Elsevier

Bioresource Technology

Volume 101, Issue 22, November 2010, Pages 8678-8685
Bioresource Technology

Robust cellulosic ethanol production from SPORL-pretreated lodgepole pine using an adapted strain Saccharomyces cerevisiae without detoxification

https://doi.org/10.1016/j.biortech.2010.06.069Get rights and content

Abstract

This study reports an ethanol yield of 270 L/ton wood from lodgepole pine pretreated with sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL) using an adapted strain, Saccharomyces cerevisiae Y5, without detoxification. The enzymatic hydrolysate produced from pretreated cellulosic solids substrate was combined with pretreatment hydrolysate before fermentation. Detoxification of the pretreatment hydrolysate using overliming or XAD-4 resin before being combined with enzymatic hydrolysate improved ethanol productivity in the first 4 h of fermentation and overall fermentation efficiency. However, detoxification did not improve final ethanol yield because of sugar losses. The Y5 strain showed excellent ethanol productivities of 2.0 and 0.8 g/L/h averaged over a period of 4 and 24 h, respectively, in the undetoxified run. The furan metabolization rates of the Y5 strain were significantly higher for the undetoxified run than those for the detoxidfied runs, suggesting it can tolerate even higher furan concentrations than those studied. Preliminary mass and energy balances were conducted. SPORL produced an excellent monomeric sugar recovery value of about 85% theoretical and a net energy output of 4.05 GJ/ton wood with an ethanol energy production efficiency of 178% before distillation.

Introduction

Efficient production of cellulosic ethanol from plant biomass remains a challenge, although much progress has been made in many areas of biomass biorefinery. Few pretreatment methods can produce high sugar recovery from plant biomass with low energy input (Zhu and Pan, 2010). This is especially true for woody biomass because of its strong physical and chemical recalcitrance. Acid-based pretreatments, such as dilute acid, acid-catalyzed steam explosion, ethanol organosolv, and sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL) (Zhu et al., 2009), have been commonly applied in research for their effectiveness in removing plant biomass recalcitrance (Yang and Wyman, 2008, Zhu and Pan, 2010). Good enzymatic cellulose saccharification efficiency from most plant biomass has been achieved using these pretreatment methods under relatively harsh conditions. However, fermentation inhibitors, such as furfural, HMF, and acetic acid, are also formed, which significantly affects ethanol production from the hemicellulose sugar stream. As a result, most studies on woody biomass using these pretreatment methods reported ethanol production only from the solid substrate cellulose fractions due to the difficulties in fermentation of inhibitor-containing pretreatment hydrolysate consisting of primarily hemicellulose sugars (Munoz et al., 2007, Sassner et al., 2008, Wyman et al., 2009). Few studies reported complete mass balance and net energy output for the process technologies examined.

Fermentation of the hemicellulosic sugar stream in the pretreatment hydrolysate is required to provide mass and energy balance process data to demonstrate the viability of a pretreatment method for practical applications. Various detoxification techniques, such as overliming and ion exchange resin, have been commonly applied before fermentation of the pretreatment hydrolysate (Cardona et al., 2010, Xavier et al., 2010, Zhu et al., 2010b). Hydrolysate detoxification adds an extra process step in practice and can result in sugar losses. Furthermore, it is effective only to certain degrees and therefore should be avoided if at all possible. Recently, a few studies reported fermentation of pretreatment hydrolysate without detoxification by using very mild dilute acid pretreatments to significantly reduce formation of fermentation inhibitors (Huang et al., 2009, Scordia et al., 2010). However, maximizing ethanol production from the more important abundant fraction, cellulose, must also be realized to maximize ethanol yield through hydrolysis and fermentation of lignocelluloses. Although alkaline-based pretreatment methods can produce less toxic or non-toxic hydrolysates, these methods have their own problems for practical applications (Zhu and Pan, 2010), such as very low sugar yield from woody biomass using ammonia-based methods. It is of great practical importance to maximize both glucose recovery and ethanol yield from the cellulose fraction using robust pretreatment and to improve fermentation efficiency of the hemicellulose sugar streams without detoxification. This presents a significant challenge to cellulosic ethanol production, especially from woody biomass because of its strong recalcitrance that limits pretreatment options.

Woody biomass is an important feedstock for the future biobased economy because of its availabilities in large quantities in many regions of the world and its advantages in transportation, storage, etc. (Zhu and Pan, 2010). Previously, we demonstrated the robust performance of SPORL pretreatment for producing readily digestible substrates from both softwood and hardwood with low energy input and low inhibitor generation (Wang et al., 2009, Zhu et al., 2009). We achieved an ethanol yield of 276 L/ton wood, or 72% theoretical yield, from lodgepole pine using a conventional Saccharomyces cerevisiae D5A (ATCC® Number 200062, incapable of fermenting xylose). This resulted in a net ethanol energy output (lignin energy excluded) of 4.55 GJ/ton wood (before distillation) when the pretreatment hydrolysate was detoxified using a resin column and the enzymatic and pretreatment hydrolysates were fermented separately (Zhu et al., 2010b). It was demonstrated that SPORL produces a lower amount of inhibitors than does dilute acid pretreatment under the same acid dosage for maximum sugar yield from the cellulose fraction (Shuai et al., 2010, Wang et al., 2009).

This study was conducted to evaluate the potential of combined fermentation of SPORL pretreatment hydrolysate and enzymatic hydrolysate from SPORL-pretreated solid substrate to maximize cellulosic ethanol production from lodgepole pine using an adapted strain S. cerevisiae Y5 without detoxification. The demonstrated tolerant inhibitor levels for the Y5 strain were 2, 0.5, and 10 g/L for furfural, HMF, and acetic acid, respectively (Li et al., 2009). Higher levels of tolerance were found based on our unpublished laboratory study. Combined fermentation of enzymatic hydrolysate with pretreatment hydrolysate can further reduce inhibitor concentrations through dilution in addition to supplementing glucose. Therefore, it is conceivable to use this combined fermentation approach to produce ethanol from SPORL-pretreated substrates without detoxification. This study is the first step towards maximizing ethanol yield while minimizing energy input from one of the most recalcitrant feedstocks, softwood lodgepole pine, without detoxification. No prior studies made such an attempt. Eliminating detoxification can avoid sugar losses and simplify the ethanol production process by using combined fermentation of the enzymatic and pretreatment hydrolysates. It can pave the way for future simultaneous enzymatic saccharification and combined fermentation with pretreatment hydrolysate. Mass and energy balance data obtained from such a study that accounts for both the cellulose and hemicellulose fractions can be used for more accurate economic analysis with commercial significance.

Section snippets

Experimental

The present experimental study from tree harvesting to fermentation was carried out according to the flow diagram shown in Fig. 1. The processes connected with dashed lines were not carried out in the present study.

Saccharide recovery and the formation of fermentation inhibitors

Saccharide recovery and the formation of fermentation inhibitors can be used to assess the effectiveness of a pretreatment as they affect downstream processing. The SPORL pretreatment retained about 88% of the glucan on solid substrate and removed about 95% of the xylan and 97% of the mannan (Table 1), suggesting the pretreatment effectively fractionated the cellulose from the hemicelluloses. Separate enzymatic hydrolysis at 10% substrate solids indicated that over 90% of the retained glucan

Conclusion

This study demonstrated the potential for robust ethanol production from softwood by SPORL pretreatment through fermenting the combined enzymatic and non-detoxified pretreatment hydrolysates using S. cerevisiae Y5. The adapted Y5 strain has good inhibitor tolerance and is capable of metabolizing furans while maintaining high ethanol productivity. Excellent ethanol yield was achieved without detailed pretreatment optimization. Future studies will focus on higher ethanol titer production using

Acknowledgements

This work is supported by the US Forest Service (USFS) and the Ministry of Science and Technology (MOST) of China through Joint Venture Agreement FS#09-CO-11111122-091. The joint venture provided financial support to S. Tian for her visiting appointment at the USFS, Forest Products Laboratory (FPL). The USFS Program of Woody Biomass, Bioenergy, and Bioproducts (WBBB, 2009) provided financial support to X. Luo for his visiting appointments at the University of Wisconsin-Madison and USFS-FPL. We

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