Technical assessment of bioenergy recovery from cotton stalks through anaerobic digestion process and the effects of inexpensive pre-treatments Mehrdad Adl a,c , Kuichuan Sheng a,⇑ , Arash Gharibi b a Bioenergy and Biomaterials Research Group, College of Biosystems Engineering and Food Science, Zhejiang University, 310058 Hangzhou, China b University of Shanghai for Science and Technology, 516 Jungong Rd., 200093 Shanghai, China c Renewable Energy Department, Niroo Research Institute (NRI), 14665 Tehran, Iran article info Article history: Received 5 May 2011 Received in revised form 20 August 2011 Accepted 27 November 2011 Available online 28 December 2011 Keywords: Biogas Lignocellulosic Hydrothermal Energy balance Methane yield Cotton abstract Methane yields from cotton stalks have been reported in the range of 0.022–0.150 Nm 3 /kg-VS in the pre- ceding studies. In this study, hydrothermal pre-treatment of cotton stalks was performed under atmo- spheric conditions prior to anaerobic digestion process through which, three agents were examined; hot water, dilute ammonia solution containing 2% NH 3 (dry matter basis), and recycled liquid from anaer- obic process. All three options increased methane yield but the highest results (over 0.200 Nm 3 /kg-VS) were obtained by utilization of recycled process liquid (PL) under an adjusted ammonia content and C:N ratio, which was achieved by dilution of PL with water as well as application of stabilized swine sludge as a nitrogen rich inoculum. The energy balance evaluation of the overall process revealed that bioenergy recovery from pre-treated cotton stalks by anaerobic digestion is feasible and viable only at feed streams with above 10% solids content, methane yield above 0.175 Nm 3 /kg-VS and with heat recov- ery from hydrothermal pre-treatment tank. A methane yield of 0.184 Nm 3 /kg-VS was observed under semi-continuous feeding of PL treated stalks to a hybrid plug-flow pilot reactor. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Cotton stalks as the on-farm post-harvesting residues of cotton shrubs are counted as a major source of agricultural biomass with a large coefficient of residues to the main crop yield (e.g. equal to 3 in China [1]) as well as globally widespread harvesting areas [2]. Comprehensive descriptions of cotton stalks have been dissemi- nated in the literature [3–8]. Depletion of water resources [9], propagation of pests and plant pathogens on the abandoned stalk piles [8] and emission of air pollutants from the on-site burning of stalks [8,10] are identified as the major environmental impacts of cotton’s agro-industry. Several technical options including parti- cle board fabrication [10], pulp and paper production [10–12], den- sification for solid bio-fuels [8], pyrolytic oil production [13–15], direct combustion [7,8,16], composting [17], gasification [16], bio- logical extraction of chemicals [10,12] and anaerobic digestion [18–24] have been employed for utilization of cotton stalks in var- ious scales. Cotton stalks like many other crop residues are potentially a major source of pentose (C-5) and hexose (C-6) sugars therefore, they can biologically produce suitable energy carriers like meth- ane, biohydrogen, bioethanol or other high value chemicals [25]. Nevertheless, they are naturally recalcitrant against enzymatic hydrolysis, the first step in any biological conversion, due to their lignocellulosic structure [26]. Pre-treatment of lignocellulosic materials aims at liberating the main structural constituents (cel- lulose, hemicellulose and lignin) and disruption of their original structure in order to facilitate biological hydrolytic breakdowns [26–29]. Several pre-treatment experiments for cotton stalks have been reported in the literature such as delignification by combina- tion of soda and organic solvents (e.g. ethanol and anthraquinone) [12] or sodium borohydride (NaBH 4 ) kraft pulping [11] targeting on pulp and paper industry, fractionation by steam explosion [24] and saccharification by alkaline (soda-based) and combined alkaline/microwave pre-treatment [30]. Silverstein et al. [26] examined pre-treatment of cotton stalks by sulfuric acid, sodium hydroxide, hydrogen peroxide, and ozone for the purpose of etha- nol production. Among numerous utilization alternatives, anaerobic digestion (AD) of cotton stalks has not been studied as deeply as some other options like solid bio-fuel or bio-based materials production. It has a background at least from two decades ago when El-Shinnawi et al. investigated the biogas generation from pulverized cotton stalks in various circumstances. Their experimental configurations comprised of a mixture of cotton stalks and partially digested cat- tle dung [18], a mixture of cotton stalks with poultry wastes including chicken manure and saw dust [19], and AD of elutriate from micro aerobically pre-digested stalks with cow dung at differ- 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.11.065 ⇑ Corresponding author. Tel./fax: +86 571 88982192. E-mail addresses: madl49@yahoo.com (M. Adl), kcsheng@zju.edu.cn (K. Sheng). Applied Energy 93 (2012) 251–260 Contents lists available at SciVerse ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy