Pretreatments to enhance the digestibility of lignocellulosic biomass

doi:10.1016/j.biortech.2008.05.027

Bioresource Technology

Volume 100, Issue 1, January 2009, Pages 10-18

https://doi.org/10.1016/j.biortech.2008.05.027 Get rights and content

Abstract

Lignocellulosic biomass represents a rather unused source for biogas and ethanol production. Many factors, like lignin content, crystallinity of cellulose, and particle size, limit the digestibility of the hemicellulose and cellulose present in the lignocellulosic biomass. Pretreatments have as a goal to improve the digestibility of the lignocellulosic biomass. Each pretreatment has its own effect(s) on the cellulose, hemicellulose and lignin; the three main components of lignocellulosic biomass. This paper reviews the different effect(s) of several pretreatments on the three main parts of the lignocellulosic biomass to improve its digestibility. Steam pretreatment, lime pretreatment, liquid hot water pretreatments and ammonia based pretreatments are concluded to be pretreatments with high potentials. The main effects are dissolving hemicellulose and alteration of lignin structure, providing an improved accessibility of the cellulose for hydrolytic enzymes.

Introduction

For a long time research is being done to enhance the digestibility of lignocellulosic biomass for mainly the efficient conversion of (hemi-) cellulose to ethanol, methane and, in the last years, also to hydrogen. It is however not clear which characteristics of the lignocellulosic biomass are important, to determine a successful pretreatment. Further more additional problems, like production of recalcitrant or inhibitory products, are to be solved.

A lot of literature is written about different pretreatment methods to enhance the digestibility of lignocellulosic material. The objective of this review is to find out which characteristics of lignocellulosic biomass determine which pretreatment method will be successful and attractive to apply. This will be done by explaining the composition of lignocellulosic material, giving an overview of the methane and ethanol production process, summarizing the effects of different pretreatment methods on lignocellulosic biomass and the consequences of these effects on ethanol and methane production. Moreover, additional problems will be analyzed, and finally, conclusions with respect to promising pretreatment techniques and needed future research are made. Hydrogen production is left out in this paper, because it is still in the R&D phase (Reith et al., 2003).

Section snippets

The composition of lignocellulosic material

Lignocellulosic material consists of mainly three different types of polymers, namely cellulose, hemicellulose and lignin, which are associated which each other (Fengel and Wegener, 1984).

Methane production by anaerobic digestion

The production of methane from lignocellulosic material can consist of three phases, namely pretreatment, anaerobic hydrolysis and methane production, and post-treatment of the liquid fraction. A product separation step is not needed during the methane production step, because methane is, under normal conditions, a gas and will separate itself from the liquid fraction.

Pretreatment can be done to improve the hydrolysis yield and total methane yield. The hydrolysis of the lignocellulose and

Ethanol production by fermentation

The production of ethanol from lignocellulosic material consists of mainly five different steps, namely pretreatment, (enzymatic) hydrolysis, fermentation, product separation, and post-treatment of the liquid fraction. The pretreatment is necessary to improve the rate of production and the total yield of monomeric sugars in the hydrolysis step. The conversion of (hemi) cellulose to monomeric sugars can be done chemically by acids or enzymatically by addition of cellulases (enzymes responsible

Factors limiting the hydrolysis

The (enzymatic) hydrolysis of lignocellulose is limited by several factors. Several researchers conclude that crystallinity of cellulose is just one of the factors. Other factors are degree of polymerization (DP), moisture content, available surface area and lignin content (Chang and Holtzapple, 2000, Koullas et al., 1992, Laureano-Perez et al., 2005, Puri, 1984). Chang and Holtzapple (2000) however mention that crystallinity affects the 1-h enzymatic hydrolysis, but not the 3-d enzymatic

Process description and mode of action

Milling (cutting the lignocellulosic biomass into smaller pieces) is a mechanical pretreatment of the lignocellulosic biomass. The objective of a mechanical pretreatment is a reduction of particle size and crystallinity. The reduction in particle size leads to an increase of available specific surface and a reduction of the degree of polymerization (DP) (Palmowski and Muller, 1999). The milling causes also shearing of the biomass.

The increase in specific surface area, reduction of DP, and the

General thermal processes in lignocellulose

During this pretreatment the lignocellulosic biomass is heated. If the temperature increases above 150–180 °C, parts of the lignocellulosic biomass, firstly the hemicelluloses and shortly after that lignin, will start to solubalize (Bobleter, 1994, Garrote et al., 1999). The composition of the hemicellulose backbone and the branching groups determine the thermal, acid and alkali stability of the hemicellulose (see Section 2.2). From the two dominant components of hemicelluloses (xylan and

Process description and mode of action

Pretreatment of lignocellulose with acids at ambient temperature are done to enhance the anaerobic digestibility. The objective is to solubilize the hemicellulose, and by this, making the cellulose better accessible.

