Off-Gassing of VOCs and Permanent Gases during Storage of

Aug 25, 2017 - Thermal treatment for upgrading of low-value feedstocks to improve fuel properties has gained large industrial interest in recent years...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/EF

Off-Gassing of VOCs and Permanent Gases during Storage of Torrefied and Steam Exploded Wood Eleonora Borén,†,‡ Fahimeh Yazdanpanah,§ Roger Lindahl,∥ Christoph Schilling,§ Richard P. Chandra,⊥ Bahman Ghiasi,§ Yong Tang,§ Shahabaddine Sokhansanj,§ Markus Broström,*,† and Sylvia H. Larsson# †

Thermochemical Energy Conversion Laboratory (TEC-Lab), Department of Applied Physics and Electronics, Umeå University, SE-901 87 Umeå, Sweden ‡ Umeå University Industrial Doctoral School for Research and Innovation, SE-901 87 Umeå, Sweden § Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada ∥ Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden ⊥ Department of Wood Science, University of British Columbia, 2424 Main Mall, Vancouver, British Columbia V6T1Z4, Canada # Biomass Technology Centre, Department of Forest Biomaterials and Technology, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden S Supporting Information *

ABSTRACT: Thermal treatment for upgrading of low-value feedstocks to improve fuel properties has gained large industrial interest in recent years. From a storage and transport perspective, hazardous off-gassing could be expected to decrease through the degradation of reactive biomass components. However, thermal treatment could also shift chemical compositions of volatile organic components, VOCs. While technologies are approaching commercialization, off-gassing behavior of the products, especially in terms of VOCs, is still unknown. In the present study, we measured off-gassing of VOCs together with CO, CO2, CH4, and O2 depletion from torrefied and steam exploded softwood during closed storage. The storage temperature, head space gas (air and N2), and storage time were varied. VOCs were monitored with a newly developed protocol based on active sampling with Tenax TA absorbent analyzed by thermal desorption-GC/MS. High VOC levels were found for both untreated and steam exploded softwood, but with a complete shift in composition from terpenes dominating the storage gas for untreated wood samples to an abundance of furfural in the headspace of steam exploded wood. Torrefied material emitted low levels of VOCs. By using multivariate statistics, it was shown that for both treatment methods and within the ranges tested, VOC off-gassing was affected first by the storage temperature and second by increasing treatment severity. Both steam exploded and torrefied biomass formed lower levels of CO than the reference biomass, but steam explosion caused a more severe O2 depletion.

1. INTRODUCTION Safety in transport and storage is an increasingly important issue in successful implementation of a biobased economy. Large-scale biomass utilization has raised concerns about issues such as self-heating, excessive dust formation, dust explosions, and hazardous off-gassing.1−6 Off-gassing from wood products reached public awareness in the early years of the 21st century when a number of fatal accidents occurred due to CO poisoning or suffocation upon discharge of concealed wood pellet cargo from trans-Atlantic ocean vessels.7,8 More recently, awareness has been raised again about toxic off-gassing, but now related to the rapidly growing number of smaller pellet storage spaces.2,9−11 Most studies have focused on hazardous CO formation and O2 depletion, but several also concern offgassing of volatile organic components (VOCs) in and around wood storage areas.12−21 Several thermal treatment methods are being developed to improve key fuel properties of biomass such as heating value, grindability, moisture uptake, and flow properties.22−25 The possibility of using thermally treated biomass as a carbonneutral substitute to fossil fuels has gained large industrial interest, and technologies such as torrefaction, steam explosion, and hydrothermal carbonization are approaching commercial© XXXX American Chemical Society

ization. End-product properties differ depending on how the treatments decompose and rearrange cell wall components. Thermally labile hemicelluloses are often completely degraded, whereas dependent on technology and treatment severity, cellulose may be partially degraded or structurally rearranged. In addition, thermally labile lignin degrades while more thermally resistant parts rearrange through dehydration, demethoxylation, and depolymerization reactions.26,27 The processes and influences of operational parameters on the end-products are well-known today, but there is little experience from large scale handling of these new energy commodities, and their off-gassing behaviors are to a large extent unknown. The underlying mechanisms for formation of permanent gases from untreated biomass are not fully understood, but in response to the fatal port incidents, a number of studies were undertaken to understand the parameters causing the excessive off-gassing. CO, CO2, CH4, and H2 have been shown to correlate positively to storage temperature and time for wood Received: July 10, 2017 Revised: August 22, 2017 Published: August 25, 2017 A

DOI: 10.1021/acs.energyfuels.7b01959 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Process Settings for Production of Thermally Treated Wood

reference steam explosion torrefaction a

name

temperature

residence time

pressure

moisture content

mass yield

severity factor

unit

°C

min

kPa

%

%

S0a

ref SE-195 °C SE-215 °C Torr-260 °C Torr-290 °C

195 215 260 290

5 5 20 20

1310 2020 atm atm

7 58 63 0.7 1

3.5 4.1 0.84 0.71

S0 = log [exp ((T − 100)/14.75)t], where T is treatment temperature [°C], and t is the residence time [min].

pellets.28−31 Storage temperature was shown to have the largest impact, followed by changes in relative humidity and head space volume. Emission rates and amounts have been found to increase more for CO2 than for CO at higher temperatures.29 CO2 has been found to increase for stored pellets with a moisture content (MC) of 4−15% at 25 °C but to remain unaffected at 35−50%. Above 15% MC, CO2 levels remained rather unaffected by increasing storage temperature (up to 60 °C).32 CO formation has been reported to increase over time with increasing temperature; if pellets had ≥15% MC, CO levels remained unaffected or decreased by increasing temperature.32,10 However, deviating CO levels for high humidity stored pellets has been reported, both higher CO levels10 and unaffected levels,11 highlighting the complexity of the underlying mechanisms. CO formation was recently suggested to result from oxidation reactions of unsaturated fatty acids and terpenes forming hydroxyl radicals, which in turn form CO when reacting with hemicellulose.33 As hemicellulose in both steam exploded and torrefied biomass is significantly decomposed already at low treatment severity, CO formation could be reduced by thermal treatment. Accordingly, steam explosion has been shown to reduce CO off-gassing from 6000 to 1000 ppm for pellets,34 and about 30−50% reduction has been found for torrefied wood chips.28,35 Moreover, CO2 off-gassing from torrefied woodchips has been found to be reduced by about 15 and 20% when compared to that of untreated chips when stored at 20 and 40 °C, respectively.35 Even though there is a consensus that thermal treatment reduces off-gassing levels of permanent gases, levels are still concerning and parameter influences and reaction mechanisms are to a great extent unknown. The main VOC groups emitted from untreated lignocellulosic biomass are terpenes13,36 and aldehydes.14 VOC offgassing is dependent on feedstock, drying temperature, drying medium, and moisture content.12,37 Monoterpene degradation has been found to be similar for air-dried and steam-dried pine wood chips, but sesquiterpenes were more degraded during airdrying.12 Aldehydes are known to be formed through oxidation of unsaturated fatty acids and resins17,19 and increase with drying temperature17 and storage temperature, but also with relative humidity.19 Exposure to biomass related VOCs is normally not acutely toxic, but certain VOCs have been recognized as a source of malodor in pellet storage.16,38,39 However, there are certain terpenes and aldehydes which cause symptoms ranging from irritation to eyes, skin, and mucous membranes to chronically reduced lung function upon human exposure.14,15 Therefore, several VOCs are subject to exposure regulations. VOC offgassing from thermally treated lignocellulosic biomass is expected to be different from that of untreated, as low boiling terpenes and aldehydes will degrade, and new VOCs derived

from degrading cell wall carbohydrates and lignin could form by the thermal treatment. In a study on steam-drying of pine for wood preservation purposes (i.e. a very mild thermal treatment), it was found that only 14 out of 41 originally identified VOCs were found to be common for treated and untreated wood, and the VOCs profile of treated wood was dominated by furfural and acetic acid.36 In contrast to VOCs traditionally associated with lignocellulosic biomass, furfural is a potentially problematic VOC emitting from thermally treated materials, being both acutely toxic upon exposure and a potential carcinogen.40,41 Furfural is a degradation product of pentose based hemicellulose polysaccharides and uronic acids42,43 but can also originate from cellulose as a result of levoglucosan degradation. 44 Both steam explosion and torrefaction are performed in a severity range that largely decomposes the hemicellulose structure of lignocellulosic biomass, and it structurally rearranges cellulose,27,45−47 potentially rendering high levels of off-gassing furfural. Thermal treatment also partially degrades the lignin fraction,27,48 and thus aromatic decomposition products could also be expected among the VOC emissions. Amounts and chemical composition of VOCs derived from degraded cell wall components will probably be affected not only by process parameters but also by how process gases are treated, if they are allowed to recondense, and how cooling and postdrying steps are designed. Currently, there is no published study on VOC off-gassing from either torrefied or steam exploded materials. Therefore, the objectives of this study were to (i) evaluate VOC off-gassing profiles and permanent gas emissions during storage of biomass thermally treated by torrefaction or steam explosion and (ii) compare off-gassing profiles with those of untreated biomass. To enable VOC measurements within a molecular size range encompassing terpenes, aldehydes, and furans, a Tenax TA absorbent (C6− C26)-based protocol for active sampling was developed. Absorbed VOCs were analyzed by thermal desorption−GC/ MS. Permanent gases were monitored by TCD and GC-FID. Off-gassing measurements were performed on softwood chips thermally treated by torrefaction and steam explosion, and with varying treatment severity, storage temperature, and storage time. In addition, the effect of purging the storage container with inert nitrogen gas was evaluated in complementary tests.

2. MATERIALS AND METHODS 2.1. Material. A mixture of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) wood chips from a saw mill in Northern Sweden, dried to 8% moisture content, was shredded in a single shaft shredder (Lindner Micromat 2000, Lindner-Recyclingtech GmbH, Spittal, Austria) with a 15 mm screen size. In total, 12 kg of wood chips was used. Raw material was stored in sealed plastic bags prior to thermal treatment and subsequent off-gassing studies. B

DOI: 10.1021/acs.energyfuels.7b01959 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 2. Experimental Matrix of Off-Gassing Analyses gas measurement

material

weight (gwb)

headspace gas (air/N2)

temperaure (°C)

time

permanent gases

ref SE-195° SE-215° Torr-260° Torr-290° ref

86.7 190 216 80.8 80.9 30.2

SE-195°

71.0

SE-215°

81.0

Torr-260°

30.3

Torr-290°

30.3

air air air air air air N2 air N2 air N2 air N2 air N2

20/40 20/40 20/40 20/40 20/40 20/40 20 20/40 20 20/40 20 20/40 20 20/40 20

56 days 56 days 56 days 56 days 56 days 112 h 112 h 112 h 112 h 112 h 112 h 112 h 112 h 112 h 112 h

VOC

a

replicatesa (no.) 2 2 2 2 2 3 2 3 2 3 2 3 2 3 2

per per per per per per

temp. temp. temp. temp. temp. temp.

per temp. per temp. per temp. per temp.

Replicates for permanent gases where measured on every second measurement. Replicates for VOCs are technical replicates.

2.2. Thermal Treatments. The wood was thermally treated by two different methods: torrefaction or steam explosion. Both treatments were performed in bench scale reactors at two temperature settings each (Table 1) to produce materials with ranges of fuel properties assumed relevant for the future fuel market. Steam explosion was performed in 350−400 g batches in a 2 L Stake Tech II batch steam gun (constructed by Stake Tech-Norvall, Ontario, Canada). At the end of the steam treatment, steam exploded material was released into a collection vessel at atmospheric pressure. Torrefaction was performed in a bench scale setup described in detail elsewhere.49 Wood chips were evenly spread out on three level trays and torrefied in an oven (Carbolitte). Nitrogen (25 L/min) was continuously fed to provide an inert atmosphere limiting oxidation reactions. The nitrogen purged fuel layers were heated at 10 °C/min to setpoint temperature, kept isothermally for 20 min, and then cooled. For the 260° setpoint, material from one batch was used, while for the 290 °C setpoint, two combined batches were used (Table 1). 2.3. Off-Gassing Measurements. All batches of steam exploded material were collected in a ventilated funnel (in total stored for up to 30 min) during production and thereafter placed in closed containers on ice until cooled to 15 °C. Torrefied material was collected from the oven after the batch was cooled and directly placed in closed containers on ice, cooled to the same temperature. Material moisture content (Table 1) was determined with a moisture analyzer (MF-50, A&D, Japan), and bottles were filled with a 30 g dry basis (d.b.) for VOC and 80 gd.b. for permanent gas analysis (Table 2). For each thermal treatment set-point, the bottling order of samples for permanent gases and VOCs was randomized to reduce any impact of filling order. Untreated reference biomass samples were weighed and placed in sample containers at room temperature. Permanent gas analyses were performed in 1 L glass containers fitted with tight sampling ports in the lids. VOC analyses were performed in 1 L glass bottles (DURAN group, VWR) fitted with a double mouthed glass tube (DURAN group, VWR) with a double screw connection. One tube reached the bottom of the sample for the purge gas inlet (10 mm i.d.), and the one with a larger mouth opening (24 mm i.d.) ended near the top of the headspace for gas sampling. Sampling bottles were stored at either room temperature (app. 20 °C) or in a heating oven (at 40 °C). A complementary set of samples at each set-point was used for a VOC reducing test where sample bottles were first left to stabilize at 40 °C for 24 h, and thereafter the head space was completely ventilated and purged with inert gas, N2. The bottles were thereafter stored at room temperature for another 24 h prior to sampling. Permanent gas composition (CO, CO2, CH4, H2, and O2) was monitored through gas sampling and analysis using a GC. For each sampling, 20 mL of gas sample was collected using an airtight syringe (25 mL SGE Gas-Tight Syringe, Luer-Lock and TOGAS Luer Lock

Adapter, Mandel Scientific Company). The syringe had a Luer lock device to help in collecting a known quantity of gas sample. The Shimadzu GC needle used with the 20 mL SGE syringe was a Togas Loop Fill Interface N711 Needle (Model number: 220-90615-00, Mandel Scientific Inc.). The composition of the sampled gas was analyzed by a Shimadzu GC-14B (Shimadzu Corporation, Japan), equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID). The GC had three packed columns in series: Porapak-N (80/100 mesh, 3m), Porapak-Q (80/100 mesh, 3m), and an MS-5A (60/80, 2.25m). For CO, CO2, and CH4, the FID detector was used, and TCD was used for N2, O2, and H2. The GC was calibrated regularly with standard calibration gases. Argon gas provided the carrier and reference gas for the TCD. Compressed air and argon were used as reference and carrier gases for the FID. For the reference material, three replicates were made, and double samplings were done on days 51 and 56 for the samples. VOCs were analyzed with ATD 90 mm × 5.0 mm (i.d.) stainlesssteel tubes (PerkinElmer Inc., USA) filled with Tenax TA absorbent (60−80 mesh). Tenax sampling tubes were conditioned in a tube conditioner (dry purge model TC-20, Markes) at 320 °C for 2 h in a nitrogen flow (70 mL/min) and stored at room temperature, sealed with gastight fittings equipped with inserts of PTFE (Swagelok Co., USA). A Tenax sampling tube was connected with a 4 mm i.d. silicon tube to the sampling port of the VOC sampling bottle. Active sampling was performed at a gas flow rate of 100 mL/min for 1 min by using a suction pump (D5 SE, Charles Austen, UK) in conjunction with a mass flow controller (GSC-A4SA-BB22, Vögtlin, Switzerland) to control sample gas flow through the Tenax sampling tube. Sampling was performed after 48 h and 112 h, and in the same order as the filling order of the bottles. The sampled tubes were kept at room temperature until analysis. The Tenax sampling tubes were analyzed by thermal desorption in an ATD autosampler injector (TurboMatrix 350 ATD, PerkinElmer), and vapors were analyzed in a GC/MS (7890A/5975C MSD, Agilent Technologies). The Tenax sampling tubes were desorbed at 280 °C for 6 min using He as a carrier gas. Primary desorbed vapors (inlet split: 19 mL/min) were collected on a cold trap filled with Tenax TA (M041-3535, PerkinElmer, CT, USA) with both ends sealed with silanized glass wool (Supelco). The cold trap was kept at −29 °C during absorbent collection and heated to 300 °C for 6 min for desorption. The desorption split was 30 mL/min and vapors passed through the heated (200 °C) valve system with an outlet split flow of 120 mL/min, with a 1 mL/min column flow, resulting in a final sample split to 1% passed onto the GC. Vapors were passed through the heated transfer line (200 °C) to the GC column (DB-5 ms, 30 m × 0.25 mm i.d., with film thickness 0.25 μm, J&W, Agilent, Sweden) and separated with a GC program: (i) 6 min hold time at 30 °C, (ii) 20 °C/min heating ramp to 300 °C, and (iii) 6 min hold time. The MS C

DOI: 10.1021/acs.energyfuels.7b01959 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 1. Heat map of VOCs sampled from the head space gas during enclosed storage of untreated (ref), steam exploded, and torrefied wood chips. Data are scaled to unit variance (0−1) and colored by intensity, red (low) to yellow (high). Brighter areas mean higher relative concentrations (average from one to three replicates) in the off-gas. Unk (unknowns) are marked by their base peak mass. If there is more than one Unk of the same base peak mass, they are separated by an increasing number stated after the base peak mass. transfer line was kept at 280 °C, MS Quad at 150 °C, electron impact ionization at 70 eV, and MS ion source at 230 °C, while scanning from 17 to 350 amu. External standards were prepared from furfural (Fluka), guaiacol (Sigma-Aldrich), and α-pinene (Sigma-Aldrich) with methanol as solvent. Standards were spiked on Tenax sampling tubes by placing the

tubes in an in-house built spiking unit where tubes were connected to a controlled flow of N2 (90 mL/min) and the standard solution injected with a syringe through a septum in the nitrogen flowdirection, and then left to absorb for 5 min. To confirm that no breakthrough of the standard compound had occurred, extra absorbent tubes were connected to some of the standard tubes. The absorbent D

DOI: 10.1021/acs.energyfuels.7b01959 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels tubes spiked with standard solutions were run as described earlier, apart from a 5 min delay and a higher scan range (40−350 amu) to avoid the solvent peak. Linear equations were achieved for furfural (R2: 0.93) and α-pinene (R2: 0.96). However, standard amounts were prepared between 0.1 and 20 μg, while the experimental results for furfural and α-pinene were higher, i.e., extrapolating the standard calibration. Peak recognition was done by the software Automatic Mass Spectral Deconvolution and Identification, AMDIS version 2.7, and when possible, compounds were manually identified in NIST (MS Search 2.0) using the NIST main libraries. To identify products specific for thermal decomposition, pyrolysis specific compound libraries for lignin50 and polysaccharides51 were used. The raw data chromatograms were converted to NetCDF format. Alignment and peak integration of the chromatograms were performed with in-house software (Swedish Metabolomics Centre (SMC) RDA (v.3.99)). Peaks were integrated by their base peak mass. Identification scores for identified compounds are seen in Supporting Information Table S1, while peaks not possible to identify were assigned as “Unknowns” followed by their base peak number, and in the cases with more than one of the same base peak, an increasing serial number. Compounds that were found as double peaks, or which had too low weight to be accurately quantified in the Tenax TA absorbent, were not integrated but left assigned as “Identified.” To detect any breakthrough of VOCs during sampling, two Tenax sampling tubes were connected in series for at least one replicate per set-point. For α-pinene, the breakthrough for several set-points was >10%, and therefore peak areas from both tubes were added together. However, since not all samples were analyzed with a control tube (one per set-point), α-pinene values were slightly underestimated. α-Pinene is known to have a low breakthrough volume on Tenax TA as compared to other terpenes,52 but extracting a smaller sample volume would have compromised analysis with other low peak VOCs. Possible contamination, carry-over, and whether compounds had too much breakthrough to be accurately quantified were checked by comparing results from the first and second series-connected Tenax sampling tubes, as well as from blank runs. Data were evaluated using a software for principal component analysis (SIMCA v.13.0.3.0, Umetrics, Sweden). For the VOC data, a heatmap overview was generated by multivariate statstical analysis where data were scaled to unit variance (UV) and assigned a value of 0−1 to give each compound equal weight (SIMCA v.13.0.3.0, Umetrics, Sweden). The heatmap was generated using heatmap.2() (R i386 3.3.1), with the variables clustered in a dendrogram by hclust() (R i386 3.3.1) using complete linkage hierarchical clustering to give the maximum distance between two clusters. Integrated compounds were analyzed as variables by principal component analysis (PCA) with all available replicates for each set-point as observations.

profiles for each treatment and comparing them to the untreated reference. 3.1. Volatile Organic Compounds Overview. VOCs were visualized in a heat map which revealed completely different VOC profiles in terms of abundance and compositions for the different thermal treatments and storage conditions (Figure 1). A large number of VOCs were found in the headspace of both the reference (Figure 1, block A) and the steam exploded wood (Figure 1, block B), whereas torrefied wood emitted considerably fewer VOCs, all at low levels (Figure 1, block C). Samples stored at 40 °C generally emitted higher levels of VOCs than those stored at 20 °C, especially illustrated by the two bright yellow lines of the reference samples stored at 40 °C. For all materials, VOC levels were higher when sampled at 48 h than again at 112 h. Although VOC levels were high for both steam exploded and reference wood, the heat map showed differences in chemical composition. For reference samples, α-pinene was the most dominant VOC, whereas steam exploded materials emitted mainly furfural. Therefore, in order to visualize also the trends of low level VOCs, data were scaled to unit variance. The scaling requires colors to be read vertically for correct interpretation, and the scheme represents a within sample (vertical) relative concentration, and not an absolute inbetween samples (horizontal) comparable concentration. By so doing, VOCs could also be hierarchically clustered in a dendrogram that revealed distance between VOCs based on the relative changes within the samples rather than by their integrated peak area. The dendrogram clusters the VOCs in two main legs (Figure 1, leg I, II). VOCs in leg I were emitted at higher levels from the reference samples and were mainly composed of terpenes (e.g., α-pinene, β-pinene, Δ3-carene, and camphene) and aldehydes (e.g., hexanal, pentanal, and octanal), Figure 1A. Instead, VOCs in leg II were emitted at higher levels from steam exploded wood and mainly comprised of furans (e.g., furfural, 5-methylfurfural, and 2-methylfuran). Although VOC composition was clearly shifted between reference and steam exploded wood, there was an overlap of emitted VOCs, displayed in a few cases: those emitted at higher levels from the reference, but also to some extent from the steam exploded wood, e.g., α- and γ-muurolene (Figure 1, block D), and those emitted predominantly from the steam exploded wood but to some extent also from the reference, e.g., γ-terpinene (Figure 1, block E−G). Furthermore, increasing steam explosion severity was found to lower the levels of terpenes (comparing SE-195 °C and SE-215 °C in Figure 1D), and furan levels were found to increase (Figure 1E,F). As stated above, VOC levels were generally higher at 40 °C than 20 °C, but some VOCs showed a higher relative increase. The less volatile but temperature sensitive VOCs clustered in leg III of the dendrogram; i.e., they were lower (more red) for the 20 °C stored samples and markedly higher (yellow) at 40 °C. Those in leg IV were emitted at high levels (yellow) already at a 20 °C storage temperature. Also included in the heat map were a number of nonidentified VOCs (integratable peaks of unknown identity, marked as Unknowns (Unk)). Even though their identities were unknown, their clustering position contributed to explaining the data, thereby enhancing interpretation of how different VOCs were affected by treatment and storage conditions.

3. RESULTS AND DISCUSSION Off-gassing profiles from wood, thermally treated by torrefaction or steam explosion, were compared to those from untreated wood by measuring changes in VOCs and permanent gases. The two thermally treated materials were produced in lab-scale set-ups aiming at settings and product properties relevant for large scale equipment and the fuel market, even though interpretations must be made while keeping in mind that resulting fuel properties are highly process dependent,22,46,53 and therefore probably also off-gassing behaviors. For instance, the torrefied material in this study was subjected to a ventilation step in the cooling phase, probably resulting in a minimum of recondensation of potentially off-gassing VOCs. The steam explosion was performed in a setup without any ventilation or drying step, which produced a material with a very high moisture content (Table 1). No efforts were put into further evaluation of these factors. Instead, focus was put on generating detailed off-gassing E

DOI: 10.1021/acs.energyfuels.7b01959 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 2. Comparison of furans emissions: (A) furfural, (B) 5-methylfurfural, and (C) 2-methylfuran; terpenes: (D) α-pinene, (E) α-murrolene, and (F) γ-terpinene; aldehydes: (G) hexanal, (H) pentanal, and (I) benzaldehyde, in the off-gas from reference and steam exploded (at 195 and 215 °C) wood chips at 20 or 40 °C storage temperatures, after 48 h and 112 h storage, using base peak areas for the respective compound. For furfural and αpinene, the concentrations are also presented in mg/m3. Average of one to three replicates.

pattern was in agreement with results from a study on mildly steam heat treated wood, intended for wood preservation.36 Furfural off-gassing was highest for the highest treatment severity (SE-215 °C) for all storage settings; levels were about twice as high for samples stored at 40 °C for both treatment severities, and although decreasing more to the second sampling time at 112 h, levels were still about twice as high for SE-215 °C. Since furfural is a VOC regulated with an occupational health threshold limit value, quantification of headspace concentrations was performed to enable comparison with limit values. A current limit value is 20 mg/m3 (5 ppm, TWA, OSHA).55 Furfural concentrations for steam exploded samples ranged from 79 to 467 mg/m3 (30 kg d.b. of material), which are higher than the limit value,41 but these values were concentrated emissions that could be ventilated and thus reduce the risk of exposure. However, risk assessment cannot be done based on these results since the product chain in the lab-scale process is not directly comparable to that of an industrial one. The decrease seen between the 48 h and 112 h samplings showed that no extensive new formation over time

Although a large number of VOCs were detected in this study, the Tenax TA absorbent is limited to C6−C26 in its volatility range.54 A number of VOCs, such as acetaldehyde, acetone, 2-methylbutane, and acetic acid, were found to be too volatile to be fully absorbed by Tenax TA, i.e., breaking through onto the second control tube. These VOCs were identified but omitted from the analysis as they were not possible to fully quantify (Supporting Information, Table S1). Another absorbent, e.g., Carboxen or Carbosieve with uptake from C3−C4, would be required in order to trap these volatile compounds.54 It is also possible that semivolatile compounds (e.g., PAHs) are present in thermally treated wood, which would require the use of a solvent based sampling technique, e.g., XAD. 3.1.1. Furans. Furans were the main chemical compound group differentiating steam exploded materials from the reference (Figure 2A−C). Furfural was by far the most abundant VOC emitted from the steam exploded wood (Figure 2A)the area of the second largest peak (α-pinene) was between 1 and 14% of the furfural peak, and the second largest furan, 5-methylfurfural, was 1−2% of that of furfural. This F

DOI: 10.1021/acs.energyfuels.7b01959 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

2D,E). This could partially be explained in the relatively high boiling points (245−298 °C) of the sesquiterpenes as compared to the monoterpenes, α-pinene (155 °C) and 3carene (168 °C), but not fully since also α-longifolene has a high boiling point of 244 °C. Terpenes undergo several chemical rearrangements during thermal degradation, e.g., dehydrogenations, epoxidations, cleavage of double bonds, and allylic reactions.61,62 o-Cymene and p-cymene, both found in in this study, are likely degradation products of 3-carene and limonene. Although most terpenes decreased, γ-terpinene was emitted at higher levels for steam exploded wood, and emission levels increased with increasing treatment severity (Figure 2F). Terpinenes together with limonene and p-cymene have been found among the degradation products of thermal decomposition of monoterpenes as myrcene,63 providing a plausible explanation for the observed increase. 3.1.3. Aldehydes. A number of aldehydes were identified among the off-gassing VOCs, e.g., hexanal, pentanal, and octanal (all alkyl-aldehydes), and were predominantly emitted from the reference (Figure 2G,H), whereas benzaldehyde was emitted at comparable levels also from steam exploded wood (Figure 2I). Alkyl-aldehydes were emitted at several times higher levels for samples stored at 40 °C than those stored at 20 °C, and in contrast to furans and terpenes, off-gassing levels did not decrease to the 112 h sampling. Pentanal and octanal were clustered close in on a subleg of leg III, while hexanal was found in another subleg of leg III, with compounds more readily emitted at already low storage temperatures. These results are in agreement with findings by others36 and show that off-gassing properties are more complex than single compound boiling points (hexanal at 130 °C, pentanal at 102 °C, and octanal at 171 °C). Especially hexanal has previously been reported to emit at problematic levels in raw wood storage areas and around wood mills,14,16,17 but as shown herein, hexanal off-gassing should not be a problem for thermally treated wood. Benzaldehyde on the other hand showed a different trend with a maximum for the SE-195 °C samples. This indicates that cell wall degradation, and consequently benzaldehyde formation, starts at lower thermal treatment, but that it is probably thermally decomposed at higher treatment severity where only lower amounts remain in the off-gas. Benzaldehyde has been suggested to be an artifact of the Tenax TA absorbent36 but has also been found to increase with the storage time of reference pellets measured with a solvent extraction protocol and not thermal desorption;16 thus, the presence of benzaldehyde is probably a true observation. 3.1.4. Means to Reduce VOC Off-Gassing. Selected samples were subjected to extra post-treatment steps during storage, aiming at reducing off-gassing by introducing nitrogen gas, considered as inert and potentially preventing oxidizing reactions contributing to the off-gas formation. The results showed that the extra treatment only slightly reduced offgassing levels compared to samples stored the full time in air (Figure 3A,B). However, not all VOCs responded in the same way; for example, furfural was unaffected (Figure 3A), while a larger reduction was seen for α-pinene (Figure 3B). Moreover, while VOCs generally decreased from the 48 h to the 112 h sampling, post-treated samples (after complete exchange of gas, replacing it with nitrogen) formed close to identical VOC levels after already 24 h to those stored untouched with air at 20 °C for 48 h.

was taking place and that emission levels could probably be lowered by ventilation. The off-gassing pattern described for furfural was also valid for other furans: 5-methylfurfural (Figure 2B), 2-methylfuran (Figure 2C), acetylfuran (Supporting Information, Figure S1A), and 2-furanmethanol (Supporting Information, Figure S1B). However, 2-methylfuran (Figure 2C) did not show the same decreasing trend from the 48 h sampling to that of 112 h. 2Methylfuran can form through decomposition of furfural, and therefore the maintained levels over time were likely a result of secondary reactions in the headspace.56 Also supporting the idea that 2-methylfuran is a secondary reaction product is its low boiling point at 63 °C (the other furans range from 161 to 187 °C; Supporting Information, Table S1); if formed during the thermal treatment, 2-methylfuran should already have been volatilized. The only identified furan that deviated completely in its offgassing pattern was 2-pentylfuran (Supporting Information, Figure S1C). It was the only furan located in the leg III (Figure 1, block D) cluster and was emitted at higher concentrations for the reference than for steam exploded wood. 2-Pentylfuran has been found in off-gassings from raw pine and spruce pellets16 and has been detected during oven drying of hardwoods, salix, and rape cakes at 105 °C.57 This explanation was further supported by 2-pentylfuran having also been found in the head space of char from cottonseed hull pyrolyzed at 200 °C, but not from chars treated at higher temperatures,58 which indicates a complete degradation already at low pyrolysis temperatures. 3.1.2. Terpenes. Terpenes were the largest compound group emitted to the headspace from reference wood, with considerably higher levels compared to those from thermally treated wood (Figure 1, block A−C). The most abundant terpenes were the bicyclic monoterpene α-pinene (Figure 2D) followed by 3-carene and β-pinene and bicyclic monoterpenes. They all clustered in dendrogram leg III (Figure 1), together with the monocyclic terpene D-limonene. VOCs in this cluster were emitted in very low amounts from steam exploded wood, as illustrated for α-pinene (Figure 2D). This is in line with previous results.36,59 α-Pinene dominated off-gassing from reference wood, and since it is a known eye and skin irritant, it was quantified with a standard in the present study. Offgassing values of α-pinene were 137−541 mg/m3 (30 kg d.b of material), bordering exposure limit values of turpentine (generic value for terpenes) at 560 mg/m3.60 Here, as well as for furans, values for bordering threshold values should be seen in the context of being very concentrated emissions occurring during closed storage. Concentrations can probably be lowered by ventilation in industrial systems. Although terpenes decreased as a group for the thermal treatments, some terpenes displayed individual differences, e.g., α-murrolene (Figure 2E) that was still emitted from steam exploded materials at levels half of the reference, i.e., a comparably small decrease. α-Murrolene was together with γmurrolene, α-cubebene, α-longipinene, and α-longifolene, the sesquiterpenes found in this study. However, α- and γmurrolene were located in block D, indicating they were still present at low levels in the steam exploded material. In accordance, α-murrolene has previously been found at higher concentrations from fresh than from steam dried samples, whereas γ-murrolene has previously been reported to increase in the off-gas from pine by steam-drying.12 For comparison, αpinene, found in leg III, increased by a factor 2.3 between 20 ° and 40 °, whereas α-murrolene increased by a factor 4.7 (Figure G

DOI: 10.1021/acs.energyfuels.7b01959 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

reduce VOCs; however, more controlled studies are also here needed to understand possible reductions. 3.1.5. Parameter Influence on VOCs Formation. The heat map comparison revealed that both process and storage conditions influence the VOC off-gases. Even though the common heat map displayed very low off-gassing for torrefied wood, samples still emitted VOCs at low levels. To evaluate which of the process parameters had most influence, and if they were consistent for the different thermal treatments, each data set of steam exploded and torrefied wood was assessed by PCA (Figure 4A−D). The PCA model revealed that VOC formation for both steam exploded and torrefied wood were affected by the same underlying order of process and storage parameters. The strongest influencing variable was storage temperature (seen along the x-axis in both score plots (Figure 4 A,C)), where all samples stored at 40 °C emitted higher levels for all VOCs (seen by all VOCs in the loading plot being superimposed to the 40 °C clusters of the score plot (Figure 4B,D)). The second strongest influencing parameter was treatment severity (seen along the score plots y-axis). The VOCs formed for both steam exploded and torrefied samples were vertically scattered depending on how they were affected by treatment severity for the respective thermal treatment (Figure 4B,D). For both treatment methods, higher treatment severity resulted in relatively higher levels of VOCs located in the upper part of the loading plot, such as furans (Figure 4B encircled, purple), while milder treatment severities caused relatively higher levels of VOCs located in the bottom part, such as terpenes and aldehydes. Also, reference wood VOCs were subjected to PCA. In accordance with thermally treated wood, the largest influencing parameter was storage temperature. However, the reference

Figure 3. Comparison of emissions of furfural in the off-gas from steam exploded (at 195 and 215 °C) wood chips (A) and of α-pinene in the off-gas from reference (B). Samples were stored at 20 ° with either air or with N2, or at 40 °C with air, sampled after 48 h and 112 h, using base peak areas for the respective compound. Average of 1−3 replicates.

At the 48 h sampling, 10% of the nitrogen atmosphere was replaced by air in the bottles by extractive sampling, yet the 112 h samples were still consistently lower, indicating that the introduction of air did not boost any VOC formation. This indicates that the VOCs emitted are the result of reactions primarily within the material and that surrounding oxygen is of minor importance. As no samples in this study were uniquely sampled at 112 h, or over any longer time period, the dilution effect of new air to levels formed in between samplings was not possible to assess. More work is needed for a detailed understanding of how inert gas affects off-gassing, but the indications of small effects in the present study, together with the practical problems with introducing new safety issues such as risk of suffocation, point at the need for other means for reducing off-gassing. For example, ventilation would probably be a more effective way to

Figure 4. PCA score plot (A) and loading plot (B) for VOCs in the head space of stored steam exploded wood chips treated at different temperatures: 195 °C (encircled dashed line in A) and 215 °C (encircled solid line in A). PCA score plot (C) and loading plot (D) for VOCs in the head space of stored torrefied wood chips treated at different temperatures: 260 °C (encircled dashed line in C) and 290 °C (encircled solid line in D). Samples were stored at 20 or 40 °C, with different purge gases (air or N2), and sampled after 48 h and again at 112 h. Model statistics, PCASteam‑explosion: significant principal components (PC) by cross validation, 4; R2Xtot, 0.86; Q2tot, 0.71. PCAtorrefaction: PC, 3; R2Xtot, 0.74; Q2tot, 0.50. The circular gray line of the score plots marks Hotelling’s T2 95% confidence interval. H

DOI: 10.1021/acs.energyfuels.7b01959 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 5. Changes in (A) CO [ppm], (B) CO2 [ppm], (C) CH4 [ppm], and (D) O2 [%] in the headspace of steam exploded, treated at 195 and 215 °C, and reference wood chips samples and (E) CO [ppm], (F) CO2 [ppm], (G) CH4 [ppm], and (H) O2 [%] in the headspace of torrefied, treated at 260 and 290 °C, and reference wood chips. Samples were stored in air at 20 or 40 °C over 56 days of storage. Values for each set-point were composed of two sample replicates measured every second time point.

were lower as a result of the extra post-treatment, but also that the treatment did not result in any new VOCs, detectable by Tenax TA. Even though the VOCs off-gassing was very low in torrefied samples, it was still possible to detect similar causalities as for steam exploded samples and deduct information not revealed in the common heat map. Also, differences related to treatment severity could be found. Torr-260 °C still emitted detectable amounts of α-pinene (Figure 4D, arrow), while it was completely absent for Torr-290 °C (Supporting Information, Figure S2A). As lignin is partially degraded and structurally rearranged during torrefaction,26 lignin derived aromatics could be expected to be found among the VOCs but were not

PCA model did not generate more than one principal component justified by statistical analysis, meaning very little separation was explained by increased sampling time. In contrast, both steam exploded and torrefied samples were possible to separate by time in their respective third component (t[1]/t[3]), which showed that storage time was the third strongest influencing parameter. The vented and nitrogen purged samples were also included in the PCA and were found to overlap with those of the air 20 °C stored samples for both steam exploded and torrefied samples (Figure 4A−D). Also notable was that all of them were located on the left side of the score plot, while all VOCs were found to the right in the loading plot, confirming that all VOCs I

DOI: 10.1021/acs.energyfuels.7b01959 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

unsaturated fatty acids,14 likely degraded in response to the thermal treatment; thus CO formation would be expected to be eliminated. However, CO formation was still comparably high. In a recent study, CO formation was found to be partly attributed to autoxidation reactions of the aldehydes but also formed as a result of reactions with hydroxyl radicals formed by autoxidation products of aldehydes.33 Although this further explains the extent of CO formation, it does not fully explain the excessive formation found in cargo-transport cases, which the authors suggest is a combinatory effect of biological activity. Although biological activity is reduced for torrefied materials, fungal growth has still been shown to take place during storage,65 which could explain the CO formation to some extent. However, other mechanisms could also affect CO formation from thermally treated materials.

detected in the present study. The overall low off-gassing observed from torrefied samples could be due to the purging and ventilation of the process, and those aspects are potentially important for the design of torrefaction processes and downstream equipment. 3.2. Permanent Gases. To complement VOC measurements, also changes in permanent gases were monitored in the head space over 56 days of storage for steam exploded, torrefied, and reference wood (Figure 5A−H). In comparison of steam exploded to reference wood, CO formation was higher for the reference at both storage temperatures than for any of the steam exploded samples. For both 20 and 40 °C stored samples, SE-215 °C had slightly higher CO levels than SE-195 °C (Figure 5A). CO2 off-gassing was higher for steam exploded wood than the reference; SE-195 °C samples at 20 °C formed the highest levels. This contrasts that, for both SE-215 °C and the reference, samples stored at 40 °C formed higher CO2 levels than their respective samples at 20 °C (Figure 5B). CH4 levels were higher for both steam exploded and reference samples stored at 40 °C, and the highest levels were observed for 40 °C stored SE-215 °C samples (Figure 5C). Both SE-195 °C and SE-215 °C stored at 40 °C caused more oxygen depletion than the reference at 40 °C (Figure 5D). H2 formation was only detected in the reference, 120 and 330 ppm at 20 and 40 °C, respectively. The CO and CH4 levels were in accordance with those previously reported for steam exploded pellets but contrast with O2 depletion as almost no depletion took place for their steam exploded pellets, while the reference reached