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Co-densification of Agroforestry Residue with Bio-oil for Improved Fuel Pellets Kang Kang, Ling Qiu, Ming-Qiang Zhu, Guo-Tao Sun, Ya-Jun Wang, and Run-Cang Sun Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03482 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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Energy & Fuels

Co-densification of Agroforestry Residue with Bio-oil for Improved Fuel Pellets

Kang Kang, *, a, Ling Qiu, a Mingqiang Zhu, a Guotao Sun, a Yajun Wang, a and Runcang Sun b a

College of Mechanical and Electronic Engineering, Northwest A&F University, Yangling, Shaanxi

712100, China. b

Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing 100083, China.

* Corresponding author at the College of Mechanical and Electronic Engineering, Northwest A&F University, Yangling, Shaanxi 712100, China Tel.: +86 029 87092391

Email: [email protected]

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ABSTRACT: Co-densification of agroforestry residue with bio-oil can improve the pellet quality with reduced energy consumption and expand the utilization of bio-oil. This study comprehensively evaluates the effect of bio-oil addition on the storage, transportation, and combustion characteristics of the pellets. The results confirm that under low pressure of 63.69 MPa, bio-oils are more efficient in improving the pellet durability than selected solid additives, due to the high viscosity which generated strong adhesion force during densification and enabled the stronger particle interlocking of the feedstock. In addition, the hydrophobicity of the apple tree branches pyrolysis oil (APO) added pellets were enhanced, due to the introduction of hydrophobic groups and the improvements in the surface morphology. Besides, APO improved the energy density and fuel characteristics of the pellets via the change of chemical compositions. Above all, the addition of the APO additive enhanced the quality of the fuel pellets. Moreover, by achieving high durability under lower pressure, the bio-oil addition could reduce the energy consumption of the densification process. Keywords Agroforestry residues, Bio-oil, Additive, Densification, Fuel pellets


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1. INTRODUCTION Agroforestry residues, which are mainly lignocellulosic biomass with global abundance, showed great potential as one of the alternatives to the depleting fossil energy.1 Bio-energy is more sustainable than fossil energy since utilization of biomass generates significantly lower CO2 emission.2 However, as received, the bulk density of the biomass is too low for economically feasible transportation, and the energy density is much lower than the fuel standards. Therefore, the densification process is typically required.3 Pellets with high physical stability and density have gained worldwide interest as it is highly effective in reducing the costs of transportation, handling, and storage of the biomass.4 To ensure reasonable pellet durability, however, severe pelleting conditions are required, e.g., temperatures of 100~130 ºC and pressures of 115~300 MPa.5,

6

Pelleting under such conditions not only increases the energy consumption but also raises the requirements for the equipment regarding mechanical reliability and maintenance complexity. Based on the above, reducing the energy input during pelleting while ensure satisfying pellet durability is critical. To address the high energy consumption and low durability issue, development of additives became an attractive direction of the current research.7 So far, various additives have been tested including alkali lignin, modified cellulose, molasses, starches, proteins, bentonite, and polymer plastic, etc.8, 9 Some of these additives were reported to improve


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the durability of the pellets, however, use of food-based materials or precious chemicals should be avoided due to serious concerns about food, environmental, and energy security.10 Above all, when developing novel additive for biomass densification, firstly, the precursor of the additive should be none-food based and cost effective. Second, the additive can improve storage and transportation characteristics (hydrophobicity, density, durability, etc.) with a minimal amount. In addition, the negative effect on combustion properties (heating value, energy density, and ignitability, etc.) and the environment should be minimized. Bio-oil is the product from pyrolysis of lignocellulosic biomass. Bio-oil mainly contains phenols, acids, alcohols, aldehydes, esters, ketones, and guaiacols.11 Although could be produced in large quantities within a short time via fast pyrolysis technology, bio-oil can not be used as a fuel due to high oxygen, moisture content and high viscosity. Therefore, expensive processes are required for the upgrading or further conversion.12 Above all, bio-oil utilization is still quite important but still very difficult. The physio-chemical properties and composition of bio-oil showed high similarity with petroleum asphalt additives and was proved as an effective modifier for asphalt for improving the fatigue performance.13, 14 Moreover, bio-oil has been successfully used as an additive for coal fines 15. Therefore, bio-oil has every potential to improve the quality biomass pellets, and the co-densification of agroforestry residue with bio-oil could


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achieve the conversion of solid waste into bio-energy and expand the utilization of bio-oil. Nevertheless, so far, no study was conducted on the effect of bio-oils addition on quality of biomass pellets. Based on the above, the objective of this study was to comprehensively investigate the co-densification of agroforestry residue with bio-oil as an additive. Systematic characterizations of the storage, transportation, and combustion characteristics of the raw material and fuel pellets were performed, with the exploration of the binding mechanism, and the optimization of additive contents. To the best of our knowledge, this is the first study to reveal the performances of the bio-oil additive on the co-densification of agroforestry residues. With comprehensiveness, the results of this study could provide a reference to the development of high quality fuel pellets with low energy input and help to expand the utilization of bio-oil.

2. EXPERIMENTAL SECTION 2.1. Materials. Apple tree branches (APB) and APB pyrolysis oil (APO) were provided by Yixin biological energy science and technology development co., LTD (Shaanxi, China). The APO was produced by pyrolysis of the APB in a batch type reactor with the processing capacity of 3t/batch. Before the pyrolysis, APB was firstly converted into hollow rods with 5 cm diameter, 50-60 cm length, and a central-through hole (2 cm diameter) and 5 ACS Paragon Plus Environment

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then loaded. Subsequently, the air in the reactor was removed by a vacuum pump and the nitrogen flow was applied to maintain the inert environment inside the reactor and to sweep out the pyrolysis vapour. Then, the reactor was heated with the ramp rate of 2 ° C/min to 500 ºC and held for 2 h. The bio-oil was collected after the condensation of the vapour facilitated by circulating water and stored in a tank before sale. Eucommia ulmoides stems (EUS) were harvested from the experimental field at Northwest A&F University (Shaanxi, China). The cotton (Gossypium hirsutum) stalks (CT) were collected from a cotton farm (Xinjiang, China). These biomass precursors are abundant agroforestry residues in the northwest part of China, represent waste from the cultivation of fruit, Chinese medicinal crop, and industrial crop, respectively. Specifically, apple tree (Malus pumila) branches is a typical orchard residue, as a huge quantity of branches were cut after fruit harvesting and left as waste every year, causing severe issues of space occupation and energy dissipation.16 For Eucommia ulmoides Oliver, the annul production of stems can reach 18.0–22.5 tons/hectare, and the planting area is increasing due to the high value of secondary metabolites in the leaves and barks, however, the utilization of the stems has rarely been reported.17 Cotton (Gossypium hirsutum) stalk is an important stalk waste in China, and the annual generation is higher than 40 million tons in recent years.18


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The EUS pyrolysis oil (EUO) was produced using a lab-scale batch type pyrolysis reactor. For each experiment, the EUS was firstly cut into pieces with the length of 7-8 cm and loaded into the reactor with the weight of 800-900 g. Then, the reactor was heated to 500 ºC with the heating rate of 2 °C/min and held for 1 h, during which a nitrogen flow of 100 mL/min was applied. The liquid products were collected after the condensation of the vapour facilitated by circulating water. After 24 h of ageing in a sealed glass bottle, layered separation of the liquid products occurred. The upper layer of the pyrolysis liquid was removed, and the lower layer was used as EUO in this study. The alkaline lignin (AL) sample in powder form derived from corncob was provided by Changsheng Biotechnology Co., Ltd, (Shandong, China). The rapeseed meal cakes (RMC) was obtained from a local oil mill. Before pelleting, the solid samples were treated with a super centrifugal grinding machine (ZM 200, Retsch, Germany) to reduce and unify the particle size to less than 0.9 mm. For the characterizations, when a small portion of the pellet was required, a slicing knife was used to cut the pellet. Samples were taken from the middle of the pellets regarding length. 2.2. Densification Process. The densification experiments were conducted using a lab scale pelletizer. The pelletizer consists of a cylindrical piston made of chromium steel (10 mm diameter, 120 mm length) and a compatible mold (110 mm length). The pelletizer also contains a


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removable intermediate plate designed for easy extrusion of the pellets. The pelletizer was connected to a universal testing machine (CSS-44300, CIMACH, China), the pelleting process was programmed and controlled using a computer. The pelletizing experiments were carried out at ambient temperature. A single pellet was made each time, and at least ten replicates were prepared for each formulation. For each pellet, 1 g of material was well stirred and loaded into the mold. The pelletizing program consists of four segments. First, the piston moves down at a speed of 80 mm/min before the change in the compression load reached 0.2 kN/s, then the piston continued to go at a load changing rate of 0.2 kN/s and stopped at 5 kN (equivalent pressure on the piston was 63.69 MPa). After holding the load of 5 kN for 1.5 min, the load was released with the rate of 0.2 kN/s. Afterward, the pellets were ejected, each pellet was sealed in a 5 cm3 centrifuge tube and numbered. All the pellets were stored separately in air-tight bags. During the process, the elastic modulus of each pellet was calculated and recorded automatically by the computer. 2.3. Characterization Methods. Proximate analysis of the samples was performed according to American Society for Testing Materials (ASTM) methods for the analysis of moisture content, fixed carbon, volatile matter, and ash.19,

20

The elemental composition of biomass samples was

analyzed using an elemental analyzer (vario Macro cube, Elementar, Germany) working


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under CHNS mode. For the bio-oils, the pH values of the samples were measured using a pH meter (PHS-3C, INESA, China). The dynamic viscosity of the sample was measured using a flow cup viscometer (LND-1, Zhejiang Lichen instrument technology co., LTD, China). The higher heating value (HHV) of the pellet was measured using a bomb calorimeter (ZDHW-9000, Hebi hongke coal evaluation equipment factory, China). The densities of the raw material and the pellets were measured separately using different methods. Specifically, for the raw material (in powder or liquid form), the bulk densities were measured using a 500 mL density cup. For the pellet, the mass density was calculated as the ratio of the sample mass to volume, and the volume was calculated based on the diameter and length of the pellet, which was measured using a digital caliper.21, 22 The energy density of the pellet was calculated by multiplying the mass density of the sample by its HHV. For the test of pellet durability, for each formulation, five pellets were dropped from 1.85 m height onto a metal plate, and the ratio of the retained and initial mass was reported as the percentage durability.23 The moisture adsorption behavior of the pellet was evaluated in a humidity incubator (MJP-80, Beijing zhongxing weiye instrument co., LTD, China). The sample was firstly dried in an oven at 105 ºC for 12 h to remove its innitial moisture, then exposed to 70% relative humidity under 30 ºC, which is in


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accordance with the method applied by other researchers.24 For each formulation, three samples were tested, the weight of the moistened pellets was measured multiple times within the 32 h of exposure time. Based on the weight change, the content of moisture uptake (wt%) was calculated. The fourier transform infrared spectroscopy (FT-IR) analysis of the biomass, additives, and pellets were performed using an FTIR spectrometer (Nicolet iS10, Thermo Scientific, USA). Samples in powder form were scanned within the range of 4000~650 cm-1 under transmittance mode and 16 accumulations were applied for each acquisition. A scanning electron microscope (SEM) (TM 3030, Hitachi, Japan) was used to study the surface morphological features of the pellets. The sliced sample of the pellet was fixed with carbon tape, and the images were recorded at 5 kV. The thermogravimetric analysis of the samples was performed using a simultaneous thermal analyzer (TGA/DSC 3, Mettler Toledo, Switzerland). For each test, a sample of approximately 15~20 mg was loaded. The experiments were performed under an oxidative atmosphere (oxygen flow at 50 mL/min). Samples were heated from room temperature to 800 ºC using a constant ramp rate of 10 ºC/min.


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3. RESULTS AND DISCUSSION 3.1. Analysis of Raw Materials. 3.1.1 Basic Physio-chemical Properties. The compositions of the solid phase feedstocks are summarized in Table 1. Based on the targeted usage, these materials are categorized into two sub-groups namely: pellet feedstocks including EUS, APB, and CT, and solid additives including RMC and AL. Pellet feedstocks are all abundant agroforestry residues which are targeted as the main components for the pellets. Solid additives are used as references for comparatively evaluating the performances of the bio-oil additives. Specifically, RMC was reported to form mechanically durable and moisture resistant fuel pellets, whereas lignin is an important binding component in the raw biomass and can be added during pelleting to improve the hardness and moisture resistance of the pellets.23, 25 The basic properties of bio-oils including APO and EUO are summarized in Table 2. In Table 1, in the pellet feedstock group, as received, the MC of the samples were all less than 10 wt%, which were all within the optimum range for pelleting treatments.5 The HHVs of different samples were quite similar, yet, the samples showed significantly different energy densities where the highest was observed with APB at 9.03 GJ/m3, whereas the lowest was observed with EUS at 3.48 GJ/m3. In the solid additive group, RMC showed highest ash content (7 wt%), therefore, using RMC as an additive may


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increase the ash content of the pellets dependent on the amount added. In pyrolysis or gasification processes, metals in ash might act as a catalyst and accelerate cracking and formation of hydrocarbons.26 However, for combustion, high ash content should be avoided due to problems such as slagging and agglomeration caused by alkali species in the ash, as well as reactor corrosion. 27 AL showed higher HHV than RMC, as it contains a higher content of carbon and less oxygen. In Table 2, APO sample showed significantly lower MC than the EUO sample (20.79 vs. 60.89 wt%), which is possibly caused by the dewatering effect by its long period of ageing, and may further result in the higher HHV of APO than EUO. The viscosity of the APO sample is much higher than that of the EUO sample. As one reason, the APO sample contains a lower content of water, therefore became more viscos. As another reason, the APO sample may contain high molecular weight polymers, which also increased its viscosity.28 Compared to pellet feedstocks listed in Table 1, the HHVs and energy densities of the bio-oils are higher. The HHVs of the bio-oils are similar to that prepared from pyrolysis of rape plant (27.15 MJ/kg) and microalgae S. dimorphus (28.52 MJ/kg).29, 30 Based on the results, as received, the densities of the feedstocks are too low to enable cost-effective transportation, and other properties are insufficient for fuel utilization. In addition, it was found that the bio-oils may not only act as an effective


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additive because it has high viscosity but may also promote the fuel quality of the pellet due to their relatively higher energy densities than the feedstock. 3.1.2. Functional Groups. The functional groups in the feedstocks and additives were detected by FT-IR technique, and the spectra are plotted in Figure 1. In Figure 1a, on the spectra of APB, EUS, and CT, the surface -OH stretching peaks in the wavelength range of around 3400~3200 cm-1 could be assigned to moisture in the samples.31 At 2920 cm-1, RMC showed one peak, which is the characteristic of C-H stretching.32 On the spectrum of AL, peaks at 1211, 1118, and 829 cm-1 are characteristics of C-O stretching of tertiary hydroxyl groups, C-O stretching of secondary hydroxyl groups, and C-H bending vibrations in the benzene rings, respectively.32, 33 Also, on RMC, AL, and APB spectra, split peaks were observed in the range of 1700~1300 cm-1. These peaks could be assigned to C=O stretching (1673 cm-1), C=C stretching (1639 and 1591 cm-1), and C-C aromatic ring stretching (1519, 1446, and 1425 cm-1). In Figure 1b, APO showed strong peaks in the range of 1800~1000 cm-1. As described before, these peaks could be attributed to C-O stretching, C=C stretching, and C=O stretching vibrations.32 On the spectra of APO, EUO and AL, other peaks were observed at 2931, 1700, 1450, 1365, and 1214 cm-1. According to the literature, these peaks are attributable to

the aliphatic methylene, carbonyl groups in carboxyl or


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aldehyde, aromatics, methyl and methylene, and hydroxyl, respectively.32,

34

The

functional groups presented in EUO and APO showed similarity with AL and phenolformaldehyde prepolymer resins.35 The results indicate that bio-oils might become effective additives during pelleting of the feedstocks due to their similar functionalcompositions with conventional additives. 3.1.3. Thermogravimetric Analysis. The

combustion

characteristics

of

the

materials

were

investigated

via

thermogravimetric (TG) and differential-thermogravimetric (DTG) analysis under an oxidizing atmosphere. From the TG profiles (Figure 2a), it was clear that the feedstocks including EUS, APB, and CT ignited at much lower temperatures than the additives, characterized by their main weight loss stage starting at 230~250 ºC.36 Secondly, the main weight loss interval of CT ended at lower temperatures than EUS and APB, indicating better combustibility of CT. Regarding the additives, their weight loss profiles featured gradual weight loss due to the loss of moisture and non-combustible compounds. Comparing the two bio-oils, APO ignited at a lower temperature than EUO (400 ºC vs. 463 ºC), indicate that APO had better ignitability. The DTG profiles (Figure 2b) confirms the excellent combustion characteristics of CT as a feedstock. Specifically, the peak height on the DTG represents maximum weight loss rate during the combustion process, whereas the peak positions regarding


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temperature could be used to evaluate the easiness of the ignition.24 CT and EUS showed stronger combustion peaks than APB. However, the ignition of CT started at an even lower temperature (252 ºC) than that of the EUS (279 ºC). Comparing different additives, AL and RMC were easier to be ignited than the bio-oils due to the lack of moisture. When comparing the two bio-oils, APO ignited earlier than EUO. Interestingly, two different peaks were observed on the DTG profile of RMC. According to the literature, the first one at around 316 ºC is possibly caused by the easy burning volatiles, whereas the second one at around 514 ºC is associated with the combustion of the carbonaceous components.37 3.2. Characterization of Fuel Pellets. 3.2.1. Properties of Pellets without Additive. To understand the effect of densification on the properties of various feedstocks, EUS, APB, and CT pellets were prepared without additive. The properties of these pellets are reported in Table 3, where the pellets are named based on their formulations, e.g., CT100 means the pellet was prepared with only CT. When compared to the raw materials (Table 1), it is evident that the pelleting process significantly enhanced the density of the material, especially for EUS (0.78 vs. 0.20 g/cm3) and CT (0.79 vs. 0.23 g/cm3), which witnessed density increments of about three times. As a result, since the chemical


