Bioethanol production from rapeseed straw at high solids loading with different process configurations Juan Carlos López-Linares, Inmaculada Romero ⇑ , Cristóbal Cara, Encarnación Ruiz, Manuel Moya, Eulogio Castro Department of Chemical, Environmental and Materials Engineering, Agrifood Campus of International Excellence, ceiA3, University of Jaén, 23071 Jaén, Spain highlights  Rapeseed straw is attracting great interest as raw material for fuel production.  Three strategies are compared for the first time in rapeseed straw.  Operation at high solids loading (20%) allows feedstock full use.  Ethanol concentrations as high as 5% are obtained. article info Article history: Received 8 November 2013 Received in revised form 4 December 2013 Accepted 8 January 2014 Available online 21 January 2014 Keywords: Bioethanol High solids loading Presaccharification Rapeseed straw SSF abstract Rapeseed is an important source of oil for biodiesel production. Nevertheless, the residues of the cultiva- tion are lacking of practical applications. As a lignocellulosic material, their conversion into ethanol can be of interest. In this work, different process configurations, separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), and prehydrolysis and simultaneous saccharifica- tion and fermentation (PSSF), were compared at high solids loading (7.5%, 15% and 20% w/v) to produce ethanol from rapeseed straw pretreated by sulfuric acid. Results show that the highest ethanol concentration (39.9 g/L) was obtained from SHF configuration at the highest substrate loading (20% w/v). This product concentration is high enough for distillation pur- poses from an economic point of view. The final ethanol concentrations and yields did not differ signif- icantly between SSF and PSSF regardless of the solids loading and, for 7.5% and 15% (w/v) solids loading were slightly higher than those attained in the SHF. However, at the highest solids loading the separate process appears to be more favorable. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Over 34 million hectares of rapeseed were cultivated word wide in 2012 [1], mainly dedicated to oil production for the biodiesel industry. Rapeseed straw is a lignocellulosic agricultural residue with relatively high sugar content – near 60% – that makes it an interesting raw material for second-generation ethanol production [2]. Ethanol from rapeseed straw would contribute to the biorefin- ery development based in this crop [3], whose concept involved the exploitation of the largest possible part of the biomass [4]. In recent years, numerous research efforts have been made in the technologies for improving the production of ethanol from different lignocellulosic materials, being bioprocesses based on enzymatic hydrolysis the most interesting alternative [5,6]. Pretreatment, hydrolysis and fermentation are the main steps involved in these processes. Increasing solids loading in both the hydrolysis and fermentation steps is one of the most important challenges to make biofuels production more economical [4] be- cause of the reduction in ethanol distillation cost [7]. However, increasing solids concentration has also associated drawbacks as larger levels of inhibiting compounds [8], end-product inhibition [9], diffusional enzyme problems [10], stirring and mixing limita- tions by viscosity increase [11] or possible mass transfer limita- tions appearing above 20% insoluble solids concentration [12]. The production of ethanol can be accomplished following sev- eral process strategies, and it is useful to compare them in order to select the best one in this particular case. They include separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF) and prehydrolysis and simultaneous sac- charification and fermentation (PSSF). SHF involves two sequential steps, enzymatic saccharification of pretreated cellulose and sugars fermentation to ethanol. This configuration allows working at opti- mal operating conditions for enzymes and microorganisms. SSF 0016-2361/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2014.01.024 ⇑ Corresponding author. Tel.: +34 953212779. E-mail address: iromero@ujaen.es (I. Romero). Fuel 122 (2014) 112–118 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel