Investigating pellet charring and temperature in ultrasonic vibration-assisted pelleting of wheat straw for cellulosic biofuel manufacturing
Introduction
The most commonly used transportation fuels are petroleum-based fuels, whose supplies are expected to decline in the future [1], [2]. Biofuels including bioethanol and biodiesel alternatives to petroleum-based transportation fuels can potentially dampen price volatility in transportation fuels, reduce economic and security concerns related to importing oil from other countries, and reduce the greenhouse gas (GHG) emissions along with associated risks of global climate change [1], [2], [3]. Currently, primary feed materials for biofuels manufacturing are sugarcane (in Brazil), corn (in U.S.), and rapeseed (in Europe). The use of edible crops for biofuels manufacturing may result in increasing of food price. Therefore, it is crucial to utilize no-food crops for biofuels manufacturing, and cellulose biomass feedstock. Cellulosic biomass includes agricultural residues, forestry and wood pulp wastes, and energy crops [5], [6]. The U.S. government expects that annual production of cellulosic biofuels will reach to 16 billion gallons by 2022, and cellulosic biomass can be available in the U.S. for biofuels manufacturing to meet 30% of the current transportation fuel consumptions [4], [7].
Fig. 1 shows major steps for cellulosic biofuels manufacturing [8], [9]. The low density of cellulosic feedstocks is a main barrier hindering large-scale and cost-effective manufacturing of cellulosic biofuels [10], [11], [12], [13]. Under certain conditions, costs of biomass collection, transportation, and storage account for more than 80% of feedstock cost [14]. Cellulosic biomass densification will increase density, improve handling efficiency [15], [16], and reduce transportation and storage costs [17], [18], [19]. Pelleting is a popular process to process cellulosic biofuels into pellets.
Traditional pelleting methods (for example, using a screw extruder, a briquetting press, or a rolling machine [18], [20], [21], [22], [23]) generally require high-temperature steam, high pressure, and/or binder materials, making it difficult to realize cost-effective pelleting on or near the field where cellulosic biomass is available. Ultrasonic vibration-assisted (UV-A) pelleting, without using high-temperature steam and binder materials, can produce pellets whose density is comparable to that processed by traditional pelleting methods [24], [25], [26]. Moreover, biomass (switchgrass) processed by UV-A pelleting has approximately 20% higher sugar yield (approximately proportional to biofuel yield) than biomass pelleted without ultrasonic vibration [24].
The reported experimental investigations in UV-A pelleting include effects of pelleting parameters on pellet quality, pelleting temperature, charring, and sugar yield [24], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. Under certain UV-A pelleting conditions, charring occurs inside pellets produced. As shown in Fig. 2, charring occurred inside the pellet, and outside of the pellet might appear to be normal before pellet being separated into halves. An experimental study on charring of cellulosic biomass in UV-A pelleting has been carried out by Feng et al. [27], however, it is still unclear about the reasons of pellets charring generation. The pellets charring may be resulted from high pelleting temperature generated by ultrasonic vibration in UV-A pelleting. Currently, there are no reported studies on the relationship between charring ratio and pelleting temperature. Therefore, it is necessary to explore the optimal pelleting temperature for a low charring ratio with further obtaining a high biofuel conversion yield.
In this paper, effects of various input parameters (pelleting duration, ultrasonic power, pelleting pressure, pellet weight, and moisture content) on charring ratio and temperatures at three different locations (at the center, on the top and bottom of a pellet) were studied. In addition, the relationship between charring ratio and pelleting temperature in UV-A pelleting was explored. The obtained results will contribute to the understanding of the influences of pelleting parameters on both charring ratio and temperature for obtaining decreased charring with high yield of biofuel.
Section snippets
Biomass material and feedstock preparation
The cellulosic biomass used in this study was wheat straw collected from a farm in western Kansas. The wheat straw was run through a combine (9600, John Deere, Moline, IL, USA). The wheat straw and chaff exited through the back of the combine. The straw chopper on the combine was disconnected to allow the straw to be baled. The longest pieces of wheat straw coming out from the combine were 28 cm long.
Before pelleting, wheat straw was milled into powder by a knife mill (SM 2000, Retsch,
Effects of ultrasonic power
Fig. 6 shows the temperature comparisons at different locations under various ultrasonic powers from 40% to 70%. Data points represent the average value and the error bars in Fig. 6, as well as in Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12, represent the maximum and the minimum values of temperature, respectively. It can be seen that at different levels of ultrasonic power, the temperature (T2) at center of pellet was always higher than the temperature (T1) at the top of pellet that was
Conclusions
In this paper, effects of different input variables including ultrasonic power, pelleting pressure, pellet duration, pellet weight, and moisture content on both charring ratio and pelleting temperature were studied. This paper, for the first time, reported the relationship between charring ratio and pelleting temperature. Major conclusions are:
- (1)
Charring always initially occurred at the center of a pellet. Also, the temperature at pellet center was always the highest among the temperatures at
Acknowledgments
This study is supported by the Foundation of the Whitacre College of Engineering and the Office of Vice President for Research at Texas Tech University, and is partially supported by U.S. National Science Foundation (NSF) through award CMMI-1538381 and National Natural Science Foundation of China (NSFC) through Grant 51275097.
References (38)
- et al.
Temperature controlled feed layer formation in biofuel pellet production
Fuel
(2012) - et al.
Fuel pellets from biomass: the importance of the pelletizing pressure and its dependency on the processing conditions
Fuel
(2011) - et al.
Internal particle size distribution of biofuel pellets
Fuel
(2011) - et al.
Characteristics of some biomass briquettes prepared under modest die pressures
Biomass Bioenergy
(2000) Analysis of extrusion compaction of fibrous agricultural residues for fuel applications
Biomass
(1990)- et al.
Effects of compressive force, particle size and moisture content on mechanical properties of biomass pellets from grasses
Biomass Bioenergy
(2006) Annual Energy Review 2010
(2011)Public Law 110-140
(2007)- et al.
Effect of kaolin and limestone addition on slag formation during combustion of wood fuels
Energy & Fuels
(2004) Short-term Energy Outlook
(2012)
Biomass as Feedstock for a Bioenergy and Bioproducts Industry: the Technical Feasibility of a Billion-ton Annual Supply
Sustainable liquid biofuels from biomass: the writing's on the walls
New Phytol.
Genomics of cellulosic biofuels
Nature
Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis for Corn Stover
Thermochemical Ethanol via Indirect Gasification and Mixed Alcohol Synthesis of Lignocellulosic Biomass
Breaking the Chemical and Engineering Barriers to Lignocellulosic Biofuels: Next Generation Hydrocarbon Biorefineries
Ethanol Faces Challenges Ahead
The United States and China: the Race to Disruptive Transport Technologies, Implication of a Changing Fuel Mix on Country Competiveness
The Pelleting Process
Cited by (11)
Effect of ultrasonic vibration-assisted pelleting of biomass on biochar properties
2021, Journal of Cleaner ProductionCitation Excerpt :The higher biochar yield was obtained from PP00, SG50, and WS50 compared to other pretreated conditions for the same biomass. The possible reason is that the lignin shell of biomass particles is destroyed by ultrasonic power (Li et al., 2016) and then are compressed into pellets to form a new structure. In addition, thermal properties also varied with type of raw materials (Theerarattananoon et al., 2011).
Comparative investigation on thermal decomposition of powdered and pelletized biomasses: Thermal conversion characteristics and apparent kinetics
2020, Bioresource TechnologyCitation Excerpt :The effect of pelletization parameters on energy consumption and pellet properties was evaluated by Li et al. (2015) and Hosseinizand et al. (2018). Li et al. (2016a) found that ultrasonic vibration-assisted pelleting can not only produce higher density but also break the lignin shell, to some extent, to increase cellulose accessibility and then increase sugar and biofuel yield. In addition, the thermal decomposition of pelletized biomass had also received the attention of researchers.
Ultrasonic vibration-assisted (UV-A) manufacturing processes: State of the art and future perspectives
2020, Journal of Manufacturing ProcessesCitation Excerpt :Moreover, ultrasonic energy could produce an intense hydro-mechanical shear force [73,74]. This impact could break the lignin shell of the particle, provide an additional bond in pellets, and increase cellulose accessibility [72,74]. A clear contrast between pelleting without ultrasonic vibration and UV-A pelleting for producing pellets is given in Fig. 7.
Predictive temperature modeling and experimental investigation of ultrasonic vibration-assisted pelleting of wheat straw
2017, Applied EnergyCitation Excerpt :Zhang [21] and Tang et al. [20] measured the temperature curve of biomass in UV-A pelleting. Li et al. [22] studied the relationship between charring ratio and temperature in UV-A pelleting. However, effects of input variables on temperature ranges (HPT and LPT) were not studied.