Lignocelluloses are often a major or sometimes the sole components in different waste streams from various sources such as industries, forestry, agriculture and municipalities. It represents an as-of-yet untapped source of fermentable sugars for significant industrial use. Many physico-chemical, structural and compositional factors hinder the hydrolysis of components present in the biomass to sugars and other organic compounds that can later be converted into fuels. During the past few years, a large number of chemical pretreatment methods including lime, acid, steam explosion, sulfur dioxide explosion, ammonia fiber explosion, ionic liquid and others have been developed for efficient pretreatment of biomass. Many pretreatment methods have shown high sugar yields i.e. more than 90% of the theoretical yield from lignocelluloses. In this review, we discuss various chemical pretreatment processes, feasibility of the processes at industrial scale in terms of the mechanisms involved, advantages, disadvantages and economic assessment. It is not possible to define the best pretreatment method as it depends on many factors such as type of lignocellulosic biomass, process parameters, environmental impact, economical feasibility, etc. However, some of these chemical pretreatments have disadvantages such as formation of inhibitory compounds especially furfural and 5-hydroxyl methyl furfural (HMF).

Importance of chemical pretreatment for bioconversion of lignocellulosic biomass

A pretreatment of lignocellulosic biomass to produce biofuels, polymers, and other chemicals plays a vital role in the biochemical conversion process toward disrupting the closely associated structures of the cellulose-hemicellulose-lignin molecules. Various pretreatment steps alter the chemical/physical structure of lignocellulosic materials by solubilizing hemicellulose and/or lignin, decreasing the particle sizes of substrate and the crystalline portions of cellulose, and increasing the surface area of biomass. These modifications enhance the hydrolysis of cellulose by increasing accessibilities of acids or enzymes onto the surface of cellulose. However, lignocellulose-derived byproducts, which can inhibit and/or deactivate enzyme and microbial biocatalysts, are formed, including furan derivatives, lignin-derived phenolics, and carboxylic acids. These generation of compounds during pretreatment with inhibitory effects can lead to negative effects on subsequent steps in sugar flat-form processes. A number of physico-chemical pretreatment methods such as steam explosion, ammonia fiber explosion (AFEX), and liquid hot water (LHW) have been suggested and developed for minimizing formation of inhibitory compounds and alleviating their effects on ethanol production processes. This work reviews the physico-chemical pretreatment methods used for various biomass sources, formation of lignocellulose-derived inhibitors, and their contributions to enzymatic hydrolysis and microbial activities. Furthermore, we provide an overview of the current strategies to alleviate inhibitory compounds present in the hydrolysates or slurries.

Physico-Chemical Conversion of Lignocellulose: Inhibitor Effects and Detoxification Strategies: A Mini Review.

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

Wood Chemistry, Fundamentals and Applications, Second Edition, examines the basic principles of wood chemistry and its potential applications to pulping and papermaking, wood and wood waste utilization, pulping by-products for production of chemicals and energy, and biomass conversion.

Wood chemistry: fundamentals and applications.

The use of technical lignins as feedstock for chemical products will require improvements in purity, molecular mass distribution, and thermal behavior. Therefore, industrial black liquors from kraf ...

Kraft lignin as feedstock for chemical products: The effects of membrane filtration

The aim of this work is to open up the structure of wood while retaining a large amount of hemicelluloses, in particular (galacto)glucomannans. The effects of pre-treatments on wood meal from spruce (Picea abies) with a reducing agent (NaBH4) combined with steam explosion at very mild conditions were investigated. The effects of steam explosion at 160 degrees C were studied for various residence times (5 to 35 min) on both water-impregnated wood meal and samples pre-treated with NaBH4. The findings showed that pre-treatment with sodium borohydride stabilized the reducing end-groups of glucomannans and that the treatment was effective both during mild steam explosion, for both long and short residence times, as well as during subsequent treatment in alkali. Extraction experiments at different pH and temperatures showed that the main part of the hemicelluloses still remained in the wood residue after treatment. The molecular weight distributions of the extracted material from the liquors indicated that there were broad molecular distributions and that the molecular weight averages were between 3 and 6 kDa.

