Liquid hot water pretreatment of lignocellulosic biomass for bioethanol production accompanying with high valuable products

Highlights

• Liquid hot water is an environmental friendly pretreatment degrading hemicellulose.

• Hemicellulose-degraded products can be extracted as valuable xylo-oligosaccharides.

• Residual lignin, solid loading, etc. can affect enzymatic conversion of cellulose.

• Enzymatic residues can be prepared as media for cultivating edible fungi.

• A technical route for full utilization of lignocellulosic components is developed.

Abstract

Pretreatment is an essential prerequisite to overcome recalcitrance of biomass and enhance the ethanol conversion efficiency of polysaccharides. Compared with other pretreatment methods, liquid hot water (LHW) pretreatment not only reduces the downstream pressure by making cellulose more accessible to the enzymes but minimizes the formation of degradation products that inhibit the growth of fermentative microorganisms. Herein, this review summarized the improved LHW process for different biomass feedstocks, the decomposition behavior of biomass in the LHW process, the enzymatic hydrolysis of LHW-treated substrates, and production of high value-added products and ethanol. Moreover, a combined process producing ethanol and high value-added products was proposed basing on the works of Guangzhou Institute of Energy Conversion to make LHW pretreatment acceptable in the biorefinery of cellulosic ethanol.

Introduction

Fuel ethanol production from lignocellulosic biomass not only relieves the demand for fossil fuel, but also reduces the greenhouse gas emission, especially makes energy renewable (Limayem and Ricke, 2012). Lignocellulose is a complex carbohydrate polymer mainly composed of cellulose, hemicellulose and lignin. Cellulose is a long-chain homopolymer of d-glucose units linked by β-1,4 glycosidic bonds. It is linear and contains amorphous and crystalline portions. Hemicellulose is a highly branched heteropolymer of pentoses (β-d-xylose and α-l-arabinose), hexoses (β-d-glucose, α-d-galactose and β-d-mannose) and sugar acids (α-d-glucuronic, α-d-galacturonic and α-d-4-O-methylgalaturonic acids) with small amounts of other sugars such as α-l-rhamnose and α-l-fucose (Girio et al., 2010). The compositional feature of hemicellulose is depended on the biomass type. Hemicellulose of hardwood and herbaceous biomass is mainly composed of xylan, while glucomannan and galactoglucomannan are the major components in the hemicellulose of softwood (Girio et al., 2010). Lignin is an amorphous and highly irregular aromatic polymer consisting of phenylpropane units, namely syringyl (S), guaiacyl (G) and p-hydroxyphenyl (H) units (Buranov and Mazza, 2008). Lignin is always connected with carbohydrates, particularly hemicellulose, through covalent bonds to build up rigid structure of lignocellulose and protect lignocellulose from being attacked by microorganisms (Buranov and Mazza, 2008). This results in low conversion of the raw lignocellulosic biomass into fuel ethanol.

The high efficient conversion of lignocellulose to fuel ethanol usually comprises three major steps: pretreatment for breaking down the lignocellulosic structure, enzymatic hydrolysis to produce fermentable sugars, and microbial fermentation for ethanol yield. Pretreatment is the essential and key step for enhancing the following enzymatic saccharification. Several methods for pretreatment have been developed, such as biological pretreatment, physical pretreatment including mechanical comminution and extrusion, chemical pretreatment with the aid of acids, alkalis, ozone, organic solvents or ionic liquids, and physico-chemical pretreatment involving steam explosion, ammonia fiber explosion (AFEX), CO₂ explosion, liquid hot water (LHW), wet oxidation, microwave and ultrasound (Alvira et al., 2010). All the pretreatments have their own advantages and disadvantages (Alvira et al., 2010), but dilute acid (DA), lime, soaking in aqueous ammonia (SAA), sulfur dioxide-impregnated steam explosion (SO₂), AFEX and LHW are the leading pretreatments which have been applied in demonstration plants (Bacovsky et al., 2013). The comparison of these six leading pretreatments was summarized in Table 1. Comparing with the other 5 pretreatments, the ethanol yield from LHW pretreatment was just higher than that from SAA pretreatment, but its total capital was the lowest. In addition, LHW pretreatment which maintains water in the liquid state at elevated temperatures (160–240 °C) does not need any chemicals except water, has little erosion on equipment, and forms lower concentration of inhibitors with little inhibition for the following enzymatic hydrolysis and fermentation (Alvira et al., 2010, Wang et al., 2012a). Hemicellulose is mostly depolymerized, and its degradation products are dissolved in the liquid phase during LHW pretreatment, while cellulose is retained completely in the solid portion. Lignin suffers from the simultaneous depolymerization and repolymerization reactions referring to the glass transition from glassy to rubbery state upon heating in the LHW process (Ko et al., 2015a). Most of insoluble lignin is retained in the solid residues. Lignin droplets were found to deposit on the surface of the pretreated lignocellulose (Wang et al., 2015). The residual lignin will put its negative effect on the subsequent enzymatic hydrolysis via physical barrier for accessibility of cellulose to cellulase, and/or nonproductive adsorption of enzymes (Rahikainen et al., 2013a, Wang et al., 2015) . Additionally, the structural characteristics of LHW-treated lignocellulosic biomass such as chemical groups, special surface area, pore size and so on also put their impacts on the enzymatic hydrolysis of cellulose (Reddy et al., 2015, Wang et al., 2012b, Zhuang et al., 2014).

Although the effect of LHW pretreatment on lignocellulose has been researched for several years, the special summary on LHW pretreatment has been not yet reported. This paper summarized the characteristic changes of lignocellulosic biomass in the LHW process based on the authors’ and other researchers’ works. The factors influencing enzymatic hydrolysis of LHW-treated lignocellulose were also discussed. Finally, the authors introduced their own work on high value-added products from the waste water and solid which were generated from the LHW and enzymatic hydrolysis process, respectively. The production of ethanol and high-value added products from lignocellulosic biomass based on LHW pretreatment was presented in Fig. 1.