Torrefaction of Larix Kaempferi C. and Liriodendron Tulipifera L

Dec 12, 2017 - Department of Wood Science & Technology, Chonbuk National University, 567 Baekjedae-ro, Deokjin-gu, Jeonju 54896, South Korea. ‡Depar...
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Torrefaction of Larix Kaempferi C. and Liriodendron Tulipifera L. Cubes: Impact of Reaction Temperature on Microscopic Structure, Moisture Absorptivity, and the Durability of Pellets Fabricated with the Cubes Seung-Won Oh,† Dae Hak Park,‡ Soo Min Lee,§ Byoung Jun Ahn,§ Sye Hee Ahn,∥ and In Yang*,‡ †

Department of Wood Science & Technology, Chonbuk National University, 567 Baekjedae-ro, Deokjin-gu, Jeonju 54896, South Korea ‡ Department of Wood and Paper Science, Chungbuk National University, 1 Chungdae-ro, Seowon-gu, Cheongju, 28644, South Korea § Division of Wood Chemistry & Microbiology, National Institute of Forest Science, 57 Hoegi-ro, Dongdaemun-gu, Seoul 02455, South Korea ∥ Department of Forest Resources, Daegu University, 201 Daegudae-ro, Gyeongsan, Gyeongbuk, 38453, South Korea ABSTRACT: Larch (LAR) and yellow poplar (YP) cubes were torrefied at 180, 220, and 260 °C for 50 min to determine the alterations in their structural and hydrophobic properties. Through microscopic observations, several cracks and crevices were found on the radial- and tangential-sectioned areas of the 260 °C-torrefied YP cubes. On the tangential-sectioned areas, horizontal resin canal for LAR and ray cells for YP were darker than vertical cells. Moisture absorptivity (MA) measured for investigating the hydrophobicity of the torrefied LAR and YP cubes decreased with the increase of torrefaction temperature. However, no obvious differences were observed between the colors of the surface and internal parts or between the internal parts of the cubes visually at each torrefaction temperature. In addition, MA of the LAR and YP cubes was not significantly influenced by the depth of the cubes. Accordingly, it can be concluded that the carbonization level of the LAR or YP cubes is affected more by torrefaction temperature than by the cube depth. On the other hand, pelletization of the torrefied LAR and YP cubes using a piston-type single pelletizer was conducted to investigate the durability of torrefied pellets. Results showed that pellets made with torrefied YP cubes were more durable than those with torrefied LAR cubes at each torrefaction temperature. Durability of the torrefied LAR and YP pellets decreased with the increase of torrefaction temperature. Therefore, torrefaction temperature of 260 °C and above might be not an effective treatment to produce strong torrefied pellets.

1. INTRODUCTION

economic transport, handling, storage, and conversion into bioenergy products. Torrefaction is one technology to solve the problem of using lignocellulosic biomass. Torrefied biomass is made with the thermal treatment at 250−300 °C in the absence of air or oxygen.4,9−13 In the 1980s, Bourgeois and Doat first had an interest to use torrefaction as an industrial process for producing a more energy-efficient fuel to replace wood charcoal as well as being a suitable reducing agent in metallurgy and gasification fuel.14 Today, torrefaction is also thought to have a potential as a pulverized fuel for offsetting or replacing fossil coal use and reducing carbon dioxide emission.10,15 Through torrefaction, several changes take place in lignocellulosic biomass. For example, moisture and some organic compounds contained in a biomass are removed as vapors, resulting in solid biofuel with a high energy density. The energy density increases closer to that of coal used for heat and power generation.16,17 Torrefaction can also improve the hydrophobic property and decay resistance of a biomass

Coal has extensively been used for the production of heat and power. For example, in South Korea, about 60 percent of electricity was provided through coal-based power plants in 2015.1 However, coal-fired power generation is one of the heaviest carbon polluters and has a negative impact on global climate changes. One way to solve this problem is to replace coal with biomass, since burning biomass emits about 1/10 of the greenhouse gas that coal does in a life-cycle basis.2 The replacement of a massive source of pollution to a cleaner energy source can minimize the impact on global climate change. From the various biomass sources available, lignocellulosic materials account for approximately 50% of global raw materials and is the most optimal source of sustainable biomass for biofuel production.3 Meanwhile, lignocellulosic biomass has several problems which include low energy density, hydrophilic property, structural heterogeneity, low decay resistance and poor grindability when used as a feedstock for biofuels. Some of these issues make lignocellulosic biomass difficult to use as an alternative, when compared to conventional fossil fuels.4−8 As a result, the use of lignocellulosic biomass has major problems for efficient and © XXXX American Chemical Society

Received: September 4, 2017 Revised: November 27, 2017 Published: December 12, 2017 A

DOI: 10.1021/acs.energyfuels.7b02623 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. Schematic of the sampling procedure for the microscopic observation and hydrophobicity measurement (A) and images of the larch (B) and yellow poplar (C) slices obtained from the procedure.

with uniform physical and chemical characteristics.18,19 In addition to the advantages, torrefied biomass can be ground into particles more easily, such as disintegrated coal powders, compared to wood chips and biomass pellets.20 The high energy density and grindability of torrefied biomass can reduce transport costs and facilitate higher rates of co-firing with coal at power plants.16,21 Other than the application, a number of papers have been published on the potential of torrefaction for its different uses, such as gasification, syn-gas production, and barbecue fuel.22−25 When torrefaction is combined with pelletization, logistic properties of torrefied biomass can be greatly improved. In other words, torrefied biomass with a random and irregular shape can be converted into consistent and uniform cylindrical-shaped solid fuel through the pelletizing process. Pelletization is the process of compressing feedstocks into the shape of a pellet with high temperature and pressure.26 As a result, torrefied pellets have high energy density and combustion efficiency.27 In addition, its small size and cylindrical shape can be suitable for long distance transport, compact storage, easy handling, and improved feeding control to burners and generator.28 On the basis of the advantages of torrefaction and pelletization of a lignocellulosic biomass, a combined torrefaction and pelletization process was introduced for the production of torrefied wood pellets by Bergman and Kiel.20 With the development of the process, an official breakthrough took place in the Netherlands in 2010, as the first commercial-scale plant to produce torrefied wood pellets was created.29 Commercialization of torrefaction and/or pelletization technologies and its widespread applications are highly relevant to the physicochemical and fuel properties of the torrefied pellets, as well as its economic feasibility. Hence, for the past decade, a lot of research has been conducted to improve the properties of lignocellulosic biomass as a source of solid biofuel. For instance, the effect of torrefaction process parameters such as reaction temperature, residence time, feedstock type and particle size on the structure, energy density, mass/energy yield, calorific value, chemical composition, grindability, and hygroscopicity of torrefied biomass has been extensively investigated.8,11,12,30−38 Along with these

variables, the impact of torrefaction on the pelletizing properties of torrefied biomass and the design of reactor for improving the fuel properties of torrefied biomass have also been investigated by several researchers.4,7,39−44 Results of the researches showed that the torrefaction and pelletization process is an innovative technology to improve fuel properties of lignocellulosic biomass for energy utilization. However, no information exists to anatomically describe how the reaction of torrefaction processes within lignocellulosic biomasses. In addition, there is a lack of knowledge on the durability of pellets fabricated with the torrefied biomass in sufficient detail. Therefore, in this study, the structural and hydrophobic properties of surface and internal parts of larch and yellow poplar cubes torrefied at 180, 220, and 260 °C for 50 min were observed through light microscope and measured for moisture absorptivity, respectively. Larch is the most used wood species for the production of wood pellets in South Korea. Yellow poplar is a major planting species due to its fast growth and high capacity for carbon absorption, so it has extensively been planted in South Korea for bioenergy production.45 That is why larch and yellow poplar were used as a raw material for this study. The durability of pellets fabricated with the torrefied larch and yellow poplar cubes using a piston-type single pelletizer was measured to investigate the pelletizing property of the torrefied cubes. On the basis of the results of this study, reaction temperature will be verified as a relevant parameter to influence the structure, hydrophobicity, and pelletability of torrefied lignocellulosic biomass. In addition, the results will elucidate how the torrefaction treatment of lignocellulosic biomasses affects the carbonization level of its surface and internal part.

2. EXPERIMENTAL SECTION 2.1. Raw Materials. Larch (Larix kaempferi C., LAR) and yellow poplar (Liriodendron tulipifera L., YP) were used as a raw material for this study. LAR lumbers were provided from the central distribution center of the National Forestry Cooperative Federation (Yeoju, South Korea). Wooden planks of YP with dimensions of 5 cm × 15 cm × 20 cm were received from the Department of Forest Sciences at the Seoul National University of South Korea. The LAR lumbers and YP planks were cut into cubes (2 cm × 2 cm × 2 cm) using a band saw. B

DOI: 10.1021/acs.energyfuels.7b02623 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 2. Microscopic images of cross-, radial-, and tangential-sectioned larch specimens (Bar: 0.5 cm = 2 μm). The specimens were obtained from the surface of larch cubes torrefied at 180, 220, and 260 °C for 50 min. The cubes were then dried at 105 °C for 24 h and used for the torrefaction study. Torrefaction treatment of the cubes was conducted in a batch torrefaction reactor as shown in the procedure by Kim et al.11 A nitrogen flow of 2 L·min−1 was used as a carrier gas throughout the experiment to eliminate the presence of oxygen, thereby avoiding oxidation and ignition. LAR and YP cubes were torrefied at three different temperatures (180, 220, and 260 °C) and at 50 min residence time. After torrefaction treatment, the cubes were cooled. The cubes were placed in air tight plastic bags and stored in a desiccator for further experiments. 2.2. Microscopic Observation. Untreated and torrefied cubes were soaked in warm water of 60 °C for 10 min, and then the cross, radial, and tangential sections of the cubes were prepared by a razor blade. The sections were observed with a stereoscopic zoom microscope (Nikon SMZ-1500, Tokyo, Japan) to examine the structural differences of LAR and YP either by torrefaction treatment or with the increase of torrefaction temperature. In addition, the untreated and torrefied cubes were sliced into eight equal-sized pieces from each sectional direction by a backsaw (Figure 1), and then sections from the top four pieces of each sectional direction were also prepared by a razor blade. The sections were observed to identify the difference of carbonization level, by the microscope as mentioned above. 2.3. Moisture Absorptivity. In order to investigate the effect of torrefaction temperature on the carbonization levels of LAR and YP cubes, moisture absorptivity of LAR and YP cubes was measured. Untreated and torrefied cubes were placed at a constant temperature in a humidity (CTH) chamber, and then maintained at 25 °C and

65% RH for 24 h and at 50 °C, 95% RH for 4 h, respectively. The cubes were weighed and placed in an oven at 105 ± 3 °C for 24 h. The oven-dried cubes were cooled to room temperature in a desiccator and then weighed again. Moisture absorptivity (MA) was obtained according to the following equation; MA (%) weight of the CTH‐treated cube − oven‐dried weight of cube = oven‐dried weight of cube × 100

In order to compare the carbonization levels between the surface and internal parts of LAR and YP cubes, the untreated and torrefied cubes were sliced into eight equal-sized pieces by a backsaw, and then MA of the top four pieces was measured. For the measurement, the pieces were maintained in a CTH chamber (50 °C and 95% RH) for 1 and 4 h, respectively. MA of the pieces was calculated according to the MA procedure of torrefied cubes as stated above. 2.4. Pelletizing Process. Untreated and torrefied cubes were chopped into particles using a chopping mill (YM-450BM, Yulim Co., Ltd., Kyeongsan, South Korea). The particles were screened through two sieves with size openings 1.41 mm (18 mesh) and 3.17 mm (8 mesh). Particles (1.41 mm < size of particles < 3.17 mm) obtained by screening were used as raw materials for the fabrication of pellets. To minimize experimental deviations resulting from the difference in pellet size, the amount of particles used for the fabrication of a pellet was adjusted to 1.2 g. Pellets were fabricated by using a single pellet press installed at the laboratory of Chungbuk National University.46 The press consisted of a 7 mm cylindrical die in C

DOI: 10.1021/acs.energyfuels.7b02623 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. Microscopic images of cross-, radial-, and tangential-sectioned yellow poplar specimens (Bar: 0.5 cm = 2 μm). The specimens were obtained from the surface of yellow poplar cubes torrefied at 180, 220, and 260 °C for 50 min. diameter, made of hardened steel, and lagged with heating elements. The end of the die was plugged by using a removable backstop. Pressure (150 MPa) was applied to wood particles using a piston made out of hardened steel and connected to a hydraulic press. The die temperature was adjusted to 180 °C, and wood particles were pressed for 3 min. The average diameter and length of the wood pellets fabricated by this process were 7 and 20 mm, respectively. The fabricated pellets were placed in an incubation room (25 °C and 50% RH) for at least 24 h before testing its durability and MC. 2.5. Moisture Content and Durability of Wood Pellets. MC of pellets fabricated with untreated or torrefied LAR and YP particles was measured according to the standard protocol of the National Institute of Forest Science (NIFOS) in South Korea for wood pellets.47 Durability was measured by placing 50 g of pellets in a tumbling can and tumbled at 0.83 Hz. Subsequently, the tumbled pellets were sieved through a 0.5 mm sieve. The durability of the pellets was calculated as the weight ratio of after tumbling to that before tumbling. Each test was repeated three times. 2.6. Experimental Design and Statistical Analysis. The statistical analyses were conducted using the SAS software package for personal computers. One-way analysis of variance (ANOVA) was used to analyze the effect of torrefaction temperature (180, 220, and 260 °C) on the moisture absorptivity of the untreated and torrefied LAR/YP cubes and slices, as well as the durability of pellets fabricated with the LAR and YP cubes at a 0.05% significance level. If a significant relationship was found for a variable, the Student t-test was used to determine any significant difference among the torrefaction temperatures (α = 0.05).

3. RESULTS AND DISCUSSION 3.1. Microstructures Observed by Light Microscopy. Figures 2 and 3 show the structural images of LAR and YP cubes observed by light microscopy, respectively. These images were prepared in accordance with different observing magnifications, sectional directions, and torrefaction temperatures. On the cross-, radial- and tangential-sectioned areas of LAR cubes, any structural changes of LAR due to either the torrefaction treatment or the increase of torrefaction temperatures were not observed. For YP, clear structural differences were not observed in low magnification images of all sectional areas, but several voids and cracks on the surface of YP cubes torrefied at a reaction temperature of 260 °C for 50 min (YP260T) were found in the images of the radial- and tangentialsectioned areas taken at a higher magnification. These results indicate that the most severe treatment (260 °C, 50 min) against YP causes damage to the anatomical structure, resulting in the structural differences.48 As shown in Figure 2, there were not many differences in the surface colors of the untreated LAR (LAR-UT) and the 180 °C-torrefied LAR cubes (LAR-180T). However, the surface of the LAR cubes torrefied with the reaction temperature of 220 °C (LAR-220T) was light brown. An increased temperature of 260 °C (LAR-260T) caused the surface to fade into a dark brown. In addition, there was no difference between the colors D

DOI: 10.1021/acs.energyfuels.7b02623 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 4. Microscopic images of cross-, radial-, and tangential-sectioned larch specimens by their depth (Bar: 0.5 cm = 2 μm). The specimens were obtained from larch cubes torrefied at 180, 220, and 260 °C for 50 min.

2.2, depending on three sectional directions. Through these observations, we can clearly ascertain that the colors of the surface and internal parts of LAR and YP cubes got darker with the increase of torrefaction temperature. However, at each torrefaction temperature, there was no difference between the colors of the surface and internal parts or between the colors of the internal parts visually. These results indicate that the carbonization level of LAR or YP cubes used in this study is not affected more by the cube depth than by torrefaction temperature. Therefore, moisture absorptivity (MA) of LAR and YP cubes or slices was determined to identify the relationship between torrefaction temperatures and carbonization level. 3.2. Moisture Absorptivity. Figure 6 shows the MA of LAR and YP cubes torrefied at three reaction temperatures, as well as the average MA of the top 4 slices obtained from the cubes. MA of LAR-UT cubes and slices stored at 25 °C and 65% relative humidity (RH) for 24 h as well as at 50 °C and 95% RH for 4 h was higher than that of the torrefied LAR cubes and slices. Most MA of the torrefied LAR cubes and slices decreased with the increase of torrefaction temperature. For YP cubes and slices, the MA values were quite similar to those of LAR cubes and slices (Figure 6). In addition, LAR-

of earlywood and latewood in the low magnification images of cross-sectioned LAR cubes visually. For YP, the color change of the surface, depending on the torrefaction temperature, was almost the same as that of LAR (Figure 3). The color changes of the LAR and YP cubes is mainly attributed to the chemical change of the increased lignin content on the surface of LAR and YP cubes.49 For instance, lignin contents of the torrefied LAR and YP cubes began to increase at a torrefaction temperature of 220 °C with the thermal degradation of hemicellulose.50 Meanwhile, on the tangential-sectioned areas of the LARand YP-260T, horizontal resin canal for LAR and ray cells for YP were darker than vertical cells (Figures 2 and 3). These results indicate that resin canal and ray cells have a higher lignin content or are more heat-sensitive than vertical cells. Another possible explanation is that the resin canal or ray cells might have a positive effect on the thermal conduction from the boundary layer to the internal part during torrefaction, resulting in a difference of torrefaction reaction rate.5 However, these speculations need to be identified in future studies observing various lignocellulosic biomasses. Figures 4 and 5 show the structure and color of the LAR and YP slices prepared with the procedure, as described in section E

DOI: 10.1021/acs.energyfuels.7b02623 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 5. Microscopic images of cross-, radial-, and tangential-sectioned yellow poplar specimens by their depth (Bar: 0.5 cm = 2 μm). The specimens were obtained from yellow poplar cubes torrefied at 180, 220, and 260 °C for 50 min.

260T or YP-260T cubes and slices showed the lowest MA at all constant temperature and humidity (CTH) conditions. These results might be due to the removal of hydroxyl groups existing on the LAR and YP during torrefaction, and thus the moisture uptake of the torrefied LAR and YP specimens decreased.7 However, MA of LAR-180T and YP-180T slices stored at the CTH conditions of 50 °C/95% RH/1 h were higher than those of LAR-UT and YP-UT, respectively. Furthermore, MA of YP-180T cubes at the CTH conditions of 25 °C/65% RH/24 h and 50 °C/95% RH/4 h were higher than that of YP-UT. These results are mainly attributed to the moisture sorption of LAR-180T and YP-180T specimens by overdrying. On the other hand, LAR- and YP-UT slices stored at the CTH conditions of 50 °C/95% RH/4 h showed higher MA than LAR- and YP-180T slices. The thickness of the slices used for the determination of MA was thin (