Short Communication Thermodynamic prediction of hydrogen production from mixed-acid fermentations Andrea K. Forrest ⇑ , Melinda E. Wales, Mark T. Holtzapple Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, United States article info Article history: Received 6 April 2011 Received in revised form 2 August 2011 Accepted 3 August 2011 Available online 10 August 2011 Keywords: Hydrogen Carboxylic acid fermentation MixAlco Gibbs free energy Thermodynamic analysis abstract The MixAlco™ process biologically converts biomass to carboxylate salts that may be chemically con- verted to a wide variety of chemicals and fuels. The process utilizes lignocellulosic biomass as feedstock (e.g., municipal solid waste, sewage sludge, and agricultural residues), creating an economic basis for sus- tainable biofuels. This study provides a thermodynamic analysis of hydrogen yield from mixed-acid fer- mentations from two feedstocks: paper and bagasse. During batch fermentations, hydrogen production, acid production, and sugar digestion were analyzed to determine the energy selectivity of each system. To predict hydrogen production during continuous operation, this energy selectivity was then applied to countercurrent fermentations of the same systems. The analysis successfully predicted hydrogen produc- tion from the paper fermentation to within 11% and the bagasse fermentation to within 21% of the actual production. The analysis was able to faithfully represent hydrogen production and represents a step for- ward in understanding and predicting hydrogen production from mixed-acid fermentations. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction With increasing energy demand, decreasing oil supply, and con- tinuously accumulating waste in landfills, interest in converting lignocellulosic biomass to liquid and gaseous fuels has grown. The MixAlco™ is a flexible and cost-effective process (Agbogbo and Holtzapple, 2007). It employs a mixed culture of naturally occurring microorganisms to ferment biomass into carboxylate salts that can be converted into a wide array of chemicals (e.g., ketones, esters, alcohols) and fuels (e.g., jet fuel and gasoline) (Granda et al., 2009). The downstream processing steps of the MixAlco™ process necessitate the addition of hydrogen. Although hydrogen is the most abundant element, generation of gaseous hydrogen for hydrogenation reactions can be costly, with steam reforming and electrolysis being the two most common methods. New technolo- gies for hydrogen production are being investigated, and microbial hydrogen production is a good alternative. Studies on fermentative hydrogen production have been conducted mostly using pure cul- tures (Evvyernie et al., 2001; Fabiano and Perego, 2002), although it is also a key intermediate from the anaerobic degradation of waste by a mixed culture (Lay et al., 1999; Mizuno et al., 2000; Sparling et al., 1997). It is difficult to predict hydrogen production from a mixed-acid fermentation. It has been reported in the literature (Chen et al., 2006; Demirel et al., 2010; Kawagoshi et al., 2005), although with wide fluctuations. Several models have been developed to predict fermentation end-products of both mixed acids and hydrogen (Offner and Sauvant, 2006; Rodriguez et al., 2006; Van Milgen, 2002; Xing et al., 2006), but with variable success. Every cellular reaction involves energy changes measured as Gibbs free energy. Because cells are very efficient, the net change in Gibbs free energy approaches zero for the sum of catabolic and anabolic reactions (Henry et al., 2006). The amount of catabolic energy microorganisms need to meet the anabolic demand depends on the specific fermentation system (e.g., temperature, pH, and substrate); therefore, hydrogen production is governed by the catabolic change in Gibbs free energy for a given system. (Heijnen, 1994; Tijhuis et al., 1993). This study uses the catabolic Gibbs free energy determined during batch fermentations of two feedstocks (paper and bagasse) to predict hydrogen production during continuous fermentations. 2. Methods 2.1. Fermentors Stainless-steel fermentors (SS) were used for all hydrogen fer- mentations. Each fermentor consisted of a 6-in.-long 4-in.-diameter Sch-10 stainless steel pipe sealed on each end with a 1/8-in.-thick stainless steel plate. The top plate of the fermentor was fitted with a quick-connect fitting (McMaster-Carr #4322K163), a 2-in. gasket (McMaster-Carr #4509K15) and cap (McMaster-Carr #4322K222) held in place by a tightened clamp (McMaster-Carr #4322K153). A 1 = 4 -in. quick-disconnect self-sealing valve was inserted into the 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.08.018 ⇑ Corresponding author. Address: Department of Chemical Engineering, Texas A&M University, 200 Jack E Brown Bldg., College Station, TX 77843, United States. Tel.: +1 979 862 1175; fax: +1 979 845 6446. E-mail address: akf8179@gmail.com (A.K. Forrest). Bioresource Technology 102 (2011) 9823–9826 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech