrefaction of biomass with different chemical composition, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.12.015

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Fig. 2. Temperature profile during torrefaction treatment for different biomass feedstocks.

torrefaction experiment. This was designed to check the background level of PCDDs and PCDFs at the operating site 2.3. Instrumental analysis Prior to instrumental analysis, the char, condensates and gas samples (trapped on PUFs) were extracted using pressurized liquid extraction, liquid-liquid extraction and Soxhlet extraction, respectively, according to methods described elsewhere [12–14]. The extracts were then subjected to a multilayer silica column clean-up procedure followed by fractionation using an AX21 carbon/celite column. A lab blank was prepared covering the complete analytical procedure including extraction and clean-up to check for interferences from all the relevant reagents and materials. A more detailed description of the extraction and clean-up procedures has been given elsewhere [14]. PCDD and PCDF analyses were performed by gas chromatography/high resolution mass spectrometry (GC/HRMS). The GC/HRMS apparatus consisted of a Hewlett-Packard 5890 gas chromatograph (Agilent Technologies, Palo Alto, CA) coupled to an Autospec Ultima mass spectrometer (Waters Corporation, Milford, MA). Separation was performed on a DB5-MS J&W fused silica capillary column (60 m × 0.25 mm i.d. × 0.25 ␮m film thickness). The mass spectrometer was tuned to a resolution of >10 000 and was operated using electron ionization and selected ion monitoring. Quantification was performed using internal standard method described elsewhere [14,15]. Positive identification was achieved in cases where the signal-to-noise ratio was above 3 and the isotope ratio was within 20% of the appropriate theoretical values. Data with recoveries outside the acceptable range (50–130%) were excluded in accordance with the EN 1948 standard method [15]. The monoCDD data was omitted in the result evaluation due to unacceptable losses during clean-up, as indicated by low recoveries for this homologue group. Table S2 in Supporting Information lists the quantification standards, internal standards, recovery standards and limit of quantification. 3. Results and discussion 3.1. Feedstock characterization The chemical properties of the five tested feedstocks are summarized in Table 1. The ash content of bark (3.9%) and cassava stems

(3.8%) was 7–10 times higher than that of stemwood and impregnated stemwood (0.4 and 0.5%, respectively). All feedstocks had low heating values within the range 17.4–19.7 MJ kg−1 , with cassava stems having the lowest heating value. Energy content was consistent with carbon content. Chlorine content was low in stemwood (<0.01%), impregnated stemwood ( < 0.01%) and bark (0.02%). However, Cl content in particle board and cassava stems was about 8 and 15 times higher than in bark (0.15% and 0.29%, respectively). Particle board had high nitrogen content (3.9%) compared with the other feedstocks (0.1–1%), which can be attributed to the presence of adhesives used as a binder. Particle board can contain up to 10% of adhesives with high nitrogen content [16]. Regarding metal content (Table 1), Cr, As and Cd contents were high in impregnated stemwood (36, 362 and 1.6 mg kg−1 , respectively), whereas particle board had the highest Cu content (11.4 mg kg−1 ). As mentioned in the description of the feedstocks, the impregnated wood sample was prepared from a discarded telephone pole. According to the literature [17], Cu and As content in a telephone pole is normally around 0.8 and 0.1 mg kg−1 , respectively. However, our impregnated wood sample had a much higher Cu (3.38 mg kg−1 ) and As (362 mg kg−1 ) content. This can be attributed to the use of preservatives formulations, such as chromated copper arsenate (CCA). Bark, impregnated stemwood and cassava stems had similar Cu content (3.39–3.56 mg kg−1 ), whereas stemwood had the lowest Cu content (0.967 mg kg−1 ). Particle board had the highest Fe content (0.071%), which is in agreement with the literature [17,18], followed by bark (0.044%), cassava stems and impregnated wood (0.006%) and stemwood (0.001%). 3.2. Temperature profile of torrefaction experiments The temperature profile of one replicate of each different feedstock during the torrefaction runs is shown in Fig. 2. An additional replicate is included for impregnated wood and bark to show the reproducibility of the experiments. Similar reproducibility was obtained for the temperature profiles of the other feedstocks (Fig. S1 in SI). The entire torrefaction process lasted about 2.5 h and gas sampling was conducted during the entire period. It took approximately 1.5 h to heat up the feedstocks from room temperature to a steady torrefaction temperature (240–250 ◦ C). However, as can be seen in Fig. 2, torrefaction of impregnated stemwood resulted in fluctuation of temperature from 300 ◦ C up to about 345 ◦ C, which exceeded the set point (290 ◦ C). One explanation to the fluctuated

