Sridhar Viamajala, Bryon S. Donohoe, Stephen R. Decker, Todd B. Vinzant, Michael J. Selig, Michael E. Himmel, and Melvin P. Tucker

Abstract Lignocellulosic biomass, a major feedstock for renewable biofuels and chemicals, is processed by various thermochemical and/or biochemical means. This multi-step processing often involves reactive transformations limited by heat and mass transport. These limitations are dictated by restrictions including (1) plant anatomy, (2) complex ultra-structure and chemical composition of plant cell walls, (3) process engineering requirements or, (4) a combination of these factors. The plant macro — and micro-structural features impose limitations on chemical and enzyme accessibility to carbohydrate containing polymers (cellulose and hemicel — lulose) which can limit conversion rates and extents. Multiphase systems containing insoluble substrates, soluble catalysts and, in some cases, gaseous steam can pose additional heat and mass transfer restrictions leading to non-uniform reactions. In this chapter, some of these transport challenges relevant to biochemical conversion are discussed in order to underscore the importance of a fundamental understanding of these processes for development of robust and cost-effective routes to fuels and products from lignocellulosic biomass.

Keywords Lignocellulose ■ Biomass ■ Biofuels ■ Heat transport ■ Mass transport

1 Introduction

The biochemical conversion of lignocellulosic biomass requires several processing steps designed to convert structural carbohydrates, such as cellulose and hemicellu — lose, to monomeric sugars, which include glucose, xylose, arabinose, and mannose. These sugars can be fermented to ethanol and other products, to varying degrees of effectiveness, by wild type and modified microbial strains. The front end of the process includes feedstock size reduction followed by a thermal chemical treatment, called pretreatment. In practice, this unit operation usually involves the exposure of

S. Viamajala (B)

Department of Chemical and Environmental Engineering, The University of Toledo, Toledo, OH 43606-3390

e-mail: sridhar. viamajala@utoledo. edu

O. V. Singh, S. P. Harvey (eds.), Sustainable Biotechnology,

DOI 10.1007/978-90-481-3295-9_1, © Springer Science+Business Media B. V. 2010 biomass to acid or alkaline catalysts at temperatures ranging from 120 to 200°C. Pretreated slurries (the hydrolysate liquor containing soluble sugars, oligosaccha­rides, and other released solubles plus the residual solids) are then enzymatically digested at 40-60°C to release sugars from the polysaccharides and oligomers remaining after pretreatment [1-9]. In both of these steps, adequate heat, mass, and momentum transfer is required to achieve uniform reactions and desirable kinetics.

Plant cell walls, which make up almost all of the mass in lignocellulosic biomass, are highly variable both across and within plant tissue types. At the macroscopic scale, such as within a stem or leaf, uneven distribution of catalyst (chemical or enzyme) due to the different properties of different tissues results in heterogeneous treatment, with only a fraction of the plant material exposed to optimal conditions [10-13]. Tissues that do not get exposed to sufficient amounts of catalyst during pretreatment are incompletely processed, resulting in decreased overall enzymatic digestibility of pretreated biomass [6]. When pretreatment severity is increased, by increasing temperature, catalyst concentration, or time of reaction, areas of biomass readily exposed to catalyst undergo excessive treatment leading to sugar degra­dation and formation of toxic by-products (furfural, hydroxymethyl furfural, and levulinic acid) that inhibit downstream sugar fermentation and decrease conversion yields [1]. This problem continues at a microscopic scale due to the compositional and structural differences between middle lamella, primary cell wall, and secondary cell wall. At even smaller scales, intermeshed polymers of cellulose, hemicellulose, lignin, and other polysaccharides present another layer of heterogeneity that must be addressed during bioconversion of plant cell walls to sugars.

Milling to fine particle sizes improves some of these mass transfer limitations, but can add significant costs [14, 15]. Size reduction, however, may not overcome heat transfer limitations associated with short time-scale pretreatments that employ hot water/steam and/or dilute acids. When such pretreatments are carried out at high solids loading (>30% w/w) to improve process efficiency and increase product con­centrations, heat cannot penetrate quickly and uniformly into these unsaturated and viscous slurries. It is thought that steam added to high-solids pretreatments can con­dense on particle surfaces impeding convective heat transfer. Depending on particle and slurry properties, the condensed steam can form temperature gradients within biomass aggregates, resulting in non-uniform pretreatment.

Besides limiting heat transfer rates, biomass slurries can pose other process­ing challenges. At high solids concentrations, slurries become thick, paste-like, and unsaturated. Limited mass transfer within these slurries can cause localized accumulation of sugars during enzymatic hydrolysis, decreasing cellulase and hemi — cellulase activity through product inhibition [16-23]. In addition, slurry transport through process unit operations is challenging at full scale. As solid concentrations increase, hydrodynamic interactions between particles and the surrounding fluid as well as interactions among particles increase. At high solids concentrations “dense suspensions” are formed and the resulting multiple-body collisional or frictional interactions and entanglement between particles creates a complex slurry rheology [24-26]. A further complicating aspect is water absorption by biomass, causing the bulk to become unsaturated at fairly low insoluble solids concentrations (~ 30-40%
w/w) and behave as a wet granular material [27]. This material is highly compress­ible and the wet particles easily “stick” to each other and agglomerate. With no free water in the system, the material becomes difficult to shear or uniformly mix.

At the ultrastructural scale of plant cell walls, catalysts must penetrate through the nano-pore structure of the cell wall matrix to access the “buried” and inter — meshed carbohydrate polymers. Based on reported average cell wall pore sizes of 5-25 nm [28-31], small chemical catalysts (<1 nm) may not face as significant a penetration barrier as do enzymes (about 10 nm). The most dominant commer­cial cellulase component enzyme, cellobiohydrolase I or Cel7A, has dimensions of ~5 x 5 x 12 nm [32, 33] which is roughly the same size as smallest of these reported nano-pores, likely restricting accessibility to primarily surface cellulose chains. Once they have penetrated the cell wall matrix, these enzymes must locate suitable substrates. For Cel7A, this implies that a region of cellulose microfibril has been sufficiently unsheathed from lignin and hemicellulose to expose the cellulose core (Fig. 1). This unsheathing process may be accomplished by the pretreatment or as an ablative effect caused by the system of cellulase enzymes which can peel away microfibrils from the surface layers. Lignin is also a major impediment to cellulase action because it is difficult to remove uniformly or modify through pretreatment. Furthermore, it is entirely unclear at this time if lignin can be effectively removed from cell walls using enzymes.

wall lumen

pit/pore

inter-cellular

space

Lignin is believed to impede enzymatic hydrolysis of cellulose by interact­ing with biomass surfaces and either blocking the path of processive hydrolases (e. g. Cel7A), preventing enzymatic access to specific binding sites, or through non­specific binding of cellulolytic enzymes [34-36] to lignin. Several low-temperature pretreatment protocols, such as alkaline peroxide [37, 38] or lime and oxygen [39], address these issues by removing substantial amounts of lignin. Although these pro­cesses are highly relevant to the pulp and paper industry, the fate of lignin and its impact on enzymatic digestibility after high-temperature acidic or neutral pretreat­ments has largely been neglected until recently [40-42]. Recent observations show that lignin undergoes significant structural changes during high temperature pre­treatments. These changes cause it to both mobilize during elevated temperatures and then coalesce upon cooling, both within the cell wall matrix and on the biomass surfaces [40]. This mobilized processed lignin, when redeposited onto cellulose sur­faces, can impede enzymatic digestion presumably due to the occlusion of substrate binding sites [42]. All of these transport limitations during lignocellulosic con­version to ethanol impact the overall process performance and thus warrant more detailed further investigation.