Impact of planetary ball mills on corn stover characteristics and enzymatic digestibility depending on grinding ball properties
Graphical abstract
Introduction
The pretreatment of lignocellulosic biomass during biofuel production disrupts the cell wall, reduces the recalcitrance of the cell structure, and ultimately improves the accessibility of carbohydrates to cellulose and hemicellulose. The pretreatment step is key to determining the efficacy of the biofuel production, typically when lignocellulosic biomass is used. The pretreatment includes physical, biological, and chemical processes (Agbor et al., 2011). Broadly used chemical pretreatment methods, especially dilute acid methods (Yang and Wyman, 2008), produce inhibitors for enzyme hydrolysis and fermentation, resulting in the need for additional processes such as washing and detoxification (Kim et al., 2013).
Mechanical methods including chipping, grinding, and milling are relatively free from these limitations; however, they usually require high levels of energy (Chiaramonti et al., 2012). Recently, combinational processes such as physicochemical or thermo-mechanical pretreatment were developed to reduce the energy requirement (Barakat et al., 2014, Chandra et al., 2016, Kim et al., 2016, Zakaria et al., 2014). A ball mill, a powerful mechanism, uses moving metallic or ceramic balls as grinding media to grind biomass particles. There are three types of ball mills, a conventional ball mill, attrition mill, and planetary mill, so named according to the movement mode of the balls. Conventional ball mills utilize solely natural gravity to exert force on the particles, while the more advanced planetary mills employ artificial gravity to apply force using a centrifugal force field. Planetary mills generate a non-uniform field of centripetal acceleration inside the grinding jar, which results in a higher impact energy (Kim et al., 2013). Attrition mills rotate the stirrer to lead the movement of the grinding balls. Conventional ball mills are usually used in conjunction with chemical treatments due to their low efficiency (Zhao et al., 2006). Planetary and attrition mills are effective in fermentable sugar production (Kim et al., 2013). Ball mill processes employ relatively long milling times associated with high energy consumption. Despite their high energy demand, ball mills are still promising methods in reducing the size and crystallinity of lignocellulosic biomass. The ball mill imposes stress and decreases the degree of polymerization of cellulose. The process produces very low levels of inhibitors and reduces biomass loss (Kim et al., 2013). Kwon et al. (2016) reported on facile biofuel production using planetary mill pretreatment. In their study, low quantities of soluble phenolic was produced after milling the Pinus rigida wood waste and it did not cause any growth inhibition of Clostridium beijerinckii NCIMB 8052. Vaidya et al. (2016) investigated the mechanism of wet ball milling of Pinus radiate. According to their study, crushing was the main mechanism for biomass size reduction when the dry mill mode was used. On the other hand, “tearing” was the main size reduction mechanism when wet milling was used. During wet milling, breakage of intact fibers into porous fragments is followed by delamination, fibrillation, and compression to disrupt the recalcitrance of biomass.
Previous studies optimized process parameters such as solid-to-liquid ratio, buffering medium, and milling time during planetary mill processes (Kim et al., 2013, Kwon et al., 2015). However, the characteristics of pretreated biomass are mainly influenced by the properties of grinding balls, rather than process conditions. Despite its importance, the relationship between the ball properties and characteristics of pretreated biomass has not been previously investigated. In this study, we used three types of commercially available grinding balls of different specific gravity and surface morphology, made out of alumina, zirconia, and steel. The characteristics and enzyme accessibility of corn stover particles were investigated after planetary ball mill pretreatment depending on the grinding ball properties. Since high energy consumption is the main drawback of using ball mill pretreatment in industrial applications, this study focused on developing a low energy consumption process for ball mill pretreatment.
Section snippets
Materials
Corn stover provided by CJ Cheil Jedang, Republic Korea, was used as biomass material in this study. The corn stover was grown and harvested in China in September 2015. Prior to pretreatment, the corn stover was ground and screened to >5 mm-size to improve the reproducibility of the result, then air-dried at 60 °C for 24 h. Three types of grinding balls were used in this study. Alumina and zirconia balls were purchased from FRITSCH GmbH (Germany). Steel balls were purchased from Taemyoung
Properties of grinding balls
The grinding balls used in this study have different properties. Alumina balls have a low specific gravity of 4.09 g/cm3 and steel balls are 1.9 times heavier than alumina balls (Table 1). Arithmetic mean height is generally used to express surface roughness. As shown in Table 1, the alumina and zirconia balls have higher surface roughness values while the steel balls have relatively smooth surfaces. The heights of the peaks on alumina balls are significantly higher than on steel balls. The
Conclusions
By planetary milling, no significant changes were detected in both C5 and C6 sugars. The use of steel balls was more effective in size reduction with all tested milling times. However, for the glucose yield, the use of alumina balls showed higher yield with 60 min milling time while that of steel balls showed higher levels with 20 min milling time. Additionally, steel balls led to higher size reduction effect but the alumina balls resulted in increased surface area of corn stover particles with 60
Acknowledgements
This work was supported by a grant from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (Grant # 20153010091990). Dr. Hyunsik Kim and Sung Hong Lee provided technical support in the use of Microscopes/3D surface profilers.
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