The pretreatment can be done with dilute or strong acids. The main reaction that occurs during acid pretreatment is the hydrolysis of hemicellulose, especially xylan as glucomannan is relatively acid stable. Solubilized hemicelluloses (oligomers) can be subjected to hydrolytic

Process description and mode of action

During alkaline pretreatment the first reactions taking place are solvation and saphonication. This causes a swollen state of the biomass and makes it more accessible for enzymes and bacteria. At ‘strong’ alkali concentrations dissolution, ‘peeling’ of end-groups, alkaline hydrolysis and degradation and decomposition of dissolved polysaccharides can take place. Loss of polysaccharides is mainly caused by peeling and hydrolytic reactions (Fengel and Wegener, 1984). This peeling is an advantage

Process description and mode of action

An oxidative pretreatment consists of the addition of an oxidizing compound, like hydrogen peroxide or peracetic acid, to the biomass, which is suspended in water. The objective is to remove the hemicellulose and lignin to increase the accessibility of the cellulose. During oxidative pretreatment several reactions can take place, like electrophilic substitution, displacement of side chains, cleavage of alkyl aryl ether linkages or the oxidative cleavage of aromatic nuclei (Hon and Shiraishi,

Thermal pretreatment in combination with acid pretreatment

A way to improve the effect of thermal steam or LHW pretreatment is to add an external acid. This addition of an external acid catalyzes the solubilization of the hemicellulose, lowers the optimal pretreatment temperature and gives a better enzymatic hydrolysable substrate (Brownell et al., 1986, Gregg and Saddler, 1996). The lignocellulose is often impregnated (soaked) with SO₂ or H₂SO₄. During steam pretreatment the SO₂ is converted to H₂SO₄ in the first 20 seconds of the process; after that,

Overview of effects of pretreatments on lignocellulose

In chapter six up to and including chapter eleven the effects of several pretreatments on the physical/chemical composition or structure of lignocellulose is reported. Table 1 summarizes the most important effects of the different pretreatment methods, discussed in this paper. The table suggests that increasing the surface area is one of the major approaches of a pretreatment by solubilization of the hemicellulose and/or lignin and/or altering the lignin. The importance of surface area is

Conclusion and final discussion

The biodegradability of lignocellulosic biomass is limited by several factors like crystallinity of cellulose, available surface area, and lignin content. Pretreatments have an effect on one or more of these aspects, as showed in Table 1. Several factors are mentioned to have a positive effect on the overall economy of the process. It is for example favourable to avoid the production of inhibitors (Ramos, 2003), because detoxification of the liquid fraction showed to be costly and/or

Acknowledgements

The authors would like to thank T. Fernandes, C. Pabon, K. Grolle, V. de Wilde, and B. Willemsen for their experimental discussion and assistance throughout the investigation.

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ReviewPretreatments to enhance the digestibility of lignocellulosic biomass

Abstract

Introduction

Section snippets

The composition of lignocellulosic material

Methane production by anaerobic digestion

Ethanol production by fermentation

Factors limiting the hydrolysis

Process description and mode of action

General thermal processes in lignocellulose

Process description and mode of action

Process description and mode of action

Process description and mode of action

Thermal pretreatment in combination with acid pretreatment

Overview of effects of pretreatments on lignocellulose

Conclusion and final discussion

Acknowledgements

Water Res.

Prog. Polym. Sci.

Bioresour. Technol.

Anim. Feed Sci. Technol.

Enzyme Microbiol. Technol.

Biomass Bioenergy

Bioresour. Technol.

Bioresour. Technol.

FEMSYeast Res.

Bioresour. Technol.

Bioresour. Technol.

Bioresour. Technol.

Biomass Bioenergy

Trends Biotechnol.

Bioresour. Technol.

Bioresour. Technol.

Bioresour. Technol.

Pretreatment of switchgrass by ammonia fiber explosion (AFEX)

Appl. Biochem. Biotechnol.

A comparison of aqueous and dilute-acid single-temperature pretreatment of yellow poplar sawdust

Ind. Eng. Chem. Res.

Increased digestibility of cedar by pretreatment with peracetic acid and steam explosion

Biotechnol. Bioeng.

The effect of the duration of alkali treatment on the solubility of polyoses

Turk. J. Agric. Forest.

Collapse phenomena in beechwood during and after NH3-impregnation

Wood Sci. Technol.

Investigation of the ethanol tolerance of Clostridium thermosaccharolyticum in continuous culture

Biotechnol. Progr.

Characteristics and adaptability of some new isolates of Clostridium thermocellum

Appl. Environ. Microbiol.

Degradation of poplar lignin by hydrothermal treatment

Cellul. Chem. Technol.

Steam explosion-hydrothermolysis-organosolv. A comparison

Steam-explosion pretreatment of wood: effect of chip size, acid, moisture content and pressure drop

Biotechnol. Bioeng.

Effect of varying crystallinity of cellulose on enzymic hydrolysis

Wood Sci.

Fundamental factors affecting enzymatic reactivity

Appl. Biochem. Biotechnol.

Lime pretreatment of crop residues bagasse and wheat straw

Appl. Biochem. Biotechnol.

Simultaneous saccharification and fermentation of lime-treated biomass

Biotechnol. Lett.

Oxidative lime pretreatment of high-lignin biomass

Appl. Biochem. Biotechnol.

Kinetics of thermochemical pretreatment of lignocellulosic materials

Appl. Biochem. Biotechnol.

Properties of cellulose and lignocellulosic materials as substrates for enzymatic conversion process

Biotechnol. Bioeng. Symp.

Pretreatments for the enhancement of anaerobic digestion of solid wastes

On the pyrolysis of wood and its components

Holz Roh Werkst.

Cellulose hydrolysis

Wood: Chemistry, Ultrastructure, Reactions

Alkaline subcritical-water treatment and alkaline heat treatment for the increase in biodegradability of newsprint waste

Water Sci. Technol.

Hydrothermal processing of lignocellulosic materials

Holz Roh Werkst.

Heat treatment and anaerobic digestion of refuse

J. Environ. Eng. Div.

Alkaline peroxide delignification of agricultural residues to enhance enzymatic saccharification

Biotechnol. Bioeng.

Review
Pretreatments to enhance the digestibility of lignocellulosic biomass

Collapse phenomena in beechwood during and after NH₃-impregnation