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composition did not change, the energy densities of the feedstocks were also improved by the pelleting process. Comparing the pellets, it was found that APB100 showed the highest energy density of 15.23 GJ/m3, but showed the lowest durability of 38.11%. The durability is a critical indicator of the pellets’ quality. Without sufficient durability, the pellets might break during the transportation due to vibrations.38 The elastic modulus of the pellets was in the order of APB100 (74.4 MPa) > CT100 (33.2 MPa) > EUS100 (19.2 MPa). The elastic modulus represents the change of the compression force with the increment of strain.24 Higher elastic modulus means higher resistance to deformation when exposed to external force. Therefore, high elastic modulus of the raw material may lead to the high energy consumption of the densification process. Due to the lowest durability, APB is not a suitable feedstock for producing fuel pellets. CT showed, if compared to EUS, higher HHV and better combustion characteristics, therefore, CT was selected as the feedstock for subsequent investigations. 3.2.2. Effects of Different Additives. 3.2.2.1. Basic Properties. To study the effects of different additives, various additives such as RMC, AL, APO, and EUO were co-pelleted with CT. The properties of these pellets are listed in Table 4. Regarding the pellets’ name, as an example, CT90-RMC10 represents that the pellet 16 ACS Paragon Plus Environment

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prepared with CT as the feedstock, RMC as the additive with contents of 90 wt% and 10 wt%, respectively. As is shown in Table 4, the addition of AL slightly decreased the MC of the pellet, specifically, 7.43 wt% for CT90-AL10 and 8.18 wt% for CT100 (Table 3). This is possibly due to the significantly lower MC of AL than that of CT (Table 1). Whereas when other additives were employed, the MCs of the composite pellets were all increased. On the one hand, retained moisture could act as a lubricant and ease the densification process by reducing the energy consumption. On the other hand, high MC is not desirable for fuel as heating of the water requires extra energy input, and may lead to low pellet density as water is not compressible.39, 40 The CT90-APO10 pellets showed the highest density, HHV, and energy density. Moreover, compared to CT100, these characteristics all improved significantly by the APO additive. It was also observed that all additives (Table 4) enhanced the durability of the composite pellets. In addition, the two bio-oil additives including APO and EUO were much more efficient than the solid ones as they both led to durability higher than 99%. 3.2.2.2. Hydrophobicity. Hydrophobicity is a critical property, as MC not only affects the durability but also the energy density and the combustibility of the pellet.5 During the pelleting process, the 17 ACS Paragon Plus Environment

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MC of the raw materials should not be too low to maintain the compressibility, whereas, for combustion, the MC should be controlled. According to the European Pellet Council standard for certified pellets, the MC should not exceed 10 wt%.41 Figure 3. shows the moisture uptake profiles of CT composite pellets. As shown in the figure, the moisture uptake within first 6 hours was quick, then slowed down and reached constant after approximately 24 hours. Based on the final MCs of the pellets, the additives could be divided into three groups. Specifically, for the first group, H2O and RMC did not affect the moisture uptake significantly. As the second group, two bio-oil additives including APO and EUO improved the hydrophobicity of the of the pellets, as the final MC decreased from 10.63 wt% (CT100) to 9.01wt% (CT90-EUO10), and 9.13 wt% (CT90APO10), respectively. For the third group, the addition of AL caused a slight improvement in the pellet hydrophobicity, but not as effective as the bio-oils. As a reasonable deduction, the higher moisture resistance of the pellets formulated with biooils is possibly caused by two reasons. Firstly, as was described before (Figure 1b), EUO and APO introduced hydrophobic functional groups into the pellets. Secondly, these additives changed the surface structure via bonding effect and hence increased the moisture resistance of the pellets. More discussion will be provided in the proceeding sections.


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3.2.2.3. Functional Groups. One critical factor affecting the moisture resistance of the pellets is its functional composition. The enrichment of hydrophobic functional groups in the pellet can effectively reduce the moisture adsorption capacity.23 The FT-IR spectra of the CT composite pellets were plotted in Figure 4. As the figure depicts, the spectrum of CT9010APO showed strong peaks at the wave numbers of 2920, 1425, 1365, and 1025 cm-1, respectively. These peaks could be assigned to the C-H stretching in aliphatic methylene, aromatic C-C stretching, C-H bending in methylene, and C-O stretching in the tertiary hydroxyl, respectively.32 Since most of these groups are hydrophobic, the introduction of these groups by adding APO additive into the pellets might increase the moisture resistance of the pellets by preventing moisture adsorption during the storage. 3.2.2.4. Binding Mechanism. Another important factor influencing both durability and hydrophobicity of the pellet is the binding mechanism. To understand the effects of different additives on the binding mechanism of CT particles, the SEM images of the cross-sections of the pellets were recorded and presented in Figure 5. As illustrated in Figure 5, the addition of H2O and AL at 10 wt% did not change the microscopic morphology of the pellet surface. The addition of RMC caused even worse particle interlocking of the feedstock, characterized


