Improving anaerobic digestion of sugarcane straw for methane production: Combined benefits of mechanical and sodium hydroxide pretreatment for process designing Leandro Janke a,b,⇑ , Sören Weinrich a , Athaydes F. Leite c , Filippi K. Terzariol a , Marcell Nikolausz c , Michael Nelles a,b , Walter Stinner a a Department of Biochemical Conversion, Deutsches Biomasseforschungszentrum gemeinnützige GmbH, Torgauer Straße 116, 04347 Leipzig, Germany b Faculty of Agricultural and Environmental Sciences, Chair of Waste Management, University of Rostock, Justus-von-Liebig-Weg 6, 18059 Rostock, Germany c Department of Environmental Microbiology, Helmholtz Centre for Environmental Research – UFZ, Permoserstraße 15, 04318 Leipzig, Germany article info Article history: Available online xxxx Keywords: Sugarcane straw Sodium hydroxide pretreatment Methane yield Process designing Economic assessment abstract Sodium hydroxide (NaOH) as an alkaline pretreatment method to enhance the degradation kinetics of sugarcane straw (SCS) for methane production was investigated with a special focus on the benefits for designing the anaerobic digestion process. For that, SCS was previously homogenized by milling in 2 mm particle size and pretreated in NaOH solutions at various concentrations (0, 3, 6 and 12 g NaOH/100 g SCS) and the methane yields were determined in biochemical methane potential (BMP) tests. The obtained experimental data were used to simulate a large-scale semi-continuous process (100 ton SCS day À1 ) according to a first-order reaction model and the main economic indicators were cal- culated based on cash flows of each pretreatment condition. The BMP tests showed that by increasing the NaOH concentration the conversion of the fibrous fraction of the substrate to methane was not only accelerated (higher a value), but also increased by 11.9% (from 260 to 291 mL CH 4 gVS À1 ). By using the experimental data to simulate the large-scale process these benefits were translated to a reduction of up to 58% in the size of the anaerobic reactor (and consequently in electricity consumption for stirring), while the methane yield increased up to 28%, if the liquid fraction derived from the pretreatment process is also used for methane production. Although the use of NaOH for substrate pretreatment has consider- ably increased the operational expenditures (from 0.97 up to 1.97 € Â 10 6 year À1 ), the pretreatment method was able to increase the profitability of methane production from SCS since a sensitivity analysis by varying the prices of anaerobic reactor, methane and NaOH showed a less attractive payback, net pre- sent value and internal rate of return for the control condition (0 g NaOH/100 g SCS) in all analyzed scenarios. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Sugarcane is the most produced agricultural commodity in the world. It is mainly cultivated in tropical and sub-tropical regions of Brazil, India, Pakistan, Thailand and China for sugar, bioethanol and bioelectricity production [1]. Sugarcane straw (SCS), also known as tops and trash, is generated by the mechanized harvest system which has been gradually implemented in substitution to the pre-harvest burning system due to different environmental, agronomic and economic reasons [2]. According to the typical Brazilian conditions SCS corresponds to approximately one third of the total primary energy of the cane [3]. Therefore, nowadays new developments are aiming to add value to such type of biomass fraction with the intention to diversify the product portfolio of the sugarcane plants in a biorefinery concept [4,5]. Anaerobic digestion (AD) is a promising strategy to treat such type of biomass, since methane and/or platform chemicals for value-added products could be produced as a result of different biochemical phases: hydrolysis, acidogenesis, acetogenesis and methanogenesis [6–8]. However, it is well known that hydrolysis is often the rate-limiting step during AD when fibrous material such as SCS is used as feedstock due to the presence of lignin, which prevents the action of microorganisms and enzymes by its hydrophobic and recalcitrance nature [9–11]. Thus, resulting in lower gas yields and longer hydraulic retention times (HRT), http://dx.doi.org/10.1016/j.enconman.2016.09.083 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Department of Biochemical Conversion, Deutsches Biomasseforschungszentrum gemeinnützige GmbH, Torgauer Straße 116, 04347 Leipzig, Germany. E-mail address: leandro.janke@dbfz.de (L. Janke). Energy Conversion and Management xxx (2016) xxx–xxx Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman Please cite this article in press as: Janke L et al. Improving anaerobic digestion of sugarcane straw for methane production: Combined benefits of mechan- ical and sodium hydroxide pretreatment for process designing. Energy Convers Manage (2016), http://dx.doi.org/10.1016/j.enconman.2016.09.083