Bioalcohol Production

Biochemical Conversion of Lignocellulosic Biomass

Keith W. Waldron

• Publisher: Woodhead Publishing
• Year: 2010
• Print ISBN: 9781845695101
• eBook ISBN: 9781845699611
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

Bioethanol is one of the main biofuels currently used as a petroleum-substitute in transport applications. However, conflicts over food supply and land use have made its production and utilisation a controversial topic. Second generation bioalcohol production technology, based on (bio)chemical conversion of non-food lignocellulose, offers potential advantages over existing, energy-intensive bioethanol production processes. Food vs. fuel pressures may be reduced by utilising a wider range of lignocellulosic biomass feedstocks, including energy crops, cellulosic residues, and, particularly, wastes.Bioalcohol production covers the process engineering, technology, modelling and integration of the entire production chain for second generation bioalcohol production from lignocellulosic biomass. Primarily reviewing bioethanol production, the book's coverage extends to the production of longer-chain bioalcohols which will be elemental to the future of the industry.Part one reviews the key features and processes involved in the pretreatment and fractionation of lignocellulosic biomass for bioalcohol production, including hydrothermal and thermochemical pretreatment, and fractionation to separate out valuable process feedstocks. Part two covers the hydrolysis (saccharification) processes applicable to pretreated feedstocks. This includes both acid and enzymatic approaches and also importantly covers the development of particular enzymes to improve this conversion step. This coverage is extended in Part three, with chapters reviewing integrated hydrolysis and fermentation processes, and fermentation and co-fermentation challenges of lignocellulose-derived sugars, as well as separation and purification processes for bioalcohol extraction.Part four examines the analysis, monitoring and modelling approaches relating to process and quality control in the pretreatment, hydrolysis and fermentation steps of lignocellulose-to-bioalcohol production. Finally, Part five discusses the life-cycle assessment of lignocellulose-to-bioalcohol production, as well as the production of valuable chemicals and longer-chain alcohols from lignocellulosic biomass.With its distinguished international team of contributors, Bioalcohol production is a standard reference for fuel engineers, industrial chemists and biochemists, plant scientists and researchers in this area. - Provides an overview of the life-cycle assessment of lignocelluloses-to-bioalcohol production - Reviews the key features and processes involved in the pre-treatment and fractionation of lignocellulosic biomass for bioalcohol production - Examines the analysis, monitoring and modelling approaches relating to process and quality control in pre-treatment, hydrolysis and fermentation

Part I

Pretreatment and fractionation processes for lignocellulose-to-bioalcohol production

1

Hydrothermal pretreatment of lignocellulosic biomass

S. Ewanick and R. Bura, University of Washington, USA

Abstract:

Lignocellulosic biomass has long been recognized as a potential sustainable source of mixed sugars fermentation to biofuels and other biochemicals. The hydrothermal pretreatments (steam explosion and hot water pretreatment) are the most effective pretreatments for a variety of biomass types and have been shown to work effectively at a commercial scale. Here, we consider the technical maturity of the hydrothermal pretreatments by looking at the process history, describing the mode of reactions and analyzing the influence of pretreatment conditions on the physico-chemical properties of pretreated biomass. Finally, we compare the effectiveness of hydrothermal pretreatments and outline the remaining challenges associated with harnessing the pretreatment for production of biochemicals.

Key words

hydrothermal pretreatment

steam explosion

liquid hot water pretreatment

lignocellulosic biomass

1.1 Introduction

Processing of lignocellulosic biomass to ethanol consists of four major unit operations: pretreatment, hydrolysis, fermentation and product separation/ purification. Pretreatment, disruption or fractionation is an important tool in the biomass to ethanol conversion process and is required to alter the structure of lignocellulosic biomass to make cellulose more accessible to the enzymes that convert the carbohydrate polymers into fermentable sugars. The goal is to break the lignin seal and disrupt the crystalline structure of cellulose. Regardless of biomass type, the pretreatment has to separate the biomass into cellulose, hemi-cellulose and lignin with high recovery of all components in pure form to allow for economical feasibility, i.e., through the separation of individual cells or destructuration of the cell wall to loosen up complexes and allow for further separation of main polymers.

It is apparent that an effective pretreatment method should be efficient on different types of lignocellulosic biomass, inexpensive (for both operating and capital costs), require a minimum of pre-pretreatment (preparation/handling) steps and affect a maximum recovery of all lignocellulosic components in usable form. In addition, if ethanol is the final product of biomass to ethanol conversion, the effective pretreatment should ensure the maximum hemi-cellulose and cellulose recovery in hydrolyzable and fermentable form. Although many pretreatment processes have been studied (biological, physical, chemical and combination of these approaches) no process currently available can provide all of these desired outcomes on all lignocellulosic materials. However, hydrothermal pretreatment is the most effective pretreatment for a variety of biomass types and has been shown to work effectively at a commercial scale. In this review, we will first analyze the technical maturity of the hydrothermal pretreatment process by looking at the history and description of the process conditions. Then, we will describe the mode of hydrothermal reactions and the influence of pretreatment conditions on the physico-chemical properties of pretreated biomass. The final section offers a comparison of the hydrothermal pretreatments (steam and hydro) and outlines the remaining challenges associated with harnessing the pretreatment for production of biochemicals.

1.2 Physical comminution

Size reduction of lignocellulosic biomass is an important factor in any pretreatment process. Mechanical means can be used to reduce particle size sufficiently so that no further pretreatment is required prior to enzymatic hydrolysis, obviating usage of chemicals and associated concerns such as corrosion, recycling, neutralization and storage. However, high energy requirements for these processes mean that they are typically not economically feasible. As particle size decreases, crystallinity is reduced, which increases enzymatic digestibility. To significantly improve hydrolysis, treatments must reduce particle size to less than 50 μm (Datta, 1981). However, due to the high energy cost of mechanical size reduction, comminution past 200 μm is not generally economically feasible (Datta, 1981).

Milling processes include dry, wet and vibratory ball milling (Millett et al., 1979; Kelsey and Shafizadeh, 1980; Fukazawa et al., 1982). Compression, hammer and disc milling are also used (Schell and Harwood, 1994). These vary widely in terms of particle size distribution, energy usage and efficacy on different feedstock types. For example, far more energy is required to mill hardwoods to a given size than agricultural residues using both hammer and knife milling (Cadoche and Lopez, 1989). In starch-to-ethanol bioconversion, wet and dry milling are the most cost effective pretreatments (Bothast and Schlicher, 2005). However, for lignocellulosic biomass, the energy demands of any physical size reduction process are high. Comminution is consequently limited to pre-pretreatment, to be followed by a chemical or thermal pretreatment process.

1.3 Hydrothermal pretreatment (liquid hot water and steam)

Physical pretreatment of lignocellulosic biomass is often inadequate in providing complete fractionation to a readily digestible and fermentable product. The cell structure of lignocellulosic biomass is by nature complex and difficult to penetrate, so fractionation requires chemical reactions in addition to physical restructuring. Pretreatments utilizing primarily steam or liquid water at high temperatures can efficiently convert biomass to a form which can be easily digested by enzymes by facilitating autohydrolysis reactions within the biomass. Processes utilizing hot water or steam as the primary chemical are known as hydrothermal pretreatments. The two forms of hydrothermal pretreatment utilize steam (steam explosion) and aqueous water (liquid hot water pretreatment). These processes are advantageous compared to chemical methods as they are regarded as safer – equipment corrosion is reduced – and more ‘environmentally friendly’, as often, no chemicals are required (Allen et al., 2001; Laser et al., 2002). Although process residence times and temperatures are similar, liquid hot water pretreatment differs from steam explosion in that water is present as a liquid instead of a gas during pretreatment. As a result, there are differences in reactor configurations, solids consistency during and after pretreatment and concentration of reaction products. In the last 80 years, there has been great progress in the development of aqueous processes to break down all types of lignocellulosic biomass. From agricultural residues to hardwoods to softwoods, hydrothermal pretreatments have the potential to sustainably generate material which can be readily converted to ethanol.

1.3.1 Process history and description

Steam explosion

Steam explosion has long been used as a means of deconstructing biomass for many purposes, from structural materials to paper to biochemicals. The first use of steam explosion to produce a commercial product was the masonite process. Developed in the 1920s, the process was used to produce a fiberboard building material (Mason, 1928) with very high yields and minimal energy usage (Overend et al., 1987; Kokta and Ahmed, 1998). However, the coarse, dark substrate, while suitable for fiberboard, was unsuitable for paper products. Asplund used a similar high temperature and pressure pr...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Woodhead Publishing Series in Energy
• Preface
• Part I: Pretreatment and fractionation processes for lignocellulose-to-bioalcohol production
• Part II: Hydrolysis (saccharification) processes for lignocellulose-to-bioalcohol production
• Part III: Lignocellulose-to-bioalcohol fermentation and separation processes
• Part IV: Monitoring and modelling processes in lignocellulose-to-bioalcohol production
• Part V: Life cycle assessment of, and multiple products from, lignocellulose-to-bioalcohol production
• Index