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Table 1 Chemical characterization and yields of the five feedstocks and char generated during torrefaction. Parameter

Moisture Ash contenta Volatile matter Fixed carbon HHVb LHVc S C H N Od H/C O/C Cl Fe Cu Cr As Cd Torrefaction product yields Char Liquid Gasg

Unit

% are % dsf % ds % ds MJ kg−1 ds MJ kg−1 ds % ds % ds % ds % ds % ds

% ds % ds mg kgds −1 mg kgds −1 mg kgds −1 mg kgds −1 % w/w initial weight

Stemwood

Impregnated stemwoodCassava stems

Particle board

Feedstock

Char

Feedstock

Char

Feedstock

Char

Feedstock

Char

Feedstock

Char

7.3 0.4 85.2 14.4 20.6 19.2 0.03 51.2 6.2 0.1 42.1 0.12 0.82 <0.01 0.001 0.967 0.093 <0.1 0.080

0.6 0.4 79.7 19.9 21.7 20.4 0.03 54.1 6.0 0.1 39.3 0.11 0.73 <0.01

7.0 3.9 73.5 23.5 21.0 19.7 n.a.h 53.0 5.8 0.4 36.9 0.11 0.70 0.02 0.044 3.39 4.9 <0.1 0.48

0.6 4.4 67.4 28.2 22.4 21.2 0.03 56.5 5.5 0.5 33.0 0.10 0.58 0.01

13 0.5 84.3 15.2 20.7 19.3 0.02 51.3 6.2 0.1 42.1 0.12 0.82 <0.01 0.006 3.38 36 362 1.6

1.2 1.0 55.5 42.5 24.4 23.4 0.03 63.9 4.5 0.3 30.3 0.07 0.47 <0.01

8.5 3.8 77.8 18.4 18.7 17.4 0.13 46.3 6.2 1.0 42.1 0.13 0.91 0.29 0.006 3.56 0.83 0.209 0.36

0.5 4.4 73.5 22.1 20.2 19.0 0.15 50.4 5.9 1.1 37.8 0.12 0.75 0.27

1.2 2.7 78.8 18.5 19.3 18.0 0.10 47.9 6.0 3.9 39.3 0.13 0.82 0.15 0.071 11.4 4.7 0.536 0.26

0.6 3.7 75.6 20.7 20.3 18.7 0.11 50.4 5.8 3.5 36.9 0.12 0.74 0.11

89 ± 2.2 6.5 ± 0.7 4.9 ± 1.9

Bar

88 ± 0.4 7.4 ± 1.0 4.1 ± 0.7

47 ± 4.3 28 ± 3.7 25 ± 1.5

85± 3.2 7.7 ± 2.0 7.2 ± 1.2

91 ± 2.5 2.8 ± 2.2 6.4 ± 1.4

Ash content: combustion at 550 ◦ C. HHV: high heating value. c LHV: low heating value. d oxygen calculated by difference,. e ar: sample as received. f ds: dry sample. g Estimated as the difference between the sum of all collected liquid and char and the amount of biomass initially inputted in the system, based on the assumption that all condensable matter was collected in the liquid fraction. h n.a.: not available. a

