Torrefaction of wheat and barley straw after microwave heating Sangram Kishor Satpathy a,b , Lope G. Tabil b,⇑ , Venkatesh Meda b , Satya Narayana Naik a , Rajendra Prasad a a Center for Rural Development and Technology, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India b Department of Chemical and Biological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9, Canada highlights  Microwave irradiation was used for the torrefaction of wheat and barley straw.  Barley straw tended to carbonize more than wheat straw under microwave irradiation.  Energy density increased in both wheat and barley straw after torrefaction.  Biomass grindability and hydrophobicity improved significantly after torrefaction. article info Article history: Received 4 November 2013 Received in revised form 28 January 2014 Accepted 29 January 2014 Available online 14 February 2014 Keywords: Torrefaction Microwave Straw Grindability Hydrophobicity abstract Microwave irradiation was used in this study for the torrefaction of wheat and barley straw. The torre- faction effect was studied by varying the microwave power level (200–300 W), reaction time (10–20 min) and moisture content of biomass (5–15%). Mass yield and energy yield of the torrefied biomass was determined. Fuel properties like H/C and O/C ratio were assessed from elemental composition. Grinding characteristics and hydrophobicity of the torrefied sample were studied and compared with the raw biomass. Barley straw tended to carbonize more under microwave irradiation with 29.1% increase in the C content against 16.2% in the case of wheat straw when torrefied at 300 W for 20 min. Both H/C and O/C ratio decreased with increase in power and reaction time. The energy density increased by 14–15% in wheat straw and 21–23% in barley straw under suitable reaction condition. Mass and energy yields were 64.0–97.8% and 73.8–98.4%, respectively for wheat straw. In barley straw, mass and energy yields were 42.7–97.4% and 52.5–97.3%, respectively. Moisture content of the biomass did not affect the reaction as much as other parameters and the mass yields were comparable between different moisture contents. Grindability of the biomass improved significantly after torrefaction. The particle size ratio between torrefied and untreated straw after grinding was 0.66 and 0.61 for wheat and barley, respectively. The torrefied biomass was more hydrophobic and the moisture uptake was reduced by 61–68% under suitable torrefaction condition. Microwave irradiation can be used effectively for torrefaction of the two biomass investigated at moderate power and short process time. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction In recent years, biomass has been sought after as a major source of renewable energy. Biomass is a source of sustainable carbon– neutral energy and has potential to play an important role in the future. Biomass is by far the most important source of renewable energy today, accounting for about 10% of total primary energy use and 78% of total renewable energy [1]. Among all the types of biomass sources, agricultural residues are the most promising in terms of their abundance and non-association with the food versus fuel problem [2]. Many agricultural residues are poorly utilized and often burnt in open fields causing massive air pollution. However, these valuable resources can be converted to green chemicals and biofuels with the use of suitable technologies. There are many thermo-chemical conversion technologies, includ- ing carbonization, torrefaction, pyrolysis, and gasification, that have been researched and developed to treat agricultural waste [3]. The products of these methods can be further upgraded into various useful biofuels to generate heat and electricity [4]. Untreated biomass materials are known to possess certain disadvantages such as high water content, hydrophilic nature, low calorific value, low energy density, poor grindability, high transportation cost due to high bulk, low combustion efficiency, and thermal instability during combustion because of high oxygen content. Torrefaction is one of the pretreatment methods which can address most of these inherent issues and upgrade untreated biomass to a higher quality and more attractive biofuel [5]. http://dx.doi.org/10.1016/j.fuel.2014.01.102 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 306 966 5317; fax: +1 306 966 4777. E-mail address: lope.tabil@usask.ca (L.G. Tabil). Fuel 124 (2014) 269–278 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel