Mechanistic Pathway of Carbon Monoxide Off-Gassing from Wood

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Mechanistic Pathway of Carbon Monoxide Off-Gassing from Wood Pellets Mohammad Arifur Rahman and Philip K. Hopke* Center for Air Resources Engineering and Science, Clarkson University, Potsdam, New York 13699, United States S Supporting Information *

ABSTRACT: The off-gassing of carbon monoxide (CO) from stored wood pellets has been identified as a significant problem, potentially resulting in adverse occupational and residential exposures. The mechanism for the production of CO from wood pellets has not been fully identified. In this study, a multiple step process has been hypothesized. The reaction is initiated by the autoxidation of unsaturated compounds, including fatty acids and terpenes, by molecular oxygen. As a byproduct of these reactions, hydroxyl radicals are formed. Then, the bulk of CO results from the reactions of hemicellulose and hydroxyl radicals. To understand the mechanistic pathway of CO off-gassing, a number of experiments were conducted in which CO was measured and evolved organic compounds were analyzed using gas chromatography−mass spectrometry (GC−MS). These studies identified a number of short- and long-chain aldehydes from the evolved gases that indicates the autoxidation mechanism. However, there is insufficient mass of these unsaturated compounds in wood to support the observed mass of off-gassed CO. However, autoxidation would form hydroxyl radicals. The role of hydroxyl radicals was investigated using a radical scavenger, and its role in CO production was confirmed. Thus, if the autoxidation initiation can be eliminated, then CO off-gassing from pellets would be substantially reduced. Destruction of the reactive compounds with ozone led to a suppression of CO formation, suggesting an approach to process the wood fiber that would result in low or no CO emission wood pellets.

1. INTRODUCTION Over the past 15 years, there have been reports of the formation of carbon monoxide (CO) from stored wood products, particularly wood pellets, resulting in serious human health outcomes. The first known incident appears to be the death of one person and injury to two others in Rotterdam, Netherlands, in 2002 during the unloading of wood pellets.1 This incident alerted the shipping industry to the potential danger of large quantities of stored pellets. However, a second shipping incident occurred during the unloading of a ship in the port of Helsingborg, Sweden, where the subsequent incident review found that the crew and dockhands had failed to follow the prescribed procedures for dealing with the unventilated cargo hold.2 Gauthier et al. then report the first deaths in wood pellet storerooms.3 The first one occurred in January 2010, in which an engineer entered a pellet bunker having a storage capacity of approximately 155 tons of pellets for a district boiler system supplying heat to about 700 households. The second death was in February 2011 in Switzerland when a woman was found dead in an 82 m3 pellet store room that provided fuel to a wood pellet heating system that supplied 60 households. Unrelated to these incidents, Svedberg and Galle4 had investigated complaints of eye irritation and odor in a pellet factory in Sweden. They identified the presence of hexanal, pentanal, methanol, acetone, and CO. A subsequent study investigated and described the presence of CO and volatile organic compounds (VOCs), particularly hexanal from the storage of wood pellets.5 They also monitored the emissions from kiln drying of wood and determined that the emissions were not specific to wood pellet production but were more general in nature. They concluded that there were sufficiently high concentrations associated with the storage of wood pellets © 2016 American Chemical Society

that they constituted both occupational and domestic health hazards. Additional laboratory studies of CO off-gassing were conducted by Kuang et al.6−8 They examined the effects of storage temperature (T), headspace volume, and relative humidity (RH) on the extent of CO off-gassing. An increased temperature produced higher CO concentrations. These studies focused on softwood pellets as commonly produced in western Canada. There were no comparable studies on the emissions from hardwood or softwood/hardwood-blended pellets. Arshadi et al.9 examined the VOC emissions from pellets made of blends of pine and spruce. The fatty and resin acid concentrations were measured using gas chromatography− mass spectrometry (GC−MS) for newly produced pellets and those aged for 2 and 4 weeks. They found a strong correlation between the pine fraction in the pellets and the fatty/resin acid content, but the influence decreased over storage time. The fatty and resin acid concentrations decreased by 40% during the 4 week storage period. The drying temperature also influenced the aldehyde and ketone emissions of fresh pellets. The amounts of emitted aldehydes and ketones generally decreased by 45% during storage as the fatty/resin acids oxidized. Thus, they conclude that it is the oxidation of the fatty acids that is the primarily mechanism for the observed off-gassing of VOCs. However, they do not relate any of these quantities to CO emissions. Received: April 13, 2016 Revised: June 1, 2016 Published: June 2, 2016 5809

DOI: 10.1021/acs.energyfuels.6b00874 Energy Fuels 2016, 30, 5809−5815

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Energy & Fuels

Figure 1. Thermograms of fresh hardwood pellets and hardwood pellets that had been aged for ∼60 days at 6−8 °C.

Soto-Garcia et al.10 examined the concentrations of CO in actively used pellet bins. Peak CO concentrations in these pellet storage locations ranged from 44 ppm in the basement residence to 155 ppm inside the storage silo at a school. The 1 h CO concentrations in the boiler rooms were typically 10− 15 ppm. The measured concentrations were compared to regulatory standards of 50 ppm and recommended guidelines of 35 and 9 ppm for work and non-working environments, respectively. The concentrations at the three locations in a middle school where their bin is actively ventilated never exceeded 35 ppm. At a museum, CO concentrations after a pellet delivery did reach a 1 h average maximum of 55 ppm. However, high concentrations remained for only 4 days as a result of natural ventilation in this location. Given these findings, storage areas for pellets in commercial buildings must be considered confined spaces and require appropriate entry procedures. Soto-Garcia et al.11,12 reported laboratory studies, in which they determined the emission rates of CO and VOCs from hardwood, softwood, and blended pellets. They found that dry pellets off-gas CO without any increase in the temperature and, thus, do not appear to be biological in origin. They found that, although there were compounds emitted that suggested autoxidation of fatty acids, the fatty acids measured in the pellets were too low in concentration to account for the mass of observed CO. On the basis of this information, the New York State Energy Research and Development Authority (NYSERDA) currently precludes support of in-building pellet bins because of the CO hazard. It represents a barrier for the increased use of pelletbased appliances. The substantial reduction of this hazard at a reasonable cost would remove an impediment to the expanded use of wood pellet fuels. The aim of the work described here was to understand the mechanism(s) by which wood pellets form and release CO and, on the basis of those findings, suggest an approach that could mitigate the CO production and lead to low or no CO off-gassing pellets.

Technology (NIST)], where the SRM recovery was 70−121%, which are typical results for this analysis. Thermogravimetric analysis (TGA) is the standard method for determining the major wood components: hemicellulose, cellulose, and lignins.14−16 TGAs were performed using Pyris 1 TGA (PerkinElmer, Waltham, MA). To mitigate the difference of heat and mass transfer, the sample weight was kept at ∼15 mg. Samples were heated to 800 °C at a constant of heating of 10 °C/min and maintained at this temperature for 3 min. Purified nitrogen (99.9995%) at a flow rate of 120 mL/min was used as the carrier gas to provide an inert atmosphere and to remove the gaseous and condensable products, thus minimizing any secondary vapor-phase interactions. The gases released in TGA were collected immediately through the outlet tubing of TGA connected to a 1.0 L Tedlar gas sampling bag. The TGA gas samples were screened with GC−MS (Thermo Electron TRACE GC with DSQ quadrupole MS). Compounds were identified by comparing their mass spectra to the NIST MS library. Off-gassing studies of stored wood pellets were conducted similarly to those of Soto-Garcia et al.12,13 The pellets were placed in 20 gallon steel drums [51 cm (20 in.) in height and 41 cm (16 in.) in diameter]. Two quick connects were inserted through each lid. The drums were sealed with a metal ring with a gasket to maintain an airtight fit. In these experiments, a Lascar CO monitor (model EL-USB-CO) was placed in each drum along with a Lascar temperature and relative humidity monitor (model EL-USB-2+). The accuracy of the sensors was verified by calibrating with calibration gas mixtures and a Thermo 146i gas calibrator coupled to a Thermo 110 zero air supply. These CO sensors have a maximum reading of 1000 ppm, and occasionally values would exceed this limit, resulting in flat line readings of 1000 ppm. Blended (∼40% softwood and ∼60% hardwood) and softwood pellets were obtained from a local manufacturer. In general, the pellet samples were ∼6 mm in diameter and 6−25 mm in length with a bulk density of ∼641 kg/m3 (40 lbs/ft3). VOC samples were collected in the laboratory studies using a 10 mL gastight syringe connected with plastic tubing to a quick release fitting in the top of the drum. A headspace sample was collected by pulling the syringe. Samples were analyzed by injecting 3 μL of sample into GC−MS. To sample for small-chain aldehydes, a small pocket pump (SKC, Inc.) was used to draw the air from the drum through a solid adsorbent tube [10% 2-(hydroxymethyl)piperdine on XAD-2, 120 mg/60 mesh]. The sampling duration was 10 min with a flow rate of 100 mL/min. After sampling, the aldehydes were desorbed with 1.0 mL of toluene and 60 min of ultrasonic agitation.17 The short-chain aldehyde samples were analyzed by GC−MS. Typical chromatograms are shown in Figures S1 and S2 of the Supporting Information. The analytical precision for replicate analyses of samples and standards were within ±10%. Five-point calibration curves were used to calculate the concentrations of the short-chain aldehyde samples.

2. MATERIALS AND METHODS Fatty acids in wood pellets were extracted in a Soxhlet apparatus with a mixture of petroleum ether [boiling point (bp) of 40−60 °C] and acetone (90:10, v/v) as the solvent for 6 h and analyzed by gas chromatography−flame ionization detector (GC−FID) according to the method of Arshadi and Gref.13 The analytical method for the determination of fatty acid was compared to Standard Reference Material (SRM) 1947 [National Institute of Standards and 5810


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Energy & Fuels A series of drum experiments were performed with blended wood pellets that was considered as freshly produced according to the manufacturer to which different compounds were added to determine their effect on CO production. The CO off-gassing was monitored as described above.

number of peaks with reasonable intensities were observed in the chromatogram of the MGN-3 hemicellulose (middle panel of Figure S5 of the Supporting Information). Yang et al.18 studied the TGA evolved gas of the pyrolysis of cellulose, hemicellulose, and lignin using Fourier transform infrared (FTIR) spectroscopy and concluded that CO was mainly released by the cracking of carbonyl and carboxyl arising from hemicellulose pyrolysis and the contribution of cellulose pyrolysis to CO was minor. The oxidation of these hemicellulose compounds might contribute to CO production. However, none of these compounds will be easily oxidized at room temperature and pressures. The GC−MS screening of the TGA evolved gas from the fresh hardwood showed mostly aldehydes and carboxylic acids. The TGA evolved gas of hemicellulose shows carboxylic acid peaks. The chromatogram of the cellulose TGA evolved gas (bottom panel of Figure S5 of the Supporting Information) shows few acids compared to hemicellulose. In addition, the TGA evolved gas from cellulose were more aromatic in nature than the products from hemicellulose. A similar finding was reported by Evans and Milne.20 3.3. Fatty Acids. The oxidation of unsaturated fatty acids gives rise to aldehyde/ketone emissions from pellets.9 Svedberg et al.5 also postulated that CO is formed as a result of autoxidative degradation of fats and fatty acids. Therefore, the concentration of fatty acids was measured from softwood, hardwood, and blended wood pellets. Figure 2 shows the

3. RESULTS AND DISCUSSION 3.1. TGA. Wood is essentially composed of hemicellulose, cellulose, lignin, and extractives. Soto-Garcia et al.11 had previously determined that storage of the pellets for extended periods of time resulted in a substantial reduction in off-gassed CO. Storage at 6−8 °C for 30 days reduced the maximum observed CO by a factor of 2. To identify the wood constituents that were reduced in concentration over that time, TGA was used.14,16 In this analysis, ground wood samples were placed in the instrument and their weight was continuously measured as the temperature is increased, so that the rate of loss of mass can be observed at a function of the temperature. Figure 1 shows a comparison of the thermograms obtained with fresh and aged hardwood pellets from the same batch. The aged pellets were stored for 6 weeks prior to TGA. The first peak in the differential thermogram is hemicellulose, and it can be seen to have decreased substantially in the aged pellets. The location of the hemicellulose peak in the fresh pellet also corresponds well with previously reported wood analysis results.16,18 The cellulose and lignin peaks were not different from those in the fresh pellets. To further characterize the wood components, lab-grade cellulose was obtained from Sigma-Aldrich and its thermogram is shown in Figure S3 of the Supporting Information. It compares well to the peak remaining in the wood sample following the aging process and again suggests that, during the storage period, much of the hemicellulose decomposed. To further test this hypothesis, hemicellulose is not commercially available like cellulose. Yang et al.18 used commercially available xylan to represent hemicellulose and display a hemicellulose peak that is well-separated from the cellulose peak. In an effort to obtain a sample that better reflected wood hemicellulose, a sample of “hemicellulose” sold as a dietary supplement (Biobran MGN-3) was acquired. The thermogram of this material is shown in Figure S4 of the Supporting Information. However, it yielded a very different thermogram (Figure S4 of the Supporting Information) from what was observed with the hardwood sample (Figure 1) and may not represent the same properties as wood hemicelluloses. On the basis of the prior literature showing the location of the hemicellulose peak in wood thermograms,16,18 it was hypothesized that the bulk of CO is produced by the oxidation of hemicellulose. 3.2. Evolved Gas Analysis. It was expected that identification of the major components in the gases evolved from TGA would help us begin to understand the oxidation/ breakdown processes. Cellulose consists of very large homopolysaccharide polymers composed of β-D-glucopyranose units linked together by 1−4 glycoside bonds. Hemicellulose consists of heteropolymers of pentose (xylose and arabinose) and hexose (glucose, galactose, and mannose) and sugar acids (acetic acid) with molecular weights between 100 and 400.19 Hardwood hemicelluloses are xylans, whereas softwood consists of glucomannans. Thus, the decomposition products in the gas evolved during TGAs were collected and analyzed. The top panel of Figure S5 of the Supporting Information shows the chromatogram of the evolved gas from a hardwood sample. A

Figure 2. Concentrations of fatty acids measured in samples of hardwood, softwood, and blended pellets.

concentrations of various fatty acids in wood pellets. The average fatty acid concentrations were between 0.02 and 2.8 ng/g. These concentrations are too low to account for the high concentrations of CO off-gassed from wood pellets. At best, the fatty acids in softwood could account for approximately 8% of CO, 2% of CO from blended pellets, and 1% of CO from hardwood pellets. To further understand the oxidation process, blended pellets were aged in a small drum for 20 days and then the VOCs were sampled and analyzed by GC−MS. Figure S6 of the Supporting Information presents the resulting chromatogram. Chromatograms of the headspace gas collected from 30 to 40 days of being in the drum showed similar results. In this case, octadecanoic and hexadecanoic acid peaks were observed with large peaks. A number of longer chain aldehydes, 5811


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Energy & Fuels Table 1. Concentration of Short-Chain Aldehydes from Two Different Types of Wood Pellets concentration (ppb) sample

formaldehyde

acetaldehyde

day day day day

3 6 9 12

108 121 135 151

± ± ± ±

12 10 12 11

575 1310 1744 3614

± ± ± ±

22 28 37 42

day day day day

3 6 9 12

614 884 1277 1757

± ± ± ±

15 18 22 45

1127 1527 1695 1813

± ± ± ±

25 33 56 72

propionaldehyde Softwood Pellets 56 ± 2 60 ± 2 73 ± 4 153 ± 7 Blended Wood Pellets 102 ± 8 124 ± 7 148 ± 8 200 ± 6

particularly decanal and nonanal, were observed similar to the results in the earlier studies.12 Previously, Svedberg et al.5 detected only nonanal inside two domestic houses adjacent to the closed pellet store. Decanal was not detected. In our VOC study, we have identified formaldehyde, acetaldehyde, propionaldehyde, buturaldehyde, valeraldehyde, and hexanal in a small drum containing wood pellets. However, the predominant peaks for both types of pellets were acetaldehyde, formaldehyde, and hexanal. These carbonyls are assumed to arise from the autoxidation of the fatty acids. Therefore, the aldehyde concentrations were measured and are presented in Table 1. However, the off-gassed CO concentration (∼1000 ppm) in the drum experiments is much higher than the aldehyde concentrations, again suggesting that they cannot represent the source of the bulk of observed CO. 3.4. Nature of the Oxidant(s). A number of studies have been performed to further understand the processes that lead to the formation of CO from the stored pellets. Because oxygen is the only oxidant present in the drums, it was assumed to be the species that lead to the off-gassing process. Thus, to confirm that oxygen was required to produce CO, a drum experiment was performed in which the container was filed with nitrogen. Figure S7 of the Supporting Information shows the results of this experiment. It is clear that oxygen is necessary to produce to measured CO. It appears that there were minor leaks at times, leading to some oxygen getting into these drums. These results are consistent with the recent work by Meier et al.21 Figure S8 of the Supporting Information shows the temperatures measured for the drums with the air and nitrogen atmospheres. There have been suggestions of self-heating occurring during the CO formation process.22 However, the results shown in Figure S8 of the Supporting Information show no differences in the temperature between the two drums, even though substantial CO is being formed in the drum containing pellets and air. To understand which constituents of the pellets oxidized by oxygen to produce CO, experiments with cellulose (2.0 g), hemicellulose (2.0 g), and biomass (8493 Monterey pine biomass) in a 1 L container were conducted. These experiments were run in normal air and nitrogen environments at room temperature. It is found that 2.0 g of hemicellulose (xylan) and 2.0 g of biomass (glucan, 43.7%; xylan, 5.94%; arabinan, 1.09%; galactan, 1.89%; mannan, 10.31%; structural sugars, 62.8%; and lignin, 28.2%) give about 40 and 15 ppm of CO, respectively, whereas hemicellulose produced no CO in nitrogen (Figure 3). In addition, cellulose produces no CO in oxygen. These results clearly indicate that both hemicellulose

butyraldehyde

valeraldehyde

hexanal

71 99 106 158

± ± ± ±

2 5 4 6

209 678 821 993

± ± ± ±

12 15 35 52

631 942 1052 1312

± ± ± ±

22 31 45 41

42 53 79 112

± ± ± ±

2 1 5 8

570 718 1068 1224

± ± ± ±

12 23 44 37

832 1131 1368 1422

± ± ± ±

12 42 36 62

Figure 3. CO concentrations in 1 L containers with cellulose, hemicellulose (in air and nitrogen), and biomass.

and oxygen have an important role to produce CO off-gassing in room temperature. The oxidation of hemicellulose was hypothesized to be the primary source of CO because the mass of CO is much greater than that of the aldehydes arising from the fatty acid oxidation. Moreover, hemicellulose will not be oxidized by molecular oxygen at room temperature. The hydroxyl radical is known as the most reactive member of the oxygen radical family.23 Thus, if this mechanism is correct, a significant impact of hydroxyl radical scavengers, such as short-chain alcohols, should be observable. Hydroxyl reacts to abstract an H from alcohol, resulting in the much weaker oxidizer alkoxy radical.24 Thus, another small drum experiment was performed, in which equal weights of bulk blended pellets were placed into two drums. To the second drum, 100 mL of 1-butanol was added to suppress the ·OH radicals. Figure 4 shows that the presence of 1-butanol substantially suppressed the formation of CO, suggesting both a role for the hydroxyl radical and supporting the hypothesis that ·OH oxidation of the hemicellulose is the primary source of evolved CO. To confirm the influence of the hydroxyl radical on CO production, another experiment with different hydroxyl scavenger concentrations (500 and 1000 ppm) was performed (Figure S8 of the Supporting Information). It can be seen the production of CO by 2 kg of pellets was delayed and reduced depending upon the concentration of the radical scavenger. 3.5. Reaction Pathway. Therefore, the next question is what process forms the hydroxyl radicals in sufficient quantity 5812


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Figure 5. CO emissions by ozonized and unreacted blended wood pellets.

Figure 4. CO emissions by blended wood pellets with and without the addition of 1-butanol.

therefore, all of the available reactive species are decomposed. Fresh fiber was obtained from the hopper of an active pellet mill before it is augured to the press. The fiber was exposed to ozone overnight. A comparison study was then conducted between the ozonized and as-received fiber. The results are shown in Figure 6. It can be observed that there is no CO produced from the ozone-passivated fiber, strongly supporting the hypothesized pathway for CO production.

to produce the observed quantities of CO. Examining the literature on the autoxidation of unsaturated fatty acids and monoterpenes suggests that this process could be the source of the hydroxyl radicals.25,26 There is extensive literature on the autoxidation of fatty acids because this process leads to spoilage of foods that contain these substances.27 Of particular interest are oleic, linoleic, and linolenic acids.28−30 The rates of autoxidation depend upon the degree of unsaturation, with linolenic acid oxidizing 10 times faster than linoleic acid that is 10 times faster than oleic acid.31 Thus, linolenic acid will be effective at starting the autoxidation process. Thus, although the fatty acids do not produce all of the observed CO, they initiate the reaction sequence that leads to the decomposition of the hemicellulose and the formation of observed CO. 3.6. Suppression of CO Formation. Thus, CO formation involves multiple processes, including autoxidation of wood components, such as linoleic and linolenic acids, as well as terpenes that would exist in softwood. If the CO formation is due to the production of hydroxyl radicals by autoxidation, then it should be possible to reduce/limit the amount of offgasing by destroying the reactive compounds that can autoxidize. Because these compounds have reactive double bonds, they will readily react with ozone and form Criegee intermediates that will then decompose into lower reactivity species while forming hydroxyl radicals.32−34 Thus, ozonolysis of the pellets should passivate them with respect to CO emissions or at least reduce the emissions substantially. Therefore, pellets were exposed to high ozone concentrations (>10 ppm and beyond our ability to measure) to passivate the surface of the pellets. Figure 5 compares the CO emanation from ozonized and unreacted pellets. Clearly, ozonation of the pellets reduced the maximum CO concentration by about a factor of 5, although some CO emissions were still observed. After ozone exposure on pellets, the evolved gas has been collected and analyzed by GC−MS. This gas contains mostly hexanal, nonanal, and decanal, indicating oxidation of oleic, linoleic, and linolenic acids from the wood pellets. CO may still be formed because of diffusion of the reactive compounds from the interior of the pellet, where ozone could not react with them. We confirmed that hydroxyl radicals were involved because the addition of butanol again reduced the CO emissions to almost zero (see Figure 5). This result suggests that it is necessary to ensure that all of the fiber going to the pellet needs to be exposed to ozone;

Figure 6. CO emissions by ozonized and unreacted blended wood fiber.

