Pilot-scale conversion of lime-treated wheat straw into bioethanol: quality assessment of bioethanol and valorization of side streams by anaerobic digestion and combustion

Ronald HW Maas¹^,4 , Robert R Bakker¹ , Arjen R Boersma² , Iemke Bisschops³ , Jan R Pels² , Ed de Jong¹^,5 , Ruud A Weusthuis¹ and Hans Reith²

¹Agrotechnology and Food Sciences Group, Wageningen University and Research Centre, PO Box 17, 6700 AA Wageningen, The Netherlands

²Energy Research Centre of The Netherlands, Biomass, Coal and Environmental Research, PO Box 1, 1755 ZG Petten, The Netherlands

³Lettinga Associates Foundation, PO Box 500, 6700 AM Wageningen, The Netherlands

⁴Nobilon Bacteriological R&D, P.O. Box 320, 5830 AH Boxmeer, The Netherlands

⁵Avantium Technologies BV, Zekeringstraat, 1014 BV Amsterdam, The Netherlands

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Biotechnology for Biofuels 2008, 1:14doi:10.1186/1754-6834-1-14


Published:	12 August 2008

Abstract

Introduction

The limited availability of fossil fuel sources, worldwide rising energy demands and anticipated climate changes attributed to an increase of greenhouse gasses are important driving forces for finding alternative energy sources. One approach to meeting the increasing energy demands and reduction of greenhouse gas emissions is by large-scale substitution of petrochemically derived transport fuels by the use of carbon dioxide-neutral biofuels, such as ethanol derived from lignocellulosic material.

Results

This paper describes an integrated pilot-scale process where lime-treated wheat straw with a high dry-matter content (around 35% by weight) is converted to ethanol via simultaneous saccharification and fermentation by commercial hydrolytic enzymes and bakers' yeast (Saccharomyces cerevisiae). After 53 hours of incubation, an ethanol concentration of 21.4 g/liter was detected, corresponding to a 48% glucan-to-ethanol conversion of the theoretical maximum. The xylan fraction remained mostly in the soluble oligomeric form (52%) in the fermentation broth, probably due to the inability of this yeast to convert pentoses. A preliminary assessment of the distilled ethanol quality showed that it meets transportation ethanol fuel specifications. The distillation residue, which contained non-hydrolysable and non-fermentable (in)organic compounds, was divided into a liquid and solid fraction. The liquid fraction served as substrate for the production of biogas (methane), whereas the solid fraction functioned as fuel for thermal conversion (combustion), yielding thermal energy, which can be used for heat and power generation.

Conclusion

Based on the achieved experimental values, 16.7 kg of pretreated wheat straw could be converted to 1.7 kg of ethanol, 1.1 kg of methane, 4.1 kg of carbon dioxide, around 3.4 kg of compost and 6.6 kg of lignin-rich residue. The higher heating value of the lignin-rich residue was 13.4 MJ thermal energy per kilogram (dry basis).