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

Abstract

Biofuels derived from cellulosic biomass offer one of the best near- to midterm alternatives to petroleum-based liquid transportation fuels. Biofuel conversion is mainly done through a biochemical pathway in which size reduction, pelleting, pretreatment, enzymatic hydrolysis, and fermentation are main processes. Many studies reveal that biomass particle size dictates the energy consumption in the size reduction. Biomass particle size also influences sugar yield in enzymatic hydrolysis, and biofuel yield in fermentation is approximately proportional to the former enzymatic hydrolysis sugar yield. Most reported studies focus on the effects of biomass particle size on a specific process; as a result, in the current literature, there is no commonly accepted guidance to select the overall optimum particle size in order to minimize the energy consumption and maximize sugar yield. This study presents a comprehensive experimental investigation converting three types of biomass (big bluestem, wheat straw, and corn stover) into fermentable sugars and studies the effects of biomass particle size throughout the multistep bioconversion. Three particle sizes (4 mm, 2 mm, and 1 mm) were produced by knife milling and were pelletized with an ultrasonic pelleting system. Dilute acid method was applied to pretreat biomass before enzymatic hydrolysis. Results suggested 2 mm is the optimum particle size to minimize energy consumption in size reduction and pelleting and to maximize sugar yield among the three particle sizes for big bluestem and wheat straw biomass. Nevertheless, there is no significant difference in sugar yield for corn stover for the three particle sizes.

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“…Size reduction is an essential step in the biomass-fuel conversion process . First, this step helps increase the bulk density of the feedstock through a smaller particle size, thereby reducing the cost of handling, storage, and transportation. − Since the feedstock surface area is increased, the conversion rate and yield are also enhanced. − Size reduction often is achieved by mechanical milling machines, such as hammer mill, knife mill, ball mill, drum/disc mill, and so on. − In particular, hammer mills have been widely used because they are relatively inexpensive, easy to operate and maintain, feasible for various biomass materials, and capable of producing feedstocks of different sizes. , However, hammer mills consume more energy than desired, and the energy consumption is exponentially increased as the screen size decreases . Furthermore, hammer mills experience difficulties in grinding of high-moisture biomass feedstocks (above around 20 wt %) because of their high toughness. , Hammer mills, because of the crushing mode, also produce a significant amount of fines that are not usable in common conversion processes.…”