b

temperature could be the effect of exothermic processes [3,19]. However, our results of TGA/DSC analyses (Fig. S2 in SI) indicated that, there are no pronounced exothermic peaks during the torrefaction process. The thermal behavior of impregnated stemwood was likely due to the high content of metal-based preservatives in the impregnated stemwood sample, which may have altered the thermal degradation behavior. For example, chromium present in the impregnated wood could enhance thermal decomposition compared to untreated wood. The thermal behavior of impregnated stemwood could also be associated with reactive tars produced in the torrefaction process, which cannot directly escape from the char surface. This could result in disproportionation reactions exothermically and yield heavily carbonized substances and oxygenated species, and meanwhile release energy to the surrounding to initiate and/or maintain a series of reactions. Nevertheless, the exceptionally high torrefaction temperature in the impregnated stemwood experiments led to a much higher degree of thermal decomposition compared to the other feedstocks. As discussed later, this was reflected by the low yield of char obtained in the impregnated stemwood tests (Table 1). The exceptional behavior of the impregnated stemwood adds an interesting aspect regarding the formation characteristics of PCDDs and PCDFs in the thermal treatment of such materials. Nevertheless, the considerable differences in torrefaction temperatures should be kept in mind when comparing the results of impregnated wood with other assortments. 3.3. Yield of torrefied products and their characterization The yields of char (Table 1) varied between 85 and 91%, except for impregnated stemwood (47%). The char yields in our study are comparable with results reported for moderate torrefaction [2]. It is well known that the yield and chemical composition of torrefied

products varies with the degree of torrefaction (i.e., temperature and treatment time). For example, moderate torrefaction at about 250 ◦ C results in the degradation of hemicellulose and lignin, with cellulose only partly degraded, whereas severe torrefaction at temperatures of 260–350 ◦ C gives rise to the de-polymerization of cellulose along with lignin and hemicellulose, and thus lower yields of char [20]. The low yield of chars from impregnated stemwood (47%) is likely associated with the exothermic reactions taking place during torrefaction, as discussed above. Torrefaction increased the heating value by 3.8%–21.2% (Table 1). This result is consistent with the increase in carbon content in all the materials. The chlorine content remained low in stemwood and was reduced by 50% in bark, 27% in particle board and 7% in cassava. As expected, the ash content was generally higher in the char than in the feedstock (Table 1). 3.4. Yields of PCDDs vs. PCDFs The concentrations of PCDFs and PCDDs in the five feedstocks varied over 2 and 3 orders of magnitude, respectively (Fig. 3). Particle board contained the highest concentrations of PCDDs (4010 ng kg−1 ) and PCDFs (275 ng kg−1 ). The concentrations of PCDDs and PCDFs in the other four feedstocks were overall low ( <58 ng kg−1 ) and within the range reported for biomass from unpolluted regions [5]. The total concentrations of PCDDs in the torrefaction products (i.e., sum of PCDDs in the gas, liquid and char) were about 2–5 fold higher than those in the respective feedstock (Fig. 3 and Table S1). Torrefaction using particle board resulted in extensive formation of PCDDs (7200 ng kg−1 ) compared to the other four feedstocks (13–27 ng kg−1 ). In contrast, there was no noticeable formation of PCDFs in torrefaction, except for the impregnated wood, for which a dramatic increase of PCDFs by 44-fold was observed

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formation of PCDDs and PCDFs. This can explain the higher yields of PCDD/Fs in impregnated wood. 3.5. Volatilization behavior of PCDD/Fs Both PCDDs and PCDFs were predominantly found in the char, accounting for 76–96% and 39–74% of the total concentration of PCDDs and PCDFs in the torrefaction products, respectively (Fig. 3). The distribution of PCDDs and PCDFs in impregnated stemwood was different from the other four feedstocks. About 55% of the PCDFs was found in the gas phase, whereas PCDDs were relatively evenly distributed among the three phases. This is likely due to the lower yield of char in this experimental run, as mentioned above, and the higher temperatures in the process. Generally, chlorinated organics may form either homogeneously in the gas phase or via heterogeneous pathways on solid surfaces, such as a degraded char or ash matrix [21]. In reactions involving (chloro)phenol precursors, formation of chlorinated organics depends strongly on precursor concentrations and residence time under temperatures sufficient for PCDD and PCDF formation [22]. The operating temperature in torrefaction may be too low to allow PCDDs and PCDFs, especially the highly chlorinated congeners, to evaporate extensively from the char surface into the gas phase. In addition, the overall high yield of chars in comparison to gas will also influence the distribution of PCDDs and PCDFs among the different phases. 3.6. Influence of feedstock composition on homologue profiles

