https://arrow.tudublin.ie/cgi/viewcontent.cgi?article=1214&context=tfschafart

Emerging technologies for the pretreatment of lignocellulosic biomass.

The implementation of biorefineries based on lignocellulosic materials as an alternative to fossil-based refineries calls for efficient methods for fractionation and recovery of the products. The focus for the biorefinery concept for utilisation of biomass has shifted, from design of more or less energy-driven biorefineries, to much more versatile facilities where chemicals and energy carriers can be produced. The sugar-based biorefinery platform requires pretreatment of lignocellulosic materials, which can be very recalcitrant, to improve further processing through enzymatic hydrolysis, and for other downstream unit operations. This review summarises the development in the field of pretreatment (and to some extent, of fractionation) of various lignocellulosic materials. The number of publications indicates that biomass pretreatment plays a very important role for the biorefinery concept to be realised in full scale. The traditional pretreatment methods, for example, steam pretreatment (explosion), organosolv and hydrothermal treatment are covered in the review. In addition, the rapidly increasing interest for chemical treatment employing ionic liquids and deep-eutectic solvents are discussed and reviewed. It can be concluded that the huge variation of lignocellulosic materials makes it difficult to find a general process design for a biorefinery. Therefore, it is difficult to define “the best pretreatment” method. In the end, this depends on the proposed application, and any recommendation of a suitable pretreatment method must be based on a thorough techno-economic evaluation.

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Pretreatment for biorefineries: a review of common methods for efficient utilisation of lignocellulosic materials

Cascade utilization of lignocellulosic biomass to high-value products

Cavitation is the sudden, unstable expansion of a void or bubble within a liquid or solid subjected to a negative hydrostatic stress. Cavitation rheology is a field emerging from the development of a suite of materials characterization, damage quantification, and therapeutic techniques that exploit the physical principles of cavitation. Cavitation rheology is inherently complex and broad in scope with wide-ranging applications in the biology, chemistry, materials, and mechanics communities. This perspective aims to drive collaboration among these communities and guide discussion by defining a common core of high-priority goals while highlighting emerging opportunities in the field of cavitation rheology. A brief overview of the mechanics and dynamics of cavitation in soft matter is presented. This overview is followed by a discussion of the overarching goals of cavitation rheology and an overview of common experimental techniques. The larger unmet needs and challenges of cavitation in soft matter are then presented alongside specific opportunities for researchers from different disciplines to contribute to the field.

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Cavitation in soft matter.

Lignocellulosic biomasses are primarily composed of cellulose, hemicelluloses and lignin and these biopolymers are bonded together in a heterogeneous matrix that is highly recalcitrant to chemical or biological conversion processes. Thus, an efficient pretreatment technique must be selected and applied to this type of biomass in order to facilitate its utilization in biorefineries. Classical pretreatment methods tend to operate under severe conditions, leading to sugar losses by dehydration and to the release of inhibitory compounds such as furfural (2-furaldehyde), 5-hydroxy-2-methylfurfural (5-HMF), and organic acids. By contrast, supercritical fluids can pretreat lignocellulosic materials under relatively mild pretreatment conditions, resulting in high sugar yields, low production of fermentation inhibitors and high susceptibilities to enzymatic hydrolysis while reducing the consumption of chemicals, including solvents, reagents, and catalysts. This work presents a review of biomass pretreatment technologies, aiming to deliver a state-of-art compilation of methods and results with emphasis on supercritical processes.

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Supercritical Fluids: A Promising Technique for Biomass Pretreatment and Fractionation.

Acoustic and hydrodynamic cavitation were examined as suitable mechanical pretreatments for lignocellulosic biomass. Microcrystalline cellulose and lime-treated sugarcane bagasse were subjected to acoustic cavitation, whereas raw and lime-treated sugarcane bagasse were subjected to hydrodynamic cavitation. Acoustic cavitation successfully increased microcrystalline cellulose enzymatic digestibility by 37% compared to no acoustic cavitation treatment; however, there was no significant effect on lime-treated sugarcane bagasse. Hydrodynamic cavitation increased the enzymatic digestibility of both raw and lime-treated sugarcane bagasse. Best results were obtained using cavitation treatment of bagasse followed by lime treatment; the 3-d enzymatic digestion increased by 46% when compared to lime treatment only.

Mechanical pretreatment of biomass – Part I: Acoustic and hydrodynamic cavitation

Enzymatic hydrolysis is a major step for cellulosic ethanol production. A thorough understanding of enzymatic hydrolysis is necessary to help design optimal conditions and economical systems. The original HCH-1 (Holtzapple–Caram–Humphrey–1) model is a generalized mechanistic model for enzymatic cellulose hydrolysis, but was previously applied only to the initial rates. In this study, the original HCH-1 model was modified to describe integrated enzymatic cellulose hydrolysis. The relationships between parameters in the HCH-1 model and substrate conversion were investigated. Literature models for long-term (> 48 h) enzymatic hydrolysis were summarized and compared to the modified HCH-1 model. A modified HCH-1 model was developed for long-term (> 48 h) enzymatic cellulose hydrolysis. This modified HCH-1 model includes the following additional considerations: (1) relationships between coefficients and substrate conversion, and (2) enzyme stability. Parameter estimation was performed with 10-day experimental data using α-cellulose as substrate. The developed model satisfactorily describes integrated cellulose hydrolysis data taken with various reaction conditions (initial substrate concentration, initial product concentration, enzyme loading, time). Mechanistic (and semi-mechanistic) literature models for long-term enzymatic hydrolysis were compared with the modified HCH-1 model and evaluated by the corrected version of the Akaike information criterion. Comparison results show that the modified HCH-1 model provides the best fit for enzymatic cellulose hydrolysis. The HCH-1 model was modified to extend its application to integrated enzymatic hydrolysis; it performed well when predicting 10-day cellulose hydrolysis at various experimental conditions. Comparison with the literature models showed that the modified HCH-1 model provided the best fit.

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Development of modified HCH-1 kinetic model for long-term enzymatic cellulose hydrolysis and comparison with literature models

Using the sugar platform, enzymes are a major cost contributor in biofuel production. Conventionally, enzymatic saccharification is performed in batch. To more efficiently use enzymes, a new continuous countercurrent method is explored. Pseudo-continuous countercurrent saccharification was performed on lime-pretreated corn stover at enzyme loadings of 1 mg CTec3/g dry biomass and (1 mg CTec3 + 1 mg HTec3)/g dry biomass and the results were compared with batch. To achieve the same glucan conversion as compared to batch, countercurrent saccharification reduced enzyme loading by 1.6 and 1.4 times at 1 mg protein/g biomass and 2 mg protein/g biomass, respectively. In batch saccharification, the effect of hemicellulase addition was investigated. In countercurrent saccharification, as compared to only 1 mg CTec3/g biomass loading, adding 1 mg HTec3/g biomass increased glucose and xylose yields by 6% and 12%, respectively. The effect of product sugars on enzyme inhibition was studied in batch saccharification.

Countercurrent saccharification of lime-pretreated corn stover – Part 1

To feed a growing population, alternative sources of animal feed (e.g., lignocellulose) are needed to replace grains (e.g., corn). Oxidative lime pretreatment (OLP) increases lignocellulose digestibility by removing lignin and hemicellulose acetyl content. Adding a mechanical pretreatment (e.g., ball milling) further improves digestibility. This study determines the effectiveness of OLP and ball milling to enhance the ruminant digestibility of lignocellulose. For forage sorghum, the 48-h in vitro TDN were 40, 64, and 84 g nutrients digested/100 g organic matter (OM) for raw, short-term OLP, and short-term OLP + ball milling, respectively. In terms of compositional changes, OLP increases NDF and decreases non-fiber carbohydrate (NFC) and crude protein (CP), all of which would normally be associated with a decrease in digestibility. However, because OLP and ball milling beneficially change composition (lignin removal) and structural features (reduced crystallinity), digestibility actually increases. Although ball milling increases digestibility according to standard laboratory assays, it reduces particle size possibly allowing fine particles to escape from the rumen before they are digested, thus limiting its practical application. Nonetheless, this study indicates that mechanical pretreatment greatly increases digestibility, and therefore it is desirable to identify an effective mechanical treatment that retains fiber integrity.

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Development of highly digestible animal feed from lignocellulosic biomass Part 1: Oxidative lime pretreatment (OLP) and ball milling of forage sorghum

Shock treatment is a novel mechanical pretreatment process that, when combined with oxidative lime pretreatment, greatly increases the digestibility of biomass. This work determined the effectiveness of shock treatment on multiple feedstocks (sugarcane bagasse, corn stover, poplar wood, sorghum, and switchgrass), determined recommended shock treatment conditions for corn stover, and compared cellulose crystallinity and copper number of raw and shock-treated samples. At an enzyme loading of 5 FPU/g raw glucan, the combination of oxidative lime pretreatment (OLP) and shock treatment increased 72-h glucan digestibility (g glucan digested/100 g glucan treated) of corn stover from 15.3 (untreated) to 84.6 (OLP + shock). The other four biomass types showed similar gains in glucan digestibility when compared to the respective untreated biomass: +55.7 (bagasse), +76.3 (poplar wood), +48.2 (sorghum), and +73.1 (switchgrass). Recommended shock treatment conditions show that a single shock at room temperature with never-frozen biomass produces the most digestible corn stover.

Chao Liang

Papers

Mechanical pretreatment of biomass – Part I: Acoustic and hydrodynamic cavitation

Development of modified HCH-1 kinetic model for long-term enzymatic cellulose hydrolysis and comparison with literature models

Countercurrent saccharification of lime-pretreated corn stover – Part 1

Development of highly digestible animal feed from lignocellulosic biomass Part 1: Oxidative lime pretreatment (OLP) and ball milling of forage sorghum

Mechanical pretreatment of biomass – Part II: Shock treatment