Optimization of Pilot Scale Mechanical Disk Refining for Improvements in Enzymatic Digestibility of Pretreated Hardwood Lignocellulosics

Abstract

Mechanical refining has potential application for overcoming lignocellulosic biomass recalcitrance to enzyme hydrolysis and improving biomass digestibility. This study highlighted the ability for a pilot scale disc refiner to improve the total carbohydrate conversion to sugars from 39% (unrefined hardwood sodium carbonate biomass) to 90% (0.13 mm gap, 20% consistency, ambient temperature) by optimizing the refining variables. The different biomass properties that changed with refining indicated the expected increase in sugar conversion. Controlling the refining parameters to narrower gaps and higher consistencies increased the resulting refined biomass hydrolysis. Positive correlations that increases in net specific energy (NSE) input and refining intensity (SEL) improved the enzymatic hydrolysis. In some severe cases, over-refining occurred when smaller gaps, higher consistencies, and more energy input reached a point of diminished return. The energy input in these scenarios, however, was much greater than realistically feasible for industrial application. Although well-established in the pulp and paper industry, gaps in understanding the fundamentals of refining remain. The observations and results herein provide the justification and opportunity for further mechanical refining optimization to maximize and adapt the mechanical refining technology for maximum efficiency within the process of biochemical conversion to sugar. Keywords: Hardwood biomass; Biochemical conversion; Pretreatment; Mechanical refining; Enzymatic hydrolysisContact information: Department of Forest Biomaterials, North Carolina State University, Raleigh, NC 27695-8005 USA; *Corresponding author: richard_venditti@ncsu.edu INTRODUCTION Biorefinery ConceptThe biorefinery concept, defined as "the sustainable processing of biomass into a spectrum of marketable products and energy," is an approach that can mitigate the negative environmental impacts of fuel and chemical production by providing alternatives to oil refining products (Cherubini 2010).Lignocellulosic biomass is an extremely diverse class of material and has a complex structure composed of cellulose, hemicellulose, and lignin naturally constructed in a multifaceted matrix. Understanding the chemical and biochemical challenges based on lignocellulosic biomass recalcitrance, which can be defined as the natural resistance to biological deconstruction, remains one of the major technical barriers to the commercialization of second generation cellulosic biorefineries (Zhao et al. 2012). Embracing the challenge as an intentional practice of engineered biomass deconstruction PEER-REVIEWED ARTICLE bioresources.com Jones et al. (2017). "Mechanical disk refining," BioResources 12(3), 4567-4593. 4568 allows for an opportunity to harvest the inherently useful properties of biomass for creating value-added products through the biorefinery concept. Many innovative technologies have been developed to address this issue and improve the biomass hydrolysis yield. The issue with these technologies...

Toward an understanding of the increase in enzymatic hydrolysis by mechanical refining

“…The refining action, especially at high refining intensity, tended to reduce particle size and increase the abundance of fines due to fiber cutting and fibrillation. Reduction of particle size was usually correlated with an increase in enzyme-accessible surface area [ 7 – 15 ]. Water retention values, dye adsorption and differential scanning calorimetry were used to access the effect of mechanical refining on biomass surface area.…”

“…Water retention values, dye adsorption and differential scanning calorimetry were used to access the effect of mechanical refining on biomass surface area. Refining processes increased cell wall porosity and surface area, which contributed to increase the exposure of carbohydrates to cellulolytic enzymes [ 9 , 10 , 12 , 13 , 15 , 16 ]. The effect of mechanical refining on cellulose crystallinity was also evaluated.…”

Fiber fractionation to understand the effect of mechanical refining on fiber structure and resulting enzymatic digestibility of biomass

“…Mechanical disk refining has emerged as one of the most promising pretreatment technologies for use in biorefinery applications due to its effectiveness and relatively low cost. Mechanical disk refining has been shown to improve enzymatic hydrolysis efficiency substantially when performed on pulp that had been subjected to only mild conventional pretreatment (de Assis et al, ; Batalha et al, ; Jones, ; Corbett, Venditti, Jameel, & Park, ; Gonzalez et al, ; Hoeger et al, ; Jones, Venditti, Park, & Jameel, ; Jones, Venditti, Park, Jameel, & Koo, ; Jones, Venditti, Park, & Jameel, ; Koo et al, ; Park et al, ; Phillips, Jameel, & Chang, ). Effective enzymatic hydrolysis depends on exposed surface area and good accessibility of carbohydrates to enzymes (Leu & Zhu, ).…”

“…Mechanical refining has been well‐optimized for the purposes of paper making, however, similar optimization efforts for improvement of fiber reactivity are still in the early stages. Recent research in this field aims to better understand the impact of mechanical refining on fiber properties in the context of fiber reactivity so as to be able to maximize the reactivity of fibers towards enzymatic digestion (or other chemistry) at minimized energy consumption (Jones, ; Corbett et al, ; Jones et al, ).…”

“…the impact of mechanical refining on fiber properties in the context of fiber reactivity so as to be able to maximize the reactivity of fibers towards enzymatic digestion (or other chemistry) at minimized energy consumption (Jones, 2017;Corbett et al, 2018;Jones et al, 2017).…”