Article pubs.acs.org/EF
Gas Adsorption Capacity of Wood Pellets F. Yazdanpanah,*,† S. Sokhansanj,†,‡ C. J. Lim,† A. Lau,† and X. Bi† †
Chemical and Biological Engineering Department, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
‡
ABSTRACT: In this study, temperature-programmed desorption (TPD) analysis was used to measure and analyze the adsorption of off-gases and oxygen by wood pellets during storage. Such information on how these gases interact with the material helps in the understanding of the purging/stripping behavior of off-gases to develop effective ventilation strategies for wood pellets. Steam-exploded pellets showed the lowest carbon dioxide (CO2) uptake compared to the regular and torrefied pellets. The high CO2 adsorption capacity of the torrefied pellets could be attributed to their porous structure and therefore greater available surface area. Quantifying the uptake of carbon monoxide by pellets was challenging due to chemical adsorption, which formed a strong bond between the material and carbon monoxide. The estimated energy of desorption for CO (97.8 kJ/mol) was very high relative to that for CO2 (7.24 kJ/mol), demonstrating the mechanism of chemical adsorption and physical adsorption for CO and CO2, respectively. As for oxygen, the strong bonds that formed between the material and oxygen verified the existence of chemical adsorption and formation of an intermediate material.
1. INTRODUCTION Wood pellets are made from compacted ground wood. Natural resins and lignin in wood help to bind loose particles together. Wood pellets are known to emit off-gases, such as carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and volatile organic compounds (VOCs), during storage, especially under high-temperature conditions.1,2 Chemical oxidation of lipids in the pellet is the source of gas emissions. This process consumes oxygen, causing oxygen depletion in the storage container. A large portion of this emission is in the form of CO, as opposed to VOC compounds such as formic acid, methanol, and aldehydes.2−4 Wood pellets are transported in large volumes in ocean vessels carrying anywhere from a few thousand metric tonne up to 40 000 tonne in a single shipment. A high level of CO was first measured in the compartments of a ship in 2002 when the ocean vessel was unloading pellets in Rotterdam, The Netherlands. The pellets were shipped from Vancouver, Canada. One person was killed and several people were severely injured as a result of exposure to high CO concentration while working around this cargo.5 Biomass can decompose over time both chemically and biologically. The degree and rate of gas generation due to microbial reaction are different depending on the access of the microorganisms to available nutrients in the material and the moisture content and the size of the material pile.6 Emissions of CO2, CH4, and N2O are reported in stored wood chips by Wihersaari.7 Svedberg et al.3 found that there is a high level of hexanal and CO associated with the stored wood pellets, which can lead to occupational and domestic health hazards. However, the hazards are not only limited to wood pellets, as the general degradation process of wood could be facilitated by drying at elevated temperature. Arshadi and Gref4 reported the emission of VOCs from stored pellets and the circumstances under which the concentration of these gases reached high levels. The emitted gases will be present in the pellet storage vessel, and although the concentrations of the off-gases could be quite © 2016 American Chemical Society
high within it, the gases generated over time may be adsorbed by the pellets and react with the pellets. This could affect the purging or stripping of the storage space during transport and unloading of pellets, depending on the degree of bonding between the gas and the wood pellet. The goal of this study was to investigate the magnitude of gas adsorption by wood pellets. The information about how these gases interact with the material is helpful toward understanding the purging/stripping behavior of off-gases in developing effective ventilation strategies for wood pellets in confined space. To measure the adsorption of gases by pellets, temperatureprogrammed desorption (TPD) analysis is used. TPD analysis was first proposed in 1967 by Amenomiya and Cvetanovic8 to measure and analyze the adsorption capacity of gases by catalysts. As temperature increases and thermal energy exceeds the adsorption energy of molecules previously adsorbed during pretreatment, these molecules desorb from the surface. The desorbed molecules are brought to a detector, such as a thermal conductivity detector (TCD) or mass spectrometer detector (MSD), by a carrier gas and quantified by the detector. Much research has been done on TPD of carbon-based samples, such as activated carbon, to better understand the surface properties of the material.9−12 Khezami et al.13 have also used TPD to characterize activated carbon produced from wood components.
2. MATERIALS AND METHODS 2.1. Materials. The experiment involved three types of pellets: white wood pellets, steam-exploded (treated) pellets, and torrefied pellets. The white wood pellets were made mostly from sawdust and shavings with no bark content. Spruce, pine, and fir (SPF) were the main sources of the sawdust. The dimensions of the pellets were 6.6 mm (diameter) and 14.2 mm (length), with a moisture content of 4.5% and a solid density of 1.17 g/cm3. The steam-exploded pellets were made in an MTI 50K press machine (Measurement Technology Received: November 18, 2015 Revised: February 2, 2016 Published: February 3, 2016 2975
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Figure 1. Temperature program and gas flow events for gas adsorption experiments using the Micromeritics device. followed by another 120 min with no CO2 flow to maximize adsorption. The recorded results showed that either this amount of sample or the duration of analysis gas passing was not enough for the desorbed amount to be detectable. By gradually increasing the amount of sample and changing the gas residence time, the optimized results were found with the operating conditions depicted in Figure 1 and summarized in Table 1. Approximately 0.7 g of the wood pellet
Inc., Roswell, GA) from sawdust that was treated with steam at 220 °C for 5 min. The dimensions of the pellets are 6.75 mm (diameter) and 22.8 mm (length), with a moisture content of 4.6% and solid density of 1.32 g/cm3. The torrefied wood pellets were made from sawdust that was heated at 270 °C for 30 min. The dimensions of the pellets are 6.6 mm (diameter) and 19.1 mm (length), with a moisture content of 5.1% and a solid density of 1.28 g/cm3. The size (weight) of the samples were chosen on the basis of the analysis gas and procedure, as explained later in this paper. Sample preparation of steam-exploded materials is described in refs 14 and 15. Details on sample preparation of torrefied materials are also available in ref 16. 2.2. Methods. A TPD analysis technique was used to measure the adsorption of gases (CO, CO2 and O2) by the pellets. In this study, a Micromeritics Auto Chem 2920 II device (Micromeritics, Norcross, GA) equipped with a thermal conductivity detector was used to quantify the amount of CO2, CO, and O2 adsorbed by the pellet samples. TCD was calibrated for CO2, CO, and O2 before conducting TPD tests through signal reading. To correlate the signal readings collected in the analysis with the volume of gas uptake at any given point in the analysis, the equipment default calibration test was run with a series of known gas concentrations. The calibration file was then associated with the signal reading to calculate the gas concentration. During the calibration test, the analyzer decreased the proportion of the analysis gas in 10% increments, beginning with 100% and ending with 0%. Using the calibration for TCD, readings were converted to cm3/min versus time. The area under the curve was calculated and divided by the mass of sample for gas uptake (cm3/g). For samples for which multiple peaks were detected, peaks were analyzed and the area under each was calculated using Originlab. 2.2.1. Carbon Dioxide TPD Procedure. TPD analysis was conducted to determine the amount of CO2 gas desorbed from the surface of the material. The differential thermal conductivity measured by the detector at any moment was proportional to the instantaneous concentration of desorbed molecules. The TPD test for CO2 adsorption by wood pellet samples started with a small portion of the wood pellet sample (0.1741 g out of ∼1 g of wood pellet), while CO2 was passed through the sample for 60 min
Table 1. Description of Events during the Gas Adsorption Process segment
thermal conditioning
measurement
A−B B−C C−D D−E E−F F−G G−H H−I
23−120 °C, 10 °C/min 120 °C; 30 min 120−40 °C; 10 °C/min 40 °C; 210 min 40 °C, 180 min baseline at 40 °C; 60 min 40−110 °C, 0.5 °C/min 110 °C, 30 min
moisture removal − sample cool down gas treatment maximize adsorption desorption baseline desorption process −
sample was loaded over quartz wool in the U-tube sample holder. The reading from the equipment was the TCD signal versus time and temperature. As seen in Figure 1, the process starts by increasing the temperature to 120 °C (10 °C/min) while helium was passed through the sample to remove any moisture in the sample (A−B). Helium was used as the carrier gas due to its very low thermal conductivity. The single species analysis gas (CO2), having a much higher thermal conductivity, was blended with helium in a fixed percentage. Temperature was maintained at 120 °C (B−C) while helium continued flowing for another 30 min. The sample was then cooled down to 40 °C (C−D). When it reached 40 °C, a stream of 10% CO2 in helium was passed through the sample. After 210 min (D−E), the inflow of the gas mixture was stopped and the sample was left for another 180 min while the sample container temperature was maintained at 40 °C (E−F). 2976
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Figure 2. Profile of effluent gases during thermal treatment of pellet samples as measured by mass spectrometer using 0.8 g of sample. For the next 60 min, a baseline was established before the desorption process starts (F−G). Then the temperature was increased to 110 °C at a heating rate of 0.5 °C/min in a helium stream (flow rate 50 cm3/min) (G−H). The sample would gradually desorb the amount of analysis gas it has previously adsorbed during this process. The evolution of carbon dioxide was monitored in the outlet stream by the TCD. The maximum desorption temperature of 110 °C (point H in Figure 1) was chosen on the basis of the results obtained from trials at different temperatures. For instance, desorption results at 60 °C showed that the maximum desorption was not achieved. Increasing the temperature gradually to a maximum of 110 °C was found to be optimal for maximum CO2 desorption from the pellets. Temperature was maintained at 110 °C before stopping the test (H−I). 2.2.2. Carbon Monoxide TPD Procedure. The test on CO TPD was similar to CO2 measurements. Initially, moisture was removed following similar steps for CO2. A stream of 10% CO in helium passed through the sample for 120 min. Once the passage of the gas mixture stopped, the sample was left for 90 min to maximize adsorption. TPD started with the same procedure with maximum desorption temperatures of 150 and 180 °C, respectively. An optimum amount of sample (∼0.7 g) was again used in the test. The analysis gas residence time and waiting time started from 60 and 120 min and increased to 210 and 180 min, respectively. At a desorption temperature of 100 °C, even after 210 min of passing CO through the material, no desorption was detected. The desorption temperature was raised to 150 and 180 °C. It can be seen that the adsorbed carbon monoxide did not fully desorb. It shall be noted that the temperature could not be raised much higher than 150 °C, as there is a possibility of sample breakdown. Although an optimum operating condition could not be found for carbon monoxide, due to much stronger bonds between CO and the sample, as compared to CO2, the maximum desorption temperature was set at 180 °C. 2.2.3. Oxygen TPD Procedure. When wood pellets are stored in confined spaces, the oxygen content in the void space can be used up by fatty acids rapidly. To study adsorption of oxygen by wood pellets, TPD was done following the same procedure as CO TPD and CO2 TPD. Initial tests started with a maximum desorption temperature of 100 °C, and no desorbed oxygen was detected in the effluent. Subsequently, the desorption temperature was increased to 120, 150, and 180 °C in various trials. Amenomia and Cvetanovic17 suggested the following relation between the heating rate and active gas molecules, assuming there is homogeneous adsorption on the surface and no readsorption or diffusion occurs ⎛T 2 ⎞ ⎛ EdA 0 ⎞ Ed p ⎟ log⎜⎜ ⎟ = 2.303RT + log⎜⎝ RC ⎟⎠ β p ⎝ ⎠
Figure 3. Typical CO2 TPD curves for (a) regular pellet, (b) torrified pellet, and (c) steam-treated pellet at a desorption temperature of 110 °C. where Tp is the desorption peak temperature (K), β is the heating rate (K/min), Ed is the energy of desorption (kJ/mol), A0 is the quantity adsorbed, C is a constant (related to desorption rate), and R is the universal gas constant (J/mol K). A change in the heating rate shifts the desorption peak temperature. Equation 1 is in the form of a linear equation, whereby the energy of desorption (Ed) can be calculated from the slope of the straight line, as Δ(log Tp2/β) divided by Δ(1/Tp).
(1) 2977
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Figure 4. Typical CO TPD curve for regular wood pellets at a desorption temperature of 150 °C.
Figure 6. Total CO uptake as a function of desorption temperature.
Table 2. Experimental Data Needed for Calculating the Energy of Desorption for CO2 and CO gas
β (K/min)
Tp (K)
log(TP2/β)
1/TP
CO2 CO2 CO CO
0.5 10 5 0.5
383 453 423 393
5.4674 5.3121 4.55 5.48
0.0026 0.0022 0.00236 0.00254
Figure 5. Typical CO TPD curve for regular wood pellets at a desorption temperature of 180 °C.
3. RESULTS AND DISCUSSION 3.1. Mass Spectrometry. A series of tests was subsequently done using mass spectrometry to verify that no gases were emitted from the sample due to material breakdown at the temperatures associated with the TPD experiments. A residual gas analyzer (RGA) (model RGA 200, SRC Stanford Research Systems) was used for the measurements. RGA is a mass spectrometer with small physical dimensions, and its function is to analyze the gases inside the vacuum chamber. During operation, a small fraction of the gas molecules is ionized, which is then separated, detected, and measured according to its molecular masses. Tests were conducted using a quartz u-tube reactor placed in a temperature-programmable muffle furnace. The mass spectrometer was connected to the reactor effluent for continuous monitoring of the products generated during the temperature-programmed desorption. About 0.8 g of sample was put in the sample holder in which a thermocouple is located and put inside the furnace. It was then purged with helium (50 cm3/min) for 10 min. The set point of the furnace was adjusted to 120 and 150 °C. Temperature was increased at the same rate (0.5 °C/min) as the TPD tests done by the Micromeritics Autochem II 2920. Partial pressure measurements were determined with the help of previously
Figure 7. Typical O2 TPD curve for regular wood pellets at a desorption temperature of 150 °C.
calculated sensitivity (calibration) factors by reference to the abundance of the individual mass numbers attributed to each gas type. As the temperature was increasing, the gas outlet was monitored for any possible gas emitting from the sample. Tests were performed for maximum desorption temperatures of 120, 150, and 180 °C with three replicates for each. The RGA results were recorded every minute as the partial pressure versus the mass of the emitted gas. Evidently, no significant amount of gas was emitted from the pellet sample during the test. Moreover, based on the TPD profiles, it was confirmed that the composition of the sample remains intact up to 180 °C. Figure 2 shows the profile of effluent gases during thermal treatment of white wood pellet samples in helium as measured by the mass spectrometer. Torrefied and steamtreated pellets showed the same results. High partial pressure 2978
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Figure 8. Two SEM micrographs of regular pellet samples before the adsorption tests.
was only observed for a mass of 4 g, which represents the flow of helium through the sample. The subsequent small peak observed for a mass of 18 g represents vaporization of the moisture inside the material. Another peak was seen for a mass of about 28 g; although the identity is not known, the partial pressure was too low to represent any substantial breakdown. 3.2. TDP Results. 3.2.1. Carbon Dioxide TPD Results. Figure 3 shows the typical curves for CO2 uptake rate for the three types of wood pellet sample used in the tests. Steamexploded pellets have the lowest CO2 uptake (0.13−0.25 cm3/g), whereas the torrefied pellets have the highest CO2 uptake (0.94− 0.98 cm3/g), as compared to the regular pellets (0.75−0.78 cm3/g). Torrefied pellets adsorbed about 20% more CO2 compared to the regular pellets. As all experiments were done at the same temperature and pressure, this may be explained by the rate of reaction between gas and solid, which is generally proportional
to the accessible surface area of the solid. The torrefaction process causes dehydration, thus initiating and propagating cracks in the lignocellulosic structure of material. Moreover, the loss of mass in the form of solid, liquid, and gas can occur, inducing changes in density and porosity of the pellets. At higher temperature and residence time, the emission of volatiles would become more intensive, also resulting in increased porosity and surface area of the material.18,19 In the steam explosion process, hemicellulose is released from the wood cell wall and becomes accessible to chemical and biochemical degradation. The degradation of hemicellulose makes wood more rigid. Cellulose and lignin may also be deconstructed by steam explosion conditions, which in turn could lead to a change in the surface properties. Figure 3c demonstrates that two peaks were detected during CO2 adsorption for the steam-treated pellets, suggesting that 2979
