Effects of Biomass Particle Size in Cellulosic Biofuel Production

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A Comprehensive Investigation on the Effects of Biomass Particle Size in Cellulosic Biofuel Production

Abstract

Today’s economy and society are dependent on liquid transportation fuels. However, more than 90% of the liquid transportation fuels used in the U.S. are petroleum-based, and more than 55% of the petroleum is imported. Furthermore, the transportation sector is responsible for about 30% of the U.S. greenhouse gas emissions. Therefore, it is imperative to develop alternative liquid transportation fuels that are domestically produced and environmentally benign. Biofuels derived from cellulosic biomass offer one of the best near- to mid-term alternatives to petroleum-based liquid transportation fuels. Biofuel conversion is mainly conducted through a biochemical pathway in which size reduction, pelleting, pretreatment, hydrolysis, and fermentation are the main processes. Many studies reveal that biomass particle size dictates the energy consumption in size reduction. Biomass particle size also influences sugar yield in hydrolysis, which is approximately proportional to biofuel yield after fermentation. It has been found that most reported studies focus on the effects of biomass particle size on a specific process; as a result, in the current literature, there are no commonly accepted guidelines to selecting the optimum particle size to both minimize energy consumption and maximize sugar yield. This study presents a comprehensive experimental investigation on the conversion of three biomass materials into fermentable sugars and studies the effects of biomass particle size on various evaluation parameters throughout the multistage cellulosic biofuel production.

Keywords

Biofuel, cellulosic, particle size, pelleting, pretreatment, size reduction

1.   Introduction

Today’s economy and society are heavily dependent on liquid transportation fuels. This reality will not change dramatically in the near future. However, more than 90% of the liquid transportation fuels used in the U.S. are petroleum-based, and more than 55% of the petroleum is imported [1-4]. Meanwhile, consuming petroleum-based transportation fuels contributes to the accumulation of greenhouse gases (CO2, SO2, and NOx) in the atmosphere [5]. Therefore, it is imperative to develop alternative liquid transportation fuels that are domestically produced and environmentally benign.

The second-generation biofuels produced from cellulosic biomass (forest and agricultural residues and dedicated energy crops) are alternatives to conventional transportation fuels. Compared to the first-generation biofuels (fuels produced from feedstocks such as corn starch and sugar cane, which can potentially be made into food/feed), producing biofuels from the inedible cellulosic biomass creates less competition with food/feed production for the limited agricultural farmland and other resources [3]. Producing and using cellulosic biofuels can turn residues and wastes into energy, reduce the nation’s dependence on foreign oil, create new jobs, and improve rural economies [6]. Cellulosic biofuel can be used on its own as a sustainable liquid transportation fuel or blended with conventional transportation fuel to meet the U.S. government’s mandate of 16 billion gallons of cellulosic biofuel produced annually by 2022 in the Renewable Fuel Standard (RFS) created under the Energy Independence and Security Act [7]. There are over 1 billion dry tons of cellulosic biomass that can be sustainably harvested in the U.S. every year. This amount is sufficient to produce 90 billion gallons of liquid fuel that can replace about 30% of the nation’s current annual consumption [8]. Furthermore, cellulosic biofuel has the potential to reduce greenhouse gas emissions. Among all the other renewable energy sources, biomass is the only sustainable carbon carrier. Biomass grows by absorbing carbon dioxide, water, and nutrients and converts them into hydrocarbons through photosynthesis. When biomass is consumed as a fuel, carbon is cycled in the atmosphere [9].

The composition of cellulosic biomass includes approximately 40-50% cellulose, 20-30% hemicellulose, and 15-20% lignin [10,11]. Surrounding cellulose fibrils (an aggregate of glucan chains consisting of many glucose units) is hemicellulose and lignin that form a matrix by bonding with cellulose and other hemicellulose molecules as shown in Fig. 1. Cellulose and hemicellulose are polysaccharides that can be hydrolyzed to sugars, which can then be fermented into biofuels. Lignin contains no sugar components, and it cannot be digested by enzymes [12]. Lignin can be used to make value-added chemicals and bio-based products or burned to produce bioenergy [13].

