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Mechanical refining is widely used in the pulp and paper industry to enhance the end-use properties of products by creating external fibrillation and internal delamination. This technology can be directly applied to biochemical conversion processes. By implementing mechanical refining technology, biomass recalcitrance to enzyme hydrolysis can be overcome and carbohydrate conversion can be enhanced with commercially attractive levels of enzymes. In addition, chemical and thermal pretreatment severity can be reduced to achieve the same level of carbohydrate conversion, which reduces pretreatment cost and results in lower concentrations of inhibitors. Refining is versatile and a commercially proven technology that can be operated at process flows of ∼ 1500 dry tons per day of biomass. This paper reviews the utilization of mechanical refining in the pulp and paper industry and summarizes the recent development in applications for biochemical conversion, which potentially make an overall biorefinery process more economically viable.

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...

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