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by large interparticle distances. The results are in coherence with the durability reported in Table 4, where the lowest was observed for the CT90-RMC10 pellets (64.48%). Compared to other pellets, CT90-EUO10 and CT90-APO10 showed significantly improved bonding patterns characterized by closely packed CT particles with minimal interparticle distances. Moreover, after the addition of bio-oil, the large vacancies on the sample surface disappeared. The bonds among biomass particles during pelleting might happen via either formation of solid bridges or interparticle attraction.42 The solid bridges are mainly formed by additives, or binding compounds softened during pelleting by external heating or internal heat generated by high pressure. After cooling, these softened compounds hardened and became solid bridges. The interparticle attraction forces are mainly caused by the physical compression with high pressure and cause the particles to be close enough to each other. It was pointed out that the viscous additives like tar can adhere to the surfaces of biomass particles and generate strong adhesion bonds and form solid bridges.43 This phenomenon could be observed in Figure 5. The intimate bonding of particles caused by bio-oils not only resulted in the high durability of CT90-EUO10 and CT90-APO10 pellets (Table 4), but also increased the moisture resistance of the pellets (Figure 3). Other than the introduction of the hydrophobic functional groups into the pellet, EUO and APO also eliminated the large vacancies on the surface of the pellets. According to the literature, the evolution of the


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surface vacancy towards smaller dimension could also contribute to the improvement of hydrophobicity.32 3.2.2.5. Thermogravimetric Analysis. Appropriate additives should not heavily affect the combustion properties of the pellets. Figure 6 illustrates the combustion characteristics of CT90 composite pellets as evidenced by TG and DTG profiles. As shown in Figure 6a, on the full view of TG profiles, the differences caused by different additives are only visible in the temperature range of 0~230 ºC, indicating that additives mainly influence the moisture removal and evaporation of low boiling point compounds of the pellets.22 It was observed that CT90H2O10 pellets maintained a higher amount of weight loss, possibly caused by its higher MC than others. The results indicate that when using only water as an additive for pelleting, excessive energy is required for the drying before the pellet could be ignited. A closer inspection could be obtained in the magnified view of the profiles in Figure 6. As shown up the temperature of about 240 ºC, the CT100 retained more weight than the additive containing pellets. Additives such as RMC, APO, and EUO, increased the MCs of the pellets, therefore, suffered more weight loss due to moisture removal. For AL, the higher percentage of weight loss might be associated with the higher content of low boiling point compounds.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

On the DTG profiles (Figure 6b), the curves are closely stacked, indicating that 10 wt% of additives did not cause any significant differences in the weight loss rates of the pellets. The magnified view showed that solid additives including AL and RMC facilitated the ignition of the pellets, as the peaking temperatures are lower than the pellets prepared with bio-oil additives. In comparison, slightly delayed ignition was observed with pellets such as CT90-H2O10 (244.3 ºC), CT90-APO10 (246.3 ºC), and CT90-EUO10 (246.5 ºC), respectively. This behavior might be attributed to MC, it is lower in AL and RMC, whereas it is higher in APO and EUO. Above all, APO was identified as the most efficient additive. As APO added pellets showed the highest density, HHV, and energy density. Also, APO led to excellent durability (99.92%) and improved the moisture resistance. Therefore, the influence of additive content was investigated using APO. 3.3. Effect of different APO contents. Since the high content of additive may raise the cost of the pellets, the minimum amount of addition which could result in promising pellet quality is preferred.5 Therefore, other than 0 and 10 wt%, the effects of four different APO contents including 2, 4, 6, and 8 wt% were further evaluated, the results are listed in Table 5. These results show, the MC, density, HHV, and energy density of the CT composite pellets increased with the increase in the APO content. A strong improvement of durability from 77.52% to 98.60%


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was observed when the APO content increased from 2 wt% to 4 wt%, however, at higher contents, the increase in pellet durability with the increase in APO content was not significant. The results indicate that addition amount lower than 2 wt% may not be sufficient to form durable pellets under the pelleting conditions used. Compared to results reported in the literature, the 77.52% durability obtained with 2 wt% APO is still reasonable.5 To study the effect of APO content on the hydrophobicity of the CT composite pellets, moisture uptake profiles are plotted in Figure 7. As illustrated by the patterns, generally, the moisture resistance increased with the increase in APO content, with a dramatic improvement of hydrophobicity observed when APO content rose from 2 wt% to 4 wt%. Above all, since we observed slightly delayed ignition of the pellets caused by the higher MC of APO than CT (Figure 6b), and considering the higher cost of the additive than the feedstock, it was concluded that 4 wt% is the optimum amount for APO addition. The durability of the CT96-APO4 reached 98.6%, which is higher than the industrial standard established by the European Pellet Council (ENPlus-A1) of 97.5% and the IWPB standards (I1 Industrial) of 97.5%.5 What’s more, the MC of the pellet was 9.35 wt%, which is within the range required by the standards mentioned above (≤ 10 wt%). The density of the pellet was 0.84 g/cm3, which is higher than the European standards (≥ 0.60 g/cm3), but lower than the Chinese standard (NY/T 1878-2010) (≥ 1.00 g/cm3).44 In


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addition, the HHV of the pellet was 17.32 MJ/kg, which was similar to the pellets made with wheat and barely straw in Canada and pellets made with pine sawdust in Spain.9, 45 Thus, co-pelleting of bio-oils with agroforestry residues could be considered as a promising alternative source of renewable energy.

4. CONCLUSIONS In this study, co-pelleting of agroforestry residues with bio-oils including EUO and APO, and selected solid additives including AL and RMC were performed. Detailed characterizations of the materials and pellets were conducted to evaluate the effect of biooil addition on the storage, transportation, and the fuel properties of the pellets. Under ambient temperature and low pelleting pressure of 63.69 MPa, bio-oils are more efficient in improving the durability of the pellets than conventional solid additives, possibly due to their high viscosity which generated strong adhesion force during densification and enabled the stronger particle interlocking of the feedstock. By achieving high durability under lower pressure, the bio-oil addition could reduce the energy required for the pelleting process. In addition, the hydrophobicity of the APO added pellets were also enhanced, possibly due to the introduction of hydrophobic groups and the improvements of the surface morphology. APO improved also the energy density and fuel characteristics of the pellets via the change of chemical compositions, enhancing the quality of the fuel pellets. 24 ACS Paragon Plus Environment

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These results reveal the potentiality of bio-oils as promising additives for the conversion of agroforestry residues into solid biofuel via co-densification, however, further pilot-scale research regarding the interaction effects between different bio-oils and feedstocks should is recommended due to the structural and compositional complexity of these materials. ACKNOWLEGEMENTS The research was supported by the Northwest A&F University (No. Z109021715) and the Fundamental Research Funds for the Central Universities (No. Z109021711).


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LIST OF TABLES Table 1. Properties of the Feedstocks and Solid Additives

Sample EUS APB CT RMC AL

Proximate analysis (ar a, wt%) MC VM ASH FC 8.12 77.27 1.48 13.13 7.82 70.20 1.48 20.50 9.03 68.40 2.00 20.57 8.77 70.29 7.00 13.94 5.79 62.94 0.78 30.49

Ultimate analysis (ar, wt%) N C H S Ob 0.17 47.44 6.56 0.02 36.21 0.35 45.43 5.57 0.05 39.30 0.99 43.74 6.04 0.07 38.14 6.58 44.73 6.34 0.18 26.41 0.83 60.12 5.50 0.06 26.92

a

ar: as received basis.

b

The content of oxygen was calculated by difference.

HHV (MJ/kg) 17.52 17.71 17.59 19.20 24.00

Bulk density (g/cm3) 0.20 0.51 0.23 0.64 0.43

Energy density (GJ/m3) 3.48 9.03 4.12 12.22 10.35

MC, VM, ASH, and FC represent moisture, volatile matter, ash, and fixed carbon, respectively. N, C, H, S, and O represent nitrogen, carbon, hydrogen, sulfur, and oxygen, respectively.

Table 2. Properties of the Bio-oils

Sample APO EUO

MC (ar, wt%) 20.79 60.89

Ultimate analysis (daf a, wt%) pH N 1.49 0.73

C 67.99 79.79

H 6.82 8.39

S 0.06 0.04

Ob 23.64 11.05

2.29 2.55

a

daf: dry and ash-free basis.

b

The content of oxygen was calculated by difference.

Viscosity (mm2/s) 466.02 61.65


HHV (MJ/kg) 29.10 26.86

Density (g/cm3) 1.14 1.09

Energy density (GJ/m3) 33.18 29.28

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Table 3. Properties of Pellets Prepared with Different Feedstocks

Pellet name EUS100 APB100 CT100

Feedstock type EUS APB CT

Pellet MC (wt%) 7.93 ± 0.22 7.00 ± 0.01 8.18 ± 0.09

Pellet density (g/cm3) 0.78 ± 0.02 0.86 ± 0.04 0.79 ± 0.03

HHV (MJ/kg) 17.52 17.71 17.59

Energy density (GJ/m3) 13.66 15.23 13.90

Durability (%) 60.59 ± 3.65 38.11 ± 5.29 59.95 ± 1.11

Elastic modulus (MPa) 19.2 ± 0.7 74.4 ± 4.3 33.2 ± 2.3

Repeated results are reported in the form of mean ± standard deviation.