Fig. 2 illustrates the major steps of converting cellulosic biomass feedstocks into biofuels. These steps are divided into the feedstock preprocessing stage conducted in a field or at a depot and the bioconversion stage performed at a biorefinery. Size reduction is a necessary process with current conversion technologies, as raw cellulosic biomass feedstocks (such as the stems of herbaceous biomass or chunks of woody biomass) cannot be directly converted into biofuels efficiently [14,15]. This process decreases cellulosic biomass particle size by mechanical methods such as milling, cutting, and chipping. The large physical size and strong structure of cellulosic biomass make size reduction a highly energy intensive process [16]. Pelleting applies mechanical forces to compress biomass particles produced by size reduction into uniformly sized pellets or briquettes. Pelleting can increase the volumetric density of biomass from 40-200 kg/m3 to 600-1400 kg/m3, which will significantly lower the cost and improve the feedstock flowability in biomass logistics [17,18]. After being transported into a biorefinery, cellulosic biomass is pretreated first. The purpose of pretreatment is to break down the shield formed by hemicellulose and lignin to make the feedstock more accessible to enzymatic hydrolysis; thus, pretreatment can speed up the bioconversion rate and increase the yield of fermentable sugars (such as glucose, xylose, arabinose etc.). Afterwards, fermentation will convert sugars into biofuels, such as bioethanol [19,20].

Biomass particle size (after size reduction) is a curial input process parameter with impacts on both the feedstock preprocessing and bioconversion stages. For example, biomass particle size dictates the energy consumption in the size reduction process [21]. In order to produce biomass with a smaller particle size, more energy is usually consumed [22-26]. Biomass particle size also influences sugar yield in hydrolysis [27-30], and biofuel yield after fermentation is approximately proportional to the former hydrolysis sugar yield [31].

Many studies have been conducted to investigate the effects of biomass particle size in cellulosic biofuel manufacturing. However, as summarized in Table 1, the reported relationships are inconsistent. More importantly, it has been found that most reported studies focus on the effects of biomass particle size on a specific process or a single output parameter; references on the effects of biomass particle size throughout both the feedstock preprocessing and bioconversion stages are quite limited. As a result, in the current literature, there are no commonly accepted guidelines on how to select biomass particle size in order to conserve energy in the feedstock preprocessing stage while still achieving good biofuel yield in the bioconversion stage. This study presents a comprehensive experimental investigation on the conversion of three biomass materials into fermentable sugars and studies the effects of biomass particle size on various evaluation parameters throughout the multistage biofuel manufacturing process.

2.   Experimental Procedures

2.1. Size reduction

The cellulosic biomass materials used in this study were wheat straw, corn stover, and big bluestem. The moisture content of the materials was conditioned at approximately 7%. All materials (whole stems) were processed through a knife mill (SM 2000, Retsch, Inc., Haan, Germany) powered by a 1.5 kW three-phase electric motor. The milling chamber of the knife mill is shown in Fig. 3. Three sieves with sizes of 4, 2, and 1 mm were used to control the particle size received. In each size reduction experiment, 100 g of biomass was gradually fed into the milling chamber. The mill was turned off after 10 seconds when all the biomass was loaded into the milling chamber. The weight of the particles collected was measured, and the size reduction time was recorded.

2.2. Pelleting

As illustrated in Fig. 4, cellulosic biomass particles were compressed into pellets using a modified ultrasonic machine (AP-1000, Sonic-Mill, Albuquerque, NM, USA). This process combines an ultrasonic generation system and a pneumatic loading system. The ultrasonic generation system includes a power supply that converts 60 Hz of conventional line electrical power into 20 kHz of electrical power, a piezoelectric converter that converts high frequency electrical energy into mechanical vibrations, and a titanium pelleting tool. The tip of the pelleting tool is a solid cylinder with a flat end. The ultrasonic motion from the converter is amplified and transmitted to the pelleting tool, which causes the pelleting tool to vibrate perpendicularly to the tool end’s surface at a high frequency. Biomass particles were compressed into a pellet inside a mold with a cylindrical cavity. Pelleting pressure is applied by a pneumatic cylinder using an air compressor. In this study, the pelleting pressure was set at 345 kPa (50 psi). Ultrasonic power determines the tool’s vibration amplitude. A larger ultrasonic power creates a higher tool vibration amplitude. The selected ultrasonic power is expressed as a percentage of the maximum ultrasonic power for the equipment, ranging from 0% (no ultrasonic vibration applied) to 100%. In this study, the ultrasonic power was set at 50%, and the pelleting duration was kept at 90 seconds. Each pellet was made of 2 g of biomass.

