Significance Statement
Cellulosic ethanol made from cellulosic biomass serves as a renewable alternative to petroleum-based liquid transportation fuels. Cellulosic ethanol production from cellulosic biomass offers various advantages in terms of little competition with limited agricultural lands, rural economic development, reduction in greenhouse gas, improvement of soil fertility and agricultural ecology.
A new study by Zhang et al. (2016) and published in Journal, Biomass Conv. Bioref. compared pellet quality, temperature, energy consumption and sugar yield of corn stover processed by ultrasonic vibration-assisted pelleting versus ring-die pelleting with two levels of sieve sizes.
Cost effectiveness due to transportation and storage of low-density cellulosic biomass has been a major challenge to cellulosic ethanol. Pelleting significantly increases density of raw cellulosic biomass ranging from 40 to 250kg/m3 to over 600kg/m3. This definitely reduces the cost of transport and storage of raw cellulosic biomass which also makes them have uniform physical features for easier handling.
Two pelleting methods for cellulosic biomass include traditional pelleting methods (press briquetting, screw extruding and ring-die) and ultrasonic vibration-assisted. The former makes use of high temperature steam, high pressure and often binder materials while the latter does not involve high temperature steam, high pressure and binder materials. Ultrasonic vibration-assisted have been known to produce pellets with same density and durability as that of traditional pelleting methods with high sugar yield.
Corn stover was baled and transported to Bioprocessing and Industrial value-added program. The chopped corn stover was milled into two particle levels using a 7.4KW hammer mill. After hammer milling, moisture content of corn stover particles was measured and adjusted to a desired level by following NREL laboratory procedure.
Ultrasonic vibration-assisted pelleting was conducted on a modified ultrasonic machine which composed mainly of three systems such as an ultrasonic generation system, a pneumatic loading system and biomass holding system, as shown in Fig.1. Five steps in making of pellet in ultrasonic vibration-assisted machine include; assembly and feeding of pelleting tool done to compress corn stover particles, turn on ultrasonic power supply and apply ultrasonic vibration, turn off ultrasonic power supply and lift up pelleting tool after pelleting duration and disassembly of mold to take out pellet.
Figure Legend 1. Ultrasonic vibration-assisted pelleting.
Experimental setup for ring-die pelleting had steam conditioning chamber by a screw feeder rotating at 7rpm. Major reasons for variables such as rotation speed of ring die, diameter and length of ring die channel include; commonly used values in literature and values that can produce pellets with high density and durability.
Pellet density was calculated as ratio of its weight over its volume. Pellets volume were obtained by measuring pellet height with diameter and each was measured three times. 100g instead of 500g of pellets were used to measure pellet durability based on ASABE standard S269.4.
Sugar yield was measured as amount of glucose obtained after pretreated enzymatic hydrolysis. Concentration of glucose solution in Autosampler vials was determined by a high-performance liquid chromatography. Measurement of temperature was achieved by using thermocouples, thermometer and a computer with data acquisition software package. Scanning Electron Microscopy was used to observe pellet microstructure of biomass.
Results from experiment showed both ultrasonic vibration-assisted and ring-die method had density higher than 900kg/m3 which is higher than that of raw cellulosic biomass, as shown in Fig 2. When smaller sieve size (3.2mm) was used, mean value of Ultrasonic Vibration-Assisted pellet density was 1100kg/m3 about 11% higher than that of ring-die pellet. When large sieve size (9.5mm) was used, mean value of Ultrasonic Vibration-Assisted pellet density was 1034kg/m3, about 6% higher than ring-die pellets.
Figure Legend 2: Comparison of pellet density between UV-A pelleting and ring-die pelleting.
Figure 3 shows that both pellets from ring-die method and ultrasonic vibration-assisted had pellet durability higher than 90%. When smaller sieve size (3.2mm) was used, pellet durability of ultrasonic vibration-assisted and ring-die pellets were 98.5% and 94.4% respectively. When larger sieve size (9.5mm) was used, pellet durability of Ultrasonic Vibration-Assisted pellets and ring-die pellets were 93.4% and 91.2% respectively.
Figure Legend 3: Comparison of pellet durability between UV-A pelleting and ring-die pelleting.
Results on sugar yield showed smaller particles (3.2mm sieve) 67.1% and 60.9% for ultrasonic vibration-assisted pellets and ring-die pellets respectively while larger particles (9.5mm sieve) sugar yield of ultrasonic vibration-assisted pellets and ring-die pellet was 59.9% and 54.9% respectively, as shown in Fig.4.
Figure Legend 4: Comparison of sugar yield between UV-A pelleting and ring-die pelleting.
The increasing rate of temperature in ultrasonic vibration-assisted pelleting was faster than that of ring-die pelleting. Energy consumption of ultrasonic vibration-assisted pelleting (296-310KWh/ton) was almost three times higher than that of ring-die pelleting (122-125KWh/ton), as shown in Fig. 5. The lab-scale setup of ultrasonic vibration-assisted was known to limit its efficiency.
Figure Legend 5: Comparison of energy consumption between UV-A pelleting and ring-die pelleting.
Scanning Electron Microscopy results inferred high pelleting temperature and shear forces during pelleting softened biomass surface and exposed more microfibrils which aids in enzymatic hydrolysis resulting in high sugar yield, as shown in Fig.6.
Figure Legend 6: Biomass microstructure after UV-A pelleting.
Zhang et al. (2016) study on comparison of corn stover pellets in ultrasonic vibration-assisted pelleting and ring-die pelleting showed that the former has a potential to be further developed and more research needs to be done to improve its pelleting efficiency and reduce energy consumption.
Journal Reference
Qi Zhang1,2, Lin Heng1, Pengfei Zhang2,3, Z. J. Pei2, Donghai Wang4, Jonathan Wilson5, JiPing Zhou1 . Comparison of two pelleting methods for cellulosic ethanol manufacturing: ultrasonic vibration-assisted pelleting vs. ring-die pelleting. Biomass Conversion and Biorefinery, March 2016, Volume 6, Issue 1, pp 13–23.
Show Affiliations- College of Mechanical Engineering, Yangzhou University Yangzhou China.
- Department of Industrial and Manufacturing Systems Engineering, Kansas State University Manhattan USA.
- Jiangsu Muyang Holdings Yangzhou China.
- School of Biological and Agricultural Engineering, Kansas State University Manhattan USA.
- Department of Grain Science and Industry Kansas State University Manhattan USA.
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