Structure and saccharification of rice straw pretreated with sulfur trioxide micro-thermal explosion collaborative dilutes alkali

Abstract

In this paper, a sulfur trioxide collaborative dilutes alkali method has been developed to pre-treat rice straw and it has been studied that the pre-treated rice straw structure affected the saccharification of the rice straw hydrolyzed by cellulose enzymatic hydrolysis. The results show that the reaction of the sulfur trioxide with rice straw resulted in the internal micro-thermal explosion, and the saccharification rate was 91% based on the pretreated rice straw with sulfur trioxide for 4 h following 1% w/v NaOH treatment for 7 h at 50 °C.

Introduction

There is an estimated annual worldwide production of 10–50 billion dry tons, accounting for about half of the global biomass yield (Zhao et al., 2009). However, straw is available worldwide in large quantities as natural waste or a nutrient for livestock but it becomes gradually a promising substrate as starting ingredient applied in fermentable sugar (Yu et al., 2009).

However, conversion of lignocelluloses to monomer sugars is a complex process, the substrate’s chemistry, heterogeneity, crystallinity, and surface area strongly affect the kinetics of the enzymatic conversion (Ahola et al., 2008). Up to now, different pretreatment methods to change lignocellulosic structure can be used to enhance the enzymatic saccharification of lignocellulosic material, such as milling (Sato et al., 2009, Zhu et al., 2009), dilute acid (Taherzadeh and Karimi, 2007), steam explosion (Mukhopadhyay and Fangueiro, 2009), ammonia fiber explosion (AFEX) (Kumar and Wyman, 2009, Taherzadeh and Karimi, 2008), and dilute alkali and so on (Das and Chakraborty, 2009, Goswami et al., 2009, Li et al., 2009). In general, lignin removal and the crystallinity reduction of lignocellulose are helpful to improve in cellulose hydrolysis and reduce the amount of enzyme preparation (Chen et al., 2010). At present, there are still weaknesses in pretreatment technologies, such as large energy consumption and high cost of investment.

In this work, a new approach to the destruction of the straw in a normal pressure has been proposed: first sulfur trioxide gas diffused into the internal structure and reacted with the water inside the straw producing the internal micro-thermal explosion; second, dilute alkali solution was used to remove the lignin. Therefore, sulfur trioxide collaborative dilutes alkali method destructed the internal structure of the lignin and increased the saccharification rate of the pretreated straw.