Fig. 3. Concentrations of PCDDs and PCDFs (ng kg−1 ) in products (stacked bars) and feedstock (non-filled columns). Error bars represent ± 1 SD of total concentrations (i.e., sum of gas, liquid and char). Note that the temperature during the treatment of impregnated stemwood was elevated (300–345 ◦ C) due to the exothermic process.

(368 ng kg−1 and 8 ng kg−1 in the torrefaction products and feedstock, respectively). A plausible explanation for the high yields of PCDDs from torrefaction of particle board is that the feedstock composition and specifically organic preservatives present in the feedstock provides favorable conditions for formation of PCDDs, as discussed in the following text. The high yield of PCDFs from torrefaction of impregnated wood could be at least partly associated with the exceptionally high temperature during treatment, which could enhance the thermal decomposition of the feedstock, as well as the generation of PCDFs. Moreover, the presence of organic preservatives in the feedstock could influence PCDF formation as well. For example, PCDFs can form de novo from PAH structures present in the organic preservatives (i.e., creosote oil), which can be reflected in the low PCDD/PCDF ratio (0.1 in char from impregnated stemwood) as shown in Table S1. Although the chlorine content of impregnated wood is similar to that in stemwood, the higher contents of Cu and Fe in impregnated wood may catalytically promote

The chars from stemwood, bark and cassava stems exhibited similar homologue profiles (Fig. 4). The relative abundance of the PCDF homologues decreased with increasing degree of chlorination, which was similar to the homologue profiles recorded for the feedstocks (Fig. S3 in SI). On the other hand, PCDD profiles in chars were generally dominated by the highly chlorinated homologues. In particular, the PCDD homologue profile in torrefied particle board was exclusively dominated by hexa- to octa-CDD, accounting for 99% of the total PCDDs. This may be related to the high abundance of OCDD in the initial feedstock (Fig. S3 in SI): the PCDD profile in the particle board feedstock was dominated by OCDD and to a lesser extent HpCDD. This profile was very similar to the previously reported profile of pentachlorophenol (PCP) preservative for wood treatment [23]. It is known that PCP preservatives normally contain high amounts of PCDDs, especially OCDD and HpCDD as impurities [24]. To investigate possible PCP contaminant in the feedstock further, the concentrations of polychlorinated phenols (PCPhs) in both untreated and torrefied particle board were determined. The results (Fig. S4 in SI) showed that the total concentration of PCPhs (mono- to penta-) in the feedstock was high (593 ␮g kg−1 ). The PCPh profile in the particle board feedstock was dominated by penta-CPh, accounting for 83% of the total PCPh concentration, which provided supporting evidence for the presence of PCP preservatives in the particle board. The presence of PCP in particle board can explain the high yield of PCDDs from torrefaction of this feedstock because of the role of PCP as PCDD precursor. As expected, less chlorinated compounds showed a higher tendency to volatilize into the gas phase, whereas highly chlorinated compounds were largely retained in the chars (Fig. 5), which was clearly observed during the torrefaction of bark, cassava stems and particle board. However, this behavior was less clear for stemwood and impregnated stemwood (Fig. S5 in SI) owing to the very low concentrations (close or below detection limit) of some homologues (i.e., TriCDF and TeCDF in the gas product from stemwood). The differences in homologue profiles of PCDDs and PCDFs in the different phases could be due to differences in the vapor pressures between the low and high chlorinated congeners. For example, less chlorinated compounds with high vapor pressure have a higher

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Fig. 4. Homologue profiles of PCDFs (upper) and PCDDs (lower) in chars. Error bars represent ± 1 SD (n = 1). Note that the temperature during treatment of impregnated stemwood was elevated (300–345 ◦ C) due to the exothermic process.