Table 4. Properties of CT Pellets Prepared with Different Additives

Pellet name CT100 CT90-H2O10 CT90-AL10 CT90-RMC10 CT90-EUO10 CT90-APO10

Additive type None H 2O AL RMC EUO APO

Pellet MC (wt%) 8.18 ± 0.09 13.83 ± 2.39 7.43 ± 0.19 9.32 ± 2.34 11.76 ± 0.20 11.39 ± 0.13

Pellet density (g/cm3) 0.79 ± 0.03 0.77 ± 0.07 0.81 ± 0.02 0.88 ± 0.01 0.83 ± 0.09 0.92 ± 0.02

HHV (MJ/kg) 17.59 14.89 16.83 17.13 17.67 18.11

Energy density (GJ/m3) 13.90 11.46 13.63 15.08 14.66 16.48

Pellet durability (%) 59.95 ± 1.11 69.10 ± 4.44 77.84 ± 14.70 64.48 ± 7.18 99.79 ± 0.03 99.92 ± 0.01

Repeated results are reported in the form of mean ± standard deviation. Table 5. Properties of CT Pellets Prepared with Different APO Contents

Pellet name CT100 CT98-APO2 CT96-APO4 CT94-APO6 CT92-APO8 CT90-APO10

APO content (wt%) 0 2 4 6 8 10

Pellet MC (wt%) 8.18 ± 0.09 8.93 ± 0.01 9.35 ± 0.35 9.62 ± 0.25 9.91 ± 0.26 11.39 ± 0.13

Pellet density (g/cm3) 0.79 ± 0.03 0.81 ± 0.02 0.84 ± 0.02 0.89 ± 0.01 0.90 ± 0.03 0.92 ± 0.02

HHV (MJ/kg) 17.59 17.22 17.32 17.37 17.43 18.11

Energy density (GJ/m3) 13.90 13.95 14.55 15.46 15.69 16.48

Repeated results are reported in the form of mean ± standard deviation. 27 ACS Paragon Plus Environment

Pellet durability (%) 59.95 ± 1.11 77.52 ± 1.57 98.60 ± 0.86 99.37 ± 0.45 99.41 ± 0.55 99.92 ± 0.01

Energy & Fuels

LIST OF FIGURES (a) 100

AL

Transmittance (%)

95

APB EUS

90

CT 85

RMC

80 75 70 4000

3600

3200

2800

2400

2000

1600

1200

800

400

Wavenumber (cm-1) (b)) 100

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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AL

95

EUO

90

APO

85 80 75 70 65 4000

3600

3200

2800

2400

2000

1600

1200

800

400

Wavenumber (cm-1)

Figure 1. FT-IR spectra of (a) the biomass feedstock and solid additives, (b) bio-oil additives with AL as the reference.


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(a) 100

AL

90

EUS

weight (%)

80

APB

70 CT

60 50

RMC

40

EUO

30

APO

20 10 0 0

100

200

300

400

500

600

700

800

Temperature (°C)

(b)

1.20

AL

Weight loss rate (wt%/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1.00

EUS APB

0.80

CT 0.60

RMC EUO

0.40

APO 0.20 0.00 0

100

200

300

400

500

600

700

800

Temperature (ºC)

Figure 2. TG (a) and DTG (b) curves of the raw biomass feedstock and additives. 29 ACS Paragon Plus Environment

Energy & Fuels

11 10

Moisture uptake (wt%)

9 8 7

CT100

6

CT90-H2O10

5

CT90-AL10

4

CT90-RMC10

3 CT90-EUO10 2 CT90-APO10

1 0 0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

Exposure time (h)

Figure 3. Moisture uptake profiles of the CT pellets prepared with different additives.

100 CT100

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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95

CT90-H2O10 CT90-EUO10

90

CT90-10APO 85

CT90-10AL CT90-10RMC

80 75 4000

3600

3200

2800

2400

2000

Wavenumber

1600

1200

800

400

(cm-1)

Figure 4. FT-IR spectra of the CT pellets prepared with different additives.


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Energy & Fuels

Figure 5. SEM images of the cross sections of different CT pellets (magnification at 50×). (a) CT100. (b) CT90-H 2 O10. (c) CT90-AL10. (d) CT90-RMC10. (e) CT90-EUS10. (f) CT90-APO10.


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Figure 6. TG (a) and DTG (b) curves of the CT pellets prepared with different additives. . 32 ACS Paragon Plus Environment

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11 10 9 8

Moisture uptake (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

7 CT100

6

CT98-APO2

5

CT96-APO4 4 CT94-APO6 3 CT92-APO8 2

CT90-APO10

1 0 0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

Exposure time (h)

Figure 7. Moisture uptake profiles of CT pellets prepared with different content of APO.


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