Pretreatment of rice straw for ethanol production by a two-step process using dilute sulfuric acid and sulfomethylation reagent
Introduction
As the world population has grown and more countries have industrialized, energy consumption has increased steadily [1]. Petroleum is currently being used as a major energy source, but it poses great concerns in terms of its future utilization because of its resources limitation, increasing costs, and the associated environmental issues [1], [2], [3]. Therefore, there is great interest in exploring alternative energy sources to maintain the sustainable growth of society [4], [5], [6]. Ethanol, as a clean and renewable energy source, which can be produced from sugars through fermentation has drawn much attention in recent years. Ethanol is now one of the most widely used transport bio-fuels in the world and its consumption as transport fuel has kept increasing steadily [3], [4]. Apart from an alternative to traditional energy source, ethanol can also be a versatile chemical and organic solvent. However, the production of ethanol through fermentation has been limited using the current sugar cane or maize starch-based technology because of raw materials shortage and high cost [4], [5], [6]. A potential method for solving this problem is to utilize lignocellulosic materials such as agricultural wastes [4], [5], [6], [7], [8]. Production of ethanol from rice straw (RS), one of the most abundant agricultural wastes, has been extensively studied, but its high production cost still prevents its commercialization based on current technology [9], [10], [11]. Therefore, it is of great importance in improving the present technology and decreasing its cost. The RS bio-refinery is an effective way to achieve this goal by fully utilizing its components and co-producing high value-added chemicals [5], [12], [13], [14].
Production of ethanol from RS generally includes three sub-processes: pretreatment, saccharification and ethanol fermentation. The RS is mainly composed of cellulose, hemicellulose and lignin. The complex structure of lignin and hemicellulose and cellulose in RS limits its effective saccharification. Hence, some pretreatment procedures need to be performed before its saccharification. Traditionally, the purpose of its pretreatment is to remove lignin and hemicellulose, to reduce cellulose crystallinity, and to increase the porosity, thus improving its saccharification efficiency [15], [16], [17]. Various methods have been used for its pretreatment to achieve this goal [18], [19], [20], [21], [22]. However, most of these pretreatment methods focus only on improving its saccharification efficiency based on its cellulose utilization, and little attention has been paid on the recovery and utilization of its hemicellulose and lignin. Currently, the most studied ethanol production process from RS is utilizing its cellulose and hemicellulose for ethanol production and burning its lignin residues for electricity [11]. This leads to high ethanol production cost and also causes some environmental problems. In order to decrease the ethanol production cost from RS, it is necessary to fully utilize its components and co-produce high value-added products by combining its pretreatment, fraction of its components and recovery of useful chemicals together, which is often called RS bio-refinery. The objective of this work is to establish such a RS bio-refinery for decreasing its ethanol production cost. In this work, RS was first treated with dilute sulfuric acid to remove its hemicellulose and recover xylose as a feedstock for xylitol production, and then the residue was treated with the standard sulfomethylation reagent to remove its lignin and recover its hydrolyzate containing lignosulfonate as a cement water reducer, finally the simultaneous saccharification and fermentation (SSF) of the two-step treated RS to produce ethanol. The acid treatment (AT) and sulfomethylation treatment (ST) conditions were optimized based on their pretreatment effectiveness and useful chemicals recovery. The SSF of the two-step treated RS for ethanol production was examined and a brief comparison between our process and the currently most studied ethanol production process was also made.
Section snippets
Materials and methods
All experiments were carried out three times, and the data reported were expressed as the mean values ± standard deviation. The RS composition or its hydrolyzed residues were expressed on a wet basis throughout this work.
The first step pretreatment: acid treatment and xylose recovery
The AT is one of the most widely-used RS pretreatment methods for ethanol production [29], [30]. In order to reduce acid consumption and achieve good pretreatment effectiveness, most of AT are carried out at relatively higher temperature and pressure, which leads to high equipment requirements and difficulty in recovery of xylose form its hydrolyzate [29], [30]. Some studies have indicated that the AT can achieve good pretreatment effectiveness under relatively mild conditions, but higher acid
Conclusions
This work investigated a novel two-step RS pretreatment process to improve its ethanol production economy by co-producing high value-added products. In the first step pretreatment, AT of RS removed its hemicellulose and xylose was recovered from the AT hydrolyzate with 83.2% yield. In the second step pretreatment, ST of the acid treated RS removed its lignin and the ST hydrolyzate containing 5.0% lignosulfonate was directly recovered as a cement water reducer. The SSF of the two-step treated RS
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 21176196) and the Collaborative Innovation Center of Catalysis Materials of Hubei Province.
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