2.3. Pretreatment

Dilute sulfuric acid pretreatment was employed in this study. Before each pretreatment test, 5 g (dry weight) of pellets and 150 mL of 2% (w/v) sulfuric acid were loaded into a 600 mL glass liner of a Parr pressure reactor (4760A, Parr Instrument Co., Moline, IL, USA). Pretreatment time was 30 minutes, and pretreatment temperature was 120°C. After pretreatment, biomass particles were collected and washed with 50-60°C distilled water using a suction filtration system with P4 grade filter paper to conduct solid-liquid separation. After filtration, the solid biomass was carefully collected from the filter paper. The moisture content of the collected solid biomass was measured.

2.4. Hydrolysis

Hydrolysis was carried out in 125-mL flasks in a water bath shaker (C76, New Brunswick Scientific, Edison, NJ, USA) with an agitation speed of 110 rpm at 50°C for 48 hours. Two independent hydrolysis flasks were prepared for each experimental condition. Each flask contained 40 mL of hydrolysis slurry, which consisted of 5% (w/v) biomass on dry weight base, sodium acetate buffer (50 mM, pH = 4.8), and 0.02% (w/v) sodium azide to prevent microbial growth during hydrolysis. Accellerase 1500TM enzyme complex (Danisco USA, Inc., Rochester, NY, USA) was used. The enzyme-to-biomass ratio was 1 mL for each gram of dry biomass. Supernatant liquid from each flask was collected after 24 and 48 hours of hydrolysis for sugar content measurement.

3. Evaluation Parameters and Measurements

3.1. Energy consumption

Energy consumption in this investigation was referred to as the electrical energy consumed in size reduction and pelleting. A Fluke 189 multimeter and a Fluke 200 AC current clamp (Fluke Corp., Everett, WA, USA) were used to conduct the measurement. Current data was collected using Fluke View Forms software. The sampling rate was two readings per second. The software recorded the average current (IAVG) in each test. The line-to-neutral voltage (V) did not fluctuate much during tests, so it was regarded as a constant of 120 V. The energy consumed during each test (that lasted for t seconds) (Et) was calculated using the following equations: size reduction (three-phase power) Et = √3IAVG·V·t (J) and pelleting (single-phase power) Et = IAVG·V·t (J). Dividing Et by the weight (w) of the biomass materials received after the tests gives energy consumption (E) per unit weight, as expressed in E = Et/w. For each experimental condition in size reduction and pelleting, eight independent energy consumption measurements were performed.

3.2. Pelleting density and durability

During the handling and transportation of biomass pellets from feedstock preprocessing facilities to biorefineries, the pellets are exposed to fragmentation and abrasion. These impacts and forces can degrade the pellets. Pellet density and durability were employed to evaluate the pellet quality in this study. Density of a pellet was calculated by the ratio of its weight to its volume. Eight pellets were randomly chosen for density measurements. Three density values (initial density, out-of-mold density, and 24-h density) were measured for each pellet. The initial density was the pellet density at the end of each test while it was still in the mold. The inner diameter of the mold cavity was taken as the pellet diameter, and the pellet height was obtained from a digital readout attached on the pelleting tool. Out-of-mold density was the pellet density right after being unloaded from the mold, and 24-h density was the pellet density measured 24 hours after being unloaded. The latter two densities were measured to reflect the amount of spring-back (volume expansion) and the stability of a pellet during storage; these pellet dimensions were measured using a digital caliper. Durability tests were performed on a pellet durability tester (Fig. 5). First, pellets (stored in ambient conditions for 24 hours) were loaded into the chamber of the tester. Then, the chamber was rotated (tumbling) at 50 rpm for 10 minutes. After each test, pellets were transferred and shaken through a U.S. No. 6 standard sieve to separate whole pellets from crumbs and loose particles. Results of the test were interpreted as a standard measure of pellet quality referred to as the durability index. The durability index was calculated as the ratio between the weight of whole pellets after tumbling and the weight of pellets before tumbling. A higher durability index indicates that the pellets have a higher ability to survive the impacts and forces of handling and transportation.

3.3. Sugar yield

Biomass-based sugar yield was employed in this study. It evaluates the glucose yield (g) per unit dry weight of biomass loaded into the hydrolysis process. Sugar yield measurement was performed on a high pressure liquid chromatograph (HPLC) system (Agilent Technologies Inc., Santa Clara, CA, USA) with a RezexTM RCM-Monosaccharide column (Phenomenex Inc., Torrance, CA, USA) and a Refractive Index Detector RID-G1362A (Agilent Technologies Inc., Santa Clara, CA, USA) at 40°C. Water was used as the mobile phase at a flow rate of 0.6 mL/min. Chromatograph temperature was maintained at 80°C. Samples were filtered through 0.2 μm hydrophilic PTFE syringe filters (EMD Millipore Corp., Billerica, MA, USA) before analysis. Two independent samples were measured under each experimental condition.

4. Results and Discussion

4.1. Size reduction energy consumption

As shown in Fig. 6, for all the biomass materials used in this study, size reduction energy consumption increased as biomass particle size decreased. This observed trend has been consistently reported in many studies with both the same size reduction equipment [21,32-35] and other machines [22-26,36-40]. In addition, to produce a specific amount of biomass particles of the same size, a size reduction machine with a higher power rating usually consumes less energy due to its higher material handling efficiency [32-36,41]. It was also noticed that when producing smaller biomass particles, the average current only increased slightly compared to that when producing larger biomass particles. The significant increase in energy consumption was mainly caused by the elongated time required to produce the same weight of biomass particles. In this stud, for wheat straw and corn stover, the energy consumed to produce 1 mm biomass particles doubled the energy consumption to produce 2 mm particles. This significant increase in energy consumption when biomass particle size decreased from 2 mm to 1 mm was also reported when conducting size reduction of other biomass materials such as switchgrass [32,33], Miscanthus [32,33], and barley straw [25]. Comparatively, for every particle size produced, big bluestem consumed the least amount of energy; producing 1 mm big bluestem only consumed 30% more energy than producing 2 mm big bluestem. It was observed that big bluestem particles had the best flowability amongst the three biomass materials. Particles smaller than the openings on the sieves could be discharged efficiently out of the milling chamber, which shortened the time required to produce the same amount of particles as required by the other two types. Contrarily, there is one report that found the opposite trend that producing larger particles consumed more energy [41]. This observation was mainly caused by the biomass flowability issue of larger particles (45 and 85 mm) in that they did not move through the grinder (HG-200, Vermeer Corp., Pella, IA, USA) as easily as the smaller particles (20 and 30 mm) [41].

4.2. Pelleting energy consumption

The effects of biomass particle size on pelleting energy consumption are shown in Fig. 7. It was noticed that pelleting wheat straw particles consumed more energy than the other two materials. Pelleting 4 and 2 mm particles consumed more energy than pelleting 1 mm particles under the same conditions for wheat straw. This observation agreed with the predicted trend of an empirical-model developed for this process [42]. Since larger biomass particles initially yield a lower bulk density of the particles in the mold than that do smaller particles; particles with a lower bulk density might require more energy to be condensed into a pellet [42]. For corn stover and big bluestem, biomass particle size did not significantly affect the pelleting energy consumption (α = 0.05). Song et al. also found that the energy consumption did not change significantly when pelleting 0.45, 1, 2, and 4 mm particles; however, pelleting 8 mm particles consumed a larger amount of energy [43]. Svihus et al. [44] studied the effects of biomass particle size on pelleting energy consumption using a pellet press (RPM350, Münch-Edelstahl GmbH, Wuppertal, Germany). The results indicated that there were no significant differences in energy consumption when pelleting 3 and 6.1 mm particles. Zhang et al. [45] also reported that the energy consumptions were approximately the same when pelleting 3.2 and 9.5 mm corn stover and sorghum stalk particles on a ring-die pelleting machine (CPM 2000, California Pellet Mill Co., Crawfordsville, IN, USA).

4.3. Pellet density

The pellet density data is shown in Fig. 8. Columns in the diagram show the out-of-mold density values. The upper bounds of the error bars are the initial density values and the lower bounds are the 24-h density values. It can be seen that pellets experienced a 15-25% spring-back in their volumes when taken out of the mold. They became more stable over the next 24 hours with less than 5% spring-back occurring. Only for wheat straw pellets, the amount of spring-back increased as the particle size became larger. Wheat straw pellets had a much higher density than the other two materials, yet the density of corn stover pellets was the lowest. In general, pellets made from 2 mm particles had a slightly higher average density than that of pellets made from 1 mm particles; while pellets made from 4 mm particles had the lowest density amongst the three particle sizes. Mani et al. [50] examined the effects of compressive force, particle size, and moisture content on the density of pellets made from corn stover, wheat straw, barley straw, and switchgrass. It was found that all these variables significantly affected the pellet density, the only exception being that no significant effect was observed regarding particle size on wheat straw pellet density. Three particle sizes, 0.8, 1.6, and 3.2, were produced on a hammer mill, and smaller particles (except wheat straw) produced denser pellets. Mani et al. explained that different viscous and elastic characteristics of biomass materials contributed to the different densification behaviors since cellulosic biomass is often regarded as a viscoelastic material [78-80].

4.4. Pellet durability

Pellet durability results are shown in Fig. 9. Pellets made from 1 and 2 mm biomass particles had similar durability, which was higher than that of pellets made from 4 mm biomass particles. Wheat straw pellets had the highest durability amongst the three biomass materials. Pellets made from 1 and 2 mm wheat straw particles had a nearly identical durability of 94%. The durability of pellets made from 4 mm wheat straw slightly decreased to 93%. Pellets made from 1 and 2 mm corn stover particles had a durability of 90%, and the durability decreased to 87% with 4 mm corn stover particles. Big bluestem pellets were less durable compared to the other two materials. Their durability results dropped from 86% to 80% as particle size increased from 1 to 4 mm. It has been broadly reported that finer particles generally correspond with greater durability as larger particles may serve as fissure points to initiate cracking or splitting in pellets [52]. With the same pelleting mechanism as used in this study, Zhang et al. by doing a full factorial design of experiments found that 2 mm wheat straw particles produced more durable pellets than 1 mm particles [64]. With conventional pelleting methods (without ultrasonic vibration), Singh and Kashyap reported that decreasing rice husk particle size from 5 to 4 mm increased pellet durability from 84% to 95% [56]. Franke and Rey found that the smallest particles in their study, ranging from 0.5 to 0.7 mm, were the most feasible in generating high quality pellets [57]. Hoover et al. reported that 4 mm corn strover particles produced pellets with a higher durability than the 6 mm particles [51]. In a study of alfalfa pellet durability, Hill and Pulkinen [58] noted that a decrease in particle size from 6.4 to 2.8 mm increased the durability by more than 15%. Several other researchers observed that optimal pellet durability was achieved with a mixture of particle sizes due to increased inter-particle bonding and the elimination of inter-particle spaces [53,59].

4.5. Sugar yield

Fig. 10 shows the sugar yield results measured after 24-h (A) and 48-h (B) hydrolysis. For the same biomass material, 48-h sugar yield was approximately 10-15% higher than 24-h sugar yield, aside from 1 and 2 mm wheat straw, whose 24 and 48-h sugar yields were almost the same. Results in this study reveal that 2 mm particles had the highest sugar yield for all three biomass materials. Big bluestem had the highest sugar yield at all three particle size levels, with the 2 mm particles yielding about 20% more sugar after 48-h hydrolysis than the other two sizes. For wheat straw, the sugar yield differences between the 2 mm particles and the other two sizes were about 10%. The sugar yield differences among the corn strover particles of three size levels were not significant (α = 0.05).

Many studies have been conducted to investigate the relationship between biomass particle size and sugar yield; however, results from these studies are inconsistent. As summarized in Table 1, three different relationships have all been reported: smaller particle size produces higher sugar yield; particle size has no significant effects on sugar yield; and larger particle size produces higher sugar yield. Two literature reviews present comprehensive summaries of reported studies regarding the effects of biomass particle size on sugar yield [22,81]. Observations from the literature reveal that the inconsistent relationships between biomass particle size and sugar yield could be a result of different (1) sugar yield definitions (sugar content vs. conversion efficiency), (2) particle size ranges (micro vs. millimeter), (3) types of biomass (pure cellulose vs. lignocellulosic biomass), and (4) different pretreatment methods (hydrothermal pretreatment vs. chemical pretreatment). It is also possible that in some cases particle size is a weak predictor of hydrolysis sugar yield, and has little effects on a substrate that has already been susceptible to hydrolysis [76].