Thus, in this study, a two-stage mechanism has been proposed for the formation of CO by stored wood pellets. The process is initiated by the autoxidation of the unsaturated compounds in the wood, leading to the formation of hydroxyl radicals. Both hardwood and softwood contain unsaturated fatty acids. Softwoods also provide monoterpenes that can also autoxidize and may explain the higher CO observed to be emitted by softwood pellets relative to hardwood pellets.11 The hydroxyl radicals can oxidize the hemicellulose, leading to the bulk of observed CO. This mechanism is consistent with the field measurements made in active pellet storage bins10,35 and in the various laboratory experiments.6−8,11 It suggests that a process such as ozonation of the fiber before pressing the pellet would produce a pellet that would produce little or no CO.36 5813


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Energy & Fuels Notes

However, this mechanism is unlikely to provide the extremely high CO concentrations that resulted in the fatalities associated with CO emissions from pellets. The highest CO concentration observed in a well-sealed bin by Soto-Garcia et al.10 was around 200 ppm, and scaling this value to larger bins would not produce the concentrations observed in the lethal situations. Several prior reports suggest the potential for spontaneous combustion, particularly when there is the potential for biological growth.37−39 Although there were no specific indications of increased temperatures associated with these events, it may be that there was sufficient moisture in these locations (several pellet bins and the holds of ships) to permit biological activity, leading to near spontaneous combustion conditions. There are numerous reports of spontaneous combustion occurring at a variety of woodhandling facilities.40 However, there have not been many studies of this effect. Ferrero et al.41 measured the temperature and gas concentrations in a newly established pine wood chip pile (20 × 15 × 6 m containing approximately 400 tons of fresh weight material) for 150 days. The temperature was measured at 10 positions within the pile, and concentrations of CO2, O2, CO, and CH4 were made at four locations. There was a substantial rise in the temperature within the first 10−12 days of storage. The analysis of collected gas samples during this period suggested that the temperature increase was caused by aerobic microbial processes. Such a process could produce sufficiently large amounts of CO to lead to lethal concentrations. Thus, it is important to keep pellets dry and well ventilated to limit the amounts of CO formed. The mechanism presented here supports the formation of CO to produce the CO concentrations that have been reported in in-use pellet bins.10,42 The results reported above with respect to the reduction in CO emission after passivating the surface of the wood fiber by exposure to sufficient ozone to react away the reactive fatty acids and terpenes suggest the potential for a process to reduce or eliminate CO off-gassing from normally stored, dry pellets. Such a process will need to be experimentally verified.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NYSERDA under Contract 63009. The authors thank Prof. Alan Rossner and Drew Walier for their assistance with TGA evolved gas collection and analyses.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00874. Additional chromatograms showing the VOCs emitted by softwood pellets measured 3 and 6 days after storage (Figures S1 and S2), chromatograms of the VOCs emitted by hardwood, hemicellulose (MGN-3), and cellulose (Figure S5), thermograms for cellulose and hemicellulose (MGN-3) along with a chromatogram of the VOCs emitted by blended pellets after 20 days of storage (Figures S3, S4, and S6), CO concentrations as a function of time for experiments with air or nitrogen and for different concentrations of added 1-butanol (Figures S7 and S8), and temperature and relative humidity as a function of time for one of the experiments with air or nitrogen in the drum (Figure S9) (PDF)

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REFERENCES

(1) Melin, S. Review of Off-Gassing from Wood PelletsA Canadian Perspective; Wood Pellet Association of Canada: Revelstoke, British Columbia, Canada, 2010; http://www.propellets.ch/fileadmin/user_ resources/proPellets.ch/downloads/Arbeitsgruppen/CO/englisch/ 2010_Melin_review%20of%20offgassing%20from%20wood%20pellets.pdf. (2) Swedish Maritime Safety Inspectorate. Bulk Carrier SAGA SPRAY, VRWW5−Fatal Accident on 16 November, 2006; Swedish Maritime Safety Inspectorate: Norrköping, Sweden, 2011; http:// www.sjofartsverket.se/upload/Listade-dokument/Haverirapporter/ EN/2006/Saga%20Spray%20470%20eng.pdf. (3) Gauthier, S.; Grass, H.; Lory, M.; Krämer, T.; Thali, M.; Bartsch, C. Lethal Carbon Monoxide Poisoning in Wood Pellet Storerooms Two Cases and a Review of the Literature. Ann. Occup. Hyg. 2012, 56, 755−763. (4) Svedberg, U.; Galle, B. Användning av FTIR teknik för bestämning av gasformiga emissioner vid träpellets-tillverkning; Värmeforsk Service AB: Stockholm, Sweden, 2001; Report 735 (in Swedish). (5) Svedberg, U.; Högberg, H.-E.; Högberg, J.; Calle, B. Emission of hexanal and carbon monoxide from storage of wood pellets, a potential occupational and domestic health hazard. Ann. Occup. Hyg. 2004, 48, 339−349. (6) Kuang, X.; Shankar, T. J.; Bi, X. T.; Lim, C. J.; Sokhansanj, S.; Melin, S. Rate and peak concentrations of off-gas emissions in stored wood pellets − sensitivities to temperature, relative humidity, and headspace volume. Ann. Occup. Hyg. 2009, 53, 789−796. (7) Kuang, X.; Shankar, T. J.; Bi, X. T.; Sokhansanj, S.; Lim, C. J.; Melin, S. Characterization and kinetics study of off-gas emissions from stored wood pellets. Ann. Occup. Hyg. 2008, 52, 675−683. (8) Kuang, X.; Shankar, T. J.; Sokhansanj, S.; Lim, C. J.; Bi, X. T.; Melin, S. Effects of headspace and oxygen level on off-gas emissions from wood pellets in storage. Ann. Occup. Hyg. 2009, 53, 807−813. (9) Arshadi, M.; Geladi, P.; Gref, R.; Fjallstrom, P. Emission of Volatile Aldehydes and Ketones from Wood Pellets under Controlled Conditions. Ann. Occup. Hyg. 2009, 53, 797−805. (10) Soto-Garcia, L.; Huang, X.; Thimmaiah, D.; Rossner, A.; Hopke, P. K. Exposures to Carbon Monoxide from Off-Gassing of Bulk Stored Wood Pellets. Energy Fuels 2015, 29, 218−226. (11) Soto-Garcia, L.; Huang, X.; Thimmaiah, D.; Denton, Z.; Rossner, A.; Hopke, P. K. Measurement and Modelling of carbon monoxide emission rates from multiple wood pellet types. Energy Fuels 2015, 29, 3715−3724. (12) Soto-Garcia, L.; Ashley, W. J.; Bregg, S.; Walier, D.; LeBouf, R.; Hopke, P. K.; Rossner, A. VOCs Emissions from Multiple Wood Pellet Types and Concentrations in Indoor Air. Energy Fuels 2015, 29, 6485− 6493. (13) Arshadi, M.; Gref, R. Emission of volatile organic compounds from softwood pellets during storage. For. Prod. J. 2005, 55, 132−135. (14) Cozzani, V.; Lucchesi, A.; Stoppato, G.; Maschio, G. A New Method to Determine the Composition of Biomass by Thermogravimetric Analysis. Can. J. Chem. Eng. 1997, 75, 127−133. (15) Grønli, M.; Várhegyi, G.; Di Blasi, C. Thermogravimetric analysis and devolatlization kinetics of wood Ind. Ind. Eng. Chem. Res. 2002, 41, 4201−4208. (16) Serapiglia, M. J.; Cameron, K. D.; Stipanovic, A. J.; Smart, L. B. High-resolution Thermogravimetric Analysis For Rapid Characterization of Biomass Composition and Selection of Shrub Willow Varieties. Appl. Biochem. Biotechnol. 2008, 145, 3−11.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 5814


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Wheeler, P.; Baxter, D.; Nikolaisen, L. Health and Safety Aspects of Solid Biomass Storage, Transportation and Feeding; IEA Bioenergy: Paris, France, 2013; http://www.ieabioenergy.com/. (40) Energy Justice Network. Biomass Industry Plays with Fire, Gets Burned; Energy Justice Network: Philadelphia, PA, Dec 22, 2014; http://www.energyjustice.net/content/biomass-industry-plays-firegets-burned. (41) Ferrero, F.; Malow, M.; Noll, M. Temperature and gas evolution during large scale outside storage of wood chips. Eur. J. Wood Prod. 2011, 69, 587−595. (42) Simpson, A. T.; Hemingway, M. A.; Seymour, C. Dangerous (Toxic) Atmospheres in UK Wood Pellet and Wood Chip Fuel Storage. J. Occup. Environ. Hyg. 2016, DOI: 10.1080/ 15459624.2016.1167279.

(17) National Institute of Occupational Safety and Health (NIOSH). Aldehydes, Screening: Method 2539. NIOSH Manual of Analytical Methods (NMAM), 4th ed.; NIOSH: Washington, D.C., 1994. (18) Yang, H.; Yan, R.; Chen, H.; Lee, D. H.; Zheng, C. Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel 2007, 86, 1781−1788. (19) Chen, H. Chemical Composition and Structure of Natural Lignocellulose. Biotechnology of Lignocellulose: Theory and Practice; Chemical Industry Press: Beijing, China, 2014; Chapter 2. (20) Evans, R. J.; Milne, T. A. Molecular characterization of the pyrolysis of Biomass. Energy Fuels 1987, 1, 123−137. (21) Meier, F.; Sedlmayer, I.; Emhofer, W.; Wopienka, E.; Schmidl, C.; Haslinger, W.; Hofbauer, H. Influence of Oxygen Availability on off-Gassing Rates of Emissions from Stored Wood Pellets. Energy Fuels 2016, 30, 1006−1012. (22) Guo, W.; Trischuk, K.; Bi, X.; Lim, C. J.; Sokhansanj, S. Measurements of wood pellets self-heating kinetic parameters using isothermal calorimetry. Biomass Bioenergy 2014, 63, 1−9. (23) Sawyer, D. T. Oxygen Chemistry; Oxford University Press: New York, 1991. (24) Nelson, L.; Rattigan, O.; Neavyn, R.; Sidebottom, H.; Treacy, J.; Nielsen, O. J. Absolute and Relative Rate Constants for the Reactions of Hydroxyl Radicals and Chlorine Atoms with a Series of Aliphatic Alcohols and Ethers at 298 K. Int. J. Chem. Kinet. 1990, 22, 1111− 1126. (25) Bolland, J. L. Kinetics of Olefin Oxidation. Q. Rev., Chem. Soc. 1949, 3, 1−21. (26) Bateman, L. Olefin Oxidation. Q. Rev., Chem. Soc. 1954, 8, 147− 167. (27) Choe, E.; Min, D. B. Mechanisms and Factors for Edible Oil Oxidation. Compr. Rev. Food Sci. Food Saf. 2006, 5, 169−186. (28) Deatherage, F. E.; Mattill, H. A. Antioxidants and the Autoxidation of Fats Autoxidation of Oleic Acid, Methyl Oleate, Alcohol, and cis-9-Octadecene. Ind. Eng. Chem. 1939, 31, 1425−1431. (29) Porter, N. A.; Wujek, D. G. Autoxidation of Polyunsaturated Fatty Acids, an Expanded Mechanistic Study. J. Am. Chem. Soc. 1984, 106, 2626−2629. (30) Hämäläinen, T. I.; Sundberg, S.; Mäkinen, M.; Kaltia, S.; Hase, T.; Hopia, A. Hydroperoxide formation during autoxidation of conjugated linoleic acid methyl ester. Eur. J. Lipid Sci. Technol. 2001, 103, 588−593. (31) Schneider, C. An update on products and mechanisms of lipid peroxidation. Mol. Nutr. Food Res. 2009, 53, 315−321. (32) Criegee, R. Mechanism of Ozonolysis. Angew. Chem., Int. Ed. Engl. 1975, 14, 745−752. (33) Stedman, D. H.; Wu, C. H.; Niki, H. Kinetics of Gas-Phase Reactions of Ozone with Some Olefins. J. Phys. Chem. 1973, 77, 2511−2514. (34) Lee, A. K. Y.; Chan, C. K. Heterogeneous Reactions of Linoleic Acid and Linolenic Acid Particles with Ozone: Reaction Pathways and Changes in Particle Mass, Hygroscopicity, and Morphology. J. Phys. Chem. A 2007, 111, 6285−6295. (35) Emhofer, W.; Pointner, C. Lagertechnik und Sicherheit bei der Pelletslagerung; Bioenergie 2020+: Graz, Austria, 2009; http://www. holzenergie.ch/fileadmin/user_resources/proPellets.ch/downloads/ Arbeitsgruppen/CO/ Studie%20Bioenergy%202020%20Lagersicherheit_final.pdf. (36) Hopke, P. K.; Rahman, M. A. Mechanistic Pathway of Carbon Monoxide Off-Gasing. Provisional Patent 62/299,835, Feb 25, 2016. (37) Blomqvist, P.; Persson, H. Self-heating in storages of wood pellets. Proceedings of the World Bioenergy Conference and Exhibition on Biomass for Energy; Jönköping, Sweden, May 27−29, 2008; pp 172− 176. (38) Stelte, W. Guideline: Storage and Handling of Wood Pellets; Energy & Climate Centre for Renewable Energy and Transport, Section for Biomass, Danish Technological Institute: Taastrup, Denmark, 2012; Resultat Kontrakt (RK) Report. (39) Koppejan, J.; Lönnermark, A.; Persson, H.; Larsson, I.; Blomqvist, P.; Arshadi, M.; Valencia-Reyes, E.; Melin, S.; Howes, P.; 5815


Mechanistic Pathway of Carbon Monoxide Off-Gassing from Wood

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