Two-step steam pretreatment of softwood by dilute H2SO4 impregnation for ethanol production
Introduction
During the past decades, global warming from the increased amount of greenhouse gases, mainly carbon dioxide, has become a major political and scientific issue. The main cause of global warming is believed to be the carbon dioxide formed by burning fossil fuels. By using biofuels, the net emission of carbon dioxide to the atmosphere can be reduced. Ethanol, a biofuel, which can be produced from various cellulosic materials, has been proposed as an alternative fuel. It can be manufactured from numerous natural materials containing cellulose or starch.
Softwood is an abundant feedstock in Sweden and can be used to produce fuel ethanol through, for example, enzymatic hydrolysis and fermentation [1], [2], [3], [4]. Softwood is mainly comprised of three polymers: natural cellulose, a crystalline polymer that is associated in a matrix with the two other polymers, lignin and hemicellulose. Because of the high lignin content, this material is very resistant to enzymatic attack. To improve the yield it is necessary to perform pretreatment prior to the enzymatic hydrolysis step.
The production cost must be competitive with that of fossil fuels for the commercial introduction of fuel ethanol. The highest costs in the conversion of biomass to ethanol are the cost of the raw material [1], and that of the enzymes. Consequently, it is very important to ensure a high degree of utilisation of all the carbohydrate components in the feedstock [5]. The overall yield has been found to be the most important parameter when evaluating the production cost of bioethanol [6].
Steam pretreatment of softwood by either H2SO4 or SO2 impregnation constitutes an effective way of hydrolysing hemicellulose and softening the structure of cellulose to facilitate enzymatic attack [2], [7], [8]. Steam pretreatment can be evaluated with the severity correlation [9], which describes the severity of the pretreatment as a function of treatment time (minutes) and temperature (°C), where .When the pretreatment is performed under acidic conditions, the effect of pH can be taken into consideration by the combined severity [10] defined asThe pH can be calculated from the amount of sulphuric acid added to the material and the water content of the material. The utilisation of the severity factor and the combined severity factor for evaluation are approximate methods as they assume that a first-order reaction is taking place. However, this is not the case in steam pretreatment of wood.
During steam pretreatment, the pentoses and hexoses formed from the hydrolysed hemicellulose and cellulose may be further degraded to furfural, 5-hydroxymethylfurfural (HMF), levullinic acid and formic acid, together with other substances. Three major groups of potential inhibitors can be found in the liquid after dilute acid steam pretreatment: aliphatic acids, furan derivatives and phenolic compounds [11]. These compounds may cause inhibition in the fermentation step.
It is well known that more severe conditions during steam pretreatment will cause greater degradation of hemicellulosic sugars [1], [5], [12], [13]. However, a high degree of severity is required to promote the enzymatic digestibility of the cellulose fibres, especially in softwood [7]. The formation of degradation products reduces the yield during the steam pretreatment step and the products may also cause inhibition in the following downstream process steps.
It is important to maximise the total sugar yield in the process and consequently it is desirable to have high yields of both glucose and hemicellulosic sugars. We have focused on hexoses, as they can be fermented by Saccharomyces cerevisae, the yeast used in this study. Previous studies have shown that maximum hydrolysis of glucose and mannose is not obtained at the same pretreatment severity. Glucan demands pretreatment of higher severity than mannan to be completely hydrolysed. This suggests two-step steam pretreatment, with the first step performed at low severity to hydrolyse the hemicellulose and the second step, where the solid material from the first step is pretreated again, at higher severity. This approach can result in higher sugar yields than one-step steam pretreatment and has been proposed in the literature several times [2], [7], [12], [14], [15].
In the present study a two-step steam pretreatment process has been investigated. The conditions in the first pretreatment step were chosen to give a high recovery of hemicellulose-derived fermentable sugars in the liquid. The solid material in the slurry was thoroughly washed with water and then pretreated in the second pretreatment step. The effect of pretreatment was assessed using both separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF). The second pretreatment step was optimised with respect to the total ethanol yield after SSF and, for SHF, to the total yield of fermentable sugars after enzymatic hydrolysis.
Section snippets
Materials and methods
The experimental procedure employed in this study is shown schematically in Fig. 1. The softwood was impregnated with dilute H2SO4 and then steam pretreated. The resulting material was separated into a solid residue and a liquid. The liquid was analysed with regard to sugars and also fermented. The solid material was washed with water and then impregnated again with dilute H2SO4 and steam pretreated in the second pretreatment step. The resulting material was evaluated by SSF of the slurry, by
First pretreatment step
The composition of the dry raw material is presented in Table 1. Sixty-two percent of the dry raw material consisted of glucan and mannan that could be used for ethanol production.
Ninety-three percent of the glucan was recovered after the first pretreatment step. Eighty-one percent was still present in the solid, whereas 12% was hydrolysed and present in the liquid as either oligomeric or monomeric sugars. Of the solubilised glucan, 87% was recovered as monomeric sugar (glucose) and the rest,
Conclusions
The ethanol yield after two-step steam pretreatment followed by SSF reached 65% of the theoretical yield. However, when using SHF the yield was increased to 69%, when the fermentation yield after enzymatic hydrolysis was assumed to be 90%, which was the yield obtained in the fermentation experiments. The SHF configuration results in higher yields than the SSF configuration. This was not the case in one-step steam pretreatment, where SSF showed the most promising results.
The severity factor and
Acknowledgements
The Swedish National Energy Administration is gratefully acknowledged for its financial support. We are grateful to Dr Robert Eklund at the Mid Sweden University, Örnskjöldsvik, Sweden for providing the raw material and performing the first pretreatment step.
References (22)
- et al.
Ethanol from lignocellulosicsa review of the economy
Bioresource Technology
(1996) - et al.
Reduced inhibition of enzymatic hydrolysis of steam-pretreated softwood
Enzyme and Microbial Technology
(2001) - et al.
Fractionation of Populus tremuloides at the pilot plant scaledoptimization of steam pretreatment conditions using the STAKE II technology
Bioresource Technology
(1991) - et al.
The influence of lactic acid formation on the simultaneous saccharification and fermentation (SSF) of softwood to ethanol
Enzyme and Microbial Technology
(2000) - et al.
Fermentability of the hemicellulose-derived sugars from steam-exploded softwood (Douglas fir)
Biotechnology and Bioengineering
(1999) - et al.
Dilute acid pretreatment of softwoods
Applied Biochemistry and Biotechnology
(1998) - et al.
Comparison of SO2 and H2SO4 impregnation of softwood prior to steam pretreatment on ethanol production
Applied Biochemistry and Biotechnology
(1998) - et al.
Modelling the enzymatic hydrolysis of dilute-acid pretreated Douglas fir
Applied Biochemistry and Biotechnology
(1999) - et al.
Optimization of steam explosion to enhance hemicellulose recovery and enzymatic hydrolysis of cellulose in softwoods
Applied Biochemistry and Biotechnology
(1999) - et al.
Two-stage dilute-acid pretreatment of softwoods
Applied Biochemistry and Biotechnology
(2000)
Steam pretreatment of lignocellulosic residues
Biotechnology and Agriculture Series
Cited by (173)
Principal factors affecting the yield of dilute acid pretreatment of lignocellulosic biomass: A critical review
2023, Bioresource TechnologySteam explosion pretreatment of willow grown on phytomanaged soils for bioethanol production
2019, Industrial Crops and ProductsEnhancing fermentable sugar yield from cassava residue using a two-step dilute ultra-low acid pretreatment process
2018, Industrial Crops and ProductsCitation Excerpt :Acid pretreatment under extreme conditions (i.e., high acid concentrations and temperature), causes greater degradation of hemicellulosic sugars and results in the production of larger amounts of potential fermentation inhibitors, such as furfural, hydroxymethylfurfural (HMF), and acetic acid. In order to maximize pretreatment efficiency, two-step dilute acid pretreatment procedures have been proposed and applied to numerous substrates (Nguyen et al., 2000; Soderstrom et al., 2005, 2003). The conditions in the first step are generally less severe and serve to hydrolyze the hemicellulose.
From lignocellulosic biomass to levulinic acid: A review on acid-catalyzed hydrolysis
2018, Renewable and Sustainable Energy Reviews