http://publications.lib.chalmers.se/records/fulltext/162373/local_162373.pdf

Mild steam explosion and chemical pre-treatment of norway spruce

Industrially chipped wood chips of Norway spruce (Picea abies) were subjected to mild steam explosion (115 - 160 degrees C) in a small-scale steam explosion reactor. This was followed by kraft cooking or extraction in alkali at 130 degrees C for two hours, or by an enzymatic treatment with a culture filtrate in order to investigate the efficiency of the process in opening wood structure. The results demonstrated that mild explosion has an effect on opening wood structure, shown by increased release of glucomannans during alkaline extraction and faster delignification in kraft cooks for steam-exploded samples. The effect was also shown by analysis of the released reducing sugars of enzymatic treated wood chips, which showed that the wood structure became accessible for enzymes even at very modest mild steam explosion conditions. This was not observed in untreated wood chips, used as reference. The enzyme activity increased with increased temperature during mild steam explosion, and the effect did not seem to be linear. The mechanical effect of steam explosion seems to be of great importance at lower temperatures, and both chemical and mechanical effects are important at higher steam explosion temperatures. Samples for enzymatic treatment were taken both from the edges of wood chips as well as from the middle part of the chips, and the effect of steam explosion was somewhat greater in samples from the middle parts.

Mild steam explosion : A way to activate wood for enzymatic treatment, chemical pulping and biorefinery processes

Acidic leaching of Scandinavian softwood, birch and eucalyptus has been studied, for the purpose of removing metal ions from the wood. Equilibrium experiments with wood powders were conducted to study the influence of pH. An increased release of metal ions was found, as expected, when the pH was lowered. For one sample of eucalyptus, the release of calcium accelerated when the pH was lowered to below 3, which led to the conclusion that a salt of low solubility could be present. Further investigations showed a high concentration of oxalate in this sample, which is likely to be an explanation for the accelerated release. Industrial cut wood chips, from Scandinavian softwood, birch and eucalyptus, were leached at different temperatures to study their leaching behavior. Birch and softwood were shown to have the same general leaching pattern for calcium and potassium. Eucalyptus showed the same leaching trend for potassium, but it was harder to remove calcium from eucalyptus than from softwood and birch. In the case of eucalyptus chips, which had a relatively high content of chloride (480 mg/kg wood), extensive removal of chloride was noted.

Removal of metal ions from wood chips during acidic leaching 1: Comparison between Scandinavian softwood, birch and eucalyptus

The major part of the intake of non-process elements (NPEs) to the pulp mill is via the wood chips. Some of the problems associated with NPEs are for example, precipitation of sparingly soluble calcium salts in the fibre line and in the recovery department. In order to investigate the possibilities of reducing the intake of NPEs via hardwood chips to the pulp mill, laboratory studies on the acidic leaching of birch and eucalypt chips were carried out. The results showed that potassium, magnesium and manganese were removed from both wood species at similar rates. The removal of calcium was however significantly slower from eucalypt than from birch. Removal of NPEs from birch wood chips prior to cooking resulted in a higher rate of delignification, a higher unbleached brightness and a higher viscosity. In the case of eucalypt, acidic leaching had no effect on the rate of delignification. The positive effect of acidic leaching of birch chips was found to be due to a lower content of calcium in the cooking stage.

Removal of Non-process Elements from Hardwood Chips Prior to Kraft Cooking

The purpose of this work was to model leaching of calcium ions from softwood chips under acidic conditions. The first step was to study the leaching of calcium ions from well-defined sawn samples of spruce wood. Leaching experiments with varying geometries under acidic conditions (pH 2.5 and 60°C) were conducted to determine the diffusion coefficients,giving a diffusion coefficient of 5.9 x 10-10 m2/s in the longitudinal direction and 0.41 x 10-10 m2/s across the grain (in the radial and tangential directions). Results show that leaching of calcium ions from spruce can be described with a model assuming that the wood piece is semi-homogenous, with different diffusion coefficients in different directions.
A model describing leaching of industrially cut wood chips,
taking into account cracks, rough surfaces and size distribution in the batch of wood chips, was developed using a model of a wood chip with an enlarged surface area. The diffusion coefficient and the chip model were then used to develop models of the leaching process that can be used to simulate and evaluate different leaching designs.

Anna Saltberg

Papers

Mild steam explosion and chemical pre-treatment of norway spruce

Mild steam explosion : A way to activate wood for enzymatic treatment, chemical pulping and biorefinery processes

Removal of metal ions from wood chips during acidic leaching 1: Comparison between Scandinavian softwood, birch and eucalyptus

Removal of Non-process Elements from Hardwood Chips Prior to Kraft Cooking

Removal of metal ions from wood chips during acidic leaching 2: Modeling leaching of calcium ions from softwood chips