tendency to evaporate into the gas phase. Moreover, the low temperatures used in torrefaction might be insufficient for the highly chlorinated compounds to volatilize to a large extent. 3.7. Mechanistic aspects in PCDD and PCDF formation Two main mechanisms have been proposed for PCDD and PCDF formation in the low temperature range: the precursor pathway and the so-called de novo synthesis from a carbonaceous matrix [7]. It has been reported that PCDD formation mostly results from the precursor pathway with only limited contribution from de novo synthesis [25]. The de novo process, on the other hand, generally yields mainly PCDFs [26,27]. Therefore, a PCDD/PCDF ratio of >1 is normally considered an indication of precursor-dominated formation [28]. The low PCDD/PCDF ratio in char from impregnated wood (Table S1) indicated that de novo synthesis could be one of the important pathways for PCDF formation. Similarly, the high PCDD/PCDF ratio in char from particle board (Table S1) provides a clear indication of precursor-based reactions taking place in torrefaction of this feedstock. It has been shown that thermal degradation of biomass at low temperatures results in the formation of monomeric phenols along with other low-molecular-weight compounds (i.e., organic acids), which is a competitive mechanism involving de-polymerization and condensation/carbonization reactions [6]. Chain de-polymerization is expected via successive formation of a new phenolic structure through cleavage of the phenolic end structure. Based on our observations of the substantial formation of PCDDs (compared to PCDFs), in combination with the presence of phenolic structures from the thermal degradation of the biomass and the possible formation mechanisms mentioned

above, it is highly likely that during the torrefaction process, phenolic PCDD precursors are formed even under a nitrogen atmosphere. A similar result was obtained in an experimental study conducted under pyrolytic conditions [29], which showed that metal catalysts were important for the reaction under a nitrogen atmosphere. Formation of PCDD and PCDFs from pyrolysis of biomass may also be mediated by the presence of OH functional groups in phenolic biomass entities as illustrated by Altarawneh and Dlugogorski [30]. Additionally, it has been reported that the high load of nitrogen in biomass can lead to formation of nitrogenated dioxin-like species such as carbazole, phenoxazine and phenazine [31]. The potential for the formation of these compounds from torrefaction of biomass requires further investigation. The similar PCDD homologue profiles in both the particle board feedstock and char generated during torrefaction indicates that the chlorinated compounds found in the torrefaction products could be the result not only of formation but also of physical transformation from the feedstock, via volatilization followed by re-condensation and/or adsorption. Alternatively, PCDDs may have been formed during torrefaction, as indicated by the increased concentrations of PCDDs in the total torrefaction products (sum of gas, liquid and char) compared to that in the feedstock. It is plausible that (chloro)phenols in the feedstock would act as precursors for PCDD formation under these conditions, and it has been reported that PCDDs (mainly OCDD) can be readily formed under the pyrolysis of PCP [32]. Moreover, it was evident from our results that dechlorination and/or degradation of highly chlorinated compounds could occur during torrefaction. For example, the relative percentage of OCDD in total PCDDs in the particle board feedstock was higher than those found in the products from torrefaction of this

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Table 2 WHO-TEQ values in torrefaction products and feedstocks.

Stemwood Bark Impregnated wood Cassava stems Particle board a

TEQa value (ng WHO-TEQ kgFUEL −1 )

Relative% of TEQ

Feedstock

Torrefaction

Gas

Liquid

Char

0.012 0.059 0.012 0.063 4.5

0.17 0.23 0.16 0.21 41

12.6 13.0 54.6 15.0 3.10

1.89 2.73 25.0 4.65 2.09

85.5 84.3 20.5 80.4 94.8

TEQs were calculated using WHO-2005 toxic equivalency factors.

has been found in combustion, chlorine content is not the only factor governing the formation of chlorinated organics [33]. Instead, the presence of transition metals, as well as organic preservatives in feedstocks, seems to play an important role in the generation of PCDDs and PCDFs [34,35]. 3.8. Toxicity equivalents (TEQs) value

Fig. 5. Relative distribution (average of triplicates) of each homologue in gas, liquid and solid products of bark, cassava stems and particleboard.

material, whereas the relative abundances of HxCDD and HpCDD in the torrefied particle board were higher than that in the feedstock. Interestingly, the PCDF homologue profile of char from impregnated stemwood was exclusively dominated by MoCDF, accounting for 94–99% of the total PCDFs (Fig. 4). The remarkable increase of PCDFs during torrefaction of impregnated stemwood compared to the concentrations in the feedstock (Fig. 3) indicates that the MoCDF observed in the products was the result of formation during torrefaction. However, it remains unknown whether or not the formation took place via precursor pathways. The high PCDF concentrations are likely the result of de novo synthesis from PAH structures in organic preservatives as discussed above. Nevertheless, additional chlorination of MoCDF to form highly chlorinated compounds could be restricted by either the reactivity of chlorine species in feedstocks or the torrefaction conditions (oxygen deficiency or low temperature) [7]. It is noteworthy that, although cassava stems contained significantly high Cl content (0.29%) compared to the other four feedstocks (up to 0.15%), we did not observe noticeable formation of PCDDs or PCDFs during torrefaction of this material. Torrefaction of impregnated stemwood and particle board with much lower Cl content (0.010% in impregnated stemwood and 0.15% in particle board) gave rise to higher formation of PCDFs and PCDDs than for cassava stems. This suggests that, similar to what

TEQs were calculated using the individual congener concentrations of PCDDs and PCDFs and WHO-2005 toxic equivalency factors [36]. The results were expressed as ng of WHO-TEQ per kg of input feedstock. Total TEQ (sum of gas, liquid and chars) in the various torrefaction products varied over two orders of magnitude (Table 2). The highest TEQ concentration was obtained for the torrefaction of particle board (41 ng TEQ kgFUEL −1 ). Torrefaction of the other four assortments resulted in much lower TEQs, ranging from 0.16 to 0.23 ng TEQ kgFUEL −1 . A similar result has been reported in a previous combustion study [37], where the TEQ from wood containing bonding agent and preservatives was significantly high in comparison with uncontaminated wood. The TEQ values of the char fractions accounted for 80.4–94.8% of the total TEQ, except for the torrefaction of impregnated wood, for which only 20.5% of the total TEQ was found for char, which is consistent with the low char yields. The obtained TEQ concentrations from torrefaction of virgin, or non-contaminated, biomass feedstocks are substantially lower than the maximum allowed thresholds (17 ng TEQ kg−1 ) established for biochar in the International Biochar Initiative guidelines [38]. Moreover, the TEQ concentrations generated in our torrefaction experiments were in general low compared to those reported from biomass combustion (7.3–22.8 ng TEQ kg−1 ) [5], and those from slow pyrolysis using waste materials [39]. In addition to the role of feedstock composition (mainly with regard to content of transition metals and chlorine) as discussed above, the oxygendeficient conditions during torrefaction could contribute to the limited formation of PCDDs and PCDFs. For example, lack of oxygen in torrefaction could inhibit the yield of chlorination agents generated via the Deacon reaction. 4. Conclusion The high variation of PCDD and PCDF concentrations observed in torrefaction using five different types of feedstocks demonstrates the influence of the biomass chemical composition on the formation of these compounds in torrefaction. The origin of the PCDDs and PCDFs found in the torrefaction products could be the result not only of formation but also of physical transformation from the feedstocks via volatilization followed by re-condensation and/or adsorption. As indicated by the relatively low PCDD and PCDF concentrations measured in the torrefaction of cassava stems, not only chlorine but also the presence of transition metals and organic contaminants in the feedstocks probably influence the formation of PCDDs and PCDFs. Further, chlorinated organics used as wood preservatives could act as precursors during thermal treatment.

