Efficient Conversion of Rice Straw to Bioethanol Using Sodium Carbonate Pretreatment S. M. Amin Salehi, † Keikhosro Karimi,* ,†,‡ Tayebeh Behzad, † and Nafiseh Poornejad † † Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran ‡ Industrial Biotechnology Group, Institute for Biotechnology and Bioengineering, Isfahan University of Technology, Isfahan 84156-83111, Iran ABSTRACT: In this study, rice straw was treated with sodium carbonate prior to enzymatic hydrolysis and fermentation. All pretreatments were performed in a high-pressure reactor at 90, 120, 150, and 180 °C by 0.25, 0.5, and 1 M sodium carbonate (Na 2 CO 3 ) solution. The reactor was designed to inject the straw to the reactor at a desired temperature without any preheating effects. The reactor content was continuously mixed, and samples were taken at different time periods. Afterward, enzymatic hydrolysis of the untreated and all treated straws were conducted at 45 °C for 72 h with enzyme loading of 20 FPU cellulase and 30 IU β-glucosidase per gram of substrate. The best pretreatment conditions were obtained to be 0.5 M Na 2 CO 3 at 180 °C for 120 min. This pretreatment improved the released glucose from 9.5 g/L for the untreated straw to 43.5 g/L for the treated one. On the other hand, the treatment showed significant improvement on ethanol production from rice straw applying simultaneous saccharification and fermentation. Ethanol production was enhanced from 90.2 to 351.4 g/L by the treatment. The analysis showed that the treatment with sodium carbonate at elevated temperature can significantly reduce the lignin and xylan contents and the cellulose crystallinity and also convert cellulose type I to type II, which is more amenable for enzymatic hydrolysis. 1. INTRODUCTION According to the Energy Information Administration, world energy consumption would increase from 505 quadrillion Btu in 2008 to 619 and 707 quadrillion Btu in 2020 and 2035, respectively, which shows almost a 53% rise. 1 To meet this demand and decrease various environmental pollutions, lignocellulosic materials are suggested as the most available promising alternative for biofuel production. 2 Rice straw is one of the most abundant lignocelluloses available in all over the world which is inexpensive and mainly unused. 3 According to a report by FAO (2011), the annual rice production would reach 721 million tons in 2011, which means an increase by 3% from 2010 to 2011. An amount of 1-1.5 kg of rice straw is produced per kilogram of harvested grain. This huge amount of straw has a high potential for bioethanol production. 4 Cellulose, the main component of lignocellulosic materials, can be enzymatically hydrolyzed to glucose and then fermented to bioethanol. However, lignocellulosic materials have a very complex structure with high crystalline cellulose protected by lignin and hemicelluloses; thus, only up to 20% yield of sugars can be achieved by enzymatic hydrolysis of native lignocellu- loses. 5 For an efficient hydrolysis, a pretreatment process should be considered as a decisive step to modify the structure and remove lignin and hemicellulose. 6-9 This pretreatment process is nowadays considered as a main key for economically feasible ethanol production from lignocelluloses. 10 Different pretreatment methods such as aqueous-ammonia soaking pretreatment, 5,11 microwave pretreatment, 12 dilute acid pretreatment, 13 ultrasonic pretreatment, 14 hot-compressed water pretreatment, 15 and alkaline pretreatment 16,17 were used to improve saccharification of rice straw. All physical methods suffer from low e fficiency and high energy consumption. In addition, despite having high yields, chemical methods including acid or alkali pretreatments have some operational problems such as corrosive characteristics, need for neutralization, and environmental hazards. To overcome the problems mentioned above, inorganic salts such as sodium carbonate can be used as a pretreatment agent. It is an inexpensive and widely available chemical with no negative environmental effects. This salt has been previously used by some researchers for pretreatment of lignocellulosic materials. 18 However, promising results were obtained only when sodium carbonate has been used in the wet oxidation method. Bjerre et al. (1996) showed that addition of sodium carbonate in wet oxidation can significantly improve the digestibility of wheat straw. 19 In another study, Schmidt and Thomson (1998) applied wet oxidation by addition of sodium carbonate to disintegrate various components of wheat straw. The optimum conditions for 60 g/L straw were 6.5 g/L Na 2 CO 3 , at 185 °C and 12 bar O 2 for 15 min reaction time. Under these conditions, 50% of lignin and 80% of hemicellulose were dissolved, while more than 95% of cellulose remained in the solid state. 20 Thus, the treatment is potentially a promising method for pretreatment of lignocelluloses. To our knowledge, no previous work was performed on rice straw which contains high amounts of ash (mainly in the form of silica) and shows di fferent behavior in pretreatment compared to other lignocelluloses. The main purpose of this study was pretreatment of rice straw using sodium carbonate in a high-pressure reactor. Effects of the pretreatment on improvement of enzymatic hydrolysis Received: September 9, 2012 Revised: October 25, 2012 Published: November 1, 2012 Article pubs.acs.org/EF © 2012 American Chemical Society 7354 dx.doi.org/10.1021/ef301476b | Energy Fuels 2012, 26, 7354-7361