5. Concluding Remarks

This study presented a comprehensive experimental investigation on the conversion of corn stover, wheat straw, and big bluestem cellulosic biomass into fermentable sugars. The three biomass materials were processed through the feedstock preprocessing stage (size reduction and pelleting) and bioconversion stage (pretreatment and hydrolysis). This study analyzed the effects of biomass particle size on various evaluation parameters throughout the multistage biofuel manufacturing process. The following conclusions were drawn:

  1. Energy consumption in size reduction increased as particle size decreased. This increase was more significant when particle size was reduced to 1 mm.
  2. For corn stover and big bluestem, biomass particle size did not significantly affect the energy consumption in pelleting. As for wheat straw, pelleting 4 and 2 mm particles consumed more energy than pelleting 1 mm particles.
  3. Pellets made from 2 mm particles had a higher average density than that of 1 mm pellets, while pellets made from 4 mm particles had the lowest density amongst the three particle sizes.
  4. Pellets made from 1 and 2 mm biomass particles had similar durability, which was higher than that of pellets made from 4 mm particles.
  5. The highest sugar yield was produced by 2 mm particles.

Based on the above discussion, 2 mm particle size was recommended under the condition employed in this study. This study serves as an example to show that in order to realize cost-effective biofuel manufacturing, research in the feedstock preprocessing stage and the bioconversion stage should not be isolated from each other. More quantitative process optimization is needed from a system engineering perspective to provide practical guidance on selecting the optimal particle size. Observed trends have to be validated on a production scale platform. On the other hand, how mechanical impacts during the feedstock preprocessing stage alter the compositional and structural features of biomass that affect the biomass susceptibility to enzymes has to be further investigated. From these two perspectives, the fundamental connection linking the two stages in biofuel manufacturing can be established.

Acknowledgement

The authors would like to acknowledge undergraduate research assistants Nick Eisenbarth, Lane Sorell, and Catharine Lei, for their help in conducting experiments and measurements for this work.

Funding

This study is supported by the U.S. National Science Foundation through Award 1562671.

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Figure Caption List

 

Fig. 1 Illustration of the structure of cellulosic biomass

Fig. 2 Major steps in cellulosic biofuel manufacturing

Fig. 3 Milling chamber of the knife mill

Fig. 4 Schematic of the pelleting process (with permission from Springer International Publishing AG)

Fig. 5 Durability tester

Fig. 6 Effects of biomass particle size on size reduction energy consumption

Fig. 7 Effects of biomass particle size on size pelleting energy consumption

Fig. 8. Effects of biomass particle size on pellet density

Fig. 9. Effects of biomass particle size on pellet durability

Fig. 10. Effects of biomass particle size on sugar yields after (a) 24-h and (b) 48-h hydrolysis

Table Caption List

 

Table 1. A summary of reported studies on the effects of biomass particle size

Fig. 1 Illustration of the structure of cellulosic biomass

Fig. 2 Major steps in cellulosic biofuel manufacturing

Fig. 3 Milling chamber of the knife mill

Fig. 4 Schematic of the pelleting process (with permission from Springer International Publishing AG)

C:UsersMengZhangDropboxBiomass and biofuel research with PeterJournal paperPhoto Jun 26, 1 41 27 PM.jpg

Fig. 5 Durability tester

Fig. 6 Effects of biomass particle size on size reduction energy consumption

Fig. 7 Effects of biomass particle size on size pelleting energy consumption

Fig. 8. Effects of biomass particle size on pellet density

Fig. 9. Effects of biomass particle size on pellet durability

Fig. 10. Effects of biomass particle size on sugar yields after (a) 24-h and (b) 48-h hydrolysis

Table 1. A summary of reported studies on the effects of biomass particle size

Size reduction energy consumption Pelleting energy consumption Pellet density Pellet durability Pretreatment sugar recovery Hydrolysis sugar yield
Smaller particle size resulted in higher____. [21-26,32-40] N.A. [47-50] [51-59] N.A. [64-72]
Larger particle size resulted in higher ____. [41] [42,43] N.A. N.A. [62,63] [73-75]
Particle size was a weak indicator of ____. N.A. [44-46] [51] [60,61] N.A. [27,28,30,76,77]

*Corresponding author. Tel.: +1 785 532 3732; fax: +1 785 532 3738.

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