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Metals used as preservatives may promote oxidation of wood char, providing the required temperature for PCDD and PCDF formation. Existing regulations currently restrict the uncontrolled burning of waste wood for energy supply, in particular those containing various types of contaminants, such as CCA and PCP. Therefore, in the torrefaction of waste wood containing metal-based preservatives, caution should be excised to control the processing parameters. Acknowledgement The authors would like to acknowledge Bio4Energy (www. bio4energy.se), a strategic research environment appointed by the Swedish government, for supporting this work. Part of the study was financed by grants from the Ångpanneföreningen’s Foundation for Research and Development and the J. Gust. Richert Memorial Fund. Torrefaction experiments were performed at the Biomass Technology Centre, SLU, Umeå, Sweden. We thank Per Liljelind for his assistance with the GC/MS analyses, Shaojun Xiong for providing the cassava stem sample and valuable information on cassava characteristics, and Mariusz Cieplik for valuable discussions. References [1] A. Nordin, L. Pommer, M. Nordwaeger, I. Olofsson, Biomass conversion through torrefaction, chapter 7, in: Technologies for Converting Biomass to Useful Energy, CRC Press, 2013. [2] M.J.C. Van der Stelt, H. Gerhauser, J.H.A. Kiel, K.J. Ptasinski, Biomass upgrading by torrefaction for the production of biofuels: a review, Biomass Bioenerg. 35 (2011) 3748–3762. [3] W.-H. Chen, J. Peng, X.T. Bi, A state-of-the-art review of biomass torrefaction, densification and applications, Renew. Sust. Energ. Rev. 44 (2015) 847–866. [4] D. Hoffmann, M. Weih, Limitations and improvement of the potential utilisation of woody biomass for energy derived from short rotation woody crops in Sweden and Germany, Biomass Bioenerg. 28 (2005) 267–279. [5] E.D. Lavric, A.A. Konnov, J. De Ruyck, Dioxin levels in wood combustion – a review, Biomass Bioenerg. 26 (2004) 115–145. [6] G.-P. Manuel, J. Metcalf, The formation of polyaromatic hydrocarbons and dioxins during pyrolysis, in: A Review of the Literature with Descriptions of Biomass Composition, Fast Pyrolysis Technologies and Thermochemical Reactions, Washington State University, Pullman, 2008. [7] M. Altarawneh, B.Z. Dlugogorski, E.M. Kennedy, J.C. Mackie, Mechanisms for formation chlorination, dechlorination and destruction of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), Prog. Energ. Combust. 35 (2009) 245–274. [8] J.G.P. Born, P. Mulder, R. Louw, Fly ash mediated reactions of phenol and monochlorophenols: oxychlorination, deep oxidation, and condensation, Environ. Sci. Technol. 27 (1993) 1849–1863. [9] L. Stieglitz, Selected topics on the de novo synthesis of PCDD/PCDF on fly ash, Environ. Eng. Sci. 15 (1998) 5–18. [10] C. Procaccini, J.W. Bozzelli, J.P. Longwell, A.F. Sarofim, K.A. Smith, Formation of chlorinated aromatics by reactions of Cl. Cl2, and HCl with benzene in the cool-down zone of a combustor, Environ. Sci. Technol. 37 (2003) 1684–1689. [11] R. Addink, E.R. Altwicker, Role of copper compounds in the de novo synthesis of polychlorinated dibenzo-p-dioxins/dibenzofurans, Environ. Eng. Sci. 15 (1998) 19–27. [12] Q. Gao, P. Haglund, L. Pommer, S. Jansson, Evaluation of solvent for pressurized liquid extraction of PCDD, PCDF PCN, PCBz, PCPh and PAH in torrefied woody biomass, Fuel 154 (2015) 52–58. [13] Q. Gao, V.L. Budarin, M. Cieplik, M. Gronnow, S. Jansson, PCDDs PCDFs and PCNs in products of microwave-assisted pyrolysis of woody biomass–distribution among solid, liquid and gaseous phases and effects of material composition, Chemosphere 145 (2016) 193–199. [14] P. Liljelind, G. Söderström, B. Hedman, S. Karlsson, L. Lundin, S. Marklund, Method for multiresidue determination of halogenated aromatics and PAHs in combustion-related samples, Environ. Sci. Technol. 37 (2003) 3680–3686.

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