Optimization of uncatalyzed steam explosion pretreatment of rapeseed straw for biofuel production

Highlights

• Ethanol production from steam exploded rape straw at high solids loading is reported.

• Operation at high solids loading (20% w/v) allows complete conversion of cellulose.

• High ethanol concentrations (5% v/v) are obtained, allowing industrial operation.

Abstract

Rapeseed straw constitutes an agricultural residue with great potential as feedstock for ethanol production. In this work, uncatalyzed steam explosion was carried out as a pretreatment to increase the enzymatic digestibility of rapeseed straw. Experimental statistical design and response surface methodology were used to evaluate the influence of the temperature (185–215 °C) and the process time (2.5–7.5 min). According to the rotatable central composite design applied, 215 °C and 7.5 min were confirmed to be the optimal conditions, considering the maximization of enzymatic hydrolysis yield as optimization criterion. These conditions led to a maximum yield of 72.3%, equivalent to 81% of potential glucose in pretreated solid. Different configurations for bioethanol production from steam exploded rapeseed straw were investigated using the pretreated solid obtained under optimal conditions as a substrate.

As a relevant result, concentrations of ethanol as high as 43.6 g/L (5.5% by volume) were obtained as a consequence of using 20% (w/v) solid loading, equivalent to 12.4 g ethanol/100 g biomass.

Introduction

Biofuels production can be a sustainable alternative to fossil energy sources as well as a solution to combat climate change (Gupta, 2014). Lignocellulose biomass like forest and agricultural residues can be considered an interesting feedstock for second generation bioethanol production without competition with food or feed uses and more environmentally friendly. They are renewable, abundant, cheap and they can be alternative renewable resources. As a source of raw materials, rapeseed opens up a multitude of possibilities in the food and feed industry, as well as in the energy and material sectors (UFOP, 2014). The land surface planted with rapeseed in the world has increased from approximately 26 million hectares in 2000 to more than 36 million hectares in 2013 (FAOSTAT, 2015). Rapeseed straw can be left on the fields to maintain soil health. Currently, the greatest demand for straw is for burning in power stations (Ryden et al., 2014). Besides, this cellulosic biomass may constitute a renewable energy resource for fuel ethanol production by biological conversion.

The first step in the conversion of lignocellulosic biomass into fuels or chemicals typically involves a biomass pretreatment step in order to make the cellulose easier to hydrolyze by removing the hemicellulose, reducing the crystallinity of cellulose, and increasing the surface area (Wettstein et al., 2012). Effective pretreatment is fundamental to a successful enzymatic hydrolysis (EH) since thus the saccharification of the pretreated feedstock is enhanced by improving the accessibility of enzymes to the cellulose fibers (Limayem and Ricke, 2012). Steam explosion is recognized as a low-cost option for agricultural residues pretreatment with significantly lower environmental impact since the addition of external chemicals is not necessary (Singh et al., 2015). Moreover, it is considered as an effective pretreatment that causes the breakdown of lignocellulose structure which results in the partial hydrolysis of hemicellulose and disruption of the lignocellulosic matrix by the sudden pressure drop (Romaní et al., 2013). Although an acid presoaking can improve the treatment, the highly acetylated nature of some lignocellulosic materials allows for uncatalyzed (or autocatalyzed) steam pretreatment due to the release of acetyl groups and formation of acetic acid (Alvira et al., 2010). In addition, steam explosion pretreatment without external catalyst addition avoid the additional chemical costs and associated problems derived of equipment corrosion. Thus, the uncatalyzed alternative means a significantly lower environmental impact for the chemical utilization (Romaní et al., 2013). Uncatalyzed steam explosion has previously been shown to be very effective for the pretreatment of others lignocellulosic materials like wheat straw (Ballesteros et al., 2006), Eucalyptus globulus (Romaní et al., 2013) or corn stover (Lu et al., 2010).

The performance of the subsequent enzymatic hydrolysis and fermentation at a high solids loading allows to increase the final ethanol concentration. It is essential to improve the economy of the process by reducing the energy consumption in the distillation step (Larsen et al., 2008). However, mixing difficulties in the reactor due to the high viscosity can occur (Sassner et al., 2006) and the yeast can be inhibited reducing ethanol yield in the simultaneous saccharification and fermentation step or even the cessation of ethanol production (Hoyer et al., 2013).

This works deals with the steam explosion pretreatment of rapeseed straw, evaluating the effect of the temperature and process time by the response surface methodology. The performance of enzymatic hydrolysis and fermentation processes at high solids loading was tested using the solid pretreated obtained under optimal steam explosion conditions as a substrate.