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was then increased to 120, 150, and 180 °C in various trials. On the basis of the temperature and TCD signal profiles, the adsorbed oxygen was observed to start desorption at about 140 °C (Figure 7). More oxygen was desorbed from the samples at 180 °C. However, due to the limitation of material breakdown at high temperature, the oxygen desorbed by the pellets cannot be quantified. Experimental results indicated that very high activation energy was needed to overcome the sorption energy between oxygen and the samples. Again, this is a clear indication of chemical adsorption of oxygen, and intermediate products could be formed as a result of material oxidation. 3.3. Material Surface Scanning Electron Microscopy (SEM). SEM analysis of the surface of wood pellets was carried out at to evaluate the surface of samples before and after adsorption. The pellet samples were mounted on specimen stubs and coated with gold under vacuum. Images were taken at 10−20 kV accelerating voltage by using a field emission scanning electron microscope (model S4700, Hitachi). Two of the SEM micrographs from the surface of wood pellets before and after CO adsorption are shown in Figures 8 and 9. As shown in the images, the structure of the pellet is made up of different layers of flakes on top of each other. There are differences in the pellet surface after gas adsorption. Images obtained from the cross section of the material after gas adsorption show clearly thicker fibers of wood standing out and that some agglomeration had occurred.
4. CONCLUSION Experiments carried out with respect to adsorption/desorption of gases by wood pellets showed the highest adsorption of carbon dioxide by torrefied wood pellets, possibly due to their higher porosity and thus surface area. Steam-exploded pellets showed minimum adsorption of carbon dioxide due to changes that resulted from the steam explosion process. The results obtained from carbon monoxide experiments showed a very high energy of desorption (∼98 kJ/mol), which was an indication of chemical adsorption. Due to a chemical reaction and therefore a strong bond between the material and carbon monoxide, quantifying the uptake of CO by pellets was challenging. For carbon dioxide, a very low energy of desorption (∼7 kJ/mol) indicated the presence of physical adsorption. Moreover, the high desorption temperature that was needed to overcome the bond energy for oxygen adsorption verified the existence of chemical adsorption of oxygen and possible formation of an intermediate material. Mass spectrometry tests confirmed that the composition of the sample remains intact up to 180 °C, and thus, no material breakdown had occurred during TPD experiments.
Figure 9. Two SEM micrographs of regular pellet samples after the CO adsorption tests.
different surface functional groups may be present after steam explosion. 3.2.2. Carbon Monoxide TPD Results. Figures 4 and 5 show the typical CO TPD curves for regular wood pellets at 150 and 180 °C, respectively. Figure 6 shows the relative differences in CO adsorption by pellets (or total amount of CO uptake in cm3/g) as a function of desorption temperature for regular wood pellet samples. The amount of gas desorbed from the material is considered as gas uptake. The values were 4.3, 7.7, and 17 cm3/g for 120, 150, and 180 °C, respectively. The maximum cumulative CO uptake of 17 cm3/g was 2 orders of magnitude higher when compared to CO2 uptake. The results from desorption at 150 °C indicate that quite a large amount of CO (7.7 cm3/g) is still attached to the sample, and the temperature was not sufficiently high to overcome the bonding energy. Table 2 lists the experimental data required for calculating the energy of desorption for the CO and CO2 gases. The calculated values were found to be 97.8 kJ/mol for CO and 7.24 kJ/mol for CO2. The large differences in the desorption energy demonstrate that CO adsorption is a chemical sorption phenomenon, while CO2 adsorption is more indicative of a physical sorption mechanism. Therefore, a much higher temperature is needed to overcome the adsorption energy of CO with respect to the surface of the material. 3.2.3. Oxygen TPD Results. Initial tests started with a maximum desorption temperature of 100 °C, and no desorbed oxygen was detected in the effluent. The desorption temperature
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[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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